JP6774372B2 - Optical element - Google Patents

Description

The present invention relates to an optical element using a nonlinear optical effect, and specifically to an optical element used in an optical communication system or an optical measurement system.

(Expectations for nonlinear optics and basic effects of nonlinear optics)
Optical application technology using nonlinear optical effects is expected in the new field of optical communication and the field of quantum information communication using light. Wavelength conversion is known as a basic effect among nonlinear optical effects. In wavelength conversion, light incident on a nonlinear optical medium can be converted into light having a different frequency. Utilizing this characteristic, it is widely known as a technology for producing light in a wavelength band that is difficult to oscillate with a single laser. In particular, a periodic polarization inversion waveguide using lithium niobate (LiNbO3), which is a material exhibiting a second-order nonlinear optical effect and has a large nonlinear constant, is a light source already on the market due to its high efficiency of the nonlinear optical effect. It is built in.

In the second-order nonlinear optical effect, light having wavelengths λ1 and λ2 is input to generate a new wavelength λ3.
1 / λ3 = 1 / λ1 + 1 / λ2 (Equation 1)
Wavelength conversion that satisfies (Equation 1) is called sum frequency generation (SFG), and when λ1 = λ2, that is, by transforming (Equation 1), (Equation 2) is obtained.
λ3 = λ1 / 2 (Equation 2)
Wavelength conversion that satisfies (Equation 2) is called second harmonic generation (SHG). further,
1 / λ3 = 1 / λ1-1 / λ2 (Equation 3)
Wavelength conversion that satisfies (Equation 3) is called differential frequency generation (DFG). Furthermore, there is also an optical parametric effect that inputs only λ1 and generates λ2 and λ3 that satisfy (Equation 3). In particular, SHG and SFG newly generate short-wavelength light, that is, high-energy light with respect to input light, and are often used for generation of visible light region and the like.

In order to efficiently cause these second-order nonlinear optical effects, it is required that the amount of phase mismatch of the interacting three wavelengths is zero. Therefore, the amount of phase mismatch can be made zero by periodically inverting the polarization of the material exhibiting the second-order nonlinear optical effect in the waveguide direction of light. Assuming that the inversion period at that time is Λ, Λ that satisfies the following (Equation 4) may be set for the wavelengths λ1, λ2, and λ3 in the sum frequency generation represented by the equation 1.
n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ = 0 (Equation 4)
Here, n1 is the refractive index at the wavelength λ1, n2 is the refractive index at the wavelength λ2, and n3 is the refractive index at the wavelength λ3.

In addition to such a periodic polarization inversion structure, high-efficiency wavelength conversion is possible by forming a waveguide. The effect of the non-linear optical effect increases as the overlapping density of light that causes the non-linear interaction increases. Therefore, highly efficient wavelength conversion becomes possible by adopting a waveguide structure capable of confining light in a small cross-sectional area and guiding light over a long distance.

(Direct junction type ridge waveguide)
In order to realize a waveguide structure using a nonlinear optical crystal, a method using Ti diffusion or proton exchange was common. However, in recent years, as described in Non-Patent Document 1 as a wavelength conversion element, the bulk characteristics of the crystal can be used as they are, and the ridge type has features such as high light damage resistance, long-term reliability, and ease of device design. Optics is being researched and developed. This ridge type optics is formed by joining two substrates, thinning one substrate, and further performing ridge processing. A direct joining technique is known as a technique for firmly joining substrates to each other without using an adhesive or the like when joining the substrates. The direct junction type ridge type waveguide using this technology can inject strong light, and as described in Non-Patent Document 2, it has succeeded in reducing the core with the progress of the waveguide technology. The efficiency of nonlinear optical effects is steadily improving.

(Applied technology and requirements)
The waveguide by the direct junction type nonlinear optical crystal with this periodic polarization inversion structure that can inject strong light is not only as a wavelength conversion element but also as a low noise optical amplifier and a generator of quantum entangled photon pairs. It is expected in recent years. In these applied techniques, it is important to align the characteristics of a plurality of nonlinear optical waveguides and to strictly control the phase of incident light. For example, as described in Non-Patent Document 3, when a phase-sensitive amplifier that operates only in one polarization is applied to the current long-distance communication, the polarization-multiplexed light is once separated and separated by nonlinear optical conduction. After amplifying the light with the waveguide, it is necessary to align the phases of the two lights again and combine them. At this time, it is ideal that the nonlinear optical waveguide has exactly the same optical characteristics. This is because crosstalk in the transmission line due to the phase shift at the time of merging and the gain difference between the polarizations eventually causes signal deterioration.

