**2. Optoelectronic scanners for position measurements**

Nowadays optoelectronic scanners are widely used for multiple applications; most of the position or geometry measuring scanners use the triangulation principle or a variant of this measurement method. There are two kinds of scanners for position measuring tasks: scan‐ ners with static sensors and scanners with rotating mirrors. Optical triangulation sensors with CCD or PSD are typically used to measure manufactured goods, such as tire treads, coins, printed circuit boards and ships, principally for monitoring the target distance of small, fragile parts or soft surfaces likely to be deformed if touched by a contact probe.

#### **2.1. Scanners with position triangulation sensors using CCD or PSD**

A triangulation scanner sensor can be formed by three subsystems: emitter, receiver, and electronic processor as shown in figure 1. A spot light is projected onto the work target; a portion of the light reflected by the target is collected through the lens by the detector which can be a CCD,CMOS or PSD array. The angle(α) is calculated, depending on the position of the beam on the detectors CCD or PSD array, hence the distance from the sensor to the tar‐ get is computed by the electronic processor. As stated by Kennedy William P. in [4], the size of the spot is determined by the optical design, and influences the overall system design by setting a target feature size detection limit. For instance, if the spot diameter is 30 μm, it will be difficult to resolve a lateral feature <30 μm.

Many devices are commonly utilized in different types of optical triangulation position scanners and have been built or considered in the past for measuring the position of light spot more efficiently. One method of position detection uses a video camera to electronically capture an image of an object. Image processing techniques are then used to determine the location of the object. For situations requiring the location of a light source on a plane, a po‐ sition sensitive detector (PSD) offers the potential for better resolution at a lower system cost[5]. However, there are other kinds of scanners used commonly in large distances meas‐ urement or in structural health monitoring tasks, these scanners will be explained in the next section.

**Figure 1.** Principle of Triangulation.

This method is based on the assumption that the signal generated by optical scanners for position measurements is a Gaussian-like shape signal. However, during experimentation it has been seen that the optoelectronic scanning sensor output is a Gaussian-like shape signal with some noise and deformation. This is due to some internal and external error sources like the motor eccentricity at low speed scanning, noise and deformation that could interfere with the wavelength of the light sources. Other phenomena could also affect such as of re‐ flection, diffraction, absorption and refraction, producing a trouble that can be minimized

The main interest of this chapter is to describe and explain a method to find the energy cen‐ tre of the signal generated by optical scanners based on a dynamic triangulation, see [3], to

Nowadays optoelectronic scanners are widely used for multiple applications; most of the position or geometry measuring scanners use the triangulation principle or a variant of this measurement method. There are two kinds of scanners for position measuring tasks: scan‐ ners with static sensors and scanners with rotating mirrors. Optical triangulation sensors with CCD or PSD are typically used to measure manufactured goods, such as tire treads, coins, printed circuit boards and ships, principally for monitoring the target distance of small, fragile parts or soft surfaces likely to be deformed if touched by a contact probe.

A triangulation scanner sensor can be formed by three subsystems: emitter, receiver, and electronic processor as shown in figure 1. A spot light is projected onto the work target; a portion of the light reflected by the target is collected through the lens by the detector which can be a CCD,CMOS or PSD array. The angle(α) is calculated, depending on the position of the beam on the detectors CCD or PSD array, hence the distance from the sensor to the tar‐ get is computed by the electronic processor. As stated by Kennedy William P. in [4], the size of the spot is determined by the optical design, and influences the overall system design by setting a target feature size detection limit. For instance, if the spot diameter is 30 μm, it will

Many devices are commonly utilized in different types of optical triangulation position scanners and have been built or considered in the past for measuring the position of light spot more efficiently. One method of position detection uses a video camera to electronically capture an image of an object. Image processing techniques are then used to determine the location of the object. For situations requiring the location of a light source on a plane, a po‐ sition sensitive detector (PSD) offers the potential for better resolution at a lower system cost[5]. However, there are other kinds of scanners used commonly in large distances meas‐ urement or in structural health monitoring tasks, these scanners will be explained in the

by taking measurement in the energy centre of the signal.

**2. Optoelectronic scanners for position measurements**

**2.1. Scanners with position triangulation sensors using CCD or PSD**

reduce errors in position measurements.

390 Optoelectronics - Advanced Materials and Devices

be difficult to resolve a lateral feature <30 μm.

next section.

#### **2.2. Scanners with rotating mirrors and remote sensing**

In the previous section, we described the operational principle of scanners for monitoring the distance of small objects, now we will describe the operational principle of scanners with rotating mirrors for large distances measurement or in structural health monitoring tasks.

There are two main classification of optical scanning: remote sensing and input/output scan‐ ning. Remote sensing detects objects from a distance, as by a space-borne observation plat‐ form. For example an infrared imaging of terrain. Sensing is usually passive and the radiation incoherent and often multispectral. Input / output scanning, on the other hand, is local. A familiar example is the document reading (input) or writing (output).The intensive use of the laser makes the scanning active and the radiation coherent. The scanned point is focused via finite-conjugate optics from a local fixed source, see [6].

