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Digest on Nonlinear Optics: Materials, Phenomena and Devices

Demonstration Of Cascadability And Spectral Bistability In Bistable Diode Laser Amplifiers

Zeqi Pan, T. N. Ding, M. Dagenais

Published Jan 1, 1990• 0 citations

Abstract

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.

DOI: 10.1109/nlo.1990.695926

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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 describe two approaches to low-energy switching and optical bistability. In one approach, which is a more fundamental approach, we use the nonlinearity associated with bound excitons in CdS to demonstrate the lowest total switching energy (4 pJ) of any optical device exhibiting two stable output states for the same input intensity.1 Bound excitons were chosen because they are known to have a very large oscillator strength (f = 7), a fast radiative decay (tspont = 500 ps), and they do not diffuse in the crystal, thus permitting a sharp focusing geometry. Furthermore, they are a good approximation to quantum dots. In a different experiment we have operated a standard diode laser structure below threshold and have demonstrated an efficient optical switch that can be used in thresholding and decision-making applications. The device switches with <400 aJ (4 × 10−16 J or <3000 photons) of optical energy incident on the device.2 During the switching operation, only 10 pJ of electrical power is dissipated. The switch operates at room temperature, is compatible with optical communication systems, and has a gain of the order of 15 dB, leading to the important properties of cascadability and large fan-out. The operation of a high contrast (5:1) AND gate was demonstrated.

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.

Bistable laser amplifiers are attractive for potential application in optical switching because of very low input optical requirement, high extinction ratio and the presence of optical gain. Optical bistability (OB) arises from gain saturation and the associated nonlinear refraction combined with the effect of feedback in the laser cavity. We present the experimental results of bistable characteristics for the case of a 1.55 /spl mu/m symmetric Fabry-Perot (F-P) laser amplifier with uncoated facets. This paper reports a particularly rich variety of behaviour for reflected signals. Three main forms of hysteresis loops are observed on reflection. The hysteresis loop changes from anticlockwise to clockwise hysteresis in the optical input-output characteristic when the drive current or initial phase detuning is changed. The butterfly hysteresis loop is observed experimentally at optical input power as low as 260 /spl mu/W for a drive current equal to 90% of the lasing threshold. The dependence of the optical bistability on drive current, initial phase detuning and injected optical power has been investigated. In the bistable light output-initial phase detuning characteristic, the optimum frequency is shifted downward with an increase in the injected power or the drive current. All measurements are in good agreement with an analytical model.

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).