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Abstract
Thin (~250 µm) crystalline layers of monoclinic Ho3+-doped KY(WO4)2 grown by the liquid phase epitaxy method on (010)-oriented undoped KY(WO4)2 substrates are promising for the development of thin-disk lasers at ~2.1 µm. Using a single-bounce pump geometry, 3 at.% and 5 at.% Ho:KY(WO4)2 thin-disk lasers delivering output powers of >1 W at 2056 nm and 2073 nm are demonstrated. The laser performance, beam quality and thermo-optic aberrations of such lasers are strongly affected by the Ho3+ doping concentration. For the 3 at.% Ho3+-doped thin-disk, the thermal lens is negative (the sensitivity factors for the two principal meridional planes, MA(B), are −1.7 and −0.7 m−1/W) and astigmatic. For higher Ho3+ doping (5-10 at.%), the effect of upconversion and end-bulging of the disk enhances the thermo-optic aberrations leading to a deteriorated laser performance.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The thin-disk laser concept consists of a disk-shaped active element with a thickness smaller than the pump and laser spot sizes attached to a cooling holder in contact with the disk rear face for unidirectional heat flow [1]. It is promising for power-scalable and efficient laser sources operating in the continuous-wave (CW) and mode-locked (ML) regimes [2]. Despite the wide spread of Yb3+ thin-disk lasers emitting at ~1 µm and using various Yb3+-doped laser crystals [3], the information about such lasers at ~2 µm is scarce [4,5] due to more strict requirements to spectroscopic and thermal properties of potential gain materials and less commercialized pump systems. Such an eye-safe emission is of interest for environmental sensing, spectroscopy and medical applications and can be achieved, e.g., with trivalent Holmium (Ho3+) due to the 5I7 → 5I8 transition.
To date, only few Ho thin-disk lasers were developed based on Ho:YAG with multiple pump bounces (up to 12, e.g. 24 passes of the pump) [5–8]. J. Speiser et al. reported on a CW 2 at.% Ho:YAG thin-disk laser (thickness: 400 µm) pumped by a Tm fiber laser at 1908 nm and utilizing 12 bounces of the pump [5]. This laser generated 15 W at ~2090 nm with a slope efficiency η of ~37%. Tuning between ~2075 and 2105 nm was also demonstrated [5]. M. Schellhorn studied a very similar Ho:YAG thin-disk with a Tm:YLF laser as a pump source and 12 bounces of the pump leading to the generation of 9.4 W of CW output at ~2090 nm with a slope efficiency reaching ~50% (with respect to the absorbed pump power) [6]. G. Renz used a 2.5 at.% Ho:YAG thin-disk (thickness: 300 µm) and a stack of InP laser diodes in combination with a commercial 12 bounce pump module to generate 22 W with η ~27% [7].
Very recently, J. Zhang et al. reported on a Kerr-lens mode-locked Ho:YAG thin-disk laser (doping: 2.5 at.% Ho, thickness: 200 µm) delivering 270 fs-long pulses at a repetition rate of 77 MHz with an average output power of 28 W [8,9]. This laser was pumped by two Tm fiber lasers at 1908 nm using a 12 bounce pump module. By soliton self-compression in a silica glass fiber, the pulse duration has been further shortened down to 15 fs [8]. All the above examples suffer from a very complex pump scheme of 12 bounces of the pump.
