Intense femtosecond optical vortices with spatially structured amplitude and spiral phase front give rise to novel phenomena in light-matter interactions and strong-field physics. However, current femtosecond vortex sources exhibit a poor power handling capability and amplification remains an open challenge due to a number of inherent technical difficulties. Here, it is demonstrated that a single-crystal fiber laser amplifier is particularly well-suited to directly amplify a femtosecond optical vortex without pulse stretching and compression in the time domain, while still maintaining the spatial properties associated with a clear central singularity and a spiral phase front, i.e., a well-defined amount of orbital angular momentum (OAM).The optical nonlinearity experienced by such twisted light is verified to be substantially weaker compared to a fundamental mode beam where supercontinuum generation and spatial distortion are observed. The simple design and straightforward power scaling capability pave the way toward ultrahigh-intensity femtosecond singular laser sources with an arbitrary topological charge. Such ultrafast OAM light sources are expected to help reveal complex physical phenomena in light-matter interactions and expand the applications to attoscience with X-ray vortices, laser plasma acceleration, and micromachining.

We present the first, to the best of our knowledge, direct generation of pulsed optical vortices in the 2.7-µ m spectral range by employing polycrystalline Fe:ZnSe as a saturable absorber (SA). A modified theoretical model taking into account the propagation features of the reshaped annular pump beam is elaborated to accurately determine the excitation conditions of the Laguerre–Gaussian (LG0,l) modes, yielding a lasing efficiency comparable to the fundamental TEM00 mode in continuous-wave (CW) regime. Nanosecond scalar optical vortices with well-defined handedness are successfully produced by taking advantages of designated mode-matching, high polarization extinction ratio (PER), and the "spatial filter" effect of the SA on other transverse modes. Such scalar vortex laser pulses in the mid-infrared region will enable new applications such as frequency down conversion to produce optical vortices at longer (far-infrared) wavelengths, structuring organic materials, novel molecular spectroscopy, etc.

We report on a semiconductor saturable absorber mirror mode-locked Tm:(Lu,Sc)2O3 ceramic laser in-band pumped by a Raman fiber laser at 1627 nm. The nonlinear refractive index (n2) of the Tm:(Lu,Sc)2O3 ceramic has been measured to be 4.66 × 10−20 m2/W at 2000 nm. An average output power up to 1.02 W at 2060 nm is achieved for transform-limited 280-fs pulses at a repetition rate of 86.5 MHz, giving an optical efficiency with respect to the absorbed pump power of 36.4%. Pulses as short as 66 fs at 2076 nm are produced at the expense of output power (0.3 W), corresponding to a spectral bandwidth of 69 nm. The present work reveals the potential of Tm3+-doped sesquioxide transparent ceramics for power scaling of femtosecond mode-locked bulk lasers emitting in the 2-µm spectral range.

The rapidly-developed high-power mid-infrared 2-5 µm laser technology requires a compact, flexible low-loss glass fiber for power delivery or laser generation. With the broadest bandwidth of low-loss transmission window in mid-infrared region amongst all mid-infrared glass fibers, chalcogenide glass fiber is the best candidate covering the whole 2-5 µm range. Multi-hundred-watt high-power delivery for 5.4-µm CO laser was previously demonstrated in a multimode chalcogenide fiber with a 1-mm-diameter large core, at the cost of giving up one of the most desirable fiber advantages, the flexibility. Indeed, chalcogenide glass fibers with decent flexibility have never exhibited hundred-watt-level power transmitting capability in the 2-5 µm range. In this paper, we have experimentally demonstrated 100-watt-level power transmission in multimode As2S3 chalcogenide fibers, using a customized high-power 2-µm thulium doped silica fiber laser source. With effective forced cooling, the multimode As2S3 fiber with 200 µm core diameter can resist incident laser power of 120 W and deliver transmitted power of 63 W. Nano-sized scattering center related laser damage mechanism and the cylindrical heat transfer model have been proposed to explain the high-power damage process of chalcogenide glass fibers. The calculation is in good agreement with the experiments. It is promising to further enhance the transmitted power above 100 W in flexible chalcogenide glass large-core fibers.

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