JPS6319602A - Light beam expander - Google Patents

Abstract

PURPOSE:To reduce the size and weight of a light beam expander by forming the expander of an optical waveguide and a concentric grating formed on the optical waveguide, and setting specific conditions among the wavelength of incident light, the effective refractive index of the optical waveguide, and the period of the concentric grating. CONSTITUTION:The light beam expander consists of a transparent substrate 1, an optical waveguide layer 2 which has a larger refractive index than it, and the concentric grating 3 formed thereupon. The incident light 4 incident on this light beam expander is converted by the concentric grating 3 into waveguide light propagated radially in the optical waveguide and further converted by the concentric grating 3 into projection light 5 which has a larger beam diameter than the original incident light 4. The period LAMBDA of the concentric grating is set to mlambda/N. Wherein lambda is the wavelength of the incident light, N is the effective refractive index of the optical waveguide, and (n) is a positive integer. The incident light 4 is therefore converted into the parallel light 5 and the light beam expander is only as thick as the transparent substrate 1 plus the optical waveguide layer 2, so the size and weight are reducible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an optical beam expander for converting the beam diameter of a laser beam in an optical system that utilizes a laser beam.

(Prior Art) In recent years, lasers have been widely used in the fields of optical information processing and optical measurement. Optical systems that utilize these laser beams generally require devices such as a light beam expander that expands the diameter of the laser beam and a collimator that collimates the laser beam that diverges from the negative point.

Conventionally, these devices are composed of multiple lenses, and in order to obtain the desired beam diameter, the diameter and focal length of the lenses must be adjusted accordingly, which poses the problem that the entire optical system cannot be miniaturized. Ta.

FIG. 12 shows an example of an optical system for recording a hologram. The light beam emitted from the laser light source 100 is converted into parallel light of a predetermined size by a light beam expander consisting of a lens 102 and a lens 1o3, and is split by a beam splitter 104 so that one side is directed to an object 107.
The illumination light 111 serves as the illumination light 111, and the other serves as the reference light 112. An interference pattern between the light 113 reflected and scattered by the object 107 and the reference light 112 is recorded on the hologram dry plate 106.

The reference light 112 and the illumination light 111 need to have a size that corresponds to the size of the hologram dry plate 106 and the object 107, respectively, so the diameter of the lens 103 must be greater than or equal to that size. Focal length f.

also becomes larger. This is the numerical aperture NA of the lens (=sinθ
) increases, spherical aberration increases in the case of a single lens, making it impossible to obtain a good plane wave. The light beam expander made up of lenses 102 and 103 ends up having a size larger than f2.

FIG. 13 shows a simplified part of an optical disc pickup optical system for an optical disc, such as a video disc or a digital audio disc (DAD). For simplicity, the optical system for signal detection is omitted. The light emitted from the semiconductor laser 120 passes through the collimation lens 1.
22 into parallel light, which is converged by an objective lens 123 and focused onto the information recording surface 125 of the optical disk substrate 124. Typically, a collimating lens 122.
For the objective lens 123, a combination of a plurality of lenses is used in order to reduce aberrations, but in order to simplify the explanation, only one lens is used here. In such a pickup optical system, a single beam of light is transmitted onto the information recording surface.
Since the light must be narrowed down to a minute spot on the order of 11rn, an objective lens 123 with a large NA (=-θ.) is required. Further, the focal length f of the objective lens 123 is also required to have a predetermined value. Although omitted here, in an actual optical disc, the information recording surface 125 and the objective lens 12
This is because a transparent substrate is inserted between the objective lens 123 and the objective lens 123, and it is necessary to maintain a required working distance between the transparent substrate and the objective lens 123.

On the other hand, NA of the collimating lens (= sfnθ
C) is determined by the radiation angle of the light emitted from the semiconductor laser 120, etc. Generally, θC is a smaller value than θ0. Therefore, the focal length of the collimating lens fc
It can't be made any smaller. For example, if the NA of the corresponding lens 123 is 0.45, the focal length is 4B, and the NA of the collimation lens 122 is 0.2, then fc is as follows. Semiconductor laser 120 and collimating lens 1
The size of the optical system including 22 is the sum of this value - and the actual size of the entire lens combined with multiple lenses,
This imposes restrictions on the miniaturization of pickups.

