Very compact and high-power CW self-Raman laser for ...

Very compact and high-power CW self-Raman laser for ophthalmological applications

Tiago A. Ortega*a,b, Alessandro D. Motaa, Giuliano Rossia, Guilherme C. de Castroa, Yuri C. Fontesa, Glauco Z. Costala, Fatima M. M. Yasuokaa, Mario A. Stefania, Andrew Leec, Helen Paskc,

Jarbas C. de Castro N.a,b

aOpto Eletrônica S/A, 1071 Joaquim A. R. de Souza, São Carlos, SP, Brazil 13560-330; bInstituto de Física de São Carlos, Universidade de São Paulo, 400 Trabalhador São Carlense, São

Carlos, SP, Brazil 13560-970; cDepartment of Physics, Macquarie University, Sydney, NSW Australia;

ABSTRACT

In this work, we present a continuous-wave yellow laser operating at 586.5nm based on self-Raman conversion in Nd:GdVO4. We report more than 4.2W CW and 5.5W instantaneous output at a 50% duty cycle regime. This is the highest CW power of a self-Raman laser to be reported so far. We also demonstrate the integration of this laser cavity into a console for applications in ophthalmology, and more specifically for retinal photocoagulation therapies.

Keywords: self-Raman laser, solid state laser, vanadate lasers, resonator design, photocoagulator, retinal treatment

1. INTRODUCTION

Stimulated Raman Scattering (SRS) in vanadate and tungstate crystals such as YVO4, GdVO4 and KGdW has enabled the extension of traditional Nd-based lasers to longer wavelengths1-3. When used in conjunction with Second Harmonic Generations (SHG), it has enabled feasible access to the yellow-orange portion of the electromagnetic spectrum4 for many applications in industry, science and medicine. Particularly, in ophthalmology, the development of a yellow laser for retina photocoagulation is of great interest. The reason is this region of the spectra is on the peak of absorption for the hemoglobin, and mainly, is not absorbed by xanthophyll.

1.1 Retinal photocoagulation

Several retinal diseases such as diabetic retinopathy, retinal detachment, retinal tears, retinal hemorrhage, vein occlusion, macular edema, etc, are widely treated with the aid of photocoagulators. These photocoagulators are laser-based ophthalmic equipment that delivers the radiation of a laser cavity into the patient’s retina. The interaction between the laser radiation and the retinal tissues that is described in many ophthalmological protocols as the responsible for the treatment is the absorption5.

It is essential that one opts for a laser whose wavelength is well absorbed by the correct molecules and not absorbed by the undesired ones. The targeted molecules in ophthalmic photocoagulation are melanin, oxyhemoglobin and reduced hemoglobin. The Xanthophyll, which are abundant molecules in the macula should never interact with the laser. The reason for that is the macula is the portion of the retina responsible for the central vision and enables the patient to read, drive, etc., and should never be damaged by the laser. Figure 1 shows the absorption spectra of the main elements that compose the retina.

*[email protected]; phone 55 16 2106-7078 ; fax 55 16 2106-7001; www.opto.com.br

Solid State Lasers XIX: Technology and Devices, edited by W. Andrew Clarkson, Norman Hodgson, Ramesh K. Shori, Proc. of SPIE Vol. 7578, 757822 · © 2010

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Figure 1. Absorption spectra of the mains retinal components.

Figure 1 shows that the peak absorption of oxyhemoglobin and reduced-hemoglobin lies in the green-yellow wavelength range, while the absorption of xanthophylls lies in the blue-green wavelength range. The yellow wavelength (586.5 nm) at which the CW laser system operates is indicated in the figure. It can be seen that the wavelength lies in the absorption peak of the hemoglobin(s) and is far from the absorption band of the xanthophylls. The system is hence ideal for application in retinal photocoagulation therapies.

1.2 The Raman effect

Raman lasers utilise the non linear effect known as Stimulated Raman Scattering (SRS), an inelastic process in which the radiation has its wavelength altered by the interaction with the media6. In liquid or gases this interaction is in the form of changes in vibrational modes. In solid media, this interaction happens via phonons.

A photon of energy h�l incident on a crystalline media may dissipate part of its energy on the lattice in the form of a phonon of energy h�p. Then, the emergent radiation will have the following energy:

�� (1)

This phenomena is known as Stokes shifting and the emitted radiation as Stokes line. The figure 2 illustrates this situation.

Figure 2. Representation of the Stimulated Raman Scattering (SRS). (a)Stokes line (b) anti-Stokes line.



Similarly, the opposite process can occur: an atom or molecule in the excited state can add energy to the incident photon. This process is known as anti-Stokes and is also illustrated in Figure 2. In this case we have:

�� (2)

Normally, the anti-Stokes line is much weaker than the Stokes one since the population of the excited state is lower than the ground state by the Boltzmann factor, exp(-(h�/kbT)), where kb is the Boltzmann Constant and T the temperature.

