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Jeremy Witzens

Publications and source records attributed to Jeremy Witzens.

At least 19 recordsLinked to original sources
arXiv2025

A focusing optical phased array for tissue interrogation with side-lobe suppression and simplified beam steering

We implement an integrated multi-electrode array on a silicon-nitride-based photonic integrated circuit for ex-vivo retinal characterization via optical stimulation. The interrogation beam formers, based on curved grating emitters and optical phased arrays, are designed to achieve transverse focusing with spot sizes in the 1 - 2 $\mu$m range to target single cells. The experimentally realized focusing optical phased arrays show suppressed side-lobes, with approximately 11.5% of the power in each side-lobe and ~60% in the main lobe, reducing unintentional cellular excitation. Additional design refinement enables further suppression of the side-lobes to a few percent of the total power. Additionally, we demonstrate a compact design of meandered thermal phase shifters implemented across the array that allow push-pull steering in the transverse direction as well as focusing and defocusing of the beam, with a total of only four control signals. Transverse angular steering by $\pm$5.1{\deg} and axial translation of the focal spot by 204 $\mu$m are demonstrated with tuning currents below 50 mA, together with longitudinal angular steering by 4.26{\deg} obtained by means of wavelength tuning in a $\pm$15 nm range centered on 525 nm.

physics.optics↗
arXiv2025

Silicon nitride PIC beam formers for light sheet fluorescent microscopy

Light sheet fluorescence microscopy (LSFM) has transformed the way we visualize biological tissues in three dimensions, offering high-resolution imaging while minimizing photo-induced damage to the samples. Recent breakthroughs in tissue-clearing methods have further improved LSFM's capabilities, making it possible to study larger, intact samples in unprecedented detail. To overcome limitations like shallow penetration and light diffraction in traditional LSFM setups, advanced beam shaping with devices such as spatial light modulators and digital micromirror arrays has been utilized. These improve the resolution and the extent of tissues that can be imaged. Advances such as Bessel-beam-based LSFM and lattice light sheet microscopy increase the field of view that can be imaged with low background noise, but often require complex and bulky equipment. Addressing these complexities, a new approach builds on silicon nitride photonic integrated circuits to create a structured light sheet in a very compact device. This system incorporates beam-steering based on wavelength control in a limited tuning range and enables the generation of light sheets with optimized characteristics in regard to thickness and diffraction-length limited penetration depth. Simulations include extensive fabrication tolerance analysis that confirm the practicability of the approach, that can be straightforwardly extended to dual wavelength excitation. This compact, chip-based LSFM system could make high-quality imaging more accessible and transform biomedical instrumentation.

physics.optics↗
arXiv2024

An efficient singlet-triplet spin qubit to fiber interface assisted by a photonic crystal cavity

We introduce a novel optical interface between a singlet-triplet spin qubit and a photonic qubit which would offer new prospects for future quantum communication applications. The interface is based on a 220 nm thick GaAs/Al-GaAs heterostructure membrane and features a gate-defined singlet-triplet qubit, a gate-defined optically active quantum dot, a photonic crystal cavity and a bot-tom gold reflector. All essential components can be lithographically defined and deterministically fabricated, which greatly increases the scalability of on-chip in-tegration. According to our FDTD simulations, the interface provides an overall coupling efficiency of 28.7% into a free space Gaussian beam, assuming an SiO2 interlayer filling the space between the reflector and the membrane. The performance can be further increased to 48.5% by undercutting this SiO2 interlayer below the photonic crystal.

physics.optics↗
arXiv2023

Modeling of an efficient singlet-triplet spin qubit to photon interface assisted by a photonic crystal cavity

Efficient interconnection between distant semiconductor spin qubits with the help of photonic qubits would offer exciting new prospects for future quantum communication applications. In this paper, we optimize the extraction efficiency of a novel interface between a singlet-triplet spin qubit and a photonic qubit. The interface is based on a 220 nm thick GaAs/AlGaAs heterostructure membrane and consists of a gate-defined double quantum dot (GDQD) supporting a singlet-triplet qubit, an optically active quantum dot (OAQD) consisting of a gate-defined exciton trap, a photonic crystal cavity providing in-plane optical confinement and efficient out-coupling to an ideal free space Gaussian beam while accommodating the gate wiring of the GDQD and OAQD, and a bottom gold reflector to recycle photons and increase the optical extraction efficiency. All essential components can be lithographically defined and deterministically fabricated on the GaAs/AlGaAs heterostructure membrane, which greatly increases the scalability of on-chip integration. According to our simulations, the interface provides an overall coupling efficiency of 28.7% into a free space Gaussian beam, assuming an SiO2 interlayer filling the space between the reflector and the membrane. The performance can be further increased by undercutting this SiO2 interlayer below the photonic crystal. In this case, the overall efficiency is calculated to be 48.5%.

physics.optics↗
arXiv2023

Chip-to-chip ODDM network with optically enabled equalization

We propose and model an optical communication scheme for short distance datacom links based on the distribution of information across a wide comb spectrum. This modulation format, orthogonal delay division multiplexing, allows the multiplexing of data streams from multiple modulators, as well as the deserialization and equalization of the data in the optical domain. A concrete communication system, that allows the transport of 400 Gb/s across a single CWDM channel with a single 80 GHz cutoff lithium niobate on insulator modulator, is modeled under consideration of all noise sources present in the system and its sensitivity to group velocity dispersion is analyzed. Data is deserialized and equalized at the receiver with a 5-tap optical equalizer. This communication architecture may provide a path forward to implement high-baud-rate signaling in short-reach optical links without requiring high-speed ADCs and electronic deserializers at the receiver, thus maintaining the in-package power consumption at manageable levels.

physics.optics↗
arXiv2023

Scalable orthogonal delay-division multiplexed OEO artificial neural network trained for TI-ADC equalization

We propose a new signaling scheme for on-chip optical-electrical-optical artificial neural networks that utilizes orthogonal delay-division multiplexing and pilot-tone based self-homodyne detection. This scheme offers a more efficient scaling of the optical power budget with increasing network complexity. Our simulations, based on a 220 nm SOI silicon photonics technology, suggest that the network can support 31 x 31 neurons, with 961 links and freely programmable weights, using a single 500 mW optical comb and an SNR of 21.3 dB per neuron. Moreover, it features a low sensitivity to temperature fluctuations, ensuring that it can be operated outside of a laboratory environment. We demonstrate the network's effectiveness in nonlinear equalization tasks by training it to equalize a time-interleaved ADC architecture, achieving an ENOB over 4 over the entire 75 GHz ADC bandwidth. We anticipate that this network architecture will enable broadband and low latency nonlinear signal processing in practical settings such as ultra-broadband data converters and real-time control systems.

cs.ET↗
arXiv2021

Silicon Nitride External Cavity Laser with Alignment Tolerant Multi-Mode RSOA-to-PIC Interface

We demonstrate an external cavity laser formed by combining a silicon nitride photonic integrated circuit with a reflective semiconductor optical amplifier. The laser uses an alignment tolerant edge coupler formed by a multi-mode waveguide splitter right at the edge of the silicon nitride chip that relaxes the required alignment to the III-V gain chip and equally splits the power among its two output waveguides. Both the ground and first order mode are excited in the coupler and reach the quadrature condition at the waveguide junction, ensuring equal power to be coupled to both. Two high-quality-factor ring resonators arranged in Vernier configuration close a Sagnac loop between the two waveguides. In addition to wideband frequency tuning, they result in a longer effective cavity length. The alignment tolerant coupler increases the alignment tolerance in the two directions parallel to the chip surface by a factor 3 relative to conventional edge couplers, making it ideal for gain chip integration via pick-and-place technology. Lasing is maintained in a misalignment range of $\pm$6 $μ$m in the direction along the edge of the chip. A Lorentzian laser linewidth of 42 kHz is achieved.

physics.optics↗
arXiv2021

Monolithic Infrared Silicon Photonics: The Rise of (Si)GeSn Semiconductors

(Si)GeSn semiconductors are finally coming of age after a long gestation period. The demonstration of device quality epi-layers and quantum-engineered heterostructures has meant that tunable all-group IV Si-integrated infrared photonics is now a real possibility. Notwithstanding the recent exciting developments in (Si)GeSn materials and devices, this family of semiconductors is still facing serious limitations that need to be addressed to enable reliable and scalable applications. The main outstanding challenges include the difficulty to grow high crystalline quality layers and heterostructures at the desired Sn content and lattice strain, preserve the material integrity during growth and throughout device processing steps, and control doping and defect density. Other challenges are related to the lack of optimized device designs and predictive theoretical models to evaluate and simulate the fundamental properties and performance of (Si)GeSn layers and heterostructures. This Perspective highlights key strategies to circumvent these hurdles and bring this material system to maturity to create far-reaching new opportunities for Si-compatible infrared photodetectors, sensors, and emitters for applications in free-space communication, infrared harvesting, biological and chemical sensing, and thermal imaging.

physics.app-ph↗
arXiv2020

Photonic integrated circuits for life sciences

We report on the use of silicon nitride (SiN) photonic integrated circuits (PICs) in high-value instrumentation, namely multi-color laser engines (MLEs), a core element of cutting-edge biophotonic systems applied to confocal microscopy, fluorescent microscopy - including super-resolution stimulated emission depletion (STED) microscopy - flow cytometry, optogenetics, genetic analysis and DNA sequencing, to name just a few. These have in common the selective optical excitation of molecules - fluorophores, or, in the case of optogenetics, light-gated ion channels - with laser radiation falling within their absorption spectrum. Unambiguous identification of molecules or cellular subsets often requires jointly analyzing fluorescent signals from several fluorescent markers, so that MLEs are required to provide excitation wavelengths for several commercially available biocompatible fluorophores. A number of functionalities are required from MLEs in addition to sourcing the required wavelengths: Variable attenuation and/or digital intensity modulation in the Hz to kHz range are required for a number of applications such as optical trapping, lifetime imaging, or fluorescence recovery after photobleaching (FRAP). Moreover, switching of the laser between two fiber outputs can be utilized for example to switch between scanning confocal microscopy and widefield illumination modes, for instance, for conventional fluorescence imaging.

physics.ins-det↗
arXiv2020

Design of a waveguide-coupled GeSn disk laser

We report on the design of a waveguide coupled GeSn microdisk-laser cavity in which the germanium virtual substrate serving as a template for GeSn growth is repurposed for the definition of passive on-chip interconnection waveguides. A main challenge resides in transferring the optical power from the upper (Si)GeSn gain stack to the underlying virtual substrate layer and is solved with laser mode engineering. Designs are based on experimentally realized layer stacks and waveguide outcoupling efficiencies as high as 27% are shown in compact resonator geometries with a small, 7 $μ$m radius, with 42% of the power being recycled in the laser cavity.

physics.app-ph↗
arXiv2020

Mode-locked laser timing jitter limitation in optically enabled, spectrally sliced ADCs

Novel analog-to-digital converter (ADC) architectures are motivated by the demand for rising sampling rates and effective number of bits (ENOB). The main limitation on ENOB in purely electrical ADCs lies in the relatively high jitter of oscillators, in the order of a few tens of fs for state-of-the-art components. When compared to the extremely low jitter obtained with best-in-class Ti:sapphire mode-locked lasers (MLL), in the attosecond range, it is apparent that a mixed electrical-optical architecture could significantly improve the converters' ENOB. We model and analyze the ENOB limitations arising from optical sources in optically enabled, spectrally sliced ADCs, after discussing the system architecture and implementation details. The phase noise of the optical carrier, serving for electro-optic signal transduction, is shown not to propagate to the reconstructed digitized signal and therefore not to represent a fundamental limit. The optical phase noise of the MLL used to generate reference tones for individual slices also does not fundamentally impact the converted signal, so long as it remains correlated among all the comb lines. On the other hand, the timing jitter of the MLL, as also reflected in its RF linewidth, is fundamentally limiting the ADC performance, since it is directly mapped as jitter to the converted signal. The hybrid nature of a photonically enabled, spectrally sliced ADC implies the utilization of a number of reduced bandwidth electrical ADCs to convert parallel slices, resulting in the propagation of jitter from the electrical oscillator supplying their clock. Due to the reduced sampling rate of the electrical ADCs, as compared to the overall system, the overall noise performance of the presented architecture is substantially improved with respect to a fully electrical ADC.

eess.SP↗
arXiv2020

Silicon Photonics DWDM NLFT Soliton Transmitter

We investigate the transmission of densely multiplexed solitons using a photonic integrated chip and the nonlinear Fourier-transform and analyze required launch conditions, the effect of (de-)multiplexing and noise on the nonlinear spectrum, and equalization techniques that can be used to enhance the transmission performance.

eess.SP↗
arXiv2019

Semiconductor laser mode locking stabilization with optical feedback from a silicon PIC

