U.S. patent application number 12/152220 was filed with the patent office on 2009-11-12 for optically amplified critical wavelength refractometer.
This patent application is currently assigned to Sealite Engineering. Invention is credited to George A. Seaver.
Application Number | 20090279074 12/152220 |
Document ID | / |
Family ID | 41266596 |
Filed Date | 2009-11-12 |
United States Patent
Application |
20090279074 |
Kind Code |
A1 |
Seaver; George A. |
November 12, 2009 |
OPTICALLY AMPLIFIED CRITICAL WAVELENGTH REFRACTOMETER
Abstract
A critical wavelength refractometer is provided. A broadband
light source (413) is optically coupled to a sensor (401), the
sensor having at least one sensing surface (407). As the light from
the broadband light source passes through the sensor, it undergoes
multiple internal reflections against the sensing surface. Due to
the index of refraction of the material in contact with the sensing
surface, a portion of the light passing through the sensor is
reflected while a second portion of the light is transmitted
through the sensing surface and into the material. A detector (421)
coupled to the sensor measures the spectral intensity of the light
that passes completely through the sensor after having undergone
the multiple internal reflections against the sensing surface. A
microprocessor (423) coupled to the detector determines the
critical wavelength based on the spectral intensity measurement,
thereby allowing the index of refraction of the material to be
determined.
Inventors: |
Seaver; George A.;
(Cataumet, MA) |
Correspondence
Address: |
PATENT LAW OFFICE OF DAVID G. BECK
P. O. BOX 1146
MILL VALLEY
CA
94942
US
|
Assignee: |
Sealite Engineering
|
Family ID: |
41266596 |
Appl. No.: |
12/152220 |
Filed: |
May 12, 2008 |
Current U.S.
Class: |
356/73 ; 356/300;
356/302 |
Current CPC
Class: |
G01N 2021/434 20130101;
G01N 21/431 20130101; G01N 21/43 20130101; G01N 2021/438 20130101;
G01N 21/4133 20130101 |
Class at
Publication: |
356/73 ; 356/300;
356/302 |
International
Class: |
G01N 21/00 20060101
G01N021/00; G01J 3/00 20060101 G01J003/00; G01J 3/40 20060101
G01J003/40 |
Claims
1. A critical wavelength refractometer comprising: a broadband
light source; a sensor, wherein an entrance face of said sensor is
optically coupled to said broadband light source, said sensor
comprised of at least one sensing surface, wherein light emitted by
said broadband light source and optically coupled to said sensor
undergoes a plurality of internal reflections within said sensor,
wherein a portion of said plurality of internal reflections are
against said at least one sensing surface, wherein said portion is
comprised of multiple reflections, and wherein for each of said
multiple reflections a first fraction of said light is reflected
from said sensing surface and a second fraction of said light
passes through said sensing surface, wherein said first and second
fractions of said light are a function of an index of refraction of
a material in contact with said at least one sensing surface; a
detector optically coupled to an exit face of said sensor, said
detector measuring spectral intensity for said first fraction of
said light passing through said sensor and exiting through said
exit face of said sensor; and a microprocessor coupled to said
detector, said microprocessor determining a critical wavelength for
said material and said index of refraction for said material from
said spectral intensity.
2. The critical wavelength refractometer of claim 1, said sensor
further comprising a sensor window, wherein said at least one
sensing surface comprises a lower surface of said sensor window,
and wherein an upper surface of said sensor window and said lower
surface are parallel to within 10 arc seconds.
3. The critical wavelength refractometer of claim 1, said sensor
further comprising a sensor window, a first prism and a second
prism, wherein said broadband light source is optically coupled to
said entrance face of said sensor via said first prism, and wherein
said detector is optically coupled to said exit face of said sensor
via said second prism.
4. The critical wavelength refractometer of claim 1, said sensor
further comprising a sensor probe, wherein said at least one
sensing surface comprises a lower surface and an upper surface of
said sensor probe, wherein an end surface of said sensor probe is
mirrored, wherein said entrance face of said sensor is comprised of
a first portion of a front surface of said sensor probe, and
wherein said exit face of said sensor is comprised of a second
portion of said front surface of said sensor probe.
