Abstract

Nd:YAG nonplanar ring lasers have been found to be useful in applications requiring the distribution of microwave signals with light and fiber-optic interferometric sensing. Nd:YAG lasers based on the nonplanar ring cavity operating at 1319 nm have the advantages of very low phase noise, inherently good frequency stability, frequency modulation of the laser at rates to 1 MHz, and single-mode operation with output powers >100 mW. By electronic feedback offset phase- locking, the lasers become mutually coherent with an offset frequency that is tunable from dc to >40 GHz, making the device an attractive source of spectrally narrow optically transmitted microwaves. The source can then be used in such applications as optical feeds for phased array microwave antennas and generation and transmission of high frequency microwave signals with light for use in testing of microwave photodiodes and for injection into microwave oscillators or transistors. The phase-lock loop method offers advantages over direct or external modulation, such as reduced harmonic distortion and higher frequency operation. Interferometric fiber sensors have been extensively researched for a wide range of measurands including acoustic, acceleration, magnetic fields, and electric fields. Semiconductor diode lasers provide a compact, reliable source of optical radiation with coherence lengths of several meters; however, they exhibit significant low frequency phase noise or frequency jitter which limits the phase shift detectability in most present-day diode laser-based interferometric sensor systems. Sensor systems using the Nd:YAG ring laser can benefit from its very low phase noise. This, combined with the FM modulation of the laser, allows the replacement of the diode laser with the Nd:YAG laser for uses requiring detection of very small phase shifts as well as systems where long measurement paths with no compensating paths are required.

© 1991 Optical Society of America