Views: 699 Author: Anna Publish Time: 2026-03-03 Origin: Site
An optical module, as a crucial component of optical fiber communication, is an optoelectronic device that performs photoelectric conversion and electro-optical conversion during optical signal transmission.
Operating at the physical layer of the OSI model, an optical module is one of the core components of an optical fiber communication system. It mainly consists of an optical transmitter, an optical receiver, functional circuits, and an optical interface. Its primary function is to perform photoelectric conversion and electro-optical conversion in optical fiber communication.
The transmitting interface receives an electrical signal of a certain bit rate. After processing by the internal driver chip, it drives a semiconductor laser (LD) or light-emitting diode (LED) to emit a modulated optical signal at the corresponding rate. After transmission through the optical fiber, the receiving interface converts the optical signal back into an electrical signal by a photodetector diode, and then outputs an electrical signal of the corresponding bit rate after passing through a preamplifier.
Average Transmitted Optical Power :Average transmitted optical power refers to the optical power output by the light source at the transmitting end of the optical module under normal operating conditions; it can be understood as the intensity of light. The transmitted optical power is related to the proportion of "1"s in the transmitted data signal; the more "1"s, the greater the optical power. When the transmitter sends a pseudo-random sequence signal, "1"s and "0"s each account for approximately half. The power measured in this case is the average transmitted optical power, measured in W, mW, or dBm. W and mW are linear units, while dBm is a logarithmic unit. In communication, we typically use dBm to represent optical power.
Extinction Ratio: The extinction ratio is the minimum ratio of the average optical power of a laser emitting all "1"s to the average optical power emitting all "0"s under full modulation conditions, measured in dB. When converting an electrical signal to an optical signal, the laser in the optical module's transmitting section converts it according to the code rate of the input electrical signal. The average optical power with all "1"s represents the average power emitted by the laser, and the average optical power with all "0"s represents the average power when the laser is not emitting light. The extinction ratio characterizes the ability to distinguish between 0 and 1 signals; therefore, the extinction ratio can be considered a measure of laser operating efficiency. The typical minimum extinction ratio ranges from 8.2 dB to 10 dB.
Center Wavelength of Optical Signal: In the emission spectrum, the wavelength corresponding to the midpoint of the line segment connecting the 50% maximum amplitude values. Different types of lasers, or two lasers of the same type, will have different center wavelengths due to manufacturing processes, etc. Even the same laser may have different center wavelengths under different conditions. Generally, manufacturers of optical devices and modules provide users with a parameter, namely the center wavelength, such as 850 nm. This parameter is usually a range. Currently, the commonly used center wavelengths of optical modules are mainly three: 850 nm band, 1310 nm band, and 1550 nm band.
Overload Optical Power: Also known as saturation optical power, it refers to the maximum average input optical power that the receiver component can receive under a certain bit error rate (BER=10⁻¹²). The unit is dBm.
It is important to note that photodetectors can experience photocurrent saturation under strong light. When this occurs, the detector requires a recovery time, during which time the receiving sensitivity decreases, potentially leading to misinterpretation and bit errors. Simply put, exceeding this overload power limit can damage the equipment. Therefore, strong light exposure should be avoided during operation to prevent exceeding the overload power limit.
Receiver Sensitivity: Receiver sensitivity refers to the minimum average input optical power that the receiving component of an optical module can receive under a given bit error rate (BER = 10⁻¹²). If transmit optical power refers to the light intensity at the transmitting end, then receive sensitivity refers to the light intensity that the optical module can detect. The unit is dBm.
Generally, higher data rates result in lower receive sensitivity, meaning a higher minimum receive optical power, and thus higher requirements for the receiving components of the optical module.
Received Optical Power: Received optical power refers to the average range of optical power that the receiving component of an optical module can receive under a given bit error rate (BER = 10⁻¹²). The unit is dBm. The upper limit of the received optical power is the overload optical power, and the lower limit is the maximum value of the receiving sensitivity.
In summary, when the received optical power is less than the receiving sensitivity, it may be impossible to receive the signal normally because the optical power is too weak. When the received optical power is greater than the overload optical power, it may also be impossible to receive the signal normally because bit errors occur.
Interface Rate: The maximum electrical signal rate that an optical device can carry for error-free transmission. Ethernet standards specify the following rates: 125 Mbit/s, 1.25 Gbit/s, 10.3125 Gbit/s, and 41.25 Gbit/s.
Transmission Distance: The transmission distance of an optical module is mainly limited by two factors: loss and dispersion. Loss refers to the loss of optical energy during transmission in the optical fiber due to absorption, scattering, and leakage by the medium. This energy is dissipated at a certain rate as the transmission distance increases. Dispersion occurs primarily because electromagnetic waves of different wavelengths propagate at different speeds in the same medium. This causes different wavelength components of the optical signal to arrive at the receiver at different times due to the cumulative effect of transmission distance, resulting in pulse broadening and making the signal values indistinguishable.
Regarding dispersion limitation in optical modules, the limitation distance is much greater than the loss limitation distance and can be disregarded. Loss limitation can be estimated using the formula: Loss-limited distance = (Transmitted optical power - Receiver sensitivity) / Fiber attenuation. Fiber attenuation is strongly correlated with the actual fiber used.
Returning to our optical modules, data center interconnection scenarios require massive information exchange between data centers. This involves larger and denser information volumes and transmission frequencies, and the distances are much greater than in a single data center. In this context, the advantages of fiber optic communication become apparent.
Furthermore, data center interconnection scenarios demand higher switching speeds, lower power consumption, and smaller size for switching equipment. One of the core factors determining whether these performance characteristics can be achieved is the optical module.
The continuous growth of data traffic in data centers and the trend towards larger and flatter data centers are also gradually driving the development of optical modules towards faster transmission rates. The transmission distance of multimode fiber is limited by the increase in signal rate, so it will gradually be replaced by single-mode fiber in data center interconnection scenarios. Currently, China is gradually evolving from 100G-400G to 400-800G, and the application of single-mode fiber is likely to increase.
5G networks are divided into three parts: access network, bearer network, and core network. The bearer network is generally divided into metropolitan area access layer, metropolitan area aggregation layer, and metropolitan area core layer/provincial trunk line, providing fronthaul and midhaul/backhaul functions for 5G services. Compared to 4G, 5G base stations have undergone some changes, adding the midhaul link, thus opening up the market for midhaul optical modules. Furthermore, the optical modules used in the 4G fronthaul and backhaul links do not have such high transmission rate requirements, using 10G optical modules, while the high bandwidth, low latency, and large connection characteristics of 5G wireless communication place higher demands on the functionality and performance of optical modules. The future demand and requirements for 25G, 50G, and even 100G optical modules will see significant increases.
We've previously discussed the principles of wavelength division multiplexing (WDM). The core principle of passive WDM is to use WDM technology to couple optical signals of different wavelengths, each carrying a series of information, into a single beam for transmission within a single optical fiber, thus enabling service transmission. At the receiving end, these different wavelength optical signals can be separated. Currently, it's widely used in campus network deployments. While ensuring high-speed, low-latency transmission, it can significantly improve the utilization rate of backbone optical cables, and passive combiners can greatly reduce network operation and maintenance workload.
