Alkali-Vapor Laser Multiplies Brightness of Diode Array
Output powers in the tens of kilowatts may one day be feasible.
Since the invention of the laser, one of its chief drawbacks has been low wall-plug efficiency. Nonetheless, early argon-ion and diffraction-limited Nd:YAG lasers, whose efficiencies seldom exceeded 10–3, were successful in some commercial applications. These successes whetted the appetites of design engineers, who envisioned many more economically practical applications if only the lasers’ efficiency could be improved. Carbon dioxide lasers provided the sought-after ~10–1 efficiencies, but their mid-infrared wavelengths excluded them from many applications.
During the 1970s, the semiconductor laser emerged from laboratories and into commercial production. Early models offered efficiencies comparable to those of CO2 lasers, and today’s semiconductor lasers readily achieve efficiencies of 50 percent. DARPA’s Super High-Efficiency Diode Sources (SHEDS) program is well on the way to providing diode lasers with 80 percent overall efficiency (see “Military Project Aims to Improve Diode Laser Efficiency,” Photonics Spectra, January 2005, page 89, and “SHEDS, Part II,” February 2005, page 59).
But the beam quality of diode lasers is too poor for the majority of laser applications. So the challenge became to develop devices capable of converting the messy output of a diode laser to a beam of the necessary quality. The first candidate was the diode-pumped solid-state laser, which emerged in the 1980s and which has been remarkably successful. Today, nearly half of solid-state lasers sold are of the diode-pumped variety.
Two more candidates came forth during the 1990s: first, the fiber laser, and somewhat later, the thin-disk laser. Both have been successful, and though in some sense they compete with each other, more often each laser’s unique characteristics open new opportunities not possible with other lasers.
Then, in the early 2000s, another candidate emerged. The alkali-vapor laser seemed to promise another set of unique characteristics that could enable it to penetrate new applications. (see “Alkali Lasers Challenge Thin Disks and Fibers,” March 2004, page 52). Early experiments at Lawrence Livermore National Laboratory in Livermore, Calif., and at the US Air Force Academy in Colorado Springs, Colo., proved the principles of alkali-vapor lasers but failed to demonstrate enhancement of the pump laser’s beam quality. Recently, experiments at Livermore have shown that the poor beam quality of a diode laser array can be enhanced many times with an alkali-vapor laser.
Figure 1. Pump light from the diode array reflected off a folding mirror into the Rb cell. Images © OSA.
The Livermore researchers coupled the pump light into the Rb cell with a folding mirror that had a small hole for the intracavity beam (Figure 1). They added He to the cell to broaden the absorbing transition to more nearly match the bandwidth of the 780.25-nm pump light, and ethane to facilitate the transfer of energy from the terminal pump level of Rb to the upper laser level. The diode array from Spectra-Physics of Mountain View, Calif., was stabilized with a volume Bragg grating that confined most of the power to a narrow peak in resonance with the Rb absorption (Figure 2).
Figure 2. The volume Bragg grating concentrated ~70 percent of the pump light into a narrow line in resonance with the 780-nm absorption in Rb. Inset: The quantum defect of Rb (dotted arrow) is only 237 cm –1, one of the inherent energy advantages of the laser.
Under optimal conditions, the researchers observed a slope efficiency of 10 percent from the Rb laser (Figure 3). But the experiment was beset with technical glitches, the most serious of which was the formation of deposits on the cell pump window, which eventually reduced its transmittance by 30 percent.
Figure 3. The input/ouput data for the Rb-vapor laser show a steadily increasing power with increasing output coupling, an indication of high intracavity loss.
Technical glitches aside, the researchers measured the brightness of the pump laser and that of the Rb-vapor laser, and concluded that the brightness had been enhanced by a factor of 2160. The brightness “enhancement” in the earlier Livermore research and the Air Force Academy experiments had been less than unity.
The scientists were gratified that their experimental results conformed closely to their theoretical predictions. Extrapolating from their model, they believe that alkali-vapor lasers eventually could be capable of generating high-quality output in the tens of kilowatts, with optical-to-optical efficiencies of 70 percent.
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