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We report the first pulsed laser operation of an Indium-modified Yb:KY(WO4)2 crystal. Indium incorporation enlarges the broadening of the Yb3+ optical bands, reduces crystal lattice parameters and increases np refractive index. A KY0.8In0.07Yb0.13(WO4)2 crystal pumped at 981 nm with a Ti-sapphire laser in a SESAM modulated resonator produces at 300 K self-starting and stable mode-locking. The shortest laser pulses achieved were centred at λ = 1041.1 nm, have a duration of 96 fs with average power of 134 mW and repetition rate of 103.5 MHz (1.3 nJ/pulse).
© 2015 Optical Society of America
1. Introduction
Lanthanide-doped “disordered” single crystals are being increasingly investigated for their use in diode-pumped lasers. In ordered laser crystals, like YAG, the trivalent lanthanide (Ln3+) and other crystal constituents each occupies a unique crystal site, therefore the Ln3+ shows spectrally narrow optical absorption (OA) and photoluminescence (PL) transitions. However, in disordered crystals Ln3+ ions experiment a spatially variable crystalline environment leading to inhomogeneous broadening of their OA and PL bands. Such broadening is useful to minimize the pump absorption changes associated either to thermal drift of the diode emission or to the narrowing of the OA in cryogenically operated laser [1]. Moreover, large bandwidths are required to generate ultrashort (< 100 fs) laser pulses.
Point defects, Ln3+ multisites, random distribution of different cations on the same crystal sites, and isovalent or aliovalent substitutions of the host cations are possible ways to induce crystal disorder. The band broadening induced by disorder is accompanied by a decrease in peak OA and PL cross sections (σABS and σEMI, respectively) which makes difficult to attain significant pump absorption over short distances, an essential feature of diode-pumped optical cavities. Therefore, disordered crystals with large σABS/EMI are particularly desired.
KLn(WO4)2 monoclinic crystals (space group C2/c, Nº 15), with Ln = Y3+, Gd3+, or Lu3+ (Ln siting at a unique eight-oxygen coordinated site, 4e Wyckoff position, with C2 local symmetry) are well established laser crystals characterized by very large OA and PL cross sections for the Nm direction of the indicatrix [2]. Laser action of Nd, Yb, Tm or Ho-doped KLn(WO4)2 crystals has been shown under continuous wave (cw), Q-switched and mode-locked operation regimes [3]. The monoclinic phase is found for Ln = Sm-Lu [4]. Crystal distortion in this host can be achieved by isovalent substitution of Ln3+ by other cations with different ionic radii. From this point of view, In3+ may have a significant influence because its ionic radius for VIII-coordination, 0.92 Å, is smaller than any other Ln3+ or Y3+, namely the smallest one for VIII-coordination corresponds to Lu3+, 0.977 Å. Additionally, Indium incorporation into KLn(WO4)2 may change the refractive index (n) and structural lattice parameters, helping to tailor the design of waveguided lasers of this crystal class [5].
In this work we show the successful growth of KY1-x-yInxYby(WO4)2 monoclinic crystals to achieve an adequate compromise between bandwidth and peak cross section. We report their optical and spectroscopic properties and show the generation of ultrashort (< 100 fs) laser pulses by passive mode-locking.
