JP2007240743A - Wavelength conversion element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a wavelength conversion device low-cost by decreasing the number of components and the assembly man-hour, and to make it easy to make the device compact. <P>SOLUTION: The wavelength conversion device has a domain inversion layer 50, an n-type first clad layer 60, a waveguide layer 70, a p-type second clad layer 80, and a cap layer 85 laminated in order on a first main surface 11 of a semiconductor substrate 10. Further, a second electrode 94 is provided on the gap layer 85 and a first electrode 92 is provided on a second main surface 13 of the semiconductor substrate. When a forward voltage is applied between the first electrode and second electrode, oscillation light is generated in the waveguide layer. The waveguide layer is provided with a periodic domain inversion structure meeting quasi-phase matching conditions among signal light, the oscillation light, and a converted light, and a difference frequency light is generated through difference frequency generation between the signal light made incident on the waveguide layer and the oscillation light, and then the difference frequency light is projected as the converted light. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a wavelength conversion element and a manufacturing method thereof.

As a wavelength conversion element constituting the wavelength conversion device, a quasi phase matching (QPM: Quasi-Phase Matching) is performed and wavelength conversion is performed by an optical waveguide having a periodic domain inversion structure formed on a semiconductor substrate. A type wavelength conversion element (hereinafter sometimes referred to as a QPM type wavelength conversion element) has been developed (see Non-Patent Document 1, for example). When pump light and signal light are incident on the wavelength conversion element, light satisfying the QPM condition, which is a difference frequency between the pump light and signal light, is obtained as converted light.
S. J. et al. Yoo et al. "Quasi-phase-matched second-harmonic generation in AlGaAs waveguides with periodic domain invented by wafer-bonding", Appl. Phys. Lett. , Vol. 66, no. 25, June 1995, pp. 3410-3412

  However, in order for the pump light to be incident on the above-described conventional wavelength conversion element, the light output of the light source must be incident on the optical fiber, and the light from the optical fiber must be incident on the wavelength conversion element. For this reason, the semiconductor laser element and the wavelength conversion element, which are light sources, are fixed to the housing of the wavelength conversion apparatus and optically coupled to the lens system and the optical fiber (hereinafter simply referred to as optical coupling), respectively, and then the semiconductor. The laser element and the wavelength conversion element are optically coupled. In this case, since it is difficult to reduce the number of parts such as a lens system and the number of assembly steps such as optical coupling, the cost cannot be reduced. In addition, since a space for optically coupling the semiconductor laser element and the wavelength conversion element is required, it is difficult to reduce the size of the wavelength conversion element.

  In order to solve the above-mentioned problems, the inventors of the present application have conducted extensive research. As a result, laser light obtained by causing laser oscillation in a waveguide for wavelength conversion is converted into a pump for wavelength conversion elements. We found that it can be used as light. If laser light generated in the waveguide of the wavelength conversion element is used as pump light, it is not necessary to enter the pump light from the outside into the wavelength conversion element.

  The present invention has been made in view of the above-mentioned problems. Therefore, the object of the present invention is to reduce the number of parts and the number of assembly steps, thereby reducing the cost and facilitating the downsizing. The object is to provide an element and a method for manufacturing the element.

  In order to achieve the above-described object, a wavelength conversion element for emitting converted light having a wavelength different from that of signal light with respect to incident signal light according to the present invention includes a semiconductor substrate, a domain inversion layer, a stacked body, , A cap layer, a first electrode, and a second electrode. The domain inversion layer is formed on the first main surface of the semiconductor substrate. The stacked body is formed by sequentially stacking an n-type first cladding layer, a waveguide layer, and a p-type second cladding layer on the domain inversion layer. The cap layer is formed on the second cladding layer. The first electrode is provided on the second main surface of the semiconductor substrate, and the second electrode is provided on the cap layer. When a forward voltage is applied to the laminate between the first electrode and the second electrode, stimulated emission light is generated in the waveguide layer. The stimulated emission light is amplified by a resonator formed between both end faces in the longitudinal direction of the waveguide layer, and oscillation light is generated. The waveguide layer is provided with a periodic domain inversion structure that satisfies a quasi-phase matching condition between the signal light, the oscillation light, and the converted light in the waveguide direction of the oscillation light, and the signal incident on the waveguide layer Difference frequency light is generated by the difference frequency generation between the light and the oscillation light. This difference frequency light is emitted as converted light.

  In the implementation of the wavelength conversion element described above, preferably, the first region and the second region are alternately set on the first main surface of the semiconductor substrate at a period satisfying the quasi-phase matching condition, A semiconductor layer and a first domain layer, which are sequentially stacked on the first main surface of the semiconductor substrate in the first region, and a second domain formed on the first main surface of the semiconductor substrate in the second region It is good to have a layer. The direction of spontaneous polarization of the second domain layer is reversed with respect to the direction of spontaneous polarization of the first domain layer.

  According to another preferred embodiment of the present invention, the wavelength conversion element includes a semiconductor substrate, a domain inversion layer, a laminate, a cap layer, a first electrode, and a second electrode. The first main surface of the semiconductor substrate is provided with a periodic structure of an inclined surface of the first region and a horizontal surface of the second region. The domain inversion layer is formed by sequentially stacking a Ge layer and a GaAs layer on a periodic structure. The stacked body is formed by sequentially stacking an n-type first cladding layer, a waveguide layer, and a p-type second cladding layer on the domain inversion layer. The cap layer is formed on the second cladding layer. The first electrode is provided on the second main surface of the semiconductor substrate, and the second electrode is provided on the cap layer. When a forward voltage is applied to the laminate between the first electrode and the second electrode, stimulated emission light is generated in the waveguide layer, and the stimulated emission light is generated between both end surfaces of the waveguide layer in the longitudinal direction. Oscillation light is generated by being amplified by the configured resonator, and the periodic structure satisfies a quasi phase matching condition among the signal light, the oscillation light, and the converted light. In the waveguide layer, the direction of spontaneous polarization in the portion of the second region is reversed with respect to the direction of spontaneous polarization in the portion of the first region. A difference frequency light is generated by the signal light incident on the waveguide layer and the oscillation light, and the difference frequency light is emitted as converted light.

