JP6714381B2 - Optical waveguide device - Google Patents

Description

The present invention relates to an optical circuit having a thin film element inserted in a substrate of an optical waveguide type device and an optical device realized by using the optical circuit.

Due to the explosive spread of smartphones and portable tablet terminals and the start of video distribution services, demands for increasing the transmission capacity of optical networks are increasing day by day. Optical communication technology is required to further develop in response to this demand, and a technology for realizing miniaturization and cost reduction of components used in the optical communication system is becoming more and more important. Waveguide devices have been mentioned as a technology that has played an important role in realizing components for optical communication systems. In the waveguide type device, by applying the principle of optical interference, various basics such as optical signal branching/coupling device, wavelength multiplexer/demultiplexer, interleave filter, optical switch, variable optical attenuator (VOA) are available. Functions have been realized. Since these devices have a waveguide type structure, they are characterized in that they have flexibility in circuit design, and that they can be easily scaled up and highly integrated. Further, since the waveguide device is manufactured by diverting the manufacturing process of semiconductor parts such as LSI, it is highly expected as a device excellent in mass productivity. Various materials such as semiconductors and polymer materials have been put to practical use as materials for the waveguide portion. In particular, a silica-based optical waveguide fabricated on a silicon substrate has the characteristics of low loss, excellent stability, and excellent compatibility with optical fibers, and is one of the most practical waveguide type devices. Is one.

In order to meet the above-mentioned demand for increasing the transmission capacity of optical networks, digital coherent optical transmission technology has become widespread. Among optical communication components that use a waveguide type device, an optical transmitter/receiver used for digital coherent optical transmission is particularly noted. This optical transmitter/receiver has achieved high-speed operation with a transmission rate per wavelength of 100 Gb/s in an optical signal subjected to wavelength division multiplexing (WDM).

The optical signal modulation method mainly used in the digital coherent optical transmission technology is phase modulation. Specifically, phase shift keying (PSK) or quadrature amplitude modulation (QAM), which is a phase modulation method combined with intensity modulation, is used. Furthermore, in the digital coherent optical transmission technology, in addition to phase modulation, by combining a polarization multiplexing system that multiplexes a plurality of phase-modulated optical signals with two orthogonal optical polarizations, the above-mentioned high transmission rate can be achieved. Has been realized.

An optical receiver in a digital coherent optical transmission system has an optical interference circuit at its front end for performing signal processing with an optical signal as it is. Interference light obtained from the optical interference circuit is detected by a light receiving element (PD: Photo Detector) and converted into an electric signal to obtain a reception signal. The received signal from the optical interference circuit is further subjected to subsequent digital signal processing to realize demodulation of the polarization multiplexed phase modulated signal.

Optical waveguide devices are widely used in the above-mentioned optical interference circuit, and include a VOA that adjusts the optical intensity of signal light, a polarization beam splitter (PBS) that separates the polarization of signal light, and a signal light. Alternatively, it is composed of basic functional elements such as a polarization rotator (polarization rotator) that rotates the polarization of the local light and a 90-degree hybrid that detects the phase difference due to the interference between the signal light and the local light. In particular, an optical waveguide type device using a quartz optical waveguide is also generally called a planar lightwave circuit (PLC). An optical receiver including a PLC is a key component for realizing further spread and increase in capacity of a digital coherent optical transmission system in the future.

FIG. 1 is a diagram showing a configuration of an optical interference circuit in a conventional optical receiver configured by a PLC, and is a top view of a substrate surface of a silicon substrate on which the optical interference circuit is configured. Although detailed explanation of the operation is omitted here, in FIG. 1, the outline of each functional element of the optical waveguide type device that realizes different functions of the optical interference circuit is roughly drawn. The optical receiver 100 includes a VOA 15, a PBS 12, a polarization rotator 13, 90-degree hybrids 16a and 16b, etc. as main functional elements. Furthermore, an input waveguide 11 for signal light, an input waveguide 14 for local light, output waveguides 18a and 18b for interference light, a signal light monitor waveguide 17, and the like are provided. In an optical interference circuit including a PLC and including a combination of functional elements that realize different functions as shown in FIG. 1, miniaturization is a very important technical issue.

In order to realize a PBS or a polarization rotator in a PLC, a configuration in which an optical wave plate is inserted in an optical interference circuit so as to cross the optical waveguide can be used. An optical wave plate is an element that causes a phase difference in an optical signal according to the polarization of an optical signal that passes through the optical wave plate, and one made of, for example, a polyimide film is widely known.

Referring again to FIG. 1, in the PBS 12 and the polarization rotator 13, a groove 3 for inserting an optical wave plate is formed so as to cross the optical waveguide. An optical wave plate is inserted inside the groove 3 so that the light propagating through each optical waveguide passes through the optical wave plate. With such a configuration in which the groove is provided on the surface of the substrate, it becomes possible to cause rotation of the polarized light transmitted through the optical wave plate. For example, in order to configure the PBS 12, a Mach-Zehnder interferometer circuit composed of two optical waveguides is used, and a λ/4 wavelength plate is inserted in each optical waveguide so that their birefringent optical axes are orthogonal to each other. Just do it. Further, in order to realize the polarization rotator 13, a λ/2 wavelength plate may be provided in the target optical waveguide in such an orientation that the birefringent optical axis is 45 degrees (Non-Patent Document 1).

Patent No. 2614365 Publication

S. Tsunashima, et. al., “Silica-based, compact and variable-optical-attenuator integrated coherent receiver with stable optoelectronic coupling system”, November 19, 2012/Vol. 20, No. 24/OPTICS EXPRESS 27174 Implementation Agreement for Integrated Dual Polarization Intradyne Coherent, IA # OIF-DPC-RX-01.2 Receivers, November 14, 2013

However, the above-mentioned optical waveguide type device having the structure in which the optical wave plate is inserted into the groove has the following problems in downsizing the device. The groove for inserting the light wave plate in the PLC has been formed by machining using a dicing device, as described in Patent Document 1, for example. The lower part of FIG. 1 shows the structure in the vicinity of the groove 3 when a cross section perpendicular to the substrate surface of the optical receiver 100 and the longitudinal direction of the groove 3 is viewed. The groove 3 has a cutting depth from the uppermost surface of the optical interference circuit to a predetermined value so as to reach the silicon substrate 1 beyond the waveguide layer 2 including the core layer and the cladding layer in which the optical waveguide is formed. It was being processed while adjusting. However, with such a method by machining, other circuits cannot be configured within the work area (work size) for performing groove processing determined by the size of the dicing blade. Considering the processing accuracy of the dicing device itself and the work size required for the processing operation, a circuit placement prohibited area is required over the area of about 1 mm×5 mm around the groove.

