JP2006047895A - Photonic crystal semiconductor device and semiconductor laser integrated device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an optical modulator which has low chirping properties and a sufficient extinction ratio and enables high speed operation even when high speed input is performed and integration with a semiconductor laser. <P>SOLUTION: A photonic crystal semiconductor device using a photonic crystal formed on a semiconductor substrate and having a periodical structure of two-dimensional refractive index in order of an optical wavelength has a light incident part (13) making light incident on the photonic crystal (12) from a prescribed direction, a light emitting part (14) taking out light from the photonic crystal and an electrode (15) applying voltage or current to the photonic crystal. A multiple quantum well structure (24) is formed in the photonic crystal (12) and the transmission direction of light in the photonic crystal (12) is changed by applying voltage or current to the crystal. In particular, an angle formed by the incident direction of light and either one symmetrical axis of the periodical structure is specified to be made larger than 0° and smaller than 15°. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a photonic crystal semiconductor device used for optical communication and a semiconductor laser integrated device in which it is monolithically integrated with a semiconductor laser.

  With the explosive increase in Internet traffic, there is a demand for an increase in transmission capacity in optical communications. In order to meet this demand, wavelength division multiplexing (WDM) is adopted, and ultra-high capacity exceeding 1 terabit / second is becoming possible at a commercial level.

  In a signal light source for transmission as a key component supporting this WDM, a method for modulating a semiconductor laser includes a direct modulation method for directly modulating the drive current of the semiconductor laser and an external light source while maintaining the laser output light at a constant intensity. There is an external modulation system that modulates using a modulator. Among these, in the direct modulation method, chirping in which wavelength variation due to the amount of carriers in the active layer of the laser occurs as a modulation current is applied becomes a problem. Chirping has been a problem particularly when high-speed modulation of the order of gigahertz is performed because the waveform distortion after transmission is caused by the dispersion characteristics of the optical fiber.

Therefore, an external modulation method is used in the case of large capacity and long distance transmission. As an example of the external modulator, there is a Mach-Zehnder type modulator using LiNbO ₃ , which can eliminate wavelength fluctuation in principle, but it is difficult to downsize the device, There was a disadvantage that monolithic integration was not possible. On the other hand, an absorption modulator (EA modulator) using a semiconductor multiple quantum well structure can control chirping by the design of the quantum well structure, and can be integrated with a semiconductor laser. It has the advantage that it can be driven and miniaturized.

  However, in the EA modulator, carriers generated by light absorption tend to be accumulated in the quantum well, so that there is a problem that high-speed modulation operation is hindered when strong light of several mW or more is incident. .

  By the way, in recent years, so-called photonic crystals in which a periodic structure of the wavelength order of light is formed using two or more kinds of materials having different refractive indexes have attracted attention, and both research and theoretical aspects have been observed. Energetic research has been carried out. In this photonic crystal, a new function that is not found in conventional optical materials has been reported. Among them, there is a super prism effect that the propagation direction varies greatly depending on the wavelength of light (Non-patent Document 1). Using this super prism effect, the position of the emitted light can be changed greatly by changing the incident light wavelength to the photonic crystal slightly, and the output angle can be changed by changing the incident angle to the photonic crystal slightly. The position of the incident light can be changed greatly. That is, the light path can be modulated with a large width.

As a device using this super prism effect, Non-Patent Document 2 discloses a device that electrically controls the super prism effect by applying a voltage to a photonic crystal using a PLZT (PbLaZrTiO ₃ ) crystal.

  However, in an optical integrated circuit, it is desirable that a light control element such as an optical modulator can be monolithically integrated with a semiconductor-based light receiving / emitting element. In this respect, the non-semiconductor device as described above is disadvantageous because it cannot be integrated with a semiconductor laser or the like.

