JP4806424B2 - Harmonic generator - Google Patents

Description

  The present invention relates to a quasi-phase matching type harmonic generating element.

  Nonlinear optical crystals such as lithium niobate and lithium tantalate single crystals have high second-order nonlinear optical constants. By forming a periodic domain-inverted structure in these crystals, quasi-phase-matched (Quasi-Phase-Matched: A second-harmonic generation (SHG) device of the QPM method can be realized. Further, by forming a waveguide in the periodically poled structure, a highly efficient SHG device can be realized, and a wide range of applications such as optical communication, medical use, photochemistry use, and various optical measurement applications are possible.

In the harmonic generation element described in Patent Document 1, a ferroelectric single crystal thin plate is bonded onto a support substrate, and an upper substrate is bonded thereto via a buffer layer and an adhesive layer. A channel type optical waveguide is formed. Then, by forming a periodically poled structure in the optical waveguide, the fundamental wave incident on the optical waveguide is wavelength-converted into a harmonic.
WO 2006/41172 A1

  A harmonic generation element such as an SHG generation element is required to operate stably even if it is repeatedly exposed to changes in environmental temperature. However, it has been found that the wavelength conversion efficiency of the element of the type described in Patent Document 1 may deteriorate after repeated exposure to a thermal cycle between −40 ° C. and + 80 ° C. did.

  When the inventor collects and observes the element in which the wavelength conversion efficiency is lowered, it is found that the periodically poled structure is deteriorated. Moreover, it has been found that such a phenomenon occurs in the X plate (Y plate) and the offset X plate (offset Y plate), but does not occur in the case of using a Z plate thin plate. Further, as described in Patent Document 1, it has been found that such a phenomenon is peculiar to an element of a type in which a ferroelectric thin plate for performing wavelength conversion is sandwiched between a support substrate and an upper substrate. .

  An object of the present invention is to form a periodic polarization reversal structure on a thin plate of an X plate or an offset X plate and sandwich a sandwiched between a support substrate and an upper substrate after being subjected to a temperature cycle. This is to prevent a decrease in wavelength conversion efficiency.

The harmonic generation element according to the present invention is
Support substrate,
A wavelength conversion layer comprising a channel-type optical waveguide comprising a ferroelectric single crystal X plate or an offset X plate and provided with a periodically poled structure;
An upper substrate provided on the upper surface side of the wavelength conversion layer,
A conductive film formed on the bottom surface of the upper substrate ;
A base adhesive layer for bonding the wavelength conversion layer and the support substrate, and an upper adhesive layer for bonding the wavelength conversion layer and the upper substrate via a conductive film are provided.
In addition, the harmonic generation element according to the present invention,
Support substrate,
A wavelength conversion layer comprising a channel-type optical waveguide comprising a ferroelectric single crystal X plate or an offset X plate and provided with a periodically poled structure;
An upper substrate provided on the upper surface side of the wavelength conversion layer,
A conductive film formed on the upper surface of the support substrate;
A base adhesive layer for bonding the wavelength conversion layer and the support substrate through a conductive film; and
An upper adhesive layer for adhering the wavelength conversion layer and the upper substrate is provided.

The inventor forms a periodically poled structure on a thin plate of an X plate or an offset X plate, and converts the wavelength after being subjected to a temperature cycle in a harmonic generation device having a structure sandwiched between a support substrate and an upper substrate. The cause of the decrease in efficiency was investigated. Then, when the periodic polarization inversion structure was confirmed by etching the thin plate of the harmonic generation element, the periodic polarization inversion structure was deteriorated or disappeared locally. This has caused a decrease in wavelength conversion efficiency.

  Further examination of the cause of the deterioration of such a periodically poled structure revealed that it was caused by pyroelectricity between both sides of the device. Based on this discovery, wavelength conversion efficiency after exposure to thermal cycle by providing a conductive film near the channel type optical waveguide with periodic polarization inversion structure sandwiched between the upper substrate and the support substrate We found that we can prevent the decline.

