Nonlinear optical element and laser light source

Abstract

The present invention provides a novel structure of a nonlinear optical element for performing wavelength conversion. This nonlinear optical element (1) comprises: a plurality of conversion units (11) that use a nonlinear optical crystal to perform wavelength conversion on incident light; and a plurality of transmissive units (12) that transmit the incident light and converted light obtained by performing wavelength conversion on the incident light. The conversion units (11) and the transmissive units (12) are formed alternately in a propagation direction PG of the incident light, and the transmissive units (12) are configured to reduce reflections of the incident light and the converted light.

Description

Nonlinear optical elements and laser light sources

[0001] This disclosure relates to a nonlinear optical element that performs wavelength conversion of incident light, and a laser light source using the nonlinear optical element.

[0002] Quasi-phase-matching (QPM) wavelength conversion elements are known as nonlinear optical elements that perform wavelength conversion of incident light (laser light) (Non-Patent Literature 1). Hereafter, quasi-phase-matching wavelength conversion elements will be referred to as "QPM elements".

[0003] For example, a QPM element that wavelength-converts the fundamental wave, which is the incident light, into a second harmonic (SH) wave has a structure in which the polarity or polarization direction of the nonlinear optical crystal that performs the wavelength conversion is periodically reversed at intervals of the coherence length Lc defined by the following equation (1). Hereinafter, such a structure will be referred to as a "periodic reversal structure". Due to the above periodic reversal structure, the converted light wavelength-converted from the incident light can be amplified inside the QPM element.

[0004] Lc = λ1 / (4 × (n2 - n1)) ... (1). In equation (1) above, λ1 is the wavelength of the fundamental wave, n1 is the refractive index of the fundamental wave propagating through the nonlinear optical crystal, and n2 is the refractive index of the SH wave propagating through the nonlinear optical crystal.

[0005] T. Suhara and M. Fujimura, “Waveguide Nonlinear-Optic Devices”, (Germany), Springer, 2003

[0006] Recently, there has been a demand for wavelength conversion elements capable of generating light in the deep ultraviolet (DUV) band, which has a wavelength range of 200 to 300 nm. However, in the case of QPM elements, as the wavelength of the incident light decreases, the coherence length decreases, making it difficult to create the periodic inversion structure described above.

[0007] Furthermore, as a nonlinear optical crystal used in the QPM element capable of generating light in the DUV band, SiO ２ (crystal), LaBGeO ５Examples include (LBGO). However, these nonlinear optical crystals have low nonlinear optical constants and poor wavelength conversion efficiency.

[0008] One aspect of this disclosure aims to provide a novel structure for a nonlinear optical element that performs wavelength conversion.

[0009] To solve the above problems, a nonlinear optical element according to one aspect of the present disclosure comprises a plurality of conversion units formed by a nonlinear optical crystal that performs wavelength conversion with respect to incident light, and a plurality of transmission units formed by a linear optical member that transmits the incident light and the converted light that has undergone the wavelength conversion with respect to the incident light, wherein the conversion units and the transmission units are formed alternately in the propagation direction of the incident light, and the transmission units are configured to reduce the reflection of the incident light and the converted light.

[0010] According to one aspect of this disclosure, a novel structure for a nonlinear optical element that performs wavelength conversion can be provided.

[0011] This is a schematic diagram of a nonlinear optical element according to one embodiment of the present disclosure. The upper panel is a graph showing the relationship between propagation distance / coherence length Lc and SH wave intensity in various nonlinear optical elements, and the lower panel is a side view showing the structure of the various nonlinear optical elements. SrB ４ O ７ This is a graph showing the refractive index of (SBO). This is a graph showing the change in reflectance for one embodiment of the above nonlinear optical element. This is a graph showing the change in reflectance for another embodiment of the above nonlinear optical element. This is a schematic diagram of a nonlinear optical element according to yet another embodiment of the present disclosure.

[0012] The embodiments of this disclosure will be described in detail below. For the sake of convenience, components having the same function as those shown in each embodiment will be denoted by the same reference numerals, and their descriptions will be omitted as appropriate.

