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在谐振腔中心位置处,通过激光光束轮廓仪(NanoScan V2,Ophir Photonics)测得经透镜聚焦后的泵浦光束腰直径约为100 μm,如图2所示。为满足模式匹配要求,结合对称双凹腔的特性和谐振腔稳态条件,设计谐振腔长为97 mm,此时谐振腔振荡光在谐振腔中的束腰直径约为96 μm,谐振腔内泵浦光与振荡光束腰比值接近1∶1。实验同时另外设计了腔长约为60 mm和80 mm的两种谐振腔,谐振腔内泵浦光束腰与振荡光束腰比值分别为1∶2.3和1∶1.8。在8.6 μm极化周期和晶体温度为27 ℃条件下,得到了这三种谐振腔的信号光功率随泵浦光功率的变化关系,如图3所示。60 mm腔长的谐振腔振荡阈值最低约为1.45 W,这是因为更小的泵浦光斑提供了更高的功率密度。97 mm腔长的谐振腔振荡阈值最高约为3.0 W,但在9 W泵浦功率下其信号光输出功率却为最高约4.8 W,相对更大的振荡光模式体积对这一现象的产生有直接影响。这些结果表明模式匹配对高功率输出有较大影响,以下工作均围绕97 mm谐振腔进行研究分析。
图 2 谐振腔内泵浦光经透镜聚焦后的光斑轮廓
Figure 2. Spatial beam profile of the pump light in the cavity after focusing by the lens
图 3 27 ℃时,不同谐振腔腔长下的信号光输出功率与泵浦功率的变化关系
Figure 3. Signal output power as function of the pump power with different kinds of laser cavity lengths at 27 ℃
由于谐振腔镜对闲频光1.3~1.5 μm波段的反射率>99.7%,实验中只检测到微弱的闲频光信号,因此文中只对信号光特性进行重点分析。图4所示为晶体温度为27 ℃,泵浦功率为13.6W时,821、837、856、879 nm的信号光分别对应的最大输出功率。同时对这4个波长的输出功率随泵浦功率变化的特性分别进行了研究,实验结果如图5所示。随着泵浦功率的增加,信号光的输出功率也在逐渐增加。当输入泵浦功率达到13.6 W时,在821 nm处,获得了6.8 W的输出,光-光转换效率达到50.0%。而当泵浦功率为5.4 W时,获得了3.1 W的821 nm激光输出,光-光转换效率达到了57.4%。极化周期为8.3、8.4、8.5 μm时,在同样泵浦功率下,分别得到了48.2%、51.7%及55.2%的高转换效率。在泵浦功率13.6 W时,在879、856、837 nm波段处获得的输出功率分别达到5.9、6.2、6.4 W,对应的光-光转换效率分别达到43.4%、45.6%和47.1%。图5(b)所示为为在0~13.6 W的泵浦功率范围内,821、837、856、879 nm波长对应的最高光-光转换效率分别为57.4%、55.2%、51.7%和48.2%。从图中可以看出,在4个波长均在泵浦功率在6 W附近时,光-光转换效率达到峰值,随着泵浦功率的进一步增加,转换效率逐渐下降,但是,OPO系统的输出仍保持着43%以上的高转换效率。随着泵浦功率的逐渐增加,输出功率也保持上升趋势,而且没有出现饱和的迹象。分析认为高转换效率的实现与单谐振结构和闲频光的高反射率有一定的关系,不过出于预防晶体损伤等原因的考虑,实验没有进一步增加泵浦功率。
图 4 在泵浦功率为13.6 W,温度为27 ℃时,不同信号光的输出功率
Figure 4. Output power of the different signal light at pump power 13.6 W and temperature 27 ℃
图 5 27 ℃时,极化周期为8.3、8.4、8.5、8.6 μm时,信号光输出功率及其转换效率随泵浦功率的变化关系。(a)信号光功率;(b)转换效率
Figure 5. Signal output power and conversion efficiency as functions with pump power when the poling periods are 8.3, 8.4, 8.5 and 8.6 μm at 27 ℃. (a) Signal output power; (b) Conversion efficiency
通过温控炉将晶体温度控制在27 ℃,移动位移平台,得到了8.3、8.4、8.5、8.6 μm 4个极化周期的周期调谐光谱曲线,分别是879、856、837、821 nm信号光及其相对应的1 352、1 411、1 464、1 515 nm闲频光,如图6所示。
如图7所示,将实验结果与根据Sellmeier色散方程[16]计算得到的理论值进行对比,图中的实线为理论计算值曲线,离散点为实验测量值。从图7中可以看出,在信号光波段,实验值与理论值高度符合,在闲频光波段,实验结果与理论值也保持较高的重合性,因此Sellmeier色散方程对工作温度为27 ℃的周期调谐具有较高的参考价值。
图 7 27 ℃时,CW MgO:sPPLT-OPO调谐曲线随周期的变化
Figure 7. Polarization periodic tuning curve of the CW MgO:sPPLT-OPO at 27 ℃
在周期调谐的基础上,通过改变MgO:sPPLT晶体的温度获得了8.3、8.4、8.5、8.6 μm 4个周期的温度调谐曲线,如图8所示。图中离散点为实验测量得到的输出光谱数据,实线为根据Sellmeier色散方程得到的理论值。通过对比发现,在27~83 ℃范围内改变这4个极化周期的温度就可以实现807~879 nm信号光和1352~1567 nm闲频光波段范围内不间断地连续可调谐输出,实验结果与理论值在低温条件下(27~50 ℃)比较吻合,而在高温条件下(55~90 ℃)实验结果与理论值有一定偏差,分析认为这是由于所采用的Sellmeier色散方程在高温条件下的精度有限所导致。根据温度调谐的结果发现周期不同,温度调谐速度也略有不同,随着极化周期的增加调谐速度逐渐降低,信号光和闲频光的变化趋势相同,如图9所示。8.3 μm周期的信号光调谐速度约为−0.4 nm/℃,闲频光调谐速度约为1.0 nm/℃,而8.6 μm周期的信号光调谐速度则降至−0.25 nm/℃,闲频光调谐速度则降至0.9 nm/℃。
