JP2018054934A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide element that mitigates polarization dependency of wavelength characteristics.SOLUTION: The optical waveguide element comprises a first waveguide 10 and a second waveguide 20 arranged apart in parallel to each other. The optical waveguide element further comprises, along the propagation direction of light, p+1 (p is an integer of one or more) couplers 101, 102 and p arm waveguide parts 201 which are series connected to each other alternately. Each arm waveguide part includes two or more phase adjustment regions having different waveguide structures from each other, and the phase adjustment region is provided in either one or both of the first waveguide and the second waveguide. A first routing waveguide and a second routing waveguide include the same structures. Phase differences between the first waveguide and the second waveguide in the arm waveguide part are equal to each other between TE polarization and TM polarization, and the amplitudes of wavelength dispersion of the phase are equal to each other between the TE polarization and the TM polarization.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a waveguide element, and more particularly to an element that functions as a wavelength multiplexing / demultiplexing filter that allocates a plurality of wavelength signals for each wavelength.

  As the amount of information transmitted increases, optical wiring technology has attracted attention. In the optical wiring technology, using an optical device using an optical fiber or an optical waveguide element as a transmission medium, information transmission between elements in information processing equipment, between boards, or between chips is performed by an optical signal. As a result, it is possible to improve band limitation due to the use of electrical wiring, which is a bottleneck in information processing equipment that requires high-speed signal processing.

As an optical device, in particular, an optical waveguide element using silicon (Si) as a waveguide material (hereinafter referred to as an Si optical waveguide element) has attracted attention. In the Si optical waveguide element, a waveguide core that substantially becomes a light transmission path is formed using Si as a material. Then, the periphery of the waveguide core is covered with a clad made of, for example, quartz (SiO ₂ ) having a refractive index lower than that of Si. With such a configuration, the refractive index difference between the waveguide core and the clad becomes extremely large, so that light can be strongly confined in the waveguide core. As a result, a small curved waveguide having a bending radius reduced to, for example, about several μm can be realized. Therefore, it is possible to create an optical circuit having a size comparable to that of an electronic circuit, which is advantageous for downsizing the entire optical device.

  In the case of a waveguide made of Si (Si waveguide), fine processing of the waveguide by high-precision photolithography or etching using a semiconductor manufacturing apparatus is possible. Also, mass production of optical devices in which various elements are monolithically integrated on the same substrate, such as modulators using the plasma carrier effect by ion implantation, and light receiving elements in which germanium (Ge) is selectively grown on Si. Is possible.

  From the above, the Si waveguide is regarded as a promising platform for optical devices, and various studies have been made (for example, see Non-Patent Documents 1 and 2).

  On the other hand, due to a large non-refractive index difference between the core and the clad as a problem of the Si waveguide, two orthogonal polarization components of Transverse Electric (TE) polarization and Transverse Magnetic (TM) polarization The difference between the equivalent refractive index and the group refractive index tends to be large. As a result, a difference in optical phase occurs between the TE polarized wave and the TM polarized wave. Therefore, in principle, in the wavelength filter that uses the optical phase interference, the divergence between the polarized waves of the wavelength response characteristic at the same wavelength tends to be large. . This problem is particularly apparent in the receiving device of the optical fiber transmission system.

  To solve this problem, a method of introducing a polarization separation functional element in an optical device has been proposed. However, for each optical functional element, two optical functional elements designed for TE polarization and TM polarization are required. For this reason, the advantage of miniaturization of the whole optical device obtained by using the Si waveguide as a platform cannot be obtained.

  Therefore, as an essential solution to the polarization dependence of the Si waveguide, it is required to align the characteristics of each optical functional element with TE polarization and TM polarization.

FIG. 8 is a diagram showing the relationship between the core width of the Si waveguide and the equivalent refractive index of the fundamental mode. Here, the wavelength is 1550 nm and the cladding is SiO ₂ . In FIG. 8A, the thickness of the waveguide core is 300 nm. In FIG. 8B, the thickness of the waveguide core is 220 nm.

