Wavelength conversion device

Abstract

A wavelength conversion device (20), to which signal light (1a) is inputted and which generates difference frequency light (1c) having a wavelength different from that of the signal light 1a, comprises: a wavelength conversion element (13) which includes an optical waveguide core (11) and a substrate (12) having a lower refractive index for the signal light than the optical waveguide core (11) and converts the wavelength of the signal light (1a); an overcladding (301) which is formed on at least part of the surface of the optical waveguide core (11) and has a lower refractive index for the signal light (1a) and the light wavelength of control light (1b) to be combined with the signal light (1a) than the optical waveguide core (11); and a temperature control element (26) for controlling the temperature of the wavelength conversion element (13).

Description

Technical Field

[0001] The present disclosure relates to a wavelength conversion device.

Background Art

[0002] Wavelength conversion technology has attracted attention in applications where light in a wavelength range that cannot be directly output by a semiconductor laser or high-power light that cannot be obtained by a semiconductor laser even in an output wavelength range is required. The production of a wavelength conversion device is realized by using an optical crystal having a second-order nonlinear effect or the like. Representative optical crystals include, for example, LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), or KTiOPO4 (potassium titanyl phosphate). In particular, an optical waveguide using periodically poled lithium niobate (hereinafter referred to as PPLN) is an element capable of realizing high wavelength conversion efficiency by increasing the light intensity and using the quasi-phase matching technique. PPLN is expected to be applied in a wide range of optical wavelength bands from the ultraviolet region to the terahertz region, which are applied to optical signal wavelength conversion, optical processing, medical treatment, biotechnology, etc. in optical communication.

[0003] Furthermore, PPLN can be used to fabricate a parametric amplification element and an excitation light generation element that constitute a phase-sensitive amplifier (PSA) capable of low-noise optical amplification. Therefore, PPLN is being considered for application as a device that realizes high-gain, low-noise optical amplification characteristics and plays an important role in the next-generation optical fiber communication field. Also, in the field of quantum computing, an optical waveguide using PPLN can be inserted into a fiber ring resonator and used as a parametric oscillation element. Regarding such a configuration, there has been a report demonstrating the realization of an optical coherent edging machine device and performing large-capacity calculations at a higher speed than a known computer. The wavelength conversion element using an optical crystal such as LiNbO3 described above is described in, for example, Patent Document 1.

[0004] Patent Document 1 discloses an example of fabricating a ridge-type optical waveguide. Patent Document 1 describes fabricating a wavelength conversion element in a ridge-type optical waveguide by bonding a first substrate of a nonlinear optical crystal having a periodic polarization reversal structure to a second substrate having a refractive index smaller than that of the first substrate, in order to improve the light confinement effect. Furthermore, Patent Document 1 describes using a nonlinear optical crystal of the same type as the first substrate as the second substrate to avoid adhesive degradation and cracking due to temperature changes, and applying heat to the first and second substrates to diffuse bond them. To further improve the performance of these technologies, it is important to realize a wavelength conversion device with higher wavelength conversion efficiency. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 3753236 [Overview of the project]

[0006] However, the ridge-type optical waveguide described in Patent Document 1 has the following problems. (a) Exposed optical waveguide core leads to accumulation of dust and other debris, increased optical loss, and malfunctions such as optical burnout. Figure 1 shows a cross-sectional view of the optical waveguide core of a known wavelength conversion element. Conventionally, known wavelength conversion elements are ridge-shaped optical waveguides in which an optical waveguide core 103 is formed on a substrate 101. Because the optical waveguide core 103 is ridge-shaped, its surface is exposed, and the area above the waveguide core 103 (opposite to the substrate 101) is the atmosphere (air). When low-intensity signal light and high-intensity control light propagate through the waveguide core, a portion of the optical electric field seeps out onto the waveguide core surface. As a result, deposits such as dust and debris that absorb light of the wavelengths of the signal light and control light are generated on the waveguide core surface. These deposits absorb the propagating light, leading to increased optical loss and decreased wavelength conversion efficiency of the wavelength conversion element.

[0007] (b) Damage to the exposed optical waveguide core Furthermore, the ridge-shaped optical waveguide core 103 can be easily damaged by being touched during the mounting of the optical conversion element. In addition, when dust or debris that absorbs the wavelength of signal light or control light adheres to the optical waveguide core 103, and especially when control light with high light intensity strikes the dust or debris attached to the core surface, very large amounts of heat are generated due to light absorption. At this time, if the debris attached to the core surface burns and carbonizes, the light absorption rate increases even further, resulting in very large localized heat generation. Such heat generation contributes to increased optical loss in the optical waveguide near the attached material, heat generation in the optical waveguide core, and damage to the optical waveguide core due to stress on the attached material.

[0008] (c) Reduced thermal conductivity due to air cladding The surface of the optical waveguide core 103 of a known wavelength conversion element is in contact with the atmosphere (air, etc.). Heat conduction to the surface above the core occurs through direct heat conduction from the substrate 101 of the wavelength conversion element, thermal radiation such as infrared light, and heat conduction due to convection of the atmosphere (air, etc.) to the surface of the waveguide core 103. Furthermore, the main heat conduction is direct heat conduction from the substrate 101. For this reason, the ridge-shaped portion of the optical waveguide core 103, as viewed from the substrate 101, is the end of the heat conduction and the boundary with the atmosphere (air, etc.). Therefore, due to the effects of thermal radiation and convection of the outside air, a temperature gradient is easily generated between the surface of the optical waveguide core 103 and the substrate 101, which is a factor that reduces temperature controllability. In addition, the wavelength conversion efficiency of the wavelength conversion element 1 is temperature-dependent, and it is necessary to control the temperature of the wavelength conversion element 1 in order to maximize the wavelength conversion efficiency. For this reason, the temperature controllability of the wavelength conversion element is important in order to quickly follow changes in ambient temperature.

