MIL-PRF-49291D
APPENDIX A
CHROMATIC DISPERSION
A.1. SCOPE
Chromatic dispersion of a Type I graded index multimode fiber can be approximated by measuring the
chromatic dispersion using a single mode launch condition and calculating the composite chromatic
dispersion based on the fiber characteristics. This method provides approximate results which are
sufficient for this specification.
A.2. MEASUREMENT
The single mode launch chromatic dispersion characteristics of a Type I (multimode) fiber shall be
determined in accordance with TIA-455-175 Methods A, B, or C, with the exception that the following
single mode launch condition shall be utilized.
For a single mode launch condition, two sections of a Type II (single mode) fiber are utilized, one at each
end of the Type I (multimode) fiber test specimen. Each Type II (single mode) fiber section shall be
greater than 2.0 meters in length. The Type II (single mode) fiber sections should have similar glass
composition as the Type I (multimode) fiber test specimen.
The light source is coupled into one of the Type II (single mode) fiber sections. This combination of light
source and Type II (single mode) fiber is now the single mode light source. The output end of the Type II
(single mode) fiber section is coupled to the input end of the Type I (multimode) fiber test specimen. The
coupling shall insure that the Type II (single mode) fiber core is centered onto the core of the Type I
(multimode) fiber test specimen.
The second Type II (single mode) fiber section is coupled to the detector system. This combination of
detector system and Type II (single mode) fiber is now the single mode detector system. The input end of
the detector system Type II (single mode) fiber is coupled to the output end of the Type I (multimode)
fiber test specimen. The coupling shall insure that the Type II (single mode) fiber core is centered onto
the core of the Type I (multimode) fiber test specimen.
Using the above described single mode light source and single mode detector system configuration,
chromatic dispersion is measured in accordance with TIA-455-175 Methods A, B, or C. The chromatic
dispersion characteristics measured this way are designated as the single mode launch chromatic
dispersion characteristics:
λoo =
single mode launch zero-dispersion wavelength
Soo =
single mode launch dispersion slope at zero-dispersion wavelength
A.3. FIBER CHARACTERISTICS
The following characteristics of the Type I (multimode) fiber test specimen are required for the
calculations:
g = index profile parameter (typically ~ 2.1)
λc = zero-dispersion wavelength of cladding material (~1273 nm for pure silica)
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