JPH087294B2 - Waveguide type optical multiplexer / demultiplexer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a waveguide type optical multiplexer / demultiplexer suitable for optical multiplexing and demultiplexing in optical frequency multiplex transmission.

[Prior Art] Optical frequency division multiplexing (FDM) transmission that wavelength-multiplexes 50 or more optical signals in one wavelength region is expected as a future optical communication system. In this FDM transmission, it is essential to develop an optical multiplexer / demultiplexer that can combine and demultiplex light waves in the GH _z order with low loss. As this optical multiplexer / demultiplexer, a periodic type demultiplexer (Mach-Zehnder multiplexer / demultiplexer) shown in FIG.
61-80109, or Miyauchi, Yamamoto: Microwave circuit for communication, PP.91-93, published by Corona, published on October 20, 1981, first edition). This is to connect the terminals of two optical directional couplers 10 and 20 with two optical waveguides 1 and 2, and to obtain the lengths l ₁ and l _{2 of the} optical waveguides.
If the relative lengths of the two are different from each other, when the same signals pass through the two optical waveguides 1 and 2, the signals of the frequencies having opposite phases are canceled out, and the signals of the frequencies having the same phase are emphasized. Therefore, the optical multiplexer / demultiplexer is configured by matching this frequency with the frequency of the input optical signal.

That is, υ ₁ = sin at the input terminal 11 of the directional coupler 10.
If the signal of ωt is incident, and the output of the demultiplexer υ ₀₁ , υ ₀₂ is obtained with the coupling coefficient of the directional couplers 10 and 20 being T, the signals appearing at the output terminals 13 and 14 of the directional coupler 10. Is υ ₁₁ = T sin ωt (1,1) υ ₂₁ = T sin (ωt + π / 2) = T cosωt (1,2) At the output terminals 21 and 22 of the waveguides 1 and 2, the waveguide length l ₁ , The phase is shifted by l ₂ , and υ ₁₂ = T sin (ωt + βl ₁ ) (1,3) υ ₂₂ = T cos (ωt + βl ₂ ) (1,4) Since υ ₁₂ and υ ₂₂ are added, the signals obtained at the output terminals 23 and 24 are However, n: Refractive index of optical waveguide f: Frequency of light C: Speed of light in vacuum l ₁ , l ₂ : Length of optical waveguide. here, That is, if 10 and 20 are 3 dB directional couplers,
Expressions (1,5) and (1,6) are expressed by the following expressions.

Therefore, f shown in the following equation is applied to the input terminal 11 of the directional coupler 10.
_When the signal of the optical frequency of ₀₁ , f ₀₂ is input, As can be seen by substituting equations (1,11) and (1,12) into equations (1,9) and (1,10), the optical frequency signal of f = f ₀₁ is output to the lower output terminal 24. Are canceled by each other and are not output, but are output to the upper output terminal 23.

On the other hand, the signal of the optical frequency of f = f ₀₂ is output from the upper output terminal 23.
Are not canceled and output, but added to the lower output terminal 24 side and output.

With the above operation, signals of optical frequencies f ₀₁ and f ₀₂ are demultiplexed.

The optical demultiplexer is for demultiplexing signals of two optical frequencies, but when demultiplexing signals of four optical frequencies f _{1 to} f ₄ , as shown in FIG. Directional coupler 10-60
Is used. The optical demultiplexer of FIG. 6 has a cascade circuit of the directional couplers 30 and 40 at one output terminal 23 of the directional coupler 20,
A cascade circuit of directional couplers 50 and 60 is connected to the other output terminal 24, and signals of optical frequencies f ₁ and f ₃ are output from the directional coupler 40 and optical frequencies f ₂ and f ₄ are output from the directional coupler 60. The signal of is demultiplexed and taken out.

