Sven Dobner

Experimental realization of femtosecond transverse mode conversion using optically induced transient long-period gratings

We present the experimental realization of transverse mode conversion in an optical fiber via an optically induced long-period grating. The transient gratings are generated by femtosecond laser pulses, exploiting the Kerr effect to translate intensity patterns emerging from multimode interference into a spatial refractive index modulation. Since these modulations exist only while the pump beam is present, they can be used for optical switching of transverse modes. As only a localized part of the grating was written at a time and the probe beam was co-propagating with the pump beam the required pulse energies could be reduced to 120 nJ which is about a factor of 600 lower than in previous quasi-continuous-wave experiments. Accompanying numerical simulations allow a better understanding of the involved effects and show excellent agreement to the experimental results.

In-line interferometric femtosecond stimulated Raman scattering spectroscopy

We present in-line interferometric femtosecond stimulated Raman scattering (II-FSRS), a new method to measure the spectral Raman intensity and phase over a broad spectral range, potentially in a single shot. An analytic model is developed, that excellently reproduces the measured spectra. Additionally, the performance of II-FSRS is directly compared in experiments to two established techniques, namely femtosecond stimulated Raman scattering and femtosecond Raman induced Kerr-effect spectroscopy.

Interferometric background reduction for femtosecond stimulated Raman scattering loss spectroscopy

We present a purely optical method for background suppression in nonlinear spectroscopy based on linear interferometry. Employing an unbalanced Sagnac interferometer, an unprecedented background reduction of 17  dB over a broad bandwidth of 60  THz (2000  cm(-1)) is achieved and its application to femtosecond stimulated Raman scattering loss spectroscopy is demonstrated. Apart from raising the signal-to-background ratio in the measurement of the Raman intensity spectrum, this interferometric method grants access to the spectral phase of the resonant χ(3) contribution. The spectral phase becomes apparent as a dispersive lineshape and is reproduced numerically with a simple oscillator model.

Electronically and rapidly tunable fiber-integrable optical parametric oscillator for nonlinear microscopy

We present a fiber-based optical parametric oscillator (FOPO) pumped by a fiber-coupled laser diode. The FOPO consisted of a photonic crystal fiber to convert the pump pulses via four-wave mixing and a dispersive resonator formed by a single-mode fiber. Via dispersion filtering, output pulses with a bandwidth of about 3 nm, a temporal duration of about 8 ps and a pulse energy of up to 22 nJ could be generated. By changing the repetition frequency of the pump laser diode by about ±1  kHz, the wavelength of the output pulses could be tuned between 1130 and 1310 nm within 8 μs, without the need to change the length of the resonator. Therewith, the FOPO should especially be suited for hyperspectral imaging, while its all-electronic control constitutes a promising approach to a turnkey and alignment-free light source.

Hyperspectral imaging with in-line interferometric femtosecond stimulated Raman scattering spectroscopy.

We present the hyperspectral imaging capabilities of in-line interferometric femtosecond stimulated Raman scattering. The beneficial features of this method, namely, the improved signal-to-background ratio compared to other applicable broadband stimulated Raman scattering methods and the simple experimental implementation, allow for a rather fast acquisition of three-dimensional raster-scanned hyperspectral data-sets, which is shown for PMMA beads and a lipid droplet in water as a demonstration. A subsequent application of a principle component analysis displays the chemical selectivity of the method.

Light source for narrow and broadband coherent Raman scattering microspectroscopy.

We present a light source that is well adapted to both narrow- and broadband coherent Raman scattering (CRS) methods. Based on a single oscillator, the light source delivers synchronized broadband pulses via supercontinuum generation and narrowband, frequency-tunable pulses via four-wave mixing in a photonic crystal fiber. Seeding the four-wave mixing with a spectrally filtered part of the supercontinuum yields high-pulse energies up to 8 nJ and the possibility of scanning a bandwidth of 2000  cm(-1) in 25 ms. All pulses are emitted with a repetition frequency of 1 MHz, which ensures efficient generation of CRS signals while avoiding significant damage of the samples. Consequently, the light source combines the performance of individual narrow- and broadband CRS light sources in one setup, thus enabling hyperspectral imaging and rapid single-resonance imaging in parallel.

Cross-polarized femtosecond stimulated Raman scattering spectroscopy

We present a method to retrieve the spectral intensity and phase of a Raman resonance over a broad spectral range. In coherent Raman scattering, the Raman transition is stimulated by two light waves (pump and probe) with a difference frequency matching the resonance. Common methods like stimulated Raman scattering (SRS) or Raman-induced Kerr effect spectroscopy (RIKES, [1]) measure the Raman intensity spectrum by scanning the difference frequency of two narrow bandwidth pulses across the region of interest. Techniques like optical heterodyne-detected RIKES [2] or the recently developed balanced-detection RIKES [3] additionally grant access to the phase, but still, the pulses need to be scanned in frequency. In femtosecond SRS (FSRS, [4]) or broadband RIKES [5], a wide intensity spectrum is measured by employing a broadband probe pulse.

Stimulated Emission Pumping Enablling Sub-Diffraction-Limited Spatial Resolution in CARS Microscopy

Suppression of CARS signal generation is demonstrated by equalization of the ground and Raman states via a control state in a theoretical investigation. Using donut-shaped control light fields for population transfer results in sub-diffraction-limited spatial resolution CARS microscopy.

Sagnac Interferometer for Background Reduction in Stimulated Raman Scattering Loss Spectroscopy

We use a Sagnac interferometer for an unprecedented background reduction of 17dB in stimulated Raman scattering (SRS) loss spectroscopy employing a 1MHz ytterbium fiber laser/amplifier system.