JP2019039959A - Planar optical waveguide device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a planar optical waveguide device capable of combining and demultiplexing light in two wavelength bands having twice and a half different wavelengths for nonlinear optical application with low loss and wideband, and extracting a part of light of both wavelengths. thing. A plane optical waveguide device including two optical waveguides 211 and 212 has an optical waveguide structure in which an additive is contained in a core portion and the refractive index is higher than that of the surroundings. The optical waveguide 211 includes a long wave input port 201 and a monitor output port 203, and the optical waveguide 212 includes a short wave input port 202 and a nonlinear optical waveguide connection port 204. The optical waveguides 211 and 212 are arranged so as to form a coupling region having an adjacent waveguide spacing d, and the length L of the coupling region is such that light having a central wavelength in a long wavelength band is completely transferred from the optical waveguide 211 to the optical waveguide 212. It is a long-wave perfect bond length that can be transferred. The adjacent waveguide interval d is set to a distance such that a part of the light in the short wavelength band is coupled to the adjacent optical waveguide. [Selection diagram] Fig. 2

Description

  The present invention relates to a planar optical waveguide device, and more particularly to a planar optical waveguide device for extracting monitor light while optimally combining light of different wavelength bands.

Optical application technology using the nonlinear optical effect is expected in the new optical communication field and the quantum information communication field using light. Among the nonlinear optical effects, wavelength conversion is known as a basic effect. In wavelength conversion using the nonlinear optical effect, light incident on the nonlinear optical medium can be converted into light having a different frequency. Using this characteristic, it is widely known as a technology that generates light in a wavelength band that is difficult to oscillate with a single laser. In particular, a periodically poled waveguide using lithium niobate (LiNbO ₃ ), which is a second-order nonlinear material and has a large nonlinear constant, is incorporated in a commercially available light source because of its high efficiency of the nonlinear optical effect. .

In the second-order nonlinear optical effect, light having wavelengths λ ₁ and λ ₂ is input to generate a new wavelength λ ₃ .
1 / λ ₃ = 1 / λ ₁ + 1 / λ ₂ (1)
The wavelength conversion satisfying the above is called sum frequency generation (SFG), and in the case of λ ₁ = λ ₂ , that is, by transforming equation (1),
_{λ 3 = λ 1/2 (} 2)
The wavelength conversion that satisfies this condition is called second harmonic generation (SHG). further,
1 / λ ₃ = 1 / λ ₁ -1 / λ ₂ (3)
Wavelength conversion that satisfies this condition is called difference frequency generation (DFG). Furthermore, there is an optical parametric effect in which only the wavelength λ ₁ is input and the wavelengths λ ₂ and λ ₃ satisfying the expression (3) are generated. In particular, SHG and SFG newly generate light having a short wavelength with respect to input light, that is, light having high energy, and are often used for generation of a visible light region.

In order to efficiently cause these second-order nonlinear optical effects, the amount of phase mismatch of the interacting three wavelengths is required to be zero. Therefore, the amount of phase mismatch can be reduced to zero by periodically inverting the polarization of the second-order nonlinear optical material. Assuming that the inversion period at that time is Λ, in the sum frequency generation shown in the equation (1), the following equations (for the wavelengths λ ₁ , λ ₂ , λ ₃ )
It is sufficient to set Λ satisfying 4).
n ₃ / λ ₃ −n ₂ / λ ₂ −n ₁ / λ ₁ −1 / Λ = 0 (4)
Here, n ₁ is the refractive index at wavelength λ ₁ , n ₂ is the refractive index at wavelength λ ₂ , and n ₃ is the refractive index at wavelength λ ₃ .

  In addition to such a periodically poled structure, a highly efficient wavelength conversion is possible by forming a waveguide. The nonlinear optical effect increases as the overlapping density of light causing nonlinear interaction increases. Therefore, highly efficient wavelength conversion becomes possible by adopting a waveguide structure in which light is confined in a waveguide having a small cross-sectional area perpendicular to the propagation direction and light is guided over a long distance.

(Direct junction ridge waveguide)
In order to realize a waveguide structure using a non-linear optical crystal, techniques using Ti diffusion and proton exchange are generally used. However, in recent years, ridge-type optical waveguides have been researched and developed as wavelength conversion elements, which can utilize the bulk characteristics of crystals as they are and have features such as high optical damage resistance, long-term reliability, and ease of device design (non- Patent Document 1).

