| 3-5?m mid-infrared laser have a wide range of applications in atmospheric pollution monitoring,remote target detection,laser medical treatment and high-resolution spectroscopy.In particular,the narrow spectral width and tunable laser system is the ideal source for such applications.For example,in atmospheric environmental monitoring field,the absorption peak of greenhouse gas N2O(nitrous oxide)is near 2.85 ?m,the absorption peak of atmospheric pollutant C2H2 is near 3.05?m,and so on.Using optical parametric oscillation(OPO)technology to achieve wide-tuned mid-infrared coherent laser output is an effective technical means.Based on this technology,solid-state laser light source has the advantages of long life,high efficiency,high reliability and stability.The periodically poled magnesium-oxide doped lithium niobate(PPMgLN)has an advantage of high-gain,wide tunning and mature 1 um compact and convenient fiber pump source,therefore,fiber-laser-pumped pulsed OPOs represent a new generation of parametric devices for their low threshold,high peak power and excellent beam quality.However,the bandwidth pulsed mid-infrared output from a free-running PPMgLN-OPO is usually broad due to broad pump bandwidth and broad emission bandwidth,it's more than ten nanometers or several tens of nanometers,obviously,it's difficult to meet the practical application.Therefore,it is significant to obtain narrow-bandwidth and wide tunning infrared pulsed laser output.This article addresses the issues of bandwidth control,wide tunning and efficiency,and we carried out theoretical and experimental studies respectively,and we have obtained a Wide tuning,narrow-bandwidth and high-efficiency infrared pulse laser output near 3 ?m.In theory,(1)the characteristic of temperature tuning and cycle tuning has been analysed and calculated to provide an effective theoretical basis for realizing a specific band infrared laser.(2)To obtain an operation of OPO with a low threshold and a high efficiency,the influence of resonant cavity parameters on oscillation mode has been analysed,and the influence of the focus parameter of pump on the gain,threshold and conversion efficiency of OPO by a double-pump has been analysed,and the cavity structure of a double-pump has been designed,which are based on the basic theory of OPO.(3)The influence of the pump bandwidth,the pump divergence angle,the parameter noise and the high-gain on the idler bandwidth of an optical parametric system has been analysed,which are based on the basic theory of dispersion.Especially,the relationship between the idler bandwidth of a free running OPO and the over threshold multiple has been analysed,which the essential factors of achieveing the narrow-bandwidth idler output has been obtained.(4)The technical parameters of optical element for bandwidth control(volume holographic grating,etalon)have been obtained,and this can provide a theoretical guidance for the experiment to obtain narrow-bandwidth,wide tunning and efficient mid-infrared idler output.In experiment,(1)according to the experiment need,a high-precision oven has been designed and producted.(2)Using a single-frequency pulsed fiber-laser as the pump source of OPO,using a meniscus mirror to reflect the pump beam and then focus it into the PPMgLN for a double-pass pump,an efficient idler output has been achieved,which laid a foundation for achieveing of high efficiency mid-infrared idler output with a narrow-bandwidth.(3)Under the technical route of passive bandwidth control,a reflective holographic grating with optimized design was used as the narrowing element of signal bandwidth,when the single longitudinal mode pump power is at 3 times the threshold,the bandwidth of oscillated signal is narrowed to 0.1 nm from about 10 nm of free running,and the idler bandwidth is indirectly narroWed to 0.42 nm from about 40 nm of free running,and the idler conversion efficiency was reached 18%,which solved the problem that the idle can not be directly narrowed,and which verifies the correctness of this scheme of achieveing a narrow-bandwidth idler output with a high efficiency.However,due to the limitations of temperature tuning insensitivity and operating temperature range of VBG,continuously tunable range of idler is only 8 nm by adjusting the temperature of the PPMgLN and VBG accordingly.(4)In order to overcome the limits of temperature tuning insensitivity and operating temperature range of VBG.In indirect access to mid-infrared idle light,the single longitudinal mode fiber laser is used to pump PPMgLN,and the etalon design is used as a mirror of OPO to effectively modulate and control the gain spectral width of the oscillating signal light.And,the self-seeding dual-solid etalon coupled cavity was used to suppress the weaker sideband of the oscillating signal light to realize the signal light oscillation with narrow spectral width,and with single longitudinal mode pump light effective interaction,the narrow spectrum of idle light was achieved.In the results,the idler bandwidth is indirectly narrowed to 0.36 nm,with an efficient conversion efficiency of 17.4%,and approximate 200 nm wide idle laser tuning from 2.85 ?m to 3.05 ?m has been achieved via adjusting the temperatures of the PPMgLN crystal and the etalon 2 accordingly. |
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