In remote sensing there is a variety of scanning methods for capturing the data needed for image formation. These methods may be classified into framing, push broom, and mechani‐ cal. In the first one, there is no need for physical scan motion since it uses electronic scan‐ ning and implies that the sensor has a two-dimensional array of detectors. At present the most used sensor is the CCD and such array requires an optical system with 2-D wide-angle capability. In push broom methods a linear array of detectors are moved along the area to be imaged, e. g. airborne and satellite scanners. A mechanical method includes one and two di‐ mensional scanning techniques incorporating one or multiple detectors and the image for‐ mation by one dimensional mechanical scanning requires the platform with the sensor or the object to be moved in order to create the second dimension of the image.

In these days there is a technique that is being used in many research fields named Hyper‐ spectral imaging (also known as imaging spectroscopy). It is used in remotely sensed satel‐ lite imaging and aerial reconnaissance like the NASA's premier instruments for Earth exploration, the Jet Propulsion Laboratory's Airborne Visible-Infrared Imaging Spectrome‐ ter (AVIRIS) system. With this technique the instruments are capable of collecting high-di‐ mensional image data, using hundreds of contiguous spectral channels, over the same area on the surface of the Earth, as shown in figure 2. where the image measures the reflected radiation in the wavelength region from 0.4 to 2.5 μm using 224 spectral channels, at nomi‐ nal spectral resolution of 10 nm. The wealth of spectral information provided by the latest generation hyperspectral sensors has opened ground breaking perspectives in many appli‐ cations, including environmental modelling and assessment; target detection for military and defence/security deployment; urban planning and management studies, risk/hazard prevention and response including wild-land fire tracking; biological threat detection, moni‐ toring of oil spills and other types of chemical contamination [7].

**Input / Output Scanning**

**Table 1.** Examples of Input / Output Scanning.

illustrates an hexagonal rotating mirror scanner.

*2.2.1. Polygonal scanners*

**Figure 3.** Polygon scanner.

**Input Output**

Image scanning / digitising Image recording / printing Bar-code reading Colour image reproduction Optical inspection Medical image outputs Optical character recognition Data marking and engraving Optical data readout Micro image recording Graphic arts camera Reconnaissance recording Scanning confocal microscopy Optical data storage Colour separation Phototypesetting

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Robot vision Graphic arts platemaking Laser radar Earth resources imaging Mensuration (Measurement) Data / Image display

These scanners have a polygonal mirror rotating at constant speed by way of an electric mo‐ tor and the radiation received by the lens is reflected on a detector. The primary advantages of polygonal scanners are speed, the availability of wide scan angles, and velocity stability. They are usually rotated continuously in one direction at a fixed speed to provide repetitive unidirectional scans which are superimposed in the scan field, or plane, as the case may be. When the number of facets reduces to one, it is identified as a monogon scanner, figure 3

**Figure 2.** The concept of hyperspectral imaging illustrated using NASA's AVIRIS sensor [7].

While remote sensing requires capturing passive radiation for image formation, active in‐ put/output scanning needs to illuminate an object or medium with a ''flying spot, '' derived typically from a laser source. In Table 1, we listed some examples divided into two principal functions: input (when the scattered radiation from the scanning spot is detected) and out‐ put (when the radiation is used for recording or displaying). Therefore, we can say that in input scanning the radiation is modulated by the target to form a signal and in the output scanning it is modulated by a signal.


**Table 1.** Examples of Input / Output Scanning.

#### *2.2.1. Polygonal scanners*

on the surface of the Earth, as shown in figure 2. where the image measures the reflected radiation in the wavelength region from 0.4 to 2.5 μm using 224 spectral channels, at nomi‐ nal spectral resolution of 10 nm. The wealth of spectral information provided by the latest generation hyperspectral sensors has opened ground breaking perspectives in many appli‐ cations, including environmental modelling and assessment; target detection for military and defence/security deployment; urban planning and management studies, risk/hazard prevention and response including wild-land fire tracking; biological threat detection, moni‐

toring of oil spills and other types of chemical contamination [7].

392 Optoelectronics - Advanced Materials and Devices

**Figure 2.** The concept of hyperspectral imaging illustrated using NASA's AVIRIS sensor [7].

scanning it is modulated by a signal.

While remote sensing requires capturing passive radiation for image formation, active in‐ put/output scanning needs to illuminate an object or medium with a ''flying spot, '' derived typically from a laser source. In Table 1, we listed some examples divided into two principal functions: input (when the scattered radiation from the scanning spot is detected) and out‐ put (when the radiation is used for recording or displaying). Therefore, we can say that in input scanning the radiation is modulated by the target to form a signal and in the output

These scanners have a polygonal mirror rotating at constant speed by way of an electric mo‐ tor and the radiation received by the lens is reflected on a detector. The primary advantages of polygonal scanners are speed, the availability of wide scan angles, and velocity stability. They are usually rotated continuously in one direction at a fixed speed to provide repetitive unidirectional scans which are superimposed in the scan field, or plane, as the case may be. When the number of facets reduces to one, it is identified as a monogon scanner, figure 3 illustrates an hexagonal rotating mirror scanner.