  In this ridge type optical waveguide, a core is formed by joining two substrates, then thinning one of the substrates, and further performing ridge processing. A direct bonding technique is known as a technique for firmly bonding substrates without using an adhesive or the like when bonding the substrates. The direct junction ridge-type waveguide using this technology can inject strong light, and has succeeded in reducing the core (thinning of the core) with the progress of waveguide technology, and its nonlinear optical efficiency is It is constantly improving (see Non-Patent Document 2).

(Applied technology and requirements)
This periodically poled direct-junction nonlinear optical crystal waveguide capable of receiving strong light is expected not only as a simple wavelength conversion element but also as a low-noise optical amplifier and generator of entangled photon pairs in recent years. . In these applied technologies, it is important to align the characteristics of a plurality of nonlinear optical waveguides and strictly control the phase of incident light. For example, when applying a phase-sensitive amplifier that operates only at one polarization to the current long-distance communication, the polarization multiplexed light is once separated into two polarized lights and amplified by separate nonlinear optical waveguides. Then, it is necessary to combine the phases of the two polarized lights again (see Non-Patent Document 3).

(Expectations for integration)
In such applied technology, integration of a quartz-based planar optical circuit and a nonlinear optical element capable of strictly controlling the phase of light in the system is expected. 2. Description of the Related Art In recent years, planar optical circuits using quartz using semiconductor microfabrication are capable of strict optical path length control due to their high processing accuracy, and have been put into practical use in current optical communication systems. Therefore, integration of a quartz-based planar optical circuit and a periodically poled lithium niobate (PPLN) array waveguide is an effective technique for nonlinear optical application techniques such as the phase sensitive amplifier described above.

(Light incident)
What is important in operating the above-described waveguide device is to efficiently couple light of a plurality of wavelengths to the nonlinear optical waveguide. Furthermore, in non-linear optics, it is necessary to combine light of completely different wavelength bands at the same time. When a phase sensitive amplifier is applied to a relay system of an optical communication network as described in Non-Patent Document 3, not only a 1.5 μm band communication wavelength band but also a 780 nm band light which is a double wave thereof is also nonlinear optical. It must be coupled to the waveguide.

  As an optical coupling technique, a technique using a dichroic mirror or a lens, or a two-wavelength band is synthesized using a planar optical circuit in which a multiplexing circuit is formed, and the planar optical circuit is abutted and joined to a nonlinear optical waveguide. A method can be considered. In the former, it is necessary to adjust the alignment between a large number of optical components and assemble them accurately, which increases the mounting cost. On the other hand, in the latter case, if a function such as multiplexing is realized in the planar optical circuit, alignment adjustment is necessary only for the junction between the planar optical circuit and the nonlinear optical waveguide, which can be implemented easily and at low cost. It is.

  Furthermore, in recent years, the processing accuracy of quartz-based planar optical circuits has increased, and the functions that can be realized on the planar optical circuits have diversified. Therefore, optical coupling using planar optical circuits has further improved the application technology of nonlinear optical devices. It has the potential to develop.

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, M. Asobe, Y. Miyamoto, and H. Takenouchi, “PDM Signal Amplification Using PPLN-Based Polarization-Independent Phase-Sensitive Amplifier,” J. Lightwave Technol., Vol. 33, No. 7, P.1326-1331, 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.

  As described above, in order to effectively use the nonlinear optical effect, it is necessary to make light of a plurality of different wavelength bands incident on the nonlinear optical medium. With respect to the multiplexing / demultiplexing of light in two greatly different wavelength bands, there have been reports on optical couplers using multimode optical interference (MMI) (see Non-Patent Document 4). However, although the MMI coupler is effective for miniaturization, there is a problem that loss due to a manufacturing error easily occurs due to the small size and it is difficult to widen the transmission band. Thus, in the prior art, there is no clear design guideline for an optical element that can multiplex / demultiplex a fundamental wave and a harmonic wave, which are essential for second-order nonlinear optical operation, into a low loss and a wide band.

  In phase sensitive amplifiers and phase conjugate converters, the nonlinear optical process is sensitive to the optical phase and light intensity, so various feedback controls can be performed while monitoring part of the incident light and output light to the nonlinear waveguide. Need to do. Therefore, it is desired to realize an optical coupler capable of multiplexing / demultiplexing at an arbitrary ratio for two different wavelength bands. Although these optical signal processing technologies handle high-power light of the watt class, it is only necessary to extract a part of the output light of the micro watt class as the monitor light. However, a clear design guideline for an optical coupler capable of extracting a part of such high output light is not currently shown.