(Expectations for integration and array waveguides)
In such an applied technique, it is expected to integrate a quartz-based planar optical circuit and a nonlinear optical element capable of aligning the characteristics of a nonlinear optical waveguide and strictly controlling the phase of incident light.

In recent years, a planar optical circuit using quartz using semiconductor microfabrication is capable of strict optical path length control due to its high processing accuracy, and has been put into practical use in current optical communication systems. Further, regarding optical signal processing using a second-order nonlinear optical effect, a technique of plane-developing optics having a second-order nonlinear optical effect has been reported as described in Non-Patent Documents 4 and 5. ing. These are optical phase levels by deploying an array of PPLN (Periodically Collected Lithium niobate) waveguides (PPLN array waveguides) and performing multiple parallel nonlinear optical operations within a single device. Strict optical signal processing is possible.

Therefore, the integration of a quartz-based planar optical circuit with high processing accuracy and a PPLN array waveguide is an effective method for a nonlinear optical application technique such as the above-mentioned phase-sensitive amplifier.

Japanese Unexamined Patent Publication No. 2015-215501 Japanese Unexamined Patent Publication No. 2014-21159

Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “Direct-bonded QPM-LN ridge waveguide with high damage resistance at room temperature,” Electronics Letters, Vol.39, No.7, p.609-611, 2003. T. Umeki, O. Tadanaga, and M. Asobe, ‘Highly Efficient Wavelength Converter Using Direct-Bonded PPZnLN Ridge Waveguide’ IEEE Journal of Quantum Electronics, Vol. 46, No. 8, pp. 1206-1213, 2010. T. Umeki, T. Kazama, O. Tadanaga, K. Enbutsu, M. Asobe, Y. Miyamoto, and H. Takenouchi, “PDM Signal Amplification Using PPLN-Based Polarization-Independent Phase-Sensitive Amplifier,” Journal of Lightwave Technology , Vol. 33, No. 7, p. 1326-1332, 2015. T. Kazama, T. Umeki, M. Asobe, and H. Takenouchi, “Single-Chip Parametric Frequency Up / Down Converter Using Parallel PPLN Waveguides,” IEEE Photonics Technology Letters, Vol. 26, No. 22, p. 2248- 2251, 2014. H. Jin, FM Liu, P. Xu, JL Xia, ML Zhong, Y. Yuan, JW Zhou, YX Gong, W. Wang, and SN Zhu, “On-Chip Generation and Manipulation of Entangled Photons Based on Reconfigurable Lithium- Niobate Waveguide Circuits, ”Physical Review Letters, Vol. 113, 103601, 2014.

(Requirements for arrayed waveguides)
Here, a phase matching wavelength can be mentioned as a characteristic to be aligned of each waveguide constituting the PPLN array waveguide. The phase matching wavelength is the wavelength at which the nonlinear optical medium emits SHG light most efficiently, and is an important parameter for determining the nonlinear optical effect. The phase matching wavelength is determined by the effective refractive index of the waveguide and the polarization inversion period. When considering the application to a phase-sensitive amplifier, it is necessary to keep the phase matching wavelength within ± 0.5 nm for the entire device.
However, it is difficult to align the phase matching wavelength within this value in the array waveguide for the following two reasons. The first is the shift of the phase matching wavelength due to the waveguide processing error, and the second is the shift of the phase matching wavelength due to the heat generation of the waveguide.