(Problems to be Solved by the Invention) As shown in the above examples, there is a limit to the miniaturization of optical systems in devices such as optical beam expanders and collimators that use conventional lenses.

An object of the present invention is to solve the above problems and provide a small and lightweight optical beam expander.

[Structure of the invention]

(Means for Solving the Problems) The present invention makes it possible to miniaturize an optical beam expander by expanding parallel light in a direction perpendicular to its traveling direction using an optical waveguide grating. be.

That is, the present invention consists of an optical waveguide and a concentric grating formed on the optical waveguide, and has the function of expanding the optical beam by setting the period of the concentric grating so that parallel light and guided light are combined. It is something that has been set.

(Function) According to the present invention, the optical system of the optical beam expander is made smaller and lighter. Furthermore, the shape of the concentric grating area allows the expanded light beam to be controlled into any shape.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure shows a first embodiment of the invention. The optical beam expander shown in this figure is composed of an optical waveguide consisting of a transparent substrate 1 and an optical waveguide layer 2 having a higher refractive index than the transparent substrate 1, and a concentric grating 3 formed thereon.

The incident light 4 entering this optical beam expander is converted into guided light that propagates radially in the optical waveguide by the concentric grating 3, and this guided light is further converted into a beam from the original incident light 4 by the same concentric grating 3. It is converted into emitted light 5 with a larger diameter. This principle will be explained with reference to FIG. The right diagram in FIG. 2 shows a pattern of concentric bracing, and the left diagram shows a cross-sectional view of the optical waveguide. The period A of this concentric grating is set by the following equation.

λ A = m K
°°° (where λ is the wavelength of the incident light, N is the effective refractive index of the optical waveguide, or m is a positive integer. The period is (11
The concentric grating given by the equation combines light traveling perpendicular to the optical waveguide surface and guided light passing through the center of the concentric circle. This can be easily understood by considering the conservation of the wave number vector in the direction parallel to the optical waveguide surface. That is, if the component of the wave number vector of the incident light parallel to the optical waveguide surface is kt, the propagation vector of the guided light is β, and the grating pertle of the optical waveguide grating is K, then −β−kt=lK (l: integer)・(2)
When the relationship holds true, the incident light and the guided light are coupled.

Since the incident light is perpendicular to the optical waveguide surface, kt = 0
(3) Also, since the direction of the grating vector is on a straight line passing through the center of the concentric circles, the directions of β and K match. Furthermore, since 1=m...(6) from (1) to (5), equation (2) holds true. In this case, the incident light and the guided light are coupled by the m-th order component of the grating.

As is clear from the above explanation, in FIG. 2, light 6. is diffracted by the concentric grating and converted into guided light beams 7 and 8 passing through the center of the concentric circles. Further, a part of the incident light becomes light 9 that goes straight without being diffracted.

On the other hand, the guided beams 7 and 8 are diffracted again by the concentric grating, and the beams 10 and 1 proceed perpendicularly to the optical waveguide surface.
It becomes 1.

As shown on the right side of Figure 2, light incident on any other point is also converted to guided light passing through the center of the concentric circles, and these are re-
and is converted into light that travels perpendicular to the optical waveguide surface. Therefore, in FIG. 1, the incident light 4 whose optical axis passes through the center of the concentric grating is converted by this light beam expander into parallel light 5 whose beam diameter is the diameter of the grating area. As is clear from a comparison with FIG. 9, the optical beam expander of FIG. 1 requires only the combined thickness of the transparent substrate 1 and the optical waveguide layer 2, so it can be made smaller and lighter. Further, since this leading waveguide grating is concentric and has a constant period of one force, it is easy to manufacture. In other words, when producing a grating using a foot mask, the period is constant, so the resist exposure conditions do not vary depending on the location, and it is easy to form a uniform grating pattern over the entire area. It is. Furthermore, since they are concentric circles, mechanical processing using an NO lathe or the like is also possible.