  As shown in FIG. 8A, when the thickness of the waveguide core is 300 nm, the equivalent refractive index of the fundamental mode TE polarized wave (TE0) and the equivalent refractive index of the fundamental mode TM polarized wave (TM0) are: This is the same when the waveguide core width is 300 nm.

  This is because the waveguide cross section has a square structure, and the effective refractive index felt from the waveguide structure is isotropic between the TE polarized light and the TM polarized light. Simply, if the cross section of the phase adjusting arm waveguide in the interferometer wavelength filter has a square structure, the polarization dependence can be eliminated. However, especially in the vicinity of the width of the waveguide core that does not depend on the polarization, the amount of fluctuation of the equivalent refractive index of the TE polarization is large. For this reason, it is easy to be affected by width variation due to manufacturing errors.

  Further, as shown in FIG. 8B, even when the waveguide core thickness is 220 nm, the fundamental mode is obtained under the condition that the waveguide cross section has a square structure as in the case where the waveguide core thickness is 300 nm. The equivalent refractive index of the TE polarized wave (TE0) matches the equivalent refractive index of the TM polarized wave (TM0) in the fundamental mode.

  However, the absolute value of the equivalent refractive index at this time is about 1.52, which is close to the mode cutoff region where the confinement in the waveguide core is weak. When close to the mode cut-off region, confinement in the waveguide core is weakened, so that it is necessary to increase the thickness of the cladding in order to ensure optical isolation from the support substrate. Further, in order to suppress the loss in the bending waveguide and the crosstalk between the parallel waveguides, the bending radius and the wiring pitch must be increased. Therefore, the advantage of downsizing the entire optical device obtained by using the Si waveguide as a platform cannot be obtained.

  As described above, when the waveguide cross section has a square structure, the thickness of the applicable waveguide core is limited when it is attempted to reduce the polarization dependence without being affected by the width variation due to the manufacturing error. For this reason, there is a need for a technique for suppressing polarization dependence that is not easily affected by fluctuations in width due to manufacturing errors and that can be applied to various waveguide core thicknesses in a phase adjustment arm waveguide.

As a technique for suppressing the polarization dependence of the interferometer wavelength filter, there is a technique using a wide waveguide resistant to manufacturing errors (for example, see Patent Document 1). The wavelength filter disclosed in Patent Document 1 is provided with a phase adjustment region A and a phase adjustment region B having two different widths in a phase adjustment arm waveguide in an interferometer wavelength filter, and is equivalent in each phase adjustment region. the polarization difference in refractive index [Delta] n _a and [Delta] n _b, the length is taken as _{L a} and _{L b} propagation direction, so that the relative relationship of its length, a _{Δn a L a + Δn b L} b = 0 Designed to.

JP 2013-57847 A

IEEE J.I. Select. Topics Quantum Electron. , Vol. 11, pp. 232-240, 2005 IEEE J.I. Select. Topics Quantum Electron. , Vol. 12, no. 6, November / December 2006 p. 1371-1379

  In the technique disclosed in Patent Document 1 described above, the polarization characteristics match at the design center wavelength, but the polarization difference of the phase-related chromatic dispersion characteristics is not taken into consideration. For example, in Dense Wavelength Division Multiplexing (DWDM) in which the wavelength interval between adjacent wavelength signals is narrow, the length of the waveguide for phase adjustment needs to be correspondingly increased in order to generate a desired wavelength grid.

  However, since the magnitude of phase chromatic dispersion is proportional to the length of the waveguide, the amount of phase change becomes more sensitive to wavelength shift. Therefore, as the distance from the center wavelength increases, the divergence between the polarizations of the wavelength characteristics increases, making it difficult to use as a polarization-independent wavelength filter.

  The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to consider the phase in the interference arm waveguide and its chromatic dispersion characteristics, and to reduce the polarization dependence of the wavelength characteristics. An object of the present invention is to provide an optical waveguide element that functions as a filter.

