JP3220003B2 - Polarization separation element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization separation element, and more particularly to a waveguide type polarization separation element having an anisotropic optical waveguide.

[0002]

2. Description of the Related Art As an optical substrate, lithium niobate has been widely used as a substrate material for a waveguide type optical functional element because of its large electro-optic effect and electromechanical coupling coefficient. Since the optical waveguide using lithium niobate as a substrate has a refractive index anisotropy, the functional element operates depending on polarization. For this reason, an element for separating polarized light (TM / TE polarized light) is required.

The prior art is disclosed in Japanese Patent Laid-Open No. 6-2892.
No. 41 discloses a technique. This conventional example is shown in FIGS. In the polarization splitting element shown in FIG. 4, a Y-branch optical waveguide including a titanium heat diffusion optical waveguide 42 and a proton exchange optical waveguide 43 is formed on a Z-cut X-axis propagating lithium niobate substrate 41.

[0004] For each of the polarized light of TE and TM inputted to the optical waveguide, a higher-order mode is excited in a region composed of two types of optical waveguides 42 and 43 spreading on a taper just before Y branch. The higher order mode of each excited polarization is based on the optical waveguide 42,
The light propagates through the optical waveguides 42 and 43 with different field distributions depending on the difference in the effective refractive index perceived with respect to the optical waveguide 43. The polarizations of TE and TM are respectively separated from the optical waveguides 42 and 43 at the branch points of the optical waveguides 42 and 43.
Is propagated in accordance with the fundamental mode of That is, T
The M-mode polarized light is guided through the proton exchange optical waveguide 43, and the TE-mode polarized light is guided through the titanium heat diffusion optical waveguide 42 to perform a polarization separation operation.

The polarization element separation shown in FIG. 5 includes a smooth S-shaped bent optical waveguide 52a and an anisotropic optical waveguide 53 that guides only a single polarized light linearly extending from the vent start portion of the optical waveguide 52a. Are formed on the substrate 51. The optical waveguide 53 has a linear titanium diffusion optical waveguide 52b.
Is connected.

The polarization splitting element shown in FIG. 5 has less occurrence of higher-order modes in the branch portion and coupling between the branched optical waveguides than the Y-branch type polarization splitting element in FIG. Separation ratios can be obtained simultaneously. In the lithium niobate substrate 51 of X-cut Y-axis propagation, the polarization of the TM mode as the ordinary ray is the S-shaped bent optical waveguide 52a due to the thermal diffusion of titanium, and the polarization of the TE mode as the extraordinary ray is anisotropic due to the proton exchange light. The polarization splitting operation is performed by propagating through the optical waveguides 53, respectively.

[0007]

Among the above-mentioned conventional polarization splitters shown in FIG. 4, since the mode perturbation is used for splitting the polarized light, (1) the polarization splitting ratio changes with the wavelength. (2) The polarization separation ratio greatly depends on the intersection angles θ ₁ and θ ₂ of the Y-branch, and the fabrication tolerance of the branch portion is small. There is.

In the polarization splitting element shown in FIG. 5, the S-shaped bent portion (return portion) of the S-shaped bent optical waveguide has
There is a problem that bending loss of the optical waveguide occurs.

SUMMARY OF THE INVENTION An object of the present invention is to provide a polarization splitting element having no wavelength dependency and having no angle dependency of a polarization splitting ratio.

[0010]

In order to achieve the above object, a polarization splitting device according to the present invention comprises a first optical waveguide and a second waveguide branched from one optical waveguide on a dielectric substrate. And a polarization splitting element for splitting two linearly polarized lights having electric field components orthogonal to each other into first and second waveguides, wherein the first optical waveguide has a switching portion in the middle.
A linearly polarized light having an ordinary ray propagated therein, having an offset, the second optical waveguide is formed in a linear shape, and an extraordinary ray among the linearly polarized light is propagated. And branches to a branch position of the first optical waveguide.
The offset has a function of shifting the field distribution of the linearly polarized light separated and polarized in the first optical waveguide to an appropriate position.

The offset is provided at a cut-back portion in the middle of the first optical waveguide.

The offset is provided at an input side branch position of the first optical waveguide in addition to a cut-back portion in the middle of the first optical waveguide.

The offset is provided at an output-side connection portion of the first optical waveguide, in addition to a turn-back portion and an input-side branch position in the middle of the first optical waveguide.

Further, lithium niobate is used as the dielectric substrate.

[0015]

The polarization splitting element of the present invention has an S-shaped bend in the second (anisotropic) optical waveguide in which only one of the linearly polarized light components propagating through the optical waveguide is guided. Due to the structure provided by branching at the branch position of the first optical waveguide, the polarization splitting action does not use the mode perturbation. Therefore, the first optical waveguide through which ordinary light propagates has no wavelength dependency and has an S-shape. Due to the bending, a characteristic having no angle dependence of the polarization separation ratio can be obtained.

