3.5.1 High Power Direct Diode Lasers

The average power from individual laser diode bars have been conting to increase, 150W per bar has been demonstrated [Hoult, et al., 2000]. In addition, an increasing number of specific wavelengths are available over a range from 690nm to 980nm. If the application process does not require a specific wavelength, temperature control of the laser becomes less demanding, and hence the laser system is less expensive. This helps reduce the costs for diode laser processing systems. For example, soldering and plastic welding do not require tight wavelength control.

To achieve high average power, all currently available high average power systems use diode bar stacks with a range of differential stacking techniques and configurations. Some reasons for the slow commercial penetration of high power direct diode lasers are cost, technical complexity and unproven reliability. Only when diode lasers can demonstrate the combination of reliability, cost per watt and brightness required for a particular process would they become widely accepted. High power direct diode lasers has found satisfactory applications in surface treating and laser forming, which don't require too high laser intensity and beam quality. For them to enter the realm of laser machining processes, however, progress in cost, reliability, working distance, beam geometry, and minimum achievable spot size must be made. It is expected that the cost of laser diodes will continue to drop, and many research works are carried out to improve its performance, one can expect its impact on LMP in the near future [Ozkan, et al., 2000].

3.5.2 Diode-Pumped Solid State Lasers

Benefits of Diode-Pumped Lasers

• Reliability: Flashlamps burn out unpredictably and often. Diode arrays degrade gracefully over time--a very long time. Many last in excess of 20,000 hours!
• Greater Efficiency: Diode arrays are far more electrically efficient than flashlamps. As an example, Nd:YAG diode array light is tuned to 808 nm ±3nm which is where the YAG host material absorbs the greatest percentage of light, versus a flashlamp which outputs a broad white light source, ~95% of which turns into waste heat(see Figure below).

• Lower Voltage Requirements: Diode-pumped lasers do not require the same high voltages necessary for an equivalent flash lamp system. The input power required for a diode-pumped laser is typically 5-10% of the equivalent flash lamp system eliminating the need for 3Ø power.
• Compact Design: Diode-pumped systems allow for smaller, more compact housings.

Figure 18: A conductively cooled electro-optically Q-switched laser (Courtesy of Cutting Edge Optronics, Inc.)

• Beam Quality: Diode-pumped lasers are well known for their superior beam quality. This is especially true when you end-pump with diodes. When radially pumped, the M² at full power is ~20 for most systems, depending on the cavity design -- as opposed to the usual >40 of most flashlamp systems.
• Less Waste Heat: With diode-pumped lasers cooling is achieved with closed loop standard distilled water, or a mixture of water and glycol -- without the need for deionization and external city or tower cooling water.
• Durability: Diode-pumped lasers are far more rugged. Complicated oscillator/amplifier units can be shipped via common carriers. When they arrive, the customer simply plugs them in and begin lasing!
• Ease of Conversion: It's a simple process to retrofit a flashlamp systems to utilize diode arrays. Most flashlamp systems can be easily converted to a diode-pumped systems.

Figure 19: A 750W diode pumping modual at 1064nm (Courtesy of Cutting Edge Optronics, Inc.)

The laser module is reliable and efficient pump cavity which can be the "engine" in new laser system development and production, or can be used to convert your existing lamp-based designs to state-of-the-art diode-pumping. The head efficiently pumps a Nd:YAG laser rod by radial arrays of efficiently coupled long lifetime laser diode bars, and delivers good pump uniformity and stable lensing performance. The laser module requires ~ 300 VDC diode bias from a reliable solid state driver and is cooled by recirculating filtered water from a simple chiller system, and central cooling water is not required.

Figure 20: Picture of a Fiber Array Packaged diode laser system (Courtesy of Coherent Inc.)

A standalone, turn-key microprocessor-controlled FAP-System™ (FAP = Fiber Array Packaged) was demonstrated by Coherent Inc. This CE marked, full-feature system delivers up to 30W of diode light using an armored, 5 meter-long, 800mm fiber. More than 100W of power may be achieved via fiber combining the outputs of several systems. The FAP-System allows users total flexibility, with built-in dynamic temporal pulsing control and quick swap out of different wavelength diode laser from the red to the infared. This power, pulsing, and wavelength control enable the system to support a variety of applications including materials processing (soldering, marking, heating, plastics welding, etc.), medical therapeutics, solid-state laser media pumping, and polarized nobel gas production through rubidium and potassium spin exchange.

Rack-mountable and air-cooled, the rugged FAP-System™ is controlled by a front-panel user interface or computer-controlled RS-232 interface. Diode laser operating wavelength is controllable with temperature tuning from 5-35 degrees Centigrade in 0.1 degree Centigrade increments.

With the compact design, high beam quality and relatively simple cooling and power supply requirements, diode pumped solid state lasers can be easily integrated with robots to realize more flexibility in spatial control.

W. C. Schwartz, et al., 1996, "A diode-pumped solid state Nd:YLF laser for micromachining," ICALEO'1996, PP. E.48-56.

P. Hoult, et al., 2000, "Preliminary processing with a novel high average power direct diode laser(HPDDL)," ICALEO'2000, pp. A.1-10

A. M. Ozkan, et al., 2000, "Diode lasers-A different route to high average power," ICALEO'2000, pp. A.50-58

C. M. Cook, et al., 2000, "High power direct diode laser applications and advanced processes," ICALEO'2000, pp. A59-68