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Transcript of Towards more efficient Praseodymium Doped Fibre Amplifiers … · Uitnodiging Tot het bijwonen van...

Uitnodiging

Tot het bijwonen van deopenbare verdediging van

mijn proefschrift

Towards more efficientPraseodymium Doped

Fibre Amplifiers forthe O-band

op dinsdag 7 november 2006

om 16:00 uur

De promotie vindt plaats inhet Auditorium van deTechnische Universiteit

Eindhoven.

Na afloop van deze plechtigheid zal er eenreceptie plaatsvinden

waarvoor u ook van hartewordt uitgenodigd.

Ronald SchimmelVoermanstraat 601973 VL IJmuiden

0255 52 31 [email protected]

PMS Yellow 012 C + zwart 266 pag. = 18 mm rug GLANS-laminaat oplage: 500 stuks

Towards more efficient Praseodymium Doped Fibre Amplifiers

for the O-Band

Ronald Schimmel

Towards m

ore efficient Praseodymium

Doped Fibre A

mplifiers for the O

-Band R

onald Schim

mel

Towards more efficient

Praseodymium Doped Fibre Amplifiers

for the O-Band

Towards more efficient

Praseodymium Doped Fibre Amplifiers

for the O-Band

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van de

Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor

Promoties in het openbaar te verdedigen

op dinsdag 7 november 2006 om 16.00 uur

door

Ronald Christiaan Schimmel

geboren te Veldhoven

Dit proefschrift is goedgekeurd door de promotoren:

prof.ir. G.D. Khoeenprof.dr.ir. R.G.C. Beerkens

Copromotor:dr.ir. H. de Waardt

CIPDATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN

Schimmel, Ronald C.

Towards more efficient praseodymium doped fibre amplifiers for the O-band / byRonald Christiaan Schimmel. Eindhoven : Technische Universiteit Eindhoven, 2006.Proefschrift. ISBN-10: 90-386-1903-0ISBN-13: 978-90-386-1903-3NUR 959Trefw.: optische versterkers / glasvezels / glastechnologie / lanthaniden /chalcogenen / optische telecommunicatiesystemen.Subject headings: optical fibre amplifiers / glass fibres / optical fibre fabrication /praseodymium / chalcogenide glasses / optical communication.

The work described in this thesis was carried out at the Department of ElectricalEngineering of the Eindhoven University of Technology and was financially supportedby TNO and the COBRA Research Institute.

Druk: Printservice Technische Universiteit Eindhoven

Aan mijn Ouders& Robin

Summary

Due to their potentially high gain efficiency (i.e. high signal gain per unit pumppower), rare earth doped amplifiers are considered as suitable travelling-wave am-plifiers for optical communication systems. In order to fully exploit the transparentwindows of the installed fibre, and particularly the 1.31 m wavelength region, apowerful amplifier technique for these wavelengths is needed in the near future.In the 12901340 nm wavelength range, the praseodymium doped fibre amplifier(PDFA) is commercially available since the late 1990s, however the efficiency of theseamplifiers (based on fluoride host glasses) is low. In this study, the objective is thedevelopment of a PDFA with improved efficiency, based on sulphide host glasses. Fur-thermore, the objective is to assess the performance (i.e. gain, saturation and noisecharacteristics) of PDFAs in telecommunication systems deploying the O-band (thewavelenght range between 1290 and 1340 nm) by means of modelling and measure-ment of e.g. the gain and noise figure.

Praseodymium doped germanium gallium sulphide host materials

Using the pure chemical elements as a starting material, germanium gallium sulphideglasses were prepared by melting at 1000 C in vitreous silica ampoules. Purifica-tion of the raw materials, especially sulphur, is essential to obtain highly transparenthost glasses, especially for the praseodymium doped fibre amplifiers pump and sig-nal wavelength ranges (i.e. 10101040 nm and 12901340 nm, respectively). Thegermanium gallium sulphide glasses are transparent in the 0.58.0 m wavelengthrange. The wavelength dependency of the refractive index of the glasses was deter-mined by ellipsometry and measurement of the Brewster angle. The guiding of lightwithin a glass fibre is determined by, among others, the refractive index of the coreand cladding glasses. The refractive index of the developed glasses, in the amplifierspump and signal wavelength ranges, is approximately 2.07 and is slightly dependentof the composition.The radiative (absorption and emission) properties of the praseodymium in the ger-manium gallium sulphide host glass were quantitatively characterised using Judd-Ofelt theory. The Judd-Ofelt parameters, for the optical transitions within thepraseodymium ions, were used to establish the signal emission cross section and ex-cited state absorption cross sections. Besides absorption or emission of photons, theenergy state of the praseodymium ion can change by non-radiative (e.g. multi-phonon)

i

ii Towards more efficient praseodymium doped fibre amplifiers for the O-band

energy transfer to the host glass. The energy difference between the 1G4 and3F4

state of praseodymium in the germanium gallium sulphide glasses is approximately2945 cm1, while the effective phonon energy of the host glass is limited to 490 cm1.The (measured) emission lifetime of the 1G4

3H5 transition is approximately 400 s.The estimated multi-phonon relaxation rate of the 1G4 level is approx. 1150 s

1 (atroom temperature), which corresponds to a non-radiative lifetime of 870 s.

Fibre drawing of chalcogenide glasses

The difference between the glass transition temperature and the crystallisation tem-perature (below this temperature no crystallisation will occur within normal timespans) is important for the stability of the germanium gallium sulphide glasses dur-ing forming processes. The glass transition temperature, of the developed glasses,is approximately 360 C and the crystallisation temperature is circa 185 C higherthan the glass transition temperature. From the measured viscosity temperaturerelation, it is concluded that the viscosity of the glass is sufficiently low to draw fibreswell below the crystallisation temperature.The relatively simple preform technique for fibre drawing was selected in favour of thecrucible drawing process, due to the availability of glass rods, fabricated by melting insealed ampoules. In this work, the methods suitable for preparation of a germaniumgallium sulphide fibre preform, based on the rod-in-tube process, were investigated.In the rod-in-tube process, the fibre preform is constructed by inserting a (thin) rodof core sulphide glass into a cladding tube of sulphide glass with slightly differentcomposition. Prior to assembly of the preform, the quality of the surface is improvedby chemical etching.A hot deformation process for production of germanium gallium sulphide claddingglass tubes is presented. In this process, the visco-elastic properties of the glass areused. Rods of cladding glass are heated inside a mould and converted into tubes bypenetration of a needle under isothermal conditions. Selection of the working tem-perature is critical to prevent cracking of the material (at low temperature and highviscosity), crystallisation (due to high temperatures) or collapse upon removal of theneedle (at low viscosity). Thin core glass rods could be prepared by stretching of coreglass rods (as melted) to the desired diameter in the fibre drawing tower.Etching of the glass surfaces, prior to assembly of the fibre preform and fibre drawing,is needed to limit the formation of defects at the core cladding interface during fibredrawing and obtain fibres with low optical losses. Aqueous, alkaline solutions can beused to etch the surface of germanium gallium sulphide glasses. In this study, thebest etching results are obtained using diluted caustic soda (0.02 M). The etching rateof more concentrated solutions (>0.5 M) is too high for controllable etching. Due tonegligible reactivity between (concentrated) acids and germanium gallium sulphideglasses, these glasses can not be etched with acids. Hence, (concentrated) acids canbe used only to clean contaminated glass surfaces, without impairments to the glasssurface.The assembly of germanium gallium sulphide preforms (with core-cladding structure)and the fibre drawing of these preforms is rather complicated and could not be realised

Summary iii

in the course of this study, due to the limited availability of suitable glass sampleswith the required dimensions.A mathematical model is used to determine suitable process conditions for fibre draw-ing of germanium gallium sulphide glasses. This twodimensional model is based onaxi-symmetry of the preform and includes the solution of the equations of continuity,motion and heat transfer and their appropriate boundary conditions for the drawingof glass fibres from a preform was presented. Furthermore, the properties of the ger-manium gallium sulphide glass are incorporated in the model.The model was used to calculate the temperature, stresses and velocity profile inthe neckdown, for various fibre drawing velocities and furnace temperatures. Theworking range (i.e. the applicable fibre drawing rate as a function of furnace temper-ature) for fibre drawing of germanium gallium sulphide glasses is derived from thesecomputed results. The accuracy of the model results possibly can be improved bydetailed modelling of both radiative heat transfer within the glass and radiative heattransfer between the glass and the furnace. It is concluded that the fibre drawing ofgermanium gallium sulphide glasses is possible, however the narrow working area isbound by crystallisation (which limits the maximum applicable furnace temperature)and the axial stresses occurring in the fibre (which limits the applicable fibre drawingspeed).Uncladded germanium gallium sulphide fibres with a diameter between 200 and300 m and a length of 17.5 m were obtained using a pilot scale fibre drawing tower.Based on the modelling results, it is expected that germanium gallium sulphide fibreswith a core diameter of less than 4 m and a cladding diameter of less than 150 mcan be drawn using this equipment by improved process control.

Optical properties of germanium gallium sulphide fibres

The attenuation losses at the amplifiers pump and signal wavelengths of these fibres,with composition Ge28.8Ga1.2S70.0, were determined, too. The applied praseodymiumdopant concentration is 370 ppm (mg/kg). The signal attenuation loss at 1300 nm ofthis fibre is 0.2 dB/cm. The attenuation at 1030 nm equals 0.43 dB/cm, which canbe partly contributed to pump ground state absorption at this wavelength. The peakwavelength of the spontaneous emission spectrum is located at 1335 nm, the 3 dBbandwidth is approximately 70 nm.

