In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.
Silicon-on-Insulator (SOI) has emerged as promising material choice for various integrated optoelectronic devices. Two
issues make SOI attractive for complex optical systems: the cost reduction due to compatibility with CMOS technology
and high refractive index contrast between core and cladding, which is an important property for good confinement of
light and efficient guiding and coupling in sub-micron waveguides. However, for those devices that are intended to be
part of broadband optical networks, for example multiplexers and de-multiplexers, it is desirable to demonstrate a high
selectivity and a tunable response. Thus, it is necessary to provide wavelength selective elements with the ability to filter
input data streams producing a large Free Spectral Range (FSR), a small Full Width at Half Maximum (FWHM), and a
high quality factor (Q), all conditions set by communication standards. Owing to the generic and adaptable operation,
ring-resonator-types of filters in SOI are often considered as candidates to meet these demands. Herein two different
designs are investigated from both experimental and modelling standpoints in order to tailor the filter transfer function.
These are mutually coupled (Vernier) resonators and cascaded resonators based on small SOI photonic wires. Fabricated
filters designed to provide a large FSR and a polarisation independent (PI) response are analysed and improvements
proposed. Issues associated with temperature control of the transfer function have also been addressed.
A theoretical investigation of thermo-optic effect in silicon-on-insulator rib waveguide arrays is presented. Two types of array arrangements are described, with the aim to find the configuration in which the heat is efficiently guided from the heater through the waveguide core without noticeably affecting the other adjacent waveguides. One of these array designs provides excellent thermal and optical separation between waveguides and efficient heat flow, allowing smaller switching power and faster response to be simultaneously achieved. Good thermal isolation for closely spaced SOI waveguide structures is very promising for the realization of dense photonic integrated circuits.
Nowadays, vertical cavity surface emitting lasers (VCSELs) provide a very exciting area of research. The unique geometry of VCSELs results in several significant advantages over their edge-emitting counterparts, including low threshold current, single-longitudinal-mode operation, circular output-beam profile. The optimization of the DBR structure is of fundamental importance to increase the performance of optical systems based on the VCSEL technology.
To this aim, sophisticated modelling techniques are needed, where only negligible or no approximations are included in the calculations. Therefore, we have used the Floquet-Bloch theory (FBT) formalism to simulate the DBR performance of VCSEL structures. In this paper we explain the general VCSEL theory and propose a number of simulations to individuate the optimal configuration of DBR mirrors with the aim to maximise the output power laser and reduce the threshold current density. The VCSEL optimization is carried out by considering the best trade off among various parameters, including period number, materials, and doping concentration and profile. It is clearly shown the superiority of the FBT approach in the prediction of the best DBR performance and VCSEL properties by comparing results (reflectivity, spectrum, peak wavelength, gain) with other well-known methods, such as the transfer matrix method
(TMM) and coupled-mode theory (CMT).
The recent interest in silicon based photonics, and the trend to reduced device dimensions in photonic circuits generally, has led to the need for mode converters to couple from optical fibres to such small devices. A range of structures have been proposed and in some cases demonstrated, including three dimensional tapers, inverted tapers and micromachined prisms. We have previously reported theoretical analyses of a Dual Grating Assisted Directional Coupler (DGADC), which promises high efficiency coupling over modest spectral linewidths. In this paper we report preliminary experimental results on the fabrication of such devices, together with an evaluation of the coupling efficiency. The approach has been to fabricate a demonstrator device for a particular arrangement of waveguide coupling parameters, i.e. we have fabricated a device that couples easily from fibre, because the input waveguide is approximately 5μm in cross sectional dimensions. The mode converter then couples to a 0.25μm silicon waveguide, primarily because comparisons exist in the literature. These results are compared with the predicted efficiency, and the results are discussed both in terms of the constituent parts of the DGADC, as well as the fabrication limitations. Whilst our device is not optimised we demonstrate that it has promise for very high efficiency coupling.