[0013] [Embodiment 1] An embodiment of the present disclosure will be described with reference to Figures 1 and 2.

[0014] Figure 1 is a schematic diagram of the nonlinear optical element 1 according to this embodiment. The nonlinear optical element 1 of this embodiment converts the wavelength of the fundamental wave, which is incident light from a laser light source (not shown), into a second harmonic (SH) wave. When the fundamental wave is input to the nonlinear optical element 1, a portion of the fundamental wave is converted into an SH wave, and the SH wave and the remainder of the fundamental wave are output from the nonlinear optical element 1.

[0015] As shown in Figure 1, the nonlinear optical element 1 comprises a plurality of conversion sections 11 formed by a nonlinear optical crystal that performs wavelength conversion with respect to the fundamental wave, and a plurality of transmission sections 12 formed by a linear optical member that transmits the fundamental wave and the SH wave (converted light) which has undergone wavelength conversion with respect to the fundamental wave. The linear optical member can also be said to be a member that does not perform wavelength conversion of the fundamental wave. The conversion sections 11 and the transmission sections 12 are formed alternately in the propagation direction PG of the fundamental wave.

[0016] Examples of the nonlinear optical crystals in the conversion unit 11 include AlN and SrB in the DUV band. ４ O ７ Examples include paraelectric crystals such as (SBO), and for the ultraviolet band with wavelengths longer than the DUV band, GaN is an example.

[0017] The length La of the propagation direction PG of the conversion unit 11 is the coherence length Lca of the nonlinear optical crystal of the conversion unit 11. Here, the coherence length Lca means the distance between the first position and the second position such that the phase difference between the converted light generated at the first position of the nonlinear optical crystal and propagated to the second position of the nonlinear optical crystal and the converted light newly generated at the second position is π.

[0018] When converting a fundamental wave to an SH wave by wavelength, the coherence length Lca of the conversion unit 11 is defined by the following equation (2): Lca = λf / (4 × (Nsa - Nfa)) ... (2). Here, λf is the wavelength of the fundamental wave, Nfa is the refractive index of the fundamental wave propagating through the nonlinear optical crystal, and Nsa is the refractive index of the SH wave propagating through the nonlinear optical crystal.

[0019] The linear optical member of the transmission part 12 is amorphous or crystalline formed of one or more linear optical materials. Examples of the linear optical material include TiO ２ , SiO ２ , ZrO ２ , HfO ２ , Al ２ O ３ , CaF ２ , MgF ２ and the like.

[0020] The length Lb of the transmission part 12 in the propagation direction PG is the coherence length Lcb of the linear optical member of the transmission part 12. Here, the coherence length Lcb means, on the assumption that the linear optical member wavelength-converts incident light into converted light, the distance between the third position and the fourth position such that the phase difference between the converted light generated at the third position of the linear optical member and propagated to the fourth position of the linear optical member, and the converted light newly generated at the fourth position is π.

[0021] When wavelength-converting a fundamental wave into an SH wave, the coherence length Lcb of the transmission part 12 is defined by the following formula (3). Lcb=λf / (4×(Nsb−Nfb)) (3). Here, Nfb is the refractive index of the fundamental wave propagating through the linear optical member, and Nsb is the refractive index of the SH wave propagating through the linear optical member. Since the refractive index varies depending on the optical material, the length La of the conversion part 11 and the length Lb of the transmission part 12 may be different.

[0022] Next, a method for manufacturing the nonlinear optical element 1 having the above configuration will be described. First, a nonlinear optical substrate made of the above nonlinear optical crystal is prepared. Next, the nonlinear optical substrate is etched to alternately form non-etched portions having a length La and etched portions having a length Lb. Then, the linear optical material is deposited on the etched portions using an atomic layer deposition apparatus or the like. Thereby, the nonlinear optical element 1 in which the non-etched portions serve as the conversion parts 11 and the etched portions serve as the transmission parts 12 is manufactured.

