We report on the fabrication, µ-Raman characterization, and continuous-wave laser operation of a channel waveguide with a hexagonal optical-lattice-like cladding fabricated in monoclinic Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. µ-Raman spectroscopy indicates preservation of the crystalline quality in the core region and an anisotropic residual stress field. When pumped by a Ti:Sapphire laser at 802 nm, the Tm:KLu(WO4)2 buried channel waveguide laser generated 136 mW at 1843.7 nm with a slope efficiency of 34.2% and a threshold as low as 21 mW, which are the record characteristics for femtosecond-laser-written Tm crystalline waveguide lasers. The variation of the output coupling resulted in discrete wavelength tuning of the laser emission from 1785 to 1862 nm. The propagation losses in the waveguide are ~1.2 ± 0.3 dB/cm.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Waveguide lasers emitting at ~2 µm are of interest for environmental and medical sensing applications because this spectral range contains absorption lines of many relevant organic molecules and gases. The 2 µm laser emission is usually obtained from Thulium (Tm3+) or Holmium (Ho3+) oscillators. In the former case, it is related to the 3F4 → 3H6 electronic transition. Tm lasers can be efficiently pumped at ~0.8 µm, they exhibit high pump quantum efficiency potentially reaching 2 due to the efficient cross-relaxation (CR) process for adjacent Tm3+ ions and they offer broad tunability of the laser emission .
An efficient continuous-wave (CW) Tm waveguide laser fabricated by the Liquid Phase Epitaxy (LPE) technique has been previously demonstrated . Low propagation losses (~0.11 dB/cm) in such devices were also detected at ~2 µm . Femtosecond (fs) direct laser writing (DLW) is an emerging technology for production of micro-scale integrated active structures in bulk transparent dielectric materials [4–9]. This method is much less complex than the LPE or pulsed laser deposition (PLD) offering a wide range of waveguide geometries, short fabrication time, high precision and good optical quality and, hence, low propagation losses for the waveguides. Fs-DLW can be easily applied to various low-symmetry crystals exhibiting natural optical anisotropy which is challenging for LPE. Even for the crystals for which the LPE method is well-developed (i.e., including two growth steps, namely growth of a bulk crystal further serving as a substrate and growth of the epitaxial layer), fs-DLW is easier as it requires only one growth step.
To date, fs-DLW channel waveguides were realized in materials doped with several rare-earth ions, such as Nd3+ [8,10–12] and Yb3+ [13,14] at ~1 µm, Er3+ at ~1.5 µm  and Tm3+ at ~2 µm [16,17]. At ~1 µm, very high CW laser slope efficiencies η were reached. In , a fs-DLW Yb:YAG waveguide laser generated 765 mW at 1.03 µm with a record η = 75% and in , a Nd:GdVO4 laser generated 256 mW at 1.06 µm with η = 70%. Watt-level output from a diode-pumped fs-DLW Nd:YAG ceramic laser was demonstrated at ~1.06 µm and at 1.3 µm . There are only sporadic reports on Tm fs-DLW waveguide lasers in the literature which were devoted to Tm3+-doped ZBLAN glasses and YAG crystals (i.e., isotropic materials) [16–20]. In , a fs-DLW Tm:ZBLAN laser generated 205 mW at 1.89 µm with η = 67%.
The monoclinic double tungstate (MDT) laser host crystals with general formula KRE(WO4)2 or shortly KREW where RE = Gd, Y or Lu are known for providing high doping concentrations , high transition cross-sections [22,23], and minimal fluorescence quenching for different rare-earth dopants at acceptable thermal properties [24,25] which makes them very suitable for efficient bulk , thermo-  and index-guided  lasers. However, only one report could be found in the literature on Yb3+-doped KGdW and KYW fs-DLW waveguide lasers  where the Yb:KGdW laser generated 18.6 mW at 1023 nm with η = 9.3%. Nd3+-doped KGdW fs-DLW waveguides were characterized in [29,30] but no lasing was demonstrated. Raman gain from the fs-DLW waveguide in undoped KGdW was observed in .
