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Fiber-optic strain sensor using a dual Mach-Zehnder interferometric configuration
Volume 8 l, number 5
OPTICS COMMUNICATIONS
1 March 1991
Fiber-optic strain sensor using a dual Mach-Zehnder interferometric configuration Satoshi Tanaka and Yoshihiro Ohtsuka Department of Engineering Science, Faculty of Engineering, Hokkaido University,Sapporo, 060 Japan Received 16 October 1990
A proposal is made of a novel fiber-optic interferometric sensor using a pair of sensing and reference birefringent single-mode fibers contacted in the length direction. A laser beam with orthogonal linearly polarized two-frequencycomponent waves is launched into each fiber in accordance with its birefringent axes. The two-frequency component waves are alternatively guided in the birefringent axes of both fibers. This alternative arrangement results in high-sensitive performance of a strain sensor. The contacting arrangement of the two fibers eliminates the output fluctuations resulting from environmental temperature disturbances.
1. Introduction In recent years, a wide variety o f fiber-optic interferometric sensors have been devised [ 1-5 ]. F r o m a practical viewpoint, however, such sensors are likely to suffer from e n v i r o n m e n t a l perturbations, since almost every site along a length o f fiber is exceedingly susceptible to the e n v i r o n m e n t a l noise factors such as t e m p e r a t u r e disturbances. F o r practical use it is essential to devise a fiber-optic interferometric configuration i m m u n e from such noise sources. F o r example, the reference arm o f fiber in a MachZ e h n d e r interferometer must be shielded from the e n v i r o n m e n t a l perturbations, or its overall system must be e q u i p p e d with a device capable o f compensating such perturbations. Alternatively, a single-fiber interferometric sensor using a length o f highly birefringent single-mode fiber would be much m o r e promising, in that orthogonal linearly polarized HEll guided modes are m a d e to interfere with each other. Since this type o f sensor needs no reference a r m o f fiber, it does not suffer from e n v i r o n m e n t a l perturbations. However, it is not as sensitive as a type o f M a c h - Z e h n d e r interferometric sensor. In a previous p a p e r [6 ] was described a doubly coiled sensor consisting o f p a i r e d highly birefringent single-mode fibers, each o f which works as a singlefiber interferometric sensor. The experiment was d e m o n s t r a t e d to show its excellent performance in
the sensing o f mechanical deformation, undergoing no e n v i r o n m e n t a l t e m p e r a t u r e disturbances. In this paper, an alternative a p p r o a c h is d e m o n strated to achieve a stable fiber-optic interferometric sensor that is much m o r e susceptible to m e a s u r a n d than a single-fiber interferometric sensor as well as being i m m u n e from e n v i r o n m e n t a l t e m p e r a t u r e disturbances. This sensor consists o f the sensing and reference fibers that are glued to each other in their length direction, but their sensing sites are loosely in contact with each other. This configuration is a i m e d at eliminating the t e m p e r a t u r e disturbances comm o n to both fibers in the interference output. In the sensor configuration, the p a i r e d eigenmodes propagated in the slow axes o f the sensing a n d reference fibers interfered with each other after a passage through both fibers, a n d also the other p a i r e d eigenmodes propagated in the axes o f the two fibers are interfered with each other. It is d e m o n s t r a t e d that this type o f sensor is conveniently fabricated as a strain sensor.
2. Illustration for interferometric configuration The sensor configuration is schematically shown in fig. 1. A b e a m o f light from a H e N e laser source is m o d u l a t e d in frequency by use o f a polarization frequency shifter ( P F S ) in such a way that it is com-
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
267
Volume 81, n u m b e r 5
OPTICS C O M M U N I C A T I O N S
Sensing fiber
\
BS 1 E - \
fiiii s,o ax,s,x, fastaxis (y)iiiiii F.s,o.
is,x,
M3
...... S3
[ .... fast axis (y) ......] ~o1
Reference fiber
r~
f
demodulator Fig. 1. Schematic diagram of a fiber-optic dual Mach-Zehnder in/erferometric configuration.
