Lide equalizer

Abstract

Provided is a gain equalizer formed by a lattice-type optical circuit having less polarization dependent loss. The gain equalizer comprises: 2M-number (M is an integer of 1 or more) of optical waveguide circuits that are formed on a substrate and that form M pairs each consisting of optical waveguide circuits having the same gain equalizing spectrum; and a turn-back connection structure in which an output waveguide and an input waveguide of one member of each of the pairs of the optical waveguide circuits are connected to each other and which turns the polarizing direction of propagating light by 90 degrees.

Description

[Technical Field]

[0001] This invention relates to a gain equalizer configured with a polarization-independent optical waveguide circuit. [Background technology]

[0002] Against the backdrop of the explosive growth of data communications, exemplified by the internet, optical communication networks are developing rapidly. In particular, Wavelength Division Multiplexing (WDM) technology, which can transmit many wavelength signals on a single optical fiber core, is considered important as a means of achieving high capacity in optical communications. Wavelength multiplexing / demultiplexing elements and optical amplifiers play important roles in realizing WDM technology. Especially when applying WDM technology to transmission over long distances of 100 km or more, it is necessary to place optical amplifiers at regular intervals in the transmission fiber, and the wavelength dependence of their gain spectrum greatly affects the Optical Signal-to-Noise Ratio (OSNR). To flatten the gain spectrum, for example, a gain equalizer disclosed in Non-Patent Document 1 has been proposed.

[0003] Incidentally, 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), which forms optical waveguides made of silica-based materials on a silicon substrate, is widely used as a method that combines multi-functionality, mass production, and low cost.

[0004] Optical circuits using silica-based glass waveguides utilize the same materials as optical fibers used in optical communications, thus enabling the realization of low-loss optical waveguides. Furthermore, because waveguides are formed on a planar substrate, various functional elements can be easily combined, allowing for the reproducible fabrication of complex optical circuits. Wavelength multiplexing / demultiplexing elements, optical switches, and other components fabricated using these technologies are indispensable for constructing optical networks.

[0005] The gain equalizer disclosed in Non-Patent Document 1 has the disadvantage of high losses because it is configured by connecting multiple Mach-Zehnder interferometers in stages. This is due to the inherent reason that in each stage of the Mach-Zehnder interferometer, the optical signal is wasted at the output port that is not connected. 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-type optical circuit consists of N-1 arm waveguides, each comprising N directional couplers and two waveguides sandwiched between them. Furthermore, by applying heat to one of the arm waveguides, a phase shifter utilizing the refractive index change due to the thermo-optic effect controls the phase of the light propagating through the waveguide. By adjusting the phase difference of the optical signals propagating through the two waveguides constituting the arm waveguides of the preceding directional coupler, the interference state in the subsequent directional coupler is adjusted, thereby controlling the transmission spectrum of the propagating light with respect to its wavelength.

[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 light intensity transmittance of the gain equalizer. The wavelength range required for the gain equalizer is Δλ1 (1525~1570 nm in Figure 1). The minimum light intensity transmittance within Δλ1 is defined as Loss. max (In Figure 1, -9dB), maximum transmittance is lost min (In Figure 1, this is -0.34 dB.) Loss max and Loss min The difference between these two values ​​will be called the transmission attenuation. The desired spectral shape only needs to be maintained within Δλ1; the spectral shape in other wavelength ranges is not relevant. In addition to the spectral shape shown in Figure 1, if a spectrum is realized in which the transmittance changes linearly with wavelength within Δλ1, for example, the gain equalizer will operate as a tilt equalizer. In this case as well, 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 composed of a lattice-type optical circuit, when the interference state differs for each polarization direction of the light propagating in the optical circuit, polarization dependence occurs in the transmission spectrum finally output from the lattice-type optical circuit, which is a problem. 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, since the PDL generated in one arm waveguide is amplified every time it passes through a stage of the lattice, the PDL of the entire lattice-type optical circuit becomes large.

