The Technologies of Next-Generation Optical Transceivers — PAM4 and 64QAM

PAM4 and 64QAM Technologies

PAM4 and 64QAM Technologies

The shift to cloud services and virtualized networks has put the data center in the middle of our world and meant that connectivity within data centers and between data centers has a huge impact on the delivery of business and personal services. Hyperscale data centers are being installed across the world and these all need connecting. To meet this demand, optical transceiver suppliers are delivering new solutions based on PAM4 and 64QAM, providing coherent modulation that will drive down the cost of connectivity and increase the bandwidth of each connection.

Connections to many servers are already 25G and links between switches in large data centers are already 100G. The introduction of SFP28 and QSFP28 transceivers integrating new technologies and built using efficient manufacturing techniques has driven down the cost of these connections and allowed massive growth in the market. The next stage is the introduction of 100G single lambda solutions and cost-effective 400G transceivers for links between switches. The PHY devices needed for this next step are already becoming available, 12.8T switch devices are in production, and the first 400G QSFP-DD and OSFP optical transceivers are sampling.


The rise of the hyperscale data center operator has dramatically changed the market. The switch to 25G and 100G from 10G and 40G has happened very quickly. The sheer scale and numbers of data centers being installed or upgraded means that the new technologies can be shipped in volume as soon as the price is right, the components have been qualified, and the production lines are operational. We are now seeing the first 400G PHY devices and optical transceivers for data centers becoming available and companies are vying for market position as we wait for the leading hyperscale operators to commit to large deployments.

Many of those companies that have benefited from 25G and 100G are putting their investments into single lambda PAM4 100G and 400G solutions for the data center. This has required new PAM4 PHY devices designed to meet the power constraints of 400G OSFP and QSFP-DD transceivers. A few companies have also invested in 50G and 200G PAM4 PHYs, enabling a cost-effective upgrade from 25G and 100G. 50G SFP56 and 200G QSFP56 transceivers are expected to be interim solutions, but it is unclear how widespread their use will be or for how long. 40G was an interim solution that lasted for many years.

Coherent technology, originally developed for 100G long-haul networks, is now widely used for long-haul connections, including subsea, metro networks, and Data Center Interconnect (DCI) between data centers. The market for DCI has grown rapidly, with many systems vendors offering solutions with 80km to 500km reach. For long-haul and metro applications, several leading equipment manufacturers continue to use in-house coherent Digital Signal Processor (DSP) designs. Coherent DSP solution is now available to optical transceiver vendors such as Gigalight that is going to ship 400G transceivers based on this design. The latest DSP ASICs are enabling 600G (64Gbaud 64QAM) solutions and CFP2-DCO transceivers. The next step is the introduction of the 7nm DSPs that will enable the cost-effective 400G ZR transceivers planned for 400G links up to 100km starting in 2020.

This continues to be a market in flux. Lumentum has completed the acquisition of Oclaro, Cisco has completed the acquisition of Luxtera, and several Chinese optical transceiver vendors have joined the charge to 400G in the data center. The PAM4 PHY devices required for 100G single lambda and 400G in the data center are proving to be very challenging to deliver. PAM4 PHY solutions in 28nm and 14/16nm technology have been sampling for more than six months and these are now being joined by 7nm solutions.

Related articles: PAM4 — The High-Speed Signal Interconnection Technology of Next-Generation Data Center

PAM4 — The High-Speed Signal Interconnection Technology of Next-Generation Data Center

What Is PAM4?

PAM4 (4-Level Pulse Amplitude Modulation) is one of PAM modulation technologies that uses 4 different signal levels for signal transmission. Each symbol period can represent 2 bits of logic information (0, 1, 2, 3), that is, four levels per unit time.

In the data center and short-distance optical fiber transmission, the modulation scheme of NRZ is still adopted, that is, the high and low signal levels are used to represent the (1, 0) information of the digital logic signal to be transmitted, and one bit of logical information can be transmitted per signal symbol period.

However, as the transmission rate evolves from 28Gb/s to a higher rate, the electrical signal transmission on the backplane will cause more severe loss to the high-frequency signal, and higher-order modulation can transmit more data in the same signal bandwidth. Therefore, the industry is increasingly calling for higher-order PAM4 modulation. The PAM4 signal uses four different signal levels for signal transmission, and each symbol period can represent 2 bits of logical information (0, 1, 2, 3). Since the PAM4 signal can transmit 2 bits of information per symbol period, to achieve the same signal transmission capability, the symbol rate of the PAM4 signal only needs to reach half of the NRZ signal, so the loss caused by the transmission channel is greatly reduced. With the development of future technologies, the possibility of using more levels of PAM8 or even PAM16 signals for information transmission is not ruled out.