  Moreover, the manufacturing method of the wavelength conversion element of this invention is equipped with the following processes.

  First, a semiconductor substrate is prepared. Next, on the first main surface of the semiconductor substrate, a domain inversion layer in which the direction of spontaneous polarization is inverted with a period satisfying a quasi-phase matching condition between the signal light, the oscillation light generated by the wavelength conversion element, and the converted light. Form. Next, an n-type first cladding layer, a waveguide layer, and a p-type second cladding layer are sequentially grown on the domain inversion layer, and the first cladding layer, the waveguide layer, and the second cladding layer are formed. A periodic domain inversion structure that satisfies the quasi-phase matching condition is formed in the stacked body formed by stacking. Next, a cap layer is formed on the second cladding layer. Next, a first electrode is formed on the second main surface of the semiconductor substrate, and a second electrode is formed on the cap layer.

  In carrying out the semiconductor element manufacturing method described above, the step of forming the domain inversion layer preferably includes the following steps. First, a first semiconductor layer and a second semiconductor layer are sequentially stacked on a first main surface of a semiconductor substrate. Next, on the second semiconductor layer, the first region and the second region are alternately set at a period satisfying the quasi-phase matching condition, and the opening is provided in the second region by photolithography and dry etching. A film mask is formed. Next, by etching using an insulating film mask, portions of the second region of the first semiconductor layer and the second semiconductor layer are removed, and the first main surface of the semiconductor substrate in the second region is exposed. Next, the portion of the second semiconductor layer remaining in the first region is used as a first domain layer, and the direction of spontaneous polarization is the direction of spontaneous polarization in the first region on the semiconductor substrate in which the second region is exposed. A second domain layer that is inverted with respect to is grown to the same height as the first domain layer. Next, the insulating film mask is removed.

  The step of forming the domain inversion layer may include the following steps. First, the first region and the second region are alternately set on the first main surface of the semiconductor substrate at a period satisfying the quasi phase matching condition, and an opening is provided in the first region by photolithography and dry etching. An insulating film mask is formed. Next, an Si layer and a GaAs layer are sequentially stacked on the first main surface of the semiconductor substrate in the first region, and the GaAs layer is used as a first domain layer. Next, the insulating film mask is removed. Next, on the first main surface of the semiconductor substrate in the second region exposed by removing the insulating film mask, a GaAs layer in which the direction of spontaneous polarization is reversed with respect to the direction of spontaneous polarization in the first region is formed. The GaAs layer is formed as a second domain layer.

  Further, in carrying out the semiconductor element manufacturing method of the present invention, a configuration including the following steps may be employed. First, a semiconductor substrate is prepared. Next, the first region and the second region are alternately formed on the first main surface of the semiconductor substrate at a period satisfying a quasi phase matching condition between the signal light, the oscillation light generated in the wavelength conversion element, and the converted light. Set. Next, a periodic structure having a horizontal surface in the first region and an inclined surface in the second region is formed on the first main surface of the semiconductor substrate. Next, a Ge layer is formed on the periodic structure, and further a GaAs layer is formed on the Ge layer, and the direction of spontaneous polarization in the second region is reversed with respect to the direction of spontaneous polarization in the first region. Get a domain inversion layer. Next, a laminated body in which a first cladding layer, a waveguide layer, and a second cladding layer are sequentially grown on the domain inversion layer, and the first cladding layer, the waveguide layer, and the second cladding layer are stacked. And the said laminated body which has the periodic domain inversion structure in which the direction of the spontaneous polarization in a 2nd area | region is reversed with respect to the direction of the spontaneous polarization in a 1st area | region is formed. Next, a cap layer is formed on the second cladding layer. Next, a first electrode is formed on the second main surface of the semiconductor substrate, and a second electrode is formed on the cap layer.

  According to the wavelength conversion element and the manufacturing method thereof of the present invention, pump light can be obtained by causing laser oscillation in the waveguide itself that performs wavelength conversion. Therefore, a light source is not necessary, and the number of components such as a lens system provided in each of the light source and the wavelength conversion element can be reduced. In addition, since it is not necessary to perform optical coupling between the light source and the wavelength conversion element as in the prior art, the number of assembling steps of the wavelength conversion device can be reduced. Furthermore, since a space for optical coupling becomes unnecessary, the wavelength converter can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangement relationships of the constituent elements are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described, but the composition (material), numerical conditions, and the like of each configuration are merely preferred examples. Therefore, the present invention is not limited to the following embodiment.

(Wavelength conversion element)
With reference to FIG. 1 (A) and (B), the wavelength conversion element of this invention is demonstrated. 1A and 1B are schematic views for explaining a configuration example of a wavelength conversion element of the present invention. FIG. 1A shows a cut surface of a cross section orthogonal to the upper surface of the wavelength conversion element and along the longitudinal direction thereof. FIG. 1B shows a cross-section cut along a direction perpendicular to the upper surface of the wavelength conversion element and perpendicular to the longitudinal direction thereof.

