Optical feedback effects on microchip laser dynamics used for determining the characteristics of the materials

In this paper we investigate experimentally the dynamical response of a class B microchip laser submitted to optical feedback. Based on the theoretical model for a laser with optical feedback, we demonstrate how the effects of the optical reinjection of the laser beam reflected by an object situated in front on the laser can be used in determining the characteristics of the object (reflectivity and distance to the laser). In this purpose we have constructed a laser system, which we further use in an optical feedback experimental setup. The laser we use in our experiment is a class B Nd:YAG microchip laser, which has a damping rate of the laser cavity γc much higer than the damping rate of the population inversion γ1. The laser has a cavity lenght L=0.8mm, operating at λ=1,064&mgr;m. In order to obtain the laser emission, the active medium is pumped by a system of laser diodes emitting at 810nm. This type of laser has the relaxation oscillations at a characteristic frequency, named relaxation frequency. For the maximum pump parameter η≈6 the maximum output power of the laser is Pout≈130mW and the relaxation frequency fr≈1MHz. The value obtained for γc/γ1≈1,7X106>>1 proves that our laser is a class B laser and also that it can be successfully utilized for optical reinjection. Based on this laser system, we develop a method for exploiting the sensivity of the Nd:YAG laser to the optical reinjection. We demonstrate that the optical feedback modifies the steady state of the laser and changes the laser characteristics. This method is based on the high sensitivity of this type of laser to the optical reinjection. In order to analyse the effects of the optical feedback on the laser behaviour, we placed an object at the distance d in front of the laser and the retro reflected beam was reinjected into the laser cavity. We use a beam splitter to send a small fraction of the laser beam to a photodiode. The signal provided by this photodetector is sent to an oscilloscope, where we observe the signal and the power spectrum of the laser. Our experimental results show that the optical feedback modifies the laser characteristics. We observe that the optical feedback induces a significant amplification of the laser signal and also a deplacement of the relaxation frequency to smaller values. Our experimental results are in good agreement with the mathematical model. By introducing the reinjected electric field into the equations for the dynamical behavior of a class B laser, it can be shown that the reinjection of a wave with the same pulsation as the laser beam determines a changement of the steady state values for the population inversion, for the electric field and also for the optical frequency of the laser. Based on the the equations for laser intensity and for the relaxation pulsation in the presence of the optical feedback, we determine experimentally the reflectivity of the object and the distance between laser and object. We conclude that the class B lasers present a high sensitivity to the optical feedback, which induces the change of the steady state of the laser. This modifications can be succesfully used for determining the characteristics of differents objects submitted to laser irradiation.