modulated to~-component wave guided along the slow axis of the sensing fiber is optically heterodyned at the photodetector D, with the to2-component wave guided along the same slow axis of the reference fiber. Alternatively, the other paired to, and o.)2 component waves, guided along the fast axes of both the fibers, are also heterodyned at 0 2. Two beat-photocurrents generated at D, and 0 2 a r e fed into a phase demodulator from which the phase difference between the two beat-photocurrents can be obtained. It should be recalled that the sensing and reference fibers must be glued to each other except for a localized sensing site in the actual sensor arrangement to which environmental perturbations are commonly given. The phase-difference measurement allows us to almost eliminate the unacceptable phase noise resulting from the perturbations. This interferometric configuration is available for a fiber-optic strain sensor [ 7-10 ]. Let L be a common length of the sensing and reference fibers, and AL be its change due to the stretching of the sensing fiber. The m, and o.)2 component waves emerging from the reference and sensing fibers, propagated along both slow x-axes, can be expressed in the form
Esx =Asx exp{ - i [to, t - ( P x L + (OOx/OL)~XL+Osx)
posed of orthogonal linearly polarized two-frequency component waves to, and 0.)2. The orthogonal birefringent x and y axes, referred to as the slow and fast axes, are schematically denoted by two dotted lines in the sensing and reference fibers. The beam from the PFS, partially reflected at the BS,, is coupled into the sensing fiber in such a way that its orthogonal m, and 0 ) 2 component waves are, respectively, in agreement in polarization direction with the x and y axes. The partially transmitted beam at the BS, enters the reference fiber in a similar way after reflection at the mirror M,, but the to, and toE component waves must be alternatively aligned in the directions of the y and x axes, respectively. This alternative coupling of the to, and (1)2 component waves to the sensing and reference fibers is essential to achieve a highly sensitive performance, as will be described later on. The to, and toE component waves propagated in the sensing fiber are modulated in phase by the measurand. As can be seen from the figure, the phase268
1 March i 991
]},
ER~=ARxeXp{--i[to2t--(flxL+OR~)]} ,
(la) (lb)
where suffix notations S and R denote sensing and reference, As~ and ARy are the amplitudes, fix is the propagation constant, OO~/OL expresses the phase sensitivity to unit extension for the to~-component wave, and 0s~ and 0R~ are almost common phase factors induced by the environmental perturbations. Also, we have for the other paired m2 and to, component waves
Esy =
A sy
exp{ - i [ to2 t
- (flyL+ (OOy/OL)AL+Osy) ]}, ERy=Aayexp{-i[to, t-(fleL+Oay)]}.
(2a) (2b)
In these equations the phases Osyand ORyare also almost the same factors. Hence, it is permissible to put
Osx~ORx, Osy~,ORy.
(3)
With this condition the signal and local oscillator waves, given by eqs. ( l a ) and ( l b ) , are photomixed
Volume 81, n u m b e r 5
OPTICS COMMUNICATIONS
at DI to generate a beat-photocurrent of intermediate frequency m ( . o = O ) I --O.) 2 (O.) 1 > O ~ 2 ) ,
jx OCcos(Atot- ( O(~x/OL )AL ) ,
(4)
whereas we obtain from D2 with help of eqs. (2a) and (2b) for the y-component jyoc cos[Amt+ (0Oy/0L) AL] .
(5)
In these expressions, the strain-induced phase terms, (OOx/OL)AL and ( 0 G / 0 L ) A L , have opposite signs, so that the phase difference measurement reduces to an additional form of them,
~( AL ) = ( OOx/ OL ) AL + (0¢y/0L)AL.
(6)
This additional resultant originates from the alternative coupling of the col and 092 component waves to the sensing and reference fibers. The propagation constants fix and fly are linearly related to the refractive indices nx and ny in the directions of the slow and fast axes, which change due to photoelastic effect by the axial strain given to the fiber. Let APx and Afly be their changes due to strain-induced refractive indices Anx and An r, and the strain-induced phase factors can be explicitly written as
OOx ( O(flxL ) Aflx o-Z- a c = \
O(,SxL )'~ AL +
=LA#x+KaL, (0(p L) ape
0(pyL)
=LAfl, +flyAL.
