We theoretically and experimentally show coherent pulse stacking (CPS) can accommodate tens-of-fs pulse durations and has negligible stacking fidelity degradation with increased pulse bandwidth. Simulations prove large number of tens-of-fs pulses can be stacked with high pre-pulse contrast. In an experiment, nine spectrally broadened and fiber amplified pulses are stacked using four cascaded cavities. CPS of pulses with different spectral bandwidths, up to 75 nm base-to-base (<50 fs transform-limited duration), are tested, showing negligible stacking degradation due to increased bandwidth. This work provides a path towards high energy, tens-of-fs pulses from ultrafast fiber lasers.
We demonstrated 55-fs pulses from spectrally combining two chirped-pulse fiber channels operating at partially-overlapped spectral bands, with a pulse shaper incorporated in each channel. The spectral intensity and phase shaping in two fiber channels are coherently-spectrally synthesized by phase-synchronizing the two channels at the overlapped spectrum. To the best of our knowledge, 55 fs is the shortest pulse duration from a spectrally combined fiber system at one-micron Yb wavelength, and this work is the first demonstration of coherent spectral synthesis of two pulse shapers. This work provides a promising path toward high-energy, tens-of-fs fiber chirped-pulse amplifier systems.
We have developed a scalable, ultrafast laser beam combination scheme, which can combine many beams using two diffractive optics. A feature of this approach is the information contained in the uncombined output beams, which can be used to derive phase error information. We show that a machine-learning algorithm can learn to stabilize beam combination with high efficiency, by finding correlations between uncombined output beam patterns and phase errors.
We demonstrate, to our knowledge, the first operation of a simultaneous spatial combining and CPSA system that provides 9mJ from two amplifier channels and coherently stacks 81 pulses to a single pulse and compresses the output to ~515fs duration. This demonstrates that CPSA with spatial coherent combining enables energy scaling of ultrashort pulses with fewer parallel channels. Future work will involve increasing energy per channel, average power per channel, the number of fiber channels and reducing the pulse duration.
KEYWORDS: Fiber amplifiers, Ultrafast phenomena, Optical filters, Amplifiers, Linear polarizers, Linear filtering, Electronic filtering, Control systems
We report demonstration of a new spectrally-controllable device, based on a sequence of linear polarizers and birefringent plates, which allows to accurately and adjustably tailor its spectral filtering properties for achieving complete gain-narrowing compensation over ~30nm of signal bandwidth in an Yb-doped fiber system with the total gain reaching 150dB. The experimental demonstration was performed in a regenerative Yb-fiber amplifier system with controllable number of passes, allowing to characterize both signal spectral-narrowing, and as well as spectral compensation at varying levels of achieved total gain. This result opens a pathway towards 100fs duration multi-mJ pulses from fiber CPSA systems.
10mJ energy extraction from a single Yb-doped 85µm core CCC fiber has been achieved using coherent pulse stacking amplification (CPSA) technique. This has been achieved by amplifying a burst of 81 stretched pulses with modulated amplitudes and phases, in a Yb-doped fiber CPA system where it is amplified to 10mJ with low nonlinearity, and coherently stacked into a single pulse with 4+4 cascading GTI cavities. The burst is generated by sending femtosecond pulses from a 1GHz repetition rate mode-locked fiber oscillator into a pair of amplitude and phase electro-optic modulators, where the burst is carved out and pre-shaped to compensate for strong saturation effect in fiber CPA system and to provide correct relative phases for coherent pulse stacking. After each pulse is stretched to approximately 1-ns, the burst is amplified through several cascading fiber amplifiers and down-counted to 1kHz repetition rate, and it extracts >90% stored energy from the last Yb-doped 85µm core CCC fiber. This multi-mJ burst of 81 pulses is then coherently stacked into a single pulse in 4+4 multiplexed GTI cavities consisting of 4 sets of 1ns-roundtrip cavities followed by 4 sets of 9-ns roundtrip cavities. After stacking, the stretched pulse is compressed to <540fs using diffraction-grating pulse compressor. CPSA enables generation of multi-mJ femtosecond pulses with one fiber amplifier channel.
We report multi-mJ energy (>5mJ) extraction from femtosecond-pulse Yb-doped fiber CPA using coherent pulse stacking amplification (CPSA) technique. This high energy extraction has been enabled by amplifying 10’s of nanosecond long pulse sequence, and by using 85-µm core Yb-doped CCC fiber based power amplification stage. The CPSA system consists of 1-GHz repetition rate mode-locked fiber oscillator, followed by a pair of fast phase and amplitude electro-optic modulators, a diffraction-grating based pulse stretcher, a fiber amplifier chain, a GTI-cavity based pulse stacker, and a diffraction grating pulse compressor. Electro-optic modulators are used to carve out from the 1-GHz mode-locked pulse train an amplitude and phase modulated pulse burst, which after stretching and amplification, becomes equal-amplitude pulse burst consisting of 27 stretched pulses, each approximately 1-ns long. Initial pulse-burst shaping accounts for the strong amplifier saturation effects, so that it is compensated at the power amplifier output. This 27-pulse burst is then coherently stacked into a single pulse using a multiplexed sequence of 5 GTI cavities. The compact-footprint 4+1 multiplexed pulse stacker consists of 4 cavities having rountrip of 1 ns, and one Herriott-cell folded cavity - with 9ns roundtrip. After stacking, stretched pulses are compressed down to the bandwidth-limited ~300 fs duration using a standard diffraction-grating pulse compressor.
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