JP5588794B2 - Substrate type optical waveguide device having grating structure, chromatic dispersion compensation element, and method of manufacturing substrate type optical waveguide device - Google Patents

Description

  The present invention relates to a substrate type optical waveguide device having a grating structure.

  In recent years, with the development of optical fiber communication systems, particularly the invention of erbium-doped optical fiber amplifiers (EDFA) and dense wavelength division multiplexing communication systems (DWDM), the amount of information transmitted in optical fiber communication networks has increased rapidly. In preparation for further increase in demand, research and development are underway for increasing the number of wavelengths to be multiplexed and modulation systems with high frequency utilization efficiency. These systems require optical components with more advanced functions such as an optical dispersion compensator that compensates for chromatic dispersion and dispersion slope of each channel more precisely than conventionally used dispersion compensating optical fiber modules. The In addition, research and development has been conducted on variable optical dispersion compensators that can cope with temporal changes and path changes in the dispersion characteristics of optical transmission lines, and polarization mode dispersion compensators that dynamically compensate for polarization mode dispersion. Yes.

  On the other hand, with the rapid expansion of the size and number of installed information communication systems, the enormous amount of power consumed by computer systems and high-end routers is becoming a problem not only from an economic perspective but also from the perspective of environmental impact. There is a need for Green ICT (Information and Communication Technology) that saves power and reduces environmental impact. If various transmission devices such as routers can be miniaturized, the efficiency of housing the equipment in the data center or communication carrier station will be improved and the space utilization efficiency will be improved, and the air conditioning power of the data center or station will be improved. It can be greatly reduced, contributing to energy saving. Therefore, power saving and downsizing are also required for optical components used in various optical transmission apparatuses.

As a technique for manufacturing a small and high-performance optical component, silicon photonics technology for manufacturing an optical waveguide device using a CMOS manufacturing process has been spotlighted and research and development are being promoted. An optical waveguide is formed using a high refractive index material such as silicon (Si) or silicon nitride (Si ₃ N ₄ ), so that various conventional silica (SiO ₂ ) -based glasses are used as the main constituent materials of the core and the clad. The waveguide device can be miniaturized. Further, by using Si as a semiconductor material by doping an impurity element, it is possible to adjust the refractive index by applying a voltage from the outside, and a variable optical property device can be realized. Since the manufacturing process is suitable for large-scale mass production, the price of optical components can be expected to be reduced in the future.

Conventionally, a planar optical waveguide device having a Bragg grating pattern, as shown in FIG. 25, and the pitch P _G is a grating structure constant equal pitch type of the projections 201 and recesses 202 provided in the side wall of the optical waveguide 200, As shown in FIG. 26, the pitches of the convex portions 301 and the concave portions 302 provided on the side wall of the optical waveguide 300 are P _G ⁱ > P _G ^j > P _G ^k > P _G ^l > P _G ^m > P _G ^n. A gradually changing chirped pitch type grating structure is known.

  Patent Document 1 is an example of an optical fiber Bragg grating rather than a substrate-type optical waveguide device. In the optical waveguide, a Bragg grating having a certain period is formed in the optical waveguide so that it overlaps with the Bragg grating. Discloses a chromatic dispersion compensation element that has a sampling structure and performs chromatic dispersion compensation in a plurality of wavelength channels. The sampling structure includes a pattern that is phase-sampled at a certain period longer than the period of the Bragg grating. Each period of phase sampling is divided into a plurality of spatial regions in a direction along the optical axis of the optical waveguide, and the phase of the Bragg grating changes discontinuously at the boundary where adjacent spatial regions are in contact with each other. FIG. As shown in 1A to 1D, there is no discontinuous change in phase within one spatial region.

  Non-Patent Document 1 is an academic paper by the inventors of Patent Document 1, and technical information that complements Patent Document 1 is disclosed. First, a Bragg grating pattern of a single channel at the center wavelength is designed using the knowledge of Patent Document 1. The grating pattern is derived from the desired reflection and chromatic dispersion spectral characteristics by the inverse scattering method. However, since the optical fiber Bragg grating has a limit in the range in which the refractive index can be changed in order to produce the grating pattern, the above spectral characteristics are inverse Fourier transformed so as not to exceed the limit, and apodized. Add the operation to do. As described above, a pattern in which the pitch of the Bragg grating continuously changes with the position is obtained. Thereafter, a Bragg grating pattern of a plurality of channels is designed by phase sampling. In optical fiber Bragg gratings, phase sampling is effective because the range of change in refractive index is limited.

  In Patent Document 2, a part of the inventors of the present invention can realize a device having complicated optical characteristics such as an optical dispersion compensator by designing and manufacturing a substrate type optical waveguide device by solving the inverse scattering problem. Is disclosed.

As examples of externally controlling the optical characteristics of the material with respect to the optical waveguide, those using temperature and those using the carrier plasma effect are known.
Patent Document 3 discloses a device using a change in optical characteristics due to a carrier plasma effect by applying a voltage to a silicon waveguide.
Patent Document 4 discloses a device that utilizes temperature change in a waveguide made of quartz glass.
On the other hand, paying attention to the structure, as an example in which a variable mechanism for optical characteristics is given to the grating structure, a split heater is used for a chirped pitch type grating element formed by fiber Bragg grating in Patent Document 5. An example is disclosed.

  Conventionally, various optical passive components such as a photonic crystal optical waveguide, a silicon fine wire optical waveguide, and an AWG have been studied as an optical component for an optical fiber communication system using silicon photonics technology, in addition to a modulator and a light receiving and emitting element. While some transceiver modules have been commercialized, research on silicon photonics technology is still in its infancy, and most of the research has been carried out using a photolithographic process using an electron beam (EB) direct lithography system. As for the photolithography process using a photomask, knowledge has not yet been accumulated. In the manufacture of a silica glass-based optical waveguide with an initial relative refractive index difference (commonly known as Δ) of about 0.3%, the core width of the optical waveguide is sufficiently thick at about 7 μm, and a photomask with an equal magnification projection is used. It was done. In a high relative refractive index difference optical waveguide used in silicon photonics technology, the effective refractive index perceived by signal light is high, so the core size of a single mode optical waveguide is one-tenth to several tenths. In addition, the interval between the periodic structures of the photonic crystal optical waveguide and the grating optical waveguide is very small. Therefore, a finer process technology is required.

  On the other hand, unlike LSIs in which electronic circuit elements such as DRAMs and CPUs are integrated, optical waveguide devices require a sufficient thickness or depth to display the peripheral structure such as the thickness of the optical waveguide core or cladding. Therefore, it is not always possible to apply the most advanced fine process, and there are many cases where it is necessary to use an older generation technology node, such as the need for thick film resist coating. In addition, in optical components for optical fiber communication systems, which have orders of magnitude less demand than DRAMs and CPUs, the use of a new generation of processes that are mass-produced using 12-inch wafers does not always lead directly to cost reduction. In many cases, the production of an appropriate amount using a 6-inch wafer or an 8-inch wafer by an old generation process results in cost reduction. For example, Non-Patent Document 2 and the like disclose a silicon photonics optical waveguide device for an optical fiber communication system manufactured using a 130 nm technology node. The 130 nm technology node is a process in which, for example, a stepper exposure apparatus (scanner) having a wavelength of 248 nm is used and a resolution is improved by using a phase shift mask.

US Pat. No. 6,707,967 JP 2004-077665 A JP-T 09-503869 Japanese Patent No. 2659293 JP 2000-235170 A

H. Li, Y. Sheng, Y. Li, and JE Rothenberg, "Phased-Only Sampled Fiber Bragg Gratings for High-Channel-Count Chromatic Dispersion Compensation," Journal of Lightwave Technology, Vol. 21, No. 9, pp. 2074-2083 (2003) T. Pinguet, V. Sadagopan, A. Mekis, B. Analui, D. Kucharski, S. Gloeckner, "A 1550 nm, 10 Gbps optical modulator with integrated driver in 130 nm CMOS," 2007 4th IEEE International Conference on Group IV Photonics, 19-21 Sept. 2007

  Conventionally known equal-pitch grating structures and chirped pitch grating structures provide advanced functionality such as optical dispersion compensation characteristics that simultaneously compensate for chromatic dispersion and dispersion slope of multiple channels as optical characteristics of substrate-type optical waveguide devices. I can't do that. In addition, when manufacturing the device using silicon photonics technology, a structure with gradually changing dimensions, such as a chirped pitch type, is not easy to manage the processing accuracy of each dimension, and a structure with easier process management. Is required.

