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Study On Multi-wavelength Coherent Beam Combination And Optical Parametric Oscillator Of Fiber Lasers

Posted on:2014-12-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:K HanFull Text:PDF
GTID:1228330422473818Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Coherent beam combination (CBC) of fiber laser is able to greatly scale the outputpower while maintaining high beam quality, which provides a new way of obtaininghigh bright laser and becomes one of the hot topics in photonics. In the field of highpower fiber laser active CBC, single frequency laser, wide spectrum laser andmulti-wavelength laser have all achieved kilowatt CBC. Multi-wavelength laser CBCsystem has relatively simpler structure and better power scaling capability, which hasachieved some good results in experiment and shows a promising prospect. In thisthesis, multi-wavelength laser CBC is systematically studied in theory.With the development of fiber laser and photonic crystal fiber (PCF), the fiber laseroptical parametric oscillator (OPO) which takes fiber laser as pump source and PCF asnonlinear medium becomes an important means of obtaining special wavelength highbrightness laser source. The relavant research is becoming more and more popular. Thisthesis focuses on a large frequency-shift fiber laser OPO system. A series of issues inthe process of optimizing system are studied.1. Some problems in the establishment of100kilowatt level fiber laser CBCsystem are analyzed. Due to the limitation of the common phase locking techniques andthe system complexity, it is difficult to build a super fiber laser CBC array whichcontains hundreds of channels. The output power of single chain should be increased tokilowatt level. The multi-wavelength laser is able to efficiently suppress the stimulatedBrillouin scattering (SBS) in fiber amplifiers and enhance the output power.Multi-wavelength laser CBC is considered to be a promising scheme to achieve highpower fiber laser CBC. The common beam combining techniques are introduced andtheir limitations are pointed out. The Cassegrain beam combining technique is presented,which is suitable for the high power CBC.2. The principle of multi-wavelength laser CBC is presented. The light waves inthe same frequency perform complex amplitude summation and the light waves indifferent frequencies perform intensity summation. The physical mechanism of the SBSsuppression in multi-wavelength fiber amplifiers is investigated and the suppressioneffects under various multi-wavelength lasers are demonstrated. Based on themulti-wavelength laser CBC principle, the dependence of the combination effect onfrequency interval is researched. By comparing the SBS suppression effect and the CBCeffect at various spectra, the optimal spectral structure for simultaneous SBSsuppression and excellent CBC effect is presented.3. Mathematical model taking Strehl ratio as evaluation criterion is established.The optical path difference (OPD) is divided into two parts: control area andnon-controlled area according to its magnitude compared with frequency interval and wavelength number. When the OPD is in control area and non-controlled area, theStrehl ratio of the coherently combined beam can be described with analytic formulas.The analytic dependences of the combination effect on OPD, frequency interval,wavelength number and channel number are revealed. In the control area, thecombination effect depends on the product of OPD, frequence interval and wavelengthnumber. In the non-controlled area, the combination effect only depends on thewavelength number and channel number. The combination effects of multi-wavelengthlaser CBC with various spectra and various channel numbers are also numericalinvestigated and the beam propagation factor (BPF) is taken as evaluation criterion. Inthe multi-wavelength laser CBC system with a few frequency components and manychannels it usually deos not need to control the OPD, while in the multi-wavelengthlaser CBC system with many frequency components and a few channels it is generallynecessary to control the OPD.4. A large frequence-shift (~140THz) all-fiber OPO system is introduced which isbased on the four wave mixing (FWM) in PCF. By detecting the backwards scatteringlight, the increase trend of signal laser is obtained and the longitudinal gain distributionin PCF is calculated. The FWM effective interaction lengths under different pump pulsewidths are studied. The system with larger pump pulse width has longer FWM effectiveinteraction length and all the effective interaction lengths are smaller than the walk offlengths. Consequently it is supposed that the walk off has not constrained theconversion efficiency. In cases of delayed pump pulse and shifted pump pulse thesystem performancs are tested, and the conversion processes from pump pulse to signalpulse are observed. A mathematical model is established for analyzing the stimulatedRaman scattering (SRS) induced by the signal laser, in which the net effective SRS gainand effective SRS interaction length are presented. By the mathematical model thethreshold condition of the SRS is investigated. To avoid serious SRS the net effectivegain should be samller than13. Various SRS mitigation approaches are discussed aswell.This theoretical research on multi-wavelength laser CBC provides good referencefor the design of the high power multi-wavelength fiber laser CBC system. The study onfiber laser OPO is helpful for the system optimization and nonlinear effect investigation.
Keywords/Search Tags:Fiber laser, Multi-wavelength laser, Coherent beamcombination, Stimulated Brillouin scattering, Optical parametric oscillator
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