JP3646828B2 - Laser beam generating optical element and barcode reading apparatus using the laser beam generating optical element - Google Patents

Description

【００７７】
【数２】

【００７８】
以下に図２を参照しつつ、上記（１１）、（１２）式の導出過程について説明する。図２に示す幾何学的関係により、下記（１３）式が成立する。
【００７９】
θ₁＝π−α，θ₂＝θ₁＋θ …（１３）
このθは上記（１０）式のθ（ｒ）である。
【００８０】
今、稜線Ｄ’に相当する曲線の関数として下記（１４）式のものを想定する。
【００８１】
ｚ＝ｆ（ｒ） …（１４）
この曲線の接線５は関数ｚ＝ｆ（ｒ）を微分すれば得られるので、下記（１５）式で表される。
【００８２】
ｚ＝ｆ′（ｒ）・ｒ＋ｍ …（１５）
但し、ｍはこの接線５とｚ軸との交点である。
【００８３】
ここで、接線５の傾きｆ′（ｒ）は、図２に示す幾何学的関係により、下記（１６）式で表される。
【００８４】
ｆ′（ｒ）＝ｔａｎα（ｒ） …（１６）
ここで、屈折率をｎとすると、スネルの法則より、下記（１７）式の関係が成立する。
【００８５】
ｎｓｉｎθ₁＝ｓｉｎθ₂ …（１７）
この（１７）式に上記（１３）式の関係を代入すると、下記（１８）式が成立する。
【００８６】
ｎｓｉｎ（π−α）＝ｓｉｎ（π−α＋θ） …（１８）
（１８）式を整理すると、下記（１９）式になる。
【００８７】
ｎｓｉｎα＝ｓｉｎ（α−θ） …（１９）
また、α＝α（ｒ），θ＝θ（ｒ）であるから、（１９）式は下記（２０）式に書き改められる。
【００８８】

（２０）式の両辺をｓｉｎα（ｒ）で割ると、下記（２１ａ），（２１ｂ）式が成立する。
【００８９】
ｎ＝ｃｏｓθ（ｒ）−｛ｔａｎα（ｒ）｝^-1・ｓｉｎθ（ｒ） …（２１ａ）
｛ｎ−ｃｏｓθ（ｒ）｝／ｓｉｎθ（ｒ）＝−｛ｔａｎα（ｒ）｝^-1 …（２１ｂ）
よって、
ｔａｎα（ｒ）＝−ｓｉｎθ（ｒ）／｛ｎ−ｃｏｓθ（ｒ）｝ …（２２）
となる。
【００９０】
（２２）式に上記（１６）式の関係を代入すると、下記（２３）式が成立する。
【００９１】
ｆ′（ｒ）＝−ｓｉｎθ（ｒ）／｛ｎ−ｃｏｓθ（ｒ）｝ …（２３）
よって、稜線Ｄ’は、上記（２３）式を積分した、上記（１１）式で表される関数となる。
【００９２】
同様に、稜線Ｄ”は、上記（１２）式で表される。
【００９３】
ここで、θ（ｒ）の与えかたによっては、式中の不定積分が実行できない場合があるが、その場合には、数値積分を行うことによつて、関数の形が決定できる。
【００９４】
一例として、ｓ＝△θ／Ｒ，ｔ＝θ₀−△θ，θ（ｒ）＝ｓｒ＋ｔとおくと、上記ｆ（ｒ）は、下記（２４）式に書き改められる。
【００９５】
ｆ（ｒ）＝−（１／ｓ）・ｌｏｇ｛ｎ−ｃｏｓ（ｓｒ＋ｔ）｝＋ｃ …（２４）
ここで、ｆ（０）＝ｚ₀とすると、下記（２５）式が成立する。
【００９６】
ｆ（０）＝ｚ₀＝−（１／ｓ）・ｌｏｇ（ｎ−ｃｏｓｔ）＋ｃ …（２５）
よって、積分定数ｃは下記（２６）式で表される。
【００９７】
ｃ＝ｚ₀＋（１／ｓ）・ｌｏｇ（ｎ−ｃｏｓｔ） …（２６）
従って、上記（１１）式は下記（２７）式に書き改められる。
【００９８】

さて、レーザビーム発生用光学素子３を出射したレーザビームは図１に示すように、稜線Ｄ’、Ｄ”で屈折され、２つの光束Ｃ’、Ｃ”に分けられる。光束Ｃ’は光軸４と交差し、かつ、光軸４外の仮想的な点Ｅ’から発散する光束となる。一方、光束Ｃ”は光軸４と交差し、かつ、光軸４外の仮想的な点Ｅ”から発故する光束となる。発散点Ｅ’とＥ”とは光軸４に対して互いに対称な位置にあるため光束Ｄ’、Ｄ”は交差することになる。互いに交差している部分では、光束Ｄ’、Ｄ”の波面が互いに干渉して、光軸４に対し垂直な面内で見たとき、動径方向に第１種０次ベッセル関数に類似した振幅分布を持つ長焦点レ‐ザビームが形成される。
【００９９】
バーコード読み取り装置においては、読み取り可能なバーコードの最小線幅は集光スポット径によりほぼ決まるので、集光スポット径をレーザビーム発生用光学素子３からの距離に応じて設定できることは実用上桓めて重要である。本発明のレーザビーム発生用光学素子３がこのような特性を持つのは以下に示すように、レーザビーム発生用光学素子３により集光スポット径がレーザビーム発生用光学素子３からの距離に応じて可変できるからである。
【０１００】
このレーザビーム発生用光学素子３を用いると、ベッセルビーム発生装置と類似の下記（２８）式、（２９）式でレーザビーム発生用光学素子３からの距離Ｚにおける集光スポット径ω₀が与えられる。
【０１０１】
ω₀＝０．７６６・λ／θ（ｒ） …（２８）
Ｚ＝ｒ・ｔａｎθ（ｒ） …（２９）
ここで、θ（ｒ）は上記（１０）式で与えられるｒのＱ次関数である。これから、距離Ｚにおける集光スポット径を決めるのは光軸４からｒの高さを通る光束の、光軸４とのなす角度θ（ｒ）である。θ（ｒ）はレーザビーム発生用光学素子３の屈折率、高さｒにおける面の傾きによって決まるので、ｒを０からＲまで変えたときのレーザビーム発生用光学素子３の面の高さｒ付近の微小な範囲における傾きを適宜設定することにより、所望の集光スポット径の光軸方向の分布を実現することができる。その詳細については、上記の作用のところで説明したので、ここでは省略する。
【０１０２】
（バーコード読み取り装置の実施形態１）
図３は実施形態１のレーザビーム発生用光学素子を用いた本発明バーコード読み取り装置の実施形態１を示す。以下にその構成を動作と共に説明する。
【０１０３】
半導体レーザ２１から出射された光（球面波）Ｆはコリメートレンズ２２により平行光に変換され、上記レーザビーム発生用光学素子３同様のレーザビーム発生用光学素子２３に入射される。このレーザビ一ム発生用光学素子２３から出た長焦点レーザビームＧは光偏向装置であるポリゴンミラー２４で±３０゜の範囲で偏向され、バーコード２５に照射される。
【０１０４】
そして、バ一コード２５により変調され、散乱された光束Ｈはレンズ２６で集光され、信号読み取り用のホトダイオード２７に入射し、ビームの出射方向制御信号（図示しない参照光）を参照することによりバーコード信号が読み取られる。上述のように、本発明のレーザビーム発生用光学素子２３で発生されたレーザビームの集光スポット径は、レーザビーム発生光学素子２３からの距離に応じて増加する。
【０１０５】
上述の作用のところで説明したように、集光スポットが形成される最も近い点はレーザビーム発生用光学素子２３の頂点の位置であり、最も遠い位置は上記（８）式で与えられる。
【０１０６】
Ｚｍａｘ＝Ｒ／ｔａｎθ（Ｒ） …（８）
ここで、ｔａｎθ（Ｒ）が負の数になるときはＺｍａｘは無限大とみなすことができる。実用的には、レーザビーム発生用光学素子２３から離れるほど光の強度が弱くなり、バーコードの読み取りには不適当となる。集光スポット径はレーザビーム発生光学素子２３からの距離に応じて増加し、その変化の大きさは図８の直線Ｚのように、傾きＰの直線となる。
【０１０７】
本実施形態では、図８に示したように、どのような最小線幅のバーコードでも必要な読み取り深度で読みとれる最も大きい傾き０．６２を選んだ。傾きが、より小さくても読み取り深度は問題ないが、長焦点レーザビームの強度が減少するため好ましくない。
【０１０８】
しかし、集光スポット径のＺ方向の変化の大きさとしては図８の直線Ｚのような関数に限ることはなく、すべての最小線幅のバーコードを読み取る必要がない場合には、傾きを大きくして強度を大きくする方が一般に有利である。また、新しい規格のバーコードが採用された場合には、新規な最小線幅のバーコードを必要な読み取り深度で読み取れる傾きになるように設計すれば良い。
【０１０９】
（レーザビーム発生用光学素子の実施形態２）
図４は本発明レーザビーム発生用光学素子の実施形態２を示す。このレーザビーム発生用光学素子３３は、実施形態１のレーザビーム発生用光学素子３と光学的に等価なホログラムである。
【０１１０】
図４に示すように、このホログラム３３は、光軸方向からみると、外側に比べて内側の間隔が詰まった同心円になっている。即ち、このホログラム３３は平行光を発散光に変換する必要があるため、外周に近付くほどピッチの粗い同心円になっている。この点で、等間隔の同心円の集合である図１３のホログラムとは明確に異なっている。
【０１１１】
光学的に等価であるため、このホログラム３３によれば、上記レーザビーム発生用光学素子２３同様の長焦点レーザビームを形成することができる。
【０１１２】
（バーコード読み取り装置の実施形態２）
図５は本発明バーコード読み取り装置の実施形態２を示す。このバーコード読み取り装置は、レーザビーム発生用光学素子として、図４のホログラム３３を用いている。
【０１１３】
他の構成については、図３に示すものと同一であるので、対応する部分に同一の符号を付して、具体的な説明は省略する。
【０１１４】
このバーコード読み取り装置においても、上記実施形態１のバーコード読み取り装置同様の効果を奏することができる。
