JP2015152729A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization-independent MMI-type optical element that performs wavelength separation.SOLUTION: An optical element includes a first width-tapered optical waveguide 12, a first MMI optical waveguide 14, and a second MMI optical waveguide 16, the first width-tapered optical waveguide 12 being connected to one end face S1 of the first MMI optical waveguide 14. The first MMI optical waveguide 14 excites at least zero and first order mode rays of input light while the second MMI optical waveguide 16 excites at least zero through second order mode rays of the input light. The first width-tapered optical waveguide 12 has width that gradually increases toward the first MMI optical waveguide 14 to a width that is no less than half a width of the first MMI optical waveguide 14 on the one end face S1, and is positioned offset from a center axis of the first MMI optical waveguide 14.

Description

  The present invention is, for example, a multimode interference (hereinafter also referred to as MMI (Multi Mode Interference)) optical waveguide type used for multiplexing / demultiplexing of component light having different wavelengths used in wavelength division multiplexing (WDM) communication. The present invention relates to an optical 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 addition, light of different wavelengths may be used for downlink communication. The mainstream at present in optical subscriber systems is GE-PON (Gigabit Ethernet (registered trademark) -Passive Optical Network) capable of two-way communication at a rate of 1 Gbps or higher. In recent years, as a next-generation technology that replaces GE-PON, WDM-PON with increased multiplicity of wavelengths used for communication has been studied. In WDM-PON, in principle, a communication speed exceeding 10 Gbps can be obtained in both directions.

  In WDM-PON, a station-side terminator (OLT: Optical Line Terminal) and a subscriber-side terminator (ONU: Optical Network Unit) are provided at the ends of the optical fiber used for communication at the station side and the subscriber side, respectively. It is done. These terminators include a light emitting element, a light receiving element, and a wavelength multiplexing / demultiplexing element that multiplexes / demultiplexes light of a plurality of wavelengths.

  Generally, a light emitting element, a light receiving element, and a wavelength multiplexing / demultiplexing element are integrated on a common substrate provided with an optical waveguide (see, for example, Non-Patent Document 1 and Patent Documents 1 to 6).

In recent years, a Si optical waveguide using a core made of silicon (Si) and a clad made of silicon oxide (SiO ₂ ) having a large refractive index difference from Si is used for connection between these elements. . Since the refractive index of the core of the Si optical waveguide is much larger than the refractive index of the clad, light confinement is strong, and a curved optical waveguide that bends light with a small curvature radius of about 1 μm can be formed. In addition, since it can be manufactured using the processing technology of the Si electronic device, an extremely fine submicron cross-sectional structure can be realized. Therefore, the element size can be reduced by using the Si optical waveguide.

Technical digest OFC / NFOEC 2010, paper OWJ3, March 2010

JP 2009-198914 A U.S. Pat. No. 4,860,294 US Pat. No. 5,764,826 US Pat. No. 5,960,135 U.S. Pat. No. 7,072,541 JP-A-8-163028

  Wavelength multiplexing / demultiplexing elements using Si optical waveguides include Mach-Zehnder interferometer type, directional coupler type, grating type, and MMI optical waveguide type.

  However, since the Mach-Zehnder interferometer type has a large wavelength dependency in the coupling coefficient, the equivalent refractive index, and the like, it is difficult to obtain the wavelength separation characteristics required by the ONU.

  In addition, the directional coupler type tends to be weak against wavelength shift of a light source or the like because the transmittance has a large wavelength dependency.

  Also, in the MMI optical waveguide type, it is difficult to obtain a periodic function propagation state in the MMI optical waveguide due to interference with unnecessary higher-order modes, and it is difficult to obtain good wavelength separation characteristics. Further, the loss of light derived from the structure tends to be large (see, for example, Optics Engineering vol. 41, pp. 723-727, March 2002).

  Further, in all types of wavelength multiplexing / demultiplexing devices described above, a large polarization dependency in the Si optical waveguide is a problem.

  The present invention has been made with such a technical background. Accordingly, an object of the present invention is to provide an optical element that operates as a polarization-independent and has an output characteristic that is periodic function while suppressing light loss, and can be used as an MMI optical waveguide type wavelength multiplexing / demultiplexing element. There is to get.

  As a result of intensive studies, the inventor has connected the above-mentioned tapered width tapered optical waveguide for light input / output and the MMI optical waveguide for synthesizing the electric field distribution of light to the above-mentioned MMI optical waveguide for wavelength separation. I was able to solve this problem.

  Therefore, the optical element of the present invention has an optical waveguide provided with a core and a clad provided around the core on the upper side of the common plane, and i pieces (i is 2 or more) having different input wavelengths. This is an optical element that multiplexes and demultiplexes the light of an integer of

  The optical element includes a first width tapered optical waveguide, a first multimode interference optical waveguide, and a second multimode interference optical waveguide along the light propagation direction.

  The first width tapered optical waveguide is a tapered optical waveguide for optical input / output, and is connected to one end face of the first multimode interference optical waveguide, and the first multimode interference of the first width tapered optical waveguide. A first optical input / output port is provided on the end surface opposite to the optical waveguide.

