JPH05204007A - Second higher harmonic generating element and second higher harmonic generator - Google Patents

Abstract

PURPOSE:To obtain the SHG element which receives the oscillated light from a semiconductor laser as basic wave light, is stable in oscillation wavelength and can output the second higher harmonic of light energy and the SHG device constituted by using this element. CONSTITUTION:The SHG element has a combined optical waveguide consisting of a linear optical waveguide 110 and nonlinear optical waveguide 120 respectively having a uniform film thickness on a substrate 100. The substrate 100 is formed of fused quartz, the linear optical waveguide 110 of a glass layer and the nonlinear optical waveguide 120 of a 2-methyl-4-nitroaniline (NMA) layer. A refractive index periodic structure 136 alternately formed with high-refractive index parts 132 and low-refractive index parts 134 by an ion exchange method is provided on the glass layer or the refractive index periodic structure formed of the high-refractive index parts of the remaining MNA layer parts and the low-refractive index parts of the space parts by periodically partially removing the MNA layer. The lengths of the high-refractive index parts and the low- refractive index parts are varied to provide as multiple periods.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a second harmonic generating element using a laser as a light source for fundamental wave light, and more particularly to a thin film waveguide type second harmonic generating element using an organic nonlinear thin film crystal. The present invention also relates to a second harmonic generation device using this second harmonic generation element.

[0002]

2. Description of the Related Art Conventionally, a thin-film waveguide type second harmonic generating element using an organic nonlinear thin-film crystal is disclosed in, for example, the document: "A.
pplied Optics, Vol. 25, no.
9, (1986), pp. 1491 to 1494 ”. Prior to the description of the present invention, in order to facilitate understanding of the present invention, the structure of the device, its manufacturing method, and the following will be described with reference to FIGS. 5, 6, and 7A and 7B. The usage method will be described. FIG. 5 is an example of a configuration for generating the second harmonic from this element. 10 is a Q-switched YAG laser, 12 is a deflector,
14 is an infrared (IR) transmission filter, 16 is a lens, 1
8 is a mirror, 20 is a prism, 22 is a 2-methyl-4-nitroaniline (MNA) single crystal thin film, 24 is a buffer region, 26 is a prism, and 28 is a glass optical formed by using Corning 7059 (product number) glass. Waveguide, 30 fused silica substrate, 32 copper sulfate aqueous solution filter, 34 diaphragm, 36 interference film filter, 38 photomultiplier tube, 40
Is a boxcar integrator, and 42 is a pen recorder.

For the optical waveguide 28, a Corning 7059 glass (refractive index for light having a wavelength of 1.604 μm is 1.54, refractive index for light having a wavelength of 0.532 μm is 1.56) is fused silica substrate ( Wavelength 1.
Refractive index for light of 064 μm is 1.45, wavelength is 0.5
It has a refractive index of 1.46) for a light of 32 μm. M on this optical waveguide 28
An NA crystal thin film 22 (having a refractive index of 1.80 for light having a wavelength of 1.064 μm and a refractive index of 2.2 for light having a wavelength of 0.532 μm) is formed.

The single crystal thin film 22 schematically shown in FIG. 4 is formed for the purpose of functioning as an optical waveguide, and has a shape in which the thickness linearly decreases depending on the location (tapered thin film). There is a feature in the point. The reason why the thin film 22 is formed in a tapered structure is to satisfy the phase matching condition for generating the second harmonic.

A method of forming the MNA single crystal thin film 22 will be described with reference to FIG. FIG. 5 shows a state in which the MNA melt 50 is prepared in the beaker 52 and the growth substrate 54 is prepared.
In this substrate 54, 56 is a fused quartz substrate (3 in FIG. 4).
0), 58 is a glass layer of an optical waveguide (corresponding to 28 in FIG. 4) formed by high frequency sputtering, 60
Is a spacer of polycarbonate. 62 is the glass layer 5 facing the glass layer 5 because the spacer 60 is sandwiched between them.
It is a wedge-shaped space formed between the opposing surfaces of No. 8.

