JP2001124944A - Optical multiplexing/demultiplexing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and accurately manufacture a quartz base array waveguide diffraction grating type optical multiplexing/demultiplexing circuit, and to miniaturize the circuit. SOLUTION: The refractive index of a part of the element waveguides of an array waveguide diffraction grating type optical multiplexing/demultiplexing circuit provided with an array waveguide composed of the plural element waveguides is made higher than remaining parts, and the length of a high refractive index part or the refractive index is made so as to be different in the respective element waveguides. When photorefractive effect is to be used, the high refractive index part can be prepared with satisfactory controllability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a wavelength division multiplexing (Wa)
The present invention relates to an optical multiplexing / demultiplexing circuit used in an optical transmission system.

[0002]

2. Description of the Related Art In a wavelength division multiplexing optical transmission system, 1
In order to multiplex and transmit a plurality of optical signals of different wavelengths to the optical fiber, an optical multiplexing / demultiplexing circuit that multiplexes light of different wavelengths and demultiplexes wavelength multiplexed light is an important device.

As an optical multiplexing / demultiplexing circuit, a substrate type arrayed waveguide diffraction grating type multiplexing / demultiplexing circuit as shown in FIG. 6 is used. This optical multiplexing / demultiplexing circuit includes a first input / output waveguide 2, a first slab waveguide 3, an arrayed waveguide diffraction grating 4, a second slab waveguide 5, and a second input / output waveguide on a substrate 1. Wavelength multiplexed light, which is constituted by a waveguide 6 and enters one of the first input / output waveguides 2, passes through a first slab waveguide 3, an arrayed waveguide diffraction grating 4, and a second slab waveguide 5. The light exits from the second input / output waveguide 6. At this time, the emitted light is emitted from different waveguides of the second input / output waveguide 6 for each wavelength. That is, it is split.

When light of different wavelengths enters each of the second input / output waveguides 6, wavelength multiplexed light is emitted from one of the first input / output waveguides 2 through a path opposite to the above. . That is, they can be multiplexed.

Further, this optical circuit has symmetry,
Even if wavelength multiplexed light is put into one of the input / output waveguides 6 of
Light having different wavelengths can be independently extracted from the individual waveguides of the first input / output waveguide 2.

The combination of an input waveguide and an output waveguide for multiplexing / demultiplexing can be determined by giving a phase difference to each of the guided lights in the arrayed waveguide diffraction grating 4. Specifically, a phase difference is given by changing the length of the waveguide.

The function of the arrayed waveguide grating type optical multiplexing / demultiplexing circuit will be described in more detail with reference to FIGS. 5 (a) and 5 (b).

FIG. 5A is a schematic diagram of a multiplexing / demultiplexing circuit using a lens and a diffraction grating. A circuit for splitting two-wavelength multiplexed light will be described for the sake of explanation. The two-wavelength multiplexed light 8 emitted from the incident fiber 7 passes through a collimator lens 9 to form a parallel light 10.
And is diffracted by the diffraction grating 11 at a diffraction angle corresponding to the wavelength,
It becomes the diffracted lights 12a and 12b. The diffracted lights 12a, 12b are incident on the condenser lens 13 at different angles for each wavelength, and as a result, the light is condensed at different positions 14a, 14b on the focal plane 14 of the lens 13 depending on the wavelength. Positions 14a, 14
b, the light of two wavelengths can be separately extracted by the output waveguides 15a and 15b.

