Spectroscopic and lasing performance of Tm^3+-doped bulk TZN and TZNG tellurite glasses operating around 19 μm

Spectroscopic and lasing performance of Tm3+-doped bulk TZN and TZNG tellurite glasses

operating around 1.9 µm F. Fusari1*, A. A. Lagatsky1, B. Richards2, A. Jha2, W. Sibbett1 and

C. T. A. Brown1

1Scottish Universities Physics Alliance, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, Scotland, UK

2Institute for Materials Research, School of Process, Environment and Materials Engineering, The University of Leeds, Leeds, LS2 9JT, UK,

* Corresponding author: [email protected]

Abstract: We report spectroscopic and bulk laser performance characteristics for Tm3+-doped tellurite glasses when used as gain media operating around 1.9 μm. Two glass hosts studied are TZN and TZNG and their performances have been compared. In each case, well-characterized cw laser performance was obtained and this has been related to detailed spectroscopic measurements of the important lasing parameters of the laser transitions around 1900 nm when pumped at 793 nm. The maximum output power achieved was 124 mW from the TZNG sample with an associated slope efficiency of 28 % with a tuning range of 135 nm. Efficiency and loss analyses yielded a calculated maximum attainable efficiency of 48 % in Tm3+:TZN compared to 28 % for the TZNG host.

©2008 Optical Society of America

OCIS codes: (140.5680) Rare earth and transition metal solid-state lasers; (140.3380) Laser materials; (140.3600) Lasers, tunable; (140.3070) Infrared and far-infrared lasers; (160.5690) Rare-earth-doped materials.

References and links

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#102251 - $15.00 USD Received 2 Oct 2008; revised 30 Oct 2008; accepted 31 Oct 2008; published 4 Nov 2008

(C) 2008 OSA 10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 19146

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1. Introduction

The development of 2-μm solid-state lasers is of significant interest for a wide range of applications that include surgery [1, 2], nonlinear optics [3], LIDAR [4] and environmental monitoring [5]. High-power and efficient laser sources around 2 μm have been successfully demonstrated using Tm3+ and Ho3+ doping in bulk crystals [5,6] and Tm3+ doping in silica fibers [7]. Glass-based active materials can also serve as an attractive solution in terms of power scalability when designed into fiber configurations, due to the large flexibility of the composition formulae and low cost production. In this case, a fundamental understanding of the bulk laser properties of such materials is important when designing the fiber element. In the work reported here, we describe the characterization of two Tm3+-based tellurite glasses which, to the best of our knowledge, have not previously been used as the gain media of bulk lasers.

Tellurium oxide (TeO2) - based glasses possess many relevant parameters for laser action around 2 μm because of the low peak phonon energy (750 cm-1 ), a transparency up to 5 μm, high optical nonlinearities compared to other glasses, high linear refractive indices and high solubility for rare earth ions without devitrification. Given that TeO2 does not form in the glass state by itself [8] and there is a requirement for one or more network modifiers (NWM), we chose to use the well known TZN (tellurium, zinc, sodium) compounds family [9].

2. Sample preparation and spectroscopy

Two samples were prepared from high purity (>99.99%) starting chemicals of TeO2, ZnO, Na2CO3, GeO2 and Tm2O3. The first sample of Tm3+:TZN with a composition 80 TeO2 – 10 ZnO – 10 Na2O (mol%) was doped with an additional 1.5 wt% of Tm2O3. The second sample of Tm3+:TZNG with a composition of 75 TeO2 – 10 ZnO – 10 Na2O – 5 GeO2 (mol%) was doped with 2 wt% of Tm2O3. The difference in concentration was intended to provide pathways for minimizing thermal and emission quenching effects. Each batch was weighed, separately mixed and thoroughly ground before being moved into respective gold crucibles. It was then melted at 800 °C for 2 hours and homogenized for 1 hour at 750 °C in an atmosphere of flowing dry O2. The melt was cast into a preheated 265 °C brass mould and then annealed for 3 hours at 285 °C before being allowed to cool slowly to room temperature in the furnace. The Tm3+ sample concentrations were calculated through density measurements and were found to be NTZN = 2.51×1020 ions/cm3 and NTZNG = 3.35×1020 ions/cm3. The samples were subsequently cut and Brewster-angle polished for spectroscopic and laser characterizations. The ground state absorption cross-section spectra of Tm3+:TZN and Tm3+:TZNG were then recorded from 400 nm to 4000 nm with a PerkinElmer Lambda 950 UV/VIS/NIR spectrophotometer and a Bruker Vertex 70 FTIR spectrophotometer. The absorption cross section peaks at 793 nm were calculated to be 4.4×10-21 cm2 for both samples with full-width at half-maximum (FWHM) around 20 nm. The OH⎯ absorption peak at ~ 3300 nm showed a slightly higher OH⎯ concentration in the Tm3+:TZN sample. The luminescence spectra of the samples were recorded with an Edinburgh Instruments FLS920 Steady State and Time Resolved Fluorescence Spectrophotometer and an InGaAs detector from 1300 nm to 2200 nm with a laser diode at 808 nm as an excitation source. The stimulated emission cross-sections of the two samples were calculated from the absorption cross-section using the McCumber theory (MC) or reciprocity method [10]:



( ) ( ) ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅⋅λλTk

hc

Z

Zλσ=λσ

ZLBU

LAE

11exp (1)

where ZL/ZU = 1.512 is the ratio of the partition function between the lower (3H6) and the upper ( 3F4 ) Stark bands as reported elsewhere [3], h is the Planck constant, c is the speed of light in vacuum, kB is the Boltzmann constant, T is the temperature in K and λZL = 1739 nm [11] is the zero-phonon line that is the energy difference between the lowest Stark multiplets of 3F4 and 3H6 energy levels. The recorded data that depend intrinsically on the excitation intensity were scaled to the calculated ones (MC) and plotted in Fig. 1.

Fig. 1. The calculated emission cross-sections of the Tm3+:TZN and the Tm3+:TZNG from 1300 nm to 2200 nm and the measured luminescence spectra scaled to the calculated values. The peaks are highlighted with the respective transitions.

The peak cross-sections of Tm3+:TZN and Tm3+:TZNG at around 1800 nm were calculated to be 5.0 × 10-21 cm2 and 6.7 × 10-21 cm2 respectively. The slight disparity (~50 nm) between the calculated and the measured data in the Tm3+:TZNG case is most likely due to the choice of parameters ZL/ZU and λZL which were originally inferred for a fluoride crystal LiYF4 [3]. Measuring such parameters for the glass, where the Tm3+ spectra are characterized by strong inhomogeneous broadening, would imply a site-selective spectroscopy that is beyond the scope of this work.

The lifetime measurements of the upper laser level 3F4 of Tm-doped TZN and TZNG were performed using the following experimental set-up. An 808 nm modulated (45 Hz, 50% duty cycle, 10 μs decay time ) laser diode output was launched in a multimode silica fiber which was brought into contact with the sample and a small portion of the induced fluorescence from the sample was then collected using the same fiber. The collected fluorescence was then filtered and analyzed by a 2 μm detector (New Focus Model 2034). The measured lifetime values were found to be 1.14±0.07 ms for the Tm3+:TZN and 1.30±0.07 ms for the Tm3+:TZNG. Using this experimental configuration ensured that fluorescence was collected directly from the sample region where it was generated by the pump beam thereby reducing the susceptibility of the measurements to reabsorption and re-emission errors. Although increasing dopant concentration might be expected to shorten the upper state lifetimes due to concentration quenching effects [12], in fact we observed a longer lifetime for the 2 wt% Tm3+:TZNG compared to the 1.5 wt% Tm3+:TZN. We believe that this



behaviour can be accounted for by the lifetime lengthening effects due to the relative OH- concentration difference between the TZN and TZNG glasses [13].

3. Continuous-wave laser performance

For the laser assessments reported here, the Tm3+:TZN and the Tm3+:TZNG samples were Brewster-cut to 7 mm and 5 mm lengths each exhibited an absorption of around 80 % at 793 nm of pump wavelength. The laser elements were wrapped in a thin sheet of high thermal conductivity indium foil and inserted into a copper mount maintained at 15 °C. The Tm3+:TZN and the Tm3+:TZNG gain elements were placed in an asymmetric astigmatic-compensated Z-folded laser cavities and were end-pumped by a cw titanium-sapphire laser operating at 793 nm. The pump beam radius was estimated to be 30 μm in the glass samples. The high reflectivity cavity mirrors were designed for high transmission (> 95 % ) at the pump wavelength and high reflectivity (> 99.95 % ) in the 1850-2000 nm range. Output couplers used in these assessments had transmissivities of 0.8 %, 2.0 %, and 4.1 % around 1950 nm. A combined output coupling of 6.1 % could also be achieved by employing simultaneously both the 4.1 % and 2 % OCs in the cavity. The cavity mode size inside the active element was calculated to be 29 μm in radius under the paraxial approximation and so there was a good overlap with the input pump beam.

The measured output powers as a function of absorbed pump power for the Tm3+:TZNG laser are plotted in Fig. 2. The maximum output power of 124 mW was obtained at 1932 nm with the 6.1 % output coupling. The Tm3+:TZN had slope efficiencies of 26.6 %, 22.3 %, 12.3 % and 6 % for output couplings of 6.1 %, 4.1 %, 2.0 %, and 0.8 %, respectively.

Fig. 2. The Tm3+:TZNG laser: output power vs. the absorbed pump power.

