JP2013156517A - Grating element and optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a grating element without polarization dependence.SOLUTION: A grating element includes an optical waveguide 12 which is constituted of: a clad 14 including a first clad 14a and a second clad 14b having a refractive index larger than that of the first clad, provided on a principal surface 8a of a substrate 8 in this order; and a core 16 provided between the first clad and the second clad and which reflects light with a wavelength λ. The core has regular projections 100L and 100R with a period Λ on both side surfaces 16L and 16R of the core and has a thickness being a length in a direction perpendicular to a principal surface equal to or larger than a width being a length in a direction perpendicular to a light propagation direction and parallel to the principal surface. The projections are arranged in line symmetry to the central axis O of the optical waveguide. The refractive index of a constituent material of the core is larger by 40% or more than that of a constituent material of the first clad. Equivalent refractive indices nTE and nTM of a TE wave and a TM wave on the optical waveguide are matched in a predetermined allowable range.

Description

  The present invention relates to a grating element that operates independently of polarization and an optical element using the 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 termination unit (ONU: Optical Network Unit), , Used in 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 an optical multiplexing / demultiplexing device using this optical waveguide (hereinafter also referred to as a waveguide type optical device), the light propagation path is limited to a pre-made optical waveguide. It is not necessary to align the optical axis of a lens or mirror. Further, in the waveguide type optical element, the light emitting element and the light receiving element may be aligned with the input / output end of the optical waveguide with reference to a mark previously created in the optical multiplexing / demultiplexing 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, a waveguide type optical element in which an optical waveguide (hereinafter also referred to as Si optical waveguide) is composed of a core made of Si and a clad made of SiO ₂ having a large refractive index difference from Si has been reported. (For example, see Non-Patent Documents 1 to 3).

  Since the refractive index of the core is much larger than that of the clad and the light 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 manufacture, a very fine submicron cross-sectional structure can be realized. For these reasons, the waveguide type optical device can be miniaturized by using the Si optical waveguide.

  Known waveguide-type optical elements that perform optical multiplexing / demultiplexing using a Si optical waveguide include those using a Mach-Zehnder interferometer, those using a directional coupler, and those using a grating.

  In the waveguide type optical element using the directional coupler, the light transmittance depends on the wavelength, and therefore the wavelength shift caused by the light source causes the transmittance fluctuation, that is, the intensity fluctuation of the transmitted light. As a result, when the directional coupler is used as an optical multiplexing / demultiplexing device, the intensity of the transmitted output varies. Further, the element length for obtaining the necessary multiplexing / demultiplexing capability is several hundred microns, and it is difficult to reduce the size. A waveguide type optical element using a Mach-Zehnder interferometer needs to connect a large number of elements in series in order to obtain a transmission band having a constant light transmittance, resulting in an increase in the length of the element.

  On the other hand, in a waveguide type optical element using a grating (hereinafter also referred to as a grating element), it is possible to prevent the invasion of light in a predetermined wavelength band into the grating by increasing the reflected light intensity of the Bragg wavelength. As a result, the light transmittance can be made constant in this wavelength band. Further, by providing a grating groove with a period of half or less of the target wavelength, light of the target wavelength can be selected with a sufficient wavelength resolution with a single element (see, for example, Non-Patent Document 4).

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 IEICE Transactions of Electronics vol. E-90-C, no. 1, p. 59, Jan 2007

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 2000-235125 A

However, since the grating element having Si as a core described in Non-Patent Document 4 has polarization dependency, there is a problem that a Bragg wavelength is different between a TE wave and a TM wave. Regarding this problem, a polarization-independent grating element has been proposed as an optical element using InP (for example, see Patent Document 6). In Patent Document 6, polarization independence is achieved by providing a low-refractive index region and a high-refractive index region in an element and making the equivalent refractive indexes of TE wave and TM wave equal in each region. That is, in Patent Document 6, an element is configured by a three-layer structure in which an InGaAsP core is sandwiched between upper and lower InP clads. However, it has been difficult to apply this three-layer structure to an optical element using Si. This is because the equivalent refractive index of the optical waveguide cannot be accurately controlled with such a layer structure because the refractive index of Si, which is the core, and SiO ₂ , which is the cladding, are significantly different.

  The present invention has been made under such a technical background. Accordingly, the object of the present invention is to provide a polarization-independent waveguide-type optical element (grating element) including a grating using Si, which has not been proposed so far and cannot be obtained by simple application of the prior art, and the element. It is to obtain an optical element including the same.