**Figure 3.** Polygon scanner.

#### *2.2.2. Pyramidal and prismatic facets*

In these types of scanners, the incoming radiation is focused on a regular pyramidal poly‐ gon with a number of plane mirrors facets at an angle, rather than parallel, to the rotational axis. This configuration permits smaller scan angles with fewer facets than those with polyg‐ onal mirrors. Principal arrangements of facets are termed prismatic or pyramidal. The pyra‐ midal arrangement allows the lens to be oriented close to the polygon, while the prismatic configuration requires space for a clear passage of the input beam.

*2.2.4. Galvanometer and resonant scanners*

electrically driven mirror transducer as shown in figure 5.

**Figure 5.** Galvanometer scanner (From http://www.yedata.com).

*2.2.5. 45° cylindrical mirror scanner*

To avoid the scan non uniformities which can arise from facet variations of polygons or holographic deflectors, one might avoid multifacets. Reducing the number to one, the poly‐ gon becomes a monogon. This adapts well to the internal drum scanner, which achieves a high duty cycle, executing a very large angular scan within a cylindrical image surface. Flatfield scanning, however, as projected through a flat-field lens, allows limited optical scan angle, resulting in a limited duty cycle from a rotating monogon. If the mirror is vibrated rather than rotated completely, the wasted scan interval may be reduced. Such components must, however, satisfy system speed, resolution, and linearity. Vibrational scanners include the familiar galvanometer and resonant devices and the least commonly encountered piezo‐

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Optical scanning systems can use coherent light emitting sources, such as laser or incoherent light sources like the lights of a vehicle. In the use of laser as light emitting source, the meas‐ urements are independent of environment lighting, so it is possible to explore during day and night, however, there are some disadvantages such as the initial cost, the hazard due to its high energy output, and that they cannot penetrate dense fog, rain, and warm air cur‐ rents that rise to the structures, interfering the laser beam, besides, it is difficult to properly align the emitter and receiver. A passive optical scanning system for SHM can use conven‐ tional light emitting sources placed in a structure to determine if its position changes due to deteriorating. Figure 6 illustrates a general schematic diagram with the main elements of the

optical scanning aperture used to generate the signals to test the proposed method.

#### *2.2.3. Holographic scanners*

Almost all holographic scanners comprise a substrate which is rotated about an axis, and utilize many of the characterising concepts of polygons. An array of holographic elements disposed about the substrate serves as facets, to transfer a fixed incident beam to one which scans. As with polygons, the number of facets is determined by the optical scan angle and duty cycle, and the elemental resolution is determined by the incident beam width and the scan angle. In radially symmetric systems, scan functions can be identical to those of the pyramidal polygon. Meanwhile there are many similarities to polygons, there are significant advantages and limitations.

**Figure 4.** Polygonal scanner (From http://beta.globalspec.com/reference/34369/160210/chapter-4-3-5-4-scannerdevices-and-techniques-postobjective-configurations).

#### *2.2.4. Galvanometer and resonant scanners*

*2.2.2. Pyramidal and prismatic facets*

394 Optoelectronics - Advanced Materials and Devices

*2.2.3. Holographic scanners*

advantages and limitations.

devices-and-techniques-postobjective-configurations).

In these types of scanners, the incoming radiation is focused on a regular pyramidal poly‐ gon with a number of plane mirrors facets at an angle, rather than parallel, to the rotational axis. This configuration permits smaller scan angles with fewer facets than those with polyg‐ onal mirrors. Principal arrangements of facets are termed prismatic or pyramidal. The pyra‐ midal arrangement allows the lens to be oriented close to the polygon, while the prismatic

Almost all holographic scanners comprise a substrate which is rotated about an axis, and utilize many of the characterising concepts of polygons. An array of holographic elements disposed about the substrate serves as facets, to transfer a fixed incident beam to one which scans. As with polygons, the number of facets is determined by the optical scan angle and duty cycle, and the elemental resolution is determined by the incident beam width and the scan angle. In radially symmetric systems, scan functions can be identical to those of the pyramidal polygon. Meanwhile there are many similarities to polygons, there are significant

**Figure 4.** Polygonal scanner (From http://beta.globalspec.com/reference/34369/160210/chapter-4-3-5-4-scanner-

configuration requires space for a clear passage of the input beam.

To avoid the scan non uniformities which can arise from facet variations of polygons or holographic deflectors, one might avoid multifacets. Reducing the number to one, the poly‐ gon becomes a monogon. This adapts well to the internal drum scanner, which achieves a high duty cycle, executing a very large angular scan within a cylindrical image surface. Flatfield scanning, however, as projected through a flat-field lens, allows limited optical scan angle, resulting in a limited duty cycle from a rotating monogon. If the mirror is vibrated rather than rotated completely, the wasted scan interval may be reduced. Such components must, however, satisfy system speed, resolution, and linearity. Vibrational scanners include the familiar galvanometer and resonant devices and the least commonly encountered piezo‐ electrically driven mirror transducer as shown in figure 5.

