JP4114694B2 - Optical wavelength conversion element and short wavelength light generator - Google Patents

Description

  The present invention relates to an optical waveguide and a wavelength conversion element used in the field of optical information processing and applied optical measurement, to which a coherent light source is applied.

  Polarization reversal, which partially inverts the polarization of a single-polarized ferroelectric crystal, enables light wave control such as nonlinear optical effect, electro-optic effect, acousto-optic effect, etc. in a wide range of fields such as communication, optical information processing, measurement, etc. Applied. Among them, application to an optical wavelength conversion element utilizing a nonlinear optical effect has been actively studied because it can realize a small short wavelength light source by wavelength conversion of a semiconductor laser.

In the conventional polarization inversion manufacturing method, polarization inversion is formed by forming electrodes in a ferroelectric and applying a voltage between the electrodes (Journal of the Institute of Electronics, Information and Communication Engineers, Kenji Kindaka et al., CI, vol.J78). -CI, No.5 pp.238-245). An electrode having a periodic structure is formed on the surface of LiNbO ₃ which is a ferroelectric, and a planar electrode is formed on the back surface. A polarization inversion structure is formed by controlling the amount of charge flowing by applying a voltage between the electrodes. A conventional polarization inversion manufacturing method is shown in FIG. The amount of charge necessary for polarization inversion is given by (spontaneous polarization Ps) × (electrode range area) × 2. Further, the spread of the domain-inverted portion is determined by the value of the ratio W / Λ of the electrode period Λ and the electrode width W, and is considered to spread by a value independent of the thickness of the substrate. The formed polarization inversion had a period of 3 μm, and the area of the region where the polarization inversion was formed was about 1 mm ² .

  As for the structure of a conventional optical wavelength conversion element, an element structure in which a periodic polarization inversion structure is formed in a nonlinear optical crystal for phase matching has been reported (for example, IEICE Transactions, Sato Manabu). Et al., C-I, vol. J78-CI, No. 8, pp. 366-372).

The structure of a conventional optical wavelength conversion element is shown in FIG. The optical wavelength conversion element is formed by forming a domain inversion layer with a period of 7.8 μm on a LiTaO ₃ substrate and phase matching with the domain inversion layer to convert the fundamental wave collected in the element to the second harmonic (hereinafter referred to as SHG). )).

Also, the fabrication method is that a comb-shaped electrode is formed on the + Z surface of a LiTaO ₃ substrate, a planar electrode is formed on the -Z surface, and a periodic polarization inversion layer is formed by applying a pulsed high voltage between the electrodes. is doing. Thereafter, the electrodes are removed, and both end faces of the optical waveguide are optically polished to form the incident / exit portions.
IEICE Transactions, Kenji Kintaka et al., CI, vol.J78-CI, No.5 pp.238-245 IEICE Transactions, Sato Manabu et al., CI, vol.J78-CI, No.8, pp.366-372

  The subject about the manufacturing method of polarization inversion is described.

The polarization inversion formed by applying an electric field requires the formation of a domain-inverted structure over a large area in order to improve conversion efficiency and productivity. However, in the conventional method, the area where the periodic polarization inversion is formed is as small as about 1 mm ^2, and when the polarization inversion is formed over a large area, the polarization inversion is not formed, and a large number of non-inversion regions are formed and uniform polarization inversion. There was a problem that the structure was not possible. In addition, only a rough periodic structure with a period of about 3 μm can be formed, and it is difficult to form a domain-inverted structure with a shorter period.

  In addition, since the relationship among the substrate thickness, electrode shape, applied charge amount and the like for uniformly forming the polarization inversion is not clear, there is a problem that it is difficult to form a fine polarization inversion shape.

  In addition, the periodic domain-inverted structure formed by applying an electric field realizes a deep structure with a short period and enables highly efficient wavelength conversion in a bulk type optical wavelength conversion element. However, since a high-voltage electric field is applied, a local refractive index change remains in the crystal, resulting in light propagation loss through the crystal. In order to reduce this, a method of annealing at a high temperature of 400 ° C. or higher has also been reported, but there is a problem in that the heat damage of the crystal increases and the instability of the SHG output increases when the heat treatment is performed at a high temperature.

Next, problems with the optical wavelength conversion element will be described. A conventional optical wavelength conversion element performs wavelength conversion by phase matching by forming a periodic polarization inversion structure in LiTaO ₃ . For this reason, the phase matching is extremely strict with a wavelength tolerance of about 0.1 nm (interaction length: about 10 mm), and changes in the refractive index of the crystal due to environmental temperature changes, generation of electric fields due to pyroelectric effects, optical damage, etc. As a result, the conversion efficiency fluctuates greatly and the output becomes unstable.

In order to solve the above-mentioned problems, the present invention has a c-plate LiTaO ₃ substrate and a periodic domain-inverted layer formed on the substrate, and the period Λ of the domain-inverted layer and the thickness T of the substrate are:
T <Λ / 0.01
This is an optical wavelength conversion element that satisfies the above relationship.

  Further, a crystal having a nonlinear optical effect, a periodic domain-inverted layer formed on the crystal, an incident surface formed on the end surface of the crystal, and an exit surface formed on the other end surface of the crystal, An optical wavelength conversion element having a metal film formed on at least a part of the front surface or the back surface of the crystal.

  A crystal having a nonlinear optical effect; a periodic domain-inverted layer formed on the crystal; an incident surface formed on an end surface of the crystal; and an exit surface formed on the other end surface of the crystal. The periodic domain-inverted layer is divided into two or more regions in a direction parallel to the propagation direction of the fundamental wave incident from the incident surface, and the phases of polarization inversion in the regions are mutually It is a different optical wavelength conversion element.

  A crystal having a nonlinear optical effect; a periodic domain-inverted layer formed on the crystal; an incident surface formed on an end surface of the crystal; and an exit surface formed on the other end surface of the crystal. The periodic domain-inverted layer is divided into two or more regions in a direction parallel to the propagation direction of the fundamental wave incident from the incident surface, and the periods of domain inversion in the regions are mutually It is a different optical wavelength conversion element.

  In addition, there are two or more crystals having a nonlinear optical effect, each of which has a periodic domain-inverted layer formed inside, an incident surface formed on one end surface, and an exit surface formed on another end surface. And the crystal is in optical contact with each other.

  Further, the optical wavelength conversion element described above, a condensing optical system, and a laser are provided, and light emitted from the laser is condensed in the optical wavelength conversion element by the optical system and is collected by the optical wavelength conversion element. This is a short-wavelength light generator that has undergone wavelength conversion.

