arXiv subjects

Wolfgang Freude

Publications and source records attributed to Wolfgang Freude.

At least 19 recordsLinked to original sources

arXiv2024

Optical Arbitrary Waveform Generation (OAWG) Using Actively Phase-Stabilized Spectral Stitching

The conventional way of generating optical waveforms relies on the in-phase and quadrature (IQ) modulation of a continuous wave (CW) laser tone. In this case, the bandwidth of the resulting optical waveform is limited by the underlying electronic components, in particular by the digital-to-analog converters (DACs) generating the drive signals for the IQ modulator. This bandwidth bottleneck can be overcome by using a concept known as optical arbitrary waveform generation (OAWG), where multiple IQ modulators and DACs are operated in parallel to first synthesize individual spectral slices, which are subsequently combined to form a single ultra-broadband arbitrary optical waveform. However, targeted synthesis of arbitrary optical waveforms from multiple spectral slices has so far been hampered by difficulties to maintain the correct optical phase relationship between the slices. In this paper, we propose and demonstrate spectrally sliced OAWG with active phase stabilization, which permits targeted synthesis of truly arbitrary optical waveforms. We demonstrate the viability of the scheme by synthesizing optical waveforms with record-high bandwidths of up to 325 GHz from four individually generated optical tributaries. In a proof-of-concept experiment, we use the OAWG system to generate 32QAM data signals at symbol rates of up to 320 GBd, which we transmit over 87 km of single-mode fiber and receive by a two-channel non-sliced optical arbitrary waveform measurement (OAWM) system, achieving excellent signal quality. We believe that our scheme can unlock the full potential of OAWG and disrupt a wide range of applications in high-speed optical communications, photonic-electronic digital-to-analog conversion, as well as advanced test and measurement in science and industry.

physics.optics↗

arXiv2024

Freeform terahertz structures fabricated by multi-photon lithography and metal coating

Direct-write multi-photon laser lithography (MPL) combines highest resolution on the nanoscale with essentially unlimited 3D design freedom. Over the previous years, the groundbreaking potential of this technique has been demonstrated in various application fields, including micromechanics, material sciences, microfluidics, life sciences as well as photonics, where in-situ printed optical coupling elements offer new perspectives for package-level system integration. However, millimeter-wave (mmW) and terahertz (THz) devices could not yet leverage the unique strengths of MPL, even though the underlying devices and structures could also greatly benefit from 3D freeform microfabrication. One of the key challenges in this context is the fact that functional mmW and THz structures require materials with high electrical conductivity and low dielectric losses, which are not amenable to structuring by multi-photon polymerization. In this work, we introduce and experimentally demonstrate a novel approach that allows to leverage MPL for fabricating high-performance mmW and THz structures with hitherto unachieved functionalities. Our concept exploits in-situ printed polymer templates that are selectively coated through highly directive metal deposition techniques in combination with precisely aligned 3D-printed shadowing structures. The resulting metal-coated freeform structures offer high surface quality in combination with low dielectric losses and conductivities comparable to bulk material values, while lending themselves to fabrication on planar mmW/THz circuits. We experimentally show the viability of our concept by demonstrating a series of functional THz structures such as THz interconnects, probe tips, and suspended antennas. We believe that our approach offers disruptive potential in the field of mmW and THz technology and may unlock an entirely new realm of laser-based 3D manufacturing.

physics.optics↗

arXiv2023

Non-sliced Optical Arbitrary Waveform Measurement (OAWM) Using a Silicon Photonic Receiver Chip

