Quartz Waveplates
Quartz Retardation Plates, more
commonly referred to as Waveplates,
in general, induce a phase retardation
between orthogonal polarization
directions. Beam propagation typically
is perpendicular to the direction
of the crystalline optic axis. Retardation
is dependent on the rotation of
the plate about the beam direction.
The maximum amount of phase retardation,
Γ, for a waveplate of thickness
t, at normal incidence, is given
by the relation Γ = 2πΔn
t/λ where Δn is the
birefringence of quartz, (ne-no),
at the wavelength of use and λ
is the wavelength.
In general, the transmitted light
is elliptically polarized. For the
special case when the retardation
Γ is π/2, or the quarter-wave
value, transmitted light is circularly
polarized; when the retardation
Γ is π, which is the half-wave
retardation value, the polarization
of the transmitted light is rotated
by 90°.
In some instances, making a zero-order
waveplate is impractical due the
thinness of the required plate.
Low order, multiple-order waveplates
are manufactured from a single plate
of a birefringent material. They
provide an effective way to alter
the polarization of laser beams.
The thickness of a multiple-order
quartz retardation plate is fixed
by the material birefringence to
within an additive multiple of a
full-wave thickness. Selection of
an appropriate thickness for a given
application is based on evaluating
the use requirements.
In general, theoretical performance
of a multiple-order waveplate is
better when it is thinnest. A thinner
waveplate shows less performance
variation due to changes in temperature,
angular tilt, and, because a smaller
overall volume of material is used,
is less likely to contain objectionable
material inclusions.
However, thicker waveplates are
easier to handle and manufacture
with a good transmitted wavefront
figure when mounted.
The following two transmission
interferograms illustrate the quality
of transmitted wavefront that can
be expected from low order, multi-order
waveplates, each of which was manufactured
by Laser Optics. The first one is
43.5 mm in diameter and 0.407 mm
thick. The second one is 30 mm in
diameter and 0.150 mm thick.
 |
 |
(Left
Image) Transmitted WaveFront
measured as 1/15 waves (0.0658
waves) PV @ 633 nm for a 43.5
mm diameter waveplate measured
over 87% of the clear aperture.
Aspect ratio of waveplate
diameter to thickness is 106:1.
(Right Image)
Transmitted WaveFront measured
as 1/21 waves (0.0460 waves)
PV @ 633 nm for a 30 mm diameter
waveplate measured over 83%
of the clear aperture. Aspect
ratio of waveplate diameter
to thickness is 200:1. |
Low-order multiple-order plates
also can be designed to perform
at two different wavelengths. For
example, when performing harmonic
generation of 1064 nm it is convenient
to use a waveplate that produces
a half-wave retardation at 532 nm
and a full wave retardation at 1064
nm. When a laser beam containing
both 532 nm and 1064 nm passes through
the waveplate, the polarization
direction of the linearly-polarized
532 nm laser beam is rotated by
90° while the polarization direction
of the 1064 nm beam is left unchanged.
Zero-order waveplates, typically
being tens of microns thick, are
used in the same way as multiple-order
plates, but the retardance is much
less dependent on variations in
wavelength, temperature, and angular
field-of-view.
Compound (zero-order) retardation
plates are made to the same tolerance
as multiple order plates, but have
the on-axis performance of true
zero-order plates. They are fabricated
from two plates, assembled so that
the individual retardances subtract,
with a thickness difference that
yields the desired retardation.
Achromatic waveplates are similar
to the compound (zero-order) waveplates
except that the two plates are made
from different materials. Since
the dispersion of the birefringence
with changing wavelength (dΔn/dλ)
can be different for the two materials,
it is possible to specify retardation
values at two separate wavelengths.
Hence, the retardation of the resulting
waveplate can be made to vary slowly
as a function of wavelength. The
temperature variation of the birefringence
for both materials is an important
design factor for achromatic waveplates
if they are used over a temperature
range.
| Summary
of Retardation Plate (Waveplate)
Types |
| TYPE |
FEATURES |
|
Low Order,
Multiple-Order Quartz |
Good for
single wavelength use. Excellent
Transmitted WaveFront Performance. |
|
Low Order,
Multiple-Order, Dual Wavelength
Quartz |
Ideal for
rotating the polarization
of one wavelength while leaving
the polarization of a different
wavelength unchanged. |
|
Zero-Order
Quartz |
Gentle
wavelength dependence. Broad
angular field-of-view.
More difficult to manufacture
at large clear apertures. |
|
Compound
Zero-Order Quartz |
Zero-order
properties, such as low T
dependence and broad wavelength
insensitivity, but with a
narrow field-of- view. |
|
Achromatic
(typically, Quartz/MgF2) |
Low sensitivity
to wavelength change. |
Laser Optics manufactures Custom
retardation plates according to
your print. Typical specification
are listed in the table below.
| Typical
Waveplate Specifications |
| TYPE |
FEATURES |
| Cross
Section |
< 1
mm to 100 mm |
| Minimum
Thickness |
60 µm |
| Retardation
tolerance |
Corresponding
to ±0.4 µm of
quartz, typically ±
λ/300 @ λ =
1064 nm |
|
Surface
Quality |
20-10 Scratch-Dig
(MIL-O-13830A) |
| Orientation
Accuracy |
better
than 0.1° |
| Transmitted
Wavefront |
< 0.1 waves @ 633 nm over
80% of aperture |
| Parallelism |
< 1 arcsec wedge |
| AR Coating |
For all wavelengths |
Quartz Retardation Plates (Waveplates)
are Custom Optics that are manufactured
to a customer-supplied print. Please,
submit a Request for Quotation.