  In order to achieve the above-described object, the optical waveguide device of the present invention is configured to include a first waveguide and a second waveguide that are spaced apart from each other and arranged in parallel. Along with the light propagation direction, there are provided p + 1 (p is an integer of 1 or more) couplers and p arm waveguides that are alternately connected in series. In each coupler, the first waveguide and the second waveguide are close to a distance that allows mutual coupling of light. In each arm waveguide portion, the first waveguide includes a first routing waveguide, and the second waveguide includes a second routing waveguide. Each arm waveguide section includes two or more phase adjustment regions having different waveguide structures, and the phase adjustment region is provided in one or both of the first waveguide and the second waveguide. The first routing waveguide and the second routing waveguide have the same structure and the same length, and the phase difference between the first waveguide and the second waveguide in the arm waveguide portion is TE polarization and TM polarization. It is the same between the waves, and the magnitude of phase chromatic dispersion is the same between the TE polarized wave and the TM polarized wave.

  According to the optical waveguide device of the present invention, in the phase adjustment region of the interferometer wavelength filter, the width dimension that is parallel to the substrate plane and perpendicular to the light propagation direction is larger than the thickness dimension that is perpendicular to the substrate plane. It can be taken big. For this reason, compared with the case where the cross section of a waveguide is made into a square structure, the characteristic fluctuation tolerance with respect to a manufacturing error can be strengthened. Also, by making the chromatic dispersion characteristics of the phase equal between the TE polarized wave and the TM polarized wave, the divergence between the polarized waves of the wavelength characteristics can be suppressed even in DWDM having a narrow wavelength grid.

It is the schematic for demonstrating a 1st optical waveguide element. It is the schematic for demonstrating the other structural example of a 1st optical waveguide element. It is the schematic for demonstrating a 2nd optical waveguide element. It is the schematic for demonstrating a 3rd optical waveguide element. It is a figure which shows the width | variety W1, W2 of a 1st phase adjustment area | region and a 2nd phase adjustment area | region, and the relationship between the polarization differences of chromatic dispersion. It is a figure which shows the calculation spectrum of the wavelength filter of the Mach-Zehnder interferometer type | mold which connected the 1st optical waveguide element in two steps. It is a figure which shows the calculation spectrum of the wavelength filter of a Mach-Zehnder interferometer type | mold which connected the 2nd optical waveguide element in two steps. It is a figure which shows the relationship between the core width of Si waveguide, and the equivalent refractive index of a fundamental mode.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(First embodiment)
An optical waveguide device (hereinafter also referred to as a first optical waveguide device) according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining the first optical waveguide device. FIG. 1 is a plan view of a waveguide core. In the following description, the expression “waveguide” includes the waveguide core and the surrounding cladding.

  The first optical waveguide element includes a first waveguide 10 and a second waveguide 20 arranged in parallel. Further, the first waveguide 10 and the second waveguide 20 constitute a first coupler 101, an arm waveguide unit 201, and a second coupler 102 in series along the light propagation direction.

  The first coupler 101 and the second coupler 102 are configured such that the first waveguide 10 and the second waveguide 20 are arranged in parallel to each other with a distance capable of optical coupling. The first coupler 101 and the second coupler 102 have equivalent branching characteristics for the TE polarization and the TM polarization. That is, the first coupler 101 and the second coupler 102 function as a polarization-independent directional coupler. A polarization-independent directional coupler can be realized by setting the width of the waveguide cores of the first waveguide 10 and the second waveguide 20 and the arrangement interval of the waveguide cores to appropriate values.

  The first waveguide 10 included in the arm waveguide unit 201 includes a first phase adjustment region 301 and a first routing waveguide that does not contribute to phase adjustment. The second waveguide 20 included in the arm waveguide unit 201 includes a second phase adjustment region 302 and a second routing waveguide that does not contribute to phase adjustment.

