JP2001311974A - Wavelength conversion element, manufacturing method therefor, and wavelength conversion module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical wavelength conversion element which permits to stably obtain a Gaussian beam form wavelength-converted wave, and has high wavelength conversion efficiency. SOLUTION: In the optical wavelength conversion element 10 wherein a cyclic domain inversion structure 14 and a channel optical waveguide 16 extending along the cyclic domain inversion structure 14 on a substrate 12 made of a non-linear optical crystal, the incident light side of the channel optical waveguide 16 is used as a 1st area 20 for converting the wavelength into a 2nd higher harmonic wave 22 by pseudo phase matching while propagating an incident fundamental wave 18 in a single mode, and the outgoing light side is used as a 2nd area 24 for propagating a 2nd higher harmonic wave 22 propagated from the 1st area 20 in a single mode. In such a manner, it is possible to stably obtain the 0-order mode 2nd higher harmonic wave 22 of a Gaussian beam form with high wavelength conversion efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a wavelength conversion element, a method for manufacturing a wavelength conversion element, and a wavelength conversion module.

[0002]

2. Description of the Related Art
2. Description of the Related Art A waveguide-type optical wavelength conversion element having an optical waveguide made of a nonlinear optical material and converting the wavelength of a fundamental wave propagating through the optical waveguide into a second harmonic or the like is known. In this type of waveguide-type optical wavelength conversion element, it is widely practiced to form a periodic domain inversion structure in an optical waveguide to achieve so-called quasi-phase matching. In the case where a wavelength-converted wave such as a second harmonic emitted from the waveguide-type optical wavelength conversion element is applied as recording light of an optical scanning recording device such as a laser printer, the recording light is focused to a smaller spot. Therefore, it is desirable that the wavelength-converted wave be a Gaussian beam, that is, a beam in which the light intensity distribution in the beam cross section has a Gaussian distribution. If the recording light is not a Gaussian beam, the recording density tends to be uneven. Further, when a color image is recorded with recording light of three colors of red, green and blue, if only a certain recording light is not a Gaussian beam, a color shift occurs.

In order to convert a wavelength-converted wave into a Gaussian beam as described above, a waveguide-type optical wavelength conversion element is phase-matched between a wavelength-converted wave propagating in the zero-order mode and a fundamental wave in the optical waveguide. It may be configured as follows.

On the other hand, according to a study by the present inventor, it has been found that the second harmonic propagated in the first-order mode has a larger overlap integral with the fundamental wave than the zero-order mode.
Therefore, from the viewpoint of wavelength conversion efficiency, it is desired to configure the waveguide type optical wavelength conversion element such that the second harmonic propagating in the first mode and the fundamental wave are phase-matched. However, the second harmonic, which propagates in the optical uniform wave path in the first mode and is emitted outside the optical waveguide, has an intensity distribution in which two lobes are arranged in the vertical direction (the thickness direction of the optical waveguide substrate). Does not become a Gaussian beam.

Japanese Patent Application Laid-Open No. Hei 8-54658 discloses that the phase difference between the two lobes is made by passing a second harmonic, which propagates in an optical waveguide in a first-order mode and then exits the optical waveguide, through a phase compensator. To convert the second harmonic into a Gaussian beam. However, in such a case, the phase difference between the two beams is not stable because the edge of the beam is unstable at the end of the beam, or there is a variation in the fabrication of the optical waveguide, so that the conditions are fixed. It is difficult to obtain a stable Gaussian beam if a certain phase compensator is used.

Japanese Patent Application Laid-Open No. 11-72810 discloses that a high refractive index cladding layer is provided on an ordinary optical waveguide without providing an external optical element, and a second harmonic and a fundamental wave propagating in the 0th mode. There is described an optical waveguide which increases the overlap with the optical waveguide and obtains the second harmonic with high conversion efficiency.
However, this optical waveguide has a problem that the emitted second harmonic is multimode in the lateral direction, and a Gaussian beam cannot be obtained stably. In this optical waveguide, the multi-mode in the horizontal direction is caused by the wavelength dispersion of the second harmonic and the fundamental wave propagating in the 0th mode (the structural dispersion of the waveguide and the dispersion of the material). This is because it is not possible to obtain an optical waveguide that becomes a single mode.

The second harmonic of the zero-order mode and the fundamental wave usually have a large overlap in the lateral direction, and the wavelength conversion wave propagating in the zero-order mode and the fundamental wave in the optical waveguide are phase-matched. If constructed, a Gaussian beam can be obtained,
The lateral beam may be in a higher mode with multiple peaks. This is presumed to be due to mode conversion due to defects or the like in the optical waveguide.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical wavelength conversion element which can obtain a Gaussian beam-shaped wavelength-converted wave stably and has high wavelength conversion efficiency. It is in.

[0009]

According to a first aspect of the present invention, there is provided a wavelength conversion element for performing wavelength conversion by quasi-phase matching while propagating an incident fundamental wave in a single mode. And a second region for transmitting the wavelength-converted wave propagated from the first region in a single mode and emitting the same.
And a region including:

In the optical wavelength conversion element according to the present invention, in the first region, the incident fundamental wave is wavelength-converted into the second harmonic by quasi-phase matching while propagating in a single mode. Since the second harmonic, which is a wavelength-converted wave propagated from the first region, is propagated in a single mode to emit the wavelength-converted wave, even if there is a defect or the like in the optical waveguide, a high level is obtained. A Gaussian beam-shaped wavelength-converted wave can be stably obtained with the wavelength conversion efficiency.