(1. Deviation due to waveguide machining error)
The phase matching wavelength is a parameter sensitive to the effective refractive index of the waveguide. FIG. 1 shows the relationship between the width of the waveguide (also referred to as the core) and the phase matching wavelength in the direct junction type PPLN waveguide. Here, the thickness of the core is 5 μm and the inversion period is 18.4 μm. A width difference of about 0.1 μm from the figure causes a wavelength shift of 3 nm. That is, in this case, in order to suppress the phase matching wavelength within ± 0.5 nm, it is necessary to suppress the waveguide width within a deviation of 20 nm. The PPLN waveguide is made by photolithography technology, and the error of the waveguide width is determined by the uniformity of the processing process. When considering the uniformity of the processing process, the error of the phase matching wavelength can be reduced by narrowing the waveguide spacing of the array waveguide. However, simply narrowing the waveguide interval will increase the effect of the phase matching wavelength shift due to the heat generation of the waveguide, which will be described later.

(2. Deviation due to heat generation of waveguide)
When it is necessary to inject high-intensity light such as phase-sensitive amplification into the waveguide, the phase-matched wavelength control of the waveguide becomes more difficult. This is because when strong light is used, the waveguide is heated by the light absorption of the waveguide base material, the refractive index changes, and the phase matching condition is not satisfied. If the optical input power is increased due to the deviation of the phase matching condition, the nonlinear optical efficiency is lowered. In addition, the heat generation becomes complicated due to the following factors, and a temperature distribution is generated in the device.

There are two patterns of light absorption: absorption of input light and absorption of converted light. The light intensity of the fundamental wave, which is the input light, is maximum on the input side, wavelength conversion occurs as it propagates in the waveguide, and the light intensity of the fundamental wave becomes smaller. Therefore, the heat generated by the absorption of the fundamental wave is large on the incident side of the waveguide and small on the outgoing side. On the other hand, the intensity of the wavelength conversion light increases as it propagates through the waveguide. As described in Non-Patent Document 1, it is generally known that when the converted light has a short wavelength, the absorption coefficient of the converted light is larger than that of the fundamental wave light, and heating of the waveguide caused by the converted light Will have a higher heat distribution on the output side. Furthermore, the heat distribution in the waveguide becomes very complicated because the heat distribution due to the fundamental wave light is also added.

In the case of an arrayed waveguide, since the distance between the used waveguides is short, there is a problem that the heat generated in the waveguide thermally interferes with the adjacent waveguide. The change in the refractive index itself due to the temperature rise due to thermal interference also affects the heat generation due to light absorption as described above, and in some cases the temperature vibrates and becomes unstable, and the optical characteristics become unstable. There is also.

In order to suppress the temperature rise of the entire element, the configuration shown in FIG. 2 is often used in the conventional PPLN element. In the conventional PPLN element shown in FIG. 2, the waveguide 2 is directly bonded on the substrate 1, and the metal plate 3 and the thermal control element 4 are formed on the surface of the substrate 1 opposite to the waveguide 2. Have. This is because the temperature rise of the entire element can be suppressed by arranging the Peltier element as a heat control element under the metal carrier that supports the element, but the effect of suppressing the heat distribution in the waveguide direction is small.

Further, a structure as shown in FIG. 3 has been proposed as a simple method for suppressing the heat distribution generated in the waveguide direction (Patent Document 1). The element of Patent Document 1 has a configuration in which the periphery of a waveguide 2 provided on a substrate 1 is covered with a conductive resin 5, and a metal plate 3 and a thermal control element 4 are further provided. In this method, the heat generated in the waveguide 2 is efficiently dissipated to the metal plate 3, the temperature gradient of the entire element can be reduced, and the temperature of the entire element can be kept constant. It is very effective from the viewpoint of suppressing heat distribution. However, this method cannot be applied to array waveguides.

In order to apply a nonlinear optical crystal array waveguide to a nonlinear optical application technology that utilizes high-power light such as a phase-sensitive amplifier, it is necessary to realize an array waveguide having a uniform phase matching wavelength. From the viewpoint of machining accuracy, it is necessary to narrow the array waveguide spacing, suppress the complicated heat distribution in each waveguide, and suppress the thermal interference with the adjacent waveguide. Here, in order to control the phase matching wavelength of each waveguide of the array waveguide, a method of installing a heater on the individual waveguide as described in Patent Document 2 has been proposed. In this case, it is possible to apply heat individually to each waveguide of the individual waveguide, but the heat cannot be dissipated and the thermal controllability is insufficient. In addition, thermal interference cannot be suppressed by this method. In addition, there is a method of forming a heater on each of the waveguides of the conventional element structure shown in FIG. 2, but a heater provided on an adjacent waveguide as compared with the distance between the heater and the thermal control element. Since the distance between them is shorter, the influence of thermal interference from the adjacent waveguide cannot be suppressed.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is an array waveguide structure capable of more accurately controlling the phase matching condition by improving the thermal controllability of optics using a nonlinear optical material. Is to provide an optical element having.

