JPH09305692A - Optical element for laser beam generation and bar code reader using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To read the bar code with minimum line width over wide range by expressing a curved line, which expresses the ridge line of a cross section including an optical axis while being rotationally symmetrical to the optical axis, as a non-linear function which can not be differentiated on the optical axis, increasing the inclination at the part of the ridge line closer to the optical axis and providing an optical element having a little inclination of the ridge line away from the optical axis. SOLUTION: The curved line of ridge lines D' and D" of the cross section cut by a plane including an optical axis 4 while being rotationally symmetrical to the optical axis 4 uses an optical element 3 for laser beam generation in the form expressed by the non-linear function which can not be differentiated on the optical axis 4. The light to be made incident to this optical element 3 for laser beam generation is divided to the optical axis 3, two light flux C' and C" emitted from the element 3 is mutually interferred and light intensity is gradually attenuated. Therefore, this element 3 generates the long focus laser beam similar to a Bessel beam. Besides, the inclination of ridge lines D' and D" of the element 3 closer to the optical axis 4 is increased but the inclination away from the optical axis 4 is decreased and a condensed light spot diameter is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam having a relatively small focused spot diameter and a deeper focal depth than a general Gaussian beam as a laser beam having the same focused spot diameter (hereinafter referred to as a long-focus laser beam). The present invention relates to a laser beam generating optical element capable of generating a laser beam and a bar code reading apparatus using the laser beam generating optical element.

[0002]

2. Description of the Related Art Conventionally, in an optical system using a laser beam, a so-called Gaussian beam has been used, which has a Gaussian intensity distribution in the radial direction in a plane perpendicular to the optical axis. When a Gaussian beam is focused using a normal focusing optical system, the distance Δ from the position where the focused spot diameter ω is the minimum to the position where the beam diameter is K times is expressed by the following equation (1).

Δ = ± (π / λ) ω ₀ ² √ (K ² -1) (1) where ω ₀ : 1 / e ² of the peak intensity when the focused spot diameter is the minimum Radius of Beam at Position λ: Wavelength From the above formula (1), it is understood that the distance Δ is almost proportional to K.

Here, the bar code having the minimum line width that can be read by the bar code reader must be wider than the spot diameter ω of the laser beam (Gaussian beam) at a certain position of the bar code. The range in which the bar code can be read is proportional to Δ in the equation (1). Due to such restrictions, the minimum line width and reading depth of the barcode that can be read by the barcode reading device cannot be freely set.

As a conventional example of a bar code reader using a Gaussian beam condensing optical system, a bar code reader under the trade name MS-710 sold by "Matsushita Intertechno Co., Ltd." (hereinafter simply referred to as MS- 710) is known.

FIG. 8 shows the relationship between the minimum line width [mm] and the reading depth [mm] of a bar code that can be read by a bar code reading device in a logarithmic log graph. The case of MS-710 is shown, the straight line connecting the plots shows the case of a bar code reader using a Bessel beam described later, and the plot + shows the required specification value of the bar code reader.

As shown in FIG. 8, in MS-710, it can be seen that the relationship between the minimum line width of a readable barcode and the reading depth can be approximated by a straight line with a slope of 1. This reflects that the distance Δ shown in the above equation (1) is almost proportional to the first power of the magnification K of the spot diameter.

[0008] By the way, in a product to which a barcode is actually attached, there are cases where the surface is uneven and the product is placed at an angle in the depth direction with respect to the barcode reading device.
In such a case, it is often the case that the barcode cannot be read because the reading depth is insufficient.

Further, when a bar code with a narrow pitch is read by a device for reading a bar code with a narrow pitch, it is necessary to read at a distance farther than when a bar code with a narrow pitch is read. Since the depth is shallow and the reading depth is not sufficient, it is inconvenient because it must be read close to the product.

Further, if it is out of reach, it is impossible to read it. Therefore, it was necessary to separately prepare a bar code reading device having a deep reading depth which can read only a bar code having a coarse pitch. Therefore, there are problems that the equipment cost is increased and the equipment cannot be installed in a narrow space.

In order to solve such a problem, a combination of optical systems having a plurality of focal lengths is used, and even an optical system using a normal Gaussian beam has an optical depth of focus corresponding to the minimum line width of a bar code. There is also a device designed to be a system.

However, even in this case, an optical system having a sufficient depth of focus for all bar codes having the minimum line width has not yet been obtained. Also, the configuration of the optical system becomes extremely complicated,
The device becomes expensive and impractical.

Further, in the case of a Gaussian beam, since the beam spreads symmetrically on both sides of the position where the minimum spot diameter is obtained, the reading range can be set wider symmetrically on both sides of the position where the minimum spot diameter is obtained. However, a barcode with the smallest minimum line width is smaller in size as a whole, so it is more natural to read it closer. Therefore, only one side of the position where the minimum spot diameter is obtained can be used.

In order to solve these problems, as a laser beam having a very large depth of focus and a relatively small spot diameter, the electric field distribution in the radial direction perpendicular to the optical axis is
A Bessel beam (non-diffractive beam) having a 0-th order Bessel function of the first kind has been devised. The details of the Bessel beam are described in the following documents and the like.