5. The critical wavelength refractometer of claim 1, wherein said
detector is a spectrograph.
6. The critical wavelength refractometer of claim 1, further
comprising an optical collimator interposed between said broadband
light source and said sensor.
7. The critical wavelength refractometer of claim 1, further
comprising a first optical fiber interposed between said broadband
light source and said sensor and a second optical fiber interposed
between said sensor and said detector.
8. The critical wavelength refractometer of claim 1, wherein said
sensor is fabricated from a material selected from the group
consisting of silica, crown glass, flint glass and germanium doped
silica.
9. The critical wavelength refractometer of claim 1, further
comprising: a beam splitter interposed between said broadband light
source and said sensor, wherein said beam splitter divides said
light from said broadband light source into a reference beam and a
sensing beam, wherein said sensing beam is optically coupled to
said sensor; and a second detector, wherein said reference beam is
optically coupled to said second detector, said second detector
measuring spectral intensity for said reference beam, and wherein
said microprocessor is coupled to said second detector.
10. The critical wavelength refractometer of claim 9, wherein said
beam splitter is a polarizing beam splitter.
11. The critical wavelength refractometer of claim 1, further
comprising: a beam splitter interposed between said broadband light
source and said sensor, wherein said beam splitter divides said
light from said broadband light source into a reference beam and a
sensing beam, wherein said sensing beam is optically coupled to
said sensor, and wherein said reference beam is optically coupled
to said detector; a first shutter interposed between said beam
splitter and said detector, wherein said first shutter controls
entry of said sensing beam to said detector; and a second shutter
interposed between said beam splitter and said detector, wherein
said second shutter controls entry of said reference beam to said
detector.
12. The critical wavelength refractometer of claim 11, wherein said
beam splitter is a polarizing beam splitter.
13. The critical wavelength refractometer of claim 11, wherein said
first shutter is interposed between said sensor and said
detector.
14. The critical wavelength refractometer of claim 11, further
comprising optical means for coupling said reference beam to said
detector, said optical means comprising at least one mirror and at
least one beam combiner.
15. The critical wavelength refractometer of claim 1, said sensor
further comprising a sensor window, wherein said at least one
sensing surface comprises a first portion of a lower surface of
said sensor window, and wherein a second portion of said lower
surface of said sensor window is mirrored, wherein said critical
wavelength refractometer further comprises: a beam splitter
interposed between said broadband light source and said sensor,
wherein said beam splitter divides said light from said broadband
light source into a reference beam and a sensing beam, wherein said
sensing beam is optically coupled to said first portion of said
lower surface of said sensor window, and wherein said reference
beam is optically coupled to said second portion of said lower
surface of said sensor window; and a second detector, wherein said
reference beam is optically coupled to said second detector, said
second detector measuring spectral intensity for said reference
beam, and wherein said microprocessor is coupled to said second
detector.
16. The critical wavelength refractometer of claim 15, wherein said
beam splitter is a polarizing beam splitter.
17. The critical wavelength refractometer of claim 1, said sensor
further comprising a sensor window, wherein said at least one
sensing surface comprises a first portion of a lower surface of
said sensor window, and wherein a second portion of said lower
surface of said sensor window is mirrored, wherein said critical
wavelength refractometer further comprises: a beam splitter
interposed between said broadband light source and said sensor,
wherein said beam splitter divides said light from said broadband
light source into a reference beam and a sensing beam, wherein said
sensing beam is optically coupled to said first portion of said
lower surface of said sensor window, and wherein said reference
beam is optically coupled to said second portion of said lower
surface of said sensor window, and wherein said reference beam is
optically coupled to said detector; a first shutter interposed
between said first portion of said lower surface of said sensor
window and said detector, wherein said first shutter controls entry
of said sensing beam to said detector; and a second shutter
interposed between said second portion of said lower surface of
said sensor window and said detector, wherein said second shutter
controls entry of said reference beam to said detector.
18. The critical wavelength refractometer of claim 17, wherein said
beam splitter is a polarizing beam splitter.