In the free-running operation, the laser output wavelengths ranged from 1920 nm to 1960 nm depending on the output coupling as a result of the quasi-three-level lasing scheme. The optical-to-optical efficiency η of the Tm3+ 3F4 --> 3H6 transition pumped through the 3H6 --> 3H4 transition can be written as [14]:

⎟⎠

⎞⎜⎝

⎛

Tη

δη+

η

η=

η

SS 11

00

(2)

where ηS is the absorption efficiency, T is the mirror transmission, δ are the resonator round-trip losses and η0 is the maximum attainable efficiency. The absorption efficiencies ηs at the



pump wavelength were 85 % and 80.6 % for Tm3+:TZN and Tm3+:TZNG samples. Figure 3 depicts the inverse slope efficiencies 1/η as functions of the inverse mirror transmissions1/T.

Fig. 3. The inverse of the output to absorbed slope efficiencies η plotted against the inverse of the transmission of the output couplers for the two samples.

Using a linear approximation of the 1/η function, the maximum attainable efficiencies η0

of 48 % and of 28 % were deduced for the Tm3+:TZN and Tm3+:TZNG sample glasses. The values obtained for η0 clearly demonstrate that the dopant concentrations are not yet fully optimized to exploit the beneficial cross relaxation effects present in 2 μm Tm3+ lasers. The quasi-resonant energy transfer among Stark-split multiplets of two neighbouring Tm3+ ions allows 2 μm Tm3+ laser systems pumped at 793 nm to overcome the ~ 40% Stokes limit ( λA/λE ) and reach the theoretical maximum of ~ 80% ( 2 λA/λE ) [15]. The slope returns the value for the overall round-trip losses of δTZN = 6.8 % and δTZNG = 1.5 %. Although the Tm3+:TZNG glass exhibits lower scattering losses than the Tm3+:TZN counterpart, it offered a lower achievable efficiency. This might be due to the effect of the concentration quenching. In order to determine the losses in the glasses due to scattering and impurities, the reflection losses from the cavity mirror and the active element surfaces need to be subtracted from the round-trip losses. The highly reflecting mirrors had an average reflectivity measured at 99.95% therefore in the Z-folded cavity this corresponded to a total loss of 0.3 %. The glass sample placed in the cavity at Brewster angle had an approximate round-trip residual reflection at its surfaces of 0.2 %. Thus the overall reflection losses were δR = 0.5 %. The Rayleigh scattering and impurities scattering could thus be calculated to be δSTZN = 6.3 %, and δSTZNG = 1 % for these Tm3+:TZN and Tm3+:TZNG glasses.

The tunability of the two lasers was obtained through the insertion of a fused silica prism in the cavity. Figure 4 depicts the tuning curves with the 0.8 % output coupler for the two materials. The maximum powers achieved with the prism inserted were 20 mW and 28 mW for the Tm3+:TZN and Tm3+:TZNG, respectively. The table in the inset also provides tuning data for laser operation with the three output couplings of 0.8 %, 2.0 % and 4.1 %. The tuning range for the Tm3+:TZNG laser was from 1830 nm to 2025 nm while the Tm3+:TZN laser could be tuned from 1876 nm to 2017 nm. The tunability of the laser generally depends on several factors including, the fluorescence linewidth of the dopant ion, the reflectivity bandwidth of the mirrors used to form the cavity, and the round-trip losses within the cavity, which in the case of the lasers described here can be mainly attributed to scattering losses within the gain media. For both the TZN and TZNG glasses, the same mirror set was used (HR 1850 nm – 2000 nm and appropriate output coupler) and the temperature was kept



constant. The mirror bandwidth was considerably less than the full fluorescence bandwidth for the 3F4

to 3H6 transition evident in Fig. 1 and we believe that the broader tunability recorded for the Tm3+:TZNG laser may be attributed to the lower optical loss in this sample compared to the Tm3+:TZN sample.

Fig. 4. The Tm3+:TZN and the Tm3+:TZNG glass tunability measured for a 0.8% output coupler and with a fused silica prism as a tuning element. The inset provides the tunability values (FWHM) for the two media.

4. Conclusions

Thulium-doped Tm3+:TZN, Tm3+:TZNG glass laser gain elements produced by the melt and quenching technique have yielded high slope efficiencies of up to 28 % and encouraging output powers (124 mW at 1932 nm) and tunability (135 nm for Tm3+:TZNG). Double-pass losses of 1 % and 6.3 % demonstrated that these were relatively high quality glass materials that could be used for the laser assessments. The broad tunability and the high nonlinear refractive indices associated with the tellurite compounds [16] offer the prospects of good performance of such glasses in Kerr-lens mode-locked laser configurations and this will be investigated further in due course.

Acknowledgments

The authors acknowledge EPSRC funding, the University of Leeds group for the production of the glass samples and the collaboration of Dr Robert Thomson at Heriot-Watt University for helping with the lifetime measurements.



Documents

Spectroscopic and lasing performance of Tm^3+-doped bulk TZN and TZNG tellurite glasses operating around 19 μm