JP2013210474A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a fixed phase difference within a prescribed wavelength range and to simplify a structure.SOLUTION: An optical element comprises an optical waveguide 22 including a core 22a and a clad 22b having the refractive index of 71.4% or less of that of the core. The optical waveguide includes two optical multiplexing/demultiplexing parts C1 and C2 and an optical interference part 16 provided between the two optical multiplexing/demultiplexing parts. The optical interference part is provided with first and second arm waveguides 12 and 14 for propagating light distributed by one of the optical multiplexing/demultiplexing parts toward the other multiplexing/demultiplexing parts. The first and second arm waveguides include first and second phase adjustment areas 18 and 20 different in equivalent refractive index from each other, respectively. The first and second phase adjustment areas 18 and 20 apply a prescribed phase difference to light propagated in the first arm waveguide and light propagated in the second arm waveguide.

Description

  The present invention relates to an optical element that includes a Mach-Zehnder interferometer and can also be used as an optical multiplexing / demultiplexing element.

  In an optical subscriber communication system in which optical transmission from the subscriber side to the station side (uplink communication) and optical transmission from the station side to the subscriber side (downlink communication) are performed using one optical fiber, uplink communication In some cases, downlink communication is performed using light of different wavelengths. In this case, an optical element (hereinafter also referred to as an optical multiplexing / demultiplexing element) that multiplexes / demultiplexes light of different wavelengths is required on both the station side and the subscriber side.

  The optical multiplexing / demultiplexing element is spatially optically aligned with the light emitting element and the light receiving element, so that an optical subscriber communication system, for example, a PON (Passive Optical Network) subscriber-side terminating device (ONU: Optical Network Unit), It is used for a station side terminal device (OLT: Optical Line Terminal). However, in recent years, an optical multiplexing / demultiplexing element constituted by an optical waveguide has been developed in order to reduce the labor for aligning the optical axis (see, for example, Patent Documents 1 to 5). In the optical multiplexing / demultiplexing device using this optical waveguide, the light propagation path is limited to the optical waveguide built in advance, so that a lens, a mirror, or the like in the conventional optical multiplexing / demultiplexing device becomes unnecessary. Further, in this optical multiplexing / demultiplexing element, the light emitting element and the light receiving element may be aligned with the light incident / exit end of the optical waveguide with reference to a mark previously formed on the element. Therefore, the labor of strict optical axis alignment of the light beams entering and exiting the light emitting element and the light receiving element is greatly reduced.

In recent years, an optical multiplexing / demultiplexing device composed of an optical waveguide using Si as a core and SiO ₂ having a large refractive index difference from Si as a clad (hereinafter also referred to as Si optical waveguide) has been reported (for example, Non-patent documents 1 to 3).

  Since the refractive index of the core is much larger than that of the clad and the optical confinement is strong, the Si optical waveguide can realize a curved optical waveguide that bends light with a small radius of curvature of about 1 μm. In addition, since a processing technique using a Si electronic device can be used at the time of manufacturing, a very fine submicron cross-sectional structure can be realized. For these reasons, various optical elements can be miniaturized by using Si optical waveguides.

  As an optical multiplexing / demultiplexing device using a Si optical waveguide, a device using a Mach-Zehnder interferometer, a device using a directional coupler, a device using a grating, and the like are known.

  An optical multiplexing / demultiplexing device using a Mach-Zehnder interferometer (hereinafter also referred to as an MZ-type device) uses a property that a phase difference generated in two lights after propagating through two arm waveguides differs depending on the wavelength. Perform separation. In general, an MZ-type element includes an input coupler that distributes light to two arm waveguides, two arm waveguides, and an output coupler having two output ports. Both arm waveguides are designed to have an optical path length difference such that a phase difference that is an integral multiple of π occurs in light of a wavelength to be separated (hereinafter also referred to as a target wavelength). Then, based on the phase difference generated in the light propagated through both arm waveguides, light of the target wavelength is output from one port of the output coupler, and the other light is output from the other port.

Photonics Technology Letters vol. 18, p. 2392, November 2006 Photonics Technology Letters vol. 20, p. 1968, December 2008 Optics Express vol. 18, p. 23891, October 2010

U.S. Pat. No. 4,860,294 US Pat. No. 5,764,826 US Pat. No. 5,960,135 US Pat. No. 7,072,541 JP-A-8-163028 JP 2005-250504 A JP 2002-40493 A

  However, the MZ type element that separates wavelengths by varying the phase difference generated depending on the wavelength has a problem that it is vulnerable to wavelength shift. That is, for example, wavelength fluctuations caused by a light source or the like cause phase difference fluctuations in the arm waveguide, which may cause unwanted intensity fluctuations in the output light.

  As one of solutions to this problem, an MZ type element in which Mach-Zehnder interferometers are connected in multiple stages is known (for example, see Patent Document 6). Certainly, in the MZ type element of Patent Document 6, it is possible to widen the wavelength width of the target wavelength light that is wavelength-selected and output from one of the output ports. However, this method requires a plurality of Mach-Zehnder interferometers, and the structure is inevitable. In addition, the MZ type element of this document cannot generate a phase difference that is an integral multiple of π, and cannot output light at an arbitrary distribution ratio from the two output ports. As another solution, an MZ type element in which a ring resonator is provided in an arm waveguide is known (for example, see Patent Document 7). However, even in this method, it is necessary to add a ring resonator to the MZ type element, and the structure is inevitable.

  The present invention has been made under such a technical background. Accordingly, an object of the present invention is to obtain an optical element that is resistant to wavelength shift, has a simple structure, and can be applied to an MZ-type element because it generates a certain phase difference in light within a predetermined wavelength range. is there.

