| Optical Reflectometer is a powerful optical instrument,widely adopted in fiber optic distributed sensor for the location of loss,measuring temperature,stress,etc.Commonly,optical reflectometers include Optical Time Domain Reflectometer(OTDR),Optical Frequency Domain Reflectometer(OFDR),and so on.OFDR uses sweeping laser as the light source.Compared with OTDR,it has higher spatial resolution and dynamic range.However,when the detection length is longer than the coherence length of the light source,the phase noise will degrade the system performance rapidly.Meanwhile,a high spatial resolution requires a wide range of sweeps of the probe light.Because of the high cost of narrow linewidth lasers and modulators,OFDR is difficult to commercialize at present.Solving the phase noise of OFDR and increasing the sweep frequency range are the main work in this paper.The distributed feedback laser(DFB)wavelength can be tuned by current and temperature.It can achieve a very wide range of frequency sweep.The recently proposed time-gated optical-frequency domain reflectometer(TGD-OFDR)is a method to suppress phase noise using the principle of fast sweeping of the pulse light,and it is possible to make the detection distance exceed the coherence length.In this paper,the DFB laser and TGD-OFDR technique are combined.Besides,the pre-distortion of DFB laser and cross-correlation calculation algorithm are applied.The system achieves a detection distance of 74 km with a spatial resolution of 10 cm at the near end and 18 cm at the far end.This is believed to be the best resolution ever reported for OFDR system over 60 km.In additional,attempts to simplify the system cost are performed.In conclusion,an OFDR with high spatial resolution over long distance is realized by using low cost devices,which has great potential for the commercialization of OFDR. |