  As described above, it has been found that the polarization is uniformly formed only when the shape of the domain-inverted portion spreads more than the minimum value (ΔWmin) in the periphery of the electrode when the domain-inversion is formed. Therefore, by producing an electrode shape that takes into account the expansion of polarization reversal (Wmin), a uniform polarization reversal structure can be formed with high accuracy, and its practical effect is great.

  Further, when forming polarization reversal, the amount of charge applied to the electrode is applied more than the value including the minimum enlarged portion of the polarization reversal part, thereby preventing formation of a non-reversal region formed at the time of polarization reversal. Since a domain-inverted structure can be formed, its practical effect is great.

  In addition, when forming a periodic domain-inverted structure, the enlargement of the domain-inverted part prevents the adjacent domain-inverted parts from coming into contact with each other to make it difficult to form the domain-inverted structure. By forming the electrode in consideration of the amount, it becomes possible to form a periodic domain-inverted structure, and its practical effect is great.

  In addition, when forming a periodic domain-inverted structure, the amount of charge applied to the electrode is applied to a value that includes the minimum enlarged portion of the domain-inverted portion, thereby preventing the formation of a non-inverted region that is formed during domain-inversion. Since a uniform domain-inverted structure can be formed, its practical effect is great.

  Moreover, since the optical wavelength conversion element having excellent light damage resistance can be formed by irradiating the substrate with plasma after the polarization inversion formation, the practical effect is great.

  Further, in the process of manufacturing the optical wavelength conversion element, the substrate is annealed at a specific temperature after the polarization inversion process, so that the light propagation loss and the optical damage can be greatly reduced, so that the practical effect is great.

  In addition, in the process of manufacturing the optical wavelength conversion element, applying a DC voltage for 2 seconds or more immediately after the application of an electric field greatly increases the uniformity of the periodic structure of polarization inversion, so that the practical effect is great.

  In addition, since the expansion of the polarization inversion portion has been found to be dependent on the substrate thickness, an element with high conversion efficiency can be configured by adopting an optical wavelength conversion element structure having a polarization inversion period limited to the substrate thickness. The practical effect is great.

  Moreover, the refractive index change by a pyroelectric effect can be reduced by mounting | wearing the surface of a light wavelength conversion element with a metal film. Furthermore, since the thermal conductivity of the crystal can be increased, the temperature control can be speeded up and stabilized, and the output of the optical wavelength conversion element can be stabilized.

  Moreover, by dividing the domain-inverted structure and changing the phase of the domain-inverted period at each divided part, it was possible to enhance the charge canceling effect due to photoexcitation causing photodamage. As a result, a stable optical wavelength conversion element with less optical damage and an output can be realized, and its practical effect is great.

  Moreover, the wavelength tolerance of the optical wavelength conversion element could be improved by dividing the domain-inverted structure and changing the domain-inverted period at each divided part. The structure of the present invention is such that the correlation of the divided portions is uniform over the length direction of the element, and the phase matching characteristics are stable even with respect to refractive index change, with distribution in the length direction such as optical damage. Therefore, an optical wavelength conversion element having a stable output against optical damage can be realized, and its practical effect is great.

  Further, by stacking nonlinear optical crystals having a domain-inverted structure, a thick substrate bulk-type wavelength conversion device can be produced. The periodic polarization reversal that can be formed by applying an electric field has a limited substrate thickness when a short-period polarization reversal structure is realized. Therefore, a thick bulk element can be realized by stacking polarization inversion structures formed on a thin substrate. The manufactured element improves conversion efficiency by increasing the element length and increases the element alignment likelihood. Further, since the optical damage and the pyroelectric effect can be reduced, the practical effect is great.

  Further, by forming two domain-inverted structures with different periods on the same substrate, higher-order harmonics can be generated. A single crystal can generate the third harmonic or the fourth harmonic, and in addition, the alignment of the element is simplified, so that the practical effect is great.

  Moreover, a short wavelength light source is realizable by converting a wavelength of a laser light source with an optical wavelength conversion element. Since a light source having a stable output can be produced by using a light wavelength conversion element having a stable characteristic, its practical effect is great.

The present invention is a method for forming a periodic domain-inverted structure necessary for an optical wavelength conversion element using second harmonic generation. As a specific method for forming domain-inversion, a single-polarized ferroelectric is used. An electrode is formed on a substrate (mainly a LiTaO ₃ substrate here), and a high voltage is applied between the electrodes to form a polarization inversion portion under the electrode. The problem here is that
-When the polarization inversion is formed over a large area, the polarization inversion cannot be uniformly formed under the electrode.

  -The shape of the domain-inverted portion is not the same as the electrode shape under a fine electrode pattern.

When forming a domain-inverted structure with a short period, adjacent domain-inverted parts are stuck together and a periodical structure cannot be formed.
Etc. occurred. Then, the result of having examined the method of solving these problems is described.

(Embodiment 1)
First, we tried to reverse the polarization of LiTaO ₃ by the method shown in the prior art. FIG. 1 shows a polarization inversion method. (a) An electrode pattern (electrode area A) was formed on the + C plane of the LiTaO ₃ substrate of the c plate, and a planar electrode was formed on the (b) -C plane. (c) Polarization was reversed by applying a pulse voltage between the electrodes on the ± C plane. The voltage was the reverse voltage of LiTaO ₃ (about 21 kV / mm), and the amount of charge flowing between the electrodes was controlled by controlling the pulse width. However, when the charge amount 2Ps · A necessary for the polarization inversion calculated from the spontaneous polarization Ps (50 μC / cm ² ) of LiTaO ₃ is applied, the polarization inversion formed under the electrode is as shown in FIG. 2 (a). Many non-inversion parts where the polarization was not inverted were formed. In addition, this tendency becomes more prominent as the substrate becomes thicker.

Therefore, as a result of studying a method of forming polarization inversion uniformly, the inventors have found that there exists a state of polarization inversion that does not form a non-inversion region. Therefore, when the polarization inversion condition in which the non-inversion region is not formed is repeatedly experimented, when the domain inversion part spreads beyond a certain value (ΔWmin) in the peripheral part of the electrode for domain inversion formed on the + C plane, FIG. It became clear that polarization inversion without non-inversion as shown in (b) can be formed. Furthermore, it has been found that the value of ΔWmin depends on the thickness T of the substrate used. This relationship is shown in FIG. ΔWmin and substrate thickness T are based on the experimental results (FIG. 3).
△ Wmin = 0.002 × T-0.2 (μm) (1)
It was found that there is a relationship.

