A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

We demonstrate methods to increase the energy incident on hollow fibers for spectral broadening by self-phase modulation. We used chirped pulses for spectral broadening, lowering the optical intensity to avoid ionization of the gaseous medium. We also used helium as a nonlinear medium and demonstrated the generation of 5.0fs, 5.0mJ pulses at a repetition rate of 1kHz using a pressure gradient hollow-fiber pulse compressor.

Generation of 5.0 fs, 5.0 mJ pulses at 1 kHz using hollow-fiber pulse compression

Carrier-envelope phase stable 4 fs near-IR pulses with 3 mJ energy were generated by spectral broadening of circularly polarized 8 mJ pulses in a differentially pumped 2 m long composite stretched flexible hollow fiber. The pulses were characterized using both second-harmonic generation frequency-resolved optical gating (SHG-FROG) and SHG d-scan methods.

https://iopscience.iop.org/article/10.1088/1612-2011/11/9/095401/pdf

Compression of CEP-stable multi-mJ laser pulses down to 4 fs in long hollow fibers

In the past two decades high-harmonic generation (HHG) has become a key process in ultrafast science due to the extremely short time structure of the underlying electron dynamics being imprinted in the emitted harmonic light bursts. After discussing the fundamental physical picture of HHG including continuum–continuum transitions, we describe the experimental progress rendering HHG a unique source of attosecond pulses. The development of bright photon sources with zeptosecond pulse duration and keV photon energy is underway. In this chapter, we describe several approaches directed toward this aim and beyond. As the main barriers for multi-keV HHG, phase-matching and relativistic drift are discussed. Routes to overcome these problems are pointed out as well as schemes to control the HHG process via alterations of the driving fields. Finally, we report on how the investigation of fundamental physical processes benefits from the continuous development of HHG sources.

Frontiers of Atomic High-Harmonic Generation

The use of multipass cells as a way to spatially homogenize self-phase modulation and distribute its accumulation over the propagation distance is analyzed in detail, with the aim to perform nonlinear temporal compression In addition to the insertion of nonlinear media at specific locations in the cell, as already demonstrated, we also propose to fill the cell with a noble gas, as is done in hollow capillary-based setups This makes the accumulation of B-integral continuous rather than discrete In this case, analytical estimates for the B-integral per round trip and scaling rules are provided as a function of cavity geometry and gas parameters Then, three-dimensional numerical simulations are performed to assess the spatiotemporal couplings in the output beam in various conditions This model is checked against experimental data presented in the literature, and used to predict our proposed scheme performance We believe that these techniques constitute a promising way to allow temporal compression at energy levels beyond 10 mJ, where capillary-based setups are difficult to implement

Nonlinear temporal compression in multipass cells: theory

Pulse compression in a differentially pumped neon-filled hollow fiber was used to generate high-power few-cycle laser pulses. The pulse compression process was optimized by adjusting gas pressure and laser chirp to produce the shortest laser pulses. Precise dispersion control enabled the generation of laser pulses with duration of 3.7 fs and energy of 1.2 mJ. This corresponds to an output of 1.5 cycle, 0.3 TW pulses at a 1 kHz repetition rate using positively chirped 33 fs laser pulses.

Generation of 1.5 cycle 0.3 TW laser pulses using a hollow-fiber pulse compressor.

By generating broadband high-harmonic pulses from neon and compensating for attosecond chirp by the material dispersion of argon, the generation of near transform-limited 63 as pulses was achieved. The spectral phase analysis showed that, without proper compensation, the attosecond chirp of the broadband harmonics caused splitting of attosecond high-harmonic pulses in addition to pulse broadening. Although it was attained only within a limited spectral range, the attosecond chirp compensation was successful in bringing out pulse compression over broad harmonics, which signifies the effectiveness of the attosecond chirp compensation by material dispersion.

/pdf/attosecond-chirp-compensation-over-broadband-high-order-59scxaafi0.pdf

Attosecond chirp compensation over broadband high-order harmonics to generate near transform-limited 63 as pulses

The influence of laser chirp on the formation of femtosecond laser filamentation in Ar was investigated for the generation of few-cycle high-power laser pulses The condition for the formation of a single filament has been carefully examined using 28-fs laser pulses with energy over 3 mJ The filament formation and output spectrum changed very sensitively to the initial laser chirp and gas pressure Much larger spectral broadening was obtained with positively chirped pulses, compared to the case of negatively chirped pulses that generated much longer filament, and compressed pulses of 55 fs with energy of 05 mJ were obtained from the filamentation of positively chirped 30-fs laser pulses in a single Ar cell

Laser chirp effect on femtosecond laser filamentation generated for pulse compression.

We have developed a practical solution to implement the direct locking method for the carrier-envelope phase (CEP) stabilization of femtosecond laser pulses and achieved 24-hour CEP stabilization without realignment of any optical components. The direct locking method realizes the CEP stabilization in the time domain by directly quenching the beat signal from an f-to-2f interferometer and, thereby, locking every pulse to a same CEP. We have accomplished the long-term CEP stabilization using commercially available standard feedback electronics, and maintained the CEP stabilization with low jitter without using any frequency-analyzing components, greatly facilitating the accessibility of the CEP stabilization.

Long-term carrier-envelope-phase stabilization of a femtosecond laser by the direct locking method

A soft x-ray microscope based on a phase-reversal zone plate was constructed and tested using high harmonic radiation as a coherent light source. The 61st harmonic centered at 13.3 nm was optimized in spectral sharpness and intensity by controlling the incident laser energy and chirp. A phase-reversal zone plate made of polymethyl methacrylate more than doubled the first-order efficiency. The nano patterns, imaged on an x-ray CCD with a magnification of 650, showed that the measured resolution of the microscope was better than 100 nm.

Jae-hwan Lee

Papers

Generation of 1.5 cycle 0.3 TW laser pulses using a hollow-fiber pulse compressor.

Attosecond chirp compensation over broadband high-order harmonics to generate near transform-limited 63 as pulses

Laser chirp effect on femtosecond laser filamentation generated for pulse compression.

Long-term carrier-envelope-phase stabilization of a femtosecond laser by the direct locking method

Soft x-ray microscope constructed with a PMMA phase-reversal zone plate