294
7 Optical Receiver Operation
quickly extract the decision threshold and determine the signal phase from a
set of overhead bits placed at the beginning of each packet burst. However, this
methodology results in a receiver sensitivity power penalty of up to 3 dB.
The key requirements of a burst-mode receiver are high sensitivity, wide dynamic
range, and fast response time. The sensitivity is important in relation to the optical
power budget, because, for example, a sensitivity improvement of 3 dB can double
the size of the power splitter so that more customers can be attached to the PON.
A wide dynamic range is essential for achieving a long network reach, that is, to be
able to accommodate users located both close and far away from the central office.
The use of a conventional ac-coupling method is not possible in a burst-mode
receiver, because the residual charge in a coupling capacitor following any particular
data burst cannot dissipate fast enough in order not to affect the initial conditions
of the next burst. The burst-mode receiver therefore requires additional circuitry
to accommodate dc-coupled operation. Such receivers now are incorporated into
standard commercially available OLT equipment.
7.5 Characteristics of Analog Receivers
In addition to the wide usage of fiber optics for the transmission of digital signals,
there are many potential applications for analog links. These range from individual
4-kHz voice channels to microwave links operating in the multiple-gigahertz region
[31–33]. The previous sections discussed digital receiver performance in terms of
error probability. For an analog receiver, the performance fidelity is measured in
terms of a signal-to-noise ratio. This is defined as the ratio of the mean-square signal
current to the mean-square noise current.
The simplest analog technique is to use amplitude modulation of the source. In
this scheme, a time-varying electric signal s(t) is used to modulate an optical source
directly about some bias point defined by the bias current I B , as shown in Fig. 7.19.
The transmitted optical power P(t) is thus of the form
P(t) = P t [1 + ms(t)]
(7.27)
where P t is the average transmitted optical power at a drive current I B , s(t) is the
analog modulation signal, and m is the modulation signal index defined by (see
Sect. 4.4)
m =
I
I
B
(7.28)
Here, I
B = I B for LEDs and I
B = I B − I th for laser diodes. The parameter I is
the variation in current about the bias point. In order not to introduce distortion into
the optical signal, the modulation must be confined to the linear region of the light
7 Optical Receiver Operation
quickly extract the decision threshold and determine the signal phase from a
set of overhead bits placed at the beginning of each packet burst. However, this
methodology results in a receiver sensitivity power penalty of up to 3 dB.
The key requirements of a burst-mode receiver are high sensitivity, wide dynamic
range, and fast response time. The sensitivity is important in relation to the optical
power budget, because, for example, a sensitivity improvement of 3 dB can double
the size of the power splitter so that more customers can be attached to the PON.
A wide dynamic range is essential for achieving a long network reach, that is, to be
able to accommodate users located both close and far away from the central office.
The use of a conventional ac-coupling method is not possible in a burst-mode
receiver, because the residual charge in a coupling capacitor following any particular
data burst cannot dissipate fast enough in order not to affect the initial conditions
of the next burst. The burst-mode receiver therefore requires additional circuitry
to accommodate dc-coupled operation. Such receivers now are incorporated into
standard commercially available OLT equipment.
7.5 Characteristics of Analog Receivers
In addition to the wide usage of fiber optics for the transmission of digital signals,
there are many potential applications for analog links. These range from individual
4-kHz voice channels to microwave links operating in the multiple-gigahertz region
[31–33]. The previous sections discussed digital receiver performance in terms of
error probability. For an analog receiver, the performance fidelity is measured in
terms of a signal-to-noise ratio. This is defined as the ratio of the mean-square signal
current to the mean-square noise current.
The simplest analog technique is to use amplitude modulation of the source. In
this scheme, a time-varying electric signal s(t) is used to modulate an optical source
directly about some bias point defined by the bias current I B , as shown in Fig. 7.19.
The transmitted optical power P(t) is thus of the form
P(t) = P t [1 + ms(t)]
(7.27)
where P t is the average transmitted optical power at a drive current I B , s(t) is the
analog modulation signal, and m is the modulation signal index defined by (see
Sect. 4.4)
m =
I
I
B
(7.28)
Here, I
B = I B for LEDs and I
B = I B − I th for laser diodes. The parameter I is
the variation in current about the bias point. In order not to introduce distortion into
the optical signal, the modulation must be confined to the linear region of the light