Tm , Ho : KLu ( WO 4 ) 2 laser mode-locked near 2 μ m by single-walled carbon nanotubes

We demonstrate passive mode-locking of a Tm,Ho-codoped crystalline laser operating on the Ho-ion transition I7→I8 near 2 μm using a single-walled carbon nanotube saturable absorber. The Tm,Ho:KLu(WO4)2 laser emits nearly transform-limited pulses with duration of 2.8 ps at a repetition rate of 91 MHz. The output power amounts to 97 mW. ©2014 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3070) Infrared and far-infrared lasers. References and links 1. F. Dausinger, F. Lichtner, and H. Lubatschowski, Femtosecond Technology for Technical and Medical Applications (Springer, 2004). 2. V. W. S. Chen, “Optical space communications,” IEEE J. Sel. Top. Quantum Electron. 6(6), 959–975 (2000). 3. T. M. Taczak and D. K. Killinger, “Development of a tunable, narrow-linewidth, cw 2.066-μm Ho:YLF laser for remote sensing of atmospheric CO2 and H2O,” Appl. Opt. 37(36), 8460–8476 (1998). 4. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2(4), 219–225 (2008). 5. 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). 6. M. Ebrahim-Zadeh and I. T. Sorokina, Mid-infrared Coherent Sources and Applications (Springer, 2008). 7. M. Eichhorn, “Quasi-three-level solid-state lasers in the near and mid infrared based on trivalent rare earth ions,” Appl. Phys. B 93(2-3), 269–316 (2008). 8. V. Sudesh and K. Asai, “Spectroscopic and diode-pumpedlaser properties of Tm,Ho:YLF; Tm,Ho:LuLF; and Tm,Ho:LuAG crystals: a comparative study,” J. Opt. Soc. Am. B 20, 1829–1837 (2003). 9. P. A. Budni, M. L. Lemons, J. R. Mosto, and E. P. Chicklis, “High-power/high-brightness diode-pumped 1.9-μm thulium and resonantly pumped 2.1-μm holmium lasers,” IEEE J. Sel. Top. Quantum Electron. 6(4), 629–635 (2000). 10. L. E. Nelson, E. P. Ippen, and H. A. Haus, “Broadly tunable sub-500 fs pulses from an additive-pulse modelocked thulium-doped fiber laser,” Appl. Phys. Lett. 67(1), 19–21 (1995). 11. 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). 12. A. A. Lagatsky, S. Calvez, J. A. Gupta, V. E. Kisel, N. V. Kuleshov, C. T. A. Brown, M. D. Dawson, and W. Sibbett, “Broadly tunable femtosecond mode-locking in a Tm:KYW laser near 2 μm,” Opt. Express 19(10), 9995–10000 (2011). 13. A. A. Lagatsky, O. L. Antipov, and W. Sibbett, “Broadly tunable femtosecond Tm:Lu2O3 ceramic laser operating around 2070 nm,” Opt. Express 20(17), 19349–19354 (2012). 14. N. Coluccelli, G. Galzerano, D. Gatti, A. Di Lieto, M. Tonelli, and P. Laporta, “Passive mode-locking of a diode-pumped Tm:GdLiF4 laser,” Appl. Phys. B 101(1-2), 75–78 (2010). 15. F. Fusari, A. A. Lagatsky, G. Jose, S. Calvez, A. Jha, M. D. Dawson, J. A. Gupta, W. Sibbett, and C. T. A. Brown, “Femtosecond mode-locked Tm and Tm-Ho doped 2 μm glass lasers,” Opt. Express 18(21), 22090– 22098 (2010). #212713 $15.00 USD Received 23 May 2014; revised 30 Sep 2014; accepted 1 Oct 2014; published 22 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.026872 | OPTICS EXPRESS 26872 16. 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 μm,” Opt. Lett. 35(2), 172–174 (2010). 17. 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). 18. K. Yang, D. Heinecke, J. Paajaste, C. Kölbl, T. Dekorsy, S. Suomalainen, and M. Guina, “Mode-locking of 2 μm Tm,Ho:YAG laser with GaInAs and GaSb-based SESAMs,” Opt. Express 21(4), 4311–4318 (2013). 19. K. J. Yang, D. C. Heinecke, C. Kölbl, T. Dekorsy, S. Z. Zhao, L. H. Zheng, J. Xu, and G. J. Zhao, “Mode-locked Tm,Ho:YAP laser around 2.1 μm,” Opt. Express 21(2), 1574–1580 (2013). 20. N. Coluccelli, A. Lagatsky, A. Di Lieto, M. Tonelli, G. Galzerano, W. Sibbett, and P. Laporta, “Passive mode locking of an in-band-pumped Ho:YLiF4 laser at 2.06 μm,” Opt. Lett. 36(16), 3209–3211 (2011). 21. A. Schmidt, P. Koopmann, G. Huber, P. Fuhrberg, S. Y. Choi, D. I. 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). 22. J. Liu, Y. Wang, Z. Qu, and X. Fan, “2μm passive Q-switched mode-locked Tm:YAP laser with single-walled carbon nanotube absorber,” Opt. Laser Technol. 