WO2024038493A1

WO2024038493A1 - Gain equalizer

Info

Publication number: WO2024038493A1
Application number: PCT/JP2022/030895
Authority: WO
Inventors: 祥江森本; 賢哉鈴木; 慶太山口
Original assignee: 日本電信電話株式会社
Priority date: 2022-08-15
Filing date: 2022-08-15
Publication date: 2024-02-22

Abstract

Provided is a gain equalizer that realizes a gain equalization spectrum having a high degree of freedom with lower power consumption. This gain equalizer is provided with: an optical waveguide circuit that is formed on a substrate and constituted from an input waveguide, N (where N is an integer of 3 or greater) two-input/two-output optical multiplexing/demultiplexing circuits, N-1 arm waveguides comprising two waveguides connecting between the optical multiplexing/demultiplexing circuits, and an output waveguide, the optical waveguide circuit including a phase shifter loaded on at least one waveguide of the arm waveguides; and a light returning part that re-outputs light inputted from the output waveguide to the output waveguide.

Description

gain equalizer

The present invention relates to a gain equalizer configured with an optical waveguide.

Optical communication networks are rapidly developing against the backdrop of the explosive increase in data communications, typified by the Internet. Among these, optical wavelength division multiplexing (WDM) technology, which allows multiple wavelength signals to be transmitted through a single optical fiber, is considered important as a means of realizing large-capacity optical communications. Wavelength multiplexing/demultiplexing elements, optical amplifiers, etc. play important roles in realizing WDM technology. In particular, in order to apply WDM technology to long-distance transmission over 100 km, it is necessary to arrange optical amplifiers at regular intervals in the transmission fiber, and the wavelength dependence of the gain spectrum is determined by the optical signal-to-noise ratio (OSNR). Signal-to-Noise Ratio). For example, a gain equalizer disclosed in Non-Patent Document 1 has been proposed for flattening the gain spectrum.

By the way, there are several methods for realizing various optical functional circuits required in optical communication networks. For example, a method called PLC (Planar Lightwave Circuit) in which an optical waveguide made of a quartz-based material is formed on a silicon substrate is widely used as a method that is multifunctional, mass-producible, and low-cost.

Optical circuits using silica-based glass waveguides use the same material as optical fibers used in optical communications, so they have the characteristic of being able to realize low-loss optical waveguides. Furthermore, since the waveguide is formed on a flat substrate, it is easy to combine various functional elements, and complex optical circuits can be manufactured with good reproducibility. Wavelength multiplexing/demultiplexing elements, optical switches, and the like manufactured using these technologies are essential components in constructing optical networks.

The gain equalizer disclosed in Non-Patent Document 1 has a configuration in which Mach-Zehnder interferometers are connected in multiple stages, so it has a drawback of high loss. This is based on the essential cause that in each Mach-Zehnder interferometer, optical signals are discarded to unconnected output ports. On the other hand, Non-Patent Document 2 discloses a gain equalizer having a configuration called a lattice circuit.

A gain equalizer using a lattice type optical circuit is composed of N-1 arm waveguides each consisting of N directional couplers and two waveguides sandwiched between them. Furthermore, by applying heat to either one of the arm waveguides, the phase of light propagating through the waveguides is controlled by a phase shifter that utilizes a change in refractive index due to the thermo-optic effect. By adjusting the phase difference between the optical signals propagating through the two waveguides that make up the arm waveguide of the directional coupler in the front stage, the interference state in the directional coupler in the rear stage can be adjusted, and the interference state of the propagating light can be adjusted. Controls the transmission spectrum for wavelength.

In a gain equalizer using a lattice type optical circuit, a phase shifter is controlled using a thermo-optic effect in arm waveguides of a plurality of directional couplers, and a wavelength spectrum that is finally output is controlled. The i-th arm of an (N-1) stage lattice type optical circuit configured by N-1 arm waveguides consisting of N directional couplers and two waveguides sandwiched between them. In the waveguide, the phase difference Θ _i occurring between the upper arm waveguide and the lower arm waveguide is expressed by equation (1).

