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The acousto-optical Q-switch often used in the laser marking makes use of mutual interaction between an ultrasonic wave and a light beam in a scattering medium. The light beam that enters in a direction forming a Bragg angle to the wave surface of the acoustic wave in the scattering medium is diffracted in accordance with periodic changes in the diffraction rate produced by the acoustic wave.
The situation is briefly explained. First of all, an RF signal is impressed to the transducer adhered to the molten quartz and thickness extensional vibration is produced. Ultrasonic shear waves are caused to advance in the molten quartz by this vibration, and phase grating formed by acoustic waves is produced. The laser beam is diffracted when it satisfies the Bragg angle with respect to this phase grating, and is separated in space from the incident light.
If the laser optical resonator is constructed against 0-dimensional diffracted light (undiffracted light), the diffracted light deviates from the laser optical resonator axis when a RF signal is impressed. As a result, loss occurs in the laser optical resonator and laser oscillation is suppressed. To make use of this phenomenon, an RF signal is impressed for a certain length of time only (status of low Q-value) to suspend laser oscillation. In the meantime, the population inversion of the Nd:YAG rod is accumulated by continuous pumping. When the RF signal is reduced to zero (status of high Q-value) and the loss to the laser optical resonator is removed, the accumulated energy is activated as laser oscillation in a pulse form within an extremely short length of time. They are Q-switch pulses.
This situation is briefly explained. When an RF signal is subjected to pulse modulation, it is possible to periodically take out a Q-switch pulse. When the period of Q-switch pulses becomes shorter than the life (about 200 ms) of the higher order of the Nd:YAG rod, however, the population inversion decreases and the peak value of Q-switch pulses decreases.
1. QS Series Q-switches at nm
A water-cooled acousto-optic Q-Switch for use in high-power Nd:YAG laser systems. Combining top grade fused silica with high quality optical finishing and in-house anti-reflection coatings, this Q-Switch exhibits very low insertion loss and high damage threshold. Through an innovative design and manufacturing process, RF powers up to 100W may be applied.
Standard options include a choice of frequencies (24 to 68MHz), active apertures (1 to 8mm), acoustic modes (compressional for linear polarisation, shear for unpolarised) and water connectors. Customised housings are available for OEM�s.
Specifications:
Model No: See "Options" below
Interaction Medium: Fused Silica
Operational Wavelength: nm
Anti Reflection Coating: Hard multi-layer dielectric
- Reflectivity: <0�2% / surface (< 0�1% typical)
- Damage Threshold: > 500MW cm-2
Insertion Loss: <10% (< 5% typical)
Active Aperture: See "Options" below
VSWR: 1�2:1
Maximum CW Drive Power: 100W
Thermal Interlock: +50�C
Water Cooling
Flow rate: 190cc / min (minimum) Acousto-Optic Q-Switch Selection Guide
Recommended water operating: <32�C
Recommended water maximum: 40�C
Water Connectors: See "Options" below
Options
QS
27
-4
S
-B
-X X n
DEVICE
FREQUENCY
ACTIVE
APERTURE
ACOUSTIC
MODE
WATER
CONNECTOR
SPECIAL
DESIGNATION
Device : QS - Q-Switch
Frequency : 24, 27, 41 - Value in MHz
Aperture : 1.6, 2, 3, 4, 5, 6�5, 8 - Value in mm (In general, the aperture of Q-switch is equal to or larger than the diameter of laser beam or YAG rod.
Acoustic Mode : C - Compressional, S - Shear
Water Connector : S - Screw-on (Swagelok etc.), B - Barbed Push-on
Special Designation : - For non-standard Q-switch models identification characters which define the configuration may be allocated.
Driver Selection Acousto-Optic Q-Switch Selection Guide
QSD series or R series drivers
2. QS Series Q-switches at -nm
Model No.
QS027-4H-xxx
Interaction material
Infrasil (water-free fused silica)
Wavelength
-nm
AR coating reflectivity
< 0.2% per surface
Damage threshold
> 100MWcm-2
Polarisation
Linear (vertical to base)
Interaction length
46.0mm
RF frequency
27.12MHz
VSWR
< 1.2 1 at 50.
Acoustic Mode
Compressional
Active aperture
5.0mm
Loss modulation
> 80% at 50W RF power
Housing
Standard QS24/27 range (Aluminium)
Water connectors
Barbed
3. QS Series Super Q-switches at nm
Model: QS2x-xD-x-xxx
High efficiency
For unpolarised, high power, high gain lasers
2 x 50W RF power handling
A new compressional mode, water-cooled, AO Q-Switch designed for use in high power unpolarised lasers giving faster switching, better pulse-to-pulse stability and higher power densities. Enhance your systems performance with greater punch and increased power, specifically for laser processing applications.
