1. Field of the Invention
The present invention relates to a laser apparatus for emitting pulsed laser beams, and more particularly, to a Q-switch optical element of a laser resonator for generating pulsed laser oscillation.
2. Discussion of the Related Art
In laser technology, a Q-switching device has been used to generate high power pulsed laser outputs. The Q-switching device controls the energy loss in the laser resonator. When the energy loss in the laser resonator becomes small (i.e., when the Q value becomes large), a laser pulse is emitted from the laser apparatus. Examples of optical elements that can be used as the Q-switching device include rotating mirrors, acoustic-optical elements (AO elements), and electrooptic elements (EO elements: electrooptic crystals).
However, rotating mirrors and acoustic-optical elements generally suffer from a drawback of having slow switching rates in changing the energy loss inside the resonator. Accordingly, in a high-gain laser apparatus and in a laser apparatus that requires a short pulse width, an electrooptic element having a high switching speed is employed as the Q switching device.
An example of the laser apparatus having such an electrooptic element as a Q switching device is described with reference to FIG. 7. This laser apparatus is equipped with a pumping light source (not shown in the figures) for emitting pumping light 75, a laser medium 71, and an output mirror 74. A reflective film 711 is coated on the light incident surface of the laser medium 71 on which the pumping light is incident. The reflective film 711 transmits light in the wavelength band corresponding to the pumping light and reflects light in the wavelength band corresponding to the laser light reflected from the output mirror 74. As a result, a laser resonator is constructed of the output mirror 74 and the reflective film 711 of the laser medium 71.
An electrooptic element 72 and a polarizer 73 are installed in the laser resonator as a Q-switching device. The refractive index of the crystal in the electrooptic element 72 varies depending on the voltage applied thereto. For example, when no voltage is applied, the electrooptic element 72 functions as a half-wave plate, and when an appropriate voltage is applied, it functions as a full-wave plate. Thus, by controlling the applied voltage, the electrooptic element can be used as a variable wave plate.
The polarizer 73 transmits P-polarized light and reflects S-polarized light towards the outside of the laser resonator. When the electrooptic element 72 is in the state of a half-wave plate, the polarization direction of light incident on the electrooptic element 72 is rotated by 90 degrees. Thus, almost all the light that has passed through the electrooptic element 72 is reflected at the polarizer 73 towards the outside of the laser resonator. Therefore, the energy loss in the resonator increases significantly. On the other hand, when the applied voltage changes such that the electrooptic element 72 assumes the state of a full-wave plate, the polarization direction of light incident on the electrooptic element 72 receives no changes. Therefore, the light from the electrooptic element 72 can pass through the polarizer 73, and the energy loss in the laser resonator is reduced. Accordingly, a pulsed laser beam 76 is emitted from the laser resonator. In other words, pulsed laser oscillation is achieved by varying the energy loss of the laser resonator using the electrooptic element 72 and polarizer 73.
Materials that can be used for the electrooptic element include LiNbO.sub.3, KDP (KH.sub.2 PO.sub.4) and LiTaO.sub.3. Usually, the applied voltage needs to be as high as several kilovolts.
Since KDP is a deliquescent substance, it is necessary to package KDP so as not to contact air. Accordingly, miniaturization of the apparatus is difficult. Furthermore, due to material deformation of the piezo-effect, LiNbO.sub.3 cannot be used at a high switching speed.
When the Q-switching device is constructed of an electrooptic element and a polarizer, as described above, a power supply capable of generating high voltages is required to drive the electrooptic element. However, it is extremely difficult to construct a power supply that can generate such high voltage pulses of several kilovolts at a repetition rate of 1 kHz or faster. Thus, it is difficult to realize such a system for high-frequency switching.