JP2018194665A - Wavelength separation element - Google Patents

Abstract

An object of the present invention is to separate light whose wavelengths are widely separated. SOLUTION: The length of a second optical waveguide 102 is LMMI, the self-image length for the wavelength of the main signal light is LMainλ, the self-image length for light other than the main signal light is Lλk, (k=1, 2... ), then "LMMI≒LMainλ and LMMI≠Lλk×M/N (N odd number, M and N are positive integers and relatively prime, M≦N)". It should be noted that because light of a plurality of wavelengths including light of the wavelength of the main signal light is input to the first optical waveguide 101, the electric field distribution generated in the first optical waveguide 101 in the direction of light propagation and the second The position where the normalized overlap integral of the electric field distribution incident on the optical waveguide 102 exceeds 0.5 and has a maximum value is defined as the center position where a self-image is formed. Further, the length from the input end of the second optical waveguide 102 to the center position where the self-image is formed is defined as the self-image length. [Selection diagram] Figure 1

Description

  The present invention relates to a wavelength separation element that separates unnecessary wavelength component light from main signal light.

A waveguide element is excellent in small size, mass productivity, and mechanical stability, and is widely used for optical communication by connecting fibers into a module. Examples of the waveguide type element include a quartz-based waveguide, a Si waveguide, a LiNbO ₃ waveguide, and a compound semiconductor waveguide.

  On the other hand, considering from the side of the optical circuit configuration, typical examples of wavelength demultiplexers (wavelength separation elements) include AWG (arrayed waveguide diffraction grating), MZI (Mach-Zehnder interferometer) and the like. For example, as shown in FIG. 11, the MZI interferometer causes interference by adding an optical path length difference ΔL between the first optical waveguide 302 and the second optical waveguide 303 formed on the substrate 301, and connects to another port for each wavelength. By outputting, wavelength separation is realized.

  In AWG, wavelength separation is performed mainly at 200 GHz (1.6 nm), 100 GHz (0.8 nm), 50 GHz (0.4 nm), 25 GHz (0.2 nm) intervals, etc. in a wavelength 1.55 μm band. In MZI, wavelength separation is realized mainly at intervals of 800 GHz (6.4 nm) to 25 GHz (0.2 nm) in a wavelength band of 1.55 μm. AWG and MZI are mainly made of silica-based glass waveguides, and have been used as wavelength multiplexing / demultiplexing devices for wavelength division multiplexing (WDM) alone or in combination (see Non-Patent Document 1).

Yasuhiro Hida et al., “400-channel AWG multiplexer / demultiplexer using quartz-based 1.5% Δ waveguide”, Proceedings of the IEICE General Conference, C-3-46, 211, 2001. Takenouchi Hirokazu et al., “Special Feature: Latest Trends in Optical Transmission Technology Controlling Optical Frequency and Phase, Phase-sensitive Amplification Using PPLN”, O plus E, vol. 37, no. 8, pp. 636-639, 2015. Yasufumi Yamada et al., “Structure of WDM transceiver module using PLC platform”, Proceedings of IEICE Electronics Society Conference, SC-1-11, p.329, 1995. Masao Kawauchi, “Current Status and Future of Planar Lightwave Circuit Technology”, NTT R & D, vol. 43, No. 11, 1273-1280, 1994. K. Okamoto, "Fundamental of Optical Waveguide second edition", Academic Press, pp. 46-50, 2006.

  However, the wavelength separation in the conventional waveguide type device has been applied when the wavelength interval is narrow. For example, in MZI, wavelength separation was at most 240 nm intervals between 1550 nm and 1310 nm.

  Here, in recent years, an optical parametric amplifier (OPA) has attracted attention because of the advantage that a multilevel signal used in the digital coherent technology can be amplified with less degradation of the SN ratio (compared to a conventional optical amplifier). For example, there is OPA using periodically poled lithium niobate (PPLN). In PPLN, light in the vicinity of a wavelength of 780 nm is input as pump light, and signal light in the vicinity of a wavelength of 1560 nm is amplified by a parametric effect. In this amplification, the signal light having a wavelength of 1560 nm and the remainder of the pump light having a wavelength of 780 nm not used for wavelength conversion (parametric effect) are mixed and output. The light having a wavelength of 780 nm becomes noise light with respect to the signal light, and thus needs to be removed.