The sampled FBG based on the phase sampling pattern described in Patent Document 1 or Non-Patent Document 1 can realize a multi-channel type optical dispersion compensator even with an optical waveguide having a relatively small effective refractive index amplitude such as FBG. This is not suitable for the purpose of shortening the element length and reducing the size.
In Patent Document 2, the dimensional change of the core pattern is too small in the substrate type optical waveguide device as disclosed in the patent, and there is a special process such as a LIGA process using X-ray lithography instead of a normal process. It is suggested that it may be necessary.

  Therefore, an object of the present invention is to provide a substrate-type optical waveguide device that can achieve a high degree of functionality, and can be shortened and miniaturized.

The invention according to claim 1 is a Bragg grating in which convex portions having a wide core width in the horizontal direction and concave portions having a narrow core width are alternately arranged only in a part of the height of the side wall of the core of the optical waveguide. A pattern structure is formed by an etching process, and a maximum core width in the Bragg grating pattern structure is larger than a core width at a height at which the Bragg grating pattern structure is not formed on the side wall. This is a substrate type optical waveguide device.
According to a second aspect of the present invention, there is provided a first structure in which a convex portion having a large core width in the horizontal direction and a concave portion having a narrow core width are alternately provided only in a part of the height of the side wall of the core of the optical waveguide. The Bragg grating pattern structure is formed by an etching process, the second Bragg grating pattern structure is located on the top surface and / or the bottom surface of the core, and the two Bragg grating pattern structures are arranged along the light guiding direction. are formed in parallel regions, a board-type optical waveguide device you wherein a.
The invention according to claim 3 is characterized in that the second Bragg grating pattern structure comprises a protrusion-like structure and / or a groove-like structure formed at the center in the width direction and at the upper part in the vertical direction of the core. A substrate-type optical waveguide device according to claim 2.
In the invention according to claim 4, the core of the optical waveguide is composed of an inner core having a rib structure and an outer core covering the three directions of the convex portion of the rib structure on the upper side of the inner core. The substrate mold according to any one of claims 1 to 3, wherein the substrate mold is made of a material having a refractive index lower than an average refractive index of the inner core, and the Bragg grating pattern structure is provided on the outer core. It is an optical waveguide device.
According to a fifth aspect of the present invention, the inner core is separated by a gap portion made of a material having a refractive index lower than that of the inner core along the light guiding direction at the center in the width direction, and the gap portion. 5. The substrate-type optical waveguide device according to claim 4, wherein the substrate-type optical waveguide device comprises two regions and a single mode optical waveguide in which a single mode propagates across the two regions.
In the invention according to claim 6, in the Bragg grating pattern structure , the length of the fin in the core width direction is not uniform along the longitudinal direction of the waveguide, and the pitch along the longitudinal direction of the waveguide is a plurality of discrete When the value is taken and P is the most frequently occurring value among all the discrete values and ΔP is the discrete step of the coordinate z along the longitudinal direction of the waveguide, each pitch is P ± NΔP (where, 6. The substrate type optical waveguide device according to claim 1, wherein N is an integer).
According to a seventh aspect of the invention, the core is made of a semiconductor material and is divided into a P type and an N type in a horizontal direction on a plane perpendicular to the light traveling direction, and each region is connected to an electrode. The substrate-type optical waveguide device according to claim 1, wherein the substrate-type optical waveguide device is a substrate-type optical waveguide device.
In the optical transmission line according to the eighth aspect of the present invention, the distance that the signal light propagates through the optical waveguide from when it enters the optical waveguide to when it is reflected differs depending on the wavelength for a plurality of wavelength channels. A chromatic dispersion compensating element for compensating chromatic dispersion and a dispersion slope, wherein the chromatic dispersion compensating element comprises the substrate type optical waveguide element according to claim 1. It is.
The invention according to claim 9 is the method for manufacturing a substrate type optical waveguide device according to any one of claims 1 to 7, wherein the Bragg grating pattern structure provided only in a part of the height of the side wall is etched. It is a manufacturing method of a substrate type optical waveguide device characterized by forming by a process.

Since the grating optical waveguide was designed by solving the inverse scattering problem using the Zakharov-Shabat equation, an optical dispersion compensator that simultaneously compensates the group delay dispersion and dispersion slope of the optical fiber transmission line for a large number of DWDM channels at once. A substrate-type optical waveguide device having such complicated optical characteristics can be made compact with a short waveguide length.
Since this can be manufactured by silicon photonics technology using a CMOS manufacturing process, large-scale mass production is possible, and future price reduction can be expected. In addition, the adoption of a high relative refractive index difference optical waveguide structure enabled a small device.
As a result of designing a grating optical waveguide by solving the inverse scattering problem using the Zakharov-Shabat equation, the grating optical waveguide is such that the grating fin length is non-uniform and the pitch becomes a plurality of discrete values with a pitch. However, since this pitch takes a plurality of discrete values, the process management becomes easy unlike the chirp type.

According to the first aspect of the present invention, by making the height of the Bragg grating a part of the height of the waveguide core, the etching depth required in the etching process at the time of manufacture is reduced, and process management becomes easy.
Further, by making the height of the Bragg grating a part of the height of the waveguide core, it is possible to reduce the dimensional difference between the upper and lower portions of the waveguide, which is caused by the deviation of the waveguide sidewall from the vertical.
Further, by setting the height of the Bragg grating as a part of the height of the waveguide core, it is possible to create a device in which the loss due to scattering that is not intended in design due to side surface roughness is reduced.

According to the invention of claim 2, in the substrate type optical waveguide device having the Bragg grating pattern, the polarization dependence of the optical characteristics can be reduced.
According to the invention of claim 3, the grating pattern on the upper part of the core can be realized more easily.
According to the invention of claim 4, the light to the inner core made of the high refractive index material is compared with the conventional high refractive index difference buried type optical waveguide made up of only the core made of the high refractive index material and the clad. Therefore, the influence (scattering loss) exerted on the optical characteristics by the inevitable roughness of the inner core side wall that occurs in the manufacturing process can be suppressed.
According to the invention of claim 5, since the mode field system of the fundamental mode is expanded, the influence (scattering loss) exerted on the optical characteristics by the inevitable roughness of the inner core side wall that occurs in the manufacturing process can be suppressed.
According to the invention of claim 6, by taking a plurality of discrete values having a local period (pitch) of the grating, process management becomes easy unlike the chirp type.
According to the invention of claim 7, by using a semiconductor material for a part of the optical waveguide core and making the refractive index adjustable by applying a voltage, it is possible to realize an optical device whose optical characteristics can be dynamically changed. it can.
According to the eighth aspect of the present invention, it is possible to realize a small chromatic dispersion compensating element that collectively compensates the chromatic dispersion and dispersion slope of a plurality of channels, and to reduce the polarization dependency thereof.

