The diode laser is the most important of all lasers, both by economic standards and by the degree of its applications. Its main features include rugged structure, small size (250 × 10 × 50 µm), high efficiency, direct pumping by low-power electric current (typically 15 milliamperes at 2 V, which makes it possible to drive it with transistor circuitry) (Pearton, 2000).
Diode laser history can be traced down to infrared welding techniques, which were based on using halogen and parabolic lamps. Robert N. Hall developed the first semiconductor diode laser, while working for General Electric in 1962. Diode lasers come under semiconductor devices; diode laser emitting semiconductors efficiently and directly change electrical power into optical power. Diode lasers have capability to transform electrical power to optical power with remarkable level of efficiency. Currently, scientists have been able to achieve more than 60% electrical-to-optical conversion efficiency that makes it efficient known light source (Sands, 2004). Diode laser chips are also extremely small, typically only 0.1 × 0.5 × 0.75 mm. The first high-powered diode laser (HPDL) had an output wavelength of 840 nm that generated considerable heat (Bachmann & Loosen, 2010). Therefore, it was only able to operate in the pulsed mode.
Operations of Diode Laser
This section discusses the operations of diode laser, enhancements in both crystal growth techniques and device designs that have increased the power available from diode lasers by several orders of magnitude while maintaining low cost. Figure presented below shows the basic components of a diode laser. With techniques similar to those used in the silicon microelectronics industry, a series of planar crystalline layers are grown to form both an optical waveguide to confine the light and a pn junction to provide current injection into the active layer (Meschede, 2008). Crystal present along the cleavage planes is divided to create two mirrors. Active layer present between these two mirrors form a laser cavity with help of passing light through the mirrors. The facets ordinarily receive high- and low-reflectivity coatings, resulting in high-power emission from a single facet.
The active layer, or light-emitting region, is composed of one or more ultrathin layers of low-band-gap material surrounded by higher-band-gap material. Since the ultrathin layers quantum-mechanically confine the carriers (electrons and holes), they are referred to as quantum wells. The thickness of a quantum well is typically 10-100 atomic layers (Zappe, 2004). The reduced density of states and improved carrier confinement properties of quantum wells lead to significant performance improvements in diode lasers. For example, threshold current densities have decreased by almost two orders of magnitude from about 1500 A/cm2 to less than 50 A/cm2. Quantum-well diode lasers also routinely exhibit very high internal quantum efficiencies of 90-95% (Silfvast, 2004).
In recent years, there has been an explosion in the number of high-quality diode laser crystal material systems. The emission wavelength of a diode laser is directly dependent on the band gap of the active-region material, and with the various new material systems the wavelengths accessible directly with diode lasers ...