A variety of guided wave optical switches have been proposed for fiber optic acoustic sensor systems. These switches include 1×1 on-off switches, 1×2 blocking switches, and 1×8 blocking switches. One characteristic requirement for all of these switches is a high extinction ratio in the off state. An extinction ratio is the ratio of two optical power levels, e.g., P 1/P 2, of a digital signal generated by an optical source, where P 1 is the optical power level generated when the light source is “on,” and P 2 is the power level generated when the light source is “off.”.
For different switch architectures, e.g., Mach Zehnder interferometers and delta beta reversal switches, an extinction ratio of up to 40 dB or more for each switch can be obtained with an appropriate voltage level. This extinction ratio degrades over time and temperature, resulting in voltage drift, unless the voltage level is adjusted to maintain a minimum null signal.
One prior art technique used to exercise an applied voltage to track and maintain the optical null requires interruption of an optical data flow in a working system. Disadvantageously, the prior art technique requires the system to be off-line.
Another prior art technique requires the application of a small dither voltage to the direct current (DC) port and the use of a hill climb servo to find the optimum DC bias point. Disadvantageously, this technique introduces excess noise into the system.