JP3070881B2 - Waveguide type optical multiplexer / demultiplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical multiplexer / demultiplexer using an optical waveguide.

[0002]

2. Description of the Related Art An optical multiplexer / demultiplexer is a key device for realizing a wavelength division multiplexing transmission system, and a micro optics type optical multiplexer / demultiplexer using an interference filter or a diffraction grating and a fiber coupler type optical multiplexer / demultiplexer are conventionally used. Has been developed. In recent years, studies have been made on a waveguide type optical multiplexer / demultiplexer using an optical waveguide. On the other hand, the performance of the wavelength division multiplexing transmission system has also been improved, and in addition to the conventional two-wavelength multiplex transmission of, for example, 1.31 μm and 1.55 μm, a wavelength of 1.31 μm is used for time division multiplex bidirectional transmission. A scheme is being developed. An optical device used in such a transmission system must have both the function of an optical multiplexer / demultiplexer and the function of a splitter. As a promising method that satisfies this requirement, a method has been reported in which an optical multiplexer / demultiplexer and a branching device are integrated on the same waveguide type substrate using an optical waveguide (ACCarter, PJWilliams, JW
Burgess, IEEE Workshop on Passive Optical Networks
for the Local Loop, 7.5 Heathrow Penta Hotel, Lon
don8th-9th May 1990).

FIG. 10 shows a conventional waveguide type optical multiplexer / demultiplexer suitable for the above purpose. In FIG. 10, 36 is a waveguide substrate, 37 is a Mach-Zehnder type optical multiplexer / demultiplexer provided on the waveguide substrate 36, 38 is a directional coupler type optical multiplexer / demultiplexer provided on the waveguide substrate 36, 39 Is the first waveguide end of the Mach-Zehnder type optical multiplexer / demultiplexer 37, 40 is the second waveguide end of the Mach-Zehnder type optical multiplexer / demultiplexer 37, 41 is the first waveguide end of the directional coupler type optical multiplexer / demultiplexer 38. Waveguide end 42 is the second waveguide end of the directional coupler type optical multiplexer / demultiplexer 38. In this prior art example, the Mach-Zehnder type optical multiplexer / demultiplexer 37 has 1.30 μm.
m and 1.53 μm, and the directional coupler type optical multiplexer / demultiplexer 38 has the function of a 1.30 μm 3 dB splitter. Applicable to the scheme.

[0004]

However, the prior art shown in FIG. 10 has the following disadvantages. 1) Since the Mach-Zehnder type optical multiplexer / demultiplexer 37 and the directional coupler type optical multiplexer / demultiplexer 38 are cascaded, the waveguide size becomes longer. 2) waveguide end 41 of directional coupler type optical multiplexer / demultiplexer 38;
In many cases, a light emitting element and a light receiving element are coupled to 42. However, since the waveguide ends 41 and 42 are adjacent to each other, mounting of the light emitting element, the light receiving element, and a transmitting circuit and a receiving circuit connected to these elements are performed. Area is limited.

SUMMARY OF THE INVENTION An object of the present invention is to provide a waveguide type optical multiplexer / demultiplexer which solves the above-mentioned drawbacks of the prior art and is small in size and can easily mount a light emitting element and a light receiving element.

[0006]

SUMMARY OF THE INVENTION A waveguide type optical multiplexer / demultiplexer according to the present invention employs a pair of three coupled optical waveguides (hereinafter abbreviated as "3-coupled waveguide"). It is characterized in that the propagation constant of the eigenmode and the equivalent coupling length thereof are set to predetermined conditions, and that it has both an optical multiplexing / demultiplexing function and a branching function.
Compared to realizing a desired function by cascade connection of an optical multiplexer / demultiplexer (or a directional coupler type optical multiplexer / demultiplexer) and a directional coupler type (2 × 2) optical multiplexer / demultiplexer, It is fundamentally different from the prior art in that a desired function is realized by an optical waveguide circuit and that the optical multiplexer / demultiplexer has an advantage that the degree of design freedom of the optical multiplexing / demultiplexing characteristics is large.

[0007]

As a result, there is an advantage that the length of the waveguide is reduced to about half as compared with the prior art. Further, since the two waveguides on both sides of the three waveguides serve as branch output ends, it is easy to arrange the branch output ends on opposite sides of the waveguide without crossing the center waveguide. The mounting area for mounting the light emitting element on one of the output ends and the light receiving element on the other is increased.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiments described below, a quartz-based single-mode optical waveguide on a silicon substrate was assumed as an optical waveguide. However, the present invention is not limited to these embodiments, and other materials are used. An optical waveguide based on the configuration can also be used. For example, an optical waveguide formed by using a multi-component glass and a thin film forming technique such as sputtering, an optical waveguide formed by using a metal ion diffusion technique on a lithium niobate single crystal substrate, and a metal ion being applied to a multi-component glass substrate by an electric field. Naturally, the present invention can be applied to a waveguide formed by thermal diffusion, a waveguide formed by growing a heterogeneous semiconductor film on a semiconductor substrate and finely processing the same, and the like.