19. A method of determining an index of refraction of a material in
contact with a sensor, the method comprising the steps of:
transmitting a light beam from a broadband light source into said
sensor, wherein said light beam undergoes multiple reflections
against a sensing surface of said sensor, wherein said material is
in contact with said sensing surface of said sensor; passing said
reflected light beam from said sensor to a detector, said detector
capable of determining spectral intensity; monitoring the spectral
intensity of said reflected light beam with said detector;
determining a critical wavelength corresponding to said reflected
light beam; and determining the index of refraction from said
critical wavelength.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to index of
refraction sensors and, more particularly, to a critical wavelength
refractometer.
BACKGROUND OF THE INVENTION
[0002] Physical oceanography studies have used the index of
refraction as a means of determining the density of seawater for
decades, although only recently have practical instruments with
suitable accuracy been developed. In general terms, these
instruments use one of three refractometry principles; critical
reflection measurements at a single wavelength, speed of light
measurements at a single wavelength, and critical reflection
measurements using a broadband source.
[0003] In conventional refractometers, the variation of the
critical angle is measured as a function of the external index of
refraction, the measurement being performed using a monochromatic
source. This method, generally accurate to the fifth decimal place,
is used in commercial laboratory instruments as well as in
industrial process control. In the 1980's an in-situ device was
made based on this principle which used a solid-state beam-position
indicator and was accurate to the sixth decimal place; however, the
mechanical nature of angular measurements make them subject to
errors from oceanic pressure and temperature changes.
[0004] The index of refraction is defined as the ratio of the speed
of light in vacuum to that in the medium in question.
Unfortunately, this parameter cannot be easily measured. However,
with the help of a reference beam with which to compare the speed
of the sensing beam, the phase difference between the two beams can
be determined and becomes a very sensitive measure of the index of
refraction. In the early 1990's a modified interferometer was used
to measure the index of refraction to the seventh decimal place in
the laboratory and to the sixth decimal place in-situ. As these
interferometric methods measure the index of refraction relative to
a fixed value in the reference beam, pressure and temperature
changes can affect the result and thus limit the overall in-situ
accuracy to the 10.sup.-6 range.
[0005] The third refractometry principle spectrally decomposes a
broadband `white` sensing beam reflected at the nominal critical
angle from a flat window and determines the wavelength at critical
reflection. This method exploits the differing dispersions of the
indices of the glass window and the water that is external to the
window. The primary benefits of this method are in its simplicity
and its ability to perform in-situ measurements with an accuracy in
the 10.sup.-6 range.
[0006] Although all of the afore-described techniques provide means
for measuring the index of refraction of seawater, none of them
provide the desired level of accuracy for an in-situ oceanographic
instrument. Accordingly, what is needed in the art is an in-situ
oceanographic instrument that can simply and reliably measure the
index of refraction of seawater to the desired level of accuracy.
The present invention provides such an in-situ instrument.
SUMMARY OF THE INVENTION
[0007] The present invention provides a method and apparatus for
determining the index of refraction of a material. The critical
wavelength refractometer of the invention employs a broadband light
source that is optically coupled to a sensor, typically with a
collimating optic interposed between the light source and the
sensor. The sensor can be fabricated from silica, germanium doped
silica, or other material. The material to be studied is in contact
with at least one sensing surface of the sensor. As the light from
the broadband light source passes through the sensor, it undergoes
multiple internal reflections against the sensing surface. Due to
the index of refraction of the material in contact with the sensing
surface, a portion of the light passing through the sensor is
reflected while a second portion of the light is transmitted
through the sensing surface and into the material. These portions
are characterized by their wavelength. A detector, e.g., a
spectrograph, coupled to the sensor measures the spectral intensity
of the light that passes completely through the sensor after having
undergone the multiple internal reflections against the sensing
surface. A microprocessor coupled to the detector determines the
wavelength, called the critical wavelength, separating the first
and second portions of light. This is based on the spectral
intensity measurement, thereby allowing the index of refraction of
the material to be determined.
[0008] In one embodiment, the sensor is comprised of a sensor
window. The sensing surface of the sensor window is preferably the
lower window surface, or a portion of the lower window surface.
Preferably the upper and lower window surfaces are parallel to
within 10 arc seconds. In one configuration a first prism is used
to couple the broadband light source to the sensor window and a
second prism is used to couple the sensor window to the
detector.