  As a result of intensive studies, the inventor optimized the dimensions of the Si core and the refractive indexes of the first and second claddings sandwiching the core so that the equivalent refractive indexes of the TE wave and the TM wave coincide with each other. I realized that it was possible to make it independent. Accordingly, the grating element of the present invention includes an optical waveguide that is constituted by a clad and a core and reflects light having a wavelength λ.

  The clad includes a first clad provided in this order on the main surface of the substrate, and a second clad having a higher refractive index than the first clad.

  Regular protrusions with a period Λ are provided on both side surfaces of the core. The thickness of the core, which is the length in the direction perpendicular to the main surface, is greater than the width that is the length in the direction perpendicular to the light propagation direction and parallel to the main surface. Further, the projecting portions are arranged in line symmetry with respect to the central axis of the optical waveguide, and the refractive index of the constituent material of the core is 40% or more higher than that of the first cladding.

  And the equivalent refractive indexes nTE and nTM of the TE wave and TM wave related to the optical waveguide coincide with each other within a predetermined allowable range.

  The present invention is configured as described above. Therefore, a polarization-independent grating element and optical element including a grating using Si can be obtained.

It is a perspective view which shows the structure of a grating element roughly. (A) is a side view which shows the schematic structure at the time of seeing a grating element from the one end part side, (B) is a schematic plan view of FIG. It is a characteristic view for demonstrating an example of the design method of the optical waveguide used for a grating element. (A) And (B) is a characteristic view which shows the operating characteristic of a grating element. It is a top view which shows schematic structure of an 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 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.

(Grating element)
Hereinafter, embodiments of the grating element will be described with reference to the drawings. FIG. 1 is a perspective view schematically showing the structure of the grating element. 2A is a side view showing a schematic structure of the grating element as viewed from one end side (indicated by 16I in FIG. 1), and FIG. 2B is an outline of the grating element shown in FIG. FIG. In FIG. 2B, illustration of the clad 14 and the substrate 8 is omitted.

  Here, referring to FIG. 1, directions and dimensions used in the following description are defined. The direction perpendicular to the light propagation direction of the input light IN (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”. . In addition, a direction perpendicular to the main surface 8a is referred to as a thickness direction, and a geometric length measured along the thickness direction is referred to as “thickness”. Similarly, a direction parallel to the light propagation direction is referred to as a length direction, and a geometric length measured along the length direction is referred to as a “length”. A section perpendicular to the light propagation direction of a predetermined structure is referred to as a “cross section”.

(Construction)
The structure of the grating element 10 will be described with reference to FIGS. The grating element 10 includes an optical waveguide 12. The optical waveguide 12 includes a clad 14 provided on the main surface 8 a side of the substrate 8 and a core 16 provided in the clad 14.

  The clad 14 includes a first clad 14a and a second clad 14b. The first and second claddings 14a and 14b are provided in this order on the main surface 8a.

The first cladding 14a is a film body having a uniform thickness formed on the major surface 8a which is a flat surface. In this example, the material constituting the first cladding 14a is SiO ₂ having a refractive index na of about 1.45. In order to prevent undesired coupling of light propagating through the optical waveguide 12 to the substrate 8, the thickness of the first cladding 14a is preferably 1 μm or more, and in this example, it is about 2 μm.

In this example, SiO ₂ is used as the first cladding 14a. However, if the constituent material of the first cladding 14a has a refractive index na satisfying na ≦ (1 / 1.4) nc (≈0.714nc) with respect to the refractive index nc of the core, There is no limit. A suitable material can be selected as the first cladding 14a according to the design.

The second cladding 14b is provided on the first cladding 14a. In this example, Si _1.6 O _1.4 N _1.1 (= SiO _{(1.4 / 1.6)} N _{(1.1 / 1.6)} ) having a refractive index nb of about 1.63 is used as the second cladding 14b. The magnitude relationship between the refractive indexes of the first and second claddings 14a and 14b is “na <nb”. The constituent material of the second cladding 14b can be suitably selected from materials having a refractive index larger than that of the first cladding 14a and smaller than that of the core 16, depending on the design.

In the condition where the core 16 is Si and the first cladding 14a is SiO ₂ (hereinafter also referred to as an example condition), the second cladding 14b has a refractive index nb in the range of 1.46 ≦ nb ≦ 2. It is preferable to select the material. If the refractive index nb is within this range, the grating element 10 can be made polarization independent by optimizing the dimensions of the core 16 under the exemplified conditions.

When Si oxynitride having a composition represented by SiO _x N _y is selected as the second cladding 14b as in this example, the composition ratios x and y of oxygen and nitrogen are set to “2 ≧ x ≧ 0 and 4/3 ≧ y ≧ 0 (except when x = 2 and y = 0) ”are preferable. By setting x and y to values within this range, the refractive index nb of the second cladding 14b can be adjusted within the above-described preferred range.

