A fiber optic based interferometer for gas and plasma density measurements

Summary form only given. A Mach-Zehnder Fiber Optic Interferometer (FOI) with <10/sup -3/ wave sensitivity has been built to measure gas and plasma densities. The FOI uses a 1550 nm CW diode laser and is contained in a 12"/spl times/12"/spl times/3" volume, which makes the system compact and easily portable. The 1550 nm laser beam is launched into a polarization maintaining optical fiber. Evanescent wave fiber couplers are used to split the beam into probe and reference paths and later recombine them. The beam is launched into and re-captured from the test region with a pair of fiber collimators. This free space test beam is 0.4 mm in diameter and 12 cm long. The relative phase between the interferometer legs is stabilized 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 fast transient measurement. Additionally, the signal-to-phase gain can be calibrated by varying the relative phase difference with the piezo wafer assembly. The second fiber coupler recombines the paths of the interferometer and provides complementary signals on its two fiber outputs. The output signals are detected with photo diodes which are differentially amplified to remove the "common mode", noninterfering signal and enhance the interference signal that is proportional to the cosine of the relative phase shift between the test and reference legs.