Fiber Optic Michelson Interferometric Sensor Research Articles

Large-Dynamic-Range and High-Stability Phase Demodulation Technology for Fiber-Optic Michelson Interferometric Sensors.

A large-dynamic-range and high-stability phase demodulation technology for fiber-optic Michelson interferometric sensors is proposed. This technology utilizes two output signals from a 2 × 2 fiber-optic coupler, the interferometric phase difference of which is π. A linear-fitting trigonometric-identity-transformation differential cross-multiplication (LF-TIT-DCM) algorithm is used to interrogate the phase signal from the two output signals from the coupler. The interferometric phase differences from the two output signals from the 2 × 2 fiber-optic couplers with different coupling ratios are all equal to π, which ensures that the LF-TIT-DCM algorithm can be applied perfectly. A fiber-optic Michelson interferometric acoustic sensor is fabricated, and an acoustic signal testing system is built to prove the proposed phase demodulation technology. Experimental results show that excellent linearity is observed from 0.033 rad to 3.2 rad. Moreover, the influence of laser wavelength and optical power is researched, and variation below 0.47 dB is observed at different sound pressure levels (SPLs). Long-term stability over thirty minutes is tested, and fluctuation is less than 0.36 dB. The proposed phase demodulation technology obtains large dynamic range and high stability at rather low cost.

Crosstalk and phase-noise reduction in time-division multiplexing of polarization-insensitive fiber optic Michelson interferometric sensors

The multiplexing system of fiber optic interferometric sensors suffers from crosstalk and noise, and the performance degrades. The crosstalk of TDM of the fiber optic interferometric sensor includes lead fiber crosstalk, sensor crosstalk for optical gate with finite extinction ratio (ER) and delay fiber crosstalk. In this paper, the laser emission time difference of each interference term of the TDM-PIFOMI system was utilized to reduce the crosstalk and noise by using laser diode high frequency modulation. The temporal characteristics of the crosstalk and the noise of the TDM-PIFOMI system were analyzed, and the theoretical sensor crosstalk, Asensor, is −4ER(dB) by using laser diode high frequency modulation. According to the PGC demodulation process, the minimum frequency of the laser diode modulation was estimated to be larger than nine times of the carrier frequency of the PGC demodulator.In our experimental results, the improvements of the sensor crosstalk were from 8.3 to 15.5dB, and the improvements of the delay fiber crosstalk were 15.7 and 14.6dB by using laser diode high frequency modulation. The phase noise was reduced from 2dB to 3.9dB. Using laser diode high frequency modulation to decrease the crosstalk and the noise of the TDM-PIFOMI system is effective and cheap.

Counting signal processing and counting level normalization techniques of polarization-insensitive fiber-optic Michelson interferometric sensors

A counting signal processing technique of the fiber-optic interferometric sensor is proposed. The technique is capable of counting the numbers of the maximum and minimum of the output interferometric signal in a specific time duration, and it can be used as the basis to distinguish the sensing phase signal. It can also be used as a signal detector on applications such as intrusion detection. All sensors are subject to aging of the optical components and bending loss, and therefore the output signal of each sensor may vary with time. We propose a counting level normalization technique to compensate for these variations and to obtain the correct counting numbers.

Suppression of double Rayleigh scattering-induced excess noise in remotely interrogated fiber-optic interferometric sensors

This letter describes experimental observations of excess noise due to coherent double Rayleigh scattering in the input fiber of a remotely interrogated fiber-optic interferometric sensor. This noise source is generally only observable when a high coherence length laser is used to interrogate the sensor and the fiber length connecting the sensor is in excess of /spl sim/10 km. We present a simplified model to explain how this noise source affects the sensor resolution and demonstrate a method based on laser source modulation to reduce this noise by /spl sim/20 dB in a fiber-optic Michelson interferometric sensor with a 25-km input fiber, at frequencies less than 10 kHz.

System design and optimization of optically amplified WDM-TDM hybrid polarization-insensitive fiber-optic Michelson interferometric sensor

We investigate the optically amplified time-division-multiplexed (TDM) polarization-insensitive fiber-optic Michelson interferometric sensor (PIFOMIS) system using erbium-doped fiber amplifier (EDFA). The EDFA was named preamplifier, in-line amplifier or postamplifier; by the position it was located. We find that the preamplifier EDFA has limited usefulness because of its unstable amplification of the optical pulse trains. Both post- and in-line cases can work successfully in the TDM-PIFOMIS system. The amplitudes of the optical pulse trains are stable after amplified by the in-line EDFA, this is a significantly advantage of the optically amplified TDM-PIFOMIS system. The MPDS of the unamplified TDM-PIPOMIS system with an extinction ratio (ER) of 33 dB of the output pulse of the optical guide wave (OGW) modulator was 2.4/spl times/10/sup -5/ rad/(Hz)/sup 1/2/ at 1 kHz. For maintaining MPDS better than 3.4/spl times/10/sup -5/ rad/(Hz)/sup 1/2/ at 1 kHz, the allowable worst ER for the post- and in-line amplified system are 20 and 17.8 dB, respectively, and the corresponding input signal peak power should be larger than -20 and -25 dBm. While employing such two post- and two in-line EDFAs in the TDM-PIFOMIS system, the allowable loss of the sensor array is 47 dB. We analyze the phase-induced intensity noise (PIIN) of the optically amplified TDM-PIFOMIS system in detail and propose methods to reduce the PIIN. The output optical pulse of an intensity modulator with high ER is a key issue to minimize the PIIN and sensor crosstalk in the system. In order to reduce the system PIIN, complexity and cost, we suggest an optimum optically amplified WDM (wavelength-division multiplexing)-TDM hybrid PIFOMIS system with four wavelengths and four eight-sensor subarrays.