FIG. 1 is a conceptual diagram illustrating the operation principle of the present invention. 1, 2, 3 are waveguides, 4 is a first non-coupling portion,
Reference numeral 5 denotes a first interval converter, 6 denotes a first coupling unit, 7 denotes a second interval converter, 8 denotes a second non-coupling unit, and 9 to 11 denote waveguides 1.
, 3 is a third interval converter, 13 is a second interval converter, 14 is a fourth interval converter, 1
5 is a third non-coupling portion. In the non-coupling portions 4, 8, and 15, the spacing between the waveguides 1 to 3 is large enough to prevent optical coupling in each section. In the space conversion units 5, 7, 12, and 14, the space between the waveguides 1 to 3 gradually decreases as approaching the coupling units 6, 13. In FIG. 1, the interval conversion units 5, 7, 12, and 14 are shown as independent portions for convenience, but the interval conversion unit appears as a transition region between the connected portion and the non-connected portion as a result. Need not be treated as a separate part. Since slight optical coupling occurs in a region close to the coupling portion of the interval conversion portion, it is customary to handle this effect by incorporating it into the coupling portion as described later. Therefore, the operation of the circuit according to the present invention can be described using the coupled portion and the non-coupled portion. The following conditions need to be satisfied for the waveguides at the coupling portion and the non-coupling portion.

(A) In the coupling portions 6 and 13, the waveguides 1 and 2
And the distance between the waveguides 2 and 3 are substantially equal. (B) The waveguides 1 and 3 have a symmetrical structure in which the widths of the waveguides 1 and 3 are equal and the widths of the waveguides 2 are substantially equal to the widths of the waveguides 1 and 3. (C) In the non-coupling portion 8, the waveguide lengths of the waveguide section 9 and the waveguide section 11 are equal, and the waveguide lengths of both are larger by a predetermined value than the waveguide length of the waveguide section 10.

The operation of the waveguide type optical multiplexer / demultiplexer having such a structure will be described below. When light is input to the waveguide 2 of the non-coupling portion 4, the three eigenpropagation modes shown in FIG. The three eigenpropagation modes are the 0th-order mode having the largest propagation constant among the eigenmodes of the three coupling waveguides, the first mode having the largest propagation constant after the 0th-order mode, and the first mode having the first propagation constant. This is the second mode that is the second largest after the next mode. Here, the input light is TE polarized or T
It is assumed that the light is M-polarized light, but when re-polarized light components are mixed, the 0th-order mode, the first mode, and the second mode exist for each of the TE-polarized light and the TM-polarized light. Is slightly different between TE polarized light and TM polarized light, strictly speaking, there are six eigenpropagation modes. In the present invention, TE polarization of the same order mode and TM
Although the difference between the propagation constants of polarized light is treated as being negligible, the basic operation of the present invention does not change even when the difference between the propagation constants cannot be ignored. Also, the attenuation constant component of the propagation constant affects the loss but does not affect the basic operation as an optical multiplexer / demultiplexer. Therefore, the following description will focus on only the phase constant component of the propagation constant. Further, since the coupling to the higher-order eigenpropagation mode and the radiation mode is small and does not affect the basic operation as the optical multiplexer / demultiplexer, the description thereof will be omitted.

In FIG. 2, I is the field distribution of the input mode, A
Is the field distribution of the 0th mode, B is the field distribution of the 1st mode,
C is the field distribution of the second mode. As a characteristic of the three-coupled waveguide having a symmetrical structure, the field distribution of the 0th-order mode and the second-order mode is symmetric, and the field distribution of the 0th-order mode has a local maximum value of the same sign in the waveguides 1 to 3. Second against
The field distribution of the next mode shows the minimum value of the same sign in the waveguides 1 and 3,
The waveguide 2 has a maximum value. On the other hand, the field distribution of the first-order mode is originally anti-symmetric and is characterized by having a node where the field distribution becomes zero in the waveguide 2, but light is input from the waveguide 2 which is the center of symmetry. It is clear that the first mode is not excited at all in the example of FIG. That is, when light is input from the waveguide 2, the 0th order and the 2nd order
It is sufficient to consider the number of eigenpropagation modes. Further, when the waveguide interval is not extremely narrow, as shown in FIG. 2, the maximum value and the minimum value of the field distribution in each mode have the following relationship. (1) The local maximum value in the waveguides 1 and 3 of the 0th-order mode is almost 1/4 of the local maximum value in the input mode, and the local maximum value in the waveguide 2 is about ^2-3-2 of the local maximum value in the input mode. is there. (2) The local minimum value in the waveguides 1 and 3 in the second mode is approximately 1/4 of the local maximum value in the input mode, and the local maximum value in the waveguide 2 is approximately ^2-3-2 of the local maximum value in the input mode. is there. The 0th-order and second-order eigenmodes having such a field distribution propagate through the coupling section 6 with different propagation constants β ₀ and β ₂ , respectively.

