Optical Module Wavelength and Transmission Distance

2023-07-20

Wavelength and transmission distance are critical parameters of optical modules. Different wavelengths correspond to different transmission distances. So what are the commonly used wavelengths, and how are they paired with transmission distances? Today, we'll examine this in detail.

The wavelength of an optical module refers to the light band used for optical signal transmission, with its unit being nanometers (nm). Common wavelengths include 850nm, 1310nm, and 1550nm. These three wavelengths feature relatively long waves with low attenuation, making them particularly suitable for fiber optic transmission. The transmission distance of optical modules can be categorized into three types: short reach, medium reach, and long reach. Generally, transmission distances of 2km and below are considered short reach, distances between 10-40km are medium reach, and distances above 60km are classified as long reach

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Multimode modules utilize either 850nm or 1310nm wavelengths (the attenuation of multimode 1310nm is approximately 0.35dB/km, which can only support Gigabit and lower rates), primarily used for short-distance transmission; multimode modules achieve a transmission distance of 550 meters at 850nm wavelength and 2km at 1310nm wavelength.

Single-mode modules utilize either 1310nm or 1550nm wavelengths for medium-to-long distance transmission; single-mode modules with 1310nm wavelength can achieve a maximum transmission distance of up to 60km (dependent on link attenuation - higher attenuation may result in shorter distances), where higher rates (155M, 1.25G, 10G, 40G, 100G, 200G, etc.) correspond to shorter transmission distances. Theoretically, 1550nm single-mode dual-fiber wavelength can reach up to 160km (limited to rates below 1.25G), as 1550nm exhibits an attenuation of approximately 0.19dB/km, thus enabling longer transmission distances compared to 1310nm.

Single-mode single-fiber optical modules achieve simultaneous bidirectional signal transmission by using different wavelengths for transmit (Tx) and receive (Rx). Conventional optical modules have two ports (one Tx port and one Rx port), whereas single-fiber BIDI modules integrate both functions into a single port through Wavelength Division Multiplexing (WDM) technology. For example:

Local port (End A) transmits at 1310nm and receives at 1550nm

Remote port (End B) transmits at 1550nm and receives at 1310nm

In simplified terms:

This port (End A): Tx1310/Rx1550nm

Opposite port (End B): Tx1550/Rx1310nm

The transmission distance of optical modules is inherently limited in practical applications, primarily due to optical signal attenuation and dispersion in fiber optic cables. Attenuation refers to the loss of optical energy caused by medium absorption, scattering, and leakage during transmission through the fiber. This energy dissipates at a certain rate as the transmission distance increases.