Optical scanning and imaging system utilizing laser diode

Photo-optical reproducing, copying/printing apparatus wherein a solid state laser diode is caused to emit a divergent beam of light which is collected and collimated. A cylindrical multi-element lens shapes the beam first in a vertical and then in a second horizontal plane for impingement upon a rotating polygonal mirror assembly. A spherical multi-element lens thereafter focusses the laser beam as scanned by the rotating mirror assembly through a cylindrical lens assembly so as to scan across the surface of a photo receptor drum assembly horizontally, rotatably disposed adjacent to a cylindrical lens producing a latent image on the drum. The scanned image is then electrophotographically developed at the drum by first toning the image and thereafter causing the image to be transferred from the drum to a fuseable item such for example as paper which is adapted to pass across the surface of the drum in contact therewith. Each of the active elements of the photo-optical system is adjustable relative to the axis of the laser light beam permitting accurate, extremely high definition data output. The photo-optical system is also made as a modular assembly which is capable of being used with a number of different speeds of associated apparatus so that only the associated apparatus need be adapted, changed or reoriented to the basic photo-optical reproducing, copying/printing apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to photo-optical scanning apparatus utilizing a 
laser diode light generating device and a polygonal mirror assembly in 
conjunction with suitable light beam collimating and focussing apparatus. 
More specifically, the invention relates to a system for precisely shaping 
the laser generating light beam into an efficient spot scanning size for 
electrophotographic printing and/or copying. 
2. Description of the Prior Art 
Many problems are associated with scanning systems wherein a modulated/or 
unmodulated light beam is caused to scan by means of a rotatable polygonal 
mirror. For example the position of each scanning line becomes difficult 
to control. This problem is a result of the angular relationship between 
adjacent facets of the polygon as well as between the facet planes and the 
rotational axis of the polygon. 
Another problem is associated with the location of the laser light 
generating apparatus and its angular relationship to the operably 
associated hardware. A further problem is that as the polygon mirror 
assembly is rotated at a constant rotational rate the speed of the 
generated spot will be constant along an arc but will not be constant with 
respect to a straight line scan. In fact, the laser beam or spot speeds up 
at the periphery of the scan line which in turn has the effect of changing 
the dimension of the output data being developed. These and other 
similarly associated problems have caused many of the prior art devices to 
be less than commercially satisfactory. 
SUMMARY OF THE INVENTION 
It is, therefore, an object for the present invention to overcome each of 
these problems in a new, novel and heretofore unobvious manner and to 
provide a photo-optical solid state laser diode scanning system wherein a 
solid state laser is caused to produce a divergent beam of visible 
electromagnetic radiation which is collected and collimated and thereafter 
optically shaped to reduce the vertical dimension while expanding the 
horizontal dimension for subsequent impingement upon a rotatable polygonal 
mirro assembly. The collimated laser beam is then focused onto a 
photoconductor, for example, a rotatable drum, through a cylindrical lens 
and a light folding mirror, passing through a spherical lens to the 
photoconductor itself. 
A novel aspect of the invention is the provision for the apparatus to be 
modularly related and to be mounted to a rigid, fixed base member. Each 
element of the novel combination is adjustably, positionable relative to 
the base as well as to the axis of the laser beam, the lenses, mirrors and 
polygonal mirror facets thereby insuring an accurate, clear and highly 
defined, latent image on the photoconductor i.e. drum. 
Another novel aspect of the present invention is the provision of a novel 
photoconductor drum charging, exposing, toning and cleaning apparatus for 
a laser diode and printing and/or copying apparatus utilizing a novel 
folded laser scanning light path in combination with a corna charging, 
discharging apparatus not heretofore available in electrophotographic 
processing apparatus. 
Other objects, features and advantages of the invention will be readily 
apparent from the following description of a preferred embodiment thereof, 
taken in conjunction with the accompanying drawings, although variations 
and modifications may be affected without departing from the spirit and 
scope of the novel concepts of the disclosure and in which;

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In one of its broadest aspects the apparatus of the present invention is 
typified by the arrangements set forth schematically in FIG. 1. The laser 
diode printer apparatus 10 is seen to comprise a laser diode 12, energized 
in a known manner to produce a beam of electromagnetic radiation, arrow 
14, which is adapted to be passed through a lens system 16 which acts to 
collimate the light beam 14 and direct the collimated light beam 18 into 
and through a pair of optical prisms 20 and 22, respectively, which act to 
change the beam from an elliptical cross section to a circular cross 
section. 
The now collimated, altered, beam 24 of light is next directed through a 
focussing lens 26 to a rotatable polygonal mirror assembly 28. The 
focussed light beam 30 is reflected off the faces or facets of the polygon 
28 as the latter is rotated by drive motor 32 in the direction of arrow 
34. The rays 36 of the reflected beam are focused onto the photoconductive 
drum surface 38 of rotatable drum 40 via a tiltable mirror assembly 42 and 
a second focussing lens system 44, FIG. 1. 
The focussed laser beam 46 is adapted to scan the cylindrical 
photoconductor surface 38 from edge to edge or side to side by means of 
the rotating polygon mirror 28 and drive motor 28. Modulation (by means 
not shown) of the laser diode 12 produces a latent electrostatic image 
upon the surface 38 of photoconductor 40. Copying and/or printing media 
48, FIGS. 5 and 6, is adapted to receive the image of the intelligence 
carried by the latent electrostatic image by means of and in a manner to 
be described later on herein. 
A physical embodiment of the apparatus 10 schematically illustrated in FIG. 
1 is seen most clearly in the side elevational view of FIG. 2, to comprise 
a rigid base member 50 on which the entire assembly is mounted and adapted 
to be slideably moveable back and forth or right to left as the case may 
be. As earlier mentioned, mirror 42 is adapted to be tilted about its 
horizontal axis mounting pivot 52 so as to fold the laser beam 36 upwardly 
toward the cylindrical lens 44, FIG. 1. Tilting adjustment of the mirror 
42 is provided by means of the threaded horizontal cross shaft 54 
adjustably moveable by means of thumb wheel 56 against the vertical mirror 
support column 58 disposed in vertical mounting pillar 60 secured to base 
50. Vertical, erectable movement of mirror 42 is provided by means of slot 
62 and pin 64, as seen most clearly in FIG. 2. 
The laser diode 12 (light generating element) is surrounded by a 
thermo-electric cooling member 66 and is gimbally mounted, as at 68 to 
support 70. A heat sink 72 of copper or similar material capable of 
rapidly and efficiently dissipating large quantities of heat abuts the 
laser diode assembly 12. The gimble pivoting arrangement 68 supports the 
heat sink 72 and cooling member 66 enabling the laser diode 12 to be 
pivoted about two orthogonal axes that pass directly through the diode 
chip. The laser diode temperature is regulated so as to be constant at 
approximately 20-21 degrees C. by a feedback controller with a thermistor 
sensor (not shown). It is necessary that the heat sink temperature, which 
is close to ambient, be somewhat above the control temperature since the 
thermoelectric device can only cool and cannot heat. 
The laser light 14 emitted by the diode 12 is collected by the objective 
lens 16 which in one embodiment comprises a microscope objective having a 
magnification of 20 times. This is necessary since the output area (of the 
diode is about two tenths micron by about five microns) from which light 
is being emitted and is extremely small. The objective lens 16 has the 
laser light at its focus. The light enters the lens as a diverging set of 
light rays 14 from the laser diode 12. The objective lens 16 collimates 
the light, as seen most clearly in FIG. 2. Since the light from the diode 
12 is diverging on the entering side of lens 16 and is collimated on the 
exiting side thereof, the beam is not generally circular but rather oblong 
or elliptical in cross section. 
In order to correct for this two separate but optically complimentary 
photo-optical elements are employed. The collimated light is first passed 
through prism 20 which is configured so as to compress the beam in the 
vertical plane or direction down to approximately one tenth inch. 
Thereafter, the beam is redirected into and through prism 22 which is 
constructed such that the light beam is expanded slightly in the 
horizontal direction. Exiting from the second prism 22 the light beam now 
has a circular cros section and is collimated before entering focussing 
lens 26. 
Except for the tiltable mirror 42, which is provided with its own separate 
individual adjustable mounting means, previously described, each of the 
lenses and prisms heretofore mentioned are provided with separate means 
for orthogonally positioning these elements relative to each other as well 
as with respect to the axis of the laser light beam. 
A rigid elevating platform 74 is secured to the base 50 to which is 
mounted, as by bolts (not shown) a second rigid mount 76. Member 76 
provides oppositely disposed parallel guiding tracks (not shown) for 
slideably moving support member 78. To one end of the member 78 is secured 
a rockably, pivoted support member 80 arcuately moveable about pivot 82 on 
the leftward projecting end of the member 78. Member 80 provides a 
tiltable support for the laser diode 12 including its heat sink 72 as well 
as for the objective lens assembly 16 and its adjusting means. Vertical, 
slideable, adjustment for diode 12 is controlled and actuated by means of 
micrometer slides and the knurled thumb wheel 84. The horizontal 
adjustment is by means of thumb wheel 86, FIG. 4. Rocking movement for 
platform 80 to axially align the laser beam 14 is provided by means of the 
threaded adjusting wheel 88 which is adapted to rockably pivot the member 
80 about pivot 82 by means of threaded shaft 90 against the rightward end 
of member 80. Sliding, focussing adjustment for objective lens assembly 16 
is provided by thumb wheel and shaft assembly 92 at the rightward end of 
member 80. 
Laser beam light rays 18 pass, as before mentioned, through a compression 
prism 20 which is angularly, adjustably mounted on a horizontal pivot 94 
and tiltable about this pivot by means of thumb wheel 96, cam 98 and 
L-shaped follower link 100. Adjustment movement of prism 72 is 
accomplished by means of thumb wheel 102, FIGS. 2 and 4, cylindrical cam 
104 and L-shaped cam follower 106 secured to prism 22. Focussing lens 26 
is threadedly, adjustable backwards and forward for accurate focussing by 
rotation within the lens support 108. 
The focussed laser light beam 30 after passing through the focussing lens 
26 is reflectively scanned across the surface 38 of photoconducting drum 
member 40 in a manner such that the data or intelligence contained in the 
modulated beam is placed upon the drum for copying/printing purposes, as 
will be hereinafter described. 
Polygon mirror 28, rotating in the direction of arrow 34 by drive motor 32 
carries 20 mirror facets and rotates at the rate of 12,558 RPM. The laser 
light is passed from the focussing lens 26 to the face of each mirror 
facet so as to scan through an angle of 36 degrees (as the polygon 
rotates) which is precisely twice the angle that the facet moves through 
during the period of time for one scan line. With the focussing lens 26 
positioned in front of or before the polygon 28, the focal point tends to 
be on the arc of a sphere. Rather than in a plane this marks for 
correction problems since the beam 24 passes through the same point of 
this lens all the time. Additionally, as the polygon scanner 28 rotates, 
the deflected beam is rotating at a constant rotational rate so the speed 
will be constant on an arc but will not be constant on the straight scan 
line. The beam or spot speeds up at the periphery, which produces a small 
effect in changing the dimension of the characters. 
In the preferred embodiment as seen in FIG. 3 a focusing lens 110 is 
positioned after the polygon 28. This lens characterized as a F-theta lens 
(fe) and avoids the variation of lineal scan velocity with the scan angle. 
Normally, the spot displacement d of a simple lens varies as the focal 
length times the tangent of theta. (d=f tan e). It is possible to make an 
F-theta (fe) lens so that the lineal displacement varies as the focal 
length times theta (fe) itself. This allows a linear relationship between 
polygon rotation and spot position. Plus, it produces a flatter field so 
that the focus is in a plane (including the scan line). The (Fe) lens is 
triplet a lens. 
As the polygon 28 turns the focussed light is reflected off tilt mirror 42 
and angularly, upwardly into and through the elongated, focussing, 
cylindrical lens 44 to be raster scanned across the photoconductor 40. 
Only about 25 degrees of the total scan is used for the printed scan line. 
Located on the right hand side of the scan line facing the drum 40 is a 
start of scan detector (not shown) which is used to time the initiation of 
printing. 
Lens 44 has a flat surface on one side and convex surface on the other side 
and is utilized to reduce the vertical or facet apex angle error, an error 
in the position of the beam due to wobbling of the facets from one facet 
to the other, i.e., the socalled change in the apex angle. (The angle 
between the axis of rotation of the polygon and the facet, varies from one 
facet to the other and this variation causes the beam to deflect slightly 
in the vertical direction). Utilization of cylindrical lens 44 reduces the 
effect of the wobble. The rotating polygon causes the light to scan a full 
raster scan length i.e. the width of the drum or the width of the line 
that is to be printed on the page one scan length for each facet on the 
polygon. Obviously, the more facets there are on the polygon the easier it 
is to reduce the RPM's required of this rotating device. For example, with 
only eight facets the device would have to be rotated at a high RPM to get 
the same number of scans per second. Scans per second is determined by the 
speed at which it is desired to print. For ninety pages per minute this is 
approximately 6,000 lines per minute. This is a lineal surface velocity on 
the drum of 17.42 inches per second. The raster line spacing is determined 
by the resolution desired for 240 dots per inch. Each raster line is 
spaced by 1 over 240. Thus, the raster scan lines are spaced 1/240th of an 
inch or 00417 inches apart. Dividing the raster line spacing by the 
velocity of the drum gives the time permitted for each scan line. The 
reciprocal of the scan line time gives the scan rate. In other words, the 
scan rate would be just equal to 17.44 inches per second by 0.00417 
inches. However, since 0.00417 is equivalent to 1 over 240, the result can 
be expressed as 17.44 times 240 dots per inch. This gives a repetition 
rate for the scan in scans per second. Each scan occurs in approximately 
239 microseconds. Obviously, the more facets on the polygon the more the 
total RPM can be reduced. The present polygon has 20 facets. The number of 
facets is tied in with the resolution that is desired to be achieved. 
To derive the desired resolution the beam must be expanded to a 
predetermined size as it is passed into the final focussing lens. The 
larger the beam going into the focussing lens the smaller the spot size. A 
reciprocal relationship exists between the spot size and beam size 
entering the focussing lens. The larger the beam going into the focussing 
lens the smaller the spot size. In other words, when the aperture is small 
at the focussing lens, the difraction is greater so the difraction limited 
spot is larger. Thus, the collimated beam size (aperture) should be larger 
to obtain a small spot. 
PRINTING/COPYING STATION 
Referring now to FIGS. 5 and 6, with emphasis first to FIG. 5, there is 
shown a highly schematic or diagrammatic side elevational view of the 
electrophotographic process station of the present invention. The block 
indentified in FIG. 5 as "optical scanning assembly" is meant, for 
purposes of illustration, to include the complete optical structural 
arrangement shown in FIG. 2 including the cylindrical focussing lens 44 
through which the modulated laser beam 46 passes to impinge upon the 
rotatable drum 40. The cylindrical lens corrects for any beam motion 
introduced by the rotating polygon and its associated vertically disposed 
mirrors. 
Drum 40 is provided with a relatively hard, long wearing, photoconductive 
coating 38 having a high infrared response, FIG. 1 and is adapted to be 
rotated in the direction of arrow 112. The size of the drum is calculated 
to accept 11 inch or 14 inch length sheets of plain paper for 
copying/printing in serial fashion, one after the other, so as to increase 
the general "throughput" of the apparatus. 
Initially the drum 40 has no surface charge on it and no toner. The charge 
coratron 114, which consists of six wires stretched across, parallel to 
but out of contact with the drum surface, is electrically energized 
placing a uniform electrical charge across the photoconductive surface 38. 
The drum 40 rotates clockwise, so that the light from the laser diode 12 
strikes the areas on the drum surface where no printing is desired which 
discharges the background. The laser diode beam is swept across the length 
of the drum and selectively modulated with the intelligence necessary to 
produce the printed matter desired. Each scan line at a resolution of 240 
dots per inch will have 240 scan lines per inch of printing. The dots will 
be generated by turning the laser diode 12 on and off to get the 
intelligence information on the drum. The drum now has selective regions 
of electrical charge and regions that are discharged or have no electrical 
charge thus forming a latent electrostatic image thereon. 
The drum next passes to the toner station 116. Toner station 116 has an 
electrical charge bias supply to the toner 118 with a polarity and 
magnitude such that the toner is attracted to the drum surfaces 38 in the 
regions corresponding to where the print will be. At this point in the 
process, the apparatus has produced a developed image. As the drum 40 
continues to rotate further, it comes into the transfer area 120 where the 
image is to be transferred from the photoconductive drum to the copy 
material e.g. paper 48. Paper 48 is moved from left to right arrow 122. 
Two implementations are employed for toner transfer. Both of them use 
electrostatic means. Nonconductive toner 118 is used. The paper 48 is 
charged by means of a transfer coratron 124. The coratron wires develop an 
electrostatic charge field which essentially causes the toner to have a 
greater attraction towards the paper 48 and the downstream (rightward) 
detac coratron 126 than it does towards the photoconductive drum 40. The 
toner effectively lifts off the drum and is applied to the paper. The 
detac coratron 126 separates the paper 48 from the drum to which is 
electrostatically attracted. Detac coratron 126 applies a DC pulse at the 
front or leading edge of the paper to lift the leading edge up. As the 
paper continues to move under member 126 and as soon as the leading edge 
is picked up off the drum, an AC electrical potential is applied to member 
126. This discharges the paper, the paper 48 thus is lifted off the drum 
with the toner intact. 
The paper carrying the toner image next passes into the fuser 128 which is 
a combination of pressure and heat produced by means of two opposing 
roller members 130 and 132, respectively. Thereafter, the paper is passed 
into the next station of the machine at which time the paper bears an 
image of the intelligence copied or printed thereon. Although greater than 
98 percent of the toner is transferred to the paper 48, in order to offset 
the problems with residual toner on the photoconductive drum, if any, a 
preclean coratron 134 and preclean lamp member 136 are used. An AC 
coratron wire is used at this point with the AC switching polarity between 
positive and negative, discharges the surface of the drum 40 and also 
discharges the toner 118. Since light also discharges the surface of the 
drum a low wattage (8 watt) florescent bar light is used to make sure that 
all of the charge is removed in addition to the toner. A vacuum cleaning 
station 138 provided with a rotating bristle brush 140 of soft bristles, 
with a vacuum applied from a source (not shown) sucks off residual toner 
which may be on the drum. At this point the drum is considered to be clean 
as far as toner is concerned. However, since toner was covering some 
surfaces of the photoconductor that the light from the drum is rotated 
past a final discharge lamp 142. Light from lamp 142 shines onto a 
completely cleaned drum removes all residual charge very effectively. The 
apparatus is now ready to start the copy process again at the charge 
coratron 114. 
If however, the paper for some reason, does not detac or lift up off the 
drum 40, oppositely disposed stripper finger members 144, which protrude 
slightly into the drum and into recessed areas at the edges i.e. opposite 
sides of the paper, catch the paper and tend to lift the paper away from 
the drum. 
There has thus been described a new, novel and heretofore unobvious 
photo-optical laser diode printing copying apparatus which provides a very 
high speed, very efficient and very cost effective combination of 
operational apparatus to provide clean, clear, crisp copies without the 
attendant problems associated with much of the prior art devices.