According to a second aspect of the present invention, in the wavelength conversion element according to the first aspect, the difference in refractive index between the second region and the surroundings is larger than the difference in refractive index between the first region and the surroundings. Is also small. The “difference in refractive index from the surroundings” refers to each region and a region adjacent to each region (however, except for the second region in the case of the first region, the first region in the case of the second region) )).

According to a third aspect of the present invention, in the wavelength conversion element according to the first or second aspect, the first region and the second region are formed by a linear optical waveguide formed on a nonlinear crystal substrate. It is characterized by comprising.

According to a fourth aspect of the present invention, in the wavelength conversion device according to any one of the first to third aspects,
The length of the region along the light propagation direction is 0.5 mm or more. By setting the length of the second region along the light propagation direction to 0.5 mm or more, offset of stray light can be prevented.

According to a fifth aspect of the present invention, in the wavelength conversion element according to any one of the first to fourth aspects, the nonlinear crystal is LiNb _x Ta _{1 -x} O ₃ (0 ≦ x ≦ 1). There is a feature.

According to a sixth aspect of the present invention, there is provided the wavelength conversion element according to any one of the first to fifth aspects, wherein the nonlinear crystal is doped with MgO, ZnO, or Sc.

According to a seventh aspect of the present invention, there is provided the wavelength conversion element according to any one of the first to sixth aspects, wherein the exit side end face is 3 ° in the light propagation direction with respect to a plane perpendicular to the optical axis direction. ~ 7
° characterized by inclination. By polishing the emission-side end face obliquely in this way, it is possible to prevent the fundamental wave from re-entering the wavelength conversion element.

According to an eighth aspect of the present invention, in the wavelength conversion device according to any one of the first to seventh aspects, an anti-reflection film having a reflectance of 0.2% or more with respect to a fundamental wave is provided on the incident side end face. And at least one of the emission-side end faces.

According to a ninth aspect of the present invention, there is provided a wavelength conversion element manufacturing method for manufacturing the wavelength conversion element according to the first aspect, wherein the first area of the nonlinear crystal substrate is provided in the first region. Forming a first proton exchange region having a predetermined depth in a corresponding region, forming a second proton exchange region shallower than the first region in a region corresponding to the second region, After the proton exchange region is annealed, a domain-inverted structure is formed to form a first region in which the incident fundamental wave can be propagated in a single mode and the fundamental wave can be converted into a wavelength-converted wave by quasi-phase matching. And annealing the second proton exchange region to form a second region capable of transmitting the wavelength-converted wave propagated from the first region in a single mode, thereby manufacturing a wavelength conversion element. And In the method for manufacturing a wavelength conversion element of the present invention,
A first region in which the incident fundamental wave is wavelength-converted by quasi-phase matching while propagating in a single mode, and a second region in which the wavelength-converted wave propagated from the first region propagates in a single mode and is emitted. Can be formed by twice proton exchange and annealing, and the wavelength conversion element of the present invention can be easily manufactured.

A wavelength conversion module according to a tenth aspect is characterized in that the wavelength conversion element according to any one of the first to eighth aspects is directly coupled to a semiconductor laser that emits a fundamental wave. Since the optical wavelength conversion module of the present invention uses the above-described optical wavelength conversion element of the present invention, the Gaussian beam shape can be stably obtained with high wavelength conversion efficiency without providing an external optical element such as a phase compensator. Can be obtained. In addition, since there is no need to provide an external optical element, the reliability of the module is improved,
Cost can be reduced.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings. (First Embodiment) First, an embodiment of the optical wavelength conversion element of the present invention will be described in detail. As shown in FIGS. 1A and 1B, the optical wavelength conversion element 1 of the present embodiment
0 is the z on the substrate 12 made of a nonlinear optical crystal.
Periodic domain reversal structure 14 in which domain reversals in which the direction of spontaneous polarization parallel to the axis is reversed are periodically formed.
And a channel optical waveguide 16 extending along the periodic domain inversion structure 14.

The nonlinear optical crystal constituting the substrate 12 is, for example, a nonlinear optical crystal such as LiNb _x Ta _{1 -x} O ₃ (0 ≦ x ≦ 1), and MgO, Z
Crystals doped with nO or Sc can be used.

The periodic domain inversion structure 14 is formed at a predetermined period Λ such that domain inversion portions are arranged in the y-axis direction of the substrate 12. The period Λ is determined so as to have a first-order period with respect to the wavelength near the fundamental wave in consideration of the wavelength dispersion of the refractive index of the nonlinear optical crystal. Such a periodic domain inversion structure 14 is disclosed in, for example,
It can be formed by the method described in the above item.

In the channel optical waveguide 16, the light incident side on which the fundamental wave 18 is incident is defined as a first region 20 having a refractive index difference Δn ₁ from the surroundings, and the second harmonic 22 as a wavelength-converted wave is emitted. The outgoing side is the second region 24 where the difference in refractive index from the surroundings is Δn ₂ . The “difference in refractive index from the surroundings” of the first region 20 refers to the difference in refractive index between the first region 20 and the air layer and the substrate 12, and the “difference in refractive index from the surroundings” of the second region 24. The “refractive index difference” is a refractive index difference between the second region 24 and the air layer and the substrate 12.