  As described above, in recent years, it is necessary to remove light components having different wavelengths from the main signal light so as to separate light having a wavelength of 780 nm from light having a wavelength of 1560 nm. For this reason, there is a demand for a waveguide type wavelength separation element that separates light having a large wavelength interval (Non-Patent Documents 2 and 3).

  The present invention has been made to solve the above-described problems, and an object of the present invention is to enable separation of light having a large wavelength interval.

The wavelength separation element according to the present invention includes a first optical waveguide, a second optical waveguide, and a third optical waveguide that are continuously formed, and the first core of the first optical waveguide and the third core of the third optical waveguide. Is made thinner than the second core of the second optical waveguide, the second optical waveguide is a multimode optical waveguide, and multi-mode interference with one input and one output by the first optical waveguide, the second optical waveguide, and the third optical waveguide. A circuit is configured, and an electric field distribution generated with respect to the traveling direction of the light in the first optical waveguide by inputting light of a plurality of wavelengths including light of the wavelength of the main signal light to the first optical waveguide; 2 The position where the normalized overlap integral of the electric field distribution incident on the optical waveguide has a maximum value exceeding 0.5 is defined as the center position for forming the self-image, and the self-image is formed from the incident end of the second optical waveguide. The length to the center position is set as the self-image length. And, the length of the second optical waveguide L _MMI, self-image length L _Mainramuda to the wavelength of the main signal light, to light other than the main signal light, the self-image length _L λk, (k = 1,2 ··· ), “L _{MMI ≈L Mainλ} and L _MMI ≠ L _λk × M / N (N odd number, M and N are positive integers and prime, M ≦ N)”.

In the above wavelength separation element, the range of L _MMI is the normalized overlap integral of the electric field distribution at the end face position on the output side of the second optical waveguide and the electric field distribution incident on the second optical waveguide with respect to the wavelength of the main signal light. It is better if the range is 50% or more of the normalized overlap integral at the center position where the self image is formed.

In the above wavelength separation element, the range of _LMMI is related to the electric field distribution at the end face position on the output side of the second optical waveguide and the second optical waveguide for all wavelengths other than the main signal light input to the first optical waveguide. It is better if the normalized overlap integral of the incident electric field distribution is in the range of 0.1 or less.

  The wavelength separation element may further include a groove formed in the clad on both sides of the third core in the light traveling direction, and a light absorbing material inserted into the groove.

  The wavelength separation element may include a plurality of multimode interference circuits, and the plurality of multimode interference circuits may be connected in series.

  As described above, according to the present invention, it is possible to obtain an excellent effect that light having a large wavelength interval can be separated.

FIG. 1 is a configuration diagram illustrating a configuration of a wavelength separation element according to an embodiment of the present invention. FIG. 2 is a characteristic diagram showing a result of calculating the electric field amplitude distribution of incident light in the multimode interference circuit of the wavelength separation element in the embodiment by a beam propagation method (BPM). FIG. 3 is a block diagram showing the configuration used for calculating the calculation result shown in FIG. FIG. 4 is a characteristic diagram showing the calculation result of the normalized overlap integral of the electric field distribution and the incident electric field distribution in the wavelength separation element according to the embodiment. FIG. 5 is a characteristic diagram showing a result of calculating the electric field amplitude distribution of incident light having a wavelength of 1560 nm in the multimode interference circuit of the wavelength separation element in the embodiment by BPM. FIG. 6 is a characteristic diagram showing the result of calculation by BPM of the electric field amplitude distribution of incident light having a wavelength of 780 nm in the multimode interference circuit of the wavelength separation element in the embodiment. FIG. 7 is a characteristic diagram showing the calculation result of the normalized overlap integral of the electric field distribution shown in FIG. 5 and the electric field distribution incident on the second optical waveguide 102. FIG. 8 is a characteristic diagram showing the calculation result of the normalized overlap integral of the electric field distribution shown in FIG. 6 and the electric field distribution incident on the second optical waveguide 102. FIG. 9 is a configuration diagram showing the configuration of another wavelength separation element according to the embodiment of the present invention. FIG. 10 is a configuration diagram showing the configuration of another wavelength separation element according to the embodiment of the present invention. FIG. 11 is a configuration diagram showing a configuration of an MZI (Mach-Zehnder interferometer).