The dicing blade is very large with respect to the optical waveguide, and due to the interference of the layout of the work area and each functional element, it was not possible to form a large number of grooves of different sizes on the substrate. If the optical waveguide that originally does not require a groove is cut due to the groove formed on the substrate, useless optical loss will occur in the groove. In the end, it was not possible to flexibly arrange the circuits of other functional elements close to each other in the working area for groove processing, except for the optical waveguide, which requires the insertion of the wave plate. In such a situation, the arrangement of the grooves is prioritized first, and as shown in FIG. 1, a single common groove is formed, and accordingly, different portions in the single groove are used to achieve different functions. It constituted the optical circuit.

Laser machining can also be used as another method for forming the groove 3 in FIG. However, in laser processing, since glass (SiO ₂ ) is melted by heat to form a groove, problems may occur such that stress is generated due to thermal contraction of the processed portion and the inner surface of the groove is roughened. It is difficult to process into a shape that minimizes the optical loss in the wave plate insertion portion, and there is concern that the optical characteristics may be deteriorated due to the distortion of the optical waveguide portion caused by the processing. Regardless of the machining method such as dicing or laser, the miniaturization of the optical circuit has been a constraint in terms of processing accuracy and work size. After all, because the thin film insertion groove formed in the optical waveguide by machining is restricted by the processing shape, it is usual that adjacent waveguides without grooves are arranged at positions 1 mm or more away from the groove. It has been a factor limiting the miniaturization of the circuit.

As a technique for forming the groove on the substrate of the optical interference circuit, a wafer process by dry etching or wet etching can also be used. These etching methods are promising as a method for forming a groove because they can be controlled at the light wavelength level in terms of both processing accuracy and shape control. However, a processing method suitable for forming a deep groove into which a thin film such as a light wave plate is inserted is required. Specifically, the processed shape is required to be perpendicular to the depth direction of the groove, and a technique is required that suppresses the roughness of the interface between the optical waveguide and the groove and that the groove opening has a processed tolerance. In the configuration example of the optical interference circuit described in the cited document 1, due to the difference in the etching ratio (selection ratio) between the horizontal direction and the depth direction, when processing a deep groove with sufficient accuracy, the groove length is long. It was necessary to make the size of the width and width larger than a certain value. In a general etching technique for producing an optical waveguide, the etching rate in the depth direction perpendicular to the substrate surface is slower than the etching rate in the horizontal direction parallel to the substrate surface. Therefore, in order to form a vertical groove deep enough to insert a light wave plate, it is necessary to make the width or length of the groove larger than the size originally required for inserting the light wave plate. It was After all, it was difficult to form a small groove for inserting the light wave plate into the groove by general dry etching or wet etching for manufacturing an optical waveguide.

Therefore, as described in Patent Document 1, for example, when the distance between adjacent optical waveguides is to be arranged close to 500 μm or less, one continuous groove is arranged over a plurality of optical waveguides. It is necessary to devise. Especially when miniaturization of the optical circuit is required, allow the loss as inevitable and form the groove to the optical waveguide that does not originally need the groove, or devise a layout to avoid the groove. However, it is necessary to dispose an optical circuit, and there are great restrictions on miniaturization of the circuit, flexibility of circuit design, and optical performance.

In recent years, a deep silicon technique called deep etching, which has an improved etching rate in the depth direction, has been realized. By improving the etching technique, it is possible to form a separate groove for each optical waveguide. Even if it is necessary to arrange the grooves in multiple optical waveguides, it is possible to arrange the grooves for each optical waveguide with the minimum groove area, so it is possible to arrange the optical waveguides closer to each other than in the past. Became. However, when the optical waveguide having a groove and the optical waveguide having no groove are arranged close to each other, in the optical waveguide having no groove (the groove does not cross), the stress in the horizontal direction of the substrate released by the adjacent groove is increased. Due to the influence of the distribution, there arises a problem that the polarization state of the light propagating in the optical waveguide having no groove changes. In particular, when the grooveless optical waveguide constitutes a circuit that propagates light of a single polarization, there arises a problem that the polarization extinction ratio of the circuit deteriorates.

Further, as in the polarization control optical waveguide device using the stress release groove disclosed in Patent Document 1, a dummy groove may be arranged in order to eliminate the imbalance of the stress released by the formation of the groove. is there. In order to maintain the symmetry of the waveguide circuit pattern with respect to the adjacent optical waveguide, even if it is attempted to form a dummy groove and a groove for inserting a thin film element at the same time, if the miniaturization is required, the arrangement of the groove itself Can be difficult. As described above, since the stress is released by the groove for inserting the thin film element, it is a new technical problem that the polarization maintaining property of the adjacent optical waveguide having no groove is impaired.

The present invention has been made in view of such a problem, and an object thereof is to provide a groove for inserting a thin film element when configuring a small optical receiver used for digital coherent communication. , Clarify the range of the distance to the adjacent optical waveguide without a groove. Under the condition of a specific circuit arrangement, the optimum range of the distance between the groove into which the thin film element is inserted and the adjacent waveguide is clarified when an optical receiver is realized by using a PLC circuit on a substrate of a predetermined size. , Give an index. We propose a structure of a groove that realizes miniaturization of an optical interference circuit having a groove into which a thin film element such as an optical wave plate is inserted, and provide an optical interference circuit and an optical waveguide device having a higher degree of integration.