Here, the refractive index difference Δn _{2 from} the periphery of the second region 24 is smaller than the refractive index difference Δn ₁ of the first region 20 and the refractive index difference Δn from the periphery of the first region 20. ₁ is a value capable of propagating the incident fundamental wave 18 in a single mode, and the refractive index difference Δn ₂ around the second region 24 is a single-mode value of the second harmonic 22 propagated from the first region 20. It is a value that can be propagated in the mode. Also, the second area 24
The length L along the light propagation direction needs a certain length in order to reduce stray light, and is preferably 0.5 mm or more, more preferably 0.75 mm or more.

Both end faces of the light wavelength conversion element 10 are optically polished. In particular, the emission side end face 26 of the optical wavelength conversion element 10 is polished obliquely so that the channel optical waveguide 1 extends in a direction perpendicular to the direction in which the channel optical waveguide 16 extends.
An inclined surface is formed that is inclined at an angle θ (3 ° ≦ θ) or more in the direction in which 6 extends. Since the emission side end face 26 is polished obliquely in this manner, a fundamental wave is generated by the channel optical waveguide 1.
6 can be prevented from re-entering. Then, an AR (anti-reflection) coat 28 having a reflectance of 0.2% or more for the fundamental wave is provided on the emission end face 26 of the optical wavelength conversion element 10.
Is provided. Further, the incident side end face 30 is also provided with an AR coat 32 having a reflectance of 0.2% or more for the fundamental wave.

Next, a method for manufacturing the optical wavelength conversion element 10 will be described. First, as shown in FIGS. 2A and 2B, photolithography, metal (for example, Cr)
Using an evaporation and a lift-off, an optical crystal substrate (for example, 87 ° Z-cut MgO 5 mol% doped LiNbO
_{On the (3} substrate) 12, a domain inversion electrode 34 having a predetermined structure is formed. The detailed structure of the polarization inversion electrode 34 is described in Japanese Patent Application Laid-Open No. 9-218431.

Next, as shown in FIGS. 3A and 3B, the substrate 1 on which the domain inversion electrodes 34 are formed by photolithography, sputtering film formation, and lift-off.
Metal mask 3 having a linear opening 38 of a predetermined width on
6 is formed. The metal mask 36 has, for example, a three-layer structure in which a Ta layer 36A, an Au layer 36B, and a Ta layer 36C are sequentially stacked from the lower layer. Also, the opening 38
Is constant in the range of 3 to 9 μm.

Next, as shown in FIGS. 4A and 4B, a first proton exchange treatment is performed using the metal mask 36 to form a proton exchange portion 39A. The conditions for the first proton exchange treatment are 15 to 60 minutes in a temperature range of 140 to 160 ° C. in pyrophosphoric acid.

Next, as shown in FIGS. 5A and 5B, photolithography, sputtering film formation, and lift-off are used to remove the T near the exit portion of the proton exchange portion 39A.
A metal mask 40 is formed in a predetermined range on the a layer 36C.
As shown in FIGS. 6A and 6B, this metal mask 4
By performing the second proton exchange treatment using 0, the proton exchange section 39B is formed by performing proton exchange on the light incident side of the proton exchange section 39A to a deeper portion.
The conditions for the second proton exchange were as follows:
It is 60 to 200 minutes in a temperature range of 0 to 160 ° C.

Next, as shown in FIGS. 7A and 7B, the Ta layer 36C is removed by etching to remove the Au layer 3C.
After exposing 6B, an annealing process is performed in the air, and the proton exchange units 39A and 39B are connected to the channel optical waveguide 1.
6 is assumed. The annealing is performed at a temperature in the range of 300 to 400 ° C. for 60 minutes. Thus, the first region 20 having a refractive index difference Δn _{1 from} the surroundings is formed on the light incident side where the deep proton exchange portion 39B is formed, and on the light emission side where the shallow proton exchange portion 39A is formed. , A second region 24 having a refractive index difference Δn ₂ (Δn ₁ > Δn ₂ ) from the surroundings is formed. The channel optical waveguide 1 is annealed by annealing.
6, the first region 20 and the second region 24 have substantially the same thickness.

The conditions for the two proton exchange treatments and the annealing treatment are such that the refractive index difference Δn _{1 from} the periphery of the first region 20 allows the incident fundamental wave 18 to propagate in a single mode. And the refractive index difference Δn _{2 from} the periphery of the second region 24 is the second refractive index difference Δn ₂ transmitted from the first region 20.
The harmonic 22 is appropriately selected so as to have a value that allows the harmonic 22 to propagate in a single mode. Further, it is preferable to select the conditions of the proton exchange treatment and the annealing treatment so that the conversion efficiency to the second harmonic and the coupling efficiency with the semiconductor laser are maximized.

Next, as shown in FIGS. 8A and 8B, the remaining metal mask is entirely removed by etching to expose the polarization inversion electrode 34, and a high voltage is applied to the polarization inversion electrode 34. Apply. For example, if the gap between the electrodes is 400μ
At the time of m, a voltage of 2 to 4 kV is applied for 1 to 10 seconds. As a result, the periodic domain inversion structure 14 is formed on the substrate 12.

