This means that a single mode fiber will allow only one mode for a wavelength of 1261nm and above.Ĭut-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. ITU-T recommendation of 1260nm is well below the 1310nm window. Popularity of 1550nm window has increased with the introduction of DWDM and availability of Non-Zero dispersion shifted fibers. Widely used single mode transmission window is 1310nm. This means the wavelength below 1260nm is acceptable for a single mode. ITU-T recommendation for single mode fibers ITU-T G.652 specify a cut-off wavelength of less than or equal to 1260nm. Thus it is clear that cut-off wavelength determines the fate of single mode transmission. 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. I have seen a sign of relief when I explain cut-off using the above analogy.Ĭut-off wavelength is important for single mode optical fibers as it is the characteristic unique to single mode optical fibers. 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. If the car width is 5.1, only one car can travel in that road at a time. Theoretically, if the car width is 5 meters, then two cars can travel in that road. To understand cut off it is better to think about the analogy of a road having a width of 10 meters. Hence cut-off is a single mode fiber characteristic. Why? The answer is simple as the very definition of cut-off wavelength itself is the wavelength below which multimode transmission starts. Cut-off wavelength is also a unique parameter of single mode fiber.įor multimode fibers cut-off wavelength is not important. Mode field diameter is one of the unique characteristic of single mode fibers.
This varies from fiber supplier to fiber supplier. Typically, single mode fibers used in telecommunication cables have a core diameter of 8 micrometers and a mode field diameter around 9 micrometers. The geometry of single mode fiber is designed to possess this property.
Single mode optical fibers are designed to have one mode for propagation. The above definition indicates that above a certain wavelength, multiple modes cease propagation. Cut-off wavelength can also be defined as the wavelength below which multimode transmission starts. In this case, the field decays exponentially along the waveguide axis and the wave is thus evanescent.Is the wavelength above which an optical fiber will allow single mode transmission. The wave equations are also valid below the cutoff frequency, where the longitudinal wave number is imaginary. As a voltage ratio this is a fall to 1 / 2 ≈ 0.707 Frequencies below the cut-off frequency are attenuated by the waveguide. Cut-off frequency can also be described as the frequency above which the waveguide offers minimum attenuation to the propagation of the signal. The signal propagation through a waveguide is dependent on the signal wavelength as. Below the cut-off frequency, waveguides fail to transfer wave energy or propagate waves. Most frequently this proportion is one half the passband power, also referred to as the 3 dB point since a fall of 3 dB corresponds approximately to half power. Frequencies below the cut-off frequency are attenuated by the waveguide. In electronics, cutoff frequency or corner frequency is the frequency either above or below which the power output of a circuit, such as a line, amplifier, or electronic filter has fallen to a given proportion of the power in the passband.