The effects of dispersion and gas flow on the operation of the ring laser gyro are summarized. The theory is shown to predict a dimple in the anomalous dispersion properties of the atomic transition. Experiments were carried out using a 43.4 cm and 21.7 cm visible laser gyro. Scale factor and nullshift results showed consistency and confirmed the existence of the dimple. The peak-peak relative magnitude of the scale factor dimple was found to be approximately 25 x 10-6. Gas flow was varied by unbalancing the discharge currents in the two legs of the laser gyro. A nullshift sensitivity of 0.0066 deg/hr/µamp of unbalance was found. Analysis of the data showed the nullshift sensitivity to velocity flow to be 4.4 deg/hr/cm/sec of velocity unbalance.
A laser gyro design approach wherein three (or more) gyros are integrated into a common structure with interwoven optical cavities achieves size reductions in excess of 50% compared to the use of three. separate single axis gyros with the same optical cavity dimensions. This makes the laser gyro package size highly competitive with conventional gyros. Characteristics of laser gyro elements most suited for the integrated 3 axis design approach are discussed, and the Sperry SLIC 7 integrated laser gyro cluster is described in detail. Performance characteristics of the SLIC 7 with supporting test data are presented, indicating that the SLIC 7 is suited for a wide range of missile applications.
The non-reciprocal bias element necessary for operation of a four-frequency differential ring laser gyroscope (RLG) is the subject of this paper. Each of the three alternative bias approaches currently under consideration, namely, Zeeman bias, a magnetic mirror, and a Faraday cell, is discussed in the context of its effect on the performance of a RLG. Relevant results of a detailed model including cross-relaxation, Zeeman effect, hole-burning, pathlength controller operation, and temperature are given.
Experimental evidence is presented on a new approach to the problem of locking in two mode ring laser gyros. The data demonstrates the new technique as potentially useful in the mechanization of a laser gyro. Some of the characteristic parameter dependencies are discussed.
The potential promise of ring laser gyro technology for low cost strapdown inertial rate sensing has attracted very significant research and development investments. The results of these investments are now becoming available in a wide range of gyro products showing applicability to many sensing requirements. The Rockwell program in the ring laser gyro technology has been directed toward understanding of the basic limitations on sensor performance, particularly under severe environments, and development of a family of sensors for applications ranging from tactical missile guidance to inertial navigation systems and re-entry vehicle guidance. This paper described the progress toward these objectives and identifies the significant limitations in performance for the current sensor candidates.
Dielectric mirrors with scatter <0.03% and loss <0.1% were developed for laser gyros. This paper discusses the current performance of laser gyro mirrors which are manufactured at Optical Coating Laboratory, Inc., and the test and diagnostic instruments which are used in their production and measurement.
The objective of this paper is to temper the recent performance claims for optical rate sensors employing passive non-resonant cavities (Sagnac effect) with some results obtained in our laboratory. Over the period from 1971-1975 the authors investigated an all-discrete component embodiment of the above rate sensor employing mirrors in the cavity, beamsplitters for input/output, and an external HeNe gas laser. Unique to the Lear Siegler effort was the fact that the countertravelling beams were cross-polarized when combined. Inertial rates could then be sensed as ellipticities in the resultant beam via a highly sensitive ellipsometer. Sources of random error signals (false rate) unique to the ellipsometer and electronics were minimized and contributed less than 80°/hr of false rate in an 8" square cavity. However, the multilayers on the mirrors and beamsplitters produced non-reciprocal phase changes between the countertravelling beams whenever the latter became even slightly non-concentric. This non-concentricity may be due to a laser beam angular deviation (wobble) or to a ring element tilt. A typical HeNe laser with a confocal or hemispherical cavity has a 3x10 radbeam wobble. This, coupled with the multilayer sensitivity, yields false rates in the 0.1-1°/sec range. In addition, the laser beam wobble directly produced a geometric non-reciprocal phase which amounted to over 25°/sec. To reduce these errors we suggested using a solid state CW laser diode source and a multiturn single mode fiber ring. Unfortunately, such elements were not available to us at that time so further experimental effort at Lear Siegler was discontinued. However, analysis performed by us showed that various error sources would still limit the performance of this hybrid model to about 1-5°/hr. What is needed, therefore, is an all-integrated-optics embodiment of the sensor.