Next, both end faces of the substrate 12 including the end faces of the optical waveguide are optically polished. In order to prevent the fundamental wave from re-entering, the emission side end face 26 is polished (hereinafter, referred to as an inclined face) inclined at a predetermined angle in the light propagation direction with respect to a plane perpendicular to the optical axis direction. , "Diagonal cut").
In addition, the inclination angle can be set to 3 to 7 °. Finally, AR coats 28 and 32 made of a single layer of SiO ₂ are respectively formed on the incident side end face 30 and the exit side end face 26 so that the reflectance with respect to the fundamental wave is 0.2% or less. The optical wavelength conversion element 10 shown is completed.

A semiconductor laser is directly coupled to the incident side end face 30 of the channel optical waveguide 16 of the above-described optical wavelength conversion element 10 and, for example, 950 nm
When the laser beam of m is incident as the fundamental wave 18, the fundamental wave 18 propagates in the first region 20 in the 0th mode, and
(So-called quasi-phase matching) in the periodic domain inversion structure 14 in the region 20 of FIG.
It is converted to a second harmonic 22 of 5 nm. The period Λ of the periodic domain inversion structure 14 is set such that quasi-phase matching is obtained between the fundamental wave 18 and the second harmonic 22 propagating in the zero-order mode. Specifically, when the effective refractive index of the channel optical waveguide 16 for the fundamental wave 18 and the effective refractive index for the second harmonic 22 are nω and n ₂ ω, respectively, and the wavelength of the fundamental wave is λF, the following equation n ₂ The period Λ may be determined so as to satisfy ω−nω = λF / 2Λ.

The second harmonic wave 22 propagated from the first region 20 to the second region 24 propagates in the second region in the 0th-order mode, and emits blue light having a wavelength of about 475 nm of 30 mW or more, for example. The light exits from the side end surface 26 in a divergent light state. The beam shape of the second harmonic 22 emitted at this time in the direction perpendicular to the substrate in the NFP (near field pattern) is shown in FIG.
(A) is shown by a solid line, and the beam shape in the direction parallel to the substrate is shown by a broken line. FIG. 9B shows a beam shape in the NFP of the second harmonic emitted after propagating in the normal mode in the primary channel optical waveguide in the first mode. Similarly to FIG. 9A, a beam shape in a direction perpendicular to the substrate is indicated by a solid line, and a beam shape in a direction horizontal to the substrate is indicated by a broken line. FIG. 9 (A)
As can be seen from a comparison between (B) and (B), the NFP of the second harmonic emitted after propagating in the first-order mode through the channel optical waveguide has a side lobe, whereas the second region has a zero-order NFP. The second harmonic NFP of the present embodiment emitted after propagating in mode showed a Gaussian distribution without side lobes.

Further, the fundamental wave 18 (0
The conversion efficiency from the second harmonic to the second harmonic 22 (the zero-order mode) depends on the conversion efficiency from the fundamental wave 18 (the zero-order mode) to the second harmonic 22.
(1st mode) is 10 to 20% of the conversion efficiency. For example, in the case of conversion to the second harmonic 22 (first-order mode), the conversion rate is 300% / Wcm ² , whereas in the case of conversion to the second harmonic 22 (0-order mode), it is 30 to 60% / Wcm ^2. W
cm ² . On the other hand, from the first region 20 to the second region 2
4, the second harmonic 22 is shifted from the first mode to 0
10-70% because mode conversion to next mode is required
The mode conversion loss in the range of? That is, in the first region 20, the fundamental wave 18 (0th mode) is converted to the second harmonic 22 (first mode), and in the second region 24, the second harmonic 22 is converted from the first mode to 0%. In the case of mode conversion to the next mode, the fundamental wave 18 (0th mode) incident on the first region 20 is converted into the second harmonic of the 0th mode with a conversion efficiency of 30 to 90%, and the second region 24 From the surface. Accordingly, a conversion efficiency 1.5 to 9 times as a whole can be obtained as compared with a case where the fundamental wave 18 (0th mode) is directly converted into the second harmonic 22 (0th mode) and emitted.

The mode conversion loss can be reduced to about 10% by matching the beam shape of the light propagating in the first area 20 with the beam shape of the light propagating in the second area 24. .

As described above, in the optical wavelength conversion device of the present embodiment, the channel optical waveguide is divided into the first region and the second region, and the incident fundamental wave is converted into the single mode in the first region. Quasi phase matching while propagating in
The wavelength is converted into a harmonic, and in the second region, the second harmonic, which is the wavelength-converted wave propagated from the first region, is propagated in a single mode to extract the second harmonic in the zero-order mode. Therefore, even when there is a defect or the like in the optical waveguide, a Gaussian beam can be obtained with high conversion efficiency.

In the above embodiment, the width of the channel optical waveguide is the same in the first region and the second region, but may be different. For example, FIG.
As shown in FIG. 10A, the width of the second region 24 is made smaller than the width of the first region 20. As shown in FIG. 10B, the width of the second region 24 is reduced to the first region. As shown in FIG. 10C, the second region 24 is wider than the width of the second region 24.
Is narrowed in a tapered shape toward the emission side. However, if the widths are different, accurate alignment is required at the time of mask formation, and the manufacturing process becomes complicated. Therefore, it is preferable that the width of the channel optical waveguide be the same in the first region and the second region. . (Second Embodiment) Next, an embodiment of an optical wavelength conversion module using the above wavelength conversion element will be described.
As shown in FIG. 11, the optical wavelength conversion module includes a semiconductor laser 110 having a rear emission end face and a front emission end face opposed to this end face, and a reflection device that forms an external resonator together with the front emission end face of the semiconductor laser 110. The optical wavelength conversion device 10 includes a mirror 112 as a member and the wavelength conversion of the fundamental wave emitted from the semiconductor laser 110 to output a second harmonic.