To achieve the object, such as this, a first aspect of the present invention, in the optical waveguide type device of silica-based material configured on a silicon substrate, and the two arm waveguides, said two arms consists waveguide across only respectively, a lambda / 4 polarization beam splitter having a insertable two grooves wavelength plate (PBS), the vertical against the arm waveguides of the two And a first waveguide provided with a groove into which the λ/2 wavelength plate can be inserted, the PBS being arranged so that a part of the two grooves face each other in a direction inclined with respect to the vertical . A polarization rotator having a second waveguide that is not present, and each including four waveguides between two optical branching devices and two optical couplers, and two waveguides inside the four waveguides. And a mixer having two 90-degree hybrids configured to intersect with each other, wherein the PBS, the polarization rotator, and the mixer are cascaded in this order, and two folds are provided to optically connect between them. The waveguides of the PBS, the polarization rotator, and the mixer are arranged so as to be substantially S-shaped or inverted S-shaped via a waveguide portion, and are configured to be parallel to each other at least in part. When the parallel direction is the Y-axis and the direction perpendicular to the Y-axis is the X-axis, in the polarization rotator, the groove on the side of the second waveguide of the groove that crosses the first waveguide. The groove-waveguide spacing d on the X axis between the end portion and the second waveguide is in the range of 0.25<d (mm), which is an optical waveguide device.

The invention of claim 2 is the optical waveguide device according to claim 1 , wherein the length of the silicon substrate in the X-axis direction is X ₀ , and the minimum value of the length of the 90-degree hybrid in the X-axis direction is X _1min. A distance in the X-axis direction between both ends of the folded waveguide portion of the second waveguide connecting one of the 90-degree hybrid on the polarization rotator side and the polarization rotator. X _4min , the first waveguide connecting the end of the groove across the first waveguide of the PBS on the second waveguide side and the output side of the PBS. The distance in the X-axis direction between both ends of the folded waveguide portion is X _3min , and the far end of the groove that crosses the other arm waveguide from the arm waveguide on the polarization rotator side of the PBS. the X-axis direction of the distance to the part and X _2min, when the _{X total = X 1min × 2 +} X 4min + X 3min + X 2min, the groove - waveguide spacing d _{is, 0.25 <d <X 0 -X} total It is characterized by being in the range of (mm).

The invention of claim 3 is the optical waveguide device according to claim 1 or 2 , wherein, when the length of the silicon substrate in the X-axis direction is X ₀ , the groove-waveguide spacing d is 0.25. <d <X ₀ -10.5 (mm).

The invention of claim 4 is the optical waveguide device according to any one of claims 1 to 3 , wherein the PBS, the polarization rotator and the mixer constitute a part of an optical receiver for digital coherent communication. It is a feature.

As described above, the present invention can realize miniaturization of an optical interference circuit having a groove into which a thin film element such as a light wave plate is inserted. It gives an index of the distance between a groove into which a thin film element is inserted and an adjacent waveguide when an optical receiver is realized by using a PLC circuit on a substrate of a predetermined size under a specific circuit arrangement condition.

FIG. 1 is a diagram showing a configuration of an optical interference circuit in a conventional optical receiver configured by a PLC. FIG. 2 is a diagram showing the configuration of an embodiment of the optical waveguide device of the present invention. FIG. 3 is a diagram showing the relationship between the PER deterioration amount and the distance between the end of the groove and the adjacent waveguides. FIG. 4 is a diagram illustrating a crossing angle in a 90-degree hybrid circuit. FIG. 5 is a diagram showing the relationship between the crossing angle of the 90-degree hybrid circuit and the distance G between the waveguides. FIG. 6 is a diagram showing the relationship between the waveguide crossing angle Θ and the loss caused by the crossing in the 90-degree hybrid circuit. FIG. 7 is a diagram for explaining the definition of the minimum value related to the functional elements in the optical receiver for digital coherent optical transmission of the present invention.

The optical waveguide type device of the present invention uses the deep etching technique for forming the groove, which has become possible with the recent progress of the wafer process technique. Wet etching or dry etching technology that has been used in optical waveguide devices has a very low vertical etching rate (depth selection ratio) compared to the horizontal etching rate on the substrate surface. It was In recent years, an etching process having a high selection ratio in the direction perpendicular to the substrate surface, that is, in the depth direction has been developed and is widely applied to a silicon substrate as a Bosch process. In the optical waveguide device of the present invention, a groove for inserting a thin film element is formed by using a deep etching technique having a large selection ratio in the depth direction with respect to SiO ₂ which is a constituent material of the waveguide. With the deep etching technique, it is possible to form a groove having a required minimum opening size for properly inserting a thin film element and a sufficient vertical depth.

In the optical waveguide device of the present invention, at least one groove crosses only one corresponding waveguide into which the thin film element is inserted, and does not cross other waveguides adjacent to this corresponding one waveguide. Composed. This groove has a substantially rectangular shape, and can be the smallest size suitable for the size of the thin film element to be inserted so that the thin film element can be stably held and fixed in the groove.

Usually, an optical circuit made of PLC is composed of a plurality of functional elements that realize different functions. By using the groove for inserting the thin film element of the present invention, it becomes possible to form the necessary groove for each waveguide in the functional element that requires the groove. Furthermore, in addition to enabling miniaturization of a functional element that requires a groove, it is also possible to reduce the size of the entire optical circuit by shortening the distance to the functional element that does not require a groove. The large groove 3 (FIG. 1) formed over a plurality of waveguides as in the prior art has a wide influence due to the distribution of stress released by the groove. In the present invention, by using the groove having the minimum required opening size and configured for each waveguide, the area itself affected by the stress distribution released by the groove is reduced. Therefore, a region of the entire optical circuit which is not affected by the stress distribution released by the groove can be minimized. If the distance between the groove and the adjacent waveguide that does not have the groove for inserting the thin film element is defined between the different functional elements of the optical circuit, the polarization-maintaining property will be maintained for the light propagating in any of the waveguides. It is possible to obtain a desired polarization characteristic without maintaining and degrading the polarization extinction ratio.

The optical waveguide type device having the groove of the present invention can effectively configure an optical receiver used for digital coherent communication. In the optical waveguide device of the present invention, at least a polarization beam splitter (PBS), a polarization rotator (polarization rotator), and two 90-degree hybrids are cascaded in this order and are optically connected to each other. It is arranged in a substantially S-shape or an inverted S-shape via the two folded waveguide portions. The two 90 degree hybrids form a mixer circuit. The waveguides included in each constituent element are arranged so as to be substantially parallel to each other within the element and between the elements. Therefore, the propagation direction of the signal light in the PBS and the propagation direction of the signal light in the polarization rotator are parallel and opposite to each other. Further, the propagation directions of the signal light in the polarization rotator and the propagation directions of the signal light and the local oscillation light (local oscillation light) in each of the two 90-degree hybrids are parallel and opposite to each other.

In the PBS, each of the two waveguides arranged in parallel is provided with a groove that crosses only the respective waveguide. Further, in the polarization rotator, the groove for inserting the thin film element is formed only in one of the two waveguides, and the other waveguide has no groove. In the following examination, a constant substrate size is set for the distance from the end of the groove of one waveguide in the polarization rotator to the adjacent waveguide without a groove (“groove-adjacent waveguide distance d” described later). Under the above condition, the optimum range is obtained in consideration of both the influence of the stress distribution in the horizontal direction of the substrate released by the groove and the demand for miniaturization. First, the configuration of an optical receiver for digital coherent optical transmission, which is most suitable as the optical waveguide device of the present invention, will be described below. In the examples below, an optical waveguide type device using a single mode optical waveguide made of a silica-based material formed on a silicon substrate will be described as an example. This is because this configuration is widely used in PLCs at present and is easily integrated, and further, it is possible to provide an optical device having excellent compatibility with a silica-based optical fiber and low loss. In the following description, what is described as a waveguide means an optical waveguide formed on a PLC circuit.

FIG. 2 is a diagram showing the configuration of an embodiment of the optical waveguide device of the present invention. The optical waveguide type device of FIG. 2 is an optical receiver (optical interference circuit) 200 for digital coherent optical transmission configured by PLC. The optical receiver 200 includes a signal light input waveguide 30, a local oscillation (local oscillation) light input waveguide 32, interference light output waveguides 33 a and 33 b, and a signal light monitor waveguide 34. Further, the optical receiver 200 includes a VOA 31, a PBS 21, a polarization rotator 22, and two 90-degree hybrid circuits 29a and 29b in the order in which the signal light propagates. The PBS 21 and the polarization rotator 22 are realized by inserting a wave plate, which is a thin film element, into each groove so as to cross each waveguide.

The PBS 21 is arranged so that the birefringence axes of the two λ/4 wave plates are orthogonal to each other in the grooves 25 and 26 formed on the two arm waveguides 23 and 24 of the Mach-Zehnder interferometer, respectively. It is configured by inserting with. The PBS 21 operates to split the input signal light into two polarized waves. The polarization rotator 22 is constructed by inserting a λ/2 wavelength plate into a groove 27 formed in one of the two waveguides 28a and 28b. The polarization rotator 22 is disposed on the rear side of the PBS 21 and rotates the polarization of one light by 90 degrees. The distance between the grooved waveguide 28a and the grooveless waveguide 28b arranged adjacently in the polarization rotator 22 will be described later as the distance 35 from the end of the groove 27 to the grooveless waveguide 28b.

The grooves 25, 26, 27 into which the thin film elements are inserted were formed using a deep etching technique optimized for deep etching of SiO ₂ . The size of the groove was designed according to the size of the wave plate inserted according to each function. The grooves 25 and 26 of the PBS 21 each had a length of 1 mm, and a λ/4 wave plate having a length of 0.75 mm was inserted. The length of the groove 27 of the polarization rotator 22 was set to 1.5 mm, and a λ/2 wave plate having a length of 1.0 mm was inserted. Each of the wave plates has a length of about 0.75 mm at a minimum in consideration of workability during assembly.

In FIG. 2, the waveguide layout is close to the image of an actual device, but it should be noted that the width and length of the groove with respect to the waveguide are different from the actual configuration. When described in terms of actual dimensions, the groove and the wave plate are not visible, so the width of the groove and the thickness of the wave plate are relatively enlarged and exaggerated.

The thin film element used as the wave plate was composed of a polyimide film and had a thickness of about 10 μm. The width of the groove was set to about 15 to 30 μm depending on the thickness of the thin film element to be inserted. By filling the gap between the thin film element inserted in the groove and the thin film element in the groove with a resin having a refractive index close to that of silica glass, and fixing it by adhesion, excess loss of the waveguide that the groove traverses is minimized. Suppressed to. The depth of the groove is 100 μm or more and 300 μm or less, the etching cross section of the waveguide portion is perpendicular to the substrate surface, and the surface is smooth. The thin film element has a height that protrudes above the substrate surface by about 500 μm in consideration of the ease of handling during assembly.

The deep etching process for forming the groove of the optical waveguide type device of the present invention is a process optimized for performing deep etching on SiO _{2 in the} waveguide portion. Therefore, strict verticality with respect to the substrate surface is not necessarily required in the etching cross section other than the waveguide portion, particularly in the etching cross section of the groove reaching the silicon substrate portion beyond the waveguide portion of SiO ₂ . If verticality is ensured in the etching cross section of the waveguide portion of SiO ₂ that causes a phase change with respect to the signal light, the thin film element can be sufficiently held by the groove of SiO ₂ . Therefore, the etching cross section of the silicon substrate deep inside the groove may be slightly deviated from the vertical state.

Referring again to FIG. 2, in the optical waveguide device of the present invention, the three types of functional elements of the PBS 21, the polarization rotator 22, and the 90-degree hybrid circuit 29 are arranged in cascade in this order, and the functional elements are connected to each other. It is optically connected via the folded waveguides 28a and 28b for connection. That is, the functional elements are continuously and smoothly connected to each other by a waveguide, and when viewed as a whole, the optical waveguide device has an S-shape or an inverted S-shape. In the optical waveguide device of the present invention, as a basic principle, the radius of the folded curve waveguide is set to the minimum value (2 mm) at which radiation loss does not occur, and the functional elements are arranged as close to each other as possible. In the optical interference circuit of the optical receiver for digital coherent optical transmission shown in FIG. 2, at least some of the waveguides of each functional element are parallel. The three functional elements are arranged side by side in a direction (X axis direction) substantially perpendicular to the parallel waveguide direction (Y axis direction). In the following description, the X-axis direction in which the three types of functional elements are arranged, that is, the direction perpendicular to the waveguide direction (Y-axis) common to the three types of functional elements is referred to as the functional element arrangement direction.

In the following, in the optical receiver for digital coherent optical transmission in FIG. 2, how each functional element affects the size of the entire chip of the optical circuit will be examined. As described above, the layout shown in FIG. 2 is close to the image of an actual device, but the width of the groove and the thickness of the wave plate are relatively enlarged and exaggerated. In each functional element, further consideration will be given together with the limitation on the optical characteristics when miniaturizing the optical circuit and the problem of the influence of the stress distribution in the horizontal direction of the substrate released by the adjacent groove.

In the PBS 21 including the two grooves 25 and 26, as described above, the lengths of the groove 25 and the groove 26 mainly determine the size of the entire PBS 21. A part of the two grooves 25, 26 is arranged to face each other in a direction substantially perpendicular to the two arm waveguides 23, 24 corresponding to the respective grooves. The two arm waveguides 23 and 24 corresponding to the arm portion of the Mach-Zehnder interferometer can be arranged close to each other so that the distance between them is 500 μm or less. As a result, in the optical receiver for digital coherent optical transmission in FIG. 2, it is possible to minimize the size in the arrangement direction (X axis) of the functional element and reduce the overall size of the optical receiver circuit. It is possible to eliminate the layout limitation of the optical circuit for avoiding the passage of the groove and the generation of an undesired loss caused by allowing the passage of the groove, which have been problems in the related art. In the case of the PBS 21, since the grooves are formed symmetrically with respect to each of the two waveguides, the stress distribution in the horizontal direction of the substrate released by the adjacent grooves does not matter.

In the polarization rotator 22, the groove 27 into which the thin film element is inserted is formed only on the side of the waveguide 28a through which the signal light of one polarization separated by the PBS 21 propagates. The waveguide 28b through which the signal light of the other polarization propagates has no groove. Therefore, in the optical receiver for digital coherent optical transmission of FIG. 2, the size of the polarization rotator 22 in the arrangement direction (X axis) of the functional elements is not only the length of the groove 27 but also the end of the groove 27. It is determined by the distance 35 between adjacent waveguides 28b without grooves. For the sake of simplicity, the distance 35 from one end of the groove 27 that crosses the waveguide 28a in the polarization rotator 22 to the waveguide 28b that is closest to the waveguide 28a and has no groove is defined as the groove-adjacent waveguide distance d. To do.

The minimum value d _min of the groove-adjacent waveguide distance 35 is based on the actual measurement value of PER in order to avoid the problem that the polarization state of light propagating in a waveguide without a groove changes due to stress release by the groove. It is determined as follows.

FIG. 3 is a diagram showing the relationship between the distance between the end of the groove and the adjacent waveguide and the PER deterioration amount. As shown in FIG. 2, the distance d from the end of the groove 27 in a state in which the waveguide 28a is slightly inclined (98°) from the vertical to the adjacent waveguide 28b having no groove is changed. The amount of deterioration of the polarization extinction ratio (PER) in the waveguide 28b having no groove is shown. That is, it shows the PER deterioration amount (dB) based on the PER value when there is no groove. Two samples in which a plurality of pairs of a groove and a waveguide to be measured were formed on the same substrate were actually measured.

As shown in FIG. 3, the PER deterioration amount is 0 when the groove-adjacent waveguide distance 35 is approximately 250 μm, and when the groove and the waveguide are close to 100 μm, PER deterioration of several dB or more occurs. Therefore, if the minimum value d _min of the groove-adjacent waveguide distance 35 is set to 250 μm, deterioration of the extinction ratio in the polarization rotator 22 of FIG. 2 is suppressed, and the polarization maintaining property of the waveguide having no groove is impaired. Optical circuit without

On the other hand, the maximum value d _max of the groove-adjacent waveguide distance 35 in the polarization rotator 22 is within each element among the three functional elements arranged in the circuit arrangement direction of the optical receiver for digital coherent communication in FIG. It is determined from each minimum distance (size) or the minimum distance between functional elements.

The maximum value d _max of the groove-adjacent waveguide distance 35 is the arrangement direction of functional elements (X-axis direction) unless the chip size for actually manufacturing the optical receiver for digital coherent optical transmission of FIG. 2 is limited. But there is no limit. If the chip size is not limited, there is a high degree of freedom in circuit layout, and it is generally impossible to uniquely determine the maximum value. However, the groove for the thin film element of the optical waveguide type device of the present invention exerts the maximum effect when it is attempted to miniaturize the optical circuit, and in particular, the optical reception in the digital coherent communication as shown in FIG. It is preferably applied to vessels.

In an optical receiver for digital coherent optical transmission, the maximum size of the substrate is set to some fixed value because it is housed in a package of a form/size determined by specifications of a standardization organization (eg, OIF: The Optical Internetworking Forum). Can be determined. If the total value of the minimum values in each functional element is cumulatively obtained, the total value of the minimum values is subtracted from the above-mentioned fixed value of the substrate size to obtain the maximum value d of the groove-adjacent waveguide distance 35 in the polarization rotator 22. _max is required. Therefore, in the following, the minimum length and the minimum distance in each functional element, which are factors that limit the miniaturization of the optical receiver for digital coherent communication in FIG. 2, are further examined and determined, and the distance between the groove and the adjacent waveguide is determined. Let's find the maximum value d _{max of} 35.

In the optical receiver 200 for digital coherent optical transmission shown in FIG. 2, the factor that limits the maximum value d _max of the groove-adjacent waveguide distance 35 of the waveguide 28b adjacent to the groove 27 of the polarization rotator 22 is First, there is a configuration of an intersection of two waveguides inside each of the 90-degree hybrid circuits 29a and 29b at the top of the receiver 200. The configuration of the 90-degree hybrid circuit is determined in consideration of the loss generated at the intersection.