**Figure 5.** Galvanometer scanner (From http://www.yedata.com).

#### *2.2.5. 45° cylindrical mirror scanner*

Optical scanning systems can use coherent light emitting sources, such as laser or incoherent light sources like the lights of a vehicle. In the use of laser as light emitting source, the meas‐ urements are independent of environment lighting, so it is possible to explore during day and night, however, there are some disadvantages such as the initial cost, the hazard due to its high energy output, and that they cannot penetrate dense fog, rain, and warm air cur‐ rents that rise to the structures, interfering the laser beam, besides, it is difficult to properly align the emitter and receiver. A passive optical scanning system for SHM can use conven‐ tional light emitting sources placed in a structure to determine if its position changes due to deteriorating. Figure 6 illustrates a general schematic diagram with the main elements of the optical scanning aperture used to generate the signals to test the proposed method.

The distance T2π is equal to the time between m1 and m1, that are expressed by the code

A Method and Electronic Device to Detect the Optoelectronic Scanning Signal Energy Centre

On the other hand, the time tα is equal to the distance between m1 andm2, could be ex‐

(1)

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397

<sup>0</sup> = × (2)

(3)

2 0 *NTf* 2 p = × p

> *N tf* a

> > <sup>2</sup> 2

a p = ×*N N* a p

a

Where f0 is a standard frequency reference. With this consideration the time variable could

All detectors (sensors) act as transducer that receive photons and produce an electrical re‐ sponse that can be amplified and converted into a form of relevant parameters to handle the input data for results interpretation. Among relevant parameters we can find spectral re‐ sponse, spectral bandwidth, linearity, dynamic range, quantum efficiency, noise, imaging properties and time response. Photon detectors respond directly to individual photons. Ab‐ sorbed photons release one or more bound charge carriers in the detector that modulates the electric current in the material and moves it directly to an output amplifier. Photon detectors can be used ina spectral band width from X-ray and ultraviolet to visible and infrared spec‐ tral regions. We can classify them as analogue waveform output and image detectors, how‐ ever, another type of classification is also possible but we will only describe this type of

Analogue waveform output detectors are used as an optical receiver to convert light into electricity. This principle applies to photo detectors, phototransistors and other detectors as photovoltaic cells, and photo resistance, but the most widely used today in position measur‐

The photodiode could convert light in either current or voltage, depending upon the mode of operation. A photodiode is based on a junction of oppositely doped regions (pn junction) in a sample of a semiconductor. This creates a region depleted of charge carriers that results

N2π as defined in equation 1.

**3. Scanner sensors**

sensors in this section.

*3.1.1. Photodiode*

Analogue waveform output detectors

ing process are the photodiode andthe phototransistors.

pressed by the code defined in equation 2.

be eliminated from equation 2obtaining equation 3,see [8].

**Figure 6.** Cylindrical Mirror Scanner.

The optical system is integrated by the light emitter source set at a distance from the receiv‐ er; the receiver is compound by the mirror E, which spins with an angular velocity ω. The beam emitted arrives with an incident angle β with respect to the perpendicular mirror, and is reflected with the same angle β, according to the reflecting principle (C L. Wyatt, 1991) to pass through a lens that concentrates the beam to be captured by the photodiode, which generates a signal "f" with a shape similar to the Gaussian function. When the mirror starts to spin, the sensor "s" is synchronized with the origin generating a pulse that indicates the starting of measurement that finishes when the photodiode releases the stop signal. This sig‐ nal is released when the Gaussian signal energetic centre has been detected.

Figure 7 shows that light intensity increments in the centre of the signal generated by the scanner. The sensor "s" generates a starting signal when tα =0, then the stop signal is acti‐ vated when the Gaussian function geometric centre has been detected.

**Figure 7.** Signal generated by a 45° cylindrical mirror scanner.

The distance T2π is equal to the time between m1 and m1, that are expressed by the code N2π as defined in equation 1.

$$N\_{2\pi} = T \mathcal{Z} \pi \cdot f\_0 \tag{1}$$

On the other hand, the time tα is equal to the distance between m1 andm2, could be ex‐ pressed by the code defined in equation 2.

$$
\Lambda \text{ Na} = t\_{\alpha} \cdot f\_0 \tag{2}
$$

Where f0 is a standard frequency reference. With this consideration the time variable could be eliminated from equation 2obtaining equation 3,see [8].

$$\alpha = 2\pi \cdot N\_{\alpha} / N\_{2\pi} \tag{3}$$

#### **3. Scanner sensors**

**Figure 6.** Cylindrical Mirror Scanner.

396 Optoelectronics - Advanced Materials and Devices

The optical system is integrated by the light emitter source set at a distance from the receiv‐ er; the receiver is compound by the mirror E, which spins with an angular velocity ω. The beam emitted arrives with an incident angle β with respect to the perpendicular mirror, and is reflected with the same angle β, according to the reflecting principle (C L. Wyatt, 1991) to pass through a lens that concentrates the beam to be captured by the photodiode, which generates a signal "f" with a shape similar to the Gaussian function. When the mirror starts to spin, the sensor "s" is synchronized with the origin generating a pulse that indicates the starting of measurement that finishes when the photodiode releases the stop signal. This sig‐

Figure 7 shows that light intensity increments in the centre of the signal generated by the scanner. The sensor "s" generates a starting signal when tα =0, then the stop signal is acti‐

nal is released when the Gaussian signal energetic centre has been detected.

vated when the Gaussian function geometric centre has been detected.

