Nonlinear optical element and laser light source

Abstract

Provided is a novel structure for a nonlinear optical element for performing wavelength conversion. A nonlinear optical element (1) comprises: a plurality of conversion sections (11) formed of a nonlinear optical crystal for performing wavelength conversion on incident light; and a plurality of transmission sections (12) formed of a linear optical member that transmits the incident light and converted light obtained by performing wavelength conversion on the incident light. The conversion sections (11) and the transmission sections (12) are alternately formed in the propagation direction PG of the incident 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 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.

[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 section is a graph showing the relationship between propagation distance / coherence length Lc and SH wave intensity in various nonlinear optical elements, and the lower section is a side view showing the structure of the various nonlinear optical elements. This is a graph showing the refractive index of a nonlinear optical crystal and a linear optical material used in one embodiment of a nonlinear optical element according to another embodiment of the present disclosure. This is a graph showing the change in wavelength conversion efficiency and reflectance per interface with respect to parameter α for the nonlinear optical element of the above embodiment. This is a graph showing the change in wavelength conversion efficiency and reflectance per interface with respect to parameter α for the nonlinear optical element of the above embodiment. 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 of paraelectric crystals include (SBO), and for the ultraviolet band with wavelengths longer than the DUV band, examples include GaN. 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.

[0017] 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.

[0018] The linear optical member of the transmission part 12 is an amorphous or crystalline body formed of one or more linear optical materials. Examples of the aforementioned linear optical materials include TiO ２ , SiO ２ , ZrO ２ , HfO ２ , Al ２ O ３ , CaF ２ , MgF ２ and the like.

[0019] The length Lb in the propagation direction PG of the transmission part 12 is the coherence length Lcb of the linear optical member of the transmission part 12. Here, the coherence length Lcb means 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, becomes π, on the assumption that the linear optical member converts the wavelength of incident light into converted light.

[0020] When converting the wavelength of 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.

[0021] 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 aforementioned nonlinear optical crystal is prepared. Next, etching is performed on the nonlinear optical substrate, so that non-etched portions having a length La and etched portions having a length Lb are formed alternately. 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.

[0022] The upper part of Figure 2 is a graph showing the relationship between propagation distance / coherence length Lc and the intensity of the SH wave in various nonlinear optical elements, and the lower part of Figure 2 is a side view showing the structures of the various nonlinear optical elements. The various nonlinear optical elements are the nonlinear optical element 1 of the present embodiment, and nonlinear optical elements 101 and 102 as comparative examples.

[0023] In the example of 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. Accordingly, the length La (=Lca) of the conversion section 11 of the nonlinear optical element 1 of the present embodiment satisfies La=Lc. Further, in the example of Figure 2, in the nonlinear optical element 1 of the present embodiment, the refractive indices for the fundamental wave of the conversion section 11 and the transmission section 12 are matched (identical), and the refractive indices for the SH wave of the conversion section 11 and the transmission section 12 are matched. As a result, the length Lb (=Lcb) of the transmission section 12 of the nonlinear optical element 1 of the present embodiment satisfies Lb=La=Lc.

[0024] In the upper graph of Figure 2, the horizontal axis represents propagation distance / Lc, and the vertical axis represents arbitrary units. The nonlinear optical element 1 of the present embodiment is indicated by a thick solid line, the nonlinear optical element 101 is indicated by a one-dot chain line, and the nonlinear optical element 102 is indicated by a thin solid line. The outlined arrows in the lower part of Figure 2 indicate the polarity or polarization direction of the nonlinear optical crystal.

[0025] The nonlinear optical element 101 is formed of a nonlinear optical crystal having a certain polarity or polarization direction. In this case, as the fundamental wave enters and propagates through the nonlinear optical crystal, the fundamental wave is successively wavelength-converted into an SH wave. As a result, the intensity of the SH wave 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 wave are different. For this reason, 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.

[0026] 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 that underwent wavelength conversion at propagation distances from 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, in the case of the nonlinear optical element 101, the SH wave does not keep increasing.

[0027] 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 is the same as that of the nonlinear optical element 101 until the propagation distance reaches the coherence length Lc. That is, as the 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 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.

[0028] When the propagation distance exceeds the coherence length Lc, the polarity or polarization direction of the nonlinear optical crystal is inverted. As a result, 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, the SH waves that underwent wavelength conversion at propagation distances from 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.

[0029] When the propagation distance exceeds the coherence length Lc×2, the polarity or polarization direction of the nonlinear optical crystal is inverted again. As a result, 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 that underwent wavelength conversion at propagation distances from Lc to 2Lc are not canceled out by the newly wavelength-converted SH waves, and as a result, the intensity of the SH wave keeps increasing.

[0030] As described above, the nonlinear optical element 1 of this embodiment has a conversion section 11 and a transmission section 12 formed alternately at coherence lengths Lc. In this case, the behavior is the same as that of nonlinear optical elements 101 and 102 until the propagation distance reaches the coherence length Lc. That is, as 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, as the SH waves propagate through the nonlinear optical crystal, the phase difference between the propagating SH waves and the SH waves newly wavelength-converted from the fundamental waves increases.