(1) J. Durnin: Vol. 4, N
o. 4 / April 1987 / J. Opt. Soc.
Am. A, p651-p654 (2) Uehara Kiyoji: Applied Physics Vol. 59, No. 6 (1
990), p746 to p750 FIG. 11 shows a conventional example of a basic Bessel beam generating optical system. In this optical system, only a semiconductor laser element and an axicon prism are used as optical elements. Since this optical system is rotationally symmetric with respect to the optical axis, in the following description, the section including the optical axis will be described as a representative.

In FIG. 11, the spherical laser light A emitted from the semiconductor laser device 11 is converted into a plane wave by the collimator lens 12, and then enters the axicon 13 which is a conical lens. The axicon 13 has an optical axis 14
When viewed in a cross section cut in a plane including, a function representing a refracting surface, that is, a ridge is two straight lines with different signs of inclination,
They are connected on the optical axis and are continuous, but non-differentiable functions. When expressed as a shape, the optical element has a shape in which continuous points are sharp.

The traveling direction of light after being refracted at the ridgelines B ', B "of the surface having the effect of refracting light on the half plane defined by the optical axis 14 does not depend on the height from the optical axis 14. Therefore, the light after refraction is a plane wave and the optical axis 14
The two half planes partitioned by means different traveling directions.

That is, within this cross section, the axicon 1
As shown in FIG. 11, the laser beam emitted from No. 3 is refracted by the ridgeline B ′ in the half plane defined by the optical axis 14 and becomes a plane wave A ′ traveling in a direction intersecting the optical axis 14 at an angle θ. . Similarly, in the opposite half plane divided by the optical axis 14, it is refracted by the ridgeline B ″ and becomes a plane wave A ″ that travels in the direction intersecting with the optical axis at the angle θ as well. However,
The traveling direction is a direction that is axisymmetric to A ′ with respect to the optical axis 14.

An optical element equivalent to the axicon 13 can also be manufactured by using a hologram. That is, for example, as shown in FIG. 12A, as in the Fresnel lens, the thickness of the refracting surface is set so that the optical path difference is 1 / wavelength λ.
Every time it becomes 2, it should be returned to the reference plane. In this case, the hologram has a sawtooth-shaped cross section, but as a simpler hologram, the optical path difference is 1 / wavelength λ, as in a Fresnel strip.
With the radius of 2 as the period, the thickness is changed as shown in FIG. 12 (b), or the transmittance is inverted as shown in FIG. 12 (c).
It suffices to change the optical characteristics. FIG. 12 (b),
In the case of (c), it is clear that the hologram will be a collection of concentric circles.

FIG. 13 shows the shape of each of the holograms shown in FIGS. 12 (a), 12 (b) and 12 (c) as seen from the direction of the optical axis 14. As is clear from the figure, they are concentric circles with equal intervals.

In order to simplify the explanation, the collimating lens 12 is used in the above conventional example, and the collimated light is converted into the axicon 13.
Although it is incident on the
The function of the collimator lens 12 can be combined. That is, it becomes possible to directly convert the spherical wave into a Bessel beam. In this case, the hologram is a concentric circle with a varying period, similar to a Fresnel lens.

Further, an elliptical light beam like a semiconductor laser device can be directly converted into a Bessel beam. For the method, see R. P. MacDonald and others are APP
LIED OPTICS / Vol. 32, no. 32 /
10 November 1993, p6470-p6.
474.

The electric field distribution in the direction perpendicular to the optical axis 14 as seen in a cross section including the optical axis 14 of the long focus laser beam is as follows.
The plane waves A ′ and A ″ interfere with each other to form a Bessel beam proportional to the 0th-order Bessel function of the first kind.

FIG. 14 shows the intensity distribution of this basic Bessel beam in the direction perpendicular to the optical axis 14. Also,
FIG. 15 shows a change in the focused spot diameter viewed in the optical axis direction.
In the case of the Bessel beam, the focused spot diameter hardly changes in the optical axis direction as indicated by the curve J in the figure, and the optical axis is smaller than that of the curve I in the figure, which is a Gaussian beam having a similar spot size. It can be seen that the spot diameter hardly changes in the direction. That is, it can be seen from FIG. 15 that the depth of focus of the Bessel beam is much deeper than the depth of focus of the Gaussian beam.

[0025]

However, although the conventional Bessel beam has a sufficiently deep depth of focus as compared with a normal Gaussian beam, it is not sufficient as a bar code reading device for the following reasons. .

That is, as the performance required for the bar code reading device, in addition to the performance of reading a bar code having a specific minimum line width in the widest possible range, it is necessary to be able to read bar codes having various minimum line widths. Is. Further, it is required that the reading range be sufficiently deep for a thick barcode having a minimum line width.

Points indicated by plots 10 in FIG. 8 are required specification values of the minimum line width of the bar code and the corresponding reading depth. In the case of a simple optical system with only one focus, the relationship between the minimum line width of a bar code that can be read and the reading depth is a straight line with an inclination of 1 when using a Gaussian beam. There was a problem that I could not be satisfied with. Therefore, in practice,
It is used with a limited reading range.