NRZ vs. PAM4: The comparison of waveforms and eye diagrams between NRZ and PAM4 signals

And then, if the optical signal can also be transmitted by using the PAM4, the clock recovery and pre-emphasized PAM4 signal can be directly realized when the electro-optical transmitting is performed inside the optical module, therefore, the unnecessary step of converting the PAM4 signal into the NRZ signal of 2 times the baud rate and then performing related processing is eliminated, thereby saving the chip design cost.

Why PAM4?

The end-to-end transmission system includes fiber optic and fiber-optic transmission systems. Since the fiber transmission can easily reach the rate of 25Gbd so that the research progress of transmitting PAM4 on the fiber has been progressing slowly. For fiber-optic transmission systems, from NRZ moving to PAM4 is considered in terms of cost. If you do not need to consider the cost, there are other related modulation technologies can be used in the long-distance range, such as DP-QPSK, which can transmit the baud rate signal above 50Gbd for several thousand kilometers. However, in the data center field, the transmission distance is generally only 10km or less. If the optical transceiver using PAM4 technology is adopted, the cost can be greatly reduced.

For 400GE, the largest cost is expected to be optical components and related RF packages. PAM4 technology uses four different signal levels for signal transmission. It can transmit 2 bits of logic information per clock cycle and double the transmission bandwidth, thus effectively reducing transmission costs. For example, 50GE is based on a single 25G optical device, and the bandwidth is doubled through the electrical layer PAM4 technology, which effectively solves the problem of high cost while satisfying the bandwidth improvement. The 200GE/400GE adopts 4/8 channel 25G devices, and the bandwidth can be doubled by PAM4 technology.

For data center applications, reducing the application of the device can significantly reduce costs. The initial goal of adopting higher order modulation formats is to place more complex parts on the circuit side to reduce the optical performance requirements. The use of high-order modulation formats is an effective way to reduce the number of optics used, reduce the performance requirements of optics, and achieve a balance between performance, cost, power, and density in different applications.

In some application scenarios, high-order modulation formats have been used for several years on the line side. However, since the client side needs are different from the line side, so other considerations are needed.

For example, on the client side, the main consideration is the test cost, power consumption and density. On the line side, spectrum efficiency and performance are mainly considered, and cost reduction is not the most important consideration. By using linear components on the client side and the PAM4 modulation format that is directly detected, companies can greatly reduce test complexity and thus reduce costs. Among all high-order modulation formats, the lowest cost implementation is PAM4 modulation with a spectral efficiency of 2 bits/s/Hz.



As a popular signal transmission technology for high-speed signal interconnection in next-generation data centers, PAM4 signals are widely used for electrical and optical signal transmission on 200G/400G interfaces. Gigalight has a first-class R&D team in the industry and has overcome the signal integrity design challenges of PAM4 modulation. Gigalight’s 200G/400G PAM4 products include 200G QSFP56 SR4, 200G QSFP56 AOC, 200G QSFP56 FR4, 400G QSFP56-DD SR8, 400G QSFP56-DD AOC, etc.

All of the PAM4 products from Gigalight can be divided into digital PAM4 products and analog PAM4 products. The digital PAM4 products adopt DSP solutions which can support a variety of complex and efficient modulation schemes. The electric port has strong adaptability and good photoelectric performance. And the analog PAM4 products simulate CDR with low power consumption and low cost. Gigalight always adheres to the concept of innovation, innovative technology, and overcomes difficulties. It invests a lot of human resources and material resources in the research and development of next-generation data center products.

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50G PAM4-based Optical Transceiver Technologies

With the PAM4 encoding technology, the amount of information transmitted on 50G PAM4-based optical transceivers within each sampling cycle doubles. A 25G optical component can be used to achieve a 50Gbps transmission rate, reducing the costs of optical transceivers.

50G PAM4 applies to multiple scenarios, such as single-lane 50GE PAM4 optical transceivers, 4-lane 200GE optical transceivers, and 8-lane 400GE optical transceivers.


This section introduces the functions of a single-lane 50GE PAM4 optical transceiver.

