Fiber Optic Repeaters Use Relevance
The use of Optical Star Couplers (active or passive) can provide flexibility in the layoutof Modbus Plus and Remote I/O networks. Figure 4 shows one example of an OpticalStar Coupler configuration. Additional repeaters can be connected in order to extendcommunication between electrical links.
If a passive Star Coupler is used, the use of 100/140 μm cable is recommendedbecause of its higher available optical power.
Selecting Fiber Optic Cable
When selecting which type of fiber optic cable to use, several factors must beconsidered, among them the cable attenuation and the cable bandwidth. Cableattenuation and bandwidth are parameters specified by the cable manufacturers, anddepend upon several factors
the wavelength and the spectral width of the optical signal;
the cable index (use graded–index only), andthe bandwidth (depends upon the size of the cable [50/125 μm, 62.5/125 μm, and100/140 μm]).
The maximum attenuation for an 820nm optical signal is:
50/125 μm cable, 3.5 dB/km; 62.5/125 μm cable, 3.5 dB/km; 100/140 μm cable, 5.0 dB/km;
The bandwidth is the parameter limited by the dispersion of the optical signal along thelength of the cable. Two types of dispersion exist: modal dispersion and materialdispersion. The modal dispersion widely depends on the fiber core size, numericalaperture (NA), and the difference in the refraction coefficient between core and claddingmaterial of fiber optic cable. This difference in refraction is called the cable index, andresults in a pulse spreading because of the different traveling distances for differentrays of light entering the multimode fiber. For example, a difference of 1% in refractioncoefficient between core and cladding material for step–index fiber produces a pulsespreading (or pulse width distortion) of 50 ns/km.
Modal dispersion (and the resultant pulse width distortion) can be reduced by using asmaller core size cable; using a graded–index fiber; and using a fiber with a smallernumerical aperture.
Material dispersion results in pulse spreading at the end of the fiber because of thedifference in velocities for different wavelengths of the light signal, and depends uponthe wavelength and the spectral width of the transmit optical signal.
If a passive Star Coupler is used, the use of 100/140 μm cable is recommendedbecause of its higher available optical power.
Selecting Fiber Optic Cable
When selecting which type of fiber optic cable to use, several factors must beconsidered, among them the cable attenuation and the cable bandwidth. Cableattenuation and bandwidth are parameters specified by the cable manufacturers, anddepend upon several factors
the wavelength and the spectral width of the optical signal;
the cable index (use graded–index only), andthe bandwidth (depends upon the size of the cable [50/125 μm, 62.5/125 μm, and100/140 μm]).
The maximum attenuation for an 820nm optical signal is:
50/125 μm cable, 3.5 dB/km; 62.5/125 μm cable, 3.5 dB/km; 100/140 μm cable, 5.0 dB/km;
The bandwidth is the parameter limited by the dispersion of the optical signal along thelength of the cable. Two types of dispersion exist: modal dispersion and materialdispersion. The modal dispersion widely depends on the fiber core size, numericalaperture (NA), and the difference in the refraction coefficient between core and claddingmaterial of fiber optic cable. This difference in refraction is called the cable index, andresults in a pulse spreading because of the different traveling distances for differentrays of light entering the multimode fiber. For example, a difference of 1% in refractioncoefficient between core and cladding material for step–index fiber produces a pulsespreading (or pulse width distortion) of 50 ns/km.
Modal dispersion (and the resultant pulse width distortion) can be reduced by using asmaller core size cable; using a graded–index fiber; and using a fiber with a smallernumerical aperture.
Material dispersion results in pulse spreading at the end of the fiber because of thedifference in velocities for different wavelengths of the light signal, and depends uponthe wavelength and the spectral width of the transmit optical signal.
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