  Hereinafter, a wavelength separation element according to an embodiment of the present invention will be described with reference to FIG. This wavelength separation element is composed of a first optical waveguide 101, a second optical waveguide 102, and a third optical waveguide 103 that are continuously formed. The first optical waveguide 101, the second optical waveguide 102, and the third optical waveguide 103 constitute a multi-mode interference circuit with one input and one output.

  Here, the first core 111 of the first optical waveguide 101 and the third core 113 of the third optical waveguide 103 are thinner than the second core 112 of the second optical waveguide 102 in a cross-sectional view. The second optical waveguide 102 is a multimode optical waveguide. A clad 104 is formed around the first core 111, the second core 112, and the third core 113.

Further, the wavelength separation element has a length of the second optical waveguide 102 as L _MMI , a self image length with respect to the wavelength of the main signal light as L _Mainλ , and a self image length with respect to light other than the main signal light as L _λk , (k = 1, 2,..., “L _{MMI ≈L Mainλ} and L _MMI ≠ L _λk × M / N (N odd number, M and N are positive integers and prime to each other, M ≦ N)”. ing.

  Here, first, when light having a plurality of wavelengths including light having the wavelength of the main signal light is input to the first optical waveguide 101, an electric field distribution generated in the traveling direction of the light in the first optical waveguide 101 The position where the normalized overlap integral of the electric field distribution incident on the second optical waveguide 102 has a maximum value exceeding 0.5 is defined as the center position where the self image is formed. The length from the incident end of the second optical waveguide 102 to the center position where the self image is formed is defined as the self image length.

  The first optical waveguide 101, the second optical waveguide 102, and the third optical waveguide 103 are silica-based glass waveguides composed of a core and a clad on a Si substrate. In this case, for example, the cross-sectional size of the first core 111 and the third core 113 is 2.0 μm × 2.0 μm. For example, the second core 112 has a thickness of 2.0 μm and a width of 30 to 50 μm. The clad 104 is made of glass, and the relative refractive index difference between the first core 111, the second core 112, the third core 113, and the clad 104 is 1.5%. These parameters and configurations are appropriately changed when the waveguide type changes.

In the case of the above-described quartz-based glass waveguide, for example, it can be produced by a method combining reactive ion etching with the well-known flame deposition method (FHD) (see Non-Patent Document 4). The first optical waveguide 101, the second optical waveguide 102, and the third optical waveguide 103 may be an optical waveguide whose core is made of Si and an optical waveguide whose core is made of LiNbO ₃ .

  Next, FIG. 2 shows the result of calculating the electric field amplitude distribution of incident light in the multimode interference circuit of the wavelength separation element in the above-described embodiment by the beam propagation method (BPM). The first optical waveguide 101 has a first core 111 having a cross-sectional size of 2 μm × 2 μm, and the second optical waveguide 102 has a second core 112 having a wide cross-sectional size of 50 μm × 2 μm. The relative refractive index difference between the core and the clad is 1.5%. In this calculation, the third optical waveguide 103 is omitted as shown in FIG. 3 in order to explain the operation of the multimode interference circuit in detail. In FIG. 2, the color difference represents the electric field amplitude distribution, black represents a place where the electric field amplitude is small, white represents a place where the electric field amplitude is large, and gray represents the middle.

In the 1 × 1 multimode interference circuit described above, the light that has passed through the first optical waveguide 101 and entered the second optical waveguide 102 is reflected by the end surfaces of the second core 112 and the cladding 104 to cause multiple interference, and the distance As the distance of L _λ ≈n _eff W ² / λ advances, the incident electric field distribution is reproduced on the same optical axis. This is called self-imaging. As can be seen from the equation, the distance _Lλ is approximately proportional to the reciprocal of the wavelength, and is approximately twice as long when the wavelength is halved.

