-
图1为无输出耦合镜外腔光谱合束结构示意图,其中半导体激光阵列水平放置,前腔面镀增透膜。空间位置不同的发光单元出射的光束经传输透镜后沿不同角度入射到衍射光栅上,部分0级衍射光和沿1级衍射方向光束再次衍射后经全反镜反馈回原发光单元,将每个发光单元锁定在不同的起振波长,起振波长满足光栅方程,合束激光沿−1级衍射方向输出。
图 1 无输出耦合镜外腔光谱合束结构示意图
Figure 1. Structure of diagram external cavity spectral beam combining without output coupler
半导体激光阵列发光面位于传输柱透镜Lens1的焦平面上,全反镜HR1位于传输透镜Lens4的焦平面上,两传输透镜Lens1和Lens4组成远焦望远镜系统,光栅到两个传输透镜的距离均为透镜焦距,在水平方向类似4f像传递系统。从半导体激光阵列上各单元发出的光束经过快轴准直镜FAC和慢轴准直镜SAC进行快慢轴准直,然后通过传输透镜Lens1叠加到衍射光栅上,−1级衍射光束作为合束光束输出,透过光栅的0级衍射光束经过传输透镜Lens4在全反镜HR1处成光源的实像,然后被全反镜HR1反射沿原光路返回,传输透镜Lens4作为返回光的变换透镜并且将光束叠加到衍射光栅上。该光束的一部分光透过光栅返回到发光单元,另一部分光沿光栅1级衍射方向传输,经过全反镜HR2反射再次返回到衍射光栅,并再次经历衍射过程,一部分透射到光栅−1级合束方向,剩余光衍射后沿光栅0级衍射方向传输。因而实现各个发光单元波长锁定的反馈光束由两部分组成:0级衍射光束经HR1反射回光栅的透射返回光和沿1级衍射方向的衍射光束再次衍射后经全反镜HR1原路返回发光单元的返回光。为滤除大偏角的1级衍射光束造成的反馈光串扰,在1级衍射光束光路中引入带有狭缝滤波器的望远镜结构(由Lens2和Lens3组成)。图中,R1、R2分别为半导体激光阵列后端面和前端面的反射率,T1、T3分别为传输透镜Lens1和传输透镜Lens4的透过率,ηg为衍射光栅的−1级衍射效率,为了计算方便,假设望远镜滤波结构的整体透过率为T2。
根据Lang-Kobayashi理论模型[17],将半导体激光阵列各子发光单元与外腔构成的谐振腔等效为如图2所示的结构,其中,R1、R2分别为半导体激光阵列的后端面和前端面的反射率,Rex1、Rex2为外腔等效反射率,ηg 为光栅衍射效率(通常为−1 级衍射效率),Ld表示半导体激光阵列内腔长度,Lex表示外腔长度。
图 2 无输出耦合镜外腔光谱合束结构等效谐振腔
Figure 2. Equivalent resonator of the external cavity spectral beam combining structure without output coupler
根据Lang-Kobayashi理论模型,在稳态工作条件下,载流子密度Nc为:
$$ {N_{\text{c}}} = {N_{\text{T}}} - \frac{{2k}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos (\omega \tau ) - \frac{{2k'}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos \left( {\omega \tau '} \right) $$ (1) 阈值载流子密度
$N_{{\rm{T}}} $ 为:$$ {N_{\text{T}}} = {N_0} + \frac{1}{{{G_{\text{N}}}{\tau _{\text{P}}}}} $$ (2) 在有光束反馈和光束串扰的情况下,从半导体激光器前端面输出光强I为:
$$ I = |E{|^2} \cdot \left( {1 - {R_2}} \right) = \frac{{R - {N_{\text{c}}}/{\tau _{\text{c}}}}}{{{G_{\text{N}}}\left( {{N_{\text{c}}} - {N_0}} \right)}} \cdot \left( {1 - {R_2}} \right) $$ (3) 进一步得到该发光单元经过光谱合束后的输出光强Ij为:
$$\begin{split} {I_{{{j}}}} =& \frac{{{\tau _{\text{c}}}R - {N_{\text{T}}} + \dfrac{{2k}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos (\omega \tau ) + \dfrac{{2k'}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos \left( {\omega \tau '} \right)}}{{\dfrac{{{\tau _{\text{c}}}}}{{{\tau _{\text{p}}}}} - \dfrac{{2k{\tau _{\text{c}}}}}{{{\tau _{\text{d}}}}}\cos (\omega \tau ) - \dfrac{{2k'{\tau _{\text{c}}}}}{{{\tau _{\text{d}}}}}\cos \left( {\omega \tau '} \right){\tau _{\text{c}}}}}\cdot \\ &\left( {1 - {R_2}} \right) \cdot \left[ {2\left( {1 - {T_1}} \right) + T_1^2(\eta _{\text{0}}^2T_3^2 + \eta _{\text{1}}^2T_2^2)} \right]\sqrt {{\eta _{{\rm{ex}}}}} \end{split}$$ (4) 式中:N0为无反馈时的载流子密度;GN为微分增益因子;ω 为发光单元的角频率;τ为外腔往返时间;τc为载流子寿命;τd为内腔往返时间;τp为光子寿命;τ′为返回到原发光单元的串扰光束外腔往返时间;k为反馈强度参数;k′为串扰光束反馈强度参数;R为单位体积载流子注入率;R2为半导体激光阵列的前端面反射率;E为光场初始振幅;ηex为外腔耦合效率;η0为光栅的0级衍射效率;η1为光栅的1级衍射效率。
在无光反馈时,即k=k′=0,则该发光单元前端面输出光强I0为:
$$ {I_0} = \left( {R{\tau _{\text{p}}} - \dfrac{{{\tau _{\text{p}}}}}{{{\tau _{\text{c}}}}}{N_{\text{T}}}} \right) \cdot \left( {1 - {R_2}} \right) $$ (5) 于是,该发光单元的合成效率ηj[13]为:
$$\begin{split} {\eta _j} =& \frac{{{I_j}}}{{{I_0}}} =\\ & \frac{{{\tau _{\text{c}}}R - {N_{\text{T}}} + \dfrac{{2k}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos (\omega \tau ) + \dfrac{{2k'}}{{{G_{\text{N}}}{\tau _{\text{d}}}}}\cos \left( {\omega \tau '} \right)}}{{\left( {R{\tau _{\text{p}}} - \dfrac{{{\tau _{\text{p}}}}}{{{\tau _{\text{c}}}}}{N_{\text{T}}}} \right)\left[ {\dfrac{{{\tau _{\text{c}}}}}{{{\tau _{\text{p}}}}} - \dfrac{{2k{\tau _{\text{c}}}}}{{{\tau _{\text{d}}}}}\cos (\omega \tau ) - \dfrac{{2k'{\tau _{\text{c}}}}}{{{\tau _{\text{d}}}}}\cos \left( {\omega \tau '} \right){\tau _{\text{c}}}} \right]}} \cdot \\ &\left[ {2\left( {1 - {T_1}} \right) + T_1^2(\eta _{\text{0}}^2T_3^2 + \eta _{\text{1}}^2T_2^2)} \right]\sqrt {{\eta _{{\rm{ex}}}}} \\[-12pt] \end{split}$$ (6) 式中:
$ k = \left( {1 - {R_2}} \right){\left( {{R_3}/{R_2}} \right)^{1/2}} $ 。$$ {R_3} = \left[ {2\left( {1 - {T_1}} \right) + T_1^2(\eta _{\text{0}}^2T_3^2 + \eta _{\text{1}}^2T_2^2)} \right]{\eta _{{\rm{ex}}}} $$ (7) 式中:R3为外腔各光学元件等效反射率。
每个发光单元经光栅-外腔合成系统之后的效率主要与内腔特性(内腔折射率n、内腔长度Ld、半导体激光器前端面反射率R2、光子寿命τp)和外腔特性(外腔长度Lex、外腔等效反射率R3、变换透镜透射率T1、光栅0级和1级衍射效率分别为η0和η1,外腔耦合效率ηex、外腔往返时间τ′)有关。此外,受到串扰光束影响,合成效率还与返回至自身串扰光的反馈强度和其他单元串扰光的反馈光有关。
故经光谱合成系统后的合成效率ηsbc可以表示为:
$$ {\eta }_{\text{sbc}}=\frac{\displaystyle\sum _{j=-m}^{m}{\eta }_{j}{I}_{0}}{\displaystyle\sum _{j=-m}^{m}{I}_{0}} $$ (8) 由公式(7)可知,对于无输出耦合镜外腔光谱合束结构,外腔长度Lex、外腔等效反射率R3、变换透镜透射率T1和T3、望远镜滤波系统整体透射率T2、光栅衍射效率η0和η1、外腔耦合效率ηex、外腔往返时间τ′均会影响反馈光束强度以及波长锁定的稳定性,进而影响合束效率及光束质量。
下文分别研究无输出耦合镜外腔光谱合束结构的两臂距离(即外谐振腔腔长)、外腔等效反射率、望远镜滤波系统及 "Smile"对反馈效率的影响。
Influence of output mirror free external cavity spectral beam combining structure on feedback efficiency
-
摘要: 半导体激光阵列无输出耦合镜外腔光谱合束技术利用光栅的0级和1级衍射光束反馈实现发光单元的波长锁定,避免了0级和1级衍射光束的转储和浪费,可以获得高的合束效率。因此,0级和1级衍射光束的反馈量高低就会决定外腔波长锁定的稳定性,进而影响合束后光束质量的高低甚至光谱合束的成败。针对此种结构,理论研究了两外腔长度、望远镜滤波结构及 "Smile"效应对0级和1级衍射光束反馈效率的影响,结果表明:(1)外腔长度会影响反馈功率以及串扰程度;(2)望远镜滤波结构可以有效滤除大偏角杂散光束以及使光束正确反馈回原发光单元;(3)"Smile"效应的程度对反馈效率以及输出光束质量影响尤为严重,需要采取措施进行抑制。
-
关键词:
- 半导体激光阵列 /
- 无输出耦合镜外腔光谱合束 /
- "Smile"效应 /
- 反馈效率
Abstract:Objective The external cavity spectral beam combining technology has two structures of open loop (without output coupler) and closed loop (with output coupler). The main difference between them is that the wavelength locked feedback beam of each light-emitting unit is different. Among them, the output coupler free external cavity spectral beam combining structure uses the beam returning from re-diffraction along the 0th-order and 1st-order diffraction direction as the feedback light, which avoids the waste of the beam and overcomes many problems of the feedback locking of the −1st order diffraction beam, and realizes the high-efficiency wavelength beam combination. For spectral beam combining technology, the feedback efficiency of the external cavity determines the stability of wavelength locking and even the success or failure of beam combination. Compared with the general closed-loop structure, the output coupler free external cavity spectral beam combining structure can only obtain enough feedback beams to achieve wavelength locking by controlling the parameters of the external cavity, and this external cavity structure is relatively complex, and the feedback efficiency variation caused by some factors in the external cavity are more obvious than the closed-loop structure. Therefore, for this structure, a simulation system is established to study the influencing factors of the external cavity feedback efficiency. Methods The efficiency model of output coupler free external cavity spectral beam combining structure is constructed, and the expression of spectral beam combining efficiency is deduced. According to the expression, the length of the external cavity, the telescope filtering system and the "Smile" effect have great influence on the feedback efficiency of this structure. The output coupler free external cavity spectral beam combining structure in Zemax is established, and the level of the feedback quantity of the two diffraction cavities in this system and the change in feedback quantity caused