[0009] (d) Occurrence of optical propagation loss due to TE-TM converted light In wavelength conversion elements, it is preferable that the optical confinement mode during the propagation of signal light and excitation light be as single as possible. Furthermore, in order to satisfy the pseudo-phase matching condition, it is desirable that the optical confinement mode propagating through the optical waveguide core be single-mode propagation as much as possible. When direct optical coupling using optical fibers (butt joint optical coupling) or optical coupling using spatial optical systems such as lenses, a simple Gaussian distribution shape single-mode propagation is more likely to yield a highly efficient optical connection in order to reduce the optical coupling loss at the input and output of light to the optical wavelength conversion element. Moreover, if multiple multi-mode propagation occurs in the optical waveguide, the effective refractive index in each mode will be slightly different, the phase matching condition during wavelength conversion will also be shifted, and the element will no longer function as a highly efficient wavelength conversion element.

[0010] Furthermore, in the case of wavelength conversion elements that form a ridge-shaped optical waveguide core on the substrate surface, the overcladding is air, which has a low refractive index. This results in a large optical confinement effect of the optical waveguide core, making multimode propagation more likely. For example, in the case of a wavelength conversion element where the anomalous refraction axis of the optical nonlinear crystal axis of the optical waveguide core is perpendicular to the substrate surface, the effective refractive index of the cladding in the polarization direction horizontal to the substrate surface is very small, approximately 1.0, compared to air. Therefore, the optical confinement effect in the polarization direction horizontal to the substrate surface becomes relatively very large, allowing propagation up to higher-order optical modes. Consequently, even if the optical waveguide core is fabricated to allow single-mode signal light propagation in the polarization direction perpendicular to the substrate surface, multiple multimode optical propagation becomes possible in the polarization direction horizontal to the substrate surface, resulting in optical propagation conditions with multiple effective refractive indices.

[0011] At this time, if the effective refractive index of the propagation mode light in the polarization direction perpendicular to the substrate and the effective refractive index of the propagation mode light in the polarization direction horizontal to the substrate are very close in value, fluctuations in the refractive index of the optical waveguide core material, as well as structural fluctuations such as core width and core thickness, occur. At this time, the polarization direction rotates, and so-called TE-TM polarization conversion of the propagation light occurs. When TE-TM polarization conversion occurs, wavelength-converted light with the required polarization cannot be obtained as output light, and the output is either a completely different polarization, or the light energy is dissipated as multimode propagation light, resulting in a loss of light energy such as light absorption in optical spectrum measurements.

[0012] While it is possible to design the optical waveguide core to prevent optical absorption due to energy transitions between waveguide modes, such as TE-TM conversion, when the bandwidth of the optical wavelength used is narrow and limited, this becomes a significant problem when wavelength conversion elements are used across a wide bandwidth of optical wavelengths. Furthermore, such TE-TM conversion is an optical energy transition caused by perturbation resulting from the overlap of the effective refractive indices in the TE-TM polarization direction of the optical waveguide. For this reason, TE-TM polarization conversion occurs similarly in wavelength conversion elements called "Type 1," where the signal light and excitation light have the same polarization direction, and in wavelength conversion elements called "Type 2," where the signal light and excitation light have perpendicular polarization directions, simply because the optical polarization direction is different. Therefore, when fabricating optical and broadband wavelength conversion elements, TE-TM conversion cannot be ignored, regardless of the optical device structure of the wavelength conversion element.

[0013] To achieve the above objective, one embodiment of the wavelength conversion device of the present disclosure is a wavelength conversion device to which signal light is input and which generates light of a different wavelength from the signal light, comprising: an optical waveguide core; a substrate having a refractive index lower than that of the optical waveguide core with respect to the signal light; a wavelength conversion element for converting the wavelength of the signal light; an overcladding layer formed on at least a portion of the surface of the optical waveguide core, having a refractive index lower than that of the optical waveguide core with respect to the wavelength of the signal light and control light combined with the signal light; and a temperature control element for controlling the temperature of the wavelength conversion element, wherein the refractive index of the overcladding layer is determined based on a wavelength range in which no TE-TM conversion loss occurs in the propagating light propagating through the optical waveguide core, and is 0% lower than the refractive index of the optical waveguide core. Higher It is characterized by being in a low range of 25% or less.

[0014] According to the above configuration, it is possible to prevent deposits from adhering to the optical waveguide core surface of the wavelength conversion element, improve temperature controllability, and broaden the optical wavelength band used. By reducing external influences on the wavelength conversion element, it is possible to reduce failures and provide a wavelength conversion device that can be used in a wide optical wavelength band. [Brief explanation of the drawing]

[0015] [Figure 1] This is a cross-sectional view of the optical waveguide core of a known wavelength conversion element. [Figure 2] This is a perspective view showing the wavelength conversion element of this embodiment. [Figure 3] Figure 2 shows a schematic cross-sectional view of an optical waveguide core, cut in a direction perpendicular to the direction of incidence of the signal light. [Figure 4] This figure shows an example of the configuration of a wavelength conversion device in which the wavelength conversion device elements shown in Figure 3 are housed in a metal casing and a temperature control element is provided. [Figure 5] This is a diagram showing the refractive index distribution in a cross-section of the optical waveguide structure in Example 1. [Figure 6]It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 1.0. [Figure 7] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 170 nm when the effective refractive index nOC of the overclad is 1.2. [Figure 8] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 1.4. [Figure 9] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 1.6. [Figure 10] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 1.8. [Figure 11] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 2.0. [Figure 12] It is a diagram showing the effective refractive indices of the propagation modes of each TE and TM polarization from a light wavelength of 1400 nm to 1700 nm when the effective refractive index nOC of the overclad is 2.1. [Figure 13] It is a diagram showing the wavelength region having an intersection point between the TM propagation fundamental mode and the TE propagation mode, which is the wavelength at which TE-TM conversion loss occurs.

