When the oscillation frequency of a laser diode is stabilized by means of the absorption line of atoms and molecules, a general method used was to apply infinitesi mel modulation directly on the bias current of the laser diode. However, this method has some disadvantages such as expanded oscillation spectral width and restriction of the stabilization frequency to the center frequency of the absorption line. In this paper, the laser light was stabilized by providing modulation indirectly instead of direct modulatin of the oscillation frequency. This is done by modulation of the reference frequency by means of the magneto‐optical effect of the absorption line of the atoms. Further, by comparison of three types of saturated absorption spectroscopy systems, the effect of the magneto‐optical phenomena on the stabilization was examined. As a result, it was found that this method provides a degree of stability comparable to or better than the direct modulation system. Also, the stabilization frequency can be swept and controlled in the range of about 1 GHz without degrading the degree of stability.

To stabilize the oscillation frequency of a semiconductor laser by using an absorption spectrum of atoms and molecules as a frequency reference, modulation of an injection current of the laser has commonly been adopted since it gives a high detection sensitivity. However, as a result, the oscillation frequency becomes broadened, which is a drawback. To overcome this difficulty, a new method has been proposed which utilizes the modulation scheme resulting from the Faraday effect of Rb atoms and thus no direct modulation is needed. In a conventional method using a normal absorption spectrum, detection sensitivity is limited by a spectrum broadening due to the Doppler effect. The stability has been improved one order of magnitude by using a saturated absorption spectroscopy method, compared to the conventional method. Also, with this method, a number of stabilized frequency points is increased. An oscillation frequency can be controlled by changing the dc magnetic field for the Faraday effect modulation for a frequency range over about 8 GHz around the frequency corresponding to Rb‐D2 lines.

No abstract

To date, frequency stabilization methods for semiconductor lasers mainly use the absorption line of an atom or molecule to modulate the frequency of laser light slightly so as to obtain a control signal. As a result, an oscillation bandwidth broadening will appear and the stabilization point will be restrained at the center frequency of the absorption line. This paper reports a frequency stabilization method using a magneto‐optical effect, the Faraday effect of an atomic absorption line, instead of direct modulation of the oscillation frequency. This stabilization method made it possible to control the stabilization point frequency by changing the dc magnetic field. With an Rb absorption line as the frequency reference, the controlled frequency range was about 1.5 GHz, which is wider than the Doppler width of the Rb absorption line at room temperature. The experimental results were consistent with the theoretical analysis.