Comb-based optical arbitrary waveform measurement (OAWM) techniques can overcome the bandwidth limitations of conventional coherent detection schemes and may have disruptive impact on a wide range of scientific and industrial applications. Over the previous years, different OAWM schemes have been demonstrated, showing the performance and the application potential of the concept in laboratory experiments. However, these demonstrations still relied on discrete fiber-optic components or on combinations of discrete coherent receivers with integrated optical slicing filters that require complex tuning procedures to achieve the desired performance. In this paper, we demonstrate the first wavelength-agnostic OAWM front-end that is integrated on a compact silicon photonic chip and that neither requires slicing filters nor active controls. Our OAWM system comprises four IQ receivers, which are accurately calibrated using a femtosecond mode-locked laser and which offer a total acquisition bandwidth of 170 GHz. Using sinusoidal test signals, we measure a signal-to-noise-and-distortion ratio (SINAD) of 30 dB for the reconstructed signal, which corresponds to an effective number of bits (ENOB) of 4.7 bit, where the underlying electronic analog-to-digital converters (ADC) turn out to be the main limitation. The performance of the OAWM system is further demonstrated by receiving 64QAM data signals at symbol rates of up to 100 GBd, achieving constellation signal-to-noise ratios (CSNR) that are on par with those obtained for conventional coherent receivers. In a theoretical scalability analysis, we show that increasing the channel count of non-sliced OAWM systems can improve both the acquisition bandwidth and the signal quality. We believe that our work represents a key step towards out-of-lab use of highly compact OAWM systems that rely on chip-scale integrated optical front-ends.

eess.SP↗

arXiv2023

Slice-Less Optical Arbitrary Waveform Measurement (OAWM) in a Bandwidth of More than 600 GHz Using Soliton Microcombs

We propose and demonstrate a novel scheme for optical arbitrary waveform measurement (OAWM) that exploits chip-scale Kerr soliton combs as highly scalable multiwavelength local oscillators (LO) for ultra-broadband full-field waveform acquisition. In contrast to earlier concepts, our approach does not require any optical slicing filters and thus lends itself to efficient implementation on state-of-the-art high-index-contrast integration platforms such as silicon photonics. The scheme allows to measure truly arbitrary waveforms with high accuracy, based on a dedicated system model which is calibrated by means of a femtosecond laser with known pulse shape. We demonstrated the viability of the approach in a proof-of-concept experiment by capturing an optical waveform that contains multiple 16 QAM and 64 QAM wavelength-division multiplexed (WDM) data signals with symbol rates of up to 80 GBd, reaching overall line rates of up to 1.92 Tbit/s within an optical acquisition bandwidth of 610 GHz. To the best of our knowledge, this is the highest bandwidth that has so far been demonstrated in an OAWM experiment.

physics.optics↗

arXiv2023

Sub-kHz-linewidth external-cavity laser (ECL) with Si$_{3}$N$_{4}$ resonator used as a tunable pump for a Kerr frequency comb

Combining optical gain in direct-bandgap III-V materials with tunable optical feedback offered by advanced photonic integrated circuits is key to chip-scale external-cavity lasers (ECL), offering wideband tunability along with low optical linewidths. External feedback circuits can be efficiently implemented using low-loss silicon nitride (Si$_{3}$N$_{4}$) waveguides, which do not suffer from two-photon absorption and can thus handle much higher power levels than conventional silicon photonics. However, co-integrating III-V-based gain elements with tunable external feedback circuits in chip-scale modules still represents a challenge, requiring either technologically demanding heterogeneous integration techniques or costly high-precision multi-chip assembly. In this work, we demonstrate Si$_{3}$N$_{4}$-based hybrid integrated ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds and facet-attached microlenses for low-loss optical coupling with relaxed alignment tolerances, thereby overcoming the need for active alignment while maintaining the full flexibility of multi-chip integration techniques. In a proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning range (1480 nm - 1570 nm) with on-chip output powers above 12 dBm and side-mode suppression ratios of up to 59 dB. We achieve an intrinsic linewidth of 979 Hz, which is among the lowest values reported for comparable feedback architectures. The optical loss of the intra-cavity photonic wire bond between the III-V gain element and the Si$_{3}$N$_{4}$-based tunable feedback circuit amounts to approximately (1.6 $\pm$ 0.2) dB. We use the ECL as a tunable pump laser to generate a dissipative Kerr soliton frequency comb. To the best of our knowledge, our experiments represent the first demonstration of a single-soliton Kerr comb generated with a pump that is derived from a hybrid ECL.

