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碘稳频He-Ne激光波长参考源采用腔内吸收方式,根据激光小信号增益模型,此时激光输出功率
$P(\omega )$ 可写为如下形式[13]:$$\begin{split} P(\omega ) \propto \Bigg\{ & G(\omega - {\omega _1}) - \\ & \left. {\alpha ^0}(\omega )\left[ {1 - \frac{{{S_0}}}{2}(1 + \frac{{{{({\gamma _s}/2)}^2}}}{{{{(\omega - {\omega _0})}^2} + {{({\gamma _s}/2)}^2}}})} \right] \right\} \end{split} $$ (1) 式中:
$\omega $ 为激光输出频率;$G(\omega - {\omega _1})$ 为激光小信号增益系数,${\omega _{\rm{1}}}$ 为增益系数中心频率;${\alpha ^0}(\omega )$ 为腔内吸收介质的吸收系数;${\omega _{\rm{0}}}$ 、${\gamma _s}$ 分别为饱和吸收峰的中心频率与半高全宽;${S_0}$ 为饱和吸收参数。公式(1)在${\omega _{\rm{0}}}$ 附近可近似写为:$$P(\omega ) = A{\omega ^2} + B\omega + C + \frac{D}{{{{(\omega {\rm{ - }}{\omega _0})}^2}{\rm{ + (}}{\gamma _{\rm{s}}}{\rm{/2}}{{\rm{)}}^2}}}$$ (2) 式中:系数A、B、C、D与
${\omega _{\rm{0}}}$ 、${\omega _{\rm{1}}}$ 、${\gamma _s}$ 、${S_0}$ 相关。公式(2)的微分函数可表示为:$${P^{(n)}}(\omega ) = \frac{{{{\rm{d}}^n}P(\omega )}}{{{\rm{d}}{\omega ^n}}},(n = 1,2,3,....)$$ (3) 则其一、二和三次微分函数可分别写为:
$${P^{(1)}}(\omega ) = 2A\omega + B - \frac{{2D(\omega {\rm{ - }}{\omega _0})}}{{{{\left[ {{{(\omega {\rm{ - }}{\omega _0})}^2}{\rm{ + (}}{\gamma _{\rm{s}}}{\rm{/2}}{{\rm{)}}^2}} \right]}^2}}}$$ (4) $${P^{({\rm{2}})}}(\omega ) = 2A + \frac{{{\rm{6}}D{{(\omega {\rm{ - }}{\omega _0})}^{\rm{2}}}{\rm{ - 2}}D{{{\rm{(}}{\gamma _{\rm{s}}}{\rm{/2)}}}^2}}}{{{{\left[ {{{(\omega {\rm{ - }}{\omega _0})}^2}{\rm{ + (}}{\gamma _{\rm{s}}}{\rm{/2}}{{\rm{)}}^2}} \right]}^3}}}$$ (5) $${P^{(3)}}(\omega ) = \frac{{24D(\omega {\rm{ - }}{\omega _0})[{{(\omega {\rm{ - }}{\omega _0})}^2} - {{{\rm{(}}{\gamma _{\rm{s}}}{\rm{/2)}}}^2}]}}{{{{\left[ {{{(\omega {\rm{ - }}{\omega _0})}^2}{\rm{ + (}}{\gamma _{\rm{s}}}{\rm{/2}}{{\rm{)}}^2}} \right]}^4}}}$$ (6) 图1给出了公式(2),(4),(5),(6)的计算曲线,计算参数为
$A,B,C,D,{\gamma _s} = 1$ ,${\omega _0} = 0$ 。由图可知,在插入吸收介质时,激光输出功率存在明显饱和吸收信号,该信号叠加在激光的输出功率的背景信号上。对该信号采用一次微分处理时,并不能完全消除背景信号,只有采用三次微分或更高阶的奇数次微分时,才能够完全消除背景信号。这种数学处理方式通过对比公式(2),(4),(6)的表达式也可以做出判断。特别在$\omega = {\omega _0}$ 时,$P(\omega )$ 与${P^{(1)}}(\omega )$ 均有非零的背景信号,而${P^{(3)}}(\omega )$ 为零无背景信号。因此$P(\omega )$ 的三次微分产生的误差信号的过零点与饱和吸收光谱的中心频率严格地对应,以此作为反馈信号的参考点能够有效地消除背景信号引入的锁定点误差,真正实现激光波长锁定至吸收介质的饱和吸收光谱线中心频率点。 -
高功率碘稳频He-Ne激光腔结构如图2所示。为了提高激光输出功率的稳定性,激光谐振腔采用低膨胀系数的石英玻璃管作为间隔器,激光高反镜与输出耦合镜通过与低膨胀系数的玻璃板研磨粘接,然后整体与石英间隔器端面垂直粘接而成。为降低激光腔内损耗,提高激光输出功率,激光增益管采用半内腔封接方式,一端封接输出耦合镜,另一端封接布鲁斯特窗。相比国外普遍采用的全外腔方式(两端均为布鲁斯特窗封接),减小了布鲁斯特窗角度误差引入的腔内损耗,同时提升了增益管与谐振腔镜的稳定性。
此外,由于采用腔内插入碘分子吸收室的方式来实现饱和光谱探测,碘分子吸收室的细微角度误差都会引起腔内损耗急剧上升,导致激光输出功率快速下降。因此实验中采用六轴精密调节结构,以精确调整碘分子吸收室布鲁斯特窗与激光增益管布斯角度重合的一致性,并通过光学粘合的方式实现碘分子吸收室与激光增益管的一体化连接,有效地降低了吸收室引入的腔内损耗,提升了光谱探测单元的稳定性。
实验中碘吸收室采用非饱和蒸气压碘分子封接而成,吸收室内无固态碘分子结晶体,完全为游离态碘分子。与传统饱和蒸气压碘分子吸收室相比,碘分子在腔内吸收损耗更小,更有利于提升激光输出功率。此外,这种吸收室不需要对冷指端和管壁进行精确控温,使得整个饱和光谱单元的结构更加简单和稳定,提高了整个激光谐振腔的可靠性。
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全封闭、一体化设计的激光腔的稳定性是评价激光长期保持锁定能力的重要指标。碘稳频激光谐振腔腔长在冷开机情况下的漂移特性如图3所示。由图可知,冷开机情况下激光器谐振腔长一直处于膨胀状态,整个预热过程中谐振腔腔长共漂移4.2 μm。前40 min几乎呈线性膨胀,膨胀率为87 nm/min,此后腔长漂移速度明显变慢,80 min以后腔长基本保持稳定,这表明此时刻开始锁定激光即可保持激光谐振腔的长期锁定。
