13.4 High-Speed Lightwave Transceivers
533
photodetection operations. These electronics include clocking functions for transmit
and receive signals, a line loopback capability for diagnostic signals, and circuitry
for multiplexing sixteen 622-Mb/s SONET/SDH inputs into a 10-Gb/s stream, which
becomes the electric signal input to the laser. In the receive direction, a photodetector
changes the incoming 10-Gb/s optical signal to an electrical format, which then is
separated back into sixteen 622-Mb/s electrical signals. External electronics need to
provide these functions for the smaller transceiver modules.
13.4.2 Transceivers for 40 Gb/s Links
New challenges in terms of transceiver response characteristics, chromatic dispersion
control, and polarization-mode dispersion compensation arise when transitioning to
higher capacity links, such as 40 Gb/s data rates [16–18]. For example, compared to a
10-Gb/s system, when using a conventional on-off keying (OOK) modulation format
a link operating at 40 Gb/s is sixteen times more sensitive to chromatic dispersion,
is four times more sensitive to polarization-mode dispersion, and needs an optical
signal-to-noise ratio (OSNR) that is at least 6 dB higher to reach an equivalent BER.
Therefore, alternate modulation schemes besides OOK have been considered.
One method is differential binary phase-shift keying, which is known as DBPSK or
simply DPSK (see Sect. 8.5). One advantage of DPSK is that when using a balanced
receiver the OSNR needed to reach a specific BER is about 3 dB lower than that
required for OOK. (A balanced receiver uses a pair of matched photodiodes to
achieve a higher sensitivity.) Because a factor of 3 dB means only half the amount of
optical power is needed for an equivalent BER, the lower OSNR that is needed for
DPSK can be allocated to reducing the optical power level at the receiver, to relaxing
the loss specifications for various link components, or to extending the transmission
distance. For example, the transmission distance could be doubled if there are no
other signal impairments, such as additional nonlinear effects. In addition, DPSK is
quite resilient to nonlinear effects, which are a problem when using OOK for data
rates greater than 10 Gb/s. This resilience is due to the fact that compared to OOK
the optical power is more evenly distributed in DPSK (that is, there is optical power
in each bit slot, as the next paragraph describes) and the optical peak power is 3 dB
lower for the same average optical power. These factors reduce nonlinear effects that
are dependent on the bit pattern and on optical power levels.
In contrast to OOK where the information is conveyed through amplitude changes,
DPSK carries the information in the optical phase. Optical power appears in each
DPSK bit slot and can occupy the entire slot for NRZ-DPSK, or it can occupy part of
the slot in the form of a pulse in an RZ-DPSK format. Binary data is encoded as either
a 0 or a π optical phase shift between adjacent slots. For example, the information
bit 1 may be transmitted by a 180° carrier phase shift relative to the carrier phase in
the previous slot, while the information bit 0 is transmitted by no phase shift relative
to the carrier phase in the previous signaling interval.
533
photodetection operations. These electronics include clocking functions for transmit
and receive signals, a line loopback capability for diagnostic signals, and circuitry
for multiplexing sixteen 622-Mb/s SONET/SDH inputs into a 10-Gb/s stream, which
becomes the electric signal input to the laser. In the receive direction, a photodetector
changes the incoming 10-Gb/s optical signal to an electrical format, which then is
separated back into sixteen 622-Mb/s electrical signals. External electronics need to
provide these functions for the smaller transceiver modules.
13.4.2 Transceivers for 40 Gb/s Links
New challenges in terms of transceiver response characteristics, chromatic dispersion
control, and polarization-mode dispersion compensation arise when transitioning to
higher capacity links, such as 40 Gb/s data rates [16–18]. For example, compared to a
10-Gb/s system, when using a conventional on-off keying (OOK) modulation format
a link operating at 40 Gb/s is sixteen times more sensitive to chromatic dispersion,
is four times more sensitive to polarization-mode dispersion, and needs an optical
signal-to-noise ratio (OSNR) that is at least 6 dB higher to reach an equivalent BER.
Therefore, alternate modulation schemes besides OOK have been considered.
One method is differential binary phase-shift keying, which is known as DBPSK or
simply DPSK (see Sect. 8.5). One advantage of DPSK is that when using a balanced
receiver the OSNR needed to reach a specific BER is about 3 dB lower than that
required for OOK. (A balanced receiver uses a pair of matched photodiodes to
achieve a higher sensitivity.) Because a factor of 3 dB means only half the amount of
optical power is needed for an equivalent BER, the lower OSNR that is needed for
DPSK can be allocated to reducing the optical power level at the receiver, to relaxing
the loss specifications for various link components, or to extending the transmission
distance. For example, the transmission distance could be doubled if there are no
other signal impairments, such as additional nonlinear effects. In addition, DPSK is
quite resilient to nonlinear effects, which are a problem when using OOK for data
rates greater than 10 Gb/s. This resilience is due to the fact that compared to OOK
the optical power is more evenly distributed in DPSK (that is, there is optical power
in each bit slot, as the next paragraph describes) and the optical peak power is 3 dB
lower for the same average optical power. These factors reduce nonlinear effects that
are dependent on the bit pattern and on optical power levels.
In contrast to OOK where the information is conveyed through amplitude changes,
DPSK carries the information in the optical phase. Optical power appears in each
DPSK bit slot and can occupy the entire slot for NRZ-DPSK, or it can occupy part of
the slot in the form of a pulse in an RZ-DPSK format. Binary data is encoded as either
a 0 or a π optical phase shift between adjacent slots. For example, the information
bit 1 may be transmitted by a 180° carrier phase shift relative to the carrier phase in
the previous slot, while the information bit 0 is transmitted by no phase shift relative
to the carrier phase in the previous signaling interval.