Optical Passive Components Series - All Optical Network

Review of Optical Fiber Communication

Based on industrial view, the development of optical fiber communication has experienced four stages and is now in the fifth stage. In 1970, the emergence of optical fiber with low loss and laser diode operating at room temperature initialized optical fiber communication.

However, the wide application of optical fiber communication was in 1990s. USA government released the plan named “National Information Infrastructure (NII)” in 1993. Optical fiber communication technologies were important parts supporting NII and were developed rapidly. The symbolic technology in the period is DWDM, which expands the transmission capacity of optical fiber communication by tens of times. The development of optical fiber communication slowed down in 2001 with the burst of internet bubble.

Then in 2004, Japan began the wide deployment of FTTH before the emergence of services and contents that require more bandwidth. With the participation of China in 2008, the development of FTTH reached the peak in ~2012. The backbone optical fibers deployed in 2001 was then fully employed.

The fourth symbolic development of optical fiber communication is related to data center. The rapid development of mobile internet promoted the construction of big data centers. Google led the application of optical fiber interconnection in data center in ~2010, which became the second grow point of optical fiber interconnection besides telecom.

Above is the past four stages of optical fiber communication. 5G is the focus of the world in recent years, which is characterized by high speed, large capacity and low delay. The wireless technologies have enabled the former two. However, the delay of 5G is related to the optical fiber network supporting the base stations. The high speed and wide connection of terminals exhausts the bandwidth of optical fiber communication and results in more delay. The optical fiber network is required to be upgraded and the focus is on metro network. Based on cost consideration, the current metro network is mainly based on CWDM and FOADM (fixed optical add/drop multiplexer) technologies. The DWDM and ROADM technologies for long haul network are expected to sink to metro network.

Structure of All Optical Network

In order to improve the efficiency and reduce the OPEX (operation expense) cost of the optical fiber network, the new generation of all optical network (AON) is required to be software defined network (SDN). The SDN network can be reconfigured based on software setting, exempting from manual operation. ROADM is the key equipment enabling SDN network, as shown in Fig.1. The ROADM-based AON includes three level of networks: long haul, metro and access network. The long haul network connects big cities and is usually constructed as a mesh network. The metro network usually employs fiber ring structure. As the telecom services become diverse and complicated, the metro network extends to be a multiple-ring network, including a core ring and many edge rings. The access network is fed by the metro rings and extends to the vicinity of the end users. The final links between the access network and the users include FTTx (to the business buildings, schools, homes, etc.) and wireless base stations.


Fig.1 Structure of all optical network

The coming 5G application promotes the upgrading of AON. As the key part for AON, ROADM market is expected to increase rapidly, especially in metro network application.


A ROADM node has a network node interface (NNI) and a user network interface (UNI). The NNI interconnects DWDM signals from/to multiple directions. The DWDM signals are switched between different directions in wavelength granularity. The UNI downloads signals designated to the node and uploads signals from the node in wavelength granularity. In order to realize non-blocking switching and adding/dropping of wavelengths, the new generation of ROADM nodes are required to be colorless, directionless and contentionless (CDC ROADM).

Considering an 8-dimentional (8D) ROADM node with 80-channel DWDM in each direction, the total wavelengths to be handled in the node is 8×80=640. However, the wavelengths to be added/dropped by the UNI of the node is usually less than 20% based on statics. Most of the wavelengths (more than 80%) are just switched by the NNI of the ROADM node. Thus 640×20%=128 add/drop ports are enough for the UNI. However, 20% reserve of add/drop ports requires each port to be versatile, which means that each add/drop port can add/drop different wavelengths from different directions (colorless and directionless) according to allocation by the control system. Meanwhile, the UNI needs to be capable to simultaneously drop the same wavelengths from different directions (contentionless).

The signals transmitting in the optical fiber may have different bit rates. In high-speed transmission system, signals with different bit rates need different channel width due to the sideband originating from modulation. As shown in Fig.2, signals with bit rates of 100G, 400G and 1T need channel width of 50GHz, 75GHz and 150GHz, which is quite different from low-speed (≤25G) signals. Low-speed signals usually occupy channel width of 50GHz or 100GHz, depending on the design of DWDM system.


Fig.2 Channel width requirement for signal of different bit rate

In order to accommodate the coming high-speed transmission, the DWDM system need to provide super-channel function. The channel width should be flexible. It can be dynamically allocated as 50GHz, 75GHz, 100GHz,150GHz, etc., according to demand. Super-channel is a term for system designer. The module designer uses another term “flexgrid” for the same meaning.