Performance and design of PDFAs for telecommunication systems

The system performance of a PDFA in an optical transmission system operating at1.3 m was investigated experimentally. The PDFA, based on a fluoride glass fi-bre, was constructed using commercially available components. The performance isanalysed for different amplifier applications (booster, in-line and pre-amplifier) andconfigurations (co-propagating and bi-directional) at bit rates up to 10 Gbit/s. Thegain of the experimental PDFA (based on fluoride fibre) is independent of the polar-isation state of the signal light. When the experimental PDFA is used in a boosterapplication, the amplifier is saturated at high signal input powers. As saturation of

iv Towards more efficient praseodymium doped fibre amplifiers for the O-band

the PDFA does not result in bit pattern dependent fluctuations of the signal gain,PDFAs are well suited for as booster amplifiers.An optical recirculating loop was used to evaluate the transmission performance of asystem incorporating multiple PDFAs (operated as in-line amplifier) and fibre spans(of 212.5 km length) at bit rates of 622 Mbit/s and 10 Gbit/s. Increasing the num-ber of roundtrips (i.e. distance) results in degradation of the signal to noise ratio.This degradation is explained by the power self regulation process i.e. the averageoutput power of the PDFA remains constant while amplified spontaneous emission(ASE) noise power increases at the expense of signal power. A total distance ofcirca 2000 km (80 roundtrips) can be bridged at a bit rate of 622 Mbit/s. How-ever, pre-amplification of the signal at the receiver is necessary. At higher bit rates(e.g. 10 Gbit/s) error free transmission is obtained only for distances up to 25 km,due to the lack of optical pre-amplification at the receiver and the limited sensitivityof the 10 Gbit/s receiver (-15.5 dBm) compared to that of the 622 Mbit/s receiver(32 dBm).Using the PDFA as a pre-amplifier, the receiver sensitivity is enhanced. The sen-sitivity of the receiver with fluoride fibre based PDFA pre-amplifier at bit rate of10 Gbit/s is approximately -30 dBm, which equals a 15 dB improvement comparedto the sensitivity of the receiver without pre-amplifier. The small signal gain is circa20 dB, while the noise figure, derived from these experiments, is 9 dB.

Modelling light amplification in praseodymium doped single-mode fibres

A spatially and spectrally resolved amplifier model, based on the four-level operationof praseodymium, is used to study amplifier operation using the optical properties ofthe praseodymium dopant and the host glasses in different configurations. The (steadystate) amplifier model describes the evolution of signal, ASE and pump power as afunction of distance within the praseodymium doped fibre. In addition, the noisefigure is calculated.The outcome of the model was validated by measurements of the gain and noise fig-ure of a PDFA assembled from commercially available components. The static gainand noise figure of this PDFA (based on fluoride host glasses) were determined usingmonochromatic continuous wave test signals, applying the interpolation-substractiontechnique on the optical output spectrum. In general, the gain as determined by theamplifier model is lower than the measured gain while the trends in the amplifiercharacteristics are well described by the model.When no pump power is applied, the wavelength dependent losses caused by signalground state absorption are slightly overestimated by the amplifier model. This re-sults in an underestimation of the gain by the model, especially at lower pump powers.The maximum measured gain is located at somewhat shorter wavelengths than themaximum gain calculated by the amplifier model. The model confirmed the smalldifferences observed in the measurements of the gain and noise figure for the amplifierin co-propagating, counter-propagating and bi-directional configurations.Taking the limited accuracy of the optical properties and the experimental errorsof the gain measurements into account, the results of the model have been found

Summary v

in trendwise agreement with the experimental data. The model is considered to besuitable for the design and development of PDFAs based on other glass compositions(such as praseodymium doped germanium gallium sulphide glasses) or geometricalconfigurations.The design of the amplifier for booster and pre-amplifier configurations, based on ger-manium gallium sulphide glasses, is optimised using the fibre amplifier model. Themain parameters involved in the optimisation of the praseodymium doped fibre itself,considered here, are numerical aperture, core radius and length. The optimum lengthof the praseodymium doped fibre, to obtain maximum gain for a given pump power,is inversely proportional to the dopant concentration. For a praseodymium dopedsulphide glass fibre with a core diameter of 4 m, which can be obtained in pilot scalefibre drawing, calculated maximum gain is obtained for a numerical aperture of 0.182and a cut-off wavelength of 950 nm. For example, the calculated saturated signaloutput power of a booster amplifier is 30 dBm using 1700 mW pump power, whilethe calculated small signal gain obtained by a sulphide based preamplifier is 30 dBfor 165 mW pump power.The calculated maximum gain is obtained at a signal wavelength of 1342 nm, whichmatches the peak of the emission spectrum of praseodymium doped germanium gal-lium sulphide glasses. The output power of a saturated PDFA, based on sulphidefibre, is nearly independent of input power, i.e. the slope of the gain curve is approx-imately -1 dB/dBm. This phenomenon allows for stabilisation of the signal powerwithin a transmission link containing several in-line amplifiers.The performance of the PDFA based on germanium gallium sulphide host materialand fluoride host materials are compared using the amplifier model. In general, thecalculated quantum conversion efficiency (defined as the increase in the number ofsignal photons as a result of amplification divided by the number of launched pumpphotons) and gain efficiency of sulphide amplifiers are much higher than the efficiencyof fluoride amplifiers. For example, the calculated quantum conversion efficiency ofa booster amplifier (signal input power 0 dBm, signal output power 20 dBm) basedon sulphide host glasses is 0.5 mW/mW which is twice as large as that of a fluoridebased booster amplifier. The calculated gain efficiency of a pre-amplifier (signal inputpower -30 dBm, gain 20 dB) based on sulphide host glasses is 0.134 dB/mW higherthan that of a fluoride based amplifier. Due to the higher efficiency of the sulphideamplifier, less pump power is needed by this amplifier to obtain high gain. Due tothe longer emission lifetime of praseodymium in the sulphide glass host compared tofluoride host glasses (i.e. about 400 versus 130 s), the efficiency of amplifiers basedon sulphide host glasses is much higher, leading to advantageous reduction of therequired pump power. The maximum gain wavelength of the sulphide amplifier is ap-proximately 1340 nm, and is shifted to longer wavelengths compared to the maximumgain wavelength of the fluoride amplifier, which is approximately 1310 nm. This shiftof the maximum gain to longer wavelengths is considered as a drawback for the useof sulphide host glass for the PDFA in telecommunication systems operating around1.31 m.

vi Towards more efficient praseodymium doped fibre amplifiers for the O-band

Conclusions

Based on the observed performance of the experimental PDFA based on fluoride hostglass and the high, calculated efficiency of the PDFA based on sulphide glass, it is ex-pected that the PDFA based on germanium gallium sulphide glass will be a promisingcandidate as fibre amplifier for the O-band (12901340 nm). However, experimentaltechniques should be improved to allow production of single mode praseodymiumdoped fibre based on germanium gallium sulphide glasses with a core diameter of lessthan 4 m and absorption and scattering losses of less than 0.1 dB/m.As powerful pump lasers (operating in the wavelength range 10101040 nm) have be-come available, the development of PDFA with high gain and high saturation outputpower is feasible. When the PDFA based on sulphide host glasses becomes available,the choice whether to apply a PDFA based on sulphide host glasses or based on flu-oride host glasses is strongly dependent on the intended application.Due to the high quantum conversion efficiency, the PDFA based on sulphide hostglasses offers a potentially better performance in booster applications (e.g. amplifi-cation of (analogue) CATV signals) than the PDFA based on fluoride host glasses.In contrast with semi-conductor amplifiers and Raman amplifiers, the PDFA is wellsuited for the amplification of amplitude modulated signals, because signal distortionby the saturated amplifier (e.g. pattern effect) is negligible.The signal output power of the saturated PDFA, based on a sulphide fibre, will benearly independent of signal input power, which is a favourable phenomenon whenused as an in-line amplifier. The ability of stabilisation of the signal power within atransmission link containing several in-line amplifiers is more pronounced for PDFAsbased on sulphide host glasses compared to PDFAs based on fluoride host glasses.Due to its high small signal gain and low noise figure, the PDFA can also be used asa pre-amplifier to enhance receiver sensitivity. In this application, the performanceof the PDFA based on either sulphide or fluoride host glasses will be similar and thedifference in gain efficiency is less decisive.

Contents

1 Introduction 1

1.1 Optical amplifiers in optical telecommunication systems . . . . . . . . 11.2 Optical amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.1 Rare-earth doped fibre amplifiers . . . . . . . . . . . . . . . . . 41.2.2 Other types of optical amplifiers . . . . . . . . . . . . . . . . . 11

1.3 General application of optical amplifiers . . . . . . . . . . . . . . . . . 121.4 Scope and objective of this thesis . . . . . . . . . . . . . . . . . . . . . 14

1.4.1 Outline of this thesis . . . . . . . . . . . . . . . . . . . . . . . . 151.5 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Praseodymium doped germanium gallium sulphide glasses 19

2.1 Preparation of the glass host . . . . . . . . . . . . . . . . . . . . . . . 202.1.1 The glass forming region . . . . . . . . . . . . . . . . . . . . . . 202.1.2 Preparation methods . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2 Optical properties of the host glass . . . . . . . . . . . . . . . . . . . . 232.2.1 IR transmittance spectra . . . . . . . . . . . . . . . . . . . . . 252.2.2 UV VIS NIR transmittance spectra . . . . . . . . . . . . . . . 282.2.3 Refractive index . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.3 Optical properties of praseodymium in the host glass . . . . . . . . . . 362.3.1 Electronic structure . . . . . . . . . . . . . . . . . . . . . . . . 372.3.2 Absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . 382.3.3 Photoluminescence spectroscopy . . . . . . . . . . . . . . . . . 392.3.4 Radiative transitions . . . . . . . . . . . . . . . . . . . . . . . . 48

2.4 Interactions between host glass and dopant . . . . . . . . . . . . . . . 552.4.1 Structure and phonon energy . . . . . . . . . . . . . . . . . . . 562.4.2 Non-radiative transitions . . . . . . . . . . . . . . . . . . . . . 57

