We report on the spectroscopic characterization, continuous-wave and continuous wave mode-locked laser performance of bulk Tm3+:GPNG fluorogermanate and Tm3+-Ho3+:TZN tellurite glass lasers around 2 μm. A slope efficiency of up to 50% and 190 mW of output power were achieved from the Tm3+:GPNG laser at 1944 nm during continuous wave operation. The Tm3+-Ho3+:TZN laser produced a 26% slope efficiency with a maximum output power of 74 mW at 2012 nm. The Tm3+:GPNG produced near-transform-limited pulses of 410 fs duration centered at 1997 nm with up to 84 mW of average output power and repetition frequency of 222 MHz when was passively modelocked using an ion-implanted InGaAsSb-based quantum well SESAM. Using the same SESAM, the Tm3+-Ho3+:TZN laser generated 630-fs pulses with 38 mW of average output power at 2012 nm. Data analysis of pulses at different intracavity pulse energies provided an estimation of n2 at 2012 nm of 2.9 × 10−15 cm2/W for the Tm3+-Ho3+:TZN
©2010 Optical Society of America
The development of near-infrared lasers operating around the 2 µm spectral region has recently attracted research interest driven by their widespread applications from LIDAR systems to medicine . The emission of Tm3+ (3F4→3H6) and Ho3+ (5I7→5I8) doped and Tm3+-Ho3+ co-doped crystalline and amorphous materials are often chosen to produce laser radiation around 2 µm [2–4]. A range of Tm and Tm-Ho doped crystalline and fiber-based 2 µm laser sources have been demonstrated producing multi-watt output powers and operating with slope efficiencies as high as 76% . Ultrashort-pulse lasers that operate around 2 μm are of particular interest for applications in time-resolved spectroscopy, nonlinear frequency down-conversion to the mid/far-infrared spectral regions, mid-IR supercontinuum generation, optical communications and photomedicine . 190-fs pulses at 1897 nm with an average output power of 1 mW have been generated from thulium fibre laser using semiconductor saturable absorber  and, alternatively, 108-fs pulses were produced at 1980 nm with an average power of 3.1 W after amplification of Raman-shifted Er-doped fiber laser in a Tm-doped fiber . The use of Tm-doped or Tm-Ho co-doped amorphous materials for ultrashort pulse generation based on bulk-glass gain elements represents an attractive option since such gain media are characterized by broadband and smooth emission spectra around 2 µm spectral region . Additionally, they can be produced with very good optical quality by using a simple melt and quenching technique that permits an extremely fast development cycle compared to growing many crystalline hosts. Optical pumping of Tm-doped glass media can be achieved by laser radiation at around 0.8 µm (3H6→3H4) but, alternatively, pumping around 1.2 µm (3H6→3H5)  and 1.6 μm (inband pumping of 3F4)  was also attained.
Here we report the spectroscopic characterization, 2 µm continuous wave (CW) and passively continuous wave mode-locked (CWML) laser operation of a Tm3+:GPNG fluorogermanate and a Tm3+-Ho3+:TZN tellurite glass lasers pumped at 792 nm. The fluorogermanate sample produced a maximum output power of 190 mW at 1944 nm with a corresponding slope efficiency of 50% during cw operation. Broadly tunable laser operation could also be observed over the 1840-2085 nm range. The Tm3+-Ho3+:TZN laser gave a 26% slope efficiency with maximum output power of 74 mW at 2012 nm. Using an InGaAsSb-based semiconductor saturable absorber mirror (SESAM)  near-transform-limited 410-fs pulses at 1997 nm with 62 mW of average output power were produced from the Tm3+:GPNG laser and 630-fs pulses were generated at 2012 nm with 38 mW of average power using Tm3+,Ho3+:TZN gain material.
2. Sample preparation and spectroscopy characterizations
The samples used in our assessments were prepared by a melt and quenching technique. Germanium dioxide GeO2 and Tellurium Dioxide TeO2 network formers were chosen and developed because of the exploitability of their broad transparency range from 0.4 - 5 µm [13,14]. Additionally, their attractive physical characteristics such as, good crystallization stability, high glass transition temperatures and good moisture resistance makes GeO2 and TeO2 compounds good candidates for production of glass laser elements. They possess high refractive indices of around 1.8 for the GeO2 and 2 for the TeO2, their phonon energies are ~900 cm−1 for the germanate and ~750 cm−1 for the tellurite . Additionally, both germanate  and tellurite glasses can be successfully drawn into optical fibers .
