Technology
Device Icons: Many types of sources and detectors exist. Icons for some of the common devices which emit or detect light are shown in Figure 1. Sources: Light is produced by many sources.
Incandescent Lamp Light is emitted from a heated metallic wire in a vacuum. The wire has a high resistance and is heated by a current. The emission spectrum is extremely broad extending from the infrared into the visible spectrum.
Arc Lamp Light is emitted from an electric arc between electrodes in a gaseous environment. The wavelengths emitted have a relatively narrow spectral range and are different for different materials.
Lasers Light is emitted due to quantum transitions in gases, liquids, or solids. These sources are characterized by high coherence and directionality over an extremely narrow spectrum. Technically useful lasers include CO2 gas lasers operating at 10,600 nm, Nd:YAG crystalline lasers operating at 1064 nm, HeNe gas lasers operating at 633 nm, and laser diodes (LDs) operating at a variety of wavelengths including the important fiber optic wavelengths of 1300 nm and 1550 nm. The Principal wavelengths for some common lasers can be seen in Figure 2 . | |
Detectors: Light irradiance must be detected or monitored in almost all optical applications. The eye is a common detector. A variety of man-made detectors exist for different optical applications.
Photographic Film Light produces a chemical change in an emulsion that is fixed in a development process. A fine spatial distribution can be recorded for image or information storage.
Thermoelectric Materials Absorbed light produces a temperature change that is detected through a resistance, voltage, or current. Examples include bolometers, thermocouples, pyroelectric detectors. These devices are often used to measure infrared radiation and to measure high power laser beams.
Semiconductor Structures Incident photons are converted by semiconductor structures into usable electrons. Solar cells convert light to electrical power. Common devices for imaging and information detection are CCDs (charge coupled devices), photodiodes, and avalanche photodiodes. The design of semiconductor photodetectors depends on the wavelength sensitivity of the materials. For instance, silicon and germanium are sensitive across the visible spectrum and near infrared spectrum. The compound semiconductors InGaAsP and GaAs are sensitive in the upper visible and near infrared. The peak quantum efficiency (electronic conductors generated per incident photon) of about 900 nm in silicon is near its upper cut-off wavelength of 1100 nm. Detectors made of materials such as InGaAsP are optimized for important optical fiber wavelengths of 1300 nm and 1550. The light sensitivity of some semiconductor structures can be seen in Figure 3. | |
Beam Control: The propagation of light can be modified in many ways. Beam control can be accomplished through changes in direction of propagation, shape of the wavefront, state of polarization, etc.
Lenses – passive elements which cause light to converge to a point or to diverge as if from a point. Mirrors – passive elements which cause a specular return of light. Prisms – passive devices which can speparate spectral components, invert images, attenuate certain polarization components, etc. Polarizers – passive devices which transmit only one direction of polarization. Retardation Plates – structures which change the relative phase between two orthogonal polarizations (the polarization state is thus modified). Waveguides – structures such as optical fibers which confine light to a selected path of propagation Stops – structures which block stray reflections and scattered light. Active Devices – a device which modulates phase, polarization, irradiance, direction, etc. in response to an external signal. These devices are often based on electro-optic, acousto-optic, or magneto-optics effects.
Thin films are often applied to lenses, mirrors, prisms, etc. to minimize reflections (anti-reflection coatings), to enhance reflection, to provide polarization or spectral selectivity, and to protect surfaces. | |
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