With the rapid development of cloud computing, 5G, and AI applications, the demand for high-speed and energy-efficient optical transceivers is growing rapidly in data centers and backbone networks. QSFP28 is the main form factor for 100G optical modules. It features low power consumption, high port density, compact size, and cost efficiency.
This article reviews QSFP28 module types and key WDM technologies like CWDM and DWDM. It also covers major modulation formats( such as NRZ, PAM4, and coherent modulation) to provide a comprehensive understanding of high-speed optical module evolution and design choices.
To address the diverse requirements in transmission distance, fiber type, and cost across various applications, 100G optical modules have evolved into multiple packaging forms and internal designs. The following introduces several commonly deployed QSFP28 module types.
The QSFP28 standard defines four 100G interfaces: 100GBASE-SR4, 100GBASE-PSM4, 100GBASE-CWDM4, and 100GBASE-LR4. They include two parallel and two serial transmission types.
The 100GBASE-SR4 QSFP28 optical module is a parallel 100G module, as shown in Figure 1(a). QSFP28 uses four independent transmit and receive channels, employing a vertical-cavity surface-emitting laser (VCSEL) array and a 12-core multimode ribbon fiber. Each channel operates at a data rate of 25Gbps, resulting in an aggregate data rate of 100Gbps.
The QSFP28 PSM4 optical module is a high-speed, low-power product specifically designed for optical interconnects in data communication applications. It utilizes four independent edge-emitting single-mode lasers and an array of photodetectors, transmitting over a single-mode ribbon fiber. Each channel supports a data rate of 25Gbps, and the module is primarily used in 100Gbps Ethernet and similar networks.
The QSFP28 CWDM4 optical module implements dual-fiber four-wavelength transmission using Coarse Wavelength Division Multiplexing (CWDM) technology, multiplexing four wavelengths—1270nm, 1290nm, 1310nm, and 1330nm—onto a single-mode fiber for transmission. It features uncooled DFB laser chips and a photodiode (PD) array, typically using a thin-film filter (TFF)-based WDM device as the multiplexer/demultiplexer. The module has a non-hermetic structure and relatively low cost, as shown in Figure 1(c).
The 100GBASE-LR4 QSFP28 optical module is designed for long-distance transmission with a maximum reach of 10 km, as shown in Figure 1(d). It transmits four optical signals and multiplexes them into a single channel using a WDM device to achieve 100G optical transmission.
The module features cooled DFB lasers and a hermetic package, meeting industrial-grade requirements with an operating temperature range of -40°C to 85°C. It is typically used in demanding scenarios such as 5G midhaul and data center interconnects.
After a deep analysis of the architecture of typical optical modules, we further deconstruct their core technological essence from the wavelength dimension—wavelength division multiplexing (WDM) technology. Based on differences in wavelength spacing and system complexity, optical modules are mainly divided into two major technological paths: coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM).
Multi-wavelength optical modules mainly categorize into CWDM and DWDM. Both utilize wavelength division multiplexing technology to combine multiple optical signals of different wavelengths onto a single optical fiber, thereby increasing fiber capacity and transmission distance.
In terms of modulation techniques, commonly used methods include intensity modulation, frequency modulation, and phase modulation, as well as more advanced schemes such as PAM4 (4-level Pulse Amplitude Modulation).
CWDM modules typically use larger wavelength spacing, such as 20 nm, which allows support for more wavelength channels. However, the transmission distance is relatively short due to limited optical power budget and lack of amplification. CWDM is commonly used in metropolitan area networks (MANs) and other short- to medium-range applications.
DWDM modules use much smaller wavelength spacing, such as 0.4 nm or 0.8 nm, enabling support for a greater number of channels and much longer transmission distances. This high spectral efficiency allows DWDM systems to carry large volumes of data over hundreds or even thousands of kilometers. As a result, DWDM is widely used in long-haul backbone networks, data center interconnects, and telecom carrier infrastructures.
Unlike the continuous lasers used in laser processing, lasers used in optical communication must be modulated to generate modulated light, allowing data signals to be carried and transmitted. There are two primary types of laser modulation methods: direct modulation and external modulation.
Directly Modulated Lasers (DML) refer to lasers used in optical communication where the modulation is achieved by directly varying the driving current of the laser. The main advantage of this approach is its simplicity and compact design. This makes it well-suited for low-frequency or short-distance transmission applications.
However, at higher frequencies, variations in the modulation current can cause fluctuations in carrier density within the laser, which in turn alters the refractive index of the laser medium. This phenomenon is known as the chirp effect. Chirp causes spectral broadening of the optical pulse. Different frequencies travel at different speeds, causing pulse spreading and distortion.
Externally Modulated Lasers (EMLs) operate by directing continuous-wave laser light into an external modulator. The modulation signal then alters properties of the external modulator—such as its electro-optic effect or phase difference—to vary the output light parameters, such as intensity, in accordance with the input data.
Because the laser itself operates in a static, continuous-wave (DC) mode and is not directly modulated by high-frequency currents, chirp effects are significantly reduced. This leads to much better signal integrity and greatly enhances transmission performance, especially over long distances.