  The first multimode interference optical waveguide is an MMI optical waveguide for wavelength separation, and has a width capable of exciting at least the 0th-order mode light and the first-order mode light of the light input to the first multimode interference optical waveguide. Have

  Further, the second multimode interference optical waveguide is an MMI optical waveguide that synthesizes the electric field distribution of light, and at least the 0th-order mode light, the first-order mode light, and the light input to the second multimode interference optical waveguide The second mode light input / output port is provided on the end surface opposite to the first multimode interference optical waveguide.

  The width of the first width taper optical waveguide is increased toward the first multimode interference optical waveguide, and at the end surface on the first multimode interference optical waveguide side, the width of the first multimode interference optical waveguide is ½ or more. The first multi-mode interference optical waveguide excites only the 0th-order mode light and the first-order mode light in the first multi-mode interference optical waveguide.

  In the optical element according to the present invention, polarization-independent wavelength separation is performed by limiting the modes excited by the first multi-mode interference optical waveguide to the 0th order and the 1st order. Furthermore, since the second multimode interference optical waveguide synthesizes the electric field of the output light into a desired distribution, loss due to radiation to the clad can be reduced.

It is a schematic diagram which simplifies and shows a 1st optical element. (A) and (B) are electric field distributions at the condensing spots of the first and second lights, respectively. (C) and (D) are the second and third of the first and second lights, respectively. It is an electric field distribution in an optical input / output port. (A) And (B) is a schematic diagram which shows the propagation state of the light in a 1st MMI optical waveguide with the optical element used for simulation, respectively by the presence or absence of a 1st width taper optical waveguide. (A) And (B) is a characteristic view which shows the wavelength separation characteristic of the optical element shown to FIG. 3 (A) and (B). It is a perspective view of a 2nd optical element. It is a principal part enlarged plan view of a 2nd optical element. FIG. 14 is a characteristic diagram showing a simulation result showing the transmittance of input light when the dimensions of the second and third MMI optical waveguides and the second width tapered optical waveguide are changed in the second optical element. (A) And (B) is a characteristic view which shows the wavelength separation characteristic of a 2nd optical element.

  Embodiments of the present invention will be described below with reference to the drawings. In each figure, the shape, size, and arrangement relationship of the components are schematically shown to such an extent that the present invention can be understood. Each of the following embodiments is a preferred example of the present invention, and the material and numerical conditions of each component are merely examples of preferred cases. Accordingly, 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.

(Summary of Invention)
The outline of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram showing a simplified first optical element according to an embodiment of the present invention. In FIG. 1, only the core of the first optical element is shown, and illustration of the cladding is omitted.

  The first optical element 10 is provided on the upper side of a common plane parallel to the drawing surface, and is connected along the light propagation direction R to the first width taper optical waveguide 12 and the first MMI optical waveguide. 14 and a second MMI optical waveguide 16.

  A first width taper optical waveguide 12 is connected to one end surface S1 of the first MMI optical waveguide 14, and the first width taper optical waveguide 12 has a first end surface opposite to the first MMI optical waveguide 14 on the first surface. An optical input / output port P1 is provided. A second MMI optical waveguide 16 is connected to the other end surface S2 of the first MMI optical waveguide 14 opposite to the one end surface S1. In addition, second and third optical input / output ports P2 and P3 are provided on the end surface S3 of the second MMI optical waveguide 16 opposite to the first MMI optical waveguide 14, respectively. Here, the region of the end surface S3 between the second and third light input / output ports P2 and P3 is referred to as a gap G. The first to third optical input / output ports P1 to P3 are connected to channel type optical waveguides 51, 52 and 53 for optical input / output, respectively.

  In this example, first and second light beams L1 and L2 having different wavelengths are input to the first optical element 10 from the channel-type optical waveguide 51. Then, the light is wavelength-separated without depending on the polarization, and the first and second light L1 and L2 are output from the second and third channel type optical waveguides 52 and 53, respectively.

  Schematically, the first width tapered optical waveguide 12 has a function of limiting the propagation modes excited in the first MMI optical waveguide 14 to the 0th order and the 1st order mode light with respect to both the lights L1 and L2. The first MMI optical waveguide 14 separates wavelengths of the first and second lights L1 and L2 having different wavelengths without depending on the polarization, and condenses them on the condensing spots F1 and F2 on the other end surface S2, respectively. The second MMI optical waveguide 16 mainly excites 0th to 2nd mode light with respect to both lights L1 and L2. Then, the loss of light leaking from the gap G to the clad is suppressed by synthesizing the electric field distributions of the two lights L1 and L2 using the inter-mode interference. As a result, the first and second light beams L1 and L2 having higher intensities are output from the channel type optical waveguides 52 and 53, respectively.

(Structure of the first optical element)
Next, the structure of the first optical element 10 will be described in more detail with reference to FIG.

  In FIG. 1, an arrow R indicates the light propagation direction for convenience. In the following description, the geometric length along the light propagation direction R is also referred to as “length”, and the geometric length along the direction perpendicular to the paper surface is referred to as “height” or “thickness”. The geometric length along the direction perpendicular to both the height direction and the length direction is also referred to as “width”. In addition, since the reverse process is established for light, the propagation directions of the first and second lights L1 and L2 are not limited to the light propagation direction R.

