3. Detectors and Receivers 85
Support system
Sensor head
Laser
Coupler
Splice
tube
Index
matching
gel
Output to
oscilloscope
Detector
Hollow core fiber
Multimode fiber
Single-mode
fiber
Adhesive
d
R
1
R
2
Figure 3.16 Extrinsic Fabry-Perot interferometric (EFPI) sensor and system.
Laser
diode
Detector
Fused fiber
Index
matching gel
2
× 2 Coupler
RS
Figure 3.17 The intrinsic Fabry-Perot interferometric (IFPI) sensor.
3.5 Noise
Any optical detection or communication system is subject to various
types of noise. There can be noise in the signal, noise created by the
detector, and noise in the electronics. A complete discussion of noise
sources has already filled several good reference books; since this is a
chapter on detectors, we will briefly discuss the noise created by detectors.
For a more complete discussion, the reader is referred to treatments by
Dereniak and Crowe [6], who have categorized the major noise sources.
The purpose of the detector is to create an electrical current in response
86 Fiber Optic Essentials
to incident photons. It must accept highly attenuated optical energy and
produce an electrical current. This current is usually feeble because of the
low levels of optical power involved, often only in the order of nanowatts.
Subsequent stages of the receiver amplify and possibly reshape the signal
from the detector. Noise is an serious problem that limits the detector’s
performance. Broadly speaking, noise is any electrical or optical energy
apart from the signal itself. Although noise can and does occur in every
part of a communication system, it is of greatest concern in the receiver
input. The reason is that receiver works with very weak signals that have
been attenuated during transmission. Although very small compared to
the signal levels in most circuits, the noise level is significant in relation
to the weak detected signals. The same noise level in a transmitter is
usually insignificant because signal levels are very strong in comparison.
Indeed, the very limit of the diode’s sensitivity is the noise. An optical
signal that is too weak cannot be distinguished from the noise. To detect
such a signal, we must either reduce the noise level or increase the
power level of the signal. In the following sections, we will describe in
detail several different noise sources; in practice, it is often assumed that
the noise in a detection system has a constant frequency spectrum over
the measurement range of interest; this is the so-called “white noise”
or “Gaussian noise,” and is often a combination of the effects we will
describe here.
There are two kinds of amplifiers discussed in fiber optics. The first is
electronic amplifiers that amplify the detector signal, which we will treat
here. The second is optical amplifiers, which like repeaters and optical
repeaters are placed standalone in the link to amplify the transmitter
signal and extend networks to very long distances. Optical amplifiers are
discussed in a later chapter.
Earlier in this chapter, we considered several circuit diagrams of detec-
tors, including Figure 3.8, which has a PIN diode and amplifier equivalent
circuit. Since we do not have all-optical communications systems, it is
necessary to consider the electronic aspects of the signal-recovery process
at the receiver.
Fiber optic systems typically send digital information, by which we
mean a stream of binary data is transmitted by modulating the optical
source such that the energy emitted during each bit period is at one of
two levels. The high corresponds to 1 and the low corresponds to 0, even
though the low is probably set to a non-zero level (as compared to “no
signal”). A pre-amplifier will typically convert the current signal from
the photodiode into a voltage signal, which may be filtered to clean it up

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