Very recently, we reported on the development and CW laser performance of fs-DLW Tm:KLuW channel waveguides with a circular cladding . In the present paper, we demonstrate a fs-DLW Tm:KLuW channel waveguide with a hexagonal optical-lattice-like [32–34] cladding. We present results of µ-Raman and emission spectroscopy, and CW laser operation at 1.78-1.86 µm. This type of fs-DLW waveguides which can be described as type II modification with type III cladding geometry  are advantageous because they can be potentially used for the creation of fully 3D fs-DLW micro-structures in anisotropic crystals allowing for light propagation in any direction (along the waveguide), and tailoring the spatial profile of the light beam, e.g. in beam splitters, for simultaneous generation and manipulation of the laser beam [9,33,34].
2. Waveguide fabrication
Depressed-index waveguides were fabricated in a bulk Tm:KLuW crystal by 3D fs-DLW. The crystal itself was grown by the Top-Seeded-Solution Growth (TSSG) Slow-Cooling method . It was doped with 3 at.% Tm3+ (NTm = 2.15 × 1020 at/cm3). A rectangular 3.0 mm-thick sample was cut from this crystal oriented for light propagation along the Ng optical indicatrix axis (KLuW is an optically biaxial crystal). Its aperture was 3.10 (Nm) × 2.85 (Np) mm2 and both Nm × Np faces were polished to laser-grade quality.
For DLW, 120-fs, 795 nm pulses from a Ti:Sapphire regenerative amplifier (Spitfire, Spectra Physics) were used employing a small fraction of the output energy at 1 kHz repetition rate. The laser beam was focused into the crystal with a 40 × microscope objective (N.A. = 0.65). The incident pulse energy on the crystal was set to 73 nJ, with the help of a half-wave plate, a polarizer and a calibrated neutral density (ND) filter. It was kept constant during the inscription. The crystal was scanned at a constant speed of 300 μm/s along the Ng-axis thus producing damage tracks. The polarization of the fs laser (E || Nm) was perpendicular to the scanning direction to avoid anisotropic effects related to the birefringence of DTs. The line scan procedure was repeated at different depths and lateral positions of the sample. The channel waveguide consisted of a 30 μm diameter core and four hexagonal-shaped groups of damage tracks forming a hexagonal cladding, see Fig. 1(a). The axis of the waveguide was located 120 μm beneath the crystal surface, Fig. 1(b). The separation between adjacent tracks was 8 μm and 3 μm (in horizontal and vertical directions, respectively).
3. Waveguide characterization
At first, we studied the luminescence from the waveguide core. For this, the output of a CW Ti:Sapphire laser (Coherent, model MIRA 900) tuned to ~802 nm (3H6 → 3H4 transition of Tm3+) was coupled into the waveguide using a 10 × microscope objective lens (Mitutoyo M Plan Apo, numerical aperture, N.A.: 0.28, focal length: 20 mm). The Tm3+ luminescence was collected from the output facet of the waveguide using a Glan-Taylor polarizer and an optical fiber, and detected with an optical spectrum analyzer (OSA, Yokogawa, model AQ6375B), Fig. 2(a). The fabricated waveguide exhibited intense luminescence related to the 3F4 → 3H6 transition of Tm3+ with a notable anisotropy for the light polarizations E || Nm and E || Np which is a prerequisite for linearly polarized laser output.
3.2 µ-Raman study
To characterize the modification of the crystal structure in the waveguide core and cladding, a µ-Raman analysis was performed. A Renishaw inVia Reflex confocal Raman microscope equipped with a 514 nm Ar+ laser and a 50 × Leica objective was used. The facet cross-section of the waveguide was studied with a spatial resolution of 0.4 μm. The polarization of the excitation laser was E || Nm. We analyzed two intense high-frequency phonon modes, namely the 907.6 cm−1 internal mode, assigned as ν(W–O)/ν1 and related to the W–O stretching vibrations in the distorted WO6 octahedrons , and the 686.5 cm−1 mode, assigned as ν(WOOW) * ν(W–O)/ν-3 and related to the double oxygen bridge stretching vibrations . While the 907.6 cm−1 mode is typical for all simple and complex tungstate crystals and it is the most intense one for KLuW, the second mode is characteristic for MDTs and it is not observed for isolated WO4 tetrahedrons, e.g. in tetragonal scheelite-type tungstates. The polarized Raman spectra measured in the core region are shown in Fig. 2(b).