  The first width tapered optical waveguide 12 has a function of limiting the propagation modes of the first and second lights L1 and L2 excited by the first MMI optical waveguide 14 to the 0th-order mode light and the first-order mode light. This simplifies inter-mode interference in the first MMI optical waveguide 14 and increases the wavelength separation capability in the first and second MMI optical waveguides 14 and 16.

  The first width taper optical waveguide 12 has a planar shape in which the width increases in an isosceles trapezoidal shape toward the first MMI optical waveguide 14. The first width taper optical waveguide 12 is provided at a position shifted from the central axis C2 of the first MMI optical waveguide 14. That is, the central axis C1 of the first width tapered optical waveguide 12 does not coincide with the central axis C2 of the first MMI optical waveguide 14. In the one end face S1 of the first MMI optical waveguide 14, the first width taper optical waveguide 12 is preferably a width that is 1/2 or more of the width of the first MMI optical waveguide 14, and more specifically, the first MMI optical waveguide A size of 50 to 80% of the width of the waveguide 14 is preferable, and 70% is more preferable. In addition, the suitable value of dimensions, such as the width | variety of the 1st width taper optical waveguide 12, was calculated | required by simulation.

  The first MMI optical waveguide 14 has a rectangular planar shape whose length is longer than the width. Then, the first MMI optical waveguide 14 separates the wavelengths of the first and second lights L1 and L2 without depending on the polarization, and collects them on the focusing spots F1 and F2 on the other end surface S2, respectively. The condensed spot F1 exists on the second light input / output port P2 side that outputs the first light L1, and the condensed spot F2 exists on the third light input / output port P3 side that outputs the second light L2.

  The first MMI optical waveguide 14 is required to have the wavelength separation capability and polarization independence described below at the same time.

  First, the wavelength separation capability will be described. In order to separate the wavelengths of the first light L1 and the second light L2, the first MMI optical waveguide 14 is designed to have a width and a length so that the evenness of the interference order differs between the first light L1 and the second light L2. Has been. Here, the interference order corresponds to the number of meanders in the first MMI optical waveguide 14 of the first and second lights L1 and L2 shown in FIG. 1 (the number of times the traveling direction is reversed in the width direction). That is, in the example of FIG. 1, the first light L1 meandering three times (interference order is odd number) in the first MMI optical waveguide 14 is condensed at the condensing spot F1 and meanders four times (interference order is even number). The second light L2 is focused on the focused spot F2. Here, if the even-oddity of the interference orders of the first and second lights L1 and L2 is reversed, the first and second lights L1 and L2 are condensed on the condensed spots F2 and F1, respectively.

  In this example, the case where the number of wavelengths i to be separated is 2 has been described. However, when the number of wavelengths i is 3 or more, the first optical element 10 can also perform wavelength separation for the first to third lights L1 to L3 having different wavelengths, for example. For example, if the even and odd orders of the interference order are different between the first and second lights L1 and L2 and the third light L3, the first and second lights L1 and L2 and the third light L3 are wavelength-separated. , And the second and third optical input / output ports P2 and P3, respectively.

  Next, polarization independence will be described. The width and length of the first MMI optical waveguide 14 are designed so as to perform polarization-independent multimode interference. Specifically, for the first light L1, the propagation constant difference between the 0th-order and first-order mode light of the TE wave and the propagation constant difference between the 0th-order and first-order mode light of the TM wave are made equal. Yes. Similarly, the second light L2 has the same propagation constant difference between the 0th-order mode light and the 1st-order mode light in both polarizations. As a result, the first MMI optical waveguide 14 can perform wavelength separation independent of polarization.

  Next, with reference to FIGS. 2A and 2B, the electric field distributions of the first and second lights L1 and L2 that have propagated through the first MMI optical waveguide 14 and reached the focused spots F1 and F2 will be described. . 2A and 2B are electric field distributions when the other end surface S2 is viewed from the one end surface S1 side. The axis in the figure represents the position in the width direction in arbitrary units. The upper and lower axes correspond to the positive and negative electric fields, respectively. Further, since the first MMI optical waveguide 14 is polarization independent, both polarizations show the same electric field distribution as in the drawing.

  FIG. 2A shows an electric field distribution on the other end surface S2 of the first light L1 having a wavelength λ1. One large peak is the electric field distribution of the zeroth-order mode light, and a sine curve is the electric field distribution of the first-order mode light. The dotted line indicates the combined electric field distribution of the 0th and 1st modes.

  The combined electric field distribution has a large positive peak on the second light input / output port P2 (condensing spot F1) side that outputs the first light L1, and has a long tail on the condensing spot F2, that is, on the gap G side. .

  FIG. 2B shows an electric field distribution on the other end surface S2 of the second light L2 having a wavelength of λ2. The electric field distribution of the second light L2 is the same as that of the first light L1 except that the positive / negative of the electric field of the primary mode light is reversed. This difference in the first-order mode light originates from the fact that the even and odd interference orders differ between the second light L2 and the first light L1. As a result, the combined electric field distribution of the second light L2 has a large positive peak on the third output port P3 (condensing spot F2) side that outputs the second light L2, and on the condensing spot F1, that is, on the gap G side. Long hem.

