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Phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR) measures the dynamic strain by extracting the phase of the scattering signal. However, the dynamic strain measurement range in conventional $\Phi $ -OTDR is restricted by the phase unwrapping algorithm. In this letter, a new phase unwrapping algorithm, combining the phase and polarization information, is proposed and demonstrated to enlarge the dynamic strain measurement range in $\Phi $ -OTDR based on ultra-weak fiber Bragg grating (UWFBG) array. By correcting the phase signal with the assistance of polarization signal, the large dynamic strain can be recovered. In experiments, the dynamic strain measurement range is demonstrated to increase by about three times, which cannot be achieved by the conventional unwrapping algorithm. Further, the proposed method has the potential to achieve a large amplitude range determined by polarization signal and a good sensitivity determined by phase signal at the same time.

The phase induced intensity noise (PIIN) may seriously reduce the signal-to-noise ratio (SNR) of continuous-wave coherent Doppler lidar (CW-CDL), thereby limiting its performance in practical applications. In this paper, we propose to use optical frequency domain reflectometry (OFDR) technique to locate the PIIN sources and evaluate their intensities. According to the OFDR information, we demonstrate that the PIIN in our CW-CDL system is mainly induced by the crosstalk of polarization beam splitter. Then, the PIIN is reduced by setting fiber delay line, polarization control, and AR-coating. Finally, a wind Doppler signal at a distance of 80 meters is obtained on a clear day, and a typical SNR of 19.4 dB is achieved.

Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in the precise acquisition of the vibration signal along the sensing optical fiber, and has attracted more and more interest. However, polarization fading and coherent fading usually lead to the phase hopping of the recovered signal, thus it is difficult to realize the continuous measurement of vibration, especially the low frequency vibration. The polarization diversity and multi-frequency detection schemes which are usually used to solve the above problems need additional hardware, thus increasing the cost and complexity of the system. In this article, we propose a statistical method based on nearest neighbor analysis. For each interrogation fiber separation, the phases of the sub-separations are evaluated and selected to reconstruct the vibration. In the experiment, the vibrations with frequencies as low as 0.2 Hz are loaded on the fiber and recovered. The results show that the method can effectively suppress the phase hopping, correct distorted vibration and improve the performance of low frequency vibration measurement of Φ-OTDR system on the basis of only numerical calculation.

Laser Doppler Velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, the signal of LDV is weaker in most applications. One challenging issue is how to effectively discover the weak characteristics from the signal submerged in noise. To address the issue, a method based on Stochastic Resonance is proposed in this paper. Using the SR method, the noise may be weakened and weak characteristics highlighted, and therefore velocity can be is obtained. FFT and the method are adopted to process simulated laser Doppler signals in circumstance of MATLAB. The input and the output signal-to-noise ratios of the bistable system demonstrate the effectiveness of the SR method.

We propose a new method which can measure large dynamic strain in $\Phi$ -OTDR based on ultra-weak FBG array, where the wrongly phase wrapping is corrected by the amplitude of the signal. © 2020 The Author(s)

Phase-sensitive optical time domain reflectometry $\Phi$ -OTDR has been widely used in vibration sensing applications due to its high sensitivity and distributed sensing capability. However, the real vibration behavior is hard to be distinguished from ambient interferences by conventional Φ-OTDR, since the responses to simultaneous behaviors are nonlinearly added together. Fortunately, if we focus on the phase information of backscattered light, it would be much easier to obtain the accurate vibration signal and separate different intrusions, due to the linear strain–phase response. Based on this technique, we present a vibration separation method by analyzing the time difference between two fiber positions in this paper. A proof of principle experiment is designed and implemented, and double vibrations influencing the same fiber zone are identified and extracted, which demonstrates the improvement of signal identification in the Φ-OTDR system.

In order to obtain the telemetry data of wind field 80 m ahead of the wind turbine, an all-fiber continuous wave coherent Doppler lidar (CW-CDL) system is demonstrated in this paper. The CW-CDL of all-fiber structure has advantages of the compact structure, good stability and low cost. However, there is inherent crosstalk in the optical circulator of the optical fiber module, the crosstalk light and the reference light enter the photon detector together to produce phase induced intensity noise (PIIN). Aiming to effectively solve the noise, we proposed a polarization control method by combining a polarization beam splitter and a quarter wave plate to reduce the PIIN in the system. The wind speed Doppler signal at 80 meters is obtained on a sunny day, and the typical signal-to-noise ratio can reach 21.7 dB.