FIG. 6 is a diagram for explaining a method of forming an MNA single crystal thin film. 62 is a motor, 64 is a growth substrate (corresponding to 54 in FIG. 5), 66 is a heater, 68 is a glass container (tube), 70 is an oil bath, 72 is a temperature control device, and 74 is a temperature detector. On the upper side of the beaker 52, two substrates 56, each having a glass layer 58 of the optical waveguide formed thereon, are prepared with the optical waveguide sides facing each other through a spacer 60. As shown in FIG. 6, the growth substrate 64 thus formed is
Immerse in NA melt 50. The temperature of the MNA melt 50 in the beaker 52 is set to 130 ° C. When two substrates 56 are immersed in the MNA melt as shown in the figure, MNA melts due to a capillary phenomenon.
The melt enters the gap between the glass layer 58 of the optical waveguide of the two substrates 56 and the spacer 60. When this is cooled, a tapered MNA polycrystalline thin film (not shown) is formed on the glass substrate 56.
Is formed on the upper side of.

The MNA polycrystalline thin film thus formed is formed as an MNA single crystal thin film by using the apparatus shown in FIG. The growth substrate 64 on which the MNA polycrystalline thin film is attached is slowly moved by the motor 26 (40 mm).
/ Day) and slowly cool. In this process, the tapered MNA polycrystalline thin film becomes a tapered single crystal thin film. After that, the two glass substrates 56 are slowly peeled off to complete an optical waveguide having a two-layer structure.

A fundamental wave light from a YAG laser is guided to the thin film type optical waveguide thus formed by the method shown in FIG. 4 to generate a second harmonic.

Now, the manner in which the fundamental wave light and the second harmonic wave propagate in the optical waveguide will be described with reference to FIGS. 7 (A) and 7 (B). FIG. 7 shows a fused quartz substrate 30 and a Corning 70.
59 In the case where the fundamental wave light ((A) of FIG. 7) and the second harmonic wave ((B) of FIG. 7) propagate in the composite optical waveguide including four layers of the glass optical waveguide 28, the optical waveguide 22, and the air, The amplitude distribution of the photoelectric field is shown respectively. The fundamental wave light is shown in Fig. 4.
As shown in FIG. 2, the structure is such that the two layers of the optical waveguide 22 and the optical waveguide 28 are guided by the prism coupler 20 to the optical waveguide 28 and propagate in the optical waveguide 28 (the substrate 30 and the optical waveguide 2).
8, a light guide 22, and a four-layer structure of air). In the four-layer structure portion, the fundamental wave light is guided by both the optical waveguides 22 and 28, as shown in FIG. While the fundamental wave light propagates through the optical waveguide 22, a part of the energy of the fundamental wave light is converted into the energy of the second harmonic wave, and as shown in FIG. 7B, the second harmonic wave is composed of four layers. It propagates through the composite optical waveguide. Eventually, it propagates through the three-layer composite optical waveguide without the optical waveguide 22 (fused quartz substrate 30, Corning 7059 glass optical waveguide 28, air) and is taken out to the outside.

At this time, the phase matching is performed by the effective refractive index of the fundamental wave light propagating in the optical waveguide 22 (also referred to as the effective refractive index or the guided refractive index) and the effective refractive index of the second harmonic. Are satisfied by matching. Specifically, the phase matching condition can be satisfied by changing the thickness of the optical waveguide. That is, by changing the thickness of the optical waveguide, the propagation modes (the shape of the intensity distribution of the photoelectric field when propagating in the optical waveguide of the fundamental wave light and the second harmonic are described. (A) and (B) of FIG. This is an example of this, and the effective refractive index can be controlled by this. The reason why the single crystal thin film has a tapered shape is to find a condition for achieving phase matching. That is, the phase matching condition is satisfied by causing the fundamental wave light to propagate through the tapered optical waveguide 22 having a thickness that allows phase matching.

[0011]

However, even if the second harmonic generation is realized by the conventional method using the second harmonic generating element having the above-mentioned conventional structure, it is industrially useful because of the following reasons. Is not so much.

Since the YAG laser is used as the light source of the fundamental wave light, the second harmonic generator cannot be downsized. Originally, the second harmonic generation (hereinafter, also referred to as SHG) element is expected to be mainly used as a compact short wavelength coherent light source, and is a semiconductor much smaller than the YAG laser. It is desired to use a laser (LD).

In the conventional SHG element, it is difficult to achieve phase matching and stable oscillation cannot be expected. LD
The oscillation wavelength fluctuates depending on the ambient temperature, the injection current value, and the like. The change in the oscillation wavelength for one oscillation longitudinal mode is L
It is a well-known fact that it usually occurs in D (even if the wavelength is particularly stable, such as in a distributed feedback LD, it is not possible to stabilize it within a range that satisfies the phase matching condition). , It is difficult even with the current technology).
Therefore, in the conventional SHG element, when the LD is used as a light source of the fundamental wave light, the phase matching condition with respect to the wavelength of the fundamental wave light is very strict, and even if the oscillation wavelength of one oscillation longitudinal mode of the LD changes, the phase matching is performed. It is out of the condition. Therefore, it is necessary to improve the structure of the SHG element corresponding to the combination with the LD so that the SHG element is different from the above-described conventional SHG element and the phase matching can be easily achieved.

