Investigation of noise effect of CMS laser lineshape and frequency fluctuations

Abstract

Lineshape and frequency fluctuations actually cause the optical field of current modulation semiconductor laser (CMS) laser beam to deviate from a pure sine wave [1]. This paper investigates the lineshape of a CMS laser for two distinct types of frequency noise. In the first case the investigation analyzes on the low-pass filter, the frequency noise level is a constant δo below a cutoff frequency but zero above this threshold. In the second case on high-pass filter the frequency noise level is a constant δo above a cutoff frequency but zero below this threshold. Investigations observed inspire us to focus for separation of the frequency noise spectrum into two regions wherein due to the effect of noise on the lineshape is radically different in the slow modulation area in a left side of the diagonal line where the noise contributes to the laser linewidth, with a Gaussian shape, and the other one the fast modulation area in a right side of the diagonal line, where the noise contributes only to the wings of the line (sidebands) and not to the linewidth, thus transforming the lineshape from Gaussian to Lorentzian.