  The present invention solves the above two problems at the same time, and an object of the present invention is to multiplex / demultiplex light of two wavelength bands whose wavelengths for nonlinear optical application are two and half times different into a low loss and wideband, and An object of the present invention is to provide a planar optical waveguide device capable of extracting part of light of both wavelengths.

  In order to solve the above problems, the present invention provides a planar optical waveguide device including two optical waveguides having a coupling region constituting a directional coupler, wherein one of the two optical waveguides is a first optical waveguide device. An optical waveguide to which light having a first wavelength in one wavelength band is input, and the other of the two optical waveguides is an optical waveguide to which light having a second wavelength in the second wavelength band is input. The second wavelength band is a band having a shorter wavelength than the first wavelength band, and the coupling length of the coupling region is that of the first wavelength band input to one of the two optical waveguides. The length of the light having the center wavelength is completely coupled to the other of the two optical waveguides, and the distance between the two optical waveguides in the coupling region is the other of the two optical waveguides. A distance at which the second wavelength light in the second wavelength band is partially coupled to one of the two optical waveguides; Characterized in that it is a distance that the transmittance of the thru port of the second wavelength light is transmittance comparable to the cross port of the first wavelength light.

  According to a second aspect of the present invention, in the planar optical waveguide device according to the first aspect, the two optical waveguides are a single mode for the light of the first wavelength and the light of the second wavelength. It is a waveguide.

  According to a third aspect of the present invention, in the planar optical waveguide device according to the first or second aspect, the optical waveguide into which the light of the second wavelength is input is a straight optical waveguide. It is characterized by that.

  A fourth aspect of the present invention is the planar optical waveguide device according to any one of the first to third aspects, wherein the second wavelength is half of the first wavelength.

  According to a fifth aspect of the present invention, in the planar optical waveguide device according to any one of the first to fourth aspects, an output port of an optical waveguide to which light of the first wavelength is input from two optical waveguides And an optical coupler for demultiplexing the light of the first wavelength and the light of the second wavelength.

  According to a sixth aspect of the present invention, in the planar optical waveguide device according to the fifth aspect, the optical coupler is another planar optical waveguide device according to the first aspect.

  According to a seventh aspect of the present invention, in the planar optical waveguide device according to any one of the first to sixth aspects, the optical waveguide is made of a dielectric or a semiconductor.

  The present invention can multiplex / demultiplex light in two wavelength bands whose wavelengths for nonlinear optical applications are different by a factor of half into a low loss and wide band, and extract a part of light of both wavelengths.

It is a figure which shows the schematic diagram of the planar optical waveguide device which concerns on one Embodiment of this invention. It is a figure which shows the top view of the planar optical waveguide device which concerns on Example 1 of this invention. The relationship between the adjacent waveguide interval obtained by light propagation simulation in the planar optical waveguide device according to Example 1 of the present invention and the complete coupling length (long wave complete coupling length) of light having a wavelength of 1.56 μm is indicated by a broken line. It is a figure which shows the transmission loss to the nonlinear optical waveguide connection port 204 of the light of wavelength 0.78 micrometer at the time of employ | adopting perfect coupling length with a continuous line. (A) is a light beam having a wavelength of 1.56 μm when the adjacent waveguide interval d is 2.7 μm and the long-wave complete coupling length L ₀ is 600 μm in the coupling region of the planar optical waveguide device according to the first embodiment of the present invention. It is a figure which shows a propagation simulation result, (b) is a figure which shows the optical propagation simulation result of the light of wavelength 0.78 micrometer on the same conditions. (A) is a figure which shows the transmittance | permeability to the nonlinear optical waveguide connection port and monitor output port of 1.56 micrometer band light, (b) is a nonlinear optical waveguide connection port and monitor of 0.78 micrometer band light. It is a figure which shows the transmittance | permeability to an output port. It is a figure which shows the top view of the planar optical waveguide device which concerns on Example 2 of this invention.

  FIG. 1 shows a schematic diagram of a planar optical waveguide device according to an embodiment of the present invention.

  The planar optical waveguide device 100 includes two optical waveguides 111 and 112, and the optical waveguides 111 and 112 are arranged so as to have a coupling region whose adjacent waveguide interval is d. Let L be the length of this coupling region (coupling length). When the light A in the long wavelength band is input from the long wave input port 101, the light A gradually moves to the adjacent optical waveguide 112 in the coupling region.