  In addition, the spread of the domain-inverted portion around the electrode occurs approximately isotropically in the C plane. For this reason, in order to form a uniform domain inversion, an electrode shape that takes into account the expansion of the domain inversion part, that is, as shown in FIG. 4, an electrode smaller than ΔWmin is formed from the periphery of the domain inversion shape to be formed, By performing polarization reversal, uniform polarization reversal can be formed with high accuracy.

Only when the polarization inversion is spread over the electrode periphery by ΔWmin or more, a uniform polarization inversion shape without a non-inversion region is formed. The amount can be calculated. If the charge amount Q is 2Ps · A based on the electrode area A and the spontaneous polarization Ps as in the conventional example, a non-inversion region is formed due to insufficient charge amount. In order to prevent this, it is necessary to add an additional charge amount in the area of the spread ΔWmin (FIG. 3) of the inversion part to the peripheral part of the electrode. That is, an extra charge amount equal to or larger than the area obtained by multiplying the outer circumference L of the electrode (the total distance of the peripheral portion of the electrode) by ΔWmin is required. Therefore, the amount of charge Q required to form a uniform polarization inversion is a value greater than the electrode area (A) plus the spread to the electrode periphery (L · ΔWmin),
Q> 2Ps (A + L · ΔWmin) (2)
It became clear that it was given in the form of

(Embodiment 2)
Next, the formation of a periodic domain-inverted structure used for an optical wavelength conversion element that requires a fine domain-inverted shape was studied. The optical wavelength conversion element can convert the wavelength of the light by half by converting the wavelength of the semiconductor laser light. Further, by integrating the semiconductor laser and the light wavelength conversion element, a small short wavelength light source can be realized, and application to many fields such as an optical disk, special measurement, medical use, and biotechnology is possible. Currently, the wavelengths of short-wavelength semiconductor lasers on the market are 800 to 900 nm, near 780 nm, and 630 to 690 nm. The period for each wavelength is Λ = 3 to 4 μm (wavelength: 800 to 900 nm), Λ = 2.8 μm (wavelength: 780 nm), and Λ = 1.5 to 1.8 μm (wavelength: 630 to 680 nm). In order to form such a fine reversal shape, it is necessary to further improve the in-plane uniformity of reversal. We proposed an insulating film loading method as a method of uniformly forming short-period polarization inversion. FIG. 5 shows the manufacturing method. (a) A periodic comb electrode pattern (electrode area A, electrode finger length Ld, width W, period Λ and distance Ls) is formed on the + C surface of the LiTaO ₃ substrate of the c-plate, (b)- A planar electrode was formed on the C surface. (d) The electrode pattern on the + C plane is covered with an insulating film (here, SiO ₂ is deposited to 200 nm). (e) A pulse voltage is applied between the electrodes on the ± C plane to invert the polarization. If an insulating film is not used, the area where polarization inversion is uniformly formed becomes 10 mm ² or less. There was a problem that it was difficult to improve the conversion efficiency. However, by using an insulating film, the region where polarization inversion is formed has been expanded to 30 mm ² or more. However, even in this case, in order to form the polarization inversion uniformly, it is necessary to expand the polarization inversion portion to the peripheral portion of the electrode as shown in (Embodiment 1). That is, in order to uniformly form the polarization inversion, it is necessary to expand the polarization inversion greater than ΔWmin shown in FIG. The periodic polarization reversal structure used for the optical wavelength conversion element needs to control the duty ratio (polarization reversal width W / polarization reversal period Λ) to about 50% as an optimum structure that maximizes the conversion efficiency. Therefore, considering the expansion of the domain-inverted portion to the electrode periphery, the width W of the electrode fingers constituting the periodic electrode is:
W <Λ / 2-2ΔWmin (3)
Must be.

Further, the value of the amount of charge necessary for forming a periodic domain-inverted structure can also be calculated in consideration of the expansion of the domain-inverted part spreading around the electrode periphery. As shown in FIG. 5, as an electrode structure for forming polarization inversion, when using a comb-shaped electrode in which electrode fingers having a length Ld and a width W are arranged over a distance Ls with a period Λ, the area of the electrode is W · Ld · Ls / Λ, the area of the enlarged portion of the polarization inversion portion is represented by 2ΔWmin · Ld · Ls / Λ. Therefore, the amount of charge to form a uniform polarization inversion is
Q> 2Ps · (W + 2ΔWmin) Ld · Ls / Λ (4)
Is required.

  Particularly difficult in the domain inversion formation is the case of forming a domain inversion structure with a short period. As shown in FIG. 3, the spread of the domain-inverted portion is a value of 1 μm or less depending on the thickness of the substrate. For this reason, there is not much problem when forming a domain-inverted shape or a periodic domain-inverted structure of 10 μm or more. However, in the case of forming a short period inversion structure or a fine structure by polarization inversion, the expansion of the polarization inversion becomes a large shape error.

  For example, in order to form a domain-inverted structure with a period of 2.8 μm, it is necessary to control the width of the domain-inverted portion to 1.4 μm in order to form an inverted structure with a duty ratio of 50%. When the thickness of the substrate is 0.2 mm, in order to make the width of the domain-inverted portion 1.4 μm, the width of the electrode must be limited to 1 μm or less in consideration of the width Wmin = 0.2 μm of the domain-inverted portion. There wasn't.

Furthermore, an attempt was made to form an inversion structure with a polarization inversion period of 2 μm or less. If an inversion structure with a polarization inversion period of about 1.7 μm can be formed, the wavelength conversion of a red semiconductor laser with a wavelength of about 680 nm becomes possible, and ultraviolet light with a wavelength of 340 nm can be generated. When ultraviolet light is used, it can be applied in a wide range of fields such as special measurement using fluorescence spectroscopy and laser printers. However, since there is currently no small light source capable of outputting in this wavelength band, the application field has not expanded. However, in order to form a domain-inverted structure with a period of 2 μm, the domain-inverted portion needs to be 1 μm or less. If a 0.5 mm thick substrate is used here, it becomes 1 μm only by expanding the domain-inverted part, and the width of the domain-inverted part becomes 2 μm or more when the electrode width W for domain-inverted and the expanded parts of both sides are combined. A periodic structure cannot be formed. In patterning by a normal photo process, the width of the electrode can only be reduced to about 0.4 μm. Therefore, in order to form a polarization inversion portion having a width of 1 μm, it is necessary to suppress the enlargement of the polarization inversion portion to 0.3 μm or less. Judging from FIG. 3, the polarization inversion cannot be formed unless the thickness of the substrate is 0.2 mm or less. Assuming that the electrode width is 0.4 μm, the relationship between the period of polarization inversion and the thickness of the substrate is calculated from the equations (1) and (3):
T <Λ / 0.01 (5)
It becomes.

