基于空间光腔的高功率布里渊频率梳 下载: 863次封底文章
In recent years, an optical frequency comb (OFC) has shown great advantages and application prospects in the fields of optical atomic clocks, laser frequency measurement, and precision spectral measurement. Limited by the resolution and response time of current optical measuring equipment, OFCs with a frequency interval of at least 20 GHz are required to calibrate the spectral measurement equipment in the field of planetary exploration. Although an OFC with an output power of mW level is sufficient to realize gas detection in a confined space, higher power OFCs in specific transmission bands are required in the application scenarios such as outside detection of hazardous chemicals and space detection. Therefore, the development of high-power OFCs with large frequency intervals in specific wavelengths is of great significance, which is also the hotspot and difficulty in the field of current laser technologies.
A scheme to achieve an OFC in a free-space optical cavity by using the stimulated Raman scattering as the intermediate process is proposed (see Fig. 1). Firstly, the pump light is injected into a Raman oscillator to form a high-power density Raman field in the cavity. Then, the Raman laser with a high power density in the cavity reaches the first-order stimulated Brillouin scattering (SBS) threshold under the action of the acoustic field in the gain medium. Finally, with the further increase of intra-cavity power density, the cascaded Stokes and anti-Stokes with equal frequency intervals are generated under the joint action of SBS and four-wave mixing.
The power and spectral characteristics of the output laser are studied by using a 1 μm quasi-continuous-wave laser as the pump. With a limited pump power, an OFC in the 1.2 μm band with up to 101 W is demonstrated with a conversion efficiency of 41% as shown in Fig. 2. Moreover, the output beam quality is also significantly improved compared with the pump beam quality (inset of Fig. 2). By optimizing the resonator parameters and pump conditions, the output spectra with single Raman frequency, first-order SBS, and OFC are obtained, respectively. As shown in Fig. 3, an OFC with a frequency interval of 71 GHz and 23 spectral lines is obtained in the 1.2 μm band corresponding to the overall bandwidth of 1.55 THz.
Here, we propose and verify the possibility of using the Raman field as an intermediate process to excite the Brillouin OFC in a free-space oscillator. As far as we know, this is the highest reported power for any Brillouin OFCs, which is four orders of magnitude higher than that of the micro-resonator-based OFC. This free-space approach provides a new path for realizing high-power OFCs in specific wavelengths.
