Thulium (Tm) lasers operating in the 2μm IR spectral range are interesting for many applications, including spectroscopy, remote sensing, photomedicine, optical communications, and metrology. For instance, in photomedicine, 2μm lasers may be used in ophthalmology to treat glaucoma (high pressure in the eye) by iridotomy, in which holes are made in the iris. Another ophthalmic 2μm laser application is to cut a hole in the back of the capsule that contains the lens of the eye (capsulotomy). Lasers at this frequency are also used in dermatology to reduce skin wrinkles. An interesting feature of such 2μm lasers is that water (the main component of human tissue) is absorbed very efficiently, more than at 1μm, which is produced by the well-known Nd:YAG (neodymium:yttrium aluminum garnet) laser.
Figure 1. Autocorrelation trace (a) and laser spectrum (b) of a mode-locked thulium in potassium lutetium double tungstate laser using a single-walled carbon nanotube-saturable absorber. a.u.: Arbitrary units. FWHM: Full width at half-maximum.
Saturable absorbers (SAs) can be employed to realize both passive mode-locking and passive Q-switching of such lasers. As the intensity increases in the laser cavity, the SAs become more and more transparent until the absorption saturates. This is called absorption bleaching. Energy absorbed by the SAs is released in a laser pulse, and the same process starts again. By using the mode-locked technique, it is possible to obtain ultrashort (femtosecond) pulse durations, depending on the breadth of the gain bandwidth of the material. For ophthalmic applications, the extremely short illumination time of a femtosecond pulse will produce less heating of ocular tissue. Consequently, a mode-locked Tm laser will be particularly suitable for iridotomy. On the other hand, passive Q-switching, which is simpler than active Q-switching, can be used to realize compact, robust, and inexpensive sources of high-energy short pulses in this wavelength range. As a result, the Q-switched Tm laser with giant energetic pulses is more suitable for skin resurfacing. In this treatment, after laser damage to the skin, new collagen is synthesized, achieving skin that is younger in appearance.
At present, semiconductor SA mirrors are used to produce mode-locking of such lasers. However, these are relatively complicated and expensive to manufacture. SWCNT-SAs exhibit unique properties and, in particular, offer intrinsically fast saturable absorption1 and relatively simple manufacturing technology at low cost. For passive Q-switching, it is also common to use an intracavity SA, such as chromium II-doped zinc sulfide (Cr2+:ZnS) or chromium II-doped zinc selenide (Cr2+:ZnSe), in some 1.9μm diode-pumped solid-state lasers.2 We considered how we could use SWCNTs, Cr2+:ZnS, and Cr2+:ZnSe as SAs for lasing at 2μm, when higher absorption in water makes the laser-tissue interaction more efficient.
We pursued the pulsed laser operation of Tm in potassium lutetium double tungstate—KLu(WO4)2, also known as KLuW—crystals. These crystals belong to a more general family of monoclinic crystals with the formula KRE(WO4)2, where RE=lutetium (Lu), gadolinium (Gd), or yttrium (Y), which are established as promising solid-state laser materials.3 The KLuW samples used were doped with 3 at% Tm (N=2.4×1020 ions·cm−3 measured in the crystal).
In the mode-locking experiments we pumped the Tm:KLuW laser with a continuous-wave titanium-doped sapphire laser tuned to 802nm. The laser set-up was very similar to that described previously,4 with an additional folding mirror of −10cm radius of curvature (RC) and an RC=−5cm end mirror to produce a second cavity waist for the SWCNT-SA. Figure 1(a) shows the autocorrelation trace recorded at an output power of >200mW. Assuming a sech2-shaped pulse, its duration is 9.7ps (full width at half-maximum, FWHM). We recorded the laser spectrum using a spectrometer with a spectral resolution of 0.2nm. The spectrum was centered near 1944nm—see Figure 1(b)—and exhibited a spectral bandwidth (FWHM) of 0.45nm. This corresponds to a time-bandwidth product of 0.347, which is close to the Fourier limit for a sech2 pulse.
In the Q-switched laser experiments, we achieved substantial improvement over previously reported results,5 in which by using co-doped ytterbium III (Yb3+), Tm3+:KY(WO4)2 samples and a Cr2+:ZnS SA, a pulse duration of 63ns and a maximum output power of 116mW at a repetition rate of 20kHz were obtained, which corresponds to a single pulse energy of 6.7μJ.6 Such high repetition rates in fact do not take full advantage of the long storage time of Tm (e.g., 1.34ms in KLuW), which intrinsically limits the pulse energy. In the present work, with a diode-pumped Tm:KLuW, we achieved single-pulse energies of 16μJ at lower (6.5kHz) repetition rate. More important, the much shorter pulse duration of 4.7ns results in peak powers of 3.4kW.
In conclusion, we realized passive mode-locking of a Tm:KLuW laser using a saturable absorber based on SWCNTs and Q-switching of the diode-pumped Tm:KLuW laser using a Cr2+:ZnSe SA. Simultaneous Raman generation is under investigation at around 2.3μm, and further work on energy scaling and pulse shortening is also in progress.
