Günther Krauss

8-fs pulses from a compact Er:fiber system : quantitative modeling and experimental implementation

We demonstrate an all-fiber turnkey source of extremely stable 2-cycle pulses at a center wavelength of 1.17 microm. Taylor-cut highly nonlinear germano-silica bulk fibers (HNFs) provide smooth supercontinua with a bandwidth of 560 nm and a spectral shape precisely controlled by the dispersion of the fiber and the phase of the 1.55 microm pump pulses. Alternatively these fibers are capable of generating pulses with central wavelengths continuously tunable from 0.9 microm up to 1.4 microm. These results are based on parameter-free simulations of nonlinear pulse propagation including higher-order dispersion as well as instantaneous Kerr and retarded Raman contributions.

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).

All-passive phase locking of a compact Er:fiber laser system

A passively phase-locked laser source based on compact femtosecond Er:fiber technology is introduced. The carrier-envelope offset frequency is set to zero via difference frequency generation between a soliton at a wavelength of 2 μm and a dispersive wave at 860 nm generated in the same highly nonlinear fiber. This process results in a broadband output centered at 1.55 μm. Subsequently, the 40 MHz pulse train seeds a second Er:fiber amplifier, which boosts the pulse energy up to 8 nJ at a duration of 125 fs. Excellent phase stability is demonstrated via f-to-2f spectral interferometry.

Femtosecond coherent seeding of a broadband Tm:fiber amplifier by an Er:fiber system.

We generate broadband pulses covering the Yb: and Tm:silica amplification ranges with a passively phase-locked front end based on Er:fiber technology. Full spectral coherence of the octave-spanning output from highly nonlinear germanosilicate bulk fibers is demonstrated. Seeding of a high-power Tm:fiber generates pulses with a clean spectral shape and a bandwidth of 50 nm at a center wavelength of 1.95 μm, pulse energy of 250 nJ, and repetition rate of 10 MHz.

Simultaneous second-harmonic generation, third-harmonic generation, and four-wave mixing microscopy with single sub-8 fs laser pulses

We demonstrate a compact pulse compression scheme that offers flexible chirp control for improved conversion efficiencies in high resolution nonlinear optical microscopy. An Er:fiber laser combined with a highly nonlinear optical fiber yields pulses centered at 1100 nm with a bandwidth of 500 nm. The compressed pulses with a duration of 7.6 fs enable simultaneous second-harmonic generation, third-harmonic generation, and four-wave mixing microscopy. The spectrum is tailored for an ideal compromise between conversion efficiency and spectral discrimination between the three types of broadband nonlinear signals. Distinct differences in structural contrast obtained from the simultaneous read-out of the three nonlinear signals are demonstrated in a biological sample.

Non-iterative characterization of few-cycle laser pulses using flat-top gates

We demonstrate a method for broadband laser pulse characterization based on a spectrally resolved cross-correlation with a narrowband flat-top gate pulse. Excellent phase-matching by collinear excitation in a microscope focus is exploited by degenerate four-wave mixing in a microscope slide. Direct group delay extraction of an octave spanning spectrum which is generated in a highly nonlinear fiber allows for spectral phase retrieval. The validity of the technique is supported by the comparison with an independent second-harmonic fringe-resolved autocorrelation measurement for an 11 fs laser pulse.

Single-cycle light pulses from a compact Er:fiber laser

We demonstrate a setup based entirely on compact Er:fiber technology which synthesizes pulses of a duration of 4.3 fs. This value corresponds to a single cycle of light in the telecom wavelength region.

Tm:fiber amplifier coherently seeded by femtosecond Er:Fiber technology

Broadband seeding of a femtosecond Tm:fiber amplifier based on passively phase-locked Er:fiber technology is demonstrated. Excellent coherence properties of the seed are observed experimentally and analyzed theoretically.

All passively phase-locked Er:fiber laser system

The generation of phase-locked pulses with a well-defined electric field is essential for applications like high harmonic pulse generation [1] or precision metrology [2]. Active and passive locking schemes have been widely demonstrated utilizing Ti:sapphire technology. However, active stabilization requires cumbersome feedback loops and a locking of fCEO = 0 is very challenging at the full repetition rate of the laser.

Coherent anti-Stokes Raman scattering microspectroscopy based on a compact Er:fiber laser

A two branch Er:fiber laser was developed for coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The compact and highly stable light source allows for fast single-frequency CARS microspectroscopy with a wide tuning range from 1150 cm-1 up to 3800 cm-1. Single-pass frequency conversion enables easy tunability. The spectral selectivity of the system is shown using polymer beads. Imaging of biological samples is demonstrated on C. elegans and yeast cells. Modification of the light source allows for broadband background-free CARS microspectroscopy. Impulsive excitation of molecular resonances is achieved using an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm-1 up to 4000 cm-1. Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal, free of nonresonant background with a resolution of 10 cm-1.