Diode-side-pumped channel waveguide laser

Geskus, D., Grivas, C., Aravazhi, S., Griebner, U., García Blanco, S.M. and Pollnau, M. (2012) Diode-side-pumped channel waveguide laser. 5th EPS-QEOD Europhoton '12, , Stockholm, Sweden. 26 - 31 Aug 2012.

Abstract

Fundamental-mode channel waveguide lasers, while offering high pump and signal intensities and excellent pump-signal mode overlap, usually require a fundamental-mode pump source, thus limiting the diode pump power that can be launched into the waveguide to typically less than 1 W, thereby also setting limitations on the achievable output power. Here we demonstrate laser emission from a highly Yb³⁺-doped, tapered channel waveguide in KGd_xLu_1-x(WO₄)₂ by diode side pumping with a high-power, multi-mode diode bar via a passive planar waveguide, offering the potential for significantly increased output powers.
We have chosen KGd_xLu_1-x(WO₄)₂:Yb³⁺ for its large pump-absorption cross-section at 981 nm. Multi-layer growth of lattice-matched layers [1] with various compositions is applied to design the refractive-index contrast and active properties of the laser device (Fig. 1). A 1.7 to 0.6-µm-high, horizontally tapered waveguide structure is etched into the 5mm-long KGd_0.447Lu_0.078Yb_0.475(WO₄)₂ active layer [2] to allow for efficient pump absorption under side pumping in its 50-µm-wide waveguide section and simultaneously ensure single-mode laser emission via its 5-µm-narrow waveguide section. The pump power is delivered from the side through a 5 to 2.5-µm-high passive KGd_0.114Lu_0.116Y_0.77(WO₄)₂ slab waveguide overgrown and surrounding the active tapered waveguide.
The multi-mode, 30-W diode bar is mounted on a water-cooled chuck without active temperature control, resulting in a pump wavelength around 980 nm (Fig. 2a). The pump beam is modulated by a chopper with 2.9% duty cycle to reduce the thermal load. Mirrors are butt-coupled to the waveguide endfacets by use of a fluorinated oil, with reflection at 1040 nm of 99.8% and 90% at the wide and narrow active-waveguide end, respectively. No additional measures are taken to cool the sample. With this first, non-optimized device a maximum output power of 10 mW (Fig. 2a) when pumped with a cold diode (operating close to the absorption peak) and laser wavelength of 1038nm (Fig. 2b) are detected. Relaxation oscillations and the mode profile of laser emission with satellite spots, indicating a non-ideal taper structure, are recorded (Fig. 2c).

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