Fiber optic gyroscope with front end polarizer

#The present invention relates generally to gyroscopes, and more particularly to a fiber optic gyroscope with a front end polarizer.

#An interferometric fiber optic gyroscope (FOG) 10, as illustrated in FIG. 1, includes an optical signal light source 12 that provides an optical signal to a fiber optic sensing coil 28. A typical optical signal light source includes a laser diode that provides pump light to a gain fiber. The gain fiber includes a dopant that absorbs pump light and then emits an optical signal that is suitable for delivery. Within the FOG 10, a multifunction integrated optic chip (MIOC) 20 is connected to the fiber optic sensing coil 28. The MIOC 20 includes components such as a polarizer 22, a phase modulator 26 and a Y-coupler (splitter/combiner 24) that are used in processing and controlling optical signals input to and from the fiber optic sensing coil 28. The output of the FOG 10 is an intensity that can be used to determine the phase difference between the two counter-propagating waves. A portion of the output is returned to the light source 12 through a splitter 16 and a second portion of the output is provided to a photodetector 14 through the splitter 16 for measuring the phase difference. The rotation rate of the coil about its sensing axis is obtained by dividing this phase difference by a scale factor of the FOG 10, referred to as the Sagnac scale factor.

#The scale factor stability of fiber optic gyroscopes (FOGs) is affected by changes in the polarization state of the light in the fiber between the optical source and the MIOC. Changes in stress within the fiber will cause the polarization state of light guided by the fiber to change. This stress may be mechanical or thermal in origin. Any change in polarization state changes the scale factor of the FOG 10 via optical filtering of the light when traversing first the section between the source and the MIOC 20 (made partly of the single mode fiber and partly of polarization maintaining fiber) then traversing the polarizing MIOC 20 itself. This optical filtering can lead to short term scale factor instability and long term degradation of the scale factor repeatability. A depolarizer 18 (e.g., Lyot depolarizer) can be placed in the optical path between the optical source 12 and the MIOC 20 for depolarizing the optical light source signal to prevent changes in its polarization state as it propagates through the FOG 10. Without the depolarizer 18, random environmental perturbations may degrade the scale factor stability via the mechanism of polarization wavelength asymmetries.

#FIG. 1 illustrates a functional block diagram of a prior art fiber optic gyroscope (FOG).

#FIG. 2 illustrates a graph of peak-to-peak PNR amplitude as a function of Lyot depolarizer cross coupling in the prior art FOG of FIG. 1.

#FIG. 3 is a graph of PNR bias error versus Lyot depolarizer length for a large sample set of prior art production gyros built with various depolarizer lengths.

#The present invention is directed to employing a depolarizer (e.g., a Lyot depolarizer) followed by a polarizer between a light source (e.g., a superluminescent diode (SLD) or broadband fiber source (BFS)) and a multifunction integrated optic chip (MIOC) in a fiber optic gyroscope (FOG). The polarizer mitigates the existence of large amplitude polarization non-reciprocity (PNR) bias error in fiber gyros due in large part to cross-coupling and birefringence temperature sensitivities introduced by the depolarizer and other front end components. Therefore, in accordance with an aspect of the present invention, a secondary polarizer is pigtailed directly to a depolarizer to enhance the polarization extinction ratio (PER) of the system with respect to front end cross couplings.

#In addition, careful placement of the secondary polarizer allows for a free segment of fiber that may be selected to optimize the optical path length to ensure a decoherent PNR bias error associated with the FOG. This additional degree of freedom can further reduce PNR bias error. Coherency is a percentage of interference of a signal as a function of optical path length. Therefore, the second polarizer can be tuned in length to select a length that is a minimum of the coherency function to assure that the PNR bias error introduced by the cross-coupling and birefringence temperature sensitivities are decoherent at the output. The secondary polarizer may take the form of a PM pigtailed inline polarizer or a distinct and separate segment of polarizing fiber. The use of polarizing fiber has the added benefits of a smaller component footprint and a reduction in design complexity. The reduced design complexity is attributed to the fact that the entirety of the polarizing element is comprised of fiber, which minimizes the likelihood of introducing spurious cross couplings.

#In either case, the predicted PNR bias suppression due to front end cross couplings is anticipated to be proportional to the square root of the PER of the secondary polarizer. Commercially available inline fiber optic polarizers and polarizing fibers are capable of achieving extinction ratios greater than −20 dB corresponding to a PNR amplitude reduction by a factor of ten. Lyot depolarizer experimentation has concluded that the careful selection of PM fiber lengths of the gyro front end may be optimized in order to realize an additional PNR reduction of twenty. In total, the anticipated PNR bias error reduction due to this configuration is anticipated to be approximately two orders of magnitude.