  As a result of intensive studies, the inventor has conceived that by providing a phase adjustment region in each of the two arm waveguides, a phase difference independent of the wavelength can be generated in the light propagated through both arm waveguides. Accordingly, the optical element of the present invention includes an optical waveguide composed of a core and a clad having a refractive index of 71.4% or less of the core. The optical waveguide includes two optical multiplexing / demultiplexing units and an optical interference unit provided between the two optical multiplexing / demultiplexing units. The optical interference unit includes first and second arm waveguides for propagating light distributed by one optical multiplexing / demultiplexing unit to the other optical multiplexing / demultiplexing unit, respectively. The first and second arm waveguides have first and second phase adjustment regions, respectively, and these first and second phase adjustment regions have different equivalent refractive indexes and propagate through the first arm waveguide. A predetermined phase difference is given to the light that propagates and the light that propagates through the second arm waveguide.

  The present invention is configured as described above. Therefore, according to the present invention, since a constant phase difference can be generated within a predetermined wavelength range in propagating light, an optical element applicable to an MZ type element that is strong against wavelength shift and has a simple structure can be obtained.

(A) is a schematic diagram schematically showing the structure of a conventional MZ type element, (B) is a characteristic diagram schematically showing the wavelength separation characteristics of (A), and (C) is an MZ of the present invention. It is a schematic diagram which shows typically the structure of a type | mold element, (D) is a characteristic view which shows the wavelength separation characteristic of (C) typically. It is a schematic diagram which shows the schematic structure of a general MZ type | mold element with output light. It is a schematic diagram which shows the schematic structure of the MZ type | mold element of this invention with to-be-selected light and 3rd light. (A) is a top view which shows schematically the structure of the MZ type | mold element of embodiment, (B) is a cut end view along the AA line of (A). (A) is a characteristic diagram showing the wavelength dependence of the equivalent refractive index of the first and second phase adjustment regions, and (B) is an interference condition in the first and second phase adjustment regions shown in (A). It is a figure which shows La and Lb which satisfy | fills with an interference order. It is a characteristic view which shows the wavelength separation characteristic of the MZ type | mold element of embodiment.

  Embodiments of the present invention will be described below with reference to the drawings. In each figure, the shape, size, and arrangement relationship of each component are schematically shown to the extent that the present invention can be understood. Moreover, although the preferable structural example of this invention is demonstrated hereafter, the material of each component, a numerical condition, etc. are only a suitable example. Therefore, the present invention is not limited to the following embodiments. Moreover, in each figure, the same code | symbol is attached | subjected to a common component and the description may be abbreviate | omitted. In addition, reference numerals of components that have a clear correspondence with other drawings may be omitted.

(Summary of Invention)
Prior to the description of the specific configuration, the features of the present invention will be outlined by comparison with a conventional MZ type element (hereinafter also referred to as a conventional element) with reference to FIG. FIG. 1A is a schematic diagram schematically showing the structure of a conventional element. FIG. 1B is a characteristic diagram schematically showing wavelength separation characteristics of a conventional element. FIG. 1C is a schematic view schematically showing the structure of the MZ type element of the present invention. FIG. 1D is a characteristic diagram schematically showing the wavelength separation characteristic of the MZ type element of the present invention. In FIGS. 1A and 1C, an MZ-type element is simply illustrated in order to help understand the invention. That is, the illustration of the substrate and the clad is omitted, the core is drawn with a simple curve, and each component is drawn with a rectangular box.

  First, a general conventional element will be briefly described with reference to FIGS. The conventional element 50 includes two optical multiplexing / demultiplexing units C1 and C2 and an optical interference unit 56. Furthermore, the conventional element 50 includes input ports in1 and in2 and output ports ot1 and ot2.

  One optical multiplexing / demultiplexing unit C1 is a so-called 3 dB coupler that equally distributes the input light IN input from the input port in1 to the first and second arm waveguides 52 and 54 and outputs the same. Hereinafter, the arm waveguide is also simply referred to as “arm”. The other optical multiplexing / demultiplexing unit C2 is a 3 dB coupler that multiplexes the light propagated through the first and second arms 52 and 54. The combined light is output from the output port ot1 or ot2 as wavelength-selected first and second output lights OUT1 and OUT2. Here, the input light IN is white light including light of all wavelengths with equal intensity.

  The optical interference unit 56 includes first and second arms 52 and 54 having different optical path lengths, and selects a wavelength by generating a phase difference corresponding to the wavelength in light propagating through the arms 52 and 54. In this example, the optical path lengths of the first and second arms 52 and 54 are such that the light having the first wavelength λ1 is output from the output port ot2, and the light having a wavelength other than λ1 is output from the output port ot1. Is set to

  Here, the “optical path length” generally indicates an optical length obtained by correcting the geometric length L of the optical waveguide with the equivalent refractive index q of the optical waveguide with respect to the propagation light having a certain wavelength λ. . Assuming that the optical path length is S, S = L × q is established between S, L, and q.

  Next, the wavelength separation characteristics of the conventional element 50 will be described with reference to FIG. The horizontal axis of FIG. 1B is the wavelength of an arbitrary unit of the second output light OUT2. The vertical axis indicates the intensity of the second output light OUT2 output after wavelength separation from the output port ot2 in arbitrary units. The intensity of the second output light OUT2 exhibits the same behavior as the phase difference φ generated in the optical interference unit 56.

  As described above, when the optical path length difference between the first and second arms 52 and 54 is optimized to the phase difference that causes the light of the first wavelength λ1 to be output from the output port ot2, the second at this wavelength λ1. The intensity of the output light OUT2 is maximized. The light of the second wavelength λ2 (≠ λ1) has a phase difference different from that of the first wavelength λ1 in the optical interference unit 56, and therefore the output intensity is significantly smaller than that of the first wavelength λ1. Light that is included in the input light IN and is not output from the output port ot2 is output from the output port ot1 as the first output light OUT1.

