JPH0756010A - Fresnel lens for infrared light - Google Patents

Abstract

PURPOSE:To provide the Fresnel lens for IR light which is shallowed in the depth of grooves, is easily produced and has a good light condensing characteristic by constituting rugged parts of a material having the refractive index of a specific value or above. CONSTITUTION:The rugged parts 2 which depends on wavelengths of incident light and meets the phase modulation quantity of the lens are composed of the material having >=3 refractive index and contg. a Ge crystal 1 or Ge. The rugged parts of the Fresnel lens are composed of the material, for example, Ge, having the high refractive index, by which the Fresnel lens having grooves of shallowed depth and exact lens shape is realized and eventually the Fresnel lens for IR light of a diffraction type having the good light condensing characteristic is easily realized according to such constitution. The constituting material may be any materials, insofar as these materials have >=3 refractive index. The similar effects are obtainable as well even with the Fresnel lens of the diffraction type constituted of the material contg. the Ge and having >=3 refractive index.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared Fresnel lens which has good condensing characteristics for infrared light and is easy to manufacture.

[0002]

2. Description of the Related Art Recently, in addition to a conventional refraction type Fresnel lens, a diffraction type Fresnel lens having small size, light weight, good reproducibility and small aberration has been attracting attention. This diffractive Fresnel lens is also called a Fresnel microlens or micro Fresnel lens because it is manufactured by fine processing such as electron beam drawing.

Since the conventional diffractive Fresnel lens is made of glass or acrylic resin having a refractive index n of about 1.5, like the refractive Fresnel lens, the depth of the groove corresponding to the phase modulation amount of the lens is In order to obtain the maximum light collection efficiency, it is necessary to make the value 1 / (n-1) times, that is, twice the wavelength of the incident light. For example, when the visible light of He-Ne laser of 0.6328 μm is used as the incident light, the groove depth is 1.3 μm, and the infrared wavelength is 10.6 μm.
The depth of the Fresnel lens groove is 21 μm
And need to.

[0004]

However, if a near-infrared diffractive Fresnel lens is made of a substance having a refractive index of around 1.5 as in the conventional example, the groove depth is deep and the lens shape is accurate. However, there is a problem in that it is difficult to obtain an infrared Fresnel microlens having a good light-condensing characteristic.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an infrared Fresnel lens which solves the above problems and has a good light converging characteristic.

[0006]

In order to achieve the above object, the present invention depends on the wavelength of incident light, and the concave and convex portion according to the phase modulation amount of the lens has a refractive index of 3 or more and Ge. Alternatively, it is made of a material containing Ge.

[0007]

The present invention realizes an infrared Fresnel lens which is easy to manufacture and highly efficient because the groove of the Fresnel lens can be made shallow because of the high refractive index of the constituent material.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. 1 (a) and 1 (b) are a sectional view and a plan view, respectively, showing a diffractive infrared Fresnel lens for infrared rays (hereinafter referred to as Fresnel lens) in one embodiment of the present invention.

In FIG. 1, reference numeral 1 is a Ge crystal, and a concavo-convex portion 2 corresponding to the phase modulation amount of a lens having a sawtooth cross section on the surface.
Has been applied. The depth t of the groove of the saw-toothed concavo-convex portion 2 is t = using the refractive index n of the substance forming the lens and the wavelength λ of the incident light in order to maximize the focusing efficiency of the lens.
It is necessary to set λ / (n-1). Infrared transparent G
The refractive index of e is n = 4.0, and in this embodiment, since CO ₂ laser light with λ = 10.6 μm was used as incident light, the groove depth was set to t = 3.5 μm. Like the conventional example, the refractive index of glass, acrylic, electron beam resist, etc. is 1.
In the case of the Fresnel lens made of about 5, the groove depth t needs to be t = 21 μm with respect to λ = 10.6 μm. Therefore, with the Fresnel lens of this embodiment, the groove depth t is It can be said that a thin Fresnel lens could be realized by about 1/6 of the conventional example. As a result, the unnecessary multiple reflection light generated in the vertical portion of the uneven portion is significantly reduced, and it is possible to prevent the diffraction efficiency from being lowered due to the multiple reflection light and to easily form a diffractive Fresnel lens having excellent light-collecting characteristics. realizable.

Further, in order to reduce reflection of incident light,
If at least either the incident side or the reflecting side of the lens is coated with a non-reflective coating, the light collection efficiency will be further improved.

Next, the manufacturing process will be described with reference to FIG.
First, an electron beam resist 3 was coated on the Ge crystal 1 of FIG. 2 (a) as shown in FIG. 2 (b), and a lens pattern was prepared by electron beam lithography as shown in FIG. 2 (c). Next, ion beam etching is performed, as shown in FIG.
As described above, the shape of the electron beam resist 3 was transferred to the Ge crystal 1 to form the uneven portion 2. At this time, electron beam resist 3
The coating thickness was controlled to optimize the groove depth t.

It is to be noted that it is the uneven portion of the surface of the Ge crystal 1 that acts as a lens, and this portion is Ge.
However, other material may be used for the portion having no unevenness.

As described above, according to this embodiment, the depth of the groove is reduced to about 1/6 of that of the conventional example.
Accurate concavo-convex shape without sagging, thin groove depth, and accurate concavo-convex shape resulted in diffractive Fresnel lens with good condensing characteristics. The time for transferring the pattern by beam etching is shortened, and the fabrication is easy.

Although the above description has been made on the diffraction type Fresnel lens using Ge, any substance having a refractive index of 3 or more may be used, and the diffraction type is made of a material containing Ge and having a refractive index of 3 or more. The same effect can be obtained with the Fresnel lens.

The transmission wavelength range of Ge is 1.8 μm
23 μm, and Ge has a refractive index of 3 or more in this region. Therefore, the same effect can be obtained regardless of the wavelength of the incident light in the transmission wavelength range of the crystal.

[0016]

As described above, according to the present invention, by forming the uneven portion of the Fresnel lens with a material having a high refractive index, for example, Ge, the groove depth is shallow and an accurate lens shape is realized. As a result, a diffractive infrared Fresnel lens with good condensing characteristics can be easily realized.

[Brief description of drawings]

1A is a cross-sectional view of an infrared Fresnel lens according to an embodiment of the present invention, and FIG. 1B is a plan view of an infrared Fresnel lens according to an embodiment of the present invention.

2 (a) to 2 (d) are manufacturing process diagrams of an infrared Fresnel lens according to an embodiment of the present invention.

[Explanation of symbols]

 1 Ge crystal 2 Uneven portion

Claims (3)

[Claims] 1. A diffractive Fresnel lens in which an uneven portion depending on the wavelength of incident light and corresponding to the phase modulation amount of the lens is formed on the lens surface, and the uneven portion has a refractive index of 3 or more. And a Fresnel lens for infrared light, which is made of Ge or a material containing Ge. 2. The concavo-convex portion has a saw-tooth shape having a groove depth of λ / (n−1) with respect to a refractive index n of a material forming the concavo-convex portion and a wavelength λ of incident light. The infrared Fresnel lens according to claim 1. 3. The Fresnel lens for infrared rays according to claim 1, wherein at least the front surface or the back surface is provided with an antireflection coating.