In this regard, as disclosed in Patent Document 1, an optical modulator using a semiconductor photonic crystal has also been proposed. The optical modulator shown in Embodiment 4 of Patent Document 1 includes a waveguide formed in a photonic crystal and an electrode that changes the photonic band gap by applying an electric field to the waveguide. The exit end face of the waveguide is formed to be inclined with respect to the waveguide direction. This optical modulator realizes modulation of a photonic band gap → modulation of a refractive index of a photonic crystal → modulation of a light emission angle from an emission end face by applying a voltage.
Toshihiko Baba and Masanori Nakamura, `` Photonic Crystal Light Deflection Devices Using the Superprism Effect '', IEEE Journal of Quantum Electronics, Vol.38, No.7, July 2002, p.909-914 David Scrymgeour, Natalia Malkovia, Sungwon Kim and Venkatraman Gopalan, `` Electro-optic control of superprism effect in photonic crystals '', Applied Physics Letters, Vol.82, No.19, 12 May 2003, P.3176-3178 JP 2002-196296 A

  However, the optical modulator described in Patent Document 1 does not actively use the super prism effect in the photonic crystal, but only uses the modulation of the emission angle accompanying the modulation of the refractive index of the semiconductor photonic crystal. Therefore, a large spatial modulation width could not be obtained, and a sufficient extinction ratio could not be expected.

  In view of the above, the present invention realizes an optical modulator that is low in chirping, capable of high-speed operation even at high input, has a sufficient extinction ratio, and can be integrated with a semiconductor laser. It was made as a purpose.

  In order to achieve the above object, the present invention provides a photonic crystal semiconductor device using a photonic crystal formed on a semiconductor substrate and having a two-dimensional refractive index periodic structure in the wavelength order of light. A light incident part for making light incident on the nic crystal from a predetermined direction; a light emitting part for extracting light from the photonic crystal; and an electrode for applying a voltage or current to the photonic crystal; The photonic crystal semiconductor device is characterized in that a multiple quantum well structure is formed in a nick crystal, and a light propagation direction in the photonic crystal is changed by application of the voltage or current.

  This photonic crystal semiconductor device changes the refractive index of the photonic crystal by applying voltage or current, and changes the propagation direction of light in the photonic crystal using the super prism effect based on this refractive index change. Therefore, a large emission angle can be modulated by modulating applied voltage or current. In addition, since the semiconductor multiple quantum well structure is formed in the photonic crystal, the refractive index of the photonic crystal can be sufficiently changed by applying the electro-optic effect of the multiple quantum well structure by applying a voltage. .

  Examples of the structure of the photonic crystal include a structure in which air hole rods are formed in a periodic array in the semiconductor, and the air hole rods have a minimum unit of a regular equilateral triangle or a square. .

  If the angle formed between the light incident direction and any one of the symmetry axes of the periodic array is greater than 0 ° and less than 15 °, the super prism effect can be fully utilized.

  Further, from the viewpoint of light input / output to / from the photonic crystal device, it is desirable that a semiconductor waveguide is connected to the incident portion and the emission portion. In this case, it is preferable that the semiconductor waveguides connected to the incident part and the emission part are arranged on non-identical axes.

  By integrating the above-mentioned photonic crystal semiconductor device monolithically with the semiconductor laser on the semiconductor substrate, an external modulator integrated laser light source can be obtained.

  Since the photonic crystal device of the present invention utilizes the super prism effect, a large emission angle can be modulated, and a high extinction ratio can be obtained. Therefore, according to the present invention, it is possible to provide a small optical modulator that is low in chirping, capable of high-speed operation even at high input, and excellent in extinction ratio.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

First, the refractive index periodic structure of the photonic crystal used in the optical modulator according to the present invention will be described. FIG. 1 is a schematic diagram of a two-dimensional photonic crystal. In Figure 1, the two-dimensional photonic crystal, in the high refractive index medium M _H, paper rod shape of the low refractive index medium M _L of circular cross-section extending in the vertical direction, periodically arranged in wavelength order of light Has a two-dimensional periodic structure. The lattice constant (arrangement period) a of the photonic crystal is defined as the distance from the center of the adjacent air hole, and the diameter of the rod is b. Although the case where the minimum unit of the array is an equilateral triangle is shown here, a square or the like may be used. The symmetry axis (Γ-M direction, Γ-K direction) of the periodic array is represented by a chain line.

