JP2003270468A - Beam diameter transformation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To decrease a beam diameter. <P>SOLUTION: A tapered waveguide 22 has a waveguide width which is gradually narrower from an incident port to an emitting port. A waveguide 24 having the same waveguide width as that of an emitting port 22b is connected to the emitting port 22b of the tapered waveguide 22. A mode waveguide 26 having a constant waveguide width narrower than that of the waveguide 24 is connected to the other end of the waveguide 24. The waveguide 22 has multimode at the incident port 22a, but has only zeroth mode, primary mode, and secondary mode at the emitting port 22b. The waveguide 24 has only zeroth mode, primary mode, and secondary mode. The waveguide 26 has only zeroth mode. The waveguide width discontinuously changes at the connected portion of the waveguide 24 and the waveguide 26. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a beam diameter conversion device for converting the beam diameter of a light wave.

[0002]

2. Description of the Related Art As an optical device for converting a light wave having a large beam diameter, such as output light of a multimode light source, into a smaller beam diameter, as shown in FIG. Waveguide 10 and waveguide 10
There is known a configuration including a single mode waveguide 12 which is connected to the emission port of and has the same waveguide width as the emission port. The refractive indexes of the claddings of the waveguides 10 and 12 are both n ₂ , and the waveguide 10
The refractive indices of 12 and 12 are both n ₁ .

In the tapered waveguide 10, at the entrance 14, the higher-order modes higher than the third order can exist as the guided mode, and at the exit 16, only the 0th-order mode can exist as the guided mode. , A waveguide parameter, for example, a waveguide width is set. When a light wave having a substantially centrosymmetrical field distribution and including a higher-order mode component is incident on the incident light 14, the light wave is propagated through the tapered waveguide 10 into a lower-order mode, specifically, a zero-order mode. Transition.

The waveguide parameter of the exit 16 of the tapered waveguide 10 matches the waveguide parameter of the single mode waveguide 12. At the exit port 16, the light wave is coupled to the single mode waveguide 12 lossless in principle.

In this way, a light wave having a large beam diameter can be reduced to a beam diameter of the 0th mode of the single mode waveguide 12.

[0006]

Generally, the output light of a multi-mode light source has power concentrated in the central portion as approximated by a Gaussian distribution, and the lower order mode has a larger power. Therefore, when converting multimode light having a higher-order mode component into a light wave of only the 0th-order mode, it is considered important that the conversion efficiency from the 1st-order mode to the 0th-order mode is high. However, in the case of a waveguide that is axially symmetric with respect to the propagation direction as shown in FIG. 21, the first-order mode that is an odd-order mode is not converted to the zero-order mode that is an even-order mode. That is, all odd modes are lost.

When the incident light is only in the even mode, the efficiency of conversion from the secondary mode to the mode of 0 is increased to increase the overall mode conversion efficiency.

The mode of mode conversion in the tapered waveguide 10 can be understood from the backward traveling property of the light wave. In the conventional example shown in FIG. 21, when the light wave is made to travel backward and the 0th-order mode of the waveguide 12 is incident on the exit 16 of the tapered waveguide 10, the tapered waveguide 10 has a secondary mode regardless of the taper angle. The conversion efficiency to a higher-order mode such as is low, and most of it propagates in the 0th-order mode.

This is because, when a higher-order mode higher than the second-order mode is incident from the entrance 14, even if the higher-order mode shifts to the second-order mode during propagation through the waveguide 10, the second-order mode changes to the zero-order mode. This means that the transition to is rare. That is, in the conventional example, most of the components of the first-order mode and higher incident on the incident light 14 are lost, and the 0th-order mode incident on the entrance 14 is simply taken out, and the mode diameter is set to 0 of the single-mode waveguide 12. Only the mode diameter of the next mode is reduced. In the conventional example, there is too much energy loss.

An object of the present invention is to present a beam diameter conversion device that reduces the beam diameter of a light wave more efficiently.

[0011]

In the mode conversion device according to the present invention, the width of the waveguide becomes narrower from the entrance end toward the exit end, and the 0th mode, the 1st mode and the 2nd mode exist at the exit end. A possible first waveguide, and a second waveguide connected to the output end of the first waveguide and having a constant waveguide width in the optical axis direction, the zero-order mode, the first-order mode, A second waveguide in which a secondary mode can exist and a third waveguide for single mode transmission which is connected to the output end of the second waveguide and has a constant waveguide width in the optical axis direction. The phase difference between the zeroth-order mode and the second-order mode in the first waveguide and the second waveguide is substantially equal to 2nπ (where n is an integer).

