A wide range fiber turbidity sensor based on single photon detection technique with an optimal optical power

A high-resolution fiber turbidity sensor has been developed to measure the wide dynamic range of turbidity. To improve the accuracy of measurements, the high sensitive single-photon detection technique is adopted in the sensor by using an avalanche photodiode (APD) to detect the total intensity of transmitted light and quantize it to the corresponding number of photons. A theoretical model for turbidity measurement is proposed by combining the B-L transmission law with the single photon counting theory. In order to cover a wide dynamic range measurement based on only one measurement system, an optimal optical power for the sensor must be chosen. Therefore, an experiment is designed to measures the different turbidity solutions by gradually changing the power of incident light. The test results show that, with the increasement of light intensity, the measured data fit better with the theoretical one when the measured turbidity is lower than 0.1NTU. However, the light intensity cannot be increased without limit in high turbidity region, especially when the turbidity is high than 1NTU, since there is a corresponding upper limit for the detection of avalanche photodiode (APD). After the processing and analysis of test data, the test results indicated the sensor can obtain the best measurement accuracy when the optical power of incident light at 11mW. The measured turbidity range is as wide as 50dB from 0.01 to 1000NTU. The proposed fiber sensor is robust and can be used for the wide dynamic turbidity detection for drinking water or some remote monitoring of water pollution.