physics.optics↗

arXiv2023

3D-printed facet-attached optical elements for beam shaping in optical phased arrays

We demonstrate an optical phased-array (OPA) equipped with a 3D-printed facet-attached element for shaping and deflection of the emitted beam. The beam shaper combines freeform refractive surfaces with total-internal-reflection (TIR) mirrors and is in-situ printed to edge-emitting waveguide facets using high-resolution multi-photon lithography, thereby ensuring precise alignment with respect to on-chip waveguide structures. In a proof-of-concept experiment, we achieve a grating-lobe free steering range of $\pm 30°$ and a full-width-halfmaximum (FWHM) beam divergence of approximately $2°$. The concept opens an attractive alternative to currently used grating structures and is applicable to a wide range of integration platforms.

physics.optics↗

arXiv2022

3D-printed facet-attached optical elements for connecting VCSEL and photodiodes to fiber arrays and multi-core fibers

Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 $μ$m, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 x 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard.

physics.optics↗

arXiv2022

Transformation-optics modeling of 3D-printed freeform waveguides

Multi-photon lithography allows to complement planar photonic integrated circuits (PIC) by in-situ 3D-printed freeform waveguide structures. However, design and optimization of such freeform waveguides using time-domain Maxwell's equations solvers often requires comparatively large computational volumes, within which the structure of interest only occupies a small fraction, thus leading to poor computational efficiency. In this paper, we present a solver-independent transformation-optics-(TO-) based technique that allows to greatly reduce the computational effort related to modeling of 3D freeform waveguides. The concept relies on transforming freeform waveguides with curved trajectories into equivalent waveguide structures with modified material properties but geometrically straight trajectories, that can be efficiently fit into rather small cuboid-shaped computational volumes. We demonstrate the viability of the technique and benchmark its performance using a series of different freeform waveguides, achieving a reduction of the simulation time by a factor of 3-6 with a significant potential for further improvement. We also fabricate and experimentally test the simulated waveguides by 3D-printing on a silicon photonic chip, and we find good agreement between the simulated and the measured transmission at $λ= 1550 \textrm{ nm}$.

physics.optics↗

arXiv2021

Ultra-broadband polarization beam splitter and rotator based on 3D-printed waveguides

Multi-photon lithography has emerged as a powerful tool for photonic integration, allowing to complement planar photonic circuits by 3D-printed freeform structures such as waveguides or micro-optical elements. These structures can be fabricated with high precision on the facets of optical devices and lend themselves to highly efficient package-level chip-chip-connections in photonic assemblies. However, plain light transport and efficient coupling is far from exploiting the full geometrical design freedom that is offered by 3D laser lithography. Here, we extend the functionality of 3D-printed optical structures to manipulation of optical polarization states. We demonstrate compact ultra-broadband polarization beam splitters (PBS) that can be combined with polarization rotators (PR) and mode-field adapters into a monolithic 3D-printed structure, fabricated directly on the facets of optical devices. In a proof-of-concept experiment, we demonstrate measured polarization extinction ratios beyond 11 dB over a bandwidth of 350 nm at near-infrared (NIR) telecommunication wavelengths around 1550 nm. We demonstrate the viability of the device by receiving a 640 Gbit/s dual-polarization data signal using 16-state quadrature amplitude modulation (16QAM), without any measurable optical-signal-to-noise-ratio (OSNR) penalty compared to a commercial PBS.

physics.optics↗

arXiv2021

Superconducting Nanowire Single-Photon Detector (SNSPD) with 3D-Printed Free-Form Microlenses

We present an approach to increase the effective light-receiving area of superconducting nanowire single-photon detectors (SNSPD) by means of free-form microlenses that are printed in situ on top of the sensitive detector area using high-resolution multi-photon lithography. We demonstrate a detector based on a niobium-nitride (NbN) nanowire with a 4.5 $\mathrm μ$m $\times$ 4.5 $\mathrm μ$m sensitive area, supplemented with a lens of 60 $\mathrm μ$m diameter. For free-space illumination at a wavelength of 1550 nm, the lensed sensor has a 100-fold-increased effective collection area, which leads to strongly enhanced system detection efficiency without the need for long nanowires. Our approach can be readily applied to a wide range of sensor types and effectively overcomes the inherent design conflict between high counting speed due to short sensor reset time, high timing accuracy, and high fabrication yield on the one hand and high collection efficiency through large effective detection areas on the other hand.