The above eigenmode has propagated through the coupling section 6
Later, the waveguides are separated into waveguides 1 to 3 and have a waveguide length difference.
After propagating through the coupling portion 13 again through the coupling portion 8, the waveguide
It is separated into 1-3 and guided to the non-coupling portion 15. At this time,
Non-coupled portion 1 for light input to waveguide 2 of non-coupled portion 4
The transmittance T of light output to the waveguides 1 to 3 of No. 5_{twenty one}, T_{twenty two}, T
_{twenty three}Is approximately given by the following equation by mode coupling theory. T_{twenty one}= 0.5 · sin^Two((Β₀−β_Two) L_c) Cos^Two(ΒL_d/ 2) (1) T_{twenty two}= Cos^Two((Β₀−β_Two) L_c) ・ Cos^Two(ΒL_d/ 2) + sin^Two(ΒL_d/ 2) (2) T_{twenty three}= 0.5 · sin^Two((Β₀−β_Two) L_c) Cos^Two(ΒL_d/ 2) (3)

[0014] Here, L _c is equivalent coupling length of the coupling section 6, 13, L _d is the waveguide length difference unbound portion 8, beta is the propagation constant of the waveguide unbound portion 8. The reason for the L _c is defined as the equivalent coupling length is that optical coupling occurs also in the space transformer unit 5,7,12,14 not only coupling section portion 6,13.
Therefore the coupling length of the coupling portion 6, 13 of the optical coupling occurring at intervals conversion unit 5,7,12,14-defined equivalent coupling length and L _c for handling as increased equivalently. In the above first to third formulas, the coupling portions 6 and 13 have a symmetric waveguide structure, and the waveguides 1 to 3 in the coupling portions 6 and 13 have substantially the same structure, and the waveguide loss is sufficiently small. Although the approximation formula is based on the assumption that the coupling is sufficiently weak, the basic operation of the present invention can be realized even in a region where the approximation accuracy deteriorates.

If the equivalent refractive indices corresponding to the propagation constants β ₀ , β ₂ , and β are replaced with the first to third equations using n ₀ , n ₂ , and n, the fourth to sixth equations are obtained. . T ₂₁ = 0.5 · sin ² ((n ₀ −n ₂ ) 2πL _c / λ) cos ² (nπL _d / λ) (4) T ₂₂ = cos ² ((n ₀ −n ₂ ) 2πL _c / λ ) · Cos ² (nπL _d / λ) + sin ² (nπL _d / λ) (5) T ₂₃ = 0.5 · sin ² ((n ₀ −n ₂ ) 2πL _c / λ) cos ² (nπL _d / λ) ) (6)

Although the wavelength dependence of n and the waveguide structure dependence are extremely small, since (n ₀ −n ₂ ) is a difference, the relative change rate is, for example, as shown in FIG. And waveguide structure dependence. The values at the first wavelength λ ₁ and the second wavelength λ ₂ are respectively (n ₀₁ −n
₂₁ ) and (n ₀₂ −n ₂₂ ), L
_c, to select the L _d. Here, N and M are integers. 2 (n ₀₁ −n ₂₁ ) · L _c = (M − ／) · λ ₁ (7) n · L _d = N · λ ₁ (8) n · L _d = (N − ／) · λ ₂ (9)

At this time, T _{21 at} the first wavelength λ ₁ ,
T ₂₂ and T ₂₃ are 0.5, 0, and 0.5 from the fourth to sixth equations, and T ₂₁ , T ₂₂ , and T _{23 at} the second wavelength λ ₂ are the first to fourth equations.
From the third equation, they are 0, 1, and 0. At the first wavelength λ ₁ , the waveguide 2 is coupled to the waveguides 1 and 3 at 3 dB,
Passes through the waveguide 2 at 0dB at a second wavelength lambda _2, i.e. 3dB branch operating in function as the first wavelength lambda ₁ of the optical multiplexer operating at the first wavelength lambda ₁ and the second wavelength lambda ₂ pass band It is clear that it has the function of a vessel. However, in order for the eighth and ninth formulas to be satisfied, the first wavelength λ ₁ and the second wavelength λ ₂ are restricted by the tenth formula. N = 0.5 · λ ₂ / (λ ₂ −λ ₁ ) (10) The present invention is effective in the combination of λ ₁ and λ ₂ that gives the solution of the integer N by the equation (10).