[Problems to be Solved by the Invention] However, in the directional coupler used in FIGS. 5 and 6, the length of the coupling portion where two optical waveguides are coupled is 5 mm.
The length of the input and output waveguides in the front and rear, in the front and rear of the coupling section is about 10 mm, and after all, one directional coupler has the X direction (the fifth
The length of () is about 15 mm. On the other hand, the width of the waveguide is around 10 μm, and the distance between the waveguides is several μm.
The width in the Y direction is 0.2 mm or less, which is a very elongated structure. Moreover, since the configuration of FIG. 5 has two directional couplers connected in series, and the configuration of FIG. 6 has four directional couplers connected in series, it becomes an optical device having an extremely elongated dimension structure. .

Therefore, when making the optical demultiplexer into a waveguide type monolithic structure, due to non-uniformity of in-plane exposure during photolithography, the patterning process of the waveguide such as etching, in-plane non-uniformity of dry etching, etc. The manufacturing deviation of the pattern dimension in the plane becomes very large. This dimensional deviation causes deviation in the coupling coefficient of each directional coupler. The deviation of this coupling coefficient increases the excess loss,
This is a fatal problem with this type of periodic demultiplexer because it reduces the amount of leakage attenuation.

Further, because of the very elongated dimension structure as described above, several photomasks for photolithography are required when the waveguide type structure is used, which leads to a significant increase in cost. Further, in that case, the substrate becomes very long and slender, which may cause the substrate to warp or cause unnecessary stress or breakage during packaging. In addition, there are problems that it is difficult to handle and that productivity is lowered.

An object of the present invention is to provide a waveguide type optical multiplexer / demultiplexer capable of solving the above-mentioned problems of the prior art.

[Means for Solving the Problems] In a waveguide type optical multiplexer / demultiplexer of the present invention, a directional coupler having two inputs and two outputs is provided in a length direction of the directional coupler on a substrate forming the waveguide. All of them are arranged in parallel at right angles to each other, and two output terminals of the first directional coupler and two input terminals of the second directional coupler are connected by a waveguide so as to have an optical path length difference. In order to expand this basic configuration, one of the input terminals of the third and fourth directional couplers is connected to a different terminal of the two output terminals of the second directional coupler. A fifth directional coupler for the third directional coupler and a sixth directional coupler for the fourth directional coupler.
The directional coupler is connected by a waveguide so as to have an optical path length difference and a basic configuration is added to enable demultiplexing or multiplexing or multiplexing / demultiplexing of four signals having different optical frequencies.

By adding a directional coupler to the above basic structure, a large number of signals having different optical frequencies can be demultiplexed or combined.

A thin film heater is provided near the waveguide between the directional couplers so that the difference in optical path length can be adjusted, and a voltage can be applied to the heater, or a ring type resonance is provided in the middle of the waveguide between the directional couplers. Vessels can be combined.

[Operation] By arranging the directional couplers in parallel, X of the waveguide substrate
A configuration with well-balanced dimensions in the Y direction and the Y direction is possible, and low excess loss and high crosstalk attenuation characteristics can be obtained. This configuration is extremely effective in realizing a waveguide type monolithic optical demultiplexer, and is also suitable for mass production.

The thin film heater heats the waveguide between the directional couplers, changes its refractive index, adjusts the optical path length difference, and outputs a signal of a desired optical frequency to the multiplexer / demultiplexer. Further, the ring resonator coupled in the middle of the waveguide between the directional couplers enhances the optical frequency selectivity.

[Example] Hereinafter, the illustrated example will be described.

FIG. 1 shows a first embodiment of the waveguide type optical demultiplexer or optical multiplexer of the present invention.

In the figure, the directions of the signals of optical frequencies f ₁ , f ₂ , f ₃ , and f ₄ are shown by the solid line and the dotted line.The solid line acts as an optical demultiplexer, and the dotted line acts as an optical multiplexer. The case is shown. If a solid line and a dotted line are used in combination, it will act as an optical multiplexer / demultiplexer. Directional coupler 10,20,30,40,50,60
It is a feature of the present invention that they are arranged in parallel. These directional couplers are configured to evenly distribute an optical signal incident from one of the input terminals to two output terminals, which are the same as conventional directional couplers.

