Yongkang Dong

Time-division multiplexing-based BOTDA over 100km sensing length

We propose and demonstrate a high-performance and long-range Brillouin optical time-domain analysis (BOTDA) based on time-division multiplexing measurement, where a probe pulse and a pump pulse are used to perform the measurement on a selected sensing section, and the measurement of the entire sensing fiber is realized by combining the series measurements over different sections through changing the delay time between the two pulses. In experiment, a 100 km sensing fiber is divided into 11 sections based on the gain-controlled principle, and spatial resolutions of 0.6 m and 2 m are obtained at the end of 75 km and 100 km, respectively.

2 cm spatial-resolution and 2 km range Brillouin optical fiber sensor using a transient differential pulse pair

We report a high-spatial-resolution and long-range distributed temperature sensor through optimizing differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). In DPP-BOTDA, the differential signal suffers from a signal-to-noise ratio (SNR) reduction with respect to the original signals, and for a fixed pulse-width difference the SNR reduction increases with the pulse width. Through reducing the pulse width to a transient regime (near to or less than the phonon lifetime) to decrease the SNR reduction after the differential process, the optimized 8/8.2 ns pulse pair is applied to realize a 2 cm spatial resolution, where a pulse generator with a 150 ps fall-time is used to ensure the effective resolution of DPP-BOTDA. In the experiment, a 2 cm spatial-resolution hot-spot detection with a 2 °C temperature accuracy is demonstrated over a 2 km sensing fiber.

Truly distributed birefringence measurement of polarization-maintaining fibers based on transient Brillouin grating.

We report on what we believe to be the first truly distributed birefringence measurement of polarization-maintaining fibers (PMFs) based on transient Brillouin grating (TBG). A TBG is created by two short pump pulses in the slow axis of the PMF, and then the birefringence-related TBG spectrum is mapped by scanning a probe pulse launched in the fast axis, where the local birefringence can be calculated using the birefringence induced frequency shift. Two types of widely used PMFs, bow-tie and panda, with a length of 8 m were measured at a spatial resolution of 20 cm, and the results show that the birefringence features a periodic variation, and their variation ranges are approximately 2.4x10(-6) and 1.3x10(-6) along the test fibers, respectively.

Distributed temperature sensing based on birefringence effect on transient Brillouin grating in a polarization-maintaining photonic crystal fiber

We demonstrate a time-domain distributed temperature sensing based on birefringence effect on transient Brillouin grating (TBG) in a polarization-maintaining photonic crystal fiber (PM-PCF), which uses two short pump pulses (2 ns) to excite a TBG and a long probe pulse (6 ns) to map the transient Brillouin grating spectrum (TBGS) associated with the birefringence. The 2 ns pump pulses defines a spatial resolution of 20 cm and a temperature measurement range of a few hundred degrees Celsius, and the long probe pulse provides a narrow TBGS with a temperature resolution of 0.07 degrees C.

High-Spatial-Resolution Fast BOTDA for Dynamic Strain Measurement Based on Differential Double-Pulse and Second-Order Sideband of Modulation

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis (BOTDA) for distributed dynamic strain measurement based on differential double-pulse and second-order sideband of modulation. The frequency-agility probe wave is obtained from the second-order sideband of the modulated light by using the microwave signal from a wideband arbitrary waveform generator (AWG), which reduces the bandwidth requirement of the AWG by half to ~ 5.5 GHz. The differential double-pulse scheme is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda polarization-maintaining fiber observing the vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is measured to be 14 με.