Here, λ is the wavelength of light, n _eff is the effective refractive index of the optical waveguide, ΔL _i is the difference in length between the upper arm waveguide and the lower arm waveguide in the i-th arm waveguide, and φ _i is i This is the phase difference added between the arm waveguides by controlling the phase shifter in the second arm waveguide. To configure a gain equalizer using a lattice type optical circuit, a desired gain equalization spectrum is obtained by appropriately controlling the phase difference φ _i added between each arm waveguide.

Examples of phase shifters include thermo-optic phase shifters that utilize heat generated by a heater and thermo-optic effects. In the case of a thermo-optic phase shifter, the heat generated is controlled by the amount of current applied to the heater, and the accompanying refractive index change and phase modulation amount are controlled. The larger the phase difference to be added, the larger the amount of drive current required.

FIG. 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 assumed to be Δλ ₁ (1525 to 1570 nm in FIG. 1). The minimum transmittance of the light intensity within Δλ ₁ is Loss _max (−9 dB in FIG. 1), and the maximum transmittance is Loss _min (−0.34 dB in FIG. 1). The difference between Loss _max and Loss _min will be referred to as an attenuation range (Att. Range). The desired spectral shape only needs to be maintained within Δλ ₁ , and the spectral shape in other wavelength ranges does not matter. In addition to the spectral shape shown in FIG. 1, for example, if a spectrum in which the transmittance changes linearly with respect to wavelength within Δλ ₁ is realized, the gain equalizer operates as a tilt equalizer. In this case as well, the slope (dB/nm) of the spectrum shape is controlled by adjusting the amount of phase shift in each arm waveguide. In the case of a tilt equalizer, Att. Range is expressed by [inclination of spectrum shape×Δλ ₁ ].

In a gain equalizer using a lattice type optical circuit, in order to control the gain equalization spectrum shape with a high degree of freedom, it is desirable that the range of the achievable Att. Range be wide. In a gain equalizer using a lattice type optical circuit, it is Θ _i expressed by equation (1) that determines the gain equalization spectrum and Att. Range. Θ _i is the difference in length ΔL _i between the upper arm waveguide and the lower arm waveguide in the arm waveguide, and the phase difference φ _i (hereinafter referred to as phase modulation amount φ) added between the arm waveguides by control of the phase shifter. _i ).

ΔL _i is fixed by the optical circuit design and determines the gain equalization spectrum in the initial state in the absence of modulation in the phase shifter. By adding the phase modulation amount φ _i to the lattice optical circuit in the initial state, the gain equalization spectrum is changed from the initial state. In order to change the Att. Range over a wide range, it is necessary to greatly change the amount of phase modulation φi, and the maximum value of the required amount of phase modulation also increases. When using a thermo-optic phase shifter as a phase shifter, the amount of phase modulation is controlled by the current applied to the heater, so the larger the amount of phase modulation to be applied, the larger the amount of drive current is required. Therefore, in order to change the Att. Range over a wide range, it is necessary to apply a large drive current to each heater, which increases power consumption.

In addition, in a gain equalizer using a lattice type optical circuit, by increasing the number of stages (N-1) of the lattice type optical circuit, it is possible to realize a more complex and highly flexible gain equalization spectrum. . However, as the number of stages increases, the number of phase shifters loaded on each arm waveguide also increases, and the number of phase shifters to which drive current is applied increases, leading to a further increase in power consumption. As described above, the conventional gain equalizer using a lattice type optical circuit has a problem in that the gain equalization spectrum cannot be controlled with a high degree of freedom within a limited power consumption.

An object of the present invention is to provide a gain equalizer using a lattice type optical circuit that can realize a gain equalization spectrum with a high degree of freedom with less power consumption.

In order to achieve such an object, one embodiment of the present invention is an optical waveguide circuit formed on a substrate, comprising N input waveguides (N is an integer of 3 or more), 2 inputs, 2 An output optical combining/distributing circuit, N-1 arm waveguides consisting of two waveguides connecting between the optical combining/distributing circuits, and an output waveguide, and at least one waveguide of the arm waveguides. The optical waveguide circuit is characterized by comprising: an optical waveguide circuit including a phase shifter loaded in the output waveguide; and an optical folding section that outputs the light input from the output waveguide to the output waveguide again.

FIG. 1 is a diagram showing an example of a gain equalization spectrum in a conventional lattice type optical circuit. FIG. 2 is a diagram showing the configuration of a gain equalizer according to the first embodiment of the present invention, FIG. 3 is a diagram showing the configuration of a gain equalizer according to a second embodiment of the present invention, FIG. 4 is a diagram showing the configuration of a gain equalizer according to a third embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First embodiment]
(composition)
FIG. 2 shows the configuration of a gain equalizer according to the first embodiment of the present invention. The gain equalizer 10 includes a lattice optical circuit 11, an input/output separation mechanism 12, and an optical folding section 13. The lattice type optical circuit 11 and the optical folding section 13 are constructed of PLC, and the circuit configuration is shown as seen from the top of the optical waveguide circuit formed on the substrate.

The lattice optical circuit 11 includes an input waveguide 111, a plurality of optical directional couplers 113-1 to 113-N and arm waveguides 114-1 to 114-(N-1), and an output waveguide 112. (N is an integer of 3 or more). The optical directional couplers 113-1 to 113-N may be any optical circuit elements as long as they are optical multiplexing/dividing circuits with two inputs and two outputs. The arm waveguides 114-1 to 114-(N-1) are composed of two waveguides that connect two optical directional couplers, and at least one of the waveguides is provided with a phase shifter. 115-1 to 115-(N-1) are loaded. The optical signal input from the input waveguide 111 passes through the optical directional couplers 113-1 to 113-N and the arm waveguides 114-1 to 114-(N-1) in order, and then exits from the output waveguide 112. Output.

The optical folding section 13 may be integrated with the lattice optical circuit 11, or may be externally connected via an optical fiber or the like. A reflecting mirror may be attached to the output end face of the output waveguide 112 of the lattice type optical circuit 11 to serve as a light return portion. Furthermore, a configuration may be adopted in which a reflecting mirror is installed at the end of the optical waveguide connected to the input/output port 131 of the optical folding section 13. As long as the light input from the input/output port 131 of the light return unit 13 is output from the input/output port 131 again, the structure of the light return unit 13 does not matter.

The optical folding unit 13 of the first embodiment includes an optical directional coupler 132, which is a 2-input and 2-output optical multiplexing/distributing circuit, and a loop waveguide 133 that connects two output ports of the optical directional coupler 132. It is configured. The light input from the input/output port 131 is separated into 50% intensities by the optical directional coupler 132. The separated lights travel in opposite directions in the loop waveguide 133, interfere again in the optical directional coupler 132, and light with 100% optical intensity is output from the input/output port 131.

The input/output separation mechanism 12 may be integrated with the lattice optical circuit 11, or may be externally connected via an optical fiber or the like. The input/output separation mechanism 12 has ports 121 to 123. Input light from port 121 is output from port 122, and input light from port 122 is output from port 123. Examples of the input/output separation mechanism 12 include a 3 dB coupler, a wavelength-independent coupler, etc. when integrated with the lattice type optical circuit 11, and an optical circulator etc. when connected externally.

(phase shifter)
At least one of the arm waveguides 114-1 to 114-(N-1) is loaded with phase shifters 115-1 to 115-(N-1), respectively. The phase shifters 115-1 to 115-(N-1) have a function of controlling the phase of the optical signal passing through them. The principle of the phase shifter does not matter as long as it can control the phase of the optical signal that has passed through it, but for example, a thermo-optic phase shifter that utilizes heat generated by a heater and thermo-optic effect may be mentioned. In the case of a thermo-optic phase shifter, the heat generated is controlled by the amount of current applied to the heater, and the accompanying refractive index change and phase modulation amount are controlled. The larger the amount of phase modulation to be applied, the larger the amount of drive current required.

(Operating principle)
With the above configuration, the operation of the gain equalizer according to the first embodiment will be explained. When an optical signal is input to port 121 of gain equalizer 10, it is input to input/output separation mechanism 12 and output from port 122. The light output from the port 122 is input to the lattice optical circuit 11 via the input waveguide 111.

The optical signal input to the lattice optical circuit 11 is subjected to gain equalization according to the gain equalization spectrum determined by the amount of phase modulation in the phase shifters 115-1 to 115-(N-1), and then It is output from the output waveguide 112. The propagation direction of the output light from the output waveguide 112 is reversed by the optical folding section 13, and the light is again input to the lattice type optical circuit 11 via the output waveguide 112.

The optical signal propagates within the lattice optical circuit 11 in the opposite direction to the direction in which it passes through the lattice optical circuit for the first time, and is output from the input waveguide 111. Output light from the lattice optical circuit 11 is input to the input/output separation mechanism 12 via a port 122 and output from a port 123.

In the gain equalizer 10 of the first embodiment, the input optical signal passes through the lattice optical circuit 11 twice, in the forward direction and in the reverse direction. In other words, it performs a reciprocating motion. Based on the principle of backward propagation of light, there are two cases: when an optical signal is input to the lattice type optical circuit 11 from the input waveguide 111 and output from the output waveguide 112, and when the optical signal is input to the lattice type optical circuit 11 from the output waveguide 112. The transmission spectrum is the same when inputting from the waveguide 112 and outputting from the input waveguide 111. Therefore, when the lattice-type optical circuit 11 of the first embodiment is operated in a reciprocating manner, it is possible to obtain the same gain equalization effect as when one lattice-type optical circuit is transmitted twice.

(Low power consumption due to reciprocating operation)
In the gain equalizer 10 of the first embodiment, since the lattice type optical circuit 11 is reciprocated, when the amount of phase modulation in each phase shifter 115-1 to 115-(N-1) is fixed, the conventional Compared to the case of one-way operation, the Att. Range of the gain equalization spectrum obtained is doubled. In other words, when a desired gain equalization spectrum exists, the gain equalizer 10 of the first embodiment has a lattice type optical circuit whose Att. Range is half that of a conventional one-way operation gain equalizer. That's fine. For example, when it is desired to operate the gain equalizer 10 of the first embodiment in the Att. Range of 0 to 8 dB, the lattice type optical circuit 11 alone has an Att. Range of 0 to 4 dB. It would be good if we could achieve this. In this way, the required Att. Range in the lattice optical circuit 11 can be narrowed, so the range of the necessary phase modulation amount can also be narrowed, and the amount of driving current for the phase shifter can be reduced. That is, power consumption can be reduced.

As an example, consider a case where a tilt equalizer is configured by 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 (when no modulation is performed) has a waveform with a slope of 0. When it is desired to output a gain equalized spectrum whose spectral waveform slope is 0.2 dB/nm (equivalent to Att. Range = 8 dB in the C band of the optical communication wavelength band), the gain equalizer 10 outputs a lattice-type optical spectrum. The circuit 11 alone may operate as a gain equalizer with a spectrum waveform slope of 0.1 dB/nm (corresponding to Att. Range=4 dB in the C band). At this time, the required power consumption can be reduced by approximately 25% compared to a gain equalizer in which a conventional lattice type optical circuit is operated only one way.

[Second embodiment]
FIG. 3 shows the configuration of a gain equalizer according to a second embodiment of the present invention. The gain equalizer 10 of the first embodiment includes one each of a lattice type optical circuit 11, an input/output separation mechanism 12, and an optical folding section 13. The number of lattice-type optical circuits is not limited to this, and a plurality of lattice-type optical circuits may be included.

