1. Introduction

The development of compact laser sources in the 2-μm wavelength region has become a topic that is intensely being studied, e.g. for sensing applications. Gases such as CO₂ and NH₃ have absorption bands in the 1.5–1.65 μm and 2 μm wavelength regions; they can act as biomarkers for diseases [1] or allow one to monitor processes in bioreactors [2,3]. Detecting these gases for environmental or medical purposes requires sensors that are able to detect amounts down to the ppb levels. In the 1.5–1.65 μm wavelength range erbium-doped fibers are used to amplify the output power of laser diodes [4–6], but this approach prevents the construction of an integrated, compact detector. Detecting CO₂ and NH₃ around 2 μm has the advantage of absorption lines increasing by a factor of 100 and 3, respectively, compared to the absorption line strengths around 1.5 μm [2]. Contemporary detection systems in this wavelength range typically use diode lasers with output powers up to tens of milliwatts [7,8]. This limited output power negatively affects the detection sensitivity and, consequently, needs to be compensated for by employing sophisticated and expensive detectors. Development of an integrated laser that operates around 2 μm and yields output powers beyond the current tens of milliwatts would, therefore, represent an important step forward toward compact, inexpensive, and cheap gas detectors operating based on optical detection principles.

The first 2-μm dielectric waveguide laser was demonstrated in a thulium-doped lead-germanate glass [9]. Planar waveguide lasers with slope efficiencies up to 68% [10] and output powers of more than 10 W [11] were operated in YAG:Tm³⁺. However, in lasers with planar geometries the output beam quality is usually hard to control. Channel waveguide geometries provide better control over the beam shape, as well as better confinement and overlap of the pump and laser modes in the doped region. Tm³⁺-doped channel waveguide lasers have previously been demonstrated in oxides [12] and glasses [13], but the output powers and slope efficiencies were inferior.

The enormous potential of the potassium double tungstates [14] for creating compact lasers has previously been demonstrated in Yb³⁺-doped KY(WO₄)₂ ( = KYW:Yb³⁺) [15] and Gd³⁺, Lu³⁺ co-doped KYW:Yb³⁺ [16] thin films, with planar and channel waveguide lasers achieving slope efficiencies as high as 82.3% [17] and 71% [18], respectively, output powers of several hundreds of milliwatts, and low thresholds. In Tm³⁺-doped KYW, planar and channel waveguide lasers have been demonstrated with maximum output powers of 32 mW and slope efficiencies of 13% [19,20]. The higher slope efficiencies of Yb³⁺-based lasers compared to the Tm³⁺-based lasers can be attributed to the usually smaller quantum defect between pump and laser wavelength and the absence of detrimental upconversion processes in Yb³⁺. Demonstrated slope efficiencies of 44% in thulium-doped bulk KGd(WO₄)₂ crystals [21] and up to 69% in YAG:Tm³⁺ [10] and fiber lasers [22], nevertheless, suggest that there is significant room for improvement in terms of slope efficiency and, consequently, output power in KYW:Tm³⁺ channel waveguides.

Here we report an efficient channel waveguide laser in a monoclinic potassium double tungstate, operating at 2 μm. The KYW:Tm³⁺(1.5at.%) ridge-type channel waveguide laser delivers up to 149 mW of output power at a slope efficiency of 31.5%, which can be further improved by optimizing the dopant concentration and increasing the outcoupling degree. Such lasers will allow for more sensitive detection of gases around 2 μm.

2. Fabrication of KY_1-x-yGd_xLu_yW:Tm³⁺ microchannels

A co-doped layer of KY_0.4Gd_0.295Lu_0.29Tm_0.015(WO₄)₂ with a thickness of several tens of micrometers was grown onto a pure, (010)-orientated, laser-grade polished KYW substrate by liquid-phase epitaxy in a K₂W₂O₇ solvent [17,23] at 920-923°C. The layer was lapped and polished to a thickness of 6.6 μm with laser-grade surface uniformity. A photoresist mask (Fujifilm OiR 908/35) was deposited onto the layer and patterned. Ar⁺-beam milling [24] at an acceleration voltage of 350 eV, resulting in an etch rate of 3 nm/min., was applied to the sample to produce ridge-type channel waveguides along the N _g optical axis with widths of 7.5–12.5 μm and an etch depth of 1.5 μm. Afterwards the channel waveguides were overgrown with a pure KYW cladding layer to prevent, firstly, additional scattering losses at the channel-air interface and, secondly, detrimental rounding effects at the end facets during end-face polishing. In addition, overgrowth with pure KYW ensures a more symmetric waveguide, which increases the spatial overlap of the pump and laser modes with the doped layer. After overgrowth, the samples were diced and end-face polished to a length of 8.4 mm.