Here, n _eff represents the effective refractive index of the second optical waveguide 102, W represents the waveguide width of the second optical waveguide 102, and λ represents the wavelength. Substituting n _eff = 1.45, W = 50 μm, and λ = 1.56 μm yields L _λ = 2320 μm (rounded to the fourth digit). In FIG. 2, L _λ = 2610 μm, which is about 12% longer, but this deviation is effective due to the Goos-HanshenShift that causes the electric field distribution to leak into the cladding 104. An effect of increasing the width of the waveguide is included (see Non-Patent Document 5).

  Hereinafter, the traveling direction of light is described as the z-axis. When actually calculated, the position of self-imaging is not clear as can be seen from FIG. The mode field is not reproduced at one point on the optical axis, and has a certain width. This width varies depending on design parameters such as the waveguide width of the second optical waveguide 102. Therefore, an overlap integral (standardized, hereinafter referred to as normalized overlap integral) of the electric field distribution generated with respect to the traveling direction of light and the incident electric field distribution of the second optical waveguide 102 is taken to exceed 0.5. First, the point at which the normalized overlap integral takes the maximum value is defined as the center position of self-imaging.

Here, the L _lambda the distance to the center position of the self-imaging the second optical waveguide 102 (second core 112) end, is again defined.

Next, the definition of the normalized overlap integral will be described. For example, when the electric field distributions of the two modes are generalized and used as vectors, and expressed as E ₁ and E ₂ , respectively, the overlap integral (POI: Power Overlap Integral) of mode E ₁ and mode E ₂ is It is represented by Formula (1).

When E ₁ = E ₂ , POI = 1, so that it is understood that it is standardized. The physical meaning of the normalized overlap integral represents how much mode E ₁ is optically coupled to mode E ₂ in terms of energy. For example, if the mode E ₁ is the electric field distribution at a position A of the second optical waveguide 102 and the mode E ₂ is the incident electric field distribution of the second optical waveguide 102, the normalized overlap integral is It represents the component ratio in terms of energy.

  In many cases, in a 1-input 1-output multimode interference circuit, the first optical waveguide 101 and the third optical waveguide 103 have the same structure. Since the incident electric field distribution of the second optical waveguide 102 is the fundamental mode of the first optical waveguide 101, it is also the fundamental mode of the third optical waveguide 103. Usually, the structures of the first optical waveguide 101 and the third optical waveguide 103 are designed in a single mode or a pseudo single mode with respect to light of a passing wavelength, and in many cases, light is transmitted in a single mode (fundamental mode). Will propagate. Therefore, POI expresses how much light is coupled when an output waveguide is connected to that portion, that is, how much light can be extracted, as a ratio to the energy of light incident on the first optical waveguide 101. .

First, the center position of self-imaging from the incident end of the second optical waveguide 102, that is, the self-imaging length was defined as _Lλ . Here, at a position where the distance z = L _λ × M / N (where M and N are positive integers and relatively prime, M ≦ N), N mode fields are reproduced centering on the z axis. I understand. Here, the z-axis is the axis (light traveling direction) that connects the center of the first optical waveguide 101 and the center of the third optical waveguide 103 and travels from the first optical waveguide 101 to the third optical waveguide 103. For example, at z = L _λ / 2, two mode fields are reproduced with the z axis as the center. The electric field amplitude distribution is distributed approximately symmetrically with z = L _λ / 2 as the center. The reason for the slight deviation from symmetry is that the energy of light is attenuated as it propagates.

Next, in z = L _λ / 3 and z = 2L _λ / 3, three mode fields are reproduced around the z axis. Furthermore, when z = L _λ / 4 and z = 3L _λ / 4, four mode fields are reproduced with the z axis as the center. In _{z = L λ / 5, L} = 2L λ / 5, L = 3L λ / 5, 5 pieces mode field is five play around the z axis. In z = 4L λ _/ 5, in order to power has decreased, the mode field has not been five playback.

  Thus, when the integer N of the denominator is an odd number, the mode field is reproduced on the z axis, and when N is an even number, the mode field is not reproduced on the z axis.

