A p-i-p configuration of an electro-optical modulator based on hydrogenated amorphous silicon (a-Si:H) is characterized and compared with an a-Si:H based p-i-n modulator. In particular, we estimate the performances in terms of optical losses, voltage-length product, and bandwidth at λ=1550 nm for waveguide-integrated p-i-p versus p-i-n configurations. Both devices are fabricated on a silicon substrate by plasma enhanced chemical vapor deposition at low temperature ensuring the back-end integration with a CMOS microchip. We demonstrate a factor of merit for the p-i-p waveguide integrated Fabry-Perot resonator of Vπ×Lπ=19 V×cm allowing the design of shorter devices with respect to p-i-n structure.
The realization of on-chip optical interconnects requires the integration of active micro-optical devices with
microelectronics. However, it is not clear yet how silicon photonics could be integrated within CMOS chips. In this
context the non-crystalline forms of silicon, such as laser-annealed polycrystalline and hydrogenated amorphous silicon
(a-Si:H), can deserve some advantages as they can be included almost harmlessly everywhere in a CMOS typical run-sheet,
yielding low-cost and flexible fabrication. In particular, a-Si:H can be deposited using the CMOS-compatible low
temperature plasma enhanced chemical vapour deposition (PECVD) technique, which brings clear advantages
particularly for a back-end photonic integrated circuit (PIC) integration. However, till now a-Si:H has been mainly
considered for the objective of passive optical elements within a photonic layer at λ=1.55 μm. Only a small number of
examples have been reported, in fact, on waveguide integrated active devices. In this paper we detail about an effective
refractive index variation obtained through an electrically induced carrier depletion in an as-deposited a-Si:H-based p-i-n
waveguiding device. For this device switch-on and switch-off times of ~2 ns were measured allowing a modulation rate
higher than 150 MHz.ÿÿ
Silicon photonics have generated an increasing interest in the recent year, mainly for optical
telecommunications or for optical interconnects in microelectronic circuits. The rationale of silicon photonics
is the reduction of the cost of photonic systems through the integration of photonic components and an IC on a common chip, or in the longer term, the enhancement of IC performance with the introduction of optics inside
a high performance chip.
In order to build a Opto-Electronic Integrated circuit (OEIC), a large European project HELIOS has been
launched two years ago. The objective is to combine a photonic layer with a CMOS circuit by different
innovative means, using microelectronics fabrication processes. High performance generic building blocks
that can be used for a broad range of applications are developed such as WDM sources by III-V/Si
heterogeneous integration, fast Si modulators and Ge or InGaAs detectors, Si passive circuits and specific
packaging. Different scenari for integrating photonic with an electronic chip and the recent advances on the
building blocks of the Helios project are presented.
A three-power-level method for obtaining efficient thermo-optical modulation in an all-silicon waveguide-integrated Fabry-Perot thermo-optic modulator is discussed by means of a thermo-optical analytical model and demonstrated. The thermal system is represented as a two-pole model where, at every time, the temperature in the waveguide core is modified by means of a heater. This temperature is calculated and used in turn for calculating the refractive index. In this way, the impact of the driving signal shape on the device speed performance is assessed. Results clearly indicate that the application of a thermal bias holding the modulator at a higher average temperature with respect to the substrate heat sink allows increasing the modulator speed. An application-specific integrated circuit has been designed and developed in order to test the new modulation logic. The system's electronics is implemented in a 0.8 µm, 5 V, CMOS process. The experimental results of this new three-power-level driver method are reported, showing the shortening of the characteristic transient times.
In this paper, the design of resonant cavity enhanced photodetectors, working at 1.55 micron and based on silicon
technology, is reported. The photon absorption is due to internal photoemission effect over the Schottky barrier at the
metal-silicon interface. A comparison is presented among three different photodetectors having as Schottky metal: gold,
aluminium or copper respectively. In order to quantify the performance of photodetector, quantum efficiency including
the image force effect, as a function of bias voltage is calculated.
In this paper, a methodology for the analysis of a resonant cavity enhanced (RCE) photodetector, based on internal
photoemission effect and working at 1.55 &mgr;m, is reported. In order to quantify the performance of photodetector, quantum
efficiency including the image force effect, bandwidth and dark current as a function of bias voltage are calculated.
We propose a comparison among three different Schottky barrier Silicon photodetectors, having as metal layers gold, silver
or copper respectively. We obtain that the highest efficiency (0.2%) but also the highest dark current is obtained with metal
having the lowest barrier, while for all devices, values of order of 100GHz and 100MHz were obtained, respectively, for the
carrier-transit time limited 3-dB bandwidth and bandwidth-efficiency.
In this work we investigate the possibility to use Zinc Oxide (ZnO) thin films, deposited by RF magnetron sputtering, for
the realization of integrated optical structures working at 1550 nm. Structural properties of sputtered zinc oxide thin
films were studied by means of X-ray Diffraction (XRD) measurements, while optical properties were investigated by
spectrophotometry and Spectroscopic Ellipsometry (SE). In particular, ellipsometric measurements allowed to determine
the dispersion law of the ZnO complex refractive index (see manuscript) = n - jk through the multilayer modeling using Tauc-Lorentz
(TL) dispersion model. We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates
from 2.5 to 3.8 Å/s. Conversely, the refractive index exhibits, from UV to near IR, a considerable and almost linear
variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm. In order to realize a waveguide
structure, a 3-&mgr;m-thick ZnO film was deposited onto silicon single crystal substrates, where a 0.5-&mgr;m-thick thermal SiO2
buffer layer was previously realized, acting as lower cladding. Dry and wet chemical etching processes have been
investigated to achieve controllable etching rate and step etching profile, with the aim to realize an optical rib waveguide.