3. Laser experiments on KY_1-x-yGd_xLu_yW:Tm³⁺ channel waveguides

Laser experiments on the Tm³⁺-doped channel waveguides were carried out at pump wavelengths of 794 nm and 802 nm with transverse-magnetic (TM, E||N _p) and transverse-electric (TE, E||N _m) polarizations, respectively. The absorption cross-section is highest for the TM polarization at 794 nm and well suited for pumping with a Ti:Sapphire pump source due to its narrow absorption peak [25]. Incoupling of pump light into the channels was optimized using cylindrical lenses with focal lengths of 40 mm and 10 mm to focus the Gaussian pump beam in the horizontal and vertical directions, respectively. An overlap efficiency of 95% between incident pump beam and channel waveguide mode at the pump wavelength was achieved in this way. Dielectric mirrors with reflectivities of 99.99% (HR), 98%, and 92% at 1900–2100 nm and high transmission at 790–810 nm were butt-coupled to the sample using index-matching fluid (Fluorinert). Different mirror combinations of HR & 98%, HR & 92%, and twice 92% reflectivity were used to assess the laser performance. The laser output power was outcoupled from the other waveguide end via a microscope objective with a numerical aperture of NA = 0.25 and measured after passing through a RG1000 high-pass filter to block residual unabsorbed pump power. For the configuration with two 8% outcoupling mirrors, the laser output power was also measured at the pump incoupling side by use of a beam splitter and a RG1000 filter, see Fig. 1 . A spectrometer (Horiba Jobin Yvon iHR550) was used to determine the lasing wavelength. The polarization of the 2-µm laser was determined by passing the beam through a polarizer with an extinction ratio of 1000:1 at wavelengths of 650–2000 nm and its beam quality was analyzed with a beam profiler (Coherent Lasercam-HR). Cavity losses at the laser wavelength were determined by measuring the relaxation-oscillation frequency as a function of pump power via an InGaAs detector (Thorlabs FGA20) connected to an oscilloscope (HP-Agilent Infinium).

 

Fig. 1 Schematic of the experimental setup. The elements enclosed by the dashed line were only used with the double 8% outcoupling mirror configuration. The blue streak of luminescence visualizes the position of the channel waveguide.

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4. Laser performance of KY_1-x-yGd_xLu_yW:Tm³⁺ channel waveguides

Laser spectra for different outcoupling mirror configurations are shown in Fig. 2 . The laser was found to operate at wavelengths around 1930 nm when a low outcoupling degree of 2% was used. When mirror configurations with higher outcoupling efficiencies of 8% and 2 × 8% were selected, the laser wavelength shifted to shorter wavelengths of 1906 nm and 1846 nm, respectively. The higher losses due to the higher outcoupling degrees increase the threshold inversion and cause the laser to shift toward shorter wavelengths where the maximum gain is higher [25]. The shape and linewidth of the laser emission peaks indicate that the laser operates on multiple longitudinal modes.

 

Fig. 2 Laser spectra for different outcoupling mirror configurations. a) HR & 2%, b) HR & 8%, and c) 2 × 8%.

