Sensors and Methods for Mobile Robot Positioning

Chapter 2: Heading Sensors 41
A typical block diagram of the “minimum-reciprocal” IFOG configuration is presented in
Figure 2.10. Polarization-maintaining single-mode fiber [Nolan and Blaszyk, 1991] is employed to
ensure the two counter-propagating beams in the loop follow identical paths in the absence of
rotation.
An interesting characteristic of the IFOG is the absence of any laser source [Burns et al., 1983],
the enabling technology allowing the Sagnac effect to reach practical implementation in the first place.
A low-coherence source, such as a super-luminescent diode (SLD), is typically employed instead to
reduce the effects of noise [Tai et al., 1986], the primary source of which is backscattering within the
fiber and at any interfaces. As a result, in addition to the two primary counter-propagating waves in
the loop, there are also a number of parasitic waves that yield secondary interferometers [Lefevre,
1992]. The limited temporal coherence of the broadband SLD causes any interference due to
backscattering to average to zero, suppressing the contrast of these spurious interferometers. The
detection system becomes sensitive only to the interference between waves that followed identical
paths [Ezekiel and Arditty, 1982; Lefevre, 1992].
The Sagnac phase shift introduced by rotation is given by [Ezekiel and Arditty, 1982]
2BLD
)N = (2.7)
8c
where
)N = measured phase shift between counter-propagating beams
L = length of fiber-optic cable in loop
D = diameter of loop
8 = wavelength of optical energy
c = speed of light in a vacuum.
The stability of the scale factor relating )N to the rotational velocity in the equation above is thus
limited to the stability of L, D, and 8 [Ezekiel and Arditty, 1982]. Practical implementations usually
operate over plus or minus half a fringe (i.e., ±B rad of phase difference), with a theoretical sensitivity
-6
of 10 radians or less of phase shift [Lefevre, 1992].
IFOG sensitivity may be improved by increasing L (i.e., adding turns of fiber in the sensing loop).
This effect peaks at an optimal length of several kilometers, after which the fiber attenuation (typically
1 dB/km) begins to degrade performance. This large amount of fiber represents a significant
percentage of overall system cost.
Source splitter
Coil splitter
Source
Polarizer
Filter
Detector
Fiber coil
Phase modulator
Figure 2.10: Block diagram of “minimum-reciprocal” integrated fiber-optic gyro. (Adapted
from [Lefevre, 1992].)