44(4), 960–962 (2012). 23. A. Schmidt, S. Y. Choi, D. Yeom, F. Rotermund, X. Mateos, M. Segura, F. Díaz, V. Petrov, and U. Griebner, “Femtosecond Pulses near 2 μm from a Tm:KLuW Laser Mode-Locked by a Single-Walled Carbon Nanotube Saturable Absorber,” Appl. Phys. Express 5(9), 092704 (2012). 24. J. Ma, G. Q. Xie, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, and J. Y. Wang, “Diode-pumped modelocked femtosecond Tm:CLNGG disordered crystal laser,” Opt. Lett. 37(8), 1376–1378 (2012). 25. A. A. Lagatsky, Z. Sun, T. S. Kulmala, R. S. Sundaram, S. Milana, F. Torrisi, O. L. Antipov, Y. Lee, J. H. Ahn, C. T. A. Brown, W. Sibbett, and A. C. Ferrari, “2μm solid-state laser mode-locked by single-layer graphene,” Appl. Phys. Lett. 102(1), 013113 (2013). 26. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). 27. V. Liverini, S. Schön, R. Grange, M. Haiml, S. C. Zeller, and U. Keller, “Low-loss GaInNAs saturable absorber mode locking a 1.3-μm solid-state laser,” Appl. Phys. Lett. 84(20), 4002–4004 (2004). 28. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–Polymer Composites for Ultrafast Photonics,” Adv. Mater. 21, 3874–3899 (2009). 29. A. K. Geim and K. S. Novoselov, “The Rise of Graphene,” Nat. Mater. 6(3), 183–191 (2007). 30. W. B. Cho, J. H. Yim, S. Y. Choi, S. Lee, A. Schmidt, G. Steinmeyer, U. Griebner, V. Petrov, D.-I. Yeom, K. Kim, and F. Rotermund, “Boosting the nonlinear optical response of carbon nanotube saturable absorbers for broadband mode-locking of bulk lasers,” Adv. Funct. Mater. 20(12), 1937–1943 (2010). 31. V. Petrov, M. C. Pujol, X. Mateos, O. Silvestre, S. Rivier, M. Aguiló, R. M. Solé, J. Liu, U. Griebner, and F. Díaz, “Growth and properties of KLu(WO4)2, and novel ytterbium and thulium lasers based on this monoclinic crystalline host,” Laser Photon. Rev. 1(2), 179–212 (2007). 32. V. Jambunathan, A. Schmidt, X. Mateos, M. C. Pujol, U. Griebner, V. Petrov, C. Zaldo, M. Aguiló, and F. Díaz, “Crystal growth, optical spectroscopy and continuous-wave laser operation of co-doped (Ho,Tm):KLu(WO4)2 monoclinic crystals,” J. Opt. Soc. Am. B 31(7), 1415–1421 (2014). 33. W. B. Cho, A. Schmidt, J. H. Yim, S. Y. Choi, S. Lee, F. Rotermund, U. Griebner, G. Steinmeyer, V. Petrov, X. Mateos, M. C. Pujol, J. J. Carvajal, M. Aguiló, and F. Díaz, “Passive mode-locking of a Tm-doped bulk laser near 2 μm using a carbon nanotube saturable absorber,” Opt. Express 17(13), 11007–11012 (2009). 34. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, “Q-switching stability limits of continuous-wave passive mode-locking,” J. Opt. Soc. Am. B 16(1), 46–56 (1999).


Introduction
In recent years ultrashort pulse crystalline lasers emitting in the 2-μm-spectral range have attracted significant attention because of their potential applications in highly precise surgery [1], free space communication [2], infrared pump-probe spectroscopy and remote sensing [3], material processing of semiconductors and polymers [4], and nonlinear frequency conversion to the mid-infrared range [5,6].The most popular lasers operating around 2 µm are based on the lanthanide ions Tm 3+ (Tm) and Ho 3+ (Ho) [7], which also holds for ultrafast lasers.Compared to the Tm-transition 3 F 4 → 3 H 6 (slightly below 2 µm), the Ho-ion emission on its 5 I 7 → 5 I 8 transition is only slightly shifted to longer wavelengths (above 2 µm) but this can have pronounced effect in medical applications due to the variation of water absorption in this range, as well as in nonlinear frequency conversion due to residual absorption in non-oxide nonlinear crystals.Due to the absorption band near 800 nm direct diode-pumping of Tmlasers is possible because such wavelengths are covered by commercially available GaAlAs laser diodes.Ho-doped lasers are usually excited by energy transfer through co-doping with Tm-ions profiting from quantum efficiency up to two via cross relaxation [8] or by direct inband pumping with Tm fiber or bulk lasers [9].