  The second MMI optical waveguide 16 has a rectangular planar shape that is wider and shorter in length than the first MMI optical waveguide 14. The second MMI optical waveguide 16 includes second and third optical input / output ports P2 and P3 on the end surface S3. The second and third optical input / output ports P2 and P3 are provided symmetrically with respect to the central axis C2 of the first MMI optical waveguide 14, and the end surface S3 between the optical input / output ports P2 and P3 has the above-described configuration. There is a gap G.

  The width of the second MMI optical waveguide 16 is designed to be large enough to excite at least the 0th-order to second-order mode light of both lights L1 and L2. However, since the mode required for the second MMI optical waveguide 16 is up to the second-order mode light, the width is preferably selected so that the excitation of the third-order or higher-order mode light is suppressed.

  The length of the second MMI optical waveguide 16 is designed so that the phase difference between the 0th-order mode light and the second-order mode light is π for both lights L1 and L2.

  Furthermore, this length is designed to be 1/4 or less, preferably 1/8 or less of the beat length of the 0th-order and 1st-order mode lights of both lights L1 and L2. When the beat length is ¼ or less, it can be considered that the phase relationship between the zero-order and first-order mode lights of both polarizations maintains the state of FIGS. 2 (A) and 2 (B). As a result, the second MMI optical waveguide 16 can be handled practically sufficiently as polarization-independent. Here, the beat length is a length at which the phase difference between both polarizations of light propagating through the second MMI optical waveguide 16 is 2π.

  By designing the second MMI optical waveguide 16 as described above, loss of light emitted from the gap G to the cladding can be reduced. Hereinafter, this point will be described with reference to FIGS. 2C and 2D show electric field distributions of the first and second lights L1 and L2 on the end face S3. Note that the direction of viewing the electric field distribution and the meaning of the axes in the figure are the same as in FIGS. 2A and 2B. 2C and 2D also show the electric field distribution of the secondary mode light excited by the second MMI optical waveguide 16.

  The electric field distributions of the first and second lights L1 and L2 shown in FIGS. 2C and 2D are the same except that the positive and negative electric fields of the primary mode light are reversed. An example will be described.

  Since the length of the second MMI optical waveguide 16 is set so that the phase difference between the 0th-order mode light and the second-order mode light is π as described above, as shown in FIG. On the left side, the electric field of 0 to 2nd mode light is positive, and in the other parts, the distribution of the electric field in each mode is antagonized.

  As a result, the combined electric field distribution of the first light L1 indicated by the dotted line has a large peak in which the positive part of the electric field of 0 to 2nd mode light overlaps at the second light input / output port P2 (left side of the figure). In other parts, the electric field in the 0 to 2nd mode cancels and the electric field strength becomes zero. That is, since the electric field in the gap G direction (right side in the figure) becomes 0, the loss of light due to the radiation from the gap G is suppressed, and most of the first light L1 is output from the second light input / output port P2. Can do.

  Referring to FIG. 2D, the same applies to the second light L2, the electric field in the gap G direction becomes 0, and the peak of the combined electric field exists only on the third light input / output port P3 side. Loss from G can be suppressed.

(About 1st width taper optical waveguide)
Next, the operation of the first width tapered optical waveguide 12 will be described in more detail with reference to FIGS.

  FIGS. 3A and 3B are schematic views showing the propagation state of light in the first MMI optical waveguide 14 with and without the first width tapered optical waveguide 12 together with the optical element used in the simulation. In the simulated optical elements 10A and 10B, the second MMI optical waveguide 16 is omitted in order to emphasize the propagation state in the first MMI optical waveguide 14. Specifically, channel-type optical waveguides 52 and 53 are connected to the other end surface S2 to which the second MMI optical waveguide 16 is to be connected via a tapered output waveguide 54. Further, the wavelength of the input light to the optical elements 10A and 10B was limited to one wavelength of 1530.5 nm.

  FIG. 3A shows a propagation state in the optical element 10A that does not have the first width tapered optical waveguide 12 and in which the channel type optical waveguide 51 is directly connected to the one end face S1. FIG. 3B shows a propagation state in the optical element 10 </ b> B including the first width tapered optical waveguide 12. In the distribution charts of FIGS. 3A and 3B, the horizontal axis indicates the length (μm) in the width direction, and the vertical axis indicates the length (μm) in the light propagation direction R. In addition, the intensity of propagating light is represented by shades of black and white, and the whiter the color, the higher the light intensity.

4A and 4B are characteristics showing wavelength separation characteristics when the first and second lights L1 and L2 having different wavelengths are input from the channel-type optical waveguide 51 to the optical elements 10A and 10B, respectively. FIG. 3 and 4 were performed by the BPM method (beam propagation method) under the following conditions.
1) The cross-sections of the channel type optical waveguides 51 to 53 were square with a side of 300 nm.
2) The width of the first MMI optical waveguide 14 was 1.61 μm, the length was 150 μm, and the thickness was 300 nm.
3) In the optical element 10B, the taper length of the first width taper optical waveguide 12 was set to 4 μm, and the width was increased from 300 nm to 1.13 μm.

