JPS58174906A - Method for preventing surface reflection of optical element - Google Patents

Abstract

PURPOSE:To eliminate the surface reflection of an optical element virtually by providing a diffraction grating of which the lattice space satisfies the specific relation with the wavelength of incident light, an incident angle, the refractive index of the medium on the incident side of the grating and the refractive index of the medium of the diffraction grating to the incident and exit face of light of an optical element such as a lens or the like. CONSTITUTION:A diffraction grating 2 is formed on, for example, the incident and exit face for light of an optical element 1 (shown by an arrow) of a lens, a prism or the like in such a way that the lattice space D satisfies the equation (lambda is the wavelength of the incident light to the optical element, thetai is the incident angle of light to the element 1, nO is the refractive index of the medium on the incident side of the grating, ng is the refractive index of the medium of the diffraction grating). The grating 2 is formed by forming a triangular diffraction grating wherein the depth (h) of the grating is roughly lambda/4 by coating or vapor deposition of a light transmittable resin or transmittable inorg. material then cutting the same with a diamond cutter or forming the grating by a holographic technique. The surface reflection of the element 1 is thus eliminated virtually.

Description

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

Conventionally, in order to prevent valuable reflections from optical elements such as lenses, prisms, beam splitters, etc., VC has been formed by coating or depositing a two-layer transparent thin film on the surface thereof. For example, in a two-layer machine, if the refractive index is nl, 2, and the warp thickness is d, d2, then the light is ζ
y) When incident perpendicularly to the thin film -2 n011n2- n, ng...
...11) λ n d = fi211 d2= .

11.・・・・・・'2) However, no: @D medium incident on the optical element, for example, air refractive index ng: refractive index of the optical element λ: wavelength of light incident on the optical element 7) so as to satisfy the relationship By doing so, the reflectance can be reduced to almost nothing.

However, it is technically quite difficult to control the thickness d1, d27) because there is no material that completely satisfies the refraction conditions (nl, n27). Moreover, the incident light is usually not monochromatic, and the angle of incidence often deviates from the vertical, making it impossible to expect a perfect antireflection effect. Currently, if the incident light wavelength width is 2tlUnm and the incident horny wavelength width is 20,
It is quite difficult to achieve a reflectance of 121 or less.

Furthermore, depending on the refractive index of the optical element, the angle of incidence, etc., anti-reflection must be designed and created in each case, and the present invention involves many gaps. The grating spacing of the diffraction grating is small,
If only the 0th-order new party exists, there will be no propagation of light in any direction other than the 0th-order light, and the reflected light will eventually disappear.Using this fact, we can use Motsutsu 2 provides an effective anti-reflection method
・.

A detailed explanation will be given below with reference to one drawing.

This invention O method original! a is as follows υ. Figure 1 shows an enlarged cross section of the diffraction grating, the medium 17) has a refractive index of n□, and the relief of the diffraction grating is 11'j! [nD
It is assumed that the refractive index is fng, and the wave λD light is incident from the four mediums (2) to the medium (n) at an incident angle θl. When the diffraction light is (b) broken by one diffraction grating and enters the medium 8, the following formula holds (ngk)2-(n,)ksinθi+m ! #)2=
(ngkeO8θ,,l...-13) However, k=2''/λ D: Diffraction grating spacing By the way, if the grating spacing is made shorter than the wavelength λ, the above equation becomes m=o Oo
D holds true for the next-next-order light, and does not hold true for higher-order diffracted light such as m=±1, ±2, . . . . When the D state is reached, the D energy of the incident light is concentrated on the ViO-order light, and other higher-order diffraction light ceases to exist.

Modifying the above formula, m = ±1, ±2... p High order (b) Clause where the new party ceases to exist? ! The following equation is obtained when calculating l-.

D〈λ/ (no sin eH+ ng) −
! 4) Figure 2 is a conceptual diagram showing an example of an optical element 7) in which reflection is prevented by the above-mentioned O diffraction grating, in which a three-shaped beam splitter is formed on the incident surface of the three-shaped IJ-7f.
The LiD linear diffraction grating 2 is shown in an exaggerated manner.