In the first optical waveguide, an appropriate offset is provided at a cut-back portion (inflection point) near the center, an input-side branch position, and an output-side connection portion.
A polarization component that does not propagate through the anisotropic optical waveguide is guided through the first optical waveguide as it is, and a polarization component that is orthogonal to the first optical waveguide is the first component.
In the (S-shaped bent) optical waveguide and the second (anisotropic) optical waveguide, the second (anisotropic) optical waveguide side is guided to perform a polarization separation function. ) Connection loss to the first (S-shaped bent) optical waveguide is reduced;
(2) In the S-shaped bent portion of the first optical waveguide, the first
(3) The effect of improving the polarization separation ratio can be obtained.

The effects of (1) and (2) are as follows. In the first optical waveguide, the field distribution of the guided light shifts to the outer peripheral side. The distribution can be shifted to an appropriate position, and qualitatively obtained.

The effect of (3) is that the extraordinary ray shifts to the second (anisotropic) optical waveguide because the boundary region where the second waveguide and the first optical waveguide come into contact increases. Can be obtained more smoothly and the bending loss of the ordinary ray is reduced, so that the spatial separation of the extraordinary ray and the ordinary ray is increased, and the polarization separation ratio is increased.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a plan view showing an embodiment of the present invention.

In the figure, the polarized light separating element according to the present invention
First optical waveguide 12 branched from one optical waveguide 12
a and a second waveguide 12b on the dielectric substrate 11, and separates two linearly polarized lights having electric field components orthogonal to each other into first and second waveguides 12a and 12b.
The first optical waveguide 12a and the second optical waveguide 12b are connected to linear optical waveguides 13a and 13b, respectively.

The first optical waveguide 12a has an S-shape having a cut-back portion (S-shaped bend) in the middle.
Out of the linearly polarized light from, and has an offset OS. Offset OS
Has a function of shifting the field distribution of the linearly polarized light (ordinary ray) separated by polarization to an appropriate position. The offset OS is provided at least in the cut-back portion in the middle of the first optical waveguide 12a. In the case of FIG. 1, the offset OS is also provided at the input side branch position and the output side connection portion of the first optical waveguide 12a. I have.

The second (anisotropic) optical waveguide 12b is
The extraordinary ray of the linearly polarized light from the optical waveguide 12 is transmitted.

In the embodiment shown in FIG. 1, a Y-cut X-axis propagating lithium niobate substrate is used as the dielectric substrate 11, and a width of 6 to 12 μm and a thickness of 60 to 20 μm are formed on the substrate 11.
Optical waveguide patterns of the optical waveguide 12, the first optical waveguide 12a, and the optical waveguides 13a and 13b are formed by a 0 nm titanium thin film. Here, 0 to 2 are set at the cut-back portion, the input side branch position, and the output side connection portion of the first optical waveguide 12a.
An offset OS pattern having an offset amount of μm is formed. This titanium thin film pattern is 950 to 11
Thermal diffusion is performed at 00 ° C. to produce optical waveguides 12, 12a, 13a, and 13b having a single mode of titanium thermal diffusion.

Subsequently, an anisotropic optical waveguide 12b branched from the optical waveguide 12 is manufactured by using a proton exchange method. That is, the surface of the lithium niobate substrate 11 is
After masking with a metal film or a dielectric film while leaving the shape of 2b, it is immersed in a benzoic acid solution or a pyrophosphoric acid solution for several minutes to tens of minutes. By further performing a heat treatment, the proton exchange optical waveguide 12b is completed.

The amounts of change in the surface refractive index of the extraordinary ray and ordinary ray due to titanium diffusion: Δne and Δno are respectively +0.0
It is about 02 to +0.004. In contrast, for example,
When it is formed by a proton exchange method using benzoic acid, it changes greatly as ne = + 0.12, Δno = −0.04 at the maximum, and only Δne changes positively. For this reason,
The ordinary ray (TM polarized light) does not guide the proton exchange optical waveguide 12b but propagates along the S-shaped bent optical waveguide 12a as it is. On the other hand, the extraordinary ray (TE polarized light) moves to the proton exchange optical waveguide 12b showing a large change in the surface refractive index and propagates.

As described above, of the randomly polarized light incident on the optical waveguide 12, the TM polarized light component is guided to the output end of the optical waveguide 12a, and the TE polarized light is guided to the output end of the optical waveguide 12b, and polarization separation is performed.

FIG. 2 is a characteristic diagram by numerical calculation for explaining the effect of the present invention. 5. The radius of curvature of the S-shaped first optical waveguide 12a is 200 mm, and the width of the optical waveguide 12a is 6.
It shows a change in the polarization separation ratio with respect to the offset amount: ΔT when the thickness is set to 0 μm. Offset amount ΔT = 0
In the range of .about.0.3 .mu.m, the polarization separation ratio has a maximum of +
A value of 30 dB is obtained by improving by 5 dB.

