高功率单共振光学参量振荡器的输出特性研究【增强内容出版】
1.5?1.8 μm and 3?5 μm infrared lasers are widely used in free-space optical communication, trace gas monitoring, environmental pollution monitoring, and biomedicine. Infrared lasers can be obtained using quantum cascade, fiber, solid-state, and Raman lasers. Compared with these methods, an optical parametric oscillator (OPO) can be used to achieve an infrared laser with a wider tuning range, higher power, and more stable operation. Based on the resonance of the pump, signal, and idler laser in the OPO cavity, the OPO can be referred to as singly resonant OPO (SRO), doubly resonant OPO (DRO), or triply resonant OPO (TRO). Compared with DRO and TRO, SRO requires only that the signal (or idler) light resonates in the cavity, has a relatively simple design, and no external servo system locking is required to obtain a stable, high-power output. Therefore, the output characteristics of a high-power SRO are investigated in this study.
First, a theoretical analysis is conducted on the influence of the SRO cavity length and nonlinear crystal thermal lens effect on the stability of the resonant cavity under standing-wave and ring-cavity structures. To ensure a stable operation of the SRO within significant changes in the resonant cavity parameters, the cavity length corresponding to |(A+D)/2|=0 is selected when the SRO is designed, that is, the standing wave cavity length is 54 mm, and the ring cavity length is 516 mm. According to the focusing factor selected for the SRO cavity, the waist spot of the signal light at the center of the MgO∶PPLN crystal is calculated to be 59.5 μm. Subsequently, based on the design of the SRO resonant cavity structure, the laser output characteristics of different SRO cavity types are theoretically analyzed. The theoretical analysis reveals that the pump light in the standing-wave cavity SRO passes through the nonlinear crystal twice; therefore the power gain of the signal light during forward and backward transmission must be considered simultaneously, whereas the pump light in the ring-cavity SRO passes through the nonlinear crystal in a single pass, and the parametric interaction during backward transmission is not considered. The threshold pump power and output power of the signal and idler light from the standing-wave cavity and ring-cavity SROs are calculated. Finally, the two mirror standing-wave cavity SROs and four mirror ring-cavity SROs based on the MgO∶PPLN crystal pumped by a high-power continuous-wave single frequency 1.06 μm laser are constructed, and the relationship between the output power of the signal and idler light with the pump power, as well as the power fluctuation and frequency drift of the signal and idler light are studied.
By controlling the temperature of MgO∶PPLN from 30 ℃ to 65 ℃, the signal wavelength can be tuned from 1550.03 nm to 1561.38 nm, and the corresponding idler wavelength can be tuned from 3394.7 nm to 3340.13 nm. When the temperature of the MgO∶PPLN crystal is controlled as 40 ℃, the signal and idler wavelengths of the SRO are 1.553 μm and 3.378 μm, respectively. The threshold pump power of the standing-wave cavity SRO is 3.2 W, and at a pump power of 14.2 W, the signal and idler powers are 5.2 W and 2.2 W, respectively. The threshold pump power of the ring-cavity SRO is 7.2 W, and at the pump power of 25 W, the signal and idler powers are 8.1 W and 3.6 W, respectively. The measured value of the ring-cavity SRO output power is in good agreement with the theoretical prediction result (Fig. 6). When the pump power is less than 15 W, the measured standing-wave cavity SRO output power agrees well with the theoretical prediction result; however, when the pump power is greater than 15 W, the measured standing-wave cavity SRO output power deviates significantly from the theoretical prediction result ( Fig. 6). According to the theoretical analysis, when the pump power is 15 W, the resonant signal power in the standing-wave cavity SRO is 260 W. The thermal lens focal length of the nonlinear crystal is 14 mm, and the corresponding stability parameter of the standing-wave cavity SRO is 0.98. An increase in the pump power results in the stability parameter of the standing-wave cavity SRO to be greater than 1; the SRO cannot operate stably, and the output power of the SRO decreases. To obtain a higher output power from the signal and idler lasers, the ring-cavity SRO is a better choice. In addition, when the signal and idler output powers from the standing-wave cavity and ring-cavity SROs are 5.0 W and 2.0 W, respectively, the power fluctuations in the signal and idler light by the standing-wave cavity SRO within 2 h are better than ±2.76% and ±2.53%, and the power fluctuations in the signal and idler light by the ring-cavity SRO within 2 h are better than ±1.24% and ±1.19%, respectively. The long term frequency drift of the signal is better than ±40 MHz and ±28 MHz, respectively.