The Tm3+:GPNG fluorogermanate was derived from an all-oxide germanate composition  via the substitution of the lead oxide with lead fluoride. In a glass matrix with a substantial amount of fluorides, Tm3+ ions tend to interact with a lower local peak phonon energy decreasing the multi-phonon relaxation rates [15,16]. High purity (>99.99%) starting chemical constituents were weighed and mixed in ambient atmosphere to form glass with a molar concentration of 56 GeO2 – 31 PbF2 – 9 Na2O – 4 Ga2O3. 2 wt% of Tm2O3 was added to a 15 g batch as dopant and the mixture was then transferred to a platinum crucible and melted at 1200 °C for 4 hours under a dry oxygen atmosphere. The melt was stirred once after 2 hours and cast on a preheated brass mould and annealed at 360 °C for 2 hours. The annealing furnace was then turned off and the glass was allowed to cool slowly to room temperature. The quality of the glass obtained was not fully optimized as lines and small casting defects could be seen in the final samples. Also, analysis of the sample between crossed polarizers showed that some material strains were present.
In the Tm3+,Ho3+:TZN sample, the host glass was designed following our previously developed process and composition . As in all Tm3+ and Ho3+ co-doped materials a balance between upconversion losses which are directly related to the Tm3+ and Ho3+ concentrations, and Tm3+→Ho3+ transfer efficiency, inversely proportional to the Ho3+/Tm3+ concentration ratio  had to be found. Different samples were produced with varying concentrations of Ho3+ and in this study we present the optimized results obtained with a sample doped with 2 wt% of Tm2O3 and 0.1 wt% of Ho2O3 in a 10g TZN batch. Both samples were cut to a length of 4.5 mm and parallel polished with an 8 mm × 8 mm aperture.
Measurements of the absorption spectrum were carried out from 400 nm to 2200 nm with a PerkinElmer Lambda 950 UV/VIS/NIR spectrophotometer and are shown in Fig. 1 . Density measurements of active ions in two glass samples returned a Tm3+ concentration of 3.1 × 1020 cm−3 in the fluorogermanate and 3.3 × 1020 cm−3 in the tellurite glasses. This in turn allowed an estimation of the peak absorption cross section of the 3H6-3H4 transition to be made from the absorption coefficient spectrum. The values found were peak absorption cross sections of 9.5 × 10−21 cm2 for the fluorogermanate glass and 8.7 × 10−21 cm2 for the tellurite glass as shown in Fig. 1. Tm3+ level 3H5 and Ho3+ level 5I6 around 1200 nm overlap and cannot be clearly resolved in our samples, a detailed spectroscopy of Ho3+ is reported by Gruber et al.  The luminescence spectra were recorded from 1300 nm to 2200 nm using an Edinburgh Instruments FLS920 Steady State spectrometer with excitation by a laser diode at 808 nm. In the fluorogermanate glass a negligible emission around 1470 nm indicated an efficient cross-relaxation (3H4 + 3H6→3F4 + 3F4) process. Using the reciprocity method and parameters reported in reference  the emission cross section for Tm3+ in the fluorogermanate glass was calculated to be 5.5 × 10−21 cm2 at 1850 nm.
Similarly, the emission cross section of the Ho3+ peak at 1950 nm in the tellurite sample was calculated to be 8 × 10−21 cm2. The calculated emission cross sections as a function of wavelength are shown in Fig. 2 . The upper laser level lifetimes of Tm3+ and Ho3+ were measured using a time-resolved fluorescence spectrophotometer and an InGaAs detector with a modulated laser diode at 808 nm as an excitation source. The fluorescence decay of the Tm3+ in germanate glass had a single exponential feature with the lifetime constant of 2.9 ms showing a negligible energy transfer upconversion (3F4 + 3F4→3H6 + 3H4). The lifetime of the 5I7 level of Ho3+ in the tellurite sample was measured to be 2.5 ms.