FIG. 4 is a diagram illustrating a crossing angle in a 90-degree hybrid circuit. The crossing angle of the 90-degree hybrid circuit is the angle at which the two inner waveguides intersect among the four waveguides arranged between the two optical branching means and the two optical coupling means. To tell. As shown in FIG. 4, in the circuit layout, the direction in which the signal light or local light propagates in each of the waveguides 41 and 42 is the Y axis, and the direction perpendicular to the four 90-degree hybrid waveguides is the X axis ( 2 corresponds to the X-axis direction). The minimum 90 degree hybrid circuit in the X-axis direction is that the two waveguides 41 and 42 each have a shape such that two arcs are continuously connected, and that the connection point of the arcs and two This is the case where the intersections 43 of the waveguides of are coincident with each other. In FIG. 4, a part where the two waveguides 41 and 42 are parallel to the Y axis is drawn around the intersection. When the intersection angle formed by the tangents of the two waveguides is Θ and the radius of the arc portion of the two waveguides is r, the inter-waveguide distance G of the two waveguides 41 and 42 that intersect each other is expressed by the following equation. It
G=2r×sin(Θ/4) Formula (1)
Expression (1) is an index for determining the length of one 90-degree hybrid circuit in the X-axis direction, that is, the length of the functional element arrangement (X-axis) direction in FIG.

FIG. 5 is a diagram showing the relationship between the crossing angle and the waveguide distance G in the 90-degree hybrid circuit. Here, the radius of the circular arc portion of the waveguide is set to r, and 2 mm which is the minimum value usually used in the current PLC circuit is used. As can be seen from FIG. 5, the larger the crossing angle Θ, the larger the inter-waveguide distance G in the 90-degree hybrid circuit. Considering miniaturization of the 90-degree hybrid circuit, naturally, the maximum crossing angle of 90 degrees is not used, and the crossing angle may be made as small as possible. In reality, the intersection angle has a minimum value determined from the optical characteristics of the 90-degree hybrid circuit, and the upper limit value and the lower limit value are determined for the distance G between adjacent waveguides.

The appropriate range of the crossing angle is defined by the phenomenon that the loss increases as the crossing angle of the two waveguides 41 and 42 in FIG. 4 decreases. Generally, when the crossing angle of the two waveguides 41 and 42 of the 90-degree hybrid circuit becomes shallow and approaches zero, the optical signal of one waveguide leaks to the other waveguide side where it crosses, and the insertion loss increases. Invite. In an actual circuit, the insertion loss is generated by exciting a higher-order mode (radiation mode) or a coupling mode (leakage to the intersecting waveguide). Since the excited states of the higher-order modes and the coupled modes largely depend on the shape of the intersection (intersection edge), it is difficult to quantitatively predict them by an analysis method such as a beam propagation method (BPM). Generally, it is obtained from the measurement result of the test circuit in the actual use state.

FIG. 6 is a diagram showing the relationship between the waveguide crossing angle Θ and the loss caused by the crossing in the 90-degree hybrid circuit. It can be seen from FIG. 6 that the loss increases as the intersection angle Θ becomes smaller (shallow), and the optical loss sharply increases in the region where the intersection angle Θ is smaller than 15° where the loss becomes 0.5 dB. The inter-waveguide distance G between the two waveguides 41 and 42 when the intersection angle Θ is 15° is approximately 250 μm from FIG. On the other hand, from FIG. 6, 50° was determined as the maximum value of the intersection angle Θ at which the loss due to the intersection is sufficiently small and the reduction amount of the loss is saturated. This is because there is no point in increasing the loss because the loss does not decrease even if the crossing angle is increased. In FIG. 6, the inter-waveguide distance G between the two waveguides 41 and 42 when the crossing angle is 50° is obtained from FIG. 5 to be 870 μm. Therefore, the distance G between the two inner waveguides in the 90-degree hybrid circuit is 250 μm to 870 μm.

FIG. 7 is a diagram showing the definition of the minimum value related to the functional elements in the optical receiver for digital coherent optical transmission of the present invention. Since one 90-degree hybrid includes four waveguides, a length of 750 μm, which is three times the minimum value 250 μm of the inter-waveguide distance G obtained above, is approximately 750 μm in the X-axis direction (function of FIG. 2). It becomes the minimum value X _1min of the element arrangement direction). Therefore, the minimum value in the X-axis direction of the hybrid circuit 29 in which the two 90-degree hybrids 29a and 29b are arranged in the X-axis direction is X _1min ×2=1.5 mm. The 90-degree hybrid peripheral part needs a certain margin between the substrate edge and the adjacent functional element, but the overlap between the minimum values X _4min and X _1min described below is set to this margin. If assigned, the value of X _1min =×2=1.5 mm in the 90-degree hybrid circuit 29 is almost appropriate. In the following, the minimum value associated with each functional element will be obtained according to the definition of FIG.

Referring again to FIG. 7, the second factor that partially overlaps the minimum value X _1min of the 90-degree hybrid circuit 29 and determines the maximum value d _max of the groove-adjacent waveguide distance 35 is the polarization rotator 22. , The distance X _4min between the front and rear portions of the folded portion 71 of the waveguide 28b that connects the lower 90-degree hybrid 29b. X _4min is twice the radius of curvature of the waveguide 28b. As described above when the inter-waveguide distance G at the crossing portion is obtained in FIG. 4, the minimum value 2 mm of the general curvature radius r in the PLC circuit is used in the present invention. Therefore, r is doubled so that X _4min =4 mm. The minimum value X _1min and X _4min of the 90-degree hybrid circuit 29 described above partially overlap. As mentioned above, this overlap can be allocated to margins for the periphery of the 90 degree hybrid, etc. When the radius of curvature r can be further reduced depending on the structure and material of the waveguide, X _4min =2r may be set.

The third factor that determines the maximum value d _max of the groove-adjacent waveguide distance 35 on the X-axis is that before and after the folded portion 72 of the waveguide 28 a that connects the polarization rotator 22 and the PBS 21. The interval X _{3 min} between the parts. Here, as shown in FIG. 7, X _3min is the distance from the upper end of the groove 27 to the waveguide 28a at the nearest portion of the PBS 21. X _3min is twice the minimum value of the radius of curvature r of the waveguide 28a, similar to X _4min , except for the protruding portion of the groove 27 to the upper side of the waveguide 28a. Also in this case, the minimum value 2 mm of the radius of curvature r generally used in the PLC circuit is used. Therefore, r is doubled so that X _3min =4 mm. Minimum X _2min and X _3min relates PBS21 described next is partially overlap, the overlapping amount is allocated to protruding portion of the waveguide 28a upper groove 27. If the radius of curvature r can be further reduced depending on the structure and material of the waveguide, X _3min =2r may be set.