【０１１５】
（レーザビーム発生用光学素子の実施形態３）
図６は本発明レーザビーム発生用光学素子の実施形態３を示す。このレーザビーム発生用光学素子４３は、実施形態１のレーザビーム発生用光学素子３と光学的に等価であり、かつコリメートレンズの機能を一体化したホログラムである。
【０１１６】
図６に示すように、このホログラム４３は、光軸方向からみると、図４のホログラム３３と同様に外側に比べて内側が詰まった同心円になっている。但し、図４のホログラム３３とは異なり、内側に向けて間隔が不均一に詰まっている。
【０１１７】
これは、このホログラム４３は球面波を平行光に変換する機能（有限の焦点距離を持つレンズと等価な機能）と、この平行光を発散光に変換する機能を併せ持つ必要があるため、前者の機能を満足するために、外周側ほどピッチの細かいホログラムとし、かつ収差を抑制するため不等間隔にする必要がある一方、図４のホログラム３３と光学的に等価にして後者の機能を発揮する必要上、外周側ほどピッチを少し粗くする必要があるため、両機能を発揮するために図示する同心円の集合になっている。
【０１１８】
（バーコード読み取り装置の実施形態３）
図７は本発明バーコード読み取り装置の実施形態３を示す。このバーコード読み取り装置は、レーザビーム発生用光学素子として、図６のホログラム４３を用いている。
【０１１９】
他の構成については、図３及び図５に示すものと同一であるので、対応する部分に同一の符号を付して、具体的な説明は省略する。
【０１２０】
このバーコード読み取り装置によれば、コリメートレンズが不要になる。このため、光学系を簡潔化できる利点がある。
【０１２１】
【発明の効果】
以上の本発明レーザビーム発生用光学素子によれば、焦点深度が深いだけでなく、集光スポット径が光学素子からの距離に応じて大きくなるレーザビームを形成することができる。このため、このレーザビーム発生用光学素子をバーコード読み取り装置に用いると、１台のバーコード読み取り装置で、全ての最小線幅のバーコードを要求される読み取り深度で読み取ることが可能となる。
【０１２２】
また、このレーザビーム発生用光学素子を用いると、光学系が、コリメートレンズとこのレーザビーム発生用光学素子だけで済むため、簡潔な構成の光学系で、全ての最小線幅のバーコードを要求される読み取り深度で読み取るとることが可能なバーコード読み取り装置を実現できる。
【０１２３】
また、特にレーザビーム発生用光学素子として、ホログラムを用いる場合は、量産性に優れているので、レーザビーム発生用光学素子及びバーコード読み取り装置のコストダウンに大いに寄与できる利点がある。
【０１２４】
また、特にレーザビーム発生用光学素子として、コリメートレンズの機能を併せ持つホログラムを用いる場合は、光学系をより一層簡潔化できる利点がある。
【図面の簡単な説明】
【図１】レーザビーム発生用光学素子の実施形態１を光源及びコリメートレンズと共に示す模式的側面図。
【図２】実施形態１のレーザビーム発生用光学素子の稜線の曲線を導き出すための幾何学的説明図。
【図３】本発明バーコード読み取り装置の実施形態１を示す斜視図。
【図４】レーザビーム発生用光学素子の実施形態２を示す、ホログラムを光軸方向からみた図。
【図５】本発明バーコード読み取り装置の実施形態２を示す斜視図。
【図６】レーザビーム発生用光学素子の実施形態３を示す、コリメートレンズの機能を併せ持つホログラムを光軸方向からみた図。
【図７】本発明バーコード読み取り装置の実施形態３を示す斜視図。
【図８】読み取るべきバーコードの最小線幅と読み取り深度との関係を表すグラフ。
【図９】ベッセルビームのビーム径が０次ベッセル関数のＪ（βρ）の第１零点の値になることを示すグラフ。
【図１０】（ａ）は強度変動大の状態を、（ｂ）は強度変動過剰抑制の状態を、（ｃ）は強度変動適正の状態をそれぞれ示す、光強度と光学素子からの距離との関係を示すグラフ。
【図１１】基本的なベッセルビーム発生光学系の一従来例を示す模式的側面図。
【図１２】（ａ）、（ｂ）、（ｃ）共に図１１のアキシコンと光学的に等価なホログラムを示す部分断面図。
【図１３】図１２のホログラムを光軸方向から見た図。
【図１４】 基本的なベッセルビームの光軸に垂直な方向での強度分布を示すグラフ。
【図１５】基本的なベッセルビームの光軸方向で見た集光スポット径の変化を示すグラフ。
【符号の説明】
１，２１ 半導体レーザ
２，２２ コリメートレンズ
３，２３ レーザビーム発生用光学素子
４ 光軸
２４ 光偏向用ポリゴンミラー
２５ バーコード
２６ バーコードで散乱された信号光を集めるレンズ
２７ 信号検出用光検出器
３３ レーザビーム発生用光学素子と光学的に等価なホログラム
４３ レーザビーム発生用光学素子と光学的に等価であり、かつコリメートレンズの機能を併せ持つホログラム[0001]
BACKGROUND OF THE INVENTION
The present invention can generate a laser beam (hereinafter referred to as a long focus laser beam) having a relatively small focal spot diameter and a deeper focal depth than a general Gaussian beam as a laser beam having the same focal spot diameter. The present invention relates to a laser beam generating optical element and a bar code reader using the laser beam generating optical element.
[0002]
[Prior art]
Conventionally, in an optical system using a laser beam, a so-called Gaussian beam in which a radial intensity distribution in a plane perpendicular to the optical axis becomes a Gaussian distribution has been used. When a Gaussian beam is condensed using a normal condensing optical system, the distance Δ from the position where the condensing spot diameter ω is minimized to the position where the beam diameter is K times is expressed by the following equation (1).
[0003]
Δ = ± (π / λ) · ω₀ ²√ (K²-1) ... (1)
Where ω₀: 1 / e of the peak intensity when the focused spot diameter is minimum²The radius of the beam at the position
λ: wavelength
From the above equation (1), it can be seen that the distance Δ is substantially proportional to K.
[0004]
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 (Gauss beam) at a position where the bar code is located. The range in which the barcode can be read is proportional to Δ in the above equation (1). Due to such restrictions, the minimum line width and reading depth of a bar code that can be read by the bar code reader cannot be set freely.
[0005]
As a conventional example of a bar code reader using a Gaussian beam condensing optical system, a bar code reader of the product name MS-710 sold by “Matsushita Inter-Techno Co., Ltd.” (hereinafter simply referred to as MS-710). )It has been known.
[0006]
