Depending on the number of the operating frequency propagating waves (modes) of the fibers and is separated into one multimode. The number of modes is dependent on the ratio of the diameter of the fiber core and the wavelength for optical fibers having step-index is calculated as follows.
α
– fiber core radius,
λ
– wavelength,
Δ – relative refractive index difference.
Since n 1 and n 2 have very similar meanings, the nominal value ∆ for most optical fibers is within
The advantage of single-mode optical fibers is the low dispersion (signal distortion), a large information-carrying capacity and range. Single-mode systems are the most actively developing and still promising direction of development of information transmission technology.
In multimode fibers pulse on the reception is broadened and distorted, which significantly limits the transmitted frequency band and the transmission distance. Multimode fibers are not used to transmit information over long distances. Generally transceivers operating on multimode fiber limited range of up to 500 meters.
For the characteristics of the optical fiber is essential in the refractive index profile in cross section. If the core of the fiber has a constant value over the radius of the refractive index, such optical fibers are called fiber optics with step refractive index profile (there is a step in the refractive index at the interface between the core and shell).
Also to fight with the broadening of pulses (that is especially true for multimode fibers) developed another type of fiber, which is found much more widely used in long-distance communications - is the optical fiber having a gradient refractive index profile. In optical fibers with a graded refractive index from the refractive index of the core center to the edge changes smoothly.
The refractive index is described by the following formula for the gradient optical fibers
r
– the current range,
n
1
– the greatest value of the refractive index of the core,
g
– coefficient determining the type of refractive index profile
Single-mode optical fibers can be divided into the following categories:
- conventional or fiber unbiased variance (SF), which are issued for equipment operating at a wavelength of 1.31 microns;
- dispersion-shifted fiber (DSF), which are available for use at a wavelength of 1.55 microns;
- fibers with non-zero dispersion-shifted (NZDSF), for transmitting a multiplex signal in the working wave range of 1,53-1,56 mm
It is also distinguished from the aligned optical fiber cladding whose refractive index corresponds to the refractive index of optical fibers and with a stepped profile matched to the refractive index of pure quartz (n = 1,4585), and the depressed cladding material which consists of two zones.
The profiles of optical fibers with refractive indices unbiased variance:
а – aligned with the shell, б – a depressed shell
The single-mode optical fibers for dispersion-shifted complex refractive index profiles determining the core diameter is substantial difficulties, so these fibers introduce the concept of the mode field diameter. fashion subfield refers to the area of the light signal passing through the cross-section of the optical fiber. This field has a minimal optical signal attenuation and optical fiber extends along minimal power losses.
ω
0
– mode field radius
2ω
0
– mode field diameter