The refractive index of the high refractive index medium M _H of the photonic crystal and 3.2 (semiconductor crystals), FIG energy band diagram of the photonic crystal structure when the 1 the refractive index of the low refractive index medium M _L 2 Shown in The vertical axis represents the frequency of incident light normalized by the lattice constant a. FIG. 2 shows the TE mode (the electric field of incident light exists only in the direction orthogonal to the air holes). Since the dispersion curve (relationship between wave vector and normalized frequency) of the lowest order energy (basic mode) in FIG. 2 is almost the same as that without the photonic crystal structure, the super prism effect cannot be expected. Therefore, in order to use the super prism effect, the energy of incident light needs to correspond to a high-order energy level. For this purpose, as shown in FIG. 2, the photonic is set so that k ₀ (= 2π / λ)> 0.3 × 2π / a (k ₀ : propagation constant in vacuum) with respect to the wavelength λ of the incident light. It is necessary to set the lattice constant a of the crystal.

Next, FIG. 3 shows a conceptual diagram of the configuration of the optical modulator according to the present invention. The optical modulator 1 includes a photonic crystal region 2, an input waveguide 3, and an output waveguide 4. The y-axis in the direction of the rod of the air hole constituting the photonic crystal (direction perpendicular to the paper surface), the z-axis in the waveguide direction in the waveguide, and the x-axis in the direction perpendicular to the y-axis and z-axis Take and explain.
The axis of the input waveguide 3 is tilted so as to have angles θ1 and θ2 other than 0 ° with respect to the symmetry axes Γ-M and Γ-K of the periodic array in the photonic crystal region 2. In other words, the photonic crystal pattern is designed so that neither the symmetry axis Γ-M nor Γ-K matches the z-axis. This is because the super prism effect cannot be obtained when light is incident parallel to one of these symmetry axes. It is preferable that either one of the angles θ1 and θ2 is greater than 0 ° and smaller than 15 °.

  The photonic crystal region 2 is provided with an electrode (not shown) for applying a voltage to change the electric field. In order to couple light propagating through the photonic crystal to the output waveguide 4 without applying an electric field, the input waveguide 3 and the output waveguide 4 have a refraction angle so as to be arranged on non-coaxial axes. In consideration of the axis deviation, it is formed with Δd.

The refractive index change caused by the quantum confined Stark effect (QCSE), which is a kind of electroabsorption effect in a semiconductor quantum well structure, is generally on the order of several percent. In the absence of a photonic crystal structure, the refraction angle follows Snell's law. Therefore, considering the case where the light beam is incident on the interface (refractive index 3.2) between the bulk semiconductor crystals at an incident angle of 5 °, the change in the refractive angle due to the change of one refractive index by 1% is only 0. .05 °.
On the other hand, in the super prism effect in the photonic crystal, when the refractive index changes by 1%, deflection of about 10 ° occurs. Therefore, if the z-direction length of the photonic crystal region 2 in FIG. 3 is 20 μm, the position of the beam after propagation is shifted by 3.5 μm in the z-axis direction due to the above deflection. In general, since the width of a semiconductor single mode waveguide is about 2 μm, the laser beam is not coupled to the output waveguide 4 by the above shift. That is, the light intensity at the output end of the output waveguide 4 is modulated by the applied voltage.

  A semiconductor laser may be disposed at the position of the input waveguide 3 of the optical modulator 1, and laser light may be incident on the photonic crystal region 2 from the waveguide region of the semiconductor laser. That is, by forming the photonic crystal region 2 on a semiconductor substrate and forming a monolithic semiconductor laser that does not require the formation of a reflecting mirror such as a DFB laser or a DBR laser in the input waveguide 3 portion, a high output A small modulator integrated semiconductor laser capable of high-speed operation and excellent extinction ratio can be obtained.

  Specific examples of the optical modulator according to the present invention will be described. As an example, a photonic crystal formed in a semiconductor having a multiple quantum well structure is used, and a refractive index is modulated by an electro-optic effect by applying a voltage. An optical modulator configured to modulate the propagation direction of light therein will be described.

  FIG. 4 is a partial cross-sectional view illustrating the optical modulator according to the first embodiment. The x, y, and z axes are defined as in FIG. The optical modulator 11 includes a photonic crystal region 12, an input waveguide region 13, and an output waveguide region 14.

  The photonic crystal region 12 is formed in a central portion of the optical modulator 11 with a size of 20 μm square, and an upper electrode 15 is formed on the upper surface of the optical modulator 11 so as to surround the photonic crystal region 12. A lower electrode 16 is formed on the lower surface of the optical modulator 11.

