High-speed railway is being developed rapidly; its safety, including infrastructure and train operation, is vital. This paper presents a railway-subgrade vibration monitoring scheme based on phase-sensitive OTDR for railway safety. The subgrade vibration is detected and rebuilt. Multi-dimension comprehensive analysis (MDCA) is proposed to identify the running train signals and illegal constructions along railway. To our best knowledge, it is the first time that a railway-subgrade vibration monitoring scheme is proposed. This scheme is proved effective by field tests for real-time train tracking and activities monitoring along railway. It provides a new passive distributed way for all-weather railway-subgrade vibration monitoring.
We demonstrate a narrow-linewidth laser source for high spatial resolution distributed optical sensing by utilizing the high-order modulation sidebands injection locking. A pair of phase-locked lasers with arbitrary frequency offset from 5 GHz to 50 GHz is generated. Meanwhile, a linearized frequency sweep covering range of 15 GHz in 6 ms with frequency errors of 240 kHz from linearity is also achieved using the same scheme, the instantaneous linewidth of the frequency-swept laser is measured to be ~2.5 kHz.
Polarization fading is a phenomenon observed often in BOTDR distributed sensors, which greatly reduces signal-to-noise ratio of the detected signal. We proposed a scheme based on injecting optical pulse probes with orthogonal polarization states, which is generated by a delay Mach-Zehnder interferometer (MZI) composed of two polarization beam splitters (PBS). The principle is analyzed and the effect of reducing polarization fading is demonstrated experimentally. The method uses simple and passive components and is suitable for practical applications.
A novel high sampling rate multi-pulse phase-sensitive OTDR (Φ-OTDR) employing frequency division multiplexing (FDM) is proposed to increase the sampling rate of the long distance sensor system. Compared with the conventional Φ- OTDR, the new system owns much higher detection bandwidth as more probe pulses are allowed simultaneously traveling in the sensing fiber. The feasibility of the technique is experimentally verified. By multiplexing four different frequencies, we realize a experimental system with 20kHz vibration detection bandwidth over 10km sensing range.
The signal noise ratio (SNR) enhancement of spontaneous Brillouin scattering spectrum on Brilloluin optical time
domain reflectometry (BOTDR) sensing system have been demonstrated experimentally through changing the pulse
shape. With the same pulse width, the SNRs of the coherent detection power spectrum for trapezoidal pulse and
triangular pulse increase relative to that of rectangular pulse. The sensing distances are also increased. This will be
helpful to improve the spatial resolution or achieve longer sensing distance in the BOTDR sensing system.
A phase-shifted double pulse method is proposed to reduce the influence of inner-pulse interference induced fading on
phase demodulation of the Φ-OTDR. The feasibility of the technique is experimentally verified and the measurement
resolution of the Φ-OTDR is minimized to as low as 0.1rad by using this technique. The experimental system
demonstrates a distributed phase monitoring capability over 4km range with SNR of >20 and detection bandwidth
A compact short-cavity fiber laser configured with
Er3+/Yb3+ highly co-doped phosphate glass fiber which has linear
polarization and single frequency output is fabricated experimentally. The threshold power of the laser is about 30mW,
and larger than 100mW output power is achieved with slope efficiency of 20% at 1549nm. At the meantime, sine
modulation, positive pulse and chaos state in the output power at different experimental condition are observed. And a
new theoretical model is proposed to describe the mechanism of the observed intensity instability behavior in the fiber
laser and the numerical results proved its feasibility. It is confirmed that the self-pulsing behavior is mainly caused by
different small external feedback. So there is an efficient way to overcome self-pulsing behavior in the compact single
polarized fiber laser by minimizing the external feedback.
The digital coherent detection method is employed into the φ-OTDR. The heterodyne detection offers very high optical
gain while the digital signal processing serves as an effective tool to rebuild the instantaneous electric field of Rayleigh
scattering light by analyzing the beating signal. Both amplitude and phase signal are obtained in our experiment. PZT
vibration measurement verifies that the phase difference signal well represents the external perturbation signal and also
with higher SNR. The proposed newφ-OTDR system shows a good application foreground in the area of distributed
A local oscillator for coherent detection of backward Brillouin scattering in Brillouin optical time domain reflectometry
(BOTDR) has been analyzed. A ring Brillouin fiber laser, whose Brillouin gain media is 70m high-nonlinear-fiber
(HNLF), is used as local oscillator of coherent detection. The BFL operates at 1549.06nm red-shifted 0.084nm from the
pump laser. As to Brillouin light, The detection frequency is reduced from ~11GHz of direct detection to ~420MHz of
heterodyne detection in this paper. Self-lasing cavity-modes of BFL impose the "burr" intervalled at 2.5MHz on the
frequency domain analysis of the beat-frequency siganl. Signal-to-noise ratio (SNR) of beat-frequency signal decreases
greatly, resulting to Lorentzian fitting with error. By adjusting variable optical attenuator (VOA) to increase the cavity
loss in the fiber ring cavity, the self-lasing cavity-modes will be eliminated and a stable Brillouin laser will be obtained.
The frequency estimation accuracy is improved greatly.
A novel scheme is proposed to generate a millimeter-wave (MMW) optical pulse by combining optical time division
multiplexing (OTDM) technology and temporal Talbot effect (TTE). A n:1 time multiplexer is used for OTDM, and an
LCFG plays a role of TTE. The basic principle is analyzed by using a Gaussian input short pulse, and its characteristics
are discussed by numerical simulation. It is shown that the proposed scheme is feasible for MMW signal generation and
has potential merits for practical application of Radio over Fiber (ROF) technology.
Non-line-of-sight (NLOS) optical scattering communication (OSC) is studied theoretically and experimentally. Making
use of single scattering propagation model, properties of NLOS optical scattering channel are simulated numerically under some
typical condition. The results show that the path loss of the channel is quite large, and becomes larger as apex angle of the transmitter
and receiver increases. The results also show that the pulse transmitted from the source is broadened significantly after propagating in
the NLOS optical scattering channel. It will limit the available bandwidth of the channel, and probably cause intersymbol interference
in digital communication systems. Moreover, some elementary experimental facilities of NLOS UV communications are constructed.
A UV digital communication system based on 254nm low pressure mercury lamp has been set up, and the BER of the system is about
~10-4 when the transmitter apex angle is 60 degree and bit rate is 1200bits/s. and NLOS light propagation experiments were conducted
by exploiting a 370nm UV light-emitting diode (LED). With the progress of devices based on semiconductor in UV band, NLOS
optical scattering communication with small volume and low power may be achieved in future.