Further, in the above-described conventional SHG element, the degree of freedom regarding the wavelength for achieving the phase matching is extremely low. In this conventional element, the phase matching condition is satisfied by changing the thickness of the nonlinear optical waveguide. However, the range of wavelengths that can be phase-matched by this method is several tens of nanometers at the maximum.

Further, in the above-described SHG element, since the optical waveguide having the ridge structure cannot be formed in principle, the SHG conversion efficiency cannot be increased, and further, the second harmonic wave to be output to the outside can be obtained. Large wavefront aberration.

The object of the present invention is to realize the above-mentioned conventional
It is an object of the present invention to provide an SHG element and a second harmonic generation device using the same, in which the above problem is solved.

[0017]

In order to achieve this object, according to a second harmonic generating element of the present invention, a second harmonic generating element using fundamental wave light is provided, which is a substrate and a substrate. A composite optical waveguide composed of a linear optical waveguide and a non-linear optical waveguide, which are sequentially stacked, is provided, and an effective refractive index for the fundamental light is alternately and periodically provided along the waveguide direction of the light in the composite optical waveguide. Different high-refractive-index portions and low-refractive-index portions are formed, and the high-refractive-index portions and the low-refractive-index portions have different lengths along the waveguide direction of the light, and the linear optical waveguide and It is characterized in that the thickness of each of the nonlinear optical waveguides is constant.

Further, according to the second harmonic generation device of the present invention, at least the semiconductor laser having the end face having a low reflection surface and the second harmonic generation element are provided. Preferably, for example, a semiconductor laser whose end face has a low reflection surface, a collimator lens, a half-wave plate, a condenser lens, and the above-mentioned second harmonic generation element are arranged in this order, and this semiconductor laser is used. Is preferably arranged so as to oscillate with the feedback light from the second harmonic generation element.

[0019]

According to the second harmonic generating device of the present invention described above, the composite optical waveguide has a structure in which two types of portions having different refractive indexes are provided alternately and periodically. With this structure, since the propagation mode is different when the fundamental wave light propagates through the optical refractive index portion and when it propagates through the low refractive index portion, the effective refractive index is different in both portions. Therefore, the energy conversion efficiency from the fundamental wave light to the second harmonic wave (hereinafter, also referred to as SHG conversion efficiency) in these two portions is also different.
Therefore, the energy (E ₂₎ and the magnitude of the energy (E ₁₎ and the second harmonic wave generated in the low refractive index portion (S ₂₎ of the second harmonic wave generated in the high refractive index portion (S ₁₎ Is different.
Further, since the second harmonics S ₁ and S ₂ have opposite phases, they interfere with each other. Therefore, the sum of energies for one cycle is the absolute value of E ₁ -E ₂ (it should be noted that there is no inconvenience even if it is argued that this energy E ₁ > E ₂ ). Also, the sum of both energies always takes a positive value). Therefore,
If the period of the refractive index periodic structure (or also referred to as a periodic refractive index distribution structure) formed in the composite optical waveguide is set as N period, for example, the second harmonic of the energy of N (E ₁ −E ₂ ). Can be effectively and stably generated, and can be taken out of the second harmonic generation element.
Since the period N can be set to a remarkably large value, the energy of the second harmonic can be remarkably increased.

As described above, according to the SHG element of the present invention, by changing the period of the refractive index periodic structure,
Phase matching can be achieved for light of virtually any wavelength. Therefore, the SHG element of the present invention can be applied not only to a small semiconductor laser (LD) but also to the purpose of realizing a small coherent light source, and can be applied to a case where the LD is not a light source of fundamental wave light.

Since the above-mentioned periodic structure of refractive index performs SHG conversion if it is provided along the light guiding direction, it may be provided in either a linear or non-linear optical waveguide with respect to the substrate, or a planar type. Or may be formed as a ridge.