In order to solve the above problems, the invention described in one embodiment is a second-order nonlinear optics having a substrate constructed of a low thermal conductive material and a periodic polarization inversion structure in the waveguide direction of light. A plurality of waveguides composed of a medium and arranged on the substrate, and each waveguide of the plurality of waveguides along the longitudinal direction of the plurality of waveguides, each of which is composed of a plurality of waveguides and a high thermal conductive material. A waveguide surrounding structure formed so as to surround the waveguide is provided, and the waveguide surrounding structure includes a heat radiating lid holding portion made of the same material as the waveguide and a heat radiating lid portion made of the high thermal conductive material. The waveguide is formed so that the distance from the waveguide to the heat radiation lid holding portion is shorter than the thickness of the substrate, and the waveguide and the waveguide surrounding structure are formed so as not to come into contact with each other. It is an optical element characterized by being

It is a figure which shows the relationship between the waveguide width and the phase matching wavelength in the direct junction type PPLN waveguide. It is a figure which shows the structural example of the conventional PPLN element. It is a figure which shows another example of the structure of the conventional PPLN element. It is a figure which shows the structural example of the optical element of this invention. It is a figure which shows the cross-sectional structure of the wavelength conversion element of Example 1. It is a figure which shows the cross-sectional structure of the wavelength conversion element of Example 2. It is a figure which shows the cross-sectional structure of the phase modulator of Example 3.

Hereinafter, embodiments of the present invention will be described in detail.

FIG. 4 is a diagram showing a configuration example of the optical element of the present embodiment. As shown in FIG. 4, the optical element of the present embodiment includes a substrate 1, a plurality of waveguides 2 arranged on the substrate 1, and a plurality of waveguides 2 along the longitudinal direction of the plurality of waveguides 2. It is configured as a wavelength conversion element having a waveguide surrounding structure 6 formed so as to separately surround the two. The waveguide 2 is formed of a second-order nonlinear optical medium having a periodic polarization inversion structure in the waveguide direction of light, the substrate 1 is constructed of a low thermal conductive material, and the waveguide surrounding structure 6 has a high thermal conductivity. It is constructed using sex materials. The waveguide 2 and the waveguide surrounding structure 6 are formed so as not to come into contact with each other.

Further, as shown in FIG. 4, the optical element may be provided with a heater 72 in a part of the waveguide 2, or a thermal control element 4 may be provided in a part of the waveguide surrounding structure 6. Further, the waveguide 2 in the optical element of the present embodiment is formed as a ridge type or rib type waveguide. The ridge type waveguide is an elongated waveguide provided on the substrate, and the rib type waveguide has a structure in which a ridge type upper structure and a lower structure wider than this upper structure extend in the longitudinal direction. It is a waveguide having. Therefore, the waveguide direction of light in the waveguide 2 is the longitudinal direction of the waveguide 2.

Since the waveguide surrounding structure 6 that separately surrounds the plurality of waveguides 2 is formed so as not to come into contact with the waveguide 2, an air layer serving as a heat insulating layer is formed around the individual waveguides 2. Will be done. Therefore, the heat generated in the waveguide 2 is conducted to the substrate 1. Since the substrate 1 is made of a low thermal conductive material, the waveguide surrounding structure 6 is made of a high thermal conductive material, so that the heat conducted to the substrate 1 is larger than that of the adjacent waveguide 2. The heat is preferentially conducted to the waveguide surrounding structure 6 existing in the foreground, and the thermal interference with the adjacent waveguide 2 is suppressed. Further, since the heat conducted to the waveguide surrounding structure 6 is dissipated from the waveguide surrounding structure 6 located on the opposite side of the substrate 1 with respect to the waveguide 2, the heat is not trapped in the waveguide 2 and the waveguide 2 By suppressing the heat distribution, the temperature controllability of the waveguide 2 is improved.

