JP5193585B2 - Optical waveguide type chromatic dispersion compensation device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a compact optical waveguide type chromatic dispersion compensation device that compensates for chromatic dispersion of an optical fiber. This device can be used for an optical fiber communication network or the like.

  In optical communication, widening and speeding up of Dense Wavelength-Division Multiplexing (DWDM) transmission are rapidly progressing. In order to perform high-speed transmission, it is desirable to use, as these transmission lines, optical fibers whose chromatic dispersion is as small as possible in the transmission band, while chromatic dispersion is not zero in order to suppress nonlinear effects. Further, in an optical fiber already laid over a wide range, it is often used in a wavelength region where the dispersion is large. For example, a standard single-mode fiber (S-SMF) having zero dispersion near a wavelength of 1.3 μm has a wavelength of 1.53 to 1.63 μm due to the practical use of an erbium-doped optical fiber amplifier. Used in obi. In addition, a dispersion shifted fiber (DSF: Dispersion Shifted Fiber) in which zero dispersion is shifted to a wavelength near 1.55 μm may be used not only in the C band but also in the S band and the L band. In addition, there are various non-zero dispersion shifted fibers (NZ-DSFs) that do not become zero dispersion at a wavelength of 1.55 μm. When these optical fibers are used in DWDM, a technique for compensating for residual dispersion over a wide wavelength range is important.

  Various techniques are used for dispersion compensation. Among them, dispersion compensation using a dispersion compensation fiber (DCF) is the most practical technique (see, for example, Patent Documents 1 and 2). The DCF is realized by controlling the refractive index distribution of the optical fiber so that a desired dispersion compensation amount can be obtained. However, the DFC normally needs to be as long as the optical fiber to be compensated. When this is modularized, not only a large installation space is required, but also propagation loss cannot be ignored. In addition, DCF requires precise control of the refractive index distribution, which is not only difficult to manufacture, but also often makes it difficult to satisfy the dispersion compensation amount required in a wide band.

  Fiber Bragg Grating (FBG) is also one of the techniques often used for dispersion compensation (see, for example, Patent Document 3). The FBG performs dispersion compensation by irradiating the fiber with UV light, thereby changing the refractive index of the fiber core and forming a grating due to the different refractive index. This makes it possible to realize a small device for dispersion compensation, but it is difficult to control the change in the refractive index, and there is a limit to the change in the refractive index of the fiber. There are also limits to device miniaturization and mass production.

  There has also been proposed a structure in which a region where dispersion compensation is performed is divided for each channel, and chirped FBGs for which dispersion compensation is performed in each channel are superposed at one place (see, for example, Patent Document 4). By using this, the required fiber length is shortened. However, in this prior art, the structure of each channel is approached because each FBG is designed so that a plurality of FBGs are simply overlapped. Since there is an influence on the characteristics, there is a limit to the characteristics that can be realized. In addition, since a change in refractive index required for superimposing FBGs cannot be obtained by UV irradiation, some structures cannot be realized.

  An optical planar circuit (PLC: Planar Lightwave Circuit) can perform dispersion compensation by using an optical circuit constructed on a plane. Lattice type PLC is one example (see, for example, Non-Patent Document 1). However, the lattice type PLC controls the dispersion by cascading coupled resonators and is based on the principle of a digital IIR (Infinite Impulse Response) filter, so that the amount of dispersion to be realized is limited.

  A mechanism is also considered in which demultiplexing is performed with an arrayed waveguide grating (AWG), a path difference is added for each channel, and the delay time is adjusted, and then multiplexed with a collimator lens (for example, Patent Document 5). Not only is the structure complicated and difficult to manufacture, but also requires a large amount of space.

A VIPA (Virtually Imaged Phased Array) type dispersion compensator is a dispersion compensation device including a wavelength dispersion element (VIPA plate) in which a reflection film is coated on both surfaces of a thin plate, and a reflection mirror (see, for example, Patent Document 6). ). This device adjusts dispersion with a three-dimensional structure, is structurally complex, and requires extremely high accuracy in manufacturing.
Japanese Patent No. 3857211 Japanese Patent No. 3819264 JP 2004-325549 A WO03 / 010586 pamphlet K. Takiguchi, et. Al, “Dispersion slope equalizer for dispersion shifted fiber using a lattice-form programmable optical filter on a planar lightwave circuit,” J. Lightwave Technol., Pp. 1647-1656, vol. 16, no. 9 , 1998 Japanese Patent No. 3852409 JP-A-2005-275101

The problems in the prior art described above are as follows.
(1) Dispersion compensation using DCF requires a large space due to the use of a long fiber and is difficult to reduce in size. In addition, there is a limit to the dispersion compensation characteristics that can be realized.
(2) Dispersion compensation using FBG has a limitation in dispersion compensation characteristics that can be realized.
(3) Dispersion compensation using FBG superposition has a limit in the dispersion compensation characteristics that can be realized.
(4) Dispersion compensation using lattice type PLC has a small amount of dispersion compensation that can be realized.
(5) The AWG-based PLC has a complicated structure, is difficult to manufacture, and increases the cost. In addition, the required space is large and it is difficult to reduce the size of the device.
(6) The VIPA type dispersion compensator has a complicated structure, is difficult to manufacture, and increases the cost.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical waveguide type chromatic dispersion compensation device that can be downsized, has excellent dispersion compensation characteristics, is easy to manufacture, and can be manufactured at low cost.