  Next, the MZ type element of the present invention will be described with reference to FIGS. According to FIG. 1 (C), the MZ-type element 10 includes the first and second arms 12 and 14 constituting the optical interference unit 16 except that the first and second phase adjustment regions 18 and 20 are provided, respectively. The configuration is the same as that of the conventional element 50.

  Here, the lights of the first and second wavelengths λ1 and λ2 related to the wavelength selection in the MZ type element 10 of the present invention are referred to as first and second lights, respectively. The wavelength range λ1 to λ2 between the first and second wavelengths λ1 and λ2 is also referred to as “selected range”, and the light in the selected range including the first and second lights is also referred to as “selected light”.

  The first and second phase adjustment regions 18 and 20 are regions for adjusting a phase difference that occurs in the light after propagation through the first and second arms 12 and 14. In order to adjust the phase difference, the equivalent refractive indexes na and nb of the first and second phase adjustment regions 18 and 20 are set to different values. Specifically, na ≠ nb is satisfied by changing the structural parameter of the optical waveguide, for example, the width. By optimizing the geometric lengths La and Lb along the light propagation direction of the first and second phase adjustment regions 18 and 20 under na ≠ nb, the MZ element 10 can select the selected range (λ1 ~ Λ2), a constant phase difference is generated in the propagating light. Here, the optical waveguide region of the first arm 12 other than the first phase adjustment region 18 is also referred to as the first propagation region 17, and the optical waveguide region of the second arm 14 other than the second phase adjustment region 20 is the second propagation region. Also referred to as 19.

  Next, the wavelength separation characteristics of the MZ element 10 will be described with reference to FIG. The vertical and horizontal axes in FIG. 1D are the same as those in FIG. As a result of providing the first and second phase adjustment regions 18 and 20 described above, the intensity of the second output light OUT2 output from the MZ type element 10 is constant in the wavelength range A including the selected range. That is, the MZ-type element 10 of the present invention provides the first and second phase adjustment regions 18 and 20 to provide a constant phase difference φ regardless of the wavelength with respect to light having a wavelength within the wavelength range A. Cause it to occur.

  Next, referring to FIG. 1C as appropriate, the operation principle of the MZ element 10, that is, the first and second phase adjustment regions 18 and 20 have a constant phase difference φ in the propagating light regardless of the wavelength. The principle to be generated will be described.

  In the MZ-type element 10, the interference condition for outputting the selected light from either one of the output ports ot1 and ot2 is given by the following equations (1) and (2). Expression (1) is an interference condition regarding the first light having the first wavelength λ1, and Expression (2) is an interference condition regarding the second light having the second wavelength λ2.

i = na _λ1 × La / λ1-nb _λ1 × Lb / λ1 (1)
i = na _λ2 × La / λ2-nb _λ2 × Lb / λ2 (2)
Here, na _λ1 and nb _λ1 are equivalent refractive indexes of the first light in the first and second phase adjustment regions 18 and 20, respectively. Similarly, na _λ2 and nb _λ2 are the equivalent refractive indices of the first and second phase adjustment regions 18 and 20 with respect to the second light, respectively.

  As will be described in detail later, i is the interference order, and when the selected light is output from either one of the output ports ot1 and ot2, it is a multiple of 1/2 including 0. When the selected light is distributed to the output ports ot1 and ot2 and output from both simultaneously, i is a positive real number excluding a multiple of 1/2. Note that 2iπ, which is obtained by multiplying the interference order i on the left side of Expression (1) by 2π, is the first phase difference that is the sum of the phase differences given by the optical interference unit 16 with the first light of the first wavelength λ1. Similarly, 2iπ obtained by multiplying the interference order i on the left side of Expression (2) by 2π is the second phase difference that is the sum of the phase differences given by the optical interference unit 16 to the second light having the second wavelength λ2. .

  In this example, the optical path lengths of the first and second propagation regions 17 and 19 are equal to each other. This is because the phase difference derived from the first and second propagation regions 17 and 19 does not occur in the selected light, and the phase difference necessary for wavelength selection is present only in the first and second phase adjustment regions 18 and 20. It means to occur. When the equivalent refractive indexes of the first and second propagation regions 17 and 19 are equal to nc, the geometric lengths of the first and second propagation regions 17 and 19 are mutually equal from the above definition “S = L × q”. It is led to be equal.

  By applying the above definition of the optical path length to the expressions (1) and (2), the following expressions (3) and (4) are obtained.

i = _Saλ1 / λ1- _Sbλ1 / λ1 (3)
i = Sa _{λ 2} / λ 2 −Sb _{λ 2} / λ 2 (4)
Here, Sa _λ1 and Sb _λ1 are optical path lengths of the first light in the first and second phase adjustment regions 18 and 20, respectively. Similarly, Sa _λ2 and Sb _λ2 are optical path lengths related to the second light in the first and second phase adjustment regions 18 and 20, respectively.

  The MZ element 10 includes the first phase adjustment region 18 in the first arm 12, but the second arm 14 does not have a region corresponding to the first phase adjustment region 18. In other words, it can be interpreted that the second arm 14 includes a virtual first phase adjustment region having a length of 0 (zero). Similarly, the first arm 12 can be interpreted as including a virtual second phase adjustment region having a length of 0 (zero).

