Near Threshold Behavior of Multimode CW Dye Lasers with an Amplitude Modulated Pump

Abstract

Most applications of lasers today require amplitude stability of the laser. The amplitude stability of the laser is affected by multiplicative fluctuations. These multiplicative fluctuations arise from technical noise sources (e.g. pump fluctuations, mirror vibrations, dust and other gain or loss fluctuations). It is possible to perform accurate studies of these multiplicative effects in lasers since the experimentalist can precisely vary control parameters such as the pump power, cavity loss, and the strength of the multiplicative fluctuations. An effective method of characterizing the response of a laser to multiplicative fluctuations is to measure the power spectrum of the intensity of a laser influenced by wideband multiplicative noise. The intensity of a single mode laser behaves as a low-pass amplifier to such amplitude fluctuations of the pump. Furthermore, the bandwidth of the response decreases as the intensity of the pump is reduced to its threshold value. This low-pass behavior of the intensity of a laser operating near its first threshold is a manifestation of a universal property of the dynamics of nonlinear systems operating near a critical point. These dynamics are present not only in the laser’s intensity but also in the inversion of the atoms and also the absorption of the pump laser. In these experiments we weakly amplitude modulate the power of the pumping argon ion laser and measure the response of a dye laser to this modulated pump. The output power, the fluorescence from the atoms in the upper lasing level, and the fraction of the pump laser power that is absorbed are all measured as a function of the modulation frequency.