Here, the coupling length L is set to a length at which the light of the center wavelength in the long wavelength band input from the long wave input port 101 can theoretically be transferred from the optical waveguide 111 to the optical waveguide 112. The coupling length is L ₀ . In this case, most of the light A in the long wavelength band is emitted from the nonlinear optical element connection port 104. However, the long-wavelength light handled is generally called signal light and has a certain wavelength width. Therefore, in the planar optical waveguide device 100, it is theoretically impossible for all of the signal light to transfer from the optical waveguide 111 to the adjacent optical waveguide 112, and light in the long wavelength band is also transmitted from the monitor port 103 at a certain rate. Is output.

The long-wave complete coupling length L ₀ has a dependency on the adjacent waveguide interval d. Generally, when the adjacent waveguide interval d is increased, the coupling coefficient to the adjacent waveguide is decreased, so that the long-wave complete coupling length L ₀ is reduced. The coupling length L ₀ becomes longer.

On the other hand, it is desired that the light B in the short wavelength band input from the shortwave input port 102 is not coupled to the adjacent optical waveguide 111 in the coupling region. Here, the coupling amount of the short wavelength light to the monitor port 103 is determined by the two parameters of the long wave perfect coupling length L ₀ and the coupling coefficient of the short wavelength light itself. Since the coupling coefficient is determined by superposition of the respective natural modes of the adjacent waveguides, the coupling coefficient has dependency on the adjacent waveguide spacing.

Accordingly, the above two parameters, the long wave perfect coupling length L ₀ and the coupling coefficient of the short wavelength light itself, are dependent on the adjacent waveguide interval d of the directional coupler. From this, the coupling amount of the short wavelength band light B to the monitor port 103 in the planar optical waveguide device 100 is also determined by the adjacent waveguide interval d. As a result of examining the dependence on the adjacent waveguide interval, the long wave perfect coupling length L ₀ of the planar optical waveguide device 100 is increased by increasing the adjacent waveguide interval d, but the light B in the short wavelength band is increased. It was found that the output from the monitor port 103 was small. This is considered to be because the electromagnetic field distribution in the waveguide in both wavelength bands is greatly different, and the dependence of the coupling coefficient on the adjacent waveguide spacing is also greatly different.

Further, by increasing the interval d between adjacent waveguides and increasing the long-wave complete coupling length L ₀ , it becomes possible to increase the bandwidth of light in a long wavelength band that is optically coupled to an adjacent optical waveguide. In order to reduce the device size, it is also effective to adjust the optical waveguide size, that is, the cross-sectional area perpendicular to the light propagation direction of the optical waveguide.

  As described above, in the planar optical waveguide device of the present invention, by adjusting the adjacent waveguide interval d and the coupling length L, low loss and wideband multiplexing / demultiplexing can be performed for light of two different wavelength bands. It can be realized and monitor light can be taken out simultaneously when handling watt-class high power light.

  The materials constituting the planar optical waveguide device of the present invention include silicon, silicon dioxide, lithium niobate, indium phosphide, polymers such as dielectrics and semiconductors, or compounds obtained by adding additives to these two wavelength bands. What is necessary is just to be transparent with respect to light.

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

Example 1
FIG. 2 shows a top view of the planar optical waveguide device 200 according to the first embodiment of the present invention. A planar optical waveguide including two optical waveguides 211 and 212 is made of a dielectric material mainly composed of SiO ₂ , and has an optical waveguide structure in which an additive is contained in the core portion and a refractive index is higher than that of the surroundings. doing. The optical waveguide 211 includes a long wave input port 201 and a monitor output port 203, and the optical waveguide 212 includes a short wave input port 202 and a nonlinear optical waveguide connection port 204. Further, the optical waveguides 211 and 212 are arranged so as to form a coupling region having an adjacent waveguide interval d, and the coupling region has a length in which the light having the center wavelength in the long wavelength band is completely transmitted from the optical waveguide 211 to the optical waveguide 212. This is the long wave perfect coupling length L ₀ that can be transferred to. The adjacent waveguide interval d is set to such a distance that a part of the light in the short wavelength band is coupled to the adjacent optical waveguide.