(Example) Next, an example of the present invention will be described. A photoresist is applied on the Y-cut X-axis propagating lithium niobate substrate 11, and the optical waveguides 12, 12a, 13a, and 13b are formed by the exposure technique into the linear optical waveguide 1
A resist pattern is formed while leaving 2b as a gap of 7 μm. Here, the connecting portion between the optical waveguide 12 and the first optical waveguide 12a, the turning back portion of the first optical waveguide 12a, and the connecting portion between the first optical waveguide 12a and the optical waveguide 13 have a thickness of 0.25 μm.
Is formed. On top of this, a film thickness of 110 nm is formed by sputtering.
A titanium thin film is deposited. Lift off using an organic solvent such as acetone to form an optical waveguide pattern using a titanium thin film. This optical waveguide pattern is subjected to thermal diffusion at 1050 ° C. for 8 hours in an oxygen atmosphere to produce single-mode titanium diffused optical waveguides 12, 12a, 13a and 13b.

Subsequently, the anisotropic optical waveguide 12b branched from the optical waveguide 12 is immersed in a benzoic acid solution at 250.degree. Optical waveguide 12 is provided on the surface of lithium niobate substrate 11.
Mask with a 200 nm aluminum thin film, leaving shape b, and immerse in benzoic acid solution for 3 minutes. further,
By performing a heat treatment at 370 ° C. for one hour in an oxygen atmosphere, the proton exchange optical waveguide 12b is completed.

FIG. 3 is a characteristic diagram based on measured values for explaining the effect of the present invention. The S-shaped radius of curvature of the first optical waveguide 12a is 50 mm, and the width of the optical waveguide 12a is 7.0 μm.
5 shows a change in the polarization separation ratio with respect to the offset amount ΔT of the offset OS in the case of. The offset amount ΔT of the offset OS is set in a range of 0 to 1.0 μm. The excess loss was significantly reduced in TM polarized light as compared with the case without offset, and the polarization separation ratio was significantly improved in TE polarized light, confirming the remarkable effect of the present invention.

[0031]

As described above, according to the present invention, an offset is provided in the S-shaped first optical waveguide through which the ordinary ray of the linearly polarized light propagates, and the electric field position of the ordinary ray is shifted to an appropriate position by the offset. Therefore, (1) the fluctuation of the separation ratio with respect to the wavelength of the guided light can be suppressed to a small value. (2) Deterioration of the separation ratio due to the manufacturing error of the intersection angle is eliminated, and a high separation ratio can be stably obtained. (3) Reduce excess loss compared to no offset,
The polarization separation ratio can be increased.

Therefore, by using the polarization beam splitting element according to the present invention, in an optical integrated circuit requiring a polarization beam splitting function, a high polarization beam splitting ratio can be ensured over a wide optical wavelength band, and a polarization beam splitting element having a large manufacturing tolerance can be supplied. The effect that can be achieved is extremely large.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating a polarization beam splitter according to one embodiment of the present invention.

FIG. 2 is a characteristic diagram for explaining an effect of the polarization beam splitter of the present invention.

FIG. 3 is a characteristic diagram for explaining an effect of the polarization beam splitter of the present invention.

FIG. 4 is a plan view for explaining a conventional technique.

FIG. 5 is a plan view for explaining a conventional technique.

[Explanation of symbols]

 Reference Signs List 11 dielectric substrate 12 optical waveguide 12a first optical waveguide 12b second optical waveguide 13a, 13b optical waveguide OS offset

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12

Claims (3)

(57) [Claims] A first optical waveguide and a second waveguide branched from one optical waveguide are provided on a dielectric substrate, and two linearly polarized light beams having electric field components orthogonal to each other are output from the first and second optical waveguides. A polarization splitting element for splitting the polarization into two waveguides, wherein the first optical waveguide has an S-shape having a cut-out portion in the middle thereof, and an ordinary ray of the linearly polarized light is transmitted, The second optical waveguide is formed in a linear shape, and transmits an extraordinary ray of the linearly polarized light, and is provided at a branch position of the first optical waveguide. The offset has a function of shifting the field distribution of linearly polarized light, which has been polarization-separated to the first optical waveguide, to an appropriate position, and is provided at a turning portion in the middle of the first optical waveguide.
In addition, it is provided at the input side branch position of the first optical waveguide.
Polarization separating element, characterized in that. 2. The method according to claim 1 , wherein the offset includes a first turn-around portion and an input-side branch position in the middle of the first optical waveguide.
2. The polarization separation element according to claim 1 , wherein the polarization separation element is provided at an output side connection portion of the optical waveguide. As claimed in claim 3, wherein said dielectric substrate, according to claim 1 or 2, characterized in that using lithium niobate
Polarization separating element according to.