JP2000305036A - Condensing optical system and image recorder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the depth of focus deep with a stable spot diameter and to prevent light quantity from being lowered. SOLUTION: By using a high-output broad area type semiconductor laser (BLD), the shallowness of the depth of focus occurring because of the broadness of an active layer being the defect of the BLD is eliminated only by the relative positional relation of a condensing optical system 16. Thus, the depth of focus can be made deep by simple constitution (condensing optical system 16).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure optical system using a broad area type semiconductor laser and an image recording apparatus.

[0002]

2. Description of the Related Art A light source,
For example, a light beam output from a semiconductor laser is focused by a focusing optical system, and a recording medium (for example,
A recording medium attached to the peripheral surface of a drum rotating at high speed) is arranged, and the drum is rotated (main scanning) while scanning (sub-scanning) the light beam in the axial direction of the drum. An image recording (exposure) device for recording (exposing) an image thereon is known.

As described above, when a semiconductor laser is used as a light source, a broad area type semiconductor laser is preferable in consideration of high output, and a desired light output intensity can be obtained.

However, in this broad area type semiconductor laser, the horizontal direction of the active layer is incoherent,
And it is relatively wide.

Therefore, in the above-described image recording apparatus, if the spot diameter is reduced to obtain high resolution, the depth of focus becomes shallow. Slight positional fluctuations when the depth of focus is shallow (fluctuations in the relative position between the exposure head and the drum)
In this case, the spot diameter on the recording medium is increased, and the image quality is reduced.

The present invention has been made in view of the above-described circumstances, and has as its object to provide a light-converging optical system and an image recording apparatus capable of increasing the depth of focus with a stable spot diameter and preventing a decrease in the amount of light. .

[0007]

According to a first aspect of the present invention, a broad area type semiconductor laser is disposed at a focal position on one side of a collimating lens, and light output from the semiconductor laser is converted by the collimating lens. In the case of forming a far-field pattern image by forming a far-field pattern at a position corresponding to the focal length on the other side of the collimating lens with an imaging lens to form parallel light, the spot diameter of the far-field pattern image,
The relative position of the semiconductor laser, the collimating lens and the imaging lens is determined so that the spot diameter of the near-field pattern image formed between the far-field pattern image and the imaging lens coincides with each other, I'm sorry.

According to the first aspect of the invention, the near field pattern image and the far field pattern are based on the relationship between the near field pattern and the far field pattern, the characteristics of the near field pattern image, and the characteristics of the far field pattern image. Determine the conditions under which the spot diameters of the images match.

The above condition is that the distance from the far field pattern position to the imaging lens can be obtained from the near field pattern diameter and the emission angle of the semiconductor laser, and the focal length of the collimating lens and the imaging lens. The relative positions of the semiconductor laser, the collimating lens, and the imaging lens can be uniquely determined.

By arranging the components at the determined relative positions in this manner, the pot diameters of the near-field pattern image and the far-field pattern image can be matched, and the depth of focus can be increased.

According to a second aspect of the present invention, in the first aspect of the present invention, an aperture is provided at the far field pattern position to block light around the light beam.

According to the second aspect of the present invention, by arranging the aperture at the far field pattern position, unnecessary peripheral light when forming a far field pattern image can be cut off. In the intensity distribution of the field pattern image, a sharp rising characteristic can be obtained.

According to a third aspect of the present invention, a light beam output from an exposure head is guided to a recording medium, and the light beam is moved relative to the recording medium by a relative movement between the light beam and the recording medium. An image recording apparatus for simultaneously performing main scanning with a plurality of light beams when an image is recorded by performing main scanning and sub-scanning, wherein the exposure head is a broad area type semiconductor laser; A collimating lens disposed at a focal position on the side and configured to collimate the light output from the semiconductor laser, and an imaging lens configured to image the parallel light output from the collimating lens. The spot diameter of the far-field pattern image formed by the lens, and the near-field pattern image formed between the far-field pattern image and the imaging lens So that the spot diameters of
The semiconductor laser, the collimating lens and the imaging lens are arranged at predetermined positions.