FIG. 8 is a bilogarithmic graph showing the relationship between the minimum bar width [mm] and the reading depth [mm] of a bar code that can be read by the bar code reader. Of these, the straight line connecting the plots ■ is MS-710. The straight line connecting the plots を indicates the case of a bar code reader using a Bessel beam described later, and the plot + indicates the required specification value of the bar code reader.
[0007]
As shown in FIG. 8, in MS-710, it can be seen that the relationship between the minimum line width of the readable barcode and the reading depth can be approximated by a straight line having a slope of 1. This reflects that the distance Δ expressed by the above equation (1) is substantially proportional to the first power of the magnification K of the spot diameter.
[0008]
By the way, a product to which a barcode is actually attached may have an uneven surface or be inclined in the depth direction with respect to the barcode reader. In such a case, the reading depth is insufficient. Therefore, it often happens that the barcode cannot be read.
[0009]
In addition, when reading a barcode with a coarse pitch with a device that reads a barcode with a narrow pitch, the Gaussian beam has a shallow depth of focus, as compared with the case of reading a barcode with a narrow pitch. Since the reading depth is not sufficient, it is necessary to read the product closer to the product, which is inconvenient.
[0010]
In addition, since it is impossible to read when it is out of reach, it is necessary to prepare a barcode reading apparatus having a deep reading depth that can read only a barcode with a coarse pitch. For this reason, there existed a problem that an installation cost increased or it could not be installed in a narrow space.
[0011]
In order to solve such problems, a composite optical system having a plurality of focal lengths is used, and even an optical system using a normal Gaussian beam has an optical system with a focal depth corresponding to the minimum line width of the barcode. There are also devices that do this.
[0012]
However, even in this case, an optical system having a sufficient depth of focus for all bar codes having the smallest line width has not been achieved. Further, the configuration of the optical system becomes extremely complicated, and the apparatus becomes expensive, which is not practical.
[0013]
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, it is possible to widen the reading range to be set symmetrically on both sides of the position where the minimum spot diameter is obtained. However, it is more natural to read the barcode with the narrowest minimum line width near because the size of the entire barcode is small. For this reason, only one side of the position where the minimum spot diameter is obtained can be used.
[0014]
In order to solve these problems, the electric field distribution in the radial direction perpendicular to the optical axis as a laser beam having a very large depth of focus and a relatively small spot diameter has a first-order zeroth-order Bessel function. Bessel beams (non-diffractive beams) have been devised. The details of the Bessel beam are described in detail in the following documents.
[0015]
(1) J.A. Dunnin: Vol. 4, no. 4 / April 1987 / J. Opt. Soc. Am. A, p651-p654
(2) Kiyoharu Uehara: Applied Physics Vol. 59, No. 6 (1990), p746-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, the following description will be made on behalf of the section including the optical axis.
[0016]
In FIG. 11, the spherical laser beam A emitted from the semiconductor laser element 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 is a function that represents a refracting surface, that is, a ridge line is two straight lines having different signs of inclination, and is connected on the optical axis when viewed in a section cut in a plane including the optical axis 14. There is a function that cannot be differentiated. When expressed in terms of shape, the optical element has a sharp point.
[0017]
The traveling direction of light after being refracted by the ridgelines B ′, B ″ of the surface having the effect of refracting light in the half plane defined by the optical axis 14 is constant regardless of the height from the optical axis 14. Therefore, the refracted light is a plane wave, and the traveling directions are different in the two half-planes defined by the optical axis 14.
[0018]
That is, in this cross section, the laser beam emitted from the axicon 13 is refracted by the ridge line B ′ in the half plane defined by the optical axis 14 and intersects the optical axis 14 at an angle θ, as shown in FIG. It becomes a plane wave A ′ traveling in the direction. Similarly, a plane wave A ″ is refracted by the ridge line B ″ in the opposite half plane defined by the optical axis 14 and travels in a direction intersecting the optical axis at an angle θ. However, the traveling direction is an axially symmetric direction with respect to the optical axis 14.
[0019]
An optical element equivalent to the axicon 13 can also be produced by using a hologram. That is, for example, as shown in FIG. 12A, like the Fresnel lens, the thickness of the refracting surface may be returned to the reference surface every time the optical path difference becomes ½ of the wavelength λ. In this case, the hologram has a sawtooth-shaped cross section, but as a simpler one, the radius of the optical path difference is ½ of the wavelength λ as in a Fresnel strip, and the period is as shown in FIG. The optical characteristics may be changed, for example, by changing the thickness or by inverting the transmittance as shown in FIG. In the case of FIGS. 12B and 12C, it is clear that the hologram is a collection of concentric circles.
[0020]
FIG. 13 shows the shapes of the holograms of FIGS. 12A, 12B, and 12C viewed from the direction of the optical axis 14. As is clear from the figure, they are concentric circles at equal intervals.
[0021]
In order to simplify the explanation, the collimator lens 12 is used in the conventional example so that parallel light is incident on the axicon 13. However, if a hologram is used, the function of the collimator lens 12 can be achieved. That is, it is possible to directly convert a spherical wave into a Bessel beam. In this case, the hologram is a concentric circle having a cycle that is similar to the Fresnel lens.
[0022]
Further, an elliptical light beam such as a semiconductor laser element can be directly converted into a Bessel beam. For the method, see R.A. P. MacDonald et al., APPLIED OPTICS / Vol. 32, no. 32/10 November 1993, p6470 to p6474.
[0023]
The electric field distribution in a direction perpendicular to the optical axis 14 as viewed in a cross section including the optical axis 14 of the long-focus laser beam is proportional to the first-order zeroth-order Bessel function due to the interference of the plane waves A ′ and A ″. A Bessel beam is formed.
[0024]
FIG. 14 shows the intensity distribution of this basic Bessel beam in the direction perpendicular to the optical axis 14. FIG. 15 shows changes in the focused spot diameter as seen in the optical axis direction. In the case of a Bessel beam, as shown by the curve J in the figure, the focused spot diameter hardly changes in the optical axis direction, and compared with the curve I in the figure, which is a Gaussian beam having the same spot size, the optical axis. 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]
[Problems to be solved by the invention]
However, the conventional Bessel beam has a depth of focus that is sufficiently deeper than that of a normal Gaussian beam, but is not sufficient as a barcode reader for the following reasons.
[0026]
That is, as the performance required for the barcode reading apparatus, it is necessary to be able to read barcodes having various minimum line widths in addition to the ability to read barcodes having a specific minimum line width in the widest possible range. Further, it is required that the reading range is sufficiently deep for a bar code having a large minimum line width.
[0027]
The points indicated by the plot 10 in FIG. 8 are the required specification values of the minimum line width of the barcode and the corresponding reading depth. In a simple optical system with only one focal point, when using a Gaussian beam, the relationship between the minimum line width of the barcode that can be read and the reading depth is a straight line with a slope of 1, so that the required specifications are completely There was a problem that we could not be satisfied. For this reason, it is practically used with a limited reading range.
[0028]
Also, in order to realize a barcode reader that can read multiple barcodes with different minimum line widths as required, an optical system with multiple focal positions is used, but a complex optical system with many lenses is used. There is a problem of becoming a system.
[0029]
On the other hand, when a conventional Bessel beam is used, the relationship between the minimum line width of the barcode that can be read even with a simple optical system using only the collimator lens 12 and the axicon 13 and the reading range is a straight line with a slope of 0 in FIG. Therefore, the required specifications for the reading depth can be satisfied for a plurality of barcodes.
[0030]
However, when the optical system is designed so that the barcode with the smallest minimum line width can be read as shown by the straight line Y in FIG. 8, the reading depth for the barcode with the smallest minimum line width is insufficient. When trying to increase the reading depth sufficiently, there is a problem that the barcode with the smallest minimum line width cannot be read as shown by the straight line X in FIG.
[0031]
The present invention has been made in view of such circumstances, and a barcode with a specific minimum line width can be read in a wide range with a simple optical system, and a plurality of barcodes having different minimum line widths are required. It is an object of the present invention to provide a laser beam generating optical element that generates a laser beam that can be read according to specifications, and a bar code reader using the laser beam generating optical element.
[0032]
[Means for Solving the Problems]
The optical element for generating a laser beam of the present invention is rotationally symmetric with respect to the optical axis, a curve representing a ridge line of a cross section cut by a plane including the optical axis is represented by a non-differentiable nonlinear function on the optical axis, and The objective is achieved by an optical element in which the slope of the ridge line near the optical axis is increased while the slope of the ridge line at a position away from the optical axis is reduced.
[0033]
The optical element for generating a laser beam according to the present invention comprises a hologram that is optically equivalent to the optical element, thereby achieving the above object.
[0034]
The optical element for generating a laser beam of the present invention comprises a hologram having both the function of the optical element and a function as a collimating lens for converting light from the light source into parallel light, thereby achieving the above object. .
[0035]
The barcode reader of the present invention is a barcode reader comprising the laser beam generating optical element according to claim 1, wherein the degree of change in the inclination of the ridge line of the laser beam generating optical element is determined. By changing the degree of change of the diameter of the focused spot diameter with respect to the distance from the laser beam generating optical element, the relationship between the minimum line width of the readable barcode and the reading depth can be adjusted. When the minimum line width of a possible barcode is represented by a logarithmic graph with the vertical axis representing the minimum line width and the horizontal axis representing the reading depth, the slope P satisfies the following condition:
0 <P <1
The above-mentioned purpose is achieved by setting.