Semiconductor mode-locked lasers can be used in a variety of applications ranging from multi-carrier sources for WDM communication systems to time base references for metrology. Their packaging in compact chip- or module-level systems remains however burdened by their strong sensitivity to back-reflections, quickly destroying the coherence of the mode-locking. Here, we investigate the stabilization of mode-locked lasers directly edge coupled to a silicon photonic integrated circuit, with the objective of moving isolators downstream to the output of the photonic circuit. A 2.77 kHz 3 dB RF linewidth, substantially improved compared to the 15.01 kHz of the free running laser, is obtained in the best case. Even in presence of detrimental reflections from the photonic circuit, substantial linewidth reductions from 20 kHz to 8.82 kHz, from 572 kHz to 14.8 kHz, and from 1.5 MHz to 40 kHz are realized.

physics.app-ph↗
arXiv2016

Stabilization and frequency control of a DFB laser with a tunable optical reflector integrated in a Silicon Photonics PIC

We investigate the effect of tunable optical feedback on a commercial DFB laser edge coupled to a Silicon Photonics planar integrated circuit in which a tunable reflector has been implemented by means of a ring resonator based add-drop multiplexer. Controlled optical feedback allows for fine-tuning of the laser oscillation frequency. Under certain conditions it also allows suppression of bifurcation modes triggered by reflections occurring elsewhere on the chip. A semi-analytical model describing laser dynamics under combined optical feedback from the input facet of the edge coupler and from the tunable on-chip reflector fits the measurements. Compensation of detrimental effects from reflections induced elsewhere on a transceiver chip may allow moving isolators downstream in future communications systems, facilitating direct hybrid laser integration in Silicon Photonics chips, provided a suitable feedback signal for a control system can be identified. Moreover, the optical frequency tuning at lower feedback levels can be used to form a rapidly tunable optical oscillator as part of an optical phase locked loop, circumventing the problem of the thermal to free carrier effect crossover in the FM response of injection current controlled semiconductor laser diodes.

physics.optics↗
arXiv2016

Silicon Photonics WDM Transceiver with SOA and Semiconductor Mode-Locked Laser

We demonstrate a complete Silicon Photonics WDM link relying on a single section semiconductor mode-locked laser and a single SOA to support up to 12 multiplexed channels with a bit error rate of 1e-12 at serial data rates of 14 Gbps without channel pre-emphasis, equalization or forward error correction. Individual channels reach error free operation at 25 Gbps and multi-channel operation at 25 Gbps is shown to be compatible with standard 7% overhead hard decision forward error correction. Silicon Photonics transmitter and receiver chips are hybridly integrated with driver and receiver electronics. A detailed link model is derived and verified. Particular emphasis is placed on accurate system level modeling of laser RIN, SOA amplified spontaneous emission noise and receiver noise. The impact of the electrical receiver bandwidth and non-Gaussian statistics on level dependent amplified spontaneous emission noise are investigated in detail. The channel count scalability as limited by SOA saturation is further analyzed taking cross gain modulation and four wave mixing into account. While semiconductor mode-locked lasers have been identified as a potential light source for low cost Datacom WDM transceivers for some time, this is, to the best of our knowledge, the first comprehensive investigation of the overall link budget in a Silicon Photonics implementation showing this technology to be a credible contender for low latency datacenter interconnects.

physics.optics↗
arXiv2016

Effect of Electrical Filtering on Level Dependent ASE Noise

We derive an analytical model describing the effect of filtering on amplified spontaneous emission noise during or after opto-electronic conversion. In particular, we show that electrical filtering results in a further reduction of the signal quality factor associated with an effective increase of the noise levels and can lead to counter-intuitive dependencies of the measured signal quality on the characteristics of the test setup. Closed form equations are compared with numerical models and experiments, showing excellent agreement.

physics.optics↗
arXiv2015

Monolithically integrated waveguide-coupled silica microtoroids

We report on the design and fabrication of a new type of microtoroid high-Q silica resonators monolithically coupled to on-chip silicon nanowire waveguides. In order to enable monolithic waveguide coupling, the microtoroid geometry is inverted such that the resonator is formed by thermal reflow at the circumference of a hole etched in a suspended SiO2 membrane. This configuration is shown to be conducive to integration with a fully functional Silicon Photonics technology platform.

physics.optics↗