Research On Technologies Of Optical Time Domain Reflectometry Based On Frequency-Shifted Heterodyne

Optical time domain reflectometry is one of the keying technologies in optical fiber link characterization and optical fiber sensing,thus it has important research value.The realization of an OTDR system with a simple structure and high performance has always been a research focus.With the advantage of high sensitivity,heterodyne detection technology is widely used in OTDR.This thesis studies the OTDR technology based on acousto-optic frequency shifts and heterodyne detection.Two main tasks were carried out:1.A method for wavelength encoding of optical signals using an arbitrary waveform generator and an acousto-optic frequency shifter is proposed.As an improvement to the traditional wavelength encoding method,this method uses two pulses with different wavelengths generated by the frequency shifter as the probe pulse light and the reference pulsed light.When the time interval between the two pulsed lights is equal to the flight time of probe pulse of the optical fiber,a beat signal appears at a frequency corresponding to the frequency difference between the two pulses,as we can tell there exists a reflection located at a distance corresponding to the interval time,by changing the interval time can realize the measurement of the entire length of the fiber.This scheme has the advantages of fast tuning time(35ns)and small line width(1MHz)of generated beat signal,as well as reducing the requirements for the bandwidth of the received signal.The feasibility of the method is verified by simulation and experiments.The 6.15 km optical fiber was measured and the results were analyzed in the time domain and frequency domain,achieving 66 dB SNR and 5m spatial resolution.2.A method of break locator using frequency-modulated pulse based on frequencyshift heterodyne is proposed.The optical signal with frequency-modulated pulse serves as both probe light and local oscillator light,the optical heterodyne detection produces two more beat signals in one cycle,and the relative time interval between them can be measured to obtain the break point location.This method does not require massive measurements for scanning the whole fiber and one measurement can do the job.Meanwhile,this scheme is also not necessary to measure the transit time of the optical pulse,only need the relative time of two beats,the continuous light can also avoid nonlinear effects and optical surges.A wavelet denoising method is proposed to denoise the signal.The experimental results show that under the coherent condition where the fiber length is less than the coherent length,a stable and continuous beat signal can be observed after wavelet denoising processing,and the 30 km fiber measurement has achieved 12.31 dB SNR with 5m spatial resolution.Under the partially coherent condition where the fiber length is close to or even exceeds the coherence length,the interference effect becomes inconspicuous and resolution deteriorates.The 70 km fiber measurement has achieved 5.33 dB SNR with 500 m spatial resolution.A wavelet threshold denoising method is proposed to process the SNR to 30.79 dB.