Embodiments for Carrying Out the Invention

[0016] [Wavelength Conversion Device] Prior to the description of an embodiment of the present disclosure, a wavelength conversion device will be described. (Explanation of the Second-Order Nonlinear Optical Effect and Phase Matching Conditions) Generally, when signal light [wavelength: λ1, frequency: ω1] and excitation light [wavelength: λ2, frequency: ω2] of different wavelengths are incident on a second-order nonlinear optical crystal, wavelength-converted light (also called idler light) [wavelength: λ3, frequency: ω3] is generated, which has wavelengths according to a relationship called the phase matching condition. Let's consider the case where sum frequency generation ω3 = ω1 + ω2. Since the momentum of a photon is expressed as hk / (2π) depending on Planck's constant h and angular wave number k, if we let the wave number mismatch be Δk, then by the law of conservation of momentum, the following relationship holds. hΔk / 2π=h(k3-k1-k2) / 2π (Formula 1) Therefore, Δk = k3 - k1 - k2 ... (Equation 2)

[0017] If the length of the second-order nonlinear optical crystal through which light propagates is L and the propagation direction is the Z direction, the nonlinear polarization Pz(ω1+ω2) changes phase exp[i(k1+k2)Z], but the phase of the generated amplitude E(ω3) is exp(ik3·Z), so there is a difference between the two. exp(ik3·Z)-exp[i(k1+k2)·Z] =exp[i(k3-k1-k2)·Z]=exp[iΔk·Z]···(Formula 3) From the above, this means that a phase difference of Δk·L will occur. When this phase difference exceeds π, the phase reverses, the direction of energy flow is reversed, and a process occurs in which the ω3 photon splits into ω1 and ω2 photons. In this way, the sum-frequency component of the light wave that was created begins to decrease. The distance at which the phase reverses. Lc = π / (|Δk|) ···(Equation 4) This is called the coherence length.

[0018] Furthermore, when this phase difference exceeds 2π (i.e., the propagation length of light exceeds twice the coherence length), the direction of energy flow returns to its original state, and it can be seen that the nonlinear polarization Pz increases or decreases with a period of twice the coherence length (increase and decrease alternate for each coherence length). Therefore, in order to increase the efficiency of generating wavelength-converted light, the coherence length at which attenuation begins must be longer than the propagation crystal length. In particular, the condition Δk=0, which eliminates wavenumber mismatch, is called the phase matching condition and is a condition for generating wavelength-converted light.

[0019] In this case, when two light waves with frequencies ω1 and ω2 are input to a second-order nonlinear material as described above, and light with frequency ω3 (=ω1+ω2) is generated, this is called sum-frequency generation (SFG). On the other hand, when two light waves with frequencies ω1 and ω3 are input to a second-order nonlinear material, and light with frequency ω2 (=ω3-ω1) is generated, this is called difference-frequency generation (DFG).

[0020] Furthermore, the phenomenon in which a high-intensity light of frequency ω3 is incident and generates two light waves of frequencies ω1 and ω2 is called the optical parametric effect. Here, considering the case where all coupled light waves travel in the same direction, the wavenumber mismatch Δk is: Δk=2π(n3 / λ3-n1 / λ1-n2 / λ2) (Equation 5) Therefore, the phase matching condition is, n3 / λ3=n1 / λ1+n2 / λ2 (Formula 6) or, ω1n1+ω2n2=ω3n3 (Equation 7) This is the result.

[0021] In the above equation, n1, n2, and n3 are the refractive indices of the second-order nonlinear material through which light of wavelengths λ1, λ2, and λ3 (each frequency: ω1, ω2, ω3) propagates. This means that (Equation 7) is equal to n3 when the weighted average of n1 and n2 is weighted by frequency. In particular, in second-harmonic generation, when the polarization of the coupled fundamental photons is the same, the phase matching condition is satisfied when the refractive indices of the fundamental wave and the second harmonic are equal. However, in reality, since materials always have refractive index wavelength dispersion, the phase matching condition is not easily satisfied.

[0022] Therefore, in a uniform medium, methods such as (1) utilizing refractive index dispersion due to the crystal orientation of birefringent crystals (anisotropy with respect to linear polarization), (2) utilizing refractive index dispersion due to optically active materials (anisotropy with respect to circular polarization), and (3) utilizing anomalous dispersion associated with resonance are being investigated.

[0023] (1) is the most widely used method because it is easy to control by angle and temperature. In the case of angle control, the phase matching condition Δk=0 is achieved by an angle matching method of non-parallel arrangement that satisfies the vector phase matching condition by setting an angle in the propagation direction of the interacting light waves, thereby generating wavelength-converted light. However, this angle matching method has the problem that it is not possible to utilize the maximum nonlinear constant of the nonlinear optical crystal. On the other hand, optical waveguides and photonic crystals that control the propagation structure of light have the advantage that the degree of freedom for phase velocity control is greatly expanded because, in addition to material dispersion based on the refractive index, there is structural dispersion that depends on the dimensions and shape of the cross section, and mode dispersion that depends on the mode order.

