4.4. Detector Arrays 143
4.4. Detector Arrays (One-dimensional Arrays and CCD and
CMOS Area Sensors)
Simple Arrays
There are several reasons why embedding a number of sensors in a single housing
might be attractive for many applications. One of the main reasons is the ability
to analyze the spatial distribution of incoming radiation without having to move
the detector. The simplest case is a two-element detector (see Fig. 4.10a) allowing
one to find the center of a light spot in the direction OX. Another example is a
four-quadrant detector (Fig. 4.10b) capable of finding the center of a light spot in
both the OX and OY directions. This is done by registering and comparing first
the A+B signals vs. C+D signals and then the A+C signals vs. B+D signals.
Due to the simplicity of signal processing this type of detector was realized first in
analog electronic circuitry and such a configuration was exploited for many years
in various optical navigation systems.
New features arise if more than two elements in one line are configured. In
this case advanced signal processing might be applied allowing one to find the
characteristic points of a light spot (a maximum or a median of the spot intensity
distribution), providing the uncertainty is smaller than the pitch p of the array
of elements (see Fig. 4.10c; details of this approach are explained in Problems
4.16 and 4.17). Usually a multi-line detector is composed of a number of pho-
todiodes separated mechanically (by grooves) on a common substrate, each one
having separate wires for voltage supply and signal output. At present detec-
tors are commercially available with 8, 16, 32, to 128 elements. Evidently
a disadvantage of such an array is the great number of wires to be handled.
This problem is solved by applying the technology of charge coupled devices
(CCDs).
Figure 4.10 Simple detector arrays: (a) two-element detector; (b) four-quadrant detector;
(c) multi-element line detector.
144 4 Detectors of Light
CCD Detectors
A CCD is an integrated circuit (chip) built of a silicon substrate above which a
number of polysilicon transparent electrodes are located (Fig. 4.11a). The elec-
trodes, isolated from the substrate by a SiO
2
layer, are divided in several groups
(three in the figure), each group being connected to a separate wire having one of
three electric potentials,
1
,
2
,or
3
. Photons of the incident radiation travel
through the electrodes and are absorbed in the upper part of the substrate generat-
ing photoelectrons. During the time, τ
exp
, to which the chip is exposed to radiation
the photoelectrons are collected in the vicinity of the electrodes where the electric
field creates potential wells (shown by dotted lines in Fig. 4.11a). As the exposure
time is ended a fast read-out procedure begins (τ
read
τ
exp
) during which the
potentials
1
,
2
, and
3
vary in such a way that the electrons collected under
each electrode are transferred (shifted) in a three-step process to the adjacent ele-
ment, all together as one block, as depicted in Fig. 4.11b where potential wells in
three sequential time intervals, t
1
, t
2
, and t
3
, are shown.
The variation of the wire potentials is then repeated, pushing the electrons
further along the array, until they finally come to the output diode and are read
out to the external electronic circuit. Thus, at the output of the CCD arrange-
ment photoelectrons are emerging as charge pulses, sequentially, one by one,
through a single wire, no matter how many elements there are in the array. The
charge values represent the spatial distribution of light intensity along the CCD line
(Fig. 4.12).
The type of detector discussed above is a one-dimensional (1-D) array. Further
development of the CCD approach results in two-dimensional (2-D) arrays widely
exploited as area sensors capable of capturing a full image in a single shot.Avariety
of possible architectures have been implemented: one of them is shown schemati-
cally in Fig. 4.13a. The image area is a 2-D array of elements (picture elements, or
a) b)
Figure 4.11 CCD detector: (a) schematic of a basic configuration; (b) potential wells and
charge transfer.

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