JP4806427B2 - Wavelength conversion element and manufacturing method thereof - Google Patents

Description

  The present invention relates to a wavelength conversion element and a method for manufacturing the same.

  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.

  The present inventor obtained a profile beam by collimating blue laser light or ultraviolet laser light (second harmonic) in the practical application research of the second harmonic generation element, and analyzed this.

  When this profile beam is viewed, low spatial frequency patterns appear above and below the main beam as viewed in the vertical direction. This is due to light leakage up and down the optical waveguide. Since the low spatial frequency pattern is separated from the main beam in the vertical direction, it can be cut relatively easily, and the intensity of the main beam does not decrease after the cut.

  On the other hand, a high spatial frequency pattern appears in the horizontal direction of the profile beam. This is due to light leakage to the slab waveguide portions on both sides of the ridge type optical waveguide portion. This is because the refractive index difference between the ferroelectric layer and the adhesive layer is large, so that the leaked light does not radiate to the substrate but propagates through the slab part and is emitted from the end face. Since such a high spatial frequency pattern overlaps with the main beam, it is difficult to separate and remove, and after the separation and removal, the intensity of the main beam is significantly reduced. Therefore, the beam quality is deteriorated.

In order to solve this problem, the present applicant covers the surface of the extension part or slab part with a metal film as described in Patent Document 1, so that the horizontal direction caused by light leakage to the extension part can be achieved. It has been disclosed to remove high spatial frequency patterns.
JP 2007-256324 A

  In the optical waveguide substrate described in Patent Document 1, a high spatial frequency pattern in the horizontal direction due to light leakage to the extending portion can be prevented, and the quality of the wavelength converted light can be improved. However, although it is necessary to bond a cap (covered substrate) to the surface of the optical waveguide substrate, it has been found that it is extremely difficult to bond the coated substrate to the surface of the optical waveguide substrate. That is, the coated substrate did not adhere to the surface of the optical waveguide substrate and was peeled off. The surface area of the optical waveguide substrate is very small, and since the adhesive strength at the interface between the metal film and the adhesive is low, it is considered that an adhesive force cannot be obtained.

  For this reason, the inventor does not provide a metal film on the surface of the extension portion of the optical waveguide substrate, and may scatter the leakage light propagating through the extension portion by roughening the surface of the extension portion. Tried. However, since it is an optical component, the surface of the extension is originally at the level of optical polishing, and it is impossible to sufficiently scatter and attenuate the leakage light propagating through the extension by roughening the surface. There was found. If a large roughness is introduced into the surface of the extended portion, the surface of the ridge-shaped optical waveguide is adversely affected, causing a propagation loss.

  An object of the present invention is to provide a beam quality of converted light after wavelength conversion in a wavelength conversion element of a type in which a wavelength conversion part is formed in a ridge type optical waveguide of an optical waveguide substrate and a coated substrate is adhered to the surface of the optical waveguide substrate. At the same time, it is possible to bond the coated substrate to the optical waveguide substrate.

The present invention provides a support substrate,
An optical waveguide substrate installed on a support substrate, made of a ferroelectric material, having a wavelength conversion function, a ridge-type optical waveguide, grooves provided on both sides of the ridge-type optical waveguide, and grooves An optical waveguide substrate having an extending portion provided on the outside;
A coated substrate provided on an optical waveguide substrate;
A first adhesive layer for adhering the support base and the bottom surface of the optical waveguide substrate , and a second adhesive layer for adhering the upper surface of the ridge-type optical waveguide of the optical waveguide substrate and the upper surface of the extending portion to the covering substrate. A wavelength conversion element comprising:
The present invention relates to a wavelength conversion element and a method for manufacturing the same, in which the upper side surface of the extending portion is roughened by a plurality of rows of elongated recesses formed by laser light irradiation.

According to the present invention, a rough surface comprising a plurality of rows of elongated concave portions is formed by scanning with laser light on the upper surface of the extending portion on both sides of the ridge-type optical waveguide on the side of the covering substrate adhesive layer. With such a concave pattern, the leakage light propagating through the extended portion was attenuated, the coherence to the wavelength converted light as the coherent light was suppressed, and a high spatial frequency pattern in the horizontal direction was successfully prevented. In addition, in the case where the concave portion formed by laser processing is used, the surface of the ridge-type optical waveguide is not adversely affected, and the adhesive force of the extending portion to the covering substrate is not reduced.

Hereinafter, the present invention will be described in more detail with reference to the drawings.
First, as shown in FIG. 1A, the buffer layer 3 is formed on the back surface of the ferroelectric material 4. Then, the ferroelectric material 4 is adhered on the support substrate 1 by the adhesive layer 3.

