JPH031641B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a triangular reflection type relief diffraction grating, and its main fields of application include wavelength dispersion elements in spectrometers, hologram media in holoscanners,
Examples include diffraction gratings in optical integrated circuits, grating couplers for optical waveguides, and wavelength division elements in optical communications.

(Prior Art) It is known that high diffraction efficiency can be obtained in a reflective surface relief diffraction grating when the shape of the diffraction grating satisfies a blaze condition.

Here, the blaze condition is obtained by making the diffraction angle of light incident on the diffraction grating match the incidence angle at the triangular slope of the blazed surface.

For example, in FIG. 1, consider the case where the diffraction grating spacing is d and light with a wavelength λ is incident on the linear grating 1 with a blaze angle θ at an incident angle α. At this time, if the diffraction angle of the first-order diffracted light is β, then the blaze condition is satisfied when the incident angle and the diffracted light have a relationship of specular reflection with respect to the blazed surface 2 (i.e., α-θ = θ-β). .

For example, in a reflective blazed diffraction grating used in a grating coupler for an optical waveguide, it is necessary to narrow the grating interval d, but in the above-mentioned reflective blazed grating, as the diffraction grating interval d becomes narrower, Due to the polarization effect of the incident light L, the diffraction efficiency of the diffracted light _L1 is not necessarily high. This tendency is particularly remarkable when the polarization direction of the incident light L is linearly polarized light parallel to the diffraction grating grooves (direction penetrating the plane of the paper in FIG. 1). Furthermore, it is not clear to what extent adjusting the blaze conditions described above is effective in improving the diffraction efficiency. Note that in FIG. 1, the symbol L ₀ indicates zero-order light.

(Purpose) The purpose of this invention is that when linearly polarized light having an electric vector parallel to the diffraction grating grooves is incident,
It is an object of the present invention to provide a reflection type relief diffraction grating that can obtain high diffraction efficiency even if the diffraction grating grooves are narrow.

In order to achieve the above object, the present invention uses a reflective surface relief diffraction grating with a grating spacing of d to transmit linearly polarized light having a wavelength λ and an electric vector parallel to the grating grooves at an angle of incidence α=sin. ^-1
When incident at an angle of (λ/2d), the diffraction grating spacing d and wavelength λ satisfy the condition of d/λ≦1.25, and the diffraction grating shape is formed in an isosceles triangle. be.

Hereinafter, a detailed explanation will be given based on one embodiment of the present invention.

(Structure) Figure 2 shows an enlarged view of the reflection type relief diffraction grating. In the figure, the width d ₁ of the triangular slope (blazed surface) is varied while the grating spacing d and the grating depth are constant. This changes the triangular shape of the diffraction grating. For example, when d ₁ /d=0, the left right triangle,
When d ₁ /d=0.5, the shape is an isosceles triangle, and when d ₁ /d=1, the shape is a right-angled triangle. In the above, d ₁ /d=
When it is 1, it is a diffraction grating shape that can satisfy the blaze condition.

Figure 3 shows the results of a simulation conducted to examine the effect of blaze on a reflective relief grating, and shows the change in diffraction efficiency when the triangular shape of the diffraction grating is changed. Third
The results shown in the figure were obtained under the following conditions.

(1) The incident light is He-Ne laser light with a wavelength λ = 0.6328 μm, and linearly polarized light parallel to the diffraction grating grooves is used.

(2) The angle of incidence is set so that the angle of incidence and the angle of diffraction match, so-called Littrow incidence. The Littrow incidence condition is α = β in Figure 1, and α = sin ^-1
(λ/2d) is satisfied.

(3) The reflector of the diffraction grating is made of vapor-deposited silver.

As shown in the figure, it can be seen that the way the diffraction efficiency changes depends on the spacing of the diffraction gratings. That is, d
> 1.00 μm, the diffraction efficiency is maximum under the blaze condition (d ₁ /d = 1.0), but when d≦0.80, the diffraction efficiency is maximum when the isosceles triangle shape (d ₁ /d = 0.5) is used. It's summery. In other words, wavelength λ=
When light of 0.6328 μm is incident, it can be said that if the diffraction grating spacing is 0.80 μm or less, the diffraction efficiency at d ₁ /d=1.0, which satisfies the blaze condition, is not maximized and there is no blaze effect.