The drift performance of an optical rotation sensor employing a passive ring cavity is presented. With a square cavity of about 17 cm on a side, and a 1 mW external laser, the rms fluctuation in the measurement of rotation was 0.45°/hour for an integration time of 1 second. This is consistent with the shot-noise-limited performance expected for the present setup.
A nuclear magnetic resonance (NMR) gyroscope senses rotation as a shift in the Larmor frequency of nuclear magnetic moments as they precess about an applied field. A sensitive optically pumped magnetometer has been developed which can sense the weak magnetic fields associated with the nuclear moments and thus allow the detection and determination of the Larmor precession frequency. The magnetometer operates on the principle that the absorption of optical pumping light by rubidium atoms is a function of the direction of the rubidium magnetic moment relative to that of the light beam and that the direction of the rubidium magnetic moment is itself a function of the magnetic field. Thus a magnetic field modulated at a nuclear Larmor frequency can cause modulations in the transmitted optical pumping light at this same frequency. A breadboard NMR gyro utilizing this magnetometer method has recently been tested. It is characterized by a high signal-to-noise ratio and a low random bias drift.
The helium isotopes 3He and 4He have properties which make them attractive in a NMR gyro application. Very long polarization relaxation times (up to 24 hours) have been observed with 3He. 4He has been used successfully for a number of years in high sensitivity magnetometers. Analysis and experimental effort have been directed toward the implementation of a NMR gyro using 3He as the rotation sensitive element and 4He to eliminate sensitivity to magnetic fields. In recent experiments, semiconductor diode lasers have been demonstrated to have good characteristics for optical pumping of helium and the simultaneous magnetic resonance of 3He and 4He in a single cell has been observed.
Elliptical and circular core low loss single mode fiber optic waveguides were fabricated in lengths exceeding 2 km using a chemical vapor deposition (CVD) technique. The typical loss measured at 0.82 pm wavelength was less than 3 dB/km in lengths approaching 2 km. A power ratio in orthogonal planes of polarization larger than 35 dB was measured on an elliptical core fiber >900 m long and 46 dB on a circular core fiber >200 m long.
The use of light in inertial rotation sensing requires the path of a light beam to enclose an area, so that clockwise and counter-clockwise degeneracy is lifted by an inertial rotation. This is presently done in laser gyros with carefully aligned mirrors. This paper reviews the way in which guided waves may be used to eliminate the use of carefully aligned mirrors and discrete optical components. The advantages of guided wave devices will be increased stability and rigidity, as well as potential decreased fabricational costs. Disadvantages are possible increases in scattering and absorption losses.
The performance of injection lasers in the last couple of years has improved substantially in terms of the reliability, linearity and the threshold current. It is now possible to make a laser that operates in a single transverse mode with a power level of over 5mW with very low thresholds. Furthermore, it has been reported that laser diodes can operate as long as 1 million hours at room temperature. Therefore, it is believed that for high data rates and long distance communications applications, the injection laser is going to be the light source used in the future.
This paper describes a laser diode suitable for integrated and fiber optic applications. Single transverse mode operation is provided by making the stripe width narrow and the length great enough. An etalon internal to the diode provides single longitudinal mode oscillation where required. Data showing the range of demonstrated operation of this type diode are presented. Stable single frequency, single transverse mode operation at output powers greater than 15 mW has been obtained.
It has been shown previously (1) that the dispersion term in the Fresnel drag coefficient can be detected using a moving single mode fiber as the beam path. This letter reports on the results of a study of optical noise sources in fiber gyroscopes as well as on determining the dispersion term with higher accuracy.
A single axis rotation sensor, utilizing a 200 m single mode optical fiber in a 30 cm diameter coil has been constructed and tested in the laboratory. A He-Ne laser is the light source for the Sagnac interferometer-mode sensor. The outputs of Si detectors viewing two complementary interference patterns are electronically differenced to obtain a rotation-dependent signal. Linear response over a rotation rate range of about 10-4 to 10-1 rad s-1 was obtained.