The influence of the SRO cavity type on the output characteristics at high pump power is investigated in this study. First, a theoretical analysis is conducted on the influence of the SRO cavity length and nonlinear crystal thermal lens effect on the stability of the resonant cavity under the standing-wave cavity and ring-cavity structures. Notably, the ring-cavity SRO can operate stably within significant changes in the resonant cavity parameters. The output characteristics of the SRO are also theoretically analyzed. Second, a two-mirror standing-wave cavity and a four-mirror ring-cavity SROs based on a MgO∶PPLN crystal are experimentally constructed. At a pump power of 14.2 W, the signal and idler output powers from the standing-wave cavity SRO are 5.2 W and 2.2 W, respectively. At a pump power of 25 W, the signal and idler output powers from the ring-cavity SRO are 8.1 W and 3.6 W, respectively. The measured value of the ring-cavity SRO output power agrees well with the theoretical prediction result. The power fluctuations in the signal and idler light from the standing-wave cavity SRO within 2 h are better than ±2.76% and ±2.53%, and the power fluctuations in the signal and idler by the ring-cavity SRO within 2 h are better than ±1.24% and ±1.19%, respectively. The long term frequency drift of signal light from the standing-wave cavity and ring-cavity SROs are better than ±40 MHz and ±28 MHz, respectively. The research results indicate that to obtain a higher output power from the signal and idler lasers, the ring-cavity SRO is a good choice.
1 引言
波长位于1.5~1.8 μm和3~5 μm的红外激光在自由空间光通信[1]、痕量气体监测[2]、环境污染监测[3]、生物医学[4]等领域中具有广泛的应用。利用量子级联激光器[5]、光纤激光器[6]、固体激光器[7]、拉曼激光器[8]等均能获得红外激光输出。与这些方式相比,光学参量振荡器(OPO)[9-12]不仅能获得调谐范围更宽的激光输出,且更容易实现高功率和高稳定性的激光输出。此外,根据OPO腔内抽运光、信号光和闲频光的共振情况,可将其分为单共振OPO(SRO)、双共振和三共振OPO。其中,SRO只需使信号光(或者闲频光)在OPO腔内共振,其设计比较简单,容易获得调谐范围更宽和输出特性更稳定的红外激光。