Further, heat conducted from the waveguide 2 to the waveguide surrounding structure 6 via the substrate 1 by a layer of air existing as a heat insulating layer between the waveguide 2 and the waveguide surrounding structure 6 is transferred to the adjacent waveguide 2. Insulated against.

The second-order nonlinear optical medium constituting the waveguide 2 is a dielectric having a small absorption loss with respect to the wavelength used for the second-order nonlinear optical effect. For example, LiNbO3 can be used, and Mg and Zn are used for this LiNbO3. , Sc, and In can be contained as an additive.

The waveguide surrounding structure 6 is supported on both sides of the waveguide 2 by a heat radiating lid holding portion 61 extending from the substrate 1 in the opposite direction of the substrate 1 and at least two heat radiating lid holding portions 61 with respect to the waveguide 2. It is composed of a heat radiating lid portion 62 that covers the waveguide 2 on the side opposite to the substrate 1. The heat radiating lid holding portion 61 and the heat radiating lid portion 62 may be made of the same material or may be different materials. Both the heat radiating lid portion 62 and the heat radiating lid holding portion 61 can be made of a material having high thermal conductivity such as metal or a heat conductive resin. As the metal, a metal having a coefficient of thermal expansion close to that of the second-order nonlinear optical medium constituting the waveguide 2 can be used, and for example, Cu can be used. Te may be contained in this Cu as an additive. As the thermosetting resin, an adhesive or the like can be used.

Further, depending on the method of manufacturing the heat radiating lid holding portion 61, the heat radiating lid holding portion 61 is not limited to the above-mentioned material, and may be formed by using the same material as the waveguide 2. For example, when the layer constituting the waveguide 2 and the substrate 1 are directly bonded and the layer constituting the waveguide 2 is etched to form the waveguide portion and the heat radiating lid holding portion 61 on the substrate 1, heat is dissipated. The lid holding portion 61 is made of the same material as the waveguide 2 made of the second-order nonlinear optical medium, and the heat radiating lid holding portion 61 is made of a different material from the heat radiating lid portion 62.

When the lid holding structure 61 is formed by etching as described above, the surface can be made flat by dicing or the like, and then the heat radiating lid can be attached to the flat surface, which is easy to manufacture.

When the heater 72 is formed on the waveguide 2, a dielectric film 71 having a refractive index lower than that of the second-order nonlinear optical medium constituting the waveguide 2 is laminated on the upper surface of the waveguide, and a metal serving as the heater 72 is laminated on the dielectric film 71. A structure in which electrodes are laminated is desirable. This is because the light propagating in the waveguide 2 can be confined and the absorption of light by the metal can be avoided. As the dielectric film 71 to be laminated, for example, SiO2 can be used.

The heater 72 may be divided in the longitudinal direction of the waveguide 2, and more detailed heat distribution control becomes possible by increasing the number of divisions.

The distance from the waveguide 2 to the heat radiating lid portion 62 may be shorter than the substrate thickness as long as an air layer is formed between the upper surface of the waveguide 2 including the heater 72 and the heat radiating lid portion 62.

It is desirable that the heat control element 4 composed of the Peltier element and the heater is installed on the surface of the heat radiating lid portion 62 opposite to the substrate 1. This is because the thermal controllability of each waveguide 2 is improved by preventing the temperature of the heat radiating lid portion 62 itself from changing. By using a heater as the heat control element 4, the influence of the environmental temperature is suppressed when the temperature outside the waveguide surrounding structure 6 is too low.

Further, since the heat generated by the heater 72 is efficiently transferred to the waveguide by the layer of air formed around the waveguide 2, the heater 72 is driven when the waveguide 2 is controlled to the same temperature. Since the power required for this is small, it is possible to control the phase matching conditions. The heat generated from the heater 72 and the heat generated by the light absorption of the nonlinear optical medium are dissipated from the heat radiating lid covering the upper surface side of the waveguide structure via the heat radiating lid holding structure. Thereby, even in the array structure as shown in FIG. 4, it is possible to suppress the heat inflow to the adjacent waveguide. The air layer also has the effect of confining the light propagating in the waveguide structure and avoiding the absorption of light by the metal constituting the surrounding portion.