(N ₀ -n ₂ ) has a wavelength dependence as shown in FIG. 3. Therefore, even when the seventh equation holds for λ ₁ , it is generally 2 (n ₀₂ -n ₂₂ ). L _c = (M−1)
/ 2) · λ ₂ does not hold, so the first term on the right-hand side of Equation 4
0.5 · sin ² ((n ₀ −n ₂ ) 2πL _c / λ) takes a finite value at λ ₂ . Even in such a case, the fourth
Since the second term cos ² (nπL _d / λ) on the right side of the equation becomes 0 at λ _{2 under} the condition of the ninth equation, T ₂₁ given by the fourth equation becomes 0, and the operation of the present invention is guaranteed. As is apparent from the above description, in order to guarantee the operation of the present invention, in the waveguide satisfying the conditions (A) to (C), the seventh to the first expressions are used.
It is sufficient to choose L _c and L _d given by equation 0, and not to impose any other constraints on the waveguide parameters.

By using the fact that (n ₀ −n ₂ ) has a wavelength dependency and a waveguide structure dependency as shown in FIG. 3, a waveguide structure satisfying the formula 11 is selected. Secondary effects can be expected. 2 (n ₀₂ −n ₂₂ ) · L _c = Mλ ₂ (11) At this time, T ₂₁ , T ₂₂ , and T _{23 at} the first wavelength λ ₁ are the fourth.
From Equations (6) and (6), 0.5, 0, and 0.5 are obtained, and T ₂₁ , T ₂₂ , and T _{23 at} the second wavelength λ ₂ are 0, 1, and
The basic function which becomes 0 is the same as the case where there is no constraint of the eleventh formula. However, it is possible to significantly improve its performance in the wavelength in the vicinity of lambda _2. Assuming the eleventh equation as λ = (1 + X) λ ₂ and rewriting the fourth and sixth equations, the following approximate equation is obtained. However, X ≒ 0. T ₂₁ = 0.5 sin ⁴ (π / 2 · X) (12) T ₂₃ = 0.5 sin ⁴ (π / 2 · X) (13)

On the other hand, when there is no constraint of the eleventh equation, the following approximate equation is obtained. T ₂₁ = K · sin ² (π / 2 · X) (14) T ₂₃ = K · sin ² (π / 2 · X) (15) where K is a constant. That is, the vicinity of X = 0 (λ = λ
_In the vicinity of ( ₂ ), the twelfth and thirteenth equations provide a fourth-order zero, whereas the fourteenth and fifteenth equations provide only a second-order zero. Therefore, it becomes clear that the amount of crosstalk in the vicinity of λ ₂ can be remarkably improved by giving the condition of Expression 11.

Embodiment 1 FIGS. 4 (a), 4 (b) and 4 (c)
Shows a specific example of claim 1 as a first embodiment of the present invention. This is because λ ₁ = 1.31 μm, λ
₂ is a configuration example of a waveguide type optical multiplexer / demultiplexer designed for two wavelengths of 1.57 μm, where (a) is a plan view, (b) and (c) are line segments B− in the plan view (a). B 'is an enlarged cross-sectional view taken along line CC'. 50 is a silicon substrate, 16 to
Reference numeral 18 denotes a quartz optical waveguide formed on a silicon substrate 50 using a quartz glass material. The waveguides 16 to 18 are made of a SiO ₂ —TiO ₂ glass core having a cross-sectional dimension of about 7 μm × 7 μm embedded in a SiO ₂ glass layer 24 having a thickness of about 50 μm. The waveguides 16 to 18 are connected to the non-coupling portions 19, 21, 2
3 and coupling portions 20 and 22. Coupling part 2
Since it is desirable that the waveguides 0 and 22 have a symmetrical structure, the waveguide 17 is substantially straight and the waveguides 16 and 18 are arranged in parallel. The waveguide width of the coupling portions 20 and 22 is also about 7 μm as described above, but the width of the waveguides 16 and 18 is increased by several percent with respect to the width of the waveguide 17 in order to obtain more desirable performance. In the non-coupling portion 21, the waveguide 16 and the waveguide 18 have the same length,
It was increased by the waveguide length difference L _d with respect to 7 the length of the. Waveguide spacing at the junction 20 and 22, the equivalent coupling length L _c and a non-coupling waveguide length difference in the portion 21 L _d were set as the following equation is established.