  Next, as shown in FIGS. 1B and 1C, the surface of the ferroelectric material 4 is irradiated with a laser beam, and the surface is scanned with the laser beam as indicated by an arrow D, whereby the grooves 7A and 7B are formed. A ridge type optical waveguide 6 is formed between the grooves 7A and 7B. Thin groove forming portions 10A and 10B remain under the grooves 7A and 7B. Extending portions 8A and 8B are formed outside the grooves 7A and 7B, respectively.

  Here, in this example, the elongated grooves 8 are formed by scanning the surfaces of the extending portions 8A and 8B as indicated by the arrow D with laser light. Preferably, the recess 9 and the ridge grooves 7A and 7B can be formed simultaneously.

  Next, as shown in FIG. 2A, the buffer layer 11 is provided so as to cover the surface of the optical waveguide substrate 5. Next, as shown in FIG. 2B, the adhesive layer 12 is provided on the buffer layer 11, and the covering substrate 13 is adhered to the buffer layer 11 and the optical waveguide substrate 5 to obtain a device.

  Also in the conventional device, as shown in FIGS. 3A and 3B, the surface of the ferroelectric material 4 is irradiated with a laser beam, and the surface is scanned with the laser beam as indicated by an arrow D, thereby forming a groove. 7A and 7B are formed, and a ridge type optical waveguide 6 is formed between the grooves 7A and 7B. Extending portions 18A and 18B are formed outside the grooves 7A and 7B, respectively. However, in the conventional device, the laser processing is not particularly performed on the surfaces of the extending portions 18A and 18B.

  As a result, in the conventional device, for example, as schematically shown in FIG. 4, in the wavelength conversion unit 6 composed of the ridge type optical waveguide, the wavelength converted light is confined in the ridge type optical waveguide 6 as 20. Output in the form. However, the ridge grooves 7A and 7B and the extending portions 18A and 18B function as slab type optical waveguides and provide interference light output of a high frequency pattern as indicated by 21. Since such a high spatial frequency pattern overlaps with the main beam, it is difficult to separate and remove, and after the separation and removal, the intensity of the main beam is significantly reduced.

  On the other hand, in the device of the present invention example, as schematically shown in FIG. 5, for example, the high frequency pattern 21 is suppressed by the action of the concave portion 9, and the quality of the wavelength converted light 20 is remarkably improved.

  In addition, in the present invention, the concave portions 9 are formed on the surfaces of the extending portions 8A and 8B, but even when the output of the high frequency pattern 21 is suppressed, the adhesive strength of the coated substrate 13 is high and practical. It was found that a sufficient strength was obtained. If the surfaces of the extending portions 8A and 8B are merely mechanically roughened, the quality of the wavelength-converted light output from the ridge-type optical waveguide deteriorates if the roughness is increased to suppress the output of the high frequency pattern 21. To do.

  The planar shape of the groove 9 is not particularly limited. For example, a large number of island-shaped grooves can be formed on the surface of the extending portion in the vertical and horizontal directions. Preferably, an elongated recess is formed on the surface of the extension. In this embodiment, the aspect ratio of the planar shape of the recess is preferably 1:50 or more.

  The width A (see FIG. 1C) of the ridge-type optical waveguide is not particularly limited, and can be selected according to the desired light output, light density, and light damage resistance of the ferroelectric material. As an example, A can be 3 μm or more, and can be 7 μm or less. The width B of the ridge grooves 7A and 7B is preferably 3 μm or more from the viewpoint of suppressing light leakage from the ridge type optical waveguide. Further, since the strength of the optical waveguide substrate 5 tends to decrease as the width B of the ridge grooves 7A and 7B increases, the width B is preferably 20 μm or less from this viewpoint.

  The width C of the elongated recess 9 is not particularly limited, but is preferably 0.1 μm or more, and more preferably 2.0 μm or more, from the viewpoint of suppressing the high frequency pattern in the extending portion.

  The cross-sectional shape of the recess is not particularly limited, and for example, a triangle, a rectangle, and a square are preferable. Moreover, the space | interval of the recessed part adjacent to the longitudinal direction D of a recessed part at right angle is not specifically limited, 0 micrometers may be sufficient as the minimum of a space | interval. However, a flat portion can also be provided between adjacent recesses. In this case, the width of the flat portion is preferably 0.1 to 2.0 μm, and more preferably 0.5 to 1.5 μm.

