Scott D. Setzler

Resonantly pumped eyesafe erbium lasers

The viability of high-power and high-energy, direct eyesafe emission from bulk erbium lasers has recently been demonstrated. In this paper, we present a review of eyesafe erbium lasers that are resonantly pumped by both fiber and diode lasers. High brightness pumping with a 1.53-/spl mu/m erbium fiber laser has yielded 60 W of continuous wave (CW) output, 10 W of repetitively Q-switched output, and as much as 16 mJ of pulse energy. Diode laser pumping has yielded 38 W of quasi-CW output and >40 mJ of Q-switched output.

Mid-infrared ZnGeP 2 parametric oscillator directly pumped by a pulsed 2 μm Tm-doped fiber laser

We have demonstrated what we believe to be the first mid-infrared optical parametric oscillator (OPO) pumped directly by a pulsed Tm-doped fiber laser. The Tm-fiber pump laser produces 30 ns pulses with a repetition rate of 30 kHz at a wavelength of 2 μm. The ZnGeP2 (ZGP) OPO produces 20 ns mid-IR pulses in the 3.4-3.9 μm and 4.1-4.7 μm spectral regions simultaneously. More than 658 mW of mid-IR output power has been generated with a total OPO slope efficiency greater than 35%.

Efficient 1645-nm Er:YAG laser

We report a resonantly fiber-laser-pumped Er:YAG laser operating at the eye-safe wavelength of 1645 nm, exhibiting 43% optical efficiency and 54% incident slope efficiency and emitting 7-W average power when repetitively Q switched at 10 kHz. To our knowledge, this is the best performance (conversion efficiency and average power) obtained from a bulk solid-state Q-switched erbium laser. At a 1.1-kHz pulse repetition frequency the laser produces 3.4-mJ pulses with a corresponding peak power of 162 kW. Frequency doubling to produce 822.5-nm, 4.7-kW pulses at 10 kHz was performed to demonstrate the lasers utility.

Resonantly pumped Tm-doped fiber laser with >90% slope efficiency.

We have demonstrated what we believe is the highest slope efficiency reported for a Tm-doped fiber laser operating in the 2-micron spectral region. Using a 1908 nm Tm-doped fiber laser as an in-band pump source, we generated 1.43 W of output power at 2005 nm with 81.25% optical efficiency and 90.2% slope efficiency, with respect to launched pump power. This resonant-pumping approach allowed us to examine the bleaching effects in Tm-doped fiber under resonant pumping. We also analytically show that this pumping method can scale to high power levels while maintaining high efficiency.

Characterization of defect-related optical absorption in ZnGeP2

A broad optical absorption band with a peak near 1 μm is present in most single crystals of ZnGeP2. These same crystals have an electron paramagnetic resonance (EPR) signal which has been assigned to singly ionized zinc vacancies. A direct correlation between the intensity of the optical absorption at 1 μm and the intensity of the EPR signal has been established using a set of ZnGeP2 crystals where this absorption varied widely. These results suggest that the singly ionized zinc vacancy acceptor plays a direct role in the electronic transition(s) responsible for the 1 μm optical absorption. In separate experiments, it was found that illuminating the ZnGeP2 crystals with a He–Ne laser (632.8 nm) while at temperatures near 25 K produces an increase in the absorption at 1 μm and an increase in the zinc vacancy EPR spectrum. These latter results provide further evidence that the absorption at 1 μm is associated with the singly ionized zinc vacancy acceptor.

