The present invention generally relates to light emitting elements, image sensors, and light receiving elements. More particularly, the present invention relates to a light emitting element, an image sensor, and a light receiving element which are suited for use in an image scanner of a facsimile machine, a copying machine and the like.
In facsimile machines, there is an increasing demand to clearly transmit fine drawings, photographs and the like. A primary factor which determines the picture quality is the resolution of the image scanner. By use of an image scanner having a high resolution, it becomes possible to transmit fine drawings, photographs and the like with a high picture quality.
Normally, in the case of the facsimile machine, a document is read by a sensor array which includes one-dimensionally arranged photoelectric conversion elements. The resolution of the sensor array increases as each sensor element confronts a smaller region of the document surface. However, as the sensor element confronts a smaller region of the document surface, a light receiving area of the sensor element consequently becomes smaller. As a result, a quantity of light received by the sensor element decreases and the signal-to-noise (S/N) ratio becomes poor.
In order to eliminate the above described problems, it is possible to consider reducing the electrical noise, improving the light sensitivity of the sensor itself and the like. However, it is most effective to improve the luminance of the light which illuminates the document surface by a light source.
A xenon lamp, a light emitting element (LED) array or the like is used as the light source for the sensor of the facsimile machine. However, these light sources have poor light converging characteristics. For example, in the case of an image scanner having a resolution of 16 lines/mm or 32 lines/mm, the region of the document surface which must be illuminated by the light from the light source is in a range of 30 to 50 .mu.m with respect to a scanning direction of the image scanner. However, the xenon lamp usually has a tube diameter of at least 1 mm, and it is difficult to converge the light to a width of 0.5 mm or less even by the use of a lens system. On the other hand, when the light from the LED array is excessively converged, there is a problem in that the light intensity become irregular because the LEDs are arranged discretely in the LED array.
When an electroluminescence (EL) element proposed in a Japanese Laid-Open Patent Application No.57-7087 is used as the light source, the width of the emitted light is determined by the thickness of a transparent substrate. But when the width of the emitted light is to be set to 30 .mu.m, for example, the transparent substrate must have an extremely small thickness of 30 .mu.m. When making an elongated light source, the substrate may warp due to a stress generated between the substrate and the EL element, and it is difficult to hold the substrate during a photolithograhy process and the like because the substrate is extremely thin. In addition, a metal reflection layer is used to trap the light. Unlike a total reflection, the reflectivity of the metal reflection layer is not 100%. But when the reflection is repeated a large number of times, the light quantity decreases and there is a problem in that the luminance of the light becomes small at an emitting edge for the light which is generated at a distant location from the emitting edge.
On the other hand, another EL element is proposed in Kun et al., "TFEL Edge Emitter Array for Optical Image Bar Applications", SID 86 DIGEST, pp.270-272. However, this EL element has an emitter layer which has a thickness in the order of 1 .mu.m and is too thin for use as the light source for the sensor. If the thickness of the light emitting layer were increased to 10 .mu.m, for example, a driving voltage of the EL element must be increased to a range of several kV to several tens of kV, and the reliability of the EL element becomes poor.
A Japanese Laid-Open Patent Application No.64-89280 proposes an EL element which is combined with a waveguide. According to this EL element, the light emitting layer is provided above or below the EL element. However, the emitted light cannot reach the substrate edge while satisfying the conditions for total reflection because the light emitting element is not provided within the waveguide, and the light intensity inevitably decreases.
On the other hand, a light emitting element having a cladding layer provided on both sides of the light emitting layer is proposed in a Japanese Laid-Open Patent Application No.1-109694. According to this structure, it is possible to prevent the light from leaking outside the light emitting layer. However, the luminance of the light emitted from the edge inevitably decreases due to the reabsorption of light within the light emitting layer.
As described above, it is essential that the sensor which reads the document has a high resolution in order to obtain a high picture quality. The resolution of the sensor may be affected by stray light existing between a light illuminating system and a light receiving system of the sensor.