[0024] (Explanation of pseudo-phase matching) The above method eliminates the wavenumber mismatch Δk=0. However, there is an alternative method called quasi-phase-matched (QPM), which allows for wavenumber mismatch and modulates the nonlinear susceptibility to cancel out the phase shift effect. This idea was proposed by Armstrong et al. in 1962 and is a technique that achieves pseudo-phase matching by periodically reversing the sign of the nonlinear susceptibility. As mentioned above, nonlinear polarization increases or decreases with a period of twice the coherence length. By setting the polarization reversal period to twice the coherence length (reversing the polarization at coherence length intervals), the nonlinear polarization waves generated from each point are added together without canceling each other out, creating an effect as if the amount of phase mismatch were pseudo-zero.

[0025] If the polarization reversal period is Λ, then from the coherence length equation (Equation 4) Λ = 2·Lc ···(Equation 8) If we consider the case where all coupled light waves travel in the same direction, then from (Equation 4), the wavenumber mismatch is not zero, Δk=2π(n3 / λ3-n1 / λ1−n2 / λ2)=2π / Λ (Equation 9) Therefore, n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ=0 (Equation 10) Thus, equation (Equation 8) is the phase matching condition for QPM. Here, n3 is the refractive index at wavelength λ3, n2 is the refractive index at wavelength λ2, and n1 is the refractive index at wavelength λ1.

[0026] Unlike the angle matching method described above, the QPM method has the advantage of being able to use the material orientation that results in the maximum component of the nonlinear susceptibility, such as a second-order nonlinear crystal, and the ability to set the operating wavelength range by selecting the inversion period. Furthermore, by creating an optical waveguide, it is possible to confine light at high density in a narrow region and propagate it over long distances, thus enabling highly efficient wavelength conversion.

[0027] Furthermore, several methods are known for fabricating wavelength conversion elements using pseudo-phase matching techniques. For example, one method involves creating a periodic polarization reversal structure from a crystal substrate that exhibits nonlinear optical effects (hereinafter referred to as a nonlinear optical crystal), and then using that periodic polarization reversal structure to fabricate a proton exchange waveguide. Another example is a method in which, similarly, a periodic polarization reversal structure is created from a nonlinear optical crystal substrate, and then a ridge-type optical waveguide is fabricated using a photolithography process and a dry etching process.

[0028] Figure 2 is a perspective view showing the basic configuration 10 of a wavelength conversion device according to one embodiment of the present disclosure. The basic configuration 10 corresponds to the wavelength conversion element of the first embodiment. The basic configuration 10 shown in Figure 2 is applied to a known wavelength conversion device that generates a difference frequency using QPM. The known wavelength conversion element is disclosed in Patent Document 1.

[0029] As shown in Figure 2, the low-intensity signal light 1a and the high-intensity control light 1b are incident on the multiplexer 14 and combined. The signal light 1a, combined with the control light 1b, travels toward a wavelength conversion element including a substrate 12 and an optical waveguide core 11 placed on the substrate 12. It is incident on one end of the optical waveguide core 11, which has a periodic polarization reversal structure and exhibits a nonlinear optical effect. As the signal light 1a passes through the optical waveguide core 11, it is converted into a difference-frequency light 1c having a different wavelength from the signal light 1a, and is emitted from the other end of the optical waveguide core 11 together with the control light 1b. The difference-frequency light 1c and control light 1b emitted from the optical waveguide core 11 are incident on the demultiplexer 15 and separated from each other. The basic configuration 10 is a wavelength conversion device that receives the signal light 1a as input and generates light with a different wavelength from the signal light 1a. The basic configuration 10 differs from known optical wavelength conversion devices in that at least a portion of the optical waveguide core 11 is provided with an overcladding layer 301, which is an overcladding layer having a lower refractive index than the optical waveguide core 11 with respect to the wavelengths of the signal light 1a and control light 1b.

[0030] In this case, as the wavelength conversion element, SHG generation and optical parametric oscillation using a wavelength conversion element with a QPM method, which has a periodic polarization reversal structure in which the polarization direction of a ferroelectric crystal or a crystal lacking a center of symmetry is periodically reversed by 180°, are used.

[0031] Generally, the refractive index of a nonlinear optical crystal exhibits wavelength dispersion, resulting in a phase difference between the velocity of the fundamental wave and the velocity of the second harmonic, as they are not equal. For this reason, the composite wave of second harmonics generated along the optical path within the crystal becomes a periodic function. Second harmonics generated at each point within the crystal propagate with phase differences between them, and the phase difference between the generated second harmonic and the second harmonic generated at a distance called the coherence length Lc is π. Beyond the coherence length Lc, the intensity of the composite harmonic decreases, and this cycle of increase and decrease repeats. QPM (Quadratic Motion) reverses the phase of the polarization wave generated from the optical nonlinear material at each of these periods, that is, it reverses the sign of the nonlinear optical constant d.

[0032] In this case, if the periodic polarization reversal period, known as the QPM condition, is set to twice the coherent length Lc, the phase of the second harmonic is reversed, correcting the phase of the composite second harmonic from the coherent length Lc. Therefore, the light intensity of the generated second harmonic is not reduced but added together, increasing the amplitude (intensity) of the second harmonic and generating second-harmonic light. This characteristic allows the use of the largest component of the nonlinear optical constant and can be utilized even with crystals having a small birefringence.

