How to Select a Mirror

Transcription

How to Select a Mirror
How to Select a Mirror
Mirrors are an essential tool in any laser system, where
they are used for beam steering and sampling, or for
delivery of pump and alignment lasers. They find equal
use in optical systems for general light routing, collection,
and delivery. A high-performance mirror needs to have
low scatter, high efficiency, and excellent durability,
often in conjunction with high laser damage threshold.
Choosing a mirror requires first identifying the wavelength
and bandwidth required, at which point more than one
type of coating may be offered. Each coating technology
offers its own advantages and limitations as regards laser
damage threshold, reflectivity, bandwidth, and transmitted
wavefront error.
CVI Laser Optics uses high quality N-BK7 substrates for
mirrors designed for use at > 400 nm and Corning 7980
UV-grade fused silica for < 400 nm. All are precisionpolished to λ/10 surface figure prior to coating, with
wedge ≤ 5 arc min, maximum diameter tolerance of +
0/- 0.25 mm, and thickness controlled to ± 0.25 mm or
better. Surface quality is 10-5 for dielectric coated mirrors
and 40-20 for metal coated mirrors, while clear aperture is
a minimum of ≥85% of the central dimension for all mirrors.
Bandwidth of operation
Mirrors for laser applications vary in spectral range
from dielectric coated laser line mirrors with only a few
nanometers of bandwidth to broadband metal mirrors
that reflect from visible wavelengths through to 20 μm.
The bandwidth needed will depend on the emission
wavelength(s) of the laser being used, as well as its stability
or repeatability from unit to unit. Most excimer lasers
emit at a single wavelength, while others like Nd:YLF and
argon-ion lasers have multiple closely spaced emission
wavelengths. Ultrashort pulsed lasers such as Ti:Sapphire
emit a range of wavelengths simultaneously, and others
like scanning dye lasers output a single wavelength at a
time from within a wide tunable range. Dielectric coated
mirrors have traditionally been used for narrowband
mirrors, while metal coatings served broadband
applications, albeit at lower reflectivity. Advances in
coating technology in both areas have blurred these lines
and increased the number of choices available for a given
application.
Coating types
The simplest method of making a mirror is to deposit a
metal coating on a glass substrate. Metal mirrors offer
broadband spectral performance, are insensitive to angle
and polarization, and are inexpensive to manufacture.
Aluminum has good reflectivity at UV and visible
wavelengths, while silver and gold are best for the visible
through infrared, depending on wavelength. Durability
can be an issue, however. Bare aluminum and silver are
prone to oxidation, and all three can be easily scratched
or damaged. This is addressed by our protected metal
coatings, in which an overcoat composed of a single hard
dielectric layer of half-wave optical thickness is applied
to the metal coating to improve tarnish and abrasionresistance without significantly affecting the mirror’s
optical properties. Another limitation of metal coatings
is the dependence of their reflection spectrum on metal
type, and the fact that reflectivity is lower overall than for
a typical dielectric coated mirror. To mitigate this, we
have developed enhanced metal coatings which use a
thin dielectric film as overcoat to both protect the metal
layer and increase the reflectance over a desired range
of wavelengths or range of incidence angles. Even with
these improvements, metal coated mirrors are limited in
their power handling and can only be used for low power
applications. All of our metal coated mirrors are inspected
to 40-20 scratch and dig surface quality.
Dielectric coatings employ quarter-wave thicknesses
of alternately high and low refractive index materials
applied to a substrate to form a multilayer stack. By
choosing the coating materials and thicknesses carefully,
the reflected wavefronts from each layer are made to
interfere constructively to produce a highly reflective
mirror. These coatings are remarkably hard, durable, and
abrasion-resistant. Over a limited wavelength range, the
reflectivity of a dielectric coating can easily be made to
exceed the most efficient metallic coating. Furthermore,
the coatings are effective for both s- and p-polarization
components, and can be designed for a wide range of
angles of incidence. As AOI moves significantly away from
the design angle, however, reflectance can be markedly
reduced.
CVI Laser Optics uses two primary technologies for
dielectric coatings. Electron beam deposition utilizes an
electron beam to vaporize the material to be deposited.
When combined with careful control of the temperature
and vacuum conditions, it creates uniform coatings with
excellent optical characteristics, high laser damage
threshold, and good reliability. Ion beam sputtering uses
a very high kinetic energy ion beam to sputter target
materials directly onto the substrate with a high level of
accuracy and repeatability over numerous coating runs. It
produces dense coating layers with almost no scatter or
absorption, which minimizes spectral shift due to moisture
absorption. In addition, the coating density and durability
allows for high damage threshold and results in fewer
pin-hole defects in the coated surface. This excellent
film quality and uniformity results in environmentally
stable optics with laser damage thresholds exceeding
40 J/cm2 pulsed at 1064 nm. When utilized by the best
filter designers, IBS coating technology can also achieve
superior optical performance, including enhanced
reflectivity, minimal polarization dependence, reduced AOI
dependence, and broader bandwidths. All of our electron
beam and IBS coated mirrors are inspected to 10-5 scratch
and dig surface quality.
The coating technique alone does not determine
performance. Control of the coating process is essential
to achieving durable, high-reflectivity coatings. We use
advanced production systems and methods to apply our
coatings, and employ optical monitoring throughout the
deposition process to check the intensity of reflected or
transmitted light until a mirror coating is complete. All
coating batches are rigorously tested and inspected to
ensure consistent, high performance. Our state-of-theart deposition facilities are able to coat large volumes
of standard catalog and custom optics, and we can also
develop and evaluate new coatings for customers’ special
requirements. In addition to our own range of N-BK7,
fused silica, and Zerodur substrates, CVI Laser Optics’
coatings can be applied to customer-supplied substrates.
Angle of incidence
Metal coatings are, by nature, equally reflective at all
angles of incidence. Dielectric coatings, however, owe
their reflectivity to interference between the reflections
from their many layers. As a dielectric coating is angled
with respect to an incident beam, the effective thickness
of the layers is altered and causes the spectrum of the
coating to shift and change. As the angle of incidence
increases from 0°, the spectrum of a typical dielectric
coating shifts toward shorter wavelengths. Two distinct
spectra also emerge, one for s-polarized light and one for
p-polarized light. At larger angles, the spectrum becomes
highly distorted, and the shift can be significantly different
for s- and p-polarized light, depending on the mirror
design. The spectral shift with angle can sometimes be
used to tune a mirror to shorter wavelength, provided that
the design allows and the change in AOI is kept relatively
small. Using a single polarization also tends to reduce the
distortion of the shifted spectrum.
Mirrors used for routing are coated with designs optimized
specifically for 45° AOI at a single laser wavelength,
and are therefore used at peak reflectance wavelengths
where polarization differences can be made negligible.
Nonetheless, it can be important to consider the
difference in reflection efficiency and spectral shape for
different polarizations when operating at non-zero angles.
It is also possible to very carefully design a dielectric mirror
for which reflectivity remains high for a wide range of
angles of incidence, from 0 - 45° or more. Our Semrock
Ultrabroadband MaxMirror® (BBDM) and MAXBRIte™
Broadband Mirrors (MPQ) are good examples. Almost
all of our other mirrors are offered in both 0° and 45°
AOI designs to ensure optimal performance for most
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applications, and custom mirrors can be made for other
angles of incidence.
Cone half angle (CHA) of the incident beam should be
considered when working with dielectric mirrors. Spectral
performance will vary over the range of incident angles
contained within the CHA, so a mirror may not meet
specification when incident light exceeds the CHA for
which the mirror was designed. This tends to be a greater
concern for optics with sharp spectral profiles.
In addition to highly reflective mirrors, we keep a wide
range of uncoated mirror substrates in stock made from
N-BK7, Corning 7980 UV-grade fused silica, and Zerodur.
These include round, square, and rectangular substrates
ranging from 10 – 152 mm in dimension, with flat, convex,
and concave surface profiles. These substrates can be
coated with any of our high reflectivity, metal and partial
reflecting coatings. We also maintain a stock of high
energy partial reflecting laser mirrors (PR1) with 10 – 99%
reflectivity in a variety of popular laser wavelengths, and
offer quick turn on semi-custom partial reflectors specified
by the user for 30 – 99% reflectivity and wavelengths from
532 – 1550 nm.
be lower for broadband dielectric mirrors. For example,
our MAXBRIte™ flat mirrors (MPQ) have a λ/4 or better
surface flatness while λ/10 is standard for our MaxMirror®
ultrabroadband mirrors (BBDM) and tunable broadband
mirrors (depending on substrate size and thickness).
Surface quality is also somewhat dependent on coating.
All of our dielectric mirrors are inspected to 10-5 scratch
and dig, while 40-20 is more typical for our metal coated
mirrors.
One final item to consider in mirror selection is the
degree of curvature of the substrate. Convex or concave
substrates can impart subtle beam-shaping to correct
collimation, or they can allow the mirror to perform dual
function as a focusing element to minimize losses within an
optical system. A number of our mirrors are offered from
stock or semi-customized with radii of curvature up to 10
m. We also offer our partial reflecting mirrors with a slight
30 arc min wedge to reduce back reflections. Whatever
your requirements may be, remember that our catalog and
semi-custom product offerings are only the beginning.