The refractive index of Si oxynitride SiO _x N _y can be adjusted by changing the oxygen content ratio x / y to nitrogen in the compound. That is, when the oxygen content ratio is decreased by decreasing x / y, the refractive index decreases, and when the oxygen content ratio is increased by increasing x / y, the refractive index increases.

The Si oxynitride having such a composition can be formed by any suitable method such as a thermal oxynitriding method, a plasma CVD (Chemical Vapor Deposition) method, and a reactive sputtering method. In particular, in Japanese Patent Application Laid-Open No. 2011-114045, a Si target is reactively sputtered with an O ₂ + N ₂ mixed gas in which the N ₂ ratio is changed, and the composition ratio x and y are controlled to obtain a Si acid having a desired refractive index. Nitride has been obtained.

  The thickness of the second cladding 14b can be any suitable value selected according to the design of the grating 10, but in order to perform the function as a cladding for the core 16 to an acceptable level in practice, 1 to The thickness is preferably in the range of 3 μm.

  The core 16 is provided between the first and second claddings 14a and 14b. On both side surfaces 16L and 16R of the core 16, regular protrusions 100R and 100L having a constant period Λ are formed. The protrusions 100R and 100L are also collectively referred to as the protrusions 100.

  The optical waveguide 12 including the core 16 reflects light having a wavelength λ corresponding to the period Λ without depending on the polarization. Hereinafter, the light reflected by the optical waveguide 12 is also referred to as reflected light RF. Further, light other than the wavelength λ is transmitted as transmitted light TR without depending on polarization. The optical waveguide 12 functions as an optical waveguide grating. In the optical waveguide 12, the dimensions of the core 16, the refractive indexes of the first and second claddings 14a and 14b, and the like are optimized so that nTE and nTM match within a predetermined allowable range Δn. Here, nTE and nTM are the equivalent refractive indexes of the TE wave and TM wave related to the optical waveguide 12, respectively.

  The TE wave indicates a polarized wave that propagates in the light propagation direction and an electric field vibrates in the width direction in a plane parallel to the main surface 8a. The TM wave indicates a polarized wave that propagates in the light propagation direction and an electric field vibrates in the thickness direction in a plane perpendicular to the main surface 8a. Further, both TE wave and TM wave are collectively referred to as both polarized waves.

In the optical waveguide 12, the period Λ is preferably set to a value satisfying the following formula (1).
Λ = λ / (2 × nTE) = λ / (2 × nTM) (1)
By setting the period Λ in this way, the optical waveguide 12 outputs the reflected light RF having a desired wavelength λ without depending on the polarization. Hereinafter, this wavelength λ is also referred to as a Bragg wavelength. In this example, the period Λ is about 377 nm. This corresponds to a case where the Bragg wavelength λ is about 1.59 μm.

  Moreover, the above-mentioned “nTE and nTM match within a predetermined allowable range Δn” means that the optical waveguide 12 satisfies the relationship of the following formula (2).

nTE = nTM ± Δn (2)
In addition, it is preferable that (DELTA) n is a magnitude | size which satisfy | fills following formula (3).

Δn <nav × Δλ / λ (3)
Here, nav is (nTE + nTM) / 2. Δλ is the full width at half maximum of the reflected light RF when the reflected light RF having the wavelength λ is output from the optical waveguide 12.

  In Δn / nav <Δλ / λ obtained by modifying Equation (3), Δλ / λ on the right side is the wavelength at which the optical waveguide 12 outputs the TE wave and the TM wave as different reflected lights having different wavelengths. Corresponds to resolution. That is, when two lights having a wavelength difference of Δλ / λ or more are input to the optical waveguide 12, these two lights are wavelength-separated and output from the optical waveguide 12 as separate reflected lights.

  On the other hand, Δn, which is the numerator on the left side, corresponds to the wavelength difference | λTE−λTM | of the reflected light RF derived from the TE wave and the TM wave, considering Equation (1). Here, λTE is the wavelength of the reflected light RF when the TE wave is the input light IN, and λTM is the wavelength of the reflected light RF when the TM wave is the input light IN. Similarly, nav as a denominator corresponds to the average wavelength ((λTE + λTM) / 2) of both polarizations of the reflected light RF.

  From these, the expression (3) is a condition for the reflected light derived from the TE wave and the reflected light derived from the TM wave to be output from the optical waveguide 12 as the reflected light RF having a single wavelength λ without being wavelength-separated. Corresponding to

  The core 16 has a rectangular cross section, and the lower surface is provided in contact with the upper surface of the first cladding 14a. Both side surfaces 16L and 16R extend perpendicular to the lower surface of the core 16. The upper and lower surfaces 16U and 16D of the core 16 are arranged in parallel to the vibration surface of the electric field of the TE wave.

