Even the specified value of cut-off wavelength is also satisfied network requirements of Cross Wavelength Division Multiplexing (CWDM), where minimum operationģ wavelength is 1290nm (O-band).
International standard specifies cabled cut-off wavelength < 1260 nm for dispersionunshifted single-mode fiber, which is well below the typical operational wavelength of 1310 nm (refer Table 1). The higher order mode will be attenuated before it can recombine with the fundamental mode. Operation of the optical fiber system at a wavelength above the specified cabled cut-off wavelength would ensure that the second order mode is not propagated on long lengths of fiber. Since both bending and length may affect the cut-off wavelength of a fiber, the cabled cut-off measurement techniques provide consistent controls for determining the cutoff wavelength of deployed cables. These cut-off requirements specify test methods 1,2, which are representative of actual field deployment conditions for optical fiber cable products. How do we avoid the second order mode? Current industry standards address cabled cut-off wavelength requirements for indoor and outdoor cables. Additionally, if a single-mode fiber is operated below a certain wavelength - the cutoff wavelength - the fiber may support the second order mode. It becomes apparent that fiber geometry as well as splicing and connectorization practices are important. The higher order mode can be generated at splice or connector junctions where significant fiber core misalignment exists. Transmitters that launch light into the fiber with overfilled conditions can initiate this mode, 2. Several conditions may cause the excitation of this higher order mode. If this secondary mode is not sufficiently attenuated or stripped out of the fiber, it may recombine with the fundamental mode at subsequent fiber connections or splices causing destructive or constructive interference. In this state, the single-mode fiber supports multimode operation.
Under certain conditions, a second, higher order mode may be excited, which has significantly higher loss, but more importantly, may exhibit a difference in optical path length. Normally, when operated above a certain wavelength, a single-mode fiber propagates one fundamental mode, supporting the information carrying light signal. The phase shifts predominantly originate from the random wavelength changes of the optical source. Modal interference results from the recombination of higher order modes exhibiting varying phase shifts with the fundamental mode. What is Modal Interference? Modal interference can occur in single-mode fiber systems causing signal degradation and potentially lower signal or carrier to noise figures.
The cutoff wavelength of a fiber is reduced when it is cabled. If a fiber is bent in a loop, the cutoff is lowered. Or the smaller the bend radius of a loop of the fiber is, the lower the effective cutoff wavelength will be. The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment and the longer the fiber, the lower the effective cutoff wavelength. An optical fiber that is single-moded at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength. Below cut-off, the fiber will transmit more than one mode. Keywords Optical fiber, cut-off wavelengthĢ What is Cut-off Wavelength? The CUTOFF WAVELENGTH of a single mode fiber is the wavelength above which the fiber propagates only the fundamental mode. This paper describes relationship between cutoff wavelength of cabled and un-cabled fibers. 1 White paper Which Cut-off wavelength to be considered Optical Fiber or Fiber Optic Cable? Author Sudipta Bhaumik Abstract Cutoff wavelength is one of the important optical characteristics of single mode optical fiber.
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