The spatial-resolved capacity of a phase-sensitive optical time-domain reflectometry ( $\varphi $ -OTDR) is limited by the optical pulse width, which increases the localization errors when measuring vibrations with range shorter than that determined by the pulse width. To overcome this problem, a high spatial-resolved $\varphi $ -OTDR with a differential pulse width sequence is proposed. The centimeter spatial resolution sensing can be realized by using meter equivalent pulses. This scheme utilizes the time-domain waveform subtraction obtained from the same pulse repetition rate and different pulse widths to form the differential vibration information at each fiber location. First, we provide a comprehensive treatment to prove that the proposed scheme can improve the spatial-resolved capacity of the $\varphi $ -OTDR system. The spatial resolution of 1.08m for 60/50ns pulse sequence is realized with a 30cm vibration source. Second, in a further proof-of-concept experiment, the spatial resolution is achieved up to 0.55m with a 55/50ns pulse sequence. It is demonstrated that the ability of spatial-resolved is enhanced greatly without any hardware modifications, compared to that required in the conventional method. This method holds great significance in accurately locating vibration signals within the narrow range of the distributed vibration sensor.

This paper reports an intracavity terahertz parametric source based on the stimulated polariton scattering in RbTiOPO4 crystal with a high repetition rate and a high average power. The side pumping for the gain medium by laser diodes is adopted to get a higher fundamental power and a larger fundamental laser beam size than those in the diode-end-pumping in order to improve the THz output power. A non-collinear convex-plane fundamental cavity is adopted so that the thermal effect can be offset in some degree and the fundamental beam size is further increased. The obtained maximum average THz output power is 367 μW at 3.88 THz when the diode pump power is 105 W and the pulse repetition frequency (PRF) is 7 kHz. The terahertz power of 367 μW is the highest ever obtained in SPS sources.

The linear demodulation range and background noise of the Michelson interferometer system are investigated with a laser phase noise measurement system. We have theoretically and experimentally analyzed the performance of the interferometer system by changing the frequency modulation amplitude of the laser and the optical path difference (OPD) of the interferometer, respectively. It is shown that the linear demodulation range of the Michelson interferometer system is finite, which depends on the parameters of the system, such as the sample frequency, the delay time between two interferometer arms, and the system bandwidth. Furthermore, the experimental results indicate that the background noise of the interferometer system can be reduced by using a sufficiently long OPD so that the smaller true phase information can be detected with the demodulation system. The parameters of the measurement system could be optimized to satisfy the demand of the phase demodulation with different levels, which is of great significance for the phase monitoring interrogator, such as fiber-optical interferometer sensors and distributed acoustic sensors.