High-quality fiber-optic gyros no longer just a dream

by Albert Tebo

Part 1 of a 2-part series

SPIE celebrated the 20th anniversary of what HervÈ C. LefÕvre called "a pioneering physics experiment" (by V. Vali and R. W. Shorthill) that developed into the fiber optic gyroscope. The occasion was SPIE's Conference 2837 on Fiber Optic Gyros: 20th Anniversary Conference, held in Denver, Colorado in August 1996.

LefÕvre, of Photonetics SA (Marly-le-Roi, France), and Eric Udd of Blue Road Research (Troutdale, OR), and Kazuo Hotate of the University of Tokyo (Tokyo, Japan), were chairs of the conference and editors of the volume.

In their introduction the chairs said the dream of very small, stable, inexpensive fiber optic gyros is a reality today, and they cited much commercial activity that confirms it.

"Litton has been in production for a few years with an attitude-heading reference system that very closely matches the 'imaginary' cube of a decade earlier. Honeywell has production fiber optic gyros flying on 777s and on Dormier commuter aircraft.

"JAE has placed fiber gyros on remotely piloted helicopters, lawn mowers for soccer fields, and robots for cleaning floors in shopping malls. Mitsubishi has flown several fiber optic gyros on launch vehicles, and has plans to use one of a penetrator flight to the Moon.

"Hitachi is producing fiber optic gyros at a rate of 3000 per month to support automobile navigation systems." Production rates of 100,000 units per month are in the offing, driving costs down to $100 each.

"Litton is near to entering production on a fiber optic gyro-based navigation system, and a number of other companies are not far behind.

"Honeywell has demonstrated a high precision fiber optic gyro with bias stability of 0.00038 deg/h that could be used to support deep space missions as well as submarine navigation systems."

"Several papers pointed out some of these emerging areas, including current sensing, fluid flow, and secure fiber optic communications."

I-FOG fundamentals

The first three papers of the conference constituted the plenary session, led off by LefÕvre's paper on the fundamentals of the interferometric fiber optic gyroscope (I-FOG). "The I-FOG is based on the Sagnac effect which produces, in a ring interferometer, a phase difference proportional to the dot product of the rotation rate vector by the area vector enclosed by the optical path:

where w is the angular frequency of the light source and c is the velocity of light in a vacuum."

Light entering the fiber optic ring of the interferometer is split by a beam splitter into two counter-propagating waves that return exactly in phase after having traveled along the same path in opposite directions.

"When the interferometer is rotating, an observer at rest in the inertial frame of reference sees the light traveling with the same vacuum velocity c in opposite directions, but during the transit time through the loop the beamsplitter has moved a distance Delta lambda around the loop.

"The observer sees that the co-rotating wave has to propagate over more than one complete turn, while the counter-rotating wave has to propagate over less than one complete turn. This difference of transit time may be measured by interferometric means."

With the I-FOG the sensitivity is enhanced by the use of a multiturn fiber coil.

LefÕvre converted this expression to one in terms of wavelength:

where is the wavelength of light, L is the total coil length (of a multiturn coil), D is the mean coil diameter, and is the rotation rate component parallel to the coil axis.

The basic response of an I-FOG is a "raised cosine," with a maximum at zero rotation. It is customary to bias the signal about an operating point with a nonzero response slope. This is obtained by a reciprocal phase modulator placed at the end of the coil that acts as a delay line.

"There is an unambiguous range of measurement of ±p radians around zero for the phase difference, which corresponds to an unambiguous range of ±Wp for the rotation rate. The sensitivity, /, has the dimension of time and is often expressed in seconds, and the rate Omegaµ which corresponds to a phase difference of a microradian is a good order of magnitude for measurement noise limit."

It is possible to scale up or down the sensitivity by changing the area of the sensing coil, a useful feature for achieving various devices without a complete redesign.

LefÕvre gave us two examples: A high-sensitivity I-FOG with L, D, and l of 1 km, 10 cm, and 1550 nm respectively that provided / of 1.35 s, an Wp of 133°/s, and an Wµ of 0.15°/h respectively; and a medium-sensitivity I-FOG with L, D, and l of 200 m, 3 cm, and 850 nm respectively that provided a

/ of 0.15 s, an Omega pi of 1220°/s, and an Omegaµ of 1.4°/h respectively.

In citing characteristics, LefÕvre stated that, through time averaging, a phase difference of 10^-7 to 10^-8 radian should be measurable, while the absolute phase accumulated by the wave along 100 m to 1 km is 10⁹ to 10¹⁰ radians.

Figure 1 shows a reciprocal (or minimum) configuration of the I-FOG.

Figure 1. Reciprocal (or minimum) configuration of the Interferometric Fiber Optic Gyro (I-FOG). (From HervÈ C. LefÕvre, in Proceedings of SPIE's Conference 2837, Denver, CO, July 1996, p. 2.)

The light is fed into the interferometer through a single-mode waveguide and the returning signal wave is filtered through this same waveguide at the output. This ensures that both returning waves have experienced exactly the same total phase despite potential defects in the splitter.

Because of the difficulties of stably coupling free-space waves into a single-mode fiber, an all-guided scheme has been developed to perform all the interferometric functions required.

A hybrid solution is preferred, using a multifunction integrated-optic circuit on an electro-optic substrate such as lithium niobate (LiNbO3).

Figure 2 depicts this hybrid configuration of an I-FOG. LefÕvre said that this technique "provides efficient low-wattage wideband phase modulators that allow one to implement powerful signal processing techniques."