Projects MAT2008-06729-C02-02/NAN, PI09/90527, DE2009-0002, TEC2010-21574-C02-02, and 2009SGR235 are acknowledged.
Xavier Mateos, Martha Yamile Segura, Maria Cinta Pujol, Joan Josep Carvajal, Magdalena Aguiló, Francesc Díaz
Rovira i Virgili University (URV)
Xavier Mateos received his PhD (2004) and is currently an associate professor at URV. He is co-author of more than 70 peer-review articles. His interests include IR laser generation in new crystalline materials in continuous-wave and pulsed regimes.
Martha Segura received her MSc in physics from the National University of Colombia. She joined the Physics and Crystallography of Materials Group at URV in 2008. Her scientific interests are diode-pumped 2μm solid-state lasers for medical applications.
Maria Cinta Pujol received a PhD in chemistry from URV (2001). She has been a member of the URV research group Physics and Crystallography of Materials and Nanomaterials since 2004. Her research focuses on the synthesis and characterization of bulk crystals and nanocrystals of lanthanide-doped materials.
Joan Josep Carvajal received a PhD in chemistry from URV (2003). He is now associate professor at URV, co-author of more than 80 papers in international journals, and collaborated on the Springer Handbook on Crystal Growth. He has guest-edited Physics Procedia and Optical Materials.
Magdalena Aguiló received a PhD in physics from Barcelona University, Spain (1983). Currently, she is professor of crystallography at URV. Her research interests include crystal growth, x-ray diffraction, x-ray texture analysis, and physical properties related to crystalline structure.
Francesc Díaz is full professor of applied physics at URV. He has published 245 peer-reviewed papers, four books, three patents, and 43 book chapters. His research interests are epitaxial layers, nanoparticles, and nanostructured materials for integrated optics, and photonic devices made from laser, nonlinear, and photonic crystals.
Won Bae Cho, Uwe Griebner, Valentin Petrov
Max Born Institute (MBI)
Won Bae Cho received his PhD degree in applied physics from Ajou University, South Korea (2010), where he subsequently worked as a postdoctoral fellow at the university's Institute of NT-IT Fusion Technology. He has been a research associate in the Engineering Department of Cambridge University (UK) since 2011.
Uwe Griebner received a PhD in physics from the Technical University of Berlin, Germany (1996). He has been at MBI since 1992, where he works on diode-pumped solid-state lasers, fiber lasers, waveguide lasers, micro-optics, micro-optics for special resonators, and ultrafast lasers. He is currently working on ultrafast diode-pumped solid-state lasers and amplifiers.
Valentin Petrov received an MSc in nuclear physics from the University of Sofia, Bulgaria (1983). He received a PhD in optical physics from Friedrich Schiller University, Jena, Germany (1988). In 1992, he joined MBI, where his research interests include ultrashort light pulses, laser physics, nonlinear optics, and optical materials. He has co-authored more than 300 papers in scientific journals.
1. 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, F. Díaz, Passive mode-locking of a Tm-doped bulk laser near 2μm using a carbon nanotube saturable absorber, Opt. Express 17, pp. 11007-11012, 2009.
2. R. D. Stultz, V. Leyva, K. Spariosu, Short pulse, high-repetition rate, passively Q-switched Er:yttrium-aluminum-garnet laser at 1.6 microns, Appl. Phys. Lett. 87, pp. 241118, 2005.
3. V. Petrov, M. C. Pujol, X. Mateos, O. Silvestre, S. Rivier, M. Aguiló, R. M. Solé, J. Liu, U. Griebner, 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, pp. 179-212, 2007.
4. X. Mateos, V. Petrov, J. Liu, M. C. Pujol, U. Griebner, M. Aguiló, F. Díaz, M. Galan, G. Viera, Efficient 2-μm continuous-wave laser oscillation of Tm3+:KLu(WO4)2, IEEE J. Quant. Electron. 42, pp. 1008-1015, 2006.
5. M. Segura, X. Mateos, M. C. Pujol, J. J. Carvajal, M. Aguiló, F. Díaz, V. Panyutin, U. Griebner, V. Petrov, Diode-pumped passively Q-switched Tm:KLuW laser with a Cr2+:ZnSe saturable absorber, OSA Opt. Photon. Congr., Adv. Solid-State Photon. Mtg., 2010. ATuA6
6. L. E. Batay, A. N. Kuzmin, A. S. Grabtchikov, V. A. Lisinetskii, V. A. Orlovich, A. A. Demidovich, A. N. Titov, V. V. Badikov, S. G. Sheina, V. L. Panyutin, M. Mond, S. Kück, Efficient diode-pumped passively Q-switched laser operation around 1.9μm and self-frequency Raman conversion of Tm-doped KY(WO4)2, Appl. Phys. Lett. 81, pp. 2926-2928, 2007.