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  • Sensitive Intrusion Detector Relies on Fiber Optic Reflectometer

Photonics Spectra
Jan 2006
Breck Hitz

Optical time-domain reflectometry (OTDR) is a well-established technique for detecting faults and discontinuities in long stretches of optical fiber. A short optical pulse is injected into one end of the fiber, and as it travels along the fiber’s length, it is partially reflected from each of the faults and discontinuities it encounters. The reflected pulse from a given fault eventually arrives back at the end of the fiber, where it is detected with a photodiode. The time between the original pulse’s injection and the reflected pulse’s return reveals the distance between the transmitter and the fault.

Recently, scientists at Texas A&M University in College Station harnessed a variation of the technique to build a sensitive intrusion detector. The system relies on a length of optical fiber buried around the perimeter of the location to be protected. They inject a laser pulse into one end of the fiber and, by monitoring the reflected light, can detect an intrusion anywhere along the perimeter.

The variation used in the Texas A&M detector is phase-sensitive OTDR. As the pulse travels through the fiber, different portions of it are backscattered from various locations and arrive simultaneously at the detector. These backscattered reflections interfere to produce a random, time-dependent signal at the detector. As long as the laser’s frequency is constant and nothing perturbs the fiber, this random signal is unchanged from one pulse to the next.

However, an intruder’s footsteps perturb the fiber’s length and/or refractive index sufficiently to change the backscattered signal. Thus, the operating principle of the intrusion detector is to compare the backscattered signals from successive pulses. Any difference between the signals indicates an intruder.

Sensitive Intrusion Detector Relies on Fiber Optic Reflectometer
Figure 1. The phase-sensitive optical time-domain reflectometer detected an intruder stepping on, or close to, the buried optical fiber. Images ©OSA.

The researchers demonstrated how their intrusion detector might protect an area with a 12-km perimeter, although they buried only 44 m of optical fiber. The rest of the fiber remained coiled on reels (Figure 1). The single-frequency fiber laser was gated with an electro-optic modulator to inject 2-µs pulses into the 12 km of fiber. The backscattered pulses were separated from the outgoing pulses with a directional coupler and directed to the detector. In a further refinement of the technique, the receiver monitored the two orthogonal polarizations simultaneously using two photodetectors.

A potential hindrance to the commercial application of such a system is the requirement for a frequency-stable, single-mode laser. In their demonstration, the scientists used a homemade, erbium-doped fiber laser in a thermally insulated box, which produced 500 µW at 1555.4 nm. The light was then amplified with an erbium-doped fiber amplifier so that 3-mW pulses were injected into the sensing fiber.

Sensitive Intrusion Detector Relies on Fiber Optic Reflectometer
Figure 2. Each of these three plots shows the backscattered signal without an intruder (blue trace) and with one (red trace). Also shown is the difference between the two. The upper two plots are for the two orthogonal polarizations, and the bottom trace is the sum of the top two; that is, the bottom trace is for unpolarized light.

A comparison of the backscattered signal with and without an intruder present clearly shows his presence (Figure 2). The value of detecting the polarizations separately is obvious in these data, as the difference is much more discernible in either polarization than in the unpolarized light.

optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
optical time-domain reflectometry (otdr)
A method for characterizing a fiber wherein an optical pulse is transmitted through the fiber, and the resulting light scattered and reflected to the input is measured (by an optical time-domain reflectometer, or OTDR) as a function of time.  
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