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Description  |
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TECHNICAL FIELD
The present invention relates to an optical amplifier and method for
amplifying optical polarization state change effects and, particularly, to
a liquid chromatography system which employs the same.
BACKGROUND ART
The detection of optical polarization state change effects is well known.
Such effects include optical rotation, circular dichroism, linear
dichroism, optical activity, and Kerr effect. For example, optical
rotation can be measured with a polarimeter for measuring concentration,
purities and specific rotation which is a fundamental property of matter
of certain compounds such as large organic molecules, pharmaceuticals,
flavors, fragrances and sugar.
The limit of measurement of optical rotation by the best known apparatus
today that are practical to build in quantity, is of the order of
0.0001.degree. of measured rotation but the most practical apparatus has a
sensitivity of only 0.001.degree.. There are many applications where a
more sensitive measurement capability is necessary or desirable, as in
high pressure liquid chromatography and other applications where the
concentration of the sample is very low or the materials constants are
very low and where the concentration cannot be increased because of the
limited volume of the sample available or in the analyzing of waste water
where there is no control of the concentration. Waste water analysis can
be used in various pharmaceutical plants to assure no contamination of
waste water with optically active materials. In the sugar industry, boiler
water analysis can be employed to determine if there are any leaks of the
reactants or purification vessels that allow sugar solution into the heat
exchangers and the cooling water.
One solution to the problem of detecting very small optical rotations has
been suggested by Yeung et al. in U.S. Pat. No. 4,498,774. However, the
apparatus disclosed therein, with present manufacturing limitations, is
not practical to produce in quantity. Thus, there is a need for an
improved apparatus and method for detecting very small optical
polarization state change effects such as optical rotations of less than
0.0001.degree., which apparatus can be produced in quantity.
DISCLOSURE OF INVENTION
The present invention solves the aforementioned problems with the known
apparatus and methods for detecting optical polarization state change
effects by providing, according to the present invention, an optical
amplifier and method for amplifying optical polarization state change
effects so that very small optical polarization state change effects can
be detected. This is accomplished according to the disclosed method by
providing polarized light and passing at least a portion of the polarized
light through a material a plurality of times such that changes in the
polarization state of the polarized light from the respective passes of
the light through the material are cumulative, and detecting the polarized
light which is passed through the material. By increasing the path length
of the polarized light through the material by passing the light through
the material a plurality of times, a very small polarization state change
effect can be amplified so that it can be detected.
The step of passing polarized light through a material a plurality of times
comprises, according to the preferred mode of carrying out the method,
reflecting light back and forth a plurality of times along an optic axis
which extends through the material. Where polarized light is reflected
back and forth through a material a plurality of times, the changes in the
polarization state of the polarized light from the respective passes of
the light through the material cancel each other. To prevent this, and to
cause the changes in the polarization state of the polarized light from
the respective passes of the light through the material to be cumulative,
according to a further feature of the method of the invention, the step of
passing polarized light through a material further comprises inverting the
polarization state of the polarized light between passes through the
material so that the changes in the polarization state in successive
passes are cumulative. Where optical rotation is the optical polarization
state change effect being detected, this permits optical rotations smaller
than 0.0001.degree., and even smaller than 1 microdegree
(0.000001.degree.), to be detected with the apparatus and according to the
method of the invention. A preferred use of the method is in detecting
weak optical activity in an eluent in high performance liquid
chromatography.
The optical amplifier of the invention for amplifying optical polarization
state change effects comprises means for providing polarized light and
means for passing at least a portion of the polarized light through a
material a plurality of times such that changes in the polarization state
of the polarized light from the respective passes of the light through the
material are cumulative. According to the preferred form of the invention,
the means for passing includes means for reflecting at least a portion of
the polarized light back and forth a plurality of times along an optic
axis of the amplifier which extends through the material. In order that
the changes in the polarization state of the polarized light from the
respective passes of the light through the material do not cancel each
other, but are cumulative, the means for passing inverts the polarization
state of the polarized light between passes through the material so that
the changes in the polarization state in successive passes are cumulative.
The means for inverting the polarization state includes a pair of quarter
wave compensators which are provided in spaced relationship on the optic
axis of the amplifier on opposite sides of the material between the
material and the means for reflecting. The retardation of each quarter
wave compensator must be maintained to a tolerance of
90.degree..+-.1.degree. and preferably is maintained to a high tolerance
of 90.degree..+-.0.3.degree.. Each quarter wave compensator has two
neutral lines. The neutral lines of each compensator are located at the
same angles with respect to a direction of polarization of the polarized
light from the means for providing polarized light.
The means for reflecting in the disclosed embodiment includes a pair of
mirrors which are provided in spaced relationship on the optic axis on
opposite sides of the material for reflecting at least a portion of the
polarized light back and forth a plurality of times along the optic axis
through the material. The mirrors each have a high reflectivity R of at
least 0.9 and preferably at least 0.99.
The optical amplifier further comprises means for analyzing the
polarization state of polarized light passed through the material. As
disclosed, the means for analyzing polarized light comprises a polarizer.
A photosensitive detector is also provided for detecting light intensity
from the means for analyzing the polarization state of the polarized light
passed through the material. Means for modulating the polarization state
of the light which is received by the analyzing means can also be provided
to aid in analyzing the output of the optical amplifier. The length of the
path along the optic axis of the amplifier along which polarized light is
reflected back and forth is also preferably selected to provide a
resonance condition for the light. The advantage of this is that intensity
of the polarized light emitted from the optical amplifier is significantly
greater when the path length along the optic axis is adjusted for the
resonance condition.
The present invention further includes a liquid chromatography system
comprising a cell for containing a liquid chromatography sample and an
apparatus for analyzing the sample in the cell for optical activity. The
apparatus includes an optical amplifier of the invention for amplifying
optical polarization state change effects, particularly, optical rotation.
Using digital signal processing, or analog circuitry optical rotations of
10.mu..degree., and even 1.mu..degree., can be detected.
These and other objects, features and advantages of the present invention
will become apparent from the following description when taken in
connection with the accompanying drawings, which show, for purposes of
illustration only, several embodiments in accordance with the present
invention.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic illustration of an optical amplifier for amplifying
optical polarization state change effects according to the present
invention, the amplifier being shown in use in a high performance liquid
chromatography system;
FIG. 2 is a schematic illustration depicting paths of the polarized light
in the optical amplifier of FIG. 1, where no modulation takes place and
the quarter wave compensators not being shown;
FIG. 3 is a schematic illustration of the optical amplifier of FIG. 1
showing paths of polarized light through the amplifier with an operable
modulator, and wherein the quarter wave compensators of the amplifier are
not shown;
FIGS. 4-12 show the wave forms of the signal at the detector of the optical
amplifier of FIGS. 1 and 3 for various values of optical rotation of the
polarization direction by a material located between the respective
mirrors and quarter wave compensators of the amplifier, in both the
no-gain case where the reflectance R of the mirrors is zero and the
transmittance T is one, and the case where R=0.99 and T=0.01;
FIG. 13 is a schematic illustration of the two quarter wave compensators of
the optical amplifier of the invention located along the optic axis of the
amplifier with their neutral lines oriented at the same angle with respect
to a direction of polarization P of the incoming light; and
FIG. 14 is a schematic illustration of a flow cell of the invention which
is formed integrally with optical elements of the amplifier.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, an optical amplifier 1 for amplifying
optical polarization state change effects according to the present
invention is illustrated in FIG. 1. The amplifier 1 is an optical rotation
parametric amplifier (ORPA) which is used in a high performance liquid
chromatography (HPLC) system 2 for detecting and analyzing optically
active organic molecules in an eluent flow from the HPLC system 2.
The liquid eluent from the HPLC column 3 of the system 2 flows by way of a
passage 4 to a flow cell 5. The eluent enters the cell 5 through an inlet
passage 14 and flows through the central bore 6 of the flow cell 5 before
exiting the flow cell via outlet passage 14. The eluent leaving the cell
is conveyed to a sump 8 via a passage 7. Such a flow cell is shown in U.S.
Pat. No. 4,498,774, for example. The ends of the bore 6 of the flow cell 5
are closed against loss of the liquid eluent by optically transparent
windows 9 which may, for example, be flat microscope cover slips which
will allow transmission of a polarized laser beam without materially
affecting the operation of the optical amplifier. Fluid pressure for
pumping the eluent from the column 3 through the flow cell 5 to the sump 8
is provided by a pump 10 which is in fluid communication with the column 3
via passage 11 and valve 12.
The optical amplifier 1 for amplifying optical polarization state change
effects which may occur when polarized light is passed through the eluent
in the bore 6 of the flow cell 5 comprises a polarizer P.sub.1 for
polarizing light from a light source 19. The light source 19 is a laser in
the illustrated embodiment. The laser directs light along an optic axis A
of the amplifier in the direction of the polarizer P.sub.1. Light from the
laser 19 which is transmitted through the polarizer P.sub.1 along the
optic axis A is linearly polarized. As shown in FIG. 13, the electric
field of the light points in a single direction P in a plane perpendicular
to the direction of propagation along optic axis A after having passed
through the polarizer P.sub.1.
Polarized light propagating in the direction of the optic axis A passes
through the material or sample under investigation, in this case the
eluent in the central bore 6 of the flow cell 5. If the eluent contains
optically active molecules, the direction of polarization P will be
rotated in the plane perpendicular to the direction of propagation of the
polarized light. Where the concentration of the optically active molecules
is very low, the change in the polarization state, that is the amount of
the rotation of the polarization direction, is very small so that it is
difficult to detect with an analyzer P.sub.2 and a detector D as shown in
FIG. 1 even with the use of a modulator 13. To avoid this difficulty of
detecting weakly optically active materials, according to the present
invention, the optical amplifier of the invention takes advantage of the
fact that the optical rotation of the polarization direction of the
polarized light is directly proportional to the path length of the light
traveling through the sample of the material being analyzed. That is,
according to the method for detecting optical polarization state change
effects according to the invention, and in the optical amplifier of the
invention, the path length of the light through the sample is increased by
passing at least a portion of the polarized light through the material
being analyzed a plurality of times such that changes in the polarization
state of the polarized light from the respective passes of the light
through the material are cumulative and thus more easily detectable.
For this purpose, the optical amplifier 1 in FIG. 1 is provided with
mirrors M.sub.1 and M.sub.2 located along the optic axis A in opposed
relationship at respective, opposite ends of the flow cell 5. The mirrors,
when they have a reflectivity R greater than zero, reflect at least a
portion of the polarized light back and forth between the mirrors a
plurality of times along the optic axis A which extends through the
material in the bore 6 of the flow cell 5. The optical polariztion state
change effects, in this case, the optical rotations of the polarization
direction, from the respective passes of the polarized light through an
optically active material would normally cancel each other if the
polarized light were simply reflected back and forth between opposing
mirrors. In order that the changes in the polarization state of the
polarized light from the respective passes of the light through the
material are cumulative, according to the method and optical amplifier for
amplifying optical polarization state change effects of the invention, the
step of passing polarized light through the material further comprises
inverting the polarization state of the polarized light between passes
through the material so that the changes in the polarization state and
successive passes are cumulative.
The inversion of the polarization state is accomplished in the optical
amplifier 1 using a pair of quarter wave compensators C.sub.1 and C.sub.2
which are provided in spaced relationship on the optic axis on opposite
sides of the material being analyzed between the material and the means
for reflecting. As shown in FIG. 1, the quarter wave compensators C.sub.1
and C.sub.2 are positioned between the flow cell 5 and the mirrors M.sub.1
and M.sub.2 at the respective ends of the flow cell. It can be shown
through a Jones analysis that the optical rotations from the respective
passes will be cumulative, that is, will be added to one another as a
result of the half wave retarding effect from the polarized light passing
through a quarter wave compensator twice, namely as the polarized light
moves toward the adjacent mirror and as it is reflected therefrom back
toward the flow cell.
The gain of the optical amplifier of the present invention and the
importance of the reflectivity of the mirrors for achieving the amplifier
gain are explained below with reference to FIG. 2 of the drawings wherein
the mirrors M.sub.1 and M.sub.2 of the amplifier are shown spaced a
distance 1 apart to form a Fabrey-Perot cavity in the amplifier. The
mirrors M.sub.1 and M.sub.2 are interferometer grade mirrors, each having
a reflectance (R) and a transmittance (T). The area between them is
assumed to be filled with a medium which has an optical rotation of
.alpha. (degrees/cm). The quarter wave compensators C.sub.1 and C.sub.2
are necessarily present, although not illustrated in FIG. 2 for
simplicity, in order that the optical rotations of the light's
polarization direction with the respective passes through the material
between the mirrors be cumulative.
Collimated monochromatic light S.sub.0 which has passed through the
polarizer P.sub.1 is incident upon the mirror M.sub.1 during operation of
the amplifier 1, as depicted in FIG. 2. The light has a Stokes vector
S.sub.0 which represents plane polarized light with an azimuth of
0.degree.. S.sub.0 is given by:
##EQU1##
where I.sub.0 is the initial intensity of the plane polarized light
incident upon the mirror M.sub.1 from the polarizer P.sub.1. The amount
transmitter through the mirror is equal to TI.sub.0, so the Stokes vector
of the light entering the active medium is:
##EQU2##
After traversing the cavity, the plane of polarization has been rotated by
.alpha.1, so the Stokes vector reaching the mirror M.sub.2 is given by:
##EQU3##
Of S.sub.2, an amount is passed through M.sub.2 to yield S.sub.3 which is
described by:
##EQU4##
This is passed through the polarizer P.sub.2, a Glan-Thompson prism
oriented at P=90.degree., which yields S.sub.4, which is described by:
##EQU5##
which will contribute an intensity of:
I.sub.D1 =T.sup.2 I.sub.0 -T.sup.2 I.sub.0 cos 2.alpha.1 (7)
The remainder of S.sub.2 is reflected back into the cavity with a Stokes
vector given by:
##EQU6##
S.sub.5 propagates through the active medium and arrives at M.sub.1 with a
Stokes vector of S.sub.6, given by:
##EQU7##
S.sub.6 is partially reflected and partially transmitted at M.sub.1. The
transmitted portion is disregarded in this analysis since it is either
lost entirely, or reflected back onto the source, adding to I.sub.0. The
reflected beam is described by:
##EQU8##
S.sub.7 is then propagated back through the optically active medium to
yield:
##EQU9##
At M.sub.2, S.sub.8 is partially transmitted yielding S.sub.9, which is
described by:
##EQU10##
which on passing through P.sub.2 yields S.sub.10, given by:
##EQU11##
This contributes an intensity at the detector D of:
I.sub.D2 =R.sup.2 T.sup.2 I.sub.0 -R.sup.2 T.sup.2 I.sub.0 cos
6.alpha.1(15)
The process of reflection is continued so that the intensity at the
detector D can be expressed as a sum of the infinite series given by:
##EQU12##
or, when .alpha.1<<1 in which case sin[(2n+1).alpha.1]=(2n+1).alpha.1, and
setting n=i-1,
##EQU13##
In comparison, if the mirrors, M1 and M2, as well as the quarterwave
plates were not present and, therefore, no amplification was possible, the
intensity at the detector would be
I.sub.D =2I.sub.0 (.alpha.1).sup.2 (17-3)
Comparing equations 17-2 and 17-3 shows that the optical amplifier in the
incoherent case enhances the intensity I.sub.D reaching the detector by
the factor G.sub.I where
##EQU14##
Using equation 17-4, the coherent case gain factor G.sub.I can be
calculated in terms of the reflectivity, R, of mirrors M.sub.I and
M.sub.2. For this purpose, R and T are assumed to be related by R=1-T,
that is absorption is neglected, but this would only show up as an
apparent slight decrease in T. The gain factor G.sub.I was evaluated for R
values of 0.9, 0.99, and 0.999. They are approximately 10, 100, and 1,000,
respectively.
The rotational gain possible with the optical amplifier is apparent from
these calculations. It is also clear from the results that the
reflectivity of the mirrors has a significant effect on the gain. These
results and the above analysis are for the incoherent case where the
length l between the mirrors in the amplifier is such that a resonant
condition does not exist. The amplifier gain, and thus the intensity seen
at the detector for a given optical rotation, can be further increased if
the length l is adjusted to obtain a resonant or coherent light condition
as discussed below.
In the analysis of the coherent case and the resonance condition one must
consider the electric field strength of the light in the preferred
direction of P1, namely 0.degree., and P2, namely, 90.degree., as opposed
to the intensity of the light beam used in the incoherent analysis. The
electric field of the light after passage through M2 in FIG. 1 is
.degree.E=tt(cos(a)+r.sup.2 e.sup.id cos(3a)+r.sup.4 e.sup.i2d cos(5a)+. .
. )E.sub.0 (17a)
and
.sup.90 E=tt(sin)a)+r.sup.2 e.sup.id sin(3a)+r.sup.4 e.sup.i2d sin(5a)+. .
. )E.sub.0 (17b)
where: a=.alpha.1, i=.sqroot.-1, d/2=phase delay due to a single passage of
the light between M1 and M2, E.sub.0 is the electric field of the light
incident on M1 in the preferred direction of polarizer P.sub.1, .degree.E
is the electric field in the preferred direction of P1 that is incident on
polarizer P.sub.2, .sup.90 E is the electric field in the preferred
direction of P2 that is incident on polarizer P.sub.2, t is the amplitude
transmission coefficient of M1 or M2, r is the amplitude reflectivity of
M1 or M2, R=r.sup.2, and T=t.sup.2. In addition, R=1-T which means that
losses in M1 and M2 are neglected. This assumption does not materially
affect operation of the optical activity amplifier since interferometer
grade mirrors do not have significant losses at the light wavelengths of
interest. Realizing that P2 only passes light with electric field parallel
to its preferred direction and rejects light with electric field
perpendicular to that direction only light described in equation (17b)
reaches the detector and needs to be analyzed further.
The amplifier will be in a resonance condition 0 whenever the cavity length
is adjusted such that d is an integer multiple of 2.pi., in which case
e.sup.ind =1(n=0,1, . . . ) and equation (17b) becomes
##EQU15##
For weak optical activities, a is small and sin[2n+1]a is approximated by
[2n+1]a. Therefore
.sup.90 E=aTE.sub.0 {(1+R)/(1-R).sup.2 }=aE.sub.0 (1+R)/T. (17e)
The intensity of light reaching the detector is obtained by squaring the
right hand portion of equation (17e)
ID=a.sup.2 I.sub.0 (1+R).sup.2 /T.sup.2 (17f)
In the case where the optical amplifier is not present and only P1, P2, and
the optically active medium are present, the intensity reaching the
detector is simply a.sup.2 I.sub.0. Thus the factor (1+R).sup.2 /T.sup.2
represents the gain achieved by the optical amplifier operated in the
resonance condition. For mirror reflectivities, R, of 0.9, 0.99, and
0.0999 the gain is 361, 39,601, and 3,996,001, respectively. This means
that if the sensitivity limit of a polarimeter apparatus without an
optical amplifier of the invention were 0.001.degree., the limit of
sensitivity with the optical amplifier and 0.99 reflectivity mirrors at
resonance condition is 2.5.times.1O.sup.-8 degrees or about 400 times more
than using the optical amplifier of the invention in nonresonance
condition.
The analysis of the gain of the amplifier of the present invention as
discussed above with respect to FIG. 2 assumed no modulation. The case
where a modulator such as modulator 13 illustrated in the optical
amplifier 1 in FIG. 1 is employed is discussed with respect to FIG. 3, for
the incoherent case. The optical amplifier of FIG. 3 is like that of FIG.
2, with the quarter wave compensators not being shown again, but with a
modulator 13, which is a Farday modulator being operated during the
operation of the amplifier. The above analysis with respect to FIG. 2
applies for FIG. 3 up to equation 5. At this point S.sub.3 is passed
through the modulator 13, generating S.sub.3 ', which is described by:
S.sub.3 '=R(2.pi.kwt)S.sub.3 (18)
where R(2.pi.k.omega.t) is the rotation matrix, k is the modulation
magnitude (radians), .omega. is the modulation frequency, and t is time in
seconds. This expands to:
##EQU16##
Upon passing through P.sub.2, S.sub.4 ' is generated, having the form:
##EQU17##
which contributes an intensity at the detector of:
I.sub.D1 =T.sup.2 I.sub.0 -T.sup.2 I.sub.0 cos(2.alpha.1+sin 2.pi.kwt)(22)
As above, the total intensity at the detector will be given by a series, in
this case:
##EQU18##
which can be expanded to yield:
##EQU19##
In this case, the signal at the detector D will be a complex AC wave form.
FIGS. 4-12 show the wave forms for various values of rotation in both the
no-gain case R=0, T=1 and the case where R=0.99 and and T=0.01. From these
results, it can be readily seen that the cavity of the optical amplifier
has a rotation gain of around 100. Using known digital signal processing,
or analog circuitry, rotations of 10.mu..degree. and even 1.mu..degree.
can be measured even in the incoherent case where the reflectivity R is
equal to or greater than 0.99.
The above-discussed capabilities of the optical amplifier to detect very
small optical rotations assumed the light from the amplifiers was
incoherent light. However, if the length l between the mirrors M.sub.1 and
M.sub.2 in the optical amplifier is selected so that a resonance condition
of the light exists within the amplifier, the gain and hence, the
intensity of the output of the amplifier to the detector can be
substantially increased. This permits increased sensitivity for the
detection of small amounts of optical rotation. To obtain the resonance
condition of the amplifier, known piezoelectric transducers can be
provided to adjust the position of the mirrors with respect to one another
so as to vary the path length l to obtain the desired constructive
interference.
The light source 19 in the optical amplifier 1 of FIG. 1 is a laser. The
laser is a preferred light source because it produces a monochromatic
light which is very intense. For example, the laser could be a 6328
angstrom helium neon laser of low power. However, other light sources
could be used with the amplifier including arc sources with specific
easily isolated wavelength bands, tungsten filaments having broad
wavelength ranges but with means for isolating the lines with a narrow
wavelength, narrow band width interference filter, etc. A bright light
source is preferred, regardless of the type of light, since with a high
reflectivity mirror M.sub.1 only a small percentage of the output of the
light source is able to pass through the mirror and before reaching the
mirror M.sub.1, light from the source 19 is halved by the polarizer
P.sub.1.
The polarizer P.sub.1 receives the unpolarized light from the laser 19 and
transmits only linear polarized light along the optic axis A in the
direction of the mirror M.sub.1. Preferably, the polarizer P.sub.1 is a
high quality birefringent crystal polarizer such as a Glan-Thompson
polarizer formed of a crystal material such as calcite, quartz, magnesium
fluoride or other suitable birefringent material. Alternatively, other
types of polarizers could be employed such as a dichroic polarizer.
The analyzer P.sub.2 in the optical amplifier 1 disclosed in FIG. 1 is a
polarizer like P.sub.1, that is, a Glan-Thompson polarizer. Both
polarizers P.sub.1 and P.sub.2 are preferably of highest quality with, for
example, an extinction ratio, the rate at which it rejects one
polarization and transmits another, being as high as a million times or
10.sup.6. The polarizers P.sub.1 and P.sub.2 are used cooperatively to
analyze the direction of the linearly polarized light transmitted through
the amplifier. The optical rotation amplifier 1, thus, is a polarimeter
amplifier. Where, for example, the preferred directions of the polarizers
P.sub.1 and P.sub.2 are crossed at 90.degree. in the plane perpendicular
to the optic axis A, and the material between the mirrors M.sub.1 and
M.sub.2 is not optically active, no polarized light passes through the
analyzer P.sub.2 to the detector D. However, where the polarized light
passes through an optically active material such as the eluent in the flow
cell 5, the direction of polarization is rotated and light will be
transmitted through the polarizer P.sub.2 to the detector in an amount
corresponding to the angle of rotation.
The equation governing the intensity I.sub.D that the detector D sees is:
I.sub.D =I.sub.0 cos .THETA. where I.sub.0 is the incident intensity upon
the polarizer P.sub.2 and the angle .THETA. is the angle between the
preferred direction of the polarizer and the actual polarization direction
of the light. When .THETA. is zero cos is 1, so there is maximum
transmission and when .THETA. is 90.degree., cos is zero and there is
nothing transmitted. Both polarizers P.sub.1 and P.sub.2 are located in a
plane perpendicular to the optic axis A. The optical amplifier 1 of the
invention acts to amplify the effect of small rotations to permit their
detection using the analyzer P.sub.2 rotationally fixed in position and
measuring the intensity at the detector D.
The liquid chromatography system 2 shown in FIG. 1 involves the use of a
carrier liquid which is flowed through the column 3, passage 4, flow cell
5, passage 7 to the sump 8 as discussed above. The carrier fluid serves as
a reference liquid. The output intensity of the amplifier 1 is monitored
over time with the detector D. A recorder can be used to plot the output
intensity versus time. The recording would show a base line for the output
intensity associated with the reference liquid and then an increase or
decrease in intensity depending on the direction of rotation caused by the
presence of an optically active material in the carrier liquid. This
information can be used to identify what the optically active material is
and the amounts or concentration thereof as is conventional in this type
of chromatography
The mirrors M.sub.1 and M.sub.2 in FIG. 1 are shown as plane mirrors
located in a plane perpendicular to the optic axis A so that they allow
theoretically an infinite number of reflections of the light back and
forth between them, depending upon the reflectivity R of the mirrors. The
mirrors form what is called a Fabrey Perot cell or cavity. Instead of
plane mirrors, the mirrors could be focusing mirrors of long focal length.
They could also have a confocal design.
From the above equations, it can be seen that the intensity at the detector
is a function of the mirror reflectivities, that is, the gain of the
amplifier is directly proportional to the reflectivity of the mirrors. The
reflectivity R of the mirrors is 0.90 or, more preferably at least 0.99
for enhanced amplifier gain. The mirrors can be formed, for example, with
a fused silica base with magnesium fluoride overcoated aluminum. Other
well known mirror materials could also be used. Piezoelectric transducers,
not shown, can be provided on the mirrors to always hold the cavity in
tune or resonance condition for high transmission and sensitivity. The
transducers also dynamically compensate for temperature changes in the
cell 5.
The quarter wave compensators or retarders C.sub.1 and C.sub.2 of the
amplifier 1 are standard polarization devices which can for example, be
made out of birefringent material such as calcite, quartz or mica. Each
retarder has a fast axis or preferred direction identified by the arrow f
in FIG. 13 and a slow axis identified by the arrows s in FIG. 13. Inside
of each compensator, light that propagates with the electrical field
parallel to the fast axis travels faster than light polarized parallel to
the slow axis so that after transmission through the quarter wave
compensator, when the two components reemerge they are no longer in phase.
The amount of the phase shift depends on the thickness d of the
compensator. With the present invention, the gain of the amplifier
directly depends upon the precision of the quarter wave compensators in
order to get close to theoretical performance for a given set of
reflectivities of the mirrors. Accordingly, the retardation of each
compensator is preferably as close as a few tenths of a degree from
90.degree. as a retarder, e.g., 90.degree..+-.0.3.degree.. As discussed
above, the effect of the quarter wave compensators is to invert the
direction of polarization on reflection, that is, after two passes of the
light through the compensator. This causes the optical rotations of the
light with respective passes of the light through the material to be
cumulative. The axes f and s, also referred to as the neutral lines of the
compensators, form the same angles with respect to the direction of
polarization P of the polarized light supplied to the polarizer P.sub.1.
If no modulation is performed as with the amplifier discussed with
reference to FIG. 2, the output of the detector d will merely be a dc
change in the intensity with respect to a base line when an optically
active material is present in the eluent. With a modulator 13 as shown in
FIG. 1, there will be a time varying intensity change that is a function
of how the modulator operates. The modulator 13 is preferably a sine or
square wave driven Faraday modulator which is placed in the beam outside
the Fabrey Perot cavity of the amplifier, although the modulator could be
placed inside the cavity. Other types of modulators such as photoelastic
modulators could also be used to continuously modulate the polarization
state of the light in terms of retardation or phase change and azimuth.
The purpose of all of these different types of modulators is to modulate
the photoelectric detected intensity in relation to the polarization state
changes. Modulation is not necessary with the optical amplifier of the
present invention but it makes it easier to detect the polarization state
change effects amplified by the amplifier as discussed above. If a
modulator is used, such as a Faraday modulator, it will be sensitive to
the sign of the rotations, that is, whether positive or negative. If no
modulator is used, the sign of the optical rotation of the polarization
direction can be detected by a slight angular displacement of the analyzer
P.sub.2.
The detector D illustrated schematically in FIG. 1 is shown as the human
eye but in automated systems the detector is preferably some kind of
photosensitive detector, a photomultiplier, photodiode or phototransistor,
for example. The detector merely detects the intensity change without the
optically active material and with the optically actively material and
then knowing other parameters associated with the liquid chromatography
system, the optically active material and its concentration can be
computed.
The flow cell 5 can be a flow cell of the type disclosed in U.S. Pat. No.
4,498,774, for example. The cell is constructed of fused quartz or some
other low expansion optical material which is chemically inert. It has an
annular central bore 6 formed therein along the length of the entire cell.
The open ends of the central bore 6 are closed by windows 9. The material
of the cell itself need not be light transmissive, only the end windows 9
have to be constructed so as to be light transmissive. Instead of a
non-metallic material, the cell could also be constructed of a metal which
is not reactive to the materials to be used therein, for example, a very
low expansion steel such as Invar could be used.
The light transmissive end windows 9 can be formed of fused silica, for
example. Microscope cover slips will work as end windows, epoxied to the
cell material, if the bore 6 is small enough. Practical lengths for the
flow cell 5 are from 1 mm to 200 mm, with typical cells being in the range
of 1-100 mm, depending upon the appl | | |
