Research On Dynamic Laser Coherence Control Theory, Technologies, And Applications

Dynamic laser coherence,referring to the preservation of high coherence during the dynamic modification of laser properties including frequency and power,has found applications in state-of-the-art research fields such as laser spectroscopy,optical frequency metrology,coherent measurement,lidar,3D-imaging,coherent communication,coherent power combining,and microwave photonics.In these fields,lasers with high dynamic coherence,namely,the capability to be phase-coherent continuously scaned with arbitrary agility within a large optical spectral range can fundamentally improve the performances and is urgently desired.However,the large spectral tunability and the high coherence during the scans are the almost on the opposite sides of the scale according to laser physics and the fundamental theory of laser technologies.In order to achieve a high coherence,the laser usually needs an ultra stable and long resonace cavity,thus compromising the dynamic properties.On the contrary,short-cavity lasers,exhibiting large optical tuning bandwidth,however,usually with poor coherence property.In a word,the fundamental physics mechanism of laser resonator has posed strict obstacles on the compromising between coherence and dynamic tunability.As a consequence,the challenge of improving the dynamic coherence has thus become a fundamental scientific issue for the relevant community.To date,optical phase-locking has been regarded as the most efficient technique in the context of dynamic laser coherence control owing to the capability of broadband feedback control of the laser phase and frequency.It is thus of great research and application signicifance and importance,and has been the focus of diverse researches for a long time.However,by far,due to the technical challegnges in aspects such as intrinsic laser phase noise,servo-control bandwidth,and other limits,the state-of-the-art dynamic coherence control techniques still embrace difficulties in achieving highly phase-coherent continuous scans with arbitrary agility.On the basis of optical phase-locking,this thesis aims at investigating new types of laser coherence control technologies.Started from the phase-coherence transfer based on the hybrid optical phase-locking,the thesis has indepthly analyzed,investigated,and elaborated the high sensitive delayed self-heterodyne in connection with wideband phase-locking and frequency traction control based on the agile frequency comb.The nonlinear mechanisms of four-wave mixing based spectral range extension has been thoroughly investigated,as well as the underlying physical picture of the phase matching conditions.And eventually,from optical domain to electrical domain,the highly phase-coherent optical frequency with low mutual linewidth has been adopted to generate low noise arbitrary microwave waveforms.The main research works and novelties of this thesis are listed below: 1.Modal analysis,theoretical simulation,and experiments of optical phase-lockingModal analysis has been conducted to elaborate the optical phase-locking system comprising multiple active servo-loops according to the loop stability theories,together with detailed discussion of its performance metrics and system limits.A systematic model has been built.Based on this model,this thesis has proposed a hybrid phase-locking scheme consists of multi-stage servo-loops including temperature control,current driver,acousto-optic frequency shifter,and phase modulator.With the action of the different characteristics of different loops,the tight phase locking is steadily accomplished with sufficient dynamic properties.The capability has been demonstrated in terms of both coherence control and coherence transfer.2.Delay self-heterodyne phase-locking for dynamic coherence control techniqueOn the basis of the theoretical model,this thesis has proposed and demonstrated dynamic coherence control technique adopting delay self-heterodyne phase-locking.To this end,an ultra-short unbalanced mach-zhender interferometer is adopted for wideband freuqnecy discrimination and feedback control,improving the dynamic coherence for several orders of magnitudes.This permits the achievement of about 60 times improvement in laser coherence length while the phase error is maintained below 0.8 degree within the 80 GHz linear frequency chirp range.The configuration can be easily applied to virtually any semiconductor laser.3.Frequency traction control based coherence control techniqueIn order to overcome the trade-offs between the phase-locking bandwidth and discrimination sensitivity,in this work,we report on a versatile and robust coherence control that allies the arbitrary agility,tunability,and coherence property of an agile frequency comb with the intensive monochromic output of a commercial CW laser.The mode spacing of the agile frequency comb can be arbitrary scanned with