We demonstrate what is to our knowledge the first passively mode-locked thin-disk Yb:KY(WO(4))(2) laser. The laser produces pulses of 240-fs duration with an average power of 22 W at a center wavelength of 1028 nm. At a pulse repetition rate of 25 MHz, the pulse energy is 0.9microJ , and the peak power is as high as 3.3 MW. The beam quality is very close to the diffraction limit, with M(2)=1.1 .

We report the first demonstration, to our knowledge, of soft-aperture Kerr-lens mode locking in a diodepumped femtosecond Yb 3+ : YVO 4 laser. Near-transform-limited pulses as short as 61 fs are generated around a center wavelength of 1050 nm with an output power of 54 mW and a pulse repetition frequency of 104.5 MHz. This is, to our knowledge, the shortest pulse generated directly from an Yb laser having a crystalline host material. The femtosecond operation has a mode-locking threshold at an absorbed pump power of 190 mW. 3 From the point of view of the development of new Yb-doped crystals for reliable and efficient femtosecond lasers, materials are preferred that possess high cross sections (in particular, the emission cross section) 4 together with broad gain spectra 5-7 and good physical and mechanical properties. 8 In this context, given the strong and broad absorption peak (FWHM of ϳ8 nm) at around 985 nm, which is compatible with optical pumping by well-developed InGaAs laser diodes, the extremely low quantum defect ͑ϳ3.5% ͒, and a relatively broad and smooth 5 (glasslike) emission spectrum, the Yb: YVO 4 crystal is a good candidate gain medium for incorporation into diode-pumped femtosecond lasers for the 1-m spectral region. Indeed, 120-fs pulses with an average power of 300 mW were generated from a diode-pumped Yb: YVO 4 laser that was passively mode locked by using a semiconductor saturable absorber mirror (SESAM). 9In fact, most of the work in the field of ultrashort Yb lasers has concentrated on the employment of SESAMs 10 for passive mode locking. Kerr-lens mode locking 11 (KLM) is another well-developed technique for the generation of ultrashort pulses from lasers that are efficient, compact, and have reduced cavity component counts. 12 Reducing the intracavity losses by excluding a SESAM (nonsaturable losses) can lead to a substantial enhancement in the optical efficiency of such a femtosecond laser. In addition, the shortest pulse generated directly from a laser was produced using the KLM technique. 13 One of the critical parameters for reliable Kerr-lens mode locking is a high value of the nonlinear refractive index n 2 of a laser medium. Although, Major et al. have shown that a number of Yb-doped crystals are characterized by relatively high n 2 and are promising for KLM operation, 14,15 only the Yb:KYW laser has been mode locked successfully by using the optical Kerr effect under the conditions of direct diode pumping. 16,17 In this Letter we describe the parameters of a new Yb-doped YVO 4 crystal, which was selected for possible exploitation in KLM solid-state lasers. Impressively, our KLM Yb: YVO 4 laser produced pulses as short as 61 fs at a center wavelength of 1050 nm with an average output power of 54 mW for an absorbed pump power of just 400 mW. The nonlinear refractive indices were measured for the Yb: YVO 4 crystal by using the z-scan technique and were found to be 19 ϫ 10 −16 cm 2 / W and 15ϫ 10 −16 cm 2 / W for and polarizations, respectively, at 1080 nm.A schematic of the laser c...

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We demonstrate, for the first time to our knowledge, femtosecond-regime mode locking of a Tm,Ho-codoped crystalline laser operating in the 2 microm spectral region. Transform-limited 570 fs pulses were generated at 2055 nm by a Tm,Ho:KY(WO(4))(2) laser that produced an average output power of 130 mW at a pulse repetition frequency of 118 MHz. Mode locking was achieved using an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror.

A diode-pumped Yb: YVO 4 laser has been passively mode locked for the first time, to our knowledge. 120 fs pulses with an average output power of 300 mW and a peak power as high as 14.5 kW are obtained by use of a semiconductor saturable-absorber mirror for passive mode locking. The optical spectrum has a 10 nm bandwidth (full width at half-maximum) and is centered at 1021 nm. having a comparatively high thermal conductivity of 11 W / mK. However, the emission bandwidth of this material limited the pulse duration to 700-800 fs, or to 340 fs in a low-power laser.3 Much shorter pulses (near 100 fs and less) were obtained with a number of crystals with a broader emission band, such as Ybdoped KYW, KGW, GdCOB, BOYS, etc., 4-10 and with Yb:glass.11 However, these materials have a low thermal conductivity of approximately 2 -3 W / mK, which severely limits their potential for high-power operation. Recently, what is believed to be the first demonstration of a femtosecond laser based on Yb-doped CaF 2 , which has a thermal conductivity near 10 W / mK, was reported.12 Pulses as short as 150 fs were obtained with this crystal. Very recently, efficient continuous-wave laser operation has been demonstrated with the new laser crystal Yb: YVO 4 , 13,14 which exhibits strong absorption near 985 nm with a bandwidth [full width at half maximum (FWHM)] of ϳ9 nm (that is suitable for pumping by commercially available laser diodes), and a broad and smooth gain spectrum comparable to that of the crystals mentioned above. The thermal conductivity of yttrium vanadate crystals is 5.23 W / mK along the c axis and 5.10 W / mK along the a axis, 15 i.e., lower than in YAG; however, approximately 40% higher than, e.g., in the well-known KGW, 4 whereas the gain spectrum is smoother than for Yb: CaF 2 . Here we report for the first time to our knowledge on femtosecond pulse generation with a diode-pumped Yb: YVO 4 laser that is passively mode locked with a semiconductor saturable-absorber mirror (SESAM). 16,17 The laser experiments were carried out with a simple delta cavity (Fig. 1). To obtain good alignment stability, the laser cavity was designed to operate in stability zone I. 18 As the gain medium we used a 2 -mm-thick Yb: YVO 4 crystal with 3-at. % ytterbium concentration at Brewster incidence. The crystal orientation was chosen for polarization ͑E ʈ c͒ where the absorption and stimulated-emission cross sections have higher values than for polarization, as shown in our previous work.13 Absorption and stimulated-emission cross-section spectra are presented in Fig. 2. An 8-W continuous-wave (cw) fibercoupled diode laser with a core diameter of 100 m and a numerical aperture of 0.22 operated around 980 nm with a spectral bandwidth of 6 nm was used for longitudinal pumping of the gain medium along the a axis. Longitudinal pumping through spherically curved mirror M1 has problems because of the narrow spectral interval (of ϳ40 nm) between the pump and the laser wavelengths. Because we used a standard / 4 coating optimized for high reflectivi...