Diode lasers

by Paul Hilton

High power diode lasers (known as HPDL's) feature a very high electrical to optical power conversion efficiency coupled with a very compact size. With suitable 'focusing' optics, today's HPDL's are suitable for some materials processing applications. The laser diodes which drive the HPDL's are also being used to replace flashlamp pumping in solid state lasers. Diode lasers also exhibit very high wall plug efficiencies which can be greater than 30% on commercially available systems.

Diode lasers consist of a p-n junction within a multi-layer semiconductor structure. For powers greater than about 4W, the only commonly used manufacturing approach produces a diode laser bar about 10mm long, with emission of radiation confined to the narrow junction region (typically 1µm thick). Along the 10mm length, many thousands of single emitters, of the order 5µm wide, produce laser output with, because of diffraction, very large beam divergence. (See Fig.1). The resulting beam with its large angular spread is characteristic of semiconductor lasers, and, compared to other types of laser, presents a drawback in terms of focusability. The beam divergence is up to 90° perpendicular to the emitting line (known as the 'fast' axis) and about 10° along the emitting line (known as the slow axis).

Fig. 1 Laser diode schematic

Powers of the order 80W and higher can be achieved from one diode bar. For high power applications, combining the power from several diode bars is required. For materials processing applications, the semiconductor material is based on InGaAs on a GaAs substrate (940nm) or InGaAlAs on a GaAs substrate (808nm). Both these wavelengths are invisible to the eye.

As a result of the rather unusual beam characteristics of the diode laser and the added complication of increasing power by adding diode bars, several different possibilities exist for beam manipulation to achieve the required power densities for material processing applications. It would appear that this is the area in which one 'diode laser' supplier may be distinguished from another.

A 3kW (highest currently available commercially is 6kW) diode laser (including beam focusing) is smaller than a shoebox and its control, power supply and cooling system is the size of a two drawer filing cabinet. As a result, a clear division can be seen between those manufacturers who would place the laser directly on the arm of a robot say and those who favour fibre optic beam delivery to a focusing head (the latter very similar to that required for a Nd:YAG laser). The approach to beam shaping and focusing is therefore different for these two cases. Two of these design configurations, suitable for material processing applications are described below. Lenses for beam shaping with diode lasers are usually manufactured from glass or fused silicon.

Individual Beam Shaping (IBS) Diode Laser

This system uses sophisticated optics to combine the beam from three individual diode bars mounted as can be seen in Fig.2a. In addition, special diodes are used where the emitting zone is confined to 5 areas 500µm wide with a centre to centre spacing of 1.5mm. This design is the basis of improved beam quality which permits the generation of focused spots about 0.25 x 0.6mm ². With its output power of about 150W, the power density ~10 ⁵W/cm ², is sufficient for conduction welding of metals.

Fig. 2a IBS diode laser schematic

Optical Fibre Delivered Diode Laser

This laser also uses a complex optical system designed to minimise the spot size from a single bar so that the beam can be launched down a silica optical fibre. After fast axis collimation with a micro lens, the slow axis is chopped into small beamlets by a special diamond machined mirror. A set of prisms then compresses these beamlets together before collimation with a cylindrical lens and final focusing via spherical optics. Fig.2b, shows how the combination of diamond machined mirror, prisms and lenses, produces a 0.8mm diameter beam for launching into the fibre. Using this configuration, a 35W single bar device can produce a power density of about 7 x 10 ³ W/cm ².

Fig. 2b Fibre delivered diode laser schematic

New Developments

Diode laser technology continues to develop at a fast pace. Its limitations continue, however, to be available spot size. Much effort has gone into this and most of the higher power (1kW+) diode lasers now use the technique of wavelength coupling in order to maximise power in a small spot.

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