  The wavelength conversion element 100 includes an n-type GaAs substrate 10 as a semiconductor substrate, a domain inversion layer 50, a stacked body 55, a cap layer 85, a first electrode 92, and a second electrode 94.

  The domain inversion layer 50 is formed on the first main surface 11 which is one main surface of the GaAs substrate.

The stacked body 55 is formed by sequentially stacking an n-type first cladding layer 60, a waveguide layer 70, and a p-type second cladding layer 80 on the domain inversion layer 50. The first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 are all made of AlGaAs. The composition formula of the first and second cladding layers 60 and 80 is Al _0.3 GaAs, and the composition formula of the waveguide layer 70 is Al _0.13 GaAs. Here, the notation of AlGaAs is used as a general term for those having different composition ratios such as Al _0.3 GaAs and Al _0.13 GaAs.

  The first cladding layer 60 is n-type by introducing, for example, Si into AlGaAs. The second cladding layer 80 is p-type by introducing, for example, Mg or Zn into AlGaAs. The waveguide layer 70 is an intrinsic semiconductor layer.

  In order to prevent diffusion of light propagating in the longitudinal direction of the wavelength conversion element 100 in the lateral direction, that is, in order to confine in the lateral direction, the second cladding layer 80 is formed in a ridge shape. As a result, the wavelength conversion element 100 has a ridge waveguide structure.

  The cap layer 85 is formed on the second cladding layer 80 of the stacked body 55. The cap layer 85 is made of, for example, GaAs.

  The first electrode 92 is provided on the second main surface 13 of the GaAs substrate 10 facing the first main surface 11. The second electrode 94 is provided on the cap layer 85.

  When a forward voltage is applied to the stacked body 55 between the first electrode 92 and the second electrode 94, carriers are injected into the waveguide layer 70, and as a result, stimulated emission light is generated in the waveguide layer 70. To do. A resonator is formed between both end faces 72a and 72b in the longitudinal direction of the waveguide layer 70, and stimulated emission light is amplified by this resonator. As a result, oscillation light is generated in the resonator of the waveguide layer 70. That is, the waveguide layer 70 functions as an active layer of a so-called semiconductor laser, and the wavelength conversion element 100 has a function as a semiconductor laser.

  The first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 are provided with a periodic domain inversion structure along the waveguide direction of oscillation light, that is, the longitudinal direction of the wavelength conversion element 100.

  The periodic domain inversion structure will be described with the GaAs substrate 10 having a rectangular parallelepiped shape in which the first main surface 11 which is the upper surface thereof is a (100) plane.

  Consider first and second regions alternately set at a period Λ on the first main surface 11 of the GaAs substrate 10.

  The domain inversion layer 50 includes a silicon (Si) layer 22 and a first domain layer 32 which are semiconductor layers sequentially formed on the first main surface 11 of the GaAs substrate 10 in the first region, and a GaAs substrate in the second region. 10 and the second domain layer 40 formed on the first main surface 11. Both the first domain layer 32 and the second domain layer 40 are made of GaAs. As the semiconductor layer, a Ge layer may be used instead of the Si layer 22.

  The direction of spontaneous polarization of the second domain layer 40 on the GaAs substrate 10 is assumed to be the [100] axis direction (direction indicated by arrow I in the figure). At this time, the direction of spontaneous polarization of the first domain layer 32 on the Si layer 22 is reversed to the [−100] axis direction (the direction indicated by the arrow II in the figure). Here, -1 represents a Miller index of 1 with a bar.

  The first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 are formed on the domain inversion layer 50 by, for example, molecular beam epitaxy (MBE) method. In the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80, the domains are inverted with a period equal to the repetition period Λ of the first domain layer 32 and the second domain layer 40 of the domain inversion layer 50. That is, the direction of spontaneous polarization of the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 located immediately above the second domain layer 40 is the same as the direction of spontaneous polarization of the second domain layer 40 [100]. Axial direction. On the other hand, the direction of spontaneous polarization of the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 located immediately above the first domain layer 32 is the same as the direction of spontaneous polarization of the first domain layer 32 [−100. ] Axial direction. Thus, the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 also have a periodic domain inversion structure having the same period Λ as the domain inversion layer 50.

  Difference frequency generation is caused by the signal light incident on the waveguide layer 70 from the outside and the oscillation light generated in the waveguide layer 70. The difference frequency light generated by the difference frequency generation is emitted from the wavelength conversion element as converted light.

Here, the wave number difference between the wave numbers k _p , k _s , and k _c of the oscillation light (sometimes referred to as pump light), the signal light, and the converted light is Δk (= k _p −k _s −k). _c ), the phase difference of each light propagated through the wavelength conversion element is ΔkL. Here, L is the element length of the wavelength conversion element. Therefore, Δk = 0 needs to be set so that no phase difference occurs during propagation.

The wave number k is expressed by k = nω / c, where n is the refractive index at the frequency ω and c is the speed of light. Using this relationship, Δk = (n _p ω _p −n _s ω _s −n _c ω _c ) / c is obtained. If the refractive index does not change depending on the frequency of light, ω _c + ω _s = ω _p , so that Δk = 0 at all times. However, actually, since the refractive index has frequency dependence, Δk is not zero. For this reason, a QPM type wavelength conversion element which is a wavelength conversion element in which a periodic domain inversion structure is formed is used. At this time, the relationship between the wave numbers is Δk = k _p −k _s −k _c −2π / Λ. Here, the period Λ of the periodic domain inversion structure is set so as to satisfy the QPM condition for the specific wavelength λ _p0 of the pump light, that is, k _p0 −k _s0 −k _c0 = 2π / Λ. Then, at the wavelength λ _p0 of this specific pump light, Δk = 0, and a large conversion efficiency is obtained.

