Application of Coherent Optical Technology in Data Centers

The research on coherent optical technology originated in the 1980s. Compared with the traditional IM-DD system (intensity modulation-direct detection), coherent optical communication has the advantages of high sensitivity, long relay distance, good selectivity, large communication capacity, and flexible modulation mode. In the Internet data center, the technical focus is increasingly shifting from DCN to DCI development, and the implementation of the national “channel computing resources from the east to the west” strategy also means that the long-distance interconnection network of data centers is more important. Therefore, coherent optical technology is a key link in this process.

Modulation Technology

The process of optical communication is actually the modulation and demodulation of signals. In order to give you a clearer understanding of coherent optical communication, we introduce two phase-related modulation methods:

PSK Modulation

PSK, also known as “phase shift keying”, transmits different digital signal streams by changing the phase value of the carrier. PSK modulation is widely used in optical communication.

According to the phase relationship of two different carriers, PSK is divided into BPSK (inverse phase) and QPSK (quadrature), which can represent 1bit and 2bit data respectively for one symbol.

QAM Modulation

In addition to the above modulation methods, QAM (quadrature amplitude) modulation is also often used in optical communication, that is, using both the phase and amplitude of the carrier to transmit data. There are m points in the quadrant, which corresponds to mQAM modulation, where m=2ⁿ, which means that in mQAM modulation, one carrier symbol transmits n bit data, which is also the concept of constellation diagram often mentioned.

Among these modulation methods, actual business scenarios often add some other technologies to increase the carrying capacity of a single channel, reduce the signal baud rate, etc. For example, the common PDM (polarization multiplexing) technology divides an optical signal into two polarization directions for modulation separately, transmitting 2 times the data. Both PSK modulation and QAM modulation use the phase of the carrier to convey information, and coherent demodulation is required at the receiving end.

Coherent Demodulation

Coherence is a phenomenon in optics: strong places are always strong, weak places are always weak, and coherent light refers to light waves that have the same frequency as the light source (here taking zero difference detection as an example), constant phase difference, and the same direction of particle vibration at the superposition point. The general process of coherent optical communication is as follows:

The baseband signal is modulated at the transmitter, and after transmission through the fiber, it is coherently demodulated at the receiver, and finally, the original electrical signal is obtained at the receiver. There are many key devices in this process, such as the digital signal processor (DSP), which plays a huge role, and we will also introduce it later. The signal changes in the whole process are as follows:

Through the above information, you must have a basic understanding of coherent optical communication. The birth of coherent transmission has changed the development of optical transmission networks. The electronic digital signal processor (DSP) introduced by it has become the key driving factor for increasing the capacity of metropolitan and long-distance WDM networks. Coherent optical technology can be said to be the basis of long-distance and large-capacity optical transmission.

400G ZR

Coherent optical technology is not a new technology, and it has experienced a long period of technological accumulation. The earliest coherent optical transceiver system was integrated into the communication equipment line card, but with the further maturity of the technology, the ability to control precision devices, and the increasing demand for optical communication bandwidth, the research on pluggable coherent optical modules gradually became on the agenda. This is especially true in the Internet industry. Based on the same equipment system, pluggable optical modules can meet different business needs. It can be said that pluggable optical modules have always been an important part of the development of Internet data centers. Pluggable coherent optical modules have been scaled up at 100G/200G rates, but they really ushered in a booming development at 400G rates.

OIF (Optical Internetworking Forum) launched the 400G ZR DCO industry standard for metropolitan area network interconnection scenarios, and more and more equipment manufacturers and optical module manufacturers began to adopt the standard and achieve heterogeneous interconnection and interoperability.

The OIF 400G ZR specification adopts a solution that combines dense wavelength division multiplexing (DWDM) and DP-16QAM, which can transmit 400G on a data center interconnect link of 80~120km (pure bare fiber to 40km, optical amplifier can reach 120km). In this standard, there are three applicable MSA packaging standards, namely: QSFP-DD, OSFP, and CFP2. In the Internet data center, the most commonly used is the QSFP-DD packaging standard. It should be noted that the OIF 400G ZR defines the DCO (digital coherent optical) module, and before that, there was also the ACO (analog coherent optical) module. The main differences between the two are as follows:

As can be seen from the figure, the core difference between the DCO module and the ACO module is that the DCO integrates the DSP chip directly on the optical device, and uses digital communication between the module and the host system. The advantage of this is that it can achieve communication among heterogeneous switch/router vendors.

