Virginia Gomez

Si moulds for glass and polymer microlenses replication

The concept of the fabrication process of glass microlenses integrated with silicon and polymer replicas is presented. These kinds of microlenses are formed using a silicon master which is wet etched in alkaline solutions (anisotropic etching) and/or in acid solutions (isotropic etching). The control of the times and the selection of the solutions, joined with the designs of the mask for conventional photolithography and the quality of the silicon wafers are the key for obtaining the desired shapes and sizes. The fabricated moulds are used to replicate microlenses in polymer by the standard well known replication technologies and also to fabricate glass microlenses integrated on a silicon frame.

Low-cost micro-optics for PCB-level photonic interconnects

One of the grand challenges in solving the interconnection bottlenecks at the Printed Circuit Board (PCB) and Multi-Chip-Module (MCM) level, is to adequately replace the PCB and intra-MCM galvanic interconnects with high-performance, low-cost, compact and reliable micro-photonic alternatives. Therefore we address the following components in this paper: 1) out-of-plane couplers for optical waveguides embedded in PCB, 2) peripheral fiber ribbons and two-dimensional single- and multimode fiber connectors for high-speed parallel optical connections, and 3) intra-MCM level optical interconnections via free-space optical modules. For the fabrication of these micro-optical interconnect modules, we are focusing at the Vrije Universiteit Brussel on the continuous development of a rapid prototyping technology, which we call Deep Proton Writing (DPW). The special feature of this prototyping technology is that it is compatible with commercial low-cost mass replication techniques such as micro injection moulding and hot embossing. Laser ablation is used at Ghent University for the fabrication of PCB-embedded waveguides and integrated micro-mirrors. The main advantage of this technology is that it is compatible with present-day PCB manufacturing. For the free-space MCM-level optical interconnect module, we furthermore give special attention to the optical tolerancing and the opto-mechanical integration of the components. We use both a sensitivity analysis to misalignment errors and Monte Carlo simulations. It is our aim to investigate the whole component integration chain from the optoelectronic device to the micro-opto-mechanical components constituting the interconnect module.

Semiconductor lasers for quantum sensing

Semiconductor lasers can be used simultaneously as optical sources and optical sensors, as they are extremely sensitive to a small amount of coherent optical feedback. We present a survey on experimental results on optical feedback in semiconductor lasers and on different approaches to describe its effect on the laser properties. We show that for long and moderate long external cavities (hundreds of meters down to centimeters) the Lang-Kobayashi delay model, multiple delays and multimode delay rate equation models are in very good agreement with experiments on edge emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs). We present examples of frequency and polarization mode hopping, periodic and quasiperiodic behavior, different routes to chaos, regular pulse packages, high frequency pulsations and stochastic and coherence resonance, that all have been experimentally and numerically demonstrated. Suitable models for studying laser diodes subject to optical feedback from extremely short external cavity, or ESEC (of the order of the wavelength) are the composite cavity and the multimode butt coupling models that either consider the field amplitudes after multiple reflections in the external cavity (EC) as stationary or treat the whole compound cavity at once. Numerical and experimental studies showed that optical feedback in ESEC leads to detectable change of the laser output power or the voltage drop over the laser for a small change of either the phase or the optical feedback strength. As an example, we discuss experimental and numerical results on spectral and polarization properties of VCSELs subject of insensitive optical feedback from ESEC. The wavelength and the current of polarization switching between the two linearly polarized fundamental modes of the VCSEL are periodically modulated with the external cavity length. High contrast polarization switching is thus possible for quarter-wavelength change of external cavity length. In the case of EEL we experimentally demonstrate that with changing the length of the EC the emitted power, the wavelength and the laser voltage are periodically modulated. We explain the longitudinal mode-hopping between the neighboring composite cavity modes followed by large jumps at the external cavity frequency splitting as a result of the spectral modulation of the effective losses of the composite cavity system.

Mach–Zehnder Interferometer for Real-Time In Situ Monitoring of Refractive Microlens Characteristics at the Fabrication Level

We present a Mach-Zehnder interferometer to monitor in situ and in real-time the geometrical and optical characteristics of refractive microlenses during their fabrication process to increase the fabrication accuracy as well as the sample-to-sample reproducibility. We demonstrate for one test case that the deviation of our developed interferometric system against commercial instrumentation is smaller than 5 % while its repeatability error is smaller than 1.5% after calibration against dedicated off-line instrumentation. To conclude, we demonstrate the practical usefulness of this interferometer by applying it to the fabrication of microlenses by deep proton writing.

Characterization of Refractive Index Distribution in Spherical Microlenses Fabricated by Deep Proton Writing

In this letter, we present the results of the refractive index measurements on deep proton writing microlenses. The measurement method used is interferometric tomography. It is a nondestructive method for the determination of the 3-D internal refractive index distributions. The influence of the different fabrication steps on the refractive index distribution is discussed.