The first rare-earth mode-locked lasers with sub-ps pulse duration near 2 µm were based on Tm-doped fibers.In 1995, Nelson et al. generated 500-fs pulses [10] using the nonlinear polarization rotation approach.Shortly afterwards, Sharp et al. demonstrated 190-fs pulses using a semiconductor saturable absorber mirror (SESAM) [11].Mode-locking based on saturable absorption had not been demonstrated with Tm-, Ho-, or Tm,Ho-doped crystalline laser media until 2009.One possible reason could be the water vapor absorption.The broad and smooth gain spectra of most Tm-doped crystalline laser media permit tunable modelocked operation or generation of sub-ps pulses in the spectral range between 1.8 and 2.1 μm (depending on the host) with a gain maximum typically between 1.90 and 1.95 µm [7].However, the strong water vapor absorption for wavelengths shorter than 2.0 µm can prevent continuous tunability and broadband mode-locking unless elaborated purging (e.g.nitrogen) or other kinds of cavity evacuation is performed.Hence, two approaches are applied.The first one is shifting the laser emission spectrum of Tm-doped media still displaying a noticeable gain in the long wavelength wing beyond 2 µm at the expense of lower gain and laser efficiency.The second approach overcomes this limitation using Ho-, or Tm,Ho-codoped crystalline laser media with highest stimulated emission cross section above 2 μm.
The SESAM [26] is a well-established device for ultrashort pulse lasers, providing a robust intracavity mode-locking mechanism.However, SESAMs only offer useful saturable absorption in a narrow spectral range and often require sophisticated manufacturing technology [27].As an alternative, single-walled carbon nanotubes (SWCNTs) and graphene have been introduced because of their unique physical and optical properties [28,29].Both are characterized by fast recovery time and can be fabricated by relatively simple processes [30].
In this work we apply a carbon nanostructure-based SA for mode-locking of the 2-µm Hoion transition 5 I 7 → 5 I 8 for the first time to our knowledge.The laser material is co-doped monoclinic Tm,Ho:KLu(WO 4 ) 2 and the SA is based on SWCNTs.

Experimental setup
As active medium served a Tm,Ho:KLu(WO 4 ) 2 (Tm,Ho:KLuW) crystal [31,32], which is the first implementation of this material for mode-locked operation.Such monoclinic potassium double tungstates, doped with active rare-earth ions, are established as promising solid-state laser materials.In this class of biaxial crystals, the gain bandwidths are very broad and the absorption and emission cross-sections are very high for selected polarizations.In particular, the KLuW crystal is very suitable as host for Yb, Tm and Ho-ions and provides very high laser efficiencies [31].The Tm,Ho:KLuW is grown by the Top Seeded Solution Growth technique with 5at.%Tm 3+ and 0.5at.%Ho 3+ -concentration in the solution.
The design of the mode-locked laser is based on a 1.6 m long X-type linear cavity (Fig. 1).It consists of four curved mirrors (M1, M2, M3 and M4) and a plane output coupler.The calculated cavity waist radii are 32 μm both at the position of the laser crystal and the SA.The latter is placed at Brewster angle near the second cavity waist formed by mirrors M1 and M2.The 3-mm long Tm,Ho:KLuW crystal is mounted in a copper holder for active water cooling and is oriented for beam propagation along the N g crystallo-optic axis and for polarization along the N m axis.Positioned at Brewster angle in the central folding section the crystal is pumped by a Ti:sapphire laser emitting at 802 nm.The pump beam is focused to a radius of ~30 μm.The mirrors M3 and M4 are highly transmitting for the pump radiation and the curved mirror M5 acts as a back-reflector for the unabsorbed pump.The single pass absorption of the Tm,Ho:KLuW active element in the lasing state was 87% and estimated to ~95% for double-pass pumping.The SA is based on arc-made single-walled carbon nanotubes, layered on a 1-mm thick quartz substrate by the spin coating technique.More about the fabrication can be found in [30].The absorption bands are spectrally shifted to longer wavelengths compared to the HiPCO (high-pressure CO conversion decomposition) SWCNT-based SA, so that the E 11 interband transition can be applied for wavelengths longer than 2 µm [23].The linear absorption of the SWCNT-SA at 2.06 μm amounts to 1.4% and the measured recovery time for such type of SA delivered a fast response of the order of 1 ps [33].The expected values for the saturation fluence and the modulation depth are <10 μJ/cm 2 and <1%, respectively [30].The samples are optimized with respect to low scattering loss by reducing SWCNT knotting and interweaving into one another compared to our previous reports [33].