2.5 Thermal properties of the host glass . . . . . . . . . . . . . . . . . . . 602.5.1 Glass transition and crystallisation temperatures . . . . . . . . 612.5.2 Thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . 642.5.3 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672.7 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 692.8 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

vii

viii Towards more efficient praseodymium doped fibre amplifiers for the O-band

3 Fibre drawing of chalcogenide glasses 77

3.1 Structure of an optical fibre . . . . . . . . . . . . . . . . . . . . . . . . 783.2 Chalcogenide glass fibre drawing techniques . . . . . . . . . . . . . . . 79

3.2.1 Preform method . . . . . . . . . . . . . . . . . . . . . . . . . . 793.2.2 Double crucible method . . . . . . . . . . . . . . . . . . . . . . 823.2.3 Germanium gallium sulphide preforms . . . . . . . . . . . . . . 823.2.4 Etching of germanium gallium sulphide glasses . . . . . . . . . 84

3.3 Fibre drawing of germanium gallium sulphide glasses . . . . . . . . . . 883.3.1 Fibre drawing model . . . . . . . . . . . . . . . . . . . . . . . . 893.3.2 Fibre drawing operating conditions . . . . . . . . . . . . . . . . 1133.3.3 Experimental fibre drawing . . . . . . . . . . . . . . . . . . . . 115

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1193.5 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1203.6 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

4 Modelling light amplification in praseodymium doped single-

mode fibres 125

4.1 Praseodymium doped fibre amplifier model . . . . . . . . . . . . . . . 1264.1.1 Operation principle of the PDFA . . . . . . . . . . . . . . . . . 1284.1.2 Propagation equations . . . . . . . . . . . . . . . . . . . . . . . 1304.1.3 Light guiding in single-mode fibres . . . . . . . . . . . . . . . . 1324.1.4 Amplifier noise analysis . . . . . . . . . . . . . . . . . . . . . . 1334.1.5 Numerical solution of the model equations . . . . . . . . . . . . 137

4.2 Model validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384.2.1 Experimental determination of gain and noise . . . . . . . . . . 1384.2.2 Experimental set-up used for amplifier model validation . . . . 1414.2.3 Co-propagating pumping scheme . . . . . . . . . . . . . . . . . 1434.2.4 Counter-propagating pumping scheme . . . . . . . . . . . . . . 1464.2.5 Bi-directional pumping scheme . . . . . . . . . . . . . . . . . . 149

4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494.3.1 Amplifier model input data . . . . . . . . . . . . . . . . . . . . 1524.3.2 Amplifier model results . . . . . . . . . . . . . . . . . . . . . . 1534.3.3 Validity of the amplifier model . . . . . . . . . . . . . . . . . . 157

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584.5 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1594.6 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

5 Performance and design of the PDFA for telecommunication

systems 163

5.1 PDFAs in communication systems . . . . . . . . . . . . . . . . . . . . 1645.1.1 Booster amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . 1665.1.2 In-line amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . 1725.1.3 Pre-amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

5.2 Optical properties of the germanium gallium sulphide glass fibre . . . 1835.2.1 Attenuation at signal and pump wavelengths . . . . . . . . . . 183

Contents ix

5.2.2 Spontaneous emission . . . . . . . . . . . . . . . . . . . . . . . 1855.3 Design of an efficient PDFA . . . . . . . . . . . . . . . . . . . . . . . . 186

5.3.1 Design of the praseodymium doped fibre . . . . . . . . . . . . . 1895.3.2 Large signal amplifier . . . . . . . . . . . . . . . . . . . . . . . 1915.3.3 In-line amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . 1975.3.4 Pre-amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1985.3.5 Performance of PDFA, based on different host glass types . . . 202

5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2085.5 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2105.6 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

6 Conclusions and recommendations 213

6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2146.1.1 Main results of this study . . . . . . . . . . . . . . . . . . . . . 2146.1.2 Prospects for a PDFA based on germanium gallium sulphide

glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2186.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

A Summary of differential operations in cylindrical coordinates 221

A.1 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

B Specifications of optical components 223

B.1 Praseodymium doped fluoride fibre modules . . . . . . . . . . . . . . . 223B.2 Dual output ytterbium fibre laser . . . . . . . . . . . . . . . . . . . . . 224

C Light guiding in single-mode fibres 225

C.1 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

D Noise in optical amplifiers 227

D.1 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

Scientific publications related to this thesis 235

List of abbreviations 237

Samenvatting 239

Dankwoord 247

Curriculum Vitae 249

x Towards more efficient praseodymium doped fibre amplifiers for the O-band

Chapter 1

Introduction

1.1 Optical amplifiers in optical telecommunication

systems

While the first optical systems deployed only a single wavelength channel for transmis-sion of the data signals, nowadays multiple wavelengths are simultaneously transmit-ted over a single fibre. The principle of wavelength division multiplexing (WDM), atechnique to combine the output of several transmitters (each operating at a differentwavelength channel) is shown in Figure 1.1. The signals are transferred throughoutthe network from transmitter to receiver, while routing of the signals (i.e. directinga specific signal towards one receiver out of a multitude receivers) is performed byelectronics incorporated in the optical part of the network [1].As a signal light pulse propagates through the transmission fibre, the duration of thepulse can change (broadening or narrowing) due to dispersion1. Furthermore, the sig-nal is attenuated by scattering and absorption. After a certain distance, the numberof photons within the signal becomes too small for detection and therefore amplifi-cation of the signal is necessary. Ideally, the amplifier must be capable of amplifyinghigh-speed WDM signals, providing uniform gain within a broad wavelength region,without distortion. An all-optical (so-called travelling-wave) amplifier can amplify allWDM channels within its bandwidth simultaneously.Historically, optical telecommunication systems operate at spectral regions (windows)where the attenuation spectrum of the used optical fibre exhibits low intrinsic losses(see Figure 1.2). The telecommunication regions around 1.31 m and 1.55 m areseparated by the presence of an absorption band around 1.4 m caused by hydroxyl(OH) impurities within the silica fibre (especially in earlier fibre types). In the1.55 m wavelength region the absorption losses and Raleigh scattering losses areminimum (approx. 0.22 dB/km). In standard single mode fibres (s-SMF), the dis-persion is crossing the zero dispersion point around 1.31 m. Near the dispersion

1Dispersion is caused by small differences in speed of the spectral components within a signal ofa finite duration t and finite spectral width [2].

1

2 Towards more efficient praseodymium doped fibre amplifiers for the O-band

1

2

n

1 2 n + + ... +

1 2 nWavelength

Pow

er

...

...

WDM

Input fibre 1 ... n Output fibre

a) b)

Figure 1.1: Wavelength division multiplexing (WDM) is a technique for increasingthe transmission capacity C in the fibre to n B bits/s where n is the numberof deployed wavelength channels and B the transmission rate in # bits/s. a) theprinciple of wavelength division multiplexing (schematically) and b) optical spectrumof the WDM-signal in the output fibre.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7-25

-20

-15

-10

-5

0

5

10

15

20

25

Wavelength [m]

Att

enuat

ion

[dB

/km

]

Dis

per

sion

[ps/

(nmk

m)]

Figure 1.2: Example of the attenuation due to absorption losses and dispersion (ex-pressed as the broadening per unit spectral width per unit length of the fibre) in(silica based) standard single mode fibre (s-SMF).

Introduction 3

1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65

Wavelength [m]

SOA

DRA

O-band E-band S+-band S-band C-band L-band L+-band

2nd window 3rd window

Silica based EDFA

Telluride based EDFA

Fluoride based EDFA

Terbium co-doped TDFATDFA

Fluoride based PDFA

Chalcogenide based PDFA

NDFA

DDFA

Figure 1.3: An overview of the naming and allocation of wavelength bands in the2nd and 3rd telecommunication windows. The (fibre) amplifiers commonly applied atthese wavelengths are also indicated.

zero-point, a higher transmission capacity can be obtained [2]. The absorption lossesin the 1.31 m wavelength region are approximately 0.3 dB/km.In the early 1980s, the application of distributed Raman amplifiers (DRA) and semi-conductor optical amplifiers (SOA) to overcome attenuation losses were investigated.In the mid 1980s rare earth doped fibre amplifiers were developed. Whereas DRA andSOA can be designed to operate at the desired wavelengths, rare earth doped fibreamplifiers are limited to the spectral regions coupled to the electron transitions whichoccur within the rare earth ions. Due to their potentially high efficiency, rare earthdoped amplifiers find a major field of application as a travelling-wave amplifier foroptical communication systems. Especially, the erbium doped fibre amplifier (EDFA)caused a breakthrough in the implementation of rare earth doped fibre amplifiers.The position of the 2nd and 3rd telecommunication windows and their partition in(wavelength) bands is depicted in Figure 1.3. The 1st window is located around 0.8 m(not shown). The O-band (12901340 nm) is positioned in the 2nd window. The 3rd

window is divided in 5 equal width wavelength bands: S+-band (14501490 nm), S-band (14901530 nm), C-band (15301570 nm), L-band (15701610 nm) and L+-band(16101650 nm). The wavelength region between the 2nd and 3rd telecommunicationwindows is covered by the E-band. Currently, no rare earth doped fibre amplifiers areavailable for this wavelength region. Due to the increasing demand for data trans-mission capacity, it is expected that the entire wavelength region between 1290 and1650 nm will be used in near future.The operating wavelengths corresponding to the most important rare earth dopedamplifiers are also shown in Figure 1.3. The gain band of the EDFA (initially de-veloped in 1987) is located in the C-band [3]. Further developments of the EDFAresulted in the L-band amplifier (1580-1620 nm) [4]. Recently, the thulium dopedfibre amplifier (TDFA, 1450-1510 nm) operating in the S and S+-bands has become

4 Towards more efficient praseodymium doped fibre amplifiers for the O-band

0

1 12 2

3 3

3Ground state

Meta-stable state

Pump excited state

Three-level system Four-level system

Ener

gy[c

m

1]

Figure 1.4: Energy level diagram indicating the principle of operation of rare earthdoped amplifiers: 1) Pump ground state absorption, 2) Signal emission, 3) Non-radiative decay.

available [5]. Praseodymium doped fibre amplifiers (PDFA), based on fluoride glassesare commercially available since 1995 [6], while alternative host glasses for the PDFAare currently being investigated. Also, neodymium doped fibre amplifiers (NDFA)and dysprosium doped fibre amplifiers (DDFA) are being developed for applicationin the O-band. In this study, the use of germanium gallium sulphide host glasses forthe PDFA is investigated in order to develop an efficient PDFA.

