Optical nonlinearities in semiconductor optical amplifier and electro-absorption modulator: their applications to all-optical regeneration

Abstract

Semiconductors have large optical nonlinearity with response speed in the several tens of picosecond range, making them ideal use as all-optical regenerators and wavelength converters. We theoretically and experimentally investigated optical nonlinearities induced by carrier dynamics both in forward biased semiconductor waveguide (SOA) and in reverse biased semiconductor waveguide (EAM). We made a detailed theoretical study of carrier dynamics in semiconductor waveguides by using the newly developed time-dependent transfer matrix method. To confirm the simulation results, we propose utilizing a polarization discriminating delayed interferometer (PD-DI) configuration as a simple technique for measuring optical nonlinearities such as cross gain modulation (XGM), cross absorption modulation (XAM), and cross phase modulation (XPM). In the first part of the paper, we reviewed SOA-based regenerators. As expected from the simulation results, we confirmed that injection of the transparent assist light was very effective in reducing of the SOA gain recovery time of down to a few tens of picoseconds. We further demonstrated 40 Gbit/s regeneration using an SOA-one-arm MZI (so-called UNI) configuration. The superior regeneration capability of two-stage UNI was successfully confirmed by a recirculating loop experiment up to 30,000 km with 150 regenerations. In the latter part of the paper, we reviewed all-optical regenerators using EAM. A bit-synchronized rf-driven XAM 3R regenerator consisting of only one EAM for both gating and timing correction was demonstrated at 20 Gbit/s. An EAM in conjunction with delayed interferometer configuration, which utilizes XPM as well as XAM in the EAM, has structurral simplicity and fast regeneration operability up to 100 Gbit/s. The fast response of EAM allows the optical regeneration with a small pattern word effect. 498Masashi Usami and Kohsuke Nishimura

A short carrier lifetime semiconductor optical amplifier with n-type modulation-doped multiple quantum well structure

“…Assuming other factors are not changed by the n-type modulation-doping, FWM wavelength conversion efficiency could be enhanced by a factor of 6.96 just due to the carrier lifetime shortening compared with undoped MQW SOA. For the XGM and XPM applications, the evolution of amplified CW probe light power and the phase at the output of the SOA as a function of time I(t) due to XGM and (t) can be expressed as [2]…”

“…However, optical nonlinearity in SOAs induced by the carrier density variation is limited by the relatively long carrier lifetime, which restricts the SOA switching speed and their scalability when a cross-gain modulation (XGM) or cross-phase modulation (XPM) mechanism is used. Therefore, an optimum switching condition to obtain the highest extinction ratio and minimal pattern effect will depend on the interplay of XPM, XGM and the gain recovery time of the SOA [2]. Even though four wave mixing (FWM) based on SOAs has the unique feature of being compatible with multichannel WDM data, with phase-and frequency-modulated signals, and with analogue signals, their conversion efficiency becomes a critical limitation for them to be used as wavelength conversion.…”