FIG. 5B is a schematic view of an arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit. The circuit for demultiplexing the four-wavelength multiplexed light has been illustrated. The wavelength division multiplexed light 16 emitted into the first slab waveguide 3 via the first input / output waveguide 2 is distributed to the array waveguide 4 almost uniformly. That is, the function of the first slab waveguide 3 is the same as the function of the collimator lens 9. The array waveguide 4 has its elementary waveguides 4a, 4b, 4c,
4d are different in length. Therefore, the array waveguide 4
a, 4b, 4c, and 4d, and the second slab waveguide 5
There is a phase difference at the slab waveguide entrance end 5a, and the phase difference differs depending on the wavelength.
The angle diffracted according to the phase difference and the wavelength changes for each wavelength. That is, the function of the array waveguide 4 is the same as the function of the diffraction grating 11. The diffracted lights 17a, 17b, 17c diffracted at the second slab waveguide incident end 5a,
17d, the shapes of the second slab waveguide ends 5a and 5b are determined so that the light is condensed at the emission end 5b of the second slab waveguide. That is, the function of the second slab waveguide is the same as the function of the condenser lens 13.

To summarize the function of the arrayed waveguide diffraction grating portion, the wavelength multiplexed light is distributed to the elementary waveguides of the arrayed waveguide 4 by the first slab waveguide 3 and guided by the arrayed waveguide to obtain a wavelength. A phase difference corresponding to the wavelength is given to each light of the multiplexed light. As a result, since the output position differs for each wavelength in the second slab waveguide 5, individual wavelengths can be extracted by arranging the output waveguide 6 at an output position of a desired wavelength.

This function utilizing the wave diffraction phenomenon is based on the same principle as that of a diffraction grating, and an array waveguide having such a function is called an array waveguide diffraction grating. The diffraction function of the arrayed waveguide depends on waveguide parameters such as the relative refractive index of the waveguide and the waveguide interval, and is designed so that the phase difference between adjacent waveguides is constant. As a typical example, in the case of a silica-based waveguide in which the relative refractive index difference of the core is 0.3% and the waveguide spacing of the arrayed waveguide is 25 μm, wavelength-division multiplexed light having a wavelength spacing of 0.8 nm is separated. The required waveguide length difference between adjacent waveguides is about 30 μm.

Next, a method of manufacturing a quartz-based arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit will be described. First, a flame hydrolysis deposition method (FHD method: Flame Hydrol
ysisDeposition) to deposit a lower cladding layer that becomes the lower cladding of quartz glass, and then deposit a core layer that becomes the core of quartz glass doped with germanium or the like as a dopant to increase the refractive index, and heat it in an electric furnace. To become transparent glass. Next, the core layer is etched using photolithography and reactive ion etching (RIE) to remove an extra core portion, thereby obtaining a waveguide circuit pattern. Furthermore, an optical multiplexing / demultiplexing circuit can be obtained by depositing the upper cladding layer by the flame hydrolysis deposition method.

The problem of this manufacturing method is that the refractive index fluctuates due to the segregation of the dopant material and the fluctuation of the density when the core layer or the clad layer is deposited or when the transparent material is heated in an electric furnace, and the pattern of the photomask is changed. Due to the accuracy of the width and the reproducibility of photolithography and reactive ion etching, the waveguide circuit dimensions fluctuate, making it difficult to fabricate as designed.In particular, it is necessary to provide the arrayed waveguide grating with the desired characteristics. It is in a difficult point.

[0014] Even if it can be performed with high accuracy, it is difficult to maintain desired multiplexing / demultiplexing characteristics due to a change in refractive index with temperature. In other words, when the temperature changes, the refractive index of the glass changes, and the designed phase difference cannot be obtained with the arrayed waveguide diffraction grating. As a result, light of the desired wavelength can be obtained from the desired output waveguide. Disappears. For this reason, when using a substrate-type arrayed waveguide diffraction grating type multiplexing / demultiplexing circuit, a temperature control device such as a heater is added, and the temperature dependence of the multiplexing / demultiplexing characteristics is utilized. And the characteristics are stabilized by stabilizing the temperature.

However, the substrate-type array waveguide type multiplexing / demultiplexing circuit is very large, several tens of mm to several hundreds of mm, compared to a normal electronic circuit having a small piece of several mm, so that the temperature is controlled. Since a control circuit such as a heater consumes a large amount of power and it is difficult to maintain a uniform temperature, there is a problem that an integrated circuit becomes large in size.

