Gain equalizer

Abstract

One embodiment disclosed herein is a gain equalizer (80) having a first optical waveguide circuit (11) and a second optical waveguide circuit (21) which are formed on a substrate. Each of the first optical waveguide circuit and the second optical waveguide circuit includes: an input waveguide; N (N is an integer of 3 or more) 2-input 2-output optical multiplexing / demultiplexing circuits; N-1 arm waveguides composed of two waveguides connecting the optical multiplexing / demultiplexing circuits; an output waveguide; and phase shifters (115-1 to 115-(N-1), 215-1 to 215-(N-1)) mounted to at least one of the arm waveguides. The gain equalizer includes: an optical folding structure (31) for inputting light output from the output waveguide of the first optical waveguide circuit to the output waveguide of the second optical waveguide circuit; and electrical wiring (811-1 to 811-(N-1), 821-1 to 821-(N-1)) that electrically connects, in series, the phase shifter of the first optical waveguide circuit and the phase shifter of the second optical waveguide circuit corresponding to the phase shifter of the first optical waveguide circuit.

Description

gain equalizer

[0001] The present disclosure relates to gain equalizers.

[0002] Optical communication networks are rapidly developing against the backdrop of the explosive growth of data communications, typified by the Internet. Wavelength Division Multiplexing (WDM), a technology that enables the transmission of multiple wavelength signals over a single optical fiber, is considered important as a means of achieving high-capacity optical communications. Wavelength multiplexing / demultiplexing elements and optical amplifiers play important roles in the realization of WDM technology. Applying WDM technology to long-distance transmissions, particularly those over 100 km, requires optical amplifiers to be spaced at regular intervals within the transmission fiber. The wavelength dependence of their gain spectrum significantly affects the optical signal-to-noise ratio (OSNR). To flatten the gain spectrum, a gain equalizer has been proposed, as disclosed in Non-Patent Document 1, for example.

[0003] There are several methods for realizing the various optical functional circuits required in optical communication networks. For example, a method called PLC (Planar Lightwave Circuit), in which optical waveguides made of quartz-based materials are formed on a silicon substrate, is widely used as a method that combines multifunctionality, mass production, and low cost.

[0004] Optical circuits using silica-based glass waveguides are characterized by the ability to realize low-loss optical waveguides because they use the same material as the optical fibers used in optical communications. In addition, because the waveguides are formed on planar substrates, it is easy to combine various functional elements, and complex optical circuits can be fabricated with good reproducibility. Wavelength multiplexing / demultiplexing elements and optical switches fabricated using these technologies are indispensable components in building optical networks.

[0005] The gain equalizer disclosed in Non-Patent Document 1 has the drawback of large loss due to its configuration of Mach-Zehnder interferometers connected in multiple stages. This is due to the essential cause that optical signals are discarded to the unconnected output port in each Mach-Zehnder interferometer. On the other hand, Non-Patent Document 2 discloses a gain equalizer with a configuration called a lattice circuit.

[0006] A gain equalizer using a lattice optical circuit is composed of N (N is an integer of 3 or greater) directional couplers and N-1 arm waveguides, each consisting of two waveguides sandwiched between them. By applying heat to one of the arm waveguides, a phase shifter utilizing a change in refractive index due to the thermo-optic effect controls the phase of light propagating through the waveguides. By adjusting the phase difference between the optical signals propagating through the two waveguides that make up the arm waveguide of the upstream directional coupler, the interference state in the downstream directional coupler is adjusted, thereby controlling the transmission spectrum for the wavelength of the propagating light.

[0007] Figure 1 shows an example of a gain equalization spectrum in a conventional lattice-type optical circuit. The horizontal axis represents the wavelength of light, and the vertical axis represents the optical intensity transmittance of the gain equalizer. The wavelength range required for the gain equalizer is defined as Δλ １ (In FIG. 1, it is 1525 to 1570 nm). １ The minimum transmittance of light intensity within ｍａｘ (-9 dB in Figure 1), the maximum transmittance is Loss ｍｉｎ (-0.34 dB in Figure 1). ｍａｘ and Loss ｍｉｎ The difference between the two is called the transmission attenuation. The desired spectral shape is given by Δλ １ The spectral shape in other wavelength ranges is not important as long as it is maintained only within the range shown in FIG. １ If a spectrum in which the transmittance changes linearly with wavelength is realized within the wavelength range, the gain equalizer operates as a tilt equalizer. In this case, too, the slope (dB / nm) of the spectral shape is controlled by adjusting the amount of phase shift in each arm waveguide.

[0008] However, in a gain equalizer configured with a lattice-type optical circuit, when the interference state differs depending on the polarization direction of the light propagating through the optical circuit, a problem occurs in that the transmission spectrum finally output from the lattice-type optical circuit becomes polarization-dependent. The polarization dependence of the transmission spectrum appears as polarization-dependent loss (PDL) when viewed as a circuit characteristic of the optical circuit. In particular, in a lattice-type optical circuit with a large number of stages, the PDL generated in one arm waveguide is amplified with each stage of the lattice, resulting in a large PDL for the entire lattice-type optical circuit.

[0009] PDL occurs when birefringence exists in an optical waveguide and there is a difference in the effective refractive index depending on the polarization of the propagating optical signal. ｅｆｆ is defined by equation (1).