3. Continuous-wave laser performance
For the assessment of CW laser performance, the glass samples were glued with thermo-conductive paint to a copper mount which was maintained at 15 °C and inserted at Brewster’s angle in a Z-fold 4-mirror laser cavity as depicted on Fig. 3 . The Ti:sapphire pumping beam was focused to a 25 µm radius spot size inside the gain material and the pump wavelength was tuned to the maximum of the 3H4 absorption line of the Tm3+ at 792 nm for the Tm3+:GPNG and 793 nm for the Tm3+,Ho3+:TZN. The two samples showed a similar absorption cross section at the pump wavelength, hence they both absorbed around ~65% of the incident radiation in a single-pass configuration. The cavity mirrors in place were designed for high transmission (>98%) at the pump wavelength and high reflectivity (>99.99%) in the 1800-2100 nm range. Four output couplers (OC) were used having transmissivities of 0.8%, 2.0%, and 4.1% around 1950 nm respectively and, by using two OCs in place of mirror M3 and M4 data for an overall 6.1% output coupling could also be obtained.
The input-output cw characteristics obtained for the two lasers are shown in Fig. 4 . The maximum slope efficiency of 50% with respect to the absorbed power was obtained using the 6.1% output coupling in the Tm3+:fluorogermanate laser (Fig. 4(a)). The maximum output power was 190 mW obtained at 1952 nm and was limited by the available pump power. The lowest laser threshold was of 43 mW of absorbed pump power with the 0.8% OC. The round-trip losses of the cavity were found to be around 1.2%/cm by plotting the inverse of the slope efficiencies against the inverse of the mirror transmissivities .
In the case of the Tm3+,Ho3+:TZN laser the highest slope efficiency reached was 26% for a 2.0% OC with corresponding maximum output power of 74 mW (Fig. 4(b)). The efficiency of this laser was lower than for singly doped Tm3+ laser system as a result of the energy transfer from the Tm3+ 3F4 level to the 5I7 laser level of Ho3+ and the presence of enhanced up-conversion losses in the Tm-Ho system . The Tm3+,Ho3+:TZN laser output was centered at 2048 nm for the 0.8% OC and operation at shorter wavelength of 2012 nm was observed for the 2.0% OC due to the quasi-three level nature of the Ho3+ 5I7-5I8 transition. With a 4.0% OC laser output was observed at 1944 nm only where Tm3+ contributed predominantly. The laser threshold was approximately 100 mW of absorbed pump power with the 0.8% OC. Interestingly, both laser systems demonstrated no thermal rollover up to the maximum incident pump power of 900 mW.
The tunability of the two lasers was measured with a fused silica prism inserted into the laser cavities. The prism losses were negligible and the output power reached 145 mW with the 0.8% OC for the Tm3+:GPNG element and 58 mW with the 0.8% OC in the case of the Tm3+,Ho3+:TZN sample.
The Tm3+:GPNG laser emission extended from 1840 nm to 2085 nm with full width at half maximum of 145 nm and the Tm3+,Ho3+:TZN laser emitted from 1870 nm to 2080 nm with a FWHM of 125 nm (Fig. 5 ).
4. Continuous wave mode-locked laser performance
The CWML laser performance of both Tm3+:GPNG and Tm3+,Ho3+:TZN lasers operating at 2 μm was evaluated when an InGaAsSb-based SESAM (described previously in Ref .) was employed. The Z-folded 4-mirror laser cavity depicted in Fig. 3 was constructed where second-order dispersion control was achieved with a pair of infrared grade fused silica prisms. The laser cavity mode size (radius) on the SESAM was 50 µm. In both laser systems, the prisms were mounted on translation stages, and had a material dispersion of −100 fs2/mm around 2000 nm. The total double-pass group velocity dispersion (GVD) that could be produced with the prism pair was varied between −2800 fs2 and −3700 fs2 depending on the amount of glass inserted. The refractive indices of both fluorogermanate and tellurite glass elements were measured with a Metricon 2010 prism coupler machine at 532 nm at 633 nm and 1321 nm. They resulted of n532 = 1.868, n633 = 1.825, and n1321 = 1.755 for the fluorogermanate and n532 = 2.068, n633 = 2.039, and n1321 = 1.988 for the tellurite glass respectively. The parameters for the Sellmeier equation fit could thus be calculated providing a measured linear refractive index dispersion curve. From the linear dispersion the second order dispersion (group velocity dispersion GVD) could be derived: the undoped fluorogermanate glass had a GVD of 280 fs2/mm at 1950 nm and the undoped tellurite glass had a positive GVD of 68 fs2/mm at 2010 nm.
