Tetragonal single crystals of NaT(WO4)2 (T = Y or Lu) co-doped with Tm3+ and Ho3+ ions have been employed for broadly tunable and efficient room-temperature laser operation at around 2 μm. With Ti:sapphire laser pumping at 795 nm, a slope efficiency and a maximum output power as high as 48% and 265 mW, respectively, have been achieved at 2050 nm from a Tm,Ho:NaY(WO4)2 crystal. Tuning from 1830 nm to 2080 nm has also been obtained using an intracavity Lyot filter.
© 2010 OSA
Inhomogeneously broadened single crystals doped with rare-earth ions have received significant attention as promising gain media for construction of compact, efficient and broadly tunable solid-state lasers operating in near-infrared region. The tetragonal (I‾4 space group) double tungstates with nominal formula NaT(WO4)2 (NaTW), where Na and T = Y,La-Lu are monovalent and trivalent cations, respectively, are a class of disordered crystals where the inhomogeneous properties arise from the quasi-random occupancy of two nonequivalent lattice sites (2b and 2d) by Na and T cations . These crystals, when doped with Yb3+ ions, have been studied comprehensively in recent past years and this has led to the demonstration of a range of sub-100 fs modelocked lasers operating in the 1-μm spectral region [2,3].
More recently, these studies have been extended to Tm3+-doped NaTW (T = Gd, La, Lu, and Y) crystals with the aim of developing efficient and broadly tunable lasers that operate around 2 μm [4–6]. Indeed, for the Tm:NaYW crystal the tunability range extended from 1850 nm to 2030 nm with spectral widths at half maximum (FWHM) of 110 nm and 142 nm for π- and σ-polarizations, respectively . It follows that such gain media can also be employed for ultrashort-pulse generation using appropriate modelocking techniques [7,8]. Ultrafast laser sources around 2 μm are of particular interest for applications in time-resolved spectroscopy, nonlinear frequency up-conversion to the mid/far-infrared spectral regions, mid-IR supercontinuum generation, optical communications and photomedicine. With Ho3+ as a co-doping active ion, tunable laser operation in a somewhat longer wavelength range extending to ~2.1 μm (5I7→5I8) can be obtained and this is preferred in remote sensing because of lower atmospheric absorption. Also, Ho3+ ions are generally characterised by higher emission cross-sections and longer upper laser level lifetimes compared to their Tm3+ counterparts and these features are especially desirable for low-threshold and efficient laser operation.
Here we report on crystal growth, spectroscopy and room-temperature continuous-wave laser operation for Tm3+ and Ho3+ co-doped NaYW and NaLuW crystals in the 2 μm spectral region. Evidence of an enhancement of the tuning range in Tm,Ho co-doped NaTW crystals as compared to their singly-doped Tm3+ counterparts has been demonstrated.
2. Crystal growth
NaYW single crystals were grown in air by the Czochralski method using platinum (Pt) crucibles. Details of the growth procedures can be found in previously reported work . Two different sets of Tm,Ho co-doped crystals were grown with 5 at% Tm3+ concentration in the melt and with two Ho3+ concentrations of 0.25 at% and 0.5 at%. Oriented (001) seeds of NaY(WO4)2 were used in both cases.
A Tm(5at%),Ho(0.5at%):NaLuW crystal was grown by the top seeded solution growth (TSSG) method using a Pt crucible. A Na2WO4-Na2W2O7 mixture in molar radio 1:1 was used as flux, and the solute-to-flux molar ratio was 1:9. The seed was a (001)-oriented Tm(5at%):NaLuW crystal. The growth started by melting and homogenization at 910 °C over a period of one day. The saturation temperature was found to be 880 °C. The crystal growth proceeded at this temperature for one week and afterwards the melt was cooled down to 842 °C at a 0.05 °C/h cooling rate. The crystal was extracted from the melt and cooled to 745 °C at 4 °C/h and later to room temperature (RT) at a rate of 15 °C/h.
For both growth methods, the Tm,Ho:NaY(Lu)W compounds and flux were synthesized using precursors of 99.5% Na2CO3, 99.8% WO3, 99.99% Y2O3 (Alfa Aesar), 99.99% Lu2O3, 99.99% Tm2O3 and 99.99% Ho2O3 (WuXi YiFeng Rare Earth Co LTD). The crystalline samples were oriented by the Laue X-ray method, then cut as (100) oriented plates and polished to optical quality.