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

[0010]

Equation

[0011] Here, n y is the effective refractive index in the Y direction, and n x is the effective refractive index in the X direction. When (effective refractive index in the Y direction) > (effective refractive index in the X direction), the birefringence Δn eff becomes a positive value, and when (effective refractive index in the Y direction) < (effective refractive index in the X direction), the birefringence Δn eff becomes a negative value. In an optical waveguide where Δn eff 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.

[0012] In a gain equalizer using a lattice-type optical circuit, the phase shifter is controlled by utilizing the thermo-optic effect in the arm waveguides of multiple directional couplers, thereby controlling the final output wavelength spectrum. In an (N-1) stage lattice-type optical circuit consisting of N-1 arm waveguides, each consisting of N directional couplers and two waveguides sandwiched between them, the phase difference Θ occurs between the upper and lower arm waveguides in the i-th arm waveguide. i This is expressed by equation (2).

[0013]

number

[0014] Here, λ is the wavelength of light, n eff ΔL is the effective refractive index of the optical waveguide. i φ is the difference in length between the upper arm waveguide and the lower arm waveguide in the i-th arm waveguide. i φ is the phase difference applied between arm waveguides by controlling the phase shifter in the i-th arm waveguide. To construct a gain equalizer using a lattice-type optical circuit, the phase difference φ applied between each arm waveguide is i By appropriately controlling this, the desired gain equalization spectrum can be obtained.

[0015] 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) will differ depending on the polarization mode, so the interference state in the subsequent directional coupler will differ depending on the polarization mode, resulting in PDL.

[0016] On the other hand, in Mach-Zehnder interferometers, polarization dependence is also generated by the asymmetrical rotation of polarization between arm waveguides. Polarization rotation in optical waveguides is generally suppressed by finite birefringence, as is known in polarization-maintaining fibers. Therefore, even if the absolute value of birefringence expressed in equation (1) is reduced and the difference in interference state caused solely by birefringence is eliminated, polarization dependence is still generated due to the effect of polarization rotation.

[0017] Thus, in the conventional lattice-type optical circuit-based gain equalizer, the problem was that polarization-dependent loss (PDL) could not be completely eliminated.

Prior Art Documents

Non-Patent Documents

[0018]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0019] An object of the present invention is to provide a gain equalizer using a lattice-type optical circuit with low polarization-dependent loss.

[0020] To achieve this objective, one embodiment of the present invention is characterized by comprising 2M (where M is an integer of 1 or more) optical waveguide circuits formed on a substrate, each pair of optical waveguide circuits having the same gain equalization spectrum, and a folded connection structure for connecting one input / output waveguide of the pair of optical waveguide circuits, the folded connection structure rotating the polarization direction of the propagating light by 90 degrees. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 shows an example of a gain equalization spectrum in a conventional lattice-type optical circuit. [Figure 2] Figure 2 shows the configuration of a gain equalizer according to the first embodiment of the present invention. [Figure 3] Figure 3 shows a first example of the folded connection structure of the gain equalizer in the first embodiment. [Figure 4] Figure 4 shows a second example of the folded connection structure of the gain equalizer in the first embodiment. [Figure 5] Figure 5 shows a third example of the folded connection structure of the gain equalizer in the first embodiment. [Figure 6] Figure 6 shows the configuration of a gain equalizer according to a second embodiment of the present invention. [Figure 7] Figure 7 shows a first example of the folded connection structure of the gain equalizer in the second embodiment. [Figure 8] Figure 8 shows a second example of the folded connection structure of the gain equalizer in the second embodiment. [Figure 9] Figure 9 shows a third example of the folded connection structure of the gain equalizer in the second embodiment. [Modes for carrying out the invention]

[0022] Embodiments of the present invention will be described in detail below with reference to the drawings.