  The thickness H of the core 16 is set to be greater than the width Wav. Here, the width Wav is obtained by averaging the width of the core 16 including the protruding portion 100 over the entire length of the core 16, as indicated by a virtual line in FIG. The core indicated by the virtual line is referred to as a virtual core 16 '. The width Wav and the thickness H of the core 16 are values satisfying the expression (1) in the range of 200 to 500 nm. If the thickness H and the width Wav of the core 16 are selected from this range, the optical waveguide 12 can be a single mode waveguide in both the width and thickness directions. In this example, the core 16 has a thickness H of about 300 nm and a width Wav of about 300 nm.

  Furthermore, in the case of the above exemplary conditions, it is preferable that the width Wav is 90% or more of the thickness H and less than 100%. By setting the width Wav within this range, the condition of the refractive index nb of the second cladding 14 required for polarization independence can be relaxed. That is, the optical waveguide 12 can be made independent of polarization in a wider range of nb.

  The left and right protrusions 100 </ b> L and 100 </ b> R provided on the core 16 are arranged symmetrically with respect to the central axis O of the optical waveguide 12. That is, as shown in FIG. 2B, the distances ER and EL from the one end 16I to the protrusions 100R and 100L closest to the one end 16I of the core 16 are equal to each other.

  Further, the number of protrusions 100 provided on the side surfaces 16L and 16R, that is, the number of periods, can be arbitrarily selected in consideration of the reflection efficiency of the optical waveguide 12, that is, the intensity of the reflected light RF. In this example, the number of periods of the protrusion 100 is 80. In order to obtain a practically acceptable reflection efficiency, the number of periods of the protrusions 100 is preferably 10 periods or more.

  Moreover, it is preferable that the protrusion amount D measured from the side surface 16L or 16R of the core 16 of the protrusion 100 is a value in the range of 10 to 100 nm. Here, the protrusion amount D is the distance between the point of the protrusion 100 projecting outward in the width direction and the side surface 16L or 16R of the core 16. In this example, the protrusion amount D is the distance between the side surface 16R and the surface of the protrusion 100R extending in parallel to the light propagation direction. The protruding amount D is related to the reflection efficiency and the half-value width of the reflected light RF in the optical waveguide 12. Increasing the protrusion amount D increases the reflection efficiency of the reflected light RF, but widens the half width of the wavelength. On the contrary, if the protrusion amount D is reduced, the reflection efficiency is reduced, but the half width of the wavelength is narrowed. Therefore, an appropriate value may be selected as the protrusion amount D according to the use of the optical waveguide 12. In this example, the protrusion amount D is about 40 nm.

  Further, when the length measured along the light propagation direction of the protrusion 100 is ML, the ratio of ML to the period Λ is 0.5. Hereinafter, this ratio (ML / Λ) is also referred to as a duty ratio. By setting the duty ratio to 0.5, the intensity of the reflected light RF can be increased.

  As described above, the core 16 is made of Si having a refractive index nc of 3.47. The refractive index nc of the core 16 is 40% or more larger than that of the first cladding 14a. The constituent material of the core 16 is not limited to Si, and any suitable material can be selected.

  Next, a mechanism for making the optical waveguide 12 independent of polarization will be described. The grating element 10 achieves polarization independence by making the equivalent refractive indices of both polarizations of the optical waveguide 12 equal by optimizing the structure of the core 16 and the refractive indexes na and nb of the first and second claddings. doing. By this optimization, the difference in both polarizations of the evanescent wave that oozes out of the core 16 from both side surfaces 16L and 16R is reduced. That is, if the difference between the polarizations of the evanescent wave is reduced, the difference between the polarizations of the interaction between the evanescent wave and the protrusion 100 is also reduced, and as a result, polarization independence is achieved.

  Next, an example of a method for designing the optical waveguide 12 will be described with reference to FIG. In the optical waveguide 12, a formulated condition for achieving polarization independence between the dimensions of the core 16 and the refractive indexes na and nb of the first and second claddings is not required. Therefore, at present, these values are determined from a simulation in which the dimensions of the core 16 and the refractive indexes na and nb of the first and second claddings are changed.

  FIG. 3 shows an example of this simulation. When the core 16 and the first cladding 14a have the above-described dimensions and materials, the refractive index nb of the second cladding 14b that makes the optical waveguide 12 independent of the polarization is shown. The decision method is shown.

