P. Melman

Linewidth Reduction Of Semiconductor Lasers Using Feedback Current Proportional To Terminal Voltage Fluctuations

We analyze the possibility of linewidth reduction of a semiconductor laser when a feedback signal is obtained directly from the noise voltage at the electrical terminal, rather than indirectly from the optical field. For sufficiently large gain in a feedback loop we find, by solving the linearized laser rate equations, that the spectral fluctuations of the instantaneous frequency are suppressed for frequencies less than a value dependent on the width of the feedback gain response. If the suppressed width is sufficiently large, the power spectrum of the field is a lorentzian of width ▵f=▵f0[1+α2/ΓIΚo/Ω 2)2], where dr() is one half the Schawlow-Townes width, a is the linewidth parameter, 0 is the relaxation oscillation frequency, Ko is the DC gain of the feedback loop and F1 is the damping rate of the photon number fluctuations. Thus, for large Ico, this method of linewidth reduction appears capable of removing the a2 contribution to the laser frequency spread. For values of the feedback response band width, ▵ w, less than that required for full suppression of the a2 contribution to the line width, the line shape may deviate considerably from a lorentzian form but with a central peak of width ▵f = ▵fo.