By the way, in FIG. 2, a part of the guided light beams 7 and 8 is converted into not only the desired output light beams 10 and 11 but also light beams 12 and 13 which proceed in the opposite direction.

To make effective use of the incident light, the amount of light traveling to the opposite side can be reduced. It is desirable to make it as small as possible.

One method for this purpose is to make the cross-sectional shape of the grating blazed to increase diffraction to one side. The fact that guided light is strongly diffracted only to one side by a blazed grating with an asymmetric cross-sectional shape is known, for example, from T. Tamr, ed., “In-tegrate.
Optics”, 2nd ed,, apringet-
Ver-1ag (1979) Chapter 3 p, 118
．． It is stated in A grating with a blazed cross section can be produced by the above-mentioned NC lathe processing or the like.

Another way to eliminate light traveling to the opposite side is to use a reflective film. FIG. 3 shows a second embodiment of the invention. In this example, on the incident light side of the transparent substrate 1,
A reflective film 20 is formed except for the incident light portion. This reflective film is composed of, for example, a metal thin film or a multilayer film. In such a light beam expander equipped with a reflective film, the grating causes the light that is diffracted in the opposite direction to the desired light to be reflected again in the same direction as the incident light.
Less light loss.

The above example is an optical beam expander that expands a circular beam into a circular beam, but the cross-sectional shape of the output beam can be arbitrarily changed depending on the shape of the grating region. FIG. 4 shows a third embodiment of the present invention, in which the concentric grating 23 has a square area. In this way, the output beam 25 also becomes a beam with a square cross section.

In the examples shown in FIGS. 1 to 4, the intensity distribution of the emitted beam is determined by the shape of the grating cross section, the grating depth, the order of diffraction, etc. If the grating is uniform, the intensity will generally be higher at the center and smaller toward the periphery. By reducing the grating depth and reducing the diffraction efficiency,
Although the change in the intensity of the diffracted outgoing light is reduced, most of the guided light is not diffracted and propagates outward as guided light. One way to prevent this is to
For example, by changing the ratio of the widths of the peaks and valleys of the grating depending on the location, the diffraction efficiency can be increased closer to the outer periphery. Another method is to provide a second concentric grating, which corresponds to a reflecting mirror, on the optical waveguide outside the concentric grating. FIG. 5 shows a fourth embodiment of the present invention, in which a second concentric grating 33 is provided as a reflecting mirror outside the concentric grating 3 that converts incident light into guided light. By this second concentric grating 33, the guided light 37 propagating to the outer region of the central concentric grating 3 is reflected and becomes a guided light 38 that heads toward the center of the concentric circle, and this guided light is converted into an output light 5 by the concentric grating 3. converted. In this way, there is no guided light propagating outside the grating region, and a highly efficient optical beam expander can be obtained. The period Δof the second concentric grating, which acts as a reflector, is given by the following equation.

A, ==-...(7) N If it's just to generate reflected light, then A! may be an integer multiple of the value of equation (7), but in order to prevent generation of invalid light other than reflected light, it is necessary to use reflection by first-order diffraction given by equation (7). If the concentric grating in the center also uses first-order diffraction, that is, m in the +1+ formula.
=l, the period A of the second concentric grating
, is the period Δ of the concentric grating in the center.

FIG. 6 shows a fifth embodiment of the present invention. In this example, the concentric grating 23 has a square area, and a second concentric grating 43 is provided outside the square area as a reflecting mirror. In this case, the emitted light becomes a beam with a square cross section and is emitted with high efficiency.

An embodiment of fa6 of the present invention is shown in FIG. This figure shows an example of use as a collimating lens for a semiconductor laser. A package 51 on which a semiconductor laser chip 50 is mounted and an optical beam expander according to the present invention are fixed to a holder 52.

FIG. 8 shows a cross section of the optical beam expander in FIG. 7 and patterns on the incident light side and the output light side. This optical beam expander is provided with a breaking lens 53 in order to once convert the emitted light 55 from the semiconductor laser into parallel light 54. In this example, to increase the diffraction efficiency,
The grating lens has a blazed cross-sectional shape.