Considering a pump laser of intensity IL passing through a Raman media and using the occupation number formulation one obtains an expression for the Stokes photon generation rate7,8:

��

��

� ��

��

!"� #$� % $�&'( ) �� ) �*+,# � � � ) !& (3)

Where ns is the Stokes photons population, N=N0/V is the number of active Raman molecules, �L and �S are the refraction indices of the Raman media for the laser and Stokes wavelength, eL and eS are the unit vectors for the laser and Stokes radiation polarization, m is the reduced mass of the molecule, c the velocity of light in vacuum, � is Dirac’s delta function and nV is the phonon population. The summation is done over all kS Stokes modes per volume interval V and solid angle ��. �q/�t is the derivative of the polarizability tensor.

One important Raman crystal is the Gadolinium Orthovanadate (GdVO4), a tetragonal crystal with two molecules per primitive cell as shown in Figure 3.

Figure 3. Structure of the GdVO4 crystal. The Gadolinium atoms are represented by the spheres and the tetrahedrons represents the VO4

-3 ions9.

This crystal lattice exhibits a so-called totally symmetric “breathing” vibration mode (Figure 4) in 882cm-1. This crystal, when doped with Nd+3 ions is capable of shifting the fundamental line at 1062.9nm to 1173nm due to this mode of lattice vibration. The Nd:GdVO4 is a relatively new laser active media, being first introduced in 1992, by Zaguniennyi10 et al. It’s application as a self-Raman laser material was proposed by Kaminskii11 demonstrated by Chen12 in 2004. The GdVO4 lattice shows better thermal proprieties then other common orthovanadate crystals such as YVO4 being more suitable to act as a host material for high power solid state lasers11.



Figure 4. Totally symmetric “breathing” vibration mode in a tetrahedral structure. In GdVO4 this frequency is 882cm-1.

Lasers in which doped, Raman –active media are employed for the generation of both the fundamental laser field and Raman-shifting are referred to as self-Raman lasers. The key advantages of self-Raman lasers in comparison with laser which employ separate laser and Raman materials is that they have less interfaces (leading to reduced internal loss), enable the generation of shorter resonators and can be inherently cheaper due to the need for a single crystal.

2. EXPERIMENTAL ARRANGEMENT

A self-Raman laser cavity was designed as illustrated in Figure 5. The cavity was longitudinally pumped by a 30W laser diode operating in 880nm (model HLU30F200-880, LIMO) coupled to a 200�m core diameter optical fiber. 880nm is utilized to directly pump the upper laser level 4F3/2 and thereby reduce the quantum defect deficit and lower the thermal loading within the laser media13. The pump laser beam pass is focused onto the active media using an optical system with magnification M=2 to a spot with diameter ~ 400�m. The active media consists of an 4x4x20mm a-cut 0.3at% doped Nd:GdVO4 crystal (CASTECH). This crystal was AR coated at 1064 and 586.5nm by ATFilms. The SHG element is a LBO crystal (NCPM, =90o, =0o) 4x4x10mm AR coated at 586.5/1064/1173nm supplied by CASTECH. The LBO crystal was maintained at a temperature of 45.5 oC.

The resonator contains three mirrors also coated by ATFilms. The input mirror is a plane substrate AR coated at 880nm and HR coated (R>99.9%) at 1064/1173nm. The intracavity mirror is a plane mirror AR coated at 1064/1173nm and HR coated at 586.5nm. The output coupler is a 50mm ROC mirror HR(R>99.9%) at 1064/1173nm and HT at 586.5nm.

Figure 5. Diagram of the self-Raman laser cavity.

A dichroic intracavity mirror, coated HT at 1063/1173 nm and HR at 586.5 nm (side facing the LBO crystal) was positioned between the Nd:GdVO4 and LBO crystals. This mirror enabled reflection of the yellow emission propagating towards the laser crystal, through the output coupler and maximized the emission and reduced thermal loading of the laser crystal.

In this work, two resonators were constructed, one on an optical bench (herein referred to as optical table model) and one designed for integration with an electronic control console (herein referred to as miniature model). The optical table



model employed large 3-axis micrometer-driven mounts and water-cooled crystal mounts, while the miniature model utilised in-house manufactured miniature mounts with fewer degrees of freedom, and also employed thermoelectric cooling of the crystals. During testing, the laser diode and feedback was controlled by an electronic console developed by Opto Eletrônica.