(7)
Use o f e q . (7) rewrites eq. (6) as
= (#x +
AL+ (a/ x +
(8)
For comparison, the strain-induced retardation for a single-fiber interferometric sensor [ 1 1 ] is given as follows:
~'(AL)=(fl~-flr)AL+(Aflx-Afly)L.
(9)
Comparing eq. (8) with eq. (9), we can see that the fiber-optic strain sensor of the present scheme, given by fig. l, is much more sensitive to AL than the single-fiber interferometric sensor.
1 March 1991
3. Experiment and discussion The experimental setup is shown in fig. 2. A light beam from a HeNe laser at a wavelength of 632.8 nm is modulated to have orthogonal linearly polarized two-frequency component waves by the PFS. The orthogonal component waves are shifted in frequency by 80.0 and 79.9 MHz, respectively. The beam from the PFS is divided at the BS, each of which is launched into a length of birefringent single-mode fiber by a couple of a half wave plate (2/2) and an objective lens (L). The orthogonal linearly polarized two-frequency component waves were carefully aligned to be in agreement with the directions of birefringent axes of the fiber. In the sensing and reference fibers the two-frequency component waves were alternatively guided, as mentioned in a previous section. The two beams from the sensing and reference fibers are aligned by the respective lenses (L) and half-wave plates (2/2) together with a polarization beam splitter (PBS), so that the paired component waves emerging from the sensing and reference fibers can be correctly heterodyned at each of the D~ and D2. The two beat-photocurrents of intermediate frequency 100 kHz are fed into a phase demodulator which can generate their phase difference. As shown in fig. 3, the strain-induced phase-change takes place in a short length of the sensing fiber, fixed with epoxy resin by 1.5 cm long to a cylindrical PZT. This PZT operated in a radially oscillating mode to give an axial strain to the sensing fiber. Hitachi elliptically-jacketed birefringent singlemode fiber, specified by a beat length of 2.6 mm, was used for the experiment. The plastic jacket protecting the fiber was stripped over the sensing site of both fibers, which were glued to each other with silicon resin with good thermal conductivity. This arrangement enabled both the fibers of 30 cm length to have almost the same response to the environmental temperature disturbances. Typical measured results are given in fig. 4. The upper shows a saw-tooth mode of the electrical signal of 17 Hz that drives the PZT, and the lower the output signal of the sensor. This output shows an excellent linear response to the sawtooth driving signal. For comparison, the strain-induced phase retardation for a single-fiber interferometric sensor was 269
Volume 8 l, number 5
He-Ne Laser
OPTICS COMMUNICATIONS
Reference fiber Sensing fiber i
L
BS~ ~
l March i 991
L A Jq ~
\M 2
PFS MI"
{
U
t)
"--\
',
~
,
-!z
Phase output Fig. 2. Experimentalarrangement for a strain sensor.
Epoxy resin ~ : e n s i n g ~_
fiber
~Reference
fiber
PZT Fig. 3. Cylindrical PZT for inducing axial strain in the sensing fiber.
i:o, 0
,J ,
O.l
0.2
Is]
t
,
0.3
0.4
860
s5o E°3 ;~'
-960
0
0. I
I
0.2
1 0.8
0.4
t [s] Fig. 4. Measured results: (a) shows the sawtooth electrical signal for driving the PZT, and (b) the output signal from the phase demodulator.
270
also measured by the experimental setup shown in fig. 5. Taking out of the output signal differs in configuration from that of fig. 2; each fiber serves as a single-fiber interferometric sensor. The signal and reference beat-photocurrents resulting from the D, and D2 are fed into the phase demodulator from which the strain-induced phase retardation, included in the signal beat-photocurrent, can be obtained. The sensitivity is given by the gradient of the straight plots. The sensitivities for A and B are, respectively, read to be A:
A ~ / A L = 1.08 X 103 [deg/~tm]
B:
A ~ ' / A L = 1.43 X 10 [deg/ktm].