In the above embodiment, the wavelength conversion element has been described as an example of the optical element, but instead of the configuration in which the heater 72 is provided on the waveguide 2, an electrode for performing optical control is formed for each waveguide 2. You may. As an example thereof, as in Non-Patent Document 5, it is possible to construct a phase modulator that applies an electric field to the waveguide 2 and utilizes a change in the refractive index due to the Pockels effect.

FIG. 5 is a diagram showing a cross-sectional structure of the wavelength conversion element according to the first embodiment. As shown in FIG. 5, the wavelength conversion element of this embodiment is composed of lithium niobate having a periodic polarization inversion structure as a waveguide 2 having a ridge-shaped structure on a substrate 1 made of lithium tantalate. Directly bonded, SiO ₂ which is a dielectric film 71 and Au 72 which is a heater 72 are formed on the upper part of the waveguide 2. In this embodiment, Ti73 is used as a heater in addition to Au. On the substrate 1 having the waveguide 2, a waveguide surrounding structure 6 is provided on the substrate 1 so that lithium niobate having the same height as the waveguide 2 serves as a heat dissipation lid holding portion 61 on both sides of the waveguide 2. A heat-dissipating lid that is joined and made of metal as a conductive material is supported by the heat-dissipating lid holding portion 61. The heat radiating lid uses a metal plate having a groove. A portion of the heat radiating lid that is not a groove is joined to the heat radiating lid holding portion 61. Further, a Peltier element is provided as the heat control element 4 on the surface (upper surface) of the heat radiating lid portion 62 opposite to the substrate 1 side.

A method of manufacturing the wavelength conversion element shown in FIG. 5 will be described. First, a substrate 1 made of lithium tantalate having a thickness of 500 μm is prepared. After bonding a flat plate of lithium niobate to this substrate 1, the flat plate of lithium niobate is thinned to 5 μm by grinding and polishing, and SiO ₂ as an overclad layer is formed in a portion of the lithium niobate that becomes a waveguide 2. After depositing 1 μm or more by sputtering, patterning is performed by dry etching to form a waveguide 2.

The material of the overclad layer may be a substance that is transparent to both the 780 nm band and the 1550 nm band, such as tantalum pentoxide, in addition to SiO ₂ . The overclad layer is formed to suppress the absorption of the photoelectric field by the metal. The waveguides 2 are provided parallel to each other, and the waveguide spacing is 100 μm. The distance from the center of the waveguide to the heat radiating lid holding portion 61 is 30 μm. Since the distance to the heat dissipation lid holding portion 61 is shorter than the distance from the adjacent waveguide 2, it is effective in suppressing thermal interference. Here, in order to realize the heat distribution suppressing effect more than the conventional structure, it is desirable that the distance from the waveguide 2 to the heat radiating lid holding portion 61 is shorter than the substrate thickness.

The heater 72 on the waveguide 2 is manufactured after forming the waveguide by dry etching. After forming the waveguide, metals are vapor-deposited in the order of Ti73 and Au72, and then the electrodes are patterned by applying and developing a resist, and finally the unnecessary metal portion is melted by wet etching to form the heater 72. The reason why Ti73 is vapor-deposited before Au72 is to improve the adhesion, but it is not limited to this configuration as long as one or more metal films are formed. An electron beam vapor deposition device was used for metal deposition.

As the metal constituting the heat radiating lid portion 62, Cu having a coefficient of thermal expansion close to that of lithium niobate or lithium tantalate or Cu with Te added can be used, but in this embodiment, the groove is finely processed by wet etching. The Cu formed in the above was covered from above the waveguide 2, and the non-grooved portion was adhered to the heat radiation lid holding portion 61. The heat radiating lid portion 62 has a role of protecting the waveguide 2 in which the waveguide 2 is structurally formed on the surface of the device, and also has an effect of suppressing the influence of disturbance factors such as the adhesion of outside air and dust.

