Laser Diode Technology Tutorial

Laser Diode Includes:
Laser diode basics     Laser diode types     Structure     How a laser diode works     Specifications     Reliability    

Other diodes: Diode types    

Semiconductor laser diode technology is in widespread use today in many areas of the electronics industry.

Laser diode technology is now well established, with laser diodes providing a cost effective and reliable means of developing laser light.

With laser diodes being lending themselves to use in many areas of electronics from CD, DVD and other forms of data storage through to telecommunications links, laser diode technology offers a very convenient means of developing coherent light.

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 background

The name laser comes from the words Light Amplification by Stimulated Emission of Radiation. Lasers operate because of a phenomenon called stimulated emission that was first postulated by Albert Einstein before 1920. Although a number of media including gases liquids and amorphous solids can be used for lasers the first ones were realised in the 1960s using rubies. A helium-neon gas laser followed this in 1961 but it was not until 1970 that semiconductor laser diodes were made to run at room temperature by Hayashi. This represented the final step in research work that had been undertaken by a number of people and organisations over the years. It had required an in depth study of the properties of gallium Arsenide, the material that is used as the basis for many laser diodes and much work on the properties of the diode structures.

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.

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.

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