JPH0370201B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel method for preventing surface reflection of optical elements using a diffraction grating.

Conventionally, in order to prevent surface reflection of optical elements such as lenses, prisms, beam splitters, etc., an antireflection film is formed by depositing a single-layer or two-layer transparent thin film on the surface of the optical element. For example, in a two-layer film, the refractive index of each layer is n ₁ , n ₂ , and the thickness is
Assuming d ₁ and d ₂ , when light is perpendicularly incident on this thin film, n ₀ · n ² ₂ = n ² ₁ n _g ...(1) n ₁ , d ₁ = n ₂ · d ₂ = λ /4 ...(2) where, n _p : refractive index of the medium on the side that enters the optical element, such as air n _g : refractive index of the optical element λ : satisfies the relationship of wavelength of light entering the optical element By doing so, the reflectance can be reduced to almost zero.

However, there is no material that completely satisfies the conditions for the refractive index n ₁ and n ₂ , and controlling the film thicknesses d ₁ and d ₂ is also technically quite difficult. Furthermore, since the incident light is usually not monochromatic and the angle of incidence often deviates from the vertical, a perfect antireflection effect cannot be expected.
At present, for example, when the wavelength width of incident light is 200 nm and the width of the incident angle is 20 degrees, it is quite difficult to reduce the reflectance to 2% or less. Furthermore, there are many other problems, such as the need to design and prepare an antireflection film each time depending on the refractive index, angle of incidence, etc. of the optical element.

In the present invention, if the grating interval of the transmission type diffraction grating becomes small and only the 0th order diffraction light exists,
This method utilizes the fact that light propagation in directions other than zero-order light disappears, and reflected light eventually disappears, and provides an anti-reflection method that is effective even when there is a certain range of wavelength, angle of incidence, etc. be.

A detailed description will be given below with reference to the drawings.

The principle of the method of the present invention is as follows. Figure 1 shows an enlarged cross section of a diffraction grating, where the refractive index of the medium is n _p , the refractive index of the medium forming the relief grating is n _g , and the wavelength from one medium to another is λ
Assume that light is incident at an incident angle θ _i . At this time, if the order of the diffracted light that is diffracted by the diffraction grating and enters the medium is m, and the exit angle is θm, the following formula holds (n _g k) ² − (n _{p k} sin θ _i + m2π/D) ² = (n _g kcosθ _n ) ² ...(3) where, k = 2π/λ D: Diffraction grating spacing By the way, if the grating spacing D is made shorter than the wavelength λ, the above equation becomes the 0th order diffracted light with m = 0. This only holds true for high-order diffracted light such as m=±1, ±2, . . . . In this state, the energy of the incident light is concentrated in the zero-order light, and other higher-order diffraction light ceases to exist.

By modifying the above equation to find the conditions under which higher-order diffracted light of m=±1, ±2, etc. no longer exists, the following equation is obtained.

D<λ/(n _p sinθ _i +n _g ) ...(4) Figure 2 is a conceptual diagram showing an example of an optical element that uses the above-mentioned diffraction grating to prevent reflection. The triangular relief type linear diffraction grating 2 formed on the incident surface is exaggerated.

The depth of the diffraction grating is h, and FIG. 3 shows the results of theoretically calculating the change in reflectance when the depth h is changed. However, the refractive index of the beam splitter is 1.52, the refractive index n _g of the medium of the diffraction grating attached to it is 1.5, the refractive index n _p of air is 1, the incident light wavelength λ is 0.550 μm, and the grating spacing D was set to 0.5λ, and the incident light was linearly polarized parallel to the grating. According to this, the reflectance can be suppressed to about 1% when the depth of the grating is approximately λ/4.

As shown in Figure 2, a triangular diffraction grating is produced by coating or depositing a transparent resin or a transparent inorganic material on the light incident surface, and then cutting it with a diamond cutter using a ruling engine as is well known. I can do it. A sinusoidal relief type diffraction grating as shown in FIG. 1 can be easily produced by a known holographic technique in which a photosensitive material such as photoresist is coated, exposed and developed using interference light of two plane waves. FIG. 4 shows an example of an optical arrangement for this purpose, in which a parallel beam 13 from a laser (not shown) is applied to a photosensitive material 12 such as a photoresist coated on an optical element 11, and is divided into two parallel beams by a beam splitter 14. Divided into 15 and 16 parts, each with 1 reflecting mirror.
7 and 18, and the interference light is made incident.
Now, if the angles at which the two parallel light beams enter the photosensitive material 12 are φ _O and φ _R , respectively, then the photosensitive material 12
Interference fringes with an interval D shown by the following equation are generated.

D=λ _O /(sinφ _O +sinφ _R ) ...(5) However, λ _O is the wavelength of the light to be recorded. If this D is within the usage conditions of the optical element 11, (4)
λ _O , φ _O , and φ _R should be selected so as to satisfy the conditions of the equation.

The antireflection effect of the present invention will be compared with that of a conventional two-layer antireflection film. Figure 5 shows the change in reflectance with respect to the change in the wavelength of incident light.
This is when the refractive index of the grating medium n _g = 1.50, and the broken line is a two-layer anti-reflection film, with center wavelength λ = 0.550 μm, n ₁ = 1.39, n ₂ = 1.714 according to equations (1) and (2). , d ₁ =
98.9 nm, d ₂ =80.2 nm. The refractive index of both optical elements is 1.52, and the angle of incidence is 0°. In a two-layer film
The reflectance varies greatly from 0% to 5% in the wavelength range of 0.4 to 0.8 μm, but when the diffraction grating of the present invention is used, the reflectance due to wavelength change is 0.4% to 0.9% in the same wavelength range. Almost no changes can be seen.

FIG. 6 shows changes in reflectance with changes in the angle of incidence θ _i on the antireflection diffraction grating of the present invention.

Two cases are shown: grating depth h = 0.3λ and h = 0.4λ, grating spacing D = 0.5λ, and the refractive index of the grating medium is
It is 1.50. In a range where the angle of incidence changes by as much as 30°, the reflectance can be suppressed to at most 1%.

In the above explanation, the diffraction grating is a linear grating, but
Depending on the purpose of use, a grating of other shapes such as a concentric grating may be used, and it goes without saying that the same effect will be produced on the light exit surface of the optical element.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the principle of the antireflection method of the present invention, Figure 2 is a conceptual diagram when applied to a beam splitter, Figure 3 is a curve diagram showing the relationship between grating depth and reflectance, and Figure 4. 5 is a layout diagram of a hologram recording optical system, and FIGS. 5 and 6 are curve diagrams showing changes in reflectance with changes in incident light wavelength and incident angle. 1... Beam splitter, 2... Diffraction grating, 1
1... Optical element, 12... Photosensitive material, 13... Incident beam, 14... Beam splitter, 17, 1
8...Reflector.

Claims (1)

[Claims] 1. A diffraction grating is provided on the light input/output surface of the optical element,
The diffraction grating spacing D is D<λ/(n _p sinθ _i +n _g ) where λ: wavelength of light incident on the optical element θi angle of incidence on the optical element, n _p : refractive index of the medium on the incident side of the grating, n _g : A surface reflection prevention method for optical elements characterized by satisfying the refractive index relationship of the medium of the diffraction grating.