Fiber optic gyroscope using a narrowband FBG filter as a wavelength reference

Fiber optic gyroscope architectures that incorporate both (i) a WDM-based wavelength control and (ii) a wavelength reference based on a narrowband fiber Bragg grating (FBG), with the latter component providing significant improvement in the stability of the wavelength reference by calibrating out wavelength errors associated with a WDM-based wavelength control scheme.

BACKGROUND

1. Field of the Invention

The present invention relates generally to interferrometric fiber optic gyroscopes (IFOGs), and more particularly to control mechanisms for improving the stability of IFOGs.

2. Background of the Invention

The scale factor stability of an IFOG is highly dependent on the stability of the wavelength observed at the system photodetector. To improve IFOG performance, especially with respect to scale factor stability, IFOGs have been provided with a wavelength control scheme based on a wavelength division multiplexer (WDM) coupler, that acts as a wavelength discriminator, and a pair of matched photodiodes. More specifically,FIG. 1shows a highly conventional IFOG architecture that includes a light source110, such as a fiber light source, a 50—50 power splitter115, and integrated optics chip118that feeds light to and from a sensing coil120, the latter two components sometimes being referred to herein as a “sensing loop assembly.” Fifty-fifty coupler115is also connected to a photodiode and associated pre-amp125that is used to detected the Sagnac effect caused by rotation of sensing coil120. IFOG loop closure electronics130bridge photodiode125and IOC118thereby providing desirable feedback, as is well-known.

To improve upon the known system ofFIG. 1, as mentioned, an architecture like that shown inFIG. 2has recently been proposed. Here, a wavelength division multiplexer (WDM) coupler210along with a pair of matched diodes220is inserted in the optical circuit. This combination is sometimes referred to herein as a “wave division multiplexer/detector assembly.” A signal representing the sum of the matched diodes is provided to IFOG loop closure electronics130and a differential signal is fed to wavelength control feedback electronics230to provide feedback directly to light source110, and thereby provide improved wavelength stability for the light source itself.

Unfortunately, the wavelength stability of this system depends substantially on the stability of WDM coupler210and matched photodiodes220, which may be subjected to external perturbations (e.g. temperature, radiation, etc.). These components may also be susceptible to long-term drift, thereby further degrading stability.

There is therefore a need for systems and methods that still further improve the wavelength stability of IFOG devices.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an IFOG with improved wavelength stability, and thus better overall scale factor performance, by providing a stable wavelength reference that can be used to monitor changes in the wavelength control components. A particularly desirable by-product of having a stable wavelength is that it makes it possible to obtain a more stable scale factor.

More specifically, the present invention provides fiber optic gyroscope architectures that incorporate both (i) a WDM-based wavelength control and (ii) a wavelength reference based on a narrowband fiber Bragg grating (FBG), with the latter component providing significant improvement in the stability of the wavelength reference.

The several embodiments described herein propose several different architectures that employ a narrowband reflection fiber Bragg grating (FBG) that is used in conjunction with a source of light and a wave division multiplexer (WDM)/Detector assembly to provide improved stability.

The features and attendant advantages of the present invention will be more fully appreciated upon a reading of the following detailed description in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3depicts an IFOG architecture that incorporates a narrow band reflection fiber Bragg grating (FBG) in accordance with a first embodiment of the present invention. In this, and other figures, the feedback lines (shown inFIGS. 1 and 2) are not shown for purposes of clarity. However, those skilled in the art will appreciate that a FOG in accordance with the several embodiments of the present invention will have such feedback lines. As shown, the architecture includes a light source assembly110, such as a fiber light source that includes, for example, a 980 nm or 1480 nm laser pump, a wave division multiplexer (WDM) coupler, an erbium doped fiber, an isolator and an optical isolator. Such FLS assemblies are well-known in the art.

The output of light source110, in this embodiment, is connected to a 50/50 power splitter115. One port of splitter115is connected to an integrated optics chip (IOC)118and associated sensing loop120. Another port of 50/50 splitter115is connected to WDM coupler210, which is connected to matched photodiodes220. In accordance with this embodiment, a fourth port of 50/50 splitter115is connected to a tap330having, for example, 90% and 10% ports. In this case, the 90% port is connected to a fiber Bragg grating (FBG)310that acts as a narrow band reflection FBG that is used to create a narrow wavelength reference from the broadband light source spectrum.

FIG. 9illustrates a typical output spectrum of a 1532 nm fiber light source (FLS) that can be employed for light source110. As indicated, a tunable range of about 350 ppm is readily achievable by tuning the input current over its full range.

Further, as shown byFIGS. 10A and 10B, if a narrow bandwidth fiber Bragg grating (FBG) having a center wavelength of about 1530 nm and a bandwidth of about 0.5 nm, along with a rejection of about 40 dB (illustrated byFIG. 9A), is inserted in the light path that is to be used as a wavelength reference, then the spectrum passing back through 50/50 splitter115is like that shown inFIG. 10B. Because of the very narrow bandwidth, an extremely stable wavelength, on the order of 1 ppm, can be achieved even when the input current to the light source is tuned over its full range. Those skilled in the art will appreciate that the center wavelength, bandwidth, and rejection of the FBG can be optimized for different light sources and stability requirements and the particular wavelength, bandwidth and rejection values noted herein are exemplary only.

