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.
An all-fiber structure detection system based on single photon detection technique(SPDT) has been developed to measure the ultra-low turbidity ofliquids. To assure the measurement accuracy,the total intensity of transmission light has been detected and quantified as number of photons by avalanche photodiode (APD) which has the advantage of high sensitivity.A fresh all-fiber structure optical fiber probe based on SPDT is applied in the system to reduce the volume and fluctuation of traditional transmission-light measurement system,in which the all-fiber structure probe is used to delivery and collection of transmission light.On the basis of Beer-Lambert (B-L) transmission law,a test system has been established and carried out a series of experiments.By combining B-Llaw with the principle of SPDT,a novel model for detecting turbidity has been proposed to explain the experimental results.The results have shown a well exponential relationship over the range of 0.01–1NTU (Nephelometric Turbidity Units).It also has showna good linear relationship with a resolution as high as 0.01NTUin the range of 0.01-0.09 NTU.When it is 1 secondofthe sampling time,the mean error of measurement result can be controlled within 5% of full scale.In addition,the new detection structure proposed in this paper, which makes the system more compact and more suitable in the small special space.