1.2 Optical amplifiers

1.2.1 Rare-earth doped fibre amplifiers

Rare earth ions that can be excited to a high energy state, which decays by emissionof radiation around the telecommunication wavelengths, can be used in optical ampli-fiers. These amplifiers are optically pumped by means of a laser to excite the rareearth dopant ions into the so-called pump excited state (see Figure 1.4). The lifetime(defined as the average time spent by ions in a specific excited state) of the pumpexcited state is short and the ions quickly decay to a lower energy manifold. Dueto its relatively long lifetime, this energy level is called the meta-stable state. Am-plification of the signal is obtained by, so-called, stimulated emission starting fromthe meta-stable state. In this process, illumination of the rare earth ion by incom-ing photons (e.g. signal), with an energy which equals the energy difference betweenthe meta-stable state and a lower energy level, results in de-excitation (decay) ofthe meta-stable state. The energy is released by emission of photons with exactlythe same wavelength as the incident photons [7, 8]. These emitted photons are inphase and radiated in a confined direction (coherent radiation [7, 8])2. Dependingon the type of rare earth ion and the radiative transition involved, the final level of

2A detailed description of the quantum mechanics related to the occurrence of spontaneous emis-sion can be found in e.g. Yariv [7] or Mandel et al. [8].

Introduction 5

Signal Amplified Signal

WSCPump

Rare EarthDoped Fibre

Figure 1.5: Schematic diagram of the rare earth doped fibre amplifier. Signal andpump light are combined by the WSC and fed into the rare earth doped fibre. (Notethat the secondary output of the WSC is not used.)

the stimulated emission transition is either an intermediate level or the ground state.By definition, the amplifier operation is designated as a four-level system, if the finallevel of the stimulated emission transition is an intermediate level, while the groundstate is the terminal state of a three-level system [9].A schematic representation of the rare earth doped fibre amplifier is shown in Figure1.5. The rare earth doped fibre is preceded by a wavelength selective coupler (WSC)which is used to combine the output of the pump laser with the input signal. Thesecondary output of the WSC can be used for monitoring the signal and pump power.The simplified energy level diagrams (i.e. only a limited number of energy levels andtransitions between them is shown) of praseodymium (Pr), neodymium (Nd), dys-prosium (Dy), erbium (Er), and thulium (Tm) are shown in Figure 1.6. The ionicenergy levels are labelled according to the Russell-Saunders notation for the angularmomentum properties of the ion (see section 2.3.1). In the host glass, the rare earthion is subjected to electric fields due to the surrounding atoms in the host lattice [10].The local electric fields around the rare earth ion, known as crystal fields, cause split-ting of the energy levels into multiplets (so-called Stark splitting). When the rareearth ions are incorporated in an amorphous material (e.g. glass host), the splittingis different for each site, i.e. at each position of the rare earth ions in the material.Stark splitting and site-to-site variations of the field result in the formation of energybands around the discrete energy levels (not shown).The main transitions (i.e. signal emission, ground state absorption (GSA), excitedstate absorption (ESA), amplified spontaneous emission (ASE) and non-radiative de-cay) between the energy levels are indicated in Figure 1.6. When an electron dropsto a lower energy level, the energy is either released by phonons (non-radiative decay,e.g. lattice vibrations) or in the form of photons (radiative decay). The efficiency ofthe amplifier is reduced by spontaneous transitions from the meta-stable state to alower energy level, as these transitions do not contribute to the amplification of thesignals by stimulated emission. The lifetime of the meta-stable state should be suffi-ciently long in order to limit the number of spontaneous transitions to lower energylevels.In the 3rd telecommunication window (around 1.55 m), an efficient amplifier can beconstructed by doping silica fibre with erbium (Er) ions. The trivalent praseodymium(Pr), neodymium (Nd) and dysprosium (Dy) ions have radiative transitions withinthe 2nd telecommunication window (around 1.31 m). Unfortunately, the radiative

6 Towards more efficient praseodymium doped fibre amplifiers for the O-band

(luminescence) properties of these ions cannot be fully utilised in a silica matrix (sil-ica) fibre. Due to non-radiative energy transfer from the meta-stable state to thenext lower energy level by lattice vibrations (phonons) of the glass host, the quantumefficiency of the radiative transitions is reduced. For praseodymium, the relaxationenergy3 (i.e. the energy difference between the levels 1G4 and next lower

3F4 level)is approx. 3000 cm1 (see Figure 1.6a). The highest phonon energy in a silica glasshost is approx. 1100 cm1. Hence, in a silicate glass only three of these phonons needto be emitted simultaneously, in order to bridge the energy gap [11]. Depopulationof the excited state by non-radiative energy transfer results in an efficiency loss. Toimprove the efficiency of these amplifiers, glass hosts with lower maximum phononenergy are required. As the number of phonons required to bridge this energy gapbecomes higher, the probability of simultaneous emission of phonons decreases, hencethe relaxation rate by non-radiative decay is lower.Fluoride and chalcogenide glasses are suitable low phonon energy host materials forefficient amplification due to their (maximum) phonon energy of 500 and 330 cm1,respectively. Silicate glasses belong to the class of oxide glasses. The chalcogenideelements sulphur (S), selenium (Se) and tellurium (Te) can adopt the role of oxygenas the main anion in the glass structure. Stable chalcogenide glasses are obtainedby vitrification starting from a glass melt. Fluoride glasses are based on the glassforming ability of heavy metal fluorides [12].Currently, commercially available PDFAs are based on fluoride host glasses. It isexpected that PDFAs, based on chalcogenide glasses, will have a better efficiency dueto the low phonon energy of these host glasses. In this thesis, the main focus will beon the development of a praseodymium doped fibre amplifier based on germaniumgallium sulphide host glasses. The preparation and properties of the germanium gal-lium sulphide glasses used in this study, will be described in chapter 2. Due to thelow phonon energy of this host glass and the resulting long lifetime of the meta-stablestate, the development of an efficient praseodymium doped fibre amplifier seems fea-sible.

Praseodymium Doped Fibre Amplifier

The amplification of telecommunication signals by the Praseodymium Doped FibreAmplifier (PDFA) around 1.3 m is based on the process of stimulated emission be-tween the 1G4 and

3H5 energy levels of the Pr3+ ions. The transitions between the

energy levels involved in the amplification process are shown in the simplified energydiagram, Figure 1.6a. The lower energy level of the stimulated emission transition inpraseodymium is the intermediate level 3H5. By definition, praseodymium effectivelyrepresents a four-level system [16], although merely three levels are involved (3H4,3H5 and

1G4).The electrons in the Pr3+ ions are pumped around 1.03 m from the ground state(3H4) into the pump excited state (

1G4). The1G4 state is also the meta-stable state.

3Photon energy and atomic energy levels relative to a zero energy reference are expressed in termsof electron volts (eV) or wavenumbers (cm1) [2]. 1

= E

hccan be used to convert the units of energy

E from electron volt (eV) into wavenumber (cm1).

Introduction 7

To a large extent the efficiency of Pr-doped fibre amplifiers is related to the longemission lifetime of the meta-stable state (approx. 110 s in fluoride and 350 s inchalcogenide glasses [17]). This permits the large population inversion (almost allions are in a meta-stable excited state, instead of the ground state) needed to achievehigh gain, in the case pumping is sufficiently strong.The radiative transition from the 1G4 to the

3H5 level, around 1.31 m, can beinduced by other photons with equal energy, e.g. telecommunication signals. Theemitted photons will have exactly the same frequency and phase as the incident sig-nal photons. Furthermore, the photons are radiated in a confined direction [7, 8].Thus, the process of stimulated emission provides amplification of the telecommuni-cation signal. After emission, the Pr3+ ion returns from the 3H5 state to the

3H4ground state by fast non-radiative decay.Processes which reduce the population of the 1G4 level (other than stimulated emis-sion) reduce the efficiency of the amplifier. The 1G4 -

3H5 transition will also takeplace spontaneously and randomly (not induced by signal photons). The uncorrelated,spontaneous emitted photons are further amplified along the amplifier, resulting inamplified spontaneous emission (ASE) noise. Pump and signal Excited State Absorp-tion (ESA, absorption of a photon by an ion which is already in the excited state) canalso cause depopulation of the 1G4 meta-stable state. Furthermore, the

1G4 level isdepopulated by non-radiative energy transfer to lower energy levels. The energy gapbetween 1G4 level and the next-lower

3F4 is approximately 3000 cm1. The efficiency

loss due to multi-phonon relaxation can be reduced by the use of a low phonon energyglass host (see chapter 2 for the measured phonon energy of the germanium galliumsulphide host glass).Due to the length of the fibre amplifier, photon densities and densities of occupied

states will vary in the longitudinal direction of the fibre. A spatially and spectrallyresolved amplifier model, to evaluate and optimise the performance of praseodymiumdoped fibre amplifiers, will be presented in chapter 4.In the next paragraphs, the properties of alternative fibre amplifiers for the 2nd

telecommunication window, based on neodymium and dysprosium are discussed,followed by a description of erbium and thulium doped fibre amplifiers for the 3rd

telecommunication window. Some properties of these rare-earth doped amplifiers aresummarised in Table 1.1.