From this interpretation, the optical path lengths Sa _λ1 , Sb _λ1 , Sa _λ2 and Sb _λ2 in the equations (3) and (4) can be replaced with optical path length differences ΔSa _λ1 , ΔSb _λ1 , ΔSa _λ2 and ΔSb _λ2 respectively. 5) and (6) are obtained.

i = ΔSa _λ1 / λ1−ΔSb _λ1 / λ1 (5)
i = ΔSa _{λ 2} / λ 2 −ΔSb _{λ 2} / λ 2 (6)
Here, ΔSa _λ1 is expressed as (Sa _λ1 −0), and the first phase adjustment region 18 of the first arm 12 and the first phase adjustment region of the virtual length 0 (zero) of the second arm 14 Is the optical path length difference with respect to the first wavelength λ1. ΔSa _λ2 is expressed as (Sa _λ2 −0), and the optical path related to the second wavelength λ2 between the first phase adjustment region 18 of the first arm 12 and the virtual first phase adjustment region of the second arm 14. It is a long difference.

Similarly, ΔSb _λ1 is expressed as (Sb _λ1 −0), and the second phase adjustment region 20 of the second arm 14 and the second phase adjustment region of the virtual length 0 (zero) of the first arm 12 Is the optical path length difference with respect to the first wavelength λ1. ΔSb _λ2 is expressed as (Sb _λ2 −0), and the optical path related to the second wavelength λ2 between the second phase adjustment region 20 of the second arm 14 and the virtual second phase adjustment region of the first arm 12. It is a long difference.

  In general, with respect to light of wavelength λ, using the relationship of ΔL = ΔS / refractive index between the optical path length difference ΔS and the actual optical path length difference ΔL, from the equations (5) and (6), Equations (7) and (8) are obtained.

ΔLa = i (na _λ2 × λ1-na _λ1 × λ2) / (na _λ2 × nb _λ1- na _λ1 × nb _λ2 ) (7)
ΔLb = i (nb _λ2 × λ1-nb _λ1 × λ2) / (− na _λ2 × nb _λ1 + na _λ1 × nb _λ2 ) (8)
ΔLa is a difference in length between the first phase adjustment regions provided in the first and second arms 12 and 14, respectively. Similarly, ΔLb is a difference in length between the second phase adjustment regions provided in the first and second arms 12 and 14, respectively.

  When providing a phase adjustment region in each of the first and second arms 12 and 14, the length may be determined according to equations (7) and (8).

(Interference order and output port)
Next, the relationship between the interference order i and the output ports ot1 and ot2 from which the selected light is output will be described with reference to FIG. FIG. 2 is a schematic diagram showing a schematic structure of a general MZ-type element not including a phase adjustment region together with bars and cross output light BAR and CRS. Note that FIG. 2 is simplified as in FIG.

  The MZ type element MZ includes two optical multiplexing / demultiplexing units c01 and c02, first and second arms a01 and a02 having different optical path lengths, input ports in01 and in02, and output ports ot01 and ot02. Yes. The optical path length difference between the arms a01 and a02 is ΔSm. At this time, in the MZ type element MZ, the interference condition regarding the light of wavelength λ is given by i = ΔSm / λ.

  Schematically, the input light IN input to the input port in01 is output from the output ports ot01 and ot02 at an intensity ratio x (x is a real number of 0 ≦ x ≦ 1) corresponding to the phase difference generated in both arms a01 and a02. Is output. If the output lights from the output ports ot01 and ot02 are the bar and cross output lights BAR and CRS, respectively, the intensity ratio between the two lights BAR and CRS is BAR: CRS = x: (1-x). Hereinafter, this ratio x is also referred to as “distribution ratio”.

  The cross output light CRS is light that is output from the output port ot02 as the power is transferred from the first arm a01 side to the second arm a02 side in the process of propagating through the MZ type element MZ. The bar output light BAR is light output from the output port ot01 without causing a power shift from the first arm a01 side to the second arm a02 side.

In such an MZ type element MZ, there is the following relationship between the interference order i and the output ports ot01 and ot02 of the output light of the wavelength λ.
<a> When i is a half integer represented by m + 1/2 (m is an integer of 0 or more) Corresponding to the case of x = 1, only the bar output light BAR is output from the output port ot01.
<B> When i is an integer represented by m Corresponding to the case of x = 0, only the cross output light CRS is output from the output port ot02.
<C> When i is a number represented by m ± 1/4, corresponding to x = 1/2, both the output ports ot01 and ot02 output the same intensity bar and cross output light BAR and CRS, respectively. The
<D> When i is a value other than <a> to <c>, both ports have a distribution ratio x corresponding to the phase difference corresponding to i for light of wavelength λ propagating through both arms a01 and a02. Bar and cross output light BAR and CRS are output from ot01 and ot02.

  If the λ in the MZ type element MZ is the average value of the first and second wavelengths λ1 and λ2 in the MZ type element 10 of the present invention, the above description can be applied to the MZ type element 10 as it is.

  When the interference order i of the MZ type element 10 of the present invention is set to <a> and <b>, i is preferably a half integer. That is, it is preferable to design so that the selected light is output as a bar from the output port ot1. This is because the bar output is slightly more resistant to dimensional errors of the optical waveguide 22 than the cross output.

(Wavelength separation of the third light of the third wavelength λ3)
Next, referring to FIG. 3, in the MZ type element 10, the third light of the third wavelength λ3 (λ3 ≠ λ1 and λ3 ≠ λ2) different from the first and second wavelengths λ1 and λ2 is selected light. And the case of wavelength separation will be described. This corresponds to a case where the MZ type element 10 is applied to an ONU or OLT optical multiplexing / demultiplexing element used in an environment where the communication wavelength fluctuates. More specifically, the MZ type element 10 is an optical subscriber communication system in which one wavelength of upstream and downstream communication light fluctuates in the selected range λ1 to λ2 and the other wavelength does not fluctuate at the third wavelength λ3. Applicable to.