  FIG. 1A is a perspective view schematically showing a conventional harmonic generation element 1, and FIG. 1B is an enlarged view showing a channel type optical waveguide 24 of the element 1 and its periphery. FIG. 2A is a perspective view schematically showing the harmonic generation element 11 of the example of the present invention, and FIG. 2B is an enlarged view showing the channel type optical waveguide of the element 11 and its periphery. FIG. 3 is a front view showing a state in which the element 11 of FIG.

  As shown in FIGS. 1 and 2, a pair of elongated grooves 6A and 6B are provided in a wavelength conversion layer 3 made of a ferroelectric single crystal X plate (Y plate, offset X plate, offset Y plate). The grooves 6A and 6B are parallel to each other, and the ridge portion 4 is formed by these grooves. A channel-type optical waveguide 24 is formed by the ridge portion 4 and the grooves 6A and 6B. Extending portions 7A and 7B are formed on the outer sides of the grooves 6A and 6B to form a thin plate.

  In the case of an X plate (Y plate), the lateral direction in FIGS. 1 and 2 is the Z direction, and the ferroelectric single crystal is polarized in the Z direction. The X axis (Y axis) is perpendicular to the upper surface 3 a of the wavelength conversion layer 3. In the case of the offset X plate and Y plate, the X axis (Y) axis is inclined from a plane perpendicular to the main surface of the wavelength conversion layer 3. This inclination angle is preferably 10 ° or less from the viewpoint of the present invention.

  In the channel-type optical waveguide 24, the light is polarized in the Z direction perpendicular to the light propagation direction, and the polarization direction is periodically reversed. As a result, the fundamental wave incident from the incident surfaces 1a and 11a of the elements 1 and 11 undergoes wavelength conversion in the optical waveguide 24, and harmonics are emitted from the emission surfaces 1b and 11b.

  The bottom surface 3 b of the wavelength conversion layer 3 is bonded to the top surface 2 a of a separate support substrate 2 by a base adhesive layer 21. The upper surface 3 a of the wavelength conversion layer 3 is bonded to the bottom surface 5 a of the separate upper substrate 5 by the upper adhesive layer 20. 11c and 11d are a pair of side surfaces extending between the entrance surface 11a and the exit surface 11b, and the side surfaces 11c and 11d are opposed to each other.

  Typically, as shown in FIG. 3, a pedestal 8 having installation surfaces 8b and 8a is provided, and an element 11 is installed on the pedestal 8 and coupled to an external line. The side surface 11d of the element 11 is brought into contact with the installation surface 8b, and the bottom surface 2b of the element 11 is brought into contact with the installation surface 8a of the base 8.

  Here, in the case of the element of FIG. 1, after being exposed to a thermal cycle, the periodic polarization inversion formed in the channel-type optical waveguide 24 due to pyroelectricity between the side surfaces 1 c and 1 d of the element 1. The structure was degraded. This has been a cause of a decrease in wavelength conversion efficiency.

  The inventor formed a conductive film 10 on the bottom surface 5a of the upper substrate 5 as shown in FIGS. Accordingly, the conductive film 10 is interposed between the upper adhesive layer 20 and the upper substrate 5 and is located in the vicinity of the optical waveguide 4. It has been found that this can prevent a decrease in wavelength conversion efficiency after thermal cycle exposure.

  FIG. 4A is a perspective view schematically showing a harmonic generation element 31 according to another embodiment of the present invention, and FIG. 4B is an optical waveguide among the elements 31 in FIG. 4 is an enlarged view showing the periphery of 4. FIG.

  In this example, the fundamental wave enters the optical waveguide from the incident surface 31a of the element, undergoes wavelength conversion by the periodic polarization inversion structure, and a harmonic is emitted from the output surface 31b. 31c and 31d are side surfaces between the entrance surface and the exit surface.

