A Q switch is a device which can be quickly switched between states where it causes very low or rather high losses, respectively, for a laser beam sent through it. Such devices are typically used within a laser resonator with the purpose of active Q switching the laser; this is a technique for generating short intense pulses, where the pulse duration is typically in the nanosecond range. Q switches 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
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.
There are various kinds of trade-offs. For example, tellurium dioxide material with its high elasto-optic coefficients requires small acoustic powers, but has a moderate damage threshold. Higher optical intensities can be tolerated by crystalline quartz or fused silica, but at the cost of higher acoustic powers (and thus also RF powers). The acoustic power required also depends on the optical aperture of the device: large aperture devices, as required for high-power lasers, require higher acoustic powers. The heat generation in the Q switch can then be so strong that water cooling is necessary. At lower power levels, conductive cooling is sufficient.
The switching speed (or modulation bandwidth) is finally limited not by the acousto-optic transducer, but by the acoustic velocity and the beam diameter.
To suppress reflections from the optical surfaces, anti-reflection coatings are frequently used. There are also Q switches where the active element is operated at Brewster's angle.
Electro-optic Q Switches
For particularly high switching speeds, as required e.g. in Q-switched microchip lasers, an electro-optic modulator 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.
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 and very robust, suitable particularly for high-power lasers with relatively long pulse durations.
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. 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, 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.
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.
See also: Q switching, Q-switched lasers, acousto-optic modulators, electro-optic modulators, saturable absorbers, semiconductor saturable absorber mirrors, cavity dumping
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