Numerical investigation into the dynamics of externally-injected, gain-switched lasers for optical comb generation

Abstract

We present simulation results of the nonlinear dynamics of externally-injected, gain-switched laser comb sources. We implement a fully-stochastic laser model and highlight the trade-off between the number of comb lines and the noise properties of the comb. Introduction The potential for high capacity arising from high spectral efficiency in the next generation of wavelength division multiplexed (WDM) optical communication systems can be unlocked through superchannel technology if each of the optical carriers are phase coherent i.e. possesses the same phase noise properties 1 . Typical optical multicarrier sources that achieve phase coherence would be: mode-locked-lasers (MLLs) 2 ; cascaded Mach-Zehnder modulators (MZM) 3 and externally-injected (EI), gainswitched (GS) lasers 4 . MLLs suffer from mode partition noise and have a fixed free spectral range (FSR); MZMs are usually bulky and require stringent control of the biasing and drive amplitudes to produce a flat comb for long-term stability; on the other hand, EI, GS lasers can be integrated, produce combs with flexible FSR with narrow linewidth on each line, without mode partition noise and provide for stable operation 4,5 . Despite the advances in the development of the EI GS laser comb source, a numerical study into their dynamical properties has been lacking. In this paper we present comprehensive simulation results detailing the dynamics of the EI, GS laser comb source. We base our model on a model for lasers with EI in 6 that considers all the Langevin noise terms and we add dynamic current injection to the model to investigate the stochastic nature of the comb generation through gain switching. Our simulation results give guidelines as to how to achieve a desired performance, i.e. trade-off between comb flatness and carrier to background ratio (CBR) for comb optimization. External-Injected, Gain-Switched Laser The use of an optical comb generator in a WDM transmission link is shown in Fig. 1(a). Each of the comb lines are separated using the arrayed waveguide grating (AWG) filter and modulated; each of the modulated comb lines are then multiplexed using the second AWG, transmitted over the entire fiber link, demultiplexed and then detected at the receiver (Rx.). The comb source in Fig. 1(a) could be an EI GS laser; the constituents of which are illustrated in Fig. 1(b). The slave laser is biased using the DC current source and external injection is carried out from a master laser. The RF source at frequency mod f is used to gain switch the EI slave. The combination of external-injection with gain switching leads to a comb similar to that shown in the inset of Fig. 1(b). The rate equations used to simulate the EI, GS laser are given by Eqs. (1)-(3) 6 ; all the symbols throughout have their usual meanings and ‘ml’ denotes master laser. The Langevin noise terms are all denoted by an F . sp R is the spontaneous carrier recombination rate and is Fig. 1 (a) Concept of using an OCG within a WDM communications link. The OCG is the source for the WDM carriers. (b) Anatomy of the externally-injected, gain-switched comb source. Light from the master laser is injected into the slave laser. The slave is gain switched by the microwave oscillator at a frequency mod f . Exemplary simulated comb is shown in the inset. Comb A W G A W G A W G Rx. Data