Lasers & Sources

Race is on to develop blue-green diode lasers

April 1, 1998

Commercial short-wavelength diode lasers could potentially quadruple optical-disk storage and make possible highly efficient lighting, but lifetimes still have to be lengthened.

Eric J. Lerner

Commercial introduction of a blue-output laser could benefit optical storage systems and consumer products such as CD-ROMs and CD audio by increasing storage capacity.

Following the removal of the particular dislocation-related defects responsible for degradation, researchers at Sony Laboratories have operated green diode lasers several hundreds of hours at room temperature.

Red-output diode lasers find wide application

Until blue and green diode lasers are commercialized, the shortest commercial wavelengths are in the red. These red diode lasers have found a variety of applications and have gradually taken over from the once-ubiquitous HeNe red lasers, which are far more expensive and less efficient. Currently, red diode lasers are used in laser pointers, for pumping solid-state lasers, and in printing, barcode reading, and optical data storage.

FIGURE 1. In ZnSe/ZnCdSe, the first system to exhibit room-temperature blue lasing, strain is required to produce a quantum well deep enough to confine both electrons and holes.

Typically, red diode lasers use aluminum gallium indium phosphide (AlGaInP) grown on GaAs substrates. Quantum-well structures are often compressively strained by adding excess indium, leading to reduced threshold current. Arrays of such red diodes have achieved impressive outputs for continuous operation, in the area of 100 W in some cases.

FIGURE 2. Electroluminescence images show how defects develop rapidly in ZnSe diode laser.

One of the most exciting applications of red diode lasers is in photodynamic therapy of cancer tumors. In this technique, a patient is administered a photosensitizer, which concentrates in the tumor. The tumor is then irradiated with the laser light, killing it. The use of red diode lasers makes possible a wider range of more-effective and less-dangerous photosensitizers and has led to remarkable cures.