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Laser Diodes

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Laser Diodes

A laser diode in a typical can package with three pins and a optical window positioned next to a penny for scale.

Public Domain Image [NASA]

Most lasers we see every day are solid state laser diodes, from laser pointers to the lasers used in CD, DVD, and Blu-ray players. The light emitting part of a laser diode is very similar to an LED and was originally developed shortly after the LED. Once the semiconductor industry started to grow and expand in the 1980's, the cost of semiconductor laser diodes started to fall and they became the most common type of laser used. Today laser diodes are available with power outputs ranging from a few milliwatts, like those found in laser pointers, to laser diodes emitting many watts of optical energy and used to cut through solid metal and in high energy physics experiments.

How they work

A laser gets its name from how it works, Light Amplification by Stimulated Emission of Radiation. All lasers, including semiconductor based laser diodes, start with a small amount of light that is sent in to an optical cavity where it bounces around and stimulates more light. Once amplified, the light then leaves the optical cavity in a tight, coherent beam. In a laser diode, the initial light that starts the light amplification process comes from what is essentially an LED and the optical cavity is typically formed by either cleaving the crystal to form mirror like ends or by optically grinding, polishing and coating the sides of the optical cavity. Laser diodes require a certain amount of current to turn on and actively lase. The turn on current is usually very close to the maximum current the laser diode can handle which makes precisely controlling the current very important.

Laser Diode performance

Laser diodes, while affordable, small, and relatively effective, have several performance limitations. The biggest offender in the laser performance is the divergence of the laser beam which is as high as 30 degrees and requires a lens to form a collimated beam (a light beam that ideally does not spread out over distance). Not only will laser diodes diverge quickly without correction, the wavelength of a laser diode will vary for a number of reasons. The manufacturing variations between one diode to another will result in a tolerance of up to tens of nanometers. Temperature will also have a large impact, with the wavelength typically changing by 0.9 nm for every 3.0 degrees Celsius. As the temperature of a laser diode increases, the optical power output will drop and the maximum laser diode power can be exceeded leading to failure if the temperature drops too far without adjusting the current to compensate. All of these factors make controlling laser diodes precisely a challenge.

Applications

As the most common type of lasers, with five times more laser diodes sold every year than all other lasers combined, laser diodes have a great number of uses. Many applications for laser diodes make use of the ability for a laser diode to be modulated on and off quickly and the directed energy of the optical beam. The laser diode used on DVD and Blu-ray disc takes advantage of both properties to effectively read or write to the disc. Optical communications make extensive use of the modulation of laser diodes to deliver high speed fiber optic communications. Other applications leverage the optical properties of the laser diode to perform low cost spectral analysis, range finding, and to control chemical reactions. The applications of laser diodes are growing every day as new products are developed that take advantage of the properties of laser diodes.

Common Wavelengths

The wavelength of laser diodes depends on two primary factors, the semiconductor material used in the diode and the optical cavity design. Some of the more common wavelengths, and the materials used, are listed below.

  • 405 nm - InGaN blue-violet laser, in Blu-ray Disc and HD DVD drives
  • 473 nm - Nd:YAG Bright blue laser pointers
  • 535 nm - AlGaAs Bright green laser pointers
  • 640 nm - High brightness red laser pointers
  • 657 nm - AlGaInP DVD drives, laser pointers
  • 670 nm - AlGaInP cheap red laser pointers
  • 785 nm - GaAlAs Compact Disc drives
  • 848 nm - GaAs laser mice
  • 1064 nm - AlGaAs fiber-optic communication, DPSS laser pump frequency
  • 1550 nm - InGaAsP, InGaAsNSb fiber-optic communication

Laser Safety

The energy in even a typical red laser pointer is enough to cause safety concerns. The 5mW a typical red laser point puts out is bright enough to see in direct sunlight on a bright day, which means that it is brighter than the sun and can cause eye damage very quickly! In fact a 5mW laser pointer delivers over 800 times the energy density to your eye that the sun does! The current laser classification system is below and relies on the standard blink response and reaction time to determine the safe power levels of a laser. Always follow laser safety rules when using lasers.

  • Class 1 - Safe.
  • Class 1M - Safe provided optical instruments are not used.
  • Class 2 - Visible lasers. Safe for accidental exposure (< 0.25 s).
  • Class 2M - Visible lasers. Safe for accidental exposure (< 0.25 s) providing optical instruments are not used.
  • Class 3R - Not safe. Low risk.
  • Class 3B - Hazardous. Viewing of diffuse reflection is safe.
  • Class 4 - Hazardous. Viewing of diffuse reflection is also hazardous. Fire risk.

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