Fiber-Optic Transducers

The role of optical fibers in telecommunication systems is now well established.

The recognition of their role in transducers, about two decades ago, led to an explosive number of applications in physical, chemical, and biochemical transducers.101–103 These transducers can be separated into extrinsic and intrinsic types. In extrinsic fiber-optic transducers, the transduction process takes place external to the fiber and the fiber itself plays a passive role as a light conduit.

In intrinsic transducers, however, the transduction process takes place within the optical fiber. The measurand modulates a parameter of the fiber, such as the refractive index, and the fiber in turn modulates the light beam propagating through its core.

Fig 5.43 Total internal reflection in optical fibers.

The optical fiber used in these transducers is of the cladded-core type used in communication links, as shown schematically in Fig. 5.43. The core is made of silica glass, and surrounding the core is a concentric silica cladding.

The light beam in an optical fiber propagates mainly in the core by total internal reflection (TIR), as shown in Fig. 5.43. A light ray, striking the core-cladding interface at S, is partly reflected (A’) and partly refracted (A”) into the cladding.

By Snell’slaw,

A ray from a laser diode propagating through an external medium of refractive index next and striking the fiber end at an angle _ext has to satisfy Snell’s law at that interface, and maintain TIR at the core-cladding interface. Consequently,

Acousto-Optic Deflection Transducers

The acousto-optic (AO) deflection transducer, aside from other applications is used to detect physical vibrations by converting them into a corresponding phase-modulated electronic signal. The primary mechanism is the interaction of an optical wave (photons) with an acoustic wave (phonons), to produce a new optical wave (photons).

Fig 5.41 Acoustooptic deflection transducer.

Figure 5.41a shows diagrammatically an acoustooptic interaction cell, also known as the Bragg cell. It consists of a solid piece of optical-quality flint glass which acts as the interaction medium. It has optical windows on two opposite sides which serve as ports for the entry and exit of the optical beam.

A high-frequency ultrasound transducer is bonded to one of the two remaining sides. The ultrasound transducer launches longitudinal waves in the interaction region and produces regions of compression and rarefaction, which are regions of varying refractive index. These regions are equivalent to a three-dimensional diffraction grating.

An optical (laser) beam entering the Bragg cell at an angle ө and passing through the length L of the grating experiences the periodic changes in the refractive index at the excitation frequency ώm of the ultrasonic transducer.

Consequently, the optical beam leaving the cell is phase-modulated and consists of an optical carrier at ώc and pairs of optical sidebands around ώc, at ώc, ±N_m, where N is an integer corresponding to the order of the sidebands.There is a unique value of ө= өB corresponding to the optical wavelength λ and the acoustic wavelength , when only one of the first-order sidebands grows by constructive interference and the other, along with all the higher sidebands, diminishes owing to destructive interference.

This is known as the Bragg effect, and өB is called the Bragg angle. The angle is given by the Bragg diffraction equation

where m is the ultrasound velocity in the medium and fm is the ultrasonic modulation frequency of the _n wave. The angle between the Bragg deflected beam and the carrier is 2B.

Fig 5.42 Noncontact vibration-measuring setup using a Bragg cell.

Figure 5.42 shows an application where the Bragg cell is used as a noncontact vibration-measuring transducer.98 A He-Ne laser is used as the source of the optical beam. The Bragg cell is used to measure the surface vibrations. On the face of the photodiode, the demodulation of the optical signals takes place.

The output current of the photodiode has a component at (2_m+_). The VHF receiver measures the sideband amplitude, which is proportional to the amplitude of the surface vibrations.

Position Sensing Transducers

Structurally modified photodiodes are used in the measurement of small angles, distances, and machine vibrations, which are related to linear position sensing.

Fig 5.40 Position sensing transducer.

The lateral effect PIN photodiode is generally used for these measurements, and its cross section is shown in Fig. 5.40a. Photons from a laser source impinge on the p-type front surface of the PIN diode and are readily absorbed within the depletion region, where electron-hole pairs are generated.