[0009] In another embodiment, the sensor is comprised of a sensor
probe in which both the upper and lower probe surfaces are sensing
surfaces, the front probe face provides both an entrance surface
and an exit surface, and the end probe face is mirrored.
[0010] In another aspect of the invention, a beam splitter is
interposed between the broadband light source and the sensor, the
beam splitter dividing the incident light into a reference beam and
a sensing beam. The sensing beam is coupled to the sensor where it
undergoes multiple internal reflections against the sensing
surface(s). The reference beam does not interact with the external
material or the sensing surface(s), thereby providing a means for
monitoring intensity and/or spectral variations in the output of
the light source. The reference beam can be coupled to a second
detector, e.g., a spectrograph, or multiplexing can be used to
couple both the sensing beam and the reference beam to a single
detector. Preferably a polarizing beam splitter is used, thus
further enhancing the sensitivity of the sensing beam.
[0011] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates a critical wavelength refractometer
according to the prior art;
[0013] FIG. 2 provides further detail regarding the prism-shaped
sensor of the critical wavelength refractometer of FIG. 1;
[0014] FIG. 3 graphically illustrates the benefits offered by the
multi-reflection approach of the present invention over the single
reflection critical wavelength refractometer of the prior art;
[0015] FIG. 4 illustrates the primary elements of a refractometer
designed in accordance with the invention;
[0016] FIG. 5 illustrates an alternate embodiment of a
refractometer designed in accordance with the invention, this
embodiment providing dual sensing surfaces;
[0017] FIG. 6 illustrates a variation of the configuration shown in
FIG. 4, this variation providing a reference beam to monitor
intensity and spectral variations in the light source;
[0018] FIG. 7 illustrates a variation of the configuration shown in
FIG. 6, this variation utilizing two spectrographs; and
[0019] FIG. 8 illustrates a variation of the configuration shown in
FIG. 4, this variation dividing the sensor into a sensing portion
and a reference portion.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0020] FIG. 1 is an illustration of a critical wavelength
refractometer in accordance with the prior art, a full description
of which is provided in U.S. Pat. No. 4,699,511, entitled
Refraction Sensor, the specification of which is incorporated
herein. As shown, refractometer 100 is comprised of a probe that
includes a prism-shaped sensor 101. Sensor 101 is coupled to a
broadband white radiant energy light source 103 by at least one
optical fiber 105, and coupled to a spectrograph detector 107 by at
least one optical fiber 109.
[0021] As shown in detail in FIG. 2, prism-shaped sensor 101
includes a ground and polished sensing face 201, a mirrored
reflecting face 203, and an incident face 205. Angle .alpha.,
measured between faces 201 and 205, is chosen along with the
wavelength range of light source 103 and the material comprising
the prism-shaped sensor 101 to cover the index of refraction range
of interest for the material to be monitored, i.e., the material in
contact with sensing face 201.
[0022] In use, the light emitted by light source 103 passes through
optical fiber 105, the light being schematically illustrated in
FIG. 2 as light ray 207. A band of broadband light ray 207 is
reflected by sensing face 201 as ray 209, this portion being
dependent upon the wavelength of the light and the index of
refraction of the material (e.g., seawater) that is in contact with
sensing face 201. Ray 209 is reflected by mirrored reflecting face
203 as ray 211. Ray 211 passes through optical fiber 109 to
spectrograph detector 107 which, in turn, determines the wavelength
band and the band edge of the light that has passed through sensor
101. By measuring the wavelength of the band edge of the reflected
ray, the index of refraction of the material in contact with the
sensing face can be determined.
[0023] The sensitivity of the critical wavelength refractometry
method increases as the optical dispersion of the sensor and the
sample (e.g., seawater) approach each other. This goal can be
accomplished by choosing an optical glass for the sensor that has a
dispersion closer to that of the sample material, for example
fabricating the sensor from Crown glass or fused quartz. This goal
can also be achieved by utilizing that part of the optical spectrum
where the dispersions of the sensor and the sample material
naturally converge, typically towards the longer red and infrared
wavelengths. Unfortunately, this approach causes the band edges to
become shallower and noisier, leaving the signal-to-noise ratio
only slightly improved.
