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 microns was stabilized under very small frequency modulation to one of the absorption lines of C2H4 in the nu(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 x 10 to the -8th. 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.