According to the invention described in claim 3, according to claim 1 of the present invention,
Alternatively, by applying the condensing optical system of claim 2 as an exposure head of an image recording apparatus, the exposure head and the recording medium relatively move to perform main scanning and sub-scanning. The deviation of the distance can be absorbed by the depth of focus, and regardless of the mechanical operation error of the device,
Appropriate image quality can be maintained.

In the case where the light-collecting optical system is applied to this image recording apparatus, the image from the near-field pattern image to the far-field pattern image is formed by another imaging lens to obtain a desired spot diameter. Good.

[0016]

1 and 2 show a laser recording apparatus 10 as an embodiment of an image recording apparatus according to the present invention.

The laser recording apparatus 10 irradiates a recording film F (recording medium) adhered on a drum 14 with a laser beam L output from an exposure head 12 to
It has a structure for recording an area-modulated image.

A two-dimensional image is formed on the recording film F by rotating the drum 14 in the arrow X direction (main scanning direction) and moving the exposure head 12 in the arrow Y direction (sub scanning direction). It is formed. The area-modulated image is obtained by controlling the on / off of the laser beam L to obtain the recording film F.
This is an image in which a plurality of pixels are formed thereon, and a predetermined gradation can be obtained depending on the area occupied by the pixels.

The exposure head 12 includes a semiconductor laser LD that outputs a laser beam L, and a condensing optical system that forms images of a near-field pattern and a far-field pattern of light output from the semiconductor laser LD on a recording film F. 16 are provided.

The semiconductor laser LD comprises, for example, a gain guide type semiconductor laser. Basically, as shown in FIG. 3, an active layer 22 is provided between a p-type semiconductor substrate 18 and an n-type semiconductor substrate 20. The laser beam L is outputted from the active layer 22 by applying a predetermined voltage between the electrodes 24 and 26 provided on the semiconductor substrates 18 and 20.

In this case, the width of the one electrode 24 is regulated, and the light emitting surface in the direction along the active layer 22 is controlled in accordance with the width. Therefore, the semiconductor laser LD
The light emission pattern of the laser beam L output from FIG.
As shown in FIG. 7, the width of the active layer 22 corresponding to the width of the electrode 24 is wide and substantially square in the direction of the bonding surface. Also,
In the thickness direction of the active layer 22, the width becomes a narrow shape corresponding to the thickness.

As shown also in FIG.
Is an optical system for forming images of a near-field pattern and a far-field pattern of the laser beam L output from the semiconductor laser LD on the recording film F. The collimator lens 28, the aperture 30, and the first Imaging lens 32 and a second imaging lens 34 for imaging a predetermined spot diameter on the film F
Are arranged in order.

Here, the condensing optical system (collimator lens 28, aperture 30, first
The relative positional relationship of the imaging lens 32) is uniquely determined by an arithmetic expression of a predetermined system (described in detail later).

That is, the range from the near-field pattern image formed by the first imaging lens 32 to the far-field pattern image (preferably at the center position) is set on the film F. During this time,
Since the spot diameter of the laser beam is substantially constant, for example, even if the laser beam is moved in a direction perpendicular to the axis (a direction including the element of the laser beam optical axis in a vector) due to vibration of the drum 14 or the like, if it is within the above range, , The spot diameter becomes constant. Therefore, the image can be prevented from being blurred or disappearing, and the image quality can be prevented from deteriorating.

In order to realize such a configuration, it is necessary to calculate based on a predetermined calculation formula such as the spread angle of the light output from the semiconductor laser LD. Table 1 shows the optical characteristics of the two semiconductor lasers applied in the present embodiment, and also shows the results in the conventional case, that is, when the image is formed only with the near-field pattern image.

[0026]

[Table 1]

In each of the lasers No. 1 and No. 2 in Table 1, the imaging spot diameter is 14 μm.
The spot diameter at a distance of μm is 19 for laser type No. 1.
μm, and reaches 25 μm for laser type No. 2.

That is, a shift of 10 μm in the optical axis direction is as follows.
This will have a significant effect on image quality.

On the other hand, in the present embodiment, as shown in FIG. 2, the arrangement (relative position) of the condensing optical system 16 is obtained by calculation.