**Figure 7.** Signal generated by a 45° cylindrical mirror scanner.

All detectors (sensors) act as transducer that receive photons and produce an electrical re‐ sponse that can be amplified and converted into a form of relevant parameters to handle the input data for results interpretation. Among relevant parameters we can find spectral re‐ sponse, spectral bandwidth, linearity, dynamic range, quantum efficiency, noise, imaging properties and time response. Photon detectors respond directly to individual photons. Ab‐ sorbed photons release one or more bound charge carriers in the detector that modulates the electric current in the material and moves it directly to an output amplifier. Photon detectors can be used ina spectral band width from X-ray and ultraviolet to visible and infrared spec‐ tral regions. We can classify them as analogue waveform output and image detectors, how‐ ever, another type of classification is also possible but we will only describe this type of sensors in this section.

Analogue waveform output detectors

Analogue waveform output detectors are used as an optical receiver to convert light into electricity. This principle applies to photo detectors, phototransistors and other detectors as photovoltaic cells, and photo resistance, but the most widely used today in position measur‐ ing process are the photodiode andthe phototransistors.

#### *3.1.1. Photodiode*

The photodiode could convert light in either current or voltage, depending upon the mode of operation. A photodiode is based on a junction of oppositely doped regions (pn junction) in a sample of a semiconductor. This creates a region depleted of charge carriers that results in high impedance. The high impedance allows the construction of detectors using silicon and germanium to operate with high sensitivity at room temperatures.

moving phonons; this allows faster switching between on and off states and Ga As also is more sensitive to the light intensity. Once charge carriers are produced in the diode materi‐

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Photodiodes are similar to regular semiconductor diodes except that they may be either ex‐ posed to detect vacuum UV or X-ray or packaged with a windows or optical fibre connec‐ tion to allow light to reach the sensitive part of the device. Many diodes designed to use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the

Spectral response: The wavelength of the radiation to be detected is an important parameter. As shown in figure 9, silicon becomes transparent to radiation of a wavelength longer than

Linearity: Current output of the photodiode is very linear with radiant power throughout a wide range. Nonlinearity remains below approximately 0.02% up to 100mA photodiode cur‐ rent. The photodiode can produce output currents of 1mA or greater with high radiant pow‐ er, but nonlinearity increases to a certain percent in this region. This excellent linearity at high radiant power assumes that the full photodiode area is uniformly illuminated. If the light source is focused on a small area of the photodiode, nonlinearity will occur at lower

Dynamic Range: Dynamic response varies with feedback resistor, using 1M resistor, the dynamic response of the photodiode can be modelled as a simple R/C circuit with a – 3dB cut off frequency of 4kHz. This yields a rise time of approximately 90μs (10% to

Noise: The noise performance of a photo detector is sometimes characterized by Noise Effec‐ tive Power (NEP). This is the radiant power which would produce an output signal equal to the noise level. NEP has the units of radiant power (watts). The typical performance curve "Noise Effective Power vs. Measurement Bandwidth" shows how NEP varies with RF and

al, the carriers reach the junction by diffusion.

**Figure 9.** Spectral responsivity and response vs. incident angle of a photodiode.

speed of response [9].

1100 nm.

radiant power.

90%). See figure 10.

measurement bandwidth.

**Figure 8.** Cross section of a typical silicon photodiode.

A cross section of a typical silicon photodiode is shown in the figure. 8. N type silicon is the starting material. A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron). The inter‐ face between the "p" layer and the "n" silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. The back contact is the cathode; the front contact is the anode. The active area is coat‐ ed with silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating. The thickness of this coating is optimized for particular irradiation wavelengths.

In semiconductors whose bandgaps permit intrinsic operation in the 1-15μm, a junction is often necessary to achieve good performance at any temperature. Because these detectors operate through intrinsic rather than extrinsic absorption, they can achieve high quantum efficiency in small volumes. However, high performance photodiodes are not available at wavelengths longer than about 1.5μm because of the lack of high-quality intrinsic semicon‐ ductors with extremely small bandgaps. Standard techniques of semiconductor device fabri‐ cation allow photodiodes to be constructed in arrays with many thousands, even millions, of pixels. Photodiodes are usually the detectors of choice for 1-6μm and are often useful not only at longer infrared wavelengths but also in the visible and near ultraviolet.

The photodiode operates by using an illumination window, which allows the use of light as an external input. Since light is used as an input, the diode is operated under reverse bias conditions. Under the reverse bias condition the current through the junction is zero when no light is present, this allows the diode to be used as a switch or relay when sufficient light is present.