[0024] In order to achieve the dynamic range necessary for
oceanographic research, preferably on the order of 2 to 3 parts in
the seventh decimal place, the present invention utilizes a
modified critical wavelength refractometry technique in which the
sensing light ray undergoes multiple reflections against the
sensing face of the optical sensor. With each additional reflection
of the incident light ray on the sensing face, further transmission
losses occur for those wavelengths that were partially transmitted
into the sample, e.g., the seawater, on earlier reflections. This
increases the resultant spectral intensity fall-off with wavelength
and, thereby, improves the steepness of the band edge.
Additionally, due to the steeper and linear band edge, the task of
fitting to resolve the critical wavelength is simpler, faster and
offering higher resolution.
[0025] FIG. 3 graphically illustrates the benefits of the present
invention. Curve 301 shows the experimental spectral intensity of
the band edge for the case of a single reflection while curve 303
shows the same measurement after eight reflections. As shown, the
optical amplification provided by the multi-reflection approach of
the present invention achieves a much sharper, and thus more easily
resolved, band edge.
[0026] FIG. 4 illustrates the primary elements of a refractometer
designed in accordance with the invention. The refractometer
includes a sensing element 401, also referred to herein as a sensor
window or simply as the sensor. In a preferred embodiment, sensor
window 401 is 2 millimeters thick with the upper and lower surfaces
parallel to within 10 arc seconds (i.e., 0.003 degrees). As the
refractometer of the invention is preferably used to determine the
index of refraction of seawater, preferred sensor materials
include, but are not limited to, silica, crown glass, flint glass
and germanium doped silica.
[0027] The upper surface 403 of sensor 401 is preferably exposed to
air, thus providing total internal reflection of light beam 405 as
it passes through the window. The lower surface 407 of sensor 401
is exposed to the material to be tested, typically seawater. It
will be appreciated that upper surface 403 can be exposed to an
environment other than air as long as light beam 405 is undergoes
total reflection. For example, a reflective coating can be applied
to surface 403 in order to achieve the desired reflectivity. In the
illustrated embodiment, the light beam impinges on, and is
reflected by, the sensing surface 407 at seven locations 409. It
will be appreciated that the invention is not limited to a sensor
with seven sensing points 409, rather the sensor can utilize more
or less sensing locations, depending upon the desired measurement
accuracy. The inventor has found, however, that for a sensing light
beam with a divergence of approximately 0.02 degrees, little is
gained by having more than eight sensing points 409.
[0028] Source 413, which provides broadband light beam 405, can be
an incandescent source, a light emitting diode (LED), or other
source type. As precise control over the beam's angle of incidence
with respect to the sensor and the sensing surface 407 is required
in order to achieve the desired level of accuracy, broadband light
beam 405 from source 413 is collimated, typically using an optical
collimating element 411 interposed between source 413 and the
sensor. Light beam 405 is preferably coupled to sensor 401 using a
small, triangular prism 415. In at least one embodiment, an optical
fiber couples source 413 to prism 415, typically with collimating
optical element 411 interposed between the exit facet of the fiber
and prism 415. After passing through sensor 401, preferably light
beam 405 exits the sensor window via a second triangular prism 417.
Although sensor window 401 and prisms 415/417 can be fabricated as
a single element, preferably they are individually fabricated and
then bonded together using an optical epoxy. Prism 415 is designed
so that light beam 405 is substantially perpendicular to prism
entrance surface 419.
[0029] Sensing beam 405, after passing through sensor window 401
and undergoing multiple reflections at the interface of the sensing
surface 407 and the external sample material, exits the sensor via
prism 417 and enters a spectrographic detector 421. The portion of
the light beam between prism 417 and spectrograph 421 can either
pass through free space, or be guided using an optical fiber. If
optical fibers are used to couple light source 413 and spectrograph
421 to the sensor, the sensor can be located a remote distance from
the support subsystems (e.g., spectrograph, microprocessor, light
source).