A one-axis, fiber optic interferometer gyro has been constructed and tested. The device, based on a 670 meter length of single-mode optical fiber, uses a GaAlAs laser as a source, and phase sensitive synchronous detection to measure fringe shifts as a function of rotation rate. Using a calibrated rate table, rotation rates from 0 to 110 deg/sec have been measured. The device easily measures rates as low as 0.05 deg/sec with a signal-to-noise ratio of ten. Error sources and potential improvements will be discussed.
Two methods are presented for sensitive measurement of nonreciprocal phase shift (NRPS) in a multi-turn fiber Sagnac interferometer for application in inertial rotation sensing. Both techniques are capable of producing an NRPS modulation of amplitude ±π/2 radians at a rate sufficiently high for shot-noise-limited performance. The first method produces an NRPS by generating a nonreciprocal refractive index using the electro-optic effect, and requires orthogonal polarization in the cw and ccw beams. The second method produces an NRPS by generating a nonreciprocal propagation frequency using two acousto-optic shifters; there is no requirement on light polarization. When either method is used in conjunction with phase-sensitive detection, feedback compensation, and adequate laser intensity, shot-noise-limited measurement becomes possible. Preliminary performance data is included.
A Sagnac effect interferometer gyro using an optical fiber solenoid ring is delineated where electronic phase detection is employed as opposed to observing an optical fringe shift indicative of rotation. A balanced double-heterodyne system is employed to detect the counter-circulating optical waves derived from a single red He/Ne laser source. The laser output is frequency shifted to provide the counter-circulating signals and the heterodyne local oscillator. Two Bragg-type elasto-optic frequency shifters are employed using Te0:Si02 centered at 70 MHz, the difference of which is the IF frequency in the 10 - 100 KHz range. Hard-limited IF amplification is employed before phase detection. The rotation-induced phase shift (Sagnac effect) appears as a balanced differential phased shift between the channels. Interferometer operation using both quasi single-mode and multi-mode optical fibers with 6328A radiation has been demonstrated. High visibility Fresnel fringes due to the Sagnac effect have been observed using a 1 Km length of multi-mode fiber and a 0.46 Km length of quasi single-mode fiber. Using the 0.46 Km long fiber at 6328A with 9dB/Km loss where its single-mode cutoff is at 9400A, rotational rates of less than 0.1 degrees rotation per second are easily discerned using the balanced double-heterodyne system with electronic phase sensing.
We are investigating a new approach to inertial rotation sensing which we refer to as an active re-entrant Sagnac system (ARS). It is aimed at rotation rates extending to very low values, as required in such amplications as navigation, guidance and geophysical measurements. It employs a multiturn fiber optic loop containing an internal optical amplifier, and uses pulsed optical signals introduced by an external laser oscillator which make multiple recirculations around the loop. Its output signal is a time-sampled sinusoidal waveform having the same frequency as the beat frequency in a standard oscillating ring laser gyro (RLG) having the same loop diameter. Like the RLG, it is basically an integrating rate gyro, in which digital cycle counting is used to determine the angle turned through. It is basically free of the mode locking effect encountered with RLG's, and has prospect for higher sensitivity to rotation. Experiments using cw light are described, which include new re-sults on removing instabilities and environmental sensitivities from fiber gyros. Initial experiments on pulsed recirculation without an amplifier are described, which affirm the basic recirculation principle. Work is beginning on development of a fiber amplifier for use in a full ARS experiment.
The optimum sensitivity limits of an optical-fiber gyro are assessed with intrinsic light scattering taken into account. Results reveal that laser wavelength of 1.1 µm is more suitable than 0.633 µm, continuous mode stripping can reduce the scattering effect by one hundredfold, and photon-noise-limited performance can be achieved with the adoption of modulation or heterodyne techniques. Numerical examples show that sensitivities of 0.0078 deg/h at 2 mW laser power and 0.0009 deg/h at 81 mW for X=0.633 Âµm, and 0.0025 deg/h at 2 mW and 0.0007 deg/h at 14.4 mW for X=1.1 pm may be achieved. These values are better than those of the current ring laser gyros and suggest that, with adequate research and development effort, the optical fiber gyroscope canlpeaviable alternative to ring laser gyros as high-performance rotational rate sensors.