Further, in order to realize a bar code reading apparatus capable of reading a plurality of bar codes having different minimum line widths according to required specifications, an optical system having a plurality of focal positions is used, but a large number of lenses are used. There is a problem that it becomes a complicated optical system.

On the other hand, when the conventional Bessel beam is used, the relationship between the minimum line width of the bar code and the reading range that can be read by a simple optical system using only the collimating lens 12 and the axicon 13 has an inclination in FIG. Since it corresponds to a straight line of 0, it is possible to satisfy the required specifications for the reading depth for a plurality of barcodes.

However, when the optical system is designed so that the bar code with the smallest minimum line width can be read as indicated by the straight line Y in FIG. 8, the reading depth for the bar code with the smallest minimum line width is insufficient, On the other hand, if the reading depth is made sufficiently wide, there is a problem that the bar code having the smallest minimum line width cannot be read like the straight line X in FIG.

The present invention has been made in view of the above circumstances, and is capable of reading a bar code having a specific minimum line width in a wide range with a simple optical system and a plurality of bars having different minimum line widths. It is an object of the present invention to provide a laser beam generating optical element that generates a laser beam that enables a code to be read according to required specifications, and a bar code reading device that uses the laser beam generating optical element.

[0032]

In the optical element for generating a laser beam of the present invention, a curve which is rotationally symmetric with respect to the optical axis and which represents a ridge line of a cross section taken along a plane including the optical axis is different from the optical axis. The optical element is represented by a possible non-linear function, and the inclination of the ridgeline near the optical axis is increased, while the inclination of the ridgeline away from the optical axis is decreased. The above object is achieved.

The laser beam generating optical element of the present invention is composed of a hologram that is optically equivalent to the optical element, thereby achieving the above object.

The laser beam generating optical element of the present invention comprises a hologram having both the function of the optical element and the function of a collimating lens for converting the light from the light source into parallel light, thereby achieving the above object. To be achieved.

A bar code reading apparatus of the present invention is a bar code reading apparatus including the laser beam generating optical element according to claim 1, wherein the inclination of the ridge of the laser beam generating optical element changes. The degree of change in the size of the focused spot diameter with respect to the distance from the laser beam generating optical element can be adjusted, and the relationship between the minimum readable barcode line width and the reading depth can be obtained. When the minimum line width of the readable barcode is plotted on the vertical axis and the reading depth is plotted on the horizontal axis in a logarithmic graph, the slope P satisfies the following condition: 0 <P <1 As a result, the above object is achieved.

A bar code reading device of the present invention is a bar code reading device equipped with the laser beam generating optical element according to claim 2, wherein the bar code has a minimum line width and a reading depth. Is expressed as a logarithmic graph with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis, the slope P satisfies 0 <P <1 is set so that the above-mentioned object is achieved.

A bar code reading device according to the present invention is a bar code reading device equipped with the laser beam generating optical element according to claim 3, wherein the bar code has a minimum line width and a reading depth. Is expressed as a logarithmic graph with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis, the slope P satisfies 0 <P <1 is set so that the above-mentioned object is achieved.

Further, the bar code reading apparatus of the present invention comprises a laser light source, a collimator lens for converting the light emitted from the laser light source into parallel light, and the laser beam generating optical element according to claim 1, which is readable. When the minimum line width of the bar code and the reading depth are expressed in a logarithmic graph with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis,
The inclination P is set to the inclination corresponding to the inclination of a straight line connecting a plurality of required specification values, and thereby the above object is achieved.

Further, the bar code reading apparatus of the present invention comprises a laser light source, a collimating lens for converting the light emitted from the laser light source into parallel light, and the hologram according to claim 2, and the minimum readable bar code. When the relationship between the line width and the reading depth is represented by a logarithmic graph with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis, the slope P has a plurality of required specifications. The inclination is set to correspond to the inclination of the straight line connecting the values, and thereby the above-mentioned object is achieved.

The bar code reader of the present invention comprises a laser light source, a collimator lens for converting the light emitted from the laser light source into parallel light, and the hologram according to claim 3, and the minimum readable bar code. When the relationship between the line width and the reading depth is represented by a logarithmic graph with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis, the slope P has a plurality of required specifications. The inclination is set to correspond to the inclination of the straight line connecting the values, and thereby the above-mentioned object is achieved.

The operation of the present invention will be described below with reference to FIGS. 1 and 8 to 10.

A curve that is rotationally symmetric with respect to the optical axis 4 and that represents the ridgelines D ′ and D ″ of the cross section cut along the plane including the optical axis 4 is represented by a non-differentiable non-linear function on the optical axis 4. When the laser beam generating optical element 3 is used, the incident light incident on the laser beam generating optical element 3 is split with respect to the optical axis 4 as shown in FIG. The two light beams C ′ and C ″ emitted from the element 3 interfere with each other, and the light intensity thereof is not easily attenuated. Therefore, the laser beam generating optical element 3 can generate a long focus laser beam similar to a Bessel beam.