[0016]

The problem to be solved by the present invention is to easily and accurately manufacture a quartz-based arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit and to reduce the size of the circuit. .

[0017]

In order to solve the above-mentioned problems, a part of the elementary waveguide of the arrayed waveguide grating type optical multiplexing / demultiplexing circuit having an arrayed waveguide composed of a plurality of elementary waveguides is made smaller than the rest. The refractive index was made high, and the length or the refractive index was different in each elementary waveguide.

[0018]

(Embodiment 1) FIG. 1 shows a first example of an optical multiplexing / demultiplexing circuit according to the present invention, showing a main part of a four-wavelength optical multiplexing / demultiplexing circuit. Although the optical waveguide has a cross-sectional structure in which a core portion is covered with a clad, a circuit pattern of the optical waveguide circuit is illustrated in FIG. The other parts than the array waveguide 22 are the same as those in the prior art, and the description is omitted. The arrayed waveguide 22 in this example is a bent waveguide, and each elementary waveguide 22a, 22b, 22c, 22d
Are sequentially longer in length, and the difference in length between adjacent elementary waveguides is constant. Elementary waveguide 2
Some of 2a, 22b, 22c and 22d have a high refractive index and the same length. The refractive indices of the high refractive index portions are different from each other so that the arrayed waveguide 22 functions properly as a diffraction grating. This high refractive index portion is formed by a photorefractive effect described later.

(Embodiment 2) FIG. 2 shows a main part of a four-wavelength optical multiplexing / demultiplexing circuit according to a second example of the optical multiplexing / demultiplexing circuit of the present invention. Although the optical waveguide has a cross-sectional structure in which a core portion is covered with a clad, a circuit pattern of the optical waveguide circuit is illustrated in FIG. In addition, array waveguide 2
Parts other than 4 are the same as in the prior art, and a description thereof will be omitted. The arrayed waveguide 24 in this example includes a straight waveguide 25 and two bent waveguides 26. The high refractive index portion is formed as a part of the linear waveguide 25, and has the same refractive index. The lengths of the individual high refractive index portions are different so that the arrayed waveguide 24 properly functions as a diffraction grating.

In this embodiment, the linear waveguide 23 is provided and a part thereof has a high refractive index. However, an arrayed waveguide is constituted only by the bent waveguide 24, and a high refractive index part is provided in a part thereof. Can be made smaller.

(Embodiment 3) FIG. 3 shows a main part of a third embodiment of the optical multiplexing / demultiplexing circuit according to the present invention. Although the optical waveguide has a cross-sectional structure in which a core portion is covered with a clad, a circuit pattern of the optical waveguide circuit is illustrated in FIG. In addition, portions other than the array waveguide are the same as those in the prior art, and thus description thereof is omitted. The high-refractive-index portion of the elementary waveguide of the arrayed waveguide includes a first high-refractive-index waveguide 29 and a second high-refractive-index waveguide 30. The first high refractive index portion and the second high refractive index portion have the same refractive index, and the sum of their lengths is adjusted so that the arrayed waveguide operates properly as a diffraction grating.

Next, the operation of the high refractive index portion in the present invention will be described. Generally, the length of an optical waveguide is called an optical path length, and the difference between the lengths of two waveguides is called an optical path length difference. As described above, the array waveguide has a function of a diffraction grating depending on the phase relationship of light propagating in the elementary waveguide. However, the second slab waveguide 5 which is a characteristic of the diffraction grating of the array waveguide is used. The wavelength dependence dx / dλ of the light condensing position can be expressed by the following equation using the optical path length difference ΔL between adjacent elementary waveguides.

[0023]

(Equation 1)

Where f is the focal length of the slab waveguide, n
c is the effective refractive index of the elementary waveguide, ns is the effective refractive index of the slab waveguide, d is the center interval of the elementary waveguide, and λ0 is the center wavelength of the wavelength band to be split.