Photodiodes are mainly made from gallium arsenide instead of silicon because silicon cre‐ ates crystal lattice vibrations called phonons when photons are absorbed in order to create electron-hole pairs. Gallium arsenide can produce electron-hole pairs without the slowly moving phonons; this allows faster switching between on and off states and Ga As also is more sensitive to the light intensity. Once charge carriers are produced in the diode materi‐ al, the carriers reach the junction by diffusion.

in high impedance. The high impedance allows the construction of detectors using silicon

A cross section of a typical silicon photodiode is shown in the figure. 8. N type silicon is the starting material. A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron). The inter‐ face between the "p" layer and the "n" silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. The back contact is the cathode; the front contact is the anode. The active area is coat‐ ed with silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating. The thickness of this coating is optimized for particular irradiation

In semiconductors whose bandgaps permit intrinsic operation in the 1-15μm, a junction is often necessary to achieve good performance at any temperature. Because these detectors operate through intrinsic rather than extrinsic absorption, they can achieve high quantum efficiency in small volumes. However, high performance photodiodes are not available at wavelengths longer than about 1.5μm because of the lack of high-quality intrinsic semicon‐ ductors with extremely small bandgaps. Standard techniques of semiconductor device fabri‐ cation allow photodiodes to be constructed in arrays with many thousands, even millions, of pixels. Photodiodes are usually the detectors of choice for 1-6μm and are often useful not

The photodiode operates by using an illumination window, which allows the use of light as an external input. Since light is used as an input, the diode is operated under reverse bias conditions. Under the reverse bias condition the current through the junction is zero when no light is present, this allows the diode to be used as a switch or relay when sufficient light

Photodiodes are mainly made from gallium arsenide instead of silicon because silicon cre‐ ates crystal lattice vibrations called phonons when photons are absorbed in order to create electron-hole pairs. Gallium arsenide can produce electron-hole pairs without the slowly

only at longer infrared wavelengths but also in the visible and near ultraviolet.

and germanium to operate with high sensitivity at room temperatures.

**Figure 8.** Cross section of a typical silicon photodiode.

398 Optoelectronics - Advanced Materials and Devices

wavelengths.

is present.

Photodiodes are similar to regular semiconductor diodes except that they may be either ex‐ posed to detect vacuum UV or X-ray or packaged with a windows or optical fibre connec‐ tion to allow light to reach the sensitive part of the device. Many diodes designed to use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the speed of response [9].

Spectral response: The wavelength of the radiation to be detected is an important parameter. As shown in figure 9, silicon becomes transparent to radiation of a wavelength longer than 1100 nm.

Linearity: Current output of the photodiode is very linear with radiant power throughout a wide range. Nonlinearity remains below approximately 0.02% up to 100mA photodiode cur‐ rent. The photodiode can produce output currents of 1mA or greater with high radiant pow‐ er, but nonlinearity increases to a certain percent in this region. This excellent linearity at high radiant power assumes that the full photodiode area is uniformly illuminated. If the light source is focused on a small area of the photodiode, nonlinearity will occur at lower radiant power.

**Figure 9.** Spectral responsivity and response vs. incident angle of a photodiode.

Dynamic Range: Dynamic response varies with feedback resistor, using 1M resistor, the dynamic response of the photodiode can be modelled as a simple R/C circuit with a – 3dB cut off frequency of 4kHz. This yields a rise time of approximately 90μs (10% to 90%). See figure 10.

Noise: The noise performance of a photo detector is sometimes characterized by Noise Effec‐ tive Power (NEP). This is the radiant power which would produce an output signal equal to the noise level. NEP has the units of radiant power (watts). The typical performance curve "Noise Effective Power vs. Measurement Bandwidth" shows how NEP varies with RF and measurement bandwidth.

Imagining Properties: The output is measured in voltage thru time, imaging like a Gaussianlike signal shape [10].

**3.2. Image sensors**

*3.2.1. CCD sensor (charge coupled device)*

**Figure 12.** CCD operating principle.

tronic images.

sor cells.

Nowadays image sensors are recognized as the most advanced technology to record elec‐

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These sensors are based on the photoelectric effect in silicon. When a photon of an appropri‐ ate wavelength (in general between 200 and 1000 nm) hits silicon, it generates an electronhole pair. If an electric field is present, the electron and the hole are separated and charge can accumulate, proportional to the number of incident photons, and therefore the scene im‐ aged onto the detector will be reproduced if a proper X-Y structure is present. Each basic element, defining the granularity of the sensor, is called a pixel (picture element) [12].

This sensor is used in scanners to capture digital Images. Typically, it is an array to perform the scanning row by row, scanning one horizontal row pixel at a time, moving the scan line down with a carriage motor. The scanners that use this CCD sensor cell use an optical lens, often like a fine camera lens, and a system of mirrors to focus the image onto the CCD sen‐

The CCD sensor cell is an analogue device, when light strikes the chip, it is held as a small electrical charge in each photo sensor. The charges are converted to voltage, one pixel at a time, as they are read from the chip. An additional circuitry is also required to convert ana‐

logue to digital signal to produce an image as shown in figure 12.

