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Optoelectronics is one of the fast emerging technology fields that deals with applying electronic devices to the sourcing, detection and control of light. This is used for numerous purposes like telecommunications, monitoring and sensing, long wavelength Li DAR, microwave photonic links, medical equipment and general science. Telecommunication using fiber optics and X ray machines in hospitals are a few illustrations of this technology. This article gives a brief insight into the basics of Optoelectronics, working principle, Optoelectronic devices, their applications and future prospects.

What is Optoelectronics

In the scientific context, Optoelectronics deals with the study and application of electronic devices that interacts with light which might be detection of light, its creation, and exploitation for several purposes. This includes Gamma rays, X ray, Ultraviolet, Infrared and visible light.It also encompasses the study, design and manufacture of hardware apparatus that facilitate the conversion of electricity into photon signals.

Optoelectronic devices are primarily transducers i.e. they can convert one energy form to another. These devices produce light by expending electrical energy. They can also detect light and  transform light signals to electrical signals for processing by a computer.

Introduction to Optoelectronics

Fig. 1 – Introduction to Optoelectronics

Optoelectronics makes use of the quantum mechanical effect of light. This property is used mainly in the materials that are used in the manufacture of semiconductors. Below mentioned are few such effects of light.

Photoelectric or Photovoltaic

Here, the light is directly converted into electricity. Solar cells make the best utilization of this direct conversion effect.

Photoconductivity

A material is made more electronically conductive by this electrical phenomenon. It is achieved by absorbing electromagnetic radiations such as UV light, infrared and visible light. Generally, it is utilized in Charge Coupled Device (CCD) imaging sensors.

Stimulated Emission

In this process, an energized molecule is made to interact with a light photon. This interaction decreases the energy level of photon and results in the liberation or emission of a matching photon. It is then transferred to an electromagnetic field. Quantum cascade lasers and laser diodes make use of this process.

Radiative Recombination

In this approach electron transfer occurs in semiconductors from valence to conducting band. This results in a recombination effect and carrier generation process that produces light. LED’s employ this principle for light production.

Optoelectronic Devices

Fig. 2 – Optoelectronic Devices

Working Principle of Optoelectronic Communication System

A typical Optoelectronic Communication System consists of components namely:

  • Light Source
  • Optical Transmitter
  • Photo Coupler
  • Optical Fiber, Wave-guide
  • Transducer
  • Optical Receiver or Detector

Light Source

The light emitted from the source acts as an input to the Optical Transmitter. LEDs and Laser Diodes are used as the light source depending upon the application. They generate input electrical signals for the communication system.

Optical Transmitter

The Optical Transmitter converts the signal received from a Laser Diode or LED to an optical output.

Photo Coupler

The Photo Couplers transfers the electrical signals between two isolated circuits through the transmission channel that may be Optical fiber, wave-guide or free space. It also provides high insulation voltage.

Optical Fiber, Wave-guide

It acts as a transmission medium and guides electromagnetic waves in optical spectrum.

Transducer

Transducer modulates the signal proportional to the incident light and the signal further undergoes coupling through the channel.

Optical Receiver or Detector

Photo-diodes and Photo-transistors are generally used as Optical Detectors. The light detector converts the incident light in to electrical signal and it is further processed or stored to receive information. The electrical signal generated is either a Photo-current or a Photo-voltage.

Working Principle of Optoelectronic Systems

Fig. 3 – Block Diagram of Optoelectronics Communication System

Optoelectronic Devices

An optoelectronic device comprises of various semiconductor alloys that lay on substrates. Different semiconductor layers are deposited sequentially on the substrate during the expansion of multi-quantum well of laser active regions.

These layers are deposited altering between barrier and well regions. Holes and electrons combine in the well region to produce laser light. Barrier regions are used for confining holes and electrons inside the well.

Optoelectronic devices include:

  • Information Displays using LED’s
  • Photo Diodes
  • Remote Sensing System
  • Solar Cells

We will now look at some of the common Optoelectronic devices used today.

Photo-diodes

This semiconductor light sensor generates electricity or voltage when light touches the junction. The Junction here is an active p-n junction, operated in the reverse bias condition. When an excited photon strikes the photo-diode, electron-hole pairs are created.

Electrons then diffuse into the p-n junction to generate an electric field. This electric field equals the negative voltage found across an unbiased diode. This process is termed as inner photoelectric effect. Photo-diodes can be used in three formats:

  • Photo-voltaic: As solar cells
  • Forward biased: As LED
  • Reverse biased: As photo detector

They are used in different types of circuitry and applications like medical instruments, cameras, communication devices, safety and industrial equipment.

Structure of Photodiode

Fig. 4 – Structure of Photo-diode

Solar Cells

This photo-voltaic cell does direct conversion of solar energy into electricity. Sunlight is composed of photons. When these photons collide with the silicon atoms of solar cell, energy transfer takes place from photons to the lose electrons. These high energy electrons then flow into external circuits.

Solar Cells consists of only two layers. The first one is laden with electrons that are always ready to jump to the second layer. The second layer has a few missing electrons and hence can accommodate electrons from the first layer.

Solar Cells are advantageous as it is cost-effective and zero fuel supply. They  demand minimal maintenance. They are used in rural electrification, ocean navigation systems and electric power generation in space.

Structure of Solar Cell

Fig. 5 – Structure of Solar Cell

Laser Diodes

Laser (Light Amplification by Stimulated Emission of Radiation) is a source of directional, coherent and highly monochromatic light. It functions under the conditions of stimulated emission. A laser beam is typically 4*0.6 mm and extends to a distance of 15 meters. Semiconductor Lasers or Injection Lasers are commonly used today.

Upon applying voltage across P-N junction, a population inversion of electrons occur. Laser beam is then available from this semiconductor region. The end points of p-n junction in a Laser Diode have a polished surface. These polished surfaces are reflective in nature and helps the emitted photons to reflect back, thereby creating more pairs of electrons. Thus the newly generated photons will have equal phase as previous photons.

Structure of Laser Diode

Fig. 6 – Structure of Laser Diode

Applications of Optoelectronics

Optoelectronic semiconductor devices have a major impact on almost all areas of Information Technology. These devices can be classified based on their functional roles like output, input, processing, transmission, memory and others.

Many technologies and physical properties are exploited by applications using Optoelectronics. Several such applications have come to our understanding and control only during the previous decade.

Below mentioned are some of the applications of Optoelectronic devices:

  • LED’s have revolutionized lighting system and used in areas like computer components, watches, medical devices, fiber optic communication, switches, household appliances, consumer electronics and 7 segment displays
  • Solar Cells are used in several solar energy based projects for measurement systems, auto irrigation system, solar power charge controller, Arduino based solar street lights, and sun tracking solar panels
  • Optical Fibers are used in telecommunication, fiber lasers, sensors, bio-medicals and other industries
  • Laser Diodes find their use in military applications, surgical procedures, optical memories, CD players, local area networks and in electrical projects like RF controlled robotic vehicles

Advantages of Optoelectronics

The advantages of Optoelectronics are:

  • Optoelectronics have helped the military and Aerospace industry immensely. The transmitted Over- Air RF links might not reach the intended receivers due to confined spaces, tunnels or in seagoing vessels and to overcome this, they use optical repeaters and fiber optic networks.
  • Optoelectronics has given a new dimension in designing satellites of future.
  • It provides a high bandwidth for communications.
  • The optoelectronic devices consume less power.

Disadvantages of Optoelectronics

The disadvantages of Optoelectronics are:

  • The optoelectronic devices are temperature sensitive.
  • Coupling requires precise alignment of Optoelectronic components which is complex.
  • Integration of Optoelectronic elements on to a substrate is difficult.

Future of Optoelectronic Devices

Optoelectronics is a vital foundation technology that is enabling seamless functioning of the information industry. Academically, Optoelectronics encompassed the study of electronic devices for transmission, emission and modulation of light signals. However, its scope has widened today and it includes electro-optics and photonics as well. Commercially significant technologies for material science, communication, computing and medicine are leaping on wards due to advancements in optoelectronics.

Optoelectronics technology is providing bigger venues for R&D today. Its effect can be seen in the areas of cost reduction, performance improvement and large volume manufacturing. Industrial and academic communities are predicting a bright future for research in Optoelectronics Technology. Ongoing advances in Photonics and Optics are expected to revolutionize the 21st century.

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