ZnSe Partial Reflectors/ Output Couplers

output_couplersRaw material properties:
Optical properties – see section M1.0. ULO Optics use only the highest quality “Lasergrade”
ZnSe material.
Dimensions/physical characteristics
ZnSe partial reflectors can be fabricated in any diameter from 4.0mm up to 250mm, and in
thicknesses from 1.0mm up to 50mm. The customer can chose the reflectivity required from around
1% up to 99.8%. ZnSe laser outcouplers and rear mirrors are generally required in standard sizes such
as 25.0 or 25.4 diameter, and with specific radii and coatings. Many such standard items are included
in the ULO Optics outcoupler Price Lists, and others are included in the ‘laser spares’
category of products (See Technical Data Section 30.0 for example).

 

Tolerances
Unless otherwise specified by the customer, the standard tolerance range is as follows:
Diameter : +0, -0.1mm.
Thickness : ±0.1mm.
Parallelism/wedge accuracy : Within 4′ arc/within ±2′ arc.
Surface form : Better than λ/20 at 10.6μm.
Reflectivity : Typically ±0.25% at R ~ 95%. Better than ±0.50%
at R ~ 85%. Better than ±1.0% at R ~ 50%

Coatings
All coatings are ultra-low absorption type for maximum laser damage threshold and lifetime.
For normal incidence coatings at 10.6μm only the coating reflectivity need be specified
(eg: “AR/70%R”). For incidence angles other than normal, and greater than about 15°, the state of
polarization of the laser beam also requires specification if the correct coating is to be applied,
(eg: “AR/50%R, 45° incidence, S-Pol”). Coatings may be applied over a limited aperture if so
required.
Absorption/laser damage
It is not possible to give complete data about the absorption levels in ZnSe partial reflectors, since
the absorption will vary with the reflectivity. In general the measured absorption will be lower than
that for an AR/AR coated window. Typical measured levels are in the range 0.07% to 0.12%.
CW laser damage thresholds are best stated as a ‘Power over beam diameter’ ratio with units of
W/mm. It has been demonstrated experimentally on lasers up to 5kW (to date) that this factor is a
constant with changing laser power. (Power density is the correct measure of laser damage threshold
for pulsed lasers.)
Examples: CW CO2 laser damage thresholds have been measured for AR/65%R as 1900W/mm and
for AR/99.25%R as 1200W/mm. Actual LIDT in use will depend on cooling, cleanliness of the cavity
and laser gas, and mechanical stresses.
Beamcombiner coatings
See Technical Data Section 3.00.
Beamsplitters
Beamsplitter components used at 45° incidence can affect the state of polarization of the two resulting
beams. This technical problem is solved in the various beamsplitter modules designed and built for
ULO Optics (See Technical Data Section 70.0).
Wedge angles on cavity components
Some lasers have cavity components, such as the output coupler, designed with a deliberate wedge
angle, typically 10′ or so. Historically this was necessary when the AR coating had a significant
reflectivity and so could affect the cavity output. The quality of modern AR coatings ensures that
wedged cavity components are rarely, if ever, required.
Cavity rear mirrors
Many laser manufacturers design rear cavity mirrors using Germanium raw material. Typically these
are coated AR/99.5%R (or similar), so that the bulk material is only required to transmit perhaps 0.5%
of the in-cavity power. Once again, this type of design is based on historical considerations and dates
from the years when Ge was significantly less expensive than the superior ZnSe.
Ge absorption: 3 %/cm
ZnSe absorption: 0.05 %/cm
Note that the laser cavity function is determined by the reflectivity, at 10.6μm, of the coating, not the
substrate material. ULO Optics often substitute ZnSe for Ge for cavity optical components.
It is a better material, and can be less expensive for small/medium batch sizes due to coating cost
considerations.