Compact, rugged, flexible fiber optic interferometer for sensitive gas and plasma density measurements

Summary form only given. A prototype fractional fringe Mach-Zehnder fiber optic interferometer (FOI) to measure gas and plasma densities has been built. The interferometer components are secured to a compact 18-cm wide/spl times/22-cm high rigid aluminum frame which makes the system easily portable. A 1550-nm IR beam from a cw diode laser is launched into the fiber system. Evanescent fiber wave couplers are used to split and later recombine the laser beam into equal length probe and reference paths. For the test region a 0.4-mm diameter, 12-cm long free space beam is launched and recaptured with a pair of fiber collimators. Relative phase between the interferometer legs is controlled by stretching and compressing a section of the reference leg fiber with a piezo wafer assembly. This is necessary for "zeroing" the phase difference between the legs prior to a measurement and for calibrating the signal-to-phase gain. The second fiber coupler recombines the paths of the interferometer providing complementary signals on its two outputs. The output signals are detected with photodiodes which are differentially amplified to produce an "ac coupled" interference signal that is proportional to the sine of the phase shift.