Fiber optical rotation sensor utilizing the Sagnac phase difference

In a fiber optical rotation sensor, a laser wave omitted from a laser diode is guided into a one directional optical coupler and is splitted into two laser waves by the coupler. The two laser waves are propagated in an optical fiber loop in an opposite directions and the phases of the laser waves are modulated in the fiber loop by a phase modulation signal having a frequency f.sub.0. The phase-modulated light waves are combined and converted into an interference laser wave and the interference laser wave is splitted in the coupler. One of the splitted interference laser waves is guided to a photodetector and is converted into an electric signal by the photodetector. A synchronous detector detects the electric signal with a frequency of an integral multiple of the frequency f.sub.0 to extract the corresponding integral multiple signal components Sn-1, Sn, Sn+1. The signal components Sn-1, Sn, Sn+1 (n.gtoreq.1) are supplied to operation circuit. In the operation circuit, the signal components Sn-1, Sn, Sn+1 are processed to obtain Sagnac phase difference .DELTA..phi. from the following equation. ##EQU1## .

BACKGROUND OF THE INVENTION 
This invention relates to a fiber optical rotation sensor, such as a fiber 
optical gyroscope, which measures a phase difference of light waves 
propagating in opposite directions within an optical fiber and senses a 
speed corresponding to an angle of rotation of a rotating body. 
A fiber optical gyroscope, for example, as a sensor for sensing a speed 
corresponding to an angle of rotation is disclosed, for example, in 
(1) International Publication No. WO 82/03456 entitled "Fiber Optical 
Rotation Sensor", published on Oct. 14, 1982; 
(2) International Publication No. WO 83/01683 entitled "Multimode Fiber 
Optical Rotation Sensor", published on May 11, 1983; and 
(3) International Publication No. WO 83/00552 entitled "Fiber Optical 
Rotation Sensor Utilizing Unpolarized Light" published on Feb. 17, 1983. 
The fiber optical gyroscope senses an amount of a phase difference, i.e., a 
Sagnac phase difference, between light waves mutually counter-propagating 
in an optical fiber loop on a rotating body as produced due to the 
rotation of the rotating body. In the fiber optical gyroscope, a whole 
light wave propagating path located between an optical source and a 
sensing section for sensing interference light waves occurring due to an 
interference between mutually counter-propagating waves can accurately be 
made up of an optical fiber and thus has a longer service file. 
The fiber optical gyroscope of the aforementioned International 
Publications uses a phase modulation system in which light waves launched 
into an optical fiber loop are phase-modulated for interference. In the 
fiber optical gyroscope the interference light wave is sensed at a 
photodetector and a sensed signal is supplied to a lock-in amplifier 
comprised of a synchronous detector and low pass filter. In the lock-in 
amplifier, the output signal of the photodetector is detected with a phase 
modulation frequency f.sub.0, only a frequency (f.sub.0) component is 
detected from the sensed signal by the synchronous detector and only a DC 
component of the detected component is delivered from the low pass filter. 
The output signal of the DC component is proportional to 
EQU J.sub.1 (.phi.m).times.sin.DELTA..phi. 
a product of Bassel function J.sub.1 (.phi.m) and sin.DELTA..phi., where 
.DELTA..phi.: a Sagnac phase difference 
.phi.m: the phase modulation amplitude 
J.sub.1 (.phi.m): the Bessel function 
If the amount of phase modulation is set at a stable point at which the 
value of a first-order Bessel function does not vary greatly, then a 
filter output of J.sub.1 .multidot.sin.DELTA..phi. becomes a sinusoidal 
function of the Sagnac phase difference .DELTA..phi. and, as shown in FIG. 
1, the Sagnac phase difference .DELTA..phi. can uniquely be determined 
from the output level through the utilization of a DC component, i.e., a 
linear range I of a plot of the output level against the Sagnac phase 
difference .DELTA..phi. in FIG. 1. 
However, this presents a narrow dynamic range problem since a measurable 
range of the Sagnac phase difference .DELTA..phi. is narrower at a lower 
level due to the use of the linear portion of sin.DELTA..phi.. 