[0023] The upper part of Figure 2 is a graph showing the relationship between propagation distance / coherence length Lc and SH wave intensity in various nonlinear optical elements, and the lower part of Figure 2 is a side view showing the structure of the various nonlinear optical elements. The various nonlinear optical elements are the nonlinear optical element 1 of this embodiment and the comparative nonlinear optical elements 101 and 102.

[0024] In the example shown in Figure 2, the nonlinear optical crystals used in the nonlinear optical elements 1, 101, and 102 are the same, and the coherence length of the nonlinear optical crystal is Lc. Therefore, the length La (=Lca) of the conversion section 11 of the nonlinear optical element 1 in this embodiment is La = Lc. Also, in the example shown in Figure 2, the refractive indices of the conversion section 11 and the transmission section 12 with respect to the fundamental wave are matched (coincided), and the refractive indices of the conversion section 11 and the transmission section 12 with respect to the SH wave are matched. As a result, the length Lb (=Lcb) of the transmission section 12 of the nonlinear optical element 1 in this embodiment is Lb = La = Lc.

[0025] In the upper graph of Figure 2, the horizontal axis represents propagation distance / Lc, and the vertical axis represents arbitrary units. Furthermore, in this embodiment, the nonlinear optical element 1 is shown by a thick line, the nonlinear optical element 101 by a dashed line, and the nonlinear optical element 102 by a thin line. The white arrows in the lower section of Figure 2 indicate the polarity or polarization direction of the nonlinear optical crystal.

[0026] The nonlinear optical element 101 is formed from a nonlinear optical crystal having a certain polarity or polarization direction. In this case, when a fundamental wave is incident and propagates through the nonlinear optical crystal, the fundamental wave is successively wavelength-converted into SH waves. As a result, the intensity of the SH waves increases. On the other hand, due to the wavelength dependence of the refractive index, the refractive index of the fundamental wave and the refractive index of the SH waves are different. Therefore, as the SH waves propagate through the nonlinear optical crystal, the phase difference between the propagating SH waves and the newly wavelength-converted SH waves from the fundamental wave increases.

[0027] When the propagation distance exceeds the coherence length Lc, the phase difference between the SH wave that has propagated the coherence length Lc and the newly wavelength-converted SH wave becomes π (180°). Therefore, when the propagation distance exceeds the coherence length Lc, the SH waves wavelength-converted over the propagation distance of 0 to Lc are successively canceled out by the newly wavelength-converted SH waves, and the intensity of the SH wave decreases. Then, when the propagation distance reaches coherence length Lc × 2, the intensity of the SH wave becomes 0. Since the above operation is repeated thereafter, the SH wave does not continue to increase in the case of the nonlinear optical element 101.

[0028] The nonlinear optical element 102 is a QPM element, and has a structure in which the polarity or polarization direction of a nonlinear optical crystal is periodically inverted for each coherence length Lc. In this case, the process until the propagation distance reaches the coherence length Lc is the same as that of the nonlinear optical element 101. That is, as the fundamental wave enters and propagates through the nonlinear optical crystal, the fundamental wave is successively wavelength-converted into SH waves. As a result, the intensity of the SH wave increases. On the other hand, as the SH wave propagates through the nonlinear optical crystal, the phase difference between the propagating SH wave and the SH wave newly wavelength-converted from the fundamental wave increases.

[0029] When the propagation distance exceeds the coherence length Lc, the polarity or polarization direction of the nonlinear optical crystal is inverted. Thereby, the phase difference between the SH wave that has propagated the coherence length Lc and the newly wavelength-converted SH wave becomes 0. Therefore, when the propagation distance exceeds the coherence length Lc, the SH waves wavelength-converted over the propagation distance of 0 to Lc are not canceled out by the newly wavelength-converted SH waves, and as a result, the intensity of the SH wave increases.

