MODEL 2010/M PRISM COUPLER
APPLICATION NOTES
BULK MATERIAL OR THICK FILM
INDEX/BIREFRINGENCE MEASUREMENT
The Model 2010/M can operate as a fully-automated
refractometer, providing high accuracy measurement of
refractive index and index anisotropy for solid or liquid
bulk materials and thick films in the index range 1.0 -2.6
without use of toxic or corrosive index matching fluids. This
bulk material index measurement is not an option, but a
standard feature provided with every system.
For bulk material index measurements, if a material with
index n (see Fig. 1) is brought into intimate contact with a
prism with index np.
Laser light directed onto the base
of the prism will be totally reflected at the prism base
until the angle of incidence
becomes
less than the critical angle
where
= arcsin(n/np) (1)
is easily measured using
the detector in Fig. 1 since the detector intensity drops
abruptly asdrops below the
critical angle and light starts to leak into the bulk
material. Since np is known, n can then be determined from
Equation (1).
This same approach can also be used to measure refractive
index for thick films. In the case of a film, the critical
angle of the prism/film interface establishes an upper limit
to the angle at which film propagation modes can occur. As a
film with fixed index gets thicker, the angle at which the
first propagation mode occurs asymptotically approaches the
critical angle as defined by equation (1) where n now refers
to the film index rather than to a bulk index. If we make the
approximation that the angle of the first film mode equals
the critical angle, the error in the measured index due to
this approximation is less than .004 for a film thickness of
3 microns, less than .001 at 5 microns, and less than .0003
at 10 microns. Thus, this approach can be applied with good
accuracy to measure the index of materials with thickness
ranging from several microns on up to "bulk"
thickness.
For measurement of liquid samples, a cell to bring the liquid
in direct contact with the prism is available.
The bulk index approach can also be used to study index
anisotropy in solid materials. In normal operation (Fig. 2a),
the Model 2010/M uses a polarized laser with TE incidence,
i.e., electric field vibrating transverse to the plane of
incidence (out of the plane of Fig. 1). Thus, if the material
in figure 1 is simply manually rotated against the prism
(Fig. 2b), the electric field vector can be made to assume
any orientation within the plane defined by the surface of
the material, and the index for electric field vibration
along any axis lying parallel to the material surface can be
measured.
In addition, with option #2010/M-TM, which allows TM incidence
(magnetic field transverse to, and electric field parallel
to, the plane of incidence), index for electric field
vibration perpendicular to the material surface is measurable
(Fig. 2c). This is because the electric field vibrates
perpendicular to the material surface at the critical angle
since the refracted ray must propagate parallel to the
surface at the critical angle.
Operation: Bulk index measurement is selected by
depressing a function key (F9) on the control PC of the Model
2010/M. In the bulk index mode, the angle of incidence (È) is
varied using a motor-driven rotary table until a sharp drop
is detected on the detector. The system software then
calculates and displays bulk index from equation (1).
Accuracy and resolution: With the high resolution
rotary table (a no-cost option), worst case index accuracy is
±.001 and resolution is ±.0001 for bulk materials and films
thicker than 10 microns. Absolute accuracy can be improved to
approximately ±.0001, however, if the user is willing to
perform a simple calibration procedure (measurement of an
absolute index standard) with each prism. Accuracy decreases
with film thickness, with measured index systematically low
by .001 at 5 microns, and by .003-.004 at 3 microns.
Measurement time: Typical measurement time is 0-20
seconds.
Index measuring range: Minimum and maximum index
measurable varies with prism type selected: 1.55-2.45
(200-P-2 prism), 1.25-2.10 (200-P-3), 1.20-2.00 (200-P-4),
and 1.00-1.80 (200-P-1). Special 200-P-2 prisms are available
to extend measuring range up to approximately 2.60.
Other wavelengths: For index measurement at other
wavelengths, the Model 2010/M can be operated with a variety of
lasers in place of, or in addition to, the standard 633 nm
HeNe. Please consult the factory for details.
Press on chart for larger view
Measurement of ordinary and extraordinary indices of bulk lithium niobate
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Other Application Notes:
MONITORING
OF PHOSPHORUS AND OTHER DOPANT CONCENTRATIONS IN SILICON
DIOXIDE
HIGH ACCURACY MEASUREMENT OF RESISTS,
POLYIMIDES, AND POLYMERS
DUAL FILM MEASUREMENTS
OPTICAL WAVEGUIDE CHARACTERIZATION
BULK MATERIAL OR THICK FILM
INDEX/BIREFRINGENCE MEASUREMENT
MEASURING INDEX ANISOTROPY/BIREFRINGENCE OF FREE-STANDING POLYMER FILMS
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Copyright © 2006 Metricon Corp. All rights reserved.
Last revised:
11/8/06