  Referring to FIG. 3A, in the first MMI optical waveguide 14, from the 0th-order mode light to the second-order or higher-order mode light is excited, so that these many modes interfere with each other, resulting in a complicated propagation state. Has occurred. On the other hand, in FIG. 3B, the first width taper optical waveguide 12 causes the first MMI optical waveguide 14 to excite only the 0th-order and first-order mode light, so that a periodic propagation state occurs. .

  4A and 4B show the wavelengths when the first light L1 having a wavelength of 1.52 μm and the second light L2 having a wavelength of 1.59 μm are input from the channel-type optical waveguide 51 to the optical elements 10A and 10B. The separation characteristics are shown. Specifically, the light intensity output from the output-side channel type optical waveguides 52 and 53 is shown. 4A and 4B, the vertical axis represents light intensity in arbitrary units, and the horizontal axis represents wavelengths in μm units.

  Comparing FIGS. 4A and 4B, FIG. 4B having the first width tapered optical waveguide 12 shows superior wavelength separation characteristics with respect to the first and second lights L1 and L2. . Specifically, the wavelength separation curves of the first and second lights L1 and L2 are bilaterally symmetric, and a good extinction ratio is shown at the peak wavelengths of the first and second lights L1 and L2. Here, the extinction ratio is the intensity ratio of the other light to the peak intensity of one light at a predetermined peak wavelength.

  However, in the optical element 10B, the peak intensity of the first light L1 having a wavelength of 1.52 μm is smaller than that of the optical element 10A. Specifically, a loss of about 2 dB occurs in the output of the first light L1. As can be seen from FIG. 3B, in the optical element 10B, since the region with a high light intensity (white region in the figure) exists in the center in the width direction of the first MMI optical waveguide 14, the other end surface S2 This is because light is emitted from the gap G between the channel type optical waveguides 52 and 53 to the clad.

(Second optical element)
Then, with reference to FIGS. 5-8, the 2nd optical element of another embodiment of this invention is demonstrated. FIG. 5 is a perspective view of the second optical element. In FIG. 5, the core constituting the optical waveguide is covered with the clad and cannot be seen directly, but is shown by a solid line for emphasis.

  The second optical element 30 is configured in the same manner as the first optical element 10 except that the second optical element 30 includes the third MMI optical waveguide 18 and the second width tapered optical waveguide 20. Therefore, the difference will be mainly described below.

  Referring to FIG. 5, the third MMI optical waveguide 18 is provided between the first and second MMI optical waveguides 14 and 16.

  The third MMI optical waveguide 18 has an isosceles trapezoidal planar shape whose width increases along the light propagation direction R. Specifically, the width of the third MMI optical waveguide 18 increases from the width of the first MMI optical waveguide 14 to the width of the second MMI optical waveguide 16. In other words, the width of the third MMI optical waveguide 18 between the first MMI optical waveguide 14 and the second MMI optical waveguide 16 is moderated. Thereby, in the 3rd MMI optical waveguide 18, the 0th order-2nd order mode light of both light L1 and L2 is mainly excited, and excitation of the unnecessary higher order mode light more than 3rd order is suppressed. Since unnecessary excitation of higher-order mode light is suppressed in this way, the electric field distribution at the second and third optical input / output ports P2 and P3 (see FIG. 6) can be easily provided by providing the third MMI optical waveguide 18. In addition, a state suitable for wavelength separation can be obtained.

  In addition, when providing the 3rd MMI optical waveguide 18, it is preferable to design by simulation etc. with the 2nd MMI optical waveguide 16. FIG.

(Structure common to the first and second optical elements)
A structure common to the first and second optical elements 10 and 30 will be described mainly with reference to FIG. The first and second optical elements 10 and 30 are constituted by an optical waveguide including a core 7 provided on the upper side of the common plane 8 a and a clad 9 provided around the core 7.

  Here, the common plane 8 a is a main surface of the lower Si layer 8 provided on the SOI (silicon on insulator) substrate 6.

The core 7 indicates the first and second width tapered optical waveguides 12 and 20 and the first to third MMI optical waveguides 14, 18 and 16. The clad 9 covers the entire periphery of the core 7. In this example, the first and second optical elements 10 and 30 are made of Si having a refractive index of the core 7 of about 3.47, and a Si optical waveguide made of SiO ₂ having a refractive index of the cladding 9 of about 1.46. It consists of Thus, by using the clad 9 having a refractive index of 71.4% or less with respect to the refractive index of the core 7, the sizes of the first and second optical elements 10 and 30 can be reduced.

  In general, in an optical element using a Si optical waveguide, the thickness of the core is preferably 0.5 μm or less, more preferably 0.3 μm or less. By setting the core thickness within this range, the first and second optical elements 10 and 30 can be single-mode optical waveguides in the thickness direction. In this example, the thickness of the core 7 is 300 nm.

  Moreover, it is preferable that the thickness t of the clad 9 provided below the core 7 is 1 μm or more. Thereby, radiation of light from the core 7 to the lower Si layer 8 can be suppressed. In this example, t is about 1.5 μm.

  In addition, since the first and second optical elements 10 and 30 operate independent of polarization, it is preferable that the channel-type optical waveguides 51 to 53 that propagate the light with the combined wavelengths are also independent of polarization. . Therefore, the cross-section of the channel type optical waveguides 51 to 53 is a square with one side of about 300 nm.