Results
At first the laser cavity is optimized in the continuous-wave (CW) regime without introducing the SWCNT-SA.Using a 1.5% transmission output coupler (T OC ) the laser delivers 187 mW of output power for 900 mW of absorbed pump power.Figure 2 shows the measured average output power versus the absorbed pump power of the laser containing the SWCNT-SA and the same output coupler.The laser threshold corresponds to ~150 mW of absorbed pump power and starts in the CW mode.Mode-locked operation disturbed by some Q-switching instabilities is observed in the pump power range between 500 mW and 700 mW detected by a small pedestal of the first beat note of the recorded radio frequency spectrum.The threshold for the self-starting CW mode-locked (ML) regime is 68 mW of output power obtained at an absorbed pump power of 700 mW.The ML threshold intracavity pulse energy of 50 nJ satisfies the condition for suppression of Q-switching instabilities, derived by Hönninger et al. [34], with the emission cross section of Tm,Ho:KLuW of 1.19 × 10 −20 cm 2 at 2060 nm [32].The maximum output power in ML operation is 97 mW at an absorbed pump power of 970 mW resulting in a slope efficiency of 12% which is about 10% larger than in the CW regime due to the saturated loss of the SA (Fig. 2).
The pulse duration (FWHM) at maximum output power is 2.8 ps, derived from the measured autocorrelation trace, assuming a sech 2 -pulse shape (Fig. 3).The measured optical spectrum has a FWHM of 1.6 nm and the laser wavelength is centered at 2059 nm (inset Fig. 3).The time-bandwidth product amounts to 0.32 corresponding to almost transform-limited pulses.It should be noted that no significant variations of the optical spectrum (FWHM), respectively pulse duration, could be achieved when two intra-cavity CaF 2 -prisms are introduced between M4 and the OC in the cavity.Self-starting and stable ML is also achieved using T OC = 0.2%, 0.5%, 3% and 5%.The shortest pulse duration of 2.4 ps is obtained applying T OC = 0.5% with an output power of 43 mW.The maximum output power of 107 mW is achieved for T OC = 3% at the expense of slightly longer pulses of 5.3 ps duration.

Conclusion
In conclusion, a Tm,Ho:KLu(WO 4 ) 2 laser operating at 2.06 μm was mode-locked using a SWCNT-SA.We demonstrate carbon nanostructure-based mode-locking of the 2-µm Hotransition for the first time.A stable pulse train was generated with average output power of 97 mW and almost Fourier-limited pulse duration of 2.8 ps at a repetition rate of 91 MHz.
A potential reason for not achieving sub-ps pulse durations as demonstrated with Tm:KLuW [23] and Tm:Lu 2 O 3 [21] when operating the SWCNT-SA mode-locked laser around 2.05 µm is the ~4 times larger emission cross section of Tm,Ho:KLuW [32].As a consequence a higher modulation depth of the SWCNT-SA is required to support even shorter pulses.This assumption is supported by applying InGaAsSb SESAM structures, typically characterized by a higher modulation depth, for mode-locking other Tm:Hocodoped double tungstates.The Tm,Ho:KY(WO 4 ) 2 [16] and Tm,Ho:NaY(WO 4 ) 2 lasers [17] delivered pulses with sub-ps duration and output powers around 150 mW.Also GaSb-based SESAMs have been applied for mode-locking of Tm,Ho:YAG [18] and Tm,Ho:YAP [19] lasers with maximum output power reaching 285 mW, however minimum pulse durations were longer than 20 ps.
Compared to the other mode-locked Tm,Ho-codoped lasers, the presented initial results with SWCNT-SA are quite encouraging.Applying SWCNT-SAs designed for slightly higher modulation depth (e.g. higher concentration of SWCNTs) in a next step one can expect further pulse shortening in the sub-ps range with Tm,Ho:KLuW.

Figure 4
Figure4shows the measured radio-frequency spectrum of the SWCNT-SA mode-locked Tm,Ho:KLuW laser output using an RF spectrum analyser (Rhode&Schwarz).It contains the fundamental beat note at 90.6 MHz, see Fig.4(a), recorded with a resolution bandwidth of 1 kHz, and 1 GHz wide-span measurement, see Fig.4(b).The high extinction ratio of 67 dBc and the absence of spurious modulation indicate very stable ML operation.