Fiber Optic Gyroscopes

Another key application for SLED is in navigation systems, primarily those in avionics and aerospace that use fiber-optic gyroscopes (FOGs) to make precise rotation measurements [11],[12]. 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. 11, are a light source, a single- mode polarization-maintaining (PM) fiber coil, a coupler 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 paths 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

 

Fig 11 FOG

 Fig. 11 Generic setup of a fiber optic gyroscope (FOG).

 

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. 
 
Benefits of EXALOS’s SLEDs for Fiber Optic Gyroscopes
The key requirements for fiber optic gyroscopes are typically low cost, reduced form factor, low power consumption, long-term stability and high reliability. In the past, wavelength stability over time was a major drawback of semiconductor based broadband light sources like SLEDs compared with fiber-based light sources such as erbium-doped fiber-amplifiers. 
 
At EXALOS, we have successfully overcome this drawback by developing SLEDs that are less sensitive to wavelength shifts induced by temperature or aging. Novel materials and material structures have made it possible to improve the wavelength stability of our devices by a factor of four compared with conventional devices. Further work is in progress with the target of achieving wavelength stabilities down to the 10 parts-per-million range. SLEDs with this performance will help to reduce the size and cost of FOGs substantially. 
 

EXALOS has a portfolio of products qualified for FOG applications that require extended operating temperature ranges, increased shock/vibrations levels and verified lifetimes due to their use in space environments. Examples of such products include uncooled free-space or fiber-coupled 820-nm SLEDs (EXS8310 or EXS8305, [2]) as well as cooled or uncooled transceiver modules with an integrated beam splitter and a receiver photodiode [13]. SLED-Transceivers are particularly interesting when realized with a polarization-maintaining (PM) fiber output as they can be directly coupled to the fiber loop of the FOG, avoiding the complexity of realizing a PM fiber coupler. EXALOS’ SLED products have been qualified according to BELLCORE GR-468-CORE.