An SOI substrate is preferably used for the production of the first and second optical elements 10 and 30. The SOI substrate, a lower Si layer 8 is a single crystal, and SiO ₂ made of BOX (Buried Oxide) layer, SOI layer is a thin film of single crystal Si are laminated in this order. Then, if the core 7 is formed in a desired shape with the SOI layer, and then the SiO ₂ to be the cladding 9 is formed by embedding the core 7 by CVD (Chemical Vapor Deposition) method or the like, the first and second optical elements are formed. 10 and 30 are obtained.

  The first and second optical elements 10 and 30 are not limited to those made of Si optical waveguides, and can be formed of various materials known as optical waveguides such as quartz-based waveguides and compound semiconductor-based waveguides.

(Second width taper optical waveguide)
Subsequently, the second width tapered optical waveguide 20 will be described with reference to FIGS. 5 and 6. FIG. 6 is an enlarged plan view of a main part of the second optical element 30, and illustration of the clad 9 is omitted.

  The second width taper optical waveguide 20 includes first and second sub-width taper optical waveguides 20a and 20b. The first and second sub-width tapered optical waveguides 20a and 20b are provided at the second and third output ports P2 and P3 of the second MMI optical waveguide 16, respectively.

  In this example, the first and second sub-width tapered optical waveguides 20 a and 20 b have a symmetrical shape and arrangement with respect to the central axis C <b> 2 of the second optical element 30.

  Referring to FIG. 6, the first sub-width tapered optical waveguide 20 a has a trapezoidal planar shape, and its central axis C <b> 20 a is inclined from the central axis C <b> 2 of the second optical element 30 by an angle θ <b> 1. On the end surface S3, the distance between the foot of the perpendicular line dropped from the midpoint of the bottom on the channel-type optical waveguide 52 side and the central axis C20a is Sb1.

  The central axis C20a of the first sub-width tapered optical waveguide 20a is the midpoint of one base of the trapezoid connected to the end face S3 and the midpoint of the other base connected to the channel type optical waveguide 52. It is a connected straight line. Similarly, in the second sub-width tapered optical waveguide 20b, the central axis C20b, the angle θ2, and the distance Sb2 are defined. In this example, θ1 = θ2 (Sb1 = Sb2). Hereinafter, θ1 and θ2 are also referred to as “inclination angles”. Sb1 and Sb2 are also referred to as “displacement”. Also, the widths of the end surfaces S3 of the first and second sub-width tapered optical waveguides 20a and 20b are W1 and W2, respectively. In this example, the widths are equal and W1 = W2.

  The central axis C20a of the first sub-width tapered optical waveguide 20a is set to be parallel to the traveling direction of the wavefront of the first light L1 propagating through the first sub-width tapered optical waveguide 20a. By setting the central axis C20a in this way, the traveling direction of the first light L1 matches the extending direction of the first sub-width tapered optical waveguide 20a, and the light from the first sub-width tapered optical waveguide 20a Loss due to radiation can be suppressed. The same applies to the second sub-width tapered optical waveguide 20b.

  However, when the wavelength difference to be separated by the second optical element 30 is 1 μm or more, the wavefronts of the first and second light beams L1 and L2 in the first and second sub-width tapered optical waveguides 20a and 20b. The direction may be different. In this case, the inclination angles θ1 and θ2 may be set to different sizes.

(Polarization independence and loss reduction)
Next, mainly referring to FIG. 6 and FIG. 7, it will be described that the second optical element 30 is substantially independent of polarization and light loss is suppressed.

  FIG. 7 is a simulation result showing the transmittance of the input light of the second optical element 30 when the dimensions of the third and second MMI optical waveguides 18 and 16 and the second width tapered optical waveguide 20 are changed.

  In FIG. 7, the horizontal axis indicates the magnitude (μm) of the displacement Sb1 (= Sb2) shown in FIG. The vertical axis represents the transmittance (dB) that is the ratio of the intensity of the output light to the input light.

  FIG. 7 shows six curves in which the length of W1 (= W2) is changed to 0.8 μm, 1 μm, and 1.2 μm for each of the TE wave and the TM wave. That is, the curves TE (0.8), TE (1), and TE (1.2) indicate the transmittance of TE waves with W1 of 0.8 μm, 1 μm, and 1.2 μm, respectively. Similarly, curves TM (0.8), TM (1), and TM (1.2) indicate the transmittance of TM waves having W1 of 0.8 μm, 1 μm, and 1.2 μm.

The simulation was performed by the FDTD (Finite-difference time-domain) method under the conditions listed below.
1) The thickness of the core 7 was 300 nm.
2) The width at the one end face S1 of the first width tapered optical waveguide 12 was 1202.5 nm. This is about 74.7% of the width of the first MMI optical waveguide 14.
3) The first MMI optical waveguide 14 has a length of 15 μm and a width of 1610 nm.
4) The length of the displacement Sb1 was changed from −0.02 μm to +0.02 μm. In addition, when the length of the displacement Sb1 is negative, it indicates that the central axes C20a and C20b of the first and second sub-width tapered optical waveguides 20a and 20b approach along the light propagation direction R. Indicates that the central axes C20a and C20b of the first and second sub-width tapered optical waveguides 20a and 20b are separated from each other along the light propagation direction R.
5) The input light to the second optical element 30 was set to 1530.5 nm so that polarization independence was achieved by the first MMI optical waveguide 14.
6) The length D18 of the third MMI optical waveguide 18 was obtained from the formula (4 × (W1-0.8) /0.6+1) in units of μm.
7) The length D16 of the second MMI optical waveguide 16 was obtained from the formula (4 × (W1-0.8) /0.6+2) in units of μm.
8) The width of the second MMI optical waveguide 16 was set to G width (300 nm) + 2 × W1. This corresponds to the width of the second MMI optical waveguide 16 being 1.7 μm to 2.9 μm.
9) The length D20 of the second width tapered optical waveguide 20 was 4 μm.
10) The width of the gap G was set to 300 nm.