2 (n ₀₁ −n ₂₁ ) L _c = １ ／ · λ ₁ (16) n · L _d = N · λ ₁ (17) n · L _d = (N − ／) · λ ₂ (18) N = 0.5 · λ ₂ / (λ ₂ −λ ₁ ) (19)

Equation (16) is obtained by selecting M = 1 in equation (7). λ ₁ = 1.31 μm, λ ₂ = 1.57 μm
Therefore, by setting N = 3, Expression 19 is almost satisfied. In addition, since a quartz-based waveguide having an equivalent refractive index of 1.46 was used, the first value was set by setting L _d = 2.7 μm.
Equations 7 and 18 are satisfied. The waveguide spacing and the equivalent length L _c at the coupling portions 20 and 22 are about 2 μm and about 66, respectively.
By setting it to 0 μm, Equation 16 is almost satisfied. In the non-coupling portions 19 and 23, the waveguide spacing was set to 250 μm in consideration of connection with an external optical fiber. A waveguide type optical multiplexer / demultiplexer having these structural parameters was fabricated by combining a linear optical waveguide pattern and an arc-shaped waveguide pattern having a curvature radius of 10 mm on a silicon substrate. For the formation and processing of the waveguide film, a combination of a glass film forming method utilizing a flame hydrolysis reaction, which is a known technique, and reactive ion etching was applied.

FIG. 5 shows the transmission loss of the waveguide type optical multiplexer / demultiplexer experimentally manufactured by the above-mentioned manufacturing technique based on the above design.
A is a transmission loss corresponding to T ₂₁ and T ₂₃ and a wavelength of 1.31 μm.
m was about 3 dB, and was 30 dB or more at a wavelength of 1.57 μm. B is a transmission loss corresponding to T _22, the wavelength 1.
It becomes almost 0 dB at 57 μm and 20 dB at a wavelength of 1.31 μm.
B or more. From these results, the waveguide type optical multiplexer / demultiplexer of this embodiment has a wavelength around 1.31 μm and a wavelength of 1.31 μm as designed.
Function and wavelength of optical multiplexer / demultiplexer operating near 57 μm
It is clear that it has the function of a 3 dB splitter operating near 1 μm.

Embodiment 2 FIG. 6 shows the transmission loss of a waveguide type optical multiplexer / demultiplexer designed as a second embodiment of the present invention. The configuration of the waveguide type optical multiplexer / demultiplexer in the present embodiment is substantially the same as that of the first embodiment, except that the waveguide parameters are set so that the thirteenth equation is satisfied in addition to the seventh to tenth equations. This is different from the first embodiment. λ ₁ = 1.31 μm,
Since two wavelengths of λ ₂ = 1.57 μm were assumed, M = 1, N = 3, and L _d = 2.7 μm as in the first embodiment. However each waveguide spacing equivalent coupling length L _c of the coupling portion about 4μ
m, about 2.3 mm. T ₂₁ from a comparison of FIG. 6 and FIG. 5, T
_The following characteristics are seen in the transmission loss corresponding to ₂₃ . λ ₁
In the vicinity of 1.31 μm, there is no significant difference between the two. On the other hand, in the vicinity of λ ₂ = 1.57 μm, it can be seen that the wavelength band in which the amount of attenuation of 30 dB or more is obtained is more than doubled. As described above, in the present embodiment using the waveguide parameter that also satisfies the formula 11, it is clear that a wide stop wavelength band can be obtained in the transmission characteristic A corresponding to T ₂₁ and T ₂₃ . Obtaining a wide stop wavelength band in the transmission characteristic A corresponding to T ₂₁ and T ₂₃ is equivalent to obtaining a wide pass wavelength band in the transmission characteristic B corresponding to T ₂₂ in the same band. The present embodiment shows that the present invention is also effective as a configuration method of a waveguide type optical multiplexer / demultiplexer having a pass wavelength band.