激光谐振腔的腔长膨胀主要来自于激光增益管发热引起的材料热膨胀。如图2所示的激光腔结构,因为激光器谐振腔镜完全与石英间隔器硬性连结,石英间隔器受增益管加热会引起轴向热膨胀,而这种热膨胀会直接传递至谐振腔,进而导致腔长漂移。由此可见,选择具有低膨胀系数的材料作为间隔器,以及优良的谐振腔设计对于激光的长期稳定性具有决定性作用。
激光输出功率漂移特性如图4所示,对比图3可知,两者之间的变化趋势高度一致。但是它们之间的影响因素却完全不同,前者主要由间隔器的轴向膨胀引起腔长漂移,而输出功率的漂移过程却主要由激光管在垂直方向的非对称形变引起。在激光器运转过程中,增益管内毛细管均匀向四周辐射热量,持续加热增益管内部密封的He、Ne气体,引起气体热对流,导致增益管上表面温度显著高于下表面,进而使得增益管在垂直方向的非对称形变。这种非对称形变有效地降低了腔内损耗,例如提高了布氏角度和毛细管与谐振腔轴向角度的一致性,进而提高了激光的输出功率。
另一方面,增益管上下表面温差通过热传递也会引起石英间隔器上下表面温差,导致间隔器在垂直方向的非对称形变,这种形变在一定程度上提高了谐振腔腔镜反射面的平行度,降低了衍射损耗,进一步提升了激光输出功率。实际测量表明,在充分预热后,石英间隔器上下表面的温差可以达到15 °C以上,以熔融石英材料单位长度的膨胀系数为8×10−7/ °C为例,由此可推算出垂直方向的非对称形变差至少为0.5 μm。
全封闭、一体化设计的激光输出功率的复现性如图5所示。两个月内共进行了22次冷开机的复现性实验,80 min预热后锁定碘稳频激光至同一吸收峰进行功率测量,测量时间为3 h,结果表明,在22次复现性测量中,激光输出功率的平均值为243 μW,极限波动小于10 μW。
Powerful iodine stabilized He-Ne laser as wavelength reference
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摘要: 为满足精密测量对高稳定激光单色光源功率的要求,研制了全封闭、一体化结构的高功率碘稳频He-Ne激光系统。对该系统所采用的饱和光谱探测原理、吸收峰识别与锁定方法以及激光波长的稳频效果进行了研究。首先,介绍了三次谐波方法探测饱和吸收光谱的基本原理,分析了其消除功率背景的方法。接着,分析了碘稳频激光中一体化谐振腔的稳定性,详细讨论了谐振腔轴向膨胀和横向非对称形变对输出功率的影响。然后,分析了激光输出功率与碘分子饱和吸收峰之间的对应关系,介绍了利用二次谐波信号实现吸收峰识别的可行性,并展示了高稳定谐振腔的长时间锁定能力。最后,分析了高功率碘稳频He-Ne激光波长的稳定度与复现性。实验结果表明:高功率碘稳频He-Ne激光波长抖动标准差为33 kHz,1 000 s稳定度达到4.1×10−13;三个月内激光波长的复现性达到3.3 kHz (7.0×10−12),与国际计量委员会推荐的频率差为3.0 kHz。Abstract: In order to meet the requirement of high output power of the laser monochromatic light source in the precision measurement, a high-power iodine stabilized He-Ne laser system with a fully enclosed, integrated structure was developed. The principle of saturation spectral detection, the method of absorption peak recognition and locking and the frequency stability of iodine stabilized laser were studied. Firstly, the basic principle of detecting saturation absorption spectrum of iodine molecular using the three harmonic method was introduced, and its mechanism of eliminating the power background was analyzed. Then, the stability of the integrated resonant cavity in the iodine stabilized laser was demonstrated, and the effects of axial expansion and lateral asymmetric deformation on the output power were discussed in detail. After that, the correspondence between the profile of laser output power and the iodine molecular saturation absorption peaks was presented, the feasibility of using the secondary harmonic signal to achieve absorption peak recognition was introduced, and the long-term locking ability of high-stability resonant cavity was demonstrated. Finally, the wavelength stability and reproducibility of high-power iodine stabilized He-Ne laser were analyzed. The experimental results showed that the standard deviation for the frequency jitter of high-power iodine stabilized He-Ne laser was 33 kHz, the stability at 1 000 s and the reproducibility in three months were 4.1×10−13 and 3.3 kHz (7.0×10−12), respectively. Its absolute frequency was 3.0 kHz lower than the recommended value by the International Committee for Weights and Measures (CIPM).
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