Optical rotation sensors have reached the level of maturity that they are strongly being considered for Air Force systems. These new inertial sensors are ideal strapped down devices and show great potential for answering the requirements of future inertial systems. This paper discusses the Various applications for these sensors and some of the major factors that could prolong their entry into the Air Force inventory.
Azimuth and astronomic latitude can be determined using inertial rotation sensors. New applications in surveying and geodesy appear as the accuracy of the determinations improve. Geophysical applications require high precision sensors operating in a carefully controlled environment. Useful determinations of polar wobble might be feasible by monitoring azimuth and astronomic latitude in geophysical observatories.
In 1960 L.I. Schiff suggested a new test of Einstein&apos's General Theory of Relativity based on measuring the precessions of the spin axes of gyroscopes in orbit around the Earth. Since 1963 a research group in Stanford Physics and Aeronautics Departments has been developing an experiment to measure the two effects calculated by Schiff. The gyro-scope consists of a uniform sphere of fused quartz 38 mm in diameter, coated with super-conductor, electrically suspended and spinning at about 170 Hz in a vacuum. Readout is by magnetic observation of the London moment in the spinning superconductor. The paper describes the proposed flight apparatus and the current state of development of the gyroscope, including techniques for manufacturing and measuring the gyro rotor and housing, generating ultra-low magnetic fields, and mechanizing the readout. The torque analysis, which indicates a limiting gyro drift-rate of 2 x 10-11 deg/hour in space, is briefly summarized.
This paper describes some new performance analysis methods, developed by the authors, for use in characterizing laser gyro system performance. Past analysis methods used by laser gyro evaluators consisted of time base averaging of pulse data over 10 to 100 second periods. When it became necessary to process data in shorter time periods such as 10 millisec and less, the time base technique introduced greater instantaneous rate deviations. This larger scattering of data results in higher processing noise, which in turn results in higher indicated random drift performance. The technique explored by the authors involves the use of Fast Fourier Transforms (FFT) to analyze per-formance in the frequency domain. In addition to examining static performance, the FFT method is also applied to dynamic tests. Finally, the FFT method provides the basis for transfer function analysis of the laser gyro.
A test program to evaluate laser gyros for Air Force applications is described. The test plan development, facilities and actual test results obtained at the Charles Stark Draper Laboratory, Inc. on a mechancially dithered laser gyro are presented. The test plan addresses methods to evaluate a laser gyro's bias and scale factor under various environments and operational modes. The test techniques are designed for use with: a mini-computer for on-line data analysis and test control, a thermal chamber mounted on an air-bearing table which allows performance evaluation with temperature profile testing, and a data acquisition system for recording available analog voltages for subsequent correlation analyses. Future test plans and additional areas for evaluation are also discussed.
In spite of the specific differences between laser gyro designs, all ring laser gyroscopes have certain similar elements such as the lasing medium itself, mirrors, windows, and optical detectors. Laser gyros may also include path length control mechanisms, dither motors, magnetic mirrors, and/or Faraday cells. Ring laser gyroscopes are very attractive for a wide range of commercial and military systems. In many military applications they must survive and continue functioning after nuclear radiation exposures. This paper identifies nuclear vulnerabilities of several components common to most ring laser gyroscopes. Particular attention is directed to radiation coloration and bleaching of optically transparent low thermal expansion materials and radiation responses of optical photodetectors. Corroborating experimental results and selected nuclear hardening design guidelines are offered.
A Sagnac Interferometer has been built with a one hundred meters monomode fiber propagating a 0.6328 µm wavelength laser light. An exploitable measurement corresponds to a rotation rate of 8.10-3 radian per second. The noise corresponds to 1,6.10-3 radian per second. This noise is about 60 dB above quantum noise. The sources of fluctuations must be analysed and reduced. One cif them is the mechanical instability which can be improved by building the interferometer with integrated optics.
In this paper, we first describe the different integrated optical devices that can be associated with single mode fiber to lead to a compact rotation sensor. Then, we present the results obtained during a simulation experiment to demonstrate the possibility of measuring very small phase shifts(10-5 fringes) with integrated optical circuits.