Frequency stabilization of 1.55-μm DFB laser diode using vibrational-rotational absorption of ¹³C₂H₂ molecules

The frequency stabilization of a 1.55-μm distributed-feedback (DFB) laser diode using vibrational-rotational absorption of carbon-thirteen acetylene (¹³C₂H₂) molecules (VRAM) at a wavelength of 1.54949 μm is discussed. The detailed investigations of the absorption characteristics of ¹³C₂H₂ molecules are also reported. It was found that ¹³C₂H₂ has many strong lines in the 1.52-1.55-μm region. These regular absorption lines can be attributed to the first overtone of vibrational-rotational absorption of H-¹³C≡¹³C-H molecules. In this experiment the absorption line at 1.54949 μm, i.e., the line closest to 1.55 μm, was used, although each separated absorption line could be used for frequency stabilization in the 1.52-1.55-μm region. The stability of the laser frequency was estimated by tracing the error signal measured with respect to the zero point of the S-curve. The peak-peak frequency fluctuation in the free-running condition was estimated to be approximately 200 MHz. With the feedback loop closed, the peak-peak frequency fluctuation never exceeded 2 MHz for a response time of 0.05 s. The square root of the Allan variance σ(2,τ) of frequency fluctuation was calculated to be approximately 7 × 10^-10 for an averaging time of 0.05 s ≤ τ ≤ 100 s.