In the beam diameter conversion device according to the present invention, the width of the waveguide becomes narrower from the entrance end to the exit end,
A first waveguide in which a 0th-order mode, a 1st-order mode, and a 2nd-order mode can exist at the emission end and an output end of the first waveguide are connected, and the waveguide width is constant in the optical axis direction. A third waveguide for single-mode transmission, wherein the phase difference between the zeroth-order mode and the second-order mode in the first waveguide is substantially equal to 2nπ (where n is an integer). Characterize.

In the beam diameter conversion device according to the present invention, the waveguide width becomes narrower from the entrance end to the exit end,
A first waveguide in which a 0th-order mode and a 1st-order mode can exist at the emission end and a second waveguide connected to the output end of the first waveguide and having a constant waveguide width in the optical axis direction. A second waveguide that is a waveguide and is capable of having a zero-order mode and a first-order mode, and a single mode that is connected to an output end of the second waveguide and has a constant waveguide width in the optical axis direction. A third waveguide for transmission, wherein the phase difference between the zeroth-order mode and the second-order mode in the first waveguide and the second waveguide is substantially nπ (where n is an integer). And the optical axis of the third waveguide is offset from the optical axis of the second waveguide.

In the beam diameter conversion device according to the present invention, the waveguide width becomes narrower from the entrance end to the exit end,
A first waveguide in which a 0th-order mode, a 1st-order mode, and a 2nd-order mode can exist at the emission end and an output end of the first waveguide are connected, and the waveguide width is constant in the optical axis direction. Second
Of the second waveguide in which the 0th-order mode, the 1st-order mode, and the 2nd-order mode can exist, and the waveguide is connected to the output end of the second waveguide, and the waveguide width is in the optical axis direction. Third that is constant
Of the third waveguide in which the 0th-order mode and the 1st-order mode can exist, and a single waveguide having a constant waveguide width in the optical axis direction, which is connected to the output end of the third waveguide. A fourth waveguide for one-mode transmission, wherein the phase difference between the zeroth-order mode and the second-order mode in the first waveguide and the second waveguide is substantially 2nπ (where n is an integer). ) And the phase difference between the 0th-order mode and the 1st-order mode in the first waveguide, the second waveguide, and the third waveguide is substantially equal to mπ (where m is an integer). The optical axis of the fourth waveguide is offset from the optical axis of the third waveguide.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings. In order to facilitate understanding, in each waveguide configuration, the cores of the respective waveguides have the same refractive index and the cladding has the same refractive index, and only the waveguide widths are different.

(First Embodiment) FIG. 1 shows a plan view of a waveguide structure according to a first embodiment of the present invention. A tapered waveguide 22, a waveguide 24 having the same waveguide width, and a waveguide 26 having the same waveguide width are serially arranged on the substrate 20.

The tapered waveguide 22 is a waveguide in which the waveguide width gradually narrows from the entrance 22a to the exit 22b. At the entrance 22a of the tapered waveguide 22, higher-order modes higher than the third order can exist, but at the exit 22b, only 0th-order, first-order, and second-order modes can exist.
There can be no more than 4th mode.

A waveguide 24 having the same waveguide width as the waveguide width of the emission port 22b is connected to the emission port 22b. That is,
In the waveguide 24, only 0th-order, 1st-order, and 2nd-order modes can exist.

A waveguide 26 having a constant waveguide width is connected to the other end of the waveguide 24. Only the 0th mode can exist in the waveguide 26. Therefore, the waveguide width of the waveguide 26 is
It is narrower than the waveguide width of the waveguide 24, and the waveguide width changes discontinuously at the connection point of the waveguide 24 and the waveguide 26.

It is assumed that a multimode light wave consisting of only even-order modes is incident on the entrance 22a of the waveguide 22. This light wave is expanded into a plurality of waveguide modes, and the tapered waveguide 22
Propagate. As it propagates through the tapered waveguide 22, a part thereof becomes a radiation mode and leaks to the outside, but a part thereof is converted into a low-order even-order mode. Finally, the exit port 22b has only the second-order mode and the zero-order mode.