2. Crystal growth and structural analysis
KY1-x-yInxYby(WO4)2 crystals have been grown by the top seeded solution growth method. Details of the growth procedures have been given previously [6]. Briefly, Indium incorporation into the monoclinic phase is limited to about x = 0.25. For larger In concentration the monoclinic phase coexists with a trigonal (space group , Nº 165) one. In order to avoid phase competition we grew KY0.9-yIn0.1Yby(WO4)2 crystals with y = 0.002 and 0.1 (10 at% In and 0.2% or 10 at% Yb). The y = 0.1 grown crystal was analyzed by single crystal X-ray diffraction. The monoclinic C2/c structure of the crystal and the Y, In and Yb incorporation in the 4e Wickoff site were confirmed. The crystal lattice parameters obtained are a = 10.625(3) Å, b = 10.334(2) Å, c = 7.5368(16) Å and β = 130.671(8)°, with cell volume 627.6(2) Å3. As expected, these lattice parameters are smaller than those of KY(WO4)2 crystal, i.e. a = 10.631 Å, b = 10.345 Å, c = 7.555 Å and β = 130.752°, V = 629.43 Å3. The structure refinement yielded the KY0.8In0.07Yb0.13(WO4)2 crystal formula, i.e. In is incorporated to the lattice (7 at%) in a lower amount that it was in the growth solution (10 at%), this deficiency is compensated by extra incorporation of Yb (13 at%), that in the solution was only at 10 at%.
3. Optical and spectroscopic characterization
Figure 1(a) shows the 300 K n(λ) dispersion of the KY0.898In0.10Yb0.002(WO4)2 crystal measured parallel to the 2-fold b crystal axis (Np direction of the indicatrix). The minimum deviation angle method and a crystal prism were used for this purpose. The limited size of the available crystals prevented prism preparation for other directions of the indicatrix. Compared to KY(WO4)2 crystal [7] np increases. Due to the low Yb concentration this change is attributed to the substitution of Y by In. The np(λ) results are fit to the Sellmeier law:
A best fit was obtained with A = 2.342, B = 1.5185, C = 193.3 nm, D = 1.306 × 10−8 nm−2. The group velocity dispersion (GVD) of the crystal was calculated as:GVD results are shown in Fig. 1(a). Although these values do not correspond to the sample and orientation later used for mode-locking, a first estimation of the In effect can be obtained. The GVD of KY0.898In0.10Yb0.002(WO4)2 crystal at λ = 1041 nm is 151 fs2/mm, i.e. slightly smaller than that of 10 at% Yb:KY(WO4)2, 200 fs2/mm [8].
Fig. 1 (a) KY0.898In0.10Yb0.002(WO4)2 crystal: 300 K np refractive index dispersion (open circles), its fit to the Sellmeir law (red continuous line) and group velocity dispersion, GVD (dashed line). To compare, the black continuous line shows np for KY(WO4)2, Ref. 7. (b) Comparison of the 2F7/2(0)→2F5/2(0´) 6 K excitation PL spectra of Yb3+ in KY0.995Yb0.005(WO4)2 and KY0.898In0.10 Yb0.002(WO4)2 crystals. λEMI = 1031 nm.
The peak Nm σABS = 1.23 × 10−19 cm2 of Yb3+ in KY1-x-yInxYby(WO4)2 crystals [6], is lower than the corresponding value for Yb:KY(WO4)2, namely σABS = 1.7 × 10−19 cm2 [10], but still significantly larger than most of Yb-doped laser crystals.
At 300 K the OA of Yb3+ is the result of the overlap of transitions from several Stark levels of the ground 2F7/2 multiplet. This hampers the identification of the bandwidth of the 2F7/2(0)→2F5/2(0´) transition and hides the Indium contribution to the inhomogeneous broadening of Yb3+. However, at 6 K the electronic population of the ground 2F7/2 multiplet is confined in its lowest energy Stark level, therefore this OA provides such information. Figure 1(b) shows the excitation spectra (formally equivalent to OA) obtained with a Spectra Physics MOPO system whose emission linewidth is lower than 0.005 nm. While the OA lineshape of 0.5 at% Yb doped KY(WO4)2 crystal is rather symmetric peaking at 981.61 nm and with a full width at half maximum FWHM = 0.070 nm, the corresponding band observed in the KY0.898In0.10Yb0.002(WO4)2 crystal shows overlapped side bands, its maximum is shifted to 981.53 nm and the bandwidth increases to FWMH = 0.139 nm. These results evidence the contribution of In to the inhomogeneous broadening the Yb3+ transitions.