by changing the cavity length, the influence of the 1st-order diffraction cavity added to the telescope system on the feedback efficiency and quality of the combined beam, and the influence of the "Smile" effect on the feedback beam intensity are studied respectively. Results and Discussions According to the output coupler free external cavity spectral beam combining simulation system, the 1st-order diffraction light feedback power accounts for 2.73% of the output power, while the 0th-order diffraction light feedback power only accounts for 0.15% of the output power (Fig.3). As the distance of the external cavity increases from 27 mm to 558 mm, the feedback power decreases from 1.76 W to about 1.45 W (Fig.4), and the feedback beam crosstalk will occur (Fig.5); When different telescope filtering systems are inserted into the 1st-order diffraction cavity, the feedback power of the external cavity remains basically unchanged and the size of the slow-axis beam spot remains about 4 mm (Fig.8); The feedback power steadily diminishes and the combined spot size gradually increases as the degree of "Smile" effect rises from 0 μm to 1 μm. The feedback power of the 500-mm long focal length cylindrical lens inserted in the fast axis direction under the impact of the "Smile" effect at 1 μm is essentially the same as that without the "Smile" effect, which lessens the influence of the "Smile" effect on the feedback power. Conclusions The factors impacting on the external cavity's feedback efficiency are analyzed using the output coupler free external cavity spectral beam combining efficiency model. The effects of the external cavity length, the filter structure of the telescope, and the "Smile" effect on the feedback efficiency of the 0th-order and 1st-order diffracted beams are studied, respectively, using the output coupler free external cavity spectral beam combining simulation system built in Zemax. The results show that: (1) The feedback beam is dominated by the 1st-order diffracted beam, and wavelength locking of the external cavity is essential. As the length of the external cavity rises, the feedback power falls and the beam crosstalk increases; (2) The telescope filter structure can effectively filter the stray beam with large deflection angle and accurately feed back the beam to the original light-emitting unit; (3) The degree of "Smile" effect has a particularly negative influence on feedback efficiency and output beam quality. Although the "Smile" effect can be lessened by inserting a long focal length cylindrical lens in the fast axis direction, the beam quality after beam combination won't be noticeably enhanced. The research on the feedback efficiency of the external cavity can be used as a guide when designing the parameters for the output coupler external cavity spectral beam combining structure. -
-
[1] Osamu Kumagai, Ikeda Masao, Yamamoto Masanobu. Application of laser diodes to optical disk systems: The history of laser diode development and mass production in three generations of optical disk systems [J]. Proceedings of the IEEE, 2013, 101(10): 2243-2254. doi: 10.1109/JPROC.2013.2275017 [2] Ning Yongqiang, ChenYongyi, Zhang Jun, et al. Brief review of development and techniques for high power semiconductor lasers [J]. Acta Optica Sinica, 2021, 41(1): 0114001. (in Chinese) doi: 10.3788/AOS202141.0114001 [3] Jiang Man, Ma Pengfei, Su Rongtao, et al. Research progress and prospect of spectral beam combining (Invited) [J]. Infrared and Laser