  The first waveguide 10 included in the arm waveguide unit 201 includes, from the first coupler 101 side, a first curved waveguide 311, a first tapered waveguide 321, a first phase adjustment region 301, a second tapered waveguide 322, The third tapered waveguide 323, the second curved waveguide 312, the third tapered waveguide 323, the second tapered waveguide 322, the first phase adjustment region 301, the first tapered waveguide 321 and the first curved waveguide 311 are sequentially arranged. Connected and configured. The second waveguide 20 included in the arm waveguide unit 201 includes a first curved waveguide 311, a first tapered waveguide 321, a second tapered waveguide 322, and a second phase adjustment region from the first coupler 101 side. 302, a third tapered waveguide 323, a second curved waveguide 312, a third tapered waveguide 323, a second phase adjustment region 302, a second tapered waveguide 322, a first tapered waveguide 321 and a first curved waveguide 311. Are connected in order.

  Of the first waveguide 10 included in the arm waveguide unit 201, a portion other than the first phase adjustment region 301, that is, the first curved waveguide 311, the first tapered waveguide 321, the second tapered waveguide 322, The three taper waveguide 323 and the second curved waveguide 312 are also referred to as a first routing waveguide. In addition, in the second waveguide 20 included in the arm waveguide unit 201, a part other than the second phase adjustment region 302, that is, the first curved waveguide 311, the first tapered waveguide 321, and the second tapered waveguide 322. The third tapered waveguide 323 and the second curved waveguide 312 are also referred to as a second routing waveguide.

  The first phase adjustment region 301 and the second phase adjustment region 302 have different waveguide core widths (core widths). The first taper waveguide 321 has a core width at one end equal to the core width of the first curved waveguide 311 and a core width at the other end equal to the core width of the first phase adjustment region 301. The second taper waveguide 322 has a core width at one end equal to the core width of the first phase adjustment region 301 and a core width at the other end equal to the core width of the second phase adjustment region 302. The third taper waveguide 323 has a core width at one end equal to the core width of the second phase adjustment region 302 and a core width at the other end equal to the core width of the second curved waveguide 312.

  The first routing waveguide and the second routing waveguide have the same structure and the same length. Accordingly, there is no phase difference between propagating light in the first routing waveguide and the second routing waveguide.

  Moreover, since the core widths of the first waveguide 10 and the second waveguide 20 are continuously changed by the first to third tapered waveguides 321 to 323, the loss of light can be suppressed.

Here, the equivalent refractive indexes for the TE polarization and the TM polarization in the first phase adjustment region 301 are _{denoted as} N _1TEi and N _1TMj , respectively. In addition, the equivalent refractive indexes for the TE polarization and the TM polarization in the second phase adjustment region 302 are _{denoted as} N _2TEi and N _2TMj , respectively. Here, i and j are integers of 0 or more, and indicate the order of the mode propagating in each phase adjustment region. Further, assuming that the total length in the propagation direction of the first phase adjustment region 301 and the second phase adjustment region 302 is L ₁ and L ₂ , respectively, between the first waveguide 10 and the second waveguide 20 in the arm waveguide section 201. (1) and (2) are given for the TE polarized wave and the TM polarized wave, respectively.

In order for the polarization characteristics of the wavelength filters to coincide, it is necessary that φ _TE = φ _TM . Therefore, substituting the above formulas (1) and (2) into φ _TE = φ _TM and rearranging gives the following formula (3).

  The above equation (3) is a condition for the polarization characteristics to be independent of the wavelength characteristic at the design center wavelength.

  Next, consider the wavelength dispersion of the phase in the phase adjustment region. When the above equations (1) and (2) are differentiated by the wavelength λ, the following equations (4) and (5) are obtained.

In order for the wavelength dispersion characteristics to match between the two polarizations, it is necessary that ∂φ _TEi / ∂λ = ∂φ _TMj / ∂λ. Therefore, when organized by substituting the above equation (4) and (5) to _{∂φ TEi / ∂λ = ∂φ TMj /} ∂λ, of formula (6) and (7) below is obtained.

Here, the above equation (7) gives an equation defining a group refractive index N _g.

  Here, since it is a premise that the polarization characteristics of the center wavelength match, the above equation (3) needs to be satisfied. Therefore, substituting the above equation (3) into the above equation (6) for rearrangement yields the following equation (8).