  The depth of the concave portion on the surface of the extending portion is preferably 0.1 μm or more, and more preferably 0.5 μm or more, in order to attenuate the leakage light of the high frequency pattern. In addition, the depth of the concave portion on the surface of the extending portion is preferably 2.0 μm or less, and more preferably 1.0 μm or less, in order to improve the adhesive force to the coated substrate.

  The wavelength conversion means in the wavelength conversion unit is not particularly limited. In a preferred embodiment, a periodic polarization inversion structure is formed in the wavelength converter, thereby converting the wavelength of the fundamental light and outputting a harmonic. The period of such a periodically poled structure is changed according to the wavelength. The method for forming the periodically poled structure is not particularly limited, but a voltage application method is preferred.

  Alternatively, a nonlinear optical crystal such as lithium potassium niobate, lithium potassium tantalate, or lithium potassium niobate-lithium potassium tantalate solid solution can be used to convert the wavelength of the incident fundamental light into a harmonic.

  The ferroelectric material constituting the optical waveguide substrate is not particularly limited as long as light modulation is possible, but lithium niobate, lithium tantalate, lithium niobate-lithium tantalate solid solution, potassium lithium niobate, KTP, GaAs And crystal can be exemplified.

  In the ferroelectric single crystal, at least one selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) in order to further improve the optical damage resistance of the optical waveguide. The metal element can be contained, and magnesium is particularly preferable. The ferroelectric single crystal can contain a rare earth element as a doping component. This rare earth element acts as an additive element for laser oscillation. As this rare earth element, Nd, Er, Tm, Ho, Dy, and Pr are particularly preferable.

  Examples of the material of the front side buffer layer 11 and the back side buffer layer 3 include silicon oxide, magnesium fluoride, silicon nitride, alumina, and tantalum pentoxide.

  The material of the adhesive layers 2 and 12 may be an inorganic adhesive, an organic adhesive, or a combination of an inorganic adhesive and an organic adhesive.

Although the specific example of an organic adhesive is not specifically limited, Aron which has a thermal expansion coefficient comparatively close to materials which have an electro-optic effect, such as an epoxy adhesive, a thermosetting adhesive, an ultraviolet curable adhesive, and lithium niobate Ceramics C (trade name, manufactured by Toa Gosei Co., Ltd.) (thermal expansion coefficient 13 × 10 ⁻⁶ / K) can be exemplified.

  The inorganic adhesive preferably has a low dielectric constant and an adhesive temperature (working temperature) of about 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.

  The processing method for forming the ridge type optical waveguide in the ferroelectric material is not limited, but laser ablation is preferable.

The laser processing used to form a recess at least on the extended portion will be described.
Preferably, the half width of the pulse of the laser beam is 10 nsec or less. Further, there is no particular lower limit on the half width of the pulse of the laser beam, but 0.5 nsec or more is preferable from the viewpoint of processing the substrate with high productivity.

  The wavelength of the laser processing light is preferably 350 nm or less, and more preferably 300 nm or less. However, from a practical viewpoint, the thickness is preferably 150 nm or more.

  The excimer laser is an ultraviolet pulsed repetitive oscillation laser. Ultraviolet light oscillated by a gaseous compound such as ArF (wavelength 193 nm), KrF (wavelength 248 nm), XeCl (wavelength 308 nm) is directed by an optical resonator. They are taken out together. As an actual light source, in addition to an excimer laser light source, a fourth harmonic of YAG (for example, a fourth harmonic of an Nd-YAG laser) and an excimer lamp are currently practical.

The light irradiation elements for laser processing include so-called batch exposure elements and multiple reflection elements. Moreover, the following three aspects can be mentioned as a method of forming a recessed part by irradiation of a laser beam.
(1) Spot scan processing (2) Batch transfer processing (3) Slit scan processing

  The material of the support base 1 and the covering substrate 13 is not particularly limited. In a preferred embodiment, the minimum value of the thermal expansion coefficient in the support base 1 and the coated substrate 13 is not less than 1/5 times the minimum value of the thermal expansion coefficient in the ferroelectric material 4, and the support base 1 and the coated substrate The maximum value of the thermal expansion coefficient at 13 is not more than 5 times the maximum value of the thermal expansion coefficient of the ferroelectric material 4.

