JP2009198651A - Wavelength converting element, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To widen the wavelength permissible width of an element which generates higher harmonics by utilizing the cyclic polarization inversion structure in a ridge type optical waveguide. <P>SOLUTION: The wavelength conversion element 1 includes a substrate 2 for wavelength conversion which consists of a ferroelectric material, and has a wavelength conversion section P for converting a basic wave A to wavelength conversion light B, the ridge type optical waveguide 4 which is formed on the surface 2a side of the substrate 2, a pair of grooves 3A, 3B, and a cyclic polarization inversion structure 9 formed in the optical waveguide 4 over the entire length of the wavelength conversion part. The width W1 of the optical waveguide 4 at the incident side end 7 of the wavelength conversion section P is greater than the width W0 of the optical waveguide 4 at the exit side end 8 of the wavelength conversion section P. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  In general optical information processing technology, in order to realize high-density optical recording, a blue light laser that stably oscillates blue light having a wavelength of about 400 to 430 nm with an output of 30 mW or more is desired, and development competition is performed. ing. As a blue light source, an optical waveguide type wavelength conversion element combining a laser that oscillates with red light as a fundamental wave and a second harmonic generation element of a quasi phase matching method is expected.

  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. In addition, by forming a waveguide in this 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 applicant has disclosed a method of forming a ridge-type optical waveguide by irradiating a base material made of an oxide single crystal with laser light (Patent Document 1).
JP-A-2002-372641

According to Patent Document 2, in a quasi-phase matching type wavelength conversion element using a periodically poled structure, a harmonic is obtained by forming a wavelength converter by forming a periodically poled structure in an optical waveguide. ing. At the same time, the periodic polarization inversion structure is not provided on the output side of the optical waveguide, and the width of the optical optical waveguide is changed on the output side. As a result, harmonics are propagated in a single mode on the exit side of the optical waveguide.
JP 2001-31974 A

  In the quasi-phase matching method using the periodically poled structure, the wavelength tolerance is narrow. That is, it is a problem that only a fundamental wave having an extremely narrow wavelength can be converted into a harmonic. The allowable wavelength width is in a very narrow range, for example, 0.1 nm. For this reason, it is difficult to cope with variations in the oscillation wavelength of the solid-state laser light source that oscillates the fundamental wave, and it is necessary to widen the allowable wavelength range of the element.

  An object of the present invention is to widen the allowable wavelength range of an element in which a harmonic is generated using a periodically poled structure in a ridge-type optical waveguide.

The present invention
A substrate made of a ferroelectric material,
Ridge type optical waveguide formed on the surface side of this substrate,
It has a pair of grooves formed on both sides of this ridge-type optical waveguide and a periodic polarization inversion structure formed in the ridge-type optical waveguide, and wavelength conversion into the ridge-type optical waveguide by this periodic polarization inversion structure A wavelength conversion element constituting a portion,
The width of the ridge type optical waveguide at the incident side end of the wavelength conversion unit is larger than the width of the ridge type optical waveguide at the output side end of the wavelength conversion unit.

The present invention also provides:
A substrate made of a ferroelectric material,
Ridge type optical waveguide formed on the surface side of this substrate,
It has a pair of grooves formed on both sides of this ridge-type optical waveguide and a periodic polarization inversion structure formed in the ridge-type optical waveguide, and wavelength conversion into the ridge-type optical waveguide by this periodic polarization inversion structure A method of manufacturing a wavelength conversion element constituting a portion,
When a groove and a ridge type optical waveguide are formed by irradiating the surface of a substrate made of a ferroelectric material with a laser beam, from the incident side end of the wavelength conversion unit toward the emission side end of the wavelength conversion unit It is characterized by slowing the scanning speed of the laser beam.

  According to the element of the present invention, the width of the ridge type optical waveguide at the incident side end of the wavelength conversion unit is made larger than the width of the ridge type optical waveguide at the output side end of the wavelength conversion unit. As a result, since the width of the ridge-type optical waveguide is relatively large at the incident side end, the effective refractive index perceived by light is also increased, and the phase matching wavelength is shortened. On the other hand, since the width of the ridge-type optical waveguide is relatively small at the emission side end, the effective refractive index perceived by light is lowered and the phase matching wavelength is lengthened. Therefore, the allowable wavelength range of the ridge optical waveguide as a whole can be increased.