Besides YAG, another family of materials very suitable for Ho3+ doping are the monoclinic double tungstates (MDTs), with a chemical formula KRE(WO4)2 (RE = Gd, Y, or Lu) [10]. In the MDT lattice, the Ho3+ ions replace the RE3+ ones in a single site (C2 symmetry, VIII-fold O2- coordination). In particular, KY(WO4)2 (shortly KYW) seems to be the best host material for Ho3+ doping among the MDT due to the closeness of ionic radii of Y3+ (1.019 Å) and Ho3+ (1.015 Å). High Ho3+ doping concentrations accompanied with a weak luminescence quenching due to the long Y3+-Y3+ interatomic distances (4.06-6.04 Å) are feasible [11]. From the point of view of spectroscopy, Ho:KYW crystals are attractive due to the high absorption, σabs, and stimulated-emission, σSE, cross-sections for polarized light (a maximum σSE ~2.6 × 10−20 cm2 at 2056 nm for light polarized along the Nm optical indicatrix axis of these biaxial crystals, for comparison Ho:YAG: σSE ~1.6 × 10−20 cm2 at 2090 nm), as well as relatively long lifetime of the upper laser level (5I7) of about 4.8 ms [11]. Anisotropic Ho3+-doped MDTs can offer an “athermal” behavior for certain crystal cuts potentially leading to almost negligible thermo-optic aberrations being weaker than those in Ho:YAG. Ho3+-doped MDTs in general and Ho:KYW in particular provided efficient lasing under in-band-pumping, e.g., by Thulium (Tm3+) ion-doped lasers [12].
In the present work, we study the effect of Ho3+-doping on the performance of Ho:KYW thin-disk lasers with a simple single-bounce (double pass) pumping scheme facilitated by the advantageous spectroscopic properties of Ho3+ in KYW. To prepare the thin-disk laser elements, we used the liquid phase epitaxy (LPE) method to grow thin layers of Ho3+-doped KYW on undoped KYW substrates [13]. The thin-disk laser performance of Tm-doped KLuW/KLuW epitaxies was previously studied by part of the authors in [14]. With 2 pump bounces, such a thin-disk laser generated 5.9 W at 1855 nm with η ~47%.
2. Experimental
The thin-disks were based on Ho3+-doped KYW layers grown on 1-mm-thick undoped KYW substrates by the LPE method from the flux using potassium ditungstate (K2W2O7) as a solvent, Fig. 1(a). The substrate itself was grown by the Top-Seeded Solution-Growth (TSSG) Slow-Cooling method and oriented with the crystallographic [010] or b-axis normal to its face. The crack- and inclusion-free active layers (KY1-xHox(WO4)2) were doped with x = 3, 5, 7 or 10 at.% Ho3+. The 0.8...1 mm-thick as-grown layers were polished down to a thickness t of 250 ± 20 μm. The substrate face was antireflection (AR)-coated and the face with the epitaxial layer was high-reflection (HR)-coated for 1.8-2.1 μm. The latter was soldered to a Cu heat-sink, see Fig. 1(b), which was water-cooled to 12 °C. The light propagation for the thin-disk laser element was along the b-axis (|| Np optical indicatrix axis).
Fig. 1 (a) Photograph of the as-grown 3 and 5 at.% Ho:KYW/KYW epitaxies, their orientation with respect to the crystallographic axes (a, b, c); (b) cut, polished, and AR/HR coated 3 at.% Ho-doped sample (top image) soldered to a Cu heat-sink (bottom image); (c) thin-disk laser set-up: CL – collimating lens, FL –focusing lens, HR and AR – high-reflection and antireflection coatings, respectively, OC – output coupler.
The plano-concave laser cavity was composed by the flat HR mirror deposited on the substrate face of the disk and a concave output coupler (OC) with a radius of curvature RoC of 75 mm and a transmission TOC of 1.5%, 3% or 5% at 1.82-2.07 µm, see Fig. 1(c). The pump source was a fiber Bragg grating (FBG) stabilized thulium-doped fiber laser (model IFL15, LISA laser products) emitting up to 12.5 W at 1960 nm (full width at half maximum: 1.5 nm). Its unpolarized output (beam quality parameter, M2 ~1) was collimated and focused onto the laser element with a pair of AR-coated CaF2 lenses (focal lengths: fCL = 11 mm, fFL = 75 mm) providing a pump spot size 2wp of 300 ± 10 μm. Thus, the condition 2wp > t for the thin-disk laser was satisfied. The total pump absorption (single-bounce = double-pass), was measured by monitoring the residual pump. It was 14% for the 3 at.% and 33% for the 5 at.% Ho3+-doped epitaxies. Optionally, the pump beam was modulated with a mechanical chopper (duty cycle: 1:2, frequency: 20 Hz).
3. Results and discussion
3.1 Laser performance
Laser operation was achieved with the 3 and 5 at.% Ho-doped KYW thin-disks in true CW as well as in the quasi-CW regime. In all cases, the laser output was linearly polarized (E || Nm) according to the anisotropy of the gain [11]. The corresponding input-output dependences are shown in Fig. 2(a,b). The best performance was observed for TOC = 3%.
Fig. 2 (a,b) Input-output dependences for the 3 and 5 at.% Ho:KYW thin-disk lasers in quasi-CW (qCW, duty cycle 1:2) (a) and CW (b) operation modes (symbols: experimental data, lines: fits for the calculation of the slope efficiency η); (c) typical laser emission spectra for the Ho:KYW thin-disk lasers for various OCs (CW regime, Pabs ~1.7 W). The laser polarization is E || Nm.
In the quasi-CW mode, the 3 at.% Ho:KYW thin-disk laser generated a peak output power of 1.10 W at 2057 nm with a slope efficiency η of 66% (with respect to the absorbed pump power, Pabs). The laser threshold was at Pabs = 0.14 W. The 5 at.% Ho:KYW thin-disk laser provided an increased output power of 1.31 W (due to its higher absorption) at 2058 nm and 2073 nm albeit at a reduced slope efficiency, η = 34%, and an increased laser threshold, Pabs = 0.40 W. The inferior characteristics are attributed to the stronger thermo-optic aberrations and higher upconversion losses associated with stronger heat load at higher Ho3+ doping. However, neither fracture of the disks nor thermal roll-over of the output dependence were observed up to at least Pabs = 4.20 W.
In the CW regime, the difference in the output performance was more pronounced: the 3 at.% Ho:KYW thin-disk laser generated 1.01 W with η = 60% whereas the maximum output from the 5 at.% Ho:KYW thin-disk laser was only 0.24 W with η = 15%. In the latter case, laser operation ceased at Pabs ~2.1 W. The slope efficiency for the CW 3 at.% Ho:KYW thin-disk laser was higher than the one for Ho:YAG [6].
The typical emission spectra of both lasers are shown in Fig. 2(c). The spectra contain multiple lines due to the etalon effects in the disk. For the 3 at.% Ho:KYW thin-disk laser, the emission wavelength was weakly dependent on the OC. However, for the 5 at.% Ho-doped one, the emission was at 2072-2076 nm for TOC = 1.5%, two emission bands were observed for TOC = 3% and again only one band at 2056-2059 nm for TOC = 5%. This spectral shift is due to the quasi-three-level nature of the Ho3+ laser [11] while the effect of Ho3+ concentration is due to the increasing reabsorption losses.
3.2 Thermo-optic effects
To characterize the thermo-optic effects in the Ho:KYW thin-disk lasers, we studied the variation of the spatial intensity profile of the laser beam with Pabs, Fig. 3(a,b). A pyrocamera SPIRICON PY-III-C-B located at 20 cm from the OC and an AR-coated 50 mm CaF2 focusing lens were used for recording the beam profiles.
Fig. 3 (a,b) Spatial profiles of the output laser beam for the 3 at.% Ho:KYW (a) and 5 at.% Ho:KYW (b) thin-disk lasers measured at different Pabs as indicated by the numbers below the figures (CW regime, TOC = 1.5%); (c) comparison of the spatial profiles of the output laser beam for the 3 and 5 at.% Ho:KYW thin-disk lasers in the CW and quasi-CW (qCW) regimes.
For the 3 at.% Ho:KYW thin-disk laser, the beam was nearly circular at the threshold. For higher Pabs, it expanded and became elliptic, Fig. 3(a). Similarly to the bulk MDTs the active media are cut along the b-axis. The orientation of the major and minor semiaxes of the elliptic beam (A and B, respectively) was related to the anisotropy of the thermal expansion of the KYW host crystal [15], namely A || X'3 and B || X'1 where X'i (i = 1, 2, 3) are the principal axes of the eigen-frame of the thermal expansion tensor αmn [16]. In this way, the elliptic laser beam was rotated with respect to the polarization direction (E || Nm) at an angle X'1^Nm ~30° [16]. For the 3 at. % Ho:KYW laser and Pabs = 1.78 W, Fig. 3(a), the measured beam ellipticity e = wL(B)/wL(A) was 0.64 and the measured beam quality factors were M2A = 3 and M2B = 1.6.