To be more specific, in FIG. 1, two directional couplers 10 and 20 forming a first functional stage are arranged in parallel in the center,
Between the two output terminals 13 and 14 of the first directional coupler 10 and the two input terminals 21 and 22 of the second directional coupler 20,
They are connected by optical waveguides 1 and ₂ having lengths l ₁ and l _{2 having} an optical path length difference. As described above, a configuration in which two directional couplers are arranged in parallel and cascaded so as to have an optical path length difference is a basic structural unit that constitutes one functional stage.

Directional couplers 10 and 2 constituting the first functional stage
There are two directional couplers 30, 40 and 50, 6 on each side of 0.
0 and 0 are arranged in parallel to form the second and third functional stages, respectively. In the second functional stage and the third functional stage, the lengths l ₃ , l ₄ between the two output terminals 33, 34 of the directional coupler 30 and the two input terminals 41, 42 of the directional coupler 40. Are connected by the optical waveguides 3 and 4, and the two output terminals 53 and 54 of the directional coupler 50 and the two input terminals 6 of the directional coupler 60 are connected.
The optical waveguides 5 and _{6 having} the lengths l ₅ and l ₆ are connected to the optical fibers 1 and 62. In the directional couplers 30 to 60 that form the second and third functional stages, the remaining terminals are the output of the directional coupler 20 in order to minimize the connection distance with the first functional stage. It is arranged so as to be on the same side as the terminals 23 and 24. Further, the terminals occupying the positions closest to the output terminals 23, 24 of the directional coupler 20 of the first functional stage, that is, the terminals 31 inside the directional coupler 30 and the terminals inside the directional coupler 50. 51 and 51 are used as input terminals of the second functional stage and the third functional stage, respectively. The input terminals 31, 51 of the directional couplers 30, 50 are connected to the directional couplers
One of the 20 output terminals 23 and 24 located on the near side is connected to each of the optical waveguides 7 and 8.

 Next, the operation of the above configuration will be described.

The outline of the operation as a whole is that the signals of the optical frequencies f ₁ , f ₂ , f ₃ , f ₄ incident on the 11 terminals are demultiplexed, respectively, and are output to the output terminal 43 at f ₁ , 44 to f ₃ and 64. It outputs a signal of optical frequency f ₄ to f ₂ and 63.

First, in the first functional stage, by setting the optical path length difference between the terminals 14 and 22 and between the terminals 13 and 21 to be l ₁ -l ₂ , signals at optical frequencies f ₁ and f ₃ are directional couplers. 20 output terminals 2
3 is output to the directional coupler 30 of the second functional stage through the input terminal 31. On the other hand, the signals of the optical frequencies f ₂ and f ₄ are output to the output terminal 24 of the directional coupler 20 and guided to the directional coupler 50 of the third functional stage through the input terminal 51.

The signals of the optical frequencies f ₁ and f ₃ guided to the second functional stage are between the terminals 33 and 41 of the second functional stage and between the terminals.
The optical path length difference between the terminals 42 by taking into l ₃ -l ₅ 34-, signals of the optical frequency f ₁ and f ₃ are demultiplexed signals f ₁ is the pin 43,
The signals at f ₃ are split to terminals 44, respectively. On the other hand, the signals of the optical frequencies f ₂ and f ₄ guided to the third functional stage are
Between terminal 53 and terminal 61 and terminal 54 and terminal of the third functional stage
By setting the optical path length difference between 62 to l ₅ −l ₆ , the optical frequency f ₂
The signals f ₄ and f ₄ are demultiplexed, the signal f ₂ is demultiplexed to the terminal 64, and the signal f ₄ is demultiplexed to the terminal 63.

When used as a multiplexer, signals of optical frequencies f ₁ , f ₃ , f ₂ , and f ₄ may be made incident from terminals 43, 44, 64, 63 of the second and third functional stages, respectively. As a result, these signals are combined and taken out from the terminal 11 of the first functional stage.

Here, the curved waveguide in each functional stage and the curved waveguide connecting between the functional stages, specifically, from the terminal 13
21, terminals 14 to 22, terminals 23 to 31, terminals 24 to 51, terminals
The curved waveguides 33 to 41, the terminals 34 to 42, the terminals 53 to 61 and the terminals 54 to 62 have a radius of curvature r of r <(2an _c / in order to minimize radiation loss in this curved portion.
It is preferable to select Δn). Here, n _c is the refractive index of the cladding portion of the optical waveguide, Δn is the relative refractive index difference between the core portion and the cladding portion of the optical waveguide, and a is the width of the core portion of the optical waveguide.