Optical characterization of semiconductor microlenses using a Mach-Zehnder interferometer in the near-infrared region

We present a Mach-Zehnder interferometer to characterize semiconductor microlenses in transmission. We therefore make use of a wavelength of 1550nm with the possibility of expansion towards the IR spectrum. In this paper, the concept of our interferometer as well as the set-up is explained. We demonstrate the working principle and measurements on fused silica and silicon microlenses and benchmark the experimental results with measurement data obtained with well established micro-optics instrumentation tools.

Interferometric method for in-situ monitoring of fiber insertion in 2D fiber connectors fabricated through Deep Proton Writing

Deep Proton Writing (DPW) is a rapid prototyping technology allowing for the fabrication of micro-optical and micro-mechanical components in PMMA, which are compatible with low-cost replication technologies. Using DPW, a high-precision 2D fiber connector featuring conically-shaped micro-holes for easy fiber insertion, was realized. When populating these fiber connectors by fiber insertion and fixation, a critical issue is the accurate control of the fiber protrusion. The use of laser interferometry to measure the fibers facet position with respect to the connector surface to within a few micrometers, is inconvenient in view of the measurement range as compared to the fiber dimensions. In this paper, we propose an interferometric method for in-situ monitoring of the fiber insertion depth, based on the phenomenon of low temporal coherence light interference in a Twyman - Green setup. In addition, achieving a few micrometers measurement range with low coherence light requires vertical scanning of the sample under test. The design of the experimental setup and the achieved measurement results are shown and discussed.

Real-time in situ sag characterization of microlenses fabricated with Deep Lithography with Protons

Today different technologies exist that allow the fabrication of individual high-quality micro-optical refractive components and more in particular spherical microlenses. In this paper we will focus on the characterization of the latter components obtained with Deep Lithography with Protons (DLP). In the past we first fabricated the DLP microlenses and secondly a full geometrical and optical characterization was performed. However, this working method is very time consuming due to the amount of experiments needed for a complete calibration of our fabrication process. Therefore, we developed an interferometer for a real-time in situ sag characterization of the microlenses. In a first step we built a Mach-Zehnder interferometer working in the visible wavelength range and demonstrated its proof-of-principle for the determination of the microlens sag. In a next step we then transferred the concept of this interferometer to the closed reactor in which the in-diffusion of monomer vapour in the irradiated zones takes place. This novel approach will allow us to continuously monitor the volume expansion of the desired areas until spherical microlenses with a specific lens sag are obtained.

Real time and in situ monitoring of microlenses fabricated with deep proton writing

Micro-optical components are of growing interest and used in very different applications such as displays, biophotonics, optical-data communication... More in particular, refractive microlenses and refractive microlens arrays are widely used. The fabrication of these components has been extensively investigated and today different technologies are already commercially available such as thermal reflow, laser ablation, reactive ion etching, microject printing... These technologies allow the fabrication of high-quality microlenses in different materials, however these fabrication methods are often too expensive and too time-consuming for prototyping. In our facilities, we implemented Deep Proton Writing (DPW) as a rapid prototyping technology to fabricate plastic refractive microlenses and microlens arrays. To reduce the calibration time and minimize the influence of uncontrollable external parameters we built a transmission Mach-Zehnder interferometer allowing to monitor in situ and in real-time the growing of the refractive microlenses. This means that we can stop the growing process of the microlenses as soon as the predefined specifications are reached. Additionally we can determine out of this interferometric data the geometrical properties and optical quality of each of the microlenses. We have studied the precision and accuracy of our interferometer for the characterization of the latter components. In this paper, we will present the latest results showing the performance of our set-up and the resulting enhancements of our technology.

Low-Cost Micro-Optical Modules for Datacommunication to Optical Interconnections from the LAN-to the PCB-Level

One of the grand challenges in solving the interconnection bottlenecks at the printed circuit board (PCB) and multi-chip-module (MCM) level, is to adequately replace the PCB and intra-MCM galvanic interconnects with high-performance, low-cost, compact and reliable micro-photonic alternatives. Therefore we address the following components in this paper: 1) out-of-plane couplers for optical waveguides embedded in PCB, 2) peripheral fiber ribbons and two-dimensional single- and multimode fiber connectors for high-speed parallel optical connections, and 3) intra-MCM level optical interconnections via free-space optical modules. For the fabrication of these micro-optical interconnect modules, we are focusing at the Vrije Universiteit Brussel on the continuous development of a rapid prototyping technology, which we call deep proton writing (DPW). The special feature of this prototyping technology is that it is compatible with commercial low-cost mass replication techniques such as micro injection moulding and hot embossing.