3. Optical spectroscopy
The optical absorption spectra of the investigated crystals in the visible and near-infrared spectral regions for σ (E⊥c,Bllc) and π (Ellc,B⊥c) polarisations were recorded using a Varian (Cary 5E) spectrophotometer. Figure 1 shows a diagram of the Ho3+ and Tm3+ energy levels relevant to the subsequent discussion of the results. For most of the spectral ranges, the Ho3+ and Tm3+ RT optical absorption bands overlap, but at around 540 nm the Ho3+ (5I8→5S2 + 5F4) absorption is free of Tm3+ lines (Fig. 2 ). Advantage was taken of this fact to evaluate the Ho3+ concentration in the crystals considering the integrated absorbance of Ho3+ in the isostructural NaGdW  crystal as a reference. By contrast, the Ho3+ absorption in the spectral region from 840 nm to 760 nm is negligible  and the corresponding Tm3+ (3H6→3H4) optical absorption band can be used for the evaluation of the Tm3+ concentration in the grown crystals using a previously investigated Tm(5at%):NaYW crystal for the reference data . In Table 1 the calculated Tm3+ and Ho3+ concentrations are summarized for the crystalline samples used.
The Tm3+ optical absorption in both crystals at around 800 nm (3H6→3H4) was characterized by a strong anisotropy, with the strongest absorption band observed for σ-polarisation centered at 795.5 nm (Fig. 2). The optical absorption for π-polarisation was less than a half that of the σ-value at around 795 nm with the existence of a secondary maximum at 781 nm.
Figure 3 shows the RT emission of Tm and Ho co-doped NaYW and NaLuW crystals when excited at 795.5 nm using a Ti:sapphire laser. It can be seen that the Tm emission covers efficiently the 1600-2000 nm region (3F4→3H6 transition; see for comparison the emission spectra of Tm:NaYW Fig. 3(b)). Additionally, two well resolved bands peaking at 1944 nm and 2049 nm were observed in all of the studied samples. These bands could be attributed to the 5I7→5I8 Ho3+ transition (the emission of Ho3+ in isostructural NaBi(WO4)2 is included in Fig. 3(a) for comparison) and these were found to be less sensitive to the polarization configurations of the samples. These transitions are excited by Tm-Ho energy transfer  which is favored by the large cut-off phonon energy of these double tungstates hosts, typically ħω = 900-1000 cm−1. It can be seen that the relative intensity of the Ho3+ emission increases as the Ho/Tm concentration ratio increases (see Fig. 3).
4. Laser results
Assessment of the lasing performance of the Tm,Ho co-doped NaYW and NaLuW crystals was undertaken with the V-type astigmatically-compensated resonator shown in Fig. 4 . This set up comprised two curved mirrors (radii of curvature of −75 mm and −100 mm) designed for high transmission at the pump wavelength (T>98%) and high reflectivity in the 1800-2100 nm range together with an output coupler OC with either 1% or 2% transmission at around 2000 nm. The cavity was configured to provide a mode radius of 25 μm inside the gain crystal. A Ti:sapphire laser providing 1.15 W of power at 794.5 nm was used as a pump source and its beam was focused into the gain medium by a 50 mm focal length lens to a spot radius of 23.5 μm (1/e2 intensity) measured in the air at the location of the gain crystal. All of the samples were (100)-cut uncoated Brewster-angled plates maintained at 20 °C by using a Peltier-cooled Cu sample holder to which samples were glued with a silver paste. The absorption of the sample was also measured in situ (during lasing) by detecting transmitted pump power. To maintain a similar absorption for the σ and π laser assessments, the sample thickness in the latter case (π) was approximately twice that of the former (σ).
4.1 Tm,Ho:NaY(WO4)2 crystals
The NaYW crystals co-doped with 5 at% Tm and 0.25 at% or 0.5 at% Ho and oriented in the cavity for pumping/lasing in σ- or π-polarizations were used during continuous-wave (cw) laser experiments. Figure 5 depicts the summary of the laser results obtained with the Tm(5at%),Ho(0.25at%):NaYW crystals. The maximum slope efficiency η = 48% was obtained in a 1.55-mm-long crystal (σ-polarisation) with a 2% output coupling (Fig. 5(a)). The corresponding maximum average output power was 240 mW and the laser threshold was reached at 55 mW of absorbed pump power. Slightly lower efficiencies were recorded when a 3.36-mm-long crystal was used in a π-configuration, namely, η = 43% and η = 39% for 2% and 1% OCs, respectively. Interestingly, the maximum output power reached 265 mW due to a higher level of absorbed pump power at π-polarisation (1.55-mm-long and 3.36-mm-long crystals absorbed in ranges of 80-70% and 85-77% of pump radiation for σ- and π-polarizations, respectively, depends on pump power level).