Recently there has been a strong trend to fabricate smaller photonic devices. In the literature, the problem of coupling optical fibres with thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances. This paper discusses a new approach, the Dual Grating-Assisted Directional Coupling (DGADC), which can result in a robust and very efficient device, with relaxed fabrication tolerances. Theoretical investigation of the coupler is presented. Coupling efficiency and device length are determined as functions of layer thicknesses and refractive indices, grating periods, depths and duty ratios, and finally wavelength. Fabrication of the coupler is also given, as well as preliminary experimental results.
There is a trend in photonic circuits to move to smaller device dimensions for improved cost efficiency and device performance. However, the trend also comes at some cost to performance, notably in the polarisation dependence of the circuits, the difficulty in coupling to the circuits, and in some cases, in increased device complexity. This paper discusses a range of Silicon-on-Insulator (SOI) based optical devices, and the advantages and disadvantages in moving to smaller waveguide dimensions. In particular optical phase modulators based upon the plasma dispersion effect and ring resonators are considered, together with a device for coupling to small waveguides, the so-called Dual Grating Assisted Directional Coupler (DGADC). The advantages of moving to small dimensions are considered, and some preliminary experimental results are given. In particular, progress of the DGADC is evaluated in the light of promising experimental results.
Waveguide based Bragg grating devices have the potential of integration with passive or active optical components. A narrow bandwidth Bragg reflection filter or Fabry-Perot resonant structures can be realised using the approach of periodic refractive index modulation in waveguide gratings to form reflective structures. Most authors have considered 1st order Bragg gratings with periods of the order of 228nm operating at 1550nm but at the expense of complexity and high cost of fabrication. This paper describes the design of Silicon-On-Insulator (SOI) rib waveguides operating in the single mode regime that exhibit low polarisation dependence. A rigorous leaky mode propagation method (LMP) has been used to investigate the influence of etch depth in 3rd order Bragg gratings on the reflectance and bandwidth in the waveguides.
A numerical method based on Floquet-Bloch formalism has been used in this paper to investigate the properties and design criteria of 1D multi-quantum-well (MQW) guided-wave planar photonic bandgap (PBG) structures. The approach allows a clear understanding of all the involved physical effects without making any restrictive assumption. 1D guided-wave PBGs including MQW can be used for microresonators and nonlinear tunable filters.
In this paper a detailed analysis and design of guided-wave 2D photonic bandgap filters are presented by using the Floquet-Bloch approach. Significant performance has been obtained in one and three cavity PBG structures.
In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect, which is a relatively slow process. Until recently, most reserachers utilized Silicon on Insulator (SOI) waveguides with cross secitonal dimensions of the order of 5 microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packaging density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. However, coupling light to such small waveguides is relatively inefficient. In the literature, the problem of coupling optical fibers to thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances, due to large difference between the fiber and the waveguide in both dimensions and refractive indices. In this paper, we discuss both the desing of small waveguide modulators (of the order of ~1 micron) together with a novel theoretical solution to the coupling problem. An example of coupling light to a thin silicon waveguide is given, as well as a discussion of a number of modulator design issues.
We present a novel type of grating-assisted coupler for coupling light from an optical fibre to a thin semiconductor waveguide. The Dual Grating-Assisted Directional Coupler (DGADC) consists of two gratings, one thick waveguide with a refractive index similar to the refractive index of the fibre, a thin semiconductor waveguide and a layer with refractive index which is between that of the thick and thin waveguides. Specifically, a coupler in silicon on insulator (SO!) has been analysed. The maximum coupling efficiency for this coupler can approach 1 00%, while the minimum coupling length can be around 2mm.