  In addition, the substance which comprises this planar optical waveguide device 200 is 2 wavelength bands to use, such as dielectrics and semiconductors, such as silicon, silicon dioxide, lithium niobate, indium phosphide, a polymer, or those which added additives to them It only needs to be transparent to the light.

  In this planar optical waveguide device 200, assuming optical communication applications such as a phase sensitive amplifier and a phase conjugate converter, the light A having a wavelength of 1.56 μm as the center wavelength of the long wavelength band and the wavelength of 0.78 μm as the center wavelength of the short wavelength band. Light B was selected. The wavelength band that can be used for the planar optical waveguide device 200 may be a transparent wavelength with respect to the optical waveguide member, and is not limited to the communication wavelength band and its double wave.

  The optical waveguide 212 for inputting the light B having a wavelength of 0.78 μm is a straight optical waveguide. This means that even if the selected optical waveguide is a single mode waveguide for light A having a wavelength of 1.56 μm, it may become a multimode waveguide for light B having a wavelength of 0.78 μm. This is to prevent higher mode coupling due to the introduction. Therefore, the optical waveguide 212 may be a bent waveguide, but it is desirable to optimize the design of the bent waveguide for two wavelengths so as to satisfy the single mode condition for both wavelengths. Further, the single mode condition may be satisfied for both wavelengths by adjusting the waveguide widths of the optical waveguides 211 and 212.

  FIG. 3 shows the relationship between the distance between adjacent waveguides obtained by light propagation simulation in the planar optical waveguide device according to the first embodiment of the present invention and the complete coupling length (long wave complete coupling length) of the light A having a wavelength of 1.56 μm. It shows with. FIG. 3 shows the transmission loss of light B having a wavelength of 0.78 μm to the nonlinear optical waveguide connection port 204 when this long-wave complete coupling length is adopted, that is, adjacent to the optical waveguide 212 of light B having a wavelength of 0.78 μm. The amount of coupling to the optical waveguide 211 is indicated by a solid line.

FIG. 3 shows that the long-wave perfect coupling length L ₀ of the planar optical waveguide device 200 is increased by increasing the adjacent waveguide interval d, but the nonlinear optical waveguide connection port 204 of the light B having a wavelength of 0.78 μm. In other words, the transmission loss of the light B having a wavelength of 0.78 μm output from the monitor port 203 is reduced.

Here, FIGS. 4A and 4B show light propagation simulation results when the adjacent waveguide interval d is 2.7 μm and the long-wave complete coupling length L ₀ is 600 μm. As shown in FIG. 4A, most of the light A having a wavelength of 1.56 μm input from the long wave input port 201 of the optical waveguide 211 moves to the adjacent optical waveguide 212, whereas in FIG. As shown, most of the light B having a wavelength of 0.78 μm input from the short wave input port 202 of the optical waveguide 212 is not coupled to the adjacent optical waveguide 211.

  FIGS. 5A and 5B show transmission spectra for the respective wavelength bands of the planar optical waveguide device according to Example 1 actually manufactured. Transmittance to the nonlinear optical element connection port 204 for each light in the 1.56 μm wavelength band and 0.78 μm wavelength band, that is, the transmittance to the cross port of the light A in the 1.56 μm wavelength band and 0.78 μm wavelength band The transmittance of the light B to the through port (solid lines in FIGS. 5A and 5B) was −0.33 dB and −0.12 dB, respectively, which were similar. This achieves low-loss multiplexing / demultiplexing comparable to that when using a general dichroic mirror, which is a spatial optical component. In addition, the present planar optical waveguide device does not have a large loss variation in the communication wavelength band of 1540 to 1580 nm, and operates sufficiently in a wide band for communication applications.

  On the other hand, the transmittance to the monitor port 203 (broken lines in FIGS. 5A and 5B) was −23 dB for the light A having a wavelength of 1.56 μm and −33 dB for the light B having a wavelength of 0.78 μm. . This indicates that when 1.0 W light is incident from the input port, 5 mW light is output from the monitor port 203 at a wavelength of 1.56 μm and 0.5 mW light is output at a wavelength of 0.78 μm. This light intensity is sufficient for a general optical receiver and can be sufficiently utilized for feedback control in a phase sensitive amplifier and a phase conjugate converter.

(Example 2)
FIG. 6 shows a top view of a planar optical waveguide device according to Example 2 of the present invention. The planar optical waveguide device 300 according to the second embodiment includes three optical waveguides 311 to 313, and the monitor output port 203 of the planar optical waveguide device 200 according to the first embodiment is used as a short wave input port of another planar optical waveguide device 200. 202 is connected.