The diffraction grating 7) has a depth of h, and FIG. 3 shows the results of theoretically calculating the D reflectance change when the depth h is changed. However, the refractive index of the beam splitter D is 1.52, and the diffraction m-ton v medium v refraction liIcn attached to this
g to 1.5. Air O refractive index n0tl, incident light wavelength λ is 0
．． 550 μm, and the grating spacing is 05λ, and the incident light is directly polarized light parallel to the grating.According to this, the grating v9
It's almost false and the reflection *ell can be suppressed to a very low level.

A triangular diffraction grating as shown in Fig. 2 is produced by coating or vapor-depositing a transparent resin or an inorganic material 'Pr' on the light incident surface, and then cutting it with a Guimond cutter using a ruling engine as is well known. I can do it. A sinusoidal relief type diffraction grating as shown in Fig. 1 is made by coating a photosensitive material such as photoresist, and transmitting 2 D, 1,000,000 waves and 1! Former
It can be easily created using the known D holographic ink technology. FIG. 4 shows an example of the D optical arrangement, in which an O-parallel beam 13 from a laser (not shown) is sent to a photosensitive material 12 such as a photoresist coated with VC on an optical element ll by a beam splitter 14. Korai 15
．． The beam is divided into 16 beams, each reflected by a reflecting mirror 17 and 18, and the interference light is made incident. Now, if the angles at which two D parallel beams are incident on the photosensitive material 12 are respectively φo1φ, interference fringes are generated on the photosensitive material 12 at intervals shown by the following equation.

D=λ0/(II l nφo−4−stnφkL
) = (5) However, λ0: Recording wavelength of light D is the optical element 11'7) Within the range of the housing 14)
λ0, φ0, and φR may be selected so as to satisfy the conditions of the equation.

The antireflection effect of the present invention is compared to that of the conventional antireflection film with two layers. Figure 5 shows the change in reflectance with respect to the change in the wavelength of incident light, and the solid line Vitm grid V # h =
0.24 μm, lattice spacing D = 9.25 μm, and the refractive index of the lattice medium i1g = 1.50.
The broken line is a two-layer anti-reflection film, and the center wavelength λ = 0.55 (Jμm1J” 1.39 n2 according to the formula ':t)L2>
= 1.714, d1 = 93.9 nm.

dε80.2 fl m. The refractive index of both optical elements υ is 152. The angle of incidence is ViO. ti04 with double layer film
~0.8μm 7) [! Reflectance ranges from 0 to 5-
However, when the diffraction grating of the present invention is used, the reflectance changes in the same wavelength range from 0.4% to 0.9%, with almost no change in reflectance due to wavelength change. Vi anti-reflection of the present invention (b) inside the incident side of the folded grating edge 1
01 shows the change in reflectance due to a change in reflectance.

The case is shown in which the grating depths h = 0.3λ and h = 04λD2, the grating spacing D = 0.5λ, and the grating grain refractive index is 1.50. In a range where the angle of incidence changes as much as 30 degrees, the reflectance can be suppressed to at most 1 inch.

In the above explanation, the diffraction grating was a linear grating, but depending on the purpose of use, it may be a concentric circular grating or a square grating.
Needless to say, a four-way effect* occurs even when the D light O is emitted rjnVr from the optical element.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the anti-reflection method of the present invention, Figure 2 is a conceptual diagram of the anti-reflection method applied to a beam splitter, and Figure 3 is a curve diagram showing the relationship between grating depth and reflectance. The figure is a layout diagram of a hologram 'D recording system, and FIGS. 5 and 6 are curved diagrams showing changes in reflectance due to changes in the incident wave and incident angle. l: Beam splitter 2: Diffraction grating ll: Optical element 12: Photosensitive material 13: Incident beam
14: Beam splinter 17.18: Reflector Special Fraud Examiner Legal Company Ricoh rating r depth h/λth, (Fig. 0.4 0.5 0.6 0.7 0
．． Fl wavelength ≠ m) 111 (1,20 co3() incident angle 11i f degrees)

Claims (1)

[Claims] A diffraction grating is provided on the light output surface of the optical element. The diffraction grating spacing is D〈λ/ (no ainθi+ng') where λ:
Wavelength of light incident on the edge of the optical element θt: Incident angle on the edge of the optical element %
"O: incidence of grating 11117) Medium υ refractive index, n: Diffraction grating υ medium 0 refractive index" optical element O surface reflection prevention method characterized by satisfying the relationship: