JPH06100693B2 - Optical waveguide lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION (1) Technical Field The present invention relates to an optical waveguide lens that collects or collimates light in an optical waveguide or an optical waveguide layer formed on a substrate.

(2) Prior Art Conventional optical waveguide lenses include geodesic lenses, Luneburg lenses, Bragg grating lenses and Fresnel lenses. Of these, the geodesic lens and the Luneburg lens are expensive because they require a highly precise polishing technique or the like to manufacture them. The Bragg grating lens has a drawback that the light collection efficiency is significantly reduced even if the incident angle or wavelength of light is slightly deviated from the optimum value.

Fresnel lenses having a stepwise refractive index distribution and a square distribution have been developed so far. The former has a drawback in that divergent light having the same intensity as the converged light is generated and high efficiency cannot be obtained. The latter is currently attracting attention because it can achieve considerably high efficiency. This type of Fresnel lens is, for example, Applied Optics Vol.2.
1, No. 11 (June 1, 1982), page 1966, "Graded
− Index Fresnel lenses for integrated Optics ”.

(3) Prior invention The refractive index square distribution Fresnel lens introduced in the above-mentioned document has a refractive index change caused by electron beam irradiation.
Due to the drawback that only chalcogenide-based amorphous semiconductors such as ₂ S ₃ can be used as a substrate, the applicant previously proposed an improved refractive index square distribution Fresnel lens (Japanese Patent Application No. 59- 230892; JP-A-61-107
304).

6 to 8 show a Fresnel lens according to the invention of the earlier application. FIG. 6 is a perspective view, FIG. 7 is a plan view, and FIG. 8 is an enlarged sectional view taken along the line VIII-VIII of FIG. 6 or 7. An optical waveguide layer (optical waveguide) 23 is formed on a substrate 21 via a buffer layer 22. The buffer layer 22 may be omitted. The Fresnel lens 20 is formed in the optical waveguide layer 23. The lens 20 is composed of 2 m zones whose width becomes narrower toward the outside with the optical axis Z as the center, and in each zone, the square distribution is centered on the optical axis Z (ie, as the origin) in the optical axis direction. A groove 24 having a length (i.e., a length in the Z-axis direction proportional to x ² ) is formed.
The groove 24 has a constant depth, which may be shallower than the thickness of the optical waveguide layer 23.

The effective refractive index of the optical waveguide layer 23 portion where the groove 24 is not formed is
Let n _{e be} the effective refractive index n _{e1 of the} portion where the groove 24 is formed. In general, n _e > n _e1 . The thickness L of the lens 20 and the length of the groove 24 in the optical axis direction are L _m (x). The thickness L of the lens 20 has the same value in all zones. L _m (x) has the above-described square distribution in the X-axis direction. That is,
L _m (x) is given by the following equation.

(| X | << f) (x _m ≤ | x | ≤x _{m + 1} ) f is the focal length of the lens 20, and λ _o is the wavelength of the incident light in vacuum. In addition, m is the number of each zone of lens 20 and is 0
And take a positive integer.

Now, such a Fresnel lens can be produced by using an electron beam drawing device. Optical waveguide layer of substrate 21
23 on top of the electron beam resist (eg PMMA
Type resist) is evenly applied. Then, the electron beam is irradiated according to the pattern for forming the groove 24. When the developing process is performed, the resist in the region irradiated with the electron beam (the region where the groove 24 is to be formed) is removed. By using the resist remaining on the optical waveguide layer 23 as a mask, the region not covered with the resist is exposed to, for example, an ion beam
Dry etch using a gas such as CF ₄ . The groove 24 is formed by this etching. Finally, the resist is removed.

As can be seen from FIGS. 6 and 7, in the outer zone, that is, in the zone where m is large, the shape of the groove 24 has a very sharp portion. On the other hand, there is a limit to reducing the beam diameter of the electron beam in the electron beam writing apparatus, and if the electron beam diameter is reduced and writing is performed, the writing time becomes extremely long.

FIG. 9 shows a groove pattern to be drawn in the electron beam drawing apparatus, and the cross section of the electron beam used for drawing is shown by a large number of circles. Very fine patterns in the outer zone cannot always be drawn sufficiently accurately with an electron beam. As a result, as shown in FIG. 10, the resist pattern after the development process has no sharp point, and the width of the zone becomes wider or narrower.

It is inevitable that the Fresnel lens made by such a manufacturing method will be inferior to the performances expected in the design stage as a matter of course.

SUMMARY OF THE INVENTION (1) Object of the Invention An object of the present invention is to provide an optical waveguide lens having a shape as accurate as possible.

(2) Structure and Effect of the Invention In the optical waveguide type lens according to the present invention, the Fresnel lens is provided on the optical waveguide. This Fresnel
The lens is composed of a groove or a space formed on the optical waveguide in each of a plurality of zones whose width becomes narrower toward the outside with respect to the optical axis. The length in the optical axis direction of the groove or space in each zone is defined by the square distribution in the optical axis direction with the optical axis as the center. The depth of the groove or space in each zone becomes deeper toward the outside with respect to the optical axis, and the maximum length in the optical axis direction of the groove or space of each zone becomes shorter as going outward from the optical axis. Has become.

The difference Δ between the Fresnel lens thickness L and the effective refractive index
The following relationship exists between n _e = n _e −n _e1 .

L = λ _o / Δn _e (2) The formula (2) is L and Δn _e for obtaining ideal lens characteristics.
It represents the relationship with.

