JPH0749407A - Fresnel lens for infrared ray - Google Patents

Abstract

PURPOSE:To provide a Fresnel lens of excellent light converging quality by constituting a rugged part which is dependent on the wavelength of the incident light and corresponds to the phase modulation of the lens of the specified material. CONSTITUTION:A rugged part 2 corresponding to the phase modulation of a lens with serrated section is provided on the surface of the Si crystal 1. The depth (t) of the groove of this rugged part 2 is set so as to meet the relationship t=lambda/(n-1) by using the refractive index (n) of the substance constituting the lens and the wavelength lambda of the incident light to maximize the light converging efficiency of the lens. The refractive index of the near-infrared and transparent InP is n=3.5, and the semi-conductor laser beam of lambda=1.55mum is used as the incident light, and the depth of the groove t=0.62mum. This constitution can realize a thin Fresnel lens whose thickness is about 1/5 of the conventional lens. As a result, the quantity of the unwanted multiply reflected light to be generated at the vertical part of the rugged part 2 is greatly reduced, the reduction of the diffraction efficiency attributed to this multiply seflected light is prevented, and a diffraction tpe Fresnel lens excellent in the light-converging efficiency can be easily realized.

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 0.6328 μm of a He-Ne laser of visible light is used as incident light, the depth of the groove is 1.3 μm, but this has a near-infrared wavelength of 1.5 μm.
When it comes to use, the groove depth of the Fresnel lens needs to be 3 μm.

[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.

The present invention solves the above problems,
It is an object of the present invention to provide an infrared Fresnel lens having good light collecting characteristics.

[0006]

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

[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 Si crystal, and a concavo-convex portion 2 corresponding to the phase modulation amount of a lens having a sawtooth cross section on its 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). The refractive index of Si which is transparent in the near infrared is n = 3.5, and in this embodiment, since the semiconductor laser light of λ = 1.55 μm was used as the incident light, the groove depth was t = 0. It was 62 μm. In the case of a Fresnel lens made of glass, acrylic, electron beam resist or the like having a refractive index of around 1.5 as in the conventional example, the groove depth t is λ = 1.55 μm, whereas t = 3.1 μm. Therefore, it can be said that the Fresnel lens of the present embodiment can realize a thin Fresnel lens with the groove depth t being about 1/5 of that 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 Si crystal 1 shown in FIG. 2A as shown in FIG. 2B, and a lens pattern was made as shown in FIG. 2C by electron beam lithography. Next, ion beam etching was performed, and the shape of the electron beam resist 3 was transferred to the Si crystal 1 as shown in FIG. At this time, the coating thickness of the electron beam resist 3 was controlled to optimize the groove depth t.

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

As described above, according to the present embodiment, the depth of the groove is reduced to about 1/5 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.

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

The transmission wavelength region of Si is 1.2 μm
16 μm, and Si 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, Si, 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 Si 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, which is made of Si or a material containing Si. 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.