Broadly tunable femtosecond Tm : Lu 2 O 3 ceramic laser operating around 2070 nm

Femtosecond mode locking of a Tm-doped Lu2O3 ceramic laser is reported. Transform-limited pulses as short as 180 fs are generated at 2076 nm with an average output power of 400 mW and a pulse repetition frequency of 121.2 MHz. An output power up to 750 mW can be reached at the somewhat longer pulse duration of 382 fs. Femtosecond pulse generation is realized in the 2030-2100 nm spectral range. Passive mode locking was achieved using an ion-implanted InGaAsSb quantum-wellbased SESAM. ©2012 Optical Society of America OCIS codes: (140.7090) Ultrafast lasers; (320.7160) Ultrafast technology; (140.3070) Infrared and far-infrared lasers; (140.4050) Mode-locked lasers; (140.5680) Rare earth and transition metal solid-state lasers; (160.3380) Laser materials. References and links 1. K. Petermann, L. Fornasiero, E. Mix, and V. Peters, “High melting sesquioxides: crystal growth, spectroscopy, and laser experiments,” Opt. Mater. 19(1), 67–71 (2002). 2. P. Klopp, V. Petrov, U. Griebner, K. Petermann, V. Peters, and G. Erbert, “Highly efficient mode-locked Yb:Sc2O3 laser,” Opt. Lett. 29(4), 391–393 (2004). 3. U. Griebner, V. Petrov, K. Petermann, and V. Peters, “Passively mode-locked Yb:Lu2O3 laser,” Opt. Express 12(14), 3125–3130 (2004). 4. C. R. E. Baer, C. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, T. Südmeyer, G. Huber, and U. Keller, “Femtosecond thin-disk laser with 141 W of average power,” Opt. Lett. 35(13), 2302– 2304 (2010). 5. P. Koopmann, S. Lamrini, K. Scholle, P. Fuhrberg, K. Petermann, and G. Huber, “Efficient diode-pumped laser operation of Tm:Lu2O3 around 2 μm,” Opt. Lett. 36(6), 948–950 (2011). 6. P. Koopmann, S. Lamrini, K. Scholle, P. Fuhrberg, K. Petermann, and G. Huber, “Long wavelength laser operation of Tm:Sc2O3 at 2116 nm and beyond,” in Advanced Solid-State Photonics, Advances in Optical Materials, Fiber Laser Applications, High-Intensity Lasers and High-Field Phenomena (ASSP/AIOM/FILAS/HILAS), (Optical Society of America, Washington, DC, 2011), ATuA5. 7. P. A. Budni, L. A. Pomeranz, M. L. Lemons, C. A. Miller, J. R. Mosto, and E. P. Chicklis, “Efficient midinfrared laser using 1.9-μm-pumped Ho:YAG and ZnGeP2 optical parametric oscillators,” J. Opt. Soc. Am. B 17(5), 723–728 (2000). 8. N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, “Octave-spanning ultrafast OPO with 2.6-6.1 μm instantaneous bandwidth pumped by femtosecond Tm-fiber laser,” Opt. Express 20(7), 7046–7053 (2012). 9. K. Scholle, E. Heumann, and G. Huber, “Single mode Tm and Tm,Ho:LuAG lasers for LIDAR applications,” Laser Phys. Lett. 1(6), 285–290 (2004). 10. R. C. Sharp, D. E. Spock, N. Pan, and J. Elliot, “190-fs passively mode-locked thulium fiber laser with a low threshold,” Opt. Lett. 21(12), 881–883 (1996). 11. K. Kieu and F. W. Wise, “Soliton thulium-doped fiber laser with carbon nanotube saturable absorber,” IEEE Photon. Technol. Lett. 21(3), 128–130 (2009). 12. M. Engelbrecht, F. Haxsen, A. Ruehl, D. Wandt, and D. Kracht, “Ultrafast thulium-doped fiber-oscillator with pulse energy of 4.3 nJ,” Opt. Lett. 33(7), 690–692 (2008). 13. F. Adler, P. Masłowski, A. Foltynowicz, K. C. Cossel, T. C. Briles, I. Hartl, and J. Ye, “Mid-infrared Fourier transform spectroscopy with a broadband frequency comb,” Opt. Express 18(21), 21861–21872 (2010). 14. M. J. Thorpe, D. Balslev-Clausen, M. S. Kirchner, and J. Ye, “Cavity-enhanced optical frequency comb spectroscopy: application to human breath analysis,” Opt. Express 16(4), 2387–2397 (2008). 