A digital signal processing algorithm for structural strain measurement by a 3 × 3 passive demodulated fiber optic interferometric sensor

A digital signal processing algorithm, the moving reference line crossing count method (MRLCCM), was developed to implement the structural strain measurement by a passive demodulated fiber optic Michelson interferometric sensor. The fiber optic Michelson sensor, which is constructed of a 3 × 3 fiber optic coupler, can give information about the magnitude and direction of the strain of structures. The beating, drifting and noise, which are caused by the longitudinal strain and the lateral strain of the fiber, bring about the counting error of the phase differences which are directly converted to the structural strain. The developed algorithm is based on the reference line crossing count method and resets the reference line during the presence of the signal drifting. The accuracy of the strain calculation was confirmed by the various simulated fiber optic signals with signal beating, drifting and noise. A passive demodulated 3 × 3 fiber optic Michelson interferometric sensor was bonded on the cantilevered aluminum beam for experimental strain sensing. The capability of real-time processing was verified by the real fiber optic signals.

Experimental investigation of an optically amplified time-division-multiplexed polarization-insensitive fiber-optic Michelson interferometric sensor system

We experimentally investigate optically amplified time-division-multiplexed polarization-insensitive fiber-optic Michelson interferometric (PIFOMI) sensor systems, using an erbium-doped fiber amplifier (EDFA) and a phase-generated carrier (PGC) demodulation technique. The influence of the EDFA on the extinction ratio (ER) of the light pulse and on the minimum phase-detection sensitivity (MPDS) is examined. We find that the EDFA acting as a preamplifier has limited usefulness because the highly amplified spontaneous emission (ASE) noise generated by the EDFA degrades the ER and the MPDS. However, both postamplifiers and in-line EDFA's can work successfully. The MPDS of the unamplified time-division-multiplexed PIFOMI system with an ER of 33 dB was 2.4 x 10(-5) rad/(Hz)(1/2) at ~1 kHz. For maintaining a MPDS of better than 3.4 x 10(-5) rad/(Hz)(1/2) at ~1 kHz, the worst ER's for the postamplified and in-line amplified systems were 20 and 17.8 dB, respectively. The corresponding input signal peak power should be larger than -20 and -25 dBm for the postamplifiers and in-line amplifiers, respectively. When two postamplifiers and two in-line amplifiers are used, an allowable sensor system loss of 47 dB and a link length of the input-output lead fiber of 108 km can be realized for this system with a 32-sensor array. Implementation of optically amplified time-division-multiplexed and wavelength-division multiplexed-time-division multiplexed PIFOMI subarray sensor systems are also addressed.

Structural strain measurement using a 3*3 passive demodulated fiber optic michelson interferometric sensor

The measuring method of structural strain by a 3*3 passive-demodulated fiber optic interferometric sensor was developed to implement the real-time monitoring of structural status. A 3*3 fiber optic Michelson interferometric sensor was constructed to sense the value and the direction of structural strain. This sensor was applied on the cantilevered aluminum beam to experiment the sensing of the structural deformation. The digital signal processing was programmed by LabVIEW to determine the structural strain from the fiber optic signals. This program was verified by various simulated fiber optic signals. Finally, the structural was well determined by this developed program from real fiber optic signals.

Analysis of distortion of a fiber optic Michelson interferometric sensor caused by the imperfect properties of its <inline-formula>3×3</inline-formula> coupler

We investigate the distortion of demodulated output signal in a fiber optic Michelson interferometric sensor in relationship to the imperfect properties of its 3 X 3 coupler. A general guide rule in choosing a 3 X 3 coupler and its connection type in the interferometer to reduce this kind of signal distortion is also suggested.

Cross-talk analysis of time-division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors with a 3 × 3 directional coupler

A time division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors (TDM-PIMI's) with a 3 x 3 directional coupler is presented. The elimination of polarization-induced fading and the output intensities of the TDM-PIMI system are described and demonstrated. The output intensity of each sensor of the system can be demodulated by a passive homodyne method to increase the sensor bandwidth significantly. The sensor cross talk of the system having an optical gate with a finite extinction ratio is analyzed. The use of a laser source with an adequate coherence length to reduce the sensor cross talk is suggested. The delay-fiber cross talk of the system by Rayleigh backscattering is analyzed and demonstrated. We further suggest some methods that could possibly reduce the effect of the Rayleigh backscattered light. Finally a sophisticated design of a TDM-PIMI system with a 3 x 3 directional coupler is described.

Crosstalk analysis and system design of time-division multiplexing of polarization-insensitive fiber optic Michelson interferometric sensors

The time-division multiplexing of polarization-insensitive fiber optic Michelson interferometric sensors (TDM-PIFOMI) with compensating interferometer is presented. The lead crosstalk in a general time-division multiplexing of the fiber optic interferometric sensors is described and demonstrated. The sensor crosstalk of TDM-PIFOMI for optical gate with finite extinction ratio is analyzed and using a laser source with adequate coherence length to reduce sensor crosstalk is suggested. The delay fiber crosstalk and noise of TDM-PIFOMI by Rayleigh backscattering is analyzed and demonstrated. We also suggest some methods that could possibly reduce the effect of the Rayleigh backscattered light. Finally, the advanced system design of TDM-PIFOMI is described.

Time-division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors

A system of time-division multiplexing of polarization-insensitive fiber-optic Michelson interferometric sensors that uses Faraday rotator mirror elements is demonstrated. This system is constructed with conventional low-birefringence single-mode fiber and is able to solve the polarization-fading problem by a combination of Faraday rotator mirrors with unbalanced Michelson interferometers. The system is lead-fiber insensitive and has potentials for practical field applications.