Prospect of Fiber array application in 40G/100G optical transceiver

Fiber Array, for the manufacturers of passive optical devices, is not unfamiliar. Fiber array is commonly said FA, that is widely used in the optical splitter and other products. By the use of different channels of the PLC chip + FA, you can make the corresponding 1:4, 1:8, 1:16, 1:32 and other splitters. When the FTTH market was launched from 2012 to 2013, it triggered a small upsurge of passive optical devices. However, as the market declined, the FA device manufacturers were hard to survive and faced many difficulties.

Since 2015, 40G, 100G Optical Transceiver module demand began to grow, the FA seems to usher in a new round of spring. Today, let’s talk about the FA in the high-speed optical communications market.

Fiber Array, generally using 4-core, 8-core, 12-core fiber ribbon, with all-quartz material engraved V-groove substrate, the assembly becomes connected optical devices and optical fiber between the important coupling components. The FA used for Splitter generally requires that the FA be flush with the end face, horizontally coupled with the PLC device, and be achieved using a common grinding process. However, the FA for 40G / 100G active optical transceiver module is mainly used for the coupling between a laser, a detector and an optical fiber, and most of them use a vertical coupling. Therefore, the FA requires that the optical fiber protrude approximately 0.2 mm from the substrate. The figure below shows the physical photos and schematic diagrams of the FA with optical path coupling using 45 ° total reflection.

For 40G and 100G optical transceiver, high density, small size has become the common goal of the industry, so the fiber array chip has become the only choice for high-speed transceivers. For long-distance transmission of DFB, FP lasers, due to material and process limitations, the processing into an array is very difficult, so very few DFB array chips or FP array chips were used. But for PD and VCSEL, it is relatively easy to process into an array.

It can be easily seen from the principle that the 45 ° FA realizes the total reflection of the light transmitted by the optical fiber and is directly guided to the surface of the optical device after being turned to 90 °. Similarly, the light emitted by the optical device can also enter the optical fiber through a 90 ° rotation angle transmission. So for the VCSEL array and PD array, it is a very suitable solution.

Due to the complex processing and high cost of the FA, there is no application in markets dominated by 10G rates. However, for 40G and 100G optical transceivers, it is difficult to find a perfectly matched plastic lens for verification in the early stages of R & D. In particular, if the market is still not full and the cost of lens development and production is hard to recover, 45° FA is a very efficient and practical optical solution.

Compared with the lens solution (the basic principle is shown below), 45 ° FA also has unparalleled advantages:

• The optical principle used at 45 ° FA is very simple and is an axiom in textbooks, so there is no patent dispute. The lens program due to the current control of international manufacturers, leading to innovative lens design difficulties, a bit inattentive may fall into international patent disputes.
• The substrates and covers used at 45 ° FA are all quartz glass with no optical aging effects on the optical path (high energy density of the laser can lead to plastic aging for long periods of time rendering the lens ineffective) and more likely It is used in scenarios where the ambient temperature is high, such as -40~+85 ℃.

Of course, the FA’s own process led to its more complicated production process, the relative cost of the lens is very high. In particular, since the principle of 45 ° FA is to achieve total reflection on the end face of the optical fiber instead of the transmission of the conventional optical fiber jumper, the processing technology is greatly different from that of the conventional optical fiber end face processing, Coupling efficiency and receiver sensitivity have a very big impact. Different manufacturers may provide the same parameters of the FA, the optical module produced is quite different. At present, most of the 45 ° FA on the market is still manufactured by passive manufacturers, and the verification of the performance of back-end optical modules is relatively weak. Therefore, more verification is needed when selecting FA.

Currently, in the industry, the 45 ° FA is widely used in array reception of 40G QSFP SR4 transceivers, 100G QSFP SR optical transceivers, 40G PSM transceivers, and 100G LR optical modules.