3. RESULTS

3.1 Optical table model

Figure 6 shows a photograph of the optical table setup (also shown in the top right is the electronic console used to control the pump diode, LBO temperature and feedback). During power scaling, the diode temperature was tuned to maintain emission at the absorption peak (~879nm) of the Nd:GdVO4 crystal (the diode exhibited a wavelength-temperature dependence of ~0.25nm/ºC).

Figure 6. Laser cavity mounted in optical table.

The power scaling performance of the laser at 586.5 nm is shown in Figure 7. In excess of 4.2W CW and 5.5W instantaneous power in a 50% duty cycle regime (100ms on/100ms off) was achieved. This represents a diode to yellow efficiency of ~14% in the CW and 17.2% in the 50% duty cycle regime. A beam quality factor of M2~ 6 was measured in both cases.



0 5 10 15 20 25 30 35

0

1000

2000

3000

4000

5000

6000

Yello

w P

ower

@58

6.5n

m (m

W)

Diode Power (W)

CW 50% Duty Cycle

Figure 7. Performance of the laser cavity in the optical table model for the CW and 50%Duty Cycle regime.

3.2 Miniature model

The miniature model was air-cooled and designed for integration within a compact console which would be commercially viable. As such, it was designed to strict weight and size specifications, and most importantly for robust operation without drift over time. Figure 8 shows a schematic of the miniature cavity and its power scaling performance is shown in Figure 9.

Figure 8. Schematic of the miniature cavity model.



0 5 10 15 20 25 30 35

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Yel

low

Pow

er @

586.

5nm

(mW

)

Diode Power (W)

CW 50% Duty Cycle

Figure 9. Performance of the laser cavity in the miniature model for the CW and 50%Duty Cycle regime.

In this miniature model we achieved more than 3.5W in CW regime @ 17W pump and more than 4.4W in the 50% duty cycle regime for the same pump power. This represents a diode to yellow optical efficiency of 11% in the CW and 13.8% in the 50% duty cycle regime. By modulating the diode laser current, the laser could operated over wide range of exposure times (10ms upwards) and duty cycles, as required for ophthalmic photocoagulation. The beam quality factor for the yellow laser output was measured to be about M2~7.

To fully integrate the miniature cavity with the console, a coupling system was designed to enable sampling a small portion of the beam (~ 3%) for active feedback control, and fibre-coupling (to a 100 µm core diameter fibre) of the yellow emission. A schematic of the coupling system is shown in Figure 10. With active feedback to control the diode current, the laser power remained constant during exposure times.

Figure 10. Image of the laser cavity with coupling system.



A photograph of the completed console is shown in Figure 11.

Figure11. Console of Opto Eletrônica’s Mitra photocoagulator.

4. CONCLUSION

In conclusion we have demonstrated more than 4.4W CW yellow emission and more than 5.5W (instantaneous) in a 50% duty cycle pulsed regime. To the best of our knowledge, this is the highest CW power for a self-Raman laser that has been reported. The diode to yellow efficiency was around 14% in the CW regime and more than 17% in the 50% duty cycle regime. These values are very satisfactory for developing a yellow laser for photocoagulation. The process of transferring this technology from the optical table to the inside of a laser console is not an easy task because the console requires fewer degrees of freedom must be air-cooled. But all these issues were overcome and we obtained a miniaturized laser cavity that operates with 80% of the power available from the optical table model. There remains some potential to engineer the miniature laser cavity in order to achieve the optical table efficiency.

Nevertheless we have clearly demonstrated a laser cavity that meets our the specifications for an ophthalmic photocoagulator. In August 2009, the Opto Eletrônica company launched in the Brazilian market the Mitra photocoagulator, the first yellow laser based photocoagulator for ophthalmological proposes produced in Latin America. Only two months later Opto Eletrônica company showed the global ophthalmological market this same photocoagulator exhibiting Mitra console in the 2009 American Academy of Ophthalmology annual meeting. Figure 11 shows the Mitra console.

5. ACKNOWLEDGEMENT

The author thanks to Opto Eletrônica and Opto Global companies for the financial support for this project and to Macquarie University, particularly to Dr Helen Pask and Dr Andrew Lee for the partnership.



REFERENCES

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969 (1968) 10. Zagumennyi, A., I., et al., Sov. J. Quantum Electron 22 (1992) 1071 11. Kaminskii, A., A., et. al., “Tetragonal vanadates YVO4 and GdVO4- new efficient �(3)-materials for Raman Lasers”,

Optics Commun. 194, 201-206 (2001) 12. Chen, Y. F., “Efficient 1521-nm Nd:GdVO4 Raman laser”, Optics Letters 29, 2632-2634 (2004)13. Lupei, V., Pavel, N., Sato, Y., Taira, T., “Highly efficient 1063-nm continuous-wave laser emission in Nd:GdVO4,”

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