340
8 Digital Optical Fiber Links
about a 6-dB coding gain. Concatenated Reed-Solomon codes (several codes used
sequentially) can provide even higher coding gains.
Current terrestrial and undersea high-speed optical communication systems use a
number of different FEC codes. For example, as part of the G.709 Digital Wrapper
Recommendation, the ITU-T has selected the (255,239) and (255,223) ReedSolomon codes.
33–35 The (255,223) code has a higher overhead (15%) compared
to the (255,239) code, but is somewhat stronger because it is able to correct 16 errors
in a block of 223 bits. The Digital Wrapper uses the same error-monitoring techniques as is employed in the earlier SDH and SONET standards. The performance
metrics that are calculated include code violations in the incoming bit stream, the
number of seconds in which at least one error occurs, the number of seconds in
which multiple errors occur (called severely errored seconds), and the total number
of seconds in which service is not available.
8.4 Coherent Detection Schemes
The basic receiver analysis in Chap. 7 considered a simple and cost-effective lightwave transmission scheme in which the light intensity of the optical source is modulated linearly with respect to the input electrical signal voltage. This scheme pays no
attention to the frequency or phase of the optical carrier, because a photodetector at
the receiving end only responds to changes in the power level (intensity) that falls
directly on it. The photodetector then transforms the optical power level variations
back to the original electrical signal format. This method is known as intensity modulation with direct detection (IM/DD). Although these IM/DD methods offer system
simplicity and relatively low cost, their sensitivities are limited by noises generated
in the photodetector and the receiver preamplifier. These noises degrade the receiver
sensitivities of square-law IM/DD transmission systems by 10 to 20 dB from the
fundamental quantum noise limit.
Around 1978 component researchers had improved the spectral purity and
frequency stability of semiconductor lasers to the point where alternative techniques using homodyne or heterodyne detection of the optical signal appeared to be
feasible. Optical communication systems that use homodyne or heterodyne detection are called coherent optical communication systems, because their implementation depends on phase coherence of the optical carrier. In coherent detection techniques the light is treated as a carrier medium that can be amplitude-, frequency-, or
phase-modulated similar to the methods used in microwave radio systems.
36–39 .
Coherent systems were examined extensively during the 1980s and early 1990s
as a method for increasing the transmission spans for long-haul links. However,
interest in these methods declined when optical amplifiers were introduced because
these amplification devices offered dramatic increases in the transmission distances
of multi-wavelength OOK systems. Fortunately research on coherent techniques
continued, because a decade later there was renewed interest as data transmission
speeds moved to 10 Gb/s and beyond. This interest was spurred by the fact that
8 Digital Optical Fiber Links
about a 6-dB coding gain. Concatenated Reed-Solomon codes (several codes used
sequentially) can provide even higher coding gains.
Current terrestrial and undersea high-speed optical communication systems use a
number of different FEC codes. For example, as part of the G.709 Digital Wrapper
Recommendation, the ITU-T has selected the (255,239) and (255,223) ReedSolomon codes.
33–35 The (255,223) code has a higher overhead (15%) compared
to the (255,239) code, but is somewhat stronger because it is able to correct 16 errors
in a block of 223 bits. The Digital Wrapper uses the same error-monitoring techniques as is employed in the earlier SDH and SONET standards. The performance
metrics that are calculated include code violations in the incoming bit stream, the
number of seconds in which at least one error occurs, the number of seconds in
which multiple errors occur (called severely errored seconds), and the total number
of seconds in which service is not available.
8.4 Coherent Detection Schemes
The basic receiver analysis in Chap. 7 considered a simple and cost-effective lightwave transmission scheme in which the light intensity of the optical source is modulated linearly with respect to the input electrical signal voltage. This scheme pays no
attention to the frequency or phase of the optical carrier, because a photodetector at
the receiving end only responds to changes in the power level (intensity) that falls
directly on it. The photodetector then transforms the optical power level variations
back to the original electrical signal format. This method is known as intensity modulation with direct detection (IM/DD). Although these IM/DD methods offer system
simplicity and relatively low cost, their sensitivities are limited by noises generated
in the photodetector and the receiver preamplifier. These noises degrade the receiver
sensitivities of square-law IM/DD transmission systems by 10 to 20 dB from the
fundamental quantum noise limit.
Around 1978 component researchers had improved the spectral purity and
frequency stability of semiconductor lasers to the point where alternative techniques using homodyne or heterodyne detection of the optical signal appeared to be
feasible. Optical communication systems that use homodyne or heterodyne detection are called coherent optical communication systems, because their implementation depends on phase coherence of the optical carrier. In coherent detection techniques the light is treated as a carrier medium that can be amplitude-, frequency-, or
phase-modulated similar to the methods used in microwave radio systems.
36–39 .
Coherent systems were examined extensively during the 1980s and early 1990s
as a method for increasing the transmission spans for long-haul links. However,
interest in these methods declined when optical amplifiers were introduced because
these amplification devices offered dramatic increases in the transmission distances
of multi-wavelength OOK systems. Fortunately research on coherent techniques
continued, because a decade later there was renewed interest as data transmission
speeds moved to 10 Gb/s and beyond. This interest was spurred by the fact that