  In this example, no conductive film is formed on the bottom surface of the upper substrate 5, and the conductive film 15 is formed on the upper surface 2 a of the support substrate 2. The conductive film 15 is interposed between the upper surface 2 a of the support substrate 2 and the base adhesive layer 21 and is located in the vicinity of the optical waveguide 4. It has been found that this can prevent a decrease in wavelength conversion efficiency after thermal cycle exposure.

  In the present invention, a conductive film is formed in the vicinity of the channel type optical waveguide. Here, the vicinity of the channel-type optical waveguide means that the position is 10 μm or less from the outer contour of the optical mode field of the optical waveguide.

In the present invention, the conductive film is provided between the bottom surface of the upper substrate and the upper adhesive layer (FIG. 2). Alternatively, a conductive film is provided between the upper surface of the support substrate and the base adhesive layer (FIG. 3). Alternatively, the conductive film is provided between the bottom surface of the upper substrate and the upper adhesive layer, and another conductive film is provided between the upper surface of the support substrate and the base adhesive layer. In these embodiments, since the conductive film is not in direct contact with the optical waveguide and the adhesive layer is interposed therebetween, light absorption by the conductive film is suppressed, and light propagation loss due to this can be suppressed.

The method for manufacturing the conductive film is not limited, and the following can be exemplified.
(1) A metal thin film is formed on the bottom surface of the upper substrate and the upper surface of the support substrate by sputtering.
(2) A conductive paste is applied to the bottom surface of the upper substrate or the upper surface of the support substrate by printing or the like and baked.
(3) A conductive tape is applied to the bottom surface of the upper substrate and the top surface of the support substrate.

  The material of the conductive film is not particularly limited, and examples thereof include metals and conductive pastes. Specifically, Al, Ti, Ta, Cu, Ag-based paste, and In-based paste are preferable.

  The thickness of the conductive film is not particularly limited, but is preferably 0.05 μm or more, more preferably 0.1 μm or more from the viewpoint of the present invention. From the viewpoint of suppressing light absorption by the conductive film, the thickness of the conductive film is preferably 5 μm or less.

  When an upper adhesive layer and / or an underlying adhesive layer is interposed between the conductive film and the optical waveguide, the thickness of the upper adhesive layer and the underlying adhesive layer prevents the wavelength conversion efficiency from being lowered due to pyroelectricity. From the viewpoint of achieving this, it is preferably 10 μm or less, and more preferably 5 μm or less. Further, the thickness of the upper adhesive layer and the thickness of the base adhesive layer are preferably 0.5 μm or more, and preferably 1.0 μm or more, from the viewpoint of suppressing absorption of propagation light by the conductive film. Further preferred.

  In the case where the conductive film is provided on the bottom surface of the upper substrate, from the viewpoint of the present invention, it is preferable that the conductive film covers 90% or more of the bottom surface of the upper substrate. The conductive film may cover the entire bottom surface of the upper substrate.

  In the case where the conductive film is provided on the upper surface of the support substrate, from the viewpoint of the present invention, the conductive film preferably covers 90% or more of the upper surface of the support substrate. The conductive film may cover the entire upper surface of the support substrate.

  The channel type optical waveguide formed in the wavelength conversion layer is not limited, and may be a ridge type optical waveguide or a diffusion type optical waveguide. The diffusion optical waveguide can be formed by metal diffusion (for example, titanium diffusion) or proton exchange. The processing method for forming the ridge structure is not limited, and methods such as machining, ion milling, dry etching, and laser ablation can be used.

Ferroelectric single crystal forming the wavelength conversion layer is not limited, lithium niobate, lithium tantalate, lithium niobate - lithium tantalate solid _solution, a _{K 3 L i2 Nb 5 O 15} , La 3 Ga 5 SiO 14 illustrate it can.

  The adhesive for bonding the wavelength conversion layer to the support substrate or the upper substrate may be an inorganic adhesive, an organic adhesive, or a combination of an inorganic adhesive and an organic adhesive.