In addition, in the laser beam generating optical element 3, the slopes of the ridgelines D ′ and D ″ of the laser beam generating optical element 3 near the optical axis 4 are increased, and conversely, from the optical axis 4. Since the slopes of the ridgelines D ′ and D ″ of the laser beam generating optical element 3 at the distance are made small, the slope of the laser beam generated near the optical axis 4 becomes large. As a result, the inclination of the laser beam generated near the laser beam generating optical element 3 also becomes large,
The focused spot diameter can be reduced. On the other hand, the inclination of the laser beam generated far from the optical axis 4 is small. As a result, the inclination of the laser beam generated at a position distant from the laser beam generating optical element 3 is also reduced, so that the focused spot diameter can be increased.

Therefore, the focused spot diameter and the focal depth can be adjusted by changing the degree of change in the inclination of the ridgelines D ', D "of the laser beam generating optical element 3. The laser beam generating optical element 3 can adjust the minimum line width and the reading depth of the readable barcode, that is, the relationship between the minimum line width of the readable barcode and the reading depth is a logarithm. When expressed graphically (see Figure 8), the slope is 0 to 1
Can be adjusted in the range.

Here, the slope P on the log-log graph is 1
When it is less than (= inclination in the case of Gaussian beam), the depth of focus can be made deeper than that of Gaussian beam, so that a bar code having a specific minimum line width can be read in as wide a range as possible. On the other hand, when the inclination P is set to be larger than 0 (= inclination in the case of Bessel beam), unlike the Bessel beam, it is possible to read a plurality of barcodes having the minimum line width. Further, it is possible to secure a sufficient reading range for a thick barcode having a minimum line width.

Further, according to the bar code reading apparatus using the laser beam generating optical element of the present invention, the laser beam generating optical element 3 is set so that the inclination P on the logarithmic graph is 0 <P <1. The inclinations of the ridgelines D ′ and D ″ of the are set.

Therefore, according to the present invention, a bar code having a specific minimum line width can be read in as wide a range as possible, and a plurality of bar codes having the minimum line width can be read, and further, the minimum line width can be read. It is possible to realize a bar code reading device that can ensure a sufficient reading range for thick bar codes.

Further, in the laser beam generating optical element of the present invention, the spot diameter becomes the smallest in the vicinity of the laser beam generating optical element 3, so that the bar having the smallest minimum line width can be obtained without sacrificing the reading range. The code can be read up close.

As can be seen from FIG. 8, in order to read the bar code having the minimum line width, the minimum focused spot diameter is 0.07 mm or less, and the maximum reading depth is 7000 m.
A straight line with m and a slope of 0.62 may be realized. That is, this straight line having a slope of 0.62 is a slope of a straight line connecting all required specification values (shown by plot + in the figure).

In order to satisfy this condition, the following may be done. That is, the case where a laser beam having a wavelength λmm is used as a light source will be described as an example. A laser beam generated at a position closest to the optical axis of the laser beam generating optical element 3 (a position where y = 0 in FIG. 1). Of the inclination θ (0) with respect to the optical axis of 0.766 × λ / 0.07ra
The minimum spot diameter is 0.07 mm by setting d or more.
The distance from the optical axis is as follows (y in FIG. 1 is y
= R), the inclination θ (R) with the optical axis of the laser beam generated is 0.766 × λ / 1.8 rad or more,
Moreover, if tan θ (R) <R / (1.2 × 7000) is satisfied, the laser beam generating optical elements 3 to 7 can be formed.
The spot diameter of the focused spot at the position of 000 mm is 1.
Since it can be set to 8 mm, the above conditions can be satisfied.

Here, the inclination θ (0) is 0.766 × λ /
If 0.07 rad or more, the minimum spot diameter is 0.0
The reason why it can be set to 7 mm or less is as follows. P. MacDonal
d etc. (Applied Optics vol32, p
6470 (1993)).

First, the long focus laser beam of the present invention is considered to be a collection of Bessel beams having different spot diameters.
Therefore, the spot diameter W of the Bessel beam formed by the light passing through the innermost circumference is determined as θ (0) so that the bar code with the smallest line width can be read.

Here, as shown in FIG. 9, the beam diameter of the Bessel beam is the value (radius) of the first zero point of J (βρ) of the 0th-order Bessel function, and is represented by the following equation (2).

W = 4.81 / β (2) However, the numerical value 4.81 is a value obtained by calculation, and the details are omitted.

On the other hand, the angle formed by the laser beam with the optical axis 4 is θ
Then, the following expression (3) is established.

Β = (2π / λ) · tan θ (3) From the expressions (2) and (3), the following expression (4) is established.

W = {4.81 / (2πtan θ)} · λ = 0.766λ / θ (4) However, tan θ≈θ From the equation (4), θ = 0.766λ / W (5) where , The spot diameter W of the laser beam as described above
Is a beam diameter equal to or smaller than the minimum line width of the bar code to be read, so W ≦ 0.07 rad
Therefore, if the inclination θ (0) is set to satisfy the condition of the following expression (6), the minimum spot diameter can be 0.07 mm or less.

Θ (0)> 0.766λ / 0.07 (6) Further, the inclination θ (R) may be set to 0.766 × λ / 1.8 rad or more for the following reason.

That is, the spot diameter W of the Bessel beam formed by the light passing through the outermost circumference may be equal to or smaller than the minimum line width of the thickest bar code having the minimum line width. Therefore, the inclination θ (R) may be set so as to satisfy the condition of the following expression (7).