[0010] Here, n ｙ is the effective refractive index in the Y direction, n ｘ is the effective refractive index in the X direction. If (effective refractive index in the Y direction)>(effective refractive index in the X direction), the birefringence Δn ｅｆｆ is a positive value, and when (effective refractive index in the Y direction) < (effective refractive index in the X direction), the birefringence Δn ｅｆｆ is a negative value. ｅｆｆ In an optical waveguide where θ is not zero, two orthogonal polarization modes, called the TM mode having an electric field component perpendicular to the substrate surface and the TE mode having an electric field component in the horizontal direction, propagate.

[0011] In a gain equalizer using a lattice optical circuit, the thermo-optic effect is used to control the phase shifters in the arm waveguides of multiple directional couplers, thereby controlling the final output wavelength spectrum. In an (N-1)-stage lattice optical circuit consisting of N-1 arm waveguides, each consisting of N directional couplers and two waveguides sandwiched between them, the phase difference Θ occurring between the upper and lower waveguides of the i-th arm waveguide is ｉ is expressed by equation (2).

[0012] where λ is the wavelength of light, n ｅｆｆis the effective refractive index of the optical waveguide, ΔL ｉ is the difference in length between the upper and lower waveguides of the i-th arm waveguide, φ ｉ is the phase difference applied between the arm waveguides by controlling the phase shifter in the i-th arm waveguide. To configure a gain equalizer using a lattice-type optical circuit, the phase difference φ applied between each arm waveguide is ｉ By appropriately controlling the gain, a desired gain equalization spectrum can be obtained.

[0013] In this case, if birefringence exists in the optical waveguide and the effective refractive index of the TE mode and the effective refractive index of the TM mode are different from each other, the phase difference expressed by equation (2) differs depending on the polarization mode, and therefore the interference state in the downstream directional coupler differs depending on the polarization mode, resulting in PDL.

[0014] On the other hand, in a Mach-Zehnder interferometer, polarization dependence also occurs due to asymmetric polarization rotation between the arm waveguides. As is generally known in polarization-maintaining fibers, polarization rotation in an optical waveguide is suppressed by finite birefringence. Therefore, even if the absolute value of the birefringence expressed by equation (1) is reduced and the difference in interference state due to birefringence alone is eliminated, polarization dependence still occurs due to the influence of polarization rotation.

[0015] As described above, the conventional gain equalizer using a lattice-type optical circuit has the problem that it is not possible to completely eliminate the polarization dependent loss (PDL).

[0016] K. Suzuki, T. Kitoh, S. Suzuki, Y. Inoue, Y. Hbbino, T. Shibata, A. Mori, and M. Shimizu. 2002), paper IThG2.TR Schlipf, MW Street, J. Pandavenes, R. McBride, and DRS Cumming, “Design and Analysis of a Control System for an Optical Delay-Line Circuit Used as Reconfigurable Gain Equalizer,” Journal of Lightwave Technology, Vol. 21, Issue 9, pp. 1944 (2003).

[0017] The present disclosure has been made in view of such problems, and its object is to provide a gain equalizer using a lattice-type optical circuit with small polarization dependent loss.

[0018] One embodiment of the present disclosure is a gain equalizer including a first optical waveguide circuit and a second optical waveguide circuit formed on a substrate. Each of the first optical waveguide circuit and the second optical waveguide circuit includes an input waveguide, N (N is an integer greater than or equal to 3) two-input and two-output optical multiplexing / branching circuits, N−1 arm waveguides each consisting of two waveguides connecting the optical multiplexing / branching circuits, an output waveguide, and a phase shifter loaded in at least one of the arm waveguides. The gain equalizer includes an optical folding structure for inputting light output from the output waveguide of the first optical waveguide circuit to the output waveguide of the second optical waveguide circuit, and electrical wiring for electrically connecting the phase shifter of the first optical waveguide circuit and the corresponding phase shifter of the second optical waveguide circuit in series.

[0019] FIG. 1 is a diagram illustrating an example of a gain equalization spectrum in a conventional lattice-type optical circuit. FIG. 2 is a diagram illustrating a basic configuration of a gain equalizer according to an embodiment of the present disclosure. FIG. 3 is a diagram illustrating a first example of an optical return connection structure of a gain equalizer having the basic configuration of an embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view. FIG. 4 is a diagram illustrating a second example of an optical return connection structure of a gain equalizer having the basic configuration of an embodiment of the present disclosure. FIG. 5 is a diagram illustrating a third example of an optical return connection structure of a gain equalizer having the basic configuration of an embodiment of the present disclosure. FIG. 6 is a diagram illustrating an example of electrical wiring of a phase shifter in a gain equalizer. FIG. 7 is a diagram illustrating an example of electrical wiring of a phase shifter in a gain equalizer. FIG. 8 is a diagram illustrating a first example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure. FIG. 9 is a diagram illustrating a second example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure. FIG. 10 is a diagram illustrating a third example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure. Fig. 11 is a diagram illustrating a fourth example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure. Fig. 12 is a diagram illustrating a fifth example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure. Fig. 13 is a diagram illustrating a sixth example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view. Fig. 14 is a diagram illustrating a seventh example of electrical wiring of a phase shifter in a gain equalizer according to an embodiment of the present disclosure.

[0020] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated description may be omitted. Furthermore, the following description is an example and does not limit the scope of the present disclosure. The embodiments of the present disclosure may be implemented by adding components, omitting some components, and / or substituting some components, without departing from the spirit of the present disclosure.

[0021] First, the basic configuration of a gain equalizer according to an embodiment of the present disclosure will be described with reference to Figures 2 to 5. Next, examples of electrical wiring of phase shifters in the gain equalizer will be described with reference to Figures 6 and 7. After that, examples of electrical wiring of phase shifters in the gain equalizer according to an embodiment of the present disclosure will be described with reference to Figures 8 to 14.