In the waveguides 22 and 24, the result of integrating the difference between the propagation constant of the 0th-order mode and the propagation constant of the 2nd-order mode in the range of the waveguides 22 and 24 is equal to 2nπ (n is an integer). Just do it. That is, ∫ ₀ ^b (β ₀ −β ₂ ) dz = 2nπ (1) where β ₀ is the propagation constant of the 0th-order mode in the waveguides 22 and 24, and β ₂ is the propagation constant of the 2nd-order mode in the waveguides 22 and 24. It is a propagation constant. Under this phase condition, the waveguide 24
Since the 0th-order mode and the 2nd-order mode excite the waveguide 26 in the same phase, it is higher than the case where the 0th-order mode and the 2nd-order mode of different phases are excited, and the case where only the 0th-order mode is excited. The 0th-order mode of the waveguide 26 can be efficiently generated.

Further, in the tapered waveguide 20, β ₀ , β ₂
Changes with the propagation, and it suffices to satisfy Expression (1) by adding the change.

(Second Embodiment) With the configuration shown in FIG. 1, the optical length L ₂ and the waveguide mode of the waveguide 24 are set so that (β ₀ -β ₂ ) L ₂ = 2nπ (2) holds. In that case, as shown in FIG. 2, the waveguide 24 may be substantially omitted.

The effect of the configuration shown in FIG. 2 was quantitatively compared with the conventional example. In the configuration shown in FIG. 2, the waveguide width of the entrance 22a is 50 μm, the waveguide width of the exit 22b is 12 μm, and the waveguide width of the waveguide 26 is 6 μm, with respect to the length L ₁ of the tapered waveguide 22. We investigated how the propagation loss changes. The relative refractive index difference between the core and the clad is 1.5
It is 0.6% at 5 μm. The height of the waveguides 22 and 26 is
All are 6 μm. Multi-mode light having a mode diameter of 50 μm in the width direction and a constant intensity in the width direction is incident on the entrance port 2.
It was incident on 2a. FIG. 3 shows the waveguide 26 with respect to the amount of incident light.
The measurement result of the loss calculated from the ratio of the 0th-order mode light amount is shown. The horizontal axis represents the optical length L (=
L ₁ ), the vertical axis represents loss. The beam propagation method (BPM) was used to estimate the loss. A minimum value appears in the loss in a cycle in which the phases of the 0th-order mode and the 2nd-order mode match.
Even if the optical length of the tapered waveguide 22 is 2000 μm or less, a coupling loss of a minimum value of 0.3 dB or less is obtained.

For comparison, with respect to the configuration shown in FIG.
The relationship between the optical length and the loss of the tapered waveguide 10 was investigated. The result is shown in FIG. The horizontal axis represents the optical length L of the tapered waveguide 10, and the vertical axis represents the loss. Parameters other than that the waveguide width of the exit of the tapered waveguide 10 is equal to the waveguide width of the single mode waveguide 12, are the same as the measurement in the configuration shown in FIG. 3, and the measurement method is also the same.

In the conventional example shown in FIG. 21, the tapered waveguide 1
The longer the length of 0, the lower the loss. This is because the radiation loss in the tapered waveguide 10 decreases as the taper slope becomes gentler. The length of the tapered waveguide is 6000
The decrease in loss saturates from around μm, and even at 10,000 μm or more, it does not fall below about 0.67 dB.

As can be seen from the above, the embodiment shown in FIG. 2 is not only higher in coupling efficiency than the conventional example, but also
The waveguide can be shortened and can be made compact.

FIG. 5 shows that the length of the tapered waveguide 20 is 506.
The relationship between the coupling loss and the waveguide length near 6 μm is shown.
Length of 5066 μm showing the minimum value of 0.2395 dB
Within the range of ± 5 μm, the increase in loss is suppressed within 0.025 dB.

(Third Embodiment) FIG. 6 shows a plan view of a waveguide structure according to a third embodiment of the present invention. Since multi-mode light generally has higher power in lower order modes,
Even if the loss is in the secondary mode or higher, the total loss does not decrease so much.

A tapered waveguide 32, a waveguide 34 having the same waveguide width, and a waveguide 36 having the same waveguide width are arranged on the substrate 30.

The tapered waveguide 32 is a waveguide in which the width of the waveguide gradually narrows from the entrance 32a toward the exit 32b. At the entrance 32a of the tapered waveguide 32, higher and higher-order modes can exist, but at the exit 32b, only 0th and 1st-order modes can exist, and 3rd-order and higher modes cannot exist.

A waveguide 34 having the same waveguide width as the waveguide width of the emission port 32b is connected to the emission port 32b. That is,
In the waveguide 34, only 0th and 1st order modes can exist.

