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8 Digital Optical Fiber Links
Drill Problem 8.2 A single-mode optical fiber link operating at a 1310nm wavelength is intended for a 1-Gb/s metro network. Suppose that the
components of the link have the following parameter values:
(a) A laser diode that emits 0 dBm of optical power from an attached fiber
flylead
(b) A pin photodiode with a −22-dBm sensitivity at 1.0 Gb/s
(c) An optical fiber with an attenuation of 0.4-dB/km at 1310 nm
(d) A 1-dB connector loss at each end of the link
(e) A required power margin of 8 dB
Show that the maximum link length is 30 km.
8.1.4 Formulating a Rise-Time Budget
A rise-time budget analysis is a convenient method for determining the dispersion
limitation of an optical fiber link. This is particularly useful for digital systems. In
this approach, the total rise time t sys of the link is the root sum square of the rise
times from each contributor t i to the pulse rise-time degradation:
t sys =
N
i=1
t
2
i
1/2
(8.3)
The four basic elements that may significantly limit system speed are the transmitter rise time t tx , the group-velocity dispersion (GVD) rise time t GVD of the fiber,
the modal dispersion rise time t mod of the fiber, and the receiver rise time t rx . Singlemode fibers do not experience modal dispersion, so in these fibers the rise time is
related only to GVD. Generally, the total transition-time degradation of a digital link
should not exceed 70% of an NRZ (non-return-to-zero) bit period or 35% of a bit
period for RZ (return-to-zero) data, where one bit period is defined as the reciprocal
of the data rate.
The rise times of transmitters and receivers are generally known from data sheets.
The transmitter rise time is attributable primarily to the light source and its drive
circuitry. The receiver rise time results from the photodetector response and the 3dB electrical bandwidth of the receiver front end. The response of the receiver front
end can be modeled by a first-order lowpass filter that has a step response [10]
g(t) =
1 − exp(−2π B e t)
u(t)
(8.4)
8 Digital Optical Fiber Links
Drill Problem 8.2 A single-mode optical fiber link operating at a 1310nm wavelength is intended for a 1-Gb/s metro network. Suppose that the
components of the link have the following parameter values:
(a) A laser diode that emits 0 dBm of optical power from an attached fiber
flylead
(b) A pin photodiode with a −22-dBm sensitivity at 1.0 Gb/s
(c) An optical fiber with an attenuation of 0.4-dB/km at 1310 nm
(d) A 1-dB connector loss at each end of the link
(e) A required power margin of 8 dB
Show that the maximum link length is 30 km.
8.1.4 Formulating a Rise-Time Budget
A rise-time budget analysis is a convenient method for determining the dispersion
limitation of an optical fiber link. This is particularly useful for digital systems. In
this approach, the total rise time t sys of the link is the root sum square of the rise
times from each contributor t i to the pulse rise-time degradation:
t sys =
N
i=1
t
2
i
1/2
(8.3)
The four basic elements that may significantly limit system speed are the transmitter rise time t tx , the group-velocity dispersion (GVD) rise time t GVD of the fiber,
the modal dispersion rise time t mod of the fiber, and the receiver rise time t rx . Singlemode fibers do not experience modal dispersion, so in these fibers the rise time is
related only to GVD. Generally, the total transition-time degradation of a digital link
should not exceed 70% of an NRZ (non-return-to-zero) bit period or 35% of a bit
period for RZ (return-to-zero) data, where one bit period is defined as the reciprocal
of the data rate.
The rise times of transmitters and receivers are generally known from data sheets.
The transmitter rise time is attributable primarily to the light source and its drive
circuitry. The receiver rise time results from the photodetector response and the 3dB electrical bandwidth of the receiver front end. The response of the receiver front
end can be modeled by a first-order lowpass filter that has a step response [10]
g(t) =
1 − exp(−2π B e t)
u(t)
(8.4)