Nonlinear Laser Spectroscopy of vibrationally excited molecules using a high-finesse cavity

第一年，我们建造了一个外腔半导体激光器，用于1.65微米区域的光谱测量。今年制造了一个精确度为300的光腔吸收池。谐振辐射在腔内形成驻波，驻波极处的辐射功率比入射功率提高150倍，便于进行饱和光谱分析。利用伺服环路，激光频率始终锁定在谐振腔的谐振频率上。在甲烷的泛音带上记录了饱和光谱。它的频谱宽度约为0.5 MHz，远小于290 MHz的多普勒宽度，从而清晰地分辨了多普勒限谱中的一些重叠谱线。另一台带有斯塔克电极的光腔吸收池已经建成，用于研究甲烷的斯塔克效应。由Stark位移测量得到的振动诱导的永久偶极矩的大小与科里奥利子谱有明显的依赖关系，这是首次观察到的，并进行了理论分析。用一对反射率在790 nm附近的反射镜建立了一个光腔吸收池。利用该池对乙炔的一个弱结合带进行了吸收光谱研究。由于辐射在腔内被多次反射，因此有效地增加了吸收长度。这种吸收池的吸收强度是单程吸收池的一千倍。腔的精细度被确定为1400，然而，这远远低于从反射镜的反射率所期望的10000。这种退化的主要原因可能是由于镜面上的斑点。钛宝石激光器的频率和光腔谐振不够稳定，无法观察到饱和光谱。

In the first year, an external-cavity semiconductor laser was constructed for spectroscopy of the 1.65-mum region. This year an optical-cavity absorption cell having a finesse of 300 was made. Resonant radiation forms a standing wave in the cavity, and the radiation power at the antinode of the standing wave is enhanced by 150 times larger than tha incident power, which makes it easy to perform saturation spectroscopy. With a servo loop, laser frequency is always locked at the resonant frequency of the cavity. Saturation spectrum has been recorded on the overtone band of methane. Its spectral Iinewidth is about 0.5 MHz, which is much smaller than Doppler width of 290 MHz, hoence some lines overlapped in Doppler-limited spectroscopy are cleary resolved. Another opical-cavity absorption cell having a pair of Stark electrodes has been built to study Stark effects of methane. The magnitude of the vibrationally-induced permanent dipole moment detarmined from easurements of the Stark shift shows a clear dependence on the Coriolis sublevels.This dependence is observed for the first time, and a theoretical anlysis has been made.An optical-cavity absorption cell was built using a pair of mirrors having a reflectivity of 99.97% around 790 nm. Using this cell, absorption spectroscopy of a weak combination band of acetylene was carried out. Because radiation is reflected many times in the cavity, an absorption length is effectively enhanced. Absorption strength in this cell is a thousand times larger than that in a single-pass absorption cell. A finesse of cavity is determined to be 1400, which is, however, far less than 10000 expected from the reflectivity of the mirrors. The main reason of this degrading is probably due to spots on the mirror surface. Frequency of a titanium-sapphire laser and the optical cavity resonance is not stable enough to observe saturation spectrum.