SANTA CLARA, CA-Toward the end of April, the third generation of forensic laser technology from Coherent began its bid for hand-held portable applications, such as illuminating fingerprints at crime scenes. The first generation, launched 30 years ago, found popularity in crime labs because the Stokes shift made fingerprints much easier to read, according to product manager David Clark. Green laser light was absorbed and would stimulate inherent fluorescence in the orange or yellow. Unfortunately, size, weight and power requirements for the large frame argon ion lasers used at that time were too high for use outside of the laboratory. So hand-held portable forensic illumination needs have been met since then primarily by lamp-based technology.
In the late 90s, Coherent introduced its second-generation laser for forensic applications, a version of the diode-pumped solid state (DPSS) Verdi laser that was dubbed “the incriminator”. The green CW laser was mounted on a cart with the chiller in a system that weighed 170 to 180 pounds. The cart made the system semi-portable, Clark said, but it remained too expensive for use by a typical sheriff’s office and too large, heavy, and cumbersome for most crime scene applications.
The current third-generation system, however, is based on optically pumped semiconductor laser (OPSL) technology that started development at Coherent about a decade ago with initial applications in bio-instrumentation. During the past 3-4 years, however, power levels have been achieved that enable use in forensic applications. Clark likened OPSL operation to a disk laser, except that the active OPSL medium is a semiconductor chip with a multiple quantum well cavity that is pumped from above with a high-power laser diode bar. Unlike a disk laser, the pump light does not have to make multiple passes through an absorptive layer, but is absorbed in an efficient single pass through a thin surface layer grown using molecular beam epitaxy upon a highly reflective mirror.
Relatively simple optics focus light onto the 1-to-2 mm2 surface area of the chip. And as long as the pump wavelength exceeds the band gap energy all of the light is absorbed without any need for close temperature control to match the absorption peak of a particular laser material, Clark said. The output wavelength is also tunable as opposed to being dependent upon natural frequency, such as the 1064-nm of neodymium-doped laser materials. Battery power enabling 1.5 hours of operation and a total system weight of 45 pounds make it portable enough to actually transport to a crime scene. In addition, the price of the laser technology has finally come down to a level comparable with lamp-illumination technology, he said.