Waveguide-coupled micro-ball lens array suitable for mass fabrication

Sub-wavelength scale characterization of on-chip coupling mirrors

Miniature free-space optical beams, originating from on-chip microstructures, are usually measured and quoted without reference to a particular polarization state. We develop an automated platform to characterize tightly focused free-space optical beams in three dimensions. We present a detailed description of each subsystem including the calibration and test procedure. We demonstrate how amplitude and phase are measured at sub-wavelength resolution using a cleaved fiber with a heterodyne reference. Further analysis provides information about the phase and intensity profile of the beam with regards to its polarization content and spatial confinement. We perform a proof-of-concept experiment for a custom waveguide-coupled micro-mirror. The work opens new possibilities for rapid analysis of micro-mirrors in prototyping and optimization of integrated optical systems.

Micro-dispenser-based optical packaging scheme for grating couplers

Due to their sub-millimeter spatial resolution, ink-based additive manufacturing tools are typically considered less attractive than nanophotonics. Among these tools, precision micro-dispensers with sub-nanoliter volumetric control offer the finest spatial resolution: down to 50 µm. Within a sub-second, a flawless, surface-tension-driven spherical shape of the dielectric dot is formed as a self-assembled µlens. When combined with dispersive nanophotonic structures defined on a silicon-on-insulator substrate, we show that the dispensed dielectric µlenses [numerical aperture (NA) = 0.36] engineer the angular field distribution of vertically coupled nanostructures. The µlenses improve the angular tolerance for the input and reduces the angular spread of the output beam in the far field. The micro-dispenser is fast, scalable, and back-end-of-line compatible, allowing geometric-offset-caused efficiency reductions and center wavelength drift to be easily fixed. The design concept is experimentally verified by comparing several exemplary grating couplers with and without a µlens on top. A difference of less than 1 dB between incident angles of 7° and 14° is observed in the index-matched µlens, while the reference grating coupler shows around 5 dB contrast.

Efficient mode coupling between a few-mode fiber and multi-mode photonic chip with low crosstalk

The mode-division multiplexing technology has been developing slowly in the field of fiber-chip-fiber optical interconnects, mainly limited by the inefficient mode coupling between few-mode fiber and multi-mode photonic chip. Here we propose and design a multi-mode coupler composed of tapered few-mode fiber and silicon integrated multistage waveguide tapers, which allows the direct coupling between high-order fiber modes and high-order waveguide modes with high coupling efficiency and low crosstalk. Based on the edge coupling scheme, the six linear polarization modes (LP01x/y, LP11a x/y, LP11bx/y) in few-mode fiber are firstly coupled into the integrated waveguide with low insertion loss, then the input modes are converted to the desired waveguide modes (TE₀, TE₁, TE₂, TM₀, TM₁, TM₂) with low mode crosstalk relying on the mode evolution principle. The obtained results show that the coupling efficiency is higher than 87%, while for the mode conversion process, the efficiency is higher than 99% and the mode crosstalk is lower than -25 dB. The favorable performance achieved may open new perspectives for diverse mode-division multiplexing applications, enabling efficient capacity scaling in fiber-chip-fiber optical interconnects and optical communication systems.

Broadband out-of-plane coupling at visible wavelengths

Photonic integrated circuits require efficient interfaces to fiber optic components for applications in optical communication, computing, and sensing. Current optical interconnects rely on edge or grating couplers with limitations in terms of alignment tolerances, efficiency, and bandwidth, respectively. Here, we present a scalable coupling concept that allows fiber-to-chip coupling to the fundamental transverse electric and transverse magnetic modes based on three-dimensional nanostructures exploiting total internal reflection. We demonstrate close to octave-spanning highly efficient coupling to nanophotonic waveguides in the visible wavelength range. Our coupling scheme can be adjusted for other wavelength regimes and fiber mode-field diameters.

An optofluidic system with integrated microlens arrays for parallel imaging flow cytometry

In recent years, high-speed imaging has become increasingly effective for the rapid analysis of single cells in flowing environments. Single cell imaging methods typically incorporate a minimum magnification of 10× when extracting sizing and morphological information. Although information content may be significantly enhanced by increasing magnification, this is accompanied by a corresponding reduction in field of view, and thus a decrease in the number of cells assayed per unit time. Accordingly, the acquisition of high resolution data from wide field views remains an unsolved challenge. To address this issue, we present an optofluidic flow cytometer integrating a refractive, microlens array (MLA) for imaging cells at high linear velocities, whilst maximizing the number of cells per field of view. To achieve this, we adopt an elasto-inertial approach for cell focusing within an array of parallel microfluidic channels, each equipped with a microlens. We characterize the optical performance of the microlenses in terms of image formation, magnification and resolution using both ray-tracing simulations and experimental measurements. Results demonstrate that the optofluidic platform can efficiently count and magnify micron-sized objects up to 4 times. Finally, we demonstrate the capabilities of the platform as an imaging flow cyclometer, demonstrating the efficient discrimination of hB and Jurkat cells at throughputs up to 50 000 cells per second.

In situ noncontact measurement system and two-step compensation strategy for ultra-precision diamond machining

Ultra-precision diamond machining is a promising technique for non-rotationally symmetrical surfaces with sub-micrometer form accuracy. The measurement and compensation processes in the fabrication process must be conducted carefully to achieve high form accuracy. However, significant challenges remain to improve the measurement accuracy and machining efficiency. Because of the remounting process, the off-machine measurements would reduce the efficiency. On the other hand, contact-type measurements can cause physical damage to some soft materials. To overcome these problems, a noncontact on-machine measurement (OMM) system is developed using two optical probes, and a two-step compensation strategy is proposed to generate a modified tool path. To verify the accuracy of the proposed measurement system, OMMs were performed on a spherical mirror using this system and were later compared with off-machine measurements. To evaluate the compensation strategy, an off-axis paraboloid mirror was diamond-machined and compensated using the proposed method. The results show that the OMM system and compensation strategy are effective for improving the form accuracy while simultaneously enhancing the machining efficiency.

Chip based common-path optical coherence tomography system with an on-chip microlens and multi-reference suppression algorithm

We demonstrate an integrated optical probe including an on-chip microlens for a common-path swept-source optical coherence tomography system. This common-path design uses the end facet of the silicon oxynitride waveguide as the reference plane, thus eliminating the need of a space-consuming and dispersive on-chip loop reference arm, thereby obviating the need for dispersion compensation. The on-chip micro-ball lens eliminates the need of external optical elements for coupling the light between the chip and the sample. The use of this lens leads to a signal enhancement up to 37 dB compared to the chip without a lens. The light source, the common-path arm and the detector are connected by a symmetric Y junction having a wavelength independent splitting ratio (50/50) over a much larger bandwidth than can be obtained with a directional coupler. The signal-to-noise ratio of the system was measured to be 71 dB with 2.6 mW of power on a mirror sample at a distance of 0.3 mm from the waveguide end facet. Cross-sectional OCT images of a layered optical phantom sample are demonstrated with our system. A method, based on an extended Fourier-domain OCT model, for suppressing ghost images caused by additional parasitic reference planes is experimentally demonstrated.