The semiconductor laser (LD) 110 is held by a mount 116 for the semiconductor laser, and the optical wavelength conversion element 10 constituted by a second harmonic generation element (SHG element) is held by a mount 118 for the optical wavelength conversion element. Have been. The semiconductor laser 110 and the optical wavelength conversion element 10 are aligned so that the emission part of the semiconductor laser 110 and the waveguide part (incident part) of the optical wavelength conversion element 10 coincide with each other while being held by the mount. LD-SHG unit 120
Is configured. Thus, the optical wavelength conversion element 10 is directly coupled to the front emission end face of the semiconductor laser 110. The LD-SHG unit 120 is fixed on a substrate 122 as shown in FIG.

The semiconductor laser 110 is a Fabry-Perot type (FP type) unimodal spatial mode (transverse single mode).
The semiconductor laser 110 has an LR (low reflectivity) coat for both ends (cleavage plane) of the semiconductor laser 110 with respect to light having an oscillation wavelength.

As shown in FIG. 11, the LD-SHG unit 120 includes a laser beam (backward emitting light) 13 emitted in a diverging light state from the rearward emitting end face of the semiconductor laser 110.
A collimator lens 136 for converting 4R into parallel light is attached. The LD-SHG unit 120 and the collimator lens 136 are packaged as a hermetic sealing member.
The inside is hermetically sealed together with an inert gas such as dry nitrogen or dry air, and is fixed in the package 138. Note that, as the collimator lens 136, any of a gradient index rod lens, an aspherical lens, and a spherical lens such as Selfoc (trade name) can be used.

The package 138 includes the semiconductor laser 11
A window hole 41A through which the backward emission light 134R from 0 is transmitted and a window hole 41B through which the forward emission light 62 from the optical wavelength conversion element 10 are formed. The window hole 41A and the window hole 41B are transparent, respectively. The window plate 42A and the window plate 42B are attached so as to maintain an airtight state. Package 13
8, a wire extraction portion 44 in which a low melting point glass or the like is fitted in a wire extraction hole in an airtight state is formed.
Two wires 46A, 46B connected to both electrodes of the ten
Are pulled out through the wire outlet 44. The package 138 includes the LD-SHG unit 120.
And the collimator lens 136 are hermetically sealed and fixed on the substrate 48 together with the mirror 112.

The mirror 112 is provided with an AR coat 50 on the surface on the laser beam incident side, and an HR coat 52 on a surface opposite to the incident side surface. By using a mirror in which the surface on the laser beam incident side is coated with an AR coating and the surface on the side opposite to the incident side is coated with an HR coating, it is possible to prevent the reflectivity of the mirror from being lowered due to the adhesion of dust. be able to.

The window plate 42A of the package 138 and the mirror 1
12, a narrow band-pass filter 56 as a wavelength selection element rotatably held by a holder 54.
A pair of total reflection prisms 58A and 58B for bending the optical path of the laser beam 134R by approximately 180 °;
The condenser lens 60 for converging on the surface of the R coat 52 is arranged in this order, and is fixed on the substrate 48. The reflectance of the mirror 112 for the fundamental wave of the HR coat 52 is 9
It is preferably set to 5%.

The resonator length of the external resonator constituted by the mirror 112 and the front emission end face of the semiconductor laser 110 (ie, the optical length from the front emission end face of the semiconductor laser 110 to the surface of the HR coat 52 of the mirror 112) is The semiconductor laser 110 and the mirror 112 so that they are longer than the coherent length of the fundamental wave emitted from the semiconductor laser.
And are arranged. The coherent length L of the fundamental wave is a coherence length unique to the laser beam, and can be calculated according to the following equation, where λ is the wavelength of the laser beam and Δλ is the spectrum width. Coherent length L of fundamental wave
Is generally about 100 mm, so that the resonator length of the external resonator can be set to, for example, a length exceeding 100 mm.

L = λ ² / 2πnΔλ Further, outside the window plate 42B of the package 138, the second harmonic 62 (including the fundamental wave 134) emitted from the front emission end face of the optical wavelength conversion element 10 is collimated. Collimator lens 64, the second harmonic 62 converted into parallel light (the fundamental wave 1
34, an infrared cut component 66, a half mirror 68, and a photodiode 7
0 is arranged and fixed on the substrate 48. As the collimator lens 64, an aspheric lens having less aberration is preferable.

As shown in FIG. 12, the substrate 48 is
It is fixed to 2. A Peltier element 74 is inserted between the substrate 48 and the mounting table 72, and each optical element fixed to the substrate 48 is adjusted to a predetermined temperature by the Peltier element 74. Each of the optical elements fixed to the substrate 48, together with the substrate 48 and the Peltier element 74, has a laser beam emitting portion covered with a transparent dustproof cover 75.

The semiconductor laser 110 is connected to the drive circuit 78 via wires 46A and 46B drawn out of the dustproof cover 75. FIG. 13 shows a schematic configuration of the drive circuit 78. The drive circuit 78 includes a DC power supply circuit 80 having an automatic output control circuit (APC), an AC power supply 8
4, and a bias T88. The bias T88 is composed of a coil 82 and a capacitor 86. The bias T88 is generated from a DC power supply circuit 80 and passes through the coil 82 to a DC power supply component. The passed high frequency is superimposed, and the high frequency superimposed current is applied to the semiconductor laser 110. In order to reduce the noise of the output second harmonic, the frequency of the superimposed high frequency is preferably 100 to 400 MHz, and the modulation degree is 1
It is preferably set to 00%.

