How do optical modules transmit information?
Optical communication
Optical communication is currently one of the mainstream communication technologies. Using optical fibers as the transmission medium to transmit information through optical signals in optical fibers has the advantages of fast transmission speed, large bandwidth, anti-interference, and high security.
Optical module
The full name of the optical module is the optical transceiver integrated module, also known as the Transceiver in English. It is an important component of the optical communication system, responsible for the photoelectric signal conversion task between the data ports at both ends of the optical fiber, and ultimately realizes the remote transmission of information.
It is mainly packaged by optical transceiver (ROSA) components, functional circuits, and optoelectronic interface components. Among them, the optical transceiver component is the core part of the optical module: TOSA is responsible for processing the received electrical signal at a certain bit rate through an internal driver chip, and driving the laser to emit a modulated optical signal at the corresponding rate; ROSA is responsible for inputting a certain bit rate optical signal into the module, converting it into an electrical signal by an optical detection diode, and outputting the corresponding bit rate electrical signal through a preamplifier.
Application of optical modules
What is a mode?
In fiber optic transmission, we decompose the light transmitted along the fiber core into two types of plane wave components: axial and cross-sectional. The plane wave transmitted along the cross-section undergoes total reflection at the interface between the fiber core and the cladding. When the phase change of each reciprocating transmission is an integer multiple of 2 * Pi, a standing wave can be formed within the cross-section, and this set of standing wave rays is called “mode”.
Optical module mode classification
Optical modules are divided into single mode optical modules and multimode optical modules according to their modes, which are actually classified according to the type of fiber connected to the optical module. By classifying the modes of the optical module, we can better understand the application scenarios of the optical module.
Multi mode optical module: The multi mode optical module is represented by MM and is only suitable for multimode fiber. The main working wavelength is 850nm. It is commonly used for short distance transmission, such as in application scenarios with many network nodes and connectors such as data centers. The transmission distance is within 2km, and the multimode optical module can be used. Its light source is a light-emitting diode or laser.
Single mode optical module: Single mode optical module is represented by SM and can operate in multimode optical fibers with multiple adaptations, but the effect cannot be guaranteed. With 1310nm and 1550nm as the main working wavelengths, it is commonly used for long-distance transmission, such as urban area network construction, with transmission distances up to 150-200km. Its light source is a laser with better beam collimation or a light-emitting diode with narrower spectral lines.
Multi mode: suitable for short distance transmission in data centers, enterprise networks, etc;
Single mode: suitable for long-distance transmission such as metropolitan area networks and wide area networks.
Optical chip selection
① VCSEL: With a small emission angle, good beam collimation, and low cost, it is generally used in multimode light module schemes for short distance transmission around 100m;
② DFB: High wavelength accuracy, suitable for medium distance transmission of 500m~2km, with moderate cost;
③ EML: Always in a luminous state during operation, with good stability and relatively pure emitted light, making it more suitable for long-distance transmission between 10km and 80km, but with higher costs compared to DFB;
④ Narrow linewidth tunable: generally used for ultra long transmission distances greater than 80km.
Channel Designing
① Single channel: Depending on the transmission rate requirements, the currently mature maximum single channel rate can reach 100G, which means that rate requirements above 100G require the use of multi-channel design;
② Multi channel: Taking the current mainstream high-speed 400G optical module as an example, various multi-channel schemes such as 100G * 4, 50G * 8, 25G * 16 are required to combine multiple lasers and receivers to achieve higher transmission rates.