  Further, according to the manufacturing method of the present invention, when the groove and the ridge type optical waveguide are formed by irradiating the surface of the material substrate made of the ferroelectric material with the laser beam, the light is emitted from the incident side end of the wavelength conversion unit. The scanning speed of the laser beam is decreased toward the side end. Accordingly, since the scanning speed of the laser light is relatively high at the incident side end, the groove becomes relatively shallow, the groove width becomes small, and the width of the ridge-type optical waveguide becomes relatively large. On the other hand, since the scanning speed of the laser beam is relatively slow at the output side end of the wavelength converter, the groove becomes relatively deep, the groove width becomes large, and the width of the ridge-type optical waveguide is relatively Get smaller.

  FIG. 1 is a cross-sectional view schematically showing a light conversion element 1 according to an embodiment of the present invention, and FIG. 2 is a top view schematically showing the element 1.

  The wavelength conversion substrate 2 made of a ferroelectric material includes a ridge-type optical waveguide 4, groove forming portions 2 a and 2 b provided on both sides of the optical waveguide 4, and an extension provided outside each groove forming portion. Part 2c, 2d is provided. Grooves 3A and 3B are formed on both sides of the ridge type optical waveguide 4, respectively. The ridge-type optical waveguide 4 includes a ridge-type protrusion 4a and a flat plate portion 4b immediately below the ridge-type protrusion 4a. A front surface side buffer layer 22 is formed on the upper surface of the substrate 2, and a rear surface side buffer layer 5 is formed on the back surface side. The substrate 2 is bonded to the support substrate 20 via the buffer layer 5 and the adhesive layer 6.

  As shown in the top view of FIG. 2, the substrate 2 has a large number of domain-inverted parts 9a, and a non-polarized-inverted part 9b is formed between adjacent domain-inverted parts. These polarization inversion portions and non-polarization inversion portions are formed with a constant period, and constitute a periodic polarization inversion structure 9.

  The ridge type optical waveguide 4 is exposed to the incident side end face 17 and the emission side end face 18 of the substrate 2, and the periodically poled structure 9 is formed over the entire length of the ridge type optical waveguide 4. In the ridge-type optical waveguide, the region where the periodically poled structure 9 is formed is the wavelength conversion part P. When the fundamental wave is incident as indicated by the arrow A, the fundamental wave undergoes wavelength conversion by the wavelength conversion unit P and is emitted as indicated by the arrow B.

  In this example, since the periodically poled structure 9 is formed over almost the entire length of the ridge-type optical waveguide 4, the optical waveguide 4 constitutes the wavelength converting portion P over the entire length. However, the periodically poled structure 9 can be formed only in a part of the ridge type optical waveguide 4. In this case, only the region of the optical waveguide 4 where the periodically poled structure 9 is formed becomes the wavelength conversion unit.

  FIG. 3A is a diagram showing the form of the ridge-type optical waveguide 4 at the incident side end 7 of the wavelength conversion section P, and FIG. 3B is an optical waveguide at the output end 8 of the wavelength conversion section P. It is a figure which shows the form of 4.

  In this example, as shown in FIG. 3A, the width of the optical waveguide 4 at the incident side end 7 of the wavelength conversion section P is WI, and the depth of the grooves 3A and 3B is DI. In addition, the width of the optical waveguide 4 at the emission side end 8 of the wavelength converting portion P is WO, and the depths of the grooves 3A and 3B are DO. Moreover, the dotted line S shown in FIG. 2 is a pair of parallel lines, and the distance between the pair of dotted lines S is constant. WI is larger than S and WO is smaller than S.

  The width of the ridge type optical waveguide is a width when a flat portion of the surface of the ridge type optical waveguide is viewed in the light traveling direction. The depth of the groove is the depth from the flat portion of the surface of the ridge type optical waveguide to the bottom surface of the groove. The width of the ridge type optical waveguide and the depth of the groove are measured by an AFM (Atomic Force Microscope).

  Here, according to the present invention, the width WI of the optical waveguide 4 at the incident-side end 7 of the wavelength converting portion P is larger than the width WO of the optical waveguide 4 at the emitting-side end 8 of the wavelength converting portion P. Further, in this example, the depth DI of the grooves 3A, 3B at the incident side end 7 of the wavelength conversion part P is smaller than the depth DO of the grooves 3A, 3B at the emission side end face 8 of the wavelength conversion part P.

  As a result, since the width WI of the optical waveguide 4 is relatively large at the incident end 7, the effective refractive index perceived by light is increased and the phase matching wavelength is shortened. On the other hand, since the width WO of the optical waveguide 4 is relatively small at the emission side end portion 8, the effective refractive index perceived by light is also lowered, and the phase matching wavelength is lengthened. Accordingly, the allowable wavelength range of the entire optical waveguide 4 can be increased.

