Regarding optical modules, just read this article

With the rapid development of artificial intelligence (AI) technology, AI data training and applications often involve massive data transmission and real-time interaction, and the demand for computing power and networks is showing explosive growth.

As the "courier" that transmits data between devices in the network, optical modules carry the sending and receiving of massive amounts of data for the "computing highway", and their importance is becoming increasingly prominent.

Today, I will show you how to understand optical modules.

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1. What is an optical module?

An optical module is a device that converts electrical signals into optical signals at the transmitting end, transmits them through optical fibers, and then converts the optical signals back into electrical signals at the receiving end.

Optical modules enable seamless connection and collaboration between various types of devices. For example, routers, switches, servers, and storage devices on the network all rely on the interconnection of optical modules, and their applications are very extensive.

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2. Composition of optical modules

An optical module is usually composed of an optical transmitting component, an optical receiving component, an optical interface, a base, a circuit board, and electrical interface gold fingers.

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3. Optical module speed

The optical module interface rate refers to the number of bits transmitted per second, and is measured in Mbps, Gbps, and Tbps.

Currently, the main transmission rates of optical modules are: 1Gbps, 10Gbps, 25Gbps, 40Gbps, 100Gbps, 200Gbps, 400Gbps, 800Gbps, etc.

4. Optical module packaging

Packaging can be simply understood as the appearance and interface form of the optical module.

Packaging standards are determined by standards organizations. These standards ensure compatibility and interoperability between optical modules manufactured by various manufacturers. The most widely used standards organizations in the optical module industry are the IEEE (Institute of Electrical and Electronics Engineers) and the Multi-Source Agreement (MSA). The MSA is a multi-vendor specification that complements the IEEE standard.

Common optical module packages currently regulated by standardization organizations include: GBIC, SFP, SFP+, SFP28, QSFP+, QSFP28, CFP, CFP2, CFP4, CFP8, QSFP-DD, OSFP, etc.

SFP

Small Form-Factor Pluggable, small package is hot-pluggable.

GBIC is the first optical module packaging standard defined by the MSA. The SFP optical module can be considered an upgraded version of the GBIC optical module. The SFP optical module is hot-swappable and half the size of the GBIC optical module. It supports both 100Mbps and 100Mbps speeds.

SFP+

Small Form-factor Pluggable Plus, enhanced small package is hot-pluggable.

SFP+ and SFP optical modules have the same appearance and size. The difference is that SFP+ optical modules consume less power and have higher speed. SFP+ supports 10 Gigabit speed.

SFP28

Small Form-factor Pluggable 28, small package hot-pluggable 28.

SFP28 is an upgraded version of SFP+. With the same appearance and size as SFP+, SFP28 optical module can support single-channel 25Gbps rate.

QSFP+

Quad Small Form-factor Pluggable Plus, four-channel enhanced small package is hot-swappable.

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The QSFP+ optical module supports 4-channel transmission at the same time. A single channel can support a rate of 10Gbps, and a rate of 40Gbps can be achieved through 4-channel transmission.

QSFP28

Quad Small Form-factor Pluggable 28, quad-channel small package hot-swappable 28.

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The QSFP28 optical module supports 4-channel transmission at the same time. A single channel can support a rate of 25Gbps to 40Gbps, and a rate of over 100Gbps can be achieved through 4-channel transmission.

QSFP28 and QSFP+ have the same form factor but different speeds.

CFP/CFP2/CFP4/CFP8

C Form-factor Pluggable, C type pluggable.

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The CFP optical module has a transmission rate range of 100Gbps to 400Gbps. CFP is designed based on SFP, but has a larger form factor than SFP.

• A single channel of CFP can support a rate of 10Gbps, and through 4×10Gbps and 10×10Gbps, it can achieve rates of 40Gbps and 100Gbps.
• The size of CFP2 is half that of CFP, and it can achieve 100Gbps and 200Gbps rates through 4×25Gbps and 8×25Gbps.
• The size of CFP4 is one-fourth of that of CFP, and it can achieve 40Gbps and 100Gbps rates through 4×10Gbps and 10×10Gbps.
• CFP8 is a 400G package type that achieves a 400Gbps rate through 16×25Gbps and 8×50Gbps.

