Table of contents

We present in this paper the vertical integration of an electro-absorption modulator (EAM) onto a vertical-cavity surface-emitting laser (VCSEL). We discuss the design, fabrication, and measured characteristics of the combined VCSEL and EAM. We previously demonstrated a standalone EAM with an optical bandwidth around 30 GHz. In this paper we present for the first time an optical bandwidth of 30 GHz for an EAM integrated onto a VCSEL. This device exhibits single-mode operation and a very low chirp, below 0.1 nm, even with a modulation depth of 70% which makes this device very competitive for high-speed communications in data centers.

Diamond has attracted great interest in the quantum optics community thanks to its nitrogen vacancy (NV) center, a naturally occurring impurity that is responsible for the pink coloration of some diamond crystals. The NV spin state with the brighter luminescence yield can be exploited for spin readout, exhibiting millisecond spin coherence times at ambient temperature. In addition, the energy levels of the ground state triplet of the NV are sensitive to external fields. These properties make NVs attractive as a scalable platform for efficient nanoscale resolution sensing based on electron spins and for quantum information systems. Integrated diamond photonics would be beneficial for optical magnetometry, due to the enhanced light–matter interaction and associated collection efficiency provided by waveguides, and for quantum information, by means of the optical linking of NV centers for long-range entanglement. Diamond is also compelling for microfluidic applications due to its outstanding biocompatibility, with sensing functionality provided by NV centers. Furthermore, laser written micrographitic modifications could lead to efficient and compact detectors of high energy radiation in diamond. However, it remains a challenge to fabricate optical waveguides, graphitic lines, NVs and microfluidics in diamond. In this Review, we describe a disruptive laser nanofabrication method based on femtosecond laser writing to realize a 3D micro-nano device toolkit for diamond. Femtosecond laser writing is advantageous compared to other state of the art fabrication technologies due to its versatility in forming diverse micro and nanocomponents in diamond. We describe how high quality buried optical waveguides, low roughness microfluidic channels, and on-demand NVs with excellent spectral properties can be laser formed in single-crystal diamond. We show the first integrated quantum photonic circuit in diamond consisting of an optically addressed NV for quantum information studies. The rapid progress of the field is encouraging but there are several challenges which must be met to realize future quantum technologies in diamond. We elucidate how these hurdles can be overcome using femtosecond laser fabrication, to realize both quantum computing and nanoscale magnetic field sensing devices in synthetic diamond.

Illumination design usually requires the collection of a large solid angle of radiation from the light source. However, it is known that elliptical reflectors in combination with extended uniform light sources result in a non-uniform irradiance profile at the secondary focus. Within this paper we propose a design method based on phase space transformations, which includes the source extension from the very beginning. We show that an analysis of the local mapping of the source to the target radiance distribution allows a profound understanding of the effects and in consequence a design concept for an additional freeform lens to correct the uniformity at the secondary focus.

We analyze numerically optical waveguiding structures created in photorefractive media by two incoherent counter-propagating 1D Airy beams under nonlinear self-focusing conditions. We then inject a Gaussian probe beam to test our waveguiding structure. By using an anti-symmetric Airy beam configuration in stationary conditions, we find rich and complex waveguiding structures with multiple input to multiple output configurations and transverse input-to-output shifts up to 13 times the guided beam's waist.

There is still the need for a compact and cost-effective solution for efficient light in-coupling in integrated waveguides employed in photonic biosensors, especially when these waveguides are of submicron dimensions and operate at visible wavelengths. The employment of a vertically stacked taper with a larger input area is proposed to meet this need. The design of the taper is divided into two stages: in the first stage, light is guided downwards by two vertically stacked tapers; in the second stage, an inverted taper directly confines the light inside the waveguide. The design parameters are optimized using commercial software, obtaining a total theoretical light coupling efficiency of 72.25%. The taper is manufactured using SU-8 polymer as the main material, employing standard photolithography techniques at wafer level. After characterization, the results show the practicality of the taper when coupling light from macrometric sources to nanometric waveguides, obtaining an experimental coupling efficiency of 55%. With this vertical taper, a compact, easy-to-couple and cost-effective solution is achieved for waveguide-based biosensors operating at visible wavelengths, opening the way for a truly portable point-of-care biosensor for low-cost and label-free diagnostics.

Gaussian mode families, including Laguerre–Gaussian, Hermite–Gaussian and generalized Hermite–Laguerre–Gaussian beams, can be described via a geometric optics construction. Ray families crossing the focal plane are represented as one-parameter families of ellipses, parametrized by curves on an analog Poincaré sphere for rays. We derive the optical path length weighting the rays, and find it to be related to the Pancharatnam–Berry connection on the Poincaré sphere. Dressing the rays with Gaussian beams, the approximation returns the Laguerre-, Hermite- and generalized Hermite–Laguerre–Gaussian beams exactly. The approach strengthens the connection between structured light and Hamiltonian optics, opening the possibility to new structured Gaussian beams.

This work demonstrates an approach for simplifying fiber-to-chip (edge coupling) packaging by virtually eliminating the longitudinal alignment procedure (also increasing compactness and efficiency) through a fiber lens embedded into the structure of the fiber itself. A parabolic lens, fabricated using focused ion beam milling, with a diameter of 15 μm and height of 5 μm, was embedded 6.5 μm (the working distance of the parabolic lens) below the endfacet of the fiber. The lens focuses a 10.4 μm fiber mode into a spot size of 2.6 μm on the surface of an SMF-28e single-mode optical fiber. The properties of the fabricated lens were studied using the three-dimensional finite-difference time-domain numerical method, and the optimal parameters for maximizing the coupling conditions were extracted. The conversion loss of the lens is estimated to be around $0.5\,\mathrm{dB}$. The insertion loss and lateral alignment of the proposed parabolic lens is comparable to a commercial lensed fiber, while directly ensuring the longitudinal alignment, easing the angular alignment, and providing additional mechanical and environmental robustness.

We propose and demonstrate a simple and highly sensitive optical microphone based on S-shaped tapered fibre (STF). The short pigtailed end of the STF is attached to the centre of a thin circular nitrile diaphragm. The applied acoustic signal deforms the nitrile diaphragm and due to the affixation, the STF structure gets modified leading to change in the bending angles of the two STF bends. As a consequence, the photodetector output, detecting the reflected light intensity of the STF, varies in accordance with the applied acoustic signal. Various properties of the proposed sensing setup can be easily tailored by changing the diaphragm diameter and thickness, and the shapes and size of the STF. For an optimized configuration, the proposed sensor achieves a sensitivity of 3.07 mV Pa⁻¹ and a minimum detectable pressure of 36.48 mPa Hz⁻¹. The sensor shows a linear behaviour up to 1300 Hz and the experimental value of its first order natural frequency is 1455 Hz.