Doubly Passively Self-$Q$-Switched ${\hbox {Cr}}^{4+}{\hbox {:Nd}}^{3+}{\hbox {:YAG}}$ Laser With a GaAs Output Coupler in a Short Cavity

By using a GaAs as both an output coupler and a saturable absorber, we present a doubly passively self-Q-switched Cr
⁴⁺
:Nd
³⁺
:YAG laser in a short cavity for the first time to our knowledge. This laser can generate more symmetric pulse shape and shorter pulsewidth in comparison with the solely self-Q-switched Cr
⁴⁺
:Nd
³⁺
:YAG laser. The output pulse energy and peak powers are higher than those in our previous doubly passively Q-switched lasers. By considering the Gaussian spatial distribution of the intra-cavity photon density and the free carrier absorption (FCA) in GaAs wafer, a set of modified rate equations have been introduced to describe the performances of the doubly Q-switched Cr
⁴⁺
:Nd
³⁺
:YAG laser with GaAs coupler. The numerical solutions of the equations and the experimental results are found to agree with each other very well. The effect of FCA process in GaAs wafer has been discussed and proved to play an important role in the pulse compression and symmetry