Abstract

The growing bandwidth demand of broadband distributive and interactive services can be met by combining wavelength division multiplexing (WDM) and optical time-division multiplexing (OTDM) to provide an even larger transmission capacity. The multichannel grating cavity (MGC) laser has the potential to generate multiwavelength picosecond pulses for such WDM/OTDM transmission systems [1]. Furthermore, the MGC laser can also be used to perform novel network functions, including space-switching and multiplexing. Integrated devices are attractive because of reduced costs, compact size, low insertion loss and improved mechanical stability. The integrated MGC laser has been demonstrated [1], but there is very little reported on design considerations of the device. We perform numerical simulations of the integrated MGC laser by combining a set of interconnectable bidirectional models comprising the master stripe, slave stripe and the transmission grating. All simulations are done for a single channel only. The numerical model we used here is based on the time-domain transmission-line laser model, which includes electrical parasitics [2]. The model is based on propagation of optical waves, in contrast to optical intensity used in the rate equations. All the important physical processes of the laser is considered, including self-phase modulation, reflections, spontaneous emission noise, the filtering effect of the dispersive element, gain saturation and wavelength dependence of gain. Nevertheless, it is computationally efficient and can be useful to identify important design factors quickly. We also propose an improved design of the integrated grating compound cavity laser by using a tapered amplifier as the slave (output) waveguide (Fig.1). Picosecond pulses are generated by active mode-locking, the shorter narrow-stripe master (input) waveguide is injected with RF power of ~20dBm at close to the cavity resonance frequency. Stable pulses with narrowest pulsewidth and highest peak power can only be achieved when the peak of the returning pulse, owing to residual reflection at the slave waveguide, coincides with the peak of gain modulation. The integrated dispersive transmission grating, which consists of a set of triangular-shaped recesses, acts like a passive filtering element. The tapered amplifier (slave) provides higher saturation output power, therefore giving less distortion during pulse amplification, resulting in lower crosstalk. Furthermore, picosecond pulses of higher peak powers can be generated by using the tapered amplifier, resulting in higher extinction ratio. The tapered active waveguide is designed to achieve quasi-adiabatic propagation with minimal loss and mode conversion. By forming a tapered waveguide with length of 900 μm and output width of 30μm, peak powers of above 450mW can be achieved (Fig. 2). The pulsewidth changes by very little with wider tapers, evidence of reduced gain saturation (Fig. 3). Simulation results of the stability and spectral behavior of the high power picosecond pulses will be presented. Device design considerations (including grating), the effect of laser material parameters, and optimum operating conditions will be discussed.

© 2000 IEEE