  More specifically, FIG. 3 is a characteristic diagram showing a relationship between the refractive index nb of the second cladding 14b and the reflectance of the reflected light RF derived from each polarization. The vertical axis represents the reflectance (non-dimensional) of the reflected light RF derived from each polarization, and the horizontal axis represents the refractive index nb (non-dimensional) of the second cladding 14b. Note that the reflectance is the intensity ratio of the reflected light RF to the input light IN.

  The calculation was performed by a three-dimensional FDTD (Finite Difference Time Domain) method. In addition, the curve TE in FIG. 3 is the reflectance of the reflected light RF derived from the TE wave, and the curve TM is the reflectance of the reflected light RF derived from the TM wave.

  Referring to FIG. 3, it can be seen that when nb = 1, that is, when the second cladding 14b is air, the optical waveguide 12 reflects only the TE wave. The reflection from the TM wave increases as nb increases. When nb = 1.45, that is, when the refractive indexes of the first and second claddings 14a and 14b are equal, the reflectance of the TM wave is about 0.5. It becomes. This reflectance (= 0.5) is approximately half that of the TE wave. When nb is about 1.63, the reflectances of both polarizations are the same at about 0.6. That is, at nb = 1.63, the optical waveguide 12 operates independent of polarization. In this way, by making the equivalent refractive indexes nTE and nTM of both polarizations with respect to the optical waveguide 12 substantially equal, and further making the reflectances of both polarizations equal, the optical waveguide 12 becomes polarization independent.

When the material of the second cladding 14b is the above-described Si oxynitride SiO _x N _y , if nb is determined, the composition ratio x and y can be obtained from nb. Thus, in this example, x = 1.4 / 1.6 and y = 1.1 / 1.6 were determined. In FIG. 3, the reflectance of both polarizations is only about 0.6 under the polarization-independent condition. However, as described above, the reflectance is 0.6 or more by increasing the number of protrusions 100 over 80 periods. Can be increased.

  Subsequently, referring to FIGS. 4A and 4B, it is shown that the grating element 10 using the nb second clad 14b determined in this way operates in a polarization-independent manner. 4A and 4B are characteristic diagrams showing the operating characteristics of the grating element 10. In both figures, the vertical axis represents the intensity ratio (dB) of the output light output from the one end 16I and the other end 16O to the input light IN, and the horizontal axis represents the wavelength (μm) of the light.

  FIG. 4A shows the intensity ratios of the transmitted light TR (curve trTE) and the reflected light RF (curve rfTE) when the input light IN is a TE wave having a wavelength range on the horizontal axis. FIG. 4B shows intensity ratios of the transmitted light TR (curve trTM) and the reflected light RF (curve rfTM) when the input light IN is a TM wave having a wavelength range on the horizontal axis. In the calculation, the above-described dimensions and materials were given to the optical waveguide 12 as conditions. For the calculation, a three-dimensional FDTD method was used. The reflected light RF and the transmitted light TR are output from the optical waveguide 12 while maintaining the same polarization as the input light IN.

  Referring to the curves rfTE and rfTM, as designed, there is a peak of reflected light RF at a wavelength of about 1.59 μm, which is a Bragg wavelength, for both polarizations. The intensity ratio at the peak of both polarizations is equal to about -8 dB. This shows that the intensity of the reflected light RF obtained from the optical waveguide 12 does not depend on the polarization.

  Referring to the curves trTE and trTM, the bottom is taken at the Bragg wavelength λ. As described above, this is because both polarized waves are output as reflected light RF at this wavelength. According to the curve trTM, the intensity ratio of the TM-wave transmitted light TR has a substantially constant value at wavelengths other than the Bragg wavelength λ. On the other hand, according to the curve trTE, the intensity ratio of the transmitted light TR of the TE wave has a large bottom near the wavelength of about 1.3 μm. This is because the TE wave is radiated to the clad 14 in the bottom wavelength band (1.2 to 1.45 μm). Referring to the curves trTE and trTM, at a wavelength other than the bottom wavelength band, that is, a wavelength exceeding 1.45 μm, the intensity ratio of the transmitted light TR of both polarizations is almost equal and polarization independence is achieved. Note that this bottom of the curve trTE hardly causes a problem in practical use. This is because the bottom wavelength band can be changed by changing the design conditions of the optical waveguide 12.

  As is apparent from FIGS. 4A and 4B, the optical waveguide 12 using the second cladding 14b having nb determined by FIG. 3 operates independent of polarization.

  Thus, according to this embodiment, the grating element 10 including the polarization-independent optical waveguide 12 using Si, which has not been proposed so far, can be obtained.

  Hereinafter, modifications of the grating element 10 will be described.