In order to achieve the above object, the present invention provides a reflection-type chromatic dispersion compensation means for reflecting an optical waveguide whose equivalent refractive index of the core varies nonuniformly in the light propagation direction by changing the physical dimension of the core embedded in the cladding. have as the optical waveguide, the wavelength region to be dispersion compensated is divided into a plurality of channels has a partial region only dispersion compensation characteristic dispersion is compensated within the wavelength region of each channel, distributed An optical waveguide type chromatic dispersion compensating device is characterized in that the distribution of the group delay with respect to the wavelength is flat in the wavelength region of the channel other than the partial region where the compensation is made .

In the optical waveguide type chromatic dispersion compensation device of the present invention, the wavelength region of the channel is close to the distribution of the group delay and the reflectance with respect to the wavelength in the wavelength region other than the partial region where the dispersion is compensated. each the distribution of distribution and reflectivity of the group delay of the partial region for dispersion compensation in succession, and each distribution channel, each distribution is adjacent farthest wavelength from a partial region for the proximate dispersion compensation And a discontinuous distribution structure.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, it is preferable that the width of the core is unevenly distributed in the light propagation direction.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, it is preferable that the width of the core is non-uniformly distributed over the light propagation direction so that both sides of the core are symmetric from the center of the core.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, the width of the core may be non-uniformly distributed over the light propagation direction so as to be asymmetric on both sides in the width direction from the core center.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, the width of the core may be configured such that only one side of both sides in the width direction from the core center is unevenly distributed in the light propagation direction.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, it is preferable that the core is provided linearly in the optical waveguide.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, the core may be provided in a meandering manner in the optical waveguide.

  In the optical waveguide type chromatic dispersion compensating device according to the present invention, the core has a distribution region in which the width change is small in the center portion in the light propagation direction, and the width change is larger on both sides of the center portion than in the center portion. It is preferable to have a local maximum.

  In the optical waveguide type chromatic dispersion compensating device of the present invention, it is preferable that the transmission end of the optical waveguide is terminated at a non-reflective end and an output is taken out at the reflection end via a circulator or a directional coupler.

  In the optical waveguide type chromatic dispersion compensation device of the present invention, the optical waveguide preferably has a dispersion compensation characteristic that cancels the chromatic dispersion of the compensated optical fiber having a predetermined length in a predetermined wavelength band.

In the optical waveguide type chromatic dispersion compensation device of the present invention, the optical waveguide has a center wavelength λ _{c in} the range of 1280 nm ≦ λ _c ≦ 1320 nm and 1490 nm ≦ λ _c ≦ 1613 nm, and an operating band ΔBW of 0.1 nm ≦ ΔBW ≦ 60 nm. In the range, the dispersion D preferably has characteristics in the range of −3000 ps / nm ≦ D ≦ 3000 ps / nm, and the ratio RDS of the dispersion slope to the dispersion is in the range of −0.1 nm ⁻¹ ≦ RDS ≦ 0.1 nm ^−1. .

  In the optical waveguide type chromatic dispersion compensating device of the present invention, the equivalent refractive index distribution of the core over the light propagation direction of the optical waveguide is an inverse scattering problem that numerically derives a potential function from the spectral data of the reflection coefficient in the Zakharov-Shabat equation. And is preferably designed by a design method for estimating a potential for realizing a desired reflection spectrum.

  The equivalent refractive index distribution of the core across the light propagation direction of the optical waveguide is derived from the logarithmic derivative of the equivalent refractive index of the optical waveguide from the wave equation that introduces a variable of the power wave amplitude propagating forward and backward of the optical waveguide. Reduced to the Zakharov-Shabat equation with potential, solved as an inverse scattering problem that numerically derives the potential function from the spectral data of the reflection coefficient, estimated the potential for realizing the desired reflection spectrum, and based on it, the equivalent refractive index Calculate the core dimensions over the optical propagation direction of the optical waveguide based on the relationship between the equivalent refractive index and core dimensions of the core with a predetermined core thickness and core-clad relative refractive index difference. Are preferably designed.

  In the optical waveguide type chromatic dispersion compensation device of the present invention, the equivalent refractive index distribution of the core in the optical propagation direction of the optical waveguide is substantially a periodic structure at the center wavelength scale of the dispersion compensation band, and at a scale larger than the center wavelength. It is preferable to have a two-layer structure with an aperiodic structure determined by the inverse scattering problem.