In FIG. 2, when the relationship between the near field pattern and the far field pattern is first considered on a paraxial axis, it can be expressed by equations (1) and (2).

W ₁ = 2α ₁ f ₁ ... (1) θ ₁ = d ₁ / 2f ₁ ... (2) where W ₁ is the diameter of the far-feel pattern position (vertical direction) α ₁ : Spread angle (half angle) of semiconductor laser f ₁ : Focal length of collimating lens 28 θ ₁ : Divergence angle (half angle) from far field pattern position d ₁ : Semiconductor laser active layer width (divergence angle from near field pattern position (half angle) Half-width).

The near-field pattern image can be expressed by the following equations (3) and (4).

D ₂ = (f ₂ / f ₂ ) d ₂ (3) α ₂ = (f ₂ / f ₂ ) α ₂ (4) where d _{2 is the} near field pattern image Size f ₂ : focal length of the first imaging lens 32

Further, the fur-feel pattern image can be expressed by a simple paraxial lens formula when the magnification is m, as shown in (5)
Expressions (8) to (8) can be used.

W ₂ = m × W ₂ (5) m = f ₂ / (af ₂ ) (6) θ ₂ = (1 / m) θ ₁ (7) b = (1 + m) f ₂ (8) where, W ₂ : the size of the far field pattern image θ ₂ : the divergence angle (half angle) of the far field pattern image θ ₁ : the divergence angle (half angle) at the far field pattern position ).

Based on the above equations (1) to (8), the condition (d ₂ = W ₂ ) that the size of the near-field pattern image matches the size of the far-field pattern image is as follows:
From the expressions (3) and (5), (f ₂ / f ₁ ) d ₁ = m × W ₁ (9) Also, from the expression (1), the expression (9) is expressed as follows: m × W ₁ = _{m × 2α 1 f 1 ··· (} 10) Therefore, the (9) from the equation and the equation _{(10), m = (f 2 / f} 1 2) (d 1 / 2α 1) ··· (11) be able to.

When the distance a from the far field pattern position (position of the aperture 30) shown in FIG. 2 to the first imaging lens 32 is obtained from the equation (11), a = a
Since b / m, a can be expressed by the equation (12).

A = f ₂ × (1 + m) / m = (1 + 1 / m) × f ₂ (12) From the equation (12) and the equation (11), a = f ₂ + f ₁ ² × (2α ₁ / d ₁ ) (13) Since the focal length f1 of the collimator lens 28 and the focal length f2 of the first imaging lens are known, all the condensing optical systems 16 Is determined.

The near-field pattern image:
The distance from the far field pattern image can be represented by m × f ₂ as shown in FIG. Therefore, (1
From the equation (1), m × f ₂ = (f ₂ / f ₁ ² ) (d ₁ / 2α ₁ ) × f ₂ = (f ₂ / f ₁ ) ² × (d ₁ / 2α ₁ ) (14) Hereinafter, the operation of the first embodiment will be described.

The laser beam L modulated according to the image information and output from the active layer 22 of the semiconductor laser LD is converted into a parallel light beam by the
A near-field pattern image and a far-field pattern image are formed by the imaging lens 32. The near-field pattern image and the far-field pattern image have a predetermined spot diameter by the second imaging lens 34 and are formed on the film F.

The focusing position of the second imaging lens 34 is
A near-field pattern image and a far-field pattern image are formed as in the case of the first imaging lens 32, and the film F wound around the drum 14 is positioned at an intermediate position between them. An image is recorded based on the image information with the spot diameter. At this time, the drum 14 rotates in the main scanning direction (X direction in FIG. 1),
As the exposure head 12 moves in the sub-scanning direction (Y direction in FIG. 1), an image is formed on a predetermined image area of the film F.