The electrons move toward the n-type region and generate a photocurrent. The operation of the lateral-effect photodiode can be represented by a variable contact resistor, as shown in Fig. 5.40b. The location of the laser spot defines that variable contact. The photocurrent I0 divides between the paths to the two edge electrodes in a ratio given by

where L is the distance between the electrodes, S is the distance of the laser spot from one of the electrodes, and I0=I1+I2. Using the above equation, S can be determined by measuring I1 and I2. This is the transduction mechanism of the position sensing transducer. It measures displacements along a single axis.

PhotoVoltaic Detectors

Photovoltaic detectors are junction-based semiconductor devices. Photovoltaic detectors can be further classified as operating in the photovoltaic mode or the photoconductive mode. When a p-n junction is formed, a carrier concentration gradient exists because of the majority carriers in the p-type and n-type regions.

Excess electrons from the n-type region diffuse into the p-type region and the excess holes diffuse in the opposite direction in order to equalize this gradient.

The movement of charges across the junction generates an electric field E within the photodiode as indicated in Fig. 5.39a, which prevents any further diffusion of carriers.

When the energy of a photon that impinges on the photodiode is equal to or greater than the energy bandgap, electron-hole pairs are generated in the regions shown in Fig. 5.39a. In the depletion region, the electrons move toward the n side and the holes toward the p side under the influence of the electric field E.

Figure 5.39 Photovoltaic detectors. (a) Energy-level diagram for a p-n photodiode showing the transduction mechanisms. (b) I-V characteristics of photovoltaic detectors.

On the p side, only the electrons move toward the n side and the hole is preventednfrom moving by the field. On the n side, the situation is reversed. The accumulation of charges on either side of the junction results in an open-circuit voltage Voc that is proportional to the photon input. Figure 5.39b shows the I-V characteristics of a photovoltaic detector. Without any photon input, the I-V plot is identical to that of a rectifying diode. 

As the photon input increases, the plot is successively displaced downward. For any photon input level, the open-circuit voltage Voc is proportional to that input. 

If the output terminals are shorted, the short circuit current Isc is also proportional to the photon input. This corresponds to operation in the photovoltaic mode, as indicated by the fourth quadrant of the I-V plot.

When a reverse bias is applied to the photovoltaic detector, the load line is displaced to the third quadrant by an amount equal to the reverse bias. In this

quadrant, the photovoltaic detector behaves as a current source and is said to operate in the photoconductive mode.

PhotoConductive Detectors

Photoconductive detectors decrease their terminal resistance when exposed to light. Figure 5.38a shows a practical representation of a photoconductive detector. It consists of a thin-film trace of a semiconductor such as cadmium sulfide deposited on a ceramic substrate. The length of the meandering trace is L, the width is W, and t is the thickness. There are two metallizations on either side of the trace to which electrical contacts can be made.

Fig 5.38 (a) Photoconductive detector. (b) Energy level diagram for an intrinsic semiconductor.

When a voltage V is applied, a current flows across the width of the trace,allalongthetrace.

The light input modulates ρc and consequently the resistance and the current
across the trace.

The transduction mechanism can be qualitatively explained with the aid of the energy level diagram for an intrinsic semiconductor shown in Fig. 5.38b.

An intrinsic semiconductor absorbs a photon when its energy barely exceeds the energy of the forbidden gap and simultaneously creates an electron-hole pair. The electron makes a single transition across the bandgap into the conduction band. This electron and the corresponding hole in the valence band contribute to the increase in total current. As a consequence, the conductivity increases and the resistance of the photoconductor decreases with increased photon input.

In extrinsic semiconductors, owing to the added impurities, donor and acceptor ionized states are created in the forbidden gap, which also contribute additional carriers to the conduction and valence band, which increase the conductivity even further. The addition of impurities increases the quantum efficiency and the long wavelength cutoff35 of photoconductive detectors.

Photoconductors made from lead sulfide (PbS) and lead selenide (PbSe) are used as infrared detectors in the range of optical wavelengths from 1 to 3 μm and 1 to 6 μm, respectively. Cadmium sulfide (CdS) cells are used in exposure meters, light dimmers, photoelectric relays, and other consumer-type applications.

Cadmium sulfide cells can be optimized in the range of 515 to 730 nm, which is close to the spectral response of the human eye.

Photoelectric Detectors

Photoelectric or photoemissive detectors operate on the photoelectric effect, which is a process in which electrons are liberated from a material surface after absorbing energy from a photon. All the energy from a photon is transferred to a single electron in a metal, and that electron, having acquired the energy in excess of Ew, will emerge from the metal surface with a kinetic energy given by

Einstein’s photoelectric equation

where v is the velocity of the emerging electron, m is the mass of an electron, and Ew is the work function. Ew is the energy required by an electron at the Fermi level to leave the metal surface with the highest kinetic energy.

A photoemissive detector is the basic component of vacuum or gas phototubes and photomultiplier tubes. A phototube consists of a semicircular photoemissive cathode concentric with a central rodlike anode in a special glass enclosure. The incoming photons impinge on the photocathode and generate photoelectrons which leave the cathode surface, and because of the electric field between the anode and cathode, these electrons are collected by the anode. This results in an anode current that is proportional to the number of photons impinging on the cathode.

The photomultiplier tube (PMT) operates on the same basic transduction mechanism as the phototube, but the PMT has provisions for realizing high current gain by current multiplication. The components of the PMT are shown schematically in Fig. 5.37. 

Figure 5.37 Photomultiplier tube.

Photoelectrons are generated as in a phototube. These photoelectrons are focused onto the first element of the PMT chain, known as a dynode.

The photoelectrons from the first dynode are directed in succession to other dynodes, which are at progressively higher potential. The electrons accelerate toward these dynodes, and on striking them, they generate many more electrons by a process known as secondary emission. As an example, if there are 10 stages and if each primary electron produced 4 secondary electrons, then the total number of electrons at the end of the dynode chain would be 410 or approximately 106 electrons. 

The electrons from the last dynode stage are collected by the anode.The resulting amplified current is proportional to the number of input photons.

Photonic Transducers

Photonic transducers measure the light intensity by measuring the impulses of finite energy content, generally known as light quanta or photons. In these transducers, the energy of the photons is converted into a proportional electrical output by means of several transduction mechanisms. In this section, in addition to others, we consider the following:

Photoemissive detectors, where the energy of a photon removes an electron from a metal surface placed in a vacuum or a gas-filled environment.

Photoconductive detectors, where the input photons energy creates electronhole pairs which change the conductivity or resistance of a semiconductor by increasing the number of available charge carriers.

Photovoltaic detectors, p-n junction devices where the input photons also generate electron-hole pairs. The electrons and holes provide additional mechanisms for current conduction. These detectors can be operated in the photovoltaic mode or the photoconductive mode.

The energy of a photon, when expressed in electron volts, is given by

Cold-Cathode Vacuum Gage

In this gage, the electrons are generated by a large electric field instead of a hot cathode. The gage consists of two plate cathodes made from zirconium or thorium.

In the space between these cathodes is placed a ring-shaped anode. A magnet provides a magnetic field normal to the plane of the cathode plates, as shown diagrammatically in Fig. 5.36. It is also known as the Penning or Philips gage.

Fig 5.36 Cold-cathode vacuum gage.

A high voltage is applied between the anode and the cathodes, producing electrons that leave the cold cathode surface. These electrons, in the presence of the magnetic field B, travel in spiral paths toward the positively biased ring anode. Only a few electrons are captured by the anode, but the rest make several passes through the ring anode, and in the process ionize the gas. 

The positiveions collected by the negatively biased cathodes produce the usual ion current proportional to the gas pressure. The effective range of this gage is 1×10-8 to 2×10-2 torr.

Hot-Cathode Vacuum Gage

The triode or thermionic emission ionization transducer is a hot-cathode vacuum gage. The top cross-sectional view is shown diagrammatically in Fig. 5.35.  

Fig5.35 Hot-cathode vacuum gage.

It consists of a tungsten filament and cathode in the center. Concentric with the cathode is the grid, which is made from a fine nickel wire helix, held in position by upright supports. Surrounding the grid is the external nickel plate electrode, which is concentric with the grid. The gage is housed in an enclosure with an opening that is connected to the chamber whose pressure is being measured.

In this configuration, the electrons are accelerated from the cathode to the grid (+180 V), but most electrons not collected by the fine helix move toward the plate (-20 V). As they move in the vicinity of the plate, the electrons get repelled by the negative potential of the plate.

These electrons undergo several oscillations between the grid and the plate, and during this process they collide with the gas molecules, creating positive ions. The ions are attracted toward the negatively biased plate collector, and an ion current flows in the plate circuit.

This ion current, within a certain pressure range, is proportional to the total pressure (vacuum) in the chamber. The effective range for the gage is 10-8 to 10–3 torr.

The Bayard Alpert vacuum transducer is an example of a hot-cathode ultra-high-vacuum gage. The collector is in the center and consists of a fine nickel wire. The filament and cathode located outside the grid. Its operation is similar to that of the triode gage.

The ion current is proportional to the gas pressure down to 10-10 torr, because the undesired x-rays generated by the grid are minimally intercepted by the wire collector. Its operating range is from 4×10-10 to 5×10-2 torr. To measure pressures lower than torr, the glass envelope is not used. The operating range of the nude gage is 2× 10–11 to 1×10–3 torr.

Ionization Transducers

Ionization transducers (vacuum gages) are used to measure low pressures (vacuum levels) below atmospheric pressure (760 torr). The basic transduction mechanism in these gages is the generation of positive ions from the gas molecules that are present in the chambers to be evacuated. The ions are generated as a result of collisions between the gas molecules and the high-energy electrons that are generated specifically for that purpose. The resulting ion current is proportional to the gas pressure in the chamber.

IC Temperature Transducers

The IC temperature (ICT) transducer is a two-terminal monolithic integrated circuit whose output is a current or a voltage that is directly proportional to temperature T/K. The transduction mechanism utilized in this transducer is the dependence of the base-emitter voltage Vbe of a silicon transistor, on the ambient temperature T/K.

Fig 5.34 Integrated-circuit temperature transducer.

If two well-matched IC transistors are connected as shown in Fig. 5.34a, then the difference between the Vbe’s82 of the transistors Q1 and Q2 is proportional to T, if the ratio of the respective emitter areas O is constant. It is made constant by designing the emitter areas of Q1 and Q2 to be in a fixed ratio and operating them at equal collector currents.

When the temperature being monitored changes, the collector currents of Q1 and Q2 also change. The control circuit as drawn in Fig. 5.34a82,83 forces the collector currents to equalize, and V0 is then proportional to the temperature being measured. The total output current IOUT through Q1 and Q2 is

 

IOUT is then proportional to T/K if R0 is constant. In practice, R0 has a very small temperature coefficient of resistance. In Fig. 5.34b, the transducer is connected in series with R1 and R2 to produce a voltage drop proportional to T/K. The ICT transducer operates in the range -55 to +150°C with very good linearity.

Crystalline Quartz Thermometers

A quartz crystal, when coupled to conventional oscillator circuitry by replacing its resonant tank circuit, results in an oscillator whose frequency is controlled by the crystal as shown schematically in Fig. 5.33. The precise control depends on the crystal geometry and orientation with respect to the crystallographic axis referred to as a cut. The AT cut, for example, is used in very stable frequency oscillators.

In a quartz thermometer, the transduction mechanism consists of the changes in the elastic and piezoelectric properties of the crystal as a function of temperature.

These changes result in corresponding changes of the oscillator frequency. The frequency of an oscillator can be represented by a third-order polynomial in temperature t80 and is expressed with reference to 25°C, as shown below.

where A, B, C are the temperature coefficients of frequency, f25 is the oscillator frequency at 25°C, and t is the temperature to be sensed. Hammond discovered a crystal cut where A was large and B, C were simultaneously zero, designated as the linear coefficient (LC) cut. This cut is used as a transduction element in quartz thermometers. Figure 5.33 shows an oscillator using the LC cut.

Fig 5.33 Crystalline quartz thermometer.

Its output is a voltage whose frequency changes linearly with temperature. The outputs from the two oscillators are mixed, and the output of the mixer is a much lower frequency but retains the linear relationship between frequency and temperature. The quartz thermometer measures temperatures in the range -80 to 250°C with an accuracy of ±0.075°C. The sensitivity is 1000 Hz/°C and the corresponding resolution is 0.0001°C.