  In this embodiment, the case where the duty ratio (ML / Λ) of the protrusion 100 is set to 0.5 has been described. However, a suitable value can be selected for the duty ratio within the range of 0 <ML / Λ <1.

  Moreover, in this embodiment, the protrusion part 100 is made into the rectangular parallelepiped which protrudes in the width direction from the side surfaces 16L and 16R. However, the shape of the protrusion 100 is not limited to a rectangular parallelepiped. For example, the planar shape may be a semicircular shape, a trapezoidal shape, or a triangular shape. Further, the side surface between the two adjacent protrusions 100 and 100 may be recessed inside the core 16.

(Optical element)
Next, an embodiment of the optical element will be described with reference to FIG. FIG. 5 is a plan view showing a schematic structure of the optical element 50. The optical element 50 includes a grating element as a constituent element. Therefore, in FIG. 5, the same components as those in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant description is omitted. Further, in FIG. 5, reference numerals may be omitted for components whose correspondence with FIGS. 1 and 2 is clear. In FIG. 5, the substrate 8 and the first and second claddings 14a and 14b are not shown.

  The optical element 50 includes first and second grating elements 10-1 and 10-2 having substantially the same configuration as that of the grating element 10. The first and second grating elements 10-1 and 10-2 are configured in the same manner as the grating element 10 except that the Bragg wavelength λ is set to about 1.31 μm. Further, the optical element 50 includes two-input two-output first and second optical couplers C1 and C2 that operate independent of polarization.

  Schematically, in the optical element 50, the first and second grating elements 10-1 and 10-2 are arranged in parallel between the first and second optical couplers C1 and C2. Then, the two outputs of the first optical coupler C1 are connected to the inputs of the first and second grating elements 10-1 and 10-2, and the first and second grating elements 10-1 and 10-2 are connected. The output is connected to two inputs of the second optical coupler C2.

  More specifically, the optical waveguide WG1 is connected to one input terminal IN1-1 of the first optical coupler C1. An optical waveguide WG2 is connected to the other input terminal IN1-2 of the first optical coupler C1. An optical waveguide WG3 is connected to one output end OUT1-1 of the first optical coupler C1. The optical waveguide WG4 is connected to the other output terminal OUT1-2 of the first optical coupler C1.

  A tapered optical waveguide L1 is connected to the input end of the first grating element 10-1, and the input end of the tapered optical waveguide L1 is connected to the optical waveguide WG3. A tapered optical waveguide R1 is connected to the output end of the first grating element 10-1, and the output end of the tapered optical waveguide R1 is connected to the optical waveguide WG5.

  A tapered optical waveguide L2 is connected to the input end of the second grating element 10-2, and the input end of the tapered optical waveguide L2 is connected to the optical waveguide WG4. A tapered optical waveguide R2 is connected to the output end of the second grating element 10-2, and the output end of the tapered optical waveguide R2 is connected to the optical waveguide WG6.

  An optical waveguide WG5 is connected to one input terminal IN2-1 of the second optical coupler C2. An optical waveguide WG6 is connected to the other input terminal IN2-2 of the second optical coupler C2. The optical waveguide WG7 is connected to one output end OUT2-1 of the second optical coupler C2. An optical waveguide WG8 is connected to the other output end OUT2-2 of the second optical coupler C2. In this example, for example, the width and thickness of the optical waveguides WG1 to WG8 are about 300 nm. Thereby, the optical waveguides WG1 to WG8 operate in a single mode.

  Note that the polarization independence is not particularly required for the optical waveguides WG1 to WG8 connecting the constituent elements of the optical element 50. This is because the optical waveguide WG1 to WG8 having the same structure are symmetrically arranged with the center line of the optical element 50 as an axis, so that the phase difference can be canceled for each polarization as the entire element.

  The first and second optical couplers C1 and C2 are required to have polarization independence. In this example, a 2 × 2 optical coupler made of an MMI (Multi Mode Interference) waveguide, which has achieved polarization independence by adjusting element dimensions, is used. More specifically, the first and second optical couplers C1 and C2 have a width of about 1600 nm and a thickness of about 300 nm, thereby achieving polarization independence in the vicinity of a wavelength of 1.55 μm.

  In this example, an optical coupler made of an MMI waveguide having a rectangular planar shape is used, but the first and second optical couplers C1 and C2 are MMI waveguides as long as they have polarization independence. It is not limited to waveguide optical couplers. For example, an optical coupler using a directional coupler or other 2 × 2 branching element may be used.

  Further, since the tapered optical waveguides L1, R1, L2, and R2 are also symmetrically arranged with the central axis of the optical element 50 as an axis, the polarization independence is not required for the reason described above.

