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Bare fiber adapters
The third method of testing fiber attenuation is to use an OTDR. The OTDR uses an indirect method of measuring loss that involves the backscatter from the fiber. Cables can be attached to the OTDR with a launch cable with a mechanical splice to connect to the fiber under test.
Bare fiber is usually tested with a launch cable with a connector on the OTDR end and bare fiber on the test end, with the bare fiber to be tested connected to the launch cable with a mechanical splice which can be reused or a fusion splice. One needs only take a trace of the fiber and use the loss between two markers a known distance apart to calculate the loss coefficient of the fiber.
OTDRs generally offer two methods of making this measurement, a simple "two point" method shown here or the "least squares" method which calculates the best fit between the two markers, reducing the effects of noise on the measurement.
Test Sources for Loss Measurements
On the test source, two factors must be controlled to minimize measurement uncertainty, the spectral output and modal characteristics. The spectral output characteristics obviously include wavelength, as seen in the spectral attenuation curve, but may also include the spectral width. A wide spectral width source suffers absorption over a larger range of wavelengths, making it more difficult to obtain precise data on spectral attenuation at any specific wavelength. Monochromators are used as sources for spectral loss testing, since the spectral width of the source can be controlled exactly.
For single wavelength measurements the source can be a fixed wavelength LED or laser. Generally, attenuation measurements will be made with a source appropriate to the fiber. Most multimode fiber systems use LED sources while singlemode fiber systems use laser sources. Thus testing each of these fibers should be done with the appropriate source. Lasers should not be used with multimode fiber, since coherent sources like lasers have high measurement uncertainties in multimode fiber caused by modal noise. The wide spectral width of LEDs sometimes overlap the singlemode fiber cutoff wavelength (the lowest wavelength where the fiber supports only one mode) at lower wavelengths and the 1400 nm OH: absorption band at the upper wavelengths.