EP0718720A1

EP0718720A1 - A printing system

Info

Publication number: EP0718720A1
Application number: EP95309223A
Authority: EP
Inventors: James M. Wilson
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1994-12-19
Filing date: 1995-12-19
Publication date: 1996-06-26
Also published as: JPH08318640A

Abstract

A printing system (40) in which an array of VCSELs as a light bar printhead (42) directly sends an array of light beams (44) onto a photoreceptor (46) without using an imaging optical element. The VCSELs are selected which have slowly diverging light beams. The photoreceptor is placed at a predetermined distance from the VCSELs where the light beam has a width equal to a desired spot corresponding to a given printing resolution.

Description

This invention relates to a printing system, and more particularly, to a printing system having a plurality of light emitting elements for emitting a plurality of light beams to a medium.
A light bar is an array of individual light emitting devices, such as, light emitting diode (LED) or electroluminescent (EL) edge emitters. For simplicity hereinafter, "light emitting devices" are called "light sources". Typically, a light bar array is utilized to produce an image on a photosensitive medium, such as, a xerographic photoreceptor used in a xerographic printer. In this kind of application, there is a need for a full width array of light sources, one per picture element or pixel, so that an array of light beams can be formed in such a manner that where they strike a photoreceptor, they generate a single line. Usually, this generated line on a photoreceptor of a scanning printing system is called a scan line. However, in this application, since the line is not scanned and each individual light source is responsible to generate one pixel of the line on the photoreceptor, hereinafter, "the generated line on the photoreceptor" will be called "line of pixels".
Each light source is individually addressed. Therefore, by applying a certain voltage selectively to the light sources, the light sources emit light beams to selectively discharge the photoreceptor in order to generate line-by-line a latent image on the moving photoreceptor.
Conventional light bar printing systems require imaging optical elements to be positioned between the photosensitive medium and the light source array. Since the output beams of the light sources diverge very fast, there is a need to focus the light from the array sources onto the line of pixels on the surface of the photoreceptor by the imaging optical elements.
A conventional imaging optical element is a Selfoc lens array. A Selfoc lens array is an array of micro-lenses which will be placed between the light bar and the photoreceptor. Each micro-lens receives multiple light beams from multiple light sources and focuses each light beam from each light source onto one spot on the photoreceptor.
Referring to Figure 1, there is shown a tangential or the fast scan view of an optical printing system 10 which utilizes a Selfoc lens and referring to figure 2, there is shown a sagittal or cross-scan view of the optical printing system 10. Referring to both Figures 1 and 2, a light bar 12 emits a plurality of light beams 14. A Selfoc lens 16, focuses each individual light beam onto an individual spot on the photoreceptor 18.
Typically, a Selfoc lens exhibits chromatic aberration problems which surface when used with a broad band emitter such as a EL edge emitter. In addition, a Selfoc lens is a significant contributor to output non-uniformity, short depth of focus, pixel placement errors and generally poor image quality.
Non-uniformity is caused by the fact that each micro-lens of a Selfoc lens array is an individual optical element and due to the manufacturing tolerances, each lens transmits the light beam in a different manner. Therefore, the light beam exiting each lens can have a different intensity causing an intensity non-uniformity over a line of pixels or it can be slightly deflected from the intended path causing a pixel placement error.
Also, due to the limitations and tolerances of the micro-lenses, the depth of focus of a Selfoc lens is very small. Depth of focus is the tolerance in which either the light source, the Selfoc lens or photoreceptor can have a positional error with respect to the other two without losing the focus. In other words, depth of focus is the tolerance of the spot size (i.e spot size + 10%) to the positional errors of the optical elements. It is desirable to improve the depth of focus in order to maintain the focus on the photoreceptor while having positional errors between the optical elements.
Considering the aforementioned problems, it is an object of this invention to eliminate the imaging optical element (typically a Selfoc lens).
In one aspect of the invention, there is provided a printing system which places a VCSEL light bar print head and a photoreceptor at the proximity of each other at a certain distance in order to generate a desired spot size corresponding to a given printing resolution. By shining an array of light beams from the light bar directly onto the photoreceptor, the imaging optical element (typically a Selfoc lens) which is required for conventional line printing systems and is the main source for various problems of a line printing systems can be eliminated.
In another aspect of the invention, there is provided a printing system comprising: a light source having a plurality of light emitting elements for emitting a plurality of light beams to a medium characterised in that said light source is located at a given distance from said medium in such a manner that each unaltered light beam strikes said medium at a spot size equal to the size of a desired spot corresponding to a given printing resolution.
The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a fast scan view of a printing system which utilizes a Selfoc lens to image the light beams of a light bar onto a photoreceptor;
Figure 2 shows a cross scan view of a printing system which utilizes a Selfoc lens to image the light beams of a light bar onto a photoreceptor;
Figure 3 shows a light beam being emitted from a small sized VCSEL;
Figure 4 shows a light beam being emitted from a large sized VCSEL;
Figure 5 shows a photoreceptor being placed at a certain distance from a large sized VCSEL in order to receive a required spot sized;
Figure 6 shows a fast scan view of a printing system of this invention;
Figure 7 shows a cross scan view of a printing system of this invention;
Figure 8 shows a chart from which the size of the required VCSEL, the distance that the photoreceptor should be placed from the VCSELs, and the depth of focus can be determined; and
Figure 9 shows the arrangement of the VCSELs in the preferred embodiment of this invention.