In this embodiment, the positions of the semiconductor laser LD, the collimating lens 28, the aperture 30, and the first imaging lens 32 are set by a predetermined arithmetic expression, and the near-field pattern image shown in FIG. Size d2
== the size of the far field pattern image W2, so that the spot diameter during this period is continuously constant. For this reason, for example, a blur due to the eccentric rotation of the drum 14, a shift in the parallelism between the moving direction of the exposure head 12 and the rotation axis of the drum 14, and a shift in the image forming position in the optical axis direction are caused. However, if this deviation is within the range of the distance between the near-field pattern image and the far-field pattern image, the spot diameter is constant, and there is no effect on image recording.

That is, even if there is some mechanical error, it is possible to optically compensate for the deterioration of the image quality caused by this error.

Table 2 below shows two examples (No. 1-A, N) of the semiconductor laser of laser type No. 1 applied in Table 1.
o.1-B) shows a change in spot diameter when the condensing optical system 16 is arranged based on the arithmetic expression described in the present embodiment. Note that Table 2 also lists actual numerical values (including symbols and units) of each variable of the condensing optical system 16 in FIG.

[0045]

[Table 2]

In Table 2, NFP means a near field pattern, and FFP means a far field pattern (the same applies hereinafter).

As shown in Table 2, the size of the NFP image coincides with the size of the FFP image at 100 μm in any of the semiconductor lasers, and the position of the condensing optical system 16 achieves the purpose. ing.

After that, the spot diameter may be set to a desired value, and this is carried by the second imaging lens. As a result, the spot diameter was 14 μm, and the laser type No. No. 1 has a depth of focus of 28 μm and laser type No. In case 2, the depth of focus is 50μm
And a relative displacement with respect to the actual focus position with respect to the film F within the range of this dimension can be allowed.

As described above, in the present embodiment, a high-output broad-area semiconductor laser (BLD) is used, and the depth of focus, which is a drawback of the BLD due to the wide active layer, is condensed to a shallow depth. This can be solved only by the relative positional relationship of the optical system 16, and the depth of focus can be increased with a simple configuration (light collecting optical system 16).

[0050]

As described above, the condensing optical system and the image recording apparatus according to the present invention have an excellent effect that the depth of focus can be deepened with a stable spot diameter and the light quantity can be prevented from lowering. Having.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an exposure head section of a laser recording apparatus according to an embodiment of the present invention.

FIG. 2 is a side view of the optical system of FIG.

FIG. 3 is a perspective view showing a structure of a broad area type semiconductor.

DESCRIPTION OF SYMBOLS 10 Laser recording device 12 Exposure head 14 Drum 16 Condensing optical system 28 Collimating lens 30 Aperture 32 First imaging lens F Recording film LD Semiconductor laser

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C362 AA03 AA48 CB66 2H045 AG09 BA02 CB24 2H052 BA02 BA06 BA13 5F073 AA03 AA61 AB27 BA04 BA09 EA18 EA19 EA20

Claims (3)

[Claims] 1. A broad area type semiconductor laser is arranged at a focal position on one side of a collimating lens, and light output from the semiconductor laser is made parallel by the collimating lens, and a focal length on the other side of the collimating lens. When a far-field pattern image is formed by forming a far-field pattern image by an imaging lens, a spot diameter of the far-field pattern image and a distance between the far-field pattern image and the imaging lens. A condensing optical system, wherein relative positions of the semiconductor laser, the collimating lens, and the imaging lens are determined so that spot diameters of the formed near-field pattern images match. 2. In the far field pattern position,
2. The light-converging optical system according to claim 1, wherein an aperture for shielding the periphery of the light beam is arranged. 3. A light beam output from an exposure head is guided to a recording medium, and the light beam is main-scanned and sub-scanned with respect to the recording medium by a relative movement between the light beam and the recording medium. An image recording apparatus for simultaneously performing main scanning with a plurality of light beams when recording an image, wherein the exposure head includes a broad area type semiconductor laser, and the semiconductor laser is disposed at a focal position on one side. A collimating lens that converts the light output from the semiconductor laser into parallel light; and an imaging lens that forms an image of the parallel light output from the collimating lens; and a far field formed by the imaging lens. The spot diameter of the pattern image matches the spot diameter of the near-field pattern image formed between the far-field pattern image and the imaging lens. As the semiconductor laser, a collimator lens and the imaging lens is disposed at a predetermined position, that the image recording apparatus characterized.