Optical substrate

Abstract

Provided is an optical substrate making it possible to further increase optical transmission capacity while also greatly reducing the number of lasers conventionally required in a high-speed high-capacity optical communication system.　This optical substrate includes a tree-shaped branch circuit on which a light source and at least 1,024 optical modulators are mounted, and in which multi-wavelength light emitted from the light source is branched into a tree shape and distributed to each of the optical modulators. In the tree-shaped branch circuit, optical amplifiers are inserted on an upstream side of respective branching nodes, and amplified light is inputted to the branching nodes, and the number of branches at all branching nodes is no greater than 16.

Description

Optical substrate

[0001] The present invention relates to an optical substrate on which a light source for high-speed and large-capacity optical communication and a plurality of optical modulators are mounted.

[0002] In the optoelectronic integration technology, in order to distribute and supply optical energy to the optical modulators on silicon photonics, there are a method using an external laser source (ELS) (external light source method) and a method using a built-in laser source (On-chip LD) (internal light source method). In the external light source method, maintenance and replacement of the light source separated from the substrate including the optical modulator are relatively easy, but the coupling loss with silicon photonics becomes large (>4 dB). On the other hand, in the internal light source method, maintenance and replacement of the light source integrated on the substrate become complicated, but the coupling loss can be made very small (~0.5 dB). Therefore, when mounting a semiconductor laser (LD) on-chip, improving the yield and reliability of the light source is an issue.

[0003] For example, in Patent Document 1, with the increase in the capacity of communication data, in the package of the internal light source method, the number of light sources increases, so measures against failures and lifetimes of individual light sources are also required. After stating this, a method of reducing the number of light sources by using a plurality of ring modulators to extract desired multi-wavelength light is disclosed. Specifically, while having a plurality of ring modulators including one or more ring resonators, each ring modulator is adjusted to have a different resonance frequency, and it is said that desired multi-wavelength light is generated and output from the input plurality of lights. By using this in combination, the communication data can be increased in capacity without increasing the number of light sources.

[0004] Patent Document 2 also shows a method of increasing the capacity of communication data by distributing light from the light source. Here, light from eight light-emitting elements with different frequencies is demultiplexed and multiplexed by a demultiplexer, and a multi-wavelength light including four different frequencies at unequal intervals is used to prevent FWM (four-wave mixing). Then, an optical branching circuit that amplifies each with eight SOAs (semiconductor optical amplifiers) and further branches and emits the light is shown.

[0005] Japanese Patent Application Laid-Open No. 2024-36924, Japanese Patent No. 7370097

[0006] By the way, optical modulators, especially the ring modulators mentioned above, require a large optical input power, making it difficult to split light from a small light source with practical power.

[0007] The present invention has been made in view of the above-described circumstances, and its objective is to provide an optical substrate that can significantly reduce the number of lasers conventionally required in high-speed, high-capacity optical communication systems while increasing the optical transmission capacity.

[0008] Conventionally, for example, when using an eight-wavelength light source, the number of branches after the semiconductor optical amplifier is assumed to be limited to about four, considering the saturation output optical power of the semiconductor optical amplifier, the losses of the optical modulator, and the losses of the optical branching circuit. Furthermore, it is assumed that the optical transmission speed that can be transmitted with one optical modulator is 100 Gb / s. For example, in the example of Patent Document 2 mentioned above, the number of wavelengths used corresponds to the number of light-emitting elements, which is eight, but the four wavelengths of light that have been decompressed are amplified using eight SOAs, and each of these is further branched into four, resulting in 100 Gb / s × (8 SOAs) × (4 wavelengths) × (4 branches) = 12.8 Tb / s, which is the limit of the optical transmission capacity, which is 128 times the optical transmission speed.

[0009] The inventors of this application have conducted extensive research to obtain a configuration that surpasses the above, and have arrived at the present invention.

[0010] The optical substrate according to the present invention is equipped with a light source and at least 1024 or more optical modulators, and includes a tree-shaped branching circuit that branches multi-wavelength light emitted from the light source in a tree-like manner and distributes it to each of the optical modulators, wherein the tree-shaped branching circuit is characterized in that an optical amplifier is inserted upstream of each branching node to amplify the light and input the amplified light to the branching node, and the number of branches at all of the branching nodes is 16 or less.