[0036]
According to another aspect of the present invention, there is provided a barcode reading apparatus including the optical element for generating a laser beam according to claim 2, wherein the relationship between the minimum line width of the readable barcode and the reading depth is obtained. When the minimum line width of the readable bar code is on the vertical axis and the reading depth is on the horizontal axis, the logarithmic graph shows that the slope P satisfies the following condition:
0 <P <1
The above-mentioned purpose is achieved by setting.
[0037]
A barcode reader of the present invention is a barcode reader comprising the laser beam generating optical element according to claim 3, wherein the relationship between the minimum line width of the readable barcode and the read depth is obtained. When the minimum line width of the readable bar code is on the vertical axis and the reading depth is on the horizontal axis, the logarithmic graph shows that the slope P satisfies the following condition:
0 <P <1
The above-mentioned purpose is achieved by setting.
[0038]
A bar code reading apparatus according to the present invention comprises a laser light source, a collimating lens for converting light emitted from the laser light source into parallel light, and the laser beam generating optical element according to claim 1, and capable of reading the bar code. When the relationship between the minimum line width and the reading depth is expressed by a log-log 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 is a plurality of logarithmic graphs. The inclination corresponding to the inclination of the straight line connecting the required specification values is set, thereby achieving the above object.
[0039]
  A barcode reader of the present invention includes a laser light source, a collimating lens that converts light emitted from the laser light source into parallel light, and2. A hologram element optically equivalent to the laser beam generating optical element according to claim 1.The relationship between the minimum line width of a readable barcode and the reading depth is represented by a log-log graph with the minimum line width of the readable barcode on the vertical axis and the reading depth on the horizontal axis. In this case, the inclination P is set to an inclination corresponding to the inclination of a straight line connecting a plurality of required specification values, thereby achieving the above object.
[0040]
  The barcode reader of the present invention includes a laser light source,A hologram having both the function as an optical element for generating a laser beam according to claim 1 and the function as a collimating lens for converting light from the laser light source into parallel lightThe relationship between the minimum line width of a readable barcode and the reading depth is represented by a log-log graph with the minimum line width of the readable barcode on the vertical axis and the reading depth on the horizontal axis. In this case, the inclination P is set to an inclination corresponding to the inclination of a straight line connecting a plurality of required specification values, thereby achieving the above object.
[0041]
The operation of the present invention will be described below with reference to FIG. 1 and FIGS.
[0042]
Generation of a laser beam having a shape that is rotationally symmetric with respect to the optical axis 4 and that represents the ridgelines D ′ and D ″ of a cross section cut by a plane including the optical axis 4 as a non-differentiable non-linear function 1, the incident light incident on the laser beam generating optical element 3 is divided with respect to the optical axis 4 as shown in FIG. The two emitted light beams C ′ and C ″ interfere with each other, and the light intensity is not easily attenuated. Therefore, according to the laser beam generating optical element 3, it is possible to generate a long focal laser beam similar to the Bessel beam.
[0043]
In addition, in this laser beam generating optical element 3, the slopes of the ridgelines D ′ and D ″ of the laser beam generating optical element 3 in the portion close to the optical axis 4 are increased, and on the contrary, away from the optical axis 4. Since the inclination of the ridgelines D ′ and D ″ of the laser beam generating optical element 3 is reduced, the inclination of the laser beam generated near the optical axis 4 is increased. As a result, the tilt of the laser beam generated near the laser beam generating optical element 3 is also increased, so that 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 far from the laser beam generating optical element 3 is also reduced, so that the diameter of the focused spot can be increased.
[0044]
Accordingly, the diameter of the focused spot and the depth of focus can be adjusted by changing the degree of change in the inclination of the ridge lines D ′ and D ″ of the optical element 3 for generating a laser beam. According to the optical element 3, the minimum line width and reading depth of a readable barcode can be adjusted, that is, the relationship between the minimum line width and reading depth of a readable barcode is represented by a log-log graph. (See FIG. 8), the slope can be adjusted in the range of 0 to 1.
[0045]
Here, if the slope P on the log-log graph is less than 1 (= the slope in the case of a Gaussian beam), the depth of focus can be made deeper than that of the Gaussian beam. Can be read. On the other hand, when the inclination P is larger than 0 (= the inclination in the case of a Bessel beam), unlike the Bessel beam, a plurality of barcodes having the minimum line width can be read. In addition, a sufficient reading range can be ensured for a barcode having a large minimum line width.
[0046]
Then, according to the barcode reader using the laser beam generating optical element of the present invention, the ridge line D of the laser beam generating optical element 3 so that the inclination P on the log-log graph is 0 <P <1. The inclination of “, D” is set.
[0047]
Therefore, according to the present invention, a barcode with a specific minimum line width can be read in as wide a range as possible, and a plurality of barcodes with a minimum line width can be read. It is possible to realize a bar code reader that can ensure a sufficient reading range for a code.
[0048]
In the laser beam generating optical element of the present invention, since the spot diameter is the smallest near the laser beam generating optical element 3, the narrowest barcode with the smallest line width is close without sacrificing the reading range. Can be read at.
[0049]
As can be seen from FIG. 8, in order to read all barcodes having the minimum line width, a straight line having a minimum focused spot diameter of 0.07 mm or less, a maximum reading depth of 7000 mm, and an inclination of 0.62 may be realized. That is, this straight line having an inclination of 0.62 is an inclination of a straight line connecting all required specification values (indicated by plot + in the figure).
[0050]
In order to satisfy this condition, the following may be performed. That is, a case where a laser beam having a wavelength of λ mm 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 (position where y = 0 in FIG. 1) Is set to 0.766 × λ / 0.07 rad or more so that the minimum spot diameter is 0.07 mm or less, and the farthest from the optical axis (in FIG. 1, y = R The inclination θ (R) with respect to the optical axis of the laser beam generated at a position of 0.766 × λ / 1.8 rad or more and tan θ (R) <R / (1.2 × 7000). If so, the spot diameter of the focused spot at a position of 7000 mm from the laser beam generating optical element 3 can be set to 1.8 mm, so that the above condition can be satisfied.
[0051]
Here, the reason why the minimum spot diameter can be reduced to 0.07 mm or less when the inclination θ (0) is 0.766 × λ / 0.07 rad or more is described in R.C. P. This will be described in accordance with MacDonald et al. (Applied Optics vol 32, p6470 (1993)).
[0052]
First, the long-focus laser beam of the present invention is considered to be a collection of Bessel beams having different spot diameters. Therefore, θ (0) is determined so that the narrowest barcode with the smallest line width can be read from the spot diameter W of the Bessel beam formed by the light passing through the innermost circumference.
[0053]
Here, as shown in FIG. 9, the beam diameter of the Bessel beam becomes the value (radius) of the first zero point of J (βρ) of the zeroth-order Bessel function, and is expressed by the following equation (2).