Here, the above equation (3) can be established by adjusting the relative relationship of the lengths regardless of the structures of the first phase adjustment region and the second phase adjustment region. On the other hand, the left side of the above equation (8) is an index obtained from the result including the relative relationship between the lengths of the first phase adjustment region and the second phase adjustment region. However, the first phase adjusting region and the second phase adjustment equivalent refractive index N in the region and the group index N _g, thus determined by the waveguide structure of the first phase adjusting region and the second phase adjusting region. For this reason, the left side of the above equation (8) is not necessarily 0, and it is necessary to separately examine an optimal combination of the waveguide structures of the first phase adjustment region and the second phase adjustment region.

Incidentally, when the coefficient of L ₁ in the above equation (3) is positive, the arrangement relationship of the first phase adjusting region and the second phase adjusting region, as shown in FIG. 1, a first phase adjustment to the first waveguide A region is provided, and a second phase adjustment region is provided in the second waveguide. In contrast, when the coefficient of L ₁ in the above equation (3) is negative, the arrangement relationship of the first phase adjusting region and the second phase adjusting region, as shown in FIG. 2, the first waveguide and the second One of the waveguides (here, the first waveguide) is provided with a first phase adjustment region and a second phase adjustment region.

Here, the coefficients of L ₁ in the above equation (3) becomes negative, in one of the phase adjustment area, greater thickness than the width of the waveguide core, a case where the thickness than the width in the smaller other. Otherwise, in both the phase adjusting region, the greater thickness than the width of the waveguide core, or in both the phase control region, when the thickness than the width of the waveguide core is small, the coefficient of L ₁ is positive Become.

(Second Embodiment)
An optical waveguide element according to a second embodiment of the present invention (hereinafter also referred to as a second optical waveguide element) will be described with reference to FIG. FIG. 3 is a schematic view for explaining the second optical waveguide device. FIG. 1 is a plan view of a waveguide core.

  The second optical waveguide element is different from the first optical waveguide element in that it includes three phase adjustment regions.

  The first waveguide 11 included in the arm waveguide unit 251 includes, from the first coupler 151 side, a first curved waveguide 311, a first tapered waveguide 321, a first phase adjustment region 301, a second tapered waveguide 322, The second phase adjustment region 302, the third taper waveguide 323, the fourth taper waveguide 324, the second curved waveguide 312, the fourth taper waveguide 324, the third taper waveguide 323, the second phase adjustment region 302, the second The second taper waveguide 322, the first phase adjustment region 301, the first taper waveguide 321, and the first curved waveguide 311 are connected in order to reach the second coupler 152. Further, the second waveguide 21 included in the arm waveguide section 251 includes the first curved waveguide 311, the first tapered waveguide 321, the second tapered waveguide 322, and the third tapered waveguide from the first coupler 151 side. 323, third phase adjustment region 303, fourth taper waveguide 324, second curved waveguide 312, fourth taper waveguide 324, third phase adjustment region 303, third taper waveguide 323, second taper waveguide 322. The first tapered waveguide 321 and the first curved waveguide 311 are connected in order to reach the second coupler 152.

  Of the first waveguide 11 included in the arm waveguide section 251, portions other than the first phase adjustment region 301 and the second phase adjustment region 302, that is, the first curved waveguide 311, the first taper waveguide 321, the first The two taper waveguide 322, the third taper waveguide 323, the fourth taper waveguide 324, and the second curved waveguide 312 are also referred to as a first routing waveguide. In addition, in the second waveguide 21 included in the arm waveguide portion 251, a portion other than the third phase adjustment region 303, that is, the first curved waveguide 311, the first tapered waveguide 321, and the second tapered waveguide 322. The third tapered waveguide 323, the fourth tapered waveguide 324, and the second curved waveguide 312 are also referred to as second routing waveguides.

