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Annual Meeting Optical Society of America

Time characteristics of bistable DFB amplifiers for all-optical switching

Riccardo A. Calvani, M. Calzavara, Renato Caponi

Published Jan 1, 1992• 0 citations
Publisher

Abstract

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).

DOI
10.1364/oam.1992.wg1

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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).
All-optical bistable switching dynamics of 1.55-/spl mu/m two-segment strained multiquantum-well (MQW) distributed-feedback (DFB) lasers were systematically studied both experimentally and theoretically. Some fundamental optical functionalities, including all-optical set-reset (flip-flop) operations, were demonstrated. The switching responses of these bistable lasers were studied, for the first time, with optical injection from a single-mode DFB laser, indicating that the switching dynamics based on gain quenching and absorption saturation are inherently different. A theoretical model including optical injection was developed to study these all-optical bistable switching characteristics in segmented bistable lasers. It was found that the nonuniform distribution of the photon density in the bistable laser cavity induced by optical injection was essential to perform the time-domain switching operations. Simulations showed a good agreement with experimental observations and indicated design improvements. Although the switching responses in the range of tens of picoseconds can be obtained with these bistable lasers, the maximum repetition frequency of the bistable systems would still be limited to the hundreds of megahertz due to the slow carrier recovery time (5 ns) of the lasers.
We report the demonstration of cascadability in bistable diode laser amplifiers operated below threshold. A schematic of the experimental set-up is shown in Figure 1. There it is shown that the optical input to the first bistable diode laser amplifier is modulated by a mechanical chopper at frequency Ω1. The output from the first bistable diode laser amplifier is then modulated at Ω2= 2Ω1using a second mechanical chopper. The light is thereafter used as the input to the second bistable device. When the input intensity to the second device is higher than a certain threshold, the output switches. These results are shown in Figure 2. For these experiments, each of the bistable diode laser amplifiers were carefully studied. Detailed bistability measurements at different bias currents and different cavity detuning's were performed. It was found that both the bistability loop and the switching power gets larger as the detuning increases. The size of the hysteresis loop also increased as the laser was operated closer to threshold. In a two beam experiment, it was found that the switching power in the data beam can be as small as 25 nW. Gains larger than 1000 have been demonstrated, indicating the large fan-out capability of these devices. The turn-on and turn-off times in these bistable diode laser amplifiers was measured to be faster than 1 ns and detector limited. These bistable diode laser amplifiers are the most efficient optical devices that have yet been demonstrated and will find applications in photonic switching networks.
We demonstrate optically bistable (light switching light) operation in a room-temperature InGaAsP Fabry-Perot amplifier. The amplifier is an InGaAsP/InP laser structure biased at ~95% of its lasing threshold. It is switched with <3 μW of optical power provided by the mechanically chopped output of a quasi-single-mode InGaAsP/InP laser. The experimental apparatus is depicted in Fig, 1. A change in the refractive Index in the amplifier of only 5 × 10−4 is sufficient to cause switching. Input and output pulses from the nonlinear amplifier are shown in Fig. 2 when the source frequency is initially detuned by 14 GHz from the closest Fabry-Perot mode of the amplifier. Smaller switching powers are required at lesser detunings. The physical mechanism of the intensity-dependent change of the refractive index is thought to be the change in carrier density resulting from stimulated recombination of free carriers.
Switching of a signal beam by another control beam at different wavelength is demonstrated experimentally using the optical bistability occurring in a 1.55 µm-distributed feedback semiconductor optical amplifier (DFBSOA) working in reflection. Counterclockwise (S-shaped) and reverse (clockwise) bistability are observed in the output of the control and the signal beam respectively, as the power of the input control signal is increased. With this technique an optical signal can be set in either of the optical input wavelengths by appropriate choice of the powers of the input signals. The switching properties of the DFBSOA are studied experimentally as the applied bias current is increased from below to above threshold and for different levels of optical power in the signal beam and different wavelength detunings between both input signals. Higher on-off extinction ratios, wider bistable loops and lower input power requirements for switching are obtained when the DFBSOA is operated slightly above its threshold value.
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.