JPH04333833A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To emit short-wavelength light with efficiency even when fundamental wave light slightly varies in wavelength. CONSTITUTION:A nonlinear optical medium 2 consists of plural domains D1-Dn whose pitch is an odd multiple of the coherence length of the short-wavelength light and the respective domains D1-Dn are inverted in polarizing direction basically by turns, but at least one domain Di among the domains D1-Dn has the same polarizing direction with its one adjacent domain Di-1 and is inverted in phase at this domain. The total intensity of the short-wavelength light increases by phase matching up to the domain Di, but decreases behind the domain Di as a border, so that even when the fundamental wave light varies in wavelength, the projection intensity of the short-wavelength light becomes nearly equal eventually regardless of the wavelength variation.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a second harmonic generation element, etc. that generates short-wavelength light having a wavelength shorter than the wavelength of the fundamental wave light by inputting a laser beam as the fundamental wave light into a nonlinear optical medium. This invention relates to a wavelength conversion element.

0002

2. Description of the Related Art A fundamental wave light from a laser light source is incident on a ferroelectric nonlinear optical medium to generate second harmonic light, that is, SHG light, having a wavelength (λ/2) that is half the wavelength λ of the fundamental wave light. A second harmonic generation element is known, and in this case,
As a laser light source, a semiconductor laser, which is suitable for miniaturization, is most often used.

By the way, since the output of a semiconductor laser is lower than that of a gas laser, in order to efficiently generate SHG light, it is necessary to
It is necessary to match the phases of the SHG lights so as not to reduce the overall intensity of the light. That is, in general, in a nonlinear optical medium, the refractive index of the fundamental wave light and the SHG light are different, so the phases of the fundamental wave light and the SHG light are not matched, and as a result, The phases of the SHG lights generated at each point are not matched. FIG. 9 is a diagram schematically showing this situation. When SHG light generated at point A due to fundamental wave light at point A in the nonlinear optical medium 71 reaches point B, for example, fundamental wave light at point B is generated. When the phase of the SHG light propagated from point A and the newly generated SHG light at point B are inverted by 180 degrees, the phase of the SHG light propagated from point A and the SHG light newly generated at point B is 180 degrees.
When these SHG lights are superimposed, their intensities cancel each other out, and the overall intensity of the SHG lights decreases. P1 in FIG. 10 indicates S in the nonlinear optical medium.
It is a diagram schematically showing changes in the intensity of SHG light at positions in the HG light propagation direction, and the overall intensity of SHG light when phase matching is not performed repeats maximum and minimum for each interference distance (coherence length) lc. Since the overall intensity of the SHG light is very small even when it is emitted from the nonlinear optical medium, it is not possible to emit the SHG light efficiently. Therefore, in order to emit SHG light efficiently, the overall intensity of SHG light must be
It is necessary to match the phases in some way so that the phase becomes as shown by the symbol P2.

0004

[Problem to be solved by the invention] However, even if the phases can be matched, the efficiency of SHG light is
There has been a problem in that when the wavelength of the fundamental light changes, it changes significantly accordingly.

[0005] That is, the wavelength of the fundamental wave light from a semiconductor laser changes greatly depending on the temperature, and when the environmental temperature changes by about 20° C., for example, the wavelength changes by about 5 nm. Figure 11 shows that the wavelength of the fundamental wave light from the semiconductor laser is 810.
FIG. 3 is a diagram showing predicted changes in the intensity of SHG light when fundamental wave lights of various wavelengths are made incident on a nonlinear optical medium designed to satisfy the phase matching condition when the wavelength is 5 nm (design wavelength). As can be seen from Fig. 11, the environmental temperature changes by only a few degrees Celsius, and as a result, the wavelength of the fundamental light changes to 8 degrees, which is the design wavelength.
With only a 1nm change from 10nm to 809nm, for example, the SHG light that was efficiently obtained at 810nm becomes 80nm.
9 nm had the disadvantage that it could not be obtained efficiently.