(A) is the partial top view of the core which shows 1st Embodiment of a substrate type optical waveguide device, (b) is sectional drawing which follows the SB-SB line in (a), (c) is SC in (a). It is sectional drawing which follows the -SC line. It is a perspective view of the part shown in FIG. It is a perspective view which shows the example of a modification which abbreviate | omitted the groove-shaped structure from the board | substrate type optical waveguide device shown in FIG. It is a perspective view which shows the modification which provided the electrode in the board | substrate type optical waveguide device shown in FIG. It is a partial top view of a core for explaining distribution of a grating pitch. It is explanatory drawing which shows an example of the form which connected the board | substrate type | mold optical waveguide device and the optical transmission line. It is sectional drawing which shows 2nd Embodiment of a substrate type optical waveguide device. It is a graph which shows an example of the change of _neff with respect to _{win in} 2nd Embodiment. It is a graph which shows an example of change of _wout accompanying change of win _in a 2nd embodiment. Is a graph showing changes in _{w in} and _{w out} for _{n eff} in the second embodiment. It is a graph which shows an example of the reflectance spectrum set to the complex reflection spectrum r ((lambda)) which is a function of a wavelength. It is a graph which shows a part of FIG. It is a graph which shows an example of the group delay spectrum set to the complex reflection spectrum r ((lambda)) which is a function of a wavelength. It is a graph which shows a part of FIG. It is a graph which shows an example of potential distribution q (z) obtained by solving an inverse scattering problem. It is a graph which shows a part of FIG. It is a graph which shows an example of effective refractive index distribution n _eff (z) obtained by converting potential distribution q (z). It is a graph which shows a part of FIG. It is a graph which shows an example of effective refractive index distribution n _eff (z) obtained by converting potential distribution q (z). It is a graph which shows a part of FIG. It is a pattern figure which shows a part of example of a side wall grating structure. It is a pattern figure which shows a part of example of an upper grooved grating structure. It is sectional drawing which shows the reference example of a board | substrate type optical waveguide device. Is a graph showing changes in _{w in} and _{w out} for _{n eff} in Reference Example. It is a top view which shows an example of the conventional single pitch type | mold grating structure. It is a top view which shows an example of the conventional chirp type | mold grating structure. (A) The partial top view of the core which shows 3rd Embodiment of a substrate type optical waveguide device, (b) is sectional drawing which follows the SB-SB line in (a), (c) is SC in (a). It is sectional drawing which follows the -SC line. It is a perspective view of the part shown in FIG. Is a graph showing a change in _{w grating} for _{n eff} in the third embodiment. It is a graph which shows an example of effective refractive index distribution n _eff (z) obtained by converting potential distribution q (z). It is a graph which shows a part of FIG. It is a graph which shows an example of distribution of a grating pitch. It is a graph which shows a part of FIG. It is a graph which shows the w _grating dimension of a part of an example of a side wall grating structure.

The present invention will be described below based on preferred embodiments with reference to the drawings.
In order to solve the above problem, the present inventors calculate a potential distribution that realizes a desired optical characteristic by solving an inverse scattering problem using the Zakharov-Shabat equation, and a grating structure that realizes this potential distribution. A new substrate-type optical waveguide device was designed, and a new process was developed to enable it to be manufactured using silicon photonics technology.
In addition, a device was designed to which a process capable of reducing the side wall roughness by reducing the grating height was applied to the problem of inevitable side wall roughness during the processing of the grating pattern.
The present invention relates to a silicon photonics technology for manufacturing a passive substrate type optical waveguide component used in an optical fiber communication system by using a CMOS device manufacturing technology, and to provide a substrate type optical waveguide device element having a grating structure. it can.
To realize an optical waveguide device with advanced functions such as an optical dispersion compensator, design a grating optical waveguide using the inverse scattering method, and realize it as a substrate-type optical waveguide using silicon photonics technology. Is preferred.

1 (a), (b), (c) and FIG. 2 schematically show a first embodiment of a substrate type optical waveguide device having a grating structure of the present invention. In FIG. 1 and FIG. 2, only the core 10 is illustrated and the cladding is not illustrated, but it is assumed that the cladding surrounds the core 10. A substrate (not shown) exists under the cladding, and the bottom surface 14 of the core 10 is parallel to the substrate surface. The horizontal direction means a direction parallel to the substrate surface, and the vertical direction means a direction perpendicular to the substrate surface.
FIG. 1A is a top view of a part of the core 10. FIG.1 (b) is sectional drawing which follows the SB-SB line | wire in Fig.1 (a). FIG.1 (c) is sectional drawing which follows the SC-SC line in Fig.1 (a). FIG. 2 is a perspective view.

In FIG. 1A, symbol C represents a single central axis in the horizontal plane of the optical waveguide core 10, and light propagates along the central axis C in the optical waveguide. The optical waveguide has a Bragg grating pattern (details will be described later), and at least one reflection band appears in the spectrum of the optical waveguide. The center wavelength λ ₀ of the reflection band is given by λ ₀ = P _G / n _eff where the period of the Bragg grating is P _G and the effective refractive index of the optical waveguide is n _eff . Here, the effective refractive index n _eff is a value when the width of a part of the core 10 of the optical waveguide is an average width w ₀ . Here, a part is a lower part of the core 10 having the side wall 12 shown in FIG.

The average width of the core 10 lower w ₀ is equal to the average value in one period of the horizontal width w _out of the lower core 10 is a constant value along the central axis C over the entire optical waveguide. Recesses 12a and the projections 12b on the core 10 lower side wall 12 of alternately formed, the width w _out to form a first Bragg grating pattern oscillates alternately every one period P _G.
On the other hand, the width w _middle of the upper stage of the core 10 including the side wall 15 is constant, and there is no change in the horizontal direction. As shown in a later manufacturing process, by reducing the height of the Bragg grating pattern, a higher quality device can be manufactured.

An optical waveguide having a rectangular cross section has its own waveguide mode when the linearly polarized electric field of light is mainly along the horizontal direction (hereinafter referred to as TE-type polarization) and mainly along the vertical direction (hereinafter referred to as TM-type polarization). Exists. And there exists a polarization dependence that there exists an effective refractive index peculiar to each waveguide mode.
The effective refractive index n _effTE of the eigen mode in the TE-type polarized light changes more sensitively to the change in the width of the optical waveguide than the effective refractive index n _effTM of the eigen mode in the TM-type polarized light. On the other hand, the effective refractive index n _effTM of the eigenmode with TM-type polarization changes more sensitively to changes in the height (ie, thickness) of the optical waveguide than the effective refractive index n _effTE of the eigenmode with TE-type polarization. To do.

  Therefore, as shown in FIG. 3, the upper surface 3 of the optical waveguide core 1 is not provided with a Bragg grating pattern, but the side walls 2 are provided with the concave portions 2a and the convex portions 2b so that only the width of the lower stage of the core 1 is periodically changed. In this case, the polarization dependency becomes large. Therefore, in order to reduce the polarization dependence of the Bragg grating, it is necessary not only to periodically change the width of the optical waveguide, but also to periodically change the height of the optical waveguide.

For this reason, in this substrate type optical waveguide device, two kinds of Bragg grating patterns are located in different regions in a cross section orthogonal to the light guiding direction.
Also, two types of Bragg grating patterns are formed in regions that are arranged in parallel along the light guiding direction. That is, the range in which each Bragg grating pattern exists along the central axis C is the same.

Accordingly, the combination of the first Bragg grating pattern and the second Bragg grating pattern can equalize the action on the TE-type polarization and the action on the TM-type polarization, thereby reducing the polarization dependence. .
In consideration of application to a rectangular waveguide (an optical waveguide having a substantially rectangular cross section), it is desirable to provide the second Bragg grating pattern on the top surface and / or bottom surface of the core. In this embodiment, in order to facilitate the formation of the core on the substrate, the first Bragg grating pattern is provided on both lower side walls of the core, and the second Bragg grating pattern is provided on the upper surface of the core. The shape of the core 10 is symmetrical in the horizontal direction with respect to the vertical plane including the central axis C (symmetric in the vertical direction with respect to the central axis C in FIG. 1A).

The core 10 may be made of a P-type semiconductor material and an N-type semiconductor material in order to make optical characteristics variable. In this case, the refractive index can be dynamically controlled by the carrier plasma effect by forming electrodes 17a and 17b and applying a voltage as shown in FIG. 4 in a part of the light guiding direction, for example. Become. Of the two semiconductor regions 10a and 10b, one is P-type and the other is N-type. The electrodes 17a and 17b and the semiconductor regions 10a and 10b of the core 10 are connected by conductive portions 16a and 16b, respectively. The conductive portions (conducting portions) 16a and 16b may be formed of the same material and / or method as the semiconductor regions 10a and 10b of the core 10.
The arrangement of the electrodes is not limited to this form, and can be arranged in a thin slab portion connected to the core as in an example described later.

