JP3033855B2 - Wavelength conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a second harmonic generation element for generating a short-wavelength light having a wavelength shorter than the wavelength of a fundamental wave by making a laser beam as a fundamental wave incident on a nonlinear optical medium. The present invention relates to a wavelength conversion element.

[0002]

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

Since the output of a semiconductor laser is lower than that of a gas laser, in order to generate SHG light efficiently, SHG generated at each point in a nonlinear optical medium is required.
It is necessary to match the phases of the SHG lights so as not to reduce the overall light intensity. That is, in general, the refractive index of the fundamental wave light and the refractive index of the SHG light are different in the nonlinear optical medium, so that the phases of the fundamental wave light and the SHG light are not matched with each other. The phases of the SHG lights generated at each point are not matched. FIG. 9 is a diagram schematically showing this state. When the SHG light generated at the point A by the fundamental wave light at the point A in the nonlinear optical medium 71 reaches the point B, for example, the fundamental wave light at the point B is obtained. Is inverted by 180 °, the phase of the SHG light propagated from the point A and the phase of the SHG light newly generated at the point B are 180 °.
When these SHG lights are superimposed on each other, the intensities cancel each other, and the overall intensity of the SHG lights decreases. The symbol P1 in FIG. 10 represents S in the nonlinear optical medium.
It is a figure which shows roughly the change of the intensity | strength of SHG light in the HG light propagation direction position, and the total intensity | strength of SHG light in the case where phase matching is not performed repeats the local minimum for every interference distance (coherence length) lc. Since the overall intensity of the SHG light is very small even at the time when the light is emitted from the nonlinear optical medium, the SHG light cannot be efficiently emitted. Therefore, in order to emit the SHG light efficiently, the overall intensity of the SHG light must be as shown in FIG.
It is necessary to match the phase in some way so that it is as shown by the symbol P2 to 0.

[0004]

However, even if the phases can be matched, the efficiency of the SHG light is
When the wavelength of the fundamental light changes, there is a problem that the wavelength significantly changes.

That is, the wavelength of the fundamental wave light from the semiconductor laser greatly changes depending on the temperature. For example, when the ambient temperature changes by about 20 ° C., the wavelength changes by about 5 nm. FIG. 11 shows that the wavelength of the fundamental wave light from the semiconductor laser is 81.
FIG. 9 is a diagram showing an expected change in the intensity of SHG light when a fundamental wave light of various wavelengths is incident on a nonlinear optical medium designed to satisfy the condition of phase matching when the wavelength is 0 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 SHG light efficiently obtained at 810 nm changes by only 1 nm from the design wavelength of 810 nm to, for example, 809 nm.
If the thickness is 09 nm, there is a disadvantage that it cannot be obtained efficiently.

In order to solve such a drawback, it is conceivable to strictly control the temperature of the semiconductor laser so that the temperature of the semiconductor laser is always constant, for example, by using a temperature control device or the like. This hinders miniaturization. Further, even if the temperature is stabilized, if the characteristics of the semiconductor laser itself change with time (for example, if the temperature is deteriorated), the wavelength changes, and there is a problem that SHG light cannot be obtained efficiently.

An object of the present invention is to provide a wavelength conversion element capable of efficiently generating short-wavelength light even when the wavelength of fundamental light slightly changes.

[0008]

In order to achieve the above object, a wavelength conversion element according to the present invention is characterized in that the nonlinear optical medium is ferroelectric and has a pitch which is an odd multiple of the coherence length of short wavelength light. The domain is composed of a plurality of domains, and the polarization direction of each domain is basically alternately inverted, but at least one of the domains is adjacent to one of the domains.
One of have the same polarization direction as the domain, that is formed so as to reverse the domain of the phase at the said domain.

Further, each domain located before the domain has a pitch that is an odd multiple of a certain coherence length of the short wavelength light, and each domain located after the domain is different from the coherence length. It is characterized by having a pitch which is an odd multiple of the coherence length of short wavelength light.

Further, 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 reduced. It is characterized in that it is arranged at a position where the overall intensity of the short-wavelength light can be made the same even if it slightly changes.