[0022] 2 shows the basic configuration of a gain equalizer according to an embodiment of the present disclosure. The gain equalizer 10 is made up of a PLC, and the diagram shows a circuit configuration of an optical waveguide circuit formed on a substrate, seen from above. The gain equalizer 10 is made up of an even number of lattice optical circuits (here, a first lattice optical circuit 11 and a second lattice optical circuit 21) and an optical turn-back connection structure 31 that connects one input / output waveguide of a pair of lattice optical circuits.

[0023] The first lattice optical circuit 11 comprises a first input / output waveguide 111, a plurality of optical directional couplers 113-1 to 113-N, arm waveguides 114-1 to 114-(N-1), and a second input / output waveguide 112 (N is an integer equal to or greater than 3). The optical directional couplers 113-1 to 113-N may be any optical circuit element as long as they are two-input, two-output optical multiplexing / branching circuits. The arm waveguides 114-1 to 114-(N-1) are composed of two waveguides connecting the two optical directional couplers, and at least one of the waveguides is loaded with a phase shifter 115-1 to 115-(N-1). An optical signal input from the first input / output waveguide 111 passes through optical directional couplers 113-1 to 113-N and arm waveguides 114-1 to 114-(N-1) in that order, and is output from the second input / output waveguide 112. Electrical wiring connected to each phase shifter is omitted here. The electrical wiring will be described later.

[0024] The second lattice optical circuit 21 has the same design as the first lattice optical circuit 11, and similarly comprises a first input / output waveguide 211, a plurality of optical directional couplers 213-1 to 213-N, arm waveguides 214-1 to 214-(N-1), and a second input / output waveguide 212, and is provided with phase shifters 215-1 to 215-(N-1). As with the first lattice optical circuit 11, the electrical wiring connected to each phase shifter is omitted here.

[0025] The phase shifters 115-1 to 115-(N-1) and 215-1 to 215-(N-1) have the function of controlling the phase of the optical signal that passes through them. The phase shifter may be based on any principle as long as it can control the phase of the optical signal that passes through it, but an example is a thermo-optic phase shifter that uses heat generated by a heater and the thermo-optic effect.

[0026] The second input / output waveguide 112 of the first lattice optical circuit 11 and the second input / output waveguide 212 of the second lattice optical circuit 21 are connected by an optical fold-back connection structure 31. With this structure, light that propagates through the first lattice optical circuit 11 and is output from the second input / output waveguide 112 is input to the second lattice optical circuit via the second input / output waveguide 212. The optical fold-back connection structure 31 has the function of rotating the polarization direction of the propagating light by 90 degrees. A specific example of the optical fold-back connection structure 31 that realizes this function will be described later.

[0027] In the above-described gain equalizer, an example has been shown in which a phase shifter is loaded into one of the two waveguides (arm waveguides) connecting the two optical directional couplers, but a phase shifter may be loaded into both of the two waveguides.

[0028] (Operating Principle) When an optical signal is input to the gain equalizer 10, the light is input to the first lattice optical circuit 11 via the first input / output waveguide 111. The input optical signal is subjected to gain equalization according to a gain equalization spectrum determined by the amount of phase modulation in the phase shifters 115-1 to 115-(N-1), and then output from the second input / output waveguide 112. At this time, the gain equalization spectrum differs depending on the polarization direction of the light due to PDL generated in the first lattice optical circuit 11, so the spectrum of the output light from the second input / output waveguide 112 has polarization dependency.

[0029] The output light from the second input / output waveguide 112 propagates through the optical folding connection structure 31 and is then input to the second input / output waveguide 212. At this time, the optical folding connection structure 31 has the function of rotating the polarization direction of the propagating light by 90 degrees, so that the light is input to the second input / output waveguide 212 in a state where the TE mode and TM mode of the optical waveguide have been converted into each other.

[0030] Light input to the second lattice optical circuit 21 via the second input / output waveguide 212 is subjected to gain equalization according to a gain equalization spectrum determined by the amount of phase modulation in the phase shifters 215-1 to 215-(N-1), and then output from the first input / output waveguide 211. At this time, it is desirable that the phase modulation conditions in the phase shifters 115-1 to 115-(N-1) of the first lattice optical circuit 11 are the same as the phase modulation conditions in the phase shifters 215-1 to 215-(N-1) of the second lattice optical circuit 21. In other words, it is desirable that the gain equalization spectrum of the first lattice optical circuit 11 is the same as the gain equalization spectrum of the second lattice optical circuit 21.

[0031] In the gain equalizer 10, an input optical signal passes through the first lattice optical circuit 11 and the second lattice optical circuit 21, which have the same design and the same phase modulation conditions. The light, which was in TE mode when passing through the first lattice optical circuit 11, undergoes a 90-degree polarization rotation in the optical turn-back connection structure 31 and passes through the second lattice optical circuit 21 in TM mode. Therefore, the PDL generated in the first lattice optical circuit cancels out the PDL generated in the second lattice optical circuit. Overall, a gain equalizer with small PDL can be realized.

[0032] When an even number (2M: M is an integer of 1 or greater) of lattice optical circuits are included, the lattice optical circuits having the same gain equalization spectrum are paired, and the second input / output waveguides of the pair of lattice optical circuits are connected to each other by an optical folding connection structure. Furthermore, the first input / output waveguides of M pairs of lattice optical circuits may be cascade-connected. That is, the first input / output waveguides of the even-numbered lattice optical circuit are connected to the first input / output waveguides of the odd-numbered lattice optical circuit, and the first input / output waveguides of the first lattice optical circuit to the first input / output waveguide of the 2Mth lattice optical circuit form one gain equalizer.

