Optical Devices

Abstract

This optical device comprises: a cladding layer (101); and a core (102) that is formed on the cladding layer (101) and is made of a crystal of a group III-V compound semiconductor. In the core (102), a plurality of first regions (102a) and second regions (102b) are periodically connected in series. In addition, the polarization of a first region (102a) and the polarization of a second region (102b) adjacent thereto are in a state of being inverted. For the plurality of first regions (102a) and second regions (102b) included in the core (102), the polarization of a first region (102a) and the polarization of a second region (102b) adjacent thereto are in a state of being inverted in a direction perpendicular a waveguide direction.

Description

[Technical Field]

[0001] The present invention relates to optical devices. [Background technology]

[0002] In realizing the generation of ultrashort pulsed light on the order of femtoseconds, the realization of short pulse lasers through the development and demonstration of the Kerr lens mode locking method was a major breakthrough, and has led to significant advances in academic research in the field of physical chemistry as well as industrial and medical applications. (3) This technology involves injecting pulsed light into an optical resonator containing a medium, and obtaining a self-focusing optical pulse that is compressed in time and space by the third-order nonlinear optical effect (Kerr effect). As mentioned above, the nonlinear dielectric constant χ (3) greatly influences its performance.

[0003] The Kerr effect is an effect that induces a change in refractive index according to the light intensity. If the refractive index dependent on the light intensity is n(ω,I), the frequency of the incident light is ω, and the light intensity is I, it can be expressed as n(ω,I)=n(ω)+n2I. Here, n2 is the nonlinear refractive index, χ (3) The larger n2 is, the larger the refractive index change can be obtained with a low incident light intensity, enabling efficient pulse compression.

[0004] Today, great progress has been made in pulse compression techniques that utilize the Kerr effect in optical fibers and combine optical fiber resonators with dispersion control elements. The generation of broadband light (supercontinuum light) using ultrashort pulse light obtained by fiber mode-locked lasers has also been proposed and demonstrated, and the development of new light source technologies using short pulse lasers is also progressing. However, the n2 of the glass that makes up optical fibers is small, and efficiency is by no means high, so further reductions in energy and improvements in efficiency are required. Furthermore, miniaturizing ultrashort pulse light sources by integrating the input pulse light source and pulse compressor will be extremely important in expanding applications.

[0005] As one method for effectively increasing n2, the cascade secondary nonlinear optics effect has been proposed, in which a third-order nonlinear optical effect is effectively obtained through a multi-stage process of the second-order nonlinear optical effect (Non-Patent Document 1). The principle of the cascade secondary nonlinear optics effect is outlined below. (2) While the fundamental wave light propagates through a material with a nonlinear optical effect, the χ (2) The second harmonic is generated by the nonlinear optical effect.

[0006] Similarly, the second harmonic wave generated by wavelength conversion generates the fundamental wave through sequential down-conversion as it propagates. This regenerated fundamental wave undergoes a large phase shift through wavelength conversion (up-conversion), propagation as a second harmonic wave, and another wavelength conversion (down-conversion). This phase shift is greater than the phase shift that the fundamental wave undergoes due to the third-order nonlinear optical effect (Kerr effect) inherent in the material, resulting in interference with the incident fundamental wave. Furthermore, because these phase shifts depend on the wavelength conversion efficiency due to the second-order nonlinear optical effect, the amount of phase shift depends on the incident light intensity.

[0007] In the cascaded second-order nonlinear optical effect described above, the effective nonlinear refractive index n 2_CSNLE is expressed as the following equation (1).

[0008]

number

[0009] In the above equation, c is the speed of sound, ε0 is the vacuum dielectric constant, L is the propagation length, λ is the wavelength of the fundamental wave, and d eff is the effective second-order nonlinear optical constant, n Fund is the refractive index at the fundamental wave, n SHG is the refractive index at the second harmonic, and ΔkL is the phase difference between the fundamental wave and the second harmonic.

[0010] In the following description, the third term on the right side of the above equation will be replaced with α, as shown in the following equation (2).

[0011]

number

[0012] For example, in lithium niobate (LN) and lithium tantalate (LT), which are the most widely used second-order nonlinear optical materials, L = 10 mm, n Fund =n SHG = 2, λ = 1 μm, d eff =10pm / V, ΔkL=2π, n 2_CSNLE is approximately 1×10 -12 cm 2 / W, and the n2 of the optical fiber (glass) is approximately 2 × 10 -16 cm 2 This is an extremely large value compared to / W. [Prior art documents] [Non-patent literature]

[0013] [Non-Patent Document 1] R. DeSalvo, et al., "Self-focusing and self-defocusing by cascaded second-order effects in KTP", Optics Letters, Vol. 17, pp. 28-30, 1992. Summary of the Invention [Problem to be solved by the invention]

[0014] The cascaded second-order nonlinear optical effect is generally achieved using ceramic materials such as LN, LT, KTP, etc. These ceramic materials have good nonlinear constants and have historically been widely used as second-order nonlinear optical materials, but there is a growing demand for even higher efficiency.