[0054]
W = 4.81 / β (2)
However, the numerical value 4.81 is a value obtained by calculation and will not be described in detail.
[0055]
On the other hand, if the angle between the laser beam and the optical axis 4 is θ, the following equation (3) is established.
[0056]
β = (2π / λ) · tan θ (3)
From the equations (2) and (3), the following equation (4) is established.
[0057]
W = {4.81 / (2πtan θ)} · λ = 0.766λ / θ (4)
However, tan θ≈θ
From equation (4)
θ = 0.766λ / W (5)
Here, as described above, the spot diameter W of the laser beam may be a beam diameter equal to or smaller than the minimum line width of the barcode to be read. Therefore, since W ≦ 0.07 rad, the inclination θ (0) is set. The minimum spot diameter can be reduced to 0.07 mm or less by setting so as to satisfy the condition of the following formula (6).
[0058]
θ (0)> 0.766λ / 0.07 (6)
The reason why the inclination θ (R) is 0.766 × λ / 1.8 rad or more is as follows.
[0059]
In other words, 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 barcode with the smallest line width. Therefore, the inclination θ (R) may be set so as to satisfy the condition of the following expression (7).
[0060]
θ (R)> 0.766λ / 1.8 (7)
The reason why tan θ (R) should be tan θ (R) <R / (1.2 × 7000) is as follows.
[0061]
First, in the laser beam generating optical element 3 shown in FIG. 1, the closest point where the focused spot is formed is the position of the apex of the ridge lines D ′ and D ″ of the optical element 3, and the farthest position is 8) It is given by the equation.
[0062]
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 necessary to devise measures to suppress the fluctuation when applied to a barcode reader. is there. 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 the case where the fluctuation of the light intensity is excessively suppressed. (C) has shown the case where the light intensity fluctuation | variation is suppressed appropriately. As can be seen from FIG. 5C, when such a device is used, the usable distance is reduced by about 20%.
[0063]
For this reason, since it is necessary to provide 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 the reading range is set to 7000 mm, Zmax≈1.2 × 7000 may be set.
[0064]
Therefore, tan θ (R) may be set from the above equation (8) so as to satisfy the condition of the following equation (9).
[0065]
tan θ (R) <R / (1.2 × 7000) (9)
As a method for optimizing the light intensity fluctuation as shown in FIG. 10C, for example, the one previously proposed by the present applicant in Japanese Patent Application No. 7-213265 is known.
[0066]
In the present invention, instead of the laser beam generating optical element 3 as described above, a hologram optically equivalent to this can be used. In the case of using a hologram, an optical element can be manufactured by using a normal photolithography method, so that mass production is possible.
[0067]
It is also possible to use a hologram integrated with a collimating lens. 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]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be specifically described below with reference to the drawings.
[0069]
(Embodiment 1 of Laser Beam Generating Optical Element)
FIG. 1 shows Embodiment 1 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 generating apparatus provided with this laser beam generating optical element is shown.
[0070]
The laser beam generator includes a semiconductor laser 1, a collimating lens 2, and a laser beam generating optical element 3. The spherical laser beam C emitted from the semiconductor laser 1 is converted into a plane wave by the collimator lens 2 and then incident on the laser beam generating optical element 3.
[0071]
Since this laser beam generating optical element 3 is rotationally symmetric with respect to the optical axis (Z axis) 4, for the sake of simplicity, the description will be limited to an arbitrary cross section viewed from the plane including the optical axis 4. Do.
[0072]
The surface having the function of refracting light of the laser beam generating optical element 3 is formed by two ridge lines D ′ and D ″ when viewed in a cross section cut by a plane including the optical axis 4. Unlike the Bessel beam, the angle θ formed with the optical axis 4 in the traveling direction of the light refracted by 'varies depending on the height r from the optical axis 4. In order to diverge outside the optical axis 4, the optical axis 4 The smaller θ is, the smaller θ becomes .. In order to emphasize that θ is a function of r, the following is written as θ (r).
[0073]
In the first embodiment, θ (r) is a function of r so that the spot diameter increases in accordance with the distance from the laser beam generating optical element 3. Due to the geometrical relationship shown in FIG. 1, θ (r) is expressed by the following (10).
[0074]
θ (r) = θ₀-Δθ (1 r / R)^Q  (10)
Where R: outer diameter (radius) of the laser beam generating optical element 3
Δθ is a constant that determines the range of divergence, and is refracted at the position of r = R from the divergence point E ′ (or divergence point E ″) and the divergence point E ′ (or divergence point E ″). Angle formed by the light refracted from and refracted at the position of r = 0
Q: A constant that determines the strength of divergence
Similarly, the relationship of the above equation (10) is established for the light refracted by the other ridgeline D ″.
[0075]
Hereinafter, in order to simplify the description, the case of Q = 1 will be described as an example. The functions f (r) and g (r) representing the ridge lines D ′ and D ″ are expressed by the following expressions (11) and (12), respectively, and become functions that cannot be differentiated on the optical axis 4.
[0076]
[Expression 1]

[0077]
[Expression 2]

[0078]
Hereinafter, the process of deriving the above expressions (11) and (12) will be described with reference to FIG. The following equation (13) is established by the geometric relationship shown in FIG.
[0079]
θ₁= Π-α, θ₂= Θ₁+ Θ (13)
This θ is θ (r) in the above equation (10).
[0080]
Now, the following equation (14) is assumed as a function of a curve corresponding to the ridge line D ′.
[0081]
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).
[0082]
z = f ′ (r) · r + m (15)
However, m is the intersection of this tangent 5 and z axis.
[0083]
Here, the inclination f ′ (r) of the tangent line 5 is expressed by the following equation (16) according to the geometrical relationship shown in FIG.
[0084]
f ′ (r) = tan α (r) (16)
Here, when the refractive index is n, the following equation (17) is established according to Snell's law.
[0085]
nsinθ₁= Sinθ₂  ... (17)
Substituting the relationship of the above equation (13) into this equation (17), the following equation (18) is established.
[0086]
nsin (π−α) = sin (π−α + θ) (18)
When formula (18) is arranged, the following formula (19) is obtained.
[0087]
nsinα = sin (α−θ) (19)
Since α = α (r) and θ = θ (r), the equation (19) is rewritten as the following equation (20).
[0088]

When both sides of the equation (20) are divided by sin α (r), the following equations (21a) and (21b) are established.
[0089]
n = cos θ (r) − {tan α (r)}^-1Sin 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)
It becomes.
[0090]
Substituting the relationship of the above equation (16) into the equation (22), the following equation (23) is established.