早在1996年,Bosenberg等[13]通过实验研究发现,利用1.06 μm单频激光器抽运基于周期性极化铌酸锂(PPLN)晶体的环形腔SRO输出的红外激光更易于保持单纵模运转。2006年,Henderson等[14]利用1.08 μm光纤激光器抽运基于掺杂氧化镁的PPLN(MgO∶PPLN)晶体的SRO,获得了输出功率为750 mW的2.80 μm闲频光输出,阈值功率为780 mW,光光转换效率达26.7%。2016年,姜洪波等[15]通过优化抽运光功率和输出镜反射率,获得了8 W的2.90 μm闲频光输出,光光转换效率为19.5%。2019年,Bae等[16]利用1.06 μm激光抽运基于扇形MgO∶PPLN晶体的OPO,当抽运光功率为1.1 W时,获得了最高功率为64 mW的3.50 μm闲频光输出,光光转化效率为5.8%。2021年,Wang等[17]采用内腔式OPO结构,抽运光波长为1.06 μm,当抽运光功率为9.1 W时,获得了1.08 W的3.19 μm闲频光输出,光光转换效率为11.88%。2022年,王海龙等[18]利用1.06 μm单频激光器抽运基于MgO∶PPLN和PPKTP晶体的环形腔SRO,当抽运光功率为21 W时,1.55 μm信号光、3.39 μm闲频光和0.775 μm倍频光的输出功率分别为2.1、1.7、1.1 W,5 h内的功率稳定性(均方根)分别优于2.5%、1.6%、0.8%,总光光转换效率为23.3%。
为了进一步优化SRO输出红外激光的转换效率和稳定性,本文研究了SRO的腔型对其输出特性的影响。首先从理论上分析了SRO腔长以及非线性晶体对腔内共振信号光的吸收导致的热透镜效应对谐振腔稳定性参数的影响,在此基础上进一步分析了SRO的阈值抽运功率以及输出激光功率随抽运光功率的变化关系。然后在实验上搭建了利用高功率全固态连续单频1.06 μm激光器抽运的两镜驻波腔和四镜环形腔SRO,其由MgO∶PPLN晶体构成,信号光共振且部分耦合输出,研究了两种腔型下SRO输出信号光和闲频光功率随抽运光功率的变化,以及输出激光的长期功率波动和频率漂移。
2 理论分析
SRO的抽运激光为高斯光束,如果选取的聚焦因子较大,虽然可以获得较低的阈值抽运功率,但晶体本身存在的热效应会导致输出激光的稳定性和光束质量变差。为了优化SRO的输出特性,选择共焦聚焦(即聚焦因子为1)设计SRO。我们将在设计SRO腔型结构的基础上,理论分析SRO腔的腔长和非线性晶体的热透镜焦距对SRO腔稳定性参数的影响以及不同腔型SRO的激光输出特性。
驻波腔SRO的结构如
环形腔SRO的结构如
首先利用谐振腔的ABCD矩阵分析计算SRO腔长和腔内MgO∶PPLN晶体的热透镜焦距对谐振腔稳定性参数的影响。在高功率抽运条件下,非线性晶体对内腔共振信号光的吸收会导致热透镜效应。由于闲频光单次穿过、抽运光双次穿过非线性晶体,忽略非线性晶体对闲频光和抽运光的吸收。假定信号光的腰斑位置与非线性晶体的中心重合,可得到MgO∶PPLN晶体的热透镜焦距(f)[19]为
式中:
由于MgO∶PPLN非线性晶体较长(实验研究中其长度l=30 mm),在理论分析过程中,将MgO∶PPLN晶体的热透镜等效为间距相同的焦距均为8f的8个薄透镜,则信号光在非线性晶体中传输一半距离(l/2)的矩阵可以表示为
设定SRO腔内共振的信号光从晶体中心处开始传输,由此可得到驻波腔和环形腔SRO的传输矩阵分别为
式中:s和r分别表示驻波腔和环形腔。
SRO稳定运转须满足谐振腔的稳定性条件 |(A+D)/2|<1。
图 3. SRO腔的稳定性参数随谐振腔腔长的变化
Fig. 3. Stability parameter of SRO cavity versus resonator cavity length
在SRO腔长确定后,由式(
图 4. SRO腔的稳定性参数随MgO∶PPLN晶体热透镜焦距的变化
Fig. 4. Stability parameter of SRO cavity versus thermal focal length of MgO∶PPLN crystal
在设计SRO谐振腔结构的基础上,下一步将理论分析不同腔型SRO的激光输出特性。驻波腔和环形腔SRO腔内各光场的分布如
式中:
驻波腔内信号光的振荡条件为
式中:
根据边界条件,驻波腔SRO输出的信号光和闲频光功率分别为
与驻波腔相比,环形腔SRO中的抽运光单次穿过非线性晶体,不需要考虑反向传输时的参量相互作用,因此环形腔中信号光的功率增益[20]为
式中:
环形腔内信号光的振荡条件为
式中:
根据边界条件,环形腔SRO输出的信号光和闲频光功率分别为
可以利用式(
3 实验装置
图 5. 利用单共振光学参量振荡器产生连续单频红外激光的实验装置
Fig. 5. Experimental setup for generating continuous-wave single-frequency infrared laser by SRO
4 实验结果
当控制MgO∶PPLN晶体的温度为40 ℃时,SRO输出的信号光和闲频光波长分别为1.553 μm和3.378 μm。驻波腔SRO输出的信号光和闲频光功率随抽运光功率变化的数据如
图 6. SRO输出信号光和闲频光的功率随抽运光功率的变化(点为实验数据,线为理论预测结果)
Fig. 6. Output power of signal and idler light from SRO versus pump power (point is experimental data and curve is theoretical prediction result)
