Longitudinal to transversal conversion of mode-locked states in an empty optical resonator

A longitudinal mode-locked state can be converted to a transverse mode-locked state by exploiting the spectral and spatial filtering of an empty optical resonator. Carrier and amplitude modulation sidebands were simultaneously transmitted by the conversion resonator, yielding phase-locked superpositions of up to five transverse modes. Equivalently, an amplitude-modulated beam was converted into a beam that periodically moved across the transverse plane. Precise control over the spatial beam shape during oscillation was gained by independently altering the set of transverse modes and their respective powers, which demonstrated an increased level of control in the generation of transverse mode-locked states.Comment: 11 pages, 4 figures. Supplement: 5 pages, 5 figure

Ultrafast, low-power, all-optical switching via birefringent phase-matched transverse mode conversion in integrated waveguides

We demonstrate the potential of birefringence-based, all-optical, ultrafast conversion between the transverse modes in integrated optical waveguides by modelling the conversion process by numerically solving the multi-mode coupled nonlinear Schroedinger equations. The observed conversion is induced by a control beam and due to the Kerr effect, resulting in a transient index grating which coherently scatters probe light from one transverse waveguide mode into another. We introduce birefringent phase matching to enable efficient all-optically induced mode conversion at different wavelengths of the control and probe beam. It is shown that tailoring the waveguide geometry can be exploited to explicitly minimize intermodal group delay as well as to maximize the nonlinear coefficient, under the constraint of a phase matching condition. The waveguide geometries investigated here, allow for mode conversion with over two orders of magnitude reduced control pulse energy compared to previous schemes and thereby promise nonlinear mode switching exceeding efficiencies of 90% at switching energies below 1 nJ

Supercontinuum generation in media with sign-alternated dispersion

When an ultrafast optical pulse with high intensity is propagating through transparent material a supercontinuum can be coherently generated by self-phase modulation, which is essential to many photonic applications in fibers and integrated waveguides. However, the presence of dispersion causes stagnation of spectral broadening past a certain propagation length, requiring an increased input peak power for further broadening. We present a concept to drive supercontinuum generation with significantly lower input power by counteracting spectral stagnation via alternating the sign of group velocity dispersion along the propagation. We demonstrate the effect experimentally in dispersion alternating fiber in excellent agreement with modeling, revealing almost an order of magnitude reduced peak power compared to uniform dispersion. Calculations reveal a similar power reduction also with integrated optical waveguides, simultaneously with a significant increase of flat bandwidth, which is important for on-chip broadband photonics.Comment: Main text and supplementary informatio

A route to sub-diffraction-limited CARS Microscopy

We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging

High-sensitivity frequency modulation CARS with a compact and fast tunable fiber-based light source

Frequency modulation coherent anti-Stokes Raman scattering (FM CARS) is presented, using a compact as well as fast and widely tunable fiber-based light source. With this light source Raman resonances between 700 cm$^{-1}$ and 3200 cm$^{-1}$ can be addressed via wavelength tuning within only 5 ms, which allows for FM CARS measurements with frame-to-frame wavelength switching. Moreover, the functionality for high-sensitivity FM CARS measurements was integrated by means of fiber optics to keep a stable and reliable operation. The light source accomplished FM CARS measurements with a 40 times enhanced sensitivity at a lock-in amplifier (LIA) bandwidth of 1 Hz. For fast imaging with frame-to-frame wavelength switching at a LIA bandwidth of 1 MHz a 18-fold contrast enhancement could be verified, making this light source ideal for routine and out-of-lab FM CARS measurements for medical diagnostics or environmental sensing