  Here, an example has been described in which a GaAs substrate is used as the semiconductor substrate and the waveguide layer and the first and second cladding layers are AlGaAs. However, the present invention is not limited thereto. Even if an InP substrate is used as the semiconductor substrate, the present invention can be similarly implemented. In addition, the composition and composition ratio of the waveguide layer, the first and second cladding layers are selected according to the wavelength of the pump light, and for example, InP can be used.

  Further, although the wavelength conversion element has been described as a ridge waveguide structure, the present invention is not limited to this. For example, when an InP substrate is used as the semiconductor substrate, the embedded waveguide is similar to the embedded semiconductor laser. It is good also as a structure.

(Method for manufacturing wavelength conversion element)
With reference to FIGS. 2-5, the manufacturing method of a wavelength conversion element is demonstrated. Here, a case where a plurality of wavelength conversion elements are formed on one wafer will be described. 2 (A) to (C), FIGS. 3 (A) to (C), FIGS. 4 (A) to (D), and FIGS. 5 (A) to (B) illustrate a method for manufacturing a wavelength conversion element. FIG.

  First, with reference to FIGS. 2A to 2C and FIGS. 3A to 3C, a process of forming a periodic domain inversion structure on a semiconductor substrate will be described.

  2 (A) to (C) and FIGS. 3 (A) to (C), the structure obtained in the main manufacturing stage is orthogonal to the upper surface of the wavelength conversion element, as in FIG. 1 (A). And it shows by the cut end of the cross section along the longitudinal direction.

  First, a GaAs substrate 10 is prepared as a semiconductor substrate. Note that an InP substrate may be used as the semiconductor substrate. Here, it is assumed that the upper surface of the GaAs substrate 10 is the (100) plane 11 and the cross section is the (011) plane 12. The GaAs substrate 10 may be an inclined substrate whose (100) plane is inclined to about 10 degrees with respect to the [011] axis (FIG. 2A).

  Next, after the Si layer 20 is grown as the first semiconductor layer on the (100) plane 11 which is the first main surface of the GaAs substrate 10, the GaAs layer 30 is grown as the second semiconductor layer. In order to cause spontaneous polarization reversal in the process described later, it is preferable to form the Si layer 20 with a thickness of 2 nm or more. The GaAs layer 30 is preferably formed to a thickness of 10 nm or more. For example, an MBE apparatus can be used for forming the Si layer 20 and the GaAs layer 30. The details of the growth conditions are device-dependent optimum conditions because of device dependency. For example, the growth temperature of the Si layer 20 is about 450 to 550 ° C., and the growth temperature of the GaAs layer 30 is 400 to 650 ° C. To the extent. Note that a Ge layer may be formed instead of the Si layer 20 (FIG. 2B).

Next, a periodic partition region is set on the upper surface of the GaAs layer 30 with two regions of the first region 5a and the second region 5b as a set. This period Λ satisfies the QPM condition between the signal light, the oscillation light generated by the wavelength conversion element, and the converted light. An SiO ₂ film is formed on the entire surface of the GaAs layer 30 by any suitable known method such as a chemical vapor deposition (CVD) method. Thereafter, the SiO ₂ film is processed to leave a portion of the SiO ₂ film in the first region 5a and remove the portion of the SiO ₂ film in the second region 5b by conventionally known photolithography and dry etching. Thus, the insulating film mask 42 is formed. Therefore, an opening 42b is formed in the second region 5b in the insulating film mask 42 formed by the remaining SiO ₂ film portion (FIG. 2C).

  Next, for example, reactive ion etching (RIE) is performed as etching using the insulating film mask 42. By this etching, the portions of the GaAs layer 30 and the Si layer 20 in the second region 5b not covered with the insulating film mask 42 are removed, and the first main surface 11 of the GaAs substrate 10 is exposed in the second region 5b. . The GaAs layer 32 and the Si layer 22 remain in the first region 5a. The GaAs layer 32 remaining in the first region 5a may be referred to as a first domain layer 32 (FIG. 3A).

  Next, after a GaAs layer is grown as a semiconductor layer on the first main surface 11 of the GaAs substrate 10 exposed in the second region, for example, by MBE, the insulating film mask 42 is removed. The GaAs layer grown here is referred to as a second domain layer 40. The second domain layer 40 is grown to the same height as the first domain layer 32. The Si layer 22, the first domain layer 32, and the second domain layer 40 constitute a domain inversion layer 50 (FIG. 3B).

  Here, when the second domain layer 40 is formed higher than the first domain layer 32, the height of the second domain layer 40 is set to the first domain by a chemical mechanical polishing (CMP) method or the like. The height may be the same as the layer 32. After the first domain layer 32 is formed, the insulating film mask 42 is removed, and a GaAs layer is grown on the first main surface 11 and the first domain layer 32 of the GaAs substrate 10, and then the first domain layer 32 is formed. The second domain layer 40 may be obtained by polishing the GaAs layer by CMP or the like until the upper surface of the layer 32 is exposed. As described with reference to FIG. 3B, if the second domain layer 40 is grown to the same height as the first domain layer 32, a polishing step such as a CMP method is not necessary, which is preferable. is there.

  Thereafter, for example, the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 are formed in this order by epitaxial growth using an MBE apparatus, and further, the cap layer 85 is formed on the second cladding layer 80 (see FIG. FIG. 3 (C)).