通过控制SRO腔内MgO∶PPLN晶体的温度可以获得波长可调谐的信号光和闲频光输出。当MgO∶PPLN晶体的温度从30 ℃调节到65 ℃时,SRO输出的信号光波长可从1550.03 nm调谐到1561.38 nm,调谐范围达13.35 nm,对应的闲频光波长可从3394.7 nm调谐到3340.13 nm,调谐范围达54.57 nm。此外,由于红外激光的光束质量对其应用具有重要的影响,当MgO∶PPLN晶体的温度为40 ℃时,SRO输出的信号光和闲频光波长分别为1.553 μm和3.378 μm。在上述SRO运转条件下,利用光束质量分析仪测量了信号光的光束质量。实测的驻波腔SRO输出的信号光在水平方向上的光束质量因子
当抽运光功率为14 W时,利用自由光谱区范围为150 MHz、精细度为380的分析腔以及数字示波器测量记录了SRO输出信号光的线宽。实测的驻波腔和环形腔SRO输出信号光的线宽分别为0.58 MHz和0.43 MHz,该测量值受到分析腔分辨率的限制。根据信号光的线宽以及信号光和闲频光的波长可以估算得到驻波腔和环形腔SRO输出闲频光的线宽分别小于2.74 MHz和2.03 MHz。可以看出,环形腔SRO输出下转换光的线宽优于驻波腔SRO输出的下转换光的线宽。
为进一步研究SRO输出激光的稳定性,利用功率计测量了信号光和闲频光的长期功率波动,并用波长计测量了信号光的长期频率漂移(由于波长计的限制,闲频光的频率无法直接测量)。当MgO∶PPLN晶体的温度为40 ℃,抽运光功率分别为13.5 W和15.0 W时,驻波腔和环形腔SRO输出的信号光和闲频光功率均分别为5.0 W和2.0 W。
图 7. SRO自由运转时信号光和闲频光的功率随时间的变化。(a)驻波腔SRO;(b)环形腔SRO
Fig. 7. Power of signal and idler light versus time when SRO is free running. (a) Standing wave cavity SRO; (b) ring cavity SRO
波长计测量记录的2 h内驻波腔和环形腔SRO输出信号光的长期频率漂移如
图 8. SRO自由运转时信号光的频率漂移曲线。(a)驻波腔SRO;(b)环形腔SRO
Fig. 8. Frequency drift curve of signal light when SRO is free running. (a) Standing wave cavity SRO; (b) ring cavity SRO
最后实验研究了在高功率抽运条件下环形腔SRO的输出特性。当MgO∶PPLN晶体温度为40 ℃、抽运光功率为25.0 W时,环形腔SRO输出的信号光和闲频光功率分别为8.1 W和3.6 W。
图 9. 环形腔SRO自由运转时信号光和闲频光的功率以及信号光的频率随时间的变化曲线。(a)信号光和闲频光的功率随时间的变化;(b)信号光的频率随时间的变化
Fig. 9. Output power of signal and idler light and frequency of signal light versus time when ring cavity SRO is free running. (a) Power of signal and idler light versus time; (b) frequency of signal light versus time
5 结论
为了获得稳定运转的高功率红外激光输出,在高抽运功率下研究了SRO的腔型对其输出特性的影响。首先从理论上分析了驻波腔和环形腔结构下SRO腔长以及非线性晶体的热透镜效应对谐振腔稳定性的影响,发现环形腔SRO在谐振腔参数变化较大的范围内均可稳定运转。进一步理论分析了SRO的阈值抽运功率以及输出激光功率随抽运光功率的变化规律。其次实验搭建了基于MgO∶PPLN晶体的两镜驻波腔和四镜环形腔SRO。当MgO∶PPLN晶体的温度从30 ℃调节到65 ℃时,SRO输出的信号光波长可从1550.03 nm调谐到1561.38 nm,对应的闲频光波长可从3394.7 nm调谐到3340.13 nm。驻波腔SRO的阈值抽运功率为3.2 W,当抽运光功率为14.2 W时,信号光和闲频光功率分别为5.2 W和2.2 W,光光转换效率为52.1%。当抽运光功率小于15 W时,驻波腔SRO输出光功率的实测值和理论预测基本一致;但当抽运光功率大于15 W时,驻波腔SRO输出光功率的实测值随抽运光功率的增大而减小,与理论预测偏差较大。这是由于随着抽运功率的增大,非线性晶体的热透镜效应导致驻波腔SRO不能稳定运转。环形腔SRO的阈值抽运功率为7.2 W,当抽运光功率为25 W时,信号光和闲频光功率分别为8.1 W和3.6 W,光光转换效率为46.8%。环形腔SRO输出光功率的实测值和理论预测基本一致。驻波腔SRO输出信号光和闲频光在2 h内的功率波动分别优于±2.76%和±2.53%,环形腔SRO输出信号光和闲频光在2 h内的功率波动分别优于±1.24%和±1.19%、信号光的长期频率漂移分别优于±40 MHz及±28 MHz。研究结果表明,为获得更高的信号光和闲频光输出功率及运转稳定性,环形腔SRO是比较好的选择。
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