  That is, when the domain inversion of the short period structure is formed, the substrate thickness is regulated by the domain inversion period. In particular, in the case of a wavelength conversion element for generating ultraviolet light that requires a domain-inverted layer with a period of 2 μm or less, the substrate thickness T is limited to the domain-inverted period Λ. When an attempt was made to form a domain inversion with a period of 1.7 μm, it was difficult to form an inversion structure with a duty ratio of 50% due to the large lateral expansion of the domain inversion part on a 0.2 mm thick substrate. The inversion structure can be formed when the substrate thickness is 0.17 mm or less, and it was confirmed that when the optical wavelength conversion element is formed, the substrate thickness of the element and the polarization inversion period must satisfy the relationship of formula (4). . When the substrate was 0.17 mm, a uniform periodic structure could be formed from the substrate surface to the vicinity of the center of the substrate, but the periodic structure was disturbed near the back surface of the substrate. When the substrate thickness was 0.15 mm or less, the polarization inversion structure was uniform from the front surface to the back surface, and an optical wavelength conversion element with high conversion efficiency could be formed. When light from a red semiconductor laser with a wavelength of 680 nm is condensed in the optical wavelength conversion element by a condensing optical system and wavelength conversion is performed with a single pass, 30 μW ultraviolet light (wavelength: 340 nm) is obtained with 50 mW input. It was. The conversion efficiency at this time was 1.2% / W. Furthermore, when 99% reflective multilayer films were formed on both end faces of the substrate to form a resonator type, ultraviolet light with an output of 3 mW was obtained. By satisfying the condition of the formula (5), it is possible to configure an optical wavelength conversion element having a short period polarization inversion structure, and thus it is possible to realize an optical wavelength conversion element for generating ultraviolet light.

(Embodiment 3)
Here, a method for manufacturing an optical wavelength conversion element having excellent light damage resistance will be described. When high-power SHG light is generated from blue light having a wavelength of about 400 nm to ultraviolet light, optical damage is a problem. For example, when generating SHG light with a wavelength of 430 nm, the beam shape of the SHG output becomes distorted when the output is about 1 mW or more. This is because the refractive index of the crystal partially changes due to optical damage and affects the beam shape of the SHG light. At shorter wavelengths, similar photodamage was observed for even lower SHG power. It was considered that the charge accumulated in the substrate when the polarization was reversed by applying a high-voltage electric field was the cause of the optical damage. Therefore, a method using plasma treatment was tried as a method of releasing the accumulated charges in the substrate. Plasma was generated in an atmosphere of Ar and oxygen, and the substrate was irradiated with plasma. When the plasma was irradiated for about 20 minutes, the accumulated charge decreased and the light damage resistance was about 1.5 times. Furthermore, in order to increase the light damage resistance, plasma was irradiated while heating the substrate. At 100 ° C, there was not much difference from the effect at room temperature. At 100 ° C. or higher, the light damage resistance gradually increased. The maximum was about 250 ° C., and the damage resistance against light was 5 times that when no plasma was irradiated. When the substrate temperature exceeded 300 ° C, the SHG conversion efficiency decreased, and the deterioration of the characteristics of the optical wavelength conversion element was observed. This is considered because high temperature plasma irradiation has some influence on the domain-inverted structure.

(Embodiment 4)
Here, a method for manufacturing a bulk SHG element having excellent light damage resistance and low waveguide loss will be described.

Periodic polarization inversion required for the optical wavelength conversion element follows the manufacturing method of FIG. What is important in the process of forming domain inversion by applying an electric field is an applied voltage waveform. In order to form the domain-inverted structure of a short period LiTaO _3, it is necessary to apply a higher voltage (approximately 21 kV / mm in LiTaO ₃ with voltage polarization is reversed) inversion voltage as a pulse voltage waveform. However, when the applied voltage was instantaneously returned to 0 after applying the pulse voltage, a phenomenon was observed in which the inverted polarization caused reinversion and the polarization inversion layer disappeared. Thus, it was found that when the CW voltage was applied for a while after applying the pulse voltage, the inverted polarization was stabilized and re-inversion could be prevented.

However, it has been clarified that the uniformity of the domain-inverted structure in which the application time of the CW voltage is formed is greatly affected. For example, after 4.2 kV was applied as a pulse voltage to LiTaO ₃ having a substrate thickness of 0.2 mm, a CW voltage was applied at 3 kV, and the relationship between the CW voltage application time and the uniformity of the polarization inversion period was measured. It has been found that the non-uniformity of the periodic structure of the polarization inversion formed increases. It is clear that the Tb time is 3 seconds or more to obtain the uniformity of the periodic structure, and a highly uniform inversion layer of 5 seconds or more is formed, so that an optical wavelength conversion element with high efficiency can be manufactured. became.

  The CW voltage was preferably between 20 and 10 kV / mm in the applied electric field. When an electric field of 20 kV / mm or more is applied, the polarization inversion further progresses, causing a problem that the polarization inversion structure deviates from the optimum shape. Further, an electric field of 10 kV / mm or less did not contribute to the homogenization of the domain-inverted structure.

  On the other hand, it was found that the polarization inversion layer formed by applying an electric field has a periodic refractive index change in the crystal, so that there is a light propagation loss, and the characteristics of the optical wavelength conversion element deteriorate. Therefore, it was found that, when annealing was performed at about 400 ° C. for several minutes, propagation loss was reduced, but optical damage increased in the optical wavelength conversion element. When a fundamental wave having a wavelength of 860 nm is incident on an optical wavelength conversion element having an element length of 10 mm and SHG having a wavelength of 430 nm is generated, a phenomenon in which the output becomes unstable due to optical damage when SHG of several hundred μW or more is emitted. Observed. As a result of various investigations on the annealing temperature, it was found that there is a correlation between the annealing temperature and photodamage. The measurement results of the relationship between annealing temperature, crystal propagation loss and optical damage are shown below.