  FIG. 4 shows the calculation result of the normalized overlap integral of the generated electric field distribution and the incident electric field distribution with the light traveling direction (z axis) as the horizontal axis. The overlap integral is integrated with the electric field distribution incident on the second optical waveguide 102, and the incident electric field distribution is a fundamental mode of the first optical waveguide 101. Here, the corresponding wavelength separation element has the configuration described with reference to FIG.

  For the wavelength of signal light extracted from the device, 3 dB degradation, that is, 50% degradation is generally used as one criterion as a measure of light intensity degradation. Therefore, if it is 50% or more of the normalized overlap integral at the center position where the self image is formed, light having a necessary intensity can be extracted. The corresponding z-axis range in FIG. 4 is 2530 μm to 2810 μm.

  In addition, with respect to the wavelength of unnecessary light other than signal light, an extinction ratio of 10 dB is one standard for separation. The extinction ratio here is an energy ratio of unnecessary light emitted from the third optical waveguide 103 to unnecessary light input to the first optical waveguide 101. If the normalized overlap integral is 10% or less, that is, 0.1 or less, light of 10 dB or more can be separated.

  The above has described the reproduction of the mode field when incident light is input to the multimode interference circuit.

Subsequently, an actual design of the wavelength separation element will be described. When W = 50 μm at a wavelength of 1560 nm, the device size becomes long as L _λ = 2000 μm or more. Therefore, W is narrowed and L _λ is shortened. The design was performed with W = 30 μm. The length of the second optical waveguide 102 is set to L _MMI = 980 μm.

  In the following, the electric field distribution characteristics of a 1-input 1-output multimode interference circuit (1 × 1 multimode interference circuit) including the third optical waveguide 103 are simulated. FIG. 5 shows a simulation result by BPM at a wavelength of 1560 nm. FIG. 6 shows a simulation result by BPM at a wavelength of 780 nm. 5 and 6, the core widths of the first core 111 and the third core 113 of the first optical waveguide 101 and the third optical waveguide 103 are 2 μm, and the core thicknesses and relative refractive index differences of all the cores are the same as the previous example. The same. The length of the first optical waveguide 101 is 100 μm, and the length of the third optical waveguide 103 is 120 μm.

5 and 6, black portions of the color indicate that the electric field amplitude increases as the color becomes white where the electric field amplitude is small. As can be seen from FIG. 5, the length of L _MMI of 980, in the light of the wavelength 1560 nm, and the electric field distribution of the incident light is reproduced in a portion connecting the third optical waveguide _103, L _MMI ≒ L Mainλ It has become. It can be seen that the electric field distribution reproduced at the output end of the second optical waveguide 102 is optically coupled to the third optical waveguide 103 with high efficiency.

  On the other hand, as can be seen from FIG. 6, in the light having a wavelength of 780 nm, two electric field distributions are reproduced symmetrically with respect to the traveling direction of the light, and M = 1 and N = 2. At the output end of the second optical waveguide 102 to which the output light is connected, the two reproduced electric field distributions are not coupled to the third optical waveguide 103 because the output positions are different.

  Accordingly, when signal light having a wavelength of 1560 nm and unnecessary light having a wavelength of 780 nm are input to the 1 × 1 multimode interference circuit having this configuration, the signal light having a wavelength of 1560 nm is transmitted and the light having a wavelength of 780 nm is attenuated. That is, according to the wavelength separation element in the embodiment, it can be understood that unnecessary light having a wavelength of 780 nm can be separated from signal light having a wavelength of 1560 nm.

  FIGS. 7 and 8 show calculation of normalized overlap integrals of the electric field distribution calculated in FIGS. 5 and 6 and the electric field distribution incident on the second optical waveguide 102 (the electric field distribution of the fundamental mode of the first optical waveguide 101), respectively. Results are shown. In FIG. 7 and FIG. 8, since the wavelengths are different, the incident electric field distribution is naturally different.

FIG. 7 shows the calculation result of the normalized overlap integral for the wavelength of 1560 nm, and shows how much of the fundamental mode component of the first optical waveguide 101 remains in terms of energy in terms of power. Since the length of the first optical waveguide 101 is 100 μm and L _MMI = 980 μm, the third optical waveguide 103 is connected at z = 1080 μm. Here, it can be seen that 72% of light having a wavelength of 1560 nm is coupled to the third optical waveguide 103. When converted to dB display, it can be seen that the signal light propagated with a loss of approximately 1.5 dB.

