Cut-off Wavelength in Singlemode Fiber
Cut-off wavelength is the wavelength above which an optical fiber will allow single mode transmission. Cut-off wavelength can also be defined as the wavelength below which multimode transmission starts.
The above definition indicates that above a certain wavelength, multiple modes cease propagation. Single mode optical fibers are designed to have one mode for propagation. The geometry of single mode fiber is designed to possess this property. Typically, single mode fibers used in telecommunication cables have a core diameter of 8 micrometers and a mode field diameter around 9 micrometers. This varies from fiber supplier to fiber supplier. Mode field diameter is one of the unique characteristic of single mode fibers. Cut-off wavelength is also a unique parameter of single mode fiber.
For multimode fibers cut-off wavelength is not important. Why? The answer is simple as the very definition of cut-off wavelength itself is the wavelength below which multimode transmission starts. Hence cut-off is a single mode fiber characteristic.
To understand cut off it is better to think about the analogy of a road having a width of 10 meters. Theoretically, if the car width is 5 meters, then two cars can travel in that road. If the car width is 5.1, only one car can travel in that road at a time. Then, 5.1 is the cut-off for this road. I know, this is not a true understanding of cut-off wavelength, but for the new comers, it is hard to understand the concept of cut-off wavelength. I have seen a sign of relief when I explain cut-off using the above analogy.
Cut-off wavelength is important for single mode optical fibers as it is the characteristic unique to single mode optical fibers. Cut-off wavelength is the minimum wavelength below which a single mode fiber will act as multimode fibers, meaning it will allow propagation of more than one mode at a time. Thus it is clear that cut-off wavelength determines the fate of single mode transmission.
ITU-T recommendation for single mode fibers ITU-T G.652 specify a cut-off wavelength of less than or equal to 1260nm. This means the wavelength below 1260nm is acceptable for a single mode. Widely used single mode transmission window is 1310nm. Popularity of 1550nm window has increased with the introduction of DWDM and availability of Non-Zero dispersion shifted fibers. ITU-T recommendation of 1260nm is well below the 1310nm window. This means that a single mode fiber will allow only one mode for a wavelength of 1261nm and above.
Cut-off wavelength is important for single mode fibers since it is the condition and criteria for a single mode fiber to allow single mode transmission.