[0031] When the propagation distance exceeds the coherence length Lc, the fundamental wave and SH wave propagate through the transmission section 12. As a result, the fundamental wave is not wavelength-converted at propagation distances Lc to 2Lc. Therefore, the SH wave that has been wavelength-converted at propagation distances 0 to Lc is not canceled out by the newly wavelength-converted SH wave, and the intensity of the SH wave is maintained.

[0032] When the propagation distance exceeds the coherence length Lc × 2, the fundamental wave and SH wave propagate through the conversion unit 11. At this time, the phase difference between the SH wave that has propagated over the coherence length Lc × 2 and the newly wavelength-converted SH wave becomes 2π (360°). As a result, the SH wave that has been wavelength-converted at propagation distances 0 to Lc is not canceled out by the newly wavelength-converted SH wave, and consequently, the intensity of the SH wave increases. The above operation is repeated thereafter, so in the case of the nonlinear optical element 1 of this embodiment, the intensity of the SH wave continues to increase.

[0033] Referring to the graph in Figure 2, it can be seen that the nonlinear optical element 1 of this embodiment has an SH wave intensity increase rate of approximately half that of the nonlinear optical element 102, which is a QPM element. On the other hand, the nonlinear optical crystals that can be used for QPM elements are mainly SiO ２ Crystallographic twins of (quartz) and LBGO (LaBGeO ５ The ferroelectric crystals are limited to nonlinear optical crystals capable of forming the above-mentioned periodic inversion structure, such as ). In contrast, the nonlinear optical element 1 of this embodiment does not require the above-mentioned periodic inversion structure, and therefore a wide variety of nonlinear optical crystals can be used.

[0034] Furthermore, the above 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.

[0035] 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.

[0036] (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).

[0037] 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.

[0038] 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.

[0039] [Embodiment 2] In another embodiment of the present disclosure, the nonlinear optical element 1 shown in Figures 1 and 2 has further matched refractive indices of the conversion section 11 and the transmission section 12. This makes it possible to minimize reflection and coupling losses at the interface between the conversion section 11 and the transmission section 12.

[0040] Normally, it is difficult to find a linear optical material that has the same refractive index as the nonlinear optical crystal of the conversion section 11. Therefore, the linear optical member of the transmission section 12 may be formed from multiple linear optical materials with different refractive indices so that their refractive indices match or substantially match those of the nonlinear optical crystal.

[0041] Specifically, the linear optical member may be formed from one or more linear optical materials having a refractive index greater than that of the nonlinear optical crystal, and one or more linear optical materials having a refractive index less than that of the nonlinear optical crystal. Alternatively, the linear optical member may have a structure in which multiple linear optical materials with different refractive indices are stacked. The stacking direction of the multiple linear optical materials may be the propagation direction PG, or it may be perpendicular to the propagation direction PG.

[0042] (Example 1) An embodiment of the nonlinear optical element 1 of this embodiment will be described with reference to Figures 3 to 5. The nonlinear optical element 1 of this embodiment is a DUV band wavelength conversion element that converts a fundamental wave with a wavelength of 460 nm to an SH wave with a wavelength of 230 nm. In this embodiment, the nonlinear optical crystal of the conversion section 11 is an AlN crystal, and the linear optical member of the transmission section 12 is ZrO ２ and SiO ２ It is formed from ZrO ２ It transmits light with a wavelength of 230 nm or more and has a higher refractive index than AlN crystal. On the other hand, SiO ２ It transmits light with a wavelength of 230 nm or more, and has a lower refractive index than AlN crystal.

[0043] Figure 3 is a graph showing the refractive indices of the nonlinear optical crystal and linear optical material used in this embodiment. Referring to Figure 3, it can be understood that the refractive indices of the nonlinear optical crystal and linear optical material depend on the wavelength of the propagating light. Therefore, it is preferable that the refractive indices of the conversion unit 11 and the transmission unit 12 match at various wavelengths.

[0044] In Figure 3, the refractive index of the AlN crystal, which is the nonlinear optical crystal mentioned above, is shown by the dashed line. Also, the linear optical material ZrO ２ and SiO ２ The refractive indices of ZrO are shown by the dashed and dotted lines, respectively. ２ and SiO ２ A mixture of αZrO ２ + (1-α)SiO ２The refractive index is shown by the solid line. Here, α is a real number satisfying 0 < α < 1. Referring to Figure 3, it can be seen that the refractive index of the mixture with α = 0.93 is approximately the same as the refractive index of the AlN crystal.

[0045] Figures 4 and 5 show that the conversion section 11 is formed of AlN crystals, and the transmission section 12 is the above mixture (linear optical member) αZrO ２ + (1-α)SiO ２ This graph shows the change in the wavelength conversion efficiency and the reflectance per interface with respect to parameter α for a nonlinear optical element 1 formed by [the specified method]. In Figures 4 and 5, the reflectance per interface for the fundamental wave is shown by a dashed line, the reflectance per interface for the SH wave is shown by a dashed line, and the wavelength conversion efficiency is shown by a solid line.