Figure 2. Optimal hybrid configuration of the I-FOG: (a) Y-coupler configuration. (b) multifunction integrated-optic (IO) circuit. (From HervÈ C. LefÕvre, in Proceedings 2837, p. 2.)

Here are some other comments from LefÕvre.

"Since an I-FOG operates about a null path difference it may use a broadband source without degradation of the signal." This concept was explained in the paper.

"The low temporal coherence of broadband sources makes them very suitable for the I-FOG, but they have to be coupled efficiently into a single-mode fiber, which requires a high spatial coherence."

LefÕvre said that a broadband source is very useful for reducing backscattering noise and also for solving the difficult problem of polarization reciprocities. "The analysis of these problems of polarization is very important in a gyroscope."

"For high-performance applications, the trend is now to use fiber sources in the

1550-nm window which are derived from erbium-doped fiber amplifier (EDFA) technology that is revolutionizing optical fiber communication."

"An Er-doped-fiber source brings, in addition, a high power (more than 10 mW) which improves the signal-over-noise ratio and an unpolarized emission which is useful to reduce polarization nonreciprocities."

"The optimal practical configuration uses a multifunction integrated optic circuit and a polarization maintaining sensing coil."

Finally, in his conclusion, LefÕvre said, "For inertial guidance and navigation, the I-FOG covers the medium accuracy range (0.1 to 10°/h) with compact sensing coils, and it becomes also a significant competitor for high-performance navigation grade (0.01°/h) with larger coils. Using mass-producible components, it brings great expectation of significant cost reduction which will extend the field of use of inertial guidance techniques."

Commercial applications of fiber optic gyros

We turn now to fiber optic gyros (FOGs) mass produced for commercial applications, as reported by Hiroshi Kajioka and colleagues of Hitachi Cable Ltd. (Ibaraki, Japan).

The authors' introduction gave a good argument for FOGs. "Because the operational principle for sensing inertial rotation is not mechanical, but optical, they are inherently quick to start, resistant to shock and vibration, light, compact, and long-lived. These are very attractive features for many users of commercial gyroscopes."

Hitachi I-FOGs are made completely of polarization-maintaining fiber (PMF) for which they have developed two types of cross-sections, an elliptical-jacket PMF for high-end applications, and an elliptical-core PMF for low-end applications. The all-PMF gyro eliminates polarization fluctuations, thus minimizing gyro bias drift.

Figure 3. Basic configuration of a minimum-reciprocal FOG. (From Hiroshi Kajioka and colleagues, in Proceedings 2837, p. 18.)

The standard configuration of the Hitachi I-FOG is the minimum reciprocal, shown in Figure 3. This is obviously equivalent to the reciprocal I-FOG shown in Figure 1, but we show it to depict the evolution of this to the configuration of the low-cost I-FOGs designed specifically for industrial and consumer applications.

In Figure 4(a) the source-detector coupler with a photodiode and two splices in the sensing coil are removed. In Figure 4(b) the polarizer has been removed. Evidently Hitachi detects the returned signal from the back of the laser.

Figure 4. Configurations of reduced minimum-reciprocal FOGs. Source-detector coupler and two splices are removed in (a). Polarizer is also removed in (b). (From Hiroshi Kajioka and colleagues, in Proceedings 2837, p. 18.)

Hitachi I-FOGs use open-loop technology, with phase modulation, which has the longest history of development and perhaps is the most well established. Open-loop I-FOGs are made solely of fiber-optic in-line devices, and are simpler than other FOGs.

The commercial applications of Hitachi's FOGs span from underground to ground surface to seaborne to airborne systems.

The authors said that there are three types of FOG usage: positioning, attitude control, and absolute direction measurement. We quote. "To determine a vehicle's position, a FOG is used as an inertial reference and provides an azimuth angle (i.e., vehicle heading). In attitude-measurement systems, FOGs also yield the inclination of the vehicle. For absolute direction measurement, a FOG is used to measure the Earth's rotation rate as a function of the direction."

Hitachi's FOGs are used on police cars in Japan as a vehicle-heading sensor as part of a police-car location system. Another type of FOG is used in Nissan's luxury cars to ensure a highly reliable map-matching operation, resulting in precise vehicle navigation systems. Production of these units has been established at a rate of 5000 per month.

FOG navigational technology is also used in dump tracks in mines and in sweeper robots in railroad stations and airports.

An airborne application is in attitude control of an unmanned radio-controlled helicopter that is used for planting seeds and for spraying chemicals. The FOGs help stabilize the flight of the helicopter.

The authors stated that FOGs have been used as underground sensing tools in urban areas to map the buried pipes for electricity and gas supplies and communication cables. "A typical underground pipe-mapping system consists of a probe with a three-axis FOG, a winch, communication and power cables, a cable measure, a cable reel, and a data processor." With three cascaded single-axis FOGs, the system's location error is less than 0.1 percent.

An opto-compass is another important FOG underground application. The authors stated, "Its operational principle is the measurement of the Earth's rotation by using the highly sensitive FOG HOFG-5000. North-finding technology using an opto-compass has been refined over the past several years." "Both north-finding and tracking accuracies are less than 0.05 degree."

For future development, Hitachi is looking toward applications that require high scale-factor accuracy and dynamic range, and is looking toward closed-loop schemes. "The Hitachi closed-loop FOG is based on all-digital serrodyne technologies developed by Photonetics. It is almost ready for mass production."

In the second part of this article we will discuss high-performance fiber optic gyros and the future evolution of fiber optic gyros.