[0030] When the propagation distance exceeds the coherence length Lc × 2, the polarity or polarization direction of the nonlinear optical crystal is inverted again. Thereby, the phase difference between the SH wave that has propagated the coherence length Lc and the newly wavelength-converted SH wave becomes 0. Therefore, even when the propagation distance exceeds the coherence length Lc × 2, the SH waves wavelength-converted over the propagation distance of Lc to 2Lc are not canceled out by the newly wavelength-converted SH waves, and as a result, the intensity of the SH wave continues to increase.

[0031] In the nonlinear optical element 1 of the present embodiment, as described above, the conversion sections 11 and the transmission sections 12 are formed alternately for each coherence length Lc. In this case, the process until the propagation distance reaches the coherence length Lc is the same as that of the nonlinear optical elements 101 and 102. That is, as the fundamental wave enters and propagates through the nonlinear optical crystal, the fundamental wave is sequentially wavelength-converted into the SH wave. As a result, the intensity of the SH wave increases. On the other hand, as the SH wave propagates through the nonlinear optical crystal, the phase difference between the propagating SH wave and the SH wave newly wavelength-converted from the fundamental wave increases.

[0032] When the propagation distance exceeds the coherence length Lc, the fundamental wave and the SH wave propagate through the transmission section 12. Thereby, the fundamental wave is not wavelength-converted in the propagation distance range of Lc to 2Lc. Therefore, the SH wave wavelength-converted in the propagation distance range of 0 to Lc is not canceled out by the newly wavelength-converted SH wave, and the intensity of the SH wave is maintained.

[0033] When the propagation distance exceeds the coherence length Lc×2, the fundamental wave and the SH wave propagate through the conversion section 11. At this time, the phase difference between the SH wave that has propagated for the coherence length Lc×2 and the newly wavelength-converted SH wave is 2π (360°). As a result, the SH wave wavelength-converted in the propagation distance range of 0 to Lc is not canceled out by the newly wavelength-converted SH wave, and as a result, the intensity of the SH wave increases. Since the above operation is repeated hereinafter, in the case of the nonlinear optical element 1 of the present embodiment, the intensity of the SH wave continues to increase.

[0034] Referring to the graph in Figure 2, it can be understood that the nonlinear optical element 1 of the present embodiment has an SH wave intensity increase rate of about 1 / 2 times that of the nonlinear optical element 102 which is a QPM element. On the other hand, nonlinear optical crystals usable for QPM elements are mainly SiO ２ (quartz) crystallographic twins, and ferroelectric crystals such as LBGO (LaBGeO ５ ), and are limited to nonlinear optical crystals capable of forming the periodic inversion structure described above. In contrast, the nonlinear optical element 1 of the present embodiment does not require the above periodic inversion structure, so a wide variety of nonlinear optical crystals can be used.

[0035] Furthermore, the aforementioned SiO２ Crystals and LBGO crystals have theoretically shortest SH wavelengths of 145 and 195 nm, respectively, and can be used for wavelength conversion in the DUV band. However, SiO ２ The crystal and LBGO crystal have nonlinear optical constants of 0.19 and 0.57 pm / V, respectively.

[0036] On the other hand, the AlN crystal, which is a nonlinear optical crystal usable in the nonlinear optical element 1 of this embodiment, has a theoretical shortest SH wavelength of 210 nm and can be used for wavelength conversion in the DUV band. Furthermore, the nonlinear optical constant of the AlN crystal is 6.3 pm / V, and SiO ２ The nonlinear optical constants are approximately 10 times greater than those of crystals and LBGO crystals. Furthermore, the SBO crystal, which is a nonlinear optical crystal usable in the nonlinear optical element 1 of this embodiment, has a theoretical shortest SH wavelength of 130 nm and a nonlinear optical constant of 3.5 pm / V. Generally, the efficiency of wavelength conversion is proportional to the square of the nonlinear optical constant. Therefore, the nonlinear optical element 1 of this embodiment using AlN crystal is SiO ２ Compared to QPM elements using either crystals or LBGO crystals, this device offers better wavelength conversion efficiency.