The other end of the waveguide 34 is connected to a single-mode waveguide 36 having a constant waveguide width and displaced from the optical axis of the waveguide 34 by a distance d. Only the 0th mode can exist in the waveguide 36. Therefore, the waveguide width of the waveguide 36 is
It is narrower than the waveguide width of the waveguide 34, and the waveguide width changes discontinuously at the connection point between the waveguide 34 and the waveguide 36.

In the waveguides 32 and 34, the result of integrating the difference between the propagation constant of the 0th-order mode and the propagation constant of the 1st-order mode in the range of the waveguides 32 and 34 is equal to nπ (n is an integer). Just do it. That is, ∫ ₀ ^b (β ₀ −β ₁ ) dz = nπ (3) where β ₀ is the propagation constant of the 0th-order mode in the waveguides 32 and 34, and β ₁ is the first-order mode of the waveguides 32 and 34. It is a propagation constant.

Under this phase condition, the 0th-order mode and the 1st-order mode of the waveguide 34 excite the waveguide 36 in the same phase or opposite phases. The offset amount d of the waveguide 36 with respect to the waveguide 34 is adjusted according to the phase relationship between the 0th-order mode and the 1st-order mode. In other words, the offset d is adjusted so that the 0th mode and the 1st mode excite the waveguide 26 with the highest total efficiency. As a result, as compared with the case of exciting only the 0th mode,
The next mode can be created.

When the waveguide width of the waveguide 34 is w, the offset d of the waveguide 36 is w / 2>d> 0. The offset d actually depends on the power ratio between the first-order mode and the zero-order mode at the exit of the waveguide 34.

The relationship between the offset amount d of the waveguide 36 and the coupling efficiency when the phases of the 0th-order mode and the 1st-order mode coincide with each other at the exit of the waveguide 34 was obtained by the BPM method. The refractive index of the clad is 1.445, and the waveguides 32 and 3 are
The refractive index of 4,36 was set to 1.456. The width of the waveguide 34 was 8 μm, and the width of the waveguide 36 was 4 μm. Waveguide 34
FIG. 7 shows the calculation results when the power ratios of the 0th-order mode and the 1st-order mode at the emission port of are 1: 1, 2: 1 and 4: 1.
Shown in. The horizontal axis represents the offset d (μm) of the waveguide 36 with respect to the waveguide 34, and the vertical axis represents the coupling efficiency from the waveguide 34 to the waveguide 36. When the optical axes of the waveguide 34 and the waveguide 36 coincide with each other, d = 0.

As is apparent from FIG. 7, the offset d showing the maximum value of the coupling efficiency depends on the power ratio between the first-order mode and the zero-order mode. If the power ratio between the first-order mode and the zero-order mode can be estimated in advance, the optimum offset d
Then, the waveguide 36 may be arranged in the waveguide 34. Even if the power ratio is unknown in advance, for example, by setting the waveguide width of the waveguide 34 to w and setting the position to w / 4 from the center of the waveguide 34 (the position of 2 μm in the case of FIG. 7), In the numerical example,
Coupling efficiency 90 in the amplitude ratio range of 1: 1 to 4: 1
% Or more is obtained.

As a result, under the proper offset d, the 0th-order mode and the 1st-order mode of the waveguide 34 have the same phase and the waveguide 3
6 is excited, therefore, in the embodiment shown in FIG. 6, the efficiency of the waveguide 36 is higher than that in the case of exciting in the 0th-order mode and the 1st-order mode having different phases and in the case of exciting only in the 0th-order mode. A 0th-order mode can be generated.

(Fourth Embodiment) As shown in FIG.
0 is separated into a substrate 30a that holds the waveguides 32 and 34 and a substrate 30b that holds the waveguide 36, and the substrate 30b is movable in a direction orthogonal to the optical axis of the waveguide 36, thereby offsetting Adjustment of d becomes easy.

(Fifth Embodiment) The propagation constant of the guided mode is
It can be adjusted by heating the waveguide. That is, the refractive index changes due to heating, and the change in the refractive index changes the propagation constant. The waveguides 22, 24, 32 in each of the above-described embodiments,
The core and / or clad of 34 is made of a material whose refractive index is easily changed by temperature, such as SiO ₂ , TiO ₂ , or Si.
The heater is made of N, SiON, an ultraviolet curable resin, polyimide or the like, and a heater made of a thin film of a metal such as Cr or a metal compound is arranged on each of them.

