Jörn P. Epping

On-chip visible-to-infrared supercontinuum generation with more than 495 THz spectral bandwidth

We report ultra-broadband supercontinuum generation in high-confinement Si3N4 integrated optical waveguides. The spectrum extends through the visible (from 470 nm) to the infrared spectral range (2130 nm) comprising a spectral bandwidth wider than 495 THz, which is the widest supercontinuum spectrum generated on a chip.

High confinement, high yield Si3N4 waveguides for nonlinear optical applications

In this paper we present a novel fabrication technique for silicon nitride (Si(3)N(4)) waveguides with a thickness of up to 900 nm, which are suitable for nonlinear optical applications. The fabrication method is based on etching trenches in thermally oxidized silicon and filling the trenches with Si(3)N(4). Using this technique no stress-induced cracks in the Si(3)N(4) layer were observed resulting in a high yield of devices on the wafer. The propagation losses of the obtained waveguides were measured to be as low as 0.4 dB/cm at a wavelength of around 1550 nm.

Two-octave spanning supercontinuum generation in stoichiometric silicon nitride waveguides pumped at telecom wavelengths

We demonstrate supercontinuum generation in stoichiometric silicon nitride (Si3N4 in SiO2) integrated optical waveguides, pumped at telecommunication wavelengths. The pump laser is a mode-locked erbium fiber laser at a wavelength of 1.56 µm with a pulse duration of 120 fs. With a waveguide-internal pulse energy of 1.4 nJ and a waveguide with 1.0 µm × 0.9 µm cross section, designed for anomalous dispersion across the 1500 nm telecommunication range, the output spectrum extends from the visible, at around 526 nm, up to the mid-infrared, at least to 2.6 µm, the instrumental limit of our detection. This output spans more than 2.2 octaves (454 THz at the -30 dB level). The measured output spectra agree well with theoretical modeling based on the generalized nonlinear Schrödinger equation. The infrared part of the supercontinuum spectra shifts progressively towards the mid-infrared, well beyond 2.6 µm, by increasing the width of the waveguides.

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.

Integrated CARS source based on seeded four-wave mixing in silicon nitride.

We present a theoretical investigation of an integrated nonlinear light source for coherent anti-Stokes Raman scattering (CARS) based on silicon nitride waveguides. Wavelength tunable and temporally synchronized signal and idler pulses are obtained by using seeded four-wave mixing. We find that the calculated input pump power needed for nonlinear wavelength generation is more than one order of magnitude lower than in previously reported approaches based on optical fibers. The tuning range of the wavelength conversion was calculated to be 1418 nm to 1518 nm (idler) and 788 nm to 857 nm (signal), which corresponds to a coverage of vibrational transitions from 2350 cm-1 to 2810 cm-1. A maximum conversion efficiency of 19.1% at a peak pump power of 300 W is predicted.

Ultra-low-power stress-optics modulator for microwave photonics

In this work, we demonstrate the first stress-optic modulator in a silicon nitride-based waveguide platform (TriPleX) in the telecommunication C-band. In our stress-optic phase modulator the refractive index of the waveguiding materials is controlled by the stress-optic effect induced by actuating a 2 μm thick PZT layer on top of the TriPleX waveguide geometry. The efficiency of the modulator is optimized by, amongst others, focusing the applied stress in the waveguide core region through a local increase of the top cladding. Using a Mach-Zehnder interferometer, we measured a half-wave voltage, Vπ, at 34 V at a wavelength of 1550 nm using a modulator with a total length of 14.8 mm. The measured static power consumption of our stress-optic modulator is in the μW-region as it is only determined by small leakage currents (< 0.1 μA), while the dynamic power consumption at a rise time of 1 ms (1 kHz excitation) is less than 4 mW per modulator. The stress optical modulator goes with an excess loss of 0.01 dB per modulator only. This is in line with the typical low loss characteristics of TriPleX waveguides, being < 0.1 dB/cm at a wavelength of 1550 nm. These specifications make stress-optic modulators an excellent choice for next generation optical beam forming networks with a large number of actuators in silicon photonics in general and in the TriPleX platform in particular.