The µ-Raman analysis results are summarized in Fig. 3. We analyzed the peak intensity, Fig. 3(a,d), full width at half maximum (FWHM), Fig. 3(b,e), and the peak position for the 907.6 cm−1 and 686.5 cm−1 Raman bands, Fig. 3(c,f). In the core region, the crystalline quality of the material is preserved because the intensity of the Raman bands in the core and in the bulk crystal volume is the same, see Fig. 3(a,d). This feature confirms the feasibility of the produced waveguide for laser operation. The cladding region consists of a slightly damaged material indicated by the decreased intensity, broadening and shift of the phonon mode. The two latter effects are especially pronounced for the 686.5 cm−1 mode which is characteristic of the MDT structure. The existence of a residual anisotropic stress field is evident from the observed phonon energy shift outside the cladding, see Fig. 3(c,f). This stress field leads to birefringence due to the photo-elastic effect and favors the polarization-selective response of the waveguide .
In Fig. 4, we analyzed the variation of the peak intensity and the peak position of the two studied Raman modes along a horizontal line going through the center of the waveguide cross-section. It supports our previous conclusion about the weak alteration of the material structure in the core and allows one to clearly see the effect of four tracks forming the cladding from both sides of the core.
4. Continuous-wave laser operation
4.1 Laser set-up
The Tm:KLuW sample containing the fs-DLW waveguide was placed on a passively-cooled flint glass substrate. The laser cavity consisted of a flat pump mirror (PM) that was antireflection (AR) coated for 0.7–1.0 μm and high-reflection (HR) coated for 1.8–2.1 μm and a flat output coupler (OC) having a transmission of TOC = 1.5%, 3%, 5%, 9%, 20% or 30% at 1.8–2.1 μm. Additionally, we tested a high-transmission (TOC = 80% at 1.78 μm) “bandpass” OC coated to support laser operation at <1.8 μm. Both PM and OC were placed close to the sample faces with minimum air gaps. No index-matching liquid was used to avoid laser-induced damage of the waveguide or mirror faces as observed in our first experiments. Laser operation without an OC relying on Fresnel reflection on an uncoated surface (TOC = 89% at 1.84 μm, the corresponding refractive index nm is 1.995) was also studied. As a pump source, we used the CW Ti:Sapphire laser (Section 3.1). The polarization of the pump beam corresponded to E || Nm in the crystal. The pump was coupled into the waveguide with a 10 × microscope objective lens (Section 3.1). The incident pump power was varied with a gradient ND filter placed before the objective. The pump beam radius wp in the focus was ~20 μm. The pump absorption under lasing conditions (single pass pumping), denoted as Abs, was ~75% (for TOC < 30%) and ~48% (for TOC = 80% and 89%), see the details below. The scheme of the laser set-up is shown in Fig. 5(a). The Tm:KLuW waveguide under lasing conditions exhibited a weak blue upconversion luminescence, Fig. 5(b).
The laser output was filtered from the residual pump with a long-pass filter (Thorlabs, FEL1000, transmission at the laser wavelength: ~83%, transmission at the pump wavelength: < 0.01%) and collimated with an aspherical uncoated plano-convex 40 mm lens. The laser output power was measured with a sensitive Ophir Nova P/N 1Z01500 powermeter, the emission spectrum was measured using the above mentioned OSA (Section 3.1) and the beam profile was captured using a FIND-R-SCOPE near-IR camera (model 85726, sensitivity: 0.4-2.2 µm). To determine the equivalent size of the laser mode at the output facet of the waveguide, it was replaced by a 1951 USAF resolution test target (Thorlabs, model R1DS1).
4.2 Laser performance
Laser operation with the developed waveguide was achieved for all OCs and without the OC. The laser output was linearly polarized, E || Nm, the polarization was naturally-selected by the anisotropy of the gain. The input-output dependences are shown in Fig. 6. The maximum output power was 136 mW at 1843.7 nm with a slope efficiency η of 34.2% (with respect to the absorbed pump power, Pabs) for TOC = 30%, Fig. 6(a). The laser threshold was as low as 21 mW and the optical-to-optical efficiency ηopt was ~12% (with respect to the incident pump power). For smaller TOC, the laser performance was inferior. Without the OC (TOC = 89%), the laser generated 46 mW at 1842.6 nm with a decreased η of 20.9% and increased laser threshold of ~35 mW, Fig. 6(b). The deterioration of the laser performance for high output coupling is probably related to stronger upconversion due to the increased population of the upper laser level. The output dependences are clearly linear indicating no detrimental thermal effects; no thermally-induced cracks or damage of the crystal faces were observed.
