Light source package incorporating thermal expansion compensating device and image forming apparatus using the same

A light source package includes a light source, a lens for collimating the light from the light source, and a support member for supporting the light source and the lens so that the optical axis of the lens is set to a desired position relative to the light source. The support member includes a compensating device for compensating the lens for deviation of the optical axis of the lens resulting from a difference in the coefficients of linear thermal expansion between the lens and the support member when the ambient temperature changes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light source package used in such 
optical systems as a photocopier, a laser printer, or similar image 
forming apparatus, and to an image forming apparatus using the light 
source package. 
2. Description of the Related Art 
Conventional image forming apparatus have a light source and a lens 
supported by a lens support member in the housing and compensate the lens 
for changes in focal length and refractive index resulting from the 
thermal expansion and contraction of the lens by utilizing the thermal 
expansion and contraction of the housing. Examples of such packages are 
disclosed in Japanese laid-open Patent Publication (Kokai) Nos. Hei 
3(1991)-179420 and Hei 3(1991)-179421. 
However, while such conventional apparatus compensate the lens for changes 
in the focal length and refractive index of the lens when an ambient 
temperature changes, the ambient temperature change also causes the 
optical axis of the lens to deviate from the normal optical axis position 
if the lens and the lens support member are made from materials with 
different thermal expansion coefficients. Such a deviation in optical axis 
prevents images from being properly formed. 
This is particularly a problem in a multi-beam image forming apparatus in 
which optical beams from plural laser sources are converted into optical 
beams for image formation by using corresponding lenses so that the plural 
laser beams simultaneously scan the object in parallel. When the optical 
axis of each lens becomes offset from the axis of the light source, the 
scanned positions on the object tend to move close to or apart from each 
other, and normal image formation is thus prevented. 
SUMMARY OF THE INVENTION 
The object of the present invention is therefore to provide, with 
consideration for these problems, a light source package wherein the 
optical axis of the lens to the light source is not caused to deviate when 
the ambient temperature changes, and to provide an image forming apparatus 
using the light source package. 
To achieve the above object, a light source package according to the 
present invention comprises a light source, a lens for collimating the 
light from the light source, and a support member for supporting the light 
source and the lens so that the optical axis of the lens is set to a 
predetermined position relative to the light source, wherein the support 
member comprises a compensation device for compensating the lens for 
deviation of the optical axis of the lens resulting from a difference in 
the coefficients of linear thermal expansion between the lens and the 
support member when the ambient temperature changes. 
A further object of the invention is to provide an image forming apparatus 
comprising a plurality of optical devices for emitting light beams 
modulated by image signals, a photoconductor, a scanning device for 
reflecting the light beams emitted from the optical devices to scan the 
photoconductor, a developer for developing the latent electrostatic image 
formed by the light beams on the photoconductor, a transfer unit for 
transferring the developed image to an output medium, and a fixing unit 
for fixing the transferred image, wherein each of the optical devices is 
the light source package of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A laser diode can be used as the light source of the present invention. 
Examples of such laser diodes include the LT025MF0 laser diode 
manufactured by Sharp Corporation, the SDL-6030-01 from Sanyo Electric 
Co., Ltd., ML6412C from Mitsubishi Electric Corporation, HL7852G from 
Hitachi, Ltd., and LN9735PS from Matsushita Electronics Corporation. 
A plastic or glass lens can be used for the lens for collimating the light 
from the light source. Examples of glass lenses include the known BK7 and 
M-Lac130 lenses. 
The support member supports the light source and the lens so that the 
optical axis of the lens is positioned in a particular manner 
corresponding to the light source. The support member is typically made of 
brass or other metal. 
A compensation device mechanically absorbs the dimensional difference 
between the lens and the support member resulting from a change in 
temperature to compensate the lens for any deviation of the optical axis 
relative to the light source. To accomplish this, the compensation device 
comprises a lens holder, a lens barrel, and a pressing member. 
The lens holder has a truncated cone or pyramid shape (a cone or pyramid 
frustum shape) with a through-hole formed coaxially therein. The lens 
holder holds the lens in this through-hole and has substantially the same 
coefficient of linear thermal expansion as the lens. The lens barrel has a 
through-hole whose inside surface has a truncated cone or pyramid shape 
for holding the lens holder so that the lens holder is guided by the 
through-hole for being fitted onto the inside surface, as shown in FIG. 3. 
The pressing member is disposed in the lens barrel for elastically 
pressing the lens holder and the lens barrel on each other so that the 
lens holder closely contacts the conical or pyramidal inside surface of 
the lens barrel. 
Materials that can be used for a lens holder having substantially the same 
coefficient of linear thermal expansion as the lens include alumina 
ceramics (with a linear expansion coefficient of 6.7 to 
8.0.times.10.sup.-6 cm/cm/.degree.C) and zirconia ceramics (with a linear 
expansion coefficient of 4.4 to 4.6.times.10.sup.-6 cm/cm/.degree.C.). 
Preferably, the lens barrel is made of metal because of such considerations 
as processability and durability. For example, the lens barrel may be 
formed of brass by cutting. 
Materials that can be used for the pressing member for elastically pressing 
the lens holder and the lens barrel on each other include rubber O-rings, 
coil springs, and wave spring washers. 
FIG. 1 is a cross-sectional side view showing a multi-beam laser printer 
according to a preferred embodiment of the invention, and FIG. 2 is a top 
view of the optical system shown in FIG. 1. 
As shown in these figures, the laser printer comprises, inside an optical 
system housing 21, a pair of light source packages 1a and 1b, cylindrical 
lenses 2a and 2b, a polygonal mirror 3, a motor 3a for rotating the 
polygonal mirror 3 in the direction of arrow A at a predetermined speed, 
an f.multidot..theta. lens 4, a photoconductive drum 5, and a plane mirror 
6. 
The pair of light source packages 1a and 1b emit beams L1 and L2, which 
correspond to the image signals inputted to each light source. The 
cylindrical lenses 2a and 2b adjust the cross-sectional shape of the two 
beams L1 and L2 emitted from the light sources 1a and 1b, and the 
polygonal mirror 3 then reflects the beams L1 and L2 from the cylindrical 
lenses 2a and 2b off the six mirror surfaces of the polygonal mirror 3 for 
scanning. The f.multidot..theta. lens 4 compensates for distortion 
aberration of the beams reflected by the polygonal mirror 3, and the plane 
mirror 6 then reflects and focuses on the photoconductive drum 5 the beams 
passed through the f.multidot..theta. lens 4. 
The laser printer includes a corona discharger 7 for uniformly charging the 
surface of the photoconductive drum 5 in advance; a developing unit 19 for 
supplying a developer by means of developer roller 8 onto the surface of 
the photoconductive drum 5; a cassette 11 for storing recording sheets 10; 
a feeding roller 12 for feeding the recording sheet from the cassette 11; 
a pair of transport rollers 13 for transporting the recording sheet; a 
pair of resistance rollers 14 for intermittently transporting the 
recording sheet toward the photoconductive drum 5 at a predetermined 
timing; an image transfer corona discharger 9 for charging the recording 
sheet transported by the resistance rollers 14 to transfer the image 
developed on the photoconductive drum 5 onto the surface of the recording 
sheet; a pair of separation rollers 15 for separating the 
image-transferred recording sheet from the photoconductive drum 5; a pair 
of fixing rollers 17 for heating the recording sheet to fix the image; a 
pair of ejection rollers 18 for ejecting the recording sheet from the 
laser printer after fixing; a tray 23 for receiving the ejected recording 
sheet; and a cleaning unit 20 for cleaning the surface of the 
photoconductive drum 5 after the image is transferred to the sheet. 
It should be noted that the f.multidot..theta. lens 4 focuses the light 
beam, incident at an angle .theta. to the optical axis of the lens, onto 
the screen disposed away by the focal length f from the lens and at a 
position away by f.multidot..theta. from the optical axis. 
The overall operation of a laser printer thus constructed is now described 
below. 
When beams L1 and L2 corresponding to the image signals are emitted from 
the light source packages 1a and 1b (FIG. 2), the beams L1 and L2 are 
reflected by the rotating polygonal mirror 3 and focused at points P1 and 
P2 on the surface of the photoconductive drum 5 via the f.multidot..theta. 
lens 4 and the plane mirror 6. Rotation of the polygonal mirror 3 thus 
allows the beams L1 and L2 to scan the surface of the photoconductive drum 
5. 
The surface of the photoconductive drum 5 uniformly charged in advance by 
the charging corona discharger 7 and rotating in the direction of arrow B 
is scanned by the beams L1 and L2 to form a latent electrostatic image. 
Application of a developer by the developer roller 8 then makes the latent 
electrostatic image appear. 
The recording sheet 10 stored in the cassette 11 is transported by the 
supply roller 12 and then by transport rollers 13, and stops temporarily 
when the leading edge of the sheet reaches the resistance rollers 14. 
Operation of the resistance rollers 14 is timed to the progress of image 
development by the photoconductive drum 5 to transport the recording sheet 
10 under and in contact with the developing area of the photoconductive 
drum 5. 
The image transfer corona discharger 9 then discharges on the back side of 
the recording sheet 10 to transfer the developer which forms a positive 
image on the surface of the photoconductive drum 5 to the recording sheet 
10. Once the image is transferred, the recording sheet 10 is separated 
from the photoconductive drum 5 by the separation rollers 15 and 
transported to the fixing rollers 17. 
The recording sheet 10 on which the developer is fixed by heating by the 
fixing rollers 17 is ejected by the ejection rollers 18 to the tray 23 to 
complete the print cycle for one recording sheet. After image transfer is 
completed, the surface of the photoconductive drum 5 is cleaned by the 
cleaning unit 20 to prepare for the next print cycle. 
