World's First Attempt at Flexible Compensation Technique for Optical Power Difference Between Spatially Multiplexed Signals:
Milestone for Realizing Beyond 1-Peta-bit Class Large Capacity Optical Transmission Proposed in IOWN

[Research Background]

Mode division multiplexing can increase transmission capacity in proportion to the number of multiplexed modes. Hence, it is considered as one of candidate technology for realizing future large capacity optical backbone network. However, a slight difference in attenuation coefficient between modes accumulates across the transmission distance, as shown as (1) Attenuation in Fig. 2. Optical amplification efficiency also differs between modes and it results in the output power discrepancy of the amplifier, as shown as (2) Amplification efficiency in Fig. 2. Since the transmitted modes are randomly coupled in the transmission line, digital signal processing is needed to demodulate the received signals. However, the processing complexity increases exponentially in proportion to the number of multiplexed modes and optical power difference between modes. Moreover, the optical power difference between modes varies with the transmission distance and operation condition of the optical amplifier. Thus, a variable compensation technique for optical power difference among the modes is essential for realizing the mode multiplexing transmission system. However, there is currently no candidate technology which satisfies compactness, low loss and variability simultaneously.

[Features of Achievements]

By utilizing a planar lightwave circuit (PLC) technology which is superior for compactness and mass productivity, NTT realized, for the first time:

1. Variable attenuation for a specific mode

Figure 3 shows the schematic of the proposed PLC configuration and operation principle. Our device consists of two cascaded optical power division/coupling circuits. A 50% of a specific mode which has higher optical power among the two is coupled from a bus waveguide to a delay line waveguide via the first optical power division circuit. A desired mode can be selected by designing a coupling length Lc and gap between waveguide g adequately. The light guided in the delay line waveguide is re-coupled into the bus waveguide via the second optical power coupling circuit. The coupled power varies depending on the delay time (phase difference) given in the delay line waveguide. Thus, the re-coupled (attenuating) power of the specific mode can vary by changing the refractive index of the delay line waveguide using a heater. Here, the other mode which has lower optical power has no excessive loss in principle since it simply passes through the bus waveguide. As a result, our device can realize compactness, low loss and variability simultaneously.

Figure 4 shows relative optical power of attenuating signal as a function of heater power. It can be seen that attenuation can be varied up to 2.3 dB by changing the heater power from 50 to 170 mW. Since a typical attenuation coefficient difference in a transmission line can be considered as less than 0.01 dB/km, the proposed device can compensate the optical power difference caused in a transmission line with more than 200 km long.

2. Wideband compensation of amplification efficiency difference

Figure 5 shows amplification efficiency difference measured with and without proposed devise. A two-mode optical amplifier for 1530 - 1565 nm wavelength region was used for this experiment. Black filled symbols show the results obtained without our device, and it shows that there are 1.5 dB or more amplifier efficiency difference over the entire operation wavelength. The amplifier efficiency difference without our PLC also depends on the operation condition of the amplifier. The operation condition means pump power, and which is shown with circles, triangles, and squares. An output power of the amplifier varies with the pump power. Red open symbols show the results obtained when we used proposed PLC device, and the results show that the amplification efficiency difference is successfully reduced to less than ±0.5 dB at all operation conditions without any noticeable influence on the wavelength dependence.

[Future Prospects]

The present achievement can be considered as an enabling technology for a large-capacity optical transmission line which uses mode division multiplexing. Our results make substantial progress in mode multiplexed optical transmission technology, together with our previous report on "Optical Transmission Property Control by Optimizing Optical Cable Structure"^*3. Effective use of spatial multiplexing is mandatory for realizing a supra-1-Petabit class optical transmission backbone as a target of IOWN^*2 proposed by NTT. We will continue our industry-academia collaboration research on effective use on the spatial modes.

[Glossary]

^*1PLC (Planar Lightwave Circuit)
Silica glass based optical waveguide technology which has been developed by NTT. PLC is superior to mass-production since the fabrication process is similar to the large scale integration.

^*2IOWN (Innovative Optical and Wireless Network)
IOWN is a future communications infrastructure to promote a smart world by using cutting-edge technologies such as photonics and computing technologies.

^*3Optical Transmission Property Control by Optimizing Optical Cable Structure
https://group.ntt/en/newsrelease/2020/03/09/200309a.html