The distance that the concave portion 12a continues in the light guiding direction of the optical waveguide is referred to as the width of the concave portion. Further, the distance that the convex portion 12b continues in the light guiding direction is referred to as the width of the convex portion. The grating pitch (P _G in FIG. 1A) at that position is a set of adjacent convex portions and concave portions, and the sum of the width of the convex portions and the width of the concave portions.
In the upper grooved grating structure, the main material forming the core 10 of the optical waveguide has a convex shape at the position corresponding to the convex part 12b, and the width of the grooved structure 13 is reduced. The main material forming the core 10 of the optical waveguide has a concave shape at the position corresponding to the concave portion 12a, and the width of the groove-like structure 13 is widened. Similarly, the concave portion 13a is formed. That is, the width of the groove-like structure 13 has an inverted relationship in which the width of the groove-like structure 13 is narrow at the convex portion 13b and the width of the groove-like structure 13 is wide at the concave portion 13a.
Although not shown in FIGS. 1 and 2, a part of the cladding is also provided in the recess 12 a and the groove-like structure 13.

  FIG. 5 shows an example of the grating pitch of the grating structure of the optical waveguide device of the present invention. In the optical waveguide device of the present invention, the grating pitch takes any value of the discrete pitch obtained as a result of solving the inverse scattering problem, and the conventionally known equal pitch grating structure, chirped pitch grating structure, Different from any of the sampled grating structures. The discretized grating pitch can be expressed as P ± NΔP, where N is an integer related to the discretization parameter when solving the inverse scattering problem.

FIG. 6 shows an example of a configuration 100 in which the substrate type optical waveguide device 101 and the optical transmission lines 103 and 105 are connected. Since the device 101 is a reflective device having a grating structure, the start end is the light input incident end and at the same time the light output end. As shown in FIG. 6, normally, an input / output optical fiber is connected via a circulator 102 and used. The circulator 102 includes an incident optical fiber 103 that propagates incident signal light, a coupling optical fiber 104 that connects the substrate-type optical waveguide device 101 and the optical circulator 102, and an outgoing optical fiber 105 that propagates outgoing signal light. Is connected.
Further, normally, this grating optical waveguide section is obtained by adding an input / output conversion section called a spot size converter to a place where the substrate type optical waveguide device 101 and the coupling optical fiber 104 are optically connected. This is preferable because the connection loss between the fiber 104 and the device 101 can be reduced.

(Device design method)
In order to obtain a substrate type optical waveguide device having a grating structure capable of obtaining desired optical characteristics, in the present invention, a potential distribution in the optical propagation direction of the optical waveguide is obtained and converted into an equivalent refractive index distribution of the core. Convert to waveguide dimensions. The potential distribution is calculated from, for example, the Zakharov-Shabat equation having a potential derived from the logarithmic derivative of the equivalent refractive index of the optical waveguide from the wave equation that introduces a variable that is the power wave amplitude propagating forward and backward of the optical waveguide. A design method that can be used to solve the inverse scattering problem that derives the potential function numerically from the complex reflection spectrum, which is the intensity and phase spectrum of the reflectance of the grating optical waveguide, and to estimate the potential to achieve the desired reflection spectrum. Can be used to design.
This makes it possible to design and manufacture a Bragg grating element having complicated optical characteristics that cannot be realized by a conventionally known equal pitch grating element or chirped pitch grating element. For example, in a DWDM optical fiber communication system 40 A device having desired optical characteristics such as an optical chromatic dispersion compensator that simultaneously compensates for chromatic dispersion and dispersion slope of a transmission line optical fiber in a channel can be realized.

(Design method of potential distribution)
The technique for designing the potential distribution from the desired complex reflection spectrum using the inverse scattering problem is as follows.
In the numerical formula in the design procedure to be described later, the mathematical formula is shown with the longitudinal direction of the grating optical waveguide, that is, the light propagation direction as the z axis. The horizontal direction in FIG. 1A and FIG. 5 is the z-axis direction. A grating region start end of the grating optical waveguide device is set to z = 0, and an end end is set to a z maximum value coordinate. The maximum z value is the region length of the grating optical waveguide portion.

  First, the electromagnetic field propagating in the optical waveguide is described in the article of Shipe (JE Sipe, L. Poladian, and C. Martijn de Sterke, “Propagation through nonuniform grating structures,” Journal of the Optical Society of America A, Vol. 11, Issue 4, pp. 1307-1320 (1994)) is formulated as follows.

  Assuming that the time variation of the electromagnetic field is exp (−iωt), the complex amplitude E (z) of the electric field in the optical waveguide and the complex amplitude H (z) of the magnetic field are expressed as follows. The following equations (1) and (2) are obtained by Maxwell's Equations.

Where E (z) is the complex amplitude of the electric field, H (z) is the complex amplitude of the magnetic field, i is the imaginary unit, ω is the angular frequency, μ ₀ is the permeability of vacuum, ε ₀ is the permittivity of vacuum, and n _eff Represents the effective refractive index of the optical waveguide.

In order to construct coupled-mode equations from equations (1) and (2), E (z) and H (z) are traveling waves as in the following equations (3) and (4). (Power wave propagating forward) Converted to amplitude A ₊ (z) and backward wave (power wave propagating backward) amplitude A ₋ (z). The device is a reflective device that realizes desired optical characteristics as a reflection spectrum. The reflected wave corresponds to the backward wave amplitude A ₋ (z).

Here, n _av is the reference refractive index (average effective refractive index) of the optical waveguide, and is a reference refractive index that serves as a basis for n _eff (z). These variables A ₊ (z) and A ₋ (z) satisfy the following expressions (5) and (6), where c _light is the speed of _light in vacuum.

Here, the wave number k (z) is expressed by the following equation (7). _Let c _{light be the} speed of _light in vacuum.

  Further, q (z) in the equation (8) is a potential distribution in the coupled mode equation.

Substituting n (z) in Equation (5) and Equation (6) with n _eff (z) in Equation (7) and Equation (8), and substituting them, Equation (5) and Equation (6) become 9) It is reduced to the Zakharov-Shabat equation shown in equation (10).

Solving the inverse scattering problem represented by the Zakharov-Shabat equation is to solve the Gel'fand-Levitan-Marchenko type integral equations, which will be described later. PV Frangos and DL Jaggard, “A numerical solution to the Zakharov-Shabat inverse scattering problem,” IEEE Transactions on Antennas and Propagation, Vol. 39, Issue. 1, pp. 74-79 (1991)).
Also, Xiao's paper (G. Xiao and K. Yashiro, “An Efficient Algorithm for Solving Zakharov-Shabat Inverse Scattering Problem,” IEEE Transaction on Antennas and Propagation, Vol. 50, Issue 6, pp. 807-811 (2002) ) Discloses an efficient solution.

  The optical characteristic of the substrate type optical waveguide device having the grating structure of the present invention is defined by the following equation (11) as the complex reflection spectrum r (k) at the input / output end of the optical waveguide.

  As shown in the following equation (12), the Fourier transform of r (k) is the impulse response R (z) of this system.

  By giving a desired group delay characteristic and reflectance distribution with respect to the wavelength as the complex reflection spectrum r (k), the potential distribution function q (z) for realizing this can be numerically solved.

In the present invention, the design is performed using an amplitude modulation type grating in which the amplitude of the grating changes and the phase changes depending on the amplitude. For this reason, in the complex reflection spectrum used as design input data, in order to improve the separation between the envelope of the amplitude of the grating and the phase of the vibration of the grating, the frequency at which a predetermined group delay time characteristic is required from the origin of the frequency, that is, 0 Hz. Include all areas.
First, the solutions of the equations (3) and (4) are expressed as the following equations (13) and (14).

A ₊ (z) and A ₋ (z) propagate in the + z direction and the −z direction, respectively. The integral term in Equation (13) and Equation (14) represents the influence of reflection. From the equations (13) and (14), the coupled mode equation is converted into the Gerphant-Levitan-Marchenko equation represented by the following equations (15) and (16).

Here, the normalization time y is y = _clight t (t is time), and z> y. R (z) is an inverse Fourier transform of the complex reflection spectrum r (k) with the wave number as a variable, and corresponds to an impulse response. The potential distribution q (z) is obtained by solving the equations (15) and (16) by giving R (z), and is given by the equation (17).

By applying the obtained potential distribution q (z) to the following equation (18), an effective refractive index distribution n _eff (z) of the grating optical waveguide is obtained.

  In the present invention, the potential distribution q (z) in the equations (8) and (17) is a real number. As a result, the calculation for converting the complex reflection spectrum r (k) into the impulse response (time response) R (z) becomes a real number type, and the amplitude changes and the phase changes depending on the amplitude.

