JP2561912B2 - Polarization separation waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A waveguide and a total reflection portion are provided on the surface of a uniaxial crystal substrate such as lithium niobate having an optical axis on the surface to separate ordinary light and extraordinary light. Optical element.

[Field of Industrial Application] The present invention relates to a polarization separation waveguide in which a polarization separation element is provided on the waveguide.

An optical fiber is used as a waveguide used for optical communication, and is used in combination with an optical circuit element such as a polarization separation element, an optical switch, and a lens.

Since these waveguides and optical circuit elements are formed separately, they require fine alignment with each other, which lacks mass productivity and is expensive.

For this reason, the practical application of an optical element in which a waveguide and an optical circuit element are integrated is desired.

[Conventional technology]

As an optical waveguide, an optical waveguide (abbreviated as a waveguide hereinafter) is made by using a transparent substrate in addition to an optical fiber made of transparent quartz or plastic.

For example, after evaporating a metal on a glass substrate to form a metal film, a window with a fine pattern is opened using a photolithography technique (photolithography), and this substrate is immersed in a molten salt for ion exchange. By performing the above, a waveguide having a higher refractive index than the substrate is formed.

In addition, a planar lens continuous with the optical waveguide is made by ion-exchanging the circular region using the same method.

Optical circuit elements are made using transparent uniaxial crystal substrates such as lithium niobate (LiNbO ₃ ) and lithium tantalate (LiTaO ₃ ).

That is, LiNbO ₃ has a large electro-optical effect and is therefore suitable as a substrate for an optical element, and a polarization separator and an optical switch are made using this.

FIG. 4 is a perspective view of the polarization separator. The conventional manufacturing method will be described. A LiNbO ₃ substrate 1 including an optical axis on the surface 1
A thin film of titanium (Ti) is formed on top of this by an electron beam evaporation method, etc., and a waveguide pattern made of a Ti thin film is formed using a photo-etching method. Then, the waveguide 2 having a refractive index larger than that of LiNbO ₃ is formed by heat treatment to diffuse Ti.

Here, the waveguide pattern is composed of two linear waveguides 3, 3 ', 4, 4'that intersect at a slight crossing angle. When laser light is incident on such a waveguide 3, the crossing angle is θ is 1
Degree, TM mode light (light whose electric field component oscillates perpendicularly to the substrate surface) bends and travels to the waveguide 4 ', while TE mode light (light whose electric field component signals in parallel with the substrate surface) is conducted. It has the property of going straight to the waveguide 3 '.

Further, when the crossing angle is 2 degrees, the TE mode light has a property of going to the waveguide 4'bending while the TM mode light goes straight to the waveguide 3 '.

Polarization can be separated by utilizing the fact that the propagation direction of polarized light changes depending on the crossing angle.

However, such a method of separating by the geometrical shape of the waveguide is unstable because it changes depending on the wavelength of laser light, the width and depth of the waveguide, etc. in addition to the crossing angle θ,
Therefore, a stable polarization separation method has been demanded.

[Problems to be solved by the invention]

Single mode laser light oscillated from a gas laser or a semiconductor laser is used for optical communication, but polarization separation is performed when operating optical circuit elements such as an optical modulator, an optical switch, an isolator, and a circulator. It is necessary.

Therefore, it is a problem to put a stable polarization separation element into a waveguide and to put it into practical use.

[Means for solving problems]

The above-mentioned problem is that the uniaxial crystal substrate has an input optical waveguide that is substantially parallel to the optical axis of the substrate, and an optical waveguide that is substantially perpendicular to the optical axis.
Two output waveguides and a total reflection portion including an end face common to the input optical waveguide and the output optical waveguide are provided, and ordinary light and extraordinary light of light incident on the total reflection portion through the input optical waveguide are reflected. This can be solved by a polarization splitting waveguide characterized in that each of the output optical waveguides is arranged at a position corresponding to a corner.

[Action]

The present invention, in a uniaxial crystal substrate having an optical axis on the surface, a waveguide is formed in parallel with the optical axis, and when the light is totally reflected, the ordinary light and the extraordinary light have different refractive indices,
Therefore, the polarization separation is performed by focusing on the different reflection.

That is, in the case of LiNbO _3, the fact that the refractive index (value of wavelength 546 mμ) between ordinary light and extraordinary light is 2.286 and 2.220 is different.

