Fiber optic gyroscope with front end polarizer

A fiber optic gyroscope (FOG) is provided. The FOG comprises a depolarizer that receives light from a light source, a multifunction integrated optic chip (MIOC) and a sensing coil coupled to outputs of the MIOC. The FOG also includes a polarizer coupled between an output of the depolarizer and an input of the MIOC. The polarizer mitigates polarization non-reciprocity (PNR) bias error and enhances the polarization extinction ratio (PER) of the FOG.

TECHNICAL FIELD

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

BACKGROUND

An interferometric fiber optic gyroscope (FOG)10, as illustrated inFIG. 1, includes an optical signal light source12that provides an optical signal to a fiber optic sensing coil28. 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 FOG10, a multifunction integrated optic chip (MIOC)20is connected to the fiber optic sensing coil28. The MIOC20includes components such as a polarizer22, a phase modulator26and a Y-coupler (splitter/combiner24) that are used in processing and controlling optical signals input to and from the fiber optic sensing coil28. The output of the FOG10is 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 source12through a splitter16and a second portion of the output is provided to a photodetector14through the splitter16for 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 FOG10, 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 FOG10via optical filtering of the light when traversing first the section between the source and the MIOC20(made partly of the single mode fiber and partly of polarization maintaining fiber) then traversing the polarizing MIOC20itself. This optical filtering can lead to short term scale factor instability and long term degradation of the scale factor repeatability. A depolarizer18(e.g., Lyot depolarizer) can be placed in the optical path between the optical source12and the MIOC20for depolarizing the optical light source signal to prevent changes in its polarization state as it propagates through the FOG10. Without the depolarizer18, random environmental perturbations may degrade the scale factor stability via the mechanism of polarization wavelength asymmetries.

However, the existence of large amplitude polarization non-reciprocity (PNR) bias error(s) in FOGs is in large part attributed to cross-coupling and birefringence temperature sensitivities introduced by the depolarizer. The severity and structure of PNR bias error due to various FOG parameters is summarized in Table I below.

FIG. 2illustrates a graph of relative PNR amplitude as a function of Lyot depolarizer cross coupling in the prior art FOG ofFIG. 1. The cross-coupling is in decibels and is the cross coupling between a first component of the Lyot depolarizer and a second component L2of the Lyot depolarizer that is coupled at about a 45° angle relative to the first component. As illustrated inFIG. 2, the PNR amplitude reduces proportional to the square-root of the cross coupling, √α. As an example, a 3 dB reduction is cross coupling is predicted to result in a 29% reduction in PNR. Extrapolation of the fit confirms that the predicted PNR amplitude is indeed 29% smaller.

In addition, tests confirm that the severity and structure of the PNR bias error is a strong function of Lyot depolarizer length.FIG. 3is 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 severity of the PNR bias error is dependent on the length of the Lyot depolarizer due to variation in the coherence function term, γ(L). Unless utilizing an alternate design, the optimization of the aforementioned parameters is highly constrained.

The parameters required to minimize cross coupling and to select a minimum of the coherence function associated with the Lyot depolarizer results in degraded depolarization performance, which is an unacceptable alternative. An additional example is the MIOC. The design and manufacturing efforts required to enhance the polarization extinction ratio (PER) of the MIOC by a meaningful amount is costly due to the technology limitations of modern integrated optics chip manufacturing and testing. In order to enhance the state of the art (SoA), it is necessary to leverage an alternate FOG configuration that is capable of manipulating PNR sensitive parameters while leaving other design considerations intact.

SUMMARY

In one embodiment of the invention, a fiber optic gyroscope (FOG) is provided. The FOG comprises a depolarizer that receives light from a light source, a multifunction integrated optic chip (MIOC) and a sensing coil coupled to outputs of the MIOC. The FOG also includes a polarizer coupled between an output of the depolarizer and an input of the MIOC. The polarizer mitigates polarization non-reciprocity (PNR) bias error and enhances the polarization extinction ratio (PER) of the FOG.

