Researchers from the University of Southampton Optoelectronic Research Centre (Southampton, England) plan to announce at CLEO/Europe in September that they have developed a high-power, tunable CW laser source at 2 µm using a double-clad thulium-doped silica fiber. The new laser has been shown to produce up to 14 W of single-mode output from 36.5 W of input power at 787 nm. The laser also can operate with wavelength tunability and has been operated between 1.85 to 2.07 µm at multiwatt power levels.
High power, all-solid-state sources of 2-µm radiation have recently attracted much interest. This spectral region is eye-safe and so is useful for remote-sensing applications such as lidar and some medical applications. It is also a useful starting point for efficient nonlinear frequency conversion to the mid-infrared (3 to 5 µm). For all these applications, good beam quality is desirable, and for some it is essential. For bulk lasers, thermal effects tend to degrade the beam quality and reduce the efficiency of the laser as higher power is pumped in.
Cladding-pumped fiber lasers offer a good alternative, with the heat generated from the surplus pump power allowed to dissipate over the whole length of the fiber. In addition, the output beam quality is determined by the waveguiding characteristics of the active-ion-doped core of the fiber, which can be tailored to produce a single-spatial-mode output.
The Southampton group produced their own fibers, pulling them from preforms fabricated in-house. Through the techniques of chemical-vapor deposition and solution doping, the preform was produced with a thulium concentration of approximately 2.2% by weight. It was machined so that the thulium-doped aluminosilicate core is slightly offset from the center of the cladding, which improves pump absorption.
The fiber has an inner core of approximately 20 µm, with a numerical aperture of 0.12. Around this core is an inner cladding 200 µm in diameter. It is this inner cladding that contains the pump light, and its diameter is chosen to be small enough to allow a high absorption of the pump light by the inner core, yet large enough to maximize the in-coupling of light from the two diode bars used as pump sources. The inner cladding is wrapped in a low-refractive-index (approximately 1.375) polymer outer cladding, resulting in a numerical aperture of 0.49.
The pump light for the fiber laser comes from two beam-shaped diode bars, both operating at 787 nm. A length of 4.5 m of fiber is used, providing efficient pump absorption. Feedback for the laser is provided by a single dichroic mirror butted onto one end of the fiber and by the 3.5% Fresnel reflection from the opposite cleaved fiber end. The focused light from the fiber laser is picked off by a dichroic mirror.
The performance measured by the group showed that the laser threshold corresponded to 5.8 W of pump power launched into the fiber. At the maximum pump power of 36.5 W launched into the fiber, the output power was 14 W at 1.998 µm with a beam quality of M21.1, confirming the single-mode nature of the output. The slope efficiency was approximately 46%.
Using a slightly different configuration the group also investigated the tunability of the thulium-doped fiber laser. In this case the two diode pump sources were polarization-coupled and launched into one end of the fiber, and a diffraction grating was included in the cavity to provide wavelength selection. This arrangement produced output between 1.85 and 2.07 µm at multiwatt power levels across the range and up to 7 W at 1.94 µm. This result was achieved with a launched pump power of 33.5 W. The researchers expect that further optimization of the optical design and thulium doping levels will bring an increase in output power.
Researcher Andy Clarkson says, "This laser gives a combination of high power, high efficiency, diffraction-limited beam quality, and wide tunability, all in a commercially interesting wavelength region. We are expecting that this will be attractive for a wide range of applications."
Bridget Marx | Contributing Editor, UK
Bridget Marx was Contributing Editor, UK for Laser Focus World.