Researchers have made great progress in optical devices. The output power of laser, linewidth, stability and noise, as well as the bandwidth of photodetectors, power capacity and common mode rejection ratio have been greatly improved. Microwave electronic devices have also been greatly improved. Then, the coherent optical communication technology has gradually become an important capacity-lifting solution for the current 100G line-side.

Market Demand for Coherent Optical Communication

One of the biggest drivers of growth in the current communications market is the transition from 10G to 100G in the metro, core and Data Center Interconnect (DCI) sectors.

With the explosive growth of information generated by the use of communication technologies such as video conferencing and the spread of the Internet, the market has proposed higher transmission performance requirements for the physical layer that is the basis of the entire communication system.

In terms of digital communication, how to expand the capacity of C-band amplifiers, overcome the deterioration of fiber dispersion effects, and increase the capacity and range of free-space transmission have become important considerations for researchers; in analog communication, sensitivity and dynamic range are key parameters of systems.

Driven by strong demand, large-scale DWDM systems are gradually depleting their wavelength resources, and the efficiency of Time-Division Multiplexing (TDM) systems through compressed optical pulses also has a large technical bottleneck. People began to consider replacing the original Wavelength Division Multiplexing (WDM) system with a coherent optical communication system.

Advantages of Coherent Optical Modules

The coherent optical communication system modulates the signal to the optical carrier by adjusting the amplitude, phase and frequency by means of external light modulation (such as DP-QPSK) at the transmitting end.

Compared with the traditional direct detection system, coherent detection can obtain more signal information through the signal light and the beat frequency of the local oscillator; after the signal reaches the receiving end, it uses high-speed Digital Signal Processing (DSP) technology to perform front-end processing such as equalization. The optical mixer and the optical signal generated by the local oscillator are coherently mixed to realize signal reconstruction and distortion compensation.

Coherent optics can be used in both 100G and 400G applications, primarily because it enables service providers to send more data over existing fiber, reducing the cost and complexity of network upgrades for bandwidth expansion.

1. Coherent detection combined with DSP technology:
   • Cleared barriers to traditional coherent reception
   • Compensate for various transmission impairments in the electrical domain, simplifying transmission links
   • Make high-order modulation formats and polarization states possible
2. At the same time, the application of high-order modulation formats enables coherent optical communication to have higher single-wavelength channel spectrum utilization compared to traditional system systems.
   Coherent receivers have no special requirements for fiber channel, so coherent optical communication can use already laid fiber lines. With the aid of digital signal processing algorithms, coherent receivers compensate for signal distortion caused by fiber dispersion, polarization mode dispersion, and carrier phase noise at a very small cost.
3. A coherent receiver is about 20 dB more sensitive than a normal receiver, so the distance that is not relayed in the transmission system becomes longer, which reduces the number of amplifications in the transmitted light path.

Based on the above reasons, coherent optical communication can reduce the cost of optical fiber erection for long-distance transmission, simplify optical path amplification and compensation design, and become the main application technology of current long-distance transmission network.

Application Scenarios of Coherent Optical Modules

At present, the coherent optical communication is mainly used on the line side of the backbone network and the metropolitan area network, and belongs to the technical research field of DWDM long-distance transmission. In the application scenarios of the metropolitan area network and the core network with distance more than 80km, the coherent optical communication features good performance of Optical Signal-to-Noise Ratio (OSNR), sensitivity, dispersion tolerance and so on.

WDM System

The operating wavelength range is C-band (1530nm to 1565nm), and the fiber type is G.652D (prefered) or G.655. The key performance index is OSNR.

Error correction coding technology can jump out of the limitations of the physical layer of transmission, and compensate for all physical transmission impairments at the logic layer, especially the effects of nonlinear effects.

5G Middlehaul/Backhaul Network

In the 5G middlehaul scenario, 100G/200G DWDM system will be deployed, and the 100G CFP-DCO and 200G CFP2-DCO optical module can be used to implement the 80km scene application; the 400G DCO product is applied in the 5G backhaul scenario with distance less than 200km.

DCI

Whether the coherent communication will be used in the DCI field of 40km to 80km depends mainly depends on the commercial cost performance and whether the market capacity is large enough.

At the current 100G rate, products such as 100G ER with EML modulation are sufficient for the use; the 100G CFP-DCO ZR series will appear in the future.

The OIF organization is developing a 400ZR specification that uses a combination of DWDM and coherent technology.

Andrew Schmitt, principal analyst at Cignal AI, said: "Coherent 400G will limit the development of existing 200G and 100G technologies by 2020, and new devices will maximize optical capacity without relying on coverage." Foreseeable Yes, more and more 400ZR products will enter the market.