FIG. 9 shows a waveguide 2 in comparison with the embodiment shown in FIG.
4 is a plan view of a modified configuration in which the refractive index 4 is made of a material whose temperature changes easily with temperature, and the heater 25 is arranged on the waveguide 24. The same components as those in FIG. 1 are designated by the same reference numerals. FIG. 10 shows a sectional view taken along the line AA of FIG. As a structural example, the height of the waveguide 24 is 6 μm, the width is 12 μm, and the distance between the waveguide 24 and the heater 25 is 30 μm.

Let T be the temperature, the refractive indices of the waveguides 22, 24, and 26 be denoted by n ₁ (T), and the refractive index of the cladding by n _1.
2 (T), the refractive index n of the cladding at a temperature of 0 ° C
₂ (0) is 1.445, and the refractive index n ₁ (0) of the waveguides 22, 24 and 26 is 1.456. The thermo-optic constant of the refractive index of the cladding and the waveguides 22, 24, 26 is set to 10
^-5 , the refractive indices of the cladding and the waveguides 22, 24, and 26 at a certain temperature T are n ₂ (T) = 1.445 + 10 ⁻⁵ T n ₁ (T) = 1.456 + 10 ⁻⁵ T, respectively. is there.

Therefore, the propagation constants β ₀ and β ₂ of the zero-order mode and the second-order mode in the waveguide 24 are functions of the temperature T, and generally (β ₂ (T) -β for any T.
₀ (T)) is not constant. In other words, since (β ₂ (T) −β ₀ (T)) can be adjusted by the heater 25,
Therefore, the waveguide 24 is made to satisfy the phase condition of the equation (1).
Can control the propagation conditions of.

As an example, the length of the waveguide 24 is 1000
In the case of 0 μm, a temperature change of about 0 to 100 ° C will result in 0 to 2
Since the phase can be changed in the range of π, it is possible to match the phase of both modes with the heater 25, regardless of the value of the phase difference between the secondary mode and the zero-order mode. is there.

Propagation constant difference (β ₂ (T) -β ₀ (T))
Therefore, the length L ₂ of the waveguide 24 can be further shortened and the temperature can be lowered by setting the condition that the difference in the propagation constant becomes larger, for example, by setting the waveguide width such that the secondary mode is near the cutoff. A phase change of 2π is possible due to the temperature change of the difference.

Further, a material having a larger thermo-optical constant tends to have a significantly different propagation constant difference. Therefore, if a material having a large thermo-optic constant is used for the waveguide 24, the length L2 of the waveguide 24 can be further shortened, and a phase change of 2π can be achieved with a temperature change having a lower temperature difference.

(Sixth Embodiment) FIG. 11 shows a plan view of a waveguide structure according to a sixth embodiment of the present invention. A tapered waveguide 42, a waveguide 44 having a constant waveguide width, and a waveguide 46 having a constant waveguide width are serially arranged on a substrate 40, and a waveguide 50 having a constant waveguide width is arranged on a substrate 48. Substrate 48, that is,
The waveguide 50 is movable with respect to the waveguide 46 in a direction orthogonal to its optical axis.

The tapered waveguide 42 is a waveguide in which the width of the waveguide gradually narrows from the entrance 42a to the exit 42b. At the entrance 42a of the tapered waveguide 42, higher and higher order modes can exist, but at the exit 42b, only 0th, 1st and 2nd order modes can exist,
There can be no more than 4th mode. The exit port 42b has a waveguide 14 having the same waveguide width as the exit port 42b.
4 connects. That is, in the waveguide 44, only 0th-order, 1st-order, and 2nd-order modes can exist.

A waveguide 46 having a constant waveguide width is connected to the other end of the waveguide 46. In the waveguide 46, the 0th mode and 1
There can be next modes. Therefore, the waveguide width of the waveguide 46 is narrower than the waveguide width of the waveguide 44, and the waveguide width changes discontinuously at the connection point of the waveguides 44 and 46.

The waveguide 50 is connected to the other end of the waveguide 46. In the waveguide 50, only the 0th mode can exist. Therefore, the waveguide width of the waveguide 50 is narrower than the waveguide width of the waveguide 46, and the waveguide width changes discontinuously at the connection point of the waveguides 46 and 50.

The waveguides 42, 44 and 46 are arranged on the substrate 40, and the waveguide 50 is arranged on another substrate 48.
The substrate 48 is movable in the direction orthogonal to the optical axis of the waveguide 50. Therefore, the coupling position of the exit of the waveguide 46 and the entrance of the waveguide 50 is variable and can be set at any position. For example, the waveguide 50 is laterally displaced from the center of the exit of the waveguide 46 by a distance d. Waveguide 46
If the waveguide width of is w, then the range of d is w / 2>d> 0
Is.

