Frequency Stabilization of a Semiconductor Laser under Direct Modulation by the PEAK method

The frequency stabilization of a semiconductor laser using an absorption line of atoms under direct frequency shift keying (FSK) is degraded because of inevitable sideband spectra. However, we devised the peak-hold method and obtained the better stability than in a traditional method under direct FSK. The stability hardly changed with the duty ratio of FSK signal in the peak-hold method. Two oscillation frequencies, i.e. mark and space frequencies of FSK signal, did not change with its duty ratio either, although they changed in the traditional method. In our modulation frequency, the oscillation frequency changed slowly and deviated from FSK in spite of rectangular wave modulation to the injection current because of the frequency chirping of the diode laser by a thermal effect. Therefore, we improved FSK signal using the modified modulation signal through the RC circuit, which improved the frequency stability and lessened the frequency deviation.