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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 2, FEBRUARY 2001
Fiber-Optic Sensor Array Based on Sagnac Interferometer with Stable Phase Bias Ki Ho Han, Wang Joo Lee, and Byoung Yoon Kim, Fellow, IEEE
Abstract—We propose and experimentally demonstrate a novel fiber sensor array based on Sagnac interferometer with very simple electronic signal processing. A stable quadrature phase bias was obtained using a phase modulator, and polarization-induced signal fading was suppressed by using a depolarizer and a broad-band at 5 kHz was source. Phase sensitivity of about 4.0 rms obtained using a two-sensor array.
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Index Terms—Fiber optic sensors, Sagnac interferometer, sensor arrays, time-division multiplexing.
I. INTRODUCTION
F
IBER-OPTIC interferometer arrays have been extensively developed for their applications as acoustic sensor arrays, and commonly used interferometers include Mach–Zehnder [1] and Michelson [2] interferometers. Some of the major problems associated with the sensor arrays stem from the fading of interference signal due to random drift in phase bias and relative polarization state of interfering optical waves. The phase biasinduced signal fading could be overcome by using relatively complex electronic signal processing techniques such as heterodyne technique that involves optical modulators [3]–[5]. In some cases, passive 3 3 couplers were used to avoid the use of modulators but nontrivial signal processing is required [6], [7]. As practical solutions for the polarization-induced signal fading problem, polarization switching [8] or dual heterodyne [9] techniques were recently proposed and successfully demonstrated. As an alternative to the Mach–Zehnder and Michelson-type interferometers, Sagnac interferometer with a 3 3 coupler has been studied for acoustic sensing [10], [11]. Sagnac interferometer configuration makes it possible to suppress the polarization-induced signal fading and provide stable phase bias [12], [13]. It is well understood that the low sensitivity of Sagnac interferometer at low acoustic frequencies matches well with the background noise in the sea environment. Recently, the use of Sagnac interferometers with a 3 3 coupler for sensor array has been theoretically analyzed [14]. In this letter, we report the first experimental demonstration, to our knowledge, of a sensor array based on a Sagnac interferometer, where an accurate phase bias is provided by a LiNbO phase modulator (PM) without of the use of a 3 3 coupler. Electronic signal processing becomes
Manuscript received May 30, 2000; revised October 25, 2000. This research was supported by the Agency for Defense Development. The authors are with the Department of Physics and Center for Electro-Optics, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea (e-mail:
[email protected]). Publisher Item Identifier S 1041-1135(01)01057-6.
trivial, and the polarization signal fading is suppressed by using an unpolarized broad-band optical source and a fiber-optic depolarizer in the interferometer. Fig. 1 shows the configuration of the time-division multiplexed (TDM) sensor array based on a Sagnac interferometer. The array is composed of Sagnac loops that share a coupler, a depolarizer, a long delay coil, and a PM located at one end of the Sagnac loop. Each loop has a corresponding sensing fiber coil. An unpolarized broad-band optical input pulse entering the first conventional 2 2 directional coupler is divided into two pulses that propagate in clockwise (CW) and counterclockwise (CCW) directions. The CW and CCW pulses pass through a PM, a depolarizer, a delay coil, and fiber sensors in fiber rungs, and the pulses that experience the same optical path length interfere with each other at the output. Coupling ratio of the th coupler in each upper and lower bus should be to achieve equal signal power from each rung. In this case, the maximum signal power reaching the detector is proportional to . Tradeoff between sensitivity and the number of sensors output inin sensor arrays can be found in [16]. There are terfered pulses and the PM operates in pulsed mode such that it phase difference between the interfering CW and provides CCW pulses. An electronic gate separates photodetector signals from different sensors, and the envelope of the pulse train from each sensor directly represents the acoustic signal without further signal processing. It is assumed that the acoustically induced phase modulation in each sensor is much smaller than 1 radian. The intensity modulator (IM) and the PM used in this scheme transmit only one polarization component and results in intensity loss of 6 dB. This can be reduced in the future by using polarization-independent IM and PM. The output signal from the Sagnac interferometer with a depolarizer inside the loop has a fringe visibility of 0.5 independently of position of the depolarizer and fiber birefringence in the Sagnac loop [12]. Since we have a polarizing PM in the Sagnac loop, the transmission axis of the PM had to be aligned with one of the axes of the depolarizer in order to guarantee the fringe visibility of 0.5. If a polarization-independent PM is deployed, no special consideration is required. In this arrangement, we have to carefully choose proper ) and the width ( ) of values for the repetition rate ( ), the width the input pulse, time delay between sensors ( amplitude, and of phase modulation pulse ( ) with the time delay in the loop ( ). Fig. 2 shows a temporal sequence of: a) the phase modulation pulse; b) the CCW; and c) the CW optical pulses at the location of the PM. Notice CW pulses that are generated by sensor that there are fiber rungs. It is arranged that the CW and CCW pulses go
1041–1135/01$10.00 © 2001 IEEE
HAN et al.: FIBER-OPTIC SENSOR ARRAY
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Fig. 1. Schematic diagram of a sensor array based on Sagnac interferometer. EDFA: erbium-doped fiber amplifier. IM: intensity modulator. PM: phase modulator. S ; 1 1 1 ; S : fiber sensors.