Also, a reflective film 2o is provided as in FIGS. 3 and 5.

The principle by which the parallel light 54 is converted into the enlarged output light 5 has already been described. The optical beam expander that also functions as this collimating lens is the semiconductor laser package 5.
1, the optical path length can be reduced compared to the case shown in FIG. 10, and a compact collimating optical system can be realized.

Furthermore, the optical beam expander shown in FIG. 8 can also be used as a window of a semiconductor laser package, in which case parallel light can be taken out directly from the package.

FIG. 9 shows a seventh embodiment of the present invention. This example is an example in which the optical beam expander according to the present invention is applied to the hologram recording optical system shown in FIG. He-Ne
A light beam 61 emitted from a laser 60 has its beam diameter expanded by a light beam expander 63 according to the present invention, and is split by a beam splitter 64, so that one becomes illumination light 71 for an object 67 and the other becomes a reference light 72. . An interference pattern between the light 73 reflected and scattered by the object 67 and the reference light 72 is recorded on the hologram dry plate 66. Since the optical beam expander can be placed immediately after the He-Ne laser 60 as the light source, the size of the optical system can be reduced and the optical axis can be easily adjusted.

FIG. 10 shows an eighth embodiment of the present invention, which is an example used as a reproduction device for the hologram created in FIG. 9. Although the light source in this case may be the same He-Ne laser as in FIG. 9, an example in which a visible light semiconductor laser 80 is used is shown here. 83 in the figure is a light beam expander according to the present invention,
The light emitted from the semiconductor laser 80 is converted into expanded parallel light. This principle is as explained in FIG. FIG. 11 shows the configuration of each component of the hologram reproduction device shown in FIG. 10. A parallel beam 84 emitted from a light beam expander 83 is diffracted by a diffraction grating 81 and becomes a parallel beam 82 tilted by θ with respect to the traveling direction, which is then applied to a hologram dry plate.
66. Since this angle θ is the same as the angle of the reference beam when recording the hologram in FIG. 9, a reconstructed image 87 of the object is observed as shown in FIG. 10. Note that the period A1 of the diffraction grating 81 is AI = λ/lhθ
is given by

High diffraction efficiency can be obtained by using, for example, a holographic grating made of dichromate gelatin as a photosensitive material, or a blazed diffraction grating made of a plastic replica. As shown in FIG. 10, this hologram reproducing device is composed of thin flat plate elements, so it has a much smaller structure than conventional hologram reproducing optical systems.

The optical beam expander described above can be applied to various fields such as optical information processing and optical measurement.

〔Effect of the invention〕

According to the present invention, a small and lightweight optical beam expander can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a light beam expander according to the present invention;
FIG. 2 is a diagram for explaining the principle of this optical beam expander, and FIGS. 3 to 11 are diagrams showing other embodiments of the present invention.
FIGS. 12 and 13 are diagrams showing conventional examples. l... Transparent substrate, 2... Optical waveguide layer, 3, 23...
Concentric grating, 33.43... Second concentric grating, 4,6.54... Incident light, 5,25
... Output light, 7.8, 37.38 ... Guided light, 20
...Reflection film, 50,80°120...Semiconductor laser, 51...Semiconductor laser package, 52...Holder, 53...Grating lens, 63.83...
...Light beam expander, 81...Diffraction grating, 60.10
0... Laser light source, 102, 103... Lens, 6
4104... Beam splitter, 65,105.
... Reflector, 66.106 ... Hologram dry plate, 67
, 107... Object, 87... Reproduction image, 122
... Collimation lens, 123 ... Objective lens, 124 ... Disk substrate, 125 ... Information recording surface.

Claims (2)

[Claims] (1) Consists of an optical waveguide and a concentric grating formed on the optical waveguide, where the wavelength of the incident light is λ, the effective refractive index of the optical waveguide is N, and m is a positive integer. An optical beam expander characterized in that the period Λ is Λ=mλ/N. (2) A second concentric grating is provided on the optical waveguide outside the concentric grating, and the period Λ_2 of the second concentric grating is Λ_2=λ/2N. The optical beam expander according to item 1.