Self-similar propagation of ultrashort, parabolic pulses in a laser resonator is observed theoretically and experimentally. This constitutes a new type of pulse shaping in mode-locked lasers: in contrast to the well-known static (solitonlike) and breathing (dispersion-managed soliton) pulse evolutions, asymptotic solutions to the nonlinear wave equation that governs pulse propagation in most of the laser cavity are observed. Stable self-similar pulses exist with energies much greater than can be tolerated in solitonlike pulse shaping, and this has implications for practical lasers.

The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.

Rapid progress in passively mode-locked fibre lasers 1-6 is currently driven by the recent discovery of new mode-locking mechanisms, namely, the self-similarly evolving pulse (similariton) 7 and the all-normal-dispersion (dissipative soliton) regimes 8,9 . These are fundamentally different from the previously known soliton 10 and dispersion-managed soliton (stretched-pulse) 11 regimes. Here, we report a fibre laser in which the mode-locked pulse evolves as a similariton in the gain segment and transforms into a regular soliton in the rest of the cavity. To our knowledge, this is the first observation of similaritons in the presence of gain, that is, amplifier similaritons, within a laser cavity. The existence of solutions in a dissipative nonlinear cavity comprising a periodic combination of two distinct nonlinear waves is novel and likely to be applicable to various other nonlinear systems. For very large filter bandwidths, our laser approaches the working regime of dispersionmanaged soliton lasers; for very small anomalous-dispersion segment lengths it approaches dissipative soliton lasers.Passively mode-locked fibre lasers are being used in a diverse range of applications, including optical frequency metrology 12,13 , material processing 14 and terahertz generation 15 . Historically, major advances in laser performance have followed the discovery of new mode-locking regimes [1][2][3][4][5][6]16 , so there is always a strong motivation to search for new regimes.The physics of mode-locked fibre lasers comprises a complex interaction of gain, dispersion and nonlinear effects 17 . Such lasers are a convenient experimental platform for the study of nonlinear waves subject to periodic boundary conditions and dissipative effects. These characteristics profoundly alter the behaviour of nonlinear waves, so this area of research is interesting in its own right. In addition to the vast literature on optical solitons . These similaritons existed in segments of the cavity without any gain and loss to avoid the large spectral broadening that is characteristic of amplifier similaritons. Formation of a self-consistent solution in a laser cavity requires the compensation of spectral broadening, which has proved to be non-trivial 5 . Despite numerical predictions of their existence dating back almost a decade 26 , amplifier similaritons had yet to be observed in a laser cavity.Here, we present our experimental and theoretical work demonstrating an entirely new mode-locking regime, in which the pulse propagates self-similarly in the gain fibre with normal dispersion, and following spectral filtering, gradually evolves into a soliton in the rest of the cavity, where the dispersion is anomalous. All mode-locked lasers to date have had a single type of nonlinear wave propagating within the cavity; however, in our laser, distinctly different similariton and soliton pulses co-exist, demonstrating that transitions between these are possible. Remarkably, this construct is extremely robust against perturbations. Although the pulse expe...

We demonstrate a high-performance, tensile-strained Ge p-i-n photodetector on Si platform with an extended detection spectrum of 650–1605 nm and a 3 dB bandwidth of 8.5 GHz measured at λ=1040nm. The full bandwidth of the photodetector is achieved at a low reverse bias of 1 V, compatible with the low driving voltage requirements of Si ultralarge-scale integrated circuits. Due to the direct bandgap shrinkage induced by a 0.20% tensile strain in the Ge layer, the device covers the entire C band and a large part of the L band in telecommunications. The responsivities of the device at 850, 980, 1310, 1550, and 1605 nm are 0.55, 0.68, 0.87, 0.56, and 0.11A∕W, respectively, without antireflection coating. The internal quantum efficiency in the wavelength range of 650–1340 nm is over 90%. The entire device was fabricated using materials and processing that can be implemented in a standard Si complementary metal oxide semiconductor (CMOS) process flow. With high speed, a broad detection spectrum and compatibility with Si CMOS technology, this device is attractive for applications in both telecommunications and integrated optical interconnects.

We present a 9 GW peak power, three-cycle, 2.2 microm optical parametric chirped-pulse amplification source with 1.5% rms energy and 150 mrad carrier envelope phase fluctuations. These characteristics, in addition to excellent beam, wavefront, and pulse quality, make the source suitable for long-wavelength-driven high-harmonic generation. High stability is achieved by careful optimization of superfluorescence suppression, enabling energy scaling.