Introduction to Q-Switching
Q-switching technique is usually utilized in solid-state laser technology to generate nanosecond high energy pulses. It creates short pulses through regulating cavity losses. Q factor (quality factor) is a definition of an oscillation damping strength measurement.
Types of Q-Switching
There are two types of q-switching: passive and active.
Active Q-Switching
Active q-switching technique uses an electrically controlled modulator (acousto-optic or electro-optic). It is applied to control optical losses which are high initially, but in the process of switching they are lowered abruptly. Pump phase and the gain-medium upper-state lifetime should be roughly the same to avoid losing energy in spontaneous emissions. Energy loss through spontaneous emissions becomes significant when laser gain is high. However, it is not the biggest concern when it comes to bulk lasers. A bigger issue is parasitic lasing because of unwanted reflections or q-switch. To avert the lasing a modulator should have a high pump phase when performing power losses.
It is possible to design smaller mode area lasers when the energy per pulse and energy stored are lowered due to high pulse repetition rates (10 kHz, 100 kHz or more). However, high repetition rates create another issue – collecting enough laser gain even when stored energy is low, because when the gain is low pulses become longer. Even in case of high average powers it can still be problematic, because it may need bigger beam areas. That is why it is better to select a crystal providing a higher laser gain, for example, Nd:YVO4.
Passive Q-Switching
Passive q-switching technique uses a saturable absorber instead of an electrical modulator. There is a high optical loss when it is in the unsaturated state. To start the lasing a laser gain has to reduce that loss. When the emission increases, it saturates losses, and the laser power grows fast, which leads to the gain saturation.
It may give an impression that it is poor because of the absorption; however this is not the case. Just a little portion of energy is required for an absorber transparency, when the laser gain medium saturation energy is higher than the absorber energy. One of the most common crystals used in this case is Nd:YAG.
The pumping in a passive q-switched laser continues up to a moment when a pulse build up begins. This process begins when there is enough energy stored in the gain medium. The main difference between passive and active q-switching technique is that in case of a passive one the pumping power change does not affect the energy pulse, it will only effect the timing. In active q-switching technique both will be affected, the energy pulse and timing.