Photonic crystal region 12 is made the air hole is a semiconductor and the low refractive index medium M _L constituting the optical refractive index medium M _H, a semiconductor has the following layer structure. That is, an n-InP lower cladding layer 22, an undoped GaInAsP lower optical confinement layer 23, an undoped GaInAsP multiple quantum well structure 24, an undoped GaInAsP upper optical confinement layer 25, and a p-InP upper cladding layer formed on the n-InP substrate 21. 26 and a p-GaInAsP contact layer 27.

The air hole is the low refractive index medium M _L so as to penetrate the non-doped GaInAsP multiple quantum well active layer 24, et provided through from an upper surface of the optical modulator 11 to the middle of the n-InP lower cladding layer 22 ing.

  The input waveguide region 13 and the output waveguide region 14 are formed on an n-InP substrate 21 common to the photonic crystal region 12, on a non-doped InP lower cladding layer 28, a non-doped GaInAsP waveguide layer 29, a non-doped InP upper cladding layer 30, The p-InP upper clad layer 26 and the p-GaInAsP contact layer 27 are formed in a layer structure. The input waveguide region 13 and the output waveguide region 14 have a mesa stripe structure embedded with an Fe-doped InP embedded layer 32 in order to confine light in the x-axis direction, and form a waveguide having a width w. Yes.

Photonic crystal pattern in the photonic crystal region 12 is a pattern as shown in FIG. 1, the lattice constant a = 651 nm, and the low refractive index medium diameter b = 391 nm of the (air holes) _{M L.} The angle between the axis of the input waveguide and the Γ-K direction of the photonic crystal was 10 °. (Note that the ratio of a to b and the angle are not necessarily accurately shown in FIG. 4).
Further, the input waveguide and the output waveguide were provided with an axis deviation of 3 μm in consideration of the refraction angle of the light emitted from the photonic crystal without applying a voltage. This refraction angle varies depending on a and b, which are structural parameters of the photonic crystal, and an angle formed between the axis of the input / output waveguide and the symmetry axis of the photonic crystal. Therefore, it is important to obtain the refraction angle in advance empirically or theoretically in accordance with the design values of the parameters and angles.

  Next, a method for manufacturing the optical modulator according to the first embodiment will be described with reference to FIGS.

First, as shown in FIG. 5A, an n-InP lower cladding layer 22, an undoped GaInAsP lower optical confinement layer 23, an undoped layer are formed on an n-InP substrate 21 by using a metal organic chemical vapor deposition (MOCVD) method. GaInAsP multiple quantum well structure 24 (band gap wavelength of quantum well layer 1.54 μm), non-doped GaInAsP upper optical confinement layer 25, thin non-doped InP layer (not shown), p-InP upper clad layer 26 are laminated in order. Is made. A SiN _x film 40 is formed on the surface of the wafer by plasma CVD, and a 20 μm square pattern photoresist film 41 is formed on the SiN _x film 40 using photolithography. The region where the photoresist film 41 is formed later becomes the photonic crystal region 12 (see FIG. 4). The SiN _x film 40 is dry-etched using the photoresist film 41 as a mask to transfer the pattern onto the SiN _x film 40 as shown in FIG. 5B, and then the photoresist film 41 is formed using oxygen plasma. Remove.

As shown in FIG. 6A, using the SiN _x film 40 as a mask, the p-InP upper cladding layer 26, the non-doped GaInAsP upper optical confinement layer 25, the non-doped GaInAsP multiple quantum well structure 24, and the non-doped GaInAsP lower optical confinement layer 23 are formed. Etching is performed and a part of the n-InP lower cladding layer 22 is also etched.

Next, as shown in FIG. 6B, using the SiN _x 40 as a selective growth mask, the etched region is a non-doped InP lower cladding layer 28, and a non-doped GaInAsP waveguide layer 29 (film thickness) having a band gap wavelength of 1.3 μm. 0.2 μm) and a non-doped InP upper clad layer 30 are sequentially laminated by using the MOCVD method.

As shown in FIG. 6C, after the SiN _x film 40 is removed by dry etching, a p-InP upper clad layer 26 and a p-GaInAsP contact layer 27 are stacked on the entire wafer by MOCVD.

