Passive optical "gyros" as well as active optical "gyros" (i.e. ring laser gyros or RLG) are based on the Sagnac effect. The Sagnac effect is the non-reciprocal phase differ-ence experienced by light propagating along opposite directions in a rotating frame. The active technique for measuring the phase difference consists of placing a gain medium within a ring cavity, i.e. a ring laser. In the presence of rotation, the non-reciprocal phase difference that is generated is automatically transformed to a difference in the laser frequencies along the counterpropagating directions in the ring laser. In the passive optical gyros, the non-reciprocal phase shift generated by rotation must be measured using external techniques. There are two types of passive gyros. One type is based on an interferometer approach in which two.light beams propagate along opposite directions around a closed path and the phase shift difference generated by rotation is measured. The sensitivity is enhanced considerably by confining the light propagation within a single mode fiber that is wrapped many times around a cylinder. The other type of passive gyro is based on a ring resonator approach in which an external light source is used to measure the difference in the resonance frequency of the cavity along opposite directions. This resonance frequency difference is proportional to the rotation-induced Sagnac phase difference. Optical cavities may be constructed using bulk-optical, fiber-optical or integrated-optical components. At present there is considerable research in progress that is aimed at the development of passive gyros for a variety of applications. The performance so far has been very encouraging, particularly with regard to short-term noise behavior. Scale factor stability and wide dynamic range are still under investigation. The development of integrated optics for use in passive gyros is progressing rapidly. The references below provide a good starting point for the interested reader.
For at least ten years now workers have been contributing techniques for modulating light signals in optical fibers1'2'3'4. These techniques have been of the phase, polarization and frequency variety. However, whereas the phase and polarization modulation techniques can be accomplished in a very efficient manner in what is referred to as the "all-fiber" approach (i.e., an unbroken or unspliced fiber), frequency modulation schemes have not been very efficient (under 2%) using an all-fiber techniques '6. To date, practical frequency modulation is limited to accomplishment in bulk Bragg Cell media external to the fiber. Hence discussion of frequency modulation within an optical fiber is premature at this time. This paper therefore will survey those signal modulation techniques which apply to demonstrated in situ optical fiber modulation techniques, i.e., all-fiber phase and polarization modulation.
Fiber optic sensor technology is explained in terms of theory and applications. Two classes of sensing devices are discussed: amplitude-modulated sensors and phase-modulated sensors. Specific examples of amplitude-modulated devices--one a pressure sensor using an optical reflection technique and another an accelerometer using a microbend technique--are described. Four types of fiber optic interferometers used in phase-modulated sensors are addressed. A highly sensitive fiber optic accelerometer using a Mach-Zehnder interferometer is detailed.
The current status of optical sensing technologies by multimode fibers in Japan is reviewed. The fiber-optic sensors are classified into three categories, and their basic configurations are outlined. The principles and performances of some successfully developed fiber-optic sensors are described in detail.
A fiber optic lever sensing technique utilizing three fibers, one transmit and two receive with different core diameters, is described. The method predicts improved sensitivity, and in some cases, larger fiber-reflector gaps than three fibers of equal cores and numerical apertures. Other features include the already established advantages of compensation for input power variations and input fiber losses. It is found that optical fibers with unequal core-cladding diameters as well as numerical apertures provide the most improved results.
A novel transducer with optical-fiber data link -- to provide electrical isolation and immunity from electromagnetic noise -- has been developed. A pulse-position (or pulse-frequency) modulated optical signal is transmitted by optical fiber from a remote converter to mainframe. Light of a different wavelength is transmitted via the same optical fiber from a laser in the mainframe to a photocell battery in the remote converter, to power it. Signal and "power" light beams are separated by dichroic mirrors. A practical thermocouple converter is described; it consumes only 84 μW, and is powered by light from a 5 mW semiconductor laser in the mainframe.
Test results from reflection-based fiber optical sensors are presented, and several examples of reflective sensor applications are discussed. Reflection sensing techniques can be used to measure the displacement of suspended mirrors for proximity and motion sensing. In addition, reflections can be used to gauge the surface finish of machined parts and accurately determine the size and shape of components in automated manufacturing applications.
To allow computer synthesis of the response of commercial multifiber optical proximity probes, an investigation was made of the response of a single pair of fibers. A theoretical and experimental analysis was made of the coupling of an illuminating and receiving fiber through reflection off a dull or specular target. A computer program was developed that superposes the responses of all fiber pairs in a probe that may contain up to 2400 fibers, with a total of 12002 pairings maximum. A comparison is made between the computed and measured response of an actual probe.
New methods for fabricating low-loss integrated-optical polarizers and analyzers are introduced for use with single-mode LiNb03 and glass waveguides. Extinction ratios in excess of 30-35 dB are experimentally obtained with no additional transmission losses.
Measurements of the change in stress-induced birefringence with temperature in single mode optical fibers are reported. The fibers examined include those with low residual stress birefringence that have circular and elliptical cores. A section of each fiber was placed under constant load with weights and heated inside a furnace. Polarized light was coupled into and out of the fiber ends outside the furnace. Two mutually perpendicular polarization components were analyzed and detected at the fiber output end. Changes in the detected signal levels were monitored as a function of the temperature of the single mode fiber stressed under constant load. Discussion of results and applications to localized stress measurements at high temperatures are presented.
The temperature dependence and fluctuation behaviour of the birefringences of two non-polarization-preserving fibers have been investigated experimentally at approximately 1 m long fiber pieces. The linear ret8rdance of one of the fibers drops to one half, when the fiber is warmed up from 20 to 50o C, whereas the other fiber shows a much lower temperature dependence. Without intentional temperature variations the linear birefringence of both fibers show fluctuations with nearly equal rms amplitude values of 2 to 3 mrad but different speed at room temperature. The influence of temperature, fiber twisting and fiber length on the fluctuations is considered.
