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IEEE Journal of Quantum Electronics

Effect of chirped gratings on reflective optical bistability in DFB semiconductor laser amplifiers

Drew N. Maywar, Govind Prasad Agrawal

Published Jan 1, 1998• 35 citations
Publisher

Abstract

We use spatial chirp of the built-in grating to improve optical bistability on reflection from distributed feedback semiconductor laser amplifiers. We show that improvements in the on-off switching ratio occur because spatial chirp greatly affects the saturation behavior of the reflectivity resonances, allowing access to states of low reflection during bistable switching. We also show that spatial chirp modifies the spectral range of the variety of hysteresis shapes that occur on reflection. In doing so, we discover a new type of hysteresis loop that reveals a qualitative difference between the loop-shaped hystereses occurring near the two edges of the stop band. With spatial chirp, the new hysteresis loop can exhibit an on-off switching ratio in excess of 100000.

DOI
10.1109/3.736108

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We use spatial chirp of the built-in grating to improve optical bistability on reflection from distributed feedback semiconductor laser amplifiers. We show that improvements in the on-off switching ratio occur because spatial chirp greatly affects the saturation behavior of the reflectivity resonances, allowing access to states of low reflection during bistable switching. We also show that spatial chirp modifies the spectral range of the variety of hysteresis shapes that occur on reflection. In doing so, we discover a new type of hysteresis loop that reveals a qualitative difference between the loop-shaped hystereses occurring near the two edges of the stop band. With spatial chirp, the new hysteresis loop can exhibit an on-off switching ratio in excess of 100000.
The distributed-feedback semiconductor optical amplifiers(DFB-SOAs) possess the dual functions of amplifier and filter due to the built-in grating.If the input power is close to saturation value,dispersive optical bistability occurs as a consequence of the positive feedback loop informed among the optical intensity in active-layer,effective mode refractive index,and resonance amplification frequency.By introducing spatial phase-shift and chirp,the filter and bistable performances can be improved.A novel configuration based on linearly variable current injection is proposed to improve the filter and bistable characteristics of the DFB-SOAs.The coupled mode equation and carrier rate equation are used to perform characteristic comparisons of this configuration with phase-shifted and chirped DFB-SOAs.The results show that,this method not only enhances the on-off switching ratio,but also has similar transmission spectra with phase-shift effect method,and similar bistable features,such as the wide-band and the flat upper branch of the hysteresis,with chirp effect method.
Dispersive optical bistability in a Fabry-Perot, a distributed feedback (DFB), or a distributed Bragg reflector (DBR), semiconductor laser amplifier has recently attracted much attention, because of its potential in the operation of highly functional optical devices. While the steady-state operation of bistable diode laser amplifiers are well understood, studies of the dynamic properties have been limited to switching time, critical slowing down and output spiking measurements[1-3]. Little attention has been paid to the dynamic frequency chirp which is associated with the turn-on spike at the onset of light injection. Such frequency chirp exists because in dispersive bistability the Fabry-Perot resonance (or the DBR/DFB transmission peak) sweeps through the injected wavelength as a result of alight induced refractive index change. In this paper, we report on a theoretical analysis and the first experimental observation of this dynamic chirp in a Fabry-Perot type nonlinear semiconductor laser amplifier. It is found that the magnitude of the chirp depends on the input power, which in turn is related to critical slowing down.
In all-optical switching systems bistable distributed-feedback (DFB) amplifiers are being used experimentally as memory cells and signal regenerators. In this frame a single section DFB amplifier (with one AR coated facet) operating around threshold at 1549 nm is pumped by a similar laser with a carefully controlled frequency detuning. The polarization controlled pump beam is modulated by a Mach-Zehnder switch (2.5 GHz bandwidth) to avoid source frequency chirp. The measurement of a pump reference and of the beam reflected from the amplifier is performed through a fiber directional coupler inserted between the two DFB diodes. Optical bistability has been observed both in transmission and in reflection for a sinewave modulated input in the 1-200 MHz frequency range. The output square wave exhibits up and down switching times lower than 1 ns and about 2 ns, respectively. The latter figure is consistent with the carrier recombination time in InGaAsP active structures, which is the main limit to the recovery time of bistable amplifiers. Work is in progress to investigate this feature in other types of amplifiers (e.g., multisection DFBs and quantum well structures).
Optical bistability (OB) behavior in quarter-wave-shift (QWS) distributed coupling coefficient (DCC) distributed feedback (DFB) semiconductor laser amplifiers (SLApsilas) is investigated. The simulations are based on a modified transfer-matrix method, which considers grating non-uniformities along the structure as well as gain saturation effect. Switching characteristics, including structural dependence of OB in QWS DCC DFB SLApsilas is demonstrated in the steady state regime. Time-dependent analysis of OB is performed as well, and the dynamics of optical switching is demonstrated.