  Next, with reference to FIG. 5, the operation when the optical element 50 is used as an ONU in an optical subscriber communication system (hereinafter also referred to as an optical subscriber system) will be described.

  Here, the optical waveguide WG1 is connected to an LD (Laser Diode) which is a light emitting element that emits the upstream optical signal UP. The wavelength λ1 of the light used for the upstream optical signal UP is about 1.31 μm generally used in the optical subscriber system. In general, the upstream optical signal UP is input from the LD to the first and second grating elements 10-1 and 10-2 via the optical waveguide WG1 and the first optical coupler C1. Then, the light is reflected by the first and second grating elements 10-1 and 10-2, returns to the first optical coupler C1, and is output from the optical waveguide WG2 and transmitted to the station side. The upstream optical signal UP is output from the LD as a TE wave due to the output characteristics of the LD.

  The optical waveguide WG7 is connected to a PD (PhotoDiode) that is a light receiving element. The PD receives the downstream optical signal DN transmitted from the station side. The wavelength λ2 of light used for the downstream optical signal DN is about 1.49 μm that is generally used in an optical subscriber system. Schematically, the downstream optical signal DN is input from the station to the first and second grating elements 10-1 and 10-2 via the optical waveguide WG2 and the first optical coupler C1. Then, the light passes through the first and second grating elements 10-1 and 10-2, is input to the second optical coupler C2, is output from the optical waveguide WG7, and is received by the PD. In addition, as a result of the polarization plane being rotated in the process of propagating the optical fiber between the station and the subscriber, the downstream optical signal DN including the polarization planes in all directions on average reaches the optical element 50.

  Further, the upstream optical signal UP propagates toward the station side and the downstream optical signal DN propagates toward the optical element 50 side inside the optical waveguide WG2. Here, it is assumed that the periods of the first and second grating elements 10-1 and 10-2 are optimized so that the wavelength λ1 of the upstream optical signal UP becomes the Bragg wavelength λ.

  First, the operation of the optical element 50 regarding the downstream optical signal DN will be described. The downstream optical signal DN having the wavelength λ2 propagating through the optical waveguide WG2 toward the optical element 50 is input from the other input terminal IN1-2 to the first optical coupler C1. In the first optical coupler C1, the downstream optical signal DN in which a plurality of modes is excited propagates while interfering with each other, and is output with equal power from one and the other output terminals OUT1-1 and OUT1-2.

  The downstream optical signal DN output from the output terminal OUT1-1 is input to the first grating element 10-1 via the optical waveguide WG3 and the tapered optical waveguide L1. Similarly, the downstream optical signal DN output from the output terminal OUT1-2 is input to the second grating element 10-2 via the optical waveguide WG4 and the tapered optical waveguide L2. Here, since the optical waveguides WG3 and WG4 and the tapered optical waveguides L1 and L2 have the same shape and symmetrical arrangement, the phase difference is canceled for each polarization, and the polarization becomes independent as a whole.

  Since the Bragg wavelength of the first and second grating elements 10-1 and 10-2 is λ1, the downstream optical signal DN having the wavelength λ2 is transmitted without being reflected, and the second optical coupler maintains its polarization. Input to C2. Here, since the optical waveguides WG5 and WG6 and the tapered optical waveguides R1 and R2 have the same shape and are symmetrically arranged, the phase difference is canceled for each polarization, and the polarization becomes independent as a whole.

  Here, the downstream optical signal DN input from the one input terminal IN2-1 to the second optical coupler C2 is referred to as a first downstream component DN1. Similarly, the downstream optical signal DN input from the other input terminal IN2-2 to the second optical coupler C2 is referred to as a second downstream component DN2. The first and second downlink components DN1 and DN2 input to the second optical coupler C2 excite a plurality of modes, respectively, and propagate while interfering between the plurality of modes and between the downlink components DN1 and DN2. As a result of the interference in the first and second optical couplers C2, the downstream optical signal DN having a phase difference of π is output only from one output terminal OUT2-1 of the second optical coupler C2, and passes through the optical waveguide WG7. Output to PD.

  Next, the operation of the optical element 50 related to the upstream optical signal UP will be described. The upstream optical signal UP of wavelength λ1 output from the LD is input from one input terminal IN1-1 to the first optical coupler C1 via the optical waveguide WG1. In the first optical coupler C1, the upstream optical signal UP in which a plurality of modes are excited propagates while interfering with each other, and is output with equal power from one and the other output terminals OUT1-1 and OUT1-2.