On the other hand, if we look at FIG. 3 from another angle, we get FIG. 7. That is, FIG. 7 shows the results of normalizing the diffraction grating spacing d by the wavelength λ (=0.6328 μm) of the incident light and determining d ₁ /d at which the maximum diffraction efficiency is obtained for each d/λ. In this figure, d/λ≧1.58 is the blaze angle, d ₁ /d=1
1.58 > d/λ > 1.25 is the region where the maximum efficiency is other than blazed angle shapes and isosceles triangles, d/λ≦1.5 is the isosceles triangle shape (d ₁ /d = 0.5)
This is the region where the efficiency is maximum, and this corresponds to the region of the present invention.

From the above study, we normalized the grating spacing by the wavelength of the incident light, and by combining the results of other simulations, we found that when d/λ≦1.25, the grating shape is isosceles for reflective relief gratings. High diffraction efficiency can be obtained when the shape is triangular.

Here, the above study result is that the incident angle α=sin ^-1
(λ/2d), we will confirm that this angle of incidence is optimal, that is, that the maximum diffraction efficiency can be obtained. FIG. 4 shows the change in diffraction efficiency with respect to the change in the incident angle of a triangular reflection type relief diffraction grating. The results shown in the figure show that the reflection type relief diffraction grating has an isosceles triangular shape with a diffraction grating interval d = 0.447 μm and a diffraction grating depth h = 0.53 μm, and has silver deposited on the diffraction grating surface. =
It was obtained by injecting a He--Ne laser beam having a diameter of 0.6328 μm and linearly polarized parallel to the diffraction grating grooves. As shown in the figure, it can be seen that the diffraction efficiency is maximum at an incident angle α=45°. That is, this is the angle of incidence α
= sin ^-1 (λ/2d) means that the diffraction efficiency is maximum. For this reason, d/
It was confirmed that for an isosceles triangular reflective relief diffraction grating with λ≦1.25, the diffraction efficiency is maximized by making light incident at an angle of incidence α=sin ⁻¹ (λ/2d).

The triangular reflection type relief diffraction grating according to the present invention can be produced by marking an aluminum substrate using a known ruling engine. FIG. 5 shows the shape of the diffraction grating produced by the above marking.

Alternatively, it can be produced by a photolithography technique in which a photosensitive resin such as a photoresist is coated on a glass substrate, exposed to two-beam mixed light, and developed. In general, the lattice shape in photolithography technology is a sine wave shape, but by performing overdevelopment, the photosensitive resin 4 on the glass substrate 3 can be made into a substantially isosceles triangular shape as shown in FIG. By forming a light-reflecting metal layer on the surface, an isosceles triangular reflective relief diffraction grating is obtained.

Furthermore, the above-mentioned scratched aluminum substrate,
A replica is created using a known duplication technique using a photosensitive resin layer treated with photolithography technology as a master, and a triangular reflective relief diffraction grating is created by forming a light-reflective metal layer on the surface of the replica. You can also.

(Effect) As described above, in the present invention, in a reflection type relief diffraction grating having a diffraction grating interval of d/λ≦1.25,
By making the diffraction grating shape an isosceles triangle, it is possible to obtain higher diffraction efficiency than a diffraction grating that satisfies the blaze condition.

[Brief explanation of the drawing]

Figure 1 shows the incident light on a reflective relief diffraction grating,
Fig. 2 is an enlarged cross-sectional view of the reflection type relief diffraction grating part explaining the relationship between diffraction grating spacing and slope width; Fig. 3 is an enlarged cross-sectional view of the diffraction grating portion explaining the relationship between diffraction grating spacing and slope width. FIG. 4 is a diagram illustrating the relationship between the triangular shape of the grating and diffraction efficiency, and FIG.
FIG. 6 is an enlarged cross-sectional view of the diffraction grating part in the process of manufacturing triangular reflection type relief diffraction gratings according to the present invention, and FIG. 7 is an enlarged cross-sectional view of the diffraction grating section in the process of manufacturing the triangular reflection type relief diffraction grating according to the present invention, and _FIG . FIG. 2 is an explanatory diagram of the relationship between the two obtained.

Claims (1)

[Claims] 1. In a reflective surface relief diffraction grating with a grating spacing of d, linearly polarized light having a wavelength λ and an electric vector parallel to the grating grooves is
When incident at an angle of = sin ^-1 (λ/2d), the diffraction grating spacing d and wavelength λ satisfy the condition of d/λ≦1.25, and the diffraction grating shape is formed into an isosceles triangle. A triangular reflective relief diffraction grating.