high sweeping rate while preserving the power and phase distribution amongst the modes.Therefore,by adopting a multi-stage phaselocking with broad loop bandwidth,the precision and resolution of the mode spacing as well as the coherence of the comb's seed laser,can be steadily transfer to the CW laser via the wellestablished link in a fully phase-coherent fashion.On top of this,the frequency excursion range of the output is determined by the high order modes of the comb and the scans can be precisely designed by controlling the comb's driving signal.This way,the proposed scheme has permitted to exhibit highly phase-coherent rapidly continuous frequency agility within a broad optical range with high precision,resolution,and spectral purity,simultaneously.The proposed system will be promising candidate for numerious potential applications.4.Generation of comb spacing agile frequency combOptical frequency comb acting as precise reference within a broad spectral range,has enabled a precision and accurate freuqnecy grid and become an ideal tool for various applications owing to their broadband highly coherence property.An optical frequency comb which could provide a phase-continuous reference is critical for frequency traction control phase-locking.However,the finite comb mode spacing has posed a fundamental limit in frequency resolution and the phase-continuous tuning range as well.In order to achieve agile tunability of the mode spacing with well-established power and phase distribution amongst the modes within the effective bandwidth,in this thesis,the propagation delay skew between the optical and electrical path has been precisely measured and accurately aligned for wideband phase matching.Thus,the phase mismatch induced flatness deterioration is effectively suppressed.A 19-line OFC with repetition rate continuously sweeping over one-octave from 8.5 to 19.0 GHz in 0.5 ms sweep time is obtained.The overall power deviation of the obtained 19 comb lines is maintained within 4 dB all through the whole repetition rate sweep range.In order to further increase the optical spectral bandwidth of the comb,four-wave mixing based fiber-optic parametric amplifer is utilized for nonlinear extension.The operation of a dual-pump phase sensitive amplifier is numerically investigated using a multi-wave model,taking into account high-order waves associated with undesired four-wave mixing processes.More accurate phase-sensitive signal gain characteristics are obtained compared to the conventional model,leading to precise optimization of the pump configuration.It permits the application-oriented arbitrary tailoring of the signal gains by manipulating the dispersion profile and pump wavelength allocation.5.Prototype of optical frequency domain reflectometryBased on the preceding agile frequency comb,in combination of the four-wave mixing based nonlinear spectrum extention,a wider optical bandwidth can be achieved.Besides,in additional to the current controlled semiconductor laser,to further achieve a broader optical spectral coverage for practical instrumental applications,multi-stage phase-locking is additionally involved to extend the tuning range to more than 200 GHz optical bandwidth.In this thesis,an optical frequency domain reflectometry prototype was demonstrated and accomplished.The detailed design description is presented.The phase-locking based dynamic coherence control has led to the robust operation with remarkable performace metrics.The prototype has achieved a 3-km range window with a guaranteed 2 mm and maximum 0.5 mm spatial resolution which outperforms the state-of-the-art commercial product in terms of maximum range window.6.Photonic generation of low phase noise linear chirped microwave waveforms withlarge time-bandwidth productPhotonic generation of arbitrary high frequency and wideband microwave waveforms has always been the essence of microwave photonics.In this thesis,a photonic approach for generating low phase noise,arbitrary chirped microwave waveforms based on heterodyne beating between high order correlated comb lines extracted from frequency-agile optical frequency comb has been demonstrated.Using the dual heterodyne phase transfer scheme,extrinsic phase noises induced by the separate optical paths are efficiently suppressed by 42-dB at 1-Hz offset frequency.Linearly chirped microwave waveforms are achieved within 30-ms temporal duration,contributing to a large time-bandwidth product.The linearity measurement leads to less than 90 kHz RMS frequency error during the entire chirp duration,exhibiting excellent linearity for the microwave and sub-THz waveforms.The capability of generating arbitrary waveforms up to sub-THz band with flexible temporal duration,long repetition period,broad bandwidth,and large time-bandwidth product is investigated and discussed.This is of profound significance for application such as microwave photonics assisted radar systems.