Embodiment 3 FIG. 7 shows a third embodiment of the present invention. 25 is a main part of the waveguide type optical multiplexer / demultiplexer, 26 to 28 are waveguides constituting a main part of the waveguide type optical multiplexer / demultiplexer, 29 is a developed part, 30 is a first fiber, 31
Is a second fiber, 32 is a light emitting element coupling optical system, 33 is a light emitting element, 34 is a dielectric interference film filter, and 35 is a light receiving element. The configuration of the main part 25 of the waveguide type optical multiplexer / demultiplexer is the third type.
This is the same as the embodiment. The unfolding part 29 is formed on the same waveguide substrate as the main part 25 of the waveguide type optical multiplexer / demultiplexer, using an arcuate waveguide pattern having a radius of curvature of 5 mm and a linear waveguide pattern. The ends of the waveguide 26 and the waveguide 28 which couple the light emitting element and the light receiving element are set at positions different from each other by a few mm on both side surfaces of the waveguide substrate, and a part of the output of the light emitting element reaches the light receiving element. Prevented that. This procedure is unnecessary if the light emitting element is not driven simultaneously with the light receiving element. Since the waveguide 27 is coupled at both ends to the first fiber 30 and the second fiber 31 via the refractive index modifier, in this embodiment, the end of the waveguide should intersect perpendicularly to the side surface of the waveguide substrate. did. On the other hand, the end portions of the waveguides 26 and 28 have inclinations of several degrees or more with respect to the normal line of the side surface of the waveguide substrate so that the reflected light on the side surfaces of the waveguide is not coupled to the waveguide mode. The dielectric interference filter 34 has a pass band near a wavelength of 1.31 μm to supplement the multiplexing / demultiplexing characteristics in the main part of the waveguide type optical multiplexer / demultiplexer, and has a wavelength of 1.55 μm.
It has a stop band in the vicinity and should be added or deleted according to the required characteristics. Further, in the present embodiment, it is bonded to the end face of the waveguide, but it can be added to a package of a light receiving element or the like.

The operation of the third embodiment shown in FIG. 7 will be described below. 1.31μ wavelength incident from the first fiber 30
As is clear from FIG. 5 of the first embodiment, the light in the m-band passes through the main part 25 of the waveguide type optical multiplexer / demultiplexer and is coupled to the light receiving element 35 with a branch loss of about 3 dB. The second fiber 31
As shown in FIG. 5 of the first embodiment, the light having a wavelength of 1.57 .mu.m incident from the main part 25 of the waveguide type optical multiplexer / demultiplexer.
Through the first fiber 30 with almost no loss.
On the other hand, the light output of the light emitting element 33 in the 1.31 μm band is guided to the first fiber 30 via the main part 25 of the waveguide type optical multiplexer / demultiplexer. Thus, by using the waveguide type optical multiplexer / demultiplexer shown in the third embodiment, the same wavelength bidirectional communication can be performed in the 1.31 μm band, and the 1.31 μm band can be used.
1.57 μm band two-wavelength division multiplexing communication can be performed, and the light emitting element 33, the light receiving element 35, and the second fiber 31 can be separated in three directions. The degree of freedom of mounting, such as mounting immediately and suppressing electrical coupling between the transmitting circuit and the receiving circuit, has been extremely widened.

Although the light-emitting element coupling optical system 32 of this embodiment uses a distributed index lens, the present invention is not limited to this embodiment, and the spot diameter of the light-emitting element and the spot diameter of the waveguide are adjusted. It is obvious that the same effect as that of the present embodiment can be obtained by using an optical system for matching, for example, a spherical lens, an aspherical lens, a front-end processed fiber, or the like, alone or in combination. In this embodiment, the light receiving element 35 is used.
Is directly coupled to the end face of the waveguide 28 via the dielectric interference filter 34, but the present invention is not limited to this embodiment, and a coupling optical system is provided between the light receiving element 35 and the waveguide 28. It is obvious that the same effect as in the present embodiment can be obtained even if this is inserted. In the present embodiment, the waveguide for coupling the light emitting element and the light receiving element is specified, but it is considered that the main part 25 of the waveguide type optical multiplexer / demultiplexer is structurally and functionally symmetrical twice. It is obvious that the arrangement of the light emitting element and the light receiving element is extremely flexible. For example, both the light emitting element 33 and the light receiving element 35 of this embodiment have a wavelength of 1.31 μm.
It is also possible to use an m-band light emitting element or light receiving element. Further, a total of four light emitting elements and light receiving elements in the 1.31 μm band can be combined and coupled to both ends of the waveguide 26 and the waveguide 28 and driven appropriately. Further, in this embodiment, optical fibers are connected to both ends of the waveguide 27, but it is obvious that one of them can be connected to a light-emitting element or a light-receiving element that operates near the wavelength band of 1.57 μm. is there.

When the specific waveguide type optical multiplexer / demultiplexer described above is actually manufactured, polarization dependence may be caused in the optical multiplexer / demultiplexer due to the influence of internal stress and the like. Such polarization dependence is not practically preferable. Therefore, a configuration example capable of eliminating the polarization dependence will be described below with reference to claim 3.