The typical laser emission spectra measured at maximum Pabs (the spectra were weakly dependent on the pump level) are shown in Fig. 7. For TOC <10%, multi-peak spectral behavior has been observed due to the relatively broad gain spectra of Tm3+ in KLuW . With the increase of TOC from 1.5% to 30%, the emission wavelength shifted from ~1862 to 1843.7 nm. Without the OC (for TOC = 89%), the laser operated at even shorter wavelength, 1842.6 nm. Such a spectral behavior is related to the quasi-three-level nature of the Tm3+ laser and it agrees with the gain spectra of Tm3+ . For the “bandpass” OC (for TOC = 80%), the laser emission was at 1784.5 nm. It should be noted that for Tm3+-doped MDT waveguides, even longer emission wavelengths (up to ~1.94 µm ) are possible.
To determine the propagation losses in the developed waveguide, we employed the Caird analysis  modified for the case of high TOC . The dependence of the slope efficiency on TOC and the internal loss L per pass is expressed as 1/η = 1/η0(1 + 2γ/γOC), where γ = – ln(1 – L) and γOC = – ln(1 – TOC), and η0 is an intrinsic slope efficiency. Figure 8(a) shows a plot of the inverse of the slope efficiency versus the inverse of γOC (excluding the data for TOC = 80% and 89% for which the laser performance is affected by strong upconversion). This fit yields η0 = 65 ± 4% and 2γ = 0.17 ± 0.03 or an equivalent propagation loss coefficient, δ = 4.34L/l = 1.2 ± 0.3 dB/cm. This value is slightly lower than that for the fs-DLW Tm:KLuW waveguides with a circular cladding (~1.4 dB/cm)  and the fs-DLW Yb:KGdW based on a pair of damage tracks (~1.9 dB/cm) .
By employing the pump-transmission measurements under non-lasing conditions at 802 nm and at 830 nm (out of the Tm3+ absorption), we determined the pump coupling efficiency, ηcoupl = 49 ± 5% and pump absorption, Abs. Here, the value of ηcoupl includes the Fresnel loss (~89%) on the uncoated input facet. The moderate pump coupling efficiency arises mostly from the factor of geometrical overlap of the waveguide core (diameter: 30 µm) and the pump spot size (2wp = 40 μm). Abs is the ratio of the absorbed pump power to the launched one. It is plotted in Fig. 8(b). Here, the blue circle is the small-signal value calculated from the spectroscopic parameters, Abs = 1- exp(-σabsNTml) = 98%, where σabs = 6.2 × 10−20 cm2 is the absorption cross-section of Tm3+ ions for E || Nm, and the red circles are the result of the pump-transmission experiment which represent the depopulation of the 3H6 ground-state and, hence, saturation of the ground-state absorption. These data were modelled using a simple rate-equation model including the ground-state (3H6), the pump level (3H4, unquenched lifetime τ30 = 240 µs ) and the upper laser level (3F4, fluorescence lifetime τ1 = 1.34 ms ). The CR for the adjacent Tm3+ ions, 3H4(Tm1) + 3H6(Tm2) → 3F4(Tm1) + 3F4(Tm2) was also accounted for using a macroscopic CR rate-constant WCR = CCRNTm , where CCR = 2.7 × 10−37 cm6/s . The results are shown in Fig. 8(b) by a black curve and they are in good agreement with the experimental data. The data from Fig. 8(b) were used to determine the pump power absorbed in the waveguide under lasing conditions for each OC.
The results on the spatial profile of the output laser beam from the waveguide laser are shown in Fig. 9. The 2D profile of the beam is nearly circular, Fig. 9(a). The comparison of the size of the laser mode at the output facet of the waveguide with the waveguide cross-section, Fig. 9(b), shows the confinement of the laser mode and its penetration into the first layer of the cladding while the mode almost completely decays at the outer hexagonal-shaped sequence of damage tracks. The 1D mode profiles were very well fitted with a Gaussian distribution, Fig. 9(c). The goodness of the Gaussian fit R2 is 0.98. A slight ellipticity of the laser beam (e = wL(p)/wL(m) = 1.06) which is expanded along the vertical direction (|| Np-axis) was detected. This is attributed to the residual anisotropic stress field formed during the fs-DLW, Fig. 3(c,f).
