130-W Single-Frequency Laser Tunes Between 1040 and 1085 nm

Breck Hitz

Wavelength-tunable, narrow-bandwidth lasers are indispensable tools for high-resolution spectroscopy and other scientific pursuits, such as radiation cooling and trapping of ions or atoms. Two years ago, researchers at the University of Southampton in the UK teamed an external-cavity diode laser with an erbium:ytterbium co-doped fiber amplifier to generate well over 150 W of single-frequency power tunable between 1546 and 1566 nm. Now scientists at Laser Zentrum Hannover eV in Germany have taken a similar approach, using an external-cavity diode laser as the seed for a pair of ytterbium-doped fiber amplifiers to generate single-frequency power in excess of 130 W in the 1-μm region, tunable between 1040 and 1085 nm.

Figure 1. An external-cavity diode laser (ECDL) provided the 1-MHz-wide input for the fiber amplifiers. (SBS = stimulated Brillouin scattering). Images reprinted with permission of Optics Letters.

The scientists used a commercial diode laser from Sacher Lasertechnik GmbH of Marburg, Germany, to generate up to 65 mW in a 1-MHz linewidth, tunable between 1020 and 1085 nm. This was sufficient power to saturate the first amplifier, a 3-m length of 6-μm-core Yb-doped fiber from Liekki Corp. of Lohja, Finland, pumped by ~7 W of 975-nm light from a Dilas fiber-coupled diode laser. The scientists positioned optical isolators based on Faraday rotators both before and after the first amplifier stage, to prevent back-coupling-induced instabilities.

Figure 2. The scientists calculated the true laser output by subtracting the measured amplified stimulated emission (ASE) from the total output of the amplifier.

The 65-nm tuning range of the diode laser was trimmed by about 10 nm at the short-wavelength end in the preamplifier. Ytterbium is a quasi-three-level system, and the terminal levels for short-wavelength transitions have a deleterious thermal population. At the center of this tuning range, the first stage produced ~1.5 W, but the power fell off to ~1.0 W at the edges.

But that was adequate power to saturate the second amplifier stage, an 8.2-m length of 20-μm core fiber from Nufern of East Granby, Conn., pumped with up to 218 W from a 976-nm diode laser module from Laserline of Mülheim-Kärlich, Germany. Amplified spontaneous emission made a non-negligible contribution to the output of the second amplifier (Figure 2). When that contribution was subtracted, the laser’s output was fairly constant at ∼130 W from 1045 to 1080 nm, but it fell to ~120 W at the edges of the tuning range.

Figure 3. The laser maintained a very narrow spectral peak across the tuning range.

By observing the reflection from an uncoated glass beamsplitter, the scientists monitored the backward-propagating light emanating from the second amplifier stage. It increased linearly with pump power, indicating an absence of stimulated Brillouin scattering in that fiber.

The laser’s spectrum at three wavelengths —1040, 1060 and 1085 nm —showed a very narrow spectral peak (Figure 3). The scientists verified single-frequency operation with a scanning Fabry-Perot etalon whose free-spectral range was 2 GHz. The output beam quality was constant at M²~1.15 across the entire wavelength range.

Optics Letters, Aug. 15, 2007, pp.