  FIG. 3 is a schematic diagram showing a schematic structure of the MZ type element together with selected light and third light. Note that FIG. 3 is simplified as in FIG.

  When the MZ type element 10 is used as such an optical multiplexing / demultiplexing element, the input light IN including the first, second, and third wavelengths λ1, λ2, and λ3 is converted into the selected range λ1 to λ2, the third wavelength λ3, and The ability to be separated is required. That is, it is necessary to output the selected light having a wavelength of λ1 to λ2 and the third light having a wavelength of λ3 from different output ports ot1 and ot2.

  The interference condition for this is given by the following ternary simultaneous equation in which the following equation (9) is added to the above equations (1) and (2).

i = na _λ1 × La / λ1-nb _λ1 × Lb / λ1 (1)
i = na _λ2 × La / λ2-nb _λ2 × Lb / λ2 (2)
(I + (j + 1/2)) = na _λ3 × La / λ3-nb _λ3 × Lb / λ3 (9)
Here, j is an integer of 0 or more. Further, na _λ3 and nb _λ3 are equivalent refractive indexes of the first and second phase adjustment regions 18 and 20 with respect to the third light, respectively.

  From equations (1), (2), and (9), it can be seen that when the interference order i is an integer, the selected light is cross-output from the output port ot2, and the third light is bar-output from the output port ot1. In addition, when the interference order i is a half integer, as shown in FIG. 3, it can be seen that the selected light is bar-output from the output port ot1 and the third light is cross-output from the output port ot2.

  The interference order (i + (j + 1/2)) in Expression (9) is a condition for outputting the third light from an output port different from the selected light. That is, the interference order (i + (j + 1/2)) set in this way generates a phase that is inverted from the interference order i. Specifically, if i is an integer, (i + (j + 1/2)) is a half integer, and the third light is output as a bar. If i is a half integer, (i + (j + 1/2)) is an integer, and the third light is cross-output.

  Note that this ternary simultaneous equation is a so-called excess linear simultaneous equation having more equations (3) than unknowns (2), so there may be no solution depending on the conditions. However, even in this case, La ′ and Lb ′, which are least square solutions of La and Lb, can be obtained by using the least square method as a next best measure. By making La 'and Lb' the lengths of the first and second phase adjustment regions 18 and 20, the light to be selected and the third light can be wavelength-separated with a practically allowable distribution ratio.

  When the MZ type element 10 is used for ONU, the input port in1 is connected to the optical fiber to the station, the light receiving element that receives the downstream optical signal is connected to one of the output ports ot1 and ot2, and the upstream optical signal is connected to the other. A light emitting element that emits light is connected. The downstream optical signal from the station is input to the input port in1 through the same path as described above, wavelength-separated by the MZ type element 10, and output from one output port ot1 or ot2. On the other hand, the upstream optical signal transmitted to the station is input from the light emitting element to the other output port ot2 or ot1 through the reverse path to that described above, multiplexed by the MZ type element 10, and transmitted from the input port in1. Coupled to optical fiber. In general, since the reverse process is established in the propagation of light, such propagation in the reverse path of the upstream optical signal is guaranteed. If the downstream optical signal and upstream optical signal of the ONU are interchanged, the MZ element 10 can also be used for the OLT.

  Further, when this MZ type element 10 is used for ONU, it is preferable to use the selected light that allows the wavelength fluctuation as the upstream optical signal. This is because the wavelength stability of the ONU light-emitting element that is the light source of the upstream optical signal is lower than that of the OLT. The wavelengths of the upstream and downstream optical signals can be about 1.49 μm and about 1.31 μm, respectively, which are common in optical subscriber communication systems.

(Optical element)
Next, with reference to FIG. 4, the structure of the MZ type | mold element 10 is demonstrated concretely. 4A is a plan view schematically showing the structure of the MZ type element 10, and FIG. 4B is a cross-sectional view taken along the line AA in FIG. 4A. In FIG. 4A, the core cannot be directly observed because it is embedded in the clad, but is shown by a solid line for emphasis.

  First, the direction and dimensions of the MZ type element 10 used in the following description are defined. The direction perpendicular to the light propagation direction of the input light IN (see arrow P in the figure) and parallel to the main surface 8a of the substrate 8 is referred to as the width direction, and the geometric length measured along the width direction is referred to as “width”. Called. In addition, a direction perpendicular to the main surface 8a is referred to as a height direction, and a geometric length measured along the height direction is referred to as “height” or “thickness”. Similarly, the geometric length measured along the light propagation direction is referred to as “length”. A section perpendicular to the light propagation direction of a predetermined structure is referred to as a “cross section”.

  The MZ element 10 includes an optical waveguide 22. The optical waveguide 22 includes a core 22a provided on the main surface 8a side of the substrate 8 and a clad 22b in which the core 22a is embedded. As described above, the optical waveguide 22 includes the two optical multiplexing / demultiplexing units C1 and C2, and the optical interference unit 16 including the first and second arms 12 and 14 between the optical multiplexing / demultiplexing units C1 and C2. And have. The first arm 12 is provided with a first phase adjustment region 18, and the second arm 14 is provided with a second phase adjustment region 20.

  Except for the first phase adjustment region 18 and the optical multiplexing / demultiplexing portions C1 and C2, the core 22a is formed in a square cross-sectional shape in which both the height H and the width W are about 300 nm. That is, in this example, the second phase adjustment region 20 has the same cross-sectional shape as the second arm 14. The first phase adjustment region 18 is formed in a rectangular cross-sectional shape having a height of about 300 nm and a width W of about 500 nm. The height H and width W of the core 22a are preferably selected from values in the range of about 200 to 500 nm. By setting the height H and the width W within this range, the optical waveguide 22 can be set to a single mode in both the height direction and the width direction. In this example, the constituent material of the core 22a is Si having a refractive index n1 of about 3.47.

