FREQUENCY CONTROL OF SEMICONDUCTOR-LASERS

This article reviews our recent works on frequency control of semicond uctor lasers. The magnitudes of quantum noise limited frequency modula tion (FM) noise, realized by the negative electrical feedback, are giv en for four methods of using an external Fabry-Perot cavity as a frequ ency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the lase r cavity and to the external Fabry-Perot cavity for negative electrica l feedback. The FM sideband technique is shown to be an effective meth od to reject the contribution of laser power fluctuations to the FM no ise detection for the negative electrical feedback system. As a candid ate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly d egenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic v apor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field sp ectrum of the laser, and also the fine detuning of the stabilized cent er frequency. For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three- level atomic vapor, obtained by using the phenomenon of coherent popul ation trapping, is shown. In order to realize a highly efficient nonli near optical frequency conversion for wideband frequency sweep of semi conductor lasers, a method of adding the output powers of several lase rs, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the v isible region) can be realized by using the techniques of nonlinear op tical frequency conversions and the optical phase locking, relevant ex perimental results of nonlinear optical frequency conversions are pres ented which are the second harmonics generation, sum and difference fr equency conversions. A highly accurate optical frequency measurement s ystem is proposed using an optical frequency comb generator with a mod ulation sidebands up to several THz. Performances of the optical frequ ency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (T(c)) oxide superconducting film are d emonstrated.