[0011]

In the wavelength conversion element having the above-described structure, a nonlinear optical medium having a plurality of domains having an odd multiple of the coherence length of the short-wavelength light and having polarization directions basically alternately inverted is provided. When the fundamental wave light is incident, the short-wavelength light generated at each point based on the fundamental wave light has a phase matching, and the overall intensity increases. In this case, although the overall intensity of the short-wavelength light can be increased, if the wavelength of the fundamental light changes, the intensity of the short-wavelength light also changes greatly. 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 domain is reversed from this domain. Of the short-wavelength light becomes the same regardless of the wavelength change even when the wavelength of the fundamental light changes.

Further, each domain located before a domain having the same polarization direction as one adjacent domain has an odd multiple of a certain coherence length of short-wavelength light, and is located behind this domain. When domains are configured so that each domain has an odd multiple of the coherence length different from the above coherence length, the overall intensity of short wavelength light based on a certain wavelength of the fundamental wave is reduced in the front domain. While increasing and decreasing in the rear domain, the overall intensity based on the fundamental light having a wavelength deviated from the above wavelength can be reduced in the front domain and increased in the rear domain, thereby increasing the fundamental light. , The intensity of the short-wavelength light can be finally made substantially the same.

[0013]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of a first embodiment of the wavelength conversion element according to the present invention. In the wavelength conversion element of FIG.
A waveguide 2 having a length L made of a ferroelectric nonlinear optical medium such as O ₃ is formed on a substrate 1. This waveguide 2
Makes a fundamental wave light from a semiconductor laser (not shown) incident thereon, and generates short-wavelength light (hereinafter referred to as SHG light) having a wavelength shorter than the wavelength of the fundamental wave light based on the fundamental wave light. In this first embodiment, the waveguide 2 has, in particular, a domain structure as shown in detail in FIG. In FIG. 2, arrows indicate the polarization direction of the ferroelectric nonlinear optical medium.

Referring to FIG. 2, each domain D _{1 to} D _n formed in the waveguide 2 has a predetermined pitch F,
Adjacent domains together, for example, the D ₁ and D ₂ are basically has assumed that the polarization direction is inverted with respect to each other, at least one domain of the domain D ₁ to D _n,
For example the polarization direction of D _i is the one before the domain D _i-1
Are not inverted with respect to the polarization direction, and have the same polarization direction.

That is, by providing at least one such domain D _i , the domains D _{1 to} D _i−1
, And the phases of the domains D _{i to} D _n are 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 generates a fundamental wave light due to a difference in refractive index from the fundamental light. And the SHG light generated at one point A and the SH generated at another point B
The phase of the G light is inverted by 180 °, and the overall intensities cancel each other. As a result, as shown by a symbol P1 in FIG. 10, the overall intensity is changed for each coherence length lc given by the following equation. The maximum and minimum will be repeated.

[0017]

Lc = λ / (4 · δN) δN = | N ₁ −N ₂ |

In the above equation, λ is the wavelength of the fundamental light, N ₁ is the refractive index of the fundamental light, and N ₂ 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 phase of the SHG light generated at each point. Conventionally, to match the phase of the SHG light, , A technique utilizing the birefringence of the waveguide 2,
A technique of forming a domain structure in which the polarization direction is alternately inverted in the waveguide 2 is known.

The inventor of the present application realizes, among such various phase matching techniques, to suppress the fluctuation of the efficiency of the SHG light even when the wavelength of the fundamental light changes due to a temperature change or the like as described later. However, it has been found that a technique for forming a domain structure in the waveguide 2 is the most suitable technique, and the technique is selected and used. This technique achieves apparent phase matching by adjusting the domain period, and is known as a quasi-phase matching technique (QPM).

[0021] For example, the electric field amplitude E _X as the fundamental wave light
Is polarized in the X direction, and the fundamental wave light is incident on the nonlinear optical medium, and the tensor component d of the nonlinear optical medium is used.
SHG having an electric field amplitude E _Z polarized in the Z direction by _ZX
When generating light, phasor of small increment along the traveling direction Z of the electric field amplitude E _Z of the SHG light (dE _Z / dZ) is expressed by the following equation.

