130NM ? 400NM OPTICAL HEADS0 pages

HEAD ASSEMBLIES
PRODUCT BULLETIN

T E C H N O L O G Y

F O R

P O L A R I Z A T I O N

M E A S U R E M E N T

PHOTOELASTIC MODULATORS

PEM photoelastic modulators change the polarization state of a light beam at frequencies between 20
and 100 kHz. The modulator optical element acts as a “dynamic waveplate” to produce an oscillating
birefringence. This can convert linear polarized light into light which oscillates between circular,
elliptical and linear states. For example, light which
oscillates between left and right circular polarized
light can be produced for use in circular dichroism
measurements.
PEM photoelastic modulators feature a “split-head”
conguration; the electronic and optical components
are housed in separate enclosures. This minimizes the
size of the unit which is placed in the optical train.
It also simplies making the optical head magnetic
eld compatible or vacuum compatible when this is
necessary.
Series I modulators use rectangular optical elements
and are useful in the ultraviolet, visible and infrared
to 1 or 2 microns.
Series II modulators use symmetrical or octagonal optical elements and are primarily intended for
use in the visible and infrared (to mid-IR) spectral regions. Special models have been used in the
ultraviolet.
Modulators are offered optical elements using various optical materials. The choice of optical material
is made primarily on the basis of the spectral transmission requirements of the instrument. A list of
commonly available materials is given in Table 1.
Compared to the Series II octagonal optical
elements, the rectangular optical elements used in
Series I modulators provide lower levels of peak
retardation for a given optical element thickness.
This is a drawback when working in the infrared,
but an asset when working in the UV, especially
the vacuum UV.

TABLE 1
SPECTRAL REGION
Vacuum UV, UV

SERIES

MATERIAL

I

Lithium Fluoride

Vacuum UV to mid-IR

I, II

Calcium Fluoride

Vacuum UV to near-IR

I, II

Mid-visible to mid-IR

II

Fused Silica
Zinc Selenide

When a PEM is used with a laser, modulated
Near- to mid-IR
II
Silicon
interference effects may occur. These can produce
spurious optical/electronic signals which may
hamper certain measurements. Hinds’ engineers should be consulted for techniques to eliminate or
minimize such signals in applications where laser light sources are used with PEMs.
Antireection coatings may be used to increase the throughput of light through the modulator, to
reduce interference effects, and to reduce the fraction of light which passes through the modulator
at an undesired peak retardation. In particular, zinc selenide and silicon modulators benet from
antireection coatings because of their high indices of refraction. (Note: An antireection coating may
signicantly reduce the usefulness of the modulator outside the spectral band of the coating.)
Octagonal (Series II) optical elements are much more efcient for a given thickness, and thus have a
signicant advantage in the infrared. Operation of Series II modulators at low retardation levels (e.g.
the deep UV) may present some problems.