Compact photonic crystal mirrors (PCM) formed in suspended InP membranes are theoretically and experimentally
studied under normal incidence. They are based on the coupling of free space waves with slow Bloch modes of the
crystal. These mirrors provide high-efficiency and broadband reflectivity (stop-band superior to 400nm), when involving
two slow Bloch modes of the crystal. They allow also for an accurate control of the polarization.
These PCMs can be used in new photonic devices, where they replace DBR mirrors. The authors report on the
demonstration of a compact and highly selective (Q>1000) tunable filter at 1.55&mgr;m, using a Fabry-Perot resonator
combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top InP/air PCM. Micromechanical tuning
of the device via electrostatic actuation is also demonstrated over a 20nm range for a maximum 4V tuning voltage. The
active version of this device is also considered: a PCM-VCSEL is studied, combining a solid 40 quarter wavelength
InP/InGaAlAs DBR with a top PCM. First experimental results show a high Q-factor (around 2000) compatible with a
laser regime. We finally demonstrate in this paper a vertical-cavity Fabry-Perot filter with ultimate compactness,
associating two PCMs.
The general aim of this project is to realize optical microsystems for NIR spectroscopy (1.5 μm to 2 μm) using the
InP/InGaAs material system. We have designed an integrated microspectrometer based on a long-wavelength strained
InGaAs quantum well RCE photodiode combined with a wavelength tunability function (MEMS concept). The weak
absorption of the QWs is enhanced by embedding the quantum wells into a micromachined tunable vertical resonator
that consists of multiple InP/air-gap alternate layers that form both the DBR reflectors and the electrostatically tunable
air-gap cavity. The devices are fabricated using a specific MOEMS process based on selective wet etching of an
InP/InGaAs epitaxial layer stack grown by MOVPE. The small size and low cost of these microsystems pave the way to
promising industrial applications, such as non-invasive biological analysis, on-line industrial process analysis and
hyperspectral imaging. The paper focuses on critical design and process issues in order to accommodate residual
stresses in the suspended membranes while preserving a suitable tuning range. We present a specific design optimized
for the monitoring of sugar concentration in water. The selected spectral range for this analysis is comprised between
1650 nm and 1750 nm.
We describe the design and fabrication of novel Fabry-Perot tunable filters based on a deformable structure composed of surface micromachined indium phosphide (InP) / Air Distributed Bragg Reflectors (DBRs). As compared to conventional designs, superior spectral selectivity has been achieved by displacing the resonant cavity into the high index material (InP) rather than in air. This configuration is expected to reduce significantly the lateral losses of the cavity. The filters feature also a novel doping structure for bi-directional electrostatic actuation. We present simulations
and experimental results that demonstrate the effectiveness of this high index cavity concept for improving the selectivity of small dimension tunable MOEMS filters. The devices are fabricated using a multiple InP-air-gap MOEMS technology based on the sacrificial etching of an InP / InGaAs stack A spectral linewidth better than 0.15 nm over a tuning range of 40 nm is experimentally demonstrated. Design improvements for doubling the tuning range are also proposed.
1D and 2D compact photonic crystal reflectors on suspended InP membranes are theoretically and experimentally studied under normal incidence. They are based on the coupling of free space waves with slow Bloch modes of the crystal. We first present monomodal 1D photonic crystal reflectors. Then, we focus on multimodal 1D reflectors, which involve two slow Bloch modes of the crystal, and thus present broadband high-efficiency characteristics. 2D broadband reflectors were also investigated. They allow for an accurate control on the polarization dependence of the reflection. A compact (50 μm x 50 μm) demonstrator was realized and characterized, behaving either as a broadband reflector or as a broadband transmitter, depending on the polarization of the incident wave (experimental stop-band superior to 200nm, theoretical stop-band of 350nm). These photonic crystal slabs can be used in new photonic devices as reflectors, where they can replace multilayer Bragg mirrors. The authors report a compact and highly selective tunable filter using a Fabry-Perot resonator combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top photonic crystal slab mirror. It is based on the coupling between radiated vertical cavity modes and waveguided modes of the photonic crystal. The full-width at half maximum (FWHM) of the resonance, as measured by microreflectivity experiments, is close to 1.5nm (around 1.55 μm). The presence of the photonic crystal slab mirror results in a very compact resonator, with a limited number of layers. The demonstrator was tuned over a 20nm range for a 4V tuning voltage, the FWHM being kept below 2.5nm.
