Frequency stabilization under very small modulation and its stability estimation of a PbSnTe diode laser

The frequency of a PbSnTe diode laser around 10.6 μm was stabilized under very small frequency modulation to one of the absorption lines of C
2
H
4
in the ν
7
band. The frequency control and modulation were performed by the injection current. The minimum modulation width in this experiment was analytically determined to be 1.1 MHz. From analysis of a linear approximate model for the experimental system, the frequency fluctuation, which represents peak-to-peak width during 5 min, including the modulation width, was estimated to be
3.8 \times 10^{-8}
. This value was about two orders larger than the value obtained by the ordinary method, which is a simple conversion of the control error signal.