Neodymium Doped Fibre Amplifier

Like praseodymium, the Neodymium Doped Fibre Amplifier (NDFA) is based on four-level operation. The neodymium ions are excited into the 4F5/2 state by pumping at0.81 m (see Figure 1.6b). Radiation around 1.32 m is obtained from the 4F3/2 -4I13/2 transition.The major disadvantages of the neodymium system are the signal excited state ab-sorption (ESA, from the 4F3/2 to the

4G7/2 level, not shown) and additional amplifiedspontaneous emission (ASE) at 1.05 m. The first process limits the operating wave-length to wavelengths longer than 1.31 m [18], while the latter reduces the amplifierefficiency due to the 1.05 m 4F3/2 -

4I11/2 radiative transition [19]. In terms of

8 Towards more efficient praseodymium doped fibre amplifiers for the O-band

a)

0

2000

4000

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16000

18000Pr

Pum

pG

SA

Sig

nal

Em

issi

on

Sig

nal

GSA

Sig

nal

ESA

Ener

gy[c

m

1]

3H4

3H5

3H6

3F2

3F3

3F4

1G4

1D2

b)

0

2000

4000

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Pum

pG

SA

Sig

nal

Em

issi

on

1.05

m

ASE

Ener

gy[c

m

1]

4F9/24F7/24F5/24F3/2

4I15/24I13/24I11/24I9/2

c)

0

2000

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6000

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14000

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Pum

pG

SA

(0.8

1m

)

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pG

SA

(0.9

1m

)

Pum

pG

SA

(1.2

5m

)

Sig

nal

Em

issi

on

Ener

gy[c

m

1] 6F5/2

6H5/2

6F7/2

6H7/2,6F9/2

6H9/2,6F11/2

6H11/26H13/2

6H15/2

Introduction 9

d)

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5000

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Er

Pum

pG

SA

(0.8

2m

)

Pum

pG

SA

(0.9

8m

)

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m

(Pum

p)

GSA

Sig

nal

Em

issi

on(1

.55

m

)

Ener

gy[c

m

1]

4F7/2

4F9/2

4S3/2

4I9/24I11/2

4I13/2

4I15/2

e)

0

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Pum

pG

SA

(1.0

5m

)

Pum

pE

SA

(1.0

5m

)

Sig

nal

Em

issi

on(1

.47

m

)

Ener

gy[c

m

1]

1G4

3F23F3

3F4

3H4

3H5

3H6

Legend:

0

Abso

rpti

on

Em

issi

on

Non

-rad

iati

ve

Dec

ay

Ener

gyLev

el

Ground state

Excited state

Figure 1.6: The simplified energy level diagrams of a) Praseodymium (Pr, [13]),b) Neodymium (Nd, [14]), c) Dysprosium (Dy, [15]), d) Erbium (Er, [9, 10]) ande) Thulium (Tm, [5]).

10 Towards more efficient praseodymium doped fibre amplifiers for the O-band

Table 1.1: Some characteristics of rare earth doped fibre amplifiers.

PDFA NDFA DDFA EDFA TDFA

Telecom. window second second second third third3/4 level operation four four three three fourSignal GSA negligible negligible possible possible negligibleLow phonon host necessary necessary necessary unnecessary necessarymaterial requiredlifetime of the 300 [17] 450 [17] 60 [17] 10800 [17] 1000 [5]meta-stable level[s]Stimulated emis- 10.5 8 38 5.7 n.a.sion cross section 1021 [cm2] [17]

the signal ESA cross section, fluoride glasses are preferable over silica glasses [20].Chalcogenide glasses are also suitable host materials for the NDFA [18].

Dysprosium Doped Fibre Amplifier

The Dysprosium Doped Fibre Amplifier (DDFA) is a three-level system. The am-plifier is pumped at 0.81, 0.91 or 1.24 m into the 6F5/2,

6F7/2 and6H9/2 -

6F11/2levels, respectively. Compared to praseodymium, dysprosium has large absorptionand emission cross sections4. This allows for a shorter fibre length at the same dopantconcentration. The transition between the 6H9/2 -

6F11/2 levels (which overlap eachother) and the ground state is situated around 1.3 m (see Figure 1.6c). The smallenergy difference between the 6H9/2 -

6F11/2 levels and the lower6H11/2 level reduces

the amplifier efficiency due to non-radiative energy transfer [21]. Low phonon energyhost materials like chalcogenide glasses are required to reduce the non-radiative en-ergy transfer [22]. The efficiency is further hampered by the short lifetime (approx.35 s) of the meta-stable level [22]. This lifetime can be increased by the addition ofCsBr to the host glass [23].

Erbium Doped Fibre Amplifier

In the 3rd telecommunication window (1.55 m), an efficient amplifier can be con-structed by doping silica fibre with erbium ions [4]. Fluoride glasses are used toimprove the width and flatness of the amplifier gain profile [24]. In telluride glassesthe gain band is shifted towards longer wavelengths.

4The (closely) related absorption and emission cross sections are defined as the emission or absorp-tion of radiation per dopant ion. The absorption cross section is given by the absorption coefficient and the dopant concentration Ndopant: abs =

Ndopant

.

Introduction 11

The amplification within the Er-doped fibre is based on transitions between threeenergy levels (three-level system). Radiation at 1.55 m is obtained from the 4I13/2 -4I15/2 transition. A high efficiency of the amplifier is obtained due to the long lifetimeof the 4I13/2 state (order of ms). Erbium is pumped at either 0.82 m, 0.98 m or1.48 m (see Figure 1.6d). In the case of pumping at 1.48 m, the erbium ions are di-rectly pumped into the upper sublevels of the meta-stable state (4I13/2 ). Commercialerbium doped fibre amplifiers are mostly pumped at 0.98 m or 1.48 m.

Thulium Doped Fibre Amplifier

Figure 1.6e shows the energy level diagram of thulium. Amplification around 1.47 mis obtained from the 3H4 -

3F4 transition (four-level operation). The long lifetime ofthe lower level 3F4 compared to the short lifetime of the upper level

3H4 has a detri-mental effect on the population inversion [25]. Fluoride and chalcogenide glass hostsare preferred for the TDFA [25, 26]. Upconversion pumping schemes (i.e. pumpingusing an intermediate energy level) are employed in order to reduce the populationof the 3F4 level while increasing the population in the meta-stable

3H4 level. TDFAswith upconversion pumping behave like three-level amplifiers, where the 3F4 level actsas virtual ground state.

1.2.2 Other types of optical amplifiers

Semiconductor optical amplifier

Like the rare earth doped fibre amplifier, signal light is amplified by the semiconductoroptical amplifier (SOA) through stimulated emission. The basic structure of a SOAchip consists of an active layer sandwiched between two cladding layers on top of asubstrate and covered by a contact layer [2]. The optical power is confined into theactive layer by the cladding layers. Within the semiconducting active layer, the signallight is amplified by stimulated emission. The energy needed for the amplificationprocess is provided by an injection current (DC), which is transferred to the activelayer of the SOA by the substrate and contact layers. An anti-reflection coating isapplied at both ends of the active region, in order to prevent undesirable reflectionscausing laser oscillations [27].In a semiconductor material, the energy levels of the charge carriers (electrons andholes) are located in the valence band and the conduction band [1]. These bands areseparated by an energy difference, the so-called band gap. Within the band gap, noenergy levels exist. The operating wavelength of the amplifier is determined by theenergy gap between the conduction and valence bands.The active region of the SOA is located at the junction of a p-type (having an excessconcentration of holes) and a n-type (excess concentration of electrons) semiconductormaterial. At the pn-junction, a region with net negative charge is formed in the p-type semiconductor and a region with net positive charge is obtained in the n-typesemiconductor due to diffusion of electrons and holes. When a sufficiently high voltage(bias) is applied to the pn-junction, the increased minority carrier concentrations (i.e.electrons in the p-type region and holes in the n-type region) result in a population

12 Towards more efficient praseodymium doped fibre amplifiers for the O-band

inversion [1]. Stimulated emission, originating from recombination between electronsand holes at the pn-junction, results in amplification of the light propagating throughthe active region [1]. The emitted photons will have exactly the same frequency andphase as the incident signal photons and these photons are radiated in a confineddirection [7, 8].Proper design of the structure of the active layer is required in order to achieveamplification characteristics that are independent of the state of polarisation of theincident beam.

Raman fibre amplifier

The principle of Raman amplification is the stimulated emission process associatedwith Raman scattering in a fibre. Whereas rare earth doped fibre amplifiers requiredopants in the fibre in order to provide gain, Raman gain is obtained in the transmis-sion fibre itself. The pump and signal light (at a longer wavelength than the pumpwavelength) are combined and fed into a fibre. In the stimulated Raman scatteringprocess, absorption of a pump photon by the host material is followed by emissionof a new (Stokes) photon at the signal frequency, whilst the residual energy (of theabsorbed pump photon) is absorbed as phonons (vibrational energy) by molecules inthe host material. As there is a wide range of vibrational states above the groundstate, gain is obtained in a broad wavelength region [28].The Raman gain strongly depends on the pump power and the frequency offset be-tween pump and signal. The operating wavelength of the amplifier can be adjustedby tuning the pump wavelength. In Raman amplifiers utilising silica fibre, the high-est gain is obtained when the difference between the pump and the signal frequency(Stokes shift) is approximately 13 THz. The gain bandwidth (i.e. the wavelengthrange where the gain of the amplifier is at least half the maximum obtainable gain) ofa Raman amplifier, pumped at a single wavelength, is about 48 nm. Multi-wavelengthpumping could be used to broaden and flatten the Raman gain profile. The gain pro-file of a multi-wavelength pumped Raman amplifier is a superposition of the gainprofiles obtained by pumping at the respective pumping wavelengths. Proper selec-tion of the pump power at the pumping wavelengths is necessary to obtain a flat gainprofile. An advantage of Raman amplification is that the same fibre used for signaltransmission can also be used for signal amplification (distributed amplification asopposed to the lumped amplification provided by other types of amplifiers). In thecase of distributed Raman amplification (DRA), the signal losses are compensatedthroughout the fibre. This results in a better signal to noise ratio (SNR) comparedto lumped amplification. [10].