[0091]
f ′ (r) = − sin θ (r) / {n−cos θ (r)} (23)
Therefore, the ridge line D ′ is a function represented by the above formula (11), which is obtained by integrating the above formula (23).
[0092]
Similarly, the ridgeline D ″ is expressed by the above equation (12).
[0093]
Here, depending on how θ (r) is given, indefinite integration in the equation may not be executed. In this case, the shape of the function can be determined by performing numerical integration.
[0094]
As an example, s = Δθ / R, t = θ₀Assuming −Δθ, θ (r) = sr + t, the f (r) is rewritten into the following equation (24).
[0095]
f (r) = − (1 / s) · log {n−cos (sr + t)} + c (24)
Where f (0) = z₀Then, the following equation (25) is established.
[0096]
f (0) = z₀=-(1 / s) .log (n-cost) + c (25)
Therefore, the integration constant c is expressed by the following equation (26).
[0097]
c = z₀+ (1 / s) · log (n-cost) (26)
Therefore, the above equation (11) is rewritten into the following equation (27).
[0098]

As shown in FIG. 1, the laser beam emitted from the laser beam generating optical element 3 is refracted at the ridgelines D ′ and D ″ and divided into two light beams C ′ and C ″. The light beam C ′ crosses the optical axis 4 and becomes a light beam diverging from a virtual point E ′ outside the optical axis 4. On the other hand, the light beam C ″ intersects with the optical axis 4 and becomes a light beam originating 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 ″ intersect each other. In the intersecting portions, the wave fronts of the light beams D ′ and D ″ interfere with each other, and are similar to the first kind 0th order Bessel function in the radial direction when viewed in a plane perpendicular to the optical axis 4. A long focal laser beam with an amplitude distribution is formed.
[0099]
In a bar code reader, the minimum line width of a bar code that can be read is substantially determined by the diameter of the focused spot, so that it is practically possible to set the focused spot diameter according to the distance from the laser beam generating optical element 3. Important. The laser beam generating optical element 3 according to the present invention has such characteristics as described below. The laser beam generating optical element 3 has a condensing spot diameter depending on the distance from the laser beam generating optical element 3. This is because it can be varied.
[0100]
When this laser beam generating optical element 3 is used, the condensing spot diameter ω at the distance Z from the laser beam generating optical element 3 according to the following equations (28) and (29) similar to the Bessel beam generating apparatus.₀Is given.
[0101]
ω₀= 0.766 · λ / θ (r) (28)
Z = r · tan θ (r) (29)
Here, θ (r) is a Q-order function of r given by the above equation (10). From this, the condensing spot diameter at the distance Z is determined by the angle θ (r) formed by the light beam passing through the height of r from the optical axis 4 and the optical axis 4. Since θ (r) is determined by the refractive index of the laser beam generating optical element 3 and the inclination of the surface at the height r, the surface height r of the laser beam generating optical element 3 when r is changed from 0 to R. A distribution in the optical axis direction of a desired condensing spot diameter can be realized by appropriately setting an inclination in a minute range in the vicinity. Since the details have been described in the above operation, they are omitted here.
[0102]
(Embodiment 1 of barcode reader)
FIG. 3 shows a first embodiment of the barcode reading apparatus of the present invention using the laser beam generating optical element of the first embodiment. The configuration will be described below together with the operation.
[0103]
Light (spherical wave) F emitted from the semiconductor laser 21 is converted into parallel light by a collimator lens 22 and is incident on a 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 in a range of ± 30 ° by a polygon mirror 24 which is an optical deflecting device, and is irradiated onto the barcode 25.
[0104]
The luminous flux H modulated and scattered by the bar code 25 is collected by the lens 26, is incident on a signal reading photodiode 27, and refers to a beam emission direction control signal (reference light (not shown)). A bar code signal is read. As described above, the focused spot diameter of the laser beam generated by the laser beam generating optical element 23 of the present invention increases according to the distance from the laser beam generating optical element 23.
[0105]
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).
[0106]
Zmax = R / tan θ (R) (8)
Here, when tan θ (R) is a negative number, Zmax can be regarded as infinite. Practically, the intensity of light decreases as the distance from the laser beam generating optical element 23 decreases, making it unsuitable for barcode reading. The diameter of the focused spot increases with the distance from the laser beam generating optical element 23, and the magnitude of the change is a straight line having an inclination P as shown by the straight line Z in FIG.
[0107]
In the present embodiment, as shown in FIG. 8, the largest gradient 0.62 that can be read at a necessary reading depth with any minimum line width barcode is selected. Even if the inclination is smaller, the reading depth is not a problem, but it is not preferable because the intensity of the long-focus laser beam is reduced.
[0108]
However, the magnitude of the change in the focused spot diameter in the Z direction is not limited to a function such as the straight line Z in FIG. 8, and if it is not necessary to read all bar codes having the minimum line width, the inclination is changed. It is generally advantageous to increase the strength by increasing it. In addition, when a new standard barcode is adopted, it may be designed so that a barcode with a new minimum line width can be read at a necessary reading depth.
[0109]
(Embodiment 2 of optical element for laser beam generation)
FIG. 4 shows Embodiment 2 of the optical element for generating a laser beam according to the present invention. The laser beam generating optical element 33 is a hologram that is optically equivalent to the laser beam generating optical element 3 of the first embodiment.
[0110]
As shown in FIG. 4, when viewed from the optical axis direction, the hologram 33 is a concentric circle having a smaller inner space than the outer surface. That is, the hologram 33 needs to convert parallel light into divergent light, so that the hologram 33 becomes a concentric circle with a coarse pitch as it approaches the outer periphery. In this respect, it is clearly different from the hologram of FIG. 13, which is a set of equally spaced concentric circles.
[0111]
Since it is optically equivalent, the hologram 33 can form a long-focus laser beam similar to the laser beam generating optical element 23.
[0112]
(Embodiment 2 of bar code reader)
FIG. 5 shows Embodiment 2 of the barcode reading apparatus of the present invention. This bar code reader uses the hologram 33 shown in FIG. 4 as an optical element for generating a laser beam.
[0113]
Since other configurations are the same as those shown in FIG. 3, the same reference numerals are given to the corresponding portions, and detailed descriptions thereof are omitted.
[0114]
This bar code reader can also achieve the same effects as the bar code reader of the first embodiment.
[0115]
(Embodiment 3 of an optical element for laser beam generation)
FIG. 6 shows Embodiment 3 of the optical element for generating a laser beam according to the present invention. The laser beam generating optical element 43 is a hologram that is optically equivalent to the laser beam generating optical element 3 of the first embodiment and integrated with the function of a collimating lens.
