VISIBLE-WAVELENGTH DIODE-LASER WITH WEAK FREQUENCY-SHIFTED OPTICAL FEEDBACK

The effects of shifting the frequency of the light fed back to a diode laser have been investigated experimentally. Effects at feedback levels lower than that required to cause coherence collapse (f(ext) less-than-or-equal-to -40 dB) are described. The frequency shift drives the system around a cycle in which the frequency is chirped. The frequency range of the chirp depends on the level of feedback and is readily adjustable over about two orders of magnitude by variation of this level. Also, the sensitivity of the laser frequency and linewidth to very small changes in the length of the external cavity, which is a feature of conventional feedback, is essentially eliminated by the frequency shift. Numerical simulation studies based on the Lang-Kobayashi equations, modified to include the frequency shift, yield results which are in excellent agreement with the experimental data as well as the results from the approximate analytical treatment. Therefore the diode laser with frequency-shifted feedback provides a very graphic illustration of this approximation. approximation.