Microwave phase noise properties of optical links

1. Microwave phase noise properties of optical links involving small signal and gain saturated optical amplifiers Vincent Auroux1,2, Arnaud Fernandez1, Olivier Llopis1, Pierre-Henri Merrer2, Alexandre Vouzellaud2 1CNRS, LAAS, Univ. de Toulouse, France 2OSAT, 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. Residual phase noise measurement setup Amplitude to phase noise conversion Noise reduction SOA EDFA • 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 • 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 Analytical model for noise floor variation Reduction of noise floor by saturating the amplifier EDFA phase noise SOA phase noise • Features : • Output saturation power : 16 dBm • Small signal gain : 25 dB • Noise figure : 6 dB • 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 • 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 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. 10.2 GHz -135 dBc/Hz @ 10 kHz Contact : vauroux@laas.fr/llopis@laas.fr