In order to comprehend the enclosed embodiment of this invention, it is necessary to study the characteristics of different size VCSELs. Small size VCSELs emit single mode light beams for any given input current applied to the VCSELs. A single mode light beam is a light beam with a Gaussian distribution. However, large size VCSELs emit single mode light beams for currents below a given current applied to the VCSELs, and if the input current to the VCSELs is increased above the given current, they will start showing a problem known as multi-mode. Multi-mode is when a light beam loses its circular shape or it generates multiple spots or in general loses its Gaussian distribution and generates a non-Gaussian distribution.
Referring to Figure 3, a small sized VCSEL 20 generates a fast diverging light beam 22. In comparison, referring to Figure 4, a larger VCSEL 24 generates a light beam 26 which diverges very slowly.
It is a common practice to use small size VCSELs in order to avoid the multi-mode problem. Contrary to the common practice, the enclosed embodiment of this invention utilizes large VCSELs. In spite of the fact that large VCSELs have a multi-mode problem at high output powers, they are quite stable and produce a single mode light beam at low output powers. Therefore, this invention utilizes large size VCSELs which will be operated at low output powers. In order to keep the output power of the VCSELs low, the VCSELs will be operated at a current above their threshold current and below the current at which large diodes start entering into multi-mode. Threshold current is a current at which a VCSEL changes from non lasing emission to lasing emission.
It should be noted that in spite of the low output power of the VCSELs of this invention, the output power of each VCSEL is sufficient to discharge a pixel on the photoreceptor.
It should also be noted that another characteristic of the large VCSELs which produce slowly diverging light beam is that each VCSEL produces a light beam in which the Full Width of the light beam at Half of its Maximum intensity (FWHM) at the light source is not less than 2.5 micron in any direction on a plane which is generally perpendicular to the axis of the light beam.
Referring to Figure 4, since the angle of divergence of the light beam emitted from a large VCSEL is very small, the width of the light beam gradually increases. As a result, for any desired spot size corresponding to a given printing resolution, the gradually increasing width of the light beam, at a certain distance from the VCSEL, will have a width equal to that desired spot size. For example, if the desired spot size on the photoreceptor is 'a', at distance 28 from the VCSEL 24, the width of the light beam will be equal to the spot size 'a'. Therefore, referring to Figure 5, if a photoreceptor 30 is placed at distance 28, the light beam 26 will generate a spot 'S' with a spot size 'a' on the photoreceptor 30. Thus, there will be no need for a selfoc lens.
In comparison, since the light beam from a small sized VCSEL diverges fast, a location at which the width of the light beam is equal to the desired spot size will be undesirably close to the photoreceptor which renders the use of small sized VCSELs impractical. In addition, the depth of focus of small sized VCSELs will be extremely small since a small movement along the path of the light beam changes the width of the light beam by a great magnitude. The extremely small depth of focus is another contributor to the impracticality of the small sized VCSELs.
However, since the light beam from a large sized VCSEL diverges slowly, a width equal to the desired spot size can be easily found. Also, since the light beam is slowly diverging, a small movement along the path of the light beam does not change the width of the light beam by far. Therefore, large sized VCSELs provide a better depth of focus.
Referring to Figures 6 and 7, there are shown a tangential or fast scan view (Figure 6) and a sagittal or cross scan view (Figure 7) of the printing system 40 of this invention. In the printing system 40, a VCSEL array light bar 42 is utilized to image an array of light beams 44 onto a photoreceptor 46 without using an imaging optical element.
By eliminating the Selfoc lens, the chromatic aberration problems, the output non-uniformity, pixel placement errors will be eliminated and the depth of focus will be greatly improved.
Referring to Figure 8, there is shown a chart from which, depending on the requirements of the printing system, the size of a VCSEL, the distance that the photoreceptor should be placed from the VCSELs, and the depth of focus can be determined. The vertical axis represents the size of the VCSEL (laser waist 1/e² diameter) and the horizontal axis represents the required distance between the VCSEL and the photoreceptor.
For example, if the printing system is a 600 dots per inch (2.54 cm/in) system, then the curve shown by 600 DPI will be used to determine the distance between the VCSELs and the photoreceptor or the size of the VCSELs. If the VCSEL size is selected to be 44 µm, then the distance between the VCSELs and the photoreceptor can be determined by drawing a horizontal line K from point 44 on the vertical axis to cross the 600 DPI curve at point 'b'. The distance from point b to the vertical axis determines the required distance between the VCSELs and the photoreceptor. In this example the distance from the VCSELs to the photoreceptor is equal to 3.07 mm (0.121 inch). The depth of focus can also be determined by measuring the distance between point c and point d where the line K crosses curve N and curve M respectively.