Summary

The coherent optical communication system is a more advanced and complex optical transmission system suitable for longer distance and larger capacity information transmission.

At present, coherent modules with the CFP form-factor are bulky and consume large power. Compact coherent modules will replace existing coherent products. The innovation of semiconductor technology and the improvement of chip technology will greatly promote the replacement of 400G coherent products.

In recent years, Gigalight, a global optical interconnect innovator, has increased its research and development of coherent modules and has achieved a series of achievements. In the next few years, it will strengthen cooperation with the industry and jointly promote the progress of related industries.

Source: Analysis of the Current Coherent Optical Module Market

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1. What Are the Benefits of Using 200G AOCs?

• To address the need for high-density, high-speed networking solutions
• To support a variety of next-generation of Ethernet applications
• The 200G AOCs are crucial to enable hyperscale data centers, service providers and enterprises to meet growing bandwidth demands
• Backward plug compatible with existing 100G-based systems and flexible port bandwidth for system design

2. What 200G AOCs Are Available on the Market?

Currently, there are only a few suppliers of 200G AOCs on the market, for example, the Gigalight company — a leader in the AOC industry. Gigalight offers a full range of 200G AOCs in both QSFP-DD and QSFP56 form-factors. The Gigalight 200G AOC product line includes 200G QSFP-DD AOC, 200G QSFP-DD to 2x 100G QSFP28 AOC, 200G QSFP-DD to 4x 50G QSFP28 AOC, 200G QSFP56 AOC, 200G QSFP56 to 2x 100G QSFP56 AOC, 200G QSFP56 to 4x 50G SFP56 AOC. All of these cable assemblies are compliant to IEEE standards and industry Multi-Source Agreements (MSAs).

Table 1: The Maximum Data Rates of Gigalight 200G AOCs

Note:

The QSFP-DD stands for a "Quad Small Form-factor Pluggable–Double Density". The QSFP-DD connector also has 8 electrical lanes similar to the QSFP form-factor, but the second row of electrical contacts has been added to the QSFP connector in order to increase the number of high-speed electrical lanes from 4 (in a QSFP) to 8 (in a QSFP-DD).

The QSFP56 stands for "Quad Small Form-factor Pluggable 56 ('56' refers to the max data rate 56Gb/s each electrical interface can handle)". The QSFP56 form-factor is a solution for 200G applications. It's a pluggable form-factor that has the same size as QSFP.

3. Can QSFP56 AOC Support QSFP-DD AOC Ports or Can QSFP-DD AOC Support QSFP56 AOC Ports?

Yes. The QSFP56 is a pluggable form-factor that has the same size as QSFP. The QSFP-DD assemblies are backward compatible with existing QSFP, so your 200G QSFP56 AOC can be used in the 200G QSFP-DD AOC system ports. Your 200G QSFP-DD AOC also can be used in the 200G QSFP56 AOC system ports.

4. What Is the Maximum Distance of 200G AOCs?

For Gigalight's 200G AOCs, a max length of 100m is supported for both QSFP-DD AOCs and QSFP56 AOCs using OM4 MMF.

Table 2: The Maximum Distances of Gigalight 200G AOCs

5. What Industry Standards Are Associated with 200G AOCs?

The QSFP-DD series AOCs are compliant with the QSFP-DD MSA while the QSFP56 series AOCs are compliant with the QSFP MSA.

6. What Is the Maximum Power Consumption of 200G AOCs?

The maximum power consumption of 200G QSFP-DD AOCs (based on NRZ) is 4w per end while that of 200G QSFP56 AOCs (based on PAM4) is 7w per end.

In general, higher power consumption levels are associated with higher data rates and longer reach.

7. Where Are 200G AOC Used?

The 200G AOCs are suitable for short distances and offer a cost-effective solution to connect within racks and across adjacent racks. While the 200G Direct Attach Cables (DAC) are a low-cost solution for 200GE high-speed interconnects within the reach up to 3 meters only, the 200G AOCs pick it up from there and can reach up to 100 meters. The 200G AOCs are widely used in High-Performance Computing (HPC) and recently became more popular in hyperscale data centers, enterprise and storage systems.

Originally article: 7 Frequently Asked Questions About 200G Active Optical Cables (AOCs)

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The Market Situation of AOC

Today’s hyperscale data centers and High-Performance Computing (HPC) markets require low-cost solutions for high-performance AOCs for the large-scale adoption of 200G and 400G data rates.

According to a recent report from LightCounting, the multi-mode AOC market will be experiencing significant growth over the next five years in the HPC and large-scale data center applications.