[0033] Furthermore, in the generation of optical difference frequency, if n3 of the wavelength conversion element is the refractive index at wavelength λ3, n2 is the refractive index at wavelength λ2, and n1 is the refractive index at wavelength λ1, and the polarization reversal period is Λ, and the coherent length is Lc, then as described above, Λ = 2·Lc ···(Equation 11) In this case, the optically nonlinear polarization wave is amplified. At this time, n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ=0 (Equation 12) As shown in the above equation (Equation 12), this is the phase matching condition for QPM. Here, n3 is the refractive index at wavelength λ3, n2 is the refractive index at wavelength λ2, and n1 is the refractive index at wavelength λ1.

[0034] Unlike the angle matching method described above, the QPM method can use the material orientation that results in the maximum component of the nonlinear susceptibility, such as that of a second-order nonlinear crystal. Furthermore, the QPM method has the advantage of being able to set the operating wavelength range by selecting the inversion period, and by creating an optical waveguide, it can confine light at high density in a narrow region and propagate it over long distances, thus enabling highly efficient wavelength conversion.

[0035] The basic configuration 10 shown in Figure 2 is known to be housed together with a multiplexer and a demultiplexer in a metal housing equipped with input / output ports capable of inputting and outputting light, in order to prevent deterioration of its characteristics due to changes in the operating environment, thereby constituting an optical conversion device. Furthermore, the wavelength conversion efficiency of the wavelength conversion element is temperature-dependent, and it is necessary to control the temperature of the wavelength conversion element in order to maximize its wavelength conversion efficiency.

[0036] Figure 3 is a schematic cross-sectional view obtained by cutting the optical waveguide core 11 shown in Figure 2 in a direction perpendicular to the incident direction of the signal light 1a. As described above, this embodiment includes an overcladding 301 on at least a portion of the optical waveguide core 11. The overcladding 301 is a layer whose refractive index for the signal light 1a and control light 1b is lower than that of the optical waveguide core 11, enabling optical confinement of the optical waveguide core 11.

[0037] The configuration shown in Figure 3 includes a substrate 12, an optical waveguide core 11 formed on the substrate 12, and an overcladding 301 formed on the upper surface 12a of the substrate 12 and a portion of the surface of the optical waveguide core 11. The refractive index of the substrate 12 with respect to signal light 1a is lower than that of the optical waveguide core 11. The overcladding 301 shown in Figure 3 is formed on the upper surface 12a of the substrate 12, the upper surface 11a and side surface 11b of the optical waveguide core 11, but not on the cross section 11c. This is to prevent impairing the transmittance of signal light 1a and control light 1b to the optical waveguide core 11.

[0038] As shown in Figure 3, the overcladding 301 does not need to cover the entire upper surface 12a of the substrate 12; it is sufficient if it covers the surface excluding the surface on which the ridge-shaped optical waveguide core 11 enters or exits. Furthermore, in this embodiment, the overcladding may be formed to cover a portion of the ridge-shaped side surface, depending on the specifications and application of the basic configuration 10. The film thickness of the overcladding 301 should be 0.5 microns or more, but a film thickness of 1 micron or more is desirable in order to completely retain the seepage electric field of the propagating light.

[0039] If the overcladding 301 reduces the leakage of the optical electric field to the surface of the ridge-shaped optical waveguide core to a negligible degree, then even if dust or other debris adheres to the surface of the overcladding 301, it is possible to reduce the increase in optical loss caused by the propagation of high-intensity light and the burning of the attached debris.

[0040] Figure 4 shows an example of the configuration of a wavelength conversion device 20, which further includes a metal housing bottom member 28, a cover member 29, and a temperature control element 26, in addition to the basic configuration 10 of Figure 3. The metal housing bottom member 28 and the cover member 29 constitute the metal housing. The metal housing is provided with an optical input port 200 and an output port 201. The wavelength conversion device 20 shown in Figure 4 further includes a support member 27 that supports the temperature control element 26. The support member 27 is a metal member for uniformly controlling the overall temperature of the wavelength conversion element 13, including the optical waveguide core 11 and the substrate 12. The temperature control element 26 is interposed between the support member 27 and the metal housing bottom member 28, and is bonded and fixed between the temperature control element 26, the support member 27, and the metal housing bottom member 28 by a bonding member (not shown) that conducts heat and is resistant to changes in its fixed position. Note that the optical waveguide core 11, substrate 12, wavelength conversion element 13, multiplexer 14, demultiplexer 15, signal light 1a, and difference frequency light 1c are the same as those described in Figure 2, so their explanation is omitted.

[0041] Furthermore, when a wavelength conversion element made of ferroelectric crystal material is used in a wavelength conversion device, a phenomenon called photodamage occurs, in which the refractive index of the wavelength conversion element changes and its properties deteriorate due to irradiation with light having a short wavelength. As a method to suppress the effects of this photodamage, it has been proposed to use the wavelength conversion element at high temperatures. For this reason, in the first embodiment, a temperature control element 26 is provided in the wavelength conversion device 20, and the temperature control element 26 operates the wavelength conversion element 13 in an environment with a temperature range from approximately 20°C or higher near room temperature, where condensation does not practically occur, to approximately 100°C or lower, where the adhesive does not deteriorate.