This expression means that when the refractive index of the waveguide is constant, the characteristic of the diffraction grating is expressed by the optical path length difference ΔL. In the present invention, this optical path length difference ΔL is given by the elementary waveguide and the high refractive index portion. That is, not only the length of the elemental waveguide but also the refractive index of the high-refractive-index portion so that the phase difference of light propagating through the elementary waveguide becomes the phase difference given by ΔL in Expression 1 in the adjacent waveguide. Give in that length. In this way, the diffraction grating function can be controlled not only by the length of the elementary waveguide but also by the refractive index and length of the high refractive index portion. As will be described later, the high refractive index portion can be manufactured using the photorefractive effect, and can be manufactured while monitoring the wavelength characteristics after forming a circuit by FHD, RIE, and photolithographic methods. A circuit can be configured efficiently.

The photorefractive effect is that when light is irradiated on a substance, the bonding state and the arrangement state of the atoms constituting the substance change to change the refractive index. In the embodiment of the present invention, a phenomenon is used in which when a quartz glass doped with Ge, B, or the like is irradiated with ultraviolet rays, the refractive index of the quartz glass at the irradiated portion increases. It is known that this effect becomes remarkable when hydrogen is contained in quartz, and that the stability over time is also enhanced. For this reason, if the quartz glass is allowed to stand in a high-pressure hydrogen atmosphere and subjected to a treatment for impregnating the quartz glass with hydrogen, and then subjected to ultraviolet irradiation, the photorefractive effect can be effectively used.

The refractive index control utilizing the photorefractive effect is performed as follows. First, the quartz glass waveguide is held in a pressurized hydrogen atmosphere of 150 atm for 10 hours, and then, as shown in FIG. 4, a wavelength multiplexing light source 19 is connected to the arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit 18. An optical spectrum analyzer 21 is connected to the output side via a 4 × 1 optical path switch 20, and the wavelength demultiplexing characteristics are measured to measure a deviation from a design value. While observing the demultiplexing characteristic, the refractive index is controlled by irradiating a part of the elementary waveguide portion of the arrayed waveguide of the arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit 18 with the refractive index, thereby obtaining the target wavelength. The irradiation is stopped when the wave characteristic is reached. Krypton fluoride (Kr
F) An ultraviolet light source having a practical irradiation intensity in the ultraviolet region, such as a laser, xenon fluoride (XeF) or a double-oscillation Ar laser, can be used. In order to selectively irradiate the portion of the elementary waveguide, a method of focusing and irradiating ultraviolet rays using a lens or the like may be used, or the ultraviolet rays may be irradiated through a light shielding plate (photomask) having a transmission window. May be collectively irradiated to perform region selection irradiation. The method using convergent ultraviolet rays requires an expensive apparatus, but the irradiation time is short because only the necessary parts are irradiated. On the other hand, the method using a light shielding plate requires a long irradiation time, but has a simple apparatus configuration. Depending on the application and purpose of the optical multiplexing / demultiplexing circuit, any one of these or a combination thereof should be appropriately selected for irradiation.

The light-shielding plate may be made of any material that does not transmit, that is, absorbs or reflects ultraviolet light. Examples of the absorbing material include metals such as Si and Cr, and examples of the reflecting material include gold and aluminum, and various materials. Multilayer reflective films can be used.

In the present invention, the photorefractive effect is used for producing a high refractive index portion. Next, the phase relationship between the amount of irradiation of ultraviolet rays and light will be described. The phase change of the light given by the irradiation of the ultraviolet rays is determined by the product of the length of the change in the refractive index of the high refractive index portion with respect to the elementary waveguide and the length. Since the amount of change in the refractive index has a correlation with the amount of ultraviolet irradiation, in other words, the phase difference of light is determined by the product of the amount of ultraviolet irradiation and the length of the irradiated portion. Therefore, the accuracy of the phase difference is determined by the product of the accuracy of the amount of ultraviolet irradiation and the accuracy of the length of the irradiation part. Can be higher.