Before the Super Q-Switch, some customers were using 2 x Compressional mode Q-Switches (like the QS27-4C-S) in the same cavity. One of the Q-Switches is rotated 90degrees to the other. Because the Compressional mode Q-Switch is more efficient for polarised light, the first Q-Switch would block one polarisation & the second Q-Switch blocks the other. This is a good solution, but takes a large space in the cavity. The Super Q-Switch gives the same performance as using 2 x Compressional Q-Switch, but they are incorporated into 1 device.
This Q-switch uses a dual channel driver to operate two orthogonal compressional mode transducers bonded to a single monolithic optical cell and mounted in one convenient housing. Our proprietary bonding techniques and power handling technology allows this device to operate up to 50W per channel giving an efficient, compact, single device for the next generation of high power, high gain, solid state lasers.
Interaction Material
Fused Silica
Wavelength
to nm
Anti-Reflection Coating
< 0.2% per surface
Damage Threshold
> 500MWcm-2 (1GWcm-2 typical)
Transmission (single pass)
> 99.6%
Frequency
24.00 or 27.12MHz
VSWR
< 1.2:1 (50. input impedance)
Active Aperture
1.6, 2, 3, 4, 5 or 6.5mm2
Clear Aperture
9 x 9mm
Acoustic Mode
Compressional (Orthogonal)
Rise-Time / Fall-Time
109ns/mm
RF Power Rating
2 x 50W cw
Water Flow Rate
190cc / minute, minimum
Maximum Water Temperature
+40�C (recommended, 22�C to 32�C)
Water Connectors
Screw-fit or Barbed (push-on)
Thermal Switch Cut-Off
+55�C � 5�C
Housing / Flow Chamber Material
Aluminium HE30TF
Driver Selection:
Aperture size 1.6D, 2D or 3D, use 25W dual channel driver (R390xx25DMzz-2CH, or R390xx-25DSzzz-2CH)
Aperture size 4D, 5D or 6.5D, use 50W dual channel driver (R390xx25DMzz-2CH, or R390xx-25DSzzz-2CH)
Options and Model
4. Stallion Series AO Q-Switches at nm Wavelength
A �Stallion� version of our industry standard water cooled Acousto-optic Q-Switch, for use in high power lamp or diode pumped Nd:YAG lasers.
The patent pending �Stallion� manufacturing technique provides superior corrosion resistance whilst maintaining optimum performance and RF power handling capabilities up to 100W.
Combining top grade fused silica with high quality optical finishing and in-house anti-reflection coatings, this Q-Switch exhibits very low insertion loss and high damage threshold.
In addition to the standard product shown, custom configurations are available for specialized applications. These include alternative housing options, wavelengths and RF frequencies.
Key Features:
Industry standard for Nd:YAG lasers
Superior corrosion resistance
Stainless steel cooling channels
High damage threshold
Push fit water-connectors
Up to 100W RF power handling
Custom configurations available
Applications:
Material processing:
Laser marking
Laser engraving
Laser cutting
Laser drilling
Medical (surgery)
Lithography
Technical Specifications:
Interaction material:
Fused silicon
Wavelength:
nm
AR coating reflectivity:
< 0.2% per surface
Damage threshold:
> 1GWcm-2
Transmission (single pass):
> 99.6%
Static insertion loss:
' 6% at 50W laser power
VSWR:
< 1.2:1 (<1.4:1 at 50W RF power)
RF power rating:
100W cw (max)
Water flow rate:
> 190cc / minute
Water-cooling channel material:
Stainless steel 316
Recommended water temperature:
+22oC to +32oC
Thermal switch cut-off:
+55oC +/- 5oC
Ordering Codes
Example: I-QS027-4S4G-N5-ST1 (Q-Switch, 27.12MHz, 4mm active aperture, shear mode, fused silica, nm, 4mm OD straight push fit water-connectors, BNC, Stallion housing with M3 mounting holes).
5. Other Q-Switches
The Q-Switches are for use in both industrial and laboratory applications. Q-Switching is used principally on high peak power solid state Nd:YAG lasers at 1.06 micrometer wavelength. The Q-Switches are divided into three categories: for use with multi-mode, un-polarized lasers, with beam sizes 5mm and larger; for use with miniature, polarized or un-polarized, solid state diode pumped lasers; and single mode, polarized, low divergence solid state lasers, with beam size of 1 to 2 mm.