[0006] In order to solve these drawbacks, it is conceivable to strictly control the temperature of the semiconductor laser so that it is always constant using, for example, a temperature control device. This poses a problem in downsizing. Further, even if the temperature is stabilized, if the characteristics of the semiconductor laser itself change over time (for example, if it deteriorates), the wavelength will change, making it impossible to efficiently obtain SHG light.

An object of the present invention is to provide a wavelength conversion element that can efficiently generate short wavelength light even when the wavelength of fundamental light changes somewhat.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the wavelength conversion element of the present invention has a nonlinear optical medium that is ferroelectric and has a pitch that is an odd multiple of the coherence length of short wavelength light. It is composed of a plurality of domains, and the polarization directions of each domain are basically alternately reversed, but at least one of each domain has the same polarization direction as one adjacent domain, It is characterized by being formed so that the phase of the domain is reversed at the domain.

[0009] or at least one of each domain
one has the same polarization direction as one adjacent domain,
The domain is formed so that the phase of the domain is reversed at the domain, and each domain located before the domain has a pitch that is an odd number multiple of a certain coherence length of short wavelength light, and Each of the domains positioned later is characterized in that it has a pitch that is an odd number multiple of the coherence length of short wavelength light, which is different from the coherence length.

Furthermore, the domain having the same polarization direction as one adjacent domain does not significantly reduce the overall intensity of the short wavelength light emitted from the final domain, and the wavelength of the fundamental light is It is characterized by being arranged at a position where the overall intensity of the short wavelength light can be maintained at the same level even if there is some change.

[0011]

[Operation] The wavelength conversion element with the above configuration converts the nonlinear optical medium into a nonlinear optical medium that has multiple domains whose length is an odd number multiple of the coherence length of short wavelength light and whose polarization directions are basically alternately reversed. When the fundamental wave light is incident, the phases of the short wavelength lights generated at each point based on the fundamental wave light are matched, and the overall intensity increases. In this case, although the overall intensity of the short wavelength light can be increased, when the wavelength of the fundamental light changes, the intensity of the short wavelength light also changes significantly. However, in this wavelength conversion element, at least one domain having the same polarization direction as one adjacent domain is formed, and the phase of the domains is reversed from this domain, so that the increased short wavelength light as described above is generated. The intensity decreases across this domain, and eventually, even when the wavelength of the fundamental wave light changes, the output intensity of the short wavelength light becomes approximately the same regardless of the wavelength change.

[0012] Furthermore, each domain located before a domain having the same polarization direction as an adjacent domain has a length that is an odd number multiple of a certain coherence length of short wavelength light, and each domain located after this domain When configuring the domains so that each domain has a length that is an odd number multiple of the coherence length, which is different from the above coherence length, the overall intensity of short wavelength light based on a certain wavelength of the fundamental wave is While increasing and decreasing in the rear domain, the overall intensity based on the fundamental light at a wavelength shifted from the above wavelength can be decreased in the front domain and increased in the rear domain, thereby causing the fundamental light to decrease in the front domain and increase in the rear domain. Even if the wavelength of the light changes, the intensity of the short wavelength light can ultimately be made to be about the same.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a first embodiment of a wavelength conversion element according to the present invention. In the wavelength conversion element of FIG. 1, LiNb
Length L made of ferroelectric nonlinear optical medium such as O3
A waveguide 2 is formed on the substrate 1. This waveguide 2
The fundamental wave light from a semiconductor laser (not shown) is incident on this, and based on this fundamental wave light, short wavelength light (hereinafter referred to as SHG light) having a wavelength shorter than the wavelength of this fundamental wave light is generated. In this first embodiment, the waveguide 2 has, in particular, a domain structure as shown in detail in FIG. Note that in FIG. 2, arrows indicate the polarization direction of the ferroelectric nonlinear optical medium.