The clad 22b is a film body provided on the main surface 8a which is a flat surface. The thickness of the clad 22b is about 4 μm in this example. The distance between the lower surface of the core 22a embedded in the clad 22b and the main surface 8a is about 2 μm. In order to prevent undesired coupling of light to the substrate 8, a cladding 22b having a thickness of 1 μm or more is preferably interposed between the core 22a and the main surface 8a. In this example, the constituent material of the clad 22b is SiO ₂ having a refractive index n2 of about 1.45. The refractive indexes n1 and n2 of the cladding 22b and the core 22a satisfy the relationship of n2 ≦ (1 / 1.4) n1 (≈0.714n1). By using the clad 22b in this refractive index range, an optical waveguide 22 having excellent light confinement properties can be obtained.

  In this example, as the optical multiplexing / demultiplexing units C1 and C2, 3 dB couplers using 2-input 2-output type MMI (Multi-Mode Interference) waveguides are used. In addition to the MMI waveguide, a directional coupler can be used as the 3 dB coupler used for the optical multiplexing / demultiplexing units C1 and C2. Further, the optical multiplexing / demultiplexing units C1 and C2 are not limited to the 3 dB couplers, and more general couplers that can distribute the light to the two output ports at an arbitrary distribution ratio can be used.

  The first arm 12 includes a first phase adjustment region 18 and a first propagation region 17. In this example, the first phase adjustment region 18 is divided into two sub-regions 18a and 18b. The geometric length La of the first phase adjustment region 18 is the sum of the lengths of these sub-regions 18a and 18b. The first propagation region 17 includes first sub-propagation regions 17 a, 17 b, and 17 c that are waveguide regions of the first arm 12 other than the first phase adjustment region 18. Each of the first sub-propagating regions 17a and 17c includes one curved optical waveguide that bends the light propagation direction at a right angle in a plane parallel to the main surface 8a. The first sub-propagating region 17b includes two curved optical waveguides having the same structure as described above.

  Similarly, the second arm 14 includes a second phase adjustment region 20 and a second propagation region 19. In this example, the second phase adjustment region 20 is divided into two sub-regions 20a and 20b. The geometric length Lb of the second phase adjustment region 20 is the sum of the lengths of these sub-regions 20a and 20b. The second propagation region 19 includes second sub-propagation regions 19 a, 19 b, and 19 c that are waveguide regions of the second arm 14 other than the second phase adjustment region 20. The second sub-propagating regions 19a and 19c include one curved optical waveguide as described above. The second sub-propagating region 19b includes two curved optical waveguides described above. In this example, the length of the first propagation region 17 and the length of the second propagation region 19 are configured to be equal.

  In this example, the case where the first and second phase adjustment regions 18 and 20 are each divided into two sub-regions has been described. However, if the total lengths La and Lb of the first and second phase adjustment regions 18 and 20 that are the sum of the lengths of the sub-regions satisfy the above expressions (1) and (2), the number of divisions may be three or more. .

  In this example, the case where the geometric lengths of the first and second propagation regions 17 and 19 are equal, that is, the case where the optical path lengths of both the regions 17 and 19 are equal, has been described. In this case, in the formulas (1) and (2), it is not necessary to consider the phase difference generated in the first and second lights by both the regions 17 and 19, so that La and Lb can be easily designed. However, the optical path lengths of both regions 17 and 19 may be different. In this case, it is necessary to design the lengths La and Lb of the first and second phase adjustment regions 18 and 20 in consideration of the phase difference generated in both regions 17 and 19.

(Operation)
Next, the operation of the MZ type element 10 will be specifically described with reference to FIGS. 5 and 6. FIG. 5A is a characteristic diagram showing the wavelength dependence of the equivalent refractive index of the first and second phase adjustment regions, and FIG. 5B is a diagram illustrating the first and second phase adjustment regions shown in FIG. It is a figure which shows La and Lb which satisfy | fill interference conditions with an interference order. FIG. 6 is a characteristic diagram showing the wavelength separation characteristics of the MZ element 10.

  Curves I and II in FIG. 5A correspond to the equivalent refractive indexes na and nb of the first and second phase adjustment regions 18 and 20 formed in the above-described cross-sectional dimensions, respectively. The horizontal axis of FIG. 5A is the wavelength (μm) of propagating light, and the vertical axis is the equivalent refractive index (dimensionless). The values of na and nb were calculated by the finite element method in which the above-described values were given to the refractive indexes n1 and n2 and dimensions of the core 22a and the clad 22b.

  From the behavior of the curves I and II, it can be seen that in the wavelength range of 1.3 to 1.6 μm, both na and nb are convex upwards, and gradually decrease as the wavelength increases. In this wavelength range, na is reduced by about 10% from about 2.4 to about 2.2, and nb is reduced by about 10% from about 2.2 to about 2.0.

If the values of the first and second wavelengths λ1 and λ2 are given, the equivalent refractive indexes na _λ1 , nb _λ1 , na _{λ2 in} the equations (1) and (2) and nb _λ2 is obtained. That is, under any interference order i, the lengths La and Lb of the first and second phase adjustment regions 18 and 20 can be determined by solving the equations (1) and (2).

  FIG. 5B illustrates La and Lb obtained in this way for several interference orders i. In obtaining FIG. 5B, the first and second wavelengths λ1 and λ2 were set to 1.54 μm and 1.56 μm, respectively.

  FIG. 5B shows cases where the interference order i is 1/2, 3/2, and 11/2. According to this figure, it can be seen that La and Lb increase as the interference order increases.

