Realization and tests of an open loop multimode fiber optic gyroscope H. Medjadbaa, S. Leclerb, L. M. Simohameda, A. Chakarib, P. Meyrueisb a Laboratory of Electronics and Optoelectronics Systems, BP 17, 16111 Bordj El-Bahri, Algeria b Photonics System Laboratory, ENSPS/UDS, BP 10413, 67412 Illkirch, Strasbourg, France ABSTRACT In this paper we report the realization of a new version of a multimode fiber optic gyroscope (MFOG). The aim our work is to demonstrate that we can improve the performances of such device by an appropriate choice of its components, in the frame of the compromise low cost/ high performances realization. Our MFOG set up uses 1031 m of step index 50/125 µm multimode fiber in the sensing coil, the first coupler is a standard fused coupler whereas the second one is a micro-optic coupler. The preliminary results show that the observed sensitivity of the prototype is ~0.05 °/sec and the measured dynamic range is ±34°/sec. Keywords: Multimode fiber, fiber optic gyroscope, fiber sensor, Sagnac effect, interference.
1. INTRODUCTION Fiber-optic gyroscopes (FOGs) continue to represent an attractive solid state alternative to the traditional mechanical one because of their high sensitivity [1]. However, the benefits of FOGs do not come cheaply. Recently, to meet the needed precision measurement the majority of companies in the world manufacture fiber gyroscopes using single-mode fiber components [2,3]. Nevertheless, the single mode approach is not the only one and the use of multimode components can give rise to very interesting solutions especially in the medium class performances. In addition, multimode solutions are cheaper than the single mode ones and require less optical components. Indeed, components such as isolators, polarizer and polarization controllers are avoided in the multimode approach and the large numerical aperture of the multimode components allows a better power coupling and solve the problems of misalignment. In another hand, researchers investigate the use of photonic crystal fibers to built gyroscopes but such fibers are very expensive and are inadequate for medium range applications [4]. Regarding the operation of an MFOG, the multimode interferences allow such devices to be relatively insensitive to environmental perturbations (temperature variations, vibrations...). Indeed, the MFOG operates as several independent mini-gyroscopes which allow the device random bias errors to be averaged over the modes. Thus, such devices should find useful applications in short time stabilization and in activities carried out in adverse environmental conditions. Until now, several works have been achieved in this domain but none has resulted in a usable device [5-7]. Thus, it still remains an area for investigations and researches. We have recently proposed a modelling of the operation of an MFOG [8] and find that good performances can be obtained using a step index 50/125 µm multimode fiber associated to a low cost LED as a light source and a photodetector with a large active area [9]. To avoid losses, the other components must be made of the same type of fiber. At this step of the study, our aim is to realize an optimized open loop fiber optic gyroscope using low cost multimode optical components. The device is dedicated to medium sensitivity applications in delicate conditions of temperature.
2. PRINCIPLE OF MFOG All FOGs are based on the Sagnac effect, first demonstrated by George Sagnac in 1913. The Sagnac effect consists in a phase shift between two contrapropagatives waves in a ring interferometer under rotation. In a ring interferometer with multimode fiber the Sagnac phase shift is the same for all modes. It can be expressed as follows:
φs =
4πRL Ω λc
*
[email protected]; phone +33 (0)3 902 44 601; fax +33 (0)3 90 24 46 19; lsp.u-strasbg.fr 20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75034E © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.834832 Proc. of SPIE Vol. 7503 75034E-1
(1)
here L is the length of the fiber, R is the radius of the coil, Ω is the rotation rate of the interferometer, c and λ are the light celerity and the wavelength in free space. In the case of MFOG constructed by a multimode fiber with a total mode groups number noted M .The output light intensity of the first fringe when using low coherent light source and large active area photodetector can be approximated by: M
M
M
M
M
M
+ − + − I ≈ ∑∑ I mn + ∑∑ I mn + 2∑∑ I mn I mn cos(φs + φmn+ − φmn− ) m =1 n =1
m =1 n =1
(2)
m =1 n =1
+ − where I mn and I mn describe the light intensity resulting from interference of light which is coupled from the nth mode in + − and φ mn are the accumulated phase shift between these modes along the fiber. the mth mode in both directions and φmn The intensity in the equation (2) can be written as follow:
I = I dc [1 + C ( M ) cos(φs + φe ( M ) )]
(3)
where Idc is a DC optical light component, C(M) is the fringe contrast and φe (M ) is the total additional phase error. In our previous work, we have shown that generally in multimode interference the fringe contrast becomes lower as the total mode groups number increase. A low contrast constitutes a limitation for the sensitivity of the device. However, the total phase error is averaged to zero ( φe (M ) ≈ 0) when total mode groups number increase which gives an independent time varying bias of the external perturbations. Therefore, it is necessary to find a compromise between sensitivity and bias stability by an appropriate choice of the total number of mode groups M and by consequent the choice of the multimode fiber. We have found that the optimum mode groups number is ten (10) [9]. To enhance the sensitivity and to force the system to operate in the linear region a dynamic phase modulation is necessary. In this case, the observed intensity becomes:
I = I dc [1 + C ( M ) cos(φm cos(2πf mt ) + φs + φe ( M ) )]
(4)
where φ m is the effective phase modulation index and fm is the frequency of the modulation signal. The measurement of the rotation rate is carried out using a lock-in demodulation by extracting the first harmonic from equation (3). The amplitude of this harmonic is given by:
A(φ s ) = 2 I dc C ( M ) J 1 (φm ) sin (φ s + φe ( M ) )
(5)
We notice that the output measured signal is sinusoidally depends on the input rotation rate which limits the linear range of the sensor.