In the first embodiment, the phase difference of the light of the elementary waveguide is given by making the refractive indexes of the individual high refractive index portions different. Further, since the refractive index is high, the circuit curvature of the bent waveguide can be reduced, and the array waveguide 22 can be shortened. Furthermore, by controlling the refractive index using the photorefractive effect, the phase difference between individual element waveguides can be adjusted with good controllability.

In the second embodiment, a phase difference is given between the elementary waveguides by the length of the high refractive index portion so that the amount of ultraviolet irradiation at the irradiated portion is constant, so that the extra high refractive index portion causes The array waveguide can be miniaturized by providing a phase difference and by changing the length of the high refractive index portion. Further, when the photorefractive effect is used, a high refractive index portion can be manufactured with good controllability. In addition, when the photorefractive effect and the light-shielding plate are used together, even if the light-shielding plate has a constant difference in the length of the irradiated portion between the elementary waveguides, the larger the irradiation amount, the larger the difference in the length of the irradiated portion. Since the phase difference given by the diffraction grating becomes large, the characteristics of the diffraction grating can be finely adjusted by adjusting the irradiation amount.

Further, a general ultraviolet laser has poor output stability, and a laser with high output stability has a problem that equipment becomes expensive. As described above, since the accuracy of the high refractive index portion is determined by the product of the ultraviolet radiation dose accuracy and the length accuracy, in the third embodiment, the length of the refractive index portion is set in the first and second embodiments.
An optical multiplexing / demultiplexing circuit can be manufactured with good controllability even with an ultraviolet laser having a poor output stability.

[0033]

According to the present invention, an arrayed waveguide composed of a plurality of elementary waveguides is provided with a waveguide in which a part of the elementary waveguide has a higher refractive index than the rest. By precisely controlling the refractive index, the desired multiplexing / demultiplexing characteristic can be obtained with high accuracy. Further, the length of the elementary waveguide can be shortened, the circuit can be reduced in size, and an optical multiplexing / demultiplexing circuit can be obtained in which temperature control is easy and power consumption of the circuit is small. Furthermore, if a high refractive index part is created using the photorefractive effect,
Since the characteristic deviation due to the refractive index and the fluctuation of the circuit at the time of fabrication can be corrected after the circuit is formed, it is possible to easily and accurately manufacture the circuit.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a main part of a first example of the present invention.

FIG. 2 is a schematic configuration diagram showing a main part of a second example of the present invention.

FIG. 3 is a schematic configuration diagram showing a main part of a third example of the present invention.

FIG. 4 is a block diagram showing a configuration of a measurement system of wavelength demultiplexing characteristics used in the manufacturing method of the present invention.

FIG. 5A is a schematic diagram for explaining the function of a multiplexing / demultiplexing circuit using a conventional lens and a diffraction grating, and FIG.
It is the schematic explaining the function of the conventional array waveguide diffraction grating type optical multiplexing / demultiplexing circuit.

FIG. 6 is a schematic diagram of a conventional arrayed waveguide diffraction grating type optical multiplexing / demultiplexing circuit.

[Explanation of symbols]

 3 1st slab waveguide 5 2nd slab waveguide 22 array waveguide 22a element waveguide 22b element waveguide 22c element waveguide 22d element waveguide

Claims (2)

[Claims] 1. An arrayed waveguide grating type optical multiplexing / demultiplexing circuit having an arrayed waveguide composed of a plurality of elementary waveguides, wherein the elemental waveguide has a high refractive index, a part of which has a higher refractive index than the rest. An optical multiplexing / demultiplexing circuit, wherein a refractive index portion is formed, and a length or a refractive index of the high refractive index portion is different in each elementary waveguide. 2. The optical multiplexing / demultiplexing circuit according to claim 1, wherein the high refractive index portion is formed using a photorefractive effect.