Demonstration of optical noise reduction using nonlinear absorption in a semiconductor laser amplifier

We propose a new way to use a semiconductor laser amplifier driven under a loss condition for a signal light that acts as an ultrafast noise reduction device using a nonlinear absorption effect. The absorption recovery time was reduced by stimulated recombination due to continuous-wave additional light (assist light) whose wavelength was longer than that of the signal. This simple method is an attractive technique to realize optical functional devices with an ultrahigh-repetition rate. We have successfully confirmed a receiver sensitivity improvement of 0.5 dB in a 5-Gb/s nonreturn-to-zero signal by using this principle. The improvement of the eye opening by passing through the noise reduction device was estimated theoretically to reveal design rules for an optimum optical noise reduction device.