Currently, two types of external modulators are widely used in long-haul optical communication systems operating at 25Gbps and beyond:
High-speed optical modules are adopting a newer transmission method called Pulse Amplitude Modulation 4 (PAM4). Within a single modulation period, PAM4 uses four distinct signal levels. Each symbol carries two bits of logical information. In simple terms, during one signal cycle, the waveform can take on four voltage levels: +V, +V/2, –V/2, and –V. This allows PAM4 to achieve twice the data rate of traditional NRZ (Non-Return-to-Zero) encoding within the same bandwidth.
However, fundamentally, PAM4 is a hybrid of digital and analog modulation, which makes it more sensitive to noise and distortion. As a result, it places higher demands on optical signal-to-noise ratio (OSNR) and signal amplitude stability. To ensure reliable transmission, PAM4 systems typically require advanced signal processing techniques such as forward error correction (FEC) and equalization.
Additionally, manufacturers commonly use coherent optical communication for modules operating above 100Gbps. as shown in Figure 2(c). Coherent modulation employs a carrier wave and a modulating signal to alter the phase and frequency of the laser light, rather than its amplitude.
Since this method is not dependent on signal amplitude, it offers several key advantages: higher bandwidth, more stable transmission, greater receiver sensitivity, and longer transmission distances. These characteristics make coherent communication ideal for long-haul and ultra-high-speed optical networks.
With the continuous increase in optical module transmission rates—from 10G and 25G to 100G, 400G, and even 800G—the modulation formats have also evolved. Traditional NRZ (Non-Return-to-Zero) modulation has gradually given way to more advanced formats such as PAM4 and coherent modulation. These different modulation schemes exhibit significant differences in terms of spectral efficiency, system complexity, transmission distance, and bit error rate. Selecting an appropriate modulation technology is crucial for improving overall communication performance.
Firstly, NRZ is a binary modulation scheme where each symbol represents only one bit of information. It has advantages such as simple structure and good noise immunity, making it suitable for lower-speed or short-distance transmission scenarios. However, with the continuous increase in transmission rates, NRZ is no longer able to meet the demands for bandwidth efficiency.
In contrast, PAM4 (four-level pulse amplitude modulation) has become the mainstream modulation format for 100G and higher-speed optical modules in data centers. PAM4 carries 2 bits of information per symbol, enabling twice the data transmission rate under the same bandwidth conditions. However, PAM4 also introduces several challenges: the reduced spacing between signal levels decreases noise immunity and imposes higher requirements on the signal-to-noise ratio (SNR), while also leading to a higher bit error rate.
To maintain signal quality, more advanced DSP algorithms and forward error correction (FEC) techniques are required. Moreover, PAM4 demands higher linearity at the transmitter, greater sensitivity at the receiver, and increases overall module power consumption.
From the perspective of laser modulation methods, directly modulated lasers (DMLs) are widely used in 10G and some short-reach 25G modules due to their simple structure and low cost. However, at high frequencies, DMLs are prone to chirp effects caused by current changes. This spectral broadening can degrade signal quality in optical fibers.
To overcome the limitations of DMLs, externally modulated lasers (EMLs) were developed. EMLs separate the laser source from the modulator and use an external modulator to impose intensity or phase modulation on the optical signal. This approach significantly reduces chirp effects, thereby improving transmission distance and data rate.
Currently, EMLs are widely used in medium- to long-reach high-speed modules such as 100G LR4 and 400G LR8. Although they come with higher cost and greater manufacturing complexity, their superior performance makes them indispensable in high-end optical modules.
In long-distance communication scenarios of 400G and beyond, coherent optical communication is gradually becoming the mainstream. Unlike amplitude modulation used in PAM4, coherent modulation encodes information by controlling the phase, frequency, or even polarization state of the optical signal, rather than relying on optical intensity. This gives coherent systems stronger resistance to signal loss and chromatic dispersion over long distances.
Coherent communication typically incorporates a local oscillator for coherent detection, along with high-speed ADCs and DSPs to demodulate complex modulation formats such as QPSK and 16QAM.
Coherent technology has higher cost and power consumption than PAM4. However, it offers key advantages in long-haul, backbone, and DCI applications.
In summary, the choice between different modulation formats reflects a balance of multiple factors, including speed, distance, power consumption, and cost. NRZ is suitable for low-speed, short-distance transmission; PAM4 is ideal for high-speed, medium-distance applications; EML enhances the transmission quality of high-speed signals; while coherent modulation supports the core capabilities of future ultra-high-speed, long-distance communication.
Since 2017, 200G and 400G optical modules have gradually entered commercial deployment. These modules are typically developed based on the 100GBASE-LR4 QSFP28 standard by changing the modulation format and increasing the number of channels.
200G optical modules generally adopt a 4×50G configuration. Their structure is similar to that of 100G modules, using PAM4 modulation to achieve a per-channel data rate of 50G. On the other hand, 400G modules build upon the 200G architecture by adding four more channels, resulting in an 8×50G transmission rate.
With the explosive growth of 5G, AI, and cloud computing, optical modules are evolving from mere “connection tools” into the “arteries of computing power.” The form factor has evolved from QSFP28 to OSFP, modulation from NRZ to PAM4, and transmission distance from CWDM to coherent DWDM. All these advancements aim to meet the growing bandwidth demands while reducing power consumption and cost.
Looking ahead, silicon photonics integration and Co-Packaged Optics (CPO) technologies will reshape the industry landscape. At this pivotal moment, deepening expertise in packaging and modulation is essential. It is key to enabling next-generation optical communications.