Further, according to the configuration of the second harmonic generation device of the present invention, when light is introduced into the composite optical waveguide in which the periodic refractive index distribution structure of the SHG element is built, a specific wavelength corresponding to this period is obtained. Is strongly reflected (Bragg reflection). Therefore, first, the oscillation threshold value is increased by coating the end surface of the LD with low reflection (AR).
The return light reflected by this Bragg is returned to the LD,
The SHG element and the LD are arranged so as to oscillate the LD. Therefore, when the oscillation light from this LD is used as the fundamental wave light by appropriately setting the period of the periodic refractive index distribution structure of the composite optical waveguide of this SHG element, as described above from this SHG element, It is possible to effectively output the second harmonic.

Therefore, according to the device of the present invention, the SHG
LD by the light returned by Bragg reflection at the element
Oscillates, the oscillation wavelength does not change due to changes in the ambient temperature and the injection current value, and therefore the LD stably oscillates. Therefore, the second harmonic can be stably extracted from the SHG element.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. It should be understood that the embodiment described below is merely a preferred example, and thus the present invention is not limited to this embodiment.

FIGS. 1A, 1B and 1C are diagrams for explaining the first embodiment of the present invention, and FIG. 1A is taken along the light guiding direction. It is a figure showing a section of a section, and (B) and (C) are figures showing a section of a plane type and a ridge section of a direction which intersects perpendicularly with a waveguide direction of light. In addition, in the cross-sectional views, hatching and the like indicating the cross-sections are omitted except for a part. The drawings are merely schematic representations of the shapes, dimensions, and arrangements of the constituent components to the extent that the present invention can be understood.

[First Embodiment of SHG Element] First, an embodiment of the second harmonic generation (SHG) element of the present invention will be described. This SHG element is an element that uses fundamental wave light. Then, the substrate 100, and the linear optical waveguide 110 and the nonlinear optical waveguide 1 that are sequentially stacked on the substrate 100.
And a composite optical waveguide 130 constituted by 20 ((A), (B) and (C) in FIG. 1). In this embodiment, it is preferable that the composite optical waveguide 130 extends in the light guiding direction and is provided in a stripe shape. Board 100
A fused quartz substrate is used as. In addition, the linear optical waveguide 11
Reference numeral 0 is a glass thin film formed on the substrate 100 by high frequency sputtering of a glass material of product number 7059 manufactured by Corning Incorporated.

In the composite optical waveguide 130, the high refractive index portion 132 and the low refractive index portion 1 in which the effective refractive index with respect to the fundamental wave light is alternately and periodically changed along the light guiding direction.
34 is formed. The lengths of the high refractive index portion 132 and the low refractive index portion 134 along the light waveguide direction are different. In this embodiment, a periodic refractive index distribution structure (refractive index periodic structure) 136 is formed on the glass thin film 110 by a known ion exchange method ((A), (B) and (C) of FIG. 1).

Therefore, in the planar type embodiment, first, a chromium (Cr) thin film is once vapor-deposited on the glass thin film 110 to have a film thickness of, for example, about 50 nm, and then the high refractive index portion is formed by using the photolithography technique. The Cr film portion above the portion to be 132 is selectively removed. After that, the remaining C
The entire structure of the substrate having the r thin film is 200 ° C to 400 ° C.
Immerse in benzoic acid at a temperature of about 30 ° C. for about 30 minutes to 1 hour. Then, this structure is pulled up in the air and returned to room temperature. Next, benzoic acid adhering to the substrate 100 is removed with ethyl alcohol or the like, and then the remaining Cr thin film is peeled off by diammonium dicerium nitrate. Then 200
Annealing is performed on the structure after the Cr thin film is removed at a temperature of about ℃ to 400 ℃. As a result, the refractive index periodic structure 136 is formed on the glass thin film 110 ((A) and (B) of FIG. 1).

On the other hand, in the ridge type embodiment, first, a ridge optical waveguide is formed on the glass thin film 110 by an ion exchange method or the like. Then, a refractive index periodic structure is formed by superposing on the ridge optical waveguide. Ions are exchanged by selectively overlapping only the high refractive index portion 132 in the ridge type optical waveguide,
A ridge-shaped optical waveguide having a periodic refractive index distribution structure 136 ((A) and (C) in FIG. 1) is used. Since the ridge type has a higher light confinement effect, it is a more excellent SHG element.

In the embodiment shown in FIGS. 1A, 1B and 1C, the high refractive index portion 132 and the low refractive index portion 134 are formed.
Let D ₁ and D ₂ (where D ₁ ≠ D ₂ and D ₁ > 0 and D ₂ > 0, respectively) have a length in the direction of the optical waveguide, and the period is Λ (that is, D ₁ ). + D ₂ ).

