In the Passive Optical Devices for 5G Application(Part II), we introduce Tunable Optical Filter (TOF) for Coherent Receiving, Optical Performance Monitoring (OPM) Module, Optical Channel Monitoring (OCM) Module. This article will show you other important passive optical devices for 5G application.
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.
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.
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.
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.14 shows the photo of a MEMS VOA . It can be seen that the MEMS-based VOA has the advantages of small size and compact structure.
HYC has more than 20 years manufacturing experience in the optical communication industry. We have powerful R&D capability and provides professional OEM and ODM services. Our products include fiber optic connectors, fiber jumpers, WDM wavelength division multiplexers, PLC splitters, and MEMS optical switch and so on. Under the large-scale construction environment of 5G base stations,HYC strengthens the production and research and development of 5G series products to help the 5G construction.
 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
 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
 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
Written by Zhujun Wan,Jianwei Feng HYC Co., Ltd