As shown in FIG. 12, each of the waveguides 42, 4
It is preferable to arrange heaters 52, 54 and 56 on 4, 46 so that the propagation conditions of the waveguides 42, 44 and 46 can be finely adjusted. 13, 14 and 15 are cross-sectional views taken along the line BB, CC and DD of FIG. 12, respectively.

It is assumed that a multimode light wave composed of only even-order modes is incident on the entrance 42a of the waveguide 42. This light wave is expanded into a plurality of waveguide modes, and the tapered waveguide 42
Propagate. As it propagates through the tapered waveguide 42, a part thereof becomes a radiation mode and leaks to the outside, but a part thereof is converted into a low-order even-order mode. Finally, at the exit 42b, only the 0th-order mode, the 1st-order mode, and the 2nd-order mode are available.

In the waveguide 44, the 0th-order mode, the 1st-order mode, and the 2nd-order mode can propagate, but in the waveguide 46, only the 0th-order mode and the 1st-order mode. β ₀ to the waveguides 42, 4
4 is the propagation constant of the 0th mode, β ₂ is the waveguide 42,
Assuming that the propagation constant of the second-order mode in 44 is ∫ ₀ ^b (β ₀ −β ₂ ) dz = 2nπ (4) in the waveguides 42 and 44, the 0th-order mode of the waveguide 46 is efficiently excited. it can. The heaters 52 and 5 are arranged so that the equation (4) is satisfied.
4 adjusts the propagation conditions of the waveguides 42 and 44.

For the same reason as described with reference to FIG. 6, when β ₁ is the propagation constant of the first-order mode in the waveguides 42, 44, 46, ∫ in the waveguides 42, 44, 46. _{When 0} ^c (β ₀ −β ₁ ) dz = nπ (5) holds, the 0th-order mode of the waveguide 50 can be efficiently excited. The heaters 52, 5 are arranged so that the equation (5) is satisfied.
4, 56 adjust the propagation conditions of the waveguides 42, 44, 56. Then, the waveguide 50 is moved in a direction orthogonal to the optical axis so that the output power of the waveguide 50 is maximized, and the offset amount of the optical axis of the waveguide 50 with respect to the waveguide 46 is adjusted.

Eventually, the heaters 52, 54 and 56 are used to guide the waveguides 42 and 4 so that the expressions (4) and (5) are satisfied.
By adjusting the propagation conditions of 4, 56 and moving the substrate 48 to adjust the offset amount of the optical axis of the waveguide 50 with respect to the waveguide 46, the loss of beam diameter conversion can be minimized.

(Seventh Embodiment) Instead of using a heater, an electro-optical effect may be used as a means for adjusting the propagation constant difference, and thus the phase difference between the two guided modes. For example, as the substrate 20 of the embodiment shown in FIG.
A Z-cut substrate of bO ₃ is used. FIG. 16 shows a plan view of the configuration of the modified embodiment. FIG. 17 shows E- of FIG.
The sectional view in the E line is shown.

Substrate 6 which is a Z-cut substrate of LiNbO ₃
0, a tapered waveguide 62 and a waveguide 64 having the same waveguide width.
And the waveguides 66 having the same waveguide width are serially arranged.
The waveguide characteristics of the waveguides 62, 64, 66 are shown in FIG.
This is the same as that of the waveguides 22, 24 and 26 of the embodiment shown in FIG. The electro-optic constant r ₃₃ of the substrate 60 is 30.8 (pm
/ V). The waveguides 62, 64 and 66 are formed by Ti diffusion, and the height of each of the waveguides 62, 64 and 66 is a single mode in the thickness direction of the substrate 60.

A SiO 2 layer 6 for electrical insulation is formed on the waveguide 64.
8 is laminated, and is made of a metal conductor thin film on the SiO 2 layer 68.
The electrodes 70, 72, 74 which are_ThreeZ-axis direction
It is arranged so that an electric field is generated in the (x3 direction). Electrode 7
2 and 74 are set to the same potential, and between the electrode 70 and the electrodes 72 and 74
Apply voltage. FIG. 18 shows an example of an electric field. Waveguide
When the electric field strength of 64 is E, the refractive index of the waveguide 64 is n = (1 / n₀ ^Two+ R₃₃E)^-1/2 Is. However, n₀Is the conduction when no electric field is applied.
The refractive index of the waveguide 64.

By adjusting the electric field strength of the waveguide 64,
As in the case of the heater, the propagation constant of the guided mode propagating in the waveguide 64 can be adjusted.

