Realization of ultrahigh coherence in semiconductor lasers by negative electrical feedback

To be useful, an ultrahigh-coherence semiconductor laser source requires high frequency stability, narrow linewidth, the capability of frequency tracking to a master laser, and stable frequency tuning. Negative electrical feedback is proposed to meet these four requirements simultaneously. Although the degree of frequency fluctuation that can be reduced by negative electrical feedback is limited by the noise contained in the feedback signal, theoretical calculations show that the fluctuations can be lower than the quantum noise limit. Experimental results obtained recently by the author are reviewed