  The first phase adjustment region 301, the second phase adjustment region 302, and the third phase adjustment region 303 have different waveguide core widths. The first taper waveguide 321 has a core width at one end equal to the core width of the first curved waveguide 311 and a core width at the other end equal to the core width of the first phase adjustment region 301. The second taper waveguide 322 has a core width at one end equal to the core width of the first phase adjustment region 301 and a core width at the other end equal to the core width of the second phase adjustment region 302. The third taper waveguide 323 has a core width at one end equal to the core width of the second phase adjustment region 302 and a core width at the other end equal to the core width of the third phase adjustment region 303. The fourth taper waveguide 324 has a core width at one end equal to the core width of the third phase adjustment region 303 and a core width at the other end equal to the core width of the second curved waveguide 312. The first routing waveguide and the second routing waveguide have the same structure and the same length, and there is no phase difference between propagating light in the first routing waveguide and the second routing waveguide. . Moreover, since the core width of the first waveguide 11 and the second waveguide 21 is continuously changed by the first to fourth tapered waveguides 321 to 324, the loss of light can be suppressed.

Assuming that the total length in the propagation direction of the first phase adjustment region 301, the second phase adjustment region 302, and the third phase adjustment region 303 is L ₁ , L ₂ , and L ₃ , respectively, The significant phase difference that occurs between the two waveguides is given by the following equations (9) and (10) for TE polarization and TM polarization, respectively.

In order for the polarization characteristics of the wavelength filters to coincide, it is necessary that φ _TE = φ _TM . Therefore, substituting the above formulas (9) and (10) into φ _TE = φ _TM and rearranging results in the following formula (11).

The above equation (11) is a condition for the wavelength characteristics at the design center wavelength to be polarization independent. Here, ΔN _k = N _kTEi− N _kTMj (k = 1, 2, 3).

Next, consider the wavelength dispersion of the phase in the phase adjustment region. When the above equations (9) and (10) are differentiated by the wavelength λ and substituted into に_{φ TEi} / ∂λ = ∂φ _TMj / ∂λ and arranged, the following equation (12) is obtained.

Here, ΔN _gk = N _gkTEi −N _gkTMj .

  By arranging the above equations (11) and (12), the following equations (13) and (14) are obtained.

As shown in the above equations (13) and (14), the lengths of the second phase adjustment region 302 and the third phase adjustment region 303 are uniquely determined based on the length L ₁ of the first phase adjustment region 301. . That is, in the second optical waveguide element having three phase adjustment regions, the waveguide structure of the phase adjustment region can be arbitrarily set.

(Third embodiment)
An optical waveguide element according to a third embodiment of the present invention (hereinafter also referred to as a third optical waveguide element) will be described with reference to FIG. FIG. 4 is a schematic diagram for explaining the third optical waveguide element. FIG. 4 is a plan view of the waveguide core.

  P + 1 (p is an integer of 1 or more) couplers 150-1 to 150-p + 1 and p arm waveguide sections 250-1 to 250-p, which are alternately connected in series. The q-th (q is an integer from 1 to p) coupler, the q-th arm waveguide section, and the q + 1-th coupler constitute a first optical waveguide element or a second optical waveguide element. That is, the third optical waveguide element is configured as a multistage Mach-Zehnder interferometer. Each arm waveguide portion can be designed in the same manner as the first or second optical waveguide element described above.

  According to the third optical waveguide element, desired wavelength characteristics can be obtained such as flattening the wavelength characteristics.

(Effect of the present invention)
According to the first to third optical waveguide elements described above, in the phase adjustment region of the interferometer wavelength filter, the thickness is perpendicular to the substrate plane and is parallel to the substrate plane and the light propagation direction. A perpendicular width dimension can be increased. For this reason, compared with the case where the cross section of a waveguide is made into a square structure, the characteristic fluctuation tolerance with respect to a manufacturing error can be strengthened. Also, by making the chromatic dispersion characteristics of the phase equal between the TE polarized wave and the TM polarized wave, the divergence between the polarized waves of the wavelength characteristics can be suppressed even in DWDM having a narrow wavelength grid.

(Example)
Hereinafter, a design using an SOI (Silicon On Insulator) substrate will be described. A BOX (Buried Oxide) layer is used as a lower cladding, and an SOI layer is used as a waveguide core. Furthermore, an upper clad is formed in which the waveguide core is embedded with a material having a refractive index equivalent to that of the BOX layer, such as SiO ₂ . In this way, a Si waveguide is formed. In the following description, for the sake of simplicity, both the TE polarized wave and the TM polarized wave propagating in the phase adjustment region will be described as a basic mode (i = j = 0).