1.3 General application of optical amplifiers

In telecommunication systems, which are limited by attenuation, the optical ampli-fier application ranges from power booster at the transmitter via in-line amplifier inlong-haul transmission links, to pre-amplifier at the receiver end. Amplification of thesignal is required to overcome intrinsic fibre absorption and scattering losses, losses

Introduction 13

Table 1.2: Some requirements for optical amplifiers in different applications.

Function booster in-line pre-amp

High gain necessary essential essentialHigh saturation output power essential essential unnecessaryLow noise characteristics unnecessary necessary essentialPolarisation independence unnecessary necessary essentialLow insertion loss unnecessary necessary essential

of additional optical components and coupling losses due to network branching. Inaddition to the gain provided by the optical amplifier, some spontaneous emissiongenerated by the optical amplifier is also added to the signal. The noise originatingfrom the amplified spontaneous emission will decrease the signal to noise ratio (SNR).At the opto-electronic receiver, the degradation of the SNR results in an increasedprobability of erroneous interpretation of the received data.The main requirements for the use of amplifiers in booster, in-line and pre-amplifierapplications are listed in Table 1.2 [27].At the transmitter, a booster amplifier is used to improve the signal power of the(amplitude, phase or frequency) modulated data signals. The amplifier is operatingin the so-called saturation regime (i.e. the input power of the amplifier is relativelyhigh and a high signal output power is required). The amplifier must provide highsaturated output power which is desirable for some applications (e.g. for distributionof common antenna television (CATV) signals to a group of subscribers). The sat-uration output power is defined as the level at which the amplifier gain is decreasedby a factor of two with respect to the small signal gain (i.e. gain at low signal inputpower). At high signal input powers, the gain of the amplifier decreases. This re-duction of gain is caused by depletion of the excited state due to stimulated emissionand a limited replenishing rate. Due to the relatively large signal input and outputpower, the noise characteristics of the booster amplifier are less important.The in-line amplifiers provide amplification of the signals along the transmission link.In a long-haul communication system, a large number of amplifiers is cascaded. Insuch a system, the performance is affected by accumulation of amplified spontaneousemission (ASE) noise. As the level of ASE grows, the amplifiers along the trans-mission line tend to saturate and the signal gain is reduced. The net result of thereduced signal level and increased ASE level is a considerable degradation of the sig-nal to noise ratio (SNR) [29]. As the state of polarisation of the signal may changeupon transmission through a fibre, this application requires polarisation independentamplifiers with a high saturation output power.At the receiver, an optical pre-amplifier can be used to improve the signal prior to de-tection and electrical amplification. The pre-amplifier operates in the so-called smallsignal gain regime. Due to the low input power, low loss between the fibre and theamplifier (insertion loss) is required. Furthermore, a relatively high gain and low noise

14 Towards more efficient praseodymium doped fibre amplifiers for the O-band

contribution by the pre-amplifier are important.A general overview of the characteristics of PDFA, SOA and DRA amplifiers oper-ating around 1.3 m are presented in Table 1.3. In this table, typical figures for thesmall signal gain5 and noise figure (pre-amplifier) as well as the saturated outputpower6 (booster) of the different types of amplifiers are summarised.The application of the praseodymium doped fibre amplifier as a booster, in-line andpre-amplifier will be discussed in chapter 5. In fibre amplifiers, the length of the fibreand the propagation direction of the pump light are important parameters to influ-ence gain and noise properties (see chapter 4). The SOA and DRA can be designedto operate at the desired wavelength by tuning the bandgap and pump wavelength,respectively. However, not all amplifier characteristics can be influenced by the de-sign of the amplifier. The possible pump and operating wavelengths of the PDFA(and other rare earth doped amplifiers) are mainly determined by the applied hostmaterial.Due to the cylindrical geometry of the fibre, both PDFA and DRA are polarisationinsensitive. In a SOA, low polarisation dependence of the amplifier characteristics isachieved by proper design of the structure of the active layer.The energy for stimulated emission in a PDFA is provided by a powerful (up to 1 W)pump laser. The (distributed) Raman amplifier requires even higher optical pumppowers. The SOA is driven by an (electric) injection current.The performance of all mentioned amplifier types is hampered by amplified sponta-neous emission (ASE) noise. The best noise performance is obtained for PDFA andDRA (noise figure of 34 dB), whereas the noise figure of SOA is approx. 3 dB higher.

1.4 Scope and objective of this thesis

The development of optical telecommunication systems for signal wavelengths around1.55 m was greatly enhanced by the development of the erbium doped fibre ampli-fier. In order to fully exploit the transparent windows of the installed fibre, andparticularly the 1.31 m wavelength region in the near future, a powerful amplifiertechnique for these wavelengths is needed. Although the praseodymium doped fibreamplifier is commercially available, the efficiency of these (fluoride based) amplifiersis low. Therefore, high pump powers are needed to achieve reasonable gain. Theobjective and motivation of this thesis is the development of a praseodymium dopedfibre amplifier with improved efficiency. As an efficient PDFA can be constructedonly using low phonon energy glass hosts (e.g. chalcogenide glasses), this study willfocus on the development of chalcogenide glasses and determination of the physical

5Gain and attenuation are often expressed on the logarithmic scale of decibels (dB). G[dB] =10 log(G[]) is used to convert the value from a linear scale to decibel. Optical power relative to a

power of 1 mW is often expressed in terms of dBm. The formula P [dBm] = 10 log(P [mW ]1mW

) can beused to convert the power P from mW into dBm.

6This power equals the maximum output power, while the 3 dB saturation output power is definedas the output power level at which the amplifier gain is decreased by a factor of 2 with respect tothe gain of the unsaturated amplifier e.g. at low signal input power.

Introduction 15

Table 1.3: Some properties of the praseodymium doped fibre amplifier (PDFA), semi-conductor optical amplifier (SOA), and distributed Raman amplifier DRA at an op-erating wavelength around 1.3 m (after [30]).

PDFA SOA DRA

Peak gain wavelength 1300-1350 nm 1280-1330 nm 1310 nmOptical bandwidth 2040 nm 60 nm 2045 nm3 dB saturation power 1018 dBm 1318 dBm 12PpumpSaturated output power 1320 dBm 1620 dBm 34PpumpSmall signal gain > 40 dB > 33 dB > 40 dBNoise figure 3.54 dB 6.59 dBm 34 dBPump wavelength 10151035 nm 1240 nmPump power 2030 dBm > 30 dBmPolarisation sensitivity none < 0.5 dB noneRadiative lifetime 110-400 s 200 ps

properties of these (praseodymium doped) chalcogenide host glasses (see chapter 2).In general, the preparation and processing of these glasses is complex. As a part ofthis work, the technology needed for melting germanium gallium sulfide glasses andfibre drawing (from a fibre preform) is investigated (see chapter 3).Furthermore, the objective of this study is to establish knowledge and understandingof the performance of praseodymium doped fibre amplifiers in telecommunication sys-tems. This system performance is evaluated in using a transmitter, an experimentalpraseodymium doped fibre amplifier, based on commercially available praseodymiumdoped fluoride fibre modules and a receiver. Due to the modular setup of the ex-perimental PDFA, the amplifier characteristics in different configurations (e.g. co-propagating, counter-propagating and bi-directional pumping schemes) can be as-sessed. Different applications (e.g. booster, in-line or pre-amplifier) put differentdemands on the design (e.g. fibre length, core radius, etc.) and configuration of thepraseodymium doped fibre amplifier. The requirements for rare earth doped fibres,used in fibre amplifiers (e.g. dispersion, absorption and scattering losses of the fibre),are not equal to those used for long-haul transmission. A mathematical amplifiermodel is used to study the amplifier characteristics of the praseodymium doped fibreusing the optical properties of both fluoride and chalcogenide glasses. The model isvalidated for praseodymium doped fluoride fibre. Furthermore, the model is used tooptimise the design of the praseodymium doped fibre amplifier based on the germa-nium gallium sulphide host glass.

1.4.1 Outline of this thesis

In the next chapter, the preparation of germanium gallium sulphide host glasses andtheir, for this study relevant, thermal, rheological and optical properties are described.

16 Towards more efficient praseodymium doped fibre amplifiers for the O-band

This information is used to select suitable core and cladding glass compositions. Inchapter 3 an overview of fibre drawing processes for chalcogenide glasses is given,followed by a description and results of the applied fibre fabrication processes forgermanium gallium sulfide glasses. The working area for fibre drawing of germaniumgallium sulphide glasses using fibre preforms is established using a mathematical fibredrawing model. In chapter 4, a detailed description of a spatially and spectrally re-solved praseodymium doped fibre amplifier model is presented. The model outcome isvalidated using the results of measurements on the experimental praseodymium dopedfluoride fibre amplifier. In chapter 5 the performance of the PDFA in telecommunica-tion systems is discussed. The experimental PDFA, based on a praseodymium dopedfluoride fibre, was used to determine the properties of the PDFA in booster, in-lineor pre-amplifier applications. Furthermore, the optical properties of a praseodymiumdoped germanium gallium sulfide fibre, prepared for this study, are described. Usingthe amplifier model of chapter 4, the optimum design (e.g. fibre length, core radius,etc.) of an efficient PDFA, based on germanium gallium sulphide host glass, is dis-cussed. At the end of chapter 5, a comparison between the praseodymium dopedamplifier based on chalcogenide and fluoride fibre is given. In the final chapter, thiswork is concluded, and some prospects for telecommunication systems incorporatingPDFAs will be given.

1.5 Bibliography

[1] R. Ramaswami and K. Siravajan, Optical networks. A practical Perspective. SanFrancisco: Morgan Kaufmann publishers, 2nd ed., 2002. ISBN 1-55860-655-6.

[2] C.-L. Chen, Optoelectronics & fiber optics. Chicago, USA: Irwin, 1996. ISBN0-256-14182-7.