Specific examples of the organic adhesive are not particularly limited, but heat that is relatively close to a material having an electro-optic effect, such as an epoxy adhesive, an acrylic adhesive, a thermosetting adhesive, an ultraviolet curable adhesive, or lithium niobate. An example is Aron ceramics C (trade name, manufactured by Toagosei Co., Ltd.) (thermal expansion coefficient 13 × 10 ⁻⁶ / K) having an expansion coefficient.

  The inorganic adhesive preferably has a low dielectric constant and an adhesive temperature (working temperature) of 600 ° C. or lower. Moreover, what can obtain sufficient adhesive strength in the case of a process is preferable. Specifically, glass in which a composition of silicon oxide, lead oxide, aluminum oxide, magnesium oxide, calcium oxide, boron oxide, or the like is used alone or in combination is preferable. Examples of other inorganic adhesives include tantalum pentoxide, titanium oxide, niobium pentoxide, and zinc oxide.

A method for forming the inorganic adhesive layer is not particularly limited, and examples thereof include a sputtering method, a vapor deposition method, a spin coating method, and a sol-gel method.
In addition, an adhesive sheet can be interposed between the wavelength conversion layer 3, the support base 1, and the upper substrate 5 for bonding. Preferably, a sheet made of a thermosetting, photocurable or photothickening resin adhesive is interposed between the wavelength conversion layer 3, the support base 1, and the upper substrate 5 to cure the sheet. As such a sheet, a film resin of 10 μm or less is suitable.

  Specific materials of the support substrate and the upper substrate are not particularly limited, and examples thereof include glass such as lithium niobate, lithium tantalate, and quartz glass, quartz, and Si. In this case, from the viewpoint of the difference in thermal expansion, it is preferable that the wavelength conversion layer, the support substrate, and the upper substrate are made of the same material, and lithium niobate single crystal is particularly preferable. Although the thickness of the upper substrate and the thickness of the support substrate are not particularly limited, 100 μm or more is preferable from the above viewpoint. Moreover, although there is no upper limit in particular of the thickness of a support substrate and the thickness of an upper board | substrate, 2 mm or less is preferable practically.

(Example 1)
An element 11 as shown in FIG. 2 was produced. Specifically, a periodically poled structure having a period of 6.6 μm was formed on a 0.5 mm-thick MgO 5% doped lithium niobate 5-degree offcut Y substrate. After applying an adhesive (acrylic adhesive) to a non-doped lithium niobate substrate having a thickness of 0.5 mm, it was bonded to the MgO-doped lithium niobate substrate 2 and the surface of the MgO-doped lithium niobate substrate was 3.7 μm thick. The thin plate was obtained by grinding and polishing until it became. Next, the ridge structure 4 (optical waveguide 24) was formed on this thin plate by a laser ablation method. After forming the optical waveguide, a 0.5 μm thick SiO ₂ overclad was formed by sputtering.

  An aluminum film 10 having a thickness of 0.1 μm was formed by sputtering on the bottom surface 5a of the upper substrate 5 made of a non-doped lithium niobate single crystal having a thickness of 0.3 mm. The bottom surface 5a of the upper substrate was covered with the aluminum film 10 over the entire surface. Thereafter, the film forming surface side was bonded to the over clad side with an adhesive. The device was cut with a dicer to a length of 9 mm and a width of 1.0 mm, and then the end face was polished and an antireflection film was applied.

  In this waveguide, optical characteristics were measured using a Nd-YAG laser. The oscillation output from the laser was adjusted to 500 mW, and the basic light was input to the end face of the waveguide with a lens, and a SHG output of 200 mW was obtained. The wavelength of the basic light at that time was 1064.3 nm.

  This device 11 was subjected to a thermal cycle test of −40 ° C./80° C. As a result of measuring the second harmonic (SHG) output again after 500 cycles, no decrease in the SHG output was observed.