4. Laser characterization
The cw laser operation at 300 K of KY0.8In0.07Yb0.13(WO4)2 was previously reported [6]. In the present work we test this crystal in the astigmatically compensated z-shaped optical cavity shown in Fig. 2(a). The cavity mode 1/e2 diameter at the sample position was 48 μm. The Ti-sapphire pump (λ = 981 nm, horizontally polarized) was focused onto the sample with a f = 62.9 mm lens to a beam waist 1/e2 diameter of 50 μm, matching well the cavity mode size. The non-coated sample was passively cooled at 291 K and sets at Brewster angle (with Nm direction also in the horizontal plane) near to the central position between a high reflector (HR, at ≈1000-1050 nm, radius of curvature ROC = −100 mm) and a pumping mirror (PM, ROC = −100 mm), both tilted ≈6°. By using further HRs with different radius, ROC = −100 mm or −75 mm, the intracavity beam was focused onto the semiconductor saturable mirror (SESAM) to a spot with 1/e2 diameter of 176 and 124 μm, respectively. A pair of SF10 prisms were used for intracavity GVD compensation and plane output couplers with transmissions TOC = 1 and 2% were tested. Two SESAM mirrors (absorbance 2%, reflectivity >96% in the 1010-1090 nm region, saturation fluence 60 μJ/cm2, damage threshold 4 mJ/cm2) purchased from BATOP Optoelectronics were used, their relaxation time was 1ps and 500 fs, respectively. Both provided similar pulse duration, τ, but the latter produced better stability. The output laser beam profile was monitored by a Gentec camera. τ was measured with a APE autocorrelator, while the pulse repetition rate and its frequency distribution were sensed with an Alphalas Si photodetector (risetime <300ps) and recorded in an oscilloscope. The spectral distribution of the laser pulses was monitored with a Wavescan spectrometer and the average output power was measured with a Gentec thermopile.
Fig. 2 (a) Astigmatically compensated optical resonator used for mode-locked laser operation. (b) Overview of laser efficiencies (open squares) and pulse duration (τ, full circles) obtained with two sample thickness and two output couplers.
We tested two samples with thickness d = 0.87 mm (≈80% pump absorption) and d = 1.215 mm (≈100% pump absorption). Figure 2(b) shows an overview of the obtained results. The shortest pulses were obtained with the thickest sample. It seems likely that higher pump absorption of this sample leads to larger intracavity powers and fluence on the SESAM. Hereafter, we will limit the laser result description to the thickest sample.
The shortest laser pulses with the KY0.8In0.07Yb0.13(WO4)2 (d = 1.215 mm) sample were obtained with a tip-to-tip prism distance of 315 mm and a total cavity length of 1490 mm. Mode-locked pulses were observed for absorbed pump powers Pabs> 600 mW, but near this threshold the operation was unstable. Stable mode-locking required Pabs> 1300 mW. In this stable regime, τ decreases and the average output power Pout increases with increasing Pabs, see Fig. 2(b). Figure 3 shows the characteristics of a representative laser pulse obtained with a fluence of ≈500 μJ/cm2 on the SESAM and a TOC = 2%. Figure 3(a) shows that the autocorrelation curve is well fit to a sech2 function, with FWHM = 148 fs, providing τ = 96 fs. Figure 3(b) shows the spectral distribution of these pulses with a maximum at λ = 1041.1 nm and FWHM = 13.9 nm, this implies a time-bandwidth product τ × Δν = 0.370, i.e. still above the Fourier limit, τ × Δν = 0.315. The repetition frequency of these pulses was 103.5 MHz, see Fig. 3(a) inset, and Pout = 134 mW, i.e. an average pulse energy of 1.3 nJ. A TEM00 beam quality was observed with an ellipticity of 94.1%, see Fig. 3(b) inset. The laser pulses were slightly tuned (≈2 nm) by varying the GVD compensation.
