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Simultaneous phases detection in multi-wavelength interferometry (MWLI) is essential for high-accuracy dynamic absolute distance measurement in high-end equipment manufacturing. To address this problem, a simultaneous phase detection scheme based on frequency-division multiplexing (FDM) technique is proposed in this paper. High-frequency sinusoidal phase modulation with non-overlapping frequencies were applied to multiple lasers individually using electro-optic modulators (EOMs), and interference phases of multiple wavelengths were extracted in parallel using phase generated carrier (PGC) demodulation technique from the FDM interference signal. Principles of FDM multi-wavelength interferometry and simultaneous phase detection scheme were illustrated. A FDM dual-wavelength interferometer was constructed to verify the performance of the proposed phase detection scheme by simulation analysis and experiments. Results of simulations show that the resolution, accuracy and nonlinear error of the two demodulated phases are all better than 0.01°. Experiments of nanometer displacement measurement further verified the good performance of the phase detection scheme and its feasibility in dynamic measurement. [ABSTRACT FROM AUTHOR]

This paper proposes a phase generated carrier (PGC) demodulation scheme for sinusoidal phase modulation interferometer (SPMI) that does not require a second harmonic carrier signal. In this scheme, an in-phase interference signal component with direct current (DC) bias is obtained by low-pass filtering the original interference signal, and a quadrature interference signal component is obtained by conventional means. The Lissajous figure formed by these two quadrature interference signal components is an ellipse whose center deviates from the origin of the I-Q coordinates. By fitting this ellipse, the major and minor axes and the center coordinates can be obtained. These elliptic parameters are then used to eliminate DC bias and normalize the amplitude of the quadrature signal components. Finally, the phase demodulation of the interference signal can be realized by using arctangent algorithm or differential cross multiplication (DCM) algorithm. The results of simulation and experiment have verified the validity and accuracy of the proposed PGC demodulation scheme, and show that it is insensitive to the phase modulation depth C. Compared with conventional PGC demodulation, this scheme requires lower depth of carrier phase modulation. The vibration measurement results indicate that the average signal-to-noise and distortion Ratio (SINAD) and total harmonic distortion (THD) values of demodulation results in the frequency range of 130 Hz to 250 Hz are 34 dB and -33 dB respectively. The proposed PGC demodulation scheme shows good performance in displacement measurement using SPMI. • A novel PGC demodulation scheme without using a second harmonic carrier signal. • Lissajous ellipse with center deviated from origin in I-Q coordinates. • Demodulation performance comparison, test for the signal-to-noise and distortion ratio and the total harmonic distortion. • PGC demodulation scheme with a low optimal phase modulation depth. [ABSTRACT FROM AUTHOR]

Primary angular vibration calibration devices based on laser interferometers play a crucial role in evaluating the dynamic performance of inertial sensing devices. Here, we propose a sinusoidal phase-modulated angle interferometer (SPMAI) to realize angular vibration measurements over a frequency range of 1–1000 Hz, in which the sinusoidal measurement retro-reflector (SMR) and the phase generation carrier (PGC) demodulation algorithm are adopted to track the dynamic angle variation. A comprehensive theoretical analysis is presented to reveal the relationship between demodulation performance of the SPMAI and several factors, such as phase modulation depth, carrier phase delay and sampling frequency. Both the simulated and experimental results demonstrate that the proposed SPMAI can achieve an angular vibration measurement with amplitude of sub-arcsecond under given parameters. Using the proposed SPMAI, the frequency bandwidth of an interferometric fiber-optic gyroscope (IFOG) is successfully determined to be 848 Hz.

In order to meet the needs of phase generated carrier (PGC) demodulation technology for interferometric fiber optic hydrophones, we proposed an optical microfiber all-optical phase modulator (OMAOPM) based on the photo-induced thermal phase shift effect, which can be used as a phase carrier generation component, so as to make the modulation efficiency and working bandwidth of this type of modulator satisfy the requirements of underwater acoustic signal demodulation applications. We analyzed the modulation principle of this modulator and optimized the structural parameters of the optical microfiber (OM) when the waist length and waist diameter of OM are 15 mm and 1.4 μm, respectively. The modulation amplitude of the modulator can reach 1 rad, which can meet the requirements of sensing applications. On this basis, the fiber optical hydrophone PGC-Atan demodulation system was constructed, and the simulated underwater acoustic signal test demodulation research was carried out. Experimental results showed that the system can demodulate underwater acoustic signals below 1 kHz.

