Progress towards enabling quantum engineering

Coronagraph designs that use photonic integrated circuits have the highest theoretical throughput for off-axis signals, and therefore the highest potential exoplanet yield for future high-contrast direct imaging campaigns. Using the rejected starlight, the photonic integrated circuit may also provide simultaneous wavefront sensing, allowing for the correction of non-common-path aberrations. This work considers how a photonic circuit should be configured to maximize its sensitivity to phase aberrations. Two cases are considered: in the first, the photonic circuit is coupled directly to an electric field in a piecewise manner, while in the second, the circuit is coupled to the field via an optical mode sorter. In either case, this work constructs a unitary matrix, which can be applied by a photonic circuit to produce maximum sensitivity.

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10 dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.

In this work, the guided modes of a photonic crystal polarization beam splitter (PC-PBS) are studied. We demonstrate that the transmission of a low-loss photonic crystal 120° waveguide bend integrated with the PBS will be influenced if the PBS is multi-moded. We propose a single-moded PC-PBS structure by introducing deformed structures, and it shows twice the enhancement of the transmission. This device with remarkable improvement of performance is promising in the use of photonic crystal integrated circuits design.

An on-chip quantum light source based on spontaneous four-wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide-based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon-pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence-to-accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence-to-accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence-count brightness occurred at an optimization length of 1/α, where α is the absorption coefficient. In contrast, the single-count brightness shows slightly different waveguide length dependence owing to loss-induced one-photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon-pair generation according to the source's application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high-fidelity quantum technologies.

Miniaturization of medical imaging devices will significantly improve the workflow of physicians in hospitals. Photonic integrated circuit (PIC) technologies offer a high level of miniaturization. However, they need fiber optic interconnection solutions for their functional integration. As part of European funded project (InSPECT) we investigate fiber bundle probes (FBPs) to be used as multi-mode (MM) to single-mode (SM) interconnections for PIC modules. The FBP consists of a set of four or seven SM fibers hexagonally distributed and assembled into a holder that defines a multicore connection. Such a connection can be used to connect MM fibers, while each SM fiber is attached to the PIC module. The manufacturing of these probes is explored by using well-established fiber fusion, epoxy adhesive, innovative adhesive and polishing techniques in order to achieve reliable, low-cost and reproducible samples. An innovative hydrofluoric acid-free fiber etching technology has been recently investigated. The preliminary results show that the reduction of the fiber diameter shows a linear behavior as a function of etching time. Different etch rate values from 0.55 μm/min to 2.3 μm/min were found. Several FBPs with three different type of fibers have been optically interrogated at wavelengths of 630nm and 1550nm. Optical losses are found of approx. 35dB at 1550nm for FBPs composed by 80μm fibers. Although FBPs present moderate optical losses, they might be integrated using different optical fibers, covering a broad spectral range required for imaging applications. Finally, we show the use of FBPs as promising MM-to-SM interconnects for real-world interfacing to PIC’s.

Photonic integrated circuits (PICs) increasingly require more advanced integrated functionalities and devices to meet the key application challenges. This evolution with the serial integration of optical functions in the same photonic circuit often results in internal reflections and optical feedback, which can specifically destabilize lasers. One solution to overcome this issue is to integrate an optical isolator at the output of the lasers. Among various proposed designs, magneto-optical-based isolators have significantly improved over the past decades in terms of compactness, insertion loss, isolation ratio, and spectral isolation bandwidth. Despite these improvements, the TE optical isolator still lacks performance. This paper introduces a new operational principle for a TE optical isolator based on modal beating in a transverse magneto-optical Kerr effect (TMOKE) coupled-waveguide system. This approach combines evanescent-coupled silicon waveguides and the magneto-optical effect, resulting in a nonreciprocal propagation that can be optimized for optical isolation. This new concept shows promise in achieving a high-performing TE optical isolator, as it does not depend on resonance and is free from constraints associated with interferometers. Based on data from magneto-optical garnet materials, the simulated device has a length of 500 and its 20 dB-isolation bandwidth is as high as 35 nm. With broadband and high isolation, this simulated device opens possibilities for miniaturizing complex photonic circuits used in optical communication, data communication, and optical sensing.