Widely tunable lasers are promising sources for future high-capacity dense wavelength divison multiplexing and photonic switching systems. These devices can be used for sparing in the cold standby mode, restoring in hot standby restoring, rerouting wavelength rerouting or conversion, or fast switching in all-optical networks. Tunable lasers need to demonstrate some featuers such as wide tunability range, optical output power of 10 dBm or more, cost and structure similar to those of commercial DFB lasers. High performance devices would require low laser chirp, high modulation speed, small size and very high reliability. For system applications, requirements on the tunable laser reliability are very stringent. Reliability studies and appropriate related testing procedures are necessary to define stability of tunable lasers and their expected lifetime. In this paper we propose some reliabilty test 'strategies' useful for qualification of tunable lasers with reference to some critical issues of the main technologies used to achieve the tunability feature.
In this paper, a detailed design and simulation of a Mach- Zehnder-type interferometric lumped modulator is presented. From the curves of relative phase shift between the two modulator arms it is possible to derive information on a set of unknown voltages, when compared with a user-controlled reference voltage, for data classification and identification purposes.
A great research effort has been spent in recent years for the use of guided-wave optoelectronic technologies for signal processing in space applications, because they allow to reduce size, weight, power consumption and system complexity with respect to electronic ones. Based on these motivations, we have conceived, designed and simulated a number of original integrated optical signal processors, such as 2D correlators for imaging reconstruction in SAR applications, both airborne and spaceborne, preprocessors for data classification and identification in satellite applications for remote sensing, beam formers and steerers in active phased array antennas, heterodyne spectrum analyzers, and so on. The substrate materials considered for these circuits are both ferroelectric and III-V semiconductor materials. Original modeling and numerical techniques have been developed to analyze and design the guided-wave components included in the optical architectures. In this paper, the most significant developments of this research activity are reviewed.
In this paper some significant developments of guided-wave devices and
recent technological advances in optoelectronics for space are described. The
performance limits of these devices are highlight in terms of technological
problems, power consumption, operating frequency, sizes and weight.
In this paper, three relevant examples related to space applications are considered of new integrated optic systems: the first one refers to on-board data pre-processing in remote sensing applications, the second one concerns a correlator for SAR processing and the third one is relevant to a beam forming network for active array antennas for satellite telecommunication applications. Exploitation of such opportunities would open new fertile terrain for commercialization of integrated optic devices.
In this work, a generalized model of acoustooptic multifrequency interaction in wave-guiding structures is described. The model, which is based on the generalized coupled model theory, can evaluate the diffraction efficiency in Bragg regime when a multifrequency signal is applied to the interdigitated transducer deposited on the optical waveguide surface. The influence of fabrication parameters of Ti:LiNbO3 waveguides on the diffraction efficiency and on the linear dynamic range of acoustooptic multifrequency Bragg cells has been also considered under monomodal propagation condition. Our results assert that the best performance in terms of linear dynamic range can be obtained on Ti:LiNbO3 waveguides having weak refractive index change and small initial titanium thickness in the medium radiofrequency range. A qualitative comparison has been performed with results obtained by other authors in the case of collinear acoustooptic interaction.
In this work we present the results of a systematic study of H:LiNbO3, H:LiTaO3 and Cs:KTP waveguides obtained by combining measurements of propagation constants, optical profiles, and Raman scattering of the exchanged layers. Also, we present direct measurements of the r33 coefficients in proton exchanged and annealed proton exchanged waveguides in LiNbO3 and LiTaO3 obtained by means of phase modulation technique and quantitative relations between the waveguide characteristics and the doping level in the Cs:KTP layers.
In this work the modeling of non collinear acoustooptic multifrequency interaction in wave- guiding structures is presented. The new model, which is based on the generalized coupled mode theory, allows us to evaluate the diffraction efficiency in Bragg regime when a multifrequency signal is applied to the interdigitated transducer deposited on the optical waveguide surface. The dependence of diffraction efficiency of the output orders on the intermodulation factors and on the transducer driving radiofrequencies has been investigated. The influence of fabrication parameters of Ti:LiNbO3 waveguides on the linear dynamic range of acoustooptic multifrequency modulators has been extensively studied under monomodal propagation condition. Our results assert that the best performance in terms of linear dynamic range (approximately equals 43 dB) can be obtained on Ti:LiNbO3 waveguides having weak refractive index change ((Delta) n < 0.001), small initial titanium thickness (< 100 angstroms) and medium enter radiofrequency (approximately equals 500 MHz).