QSFP-DD

Quad Small Form-factor Pluggable-Double Density, double-density quad-channel small pluggable package.

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The QSFP-DD package is compatible with QSFP packages such as QSFP+/QSFP28. It adds a row of channels to the 4 channels of QSFP, supporting 8-channel transmission at the same time. The single-channel rate can reach 25Gbps, 50Gbps, and 100Gbps. Therefore, the QSFP-DD optical module can support 200Gbps, 400Gbps, or 800Gbps.

OSFP

Octal Small Form-factor Pluggable, eight-channel small package hot pluggable.

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The OSFP optical module has 8 high-speed electrical channels, each with a rate of up to 100Gbps. The total bandwidth can support rates of 200Gbps, 400Gbps, 800Gbps and 1.6Tbps. Its size is slightly larger than QSFP-DD.

As the performance and transmission bandwidth of optical modules gradually improve, the packaging method of optical modules is also continuously evolving, developing in the direction of higher transmission rate, smaller size, lower power consumption and higher density.

5. Transmission distance of optical module

The transmission distance of optical modules is divided into three types: short distance, medium distance, and long distance. Generally speaking, 2 km or less is considered short distance, while 30 km or more is considered long distance.

In actual use, the transmission distance of the optical module is limited, mainly because the optical signal will have certain loss and dispersion when transmitted in the optical fiber.

• Loss refers to the gradual decrease in optical signal intensity as it travels through an optical fiber. It's expressed in dB/km. The main sources of optical signal loss include fiber material absorption loss, scattering loss, bending loss, and connector/splice loss. Generally, single-mode fiber has lower loss than multimode fiber.
• Chromatic dispersion refers to the phenomenon in which optical signals of different frequencies or modes propagate at different speeds in optical fibers, causing pulse broadening and signal distortion. It is expressed in ps/(nm·km). Chromatic dispersion causes overlapping of adjacent pulses, increasing the bit error rate and limiting the maximum transmission rate and unrelayed transmission distance of optical fibers.

6. Transmission mode of optical module

Optical fibers can be categorized as single-mode fiber (SMF) or multimode fiber (MMF) based on the transmission mode of optical signals. To accommodate different types of optical fibers, optical modules are also categorized as single-mode and multimode.

• Single-mode optical modules are used with single-mode optical fibers. Single-mode optical fibers have a thin core and use a single mode of light to transmit signals. This results in minimal dispersion and high transmission capacity, making them commonly used for long-distance transmission.
• Multimode optical modules are used with multimode optical fibers. Multimode optical fibers have a thicker core and use multiple light modes to transmit signals. This results in greater dispersion during transmission, resulting in lower transmission performance than single-mode optical fibers. However, they offer lower costs and are suitable for smaller capacity and shorter distance transmissions.

7. Central wavelength of the optical module

The center wavelength refers to the optical wavelength band used for optical signal transmission, measured in nanometers (nm). The longer the center wavelength, the less loss the optical signal experiences in the optical fiber, and the longer the transmission distance.

There are three main central wavelengths of commonly used optical modules: 850 nm, 1310 nm, and 1550 nm.

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In addition to the three bands mentioned above, there are also CWDM and DWDM bands used in wavelength division systems.

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Whether using CWDM or DWDM, these wavelengths appear on the equipment as "colored" optical modules. These modules transmit data using different colors of light (i.e., different wavelengths), with each color representing an independent data channel. This enables the transmission of multiple wavelengths on a single optical fiber. This technology significantly improves the transmission capacity and efficiency of optical fibers.

