Definition: optical switches which are typically used for generating nanosecond pulses in lasers
Alternative term: Q-switching devices
More specific term: acousto-optic Q switches
A Q switch (or Q-switching device) is a fast optical switch, i.e., a device which can be quickly switched between states where it causes very low or rather high power losses, respectively, for a laser beam sent through it. Q switches are used within a laser resonator with the purpose of active Q switching the laser; this is a technique for generating short intense light pulses, where the pulse duration is typically in the nanosecond range.
Similar devices can also be used for pulse generation with cavity dumping, but the detailed requirements on the optical switch are actually somewhat different in that case.
Types of Q Switches
Acousto-optic Q Switches
Figure 1: Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted.
The most common type is an acousto-optic modulator. The transmission losses through some crystal or glass piece are small as long as the acoustic wave is switched off, whereas strong Bragg reflection occurs with the acoustic wave switched on, so that the losses are typically of the order of 50% per pass, corresponding to 75% per double pass in a linear laser resonator. For generating the acoustic wave, an electronic driver is required with an RF power of the order of 1 W (or several watts for large-aperture devices) and a radio frequency (RF) of the order of 100 MHz.
The switching speed (or modulation bandwidth) is finally limited not by the acousto-optic transducer itself, but by the acoustic velocity and the beam diameter. The latter implies that the switching speed becomes tentatively lower for high-power lasers, which have larger beams.
For more details, see the article on acousto-optic Q-switches.
Electro-optic Q Switches
For particularly high switching speeds, as required e.g. in Q-switched microchip lasers, an electro-optic modulator (a Pockels cell) can be used. Here, the polarization state of light can be modified via the electro-optic effect (or Pockels effect), and this can be turned into a modulation of the losses by using a polarizer. Compared with an acousto-optic devices, much higher voltages are required (which need to be switched with nanosecond speeds), but on the other hand no radiofrequency signal.
A typical application area for electro-optic Q switches is in lasers with rather high gain, where the diffraction efficiency of an AOM would be insufficient. A high laser gain is needed for achieving short pulses. The laser gain may also get high if one requires a high pulse energy but wants to limit the mode area, e.g. due constraints of the resonator design.
Mechanical Q Switches
Particularly in the early days of Q-switched lasers, mechanical Q switches were often used – mostly in the form of rotating mirrors. Here, a small laser mirror is mounted on a quickly rotating device. The mirror is used as an end mirror in a linear laser resonator. A pulse builds up when the mirror is in a position where it closes the laser resonator. This approach is simple, applicable in a wide range of spectral regions without requiring special parts, and is quite robust (e.g. in terms of damage threshold), suitable particularly for high-power lasers with relatively long pulse durations. It is still used in a few cases.
Passive Q Switches
Passive Q switches are saturable absorbers which are triggered by the laser light itself. Here, the losses introduced by the Q switch must be small enough to be overcome by the laser gain once sufficient energy is stored in the gain medium; otherwise, the laser could not start operation. The laser power then first rises relatively slowly, and once it reaches a certain level, the absorber is saturated, so that the losses drop, the net gain increases, and the laser power can sharply rise to form a short pulse.
For a passively Q-switched YAG laser operating in the 1-μm spectral region, a Cr4+:YAG crystal typically serves as the passive Q switch. There are other possible materials, such as various doped crystals and glasses, and semiconductor saturable absorber mirrors are particularly suitable for small pulse energies.
Key Properties
For the selection of a suitable Q switch, the following aspects have to be considered:
the operation wavelength, which influences e.g. the required anti-reflection coating
the open aperture
the losses in the high-loss state (particularly for high gain lasers) and low-loss state (influencing the power efficiency)
the switching speed (particularly for short pulse lasers)
the damage threshold intensity
the required RF power
the cooling requirements
the size of the setup (particularly for compact lasers)
Of course, the electronic driver must be selected to fit to the Q switch in various respects.