Here, when each electro-optic material constituting the ferroelectric material 4, the support base 1, and the coated substrate 13 has no anisotropy of thermal expansion coefficient, the ferroelectric material 4, the support base 1, and the coated substrate. In FIG. 13, the minimum thermal expansion coefficient and the maximum thermal expansion coefficient coincide. When each electro-optic material constituting the ferroelectric material 4, the support base 1, and the coated substrate 13 has anisotropy in thermal expansion coefficient, the thermal expansion coefficient may change for each axis. For example, when each electro-optic material constituting the ferroelectric material 4 is lithium niobate, the thermal expansion coefficient in the X-axis direction and the Y-axis direction is 16 × 10 ⁻⁶ / ° C., which is the maximum value. Become. The thermal expansion coefficient in the Z-axis direction is 5 × 10 ⁻⁶ / ° C., which is the minimum value. Accordingly, the minimum value of the thermal expansion coefficient of the support substrate 1 is set to 1 × 10 ⁻⁶ / ° C. or more, and the maximum value of the thermal expansion coefficient of the support substrate 1 is set to 80 × 10 ⁻⁶ / ° C. or less. For example, the thermal expansion coefficient of quartz glass is 0.5 × 10 ⁻⁶ / ° C., which is less than 1 × 10 ⁻⁶ / ° C.

  From this point of view, it is more preferable that the minimum value of the thermal expansion coefficient of the support base 1 and the coated substrate 13 is at least 1/2 times the minimum value of the thermal expansion coefficient of the ferroelectric material 4. Further, it is more preferable that the maximum value of the thermal expansion coefficient of the support base 1 and the coated substrate 13 is not more than twice the maximum value of the thermal expansion coefficient of the ferroelectric material 4.

  Specific materials of the support base 1 and the covering substrate 13 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 ferroelectric material, the supporting substrate, and the covering substrate are made of the same material, and lithium niobate single crystal is particularly preferable.

(Example)
According to the method described with reference to FIGS. 1 and 2, a device as shown in FIG. Specifically, a comb-like periodic electrode with a period of 4.20 μm was formed on a 0.5 mm-thick MgO 5% doped lithium niobate 5-degree offcut Y substrate 4 by a photolithography method. An electrode film was formed on the entire back surface of the substrate 4 and then a pulse voltage was applied to form a periodically poled structure. After forming the periodically poled structure on the substrate 4, a 0.4 μm thick SiO ₂ underclad 3 was formed by sputtering.

  After the adhesive 2 is applied to the non-doped lithium niobate substrate 1 having a thickness of 0.5 mm, it is bonded to the MgO-doped lithium niobate substrate 4 until the surface of the MgO-doped lithium niobate substrate 4 has a thickness of 3.7 μm. Grinded and polished.

  A ridge type optical waveguide 7 was formed on the substrate 4 by a laser ablation processing method. At this time, a mask was prepared so that the beam shape on the surface of the extended portion during laser processing became the shape shown in FIG.

After forming the ridge-type optical waveguide 7 and the recess 9, a 0.5 μm thick SiO ₂ overclad 11 was formed by sputtering. After the adhesive 12 was applied on the over clad 11, a non-doped lithium niobate substrate 13 having a thickness of 0.5 mm was adhered. After the adhesive 12 was thermally cured, the coated substrate 13 was ground to a thickness of 0.1 mm. The device was cut with a dicer to a length of 9 mm and a width of 1.0 mm, the end face was polished, and an antireflection film was applied.

  The optical characteristics of this optical waveguide chip were measured using a titanium sapphire laser. As a result of adjusting the oscillation output from the laser to 100 mW and condensing the basic light on the end face of the waveguide with a lens, 80 mW could be coupled to the waveguide. When the wavelength of the titanium sapphire laser was varied and adjusted to a phase matching wavelength, a maximum SHG output of 13 mW was obtained. The wavelength of the fundamental light at that time was 919.8 nm.

  Further, when the M2 value of the outgoing SHG light was measured by a beam profiler, it was 1.05, and good beam quality was obtained. The M2 value is 1.0 for an ideal Gaussian beam. The larger the beam profile, the greater the value. This is a result of a decrease in the scattered light of SHG emitted from the extended portion (slab portion).

  Moreover, the adhesion state of the coated substrate and the optical waveguide substrate 5 was good, and no peeling was observed between the two even after applying a thermal cycle between -40 ° C and + 85 ° C.

(Comparative Example 1)
In the example, the recess 9 as shown in FIGS. 1B and 1C was not provided. Otherwise, an optical waveguide chip was fabricated in the same manner as in Example 1.

  The optical characteristics of this chip were measured using a titanium sapphire laser. As a result of adjusting the oscillation output from the laser to 100 mW and condensing the basic light on the end face of the waveguide with a lens, 80 mW could be coupled to the waveguide. When the wavelength of the titanium sapphire laser was varied and adjusted to a phase matching wavelength, a maximum SHG output of 13 mW was obtained. At this time, the wavelength of the basic light was 919.9 nm. The M2 value of the outgoing SHG light was 1.25.