Maskless selective area cw Nd:YAG laser annealing of GaInAsP/InP quantum well (QW) structures has been investigated as a possible route towards the fabrication of monolithically integrated photonic circuits. Laser irradiation of a 5 QW laser structure, originally designed for lasers emitting at 1.52 μm, yielded material having a continuously changing band gap ranging from 1.52 to 1.4 μm over the distance of about 3 mm. Bars with arrays of ridge waveguide lasers, having cavity lengths from 300 to 600 μm, were fabricated from the processed material. An individual bar, 2–3 mm long, comprised lasers operating typically between 1.4 and 1.52 μm. The lasers showed stable threshold current density and high quantum efficiency at all operating wavelengths. The results indicate that the applied approach has the potential to realize the cost-effective fabrication of advanced photonic devices and photonic integrated circuits.

In this paper, we present the development of a miniaturized Laser Doppler Vibrometer (LDV) system, based on the 3D hybrid integration of the Si3N4 platform of LioniX (TriPleX) and the polymer platform of FhG-HHI (PolyBoard). The photonic integrated circuit (PIC) supports all the functionalities of an LDV system including the splitting of the input light to the measurement and the reference beam, the introduction of an optical frequency shift up to 100 kHz, polarization handling and detection of the reflected measurement beam, using a heterodyne detection technique. The optical frequency shift is accommodated in the TriPleX section of the PIC based on a simple serrodyne scheme, where a phase modulator is driven with a sawtooth signal with the desired frequency. The modulation of the optical field is based on the stress-optic effect utilizing thin-films of PZT deposited on top of the waveguide structures of the TriPleX platform, capable of supporting modulation frequencies up to several MHz. The PolyBoard part enables polarization handling and heterodyne detection of the reflected beam using micro-optic elements on chip, including a polarization beam splitter (PBS), a half wave plate (HWP), and a pair of balanced detectors with four photodiodes that are flip chip bonded on the top. The TriPleX and the PolyBoard platform were brought together based on the 3D hybrid integration, using mode size converters and vertical directional couplers with coupling losses lower than 15 dB. On-chip beating, using the integrated photodiodes is experimentally demonstrated.

A design framework to implement non-unitary input–output operations to a practical unitary photonic integrated circuit is described. This is achieved by utilising the cosine-sine decomposition to recover the unitarity of the original operation. The recovered unitary operation is decomposed into fundamental unitary building blocks, forming a photonic integrated circuit network based on directional couplers and waveguide phase shifters. The individual building blocks are designed and optimised by three-dimensional full-wave simulations and scaled up using a circuit approach. The paper investigates the scalability and robustness of the design approach. Our study demonstrates that the proposed approach of performing unitary matrix completion can be applied to any arbitrary matrices. This design approach allows for implementation of non-unitary operations to perform various linear functions in neuromorphic photonics for computing, sensing, signal processing and communications.

For stimulates great developments in high-speed and high-capacity Cloud Computing Technologies (CCT) systems in the future. The all-optical logic gate based on planar photonic crystal (PPC) waveguide is a promising technology. We design an all-optical AND gate in PPC, as an ultracompact component for planar lightwave circuit integration with suitable choice of parameters, perform this task. The PPC waveguides are composed of circular dielectric rods set in two-dimensional triangular lattice. To realize the AND logic function, as predicted using 2D finite-difference time-domain simulations. The combination of the ring and line defect coupler waveguides forms the device. On the basis of our simulations, we found that the optimized scheme maximizes the power transmission above 80% at a wavelength of 1.55 um. Besides, this device can apply to all-optical Arithmetic Logic Unit (ALU) in all-optical computing and potentially applicable for photonic integrated circuits (PICs) in the future.

We propose and numerically demonstrate operation of a processing-free functional photonic integrated circuit based on a semiconductor quantum template. This template combines monolayer engineering of the well/barrier interfaces in quantum-well structures with their coherent nonlinear effects to spatially form desired patterns of refractive index, gain and absorption without any need for top-down processes such as lithography, etching, postprocessing epitaxial overgrowth, etc. As an example, we investigate an integrated circuit that includes a distributed feedback laser and a multitask functional section that can act as an ultra-fast attenuator, modulator or amplifier on demand. The distributed feedback laser is based on gain-without inversion to overcome various losses and induce single mode operation. The photonic circuit proposed in this paper is optically activated and holds the promise of highly functional multicomponent integrated photonic circuits fabricated in a single epitaxial growth.