[0015] The present invention has been made to solve the above problems, and has as its object to make it possible to obtain the cascaded second-order nonlinear optical effect with even higher efficiency. [Means for solving the problem]

[0016] The optical device according to the present invention comprises a cladding layer and a core formed on the cladding layer and made of a III-V compound semiconductor crystal, wherein the core has a plurality of regions periodically connected in series, with the polarization of adjacent regions being reversed. [Effects of the Invention]

[0017] As explained above, according to the present invention, the cascaded second-order nonlinear optical effect can be obtained with even higher efficiency. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of an optical device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a characteristic diagram showing the calculation results of the α dependency of the effective nonlinear refractive index n2_CSNLE when the wavelength of the fundamental wave is 1.55 μm and the propagation length of the core 102 is varied from 1 mm to 10 mm. [Figure 3A] FIG. 3A is a cross-sectional view showing a state of an optical device in the middle of a process for explaining a method for fabricating an optical device according to an embodiment of the present invention. [Figure 3B] FIG. 3B is a cross-sectional view showing a state of the optical device in the middle of a process for explaining the method for fabricating the optical device according to the embodiment of the present invention. [Figure 3C] FIG. 3C is a cross-sectional view showing a state of the optical device in the middle of a process for explaining the method for fabricating the optical device according to the embodiment of the present invention. [Figure 3D] FIG. 3D is a cross-sectional view showing a state of the optical device in the middle of a process for explaining the method for fabricating the optical device according to the embodiment of the present invention. [Figure 3E]FIG. 3E is a cross-sectional view showing a state of the optical device in the middle of a process for explaining the method for fabricating the optical device according to the embodiment of the present invention. [Figure 3F] FIG. 3F is a cross-sectional view showing a state of the optical device in the middle of a process for explaining the method for fabricating the optical device according to the embodiment of the present invention. [Figure 4A] FIG. 4A is a plan view showing the configuration of another optical device according to an embodiment of the present invention. [Figure 4B] FIG. 4B is a cross-sectional view showing the configuration of another optical device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] An optical device according to an embodiment of the present invention will now be described with reference to Fig. 1. This optical device includes a cladding layer 101 and a core 102 formed on the cladding layer 101 and made of a III-V compound semiconductor crystal.

[0020] The core 102 has a plurality of first regions 102a and second regions 102b periodically connected in series. The polarization of adjacent first regions 102a and second regions 102b is inverted. The plurality of first regions 102a and second regions 102b constituting the core 102 are inverted in the direction perpendicular to the waveguiding direction between adjacent first regions 102a and second regions 102b.

[0021] 1, a structure can be used in which the polarization of adjacent first regions 102a and the polarization of adjacent second regions 102b are reversed in a direction perpendicular to the waveguide direction and parallel to the plane of the cladding layer 101. Alternatively, a structure can be used in which the polarization of adjacent first regions 102a and the polarization of adjacent second regions 102b are reversed in a direction perpendicular to the waveguide direction and perpendicular to the plane of the cladding layer 101. These can be set appropriately depending on the polarization direction of the target light (wavelength-converted light).

[0022] The cladding layer 101 can be made of, for example, SiO2. The core 102 (first region 102a, second region 102b) can be made of, for example, AlGaAs (Al composition 0.2 or less). The band gap of this AlGaAs is appropriately designed so that SHG light can pass through the core 102. The band gap of AlGaAs can be controlled by the Al composition. Designing the band gap to allow SHG light to pass through also has the effect of reducing optical loss due to two-photon absorption in the semiconductor.

[0023] Although not shown in FIG. 1, an upper cladding can be provided on the cladding layer 101 to cover the core 102. The upper cladding can be made of an insulating material such as SiO2. The refractive index of AlGaAs at a wavelength of 1.55 μm is 3.28, and the refractive index of SiO2 at a wavelength of 1.55 μm is 1.44. Therefore, a large refractive index difference is obtained between the cladding layer 101 (upper cladding) and the core 102, achieving high optical confinement to the core 102. In addition, the second-order nonlinear optical constant of AlGaAs is approximately 120 pm / V, which is significantly larger than the values ​​(10 to 30 pm / V) of lithium niobate (LN) and lithium tantalate (LT).