Referring to FIG. 2, each domain D1 to Dn formed in the waveguide 2 has a predetermined pitch F, and adjacent domains, for example D1 and D2, basically have polarization directions that are different from each other. However, the polarization direction of at least one of the domains D1 to Dn, for example Di, is not reversed with respect to the polarization direction of the previous domain Di-1,
They have the same polarization direction.

That is, by providing at least one such domain Di, domains D1 to D
The phase of the domains up to i-1 and the domains Di ~ D
The phase of the domains up to n is reversed.

When the waveguide 2 does not have any domain structure, as described above, the SHG light generated based on the fundamental light propagating in the waveguide 2 becomes the fundamental light due to the refractive index difference with the fundamental light. Therefore, the SHG light generated at a certain point A and the SH generated at another point B
The phase of the G light is reversed by 180°, and the overall intensity cancels each other out.As a result, the overall intensity is, as shown by the symbol P1 in Fig. 10, for each coherence length lc given by the following equation. It will repeat the maximum and minimum.

[0017]

[Equation 1] lc=λ/(4・δN) δN=|N1-N2|

In the above equation, λ is the wavelength of the fundamental light, N1 is the refractive index of the fundamental light, and N2 is the refractive index of the SHG light.

In order to prevent such a decrease in the overall intensity of the SHG light, it is necessary to match the phases of the SHG lights generated at each point. , technology that utilizes the birefringence of the waveguide 2,
A technique is known in which a domain structure in which the polarization direction is alternately reversed is formed in the waveguide 2.

[0020] Among these various phase matching techniques, the inventor of the present application has realized the suppression of fluctuations in the efficiency of SHG light even when the wavelength of the fundamental wave light changes due to temperature changes, etc., as described later. However, they found that the most suitable technique was to form a domain structure within the waveguide 2, and decided to use this technique. This technique achieves apparent phase matching by adjusting the domain period, and is known as quasi-phase matching technique (QPM).

For example, as a fundamental wave light, its electric field amplitude EX
is polarized in the X direction, and when this fundamental wave light is incident on a nonlinear optical medium to generate SHG light having an electric field amplitude EZ polarized in the Z direction by the tensor component dZX of the nonlinear optical medium, the electric field of the SHG light is Amplitude EZ
The phasor representation of the minute increment (dEZ/dZ) along the traveling direction Z is expressed by the following equation.

[0022]

[Mathematical 2] dEZ /dZ=-iω(μ0/ε)1/2・[τ・D
] [Ex (Z)] 2 ・exp(i・δk・Z)

[
Further, the electric field amplitude EZ of SHG light is given by the following equation.

[0024]

[Math. 3] EZ (Z) = ∫τ・(dEz /dZ)・d
Z

In Equations 2 and 3, ω is the angular frequency of the fundamental light, μ0 is the magnetic permeability in vacuum, ε is the dielectric constant, D is the nonlinear optical coefficient, and δk is the amount of phase mismatch. In addition, τ represents the sign of the nonlinear optical coefficient, (+1) or (−
Takes the value of 1).

In the quasi-phase matching technique, when the domain is inverted, the sign of the nonlinear optical coefficient D, that is, τ, is inverted, and in this case, in Equation 3, the electric field amplitude EZ of the SHG light is
It takes advantage of the fact that the sign of is reversed.

That is, in the example of FIG. 1, when the waveguide 2 does not have a domain structure, at the coherence length lC, the slight increase in the electric field amplitude of the SHG light (
dEZ /dZ) becomes negative and the electric field amplitude EZ tends to decrease, but if the sign τ is reversed at this time, the electric field amplitude EZ can be made to increase. This is possible by providing a nonlinear optical medium with a domain structure in which the polarization direction is alternately reversed, and in this case, the pitch F of the domains D1 and D2 is set to an odd multiple of the coherence length lC as shown in the following equation. If set, SH
SHG light can be efficiently generated without reducing the overall intensity of G light.

[0028]

[Math. 4] F=(2m+1)・lC=(2m+1)
・λ/(4・δN)

[0029] In Equation 4,
m is an integer and can be regarded as the degree of the domain. Figure 3 shows an example of calculation of the overall intensity of SHG light when each domain D1 to Dn is formed with a pitch F of several 4, and the polarization directions of these domains D1 to Dn are alternately reversed. It is a figure shown for comparison. In FIG. 3, the symbol Q1 indicates the overall intensity of the SHG light when it does not have a domain structure, and the symbols Q2 and Q3 indicate the order m of the domain "0" and "
1", and the symbol Q4 indicates the overall intensity of the SHG light when the perfect phase matching condition is satisfied under the same nonlinear optical coefficient (which cannot be realized). Now, δN is 0.025.
If m is "0", the domain pitch F needs to be 10 times the wavelength λ of the fundamental light, and m
The pitch F of the domain when m is "1" needs to be three times the pitch when m is "0".

By providing the waveguide 2 with a domain structure in this way, it is possible to efficiently generate SHG light, but if the polarization directions of the domains D1 to Dn are simply alternately reversed, the As shown in Figure 11, the overall intensity of the SHG light that is finally emitted varies greatly depending on the wavelength change of the fundamental wave light, and if the wavelength of the fundamental wave light deviates even slightly from the design wavelength, the SHG light cannot be obtained efficiently. That is, when the wavelength of the fundamental light is the design wavelength, the coherence length lC of the SHG light matches the domain pitch F (for simplicity, let m = "0"), and the phase of the SHG light generated at each point is Apparently matched, the SHG lights overlap in a direction that increases their intensity and are efficiently emitted. However, if the wavelength of the fundamental light deviates from the design wavelength, the coherence length lC of the SHG light changes to the domain pitch F. As a result, the phases of the SHG light generated at each point no longer match, resulting in a decrease in efficiency.

[0031] The inventor of the present application has proposed that the SHG at the design wavelength
In order to effectively prevent the change in the efficiency of SHG light that changes the wavelength of the fundamental wave light, even if the maximum efficiency of light is sacrificed to some extent,
, as shown in FIG.
The polarization direction of the domain Di-1 is the same as that of the previous domain Di-1, and the phase of the domains Di-Dn is reversed from that of the domains D1-Di-1.

Specifically, the wavelength conversion elements shown in FIGS. 1 and 2 are manufactured, for example, by the following manufacturing process. That is, the substrate 1 is a ferroelectric nonlinear optical medium, for example, L
A periodic pattern of Ti is formed on the +C plane of the iNbO3 crystal, and after heat treatment for about 1 hour at a temperature slightly lower than the Curie temperature Tc of LiNbO3 (approximately 1035° C.), it is rapidly cooled. In LiNbO3 crystal, the ferroelectric domain structure has a 180° inversion region in the C-axis direction, especially in the +C plane.
Since inversion occurs due to various external factors, a polarization inversion pattern, that is, a domain structure, can be formed to a thickness of about 2 μm from the surface of the substrate 1 by the above-mentioned rapid cooling. Note that the waveguide 2 is created at a low temperature by a proton exchange method, and a mask as shown in FIG. 4 may be used for patterning the polarization inversion at that time. The mask shown in FIG. 4 is a perforated mask assuming a positive resist.
A mask suitable for the type of resist may be used.

Next, the operation of the wavelength conversion element of the first embodiment having such a configuration will be explained. Now, when fundamental wave light from a semiconductor laser is made incident on a nonlinear optical medium having a domain structure as shown in Fig. 5(a) (same as in Fig. 2), this fundamental wave light is Based on this, SHG light is generated.

By the way, in the domain D1 to Di-1, as shown in FIG. 3, the overall intensity (superimposed intensity) of the SHG light increases depending on the wavelength change of the fundamental light. Change. That is, when the wavelength of the fundamental light is the design wavelength λ, the coherence length lC of the SHG light matches the domain pitch F (for simplicity, m
is set to “0”), and apparent phase matching can be achieved, so
The SHG light has the highest overall intensity, but if the wavelength of the fundamental light deviates from the design wavelength λ by δλ, the coherence length lC of the SHG light will not match the domain pitch F, and the phase matching condition will not be satisfied. , the overall intensity of the SHG light is reduced compared to the case of the design wavelength. However, even in this case, when the deviation from the design wavelength is as small as several nanometers, the overall intensity of the SHG light is very large compared to the case where there is no domain structure.

In this way, domains D1 to Di-1
Until then, when the wavelength of the fundamental wavelength changes due to environmental temperature changes, etc., the overall intensity of the SHG light also changes accordingly. However, in the domains Di to Dn, the phase is reversed with respect to the phase of the domains D1 to Di-1, so this domain Di to Dn
The phase of the SHG light generated in domain D1 ~
Phase and 180° of SHG light generated at Di-1
It becomes the reverse. As a result, domains D1 to D
The overall intensity of the SHG light generated at i-1 is
The intensity of the SHG light generated in the domains Di to Dn cancels out and decreases. In particular, when the wavelength of the fundamental light is the design wavelength λ, the domains D1 to Di-1
Since the phase matching was achieved in the domain Di ~ Dn
In this case, phase matching cannot be achieved, and the overall intensity of the SHG light decreases to a greater degree than in the case where the wavelength of the fundamental wave light deviates from the design wavelength. As a result, the domain Di-
1. The overall intensity of the SHG light, which sometimes differs greatly due to the wavelength change of the fundamental light, is in the domain Di ~ Dn.
The difference decreases as the domain Dn progresses, and in the domain Dn, the difference becomes almost the same regardless of the wavelength change of the fundamental wave light.

FIG. 5(b) shows that when the domain structure is as shown in FIG. 5(a), the wavelength of the fundamental light is the design wavelength λ (=810 nm) and the wavelength shifted by ±1 nm from the design wavelength (809 nm). , 811 nm)
It is a figure which shows the calculation result of the total intensity of G light. Figure 5 (
As can be seen from b), if the domain structure is made as shown in FIG.
Even so, the intensity of the SHG light finally emitted from the domain Dn can be made almost the same as when the wavelength of the fundamental wave is 810 nm. In this case, the intensity of the SHG light is in the domain Di-1
Although the intensity is lower than that in the original case, it can be maintained at a higher intensity than in the case without a domain structure, and SHG light can be obtained efficiently. In this way, even if the wavelength of the fundamental light changes somewhat, the SHG light can be efficiently emitted.

The position where the domain Di is provided is determined so that the overall intensity of the SHG light emitted from the final domain Dn is not significantly lowered, and the overall intensity is maintained even if the wavelength of the fundamental wave light changes somewhat. In the example of FIG. 5(b), the length from the domain D1 to the domain Dn, that is, the length L of the waveguide 2, may be relatively "1". ”, and the domain Di is provided at a point “0.84” from the domain D1. In addition, in this case, the waveguide 2
When the length of L is made longer than L and more domains are provided, phase matching begins to be achieved again, and the overall intensity of the SHG light increases again, as shown by the broken line in Fig. 5(b). Since the overall intensity of the SHG light changes according to a change in the wavelength of the fundamental wave light, the length of the waveguide 2 is preferably set to a length L that satisfies the above conditions.

FIG. 6 is a block diagram of a second embodiment of the wavelength conversion element according to the present invention, and FIG. 7 is a diagram showing a domain structure formed in the waveguide of the wavelength conversion element of FIG. 6. Similar to the wavelength conversion elements shown in FIGS. 1 and 2, the wavelength conversion elements shown in FIGS. 6 and 7 are made of a ferroelectric nonlinear optical medium such as LiNbO3, and have a domain structure with a length L waveguide 12.
are formed on the substrate 11, and the polarization directions of the domains E1 to En formed in the waveguide 12 are basically alternately reversed, but at least one of the domains E1 to En is , for example, the polarization direction of Ei is not reversed with respect to the polarization direction of the previous domain Ei-1, but is the same polarization direction.