In this paper recent advances in guided-wave acousto-optic interaction and modeling of SAW devices for high signal processing are presented. SAW devices are extremely attractive for reducing hardware complexity problems and power consumption and improve resolution and efficiency in signal processing operations, involving both acoustical and optical field interaction, or only surface acoustic field propagation.
In this work are presented the design parameters and performance of a guided-wave spectrum analyzer based on a multilayered ZnO/AlxGa1-xAs/AlyGa1-yAs/GaAs structure for heterodyne detection of spread spectrum signals. The adopted circuit configuration includes a double integrated collimating grating having non linear groove profiles, an acousto-optic Bragg cell, a concave frating lens, a focusing grating lens and an output photodiode array. The optical structure has been optimized as a function of the layer thickness, and Al title in order to obtain improved performance of the circuit in terms of 3-dB Bragg bandwidth (up to 185 MHz), surface acoustic wave power consumption (less than 3 mW), frequency resolution (less than 1.5 MHz), and single-tone dynamic range (about 56 dB). The number of channels has been found equal to 112 and 125 in the two structures under investigation, respectively. The calculated single- and double-tone dynamic range are reduced of about 40% with respect to the corresponding dynamic ranges of the homodyne architecture. All the design parameters have been calculated for each integrated component of the circuit at the free-space optical wavelength of 0.85 micrometers .
Correlation filters with three transmittance levels ( +1 , 0, and -1) are of interest in optical pattern recognition because they can be implemented on available spatial light modulators and because the zero level allows us to include a region of support (ROS). The ROS can provide additional control over the filter's noise tolerance and peak sharpness. A new algorithm based on optimizing a compromise average performance measure (CAPM) is proposed for designing three-level composite filters. The performance of this algorithm is compared to other three-level composite filter designs using a common image database and using figures of merit such as the Fisher ratio, error rate, and light efficiency. It is shown that the CAPM algorithm yields better results.
In this work the modeling of multifrequency acoustooptic interaction in guiding structure is presented. This model, based on a generalized coupled mode theory, allows us to evaluate the Bragg diffraction efficiency when a multifrequency signal is applied to the interdigited transducer. The dependence of diffraction efficiency of the output orders and of the intermodulation factors among the same orders on both the applied voltage amplitudes and transducer driving radiofrequencies has been extensively investigated. Planar waveguide structures on lithium niobate have been considered under monomodal propagation condition.
In this paper we present the theoretical study and design of a GaAs-based guided-wave grating device for uniform out-coupling of optical power, to be detected on an external 2D plane. The model of diffraction is based on the exact solution of the relevant boundary-value problem. The device is formed by a number of cascaded gratings, each of them having different geometric characteristics. Simulations have been carried out at different values of grating thickness and number of cascaded gratings, in order to obtain the best performance in terms of diffraction efficiency and device transmissivity. Uniform distribution of the out-coupled power on a 2D region has been achieved by varying the groove depth of each cascaded gratings. This device can be successfully used to obtain uniform illumination of coupled-charge-devices in optical computing and signal processing applications, such as image restoring in synthetic aperture radar systems, spectral analysis, matrical multiplication, and so on.
Two configurations of guided-wave planar devices, one on LiNbO3 and one on GaAs, involving noncollinear acousto-optic interaction, are presented, which satisfy the narrowband and wide tunable range filtering requirements. The analysis of these configurations is accomplished by a sophisticated fully general model of the acousto-optic interaction in multilayer planar guiding structures, which can take into account the electro-mechanical losses at any diffraction regime. WDM devices have been investigated by considering multimodal optical propagation.