[0011] These characteristics make it possible to significantly reduce the number of light sources while increasing the optical transmission capacity.

[0012] In the invention described above, the light source may be characterized by including a set of multiple light-emitting elements and an optical circuit that selects one of these sets of light-emitting elements to emit the multi-wavelength light. With this feature, the complexity of maintenance due to failure of the onboard light source can be greatly reduced.

[0013] In the invention described above, the optical amplifier may be characterized as a semiconductor optical amplifier. With this characteristic, it is possible to increase the optical transmission capacity while using a general-purpose optical amplifier.

[0014] In the above-described invention, the optical amplifier may be characterized by including a plurality of independent optical amplifiers and an amplification element of one of them, and the light source may be characterized by including a plurality of light-emitting elements and an optical circuit that selects one of the light-emitting elements to emit the multi-wavelength light. With such features, the complexity of maintenance due to failure of the onboard optical amplifier can be greatly reduced.

[0015] This is a diagram of the optical substrate in one embodiment of the present invention. This is a graph showing the relationship between the number of branching stages and the number of phase output branches. This is a block diagram of a verification device for the quality of the branched optical waveform. This is an oscilloscope screen of the branched optical waveform. This is a diagram of the optical modulation unit in another embodiment of the present invention. This is a diagram of the optical substrate in the same embodiment.

[0016] As shown in Figure 1, in one embodiment of the present invention, the optical substrate 10 includes a light source 1 that emits multi-wavelength light, a branching circuit 20 that branches the multi-wavelength light emitted from the light source 1 in a tree-like manner, and an optical modulation unit 30 into which the distributed multi-wavelength light is incident. The optical substrate 10 is used to perform optical transmission using the Wavelength Division Multiplexing (WDM) method.

[0017] Light source 1 is capable of emitting multi-wavelength light, and here we illustrate a light source 1 that emits eight wavelengths of light. Light source 1 includes a demultiplexing light source 1a and a demultiplexing light source 2a that divide the eight wavelengths of light into two groups of four wavelengths each and emit them. Light source 1 can, for example, include at least eight light-emitting elements (not shown) that emit light of different wavelengths from each other, and these can be divided into groups of four wavelengths and combined in each group to emit four wavelengths of light from demultiplexing light source 1a and demultiplexing light source 1b, respectively. In other words, demultiplexing light sources 1a and 1b each include a set of four light-emitting elements. For example, a semiconductor laser can be suitably used as the light-emitting element.

[0018] Furthermore, regarding the grouping into four-wavelength groups, it is preferable to use a combination of wavelengths that are unequally spaced in order to suppress interference caused by four-wave mixing (FWM).

[0019] The branching circuit 20 is constructed by combining multiple stages of branching groupings, each consisting of multiple branching sections 2 arranged in parallel to amplify and branch the incident light. The branching circuit 20 includes a first-stage branching group 2-1 that receives multi-wavelength light from the light source 1, and a second-stage branching group 2-2 connected downstream of branching group 2-1 that further branches the multi-wavelength light. It also includes a final-stage branching group 2-F connected downstream of these branching groups that further branches the multi-wavelength light and inputs it to the optical modulation unit 3. In other words, the branching circuit 20 has a three-stage configuration consisting of two stages (previous branching groups 2-1 and 2-2 and the final branching group 2-F) plus the final stage. The first-stage branching group 2-1 includes two branching sections 2 corresponding to two groups of four wavelengths each. Subsequent branching groups include the number of branching sections 2 included in the previous branching group, plus a number of branching sections 2 corresponding to the number of branches in each group. For example, the number of branch sections 2 included in branch group 2-2 may be equal to the product of the number of branch sections 2 included in branch group 2-1 and the number of branches. In other words, the branch circuit 20 can branch multi-wavelength light in a tree-like manner. Note that the preceding branch group 2-1 etc. is shown as two stages here, but it may be one stage or a multi-stage configuration of three or more stages.