In contrast, the 850 nm, 1310 nm, and 1550 nm bands are also known as "gray light" or "black and white light" due to their relatively single central wavelength. Unlike color optical modules, gray light modules do not utilize complex wavelength division multiplexing technology, instead focusing on providing stable and reliable single-wavelength transmission. This makes them more suitable for short-distance, low-cost network connections, such as IP network scenarios.

8. Optical power of optical module

Optical power of an optical module is one of the core parameters for measuring its performance. It includes indicators such as transmitted optical power, received optical power, overload optical power, and receiving sensitivity, which directly affects the stability and transmission quality of the optical fiber communication system.

• Transmitted optical power: refers to the light intensity emitted by the transmitting light source of the optical module, measured in dBm. Transmitted optical power must remain stable to ensure signal transmission quality.
• Received optical power: indicates the average optical power range that the receiver can detect. Its lower limit is the maximum receiver sensitivity, and its upper limit is the overload optical power. The unit is dBm.
• Overload optical power, also known as saturation optical power, refers to the maximum input optical power that the receiving end of an optical module can withstand. It is measured in dBm. When the received optical power exceeds the overload optical power, bit errors may occur, or even equipment damage may occur.
• Receive sensitivity: refers to the minimum optical power that an optical module can receive while meeting a certain bit error rate (BER). The unit is dBm.

9. Interface type of optical module

Refers to the physical connector type used when connecting an optical module to an optical fiber. Common connector types include SC, LC, and MPO.

• SC (Square Connector): A standard square fiber optic connector with excellent stability and durability. The SC interface was originally designed for user-end equipment in access networks, but is now widely used in various network environments.

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• LC (Little Connector): A miniaturized fiber optic connector with a compact size and high precision. The LC interface can be used with single-mode or multimode fiber and is widely used in high-density cabling environments such as data centers and telecommunications networks.

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• MPO (Multi-fiber Push On): A multi-fiber connector that can connect multiple fibers simultaneously. MPO interfaces are commonly used for parallel data transmission in high-density cabling environments, such as server interconnections within data centers or high-speed links between switches.

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10. Naming of optical modules

Different manufacturers have their own naming conventions for optical modules. Organizations like IEEE and MSA also provide standards for optical module naming. For example, the naming convention defined by IEEE 802.3 for 100G optical modules is shown in the figure below.

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The following table lists the parameter values for 100G optical modules.

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For example, a 100G optical module is named 100GBASE-LR4, which means that the optical module has a rate of 100 Gbps, supports a transmission distance of 10 km, and supports four channels.

In actual use, some manufacturers also include the package type in the name of the optical module, such as QSFP28-100G-S40K. QSFP28 indicates the package type is QSFP28, 100G indicates the interface rate of the optical module, S indicates single mode, and 40K indicates the supported transmission distance is 40 km.

In short, the naming of optical modules varies from manufacturer to manufacturer, but the naming rules usually include information such as package type, transmission rate, fiber type, transmission distance, and operating wavelength.

11. New technologies for optical modules

The core of optical module manufacturing lies in packaging technology. Currently, COB (Chip on Board) is the mainstream packaging solution for high-speed optical modules.

COB significantly improves the integration by directly mounting bare chips (optical chips and electrical chips) on the PCB substrate and using wire bonding to achieve electrical connection. It also has the advantages of small size, good heat dissipation and low cost, and is widely used in high-speed modules such as 400G and 800G.

To meet the network's demand for higher bandwidth and lower power consumption, optical module technology is evolving along a clear path, with the main directions including the following three key technologies.

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END

Currently, with the acceleration of computing infrastructure construction brought about by AI, the demand for high-speed optical modules used for data center optical interconnection has increased significantly. 400G optical modules have been widely used, 800G optical modules have been commercialized on a large scale, and 1.6T optical modules have entered the mass production stage.

In the future, driven by AI and computing power networks, optical modules will inevitably accelerate their development towards the stage of "higher speed, lower power consumption, smaller size, and more intelligent integration". CPO and silicon photonics technology may become the core engines for the future development of optical modules.