Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H 2 CO [ 5_{1,5}(\upsilon = 0) \rightarrow 6_{0,6}(\upsilon_{5}= 1) ] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H 2 CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H 2 CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was 1.0\times10^{-14} at \tau = 100 s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as 8.0\times10^{-13} \cdot \tau^{-1} for 10 ms \leq \tau \leq 100 s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H 2 CO spectrum.