In addition, a material whose refractive index can be changed by injecting a current is used as the substrate and / or the waveguides 22, 24, 3
By adopting 2, 34, 42, 44, 62, 64,
The phase difference between the guided modes can be controlled.

(Eighth Embodiment) The present invention can also be applied to an optical fiber. 19 is a perspective view of an embodiment applied to an optical fiber, and FIG. 20 is a central sectional view.

The optical fiber 110 includes a core 112 having a smaller radius in the propagation direction and a clad 11 surrounding the core 112.
It consists of 4. At the entrance 112a of the core 112, the LP
_Although there can be a plurality of guided modes including the ₀₁ mode (corresponding to the 0th-order mode) and the LP ₁₁ mode (corresponding to the 1st-order mode), LP ₀₁ is generated at the exit 112b.
Only modes and LP ₁₁ modes can exist. Therefore, while propagating from the entrance 112a to the exit 112b, mode components higher than the LP ₁₁ mode are radiated to the outside.

The optical fiber 120 comprises a core 122 having a constant radius in the propagation direction, and a clad 124 surrounding the core 122. The entrance 122a of the core 122 has the optical fiber 1
10 facing the emission port 112b of the core 112,
The optical axis of 2 and the optical axis of the core 112 are located on the same straight line. The optical fiber 120 is a single mode optical fiber in which only a single mode (LP ₀₁ mode) can exist.

In principle, as in the case of the planar waveguide structure shown in FIG. 2, the optical fiber 110 is introduced from the entrance 112a.
The light incident on is incident on the optical fiber 110 and is expanded into a guided mode. Part of the incident light is radiated to the outside, and only the 0th-order mode and the 1st-order mode are present at the exit 112b.

By mechanically extending the optical fiber 110 in the optical axis direction, the phase difference between the 0th-order mode and the 1st-order mode in the optical fiber 110 can be adjusted. The phase difference is 2nπ
The zero-order mode of the optical fiber 120 can be efficiently excited by adjusting so that

[0068]

As can be easily understood from the above description, according to the present invention, the higher-order mode component of the light wave can be efficiently converted into the lower-order mode, and the post beam can be effectively reduced. For example, an erbium-doped fiber amplifier (hereinafter, E
It is suitable as a means for focusing the output light of the excitation light source of (DFA).

[Brief description of drawings]

FIG. 1 is a plan view of a waveguide structure according to a first embodiment of the present invention.

FIG. 2 is a plan view of a waveguide structure according to a second embodiment of the present invention.

FIG. 3 is a result of measuring the loss of the embodiment shown in FIG.

FIG. 4 is a measurement result of loss in the conventional example shown in FIG.

FIG. 5 is a diagram showing the relationship between the coupling loss and the waveguide length when the length of the tapered waveguide 20 is near 5066 μm in the second embodiment.

FIG. 6 is a plan view of a waveguide structure according to a third embodiment of the present invention.

FIG. 7 is a calculation example of the relationship of the coupling efficiency of the offset d in the third embodiment.

FIG. 8 is a plan view of a waveguide structure according to a fourth embodiment of the present invention.

FIG. 9 is a plan view of a fifth embodiment of the present invention.

10 is a cross-sectional view taken along the line AA of FIG.

FIG. 11 is a plan view of a waveguide structure according to a sixth embodiment of the present invention.

FIG. 12 is a plan view of a configuration in which a heater is added to the sixth embodiment.

13 is a cross-sectional view taken along the line BB of FIG.

FIG. 14 shows a cross-sectional view taken along the line CC of FIG.

FIG. 15 is a sectional view taken along line DD of FIG.

FIG. 16 is a plan view of a waveguide structure according to a seventh embodiment of the present invention.

FIG. 17 is a sectional view taken along line EE in FIG.

FIG. 18 is an example of an electric field.

FIG. 19 is a perspective view of an example applied to an optical fiber.

20 is a central cross-sectional view of the embodiment shown in FIG.

FIG. 21 is a plan view showing a conventional waveguide structure.