The present invention also provides a lower cladding layer of the optical waveguide,
Next, a core layer having a refractive index larger than that of the lower cladding layer is provided on the lower cladding layer,
Next, the core layer is left with a predetermined core shape designed so that the equivalent refractive index of the core varies nonuniformly in the light propagation direction, and a core is formed by removing the other portions.
Next, an optical waveguide is manufactured by providing a clad covering the core, and an optical waveguide type chromatic dispersion compensation device according to the present invention described above is manufactured. To do.

Since the optical waveguide type chromatic dispersion compensation device of the present invention has an optical waveguide in which the equivalent refractive index of the core embedded in the cladding changes nonuniformly over the light propagation direction as a reflection type chromatic dispersion compensation means, Compared to the conventional technique using a dispersion compensating fiber or the like, the size can be reduced and installation space can be reduced.
In addition, the optical waveguide type chromatic dispersion compensation device of the present invention can obtain excellent dispersion compensation characteristics such as a wider dispersion compensation characteristic that can be realized as compared with dispersion compensation using a conventional FBG.
In addition, the optical waveguide type chromatic dispersion compensation device of the present invention has a simple structure and can be manufactured at a lower cost than conventional dispersion compensation devices such as PLC and VIPA.
The optical waveguide type chromatic dispersion compensation device of the present invention has a dispersion compensation characteristic in which the wavelength region for dispersion compensation is divided into a plurality of channels, and dispersion is compensated only in a partial region of the wavelength regions of each channel. Therefore, the required maximum delay amount can be shortened, the required length of the optical waveguide can be shortened, the device can be miniaturized, and the transmission loss of the optical waveguide can be reduced.

  According to the method for manufacturing an optical waveguide type chromatic dispersion compensation device of the present invention, a small dispersion compensation device having excellent dispersion compensation characteristics as described above can be efficiently manufactured at low cost.

  The dispersion compensation device of the present invention has an optical waveguide in which the equivalent refractive index of the core embedded in the cladding varies non-uniformly in the light propagation direction as a reflection type chromatic dispersion compensation means, and this optical waveguide compensates for dispersion. The wavelength region is divided into a plurality of channels and has a dispersion compensation characteristic in which dispersion is compensated only in a partial region of the wavelength regions of each channel.

Embodiments of an optical waveguide type chromatic dispersion compensation device (hereinafter abbreviated as dispersion compensation device) of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic perspective view showing an embodiment of an optical waveguide which is a main component of the dispersion compensation device of the present invention. The optical waveguide of the present embodiment is a planar waveguide (Non) having a non-uniform width obtained by changing the width w of the core in the longitudinal direction (z) as means for changing the equivalent refractive index of the core non-uniformly in the light propagation direction. -uniform Planar WaveGuide; hereinafter referred to as NPWG). In FIG. 1, reference numeral 10 is an NPWG, 11 is a core, and 12 is a cladding.

The NPWG 10 of this embodiment has a structure having a core 11 in a clad 12. The core 11 has a constant height h _3, the width w is the longitudinal direction (z) unevenly varied over direction and by changing the local equivalent refractive index of the propagating mode of the waveguide, reflected by this Type chromatic dispersion compensation function.

The operation principle of the NPWG 10 is somewhat similar to that of the FBG grating. However, the FBG changes the refractive index of the core medium with respect to the change of the equivalent refractive index, whereas the NPWG 10 of the present embodiment changes the refractive index of the core medium. The equivalent refractive index is changed by changing the width of the core 11 along the longitudinal direction, and the two are completely different with respect to the change of the equivalent refractive index.
The variation rate of the equivalent refractive index obtained by changing the width of the core 11 along the longitudinal direction is larger than that of the FBG, and fine and accurate control is easy.
Further, since the structure of the NPWG 10 is planar, it can be manufactured in large quantities by a known manufacturing process, and the cost can be reduced.

  This NPWG 10 can use a quartz glass-based material. In that case, for example, the clad may be made of pure quartz glass, and the core may be made of germanium-added quartz glass. It is also possible to use a resin material.

  In the dispersion compensation device using the NPWG 10, as shown in FIG. 2, the band to be subjected to dispersion compensation can be divided into a plurality of channels, and dispersion compensation can be performed for each channel. 2A is a graph showing the group delay characteristics of the NPWG 10, and FIG. 2B is a diagram schematically showing the relationship between the wavelength of light and the reflection in the core 11. As shown in FIG.

When compensating for the chromatic dispersion of an optical fiber, the required dispersion characteristics often change monotonously. As a result, the wider the band to be compensated, the higher the absolute delay amount (τmax−τmin in FIG. 2) within the band. As shown in the figure, the NPWG reflects a signal in a wavelength band with a fast delay (λ _{1 in} FIG. 2) in front of the device and reflects a signal in a wavelength band with a slow delay (λn in FIG. 2) at the back of the device. Therefore, when the absolute delay amount is high, the required device length becomes long. That is, the required device length L can be estimated by the following equation (A).

