Q-switches
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: optical switches which are typically used for generating nanosecond pulses in lasers
Alternative term: Q-switching devices
Related: optical switchesQ-switchingQ-switched lasersacousto-optic Q-switcheselectro-optic modulatorssaturable absorberssemiconductor saturable absorber mirrorscavity dumpingmodulator drivers
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DOI: 10.61835/9a1 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What is a Q-switch?
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 (i.e., switching the Q-factor of its resonator); 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
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 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 lasers for very short pulse durations, 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 to constraints of the resonator design.
For more details, see the article on electro-optic Q-switches.
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. Initially, the laser power rises relatively slowly. But 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 of Q-switches
For the selection of a suitable Q-switch, the following aspects have to be considered:
- the operating 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 the Q-switch in various respects.
Timing Synchronization and Jitter
A key distinction between active and passive Q-switches is their ability to synchronize pulse emission with an external signal.
- Active Q-switches (acousto-optic or electro-optic) are triggered by an electronic signal. They allow for precise timing of the pulse emission with very low jitter (often in the nanosecond or sub-nanosecond range), which is essential for applications requiring synchronization with other devices (e.g., in LIDAR or time-resolved spectroscopy).
- Passive Q-switches (saturable absorbers) are triggered automatically when the intracavity intensity reaches a certain threshold. The exact timing depends on the pump power, the recovery time of the absorber, and the noise in the initial spontaneous emission. This results in significant timing jitter (fluctuations in the pulse repetition period) and makes them generally unsuitable for applications requiring tight temporal synchronization, although they offer the advantage of not requiring any driving electronics.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a Q-switch?
A Q-switch is a fast optical switch used inside a laser resonator for Q-switching. It can be rapidly switched between a state of high optical loss, which prevents lasing, and a state of low loss, which allows a short and intense light pulse to build up.
What are the main types of Q-switches?
The main types are active Q-switches, which are controlled externally, and passive Q-switches. Active types include acousto-optic, electro-optic, and mechanical devices. Passive Q-switches are based on saturable absorbers triggered by the laser light itself.
When are electro-optic Q-switches used?
Electro-optic Q-switches, usually Pockels cells, are used for very high switching speeds to generate extremely short pulses. They are also preferred for high-gain lasers where the loss modulation of an acousto-optic device would be insufficient to prevent premature lasing.
How does a passive Q-switch work?
A passive Q-switch is a saturable absorber with intensity-dependent loss. It initially has high loss, allowing energy to be stored in the gain medium. When the intracavity light intensity becomes high enough, the absorber saturates (becomes transparent), causing a sudden increase in the resonator Q-factor and the emission of a powerful pulse.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 52 suppliers for Q-switches. Among them:
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.
Our acousto-optic Q-switches are rugged, reliable, and long-lasting, backed by millions of hours of service in the field.
We offer low insertion loss, highly efficient acousto-optic Q-switches capable of handling very high peak power, and will draw on our 35 years of experience to match the cavity length, repetition rate, wavelength, beam diameter, polarization state and output power of a laser to the best acousto-optic Q-switch solution.
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.
Hangzhou Shalom EO offers standard and custom Q-switches made of DKDP, BBO crystals, and LiNbO3 or MgO:LiNbO3 crystals.
Shalom EO’s DKDP Pockels cells feature high deuteration (> 98%), low capacitance and fast rise time, and high transmission, high extinction ratio, with a maximum aperture of 50 mm. Recently, we have succeeded in developing a 3-pin connector DKDP Pockels cells for 755 nm alexandrite lasers manufactured using 1 pce of 99% deuteration DKDP crystal with 3 gold plating.
Shalom EO’s 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 especially preferable for Er:YAG, Ho:YAG and Tm:YAG lasers.
Our BBO Pockels cells have significant advantages in terms of high damage threshold, low insertion loss, high extinction ratio, minimal piezoelectric ringing, and competitive price. BBO Pockels cells with both Single and double BBO crystal designs and low-voltage geometries are available upon request.
Hangzhou Shalom EO also offers polished and AR-coated DKDP, BBO and LiNbO3, and MgO:LiNbO3 crystals with Cr–Au electrodes, which can be used as central components of Pockels cells.
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.
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.
Artifex Engineering offers customised Pockels cells, available as DKDP (KD*P) 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.
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.
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.