The effective refractive index distribution n _eff (z) thus obtained is such that the high refractive index value and the low refractive index value appear alternately at a short pitch (period), and shows a grating optical waveguide structure. It has become. This grating structure is uneven and the refractive index difference between the adjacent high refractive index value and low refractive index value corresponding to the concave and convex portions of the optical waveguide core is not constant but gradually changes. The pitch at which the refractive index changes takes a limited discrete value, and has a novel structure that does not match any of the conventionally known equal pitch grating optical waveguide, chirped pitch grating optical waveguide, or sampled grating optical waveguide. Have.

  The grating optical waveguide of the present invention forms a grating pattern by changing the amplitude of the Bragg grating, and is an amplitude modulation type in which the sign of the gradient of the envelope of the amplitude of the grating is inverted. The sampled grating optical waveguide is characterized in that an optical waveguide region in which the amplitude is continuously zero is interposed between two points where the sign is inverted. On the other hand, such a structure does not appear in the amplitude modulation type grating optical waveguide of the present application. The inversion of the sign indicates a steep steepness or discontinuity that occurs at an isolated single coordinate point. That is, the sign of the envelope gradient is inverted at a certain z coordinate. Since the amplitude becomes zero only at an isolated coordinate point where the sign of the envelope slope is reversed, there is virtually no region where the amplitude remains constant at zero. This makes it possible to shorten the waveguide length as compared with the sampled Bragg grating.

There are a plurality of isolated coordinate points on the waveguide where the sign of the envelope gradient is reversed. Each coordinate point is accompanied by a phase discontinuous change. Since the local period (pitch) changes when the phase changes discontinuously, the pitch is different from half of the value obtained by dividing the center wavelength in the spectrum of interest at the coordinate point by the average value n _av of the effective refractive index of the optical waveguide. Takes a value. The accuracy of specifying the coordinate point where the sign of the envelope gradient is inverted depends on the discrete step of the waveguide coordinate z on the horizontal axis. If the step is ΔP, the accuracy of specifying the coordinate point is in the range of ± ΔP. As described above, in the amplitude modulation type grating optical waveguide of the present invention, the sign of the slope of the envelope of the amplitude of the grating is inverted, and as a result, there are coordinate points at which the pitch changes discretely. The discretized grating pitch can be expressed by ΔP as P ± NΔP, where N is an integer related to the discretization parameter when solving the inverse scattering problem.

The discrete change in pitch is a feature not found in chirped Bragg gratings. In the chirped Bragg grating, the pitch continuously changes along the optical waveguide direction. In the chirp Bragg grating, the amplitude of the Bragg grating also changes at the same time, but the change in amplitude is only used to realize secondary characteristics such as apodization. This characteristic is achieved by changing the frequency of the Bragg grating along the light guiding direction. With the procedure disclosed here, a chirped grating cannot be constructed. In order to construct a chirped grating, it is necessary to switch the conversion from the complex reflection spectrum r (k) to the time response (impulse response) R (z) to the complex type. As a result, the potential distribution q (z) obtained by Expression (17) is a complex number. When q (z) is a complex number, _neff (z) is a real number in determining the effective refractive index distribution n _eff (z) from q (z), and therefore only the real part of q (z) is taken. is necessary. Therefore, the amplitude modulation type grating structure of the present invention and the conventionally known chirped grating structure are classified into different categories by different design methods. Since it is opposed to the amplitude modulation type, the chirped grating structure is classified as a frequency modulation type.

In the present invention, including all the other embodiments, the calculation used for conversion from the complex reflection spectrum to the impulse response is a real number type, and is intended for an amplitude modulation type Bragg grating. The conditions for selecting the amplitude modulation type Bragg grating are summarized as follows.
(I) The frequency range of the specified spectral characteristic is all included from the origin (frequency zero) to the region where the corresponding spectral channel exists.
(II) The real type is selected in the conversion from the above complex reflection spectrum to the impulse response.

In the actual calculation procedure, first, the maximum value of z is specified by determining the total length of the grating optical waveguide element. For example, in the case of an optical dispersion compensator, the maximum value of the group delay time to be generated in the grating optical waveguide is determined from the group delay dispersion value to be compensated and the channel bandwidth. By multiplying by the speed c _light and further dividing by the average value n _av of the effective refractive index, the minimum required element length can be determined. The total length of the element is obtained by adding a certain extra length to this. Subsequently, the increment of difference is determined. As an example, if the total element length is set to 17,200λ with the design center wavelength λ as a reference, and the difference increment at the z position is set to λ / 40, the potential distribution of the optical dispersion compensator at 688,000 points from z ₀ to z _688000. q (z) will be calculated. As an example of desired optical properties for a wavelength given as a complex reflection spectrum r (k),
FIGS. 15 and 16 show the potential distribution q (z) obtained by calculation when the reflectance distributions are FIGS. 11 and 12 and the group delay characteristics are FIGS. 13 and 14, respectively.

(Calculation of optical waveguide dimensions)
Based on the optical waveguide cross-sectional structure obtained in advance, specifically, the relationship between the core dimension and the equivalent refractive index, the potential distribution q (z) obtained by solving the inverse scattering problem is converted into the effective refractive index distribution n _eff (z). Next, the core size distribution in the light propagation direction (longitudinal direction) of the optical waveguide is calculated.
Here, as the second embodiment, in order to solve the problem of polarization dependence of optical characteristics and to suppress the influence (scattering loss) on the optical characteristics due to the inevitable roughness of the inner core side wall that occurs in the manufacturing process, silicon is used. An example in which a dual core structure in which (Si) is disposed inside the core and silicon nitride (Si ₃ N ₄ ) is disposed outside the core will be described, and the dimensions of the optical waveguide are calculated in the case of the second embodiment. .

In the case of this optical dispersion compensator for a substrate type optical waveguide, in order to eliminate the problem of polarization dependence of optical characteristics, a grating structure is provided on a part of the optical waveguide core side wall and a grooved grating structure is provided on the upper part of the core. Furthermore, in order to make the optical characteristics variable by changing the carrier density, silicon, which is a semiconductor material having conductivity by adding a dopant, is used as the material. Further, the clad material of the waveguide is silica (SiO ₂ ), and a double core structure in which silicon (Si) is disposed inside the core and silicon nitride (Si ₃ N ₄ ) is disposed outside the core is employed. The cross-sectional structure of a light waveguide designed according to the structure shown in FIG. 7, _{w in} dependence of the effective refractive index for the TE polarization shown in FIG. 8 (mode1) and TM polarization (mode2), _{w in} and w shown in FIG. 9 the relationship between the _out, to determine the correspondence between the _{w in} and _{w out} to the effective refractive index of the optical waveguide shown in FIG. 10.

Structure shown in Figure 7, along with the portion shown in w _out and t _out of the optical waveguide core sidewall has a similar sidewall grating structure with reference numeral 12 in FIG. 1, the portion indicated in the core upper w _in and t _in the It has a grooved grating structure similar to reference numeral 13 in FIG. Further, the portion indicated by w _middle and t _middle on the side wall of the optical waveguide core has a structure in which the core width is the same as that of the side wall upper portion 15 shown in FIG.
The meanings of w _out , t _out , win _, t _in , w _{middle, and} t _{middle in} the outer core 24 shown in FIG. 7 are the same as those in the core 10 shown in FIG.

The first rib 21 and the second rib 22 are made of silicon (Si), the central gap 23 is made of silica glass (SiO ₂ ), the outer core 24 is made of silicon nitride (Si ₃ N ₄ ), the substrate 25 is made of silicon (Si), and the lower part The clad 26 is made of silica glass (SiO ₂ ), and the upper clad 27 is made of silica glass (SiO ₂ ), t ₁ = 240 nm, t ₂ = 50 nm, w ₁ = 257 nm, w ₂ = 160 nm, w _middle = 1000 nm, t _The calculation was performed when _out = 250 nm, t _middle = 200 nm, t _in = 100 nm, the thickness of the lower cladding 26 was 2,000 nm, and the maximum thickness of the upper cladding 27 was 2,000 nm.