Where c is the speed of light, n _eff is the equivalent refractive index of the waveguide, and τmax and τmin are the maximum delay amount and the minimum delay amount, respectively.

As shown in FIG. 3, the present invention does not perform dispersion compensation for the entire divided channel (for example, [λ _{1 to} λ ₂ ] in the figure), but as shown in FIG. Is compensated only for the wavelength region that is really used (for example, [λ ′ _{1 to} λ ′ ₂ ] in FIG. 4). As shown in FIG. 3, rather than compensating for dispersion across the channel [λ ₁ ~λ _2], as shown in FIG. 4, it suffices to compensate for the part [λ _'1 ~λ' _2] only For example, the required maximum delay amount in the device can be shortened from the range of [τmax to τmin] to the range of [τ′max to τ′min], and a shorter device can be realized.

Actually, outside the compensation band ([λ _{1 to} λ ′ ₁ ] or [λ _{2 to} λ ′ ₂ ] in FIG. 4), the group delay may be prevented from exceeding the range of τ′max and τ′min. However, when the reflection spectrum at the boundaries λ ′ ₁ and λ ′ ₂ becomes discontinuous, the characteristic is disturbed due to the discontinuity within the compensation band [λ ′ _{1 to} λ ′ ₂ ]. Therefore, in the present invention, as shown in FIG. 5, the group delay outside the compensation band is flat and continuous at the boundaries λ ′ ₁ and λ ′ ₂ of the compensation band. By doing so, the influence which a discontinuity has on an actual use band can be suppressed.

The device design uses the inverse scattering problem approach to obtain the required width distribution from the desired reflection spectrum.
First, the electromagnetic field propagating in the waveguide is formulated as follows (reference: JE Sipe, L. Poladian, and C. Martijn de Sterke, “Propagation through nonuniform grating structures,” J. Opt. Soc. Am A, vol. 11, no. 4, pp. 1307-1320, 1994). Assuming that the time variation of the electromagnetic field is exp (−iωt), the following equations (1) and (2) are obtained according to the Maxwell equation.

  Here, E and H represent the complex amplitudes of the electric field and magnetic field, respectively, and n represents the refractive index of the waveguide. Here, the following equations (3), (4)

The power wave amplitude A ₊ (z) propagating forward of z and the power wave amplitude A ₋ (z) propagating backward are defined. However, Z ₀ = √μ ₀ / ε ₀ represents the impedance in vacuum, and n ₀ represents the reference refractive index. These variables satisfy the following equations (5) and (6):

Here, c represents the speed of light in vacuum. This equation is expressed by the following equation (7)

If the variable conversion is performed in, it is reduced to the Zakharov-Shabat equation shown in the following equations (8) and (9):

However, ω ₀ represents the reference angular frequency.
The Zakharov-Shabat equation can be solved as an inverse scattering problem. That is, the following formula (10)

The potential function u (x) can be solved numerically from the spectral data of the reflection coefficient defined in (Reference: PV Frangos and DL Jaggard, “A numerical solution to the Zakharov-Shabat inverse scattering problem,” IEEE Trans. Antennas and Propag., Vol. 39, no. 1, pp. 74-79, 1991). When this is applied to the above problem, a potential for realizing a desired reflection spectrum can be obtained. Here, the reflection spectrum refers to complex reflection data obtained from the group delay amount and the reflectance with respect to the wavelength. Furthermore, non-uniform means that the physical dimension changes with the location of the waveguide in the traveling direction.
If the potential u (x) is obtained, the local equivalent refractive index n (x) can be obtained by the following equation (11).

  Further, the core at a predetermined position in the light propagation direction is obtained from the relationship between the equivalent refractive index to the core width, which is obtained from the thickness of the core, the refractive index of the core, and the refractive index of the cladding in the waveguide to be actually manufactured. The width w (x) can be determined.

  Considering the wavelength, bandwidth, and length of the compensated fiber, create spectral data that is opposite to the dispersion of the compensated fiber (so that dispersion can be compensated), and use the above design method to solve the inverse problem. Solving and creating an optical planar waveguide realizes a compact dispersion compensation device.

Then, in consideration of the wavelength used, the band used, and the length of the optical fiber to be compensated, spectral data is created so as to be opposite to the dispersion of the optical fiber to be compensated (so that dispersion compensation can be performed), and the design method is used. If the inverse problem is solved and the NPWG 10 is manufactured, a small and high-performance dispersion compensation device can be realized.
What is characteristic of the present invention is that the reflection spectrum indicated by the desired dispersion compensation device is divided into wavelength bands (channels) that require dispersion compensation, and is set so as to indicate the necessary compensation group delay only within each wavelength band. It is to be. Further, in the wavelength band that does not require dispersion compensation, the reflection spectrum is set so that the group delay with respect to the wavelength is constant at least in a predetermined wavelength band that is continuous with the wavelength band that requires dispersion compensation. The constant value of the group delay with respect to this wavelength is a wavelength band that requires dispersion compensation, and is preferably the same as a boundary value between a wavelength band that does not require dispersion compensation and a continuous band. Further, the reflection spectrum is set so that the group delay with respect to the wavelength becomes discontinuous at a wavelength exceeding the predetermined wavelength band.

In the above embodiment, the NPWG 10 having a structure in which the core 11 whose height (thickness) is constant and whose width varies non-uniformly in the longitudinal direction is embedded in the clad 11, but the optical waveguide used in the present invention is The present invention is not limited to this example, and various modifications are possible.
For example, as shown in FIG. 6A, the width distribution of the core 11 is not uniformly distributed over the light propagation direction so that both sides in the width direction are symmetric from the center of the core 11, as shown in FIG. As shown in B), it may be a structure that is unevenly distributed over the light propagation direction so that both sides in the width direction are asymmetric from the center of the core 11.
In addition to the structure in which the core 11 is provided linearly along the longitudinal direction (z) of the NPWG 10 as shown in FIG. 1, the core 11 may be provided in a meandering manner as shown in FIG. By adopting such a structure in which the core 11 is provided in a meandering manner, the NPWG 10 can be further downsized.

  FIG. 8 is a block diagram showing an embodiment of the dispersion compensation device of the present invention. The dispersion compensation device 20 of the present embodiment is configured to include the NPWG 10 described above and a circulator 15 connected to the reflection end 13 side, and the transmission end 14 of the NPWG 10 is a non-reflection termination 16. The circulator 15 is connected to an optical fiber to be compensated (not shown) on the input side (input), and an optical fiber on the downstream side is also connected to the output side (output), and is used in the optical transmission line. The dispersion compensation device 20 of the present invention is a reflection type device, and an optical signal input from the compensated optical fiber to the input side of the circulator 15 enters the NPWG 10 and is reflected, and the reflected wave is output through the circulator 15. It has become so.

  As described above, the NPWG 10 of the dispersion compensation device 20 has reflectivity characteristics that can compensate for the chromatic dispersion of the compensated optical fiber. Therefore, the optical signal output from the compensated optical fiber is reflected by the NPWG 10. In this case, the chromatic dispersion of the optical signal is corrected and output. The optical signal output from the dispersion compensation device 20 is input to the downstream optical fiber connected to the output side of the circulator 15 and propagates through the fiber.

The NPWG 10, which is a main component of the dispersion compensation device 20 of the present invention, is manufactured as follows, for example.
(A) First, a lower cladding layer of the NPWG 10 is provided,
(B) Next, a core layer having a refractive index larger than that of the lower cladding layer is provided on the lower cladding layer,
(C) Next, the core layer 11 is processed by leaving a predetermined core shape designed so that the equivalent refractive index of the core varies nonuniformly in the light propagation direction, and removing the other portions. Form the
(D) Next, the cladding 12 covering the core 11 is provided, and the NPWG 10 is manufactured.

  The dispersion compensation device 20 of the present invention is manufactured by manufacturing the NPWG 10 as described above, terminating the transmissive end 14 of the NPWG 10 with a non-reflective terminal 16, and connecting the circulator 15 or the directional coupler to the reflective end 13. The dispersion compensation device 20 shown in FIG.

  When the core 11 of the NPWG 10 is formed, the core 10 is formed by a photolithography method using a mask having a predetermined core shape designed so that the equivalent refractive index of the core 10 changes nonuniformly in the light propagation direction. It is preferable. The material and procedure used for this photolithography method can be implemented using the material and procedure used for the photolithography method well-known in the semiconductor manufacturing field etc. Moreover, the film-forming method of a clad layer or a core layer can be implemented using the well-known film-forming technique currently used in manufacture of a general optical waveguide.

In the wavelength region [154.14 nm to 1558.17 nm], a dispersion compensation device that realizes chromatic dispersion compensation in which the dispersion amount D = −1000 ps / nm and the ratio of dispersion slope to dispersion RDS = 0.0036 nm ⁻¹ was designed. . However, in this embodiment, the wavelength region is divided into 20 channels satisfying the frequency f of 193.4 + 0.1 nTHz ≦ f ≦ 193.5 + 0.1 nTHZ, and the band 0. Dispersion compensation is performed only in the 02 THz region. Here, n represents an integer satisfying −10 ≦ n ≦ 9. These channels satisfy the ITU grid interval. The dispersion compensation device according to the present embodiment can compensate for the residual dispersion of the S-SMF having a length of 58 km.

  FIG. 9 is a graph showing the NPWG potential distribution of the dispersion compensation device manufactured in this example. The horizontal axis in the figure represents a location standardized at a center wavelength of 1550.12 nm. When this potential is used, the group delay characteristic shown in FIG. 10 and the reflectance characteristic shown in FIG. 11 are obtained. In both figures, spectral data (designed) used for design and obtained spectral data (realized) are shown.