**Figure 10.** Small and large signal dynamic response of a photodiode.

#### *3.1.2. Phototransistors*

The Phototransistor is similar to the photodiode except for an n-type region added to the photodiode configuration. The phototransistor includes a photodiode with an internal gain. A phototransistor can be represented as a bipolar transistor that is enclosed in a transparent case so that photons can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and the photodiode cur‐ rent is then amplified by the transistor's current gain β (or hfe). Unlike photodiode photo‐ transistor cannot detect light any better, it means that they are unable to detect low levels of light. The drawback of a phototransistor is the slower response time in comparison to a pho‐ to diode. If the emitter is left unconnected, the phototransistor becomes a photodiode [11].

**Figure 11.** Relative spectral sensitivity and collector current vs. angular displacement of a phototransistor.

#### **3.2. Image sensors**

Imagining Properties: The output is measured in voltage thru time, imaging like a Gaussian-

The Phototransistor is similar to the photodiode except for an n-type region added to the photodiode configuration. The phototransistor includes a photodiode with an internal gain. A phototransistor can be represented as a bipolar transistor that is enclosed in a transparent case so that photons can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and the photodiode cur‐ rent is then amplified by the transistor's current gain β (or hfe). Unlike photodiode photo‐ transistor cannot detect light any better, it means that they are unable to detect low levels of light. The drawback of a phototransistor is the slower response time in comparison to a pho‐ to diode. If the emitter is left unconnected, the phototransistor becomes a photodiode [11].

**Figure 11.** Relative spectral sensitivity and collector current vs. angular displacement of a phototransistor.

like signal shape [10].

400 Optoelectronics - Advanced Materials and Devices

*3.1.2. Phototransistors*

**Figure 10.** Small and large signal dynamic response of a photodiode.

Nowadays image sensors are recognized as the most advanced technology to record elec‐ tronic images.

These sensors are based on the photoelectric effect in silicon. When a photon of an appropri‐ ate wavelength (in general between 200 and 1000 nm) hits silicon, it generates an electronhole pair. If an electric field is present, the electron and the hole are separated and charge can accumulate, proportional to the number of incident photons, and therefore the scene im‐ aged onto the detector will be reproduced if a proper X-Y structure is present. Each basic element, defining the granularity of the sensor, is called a pixel (picture element) [12].

#### *3.2.1. CCD sensor (charge coupled device)*

This sensor is used in scanners to capture digital Images. Typically, it is an array to perform the scanning row by row, scanning one horizontal row pixel at a time, moving the scan line down with a carriage motor. The scanners that use this CCD sensor cell use an optical lens, often like a fine camera lens, and a system of mirrors to focus the image onto the CCD sen‐ sor cells.

The CCD sensor cell is an analogue device, when light strikes the chip, it is held as a small electrical charge in each photo sensor. The charges are converted to voltage, one pixel at a time, as they are read from the chip. An additional circuitry is also required to convert ana‐ logue to digital signal to produce an image as shown in figure 12.

**Figure 12.** CCD operating principle.

The basic concept o CCDs is a simple series connection of Metal-Oxide-Semiconductor ca‐ pacitors (MOS capacitors). The individual capacitors are physically located very close to each other. The CCD is a type of charge storage and transport device: charge carriers are stored on the MOS capacitors and transported. To operate the CCDs, digital pulses are ap‐ plied to the top plates of the MOS structures. The charge packets can be transported from one capacitor to its neighbour capacitor. If the chain of MOS capacitors is closed with an out‐ put node and an appropriate output amplifier, the charges forming part of a moving charge packet can be translated into a voltage and measured at the outside of the device. The way the charges are loaded into the CCDs is application dependent.

The advantages of CCDs are size, weight, cost, power consumption, stability and image quality (low noise, good dynamic range, and colour uniformity). A disadvantage is that it is susceptible to vertical smear from bright light sources when the sensor is overloaded [13].

**Figure 14.** CMOS sensor.

the best QE.

*3.2.3. Position sensing detector*

Typically the sensitivity of the sensor is evaluated based on the quantum efficiency, QE, or the chance that one photon generates one electron in the sensor at a given wavelength. This is a good indicator. This gives the minimum amount of light you can see.In general, CMOS sensors have a higher QE in the sensitivity due to their design structure, and this can be fur‐ ther optimized by producing the sensor using a thicker epitaxial layer (shown as CMOS 1-b in Figure 15 below).Hence, at 800nm, the CMOS sensor with the thicker epitaxial layer has

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**Figure 15.** Relative radiant sensitivity vs. angular displacement and CCD vs. CMOS sensitivity.

Other device widely used as triangulation position sensor is the PSD (Position Sensing De‐ tector), which converts an incident light spot into continuous position data(figure 16)and is

**Figure 13.** Responsivity and quantum efficiency vs. wavelength of a CCD.

As light enters the active photo sites in the image area, electron hole pairs are generated and the electrons are collected in the potential wells of the pixels. The wells have a finite charge storage capacity determined by the pixel design [14].

#### *3.2.2. CMOS sensor (complementary metal oxide semiconductor)*

This is an active pixel sensor or image sensor fabricated with an integrated circuit that has an array of pixel sensors, each pixel containing both a CMOS component and an ac‐ tive amplifier. Extra circuitry next to each photo sensor converts the light energy to a voltage and additional circuitry is also required to convert analogue to digital signal. In CMOS sensor the incoming photons go through colour filters, then through glass layers supporting the metal interconnect layers, and then into silicon, where they are absorbed, exciting electrons that then travel to photodiode structures to be stored as signal. These are commonly used in cell phone cameras and web cameras. They can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCDs, scaling to high resolution formats. CMOS sensors are cheaper to manufac‐ ture than CCD sensors. However, a disadvantage is that they are susceptible to unde‐ sired effects that come as a result of rolling shutter [15].

**Figure 14.** CMOS sensor.

The basic concept o CCDs is a simple series connection of Metal-Oxide-Semiconductor ca‐ pacitors (MOS capacitors). The individual capacitors are physically located very close to each other. The CCD is a type of charge storage and transport device: charge carriers are stored on the MOS capacitors and transported. To operate the CCDs, digital pulses are ap‐ plied to the top plates of the MOS structures. The charge packets can be transported from one capacitor to its neighbour capacitor. If the chain of MOS capacitors is closed with an out‐ put node and an appropriate output amplifier, the charges forming part of a moving charge packet can be translated into a voltage and measured at the outside of the device. The way

The advantages of CCDs are size, weight, cost, power consumption, stability and image quality (low noise, good dynamic range, and colour uniformity). A disadvantage is that it is susceptible to vertical smear from bright light sources when the sensor is overloaded [13].

As light enters the active photo sites in the image area, electron hole pairs are generated and the electrons are collected in the potential wells of the pixels. The wells have a finite charge

This is an active pixel sensor or image sensor fabricated with an integrated circuit that has an array of pixel sensors, each pixel containing both a CMOS component and an ac‐ tive amplifier. Extra circuitry next to each photo sensor converts the light energy to a voltage and additional circuitry is also required to convert analogue to digital signal. In CMOS sensor the incoming photons go through colour filters, then through glass layers supporting the metal interconnect layers, and then into silicon, where they are absorbed, exciting electrons that then travel to photodiode structures to be stored as signal. These are commonly used in cell phone cameras and web cameras. They can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCDs, scaling to high resolution formats. CMOS sensors are cheaper to manufac‐ ture than CCD sensors. However, a disadvantage is that they are susceptible to unde‐

the charges are loaded into the CCDs is application dependent.

402 Optoelectronics - Advanced Materials and Devices

**Figure 13.** Responsivity and quantum efficiency vs. wavelength of a CCD.

storage capacity determined by the pixel design [14].

*3.2.2. CMOS sensor (complementary metal oxide semiconductor)*

sired effects that come as a result of rolling shutter [15].

Typically the sensitivity of the sensor is evaluated based on the quantum efficiency, QE, or the chance that one photon generates one electron in the sensor at a given wavelength. This is a good indicator. This gives the minimum amount of light you can see.In general, CMOS sensors have a higher QE in the sensitivity due to their design structure, and this can be fur‐ ther optimized by producing the sensor using a thicker epitaxial layer (shown as CMOS 1-b in Figure 15 below).Hence, at 800nm, the CMOS sensor with the thicker epitaxial layer has the best QE.

**Figure 15.** Relative radiant sensitivity vs. angular displacement and CCD vs. CMOS sensitivity.

#### *3.2.3. Position sensing detector*

Other device widely used as triangulation position sensor is the PSD (Position Sensing De‐ tector), which converts an incident light spot into continuous position data(figure 16)and is more accurate and faster than CCD because the PSD is a continuous sensor, while CCD is a matrix of dots switched on and off and its resolution depends on how many dots are located on the sensor. Typically a linear CCD has 1024 or 2048 dots.

**Figure 16.** PSD Operating principle.

PSD has an infinite resolution because it is a continuous sensor, therefore the digital resolu‐ tion of a PSD depends not on the PSD itself. Alignment sensors using CCDs have to be pro‐ grammed to do multiple measurements at every step to improve accuracy and to lower noise because linear CCDs have a low resolution. To have the same accuracy of a PSD, CCD should perform no less than 32 measurements and hence calculate the average measure‐ ment. However, CCD is generally preferred to PSD because PSD needs an expensive circuit design including Analogue-to-Digital conversion [17].

**Figure 17.** Ideal photon distribution on CCD and PSD sensors.

mirror.

**Figure 18.** Real photon distributions as a function of the detector for different diffraction patterns.

Based on A.M. van Oijen and J.Köhler study, we can observe that the spatial distribution function of light has an Airy-function-like shape, see [20]. It is well known that CCD, CMOS and SPD use the light quantity distribution of the entire beam spot entering the light receiv‐ ing element to determine the beam spot centre or centroid and identifies this as the target position. However, they are not the only sensors that generate a similar Gaussian-like shape, there are still a lot of sensors to be further investigated. For example, a simple photodiode can also originate a similar Gaussian-like shape, when it is used as a sensor on a scanner with a rotating mirror, [21]. Figure 19 below illustrates a hypothetical spot model, and at‐ tempts to explain how the signal is created by the photodiode on a scanner with a rotating

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