Tuned Integrated Optic Modulator on a Fiber Optic Gyroscope

1. Field of the Invention

The present invention relates to tuning of fiber optic gyroscope modulators and more specifically to tuning out a newly discovered imperfection in an integrated optic modulator.

2. Description of the Prior Art

Many different implementations of Fiber Optic Gyroscopes (FOGs) have been developed using integrated Optic Phase Modulators, typically of lithium niobate or other electro-optic materials. Three recent patents on this subject, assigned to the same assignee as the present invention, are U.S. Pat. Nos. 5,309,220, 5,280,339 and 5,278,631. All of the implementations described to date assume that the integrated optic modulator itself is a perfect device, with only a small shunt capacitance (a few picofarads) to distort the modulating signal. Integrated optic modulators used in FOG applications having a serrodyne feedback require a stable scale factor with flat frequency response. These devices typically have flat frequency responses over very wide bandwidths, far exceeding the requirements of most FOG applications. Analysis of the detected waveforms transmitted from the fiber optic gyro indicates that a very small frequency effect exists in the kilo hertz region, which can affect the scale factor and linearity of a FOG by very small amounts. This very subtle additional distortion is not measurable by normal means and is important only for high accuracy applications. This effect has been isolated to the integrated optic modulator and is not related to the external electronics.

The present invention provides a system for tuning out a newly discovered imperfection in integrated optic modulators which allows the elimination of an error source in high performance fiber optic gyroscopes. The integrated optic modulator has an internal pole and zero, which almost cancel each other at a frequency of a few kilohertz. The pole and zero are so close together that they are not detectable by ordinary means. The observed error effect can be compensated by an electronic network in the modulator drive electronics which has the opposite pole and zero (a lead-lag network). The network, a resistor capacitor combination, must be adjusted to achieve nearly complete cancellation of the observed error effect. Any network having the necessary pole and zero, including both active and passive filters, can be used to cancel this error effect.

FIG. 1 illustrates a block diagram of a closed loop fiber optic gyroscope configured in accordance with the present invention.

FIG. 2 illustrates an integrated optic phase modulator utilized in the present invention.

FIG. 3 illustrates the ideal detected error signal of the prior art.

FIGS. 4 and 5 illustrate actual gyroscope signals without the compensation network of the present invention.

FIG. 6 illustrates a schematic of one embodiment of the present invention.

FIG. 7 illustrates a schematic of another embodiment of the invention.

FIGS. 8 and 9 illustrate actual gyroscope signals with the compensation network of the present invention.

FIG. 1 illustrates a minimum reciprocal fiber optic gyroscope as contemplated by the invention and designated by the numeral 2. Gyro 2 includes a fiber optic coil 6 having an end 6A coupled through a directional coupler 8, a polarizer 10 and a directional coupler 12 to a light source 14. Another end 6B of fiber optic coil 6 is coupled through a phase modulator 15, directional coupler 8, polarizer 10 and directional coupler 12 to a photo detector 16. The arrangement is such that a light beam from light source 14 is split into two beams. One of the two beams travels around coil 6 in a clockwise direction and the other beam travels around coil 6 in a counterclockwise direction. The phase shift between the clockwise and counterclockwise traveling beams is detected by detector 16 which provides a corresponding square wave analog electrical output. The output of detector 16 is passed through amplifier 18.

The amplified analog square wave output is applied to an analog to digital converter 20 which digitizes the square wave output, and therefrom to signal processing implementation 4, which is a digital implementation and includes a square wave detector 22, an integrator 24 and a ramp generator 26. Thus, the square wave digital output from A/D converter 20 is applied to square wave detector 22 and therefrom to integrator 24. The output from integrator 24 which corresponds to the rotational rate of gyro 2 is applied to ramp generator 26. The output of ramp generator 26 corresponds to the rotational angle of the gyro.

The output from ramp generator 26 is applied to a digital to analog converter 28, and the analog output therefrom is applied to summing means 30. The output from a square wave generator 32 is applied to summing means 30 and summed thereat with the analog output from D/A converter 28 to provide a summed output. Summing means 30 with its square wave generator input may be located between ramp generator 26 and D/A converter 28. The summed output is applied to modulator driver 34 which is connected to compensation network 35 utilized in the present invention. Compensation network 35, a resistor capacitor combination, achieves a nearly complete cancellation of the error effect discussed in the Background of the Invention. The output of compensation network 35 connects to phase modulator 15 to provide a closed loop gyro/signal processing configuration.

Gyro 2 acts as an optical interferometer with the desired optical phase shifts being in accordance with the Sagnac phase shift from a rotational rate imposed on coil 6 and phase shifts imposed by phase modulator 15. The Sagnac phase shift (.DELTA..sub..chi.) is expressed as:

where (.lambda.) is the wavelength of the light from light source 14, (D) and (L) are the diameter and length, respectively, of coil 6, and (.OMEGA.) is the input rotation rate about the axis of coil 6 as seen by gyro 2.