In order to improve this defect, a method is proposed in Electronics 
Letters 10th Nov. 1983 Vol. 19 No. 23 P 997 "Direct Rotation-Rate 
Detection with a Fiber-Optic Gyro By Using Digital Data Processing." That 
is, as the output signal of the lock-in amplifier detected by a phase 
modulation system, the fundamental and the high harmonic wave components 
(2f.sub.0, 3f.sub.0, 4f.sub.0 . . . ) of a modulation frequency f.sub.0 
appear, as set out below, which include a Sagnac phase difference 
.DELTA..phi. as a factor: 
##EQU2## 
noting that .DELTA..phi. is proportional to the detection rate and that Jn 
and .phi.m represent an n-order Bessel function and phase modulation 
amplitude, respectively. 
If, out of the output components, a ratio is taken for the 1st and 2nd 
harmonic waves S.sub.1 and S.sub.2 of the modulation frequency which are 
detected simultaneously, then a relation 
##EQU3## 
is established. Thus the Sagnac phase difference .DELTA..phi. becomes 
##EQU4## 
From this it will be appreciated that the Sagnac phase difference 
.DELTA..phi. is detected from the 1st and 2nd components S.sub.1 and 
S.sub.2 of the modulation frequency contained in the output signal. 
In order to accurately detect the Sagnac phase difference .DELTA..phi. it 
is required that the factor J.sub.2 (.phi.m)/J.sub.1 (.phi.m) in Equation 
(3) be made constant irrespective of the phase modulation amplitude 
.phi.m. For the 4-th harmonic waves detected, therefore, the phase 
modulation amplitude should be so controlled as to make constant a ratio 
EQU S.sub.2 /S.sub.4 =J.sub.2 (.phi.m)/J.sub.4 (.phi.m) (4) 
This method employs the ratio S.sub.1 /S.sub.2 of the two modulation 
frequency components and thus a light amount variation resulting from an 
interference noise is eliminated. It is, therefore, possible to obtain a 
broader dynamic range through the calculation of tan.sup.-1. 
In this method, however, if as shown in FIG. 2 a better phase modulation 
amplitude .phi.m1 or .phi.m2 is selected for the 1st or 2nd Bessel 
function J.sub.1 or J.sub.2, then the variation of the factor J.sub.1 
(.phi.)/J.sub.2 (.phi.m) is increased with respect to the variation of the 
phase modulation amplitude .phi.m. In order to achieve a better 
sensitivity level, it will be necessary to obtain as great an output 
amplitude as possible. As shown in FIG. 2, therefore, if the phase 
modulation amplitude .phi.m1 or .phi.m2 is set at a level at which the 1st 
or 2nd Bessel function J.sub.1 or J.sub.2 is maximized, then the 
differential coefficient J.sub.1 ' or J.sub.2 ' of the 1st or 2nd Bessel 
function J.sub.1 or J.sub.2 becomes greater. The variation of the phase 
modulation amplitude causes a greater variation in the 1st or 2nd Bessel 
function J.sub.2 or J.sub.1 and hence a greater variation in J.sub.1 
(.phi.m)/J.sub.2 (.phi.m). 
For the phase modulation amplitude .phi.m1 or .phi.m2 the component of the 
4-th Bessel function J.sub.4 is at a smaller level and it is, therefore, 
difficult to make the phase modulation amplitude constant. As a result, 
J.sub.2 (.phi.m)/J.sub.1 (.phi.m) is not stabilized, thus lowering the 
detection amplitude level. 
SUMMARY OF THE INVENTION 
It is accordingly the object of this invention to provide a fiber optical 
rotation sensor which can stably detect a rotation rate with high 
accuracy. 