[0033] (Optical Fold-Back Connection Structure) Figure 3 shows a first example of an optical fold-back connection structure of a gain equalizer having the basic configuration of an embodiment of the present disclosure. Figure 3(a) shows the circuit configuration seen through from above, and Figure 3(b) shows a cross section taken along IIIb-IIIb in Figure 3(a). The optical fold-back connection structure 31 connects the second input / output waveguide 112 of the first lattice optical circuit 11 and the second input / output waveguide 212 of the second lattice optical circuit 21 via an optical waveguide 311. A groove 312 is formed along part of the optical waveguide 311 on one side of the substrate plane.

[0034] 3(b), an optical waveguide core 3113 is embedded in an optical waveguide clad 3112 formed on a substrate 3111. The clad layer and core layer of the optical waveguide may be formed by any method as long as they can form uniform and smooth layers, and examples of such methods include flame deposition, chemical vapor deposition (CVD), and sputtering.

[0035] Typically, in silica-based optical waveguide circuits, even if the structural birefringence due to the optical waveguide structure is zero, stress birefringence occurs because compressive stress in the horizontal direction of the substrate acts on the core due to the differences in the thermal expansion coefficients of the substrate, cladding material, and core material. In particular, when a flame deposition method or the like is used to form the cladding layer or core layer of an optical waveguide, high-temperature processes are included in the fabrication of the optical waveguide, resulting in large compressive stress at room temperature and a large increase in birefringence. Thus, birefringence with its principal axis perpendicular or horizontal to the substrate occurs in the core of a silica-based optical waveguide.

[0036] When a groove is formed in the cladding on one side of the core of an optical waveguide that has birefringence, asymmetric stress is applied to the optical waveguide core, tilting the principal axis of birefringence. The degree to which stress is applied depends on the distance from the waveguide core to the groove, and adjusting this distance can change the degree of tilt of the principal axis of birefringence.

[0037] When linearly polarized light passes through such a birefringent object with an inclined principal axis, the angle θ between the polarization direction of the input light and the principal axis of the birefringent object, and the magnitude of birefringence Δn ｅｆｆ The polarization state of light propagating through a birefringent object changes depending on the propagation distance L in the birefringent object. When these relationships satisfy equation (3), a linearly polarized wave is output whose polarization direction is rotated by 2θ relative to the input linearly polarized wave.

[0038] Here, k is the wave number of the input light. ｅｆｆ , L, a desired amount of polarization rotation can be achieved for an input linearly polarized wave.

[0039] In the first example of the optical folding connection structure 31, by forming a groove 312 in the cladding portion on one side of the core in a partial region of the optical waveguide 311, it is possible to rotate the polarization of transmitted light with respect to the optical waveguide 311-2 in this region. By adjusting the distance from the optical waveguide 311 to the groove 312, the amount of polarization rotation in the optical waveguide 311-2 can be adjusted, and the amount of polarization rotation can also be adjusted to 90 degrees. Light that was in TE mode in the optical waveguide 311-1 before passing through the region where the groove 312 is formed undergoes polarization rotation of approximately 90 degrees while propagating through the optical waveguide 311-2, and is converted to TM mode in the optical waveguide 311-3 after passing through this region. Therefore, light input from the input / output waveguide 112 to the optical folding connection structure 31 is output from the input / output waveguide 212 with its polarization direction rotated by approximately 90 degrees. It is preferable to set the polarization rotation to 90 degrees, but considering manufacturing errors, if the polarization rotation is around 90 degrees, the polarization dependent loss can be made small enough for practical use.

[0040] 4 shows a second example of an optical folding connection structure of a gain equalizer having a basic configuration according to an embodiment of the present disclosure. The optical folding connection structure 31 connects the second input / output waveguide 112 of the first lattice-type optical circuit 11 and the second input / output waveguide 212 of the second lattice-type optical circuit 21 via an optical waveguide 311. The optical waveguide 311 has a wave plate 314 inserted at at least one location along its path. A typical wave plate is a half-wave plate, which shifts the phase difference between the major axis and the slow axis by π. By installing the half-wave plate with its major axis tilted at 45° with respect to the substrate surface of the optical waveguide 311, the polarization direction of light propagating through the optical waveguide 311 is rotated by 90°. In this way, the light input from the input / output waveguide 112 to the optical turn-back connection structure 31 is output from the input / output waveguide 212 with its polarization direction rotated by 90 degrees.

[0041] 5 shows a third example of an optical return connection structure of a gain equalizer having the basic configuration of an embodiment of the present disclosure. The second input / output waveguide 112 of the first lattice-type optical circuit 11 and the second input / output waveguide 212 of the second lattice-type optical circuit 21 are formed up to the end face of the PLC substrate, and the end faces of the respective waveguides are connected by a polarization-maintaining fiber 315. The polarization-maintaining fiber 315 maintains the polarization state of light propagating through the fiber.

[0042] The connection point between the second input / output waveguide 112 and the polarization-maintaining fiber 315 is designated as connection point 316, and the connection point between the second input / output waveguide 212 and the polarization-maintaining fiber 315 is designated as connection point 317. The polarization-maintaining fiber 315 is connected so that the main axis direction of the polarization-maintaining fiber 315 at connection point 316 and the main axis direction of the polarization-maintaining fiber 315 at connection point 317 differ by approximately 90° from each other. As a result, light that was in TE mode in the input / output waveguide 112 propagates through the polarization-maintaining fiber 315 and is input to the input / output waveguide 212 as TM mode. In this way, the light input from the input / output waveguide 112 to the optical folding connection structure 31 is output from the input / output waveguide 212 with its polarization direction rotated by approximately 90°.