[0024] The first region 102a and the second region 102b have a structure (periodically poled structure) in which domains (polarization) are periodically reversed in the light propagation direction to satisfy the quasi-phase matching (QPM) condition in the core 102. The period at which the polarization is reversed, in other words, the length (thickness) of the first region 102a and the second region 102b in the waveguiding direction, can be set to a value that matches the quasi-phase matching condition, and can be set to, for example, 10 μm or less. The cross-sectional dimensions of the core 102 are appropriately designed to satisfy the single-mode condition and the desired phase matching condition together with the QPM period.

[0025] When the wavelength of the propagating light is 1.55 μm and the propagation length is 10 mm, the effective nonlinear refractive index n 2_CSNLE is approximately 3.7cm 2 / W, a value approximately 50 times larger than that of LN or LT. It also achieves a value larger than the third-order nonlinear refractive index n2 of an AlGaAs optical waveguide. That is, the core 102, which has a periodically poled structure consisting of the first region 102a and the second region 102b, achieves an extremely excellent cascaded second-order nonlinear optical effect with high efficiency.

[0026] FIG. 2 shows the effective nonlinear refractive index n 2_CSNLE The calculation results of the α dependence of n are shown below. α is given as shown in equation (2). Semiconductor materials (AlGaAs) generally have a higher refractive index than ceramic materials such as LN and LT. That is, n Fund and n SHG becomes large, so to increase α, d eff Therefore, it is important to select a second-order nonlinear optical material system with a sufficiently large

[0027] Next, a method for fabricating an optical device according to an embodiment of the present invention will be described with reference to FIGS. 3A to 3F. First, as shown in FIG. 3A, a Ge layer 122 made of Ge and a buffer layer 123 made of GaAs are crystal-grown on a growth substrate 121 made of GaAs. The growth substrate 121 made of GaAs may have a principal surface orientation of the (100) plane. Alternatively, the growth substrate 121 made of GaAs may have a principal surface orientation offset by 1 degree from the (100) plane.

[0028] Furthermore, a domain inversion layer 124 made of AlGaAs is grown on the buffer layer 123. When a Ge layer 122 is formed on a GaAs growth substrate 121, the domain (polarization direction) of the GaAs crystal grown on this is inverted from that of the growth substrate 121. Furthermore, if AlGaAs is grown directly on the Ge layer 122, the quality of the grown crystal cannot be improved due to lattice mismatch. For this reason, a buffer layer 123 made of GaAs is formed, and then AlGaAs is grown to form the domain inversion layer 124. The buffer layer 123 is also inverted.

[0029] Next, by using known lithography and etching techniques, the Ge layer 122, the buffer layer 123, and the domain inversion layer 124 are patterned into a line-and-space structure with a period that matches the QPM condition, thereby forming multiple first regions 102a in the domain inversion layer 124, as shown in Fig. 3B. In patterning the Ge layer 122, the buffer layer 123, and the domain inversion layer 124 into a line-and-space structure, the surface of the growth substrate 121 is exposed in the spaced portions.

[0030] Next, AlGaAs is grown on the growth substrate 121 exposed in the spaces described above to form multiple second regions 102b, as shown in FIG. 3C. The second regions 102b grow to fill the spaces. Next, chemical mechanical polishing (CMP) is performed to reduce the surface irregularities of the domain inversion layer 124, and the surfaces of the multiple alternatingly arranged first regions 102a and multiple second regions 102b, as seen from the growth substrate 121, are flattened, as shown in FIG. 3D. In this case, the first regions 102a and second regions 102b are grown alternately in the

[0011] direction and the [01-1] direction toward the waveguide direction.

[0031] Next, as shown in Fig. 3E, the planarized surfaces of the alternatingly arranged first regions 102a and second regions 102b are attached to a cladding layer 101 previously formed on a Si substrate 111. This attachment can be performed by, for example, direct bonding. After this bonding, the growth substrate 121 is removed.

[0032] Next, the Ge layer 122, buffer layer 123, first region 102a, and second region 102b, which have been processed into a space pattern, are polished by CMP to remove the Ge layer 122 and buffer layer 123, and the first region 102a and second region 102b are formed to a thickness corresponding to the predetermined core height, and the surface is flattened (FIG. 3F). Thereafter, the core 102 is formed by patterning using known lithography and etching techniques, thereby obtaining the optical device described with reference to FIG.

[0033] The optical device described above with reference to Figures 3A to 3C can be fabricated using the technology of Reference 1. Furthermore, the planarized surfaces of the alternatingly arranged first regions 102a and second regions 102b can be bonded to the cladding layer 101 by forming an interface layer made of SiO2, Al2O3, or the like on the planarized surfaces of the first regions 102a and second regions 102b, and then directly bonding the formed interface layer to the cladding layer 101. Commonly used hydrophilic bonding or surface activated bonding can be used for this direct bonding.