[0023] [First Embodiment] (composition) Figure 2 shows the configuration of a gain equalizer according to the first embodiment of the present invention. The gain equalizer 10 is composed of a PLC and shows the circuit configuration as viewed from above of the optical waveguide circuit formed on the substrate. The gain equalizer 10 is composed of an even number of lattice-type optical circuits (here, a first lattice-type optical circuit 11 and a second lattice-type optical circuit 21) and a folded connection structure 31 that connects one input / output waveguide of a pair of lattice-type optical circuits.

[0024] The first lattice-type optical circuit 11 consists of a first input / output waveguide 111, a plurality of optical directional couplers 113-1 to 113-N and arm waveguides 114-1 to 114-(N-1), and a second input / output waveguide 112 (where N is an integer of 3 or more). The optical directional couplers 113-1 to 113-N can be any optical circuit element as long as they are 2-input 2-output optical multiplication and shunt circuits. The arm waveguides 114-1 to 114-(N-1) consist of two waveguides connecting two optical directional couplers, and at least one of these waveguides is loaded with a phase shifter 115-1 to 115-(N-1). The optical signal input from the first input / output waveguide 111 passes sequentially through the optical directional couplers 113-1 to 113-N and the arm waveguides 114-1 to 114-(N-1), and is output from the second input / output waveguide 112.

[0025] The second lattice-type optical circuit 12 has the same design as the first lattice-type optical circuit 11, and similarly consists of a first input / output waveguide 211, a plurality of optical directional couplers 213-1 to 213-N and arm waveguides 214-1 to 214-(N-1), and a second input / output waveguide 212, and is equipped with phase shifters 215-1 to 215-(N-1).

[0026] 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. As a phase shifter, the principle is not limited as long as it can control the phase of the optical signal that passes through it, but examples include thermo-optic phase shifters that utilize heat generated by a heater and the thermo-optic effect.

[0027] 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 connected by a folded connection structure 31. This structure allows light propagating within the first lattice-type optical circuit 11 and output from the second input / output waveguide 112 to be input to the second lattice-type optical circuit via the second input / output waveguide 212. The folded connection structure 31 has the function of rotating the polarization direction of the propagating light by 90 degrees. Specific examples of folded connection structures 31 for achieving this function will be described later.

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

[0029] The output light from the second input / output waveguide 112 propagates through the folded connection structure 31 before being input to the second input / output waveguide 212. At this time, since the folded connection structure 31 has the function of rotating the polarization direction of the propagating light by 90 degrees, the TE mode and TM mode of the optical waveguide are converted to each other before being input to the second input / output waveguide 212.

[0030] The light input to the second lattice-type optical circuit 21 via the second input / output waveguide 212 undergoes gain equalization according to the gain equalization spectrum determined by the phase modulation amount in the phase shifters 215-1 to 215-(N-1), and is 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-type optical circuit 11 and the phase modulation conditions in the phase shifters 215-1 to 215-(N-1) of the second lattice-type optical circuit 21 be the same. That is, it is desirable that the gain equalization spectrum of the first lattice-type optical circuit 11 and the gain equalization spectrum of the second lattice-type optical circuit 21 be the same.

[0031] In the gain equalizer 10, the input optical signal passes through a first lattice-type optical circuit 11 and a second lattice-type 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-type optical circuit 11, undergoes a 90-degree polarization rotation in the folded connection structure 31 and passes through the second lattice-type optical circuit 21 as TM mode. Therefore, the PDL generated in the first lattice-type optical circuit is canceled out by the PDL generated in the second lattice-type optical circuit. Overall, a gain equalizer with a small PDL can be realized.

[0032] If an even number (2M: M is an integer greater than or equal to 1) of lattice-type optical circuits are included, a pair of lattice-type optical circuits with identical gain equalization spectra are formed, and the second input / output waveguides of the pair of lattice-type optical circuits are connected using a folded connection structure. Furthermore, the first input / output waveguides of the M pairs of lattice-type optical circuits can be connected in cascaded order. That is, the first input / output waveguides of the even-numbered lattice-type optical circuits are connected to the first input / output waveguides of the odd-numbered lattice-type optical circuits, so that the first input / output waveguide of the 1st lattice-type optical circuit to the first input / output waveguide of the 2Mth lattice-type optical circuit constitute a single gain equalizer.

[0033] (Folded connection structure) Figure 3 shows a first example of the folded connection structure of the gain equalizer according to the first embodiment. Figure 3(a) shows the circuit configuration viewed from above, and Figure 3(b) shows the cross section IIIb-IIIb' of Figure 3(a). The folded 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 12 by an optical waveguide 311. In a part of the optical waveguide 311 between these two points, a groove 312 is formed along a portion of the optical waveguide on one side with respect to the substrate plane.

[0034] As shown in Figure 3(b), an optical waveguide core 3113 is embedded in the cladding 3112 of the optical waveguide formed on the substrate 3111. The method for forming the cladding layer and core layer of the optical waveguide is not limited as long as a uniform and smooth layer can be formed, but examples include flame deposition, chemical vapor deposition (CVD), and sputtering.

[0035] Normally, in silica-based optical waveguide circuits, even when structural birefringence due to the structure of the optical waveguide is zero, stress birefringence occurs because compressive stress acts on the core in the horizontal direction of the substrate due to the different thermal expansion coefficients of the substrate material, cladding material, and core material. In particular, when flame deposition or similar methods are used to form the cladding or core layer of the optical waveguide, high-temperature processes are involved during the fabrication of the optical waveguide, resulting in large compressive stresses at room temperature and a significant increase in birefringence. Thus, birefringence occurs in the core of silica-based optical waveguides, with the main axis being either perpendicular to the substrate or horizontal to the substrate.

[0036] When a groove is formed in the cladding along one side of the core of an optical waveguide exhibiting birefringence, asymmetric stress is applied to the waveguide core, causing the principal axis of birefringence to tilt. The way in which the stress is applied depends on the distance from the waveguide core to the groove, and by adjusting this distance, the degree of tilt of the principal axis of birefringence can be changed.

[0037] When linearly polarized light passes through a birefringent object with a tilted 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 are considered. eff The polarization state of light propagating through a birefringent object changes depending on the propagation distance L within the birefringent object. When these relationships satisfy equation (3), a linearly polarized signal is output with its polarization direction rotated by 2θ relative to the input linearly polarized signal.

[0038]

number

[0039] Here, k is the wavenumber of the input light. Using this characteristic, θ, Δn eff By adjusting L respectively, it is possible to obtain the desired amount of polarization rotation for an input linearly polarized wave.

[0040] In the first example of the folded connection structure 31, by forming a groove 312 in the cladding on one side of the core in a portion of the optical waveguide 311, the polarization of the transmitted light in the optical waveguide 311-2 in this region can be rotated. 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 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 a 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, the light input to the folded connection structure 31 from the input / output waveguide 112 is output from the input / output waveguide 212 with its polarization direction rotated by approximately 90 degrees. While a polarization rotation of 90 degrees is preferable, considering manufacturing tolerances and other factors, a rotation of around 90 degrees is sufficient to keep polarization-dependent losses sufficiently small in practical operation.

[0041] Figure 4 shows a second example of the folded connection structure of the gain equalizer of the first embodiment. The folded 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 12 by an optical waveguide 311. The optical waveguide 311 has a waveplate 314 inserted at least at one point along its path. A half-waveplate that shifts the phase difference between the principal axis and the lagging axis by π is a promising choice for the waveplate. By installing the half-waveplate at an angle such that its principal axis is at 45° with respect to the substrate surface of the optical waveguide 311, the polarization direction of the light propagating through the optical waveguide 311 is rotated by 90 degrees. In this way, the light input from the input / output waveguide 112 to the folded connection structure 31 is output from the input / output waveguide 212 with its polarization direction rotated by 90 degrees.

[0042] Figure 5 shows a third example of the folded connection structure of the gain equalizer in the first embodiment. 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 12 are formed up to the end face of the PLC substrate, and the ends of each waveguide are connected by a polarization-maintaining fiber 315. The polarization-maintaining fiber 315 maintains the polarization state of the light propagating through the fiber.

[0043] 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 such that its main axis direction at connection point 316 and its main axis direction at connection point 317 differ from each other by approximately 90°. 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 in TM mode. In this way, the light input from the input / output waveguide 112 to the folded connection structure 31 is output from the input / output waveguide 212 with its polarization direction rotated by approximately 90°.

[0044] According to the gain equalizer 10 of the first embodiment, the PDL of the gain equalizer composed of a lattice-type optical circuit can be eliminated by any of the first to third examples of the folded connection structure 31.

[0045] [Second Embodiment] Figure 6 shows the configuration of a gain equalizer according to a second embodiment of the present invention. The gain equalizer 60 consists of a lattice-type optical circuit 61, an input / output separation mechanism 62, and an optical folding section 63. The lattice-type optical circuit 61 and the optical folding section 63 are composed of a PLC, and the circuit configuration is shown as viewed from above of the optical waveguide circuit formed on the substrate. The lattice-type optical circuit 61 has the same configuration as the first lattice-type optical circuit 11 in the first embodiment, so its description is omitted.

[0046] As mentioned above, the phase shifters 615-1 to 615-(N-1) can be of any principle as long as they can control the phase of the passed optical signal. For example, a thermo-optic phase shifter that utilizes the heat generated by a heater and the thermo-optic effect can be used. In the case of a thermo-optic phase shifter, the amount of heat generated is controlled by the amount of current applied to the heater, and the resulting change in refractive index and the amount of phase modulation are controlled. The larger the amount of phase modulation to be applied, the larger the driving current required.

[0047] The optical folding section 63 may be integrated with the lattice-type optical circuit 61, or it may be connected externally via an optical fiber or the like. The structure of the optical folding section 63 is not limited as long as the light input from the input / output port 631 of the optical folding section 63 is output again from the input / output port 631.

[0048] The optical folding section 63 of the second embodiment consists of an optical directional coupler 632, which is a 2-input, 2-output optical combining and splitting circuit, and a folding connection structure 633 that connects the two output ports of the optical directional coupler 632. The light input from the input / output port 631 is separated into 50% intensity portions in the optical directional coupler 632. The separated light travels in opposite directions in the folding connection structure 633, interferes again in the optical directional coupler 632, and light with 100% intensity is output from the input / output port 631.

[0049] Figures 7-9 show the first to third examples of the folded connection structure of the gain equalizer in the second embodiment. The folded connection structure 633 has the function of rotating the polarization direction of the propagating light by approximately 90°. The configuration of the first to third examples of the folded connection structure 633 shown in Figures 7-9 is the same as the configuration of the first to third examples of the folded connection structure 31 shown in Figures 3-5.

[0050] In the folded connection structure 633 shown in Figure 7, a groove 635 is formed along a portion of the folded optical waveguide 634, on one side relative to the substrate plane. The groove 635 needs to be formed at a position where the path lengths from the two output ports of the optical directional coupler 632 are approximately equidistant.

[0051] The folded connection structure 633 shown in Figure 8 has a waveplate 636 inserted at least at one point along the path of the folded optical waveguide 634. The half-waveplate 636 needs to be located at a position where the path lengths from the two output ports of the optical directional coupler 632 are approximately equidistant.

[0052] In the folded connection structure 633 shown in Figure 9, the two output ports of the directional coupler 632 are formed up to the substrate end face of the PLC in the optical folding section 63, and the two waveguide end faces are connected by polarization-maintaining fibers 637.

[0053] The input / output separation mechanism 62 may be integrated with the lattice-type optical circuit 61, or it may be connected externally via an optical fiber or the like. The input / output separation mechanism 62 has ports 621 to 623. Input light from port 621 is output from port 622, and input light from port 622 is output from port 623. Examples of the input / output separation mechanism 62 include a 3dB coupler and a wavelength-independent coupler when integrated with the lattice-type optical circuit 61, and an optical circulator when connected externally.

[0054] (Operating principle) The operation of the gain equalizer according to the second embodiment will now be explained based on the above configuration. When an optical signal is input to port 621 of the gain equalizer 60, it is input to the input / output separation mechanism 62 and output from port 622. The light output from port 622 is input to the lattice-type optical circuit 61 via the input waveguide 611.

[0055] The optical signal input to the lattice-type optical circuit 61 undergoes gain equalization according to the gain equalization spectrum determined by the phase modulation amount in the phase shifters 615-1 to 615-(N-1), and is then output from the output waveguide 612. The output light from the output waveguide 612 is reversed in propagation direction by the optical folding section 63 and is input back into the lattice-type optical circuit 61 via the output waveguide 612.

[0056] The optical signal propagates through the lattice-type optical circuit 61 in the opposite direction to when it passed through the lattice-type optical circuit the first time, and is output from the input waveguide 611. The output light from the lattice-type optical circuit 61 is input to the input / output separation mechanism 62 via port 622 and output from port 623.

[0057] In the gain equalizer 60 of the second embodiment, the input optical signal passes through the lattice-type optical circuit 61 twice, once in the forward direction and once in the reverse direction. That is, it performs a back-and-forth operation. Due to the principle of backward propagation of light, the transmission spectrum is the same whether the optical signal is input to the lattice-type optical circuit 61 from the input waveguide 611 and output from the output waveguide 612, or whether the optical signal is input to the lattice-type optical circuit 61 from the output waveguide 612 and output from the input waveguide 611. Therefore, when the lattice-type optical circuit 61 of the second embodiment is operated back and forth, the same gain equalization effect as when the signal is transmitted through one lattice-type optical circuit twice can be obtained.

[0058] In the gain equalizer 60, when the lattice-type optical circuit is operated back and forth, the light, which was in TE mode on the forward path, undergoes polarization rotation in the optical folding section 63 and passes through the lattice-type optical circuit as TM mode on the return path. Therefore, polarization-dependent characteristics are eliminated by the back and forth operation, and a gain equalizer with low polarization-dependent loss (PDL) can be realized overall.

[0059] (Reduced power consumption through reciprocating motion) In the gain equalizer 60 of the second embodiment, since the lattice-type optical circuit is operated in a reciprocating manner, when the phase modulation amount in each phase shifter 615-1 to 615-(N-1) is fixed, the transmission attenuation of the obtained gain equalization spectrum is doubled compared to the case of conventional one-way operation. In other words, when a desired gain equalization spectrum exists, the gain equalizer 60 of the second embodiment has half the transmission attenuation of one lattice-type optical circuit compared to a conventional one-way gain equalizer. For example, if the gain equalizer 60 of the second embodiment is to be operated in a transmission attenuation range of 0 to 8 dB, the lattice-type optical circuit 61 alone only needs to achieve a transmission attenuation range of 0 to 4 dB. In this way, the range of transmission attenuation required for the lattice-type optical circuit 61 can be narrowed, so the range of required phase modulation amount can also be narrowed, and the driving current amount of the phase shifter can be reduced. That is, power consumption can be reduced.

[0060] As an example, consider the case where a tilt equalizer is constructed using a lattice-type optical circuit with N=6. In this tilt equalizer, the optical circuit is designed so that the gain equalization spectrum in the initial state (unmodulated) has a slope of 0. When it is desired to output a gain equalization spectrum with a spectral waveform slope of 0.2 dB / nm (corresponding to a transmission attenuation of 8 dB in the C band of the optical communication wavelength band), the gain equalizer 60 states that the lattice-type optical circuit 61 alone should perform a gain equalization operation with a spectral waveform slope of 0.1 dB / nm (corresponding to a transmission attenuation of 4 dB in the C band). In this case, the required power consumption can be reduced by approximately 25% compared to a conventional gain equalizer that simply operates a lattice-type optical circuit in one direction.

[0061] According to the gain equalizer 60 of the second embodiment, it is possible to provide a gain equalizer that can realize a gain equalization spectrum with a high degree of freedom with low power consumption while suppressing the PDL of a gain equalizer composed of a lattice-type optical circuit.

[0062] [Other embodiments] The gain equalizer 60 of the second embodiment has one lattice-type optical circuit 61, one input / output separation mechanism 62, and one optical folding section 63. The number of lattice-type optical circuits is not limited to this, and multiple circuits may be included. For example, when n gain equalizers 60 (where n is an integer of 2 or more) are connected in cascading order, the k-th (1

Claims

1. A substrate has 2M (M is an integer of 1 or more) lattice-type optical circuits, each consisting of M pairs of lattice-type optical circuits with identical gain equalization spectra, A folded connection structure for connecting one input / output waveguide of a pair of lattice-type optical circuits, wherein the folded connection structure rotates the polarization direction of the propagating light by 90 degrees. A gain equalizer characterized by being equipped with the following features.

2. Each of the aforementioned lattice-type optical circuits is It consists of a first input / output waveguide, N (where N is an integer of 3 or more) 2-input 2-output photomultiplier / divistor circuits, N-1 arm waveguides consisting of two waveguides connecting the photomultiplier / divistor circuits, and a second input / output waveguide. The gain equalizer according to claim 1, characterized in that it includes a phase shifter loaded in at least one of the waveguides of the arm waveguide.

3. A lattice-type optical circuit formed on a substrate, It consists of an input waveguide, N (where N is an integer of 3 or more) 2-input 2-output photomultiplier / divistor circuits, N-1 arm waveguides consisting of two waveguides connecting the photomultiplier / divistor circuits, and an output waveguide. A lattice-type optical circuit including a phase shifter loaded in at least one waveguide of the arm waveguide, An optical folding section that outputs light input from the output waveguide back to the output waveguide, wherein the optical folding section rotates the polarization direction of the propagating light by 90 degrees. A gain equalizer comprising the lattice-type optical circuit, wherein the light that has traveled back and forth through the lattice-type optical circuit is output from the input waveguide.

4. The aforementioned folded connection structure includes an optical waveguide that connects the two input / output waveguides, In a portion of the optical waveguide, a groove is formed along the portion of the optical waveguide on one side relative to the substrate plane. A gain equalizer according to claim 1 or 2, characterized by including the following:

5. The aforementioned folded connection structure includes an optical waveguide that connects the two input / output waveguides, A waveplate inserted into the optical waveguide and A gain equalizer according to claim 1 or 2, characterized by including the following:

6. The gain equalizer according to claim 1 or 2, characterized in that the folded connection structure consists of polarization-maintaining fibers connecting the two input / output waveguides.

7. The aforementioned light reflection portion is, A second photomultiplier / divider circuit with two inputs and two outputs connected to the output waveguide, A folded optical waveguide connecting the two output ports of the second photomultiplier / division circuit, In a portion of the folded optical waveguide, a groove is formed along the portion of the folded optical waveguide on one side with respect to the substrate plane. The gain equalizer according to claim 3, characterized by including the following:

8. The aforementioned light reflection portion is, A second photomultiplier / divider circuit with two inputs and two outputs connected to the output waveguide, A folded optical waveguide connecting the two output ports of the second photomultiplier / division circuit, A waveplate inserted into the aforementioned folded optical waveguide and The gain equalizer according to claim 3, characterized by including the following:

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