  FIG. 12 is a partially enlarged view of FIG. 10. As shown in the figure, the influence due to the spectrum discontinuity brings about the ripple of the group delay, but the influence on the band where dispersion compensation is performed is limited.

When an NPWG has a waveguide structure in which a core having a height of h ₃ = 6 μm and a relative refractive index difference Δ = 0.6% is embedded in a clad made of quartz glass, the equivalent refractive index and waveguide at a wavelength of 1550 nm The relationship with the width of is as shown in FIG. In this case, the thickness of the clad is sufficiently larger than that of the core.

  When this waveguide structure is used, the core width distribution of the NPWG that realizes the characteristics of FIGS. 10 and 11 has a distribution region with a small width change at the center in the light propagation direction, as shown in FIG. A width change maximum portion having a width change larger than that of the center portion is provided on both sides of the center portion. The distribution of the equivalent refractive index of the NPWG at that time is as shown in FIG.

  FIG. 16 is an enlarged view of a part of FIG. 14, and FIG. 17 is an enlarged view of a part of FIG. As shown in these figures, the NPWG according to the present embodiment has a periodic structure in which the period is approximately ½ of the center wavelength on the scale of the center wavelength of the band for dispersion compensation. In other words, this NPWG has a two-layered structure such as a periodic structure at the center wavelength scale and a non-periodic structure determined by an inverse problem at a scale much larger than the wavelength.

  Even if waveguide structures of the same material are used, if the reference refractive index n (o) indicating the average equivalent refractive index of the entire waveguide is set according to the thickness and material of the waveguide, waveguides having different core widths can be used. The same characteristics can be realized. FIG. 18 shows the distribution in the width direction when high n (0) is used. The distribution of the equivalent refractive index of the waveguide at that time is as shown in FIG.

  The material of the core and the clad is not limited to quartz glass, and other transparent materials that are conventionally known in the optical field such as silicon compounds and polymers can also be used. In particular, if a material having a high refractive index is used, the device can be further reduced and the transmission loss can be reduced.

In the wavelength region [153.25 nm to 1566.31 nm], a dispersion compensation device that realizes chromatic dispersion compensation with a dispersion amount D = −1000 ps / nm and a ratio of dispersion slope to dispersion RDS = 0.0036 nm ⁻¹ was designed. . However, in this embodiment, the wavelength region is divided into 40 channels satisfying the frequency f of 193.4 + 0.1 nTHz ≦ f ≦ 193.5 + 0.1 nTHZ, and the band 0. Dispersion compensation is performed only in the 02 THz region. Here, n represents an integer satisfying −20 ≦ n ≦ 19. These channels satisfy the ITU grid interval. The dispersion compensation device according to the present embodiment can compensate for the residual dispersion of the S-SMF having a length of 58 km.

  FIG. 20 is a graph showing the NPWG potential distribution of the dispersion compensation device manufactured in this example. The horizontal axis in the figure represents a location standardized at a center wavelength of 1550.12 nm. When this potential is used, the group delay characteristic shown in FIG. 21 and the reflectance characteristic shown in FIG. 22 are obtained. In both figures, spectral data (designed) used for design and obtained spectral data (realized) are shown.

  FIG. 23 is a partially enlarged view of FIG. 21, and as shown in the figure, the influence of the spectral discontinuity causes the ripple of the group delay, but the influence on the band where dispersion compensation is performed is limited. .

In the case of a waveguide structure in which a core having a height of h ₃ = 6 μm and a relative refractive index difference Δ = 0.6% is embedded in a clad made of quartz glass, NPWG that realizes the characteristics shown in FIGS. As shown in FIG. 24, the core width distribution has a distribution region with a small width change at the center in the light propagation direction, and has a width change maximum portion where the width change is larger than the center at both sides of the center. become. The distribution of the equivalent refractive index of the NPWG at that time is as shown in FIG.

  Compared with the first embodiment, the amount of fluctuation in the potential is larger than that of the first embodiment, and the amount of change in the core width of the NPWG that realizes this is increased as the compensation band (channel) is increased. However, the length of the device is the same as in the first embodiment.

In the wavelength region [1530.33 nm to 1570.42 nm], a dispersion compensation device that realizes chromatic dispersion compensation in which the dispersion amount D = −1700 ps / nm and the ratio of the dispersion slope to the dispersion RDS = 0.0036 nm ⁻¹ was designed. . However, in this embodiment, the wavelength region is divided into 50 channels in which the frequency f satisfies 193.4 + 0.1 nTHz ≦ f ≦ 193.5 + 0.1 nTHz, and the band 0. Dispersion compensation is performed only in the 02 THz region. Here, n represents an integer satisfying −25 ≦ n ≦ 24. These channels satisfy the ITU grid interval. The dispersion compensation device according to the present embodiment can compensate for the residual dispersion of the S-SMF having a length of 100 km and can cover the entire C band.