According to this invention, a fiber optical rotation sensor is provided 
which comprises: 
a light source for producing a light wave; 
an optical fiber forming a loop for sensing a rotation in accordance with 
the Sagnac effect; 
means for coupling the light source to the optical fiber loop to provide a 
pair of light waves which propagate around the loop in opposite 
directions, and combining the pair of light waves to form an optical 
output signal; 
means for generating a phase modulation signal having a frequency f.sub.0 
and modulating the phase of the light waves propagating in the fiber 
optical loop with the phase modulation signal; 
means for detecting the optical output signal and converting the output 
signal to an electric signal; 
means for extracting from the electric signal a frequency-component S(n-1) 
of (n-1) times the modulation frequency f.sub.0, frequency-component Sn of 
n-times the modulation frequency f.sub.0 and frequency component S(n+1) of 
(n+1) times the modulation frequency f.sub.0, where n represents an 
integer of 1 or more; and 
means for processing the extracted components Sn-1, Sn and Sn+1 
corresponding to (n-1)-, n- and (n+1)-times the modulation frequency 
f.sub.0, respectively, to evaluate 
##EQU5## 
proportional to a phase difference .DELTA..phi. between the light waves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 is a block diagram showing a fiber optical gyroscope according to 
one embodiment of this invention. In the fiber optical gyroscope shown in 
FIG. 3 a light source, such as laser unit 12, is optically coupled to 
optical fiber 101 so that a laser wave is emitted from laser unit 12 into 
optical fiber 101. One-directional optical coupler 131 is coupled to 
optical fiber 101 to permit the laser wave to be passed. 
As well known in the art, one-directional optical coupler 131 is comprised 
of, for example, two optical fibers with their cores joined with an 
adequately thin cladding therebetween to allow the incoming laser wave to 
be split by an evanescent wave coupling into two waves, one of which is 
propagated into optical fiber 102 and the other of which is propagated 
into optical fiber 103 with a nonreflective end. 
Optical fiber 102 is coupled through another one-directional optical 
coupler 132 to optical fiber loop 14. Optical coupler 132 allows the 
incoming laser wave to be split into the two laser waves, which are 
mutually oppositely propagated in optical fiber loop 14. Optical phase 
modulator 15 is inserted in optical fiber loop 14 and the two mutually 
counter-propagating laser waves upon a passage through phase modulator 15 
are phase modulated with a modulation wave with a phase modulation 
amplitude .phi.m and modulation frequency f.sub.0, as shown, for example, 
in FIG. 4, which has been oscillated by oscillator 19. Phase modulator 15, 
as seen from the aforementioned Publications, is comprised of a 
piezoelectric element on which an optical fiber is wound. When the 
piezoelectric element is oscillated by an oscillation signal of the phase 
modulation frequency f.sub.0 coming from oscillator 19, the optical fiber 
on the piezoelectric element minutely expands and contracts, resulting in 
a variation in its optical path. The laser wave is phase modulated through 
the utilization of this phenomenon. 
One-directional optical coupler 132 allows an interference to occur between 
the mutually counter-propagating waves. The interference laser beam is 
split into two laser beams, one of which is propagated into optical fiber 
104 with a nonreflective end and the other of which is transmitted through 
optical fiber 102 into one-directional optical coupler 131 where it is 
split into two waves for interference. One of the interference waves so 
split is guided through optical fiber 105 to photodetector 16 where it is 
detected. A detection signal from photodetector 16 as shown in FIG. 4 is 
supplied to synchronous detector 30 supplied with an oscillation signal 
from oscillator 19. Synchronous detector 30 detects the detection signal 
with the phase modulation frequency f.sub.0 and its harmonic waves and 
delivers a predetermined output signal to operation circuit 40 where it is 
processed by an operation method as set forth below and the Sagnac phase 
difference .DELTA..phi. is calculated. A feedback signal is supplied to 
oscillatcr 19 so as to maintain the phase modulation amplitude .phi.m 
constant. 
In the aforementioned embodiment, the amplitude (i.e., an extent of 
modulation) of the modulation signal with the phase modulation frequency 
f.sub.0 is so set as to substantially satisfy a Bessel function given 
below: 
EQU Jn-1(.phi.m)=Jn+1(.phi.m) 
The laser wave subjected by the modulation signal to the phase modulation 
is split by one-directional coupler 132 into two waves, one of which is 
propagated into optical fiber 102 and the other of which is propagated 
into optical fiber 104. In this connection it is to be noted that light 
waves from one-directional optical coupler 132 are mutually 
counter-propagated in optical fiber loop 14 and are converted into two 
interference light waves. 
One of the interference light waves from optical fiber 102 is coupled by 
optical coupler 131 to optical fiber 105, then detected by photodetector 
16 and delivered as an output electric signal to synchronous detector 30. 
