Robert V. Steele
In recent years, diode-pumped solid-state lasers (DPSSLs) have established a strong position in the low-power (<20 W) solid-state-laser market. As the prices of pump diodes have fallen and DPPSLs have demonstrated their advantagescompactness, power stability, beam quality, and operational cost savingslow-power markets have opened up. In application areas such as semiconductor processing, marking, ophthalmology, and instrumentation, DPSSLs have established a solid position and are moving toward becoming the dominant laser source.
Despite these developments, the use of DPSSLs in high-power industrial materials-processing applications has always appeared to be a longer-term proposition because of the high cost burden of the pump diodes as the lasers are scaled to higher (kilowatt) power levels. To date, DPSSLs have captured less that 1% of the $573 million metals-processing market (see Laser Focus World, Feb. 1999, p. 52). Nevertheless, within the past year, there has been a significant movement toward the use of DPSSLs in multiple-kilowatt industrial applications. While actual sales to date have been modest, momentum has been slowly building, and several major laser companies are offering, or will soon offer, such systems.
The motivation for the use of DPSSLs in preference to lamp-pumped lasers is much the same as for low-power DPSSLs: high peak-to-peak pulse stability at high pulse frequency; excellent beam quality; compact size; long, maintenance-free operating life; and low operating costs arising from high electrical-to-optical conversion efficiency. Of particular importance in the development of high-power DPSSLs for the industrial materials-processing market are the issues of beam quality and operating cost. Beam quality is important in applications such as cutting and welding because it determines the amount of laser power that must be applied to the workpiece, as well as the system tolerance related to the exact positioning of the focal point (depth-of-field considerations, for example).
*M2 is the measure of the beam quality in relation to a diffraction-limited beam; that is, the M2 of a diffraction-limited beam is 1. Beam parameter product (BPP) is related to M2 as follows BPP = (4lambda/pi)M2, where lambda is the laser wavelength.
** Applies to central portion (containing 50%-%6 of total beam power), of a multilobed beam.
For most cutting and welding applications, a beam parameter product (BPP) of less than 60 mm-mrad is required. The BPP is defined as the product of the beam diameter and the beam divergence angle. An alternative definition, most commonly used in Europe, equates the BPP with the product of the beam radius and one-half of the divergence angle. With this definition, the BPP is one-fourth the value defined above.
Traditionally, carbon dioxide (CO2) lasers have been used for most high-power cutting and welding applications because of their high beam quality and low operating costs. However, in applications requiring fiber delivery of the beam to the workpiece, Nd:YAG lasers are required. High-power lamp-pumped solid-state lasers (LPSSLs) typically have beam quality on the order of 100 mm-mrad. Beam quality can be improved by aperturing the beam, but this results in a reduction in efficiency. Overall electrical-to-optical efficiency is reduced from about 3% to around 2% to obtain beam qualities of less than 60 mm-mrad.
However, DPSSLs readily achieve beam qualities of 60 mm-mrad or less with no additional modification of the beam and no reduction in efficiency. Moreover, DPSSL efficiencies are inherently higher (on the order of 10%-20%). The table compares the beam quality available from typical CO2 lasers and LPSSLs in the 500-W to 10-kW range with those attainable in the latest generation of high-power DPSSLs.
Although DPPSLs exhibit superior operational characteristics compared to LPSSLs, cost remains a significant issue. Clearly, the continuing decrease in the price of pump diodes will help to narrow the gap. Also, the fact that lower power is required for a DPSSL to do the same job as a LPSSL helps to reduce the price differential for any given application. Moreover, operational-cost considerations also can play an important role in assessing the overall economics of LPSSLs versus DPSSLs.
A recent analysis by Rofin-Sinar (Hamburg, Germany) has shown that the cost of electricity for operating a high-power DPSSL is just 25% of that required for a LPSSL in carrying out the same function, due to the much higher efficiency of a DPSSL. The energy cost saving is substantial and weighs the economics favorably toward the DPPSL on a life-cycle basis.
According to the Rofin-Sinar analysis, the capital cost increment of the DPSSL over the LPSSL is recovered in just the first year of operation due to the lower operating costs of a DPSSL. Although the cost analysis is highly sensitive to the assumed cost of the diode pumps (a relatively low cost was used by Rofin-Sinar), it does illustrate the substantial operating cost savings that are possible with high-power DPPSLs.
Commercial activities
Based on operational cost savings, the higher beam quality of DPSSLs, and other performance benefits (such as elimination of downtime caused by the statistical failure of lamps in multiple-kilowatt LPSSL systems), several companies have begun to offer, or will soon offer, high-power DPSSLs on a commercial basis. In the USA, TRW (Redondo Beach, CA), as a result of experience gained in the DARPA-sponsored Precision Laser Machining Program, has developed a high-power slab laser pumped by laser diode stacks, which it is now offering on a commercial basis. This laser is capable of delivering average and peak-power densities up to five times higher than those of LPSSLs of equivalent powers.
TRW's current standard product is a 500-W DPSSL, designed for cutting and drilling applications, which exhibits beam quality of about three times diffraction limit in quasi-CW operation. The company has sold two of these systems to dateone to Cummins Engine and one to the University of Michigan. TRW has also developed a 3-kW DPSSL with similar beam parameters, and its ultimate goal is the development of a 6-kW product. A major targeted application for these lasers is drilling fine holes for the automotive, aerospace, and defense industries.
At LASER 99 (June 1999; Munich, Germany), Rofin-Sinar exhibited its 4.4-kW DPSSL. This laser is based on a folded-optical-path design using eight 550-W stages in a side-pumped rod design, so that it is relatively compact. The laser was developed in conjunction with the Fraunhofer Institute for Laser Technology (Aachen, Germany). Rofin-Sinar has sold several of these units since the beginning of the year, mostly for automotive-welding applications.
One of the advantages of the high beam quality available from this DPSSL is that long focal distances can be obtained for three-dimensional welding applications, which are becoming common in the automotive industry. Rofin-Sinar obtains packaged laser-diode pump bars from its subsidiary DILAS Diodenlaser (Mainz-Hechsheim, Germany), which it purchased two years ago.
Also at LASER 99, the Haas subsidiary of Trumpf (Ditzigen, Germany) exhibited its high-power DPSSL, although the laser is not yet available for sale. Trumpf has developed a 1-kW DPSSL based on a unique Yb:YAG disk-laser design pioneered by the Institute fur Strahlwerkzeuge at the University of Stuttgart. A high-power DPSSL based on this design is expected to be introduced commercially in late 1999 or early 2000 for heavy-duty industrial metal-cutting applications formerly handled by CO2 lasers. Its beam parameter product is on the order of 15 mrad-mm.
All of the high-power DPSSL developments discussed here have been carried out with the support of government programs in the USA and Germany. A government-sponsored high-power DPSSL program is also underway in Japan. However, commercial firms have now taken the initiative to get these lasers into the marketplace. With the DPPSL no longer constrained by its image as a low-power device, it may be fair to say that the age of the DPSSL has only just begun.
ROBERT V. STEELE is director of optoelectronics at Strategies Unlimited, 201 San Antonio Circle #205, Mountain View, CA 94040; e-mail: [email protected].