  Referring to FIG. 7, for example, as shown by curves TE (0.8), TE (1) and TE (1.2), the width W1 of the first and second sub-width tapered optical waveguides 20a and 20b and It can be seen that the loss of light decreases as W2 increases. This is presumably because the ratio of the widths W1 and W2 that output light increases with respect to the width (300 nm) of the gap G that causes light loss. However, although not shown, when W1 exceeds 1.5 μm, the transmittance loss tends to increase.

  In particular, in the curve TM (1.2) where W1 is 1.2 μm, the loss of input light is suppressed to about 0.8 dB when the displacement Sb1 is −0.02 μm. Further, in the same Sb1 (= −0.02 μm), TE (1.2) having different polarizations suppresses the loss of input light to about 1.15 dB.

  Since the loss of input light in the optical element 10B not having the second and third MMI optical waveguides 16 and 18 and the first width tapered optical waveguide 12 shown in FIG. 4B is about 2 dB, the second The optical element 30 includes the second and third MMI optical waveguides 16 and 18 and the second width tapered optical waveguide 20, and the loss of input light can be further reduced by performing optimization through simulation.

  Further, in order to operate the second optical element 30 practically without polarization dependence, it is required that the difference in transmittance between both polarizations be within 0.5 dB. In this sense, the difference in transmittance between the curves TE (1.2) and TM (1.2) at the displacement Sb1 = −0.02 μm is about 0.3 dB, and the second optical element 30 is The condition is met.

(Application to ONU and OLT)
Subsequently, referring to FIGS. 8A and 8B, the wavelength separation of the second optical element 30 optimized for wavelength separation of light having wavelengths of 1.3 μm and 1.49 μm used in the subscriber system. The characteristics will be described.

In FIG. 8 (A), simulation was repeated under the condition that the thickness of the core 7 was 300 nm until a practically sufficient characteristic was obtained by the FDTD method, and the following conditions 1) to 6) were determined. In the simulation, white light including all wavelengths was input to the second optical element 30.
1) In order to achieve polarization-independent wavelength separation at the two wavelengths described above, the width of the first MMI optical waveguide 14 was 1850 nm and the length was 50.5 μm.
2) The width at the one end face S1 of the first width tapered optical waveguide 12 was 1365 nm. This width is about 65% of the width of the second MMI optical waveguide 16. The taper length of the first width taper optical waveguide 12 is 2 μm.
3) The width of the second MMI optical waveguide 16 was 2100 nm and the length was 2 μm.
4) The length of the third MMI optical waveguide 18 was 4 μm.
5) The widths W1 and W2 of the first and second sub-width tapered optical waveguides 20a and 20b were 900 nm and the displacement Sb1 was −100 nm. That is, the first and second sub-width tapered optical waveguides 20a and 20b are provided to be inclined in the direction of the central axis C2.
6) The width of the gap G was 220 nm.

  FIG. 8A shows four curves. Curves L1 (TE) and L1 (TM) indicate the wavelength separation characteristics of the TE wave and the TM wave of the first light L1 in which the target wavelength is set to about 1.49 μm. Similarly, the curves L2 (TE) and L2 (TM) indicate the wavelength separation characteristics of the TE wave and TM wave of the second light L2 whose target wavelength is set to about 1.3 μm.

  Referring to FIG. 8A, the second optical element 30 has a wavelength separation characteristic that is a periodic function. In the vicinity of the target wavelengths of 1.3 μm and 1.49 μm, the loss of transmittance for both the TE wave and the TM wave is suppressed to within 0.7 dB.

  Further, near the target wavelength, the curves of the TE wave and the TM wave substantially coincide with each other, and the polarization independence is achieved sufficiently for practical use.

In FIG. 8 (B), the simulation was repeated under the condition that the thickness of the core 7 was 220 nm, and practically sufficient characteristics were obtained by the FDTD method, and the following conditions 1) to 6) were determined. Note that white light including all wavelengths was input to the second optical element 30.
1) The width at one end surface S1 of the first width tapered optical waveguide 12 was 437 nm. This width is about 46% of the width of the second MMI optical waveguide 16.
2) In order to achieve polarization-independent wavelength separation at the two wavelengths described above, the width of the first MMI optical waveguide 14 was 950 nm and the length was 12 μm.
3) The width of the second MMI optical waveguide 16 was 1700 nm and the length was 2 μm.
4) The length of the third MMI optical waveguide 18 was 1.3 μm.
5) The widths W1 and W2 of the first and second sub-width tapered optical waveguides 20a and 20b were 700 nm and the displacement Sb1 was −20 nm. That is, the first and second sub-width tapered optical waveguides 20a and 20b are provided to be inclined in the direction of the central axis C2.
6) The width of the gap G was 250 nm.