For the 5 at.% Ho:KYW thin-disk laser, the beam was already slightly distorted even at the laser threshold. With the increase of Pabs, it was confined and became extended along the vertical direction so that the A and B semiaxes were oriented along the vertical and horizontal directions, respectively, Fig. 3(b). It was not possible to associate them with the X'i axes which might indicate a strong effect of the pump geometry. The beam ellipticity e was 0.60 and M2A = 1.5, M2B = 1.4 (at Pabs = 1.75 W). For Pabs > 1.9 W, a multimode output was observed.
In the quasi-CW pumped regime the distortions of the beam profiles were reduced, Fig. 3(c). As a consequence the 5 at.% Ho:KYW thin-disk laser could be operated up to much higher pump levels.
The observed distortions of the output beam were modeled within the ray transfer matrix formalism (ABCD law) assuming a thin astigmatic thermal lens (TL) located in the active layer and are characterized by the optical (refractive) power of DA (DB) [15]. The results are shown in Fig. 4(a). For the designed cavity, according to the ABCD modelling, a negative (defocusing) lens will lead to the beam expansion in the far-field and a positive (focusing) lens – to a beam compression. For the 3 at.% Ho:KYW thin-disk laser, a purely negative TL (e.g., with DA(B) < 0 for both principal meridional planes containing the A and B directions [17]) was determined with the so-called sensitivity factors (MA(B) = dDA(B)/dPabs) of MA = −1.7 and MB = −0.6 m−1/W. The astigmatism degree S/M = |MA – MB|/|MA| is then 59%.
Fig. 4 (a) Optical power of the TL in the 3 at.% Ho:KYW thin disk along A(B) directions (symbols: experimental data, lines: Linear fits for the calculation of the sensitivity factors M); (b) upconversion luminescence (UCL) spectra of the 3 - 10 at.% Ho:KYW/KYW epitaxies, λexc = 1960 nm; (c) scheme of the energy levels of the Ho3+ ion showing relevant processes: GSA – ground-state absorption, ETU – energy-transfer upconversion, CR – cross-relaxation, NR – nonradiative relaxation.
For the plane stress approximation suitable for laser disks [17], the sensitivity factors of an astigmatic TL can be represented as MA(B) = [ηh/(πκwp2)] × ΔA(B), where ηh is the fractional heat loading estimated as ηh ≈1 – ηSt = 1 – λp/λL, κ is the thermal conductivity, and ΔA(B) is the “generalized” thermo-optic coefficient (TOC), namely ΔA(B) = dn/dT + PPEA(B) + (1 + ν*A(B))(n – 1)α [17]. Here, the three terms stand for the temperature dependence of the refractive index (expressed by TOC, dn/dT), the photo-elastic effect PPE, and the macroscopic bulging of the surface of the laser disk (ν*A(B) is the “generalized’ Poisson ratio and n is the refractive index).
The TOC of KYW is negative at ~2 µm, dnm/dT = −8.9 × 10−6 K−1 [18]. However, the photo-elastic term and the end-bulging one are known to be positive for b-cut MDT laser elements [15] and thus a positive ΔA(B) is typically observed for at least one principal meridional plane (typically, along the X'1-axis). For the b-cut 3 at.% Ho:KYW/KYW thin-disk, the substrate acting as an undoped cap efficiently diminishes the end-bulging and thus a negative TL is observed for both principal meridional planes, as expressed by the calculated ΔA = −6.5 and ΔB = −2.3 × 10−6 K−1.