However, the market for 100G AOC is continually growing in the HPC and large-scale data center applications now, and it is still too early to adopt 400G AOC owing to cost and others so that the adoption of 200G AOC is expected to grow in the next years.

AOC and EOM to see revenue growth through 2023 (Source: LightCounting)

In the 200G AOC market, the 200G QSFP-DD AOC is a kind of parallel transceiver optics assembly. It will be the huge potential market and it is possible to replace copper technology in HPC and data center, the reasons include form factor, cost and so on. Next, we will explore them together.

Why Is 200G QSFP-DD AOC More Likely to Be Popular?

The 200G QSFP-DD AOC is a kind of 200G AOC that adopts the QSFP-DD form factor.

QSFP-DD is an eight-channel electrical interface with an additional row of contacts. It is being developed by the QSFP-DD MSA as a key part of the industry’s effort to enable high-speed solutions. The 200G QSFP-DD AOC meets the requirements of QSFP-DD MSA specification.

The QSFP-DD modules are similar to current QSFP. The systems designed with QSFP-DD modules can be backward compatible, allowing them to support existing QSFP modules and provide flexibility for end users and system designers. The 200G QSFP-DD AOC is convenient for end users and system designers.

The Introduction of Gigalight 200G QSFP-DD AOC

Gigalight is one of the rare providers for 200G QSFP-DD AOC. Its 200G QSFP-DD AOC is driving from its innovative optical packaging and the key manufacturing technologies enable scalability, reduced power consumption, increased reliability, and superior module performance for optical communications.

Gigalight 200G QSFP-DD AOC

Features of Gigalight 200G QSFP-DD AOC

• 8 channels full-duplex 850nm parallel active optical cable
• Transmission data rate up to 25.78Gbps per channel with integrated CDR
• Hot-pluggable QSFP-DD form-factor connectors
• Low power consumption < 4W per end
• Operating case temperature range 0°C to +70°C

The module block diagram of Gigalight 200G QSFP-DD AOC

Gigalight 200G QSFP-DD AOC adopts self-developed COB (Chip on Board) high-precision technology. The cost of the product is lower and the volume is smaller, which can provide a new generation solution with low cost, low power consumption, high density and high speed for the data center.

Originally article: An Overview of 200G QSFP-DD AOC

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According to data disclosed by Google, Facebook, etc., the internal traffic of these Internet giant data centers is increasing by nearly 100% every year. Currently, some Internet giants deploying 100G earlier have begun to seek higher-speed solutions, and the choice of next-generation data centers has become A topic that everyone is enthusiastic about.

The 400G Ethernet standard is preceded by the 200G Ethernet standard, which may reflect the industry’s mindset—more optimistic about 400G, or 200G is just a transition solution for 400G.

But directly from 100G to 400G is actually not very scientific.

1. First of all, from the data center side, we need to rebuild the ultra-large-scale data center and define a new specification architecture. The requirements for rack power in the 400G era switch will be quite high, and the traditional air-cooling heat dissipation is more difficult.
2. Furthermore, the 400G data center will use PAM4 technology, and the PAM4 technology will make the system less transparent and difficult to manage. The traditional NRZ technology together with the parallel technology can make the data center easy to manage.

In order to more flexibly adapt to the needs of the future data center and achieve a perfect transition to the 400G data center, Gigalight recently completed a low-cost data center internal parallel optical interconnection solution based on 200G NRZ transmission. This paper mainly compares 200G NRZ—Two parallel technologies in the solution, and two products as an example for simple analysis.

Fiber Parallel Solution—Is It Single- or Multi-Mode?

The traditional parallel optical module products are mainly based on optical interconnect technology of multimode fiber, and have the advantages of high bandwidth, low loss, no crosstalk and matching and electromagnetic compatibility problems. They have gradually replaced copper-based electrical interconnection products and are used in cabinets. High-speed interconnection between the boards, the connection distance is up to 300 meters under the OM3 fiber.

At the same time, in order to apply to longer-distance transmission solutions, Parallel Single-Mode (PSM) optical modules have emerged, mainly using FP lasers to transmit 2km in single-mode fiber and DFB to transmit 10km applications, which is more difficult than multi-mode interconnection technology.

Data center cabling is a very complicated problem. The choice of multimode fiber or single-mode fiber has been the subject of heated discussion in the industry. There are also choices in major data centers. For example, in the 100G era, Facebook chooses single mode, Google chooses both multimode and single mode. At the same time, BAT (Baidu, Alibaba, Tencent) chooses multimode. From the perspective of cost, multimode fiber is expensive and multimode optical module is cheap. Single mode fiber is cheap and single mode optical module is expensive. Therefore, it is easy to combine the cost of fiber and optical module to obtain the relationship between distance and cost. Taking the 100G solution as an example, the cost advantage of a multimode solution is very obvious when the fiber distance is within 100 meters.