[0022]

[Number 2] _{dE Z / dZ = -iω (μ} 0 / ε) 1/2 · [tau · D] [E _x (Z)] ^{2 · exp (i · δk ·} Z)

Further, the electric field amplitude E _Z of the SHG light is given by the following equation.

[0024]

[Equation 3] E _Z (Z) = ∫τ · (dE _z / dZ) · dZ

In Equations 2 and 3, ω is the angular frequency of the fundamental light, μ ₀ 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. Τ represents the sign of the nonlinear optical coefficient, and is (+1) or (−1)
Take the value of

[0026] In quasi-phase matching techniques, the domains are inverted, the sign of the nonlinear optical coefficient D, i.e. τ is inverted, the number 3 in this case, the SHG light field amplitude E _Z
The sign is inverted.

That is, in the example of FIG. 1, when the waveguide 2 has no domain structure, a small increase (d) in the electric field amplitude of the SHG light at the coherent length l _C.
E _Z / dZ) is negative, but field amplitude E _Z goes to the decreasing direction, by reversing the sign τ in this case, the electric field amplitude E
_Z can be moved in the increasing direction. This means
This can be achieved by providing a domain structure in which the polarization directions are alternately inverted to a nonlinear optical medium. At this time, the pitch F of the domains D ₁ and D ₂ is set to an odd multiple of the coherent length l _C as shown in the following equation. Then, the SHG light can be efficiently generated without reducing the overall intensity of the SHG light.

[0028]

F = (2m + 1) · l _C = (2m + 1) · λ / (4 · δN)

In Equation 4, m is an integer and can be regarded as the order of the domain. 3 forming each domain D ₁ to D _n by the number 4 in the pitch F, the overall intensity of the SHG light when the polarization direction is assumed to have reversed alternately in these domains D ₁ to D _n FIG. 6 is a diagram showing a calculation example of the above for comparison. In FIG. 3, reference numeral Q1 indicates the overall intensity of the SHG light in the case where the domain structure is not provided, and reference numerals Q2 and Q3 indicate the case where the order m of the domain is “0” and “1”, respectively. Indicates the overall intensity of the SHG light, and the symbol Q4 indicates the overall intensity of the SHG light when the perfect phase matching condition is satisfied (unrealizable) under the same nonlinear optical coefficient. ing. Now, assuming that δN is 0.025, the domain pitch F when m is “0” needs to be 10 times the wavelength λ of the fundamental light, and the domain pitch F when m is “1”. The pitch F needs to be three times the pitch when m is "0".

By providing the waveguide 2 with the domain structure as described above, SHG light can be efficiently generated. However, the waveguide D can be generated simply by alternately inverting the polarization directions of the domains D _{1 to} D _n. As shown in FIG. 11, the overall intensity of the SHG light finally emitted from 2 greatly changes according to the change in the wavelength of the fundamental light, and if the wavelength of the fundamental light deviates even slightly from the design wavelength, SHG light cannot be obtained efficiently. That is, when the wavelength of the fundamental light is the design wavelength, the coherence length l _C of the SHG light matches the pitch F of the domain (for simplicity, m =
“0”), the phases of the SHG lights generated at each point are apparently matched, and the respective SHG lights are superposed in a direction of increasing the intensity thereof, and are efficiently emitted. If the wavelength deviates from the design wavelength, the coherence length l _C of the SHG light will not match the pitch F of the domain, the phase of the SHG light generated at each point will not match, and the efficiency will decrease.

The inventor of the present application has found that SHG at the design wavelength
Even if the maximum efficiency of the light is somewhat sacrificed, in order to effectively prevent the change in the efficiency of the SHG light that changes the wavelength of the fundamental light, the light is basically alternately inverted as shown in FIGS. The polarization direction of at least one of the domains D _{1 to} D _n , for example, D _i is the same as that of the preceding domain D _i−1, and the phases of the domains D _{i to} D _n are changed to the domains D ₁ to D _n.
Reversed the phase of _i-1 .

