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Development Direction of Data Center Optical Transceivers

With the commercial use of cloud computing, big data and other new technologies, data center flow and bandwidth have an index incensement. According to the LightCounting forecast, by 2019, the sales volume of data center optical transceivers will be over $50 million and the market scale is hopeful to reach $4.9 billion, which will be a huge opportunity for optical transceiver vendors. At the same time, we can see that there are some difference in applications of optical transceivers between data center and Telecom. In the post, we will discuss the technology development direction of data center optical transceivers in detail.

On the macro level, the data center optical transceiver market is a market that reasonably defines the life and working conditions of optical transceivers according to the actual requirements, and fully optimizes the market for the cost performance of optical modules. Due to the open trend of several networks, this market has the characteristics of positivity and open, welcoming the characteristics of new technologies and the atmosphere of exploring new standards as well as application conditions. All of these provide excellent conditions for the development of data center optical transceiver technology. Here we are trying to enumerate the development direction of some data center optical transceiver technology for your reference.

Non-hermetic Package

As the cost of optical components (OSA) accounts for over 60% of the cost of optical transceivers, and the space for cost reduction of optical chips becomes smaller and smaller, the most likely cost reduction is the packaging cost. While ensuring the performance and reliability of optical transceivers, it is necessary to promote the packaging technology from the expensive hermetic package to the low-cost non-hermetic package. The key points of the non-hermetic package include the non-air tightness of the optical device itself, the optimization of the design of the optical components, the packaging materials and the improvement of the process. Among them, optical devices, especially lasers, are the most challenging. This is because if the laser device is not hermetic, expensive hermetic package is not needed. Fortunately, in recent years several laser manufacturers have avowed that their lasers can be applied to non-airtight applications. In view of the large number of shipping data center optical transceivers, most of them are mainly non-hermetic package. It seems that the non-hermetic packaging technology has been well received by the data center optical transceiver industry and customers.

Hybrid and Integrated Technology

Under the drive of multi-channel, high speed and low power consumption demand, the same volume optical transceivers need to have more data transmission, and the photonic integration technology gradually becomes a reality. Photonic integration technology has a broader meaning: for example, based on the integration of silicon-based (planar optical waveguide hybrid integration, silicon photonics, etc.), based on the integration of indium phosphide. The hybrid and integrated technology usually refers to the integration of different materials. There are also the construction of partially free space optics and partially integrated optics called hybrid integration. The typical hybrid integrated active optical devices (laser, detector, etc.) are integrated into the passive optical path connection or some other function (points or wave, etc.) of the substrate (planar optical waveguide and silicon light, etc.). Hybrid integrated technology of optical components can be done very compactly, complying with the trend of miniaturization of optical transceivers, easy to use mature IC encapsulation process automation. It is beneficial to mass production, which is an effective technical method for recent data center optical transceivers.

Flip Chip Technology

Flip chip is a high-density chip interconnection technology from IC packaging industry. In the rapid development of optical transceivers today, the interconnection between short – shrinking chips is a valid option. It is better to weld optical chip directly onto the substrate through gold-gold welding or eutectic welding, which is much better than the high frequency effect of gold wire bonding (short distance, small resistance, etc.). In addition to the laser, the heat generated by the laser is easily transferred from the solder to the substrate due to the proximity of the source area to the solder, which is helpful for improving the efficiency of the laser at high temperature. Because the backward welding is the mature technology of IC packaging industry, there are many kinds of commercial automatic reverse welding machines used in IC packaging. Optical components require optical path coupling, so the accuracy requirements are high. These years optical components processing with high precision inversion welding machine are very eye-catching and in many cases have realized the passive light, greatly improving the productivity. Due to the characteristics of high precision, high efficiency and high quality, the flip chip technology has become an important technology in the data center optical module industry.

Chip On Board Technology

COB (chip on board) technology also comes from the IC packaging industry, whose principle is through the rubber patch technology (epoxy die bonding) to fix chips or optical components on the PCB, and then gold wire bonding (wire bonding) uses electrical connection, and lastly drip glue sealing on the top. Obviously, this is a non-hermetic package. The advantage of this process is that it can be automated. For example, the optical components can be viewed as a „chip“ after it has been integrated by back loading and welding. Then the COB technology is used to fix it on the PCB. At present, COB technology has been widely adopted, especially in the use of VCSEL arrays in short distance data communication. The integrated silicon photonics can also be packaged by using COB technology.

Silicon Photonics Technology

Thh silicon photonics is a technology that discusses the technology and technique of optoelectronic devices and silicon-based integrated circuits, and a science integrated into on the same silicon substrate. Silicon photonics technology will eventually go to photoelectric integration (OEIC: Opto – Electric Integrated Circuits), making the current separated photoelectric conversion (optical transceivers) into local photoelectric conversion of photoelectric integration, further pushing the system integration. Silicon photonic technology can certainly do a lot of things, but for now it’s the silicon modulator. From the industry, the threshold of a new technology into the market must be the performance and cost is competitive and the need for huge upfront costs of silicon photonics technology is really a big challenge. The data center optical transceiver market, due to the large demand concentration within 2 kilometers, with the strong requirements of low cost, high speed and high density, is suitable for a large number of applications of silicon photonics.

In my opinion, the traditional 100G optical transceivers have been very successful, and they are not easy to get a lot of silicon photons. However, at the rate of 200G or 400G, since the traditional direct modulation type is close to the limit of bandwidth, the cost of EML is relatively high, which will be a good opportunity for the silicon photonics. The large number of applications of silicon photons also depends on the openness and acceptance of technology in the industry. If taking into account the characteristics of silicon photonics when setting the standards and agreements or relaxing some indicators (wavelength, extinction ratio, etc.) on the premise of meeting the transmission condition, they will greatly promote the development and application of silicon photonics.

On Board Optics

If OEIC is the ultimate photoelectric integration scheme, on board optics is a technology between OEIC and optical transceivers. On board optics moves the photoelectric conversion function from the panel to the motherboard processor or to the associated electrical chip. By saving space and increasing the density, it also reduces the distance of the high frequency signal, thus reducing the power consumption. On board optics is primarily focused on the short-range multimode fiber used in the VCSEL array, but recently there is a scheme for using silicon photonic technology in single-mode fiber. In addition to the composition of the simple photoelectric conversion function, there are also the forms (co-package) that encapsulate the photoelectric conversion function (I/O) and the associated electrical chip (processing). Although on board optics has the advantages of high density, the manufacturing, installation and maintenance costs are relatively high, and are currently used in the field of supercomputing. It is believed that with the development of technology and the need of the market, on board optics will gradually enter into the field of optical interconnection of data center.

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