The sensitivity for A is 76 times as high as for B (see fig. 6). To ensure stable performance, the fabricated sensor was exposed to temperaiiare disturb~ces made with a heated nichrome wire. As shown in fig. 7, the disturbance-induced phase change is almost canceled for A, but clearly appears for the only sensing fiber, indicated by B. Note that the temperature disturbance takes place from a time a and is removed at a time b. In this sensor arrangement given by fig. 2, the linearity of the output as well as the ultimate sensitivity of the sensor depend seriously on the polarizationdependent optical components and cross talk taking place in the both fibers. This fact would need further improvement of the sensor system.
Volume 8 l, number 5
OPTICS COMMUNICATIONS
He-Ne Laser
.I
-ff
L
Reference fiber i : Sensing fiber
1 March i 991
L
A
k
A
PFS
2
Phase demodulator
)
Phase output
Fig. 5. Experimental arrangement for the paired single-fiberinterferometnc sensor.
A/
400
L
300
a b
T
o z < c.) co <
200
A a
A 1 0 [s]
100 t Is] B
0
i
I
o. 1
0.2
i
¢
•
N
o.a
:
:
:-
0.
,~L Ix i C % m ] Fig. 6. Measured results as a function AL. The straight line A shows the result obtained with the dual Mach-Zehnder interferometric systemand B the result with the paired single-fiberinterferometric system.
4. Concluding remarks This paper has proposed a novel fiber-optic intcrferomctric sensor using sensing a n d reference highly birefringent single-mode fibers. It incorporates a dual Mach-Zehnder interferometric scheme. Since both fibers are in touch with each other in the length direction, the e n v i r o n m e n t a l perturbations are almost c o m m o n to them a n d can be eliminated in the measurement o f phase difference. A laser b e a m with orthogonal linearly polarized two-frequency component waves is used for optical heterodyne detection
Fig. 7. Effects of temperature induced-disturbances.The cancellation of the temperature-induced phase change is demonstrated in A. The traced-curve B is due to only the sensing fiber. processes. These two-frequency component waves are alternatively coupled into the directions of the birefringent axes o f the sensing a n d reference fibers. The configuration of the dual Mach-Zehnder interferometric scheme enables us to achieve a highly sensitive operation o f the sensor. It is demonstrated that the fabricated sensor is 76 times as sensitive as a conventional single-fiber interferometric sensor.
References [ 1] T.G. Giallorenzi, J.A. Bucaro, A. Dandridge, G.H. Sbigel, Jr., J.H. Cole, S.H. Rashleigh and R.G. Priest, Quantum Electron. QE-I 8 (1982) 626. [2] B. Culshaw, J. Phys. E 16 (1983) 978. [3] B.E. Jones, J. Phys. E 18 (1985) 770. 271
Volume 81, number 5
OPTICS COMMUNICATIONS
[4] D.A. Jackson and J.D.C. Jones, Optical Laser Technol. 18 (1986) 243 (Part 1 ), 299 (Part 2). [ 5 ] D.A. Jackson and J.D.C. Jones, Optica Acta 33 (1986) 1469. [ 6 ] Y. Ohtsuka, M. Kamiashi and Y. Imai, Int. J. Optelectron. 3 (1988) 371. [ 7 ] C.D. Butter and G.B. Hocker, Appl. Optics 17 ( 1978 ) 2867. [8] M.P. Varnham, A.J. Barlow, D.N. Pane and K.Okamoto, Electron. Lett. 19 ( 1983 ) 669.
272
1 March 1991
[9] P.A. Leilabady, J.D.C. Jones and D.A. Jackson, Optics Lett. 20 (1984) 67. [ 10] T.M. Taylor, D.J. Webb and J.D.C. Jones, Optics Lett. 12 ( 1987 ) 744. [ 11 ] Y. Ohtsuka, T. Ando, Y. Imai and M. Imai, J. Lightwave Technol. LT-5 ( 1987 ) 602.