[0042] In this embodiment, there is no restriction on the refractive index as long as it is sufficient to prevent the adhesion of dust and other deposits to the surface of the optical waveguide core 11. However, in practice, since it is necessary to propagate the signal light 1a and control light 1b through the optical waveguide core 11, an overclad 301 with a lower refractive index than the optical waveguide core 11 is desirable in order to confine the light at the wavelengths of the signal light and control light. Furthermore, since light leakage of the signal light and control light from the optical waveguide core to the overclad occurs, it is desirable that the overclad be made of a material with excellent light transmittance at the wavelengths of the signal light and control light.

[0043] Here, we will explain how limiting the refractive index range of the overcladding reduces TE-TM optical coupling and broadens the usable optical bandwidth. In particular, when a nonlinear crystal is used as the optical waveguide core of a wavelength conversion element, the refractive index of the optical waveguide core for each TE polarization and TM polarization is generally sufficiently larger than the refractive index of air, which is approximately 1.0. Therefore, the specific refractive index of optical confinement in TE polarization parallel to the substrate becomes large, making it easier for the propagation modes in TE polarization to become multimode, and allowing optical confinement modes of TE polarization with a very large number of effective refractive indices to propagate. For this reason, even if the design is made to approach a single mode (0th-order mode) with respect to the propagation mode of signal light or excitation light in TM polarization mode, a propagation mode light of TE polarization equal to the effective refractive index of TM polarization can still exist.

[0044] Therefore, fluctuations in the refractive index and structure of the optical waveguide make it easier for optical loss of TM polarization caused by TE-TM polarization conversion to occur. For this reason, by bringing the refractive index of the overcladding closer to that of the optical waveguide core, not only for TM polarization, but also for TE polarization, the number of propagation modes in TE polarization can be reduced, thereby reducing the optical loss of TE-TM polarization conversion and enabling broader wavelength ranges. To achieve such a configuration, it is desirable that the refractive index of the overcladding be 0% to 25% lower than that of the optical waveguide core. More specifically, it is desirable that the refractive index of the overcladding be within a range of 0% to 6% lower than that of the optical waveguide core. Here, refractive index refers to the refractive index of the signal light and control light relative to the overcladding, or the refractive index relative to the optical waveguide core. "Refractive index 0% lower than that of the optical waveguide core" means that the refractive index is equal to that of the optical waveguide core.

[0045] Next, the material of the overcladding will be described. As the overcladding material, since signal light and highly intense excitation light are incident on the optical waveguide core of the wavelength conversion element, a material that does not degrade easily with respect to the optical wavelength used is desirable. Furthermore, since the overcladding is fabricated adjacent to the optical waveguide core, a material with a similar coefficient of linear thermal expansion to the optical waveguide core is desirable. This includes inorganic materials similar to the optical nonlinear crystalline materials used in the optical waveguide core, specifically LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1-x)O3 (0≦x≦1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and further, It is desirable that these materials be inorganic materials containing at least one oxide selected from Zr (zirconium), Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium), Zr (zirconium), Nb (niobium), Ta (tantalum), Hf (hafnium), Mg (magnesium), Zn (zinc), Sc (scandium), Ti (titanium), Y (yttrium), Al (aluminum), In (indium), and Si (silicon).

[0046] Furthermore, depending on the material of the optical waveguide core, if an optically nonlinear crystal or the like is used as the inorganic material, it may have a relatively large linear thermal expansion coefficient of 10 ppm or more. In such cases, an organic material with a relatively large linear thermal expansion coefficient can also be used. More specifically, polyolefins such as polyethylene, polypropylene, and polybutylene, polydienes such as polybutadiene and natural rubber, vinyl polymers such as polystyrene, polyvinyl acetate, polymethyl vinyl ether, polyethyl vinyl ether, polyacrylic acid, polymethyl polyacrylate, polymethacrylic acid, polymethyl methacrylate, polybutyl methacrylate, polyhexyl methacrylate, and polydodecyl methacrylate, linear olefin-based polyethers, polyphenylene oxide (PPO), and its copolymers and blends, polyethersulfone (PES) which contains a mixture of ether groups and sulfone groups, and ethersulfone. Polyethers such as polyether ketones (PEK) containing both thiol and carbonyl groups, polyphenylene sulfide (PPS) and polysulfone (PSO) containing thioether groups, and their copolymers and blends; polyolefins having at least one substituent such as an OH group, thiol group, carbonyl group, or halogen group at their termini, such as HO-(CCCC-)n-(CC-(CC-)m)-OH, polyoxides such as polyethylene oxide and polypropylene oxide, polymer materials such as polybutyl isocyanate and polyvinylidene fluoride; and crosslinked products of epoxy resins, oligomers and curing agents. Furthermore, mixtures of two or more of these materials may also be used.

[0047] Furthermore, polysiloxane or crosslinked polysiloxane (commonly known as silicone resin) may be used. This material not only has a large temperature coefficient of refractive index, but also excellent water resistance and long-term stability, making it the most suitable material for light intensity compensation in the present invention.

[0048] Polysiloxanes are represented by the following general formula. R1-((R4)Si(R3)-O)-((R4)Si(R3)-O) n-((R4)Si(R3)-O)-R2 In the above formula, R1 and R2 at both ends represent terminal groups, and consist of one of the following: hydrogen, alkyl group, hydroxyl group, vinyl group, amino group, aminoalkyl group, epoxy group, alkyl epoxy group, alkoxy epoxy group, methacrylate group, chlorine group, or acetoxy group.