[Embodiment 4] FIG. 8 shows a fourth embodiment of the present invention in which the above-described Embodiment 3 is improved to a configuration capable of eliminating the polarization dependence. (B),
(C) is a line segment BB ′ and a line segment C− in the plan view (a).
FIG. 4 is an enlarged cross-sectional view cut along C ′. A portion of the central waveguide between the two directional couplers is reduced to a narrow tapered waveguide 60.
The third embodiment is different from the third embodiment in that In such a configuration, the difference between the effective refractive index felt by the TE-polarized light and the TM-polarized light is given in reverse by reducing the width of the central waveguide, and the polarization dependence can be canceled as a whole. In addition,
It should be noted that the central waveguide width may need to be increased as shown in FIG. 9 depending on the parameter conditions of the optical multiplexer / demultiplexer, the manner of applying internal stress, and the like.

In the first to fourth embodiments described above, a quartz single-mode optical waveguide on a silicon substrate is assumed as the optical waveguide, but the present invention is not limited to these embodiments. An optical waveguide can be configured based on other material configurations. For example, an optical waveguide formed using a multi-component glass and a thin film forming technique such as sputtering,
An optical waveguide formed on a lithium niobate single crystal substrate using metal ion diffusion technology, a waveguide formed by electrothermal diffusion of metal ions in a multi-component glass substrate, and a heterogeneous semiconductor film grown on a semiconductor substrate The present invention is also applicable to a waveguide or the like formed by fine processing. In particular, the third embodiment can be expected to exhibit further effects by forming a semiconductor waveguide, a light emitting element, and a light receiving element on a semiconductor substrate.
In each embodiment, a single waveguide type optical multiplexer / demultiplexer is used. However, a plurality of waveguide type optical multiplexer / demultiplexers can be formed on the same waveguide substrate, and a plurality of waveguide type optical multiplexer / demultiplexers can be used. Naturally, it is possible to combine a wave circuit with a plurality of optical fibers, light-emitting elements, and light-receiving elements, and the function of a waveguide-type optical multiplexer / demultiplexer can exert a great effect at such an integration ratio. Can be expected.

[0032]

The prior art is realized by, for example, a cascade connection of a Mach-Zenda optical interference type (2 × 2) optical multiplexer / demultiplexer and a directional coupling type (2 × 2) 3 dB splitter, for example. In the present invention, two functions of a two-wavelength multiplexing / demultiplexing function and a 3 dB branching function are used in the present invention.
3) It can be realized with a single optical waveguide circuit of the same scale as the optical multiplexer / demultiplexer. As a result, the scale of the optical waveguide is reduced by half, and the area of the optical waveguide can be reduced by half. By the way, in the design of the optical waveguide, in order to suppress the loss of the bent part,
When the functional blocks of the optical waveguide are large, they are usually arranged in the optical axis direction. Therefore, there has been a problem that the length of the optical waveguide substrate increases as the optical waveguide scale increases. The fact that the length of the waveguide substrate can be reduced by half with the reduction of the scale of the optical waveguide by the present invention is a great advantage in practical use.

With respect to the two-wavelength multiplexing / demultiplexing function, it is possible to increase the stop bandwidth or pass bandwidth of one wavelength as compared with the prior art. Since a waveguide type optical multiplexer / demultiplexer generally has a basic sinusoidal periodic transmission characteristic, it has been difficult to obtain a wide stopband or passband by itself. For this reason, it has been customary to connect a waveguide type optical multiplexer / demultiplexer in multiple stages or add an interference film filter to increase the stop bandwidth. However, the multi-stage connection of the waveguide type optical multiplexer / demultiplexer causes an increase in the length of the waveguide substrate, and the addition of the interference filter increases only the stop bandwidth and does not contribute to the corresponding increase in the pass bandwidth. . The invention has the advantage that the passband corresponding to the stopband can be increased without increasing the waveguide substrate length.

In the prior art, for example, when a Mach-Zenda optical interference type (2 × 2) optical multiplexer / demultiplexer and a directional coupling type (2 × 2) 3 dB splitter are successively connected, a 3 dB splitter is used. In the present invention, the output ports of the 3 dB branch are separated from each other on both sides of the multiplexing / demultiplexing output port, whereas the light emitting element and the light receiving element are coupled to the 3 dB branch. And a transmission circuit and a reception circuit connected thereto are easily mounted. The necessity of arranging a transmitting circuit and a receiving circuit in the immediate vicinity of the light-emitting element and the light-receiving element is increasing with the speeding up of the transmission signal. It is expected that satisfying this requirement will be difficult. Furthermore, assuming simultaneous bidirectional communication in which the light emitting element and the light receiving element are simultaneously driven, it is indispensable to electromagnetically shield the transmission circuit and the reception circuit in order to suppress the electrical coupling between the transmission circuit and the reception circuit. Electromagnetic shielding is extremely difficult with the prior art configuration in which the arrangement of the element and the light receiving element are close to each other. In the configuration of the present invention, since the output port of the 3 dB branch is separated on both sides of the multi / demultiplex output port,
The 3 dB branch output port can be arranged at any position without intersecting with other waveguides, which has great advantages in high speed and simultaneous bidirectional communication.