  Referring to FIG. 8B, the second optical element 30 has a wavelength separation characteristic that is a periodic function. In addition, in the vicinity of the target wavelengths of 1.3 μm and 1.49 μm, the loss of transmittance of both the TE wave and the TM wave is smaller than 1 dB, and it can be seen that the polarization independence is achieved sufficiently practically.

  Further, the degree of coincidence between the TE wave and TM wave curves is slightly inferior to that in FIG.

  This is because when the thickness of the core 7 is 220 nm, the width of the first MMI optical waveguide 14 approaches the cutoff of the primary mode light of the target wavelength of 1.5 μm. This seems to be because the dimensional constraints are increased, for example, the width of the one end surface S1 must be narrowed.

7 Core 8 Si substrate 8a Common plane (Main surface of Si substrate 8)
9 Cladding 10 First optical element 12 First width tapered optical waveguide 14 First multimode interference optical waveguide 16 Second multimode interference optical waveguide 18 Third multimode interference optical waveguide 20 Second width tapered optical waveguide 20a First sub-width Tapered optical waveguide 20b Second sub-width tapered optical waveguide 30 Second optical elements 51, 52, 53 Channel type optical waveguide 54 Output waveguide

Claims (9)

An optical waveguide having a core and a clad provided around the core is provided on the upper side of the common plane, and input light of i pieces (i is an integer of 2 or more) having different wavelengths is polarization independent. An optical element that multiplexes and demultiplexes at
When the width is perpendicular to the light propagation direction and parallel to the common plane,
The optical element is
A first width tapered optical waveguide, a first multimode interference optical waveguide, and a second multimode interference optical waveguide are provided along the light propagation direction,
The first width tapered optical waveguide is connected to one end face of the first multimode interference optical waveguide, and has a first optical input / output port on an end face opposite to the first multimode interference optical waveguide,
The first multimode interference optical waveguide has a width capable of exciting at least the 0th-order mode light and the first-order mode light of the light input to the first multimode interference optical waveguide;
The second multimode interference optical waveguide has a width capable of exciting at least 0th-order mode light, first-order mode light, and second-order mode light of light input to the second multimode interference optical waveguide, A second and third optical input / output port on the end surface opposite to the one multimode interference optical waveguide;
The width of the first width-tapered optical waveguide is increased toward the first multimode interference optical waveguide, and 1 / of the width of the first multimode interference optical waveguide at the end surface on the first multimode interference optical waveguide side. Having a width of 2 or more, provided at a position shifted from the central axis of the first multimode interference optical waveguide,
In the first multimode interference optical waveguide, only the zeroth-order mode light and the first-order mode light are excited.   The width of the first width taper optical waveguide at the end face on the first multimode interference optical waveguide side is a value in the range of 50 to 80% of the width of the first multimode interference optical waveguide. The optical element according to claim 1. The first multimode interference optical waveguide includes a propagation constant difference between a TE wave 0th-order mode light and a first-order mode light, a TM wave 0th-order mode light, and a first-order light for each of the i input lights. It has a width that makes the propagation constant difference of mode light equal,
An even or odd interference order is given to each of the input i light beams, and an odd interference order is output when light having an even interference order is output from the second optical input / output port. The light having the interference order is output from the third light input / output port, and the light having the even interference order is output from the third light input / output port, the light having the odd interference order is output from the second light input / output port. The optical element according to claim 1, wherein the optical element is output from an input / output port. A third multimode interference optical waveguide is provided between the first and second multimode interference optical waveguides;
The width of the third multimode interference optical waveguide increases from the first multimode interference optical waveguide toward the second multimode interference optical waveguide.
The third multimode interference optical waveguide has a width capable of exciting at least the 0th-order mode light, the first-order mode light, and the second-order mode light of the i pieces of light input to the third multimode interference optical waveguide. The optical element according to any one of claims 1 to 3, wherein:   The sum of the lengths along the light propagation direction of the second and third multimode interference optical waveguides is the zero-order mode light of the i number of lights input to the second and third multimode interference optical waveguides. 5. The beat length of the first-order mode light is ¼ or less, and the phase difference between the zero-order mode light and the second-order mode light is set to be π. An optical element according to 1. A second width tapered optical waveguide having first and second sub-width tapered optical waveguides is connected to the second and third optical input / output ports,
The first and second sub-width tapered optical waveguides are reduced in width as they are separated from the second multimode interference optical waveguide,
Each central axis is provided so that it may correspond with the direction of the wave front of the light which propagates the said 1st and 2nd sub width taper optical waveguide. The optical element as described. The first multimode interference optical waveguide includes first and second side surfaces that connect between the one end surface and the other end surface;
The distance between the end of the first width taper optical waveguide on the one end surface and any one of the first and second side surfaces is 0 (zero). An optical element according to 1.   The core is Si, and the clad is made of a material having a refractive index of less than 71.4% of the refractive index of the core. Optical element.   The optical element according to claim 1, wherein i is 2. 9.

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