[0043] According to the gain equalizer 10 having the basic configuration of one embodiment of the present disclosure, the PDL of the gain equalizer configured with a lattice-type optical circuit can be eliminated by using any of the first to third examples of the optical foldback connection structure 31.

[0044] (Electrical Wiring) As described above, in the configuration of a gain equalizer having the basic configuration of the present disclosure, it is desirable that the first lattice optical circuit 11 and the second lattice optical circuit 21 have the same design and the same optical characteristics. It is also desirable that the phase modulation conditions in the phase shifters 115-1 to 115-(N-1) of the first lattice optical circuit 11 and the phase modulation conditions in the phase shifters 215-1 to 215-(N-1) of the second lattice optical circuit 21 are the same.

[0045] Generally, each phase shifter is driven by connecting it to a power source via electrical wiring formed on the optical waveguide chip and supplying power. In this case, if the phase shifters 115-1 to 115-(N-1) and the phase shifters 215-1 to 215-(N-1) have the same design and are thermo-optic phase shifters using heaters, the condition for making the phase modulation conditions the same is that the amount of current flowing through the thermo-optic phase shifters be the same.

[0046] Fig. 6 shows an example of electrical wiring of phase shifters in a gain equalizer. The gain equalizer 60 in Fig. 6 has electrical wiring 611-1 to 611-(N-1) that connects the phase shifters 115-1 to 115-(N-1) of the first lattice optical circuit 11 to power supplies 651-1 to 651-(N-1). Furthermore, the gain equalizer 60 has a common wiring 631 that connects the phase shifters 115-1 to 115-(N-1) to ground (GND) 650, which serves as a predetermined potential. The gain equalizer 60 also has electrical wiring 621-1 to 621-(N-1) that connects the phase shifters 215-1 to 215-(N-1) of the second lattice optical circuit 21 to power supplies 652-1 to 652-(N-1). The phase shifters 215-1 to 215-(N-1) are connected to a ground (GND) 650, which is a predetermined potential, by a common wiring 631. However, when each phase shifter is individually connected to a different power supply by an electric wiring as in Fig. 6, variations in the voltage of each power supply, as well as differences in the length of the electric wiring and variations in the resistance value of the electric wiring, occur in the amount of current that flows.

[0047] FIG. 7 shows another example of electrical wiring for phase shifters in a gain equalizer. The gain equalizer 70 of FIG. 7 differs from the wiring example shown in FIG. 6 in that the phase shifters 115-1 to 115-(N-1) of the first lattice-type optical circuit 11 and the phase shifters 215-1 to 215-(N-1) of the second lattice-type optical circuit 21 are connected to the same power supplies 750-1 to 750-(N-1). In the gain equalizer 70 of FIG. 7, the same voltage is applied to each phase shifter. However, as in the wiring example of FIG. 6, the gain equalizer 70 of FIG. 7 has variations in the amount of voltage drop due to differences in the length of the electrical wiring and variations in the resistance values ​​of the electrical wiring, making it difficult to uniform the amount of current flowing through the phase shifters.

[0048] Therefore, in order to make the amount of current flowing through phase shifters 115-1 to 115-(N-1) and phase shifters 215-1 to 215-(N-1) the same and to make the phase modulation conditions the same, in one embodiment of the present disclosure, a gain equalizer having the basic configuration described above uses the wiring example described below.

[0049] (First Embodiment) Fig. 8 shows a first example of electrical wiring of a phase shifter in a gain equalizer according to the first embodiment of the present disclosure. The gain equalizer 80 in Fig. 8 includes a lattice optical circuit 81 connecting a first lattice optical circuit 11 and a second lattice optical circuit 21. The gain equalizer 80 in Fig. 8 includes electrical wiring 811-1 to 811-(N-1) connecting the phase shifters 115-1 to 115-(N-1) of the first lattice optical circuit 11 and the phase shifters 215-1 to 215-(N-1) of the second lattice optical circuit 21. Furthermore, the gain equalizer 80 includes electrical wiring 821-1 to 821-(N-1) connecting the phase shifters 215-1 to 215-(N-1) to ground (GND) 650, which serves as a predetermined potential. Electrical wiring 811 connects a pair of phase shifters 115 of the first lattice optical circuit 11 and corresponding phase shifters 215 of the second lattice optical circuit 21 to corresponding power supplies 750. Connecting two corresponding phase shifters 115 and 215 to one power supply 750 makes it possible to make the amount of current flowing the same. In the gain equalizer 80 of Fig. 8, the phase modulation conditions in the phase shifters 115-1 to 115-(N-1) of the first lattice optical circuit and the phase modulation conditions in the phase shifters 215-1 to 215-(N-1) of the second lattice optical circuit are the same.

[0050] The electrical wiring for connecting the two corresponding phase shifters 115 and 215 and one power supply 750 in series is not limited to the form shown in FIG. 8, and the same effect can be achieved with any shape.

[0051] As mentioned above, the number of lattice optical circuits constituting the gain equalizer does not have to be two, as long as there is an even number. The gain equalizer 80 shown in Fig. 8 has two phase shifters connected in series, corresponding to each stage of two lattice optical circuits, with the right side of the upper phase shifter 115 connected to a power supply 750 and the left side of the lower phase shifter connected to ground 650. By passing electrical wiring between the two lattice optical circuits constituting the gain equalizer, the space required for arranging the electrical wiring can be reduced.