The parallel technology route is characterized in that each pair of multimode fibers respectively carries one optical signal. At present, IEEE’s 400G SR16 standard is a 16x 25G parallel solution, which requires 16 pairs of multimode fiber. It is far more than the 12-core MPO widely used in the 100G era, which will lead to a significant increase in cost; more importantly, multimode optical modules rely on The low-cost VCSEL optical chip solution, 2020, is likely to still require more than 12-core MPO’s 8-pair multimode fiber. The 400G SR4 that the existing 12-pin MPO can accommodate seems to be in the foreseeable future.

Therefore, in 2020, if there is no open and standardized multi-mode wavelength multiplexing technology (such as SWDM technology), low-cost VCSEL 100G technology can not achieve breakthrough, 400G multi-mode fiber solution cost advantage will no longer be obvious, single-mode fiber It may become mainstream in large-scale data centers, and short- and medium-range single-mode parallel solutions will be a cost-effective alternative to multi-mode parallel solutions.

——Yang Zhihua, "Top Ten Hotspots of Data Center Network Technology in 2020"

200G PSM8 vs. 200G SR8

Based on Gigalight’s unique PSM series product line, Gigalight recently released a new product—200G QSFP-DD PSM8, a high-speed product of single-mode parallel technology.

To achieve long-distance transmission, single-mode fiber with low dispersion loss must be used. To achieve high coupling efficiency between single-mode fiber and semiconductor, it is necessary to shape the light field emitted by the semiconductor laser to maximize the incident light field and the intrinsic optical field of the fiber.

And the 200G QSFP-DD SR8 uses an 8-channel 850nm VCSEL array that complies with the 100GBASE-SR4 protocol standard. The 200G QSFP-DD SR8 is a multimode parallel product. With the traditional VCSEL advantage platform, Gigalight uses a simple, efficient and reliable fiber coupling process technology to add a 45° prism between the laser and the fiber. The special material treatment of the fiber surface increases the coupling efficiency of the fiber to over 80%.

The two products are similar in that they belong to the optical modules in the 200G data center solution, and all use the QSFP-DD package, which can use the 16-core MTP.

The advantage of QSFP-DD is that the 1U panel can achieve a density of 36x 200G/400G, and it is forward- and backward compatible with QSFP, and is compatible with existing QSFP28 optical modules and AOC/DAC.

The main difference is that the 200G QSFP-DD PSM8 adopts an 8-way 1310nm single-mode fiber parallel solution with a transmission distance of up to 10km. The 200G QSFP-DD SR8 adopts a multi-mode fiber parallel solution and can travel over the OM4 fiber link. Up to 100m.

Summary

The multi-mode parallel solution is the core of the current data center development, and the transmission distance between the switch and the core switch is just within the scope of the multi-mode fiber.

Corning has introduced OM5 fiber in the past few years, but it has not caused the expected market reaction. The SWDM short-range wavelength division multiplexing scheme is only promoted by a few manufacturers—it is indeed lacking in the market.

In the near future, if a general enterprise data center wants to continue to use standard-certified solutions and reduce the cost of optical components, you can choose multi-mode parallel optics—after all, SMBs do not need as large a capacity as 400G.

However, if it is in the construction and deployment process of a very large-scale data center, especially considering the scalability of the system and the flexibility of the system, we should probably consider the single-mode parallel solution.

In the eyes of some people of insight, the single-mode parallel solution increases the number of fiber cores, but overall reduces the maintenance complexity, is easier to manage, and is easier to upgrade from 100G to 400G later. Without increasing fiber resources, the current 100G CWDM4 based on wavelength division multiplexing can only evolve to 200G FR4, and 100G PSM4 can be upgraded to 400G DR4).

——Li Mofei, "Review of Data Center: Cost Technology is Concise and Reconfigurable"

In general, the technology roadmap for major switch and transceiver vendors shows a very clear and simple migration path for customers deploying parallel optics. So when optics are available and migrated from 100G to 200G or 400G, their fiber infrastructure still exists and no upgrades are required.

Reliability, product life and maintenance costs are all interrelated. The parallel single-mode solution represented by 200G QSFP-DD PSM8 in total cost should be the cabling guide for large-scale data centers in the future.

Originally article: Comparison of Two Parallel Technologies in 200G Optical Modules

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