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Where There's Smoke, It's Fiber to the Rescue

Photonics Spectra
Mar 2002
Brent D. Johnson

A research team at the St. Petersburg State University of Aerospace Instrumentation, led by Sergei V. Kulakov, has demonstrated the feasibility of a fiber optic fire sensor that can be used in environments where radiation or chemicals may pose a significant danger. The device is intended for use in facilities that produce and store explosives and/or flammable liquids such as oil and gas, and in areas that may contain explosive dust and gas mixtures. Optical fiber is well-suited to these environments because it is immune to radiation and aggressive chemical substances.

Researchers in Russia are studying the feasibility of a fiber optic sensor for detecting fires in hazardous environments.

The fire-sensing system uses a modulated optical beam that is emitted from a Volga Technology ILPN 820 semiconductor laser at a wavelength of 0.82 µm. It travels across open space in the high ceilings of warehouses and is intercepted by a photodiode with a band preamplifier that has a maximum gain of 70 dB. If smoke appears, the intensity of the optical signal decreases, triggering the alarm.

An advantage of this system is that it can be used with optical reflectors to cover areas as large as 1800 square meters. Also, the emitting and receiving modules can be isolated from the amplifier, comparator and alarm signal trigger, which could be affected by emissions from the hazardous environment or from explosions. These components can be kept kilometers from the site, isolating them from possible contamination or damage that could interfere with their performance.

Kulakov and his colleagues, Oleg D. Moskaletz and Leonid N. Preslenev, selected the ILPN 820 semiconductor laser because of the high efficiency of the connection with the optical fiber. When power in the optical fiber was approximately 5 mW, the output amplitude of the receiving module was 2.4 V. Visually detected "fast" amplitude fluctuation of the carrier was less than 0.1 V, approximately 4 percent.

The only drawback to the arrangement is that, when the optical beam propagates in free space, it becomes distorted by the airflow, thermal gradients and dust. After passing a considerable distance through free space, the beam must be sent through a large-aperture multimode optical fiber.

Although the device is not intended as a replacement for conventional alarm systems, Kulakov found that, based on his prototype, an effective explosion-safe fiber optic smoke detector with an optical beam length of 100 m and a sensitivity similar to point smoke fire detectors is feasible.

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