We report a 2µm ultrafast solid-state Tm:Lu 2 O 3 laser, mode-locked by single-layer graphene, generating transform-limited∼410fs pulses, with a spectral width∼11.1nm at 2067nm. The maximum average output power is 270mW, at a pulse repetition frequency of 110MHz. This is a convenient high-power transformlimited laser at 2µm for various applications, such as laser surgery and material processing.Ultrafast lasers operating at∼2µm are of great interest due to their potential in various applications, e.g. telecoms 1 , medicine 2,3 , material processing 3,4 and environment monitoring 5 . They can be used for light detection and ranging measurements 5 and free-space optical communications 5 , due to the 2-2.5µm atmospheric transparency window 5 . Because water (main constituent of human tissue) absorbs more strongly at∼2µm (∼100/cm) 3 than at other conventional laser wavelengths (e.g.∼10/cm at∼1.5µm, and∼1/cm at∼1µm) 3 , sources working at∼2µm are promising for medical diagnostic 3 and laser surgery 3 . Currently, the dominant technique for ultrafast pulse generation at 2µm relies on semiconductor saturable absorber mirrors (SESAMs) 6,7 . In-GaAsSb quantum-well-based SESAMs have been used to mode-lock Tm,Ho:NaY(WO 4 ) 2 8 and Tm:Sc 2 O 3 9 lasers, generating 258fs pulses with 155mW output power at 2µm 8 , and 246fs pulses with 325mW output at 2.1µm 9 . However, SESAMs require complex growth techniques (e.g. molecular beam epitaxy 6 ), often combined with ion implantation 8,9 to reduce recovery time 6,7 .Nanotubes and graphene have emerged as promising saturable absorbers (SA), due to their low saturation intensity 10-14 , low-cost 10 and easy fabrication 12,14,15 . With nanotubes, broadband operation can be achieved by using a distribution of tube diameters 10,16 . With graphene, this is intrinsic, due to the gapless linear dispersion of Dirac electrons 12,14 . Ultrafast pulse generation at 0.8 17 , 1 18 , 1.3 19 and 1.5µm [10][11][12]14,[20][21][22][23] was demonstrated with graphene-based SAs (GSAs). Ref.25 reported a 1.94µm Tm-doped fiber laser mode-locked by a polymer composite with graphene produced by liquid phase exfoliation of graphite 14,24 . Compared to solidstate lasers, fiber lasers have some advantages, such as compact geometry and alignment-free operation. However, their output power is typically very low (∼mW 26 ) and their output spectrum generally has side-bands 26 . Solid-state lasers have the advantage, compared to fibre lasers, of sustaining ultrafast pulses with higher output power (typically≥100mW) 6,7 and better pulse quality (e.g. transform-limited with sideband-free profile in

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Femtosecond mode locking of a Tm-doped Lu 2 O 3 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. 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," ©2012 Optical Society of AmericaLaser 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.

An overview on current trends in stimulated Brillouin scattering and optical phase conjugation is given. This report is based on the results of the "Second International Workshop on stimulated Brillouin scattering and phase conjugation" held in Potsdam/Germany in September 2007. The properties of stimulated Brillouin scattering are presented for the compensation of phase distortions in combination with novel laser technology like ceramics materials but also for e.g., phase stabilization, beam combination, and slow light. Photorefractive nonlinear mirrors and resonant refractive index gratings are addressed as phase conjugating mirrors in addition.

We quantify the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and, for the first time, report details of the effect dynamics. The RIC dynamics is shown to follow that of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD). The absolute PD value is evaluated in the spectral range of 1460-1620 nm for different fiber samples and is found to be independent on the fiber geometry and on the ion concentration. The PD dispersion profile indicates to a predominant far-resonance UV rather than near-resonant IR transitions contribution to the RIC.

Spatio-temporal instability of the fundamental mode in Yb(3+)-doped few-mode PM fiber amplifiers with a core diameter of 8.5 μm was registered at 2-30 Watts pump power. Both experimental and theoretical analysis revealed the nonlinear power transformation of the LP(01) fundamental mode into high-order modes. Numerical simulation revealed self-consistent growth of the higher-order mode and traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb(3+) ions). Experimental results and numerical calculations showed the increase of the instability threshold along with an increase of the signal frequency bandwidth.

Highly efficient laser oscillations at 2 μm were investigated in Tm:Lu₂O₃ ceramics in-band pumped at 1670 nm by a Raman-shifted erbium fiber laser. Both 23 W CW and 15 W active Q-switched oscillations with 40 ns pulse duration and 15-30 kHz repetition rate were achieved in a high-quality beam. The evolution of two generated waves at 1966 and 2064 nm in dependence on pump power was studied.

Refractive-index changes accompanying changes in population of electronic levels of Yb3+ ions in a Yb:YAG laser disk under intense diode and laser pumping have been studied by use of both a highly sensitive polarization interferometer and transient grating testing at 633 nm. The electronic change of the index that is due to excitation of the Yb3+ ions (which have different polarizability of ground state 2F7/2 and excited level 2F5/2) is strongly predominant over the thermal component. The polarizability differences are deltap (1.9 +/- 0.8) x 10(-26) and (1.95 +/- 0.25) x 10(-26) cm3 as measured at 633 nm in interferometric and transient grating experiments, respectively.