Our technical staff is on hand to assist you in selecting or
creating the optimum mirror for your application.
Laser damage threshold
Coating type is the primary factor to consider when
choosing a mirror based on laser damage threshold
(LDT). Metal coatings have the lowest damage thresholds.
Broadband dielectric coatings such as MAXBRIte™ are
better, but single-wavelength or laser-line coatings are
better still. If even higher damage thresholds are needed,
ion beam sputtering creates dense, robust coatings with
low scatter and absorption to achieve maximum LDT.
Our IBS Nd:YAG laser mirrors (YxS) are rated to 40 J/cm2
pulsed at 1064 nm. The substrate material is also a factor;
higher damage thresholds can be achieved using fused
silica instead of BK7.
Making the final decision
Bandwidth, center wavelength, laser damage threshold
and AOI will largely determine the best mirror for your
application, but a few other factors may also influence
the decision. Surface figure or flatness of the mirror will
determine the distortion of your beam profile. This
parameter is held tightly to λ/10 @ 633 nm before
coating and often maintained through coating, but can
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Selection Guide:
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Product Code Description
Wavelengths
AOI
Coating type
AR1
Argon-Ion Laser Mirrors for
488-515 nm
488 - 515 nm
0° or 45°
e-beam
dielectric
Additional features
AR2
Argon-Ion Laser Mirrors for
458-529 nm
454 - 529 nm
0° or 45°
e-beam
dielectric
AR3
Argon-Ion Laser Mirrors for
351-364 nm
351 - 364 nm
0° or 45°
e-beam
dielectric
AR4
Argon-Ion Laser Mirrors for
244-257 nm
244 - 257 nm
0° or 45°
e-beam
dielectric
ARF
Excimer Laser Mirrors for
193 nm / ArF
193 nm
45°
e-beam
dielectric
BBDM
Semrock MaxMirror®
Ultrabroadband Mirrors
350 - 1100 nm
0° to 45°,
variable
IBS dielectric
▪ High reflectivity at any AOI from 0° to 45°
DPY
Diode Pumped Resonator
Mirrors
1064/808 nm
0°
e-beam
dielectric
▪ R > 99.5% @ 1064, T > 95% @ 808 nm
▪ For pump delivery to Nd:YAG cavity; concave
optional
DUVA
Deep UV Aluminum Mirrors
193 - 1200 nm
Any
high density
aluminum
EAV
Enhanced Aluminum Mirrors 450 -650 nm
Any
high density
aluminum
▪ Protective dielectric coating optimized for visible
FLM
Fiber Laser Mirrors
CWLs from 780 1900 nm
0° or 45°
e-beam
dielectric
▪ 1030, 1064, 1070, 1550 nm standard, others
custom
HC2
Helium Cadmium Laser
Mirrors for 325 nm
325 nm
0° or 45°
e-beam
dielectric
HM
Nd:YAG 1064/532 nm Dual
Wavelength Mirrors
1064/532 nm
0° or 45°
e-beam
dielectric
HN
Helium Neon Laser Mirrors
for 633 nm
633 nm
0° or 45°
e-beam
dielectric
KRF
Excimer Laser Mirrors for
248nm / KrF
248 nm
0° or 45°
e-beam
dielectric
LDM
Laser Diode Mirrors
670, 780, 980,
1300, or 1550
nm
0° or 45°
e-beam
dielectric???