physics.ins-det↗

arXiv2020

Hybrid Electro-Optic Modulator Combining Silicon Photonic Slot Waveguides with High-k Radio-Frequency Slotlines

Electro-optic (EO) modulators rely on interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced by using dielectric slot-waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherent RC-limitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot-waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on BaTiO3 as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm-long Mach-Zehnder modulator that offers a 3 dB-bandwidth of 76 GHz and a 6 dB-bandwidth of 110 GHz along with a small π-voltage of 1.3 V (UπL = 1.3 V mm). To the best of our knowledge, this represents the largest EO bandwidth so far achieved with a silicon photonic modulator based on dielectric waveguides. We further demonstrate the viability of the device in a data transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

physics.app-ph↗

arXiv2020

Silicon-Organic Hybrid (SOH) Mach-Zehnder Modulators for 100 GBd PAM4 Signaling With Sub-1 dB Phase-Shifter Loss

We report on compact and efficient silicon-organic hybrid (SOH) Mach-Zehnder modulators (MZM) with low phase shifter insertion loss of 0.7 dB. The 280 $μ$m-long phase shifters feature a $π$-voltage-length product of 0.41 Vmm and a loss-efficiency product as small as $aU_πL = 1.0\space\rm{VdB}$. The device performance is demonstrated in a data transmission experiment, where we generate on-off-keying (OOK) and four-level pulse-amplitude modulation (PAM4) signals at symbol rates of 100 GBd, resulting in line rates of up to 200 Gbit/s. Bit error ratios are below the threshold for hard-decision forward error correction (HD-FEC) with 7 % coding overhead, leading to net data rates of 187 Gbit/s. This is the highest PAM4 data rate ever achieved for a sub-1 mm silicon photonic MZM.

physics.app-ph↗

arXiv2019

Hardwire-Configurable Photonic Integrated Circuits Enabled by 3D Nano-Printing

Photonic integrated circuits (PIC) currently rely on application-specific designs that are geared towards a particular functionality. Development of such application-specific PIC requires considerable effort and often involves several design, fabrication, and characterization iterations, each with cycle times of several months, to reach acceptable performance. At the same time, the number of chips that is eventually needed to serve a certain application is often too small to benefit from the scalability of advanced photonic integration platforms. As a consequence, large-scale photonic integration has hitherto been unable to unfold its full economic advantages within the highly fragmented application space of photonics. Here we introduce a novel approach to configurable PIC that can overcome these challenges. The concept exploits generic PIC designs consisting of standard building blocks that can be concatenated to provide an application-specific functionality. Configuration of the PIC relies on establishing hardwired optical connections between suitable ports of an optical wire board using highly flexible 3D direct-write laser lithography. Hardwire-configurable PIC allow to exploit high-throughput production of generic chips in large-scale fabrication facilities for serving a wide variety of applications with small or medium volumes of specifically configured devices.

physics.app-ph↗

arXiv2019

A verified equivalent-circuit model for slotwaveguide modulators

We formulate and experimentally validate an equivalent-circuit model based on distributed elements to describe the electric and electro-optic (EO) properties of travellingwave silicon-organic hybrid (SOH) slot-waveguide modulators. The model allows to reliably predict the small-signal EO frequency response of the modulators exploiting purely electrical measurements of the frequency-dependent RF transmission characteristics. We experimentally verify the validity of our model, and we formulate design guidelines for an optimum trade-off between optical loss due to free-carrier absorption (FCA), electro-optic bandwidth, and π-voltage of SOH slot-waveguide modulators.