[0020] The multi-wavelength light emitted from branch 2 of the final branching group 2-2 is incident on multiple optical modulation units 3. Each optical modulation unit 3 includes eight optical modulators 31. Microring modulators can be suitably used as optical modulators 31. Furthermore, each of these eight optical modulators 31 has a different resonance frequency. These resonance frequencies correspond to the eight wavelengths of the multi-wavelength light described above. In other words, the optical modulation unit 3 comprises a modulator group 31a having four optical modulators 31 corresponding to four wavelengths of the multi-wavelength light from the demultiplexing light source 1a, and a modulator group 31b having four optical modulators 31 corresponding to four wavelengths of the multi-wavelength light from the demultiplexing light source 2a. The four optical modulators 31 of modulator group 31a are connected to one of the branches from the demultiplexing light source 1a via an input waveguide 32a, and the four optical modulators 31 of modulator group 31b are connected to one of the branches from the demultiplexing light source 1b via an input waveguide 32b. The optical modulation unit 3 also includes an output waveguide 33 connected to eight optical modulators 31. Each of the optical modulators 31 is also connected to an electronic circuit (not shown) that can receive electrical signals individually.

[0021] With this configuration, the optical modulation unit 3 receives multi-wavelength light divided into two groups of four wavelengths, and inputs constant-intensity light from input waveguides 32a and 32b to the optical modulators 31a and 31b. Each optical modulator 31 then applies intensity modulation or phase modulation corresponding to the signal received from the electronic circuit to the input light and outputs it as an optical signal. Furthermore, the output waveguide 33 can output the optical signals from the eight optical modulators 31 with different corresponding wavelengths while multiplexing them.

[0022] Incidentally, as mentioned above, an optical modulation unit 3 using such an optical modulator 31 requires a large optical input power. On the other hand, in order to increase the optical transmission capacity while reducing the number of light sources and light-emitting elements, it is considered to increase the number of branches and make them multi-stage. However, simply increasing the number of branches makes it difficult to supply a large optical input power to the optical modulator.

[0023] In contrast, referring to Figure 1(b), in the optical substrate 10, the branching section 2 includes a branching node 22 and an optical amplifier 21 inserted upstream of the branching node 22. That is, the branching section 2 can amplify the intensity of the incident multi-wavelength light with the optical amplifier 21 and then branch it with the branching node 22. This increases the number of branches in a single stage, and even with multiple branching stages, it is possible to input light with sufficient optical input power for practical use. A semiconductor optical amplifier (SOA) can preferably be used as the optical amplifier 21. Also, an optical coupler can preferably be used for the branching node 22.

[0024] As described above, the optical substrate 10 has a two-stage branching configuration consisting of a first-stage branching group 2-1 and a second-stage branching group 2-2, and further comprising a final-stage branching group 2-F. Here, let N be the number of branches in the first and second stages, and M be the number of branches in the final stage. If N = 16 and M = 4, then the total number of branches from the demultiplexing light source 1a plus the final stage is 16. ２ ×4 = 1024. In this case, the same number of branches are also made from the demultiplexing light source 2a, so 1,024 optical modulation units, the same number as the number of branches, will be corresponding. Furthermore, since there are 8 optical modulators 31 in one optical modulation unit 3, 8,192 optical modulators 31 will be corresponding. In other words, 8,192 optical signals can be generated simultaneously from 8 light-emitting elements. For example, if the optical modulation speed of the optical modulator 31 is 100 Gbps, an optical transmission capacity of 819.2 Tbps can be obtained.

[0025] In the case of a single stage plus a final stage, the number of branches becomes 16 × 4 = 64, and the same number of optical modulation units 3 correspond to this, with 512 optical modulators 31 corresponding to these units. In other words, 512 optical signals can be generated simultaneously from 8 light-emitting elements, and 512 optical signals can be generated simultaneously. That is, even with a two-stage configuration of a single stage plus a final stage, an optical transmission capacity of 51,200 Tbps can be obtained by setting the modulation speed of the optical modulator 31 to 100 Gbps.

[0026] In the preceding branching groups 2-1 and 2-2, the optical amplifier 21 of the next branching group 2 is connected afterwards, so the number of branches (N) can be made relatively large, for example, 16 branches as described above. On the other hand, in the branching group 2-F of the final stage, the number of branches (M) cannot be made very large due to the relationship between the saturation output power of the optical amplifier 21 and the output power required by the standard specifications as the output to the optical modulator 31, so it is set to 4 branches as described above.

[0027] Figure 2 shows the total number of output branches in a multi-stage configuration, with 4 branches in the final stage and 16 branches in the other stages, in relation to the number of stages excluding the final stage. For example, with 3 stages + the final stage, the total number of output branches is 16. ３ × 4 = 16,384.