光学频率梳是由频域中若干个等间隔的频率分量组合而成的光谱,可用于实现超高分辨率的距离、频率和时间的测量,被誉为“最高精度的测量手段”,其应用已经覆盖了光钟、空间相干测量、化学探测、光学传感等领域,推动了超精密计量和光谱学的革命性发展[1-5]。为了满足多元的应用场景要求,锁模超短脉冲激光技术、光学微腔、电光调制技术、四波混频(FWM)和受激布里渊散射(SBS)等诸多手段已经被用于产生光学频率梳[6-8]。受限于目前光学测量设备的分辨率和响应时间,在测量频率应用中,通常要求光学频率梳的频率间隔在百MHz以上,以便波长计数器实现波数计量;而在行星探测等空间中,通常要求光学频率梳的频率间隔在20 GHz以上以便于对光谱测量设备进行定标[1,3,7]。在密闭环境下,光频梳功率在mW至W量级即可实现气体检测等应用,但是在危险化学品防区外检测、空间探测等领域则需要功率得到进一步提升[9]。因此,开展具有大频率间隔、特定波长(如大气窗口、人眼安全)的高功率光学频率梳研究,对拓展其在遥感、空间和海洋探测等领域的应用范围具有重要的意义,也是当前激光技术领域研究的热点和难点。
锁模超短脉冲激光技术是最早获得光学频率梳的方法,目前在精密测量中的应用最为广泛,但是受到锁模振荡器固有腔长的制约,其本征频率间隔只有MHz量级,往往需要结合复杂的频率倍增装置才能增大频率间隔[7,10]。基于光学微腔能够产生频率间隔在GHz甚至THz量级的光学频率梳,虽然高品质因子能够有效降低产生阈值,但其输出功率也严重受限[6,11]。SBS作为一种基于光波场和声波场互作用增益机制的三阶非线性光学效应,在光束整形、集成光子学等领域中有着广泛的应用[8,12]。尤其是布里渊增益介质固有的声波特征频率(>10 GHz)和高的时间相干特性(~MHz),使得获得低噪声和窄线宽的激光输出以及产生频率间隔在10 GHz量级的光学频率梳成为了可能。但是,目前关于布里渊频率梳的报道仅限于基于导波结构的CaF2、二氧化硅、硫系化合物等材料,其输出功率通常仅有百mW量级。金刚石晶体是一种宽频带隙材料,具有高布里渊增益系数(~80 cm/GW)、极高的热导率[>2000 W/( m·K)]和极宽的光谱透过范围(>0.2 μm),因此有望突破传统导波结构布里渊频率梳的功率和光谱极限,大幅度提升布里渊器件的性能[13-14]。本文报道了以拉曼场作为媒介在空间光腔中直接获得布里渊频率梳的方法,以金刚石晶体作为增益介质实现了频率间隔高达71 GHz、光谱带宽为1.55 THz的频率梳输出。
实验装置及原理如
图 1. 基于空间光腔的布里渊频率梳装置示意图(插图:工作原理)
Fig. 1. Schematic of Brillouin frequency comb based on free-space optical cavity (inset: working principle)
对于固定的增益介质,其拉曼和布里渊频移均为常数,因此当泵浦光频率取固定值时,腔内的拉曼光频率ωR和激发的SBS频率ωB均为常数,产生的光频梳以ωR作为中心频率,该频率两侧所对应的第n阶梳齿线频率为ωR-nΔωB,其中n为梳齿模数,n>0时梳齿线频率由级联SBS激发产生,当n<0时梳齿线频率由FWM激发产生。此外,由于光学材料的ΔωR≫ΔωB(例如,金刚石晶体ΔωR≈1330 cm-1,ΔωB≈2 cm-1),因此相比于微腔振荡器采用的倏逝波耦合,可以通过腔镜镀膜即可实现泵浦光的直接耦合。
实验利用波长为1064 nm的准连续固体激光器作为泵浦源[15],其脉冲宽度为250 μs,重复频率为40 Hz,稳态功率最高为246 W。泵浦光首先经过磁光隔离器以保证泵浦光的单向传输,然后通过焦距为100 mm的凸透镜耦合至振荡器,并聚焦在增益介质单晶金刚石的中心。金刚石晶体的尺寸为8 mm×4 mm×1.2 mm,两端镀有损伤阈值>500 MW/cm2的1.24 μm波长的增透膜。振荡器的输入镜和输出镜的曲率半径分别为-100 mm和-50 mm,其中输入镜内表面镀有增透膜(@1.06 μm泵浦光)以及对应的高反射膜(@1.24 μm一阶拉曼频移);输出镜表面镀有高反射膜(@1.06 μm)和1.24 μm波段反射率约为99.5%的反射膜。为了降低光频梳的产生阈值,金刚石晶体置于振荡器的共心处,振荡器长度约为152 mm,对应冷腔的拉曼频率和一阶SBS频率处的光束束腰半径约为60 μm。同时,振荡器的腔镜置于压电陶瓷精密位移平台上,通过调节腔长可实现腔内拉曼光谱频率和一阶SBS光波频率的匹配,进而达到功率增强的效果。置于输出端的滤光片用于过滤残余泵浦光以获得纯净的布里渊频率梳输出。
图 2. 输出光功率随泵浦光功率的变化(插图:泵浦光和Stokes光的近场光斑)
Fig. 2. Output power versus pump power(inset: near-field profiles of pump and Stokes beams)
图 3. 输出的Stokes光光谱图。(a)单一的拉曼频率;(b)一阶SBS频率;(c)布里渊频率梳
Fig. 3. Output spectra of Stokes beams. (a) Single Raman frequency; (b) first-order SBS frequency; (c) Brillouin frequency comb
本文创新性地提出以拉曼场作为媒介在空间光腔中激发SBS的方案,并利用具有高拉曼和布里渊增益系数的金刚石作为增益介质获得了频率间隔高达71 GHz、稳态功率大于100 W的连续波布里渊频率梳输出。由于增益介质的固有拉曼和布里渊频移均为常数,因此对于该空间光腔结构,仅需要控制泵浦光波长和振荡器参数,即可解决微腔布里渊振荡腔固有的制备困难和输出功率过低的问题,同时借助拉曼转换使得大幅度拓展光学频率梳的工作波长范围成为可能。空间光腔布里渊频率梳具有独特优势,本课题组将针对空间相干测量、精密测量等领域所需的高功率、大频率间隔的光学频率梳开展研究工作,进一步提升布里渊频率梳的频率稳定性、提高转换效率并拓展输出波段范围。
[1] Fortier T, Baumann E. 20 years of developments in optical frequency comb technology and applications[J]. Communications Physics, 2019, 2: 153.