  FIG. 8 shows the calculation result of the normalized overlap integral for the wavelength of 780 nm. It can be seen that 0.101% of light having a wavelength of 780 nm is coupled to the third optical waveguide 103 at z = 1080 μm. When converted to dB display, it can be seen that light having a wavelength of 780 nm can be separated at an extinction ratio of 30 dB.

  As described above, it can be seen that by appropriately designing the length of the second optical waveguide 102, it is possible to design a wavelength separation element that propagates light having a wavelength of 1560 nm and separates light having a wavelength of 780 nm.

  Generalize the above. To repeat, the self-image is formed at the position where the normalized overlap integral of the electric field distribution generated in the second optical waveguide 102 and the electric field distribution incident on the second optical waveguide 102 has a maximum value exceeding 0.5. Defined as the center position. The length from the incident end of the second optical waveguide 102 to the center position where the self image is formed is defined as the self image length.

When the length L _MMI of the second optical waveguide 102 is close to the self image length L _Mainλ with respect to the wavelength of the main signal light, the main signal light is transmitted. Further, there is no mode field on the optical axis for a plurality of wavelengths of light to be removed, that is, z ≠ L _λk × M / N (N odd number, M and N are positive integers and prime to each other, M ≦ N) If the length of the second optical waveguide 102 is designed so as to be, an element for wavelength-separating light other than the main signal light is obtained. Note that L _λk represents the self-image length for light of wavelengths λ1, λ2,... Λk other than signal light.

Furthermore, with respect to the wavelength of the main signal light, the value of the normalized overlap integral between the electric field distribution on the output side end face of the second optical waveguide 102 and the electric field distribution incident on the second optical waveguide 102 is the center position where the self image is formed. If the range of the length L _MMI of the second optical waveguide 102 is set so as to be 50% or more of the value of the normalized integration at, the power of the main signal light can be sufficiently extracted. In other words, for the range of _LMMI , the normalized overlap integral of the electric field distribution at the end face position on the output side of the second optical waveguide 102 and the electric field distribution incident on the second optical waveguide 102 is self-related with respect to the wavelength of the main signal light. It suffices if the range is 50% or more of the normalized overlap integral at the center position where the image is formed.

Further, for all wavelengths other than the main signal light input to the first optical waveguide 101, the normalized overlap of the electric field distribution on the output side end face of the second optical waveguide 102 and the electric field distribution incident on the second optical waveguide 102 If the integration value falls within 0.1 or less, unnecessary light can be separated with an extinction ratio of 10 dB or more. In other words, with respect to all the wavelengths other than the main signal light input to the first optical waveguide 101 in the range of _LMMI , the electric field distribution at the end face position on the emission side of the second optical waveguide 102 and the second optical waveguide 102 The normalized overlap integral of the incident electric field distribution may be in the range of 0.1 or less.

  Next, another wavelength separation element in the embodiment will be described. As shown in FIG. 9, the wavelength separation element may be provided with light absorbing portions 114 made of a light absorbing material on both sides of the third core 113 in the light traveling direction. For example, the light absorbing portion 114 may be formed by forming a groove in the clad 104 on both sides of the third core 113 with respect to the light traveling direction and inserting a light absorbing material into the groove.

  For example, at the end face of the second optical waveguide 102 where the third optical waveguide 103 is optically connected, most of the light goes out of the waveguide without being coupled to the third optical waveguide 103, but becomes stray light. There is light having a wavelength of 780 nm which is reflected and returned to the third optical waveguide 103 again. This contributes to lowering the extinction ratio. On the other hand, stray light can be absorbed by the light absorption unit 114 by providing the light absorption unit 114. On the other hand, most of the light having a wavelength of 1560 nm is propagated to the third optical waveguide 103, so that there is no influence of the light absorber 114. Therefore, by providing the light absorbing portion 114, light with a wavelength of 1560 nm can be propagated with the same loss as described above, and light with a wavelength of 780 nm can be wavelength-separated with a higher extinction ratio.