The wavelength conversion element shown in FIGS. 1 and 2 is specifically manufactured by, for example, the following manufacturing process. That is, a periodic pattern of Ti is formed on the + C plane of a ferroelectric nonlinear optical medium, for example, LiNbO ₃ crystal as the substrate 1, and heat-treated at a temperature slightly lower than the Curie temperature Tc of LiNbO ₃ (about 1035 ° C.) for about 1 hour. Then, quench. In the LiNbO ₃ crystal, the ferroelectric domain structure has a 180 ° inversion region in the C-axis direction.
Since inversion is caused by various external factors, the above-described rapid cooling can form a domain inversion pattern, that is, a domain structure with a thickness of about 2 μm from the surface of the substrate 1. The waveguide 2 is formed at a low temperature by a proton exchange method, and the patterning of the polarization inversion at this time may be performed using a mask as shown in FIG. The mask shown in FIG. 4 is a perforated mask assuming a positive resist,
A mask corresponding to the type of the resist may be used.

Next, the operation of the wavelength conversion element of the first embodiment having such a configuration will be described. Now, when a fundamental wave light from a semiconductor laser is incident on a nonlinear optical medium having a domain structure as shown in FIG. 5A (same as FIG. 2), from each point of the nonlinear optical medium, the fundamental wave light The SHG light is generated based on the SHG light.

By the way, in the domain D _{1 to} D _{i -1} , as shown in FIG. 3, the overall intensity of the SHG light (the superimposed intensity) depends on the wavelength change of the fundamental light. Change greatly. That is, when the wavelength of the fundamental light is the design wavelength λ, the coherence length l _C of the SHG light matches the pitch F of the domain (m is set to “0” for simplicity).
), The apparent phase matching can be achieved, so that the overall intensity of the SHG light is the highest. However, if the wavelength of the fundamental light deviates from the design wavelength λ by δλ, the coherence length l _C of the SHG light becomes the pitch of the domain. F does not match, and the condition of phase matching is not satisfied, so that the overall intensity of the SHG light is
It is lower than the case of the design wavelength. However, even in this case, when the deviation from the design wavelength is as small as several nm, the overall intensity of the SHG light is very large as compared with the case where the domain structure is not provided.

As described above, when the wavelength of the fundamental wavelength changes due to a change in the temperature of the environment or the like, the overall intensity of the SHG light also changes between the domains D _{1 to} D _i-1 . However, in domains D _i -D _n ,
Since the phase is inverted with respect to the domain D ₁ ~D _i-1 phase, S occurring in the domain D _i to D _n
The phase of the HG light is 180 ° inverted from the phase of the SHG light generated in the domains D _{1 to} D _i-1 . As a result, SHG generated in domains D _{1 to} D _i-1
Overall intensity of the light is canceled out by the intensity of the SHG light generated in the domain D _i to D _n, decreases. In particular,
When the wavelength of the fundamental light is the design wavelength λ, the partial had well-phase matching in domain D ₁ ~D _i-1, domain D _i
At ~ D _n , phase matching cannot be achieved, and the degree of reduction in the overall intensity of the SHG light is greater than in the case where the wavelength of the fundamental light deviates from the design wavelength. As a result, the overall intensity of the SHG light, which greatly differs due to the wavelength change of the fundamental light at the time of the domain D _i−1 , becomes smaller as it progresses to the domains D _{i to} D _n, and at the time of the domain D _n ,
It is almost the same regardless of the wavelength change of the fundamental light.

FIG. 5B shows the case where the wavelength of the fundamental light is the design wavelength λ (= 810 nm) and the wavelength shifted from the design wavelength by ± 1 nm (809 nm) when the domain structure is as shown in FIG. , 811 nm) is a diagram showing calculation results of the overall intensity of SHG light. As can be seen from FIG. 5B, if the domain structure is as shown in FIG. 5A, even if the wavelength of the fundamental wave changes by about 1 nm from the design wavelength (810 nm) and reaches 809 nm or 811 nm, it can be wavelength of the fundamental wave the intensity of the SHG light is finally emitted to the ones substantially the same as the case of the 810nm from the domain D _n when. In this case, SHG
Although the light intensity is lower than the intensity at the time of the domain D _i−1 , the light intensity can be maintained higher than the case without the domain structure, and the SHG light can be obtained efficiently. In this way, SHG light can be efficiently emitted even when the wavelength of the fundamental light slightly changes.

The position where the domain D _i is provided does not significantly reduce the overall intensity of the SHG light emitted from the final domain D _n , and furthermore, even if the wavelength of the fundamental light slightly changes. the intensity may be any position that can be of the same degree, in the example of FIG. 5 (b), the length of the domain D ₁ until the domain D _n, i.e. the relative length L of the waveguide 2 The case where a domain D _i is provided at a point “0.84” from the domain D ₁ when “1” is specifically set is shown. In this case, if the length of the waveguide 2 is made longer than L and more domains are provided,
As shown by the broken line in FIG. 5B, the overall intensity of the SHG light increases again, and the overall intensity of the SHG light also changes according to the change in the wavelength of the fundamental light, as shown by the broken line in FIG. Therefore, the length of the waveguide 2 is a length L satisfying the above condition.
It is good to set.

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 view showing a domain structure formed in the waveguide of the wavelength conversion element of FIG. The wavelength conversion element shown in FIGS. 6 and 7 is, like the wavelength conversion element shown in FIGS. 1 and 2, made of a ferroelectric nonlinear optical medium such as LiNbO ₃ and has a length L of a waveguide 12 having a domain structure. There are formed on the substrate 11, the polarization direction of the waveguide 12 to form domains E ₁ to E _n are fundamentally as inverted alternately, but out of the domain E ₁ to E _n The polarization direction of at least one domain, for example, E _i is not inverted with respect to the polarization direction of the immediately preceding domain E _i−1 , and has the same polarization direction.

[0039] Incidentally, in this second embodiment, further, the domain E ₁ ~E _i-1 of the pitch F ₁ and Domain E _i
ＥE _{n from} the pitch F ₂ . That, combined pitch F ₁ domain E ₁ ~E _i-1 to the coherence length lc of the SHG light based on the design wavelength of the fundamental wave light lambda (lambda), in the domain E ₁ ~E _i-1, in that each point while SHG light phase generated on the basis of the fundamental wave light of the wavelength λ is so aligned, the pitch F ₂ domains E _i to E _n
Based on the fundamental light shifted from the wavelength λ by δλ.
Suit HG coherence length of the light lc (λ + δλ), in the domain E _i to E _n, in that each point wavelength (lambda +
The phase of the SHG light generated based on the fundamental light of δλ) is matched. In order to achieve phase matching, it is sufficient that the domain pitch is an odd multiple of the coherence length of the SHG light. In the above example, the domain pitch and the coherence length of the SHG light match each other for simplicity. Let me.

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

In the second embodiment, when the wavelength of the fundamental light is λ, the phases of the SHG lights generated at the respective points in the domains E _{1 to} E _i−1 are apparently matched. In the case of the domain E _i−1, the overall intensity greatly increases as a result of the superposition. However, in the domain E _i to E _n does not take a phase matching, the overall intensity of the SHG light was greatest in time domain E _i-1 is reduced in the domain E _i to E _n.

When the wavelength of the fundamental wave light is (λ + δλ), on the contrary, the SHG light generated at each point in the domains E _{1 to} E _i-1 does not have the same phase, so that the domain E _{At i-1,} its overall strength is low. However, since the phase matching take in Domain E _i to E _n, overall intensity of the SHG light in time domain E _i-1, increased in the domain E _i to E _n, in the final domain E _n, The overall intensity can be made similar to the case where the wavelength of the fundamental light is λ.

FIG. 8B shows the domain structure shown in FIG.
When the wavelength of the fundamental light is the design wavelength λ (= 810 nm) and the wavelength (809 nm, 811 nm) shifted from the design wavelength by ± 1 nm, respectively,
It is a figure showing the calculation result of the total intensity of HG light. FIG.
As can be seen from (b), the wavelength of the fundamental wave is the design wavelength (8
10 nm) from the change about 1 nm, 809 nm or even a 811 nm, that wavelength of the fundamental wave light intensity of the SHG light is finally emitted from the domain D _n at this time is approximately the same as the case of 810 nm, Can be In this case, the intensity of the SHG light is lower than the maximum intensity at the time of the domain _Di-1 when the wavelength of the fundamental light is the design wavelength. SHG
Light can be obtained efficiently. In this way, SHG light can be efficiently emitted even when the wavelength of the fundamental light slightly changes.

The provision of the domain E _i positions, as in the first embodiment, SH emitted from the final domain E _n
Any position may be used as long as the overall intensity of the G light is not reduced so much and the overall intensity can be made the same even if the wavelength of the fundamental light slightly changes.

Although LiNbO ₃ is used as the nonlinear optical medium in each of the above embodiments, LiTaO ₃ or KTP may be used instead of LiNbO ₃ . Further, the wavelength conversion element of each of the above-described embodiments is of a waveguide type in which a waveguide is formed on a substrate, but the present invention can be similarly applied to a bulk type.

[0046]

According to the present invention as described above,
The nonlinear optical medium is ferroelectric and is composed of a plurality of domains having a pitch of an odd multiple of the coherence length of short-wavelength light, and the polarization direction of each domain is basically alternately inverted. However, since at least one of the domains has the same polarization direction as one of the adjacent domains and the domains are formed so as to reverse the phase of the domain at the domain, the wavelength of the fundamental light Can be generated efficiently even when the light amount slightly changes.

In addition, at least one of the domains has the same polarization direction as one of the adjacent domains, is formed so as to reverse the phase of the domain at the domain, and is further formed before the domain. Have a pitch that is an odd multiple of a certain coherence length of the short wavelength light, and each domain located after the domain has an odd multiple of a coherence length different from the coherence length of the short wavelength light. Even when there is a pitch, short-wavelength light can be generated efficiently regardless of a slight change in the wavelength of the fundamental light.

[Brief description of the 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 illustrating a domain structure formed in a waveguide of the wavelength conversion element in FIG. 1;

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

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

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

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

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

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

FIG. 9 is a diagram for explaining a phenomenon in which SHG light cannot be phase-matched in a nonlinear optical medium.

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

FIG. 11 is a diagram showing an expected change in the intensity of SHG light when a fundamental wave light having a moderate wavelength is incident on a nonlinear optical medium satisfying the condition of phase matching.

[Explanation of symbols]

1 substrate 2 waveguide 11 substrate 12 waveguide D ₁ to D _n domain E ₁ to E _n domain

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-98227 (JP, A) JP-A-2-63026 (JP, A) Optics Letters, Vol. 16 No. 6 (March 15, 1991) pp. 375-377 IEEE Journal of Quantum Electronics, Vol. 26 No. 7 (July 1990) pp. 1265-1276 Applied Physics Letters, Vol. 56 No. 18 (30 April 1990) pp. 1725-1727 (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/35-1/39 H01S 3/108-3/109

Claims (2)

(57) [Claims] 1. A fundamental wave light is made incident on a nonlinear optical medium,
In the nonlinear optical medium, in a wavelength conversion element adapted to generate a short-wavelength light having a shorter wavelength than the wavelength of the fundamental wave light based on the fundamental wave light, the nonlinear optical medium is ferroelectric, The domain is composed of a plurality of domains, and the polarization direction of each domain is basically alternately reversed.
One domain has the same polarization direction as one adjacent domain and is formed to reverse the phase of the domain at the domain, and the domain located before the domain is an odd number of coherence lengths of short wavelength light. A wavelength conversion element having a double pitch and a domain located after the domain having an odd multiple of a coherence length of short-wavelength light different from the coherence length. 2. 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 reduced. the wavelength conversion element according to claim 1 Symbol mounting characterized by being arranged more or less the overall strength of the short-wavelength light be varied in a position such that can be of the same degree.