JPS63147114A - Waveguide type optical phase plate - Google Patents

Abstract

PURPOSE:To effectively obtain the functions of a birefringent crystal plate by forming a stress releasing groove in part of a clad layer to specific length along a core part so that the birefringence main axis direction of an optical waveguide is perpendicular to a substrate surface or slants from the horizontal direction. CONSTITUTION:A quartz-based glass single mode optical waveguide core part 22 is provided to a silicon substrate 21 and the stress releasing groove 31 is formed in a clad layer 23 on one side of the core part 22 along the core part. Therefore, part of stress operating on the optical waveguide from the silicon substrate 21 is released by groove 31, so right-left symmetry about the core part 22 is lost and the birefringence main axes 34a and 34b slant by an angle thetaas compared with there is not the stress releasing groove 31. Consequently, the optical waveguide provided with the stress releasing groove 31 on one side serves equally as a birefringent crystal plate.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a waveguide optical phase plate used for controlling the polarization plane of an optical signal in the fields of optical communications and optical sensors.

[Conventional technology and problems]

In technical fields such as optical communications and optical sensors, optical elements called phase plates are often used to control the polarization plane of signal light. Conventionally, a birefringent crystal plate has been used as a phase plate.

FIG. 7 is a diagram showing the principle of construction of a conventional phase plate called a 172-wave plate. When linearly polarized light 3 is incident and transmitted by tilting the birefringent crystal plate 1 by an angle θ in the birefringent principal axis direction 2,
The light wave is divided into two polarized components, one in the main direction, the other in the direction perpendicular to each other, and the phase of one light wave advances later than the other, and the two are combined when emitted. Polarized light roars. This phase delay is called retardation, and R
It is expressed as If the birefringence value is 81 and the thickness of the plate is j, then R is given by B-J and is usually expressed in units of wavelength 1".

1 when the retardation is 172 of the wavelength λ of the incident light
It is well known that linearly polarized light used in a 172-wave plate, called a 72-wave plate, is emitted as linearly polarized light 4 tilted by 2θ.

However, the configuration shown in FIG. 7 requires a lens system to make the input light 3 incident perpendicularly to the crystal plate 1, and when constructing the intended optical device by incorporating a phase plate, it is difficult to miniaturize the optical device. There were problems such as the lack of stability for light waves to propagate through space.

On the other hand, recent trends in the technological development of optical devices for optical communications and optical sensors have shifted from so-called bulk optical components that combine lens systems and prisms to planar substrates in pursuit of miniaturization, stability, and economy. There is a trend toward waveguide-type optical components and optical integrated circuits that are based on guiding waveguides formed above. especially,
Considering the compatibility with optical fiber, optical waveguide etc.
There are many practical advantages to using a silica-based optical waveguide made of the same material as the optical fiber.

FIG. 8 shows an explanatory diagram of a cross-sectional structure of a silica-based glass single mode guiding waveguide formed on a silicon substrate 21. This optical waveguide is constructed by forming on a silicon substrate 21 a cladding layer 23 in which a core portion 22 is embedded in the inner layer. The thickness of the cladding layer 23 is approximately 50 μm, and the cross-sectional dimension of the core portion 22 is selected to be approximately 6 to 12 μm in accordance with the core diameter of the single mode optical fiber. In this optical waveguide, the signal light is confined in the core part 22 and propagates on the silicon substrate 21.
By appropriately selecting the optical waveguide structure, optical circuit functions such as optical branching and coupling can be obtained.

In FIG. 8, compressive stress is generated in the core portion 22 in a direction parallel to the Ij plate surface due to the difference in thermal expansion coefficient between the silica-based glass optical waveguide and the silicon substrate 21, and due to the photoelastic effect, the leading The wave path exhibits birefringence. It is well known that in such an optical waveguide, the principal axis directions of birefringence exist in two directions: a direction 24a perpendicular to the substrate surface and a direction 24b parallel to the substrate surface. Incident linearly polarized light parallel to the principal axis of birefringence 24a or 24b propagates along the core portion 22 of the optical waveguide with its polarization direction preserved, but in the conventional optical waveguide structure illustrated in FIG. It was difficult to provide a function such as a 172-wave plate that rotates the wave direction midway along the optical waveguide.

The reason for this is that in conventional optical waveguide structures, the principal axis directions of birefringence are limited to two directions, perpendicular and parallel to the substrate surface.

SUMMARY OF THE INVENTION An object of the present invention is to provide a wide-wave optical phase plate that solves the above-mentioned problems.

