Common Mode Compensation in a Fiber 1.5 Mach-Zehnder Interferometer.

Common mode compensation, which is defined as minimizing sensitivity to environmental variables that affect all paths in the optical circuit while retaining high sensitivity to the measurand, is achieved by the feedback 1.5 Mach-Zehnder interferometer proposed and demonstrated in this work. The feedback 1.5 MZ interferometer consists of an optical compensating circuit and a sensing circuit. The compensating circuit is optically connected between the sensing circuit and the light source. This minimizes the sensor measurand errors induced by random fluctuations in the light source wavelength, ambient temperature, or any other environmental fluctuations. The sensing circuit converts changes due to measurand in the phase or the intensity of the light to changes in an output voltage. The output is obtained by taking a ratio of the reference signal (input) to the signal of the output terminal of the interferometer. The ratio is obtained through a division circuit. The output signal is fed back to the phase shifter in the sensing circuit through an amplifier. It is shown numerically and experimentally that with fine adjustments to the feedback gain and the initial phase biases, the operating point of the interferometer to achieve common mode compensation can be made to lie in a region where the measurand sensitivity is greater than it would be in a conventional Mach-Zehnder interferometer. Furthermore, even if the source is less coherent, high sensitivity can be achieved by adjusting the feedback gain to a higher value. In all of the cases we have simulated, it is possible to find a set of electronically controllable parameters, external voltages to PZT and amplifier gain, which lead to high measurand sensitivity and low common mode sensitivity. This capability to electronically tune the interferometer is an advantage in its manufacture.