Thermal effects, which limit the average power, can be minimized by using low-doped, longer gain fibers, whereas the presence of nonlinear effects requires use of high-doped, shorter fibers to maximize the peak power. We propose the use of varying doping levels along the gain fiber to circumvent these opposing requirements. By analogy to dispersion management and nonlinearity management, we refer to this scheme as doping management. As a practical first implementation, we report on the development of a fiber laser-amplifier system, the last stage of which has a hybrid gain fiber composed of high-doped and low-doped Yb fibers. The amplifier generates 100 W at 100 MHz with pulse energy of 1 μJ. The seed source is a passively mode-locked fiber oscillator operating in the all-normal-dispersion regime. The amplifier comprises three stages, which are all-fiber-integrated, delivering 13 ps pulses at full power. By optionally placing a grating compressor after the first stage amplifier, chirp of the seed pulses can be controlled, which allows an extra degree of freedom in the interplay between dispersion and self-phase modulation. This way, the laser delivers 4.5 ps pulses with ~200 kW peak power directly from fiber, without using external pulse compression.

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We report on the generation of 42 fs pulses at 1 μm in a completely fiber-integrated format, which are, to the best of our knowledge, the shortest from all-fiber-integrated Yb-doped fiber lasers to date. The ring fiber cavity incorporates anomalous-dispersion, solid-core photonic crystal fiber with low birefringence, which acts as a broadband, in-fiber Lyot filter to facilitate mode locking. The oscillator operates in the stretched-pulse regime under slight normal net cavity dispersion. The cavity generates 4.7 ps long pulses with a spectral bandwidth of 58.2 nm, which are dechirped to 42 fs via a grating pair compressor outside of the cavity. Relative intensity noise (RIN) of the laser is characterized, with the integrated RIN found to be 0.026% in the 3 Hz-250 kHz frequency range.

We report on the development of, to the best of our knowledge, the first ultrafast burst-mode laser system operating at a central wavelength of approximately 2 μm, where water absorption and, consequently, the absorption of most biological tissue is very high. The laser comprises a harmonically mode-locked 1-GHz oscillator, which, in turn, seeds a fiber amplifier chain. The amplifier produces 500 ns long bursts containing 500 pulses with 1 GHz intra-burst and 50 kHz inter-burst repetition rates, respectively, at an average power of 1 W, corresponding to 40 nJ pulse and 20 μJ burst energies, respectively. The entire system is built in an all-fiber architecture and implements dispersion management such that output pulses are delivered directly from a single-mode fiber with a duration of 340 fs without requiring any external compression. This gigahertzrepetition-rate system is intended for ablation-cooled laser material removal in the 2 μm wavelength region, which is interesting for laser surgery due to the exceptionally high tissue absorption at this wavelength. Fiber lasers with all-fiber architecture, guiding light through optical fibers to the point of exit from the laser system, continue to attract attention as practical and robust sources for various applications such as material processing, spectroscopy, and metrology, to name a few. Material processing with ultrafast pulses has superb aspects such as minimal collateral damage and high precision with the downsides being slow process speed and the requirement of complex laser systems [1]. The ablation-cooled laser material regime demonstrated recently [2] opened the door to transcending the limited ablation rates and the need for tens to hundreds of microjoules which have been impairing ultrafast laser material processing for industrial usage. In this regime, the repetition rate has to be high enough so that there is insufficient time for the targeted spot size to cool down substantially by heat conduction into the rest (bulk) of the target material by the time the next pulse arrives. As a result, the individual pulse energy ablation threshold is scaled down by several orders of magnitude if the repetition rate is simultaneously increased, while the thermal effects to the bulk of the target are also reduced. Furthermore, the simultaneous reduction of pulse energy and the pulse-to-pulse spacing reduces the plasma shielding effects. This is a breakthrough which opens the door to the simplification of ultrafast lasers built for material processing and, hence, boosts the usage of ultrafast fiber lasers outside the laboratory and, consequently, the proliferation of such lasers.Among the many applications, laser surgery is a promising area for ultrafast laser ablation because it can enable precise cutting with minimal collateral damage and improved after-operation healing process [3]. One can foresee that, with laser systems operating around 2 μm, the ablation cooling regime will take its effect on water-rich soft tissues since laser-tissue interaction is strongly ...

We report on the experimental characterization and theoretical prediction of pulse-to-pulse intensity fluctuations, namely, intensity noise, for ultrafast fiber amplifiers. We present a theoretical model with which the intensity noise of a Yb-doped fiber amplifier can be predicted with high accuracy, taking into account seed and pump noise, as well as generation of amplified spontaneous emission. Transfer of pump and seed signal modulations to the amplified output during fiber amplification are investigated thoroughly. Practically, our model enables design and optimization of fiber amplifiers with regards to their intensity noise performance. As a route to reducing noise imparted by pump diodes in a double-clad amplifier, we show the use of multiple, low-power diodes is more beneficial compared to a single, high-power diode due to the largely uncorrelated nature of their individual noise contributions. © 2014 AIP Publishing LLC