  Depending on the presence or absence of the Si layer 22 on the (100) surface 11 of the GaAs substrate 10, the direction of spontaneous polarization of the first domain layer 32 and the second domain layer 40 of the domain inversion layer 50 is inverted. The direction of spontaneous polarization of the first clad layer 60, the waveguide layer 70, and the second clad layer 80 grown on the domain inversion layer 50 is the region on the first domain layer 32, that is, the first region 5a and the second region. In the region on the domain layer 40, that is, the second region 5b, they are mutually inverted. Here, it is assumed that the direction of spontaneous polarization of the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 in the second region 5b is the [100] axis direction. At this time, the directions of spontaneous polarization of the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 in the first region 5a are reversed to the [-100] axis direction.

  Here, the first region 5a, that is, the domain in which the direction of spontaneous polarization is the [-100] axis direction is referred to as a first domain. Further, the second region 5b, that is, the domain in which the direction of spontaneous polarization is the [100] axis direction is referred to as a second domain.

  With reference to FIG. 4 (A)-(D), the process after obtaining the periodic domain inversion structure of a 1st domain and a 2nd domain is demonstrated. 4A is a plan view, and FIGS. 4B to 4D are views partially showing a cut surface of a cross section taken along a line AA in FIG. 4A. is there.

  After obtaining the periodic domain inversion structure of the first domain and the second domain, a ridge waveguide structure is formed by any suitable well-known reactive ion etching (RIE). When a plurality of wavelength conversion elements are formed on one wafer, a plurality of ridge protrusions 82 are formed in parallel with each other (FIGS. 4A and 4B).

  Thereafter, an insulating film 44 for protecting the surface is formed on the upper surface of the second cladding layer 80 in order to prevent oxidation of the exposed portion by RIE. As this insulating film 44, for example, an SiN film formed by any suitable known CVD method or the like is used. (FIG. 4C).

  Next, after the portion of the insulating film 44 on the ridge protrusion 82 is removed, a second electrode 94 is formed on the cap layer 85 remaining on the ridge protrusion 82 by vapor deposition. Thereafter, the GaAs substrate 10 is polished to a thickness that can be easily cleaved, and then the first electrode 92 is formed on the second main surface 13 of the GaAs substrate 10 by vapor deposition (FIG. 4D).

  After forming the first electrode 92 and the second electrode 94, the wafer is separated into chips to obtain a wavelength conversion element. With reference to FIGS. 5A and 5B, a process of dividing the wafer into individual pieces will be described. FIGS. 5A and 5B are plan views of the wafer and the bar obtained by cleaving the wafer, respectively.

  A structure in which electrodes are formed on both surfaces of the substrate (FIG. 5A) is cleaved on a surface orthogonal to the longitudinal direction of the second electrode 94 (indicated by symbol I in FIG. 5A). Get bar 102. This cleavage is performed along the (0-11) plane of the GaAs substrate. One bar 102 is provided with a plurality of second electrodes 94, that is, ridge waveguides arranged in parallel (FIG. 5B).

  Next, a dielectric multilayer film is formed as a low-reflectance reflective film on the (0-11) plane of the bar 102 obtained by this cleavage.

  Thereafter, the bar 102 is cleaved at a surface along the longitudinal direction of the second electrode 94 of the ridge waveguide (indicated by reference numeral II in FIG. 5B) to obtain the wavelength conversion elements 100 that are individually separated. .

(First Example of Method for Forming Periodic Domain Inversion Structure)
The method for forming the periodic domain inversion structure is not limited to the method described above, and other methods may be used.

  With reference to FIGS. 6A to 6E, a first example of obtaining a periodic domain inversion structure will be described. 6A to 6E are schematic views for explaining a method of forming a periodic domain inversion structure. 6 (A) to 6 (E) show the structures obtained in the main manufacturing stage, as in FIG. 1 (A), perpendicular to the upper surface of the wavelength conversion element and cut in a section along the longitudinal direction thereof. Is shown.

  First, a GaAs substrate 10 is prepared as a semiconductor substrate. Here, the GaAs substrate 10 is described as a rectangular parallelepiped shape whose upper surface is the (100) plane 11. 6A to 6E, the cross section is taken as the (011) plane 12. The GaAs substrate 10 may be a tilted substrate in which the (100) plane 11 is tilted to about 10 degrees with respect to the [011] axis (FIG. 6A).

Next, a periodic partition region is set on the upper surface of the GaAs substrate 10 with a pair of two regions of the first region 5a and the second region 5b as a set. This period Λ satisfies the QPM condition between the signal light, the oscillation light generated by the wavelength conversion element, and the converted light. Thereafter, a SiO ₂ film is formed on the entire surface of the (100) surface 11 of the GaAs substrate 10 by any suitable known method such as a CVD method. Thereafter, the well-known photolithography and dry etching, with leaving the portion of the SiO ₂ film in the second region 5b by processing a SiO ₂ film, part of GaAs substrate of SiO ₂ film in the first region 5a 10 An insulating film mask 19 is formed by removing the first main surface 11 until the first main surface 11 is exposed. Therefore, the insulating film mask 19 formed by the remaining SiO ₂ film portion has the opening 19a in the first region 5a (FIG. 6B).

  Next, the Si layer 22 and the first domain layer 32 are sequentially grown as the domain inversion layer 50 on the (100) surface 11 of the GaAs substrate 10 using the insulating film mask 19. The Si layer 22 is preferably formed to a thickness of 1 nm or less. The first domain layer 32 is preferably formed of GaAs with a thickness of 10 nm or more. For example, an MBE apparatus can be used to form the stacked layers of the Si layer 22 and the first domain layer 32. The details of the growth conditions are device-dependent optimum conditions because of device dependency. For example, the growth temperature of the Si layer 22 is about 450 to 550 ° C., and the growth temperature of the first domain layer 32 is as shown in FIG. Similar to the GaAs layer (indicated by reference numeral 30 in FIG. 2B) described with reference to (B), the temperature is set to about 400 to 650 ° C. (FIG. 6C).

  Next, the insulating film mask 19 is removed (FIG. 6D).

  Thereafter, for example, GaAs is grown on the (100) surface 11 of the GaAs substrate 10 in the second region 5b by epitaxial growth using an MBE apparatus, and the second domain layer 40 is obtained. The direction of spontaneous polarization of the first domain GaAs layer 32 and the second domain GaAs layer 40 of the domain inversion layer 50 is inverted depending on the presence or absence of the Si layer 22 on the (100) plane 11 of the GaAs substrate 10. The Si layer 22, the first domain layer 32, and the second domain layer 40 constitute a domain inversion layer 50 (FIG. 6E).

  Since the process after forming the domain inversion layer 50 is performed in the same manner as described with reference to FIGS. 3C, 4A to 4D, and FIGS. 5A to 5B. The description is omitted.

(Second Example of Method for Forming Periodic Domain Inversion Structure)
With reference to FIGS. 7A to 7D and FIGS. 8A to 8C, a second example of obtaining a periodic domain inversion structure will be described. FIGS. 7A to 7D and FIGS. 8A to 8C are schematic views for explaining a method of forming a periodic domain inversion structure. 7 (A) to (D) and FIGS. 8 (A) to (C), the structure obtained in the main manufacturing stage is orthogonal to the upper surface of the wavelength conversion element, as in FIG. 1 (A). And it shows by the cut end of the cross section along the longitudinal direction.

  First, a GaAs substrate 10 is prepared as a semiconductor substrate. Note that a Si substrate, a Ge substrate, or an InP substrate may be used as the semiconductor substrate. Here, the GaAs substrate 10 is described as a rectangular parallelepiped shape whose upper surface is the (100) plane 11. 7A to 7D and FIGS. 8A to 8C, the cross section is taken as the (011) plane 12. The GaAs substrate 10 may be a tilted substrate with the (100) plane tilted to about 10 degrees with respect to the [011] axis (FIG. 7A).

Next, a silicon oxide (SiO ₂ ) film 15 is formed on the (100) surface 11 of the GaAs substrate 10 by any suitable known method such as a CVD method. Thereafter, a photoresist 17 is applied on the SiO ₂ film 15 (FIG. 7B).

  Next, the photoresist 17 is patterned by photolithography using a gradation mask, and processed into an inclined resist 18 having an inclined surface 18a in the first region 5a and a horizontal surface 18b in the second region 5b periodically. The first region 5a and the second region 5b are a set of continuous regions, and are periodically formed on the basis of this set. This period Λ satisfies the QPM condition among the signal light, the oscillation light, and the converted light (FIG. 7C).

Next, dry etching is performed on the SiO ₂ film 15 using the inclined resist 18 as a mask. By this dry etching, a part of the SiO ₂ film 15 is removed, and the inclined insulating film mask 16 is obtained. In this dry etching, the inclined resist 18 is removed together with a part of the SiO ₂ film 15. Since the inclined resist 18 has a uniform thickness portion and a thickness continuously decreasing portion, the SiO ₂ film 15 is also etched in accordance with the shape of the inclined resist 18. Therefore, the mask shape of the inclined insulating film mask 16 is transferred to the etched SiO ₂ film 15. Therefore, the SiO ₂ film 15 becomes an inclined insulating film mask 16 having the same shape as that of the inclined resist 18 by this etching. Therefore, the inclined insulating film mask 16 periodically has the inclined surface 16a in the first region 5a and the horizontal surface 16b in the second region 5b (FIG. 7D).

  Next, dry etching using the inclined insulating film mask 16 is performed on the underlying GaAs substrate 10 to etch the upper portion of the GaAs substrate 10. At this time, the inclined insulating film mask 16 is also removed together with the upper portion of the GaAs substrate 10. The shape of the upper portion of the GaAs substrate 10 is a shape obtained by transferring the shape of the mask 16 in the same manner as the shape of the inclined insulating film mask 16. Therefore, the upper surface of the GaAs substrate 10 periodically has the inclined surface 11a in the first region 5a and the horizontal surface 11b in the second region 5b. That is, the upper surface of the GaAs substrate 10 has a periodic structure of the inclined surface 11a and the horizontal surface 11b. Here, the inclination direction of the inclined surface 11a is formed with reference to the [0-11] axis. Further, the inclination angle θ of the inclined surface 11a with respect to the horizontal surface 11b, that is, the [0-11] axis of the inclined surface 11a is set to about 1 to 10 degrees (FIG. 8A).

  A Ge layer 24 and a GaAs layer 34 are sequentially grown as a buffer layer on the periodic structure formed on the (100) surface 11 of the etched GaAs substrate 10. In order to cause reversal of spontaneous polarization in the process described later, it is preferable that the Ge layer 24 be formed to a thickness of 2 nm or more. The GaAs layer 34 is preferably formed to a thickness of 10 nm or more. For example, an MBE apparatus can be used to form the domain inversion layer 52 including the Ge layer 24 and the GaAs layer 34. The details of the growth conditions are device-dependent optimum conditions because of device dependency. For example, the growth temperature of the Ge layer 24 is about 400 to 500 ° C., and the growth temperature of the GaAs layer 34 is 400 to 650 ° C. It is set to a degree (FIG. 8B).