In the present invention, the thickness L is changed for each zone of the Fresnel lens. This is represented by L (m). It can be said that L (m) is the maximum length of the groove in each zone in the optical axis direction. The length L (m) is set smaller as it goes outward from the optical axis, that is, as m increases.

Formula (2) (L is replaced by L (m)) must be established for each zone. Therefore, it is necessary to change the difference Δn _e between the effective indices for each zone.
The effective refractive index depends on the thickness of the optical waveguide layer (path). Since the effective refractive index n _e is constant, the same n _e1 changes in each zone, and the effective refractive index n _{e1 of the} grooved portion becomes smaller as it goes to the outer zone, that is, as m increases. Is set. That is, the groove is formed deeper in the outer zone.

According to the present invention, the above-mentioned length L (m) is set smaller toward the outer zone, so that the shape of the groove in the outer zone is not as sharp as that of the conventional one. Therefore, even if the diameter of the electron beam has a certain limit value, it is possible to perform writing in a shape close to the pattern designed by the electron beam writing method, and the completed Fresnel lens is different from conventional ones. It can be expected that the performance will be improved.

Description of Embodiments FIGS. 1 to 3 show an embodiment of the present invention. 1 is a perspective view showing a Fresnel lens formed on a substrate, FIG. 2 is an enlarged plan view thereof, and FIG. 3 is III-I of FIG.
It is a cross-sectional view taken along line II, and the thickness direction is emphasized.
In these, the same components as those described above are designated by the same reference numerals. Also, the Fresnel lens according to the present invention is shown at 30, and its groove is shown at 34.

The length L (m) of the groove 34 in the optical axis direction in the m-th zone
Is expressed by the following equation.

As can be seen from FIGS. 1 and 2 and the equation (3), the maximum length L (m) of the groove 34 in each zone becomes smaller toward the outside with the optical axis Z as the center. There is. Further, as can be seen from FIG. 3, the depth of the groove 34 in each zone becomes deeper toward the outside with the optical axis Z as the center, and the depth satisfies the above equation (2). It is set. The groove depth is uniform in each zone.

Such a Fresnel lens 30 can also be produced by the electron beam drawing and etching described above. In order to change the depth of the groove 34 in each zone, the dose amount is changed for each zone in electron beam writing, and the dose amount increases toward the outer zone. The dose amount can be controlled by changing the scanning speed or the number of scanning times of the electron beam.

When the development process is performed after electron beam writing, a resist having a thickness corresponding to the dose amount remains. When a positive type resist is used, all the resist remains because the electron beam drawing is not performed on the portion where the groove is not formed. In the region where the groove is to be formed, the thickness of the residual resist becomes thicker in the portion closer to the optical axis, and becomes thinner as the distance from the optical axis increases. It is preferable that no resist remains in the outermost zone region.

Dry the optical waveguide layer on the substrate from above such a resist.
The etching yields a Fresnel lens with grooves 34 as shown in FIGS.

Negative type resists can be used and wet etching is also possible.

4 and 5 show views corresponding to FIGS. 9 and 10. In the present invention and the conventional example, the widths of the zones are equal to each other. In the present invention, the maximum zone length (thickness of each lens zone) becomes smaller toward the outer zone. The width of the outer zone is narrower, but the maximum length is also shorter, preventing sharpening of the corners of the groove pattern,
An electron beam writing pattern or a resist pattern having a shape close to the designed one can be obtained.

FIG. 11 shows another embodiment. A Fresnel lens 30 is loaded on the optical waveguide layer 23. That is, instead of forming the above-mentioned groove 34 in the optical waveguide layer, Fresnel
A portion other than the groove 34 of the lens 30 is formed on the optical waveguide layer. For example, using LiNbO ₃ or GaAs as the substrate 21,
Loaded with SiO ₂ layer at a position to form the Fresnel lens on the optical waveguide layer 23 formed on the substrate 21, in which a portion corresponding to the groove 34 is removed by etching in the SiO ₂ layer.

[Brief description of drawings]

1 to 3 show an embodiment of the present invention. FIG. 1 is a perspective view, FIG. 2 is a plan view, and FIG. 3 is III of FIG.
A cross-sectional view taken along line III, FIG. 4 is a plan view showing a state of electron beam writing, FIG. 5 is a plan view showing a formed resist pattern, and FIGS. 6 to 8 are embodiments of the prior invention. FIG. 6 is a perspective view, FIG. 7 is a plan view, FIG. 8 is a cross-sectional view taken along the line VIII-VIII of FIG. 6 or 7, and FIG. 9 and FIG. FIGS. 4 and 5 are plan views corresponding to FIGS. 4 and 5, and FIG. 11 is a side view showing another embodiment of the present invention. 21 ... Substrate, 23 ... Optical waveguide layer (optical waveguide), 30 ... Fresnel lens (optical waveguide lens), 34 ... Grooves formed in Fresnel lens

Claims (1)

[Claims] 1. A Fresnel lens is provided on the optical waveguide, and the Fresnel lens has a groove or space formed on the optical waveguide for each of a plurality of zones whose width becomes narrower toward the outer side with respect to the optical axis. The length in the optical axis direction of the groove or space in each zone is defined by the square distribution in the optical axis direction centered on the optical axis, and the depth of the groove or space in each zone is centered on the optical axis. An optical waveguide lens in which the depth becomes deeper toward the outside and the maximum length in the optical axis direction of the groove or space in each zone becomes shorter toward the outside around the optical axis.