We propose an electrical I/Q demodulation scheme to decrease the sampling rate and computational cost in coherent phase-sensitive Optical Time-Domain Reflectometry systems. The IF signal from the photon detector and the local-oscillator from the acoustic-optic modulator driver are splited into two parts, respectively. One of local-oscillators is 90°shifted, then the signals and LOs are cross-mixed and the outputs are low-pass filtered to obtain the I/Q signal. Besides of the save of computational cost, the sampling rate and data storage is reduced at least 1.6 times. At last, a quantitative measurement of a 50 Hz vibration is successfully demonstrated in the experiment.

As a kind of distributed optical fiber sensor, Optical Frequency-Domain Reflectometry (OFDR) can realize high spatial resolution distributed strain/temperature measurement. A method of measuring Rayleigh backscatter spectrum shift by cross-correlation calculation is widely adopted in OFDR sensor system. The other approach is based on the phase shift induced by the strain/temperature variation. In this paper, we propose a digital demodulation method to achieve it. Firstly the output of the photon detector is Fourier transformed and the phase information is obtained. The cross-correlation method and phase demodulation method are compared based on the theoretical and numerical analysis. The result shows that the spatial resolution (SR) of strain/temperature sensing is decided by the sweep range of the tunable laser source, while this parameter is much larger in traditional scheme. However, better stability can be achieved in cross-correlation scheme for sharp varying strain/temperature.

Phase sensitive optical time domain reflectometry is suitable for vibration detection due to its high sensitivity. However, for conventional amplitude-measuring method, the strain-amplitude relation is nonlinear and the monotonic interval of strain-amplitude transfer function is very small (corresponding to less than $2\pi $ phase shift in light duration). Thus, the vibration detection is qualitative and it is difficult to distinguish different kinds of vibrations. In this letter, a quantitative vibration measurement by analyzing the differential phase in the interrogated fiber separation is proposed. A statistics calculating method is present to suppress the uncertainty in phase-measuring. Dynamic strain of 400 $\text{n}\varepsilon $ (peak-to-peak, corresponding to phase shift of 17 rad) was reproduced with high quality by proposed method.

The random noise and the slow drift, which respectively limit the signal-to-noise ratio and the long-term stability, of a wavelength modulated tunable diode laser absorption spectroscopy system, are critically analyzed and evaluated experimentally. The noise sources considered in detail include the noise contributions from the lock-in amplifier, detector-preamplifier combination noise, excess noise from the DFB laser, and detection cell instabilities, especially in multiple reflection long-path cells. We also analyze contributions to long-term drift and find that the drift may be significantly compensated by the use of an optical reference path. We find experimentally that a white cell with a path length of 17.5 m introduces excess noise resulting in its having a performance equivalent to 6-m single pass absorption path. The experimental system optimized using the results of this analysis and using the 17.5-m white cell is found to operate at a sensitivity of 1.5-ppmv carbon monoxide in a bandwidth of 4 Hz. The long-term zero offset drift levels measured over 8 h are found to be $\sim 120$ ppmv without the reference optical path and are reduced to 20 ppmv using the balance path technique.

We proposed and experimentally demonstrated a new scheme for enhancing the sensitivity of a fiber laser sensor using Brillouin slow light. The Brillouin laser was exposed to environmental vibrations, producing fluctuations at 408 kHz frequency, which were then interrogated using a Mach-Zehnder interferometer. By introducing Brillouin slow light into one arm of the interferometer, the sensitivity increased by 1.57 times that of a device without slow light. We believe this scheme may provide a new way of using Brillouin slow light and that it has some important implications regarding the use of fiber sensors for measuring the vibration, temperature, strain and so on.

A novel technique to perform dynamic strain measurement with a Brillouin optical time-domain reflectometry (BOTDR) sensing system based on short-time Fourier transform (STFT) is proposed. Unlike traditional frequency scanning methods, the proposed system samples the Brillouin backscattering signal directly after it has been downconverted to the intermediated frequency (IF) region. The Brillouin frequency shift (BFS) of the sensing fibre was reconstructed from the recorded time-domain data continuously without dead-time, which provided a much higher refreshing rate of the strain measurement. Therefore, dynamic strain variation over an optical fibre can be measured with a single-end structure that is more flexible and reliable. A 16.7 Hz strain variation on a 12 m section was successfully detected at the end of a 270 m sensing fibre with 4 m spatial resolution and 45 μe uncertainty.

