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Exploring the Correlation Between Optical Module Wavelength and Transmission Distance

Posted on Dec 12, 2023 by
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FS 25G SFP28 Optical Modules

The wavelength and transmission distance are important parameters of optical modules, and the transmission distance varies with different wavelengths. So, what is the relationship between wavelength and transmission distance? Is wavelength a factor affecting the transmission distance of optical modules? This article delves into the correlation between optical module wavelength and transmission distance, shedding light on the complexities that impact the efficiency of data transmission.

Optical Module Wavelength Explained

The operating wavelength of an optical module is a range measured in nanometers (nm). Optical modules can be broadly categorized into two types based on the wavelength of light they utilize: gray optical modules and colored optical modules.

Gray Optical Modules

Gray optical modules typically operate in the range of 850nm to 1550nm. Common center wavelengths for gray optical modules include:

  • 850nm (with MMF): Low cost but short transmission distance. Can transmit up to 2 km at 100M rate, 550m at 1G rate, 300m at 10G rate, 400m at 40G rate, and 100m at 25G/100G/200G/400G rates.

  • 1310nm (with MMF): Maximum transmission distance is 2 km, such as 1000BASE-SX SFP.

  • 1310nm (commonly with SMF): High loss during transmission but low dispersion, generally used for transmissions within 40 km.

  • 1550nm (with SMF): Low loss during transmission but high dispersion, typically used for long-distance transmissions of 40 km and above, with a maximum direct transmission distance of 120 km.

Wavelength

Colored Optical Modules

Colored optical modules carry light of various center wavelengths and are divided into Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM) modules. CWDM module wavelengths range from 1270 to 1610nm, while DWDM module wavelengths are 1525 to 1565nm (C-band) or 1570 to 1610nm (L-band).

CWDM & DWDM

In the same wavelength band, DWDM optical modules have more types, making better use of wavelength resources. Lights with different center wavelengths in the same optical fiber can be transmitted without interference. Therefore, by using a passive combiner to combine lights with different center wavelengths from multiple color optical modules into one path for transmission, and at the far end, a demultiplexer separates the light into multiple paths based on different center wavelengths, effectively saving optical fiber lines. Therefore, colored optical modules are mainly used for long-distance transmission lines.

Understanding these wavelength variations is vital as they directly impact the transmission characteristics of optical modules, influencing factors such as attenuation, dispersion, and transmission distance.

Optical Module Transmission Distance Explained

The transmission distance of optical modules refers to the distance over which optical signals can be transmitted without the need for relay amplification. It is divided into short, medium, and long distances. Generally, distances of 2 km and below are considered short, 10 to 20 km are medium, and 30 km, 40 km, and above are long distances. Different wavelengths of optical modules paired with different types of optical fibers correspond to different transmission distances.

The transmission distance of optical modules is primarily constrained by two factors: signal loss and dispersion.

Signal Loss

As light travels through optical fibers, it experiences attenuation or signal loss. This loss is influenced by the inherent properties of the fiber, connector quality, and other environmental factors. Optical modules with shorter wavelengths often experience higher attenuation, limiting their effective transmission distance. Conversely, longer wavelengths exhibit lower attenuation, enabling data to traverse greater distances without significant signal degradation.

The transmission loss of light in different wavelength bands is different, with decreasing order 850nm > 1310nm > 1550nm. The smaller the loss, the longer the transmission distance of the optical module.

Dispersion

Dispersion refers to the spreading of light pulses as they travel through the optical fiber. Two common types of dispersion are chromatic dispersion and modal dispersion. Chromatic dispersion occurs due to the varying speeds of different colors of light, causing the pulse to spread over distance. Modal dispersion, on the other hand, arises from different propagation modes within multimode fibers. Both types of dispersion can impact the clarity and integrity of transmitted data.

Dispersion in an Optical Fiber

In general, single-mode transmission does not produce intermodal dispersion, while multimode transmission, supporting multiple transmission modes, causes light to refract multiple times, resulting in intermodal dispersion. The greater the dispersion, the shorter the transmission distance of the optical module.

Correlation Between Wavelength and Transmission Distance

Contrary to a direct and linear correlation between optical module wavelength and transmission distance, the relationship is more nuanced. The wavelength of light used by an optical module is not inherently tied to its transmission range. Instead, it is the unique transmission characteristics associated with different wavelengths that influence the range.

For instance, shorter wavelengths, such as those used in gray optical modules for short-range multimode applications (e.g., 850nm), may experience higher attenuation, limiting their effective transmission distance. This makes them suitable for local area networks (LANs) and shorter connections.

Conversely, longer wavelengths, typically in the range of 1310nm and 1550nm for gray optical modules and specific colored optical modules, exhibit lower attenuation. This characteristic allows them to cover longer distances, making them ideal for applications like wide-area networks (WANs) and long-distance connections.

The choice of wavelength, therefore, becomes a strategic decision based on the intended application and the desired transmission distance. Network planners must carefully consider the trade-offs between wavelength and distance to optimize the performance and reliability of their optical networks.

Key Takeaways

Understanding the correlation between optical module wavelength and transmission distance is essential for designing efficient and reliable optical networks. The wavelength of light used by an optical module is not a standalone determinant of its transmission range. Instead, it is the transmission characteristics associated with different wavelengths that influence the achievable distance. Network engineers and designers must carefully evaluate the specific requirements of their applications, considering factors such as attenuation and dispersion, to select the most suitable optical modules.

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