The Research On The Analysis And Compensation Of Laser Phase Noise In Optical Frequency Domain Reflectometry

Along with the development of industry of photoelectronic,the distributed fiber sensor and fiber measurement has been becoming a fresh technological field.One of most significant techniques in this field is optical frequency-domain reflectometry(OFDR).It can provide a higher sensitivity and spatial resolution compared to the traditional optical time-domain reflectometry(OTDR),and has wide potential applications,thus it has got a considerable attention.However,due to the phase noise of the light source in OFDR,the performance indexes including spatial resolution and measurement range are limited.Two approaches are often taken to handle the phase noise in OFDR,which are coherence enhancement at emitting part,and phase noise compensation at receiving part.In this thesis,for the aspect of coherence enhancement,we proposed novel methods for phase noise evaluation,measurement,and analyzed the characteristics of phase noise in OFDR.For the aspect of phase noise compensation,we also proposed a novel phase noise compensation method based on the delay-matched sampling.The key innovations in this thesis are listed as follows:1.The power-area method to precisely estimate linewidth for arbitrary frequency-noise spectrum.While the narrowed linewidth laser and linewidth reduction are widely used,the high-precision estimation of linewidth is urgently required.Numerical calculation from a theoretical relationship is available but the computation is complicated,thus a simple and intuitive approach to estimate the laser spectrum lineshape and linewidth becomes the research subject of this field.A linewidth estimation method called ?-separation line method has been reported in a previous work to observe the influence on lineshape and linewidth from frequency-noise spectrum simply and intuitively,however its precision was not enough,and its application was limited.We have proposed a novel linewidth estimation method named power-area method(PAM),and realized the precise linewidth estimation for arbitrary frequency-noise spectrum.It is not only simple and intuitive,but also provides a better precision compared.The linewidth for the servo-controlled laser spectrum and the delayed self-homodyne/heterodyne beat-note spectrum can also be evaluated with this method.2.A short-delayed self-heterodyne method to measure a large-frequency-range frequency-noise spectrum.While the development of narrow-linewidth-laser technique,the measurement of frequency-noise spectrum requires a large frequency range.The previous measurement methods have the drawbacks such as high system complexity,the serious environment disturbance introduced by the long fiber,limited Fourier frequency range,etc..In this thesis,we proposed a short-delayed self-heterodyne method(SDSH)to measure frequency-noise spectrum.Using a short delay fiber(15.5 m),a frequency-noise spectrum with Fourier frequency range from 10 Hz to 50 MHz is precisely measured.The frequency range is actually limited by the sampling rate and sampling depth of digital acquisition card.The measurement system and digital process are both quite simple in this method,thus this method has a high practical application value.3.A dynamic frequency-noise spectrum measurement method for frequency-swept laser based on the extraction and restoration of laser intrinsic frequency noise.For the frequency-swept laser in OFDR,the traditional frequency-noise spectrum measurement method is not enough to reveal its dynamic characteristics.In this thesis,we proposed an averaging method to extract the laser's intrinsic frequency noise,and restored the frequency-noise spectrum using SDSH method in fractional time bands,thus obtained a 3-D dynamic frequency-noise spectrum figure of frequency-swept laser.Moreover,we measured and analyzed the dynamic frequency-noise spectrum of servo-controlled frequency-swept laser.The dynamic frequency-noise spectrum is able to provide more precise information on the laser instaneous characteristic compared with the traditional measurements,thus can be employed in the application of frequency-swept laser and highly stable laser.4.A novel OFDR phase noise compensation method with delay-matched sampling.In a high-precision OFDR system,the compensation of phase noise is necessary.The methods reported in previous works have the drawbacks of imprecise estimation of phase noise and the environment disturbance introduced by long delay fiber.We proposed a OFDR phase noise compensation method based on delay-matched sampling with a short delay fiber,which can estimate and compensate the phase noise with a better precision.With this method,the OFDR' performance is highly improved.