In this case, the film thickness of the glass thin film 110, which is a linear optical waveguide, is constant along the optical waveguide direction.

Next, a nonlinear optical waveguide 120 is formed on the glass thin film 110 on which the refractive index periodic structure 136 is formed. In this embodiment, the nonlinear optical waveguide 120
Is formed of a thin film of an organic nonlinear optical material, for example, 2-methyl-4-nitroaniline (MNA), as in the conventional case.
The MNA thin film 120 is manufactured by using the conventional method described in FIGS. 5 and 6. In the present invention, it is necessary to make the film thickness of the MNA thin film 120, which is a nonlinear optical waveguide, uniform. Therefore, instead of the conventional tapered spacer,
By using two spacers of the same size, the space into which the MNA melt enters by capillary action is made uniform. Further, as in the conventional case, two substrates on which the linear optical waveguides are already formed are prepared. Then, after the MNA melt enters between the spacers to form an MNA polycrystalline thin film, the polycrystalline thin film is
In the same manner as in the conventional method, the MNA single crystal thin film is changed to two substrates which are opposed to each other via a spacer and are carefully separated. As a result, the MNA thin film remains attached to the linear optical waveguide of one of the substrates. Therefore, a desired three-layer structure, that is, a planar SH composed of the fused quartz substrate 100, the glass thin film 110 and the MNA thin film 120.
Obtain the G element.

[Second Embodiment of SHG Element] Next, another embodiment of the SHG element of the present invention will be described. In this embodiment, the high refractive index portion 232 and the low refractive index portion 234 are formed in the nonlinear optical waveguide 220. The structure of this embodiment is shown in cross section in FIG. Also in this embodiment, as in the case of the embodiment shown in FIG. 1, a glass thin film is formed on a fused silica substrate 200 by a high frequency sputtering technique using a glass material of product number 7059 manufactured by Corning Co., Ltd.
Form as 10. On this glass thin film 210, the MNA thin film 220 having a uniform film thickness is formed as a nonlinear optical waveguide thin film by the same method as in the case of FIG. 1 described above.

After that, the MNA thin film 220 is formed of a heteropolytungstic acid (H
PA) or the like is used to pattern the MNA thin film 220 in a grid pattern. As a result, the SHG having the structure shown in FIG.
Get the element. In this case, the size of this lattice is such that the remaining MNA thin film portion 232 constitutes the high refractive index portion,
Let its length be D ₁ . Further, a portion (referred to as a space portion) 234 which has been removed to form a space constitutes a low refractive index portion, and its length is defined as D ₂ . The period Λ of the lattice is D ₁ + D ₂ .

Also in the case of the embodiment shown in FIG. 2, the glass thin film 210 may be formed as a planar type optical waveguide as in the case described in FIGS. 1B and 1C, or Preferably, it may be previously formed as a ridge-shaped optical waveguide by an ion exchange method or the like. Also in this embodiment, it is preferable that the composite optical waveguide 230 is provided in a stripe shape by extending in the light guiding direction.

After forming the glass thin film 210 as a ridge type optical waveguide, a lattice-shaped MNA single crystal thin film 232 is formed to obtain an SHG element having this structure. This ridge structure has an advantage that the light confinement effect is higher than that of the planar structure and the SHG conversion efficiency is correspondingly increased as in the case described above with reference to FIG. 1B.

In the case of the structure shown in FIG. 2, the effective refractive index for light propagating through the linear optical waveguide, that is, the glass thin film 210 is larger than that of the space portion 234 formed by removing the remaining MNA thin film portion 232. growing. Therefore, when the former refractive index is N ₁ and the latter refractive index is N ₂ , the design policy regarding the above-mentioned dimensions D ₁ and D ₂ is the same as that of the structure shown in FIG. That is, there is a difference in the effective refractive index between the remaining MNA thin film portion 232 and the portion 234 which is selectively removed and becomes a space. Moreover, in the remaining portion 232, while the second harmonic is generated, the space portion 23
At 4, no second harmonic is generated. Therefore, by appropriately setting the period of the lattice structure of the nonlinear optical waveguide, the first
As in the case of the embodiment, phase matching can be achieved, the fundamental wave light can be efficiently converted into the second harmonic, and this can be efficiently output to the outside. The effective refractive index for the fundamental wave is large in the portion 232 where the nonlinear optical waveguide remains and is small in the removed portion 234, so
The energy of the harmonic (S ₁ ) is E ₁ , and the energy of the latter is 0.