In contrast, for the 5 at.% Ho-doped thin-disk, the beam compression (Fig. 3(b)) and even laser ceasing for Pabs ~2.1 W (CW regime) can be explained only by the action of a strong positive TL. The corresponding sensitivity factors are estimated to be MB = 5.2 and MA = 0.5 m−1/W. We attribute this to a strongly localized heat load leading to an increased disk bulging which acts as a positive lens counteracting the negative dn/dT. The disk bulging is facilitated by the inclination of the pump beam (angle of incidence: ~10°) and the strongly anisotropic thermal expansion in the disk plane (e.g., the crystallographic a-c plane of KYW) [16]. The positive TL leads to the cavity instability and to the observed termination of the laser operation. Moreover, we believe that the same effect prevented laser operation of the 7 and 10 at.% Ho:KYW thin disks.
ML of a thin-disk laser requires the lowest order transverse mode operation. In our case, the beam quality was limited by a relatively strong astigmatic thermal lens arising from (i) small pump spot size, (ii) asymmetric pump geometry and (iii) light propagation along the b (Np) axis. As the M-factor of the thermal lens is proportional to 1/wp2, an increase of the pump spot size will efficiently reduce the thermo-optic aberrations. Moreover, anisotropic MDTs offer an “athermal” behavior for certain directions of light propagation [16,18] (e.g., along the Ng axis), and both the unwanted negative sign of the thermal lens and its astigmatism can be neglected for “athermal”-cut laser elements [12].
The effect of Ho3+ concentration was also monitored by measuring the upconversion luminescence (UCL) spectra, Fig. 4(b). For low (3 at.% Ho) doping, the dominant emission is from the 5I5 state (at ~913 nm), which is populated by the phonon-assisted energy-transfer upconversion (ETU) process ETU1 (5I7 + 5I7 → 5I5 + 5I8) [19], Fig. 4(c). For higher Ho3+ doping, the dominant emissions occur from the higher-lying thermally coupled 5S2 + 5F4 states (at ~545 nm, to the 5I8 ground-state, and at ~725 nm, to the 5I7 excited-state) and from the 5F5 state (at ~660 nm, to the ground-state) which are populated through different ETU and cross-relaxation (CR) processes whose efficiency is enhanced with the Ho3+ doping level, Fig. 4(b).
KYW has a maximum phonon energy hνph of 905 cm−1 which is higher than that of YAG (~800 cm−1). The shortest Y3+-Y3+ distance for KYW (4.06 Å) is longer than that for YAG (3.67 Å). Thus, one can expect weaker upconversion for Ho:KYW. Focusing on KYW, Ho3+ doping with a concentration of >5 at.% seems to produce a relatively strong upconversion. Note that for Ho:YAG thin-disks [6], the Ho3+ doping is typically limited to about 2 at.% because of the same reason.
4. Conclusion
We demonstrated successful operation of Ho:KYW thin-disk lasers based on a Ho:KYW/KYW epitaxial structure grown by the LPE technology. The lasers are capable of delivering >1 W of output power at ~2056 or ~2073 nm with a slope efficiency as high as ~60% using a simple single-bounce pump geometry. Our study reveals the key role of the Ho3+ concentration on the output characteristics of such lasers (slope efficiency, beam profile and emission wavelength). The degradation of the laser output for >3 at.% Ho3+ doping is attributed to increasing thermo-optic distortions caused by disk bulging and a strong interaction of the Ho3+ ions leading to upconversion losses. Power scaling of these Ho:KYW thin-disk lasers is feasible by optimizing the Ho3+ doping (1-3 at.%), simple pump retro-reflection with readjusted polarization for the second bounce, or alteration of the disk orientation (e.g., for light propagation along the Ng-axis). In the latter case, efficient laser operation with higher Ho3+ doping levels (5-7 at.%) can be expected.
Funding
Spanish Government (projects No. MAT2016-75716-C2-1-R (AEI/FEDER,UE); TEC 2014-55948-R); Generalitat de Catalunya (2014SGR1358); Horizon 2020 (657630); R&D RAS program (I. 56).
Acknowledgments
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. P.L. acknowledges financial support from the Government of the Russian Federation (Grant 074-U01) through ITMO Post-Doctoral Fellowship scheme. S.V. and I.V. acknowledge financial support from R&D RAS program No.I.56 “Fundamentals of Breakthrough Double Technologies for National Security”.
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