  FIG. 26 is a graph showing the potential distribution of NPWG of the dispersion compensation device manufactured in this example. The horizontal axis in the figure represents a location standardized at a center wavelength of 1550.12 nm. When this potential is used, the group delay characteristic shown in FIG. 27 and the reflectance characteristic shown in FIG. 28 are obtained. In both figures, spectral data (designed) used for design and obtained spectral data (realized) are shown.

  FIG. 29 is a partially enlarged view of FIG. 27. As shown in FIG. 29, the influence due to the spectrum discontinuity causes the ripple of the group delay, but the influence on the band where dispersion compensation is performed is limited. .

In the case of a waveguide structure in which a core having a height of h ₃ = 6 μm and a relative refractive index difference Δ = 0.6% is embedded in a clad made of quartz glass, NPWG that realizes the characteristics shown in FIGS. As shown in FIG. 30, the core width distribution of FIG. 30 has a distribution region with a small width change at the central portion in the light propagation direction, and has a width change maximum portion where the width change is larger than the central portion on both sides of the central portion. become. The distribution of the equivalent refractive index of NPWG at that time is as shown in FIG.

It is a schematic perspective view which shows the structure of NPWG as one Embodiment of the optical waveguide which is the main components of the dispersion compensation device of this invention. In NPWG, the wavelength band to be compensated is divided into a plurality of channels, and dispersion compensation is performed in each channel. (A) is a graph showing group delay characteristics of NPWG, and (B) is a core. It is a graph which shows typically the relationship between the wavelength of the light in, and reflection. It is a graph which shows the maximum amount of delay in the case where the wavelength band to be compensated is divided into a plurality of channels and dispersion compensation is performed for the entire channels. In the present invention, the wavelength band to be compensated is divided into a plurality of channels, and is a graph showing the maximum delay amount when dispersion compensation is performed only in a partial region of the wavelength region of each channel. In the present invention, it is a graph illustrating a group delay characteristic of NPWG that performs dispersion compensation for each channel maintaining continuity. It is a schematic plan view which illustrates the distribution shape of the core width. It is a schematic plan view which illustrates the case where a core is provided in a meandering shape. It is a block diagram which shows one Embodiment of the dispersion compensation device of this invention. 3 is a graph showing a potential distribution of the NPWG of Example 1. 3 is a graph showing group delay characteristics of the NPWG of Example 1. 3 is a graph showing the reflectance characteristics of the NPWG of Example 1. FIG. 11 is a partially enlarged view of FIG. 10. 6 is a graph showing the relationship between the equivalent refractive index and the core width at a wavelength of 1550 nm when a core having h ₃ = 6 μm and a relative refractive index difference Δ = 0.6% is used. 3 is a graph showing a core width distribution of the NPWG of Example 1. 3 is a graph showing an equivalent refractive index distribution of the NPWG of Example 1. FIG. 15 is a partially enlarged view of FIG. 14. FIG. 16 is a partially enlarged view of FIG. 15. 6 is a graph showing a core width distribution of an NPWG when a high initial refractive index is used in the NPWG of Example 1. FIG. 3 is a graph showing an equivalent refractive index distribution of NPWG when a high initial refractive index is used in the NPWG of Example 1. 6 is a graph showing a potential distribution of NPWG in Example 2. 10 is a graph showing group delay characteristics of the NPWG of Example 2. 6 is a graph showing the reflectance characteristics of the NPWG of Example 2. FIG. 22 is a partially enlarged view of FIG. 21. 6 is a graph showing a core width distribution of the NPWG of Example 2. 6 is a graph showing an equivalent refractive index distribution of the NPWG of Example 2. 10 is a graph showing a potential distribution of NPWG in Example 3. 6 is a graph showing group delay characteristics of the NPWG of Example 3. 6 is a graph showing the reflectance characteristics of the NPWG of Example 3. It is a partially enlarged view of FIG. 6 is a graph showing a core width distribution of the NPWG of Example 3. 10 is a graph showing an equivalent refractive index distribution of the NPWG of Example 3.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... NPWG, 11 ... Core, 12 ... Cladding, 13 ... Reflection end, 14 ... Transmission end, 15 ... Circulator, 16 ... Non-reflection termination, 20 ... Dispersion compensation device

Claims (16)