Next, as shown in FIG. 7A, an SiN _x film 42 is formed on the wafer surface by a plasma CVD method, an input / output waveguide pattern is formed by photolithography, and the photonic crystal region 12 (see FIG. 7). 4), a rectangular pattern of the photoresist film 43 is formed.

Next, the SiN _x film 42 is dry-etched using the photoresist film 43 as a mask to transfer the above pattern to the SiN _x film 42 as shown in FIG. 7B, and then using oxygen plasma. The photoresist film 43 is removed.

Subsequently, as shown in FIG. 8A, using the SiN _x film 42 as a mask, the p-GaInAsP contact layer 27, the p-InP upper clad layer 26, the non-doped InP upper clad layer 30, the non-doped GaInAsP waveguide layer 29, The non-doped InP lower cladding layer 28 is etched, and a part of the n-InP lower cladding layer 22 is also etched.

  Next, as shown in FIG. 8B, using the MOCVD method, a high resistance Fe film is formed so that the regrowth surface is located at the same height as the p-GaInAsP contact layer 27 in the etched region. A doped InP buried layer 32 is grown.

After removing the SiN _x film 42 used as a mask, as shown in FIG. 9A, an SiN _x film 44 is formed on the entire surface of the wafer by plasma CVD, and an electron beam lithography apparatus is used. A patterned electron beam resist film 45 is formed in the photonic crystal region 12.

Using this electron beam resist film 45 as a mask, the SiN _x film 44 is dry-etched to transfer the photonic crystal pattern to the SiN _x film 44 as shown in FIG. 9B.

Next, etching is performed to a depth (see FIG. 4) reaching the n-InP lower cladding layer 22 by reactive ion etching (RIE). Thereafter, the SiN _x film 44 is removed by dry etching.

  Finally, after the upper electrode 15 is formed using the lift-off method so as to contact the outer peripheral portion of the photonic crystal pattern, the lower electrode 16 is formed on the back surface of the n-InP substrate 21, and the light shown in FIG. The modulator 11 is completed.

  The relationship between the voltage applied to the photonic crystal region 12 of the optical modulator 11 and the light output from the output end in the output waveguide region 14 is shown in FIG. Incident light was laser light having a wavelength of 1.55 μm, and was coupled to the TE mode of the optical modulator using a polarization maintaining fiber. The light output from the output end when no voltage is applied is 10 mW. An extinction ratio of 20 dB was obtained at an applied voltage of 3V.

  As described above, the optical modulator according to the present invention uses the super prism effect that occurs in the photonic crystal based on the change in the refractive index of the multiple quantum well structure, so that conventional absorption modulation with generation of carriers is performed. Compared to a device, it has the advantages of being capable of high-speed operation even at high input and providing a high extinction ratio.

  In this embodiment, a photonic crystal structure having a triangular lattice in which the minimum unit of the array is a regular triangle is used. However, the minimum unit of the array may not be a strict regular triangle, and the minimum unit may be a square (or Even when a square lattice photonic crystal having a substantially square shape is used, an optical modulator having similar performance can be realized.

  Also, when the incident light is TM polarized (when the incident light is controlled to TM polarized by a polarization maintaining fiber, or when integrated with a semiconductor laser that oscillates with TM polarization such as a tensile strain quantum well laser) However, it is possible to realize an optical modulator having similar performance by designing photonic crystal parameters based on the photonic crystal band structure with TM polarization.

  In this embodiment, there is one output waveguide, but by using two or more output waveguides, a waveguide to which light propagated through the photonic crystal region is coupled can be selected by applying an electric field. It can also be used as an optical switch.

  Further, instead of providing an output waveguide as shown in FIG. 3, a light output surface may be formed on the photonic crystal so that the emitted light is coupled to an external light extraction means. A schematic diagram in this case is shown in FIG. The light incident on the photonic crystal region 2 from the input waveguide 3 propagates through the photonic crystal and is emitted from the light emitting end face 50 provided on the emission side of the photonic crystal region 2. Near the light emitting end face 50, a light extraction means 51 such as an optical fiber or a lens is provided at a position in consideration of the refraction angle when no voltage is applied. By applying a voltage to the photonic crystal, the refractive index of the photonic crystal changes, and in accordance with the change in the refractive index, both a change in the propagation direction due to the super prism effect and a change in the emission angle from the light exit end face 50 occur. Then, the coupling efficiency to the light extraction means 51 changes.

