Scaling rules for high quality soliton self-compression in hollow-core fibers.

Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber.

Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient nonlinear temporal compression of femtosecond laser pulses, two main schemes being direct soliton-effect self-compression and spectral broadening followed by phase compensation. To obtain stable compressed pulses, it is crucial to avoid decoherence through modulational instability (MI) during spectral broadening. Here, we show that changes in dispersion due to spectral anti-crossings between the fundamental-core mode and core wall resonances in anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum, enabling MI-free pulse compression for optimized fiber designs. The results are important, since MI cannot always be suppressed by pumping in the normal dispersion regime.

Carrier-envelope-phase-stable soliton-based pulse compression to 4.4 fs and ultraviolet generation at the 800 kHz repetition rate.

In this Letter, we report the generation of a femtosecond supercontinuum extending from the ultraviolet to the near-infrared spectrum and detection of its carrier-envelope-phase (CEP) variation by f-to-2f interferometry. The spectrum is generated in a gas-filled hollow-core photonic crystal fiber, where soliton dynamics allows the CEP-stable self-compression of the optical parametric chirped-pulse amplifier pump pulses at 800 nm to a duration of 1.7 optical cycles, followed by dispersive wave emission. The source provides up to 1 µJ of pulse energy at the 800 kHz repetition rate, resulting in 0.8 W of average power, and it can be extremely useful, for example in strong-field physics, pump-probe measurements, and ultraviolet frequency comb metrology.

Broadband electric-field-induced LP01 and LP02 second harmonic generation in Xe-filled hollow-core PCF.

Second harmonic (SH) generation with 300 fs pump pulses is reported in a xenon-filled hollow-core photonic crystal fiber (PCF) across which an external bias voltage is applied. Phase-matched intermodal conversion from a pump light in the LP01 mode to SH light in the LP02 mode is achieved at a particular gas pressure. Using periodic electrodes, quasi-phase-matched SH generation into the low-loss LP01 mode is achieved at a different pressure. The low linear dispersion of the gas enables phase-matching over a broad spectral window, resulting in a measured bandwidth of ∼10 nm at high pump energies. A conversion efficiency of ∼18%/mJ is obtained. Gas-filled anti-resonant-reflecting hollow-core PCF uniquely offers pressure-tunable phase-matching, ultra-broadband guidance, and a very high optical damage threshold, which hold great promise for efficient three-wave mixing, especially in difficult-to-access regions of the electromagnetic spectrum.

An advanced algorithm for dispersion encoded full range frequency domain optical coherence tomography.

Dispersion encoded full range (DEFR) optical coherence tomography (OCT) has become highly attractive as it is a simple way to increase the measurement range of OCT systems. Full range OCT is especially favorable as it does not only increase the measurement range but also shifts the highest sensitivity into the center of the measurement range. While the early versions of DEFR were highly computational expensive, new versions reduce the number of necessary Fourier transforms. Recently it has been shown that a GPU based algorithm can perform DEFR with more than 20,000 A-lines per second. We present a new version of the DEFR algorithm that requires only one Fourier transform per A-scan and uses convolution in z-space instead of multiplication in k-space, therefore reducing the computational effort considerably. While dispersion encoding has so far only been used to suppress mirror artifacts, we show that, with dispersion encoding and only one more Fourier transform, autocorrelation terms can be removed likewise. Since very high values of dispersion reduce the effective measurement range in dispersion encoded OCT, we present an estimate for a sufficient amount of dispersion for a successful image recovery, which is depending on the thickness of the scattering layers. Furthermore, we demonstrate the usability of ZnSe as a new dispersive material with a very high dispersion and describe a simple method to extract the dispersive phase from the measurement of a single reflex of a glass surface. Using a standard consumer PC, an artifact-free recovery of 1000 - 2000 A-scans per second with 2048 depth values including autocorrelation removal was achieved. The dynamic range (sensitivity) is not reduced and the suppression ratio of mirror artifacts and autocorrelation signals is more than 50dB using ZnSe.