In this paper we describe a new class of tunable interferometric MOEMS devices able to perform tunable wavelength selective beam steering. These GEMOEMS (Grating Enhanced Micro Opto Electro Mechanical Systems) devices comprise a diffraction grating etched on top of a selective and tunable vertical interferometric MOEMS filter. The underlying filter reflects the transmitted orders with controllable amplitude and phase so that they interfere with the reflected orders and modulate the diffraction efficiency distribution into these reflected orders. Our first demonstrator is a 1X2 optical switch designed for operation over the C-band (1530-1560 nm). The diffraction efficiency in the reflected orders is controlled by changing the distance from the grating to a Bragg mirror via an electrostatic actuation of the suspended grating membrane. The devices are fabricated using a multiple InP-air-gap MOEMS technology based on the sacrificial etching of an InP / InGaAs stack. The grating is realized using an electron-beam lithography step. The simulated performances on the C-band show low insertion loss (less than 0.3dB), low ripple (0.15dB), reasonable cross-talk (-15dB), and an estimated switching time around 10μs. These characteristics make such an optical switch a solid contender of a rotating mirror, with the advantage of a much faster response. In this presentation, we introduce the general physical principles of GEMOEMS and describe the design, simulation, fabrication and preliminary experimental results for a simplified 1x2 optical switch. We also propose other prospective devices such as add-drop filters.
The general objective of this presentation is to demonstrate the great potential of InP-membrane photonic devices, with a special emphasis on applications for Optical Communications. Various classes of devices will be presented, which are based on MOEMS (Micro-Opto-Electro-Mechanical) structures, or 2 dimensional (2D)photonic crystals (PC) or a combination of both, according to a '2.5-dimensional' approach, which should broaden considerably the combinations of functionality beyond those presently contemplated with the two first classes. For the MOEMS devices, the basic building block consists in a multi-air-gap/suspended-membrane structure, which can be micro-machined using multi-layered III-V semiconductor based heterostructures : tunable filters will be presented for illustration. For PC devices, the basic building block consists in an InP (and related material) membrane including a 2D PC formed by a lattice of holes : the membrane is either suspended in air or bonded onto low index material, e.g. silica on silicon substrate, in the prospect of heterogeneous integration with silicon based microelectronics. Examples of devices will be presented, specifically micro-lasers based on 2D PC micro-cavities as well as on 2D in plane Bloch modes (2D Distributed-Feed-Back micro-laser). For the '2.5-dimensional' photonic structures, it will be shown that the multi-layer membrane approach, where individual layers or combinations of layers may be structured across to form a 2D PC, is naturally suitable for this purpose. Examples of devices will be presented (2D PC surface emitting micro-sources, switching devices combining vertical MOEMS multi-layer membrane structures and 2D PC).
In this paper, we present a new concept of tunable active Micro-Opto-Electro-Mechanical System (MOEMS) microdevice for specific applications in the 1.3-2.5 micrometers wavelength range such as near infrared spectroscopy or optical telecommunications. The proposed optical structure can be used for the realization of tunable wavelength selective devices (photodetectors or emitters). The device uses a radically new optical design which separates the detector (or emitter) from the filter but place it on top of the filter. As compared to the existing micro-mechanical tunable devices, this concept does present two main advantages such as the improvement of the optical spectral response (forward and backward traveling of the optical waves through the active part) and relaxation of the technological constraints for fabrication (planar monolithic integration of the active component with post-process micromachining). We present here the design, the optical simulations and the fabrication procedure of a first demonstrator consisting in an optically pumped wavelength selective and tunable light emitting diode. The gain active region comprises InAs quantum wires designed for light emission aro7und 1500 nm. The MOEM structure is made of InP/air gap layers. We have obtained an increase of the spontaneous emission by a factor of about 40 and tuning range about 60 nm for actuation voltages up to 15 V.