Θ (R)> 0.766λ / 1.8 (7) Further, tan θ (R) is tan θ (R) <R / (1.2
The reason why x7000) may be set is as follows.

First, in the laser beam generating optical element 3 of FIG. 1, the closest point at which the focused spot is formed is
The positions of the vertices of the ridgelines D ′ and D ″ of the optical element 3 and the furthest positions are given by the following equation (8).

Zmax = tan θ (R) (8) Here, as shown in FIG. 10, since the light intensity of the Bessel beam fluctuates far from the laser beam generating optical element 3, it is applied to a bar code reader. If you do, it is necessary to devise to suppress the fluctuation. That is, FIG. 10A shows that the fluctuation of the light intensity is large at a position far from the laser beam generating optical element 3, and FIG. 10B shows a case where the fluctuation of the light intensity is excessively suppressed. (C) shows the case where the light intensity fluctuation is appropriately suppressed. As can be seen from FIG. 7C, the distance that can be used is reduced by about 20% when such a device is devised.

For this reason, since it is necessary to have a margin for the usable distance, in the present invention, a margin of about 20% is provided. Therefore, the practical maximum distance with a constant beam diameter is about Zmax / 1.2. Therefore, when setting the reading range to 7000 mm, Zmax≈
It may be 1.2 × 7000.

Therefore, from the above equation (8), tan θ
(R) may be set so as to satisfy the condition of the following expression (9).

Tan θ (R) <R / (1.2 × 7000) (9) As a method of optimizing the light intensity variation as shown in FIG. Flat 7-213
There is one previously proposed in No. 265.

In the present invention, instead of the laser beam generating optical element 3 as described above, it is possible to use a hologram optically equivalent thereto. When a hologram is used, an optical element can be produced by using a normal photolithography method, and thus mass production becomes possible.

It is also possible to use a hologram in which a collimator lens is integrated. In this case, since the number of parts of the optical system can be reduced, the configuration of the optical system can be further simplified.

[0068]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings.

(First Embodiment of Laser Beam Generating Optical Element) FIG. 1 shows a first embodiment of a laser beam generating optical element of the present invention together with a light source and a collimating lens. That is, a laser beam generator including this laser beam generating optical element is shown.

This laser beam generator comprises a semiconductor laser 1, a collimator lens 2 and a laser beam generating optical element 3. The laser beam C of the spherical wave emitted from the semiconductor laser 1 is converted into a plane wave by the collimator lens 2 and then enters the laser beam generating optical element 3.

Since this laser beam generating optical element 3 is rotationally symmetric with respect to the optical axis (Z axis) 4, it is limited to an arbitrary cross section viewed from the plane including the optical axis 4 for simplicity in the following. And explain.

The surface of the optical element 3 for generating a laser beam, which has a function of refracting light, is formed by two ridges D ′ and D ″ when viewed in a cross section taken along a plane including the optical axis 4.
The angle θ formed by the ridgeline D ′ with the optical axis 4 in the traveling direction of the light is different from the Bessel beam, and the height r from the optical axis 4 is r.
It changes with. In order to diverge outside the optical axis 4, θ becomes smaller as the distance from the optical axis 4 increases. To emphasize that θ is a function of r, it is written as θ (r) below.

In the first embodiment, θ (r) is a power function of r so that the spot diameter is increased according to the distance from the laser beam generating optical element 3. Due to the geometrical relationship shown in FIG. 1, θ (r) is represented by the following (10).

Θ (r) = θ ₀ −Δθ (11−r / R) ^Q (10) where R: outer diameter (radius) of the laser beam generating optical element 3 Δθ: divergence range It is a constant, and the light emitted from the divergence point E ′ (or the divergence point E ″) and refracted at the position of r = R and the light radiated from the divergence point E ′ (or the divergence point E ″) and r = 0 The angle formed by the light refracted at the position Q: A constant that determines the intensity of divergence The same formula (10) holds for the light refracted on the other edge D ″.

In order to simplify the description, the case of Q = 1 will be described below as an example. The functions f (r) and g (r) representing the above ridge lines D ′ and D ″ are respectively expressed by the following equation (11):
It is expressed by the equation (12) and is a function that cannot be differentiated on the optical axis 4.

[0076]

[Equation 1]

[0077]

[Equation 2]

With reference to FIG. 2 below, the above (11),
The process of deriving the equation (12) will be described. The following equation (13) is established by the geometrical relationship shown in FIG.

Θ ₁ = π−α, θ ₂ = θ ₁ + θ (13) This θ is θ (r) in the above equation (10).

Now, assume that the function of the curve corresponding to the ridge D'is expressed by the following equation (14).

Z = f (r) (14) Since the tangent line 5 of this curve is obtained by differentiating the function z = f (r), it is expressed by the following equation (15).

Z = f '(r) .r + m (15) where m is the intersection of the tangent 5 and the z axis.

Here, the slope f '(r) of the tangent 5 is as shown in FIG.
It is expressed by the following equation (16) according to the geometrical relationship shown in.

F ′ (r) = tan α (r) (16) Here, assuming that the refractive index is n, the following equation (17) is established according to Snell's law.

