- Features of single mode fibers are :
- Signal transfer quality is
- Modal noise is
- Largest BW-distance
- Basic design – optimization includes the following :
- Mode field
- Refractive index profile.
Refractive Index Profile
- Dispersion of single mode silica fiber is lowest at 1300 nm while its attenuation is minimum at 1550 nm. For archiving maximum transmission distance the dispersion null should be at the wavelength of minimum attenuation. The waveguide dispersion is easier to control than the material Therefore a variety of core-cladding refractive
idex configuration fivers. Such as 1300 nm – optimized fibers, dispersion shifted fibers, dispersion – flattened fibers and large effective core area fibers.
1. 1300 nm – Optimized Fibers
- These are most popularly used The two configurations of 1300 nm – optimized single mode fibers are :
- Matched cladding
- Dressed cladding
- Matched cladding fibers have uniform refractive index throughout its Typical diameter is 9.0 µm and ∆ = 0.35 %.
- Dressed cladding fibers have the innermost cladding portion has low refractive index than outrcladding Typical diameter is 8.4 µm and ∆1 = 0.25 %, ∆2 = 0.12 %.
Fig 2.9.1 shows both types of fibers.
2. Dispersion Shifted Fibers
- The addition of wavelength and material dispersion can shift the zero dispersion point of longer wavelength. Two configurations of dispersion shifted fibers are :
- Step index dispersion shifted
- Triangular dispersion shifted
3. Dispersion Flattened
- Dispersion flattened fibers are more complex to It offers much broader span of wavelengths to suit desirable characteristics. Two configurations are :
- Fig 9.4 shows total resultant dispersion.
- The total dispersion consists of material and waveguide The resultant intermodal dispersion is given as,
where, τ is group delay per unit length of fiber.
- The broadening σ of an optical pulse is given as,
σ = D (λ) Lσ λ
where, σλ is half power spectral width of source.
- As the dispersion varies with wavelength and fiber Different formulae are used to calculate dispersions for variety of fiber at different wavelength.
- For a non – dispersion shifted fiber between 1270 nm to 1340 nm wavelength, the expression for dispersion is given as :
λ0 is zero dispersion wavelength. S0 is value at dispersion slop at λ0.
- Fig 9.5 shows dispersion performance curve for non-dispersion shifted fibers in 1270 – 1340 nm region.
- Maximum dispersion specified as 5 ps/(nm . km) marked as dotted line in Fig. 2.9.5.
Cut-off Frequency of an Optical Fiber
- The cut-off frequency of an optical fiber is determined not only by the fiber itself (modal dispersion in case of multimode fibers and waveguide dispersion in case of single mode fibers) but also by the amount of material dispersion caused by the spectral width of
Bending Loss Limitations
- The macrobending and microbending losses are significant in single mode fibers at 1550 nm region, the lower cut-off wavelengths affects more. Fig. 2.9.6 shows macrobending
- The bending losses are function of mode-filed diameter, smaller the mode-field diameter, the smaller the bending Fig. 2.9.7 shows loss due to mode-field diameter.
- The bending losses are also function of bend-radius of curvature. If the bend radius is less, the losses are more and when the radius is more, the bending losses are
- Briefly explain material dispersion with suitable
- Give expression of pulse broadening in graded index
- State the significance of mode coupling in optic fiber communication.
- Explain in detail the design optimization of single mode
- Elaborate dispersion mechanism in optical