Sensitivity of fiber optic sensor is critical concern, in case of concealed application such as buried fiber-optic sensor,
because ground soil behaves as buffer to isolate from outside disturbance by intruder. Thus coherent intrusion sensor is
getting popularity where the sensitivity is primary concern, since the superior sensitivity comes from optical phase
relationship of two received light signal. The performance of coherent sensor depends on the coherent degree of light
source. Conventional laser light source is not sufficient for this application, thus enhancing technique to increase
coherence length is required to have stable sensor response. Fiber laser is the suitable candidate because emission spectra
can be adjusted to the wavelength where transmission loss through glass fiber medium is at its minimum. The
performance of erbium doped fiber laser with single optical feedback loops are compared with the one using selfinjection
locked DFB laser experimentally in Mach-Zehnder interferometer configuration. The response of coherent
fiber-optic sensor, in Mach-Zehnder interferometer configuration, using erbium doped fiber laser with single optical
feedback loops as a light source is attempted to the various stimulus, including pressure, open flame and vehicle passing,
for the purpose of evaluating sensor application. The possible application of this fiber laser with enhanced coherence is
in zone perimeter security within ~20km where buried fiber-optic sensor installation is necessary.
The optical fiber speckle sensor is attractive candidate in the security area involving wire net fence. A single fiber detection zone can be as long as few kilometers and also high sensitivity can be obtained using a simple optical configuration. The drawback in the application using wire net fence is its high false alarm rate caused by the environment effect such as wind. In this paper, a new algorithm to reduce the false alarm rate in optical fiber speckle sensor for wire net fence application was proposed. The generated signal from speckle sensor was compared with expected values in time domain response. Also the event number was compared with the probability of detection. The optical fiber speckle sensor having signal filter and signal processor has been fabricated and investigated experimentally. The custom filter was found to be effective in environmental noise reduction, however the signal processor played the key role to prevent fault burglar detection crossing the wire net fence. The monitored result using calibrated hardware and algorithm showed effective blocking of false signals while showing the reliable burglar detections. The possible application of this optical fiber speckle sensor using the proposed algorithm is expected in security area where vibration or strain is the major impact to the sensor.
The use of an optical fiber as a distributed sensor for detecting, locating, and (with suitable signal processing) classifying intruders is proposed. Phase changes resulting from either the pressure of the intruder on the ground immediately above the buried fiber or from seismic disturbances in the vicinity are sensed by a phase-sensitive optical time-domain reflectometer (Φ-OTDR). Light pulses from a cw laser with a narrow (kHz range) instantaneous linewidth and low (MHz/min range) frequency drift are injected into one end of the single mode fiber, and the backscattered light is monitored with a photodetector. Results of analyses and experimental studies to establish the feasibility of the concept are described. Simulations predict a range of 10 km with 35 m range resolution and 30 km with 90 m range resolution. Experiments indicate adequate (several π-rad) phase changes are produced by intruders on foot for burial depths in the 20 - 40 cm range in sand and in clay soils. A phase perturbation in a fiber has been detected and located in a laboratory demonstration of the Phi-OTDR using an Er:fiber laser as the light source. This technology could in a cost-effective manner provide enhanced perimeter security for nuclear power plants, electrical power distribution centers, storage facilities for fuel and volatile chemicals, communication hubs, airports, government offices, military bases, embassies, and national borders.
A cross-correlation technique for the measurement of timing jitter of femtosecond light pulses in time domain is presented. This measurement technique uses an optical mixing process in a scanned Michelson interferometer, which is simple in application but has femtosecond temporal resolution. The width of autocorrelated pulse and the width of pulse by cross- correlating with the next pulse following after the external cavity round trip time were measured. Timing jitter of the light pulses from the actively mode-locked semiconductor laser was measured by comparing the above two pulse widths, and the timing jitter was found to be approximately 15 fs for a pulse having approximately 270 fs FWHM width.