Engineering, 2020, 49(12): 20201053. (in Chinese) doi: 10.3788/IRLA20201053 [4] Daneu V, Sanchez A, Fan T Y, et al. Daneu. Spectral beam combining of a broad-stripe diode laser array in an external cavity [J]. Optics Letters, 2000, 25(6): 405-407. [5] Chann B, Huang R-K, Missaggia L-J, et al. Near-diffraction-limited diode laser arrays by wavelength beam combining [J]. Optics Letters, 2005, 30(16): 2104-2106. doi: 10.1364/OL.30.002104 [6] Zhu Zhanda, Gou Long, Jiang Menghua, et al. High beam quality in two directions and high efficiency output of a diode laser array by spectral-beam-combining [J]. Optics Express, 2014, 22(15): 17804-17809. doi: 10.1364/OE.22.017804 [7] Zhang J, Peng H, Fu X, et al. CW 50 W/M2 = 10.9 diode laser source by spectral beam combining based on a transmission grating [J]. Optics Express, 2013, 21(3): 3627-3632. doi: 10.1364/OE.21.003627 [8] Zhang Jun, Peng Hangyu, Wang Jingbo, et al. Dense spectral beam combining of quantum cascade lasers by multiplexing a pair of blazed gratings [J]. Optics Express, 2022, 30(2): 966-971. doi: 10.1364/OE.446124 [9] Lin Xunchun, Lin Guyi, Zhao Pengfei, et al. Generation of high brightness diode laser by using spectral and polarization beam combination [J]. Optics & Laser Technology, 2019, 116: 219-223. doi: doi.org/10.1016/j.optlastec.2019.03.042 [10] Meng Huicheng, Sun Tangyou, Tan Hao, et al. High-brightness spectral beam combining of diode laser array stack in an external cavity [J]. Optics Express, 2015, 23(17): 21819-21824. doi: 10.1364/OE.23.021819 [11] Wu Z, Yang L, Zhang B. Effect of crosstalk on spectrally combined beam properties and efficiency in external cavities [J]. Applied Optics, 2017, 56(10): 2834-2842. doi: 10.1364/AO.56.002834 [12] Yang Lei, Wu Zhen, Zhong Zheqiang, et al. Effect of crosstalk on combined beam characteristics in spectral beam combining systems [J]. Optics Communications, 2017, 384: 30-35. doi: doi.org/10.1016/j.optcom.2016.09.060 [13] Zhong Zheqiang, Yang Lei, Hu Xiaochuan, et al. Light crosstalk and its effect on combining efficiency of diode laser array grating-external cavity spectral beam combining system [J]. Chinese Journal of Lasers, 2015, 42(10): 1002010. (in Chinese) doi: 10.3788/CJL201542.1002010 [14] Huang Robin-K, Chann Bien, Burgess James, et al. Teradiode's high brightness semiconductor lasers[C]// Components and Packaging for Laser Systems II, Proceedings of SPIE, 2016, 9730: 97300C. [15] Sun Shujuan, Tan Hao, Meng Huicheng, et al. High brightness diode laser by coupler free grating external cavity spectral beam combining [J]. Infrared and Laser Engineering, 2019, 48(3): 0306006. (in Chinese) doi: 10.3788/IRLA201948.0306006 [16] Sun Shujuan, Tan Hao, Meng Huicheng, et al. Coupler free grating external cavity spectral beam combining of diode laser stacks [J]. Chinese Journal of Lasers, 2018, 45(10): 1001007. (in Chinese) doi: 10.3788/CJL201845.1001007 [17] Lang R, Kobayashi K. External optical feedback effects on semiconductor injection laser properties [J]. IEEE Journal of Quantum Electronics, 1980, 16(3): 347-355. doi: 10.1109/JQE.1980.1070479 [18] Li Jing, Cao Yinhua, Liu Youqiang et al. Smile effect measurement of laser diode line arrays based on external cavity [J]. Chinese Journal of Luminescence, 2017, 38(10): 1302-1306. (in Chinese) doi: 10.3788/fgxb20173810.1302 [19] Gopinath J T, Chann B , Fan T Y, et al. 1450-nm high-brightness wavelength-beam combined diode laser array [J]. Optics Express, 2008, 16(13): 9405-9410. doi: 10.1364/OE.16.009405