In order to obtain these correspondences, the values of the width w _in and the core width w _out of the groove-like structure are changed, and the electromagnetic field distribution of the eigenpropagation mode is determined from the cross-sectional structure of each optical waveguide by the mode matching method and the finite element method. Alternatively, it is obtained by a mode solver program employing various methods such as a beam propagation method, and is obtained by calculating its effective refractive index n _eff .
Figure 10 is set to obtain a _{w in} the _{w out} corresponding to _{n eff} in correspondence between _{w in} and _{w out} to the effective refractive index of the optical waveguide shown, devices designed in polarization-independent is there.
From the effective refractive index distribution n _eff (z) and FIG. 10, the width w _in and the core width w _out of the groove-like structure at each z coordinate can be obtained. From FIG. 10, the reference refractive index (average effective refractive index) n _av is set to, for example, 2.2164 by taking approximately the center of the range in which the relationship between the effective refractive index and the structural dimension is examined.

Based on the second embodiment, the reference refractive index (average effective refractive index) n _av is set to 2.2164 from FIG. 10, and the center frequency is 188.4 THz, that is, the center wavelength is 1,591.255 nm, as a device for L-Band. The example designed in is shown below. By converting the reference refractive index (average effective refractive index) n _av to 2.2164 and the center wavelength to 188.4 THz for the potential distribution q (z) shown in FIGS. 15 and 16, FIG. 17 and FIG. The effective refractive index distribution n _eff (z) shown in FIG. 18 is obtained. This is integrated (averaged) so that a convex part with a constant amplitude and a concave part with a constant amplitude are alternately repeated with a steep change, and n _eff is used using the relationship of FIG. the core width in terms of (the width w _in the groove of the grooved grating structure of the core upper, core width w _out of the grating structure of the optical waveguide core sidewall _bottom), required dimensions of the grating structure of a specific optical waveguide It is done.
In the present invention, the grating structure thus obtained has a non-uniform length of the grating fin (a length corresponding to the difference between the convex portion and the concave portion), and the grating pitch takes a limited discrete value. Has characteristics.

19 and 20 show an example of the grating pitch distribution of the optical dispersion compensator having an element length of about 12.3 mm and about 17,200 periods. This is an example in which the optical waveguide dimensions are calculated by designing for L-Band with a reference refractive index (average effective refractive index) n _av of 2.2164 and a center wavelength of 1,591.955 nm.
Since the inverse scattering problem is solved and the potential distribution q (z) is obtained, the difference in the z position is subdivided into λ / 40. Therefore, ΔP is 17 nm, and the main grating pitch P is λc / P = 370 nm, which is a value close to (n _av × 2) = 359 nm, and then a large pitch of 370 nm, which is close to P + ΔP, is also seen, and these two occupy 99% of the total pitch. FIG. 19 shows P-10ΔP, P-8ΔP, P-7ΔP, P-6ΔP, P-5ΔP, P-4ΔP, P-3ΔP, P-2ΔP, P-ΔP, P, P + ΔP, P + 2ΔP, P + 3ΔP, P + 4ΔP, The grating pitches corresponding to P + 5ΔP, P + 6ΔP, P + 7ΔP, P + 8Δ, P + 10ΔP are 166 nm, 203 nm, 222 nm, 240 nm, 259 nm, 277 nm, 296 nm, 314 nm, 333 nm, 351 nm, 370 nm, 388 nm, 406 nm, 425 nm, 443 nm, 462 nm, 480 nm, 499 nm, There is a pitch of 536 nm.
There are no grating pitches equal to or less than P-11ΔP and P + 11ΔP, and no grating pitches corresponding to P-9ΔP and P + 9ΔP. FIG. 20 shows an enlarged view of the vicinity of 3.390 mm in FIG. In this region, many pitches are distributed at 351 nm and 370 nm.

In general design cases, P that is closest to λc / (n _av × 2) is the largest in the difference increments, and thereafter the frequency of appearance of the corresponding grating pitch decreases as N of P ± NΔP increases. For example, in a design example of a single channel optical filter, almost all grating pitches are P, only a few P ± ΔP are observed, and P ± NΔP where N is 2 or more does not appear. There are cases. There are also cases where some pitches do not appear, such as P-10ΔP appears but P-9ΔP, P-8ΔP, and P-7ΔP do not appear.

  Taking discrete values of a limited number (small number) of pitches is effective in maintaining the processing accuracy in the CMOS manufacturing process. In the CMOS manufacturing process, it is a common process control technique to measure dimensions using a scanning electron microscope (SEM) such as DICD (Development Inspection Critical Dimension) and FICD (Final Inspection Critical Dimension). It is difficult to control the pitch accuracy with a structure having a gradually changing pitch such as a mold grating, but the process management is easy when taking a small number of discrete values such as an equal pitch type or the present invention. .

  FIGS. 21 and 22 are examples of the sidewall grating structure and the upper groove grating structure of the optical dispersion compensator designed as described above, and correspond to FIG.

(Embodiment having a grating structure only on the side wall)
FIGS. 27 (a), (b), (c) and FIG. 28 schematically show examples of a substrate type optical waveguide device having a grating structure only on the side wall of the core. 27 and 28, only the core 1 is illustrated and the cladding is not illustrated, but the cladding surrounds the core 1. A substrate (not shown) exists under the cladding, and the bottom surface 4 of the core 1 is parallel to the substrate surface. The horizontal direction means a direction parallel to the substrate surface, and the vertical direction means a direction perpendicular to the substrate surface.
FIG. 27A is a top view of a part of the core 1. FIG.27 (b) is sectional drawing which follows the SB-SB line | wire in Fig.27 (a). FIG.27 (c) is sectional drawing which follows the SC-SC line in Fig.27 (a). FIG. 2 is a perspective view.

In FIG. 27A, symbol C represents a single central axis in the horizontal plane of the optical waveguide core 1, and light propagates along the central axis C in the optical waveguide. This optical waveguide has the same Bragg grating pattern as the sidewall grating structure 12 in the first embodiment. That is, the concave portion 2a and the convex portion 2b in the core 1 the lower side wall 2 are alternately formed, the width w _grating forms a Bragg grating pattern oscillates alternately every one period P _G.
On the other hand, it is assumed that the width w _{fix at the} upper side wall 5 of the core 1 is constant and the width does not change along the longitudinal direction. Thus, by reducing the height of the Bragg grating pattern, it is possible to manufacture a higher quality device.

When the grating structure is formed on a part of the side wall of the core, the constant core width w _fix in the side wall 5 may be larger than the maximum value of the core width w _grating in the side wall grating structure 2, and the constant core width w _fix is The core width w _{grating in} the _grating may be smaller than the minimum value or, as shown in FIG. 3, the constant core width in the side wall 5 may take an intermediate value between the core width in the concave portion 2a and the core width in the convex portion 2b. .

In the case of this embodiment shown in FIG. 27, the width w _fix of the core upper portion 5 is smaller than the core width in the recess 2a, and the side wall grating structure 2 has a slab shape. Such a structure can be formed by the following method, for example.
(1) forming a recess 2a and convex portions small sidewall grating structure 2 heights _{t grating} having 2b, if necessary, the upper surface to form a cladding of the height _{t grating} around the sidewall grating structure 2 flattened After that, a method of increasing the height of the core 1 stepwise by laminating a top portion of the core having a height t _fix having the side wall 5 having a constant width thereon.
(2) After forming a high refractive index layer having a final core 1 thickness (t _grating + t _fix ) and etching the depth corresponding to t _fix on both sides of the core 1 to form the sidewall 5, Further, a method of forming the concave portion 2a and the convex portion 2b by etching.
In a waveguide using silicon, in general, when single crystal silicon is used, a waveguide having a light propagation loss smaller than that in the case of using silicon laminated by deposition can be formed. Therefore, in the case of silicon, in addition to forming the core 1 by film formation, a method of cutting based on single crystal silicon is often preferred. In the structure of this embodiment, when silicon is used as the core material, for example, an SOI substrate is used to adjust the thickness by etching based on the single crystal silicon layer above the BOX layer, and a waveguide can be formed by this technique. it can.
In this embodiment, the upper surface 3 of the core 1 is flat, but a grooved grating structure can be provided on the upper surface 3 as in FIG.

