Laser Diode

                            Laser Diode

The laser diode is a further development upon the regular light-emitting diode, or LED. The term “laser” itself is actually an acronym, despite the fact its often written in lower-case letters. “Laser” stands for Light Amplification by Stimulated Emission of Radiation, and refers to another strange quantum process whereby characteristic light emitted by electrons falling from high-level to low-level energy states in a material stimulate other electrons in a substance to make similar “jumps,” the result being a synchronized output of light from the material. This synchronization extends to the actual phase of the emitted light, so that all light waves emitted from a “lasing” material are not just the same frequency (color), but also the same phase as each other, so that they reinforce one another and are able to travel in a very tightly-confined, nondispersing beam. This is why laser light stays so remarkably focused over long distances: each and every light wave coming from the laser is in step with each other.

Laser diode overview

Laser diodes are used in all areas of electronics from domestic equipment, through commercial applications to hash industrial environments. In all these applications laser diodes are able to provide a cost effective solution while being rugged and reliable and offering a high level of performance.

Laser diode technology has a number of advantages:

• Power capability:   Laser diodes are able to provide power levels from a few milliwatts right up to a few hundreds of watts.
• Efficiency:   Laser diode efficiency levels can exceed 30%, making laser diodes a particularly efficient method of generating coherent light.
• Coherent light:   The very nature of a laser is that it generates coherent light. This can be focussed to a diffraction limited spot for high density optical storage applications.
• Rugged construction:   Laser diodes are completely solid state and do not require fragile glass elements or critical set-up procedures. Accordingly they are able to operate under harsh conditions.
• Compact:   Laser diodes can be quite small allowing for laser diode technology to provide a very compact solution.
• Variety of wavelengths:   Using the latest technology and a variety of materials, laser diode technology is able to generate light over a wide spectrum. The use of blue light having a short wavelength allows for tighter focussing of the image for higher density storage.
• Modulation:   It is easy to modulate a laser diode, and this makes laser diode technology ideal for many high data rate communications applications. The modulation is achieved by directly modulating the drive current to the laser diode. This enables frequencies up to several GHz to be achieved for applications such as high-speed data communications.

Laser diode symbol

the laser diode symbol used for circuit diagrams is often the same one used for light emitting diodes. This laser diode circuit symbol uses the basic semiconductor diode symbol with arrows indicating the generation and emanation of light.

Laser diode circuit symbol

When used within a circuit, they are often denoted as being a laser diode to distinguish them from other forms of light emitting diode.

Laser diode basics

There are two maintypes of semiconductor laser diodes. They operate in quite different ways, although many of the concepts used within them are very similar.

• Injection Laser Diode:   The Injection laser diode, ILD, has many factors in common with light emitting diodes. They are manufactured using very similar processes. The main difference is that laser diodes are manufactured having a long narrow channel with reflective ends. This acts as a waveguide for the light.
  
  In operation, current flows through the PN junction and light is generated using the same process that generates light in a light emitting diode. However the light is confined within the waveguide formed in the diode itself. Here the light is reflected and then amplified before exiting though one end of the laser diode.
• Optically Pumped Semiconductor Laser:   Optically pumped semiconductor laser, OPSL uses a III-V semiconductor chip as its basis. This acts as an optical gain medium, and another laser which may be an ILD is used as the pump source. The OPSL approach offers several advantages, particularly in wavelength selection and lack of interference from internal electrode structures.

A more complete explanation of laser diode theory and operation can be found in another page within this tutorial.

The laser diode is now well established, and used in a wide variety of applications. Although not nearly as cheap as many other forms of diode, laser diodes are still produced in vast quantities and at a relatively low cost, as demonstrated by the fact that laser diodes are even used in the light pencils used for illustrating overhead projector slide presentations. At the other end of the market, laser diodes for use in optical communications systems have been shown with data rates in excess of 20 Gbits per second. With performance levels in this region, they are being used increasingly in many communications applications.

Laser diode major categories

There are two major categories of semiconductor laser diodes. They operate in quite different ways, although many of the concepts used within them are very similar.

• Injection Laser Diode:   The Injection laser diode, ILD, has many factors in common with light emitting diodes. They are manufactured using very similar processes. The main difference is that laser diodes are manufactured having a long narrow channel with reflective ends. This acts as a waveguide for the light.
  
  In operation, current flows through the PN junction and light is generated using the same process that generates light in a light emitting diode. However the light is confined within the waveguide formed in the diode itself. Here the light is reflected and then amplified as a result of stimulated emission before exiting though one end of the laser diode as the external beam.
• Optically Pumped Semiconductor Laser:   Optically pumped semiconductor laser, OPSL uses a III-V semiconductor chip as its basis. This acts as an optical gain medium, and another laser which may be an ILD is used as the pump source. The optical gain is provided by stimulated emission. The OPSL approach offers several advantages, particularly in wavelength selection and lack of interference from internal electrode structures.

Laser diode theory basics

There are three main processes in semiconductors that are associated with light:

• Light absorption:   Absorption occurs when light enters a semiconductor and its energy is transferred to the semiconductor to generate additional free electrons and holes. This effect is widely used and enables devices like to photo-detectors and solar cells to operate.
• Spontaneous emission:   The second effect known as spontaneous emission occurs in LEDs. The light produced in this manner is what is termed incoherent. In other words the frequency and phase are random, although the light is situated in a given part of the spectrum.
• Stimulated emission:   Stimulated emission is different. A light photon entering the semiconductor lattice will strike an electron and release energy in the form of another light photon. The way in which this occurs releases this new photon of identical wavelength and phase. In this way the light that is generated is said to be coherent.

The key to the laser diode operation occurs at the junction of the highly doped p and n type regions. In a normal p-n junction current flows across the p-n junction. This action can occur because the holes from the p-type region and the electrons from the n-type region combine. With an electromagnetic wave (in this instance light) in passing through the laser diode junction diode junction it is found that the photo-emission process occurs. Here the photons release further photons of light occurs when they strike electrons during the recombination of holes and electrons occurs.

Naturally there is some absorption of the light, resulting in the generation of holes and electrons but there is an overall gain in level.

The structure of the laser diode creates an optical cavity in which the light photons have multiple reflections. When the photons are generated only a small number are able to leave the cavity. In this way when one photon strikes an electron and enables another photon to be generated the process repeats itself and the photon density or light level starts to build up. It is in the design of better optical cavities that much of the current work on lasers is being undertaken. Ensuring the light is properly reflected is the key to the operation of the device.