Highly efficient and widely tunable Si3N4 waveguide-based optical parametric oscillator.

We demonstrate an efficient and widely tunable synchronously pumped optical parametric oscillator (OPO) exploiting four-wave mixing (FWM) in a silicon nitride (Si3N4) waveguide with inverted tapers. At a pump pulse duration of 2 ps, the waveguide-based OPO (WOPO) exhibited a high external pump-to-idler conversion efficiency of up to -7.64 dB at 74% pump depletion and a generation of up to 387 pJ output idler pulse energy around 1.13 µm wavelength. Additionally, the parametric oscillation resulted in a 64 dB amplification of idler power spectral density in comparison to spontaneous FWM, allowing for a wide idler wavelength tunability of 191 nm around 1.15 µm. Our WOPO represents a significant improvement of conversion efficiency as well as output energy among χ3 WOPOs, rendering an important step towards a highly efficient and widely tunable chip-based light source for, e.g., coherent anti-Stokes Raman scattering.

Toward integrated tantalum pentoxide optical parametric oscillators.

We present a hybrid waveguide-fiber optical parametric oscillator (OPO) exploiting degenerate four-wave mixing in tantalum pentoxide. The OPO, pumped with ultrashort pulses at 1.55 µm wavelength, generated tunable idler pulses with up to 4.1 pJ energy tunable center wavelength between 1.63 µm and 1.68 µm. An upper bound for the total tolerable cavity loss of 32 dB was found, rendering a chip-integrated OPO feasible as a compact and robust light source.

Multi-color stimulated Raman scattering with a frame-to-frame wavelength-tunable fiber-based light source.

We present multi-color imaging by stimulated Raman scattering (SRS) enabled by an ultrafast fiber-based light source with integrated amplitude modulation and frame-to-frame wavelength tuning. With a relative intensity noise level of -153.7 dBc/Hz at 20.25 MHz the light source is well suited for SRS imaging and outperforms other fiber-based light source concepts for SRS imaging. The light source is tunable in under 5 ms per arbitrary wavelength step between 700 cm-1 and 3200 cm-1, which allows for addressing Raman resonances from the fingerprint to the CH-stretch region. Moreover, the compact and environmentally stable system is predestined for fast multi-color assessments of medical or rapidly evolving samples with high chemical specificity, paving the way for diagnostics and sensing outside of specialized laser laboratories.

Toward integrated synchronously pumped optical parametric oscillators in silicon nitride.

We present a tunable, hybrid waveguide-fiber optical parametric oscillator (OPO) synchronously pumped by an ultra-fast fiber laser exploiting four-wave mixing (FWM) generated in silicon nitride waveguides. Parametric oscillation results in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM, with the ability of wide wavelength tuning over 86 nm in the O-band. Measurements of the oscillation threshold and the efficiency of the feedback loop reveal how an integration of the OPO on a single silicon nitride chip can be accomplished at standard repetition rates of pump lasers in the order of 100 MHz.

Optical parametric amplification in silicon nitride waveguides for coherent Raman imaging.

We present tunable waveguide-based optical parametric amplification by four-wave mixing (FWM) in silicon nitride waveguides, with the potential to be set up as an all-integrated device, for narrowband coherent anti-Stokes Raman scattering (CARS) imaging. Signal and idler pulses are generated via FWM with only 3 nJ pump pulse energy and stimulated by using only 4 mW of a continuous-wave seed source, resulting in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM. By using waveguides with different widths and tuning the wavelength of the signal wave seed, idler wavelengths covering the spectral region from 1.1 µm up to 1.6 µm can be generated. The versatility of the chip-based FWM light source is demonstrated by acquiring CARS images.

Higher-order mode supercontinuum generation in dispersion-engineered liquid-core fibers.

Supercontinuum generation enabled a series of key technologies such as frequency comb sources, ultrashort pulse sources in the ultraviolet or the mid-infrared, as well as broadband light sources for spectroscopic methods in biophotonics. Recent advances utilizing higher-order modes have shown the potential to boost both bandwidth and modal output distribution of supercontinuum sources. However, the strive towards a breakthrough technology is hampered by the limited control over the intra- and intermodal nonlinear processes in the highly multi-modal silica fibers commonly used. Here, we investigate the ultrafast nonlinear dynamics of soliton-based supercontinuum generation and the associated mode coupling within the first three lowest-order modes of accurately dispersion-engineered liquid-core fibers. By measuring the energy-spectral evolutions and the spatial distributions of the various generated spectral features polarization-resolved, soliton fission and dispersive wave formation are identified as the origins of the nonlinear broadening. Measured results are confirmed by nonlinear simulations taking advantage of the accurate modeling capabilities of the ideal step-index geometry of our liquid-core platform. While operating in the telecommunications domain, our study allows further advances in nonlinear switching in emerging higher-order mode fiber networks as well as novel insights into the sophisticated nonlinear dynamics and broadband light generation in pre-selected polarization states.

Spontaneous four-wave mixing in silicon nitride waveguides for broadband coherent anti-Stokes Raman scattering spectroscopy.

We present a light source for coherent anti-Stokes Raman scattering (CARS) based on broadband spontaneous four-wave mixing, with the potential to be further integrated. By using 7 mm long silicon nitride waveguides, which offer tight mode confinement and a high nonlinear refractive index coefficient, broadband signal and idler pulses were generated with 4 nJ of input pulse energy. In comparison to fiber-based experiments, the input energy and the waveguide length were reduced by two orders of magnitude, respectively. The idler and residual pump pulses were used for CARS measurements, enabling chemically selective and label-free spectroscopy over the entire fingerprint region, with an ultrafast fiber-based pump source at 1033 nm wavelength. The presented simple light source paves the path towards cost-effective, integrated lab-on-a-chip CARS applications.

Low-power broadband all-optical switching via intermodal cross-phase modulation in integrated optical waveguides.

We demonstrate the potential of all-optical switches in integrated waveguides based on intermodal cross-phase modulation between transverse modes. For this purpose, the differential phase between two transverse modes of a probe beam was altered by cross-phase modulation with a control beam propagating only in the fundamental mode. A switching behavior was accomplished by spatially filtering the resulting multimode interference of the probe modes, which changed depending on the control beam power. All-optical switching with a contrast of 82% at 1280 nm over a frequency range of 4.4 THz at 1.6 nJ was achieved, representing an improvement of the product of necessary power and waveguide length by a factor of nearly 2000 compared to similar experiments in graded-index fibers. Additionally, we show that the center wavelength of the switch can be tailored by changing the cross-sectional geometry of the waveguide or the involved probe modes.