15. B. Shan and Z. Chang, “Dramatic extension of the high-order harmonic cutoff by using a long-wavelength driving field,” Phys. Rev. A 65(1), 011804 (2001). 16. P. B. Corkum and F. Krausz, “Attosecond science,” Nat. Phys. 3(6), 381–387 (2007). #172169 $15.00 USD Received 9 Jul 2012; revised 1 Aug 2012; accepted 1 Aug 2012; published 8 Aug 2012 (C) 2012 OSA 13 August 2012 / Vol. 20, No. 17 / OPTICS EXPRESS 19349 17. A. A. Lagatsky, P. Koopmann, P. Fuhrberg, G. Huber, C. T. A. Brown, and W. Sibbett, “Passively mode locked femtosecond Tm:Sc2O3 laser at 2.1 μm,” Opt. Lett. 37(3), 437–439 (2012). 18. A. Schmidt, P. Koopmann, G. Huber, P. Fuhrberg, S. Y. Choi, D.-II Yeom, F. Rotermund, V. Petrov, and U. Griebner, “175 fs Tm:Lu2O3 laser at 2.07 μm mode-locked using single-walled carbon nanotubes,” Opt. Express 20, 5313–5318 (2012). 19. J. Lu, J. F. Bisson, K. Takaichi, T. Uematsu, A. Shirakawa, M. Musha, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Yb:Sc2O3 ceramic laser,” Appl. Phys. Lett. 83(6), 1101–1103 (2003). 20. K. Takaichi, H. Yagi, A. Shirakawa, K. Ueda, S. Hosokawa, T. Yanagitani, and A. A. Kaminskii, “Lu2O3:Yb 3+ ceramics a novel gain material for high-power solid-state lasers,” Phys. Status Solidi 202(1), R1–R3 (2005). 21. M. Tokurakawa, K. Takaichi, A. Shirakawa, K. Ueda, H. Yagi, S. Hosokawa, T. Yanagitani, and A. A. Kaminskii, “Diode-pumped mode-locked Yb:Lu2O3 ceramic laser,” Opt. Express 14(26), 12832–12838 (2006). 22. M. Tokurakawa, K. Takaichi, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Diodepumped 188 fs mode-locked Yb:Y2O3 ceramic laser,” Appl. Phys. Lett. 90(7), 071101 (2007). 23. M. Tokurakawa, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Diode-pumped sub-100 fs Kerr-lens mode-locked Yb:Sc2O3 ceramic laser,” Opt. Lett. 32(23), 3382–3384 (2007). 24. G. A. Newburgh, A. Word-Daniels, A. Michael, L. D. Merkle, A. Ikesue, and M. Dubinskii, “Resonantly diodepumped Ho:Y2O3 ceramic 2.1 μm laser,” Opt. Express 19(4), 3604–3611 (2011). 25. O. L. Antipov, S. Yu. Golovkin, O. N. Gorshkov, N. G. Zakharov, A. P. Zinov’ev, A. P. Kasatkin, M. V. Kruglova, M. O. Marychev, A. A. Novikov, N. V. Sakharov, and E. V. Chuprunov, “Structural, optical, and spectroscopic properties and efficient two-micron lasing of new Tm:Lu2O3 ceramics,” Quantum Electron. 41(10), 863–868 (2011). 26. O. L. Antipov, A. A. Novikov, N. G. Zakharov, and A. P. Zinoviev, “Optical properties and efficient laser oscillation at 2066 nm of novel Tm:Lu2O3 ceramics,” Opt. Mater. Express 2(2), 183–189 (2012). 27. A. A. Lagatsky, F. Fusari, S. Calvez, S. V. Kurilchik, V. E. Kisel, N. V. Kuleshov, M. D. Dawson, C. T. A. Brown, and W. Sibbett, “Femtosecond pulse operation of a Tm,Ho-codoped crystalline laser near 2 microm,” Opt. Lett. 35(2), 172–174 (2010). 28. A. A. Lagatsky, X. Han, M. D. Serrano, C. Cascales, C. Zaldo, S. Calvez, M. D. Dawson, J. A. Gupta, C. T. A. Brown, and W. Sibbett, “Femtosecond (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm,” Opt. Lett. 35(18), 3027–3029 (2010). 29. H. A. Haus, “Mode-locking of lasers,” IEEE J. Quantum Electron. 6(6), 1173–1185 (2000). 30. Y. Senatsky, A. Shirakawa, Y. Sato, J. Hagiwara, J. Lu, K. Ueda, H. Yagi, and T. Yanagitani, “Nonlinear refractive index of ceramic laser media and perspectives of their usage in a high-power laser-driver,” Laser Phys. Lett. 1(10), 500–506 (2004). 31. J. Ma, G. Q. Xie, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene mode-locked femtosecond laser at 2 μm wavelength,” Opt. Lett. 37(11), 2085–2087 (2012).


