Phase Generated Carrier Scheme Research Articles

Effect of three-beam interference on phase-generated carrier demodulation of a fiber-optic interferometric sensor.

Homodyne demodulation using a phase-generated carrier (PGC) has been applied in fiber-optic interferometric sensors to overcome the signal fading and distortion due to the drift of the operating point. An assumption needed for the PGC method to be valid is that the sensor output is a sinusoidal function of the phase delay between the arms of the interferometer, which is readily achieved by a two-beam interferometer. In this work, we theoretically and experimentally study the effect of three-beam interference, whose output deviates from a sinusoidal function of the phase delay, on the performance of the PGC scheme. The results show that the deviation could lead to additional undesirable terms in the in-phase and quadrature components in PGC implementation, which may result in significant signal fading with the drift of the operating point. The theoretical analysis leads to two strategies for eliminating these undesirable terms so that the PGC scheme is valid for three-beam interference. The analysis and the strategies were validated experimentally using a fiber-coil Fabry-Perot sensor with two fiber Bragg grating mirrors, each having a reflectivity of 26%.

Experiment on a distributed fiber optic interferometric sensing system to monitor and locate urban high-density polyethylene gas pipe leakage

A distributed fiber optical interferometric sensing system was built to monitor and detect urban high-density polyethylene gas pipe leakage. To eliminate the phase drift, the digital phase generated carrier scheme was designed to demodulate the phase difference of interferometers induced by high-density polyethylene gas pipe leakage. Then the empirical mode decomposition and approximate entropy were combined to process fiber optic interferometric signals. Finally, leakage points were located by the null-frequency spectrum after empirical mode decomposition filter denoising. The lab-scale test validated the distributed fiber optical interferometric sensing system leakage location performance and demonstrated a reduced leakage accident missed alarm rate.

Correction of nonlinear errors from PGC carrier phase delay and AOIM in fiber-optic interferometers for nanoscale displacement measurement.

In fiber-optic interferometers with laser frequency modulation, carrier phase delay and accompanied optical intensity modulation (AOIM) in phase-generated-carrier (PGC) demodulation inevitably produce nonlinear errors that can seriously hamper displacement measurement accuracy. As for the existing improved PGC scheme, they are only capable to compensate for one of these effects. As the only method that is effective in eliminating the two effects simultaneously, typical ellipse fitting methods require target movements λlaser/4, and fail when the PGC carrier phase delay is proximate to certain values (e.g., nπ +π/4, nπ +π/2). Herein, a modified nonlinear-error correction method for errors due to PGC carrier phase delay and AOIM is proposed. Active laser-wavelength scanning by constant variation of the laser drive temperature is used to replace the target movement. A fiber-optic Michelson interferometer is constructed and experiments are performed to verify the feasibility of the proposed method. The experimental results show that after correction, the nonlinear error is reduced to less than 1nm, and nanoscale displacement measurement is achieved.

PGC-Atan Demodulation Scheme With the Carrier Phase Delay Compensation for Fiber-Optic Interferometric Sensors

Phase generated carrier (PGC) demodulation schemes are widely used in fiber-optic interferometric sensors. All known PGC schemes use carrier signals to obtain in-phase and quadrature components from the interference signal. The unwanted phase delay between the reference carrier signal and the modulation signal in the interference signal might exist. This effect might lead to the incorrect measurement of the phase signal in the PGC scheme. In this paper, the original PGC demodulation scheme with the carrier phase delay compensation based on the basic PGC-Atan scheme is presented. The proposed scheme provides the real-time carrier phase delay compensation over the all possible phase delay range from 0° to 360° without using phase adjustments of the carrier signal. Theoretical analysis and mathematical modeling of the proposed scheme and the basic PGC-Atan demodulation scheme were performed to compare their performance under the same conditions. The proposed scheme, the basic PGC-Atan demodulation scheme, and the PGC synchronous carrier restoration scheme were implemented in the FPGA-based electronic processing circuit of the single interferometric fiber Bragg grating sensor. Experimental results showed that the proposed scheme provides carrier phase delay compensation without using phase adjustments of the carrier signal.

Crosstalk Reduction and Demodulation Stability Promotion in Inline Fiber Fabry–Pérot Sensor Array Using Phase Generated Carrier Scheme

An inline time-division multiplexing (TDM) fiber Fabry–Perot sensor array based on the low-reflectivity fiber Bragg gratings (FBGs) is promising for lots of sensing applications. The intrinsic multireflection (MR) between the FBGs with the same Bragg wavelength is a serious problem, which induces crosstalk and generates false detection information. Furthermore, the amplitude of the demodulated signal cannot reach a stable state due to the MR. Here, we presented a full theoretical demonstration of the MR-induced crosstalk fluctuation and demodulation instability, where the phase generated carrier method was used as the interrogation scheme. An interference synthesis method was presented to reduce the MR-induced crosstalk and to promote the demodulation stability simultaneously. The experimental results showed that a crosstalk reduction of 18 dB and a demodulation stability promotion of 1.42 dB could be achieved with the FBG reflectivity of 5%. The principle of the method can be extended to the array with more TDM channels.

Transmission-link-induced intensity and phase noise in a 400-km interrogated fiber-optics hydrophone system using a phase-generated carrier scheme

Transmission-link induced-intensity noise, as well as the con- version of intensity fluctuation to demodulated phase noise in a 400-km interrogated hydrophone array with a phase-generated carrier (PGC) scheme are investigated. Theoretical and experimental results all show that a PGC scheme can increase the erbium-doped fiber amplifiers- induced phase noise level by ∼3.7 dB in intensity to phase noise conver- sion. However, it can significantly reduce the coherent double Rayleigh scattering (DRS)-induced phase noise by ∼11.7 dB when compared with the unmodulated DRS intensity noise. Considering the effects of these two kinds of noises, a PGC scheme still has significant advantages for noise suppression in the remote system, and the total phase noise of the 400-km hydrophone system is shown to be only 11 to 14 μrad∕ sqrt ðHzÞ. Moreover, it is demonstrated that the spectra of DRS-induced relative intensity noise and phase noise all have Lorentzian line shapes and the full widths at half maximum are twice the width of the laser source. This result is helpful for us to choose a suitable laser source in long- distance hydrophone systems. © 2013 Society of Photo-Optical Instrumentation

Phase-generated carrier demodulation scheme for fiber Fabry–Pérot interferometric sensor with high finesse

This paper presents a demodulation scheme using phase-generated carrier (PGC) for a fiber Fabry-Pérot interferometric (FFPI) sensor with high finesse. The FFPI is constructed by a polarization maintaining fiber ring resonator with dual-coupler (PMDC-FRR), which can eliminate the polarization induced fading phenomenon. Compared with the former phase demodulation methods, the PGC scheme in this paper does not assume a two-beam interferometric approximation for the Fabry-Pérot cavity, and can work at arbitrary value of finesse in theory. Two PMDC-FRRs with reflective coefficients of 0.5 and 0.9 are made in experiments for demodulation. Both the single-frequency and the wideband signals are successfully demodulated from the transmission intensities using the PGC demodulation scheme. The experimental results demonstrate that the PGC demodulation scheme is feasible for the FFPI sensor with high finesse. The effects of the reflective coefficient and the intensity loss to the finesse are also discussed.

Phase Mode-Matching Demodulation Scheme for Interferometric Fiber-Optic Sensors

A novel digital demodulation scheme for an interferometric optical fiber sensor is developed, which uses only an interferometer with zero optical path difference, and has a larger dynamic range and a faster operation speed than that of a digital phase generated carrier scheme