In the case of this embodiment, since there is only one type of width w _grating that changes in the grating structure, when “calculating the dimensions of the optical waveguide”, the effective refractive index n _effTE of the optical waveguide is shown in FIG. The correspondence relationship of the core width w _grating of the _grating is used. That is, in the second embodiment described above, when the "calculation of the dimensions of the optical waveguide", has been determined a relationship between w _in and w _out to the effective refractive index of the optical waveguide shown in FIG. 10, the present embodiment In this case, it is only necessary to consider the change in the core width w _grating . Otherwise, the grating structure can be designed in the same manner as in the above embodiment.

29 corresponds to the core 1 made of silicon nitride (Si ₃ N ₄ ) having a refractive index n of about 2.0, and the cladding (not shown) made of silica (SiO ₂ ) having a refractive index n of about 1.45. It is configured and obtained for the case where w _grating is changed between 1 to 1.8 μm with w _fix = 600 nm, t _grating = 150 nm, and t _fix = 350 nm.

In order to obtain these correspondences, various methods such as mode matching, finite element method, or beam propagation method are used to change the electromagnetic field distribution of the eigenpropagation mode from the cross-sectional structure of each optical waveguide by changing the slab width w _grating. The effective refractive index n _eff is calculated from the obtained mode solver program. As a result, as shown in FIG. 29, the value of the slab width w _grating with respect to the effective refractive index of the optical waveguide is obtained.
From the effective refractive index distribution n _eff (z) and FIG. 29, the slab width w _grating at each z coordinate can be obtained. From FIG. 29, the reference refractive index (average effective refractive index) n _av is set to 1.631, for example, by taking about the center of the range in which the relationship between the effective refractive index and the structural dimension is examined.

Based on the third embodiment, the reference refractive index (average effective refractive index) n _{av is set} to 1.631 from FIG. 29, and the center frequency is 188.4 THz, that is, the center wavelength is 1,591.955 nm, as a device for L-Band. The example designed in is shown below. As the potential distribution q (z), the one shown in FIGS. 15 and 16 was used as in the second embodiment.

By converting the reference refractive index (average effective refractive index) n _av to 1.631 and the center wavelength to 188.4 THz with respect to the potential distribution q (z), the effective refractive index distribution shown in FIGS. 30 and 31 is obtained. n _eff (z) is obtained. This is integrated (averaged) so that a convex part with a constant amplitude and a concave part with a constant amplitude are alternately repeated with a steep change, and n _eff is used using the relationship of FIG. Is converted into the slab width w _grating , the specific dimensions of the grating structure of the optical waveguide are obtained.
In the present invention, the grating structure thus obtained has a non-uniform length of the grating fin (a length corresponding to the difference between the convex portion and the concave portion), and the grating pitch takes a limited discrete value. Has characteristics.

32 and 33 show an example of the distribution of the grating pitch of the optical dispersion compensator having an element length of about 12.3 mm and about 17,200 cycles. This is an example in which the optical waveguide dimensions are calculated by designing for L-Band with a reference refractive index (average effective refractive index) n _av of 1.631 and a center wavelength of 1,591.955 nm.
Since the inverse scattering problem is solved and the potential distribution q (z) is obtained, the difference in the z position is subdivided into λ / 40. Therefore, ΔP is 24 nm, and the main grating pitch P is λc / P = 477 nm, which is close to (n _av × 2) = 488 nm, and a close pitch of 502 nm, which is close to P + ΔP, is also often observed, and these two occupy 99% of the total pitch.
FIG. 32 shows P-10ΔP, P-8ΔP, P-7ΔP, P-6ΔP, P-5ΔP, P-4ΔP, P-3ΔP, P-2ΔP, P-ΔP, P, P + ΔP, P + 2ΔP, P + 3ΔP, P + 4ΔP, The grating pitches corresponding to P + 5ΔP, P + 6ΔP, P + 7ΔP, P + 8Δ, P + 10ΔP are 221 nm, 276 nm, 301 nm, 327 nm, 352 nm, 377 nm, 402 nm, 427 nm, 452 nm, 477 nm, 502 nm, 528 nm, 553 nm, 578 nm, 603 nm, 628 nm, 653 nm, 678 nm, There is a 692 nm pitch.
There are no grating pitches of P-11ΔP or less and P + 11ΔP or more and grating pitches corresponding to P-9ΔP and P + 9ΔP. FIG. 33 shows an enlarged view around 4.577 mm in FIG. In this region, many pitches are distributed at 477 nm and 502 nm.

In general design cases, P that is closest to λc / (n _av × 2) is the largest in the difference increments, and thereafter the frequency of appearance of the corresponding grating pitch decreases as N of P ± NΔP increases. For example, in a design example of a single channel optical filter, almost all grating pitches are P, only a few P ± ΔP are observed, and P ± NΔP where N is 2 or more does not appear. There are cases. There are also cases where some pitches do not appear, such as P-10ΔP appears but P-9ΔP, P-8ΔP, and P-7ΔP do not appear.

  Taking discrete values of a limited number (small number) of pitches is effective in maintaining the processing accuracy in the CMOS manufacturing process. In the CMOS manufacturing process, it is a common process management technique to measure dimensions using a scanning electron microscope (SEM) such as DICD (Development Inspection Critical Dimension) and FICD (Final Inspection Critical Dimension). It is difficult to control the pitch accuracy with a structure having a gradually changing pitch such as a mold grating, but the process management is easy when taking a small number of discrete values such as an equal pitch type or the present invention. .

  FIG. 34 is an example of the side wall grating structure and the upper groove grating structure of the optical dispersion compensator designed as described above, and corresponds to FIG.

Example 1
An optical waveguide having a double core structure in which silicon (Si) is disposed on the inner side of the core and silicon nitride (Si ₃ N ₄ ) is disposed on the outer side of the structure shown in FIG. 7, and silica glass (SiO ₂ ) is used as the cladding. It was fabricated designed the optical dispersion compensator planar optical waveguide having a grooved grating structure portion shown in the core upper w _in and t _in conjunction with a grating structure to the portion shown in w _out and t _out of the core side wall.
The cross-sectional structure of a light waveguide designed according to the structure of FIG. 7, _{w in} dependence of the effective refractive index for the TE polarization shown in FIG. 8 (mode1) and TM polarization (mode2), _{w in} and _{w out} shown in Figure 9 the relationship between the calculated and further, to determine the correspondence between the w _in and w _out to the effective refractive index of the optical waveguide shown in FIG. 10.

The first rib 21 and the second rib 22 are made of silicon (Si), the central gap 23 is made of silica glass (SiO ₂ ), the outer core 24 is made of silicon nitride (Si ₃ N ₄ ), the substrate 25 is made of silicon (Si), and the lower part The clad 26 is made of silica glass (SiO ₂ ), and the upper clad 27 is made of silica glass (SiO ₂ ). T ₁ = 240 nm, t ₂ = 50 nm, w ₁ = 257 nm, w ₂ = 160 nm, w _middle = 1000 nm, t _out = 250 nm, t _middle = 200 nm, t _in = 100 nm, the thickness of the lower cladding 26 is 2,000 nm, and the maximum thickness of the upper cladding 27 is 2,000 nm.

Subsequently, a grating pattern was designed. The design center frequency was 188.4 THz. That is, the design center wavelength is 1591.255 nm. ITU-T G.L over 100 channels with L-Band and 45 channels with channel bandwidth of 50 GHz. It is assumed that the dispersion velocity single-mode optical fiber (DSF) 100 km defined in 653 is compensated for the group velocity dispersion and dispersion slope, and the optical characteristics of the optical fiber line to be compensated are as follows: group velocity dispersion −295 ps / nm, dispersion slope Dispersion Slope, RDS) 0.018 / nm was assumed. The amplitude intensity reflectance in the channel band was 93.5%. 11 and 12 show the reflectance spectrum of the complex reflection spectrum r (λ) prepared based on these set values, and FIGS. 13 and 14 show the group delay spectrum. This was set to an element total length of 17,200λ, and the difference increment of the z position was set to λ / 40, and the inverse scattering problem was solved to obtain the potential distribution q (z). The results are shown in FIGS.
Subsequently, the optical waveguide dimensions shown in FIG. 10 are selected from the vicinity of the center of the effective refractive index range, the reference refractive index (average effective refractive index) n _av is set to 2.2164, the center frequency is 188.4 THz, that is, the center wavelength is 1. , 590.255 nm, the potential distribution q (z) was converted into an effective refractive index distribution n _eff (z).
The optical waveguide dimensions related to the formation of the sidewall grating structure were determined from the effective refractive index distribution n _eff (z) and w _{out in} FIG. 10, and a photomask for processing the sidewall grating structure was manufactured based on this dimension.
Further, the effective refractive index distribution n _{eff (z),} and a w _in FIG. 10, the upper grooved grating determines the optical waveguide dimensions on the formation of the structure, a photomask for the upper grooved grating structure processed in this dimension Was made.
An optical waveguide was manufactured using these two sets of photomasks. A stepper exposure apparatus having a wavelength of 248 nm was used.
As a result, a grating structure having a desired shape could be formed.