OPTICS COMMUNICATIONS
1 March 1991
Fiber-optic strain sensor using a dual Mach-Zehnder interferometric configuration Satoshi Tanaka and Yoshihiro Ohtsuka Department of Engineering Science, Faculty of Engineering, Hokkaido University,Sapporo, 060 Japan Received 16 October 1990
A proposal is made of a novel fiber-optic interferometric sensor using a pair of sensing and reference birefringent single-mode fibers contacted in the length direction. A laser beam with orthogonal linearly polarized two-frequencycomponent waves is launched into each fiber in accordance with its birefringent axes. The two-frequency component waves are alternatively guided in the birefringent axes of both fibers. This alternative arrangement results in high-sensitive performance of a strain sensor. The contacting arrangement of the two fibers eliminates the output fluctuations resulting from environmental temperature disturbances.
1. Introduction In recent years, a wide variety o f fiber-optic interferometric sensors have been devised [ 1-5 ]. F r o m a practical viewpoint, however, such sensors are likely to suffer from e n v i r o n m e n t a l perturbations, since almost every site along a length o f fiber is exceedingly susceptible to the e n v i r o n m e n t a l noise factors such as t e m p e r a t u r e disturbances. F o r practical use it is essential to devise a fiber-optic interferometric configuration i m m u n e from such noise sources. F o r example, the reference arm o f fiber in a MachZ e h n d e r interferometer must be shielded from the e n v i r o n m e n t a l perturbations, or its overall system must be e q u i p p e d with a device capable o f compensating such perturbations. Alternatively, a single-fiber interferometric sensor using a length o f highly birefringent single-mode fiber would be much m o r e promising, in that orthogonal linearly polarized HEll guided modes are m a d e to interfere with each other. Since this type o f sensor needs no reference a r m o f fiber, it does not suffer from e n v i r o n m e n t a l perturbations. However, it is not as sensitive as a type o f M a c h - Z e h n d e r interferometric sensor. In a previous p a p e r [6 ] was described a doubly coiled sensor consisting o f p a i r e d highly birefringent single-mode fibers, each o f which works as a singlefiber interferometric sensor. The experiment was d e m o n s t r a t e d to show its excellent performance in
the sensing o f mechanical deformation, undergoing no e n v i r o n m e n t a l t e m p e r a t u r e disturbances. In this paper, an alternative a p p r o a c h is d e m o n strated to achieve a stable fiber-optic interferometric sensor that is much m o r e susceptible to m e a s u r a n d than a single-fiber interferometric sensor as well as being i m m u n e from e n v i r o n m e n t a l t e m p e r a t u r e disturbances. This sensor consists o f the sensing and reference fibers that are glued to each other in their length direction, but their sensing sites are loosely in contact with each other. This configuration is a i m e d at eliminating the t e m p e r a t u r e disturbances comm o n to both fibers in the interference output. In the sensor configuration, the p a i r e d eigenmodes propagated in the slow axes o f the sensing a n d reference fibers interfered with each other after a passage through both fibers, a n d also the other p a i r e d eigenmodes propagated in the axes o f the two fibers are interfered with each other. It is d e m o n s t r a t e d that this type o f sensor is conveniently fabricated as a strain sensor.
2. Illustration for interferometric configuration The sensor configuration is schematically shown in fig. 1. A b e a m o f light from a H e N e laser source is m o d u l a t e d in frequency by use o f a polarization frequency shifter ( P F S ) in such a way that it is com-
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
267
Volume 81, n u m b e r 5
OPTICS C O M M U N I C A T I O N S
Sensing fiber
\
BS 1 E - \
fiiii s,o ax,s,x, fastaxis (y)iiiiii F.s,o.
is,x,
M3
...... S3
[ .... fast axis (y) ......] ~o1
Reference fiber
r~
f
demodulator Fig. 1. Schematic diagram of a fiber-optic dual Mach-Zehnder in/erferometric configuration.