Introduction
Rare-earth ion doped crystalline sesquioxides RE 2 O 3 (Re = Y, Sc, and Lu) offer attractive gain media options for high-power solid-state laser developments because of their superior thermo-mechanical properties. Indeed, the thermal conductivities of Sc 2 O 3 and Lu 2 O 3 crystals are 16.5 W/m·K and 12.5 W/m·K, respectively, and this is essentially unaffected with the rare-earth ion doping level in the case of Lu 2 O 3 . Additionally, due to strong electron-phonon interaction, a large splitting of the ground-state is observed and a moderate spectral broadening of the optical transitions takes place. These features are especially well suited to efficient laser operation under the quasi-three-level condition, broad tunability and further ultrashort-pulse generation [1][2][3][4]. Recently, efficient, high-power and broadly tunable (~1930-2150 nm) laser operation was demonstrated from Tm 3+ -doped Lu 2 O 3 and Sc 2 O 3 continuous-wave (cw) lasers [5,6]. Operation at these extended wavelengths from Tm-doped sesquioxides makes them attractive for a range of applications in molecular spectroscopy, as pump sources for optical parametric oscillators based on ZnGeP 2 [7] or GaAs [8] and for incorporation into LIDAR systems [9]. Moreover, it is also advantageous for stable passive mode locking because the presence of strong water vapor absorption bands in the ~1800-2000 nm region can prevent broadband mode locking and give rise to Q-switching instabilities.
The work on the development of ultrashort-pulse lasers that operate in the 2-µm spectral region is rather limited and based predominantly, to date, on fiber laser technology. Although Tm-doped or Tm-Ho co-doped fibre lasers operating around 2 µm exhibit excellent power scalability and efficiency in the cw regime, their performance characteristics during mode locking are rather restricted [10][11][12]. In this respect, Tm-doped sesquioxides represent excellent alternatives in the development of high-efficiency and high-power ultrashort-pulse oscillators operating in the 2-µm region. This is of particular interest for further developments of mid-IR ultrafast broadband laser systems for molecular fingerprinting [13], trace gas detection [14], high-harmonic and X-ray generation [15] and attosecond science [16].
In our recently reported work [17], we have demonstrated passive mode locking of a Tm:Sc 2 O 3 laser around 2.1 µm using InGaAsSb-based semiconductor saturable absorber mirror (SESAM) where pulses as short as 218 fs were produced with an average power of 210 mW. This was followed up by the passive mode locking of a Tm:Lu 2 O 3 crystalline laser using a carbon nanotube saturable absorber from which 175-fs pulses were produced with an average power of 36 mW [18].
Interestingly, the attractive characteristics of rare-earth ion doped crystalline sesquioxides gain media have also led them to being studied extensively as ceramics. Currently, high optical quality sesquioxide ceramics can be produced by nanocrystalline and vacuumsintering technologies. Such ceramic gain media possess stronger fracture toughness than single crystals and afford a high potential for size scalability thereby offering practical advantages in high-power laser implementations. Previously, a range of diode-pumped cw and ultrashort-pulse Yb-doped sesquioxides ceramic lasers have been demonstrated [19][20][21][22][23] and more recently high-power Ho:Y 2 O 3 (cw operation) [24] and Tm:Lu 2 O 3 (cw and Q-switched operation) [25,26] ceramic lasers operating around 2100 nm and 2070 nm, respectively, were reported.