  It has been found that the propagation loss can be improved by annealing at a low temperature of about 150 ° C. In addition, it was observed that photodamage tends to increase above 350 ° C. Therefore, it was found that heat treatment must be performed at a temperature of 150 ° C. to 350 ° C. in order to form an element with small propagation loss and small optical damage. In particular, in order to form a light wavelength conversion element having a high light damage strength, it was desirable to perform heat treatment at about 180 ° C. to 280 ° C. When SHG generation at a wavelength of 430 nm was performed by an optical wavelength conversion element heat-treated in this temperature range, no output fluctuation due to optical damage was observed even at an SHG output of 10 mW or more, and an element capable of generating high output SHG, which was difficult in the past Was made.

  As a usage method in which the optical wavelength conversion element formed according to the present embodiment is particularly effective, there is a resonator type optical wavelength conversion element. By inserting an optical wavelength conversion element in the resonator, a fundamental wave having a high power density can be used, and conversion efficiency can be greatly improved. However, if there is a loss of light that passes through the optical wavelength conversion element, the resonator characteristics deteriorate, and the power density of light in the resonator does not increase. It is necessary to suppress the propagation loss of light to several percent or less. The optical wavelength conversion element manufactured by the method of the present embodiment has a propagation loss of 3% or less, and the conversion efficiency can be greatly improved by inserting it into the resonator.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) NbxO ₃ (0 ≦ x ≦ 1) A similar device can be fabricated using a substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

Next, the bulk type optical wavelength conversion element of the present invention for the purpose of stabilizing the output will be described. As a specific element structure,
A structure in which a metal film is deposited on the surface of the light wavelength conversion element to achieve temperature uniformity of the element and prevent a pyroelectric effect.

  A structure that realizes reduction of optical damage and expansion of tolerance of optical wavelength conversion elements by changing the phase distribution of the polarization inversion period.

  A structure that realizes an increase in the tolerance of the optical wavelength conversion element by changing the distribution of the period of polarization inversion.

A structure that prevents the pyroelectric effect by increasing the efficiency of the element by bonding crystals having a domain-inverted structure.
It is. In the following embodiments, characteristics of each optical wavelength conversion element will be described.

(Embodiment 5)
Here, the result of reducing the pyroelectric effect by adding a metal film to the bulk type optical wavelength conversion element will be described.

The structure of the optical wavelength conversion element will be described with reference to FIG. As shown in FIG. 6, a periodic domain-inverted layer 4 is formed in the LiTaO ₃ crystal 1 (plane perpendicular to the C-axis of the crystal) of the C plate. Further, the surface is covered with an Al film 5. The end face of the crystal 1 is optically polished, and the fundamental wave 6 having a wavelength of 860 nm incident from the incident end is converted into a second harmonic (SHG) 7 having a wavelength of 430 nm in the crystal.

  In the conventional bulk type light wavelength conversion element, when the temperature is changed, charges are accumulated on the surface due to the pyroelectric effect, and the SHG output is changed due to the change in the refractive index caused by this. Therefore, a metal film was formed on the substrate surface to cancel the generated pyroelectric charge. As a result, the surface charge generated on the substrate surface can be eliminated, and a stable output was obtained with no change in output due to the pyroelectric effect observed at a temperature change of 0 to 60 ° C.

In addition, by forming a metal film on the surface, the crystal temperature can be easily controlled. Since the wavelength tolerance of the optical wavelength conversion element is narrow, the phase matching condition changes due to the change in the refractive index of the crystal due to the temperature change, and the output decreases. In order to prevent this, it is necessary to control the temperature of the optical wavelength conversion element. However, since the LiTaO ₃ crystal has low thermal conductivity, there is a problem that it is difficult to keep the crystal temperature uniform over the element length of 10 mm. However, covering the crystal with a metal film increases the thermal conductivity of the optical wavelength conversion element, and makes it easy to equalize the temperature over the entire length of the element. In addition, temperature control can be performed at high speed, and a stable output can be obtained because a pyroelectric effect does not occur even when the temperature changes suddenly.

  An experiment was also conducted in which a current was passed through the metal film and the metal film was used as a heater. Ti was deposited as a metal film with a thickness of 30 nm, processed into a stripe shape, and an electric current was passed through this to use it as a thin film heater. The temperature of the crystal was controlled at 50 ° C. with a heater, and it was confirmed that the output fluctuation of SHG could be reduced to ± 5% or less with respect to the change in atmospheric temperature from 0 to 50 ° C. By using the metal film as a heater, the pyroelectric effect of the crystal was reduced and at the same time the crystal temperature was stabilized.

  In addition, by adding a metal film to the crystal surface, it is possible to prevent deterioration of characteristics due to crystal contamination. The pyroelectric charge due to the change in the atmospheric temperature adsorbs dust in the crystal atmosphere and, when used for a long time, a lot of dust adheres to the crystal surface and the optical wavelength conversion element characteristics deteriorate. Such dust adhesion could be prevented by covering the crystal surface with a metal film.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) NbxO ₃ (0 ≦ x ≦ 1) A similar device can be fabricated using a substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 6)
Here, the result of improving the light damage resistance by changing the periodic structure of polarization inversion will be described.

  The electric charge generated by photoexcitation moves in the polarization direction of the crystal, and the electric charge distribution is biased in the crystal. The electric field generated thereby causes a refractive index change via the electro-optic effect, resulting in optical damage. Since the movement of charges moves along the polarization direction of the crystal, the movement of charges is reversed between the polarization inversion layer and the non-inversion layer portion in the crystal. Therefore, an electric field generated by photoexcitation can be canceled by forming a short period domain-inverted structure. Here, in order to further enhance the effect of canceling the electric field due to photoexcitation, the periodic domain-inverted structure is divided in a direction parallel to the light propagation direction, and the phase of domain-inverted phases in each part is shifted from each other. . As shown in FIG. 7A, the phase is divided into A, B, C, D, and E regions parallel to the propagation direction, and the phase of the polarization inversion period between the regions is different from each other as shown in FIG. 7B. It is formed as follows. By forming a phase difference in the domain-inverted structure, the electric field was canceled by photoexcitation between the regions, and optical damage could be reduced.

  Further, the wavelength tolerance of the optical wavelength conversion element can be increased by adjusting the phase difference. It has been reported that the wavelength tolerance of the optical wavelength conversion element is expanded by dividing the domain-inverted structure into several segments in the propagation direction and controlling the phase of each segment (described in the electronics letter, MLBortz, M Fujimura, and MMFejer, Electronics Letters, vol.30, pp.34-35, 1994). However, when the inversion structure is divided in the propagation direction, when the distribution of optical damage is formed in the length direction (SHG increases in square with respect to the propagation distance, the distribution of optical damage is formed). As a result, the effect of expanding the wavelength tolerance does not appear, and the output of SHG is reduced. Therefore, in the configuration of the present invention, the polarization inversion structure is divided in parallel with the propagation direction. In the configuration of the present invention, even when a refractive index distribution occurs in the propagation direction, the mutual relationship between the segments is always maintained, so that the SHG output can be stabilized.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) NbxO ₃ (0 ≦ x ≦ 1) A similar device can be fabricated using a substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 7)
Here, the expansion of the wavelength tolerance of the optical wavelength conversion element by changing the polarization inversion structure will be described.

  An optical wavelength conversion element using a periodic domain-inverted structure is capable of highly efficient wavelength conversion, but on the other hand, because the phase matching wavelength tolerance is narrow, the output is greatly reduced due to wavelength fluctuation of the fundamental wave to be excited. There is a problem. Therefore, it is necessary to stabilize the output by expanding the phase matching wavelength.

  In this embodiment, as shown in FIG. 8 (a), the domain-inverted structure is divided into two parts A and B parallel to the light traveling direction, and the domain-inverted periods Λ1 and Λ2 are formed differently. For this reason, the fundamental wavelength dependency of the SHG output is slightly shifted between the segment A and the segment B as shown in FIG. 8B, and the wavelength dependency of A and B is added to the entire optical wavelength conversion element. The wavelength tolerance increases.

  As a conventional configuration, there is a method of dividing the domain-inverted structure in the propagation direction and changing the periodic structure in each segment to expand the tolerance of the phase matching wavelength by the interaction between the segments. However, in the conventional configuration, when a refractive index distribution due to optical damage occurs in the propagation direction, there is a problem that the correlation between the segments changes and the phase matching tolerance does not always increase.

  On the other hand, when this configuration is used, a uniform periodic structure is formed in the light propagation direction. Therefore, even when a refractive index distribution due to light damage occurs in the propagation direction, the interrelationship between the segments does not change. Therefore, stable output characteristics can be obtained even with respect to a change in phase matching wavelength due to optical damage.

  Furthermore, the fundamental wave decreases as it propagates due to waveguide loss and power transfer from the fundamental wave to SHG. The element design is facilitated because the correlation with the domain-inverted structure does not change even when the fundamental wave power changes.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) NbxO ₃ (0 ≦ x ≦ 1) A similar device can be fabricated using a substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 8)
Here, the results of improving the conversion efficiency and stabilizing the output by stacking the domain-inverted structures will be described.

A deep domain inversion layer can be formed by applying an electric field to LiTaO ₃ and LiNbO ₃ crystals. For example, it has been reported that a domain-inverted layer having a period of 3 to 4 μm is formed on these crystals over a thickness of 200 μm. However, there is a limit to the thickness of the substrate that can form such a short-period polarization inversion uniformly in the depth direction. For example, the currently reported thickness is about 150 to 200 μm, and if it exceeds 300 μm, the non-uniformity of inversion increases. Therefore, the thickness of the crystal forming the bulk type light wavelength conversion element is limited to about 200 μm.

  When such an optical wavelength conversion element is used as a bulk type, several problems arise. First, the beam diameter of the fundamental wave passing through the crystal is restricted by the thickness of the crystal. This is because when the beam diameter is larger than the thickness of the crystal, the beam is distorted and the wavefront characteristics of the obtained SHG output are deteriorated, and sufficient condensing cannot be obtained. When the beam diameter is limited, the element length is limited. For example, when the thickness is about 200 μm, the element length is about 10 mm. Second, since the area where the fundamental wave is incident is small, fine alignment is required for alignment of the optical system. As a method for solving these problems, the present invention has found a method for increasing the thickness of the substrate by optically contacting the polarization-reversed substrate (optical contact).

As shown in FIG. 9, the structure of the optical wavelength conversion element is formed by bonding LiTaO ₃ substrates 8 and 9 on which a periodic domain-inverted structure 4 is formed. When a plurality of substrates are bonded together, the thickness can be further increased.

Next, the result of having made it possible to increase the conversion efficiency by the configuration of the present embodiment will be shown. LiTaO ₃ crystals form a periodic domain-inverted structure from the + C plane. Therefore, the uniformity of the domain-inverted structure is the best on the + C plane and degrades as it approaches the -C plane. Therefore, as shown in FIG. 9, a configuration is adopted in which the + C surfaces are brought into contact with each other. Propagating the fundamental wave around the contact portion of the substrate to another configuration (for example, when using one substrate, when contacting the + C surface and the −C surface, or when contacting the −C surfaces) In comparison, 1.5 to 2 times higher efficiency can be achieved. Furthermore, the element length can be increased twice as compared with the case of using a conventional single substrate, and the conversion efficiency can be further increased twice.

Next, when bonding a board | substrate with an adhesive agent, the material whose refractive index is higher than a board | substrate was used for the adhesive agent. The adhesive is a material that is transparent to the fundamental and harmonics. For example, a TiO ₂ sol-gel solution was used as the material. The substrate could be adhered by pouring TiO ₂ sol-gel between the substrates and sintering at about 500 ° C. When an adhesive material having a high refractive index is used, an optical waveguide having a symmetrical structure in which a high refractive index portion is sandwiched between substrates can be formed. Since the fundamental wave becomes a waveguide mode and propagates around the adhesive portion, the power density of light can be increased. Further, since the propagation distance can be increased, the interaction length is increased and the conversion efficiency is greatly improved.

  Next, it is shown that output instability due to optical damage and pyroelectric effect can be reduced. In the configuration of the optical wavelength conversion element of FIG. 9, the polarization inversion layer of the substrate 8 and the non-inversion layer of the substrate 9 overlap. However, if the inversion layers of the two substrates overlap each other by shifting the phase, the polarization of the crystal The direction will conflict between the two substrates. As a result, charges generated by optical damage and charges generated by the pyroelectric effect are canceled out because positive and negative charges are generated at the bonded portion of the crystal, and the refractive index change due to the electric field does not occur in the crystal. That is, the refractive index fluctuation due to optical damage and pyroelectric effect did not occur, and a stable SHG output was obtained.

  Next, a description will be given of a shift in the period of the polarization inversion structure when the crystal is formed in a laminated structure. When an optical wavelength conversion element is configured by stacking crystals with a periodic polarization reversal structure, the phase matching of the light passing through each of them must be equal unless the period of polarization reversal with respect to the traveling direction of the fundamental wave passing through the crystal is substantially equal. High-efficiency wavelength conversion cannot be performed under different conditions. Therefore, the deviation of the domain-inverted structure that satisfies the phase matching condition was examined. For high efficiency, it is desirable that the deviation between the polarization inversion structures is 0, but it is difficult to actually eliminate the deviation. It was found that when calculating the deviation of the polarization inversion period that does not decrease the efficiency, Λav / L> ΔΛn (n = 1, 2, 3,...).

Where Λav is the average value of the period of polarization inversion, ΔΛn (n = 1, 2, 3,...) Is the absolute value of the difference between the period of each polarization inversion layer and Vav, and L is the interaction length. For example, when two crystals are stacked as shown in FIG. 9, Λav = (Λ1 + Λ2) / 2, and ΔΛ1 = | Λ1−Λav | and ΔΛ2 = | Λ2−Λav |. When the period is 3.6 μm and the action length is 10 mm, the deviation of the polarization inversion period needs to be 3.6 × 10 ⁻⁴ μm or less.

  In the case of stacking elements having a domain-inverted structure in practice, the substrate was adjusted with a fine moving table while the fundamental wave was incident on the crystal, and the substrate was bonded after adjusting the conversion efficiency to be maximum. The substrates are bonded together by heating in a state where the substrates are in optical contact. When using an adhesive, the wavefront aberration of SHG light could be reduced by using an adhesive having a refractive index close to that of the substrate. In addition, by fixing the substrates in an optical contact state without using an adhesive, it is possible to eliminate almost the wavefront aberration of SHG light, and it is possible to realize a configuration of an optical wavelength conversion element with excellent light collecting characteristics. It was.

  There is also a method of positively utilizing the difference in polarization inversion period of the stacked substrates. As shown in the third embodiment, it is also possible to increase the tolerance of the phase matching wavelength by using the polarization inversion layers having different periods next to each other and passing the fundamental wave therethrough. For example, a structure in which different inversion layers having different polarization inversion periodic structures are stacked by providing a difference between the substrates in the period with respect to the traveling direction of light by overlapping and slightly rotating substrates using the same period. Realized. With this method, the wavelength tolerance of phase matching can be expanded. By expanding the phase matching wavelength tolerance, a stable output can be obtained even with respect to the wavelength fluctuation of the fundamental wave, which is effective.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) NbxO ₃ (0 ≦ x ≦ 1) A similar device can be fabricated using a substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 9)
Here, a bulk type optical wavelength conversion element for generating a third or fourth harmonic using a periodically poled structure will be described.

  In the above-described embodiment, the optical wavelength conversion element using the bulk type second harmonic generation using the periodic domain-inverted structure has been described. When periodic polarization reversal is used, higher order third harmonics and fourth harmonics can be generated. In the present embodiment, an element capable of generating higher-order harmonics using a single element will be described.

  Conventionally, the third harmonic generation and the fourth harmonic generation using the nonlinear optical effect are performed by using the nonlinear optical crystal to generate the second harmonic. Furthermore, the third or fourth harmonic generation using the second harmonic was performed using another nonlinear optical crystal. In these optical systems, a plurality of nonlinear optical crystals are required, and complicated adjustment of the optical system is required.

Therefore, in the present embodiment, the configuration of the optical wavelength conversion element shown in FIG. In FIG. 10A, the LiTaO ₃ substrate 1 is divided into two segments A and B. The segment A converts the fundamental wave 6 into the second harmonic, and the segment B has a fundamental wave that has passed through the segment A. A third harmonic 16 is generated by the second harmonic. The period of each polarization inversion is segment A,
Λ1 = λ / 2 / (N2-N1)
In segment B,
Λ2 = λ / (3N3-N1-2N2)
It has become. Where λ is the wavelength of the fundamental wave, N1 is the refractive index of the crystal for light of wavelength λ, N2 is the refractive index of the crystal for light of wavelength λ / 2, and N3 is the refraction of the crystal for light of wavelength λ / 3. Rate.

  By forming different domain-inverted structures in one nonlinear material, third harmonic generation can be achieved with a single crystal. Furthermore, since the domain-inverted structure is formed on the substrate, there is an advantage that a complicated output adjustment is unnecessary and a stable output can be obtained. This structure is effective in that short wavelength light can be generated with a simple configuration and a stable output can be obtained. For example, by using a YAG laser with a wavelength of 1.06 μm as a light source and converting the wavelength of the laser light, ultraviolet light of 0.35 μm was generated.

It is possible to generate higher-order fourth harmonics with the same configuration. In this case, the second harmonic is generated in the segment A, and the fourth harmonic is generated in the segment B from the second harmonic. At this time, the polarization inversion period is segment A,
Λ1 = λ / 2 (N2-N1)
And in segment B,
Λ2 = λ / 4 (N4−N2)
It is. Where λ is the wavelength of the fundamental wave, N1 is the refractive index of the crystal for light of wavelength λ, N2 is the refractive index of the crystal for light of wavelength λ / 2, and N4 is the refraction of the crystal for light of wavelength λ / 4. Rate.

  Next, FIG. 10B also shows an optical wavelength conversion element configuration to which an adjustment mechanism is added when a phase matching shift of the optical wavelength conversion element occurs due to a temperature change or the like. When the refractive index changes due to the temperature change of the crystal, the phase matching condition changes and the harmonic output decreases. At this time, the generation of the second harmonic can be adjusted to the maximum in the segment A by slightly rotating the optical wavelength conversion element to modulate the polarization inversion period with respect to the traveling direction of the light. However, since the phase matching condition in segment B is also shifted, the third harmonic phase matching condition may not be satisfied. In order to adjust this, the segment B is formed so that the polarization inversion period slightly differs depending on the position of the element. If the position of the optical wavelength conversion element is adjusted to the left and right, the phase matching state in segment B can be adjusted optimally, and the third harmonic can be extracted efficiently.

  If this configuration is used, it can be applied to parametric oscillation or the like, and a segment can be further added to generate higher-order harmonics.

(Embodiment 10)
Here, a short wavelength light source using the optical wavelength conversion element of the above-described embodiment will be described.

  A short wavelength light source can be configured using a laser light source and an optical wavelength conversion element. FIG. 12 shows a short wavelength light source of this embodiment. The fundamental wave 6 emitted from the laser 12 is wavelength-converted by the optical wavelength conversion element 14 and emitted as SHG7. For example, when a semiconductor laser with a wavelength of 800 nm is used, blue SHG light with a wavelength of 400 nm can be obtained, and a compact blue light source can be realized. The phase matching condition was achieved by adjusting the effective polarization inversion period by rotating the angle of the substrate with respect to the optical axis. It is easy to adjust the angle of phase matching alignment, and a stable light source can be realized by stabilizing the temperature.

  A stable small short wavelength light source can be applied in a wide range of fields such as high-density optical recording, color laser printers, medical use, and biotechnology. By using a red semiconductor laser having a wavelength of 680 nm as a fundamental wave, it is possible to generate ultraviolet light having a wavelength of 340 nm, and a small ultraviolet light source that is difficult to manufacture can be realized. Application to biotechnology, fluorescence lifetime measurement, special measurement, etc. becomes possible. Further, when the laser is pulse-driven, a fundamental wave having a high peak power can be obtained, so that highly efficient wavelength conversion is possible. For example, even a semiconductor laser having a maximum output of about 40 mW in CW drive can generate a high peak power of several hundreds mW by pulse driving, and an SHG output of several tens mW can be obtained. Impurity detection and the like can be performed by applying SHG light having high peak power to fluorescence lifetime measurement and the like. In addition, by driving the semiconductor laser with high frequency RF, pulse train oscillation with high peak power is possible, and the conversion efficiency can be improved by 5 times or more compared with the semiconductor laser driven by CW with average power. Excellent characteristics as a high output compact light source.

  When high output SHG light is generated, output instability due to optical damage becomes a problem. Even in the element shown in this embodiment, when an SHG output exceeding 10 mW is generated, instability of the output may be observed. In order to solve this, as shown in FIG. 11, the optical wavelength conversion element is fixed to the fine movement table 15 and the position of the optical wavelength conversion element with respect to the fundamental wave is changed. Since optical damage occurs at a relatively slow speed (several seconds or more), the power of the light irradiated in the crystal is dispersed by changing the position of the optical wavelength conversion element at a speed of several tens of Hz or more. As a result, the power density of light can be reduced. By this method, the strength of light damage resistance was improved more than twice, and stable generation of high output SHG became possible.

  On the other hand, a decrease in the conversion efficiency of the optical wavelength conversion element due to changes in the phase matching condition due to temperature was observed. This occurred because the refractive index of the substrate crystal changed with temperature and the phase matching condition shifted. Therefore, as shown in FIG. 12, the optical wavelength conversion element was controlled by the rotary fine movement table 15. An optical wavelength conversion element having a periodic domain-inverted layer can vary the substantial domain-inverted period for light by tilting the element with respect to the optical axis. A stable SHG can be generated by adjusting this rotary fine movement table to always optimally adjust the phase matching condition of the optical wavelength conversion element.

  The wavelength conversion element according to the present invention is useful as an optical wavelength conversion element having high conversion efficiency and stable output.

(A)-(c) Production process perspective view showing method for producing polarization inversion (A) Surface view of polarization inversion with non-inversion region formed (b) Surface view of uniform polarization inversion Characteristic factor diagram showing the relationship between the expansion (Wmin) distance of the domain-inverted part and the substrate thickness Surface view showing the positional relationship between the electrode and the domain-inverted part (A)-(d) The manufacturing process perspective view of the optical wavelength conversion element of this invention Configuration perspective view of optical wavelength conversion element of the present invention (A) Configuration perspective view of optical wavelength conversion element of the present invention (b) Characteristic factor diagram showing phase relationship of polarization inversion structure in each region (A) Configuration perspective view of optical wavelength conversion element of the present invention (b) Characteristic factor diagram showing phase matching characteristics in each region Configuration perspective view of optical wavelength conversion element of the present invention (A) Configuration perspective view of optical wavelength conversion element for high-order harmonic generation (b) Configuration perspective view of optical wavelength conversion element with phase matching adjustment mechanism Configuration perspective view of the short wavelength light source of the present invention Configuration perspective view of the short wavelength light source of the present invention Fabrication perspective view showing a conventional method of manufacturing polarization inversion Configuration perspective view of conventional optical wavelength conversion element

Explanation of symbols

1 C-plate LiTaO ₃ substrate 4 Polarization inversion layer 5 Al film 6 Basic light 7 SHG
8 First LiTaO ₃ substrate 9 Second LiTaO ₃ substrate 11 SiO ₂
DESCRIPTION OF SYMBOLS 12 Laser 13 Condensing optical system 14 Optical wavelength conversion element 15 Fine moving table 16 Harmonic wave 17 Comb electrode 18 Planar electrode 19 Insulating film 20 Electrode 21 Polarization inversion part 22 Non-inversion part 23 LiNbO ₃ substrate 24 Comb electrode 25 Plane electrode

Claims (6)

A single-polarized ferroelectric crystal;
Anda periodic polarization inversion layer formed in the crystal,
The periodic domain-inverted layer is formed by applying a voltage to a comb-shaped electrode in which electrode fingers with a width W produced on the crystal surface are arranged with a period Λ,
And the period Λ of the domain inverted layer, the thickness T of the substrate,
Satisfies the relationship of T <Λ / 0.01 and W <Λ / 2-2 (0.002 × T-0.2)
An optical wavelength conversion element characterized by the above .   The optical wavelength conversion element according to claim 1, wherein the polarization inversion period is 2 μm or less. The optical wavelength conversion element according to claim 1 or 2 ,
A condensing optical system;
A semiconductor laser ,
A short-wavelength light generator in which light emitted from the laser is condensed in the optical wavelength conversion element by the optical system and wavelength-converted by the optical wavelength conversion element. The short-wavelength light generator according to claim 3, wherein the laser is pulse-driven. Equipped with a fine motion table,
The optical wavelength conversion element is fixed to the fine movement table;
The light emitted from the laser is condensed inside the optical wavelength conversion element by the condensing optical system,
4. The short wavelength light generating device according to claim 3, wherein a position of the crystal with respect to the laser light is varied by the fine movement table. Equipped with a rotating fine table,
The optical wavelength conversion element is fixed to the rotary fine movement table;
The light emitted from the laser is condensed inside the optical wavelength conversion element by the condensing optical system,
4. The short-wavelength light generator according to claim 3 , wherein a phase matching condition of the crystal with respect to the laser beam is controlled by the turntable.

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