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Lasers & Sources

Novel ytterbium and thulium lasers based on a double tungstate crystalline host

Yb3+- and Tm3+-doped KLu(WO4)2 crystals are highly efficient laser media that can be pumped by laser diodes and operate in continuous-wave q-switched and mode-locked regimes.
30 September 2007, SPIE Newsroom. DOI: 10.1117/2.1200709.0765

Among the family of monoclinic double tungstate crystals, KLu(WO4)2—which is known as KLuW—is a very attractive laser host material. Recent studies have shown that Yb3+:KLuW crystals are highly efficient laser media in the 1μm spectral range for continuous-wave (CW)\ room-temperature operation and very suitable for pumping with InGaAs laser diodes.1,2 This is because lutetium is the ion closest to ytterbium in the lanthanide series of the periodic table, which facilitates its incorporation. Besides achieving up to 100% doping without affecting the quality of the grown crystals, the small lattice mismatch between KLuW and KYbW is also advantageous for the epitaxial growth of thin doped layers on undoped substrates. Furthermore, the thermal conductivity of KLuW is least affected by Yb-doping and its good mechanical properties also facilitate high-quality polishing and coating.

The next closest lanthanide to lutetium is thulium: hence KLuW also represents a promising host for this dopant. In contrast to Yb3+ however, Tm3+ exhibits quenching at high doping concentrations. Nevertheless, the attractive properties of double tungstate hosts, especially their high absorption and emission cross sections and relatively broad bandwidths, still make them suitable both for tuning and mode-locking when doped with Tm3+.

Figure 1. (a) Morphology and single crystals of (b) undoped, (c) 5% Yb-doped, and (d) 3% Tm-doped KLu(WO4)2. (Figure courtesy of Universitat Rovira I Vergili, Tarragona, Spain.)

An important prerequisite in laser design is power scalability and both Yb3+ and Tm3+ have absorption peaks in KLuW that overlap very well with the emission maxima of commercial InGaAs and AlGaAs diode lasers. The properties of KLuW are now well characterized as is the spectroscopy of Yb3+ and Tm3+. While the strong anisotropy of the KLuW spectroscopic properties is considered an important advantage for laser applications, the anisotropy of its thermo-mechanical properties can introduce some limitations. However, these issues can be addressed by careful design of the active elements and efficient thermal management.

For instance, two polarizations parallel to the Nm and Np principal optical axes have been shown to be useful for laser operation with Yb3+- and Tm3+-doped KLuW.2 Both ions can be used for pumping, either separately or together when pumping with unpolarized diode lasers, which is important for power scaling. While the gain is normally higher for E//Nm, the E//Np polarization can have some advantages related to spectral features such as absorption or emission. The choice of any of these two polarizations therefore allows more propagation direction flexibility because KLuW is a biaxial crystal, meaning that it has two optical axes along which an incoming beam can propagate at the same speed. This feature can be used to optimize the thermal management or the nonlinear properties in the case of stimulated Raman scattering (SRS) or mode-locking.

We achieved extremely high slope efficiencies, approaching the theoretical limits, for CW laser operation with both Yb3+- and Tm3+-doped KLuW crystals.2 In both cases, diode pumping successfully achieved multiwatt output powers. While the slope efficiencies with epitaxial composites were rather high, problems related to non-radiative and heat-generating processes at high doping levels remain to be addressed. The purity of the materials used for the flux growth process will be an important factor for advances in this direction. Once the impurity problem in the KLuW host solved, we expect that such epitaxies could lead to a breakthrough in thin disk laser design by allowing the reduction of its geometry to a single pump pass with extremely efficient one-dimensional heat removal.

We demonstrated impressive tunability in Tm-lasers, both bulk and epitaxial, in the 2μm spectral range. The gain bandwidths are quite large, both for Yb and Tm, meaning that passive mode-locking can produce sub-100fs light pulses. However, since semiconductor saturable absorber mirrors are at present only available for the 1μm spectral range, we were only able to demonstrate this feature for Yb3+:KLuW lasers, both bulk and epitaxial. The KLuW host is also attractive for SRS, which can be realized in a single cavity using the same doped crystal and passive Q-switching. We achieved this regime with Yb3+:KLuW and a saturable absorber producing sub-nanosecond pulses.

Future work will be focused on improving heat removal using special geometries for the active elements (thin slabs or disks) as well as implementing the use of reflective and antireflection coatings to simplify the cavity design and further scale the output power. However, the use of thermally insensitive orientations for power scaling will require knowledge of the KLuW thermo-optic coefficients. These crystal parameters, together with the nonlinear refractive index, are important for mode-locked and high-power operation and are presently under study. Special coatings of the cavity mirrors are also expected to allow the generation of even shorter mode-locked pulses with Yb3+:KLuW by supporting spectrally broader gain, extending closer to the pump wavelength. This feature characterizes quasi-three-level laser systems where the gain bandwidth depends on the achievable inversion. The search for suitable passive elements with saturable absorption for Q-switching and mode-locking in the 1.9μm spectral range will also continue. Both regimes are presently investigated with Tm3+:KLuW.

This work was supported by EU Project DT-CRYS (NMP3-CT-2003- 505580) funding.

Valentin Petrov
Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy
Berlin, Germany

Valentin Petrov received a MS in nuclear physics from the University of Sofia in 1983 and a PhD in optical physics from the Friedrich-Schiller-University, Jena, in 1988. He joined the Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy in Berlin in 1992. His research interests include ultrashort light pulses, laser physics and nonlinear optics and he has co-authored more than 180 papers in scientific journals.