The Optical Time Domain Measurement By Using Photon Counting Technology

Quantum Cryptography plays an important role not only in the areas of national defense, but also in the areas of bank business, electronic commerce and daily correspondence. Using the smallest unit of quantum systems of single-photon as information carrier in quantum cryptography, the combination of cryptography and quantum mechanics can perfectly eliminate the problem of classical cryptography where the key distribution is settled by with mathematic method. Quantum non-cloning principle, single-photon non-splitting principle and the quantum uncertainty principle can ensure the key transmission to be absolutely safe and eavesdropping detectable.Optics time domain reflection (OTDR) can realize the rapid measurement of the transmission characteristics of optic fiber and position the breakdown sites exactly through the detection of back scattered signals during the optical fiber transmission, which can be used in Quantum Cryptography for analysis the bit error rate and identifying the wiretapping.In this paper, after introducing the OTDR principle and the present research development, we have shown the detection of OTDR based on the single photon detection, and have measured the characteristics of fiber transmission by using the photon counting and intensity modulation spectroscopy respectively, in which we have studied how to make improvement of the signal to noise ratio and quantum efficiency of the OTDR system. What we have done could be mainly shown in the following several aspects:(1) We have measured the pulsed light OTDR with and without the Faraday Rotator Mirror. The relationship between the pulse width and the measurement precision was discussed.(2) The photon counts modulation technology was applied in OTDR measurement system. The back reflected signals are measured with different modulation frequency and different modulation depth. It is found that the intensity modulation spectroscopy can greatly reduce the quantum statistics noise. The improvement for signal-to-noise ratio is up to 28.3dB in a typical measurement.(3) By use of the directly demodulated the single photon event, we found that the FFT spectrum of photon counting is a typical white noise style, and there was not the normal low frequency noise characteristics existing in electronic measurements. By decreasing the voltage threshold of the discriminator, we achieved the 1.87 times bigger quantum efficiency compared with the single photon counting detection.