图 8 极化周期为8.3、8.4、8.5、8.6 μm时,信号光和闲频光光谱随温度的变化关系
Figure 8. Signal and idler wavelength tuning varies with temperature when the grating periods are 8.3, 8.4, 8.5 and 8.6 μm
图 9 极化周期为8.3、8.4、8.5 、 8.6 μm时,信号光和闲频光光谱的温度调谐速度
Figure 9. Temperature tuning rates of signal and idler spectra at poling periods of 8.3, 8.4, 8.5 and 8.6 μm
当泵浦功率为5.4 W时,在距离OPO输出腔镜M2约150 mm处,利用光束轮廓仪测试了信号光光斑的轮廓,获得的光斑强度分布图像如表1所示。从测试结果可以看出,不同信号光在X和Y方向的光斑大小均接近1∶1,且都表现为近高斯分布。
表 1 不同周期输出的信号光光斑
Table 1. Signal beam profile of different periods
Grating period 8.3 μm
X:2.3 mm
Y:2.4 mm8.4 μm
X:2.3 mm
Y:2.6 mm8.5 μm
X:2.4 mm
Y:2.6 mm8.6 μm
X:2.4 mm
Y:2.5 mm
High-conversion-efficiency continuous-wave near-infrared singly resonant optical parametric oscillator
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摘要: 提出了一种高功率及高转换效率的可调谐连续波近红外外腔单谐振光学参量振荡器。为了获得短波段近红外可调谐激光光源,基于准相位匹配晶体的光学参量振荡技术是其中一项有效的技术手段。光学参量振荡器采用连续波532 nm激光器作为泵浦源,掺杂氧化镁的周期性极化化学计量比钽酸锂(MgO:sPPLT)晶体作为准相位匹配晶体,通过在周期调谐的基础上再结合温度调谐的组合调谐方式,在8.3~8.6 μm的4个极化周期内实现了信号光807~879 nm和闲频光1352~1567 nm近红外宽波段的无跳模可调谐激光输出。通过闲频光单谐振设计,当泵浦功率5.4 W时,在8.6 μm周期处,获得了3.1 W的821 nm的近红外信号光输出,实现了57.4%的信号光光-光转换效率。当泵浦功率达到13.6 W时,在8.6 μm周期处,获得了6.8 W的高功率输出。
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关键词:
- 光学参量振荡器 /
- MgO:sPPLT晶体 /
- 近红外激光 /
- 连续波
Abstract: High-power, high-conversion-efficiency and tunable continuous-wave (CW) near-infrared external-cavity pumped singly resonant optical parametric oscillator (SRO) was proposed. OPO based on a quasi-phase-matched (QPM) nonlinear crystal was a very effective technology to obtain the short-wave near-infrared tunable laser sources. CW laser at 532 nm was used as the fundamental laser source to drive the OPO in the cavity. The QPM crystal was a multi-grating MgO-doped stoichiometric periodically poled LiTaO3(MgO:sPPLT). The widely tunable SRO output signal wavelength ranging from 807 to 879 nm and idler wavelength ranging from 1352 to 1567 nm were achieved by combination of poling period tuning and temperature tuning with four different periodically poled gratings from 8.3 to 8.6 μm. By means of using single resonant of idler light, the output power of the signal (821 nm) was 3.1 W at a pump power of 5.4 W with the efficiency of 57.4% was achieved. Under an incident pump power of 13.6 W, a maximum signal output power of 6.8 W at 821 nm was obtained with the period of 8.6 μm.-
Key words:
- optical parametric oscillator /
- MgO:sPPLT crystal /
- near-infrared laser /
- continuous-wave
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表 1 不同周期输出的信号光光斑
Table 1. Signal beam profile of different periods
Grating period 8.3 μm
X:2.3 mm
Y:2.4 mm8.4 μm
X:2.3 mm
Y:2.6 mm8.5 μm
X:2.4 mm
Y:2.6 mm8.6 μm
X:2.4 mm
Y:2.5 mm -
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