Fiber Optic Gyroscopes

A A key application for SLED is in navigation systems, such as those in avionics, aerospace, sea, terrestrial, and subsurface, that use fiber-optic gyroscopes (FOGs) to make precise rotation measurements [ 1 ], [ 2 ]. FOGs measure the Sagnac phase shift of optical radiation propagating along a fiber-optic coil when it rotates around the winding axis. When a FOG is mounted within a navigation system, it tracks changes in orientation.

The basic components of a FOG, as shown in Fig. 1, are a light source, a single-mode fiber coil (could be polarization-maintaining), a coupler, a modulator, and a detector. Light from the source is injected into the fiber in counter-propagating directions using the optical coupler.

When the fiber coil is at rest, the two light waves interfere constructively at the detector and a maximum signal is produced at the demodulator. When the coil rotates, the two light waves take different optical path lengths that depend on the rotation rate. The phase difference between the two waves varies the intensity at the detector and provides information on the rotation rate.

SLED-based gyroscopes rely on the large bandwidth of the source to reduce both the large Kerr- induced drift and the high coherent backscattering noise along the fiber as well as to minimize noise from reflections at the facets of the internal optical components, which could decrease the sensitivity at very-low rotation rates.

Furthermore, SLEDs with high polarization extinction ratio (PER) are desired in order to reduce the insertion loss of the polarization-dependent optical components. Amplitude noise is also of concern for those applications, so broadband light sources with relative intensity noise (RIN) values of -120 dBc/Hz or less are required. The amplitude noise requirement also places stringent requirements on the low-noise performance of the electrical current driver of the SLED.