Q Switches
Author: the photonics expert Dr. Rüdiger Paschotta
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
DOI: 10.61835/9a1 Cite the article: BibTex plain textHTML Link to this page LinkedIn
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
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.
More to Learn
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains 56 suppliers for Q switches. Among them:
Laserton
Laserton offers electro-optic Q switches based on KTP, RTP, BBO or KD*P Pockels cells. We also make devices with a thermally compensated double crystal design.
Besides, Laserton offers Er:Yb:glass gain chips with integrated passive Q switches (saturable absorbers).
ALPHALAS
ALPHALAS offers a combination of high-speed, high voltage Pockels cell drivers with a Pockels cell for active Q-switching, designed for all standard laser wavelengths. Rise times below 1 ns and amplitudes > 10 kV cover even the most demanding applications. Repetition rate can be as high as 100 kHz.
The acousto-optic Q-switches from ALPHALAS have the advantage to operate in the MHz repetition rate range where the electro-optical Q-switches can operate only at the cost of considerable technical efforts and complex water-cooled design.
The passive Q-switch alternative has the advantage of simplicity, durability and low cost when compared with the active Q-switching alternative. Cr4+:YAG, V3+:YAG and Co-spinel (Co2+:MgAl2O4) cover the spectral range from 900 nm to 1600 nm. Most Q-switching crystals are available from stock.
Shalom EO
Shalom EO offers Pockels cells made from DKDP, BBO crystals and LiNbO3 or MgO:LiNbO3 crystals:
- Our DKDP Pockels cells have a high damage threshold and can be used in a wide spectral range from 0.3 to 1.1 μm; it is often used in medical, cosmetic and industrial lasers. Apertures up to 50 mm are available.
- Our LiNbO3 and MgO:LiNbO3 Pockels cells are excellent choices for ultraviolet (UV) lasers to infrared lasers with long operating wavelengths up to 4 μm. LiNbO3 Pockels cells are specially preferable for Er:YAG, Ho:YAG and Tm:YAG lasers.
- Our BBO Pockels cells have significant advantages in terms of laser power handling abilities, temperature stability, and substantial freedom from piezoelectric ringing. BBO Pockels cells are specially suitable for high repetition rate lasers up to 1 MHz.
Hangzhou Shalom EO also offers polished and AR-coated DKDP, BBO and LiNbO3 and MgO:LiNbO3 crystals with Cr-Au electrodes used in EO and AO Pockels cells.
RefleKron
Our customized semiconductor saturable absorber mirrors (SESAMs) can be used as passive Q-switches in applications where short (max. few nanoseconds), high repetition rate and relatively low-energy pulses are required.
Contact us for the optimal customized SESAM for your application.
Artifex Engineering
Artifex Engineering offers customised Pockels cells, available as DKDP (KD*P), MgO:LiNbO₃ and BBO. Our Pockels cells feature low insertion loss, high damage threshold. These are cost effective units at OEM prices. We also offer a range of standard Pockels cells with an aperture of 6 mm to 12 mm diameter. Visit our product page for more information. We look forward to your inquiry.
Raicol Crystals
Raicol Crystals offers a wide range of electro-optic solutions, including RTP, BBO, i-RTP, and LN, catering to diverse electro-optic applications. Some of those are particularly suited for Q switching of lasers:
- RTP (rubidium titanyl phosphate) is an excellent choice for electro-optic applications, such as Pockels cells, shutters, Q-switches, phase modulators, pulse pickers, and more. The outstanding electro-optical parameters enable extremely fast switching with rise/fall times under 1 nanosecond and high repetition rates exceeding 1 MHz, all without ringing.
- BBO (beta barium borate) crystals combine a wide transparency range, high damage threshold, and excellent chemical and mechanical properties, making them ideal for various electro-optic applications.
EKSMA OPTICS
We produce KTP, KD*P and BBO Pockels cells for high repetition rate Q-switching. Our Pockels cells can be supplied with mounting stages, drivers and power supplies.
Questions and Comments from Users
2020-08-31
Is the deflection typically limited to under 5 degrees from these types of devices?
The author's answer:
Yes, assuming an acousto-optic device.
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2020-05-16
Why do we place the Q-switch between the active medium and the partially reflecting mirror?
The author's answer:
It is not necessary to put it there. It is in fact often placed near the highly reflecting mirror. Depending on the concrete circumstances, different positions may be ideal.