Fiber optic temperature sensor with duplex Michleson interferometric technique

https://doi.org/10.1016/S0924-4247(00)00357-5Get rights and content

Abstract

An interferometric fiber optic temperature sensing system employing a light emitting diode (LED) as the broadband optical source and a single mode fiber coil, which was fabricated in definite length as the sensor head, is described. The fiber transmitting line back and forth along the same path is used, so the changing caused by the environmental temperature fluctuation of the fiber path can automatically be compensated. The sensitivity of the sensing system can be easily improved by using the long length of the sensing fiber. The experimental result of the sensing gauge length vs. the sensitivity is given. A typical wavelength 1300-nm LED source is used in the experiments. The experimental curve of the resolution characteristic of the system related with the length of sensing fiber coil is also discussed.

Introduction

Fiber optic sensors are being widely developed because they have well known advantages over conventional sensor [1]: immunity to electromagnetic interference, ability to operate in hostile environment, high sensitivity, and potential for multiplexing. The fiber's small dimension and high mechanical strength also enable them to be used as embedded sensors for concrete structures [2] or other composite materials [3]. Recently, semiconductor lasers are used as sources of interferometric fiber sensors since they provide small size, low cost and lower electrical power consumption than other type of lasers. However, from the standpoint of practical application there are some serious drawbacks in using semiconductor lasers. It may be necessary to insert an expensive Faraday isolator between the laser and the sensing system to eliminate feedback-induced mode hopping and line broadening. Also thermally induced wavelength shifts in the laser emission cause changes in the interferometer operating point that must be compensated in some way.

This paper reports an optical path automatic compensated Michelson interferometer fiber optic temperature sensor with a low coherence light source such as light emitting diode (LED) or Superluminescent Diode (SLD). A typical wavelength, 1300 nm LED source and a series different length fiber as sensing head were used to investigate the sensitivity and resolution characteristics of the system. The system uses a reference fiber, which was connected to a 3-dB coupler loop, the loop length nearly twice the sensing fiber length. Also, the fiber transmitting line goes back and forth along the same path. Therefore, it becomes possible to eliminate the environmental temperature variation and other influence caused by transmitting fiber line.

Section snippets

Duplex fiber optic Michelson interferometer for temperature measurement

In a duplex Michelson white light interferometer arrangement, light from an LED is split into two beams as shown in Fig. 1. One of the beams travels along the sensing arm L1 runs into the first reflective surface of the sensor and then an optical signal 1 is reflected, while the path length of the other beam called the referencing arm L2 is adjusted by a scanning mirror (see Fig. 1(b)). Then, the optical signals reflected from the scanning mirror and the sensor's first reflective surface are

Reflective signal from sensing branch:

For the fiber optic sensing system, assume that the light intensity from the LED source lunched into the fiber is I0. A 3-dB coupler splits it into two branches. In the sensing branch, the light intensity is I0α1/2. Here α1 represents the 3-dB coupler insertion loss parameter definite by αi=10δi/10 and δi is insertion losses of the coupler. This intensity incident into the sensing part: a segment fiber. At fiber end surface, the light partly reflects and partly transmits. If the reflectively

Sensing system configuration

A diagram of the duplex white light source Michelson interferometer fiber optic temperature sensor system is shown in Fig. 4. The system is comprised of an LED, a fiber optic coupler for separating and recombining the light, two optical fiber arms with reflective surface ends, and a scanning mirror mounted on a stepper moving stage.

The length of the fiber optic sensor arm is fixed, and the reference arm is made slightly shorter than the sensing arm. Separate beams of light travel through the

Experimental results

To test the sensor performance, the coupler loop with reference fiber length L was coiled and maintained in a chamber at a constant temperature, T0=38.5±0.1°C, while the fiber sensor coil was heated in an oven. A thermocouple located near the fiber sensor was used to independently monitor its temperature. As the sensor coil was heated or cooled, the optical path variation was recorded, as shown in Fig. 5. For the fiber sensor, with a coil length of 925 mm and a wavelength of 1300 nm, the linear

Conclusion

Generally LED provides an advantage in short-distance fiber optic communications over semiconductor laser light sources. Similar benefits may be realized through the use of LED in interferometric fiber optic sensor system. Interferometric performance has been demonstrated in fiber optic temperature sensor with an interferometer cavity length much greater than the coherent length of the LED light source. The sensor makes use of duplex fiber optic Michelson interferometer, which demodulates the

Acknowledgements

This work was supported by both the National Natural Science Foundation of China (Grant No. 5987903) and the Research Grant Council of Hong Kong (Grant No. Polyu5121/98E).

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