[Means for Solving the Problems] The present invention provides a stress relief groove in a part of the cladding layer near the guiding waveguide core, thereby changing the principal axis of birefringence of the optical waveguide in a direction perpendicular or parallel to the substrate surface. Its most important feature is that it effectively functions as a birefringent crystal plate. The optical phase plate of the present invention is completely different from the prior art in that it is a bulk type phase plate, and can be constructed as a waveform and continuously incorporated into a part of an optical waveguide.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

〔Example〕

FIG. 1 is a diagram illustrating the principle of the present invention, in which reference numeral 21 denotes silicon 3. (The plate, 22 is a silica-based glass TG-mode optical waveguide core part, 23 is a cladding layer, and 31 is a groove provided in the cladding layer on one side of the core part 22 along the core part. Part of the stress that was acting on the optical waveguide from the silicon substrate 2'1 is released by the groove 31, so the left-right symmetry about the core part 22 is broken, and the birefringent principal axis 3
4a and 34b were found to be tilted by an angle θ compared to the case without the stress release groove 31. Therefore, the optical waveguide provided with the stress release groove 31 on one side can play the same role as the birefringent crystal plate shown in FIG. 7.

The angle θ is mainly defined by the thickness of the cladding layer 23, the height of the core portion 22, and the distance 9lS to the stress release groove 31.

FIG. 2 shows an example of a relationship curve between the main shaft angle θ and the groove path 1111s. The results shown in Figure 2 were measured using the finite element method, and the thickness of the cladding layer 23 was 50 μm.
m, and the height of the center of the core portion 22 from the substrate surface is assumed to be 25 μm. It can be seen that as the bulk distance S becomes smaller, the principal axis angle θ increases. Further, in FIG. 2, the main axis direction 34a is shown.

The refractive index difference between 34b, that is, the birefringence value B, is shown in a form normalized by the birefringence value So when the stress release groove 31 is not provided or when 3==o. S.

The value of B varies somewhat depending on the composition of the silica glass that constitutes the optical waveguide, but is usually B. It is about #4×10-.

Next, a specific example will be shown.

(Example 1) FIG. 3 is an explanatory diagram showing an example of the present invention, and is a plan view of a 1/2 wavelength plate. A stress release groove 31 is provided in the vicinity of a core portion 22 embedded in a silica-based glass cladding layer 23 with a thickness of 50 μm formed on a silicon substrate 21 with a thickness of 0.7M.

The dimensions of the core portion 22 were 8 μm square, and the relative refractive index difference with the cladding layer was 0.25%. In Fig. 3, the groove path l11tS explained in Fig. 1 is set to S''; 35 μm using the relationship in Fig. 2 so that the main axis angle 0 is 22.5°. Stress release The width of the groove 31 was approximately 200 μm. At this time, 8/EL = 0.63, and BO#4X10'
Taking into consideration the tilted main axis angle θ of 3# 2.5X 10″4
A birefringence of −225° can be obtained. Stress release groove 3
The length 1 of 1 is Bi = 1/2λ, that is,
The wavelength used was λ=1.3 μm, and J=2.6 m. The structure of such an optical waveguide and stress release groove is S
Formed by a well-known processing method that combines a glass film deposition technique using a flame hydrolysis reaction of a glass-forming raw material gas such as iC14, Tice<, etc., and a dry etching process of which reactive ion etching is a typical example. I can do it.

In FIG. 3, when linearly polarized light (TM wave) perpendicular to the substrate surface is incident on the core part 22 from the left, it passes through the stress release groove 31 forming part, that is, the waveguide 172 wavelength plate part, It was confirmed that the plane of polarization was rotated by 20=45 degrees.

Actually, in FIG. 3, after passing through the 172-wavelength plate, the polarized light enters the normal optical waveguide region without stress release fRs3i, so the linearly polarized light is While repeating the state and circularly polarized state,
A phenomenon propagating through the core portion 22 was observed, and the stress release groove 31
However, the 172 wavelength plate action of the forming part was mg.

(Example 2) FIG. 4 is a plan view of an optical waveguide showing a second example of the present invention. Unlike the case of the first embodiment shown in FIG. 3, two stress release grooves 41 and 42 are continuously formed on both sides of the core portion 22. Each stress relief groove 41, 42 has the same full distance S and length as in the first embodiment.