  First, the first optical waveguide element will be described. The thickness of the waveguide core is, for example, 300 nm. If the core widths of the first waveguide and the second waveguide are equal to 288 nm and the distance between the centers of the first waveguide and the second waveguide is 350 nm, the branch characteristic of the directional coupler can be made polarization independent. Are known. The output branching ratio in each coupler is adjusted by the length of the coupler.

  As means for giving different structures to each other in the phase adjustment region, the thickness of the waveguide core and the difference in the surrounding cladding can be used. However, the core width is changed here in order not to add an extra step to the manufacturing process.

  The optimal combination of the core widths of the first phase adjustment region and the second phase adjustment region, that is, the left side of the above equation (8) is closest to 0 under the conditions of the analysis center wavelength of 1550 nm and the waveguide core thickness of 300 nm. Considered the combination.

  FIG. 5 is a diagram illustrating the relationship between the core widths W1 and W2 of the first phase adjustment region and the second phase adjustment region and the difference between the polarizations of chromatic dispersion. In FIG. 5, the horizontal axis represents the core width W2 of the second phase adjustment region, and the vertical axis represents the difference between the polarizations of chromatic dispersion. The difference between the polarizations of chromatic dispersion is given by the left side of the above equation (8).

As shown in FIG. 5, when W1 is 600 nm and W2 is 550 nm, the difference between polarizations is closest to zero. In this combination, the relative relationship between the lengths of the first phase adjustment region and the second phase adjustment region is L ₂ / L ₁ = 1.049 from the above equation (3).

  Reflecting the above parameters, a two-stage Mach-Zehnder interferometer type wavelength filter was designed. FIG. 6 is a diagram showing a calculated spectrum at this time. In FIG. 6, the horizontal axis indicates the wavelength [unit: μm] and the vertical axis indicates the signal intensity [unit: dB] of the output of the wavelength filter. Here, TE-BAR and TM-BAR indicate a case where TE polarization and TM polarization input to the first waveguide are output from the first waveguide, and TE-CROSS and TM-CROSS are The case where the TE polarization and TM polarization input to one waveguide are output from the second waveguide is shown. 6A represents the range of wavelengths from 1.545 μm to 1.555 μm, and FIG. 6B represents the range of wavelengths from 1.545 μm to 1.547 μm.

  Here, assuming DWDM, the branching ratio in each coupler, the arrangement relationship and the length of the phase adjustment region were adjusted so that the wavelength grid would be 1.0 nm. As shown in FIG. 6A, even when the center wavelength is away from 1.550 μm, the deviation from the TE polarization and TM polarization of the wavelength characteristics can be suppressed. Therefore, DWDM with a relatively narrow wavelength interval can be sufficiently applied. As shown in FIG. 6B, when the center wavelength is away from 1.550 μm, although there is a deviation between the TE polarization and the TM polarization, the correlation coefficient between the TE polarization and the TM polarization is 0 as a whole. .75 to 0.76, indicating a good correlation.

  Next, the second optical waveguide element will be described. In the second optical waveguide element, the trouble of optimizing the combination of structures can be omitted.

First, the waveguide widths W1 to W3 of the first to third phase adjustment regions are provisionally determined as W1 = 600 nm, W2 = 350 nm, and W3 = 450 nm, respectively. Thereby, the equivalent refractive index and the group refractive index in the first to third phase adjustment regions are uniquely determined. , L _{1 to} L ₃ are expressed as L ₂ / L ₁ = 0.7538 and L ₃ / L ₁ = 1.6440 from the above equations (13) and (14). That is, L ₂ and L ₃ can be determined based on L ₁ .

The design freedom in each arm waveguide is only L _1, but the length of L ₁ can be adjusted, and if a multi-stage configuration is used, an arbitrary phase is given to each arm waveguide section. Can do. Therefore, desired characteristics can be obtained.