(Comparative Example 2)
In the example, the recess 9 as shown in FIGS. 1B and 1C was not provided. Instead, the surfaces of the extending portions 8A and 8B were roughened by an ion milling method, and the center line average surface roughness was increased to 0.2 um. Other than that, an optical waveguide chip was fabricated in the same manner as in the example.

  With respect to this chip, the oscillation output from the titanium sapphire laser was adjusted to 100 mW, and the basic light was condensed on the end face of the waveguide with a lens. As a result, 80 mW could be coupled to the waveguide. When the wavelength of the titanium sapphire laser was varied and adjusted to a phase matching wavelength, a maximum SHG output of 12 mW was obtained. At this time, the wavelength of the basic light was 919.8 nm. The M2 value of the outgoing SHG light was 1.18.

(A) is sectional drawing which shows the state which adhered the ferroelectric material 4 to the support substrate 1, (b) is a cross section which shows the optical waveguide board | substrate 5 obtained by carrying out the laser processing of the ferroelectric material 4 It is a figure and (c) is a top view which shows the planar form of the optical waveguide board | substrate 5 surface. (A) is sectional drawing which shows the state which formed the buffer layer 11 in the surface of the optical waveguide board | substrate 5, (b) is sectional drawing which shows the state which adhere | attached the coating substrate 13 to the optical waveguide board | substrate 5. . (A) is sectional drawing which shows the conventional wavelength conversion element, (b) is a top view which shows the planar shape of the ridge type | mold optical waveguide 6 and extension part 18A, 18B in the element of (a). It is a schematic diagram which shows the main beam 20 and the high frequency pattern 21 of the wavelength conversion light in the conventional wavelength conversion element. It is a schematic diagram which shows the main beam 20 of the wavelength conversion light in the wavelength conversion element of this invention example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Support substrate 2,12 Adhesive layer 3,11 Buffer layer 4 Ferroelectric material 5 Optical waveguide board | substrate 6 Ridge type | mold optical waveguide 7A, 7B Ridge groove | channel 8A, 8B, 18A, 18B Extension part 9 Concave part 20 Main of wavelength conversion light Beam 21 Light leakage of high frequency pattern A Width of ridge-type optical waveguide B Width of ridge groove C Width of recess D Scanning direction of laser beam

Claims (10)

Support substrate,
An optical waveguide substrate placed on the support substrate, made of a ferroelectric material, having a wavelength conversion function, a ridge type optical waveguide, grooves provided on both sides of the ridge type optical waveguide, and An optical waveguide substrate having an extending portion provided outside the groove;
A coated substrate provided on the optical waveguide substrate;
A first adhesive layer for bonding the support base and the bottom surface of the optical waveguide substrate; and an upper surface of the ridge-type optical waveguide and an upper surface of the extending portion of the optical waveguide substrate and the covering substrate. A wavelength conversion element comprising a second adhesive layer,
The wavelength conversion element according to claim 1, wherein the upper side surface of the extending portion is roughened by a plurality of rows of elongated concave portions formed by laser light irradiation. Characterized in that said groove and said recess extends in parallel, the wavelength conversion element according to claim 1, wherein. The wavelength conversion element according to claim 1, wherein the plurality of recesses are adjacent to each other without a gap. The wavelength conversion element according to claim 1, wherein a flat portion is provided between the adjacent concave portions. The wavelength conversion element according to claim 4, wherein a width of the flat portion is 0.1 to 2.0 μm. Support substrate,
An optical waveguide substrate placed on the support substrate, made of a ferroelectric material, having a wavelength conversion function, a ridge type optical waveguide, grooves provided on both sides of the ridge type optical waveguide, and An optical waveguide substrate having an extending portion provided outside the groove;
A coated substrate provided on the optical waveguide substrate;
A first adhesive layer for bonding the support base and the bottom surface of the optical waveguide substrate; and an upper surface of the ridge-type optical waveguide and an upper surface of the extending portion of the optical waveguide substrate and the covering substrate. A method for producing a wavelength conversion element comprising a second adhesive layer,
A method for manufacturing a wavelength conversion element, characterized in that a plurality of rows of elongated recesses are formed on the upper side surface of the extension part by laser light irradiation to roughen the surface . The method according to claim 6 , wherein the groove and the recess are simultaneously formed by scanning with the laser beam. The method according to claim 6, wherein the plurality of recesses are adjacent to each other without a gap. 8. A method according to claim 6 or 7, characterized in that a flat part is provided between the adjacent recesses. The method according to claim 9, wherein a width of the flat portion is 0.1 to 2.0 μm.

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