The linearly polarized light (TM wave) that enters the leading waveguide core part 22 from the input D'522a or is perpendicular to the plate surface is transmitted through the stress release groove 41.
When passing through the area provided with the stress release groove 42, the plane of polarization becomes linearly polarized light with a tilt of 45°.When passing through the area provided with the stress release groove 42, the plane of polarization is further inclined by 450 degrees, and the plane of polarization becomes linearly polarized light (TE wave) parallel to the substrate 21. ) and was confirmed to be emitted from the emission end 22b. Conversely, when a TE wave is input to the input terminal 22a, a TM wave is obtained at the output end 22b, confirming that the configuration shown in FIG. 4 functions as a TE/TM mode exchanger.

In the above embodiments, a 172-wavelength plate and its combination have been described, but the present invention is not limited thereto. Of course, it is possible to construct various optical phase plate polarization control elements including plates.

(Example 3) FIG. 5 is an explanatory diagram showing a third example of the present invention,
FIG. 4 is a plan view of a single mode optical waveguide provided with stress relief 1f451. The stress release groove 51 of this embodiment is characterized in that the groove path ms gradually changes along the longitudinal direction of the optical waveguide core portion 22.

That is, in the region 51a where the full distance S is large, the core portion 2
The principal axis direction of birefringence in No. 2 is perpendicular (parallel) to the substrate surface, but as the groove distance S decreases toward the region 51b,
The direction of the principal axis of birefringence gradually tilts. In such a structure, for example, when a TM wave passes from the left, the polarization direction can be gradually rotated while maintaining a linearly polarized state, and is useful as a means for -1, I+ 1211 polarization planes.

In this case, since the polarization plane cannot follow the sudden change in the principal axis angle θ, from region l[151a to region 51b,
The 17th measure is to set the distance to be relatively long, for example, about 5M or more.

(Example 4) In each of the above examples, the stress relief grooves all reached the substrate surface, but in some cases, as illustrated in FIG. Set the depth in the middle and corner.

It is also possible to adopt a method of tilting the principal axis of birefringence of the core portion 22 by utilizing the strong stress birefringence generated starting from the portion 61a.

The present invention has been described in detail using a quartz-based optical waveguide on a silicon substrate as an example, but the present invention is not limited to the above-described combination of a substrate and an optical waveguide, and is capable of receiving stress birefringence from the substrate. As long as it is a guiding waveguide, it can also be applied to optical waveguide systems made of other types of substrates or intermediate materials.

〔Effect of the invention〕

As explained above, according to the present invention, by providing stress release grooves along the optical waveguide, it is possible to change the principal axis of birefringence of the leading waveguide, which was fixed in a direction perpendicular or parallel to the plane substrate surface. Therefore, it is possible to provide a waveguide optical phase plate and a polarization plane control element that have excellent compatibility with waveguide optical components and optical integrated circuits.

Furthermore, if the optical phase plate of the present invention is applied to fields such as single-mode optical communication, optical sensors, and optical information processing where polarization control is a serious concern, optical devices can be made smaller, more stable, and more economical. You can get the benefits of

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the principle of the present invention, Fig. 2 is an explanatory diagram showing the action of the stress release groove in the present invention, and Fig. 3 is an explanatory diagram of the principle of the present invention.
Example, FIG. 4 is an explanatory diagram showing an example of M2, FIG. 5 is a third example, and FIG. 6 is an explanatory diagram showing a fourth example. FIG. FIG. 8 is an explanatory diagram showing the effect of a conventional silica-based glass single-mode optical waveguide structure on a silicon substrate. 1... Birefringent crystal plate, 2... Birefringence principal axis direction, 3.4... Linearly polarized light, 21...
...Silicon substrate, 22...Core part, 23...
... Cladding layer, 24a, 24b ... Birefringence main axis, 31 ... Stress release groove, 34a, 34b ... Birefringence main axis, 41.42.
...Stress release groove, 51.61...Stress release groove. Applicant: Nippon Telegraph and Telephone Corporation 21--C) 11 anti-22-11f 7th edition 23-1-kura...) layer 3+----5 beak (Nari Gashima Sohiroji beak) 31 ---U
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Claims (2)

[Claims] (1) A part of the cladding layer of a single mode optical waveguide in which a core portion is embedded in a cladding layer formed on a substrate is arranged so that the birefringent principal axis direction of the optical waveguide is perpendicular or parallel to the substrate surface. A waveguide optical phase plate characterized in that a stress release groove of a predetermined length is formed along the core portion so as to be inclined. (2) The waveguide optical phase plate according to claim 1, wherein the substrate is a silicon substrate and the single mode optical waveguide is a silica glass single mode optical waveguide.