Fiber Optic Bandwidth

Dispersion reduces
fiber optic bandwidth
, or information-carrying capacity, of an optical fiber. Dispersion causes the spreading of the light pulse as it travels along the fiber.

Fiber dispersion mechanisms include intramodal (chromatic) dispersion and intermodal (modal) dispersion. Multimode bandwidth is a measure of the intermodal dispersion of the multimode fiber.

Intermodal dispersion is maximum when all fiber modes are excited. The source used for intermodal dispersion measurements must overfill the fiber. The optical source must also have a narrow spectral fiber optic bandwidth to reduce the effects of chromatic dispersion in the measurement.

There are two basic techniques for measuring the modal fiber optic bandwidth of an optical fiber. The first technique characterizes dispersion by measuring the impulse response h(t) of the fiber in the time domain. The second technique characterizes modal dispersion by measuring the baseband frequency response H(f) of the fiber in the frequency domain. H(f) is the power transfer function of the fiber at the baseband frequency (f). H(f) is also the Fourier transform of the power impulse response h(t). Only the frequency response method is described here.

The test method for measuring the bandwidth of multimode fibers in the frequency domain is EIA/TIA-455-30. Signals of varying frequencies (f) are launched into the test fiber and the power exiting the fiber at the launched fundamental frequency measured. This optical output power is denoted as Pout(f).The test fiber is then cut back or replaced with a short length of fiber of the same type. Signals of the same frequency are launched into the cut-back fiber and the power exiting the cut-back fiber at the launched fundamental frequency measured. The optical power exiting the cutback or replacement fiber isdenoted as Pin (f). The magnitude of the optical fiber frequency response is

The fiber optic bandwidth is defined as the lowest frequency at which the magnitude of the fiber frequency response has decreased to one-half its zero-frequency value. This is the -3 decibel (dB) optical power frequency (f3dB). This frequency is referred to as the fiber bandwidth.

Bandwidth is normally given in units of megahertz-kilometers (MHz-km). Converting the -3 dB fiber bandwidth to a unit length assists in the analysis and comparison of optical fiber performance. For long lengths of fiber (>1km), the method for normalization is to multiply the length times the measuredbandwidth.

Chromatic Dispersion

Chromatic, or intramodal, dispersion occurs in both single mode and multimode optical fibers. Chromatic dispersion occurs because different colors of light travel through the fiber at different speeds.

Since the different colors of light have different velocities, some colors arrive at the fiber end before others. This delay difference is called the differential group delay t(l) per unit length. This differential group delay leads to pulse broadening.

Chromatic dispersion is measured using EIA/TIA-455-168 in the time domain. Chromatic dispersion is also measured in the frequency domain using EIA/TIA-455-169 and EIA/TIA-455-175. These methods measure the composite optical fiber material and waveguide dispersion. To understand the contribution that material and waveguide dispersive mechanisms have on multimode and single mode fiber dispersion.

The chromatic dispersion of multimode graded-index and single mode fiber is obtained by measuring fiber group delays in the time domain. These measurements are made using multiwavelength sources or multiple sources of different wavelengths. A multi-wavelength source could be a wavelength-selectable laser.

The pulse delay for both a long test sample fiber and a short reference fiber are measured over a range of wavelengths. The pulse delay for the reference fiber as a function of wavelength is tin(l). The pulse delay for the test fiber as a function of wavelength is tout(l). The group delay t(l). per unit length ateach wavelength is

where Ls is the test sample fiber length in kilometers (km) and Lref is the reference sample length in km.

The fiber chromatic dispersion is defined as the derivative, or slope, of the fiber group delay curve with respect to wave-length. Generally, the group delay as a function of wavelength is fit to a simple mathematical function and the derivative calculated. The range of wavelengths over which meaningful data is obtained depends on the wavelength range of optical source(s) used. The zero-dispersion wavelength (l 0) and the zero dis-persion slope (S0) are determined from the chromatic dispersion curve.

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