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Figure 3 displays the laser output power as a function of absorbed pump power. When pumping at 794 nm in TM polarization, see Fig. 3(a), a slope efficiency as high as 31.5% and a maximum output power of 149 mW were measured when two mirrors with an outcoupling efficiency of 8% were used. Output powers of 80 mW and 41 mW and slopes of 15.1% and 8.5% were measured for 8% and 2% outcoupling efficiencies, respectively. When pumping at 802 nm in TE polarization, see Fig. 3(b), the output powers and slope efficiencies decreased to 76 mW, 54 mW, and 18 mW and 17.0%, 8.9%, and 4.9% for outcoupling efficiencies of 2 × 8%, 8%, and 2%, respectively. A zoom-in on the threshold region when pumping at 794 nm in TM polarization is shown in Fig. 3(c). The lowest threshold of approximately 7 mW of absorbed pump power was achieved in the configuration when pumping at 794 nm in TM polarization and choosing a 2% outcoupling degree. With higher outcoupling degrees, the threshold increased, but the actual onset of lasing was difficult to assess due to the fluctuations of the thermal detector at these low powers. The low threshold values demonstrated here compared to previously reported values of 75 mW [19] and 42 mW [20] are due to the better horizontal confinement in the channel compared to a planar waveguide [19], the lower output coupling compared to a mirror-less laser [20], as well as the better vertical confinement in the highly co-doped layer compared to both previous results. The low threshold will facilitate diode pumping of a KY_1-x-yGd_xLu_yW:Tm³⁺ waveguide laser.

 

Fig. 3 Laser output power versus absorbed pump power for two different pump wavelengths and polarizations: a) pumped at 794 nm in TM polarization (E||N _p); b) pumped at 802 nm in TE polarization (E||N _m); c) zoom-in on the threshold region when pumped at 794 nm in TM polarization (E||N _p).

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The cavity losses were determined by measuring the laser relaxation-oscillation frequency as a function of the ratio of pump power over pump threshold. For a three-level laser, the relaxation-oscillation frequency is given by [26]:

where γ_c = −2ℓ _opt/cln[R ₁ R ₂(1-L)] is the cavity decay rate, 2ℓ _opt = 33.9 mm is the optical path length, c is the speed of light, R ₁ = 0.9999 and R ₂ = 0.92 are the mirror reflectivities, L is the intrinsic roundtrip loss, τ_l = 1.7 ms is the ³ F ₄ upper laser level lifetime for this Tm³⁺ concentration [27], P/P_thr is the ratio of pump power over pump threshold, N = 0.951 × 10²⁰ cm⁻³ is the Tm³⁺ concentration, σ_al = 6.68 × 10⁻²² cm² is the effective absorption cross-section at the lasing wavelength [25], and n = 2.006 is the refractive index of the guiding layer at the lasing wavelength. The measured relaxation-oscillation frequency versus the quantity P/P_thr–1 for the laser with 8% outcoupling degree is displayed in Fig. 4 . A linear fit of the measured data yields a value for the intrinsic roundtrip loss of L = 7.9 ± 2.5%, which provides an upper limit for the waveguide propagation loss of 0.11 ± 0.04 dB/cm. This value is lower than the value of 0.34 dB/cm reported for Gd³⁺-Lu³⁺-Yb³⁺ co-doped KYW channel waveguide lasers [24], which can be partially explained by the longer operating wavelength (~2 µm in Tm³⁺ instead of ~1 µm in Yb³⁺) of our laser, resulting in lower scattering losses. We assume that the largest, potentially wavelength-independent contribution to the present intrinsic roundtrip losses of ~0.11 dB/cm stems from butt-coupling of the dielectric mirrors.

 

Fig. 4 Measured relaxation-oscillation frequency ω² as a function of the quantity P/P_thr–1 for the laser with an outcoupling degree of 8% and linear fit providing the intrinsic roundtrip loss. The error bars represent the standard deviation given by the oscilloscope. The relaxation oscillations became first visible at a threshold pump power of 7 mW.

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The measured near-field output profile of the laser is displayed in Fig. 5 . A Gaussian fit of the beam profile in horizontal and vertical direction results in 1/e² intensity radii of 6.1 × 3.5 μm². The ridge-type waveguide supports only the fundamental mode at lasing wavelengths of 1846 nm and beyond. The laser output polarization measured by comparing the power ratios reveals a preferential quasi-TE output ratio of 12:1.

 

Fig. 5 Measured mode profile and 1/e² Gaussian fit of the laser output beam generated with the double 8% outcoupling mirror configuration.

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5. Conclusions

We have demonstrated a record-high output power of 149 mW, slope efficiencies up to 31.5%, and threshold as low as 7 mW in Gd³⁺-Lu³⁺-Tm³⁺ co-doped KYW channel waveguides. These high output powers will allow for more sensitive detection of gases with absorption bands around 2 μm, while the low threshold will facilitate diode pumping. Further improvement in slope efficiency and output power seems feasible when optimizing the Tm³⁺ concentration and further increasing the outcoupling degree.