  Reflecting the above parameters, a two-stage Mach-Zehnder interferometer type wavelength filter was designed. FIG. 7 is a diagram showing a calculated spectrum at this time. In FIG. 7, the horizontal axis indicates the wavelength [unit: μm], and the vertical axis indicates the signal intensity [unit: dB] of the output of the wavelength filter. Here, TE-BAR and TM-BAR indicate a case where TE polarization and TM polarization input to the first waveguide are output from the first waveguide, and TE-CROSS and TM-CROSS are The case where the TE polarization and TM polarization input to one waveguide are output from the second waveguide is shown. FIG. 7A shows a wavelength range of 1.545 μm to 1.555 μm, and FIG. 7B shows a wavelength range of 1.545 μm to 1.547 μm.

  Assuming DWDM, the branching ratio in each coupler, the arrangement relationship and the length of the phase adjustment region were adjusted so that the wavelength grid would be 1.0 nm.

  As shown in FIG. 7A, the deviation from the TE polarization and TM polarization of the wavelength characteristics can be suppressed even if the center wavelength is away from 1.550 μm. Therefore, DWDM with a relatively narrow wavelength interval can be sufficiently applied. As shown in FIG. 7B, when the distance from the center wavelength is 1.550 μm, although the deviation between the TE polarization and the TM polarization is observed, the degree of the deviation is smaller than that of the first optical waveguide element. Further, as a whole, the correlation coefficient between the TE polarization and the TM polarization is about 0.86 to 0.88, indicating a better correlation than the first optical waveguide element.

10, 11, 20, 21 Waveguide
101, 102, 150, 151, 152 coupler
201, 250, 251 Arm waveguide section
301, 302, 303 Phase adjustment region 311, 312 Curved waveguide 321, 322, 323, 324 Tapered waveguide

Claims (4)

A first waveguide and a second waveguide disposed in parallel and spaced apart from each other;
P + 1 (p is an integer of 1 or more) couplers and p arm waveguide portions, which are alternately connected in series along the light propagation direction, are provided.
In each of the couplers, the first waveguide and the second waveguide are close to a distance capable of mutual coupling of light,
In each of the arm waveguide portions, the first waveguide includes a first routing waveguide,
In each of the arm waveguide portions, the second waveguide includes a second routing waveguide,
Each of the arm waveguide portions includes two or more phase adjustment regions having different waveguide structures,
The phase adjustment region is provided in one or both of the first waveguide and the second waveguide,
The first routing waveguide and the second routing waveguide have the same structure and the same length.
The phase difference between the first waveguide and the second waveguide in the arm waveguide portion is the same between the TE polarized wave and the TM polarized wave, and the phase chromatic dispersion is equal to the TE polarized wave and the TM polarized wave. An optical waveguide device characterized by being equal between waves. Each of the arm waveguide portions includes a first phase adjustment region and a second phase adjustment region,
The equivalent refractive indexes for the TE polarization and the TM polarization in the first phase adjustment region are N _1TEi and N _1TMj , respectively, and the equivalent refractive indexes for the TE polarization and the TM polarization in the second phase adjustment region are N _2TEi and _Assuming that N _2TMj and the total length in the propagation direction of the first phase adjustment region and the second phase adjustment region are L ₁ and L ₂ , respectively, the orders i and j (i and j are 0 or more) of the modes propagating through the phase adjustment regions. The optical waveguide device according to claim 1, wherein the following formulas (1) to (3) are satisfied:

Each of the arm waveguide portions includes first to third phase adjustment regions,
The equivalent refractive indices for the TE polarization and TM polarization in the k-th (k = 1, 2, 3) phase adjustment region are N _kTEi and N _kTMj , respectively, and the total propagation length of the k-th phase adjustment region is L _k. , ΔN _gk = N _gkTEi −N _gkTMj , the following formulas (4) and (5) are satisfied for the orders i and j (i and j are integers of 0 or more) of the modes propagating through the respective phase adjustment regions. The optical waveguide device according to claim 1.

The plurality of phase adjustment regions are different from each other in at least one of a width of the waveguide core, a thickness of the waveguide core, and a cladding around the waveguide core. 4. The optical waveguide device according to any one of 3.

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