[0046] Furthermore, the length La of the conversion portion 11 of the nonlinear optical element 1 is 313 nm, which corresponds to the coherence length Lca of the AlN crystal, and the propagation length of the nonlinear optical element 1 is 3 mm. The length Lb of the transmission portion 12 of the nonlinear optical element 1 corresponds to the coherence length Lcb of the mixture, and the coherence length Lcb of the mixture depends on the parameter α of the mixture.

[0047] Referring to Figure 4, it can be seen that the efficiency of wavelength conversion increases sharply around α = 0.9. Therefore, in the graph of Figure 5, the horizontal axis α is limited to the range of 0.70 to 1.00, and one of the vertical axes, the reflectance per interface, is changed to a logarithmic scale.

[0048] Referring to Figure 5, the reflectance per interface for both the fundamental wave and the SH wave decreases sharply as α approaches 0.93, and at α = 0.93 it is 10 －７ It can be seen that the following minimum values ​​are obtained. Also, referring to Figure 5, it can be seen that the wavelength conversion efficiency increases rapidly as α approaches 0.93, and reaches a maximum value of approximately 0.35 at α = 0.93.

[0049] Therefore, ZrO ２ SiO ２By forming the linear optical member of the transmission portion 12 such that the ratio is 0.93:0.07, a nonlinear optical element 1 with good wavelength conversion efficiency can be realized. In this case, the length Lb of the transmission portion 12 of the nonlinear optical element 1 is 288 nm, which corresponds to the coherence length Lcb. The coherence length Lcb (288 nm) of the above mixture is different from the coherence length Lca (313 nm) of the AlN crystal. This is because the refractive index of the above mixture does not perfectly match that of the AlN crystal. Thus, the refractive indices of the above mixture and the AlN crystal do not need to perfectly match; they only need to match to the extent that the fundamental wave and SH wave propagate sufficiently through the nonlinear optical element 1.

[0050] Although Figures 3 to 5 utilize two linear optical materials, the transparent portion 12 may be formed using three or more linear optical materials.

[0051] (Example 2) Another embodiment of the nonlinear optical element 1 of this embodiment will be described. The nonlinear optical element 1 of this embodiment is a wavelength conversion element in the visible light band (380 nm to 780 nm) that converts a fundamental wave with a wavelength of 1064 nm to an SH wave with a wavelength of 532 nm. In this embodiment, the nonlinear optical crystal of the conversion section 11 is a GaN crystal, and the linear optical member of the transmission section 12 is TiO ２ and SiO ２ It is a mixture formed from TiO ２ It transmits light with a wavelength of 532 nm or higher and has a higher refractive index than GaN crystal. On the other hand, SiO ２ It transmits light with a wavelength of 532 nm or higher and has a lower refractive index than GaN crystal.

[0052] Then, as a result of the same investigation as in the examples shown in Figures 3 to 5, TiO ２ SiO ２ The wavelength conversion efficiency was maximized when the ratio was 0.815:0.185. Therefore, TiO ２ SiO ２ By forming the linear optical member of the transmission portion 12 such that the ratio is 0.815:0.185, a nonlinear optical element 1 with good wavelength conversion efficiency can be realized.

[0053] (Additional Notes) In the above embodiment, the case in which the refractive indices of the conversion section 11 and the transmission section 12 are matched in the nonlinear optical element 1 has been described, but this disclosure is not limited thereto. For example, an anti-reflective coating material may be used as the linear optical material forming the transmission section 12. In this case as well, reflection and coupling losses at the interface between the conversion section 11 and the transmission section 12 can be minimized.

[0054] [Embodiment 3] Yet another embodiment of the present disclosure will be described with reference to Figure 6.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] (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.

[0059] 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.

[0060] 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.

[0061] [Summary] A nonlinear optical element according to one aspect of the present disclosure comprises 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.

[0062] 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.

[0063] In the conversion section of the nonlinear optical element, the propagation direction length La of the conversion section 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 section. The propagation direction length Lb of the transmission section 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 assuming that the linear optical member of the transmission section performs the wavelength conversion.

[0064] In the aforementioned nonlinear optical element, the refractive index of the conversion portion and the refractive index of the transmission portion may be matched.

[0065] In the nonlinear optical element, the transmitting portion may be formed of a plurality of linear optical materials having different refractive indices so that the refractive index of the conversion portion matches the refractive index of the transmitting portion.

[0066] 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.

[0067] 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.

[0068] 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.

2. 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.

3. 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 greater than or equal to 0, 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 greater than or equal to 0, and Lcb is the coherence length when it is assumed that the linear optical member of the transmission section performs the wavelength conversion.

4. The nonlinear optical element according to claim 1, wherein the refractive index of the conversion section and the refractive index of the transmission section are matched.

5. The nonlinear optical element according to claim 4, wherein the transmitting portion is formed of a plurality of linear optical materials having different refractive indices so that the refractive index of the conversion portion matches the refractive index of the transmitting portion.

6. 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.

7. 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 6 that performs wavelength conversion using the laser light as incident light.

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