[0049] R3 and R4 of the siloxane bond represent side chain groups and consist of hydrogen, alkyl, alkoxy, hydroxyl, vinyl, amino, aminoalkyl, epoxy, methacrylate, chlor, acetoxy, phenyl, fluoroalkyl, alkylphenyl, and cyclohexane groups. The polysiloxane used may be one type or a mixture of multiple types.

[0050] On the other hand, crosslinked polysiloxanes are produced by reacting a reactive polysiloxane having vinyl, hydrogen, silanol, amino, epoxy, or carbinol end groups with a polysiloxane in the presence of a platinum catalyst, radicals, acid, or base. In addition, polysiloxanes can be used in the form of a soft gel, a composite material containing low molecular weight polysiloxanes in a gel-like state, or a mixture of high molecular weight polysiloxanes and low molecular weight polysiloxanes that has been crosslinked.

[0051] Next, we will explain the method for fabricating the optical conversion element described above. First, the method for fabricating the optical waveguide core shown in Figure 2, etc., will be explained. As a method for fabricating the wavelength conversion element, first, a metal electrode film is fabricated at a desired position on a wafer substrate made of a nonlinear optical crystal, which is the wavelength conversion material, using photolithography to create a periodic polarization reversal structure that satisfies the pseudo-phase matching condition. Periodic polarization reversal is formed by applying a DC high electric field, and the metal electrode film and insulating film are removed to fabricate a wafer for the optical waveguide core.

[0052] Next, the wafer for the optical waveguide core is bonded to the substrate using a plasma discharge surface activation method or a thermal bonding method, and then processed to the desired core thickness by grinding and polishing to the desired film thickness. Furthermore, a pattern of the optical waveguide core is formed on the surface of the optical waveguide core layer on the substrate using a photoresist material, and the core layer is processed into an optical waveguide core with the desired ridge shape by a dry etching method under vacuum using Ar plasma, etc. Finally, resist residue and other materials on the surface of the optical waveguide core are cleaned and removed by methods such as piranha washing.

[0053] Next, the method for fabricating the overcladding formed on the surface of the optical waveguide core in this embodiment will be described. Subsequently, in this embodiment, overcladding is formed on the surface of the optical waveguide core of the ridge-shaped wavelength conversion element. As for the method of forming the overcladding, for materials that do not undergo solvent dilution or liquefaction, sputtering, chemical vapor deposition (CVD), or vacuum deposition can be used in an air environment. For materials that are soluble in solvents or that can be made non-fluid by thermal melting or chemical reaction, spin coating or casting in a solution state can also be used. [Examples]

[0054] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples. (Example 1) Figure 5 shows the refractive index distribution diagram of the cross-section of the optical waveguide structure used in Example 1 of the present invention. Figure 5 shows the effective refractive index n of the overcladding to make the structure and refractive index of the overcladding easier to understand. OC Figure 5 illustrates the refractive index distribution configuration of the cross-section of an optical waveguide structure when the refractive index is 1.6. Figure 5 shows the refractive index of the cross-section of the optical waveguide core using the shades of gray in the image. The relationship between the refractive index and the shades of gray is shown by the bar on the right side of the figure.

[0055] In this embodiment of the wavelength conversion device, in order to verify whether TE-TM conversion occurs in the S-band (Short-wavelength-band) from 460 nm to 1530 nm, the C-band (conventional-band) from 1530 nm to 1565 nm, and the L-band (Long-wavelength-band) from 1565 nm, which are optical communication wavelengths, the optical propagation modes in each TE and TM mode were analyzed over a broadband of optical wavelengths from 1400 nm to 1700 nm, and the wavelength bands in which TE-TM mode conversion loss occurs were estimated.

[0056] As an analysis method for optical propagation modes, the Mode Solver of OptiWave's BPM-CAD was used, and the analysis was performed using the finite element method. For the optical waveguide cross-sectional structure, the optical waveguide core 11 shown in Figure 5 was made of LiNbO3 (LN), with a Z-cut crystal axis having anomalous refraction in a direction perpendicular to the substrate surface, and a core width of 5.3 μm and a core thickness of 5.0 μm. The substrate 12 was made of an LN crystal using LiTaO3 (LT), and in order to reduce the influence of the refractive index of the adjacent substrate, the optical waveguide core was set to be the same width as the core (5.3 μm) but with a height of 1.0 μm away from the substrate 12, and a convex rib structure was formed on the surface of the substrate 12. The core width and core thickness in this embodiment were set so that single-mode propagation of signal light with TM polarization is possible, and it is possible to fabricate it with a core width of 5.3 μm and a core thickness of 5.0 μm or less. An overcladding 301 with a thickness of 1.0 μm was formed on the surface of the optical waveguide core 11. The surface of the substrate 12 and the region surrounding the overcladding 301 were assumed to be air (vacuum), and the refractive index was assumed to be 1.0.

[0057] Furthermore, in the optical waveguide cross-sectional structure of this embodiment, the effective refractive index n of the overcladding 301 OC =1.0, the n adjacent to the refractive index in the TM direction (Z axis direction) of the optical waveguide core. OCThe value was varied up to 2.1, and propagation modes in the optical wavelength range of 1400 to 1700 nm were calculated, along with the wavelength dependence of the effective refractive index. Assuming a type 1 wavelength conversion element where the polarization direction of the signal light and excitation light are the same, the TM polarization mode perpendicular to the substrate surface and the TE polarization mode parallel to the substrate were analyzed. When the signal light and excitation light propagating in TM polarization have the same effective refractive index as the TE polarization mode, TE-TM polarization conversion occurs, and the wavelength of that optical conversion was determined.

