The round trip time of the light pulse limits the maximum detectable frequency response range of vibration in phase-sensitive optical time domain reflectometry (φ-OTDR). We propose a method to break the frequency response range restriction of φ-OTDR system by modulating the light pulse interval randomly which enables a random sampling for every vibration point in a long sensing fiber. This sub-Nyquist randomized sampling method is suits for detecting sparse-wideband- frequency vibration signals. Up to MHz resonance vibration signal with over dozens of frequency components and 1.153MHz single frequency vibration signal are clearly identified for a sensing range of 9.6km with 10kHz maximum sampling rate.
An in-fiber whispering gallery mode resonator fabricated by femtosecond laser micromaching is demonstrated. The cylinder resonator cavity is fabricated by scanning the D-fiber cladding with infrared femtosecond pulses along a cylindrical trace with radius of 25μm and height of 20μm. Quality factor on the order of 103 is achieved by smoothing the cavity surface with ultrasonic cleaner, which is mainly limited by the surface roughness of hundreds nanometers. Resonant characteristics and polarization dependence of the proposed resonator is also studied in detail. Our method takes a step forward to the integration of whispering gallery mode resonators.
A core-cladding-mode interferometer (CCMI) is fabricated by splicing a single-mode fiber (SMF) to an endlessly singlemode photonic crystal fiber (ESM-PCF). The optimum collapsed length of ESM-PCF is investigated to ensure a higherorder cladding mode of ESM-PCF to be excited, serving as a sensing beam. At the end of ESM-PCF, a spherical endfacet with big radius is proposed to ensure from where the higher-order cladding mode to be reflected and then it interferes with the core mode serving as a reference beam. Such a device has an enhanced refractive index sensitivity of ~199 nm/RIU, offering potentials in biological and chemical applications.
In conventional phase-sensitive optical time domain reflectometry (φ-OTDR), the length of sensing fiber mainly
determines the repetition rate of probe light pulses, which limits the extent of detectable frequency range. Moreover,
averaging method, which is adopted to enhance the location signal-to-noise-ratio (SNR), further decreases the maximum
detectable frequency. This paper demonstrates a distributed vibration sensor with satisfied location SNR and extended
frequency response range by using a probe pulse pair with a frequency difference. Experimental results show that this
method is able to break the trade-off between the given sensing fiber length and the traditional maximum detectable
frequency response of φ-OTDR system.