As shown in FIG. 3, the gain equalizer 20 includes a plurality of lattice-type

optical circuits

11, 21, and 31 connected in cascade to each other. The output port 123 of the first input/output separation mechanism 12 is connected to the input port 221 of the second input/output separation mechanism 22. After the first lattice optical circuit 11 is reciprocated, the second lattice optical circuit 21 can be reciprocated. Furthermore, the output port 223 of the second input/output separation mechanism 22 is connected to the input port 321 of the third input/output separation mechanism 32. After the second lattice optical circuit 21 is reciprocated, the third lattice optical circuit 31 can be reciprocated. When the gain equalizer 20 is configured by three lattice

optical circuits

11, 21, and 31, the gain equalization spectrum of the entire gain equalizer is further A large Att. Range can be achieved.

[Third embodiment]
FIG. 4 shows the configuration of a gain equalizer according to a third embodiment of the present invention. The gain equalizer 40 includes a lattice optical circuit 11, an input/output separation mechanism 12, and an optical folding section 13. The lattice type optical circuit 11 and the optical folding section 13 are constructed of PLC, and the circuit configuration is shown as seen from the top of the optical waveguide circuit formed on the substrate. The difference from the first embodiment is that a polarization rotation mechanism 134 is installed at the center of the loop waveguide 133 of the optical folding section 13.

The polarization rotation mechanism 134 is an element that rotates the polarization direction of light propagating through the loop waveguide 133 by 90°. As the polarization rotation mechanism 134, there is a wavelength plate, for example, and a half-wave plate that shifts the phase difference between the main axis and the slow axis by π is particularly effective. By installing the half-wave plate at an angle of 45° with respect to the substrate surface of the loop waveguide 133, the polarization direction of the light propagating through the loop waveguide 133 is rotated by 90°. . In this way, the optical folding section 13 can convert the TE mode and TM mode of the optical waveguide into each other.

In the gain equalizer 40 of the third embodiment, when the lattice optical circuit 11 is reciprocated, the light that was in the TE mode on the outward path undergoes polarization rotation in the optical folding section 13, and becomes the TM mode on the return path. Transmits through a lattice optical circuit. Therefore, by reciprocating the lattice-type optical circuit 11, polarization-dependent characteristics are eliminated, and a gain equalizer with small polarization-dependent loss (PDL) can be realized as a whole.

As described above, according to the present embodiment, a gain equalization spectrum with a high degree of freedom can be realized with less power consumption in a gain equalizer using a lattice type optical circuit.

Claims

An optical waveguide circuit formed on a substrate,
an input waveguide, N (N is an integer of 3 or more) two-input, two-output optical multiplexing/distributing circuits, N-1 arm waveguides each consisting of two waveguides connecting the optical multiplexing/distributing circuits; It consists of an output waveguide,
an optical waveguide circuit including a phase shifter loaded in at least one waveguide of the arm waveguide;
A gain equalizer comprising: an optical folding section that outputs the light input from the output waveguide to the output waveguide again.
The gain equalizer according to claim 1, wherein the optical folding section is a reflection mirror installed at the output end face of the output waveguide.
The gain equalizer according to claim 1, wherein the optical folding section is an optical waveguide connected to the output waveguide and a reflection mirror installed at the end of the optical waveguide.
The optical folding section includes a 2-input 2-output second optical multiplexing/distributing circuit connected to the output waveguide, and a loop waveguide connecting two output ports of the second optical multiplexing/distributing circuit. A gain equalizer according to claim 1.
4. The gain equalizer according to claim 1, further comprising an input/output separation mechanism connected to the input waveguide and comprising an optical circulator.
The gain equalizer according to claim 1, 2 or 3, wherein the optical waveguide circuit is made of a quartz-based material formed on a silicon substrate.
The gain equalizer according to claim 1, 2 or 3, wherein the phase shifter utilizes a thermo-optic effect.