Multi-wavelength semiconductor lasers using waveguide grating routers

By combining gain elements with a waveguide grating router (WGR), one can make a multi-wavelength laser. Such multifrequency lasers (MFLs) consist of an array of N semiconductor optical amplifiers (SOAs), a WGR, and an optional shared SOA, all between a set of mirrors. Turning on a particular SOA induces laser oscillation at one of the N wavelengths. The most influential feature on the performance of MFLs is their long cavity. MFL cavities typically have cavity-mode spacings of 3 GHz, more than 30 times longer than typical DFB and DBR lasers. The round-trip time in MFLs approaches the spontaneous emission time of the SOA carriers. Subsequently, MFLs behave substantially differently than DFB and DBR lasers in three main ways. First, while the direct modulation speed is largely determined by the photon lifetime in short-cavity lasers, it is largely determined by the cavity round-trip time in MFLs. Second, the linear concept that the laser oscillation frequencies are in the cavity modes nearest the intracavity filter peak does not apply to MFLs. Instead nonlinearities result in a strong hysteresis of the oscillation frequency. It is quite possible for an MFL to oscillate in a single-longitudinal mode which is multiple cavity modes away from the filter peak. With proper design, the hysteresis can guarantee stable, single-longitudinal-mode behavior in MFLs without the need for external observation. Third, the narrow cavity-mode spacing, combined with the fact that the WGR is passive, implies that the oscillation wavelength shifts very little with amplifier changes, and may eliminate the need for external filter stabilization. Some of the main applications of these MFLs are discussed.