When the above-mentioned waveguide type optical multiplexer / demultiplexer is actually manufactured, polarization dependence may be caused in the optical multiplexer / demultiplexer by the influence of internal stress and the like. Not practically preferable. By making a part of the central waveguide between the two directional couplers a tapered waveguide,
By giving the difference between the effective refractive indices felt by the TE polarized light and the TM polarized light in advance, the polarization dependence can be canceled as a whole.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating the operation principle of the present invention.

FIG. 2 is a characteristic diagram showing an eigenpropagation mode excited when light is input to a central waveguide of the three coupling waveguides.

FIG. 3 is a characteristic diagram showing the wavelength dependence and the waveguide structure dependence of the difference (n ₀ −n ₂ ) between the propagation constants of the zero-order mode and the second-order eigenpropagation mode of the three-coupled waveguide.

FIG. 4 is a configuration diagram showing a first embodiment of the present invention.

FIG. 5 is a characteristic diagram showing a characteristic example of the first embodiment.

FIG. 6 is a characteristic diagram showing a characteristic example of the second embodiment.

FIG. 7 is a configuration diagram showing a third embodiment of the present invention.

FIG. 8 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a modification of the fourth embodiment.

FIG. 10 is a configuration diagram showing a conventional waveguide type optical multiplexer / demultiplexer.

[Explanation of symbols]

 1,2,3 waveguide 4,8,15 non-coupling part 5,7,12,14 spacing conversion part 6,13 coupling part 9,10,11 waveguide section 16,17,18 waveguide 19,21,23 Non-coupling part 20,22 Coupling part 25 Main part of waveguide type optical multiplexer / demultiplexer 26,27,28 Waveguide 29 Expansion part 30,31 Fiber 32 Light emitting element coupling optical system 33 Light emitting element 34 Dielectric interference film filter 35 Light receiving Element 36 Waveguide substrate 37 Mach-Zehnder type optical multiplexer / demultiplexer 38 Directional coupler type optical multiplexer / demultiplexer 39, 40, 41, 42 Waveguide end 50 Waveguide substrate 60 Tapered waveguide

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroshi Terui 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-4-353804 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12-6/14 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A pair of three coupled waveguides which are three coupled waveguides, a pair of three coupled waveguides are connected by three waveguides, and a pair of three coupled waveguides and The waveguide type optical multiplexer / demultiplexer, characterized in that the three waveguides that connect the optical waveguides satisfy the following conditions. (A) The three-coupled waveguide has a symmetric structure in which the waveguide spacing on both sides and the waveguide width on both sides are substantially equal to the center waveguide, and has an equivalent coupling length Lc given by (c). (B) The three waveguides connecting the three coupling waveguides have substantially equal waveguide widths on both sides with respect to the central waveguide, and have substantially equal waveguide lengths on both sides, and the length of the central waveguide is equal to that of the central waveguide. having a waveguide length difference L _d given for waveguide length in section (c). (C) When the first wavelength is λ ₁ and the second wavelength is λ ₂ ,
Equivalent coupling length Lc and the waveguide length difference L _d is substantially satisfy the following equation. 2 (n ₀₁ −n ₂₁ ) L _C = (M− ・) · λ ₁ n · L _d = N · λ ₁ n · L _d = (N − ／) · λ ₂ where n ₀₁ is The equivalent refractive index of the lowest-order eigenpropagation mode of the three coupling waveguides at the first wavelength, and n ₂₁ is the equivalent refractive index of the third eigenpropagation mode counted from the lowest order of the three coupling waveguides at the first wavelength. Here, n is an equivalent refractive index of three waveguides connecting the three coupling waveguides, and N and M are integers. 2. The waveguide type optical multiplexer / demultiplexer according to claim 1, wherein the following expression is almost satisfied. 2 (n ₀₂ −n ₂₂ ) L _C = M · λ ₂ where n ₀₂ is the equivalent refractive index of the lowest-order eigenpropagation mode of the three-coupled waveguide at the second wavelength, and n ₂₂ is 3 at the second wavelength. This is the equivalent refractive index of the third eigenpropagation mode counted from the lowest order of the coupling waveguide. 3. The width of a part of a central waveguide or an outer waveguide in a coupling region or in a Mach-Zehnder interferometer is different from a width of an input / output waveguide. Waveguide type optical multiplexer / demultiplexer.