Laser Zentrum Hannover (LZH; Hannover, Germany) researchers have, to the best of their knowledge, achieved the highest extracted pulse energy from an all-fiber amplification system seeded by a gain-switched laser diode: 270 kW at a repetition rate of 1 MHz.1 Although sub-100-ps gain-switched laser diodes have been amplified by other research groups using all-fiber techniques, output power levels were typically in the hundreds of watts at repetition rates of tens of megahertz. Furthermore, the effects of amplified spontaneous emission (ASE) generated in the fiber amplifier—which can cause inefficient amplification of the signal and a continuous-wave (CW) power background—were not carefully considered in these prior demonstrations.
Three-stage fiber amplification
In the setup, a fiber-coupled low-power (20 μW at 1 MHz) gain-switched laser diode from PicoQuant (Berlin, Germany) was fusion spliced into a three-stage ytterbium (Yb) fiber amplifier (see figure). The seed laser had a 37 ps pulse duration at 1040 nm wavelength with 20 nm bandwidth. Repetition rates could be controlled by the drive electronics to between 31.25 kHz and 80 MHz. To minimize nonlinear effects such as stimulated Raman scattering and self-phase modulation, the optical power density within the fiber was minimized through the largest fiber core diameter that could maintain singlemode propagation.
The first and second amplification stages used a single pump laser diode, while the third stage used three pump-combined laser diodes. In the first stage, the output of an up to 600 mW average power, 976 nm pump laser diode was combined via a wavelength-division multiplexer (WDM) with the seed laser and input to a 60 cm long section of single-clad Yb-doped fiber with a 4.4 μm mode-field diameter (MFD). This first-stage output was combined via WDM with a second pump laser diode and coupled into an 80 cm length of 10 μm MFD Yb fiber using a homemade mode-field adapter (MFA) that enabled efficient coupling between the 4.4 μm and 10 μm MFD fibers. The three third-stage pump laser diodes were fed to a multimode pump combiner and into a 2.5 m long double-clad large-mode-area (30 μm MFD) Yb fiber via another homemade MFA.
With minimized nonlinear effects and optimized signal-to-ASE ratio, average output power levels of 4 mW and 90.5 mW for the first and second gain stages, respectively, were observed at the 1 MHz seed-laser repetition rate; no spectral deformation of the 90.5 mW output pulse was observed. Final output from the third stage was recorded as a function of increasing pulse energy. At the highest extracted pulse energy (13 μJ), which corresponded to a 14.9 W average output power with signal-to-ASE ratio of 88%, peak power was 270 kW—the highest-achieved peak power for fiber-based amplification of a gain-switched laser diode. "The intention of the all-fiber based sub-100-ps laser was to achieve a compact, stable, and versatile laser source for micromachining processes such as engraving and solar cell manufacturing," says Sebastian Kanzelmeyer, Dipl.-Phys. at LZH. "The regime of sub-100-ps is promising because of the versatility of laser sources and the chance of very precise micromachining results. The laser is already successfully used for materials processing."
1. S. Kanzelmeyer et al., Opt. Exp., 19, 3, 1854–1859 (Jan. 31, 2011).