Dispersion engineering silicon nitride waveguides for broadband nonlinear frequency conversion

In this thesis, we investigated nonlinear frequency conversion of optical wavelengths using integrated silicon nitride (Si3N4) waveguides. Two nonlinear conversion schemes were considered: seeded four-wave mixing and supercontinuum generation. The first—seeded four-wave mixing—is investigated by a numerical study and proposed as light source for coherent anti-Stokes Raman scattering (CARS). The compatibility of the silicon nitride-based integrated waveguide with microfluidic channels enables potential applications for on-chip CARS spectroscopy. Both four-wave mixing and supercontinuum generation require waveguides with a large core area to obtain the dispersion required for phase-matched nonlinear frequency conversion. A novel fabrication technique for manufacturing large-core Si3N4 waveguides was investigated. The main advantage of this novel technique is the ability to manufacture crack-free Si3N4 waveguides with sufficient thickness to phase match nonlinear optical processes, while simultaneously realizing a high device yield. To demonstrate that such waveguides can be dispersion engineered and, i.e., allow for phase matching for nonlinear frequency conversion, we experimentally investigated supercontinuum generation in these waveguides using two different pump wavelengths. For a pump wavelength of 1560 nm, the waveguide was dispersion engineered to have a zero-dispersion wavelength just above 1600 nm, such that the pump wavelength experiences anomalous dispersion. The generated supercontinuum spanned more than 700 nm (at -30 dB), limited by the available pump energy. Theoretical modeling showed an exceptionally good agreement with the measured spectrum, and that an octave-spanning supercontinuum is possible if the pump energy is increased. The pulse-to-pulse coherence was calculated for this case and we found that the supercontinuum was fully coherent over its bandwidth (-30 dB), showing that Si3N4 waveguides could be used to generate an optical frequency comb. For a pump wavelength of 1064 nm, the waveguide was designed to have a zero-dispersion wavelength just below 1000 nm to have, again, anomalous dispersion for the pump wavelength. At this pump wavelength, the generated supercontinuum spanned an ultrabroad-bandwidth of nearly 500 THz covering nearly the whole transparency window of Si3N4/SiO2 waveguides.

Enhanced coverage though optical beamforming in fiber wireless networks

Integrated microwave photonics (IMWP) is a novel field in which the fast-paced progress in integrated, on-chip, optics is harnessed to provide breakthrough performances in well-established microwave photonic processing functions, which are traditionally realized using discrete optoelectronic components. A field where IMWP can have a strong impact is the one of Antenna Arrays for 5G networks. Such arrays offer a number of attractive characteristics, including a conformal array profile, electronic beamforming (beam shaping and beam steering), interference nulling and the capability to generate multiple antenna beams simultaneously. In many cases, however, the performance of a phased array is limited by the characteristics of the beamforming network (BFN) used. It is generally desired to realize beamformers with broad instantaneous bandwidth, continuous amplitude, and delay tunability while, at the same time, capable of feeding large arrays. This, however, is very challenging to achieve using only electronics. For this reason, in the last few years, an increasing amount of research has been directed to beamforming in the optical domain using, integrated microwave photonics solutions. Besides antenna array applications, opportunities for cost effective use of IMWP in switched delay lines has become feasible due to the continuous improvement of optical chips, particularly the achieved record-low propagation losses in Si3N4/SiO2-based-chips combined with the high integration density.

Ultra-low-power stress-based phase actuator for microwave photonics

The next generation of applications in integrated microwave photonics[1], such as optical beam forming networks or programmable photonic processors[2], requires complex chip designs with hundreds or even thousands of actuators and low propagation losses. While passive material platforms such as silicon nitride possess ultra-low losses in the telecommunication C-Band, today their on-chip phase actuation, however, is solely based on thermo-optic tuners and, hence, suffering from huge power dissipation.

Second-harmonic generation in stoichiometric silicon nitride glass waveguides

Integrated optical waveguides based on stoichiometric silicon nitride (SÌ3N4) grown with low pressure chemical vapor deposition (LPCVD) are of high importance for applications, e.g., in microwave photonics [1] and quantum optics [2] due to their record-low loss and broad spectral transparency range. The materials third-order nonlinearity and wide transparency have enabled efficient nonlinear conversion, specifically, supercontinuum generation with record-wide bandwidth [3, 4] and are promising for all-optical switching [5].