Synchronous detector 30 synchronously detects the output of photodetector 
16 with a frequency of an integral multiple of the phase-modulated output 
wave from oscillator 19 to extract the corresponding integral-multiple 
signal components Sn-1, Sn, Sn+1. The signal components Sn-1, Sn, Sn+1 
(n&gt;1) are supplied to operation circuit 40 where the aforementioned Sagnac 
phase difference .DELTA..phi. given by 
##EQU6## 
is gained. 
##EQU7## 
At this time, the factor 
##EQU8## 
can be determined by controlling the amplitude .phi.m so that the 
components (Sn-1) and Sn+1 corresponding to (n-1)- and (n+1)-times the 
phase modulation wave are so set as to be maintained as Sn-1=Sn+1, or 
finding the phase modulation amplitude .phi.m from Sn-1 and S+1. 
The advantage of the Sagnac phase difference .DELTA..phi. lies in that a 
better detection accuracy is assured since, in the Bessel function 
Jn-1=Jn+1, the differential coefficient Jn'=0 (the property of the Bessel 
function), since with respect to the phase modulation amplitude .phi.m a 
variation of the factor 
##EQU9## 
is small through the use of Jn-1+Jn+1 due to the establishment of 
##EQU10## 
With J'n-1, J'n+1 and Jn-1+Jn+1 each representing a differential 
coefficient and since Jn-1, Jn, Jn+1 components are of relatively the same 
sensitivity level. 
An explanation will be given below about utilizing 2nd, 3rd and 4th 
harmonic waves of the phase modulation frequency as one practical variant. 
In this case, the phase modulation amplitude .phi.m is so set that as shown 
in FIG. 2 J.sub.2 (.phi.m)=J.sub.4 (.phi.m), that is, J.sub.3 (.phi.m) is 
maximal. This is achieved by detecting the 2nd frequency modulation 
component S.sub.2 =cos.DELTA..phi..multidot.J.sub.2 (.phi.m) and 4th 
frequency modulation component S.sub.4 =cos.DELTA..phi..multidot.J.sub.4 
(.phi.m) by synchronous detector 30, detecting, for example, a difference 
between S.sub.2 and S.sub.4 by operation circuit 40 and controlling the 
output amplitude (phase modulation amplitude .phi.m) of oscillator 19 so 
that the aforementioned difference becomes zero, that is, S.sub.2 
=S.sub.4. Here, unless .DELTA..phi.=90.degree. the modulation frequency 
components S.sub.2 and S.sub.4 can be detected due to the Sagnac phase 
difference. Furthermore, the Sagnac phase difference .DELTA..phi. can be 
found by detecting the 3rd modulation frequency S.sub. 
=sin.DELTA..phi..multidot.J.sub.3 (.phi.m) and evaluating Equation (5) 
##EQU11## 
by operation circuit 40. In this case, as will be seen from FIG. 2 the 
differential coefficient J'.sub.3 =0 (J.sub.3 is maximal) for the phase 
modulation amplitude .phi..sub.m3 and the differential coefficient 
(J.sub.2 +J.sub.4)' is relatively flat, as shown in FIG. 2, with respect 
to the variation of the phase modulation amplitude .phi..sub.m3, thus 
obtaining a stable fact 
##EQU12## 
Similarly, it is also possible to detect the aforementioned difference 
.DELTA..phi. through the utilization of the modulation frequency 
components S.sub.1, S.sub.2 and S.sub.3 and of the modulation frequency 
components S.sub.0, S.sub.1 and S.sub.2. In this connection it is to be 
noted that in the former case it is necessary to detect the modulation 
frequency components S.sub.1, S.sub.2 at .DELTA..phi.=0 (a very small 
rate) whereas in the latter case a circuit is required for detecting the 
modulation frequency components S.sub.0. 
In either case, the stable scale factor is obtained according to this 
invention in comparison with the conventional method for evaluating the 
Sagnac phase difference .DELTA..phi. from S.sub.1 /S.sub.2 through the use 
of the modulation frequency components S.sub.1, S.sub.2. It is, therefore, 
possible to provide a gyroscope which can stably detect a rotation rate 
with high accuracy. 
As set out above, the optical fiber gyroscope of this invention assures a 
high-performance gyroscope with a stable scale factor.