In the second embodiment, the pitch F1 of the domains E1 to Ei-1 is different from the pitch F2 of the domains Ei to En. That is, the pitch F1 of domains E1 to Ei-1
is adjusted to the coherence length lc(λ) of the SHG light based on the design wavelength λ of the fundamental light, and the domains E1 to Ei-1 are
In this case, the phases of the SHG light generated based on the fundamental wave light of the wavelength λ are matched at each point, while the pitch F2 of the domains Ei to En is shifted by δλ with respect to the wavelength λ. Domain Ei ~ En according to the optical coherence length lc (λ + δλ)
In this case, the phases of the SHG light generated based on the fundamental wave light of wavelength (λ+δλ) are matched at each point. Note that in order to achieve phase matching, the domain pitch only needs to be an odd number multiple of the coherence length of the SHG light, and in the above example, to simplify the explanation, the domain pitch and the coherence length of the SHG light are set to match. I'm letting you do it.

Next, the operation of the second embodiment having such a configuration will be explained. Now, when fundamental wave light from a semiconductor laser is made incident on a nonlinear optical medium having a domain structure as shown in FIG. 8(a) (same as in FIG. 7), this fundamental wave light is Based on this, SHG light is generated.

By the way, in this second embodiment, when the wavelength of the fundamental wave light is λ, the domains E1 to Ei-1
Since the phases of the SHG lights generated at each point in the domain Ei-1 are apparently matched, their overall intensity greatly increases as a result of superposition in the domain Ei-1. However, since phase matching cannot be achieved in domains Ei to En, the overall intensity of SHG light, which was maximum in domain Ei-1, is
decreases in

Further, when the wavelength of the fundamental wave light is (λ+δλ), contrary to the above, the phases of the SHG lights generated at each point in domains E1 to Ei-1 are not matched, so Sometimes its overall strength is low. However, domain Ei
Since phase matching can be achieved in ~En, the domain E
The overall intensity of the SHG light at time i-1 increases in the domains Ei ~ En and the final domain E
n, the overall intensity can be made comparable to that when the wavelength of the fundamental light is λ.

FIG. 8(b) shows the structure of the domain as shown in FIG. 8(a).
S when the wavelength of the fundamental wave light is the design wavelength λ (=810nm) and the wavelength (809nm, 811nm) shifted by ±1 nm from the design wavelength, respectively.
It is a figure which shows the calculation result of the total intensity of HG light. Figure 8
As can be seen from (b), the wavelength of the fundamental wave is the design wavelength (8
Even if the wavelength changes by about 1 nm from 10 nm) to 809 nm or 811 nm, the intensity of the SHG light finally emitted from domain Dn at this time will be approximately the same as when the wavelength of the fundamental light is 810 nm. can do. In this case, the intensity of the SHG light is the domain Di-1 when the wavelength of the fundamental light is the design wavelength.
Although the maximum strength is lower than the maximum strength when the
SHG light can be obtained efficiently. In this way, even if the wavelength of the fundamental wave light changes slightly, the SHG
Light can be emitted efficiently.

Similar to the first embodiment, the position where the domain Ei is provided is such that the overall intensity of the SHG light emitted from the final domain En does not decrease significantly, and
Any position may be used as long as the overall intensity can be maintained at the same level even if the wavelength of the fundamental light changes somewhat.

In each of the above-mentioned examples, LiNbO3 was used as the nonlinear optical medium, but LiNbO3
Instead, it is also possible to use LiTaO3 or KTP. Furthermore, although the wavelength conversion elements of the above embodiments are of a waveguide type in which a waveguide is formed on a substrate, the present invention can be similarly applied to a bulk type.

