Vincent Auroux1,2, Arnaud Fernandez1, Olivier Llopis1, Pierre-Henri Merrer2, Alexandre Vouzellaud2

1 CNRS, LAAS, Univ. de Toulouse, France2 OSAT, 16 avenue Didier Daurat, Toulouse, France

Phase noise of optical links involving optical amplifiers are presented. Noiseless optical links are required in several telecommunication applications. The amplifier appears to be a significant contributor to the link phase noise and the purpose is to determine in which conditions the amplifier is noiseless. The optical amplifiers investigated are of two types: Semiconductor Optical Amplifier (SOA) and Erbium Doped Fiber Amplifier (EDFA). A noise reduction technique based on optical amplitude to RF phase conversion suppression is also described. Residual phase noise measurements for both amplifiers are compared and the evolution of noise floor is related to noise figure measurements. The choice of the suitable amplifier applications is important for high quality frequency sources like coupled optoelectronic oscillators (COEO). And the first device measurements are presented.

Microwave phase noise properties of optical linksinvolving small signal and gain saturated optical amplifiers

Contact : vauroux@laas.fr / llopis@laas.fr

Residual phase noise measurement setup

3,5 GHz low phase noise and low amplitude noise dielectric resonant oscillator (DRO)

Cross correlation technique RF delay to compensate the optical delay introduce by the link

=> No parasitic DRO phase noise detection

Amplitude to phase noise conversion Noise reduction

Optical power on photodiode : 10 mW Suppression of amplitude to phase conversion on

the photodiode

10 dB phase noise reduction for the EDFA No reduction for the SOA

linear

saturated

Measurement floor

CNR linearCNR saturated

SOA phase noise

Analytical model for noise floor variation

Low 1/f close to the carrier phase noise Reduction up to 10 dB in saturated regime Good fit with the analytical calculation of

CNR

𝐹=2𝑃 𝐴𝑆𝐸

𝐺 ∙ h𝜈 ∙ 𝛥𝜈𝑆𝜙=

2h𝜈 ∙ 𝐼𝑃𝐷2 ∙𝐹

𝑃𝑖𝑛

𝐶𝑁𝑅=10 𝑙𝑜𝑔( 𝐼 𝑃𝐷2𝑚2

2(𝑅𝐼𝑁 ∙ 𝐼𝑃𝐷2+4 𝑘𝑇𝑅𝑑

+2𝑞 𝐼𝑃𝐷+2h𝜈 ∙ 𝐼 𝑃𝐷

2 ∙𝐹𝑃 𝑖𝑛

))

EDFA phase noise

Reduction of noise floorby saturating the amplifier

Features :• Output saturation power : 16 dBm• Small signal gain : 25 dB• Noise figure : 6 dB

Features :• Output saturation power : 17 dBm• Small signal gain : 25 dB• Noise figure : 4,5 dB

Phase noise important close to the carrier Reduction up to 15 dB in saturated regime Variations of noise floor are well predicted

but the model does not fit well

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1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06

Phas

e no

ise

(dBc

/Hz)

Frequency (Hz)

COEO 400m (SOA)

10.2 GHz-135 dBc/Hz @ 10 kHz

SOA based COEO Conclusions

Phase noise of optical amplifiers in optical links have been investigated :

There is an optical amplitude to RF phase conversion on the photodiode which can be suppressed by adjusting the optical power on the photodetector

An analytical model has been used to predict variations of noise floor while saturating the amplifier

As SOA has good performance close to the carrier, EDFA presents better noise floors due to its lower noise figure : there is a tradeoff

A coupled optoelectronic oscillator has recently been realized with the SOA and presents good performances, close to the state of the art.

SOA EDFA