We will start by examining the quantum detectors, since they offer the best performance for detection of optical radiation. In all of the quantum detectors, the photon is absorbed and an electron is liberated in the structure with the energy of the photon. This process is very complicated, and we will not examine it in detail. It is important to recognize that semiconductors feature the basic property that electrons are allowed to exist only at certain energy levels. If the device being used to detect the radiation does not allow electrons with the energy of the incident photon, the photon will not be absorbed, and there will be no signal. On the other hand, if the photon carries an amount of energy which is “allowed” for an electron in the semiconductor, it can be absorbed. Once it is absorbed, the electron moves freely within the device, subject to electric fields (due to applied voltages) and other effects. Many such devices have a complicated “band structure” in which the allowed energies in the structure change with location in the device. One example of such a “band structure” is that offered by a p-n diode. In a diode, the p-n junction produces a step in the allowed energy levels, resulting in a direction in which currents flow easily and the opposite direction in which current flow is greatly reduced. A photodiode is simply a diode, biased against its easy flow direction (“reverse-biased”) so that the current is very low. If a photon is absorbed and an electron is freed, it may pass over the energy barrier if it possesses enough energy. In this respect, the photodiode only produces a current if the absorbed photon has more energy than that needed to traverse the p-n junction. Because of this effect, the p-n photodiode is said to have a cutoff wavelength—photons with wavelength less than the cutoff produce current and are detected, while photons with wavelength greater than the cutoff do not produce current and are not detected. Photodiodes may be biased and operated in two basic modes: photovoltaic and hotoconductive. In the photovoltaic mode, the diode is attached to a virtual ground preamplifier as shown in Figure, and the arrival of photons causes the generation of a voltage which is amplified by the op-amp. The primary feature of this approach is that there is no dc-bias across the diode, and so there is no basic leakage current across
the diode aside from thermally generated currents. This configuration does suffer from slower response because the charge generated must charge the apacitance of the diode, causing an R-C delay. In the photoconductive mode, the diode is biased, and the current flowing across the diode is converted to a voltage (by a resistor), and amplified. A photoconductive circuit is shown in Figure 14.1.2. The primary advantage of this approach is that the applied bias decreases the effective capacitance of the diode (by widening the depletion region), and allows for faster response. Unfortunately, the dc bias also causes some leakage current, so detection of very small signals is compromised. In addition to making optical detectors from diodes, it is also possible to construct them from transistors. In this case, the “photocurrent” is deposited in the base of a bipolar junction transistor. When subjected to a collector-emitter bias (for npn), the current generated by the photons flows from the base to the emitter, and a larger current is caused to flow from the collector to the emitter. For an average transistor, the collector-emitter current is between 10 and 100× larger than the photocurrent, so the phototransistor is fundamentally more sensitive than the diode. Photodiodes and phototransistors are very widely available. Most semiconductor device manufacturers also offer photodiodes and transistors, so there are nearly 100 suppliers. More than 10 manufacturers specialize in photosensors. As a result, optimized photodiodes and transistors are available at very low cost. These devices are also available in packages designed for particular applications. For example, it is common to use a light-emitting diode and a detector mounted in a pair so that passing objects can interrupt the optical beam between them. Opto-interruptors consisting of such emitter-detector pairs are available in a wide variety of configurations. Proximity detectors situated side by side sense the presence of a reflecting surface by causing reflected light to strike the detector. Other applications of optical detector-emitter airs include measurement of the rotation rate of electric motors. In this case, a disk is mounted on the shaft of the motor with a large number of slits cut through it. The detector emitter pair is mounted so that the slits cause an oscillation in the signal—and the rotary position can be determined by counting the peaks in the signal. This is called an optical encoder, or an incremental ncoder, and it is widely used in electric motors, as shown in Figure.
optical encoder
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