How much dB is lost per kilometer of optical cable?

　　The loss value (dB) of one kilometer of optical cable is affected by factors such as fiber type, operating wavelength, and manufacturing process, as follows:

　　First, single-mode fiber loss

　　typical value

　　1310Nm wavelength: loss of about 0.35dB/km (typical value specified by the International Telecommunication Union ≤ 0.35dB/km).

　　1550Nm wavelength: loss of about 0.25dB/km (modern technology can approach the theoretical limit of 0.15dB/km).

　　Project case: A 10km single-mode fiber link (including 2 connectors and 1 fusion head), with a total loss of 6.8dB, converted to 0.68dB per kilometer (including joint loss).

　　low loss scenario

　　The measured cross-sea optical cable: the average loss of the 40-kilometer relay section is 0.198dB/km (the peak loss of the local micro-bending point is 0.35dB/km).

　　Advanced process: By optimizing the deposition process, the attenuation can be reduced to less than 0.2 dB/km.

　　Second, multimode fiber loss

　　1310Nm wavelength: loss of about 0.36dB/km (short tranSMission distance, inter-mode dispersion limiting performance).

　　Applicable scenarios: It is mostly used for short-distance transmission (e.g. within a data center), and requires relay equipment over long distances.

　　Third, the relationship between loss and operating wavelength

　　Overall trend: Loss decreases with increasing wavelength, but there are exceptions.

　　850Nm: loss of about 2.5dB/km (short wavelength, strong absorption).

　　1310Nm: loss of about 0.35dB/km (common window, balance attenuation and dispersion).

　　1550Nm: loss of about 0.2dB/km (lowest loss window, suitable for long-distance transmission).

　　Special range: 900~ 1300nm and 1340~ 1520nm due to hydroxyl absorption, high loss.

　　IV. Other factors affecting wear and tear

　　Fiber quality

　　Material purity: Impurities (e.g. transition metal ions) can cause increased absorption loss.

　　Manufacturing process: preform refractive index fluctuation, drawing temperature control effect attenuation.

　　Environmental factors

　　Bending loss: When the bending radius is less than 30mm, the 1550nm wavelength signal attenuation increases exponentially.

　　Temperature effect: For every 10 ° C decrease in temperature, the macrobending loss increases by 0.02 dB/km.

　　Connection and connection

　　Connector loss: Each ST/sc connector is about 0.5 dB, and APC connectors need to be cleaned regularly.

　　Welding loss: Typical value of 0.1dB/point, mechanical micro-bending point may produce a peak of 0.35dB.

　　V. Summary and Selection Suggestions

　　Selection suggestions:

　　Long-distance/high bandwidth requirements: choose single-mode fiber (1550nm wavelength preferred).

　　Short range/low budget: choose multimode fiber (pay attention to inter-mode dispersion limitations).

　　Special environment: Consider bending-resistant and radiation-resistant optical fibers (e.g. germanium-doped core fibers for space communications).

　　In practical engineering, it is recommended to comprehensively calculate the total loss based on the specific wavelength, fiber quality, and link design (including connectors and fusion joints), and reserve 10% to 20% of the margin to cope with aging and environmental changes.