[0052] Second Embodiment As described above, there are cases where phase shifters are loaded on both arm waveguides in each stage of a lattice optical circuit. In this case, the phase shifters installed in the upper arm waveguides and the phase shifters installed in the lower arm waveguides are connected in series, so that the phase modulation conditions of the phase shifters in the lattice optical circuit can be made the same.

[0053] Fig. 9 shows a second example of electrical wiring of a phase shifter in a gain equalizer according to the second embodiment of the present disclosure. The gain equalizer 90 in Fig. 9 includes a lattice optical circuit 91 in which a first lattice optical circuit 11 and a second lattice optical circuit 21 are connected. In the gain equalizer 90 in Fig. 9, phase shifters are loaded on both of the arm waveguides at each stage of the first lattice optical circuit 11 and the second lattice optical circuit 21. The first lattice optical circuit 11 includes phase shifters 115-1 to 115-(N-1) and phase shifters 915-1 to 915-(N-1). The second lattice optical circuit 21 includes phase shifters 215-1 to 215-(N-1) and phase shifters 925-1 to 925-(N-1). Gain equalizer 80 also has power supplies 950-1 to 950-(N-1) corresponding to phase shifters 915-1 to 915-(N-1) and phase shifters 925-1 to 925-(N-1). Gain equalizer 80 also has electrical wiring 921-1 to 921-(N-1) connecting phase shifters 925-1 to 925-(N-1) with ground (GND) 650, which serves as a predetermined potential.

[0054] 8 , in the gain equalizer 90, two corresponding phase shifters 115 and 215 loaded into the upper waveguides of the arm waveguides are connected in series by electrical wiring 811. Furthermore, in the gain equalizer 90, two corresponding phase shifters 915 and 925 loaded into the lower waveguides of the arm waveguides are connected in series by electrical wiring 921. In this way, by arranging the phase shifters loaded into both waveguides of the arm waveguides, it becomes possible to control the optical path length difference between the two waveguides of the arm waveguides over a wider range.

[0055] (Third Embodiment) Fig. 10 shows a third example of electrical wiring of phase shifters in a gain equalizer according to a third embodiment of the present disclosure. The gain equalizer 1000 in Fig. 10 includes a lattice optical circuit 1001 in which a first lattice optical circuit 11 and a second lattice optical circuit 21 are connected. In the gain equalizer 900 in Fig. 9, electrical wiring 821-1 to 821-(N-1) connecting the phase shifters 215-1 to 215-(N-1) to GND 650 as a predetermined potential, and electrical wiring 921-1 to 921-(N-1) connecting the phase shifters 925-1 to 925-(N-1) to ground (GND) 650 as a predetermined potential are provided separately. Gain equalizer 1000 in FIG. 10 differs from gain equalizer 900 in FIG. 9 in that phase shifters 215-1 to 215-(N-1) and phase shifters 925-1 to 925-(N-1) are connected to GND 650 by common electrical wiring 1011-1 to 1011-(N-1).

[0056] In this way, the electrical wiring for connecting the phase shifters loaded on both waveguides of the arm waveguide to GND 650 can be made common, thereby simplifying the electrical wiring.

[0057] 11 shows a fourth example of electrical wiring of a phase shifter in a gain equalizer according to a fourth embodiment of the present disclosure. The gain equalizer of this embodiment is formed by arranging a plurality of gain equalizers 1000 shown in FIG. 10 side by side. According to the gain equalizer of this embodiment, electrical wiring for connection to GND can be shared even among a plurality of gain equalizers, thereby further simplifying the electrical wiring during integration.

[0058] The gain equalizer 1100 of Fig. 11 is a lattice-type optical circuit 1101 in which two gain equalizers 1000 of Fig. 10 are arranged. Phase shifters 215a-1 to 215a-(N-1) and phase shifters 925a-1 to 925a-(N-1) in the lower gain equalizer of the gain equalizer 1100, and phase shifters 215b-1 to 215b-(N-1) and phase shifters 925b-1 to 925b-(N-1) in the upper gain equalizer are connected by common electrical wiring 1011-1 to 1011-(N-1). Three or more gain equalizers 1000 of Fig. 10 can also be arranged.

[0059] Fifth Embodiment Fig. 12 shows a fifth example of electrical wiring of phase shifters in a gain equalizer according to the fifth embodiment of the present disclosure. The gain equalizer 1200 in Fig. 12 is a modified version of the gain equalizer 90 in Fig. 9. The gain equalizer 1200 has a lattice optical circuit 1201 in which a first lattice optical circuit 11 and a second lattice optical circuit 21 are connected. The gain equalizer 1200 has dummy electrical wirings 1211-1 to 1211-(N-1) arranged adjacent to the phase shifters 215-1 to 215-(N-1), and dummy electrical wirings 1221-1 to 1221-(N-1) arranged adjacent to the phase shifters 925-1 to 925-(N-1).