It should be noted that dual wavelength operation was observed in the case of the 1.55-mm-long Tm,Ho:NaYW crystal (σ-polarization). Additional to the output at 2050 nm, laser emissions at 1935 nm or 1970 nm were observed when the pump power reached a sufficiently high level (300 mW for 1% OC). This can be explained by the quasi-three-level nature of the Ho3+ 5I7→5I8 transition - see Fig. 1. The 5I8 Ho ground multiplet has a first set of Stark levels at 0-50 cm−1, an isolated level at 79 cm−1, followed by an energy gap of ≈120 cm−1 . At low pump powers the laser transition favors the high-energy Stark levels of the 5I8 manifold (low reabsorption losses), but at increased pump power the lower Stark levels become saturated thereby permitting lasing in these channels. This explains why an additional lasing band did not arise around 1935 nm or 1970 nm in the case of 3.36-mm-thick samples were reabsorption losses are larger such that higher pump power levels would be required to reach transparency.
A similar laser performance was observed for the Tm(5at%),Ho(0.5at%):NaYW crystal as compared to the 0.25 at% Ho-doped counterpart. Notably, slope efficiencies as high as 37% were achieved with corresponding maximum output powers of up to 230 mW at 2058 nm (Fig. 6 ). A more pronounced roll-over of input-output characteristics can be observed with the 0.5at% Ho-doped samples compared to the lower-doped counterparts. This fact can be explained by the presence of up-conversion losses in the Tm-Ho system . Indeed, during quasi-cw operation (10% duty cycle) involving the Tm(5at%),Ho(0.5at%):NaYW crystal a slope efficiency as high as 43% was measured for the π-polarization and this is comparable to the values obtained for lower Ho-doped samples during cw operation.
Tunable operation of the Tm,Ho:NaYW lasers was obtained by inserting a 2-mm thick Lyot filter into the long cavity arm and the results obtained with the Tm(5at%), Ho(0.25at%):NaYW crystal are as summarized in Fig. 7 . Tuning to ~2080 nm was observed whereas a high energy stop-band varied from 1825 nm for 1.55-mm-long crystal to 1880 nm for 3.5-mm-long. All crystals with higher Ho3+-doping and 3.4-mm-long 0.25 at% Ho-doped element produced maximum output powers at around 2050 nm whereas the σ-polarized 0.25 at% Ho-doped crystal was characterized by a flatter tuning profile in the 1950-2050 nm range and this could be advantageous for ultrashort pulse generation.
4.2 Tm,Ho:NaLu(WO4)2 crystals
All Tm(5at%),Ho(0.5at%):NaLuW crystal samples (1.46-mm and 2.7-mm long for σ and π, respectively) investigated produced laser emission only around 2060 nm in the absence of any tuning element in the cavity. The performance was characterized by quite pronounced thermal effects, more evident as the absorbed pump power increased. As a representative example, Fig. 8(a) depicts an output power vs pump characteristic for a cw and a quasi-cw operation (10% duty cycle) for σ-polarization. Only 50 mW of average power was achieved during cw operation, whereas a slope efficiency up to 47% was reached with a chopped pump.
A similar tuning range of 2020-2070 nm was demonstrated for all Tm,Ho:NaLuW samples. This is much narrower than the tuning ranges observed for Tm,Ho:NaYW crystals (see Figs. 7 and 8(b)). The difference between both cases is due first to a more efficient Tm-Ho energy transfer associated to the higher Tm and Ho concentrations in NaLuW, which prevents optical gain at Tm3+ (1850-2020 nm lasing range). Secondly, pronounced thermal roll-over in input-output characteristics of Tm,Ho:NaLuW indicates about higher heat load in this laser crystal, likely due to enhanced probability of up-conversion in the Tm-Ho system, the nature of which should be further investigated. On this reason the tunability range is limited to ~2020-2070 nm since the lower Ho3+ levels at ~0-50 cm−1 and ~80 cm−1 become thermally populated making the Ho laser transitions in ~1900-2000 nm range too lossy.
Continuous-wave and broadly tunable laser operation of the Tm,Ho co-doped NaY(WO4)2 crystals around 2 μm has been demonstrated with Ti:sapphire laser pumping at 795 nm. Maximum slope efficiencies of 48% and 43% with corresponding output powers of 240 mW and 265 mW were realized around 2050 nm using a Tm(5at%),Ho(0.25at%) co-doped crystal in σ- and π-configurations, respectively. It has also been deduced that the specific distribution of the 5I8 Ho3+ Stark energy levels in Tm,Ho:NaYW crystals leads to competition between several 5I7→5I8(n) laser channels. While emission at 2050 nm is always observed, a second laser band (either at 1970 nm or 1935 nm) can be obtained simultaneously when the lowest levels becomes partially depopulated at high pump levels, thereby producing a dual- wavelength operation. When an intracavity Lyot filter was used for tuning experiments an output from 1830 nm to 2075 nm with a FWHM of 175 nm and a top-hat profile was observed that favors femtosecond pulse generation when a laser of this type is modelocked.
Work supported by projects (Spain) MAT2008-06729-C02-01 and (UK) EP/D04622X/1.
References and links
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