Flexible Laser Design Targets Low-Volume Material Processing Needs

Coherent, Inc., Santa Clara, California

Q-switched, diode-pumped solid-state lasers with an end-pumped cavity design are now widely used in micromachining, materials processing, marking, and related applications. They are used to process a broad range of materials including metals, glass, plastics, and semiconductors. But this application diversity creates a concomitant need for laser diversity. Namely, while each application requires superior reliability and performance, the definition of “superior performance” is very application-specific. For example, some metal ablation applications may benefit from a long laser pulse, whereas semiconductor scribing needs a short pulse and a very high pulse repetition rate.

Table 1. Relationship between pulsewidth and output power to other resonator parameters in a q-switched DPSS laser.

The three main output parameters for a diode-pumped laser are overall power, pulse duration, and pulse repetition rate. However, these cannot be varied independently (see Table 1). For a given pump power, resonator length, and cavity configuration, optimizing one of these parameters involves tradeoffs in the others. So the resonator must be carefully designed to deliver the required combination of output specifications for each application.

Figure 1. Soft Wafer Marking optimally requires a 1,064-nm Nd:YAG laser with a high pulse energy, a pulse length in the 200 ns range, and a repetition rate of around 1 kHz. This provides the necessary process throughput, since each mark consists of only a small number of individual holes. a) Characters produced on a silicon wafer using soft marking, and b) a close-up of the individual laser-produced bumps that make up the mark.

In the case of high-volume applications, laser manufacturers have developed automated manufacturing methods such as robotic assembly and soldered mounting of optics that combine product consistency and reliability with low-cost assembly. Here, the laser resonator has been designed to deliver the optimum output parameters needed for the application, with no flexibility or options that would increase production costs.

But there are many niche applications, as well as newly emerging applications, where the unit volume does not justify the engineering costs associated with designing a laser of this type. Coherent has answered this market need with the PRISMA family of diode-pumped lasers, which utilize a flexible design approach. For example, every laser is built on an identical resonator baseplate that accommodates a number of different, predetermined optical configurations. This is accomplished by placing a series of precisely positioned holes, threads, and locating pins on the baseplate. Various sets of individual optical components are placed in standardized mounts and then attached to this baseplate as needed to produce the desired resonator configuration, and hence, the specific set of required output characteristics. This minimizes the fabrication cost of the laser and provides maximum design flexibility without requiring that each unit be manually aligned during production, eliminating unit-to-unit variation.

This flexibility also supports both Nd:YVO₄ laser crystals, which enable high pulse repetition rate (up to 150 kHz) and Nd:YAG crystals, which are optimum for high pulse energy at modest (<10 kHz) repetition rates. Furthermore, the cavity design enables the laser crystal to be end-pumped at one or both ends, providing flexibility in using significantly different levels of pump power. In addition, frequency-doubling crystals provide the option of 1,064-nm or 532-nm output.

As a result of this flexibility, PRISMA lasers have been optimized to successfully enable applications as diverse as soft wafer marking (see Figure 1), steel and ceramic mold form manufacturing (see Photonics Tech Briefs, April 2006, page 58), solar cell scribing, and ID card marking.

This article was written by Joerg Heller of Coherent Inc. For more information, contact Mr. Heller at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/10960-204 .