Four port single mode fiber optic directional couplers have been fabricated exhibiting coupler losses approaching 0.1% (.005 dB). A new experimental technique with an integrating sphere was used to measure these low losses. The couplers, which can be tuned for any coupling ratio, have coupled as much as 99.8% of the total input power in one fiber to another fiber. In this high coupling, low loss regime new polarization dependent effects were observed in the couplers.
Oscilloscopes based on the Cathode Ray Tube technology have been the basis of conventional high speed data recording at analog bandwidths greater than 200 MHz. With the advent of high bandwidth optical fibers and fiber sensors, an alternate and eventually more economical method for high speed data recording is possible. In this approach, the combined broad band characteristics of the optical fiber and the electrooptical streak camera are utilized to extend bandwidth, to limit signal degradation, and to permit synchronous multi-channel high speed data recording.
Magnetic sensors have been used in applications ranging from keyboard switches to submarine detection. Recently developed fiber optic magnetometers promise to provide high performance/moderate cost alternative to existing sensors. The special properties of fiber magnetometers such as directional response, high sensitivity, room temperature operation are discussed in the context of other magnetometers. Fiber magnetometer design considerations, fabrication procedures, and performance measurements using magnetostrictive alloy coatings are presented. Optimum performance is obtained with a 36% - 64% nickel-iron alloy electroplated over the polymer jacket on a single mode fiber. Sensor designs for fiber optic magnetometers are shown.
Single-sideband frequency shifting has been demonstrated in birefringent fiber by using a traveling acoustic wave to couple the two orthogonal polarizations of the fiber. Both surface and bulk acoustic waves have been used. Frequency shifts as high as 15 MHz have been observed, with carrier and unwanted sideband suppressions of 25-30 dB, using acoustic powers of a few watts.
A laboratory study has been made of the characteristics of an interferometric type optical fiber pressure gradient hydrophone. The optical system is configured as an all fiber Mach-Zehnder interferometer excited by a single mode gallium arsenide diode laser. A pair of identical fiber coils, one in each arm of the interferometer, forms the sensing portion of the gradient hvdrophone. Each coil consists of 10 meters of polyethylene jacketed, single mode fiber, wound in a toroidal element of mean diameter 4 cm and thickness 3 mm. A standing wave type acoustic calibrator was constructed and used to determine the sensitivity of each individual coil. The pair then was aligned coaxially and separated by 10 cm to operate as a pressure gradient device. Details of the construction of the system, calibration procedure, and hydrophone sensitivity data are presented.
A passive high finesse fiber ring resonator has been demonstrated using ultra low loss directional couplers. The finesse of the resonator was measured as high as 500. Resonator loss were dominated by the fiber attenuation rather than insertion loss of a directional coupler as previously reported elsewhere in recent literature. Further improvement of resonator finesse is feasible and applications of the this device are also discussed. Polarization effects in the resonator have been observed which indicate polarization dependent phenomena are occurring in the directional coupler.
Nonplanar active ring laser cavities and their uses for the gyro builder are discussed and it is shown how a slightly nonplanar configuration may create magnetic sensitivities and how they may be controlled. Another origin of magnetic biases in both square and triangular ring laser gyros, namely stress-induced linear birefringence in the gyro mirrors, is examined and possible experimental arrangements for measuring the size of birefringences produced by stress in mirrors suggested.
A description of the elements of a 7.62 m x 7.62 m Passive Ring Resonator Laser Gyro (PRRLG) is given. Predicted performance of the device is shown to be in the 3 x 10-8 to 4 x 10-10 earth rate unit (ERU) range.
In this paper we summarize recent developments in passive resonator gyros in our laboratory. The resonators investigated include a mirror resonator and also an all-fiber resonator. Preliminary performance data and important sources of error are presented.
A fiber optic gyroscope utilizing a passive, non-reciprocal phase shifter is presented here. This fiber optic gyroscope, when combined with one of the proposed modulation schemes, is expected to achieve, in an all-fiber implementation, high sensitivity, large dynamic range, low drift and low cost.
Means to correct scale factor in the phase-nulling optical gyro are presented. In particular, methods are described that allow the wavelength of the light source to be tracked to high accuracy. A derivative of this technique, allowing the phase-nulling optical gyro to execute an in-flight cold start, is also described.
Direct measurement of Sagnac phase shift from an open-loop, all-fiber-optic gyroscope using signal processing is described. The photodetector output from a phase modulated gyro is amplitude modulated at the phase modulation frequency to transpose the optical phase shift into a low frequency electronic phase shift, which is measured using a digital time interval counter. A wide dynamic range and linear scale factor are demonstrated using this approach.
Fiber optic gyroscope 1984 development status is reviewed and summarized. The compan-ies, agencies and universities involved in this development are listed, and leaders are discussed. Included in the review are these gyro categories: All-fiber Fiber - plus - bulk optics Fiber - plus IOC The US funding of development of fiber optic gyros, segmented into internal R&D and government funding, is forecasted, 1984-1994. The US production of fiber optic gyroscopes is forecasted, year by year, 1984-1994. The forecast is segmented into repeatability rate categories and related applications, and includes both quantity and value data. Fiber optic gyro forecasted production is contrasted to spinning mass and ring laser gyroscope demand. Currently available fiber optic components for gyroscope use are reviewed, and performance advances of these components are forecasted. Component price trends are fore-casted. Components discussed include: Single mode fiber - Unipolarized fiber - Conventional single mode - Polarization maintaining Emitters and detectors Couplers Other fiber components Integrated optic circuits The trend to operation at 1.3-1.6 microns is forecasted. Component performance in the 1.3-1.6 micron range is projected. Component suppliers are listed.