[0037] (Additional Notes) In this embodiment, the length of the conversion section 11 is set to the coherence length Lca, but some error in the above length is permissible. That is, the length La of the conversion section 11 may be La = Lca × (1 ± 0.1) or La = Lca × (1 ± 0.05). Similarly, the length Lb of the transmission section 12 may be Lb = Lcb × (1 ± 0.1) or Lb = Lcb × (1 ± 0.05).

[0038] Furthermore, as can be seen by referring to Figure 2, the SH wave can be increased even if the length of the conversion unit 11 is an odd multiple of the coherence length Lca. That is, the length La of the conversion unit 11 may be La = (2ma + 1) × Lca × (1 ± 0.1) or La = (2ma + 1) × Lca × (1 ± 0.05). Here, ma is a non-negative integer. Similarly, the length Lb of the transmission unit 12 may be Lb = (2mb + 1) × Lcb × (1 ± 0.1) or Lb = (2mb + 1) × Lcb × (1 ± 0.05). Here, mb is a non-negative integer.

[0039] Furthermore, referring to Figure 2, it can be understood that in the region where the conversion section 11 and the transmission section 12 are formed, the transmission section 12 may be formed in at least a portion of the region, and the conversion section 11 may be formed in the remaining region. In this case, since no cancellation of SH waves occurs in the transmission section 12, SH waves remain at a propagation distance of 2Lc. In other words, if multiple conversion sections 11 and multiple transmission sections 12 are formed alternately so that the phase of the SH waves in the upstream conversion section 11 connected to the upstream side of the transmission section 12 matches the phase of the SH waves in the downstream conversion section 11 connected to the downstream side of the transmission section 12, the number of SH waves can be increased.

[0040] [Embodiment 2] Another embodiment of the present disclosure will be described with reference to Figures 3 to 5.

[0041] To minimize reflection at the interface between the conversion section 11 and the transmission section 12, it is conceivable to make the refractive indices of the conversion section 11 and the transmission section 12 the same. However, it is difficult to find a linear optical material that has the same refractive index as the nonlinear optical crystal in the conversion section 11.

[0042] Therefore, in the nonlinear optical element 1 of this embodiment, as shown in Figures 1 and 2, the transmission portion 12 is configured to reduce the reflection of fundamental waves and SH waves. Specifically, the linear optical member of the transmission portion 12 is formed to satisfy the following conditions 1 and 2. Condition 1: The reflected light of the fundamental wave at the downstream interface between the transmission portion 12 and the downstream conversion portion 11 connected downstream thereto is in opposite phase to the reflected light of the fundamental wave at the upstream interface between the transmission portion 12 and the upstream conversion portion 11 connected upstream thereto. Condition 2: The reflected light of the SH wave at the downstream interface is in opposite phase to the reflected light of the SH wave at the upstream interface.

[0043] By satisfying condition 1 above, the reflected light of the fundamental wave at the downstream interface and the reflected light of the fundamental wave at the upstream interface cancel each other out, and as a result, the reflection of the fundamental wave can be reduced. Furthermore, by satisfying condition 2 above, the reflected light of the SH wave at the downstream interface and the reflected light of the SH wave at the upstream interface cancel each other out, and as a result, the reflection of the SH wave can be reduced. Therefore, the reflection of the fundamental wave due to the difference in refractive index of the conversion unit 11 and the transmission unit 12 with respect to the fundamental wave can be reduced, and the reflection of the SH wave due to the difference in refractive index of the conversion unit 11 and the transmission unit 12 with respect to the SH wave can be reduced.

[0044] Furthermore, the linear optical member of the transmission section 12 may be formed from a plurality of linear optical materials having different refractive indices so as to satisfy conditions 1 and 2 above. One of the plurality of linear optical materials may be air. For example, the linear optical member of the transmission section 12 may be a mixture of one or more linear optical materials and air. The mixture may have a nanoporous structure. Also, the linear optical member of the transmission section 12 may have a structure in which a plurality of linear optical materials having different refractive indices are stacked. The stacking direction of the plurality of linear optical materials may be the propagation direction PG or a direction perpendicular to the propagation direction PG.