In accordance with another embodiment of the invention, a FOG is provided that comprises a Lyot depolarizer that receives light from a light source. The Lyot depolarizer is formed from two polarization maintaining (PM) fiber elements whose orthogonal transmission axes are spliced at a 45° angle from one another. The FOG also comprises a MIOC, a sensing coil coupled to outputs of the MIOC, and a polarizing component with PM fiber pigtails coupled between an output of the depolarizer and an input of the MIOC. The input PM fiber of the polarizing component may double as the second PM fiber segment of the Lyot depolarizer. The length of the output PM fiber of the polarizing component pigtail entering the MIOC is selected to optimize the optical path length to ensure a decoherent PNR bias error associated with the FOG. The polarizing component also enhances the PER of the FOG.

In accordance with another embodiment of the invention, a FOG is provided that comprises a Lyot depolarizer that receives light from a light source. The Lyot depolarizer is formed from two polarization maintaining (PM) fiber elements whose orthogonal transmission axes are spliced at a 45° angle from one another. The FOG also comprises a MIOC, a sensing coil coupled to outputs of the MIOC, and a polarizing fiber coupled between an output of the depolarizer and an input of the MIOC. The length of the polarizing fiber is selected to optimize the optical path length to ensure a decoherent PNR bias error associated with the FOG. The polarizing fiber also enhances the PER of the FOG.

In accordance with yet a further embodiment of the invention, a method of forming a FOG is provided. The method comprises pigtailing a sensing coil to outputs of a multifunction integrated optic chip (MIOC), selecting a polarizer with a length that optimizes the optical path length to ensure that PNR bias error associated with the FOG is decoherent, and pigtailing a second end of the polarizer to an input of the MIOC, wherein the polarizer enhances the polarization extinction ratio (PER) of the FOG. The method further comprises splicing a first end of the polarizer to a second polarization maintaining (PM) fiber element, splicing a first polarization maintaining (PM) fiber element to the second PM fiber element with orthogonal transmission axes disposed at a substantially 45° angle from one another to form a Lyot depolarizer, and splicing the first PM fiber element to an optical splitter that provides light from a light source and receives return light to be provided to a photodetector.

DETAILED DESCRIPTION

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.

FIG. 4illustrates a functional block diagram of a FOG40in accordance with an embodiment of the present invention. The FOG40includes a light source/splitter/photodetector arrangement42similar to the arrangement of the light source12, splitter16and photodetector14illustrated inFIG. 1. The light source can be a broadband light source with a low degree of coherence. The light source/splitter/photodetector arrangement42is coupled to a single mode (SM) fiber44via a pigtail. Splice coupling is indicated by X's, while pigtail coupling is indicated by dashed boxes. The photodetector of the light source/splitter/photodetector arrangement42can be a semiconductor device capable of converting optical energy into electrical energy. The photodetector can be coupled to a single mode (SM) fiber. The SM fiber44and the SM fiber coupled to the photodetector are spliced to an optical splitter that can be a bidirectional fused taper fiber coupler that includes an additional end coupled to the light source. The optical splitter of the light source/splitter/photodetector arrangement42is also coupled to a Lyot Depolarizer46through the SM fiber44.

The Lyot depolarizer46includes a first leg L1and a second leg L2. The Lyot depolarizer46can be formed from two high birefringent (HB) polarization maintaining (PM) fiber elements whose orthogonal transmission axes are spliced at a 45° angle from one another. Alternatively, the Lyot depolarizer46can be formed of two HB photonic crystal (PC) fibers to further reduce birefringence temperature sensitivity. Furthermore, the Lyot depolarizer46can be formed of non-fiber components or the Lyot depolarizer46can be a mixed mode Lyot-type depolarizer that utilizes the Lyot effect produced by the combination of fiber and non-fiber components, such as employing crystal substrate material that exhibits substantial birefringence properties.