FIG. 3 is a detailed cross section of the light source package 1a shown in 
FIG. 2. Note that the other light source package 1b is constructed in a 
manner similar to the light source package 1a, and further description 
thereof is thus omitted below. 
Referring to FIG. 3, an inner lens barrel 32 is housed in an outer lens 
barrel 31 with the position fixed by a screw 33. A laser diode 34 is 
housed in the outer lens barrel 31 coaxially with the inner lens barrel 32 
and is secured by a tubular screw 42. 
A collimator lens 35 for collimating a light from the laser diode 34 is 
housed in a lens holder 36. As shown in FIG. 4, the lens holder 36 has a 
truncated cone shape or cone frustum shape with a through-hole 37 formed 
coaxially therein. The collimator lens 35 is housed inside the 
through-hole 37 and is fixed to the lens holder 36 by a quick-setting 
adhesive. 
The inner lens barrel 32 has a through-hole 39 whose inside surface has a 
truncated cone or cone frustum shape that is fitted onto the outside 
surface of the lens holder 36. The lens holder 36 is housed in the 
through-hole 39 of the inner lens barrel 32 with the vertex end of the 
lens holder 36 seated in the vertex end of the through-hole 39 as shown in 
FIG. 3. The lens holder 36 is fixed in close contact with the conical 
inside surface 38 of the through-hole 39 by means of elastic pressure 
applied thereto by a tubular screw 41 via a rubber O-ring 40. The rubber 
O-ring 40 is compressed by 10% to 20% at this time, and thus the lens 
holder 36 is urged in the direction of the laser diode 34 (the direction 
of arrow C) by elastic force of the rubber O-ring 40. 
The materials used for the component elements described above and the 
coefficients of linear thermal expansion (cm/cm/.degree.C.) of those 
materials are shown in the table below. 
______________________________________ 
collimator lens 35 
M-Lac130 6.9 .times. 10.sup.-6 
lens holder 36 alumina ceramic 
6.7 to 8.0 .times. 10.sup.-6 
outer lens barrel 31 
brass 18 to 23 .times. 10.sup.-6 
inner lens barrel 32 
brass 18 to 23 .times. 10.sup.-6 
threaded parts 33, 41, 42 
brass 18 to 23 .times. 10.sup.-6 
diode package 34 
brass 18 to 23 .times. 10.sup.-6 
______________________________________ 
FIG. 6 and FIG. 7 are side views of the tubular screws 41 and 42 which 
include grooves 41a and 42a in the end thereof, respectively. The grooves 
41a, 42a are used to tighten the tubular screws 41 and 42 using, for 
example, a screwdriver. 
In this configuration, when the ambient temperature rises, the outer lens 
barrel 31, the inner lens barrel 32, the screws 33, 41 and 42, and the 
laser diode package 34 all expand by the same amount in dimension because 
they all have substantially the same thermal expansion coefficient. The 
collimator lens 35 and the lens holder 36 similarly have substantially the 
same thermal expansion coefficient, and therefore also expand 
substantially by the same amount. 
The thermal expansion coefficient of the inner lens barrel 32, however, is 
greater than that of the lens holder 36. As a result, the inside diameter 
of the conical inside surface 38 of the inner lens barrel 32 increases by 
an amount greater than the outside diameter of the lens holder 36 held 
therein, and a gap or clearance thus develops therebetween. The pressure 
of the rubber O-ring 40 urging in the direction of arrow C, however, 
causes the lens holder 36 to move in the direction of arrow C, and thus 
the lens holder 36 remains in contact with the conical inside surface 38 
of the inner lens barrel 32. The optical axis of the collimator lens 35 
therefore remains correctly aligned with the light source 34 and does not 
deviate from the preset position. 
Conversely, when the ambient temperature drops, thermal contraction causes 
the conical inside surface 38 of the inner lens barrel 32 to compress the 
lens holder 36. This compressive force causes the lens holder 36 to move 
in the direction of arrow D, thereby compressing the rubber O-ring 40. The 
rubber O-ring 40 then absorbs the movement of the lens holder 36 caused by 
contraction of the conical inside surface 38 of the inner lens barrel 32. 
The optical axis of the collimator lens 35 thus remains correctly aligned 
and does not deviate from the preset position. The lens holder 36 and the 
collimator lens 35 are also not mechanically distorted or damaged. 
It should be noted that the lens holder 36 can be manufactured integrally 
with the collimator lens 35 and from the same material as the collimator 
lens 35, as shown in FIG. 5. A coil spring or a wave spring washer can 
also be used in place of the rubber O-ring 40. 
It is therefore possible by means of the light source package of the 
present invention to compensate, with good precision, for potential shift 
(deviation) of the optical axis of the lens relative to the light source 
caused by changes in the ambient temperature. Therefore, by using a 
plurality of such light source packages in a multi-beam image forming 
apparatus, it is possible to consistently assure normal image formation 
without being affected by temperature changes. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to those skilled in the art are intended to be included 
within the scope of the following claims.