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Author: the photonics expert Dr. Rüdiger Paschotta
Acousto-optic modulators (AOMs) can be optimized for the particular application of Q switching lasers.
Such an acousto-optic Q switch is placed inside a laser resonator. While the laser is pumped, the RF input of the AOM is switched on, on that the diffraction losses of light circulating in the resonator are high ( because the diffracted beams leave the resonator), and lasing is suppressed. When the RF input is suddenly switched off, an intense laser pulse is generated.
Most Q-switched solid-state lasers contain an acousto-optic Q switch; only few lasers are built with an electro-optic Q switch, where highest switching speeds and/or very high loss modulations are required.
Figure 1:
A compact acousto-optic Q-switch. Source: G & H.
Requirements on Acousto-optic Q switches
General requirements on AOMs are discussed in the article on acousto-optic modulators. Specifically for Q switching of lasers, the following aspects are relevant for proper performance:
- The insertion loss for the zero-order (non-diffracted) beam should be very low in order to avoid power losses and thermal effects. Therefore, a loss-absorption acousto-optic medium should be used (frequently fused silica) and prepared with high surface quality. 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; that enforces linear polarization of the laser beam. In some cases (e.g. for lasers where strong depolarization loss would occur), the Q switch should be polarization-independent.
- The input and output polarization should usually be identical. (Q switches are normally based on isotropic diffraction, even when non-isotropic acousto-optic media are used.)
- The damage threshold of the modulator must be high enough to withstand the intense laser pulses.
- The diffraction losses in the &#;on&#; state must be high enough to safely suppress lasing. The required losses depend on the gain of the laser gain medium.
- The switching speed must be high enough to obtain a clean pulse build-up. That requirement essentially depends on the round-trip time of the laser resonator (thus on its length) and on the laser gain. The achievable switching speed is essentially limited by the acoustic velocity and the beam radius in the Q switch. For usual solid-state lasers, the required switching speed is easily obtained, but for particularly compact short-pulse lasers, this may be challenging. In some cases, an electro-optic modulator is required due to the speed limitations of AOMs.
- The device should be suitable for a high duty cycle, as is typical for such laser applications.
There are various kinds of trade-offs. For example, tellurium dioxide (TeO2) 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.
For high-gain lasers (for example, fiber lasers), one may use the first-order diffracted beam under lasing conditions, so that very high resonator losses result when the AOM is turned off. However, the losses in the lasing state are then also fairly high, and a frequency shift occurs during pulse generation. Therefore, that configuration is not often used.
RF Drivers for Acousto-optic Q switches
The used electronic driver is usually a device operating with a fixed modulation frequency and a digital input for rapid on/off switching of the RF output.
The required RF drive power is normally substantial (sometimes even well above 10 W) for several reasons:
- The laser gain is often (but not always) quite high, so that substantial loss modulation is necessary for Q switching.
- The acousto-optic medium must be optimized for low losses, tentatively leading to a lower acousto-optic figure of merit.
- If polarization-independent operation is required, one must use shear waves, leading to a still lower figure of merit.
- Particularly for high-power lasers, a large aperture and correspondingly large acoustic beam diameter is required.
As the RF power is finally converted to heat, water cooling of AOMs is often necessary.
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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
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
For more Acousto-Optic Q-Switch Driverinformation, please contact us. We will provide professional answers.
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
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
150/630
80/400
Acoustic Velocity
(m/s)
5.96E+3
5.96E+3
5.96E+3
4.2E+3
4.2E+3
Wave 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
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