Abstract:We demonstrate the first Tm-doped yttria planar waveguide laser to our knowledge, grown by pulsed laser deposition. A maximum output power of 35 mW at 1.95 µm with 9% slope efficiency was achieved from a 12 µm-thick film grown on a Y 3 Al 5 O 12 substrate.
In this paper, we present the underlying advantages that make the crystalline planar waveguide (PW) the key ingredient in power-scaling difficult or "weak" laser transitions, especially those which are extremely challenging to operate in other gain medium configurations. The planar waveguide architecture is shown to enable efficient laser operation of low-gain and/or quasi-four-level transitions that suffer reabsorption losses. Exemplar configurations are reported to make this case, for example, 1.4 W at 1.8 μm from a Nd:YAG double-clad planar waveguide laser (PWL), in addition to 0.5 W at 2.7 μm from a similar highly doped Er:YAG PWL. New laser performance levels from sesquioxide PWs fabricated by pulsed laser deposition are also presented for the first time, with >1 W obtained from a Yb:Y 2 O 3 PWL. Current performance and future prospects are discussed for this laser architecture.
Lasers operating in the 2 micron wavelength region are of particular interest for various applications in remote sensing/LIDAR, materials processing, and medical treatments. Thulium-doped media have several attractive features for generating light in this wavelength band, including a broad emission bandwidth, long-lived metastable states, absorption bands matched to high-power 0.8 µm diode-pump sources coupled with the potential for high quantum-efficiency due to a 2-for-1 cross-relaxation process. The sesquioxide crystal family is of considerable interest as a potential host due to their excellent thermo-optic characteristics and spectroscopic properties. A key challenge for this host material is its high-temperature growth requirements (some in excess of 2500 K for bulk crystals); as such there has been limited success in fabricating these crystals commercially. Here we report the first growth and lasing results (to the best of our knowledge) of a crystalline Tm:Y 2 O 3 waveguide, fabricated via pulsed laser deposition (PLD).A Tm:Y2O3 film of thickness ~12 µm was deposited on a 1 cm 2 Y 3 Al 5 O 12 substrate. The PLD source was a KrF excimer laser, producing ~2 Jcm -2 pulses incident upon a Tm:doped ceramic yttria target in an oxygen atmosphere at a gas pressure of 4×10 -2 mbar. During deposition the substrate was heated to ~1000 ºC via a CO 2 laser at a wavelength 10.6 µm, leading to crystalline growth, highly textured in the (222) orientation, as determined via X-ray diffraction. The Tm concentration in the film was determined to be ~2.5 at.% by energy dispersive X-ray analysis. Two opposing facets were polished plane and parallel for subsequent laser experiments, leading to a final waveguide length of 8mm. The fluorescence spectrum was measured from the film's end facet when face pumped by a 795 nm diode laser, Fig. 1, consistent with that obtained from bulk Tm:Y2O3 [1].Longitudinally pumped by a Ti:sapphire laser tuned to an absorption peak at 797nm, the Tm:Y2O3 waveguide laser operated around 1951 nm, depending upon the cavity arrangement. An aspheric-lens was used to couple the pump light into the active layer, generating a spot radius of ~4 µm at the waveguide facet. Using a thin pump-in-coupling high-reflectance (HR) mirror attached to the input facet and various bulk output coupling mirrors, the best laser performance is shown in Fig. 2. The highest output power was achieved with a bulk 15% transmitting output coupler mirror positioned in close proximity to the waveguide end facet, giving 35 mW out for 600 mW of incident pump power on the pump-coupling lens, and a slope efficiency of 9 %. The waveguide propagation loss was determined to be ~2 dBcm -1 by measuring the laser's relaxation oscillation frequency at various pump power levels. We will discuss the laser performance and growth of this first Tm:Y2O3 crystalline waveguide, and further progress in reducing losses and improving the output powers, including growth of multilayer structures for high power operation.
High resolution, absolute excited-state absorption (ESA) spectra, at room temperature, from the long-lived 3 F 4 energy level of several crystals doped with trivalent thulium (Tm 3+ ) ions have been measured employing high-brightness narrowband (FWHM <30 nm) light emitting diodes (LEDs) as a probe wavelength. The aim of this investigation was to determine the strength of ESA channels at wavelengths addressable by commercially available semiconductor laser diodes operating around 630-680 nm. The favourable lifetime of the 3 F 4 manifold and negligible ground-state absorption (GSA) for the red-wavelength second-step excitation, ensures a direct and efficient route for a dual-wavelength pumping scheme of the thulium ion, which will enable blue-green laser emission from its 1 G 4 upper-laser level.
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