Then, the polarization splitting element integrated with the waveguide is formed by making the total reflection condition on the end face of the crystal substrate or inside the crystal substrate.

〔Example〕

1 and 2 are illustrations of the polarization separator according to the present invention.

That is, a LiNbO ₃ substrate 5 including the optical axis on its surface has a total reflection surface 6 with one end cut off obliquely, and when light is injected through this into an incident waveguide 7 parallel to the optical axis, birefringence occurs. Therefore, different reflection is performed and the TM mode light 8 of ordinary light and the TE mode light 9 of extraordinary light are used for separation, and a waveguide is formed at that position for separation.

The condition for total reflection is that the sine of the incident angle θ is LiNbO ₃
It is larger than the reciprocal of the refractive index n.

Sin θ> 1 / n The total reflection condition can be created not only on the end surface of the LiNbO ₃ substrate 5 but also by etching on the substrate 5.

Next, the reason why the ordinary ray and the extraordinary ray can be separated by the total reflection is as follows.

Letting the refractive index of the ordinary ray be n _o , the refractive index of the extraordinary ray be n _e, and the incident angle of the incident light on the total reflection surface be θ _i , the ordinary ray reflection angle θ _o and the extraordinary ray reflection angle θ _e Can be expressed as θ _o = θ _i (1) θ _e = sin ⁻¹ (n _o / n _e · sin θ _i ) (2) and can be separated because the reflection angles are different. it can.

Next, as a method for forming the separation waveguide, a photolithography technique is used, and dry etching is linearly performed at an angle satisfying the above-described total reflection condition to form a groove 10 having a total reflection surface, which is obliquely crossed. In this case, the incident waveguide 7, the TM mode light 8 and the TE mode light 9 may be formed by patterning a waveguide.

The magnitude of the polarization separation angle 11 of both modes of light is about 1.7 degrees.

Next, FIG. 2 shows a method of separating the incident light propagating through the incident waveguide 7 into TM mode light 8 and TE mode light 9 which are parallel to the incident light. The etching groove 12 thus formed and the etching groove 13 for totally reflecting the TM mode light 8 and the TE mode light 9 separated by the reflection on the total reflection surface may be provided in parallel.

The waveguide 7 can be formed in a thin or thick pattern by the total reflection portion 6 as shown in FIGS. 3A and 3B when the pattern is formed, but the total reflection surface 6 can be adjusted by adjusting the pattern width. If the light in (1) is brought close to a plane wave, the TM mode light 8 and the TE mode light 9 will be completely separated, and the crosstalk between the two can be reduced.

〔The invention's effect〕

As described above, the polarization separation can be stably performed by implementing the present invention, and the polarization separation waveguide combined with the waveguide can be put into practical use.

[Brief description of drawings]

1 is a plan view for explaining the present invention (No. 1) FIG. 2 is a plan view for explaining the present invention (No. 2) FIG. 3 is a plan view for explaining the present invention (No. 3) FIG. 4 is conventional FIG. 6 is a perspective view illustrating the polarized light separation method of FIG. In the figure, 1, 5 is a LiNbO ₃ substrate, 2 is a waveguide, 6 is a total reflection surface, 7 is an incident waveguide, 8 is TM mode light, 9 is TE mode light, 10, 12 and 13 are etching grooves. .

Claims (4)

(57) [Claims] 1. A uniaxial crystal substrate, an input optical waveguide substantially parallel to an optical axis of the substrate, two output optical waveguides substantially perpendicular to the optical axis, and a common input optical waveguide and output optical waveguide. Provided with a total reflection portion including an end face, of the light incident on the total reflection portion through the input optical waveguide,
A polarization splitting waveguide, wherein each of the output optical waveguides is arranged at a position corresponding to the reflection angle of ordinary light and extraordinary light. 2. The polarization splitting waveguide according to claim 1, wherein the uniaxial crystal substrate is made of lithium niobate. 3. The output waveguide has a total reflection portion different from the total reflection portion in the middle thereof, and the ordinary light enters the other total reflection portion through the front half of the output optical waveguide. And each of the latter half of the output optical waveguide is provided at a position corresponding to the reflection angle of extraordinary light, and the latter half of the input optical waveguide and the output optical waveguide are arranged substantially in parallel. The polarization splitting waveguide according to claim 1. 4. The size of either the input optical waveguide or the output optical waveguide is changed in the vicinity of the total reflection section or the another total reflection section. A polarization splitting waveguide according to claim 1.