Here we present, for the first time to our knowledge, research results on a passively modelocked Tm:Lu 2 O 3 ceramic laser. Transform-limited pulses of 180 fs duration were produced at 2070 nm with an average power up to 400 mW. The maximum output power of 750 mW was obtained during mode locking when the corresponding pulse duration was 382 fs. Tunability in the range of 2030-2100 nm was achieved for this Tm:Lu 2 O 3 mode-locked laser .

Tm:Lu 2 O 3 laser experimental setup and its cw performance
The experimental assessments were carried out with a Ti:sapphire pump laser that produced up to 2.6 W of output power at 796 nm. The gain medium was a 5-mm-long, Brewster-cut, 2 at % Tm-doped Lu 2 O 3 ceramic rod. Its optical and spectroscopic properties have been previously reported [25,26]. An asymmetric, astigmatically-compensated Z-fold resonator, described elsewhere [27,28], was configured with two high-reflectivity (R > 99.8% from 1900 to 2100 nm) folding (full angle of 14°) mirrors having radii of curvature of 100 mm, an output coupler (OC) with 1.2%, 2.3%, or 4.3% transmission at around 2070 nm and a SESAM structure. The resonator was designed to operate in the middle of stability zone II to ensure a second mode waist on the saturable absorber. The SESAM used for initiating and stabilizing the passive mode locking is based on an ion-implanted InGaAsSb quantum-well structure having an initial reflectivity of 99.5-98.1% in the 2-2.1 µm range. To support a long-term stable laser operation both the gain element and the SESAM were mounted onto copper heat sinks that were maintained at 20°C using thermoelectric coolers. A p-polarized pump beam was focused into the gain medium via an 80-mm focal length lens and the folding mirror (T > 99% at 976 nm) to a spot radius of 26 µm (1/e 2 intensity) measured in air at the location of the input facet of the crystal. The laser beam mode radii inside the gain medium were calculated to be 32 × 61 µm 2 .
During cw operation (a high-reflectivity plane mirror was used instead of SESAM and without the prism pair) the Tm:Lu 2 O 3 ceramic laser produced a maximum output power of 930 mW at 2067 nm when the 4.3% OC was used and this corresponded to a slope efficiency of 54% (with respect to the absorbed pump power). With output couplings of 2.3% and 1.2% the average output powers reached 920 mW (52% slope efficiency) and 760 mW (43% slope efficiency), respectively, for the laser wavelengths around 2068 nm. The gain element absorbs about 70% of pump radiation during laser operation.
A pair of infrared-grade fused silica prisms with tip-to-tip separation of 12 cm was used to control the intracavity net group velocity dispersion (GVD). Each prism was placed at a minimum deviation to minimize the insertion losses. In particular, with the prism pair, the laser produced 900 mW, 820 mW and 640 mW with output couplings of 4.3%, 2.3% and 1.2%, respectively.

Mode locking results and analysis
Stable and self-starting passive mode locking of the Tm:Lu 2 O 3 laser was realized when the intracavity beam mode size on the SESAM was set appropriately to overcome Q-switching instabilities on the one hand and to avoid multiple pulsing under maximum pump power conditions on the other. Figure 1 depicts the input-output characteristics of the Tm:Lu 2 O 3 laser for three different output couplings with the SESAM in place. The maximum output power during mode locking of 750 mW was realized with the 4.3% OC and the laser operated with a slope efficiency of 46%. The cavity mode radii on the SESAM were set to be 135 µm, 158 µm and 175 µm for the 4.3%, 2.3% and 1.2% OCs respectively. Under these conditions, the thresholds for mode locking were similar for different output couplers, typically in the range of 100-107 µJ/cm 2 for the intracavity fluence on the SESAM, and single pulse stable operation was observed when the pump power was increased up to its maximum available level.