FIG. 2 shows another embodiment of the optical multiplexer / demultiplexer of the present invention. This is an optical demultiplexer that demultiplexes _four optical frequencies f ₁ , f ₂ , f ₃ and f ₄ as in the case of FIG.
It is similar to the case of the figure. The difference from FIG. 1 is that the input terminals 11, 31, and 51 of the first, second, and third functional stages are all provided on the same side, and the lengths l ₁ and l ₂ of the optical waveguides ₁ and ₂ are respectively set. The optical demultiplexer input terminal 110, which is shorter and is located on the opposite side instead
To the input terminal 11 of the directional coupler 10 are connected by the optical waveguide 9 to increase the length.

71, 72, 73 are thin film heaters for heating the optical waveguides 1, 3, 5 and are connected to respective variable voltage sources 81, 82, 83,
The temperature is adjusted by the voltages V ₁₂ , V ₃₄ and V ₅₆ . These thin-film heaters 71, 72, 73 use the above-mentioned voltages V ₁₂ , V ₃₄ , V for the optical path length difference between l ₁ and l _{2, the} optical path length difference between l ₃ and l ₄ , and the optical path length difference between l ₅ and l ₆ , respectively. Adjusted by ₅₆ , desired optical frequencies f ₁ , f ₂ , f ₃ , and f ₄ are output to the respective output terminals 43, 64, 44, 63. That is, the variable voltage source 8
By changing the voltage of 1,82,83, the temperature of the optical waveguides 1,3,5 is changed, which changes the refractive index of the optical waveguides 1,3,5, and equivalently the waveguides 1,3,5 It utilizes the fact that the length of can be changed.

In the configuration of FIG. 2 described above, the thin film heater and the voltage application terminal can all be arranged on the same left side, so that they are easy to make and the operability when mounted in a package is good. Also, since these are on the opposite side of the output terminal for the optical frequency signal, they are easy to mount and handle.

FIG. 3 shows another embodiment of the optical demultiplexer of the present invention. This is a modification of the configuration of FIG. That is, in order to reduce the transmission loss by shortening the length of the optical waveguide 9 from the input terminal 110 of the optical demultiplexer in FIG. 2 to the input terminal 11 of the first directional coupler 10 as much as possible, As shown in FIG. 3, the optical demultiplexer input terminal 110 is on the same side as the input terminal 11 of the directional coupler 10, the optical waveguide 9 from the terminals 110 to 11 and the output terminal 23 to the second terminal of the first functional stage. The optical waveguide 7 up to the input terminal 31 of the functional stage is crossed as indicated by 90. In this way, when the two optical waveguides 7 and 9 are formed in a cross shape to form an orthogonal waveguide,
Since it is well known that the signal crosstalk between the two optical waveguides is extremely small, the optical frequencies f ₁ , f ₂ and f incident on the optical demultiplexer input terminal 110 are configured even with the configuration shown in FIG. The signals ₃ and f ₄ are transmitted to the optical waveguide 7 of the output terminal 23 of the first functional stage.
Rarely leaks to the optical waveguide 9 of the input terminal 31 of the first functional stage.

FIG. 4 shows still another embodiment of the optical demultiplexer of the present invention. This is the optical transmission line in the configuration of FIG.
By coupling the ring type resonators 91, 92, 93 in the middle of 1, 3, 5 the optical frequency selectivity is further enhanced. As a result, it is possible to further reduce crosstalk between the optical frequencies f ₁ and f ₃ , f ₂ and f ₄ , and f ₁ , f ₃ , f ₂ , and f ₄ . Resonator length of ring resonator 91, 92, 93 l
_r1 , l _r2 , l _r3 are selected as in the following equation.