In this paper, we present the theoretical investigation, design, and simulation of a new LiNbO3 guided-wave optical correlator suitable for real-time SAR applications. It is based on a complex interferometric structure, involving four aperiodic phase-reversal traveling wave modulators. The electrode structure is designed in order to reproduce the product signal between the received and reference voltages, which is then time-integrated by a suitable photodetector. The filtered signal outgoing from the detector is proportional to the final correlation function, which can be electronically registered and multiplexed on a two- dimensional matrix by sum-and-shift procedure. Thus, the processor performs the correlation function between the reference signal and the received signal when they are applied to the laser diode and to the electrodes as driving voltage, respectively. Comparisons between two different LiNbO3 waveguide fabrication techniques, i.e., proton exchange and titanium indiffusion, have been carried out in terms of circuit performances in order to reconstruct the SAR images.
The design criteria of an all-photonic error control system are presented. It is implemented by cascading a number of iterative cells. The performance is evaluated and optimized by simulating its behavior with the Beam Propagation Method.
Optical interconnection networks for linking a large number of users require the use of particular switching elements. For the classical 2 X 2 switching elements, in order to maintain the control voltage as low as possible, the length of the element cannot be less than a few millimeters. For this reason, the total number of stages implementable on a single LiNbO3 crystal must be contained and the number of users N cannot be above 8. In order to extend the network dimensions, a different network architecture must be chosen which requires a new type of (alpha) X (beta) switching element. Generally speaking, (alpha) equals 2n and (beta) equals 2m with n does not equal m. In this paper we present a 4 X 4 switching element for which two different solutions are considered. The design criteria of these structures are presented and the insertion loss and crosstalk are evaluated.
In this paper we present the theoretical investigation and design of a guided-wave optical processor suitable for beam forming of a linear phased array antenna and based on a planar structure involving two acousto-optic transducers (AOT) having contra-directional surface acoustic wave (SAW) propagation. By applying two different periodic microwave signals to the transducers, the phase content of the output field distribution in the transverse direction with respect to the propagation one is a linear function of the applied frequency difference and can be used to drive a photodiode array connected to the phased array antenna by a RF circuit. The performance of the circuit has been also defined with respect to the resulting beam forming and to the radiation pattern characteristics of the driven microwave array. In this study, acousto-optic interaction on a lithium niobate planar-waveguide has been considered.
Acousto-optic (AO) devices based on surface acoustic wave (SAW) propagation are widely studied for signal processing and optical computing systems. Among them, devices formed by a multilayer acoustic structure of III/V semiconductor materials are of particular interest because of the acoustic dispersion characteristics, which increase the number of design parameters to be used. In this paper, we present an accurate model of the acousto-optic interaction in III/V multilayer guiding structures having N layers of different materials, taking into account electro-mechanical and optical losses. By considering the full SAW field distribution, we calculate the overlapping integrals of diffracted modes in the grating region and compare the results in terms of diffraction efficiency with a well-known approximated design expression. A number of different cases have been analyzed, concerning proton exchanged and titanium indiffused lithium niobate waveguides and III/V semiconductor guiding structures, such as ZnO/AlGaAs/AlGaAs. The AO transducer behavior is defined in terms of diffraction efficiency as a function of the acoustic frequency and geometric parameters. The typical behavior of Bragg-regime diffraction in acoustooptic cells on LiNbO3 and GaAs planar waveguides has been confirmed, when both +1 diffraction order and higher orders are produced by the acoustooptic grating as output guided modes. Interesting diffraction properties have been derived as a function of waveguide structure, transducer acoustic power, grating length and applied acoustic frequency.
In this paper a review of recent advances in the field of guided-wave correlators for real-time optical processing is presented. These advances refer to some configurations which are extremely attractive for reducing hardware complexity problems and power consumption in on-board applications, involving airborne or satellite platforms and focused on remote sensing and synthetic aperture radar (SAR) systems.