• A novel precision vibration measurement using differential phase-modulated homodyne interferometry is presented, in which an orthogonal linearly polarized laser source is constructed by using a DFB fiber laser and a fiber-coupled electro-optic phase modulator to improve the measurement range of amplitude and frequency. • The optical path drift and common-mode noise of the fiber can be differentially compensated by using an auxiliary monitoring signal. • By pre-scanning the wavelength of DFB laser, the ellipse fitting method can be realized to compensate the periodic nonlinearity of PGC demodulation without needing the object to move more than half a wavelength. A novel precision vibration measurement using differential phase-modulated homodyne interferometry is presented. The orthogonal linearly polarized phase-modulated single frequency laser source is constructed by using a distributed feedback (DFB) fiber laser and a fiber-coupled electro-optic phase modulator (EOM). An auxiliary monitoring signal is used to differentially compensate the optical path drift and common-mode noise of the fiber. By pre-scanning the wavelength of DFB laser to introduce enough phase changes, ellipse fitting can be realized without moving the object more than half a wavelength. Therefore, the periodic nonlinearity of the phase generated carrier (PGC) demodulation is compensated. Experimental setup was built and the real-time signal processing was realized based on field programmable gate array (FPGA). The experiments of pulse vibration, sinusoidal vibration and comparison with commercial laser vibrometer were performed to verify the feasibility of the proposed method. The experimental results show that the average deviation of vibration amplitude is about 0.97 nm in the frequency range of 0.5 kHz to 10.0 kHz, which indicates that the proposed method has significant application for precision vibration measurement. [ABSTRACT FROM AUTHOR]

Abstract: The phase carrier signal of the PGC demodulation in fiber optic interferometric sensors (FOIS) may be generated by using laser current modulation to achieve all-fiber structures in long-distance and outdoor applications. An optical intensity modulation with carrier frequency induced from the laser modulation will cause the instability and distortion in the PGC demodulation output. Additionally, since the optical intensity attenuation of the FOIS will degrade the PGC demodulation output, proper sensitivity compensation on the PGC demodulator is necessary for the field applications. In this paper, we propose an improved technique by using a balanced photoreceiver in the FOIS to attain significant improvements in the instability and distortion of the PGC demodulation output. Moreover, we choose a small phase scrambler signal amplitude of 1rad to achieve improved the minimum phase detection sensitivity (MPDS) as well as sensitivity normalization because the improvement of the MPDS is very important to the advanced FOIS. The experimental results show that the improved values of the best MPDS are 5.7dB (for DFB laser diode) and 4.3dB (for EP1550 laser diode), respectively, by using the improved technique. From the sensitivity normalization experiment results, the relative error is shown to be less than 0.5% when the optical intensity attentuation is no more than 15dB. It verifies the effectiveness of the sensitivity normalization technique proposed in our improved PGC demodulation system. [Copyright &y& Elsevier]

Abstract: The phase carrier signal of the PGC demodulator can be generated by using laser current modulation to achieve all-fiber structures for long-distance and outdoor applications. However, an optical intensity modulation with carrier frequency induced from the laser modulation may cause the instability and distortion in the PGC demodulation output. In addition, optical loss may reduce the PGC demodulation output so sensitive that sensitivity compensation on demodulation circuit is needed. In this paper, we propose a novel phase scramble technique by using low frequency phase signal with amplitude 2.405rad to achieve the sensitivity normalization, and improve the instability and distortion of the PGC demodulation output simultaneously. Experiment results show that without applying scramble signal, the maximum value of total harmonic distortion is very close to the theoretic value and it is large. After applying scramble signal which matches the theorectical optimum condition J 0(ϕ L )=0, the instability and distortion of the PGC demodulation output are reduced obviously. According to the sensitivity normalization experiment results, the relative error is less than 0.6% when the optical intensity attentuation is no more than 15dB. Therefore, the sensitivity normalization technique proposed in our study is effective. [Copyright &y& Elsevier]

Primary angular vibration calibration devices based on laser interferometers play a crucial role in evaluating the dynamic performance of inertial sensing devices. Here, we propose a sinusoidal phase-modulated angle interferometer (SPMAI) to realize angular vibration measurements over a frequency range of 1–1000 Hz, in which the sinusoidal measurement retro-reflector (SMR) and the phase generation carrier (PGC) demodulation algorithm are adopted to track the dynamic angle variation. A comprehensive theoretical analysis is presented to reveal the relationship between demodulation performance of the SPMAI and several factors, such as phase modulation depth, carrier phase delay and sampling frequency. Both the simulated and experimental results demonstrate that the proposed SPMAI can achieve an angular vibration measurement with amplitude of sub-arcsecond under given parameters. Using the proposed SPMAI, the frequency bandwidth of an interferometric fiber-optic gyroscope (IFOG) is successfully determined to be 848 Hz. [ABSTRACT FROM AUTHOR]

In order to meet the needs of phase generated carrier (PGC) demodulation technology for interferometric fiber optic hydrophones, we proposed an optical microfiber all-optical phase modulator (OMAOPM) based on the photo-induced thermal phase shift effect, which can be used as a phase carrier generation component, so as to make the modulation efficiency and working bandwidth of this type of modulator satisfy the requirements of underwater acoustic signal demodulation applications. We analyzed the modulation principle of this modulator and optimized the structural parameters of the optical microfiber (OM) when the waist length and waist diameter of OM are 15 mm and 1.4 μm, respectively. The modulation amplitude of the modulator can reach 1 rad, which can meet the requirements of sensing applications. On this basis, the fiber optical hydrophone PGC-Atan demodulation system was constructed, and the simulated underwater acoustic signal test demodulation research was carried out. Experimental results showed that the system can demodulate underwater acoustic signals below 1 kHz. [ABSTRACT FROM AUTHOR]