[0030] Spectrograph 421 measures the intensity as a function of
wavelength for the light beam that has passed through sensor 401,
this quantity also referred to herein as spectral intensity. This
data is sent to a microprocessor 423 which, in turn, determines the
band edge, also referred to as the critical wavelength. The band
edge is the wavelength at which the intensity of the reflected
light beam undergoes an abrupt change. For example, in the graph
given in FIG. 3, the critical wavelength is 587.8 nanometers. The
microprocessor then calculates the index of refraction of the
external material (e.g., seawater) based on the critical
wavelength, the angle of incidence of light beam 405 on sensing
surface 407, and the known index of refraction of the sensor using
Snell's law. The microprocessor can also calculate the salinity of
the seawater, given the temperature and pressure, as well as the
density of the seawater, given the conductivity of the material.
Note that microprocessor 423 can be either external or internal to
spectrograph 421. Additionally and as previously noted,
microprocessor 423 can be either co-located with, or remote from,
sensor 401.
[0031] Another advantage of an instrument designed to utilize the
elements of the invention as described relative to FIG. 4 is that
the sensor window (e.g., sensor 401) can be mounted flush within a
probe housing or device structure, not shown. As a result, only a
flat surface need be placed within the fluid to be tested, thereby
discouraging the accumulation or formation of unwanted substances
along corners, edges and crevices of the sensing element. This
configuration also simplifies cleaning of the sensing surface.
[0032] Although the embodiment shown in FIG. 4 utilizes a single
sensing surface 407, it should be appreciated that the invention is
not limited to single sensing surface configurations. For example,
a second preferred embodiment of the invention shown in FIG. 5
utilizes a probe 501 with dual sensing surfaces 503 and 505. During
use, probe 501 is inserted into the medium, e.g., seawater, to be
investigated. In this configuration a single probe surface 507
provides both the entrance and exit surfaces for the probe. In
order to provide the desired level of probe mobility, preferably
the probe is coupled to light source 413 and the spectrograph
detector 421 via a pair of optical fibers 509 and 511,
respectively. Preferably optical fibers 509 and 511 are single mode
optical fibers. To provide the desired level of beam collimation,
preferably lenses (not shown) are interposed between probe 501 and
one or both fibers 509/511.
[0033] In the embodiment illustrated in FIG. 5, the input light
beam is reflected once by sensing surface 503 and twice by sensing
surface 505 before being totally reflected by probe end mirror 513.
On the return path of the light beam, it is reflected twice by
sensing surface 503 and once by sensing surface 505 before exiting
the probe. Accordingly, the light beam within probe 501 interacts
with the external material to be tested six times at sensing
locations 515. It will be appreciated that a probe can be designed
for use with the invention that has fewer or greater numbers of
sensing locations.
[0034] In a device configuration such as that shown in FIG. 5, it
is critical that the angle at which the light beam enters the
sensor remains constant since changes in the angle of incidence
will affect the angle of incidence at the sensing locations and
thus the accuracy of the device. Accordingly, in a preferred
embodiment the materials comprising optical fiber 509, probe 501
and any optical elements (e.g., collimating lenses) interposed
between fiber 509 and prove 501 are all selected to insure that
their thermal expansion coefficients match.
[0035] In one configuration of probe 501, the probe has a width of
1 millimeter and a length of 7 millimeters. Preferably surfaces 503
and 505 are parallel to one another to within 10 arc seconds.
Suitable materials for probe 501 include fuzed quartz, Crown glass
and Flint glass, depending upon the desired sensitivity and
range.
[0036] In another preferred embodiment of the invention, a
reference beam is used to monitor variations in light source
intensity and/or the spectral distribution of the light source. In
the exemplary embodiment illustrated in FIG. 6, which is based on
the configuration shown in FIG. 4, a reference beam 601 of primary
light beam 603 is split off, preferably using a polarizing beam
splitter 605. Reference beam 601, after bypassing sensor 401, is
directed to spectrograph 421, aligning the reference beam with
sensor beam 607. Preferably the alignment of reference beam 601 is
accomplished with the combination of a mirror 609 and a second
polarizing prism 611.