[2] Udem T, Holzwarth R, Hänsch T W. Optical frequency metrology[J]. Nature, 2002, 416(6877): 233-237.
[3] Chembo Y K. Kerr optical frequency combs: theory, applications and perspectives[J]. Nanophotonics, 2016, 5(2): 214-230.
[4] 吴冠豪, 周思宇, 杨越棠, 等. 双光梳测距及其应用[J]. 中国激光, 2021, 48(15): 1504002.
[5] 杨明哲, 孟飞, 林弋戈, 等. 基于光纤光梳的传递振荡器技术研究[J]. 激光与光电子学进展, 2020, 57(7): 070602.
[6] Kippenberg T J, Holzwarth R, Diddams S A. Microresonator-based optical frequency combs[J]. Science, 2011, 332(6029): 555-559.
[7] Cundiff S T, Ye J. Colloquium: femtosecond optical frequency combs[J]. Reviews of Modern Physics, 2003, 75(1): 325.
[8] Bai Z X, Yuan H, Liu Z H, et al. Stimulated Brillouin scattering materials, experimental design and applications: a review[J]. Optical Materials, 2018, 75: 626-645.
[9] Lu Q Y, Razeghi M, Slivken S, et al. High power frequency comb based on mid-infrared quantum cascade laser at λ ~9 μm[J]. Applied Physics Letters, 2015, 106(5): 051105.
[10] Ye J, Schnatz H, Hollberg L W. Optical frequency combs: from frequency metrology to optical phase control[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2003, 9(4): 1041-1058.
[11] Gundavarapu S, Brodnik G M, Puckett M, et al. Sub-hertz fundamental linewidth photonic integrated Brillouin laser[J]. Nature Photonics, 2019, 13(1): 60-67.
[12] Eggleton B J, Poulton C G, Rakich P T, et al. Brillouin integrated photonics[J]. Nature Photonics, 2019, 13(10): 664-677.
[13] Bai Z X, Williams R J, Kitzler O, et al. Diamond Brillouin laser in the visible[J]. APL Photonics, 2020, 5(3): 031301.
[14] Williams R J, Kitzler O, Bai Z X, et al. High power diamond Raman lasers[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2018, 24(5): 1602214.
[15] Bai Z, Zhang Z, Wang K, et al. Comprehensive thermal analysis of diamond in a high-power Raman cavity based on FVM-FEM coupled method[J]. Nanomaterials, 2021, 11(6): 1572.
[16] Bai Z X, Williams R J, Jasbeer H, et al. Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion[J]. Optics Letters, 2018, 43(3): 563-566.
[17] Rodgers B C, Russell T H, Roh W B. Laser beam combining and cleanup by stimulated Brillouin scattering in a multimode optical fiber[J]. Optics Letters, 1999, 24(16): 1124-1126.
[18] 白振旭, 杨学宗, 陈晖, 等. 高功率金刚石激光技术研究进展(特邀)[J]. 红外与激光工程, 2020, 49(12): 20201076.
[19] Li Y L, Ding J, Bai Z X, et al. Diamond Raman laser: a promising high-beam-quality and low-thermal-effect laser[J]. High Power Laser Science and Engineering, 2021, 9: e35.
[20] Antipov S, Sabella A, Williams R J, et al. 1.2 kW quasi-steady-state diamond Raman laser pumped by an M2=15 beam[J]. Optics Letters, 2019, 44(10): 2506-2509.
[21] Granados E, Spence D J, Mildren R P. Deep ultraviolet diamond Raman laser[J]. Optics Express, 2011, 19(11): 10857-10863.
[22] Li Y Q, Bai Z X, Chen H, et al. Eye-safe diamond Raman laser[J]. Results in Physics, 2020, 16: 102853.
[23] Sabella A, Piper J A, Mildren R P. Diamond Raman laser with continuously tunable output from 3.38 μm to 3.80 μm[J]. Optics Letters, 2014, 39(13): 4037-4040.
白振旭, 陈晖, 丁洁, 齐瑶瑶, 王雨雷, 吕志伟. 基于空间光腔的高功率布里渊频率梳[J]. 中国激光, 2022, 49(4): 0415001. Zhenxu Bai, Hui Chen, Jie Ding, Yaoyao Qi, Yulei Wang, Zhiwei Lü. High-Power Brillouin Frequency Comb Based on Free-Space Optical Cavity[J]. Chinese Journal of Lasers, 2022, 49(4): 0415001.