Optical fiber displacement sensor and its application to tuning fork response measurement

Optical sensors based on a knife edge obstruction represent an inexpensive method for stable, high signal to noise ratio (SNR), variable measurable range, low moving mass, compact size and high bandwidth displacement measurement. Two displacement sensor implementations have been evaluated in terms of noise and sensitivity as a function of the proximity of the knife edge relative to the source. Using a pigtailed laser diode and a photodiode as detector, the displacement of a knife blade across the output from the fiber varies the intensity of the laser transmitted to the photo detector. With this design, the range of the sensor is determined by the fiber diameter. This is compared to an inexpensive, LED opto-interruptor and knife edge. The results show that laser diode and fiber illumination source sensor having an SNR of 40,000 at 1 kHz band width with working ranges as defined in this paper of between 4 and 90 μm can be readily implemented. This is compared to a photo-interrupter based sensor for which the SNR is about 8000 with a displacement range of larger than 100 μm. Based on these results, this sensing method is applied to the dynamic and static measurement of a tuning fork oscillator.