Fig. 3 Characteristics of the mode-locked laser pulses obtained with KY0.8In0.07Yb0.13(WO4)2 crystal. (a) Autocorrelation trace (open circles) and its fit to a sech2 function (line). The inset shows a mode-locked pulse train. (b) Spectral distribution of the mode-locked pulses. The inset shows the cross section of the beam intensity.
Table 1 summarizes the most significant results of mode-locked operation obtained with Yb:KY(WO4)2 crystals. The comparison of the laser performance of our In-modified Yb:KY(WO4)2 crystal with those previously reported for Yb:KY(WO4)2 crystal is not straightforward because of the large variety of experimental conditions. Laser characteristics depend strongly on the inversion ratio, i.e. on 2F5/2 Yb3+ lifetime and temperature. The former decreases with Yb concentration, while the temperature rise increases the population of the 2F7/2(n≠ 0) Stark levels reducing the inversion ratio. For the nominal 10 at% Yb concentration used in the present work pulses obtained directly from the oscillator were shorter only by Kerr lens mode-locking when the geometry of the sample allowed Np polarization [10], since in this case the optical bandwidth is larger than for Nm polarization. Pulse duration of Nm polarized pulses reported by Kerr lens mode-locking of 10 at% Yb:KY(WO4)2 [11] is in fact slightly longer than that here reported for the In-modified Yb:KY(WO4)2 crystal. Pulses shorter than those here presented and better laser efficiency were obtained only with 5 at% Yb concentration using diffusion bonded KY0.95Yb0.05W(WO4)2/KY(WO4)2 crystals [12]. This likely corresponds to a better dissipation of the heat generated in the sample, lower reabsorption losses, and to a longer Yb lifetime. In fact, for cw operation it was observed that the laser threshold increases with Yb concentration. For the same absorbed pump power the laser output power decreases only slightly from 5 at% to 10 at% Yb, but much significantly in the 10 at% to 20 at% Yb concentration range [13]. Other works on Yb:KY(WO4)2 mode-locked lasers [14–16] report longer pulse durations than those here obtained with the In-modified Yb:KY(WO4)2 crystal.
Table 1. Summary of mode-locking laser results obtained with Yb-doped KY(WO4)2 crystals. λPUMP = Pump wavelength. λEMI = Laser emission wavelength. pol = Polarization. τ = Pulse duration. <E> = Average output power. f = Repetition frequency. τ × Δν = Time bandwidth product. KL = Kerr lens. SESAM = Semiconductor saturable mirror. Ti-sa = Ti-sapphire laser. DL = diode laser. Np, Nm and Ng refer to the directions of the indicatrix.
5. Summary
In3+, with small ionic radius, is incorporated to the KY(WO4)2 monoclinic structure up to ≈25 at% with regards to the Y position. KY1-x-yInxYby(WO4)2 crystals have been grown by the top seeded solution growth method. In incorporation reduces the crystal lattice parameters and enlarges np refractive index with regards to the parent KY(WO4)2 crystal, therefore it may help to design waveguided lasers based on this monoclinic structure. In also increases the bandwidth of the 2F7/2(0)→ 2F5/2(0´) Yb3+ transition. The KY0.8In0.07Yb0.13(WO4)2 grown crystal tested in a SESAM modulated z-shaped oscillator produced laser pulses as short as 96 fs at 1041.1 nm with average output power of 134 mW. Further increase of the inhomogeneous broadening of Yb3+ bands and the corresponding reduction of the pulse duration are envisaged by increasing the In3+ incorporation up to the stability limit of the monoclinic phase.
Acknowledgments
Work supported by the Spanish Ministry of Economy and Competitiveness and by the European Regional Development Fund through MAT2011-29255-C02-01, MAT2014-56607, IPT2011-1121-020000, FIS2013-47741 and Laserlab-III-Ref. 284464 projects. E. C. is supported by the BES-2012-060296 grant.
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