  The upstream optical signal UP output from the output terminal OUT1-1 is input to the first grating element 10-1 via the optical waveguide WG3 and the tapered optical waveguide L1. Similarly, the upstream optical signal UP output from the output terminal OUT1-2 is input to the second grating element 10-2 via the optical waveguide WG4 and the tapered optical waveguide L2. For the reasons described above, the optical waveguides WG3 and WG4 and the tapered optical waveguides L1 and L2 are independent of polarization as a whole structure.

  The upstream optical signal UP having the wavelength λ1 equal to the Bragg wavelength of the first and second grating elements 10-1 and 10-2 is reflected as described in the embodiment. As a result, the upstream optical signal UP is input again to the first optical coupler C1 from one and the other output terminals OUT1-1 and OUT1-2 of the first optical coupler C1.

  Here, the upstream optical signal UP input from the one output terminal OUT1-1 to the first optical coupler C1 is referred to as a first upstream component UP1. Similarly, the upstream optical signal UP input from the other output terminal OUT1-2 to the first optical coupler C1 is referred to as a second upstream component UP2. The first and second upstream components UP1 and UP2 input to the first optical coupler C1 respectively excite a plurality of modes and propagate while interfering between the plurality of modes and between the upstream components UP1 and UP2. In the process of reciprocating through the first optical coupler C1, the upstream optical signal UP having a phase difference of π is output only from the other input terminal IN1-2 of the first optical coupler C1, and is transmitted to the station side via the optical waveguide WG2. Sent.

  Thus, the optical element 50 using the first and second grating elements 10-1 and 10-2 can multiplex and demultiplex light having different wavelengths without depending on the polarization.

8 Substrate 8a Main surface 10 Grating element 10-1 First grating element 10-2 Second grating element 12 Optical waveguide 14 Cladding 14a First cladding 14b Second cladding 16 Core 16 'Virtual core 16U Upper surface 16D Lower surface 16L Left side surface (side surface) )
16R Right side (side)
50 Optical elements 100, 100L, 100R Protrusion C1 First optical coupler C2 Second optical coupler IN1-1, IN2-1 One input terminal IN1-2, IN2-2 The other input terminal OUT1-1, OUT2-1 Output terminal OUT1-2, OUT2-2 The other output terminal WG1, WG2, WG3, WG4, WG5, WG6, WG7, WG8 Optical waveguide L1, R1, L2, R2 Tapered optical waveguide

Claims (11)

A clad comprising a first clad provided in this order on the main surface of the substrate, a second clad having a higher refractive index than the first clad, and a core provided between the first and second clads Comprising an optical waveguide that reflects light of wavelength λ,
Protruding portions having a period Λ are provided on both side surfaces of the core, and the core has a thickness that is a length in a direction perpendicular to the main surface, in a direction perpendicular to the light propagation direction and parallel to the main surface. The projecting portion is arranged in line symmetry with respect to the central axis of the optical waveguide, and the refractive index of the constituent material of the core is higher than that of the constituent material of the first cladding. Is over 40% larger,
A grating element characterized in that the TE-wave and TM-wave equivalent refractive indexes nTE and nTM relating to the optical waveguide coincide within a predetermined tolerance.   The grating element according to claim 1, wherein the period Λ is set so as to follow a relationship of Λ = λ / (2 × nTE).   When nav = (nTE + nTM) / 2 and the full width at half maximum of the light having the wavelength λ reflected by the optical waveguide is Δλ, the allowable range is a value smaller than nav × Δλ / λ. The grating element according to claim 2.   The grating element according to any one of claims 1 to 3, wherein the protrusions are provided on the both side surfaces for 10 cycles or more. The grating element according to claim 1, wherein the core material is Si, and the first cladding material is SiO ₂ .   The grating element according to claim 5, wherein an amount of protrusion measured from a side surface of the core of the protrusion is a value in a range of 10 to 40 nm. The material of the second cladding is SiO _x N _y (where x and y are excluding (x = 2 and y = 0) (2 ≧ x ≧ 0 and 4/3 ≧ y ≧ 0)). The grating element according to claim 5 or 6, wherein:   The grating element according to claim 7, wherein x is (1.4 / 1.6), and y is (1.1 / 1.6).   The grating element according to claim 5, wherein a refractive index of the second cladding is set to a value of 1.46 or more and 2 or less.   The grating element according to any one of claims 5 to 9, wherein the width is 90% or more of the thickness. Two grating elements according to any one of claims 1 to 10,
A two-input two-output first and second optical coupler that operate without polarization;
Two outputs of the first optical coupler are respectively connected to one end of the two grating elements, and two inputs of the second optical coupler are respectively connected to the other ends of the two grating elements. A characteristic optical element.

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