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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.
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