[Explanation of symbols]

10: Tapered waveguide 12: Single mode waveguide 14: entrance of tapered waveguide 10 16: Outlet of taper waveguide 10 20: substrate 22: Tapered waveguide 22a: entrance 22b: exit port 24, 26: Waveguide 25: heater 30, 30a, 30b: substrate 32: Tapered waveguide 32a: entrance 32b: exit port 34: Waveguide 36: Waveguide 40: substrate 42: Tapered waveguide 42a: entrance 42b: exit port 44: Waveguide 46: Waveguide 48: Substrate 50: Waveguide 52, 54, 56: heater 60: substrate 62: Tapered waveguide 64, 66: Waveguide 68: SiO2 layer 70, 72, 74: electrodes 110: optical fiber 112: Core 112a: entrance 112b: exit port 114: Clad 120: optical fiber 122: Core 124: Clad

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Komura Nishimura             2-15 Ohara Stock Exchange, Kamifukuoka City, Saitama Prefecture             Company KDI Research Institute F term (reference) 2H047 KA03 KA13 LA23 TA35 TA36

Claims (7)

[Claims] 1. A first waveguide (22) in which a waveguide width becomes narrower from an incident end toward an emitting end, and a 0th-order mode, a first-order mode, and a second-order mode can exist at the emitting end, It is a second waveguide connected to the output end of the first waveguide (22) and having a constant waveguide width in the optical axis direction, and a 0th mode, a 1st mode and a 2nd mode can exist. A second waveguide (24) and a third waveguide (26) for single mode transmission, which is connected to the output end of the second waveguide (24) and has a constant waveguide width in the optical axis direction. And a phase difference between the zeroth-order mode and the second-order mode in the first waveguide and the second waveguide is substantially equal to 2nπ (where n is an integer). Beam diameter conversion device. 2. A phase difference adjusting means (2) for adjusting a phase difference between predetermined waveguide modes of the second waveguide (24).
5; 70, 72, 74), The beam diameter conversion device according to claim 1. 3. A first waveguide (22), wherein the width of the waveguide is narrowed from the entrance end toward the exit end, and a 0th-order mode, a first-order mode, and a second-order mode can exist at the exit end, A third waveguide (26) for single-mode transmission, which is connected to the output end of the first waveguide (22) and has a constant waveguide width in the optical axis direction. (22) The beam diameter conversion device, wherein the phase difference between the 0th mode and the 2nd mode is substantially equal to 2nπ (where n is an integer). 4. A first waveguide (32) having a narrower waveguide width from an entrance end toward an exit end, in which a zero-order mode and a first-order mode can exist at the exit end, and the first waveguide (32). A second waveguide (34) which is connected to the output end of the waveguide (32) and has a constant waveguide width in the optical axis direction and in which a 0th-order mode and a 1st-order mode can exist. And a third waveguide (36) for single-mode transmission, which is connected to the output end of the second waveguide (34) and has a constant waveguide width in the optical axis direction. The phase difference between the zeroth-order mode and the second-order mode in the waveguide (32) and the second waveguide (34) is substantially equal to nπ (where n is an integer), and the third waveguide Beam diameter conversion wherein the optical axis of (36) is offset from the optical axis of the second waveguide (34) device. 5. A waveguide holding means (30b) for holding the third waveguide (34) movably in a direction orthogonal to the optical axis of the third waveguide. Item 4. The beam diameter conversion device according to item 4. 6. A first waveguide (42), wherein the width of the waveguide becomes narrower from the entrance end toward the exit end, and a 0th-order mode, a first-order mode, and a second-order mode can exist at the exit end, A second waveguide connected to the output end of the first waveguide (42) and having a constant waveguide width in the optical axis direction, in which a 0th-order mode, a 1st-order mode and a 2nd-order mode can exist. A second waveguide (44) and a third waveguide connected to the output end of the second waveguide (44) and having a constant waveguide width in the optical axis direction. Third waveguide (46) in which the first mode can exist
And a fourth waveguide (50) for single-mode transmission, which is connected to the output end of the third waveguide (46) and has a constant waveguide width in the optical axis direction. The phase difference between the zeroth-order mode and the second-order mode in the waveguide (42) and the second waveguide (44) is substantially equal to 2nπ (where n is an integer), and the first waveguide ( 42), the second waveguide (44)
And the phase difference between the zeroth-order mode and the first-order mode in the third waveguide (44) is substantially equal to mπ (where m is an integer), and the optical axis of the fourth waveguide (50) is A beam diameter conversion device, wherein the beam diameter conversion device is offset from the optical axis of the third waveguide (46). 7. A first propagation constant adjusting means (52) for adjusting a guided mode propagation constant in the first waveguide (42), and a guided mode propagation in the second waveguide (44). Second propagation constant adjusting means (5) for adjusting the constant
4) and third propagation constant adjusting means (5) for adjusting the propagation constant of the guided mode in the third waveguide (46).
The beam diameter conversion device according to claim 6, comprising at least one of 6).