[0028] Next, we will explain the results of an experiment that verified whether there was any discrepancy in the frequency or amplitude of the branched light when the light from light source 1 was branched in this way, that is, whether the quality of the light waveform was good.

[0029] As shown in Figure 3, an experimental apparatus 40 was constructed for verification. In the experimental apparatus 40, light emitted from a tunable laser light source was amplified by a first-stage semiconductor optical amplifier (SOA(1)) and then attenuated by a 24 dB optical attenuator equivalent to 256 branches (1 / 256 of the optical power). Here, the saturation output optical power of SOA(1) is +17 dBm (50 mW). In the verification experiment, the light was amplified to 50 mW at one wavelength, but if we assume multi-wavelength light with four wavelengths as described above, the optical power per wavelength becomes 1 / 4 (+11 dBm). In other words, it is essentially equivalent to 256 / 4 = 64 branches. Furthermore, after amplified by a second-stage semiconductor optical amplifier (SOA(2)), it was attenuated by a 6 dB optical attenuator equivalent to 4 branches. In other words, the total number of branches is equivalent to 64 branches × 4 branches = 256 branches. Furthermore, the light was converted into an optical signal by an optical modulator, and the waveform of this optical signal was obtained with an oscilloscope. The optical modulator uses PAM4 (quadriple pulse amplitude modulation) transmission and has an optical transmission speed of 100 Gbps. The laser light source sequentially emits light at each of the eight wavelengths corresponding to the multi-wavelength light described above, and the waveform of the optical signal is obtained similarly for each wavelength.

[0030] As shown in Figure 4, the waveforms exhibited good eye patterns at all eight wavelengths, confirming that the waveform quality remained good even with 256 branches. Furthermore, the 400 Gigabit Ethernet standard specifies a TDECQ (Transmitter Dispersion and Eye Closure Quaternary) value for the PAM4 optical signal waveform, and all eight wavelengths satisfied the specified value of TDECQ < 3.4 dB. In other words, it was demonstrated that it is possible to achieve 256 branches, which is 16 times the number of branches shown in Patent Document 2 for 16 branches of light from a single light-emitting element, while maintaining the quality of the light waveform input to the optical modulation unit 3. When an optical substrate using four wavelengths of light as a light source is configured as described above, optical signals can be generated simultaneously from 256 × 4 = 1024 optical modulators.

[0031] As described above, the branching circuit 20, by inserting the optical amplifier 21 upstream of the branching node 22 in the branching section 2, enables numerous and multi-stage branching, significantly increasing the number of optical modulators relative to the number of light-emitting elements and thus increasing the optical transmission capacity. In other words, the number of light sources 1, i.e., the number of light-emitting elements, relative to the number of optical modulators 31, was drastically reduced. This means that the number of components relative to the optical transmission capacity, especially the number of light sources, can be significantly reduced. This can also improve the reliability of the optical substrate.

[0032] When using an optical isolator (SOA) as the optical amplifier 21, a problem may arise where the reflected light from the SOA destabilizes the laser oscillation of the semiconductor laser, which is the light-emitting element. In such cases, it is preferable to insert an optical isolator upstream of the SOA to suppress the reflected light. Alternatively, it is preferable to use a quantum dot laser, which has high resistance to reflected light, as the light-emitting element. Furthermore, a quantum dot optical amplifier may be used as the optical amplifier 21.

[0033] Furthermore, amplification by the optical amplifier 21 may result in nonlinear optical phenomena or noise due to spontaneous emission. For this reason, it is preferable to incorporate automatic gain control to adjust the amplification level of the optical amplifier 21, or to insert an optical filter element downstream of the optical amplifier 21 to reduce noise. In addition, automatic power control may be incorporated to maintain a constant intensity of the output optical signal. Other elements such as wavelength dispersion compensators, gain flattening elements, and optical switches may be appropriately combined to improve the quality of the obtained optical signal.

[0034] Incidentally, as shown in Figure 5, the optical modulation unit 3 emits excess light that was not incident on the optical modulator 31 from the end 32c of the input waveguides 32a and 32b. Therefore, it is also preferable to connect a photodetector 34, such as a photodiode, to the end 32c of a part of the optical modulation unit 30 and monitor the intensity of the light emitted from the end 32c. This makes it possible to detect faults in the light source 1 or the branch circuit 20.

[0035] Furthermore, as shown in Figure 6, spare elements may be selected for the light source 1 and optical amplifier 21 depending on the detection result of the photodetector 34. That is, multiple independent light sources 1 are connected in parallel to the same optical amplifier 21 in advance, and / or multiple independent optical amplifiers 21 are connected in parallel to the same branch node 22, so that when an abnormality is detected by the photodetector 34, it switches to another light source or another optical amplifier connected in parallel.

[0036] In detail, the main element, the demultiplexing light source 1a, and its secondary element, the auxiliary demultiplexing light source 1a', are connected in parallel at a merging / branching node 22' such as an optical coupler. Similarly, the main element, the optical amplifier 21, and its secondary element, the auxiliary optical amplifier 21', are connected in parallel at a merging / branching node 22'. It is preferable that the optical amplifiers 21 and 21' connected in parallel are adjusted so that they can obtain saturation output power at half the output of the light input to the preceding merging / branching node 22'. Such auxiliary elements can be applied to one or both of the light sources and optical amplifiers. Also, for the sake of simplicity in the diagram, only one of the four-wavelength light sources, demultiplexing light source 1a, is shown here. That is, the optical modulation unit 30 is assumed to have input from demultiplexing light source 1b, which is not shown in the diagram. Furthermore, since demultiplexing light source 1a emits four-wavelength light, it includes a set of four light-emitting elements as described above. Here, the anomaly detected by the photodetector 34 is for four-wavelength light. Therefore, the above-mentioned switching to the backup will be performed for each set of four light-emitting elements. Furthermore, the backup elements may be provided not just as one or one set, but as two or more sets.

[0037] The photodetector 34 of the optical modulation unit 30 is provided in one of the optical modulation units 30 that are branched and connected from a light source or optical amplifier that requires monitoring, and is connected to the monitor unit 41. The monitor unit 41 monitors the output from the photodetector 34, and if the output decreases, it assumes that there is a fault upstream of the photodetector 34 with the decreased output, and switches the driving element from the demultiplexing light source 1a or optical amplifier 21 to a backup demultiplexing light source 21a' or optical amplifier 21'. More specifically, if the monitor unit 41 assumes a fault, it sends a signal to the corresponding drive unit 42 or 43 to switch the driving element to the backup. Based on the received signal, the drive unit 42 or drive unit 43 injects or stops current into the corresponding element, thereby switching the driving element. If multiple elements correspond to one of the photodetectors 34, it is preferable to switch the drive at each corresponding location to identify the fault location and then determine which element to switch the drive of.

[0038] In this way, by connecting spare elements in parallel and switching them based on the output from the photodetector 34, the optical substrate can maintain its function in response to element failure. This significantly reduces the complexity of maintenance due to failures of the onboard light source and / or optical amplifier, and also improves the reliability of the optical substrate.

[0039] Although embodiments and modifications based thereon have been described, the present invention is not necessarily limited to these examples. Furthermore, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the attached claims.

[0040] 1 Light source 2 Branching section 20 Branching circuit 21 Optical amplifier 22 Branching node 30 Optical modulation unit 31 Optical modulator

Claims

1. An optical substrate comprising a light source and at least 1024 or more optical modulators, and including a tree-like branching circuit that branches multi-wavelength light emitted from the light source in a tree-like manner and distributes it to each of the optical modulators, wherein the tree-like branching circuit is characterized in that an optical amplifier is inserted upstream of each branch node to amplify the light and input the amplified light to the branch node, and the number of branches at all branch nodes is 16 or less.

2. The optical substrate according to claim 1, characterized in that the light source includes a set of multiple light-emitting elements and includes an optical circuit that selects one set of the light-emitting elements from which to emit the multi-wavelength light.

3. The optical substrate according to claim 1, characterized in that the optical amplifier is a semiconductor optical amplifier.

4. The optical substrate according to claim 1 or 2, characterized in that the optical amplifier includes a plurality of independent optical amplifiers and an amplification element of one of them, and the light source includes a plurality of light-emitting elements and an optical circuit that selects one of the light-emitting elements to emit the multi-wavelength light.

Images