The divergence of the output laser beam θ (half-angle) was similar along the directions of Nm and Np axes and amounted to 0.050 ± 0.005 (as measured for TOC = 30% at the maximum pump power). Using the approximation of a step-index waveguide, N.A.2 ≈2ncoreΔn, where N.A. = sinθ is the numerical aperture of the waveguide and ncore = nm is its refractive index in the core region [39,40], we estimated the variation of the refractive index in the cladding region Δn as ~6 × 10−4. This allowed us to conclude about the transverse mode structure. Using the step-index approximation, we calculated the normalized frequency V = 2πa·N.A./λL  as 2.56 for λL = 1.84 µm and a waveguide core radius a of 15 µm. The cut-off V parameters for the LPlm = LP01, LP11 and LP02 modes are 0, 2.405 and 3.832, respectively, so that for a linear laser polarization, three modes are expected (LP01 and LP11, note that the latter has a degeneracy of 2 because of its azimuthal index l > 0). This explains the determined values of the mode radii wL, Fig. 9(c), which are larger than those expected for the LP01 mode.
The slope efficiency of the developed laser can be represented as ηcalc = ηmodeηStηqηout  (further analysis applies to TOC = 9% for which the detrimental effect of upconversion is expected to be weak), where ηmode < 1 is the mode-matching efficiency, ηSt = λp/λL = 0.43 is the Stokes efficiency, ηq = 1.75 for the given NTm is the pump quantum efficiency accounting for CR, and ηout = ln[1-TOC]/ln[(1-TOC)·(1-2L)] = 0.34 is the cavity out-coupling efficiency. Thus, the theoretical ηcalc = 26% which is in agreement with the observed value, Fig. 6(a), indicating very high mode-matching efficiency. For higher TOC, stronger discrepancy between the calculated and measured slope efficiency is observed due to the upconversion losses being enhanced with the inversion ratio and, consequently, with TOC.
In Table 1, we summarized the results achieved recently with the fs-DLW Tm waveguide lasers. The output characteristics (output power, laser threshold, and slope efficiency) achieved in the present work for hexagonal cladding fs-DLW Tm:KLuW waveguide are superior with respect to the previously reported data for the fs-DLW Tm waveguide lasers based on crystalline/ceramic materials.
MDTs doped with Tm3+ ions are suitable for the development of fs-laser-written waveguide lasers. In the present work, we successfully fabricated, characterized and demonstrated CW laser operation of a fs-laser-written Tm:KLuW buried channel waveguide laser with a specific hexagonal optical-lattice-like cladding. This laser delivered a maximum output power of 136 mW at 1843.7 nm with a slope efficiency of 34.2% with respect to the absorbed pump power corresponding to a nearly-Gaussian laser mode. The developed waveguide is characterized by a low laser threshold 21 mW and moderate propagation losses, ~1.2 ± 0.3 dB/cm. Moreover, by applying variable output coupling, a discrete wavelength tuning from ~1785 to 1862 nm was observed. Further improvement of the slope efficiency and power scaling of fs-laser-written Tm3+-doped MDT waveguide lasers is possible with the optimization of the waveguide geometry (e.g., number of track layers between 1 and 4) and fabrication conditions (e.g., pulse energy, scanning speed and track separation, which can be critical as Tm3+ ions absorb at the emission wavelength of the fs laser), and the use of butt-coupled mirrors. Since efficient laser operation is possible with highly-doped (up to 15 at.%) Tm3+-doped MDTs , the fabrication of very compact integrated lasers is feasible.
Spanish Government (MAT2016-75716-C2-1-R (AEI/FEDER,UE); MAT2013-47395-C4-4-R, TEC 2014-55948-R, FIS2013-44174-P, FIS2015-71933-REDT); Junta de Castilla y León (UIC016, SA046U16); Generalitat de Catalunya (2014SGR1358).
E. K. acknowledges financial support from the Generalitat de Catalunya under grants 2016FI_B00844 and 2017FI_B100158. F.D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. X. M. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 657630. A. R. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Individual Fellowship Grant Agreement No. 747055. P. L. acknowledges financial support from the Government of the Russian Federation (Grant 074-U01) through ITMO Post-Doctoral Fellowship scheme.
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