A novel tunable and stable frequency fiber laser with ~4 kilohertz linewidth based on stimulated Rayleigh scattering
(STRS) in non-uniform optical fiber by using a broadband light source and a tunable filter is proposed in this paper for
the first time to our knowledge without ring laser configuration. Because of the variable core size and dispersion along
the fiber for non-uniform optical fiber, the threshold value of stimulated Brillouin scattering (SBS) was increased by
~7 dB; hence the phenomena of STRS can be clearly demonstrated. The frequency stability of the tunable laser only
depends on the tunable filter because the frequency of Rayleigh scattering in optical fiber is not sensitive to the change
of environmental conditions. Such kind of narrow tunable laser sources could find widely applications in optical
communication, optical sensing, optical precision metrology, and high-resolution spectroscopy.
KEYWORDS: Signal to noise ratio, Signal detection, Ferroelectric materials, Heterodyning, Signal processing, Modulation, Data acquisition, Acoustic emission, Refractive index, Bridges
A coherent phase-sensitive OTDR system to detect distributed weak vibration is reported via heterodyne detection
combined with moving averaging and moving differential signal processing. The location and high frequency response
of weak vibration signal, such as pencil-break vibration which would match the characteristics of crack as a source of an
acoustic emission signals, are easily monitored. Our experimental results show that spatial resolution of vibration is
about 5m with 50 ns pump pulses, and the signal to noise ratio could be up to 6.5 dB. Also the frequency response could
be up to 1 KHz. Both consecutive multiple events at the same location and multiple simultaneous events at different
location with frequency components are identified which means our vibration testing system could be used for
distributed multiple vibration events detection.
The linewidth and the threshold 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 2km
SMF-28e, 7km LEAF, and 100m PMF are 11dBm, 4.5dBm and 16.5dBm, respectively. The measured Rayleigh gain
coefficient is a 2×10-13m/W for SMF-28e. 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.
A novel tunable Fabry-Perot (F-P) filter is reported in this paper. The interference cavity is the hollow core of hollowcore
photonic bandgap fiber (HC-PBF), which supports fewer modes than the conventional cavities. One of the reflection
mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other one is a cleaved end of a
section of micro-fiber, which is inserted into the hollow core of HC-PBF. The cavity length of the F-P device can be
tuned by adjusting the position of the micro-fiber in the hollow core. Because of the low loss of HC-PBF, the F-P cavity
length can reach several millimeters, even up to the order of centimeters, which results in its very narrow linewidth and
high multiplexing capability. The experimental results show that the F-P filter has a fringe contrast of over 4 dB over a
wide wavelength range, which can be used as the tunable filter in a fiber laser to obtain a very narrow linewidth.
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