The depth of the metal groove used as the heat radiating lid portion 62 is set to be the thickness of the overclad and the heater 72 + 1.0 μm or more. This is to insulate the heater 72 and the heat dissipation lid made of metal. A conductive resin having high thermal conductivity was used for bonding the heat radiating lid portion 62 and the heat radiating lid holding portion 61. The microfabrication of the Cu or Te-added Cu plate may be performed by dicing or dry etching. Since the heat radiation lid holding portion 61 formed by the secondary nonlinear optical medium and the substrate 1 are transparent to the visible region, the connection with the metal carrier can be aligned by observing from the back surface of the substrate using an optical microscope. Is. A Peltier element is attached as a heat control element 4 on the surface of the heat radiating lid portion 62 opposite to the substrate 1.

The polarization reversal period of the wavelength conversion element is set to a pitch period that converts light of 1560 nm into light of 780 nm with high efficiency. The element length was 4 cm. Since the nonlinear optical effect such as wavelength conversion is proportional to the square of the light wave interaction length, the longer the element length, the higher the efficiency.

When the temperature of the waveguide 2 was raised by + 1.0 ° C. by the heater 72, the temperature rise due to the thermal interference of the adjacent waveguides 100 μm apart was investigated.

In the wavelength conversion element having the conventional structure shown in FIG. 2, Cu having high heat dissipation is adhered to the lower surface of the substrate 1 made of lithium tantalate, and the heat of the waveguide is dissipated through the substrate 1 of 500 μm. When the temperature rise of the adjacent waveguide 2 was examined when the temperature of the waveguide increased by + 1.0 ° C., the wavelength conversion of the present embodiment was performed while the temperature of the wavelength conversion element having the conventional structure increased to + 0.45 ° C. or higher. The temperature of the device increased by + 0.1 ° C or less. Therefore, it can be said that the effect of this measure has succeeded in reducing the thermal interference to the adjacent waveguide.

FIG. 6 is a diagram showing a cross-sectional structure of the wavelength conversion element according to the second embodiment. The wavelength conversion element of this embodiment is an array waveguide having four waveguides 2 made of the same medium as that of the first embodiment. The waveguide spacing was 100 μm, and the width of the heat dissipation lid holding structure was 70 μm. In this embodiment, unlike the first embodiment, a heat radiating lid portion 62 and a heat radiating lid holding portion 61 of the waveguide surrounding structure 6 are integrally formed. The waveguide surrounding structure 6 is constructed by using a metal which is a heat conductive material.

In the wavelength conversion element shown in FIG. 6, the substrate 1 made of lithium tantalate has a thickness of 500 μm. Lithium niobate as the waveguide 2 is bonded to a lithium tantalate substrate, then thinned to 5 μm by grinding and polishing, and has a waveguide structure patterned by dry etching.

As the metal as the heat conductive material constituting the waveguide surrounding structure 6, Cu having a coefficient of thermal expansion close to that of lithium niobate or lithium tantalate or Cu with Te added is used. This is because ferroelectric crystals such as lithium niobate and lithium tantalate have less resistance to strain and are more fragile than SiO ₂ , which is known as a general waveguide medium, so that the coefficient of thermal expansion of the heat conductive material is high. This is because if it is significantly different from the array waveguide, the waveguide 2 will be destroyed due to the difference in the expansion coefficient during temperature control. A Peltier element is attached as a heat control element 4 on the surface (upper surface) of the heat radiating lid opposite to the substrate 1 side.

In this embodiment, as the waveguide surrounding structure 6, Cu which has been finely grooved by wet etching is covered from above the waveguide. The depth of the groove is set to be the height (ridge height) of the waveguide 2 from the substrate 1 + 1.0 μm or more. This is to prevent the evanescent component of light passing through the waveguide 2 from being captured by the waveguide surrounding structure 6 made of metal. Adhesion between the waveguide surrounding structure 6 and the substrate 1 was carried out with a conductive resin having high thermal conductivity. The microfabrication of the Cu or Te-added Cu plate may be performed by dicing or dry etching. The polarization reversal period of the wavelength conversion element is set to a pitch period that converts light of 1560 nm into light of 780 nm with high efficiency. The element length was 4 cm.