[0045] Figure 3 is a graph showing the refractive index of SBO. Referring to Figure 3, it can be understood that the refractive index of a crystal depends on the wavelength of the propagating light. Therefore, the refractive index Nfb of the linear optical member of the transmission part 12 should be determined such that it satisfies condition 1 at the wavelength of the fundamental wave incident on the nonlinear optical element 1. Furthermore, the refractive index Nsb of the linear optical member should be determined such that it satisfies condition 2 at the wavelength of the SH wave obtained by wavelength conversion of the fundamental wave. Then, one or more linear optical materials should be selected such that the linear optical member has the refractive index Nfb at the wavelength of the fundamental wave and the refractive index Nsb at the wavelength of the SH wave, and the linear optical member should be formed using the selected linear optical materials.

[0046] Incidentally, the conversion section 11 and the transmission section 12 may be formed on a substrate. In this case, by making the conversion section 11 and the transmission section 12 function as waveguides, the power density of the fundamental wave increases, and the conversion efficiency from the fundamental wave to the SH wave is improved. It is desirable that the refractive index of the substrate is lower than that of the linear optical member of the transmission section 12. For example, if the linear optical member of the transmission section 12 is SiO ２ If formed by SiO ２ The above substrate may be formed using Cytop®, an amorphous fluororesin with a lower refractive index than the above.

[0047] (Example 1) Figure 4 is a graph showing the change in reflectance for one embodiment of the nonlinear optical element 1. In this embodiment, the nonlinear optical crystal of the conversion section 11 is an SBO crystal, and the linear optical member of the transmission section 12 is a βSiO ２ Nanoporous SiO represented by + (1-β) air ２ It is a film. In Figure 4, the horizontal axis is the parameter β, and the vertical axis is the reflectance of the nonlinear optical element 1. In Figure 4, the fundamental wave is shown by a dashed line, and the SH wave is shown by a solid line.

[0048] The nonlinear optical element 1 of this embodiment converts a fundamental wave with a wavelength of 386 nm into an SH wave with a wavelength of 193 nm, and the length of the conversion section 11, which corresponds to the coherence length of the SBO crystal, is 820 nm. The length of the transmission section 12, which corresponds to the coherence length of the linear optical element, depends on the parameter β. In addition, in the nonlinear optical element 1 of this embodiment, 1000 sets are formed, with adjacent conversion sections 11 and transmission sections 12 forming one set.

[0049] Referring to Figure 4, it can be seen that the reflectivity of the fundamental wave and SH wave becomes close to 0 around β = 0.76 and 0.88. Therefore, SiO ２ :Air = 0.76:0.24, or SiO ２ By forming the linear optical member of the transmission portion 12 such that β = air = 0.88:0.12, a nonlinear optical element 1 can be realized in which the reflection of fundamental waves and SH waves is reduced. The length of the transmission portion 12 at β = 0.88 is 1229 nm.

[0050] (Example 2) Figure 5 is a graph showing the change in reflectance for another embodiment of the nonlinear optical element 1. In this embodiment, the nonlinear optical crystal of the conversion section 11 is an SBO crystal, and the linear optical member of the transmission section 12 is a βSiO ２ Nanoporous SiO represented by + (1-β) air ２ It is a film. In Figure 5, the horizontal axis is the parameter β, and the vertical axis is the reflectance of the nonlinear optical element 1. In Figure 5, the fundamental wave is shown by a dashed line, and the SH wave is shown by a solid line.

[0051] The nonlinear optical element 1 of this embodiment converts a fundamental wave with a wavelength of 340 nm into an SH wave with a wavelength of 170 nm, and the length of the conversion section 11, which corresponds to the coherence length of the SBO crystal, is 494 nm. As mentioned above, the length of the transmission section 12, which corresponds to the coherence length of the linear optical element, depends on the parameter β. In addition, in the nonlinear optical element 1 of this embodiment, 1000 sets are formed, with adjacent conversion sections 11 and transmission sections 12 forming one set. Therefore, the nonlinear optical element 1 of this embodiment can output SH waves in the vacuum ultraviolet band, which is in the wavelength range of 10 to 200 nm.