As described above, the optical demultiplexer, the optical multiplexer or the optical multiplexer / demultiplexer of the present invention has all the directional couplers arranged in parallel, so that the length × width: 30 mm × 30 mm substrate (for example, Si , SiO
₂ ) Can fit well on top. This is in contrast to the very elongated dimension structure, which is 120 mm x 10 mm in length x width under conventional configurations.

Although the preferred embodiment has been described above, the present invention is not limited to the above embodiment. First, the directional coupler and the transmission line structure for connecting the directional couplers may be any of a buried type, a ridge type, a channel type, a loaded type, or a combination thereof. Further, although only the case of four optical frequencies is described, the number M of frequencies for optical demultiplexing is: M = 2 ^P However, as shown by P = 1,2,3 ... Even for waves, ..., By similarly increasing the number of functional stages, it is possible to construct a desired optical multiplexer / demultiplexer.

EFFECT OF THE INVENTION Since the present invention is configured as described above,
The following effects are achieved.

By placing all directional couplers in parallel,
The waveguide type monolithic optical multiplexer / demultiplexer can balance the size and size in the length direction and the width direction. The monolithic optical multiplexer / demultiplexer of the present invention is, for example,
It can be formed on a 30 mm square Si substrate, but the conventional structure has a substrate size of 120 mm × 10 mm.When such a very long substrate is used, the optical waveguide film is formed and the optical waveguide is formed by the semiconductor process. The deviation of the optical waveguide size at the time of patterning by photolithography and dry etching becomes large, resulting in an increase in excess loss and a decrease in crosstalk attenuation. In the case of the present invention, such a problem hardly occurs.

Further, the optical multiplexer / demultiplexer of the present invention has a configuration suitable for mass productivity, and is easy to handle and mount. Further, in the conventional case, about three photomasks for photolithography are required, whereas in the case of the present invention, only one is required. Therefore, from this aspect as well, the optical multiplexer / demultiplexer of the present invention is low in cost. Can be realized.

In the optical multiplexer / demultiplexer according to claim 2, since the thin film heater heats the waveguide between the directional couplers to change the refractive index of the transmission line and adjust the optical path length difference, a signal of a desired optical frequency is generated. Obtained at the output of the multiplexer / demultiplexer.

In the optical multiplexer / demultiplexer according to the third aspect, the ring resonator is coupled in the middle of the transmission path between the directional couplers, so that the selectivity of the optical frequency can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an embodiment of a waveguide type optical multiplexer / demultiplexer according to the present invention, and FIGS. 2, 3, and 4 are respectively the waveguide type optical multiplexer / demultiplexer according to the present invention. FIG. 5 is a principle diagram of a conventional duplexer for 2-frequency demultiplexing, and FIG. 6 is a configuration diagram of a conventional duplexer for 4-frequency demultiplexing. In the figure, 10, 20, 30, 40, 50 and 60 are directional couplers, 1 to 9 are optical waveguides, 71, 72 and 73 are thin film heaters, 81, 82 and 83 are variable voltage sources, 91, 92, 93 indicates a ring resonator.

Claims (3)

[Claims] 1. A directional coupler having two inputs and two outputs is arranged in parallel in a direction perpendicular to a length direction of the directional coupler on a substrate forming a waveguide to obtain a first directional coupler. The two output terminals of the second directional coupler have a basic structure in which the two output terminals of the coupler and the two input terminals of the second directional coupler are connected by a waveguide so as to have an optical path length difference. One of the input terminals of the third and fourth directional couplers is connected to a different terminal of the terminals, and the fifth terminal is connected to the third directional coupler.
For the fourth directional coupler, the sixth directional coupler is connected to the fourth directional coupler by a waveguide so as to have a difference in optical path length, and a basic configuration is added to add four different directional couplers. A waveguide type optical multiplexer / demultiplexer, which is configured to perform demultiplexing, multiplexing or multiplexing / demultiplexing of signals. 2. A thin film heater is provided near the waveguide between the directional couplers so as to adjust the optical path length difference, and a voltage can be applied to the heater. Waveguide type optical multiplexer / demultiplexer. 3. A waveguide type optical multiplexer / demultiplexer according to claim 1, wherein a ring type resonator is coupled in the middle of the waveguide between the directional couplers.