Compact Raman Laser Generates Eye-Safe, Nanosecond Pulses

Breck Hitz

Eye-safe lasers, whose wavelengths are not transmitted through the human cornea and thus are incapable of damaging the retina, are required in many applications where accidental eye exposure is a possibility. Recently, researchers in China designed and operated a compact, eye-safe laser that relies on the same Nd:KGW crystal to generate the fundamental 1351-nm radiation as it does the Raman output at 1538 nm. The scientists believe that their laser produces more energy in shorter pulses — 31.8 mJ in 2 ns — than any Nd:KGW eye-safe laser previously reported.

Figure 1. Whereas the 1351-nm fundamental-wavelength resonator was defined by mirrors M1 and M2, the 1538-nm Raman resonator was defined by M1 and a high-reflectivity coating on the left-hand end of the laser rod. Images reprinted with permission of Optics Letters.

The researchers, at the Chinese Academy of Sciences in Fuzhou, started with five Nd:KGW laser rods that they had grown, all of which were 3.5 mm in diameter and about 62 mm long. They evaluated the crystals, which had doping ranging from 1.6 to 5.0 atomic percent, to select the one with the best performance. The best one turned out to have 3 percent doping, but its superiority may have resulted from its better optical quality rather than its doping level.

Rod characterization

The researchers characterized this rod’s performance in a Q-switched resonator (Figure 1). The 32 pump-diode bars — manufactured by colleagues at another laboratory of the Chinese Academy — were arranged in eight sections of four bars each around the sides of the rod and generated a total of 3200 W of pump power.

Figure 2. Operating as a simple 1351-nm laser (i.e., no feedback at the Raman wavelength), the laser produced the highest energy output pulses when pumped with the longest (385-μs) pulses from the diode pump lasers.

In the laser experiments, the researchers pulsed the diode bars at 10 Hz, with pulse durations between 115 and 385 μs.

The polarizer in Figure 1 reflected only the S-polarization, forcing the laser to oscillate in that polarization. Both resonator mirrors (M1 and M2) were antireflection-coated at 1067 nm to suppress the strong neodymium line at that wavelength. The back mirror (M2) was a maximum reflector at the 1351-nm laser wavelength, and the output coupler (M2) transmitted 90 percent at the laser wavelength. The intracavity components were antireflection-coated at the laser wavelength.

Pump-pulse duration

The laser generated up to 52 mJ in 33-ns Q-switched pulses (Figure 2). The spontaneous lifetime of Nd:KGW is ∼110 μs, so it is perhaps surprising that laser pulse energy increased with pump-pulse duration all the way to 385 μs. (One might expect that, because the pump pulse exceeds the spontaneous lifetime, the population inversion would lose energy to spontaneous emission as fast as it gained energy from the pump.) One possible explanation for the observed behavior is the slow rise time of the pump pulse.

To generate output at the first Stokes Raman line, the researchers substituted an output mirror (M1) that reflected nearly all the 1351-nm fundamental wavelength and transmitted 90 percent of the 1538-nm Stokes wavelength. They added a high-reflectivity coating at the Stokes wavelength to one end of the laser rod, so the 7-cm-long Raman laser resonator included only the laser rod and the air between the rod and M1.

Figure 3. When the intracavity Raman resonator was present, the laser produced 2-ns pulses containing up to 31.8 mJ at 1538 nm. All data displayed were taken at 10 Hz.

In other words, the Raman laser was shorter than the fundamental-wavelength laser, although both used the same Nd:KGW crystal as their gain medium. This arrangement minimized the intracavity loss for the Raman laser and protected the Q-switch and mirror M2 from the intracavity power of the Raman laser.

As was the case for the fundamental wavelength alone — before feedback was added for the Raman resonator — the best results occurred with the longest pump pulses (Figure 3). With a pump-pulse energy of 1.23 J, the laser produced 2-ns output pulses containing 31.8 mJ at 1538 nm.

Optics Letters, May 1, 2007, pp. 1096-1098