  From this viewpoint, the ratio of the width WI of the optical waveguide 4 to the WO (WI / WO) is preferably 1.05 or more, and more preferably 1.1 or more. However, if the ratio (WI / WO) of the width WI of the optical waveguide 4 to the WO becomes too large, the overall wavelength conversion efficiency tends to decrease. For this reason, from the viewpoint of keeping the wavelength conversion efficiency high, the ratio of the width WI of the optical waveguide 4 to the WO (WI / WO) is preferably 2.0 or less, and preferably 1.5 or less. Further preferred.

  Further, in this example, the depth DI of the grooves 3A, 3B at the incident side end 7 of the wavelength conversion part P is smaller than the depth DO of the grooves 3A, 3B at the emission side end face 8 of the wavelength conversion part P. This also contributes to the expansion of the allowable wavelength range based on the principle described above. From this viewpoint, the ratio of DI to DO (DI / DO) is preferably 0.95 or less, and more preferably 0.8 or less. However, if the ratio of DI to DO (DI / DO) becomes too small, the overall wavelength conversion efficiency tends to decrease. For this reason, from the viewpoint of maintaining high wavelength conversion efficiency, the ratio of DI to DO (DI / DO) is preferably 0.3 or more, and more preferably 0.5 or more.

  In a preferred embodiment, the width of the ridge type optical waveguide monotonously decreases from the incident side end to the output side end. As a result, the allowable wavelength range can be greatly increased, and a decrease in wavelength conversion efficiency can be easily suppressed. At this time, the width of the ridge optical waveguide preferably decreases monotonically in a linear function over the entire length of the wavelength conversion unit, but is not limited thereto.

The widths WI and WO of the optical waveguide 4 are not particularly limited, but can generally be 3.0 to 7.0 μm.
The depths DI and DO of the grooves are not particularly limited, but can generally be 0.5 to 3.0 μm.

  The material constituting the light conversion 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 quartz Etc. can be illustrated.

  In the ferroelectric single crystal, at least one selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) is used 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 and the back side buffer layer include silicon oxide, magnesium fluoride, silicon nitride, alumina, and tantalum pentoxide.

  The material of the bonding layer may be an inorganic adhesive, an organic adhesive, or a combination of an inorganic adhesive and an organic adhesive.

  The specific material of the support base 20 is 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 thermal expansion difference, it is preferable that the ferroelectric layer and the support substrate are made of the same material, and lithium niobate single crystal is particularly preferable.

The processing method for forming the ridge-type optical waveguide in the ferroelectric material is not limited, but laser processing is preferable. In this case, the groove and the ridge type optical waveguide are formed by irradiating the surface of the material substrate made of a ferroelectric material with laser light. At this time, the scanning speed of the laser light is decreased from the incident side end of the wavelength conversion unit toward the emission side end surface of the wavelength conversion unit.
When the scanning speed is slow, the groove becomes deeper and the width of the ridge-type optical waveguide becomes wider. When the scanning speed is high, the groove becomes shallow and the width of the ridge type optical waveguide becomes narrow.

  The laser beam for processing is not particularly limited, and is selected according to the material to be processed. In a preferred embodiment, the half width of the laser light pulse 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, and ultraviolet light emitted from 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, as a method of forming a groove | channel by irradiation of a laser beam, the following two aspects can be mentioned.
(1) Spot scan processing (2) Slit scan processing

  The scanning speed of the laser beam can be changed depending on the moving speed of the processing stage.

  The scanning speed of the laser beam is selected according to the material to be processed, and can be, for example, 0.005 mm / second to 1.0 mm / second.

Example 1
The wavelength conversion element 1 shown in FIGS. 1 to 3 was produced. Specifically, a comb-like periodic electrode having a period of 5.06 μm was formed on a 0.5 mm-thick MgO 5% doped lithium niobate 5-degree offcut Y substrate by a photolithography method. After an electrode film was formed on the entire back surface of the substrate, a pulse voltage was applied to form a periodically poled structure 9. Next, a 0.4 μm thick SiO ₂ underclad 5 was formed by sputtering.

  After applying an adhesive to the non-doped lithium niobate substrate 20 having a thickness of 0.5 mm, it is bonded to the MgO-doped lithium niobate substrate, and the surface of the MgO-doped lithium niobate substrate is ground to a thickness of 3.5 μm. Polished.

  Next, a ridge type optical waveguide 4 was formed on the surface side of the substrate 2 by laser ablation processing. A mask was produced so that the beam shape on the surface of the substrate 2 during laser processing was as shown in FIG. The length E of the beam is 150 μm, and the beam interval F is 6 μm. This beam pattern was scanned on the surface of the substrate 2 to produce a ridge type optical waveguide 4 having an element length of 9 mm. At this time, the scanning speed was 0.25 mm / sec at the incident end and continuously decreased, and 0.15 μm / at the emission end.