  Depending on the inclination direction of the upper surface of the GaAs substrate 10, the direction of spontaneous polarization of the GaAs layer 34 grown thereon is reversed. Here, the direction of spontaneous polarization in the second region 5b, that is, the direction of spontaneous polarization of the portion of the GaAs layer on the horizontal surface 11b is the [100] axis direction, and the spontaneous polarization in the first region 5a, that is, formed by etching. It is assumed that the direction of spontaneous polarization on the inclined surface 11a is the [-100] axis direction. In the following description, the first region 5a, that is, the domain in which the direction of spontaneous polarization is the [-100] axis direction may be referred to as the first domain. In addition, the second region 5b, that is, a domain in which the direction of spontaneous polarization is the [100] axis direction may be referred to as a second domain.

  The inclination angle θ of the inclined surface 11a with respect to the horizontal surface 11b described with reference to FIG. 8A is set so that the direction of the spontaneous polarization of the second domain is reversed with respect to the spontaneous polarization of the first domain. It is supposed to be.

  Next, for example, the first cladding layer 60, the waveguide layer 70, the second cladding layer 80, and the cap layer 85 are formed in this order by epitaxial growth using an MBE apparatus.

  The direction of spontaneous polarization of the first cladding layer 60, the waveguide layer 70, and the second cladding layer 80 is equal to the direction of spontaneous polarization of the GaAs layer 34 in each region for each of the first region 5a and the second region 5b. (FIG. 8C).

  The steps after the formation of the periodic domain inversion structure are performed in the same manner as described with reference to FIGS. 4A to 4D and FIGS. To do.

(Operation of wavelength conversion element)
When a forward voltage is applied to the stacked body 55 between the first electrode 92 and the second electrode 94 of the wavelength conversion element formed in the above-described process, stimulated emission light is generated. This stimulated emission light is amplified by a resonator formed between the two cleavage surfaces, and so-called laser light is obtained. This laser light is used as pump light.

When signal light is input in a state where a forward voltage is applied to the wavelength conversion element, that is, in a state where laser light used as pump light is generated, signal light of frequency ω _s and pump light of frequency ω _p nonlinear optical effect and the converted light of frequency omega _c is generated. For example, corresponding to the frequency omega _s, the wavelength lambda _s of the signal light is 1.55 .mu.m, corresponding to the frequency omega _p, and the wavelength lambda _p of the pump light and 0.77 .mu.m, the converted light, ω _c + ω _s = The difference frequency light having the frequency ω _c that satisfies ω _p is obtained, and the wavelength λ _{c of the} converted light is 1.53 μm from the expression 1 / λ _c + 1 / λ _s = 1 / λ _p .

  According to the wavelength conversion element of the present invention, laser light can be generated in the waveguide itself that performs wavelength conversion, and pump light can be obtained as oscillation light. Therefore, unlike the prior art, it is not necessary to mount the semiconductor laser element and the wavelength conversion element on a module and perform optical coupling. As a result, the assembly man-hours and size of the wavelength conversion device can be improved.

  In addition, since optical coupling between the semiconductor laser element and the wavelength conversion element is not necessary, it is possible to efficiently convert the optical output of the pump light. As a result, it is possible to reduce the power required to generate the pump light.

It is a figure for demonstrating a wavelength conversion element. It is FIG. (1) for demonstrating the manufacturing method of a wavelength conversion element. It is FIG. (2) for demonstrating the manufacturing method of a wavelength conversion element. It is FIG. (3) for demonstrating the manufacturing method of a wavelength conversion element. It is FIG. (4) for demonstrating the manufacturing method of a wavelength conversion element. It is a figure for demonstrating the 1st example of the formation method of a periodic domain inversion structure. It is FIG. (1) for demonstrating the 2nd example of the formation method of a periodic domain inversion structure. It is FIG. (2) for demonstrating the 2nd example of the formation method of a periodic domain inversion structure.

Explanation of symbols

5a 1st area | region 5b 2nd area | region 10 GaAs substrate 11 (100) surface (1st main surface)
11a, 16a, 18a Inclined surfaces 11b, 16b, 18b Horizontal surface 12 (011) surface 13 Second main surface 15 SiO ₂ film 16 Inclined insulating film mask 17 Photoresist 18 Inclined resist 19, 42 Insulating film mask 19a Opening 20 , 22 Si layer 24 Ge layer 30, 34 GaAs layer 32 First domain layer 40 Second domain layer 44 Insulating film 50, 52 Domain inversion layer 55 Laminate 60 First cladding layer 70 Waveguide layer 80 Second cladding layer 82 Convex Part 85 cap layer 92 first electrode 94 second electrode 100 wavelength conversion element 102 bar

Claims (7)