Two wires 71 A and 71 B are connected to both electrodes of the photodiode 70, and the wires 71 A and 71 B are drawn out of the dustproof cover 75. The photodiode 70 includes the dustproof cover 7.
5 is connected to a DC power supply circuit 80 having an APC via wires 71A and 71B drawn out.
The amount of current applied to the semiconductor laser 110 is controlled by the APC so that the optical output of the second harmonic 62 becomes a predetermined value. The Peltier device 74 is connected to the temperature controller 90. Further, a thermistor (not shown) for adjusting the temperature inside the device is provided inside the device covered with the dustproof cover 75, and this thermistor is also connected to the temperature controller 90. The temperature controller 90 controls the Peltier element based on the output of the thermistor so that the inside of the device is maintained in a temperature range where the optical system does not dew in the use environment (for example, 30 ° C. or more if the use environment temperature is 30 ° C.). 74 is controlled.

Since the optical wavelength conversion module according to the present embodiment uses the optical wavelength conversion element according to the first embodiment, a high wavelength light can be obtained without providing an external optical element such as a phase compensator. It is possible to stably obtain a Gaussian beam-shaped second harmonic of the 0th mode with a conversion efficiency. Further, since there is no need to provide an external optical element, the reliability of the module is improved and the cost can be reduced.

[0052]

Embodiments Hereinafter, specific embodiments will be described. (Example 1) First, at 87 ° Z
An electrode for polarization inversion was formed on the cut MgO 5 mol% doped LiNbO ₃ substrate using photolithography, Cr vapor deposition, and lift-off. The detailed structure of the electrode is disclosed in JP-A-9-218431. The length of the polarization inversion grating, that is, the electrode length is 8 m.
m. Next, a metal mask having a three-layer structure of Ta / Au / Ta was formed using photolithography, sputtering film formation, and lift-off. The thickness of each layer was 300 °, 1000 °, and 300 °, respectively, and the line width of the opening was 7 μm. Using this metal mask, the first proton exchange treatment was performed in pyrophosphoric acid at 140 ° C. for 32 minutes.

Next, a metal mask having a three-layer structure of Ta / Au / Ta was formed in a region from the emission end to 1 mm inside along the optical waveguide by using photolithography, sputtering film formation, and lift-off. . The thickness of each layer was 300 °, 1000 °, and 300 °, respectively. Using this metal mask, 140 ° C., 17 ° C.
The second proton exchange treatment was performed under the condition of 6 minutes.

Next, after the upper Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2), annealing was performed in air at 350 ° C. for 60 minutes. After that, the Au layer was removed by etching with a mixed solution of KI, I and H ₂ O, and then the Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2).

Next, in a vacuum or Fluorinert,
In the pole, a high voltage of 3 kV when the electrode gap is 400 μm
Polarization inversion was performed by applying a voltage for 1 to 10 seconds. Obtained wavelength
Both ends of the conversion element were optically polished. In particular, the emission side end face 26
Is 3 to 3 degrees to prevent the fundamental wave from re-entering.
The oblique cut was 7 °. Finally, the incident side end face 30 and
The output side end face 26 has a reflectance of 0.2% or less for the fundamental wave.
SiO below _TwoAR coat consisting of a single layer thin film
Formed.

When a laser beam (fundamental wave) having a wavelength of 950 nm is input to the completed wavelength conversion element using a semiconductor laser, a blue light (wavelength
Harmonic). When the conversion efficiency to the second harmonic was examined, the conversion efficiency to the second harmonic in the first region was 300% / Wcm ² , and the conversion efficiency when entering from the first region to the second region was determined. The mode conversion loss is 40%. Therefore, the conversion efficiency to the second harmonic through the first region and the second region is 180% / Wcm ² .

The beam shape in the direction perpendicular to the substrate in the NFP of the second harmonic emitted from the wavelength conversion element is as follows:
As shown by a solid line in FIG. 9A, a Gaussian distribution is shown. (Embodiment 2) First, at 87 ° Z
An electrode for polarization inversion was formed on the cut MgO 5 mol% doped LiNbO ₃ substrate using photolithography, Cr vapor deposition, and lift-off. The detailed structure of the electrode is disclosed in JP-A-9-218431. The electrode length was 8 mm. Next, using photolithography, sputter deposition, and lift-off, Ta / A
A three-layer metal mask made of u / Ta was formed. The thickness of each layer was 300 °, 1000 °, and 300 °, respectively, and the line width of the opening was 6 μm. Using this metal mask, the first proton exchange treatment was performed in pyrophosphoric acid at 150 ° C. for 34 minutes.

Next, a metal mask having a three-layer structure of Ta / Au / Ta was formed in a region from the emitting end to 1 mm inside along the optical waveguide by using photolithography, sputtering film formation, and lift-off. . The thickness of each layer was 300 °, 1000 °, and 300 °, respectively. Using this metal mask, 170 ° C., 53 ° C.
The second proton exchange treatment was performed under the conditions of minutes.

Next, after the upper Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2), annealing was performed in air at 370 ° C. for 60 minutes. After that, the Au layer was removed by etching with a mixed solution of KI, I and H ₂ O, and then the Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2).

