Maximilian Brinkmann

Ultrafast two-color all-optical transverse mode conversion in a graded-index fiber

We demonstrate ultrafast, all-optical conversion of transverse modes in a novel dual-wavelength setup. Mode conversion of picosecond probe pulses by optically induced transient long-period gratings using sub-picosecond control pulses is directly observable for the first time.

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.

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.

Phase-dependent spectral control of pulsed modulation instability via dichromatic seed fields

We investigated experimentally and numerically the spectral control of modulation instability (MI) dynamics via the initial phase relation of two weak seed fields. Specifically, we show how second-order MI dynamics exhibit phase-dependent anti-correlated growth rates of adjacent spectral sidebands. This effect enables a novel method to control MI-based frequency conversion: in contrast to first-order MI dynamics, which exhibit a uniform phase dependence of the growth rates, second-order MI dynamics allow to redistribute the spectral energy, leading to an asymmetric spectrum. Therefore, the presented findings should be very attractive to different applications, such as phase-sensitive amplification or supercontinuum generation initiated by MI.

Electronically tunable femtosecond all-fiber optical parametric oscillator for multi-photon microscopy

We present a femtosecond fiber-based optical parametric oscillator (FOPO) for multiphoton microscopy with wavelength tuning by electronic repetition rate tuning in combination with a dispersive filter in the FOPO cavity. The all-spliced, all-fiber FOPO cavity is based on polarization-maintaining fibers and a broadband output coupler, allowing to get access to the resonant signal pulses as well as the idler pulses simultaneously. The system was pumped by a gain-switched fiber-coupled laser diode emitting pulses at a central wavelength of 1030 nm and an electronically tunable repetition frequency of about 2 MHz. The pump pulses were amplified in an Ytterbium fiber amplifier system with a pulse duration after amplification of 13 ps. Tuning of the idler (1140 nm - 1300 nm) and signal wavelengths (850 nm - 940 nm) was achieved by changing the repetition frequency of the pump laser by about 4 kHz. The generated signal pulses reached a pulse energy of up to 9.2 nJ at 920 nm and were spectrally broadened to about 6 nm in the FOPO by a combination of self-phase and cross-phase modulation. We showed external compression of the idler pulses at 920 nm to about 430 fs and appleid them to two-photon excitation microscopy with green fluorescent dyes. The presented system constitutes an important step towards a fully fiber-integrated all-electronically tunable and, thereby, programmable light source and already embodies a versatile and flexible light source for applications, e.g., for smart microscopy.

Rapid and lossless bandwidth-switching of a fiber-based optical parametric oscillator for multimodal nonlinear microscopy

A concept to adjust the output spectral bandwidth of a wavelength-tunable fiber optical parametric oscillator (FOPO) is presented. By adjusting the chirp of the pump pulses relative to the chirp of the resonant pulses, the energy of the output pulses can be reorganized into a wide or narrow spectral bandwidth. We present numerical simulations of a FOPO, which is able to generate pulses with an adjustable bandwidth between 12 and 0.8 nm. Such a FOPO should allow the cost-efficient application within two-photon excited fluorescence microscopy, which benefits from high peak powers, combined with coherent Raman microscopy, which requires spectrally narrow pulses.

Rapid spectro-polarimetry to probe molecular symmetry in multiplex coherent anti-stokes Raman scattering

We present our recent advances on rapid spectro-polarimetry to identify otherwise indistinguishable molecules by polarization effects in multiplex coherent anti-Stokes Raman scattering spectroscopy (CARS).

Electronically tunable dual-color all-fiber optical parametric oscillator

Parametric light generation has been utilized very successfully in the past as a means of generating wavelength-tunable light pulses in a variety of systems. Especially fiber optical parametric oscillators (FOPO) based on four-wave mixing in a photonic crystal fiber [1, 2] have the potential to benefit from the inherent mechanical stability and robustness of a fiber-based setup. However, the presented concepts inherently prevent an all-fiber integrated setup by relying on a mechanical tuning of the resonator length or pump wavelength. In addition, such a tuning mechanism limits the achievable tuning speed, thereby, reducing the flexibility of the light source as well as being prone to mechanical abrasion and errors.

Rapid spectro-polarimetry to probe molecular symmetry in multiplex coherent anti-Stokes Raman scattering

We present our recent advances on rapid spectro-polarimetry to identify otherwise indistinguishable molecules by polarization effects in multiplex coherent anti-Stokes Raman scattering spectroscopy (CARS).