Working principle of a 50GE PAM4 optical transceiver

Working principle of a 50GE PAM4 optical transceiver

The working principle of a 50GE PAM4 optical transceiver is described as follows:

    • In the transmit direction, the PAM4 encoding chip aggregates two 25Gbit/s NRZ signals into one 25GBaud PAM4 signal. The laser drive chip amplifies the PAM4 signal, and the 25Gbps laser converts the electrical signal into a 25GBaud(50Gbps) single-wavelength optical signal.
    • In the receive direction, the detector converts the 25GBaud single-wavelength optical signal into an electric signal. The electric signal is shaped and amplified, and then output to the PAM4 decoding chip. The PAM4 decoding chip converts the signal into two 25Gbps NRZ signals.

The 50GE PAM4 optical transceiver uses the QSFP28 encapsulation mode, LC optical interfaces, and single-mode optical fibers. The transmission distance is 10km or 40km, and the maximum power consumption is 4.5W.


The performance of transmitters and receivers on optical interfaces of 50GE PAM4 optical transceivers must comply with the IEEE 802.3bs and IEEE 802.3cd standards.

An optical transceiver provides N 25Gbps electrical interfaces. For a 50GE optical transceiver, the two electrical lanes transmit TX1/RX1 and TX2/RX2 signals specified in the SFF-8436_MSA standards. The performance of electrical interfaces must comply with the CEI-28G-VSR LAUI-2 standard.

The optical transceiver with a transmission rate of 50Gbps on a single wavelength supports 50GE, 200GE, and 400GE interfaces. The following table lists the parameters for the 50GE, 200GE, and 400GE technical solutions.

The parameters for the 50GE, 200GE, and 400GE technical solutions

Technical Solutions

Optical Component and Drive Chip

50G PAM4 optical transceivers use mature 25Gbps optoelectronic chips to deliver cost-effective solutions. In 50GBASE-LR 10 km scenarios, uncooled Direct Modulated Laser (DML) Transmitter Optical Sub-Assemblies (TOSAs) with TO packaging are used. Such a solution features mature technologies, low costs, low power consumption, and easy mass production. The linear DML driver chip can convert input PAM4 voltage electric signals into current signals that can directly drive lasers. Such chips deliver a high bandwidth and output large drive current. Their maximum working rate can reach 28GBaud. At the receive end, Receiver Optical Sub-Assemblies (ROSAs) with TO packaging are used. 25Gbps pins and linear Trans-Impedance Amplifier (TIA) chips are integrated to the ROSAs.

Optical components in 50GBASE-LR scenarios

Optical components in 50GBASE-LR scenarios

In 50GBASE-ER 40 km scenarios, 25Gbps Electro-absorption Modulated Laser (EML) TOSAs with BOX packaging are used. External cavity modulated Distribution Feed-Back (DFB) lasers, isolators, monitoring diodes, thermistors, and EML components are integrated to the TOSAs and driven by voltage signals. Such a solution features wide linear domains, high ER, high output optical power, and low TDECQ. Linear EML drive chips can amplify input PAM4 signals and output them to next EMLs. These chips provide a high bandwidth, a small jitter, an adjustable output gain, and a working rate up to 28GBaud. At the receive end, APD ROSAs with TO packaging are used. 25Gbps APDs and linear TIA chips are integrated into the ROSAs. Such ROSAs feature high sensitivity and apply to 40km long-distance transmission.

Optical components in 50GBASE-ER scenarios

Optical components in 50GBASE-ER scenarios

PAM4 Chip

PAM4 codec chips perform conversion between NRZ signals and PAM4 signals inside transceivers. In the transmit direction, PAM4 chips shape, amplify, and convert two 25Gbps NRZ signals output by boards into one 25GBaud PAM4 signal. In the receive direction, PAM4 chips use the Analog to Digital Converter (ADC) and Digital Signal Processing (DSP) technology to decode the one 25GBaud signal to two 25Gbps NRZ signals.

Differences Between Solutions of NRZ and PAM4 Transceivers

The optical components and chips of PAM4 transceivers are very different from those of NRZ transceivers. The following table lists the differences between 50G QSFP28 LR and 25G SFP28 LR.

The differences between 50G QSFP28 LR and 25G SFP28 LR

The main difference lies in laser drive chips, TIA chips, and data processing chips.

  • Since PAM4 code has four types of level logic, the laser drive chips and TIA chips are capable of linear outputs. NRZ transceivers output signals in amplitude limiting mode.
  • PAM4 transceivers use DSP to implement conversion between a 50G PAM4 signal and two 25Gbps NRZ signals. NRZ transceivers transmit data using Clock & Data Recovery (CDR) chips only.

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