[0052] Referring to Figure 5, it can be seen that the reflectivity of the fundamental wave and SH wave becomes close to 0 around β = 0.80 and 1.00. Therefore, SiO ２ By forming the linear optical member of the transmission portion 12 such that :air = 0.80:0.20, a nonlinear optical element 1 can be realized in which the reflection of fundamental waves and SH waves is reduced. ２ :Air = 1.00:0.00, that is, the linear optical member of the transmission section 12 is SiO ２ By forming it in this way, a nonlinear optical element 1 can be realized in which the reflection of fundamental waves and SH waves is reduced. The length of the transparent portion 12 at β = 1.00 is 688 nm.

[0053] (Additional Notes) In the above examples 1 and 2, examples where the reflectance is close to 0 were described. As can be seen from this description, the lower the reflectance of the entire nonlinear optical element 1 for the fundamental wave and SH wave (hereinafter referred to as "element reflectance"), the better. However, the nonlinear optical element 1 will function even if it has a certain degree of element reflectance for each wavelength of the fundamental wave and SH wave.

[0054] For example, without considering the effects of interference, we can estimate the SH wave intensity from the fundamental wave intensity. If the element reflectance for each wavelength is 20%, then compared to an ideal nonlinear optical element with reflectance of 0, the fundamental wave intensity will be 4 / 5, so the SH wave intensity generated from the fundamental wave will be 16 / 25. Since 20% (1 / 5) of the generated SH wave is reflected, the SH wave intensity output from nonlinear optical element 1 will be (16 / 25) × (4 / 5) = 0.512 (51.2%). Thus, if the element reflectance is 20% or less, the degradation of element performance can be estimated to be kept to less than half, which is a desirable range.

[0055] [Embodiment 3] Yet another embodiment of the present disclosure will be described with reference to Figure 6.

[0056] Figure 6 is a schematic diagram of the nonlinear optical element 1 of this embodiment. The nonlinear optical element 1 of this embodiment differs from the nonlinear optical element 1 shown in Figures 1 and 2 in that one or more thin plates 13 (nanowalls) of a nonlinear optical crystal with the propagation direction PG as the normal direction are formed in the transmission portion 12, but the other configurations are the same.

[0057] As described above, the transparent portion 12 is formed by depositing the linear optical material using an atomic layer deposition apparatus or the like. Therefore, the formation of the transparent portion 12 takes time.

[0058] In contrast, in this embodiment, the linear optical material can also be deposited from the thin plate 13. If the number of thin plates 13 formed in the transmission section 12 is n (where n is an integer of 1 or more), the deposition time of the linear optical material can be shortened to approximately 1 / (n+1). It is desirable that the thin plate 13 of the nonlinear optical crystal be thin enough not to affect the SH waves whose wavelength has been converted in the conversion section 11.

[0059] (Additional Notes) The nonlinear optical element 1 in the above embodiment generates SH waves from fundamental waves, but this disclosure is not limited thereto. For example, this disclosure can also be applied to nonlinear optical elements 1 that perform functions such as generating third or higher harmonics, generating sum frequencies, parametric oscillation, spontaneous parametric downconversion, and generating difference frequencies.

[0060] Furthermore, the above-described effects can be achieved if the laser light source comprises a laser element that outputs laser light and a nonlinear optical element of the above embodiment that performs wavelength conversion using the laser light as incident light.

[0061] This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure.

[0062] [Summary] A nonlinear optical element according to one aspect of the present disclosure comprises a plurality of conversion units formed by a nonlinear optical crystal that performs wavelength conversion with respect to incident light, and a plurality of transmission units formed by a linear optical member that transmits the incident light and the converted light that has undergone the wavelength conversion with respect to the incident light, wherein the conversion units and the transmission units are formed alternately in the propagation direction of the incident light, and the transmission units are configured to reduce the reflection of the incident light and the converted light.

[0063] In the nonlinear optical element, the transmission portion may be formed of a plurality of linear optical materials with different refractive indices to reduce the reflection of the incident light and the converted light.