As a reflection-type wavelength dispersion compensation means, an optical waveguide in which the equivalent refractive index of the core changes nonuniformly in the light propagation direction by changing the physical dimensions of the core embedded in the clad,
In the optical waveguide, a wavelength region for dispersion compensation is divided into a plurality of channels,
It has a dispersion compensation characteristic in which dispersion is compensated only in a partial region of the wavelength region of each channel ,
An optical waveguide type chromatic dispersion compensation device , wherein a distribution of group delay with respect to a wavelength is flat in a wavelength region of the channel other than the partial region in which dispersion is compensated. Wavelength region of the channel, the dispersion of wavelength region other than the partial area to be compensated, the distribution of the group delay distribution and reflectance with respect to wavelength, the same group delay of the partial area to be close to each dispersion compensation A distribution structure in which each distribution is discontinuous with each distribution of adjacent channels at a wavelength most distant from a partial region where dispersion compensation is performed, which is continuous with the distribution of the distribution and the reflectance. The optical waveguide type chromatic dispersion compensating device according to claim 1 . 3. The optical waveguide type chromatic dispersion compensating device according to claim 1, wherein the width of the core is unevenly distributed in the light propagation direction. 4. The optical waveguide type chromatic dispersion compensating device according to claim 3 , wherein the width of the core is unevenly distributed over the light propagation direction so that both sides in the width direction are symmetrical from the center of the core. 4. The optical waveguide type chromatic dispersion compensation device according to claim 3 , wherein the width of the core is non-uniformly distributed over the light propagation direction so as to be asymmetric on both sides in the width direction from the core center. 4. The optical waveguide type chromatic dispersion compensation device according to claim 3 , wherein the width of the core is non-uniformly distributed over the light propagation direction only on one side in the width direction from the core center. 5. Wherein the core, the optical waveguide-type wavelength dispersion compensation device according to any one of claims 1 to 6, characterized in that provided linearly in the optical waveguide. Wherein the core, according to claim 1 to 7 optical waveguide-type wavelength dispersion compensation device according to any of characterized in that provided in a meandering shape in the optical waveguide. 2. The core has a distribution region having a small width change at a central portion in a light propagation direction, and has a width change maximum portion having a width change larger than that of the central portion on both sides of the central portion. The optical waveguide type chromatic dispersion compensating device according to any one of to 8 . Transmission end of the optical waveguide is terminated in reflection-free termination, according to any one of claims 1 to 9, characterized in that it is configured to retrieve the output via the circulator or a directional coupler at the reflecting end Optical waveguide type chromatic dispersion compensation device. The optical waveguide wavelength according to any one of claims 1 to 10 , wherein the optical waveguide has a dispersion compensation characteristic that cancels chromatic dispersion of a compensated optical fiber having a predetermined length in a predetermined wavelength band. Dispersion compensation device. The optical waveguide has a dispersion D of −3000 ps / nm ≦ D when the center wavelength λ _c is in the range of 1280 nm ≦ λ _c ≦ 1320 nm and 1490 nm ≦ λ _c ≦ 1613 nm, the operating band ΔBW is in the range of 0.1 nm ≦ ΔBW ≦ 60 nm. ≦ 3000 ps / nm range, according to any one of claims 1 to 11, the ratio RDS dispersion slope is characterized by having the properties of a range of -0.1nm-1 ≦ RDS ≦ 0.1nm- 1 for dispersion Optical waveguide type chromatic dispersion compensation device. The equivalent refractive index distribution of the core over the light propagation direction of the optical waveguide is solved as an inverse scattering problem that numerically derives a potential function from the spectral data of the reflection coefficient in the Zakharov-Shabat equation, and realizes a desired reflection spectrum. the optical waveguide-type wavelength dispersion compensation device according to any one of claims 1 to 12, characterized in that it is designed in the design process to infer potential. The equivalent refractive index distribution of the core over the light propagation direction of the optical waveguide is derived from the logarithmic derivative of the equivalent refractive index of the optical waveguide from the wave equation that introduces a variable of the power wave amplitude propagating forward and backward of the optical waveguide. Reduced to the Zakharov-Shabat equation with potential, solved as an inverse scattering problem that numerically derives the potential function from the spectral data of the reflection coefficient, estimated the potential for realizing the desired reflection spectrum, and based on it, the equivalent refractive index Calculate the core dimensions over the optical propagation direction of the optical waveguide based on the relationship between the equivalent refractive index and core dimensions of the core with a predetermined core thickness and core-clad relative refractive index difference. The optical waveguide type chromatic dispersion compensation device according to claim 13 , wherein the device is designed as follows. The equivalent refractive index distribution of the core over the light propagation direction of the optical waveguide has a substantially periodic structure on the scale of the center wavelength of the band for dispersion compensation, and on the scale larger than the center wavelength, the equivalent refractive index distribution of the non-periodic structure determined by the inverse scattering problem. The optical waveguide type chromatic dispersion compensating device according to claim 13 or 14 , wherein the optical waveguide type chromatic dispersion compensating device has a hierarchical structure. Provide a lower cladding layer of the optical waveguide,
Next, a core layer having a refractive index larger than that of the lower cladding layer is provided on the lower cladding layer,
Next, the core layer is left with a predetermined core shape designed so that the equivalent refractive index of the core varies nonuniformly in the light propagation direction, and a core is formed by removing the other portions.
Next, an optical waveguide is manufactured by providing a clad covering the core, and the optical waveguide type chromatic dispersion compensation device according to any one of claims 1 to 15 is manufactured. Manufacturing method.

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