N sin θ ₁ = sin θ ₂ (17) Substituting the relationship of the above equation (13) into this equation (17),
The following expression (18) is established.

Nsin (π−α) = sin (π−α + θ) (18) When the formula (18) is rearranged, the following formula (19) is obtained.

Nsin α = sin (α−θ) (19) Since α = α (r) and θ = θ (r), (1
Equation (9) can be rewritten as equation (20) below.

N sin α (r) = sin {α (r) −θ (r)} = sin α (r) cos θ (r) −cos α (r) sin θ (r) (20) (20) Both sides of sin α Dividing by (r) gives (2
Equations 1a) and (21b) are established.

N = cos θ (r) − {tan α (r)} ⁻¹ · sin θ (r) (21a) {n-cos θ (r)} / sin θ (r) = − {tan α (r)} ⁻¹ (21b) Therefore, tan α (r) = − sin θ (r) / {n−cos θ (r)} (22)

By substituting the relationship of the above expression (16) into the expression (22), the following expression (23) is established.

F ′ (r) = − sin θ (r) / {n−cos θ (r)} (23) Therefore, the ridge line D ′ is represented by the above formula (11) by integrating the above formula (23). Is a function that is

Similarly, the ridgeline D ″ is represented by the above equation (12).

Here, depending on how to give θ (r),
In some cases, the indefinite integral in the formula cannot be executed. In that case, the form of the function can be determined by performing the numerical integration.

As an example, s = Δθ / R, t = θ ₀ −Δ
When θ and θ (r) = sr + t are set, the above f (r) can be rewritten as the following equation (24).

F (r) = − (1 / s) · log {n−cos (sr + t)} + c (24) Here, if f (0) = z ₀ , the following equation (25) is established. .

F (0) = z ₀ = − (1 / s) · log (n-cost) + c (25) Therefore, the integration constant c is expressed by the following equation (26).

C = z ₀ + (1 / s) · log (n-cost) (26) Therefore, the above equation (11) can be rewritten as the following equation (27).

Z = f (r) = − (1 / s) · log {n-cos (sr + t)} + (1 / s) · log (n-cost) + z ₀ (27) Now, laser beam generation As shown in FIG. 1, the laser beam emitted from the use optical element 3 is refracted at the ridgelines D ′ and D ″ and divided into two light beams C ′ and C ″. The light flux C ′ is a light flux that intersects the optical axis 4 and diverges from a virtual point E ′ outside the optical axis 4. On the other hand, the luminous flux C ″ intersects the optical axis 4,
Further, it becomes a light beam that originates from a virtual point E ″ outside the optical axis 4. Since the divergence points E ′ and E ″ are symmetrical with respect to the optical axis 4, the light beams D ′ and D ″ are You will cross.
When the wavefronts of the light beams D ′ and D ″ interfere with each other at the intersecting portions and are viewed in a plane perpendicular to the optical axis 4,
A long-focus laser beam having an amplitude distribution similar to the 0th-order Bessel function of the first kind is formed in the radial direction.

In the bar code reading apparatus, since the minimum line width of a readable bar code is substantially determined by the focused spot diameter, the focused spot diameter can be set according to the distance from the laser beam generating optical element 3. It is important for practical use. The laser beam generating optical element 3 of the present invention has such characteristics as described below.
This is because the focused spot diameter can be changed by the laser beam generation optical element 3 according to the distance from the laser beam generation optical element 3.

Using this laser beam generating optical element 3, the following (28) similar to the Bessel beam generating device is used.
The focused spot diameter ω _{0 at} the distance Z from the laser beam generating optical element 3 is given by the equations (29).

Ω ₀ = 0.766 · λ / θ (r) (28) Z = r · tan θ (r) (29) Here, θ (r) is the value of r given by the above equation (10). Q
It is the next function. From this, it is the angle θ (r) formed by the light beam passing through the height r from the optical axis 4 and the optical axis 4 that determines the focused spot diameter at the distance Z. Since θ (r) is determined by the refractive index of the laser beam generation optical element 3 and the inclination of the surface at the height r, the height r of the surface of the laser beam generation optical element 3 when r is changed from 0 to R. By appropriately setting the inclination in a minute range in the vicinity, it is possible to realize a desired distribution of the focused spot diameter in the optical axis direction. Since the details thereof have been described in the above-mentioned operation, they are omitted here.

(First Embodiment of Bar Code Reader)
FIG. 3 shows Embodiment 1 of the bar code reading apparatus of the present invention using the laser beam generating optical element of Embodiment 1.
The configuration will be described below together with the operation.

The light (spherical wave) F emitted from the semiconductor laser 21 is converted into parallel light by the collimator lens 22 and is incident on the laser beam generating optical element 23 similar to the laser beam generating optical element 3. The long-focus laser beam G emitted from the laser beam generating optical element 23 is deflected within a range of ± 30 ° by a polygon mirror 24 which is an optical deflector, and is irradiated onto the bar code 25.

Then, it is modulated by the bar code 25,
The scattered light beam H is condensed by the lens 26, enters the photodiode 27 for signal reading, and the barcode signal is read by referring to the beam emission direction control signal (reference light not shown). As described above, the focused spot diameter of the laser beam generated by the laser beam generating optical element 23 of the present invention is determined by the laser beam generating optical element 2
Increases with distance from 3.

