An Evolution with Indigo pulsed single-frequency Ti:sapphire laser attached.

As little as five years ago, if you wanted a laser producing watts of green light, you could choose from the following:

Then along came diode-pumped solid-state (DPSS) lasers, and suddenly 10 watts of green light could be packaged in about 4 ft.³ of real estate, consuming a couple of kilowatts, including cooling. Of equal importance, the DPSS laser promised vastly improved reliability and much more stable performance than the technologies it was displacing. There was unanimous approval from the scientific laser community. The response from researchers attempting to take the earlier lasers out of the laboratory and into industrial, mobile, and airborne environments was similar: a collective sigh of relief could be heard from JPL to Goddard.

In the five or so years since their widespread introduction commercially, DPSS lasers have more than lived up to their hype, and now stand ready to make significant contributions in research areas that include LIDAR, spectroscopy, and laser remote sensing. A close look at evolving DPSS technology yields even more reason for optimism, as costs, lifetimes, and performance all continue in directions favoring consumers.

Figure1: Schematic of the Evolution diode-pumped laser cavity

 

Positive Light's Evolution

An example is Positive Light's Evolution. The Evolution-X is an Nd:YLF laser producing 10 W at 527 nm. The single Nd:YLF rod is transversely pumped by 4 40-W diode bars arranged around the water-cooled rod (see Figure 1, above). The pump diodes operate continuous-wave, with the slightly lower gain compared to pulsed operation compensated for by the improved diode lifetime. The resonator design produces a multimode output with a flat beam profile. An acousto-optic modulator serves as the Q-switch, and the intracavity LBO frequency-doubling crystal features noncritical phase-matching. By using the LBO crystal at 90^o with respect to the optical axis, the angular acceptance is greatly increased, thus promoting long-term stability without the need for continuous tweaking. The result of this design is that a strip-chart recording (see Figure 2, below) shows the Evolution's output stability is 0.2 percent RMS over a 24-hour period.

Because Nd:YLF exhibits negligible thermal lensing, the Evolution's repetition rate can be easily changed from 1 kHz to 5 kHz without affecting beam quality. Once again addressing the need for long-term reliability and performance stability, the Evolution is mounted onto a monolithic platform that shares water-to-air cooling with the Nd:YLF rod.

 

A Sharp Drop in Price

It is worth noting that while DPSS lasers have been around for more than a decade, the cost associated with diode pumping precluded them from high-power commercial laser viability until quite recently. Within the last two years, diode bar arrays producing in excess of 40 W at about 0.8 mm, a neodymium pump line, have fallen in price dramatically. Somewhat more gradually, the lifetimes for diodes have risen, and Positive Light now guarantees the Evolution's performance for 5000 hours of operation.

Though much can be accomplished using green light at a fixed wavelength (i.e., measurements of aerosol scattering, laser ranging), the market potential for tunable lasers has considerably more appeal. Since the late 70s, argon and frequency-doubled Nd:YAG lasers were put to work as the pump sources for tunable dye lasers. Just as these older pump lasers gave way to DPSS lasers, the dye laser is inexorably being replaced by solid-state alternatives, most notably titanium-doped sapphire (Ti:sapphire).

Figure 3: Schematic of the Indigo Ti:sapphire single-frequency laser cavity.

Last year, under a licensing agreement with Stanford University, Positive Light introduced Indigo, the first all-solid-state pulsed laser combining narrowband (single-longitudinal-mode) performance with wide tunability from the UV to the IR. When paired with Evolution-X, the Indigo produces a 1-kHz output of more than 800 microjoules at 800 nm. The transform-limited pulses are about 10 ns in duration, and pulse-to-pulse energy stability is better than 2 percent over the fundamental tuning range from 690 to 920 nm. The Stanford/Positive Light design features a unique cavity configuration, shown in Figure 3, dubbed "MDW" for mode-discrimination walk-off.

A Ti:sapphire rod 3 mm in diameter and 15 mm in length is longitudinally pumped with an Evolution at 527 nm. The lasing output is gain-switched -- the 150-ns pump pulses from the Evolution assure a low peak-power level in the Ti:sapphire, and a comfortable safety cushion against damage on all the optical surfaces.

Three of the main areas of commercial interest in the Indigo system are light detection and ranging (LIDAR), resonant ion mass spectroscopy (RIMS), and ozone detection. Because the stability of the system means it is well suited to harmonic generation, the Indigo can be specified with an output as short as 193 nm, where its approximately 5-mW output complements excimer lasers in low-power applications, including metrology and optical system alignment. The entire Indigo-193-nm system consumes less than 2 kW of power, and the system footprint is a paltry 24-x-31-x-6 in. for the laser heads, and 20-x-17-x-13 in. for the power supply.

In offering a complete turnkey system for tunable narrowband laser radiation, Positive Light hopes to extend the operating platform for laser systems that have been somewhat notorious in their attendant needs. Higher-power versions of the Evolution are already under development; >20 W output at 527 nm is expected before the end of the year.

For more information contact Leigh Bromley, R&D manager at Positive Light, 103 Cooper Court, Los Gatos, CA 95030; (408) 399-7744; fax: (408) 354-4695; poslight@aol.com.