The fourth factor that further determines the maximum value d _max of the groove-adjacent waveguide distance 35 on the X-axis is the size of the PBS 21 on the X-axis, and as shown in FIG. The distance between the upper waveguide 23 and the lower end of the groove 26 is X _{2 min} . The distance between the two waveguides of the PBS 21 can be brought close to 500 μm by arranging the two grooves 25 and 26 so as to overlap each other in the X-axis direction so that the two parts face each other. In addition, the grooves 25 and 26 each have a length of 1 mm at the shortest in consideration of easy handling of the wave plate inserted in the grooves. Since the groove 26 is located in the area between the two arm waveguides 23 and 24, X _2min is 1 to 1.5 mm, but considering the overlapping portion of X _2min and X _3min described above, X _2min was 1 mm. Since grooves are inserted in both waveguides in the PBS 21, the influence of stress released by the grooves symmetrically affects the mutual waveguides. Therefore, the distance between one groove and the other waveguide can be made closer than the minimum value 250 μm of the polarization rotator 22.

Minimum X _1min between the functional elements in or functional elements in the X-axis direction described above, X _4min, X _3min, if the X _2min obtained, the total X _total of these minimum values from the length X ₀ of the substrate The maximum value d _max of the groove-adjacent waveguide distance 35 in the polarization rotator 22 can be determined by subtracting it.
_{X total = X 1min × 2 +} X 4min + X 3min + X 2min formula (2)
d _max =X ₀ −X _total formula (3)

In equation _{(3), X 1min = 1.5mm} , X 4min = 4mm, X 3min = 4mm, by substituting each value of X _2min = 1 mm, the grooves - the maximum value d _max of the adjacent waveguide distance 35, the following It is calculated by the formula.
d _max =X ₀ -10.5 (mm) Formula (4)

In FIG. 2, the length X ₀ in the X-axis direction of the substrate for manufacturing the optical receiver for digital coherent light transmission is 27 mm for the Type 1 package of the OIF specification (Non-Patent Document 2), and the length of the Type 2 is It is specified as 16 mm in the package. For example, when the receiver 200 of FIG. 2 is accommodated in the Type 2 package so as to fit the size of the Type 2 package, if the substrate size X ₀ is 15 mm, the groove-adjacent waveguide distance 35 in the polarization rotator 22 is 35. The maximum value d _max that can be taken is 15-10.5=4.5 mm. The maximum value d _max, using the groove configuration of the present invention, in the functional elements described above, the minimum value X _1min between functional elements, X _4min, X _3min, a value when the X _2min occur simultaneously Yes, the optical receiver 200 for digital coherent optical transmission according to the arrangement of the functional elements of FIG. After all, when the minimum value of the groove-adjacent waveguide distance 35 obtained from FIG. 3 is combined with the equation (4), the size of the substrate is defined as X ₀ (mm) in the arrangement direction (X axis) of the functional element. At this time, the groove-adjacent waveguide distance 35, that is, the range of d, in the polarization rotator 22 is as follows.
0.25 <d <X ₀ -10.5 (mm) Formula (5)

Therefore, according to the present invention, in an optical waveguide type device formed on a substrate, the two arm waveguides 23 and 24 and the two arm waveguides are configured to traverse only the respective λ/4 wavelength plates. A polarization beam splitter (PBS) 21 having two grooves 25 and 26 into which the two grooves can be inserted, and the two beams are arranged so that a part of the two grooves face each other in a direction perpendicular to the two arm waveguides. Further, a polarization rotator 22 having a PBS, a first waveguide 28a having a groove 27 into which a λ/2 wavelength plate can be inserted, and a second waveguide 28b having no groove, and a polarization rotator 22 each having two lights A mixer having two 90-degree hybrids 29a, 29b, which is configured to include four waveguides between a branching device and two optical couplers, and two inner waveguides of the four waveguides are crossed with each other. The PBS, the polarization rotator, and the mixer are cascaded in this order, and are generally S-shaped or inverted S-shaped via two folded waveguide portions 71 and 72 that optically connect between them. And the waveguides of the PBS, the polarization rotator, and the mixer are configured to be parallel to each other at least in part, and the parallel direction is defined as the Y-axis. When the direction perpendicular to the Y axis is taken as the X axis, in the polarization rotator, the second waveguide side end of the groove crossing the first waveguide and the second waveguide are formed. The distance d between the groove on the X-axis and the waveguide is in the range of 0.25<d (mm), which can be implemented as an optical waveguide device.

In the above-mentioned examination, the groove 27 in the polarization rotator 22 has been described as being on the waveguide 28a on the PBS side, but a groove is formed on the waveguide 28b on the 90-degree hybrid side and the waveguide 28a is formed. You may comprise as a waveguide without a groove|channel. In this case, if X _3min and X _4min are interchanged, the relations of the equations (2) to (4) regarding the minimum value of the functional element defined in FIG. 7 are similarly established.

In the optical waveguide type device of the present invention, as the size of the optical circuit becomes smaller, the light reflection generated at the interface between the waveguide and the groove into which the thin film is inserted may affect the performance as an optical receiver. There is. In order to sufficiently suppress the light reflection attenuation amount, as shown in FIG. 2, the grooves were formed such that the angle between the waveguides of the grooves 25, 26 and 27 and the boundary surface between the grooves was 98 degrees. In the PBS 21, by arranging the grooves at a predetermined angle rather than a right angle, the adjacent grooves 25 and 26 are arranged so that some of them face each other on the axis perpendicular to the waveguide, and each groove crosses each other. It can be laid out so that it crosses only the waveguide to be used. One groove is configured to cross only one waveguide. In other words, the groove according to the present invention traverses only the corresponding waveguide, and does not traverse other closely arranged waveguides.