One of the key challenges in Silicon based optical interconnect system remains to be the efficient coupling of optical signals from the submicron size on-chip waveguides to standard single mode (SM) fibers with low insertion loss (IL) and relaxed alignment tolerance. Large optical alignment tolerance allows optical connectors to be attached to on-chip waveguides passively using standard semiconductor pick-and-place assembly tools that have placement accuracies of 10- 15μm. To facilitate the assembly, optical fiber coupling elements need to be modular and compact. They have to also have low profile to avoid blocking air flow or mechanical interference with other elements of the package. In this paper we report the development of a two-dimensional (2D) SM optical fiber coupling architecture that consists of Si based photonic lightwave circuit (PLC) substrate and a high-density micro-lensed fiber optic connector. The system is compact, efficient and has large optical alignment tolerance. At 1300nm an insertion loss of 2.4dB and 1.5dB was measured for the PLC module and the fiber optic connector, respectively. When the PLC module and connector was aligned together, a total insertion loss of 7.8dB was demonstrated with x,y alignment tolerance of 40μm for 1dB optical loss. The SM optical coupling architecture presented here is scalable, alignment tolerant and has the potential to be manufactured in high volume. To our knowledge, such a system has not been reported in the literature so far.

High-density photonic integrated circuits (PICs) are expected to replace their current electronic counterparts in the future. The most crucial prerequisite for realizing successful PICs is to develop a low-loss coupling technique between active and passive photonic components based on various nanoscale materials and devices. Here we propose and demonstrate an on-chip integration technique in which a high-refractive-index layer constitutes the coplanar structural backbone across the entire PIC chip. To prove the concept, patterns of a two-dimensional photonic crystal (PhC) band-edge laser and grating couplers are engraved into the backbone layer, and colloidal quantum dots (CQDs) for optical gain are selectively deposited in the PhC area by a conventional lift-off process. Using optical excitation, we observe that the CQD–PhC structure emits coherent single-mode laser light, which is subsequently coupled to and propagates through an adjacent slab waveguide in the well-defined directions corresponding to the selected band-edge point, finally emerging through the grating coupler. Our study demonstrates a simple but highly suggestive PIC platform that automatically guarantees high coupling efficiencies between the micro- and nanophotonic devices to be integrated (through high degrees of modal matching in the vertical direction) and will therefore advance the development of high-density PIC technologies.

Efficient photonic circuit simulation tools are essential for the design and optimization of photonic integrated circuits (PICs). However, the computational speed of current commercial simulators remains insufficient to support large-scale circuit optimization and statistical verification. In this paper, a multimode scattering matrix computation method based on a multimode subnetwork growth approach (referred to as MSG) for efficiently calculating the frequency-domain response of photonic circuits is proposed. The presented method starts with accurate photonic devices modeling leveraging the multimode scattering matrix. It then utilizes the established multimodal models to precisely characterize component parameters and topology information of the arbitrarily interconnected multiport photonic circuit. By employing an iterative strategy that connects only two subnetworks at each step, the global response is progressively computed, which significantly reduces the complexity of matrix inversion. The sequence of network connections is optimized using a suboptimal ordering algorithm, further minimizing the total computational cost. Benchmark results demonstrate that our approach enables fast multimode simulation, delivering over twice the speed of commercial simulators and strong engineering applicability.

Silicon photonics (SiPh) is a disruptive technology in the field of integrated photonics and has experienced rapid development over the past two decades. Various high-performance Si and Ge/Si-based components have been developed on this platform that allow for complex photonic integrated circuits (PICs) with small footprint. These PICs have found use in a wide range of applications. Nevertheless, some non-native functions are still desired, despite the versatility of Si, to improve the overall performance of Si PICs and at the same time cut the cost of the eventual Si photonic system-on-chip. Heterogeneous integration is verified as an effective solution to address this issue, e.g. through die-wafer-bonding and flip-chip. In this paper, we discuss another technology, micro-transfer printing, for the integration of non-native material films/opto-electronic components on SiPh-based platforms. This technology allows for efficient use of non-native materials and enables the (co-)integration of a wide range of materials/devices on wafer scale in a massively parallel way. In this paper we review some of the recent developments in the integration of non-native optical functions on Si photonic platforms using micro-transfer printing.