Laser diffraction particle sizing method using a monomode optical fiber

A fiber optic spatial filter assembly for laser diffraction particle sizing apparatus utilizing a laser to generate a monochromatic light beam which is coupled to an optical fiber operating substantially in a monomode and creating a beam of light having a high degree of spatial coherence which is then passed through collimating lenses to interrogate and impinge upon the particles of matter through which the laser diffracted light passes. The light scattered by the particles is focused onto a Fourier plane and thereafter impinges upon a photooptical detector array, positioned coincident with the Fourier plane, for measuring the light intensities of the scattered light by scattering angle, thus enabling the computation of particle size.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to apparatus and method for 
particle sizing of the type which employs laser diffraction to measure 
particle size. The present invention, more specifically, uses, with other 
components, a monomode optical fiber for producing a beam of light having 
a high degree of spatial coherence in a spatial filter that is easily 
aligned, replaceable, rugged and cost effective. 
2. Description of Prior Art 
The use of laser light diffraction to measure particle size is a widely 
known technique. Laser diffraction is a particle sizing method which uses 
the average relative angular intensity of scattered light. Instruments 
that use laser light diffraction to measure particle size have been 
available for many years from a number of different manufacturers. All 
laser diffraction instruments use the same basic method to measure 
particle size. All laser diffraction instruments require a beam of 
monochromatic light with a very uniform wave front. This beam of laser 
light is directed at the sample particles to be measured. When the light 
hits the particles, the light is diffracted or scattered from the 
particles. Detectors are used to measure the relative average intensity of 
the light scattered at various angles from the sample material. Once the 
relative intensity of light scattered at several different angles from the 
particles is known, the particle size and size distribution can be 
calculated. 
The ability to make accurate measurements of particle size is directly 
related to the quality of the beam, its spatial coherence, which 
illuminates the sample particles. This monochromatic light beam must be 
highly collimated, meaning that all the rays of light traveling in the 
beam are parallel to one another. 
In order for a beam of light to be highly collimated, the light must have a 
very uniform wave front from the light source. Ideally the light source 
would be a perfect point source of light, infinitesimally small. Also, the 
light source must be free of diffraction, which could be caused by dust 
particles in the air, or because the light beam is partially obstructed. 
In addition, any optical lenses used to collimate the beam of light must 
be free of surface and material imperfections which would also cause light 
diffraction. Finally, any optical lenses use to collimate the beam must be 
designed to minimize any aberrations caused by the lens itself. These 
characteristics are necessary to achieve high resolution size 
measurements. 
Apparatus and method using laser diffraction to measure particles is 
importantly different from dynamic light scatter apparatus and method for 
particle analysis. Dynamic light scatter requires time fluctuation, or 
power spectral measurement of the scattered light. Whereas, laser 
diffraction requires measurement of average, relative angular intensity of 
the scattered light at a number of detection angles, which is not a time 
or frequency based measurement. The basic differences between structures, 
methods, and optical requirements of laser diffraction versus dynamic 
light scatter are known to those in these fields. However, some 
sophisticated and subtle differences of laser diffraction might not be 
appreciated by those knowledgeable in dynamic light scatter technology. 
In laser diffraction devices, such as the COULTER.RTM. LS and competitive 
devices, spatial filtering of the laser beam is used to create the above 
discussed high spatial coherence quality beam and is one of the most 
important aspects of the instrument. In the COULTER LS, in order to 
measure the small angular deflection of the laser beam caused by 
diffraction from very large particles such as nine hundred micrometers 
(900 .mu.m), light scattered at angles as small as 0.5 milliradians (mR) 
must be measured and an angular resolution of approximately 0.05 mR is 
desired. To achieve this level of beam quality, the laser beam must be 
expanded to about thirteen millimeters (13 mm) and a diffraction limited 
beam of this diameter must be formed by the collimating optics. A 
diffraction limited Gaussian beam of thirteen (13) mm diameter, with a 
wavelength of seven hundred and fifty nanometers (750 nm), has a 
divergence of .about.0.04 mR. Any serious discrepancy between this desired 
level of spatial coherence and collimation and the actual performance 
leads to degradation in the resolution of the instrument. 
All particle sizing instruments based on laser diffraction techniques use a 
spatial filter to provide this very high quality beam of laser light. All 
of these spatial filters use a pinhole in combination with other optical 
elements to create the required quality light beam. A pinhole is a small, 
circular hole in a thin, flat piece of rigid, opaque material. A typical 
pinhole spatial filter is configured in the following manner. A source of 
light, such as a laser diode, illuminates a circular beam stop, which 
makes the light beam circular. The circular light beam then passes through 
a system of optical lenses. These lenses focus the laser beam down to the 
pinhole, which is typically between twenty to fifty (20-50) .mu.m in 
diameter, allowing most of the light beam to pass through the pinhole. Any 
impurities in the laser light, caused by diffraction or lens aberration do 
not pass through the pinhole, but are blocked by the opaque material 
surrounding the pinhole. The light that passes through the pinhole is then 
"clean," except for some diffraction rings caused by the pinhole itself. 
These diffraction rings are removed by another beam stop placed at the 
exact minimum of the first diffraction ring. Finally, a lens collimates 
this diverging, circular beam of light at the point the desired beam 
diameter, thirteen (13) mm in the case of the COULTER LS, is reached, 
creating a highly collimated, uniform wave front beam of light, which is 
useful for laser diffraction particle sizing. 
While the pinhole method of creating this beam of light works effectively, 
in practice it has many problems. First, in order to pass most of the 
light from the laser source through the pinhole, the optical elements 
including the source, the first beam stop, the lenses and the pinhole, 
must be precisely focused and aligned to within a few micrometers. This 
requires the use of very complicated and expensive mechanical elements to 
provide the fine resolution these adjustments require. Additionally, the 
time required to sufficiently adjust the assembly can be many hours. 
Secondly, once the assembly is fully aligned, it can be easily misaligned 
by mechanical distortions from clamping, or from temperature changes, 
which cause the various components to expand per their respective 
coefficients of thermal expansion. Also, shock and vibration during 
shipment of the instrument can cause the pinhole assembly to become 
misaligned, causing expensive, time consuming field service. Once in use 
in the laboratory, if a component of the spatial filter optical train 
burns out or is damaged, the entire optical assembly must be returned to 
the factory for parts replacement and then the time consuming, expensive 
optical realignment. 
Thus, it would be advantageous for laser diffraction particle analysis 
apparatus to improve upon the pinhole style of spatial filter assembly to 
reduce or eliminate the above mentioned drawbacks. Alternatively, if the 
pinhole and other associated components could be replaced to provide an 
assembly that is much more rugged, much more immune to distortions caused 
by thermal effects, shock and vibration, requires very little alignment, 
and is lower cost, such replacement would solve a longstanding need. 
Many devices, for example those described in one or more of the hereinafter 
listed publications, utilize various forms of optical fibers, including 
monomode and multimode fibers, in light transmitting and light detecting 
arrangements. However, none of the prior art devices describe a monomode 
optical fiber apparatus in a spatial filter capable of providing the high 
quality light beam required for particle sizing using laser diffraction 
techniques. 
U.S. Pat. No. 4,953,978, Steven E. Bott et al., Coulter Electronics of New 
England, Inc., TICLE SIZE ANALYSIS UTILIZING POLARIZATION INTENSITY 
DIFFERENTIAL SCATTERING. 
U.S. Pat. No. 4,975,237, Robert G. W. Brown, The Secretary of State for 
Defence in Her Britannic Majesty's Government of the United Kingdom of 
Great Britain and Northern Ireland, DYNAMIC LIGHT SCATTERING APATUS. 
U.S. Pat. No. 5,056,918, Steven E. Bott et al., Coulter Electronics of New 
England, Inc., METHOD AND APATUS FOR TICLE SIZE ANALYSIS. 
Juskaitis, R., et al., 1992, Electronics Letters Vol. 28(11), FIBRE-OPTIC 
BASED CONFOCAL SCANNING MICROSCOPY WITH SEMICONDUCTOR LASER EXCITATION AND 
DETECTION. 
Brown, R. G. W., et al., 1987, J. Physics E, Vol. 20, MONOMODE FIBRE 
COMPONENTS FOR DYNAMIC LIGHT SCATTERING. 
Brown, R. G. W., 1988, HMSO, MINIATURE INSTRUMENTATION FOR LASER LIGHT 
SCATTERING EXPERIMENTS. 
Knuhtsen, J., et al., 1982, The Institute of Physics, FIBRE-OPTIC LASER 
DOPPLER ANEMOMETER WITH BRAGG FREQUENCY SHIFT UTILISING 
POLARISATION-PRESERVING SINGLE-MODE FIBRE. 
Brown, R. G. W., 1987, Applied Optics, Vol. 26(22), DYNAMIC LIGHT 
SCATTERING USING MONOMODE OPTICAL FIBERS. 
Dabbs, T., et al., 1992, Applied Optics, Vol. 31(16), FIBER-OPTIC CONFOCAL 
MICROSCOPE: FOCON. 
Brown, R. G. W., 1987, DESIGNS OF FIBRE OPTIC PROBES FOR LASER ANEMOMETRY: 
Paper 9, Second International Conference on Laser Anemometry--Advances and 
Applications, Strathclyde, UK. 
U.S. Pat. No. 4,975,237 to Brown relates to the use of monomode optical 
fibers in a light detector assembly, in a dynamic light scatter apparatus. 
Brown describes the substitution of a pinhole in front of a photo detector 
with a monomode optical fiber, the purpose of which is to isolate a small 
area of light from a large amount of scattered light from the particles. 
Brown uses a monomode optical fiber simply because the core diameter of 
the monomode fiber is of approximately the correct size to isolate a 
single coherence area of scattered light. Brown does not use an optical 
fiber as a light delivery and filtering device suitable for laser 
diffraction. In FIG. 1 of Brown, a monomode optical fiber is shown in a 
beam delivery path. Brown does not, however, teach or suggest benefits of 
spatial filtering employing the monomode optical fiber, because the 
Dynamic Light Scattering method of his device does not require the beam 
quality required of the laser diffraction sizing apparatus. 
Other apparatus, such as the confocal microscope of Juskaitis et al., use 
monomode optical fibers for both delivery and detection of light. Such 
devices and their methods are not related to laser diffraction particle 
sizing and do not teach the use of monomode optical fibers in spatial 
filter assemblies for such. 
SUMMARY OF THE INVENTION 
It has been discovered and demonstrated that a monomode optical fiber can 
be used in a spatial filter assembly to provide the filtering benefits of 
conventional, pinhole-based spatial filter assemblies, for producing the 
spatially clean, uniform wave front, point source of monochromatic light 
that is required for laser diffraction techniques of particle sizing. The 
obtained point source of light is closer to the ideal point source. The 
spatial coherence is especially of high degree. The optical fiber-based 
filtering assembly provides the additional benefits of being less 
expensive, more rugged, easier to align and more resistant to thermal and 
vibration effects.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
As illustrated in the diagrammatic view of FIG. 1 of the drawing, a 
conventional, prior art pinhole-based spatial filter 10, generally 
includes: a light generating laser diode 12, the beam 14 of which is 
passed to a first circular beam stop 16, which acts to eliminate unwanted 
peripheral light rays and makes the light beam circular in shape; focusing 
lenses 18 and 20; a pinhole member 22; a second beam stop 24 to remove 
diffraction rings caused by parts of the beam 14 hitting the edges of the 
pinhole 22; and finally, the laser light beam is collimated by a lens 26 
and passed on into a sample containing cell, passageway or flow path (not 
shown) for sizing particulate matter contained therein. The second beam 
stop 24 and the collimating lens 26 are required parts of the prior art 
spatial filter arrangement. Imperfections in the lenses 18 and 20 and dust 
particles between the laser source and the pinhole can cause some 
diffraction of the beam 14 and form unwanted or "dirty" light rays which 
do not pass through the pinhole 22, but are blocked. 
In contrast to the foregoing prior art, the optical fiber-based spatial 
filter of the laser diffraction particle sizing apparatus embodying the 
present invention is illustrated in FIG. 2 of the drawings, and will be 
described presently. Laser diffraction particle sizing apparatus 
incorporating the novel optical fiber-based spatial filter assembly of 
this invention are illustrated in FIGS. 3 and 4 of the drawings. 
As shown in FIG. 2, a monomode optical fiber-based spatial filter assembly 
30 according to the preferred embodiment includes many elements. A first 
rigid, vertical support member 32 supports a laser diode to optical fiber 
coupler 34, which couples the light from a laser diode (35) into a 
monomode optical fiber cable 36. The construction of optical fiber 
couplers are widely available, as used in the telecommunications industry. 
The optical fiber cable 36 is cut to approximately six meters in length. 
The specific length is not critical to the invention, but must be 
sufficiently long so that the extraneous light traveling through the 
cladding (not shown) of the fiber is attenuated to the extent that it is 
not detrimental to the output of a clean, spatially filtered beam. The 
fiber optic coupler 34 is supported by a printed circuit board 38, which 
provides a mounting member for individual circuit components 40, not 
otherwise described, which apply electrical power to the laser diode (not 
shown). The printed circuit board 38 is supported by the first rigid 
support member 32 which itself is disposed on a rigid base member 42. The 
support member 32 also supports a short cylindrical form or tube 44. The 
six meter length of the optical fiber cable 36 is wrapped in a coil around 
the cylindrical tube 44, to provide a relatively small, compact assembly 
46, which is thereafter covered by a shrink-wrap cover 48, which protects 
the cable from damage due to handling. A forwardly extending end 50 of the 
optical fiber cable 36 is terminated with an optical fiber connector 52, 
to permit the assembly 46 to be replaced in the field, if parts thereof 
are damaged or burn out. 
The optical fiber 36 is a monomode (single mode) optical fiber with a core 
diameter of approximately five (5) .mu.m and a cladding diameter of one 
hundred and twenty-five (125) .mu.m. An optical fiber is defined as being 
single mode for a given wavelength of light when the Normalized Frequency 
(V-number) of the fiber is less than or equal to 2.405. The V-number is 
calculated as follows: 
##EQU1## 
where: NA=numerical aperture of the fiber (dimensionless) 
a=fiber core radius (.mu.m) 
.lambda.=light wavelength (.mu.m) 
In an embodiment of the present invention, the numerical aperture of the 
optical fiber was 0.11, the wavelength of the laser light 750 nm and the 
fiber core radius 2.5 .mu.m. This design yields a V-number of 2.30. 
Fiber couplers, such as the coupler 34, typically have means to secure the 
laser diode in place relative to a lens which focuses the light onto the 
input face of the optical fiber. Fiber couplers can be purchased with 
different degrees of coupling efficiency, but generally less than 45%; 
meaning that only 45% of the output power of the laser diode is coupled 
into the optical fiber. However, this coupling efficiency is one to three 
times better than the efficiency found in a typical pinhole-based 
filtering assembly. In the preferred embodiment of the present invention, 
a laser diode with five milliwatt (5 mW) output power is coupled into the 
optical fiber 36 with approximately 20% efficiency, for producing output 
power of the fiber of approximately one (1) mW. 
Most of the light that is coupled into the optical fiber travels through 
the core of the optical fiber, but a portion of the light is coupled into 
the cladding of the optical fiber. This "cladding light" can be a source 
of very poor performance of the spatial filter if it is not adequately 
extinguished. In the embodiment of the present invention, a relatively 
long length of optical fiber is used, approximately six meters, to allow 
the light traveling through the cladding to be extinguished due to the 
inherently high attenuation losses of the cladding. Alternatively, this 
cladding mode light could be removed from the cladding through the use of 
an index matching gel surrounding the cladding along the fiber. 
After the cladding light traveling through the fiber has been sufficiently 
attenuated, the light leaves the end of the fiber through its core (5 
.mu.m diameter). This core diameter used with seven hundred and fifty 
(750) nm light caused the light output of the monomode optical fiber to be 
closer to an ideal point source of light than the typical pinhole spatial 
filter, which uses a pinhole of twenty to fifty (20-50) .mu.m diameter. 
Also, due to the core diameter and limited numerical aperture of the light 
output from a monomode optical fiber, there is no need for the second beam 
stop 24 required in the typical prior art pinhole type filter of FIG. 1. 
The optical fiber connector 52 is mounted through a second, rigid, vertical 
support member 54, which also is secured to the base member 42. The laser 
light beam 56 exits the end or tip 58 of the optical fiber 36. Due to the 
nature of the monomode optical fiber, the light exiting the optical fiber 
has wave front distortion typically less than .lambda./10, is circular in 
shape, has a Gaussian intensity profile, and diverges according to the 
numerical aperture of the fiber, which in the preferred embodiment was 
0.11. In the preferred embodiment, the output beam 56 has a high degree of 
spatial coherence and is allowed to expand to approximately thirteen (13) 
mm in diameter before being collimated. 
The optical fiber connector 52 attaches to a female connector member 60, 
which in turn is attached to a rigid positioning disk 62. During assembly, 
the disk 62 allows the tip 58 of the optical fiber 36 to be statically 
centered to the nominal center position of a pair of collimating or beam 
forming lenses 64 and 66. The positioning disk 62, by way of locking 
screws 67, is held in place on a second vertical support member 68, which 
also is mounted to the base member 42. 
The positioning disk 62 is located inside of a positioning ring 70, which 
provides mounting for two adjustment screws 72 (only one of which is 
shown). The adjustment screws 72 move the positioning disk 62 and thereby 
the fiber tip 58 in both an X and Y axis direction, allowing the beam of 
light 56 to be centered at the nominal center of the beam forming lenses 
64 and 66. Once the positioning disk 62 is centered, the locking screws 67 
are tightened, securing the position of the disk 62 to the vertical 
support member 68 and the base member 42. 
The lenses 64 and 66 are disposed in a lens mount 74, which is secured to a 
lens tube 76, which is slidable along the Z axis through a block-like 
support member 78, such sliding permitting the lenses to be focused. Once 
focusing is accomplished, a set screw (not shown) retains the lens tube 76 
in place with respect to the support block member 78. 
The lenses 64 and 66 are positioned relative to the X and Y axes by moving 
the block 78. This block 78 is flexibly supported from the support member 
54 by four equidistant, corner disposed, flexible wire-like members 80 
extending from the first support member 54 forwardly through clearance 
holes in the second vertical support member 68 to the block 78. Through 
the use of two stepper motors 82 (only one shown), the block 78 is 
positioned in an X-Y orientation and combinations thereof. This flexible 
construction allows the laser diffraction apparatus to dynamically align 
the light beam 56 with respect to the rest of the apparatus. 
As seen in the diagrammatic view of FIG. 3, the laser diffraction particle 
sizing apparatus embodying the present single mode optical fiber-based 
filter assembly 30 is illustrated with the collimated laser light beam 56 
passing into and through a sample passageway 84, through which particulate 
matter 86 of varying size particles moves or flows in the direction of the 
arrow 88. The passageway 84 can be a flow cell, a sample stream flowing in 
air, or a stream of sample sheathed by another media. It is not essential 
for the particle sample to be flowing. The particles 86 (shown in this 
figure to be the same size for simplification) diffract some of the 
impinging laser light beam in accordance with the well-known Fraunhofer 
diffraction and other scattering theory. The light diffracted from the 
beam becomes a plurality of diffracted rays 90 of light spreading away 
from the particles, as illustrated in FIG. 3. The angle of the diffracted 
light 90 relative to the collimated light beam 56 is roughly inversely 
proportional to the size of the particles 86. 
The diffracted light 90 then passes through a Fourier lens 92, which causes 
light of a given angle 94, incident on the lens, to be focused onto a 
Fourier surface or plane 96, which is displaced from the Fourier lens 92 
by a distance equal to the focal length of that lens. A photo detector 98 
is positioned coincident with the Fourier plane 96. The photo detector 
array 98 is made up a large number of individual photo detectors which 
measure light intensity. By measuring the light intensity at a large 
number of detector locations on the photo detector array 98, a precise 
profile of scattered light intensity versus scattering angle is obtained. 
A computer (not shown) operably associated with the laser diffraction 
sizing apparatus can determine the actual size and size distribution of 
the sample particles 86. 
FIG. 4 illustrates an embodiment of the invention in which the collimating 
lens 64 and 66 are replaced by a reverse Fourier lens 100, itself well 
known in the art. The reverse Fourier lens 100 obviates the need for the 
Fourier lens 92 of FIG. 3, but has as its Fourier plane the same plane 96 
as the Fourier lens 92 in FIG. 3. The reverse Fourier lens 100 produces a 
convergent beam 102, which converges at a point 104 on the Fourier plane; 
the convergence point 104 also being the same for the Fourier lens 92. The 
reverse Fourier lens 100 and the collimating lenses 64 and 66 are 
generically identified as "beam forming means". 
In summary, the present invention provides the same functions as the 
conventional pinhole-based spatial filter assemblies, but without its 
second beam stop, expensive alignment assemblies, the high cost associated 
with the time-consuming alignment procedure and the expensive field 
service problems associated with shipping the delicate pinhole-based 
filter. The laser light output beam of the monomode optical fiber-based 
filter is of better quality, spatial coherence, than the typical 
pinhole-based filter because the size of the effective light emitting 
source is closer to the ideal point source of light. For example, with 
optical fiber-based technology, a five .mu.m core diameter can be achieved 
quite readily; whereas, a pinhole-based filter of equivalent size would be 
practically impossible to align and successfully ship in a commercial 
product. 
It is understood that the illustrative embodiments constitute examples of 
the principles of the present invention, but that alternatives will occur 
to those of ordinary skill in the art, without departure from the scope of 
this invention.