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利用偏振光谱稳频技术将激光器的频率精准锁定到铯原子D1线的F=4(62S1/2)→F′=3(62P1/2)超精细跃迁能级上[26],以保证半导体激光器的稳定输出。在其它条件相同,入射腔镜透射率分别为5%和10%时,逐渐增大注入基频光功率,并调节锁相放大器的参数及晶体的温度,使得倍频腔运转在最佳工作状态,输出蓝光功率变化情况如图 3所示。其中,实线为理论值,图 3a中的圆点和图 3b中的方块分别为使用两种透射率腔镜时测得蓝光实际功率。
倍频所得蓝光功率理论值的计算方法如下[11]:
$ \begin{gathered} \sqrt{\eta}\left[2-\sqrt{1-T_1}\left(2-L-\mathit{\Gamma} \sqrt{\frac{\eta P_1}{E_{\mathrm{NL}}}}\right)\right]^2- \\ 4 T_1 \sqrt{E_{\mathrm{NL}}} P_1=0 \end{gathered} $
(1) 式中,$\sqrt{\eta}=P_2 / P_1 $为倍频转换效率,P1和P2分别为基频光及倍频光功率,T1代表入射腔镜对基频光的透射率,L代表内腔损耗(实际测量值为2.7%),非线性损耗Γ=ENL+Γabs,ENL为晶体的单次穿过效率(实际测量值为0.89%/W),Γabs为非线性晶体对产生蓝光的吸收,根据参考文献[18],这里取Γabs=0.1ENL。
从图 3可以看出,在注入基频光功率较低时,实际获得蓝光功率与理论值基本相同。然而随着基频光功率的增加,晶体对蓝光和基频光的吸收变强,导致输出蓝光功率越来越低于理论值。在注入基频光功率最大为350mW时,采用透射率T为5%和10%时的入射腔镜分别获得178mW(转换效率50.8%)和131mW(转换效率37.4%)蓝光。理论上,在注入基频光功率最大时,利用两种透射率腔镜倍频所得的蓝光功率应该基本相等,而实际产生的蓝光功率却相差了47mW,转换效率相差13.4%。这种情况的出现,一方面是由于入射腔镜的透射率为5%时更接近最佳透射率[14],另一方面可能是透射率为10%的腔镜的镀膜不够理想,影响了倍频效率。
图 4显示了在最大注入光功率时,倍频腔输出蓝光功率随时间的起伏。采用透射率T为5%和10%时的倍频腔, 获得蓝光的0.5h功率起伏均方根(root mean square, RMS)分别为1.4%和0.7%,具有较好的功率稳定性。输出蓝光功率下降主要是由于在较高功率密度的蓝光照射下,PPKTP晶体会被损伤,导致晶体的损耗随着时间的增加而变大,但是这种损伤大部分是可逆的,通常几个小时即可自行恢复[27]。
两种透射率入射腔镜对894.6nm倍频腔转换效率的比较
Comparison of the conversion efficiency of 894.6nm frequency doubling cavity with different transmission input coupler
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摘要: 为了在对基频光透射率分别为5%和10%的两种入射腔镜(其它参数相同)中择优, 利用波长对应于铯原子D1线的894.6nm半导体激光作为基频光, 抽运以周期极化磷酸氧钛钾(PPKTP)晶体作为非线性介质的两镜驻波倍频腔, 通过外腔倍频过程产生447.3nm蓝光, 对利用这两种腔镜搭建倍频腔所产生的蓝光进行了对比。结果表明, 在注入350mW基频光、倍频腔采用透射率为5%的入射腔镜时, 制备了178mW蓝光, 倍频效率为50.8%, 0.5h功率起伏为1.4%;采用10%透射率腔镜的倍频腔获得131mW蓝光, 倍频效率为37.4%, 0.5h功率起伏为0.7%;使用5%透射率入射腔镜的倍频效果更好。该研究对产生铯原子D1线非经典光场所需高质量抽运源的制备具有重要指导意义。Abstract: In order to choose a better input coupler, the conversion efficiency of the frequency doubling cavity were studied in the case of the transmittance of the input coupler to the fundamental light of 5% and 10%, respectively. 447.3nm blue light was obtained by external-cavity frequency doubling of a tapered amplifier-boosted continuous-wave diode laser at cesium D1 line. The frequency doubling cavity consists of a two-mirror standing wave cavity with a periodically poled KTiOPO4 (PPKTP) crystal inside. With a maximum fundamental power around 350mW, the frequency doubling cavity with a 5% transmittance input coupler generate 178mW blue light, corresponding to a conversion efficiency of 50.8%. With the input coupler with a 10% transmittance at the fundamental wavelength, 131mW of blue light is obtained, and the corresponding conversion efficiency is 37.4%. With a maximum input fundamental power, the output blue power was measured for 0.5h. In the frequency doubling cavity with 5% transmittance input coupler, the root mean square fluctuation is 1.4%, while the other is 0.7%. The result shows that the input coupler with 5% transmittance is better. This research is helpful for preparing high quality pump light resources to generate nonclassical light at cesium D1 line.
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Key words:
- laser technique /
- external-cavity frequency doubling /
- input mirrors /
- transmittance
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