  In general, chirping that is a problem in the EA modulator is caused by phase modulation on the exit surface due to a change in refractive index. However, in the above-described configuration of the present invention, the light propagation length is 1 / in the case of the EA modulator. Since it is as short as about 10, chirping is very small.

  When the present invention is applied to an optical integrated circuit, if a relatively low speed (response time of 1 ns or more) is sufficient, the plasma effect by current injection is used without applying voltage. A change in refractive index or a change in refractive index due to heat injection may be used.

It is a schematic diagram of a two-dimensional photonic crystal. FIG. 2 is an energy band diagram of the photonic crystal structure shown in FIG. 1. It is a conceptual diagram of the structure of the optical modulator which concerns on this invention. It is a fragmentary sectional perspective view which shows the optical modulator which concerns on Example 1 of this invention. It is a perspective view which shows the manufacturing method of the optical modulator which concerns on Example 1 of this invention. It is a perspective view which shows the manufacturing method of the optical modulator which concerns on Example 1 of this invention. It is a perspective view which shows the manufacturing method of the optical modulator which concerns on Example 1 of this invention. It is a perspective view which shows the manufacturing method of the optical modulator which concerns on Example 1 of this invention. It is a perspective view which shows the manufacturing method of the optical modulator which concerns on Example 1 of this invention. It is a graph which shows the relationship between the applied voltage and optical output of the optical modulator which concerns on Example 1 of this invention. It is a schematic diagram which shows the modification of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical modulator 2 Photonic crystal region 3 Input waveguide 4 Output waveguide 5 Electrode 11 Optical modulator 12 Photonic crystal region 13 Input waveguide region 14 Output waveguide region 15 Upper electrode 16 Lower electrode 21 n-InP substrate 22 n-InP lower cladding layer 23 non-doped GaInAsP lower optical confinement layer 24 non-doped GaInAsP multiple quantum well structure 25 non-doped GaInAsP upper optical confinement layer 26 p-InP upper cladding layer 27 p-GaInAsP contact layer 28 non-doped InP lower cladding layer 29 non-doped GaInAsP conducting waveguide layer 30 non-doped InP upper clad layer 32 Fe-doped InP burying layer 40 SiN _x film 41 a photoresist film 42 SiN _x film 43 a photoresist film 44 SiN _x film 45 electron beam resist film 50 light emitting Surface 51 the light extraction means

Claims (9)

In a photonic crystal semiconductor device using a photonic crystal formed on a semiconductor substrate and having a two-dimensional refractive index periodic structure in the wavelength order of light,
A light incident part that makes light incident on the photonic crystal from a predetermined direction; a light emitting part that extracts light from the photonic crystal; and an electrode that applies a voltage or current to the photonic crystal; and A multiple quantum well structure is formed in the photonic crystal,
A photonic crystal semiconductor device, wherein a propagation direction of light in the photonic crystal is changed by application of the voltage or current. The photonic crystal semiconductor device according to claim 1, wherein the photonic crystal is formed by air hole rods having a periodic arrangement. The photonic crystal semiconductor device according to claim 1, wherein the periodic arrangement of the air hole rods has a substantially equilateral triangle as a minimum unit. 3. The photonic crystal semiconductor device according to claim 1, wherein the periodic arrangement of the air hole rods has a substantially square as a minimum unit. 5. The photonic crystal according to claim 2, wherein an angle formed between the incident direction of the light and any one axis of symmetry of the periodic array is greater than 0 ° and smaller than 15 °. Semiconductor device. The photonic crystal semiconductor device according to claim 1, wherein a first semiconductor waveguide is connected to the incident portion. The semiconductor laser integrated device according to claim 1, wherein a second semiconductor optical waveguide is formed in the light emitting portion. 8. The semiconductor laser integrated device according to claim 7, wherein the first semiconductor waveguide and the second semiconductor waveguide are arranged on non-coaxial axes. 9. A semiconductor laser integrated device, wherein a semiconductor laser is formed monolithically with the photonic crystal device according to claim 1 on a semiconductor substrate.

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