FIG. 7 is a diagram showing a cross-sectional structure of the optical element according to the third embodiment. In the phase modulator of this embodiment, in the wavelength conversion element of Example 1, the waveguide 2 and the heat dissipation lid holding portion 61, which are configured by using lithium niobate, are composed of lithium niobate doped with MgO. ing. The optical element of this embodiment is an array waveguide having two waveguides 2. The waveguide interval was 100 μm, and the width of the heat dissipation lid holding portion 61 was 30 μm.

Further, an electrode in which SiO ₂ , Ti 73, and Au 72 are laminated is formed on the waveguide 2 as in the first embodiment, and the heat radiation lid holding portions 61 on both sides of the waveguide 2 and the waveguide 2 are formed. An electrode in which Ti73 and Au72 are laminated is formed between the two. With these electrodes, the optical element of this embodiment can operate as a phase modulator that exhibits an electro-optical effect on the waveguide 2 and operates even at a high optical power.

The substrate 1 made of lithium niobate has a thickness of 500 μm. Lithium niobate doped with MgO as a waveguide is bonded to the substrate 1, thinned to 5 μm by grinding and polishing, and patterned by dry etching to form a waveguide 2. There is.

Electrodes for applying an electric field to the waveguide 2 are formed on the waveguide 2 and on the side of the waveguide 2 after the waveguide is formed by dry etching. After forming the waveguide, metals are vapor-deposited in the order of Ti73 and Au72, and then the electrodes are patterned by resist coating and development, and finally the unnecessary metal portion is melted by wet etching to form the heater 72. Ti73 is vapor-deposited before Au72 in order to improve the adhesion between Au72 and the substrate 1 or SiO ₂ , but it is limited to this configuration as long as one or more metal films (Au) are formed. Absent. An electron beam vapor deposition device was used for metal deposition.

Further, a Peltier element is attached as a heat control element 4 on the surface (upper surface) of the heat dissipation lid opposite to the substrate 1 side.

As the metal as the heat conductive material constituting the heat radiating lid portion 62, Cu having a coefficient of thermal expansion close to that of lithium niobate or lithium tantalate or Cu with Te added is used. In this embodiment, similarly to the first embodiment, Cu that has been finely processed by wet etching is covered from above the waveguide 2, and the non-grooved portion is adhered to the heat radiating lid holding portion 61.

The optical element of this embodiment can operate as a phase modulator that exhibits an electro-optical effect on the waveguide and operates even at a high optical power.

1 Substrate 2 waveguide 3 metal plate 4 thermal control element 5 conductive resin 6 waveguide surrounding structure 61 heat dissipation lid holding part 62 heat dissipation lid part 71 dielectric film 72 heater (Au)
73 Ti

Claims (5)

A substrate constructed using a low thermal conductivity material,
It is composed of a second-order nonlinear optical medium having a periodic polarization inversion structure in the waveguide direction of light.
A plurality of waveguides arranged on the substrate and
It is provided with a waveguide surrounding structure which is constructed by using a high thermal conductive material and is formed so as to separately surround each of the waveguides of the plurality of waveguides along the longitudinal direction of the plurality of waveguides.
The waveguide surrounding structure has a heat radiating lid holding portion made of the same material as the waveguide and a heat radiating lid portion made of the high thermal conductive material, from the waveguide to the heat radiating lid holding portion. An optical element characterized in that the distance is set to be shorter than the substrate thickness, and the waveguide and the waveguide surrounding structure are formed so as not to come into contact with each other. The optical element according to claim 1, further comprising a heater arranged so as not to come into contact with the waveguide surrounding structure on a surface of the waveguide opposite to the substrate. The optical element according to claim 2, wherein a dielectric layer having a refractive index lower than that of the waveguide is provided between the waveguide and the heater. The optical element according to claim 3 , wherein the heat radiating lid portion is made of Cu or Cu to which Te is added. The invention according to any one of claims 1 to 4 , wherein the waveguide is composed of a material containing at least one of Mg, Zn, Sc, and In as an additive in LiNbO3 or LiNbO3. Optical element.

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