Curve N is the preceding curve and curve M is the succeeding curve to curve 600 DPI. In this example the depth of focus is 3.6 mm (0.142) - 2.62 mm (0.103) = 0.99 mm (0.039 inch).
Alternatively, in a 600 DPI printing system, if the VCSEL size is selected to be 65 µm, a horizontal line K' from point 77 on the vertical axis drawn to cross curve 600 DPI at point b' determines the distance from the VCSELs to the photoreceptor which in this example is equal to 2.54 mm (0.1 inch). As it can be observed, if the VCSEL size is selected to be 65 µm, the depth of focus (the distance between points c' and d') will be equal to 3.73 mm (0.147) - 0.91 mm (0.036) = 2.82 mm (0.111 inch) which is larger than the depth of focus for the 44 µm VCSELs.
In this invention, depending on the requirements of the printing systems, the VCSEL size can be selected in such a manner to achieve a certain depth of focus or a certain distance between the VCSELs and the photoreceptor. In addition, the printing system of this invention provides an improved depth of focus. Referring back to the aforementioned examples, the depth of focus for a 44 µm VCSEL is 0.99 mm (0.039 in.) and the depth of focus for a 65 µm VCSEL is 2.82 mm (0.111 inch). However, in a 600 dot per inch (DPI) printing system with a Selfoc lens, the depth of focus is in the range of 0.41 mm (0.016 inch). Therefore, in this invention, not only the depth of focus can be modified by selecting a different size VCSEL but also the depth of focus is improved.
It should be noted that the chart shown in Figure 8 is based on a a VCSEL emitting a light beam with a 657 nm wavelength. For VCSELs with different wavelengths, different charts should be used.
It should also be noted that the printing system of this invention is more suitable for high resolution printing systems which require smaller spot sizes. The maximum desired spot size is at a printing resolution of 300 dot per inch (2.54 cm.in).
Referring to Figure 9, there is shown the arrangement 50 of the VCSELs in the preferred embodiment of this invention. In the preferred embodiment of this invention, for the purpose of improving the VCSEL density, the VCSELS are staggered onto three rows R₁, R₂ and R₃. In high resolution printing systems due to a higher number of pixels, a higher number of VCSELs are needed. However, VCSELs can not be placed too close to each other. Therefore, in order to have a high density of VCSELs in a limited space, the VCSELs can be staggered as shown in Figure 9. The exposures from VCSELs in multiple rows are aligned in the tangential direction on the photoreceptor by delaying the emission of the light beam of the successive rows R₂ and R₃ relative to the first row R₁ until the photoreceptor has moved sufficiently for the pixel line to be exposed to the light beams from the rows R₂ and R₃ respectively.
It should be noted that different variation of VCSEL arrangement can replace the VCSEL arrangement of this invention. For example, the VCSELs can be arranged to be all on one line or they can be arranged to form a staggered matrix.
This invention utilizes vertical cavity surface emitting laser (VCSEL) array for a light bar print head, in order to eliminate the need for an imaging optical element (typically a Selfoc lens).
It should also be noted that the VCSEL light bar of this invention can be replaced by any light bar which has a slowly diverging light beam. This type of light bar will have a characteristic which will produce a light beam in which the Full Width of each light beam at Half of its Maximum intensity (FWHM) at the light source is not less than 2.5 µm in any direction on a plane which is generally perpendicular to the axis of the light beam.

Claims

A printing system (40) comprising:
a light source (42) having a plurality of light emitting elements for emitting a plurality of light beams (44) to a medium (46),

characterised in that said light source is located at a given distance (28) from said medium in such a manner that each unaltered light beam strikes said medium at a spot size (a) equal to the size of a desired spot corresponding to a given printing resolution.
The printing system as claimed in claim 1, wherein the maximum desired spot size is at a printing resolution of 762 dots per cm (300 dots per inch).
The printing system as claimed in claims 1 or 2, wherein said plurality of light emitting elements is a vertical cavity surface emitting laser (VCSEL) array.
The printing system as claimed in claim 3, wherein said vertical cavity surface emitting laser array emit light beams having Gaussian distribution when current applied to said light emitting elements is less than a given current and emit light beams having non-Gaussian distribution when current applied to said light emitting elements is greater than the given current, said plurality of light emitting elements receiving a current below said given current.
The printing system as claimed in any preceding claim, wherein said plurality of light emitting elements are light sources which produce light beams in which the Full Width of each light beam at Half of its Maximum intensity at the light source is greater than 2.5 µm in any direction on a plane which is generally perpendicular to the axis of the light beam.