The light which illuminates the document surface is originally reflected depending on the tone of the document and the reflected light reaches the light receiving element. However, a portion of the light emitted from the light source is reflected at an interface of a thin film which is interposed between the document and the light receiving element, and this portion of the light reaches the light receiving element directly. This portion of the light is referred to as the stray light.
FIG. 1 is a cross sectional view for explaining the stray light generated in the conventional sensor. FIG. 1 shows a light source 100, a document 101, a substrate 102, a photoelectric conversion element 103, stray light 104, and interfaces 105 of thin films.
When the stray light exists, the photocurrent of the light receiving element is no longer solely dependent on the tone of the document, and the resolution of the sensor deteriorates.
For example, Japanese Laid-Open Patent Applications No.61-100073 and No.58-106947 propose arrangements for preventing deterioration of the resolution due to the stray light, by providing an optical waveguide in the light illuminating system and the light receiving system. However, according to these proposals, there are problems in that the light source and the waveguide must be assembled, the production process is complex because of the need to form the light receiving element on the edge of the waveguide so as to cover the edge and the like. For these reasons, it is difficult to reduce both the size and production cost of the sensor.
Next, a description will be given of an edge receiving type image sensor (image reading element) which uses an optical waveguide in the light receiving system. Compared to the image sensors of the conventional facsimile machines using a reduction optical system or a full-size imaging system, the image sensor which uses the optical waveguide is advantageous in that a light illuminating system for illuminating the document and the light receiving system for receiving the light at the photoelectric conversion part can be isolated optically. For this reason, the image sensor which uses the optical waveguide is developed for applications where a high resolution is required, such as the case where fine graphics, drawings, photographs and the like are to be read.
In the edge receiving type image sensor, the light received from the edge of the optical waveguide reaches the photoelectric conversion part, and an incident angle of the light at the edge of the optical waveguide is uniquely determined depending on a ratio of refractive indexes of a core layer and a cladding layer. In other words, when the ratio of the refractive indexes of the core layer and the cladding layer of the optical waveguide is small, the incident angle of the light which is trapped within the optical waveguide and is transmitted to the photoelectric conversion part is small. On the other hand, when the ratio of the refractive indexes of the core layer and the cladding layer of the optical waveguide is large, the incident angle of the light which is transmitted to the photoelectric conversion part is large. Accordingly, in the edge receiving type image sensor, it cannot be determined unconditionally whether the ratio of the refractive indexes of the core layer and the cladding layer should be large or small. A more detailed description will now be given in conjunction with FIGS. 2A and 2B.
FIG. 2A shows an example of a case where a ratio of a refractive index n.sub.core of a core layer 211 and a refractive index n.sub.clad of a cladding layer 212 is small. In this case, the incident angle of the light is small as indicated by hatchings, and regions of a document 202 which are read by two mutually adjacent optical waveguide edges of an image sensor 201 do not overlap between the document 202 and the image sensor 201, that is, the edges of the optical waveguides to be more accurate. As a result, the resolution of the image sensor 201 is high, and a change in the resolution of the image sensor 201 with respect to a change in the distance between the document 202 and the sensor 201 is small. But on the other hand, the absolute quantity of the light input to the optical waveguide edges from the document 202 is small because of the small incident angle of the light. For this reason, a large photocurrent cannot be obtained, and the S/N ratio of the image sensor 1 is deteriorated thereby.
On the other hand, FIG. 2B shows an example of a case where the ratio of the refractive index n.sub.core of the core layer 211 and the refractive index n.sub.clad of the cladding layer 212 is large. In this case, the incident angle of the light is large as indicated by hatchings, and the absolute quantity of the light input to the optical waveguide edges from the document 202 is large. However, the regions of the document 202 which are read by two mutually adjacent optical waveguide edges of the image sensor 201 overlap between the document 202 and the image sensor 201. As a result, the resolution of the image sensor 201 is poor, and a change in the resolution of the image sensor 201 with respect to a change in the distance between the document 202 and the sensor 201 is large.
Therefore, according to the conventional image sensor, it is impossible to simultaneously obtain satisfactory resolution and operation stability and satisfactory light quantity which is input to the optical waveguide edges, where the operation stability refers to the stability with respect to the change in the distance between the document and the image sensor.