Phase modulator 15 imposes phase shifts by virtue of a time delay (.tau.) for travel of the light beam from light source 14 through coil 6. Thus, for gyro 2, the light beam that travels counter-clockwise through coil 6 will reach phase modulator 15 (.tau.) seconds earlier than the clockwise traveling light beam. If phase modulator 15 changes state in those (.tau.) seconds, then the phase modulator has imposed a non-reciprocal phase shift between the clockwise and counterclockwise light beams.

Compensation network 35 eliminates an error source affecting highly accurate fiber optic gyroscopes, in which a scale factor error of approximately 0.1%, and a linearity error of a few parts per million, can be eliminated. This error source is internal to an integrated optic modulator. As shown in FIG. 2, an integrated optic phase modulator 40 (15 of FIG. 1) consists of an optical waveguide 42 and electrodes 44 connected to wires 46. A voltage applied to electrodes 44 modifies the index of refraction and imparts a phase shift. The modulator is typically part of a larger structure, as shown in FIG. 1, containing other optical waveguide components such as Y-branches and polarizers and is typically fabricated in a slice of monocrystalline lithium niobate or lithium tantalate.

The error effect discussed in the Background of the Invention distorts the detected FOG error signal. As shown in FIG. 3, the ideal error signal looks like a square wave in synchronism with the modulation signal. The value or the error signal in the half cycle after the reset of the modulation waveform will vary according to the reset amplitude. An example of actual error signals is shown in FIG. 4 and FIG. 5. It can be seen that the error signal has been distorted significantly as follows: the square wave tops and bottoms have a very noticeable tilt, and additional distortion is seen for several cycles after the reset of the modulation waveform. This distortion in the error signal will cause errors in the FOG output.

The observed effect gives the appearance of a transfer function having a pole and a zero very close together, of the form ##EQU1## which is similar to a lag-lead network. By utilizing simulation, it has been confirmed that a transfer function of this sort can generate the observed waveform and that it is possible to determine approximate values for T1 and T2. This analysis indicates that the time constants are in the neighborhood of 15 to 30 microseconds and the time constants are very close together (within approximately 0.1 percent of each other). Therefore the time constants are not readily detectable by normal means. It should be noted that this type of error effect has never been reported by the manufacturers of integrated optic modulators.

The observed error effect can be compensated by an electronic network in the modulator drive electronics which has the opposite pole and zero (a lead-lag network). The network shown in FIG. 6 will accomplish this purpose. In FIG. 6, Basic Compensation Circuit, modulation voltage is connected to resistor 52. Capacitor 54 is connected across resistor 52. Resistor 56 is connected to capacitor 54, resistor 52 and across integrated optic modulator 58. FIG. 7 illustrates a balanced compensation circuit. This has been implemented with a working gyro using a balanced twisted pair cable between the compensation network and the integrated optic modulator. In FIG. 7, modulation voltage is connected to resistors 62 and 63. Capacitors 64 and 65 are connected across resistors 62 and 63 respectively. Resistor 66 is connected between capacitors 64 and 65, resistors 62 and 63 and across integrated optic modulator 68. The network values can be adjusted to achieve nearly complete cancellation of the observed effects. The testing that has been performed served also to confirm that the interpretation of the observed effects was valid. The actual values in the compensation network to achieve this cancellation, indicate that the time constants internal to the integrated optic modulator are on the order of 20 microseconds and the actual time constants are only 0.1% apart. Any network, including both active and passive filters, having the necessary pole and zero, can be used to cancel the error effect. The FOG error signal observed after the cancellation network of the present invention was tuned is shown in FIGS. 8 and 9. It can be seen from FIGS. 8 and 9 that the distortions are removed.

The optimum component values for the tuning network of the present invention can be quickly determined using the procedure described below. Assuming the compensation network of FIG. 6, start with a value of resistor 52 which is excessively large. Adjust capacitor 54 to get a level slope on the detected square wave error signal. Then adjust resistor 56 to make the error signal on the second half cycle after the reset, and all subsequent cycles, to look exactly like the signal before the reset. (The half cycle immediately after the reset may be different, depending upon the accuracy of setting the reset value to 2-pi radians of optical phase shift). This method is successful because the value of capacitor 54 affects the slope of the error signal directly but the value of resistors 52 and 56 cancels in its effect on the slope, making it possible to adjust them independently. Due to small interactions, it may be necessary to repeat the above operation for both the capacitor and the resistors to fine tune it, especially if the modulation frequency is within an order of magnitude of the pole and zero being compensated.

Implementation of the present invention was performed using lithium niobate modulators, however, similar effects probably occur in other materials as well, and the methods described herein are equally applicable in those materials. The present invention pertains primarily to interferometric FOGs with serrodyne feedback, although the compensation method can be used with any application of integrated optic modulators where the error terms discussed above are important.

It is not intended that this invention be limited to the hardware or operational procedures shown disclosed. This invention includes all of the alterations and variations thereto as encompassed within the scope of the claims as follows.