Ultimately, and in accordance with the present invention, the wavelength reference provided by FBG310is used to calibrate the WDM coupler210and photo diodes220. The WDM coupler acts as a wavelength discrimator, i.e. it splits the longer and shorter wavelengths components of the light between its two output ports. The photodetectors measure the total power at each of the output ports of the WDM coupler. A change in the input wavelength will change the power split between these two ports. Therefore, the ratio of the DIFF signal (P2−P1) to the SUM signal (P2+P1) can be used to monitor this wavelength shift.

If one assumes that the WDM coupler and photodetectors are perfectly stable, then a change in this ratio is an indication of a wavelength shift. However, if the center wavelength of the WDM coupler and/or the responsitivity of the photodetectors changes over time this will also cause a change in the ratio even though the mean wavelength of the input light does not change. The stable wavelength reference provide by the FBG is used to measure the shift in the center wavelength of the WDM coupler. When used in conjuction with a 2×1 switch (as shown, for example, inFIG. 4) it can also compensate for the differential change in the photodetector responsitivities as well. In this way, one can distinguish changes in the input wavelength from changes in the WDM coupler center wavelength and the photodetector differential responsitivity.

Preferably, a photodetector/preamplifier320is provided to amplify the reference signal that is provided by wavelength reference FBG310. The wavelength reference will have its own relative intensity noise (RIN) which is added to the IFOG signal at the 50/50 coupler115before it enters the WDM coupler210. This noise can degrade the IFOG Angle Random Walk performance unless it is compensated for. The FBG Reference RIN signal can be used to remove the added noise from the wavelength reference in the SUM signal which is used to close the IFOG loop. Note that this FBG Reference Signal is not needed if a switch is used to turn off the wavelength reference signal as shown inFIG. 4.

More specifically, tap330and photodetector/preamplifier320are employed to compensate for RIN noise at matched or dual photo detectors220that is induced by the narrow band reference.

In an alternative embodiment, wavelength reference FBG310can be replaced with a different type of wavelength selective filter to create a wavelength reference. One possibility is to use a molecular reference such as an acetylene cell. It should be understood that while the potential wavelength stability of a molecular reference is much greater (˜2 orders of magnitude) than an FBG based wavelength reference, an advantage of the FBG architectures described herein is that they are much less complicated to implement.

FIG. 4shows a second embodiment of the present invention in which an on/off switch410is employed to turn wavelength reference FGB310on and off, thereby effectively replacing tap330and additional photo diode and preamplifier320. With switch410in an ON position the desired narrowband reference is supplied to WDM coupler210and matched photo diodes220. In addition, an optional 2×1 switch420can be used to switch input legs of WDM coupler210to eliminate photo detector responsivity sensitivities.

FIG. 5illustrates a third embodiment of the present invention that is similar to the first embodiment, but here includes a separate photo diode and pre amp510that is used for loop closure. A second 50/50 coupler520is employed to tap the light signal returning from IOC118. Here, of course, wavelength reference FBG310is still employed, consistent with the principles of the present invention. It is noted that the optical switch410of the second embodiment is no longer necessary because of the separate loop closure photo diode510.

FIG. 6shows yet another embodiment of the present invention in which a circulator610is used instead of a 50/50 splitter as was shown in the first three embodiments. Here, wavelength reference FBG310is connected to photo diode and pre-amplifier320via tap630(here shown having 90% and 10% ports). Light source110is in communication with wavelength reference FGB310via tap620(here shown with 80% and 20% ports) and tap630. As before, narrowband reflection FBG310is used to create a narrow wavelength reference from broadband light from light source110. This wavelength reference is then used to calibrate WDM coupler210and matched photo diodes220consistent with the overriding goal of the present invention. In accordance with this embodiment, and as before, tap630and photo diode/preamplifier320are employed to compensate for RIN noise at the dual photo detectors induced by the narrowband reference. Those skilled in the art will appreciate that tap split ratios other than those illustrated can be used.

FIG. 7shows a fifth embodiment of the present invention that employs a switch710to turn wavelength reference FBG310on/off. Wavelength reference FBG310is connected to circulator610via a tap720. As before, split ratios other than the ones shown can be implemented. Also, it should be pointed out that tap720and circulator610replace 50/50 splitter115, like that shown inFIG. 1. This may be significant since double pass loss of a 50/50 splitter is 6 dB, while loss of a circulator/tap coupler pair is about 3 dB, depending on the tap split ratio.

As in other embodiments, this fifth embodiment may also optionally include a 2×1 switch420to switch input legs of WDM coupler210to eliminate photo detector responsivity sensitivities.

Finally,FIG. 8depicts a sixth embodiment of the present invention that employs a circulator610and three taps810,820and830to implement a separate loop closure photo detector840. Thus here, like in other embodiments, a switch is unnecessary to obtain the narrowband reference signal generated by wavelength reference FGB310.

As will be appreciated from the foregoing, the present invention is directed to adding a wavelength reference to an overall IFOG architecture that can be used to calibrate out wavelength errors associated with a WDM-based wavelength control scheme.