The general objective of this presentation is to demonstrate the potential of Micro-Opto-Electro-Mechanical (MEOMS) devices based on III-V semiconductor materials, with a special emphasis on applications for Telecommunications. Unlike more classical MOEMS devices, such as shutters, rotating mirrors, etc..., which utilize the concept of geometrical optics, III-V semiconductor MOEMS structures presented here operate via the manipulation of optical interferences. The basic building block consists in a multi-air-gap/suspended membrane structure which can be micromachined using multi-layered III-V semiconductor based heterostructures. This building block is very generic in that it can be designed in a variety of manners allowing for the production of a wide range of optical functions. As a matter of fact the wavelength dependence of the transmittance or of the reflectance depends strongly on the number, the thickness and the successive air- gap/semiconductor pairs, and, given the high index contrast between air and semiconductor, a wide choice of spectral responses can be obtained with very few of them. In addition, a wide choice of electro-opto-mechanical modulations of the spectral responses can be produced by moving vertically, via electrostatic actuation, one or several suspended membranes independently. One single device can be designed in order to achieve one or several functions. Such devices as tunable filters for WDM systems, tunable photodetectors, tunable VCSEL, which are based on this generic building block, will be presented.
It has been shown that it is possible to produce highly selective and continuously tunable filters based on InP material using surface micro-machining. One interesting issue for this kind of device is NIR absorption spectroscopy for gas analysis. In this work, we present the design of a Resonant Cavity Enhanced tunable photodiode for operation around 1.6 micrometer near the C-H stretching frequency for organic molecules such as benzene. For this type of application, the required performances are a large tunability, a high selectivity, a weak temperature dependence and a constant absorption level over the tuning range. To meet these requirements the micro-system must be optimized from the optical and mechanical point of view. The RCE photodiode structure is composed of an air/InP bottom Bragg mirror and a dielectric top Bragg mirror. The cavity includes an air-gap and the InP layer containing a p.i.n. photodiode with absorption in a few strained InGaAs Quantum Wells (QWs). Tuning is obtained by actuating electrostatically the air micro-cavity thickness. A prospective device meeting the optical requirements has been designed. It is based on an absorption region composed of three InGaAS QWs conveniently located in the cavity standing wave pattern in order to optimize the resonant absorption over the tuning range. Optical simulation shows that an absorption level greater than 50% can be achieved. The temperature dependence of the resonance wavelength can be kept below 0.08 nm/(Delta) T(C degrees) at room temperature. The mechanical properties of the micromachined structure has been investigated using finite element analysis.
We demonstrate NIR (1.8 micrometer - 2.3 micrometer) resonant photo-detectors based on inter-band (Ecl- Ehhl) absorption in strain compensated, indium rich, InGaAs quantum wells (QW). Extremely low room temperature dark current densities are achieved by reduction of the active layer thickness combined with low defect density of the pseudomorphic strain compensated QWs. The weak absorption of the QW is enhanced by embedding the quantum well into a vertical resonant cavity. We present the experimental results for a demonstrator designed for a wavelength of 2 micrometer. The device, based on a single In0.83Ga0.17As quantum well and tensile strained barriers for strain compensation, exhibits a selectivity of 9 nm and 18% quantum efficiency. InP/InGaAs and Si/SiO2 material systems are used for the bottom and top distributed Bragg reflectors (DBR) of the cavity, with 20 pairs and 2 pairs respectively. The semiconductor structure is grown by MOCVD. The top Si/SiO2 DBR is deposited after fabrication of p-i-n planar photodiodes. Typical dark current densities are lower than 10-7 A/cm2 at -2 V bias. Conditions for extension of the operating wavelength up to 2.3 micrometer have been obtained experimentally using InAs/GaAs superlattice deposition to increase the thickness of the strained QW. A prospective tunable detector based on an actuable micro-machined air cavity and air/InP bottom DBR is proposed.