As described above, the SHG element of the present invention is a composite waveguide consisting of a linear waveguide and a non-linear waveguide, and the effective refractive index of the fundamental wave light propagating through this waveguide changes periodically, and The second harmonic generation efficiency is also changed by linking to this periodic change.

It is obvious that the SHG element of the present invention is not limited to the above-mentioned two embodiments but can be modified and changed in many ways. For example, instead of using a fused quartz substrate as the substrates 100 and 200, a substrate made of a ferroelectric material such as LiNbO ₃ or LiTaO ₃ may be used. Instead of using glass thin films as the linear optical waveguides 110 and 220, the ferroelectric thin film described above may be used, and ridge optical waveguides may be formed on the ferroelectric thin films by an ion exchange method. In addition, the nonlinear optical waveguide 12
As 0 and 220, a non-linear optical material other than MNA, which is a material having the same properties as MNA, may be used. For example, 2- (α-methylbenzylamino) -5-
Nitropyridine (abbreviated as MBANP) and DAN
Alternatively, 1-nitropyrene or the like can be used.

[Second Harmonic Generator Using SHG Element] Next, the second harmonic generator of the present invention will be described. FIG. 3 schematically shows the arrangement structure of each component of this SHG device. This SHG device has a low reflection surface (A
R) semiconductor laser 300 and collimator lens 3
10 and a half-wave plate 312 for controlling the polarization direction
, The condenser lens 314, and the above-described second harmonic generation element 320 in this order, and the semiconductor laser 300.
Is configured to oscillate with the feedback light from the second harmonic generation element 320. In the figure, 302 and 3
Reference numeral 04 denotes an AR coating film, which is preferably formed as a multilayer film structure of magnesium fluoride (MgF ₂ ) and silicon oxide film (SiOx). Further, 322 is a substrate, 324 is a linear optical waveguide, 326 is a non-linear optical waveguide, and in this case, a refractive index periodic structure is incorporated in this non-linear optical waveguide. Reference numerals 328 and 330 are AR reflection films similar to those described above.

With this arrangement, when a semiconductor laser (LD) whose end face is AR-coated is used as a light source of fundamental wave light in the SHG element of the present invention, the composite optical waveguide described above is used. Bragg reflected light is L
Return to D. Then, by this feedback light, the fundamental wave light satisfying the phase matching condition in the SHG element can be automatically and stably obtained from this LD.

This point will be described below. Usually LD3
No. 00 does not oscillate because the AR coating films 302 and 304 are formed even if a drive current (injection current) is applied in a single state without the arrangement as shown in FIG. The oscillation threshold is extremely high. However, if a driving current is passed in the arrangement configuration shown in FIG. 3, the LD 300 is returned due to the return light from the optical waveguide 324.
Oscillates at a specific wavelength λ corresponding to the period Λ of the refractive index periodic structure of the optical waveguide 326. By setting the period Λ so that the oscillation wavelength λ satisfies the phase matching condition of the second harmonic generation element 320, the second harmonic generation device as shown in FIG. 3 is realized.

Therefore, a method of determining the period Λ will be described. Now, let the wavelength of the fundamental wave light be λ, and the wavelength of the second harmonic wave be (1/2) λ. Further, p, q, s, and t are positive integers, N _H1 and N _H2 are high refractive index portions of the effective refractive index for the fundamental light and the second harmonic, and N _L1 and N _L2 are low refractive index portions. Is the effective refractive index for the fundamental wave light and the second harmonic, D ₁ and D ₂ are the lengths of the light refractive index portion and the low refractive index portion in the waveguide direction of the light, and Λ is the period.

A positive integer p, which simultaneously satisfies the following equations (1) to (4) under the condition that the wavelength λ exists in the gain range of the LD 300 as the light source of the fundamental wave light,
Find q, s, t.

As D ₁ = (2p−1) λ / [4 (N _H2 −N _H1 )] (1) D ₂ = (2q−1) λ / [4 (N _L2 −N _L1 )] (2) , D ₁ = s (λ / 4N _H1 ) (3) D ₂ = t (λ / 4N _L1 ) (4) and Λ = D ₁ + D ₂ (5) Each value of p, q, s, and t found here Should be as small as possible. The reason for this is that the second harmonic generation efficiency is proportional to the square of the number of repetitions of this period, so the smaller the period Λ of the periodic refractive index distribution structure, the greater the number of repetitions of the unit length, As a result, high second harmonic conversion efficiency can be realized.
Actually, assuming that the values of D ₁ and D ₂ are matched with about 3 significant digits, p, q (in many cases,
If p = q) is about 1 to 3, s and t (in many cases, s = t) that satisfy the equations (1) to (4) can be obtained (in many cases, D ₁ = D _2). However, in the present invention, the case where D ₁ = D ₂ is excluded). In reality, the present invention can be realized with this degree of agreement.