[0060] The distance between the phase shifters 115-1 to 115-(N-1) and the dummy electrical wirings 1211-1 to 1211-(N-1) is d. The distance between the phase shifters 215-1 to 215-(N-1) and the electrical wirings 811-1 to 811-(N-1) is also d. The distance between the phase shifters 915-1 to 915-(N-1) and the electrical wirings 911-1 to 911-(N-1) is d'. The distance between the phase shifters 925-1 to 925-(N-1) and the dummy electrical wirings 1221-1 to 1221-(N-1) is also d'. Therefore, the arrangement of the phase shifters, the electrical wiring between two corresponding phase shifters, and the dummy electrical wiring is such that the corresponding phase shifters 115-1 to 115-(N-1) and 215-1 to 215-(N-1), and the phase shifters 915-1 to 915-(N-1) and 925-1 to 925-(N-1) between the first lattice optical circuit 11 and the second lattice optical circuit 21 are at equal distances to the adjacent electrical wiring or dummy wiring, and are equally affected by these electrical wirings or dummy wirings.

[0061] The cladding of a waveguide is generally made of a dielectric, and the electrical wiring is made of a material different from the dielectric. Therefore, when electrical wiring is placed near a waveguide, stress is generated in the nearby waveguide. As a result, the characteristics of the waveguide change. In a gain equalizer having the basic configuration of this disclosure, it is important to match the characteristics of the two lattice optical circuits. Therefore, it is desirable to place the electrical wiring near the waveguides in the lattice optical circuits symmetrically with respect to the two lattice optical circuits.

[0062] When two lattice optical circuits are placed close to each other to increase the integration density of the gain equalizer, the electrical wiring arrangement between the first and second lattice optical circuits becomes asymmetric due to the electrical wiring placed between the two lattice optical circuits. Specifically, the upper first lattice optical circuit has electrical wiring near the waveguide below the arm waveguide, while the lower second lattice optical circuit has electrical wiring near the waveguide above the arm waveguide. Therefore, as described above with reference to FIG. 12 , symmetry can be improved by eliminating dummy electrical wiring near the waveguide above the arm waveguide in the first lattice optical circuit and placing dummy electrical wiring near the waveguide below the arm waveguide in the second lattice optical circuit. In other words, the symmetry of the stresses generated in the waveguides of the arm waveguides of the first and second lattice optical circuits is improved, and the characteristics of the two lattice optical circuits become consistent.

[0063] Sixth Embodiment Fig. 13 shows a sixth example of electrical wiring of a phase shifter in a gain equalizer according to a sixth embodiment of the present disclosure. Fig. 13(a) is a top view, and Fig. 13(b) is a cross-sectional view taken along line XIIIb-XIIIb in Fig. 13(a). The gain equalizer 1300 in Fig. 13 is a variation of the gain equalizer 1200 in Fig. 12. The gain equalizer 1300 differs from the gain equalizer 1200 in Fig. 12 in that it further includes a thermal insulating groove 1311 formed between the two waveguides of the arm waveguide of the first lattice optical circuit 11 and near both sides of the arm waveguide, and a thermal insulating groove 1321 formed between the two waveguides of the arm waveguide of the second lattice optical circuit 21 and near both sides of the arm waveguide. Fig. 13(b) shows a cross section of the right arm waveguide of the second lattice optical circuit 21.

[0064] When the phase shifter in the gain equalizer is a thermo-optic phase shifter that uses a heater, the phase shifter heats the waveguide and reaches a high temperature. Furthermore, metals with high thermal conductivity are generally used for electrical wiring. Therefore, the heat generated by the phase shifter heats the electrical wiring. The electrical wiring connecting two heated phase shifters in particular rises in temperature because it has limited heat dissipation paths. Heating the waveguide in the lattice-type optical circuit due to the electrical wiring heats the gain equalizer's characteristics, which can cause fluctuations. In particular, if the heat distribution is asymmetric between the two corresponding lattice-type optical circuits, the polarization-dependent compensation capability deteriorates.

[0065] In the gain equalizer 1300 of this embodiment, as a solution to the problem of overheating of the electrical wiring, the above-mentioned thermal insulating groove is arranged to block the flow of heat between the electrical wiring connecting the two phase shifters and the waveguides of the lattice-type optical circuit. The thermal insulating groove forms an area where the cladding is removed between the arm waveguide and the electrical wiring connecting the two phase shifters, and by utilizing this area as a thermal insulating groove, it is possible to suppress the influence of heat between the electrical wiring on the arm waveguide. To maintain the above-mentioned symmetry, thermal insulating grooves are formed near both sides of the arm waveguide. Furthermore, to suppress the transfer of heat between the phase shifters loaded in the two waveguides of the arm waveguide, thermal insulating grooves are also formed in the two waveguides of the arm waveguide. In this case, by configuring the thermal conductivity of the substrate 1330 to be higher than that of the cladding 1331, heat can be dissipated toward the substrate, suppressing the influence on the phase shifters and the waveguides.

[0066] Seventh Embodiment Fig. 14 shows a seventh example of electrical wiring of a phase shifter in a gain equalizer according to a seventh embodiment of the present disclosure. The gain equalizer 1400 in Fig. 14 is a modified version of the gain equalizer 1200 in Fig. 12 or the gain equalizer 1300 in Fig. 13. In Fig. 14, for each of the phase shifters 115-1 to 115-(N-1), of the two dummy electrical wirings 1211 arranged at adjacent positions, one closer to the phase shifter is designated 1211a, and the other is designated 1211b. Similarly, for each of the phase shifters 925-1 to 925-(N-1), one closer to the phase shifter is designated 1221a, and the other is designated 1221b. Gain equalizer 1400 in FIG. 14 differs from gain equalizer 1200 in FIG. 12 or gain equalizer 1300 in FIG. 13 in that dummy electrical wiring 1211a is connected to electrical wiring 811 and dummy electrical wiring 1221a is connected to electrical wiring 911.