MPQ
MAXBRIte™ Flat Mirrors
"Bands from
245 - 850 nm"
0° to 45°,
variable
IBS dielectric
▪ High reflectivity at any AOI from 0° to 45°
▪ Bandwidths from 145 - 220 nm
▪ Round or square substrates, λ/4 surface flatness
PAUV
UV Enhanced Aluminum
Mirrors
250 - 600 nm
Any
high density
aluminum
▪ Protective dielectric coating optimized for UV
PAV
Protected Aluminum Mirrors 400 - 800 nm
Any
high density
aluminum
▪ Protective dielectric coating optimized for visible
& NIR
PG
Protected Gold Mirrors
650 - 20,000 nm
Any
gold
▪ Protective dielectric coating optimized for visible
through IR
PLFM
Semrock® Ultrabroadband
Femtosecond Mirrors
650 - 1100 nm
0° or 45°
IBS dielectric
▪ Low GVD for 40 - 120 fs Ti:sapph laser use
▪ High reflectivity at 1064 & 532 nm
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PM
Plane Round Mirror Blanks
See material
Any
none
▪ Wide range of diameters in N-BK7, fused silica,
or Zerodur, others custom
PR1
High Energy Partial
Reflecting Laser Mirrors
CWLs from
193 - 1064 nm
0°
e-beam
dielectric
▪ User-specified reflectivity, wavelength, curvature/
wedge
▪ AR-coated second surface to minimize loss
PS
Protected Silver Mirrors
400 - 20,000 nm
Any
silver
▪ Protective dielectric coating optimized for visible
through IR
RM
Plane Rectangular Mirror
Blanks
See material
Any
none
▪ Range of dimensions in fused silica, others
custom
SMCC
Concave Spherical Mirror
Blanks
See material
Any
none
▪ Wide range of radii in N-BK7 or fused silica,
others custom
SMCX
Convex Spherical Mirror
Blanks
See material
Any
none
▪ Wide range of radii in N-BK7 or fused silica,
others custom
SQM
Plane Square Mirror Blanks
See material
Any
none
▪ Wide range of sizes in N-BK7 or fused silica,
others custom
TLM1
Tunable Laser Line Mirrors
CWL's from
190 - 1550 nm
0° or 45°
e-beam
dielectric
▪ User-specified reflectivity, wavelength, curvature/
wedge
▪ Flat, wedged, or concave substrate option
TLM2
Tunable Broadband Mirrors
CWLs from
450 - 2100 nm
0° or 45°
e-beam
dielectric
▪ User-specified center wavelength
▪ Bandwidth of 90 - 250 nm @ 0°, depending on
CWL
TLMB
High Energy Ti:Sapphire
Mirrors
740 - 860 nm
0° or 45°
???
▪ Ultralow GVD for femtosecond laser use
TLMW
Enhanced Ti:Sapphire
Mirrors
720 - 900 nm
0° or 45°
e-beam
dielectric
▪ Low GVD for ≥ 15 fs Ti:sapph laser use
VUVA
Vacuum UV Aluminum
Mirrors
157 - 190 nm
Any
high density
aluminum
XeCl
Excimer Laser Mirrors for
308 nm / XeCl
308 nm
45°
e-beam
dielectric
Y1
High Energy Nd:YAG Laser
Mirrors for 1064 nm
1064 nm
0° or 45°
e-beam
dielectric
Y2
High Energy Nd:YAG Laser
Mirrors for 532 nm
532 nm
0° or 45°
e-beam
dielectric
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Y3
High Energy Nd:YAG Laser
Mirrors for 355 nm
355 nm
0° or 45°
e-beam
dielectric
Y4
High Energy Nd:YAG Laser
Mirrors for 266 nm
266 nm
0° or 45°
e-beam
dielectric
Y5
High Energy Nd:YAG Laser
Mirrors for 213 nm
213 nm
45°
e-beam
dielectric
Y13
High Energy Nd:YAG Laser
Mirrors for 1319 nm
1319 nm
45°
e-beam
dielectric
Y1S
Ion Beam Sputtered
Nd:YAG Laser Mirrors for
1064 nm
1064 nm
0° or 45°
IBS dielectric
Y2S
Ion Beam Sputtered
Nd:YAG Laser Mirrors for
532 nm
532 nm
0° or 45°
IBS dielectric
Y3S
Ion Beam Sputtered
Nd:YAG Laser Mirrors for
355 nm
355 nm
0° or 45°
IBS dielectric
Y4S
Ion Beam Sputtered
Nd:YAG Laser Mirrors for
266 nm
266 nm
0° or 45°
IBS dielectric
YH
Nd:YAG 1064/633 nm Dual
Wavelength Mirrors
1064/633 nm
0° or 45°
e-beam
dielectric
YL1
Nd:YLF Laser Mirrors for
1047-1053 nm
1047 - 1053 nm
0° or 45°
e-beam
dielectric
YL2
Nd:YLF Laser Mirrors for
524-527 nm
524 - 527 nm
45°
e-beam
dielectric
YL3
Nd:YLF Laser Mirrors for
349-351 nm
349 - 351 nm
45°
e-beam
dielectric
YL4
Nd:YLF Laser Mirrors for
262-263 nm
262 -263 nm
45°
e-beam
dielectric
YL5
Nd:YLF Laser Mirrors for
209-211 nm
209 - 211 nm
45°
e-beam
dielectric
▪ R > 99% @ 1064, R > 80% @ 633 nm
▪ For Nd:YAG alignment using a HeNe laser
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