With the rise of 5G technologies and massive deployment of 5G base stations, wireless access of terminals with high speed and large capacity is realized. Meanwhile, the traffic in optical fiber network increases rapidly. It is predicted that the current optical fiber network will become the bottleneck of information exchange in the future 12-18 months. The upgrading of optical fiber network is urgent. The representative trend is that the technologies for long-haul network (LHN) will be sunk to metropolitan area network (MAN), including DWDM (Dense Wavelength Division Multiplexing), ROADM (Reconfigurable Optical Add-Drop Multiplexer) and coherent receiving techniques. This paper discusses some of the passive optical devices for the coming 5G applications.
7. Dynamic Gain Equalization (DGE) Filter
For the complicated fiber links transmitting DWDM signals, OPM and OCM just provide solutions for monitoring of OSNR and channels. However, the DWDM signals need to be equalized before they leave each ROADM node or relay station. In the ROADM-based optical network, the power levels of the DWDM channels are always changing. Thus DGE is required to provide dynamic equalization of the DWDM channels, which is different from the fixed GFF (Gain Flattening Filter) for an EDFA.
There are two ways to equalize the DWDM channels. The first is to adjust the power levels of the channels on by one. The second is to construct a spectral curve of attenuation which is reverse to the measured power spectrum.
For the scheme of independent control of each channel, free space optical structure is generally adopted, and the attenuation level of each channel is controlled by SLM.As shown in Fig.11 [1].
Fig.11 Schematic diagram of DGE with free space optical structure[1]
SLM can be fabricated by MEMS micro mirror array or grating light valve (GLV). The latter is also based on MEMS technology, which uses the diffraction of light to work.As shown in Fig.12[2].
Fig.12 Structural schematic diagram of grating light valve[2]
For the scheme of adjusting transmission spectrum as a whole, Sinusoidal Filter Based on multi-level series structure is generally used. The whole spectrum is adjusted by the phase adjustment of each stage.
8. Variable Optical Attenuator (VOA) for Dynamic Optical Network
In the dynamic optical network based on ROADM nodes, optical power in the fiber links need to be well managed.As one of the most widely employed basic optical devices, variable optical attenuator (VOA) is used for management. The approaches for VOAs are variable.
VOA based on MEMS mirror has the advantages of simple structure and low cost, and is the mostly employed VOA approach. Fig.13 shows the structure of VOA based on a MEMS mirror. The pigtails of a dual-fiber collimator are employed as the input/output ports. The collimated beam is reflected by the MEMS mirror and thus the input/output ports are connected. Torsion of the mirror deflects the beam and results in attenuation.
Fig.13 Structure of VOA based on a MEMS mirror
Fig.14 shows the photo of a MEMS VOA [3]. It can be seen that the MEMS-based VOA has the advantages of small size and compact structure.
Fig.14 Final assembly of VOA based on MEMS mirror [3]
Reference
[1] T. Loukina, R. Chevallier, J. L. de Bougrenet de la Tocnaye, M. Barge, Dynamic spectral equalizer using free-space dispersive optics combined with a polymer dispersed liquid crystal SLM, Journal of Lightwave Technology, 21(9): 2069-2073, 2003
[2] A. Pothisorn, A. J. Hariz, B. Wedding and O. Trithaveesak, Fabrication of a grating light modulator using standard CMOS processes, 2011 6th IEEE International Conference on Nano/Micro Engineered and Molecular Systems, Kaohsiung, pp.1229-1232, 2011
[3] Huangqingbo Sun, Wei Zhou, Zijing Zhang, and Zhujun Wan, A MEMS Variable Optical Attenuator with Ultra-Low Wavelength-Dependent Loss and Polarization-Dependent Loss, Micromachines, 9(12): 632, 2018
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