A wavelength conversion element that emits converted light having a wavelength different from that of the incident signal light,
A semiconductor substrate;
A domain inversion layer formed on the first main surface of the semiconductor substrate;
A laminate formed by sequentially laminating an n-type first cladding layer, a waveguide layer, and a p-type second cladding layer on the domain inversion layer;
A cap layer formed on the second cladding layer;
A first electrode provided on a second main surface of the semiconductor substrate;
A second electrode provided on the cap layer,
When a forward voltage is applied to the stacked body between the first electrode and the second electrode, stimulated emission light is generated in the waveguide layer,
The stimulated emission light is amplified by a resonator configured between both end faces in the longitudinal direction of the waveguide layer to generate oscillation light,
The waveguide layer is provided with a periodic domain inversion structure that satisfies a quasi-phase matching condition between the signal light, the oscillation light, and the converted light in the waveguide direction of the oscillation light. A wavelength conversion element, wherein a difference frequency light is generated by the incident signal light and the oscillation light, and the difference frequency light is emitted as the converted light. On the first main surface of the semiconductor substrate, the first region and the second region are alternately set at a period satisfying the pseudo phase matching condition,
The domain inversion layer includes
A semiconductor layer and a first domain layer, which are sequentially stacked on the first main surface of the semiconductor substrate in the first region;
A second domain layer formed on the first main surface of the semiconductor substrate of the second region,
2. The wavelength conversion element according to claim 1, wherein the direction of spontaneous polarization of the second domain layer is reversed with respect to the direction of spontaneous polarization of the first domain layer. A wavelength conversion element that emits converted light having a wavelength different from that of the incident signal light,
A semiconductor substrate provided with a periodic structure of an inclined surface of the first region and a horizontal surface of the second region on the first main surface;
A domain inversion layer formed by sequentially laminating a Ge layer and a GaAs layer on the periodic structure;
A laminate formed by sequentially laminating an n-type first cladding layer, a waveguide layer, and a p-type second cladding layer on the domain inversion layer;
A cap layer formed on the second cladding layer;
A first electrode provided on a second main surface of the semiconductor substrate;
A second electrode provided on the cap layer,
When a forward voltage is applied to the stacked body between the first electrode and the second electrode, stimulated emission light is generated in the waveguide layer,
The stimulated emission light is amplified by a resonator configured between both end faces in the longitudinal direction of the waveguide layer to generate oscillation light,
The periodic structure satisfies a quasi phase matching condition between the signal light, oscillation light, and converted light,
In the waveguide layer, the direction of spontaneous polarization of the portion of the second region is reversed with respect to the direction of spontaneous polarization of the portion of the first region, and the signal light incident on the waveguide layer; A wavelength conversion element characterized in that difference frequency light is generated by the oscillation light, and the difference frequency light is emitted as the converted light. In manufacturing a wavelength conversion element that emits converted light having a wavelength different from that of the signal light with respect to the incident signal light,
Preparing a semiconductor substrate; and
On the first main surface of the semiconductor substrate, a domain inversion layer in which the direction of spontaneous polarization is inverted with a period satisfying a quasi phase matching condition between the signal light, the oscillation light generated by the wavelength conversion element, and the converted light is formed. And a process of
An n-type first cladding layer, a waveguide layer, and a p-type second cladding layer are sequentially grown on the domain inversion layer, and the first cladding layer, the waveguide layer, and the second cladding layer are stacked. Forming a periodic domain inversion structure that satisfies the quasi-phase matching condition in the formed laminate;
Forming a cap layer on the second cladding layer;
Forming a first electrode on the second main surface of the semiconductor substrate, and forming a second electrode on the cap layer. The step of forming the domain inversion layer includes:
Forming a first semiconductor layer and a second semiconductor layer on the first main surface of the semiconductor substrate by sequentially laminating;
Insulation in which the first region and the second region are alternately set on the second semiconductor layer at a period satisfying the quasi-phase matching condition, and an opening is provided in the second region by photolithography and dry etching Forming a film mask; and
Removing a portion of the second region of the first semiconductor layer and the second semiconductor layer by etching using the insulating film mask to expose a first main surface of the semiconductor substrate in the second region;
The portion of the second semiconductor layer remaining in the first region is used as a first domain layer, and the direction of spontaneous polarization is the direction of spontaneous polarization in the first region on the semiconductor substrate where the second region is exposed. Growing a second domain layer that is inverted with respect to the same height as the first domain layer;
The method for manufacturing a wavelength conversion element according to claim 4, further comprising a step of removing the insulating film mask. The step of forming the domain inversion layer includes:
On the first main surface of the semiconductor substrate, first regions and second regions are alternately set at a period satisfying the quasi phase matching condition, and openings are provided in the first region by photolithography and dry etching. Forming an insulating film mask,
Forming a Si layer and a GaAs layer on the first main surface of the semiconductor substrate in the first region, and forming the GaAs layer as a first domain layer;
Removing the insulating film mask;
On the first main surface of the semiconductor substrate in the second region exposed by removing the insulating film mask, a GaAs layer in which the direction of spontaneous polarization is reversed with respect to the direction of spontaneous polarization in the first region The method of manufacturing a wavelength conversion element according to claim 4, further comprising: forming the GaAs layer as a second domain layer. In manufacturing a wavelength conversion element that emits converted light having a wavelength different from that of the signal light with respect to the incident signal light,
Preparing a semiconductor substrate; and
On the first main surface of the semiconductor substrate, the first region and the second region are alternately arranged at a period satisfying a quasi phase matching condition between the signal light, the oscillation light generated by the wavelength conversion element, and the converted light. A setting process;
Forming a periodic structure having a horizontal surface in the first region and an inclined surface in the second region on the first main surface of the semiconductor substrate;
A Ge layer is formed on the periodic structure, and a GaAs layer is further formed on the Ge layer, and the direction of spontaneous polarization in the second region is reversed with respect to the direction of spontaneous polarization in the first region. Obtaining a domain inversion layer,
A laminated body in which a first cladding layer, a waveguide layer, and a second cladding layer are sequentially grown on the domain inversion layer, and the first cladding layer, the waveguide layer, and the second cladding layer are stacked. Forming the laminate having a periodic domain inversion structure in which the direction of spontaneous polarization in the second region is reversed with respect to the direction of spontaneous polarization in the first region;
Forming a cap layer on the second cladding layer;
Forming a first electrode on the second main surface of the semiconductor substrate, and forming a second electrode on the cap layer.

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