The Tm3+:GPNG laser with the two prisms inserted and the high reflectivity mirror M4 in place produced 140 mW of output power with corresponding slope efficiency of 29% with the 0.8% OC. Upon insertion of the SESAM the efficiency dropped to 19%, Fig. 6(a) .
Stable self-starting mode locking operation was achieved at 410 mW of incident pump power (275 mW of absorbed power) with 37 mW of average output power. A maximum average output power of 84 mW was achieved in the mode locking mode.
The pulse repetition frequency was 222 MHz and the modelocking threshold fluence on the absorber was estimated to be 265 µJ/cm2. Near-transform-limited pulses were produced at 1997 nm center wavelength with pulse duration ranging from 520 fs at modelocking threshold conditions to 410 fs at 62 mW and higher values of average output power, the autocorrelation trace is shown in Fig. 7(a) . This corresponded to a maximum achieved peak power for these pulses of 680 W. The FWHM output spectrum for the pulses ranged from 8.1 nm to 10.4 nm depending on the intracavity pulse energy with corresponding time-bandwidth products ΔνΔτ ranging from 0.32 to 0.36. Figure 7(b) shows the output spectrum obtained for the 410-fs pulse. The Tm3+,Ho3+:TZN laser was modelocked using the same cavity arrangements. As shown in Fig. 8 , the shortest measured pulse duration was 630 fs with corresponding spectral bandwidth of 6.8 nm at maximum average output power of 38 mW. Starting from modelocking threshold and increasing the incident power in the TZN laser, and therefore the intracavity power, the pulses shortened from 1010 fs to 630 fs when the SESAM dynamic saturable losses were fully saturated.Fig. 9 the δL = 2.33·10−6 W−1 could be extracted taking into account that D value was estimated to be −3300 fs2.
The SPM coefficient, or Kerr coefficient, δL is directly connected to the nonlinear refractive index of the gain material, n2, in this case the alkali tellurite TZN glass, by the formula :23].
In conclusion, we have presented spectroscopic, CW and CWML femtosecond laser characterization of the Tm3+:GPNG bulk fluorogermanate and a doubly doped Tm3+,Ho3+:TZN bulk tellurite based glasses pumped by a Ti:Sapphire laser at around 800 nm. In case of the Tm3+:GPNG laser a maximum slope efficiency of 50% with the corresponding maximum output power of 190 mW were achieved during continuous wave operation at 1952 nm. This confirms that germanate glass represents a good candidate for 2 μm gain media in agreement with the work of G. Turri et al . The Tm3+-Ho3+:TZN laser operated with maximum slope efficiency of 26% and produced a maximum output power of 74 mW at 2012 nm. Modelocking pulsed assessments were carried out using an ion-implanted InGaAsSb-based quantum well SESAM. The Tm-doped fluorogermanate glass produced near-transform-limited pulses of 410 fs duration centered at 1997 nm with up to 84 mW of average output power and a repetition frequency of 222 MHz. The Tm3+,Ho3+:TZN laser yielded 630 fs pulses centered at 2012 nm at 38 mW average output power and repetition frequency 143 MHz. Data analysis of pulses at different intracavity pulse energies provided an estimation of n2 parameter at 2012 nm of 2.9 × 10−15 cm2/W for the Tm3+,Ho3+:TZN. This is the first time, to the authors’ knowledge, that a femtosecond-pulse operation has been demonstrated using bulk glass lasers in the 2 μm spectral band.
The authors acknowledge funding from the UK Engineering & Physical Sciences Research Council under a Photon Flow Basic Technology grant EP/D04622X/1. Access to the specialist equipment in the laboratories of colleagues Professors Andrew P. Mackenzie and Thomas F. Krauss at St Andrews University is also gratefully acknowledged.
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