Dual-cavity $Nd^{3+}$:YAG laser is studied theoretically and experimentally. To construct a dual-cavity laser, an auxiliary cavity for a passive Q-switch is inserted into a conventional passively Q-switched $Nd^{3+}$:YAG laser. The $Nd^{3+}$:YAG laser using a dual-cavity configuration enables its pulse duration to be varied between 24 ns and 32 ns. The variation of the pulse duration can be done by changing the auxiliary cavity length.
We use a LiF:$F_{2}^{-}$ color-center crystal for passive Q-switching and an auxiliary cavity for lasing the emitted light from the LiF:$F_{2}^{-}$ crystal. The lasing in the auxiliary cavity cause the LiF:$F_{2}^{-}$ to reclose and a long tail part of the Q-switched pulse to be cut out. In other words the lasing in the auxiliary cavity restores the saturable loss and turns off the Q-switched pulse. When the auxiliary cavity length is reduced, the pulse duration is decreased. It is explained that the lasing of the 1120 nm photons occurs more easily and the reclosing more quickly at a smaller auxiliary cavity length than at a larger one. The range of the auxiliary cavity length for the main cavity length of 90 cm is limited between 15 cm and 30 cm due to a geometrical limit in our experimental setup. If we use other types of saturable absorber crystals and modify the geometrical structure of the cavity, the tuning range of the pulse duration can be extended more. It was verified by the results of the computer simulation and the experiment. This reclosing mechanism of the saturable absorber was found and analyzed for the first time in this study. This method of a control of the pulse duration can be applied to an industrial and an medical laser and other applications due to an easy control and cheap costs.
In the experiment the LiF crystal has the thickness of 4.5 cm and the initial transmittance of 50%. The $Nd^{3+}$:YAG rod has the length of 10 cm and the diameter of 8.0 mm. The full mirror has the radius of curvature of 5.0 m and reflectivities of 100% at emission wavelengths of both the $Nd^{3+}$:YAG and the LiF:$F_{2}^{-}$ crystals. The output coupler for the $Nd^{3+}$:YAG cavity has reflectivities of 20% at 1064 nm and 3% at 1120 nm. The dichroic mirror is inserted between the $Nd^{3+}$:YAG and the LiF:$F_2^{-}$ crystals and have reflectivities of 3% at 1064 nm and 90% at 1120 nm. The dichroic mirror is moved to change the auxiliary cavity length. To measure the pulse profile from the laser, we use a PIN photodiode with risetime of 0.5 ns and a 2 GHz real-time digitizer (RTD720, Tektronix).
The rate equations are used and the theoretical computer simulations are executed to investigate the output characteristics from a Q-switched laser with the dual-cavity configuration. The experimental results were in good agreement with the results of the theoretical computer simulation.
Also we have modeled and simulated an x-ray generation from the laser-produced plasma. To develop the temporal evolutions of the temperatures and densities of the electrons and ions, we have used the hydrodynamic equations including the charge density continuity, the momentum conservation, and the energy conservation equations and the average charge model. The inverse bremsstrahlung absorption is used to know the attenuation of the incident laser power and the classical Spitzer heat conduction model is used to calculate heat conductivity of the electron plasma. To simplify the calculation of the emission from the laser plasma, we consider one dimensional treatment and only the bremsstrahlung radiation which is the free-free transition as a radiation. To satisfy the Courant condition in the partial differential equation, we have to partition the temporal and spatial steps much finely and use the C-language and the supercomputer of CRAY YMP model.
We, after the simulation, found that the electron temperature rose to keV level at $10^15$ to $10^16$ W/㎠ and the x-ray conversion efficiency from the laser to the x-ray energies in the range between 1 eV and 100 eV had a maximum at $5x10^15 W/㎠. Relating to the bremsstrahlung radiation we also found that the major evolutions of the electron temperature and density and the bremsstrahlung radiation were lived only during the duration of the laser pulse.