[0046]

[Effects of the Invention] As explained above, according to the present invention,
The nonlinear optical medium is ferroelectric and consists of multiple domains with a pitch that is an odd multiple of the coherence length of short wavelength light, and the polarization direction of each domain is basically alternately reversed. However, at least one of the domains has the same polarization direction as one adjacent domain, and the domains are formed so that the phase of the domains is reversed at the domain, so that the wavelength of the fundamental light is Even if there is a slight change in the wavelength, short wavelength light can be efficiently generated.

[0047] Furthermore, at least one of the domains has the same polarization direction as one adjacent domain, and is formed so that the phase of the domain is reversed at the domain, and Each domain located at has a pitch that is an odd multiple of a certain coherence length of short wavelength light, and each domain located after this domain has a pitch that is an odd multiple of a coherence length different from the coherence length of short wavelength light. Even when there is a pitch, short wavelength light can be efficiently generated without depending on a slight change in the wavelength of the fundamental wave light.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a first embodiment of a wavelength conversion element according to the present invention.

FIG. 2 is a diagram showing a domain structure formed in a waveguide of the wavelength conversion element in FIG. 1;

FIG. 3 is a diagram showing the overall intensity of SHG light when the polarization directions of domains are alternately reversed.

FIG. 4 is a diagram illustrating an example of a mask used to create the domain structure of FIG. 2;

5A and 5B are diagrams for explaining the overall intensity of SHG light in the wavelength conversion element of FIG. 1. FIG.

FIG. 6 is a configuration diagram of a second embodiment of a wavelength conversion element according to the present invention.

7 is a diagram showing a domain structure formed in the waveguide of the wavelength conversion element in FIG. 6. FIG.

8A and 8B are diagrams for explaining the overall intensity of SHG light in the wavelength conversion element of FIG. 6;

FIG. 9 is a diagram for explaining a phenomenon in which phase matching of SHG light cannot be achieved in a nonlinear optical medium.

FIG. 10 is a diagram for comparing the intensity of SHG light when phase matching is achieved and when phase matching is not achieved.

FIG. 11 is a diagram showing a predicted change in intensity of SHG light when fundamental wave light of an appropriate wavelength is made incident on a nonlinear optical medium that satisfies phase matching conditions.

[Explanation of symbols]

1　　　　Substrate 2 Waveguide 11　　　　Substrate 12 Waveguide D1 ~ Dn Domain E1 ~ En Domain

Claims (3)

[Claims] 1. A wavelength conversion element configured to make fundamental wave light incident on a nonlinear optical medium and generate short wavelength light having a wavelength shorter than the wavelength of the fundamental wave light based on the fundamental wave light in the nonlinear optical medium, The nonlinear optical medium is ferroelectric and consists of multiple domains with a pitch that is an odd multiple of the coherence length of short wavelength light, and the polarization direction of each domain is basically alternately reversed. A wavelength conversion method characterized in that at least one of the domains is formed to have the same polarization direction as one adjacent domain, and the phase of the domains is reversed at the domain. element. 2. A wavelength conversion element configured to make fundamental wave light incident on a nonlinear optical medium, and to generate short wavelength light having a wavelength shorter than the wavelength of the fundamental wave light based on the fundamental wave light in the nonlinear optical medium, The nonlinear optical medium is ferroelectric and is composed of a plurality of domains, and the polarization direction of each domain is basically alternately reversed, but at least one of the domains is
One domain has the same polarization direction as one adjacent domain, and is formed so that the phase of the domain is reversed at this domain, and the domain located before this domain has an odd coherence length with short wavelength light. 1. A wavelength conversion element, wherein a domain located after the domain has a pitch that is an odd number multiple of a coherence length of short wavelength light different from the coherence length. 3. The domain having the same polarization direction as one adjacent domain does not significantly reduce the overall intensity of the short wavelength light emitted from the final domain, and the wavelength of the fundamental light is 3. The wavelength conversion element according to claim 1, wherein the wavelength conversion element is arranged at a position where the overall intensity of the short wavelength light can be maintained at the same level even if there is some change.