  A plurality of multimode interference circuits may be provided, and the plurality of multimode interference circuits may be connected in series. For example, as shown in FIG. 10, the second optical waveguide 102a is newly connected to the emission side of the third optical waveguide 103a connected to the emission side of the second optical waveguide 102, and is connected to the emission side of the second optical waveguide 102a. The third optical waveguide 103b is connected. In this example, two multimode interference circuits are connected in series. In this example, the output optical waveguide of the incident-side multimode interference circuit and the incident optical waveguide of the output-side multimode interference circuit are shared by the third optical waveguide 103a. Other configurations are the same as those of the wavelength separation element described with reference to FIG.

  As described above, when two multimode interferometers are connected in series, the second optical waveguide 102a is added, so that the transmission loss of light having a wavelength of 1560 nm increases by 1.5 dB. However, since the light having a wavelength of 780 nm passes through the other 1 × 1 multimode interference circuit, it is calculated that the extinction ratio of wavelength separation is further improved by 30 dB. The configuration described above is effective when it is desired to further suppress light having a wavelength of 780 nm as compared with light having a wavelength of 1560 nm.

As described above, in the present invention, the length of the second optical waveguide, which is the multimode optical waveguide of the multimode interference circuit, is L _MMI , the self-image length with respect to the wavelength of the main signal light is L _Mainλ , and other than the main signal light self-image length L _.lamda.k of to light, when (k = 1,2 ···) _to, L _MMI ≒ L Mainλ _{and, L MMI ≠ L λk × M} / N (N odd, M and N are positive integers And M ≦ N). As a result, according to the present invention, it becomes possible to separate light having a large wavelength interval.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... 1st optical waveguide, 102 ... 2nd optical waveguide, 103 ... 3rd optical waveguide, 104 ... Cladding, 111 ... 1st core, 112 ... 2nd core, 113 ... 3rd core.

Claims (5)

Comprising a first optical waveguide, a second optical waveguide, and a third optical waveguide formed in succession;
The first core of the first optical waveguide and the third core of the third optical waveguide are thinner than the second core of the second optical waveguide,
The second optical waveguide is a multimode optical waveguide;
The first optical waveguide, the second optical waveguide, and the third optical waveguide constitute a multi-mode interference circuit with one input and one output,
An electric field distribution generated with respect to the traveling direction of light in the first optical waveguide by the input of light having a plurality of wavelengths including the light of the wavelength of the main signal light to the first optical waveguide, and the second light The position where the normalized overlap integral of the electric field distribution incident on the waveguide has a maximum value exceeding 0.5 is defined as the center position forming the self-image,
The length from the incident end of the second optical waveguide to the center position where the self image is formed is defined as the self image length,
L _{MMI is} the length of the second optical waveguide,
The self image length with respect to the wavelength of the main signal light is L _Mainλ ,
The self image length for light other than the main signal light is L _λk , (k = 1, 2,...)
L _{MMI ≈L Mainλ} and L _MMI ≠ L _λk × M / N (N odd number, M and N are positive integers and prime to each other, M ≦ N). The wavelength separation element according to claim 1,
The range of _LMMI is related to the wavelength of the main signal light, and the normalized overlap integral of the electric field distribution at the end face position on the output side of the second optical waveguide and the electric field distribution incident on the second optical waveguide is the self-reflection. A wavelength separation element characterized by being in a range of 50% or more of a normalized overlap integral at a center position where an image is formed. The wavelength separation element according to claim 2,
The range of L _MMI is the electric field distribution at the end face position on the output side of the second optical waveguide and the incident light on the second optical waveguide with respect to all wavelengths other than the main signal light input to the first optical waveguide. The wavelength separation element, wherein the normalized overlap integral of the electric field distribution is in a range of 0.1 or less. In the wavelength separation element according to any one of claims 1 to 3,
Grooves formed in the clad on both sides of the third core in the light traveling direction;
And a light absorbing material inserted into the groove. In the wavelength separation element according to any one of claims 1 to 4,
A plurality of the multi-mode interference circuits,
The plurality of multimode interference circuits are connected in series.