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686 OPTICS LETTERS / Vol. 11, No. 10 / October 1986
Self-pumped phase-conjugate fiber-optic gyro
Ian McMichael and Pochi Yeh
Rockwell International Science Center, 1049 Camino Dos Rios,
Thousand Oaks, California
91360
Received June 2, 1986; accepted July 30, 1986
We describe a new type of phase-conjugate fiber-optic gyro that uses self-pumped phase conjugation. The self-
pumped configuration
is simpler than externally pumped configurations
and permits the use of sensing fibers
longer
than the coherence length of the laser. A proof-of-principle demonstration of rotation sensing with the device is
presented.
Several types of phase-conjugate gyro
are described in
the literatures and we recently reported on the first
demonstration to our knowledge of rotation sensing
with a phase conjugate gyro.
5The passive phase-
conjugate fiber-optic gyros described in Refs. 3 and 5
are Michelson interferometers in which the arms con-
tain fiber-optic coils that are terminated by externally
pumped phase-conjugate mirrors. Since the phase-
conjugate mirrors produce time-reversed waves, all
reciprocal phase changes in the optical paths are com-
pensated for and do not effect the output of the inter-
ferometer. However, since the phase shift produced
by the Sagnac effect is nonreciprocal, the output of the
interferometer is sensitive to rotation and can be used
as a gyro.
Standard fiber-optic gyros6are Sagnac interferome-
ters that are inherently insensitive to reciprocal phase
changes and sensitive to nonreciprocal phase changes.
This is true only when their operation is restricted to a
single polarization mode,7and the best fiber-optic gy-
ros use polarization-preserving fibers and couplers.8
However, if the phase-conjugate mirrors in the phase-
conjugate fiber-optic gyro
preserve polarization,9then
nonpolarization-preserving single-mode fibers, and
even multimode fibers, can be used in the gyro.
In the externally pumped configurations described
in Ref. 3 and 5, the fiber-optic coils can be no longer
than the coherence length of the laser. This limits the
sensitivity of the device. It is true that longer coils
can
be used if a polarization-preserving fiber of equal
length is used to carry the pumping waves to the
phase-conjugate mirrors. However, this defeats the
above-mentioned advantage in that the phase-conju-
gate gyro can use inexpensive multimode fibers and
couplers. In this Letter we describe and demonstrate
a self-pumped configuration of the phase-conjugate
fiber-optic gyro
that is not only simpler than the exter-
nally pumped configurations but also allows for the
use of fiber-optic coils that are longer than the coher-
ence length of the laser.
Figure 1 shows a schematic of a self-pumped phase-
conjugate fiber-optic gyro. Light from a laser is split
by beam splitter BS into two fibers, Fl and F2. Fibers
F1 and F2 are coiled such that light travels clockwise
in F1 and counterclockwise in F2. Light waves tra-
versing fibers F1 and F2 experience reciprocal phase
shifts
O,= J k1d1
1, ¢kr
2= J k2d12,(1)
respectively, where d1l and d1
2are elements of length
along Fl and F2, and kl,2= 27rnl,
2/X. In addition, the
nonreciprocal phase shifts
qnrl =+27rR1LW/Xc, q5n,2
= -27rR 2L2Q/XC (2)
are due to the Sagnac effect, where R1,2and L1,2are the
radii and lengths of the fiber loops, respectively, and Q
is the rotation rate. The net phase shifts are then Gr1
+ 0,,, and 0r2 + Cknr2.On reflection of the light from
the phase-conjugate mirror, the phase shifts become
-br1-knr 1and -0r2-,nr 2, where we have dropped the
phase shift of the phase conjugator
1 0"' since it is com-
mon to both waves and we are interested only in the
phase difference. It should be noted that the phase
shift of the phase conjugator is common to both waves
D
/PCM
M
Fig. 1. Schematic of a self-pumped phase-conjugate fiber-
optic gyro. Light from a laser is split by beam splitter BS
into two fibers F1 and F2 that are coiled such that light
travels clockwise in F1 and counterclockwise in F2. Light
traversing the fibers experiences phase shifts due to thermal,
mechanical, and rotational effects. The self-pumped
phase-conjugate mirror PCM produces time-reversed waves
that compensate for the reciprocal phase changes produced
by thermal and mechanical effects but do not compensate
for the nonreciprocal phase shift produced by rotation (Sag-
nac effect). Therefore rotation can be sensed by measuring
the interference between the recombining waves at detector
D.
0146-9592/86/100686-03$2.00/0 © 1986, Optical Society of America