19.1 INTRODUCTION

As the name implies, an optical limiter is a device designed to keep the power, irradiance, energy, or fluence transmitted by an optical system below some specified maximum value, regardless of the magnitude of the input. It must do this while maintaining high transmittance at low input powers. The most important application of such a device is the protection of sensitive optical sensors and components from laser damage. There are many other potential applications for such devices, including laser power regulation or stabilization, or restoration of signal levels in optical data transmission or logic systems, but this chapter will primarily concentrate on devices for sensor protection.

Perhaps the most obvious way of achieving optical limiting is by active control, where input light levels are monitored by a sensor, which through some processor activates a modulator or shutter that in turn limits the transmitted light. The best-known examples of these are the iris and blink response of the eye. However, these are limited in speed to about 0.1 seconds, so that any intense pulse of light shorter than this can get past these defense mechanisms and damage the retina before they can respond. Speed is an issue with most active control systems for optical limiting. To protect sensors, the transmittance must be reduced in a time much shorter than the width of the potentially damaging pulse. Even...