physics.app-ph↗

arXiv2019

Lasing in $\mathrm{Si}_3\mathrm{N}_4$-organic hybrid (SiNOH) waveguides

Lasers that operate in the visible wavelength range and can be easily integrated on a chip together with photonic integrated circuits are of particular interest for biophotonic applications. We present a new class of laser sources that emit at 600 nm wavelength and can be efficiently integrated on the $\mathrm{Si}_3\mathrm{N}_4$ platform using low-cost fabrication techniques. The $\mathrm{Si}_3\mathrm{N}_4$-organic hybrid lasers are optically pumped from top without the need of a precise alignment. We investigate different laser geometries based on spiral shaped ring resonators and distributed feedback resonators. We discuss the laser properties with a focus on their application in the field of bioscience.

physics.optics↗

arXiv2019

Coherent WDM transmission using quantum-dash mode-locked laser diodes as multi-wavelength source and local oscillator

Quantum-dash (QD) mode-locked laser diodes (MLLD) lend themselves as chip-scale frequency comb generators for highly scalable wavelength-division multiplexing (WDM) links in future data-center, campus-area, or metropolitan networks. Driven by a simple DC current, the devices generate flat broadband frequency combs, containing tens of equidistant optical tones with line spacings of tens of GHz. Here we show that QD-MLLDs can not only be used as multi-wavelength light sources at a WDM transmitter, but also as multi-wavelength local oscillators (LO) for parallel coherent reception. In our experiments, we demonstrate transmission of an aggregate data rate of 4.1 Tbit/s (23x45 GBd PDM-QPSK) over 75 km standard single-mode fiber (SSMF). To the best of our knowledge, this represents the first demonstration of a coherent WDM link that relies on QD-MLLD both at the transmitter and the receiver.

physics.app-ph↗

arXiv2019

Electrically packaged silicon-organic hybrid (SOH) I/Q-modulator for 64 GBd operation

Silicon-organic hybrid (SOH) electro-optic (EO) modulators combine small footprint with low operating voltage and hence low power dissipation, thus lending themselves to on-chip integration of large-scale device arrays. Here we demonstrate an electrical packaging concept that enables high-density radio-frequency (RF) interfaces between on-chip SOH devices and external circuits. The concept combines high-resolution $\mathrm{Al_2O_3}$ printed-circuit boards with technically simple metal wire bonds and is amenable to packaging of device arrays with small on-chip bond pad pitches. In a set of experiments, we characterize the performance of the underlying RF building blocks and we demonstrate the viability of the overall concept by generation of high-speed optical communication signals. Achieving line rates (symbols rates) of 128 Gbit/s (64 GBd) using quadrature-phase-shiftkeying (QPSK) modulation and of 160 Gbit/s (40 GBd) using 16-state quadrature-amplitudemodulation (16QAM), we believe that our demonstration represents an important step in bringing SOH modulators from proof-of-concept experiments to deployment in commercial environments.

physics.app-ph↗

arXiv2019

Generalized Kramers-Kronig Receiver for Coherent THz Communications

High-speed communication systems rely on spectrally efficient modulation formats that encode information both on the amplitude and on the phase of an electromagnetic carrier. Coherent detection of such signals typically uses rather complex receiver schemes, requiring a continuous-wave (c.w.) local oscillator (LO) as a phase reference and a mixer circuit for spectral down-conversion. In optical communications, the so-called Kramers-Kronig (KK) scheme has been demonstrated to greatly simplify the receiver, reducing the hardware to a single photodiode. In this approach, an LO tone is transmitted along with the signal, and the amplitude and phase of the complex signal envelope are reconstructed from the photocurrent by digital signal processing. This reconstruction exploits the fact that the real and the imaginary part, or, equivalently, the amplitude and the phase of an analytic signal are connected by a KK-type relation. Here, we transfer the KK scheme to high-speed wireless communications at THz carrier frequencies. We use a Schottky-barrier diode (SBD) as a nonlinear element and generalize the theory of KK processing to account for the non-quadratic characteristics of this device. Using 16-state quadrature amplitude modulation (16QAM), we transmit a net data rate of 115 Gbit/s at a carrier frequency of 0.3 THz over a distance of 110 m.

physics.app-ph↗