J. Jasapara

Full-field characterization of femtosecond pulses by spectrum and cross-correlation measurements

We present a practical and accurate technique for retrieving the amplitude and the phase of ultrashort pulses from a nonlinear (second-order) intensity cross correlation and the spectrum that overcomes shortcomings of previous attempts. We apply the algorithm to theoretical and experimental data and compare it with frequency-resolved optical gating.

Refractive index perturbations in optical fibers resulting from frozen-in viscoelasticity

We show that frozen-in viscoelasticity has a significant impact on the refractive index profile of optical fibers. Viscoelastic strains can be frozen into an optical fiber during draw, or by cooling down a drawn fiber from its fictive temperature while under tension. The resulting refractive index perturbation is concentrated in the highest viscosity region of the fiber, exhibits minimal birefringence, and can significantly alter critical performance characteristics. Our quantitative theory for the refractive index perturbation is in excellent agreement with measured data. Frozen-in viscoelasticity can also be harnessed to serve as the basis for optical fiber devices.

On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.

Characterization of sub-10-fs pulse focusing with high-numerical-aperture microscope objectives

Dispersion precompensation with a prism sequence and a third-order dispersion mirror resulted in negligible broadening of sub-10-fs pulses at subwavelength spot sizes when the pulses were focused with microscope objectives and moderate apertures. At larger apertures, lens chromaticity and spherical aberration led to an effective pulse broadening of up to 1.3x , depending on the aperture size and the detector position. The data suggest that intensities exceeding 10(14) W/cm(2) can be produced directly from femtosecond pulse oscillators.

High-energy (nanojoule) femtosecond pulse delivery with record dispersion higher-order mode fiber

Delivery of high peak-power femtosecond pulses with fibers is constrained by nonlinear distortions accumulated during pulse propagation. We address this problem with a novel, to our knowledge, fiber schematic, where the pulse propagates in a small Aeff (18 microm2) but highly dispersive (record value of approximately -900 ps/nm km) medium, enabled by transmission in the LP02 mode of a few-mode fiber. The novel fiber yields a low dispersion-to-nonlinear-length ratio (due to its large dispersion) despite its small Aeff, hence enabling mitigation of nonlinearities. This enables fiber delivery of distortion-free <150 fs, approximately 1 nJ, and 840 nm pulses--an order-of-magnitude improvement over single-mode fibers of similar Aeff.

Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber

Amplification in a single-clad, large-mode-area erbium fiber as an alternative to double-clad Er-Yb amplifiers is presented. Both signal and pump are coupled through a mode-matched splice into the fundamental mode, which ensures preferential gain in the fundamental mode while minimizing the amplified spontaneous emission (ASE). The 875 microm(2) effective area of the Er fiber enables amplification of 6 ps pulses at 1.55 microm wavelength by approximately 33 dB in a single stage to >25 kW peak power with low nonlinear pulse distortion and a diffraction-limited output beam with M(2)<1.1.

Effect of heat and H 2 gas on the photo-darkening of Yb +3 fibers

Heating a photo-darkened ytterbium fiber causes a recovery in spectral transmission to the pre-darkened state. Exposure to H2 at high temperatures causes a permanent change in the visible transmission identical to that caused by photo-darkening.

Unbalanced third-order correlations for full characterization of femtosecond pulses.

Phase and intensity from correlation and spectrum only (PICASO) is a simple and accurate method for fully characterizing femtosecond pulses. We extend this technique to make use of third-order intensity correlations. Unbalancing the interferometer makes the retrieval algorithm sensitive to the direction of time and improves the retrieval of certain pulse shapes. We investigate the sensitivity of the retrieval to direction of time as a function of the degree of imbalance in the correlator.

Accurate noncontact optical fiber diameter measurement with spectral interferometry.

A new technique using spectral interferometry to measure optical fiber dimensions with interferometric accuracy better than 0.01 micrometer is presented. The method can potentially be used for online monitoring and control during fiber draw.

Large-mode-area Er-doped fiber chirped-pulse amplification system for high-energy sub-picosecond pulses at 1.55 μm

We demonstrate a chirped-pulse amplification system generating 25 μJ compressed pulses at a center wavelength of 1552.5 nm. The seed module and the amplifier chain are all in-fiber (with a few small fiber-pigtailed free-space components), followed by a free-space diffraction grating pulse compressor. The amplifier chain contains a pre-amplifier and a booster whose gain fibers are 45/125 μm core/cladding-diameter, core-pumped Er-doped fibers. The pump lasers for both amplifiers are single-mode 1480 nm Raman lasers capable of up to 8 W output. The seed module generates up to 2 ns chirped pulses that are amplified and subsequently compressed to <800 fs duration. At a repetition rate of 50 kHz, the 2 ns pulses from the seed module were amplified to 72 μJ, resulting in 25 μJ after pulse compression. The corresponding peak power levels after the amplifier chain and compressor were 36 kW and 31 MW, respectively.