Optoelectronics

Optoelectronics deals with the study of all the electrically driven light-emitting devices, all devices which produce electrical effects when exposed to light, and all methods of conducting light through fixed pathways.

Photometry units

When a current is passed through a light source, a portion of the power dissipated appears in the form of light radiated outward from the source; the frequencies radiated will depend on the source used, and usually on the current used. The energy radiated, i.e. for the visible light is expressed in a special unit of luminous flux, the lumen (lm). When light falls on a surface to be illuminated, the illumination is measured in lumen per square metre of surface. The luminance or brightness of source of light is also measured in lm/m² of emitting surface.

Light Emitting Diodes

When an ordinary PN diode is forward biased, the energy lost when holes and electrons combine at the junction is dissipated as heat. Certain types of semiconductor material such as Gallium arsenide yield up some of this energy in the form of emitted light; since Gallium arsenide is a transparent material, the light from such a PN junction is visible externally. The wavelength of this light depends on the diode voltage, in the forward conducting direction, a junction between N-type GaAs and P-type material starts to conduct at about 0.8 V, and emits only infra-red light at a wavelength of 900 nm. Visible light may be produced by the use of materials having a higher forward voltage; a Gallium arsenide phosphide (GaAsP) junction emits red light at 670 nm, and a Gallium phosphide (GaP) junction emits green light at 500 nm. The visible light diodes are progressively less efficient as the wavelength falls, and their cost rises rapidly; however, red-emitting diodes are quite efficient and quite cheap.

Light emitting diodes are operated by passing current in the forward direction. A typical example of a red-emitting diode, the Monsanto MV10 has a brightness of about 5000 lm/m² for a current of 50 mA and requires 1.65 V across it at this current. LED operate at low voltages & currents, they are highly efficient, very small, and have very long life expectancy. The light produced is directly proportional to the current passed through them.

Photocells

There are 3 common types of light-sensitive devices:

• Photo-transistor
• Photo-diode
• Photomultiplier

The photo-transistor and the photo-diode are solid state devices and depend on the principle that light falling on a PN junction causes the formation of electron-hole pairs, and this produces the same effect as a forward bias.

The photomultiplier tube is a vacuum tube device, depending on electron emission from a suitable material exposed to light.

Photo-transistor

The photo-transistor consists of NPN silicon transistor with a window to permit light to strike its base-to-emitter junction; this window is usually in the form of lens to focus on incoming parallel light beam on to the junction. The base connection is usually left to open, so the transistor will be non-conducting form collector to emitter in the absence of illumination. When incident light strikes the base-to-emitter junction, electron-hole pairs are formed and the collector commences to draw current.

Silicon photo-transistors have a maximum sensitivity to light in the near infra-red region at about 830nm. The spectral peak is quite broad, and the transistors respond well to both GaAsP and Infra-red GaAs sources, and to light from incandescent lamps.

Photo-diode

Photo-diode is a silicon diode with a window opening to the PN junction. These diodes are usually operated with a reverse bias of about 20V, and act as sources delivering a current dependent on the incident light. A typical diode will deliver a current of 30 μA for an illumination of 2000 lux; in the absence of illumination its dark current will be 0.5 μA.

Photo-diodes are much faster in response than photo-transistor, and so maybe used where rise time is a consideration; they can also be made much smaller than photo-transistors.

You can also read: The Operation of Silicon Controlled Rectifiers

Photomultiplier

This is an extremely sensitive and precise light detecting device, and used for nearly all quantitative measurement of light. The tube which is highly evacuated contains a photo-cathode that is coated with a material that readily emits electrons, when illuminated. This is followed by a chain of electrodes (dynodes) at progressively higher voltages and at the far end is an anode, at the highest voltage of all.

Any electron emitted from the cathode is attracted to the first dynode, which it hits with sufficient energy to expel several secondary electrons (up to about 10). These in turn are attracted to the second dynode, where each expels several electrons and so on. These anode may finally collect millions electrons for each primary electron emitted from the photo-cathode.

A total working voltage of about 1000 V is usual; the tube may contain up to about 14 dynodes. The electrons arriving at the anode represent a current flow through the anode load resistor; the anode voltage, and hence the output voltage falls when the cathode is illuminated. The sensitivity of this instrument depends on the applied voltage, so highly stable supplies are required for quantitative measurement.

Photomultiplier tubes are extremely sensitive and are operated in total darkness except for the admission to the photo-cathode of the small amounts of light to be measured. A tube will be instantly destroyed if room lighting is permitted to enter while the e.h.t is on.

Related: Solid-state Relays (SSR) Features and Operation

Light Pipes

Light can be transmitted for considerable distance, even around curves, by means of a transparent rod. The ends of the rod are polished, and the light fed into one end from suitable source; a considerable proportion of the light will then be delivered from the other end. The transmission is based on the fact that, if light travelling in a substance of higher refractive index strikes obliquely on the interface with a substance of lower refractive index, it will tend to be refracted towards the interface; if the angle is sufficiently oblique (greater than a certain critical angle) the light will be unable to leave the first medium at all, and it will be totally reflected internally.

If the light must be flexible, it may be made of a bundle of very thin uniform rods, usually of glass having a high refractive index.

If the light pipe must be flexible, it may be made of a bundle of very thin uniform rods, usually of glass having a high refractive index.

Isolators

You can avoid direct switching of an electrical connection by using a light-emitting diode optically coupled to a photo-transistor either directly or through a light pipe. This kind of an arrangement is what is termed as an isolator in electrical and electronics systems.

Related articles:

• Semiconductor Diodes
• The Different Applications of Diodes
• Principle of Operation and Applications of a Laser Diode
• Gunn Diode – Features, Operation & Applications
• PIN Diode – Features & Applications
• Photoelectric Pressure Transducer Operation