Extending the Sensing Range of Brillouin Optical Time-Domain Analysis Combining Frequency-Division Multiplexing and In-Line EDFAs

We demonstrate a high-performance Brillouin optical time-domain analysis (BOTDA) system with an extended sensing range by combining frequency-division multiplexing and in-line Erbium doped fiber amplifiers (EDFAs). The frequency-division multiplexing BOTDA features multiple sections of fibers with different Brillouin frequency shifts, and it reduces the effective Brillouin interaction length to one resonant Brillouin frequency section rather than the entire length of the sensing fiber, so that the power of CW probe of BOTDA can be increased to enhance the Brillouin signal within individual sections and consequently extend the sensing range combined with high strain or temperature resolution with negligible pump depletion. In addition, in-line EDFAs placed between spans are used to compensate the fiber loss for similar Brillouin gains in each span. In experiment, a 150-km sensing range is achieved by dividing the sensing fibers into two spans of equal length and using two types of fibers in each span. Using the differential pulse-width pair technique, a 100/120 ns pulse pair is used to realize a 2-m spatial resolution and a measurement accuracy of 1.5°C/30 με at the end of the sensing fibers.

High-Spatial-Resolution Time-Domain Simultaneous Strain and Temperature Sensor Using Brillouin Scattering and Birefringence in a Polarization-Maintaining Fiber

We report on a high-spatial-resolution simultaneous strain and temperature measurement in time domain through measuring Brillouin frequency shift (BFS) and birefringence-induced frequency shift (BireFS) in a polarization-maintaining fiber. High-spatial-resolution BFS and BireFS measurement are obtained through differential pulsewidth pair Brillouin optical time-domain analysis and a local Brillouin grating, respectively. Simultaneous strain and temperature measurement with a spatial resolution of 20 cm is demonstrated in a 6-m Panda fiber. The temperature and strain accuracy is 0.4°C and 9 με, and their range can be up to 700°C and 14 mε, respectively.

Slow light in multi-line Brillouin gain spectrum

We present a method to achieve flat-top gain spectrum through overlapping multiple gain lines, which can be used to increase slow light bandwidth and relative pulse delay. A tunable gain bandwidth can be realized by changing the number of spectral lines and frequency separation between adjacent spectral lines. We demonstrate the method in a SBS-based slow light system. A phase modulator is used to modulate the phase of the pump wave, generating a pump wave with multi-line spectrum and achieving a Brillouin gain bandwidth of ~ 330 MHz.

Differential Brillouin gain for improving the temperature accuracy and spatial resolution in a long-distance distributed fiber sensor

We demonstrate a 12 km differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) using 40 ns and 50 ns pulses with DC-coupled detection. A spatial resolution of 1 m and a narrowband Brillouin gain spectrum of 33 MHz are obtained simultaneously compared with 88 MHz with the use of 10 ns pulses in a conventional BOTDA. The experimental results show that the differential Brillouin gain of a 40/50 ns pulse pair is 7 times stronger than the direct Brillouin gain of BOTDA with the use of a 10 ns pulse, and the temperature uncertainty is 0.25 degrees C compared with 1.8 degrees C for a 10 ns pulse. As the pulse-width difference decreases from 10 ns to 1 ns, corresponding to a spatial resolution from 1 m to 10 cm, the prediction of temperature uncertainty will only increase from 0.25 degrees C to 0.8 degrees C for DPP-BOTDA over a 12 km long single-mode fiber.

Experimental study on stimulated Rayleigh scattering in optical fibers

The linewidth, the threshold, and frequency shift of the stimulated Rayleigh scattering (STRS) in single mode fiber (SMF-28e), large effective area fiber (LEAF) and polarization maintaining fiber (PMF) have been studied using heterodyne detection to separate the Brillouin scattering with a fiber laser for the first time to the best of our knowledge. Experimental results show that the linewidth of STRS and spontaneous Rayleigh scattering are ~9 kHz, ~10 kHz, and ~11 kHz, and ~25 kHz, ~30 kHz, and ~27 kHz for SMF-28e, LEAF and PMF, respectively. The threshold power for STRS for 2 km SMF-28e, 7 km LEAF, and 100 m PMF are 11 dBm, 4.5 dBm and 16.5 dBm, respectively. The measured Rayleigh gain coefficient is a 2 × 10(-13) m/W for SMF-28e. Also, weak frequency shift could be observed when input power is large enough before SBS occurred. Because of the properties of narrower bandwidth and lower threshold power of STRS in fibers, some of applications, such as narrower filter, could be realized.