Study of the frequency fluctuations of a semiconductor diode laser

Abstract

Frequency fluctuations of an inexpensive single-mode semiconductor diode laser, which operates in the 822 nm region, are investigated by direct measurement of the error signal. The linear slope of first derivative signal of a transition in the (2,1,1) vibration-rotation band of water vapour is used as a frequency discriminator. A balanced photodetector is used to reduce the intensity noise and to improve the S/N ratio. Frequency stability of the diode laser is investigated when the laser is under a free-running condition and is locked to the line center of the reference transition. An integrator is used to provide feedback voltage to the laser current driver. After frequency stabilization, a more than 60-fold improvement in long-term laser-frequency stability is attained over the performance provided by the free-running semiconductor diode laser. The frequency-noise power spectrum of the diode laser is extracted from the error signal for the Fourier-frequency range ~100 Hz. The Allan variance curve for the laser system is obtained from the frequency-noise power spectrum of the error signal by using a suitable mathematical relation under certain approximations. The extracted values of the Allan variance are compared with the theoretical τ–1 model. The experimental setup is easy to implement in graduate laboratory classes. PACS Nos.: 42.55.Px, 42.62.Fi, 33.70.Jg