[0064] In the aforementioned nonlinear optical element, the nonlinear optical crystal of the conversion unit is SrB ４ O ７ It is a crystal, and the permeable portion is nanoporous SiO ２ It can also be a membrane.

[0065] The nonlinear optical element further comprises a substrate, the conversion portion and the transmission portion are formed on the substrate, and the refractive index of the substrate may be lower than the refractive index of the transmission portion.

[0066] In the nonlinear optical element, a plurality of conversion units and a plurality of transmission units may be formed such that the phase of the converted light in the upstream conversion unit connected to the upstream side of the transmission unit matches the phase of the converted light in the downstream conversion unit connected to the downstream side of the transmission unit.

[0067] In the nonlinear optical element, the propagation direction length La of the conversion portion is given by La = (2 × ma + 1) × Lca × (1 ± 0.1), where ma is an integer greater than or equal to 0, and Lca is the coherence length of the nonlinear optical crystal of the conversion portion. The propagation direction length Lb of the transmission portion is given by Lb = (2 × mb + 1) × Lcb × (1 ± 0.1), where mb is an integer greater than or equal to 0, and Lcb may be the coherence length when it is assumed that the linear optical member of the transmission portion performs the wavelength conversion.

[0068] In the nonlinear optical element, one or more thin plates of a nonlinear optical crystal whose propagation direction is normal to the transmission direction may be formed in the transmission portion.

[0069] A laser light source according to another aspect of the present disclosure comprises a laser element that outputs laser light and a nonlinear optical element having the above configuration that performs wavelength conversion using the laser light as incident light.

[0070] 1 Nonlinear optical element 11 Conversion section 12 Transmission section 13 Thin plate

Claims

1. A nonlinear optical element comprising: a plurality of conversion sections formed by a nonlinear optical crystal that performs wavelength conversion with respect to incident light; and a plurality of transmission sections formed by a linear optical member that transmits the incident light and the converted light that has undergone the wavelength conversion with respect to the incident light, wherein the conversion sections and the transmission sections are formed alternately in the direction of propagation of the incident light, and the transmission sections are configured to reduce reflection of the incident light and the converted light.

2. The nonlinear optical element according to claim 1, wherein the transmitting portion is formed of a plurality of linear optical materials having different refractive indices to reduce the reflection of the incident light and the converted light.

3. The nonlinear optical crystal in the conversion unit is SrB ４ O ７ It is a crystal, and the permeable portion is nanoporous SiO ２ A nonlinear optical element according to claim 2, which is a film.

4. The nonlinear optical element according to claim 1, further comprising a substrate, wherein the conversion portion and the transmission portion are formed on the substrate, and the refractive index of the substrate is lower than the refractive index of the transmission portion.

5. The nonlinear optical element according to claim 1, wherein a plurality of conversion units and a plurality of transmission units are formed such that the phase of the converted light in the upstream conversion unit connected to the upstream side of the transmission unit matches the phase of the converted light in the downstream conversion unit connected to the downstream side of the transmission unit.

6. The nonlinear optical element according to claim 1, wherein the length La in the propagation direction of the conversion section is given by the following equation (1): La = (2 × ma + 1) × Lca × (1 ± 0.1) ... (1) where ma is an integer of 0 or more, and Lca is the coherence length of the nonlinear optical crystal of the conversion section, and the length Lb in the propagation direction of the transmission section is given by the following equation (2): Lb = (2 × mb + 1) × Lcb × (1 ± 0.1) ... (2) where mb is an integer of 0 or more, and Lcb is the coherence length when it is assumed that the linear optical member of the transmission section performs the wavelength conversion.

7. The nonlinear optical element according to claim 1, wherein one or more thin plates of a nonlinear optical crystal with the propagation direction as the normal direction are formed in the transparent portion.

8. A laser light source comprising a laser element that outputs laser light, and a nonlinear optical element according to any one of claims 1 to 7 that performs wavelength conversion using the laser light as incident light.

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