328
8 Digital Optical Fiber Links
photodetectors with nonuniform responsivity across the photosensitive area. Noise
is generated when the speckle pattern changes in time so as to vary the optical power
transmitted through the particular loss element. The continually changing speckle
pattern that falls on the photodetector thus produces a time-varying noise in the
received signal, which degrades receiver performance.
The modal distortion resulting from interference between a single pair of modes
will appear as a sinusoidal ripple of frequency
ν = δT
dν source
dt
(8.23)
where dν source /dt is the rate of change of optical frequency.
The performance of a high-speed, laser-based multimode fiber link is difficult to
predict because the degree of modal noise that can appear depends greatly on the
particular installation. Thus the best policy is to take steps to avoid it. This can be
done by the following measures:
1. Use LEDs (which are incoherent sources). This totally avoids modal noise.
2. Use a laser that has a large number of longitudinal modes (10 or more).
This increases the graininess of the speckle pattern, thus reducing intensity
fluctuations at mechanical disruptions in the link.
3. Use a fiber with a large numerical aperture because it supports a large number
of modes and hence gives a greater number of speckles.
4. Use a single-mode fiber because it supports only one mode and thus has no
modal interference.
8.2.5 Power Penalties Due to Mode-Partition Noise
Mode-partition noise is associated with intensity fluctuations in the longitudinal
modes of a multimode laser diode [25]; that is, the side modes are not sufficiently
suppressed. This is the dominant noise in single-mode fibers when using multimode devices, such as Fabry–Perot lasers. Intensity fluctuations can occur among
the various modes in a multimode laser even when the total optical output is constant,
as exhibited in Fig. 8.11. This power distribution can vary significantly both within
a pulse and from pulse to pulse.
Because the output pattern of a laser diode is highly directional, the light from
these fluctuating modes can be coupled into a single-mode fiber with high efficiency.
Each longitudinal mode that is coupled into the fiber has a different attenuation
and time delay because each mode is associated with a slightly different wavelength.
Because the power fluctuations among the dominant modes can be quite large, significant variations in signal levels can occur at the receiver in systems with high fiber
dispersion.
The signal-to-noise ratio due to mode-partition noise is independent of signal
power, so that the overall system error rate cannot be improved beyond the limit
8 Digital Optical Fiber Links
photodetectors with nonuniform responsivity across the photosensitive area. Noise
is generated when the speckle pattern changes in time so as to vary the optical power
transmitted through the particular loss element. The continually changing speckle
pattern that falls on the photodetector thus produces a time-varying noise in the
received signal, which degrades receiver performance.
The modal distortion resulting from interference between a single pair of modes
will appear as a sinusoidal ripple of frequency
ν = δT
dν source
dt
(8.23)
where dν source /dt is the rate of change of optical frequency.
The performance of a high-speed, laser-based multimode fiber link is difficult to
predict because the degree of modal noise that can appear depends greatly on the
particular installation. Thus the best policy is to take steps to avoid it. This can be
done by the following measures:
1. Use LEDs (which are incoherent sources). This totally avoids modal noise.
2. Use a laser that has a large number of longitudinal modes (10 or more).
This increases the graininess of the speckle pattern, thus reducing intensity
fluctuations at mechanical disruptions in the link.
3. Use a fiber with a large numerical aperture because it supports a large number
of modes and hence gives a greater number of speckles.
4. Use a single-mode fiber because it supports only one mode and thus has no
modal interference.
8.2.5 Power Penalties Due to Mode-Partition Noise
Mode-partition noise is associated with intensity fluctuations in the longitudinal
modes of a multimode laser diode [25]; that is, the side modes are not sufficiently
suppressed. This is the dominant noise in single-mode fibers when using multimode devices, such as Fabry–Perot lasers. Intensity fluctuations can occur among
the various modes in a multimode laser even when the total optical output is constant,
as exhibited in Fig. 8.11. This power distribution can vary significantly both within
a pulse and from pulse to pulse.
Because the output pattern of a laser diode is highly directional, the light from
these fluctuating modes can be coupled into a single-mode fiber with high efficiency.
Each longitudinal mode that is coupled into the fiber has a different attenuation
and time delay because each mode is associated with a slightly different wavelength.
Because the power fluctuations among the dominant modes can be quite large, significant variations in signal levels can occur at the receiver in systems with high fiber
dispersion.
The signal-to-noise ratio due to mode-partition noise is independent of signal
power, so that the overall system error rate cannot be improved beyond the limit