As described in the above operation, the closest point where the focused spot is formed is the position of the apex of the laser beam generating optical element 23, and the farthest position is given by the above equation (8). .

Zmax = R / tan θ (R) (8) Here, when tan θ (R) is a negative number, Zmax
Can be considered infinity. Practically, the farther away from the laser beam generating optical element 23, the weaker the light intensity becomes, and it becomes unsuitable for reading a barcode. The focused spot diameter increases in accordance with the distance from the laser beam generating optical element 23, and the magnitude of the change is a straight line with an inclination P as shown by a straight line Z in FIG.

In the present embodiment, as shown in FIG. 8, the largest inclination of 0.62 that can be read at the required reading depth for a bar code having any minimum line width is selected. Even if the inclination is smaller, the reading depth does not matter, but the intensity of the long-focus laser beam decreases, which is not preferable.

However, the magnitude of the change in the condensed spot diameter in the Z direction is not limited to the function shown by the straight line Z in FIG. 8, and when it is not necessary to read the bar codes of all the minimum line widths. It is generally advantageous to increase the inclination to increase the strength. Further, when a barcode of a new standard is adopted, the barcode with a new minimum line width may be designed so that it can be read at a required reading depth.

(Second Embodiment of Laser Beam Generating Optical Element) FIG. 4 shows a second embodiment of the laser beam generating optical element of the present invention. The laser beam generating optical element 33 is
It is a hologram optically equivalent to the laser beam generating optical element 3 of the first embodiment.

As shown in FIG. 4, this hologram 33
When viewed from the direction of the optical axis, is a concentric circle in which the inner space is closer than the outer space. That is, this hologram 33
Since it is necessary to convert parallel light into divergent light, the concentric circles with a coarser pitch become closer to the outer circumference. In this regard,
It is clearly different from the hologram of FIG. 13, which is a set of concentric circles with equal intervals.

Since the holograms 33 are optically equivalent, a long focus laser beam similar to the laser beam generating optical element 23 can be formed by the hologram 33.

(Second Embodiment of Bar Code Reading Device)
FIG. 5 shows Embodiment 2 of the barcode reading apparatus of the present invention. This bar code reader uses the hologram 33 of FIG. 4 as a laser beam generating optical element.

Since the other structure is the same as that shown in FIG. 3, the corresponding parts are designated by the same reference numerals and the detailed description thereof will be omitted.

Also in this bar code reader,
The same effects as those of the barcode reading device of the first embodiment can be obtained.

(Third Embodiment of Laser Beam Generating Optical Element) FIG. 6 shows a third embodiment of the laser beam generating optical element of the present invention. The laser beam generating optical element 43 is
It is a hologram that is optically equivalent to the laser beam generating optical element 3 of the first embodiment and has the function of a collimating lens integrated.

As shown in FIG. 6, this hologram 43
When viewed from the optical axis direction, is a concentric circle in which the inner side is smaller than the outer side, like the hologram 33 in FIG. However, unlike the hologram 33 of FIG. 4, the gaps are unevenly packed toward the inside.

This is because the hologram 43 needs to have both a function of converting a spherical wave into parallel light (a function equivalent to a lens having a finite focal length) and a function of converting this parallel light into divergent light. , To satisfy the former function,
It is necessary to make holograms having a finer pitch on the outer peripheral side and to have unequal intervals in order to suppress aberrations. On the other hand, it is necessary to make the latter function optically equivalent to the hologram 33 of FIG. Since it is necessary to make it a little rough, it is a set of concentric circles shown in order to exert both functions.

(Third Embodiment of Bar Code Reader)
FIG. 7 shows Embodiment 3 of the barcode reading apparatus of the present invention. This barcode reader uses the hologram 43 of FIG. 6 as an optical element for generating a laser beam.

Since the other structure is the same as that shown in FIGS. 3 and 5, the same reference numerals are given to the corresponding portions and the detailed description thereof will be omitted.

According to this bar code reader, a collimating lens is unnecessary. Therefore, there is an advantage that the optical system can be simplified.

[0121]

According to the above laser beam generating optical element of the present invention, it is possible to form a laser beam which not only has a deep depth of focus but also has a focused spot diameter which increases with the distance from the optical element. . For this reason, when this laser beam generating optical element is used in a bar code reading device, it becomes possible to read bar codes of all minimum line widths with a required reading depth with one bar code reading device.

When this laser beam generating optical element is used, the optical system only needs the collimating lens and the laser beam generating optical element. A bar code reader capable of reading a code at a required reading depth can be realized.

Further, particularly when a hologram is used as the laser beam generating optical element, the mass productivity is excellent, and there is an advantage that it can greatly contribute to the cost reduction of the laser beam generating optical element and the bar code reading apparatus.

In particular, when a hologram having a function of a collimating lens is used as the laser beam generating optical element, there is an advantage that the optical system can be further simplified.

[Brief description of drawings]

FIG. 1 is a schematic side view showing a first embodiment of a laser beam generating optical element together with a light source and a collimating lens.

FIG. 2 is a geometrical explanatory diagram for deriving a curve of a ridgeline of the laser beam generating optical element according to the first embodiment.