  FIG. 6 is a characteristic diagram obtained by analyzing and calculating the wavelength separation characteristics of the MZ element 10 given La and Lb in FIG. The vertical axis in FIG. 6 is the intensity of the bar output light BAR, and the horizontal axis is the wavelength (μm) of the bar output light BAR. The intensity on the vertical axis is the intensity ratio of the bar output light BAR to the input light IN. Curves I, II, and III in FIG. 6 correspond to i = 1/2, 3/2, and 11/2 in FIG. 5B, respectively.

  Referring to FIG. 6, it can be seen that the curves I, II, and III are all constant at a wavelength of 1.5 to 1.6 μm and an intensity of 1. That is, it can be seen that the phase difference generated in both arms 12 and 14 does not change at least in this wavelength range including the first and second wavelengths λ1 and λ2.

  It can also be seen that the wavelength range in which the phase difference does not change decreases as the interference order i increases. More specifically, i = 1/2 is in the range of about 1.45 to about 1.65 μm, i = 3/2 is in the range of about 1.48 to about 1.62 μm, and i = 11/2. The phase difference is constant in the range of about 1.5 to about 1.6 μm.

  In addition, in the wavelength range outside the range where the phase difference is constant, the MZ element 10 exhibits the trigonometric function of wavelength separation characteristics as in the conventional element 50. This is because the larger the wavelength difference between the first and second wavelengths λ1 and λ2, the greater the deviation from the equations (1) and (2). By utilizing this trigonometric characteristic, the MZ type element 10 can wavelength-separate the selected light and the third light as described above. For example, in the curve III (i = 11/2), the intensity of the bar output light becomes 0 at a wavelength of about 1.37 μm. This means that light having a wavelength of about 1.37 μm is cross-output. Therefore, in this MZ type element 10, if the third wavelength λ3 is 1.37 μm, the third light can be cross-output and the selected light of about 1.5 to about 1.6 μm can be bar-output.

  As described above, the MZ type element 10 generates a certain phase difference in the light propagated through the first and second arms 12 and 14 in the selected range by the first and second phase adjustment regions 18 and 20. it can. As a result, the MZ-type element 10 can output output light whose intensity does not change in this wavelength range even when the wavelength of light to be wavelength fluctuates in the range of λ1 to λ2.

  In addition, the MZ type element 10 corresponds to the one in which the first and second phase adjustment regions 18 and 20 are formed by changing the widths of the partial regions of the first and second arms 52 and 54 of the conventional device 50. To do. That is, unlike Patent Documents 6 and 7 in which the phase difference is made constant in a predetermined wavelength range by adding a configuration such as a ring waveguide, the MZ element 10 of the present invention can be obtained without adding a configuration. Works the same as the literature.

(Deformation of interference condition)
Since the curves I and II in FIG. 5A are smooth, when the wavelength difference dλ (= λ2−λ1) between the first and second lights is sufficiently small, the following formula (10 ) And (11) can be considered to hold true. In this case, the interference conditions of the equations (1) and (2) can be further modified.

na _λ2 = na _λ1 + dλ (dna / dλ) _λ1 (10)
nb _λ2 = nb _λ1 + dλ (dnb / dλ) _λ1 (11)
Here, (dna / dλ) _λ1 represents the rate of change of the equivalent refractive index na of the first phase adjustment region 18 with respect to the wavelength λ, and corresponds to the slope of the curve I in FIG. 5A at the wavelength λ1. Further, (dnb / dλ) _λ1 represents the rate of change of the equivalent refractive index nb of the second phase adjustment region 20 with respect to the wavelength λ, and corresponds to the slope at the wavelength λ1 of the curve II in FIG.

  Note that the above-mentioned “linearity is satisfied” means that the variation in the slopes of na and nb in the selected range is within 20% of the average value of the slopes of na and nb in the same range. Show. In this case, it can be considered that the above formulas (10) and (11) hold.

  When Expression (2) is transformed using Expressions (10) and (11), the following Expression (12) is obtained.

(I + di) (λ1 + dλ) = (na _λ1 + dλ (dna / dλ) _λ1 ) La + (nb _λ1 + dλ (dnb / dλ) _λ1 ) Lb (12)
In order to obtain Expression (12), it was assumed that the interference order i changes to (i + di) due to a minute change dλ of the first wavelength λ1. Here, di indicates a minute change in the interference order.

  Further, taking the difference between both sides of the equations (12) and (1) and considering that the interference order i is invariant (di = 0) even if the wavelength changes by dλ, the following equation (13) is obtained. It is done.

i = (dna / dλ) _λ1 La + (dnb / dλ) _λ1 Lb (13)
Meanwhile, the first and second group refractive indexes nag and nbg of the light including the first and second wavelengths λ1 and λ2, that is, the first and second phase adjustment regions 18 and 20 of the selected light, are well-known below. It represents with Formula (14) and (15).

nag = na _AV −λ _AV (dna / dλ) _AV (14)
nbg = nb _AV −λ _AV (dnb / dλ) _AV (15)
Here, na _AV = (na _λ1 + na _λ2 ) / 2 and nb _AV = (nb _λ1 + nb _λ2 ) / 2. In addition, λ _AV = (λ1 + λ2) / 2. Also, (dna / dλ) _AV = ((dna / dλ) _λ1 + (dna / dλ) _λ2 ) / 2, and (dnb / dλ) _AV = ((dnb / dλ) _λ1 + (dnb / dλ) _λ2 ) / 2.

  The group refractive index is an amount obtained by dividing the traveling speed of light pulses (group speed) in a uniform medium by the traveling speed of light pulses in a vacuum (group refractive index = light speed / group speed). ).