modulated to~-component wave guided along the slow axis of the sensing fiber is optically heterodyned at the photodetector D, with the to2-component wave guided along the same slow axis of the reference fiber. Alternatively, the other paired to, and o.)2 component waves, guided along the fast axes of both the fibers, are also heterodyned at 0 2. Two beat-photocurrents generated at D, and 0 2 a r e fed into a phase demodulator from which the phase difference between the two beat-photocurrents can be obtained. It should be recalled that the sensing and reference fibers must be glued to each other except for a localized sensing site in the actual sensor arrangement to which environmental perturbations are commonly given. The phase-difference measurement allows us to almost eliminate the unacceptable phase noise resulting from the perturbations. This interferometric configuration is available for a fiber-optic strain sensor [ 7-10 ]. Let L be a common length of the sensing and reference fibers, and AL be its change due to the stretching of the sensing fiber. The m, and o.)2 component waves emerging from the reference and sensing fibers, propagated along both slow x-axes, can be expressed in the form
Esx =Asx exp{ - i [to, t - ( P x L + (OOx/OL)~XL+Osx)
posed of orthogonal linearly polarized two-frequency component waves to, and 0.)2. The orthogonal birefringent x and y axes, referred to as the slow and fast axes, are schematically denoted by two dotted lines in the sensing and reference fibers. The beam from the PFS, partially reflected at the BS,, is coupled into the sensing fiber in such a way that its orthogonal m, and 0 ) 2 component waves are, respectively, in agreement in polarization direction with the x and y axes. The partially transmitted beam at the BS, enters the reference fiber in a similar way after reflection at the mirror M,, but the to, and toE component waves must be alternatively aligned in the directions of the y and x axes, respectively. This alternative coupling of the to, and (1)2 component waves to the sensing and reference fibers is essential to achieve a highly sensitive performance, as will be described later on. The to, and toE component waves propagated in the sensing fiber are modulated in phase by the measurand. As can be seen from the figure, the phase268
1 March i 991
]},
ER~=ARxeXp{--i[to2t--(flxL+OR~)]} ,
(la) (lb)
where suffix notations S and R denote sensing and reference, As~ and ARy are the amplitudes, fix is the propagation constant, OO~/OL expresses the phase sensitivity to unit extension for the to~-component wave, and 0s~ and 0R~ are almost common phase factors induced by the environmental perturbations. Also, we have for the other paired m2 and to, component waves
Esy =
A sy
exp{ - i [ to2 t
- (flyL+ (OOy/OL)AL+Osy) ]}, ERy=Aayexp{-i[to, t-(fleL+Oay)]}.
(2a) (2b)
In these equations the phases Osyand ORyare also almost the same factors. Hence, it is permissible to put
Osx~ORx, Osy~,ORy.
(3)
With this condition the signal and local oscillator waves, given by eqs. ( l a ) and ( l b ) , are photomixed
Volume 81, n u m b e r 5
OPTICS COMMUNICATIONS
at DI to generate a beat-photocurrent of intermediate frequency m ( . o = O ) I --O.) 2 (O.) 1 > O ~ 2 ) ,
jx OCcos(Atot- ( O(~x/OL )AL ) ,
(4)
whereas we obtain from D2 with help of eqs. (2a) and (2b) for the y-component jyoc cos[Amt+ (0Oy/0L) AL] .
(5)
In these expressions, the strain-induced phase terms, (OOx/OL)AL and ( 0 G / 0 L ) A L , have opposite signs, so that the phase difference measurement reduces to an additional form of them,
~( AL ) = ( OOx/ OL ) AL + (0¢y/0L)AL.
(6)
This additional resultant originates from the alternative coupling of the col and 092 component waves to the sensing and reference fibers. The propagation constants fix and fly are linearly related to the refractive indices nx and ny in the directions of the slow and fast axes, which change due to photoelastic effect by the axial strain given to the fiber. Let APx and Afly be their changes due to strain-induced refractive indices Anx and An r, and the strain-induced phase factors can be explicitly written as
OOx ( O(flxL ) Aflx o-Z- a c = \
O(,SxL )'~ AL +
=LA#x+KaL, (0(p L) ape
0(pyL)
=LAfl, +flyAL.