FIG. 3 is a perspective view showing Embodiment 1 of the barcode reading apparatus of the present invention.

FIG. 4 is a view showing a second embodiment of an optical element for generating a laser beam, as seen from the optical axis direction of a hologram.

FIG. 5 is a perspective view showing a second embodiment of the barcode reading device of the present invention.

FIG. 6 is a view showing a hologram having a function of a collimator lens as seen from the optical axis direction, showing a third embodiment of an optical element for generating a laser beam.

FIG. 7 is a perspective view showing a third embodiment of the barcode reading apparatus of the present invention.

FIG. 8 is a graph showing the relationship between the minimum line width of a barcode to be read and the reading depth.

FIG. 9 is a graph showing that the beam diameter of the Bessel beam becomes the value of the first zero point of J (βρ) of the 0th-order Bessel function.

FIG. 10A shows a state of large intensity fluctuation, FIG. 10B shows a state of excessive intensity fluctuation suppression, and FIG. 10C shows a state of proper intensity fluctuation. A graph showing the relationship.

FIG. 11 is a schematic side view showing a conventional example of a basic Bessel beam generating optical system.

12 (a), (b), and (c) are partial cross-sectional views showing holograms optically equivalent to the axicon of FIG.

13 is a diagram of the hologram of FIG. 12 viewed from the optical axis direction.

FIG. 14 is a graph showing the intensity distribution of a basic Bessel beam in a direction perpendicular to the optical axis.

FIG. 15 is a graph showing changes in the focused spot diameter as seen in the basic optical axis direction of the Bessel beam.

[Explanation of symbols]

1, 21 Semiconductor laser 2, 22 Collimating lens 3, 23 Laser beam generating optical element 4 Optical axis 24 Polygon mirror for light deflection 25 Bar code 26 Lens for collecting signal light scattered by bar code 27 Photodetector for signal detection 33 Hologram Optically Equivalent to Laser Beam Generating Optical Element 43 Hologram Optically Equivalent to Laser Beam Generating Optical Element and Having Function of Collimating Lens

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G06K 7/12 G06K 7/12 B H01S 3/18 H01S 3/18

Claims (9)

[Claims] 1. A curve which is rotationally symmetric with respect to the optical axis and which represents a ridgeline of a cross section taken along a plane including the optical axis is represented by a non-differentiable non-linear function on the optical axis, and the optical axis of the ridgeline. An optical element for generating a laser beam comprising an optical element in which the inclination of a portion near the optical axis is increased while the inclination of a portion near the optical axis is increased. 2. An optical element for generating a laser beam, which comprises a hologram optically equivalent to the optical element. 3. A laser beam generating optical element comprising a hologram having the function of the optical element and the function as a collimating lens for converting light from a light source into parallel light. 4. A bar code reading apparatus comprising the laser beam generating optical element according to claim 1, wherein the laser beam generating optical element changes the degree of inclination of the ridge of the laser beam generating optical element. The degree of change in the size of the focused spot diameter with respect to the distance from the beam generating optical element can be adjusted, and the relationship between the minimum line width of a readable barcode and the reading depth can be determined by the minimum readable barcode. A bar code reading apparatus in which the slope P satisfies 0 <P <1 so that the slope P satisfies the following conditions when the line width is plotted on the vertical axis and the reading depth is plotted on the horizontal axis. 5. A bar code reading apparatus equipped with the optical element for generating a laser beam according to claim 2, wherein the relationship between the minimum line width of the readable bar code and the reading depth is indicated by the readable bar code. A bar code reading apparatus in which a slope P satisfies 0 <P <1 when the logarithmic graph is represented with the minimum line width of the code as the vertical axis and the reading depth as the horizontal axis. 6. A bar code reading apparatus equipped with the laser beam generating optical element according to claim 3, wherein the bar code readable by the minimum line width and the reading depth of the bar code is readable. A bar code reading apparatus in which a slope P satisfies 0 <P <1 when the logarithmic graph is represented with the minimum line width of the code as the vertical axis and the reading depth as the horizontal axis. 7. A laser light source, a collimator lens for converting light emitted from the laser light source into parallel light, and
The laser beam generating optical element according to claim 1, wherein the minimum line width of a readable barcode and the reading depth are represented by the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis. For example, the bar code reading device in which the slope P is set to a slope corresponding to the slope of a straight line connecting a plurality of required specification values when represented by a logarithmic graph. 8. A laser light source, a collimator lens for converting light emitted from the laser light source into parallel light, and
The above-mentioned hologram, and a logarithmic graph showing the relationship between the minimum line width of a readable barcode and the reading depth, with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis. When expressed by
However, the bar code reading device is set to an inclination corresponding to the inclination of a straight line connecting a plurality of required specification values. 9. A laser light source, a collimator lens for converting light emitted from the laser light source into parallel light, and
A logarithmic graph showing the relationship between the minimum line width of a readable barcode and the reading depth, including the hologram described in the above, with the minimum line width of the readable barcode as the vertical axis and the reading depth as the horizontal axis. When expressed by
However, the bar code reading device is set to an inclination corresponding to the inclination of a straight line connecting a plurality of required specification values.