Which loss measurement wavelengths do I need?
Fiber optic loss testing is usually performed at expected current and future operating wavelengths, since optical loss can vary widely across the range of potential operating wavelengths.
|Fiber Type||Most common
operating λ nm
operating λ nm
tests λ nm
|Other λ nm||Out of band
loss tests λ nm
|Plastic 1 mm||650||650||LED|
|PCS 200u||650||850||650 / 850||LED|
|Glass multimode 50/125, 62.5/125||850||1300||850 / 1300||LED||1270 - 1610 CWDM|
|Glass single mode 9.5/125, Traditional||1310||1550||1310 / 1550||Laser||>1580|
|Glass single mode 9.5/125, DWDM||C band
1525 - 1565
1570 - 1610
|1550 / 1610||Laser||S band
≈ 1420 - 1530
|Glass single mode 9.5/125, CWDM||1310, 1490, 1550||1270 - 1610||1310, 1490, 1550||Laser||1383||1625|
It has been standard practice for many years to perform single mode fiber tests at 1550 nm (in addition to 1310 nm), to help find identify cabling stress points. Typically, a kinked cable may pass at 1310 nm, but fail at 1550 nm.
Laser sources are unsuitable for work on multimode fiber, since very unstable power meter readings are obtained. For optimum multimode accuracy, a mandrel wrap and a LED source with standards compliant characteristics are required.
C band systems may only be loss tested at 1550 nm. C & L band systems are typically loss tested at 1550 / 1625 nm. The S band is currently somewhat ill-defined, and may run into the fiber water absorption peak, so loss measurement will need to be at the shortest relevant wavelength, and 1550 nm.
Attenuation in CWDM systems is usually measured at only a few wavelengths, and varies quite substantially depending on the application and fiber type. The water absorption peak at 1383 nm may require evaluating, and a 1390 nm laser is adequate for this task. Most current systems avoid the water peak wavelength area due to this potential loss problem. Loss checking of passive CWDM channel filters requires specific CWDM compliant light sources.
"Out of band" single mode
Has been traditionally specified as "1625" nm. This has traditionally used Fabry-Perot lasers with poor spectral accuracy. New DFB lasers have much better properties for this application, since their actual operating wavelength is more tightly controlled (see table below). For true WDM compliance, the out of band wavelength must be <1620 mn.
There is also an emerging class of "1650" nm devices, however their poor wavelength accuracy and high cost makes them less useful.
|Laser Type||Center λ Tolerance
@ 25 °C
Range @ 25 °C
|Typ Center λ
/ °C coefficient
Range 0 - 50 °C
|Typ λ Width
|Total λ Range
0 - 50 °C, FWHM
|1310 nm||Fabry-Perot||± 30 nm||1280 - 1340||± 0.4||1270 - 1350||3||1268.5 - 1351.5||Ge or InGaAs|
|1550 nm||Fabry-Perot||± 30 nm||1520 - 1580||± 0.4||1510 - 1590||3||1508.5 - 1591.5||Ge or InGaAs|
|CWDM||DFB||± 3 nm||± 0.1||±2.5 nm||0.1||±5.5 nm||InGaAs|
|1610 nm||DFB||± 3 nm||1607 - 1613||± 0.1||1604.5 - 1615.5||0.1||1604.5 - 1615.5||InGaAs|
|1625 nm||DFB||± 5 nm||1620 - 1630||± 0.1||1617.5 - 1632.5||0.1||1617.5 - 1632.5||InGaAs|
|1625 nm||Fabry-Perot||± 30 nm||1595 - 1655||± 0.4||1585 - 1665||3||1583.5 - 1667.5||InGaAs|
|1650 nm||Fabry-Perot||± 30 nm||1620 - 1680||± 0.4||1610 - 1690||3||1608.5 - 1691.5||InGaAs|
This graph shows how typical single mode fiber attenuation varies with wavelength.
Modern fibers may have a negligible water absorption peak at 1.38 µ
This clearly shows that wavelength uncertainly of tests above 1600 nm has a major impact on loss measurement
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