  A first-stage planar optical waveguide device structure 321 corresponding to the first planar optical waveguide device 200 includes an optical waveguide 311 including a long-wave input port 301, a short-wave input port 302, and a nonlinear optical waveguide connection port 305. 3, and includes a coupling region in which light having a central wavelength in the long wavelength band can completely transfer from the optical waveguide 311 to the optical waveguide 312.

  The second-stage planar optical waveguide device structure 322 corresponding to the second planar optical waveguide device 200 includes a long-wave monitor output port 303 and a short-wave monitor output port 304, and the light at the center wavelength in the long wavelength band is completely transmitted. A coupling region that can be transferred from the optical waveguide 311 to the optical waveguide 313 is included.

  The monitor light extracted by the first-stage planar optical waveguide device structure 321 can be divided into a long-wave monitor output port 303 and a short-wave monitor output port 304 that are different for each wavelength by the second-stage planar optical waveguide device structure 322. It is. This makes it possible to apply feedback to both wavelengths.

  In the planar optical waveguide device 300 shown in FIG. 6, the planar optical waveguide device structures 321 and 322 share the optical waveguide 311 and are integrally formed. However, the planar optical waveguide device structures 321 and 322 are separately formed. The planar optical waveguide device structure 322 in the second stage may be a general MMI coupler. However, when manufacturing errors are taken into account, it is easier to apply manufacturing feedback if the first and second stages have the same structure.

  In the planar optical waveguide device 300 according to the second embodiment, an optical communication application such as a phase sensitive amplifier or a phase conjugate converter is assumed, and the center wavelength of light in the short wavelength band is 1.56 μm as the center wavelength of the light in the long wavelength band. A wavelength of 0.78 μm was selected. Note that the wavelength band that can be used for the planar optical waveguide device 300 according to the second embodiment is not limited to the communication wavelength band and the double wave thereof, as long as it is a transparent wavelength with respect to the optical waveguide member. When 1.0 W light was incident on the actually produced multiplexer / demultiplexer, 4.0 mW was output from the long wave monitor output port and 0.33 mW was output from the short wave monitor output port.

100, 200, 300 Planar optical waveguide device 101, 201, 301 Long wave input port 102, 202, 302 Short wave input port 103, 203 Monitor output port 104, 204, 305 Nonlinear optical waveguide connection port 111, 112, 211, 212, 311, 312, 313 Optical waveguide 303 Long wave monitor output port 304 Short wave monitor output port 321, 322 Planar optical waveguide device structure

Claims (7)

A planar optical waveguide device including two optical waveguides having a coupling region constituting a directional coupler,
One of the two optical waveguides is an optical waveguide to which light having a first wavelength in the first wavelength band is input,
The other of the two optical waveguides is an optical waveguide to which light having a second wavelength in the second wavelength band is input.
The second wavelength band is a shorter wavelength band than the first wavelength band,
The coupling length of the coupling region is a length at which the light having the center wavelength of the first wavelength band input to one of the two optical waveguides is completely coupled to the other of the two optical waveguides.
The distance between the two optical waveguides in the coupling region is such that the light of the second wavelength in the second wavelength band input to the other of the two optical waveguides enters one of the two optical waveguides. It is a partly coupled distance, and is a distance at which the transmittance of the light of the second wavelength to the through port is approximately the same as the transmittance of the light of the first wavelength to the cross port. Planar optical waveguide device.   2. The planar optical waveguide device according to claim 1, wherein the two optical waveguides are single-mode waveguides for the light having the first wavelength and the light having the second wavelength.   3. The planar optical waveguide device according to claim 1, wherein an optical waveguide to which light having the second wavelength is input is a straight optical waveguide among the two optical waveguides.   4. The planar optical waveguide device according to claim 1, wherein the second wavelength is half of the first wavelength.   An optical coupler that demultiplexes the light of the first wavelength and the light of the second wavelength into an output port of the optical waveguide into which the light of the first wavelength is input out of the two optical waveguides. The planar optical waveguide device according to claim 1, further comprising: a planar optical waveguide device according to claim 1.   The planar optical waveguide device according to claim 5, wherein the optical coupler is another planar optical waveguide device according to claim 1.   The planar optical waveguide device according to any one of claims 1 to 6, wherein the optical waveguide is made of a dielectric or a semiconductor.

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