Next, in a vacuum or florinate,
In the pole, a high voltage of 3 kV when the electrode gap is 400 μm
Polarization inversion was performed by applying a voltage for 1 to 10 seconds. Obtained wavelength
Both ends of the conversion element were optically polished. In particular, the emission side end face 26
Is 3 to 3 degrees to prevent the fundamental wave from re-entering.
The oblique cut was 7 °. Finally, the incident side end face 30 and
The output side end face 26 has a reflectance of 0.2% or less for the fundamental wave.
SiO below _TwoAR coat consisting of a single layer thin film
Formed.

When a laser beam (fundamental wave) having a wavelength of 950 nm is input to the completed wavelength conversion device using a semiconductor laser, blue light having a wavelength of 475 nm (second
Harmonic). When the conversion efficiency to the second harmonic was examined, the conversion efficiency to the second harmonic in the first region was 500% / Wcm ² , and the conversion efficiency when entering from the first region to the second region was determined. The mode conversion loss is 60%. Therefore, the conversion efficiency to the second harmonic through the first region and the second region is 200% / Wcm ² .

The beam shape of the second harmonic emitted from the wavelength conversion element in the direction perpendicular to the substrate in the NFP is as follows:
As shown by a solid line in FIG. 9A, a Gaussian distribution is shown. (Embodiment 3) First, at 87 ° Z
An electrode for polarization inversion was formed on the cut MgO 5 mol% doped LiNbO ₃ substrate using photolithography, Cr vapor deposition, and lift-off. The detailed structure of the electrode is disclosed in JP-A-9-218431. The electrode length was 8 mm. Next, using photolithography, sputter deposition, and lift-off, Ta / A
A three-layer metal mask made of u / Ta was formed. The thickness of each layer was 300 °, 1000 °, and 300 °, respectively, and the line width of the opening was 7 μm. Using this metal mask, the first proton exchange treatment was performed in pyrophosphoric acid at 150 ° C. for 32 minutes.

Next, a metal mask having a three-layer structure of Ta / Au / Ta was formed in a region from the emission end to 1 mm inside along the optical waveguide by using photolithography, sputtering film formation, and lift-off. . The thickness of each layer was 300 °, 1000 °, and 300 °, respectively. Using this metal mask, in pyrophosphoric acid at 150 ° C., 84
The second proton exchange treatment was performed under the conditions of minutes.

Next, the upper Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2), and then an annealing treatment was performed in air at 370 ° C. for 60 minutes. After that, the Au layer was removed by etching with a mixed solution of KI, I and H ₂ O, and then the Ta layer was removed by etching with a mixed solution of hydrofluoric acid and nitric acid (ratio 1: 2).

Next, in a vacuum or Fluorinert,
In the pole, a high voltage of 3 kV when the electrode gap is 400 μm
Polarization inversion was performed by applying a voltage for 1 to 10 seconds. Obtained wavelength
Both ends of the conversion element were optically polished. In particular, the emission side end face 26
Is 3 to 3 degrees to prevent the fundamental wave from re-entering.
The oblique cut was 7 °. Finally, the incident side end face 30 and
The output side end face 26 has a reflectance of 0.2% or less for the fundamental wave.
SiO below _TwoAR coat consisting of a single layer thin film
Formed.

When a laser beam (fundamental wave) having a wavelength of 950 nm was input to the completed wavelength conversion device using a semiconductor laser, an output of blue light (second harmonic) having a wavelength of 475 nm of 9 mW or more was obtained. When the conversion efficiency to the second harmonic was examined, the conversion efficiency to the second harmonic in the first region was 300% / Wcm ² , and the conversion efficiency when entering from the first region to the second region was determined. The mode conversion loss is 70%. Therefore, the conversion efficiency to the second harmonic through the first region and the second region is 90% / Wcm ² .

The beam shape in the direction perpendicular to the substrate in the NFP of the second harmonic emitted from the wavelength conversion element is as follows:
As shown by a solid line in FIG. 9A, a Gaussian distribution is shown. (Example 4) A wavelength conversion element having the same configuration as that of Example 1 was produced except that the length L of the second region was changed variously, and NA0 was placed on the light emission side of the wavelength conversion element as shown in FIG. The second harmonics emitted by disposing the lens A and the aperture B of 0.5 were collected, and the stray light level of each wavelength conversion element was measured. The stray light level here is a level of the background light intensity P2 with respect to the peak intensity P1 of the output light, as shown in FIG. 15B, and is represented by 10 × Log (P2 / P1).

FIG. 15A shows the relationship between the length L of the second region and the stray light level. Stray light level is practically -10dB
It is necessary to set it as below, and it is more preferable to be -15 dB or less. Therefore, the length L of the second region is obtained from FIG.
Is preferably 0.5 mm or more, more preferably 0.75 mm or more.

At the junction between the second region and the first region, light not converted from the second region to the first region is radiated into the space or the wavelength conversion element. This emitted light becomes stray light and overlaps with the light actually used. For this reason, conventionally, when the light emitted from the wavelength conversion element is imaged through a lens, an unnecessary portion caused by stray light is cut by an aperture. However, if the junction is too close to the end face of the element, there is a problem that beams are not separated and stray light is offset. The stray light level indicates the degree of the offset, and the stray light level is −
If it is larger than 10 dB, that is, if the length L of the second region is less than 0.5 mm, the offset of stray light is large and is not practically preferable. The aperture becomes unnecessary by selecting the length L of the second region so that the stray light level has no practical problem.