3. EXPERIMENTAL SETUP DESCRIPTION A schematic diagram of our MFOG is illustrated in Fig. 1(a). All the optical components are made with the same fiber type which is SI 50/125 µm. An GaAlAs surface emitting LED with a multimode fiber pigtail was chosen as a light source. It operates at 850 nm and its bandwidth is 50 nm with an output power of 45 µW at the fiber end. To improve the light power budget of the MFOG, we have used a 3dB Y directional coupler type for the two couplers. The first one is a standard fused multimode coupler and to get a symmetrical modal distribution, a micro-optic coupler is chosen as a second coupler. The sensing coil consists of 1031 m of SI 50/125 µm multimode fiber wounded around an aluminum spool of 10 cm of diameter. In order to operate in the sensitive and linear region of the interferometer response, a sinusoidal phase modulation is applied to the light path. This modulation is provided by a PZT tube of 20 mm of diameter driven by a 3,8 Volts sinusoidal signal at a frequency of 6,7 kHz. The demodulation technique uses a synchronous detector (lock-in) at the modulation frequency. The photodetector (PD) is a silicon PIN photodiode which is suitable to operate at 850 nm used with a differential transimpedance amplifier circuit. To avoid losses, the connections between several optical fiber components are carried out by the fiber fusion splicer unit. To test the setup the MFOG is placed on a motorized rotating table controlled by a computer as shown in Fig. 1(b).
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Ω
Pigtailed Fused Micro-optic coupler coupler LED current LED driver PD PZT Amplifier & filter
PZT modulator Fiber coil
Signal generator
Lock –in Amplifier
LED
Electronics
Fiber sensing coil Couplers
Photodetector Rotation rate measurement
DAQ
Rotation table
Fig. 1. Experimental setup, (a) Schematic block diagram of our open loop MFOG, (b) Photograph of the prototype.
4. RESULTS AND DISCUSSIONS The first tests carried out are those related to the optical interference signal and to the measurement of the fringe contrast before the lock-in detection. The Fig. 2 (a) shows the obtained multimode interference signal at rest whose apparent frequency is twice of the modulation one. But when the MFOG is rotating (15°/sec), we can see from Fig 2 (b) the appearance of the first harmonic whose amplitude is proportional to the applied rotation rate. The measured contrast is about 0,025 and due to this low contrast DC component of the signal shown here is filtered. The demodulated signal for different rotation velocities and directions (+/-) are shown in Fig. 4. The result of Fig. 3 (a) is obtained when applying a rotation rate of 10°/s in both directions. That of the Fig. 3(b) is obtained for rotation rate of +3, +5 and +10°/sec. these tests are obtained using 30 ms as time constant. 5%
Interference signal at rest
100
Interference signal in rotation
g 0) (0
0
> 0
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-100
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Fig. 2. Interference signals observed, (a) at rest and (b) in rotating of the open loop MFOG.
+100/
+10 °Is
LJ
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Voltage (Volts
4
-6
(b)
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6
-1O°Is
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Fig. 3. Tests of the MFOG in rotation, (a) response for direction +/- of the rotation, (b) response for the amplitude of rotation rates.
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We report in the Fig. 4 the input/output characteristic of our MFOG which represents the demodulated voltage versus the applied rotation rate. We can see the sinusoidal dependence between the input and the output. This dependence yields a non-linear response of the MFOG for high rotation rate. The range inferred from this response is ± 34°/sec. The minimum measured rotation rate using 30 ms time constant is 0.05 °/s.
-Fit (_
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irmn+ I II IL I
Co
>
-6
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Fig. 4. Input/output characteristic of the open loop MFOG.
5. CONCLUSION An open loop MFOG has been realized by using a SI 50/125µm fiber as sensing coil and low cost components such as LED and fused coupler. The system presents the advantage to be relatively insensitive to external perturbations. The preliminary tests of this configuration show that a sensitivity of ~0.05°/s was obtained with a full dynamic range of ±34°/sec. This sensitivity can be be determined precisely and improved in future tests by finding and optimizing the modulation. The MFOG is mainly directed to low cost and medium performances applications. We expect that in the future this technology can compete the single mode FOG in several fields.
6. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
Burns, W. K., "Current status of fiber optic gyroscope," Proc.of Optical Communication conference (OFC’98 Technical Digest Series), 2, 370-370 (1998). Lee, B., "Review of the Present Status of Optical Fiber Sensors, "Optical Fiber Tech. 9(2), 57-79 (2003). Bergh, R., A., Lefevre, H., C., and Shaw, H., J., "All single mode fiber-optic gyroscope," Opt. Lett. 6(4), 198-200 (1981). Kim, H. K., Digonnet, M., Kino, G. S., "Air-core photonic band gap fiber optic gyroscope," J. of Lightwave technology, 24(8), 3169-3174 (2006). Alekseev, E. I., et al., "Ring interferometer with a multimode waveguide," Sov. J. Quantum Electronics, 14(11), 1436-1442 (1984). Fredricks, J. R., Johnson, D., R., "Low cost multimode fiber optic rotation sensor," Proc. of the SPIE on Fiber Optic and Laser Sensors V, 985, 106-116 (1987). Bouamra, M., and Meyrueis, P., "Multimode fiber optic gyrometer," Proc. Of the 15th Anniversary Conference on fiber optic gyros, 1585, 309-321 (1991). Medjadba, H., Lecler, S., Simohamed, L. M., Chakari, A., "Modeling a multimode Sagnac interferometer: application for an embarked fiber optic gyroscope," Proc. of the SPIE on optical fiber sensor, 7003, 700310 (2008). Medjadba, H., Lecler, S., Simohamed, L. M., Chakari, A., Javahilary, N., "Optimizing the optical components choice for performances improvement of multimode fiber gyroscope," In Defense, Security and Sensing SPIE 2009)
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