In the rapidly developing field of high-speed data communication networks, optical modules play an indispensable and key role. As the connecting "bridge" between switches, routers and other devices, these small and easily replaceable modules are responsible for the conversion between electrical signals and optical signals. From connector types to external dimensions, optical modules show rich diversity. Before exploring its details in depth, mastering the basic knowledge of optical modules can help us better distinguish the differences between different optical modules.

1. Core Functions and Working Principles of Optical Modules

Optical module, also known as Optical Module in English, is the "heart" of the optical fiber communication system. Its main function is to realize the conversion of optical and electrical signals. In practical applications, it is usually inserted into the optical interface of switches, routers and other devices to convert the electrical signals output by the device into optical signals for long-distance transmission through optical fibers; or to restore the received optical signals into electrical signals for processing at the device end.

From the internal structure point of view, the optical module is composed of optoelectronic devices (including optical transmitters and optical receivers), functional circuits, and optical interfaces. Its working principle is based on the photoelectric conversion and electro-optical conversion process, as follows:

(1). Transmitter (TX: electrical signal → optical signal)

The input electrical signal is first processed by the driver chip, and then the laser (such as VCSEL, DFB, etc.) converts the electrical signal into an optical signal of a specific wavelength, and finally the optical signal is emitted through the optical fiber.

(2). Receiving end (RX: optical signal → electrical signal)

When the optical signal in the optical fiber enters the optical module, the photodetector (such as PIN, APD) converts the optical signal into a weak current, and then the transimpedance amplifier (TIA) amplifies the current signal and restores it to an electrical signal.

Since the operation of optical modules involves many factors, such as transmission rate, wavelength, transmission distance, etc., these factors directly affect the transmission quality and transmission distance of the optical fiber communication system, so the performance indicators of optical modules are crucial.

2. Analysis of the External Structure of Optical Modules

Taking the common SFP packaging products as an example, although there are many types of optical modules with different appearances, the basic structure generally includes the following parts:

• Dust cap: Like a "protective cover", it can effectively protect the optical interface of the optical fiber connector, optical fiber adapter and optical module to avoid contamination and damage from the external environment.

• Pull ring: It is convenient for users to plug and unplug optical modules . The pull rings of different bands have different colors for quick identification.

• EMI conductive spring: This is a unique part of the SFP packaged optical module, which can ensure a stable connection between the optical module and the optical port of the device .

• Label: The key parameters of the optical module and manufacturer information are clearly marked on it.

• SFP socket: responsible for the connection between the optical module and the single board, which can transmit signals and provide power for the optical module.

• Shell: protects the components inside the optical module.

• Receiving interface RX: used to receive optical signals transmitted from optical fiber.

• Transmitting interface TX: sends out the optical signal processed by the optical module.

3. Diverse Classification of Optical Modules

In the optical fiber communication system, choosing the right optical module plays a decisive role in the system performance and stability, and understanding the classification of optical modules is the basis for selection. Optical modules can be divided into many types according to different standards:

1). Package type: GBIC, X2, XENPAK, XFP, SFP, SFP+, SFP28, QSFP+ , QSFP28, CFP, CFP2, CFP4, QSFP56, QSFP-DD, OSFP, etc.

2). Transmission rate: 10Mbps, 100Mbps, 1.25Gbps, 10Gbps, 40Gbps, 100Gbps, 25Gbps, 200Gbps, 400Gbps, 800Gbps.

3). Wavelength: 850nm, 1310nm, 1490nm, 1550nm, WDM, etc.

4). Mode: Single mode, Multi mode.

5). Transmission distance: 100m, 300m, 550m, 10km, 20km, 40km, 80km, 120km, 160km, etc.

6). Modulation formats: NRZ, PAM4, DP-QPSK/n-QAM, etc.

7). WDM support: gray light module (does not support WDM), color light module (supports WDM).

8). Optical interface working mode: dual- fiber bidirectional (Duplex), single-fiber bidirectional ( BiDi ).

9). Laser types: VCSEL, FP, DFB, EML, DML, etc.

10). Photodetector types: PIN junction diode (PIN), avalanche photodiode (APD).

11). Module interface: LC, SC, MPO/MTP, etc.

12). Usage features: Support hot plugging.

13). Operating temperature: Commercial (0-70°C), Extended (-20-85°C), Industrial (-40-85°C).

Among these classification methods, the three most common ones are by package, by rate, and by transmission distance. Generally speaking, the higher the transmission rate of the optical module, the more complex its internal structure. Currently, most switches are suitable for packaging types such as QSFP-DD, QSFP28, QSFP+, SFP28, SFP/ eSFP , SFP+, CXP, CFP, etc.