(Reference example)
As a reference example, the structure shown in FIG. 23 is a double core structure in which silicon (Si) is disposed inside the core and silicon nitride (Si ₃ N ₄ ) is disposed outside the core, and silica glass (SiO ₂ ) is clad. An optical dispersion compensator for a substrate-type optical waveguide having a grating structure on the optical waveguide core side wall 24b and a grooved grating structure 24c on the core upper portion 24a was designed and manufactured.
The cross-sectional structure of a light waveguide designed according to the structure of FIG. 23, _{w in} dependence of the effective refractive index for the TE polarization (mode1) and TM polarization _(mode2), the relationship between _{w in} and _{w out} are calculated, Figure to determine the relationship between the _{w in} and _{w out} to the effective refractive index of the optical waveguide of 24. The first rib 21 and the second rib 22 are made of silicon (Si), the central gap 23 is made of silica glass (SiO ₂ ), the outer core 24 is made of silicon nitride (Si ₃ N ₄ ), the substrate 25 is made of silicon (Si), and the lower part The clad 26 is made of silica glass (SiO ₂ ), and the upper clad 27 is made of silica glass (SiO ₂ ), t ₁ = 250 nm, t ₂ = 50 nm, w ₁ = 280 nm, w ₂ = 160 nm, t _out = 600 nm, t _It was calculated when _in = 100 nm, the thickness of the lower cladding 26 was 2,000 nm, and the maximum thickness of the upper cladding 27 was 2,000 nm.
Subsequently, a grating pattern was designed. The design center frequency was 188.4 THz. That is, the design center wavelength is 1591.255 nm. ITU-T G.L over 100 channels with L-Band and 45 channels with channel bandwidth of 50 GHz. It is assumed that the dispersion velocity single-mode optical fiber (DSF) 100 km defined in 653 is compensated for the group velocity dispersion and dispersion slope, and the optical characteristics of the optical fiber line to be compensated are as follows: group velocity dispersion −295 ps / nm, dispersion slope Dispersion Slope, RDS) 0.018 / nm was assumed. The amplitude intensity reflectance in the channel band was 93.5%. The total length of the element was 18,000λ, and the difference increment of the z position was set to λ / 40 to solve the inverse scattering problem and obtain the potential distribution q (z).
Subsequently, the optical waveguide dimensions in FIG. 24 are selected from the vicinity of the center of the effective refractive index range, the reference refractive index (average effective refractive index) n _av is set to 2.348, the center frequency is 188.4 THz, that is, the center wavelength is 1. , 590.255 nm, the potential distribution q (z) was converted into an effective refractive index distribution n _eff (z).
The optical waveguide dimensions related to the formation of the sidewall grating structure were determined from the effective refractive index distribution n _eff (z) and w _{out in} FIG. 24, and a photomask for processing the sidewall grating structure was manufactured based on this dimension. Furthermore, an effective refractive index distribution n _{eff (z),} and a w _in FIG. 24, the upper grooved grating determines the optical waveguide dimensions on the formation of the structure, a photomask for the upper grooved grating structure processed in this dimension Was made.
An optical waveguide was manufactured using these two sets of photomasks. A stepper exposure apparatus having a wavelength of 248 nm was used.

In this reference example, the height of the side wall grating is 600 nm, while in Example 1, the height of the side wall grating is 250 nm. In an etching process such as a grating pattern, it is known that as the depth is increased, the side wall becomes rougher and the inclination of the angle appears, leading to a reduction in waveguide quality.
In the present invention, by making the side wall grating pattern a part of the core that forms the side wall grating pattern, the quality of the waveguide can be reduced by the manufacturing process as described here while retaining the original function of the grating. It is possible to provide a substrate-type optical waveguide device having a grating pattern with reduced characteristics.

DESCRIPTION OF SYMBOLS 1,10 ... Core, 10a, 10b ... Semiconductor region with different polarity, 2, 12 ... Side wall grating structure, 2a, 12a ... Concave part, 2b, 12b ... Convex part, 3, 11 ... Upper surface, 13 ... Groove grating structure ( (Groove-like structure), 13a ... concave portion, 13b ... convex portion, 4, 14 ... bottom surface, 5, 15 ... upper side wall, 16a, 16b ... current-carrying portion, 17a, 17b ... electrode, 20 ... substrate type optical waveguide device, 21, 22 ... inner core, 21a, 22a ... flat plate part, 21b, 22b ... rectangular parallelepiped part, 23 ... central gap, 24 ... outer core, 24a ... upper surface, 24b ... side wall, 24c ... grooved structure, 25 ... substrate (support substrate) , 26 ... lower clad, 27 ... upper clad.

Claims (9)

A Bragg grating pattern structure in which convex portions having a wide core width in the horizontal direction and concave portions having a narrow core width are alternately formed in only a part of the height of the side wall of the core of the optical waveguide is formed by an etching process. A substrate type optical waveguide device , wherein a maximum core width in the Bragg grating pattern structure is larger than a core width at a height at which the Bragg grating pattern structure is not formed on the side wall . A first Bragg grating pattern structure in which convex portions having a wide core width in the horizontal direction and concave portions having a narrow core width are alternately etched only in a part of the height of the side wall of the core of the optical waveguide is etched. A second Bragg grating pattern structure is formed on a top surface and / or a bottom surface of the core, and the two Bragg grating pattern structures are formed in parallel regions along the light guiding direction. board type optical waveguide device you wherein a.   3. The substrate mold according to claim 2, wherein the second Bragg grating pattern structure includes a protrusion-like structure and / or a groove-like structure formed at the center in the width direction and at the upper part in the vertical direction of the core. Optical waveguide element. The core of the optical waveguide includes an inner core having a rib structure and an outer core covering the three directions of the convex portion of the rib structure on the upper side of the inner core, and the outer core has an average refraction of the inner core. The substrate-type optical waveguide device according to any one of claims 1 to 3, wherein the substrate-type optical waveguide device is made of a material having a refractive index lower than the refractive index, and the Bragg grating pattern structure is provided on the outer core.   The inner core has a gap portion made of a material having a refractive index lower than that of the inner core along the light guiding direction at the center in the width direction, and two regions separated by the gap portion. 5. The substrate type optical waveguide device according to claim 4, wherein a single mode optical waveguide in which the mode is propagated across the two regions is constituted. In the Bragg grating pattern structure , the length of the fin in the core width direction is not uniform along the longitudinal direction of the waveguide, and the pitch along the longitudinal direction of the waveguide takes a plurality of discrete values, and all discrete values Of these, when P is the most frequently occurring value and ΔP is the discretization step of the coordinate z along the longitudinal direction of the waveguide, each pitch can be expressed as P ± NΔP (where N is an integer). The substrate-type optical waveguide device according to any one of claims 1 to 5, wherein the substrate-type optical waveguide device is possible.   The core is made of a semiconductor material and is divided into a P-type and an N-type in a horizontal direction on a plane perpendicular to the light traveling direction, and each region is connected to an electrode. Item 7. The substrate type optical waveguide device according to any one of Items 1 to 6.   Compensates for chromatic dispersion and dispersion slope in the optical transmission line by varying the distance that the signal light propagates through the optical waveguide from the time it enters the optical waveguide until it is reflected for multiple wavelength channels. A chromatic dispersion compensating element, wherein the chromatic dispersion compensating element comprises the substrate-type optical waveguide element according to claim 1. 8. The method of manufacturing a substrate type optical waveguide device according to claim 1, wherein a Bragg grating pattern structure provided only at a part of the height of the side wall is formed by an etching process. A method for manufacturing a substrate-type optical waveguide device.

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