[0070]

The optical wavelength conversion device of the present invention has an effect that a Gaussian beam-shaped wavelength converted wave can be stably obtained with high wavelength conversion efficiency.

Further, according to the method for manufacturing an optical wavelength conversion device of the present invention, the optical wavelength conversion device of the present invention can be easily manufactured.

Further, the optical wavelength conversion module of the present invention comprises:
Since the optical wavelength conversion element of the present invention is used, it is possible to stably obtain a Gaussian beam-shaped wavelength conversion wave with high wavelength conversion efficiency without providing an external optical element such as a phase compensator. The effect is that the reliability of the device is improved.

[Brief description of the drawings]

FIGS. 1A and 1B are schematic diagrams of an optical wavelength conversion element according to an embodiment, wherein FIG. 1A is a cross-sectional view along an optical waveguide, and FIG. 1B is a plan view.

2A and 2B are diagrams illustrating a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 2A is a plan view and FIG. 2B is a cross-sectional view along the optical waveguide.

3A and 3B are diagrams illustrating a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 3A is a plan view and FIG. 3B is a cross-sectional view along the optical waveguide.

4A and 4B are diagrams showing a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 4A is a plan view and FIG. 4B is a cross-sectional view along the optical waveguide.

5A and 5B are diagrams showing a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 5A is a plan view and FIG. 5B is a cross-sectional view along the optical waveguide.

6A and 6B are diagrams showing a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 6A is a plan view, and FIG. 6B is a cross-sectional view along the optical waveguide.

7A and 7B are diagrams showing a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 7A is a plan view and FIG. 7B is a cross-sectional view along the optical waveguide.

8A and 8B are diagrams showing a manufacturing process of the optical wavelength conversion element according to the present embodiment, wherein FIG. 8A is a plan view and FIG. 8B is a cross-sectional view along the optical waveguide.

FIG. 9A is a diagram illustrating a near-field pattern of output light of the optical wavelength conversion element according to the present embodiment;
(B) is a diagram showing a near-field pattern of output light of a conventional optical wavelength conversion element.

FIGS. 10A to 10C are schematic diagrams showing modified examples of the optical wavelength conversion element according to the present embodiment.

FIG. 11 is a plan view of the optical wavelength conversion module according to the present embodiment.

FIG. 12 is a side view showing a mounting form of the optical wavelength conversion module according to the present embodiment.

FIG. 13 is a circuit diagram showing a drive circuit of the optical wavelength conversion module according to the present embodiment.

FIG. 14 is a schematic diagram showing a device configuration for measuring a stray light level.

15A is a diagram illustrating a relationship between a length L of a second region of the optical wavelength conversion element and a stray light level, and FIG. 15B is a diagram illustrating a stray light level.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical wavelength conversion element 12 Substrate 14 Periodic domain inversion structure 16 Channel optical waveguide 18 Fundamental wave 20 1st region 22 2nd harmonic 24 Second region 26 Emission side end surface 28, 32 AR coat 30 Incident side end surface

Claims (10)

[Claims] 1. A first region in which an incident fundamental wave is wavelength-converted by quasi-phase matching while propagating in a single mode, and a wavelength-converted wave propagated from the first region is propagated in a single mode and emitted. A wavelength conversion element comprising: 2. The method according to claim 1, wherein a difference in refractive index between the second region and the surroundings is smaller than a difference in refractive index between the first region and the surroundings.
3. The wavelength conversion element according to 1. 3. The wavelength conversion device according to claim 1, wherein the first region and the second region are each formed of a linear optical waveguide formed on a nonlinear crystal substrate. 4. The wavelength conversion element according to claim 1, wherein the length of the second region along the light propagation direction is 0.5 mm or more. 5. The method according to claim 1, wherein the nonlinear crystal is LiNb _x Ta _1-x O
₃ (0 ≦ x ≦ 1) a wavelength conversion element according to any one of claims 1 to 4,. 6. The wavelength conversion element according to claim 1, wherein said nonlinear crystal is doped with MgO, ZnO, or Sc. 7. The wavelength conversion element according to claim 1, wherein the output side end face is inclined by 3 ° to 7 ° in the light propagation direction with respect to a plane perpendicular to the optical axis direction. 8. The wavelength converter according to claim 1, wherein an antireflection film having a reflectance of 0.2% or more for a fundamental wave is provided on at least one of the incident end face and the output end face. element. 9. A method for manufacturing a wavelength conversion element for manufacturing a wavelength conversion element according to claim 1, wherein a first proton having a predetermined depth is formed in a region corresponding to the first region of the nonlinear crystal substrate. Forming an exchange area, the second
A second region shallower than the first region in a region corresponding to the region
Forming a proton exchange region, annealing the first proton exchange region, and forming a domain-inverted structure so that the incident fundamental wave can be propagated in a single mode, and the fundamental wave can be wavelength-converted by quasi-phase matching. Forming a first region that can be converted into a converted wave; annealing the second proton exchange region to form the first region;
A method of manufacturing a wavelength conversion element, comprising forming a second area in which a wavelength-converted wave propagated from the area (1) can propagate in a single mode, and manufacturing the wavelength conversion element. 10. An optical wavelength conversion module in which the wavelength conversion element according to claim 1 is directly coupled to a semiconductor laser that emits a fundamental wave.