波长可调的低噪声全固态黄绿光激光器(特邀)

Wavelength-adjustable All-solid-state Yellow-green Laser with Low Noise(Invited)

报道了一种808 nm激光二极管端面泵浦Nd:YAG激光晶体/腔内倍频的波长可调、低噪声全固态黄绿激光器。基于L型折叠谐振腔结构的优化设计,不但使谐振腔对热焦距具有动态热不灵敏性,而且补偿了折叠型结构和凹面反射镜引起的光束像散。基于布儒斯特片的布儒斯特角调节,实现对Nd:YAG晶体受激辐射产生的1112.62 nm、1116.70 nm和1123.24 nm三条谱线的选频和单一波长基频光腔内振荡;调节单一波长基频光的p偏振方向与双折射单晶玻璃光轴之间的夹角,通过压缩纵模个数,实现对该波长基频光的滤波。在此基础上,通过Ⅰ类角度相位匹配LBO晶体腔内倍频,分别获得了556.31 nm、558.35 nm和561.62 nm三个波长可调、高稳定性、低噪声的黄绿激光输出,对应最大连续输出功率分别达到了678 mW、653 mW和606 mW,光光转换效率分别为8.47%、8.16%和7.58%。在输出功率均为500 mW时,功率不稳定度分别为±0.42%、±0.38%和±0.49%,对应激光噪声分别为0.69%、0.51%和0.96%。

A Laser Diode(LD)pumped Nd:YAG crystal low-noise all-solid-state yellow-green laser in the wavelength range of 555~561 nm is reported,which becomes as a research hotspot because of its great application potential in industry,atmospheric remote sensing,communication,information storage,food and drug detection and other fields.As we all know,noise is one of the key indicators to measure the stability of laser output.The power stability of the low noise laser beam is higher because there are no power transient spikes.However,the Nd:YAG crystal excites three spectral lines simultaneously at 1112.62 nm,1116.70 nm and 1123.24 nm.The mode competition among the three spectral lines,the mode competition among the different longitudinal modes in each spectral line and the three-wave coupling effect of the frequency doubling process all make that the frequency doubled yellow-green laser has strong noise and low power stability.It is very difficult to realize that any one of the above-mentioned three spectral lines with very short wavelength interval oscillates individually in the cavity by the method of coating a narrow-band reflective film.At present,the common method to reduce the noise of all-solidstate yellow-green laser is to insert an etalon with the function of selecting a single longitudinal mode or a Birefringent Crystal(BC)with a filtering function into the linear cavity.However,the fundamental frequency laser oscillating in the linear cavity has the inherent defect of small mode volume,which is not conducive to obtaining high power frequency doubled yellow-green laser.The use of etalons to eliminate mode competition among multiple longitudinal modes usually comes at the expense of the output power and the optical-to-optical conversion efficiency of the laser.When performing frequency selection and filtering with a BC placed according to the Brewster angle,it is usually necessary to precisely adjust the size of Brewster angle and the angle between the optical axis of the fundamental frequency laser and the surface of BC.Therefore,it is extremely difficult to obtain a high-power yellow-green laser using a compact cavity.In this paper,an all-solid-state yellow-green laser with wavelength-adjustable in the range of 555~561 nm,high power and low noise is reported.A folded cavity structure based on an 808 nm LD endpumped Nd:YAG crystal and a type-I angle-matched LBO crystal intracavity frequency-doubling is used.After optimizing the structural parameters of the cavity,the coupling rate between the mode volume of the fundamental frequency laser and the mode volume of the LD pump laser is improved,and the cavity had the thermal insensitivity to the dynamic change of thermal focal length.In addition,the beam astigmatism caused jointly by the folded cavity structure and the concave mirror is also effectively compensated.A Brewster Polarizer(BP)and a BC are successively inserted into the cavity to form a Birefringent Filter(BF).After precisely adjusting the corresponding Brewster angle of the BP,respectively,the frequency selection for the three spectral lines with wavelengths of 1112.62 nm,1116.70 nm and 1123.24 nm generated by Nd:YAG crystal is completed.Meanwhile,the oscillation of a single wavelength fundamental frequency laser is realized in the cavity on the basis of precisely controlling the pitch angle of the cavity mirror.Then,the filtering of the fundamental frequency laser(i.e.the number of compressed longitudinal modes)is realized by adjusting the angle between the p-polarization direction of the fundamental frequency laser and the optical axis of BC.Finally,high-power,high-stability,and low-noise yellow-green laser with central wavelengths of 556.31 nm,558.35 nm,and 561.62 nm is obtained,respectively,which is based on the I-type angle-matched LBO crystal intracavity frequency doubling.When the highest pumped power of LD is 8.0 W,the maximum independent CW output powers of the yellowgreen laser beams with central wavelengths of 556.31 nm,558.35 nm and 561.62 nm reach 678 mW,653 mW and 606 mW,respectively,corresponding to the optical-to-optical conversion efficiency are 8.47%,8.16%and 7.58%,and the line widths are 0.34 nm,0.42 nm and 0.37 nm,respectively.At an output power of 500 mW,the power instability of the three yellow-green laser beams at 556.31 nm,558.35 nm and 561.62 nm are±0.42%,±0.38%and±0.49%,respectively,the corresponding SMR noise are 0.69%,0.51%and 0.96%.Meanwhile,the beam quality factor are(M^(2)_(x-556.31)=3.943,M^(2)_(y-556.31)=4.301),(M^(2)_(x-558.35)=3.409,M^(2)_(y-558.35)=3.584)and(M^(2)_(x-561.62)=3.732,M^(2)_(y-561.62)=3.971),respectively.The experimental results show that the frequency selective filtering of“BP+BC”is an effective method to realize high-power,high-stability,low-noise,wavelength-adjustable yellow-green laser.Our research provides a novel source for future potential applications in biomedicine,laser measurement,pollution monitoring and spectral analysis.