[0037] In order to utilize the reference beam of the embodiment
shown in FIG. 6, means such as a shutter are used to control both
the reference beam (e.g., shutter 613) and the sensing beam (e.g.,
shutter 615). Under normal conditions when the system is being used
to measure the index of refraction of seawater or some other
substance, reference beam 601 is prevented from entered
spectrograph 421, for example by closing shutter 613, while sensing
beam 607 passes unimpeded to spectrograph 421, for example by
opening shutter 615. Conversely, when a normalizing spectrum is
desired, the sensing beam shutter is closed and the reference beam
shutter is opened.
[0038] As previously noted, preferably if the system of the
invention uses a reference beam in order to compensate for light
source drift as described relative to system 600, the reference
beam is generated using a polarizing beam splitter (e.g.,
polarizing beam splitter prism 605). By using polarizing beam
splitters, the p-polarization (i.e., the polarization parallel to
the plane of incidence) passes through polarizing prism 605 and is
transmitted to sensor 401 and thus sensing surface 407 while the
s-polarization (i.e., the polarization perpendicular to the plane
of incidence) becomes the reference beam 601. Since the slope and
the rate of beam intensity change near the critical reflection
wavelength of a p-polarized beam is twice that of an s-polarized
beam, an additional benefit of system 600 using polarizing beam
splitters is that the sensitivity of the index of refraction
measurement is increased by a factor of two over the non-polarized
approach. It will be appreciated that the use of a p-polarized beam
can also be used to increase the sensitivity of a conventional,
i.e., single reflection, critical wavelength refractometer.
[0039] In system 600, shutters 613 and 615 allow multiplexing of
the reference beam and the sensing beam with single spectrometer
421. If desired and as illustrated in FIG. 7, a pair of
spectrometers 701 and 703 can be used with the reference and
sensing beams, respectively. Accordingly, the use of dual
spectrometers allows light source intensity and wavelength
variations to be continuously monitored, thereby providing means
for continuously compensating for the effects of such variations on
the measured data.
[0040] It is understood that there are numerous optical techniques
that can be used to direct the light beams; both the sensing light
beam before and after it passes through the sensor and the
reference light beam, assuming the configuration in question
employs a reference light beam. For example, if the desired device
configuration utilizes a reference beam to compensate for light
source variations as described relative to FIGS. 6 and 7 and to
compensate for some environmental variations such as temperature,
the reference beam optical path can be directed through a
non-sensing portion of the sensor. FIG. 8 shows a top view of the
primary elements of such a configuration.
[0041] As shown in FIG. 8, the broadband light from source 413
passes through a beam splitting element 801, thereby dividing
incident beam 803 into sensing beam 805 and reference beam 807.
Sensing beam 805 passes through portion 809 of sensor 811,
undergoing multiple reflections (not shown) against the sensing
surface(s) of sensor 811, for example as shown relative to the
multi-reflection paths shown in FIGS. 6 and 7. Reference beam 807
is directed, for example by a mirror 813, into a second portion 815
of sensor 811. Instead of having a sensing surface, portion 815 is
mirrored so that reference beam 807 passes through sensor 811
unaffected by the seawater or other material in contact with the
sensor. After passing through sensor 811, reference beam 807 and
sensing beam 805 are directed into a pair of spectrographs 701 and
703, respectively. Alternately, both the reference beam and the
sensing beam can use a single spectrograph (not shown), for example
by using shutters as described relative to FIG. 6.
[0042] Preferably in the embodiment illustrated in FIG. 8 beam
splitting element 801 is a polarizing splitter. In such a
configuration, and as described above, preferably after passing
through element 801, sensing beam 805 is p-polarized while
reference beam 807 is s-polarized.
[0043] It will be appreciated that a refractometer designed and
fabricated in accordance with the invention can be used alone, or
in combination with other test instruments to provide further
information about the material (e.g., seawater) under test. For
example, a refractometer in accordance with the invention can be
combined with a conductivity cell to provide information regarding
water contamination, etc.
[0044] As used herein, the terms light and light beam refer to
electromagnetic radiation comprised of ultraviolet and/or visible
and/or infrared radiation.
[0045] As will be understood by those familiar with the art, the
present invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof.
Accordingly, the disclosures and descriptions herein are intended
to be illustrative, but not limiting, of the scope of the invention
which is set forth in the following claims.
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