Reaserch Of Linear Frequency-Swept Laser Source Generation In Optical Frequency Domain Reflectometer

The distributed optical fiber measurement technology,because of its advantages of large measurement range,high spatial resolution,and convenient to application,has been widely used in a number of national defense and peopleundefineds livelihood related fields,such as aerospace,optical fiber communication networks,and military security,etc.The optical frequency domain reflectometer(OFDR)is particularly concerned with its outstanding performance.The key componont of the OFDR is a linear frequency-swept laser source,the measurement distance of the system is determined by the coherence of the light source,and the spatial resolution is determined by the frequency tuning range.In general,laser source that can achieve a large-range frequency sweep typically has poor phase noise performance,and high coherent light source generally does not have good frequency tuning ability.In addition,the existence of the frequency sweep nonlinearity of the laser source restricts the measurement distance and the spatial resolution,and seriously deteriorates the performance of the system.Thus the linear frequency-swept laser source generation with high coherence and high linearity,and has the frequency sweep range satisfying the application requirement is the key problem in the research of the OFDR technology.The optical phase-locked loop(OPLL)plays an important role in the generation of frequency-swept laser source with high linearity and coherence.In this thesis,concerning the high spatial resolution OFDR and ultra-long measurement distance OFDR application background,the DFB laser and the fiber laser are used as the tunable laser source for high-quality linear frequency-swept laser source generation.The main research contents and innovation points of this thesis are reflected in the following several aspects:(1)A method for suppressing the frequency sweep nonlinearity and broadband phase noise of a DFB laser by adopting a composite OPLL structure is provided.The DFB laser current frequency modulation response has a phase reversal problem at high frequency,which limits the loop bandwidth that ordinary OPLL can obtain.In this dissertation,a new composite OPLL structure is proposed,in which the frequency sweep nonlinearity at the low frequency is suppressed by the current feedback loop,and the broadband phase noise is suppressed by the acoustic optical frequency shifter loop.The control bandwidth of the loop is extended from 500 k Hz to 1 MHz compared to a conventional single feedback loop,and the time domain frequency error of the DFB laser during the sweep is reduced from 153 k Hz in the single loop to 89 k Hz in composite OPLL configuration.(2)A method for broadband high linear frequency sweep generation of a DFB laser using a combination of temperature tuning and current tuning is proposed.In the case of the temperature tuning of the DFB laser,broadband frequency sweep of hundreds of GHz can be realized,however the frequency sweep nonlinearity is more serious due to the slow response of temperature tuning.In this dissertation,a method for compensating the non-linear effect in the process of temperature frequency tuning by using current frequency tuning is proposed,and a high linear broadband frequency sweep with a tuning range of 200 GHz is realized.The frequency error of the DFB laser during the temperature tuning frequency sweep is reduced from 700 MHz in the open loop to 1.3MHz in the closed-loop state.And a high spatial resolution of 0.5 mm is obtained in OFDR system measurement.(3)A method for realizing ultra-low frequency noise sweep of a fiber laser is proposed by combining the PZT driving voltage pre-distortion scheme and the OPLL structure.In this dissertation,a tunable fiber laser is used as a light source,the driving voltage of the PZT is firstly pre-distorted by an auxiliary MZI interferometer,and then the frequency sweep rate of the swept laser is locked to a reference MZI interferometer,thus a high linear ultra-low-noise frequency sweep of 1 GHz in the time duration of 25 ms is realized.In order to show the dynamic characteristics of the fiber laser in the frequency sweep process,a method for measuring the dynamic frequency noise spectrum of the laser is also given,and the process of loop locking and the laser dynamic frequency noise spectrum in the whole frequency sweep process are presented.(4)An ultra-long range OFDR system with a sub-meter level spatial resolution over200 km fiber link is realized for the first time.Along with the transmission distance of the optical fiber communication is increasing,the demand for long-distance OFDR is also growing.In previous report,as the sweep linearity and coherence of the laser source are limited,the maximum measurement distance of the OFDR system is 170 km.In this thesis,OFDR system with a measuring distance of 200 km and dynamic range of 50 d B is realized.With a measurement time of25 ms,a spatial resolution of 10 cm is obtained at a measurement range of 20 km;at a measurement time of 5 ms,the measured spatial resolution is maintained at 0.72 m over the entire 200 km fiber link.