(7)
Use o f e q . (7) rewrites eq. (6) as
= (#x +
AL+ (a/ x +
(8)
For comparison, the strain-induced retardation for a single-fiber interferometric sensor [ 1 1 ] is given as follows:
~'(AL)=(fl~-flr)AL+(Aflx-Afly)L.
(9)
Comparing eq. (8) with eq. (9), we can see that the fiber-optic strain sensor of the present scheme, given by fig. l, is much more sensitive to AL than the single-fiber interferometric sensor.
1 March 1991
3. Experiment and discussion The experimental setup is shown in fig. 2. A light beam from a HeNe laser at a wavelength of 632.8 nm is modulated to have orthogonal linearly polarized two-frequency component waves by the PFS. The orthogonal component waves are shifted in frequency by 80.0 and 79.9 MHz, respectively. The beam from the PFS is divided at the BS, each of which is launched into a length of birefringent single-mode fiber by a couple of a half wave plate (2/2) and an objective lens (L). The orthogonal linearly polarized two-frequency component waves were carefully aligned to be in agreement with the directions of birefringent axes of the fiber. In the sensing and reference fibers the two-frequency component waves were alternatively guided, as mentioned in a previous section. The two beams from the sensing and reference fibers are aligned by the respective lenses (L) and half-wave plates (2/2) together with a polarization beam splitter (PBS), so that the paired component waves emerging from the sensing and reference fibers can be correctly heterodyned at each of the D~ and D2. The two beat-photocurrents of intermediate frequency 100 kHz are fed into a phase demodulator which can generate their phase difference. As shown in fig. 3, the strain-induced phase-change takes place in a short length of the sensing fiber, fixed with epoxy resin by 1.5 cm long to a cylindrical PZT. This PZT operated in a radially oscillating mode to give an axial strain to the sensing fiber. Hitachi elliptically-jacketed birefringent singlemode fiber, specified by a beat length of 2.6 mm, was used for the experiment. The plastic jacket protecting the fiber was stripped over the sensing site of both fibers, which were glued to each other with silicon resin with good thermal conductivity. This arrangement enabled both the fibers of 30 cm length to have almost the same response to the environmental temperature disturbances. Typical measured results are given in fig. 4. The upper shows a saw-tooth mode of the electrical signal of 17 Hz that drives the PZT, and the lower the output signal of the sensor. This output shows an excellent linear response to the sawtooth driving signal. For comparison, the strain-induced phase retardation for a single-fiber interferometric sensor was 269
Volume 8 l, number 5
He-Ne Laser
OPTICS COMMUNICATIONS
Reference fiber Sensing fiber i
L
BS~ ~
l March i 991
L A Jq ~
\M 2
PFS MI"
{
U
t)
"--\
',
~
,
-!z
Phase output Fig. 2. Experimentalarrangement for a strain sensor.
Epoxy resin ~ : e n s i n g ~_
fiber
~Reference
fiber
PZT Fig. 3. Cylindrical PZT for inducing axial strain in the sensing fiber.
i:o, 0
,J ,
O.l
0.2
Is]
t
,
0.3
0.4
860
s5o E°3 ;~'
-960
0
0. I
I
0.2
1 0.8
0.4
t [s] Fig. 4. Measured results: (a) shows the sawtooth electrical signal for driving the PZT, and (b) the output signal from the phase demodulator.
270
also measured by the experimental setup shown in fig. 5. Taking out of the output signal differs in configuration from that of fig. 2; each fiber serves as a single-fiber interferometric sensor. The signal and reference beat-photocurrents resulting from the D, and D2 are fed into the phase demodulator from which the strain-induced phase retardation, included in the signal beat-photocurrent, can be obtained. The sensitivity is given by the gradient of the straight plots. The sensitivities for A and B are, respectively, read to be A:
A ~ / A L = 1.08 X 103 [deg/~tm]
B:
A ~ ' / A L = 1.43 X 10 [deg/ktm].