Based on weak fluctuation theory, the expression of bit-error rate (BER) of Multiple Quadrature Amplitude Modulation (MQAM) with the consideration of detector noise in the downlink of space communication system is discussed in this paper. According to the expression, the performance of three typical modulation schemes, which are 4QAM, 16QAM, 64QAM, are specially analyzed. It is known that the higher the order of the modulation scheme is, the more bits of information per symbol can carry. However, when the transmission power is 1 W and the receiver diameter Dr is 1 m, the BER is 2.12×10 -13 for 4QAM, 5.98×10 -8 for 16QAM and 6.22×10 -5 for 64QAM, which means that a higher order modulation scheme shows a higher bit-error rate (BER). Thus considering bandwidth efficiency as well as bit error rate, 16QAM is highly recommended in the real space optical communication system. In addition, the relationships between BER and optimum divergence angle, transmitter beam radius, receiving aperture for downlink are also suggested respectively in this paper, which has important reference significance for the design of the ground-to-satellite laser communication system.

Space optical communication technique is attracting increasingly more attention because it owns advantages such as high security and great communication quality compared with microwave communication. As the space optical communication develops, people have already achieved the communication at data rate of Gb/s currently. The next generation for space optical system have goal of the higher data rate of 40Gb/s. However, the traditional optical communication system cannot satisfy it when the data rate of system is at such high extent. This paper will introduce ground optical communication system of 40Gb/s data rate as to achieve the space optical communication at high data rate. Speaking of the data rate of 40Gb/s, we must apply waveguide modulator to modulate the optical signal and magnify this signal by laser amplifier. Moreover, the more sensitive avalanche photodiode (APD) will be as the detector to increase the communication quality. Based on communication system above, we analyze character of communication quality in downlink of space optical communication system when data rate is at the level of 40Gb/s. The bit error rate (BER) performance, an important factor to justify communication quality, versus some parameter ratios is discussed. From results, there exists optimum ratio of gain factor and divergence angle, which shows the best BER performance. We can also increase ratio of receiving diameter and divergence angle for better communication quality. These results can be helpful to comprehend the character of optical communication system at high data rate and contribute to the system design.

As the technique of space optical communication is developing, the requirement of higher data rate has attracted more attention. Currently, low data rate has been successfully applied in communication system. The next generation for space optical system have goal of higher data rate of 10Gb/s. In this paper, bit error rate (BER) performance on minimum shift keying (MSK) scheme at 10Gb/s data rate in uplink of space optical communication system is analyzed. From simulation results, probability density function (PDF) of received optical power shows left shift with increase of divergence angle at 10Gb/s data rate. Besides, we can increase ratio of receiving diameter and transmission diameter or decrease ratio of zenith angle and transmission diameter for better communication quality. These results are helpful for comprehending characters of the uplink optical communication system at 10Gb/s and the design of communication system.

This paper designs and implies a high precision FBG demodulation system which based on F-P etalon. In order to reduce the influence of the temperature drift effect, the peristaltic effect, and the nonlinear effect of F-P filter in traditional tunable filter method, F-P etalon is added as dynamical calibration and wavelength reference. Meanwhile segmentation demodulation which uses ASE spectral characteristics is applied to achieve high accuracy of the center wavelength of FBG. The experiment shows that the stability, resolution are 0.65pm, 0.23pm, respectively. Key words: fiber optics; fiber Bragg grating sensor system; tunable Fabry-Perot filter; F-P etalon; spectrum segmentation demodulation

There have been a number of papers focusing on fiber distributed sensing with coherent Rayleigh backscattering published. However, up to now, very limited research articles on investigation of coherent Rayleigh backscattering signal waveform and its physical mechanism have been reported. This paper first proposes a theoretical derivation to illustrate coherent Rayleigh backscattering waveform. The theoretical model is then proved with numerical simulation and experimental measurement. In addition, signal processing method is an important factor on the performance of a phasesensitive OTDR system. An improvement of signal processing method, which is consisted of digital average, moving average and interval subtraction, with good effect on locating external perturbation is also introduced.

A novel method for online correction of light intensity fluctuation in a practical tunable diode laser absorption spectroscopy (TDLAS) system with wavelength modulation is presented. The proposed method is developed according to the linear relation between peaks at multiple frequencies of sine modulation in the power spectral density of the demodulated second-harmonic (2f) signal and the incident light intensity. Those peaks are demonstrated experimentally and explained as residual power at the first-harmonic and third-harmonic frequencies after 2f demodulation of the residual amplitude modulation signal due to the limited integrating time constant of the lock-in-amplifier. This method can achieve real-time correction of light intensity fluctuations with only little calculation. It can work well in a very large range of light intensity and has great potential applications in the wavelength modulation spectroscopy system.