A ROADM capable of colorless, directionless and contentionless functions is called CDC ROADM. It can be further defined as a CDC-F ROADM when flexgrid function is also supported.

ROADM Structures

A ROADM node is usually constructed with wavelength selective switches (WSSs) and other modules. The CDC functions depends on the configuration of the ROADM node, while the flexgrid function depends on the key module WSS. There are currently three types of WSSs, MEMS, LC (liquid crystal) and LCOS (liquid crystal on silicon). LCOS WSS supports flexgrid function endogenously and LC WSS supports the function after optimization, while MEMS WSS doesn’t support the function. The following are five types of ROADM structures. The focus is on how they support CDC functions.

Fig.3 shows ROADM structure #1. The NNI is constructed with 1×N WSSs. The number of WSSs for a M-dimensional ROADM is 2M. The figure shows only three dimensions. The UNI includes several add/drop modules (the figure shows only two of them). Each add/drop module has two 1×N WSSs linked back-to-back for signal dropping and a 1×N WSS linked to a power splitter for signal adding. According to the function of 1×N WSS, the ROADM structure can support colorless and directionless add/drop of signals. However, when the dimension of the ROADM node is more than the number of add/drop modules, contention happens at the links in the red circle. Maybe we can add the add/drop modules. But the cost is not cheap. Thus this ROADM structure is defined as a CD ROADM and doesn’t meet the CDC requirement.


Fig.3 ROADM structure #1

The second ROADM structure is shown in Fig.4. Just as the ROADM structure #1, the NNI is also constructed with 1×N WSSs, while the UNI 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 (PS) 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, ROADM structure #2 can realize CDC function. However, too much loss happens when the PS in the MCS split and broadcast the signal. Thus the arrays of optical amplifiers are necessary to supply the optical power. It is not cheap to deploy the optical amplifier arrays.


Fig.4 ROADM structure #2

Fig.5 shows ROADM structure #3. The difference is also in the UNI. It employs two M×N WSSs to realize CDC add/drop of signals. A M×N WSS has M inputs and N outputs. It can switch any wavelength group form any input to any output. The insertion loss of a M×N WSS is much less than that of a MCS, thus no optical amplifier is demanded.


Fig.5 ROADM structure #3

The ROADM structure #4 is shown in Fig.6. The UNI employs arrayed waveguide gratings (AWGs) and large scale optical switches. The large scale optical switch is usually implemented through 3D MEMS mirror arrays and free-space optics. Thus it is also called 3D MEMS optical switch (OSW). The 3D MEMS OSW can realize very large scale such as 512×512.

In the dropping module, all the wavelengths are first de-multiplexed by the AWGs. Then the de-multiplexing ports are cross connected to the dropping ports through the 3D MEMS OSW. The principle of the adding module is the same. ROADM structure can realize CDC functions and reserve 100% add/drop ports for the wavelengths addressing the ROADM node.


Fig.6 ROADM structure #4

The fifth ROADM structure is shown in Fig.7. It is similar to the structure in Fig.5, while the M×N WSSs are substituted by M×N adWSSs. The acronym adWSS is fully named add/drop wavelength selective switch. It has M inputs and N outputs. The inputs are DWDM ports, while the outputs are single wavelength ports. It can switch any single wavelength from any input to any output.


Fig.7 ROADM structure #5

Summary of ROADM Structures

Let’s review the five ROADM structures mentioned above. All the structures use 1×N WSSs in the NNI side. The differences are in the UNI side. ROADM #1 support only colorless and directionless functions and is defined as a CD ROADM. ROADM #2 is a CDC ROADM, while the cost is high due to insertion loss of power splitter and employment of optical amplifier arrays. ROADM #3 introduces M×N WSSs in the UNI side and fully support CDC functions, while there is only one supplier around the world. Meanwhile, the dropping ports in one M×N WSS (typically have 8×24 ports) are not enough for a high dimensional ROADM node. ROADM #4 is a CDC ROADM with 100% reserve of add/drop ports. However, the 3D MEMS OSW is quite expensive. Meanwhile, the 100% reserve of Rx and Tx modules is not cost effective. ROADM #5 needs adWSS in the UNI side. An typical adWSS has 8×128 ports, which means that two adWSSs are enough for a 8D ROADM node. However, adWSS technology is still not mature for commercial application.

Based on above consideration, ROADM #2 is currently the main approach for a CDC ROADM node. ROADM #3 is a potential candidate when it is cost down and more suppliers emerge. ROADM #5 will be the best solution when it is technically mature.

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