  When Expression (13) is transformed using Expressions (14) and (15), the following Expression (16) is obtained.

i = (na _AV −nag) La / λ _AV + (nb _AV −nbg) Lb / λ _AV (16)
For the transformation to the equation (16), it is used that (dna / dλ) _λ1 = (dna / dλ) _AV and (dnb / dλ) _λ1 = (dnb / dλ) _AV holds based on the linearity described above. doing.

  By the way, the following formula (17) is obtained from the formula (1) based on the linearity described above.

i = na _AV × La / λ _AV + nb _AV × Lb / λ _AV (17)
Here, the following formula (18) is obtained by subtracting (17) from the formula (16).

nag × La = nbg × Lb (18)
In order to obtain a constant phase difference in the selected range, equation (18) indicates that the product of the length La of the first phase adjustment region 18 and the first group refractive index nag and the length Lb of the second phase adjustment region 20 are obtained. And the product of the second group refractive index nbg should be equal. As described above, La and Lb may be determined using the equation (18) instead of the equations (1) and (2).

  In order to obtain Equation (18), it is necessary to make the optical path lengths of the first and second propagation regions 17 and 19 equal.

(Manufacturing method of MZ type element)
Finally, a method for manufacturing the MZ element 10 will be briefly described. The MZ type element 10 is manufactured using an SOI (Si On Insulator) substrate in which a SiO ₂ layer and a Si layer are stacked in this order on a Si substrate. That is, the core 22a is formed using the uppermost Si layer, and the SiO ₂ layer, which is a BOX (Buried-OXide) layer, is used as the lower layer of the cladding 22b. More specifically, the uppermost Si layer is patterned by a conventionally known dry etching method or the like to form the core 22a. Then, an SiO ₂ layer corresponding to the upper layer of the cladding 22b is formed by a CVD (Chemical Vapor Deposition) method or the like so as to embed the core 22a, and the MZ type element 10 is obtained.

8 Substrate 8a First main surface 10, 50, MZ MZ type element (optical multiplexing / demultiplexing element using Mach-Zehnder interferometer)
12, 52, a01 First arm (first arm waveguide)
14, 54, a02 Second arm (second arm waveguide)
16, 56 Optical interference unit 17 First propagation region 17a, 17b, 17c First sub propagation region 18 First phase adjustment region 18a, 18b, 20a, 20b Sub region 19 Second propagation region 19a, 19b, 19c Second sub propagation Region 20 Second phase adjustment region 22 Optical waveguide 22a Core 22b Clad C1, C2, c01, c02 Optical multiplexing / demultiplexing portions in1, in2, in01, in02 Input port ot1, ot2, ot01, ot02 Output port

Claims (9)

An optical waveguide comprising a core and a clad having a refractive index of 71.4% or less of the core;
The optical waveguide includes two optical multiplexing / demultiplexing units, and an optical interference unit provided between the two optical multiplexing / demultiplexing units,
The optical interference unit includes first and second arm waveguides for propagating light distributed by one of the optical multiplexing / demultiplexing units to the other optical multiplexing / demultiplexing unit, respectively.
The first and second arm waveguides include first and second phase adjustment regions, respectively;
The first and second phase adjustment regions have different equivalent refractive indexes, and give a predetermined phase difference between the light propagating through the first arm waveguide and the light propagating through the second arm waveguide. An optical element. The sum of the phase differences that the first light of the first wavelength is applied by the optical interference unit is defined as the first phase difference, and the second light of the second wavelength that is different from the first wavelength is applied by the optical interference unit. When the total phase difference is the second phase difference,
The geometric length along the light propagation direction of the first and second phase adjustment regions is configured to make the first and second phase differences equal to each other. Optical element.   3. The optical element according to claim 2, wherein the first and second phase differences are both 2iπ (where i is a multiple of 1/2 including 0). When the third phase difference is the sum of the phase differences given by the optical interference unit with the third light of the third wavelength different from both the first and second wavelengths,
The optical element according to claim 3, wherein the third phase difference is 2 (i + (j + 1/2)) π (j is an integer of 0 or more).   3. The optical element according to claim 2, wherein the first and second phase differences are both 2 (i ± 1/4) π (i is a multiple of 1/2 including 0). When a region of the first arm waveguide other than the first phase adjustment region is a first propagation region, and a region of the second arm waveguide other than the second phase adjustment region is a second propagation region,
The optical element according to claim 1, wherein the optical path lengths of the first and second propagation regions are equal. An optical waveguide comprising a core and a clad having a refractive index of 71.4% or less of the core;
The optical waveguide includes two optical multiplexing / demultiplexing units, and an optical interference unit provided between the two optical multiplexing / demultiplexing units,
The optical interference unit includes first and second arm waveguides for propagating light distributed by one of the optical multiplexing / demultiplexing units to the other optical multiplexing / demultiplexing unit, respectively.
The first and second arm waveguides are provided with first and second phase adjustment regions, respectively.
The light includes a first light having a first wavelength and a second light having a second wavelength different from the first wavelength, and the group refractive indexes of the light in the first and second phase adjustment regions are respectively When the first and second group refractive indexes nag and nbg and the geometric lengths along the light propagation direction of the first and second phase adjustment regions are La and Lb, respectively,
La × nag = Lb × nbg is satisfied, and
When a region of the first arm waveguide other than the first phase adjustment region is a first propagation region, and a region of the second arm waveguide other than the second phase adjustment region is a second propagation region,
An optical element characterized in that the optical path lengths of the first and second propagation regions are equal.   The optical element according to claim 1, wherein a 3 dB coupler that divides the input light into two and outputs the divided light is used as the optical multiplexing / demultiplexing unit. The optical element according to claim 1, wherein Si is used as the core and SiO ₂ is used as the clad.