Polymer waveguide tunable transceiver for photonic front-end in the 5G wireless network

Recent explosive expansion of web-based video platform services pushes the data traffic increase enormously, and has become a driving force in the development of the next-generation communication technologies. The number of communication channels in optical communication is upgraded by using the wavelength division multiplexing (WDM) technology. The bottleneck of communication capacity occurs at the fronthaul connecting radio-access networks of 5G communication.

Unlike core networks, fronthaul WDM networks require cost-effective network equipment, for which low-cost WDM transceiver is crucial. For this purpose, tunable laser technology has matured, while tunable filters are not yet available, making it difficult to find inexpensive tunable transceivers.

To solve the above problems, the research group led by Prof. Min-Cheol Oh from Pusan National University, South Korea have demonstrated an integrated optic tunable transceiver suitable for the practical 5G fronthaul network. The research results are published in Photonics Research, Vol. 9, No. 2, 2021 (Tae-Hyun Park, Sung-Moon Kim, Eun-Su Lee, Min-Cheol Oh. Polymer waveguide tunable transceiver for photonic front-end in the 5G wireless network[J]. Photonics Research, 2021, 9(2): 02000181)

Schematic diagram of tunable transceiver consisting of tunable laser and tunable wavelength filter based on polymer waveguide Bragg reflectors. The tunable laser consists of an SLD gain chip attached at the end of the polymer waveguide with a Bragg grating at the other end. The tunable wavelength filter has two stages: an inclined Bragg grating as well as the mode-sorting Y branches

For the purpose of compact tunable transceiver device integrating the tunable laser along with the tunable filter, researchers exploit a polymeric optical waveguide technology. Because the polymer has high thermo-optic effect and strong thermal confinement, wide tuning of Bragg wavelength is available, and because of the simple device structure, wavelength control is easy and high production yield is achievable. Both polymer lasers and filters include Bragg gratings with different structures, and they are fabricated simultaneously through a compatible fabrication process.

In order to demonstrate a high performance tunable filter device for WDM systems, researchers have been working on various approaches. To satisfy all the requirements for the practical 5G fronthaul application, asymmetric Y-branches and tilted Bragg gratings are adopted. The crosstalk from adjacent channel is greatly suppressed by using a two-stage tilted Bragg grating structure. Device performance levels suitable for 5G fronthaul optical communication networks are clearly demonstrated.

The group has been working on polymeric integrated optic devices which has the advantage of supplying low cost mass producible photonic devices. Especially the Bragg grating device made of polymer waveguide is very effective to tune the reflection wavelength by applying small amount of heating power through an integrated heater. A tunable laser is realized by using a broadband light source aligned to the Bragg reflection polymer grating as an external cavity structure, and a tunable filter is consisting of a Bragg grating with a tilt angle for the coupling of orthogonal modes and an asymmetric Y-branch for the mode sorting.

Prof. Min-Cheol Oh from Pusan National University said that this work presented the excellent performance of tunable transceiver devices made of polymer waveguides, which can be produced in mass amount with low cost. The tunable transceiver will be adopted in the 5G fronthaul network in a near future, and the WDM system performance will be evaluated in order to deploy the novel polymeric tunable transceiver for the next generation WDM communication network.