[0034] Note that the material structure used above is just one example; any compound semiconductor material system that can similarly produce a highly efficient cascaded second-order nonlinear optical effect and that can fabricate a domain-inverted structure can be used. Furthermore, this technology is also useful for developing light source technology in the mid-infrared region, which has attracted attention in recent years, from wavelengths of 2000 nm to several tens of microns. However, since SiO2 exhibits significant loss at wavelengths exceeding 4 microns, by constructing the cladding layer 101 from a material with high transmittance in the desired wavelength band, such as SiN, Al2O3, or air, loss can be suppressed up to wavelengths of approximately 7 microns.

[0035] The optical device described above may include a waveguide-type semiconductor laser 103 formed on a cladding layer 101 and emitting pulsed light (FIGS. 4A and 4B). The semiconductor laser 103 includes an active layer 132 formed on a semiconductor layer 131 made of a III-V compound semiconductor such as InP, and a p-type p-semiconductor layer 133 and an n-type n-semiconductor layer 134 formed on the semiconductor layer 131 with the active layer 132 sandwiched therebetween. A diffraction grating (not shown) is formed on the active layer 132 to form a resonator. A current injection structure is formed by the p-semiconductor layer 133 and the n-semiconductor layer 134. A p-electrode 135 is formed on the p-semiconductor layer 133, and an n-electrode 136 is formed on the n-semiconductor layer 134. The semiconductor laser 103 may have a general laser structure as described in Reference 2. For example, a short-pulse light can be emitted by performing gain switching as described in Reference 3.

[0036] Pulsed light emitted from semiconductor laser 103 is emitted into an optical waveguide formed by laser core 137 made of InP. The optical waveguide formed by laser core 137 is optically coupled to the optical waveguide formed by core 102 at optical coupling section 104 with an opposed tapered structure. An upper cladding layer 138 is formed on cladding layer 101 to cover core 102, semiconductor laser 103, and laser core 137. The pulsed light emitted into the optical waveguide formed by laser core 137 is incident with low loss on the optical coupling section 104 into the optical waveguide formed by core 102.

[0037] As described above, the optical device according to the embodiment allows a nonlinear optical element capable of performing wavelength conversion using the core 102 with a periodically poled structure, and a semiconductor laser serving as a light source, to be integrated on the cladding layer 101. Conventionally used ceramic materials such as LN, LT, and KTP can realize a cascaded second-order nonlinear optical effect process, but require a separate external input pulse light source, making it difficult to miniaturize and highly integrate a pulse light source module. In contrast, the optical device according to the embodiment allows for easy miniaturization and high integration of a pulse light source module.

[0038] As described above, according to the present invention, the core is made of a III-V compound semiconductor crystal, and further, a plurality of regions are periodically connected in series so that the polarization of adjacent regions is reversed, thereby enabling the cascaded second-order nonlinear optical effect to be obtained with even higher efficiency.

[0039] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0040] [Reference 1] X. Yu et al., "Efficient continuous wave second harmonic generation pumped at 1.55 μm in quasi-phase-matched AlGaAs waveguides", Optics Express, vol. 13, no. 26, pp. 10742-10748, 2005. [Reference 2] T. Fujii et al., "Multiwavelength membrane laser array using selective area growth on directly bonded InP on SiO2 / Si", Optical Society of America, vol. 7, no. 7, pp. 838-846, 2020. [Reference 3] Z. Liu et al., "50-GHz Repetition Gain Switching Using a Cavity-Enhanced DFB Laser Assisted by Optical Injection Locking", Journal of Lightwave Technology, vol. 38, no. 7, pp. 1844-1850, 2020. [Explanation of symbols]

[0041] 101...cladding layer, 102...core, 102a...first region, 102b...second region.

Claims

1. A clad layer made of any one of SiO 2 , SiN, and Al 2 O 3 ; a core formed on the cladding layer and made of a III-V compound semiconductor crystal; Equipped with The core has a plurality of regions periodically connected in series, and the polarization of adjacent regions is reversed. An optical device characterized by:

2. 2. The optical device according to claim 1, The plurality of regions constituting the core are in a state in which the polarization of adjacent regions is reversed in a direction perpendicular to the waveguiding direction. An optical device characterized by:

3. 3. The optical device according to claim 1, a waveguide-type semiconductor laser formed on the cladding layer and emitting pulsed light; An optical device, wherein pulsed light emitted from the semiconductor laser is incident on an optical waveguide formed by the core.

4. 4. The optical device according to claim 3, The semiconductor laser is an active layer made of a III-V group compound semiconductor; a current injection structure formed by a p-semiconductor layer made of a p-type III-V group compound semiconductor and an n-semiconductor layer made of an n-type III-V group compound semiconductor, the p-semiconductor layer and the n-semiconductor layer being disposed on either side of the active layer; An optical device comprising:

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