[0067] As mentioned above, temperature rise in electrical wiring with limited heat dissipation paths can cause fluctuations in the characteristics of the gain flattener. In particular, if the heat distribution is asymmetric between two corresponding lattice-type optical circuits, the polarization-dependent compensation ability will deteriorate. Therefore, in the gain flattener 1300 of this embodiment, the electrical wiring connecting the two phase shifters is connected to a dummy electrical wiring, thereby symmetricalizing the influence of heat.

[0068] As shown in Figure 14, by connecting a dummy wiring arranged to improve stress symmetry to the electrical wiring connecting two phase shifters, the temperature of the dummy wiring can be intentionally increased, thereby improving thermal symmetry. For example, in the gain equalizer 1400 shown in Figure 14, heat dissipation from the phase shifter 115 loaded on the upper waveguide of the arm waveguide of the first lattice optical circuit 11 will increase the temperature of the electrical wiring 811 located near the upper waveguide of the arm waveguide of the second lattice optical circuit 21 unless countermeasures are taken. Therefore, by thermally connecting the dummy electrical wiring 1211a located near the upper waveguide of the arm waveguide of the first lattice optical circuit 11 to the electrical wiring 811 connecting the two phase shifters, the temperature of this dummy electrical wiring 1211a will increase, thereby improving thermal symmetry. Alternatively, the dummy electrical wiring 1211a can be used as a heat dissipation path for the electrical wiring 811.

[0069] 14, if no countermeasures are taken, heat dissipation from the phase shifter 925 loaded on the lower waveguide of the arm waveguide of the second lattice optical circuit 21 will raise the temperature of the electrical wiring 911 located near the lower waveguide of the arm waveguide of the first lattice optical circuit 11. Therefore, by thermally connecting the dummy electrical wiring 1221a located near the lower waveguide of the arm waveguide of the second lattice optical circuit 21 to the electrical wiring 911 connecting the two phase shifters, the temperature of this dummy electrical wiring 1221a will rise, thereby improving thermal symmetry. Alternatively, the dummy electrical wiring 1221a can be used as a heat dissipation path for the electrical wiring 911.

[0070] Here, by connecting the electrical wirings 811, 911 connecting the two phase shifters to the dummy electrical wirings 1211a, 1221a, respectively, the temperatures of the electrical wirings connecting the two phase shifters and the dummy electrical wirings connected thereto can be made approximately the same, thereby enhancing the effect of symmetricalizing the influence of temperature on the electrical wirings. By setting the positions of the dummy electrical wirings 1211a, 1221a in the same way as in the fifth embodiment described with reference to Figure 13, the influence of heat in the first lattice-type optical circuit and the second lattice-type optical circuit can be made the same.

[0071] According to an embodiment of the present disclosure, it is possible to provide a gain equalizer using a lattice-type optical circuit with small polarization dependent loss.

[0072] 10, 60, 70, 80, 90, 1000, 1100, 1200, 1300, 1400 Gain equalizer 11, 21, 61, 71, 81, 91, 1001, 1101, 1201, 1301, 1401 Lattice optical circuit 111, 112, 211, 212 Input / output waveguide 113, 213 Optical directional coupler 114 Arm waveguide 115, 215 Phase shifter 214 Arm waveguide 31 Optical return connection structure 311 Optical waveguide 312 Groove 3111 Substrate 3112 Cladding 3113 Optical waveguide core 314 Wave plate 315 Polarization-maintaining fiber 316 Connection point 317 Connection point 611, 621, 811, 911, 921, 1011, 1111 Electrical wiring 631 Common wiring 650 GND 651, 652, 750, 950 Power supply 915, 925 Phase shifter 1211, 1211a, 1211b, 1221, 1221a, 1221b Dummy electrical wiring 1311, 1321 Heat insulating groove 1330 Substrate 1331 Cladding 1332 Core

Claims

1. A gain equalizer comprising: a first optical waveguide circuit and a second optical waveguide circuit formed on a substrate, each of the first optical waveguide circuit and the second optical waveguide circuit comprising: an input waveguide; N (N is an integer of 3 or greater) two-input and two-output optical multiplexing / branching circuits; N-1 arm waveguides each consisting of two waveguides connecting the optical multiplexing / branching circuits; an output waveguide; and a phase shifter loaded in at least one of the arm waveguides; an optical folding structure for inputting light output from the output waveguide of the first optical waveguide circuit to the output waveguide of the second optical waveguide circuit; and electrical wiring for electrically connecting the phase shifter of the first optical waveguide circuit and the phase shifter of the second optical waveguide circuit corresponding to the phase shifter of the first optical waveguide circuit in series.

2. The gain equalizer according to claim 1, wherein the phase shifter is a thermo-optic phase shifter, and further comprising a heat insulating groove formed between the electrical wiring and the phase shifter.

3. The gain equalizer according to claim 1, further comprising another electrical wiring arranged at a position symmetrical with respect to the two waveguides of the arm waveguide with respect to the position of the electrical wiring arranged between the phase shifter of the first optical waveguide circuit and the corresponding phase shifter of the second optical waveguide circuit.

4. A gain equalizer according to claim 3, wherein the other electrical wiring is connected to the electrical wiring.

5. A gain equalizer according to claim 1, wherein said first optical waveguide circuit and said second optical waveguide circuit are lattice filter type circuits.

6. A gain equalizer according to claim 1, wherein the optical folding structure rotates the polarization direction of light output from the output waveguide of the first optical waveguide circuit and input to the output waveguide of the second optical waveguide circuit by approximately 90°.

Images