After forming the ridge type optical waveguide 4 and the grooves 3A and 3B in this manner, a SiO ₂ overclad 22 having a thickness of 0.5 μm was formed by sputtering. The device was cut with a dicer with a length (L) of 9 mm and a width of 1.0 mm, the end surface was polished, and an antireflection film was applied to the end surface.

  The width WI of the ridge-type optical waveguide 4 at the incident side end 7 is 5 μm, and the depth DI of the grooves 3A and 3B is 1.5 μm.

  In this optical waveguide, the optical characteristics 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 optical waveguide. When the wavelength of the titanium sapphire laser was varied and adjusted to a phase matching wavelength, a maximum SHG output of 7 mW was obtained. The wavelength of the fundamental light at that time was 976.2 nm. In addition, the phase matching wavelength allowable width at that time was 0.25 nm in terms of the full width at half maximum.

(Comparative Example 1)
In the same manner as in Example 1, the wavelength conversion element 1 shown in FIGS. However, the beam pattern shown in FIG. 4 was scanned on the substrate surface at 0.2 mm / second to form optical waveguides and grooves. The scanning speed of the laser beam was not changed.

  In this optical waveguide, the optical characteristics 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, 70 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 14 mW was obtained. At this time, the wavelength of the fundamental light was 976.3 nm. In addition, the phase matching wavelength allowable width at that time was 0.12 nm in terms of the full width at half maximum.

  Note that the width WO of the ridge-type optical waveguide 4 at the exit-side end 8 was 4 μm, and the depth DO of the grooves 3A and 3B was 2.0 μm.

1 is a cross-sectional view schematically showing a wavelength conversion element 1 according to an embodiment of the present invention. 3 is a top view schematically showing an element 1. FIG. (A) is a figure which shows the form of the optical waveguide 4 and groove | channel 3A, 3B in the incident side edge part 7 of the wavelength conversion part P, (b) is an optical waveguide in the output side edge part 8 of the wavelength conversion part P 4 is a diagram showing the form of 4 and grooves 3A, 3B. It is a top view which shows the beam pattern in an Example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Wavelength conversion element 2 Wavelength conversion board | substrate 2a, 2b Groove formation part 2c, 2d Extension part 3A, 3B Groove 4 Ridge type | mold optical waveguide 4a Ridge part 5,22 Buffer layer 6 Bonding layer 7 Incident side edge of wavelength conversion part P Portion 8 Emission side end of wavelength conversion unit 9 Periodic polarization inversion structure 9a Polarization inversion unit 9b Non-polarization inversion unit A Fundamental wave B Wavelength converted light P Wavelength conversion unit WI Ridge type optical waveguide at the end of incident side of wavelength conversion unit Width WO width of the ridge-type optical waveguide at the output side end of the wavelength conversion unit DI depth of the groove at the input side end of the wavelength conversion unit DO depth of the groove at the output side end of the wavelength conversion unit

Claims (4)

A substrate made of a ferroelectric material,
Ridge type optical waveguide formed on this substrate,
A pair of grooves formed on both sides of the ridge-type optical waveguide and a periodic polarization inversion structure formed in the ridge-type optical waveguide are provided. A wavelength conversion element constituting a wavelength conversion unit,
The wavelength conversion element, wherein a width of the ridge-type optical waveguide at an incident side end of the wavelength conversion unit is larger than a width of the ridge-type optical waveguide at an output side end of the wavelength conversion unit.   The wavelength conversion element according to claim 1, wherein a depth of the groove at an incident side end of the wavelength conversion unit is smaller than a depth of the groove at an output side end of the wavelength conversion unit.   3. The wavelength conversion element according to claim 1, wherein the width of the ridge-type optical waveguide monotonously decreases from the incident side end portion toward the emission side end portion. A substrate made of a ferroelectric material,
Ridge type optical waveguide formed on the surface side of this substrate,
A pair of grooves formed on both sides of the ridge-type optical waveguide and a periodic polarization inversion structure formed in the ridge-type optical waveguide are provided. A method of manufacturing a wavelength conversion element constituting a wavelength conversion unit,
When forming the groove and the ridge-type optical waveguide by irradiating the surface of the substrate made of the ferroelectric material with a laser beam, from the incident side end of the wavelength conversion unit to the emission side end of the wavelength conversion unit A method of manufacturing a wavelength conversion element, wherein the scanning speed of the laser beam is decreased toward the portion.

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