If the refractive index periodic structure is formed so as to satisfy the equations (3) and (4), the light of wavelength λ satisfies the Bragg reflection condition, and therefore the light of this wavelength is strongly reflected. Therefore, with the arrangement shown in FIG. 3, the light of wavelength λ is strongly and selectively reflected from the optical waveguide 324, and as a result, the LD 300 oscillates light of wavelength λ. Further, since this refractive index periodic structure simultaneously satisfies the equations (1) and (2), the oscillation light of the wavelength λ is the fundamental wave light that satisfies the phase matching condition.

Since the LD 300 oscillates by the feedback from the optical waveguide 324 having a periodic refractive index distribution as an external resonator, the oscillation wavelength may change depending on the variation of the injection drive current to the LD 300 and the variation of the ambient temperature. Does not change. That is, it is possible to stably generate the second harmonic. The SHG element used here may be the SHG element having any structure of the above-described embodiments.

Since the half-wave plate 312 is inserted in the optical system to control the polarization direction of the light incident on the optical waveguide 324, the fundamental wave light is incident on the SHG element 320 in the TE mode. The wave plate 312 is rotationally adjusted. In this case, the LD 324, the collimator lens 310, the half-wave plate 312, and the condenser lens 31 are arranged so that the oscillation light of the LD 300 enters the optical waveguide 324 with the highest efficiency.
4 and SHG element 320 are placed, and then 1/2
It is preferable to adjust the second harmonic generation efficiency (the intensity of the second harmonic) to the maximum by rotating the wave plate 312. Therefore, unlike the conventional device, the LD
It is not necessary to adjust the incident position of the fundamental wave on the SHG element so that the phase matching can be achieved each time when the oscillation wavelength of is changed.

In this way, it is possible to form the second harmonic generation device having the arrangement as shown in FIG. In this device, since a semiconductor laser (LD) can be used as a light source of fundamental wave light, a small coherent light source can be realized.

Further, although the embodiment of this apparatus has a structure in which the LD and the SHG element are optically coupled by using two lenses, it is obvious that the invention is not limited to this structure. For example, the LD and the SHG element may be combined by using a prism coupler in the optical system. Also,
The LD and the SHG element may be directly coupled to each other. Alternatively, the LD and the SHG element may be coupled using a polarization direction preserving (polarization plane preserving) fiber. In these cases, the polarization direction of the incident light on the SHG element may be adjusted to a direction in which the second harmonic can be generated by some method.

[0051]

As is apparent from the above description, according to the second harmonic generation (SHG) element of the present invention, a semiconductor laser (LD) can be used as a light source of fundamental wave light. As a result, a device using this SHG element can be downsized.

Further, according to the SHG element of the present invention, the composite optical waveguide is provided with the refractive index periodic structure. Therefore, if the cycle of each SHG element is set in advance so that the LD gain can be obtained, phase matching can be achieved for light of virtually any wavelength. As a result, the SHG element of the present invention can be used for purposes other than the purpose of realizing a small coherent light source using an LD as a fundamental wave light source.

Further, since the SHG element of the present invention has a ridge structure, the energy density in the optical waveguide of the fundamental wave light can be increased, so that the SHG conversion efficiency can be increased and the SHG element can be transferred to the outside. It is possible to reduce the wavefront aberration of the extracted second harmonic.

Further, according to the device in which the SHG element and the LD of the present invention are combined, the LD oscillates due to the feedback light due to the Bragg reflection from the SHG element. There is no risk of oscillation wavelength fluctuation due to Therefore, this oscillated light becomes the fundamental wave light and enters the SHG element, and the fundamental wave light is converted into the second harmonic wave with high efficiency based on the good phase matching in the SHG element. Therefore, according to the SHG device of the present invention, it is possible to output the high-energy, wavelength-stabilized second harmonic.

[Brief description of drawings]

1 (A), (B) and (C) are cross-sectional views for explaining a first embodiment of a second harmonic generating element of the present invention.