The temporal and spectral characteristics of the mode-locked Tm:Lu 2 O 3 laser are as summarized in Fig. 2. The pulse duration was measured using intensity autocorrelator based on two-photon absorption effect in a Si-based detector, optical spectra were detected with a laser spectrometer having a resolution of 0.3 nm around 2-µm region (APE WaveScan) and the pulse repetition frequency was characterised by a fast (>7 GHz cut-off frequency) InGaAs photodetector in combination with a radio frequency spectrum analyzer (Rohde&Schwarz, FSP40). The shortest pulse duration of 180 fs was obtained with the 1.2% OC at a maximum average output power of 400 mW. The corresponding optical spectrum was centered at 2076 nm having a spectral width of 25.7 nm such that a time-bandwidth product of 0.32 was implied. When the output coupling was increased, longer pulse durations were observed. In particular, nearly transform-limited pulses of 213 fs and 382 fs duration were measured with the 2.3% and 4.3% OCs, respectively, for which optical spectra centered at 2072 nm and 2068 nm had the corresponding bandwidths of 22 nm and 11.7 nm. The average output powers reached of 608 mW (2.3% OC) and 750 mW (4.3% OC) that corresponds to generated pulse energies of 4.9 nJ and 5.9 nJ, respectively. The pulse repetition frequencies varied slightly in the range of 121.2-127.3 MHz as a result of different cavity lengths applying for specific output couplings that facilitated a required spot size on the SESAM.
A soliton mode locking regime was observed for all of the output coupling levels used and this is consistent with the dependence of the pulse duration on intracavity pulse energy (output power) and the net GVD. As a representative example, Fig. 3(a) depicts the variation of the pulse duration on the intracavity round-trip GVD which was increased from its initial value D 0 by insertion of a fused silica prism (−116 fs 2 /mm at 2070 nm) at a fixed intracavity pulse energy of 214 nJ (2.3% OC, 608 mW average output power). From the linear fit to the experimental data the self-phase modulation coefficient δ could be deduced to be 4.6 × 10 −7 W −1 using the well-known relationship between soliton pulse duration, pulse energy and GVD [29]. Complementarily, using the extracted value for δ and making the fit to the experimental data of the pulse duration versus the intracavity pulse energy at a fixed GVD (Fig. 3(b)) the value for D 0 was found to be −2980 fs 2 . This is the total round-trip cavity GVD which, in this experimental setup, originates from the insertion of the prisms (glass material dispersion, -1392 fs 2 ), the gain medium itself (-151 fs 2 ) and the angular dispersion of the prism pair (-1437 fs 2 ). Additionally, data deduced for the δ coefficient can provide an estimate for the nonlinear refractive index, n 2 , of the gain medium according to the expression of δ = (2π/λ)·(2L/A eff )·n 2 , where L is the length of the gain medium and A eff if the effective mode area in the gain medium. The n 2 of the Tm:Lu 2 O 3 ceramic at 2070 nm was thus calculated to be 3.3 × 10 −16 cm 2 /W. Previously, the nonlinear refractive index of the Lu 2 O 3 ceramic was reported to be 8.6 × 10 −16 cm 2 /W at 1.06 µm [30]. The tunability of the Tm:Lu 2 O 3 ceramic laser during ultrashort pulse generation was measured with the 1.2% OC and using a single intracavity prism as a dispersive element. Stable and self-starting mode locking was realized in the 2030-2100 nm spectral range (Fig.  4) where the pulse durations varied from 370 fs at 2100 nm to 282 fs at 2030 nm with the maximum average output power of 450 mW achieved at around 2040 nm.

Conclusion
In conclusion, efficient mode locking of a Tm:Lu 2 O 3 ceramic laser in the femtosecond regime has been demonstrated for the first time to the authors' knowledge. Nearly transform-limited pulses as short as 180 fs were produced at 2076 nm with an average output power of 400 mW at a pulse repetition frequency of 121.2 MHz. When the average output power reached a maximum value of 750 mW during mode locking a pulse energy of 5.9 nJ was implied. We believe that further power and energy scaling from a femtosecond Tm:Lu 2 O 3 ceramic laser is feasible with high-power laser diode pumping around 800 nm in combination with thin-disk cavity configurations. Also, it is expected that shorter pulse durations can be generated with optimized SESAM parameters or using more broadband and ultrafast absorbers based on graphene [31].