Fig. 2. Temporal relationship between the waveforms of phase modulation and CW, CCW optical pulses. CW: clockwise. CCW: counterclockwise.
through the PM when the amplitude is 0, and , respecphase bias between CW and tively. It provides a stable CCW pulses. In this case, the output intensity of each pulse , where is takes the form determined by input intensity and loss in the array structure, represents fringe visibility, and is phase difference caused by acoustic signal. The ac component of the output is (when ). Since the , and , unambiguous phase dc component modulation signal induced by acoustic wave can be obtained . In order to avoid unwanted pulse overlap, by the time interval between input pulses should satisfy (1) and . The time delay in the loop is the where minimum transit time for the light to travel from the location of the PM to the symmetric point on the opposite side of the loop. is a function of the position of the PM. For a given , the position of the PM should be chosen such that satisfies
(2) Here, value of
corresponds to the case of minimum allowable where the PM should be located the farthest from the
loop coupler. The maximum value of corresponds to the case of maximum , and therefore, the PM should be positioned closest to the loop coupler. The value of is determined by the and given as condition where is the length of the first loop and is light velocity in means the maximum integer that does fiber, and the bracket not exceed a number . A sensor array with two sensors was constructed, as shown in Fig. 1, and experimentally evaluated to demonstrate the operating principle. The light source was amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier with a center wavelength of 1539 nm and optical power of 5 mW. The amplifier was pumped by a laser diode at 980-nm wavelength. Optical pulses with 150-ns duration and 400-kHz repetition rate were generated by a LiNbO integrated optic intensity modulator (IM). The PM was also a LiNbO -integrated optic component. Because both IM and PM transmitted only one polarization component, a depolarizer was placed between the two in order to prevent intensity variations. Another depolarizer in the Sagnac loop was used to suppress polarization-induced signal fading. The depolarizers were constructed by splicing two high-birefringence fiber sections of 1 : 2 length ratio with their axes rotated by 45 [15]. The rest of the fiber was conventional single-mode fiber, and the length of the delay coil was 1 km. A piezoelectric (PZT) cylinder, placed in each rung, with 70 turns of fiber wrapped around acted as an acoustic sensor. The optical , was 250 transit time difference between the adjacent rungs, ns, which corresponds to about 50-m-length difference. For the measurement of phase sensitivity, two harmonic signals at 7 kHz and 5 kHz with the same phase modulation amplitude of 100 mrad were applied to the sensors 1 and 2 in Fig. 1, respectively. An electronic gate selected one of the pulses and sent it to a low-pass filter to reproduce the applied acoustic signal without any complex signal processing. Fig. 3 shows the waveforms of the applied input signal and the output signal from sensor 2. Fig. 4 shows the same output signal on a RF spectrum analyzer with noise-equivalent bandwidth of 77 Hz in the presence of 100-mrad signals for both sensors. The measured signal-to-noise ratio of about 66 dB corresponds to a noiseat 5 kHz when the equivalent phase shift of 4.0
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Fig. 5. Phase sensitivity measured in the frequency range of 1–20 kHz. Fig. 3. Oscilloscope traces of a harmonic signal at 5 kHz applied to PZT and the output signal recovered from the optical output.
processing, a high phase sensitivity of about 4.0 at 5 kHz, limited by source intensity noise, and a crosstalk dB were demonstrated. This work demonstrated the of possibility of a very simple interferometric sensor array with high sensitivity. REFERENCES
Fig. 4. Spectrum of the measured output signal of sensor 2 when two 100-mrad sine waves at 7 and 5 kHz were applied to sensors 1 and 2, respectively. Noise-equivalent bandwidth was 77 Hz.
received average optical power on the detector was 3.2 W. Crosstalk was measured to be about 40 dB, which is limited mainly by the limited extinction ratio of the IM that can be improved. Fig. 5 shows the measured phase sensitivities of apat frequencies up to 20 kHz. proximately 3.9–4.1 Below 1 kHz, the sensitivity became poor, limited by the room and spectrum analyzer noise, which can be seen in Fig. 4. For comparison, at 3.2 W received optical power, the shot noise , and the source relative intensity should be 0.8 corresponding noise was measured to be about of phase noise in our sensor. to 3.4 In conclusion, we proposed and demonstrated a novel fiber-optic sensor array based on a Sagnac interferometer with phase bias and freedom from the polarization-ina stable duced signal fading. The position of a PM in a Sagnac loop should be carefully chosen to maximize the total duty cycle of the sensor array. With a much simplified electronic signal
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