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The oscillation frequency of semiconductor lasers fluctuates by a temperature or an injection current change. Semiconductor lasers are now used in many application fields such as optical fiber communications systems, compact disk and laser disk systems, and others, because these application fields don't' need the high frequency stability in their light source. Therefore, we must stabilize the oscillation frequency of semiconductor lasers if we use them in a coherent optical communications or other coherent systems. Some frequency stabilization experiments of semiconductor lasers using external frequency references have been reported in recent years. We have also stabilized them using the Rb-D2 absorption lines as an external frequency reference. Applying a small modulation directly to the injection current usually performs these stabilization methods. But the oscillation spectrum of the semiconductor laser is broadened by the modulation in this method. So we devised two stabilization methods without any direct modulation, which use the magneto-optical effects called the Zeeman and the Faraday effects, and stabilized the oscillation frequency of the semiconductor laser. As a result, we got high frequency stability without any spectrum broadening caused by the modulation. The stabilized frequency was tuned in a wide range at almost the same stability conditions. Moreover, we devised an optical setup, which produces a high sensitive error signal between the reference frequency and the laser oscillation frequency. This method, which we named `PEAK method', is also combined with the Zeeman effect to improve the frequency controllability.
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