Study On All-solid-state CW Single-frequency Laser Technique Based On Birefringent Filter

Laser interferometry has been widely used in advanced manufacturing,aerospace,scientific research and military reconnaissance because of its high measurement accuracy,wide measurement range and so on.For example,laser interference is the main method to measure the surface precision of optical elements with a large aperture and a long focal length.It has strict requirements on the output power,power stability,laser coherence length,laser structure and cost of single-frequency laser,but the current available lasers cannot meet these requirements at the same time.Therefore,this paper aims to study and explore a single frequency laser technology which can obtain output power of hundreds of milliwatts,good power stability,narrow laser linewidth,simple structure,low cost and output wavelength from visible light to infrared.Based on the comprehensive analysis of the advantages and disadvantages of all kinds of single frequency technology at home and abroad,the research technical route of the birefringent filter to get single frequency laser was established.The novel birefringent filter containing a wedged Nd:YVO₄ crystal and a wave plate was proposed,which overcomed that the traditional birefringent filter needed an additional insertion of a Brewster plate or a polarizer in the resonator.A series of theoretical and experimental reserach based on frequency selection of birefringent filter have been carried out,and several design scheme have been achieved and meet the above requirements.The main works in this dissertation are as follows:1.In a standing wave cavity with one end of laser crystal as a resonator mirror,the influence of the hole-burning effect in LD end-pumped continuous-wave（CW）laser on the population inversion distribution and the gain of each mode was analyzed theoretically,and the effect of gain medium length,doping concentration,resonator length and frequency difference among the oscillatiing modes on the single longitudinal mode operation of the laser was numerically calculated.The principle of birefringent filters in different applications was analyzed,the transmittance curves of two birefringent filter schemes including a Brewster plate+wave plate and a polarizer+wave plate were compared,and a method of forming a birefringent filter with a wedged Nd:YVO₄ laser crystal and a wave-plate was proposed.The influence of mode matching between pump spot and the oscillating mode on laser output was summarized,which provided a theoretical guidance for the design of pump coupling system and the resonat cavity in the following experiments.2.A beam shaping system based on a broad-area laser diode（LD）was designed as the end-pump coupling system of the laser.By analyzing the beam characteristics of a broad-area laser diode the far-field distribution of spot was simulated,the calcutation method of astigmatism was determined,and influence of LD current and temperature on output power,the center output wavelength,the divergence angle and the far-field spot of beam was investigated.Based on the analysis and summary of the current end-pump coupling system,two different pump coupling systems were designed for the laser.The simulated focal spot sizes were 150μm×170μm and 71μm×52μm,respectively.The experimental results show that the latter was more suitable for the requirements of mode matching.3.An a-cut Nd:YVO₄crystal with a wedge angle of 10°was selected as as a polarization component,and a YVO₄crystal with an especial design was used to produce a phase difference,so a birefringent filter composed of a wedged Nd:YVO₄crystal and a YVO₄crystal was constructed in the resonator.Its principle was explained and the corresponding formula of the filter loss was derived in theory.Effect of the length and temperature of YVO₄ crystal on the filter transmission spectrum was analyzed.An LD end-pump wedged Nd:YVO₄/YVO₄ laser was set up to investigate the influence of crystal temperature on laser output wavelength and power at different pump powers.When the laser was operating in single longitudinal mode,the wavelength tuning temperature range was greater than 15℃,realizing the single-frequency line polarized light output with a wide temperature tunable range and a maximum single-frequency output power of 762m W,with a slope efficiency of 40%.4.The effect of linear loss and nonlinear loss on the small signal gain of the non-lasing mode was studied theoretically for the multi-mode oscillation in the free-runing frequency-doubling laser.The results show that it is very difficult for an intracavity frequency-doubling laser to achieve a single frequency laser without inserting an additional frequency-selecting components.The gain and filter loss of different modes was calculated numerically when laser was in a single-longitudinal-mode and two-longitudinal-mode operation after introducing a birefringent filter into the cavity.An a-cut Nd:YVO₄crystal with a wedge angle of 10°and a YVO₄crystal form a birefringent filter in a V-shaped resonator.Influences of the KTP length,KTP temperature,and incident angle of the fundamental mode in KTP on the electing-mode function of the birefringence filter were investaged experimentally.By optimizing the radius of curvature of the output mirror,the length of the resonator,and the resonator temperature,output power of the single-frequency laser was increased from 120 m W to 290 m W.The optical-to-optical conversion efficiency was 14.5%,and the single-frequency operating temperature range of the laser was approximately 6℃.In this paper,by studying the single-frequency laser technology based on birefringent filtering,1064nm and 532nm single-frequency lasers were successfully obtained in the Nd:YVO₄ laser.The birefringent filter formed by a wedged Nd:YVO₄laser crystal and a wave plate can also be used in 1342nm,671nm,914nm lasers and so on.This kind of filter provides a new method and idea for the construction of birefringent filters in single-frequency laser.