Fiber Optic Temperature Sensors

Fiber optic temperature sensors are used where the temperature measurement is made in hostile environments, in the presence of electromagnetic, chemical and mechanical disturbances.

Why use fiber optic sensors?

Transducers, such as thermocouples and resistance temperature detectors (RTD), do not always produce satisfactory performance, for example, when the fluid temperature must be measured in hostile environments, in the presence of electromagnetic, chemical and mechanical disturbances.

Under circumstances such as extreme marine environments and underground geological sites where accurate long distance measurement is required.  For such applications, fibre optical sensors offer a better alternative since the optical signal does not suffer from interference by electromagnetic fields and can be transmitted over extremely long distances without any significant loss.

Measurement using fiber optic displacement sensor:

 

The sensor is essentially a FODS with a 3 dB multimode fiber coupler as a probe. A 594 nm He-Ne beam is launched to port 1 of the coupler. The light travels to port 3 and disperses when it leaves the end of the fiber. It is then reflected by the upper surface of an aluminum rod with dimensions of 0.5 cm in diameter and 7 cm in length.

The port 3 probe is held in the approximately 1 mm position perpendicular to the upper surface of the aluminum rod, so that the reflected light can easily be reintroduced into the same port. The collected light is sent to port 2 via the 3dB coupler and measured with a silicon photodetector. The detector converts the light into an electrical signal, which is then processed by the blocking amplifier and finally visualized and stored in the computer.

 

Fiber optic temperature sensor based on lifetime measurement:

Fluorescence-based sensors are widely used for measuring various parameters due to its relatively independent of ambient conditions.

The 980nm laser pump beam is launched into a piece of 90 cm long Erbium-doped fiber (EDF) via a wavelength division multiplexing (WDM) coupler. The EDF is placed in a vacuum oven which allows us to vary the fiber temperature within 25 to 200oC interval.

The fluorescence signal from the forward pumped EDF is detected by a Ge photo-detector and processed with a digital oscilloscope. The 980 nm pump beam is chopped so as to generate a square-wave modulated signal with pulse width of 2.2ms.

The temperature sensing mechanism in this work is based on the temperature dependence of the Erbium fluorescent lifetime decay. The Erbium fluorescence lifetime is measured using a modulated pump laser with power of approximately 2 mW and the results for temperature measurements of 85°C and 130°C

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