(Example 2)
2 was produced in the same manner as in Example 1. However, unlike Example 1, 90% of the bottom surface 5 a of the upper substrate was covered with the aluminum film 10.

  This device 11 was subjected to a thermal cycle test of −40 ° C./80° C. As a result of measuring the second harmonic (SHG) output again after 500 cycles, no decrease in the SHG output was observed.

(Example 3)
2 was produced in the same manner as in Example 1. However, unlike Example 1, 80% of the bottom surface 5 a of the upper substrate was covered with the aluminum film 10.

  This device 11 was subjected to a thermal cycle test of −40 ° C./80° C. As a result of measuring the second harmonic (SHG) output again after 500 cycles, the SHG output decreased to 170 mW.

(Comparative Example 1)
1 was produced in the same manner as in Example 1. Unlike Example 1, the aluminum film 10 was not provided.

  In this waveguide, optical characteristics were measured using a Nd-YAG laser. The oscillation output from the laser was adjusted to 500 mW, and the basic light was input to the end face of the waveguide with a lens, and a SHG output of 200 mW was obtained. The wavelength of the basic light at that time was 1064.3 nm.

  The device 1 was subjected to a thermal cycle test of −40 ° C./80° C. As a result of measuring the second harmonic (SHG) output again after 500 cycles, the SHG output decreased to 65 mW.

(A) is a perspective view schematically showing a harmonic generation element 1 of a conventional example, and (b) is an enlarged view showing a channel type optical waveguide 24 of the element 1 and its periphery. (A) is a perspective view schematically showing a harmonic generation element 11 of an example of the present invention, and (b) is an enlarged view showing a channel type optical waveguide of the element 11 and its periphery. FIG. 3 is a front view showing a state in which an element 11 of FIG. 2 is installed on a base 8. (A) is a perspective view which shows typically the harmonic generation element 31 which concerns on the other example of this invention, (b) is an enlarged view which shows the channel type | mold optical waveguide of the element 31, and its periphery.

Explanation of symbols

2 Support substrate 2a Upper surface of support substrate 3 Wavelength conversion layer 4 Ridge part 5 Upper substrate 5a Bottom surface of upper substrate 6A, 6B Ridge groove 10, 15 Conductive film 11, 31 Harmonic wave generation element 11a, 31a Incident surface 11b, 31b Output surface 20 Upper adhesive layer 21 Base adhesive layer

Claims (5)

Support substrate,
A wavelength conversion layer comprising a channel-type optical waveguide comprising a ferroelectric single crystal X plate or an offset X plate and provided with a periodically poled structure;
An upper substrate provided on the upper surface side of the wavelength conversion layer,
A conductive film formed on the bottom surface of the upper substrate ;
A harmonic comprising: a base adhesive layer for bonding the wavelength conversion layer and the support substrate; and an upper adhesive layer for bonding the wavelength conversion layer and the upper substrate through the conductive film. Generating element. It comprises another conductive film formed on the upper surface of the support substrate, and the wavelength conversion layer and the support substrate are bonded via the other conductive film by the base adhesive layer. , harmonic generation device of claim 1, wherein. Support substrate,
A wavelength conversion layer comprising a channel-type optical waveguide comprising a ferroelectric single crystal X plate or an offset X plate and provided with a periodically poled structure;
An upper substrate provided on the upper surface side of the wavelength conversion layer,
A conductive film formed on the upper surface of the support substrate;
A harmonic comprising: a base adhesive layer that bonds the wavelength conversion layer and the support substrate through the conductive film ; and an upper adhesive layer that bonds the wavelength conversion layer and the upper substrate. Generating element. The pair of elongate grooves are formed in the wavelength conversion layer, and the channel type optical waveguide is formed by the pair of grooves, according to any one of claims 1 to 3. Harmonic generation element. The harmonic generating element according to any one of claims 1 to 4, wherein a thickness of the upper adhesive layer and the base adhesive layer is 0.5 µm or more and 10 µm or less.

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