JPS58198006A - Transmission type surface relief diffraction grating - Google Patents

Abstract

PURPOSE:To enhance diffraction efficiency by forming a relief layer of no refractive index and a coat layer of (n) refractive index flat on the surface with respect the wavelength of light incident on the diffraction grating, and specifying the refractive indices no and (n). CONSTITUTION:The relief layer 2 made of transparant resin changed in thickness like a sine wave is formed on a transparent flat glass 1, and a coat layer 3 made of transparent resin of (n) refractive index is formed on the layer 2, with the surface 4 of the layer 3 flattened. The diffractive grating thus obtained is raised in diffraction efficiency, with its depth kept constant, by setting the difference of refractive index between the layers 2, 3. The diffraction efficiency can be raised by forming the layer 2 of no refractive index and the layer 3 of (n) so as to satisfy n>2no-1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a relief 1 refractive grating in which diffraction efficiency is improved by disposing a high refractive index layer on the relief surface.

A diffraction grating in which a transparent medium with a uniform refractive index has a thickness that varies depending on the location, and relief-like unevenness is created on the surface of the transparent medium is called a narrow-surface relief diffraction grating. In particular, diffraction gratings made by recording interference fringes on photoresist have sinusoidal irregularities and have high diffraction efficiency for incident light, and are used in optoelectronics such as optical scanning holograms, grating couplers in optical waveguides, and optical communications. Its uses are expanding.

Figure 1 1-i Transparent flat glass l ry! - This figure shows a relief diffraction grating in which a transparent resin layer 2 with a sinusoidal curve is formed. The optical system for such a transmission type IJ-F (diffraction grating is manufactured by porographic technology is shown in FIG. 2).

Light from a laser light source 5 is converted into a traveling light by a lens 6, and focused onto a pinhole 8 by an objective lens 7. The source light that has passed through the pinhole 8 becomes a traveling light again by the lens 9. This traveling light has an almost Gaussian intensity distribution due to the filtering effect of the pinhole 8. Distribution correction vapor deposition filter i.

has a transmittance with an inverse Gaussian distribution, and when a traveling light with a Gaussian intensity distribution is transmitted, it becomes a traveling light with a uniform distribution. This uniformly distributed light is split into two beams by a beam splitter 11, 12 is a reflecting mirror 12, and 12 is reflected by a reflecting mirror 13, both of which are reflected by a photoresist 2 coated on a glass substrate 1.
The beams are irradiated so that they overlap. If you do this,
The two light beams interfere on the photoresist 2 to form bright and dark stripes, and the photoresist 2 is exposed to light according to the bright and dark stripes. If a positive type photoresist is used, after irradiation with light, the exposed area will be removed and a sinusoidal relief as shown in Figure 1 will be formed. The diffraction grating to be produced becomes an inner cone by customizing the photoresist material so that as the grating spacing becomes smaller, the grating depth hfR increases.

The inventor explained that in order to obtain high diffraction efficiency with a transmission type narrow-surface relief diffraction grating, the depth h of the grating satisfies the condition b 7λ 〉o4 with respect to the wavelength λ of the light incident on the diffraction grating V. I found that this is necessary.

However, even when using a positive photoresist with a high degree of resolution, even if the wavelength of the hologram recording light is λ=0.
In the case of 4416 μm, the above-mentioned kettle F! Therefore, when the grating spacing is 0.3 μm or less, it is impossible to obtain a high diffraction efficiency.

The present invention aims to obtain high diffraction efficiency even in a transmission type relief diffraction grating having a surface relief with a small depth h of the grating as described above.

That is, a transparent type garment [m relief 1] as shown in Fig.
In a diffraction grating, the diffraction efficiency increases as the grating depth h increases, but on the other hand, when the depth h is constant, the diffraction efficiency increases as the refractive index nQ of the medium forming the diffraction grating increases. climb.

Taking advantage of this effect, in the embodiment shown in FIG. A coating layer 3 made of a transparent material is provided, and its surface 4
is held flat. In a diffraction grating having such a structure, in order to increase the diffraction efficiency while keeping the grating'R and height h constant, the refractive index difference Δn at the relief interface can be increased.

In the conventional grating structure as shown in FIG. 1, the refractive index difference Δn11d of the relief layer 2 is n□ v
It becomes no-1 for C. On the other hand, in the embodiment of the present invention shown in FIG. 3, the refractive index difference Δn2 is n - n (.Therefore, in order to increase the diffraction efficiency by providing it in the coating layer 3', Δn2)Δn1 A In other words, the refractive index n of the coating layer should be selected so as to satisfy n −n(,:) n□−1 ,', n > 2 nQ−1 0 When a positive photoresist layer is used as the relief layer 2 No# 1.65. Therefore covered! The refractive index n of -3 needs to be 23 or more.

Such high refractive index substances are not found in polymeric materials. An example of an inorganic material that is easy to manufacture is Aa2S, where n is about 2.6. A
s25sVi is an amorphous semiconductor, and a coating layer can be easily formed on the surface of the diffraction grating by vapor deposition.

The effect of the coating layer is the same even when the grating depth h is large.b5 As mentioned above, when the grating spacing is 03 μm or less, the V'i depth h cannot be increased, so the improvement in diffraction efficiency by this invention is This is a particularly necessary target. In this case, if the thickness of the coating layer 3 is 114'&iff .mu.m or more, the uneven shape of the relief layer 2 will be buried, resulting in a flat surface like the surface 4 in FIG. 3.

To give one example, the grid spacing D = 0.447 μm grid (7
) A flat surface 4 was obtained by depositing Aa2Ss to a thickness of about 2 μm, and the diffraction efficiency of 25% before deposition was improved to 45% by the coating layer. .

The present invention has been described above using as an example a case where the present invention is applied to a sinusoidal residual ui direction relief diffraction grating manufactured by holographic ink technology, but the relief shape may be any arbitrary shape such as triangular or rectangular. Regardless of the technique, it is equally effective for diffraction gratings manufactured by injury.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the structure of a normal sinusoidal relief diffraction grating, Figure 2 is an optical path diagram of a holographic optical system, and Figure 3 is a diagram showing the optical path of a holographic optical system.
The figure is a cross-sectional view showing the reconstruction of an embodiment of the present invention. 1. Niglass substrate 2: Photoresist relief J-3: Covering layer 4: Coating layer surface % Junto Kende, Ricoh Co., Ltd. Applicant's agent Patent attorney Aya Sato Man (1 other
name) 1st diagram 3 diagram

Claims (1)

[Claims] It has a relief layer with a refractive index no for the wavelength of light incident on the diffraction grating, and a covering layer with a refractive index n having a thousand degrees of shading on the relief layer, and the refractive indexes nQ, n of both these layers. A transmission type surface relief diffraction grating characterized in that n>2n, )-1.