[3] R. Mears, L. Reekie, I. Jauncey, and D. Payne, Low-noise erbium-doped fibreamplifier operating at 1.54 m, Electron. Lett., vol. 23, no. 19, pp. 10261028,1987.

[4] E. Desurvire, Erbium-doped fiber amplifiers. Principles and applications. NewYork, NY.: John Wiley & Sons, Inc., 1994. ISBN 0-471-58977-2.

[5] F. Roy, D. Bayart, A. le Sauze, and P. Baniel, Noise and gain band managementof thulium-doped fiber amplifier with dual-wavelength pumping schemes, IEEEPhotonics Technol. Lett., vol. 13, pp. 788790, August 2001.

[6] T. Whitley, A review of recent system demonstrations incorporating 1.3-mpraseodymium-doped fluoride fiber amplifiers, J. Lightwave Technol., vol. 13,pp. 744760, May 1995.

[7] A. Yariv, Optical electronics. London: Saunders College, 4th ed., 1991. ISBN0-03-047444-2.

Introduction 17

[8] L. Mandel and E. Wolf, Optical coherence and quantum optics. Cambridge:Cambridge University Press, 1995. ISBN 0-521-41711-2.

[9] S. Sudo, Optical fiber amplifiers: materials, devices, and applications. Norwood,MA: Artech House, Inc., 1997. ISBN 0-89006-809-7.

[10] A. Bjarklev, Optical Fiber Amplifiers, Design and system applications. Norwood:Artech House, 1993. ISBN 0-89006-659-0.

[11] K. Wei, D. Machewirth, J. Wenzel, E. Snitzer, and G. Sigel Jr., Pr3+-doped Ge-Ga-S glasses for 1.3 m optical fiber amplifiers, J. Non-Cryst. Solids, vol. 182,pp. 257261, 1995.

[12] D. Aggarwal and G. Lu, Fluoride glass fiber optics. San Diego, CA: AcademicPress, 1991. ISBN 0-12-044505-0.

[13] D. Simons, A. Faber, and H. d. Waal, GeSx glasses for Pr3+-doped amplifiers

at 1.3 m, J. Non-Cryst. Solids, vol. 185, pp. 283288, 1995.

[14] I. Mitchell, V. Bogdanov, and P. Farell, Energy transfer in praseodymium andneodymium co-doped fluorozirconate glass, Opt. Communications, vol. 155,pp. 275280, October 1998.

[15] D. Machewirth, K. Wei, V. Krasteva, R. Datta, E. Snitzer, and G. Sigel Jr.,Optical characterisation of Pr3+- and Dy3+- doped chalcogenide glasses, J.Non-Cryst. Solids, vol. 213&214, pp. 295303, 1997.

[16] S. Fleming, Crosstalk in 1.3 m praseodymium fluoride fiber amplifiers, J.Lightwave Technol., vol. 14, pp. 6671, January 1996.

[17] D. Simons, Germanium Gallium Sulfide Glasses for Pr-Doped Fiber Amplifiersat 1.3 m. PhD thesis, Technische Universiteit Eindhoven, 1995. ISBN 90-386-0496-3.

[18] J.-L. Adam, J.-L. Doualan, L. Griscom, S. Girard, and R. Moncorge, Excited-state absorption at 1.3 m in Nd3+-doped fluoride and sulfide glasses, J. Non-Cryst. Solids, vol. 256&257, pp. 276281, 1999.

[19] A. Jha, M. Naftaly, E. Taylor, B. Samson, D. Marchese, and D. Payne, 1310-1320 nm emission in Nd3+-ion doped fluoroaluminate glasses, in Conference onInfrared Glass Optical Fibers and Their Applications, vol. 3416, pp. 115123,SPIE, July 1998.

[20] T. Sugawa, Y. Miyajima, and T. Komukai, 10 dB gain and high saturationpower in a Nd3+-doped fluorozirconate fibre, Electron. Lett., vol. 26, pp. 20422044, November 1990.

[21] D. Hewak, B. Samson, J. Medeiros Neto, R. Lamming, and D. Payne, Emis-sion at 1.3 m from dysprosium-doped Ga-La-S glass, Electron. Lett., vol. 30,pp. 968970, June 1994.

18 Towards more efficient praseodymium doped fibre amplifiers for the O-band

[22] K. Wei, D. Machewirth, J. Wenzel, E. Snitzer, and G. Sigel Jr., Spectroscopyof Dy3+ in Ge-Ga-S glass and its suitability for 1.3-m optical amplifiers appli-cations, Opt. Lett., vol. 19, pp. 904907, June 1994.

[23] Y. Shin, J. Heo, and H. Kim, Modification of the local phonon modes andelectron-phonon coupling strengths in Dy3+-doped sulfide glasses for efficient 1.3m amplification, Chem. Phys. Lett., vol. 317, pp. 637641, February 2000.

[24] H. Ono, M. Yamada, T. Kanamori, S. Sudo, and Y. Ohishi, 1.58 m bandfluoride-based Er3+-doped fibre amplifier for WDM transmission systems, Elec-tron. Lett., vol. 33, pp. 14711472, August 1997.

[25] S. Aozaso, T. Sakamoto, T. Kanamori, K. Hoshino, K. Kobayashi, andM. Shimizu, Tm-doped fiber amplifiers for 1470-nm-band WDM signals, IEEEPhotonics Technol. Lett., vol. 12, pp. 13311333, October 2000.

[26] K. Kandono, T. Yazawa, M. Shojiya, and Y. Kawamoto, Judd-Ofelt analysisand luminescence property of Tm3+ in Ga2S3-GeS2-La2S3 glasses, J. Non-Cryst.Solids, vol. 274, pp. 7580, 2000.

[27] S. Shimada and H. Ishido, Optical amplifiers and their applications. Chichester:John Wiley & Sons, Inc., 1994. ISBN 0-471-94005-4 transl. by F.R.D. Apps.

[28] M. Nissov, Long-haul optical transmission using distributed Raman amplification.PhD thesis, Technical University of Denmark, December 1997.

[29] G. Agrawal, Fiber-optic communication systems. Microwave and Optical Engi-neering, New York: John Wiley, 2nd ed., 1997. ISBN 0-471-17540-4.

[30] J. Jennen, Noise and saturation effects in high-speed transmission systems withsemiconductor optical amplifiers. PhD thesis, Eindhoven University of Technol-ogy, 2000. ISBN 90-386-1760-7.

Chapter 2

Praseodymium doped

germanium gallium sulphide

glasses

In this chapter, the melting process and relevant properties determined for praseody-mium doped germanium gallium sulphide glasses are described. The germanium gal-lium sulphide glasses are studied as alternative host materials for the praseodymiumdoped fibre amplifier (PDFA). The PDFA is used for amplification of telecommu-nication signals around 1.31 m and is optically pumped with laser light around1.03 m.In the first section, an overview of the glass forming region of the germanium galliumsulphide system and the preparation techniques of the glasses is given. Purificationof the glass or its raw materials is needed to obtain highly transparent host glasses,especially around the amplifiers pump and signal wavelengths.In the following section, the optical properties of the germanium gallium sulphidehost glass and the applied measuring techniques are summarised. The germaniumgallium sulphide glasses are transparent in a broad wavelength range. The transmit-tance properties of the glasses in the ultra violet (UV), visible (VIS) and infrared(IR) wavelength range are presented in this section. Absorption phenomena causedby impurities are observed in the IR spectra. The source and amount of the impu-rities incorporated in the host glass and their influence on the optical properties arediscussed.The light guiding properties of a glass fibre depend on both the core diameter and therefractive index difference between the core and the cladding. In an optical fibre, therefractive index of the cladding glass is usually less than one percent lower than therefractive index of the core. Therefore, precise measurement of the refractive index ofthe glasses is necessary to select suitable core and cladding glasses. In section 2.2, thedetermination of the refractive index (as a function of wavelength) of the germaniumgallium sulphide glasses by ellipsometry and the measurement of the Brewster angle

19

20 Towards more efficient praseodymium doped fibre amplifiers for the O-band

is discussed, also.In section 2.3, the optical properties of the praseodymium dopant in germaniumgallium sulphide host glasses are described with respect to their use in the fibre am-plifier. Trivalent praseodymium ions are optically active in the IR wavelength range(e.g. around 1.03 and 1.31 m), via electronic transitions between its 4f levels. Ab-sorption and photoluminescence spectroscopy are used to measure the absorption andemission (as a function of wavelength) and the emission lifetimes. Then, the radia-tive properties (e.g the radiative lifetime, absorption and emission cross sections) ofthe praseodymium in the germanium gallium sulphide host glass are quantitativelycharacterised, using the Judd-Ofelt theory.Besides absorption or emission of photons, the energy state of the praseodymium ioncan change by non-radiative (e.g. multi-phonon) energy transfer to the host glass.This interaction between the praseodymium dopant and the host glass, which reducesthe efficiency of the praseodymium doped fibre amplifier, is discussed in section 2.4.In section 2.5, the thermal properties of the germanium gallium sulphide glasses arereviewed. These properties of the host glass (e.g. the glass transition and crys-tallisation temperatures, thermal expansion and rheological properties) are of majorimportance for the fibre drawing process.In the last section of this chapter, the main conclusions concerning the optical, me-chanical, structural and thermal properties of the praseodymium doped germaniumgallium sulphide glasses, relevant for the preparation and application of glass fibresof these materials, are given.

2.1 Preparation of the glass host

In this section, an overview of the compositions of germanium gallium sulphide com-positions with glass forming ability is given. Here, the preparation techniques, whichhave been used to melt the germanium gallium sulfide glasses from their raw materials,are described. Also the purification of the starting materials is discussed briefly.

2.1.1 The glass forming region

The glass forming region (i.e. the chemical composition of the materials that canform glasses upon cooling of the melt) of the germanium gallium sulphide system isshown in the ternary diagram 2.1, which summarises the results of several investiga-tors [16]. The binary germanium sulphide glasses (Ge-S) were studied thoroughlyby Kawamoto et al. [1, 2], while Loireau-Lozach et al. [3] investigated stoichiomet-ric1GeS2 Ga2S3 compositions. The stoichiometric tie-line composition for the Ge-Ga-S system is defined as the line joining the stoichiometric compounds GeS2 andGa2S3. Any composition with germanium (Ge) content greater than those on the

1Stoichiometry refers to the amounts of substances that satisfy the balanced chemical reactionequation (e.g. Ge+2S GeS2) for a particular chemical reaction [7]. The composition of theproduct, formed in this chemical reaction (e.g. GeS2), is called stoichiometric.

Praseodymium doped germanium gallium sulphide glasses 21

100

80

60

40

20

0 100

80

60

40

20

0

S

Ga Ge100806040200

GaS1.5

GeS2

glass formingnon-glasforming

Stochimetric tie-lineSimons

This work

Figure 2.1: Ternary diagram for germanium (Ge), gallium (Ga) and sulphur (S)indicating the glass forming compositions. The fractions of Ge, Ga and S are expressedin mol%.

tie-line (i.e. non-stoichiometric composition) is referred to as Ge-rich (or equivalentlysulphur deficient) composition and that with more sulphur (S) content is referred toas S-rich composition (or germanium deficient). The properties of non-stoichiometric,sulphur rich and sulphur deficient compositions were systematically studied by Abeet al. [4] and Saffarini [5].Binary germanium sulphide glasses are formed by air quenching between 6690 at.% S[6]. Ternary glasses are obtained containing up to 15 at.% Ga and 55-85 at.% S, seeFigure 2.1. However, the range of composition of glasses which can be drawn intofibre is limited. This is mainly caused by crystallisation and decomposition sensitivityof some glass compositions.The solubility of praseodymium in germanium sulphide glasses is enhanced by a smallamount of gallium in the glass host [8]. Simons investigated glasses with general com-position (GeSx)100y(Ga2S3)y where x=24 and y=020. The glasses were meltedfrom germanium, gallium metals and sulphur as starting materials. In this work,glasses in the region x=2.252.6 and y=25 were investigated.

22 Towards more efficient praseodymium doped fibre amplifiers for the O-band

2.1.2 Preparation methods

Purification

Using the elements as a starting material, allows the production of stoichiometric,sulphur rich and sulphur deficient glasses. Another way to prepare, in particular stoi-chiometric, chalcogenide glasses is to start from the (technical grade) metal-sulphides.As impurities affect the transmittance of the glass material at certain wavelengths,high purity starting materials are needed to obtain a highly transparent host glass.Furthermore, impurities (especially hydroxyl) can cause a decrease in the fluorescencelifetime of the excited states of praseodymium, due to an increase of the non-radiativetransition rates [9]. Several chalcogenide raw material purification techniques havebeen developed. Vacuum distillation is proposed to separate both poorly volatileimpurities (e.g. suspended particles, which become a residu) and volatile impurities(like water, carbon oxides, sulphur oxides and hydrocarbons) from the sulphur, due todifference in vapour pressure between sulphur and the impurities [4, 10]. In addition,heating the sulphide for several hours in the presence of magnesium (submerged insulphur melt) can be used to reduce the oxide impurities [11]. Removal of OH andSH impurities using a reactive gas atmosphere (e.g. S2Cl2 vapour) was reported byShibata et al. [12].

Melting

In this study, the starting materials were commercially available germanium (Ge, 99.9999% pure, semiconductor grade ingots) supplied by Highways2, gallium (Ga, 99.999% pure, ingots) by Johnson Matthey3, and sulphur (S, 99.999% pureflakes) by Cerac4. Praseodymium (as a sulphide) is added as 99.99% pure Pr2S3,supplied by Cerac. The quoted purity levels are based on the total metallic contentonly (spectrographic analysis, performed by the supplier). In this figure, carbon, oxy-gen, nitrogen and hydrogen and surface deposits are not taken into account .In this study, the sulphur was purified by sublimation in vacuum at 120 C in or-der to remove H2O, H2S, SO2, and SO3 [13] and some less volatile impurities. Inthis process, the sulphur is heated, under vacuum, inside a small vessel. The sulphurevaporates, and subsequently sublimates on a cold surface, mounted within the vessel.The sublimated sulphur was used to prepare the germanium gallium sulphide glasses.All materials were stored and handled in a glovebox (MBraun MB-150) under argonatmosphere ( 1 ppm H2O and O2). Inside this glove box, the germanium ingotswere crushed and ground in a mortar. Sufficiently small particles of gallium were pre-pared by melting the gallium at 35 C, and subsequent separation of small amountsof gallium (droplets) from the melt using a small spoon. These small droplets werecooled at room temperature in order to solidify.Fused silica ampoules were used in the melting experiments. The ampoules have antypical length of 200 mm and an inner diameter of 8 and 10 mm. In order to withstand

2Highways International, PO box 153, 3740 AD Baarn, NL3Johnson Matthey Plc. 24 Cockspur Street, London SWIY 5BQ, UK4Cerac Inc., PO box 1178, Milwaukee, WI 53201, USA

Praseodymium doped germanium gallium sulphide glasses 23

the internal pressure of several MPas [13] (caused by the high vapour pressure of thechalcogenide glass batch upon heating) at elevated temperatures the wall thickness is3.5 and 4.0 mm, respectively.The ampoules were first etched with HNO3 acid, rinsed with distilled water and ace-tone, before being vacuum treated at 1000 C for 8 hours. The purpose of this vacuumtreatment was dehydration of the ampoule.In the experiments, batches of approximately 20 g mixed raw materials were pre-pared. The ampoules, filled with the raw materials mixture, were vacuum-sealed(at p 104 mmHg) using an oxygen-natural gas torch. After being mounted ona support, the ampoules were placed horizontally in an electrical chamber furnace.The temperature was gradually heated to melting temperature and the glasses weremelted overnight at 1000 C. At the beginning of the melting process, the raw mate-rials inside the ampoule tend to segregate due to the different melting temperaturesand the large difference in density of the materials. This segregation was minimisedby placing the ampoules in a horizontal position. The melt was homogenised by nat-ural convection of the melt. If necessary, the support axis could be rotated in orderto mix the materials inside. At the end of the melting time, the ampoules were gentlyput into a vertical position in order to prepare glass rods directly by solidification ofthe glass melt. In this position the material gathers in the ampoules bottom end.During this stage, confinement of gas bubbles into the melt phase could occur5. Theampoules were withdrawn from the furnace, quenched in air (typically 90 120 s) andannealed. Due to the higher thermal expansion coefficient of the germanium galliumsulphide glasses compared to that of silica, the glass shrinks more than the ampoulewall during quenching and cooling. Therefore, after breaking the ampoule, the (de-tached) rods can easily be removed from the ampoule. To determine the physicalproperties of the germanium gallium sulphide glass, pieces of at least 10 mm lengthare needed, while rods of at least 50 mm long are required for fibre drawing. Piecesof glass containing defects (e.g. bubbles, crystals or cracks) are rejected for furtherprocessing.

2.2 Optical properties of the host glass

In this section, various optical techniques are described, that are used to determinethe properties (transmittance, reflectance and refractive index) of the germanium gal-lium sulphide host glasses. These glasses are highly transparent in the 0.5 8.0 mregion. A typical transmittance spectrum is shown in Figure 2.2.The optical properties of the host glass are dependent of the wavelength . In thischapter, the wavelength dependence in the notation of the optical functions is omittedfor the purpose of convenience.

5Gas composition measurements using mass spectrometry indicated that the bubble containsargon and a species with a mass/charge of 76 (probably CS2). The ratio Ar:CS2 is approximately1:3. Other species are below detection limits. The argon must be a remainder of the atmospherein the glove-box. The CS2 is probably a result of the reaction between carbon impurities from theraw materials and sulphur. The measurement confirms that bubble formation is probably caused byenclosure of gas in the melt during the procedure of turning the furnace.

24 Towards more efficient praseodymium doped fibre amplifiers for the O-band

0

0.2

0.4

0.6

0.8

1.0

0 2000 4000 6000 8000 10000 12000

Tra

nsm

ittan

ce [

-]

Wavelength [nm]

Figure 2.2: Total transmittance spectra (i.e. including the losses due to reflection(at normal incidence) at both glass interfaces) of praseodymium doped germaniumgallium sulphide glass (Ge27.7Ga1.1S71.1) from the UV to IR range. The absorptionpeaks are discussed in section 2.2.1 and 2.2.2.

The glasses are characterised by their wavelength dependent complex index of refrac-tion

N = n jk (2.1)where the real part of the index of refraction N (i.e. N()) is called the refractiveindex n (i.e. n()) and the imaginary part k (i.e. k()) is the extinction coefficient.The refractive index n is an inverse measure of the phase velocity v in the materialrelated to the speed of light c in vacuum

n =c

v(2.2)

In an absorbing material, the internal transmittance is related to the absorptioncoefficient and optical path length d (e.g. the thickness of the sample), accordingto Lambert-Beers law

= ed (2.3)

The internal transmittance is determined from the measured total transmittance ofincident radiation Ttr, after correcting for reflection losses. The extinction coefficientk is related to the absorption coefficient by

k =

4 (2.4)

The presence of impurities incorporated in the host glass is indicated by absorptionbands in the infrared (IR) transmittance spectra. In section 2.2.1, the concentrations

Praseodymium doped germanium gallium sulphide glasses 25

of impurities present in the glass are estimated from the absorption spectra. Impuri-ties can affect the radiative properties of praseodymium (see section 2.4).In section 2.2.2, the transmittance of the host glass in the ultraviolet (UV) to nearinfrared (NIR) is discussed.The wavelength dependent refractive index in the UV to NIR range is determinedfrom ellipsometry and from measurements of the Brewster angle. The determinationof the refractive index of the host glasses is discussed in section 2.2.3.

2.2.1 IR transmittance spectra

The absorption spectra in the infrar