Increasing the Laser Induced Damage Threshold of Single Crystal ZnGeP2

The laser-induced damage threshold (LIDT) of single-crystal zinc germanium phosphide (ZGP), ZnGeP2, was increased to 2J/cm2 at 2.05μm and a 10 kHz pulse rate frequency (double the previously measured value of 1 J/cm2). This increased LIDT was achieved by improving the polishing of ZGP optical parametric oscillator crystals. Two different polishing techniques were evaluated. Surfaces were characterized using scanning white-light interferometry to determine rms surface roughness and sample flatness. The photon backscatter technique was used to determine the degree of surface and subsurface damage in the sample induced through the fabrication process. The effect of subsurface damage in the samples was studied by removing different amounts of material during polishing for otherwise identical samples. Statistical LIDT was measured using a high-average-power, repetitively Q-switched Tm,Ho:YLF 2.05μm pump laser. On average, lower surface roughness and photon backscatter measurements were a good indicator of ZGP samples exhibiting higher LIDT. The removal of more material during polishing significantly improved the LIDT of otherwise identical samples, indicating the importance of subsurface damage defects in the LIDT of ZGP.

5-W repetitively Q-switched Er:LuAG laser resonantly pumped by an erbium fiber laser

We report a high-average-power, near-diffraction-limited Er:LuAG laser generating 5 W of power at 1.648 microm in either cw or repetitively Q-switched operation. When the laser is Q switched at 9 kHz, we measure 0.52 mJ/pulse. The laser is end pumped by a 20-W erbium fiber laser and achieves >30% optical conversion efficiency and >40% incident slope efficiency. This is, to our knowledge, the highest performance (average power and conversion efficiency) obtained from a bulk solid-state erbium laser.

High power resonant pumping of Tm-doped fiber amplifiers in core- and cladding-pumped configurations

We have demonstrated ultra-high efficiency amplification in Tm-doped fiber with both core- and cladding-pumped configurations using a resonant tandem-pumping approach. These Tm-doped fiber amplifiers are pumped in-band with a 1908 nm Tm-doped fiber laser and operate at 1993 nm with >90% slope efficiency. In a core-pumped configuration, we have achieved 92.1% slope efficiency and 88.4% optical efficiency at 41 W output power. In a cladding-pumped configuration, we have achieved 123.1 W of output power with 90.4% optical efficiency and a 91.6% slope efficiency. We believe these are the highest optical efficiencies achieved in a Tm-doped fiber amplifier operating in the 2-micron spectral region.

Electron paramagnetic resonance and electron–nuclear double-resonance study of Ti3+ centres in KTiOPO4

Electron paramagnetic resonance and electron–nuclear double resonance have been used to characterize four Ti3+ centres in undoped crystals of potassium titanyl phosphate (KTiOPO4 or KTP). These 3d1 defects (S = 1/2) are produced by ionizing radiation (either 60 kV x-rays or 355 nm photons from a tripled Nd:YAG laser), and form when the regular Ti4+ ions in the crystal trap an electron. Two of these trapped-electron centres are only observed in hydrothermally grown KTP and the other two are dominant in flux-grown KTP. Both of the Ti3+ centres in hydrothermally grown crystals have a neighbouring proton (i.e. an OH− molecule). In the flux-grown crystals, one of the Ti3+ centres is adjacent to an oxygen vacancy and the other centre is tentatively attributed to a self-trapped electron (i.e. a Ti3+ centre with no stabilizing entity nearby). The g matrix and phosphorus hyperfine matrices are determined for all four Ti3+ centres, and the proton hyperfine matrix is determined for the two centres associated with OH− ions. These Ti3+ centres contribute to the formation of the grey tracks often observed in KTP crystals used to generate the second harmonic of high-power, near-infrared lasers.

Resonantly diode-pumped eyesafe Er:YAG lasers (Invited Paper)

We have demonstrated efficient operation of the eyesafe laser transition (4I13/2 -> 4I15/2) in Er:YAG by resonantly pumping with 1470nm diodes. Quasi-cw powers in excess of 30W have been achieved at 10% duty cycle with 47% slope efficiency, 26% conversion efficiency, and beam quality of M2=1.4 x 2.2. In energy storage mode, we have generated near-diffraction-limited 41mJ / 58ns pulses, more than 700kW of peak power, at 10Hz. Storage lifetimes in the range of 5 to 7msec have been measured, and pulses as short as 25ns have been obtained at reduced energy. We believe this to be the first-ever demonstration of a resonantly diode pumped (bulk) erbium laser.