FIG. 2 is a sectional view for explaining a second embodiment of the second harmonic generation element of the present invention.

FIG. 3 is an arrangement configuration diagram of each component used for explaining an embodiment of a second harmonic generation device using the second harmonic generation element of the present invention.

FIG. 4 is an explanatory diagram of a conventional second harmonic generator.

FIG. 5 is an explanatory diagram of a main part of a first half step of a conventional method for manufacturing a second harmonic generation element.

FIG. 6 is an explanatory diagram of a main part of a latter half step of a conventional method for manufacturing a second harmonic generation element.

7 (A) and 7 (B) show a fundamental wave light and a second wave in a composite optical waveguide including a linear optical waveguide and a nonlinear optical waveguide.
It is an explanatory view for explaining a mode of propagation of a harmonic.

[Explanation of symbols]

 100, 200, 322: Substrate 110, 210, 324: Linear optical waveguide 120, 220, 326: Non-linear optical waveguide 132, 232: High refractive index portion 134, 234: Low refractive index portion 136, 236: Refractive index periodic structure 300 : Semiconductor laser 302, 304, 328, 330: AR coating film 310: collimator lens 312: 1/2 wavelength plate 314: condenser lens

Claims (12)

[Claims] 1. A second harmonic generation element using fundamental wave light, comprising: a substrate; and a composite optical waveguide configured by a linear optical waveguide and a nonlinear optical waveguide, which are sequentially stacked on the substrate. In the optical waveguide, a high refractive index portion and a low refractive index portion having different effective refractive indices for the fundamental light are periodically formed along the light guiding direction. The high refractive index portion and the low refractive index portion are formed. 2. The second harmonic generation element, wherein the lengths of the portions along the waveguide direction of the light are different, and the thicknesses of the linear optical waveguide and the nonlinear optical waveguide are constant. 2. A second harmonic generation element, wherein the high refractive index portion and the low refractive index portion according to claim 1 are formed in a linear optical waveguide. 3. A second harmonic generation element, wherein the high refractive index portion and the low refractive index portion according to claim 1 are formed in a nonlinear optical waveguide. 4. The second harmonic generation element according to claim 1, wherein the periods of the high refractive index portion and the low refractive index portion satisfy the following expressions (1) to (5). D ₁ = (2p−1) λ / [4 (N _H2 −N _H1 )] (1) D ₂ = (2q−1) λ / [4 (N _L2 −N _L1 )] (2) D ₁ = s (Λ / 4N _H1 ) (3) D ₂ = t (λ / 4N _L1 ) (4) Λ = D ₁ + D ₂ (5) where p, q, s, and t are positive integers, and N _H1 , N _H2 Is the effective refractive index of the high refractive index portion with respect to the fundamental wave light and the second harmonic wave, and N _L1 and N _L2 are the effective refractive indices of the low refractive index portion with respect to the fundamental wave light and the second harmonic wave, respectively D ₁ and D ₂ Is the length of the light refractive index portion and the low refractive index portion in the waveguide direction of the light, and Λ is the period. 5. A second harmonic generation element, wherein the composite optical waveguide according to claim 1 is provided on the substrate in a stripe shape so as to extend in the light guiding direction. 6. A second harmonic generation element, wherein the composite optical waveguide according to claim 1 is provided in a stripe shape in the substrate so as to extend in the light guiding direction. 7. A second harmonic generation element, wherein the high refractive index portion according to claim 3 is formed of a non-linear material layer, and the low refractive index portion is a space portion. 8. A second harmonic generation element, wherein the nonlinear optical waveguide according to claim 1 is formed of an organic nonlinear optical material. 9. A second harmonic generating element, wherein the nonlinear optical waveguide according to claim 1 is formed of a 2-methyl-4-nitroaniline (MNA) single crystal thin film. 10. The substrate of claim 1 is fused silica, the linear waveguide is a glass layer formed by sputtering, and the nonlinear optical waveguide is 2-methyl-4-nitroaniline (MN).
A) A second harmonic generation element formed by a single crystal thin film. 11. A second harmonic generation device comprising at least a semiconductor laser having an end face having a low reflection surface, and the second harmonic generation element according to claim 1. 12. A semiconductor laser having an end face having a low reflection surface, a collimator lens, a half-wave plate, a condenser lens, and the second harmonic generation element according to claim 1, in this order. A second harmonic generation device characterized in that the semiconductor laser is arranged to oscillate with the feedback light from the second harmonic generation element.