In the last few years, a considerable effort in the optoelectronics research field has been spent for the development of a number of guided-wave active and passive components, such as laser diodes, electrooptic modulators, acoustooptic transducers, photodetectors, microlens array, and so on, for fabricating optical devices and circuits for signal processing and computing. The interest related to optical processors is particularly due to a lower power consumption, reduced size, cost and weight, and high throughput with respect to the corresponding electronic processors. In particular, synthetic aperture radar (SAR) applications are well suited for an optics-based processing technique implementation, because the synthesis of the object image, performed by correlating the received radar signal with a reference signal, is equivalent to the optical reconstruction of the Fresnel diffraction pattern of the same object, illuminated with coherent light. Guided-wave optical processors, including acousto-optic transducers and CCD cells, can be successfully applied to the reconstruction of two- dimensional images by using both spatial and time integration. In this paper, we present the theoretical investigation, design, and simulation of a new LiNbO3 guided-wave optical correlator suitable for real-time SAR applications. It is based on a complex interferometric structure, involving four aperiodic phase-reversal traveling wave modulators. The electrode structure is designed in order to reproduce the product signal between the received and reference voltages, which is then time-integrated by a suitable photodetector. The filtered signal coming from the detector is proportional to the final correlation function, which can be electronically registered and multiplexed on a two-dimensional matrix by sum-and-shift procedure. Thus, the processor performs the correlation function between the reference signal and the received signal when they are applied to laser diode and to the electrodes as driving voltage, respectively.
The acousto-optic transducer (AOT) is a fundamental component of a number of guided-wave devices and circuits suitable to the signal processing and optical computing. Most of such devices has been fabricated by using the LiNb03 technology, but some effort has been also spent on components based on lll/V semiconductor technology, i.e. GaAs and related compounds. However, because of the weak piezoelectric effect in these materials, it needs to analyze complex multilayer structures, including an additional ZnO superstrate, which shows an acoustic mode dispersion, due to the dependence of the surface acoustic wave (SAW) propagation speed on the acous.ic frequency applied to the transducer . In this paper a generalized model of the SAW propagation into multilayered structures has been applied to some typical acoustic waveguides, a simple semi-infinite layer of AlxGa-1 _xAs, as a function of the Al title x, a ZnO/AIGaAs structure, and a more complex ZnO/Alx1Ga-|_xiAs/Alx2Gai_X2As/GaAs structure, as a function of the intermediate layer thicknesses, frequency and Al titles x1 and x2.
In this paper, the modeling of the acoustooptic interaction in multilayered guiding structures is presented. The mathematical analysis has been performed on the basis of the generalized coupled-mode theory and applied for the first time to ZnO/AIGaAs/AIGaAs/GaAs complex structures.
In this paper, the design and the simulation of a GaAs acousto-optic correlator for synthetic aperture radar (SAR) data processing are reported. The proposed integrated circuit is available for airborne applications and is able to operate with side-looking focused radar. The range compression is performed by the acousto-optic correlator, driven by the received signal, and the azimuth data compression is obtained by using a transmission mask/charge coupled device (CCD) system. Data for coherent compression are collected according to the pulse repetition frequency (PRF) of the reference chirp signal which modulates the laser beam. The proposed integrated circuit allows SAR signal processing on a substrate with very small sizes (37 X 17 mm2) and low power consumption. A range resolution of 126 cm and an azimuth resolution of 68 cm have been theoretically achieved.
The analysis and design of a Fresnel microlens array are developed in order to couple a large incident guided beam into a channel waveguide array for performing optical computing and signal processing operations. The device includes a planar waveguide region constituted by an AlGaAs/AlGaAs/GaAs epitaxial layer structure, with appropriate Al title and thickness, allowing the propagation of both the TE and TM polarizations of the guided beam. The channel waveguide region can be obtained by the diffusion of zinc in the planar waveguide. Each Fresnel lens of the array is assumed to be of the graded thickness type, and it can be fabricated on an additional AlGaAs layer by zinc diffusion. The designed array exhibits good theoretical properties, including very short focal length and very small focal spot size. It also makes it possible to couple only one polarization of the incident planar wave, i.e., TM polarization. A detailed simulation of the electric field distribution in the focal plane of each lens is also performed.