Digital Signal Processor (DSP)

The DSP chip, as a part of the DCO module, is of paramount importance. How was the DSP born? In simple terms, optical signals are easily distorted when transmitted over long distances, making it difficult for the receiver to accurately restore the data. However, digital signals are easier to process than optical signals and can counteract and compensate for distortion, thereby reducing the impact of distortion on the system bit error rate. It can be said that the emergence of DSP opened the digital era of optical communication, and DSP is an important support for coherent optical communication. Let’s take a look at the role of DSP in the DCO module through a figure:

As shown in the figure, the functional modules with brown-red backgrounds are all carried by the DSP chip. We summarize some of the core functions of the DSP:

1. IQ orthogonal: compensate for the IQ non-orthogonal caused by the modulator, mixer
2. Clock recovery: compensate for sampling errors
3. Dispersion compensation
4. Polarization equalization: compensate for polarization-related impairment, polarization demultiplexing
5. Frequency estimation: carrier frequency offset estimation and compensation between transmitter and receiver
6. Phase estimation: carrier phase noise estimation and compensation
7. Decision output: soft/hard decision, channel decoding, source decoding, bit error rate estimation

Because DSP carries too many functions, the initial DSP also faced problems such as large size and high power consumption. Therefore, the technological progress around the DSP chip is also constantly explored: l

• At the current stage, most DSPs are 7nm process, and the main packaging forms of DCO modules are QSFP-DD, OSFP, and CFP2, with a rate of 400G/200G l
• In the 2022-2025 stage, 5nm process DSPs will be launched, and the target rate will be 1.6T/800G

In the Internet industry, 400G DCO will be a typical scenario for the large-scale application of coherent optics. As a leader in the 400G field, H3C will naturally actively promote the construction of coherent optics. In fact, as soon as 400G ZR was born, H3C conducted joint tests with the industry’s top DCO manufacturers and launched the IPoverDWDM solution:

This solution directly inserts a 400G ZR/OpenZR+ QSFP-DD coherent optical module into the H3C switch 12500R and realizes the IPoverDWDM transmission of DCI through the optical layer transmission. The launch of this solution helps to reduce the network complexity of data center interconnection, increase the reliability of the transmission system, achieve large-capacity transmission, and reduce system power consumption and cost.

DCI Solution Application Case

Company:      Distributor        

Location:       France

Item Year:       May 2022

Application:     Data Center

Background: The customer in this case is a distributor. They were helping their client to expand the existing network of two adjacent 80km data centers in France, and only a few unused wavelengths could be utilized. The end-user service was then 100G and 10G, and there will be multiple 10G, 100G, and 400G hybrid transmissions in the future.

The picture is kindly provided by the customer from the application site

Solution: FiberMall’s R&D team uniquely developed the 2Q28-10SFP-200G 200G Muxponder, which can support the conversion of 1*100G+10*10G (or 2*100G) to 1*200G CFP2-DCO, matching the FM-3200 DCI-8 2U platform with 8 services slots to best utilize the limited wavelengths to meet customer needs and future expansion plan perfectly.

Company:      System Integrator        

Location:       Netherlands

Item Year:       June 2022

Application:     Cloud Computing

Background: The customer in this case is a system integrator specializing in various Internet solutions for local users in Europe. They were looking for a highly integrated and relatively low-cost OTN solution for 350 km of fiber that had just been leased by a cloud computing company in the Netherlands and the optical layer has been deployed. The end-user service at the time was a 4x100GE service, with more 100G/400G expansion to come.

The picture is kindly provided by the customer from the application site

Solution: FiberMall’s technical team fully considered the customer’s demands, the electrical layer using 4Q28-CFP2-400G 400G Muxponder which supports 1*400G or 4*100G at client-side conversion in FM-3200 DCI-8 2U platform to minimize equipment space, deploy on demand and save further investment.

Company: Reseller       

Location: Vietnam

Item Year: May 2023

Application: ISP

Solution: FiberMall’s R&D team configured the 2 pcs 4Q28-2CFP2-200G 200G Muxponder, which can support conversion of 4*100G to 2*200G CFP2-DCO, matching the FM-1600 DCI-4 1U platform with 1x optical line protection card and 1x slot for expansion, traditional FM-3200 II 2U chassis is equipped with dual MuxDemux and optical amplifier for better protection.

Optical channel protection: Two 200G services were divided into two identical services through the OLP module, respectively into the main and backup routing of the Mux and demux, transmitted to the opposite end. It can make the two services of the main and backup routing on the same line; can also make the two services of the main and backup on a different routing.

The picture is kindly provided by the customer from the application site

Schematic Diagram

Company: Distributor        

Location: USA

Item Year: July 2023

Application: ISP

Background: The customer in this case is a distributor and the final user already built the optical layer, and needs to expand the new 16x100G service on the original network.

Solution: FiberMall’s R&D team uniquely developed the 4Q28-CFP2-400G 400G Muxponder, which can support the conversion of 4*100G (or 1*400G) to 1*400G CFP2-DCO, matching the FM-1600 DCI-4 1U platform with 4 services slots to maximize the use of chassis space and save customer costs.

The picture is kindly provided by the factory