A PM pigtailed inline fiber polarizer48is coupled between the Lyot depolarizer46and inputs of a MIOC50. In an aspect of the invention, the input leg of the inline fiber polarizer48doubles as the second leg L2of the Lyot depolarizer46and its length is determined by the Lyot design. An separate output leg L3of the inline fiber polarizer48is pigtailed directly to the MIOC50such that the length of output leg L3can be varied as necessary to minimize PNR bias error while the inline fiber polarizer48remains connected between the Lyot depolarizer46and the output leg L3. Outputs of the MIOC50are pigtailed to opposite ends of a coil52. The coil52is a sensing coil having first and second ends for coupling to the MIOC50and for receiving light from the light source42that has been split and provided to opposite ends of the coil52by the MIOC50. The sensing coil can be made of polarization maintaining fiber.

The MIOC50may comprise any of a number of commercially available optical circuit devices having a birefringent crystal substrate or waveguide. The MIOC50can be built on a crystal substrate or waveguide of lithium niobate (LiNbO3) material or any other non-isotropic material suitable for integrated-optic waveguides, and has a housing with three ports providing external connection to an internal waveguide coupler/beam splitter, such as a Y-type waveguide coupler and phase modulator disposed within the housing.

FIG. 5illustrates a functional block diagram of a FOG70in accordance with another embodiment of the present invention. The FOG70includes a light source/splitter/photodetector arrangement72. The light source of the light source/splitter/photodetector arrangement72can be a broadband light source with a low degree of coherence, as described inFIG. 4. The photodetector of the light source/splitter/photodetector arrangement72can be a semiconductor device capable of converting optical energy into electrical energy. The photodetector can be coupled to a single mode (SM) fiber. The SM fiber74and the SM fiber coupled to the photodetector are also spliced to an optical splitter that can be a bidirectional fused taper fiber coupler that includes an additional end coupled to the light source. The SM fiber74is also spliced to a Lyot Depolarizer76. Splice coupling is indicated by X's, while pigtail coupling is indicated by dashed boxes. The Lyot depolarizer76includes a first leg L1and a second leg L2. The Lyot depolarizer76can be formed from two high birefringent (HB) polarization maintaining (PM) fiber elements, two HB photonic crystal (PC) fibers to further reduce birefringence temperature sensitivity, formed of non-fiber components, or the combination of fiber and non-fiber components, as previously described with respect toFIG. 4.

A single polarization fiber L378is coupled between the Lyot depolarizer76and inputs of a MIOC80. A first end of the single polarization fiber78can be spliced to an end of the second leg L2of the Lyot depolarizer76and a second end of the single polarization fiber78can be pigtailed to an input of a MIOC80. In an aspect of the invention, the length of the polarization fiber78is selected to tune the optical path length to ensure decoherent PNR bias error of the FOG70. Outputs of the MIOC80are pigtailed to opposite ends of a coil82. The coil82is a sensing coil having first and second ends for coupling to the MIOC80and for receiving light from the light source72that has been split and provided to opposite ends of the coil82by the MIOC80. The sensing coil82can be made of polarization maintaining fiber.

In view of the foregoing structural and functional features described above, an example methodology will be better appreciated with reference toFIG. 6. While, for purposes of simplicity of explanation, the methodology ofFIG. 6is shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some actions could in other examples occur in different orders and/or concurrently from that shown and described herein.

FIG. 6illustrates a flow diagram of a method100for forming a FOG in accordance with an embodiment of the present invention. The method100begins at102where a first and a second end of a sensing coil are pigtailed to respective first and second output ends of a MIOC. At104, a polarizer is selected with a length to optimize the optical path length to ensure decoherent PNR bias error associated with the FOG. At106, a second end of the polarizer is pigtailed to an end of an input to the MIOC. At108, a first end of the polarizer is spliced to a second PM fiber element. At110, a first PM fiber element is spliced to the second PM fiber element at a 45° angle relative to the first PM fiber element. At112, the first PM fiber element is coupled to an optical splitter that provides light from a light source and receives return light to be provided to a photodetector. The polarizer mitigates PNR bias error and enhances the PER of the FOG. The polarizer can be an inline polarizer or a single polarizing fiber element.

What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims.