Tunable wavelength filter and control method thereof

Abstract

Since conventional variable wavelength filters each comprising a lattice-type optical circuit are optical circuits in which a plurality of MZIs are connected in multiple stages, the conventional variable wavelength filters have a problem that the power consumption of phase shifters is high. In order to provide a wavelength filter comprising a lattice-type optical circuit with which it is possible to make power consumption in the phase control of signal light lower than in the prior art, and a method for controlling the same, the present disclosure proposes a variable wavelength filter comprising a lattice-type optical circuit wherein a waveguide length difference (ΔL) between two arm waveguides (131, 132) in an MIZ is adjusted such that the phase shift amounts of phase shifters (133a-133e) are reduced while the fluctuation ranges thereof are maintained, and a method for controlling signal light using the variable wavelength filter comprising the lattice-type optical circuit.

Description

[Technical field]

[0001] The present invention relates to a tunable wavelength filter formed of a waveguide, and more particularly to a tunable wavelength filter using a lattice-type optical circuit capable of reducing power consumption, and a method for controlling the same. [Background technology]

[0002] Optical communication networks are rapidly developing against the backdrop of the explosive increase in data communications, as typified by the Internet. In particular, Wavelength Division Multiplexing (WDM), a technology that allows the transmission of multiple signal light with different wavelengths over a single optical fiber, is a major driving force behind the increase in capacity of optical communications. To support such optical communication networks, there has been an increasing demand in recent years for high-performance optical circuits, such as wavelength multiplexing / demultiplexing elements and tunable wavelength filters.

[0003] A representative example of such high-performance optical circuits required in optical communication networks is the Planar Lightwave Circuit (hereinafter referred to as PLC). PLC is a waveguide-type optical circuit that uses quartz on a planar substrate, and is widely used in current optical communication networks due to its advantages in terms of functionality, mass production, and cost.

[0004] PLC-based waveguide-type optical circuits use quartz, which is the same material as the optical fibers used in optical communication networks, and thus have the advantage of being able to realize low-loss waveguides. In addition, because the waveguide is formed on a planar substrate, it is easy to combine various functional elements, and complex optical circuits can be fabricated with good reproducibility. Therefore, it is widely used in wavelength multiplexing / demultiplexing elements and optical switches, and is indispensable in constructing modern optical communication networks.

[0005] Meanwhile, in recent years, with the advancement of optical communication networks, there is an increasing demand for more complex optical circuits. One example is a lattice-type optical circuit (see, for example, Non-Patent Document 1). A lattice-type optical circuit is an optical circuit in which a directional coupler and a number of asymmetric Mach-Zehnder interferometers (hereinafter, referred to as MZIs) are connected in a lattice-like multistage configuration. A phase shifter is connected to the waveguide in each MZI, and this phase shifter adjusts the interference state in each MZI by controlling the phase of light propagating in the waveguide. In addition, as a method for the phase shifter to control the phase of light, for example, a change in refractive index based on the thermo-optical effect caused by heating can be cited. The application of such a lattice-type optical circuit to devices such as a tunable wavelength filter that controls the transmission spectrum for signal light having a specific wavelength and a gain equalizer for an optical amplifier (see, for example, Non-Patent Document 2).

[0006] However, the tunable wavelength filter using a lattice-type optical circuit described in the prior art documents has a problem of high power consumption. As mentioned above, each MZI needs to control a phase shifter, but in a tunable wavelength filter using a lattice-type optical circuit in which multiple MZIs are connected in multiple stages, the driving power for controlling this phase shifter becomes high. Usually, the power consumption for driving a tunable wavelength filter using a lattice-type optical circuit is the sum of the power consumption generated by each phase shifter installed in each MZI. For example, when the method for controlling the phase of the signal light using the phase shifter is heating as mentioned above, multiple heating mechanisms are required to properly heat each phase shifter, and the power consumption increases accordingly.

[0007] In addition, when using a lattice optical circuit as a tunable wavelength filter, the maximum power consumption during use is important, but this is determined by the transmission spectrum shape with the maximum power consumption and the power consumption at that time by modifying the filter shape (transmission spectrum shape) of the expected tunable wavelength filter. Therefore, in a tunable wavelength filter using a lattice optical circuit, it is important to design each MZI or control the phase shifter so that the power consumption at the transmission spectrum shape with the maximum power consumption is minimized. However, at present, such a control method has not been established, and it has not been possible to reduce the power consumption in a tunable wavelength filter using a lattice optical circuit. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] M. Oguma et al., "Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide," Journal of Lightwave Technology, vol. 22, no. 3, pp. 895-902, March 2004, doi 10.1109 / JLT.2004.824544. [Non-Patent Document 2] K. Suzuki, T. Kitoh, S. Suzuki, Y. Inoue, Y. Hbbino, T. Shibata, A. Mori, and M. Shimizu, "Ultra wide range dynamic gain equalizer with high contrast silica planar lightwave circuit," Integrated Photonics Research, Vol. 78 of OSA Trends in Optics and Photonics, paper IThG2, 2002, doi 10.1364 / IPR.2002.ITHG2 Summary of the Invention

[0009] The present disclosure has been made in consideration of these problems, and has as its objective to provide a wavelength filter using a lattice-type optical circuit that makes it possible to reduce the amount of power consumption in phase control of signal light compared to conventional methods, and a method for controlling the same.

[0010] In order to solve the above problems, the present disclosure provides a tunable wavelength filter using a lattice-type optical circuit including a plurality of Mach-Zehnder interferometers (MZIs) that has a transmission spectrum shape in which the transmission intensity of signal light changes linearly with respect to wavelength and controls the slope of the linear relationship, the MZI including a first arm waveguide and a second arm waveguide, and a phase shifter connected to the first arm waveguide that controls the phase of the signal light in order to control the slope of the transmission spectrum shape, and the tunable wavelength filter is designed such that the waveguide length difference between the first arm waveguide and the second arm waveguide reduces the amount of phase shift generated in each of the phase shifters while maintaining the fluctuation range of the phase shift amount.

[0011] Furthermore, the present disclosure provides a control method for a tunable wavelength filter, the control method including controlling a phase shift amount of a phase shifter so that the transmission intensity of a signal light has a transmission spectrum shape that changes linearly with respect to the wavelength, and controlling the phase shift amount includes not controlling the phase shift amount in at least one of a plurality of MIZs. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a plan view showing the configuration of a tunable wavelength filter using a lattice-type optical circuit in one embodiment of the present invention. [Diagram 2] FIG. 13 is a diagram showing the shape of the transmission spectrum when the slope of the linear relationship is changed. [Diagram 3] FIG. 13 is a diagram showing the amount of phase shift adjusted by each phase shifter in order to obtain a desired slope in the linear relationship of the transmission spectrum shape. [Figure 4] FIG. 11 is a diagram showing the amount of phase shift in each phase shifter when a tunable wavelength filter using a lattice-type optical circuit is used in one embodiment of the present invention. [Diagram 5] FIG. 1 is a plan view showing the configuration of a tunable wavelength filter using a lattice-type optical circuit in one embodiment of the present invention. [Figure 6] 11 is a graph illustrating an example of a phase shift amount in each phase shifter with respect to a slope of a transmission spectrum shape. [Figure 7] 7 is a graph showing the relationship between the slope of the transmission spectrum shape and the amount of phase shift in the case where the amount of phase shift in each of the phase shifters shown in FIG. 6 is individually controlled. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. One embodiment of the present invention is different from a tunable wavelength filter using a conventional lattice-type optical circuit in that the difference in the waveguide lengths of two arm waveguides in the MIZ is adjusted so as to reduce the value of the phase shift amount itself while maintaining the fluctuation range of the phase shift amount in the phase shifter. Note that the fluctuation range used in this specification corresponds to the difference between the maximum and minimum values ​​of the phase shift amount in each phase shifter with respect to the slope of the transmission spectrum shape described later.

[0014] First Embodiment A first embodiment of the present invention will be described below with reference to the drawings. In this embodiment, as an example, a tunable wavelength filter using a lattice-type optical circuit in which a waveguide chip having one input and one output (hereinafter referred to as 1×1) port including five MZIs is formed on a substrate. The tunable wavelength filter according to this embodiment is a PLC using quartz.

[0015] FIG. 1 is a plan view showing the configuration of a tunable wavelength filter using a lattice-type optical circuit according to an embodiment of the present invention. The tunable wavelength filter using a lattice-type optical circuit in this embodiment is a device in which a waveguide chip 12 is formed on a substrate 11. In the waveguide chip 12, five MZI units 13a to 13e are repeatedly connected, and an input port 14 is connected to the input end of the MZI unit 13a, and an output port 15 is connected to the output end of the MZI unit 13e, forming a 1×1 port optical circuit. Each of the MZI units 13a to 13e has a first arm waveguide 131 and a second arm waveguide 132, and the first arm waveguide 131 and the second arm waveguide 132 form a directional coupler that splits the signal light into two on the input side, and form a directional coupler that multiplexes the propagated signal light on the output side. Phase shifters 133a to 133e that control the phase of the signal light are connected to the first arm waveguide 131. Then, the phase shifters 133a to 133e adjust the phase of the propagating signal light so that only the signal light of a desired wavelength band has high transmission intensity. In this embodiment, the phase shifters 133a to 133e adjust the phase of the signal light by changing the refractive index using a thermo-optic effect caused by heating using a heater. In this embodiment, the branching ratio of the signal light branched to each of the first arm waveguide 131 and the second arm waveguide 132 is 50:50.

[0016] In the tunable wavelength filter using a lattice-type optical circuit in this embodiment configured as described above, the difference ΔL in the waveguide lengths of the first arm waveguide 131 and the second arm waveguide 132 is designed to satisfy (Equation 1). ΔL=(λ Δθ) / nef (Equation 1)

[0017] Here, λ is the wavelength of the signal light, and nef is the effective refractive index of the first arm waveguide 131 and the second arm waveguide 132. Δθ is the difference between the minimum value of the phase shift in each of the phase shifters 133a to 133e and 0, or the difference between the maximum value and the fluctuation range of the phase shift amount, and corresponds to the reduction amount of the phase shift amount to be changed in order to reduce the power consumption. That is, in this embodiment, the power consumption is reduced by reducing the value of the phase shift amount itself while maintaining the fluctuation range of the phase shift amount in each of the phase shifters 133a to 133e. The reduction amount of the phase shift amount is set so that the minimum value of the phase shift amount in each of the phase shifters 133a to 133e becomes 0, and the reduction of the phase shift amount is achieved by adjusting the waveguide lengths of the first arm waveguide 131 and the second arm waveguide 132.

[0018] By designing the difference in the waveguide length between the first arm waveguide 131 and the second arm waveguide 132 in this way, the amount of phase shift in each of the phase shifters 133a to 133e can be reduced, and accordingly, the amount of power consumption can be reduced. The detailed principle will be described below.

[0019] In general, in the phase shifters 133a to 133e, the power consumption increases as the amount of phase shift of the signal light increases. For example, in the phase shifter using the thermo-optic effect adopted in this embodiment, the phase of the signal light is controlled by changing the refractive index of the waveguide in the tunable filter by heating with a heater. In this case, in order to increase the amount of phase shift of the signal light, the set temperature of the heater needs to be increased, which increases the power consumption.

[0020] In this embodiment, a tunable filter is assumed in which the transmission intensity of the signal light has a transmission spectrum shape that changes linearly with respect to the wavelength, and the slope of the transmission spectrum shape is continuously controlled. In such a tunable filter, the slope of the transmission spectrum shape is usually controlled by adjusting the phase shift amount in each of the phase shifters 133a to 133e.

[0021] FIG. 2 shows the transmission spectrum shape with the slope of the linear relationship changed, and FIG. 3 shows the phase shift amount adjusted by the phase shifters 133a to 133e to obtain a desired slope in the linear relationship of the transmission spectrum shape. As shown in FIG. 2, by adjusting the phase shift amount in the phase shifter 133, the transmission spectrum shape becomes a linear relationship in a specific wavelength region, and the slope can be controlled. Also, as shown in FIG. 3, the phase shift amount adjusted by the phase shifters 133a to 133e to obtain a desired slope changes continuously. In such a variable wavelength filter, if the phase shift amount in the phase shifters 133a to 133e is reduced while maintaining the fluctuation range of the phase shift, that is, the range between the minimum and maximum values ​​of the phase shift amount in each phase shifter in the correlation between the slope and the phase shift amount shown in FIG. 3, the power consumption in each phase shifter 133a to 133e can be reduced. To achieve this, as described above, the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 is designed to satisfy (Equation 1). With this configuration, the value of the phase shift amount changes so that the phase difference generated by the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 becomes the lower limit or condition of the fluctuation range of the phase shift amount. In other words, the value of the phase shift amount is reduced overall while maintaining the fluctuation range of the phase shift.

[0022] In particular, when utilizing the phase control method adopted in this embodiment, which utilizes the heat generated by the heater and the thermo-optical effect, it is preferable that the maximum phase shift amount from when the phase shift amount is 0 becomes the same as the value of the phase shift fluctuation range described above.

[0023] Fig. 4 is a diagram showing the amount of phase shift in each of the phase shifters 133a to 133e when a tunable wavelength filter using a lattice-type optical circuit according to this embodiment is used. It can be seen that the behavior of the amount of phase shift generated in the phase shifters 133a to 133e is such that while the fluctuation range in Fig. 3 is maintained, the amount of phase shift is generally reduced, and the minimum value of the amount of phase shift in each of the phase shifters 133a to 133e is approximately 0. As described above, the larger the amount of phase shift, the higher the amount of power consumption. Therefore, it can be seen that the tunable wavelength filter using the lattice-type optical circuit in this embodiment has the effect of reducing power consumption more than a tunable wavelength filter using a conventional lattice-type optical circuit.

[0024] In this embodiment, a lattice-type optical circuit is used in which five MZIs 13a to 13e are connected, but the same effect can be achieved if two or more MZIs are connected.

[0025] Furthermore, in this embodiment, the branching ratio of the signal light branched to the first arm waveguide 131 and the second arm waveguide 132 is set to 50:50, but the same effect can be achieved regardless of the branching ratio.

[0026] In this embodiment, heating by a general heater is used as a method for controlling the phase of the signal light, but this method has the disadvantage that the direction of the phase shift of the signal light is limited to one direction. To compensate for this, a method that can control the phase in both directions may be used. For example, even if the same thermo-optic effect is used, it is possible to further reduce the maximum power consumption by using a method that has both heating and cooling functions such as a Peltier element. In this case, the reduction amount Δθ in (Equation 1) is set to the phase shift amount up to the intermediate value of the fluctuation range of the phase shift amount generated in the phase shifters 133a to 133e, thereby achieving the same effect.

[0027] Second Embodiment A second embodiment of the present disclosure will be described below with reference to the drawings. This embodiment is an example in which a phase shifter is also connected to the second arm waveguide 132 in the lattice-type optical circuit tunable wavelength filter of the first embodiment.

[0028] 5 is a plan view showing the configuration of a tunable wavelength filter using a lattice-type optical circuit in one embodiment of the present invention. The tunable wavelength filter using a lattice-type optical circuit in this embodiment has a structure in which phase shifters 501a to 501e are also connected to the second arm waveguide in addition to the configuration of the tunable wavelength filter using a lattice-type optical circuit described in the first embodiment. Note that the method for controlling the phase of the signal light in the phase shifters 133a to 133e and the phase shifters 501a to 501e is heating by a heater, as in the first embodiment. In this embodiment, the branching ratio of the signal light branched to each of the first arm waveguide 131 and the second arm waveguide 132 is 50:50, as in the first embodiment.

[0029] In the tunable wavelength filter using the lattice-type optical circuit of this embodiment configured as described above, the first arm waveguide 131 and the second arm waveguide 132 are designed so that the difference in their waveguide lengths satisfies (Equation 1). Note that, for the reduction amount Δθ in (Equation 1), the median value of the fluctuation range of the phase shift in the phase shifters 133a to 133e is used, as in the case where a Peltier element is used as well as described in the first embodiment.

[0030] In this embodiment, when the phase shift amount is changed to the positive side, the phase shifters 133a to 133e are driven, and when the phase is shifted to the negative side, the phase shifters 501a to 501e are driven. Therefore, the tunable wavelength filter using the lattice type optical circuit in this embodiment can control the phase in both directions even if a phase control method that can control the phase of the signal light only in one direction, such as a heater, is adopted. That is, the phase control using the Peltier element described in the first embodiment can be realized even when a heater is used.

[0031] As in the first embodiment, in this embodiment, a lattice-type optical circuit is used in which five MZIs 13a to 13e are connected, but the same effects can be achieved if there are two or more MZIs.

[0032] Furthermore, in the present embodiment, as in the first embodiment, the branching ratio of the signal light branched to each of the first arm waveguide 131 and the second arm waveguide 132 is set to 50:50, but the same effect can be obtained regardless of the branching ratio.

[0033] (Third embodiment) A third embodiment of the present disclosure will be described below with reference to the drawings. The tunable wavelength filter using a lattice-type optical circuit in this embodiment is a device having the configuration shown in Fig. 5, similar to the second embodiment. However, in this embodiment, the difference in the waveguide lengths between the first arm waveguide 131 and the second arm waveguide 132 is not designed so that the minimum value of the phase shift amount is zero.

[0034] In the first and second embodiments, the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 is uniquely determined based on the fluctuation range of the phase shift amount in the phase shifters 133a to 133e and the phase shifters 501a to 501e so that the minimum value of the phase shift amount is 0 or the maximum value is the same as the fluctuation range. However, depending on the desired transmission spectrum shape, the minimum value of the phase shift amount for appropriately reducing the power consumption is not necessarily 0 in all the phase shifters 133a to 133e and the phase shifters 501a to 501e. That is, in order to efficiently reduce the power consumption in the tunable wavelength filter using a lattice-type optical circuit, it may be better to individually control the reduction amount of the phase shift amount in the phase shifters 133a to 133e. The present embodiment is a form for efficiently reducing the power consumption of the tunable wavelength filter using a lattice-type optical circuit under such circumstances.

[0035] The tunable wavelength filter using a lattice-type optical circuit in this embodiment has a circuit configuration as shown in Fig. 5, similar to the second embodiment. However, the design of the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 in each of the phase shifters 133a to 133e and the phase shifters 501a to 501e is different from that in the first and second embodiments. That is, in this embodiment, as described above, the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 is not designed so that the minimum value of the phase shift amount in each of the phase shifters 133a to 133e and the phase shifters 501a to 501e is 0.

[0036] The phase control method of the signal light in the phase shifters 133a to 133e and the phase shifters 501a to 501e is heating by a heater, similar to the first and second embodiments. In this embodiment, the branching ratio of the signal light branched to the first arm waveguide 131 and the second arm waveguide 132 is 50:50, similar to the first and second embodiments.

[0037] FIG. 6 is a graph illustrating the phase shift amount of each of the phase shifters 133a to 133e with respect to the slope of the transmission spectrum shape. FIG. 7 is a graph showing the relationship between the slope of the transmission spectrum shape and the phase shift amount when the phase shift amount of each of the phase shifters 133a to 133e shown in FIG. 6 is individually controlled. As can be seen from FIG. 6 and FIG. 7, the minimum value of each phase shift amount in the phase shifters 133a to 133e is not necessarily 0. Also, it can be seen that the phase shifter 133d hardly changes between FIG. 5 and FIG. 6. Thus, in the tunable wavelength filter using the lattice-type optical circuit in this embodiment, the change amount of the phase shift amount is individually adjusted for each of the phase shifters 133a to 133e and the phase shifters 501a to 501e. However, the fluctuation range of the phase shift of the phase shifters 133a to 133e and the phase shifters 501a to 501e is maintained.

[0038] Furthermore, in the tunable wavelength filter using the lattice-type optical circuit in this embodiment, the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 in each of the phase shifters 133a to 133e and the phase shifters 501a to 501e is designed by applying (Equation 1) so that the total power consumption is minimized. For example, in all of the phase shifters 133a to 133e and the phase shifters 501a to 501e, the power consumption is proportional to the phase shift amount, and the power consumption required to generate the same phase shift amount is the same for all the phase shifters. In this case, it is only necessary to select the reduction amount Δθ of the phase shift amount such that the sum of the maximum values ​​of the phase shift amounts generated in all the phase shifters is minimized so as to obtain a desired slope of the transmission spectrum shape.

[0039] In this way, by individually adjusting the waveguide length difference between the first arm waveguide 131 and the second arm waveguide 132 in the phase shifters 133a to 133e and the phase shifters 501a to 501e, the power consumption of the tunable wavelength filter using a lattice-type optical circuit can be efficiently reduced.

[0040] As in the first and second embodiments, in this embodiment, a lattice-type optical circuit is used in which five MZIs 13a to 13e are connected, but the same effects can be achieved if there are two or more MZIs.

[0041] Furthermore, in the same manner as in the first and second embodiments, in this embodiment, the branching ratio of the signal light branched to each of the first arm waveguide 131 and the second arm waveguide 132 is set to 50:50, but the same effect can be obtained regardless of the branching ratio.

[0042] In addition, in this embodiment, since the phase shifters 133a to 133e and the phase shifters 501a to 501e are individually controlled, it is not necessary to control all the installed phase shifters 133a to 133e and the phase shifters 501a to 501e. In other words, if at least one of the phase shifters 133a to 133e and the phase shifters 501a to 501e controls the phase shift amount, the effect of reducing the power consumption can be achieved in the same way. [Industrial Applicability]

[0043] As a tunable wavelength filter that can reduce power consumption compared to conventional filters, it is expected to be applied to optical communication networks such as WDM.

Claims

1. A tunable wavelength filter using a lattice-type optical circuit including a plurality of Mach-Zehnder interferometers (MZIs) that has a transmission spectrum shape in which the transmission intensity of a signal light changes linearly with respect to the wavelength and controls the slope of the linear relationship, The MZI includes a first arm waveguide and a second arm waveguide, a first phase shifter connected to the first arm waveguide and configured to control a phase of the signal light in order to control the slope of the transmission spectrum shape; Equipped with a waveguide length difference ΔL between the first arm waveguide and the second arm waveguide is designed based on (Equation 1) expressed using a wavelength λ of the signal light, an effective refractive index n of the first arm waveguide and the second arm waveguide, and a reduction amount Δθ, so as to reduce a phase shift amount in each of the first phase shifters by the reduction amount Δθ while maintaining a fluctuation range of the phase shift amount, The reduction amount Δθ is the difference between the minimum value of each of the first phase shifters and zero, or a median value of the fluctuation range in each of the first phase shifters; Either Tunable wavelength filter. [0010]

2. The MZI further includes a second phase shifter connected to the second arm waveguide and configured to control a phase of the signal light to control the slope of the transmission spectrum shape; the waveguide length difference ΔL is designed to be reduced by the reduction amount Δθ while maintaining a fluctuation range of the phase shift amount in each of the first phase shifter and the second phase shifter, based on the (Equation 1) expressed using the wavelength λ of the signal light, the effective refractive index n of the first arm waveguide and the second arm waveguide, and the reduction amount Δθ; 2. The tunable wavelength filter according to claim 1.

3. 3. The tunable wavelength filter according to claim 1, wherein the first arm waveguide and the second arm waveguide are made of quartz.

4. 4. The tunable wavelength filter according to claim 1, wherein the first phase shifter includes a heater for heating or a Peltier element for heating or cooling.

5. A variable wavelength filter as described in claim 2, wherein the second phase shifter includes a heater for heating or a Peltier element for heating or cooling.

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