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Journal of the Optical Society of America B

Cross talk between bistable elements on an InSb étalon

James Young, David J. Hagan, Harvey Richardson, Hugh Alexander MacKenzie, E. Abraham

Published Jan 1, 1988• 2 citations

Abstract

Cross talk due to transverse carrier diffusion between identical bistable elements on an InSb étalon at 80 K is studied both theoretically and experimentally. Measurements of cross talk between two such elements show that typically, for a practical system, a critical separation of a few diffusion lengths between beam centers is necessary for independent operation. When the difference between holding and switching powers approaches zero, the results show that the critical distance diverges. Delays as great as 4 μsec have been observed when one element is switched and its neighbor is induced to switch through the cross-talk mechanism. These delays can be partially attributed to critical slowing down, and, in principle, they diverge as the interelement distance is increased toward the critical separation. A theoretical model based on the diffusion equation for the single-pass nonlinear phase shift shows how the critical separation depends on physical parameters and the number of elements forming an array. We found that the relevant interaction is essentially limited to nearest neighbors. When the theory is applied to two bistable elements, good agreement is obtained with the experiments in both the steady and the dynamic states.

DOI: 10.1364/josab.5.000003

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Cross talk due to transverse carrier diffusion between identical bistable elements on an InSb étalon at 80 K is studied both theoretically and experimentally. Measurements of cross talk between two such elements show that typically, for a practical system, a critical separation of a few diffusion lengths between beam centers is necessary for independent operation. When the difference between holding and switching powers approaches zero, the results show that the critical distance diverges. Delays as great as 4 μsec have been observed when one element is switched and its neighbor is induced to switch through the cross-talk mechanism. These delays can be partially attributed to critical slowing down, and, in principle, they diverge as the interelement distance is increased toward the critical separation. A theoretical model based on the diffusion equation for the single-pass nonlinear phase shift shows how the critical separation depends on physical parameters and the number of elements forming an array. We found that the relevant interaction is essentially limited to nearest neighbors. When the theory is applied to two bistable elements, good agreement is obtained with the experiments in both the steady and the dynamic states.

We study theoretically static and dynamic effects of cross talk between two bistable elements in the same interference filter. The nonlinearity is assumed to be thermal. In the steady state we find that an element in the ON state causes a thermal detuning in another in the OFF state, thereby decreasing its threshold power and narrowing its bistable region. We discuss various ON/OFF combinations in terms of interpixel separation. Starting with two elements in the OFF state, transients are studied by switching one of them with a pulse while the other one remains unaddressed. As the separation approaches a critical value from below, the switching time of the second element diverges. We interpret this effect as critical slowing down. Extension of these results to any array is also discussed.

We demonstrate the simultaneous, independent, and nearly identical operation on nanosecond time scales of GaAs optical bistable devices. A Wollaston prism inserted into the input beam produced two equal-intensity beams which were focused to <10-μm diam spots centered ~40 μm apart. Achievement of equal switch-on intensities for the two spots was straightforward and easy. Elimination of either beam by a polarizer revealed no visible change in the response to the other; thus the switches operated independently. Operation of four spots did not yield uniform response in this initial attempt. The device consists of sixty-one periods of 299 Å GaAs and 98 Å AIGaAs multiple quantum well structure capped by ~2000-Å AIGaAs windows and sandwiched between 94% reflectance mirrors. Using two independently controlled beams we could vary continuously the distance between two spots. Diffusive crosstalk was then identified by a decrease in switch-on intensity of one of the beams when the other beam was brought sufficiently close. The extent of the carrier diffusion varied with position on the sample, orientation of the spots, and the switch-on intensities. The critical spacing at which crosstalk became noticeable ranged from 35 μm to more than 100 μm.

The dependence of switching power upon input beam spot radius for an optically bistable InSb étalon has been measured. The results show that the irradiance required for switching increases as the spot radius is reduced. There is no evidence of any lower limit to the switching power, even for spot radii much less than the ambipolar diffusion length. Studies of the transmitted beam intensity profile in the near-field demonstrate the role of diffusion in our observations. The implications of our results for digital optical processing are briefly discussed.

This Letter is concerned with the parallel operation of optical bistable devices. The two-beam side-by-side case has been simulated numerically. Diffraction coupling from one device operating in the upper branch can cause the other one, operating in the lower branch, to switch to the upper branch if the separation of the two beams is too small. The separation required for independent operation is usually a few beamwidths. The results give promise for performing parallel operations with optical bistable devices on the same étalon.

We report the first known direct observation of the critical slowing-down effect and transient bimodality in an intrinsic semiconductor all-optically bistable device with a cw CO laser holding beam and a PbSSe diode laser switching beam. The measurements of critical slow-down effects show good agreement with the numerical model using a modified Debye dynamic equation. The switching profile has been shown to depend on both the size of the hysteresis loop and the diode pulse parameter. A scaling law for the delay time of the switching when external parameter P changes close to the critical value Pc has been indicated to fit (P − Pc)−1/2.

We report the first observation of quasi-steady-state optical bistability in an InSb etalon at room temperature as a result of the generation of free carriers through two photon absorption of 10 μm radiation. Nonlinear tuning of the etalon through two free spectral ranges has been achieved, with bistability observed around the two cavity resonances. Switching from a low to high transmission state has been achieved in times less than 10 ns while switching in the opposite direction occurs within 50-100 ns. The experimental results have been successfully fitted to a numerical model of the device.