Silicon Photonics and Lasers Technologies in 100G QSFP28 Transceivers

The 100G network architecture still occupies the market mainstream, and the related technology is also developing rapidly. Traditional laser technology applied in 100G QSFP28 is very popular in the market, while silicon photonics technology has been attracting attention over the years of exploration, and got some breakthroughs in the optical module field. This article will discuss silicon photonics and laser technology in QSFP28 100G together.

Silicon Photonics in 100G QSFP28 Modules

Silicon photonics is a breakthrough optical technology. It mainly uses silicon-on-insulator wafers as semiconductor substrate materials and applies CMOS manufacturing technology, reducing power while enhancing the transmission performance of optical modules. Currently, two types of silicon photonics modules are available on the market: those designed for short-distance transmission and those for long-distance transmission.

100G QSFP28 FR1 (SiPh) can replace 100G QSFP28 CWDM4 to achieve a 2km short-distance transmission. In addition, 100G QSFP28 DR1 (SiPh) has two short-distance application scenarios:

• Replacing 100G QSFP28 PSM4 500m, the cabling is optimized and cost-reduced from the MPO8-core jumper of PSM4 to the dual-core LC fiber.

• Replacing 100G QSFP28 CWDM4 500m (OCP) with unchanged fiber optic cable.

The 100G QSFP28 LR1 (SiPh) can directly replace the 100G QSFP28 LR4 to achieve 100G long-distance transmission of 10km and 20km.

Pros & Cons of Silicon Photonics in 100G QSFP28

Advantages of Silicon Photonics in 100G QSFP28

Today, the technology route of optical integration commercial products is mainly divided into InP and Si. Laser technology used for 100G QSFP28 transceivers, such as DFB, DML, EML lasers, etc., belongs to InP, and the technology is relatively mature. But these lasers are expensive, incompatible with complementary metal-oxide-semiconductor processes, and their substrate materials only double every 2.6 years. The Silicon photonics technology (SiPh) can be integrated on a large scale by passive optoelectronic devices and integrated circuits with the CMOS process. It is featured by its high density and the substrate material doubles every year.

Compared with traditional electronic circuits, photonic integrated circuits (PICs) made of silicon photonics consume less power and have low heat, and are used in 100G modules to reduce costs. Waveguides are made of silicon cores and are used to connect photonics devices in circuits. And, the silicon photonics PIC process will build additional waveguides with silicon nitride as the core material, enabling wavelengths of 100G QSFP28 transceivers to reach longer distance transmission.

Challenges of Silicon Photonics in 100G QSFP28

Although silicon photonics technology brings relatively good advantages to 100G QSFP28 transceivers, there are some challenges to achieving effective utilization.

Packaging: There are still some technical difficulties in the packaging process from silicon photonics chips to optical modules, and the packaging yield and cost still need to be optimized.

Laser: The approach to integrating photonics with silicon-based microelectronics should be achieved. Silicon is an indirect bandgap semiconductor, unable to emit light efficiently, thus independent light is required, which, however, does not comply with Moore's law. The more coupled integration, the higher the cost, which eventually offsets the cost advantages of silicon materials and process integration.

Power: Chips made of silicon photonics are more sensitive to temperature. Therefore, optical modules using silicon photonics technology need to continue to optimize power consumption control and structural characteristics to achieve better cost-effectiveness.

Lasers of 100G QSFP28 Transceivers

From the above, we understand the impact of silicon photonics technology on 100G transceivers and future 400G networks, but the application of other traditional laser technologies in transceivers relative to silicon photonics technology also occupies a certain market position. We can learn more about laser technology for QSFP28 100G transceivers here.

Typical Laser Types

Lasers are the core devices of optical transceivers, which inject current into semiconductor materials and inject laser light through the photon oscillations and gains in the resonator. The laser occupies 60% of the cost of the transceiver module, closely relating to the transmission distance of the transceiver. Typical laser types in the market include VCSEL, FP, DFB, DML, and EML. The table below shows their wavelengths, working patterns, and applications.

Lasers of 100G QSFP28 Transceivers

Here are three typical types of lasers commonly seen in 100G QSFP28 transceivers: VCSEL, EML, and DML.

• VCSEL laser features a small size, high coupling rate, low power consumption, and low price, thus favored by the 100G QSFP28 SR4 optical transceiver that is mainly deployed in the 100G multi-mode fiber network.

• The EML laser has less wavelength dispersion and a stable wavelength at high-speed operation. The frequency response of the EML depends on the capacitance of the EAM section and can achieve high operating speeds even beyond 40 GHz. It is often adopted by 100G QSFP28 ER4 and 100G QSFP28 LR4 transceivers which are designed for applications over single-mode fiber (SMF) with transmission distances of up to 10km.

• DML laser is mainly used for relatively lower speeds (≤25Gbps) and shorter reaches (2-10km) in telecom and datacom applications, due to constraints such as larger chromatic dispersion, lower frequency response, and relatively low extinction ratio, compared to EML. 100G QSFP28 CWDM4 optics for CWDM programs over 2km frequently adopt the DML laser.

DML and EML laser technologies are used in various transceivers, and the main difference between them is that DML laser uses a single chip with a simple circuit design whereas EML laser has an electro-absorption modulator (EAM) integrated into a single chip. Learn more about laser differences here: EML vs. DML: Essential Laser Technologies in 100G/200G/400G/800G Optics.

Exploring The Future of Laser Technology

In the current industry view, 100G silicon photonics technology has significant impact on parallel solutions, such as 100G PSM4 products. Silicon photonics has significant advantages in 400G DR4 applications with 500m distance transmission, but long distances require the use of EML modulation or coherent technology. According to the latest research from vantage market research in 2022, the market size of silicon photonics technology will continue to grow at a CAGR of 25.8% until around 2028.

On the other hand, Intel's silicon photonics module industry has entered a period of rapid development. From this trend, silicon photonics technology will surpass traditional optical modules in terms of its peak speed per second, energy consumption, and cost. It also will have a huge impact on the 400G transceiver market. Existing market 100G transceivers use silicon photonics technology to achieve more cost-effective transmission solutions and combined solutions with 400G interconnection.

Both the application of lasers and silicon chips in 100G QSFP28 transceivers is very important. But it is undeniable that silicon photonics technology has indeed set off a wave in the networking industry. While the development and deployment of optical technologies will continue to drive changes in data center networks, the challenges posed by these technologies remain to be solved.