The optical waveguide type device of the present invention enables reduction of the size of the optical circuit by simply changing the shape of the groove while using the deep etching technique while using the conventional optical waveguide design technique. The optical characteristics of the optical interference circuit, that is, the insertion loss, the polarization extinction ratio in PBS, the phase error in the 90-degree hybrid, the common-mode signal rejection ratio, etc., can achieve the same performance as that of the optical circuit according to the prior art. Is.

As described above, in the optical waveguide type device of the present invention, by providing the above-mentioned characteristic groove configuration, it is possible to eliminate a large constraint on the flexibility and the optical performance of the circuit design regarding the groove configuration, The size of the optical circuit is greatly reduced. Further, in the polarization rotator, the distance between the groove and the adjacent waveguide having no groove is set to 250 μm or more, which is the minimum value d _min , so that the deterioration of the PER is suppressed and the polarization is maintained in the waveguide having no groove. There is no loss of sex. Further, the optical waveguide device of the present invention is within the range of the maximum value d _max of the distance between the groove and the adjacent waveguide having no groove in the polarization rotator under the condition of the predetermined substrate length X ₀ . Thus, an optical receiver for digital coherent optical transmission having a groove unique to the present invention can be constructed. An index of the distance between a groove into which a thin film element is inserted and an adjacent waveguide is given when an optical receiver is realized by using a PLC circuit on a substrate of a predetermined size under a specific circuit arrangement condition. To be

In the examination of the maximum value d _max of the groove-waveguide spacing d, the maximum value is assumed to be 2 mm, assuming that the minimum curvature radius r of the arc portion of the optical waveguide implemented in the PLC circuit at the present time is 2 mm. Decided. However, depending on the structure and material of the optical waveguide, a smaller radius of curvature may be realized in the future. In that case, the minimum values X _1min , X _4min , and X _3min within each functional element or between the functional elements in the X-axis direction can be changed based on the value of the smaller radius of curvature r. Therefore, in such a case, the equations (2) to (4) can be modified by the changed minimum values X _1min , X _4min , and X _3min , and the maximum value d _max of the groove-waveguide spacing d is It can be set larger.

In the above-described embodiments, the silica-based glass waveguide type device formed on the silicon substrate is described as an example, but the present invention is not limited to other materials forming the waveguide, such as polymer, semiconductor, silicon, and ion diffusion. It is also applicable to an optical waveguide type device using a lithium niobate type or the like. For each waveguide material other than SiO ₂ , it is possible to use the deep etching technique having a high selection ratio in the depth direction perpendicular to the horizontal direction of the substrate. By using the deep etching technology, the corresponding groove is configured in a one-to-one correspondence with the waveguide that requires the groove, thereby eliminating the restrictions on the circuit design flexibility and the optical performance regarding the groove structure. In addition, the optical circuit can be downsized. The optical waveguide device of the present invention is very effective for an optical transmitter/receiver used for digital coherent optical transmission including an optical interference circuit.

As described above in detail, the optical waveguide device of the present invention can realize miniaturization of an optical interference circuit having a groove into which a thin film element such as an optical wave plate is inserted.

The present invention can be generally used in communication systems. In particular, it can be used for an optical waveguide type device of an optical communication system.

1 substrate 2 waveguide layer 3, 25, 26, 27 groove 11, 14, 30, 32 input waveguide 12, 21 PBS
13,22 Polarization rotator 15,31 VOA
16a, 16b, 29, 29a, 29b 90 degree hybrid circuit 17, 34 Signal light monitor waveguide 18a, 18b, 33a, 33b Output waveguide 23, 24 Arm waveguide 28a, 28b Waveguide 35 Groove-waveguide distance 71 , 72 Folded waveguide 100, 200 Optical receiver

Claims (4)

In an optical waveguide type device made of a silica-based material formed on a silicon substrate,
A polarization beam splitter (PBS) having two arm waveguides and two grooves configured to traverse only the two arm waveguides, respectively, into which a λ/4 wave plate can be inserted. A PBS which is tilted with respect to the two arm waveguides rather than perpendicular and is arranged such that a part of the two grooves face each other in a direction tilted with respect to the perpendicular.
a polarization rotator having a first waveguide having a groove into which a λ/2 wave plate can be inserted, and a second waveguide having no groove;
Two 90 degree hybrids each comprising four waveguides between two optical splitters and two optical couplers, the two inside of said four waveguides being configured to intersect. And a mixer having
The PBS, the polarization rotator, and the mixer are cascaded in this order, and are arranged in an approximately S-shape or an inverted S-shape via two folded waveguide portions that optically connect between them. The waveguides of each of the PBS, the polarization rotator, and the mixer are configured to be parallel to each other at least in part.
When the parallel direction is the Y-axis and the direction perpendicular to the Y-axis is the X-axis, in the polarization rotator, an end portion of the groove crossing the first waveguide on the second waveguide side is formed. The distance d between the groove and the waveguide on the X axis with respect to the second waveguide is
0.25 <d (mm)
An optical waveguide type device characterized in that The length of the silicon substrate in the X-axis direction is X ₀ ,
The minimum value of the length of the 90-degree hybrid in the X-axis direction is X _1min ,
A distance in the X-axis direction between both ends of the folded portion of the folded waveguide portion of the second waveguide that connects between one side of the 90-degree hybrid on the polarization rotator side and the polarization rotator. X _4min ,
The folded waveguide part of the first waveguide that connects between the end of the groove that crosses the first waveguide of the PBS on the second waveguide side and the output side of the PBS. The distance in the X-axis direction between both ends of the turn back is X _3min ,
The distance in the X-axis direction from the arm waveguide on the polarization rotator side of the PBS to the far end of the groove that crosses over the other arm waveguide is X _2min ,
_{X total = X 1min × 2 +} X 4min + X 3min + X 2min
When the said groove - between the waveguides spacing d is 0.25 <optical waveguide device according to claim 1, characterized in that the range of _{d <X 0 -X total (mm} ). When the length of the silicon substrate in the X-axis direction is X ₀ , the groove-waveguide spacing d is in the range of 0.25<d<X ₀ -10.5 (mm). optical waveguide device according to claim 1 or 2. The PBS, the polarization rotator and the mixer, an optical waveguide device according to any claims 1 to 3, characterized in that it constitutes a part of the digital coherent communication optical receivers.