Fiber-Optic Temperature Sensor Using Fluorescence Decay Time

Many different principles for temperature measurement with fiber-optic sensors have been suggested and brought about experimentally. If we restrict ourselves to the use of sensors employing fluorescence decay time, the first proposal was made by K.A. James et al. (1). They suggest exciting fluorescence by a flash and measuring the intensity of the decaying fluorescent light and determining the point of time at which the intensity has decreased to 1/e of the starting value. Except if laser flashes are used for excitation, measurement according to this principle will surely be very inaccurate as a result of noise. McCormack (2) sinusoidally modulates the exciting light with a fixed frequency and measures the phase shift of the fluorescent light sinusoidally oscillating at the same frequency. The accuracy achievable referred to the intensity of the exciting light is probably better; however, it is difficult to measure any phase shift very exactly.