Influence Of Pump Laser Line Width In Frequency Conversion

Frequency conversion by making use of nonlinear optical crystal is an effectivemeans to get the wavelength which can not produced by laser directly. Among thefactors that influence frequency conversion, the influence of pump source's line widthhasn't got enough valued and researched. All nonlinear optical crystals used forfrequency conversion have an allowable range of line width. If the line width of laser iswider than allowable line width of crystal, the efficiency will seriously decrease. Thefiber laser has been becoming very important pump source of frequency conversionsince its rapid development, however, whose beneficial bandwidth is very wide.Affected by nonlinear effect, the line width of fiber laser will get broadened while theoutput power of pump source is rising, which is contradictive to the request that the linewidth of should be narrow. So the fiber laser chosen as the pump source should have areasonable line width, which wouldn't lead to serious decline of efficiency. Studying themechanism of the influence of line width in frequency conversion can provide scientificbasis to choose the fiber laser as the pump source whose bandwidth should bereasonable. The thesis takes frequency doubling as an example, studying the influenceof line width in frequency doubling through the theory and experiment.Firstly the departure of spectral line from the best matching point which wouldlead to the decline of frequency doubling efficiency was analyzed in theory; Then basedon the work of foregoing people, the frequency doubling efficiency being confined bythe bandwidth of laser, which has a certain line width as a pump source, was researchedin more quantification；Furthermore, the departure of frequency from the best matchingpoint would be induced by the change of temperature, by which the efficiency would bereduced too.The output identity of 1064nm fiber laser was studied in the experiment, whoseline width would be broadened along with rise of average power, as well influence ofbandwidth in frequency doubling. Then the crystal was pumped by the fiber laser aspump source to produce 532nm green light. The average power of fiber laser beinginvariable, the line width at -3dB was broadened from 0.639nm to 1.226nm throughchanging the bandwidth of seminal light, and the frequency doubling efficiencydeclined from 12.39% to 9.11%. The results indicated that, the efficiency reducedgradually when the line width was broadened. In the end the experiment result contrasted the theory, the reason why the efficiency would descend was analyzed. Thefiber laser which has moderate line width should be chosen as the pumped source, to geteffective output of high average power in frequency conversion.