The sensitivity for A is 76 times as high as for B (see fig. 6). To ensure stable performance, the fabricated sensor was exposed to temperaiiare disturb~ces made with a heated nichrome wire. As shown in fig. 7, the disturbance-induced phase change is almost canceled for A, but clearly appears for the only sensing fiber, indicated by B. Note that the temperature disturbance takes place from a time a and is removed at a time b. In this sensor arrangement given by fig. 2, the linearity of the output as well as the ultimate sensitivity of the sensor depend seriously on the polarizationdependent optical components and cross talk taking place in the both fibers. This fact would need further improvement of the sensor system.
Volume 8 l, number 5
OPTICS COMMUNICATIONS
He-Ne Laser
.I
-ff
L
Reference fiber i : Sensing fiber
1 March i 991
L
A
k
A
PFS
2
Phase demodulator
)
Phase output
Fig. 5. Experimental arrangement for the paired single-fiberinterferometnc sensor.
A/
400
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300
a b
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o z < c.) co <
200
A a
A 1 0 [s]
100 t Is] B
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i
I
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0.2
i
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o.a
:
:
:-
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,~L Ix i C % m ] Fig. 6. Measured results as a function AL. The straight line A shows the result obtained with the dual Mach-Zehnder interferometric systemand B the result with the paired single-fiberinterferometric system.
4. Concluding remarks This paper has proposed a novel fiber-optic intcrferomctric sensor using sensing a n d reference highly birefringent single-mode fibers. It incorporates a dual Mach-Zehnder interferometric scheme. Since both fibers are in touch with each other in the length direction, the e n v i r o n m e n t a l perturbations are almost c o m m o n to them a n d can be eliminated in the measurement o f phase difference. A laser b e a m with orthogonal linearly polarized two-frequency component waves is used for optical heterodyne detection
Fig. 7. Effects of temperature induced-disturbances.The cancellation of the temperature-induced phase change is demonstrated in A. The traced-curve B is due to only the sensing fiber. processes. These two-frequency component waves are alternatively coupled into the directions of the birefringent axes o f the sensing a n d reference fibers. The configuration of the dual Mach-Zehnder interferometric scheme enables us to achieve a highly sensitive operation o f the sensor. It is demonstrated that the fabricated sensor is 76 times as sensitive as a conventional single-fiber interferometric sensor.
References [ 1] T.G. Giallorenzi, J.A. Bucaro, A. Dandridge, G.H. Sbigel, Jr., J.H. Cole, S.H. Rashleigh and R.G. Priest, Quantum Electron. QE-I 8 (1982) 626. [2] B. Culshaw, J. Phys. E 16 (1983) 978. [3] B.E. Jones, J. Phys. E 18 (1985) 770. 271
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OPTICS COMMUNICATIONS
[4] D.A. Jackson and J.D.C. Jones, Optical Laser Technol. 18 (1986) 243 (Part 1 ), 299 (Part 2). [ 5 ] D.A. Jackson and J.D.C. Jones, Optica Acta 33 (1986) 1469. [ 6 ] Y. Ohtsuka, M. Kamiashi and Y. Imai, Int. J. Optelectron. 3 (1988) 371. [ 7 ] C.D. Butter and G.B. Hocker, Appl. Optics 17 ( 1978 ) 2867. [8] M.P. Varnham, A.J. Barlow, D.N. Pane and K.Okamoto, Electron. Lett. 19 ( 1983 ) 669.
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[9] P.A. Leilabady, J.D.C. Jones and D.A. Jackson, Optics Lett. 20 (1984) 67. [ 10] T.M. Taylor, D.J. Webb and J.D.C. Jones, Optics Lett. 12 ( 1987 ) 744. [ 11 ] Y. Ohtsuka, T. Ando, Y. Imai and M. Imai, J. Lightwave Technol. LT-5 ( 1987 ) 602.