Passive Optical Devices for 5G Application Part I

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.

1.    CDC ROADM Based on MCS

The demand for increasing bandwidth promotes the upgrading of all-optical network (AON). As key parts of AON, the market of ROADM and related passive optical devices is expected to grow rapidly. In the nodes of optical fiber network, any wavelengths can be downloaded/uploaded via ROADMs. With the rapid growth of Internet traffic, the traditional ROADM nodes can’t meet the requirements. The new generation of ROADMs are required to be colorless, directionless and contentionless (CDC ROADM).

The most popular ROADM structure is shown in Fig.1. The NNI (Network to Network Interface, for interconnection between ROADM nodes) is constructed with 1×N WSSs, while the UNI (User Network Interface, for local adding/dropping of wavelengths) is constructed with multicast switches (MCSs). A M×N MCS has M inputs and N outputs. It is constructed with M 1×N power splitters (PSs) and N M×1 optical switches (OSWs). The signal from one input is first split by the corresponding PS and broadcast to all the N OSWs. Then the OSW corresponding to the destination output selects the signal it receives, while the other OSWs neglect the signal.

According to the functions of 1×N WSS and MCS, this ROADM structure can realize CDC functions. However, too much loss happens when the PSs in the MCS split and broadcast the signals. Optical amplifierarrays are necessary to supply the optical power. It is not cheap to deploy the optical amplifier arrays. Thus the cost and size reduction of EDFA is demanded. The components in EDFA, such as WDM, optical isolator, tap coupler, VOA (Variable Optical Attenuator) and PD (Photon Detector), are required for hybrid integration to reduce the size and cost.


Fig.1 Structure of CDC ROADM based on MCS


2.    1×N Port Optical Switch for MCS Module

As shown in Fig.2, a M×N port MCS optical switch requires N M×1 port optical switches (OSWs). Due to the large number of 1×N port OSWs needed in one MCS module, and the large number of ports in each OSW, the traditional mechanical OSWs can’t meet the size and loss requirements. Thus MEMS OSWs become the first choice.


Fig.2 Structure of MCS module


The structure of a 1×N port MEMS OSW is shown in Fig.3. In order to accommodate more ports in the deflection range of the MEMS mirror and reduce the loss caused by optical aberration, the off-axis distance of the edge ports can be cut downby reducing the outer diameter of the optical fibers through chemical corrosion and configuring appropriate optical fiber alignment.


Fig.3 1 × N ports MEMS optical switch


3.    Y-branch Power Splitter

As shown in Fig.2, a M×N port MCS switch requires M 1×N port power splitters (PSs). The basic elements for a 1×N port PS is Y-branches whose principle is shown in Fig.4. Optical mode is adiabatically converted at the branching area and thus optical power is equally split into two waveguides. Cascade of multiple Y-branches constitute a PS with a large port number. The Y-branches are usually made of glass waveguide, as shown in Fig.5.


Fig.4 Schematic diagram of Y-branch


Fig.5 Power splitter based on cascade of Y-branch


PS has been widely used in the PON (Passive Optical Network) network for FTTH (Fiber to the Home). However, its application in MCS requires more on reliability because the MCS modules are employed in the ROADM nodes of long haul network or metro network, while not in the access network.

To be continued.

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