Wavelength(nm)
Q-Switch Model
Driver Power
Loss
Modulation (%@Watts) @nm
Polarization
Active Aperture
(mm)
Center Frequency
(MHz)
Rise Time
(ns / mm beam dia.)
Optical Power Density /(cm2)
Ave./Pk
Interaction Material
Outlook
drawing
10.6um
-3
30
85
3
27.12
120
500
Ge
53B
10.6um
-5
30
For more information, please visit Acousto-Optic Q-Switch Driver.
75
5
27.12
120
500
Ge
53B
-50-4
50
55
4x13
24
175
50K/500M
Fused Silica
53B
-50-4
50
55
4x13
27.12
175
50K/500M
Fused Silica
53B
-50-5-I-HGM-W
50
70
5x10
24
115
50K/500M
Crystal Quartz
53B
-50-5-I-HGM-CMS
50
70
5x10
24
115
50K/500M
Crystal Quartz
53B
-50-5-I-HGM-W
50
85
70
5x10
27.12
115
50W/500M
Crystal Quartz
53B
-50-5-I-HGM
50
70
5x10
27.12
115
50W/500M
Crystal Quartz
53B
-50-5-I-HGM-CMS
50
70
5x10
27.12
115
50W/500M
Crystal Quartz
53B
-50-5-I-M3
50
85
70
5x10
27.12
115
Crystal Quartz
53C
-70-7
70
55
7x13
24
175
50K500M
Fused Silica
53B
-70-7
70
55
7x13
27.12
175
50K/500M
Fused Silica
53B
-70-7-I-HGM-W
70
85
7x10
24
115
50K/500M
Crystal Quartz
53B
-70-7-I-HGM-CMS
70
85
7x10
24
115
50K/500M
Crystal Quartz
53B
-70-7-I-HGM-W
70
85
7x10
27.12
115
50K/500M
Crystal Quartz
53B
-70-7-I-HGM-CMS
70
85
7x10
27.12
115
50K/500M
Crystal Quartz
53B
-100-4-HGM-W
100
90
4x13
24
175
50K/500M
Fused Silica
53B
-100-4-HGM-W
100
90
4x13
27.12
175
50K/500M
Fused Silica
53B
-100-4-HGM-CMS
100
90
4x13
27.12
175
50K/500M
Fused Silica
53B
Remark: EO Q-switches operating at 10.6um (CO2 lasers) available.
6. STBR series free space Q-switch
STBR series Acousto-Optic Q-switching systems for industrial and laboratory applications. The STBR free space Q-switches are designed for the highest conversion efficiency of RF energy into acoustic energy by attaching the transducer to the crystal with an advanced vacuum metallized process. Q-switches are special modulators designed for use inside laser cavities. They are fabricated from high optical quality Fused Quartz, Flint Glass, and Tellurium Dioxide, or other acousto-optic materials with Brewster cut optical faces or durable hard oxide AR coatings for high optical power applications.
Model # FSQ-24-2-BC FSQ-27-5-BC FSQ-80-5-BC TEQ-27-4-BC TEQ-80-20-BC Substrate SiO2 SiO2 SiO2 TeO2 TeO2 Brewster cut yes yes ye yes yes Laser Wavelength (nm) 800 Active Aperture (mm) 2 2 1 1.5 3 Center Frequency (MHz) 2 27 80 27 80 Digital Modulation Bandwidth (MHz) 2 5 6.5 4 (3dB Bandwidth) 20 (3dB Bandwidth) Optical Transmission (%) 99.8 99.8 >99.5 >99.5 >99.5 Maximum Diffraction Efficiency (%) 30 30 25 >50 >65 Rise Time (nsec) 100 100 85 150/630Wave Front Distortion
λ/10
λ/10
λ/10
λ/10
λ/10
Separation Angle
5 mrad @ nm
5 mrad @ nm
5 mrad @ nm
1 deg @ nm
0.9 deg @ 800nm
Input Impedance
50 ohms
50 ohms
50 ohms
50 ohms
50 ohms
Optical Polarization
Linear (perpendicular to acoustic wave)
Linear
Perpendicular to acoustic wave
VSWR
2.1:1
For the associated RF drivers, please refer to �RF Drivers for STBR series�
Air-cooled Q-switch Q-switch drivers Fiber Q
Basics of Acousto-Optic Q-switch Frequently Asked Questions
Ask RP Photonics for advice concerning the selection of a suitable Q switch. RP Photonics can do numerical simulations for investigating exact requirements and possible limitations.
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Author: the photonics expert Dr. Rüdiger Paschotta
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.
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.
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.
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 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.
For the selection of a suitable Q switch, the following aspects have to be considered:
Of course, the electronic driver must be selected to fit to the Q switch in various respects.
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