[0116]
As shown in FIG. 6, when viewed from the optical axis direction, the hologram 43 is a concentric circle whose inner side is clogged as compared with the outer side, like the hologram 33 of FIG. However, unlike the hologram 33 in FIG. 4, the intervals are not uniform toward the inside.
[0117]
This is because the hologram 43 needs to have a function of converting spherical waves 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. In order to satisfy the function, it is necessary to make a hologram with a finer pitch toward the outer peripheral side and to make it an unequal interval in order to suppress aberrations, while exhibiting the latter function optically equivalent to the hologram 33 in FIG. Since it is necessary to make the pitch a little rougher on the outer peripheral side, it is a set of concentric circles shown in order to exhibit both functions.
[0118]
(Embodiment 3 of Barcode Reader)
FIG. 7 shows Embodiment 3 of the barcode reading apparatus of the present invention. This bar code reader uses the hologram 43 shown in FIG. 6 as an optical element for generating a laser beam.
[0119]
Since other configurations are the same as those shown in FIGS. 3 and 5, the same reference numerals are given to the corresponding portions, and detailed descriptions thereof are omitted.
[0120]
According to this barcode reader, a collimating lens is not required. For this reason, there exists an advantage which can simplify an optical system.
[0121]
【The invention's effect】
According to the above optical element for generating a laser beam of the present invention, it is possible to form a laser beam that not only has a deep focal depth, but also has a condensing spot diameter that increases in accordance with the distance from the optical element. For this reason, when this laser beam generating optical element is used in a barcode reader, all barcodes with the minimum line width can be read at a required reading depth with one barcode reader.
[0122]
In addition, when this laser beam generating optical element is used, the optical system only needs to be a collimating lens and this laser beam generating optical element, so a bar code with all minimum line widths is required with an optical system with a simple configuration. Therefore, it is possible to realize a bar code reader capable of reading at a read depth.
[0123]
In particular, when a hologram is used as an optical element for generating a laser beam, it is excellent in mass productivity, and thus has an advantage that it can greatly contribute to cost reduction of the optical element for generating a laser beam and a barcode reading apparatus.
[0124]
In particular, when a hologram having a collimating lens function is used as the laser beam generating optical element, there is an advantage that the optical system can be further simplified.
[Brief description of the drawings]
FIG. 1 is a schematic side view showing Embodiment 1 of a laser beam generating optical element together with a light source and a collimating lens.
FIG. 2 is a geometric explanatory diagram for deriving a ridge line curve of the optical element for generating a laser beam 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 diagram showing a hologram viewed from an optical axis direction, showing a second embodiment of the optical element for generating a laser beam.
FIG. 5 is a perspective view showing Embodiment 2 of the barcode reading apparatus of the present invention.
FIG. 6 is a diagram showing a hologram having a function of a collimating lens, viewed from the optical axis direction, showing Embodiment 3 of the optical element for generating a laser beam.
FIG. 7 is a perspective view showing Embodiment 3 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.
FIGS. 10A and 10B show a state where the intensity fluctuation is large, FIG. 10B shows a state where the intensity fluctuation is excessively suppressed, and FIG. 10C shows a state where the intensity fluctuation is appropriate. A graph showing the relationship.
FIG. 11 is a schematic side view showing a conventional example of a basic Bessel beam generating optical system.
FIGS. 12A, 12B, and 12C are partial cross-sectional views showing holograms that are 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 an intensity distribution in a direction perpendicular to the optical axis of a basic Bessel beam.
FIG. 15 is a graph showing a change in a focused spot diameter as seen in the optical axis direction of a basic Bessel beam.
[Explanation of symbols]
1,21 Semiconductor laser
2,22 Collimating lens
3,23 Laser beam generating optical element
4 optical axes
24 Light deflection polygon mirror
25 Barcode
26 Lens that collects signal light scattered by barcode
27 Photodetector for signal detection
33 Hologram optically equivalent to laser beam generating optical element
43 A hologram that is optically equivalent to a laser beam generating optical element and also has the function of a collimating lens

Claims (9)

  A curve that is rotationally symmetric with respect to the optical axis and represents a ridgeline of a cross section cut by a plane including the optical axis is represented by a non-differentiable nonlinear function on the optical axis, and the slope of the ridgeline at a location close to the optical axis An optical element for generating a laser beam comprising an optical element in which the inclination of a portion away from the optical axis of the ridge is reduced. 2. The optical element for generating a laser beam according to claim 1, comprising a hologram optically equivalent to the optical element. 2. The optical element for generating a laser beam according to claim 1, comprising a hologram having both a function of the optical element and a function as a collimating lens for converting light from a light source into parallel light. A bar code reader comprising the laser beam generating optical element according to claim 1,
By changing the degree of change in the inclination of the ridge line of the laser beam generating optical element, the degree of change in the diameter of the focused spot with respect to the distance from the laser beam generating optical element can be adjusted and read. When the minimum line width of the barcode and the reading depth are represented by a log-log 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 is 0 <P <1 to satisfy the following conditions
Bar code reader set. A barcode reader comprising the laser beam generating optical element according to claim 2,
The relationship between the minimum line width of a readable barcode and the reading depth is represented by a logarithmic graph with the minimum line width of the readable barcode on the vertical axis and the reading depth on the horizontal axis. 0 <P <1 so that P satisfies the following conditions
Bar code reader set. A barcode reader comprising the laser beam generating optical element according to claim 3,
The relationship between the minimum line width of a readable barcode and the reading depth is represented by a logarithmic graph with the minimum line width of the readable barcode on the vertical axis and the reading depth on the horizontal axis. 0 <P <1 so that P satisfies the following conditions
Bar code reader set.   A laser light source, a collimating lens that converts light emitted from the laser light source into parallel light, and the optical element for generating a laser beam according to claim 1, and the relationship between the minimum line width of a readable barcode and the reading depth Is represented by a log-log 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 corresponds to the slope of a straight line connecting a plurality of required specification values. Bar code reader set to tilt. A bar code minimum line comprising: a laser light source; a collimating lens for converting light emitted from the laser light source into parallel light; and a hologram element optically equivalent to the laser beam generating optical element according to claim 1. When the relationship between the width and the reading depth is represented by a log-log 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 specification values. Bar code reader set to an inclination corresponding to the inclination of the straight line connecting the two. A readable barcode comprising a laser light source and a hologram having both a function as a laser beam generating optical element according to claim 1 and a function as a collimating lens for converting light from the laser light source into parallel light . When the relationship between the minimum line width and the reading depth is represented by a log-log graph with the minimum line width of the bar code being readable as the vertical axis and the reading depth as the horizontal axis, the slope P has a plurality of requirements. Bar code reader set to an inclination corresponding to the inclination of the straight line connecting the specification values.

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