[0058] Figures 6 to 12 show the effective refractive index n of the overcladding. OC These figures show the effective refractive indices of the propagation modes of each TE and TM polarization at optical wavelengths from 1400 nm to 1700 nm, respectively, when the refractive indices are 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, and 2.1. In this case, the combination of dashed lines and dots with a relatively large pitch indicates the effective refractive index of the 0th (fundamental) mode of TM polarization, which corresponds to the effective refractive index of the signal light. In Figures 6 to 11, except for the optical wavelength dependence of the 0th mode of TM light, these are the optical wavelength dependences of the effective refractive indices of each higher-order mode of TE polarization. As shown in Figures 6 to 12, near the intersection of the effective refractive indices of the 0th-order TM mode and each higher-order TE mode, optical energy conversion between TE mode light and TM mode light occurs due to perturbations in the propagation modes of the optical waveguide, resulting in TE-TM conversion loss. In other words, the intersection of the effective refractive indices of TE mode light and TM mode light corresponds to the optical wavelength of the TE-TM conversion loss.

[0059] As a result, the refractive index n of the overcladding OC As the value increases from 1.0 towards 2.1 near the core refractive index, the wavelength band without intersection, i.e., where TE-TM conversion does not occur, expands.

[0060] Figure 13 shows the region where the TM propagation zeroth mode and TE propagation mode intersect, which is the wavelength at which TE-TM conversion loss occurs, with the longer wavelength side indicated by a triangle (▲) and the shorter wavelength side indicated by a square (■). In other words, the solid line region in Figure 13 is the region where TE-TM conversion loss does not occur. Figure 13 also shows the wavelength bands of the S band, C band, and L band used in optical communication.

[0061] As shown in Figure 13, in order to use the wavelength changing element of this embodiment across the entire C band, the refractive index n of the overcladding must be OC The refractive index should be greater than 1.6, meaning it needs to be between 0% and 25% smaller than the effective refractive index of the optical waveguide core. Furthermore, in order to use the wavelength conversion element across the entire wavelength band of the S, C, and L bands, the refractive index of the overcladding must be n OC The value should be greater than 2.0, meaning the refractive index needs to be 0% to 6% smaller than the effective refractive index of the optical waveguide core. Thus, according to this embodiment, it was found that overcladding formation and refractive index control are necessary for broadening the optical wavelength range of the wavelength conversion element. [Explanation of Symbols]

[0062] 1a signal light 1b Control light 1c difference frequency light 10 Basic configuration 11 Optical waveguide core 11a Top surface 11b Side 11c cross section 12 circuit boards 12a Top side 13 Wavelength conversion element 14 Multiplexer 15 Duplexer 20 Wavelength conversion device 26 Temperature control elements 27 Support Member 28 Metal housing bottom member 29 Lid Member 101 circuit board 103 Optical waveguide core 301 Overclad

Claims

1. A wavelength conversion device that receives a signal light and generates light of a different wavelength from the signal light, A wavelength conversion element comprising an optical waveguide core and a substrate having a lower refractive index than the optical waveguide core with respect to the signal light, which converts the wavelength of the signal light, The optical waveguide core is formed on at least a portion of its surface and includes an overcladding layer having a lower refractive index than the optical waveguide core with respect to the wavelength of the signal light and the control light combined with the signal light. A wavelength conversion device characterized in that the refractive index of the overcladding layer is determined based on the wavelength range in which no TE-TM conversion loss occurs in the propagating light propagating through the optical waveguide core, and is in a range that is greater than 0% and less than or equal to 25% lower than the refractive index of the optical waveguide core.

2. The overcladding layer is LiNbO 3 (Lithium niobate), KNbO 3 (Potassium niobate), LiTaO 3 (Lithium tantalate), LiNb(x)Ta(1-x)O3 (0≦x≦1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and furthermore, at least one oxide selected from Zr (zirconium), Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium), Zr (zirconium), Nb (niobium), Ta (tantalum), Hf (hafnium), Mg (magnesium), Zn (zinc), Sc (scandium), Ti (titanium), Y (yttrium), Al (aluminum), In (indium), Si (silicon), Furthermore, polyolefins such as polyethylene, polypropylene, and polybutylene; polydienes such as polybutadiene and natural rubber; vinyl polymers such as polystyrene, polyvinyl acetate, polymethyl vinyl ether, polyethyl vinyl ether, polyacrylic acid, polymethyl polyacrylate, polymethacrylic acid, polymethyl methacrylate, polybutyl methacrylate, polyhexyl polymethacrylate, and polydodecyl methacrylate; linear olefin-based polyethers; polyphenylene oxide (PPO), and copolymers and blends thereof; ether groups and sulfones. The wavelength conversion device according to claim 1, comprising: polyethersulfone (PES) with mixed groups; polyetherketone (PEK) with mixed ether and carbonyl groups; polyethers such as polyphenylene sulfide (PPS) and polysulfone (PSO) having thioether groups; copolymers and blends thereof; polyolefins having at least one substituent such as an OH group, thiol group, carbonyl group, or halogen group at their terminals; epoxy resin; crosslinked products of oligomers and curing agents; or mixtures of two or more of the above materials.

3. The optical waveguide core is LiNbO 3 (Lithium niobate) is used, and as the substrate, LiTaO 3 The wavelength conversion device according to claim 1 or 2, characterized in that (lithium tantalate) is used and the overcladding layer is provided on the surface of the optical waveguide core.

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