The etched surfaces were inspected using scanning electron microscopy (SEM) and optical microscopy. Moreover, we
carried out the experimental measurements of the fringes pattern and Free Spectral Range (FSR) of an integrated Fabry-
Perot etalon, obtained by cleaving of a single mode rib waveguide.
Infrared absorption photoinduced by visible light in a-SixC1-x:H is characterized by in guide pump and probe measurements in order to test its applicability to a low-cost micromodulator, fully integrable as a post-processing on-top of a standard microelectronic chip. The Photoinduced Absorption phenomenon in amorphous silicon arises from an alteration of the defect state population by decay of carriers photogenerated by visible light. These levels, deep in the gap, are strongly involved in interactions with IR radiation, and then the VIS illumination modifies their optical properties by increasing the IR absorption coefficient value. Test waveguiding devices are fabricated by Plasma Enhanced Chemical Vapour Deposition on silicon wafers, at temperatures lower than 180°C, and consist of a a-SiC:H/oxide stack. In particular, devices having a-SixC1-x:H cores with different doping and different carbon concentration are characterized. The 1.55 μm probe radiation generated by a DFB laser diode is efficiently transmitted through the a-SixC1-x core thanks to the step index waveguide structure. The pump system consists of low cost AlInGaP LEDs pulsed by a function generator, for an illumination intensity ranging from 0.15 up to 0.85 mW/mm2. Results show that the modulation effect increase for longer pump penetration depth and for higher doping concentration. The phenomenon strongly depends on the carbon introduction in a-Si:H. Digital transmissions tests at 300 kbit/s were performed.
A characterization of low temperature silicon-glass anodic bonding (AB) parameters is presented here. Silicon-glass couples are bonded at temperature and voltage in the ranges of (200-430)°C and (0.2-2.5)kV, respectively. Two different electrodes are used for applying voltage, single point and planar. Low voltage, low temperature and short bonding time are investigated for different glass thickness and electrode type. The results show that the planar electrode provides a bonding time reduced to less than 5min against the few hours obtained by point electrode, and only slightly dependent on glass thickness. The bond strength of the bonded couples starts to be over the bulk glass strength at 300°C, when using planar electrode, and the high quality bond does not show voids. These results are particularly interesting in case of low temperature, and can be considered better than others presented in literature considering the simpler set-up and the novel electrode type used here.
In addition, employment of above mentioned works are demonstrated for the fabrication of sensing microcomponents in lab-on-chip applications. The compatibility of porous silicon (PSi) and the very quick AB process, performed at low temperatures in order to prevent silicon pore filling with thermal oxides, is confirmed here. Satisfactory strength and bond quality was obtained at temperatures as low as 200°C, at voltages of 2500V, with process times lower than 1,5 minutes.
The performances of a digital optical switch, operating at the infrared communications wavelength of 1550 nm and based on the thermo-optic effect in amorphous silicon, are investigated for the first time. We prove that the strong thermo-optic effect of amorphous silicon, combined with the possibility of realizing micrometric integrated structures, allows the design of promising integrated switches. The device, designed for low-cost photonic applications, could be easily integrated in silicon optoelectronic circuits.
The performances of a digital optical switch, based on the thermo-optic effect in amorphous silicon, are investigated for the first time to our knowledge. Numerical simulations shows that the strong thermo-optic effect of amorphous silicon combined with the possibility of realizing micrometric integrated structures, allow the realization of efficient waveguide devices capable of microsecond switching times. The switch, operating at the infrared communications wavelength of 1550 nm, can be easily integrated in silicon optoelectronic circuits and is thought for low-cost photonic applications.
Photoinduced absorption by VIS radiation in a-Si:H has been studied in-guide, in order to realise a novel all-optical waveguide micromodulator for application at 1.3 and 1.55 μm fiber communication wavelengths. In a-Si:H the photoinduced effects and the NIR absorption both involve dangling bonds states. The density of these states, deep in the gap, can be varied with doping. Therefore three waveguide prototypes have been fabricated by Plasma Enhanced Chemical Vapour Deposition on a silicon wafer. Their structure consist of a a-Si:H/SiO2 stack where the a-Si:H cores have different doping. The upper cladding is air. Optical measures on the core materials and signal transmission analysis in-guide at bit rates up to 200 kBit/s have been carried out. The excitation source of the VIS pump system for in-guide analysis consisted of simple, low cost AlInGaP LED’s controlled by a pulse generator. The pump and probe measures have been performed with different pump wavelengths and by varying the illumination intensity. LED’s with wavelengths of 644, 612, 590 and 571 nm have been alternatively used. For each pump wavelength, the light intensity was varied between 0,15 and 0,85 mW/mm2.
The results confirms that the optical modulation of the NIR signal enhances at high doping levels and for longer wavelengths. The modulation speed is probably limited by recombination phenomena.
The performances of a digital optical switch, based on the thermo-optic effect in amorphous silicon, are investigated for the first time to our knowledge. Numerical simulations show that the strong thermo-optic effect of amorphous silicon, combiend with the possibiltiy of realizing micrometric integrated structures, allows the realization of efficient waveguide devices capable of microsecond switching times. The switch, operating at the IR communications wavelength of 1550 nm, can be easily integrated in silicon optoelectronic circuits and is thought for low-cost photonic applications.
In this work the properties of an a-Si:H/GaAs heterojunction are discussed and the analysis of advantages that may result from its use in bipolar devices compatible with the GaAs homojunction technology is performed. Experimental and theoretical results are presented concerning the application of a wide gap amorphous silicon layer to improve the injection efficiency into GaAs regions. Fundamental DC and high frequency characteristics of an a-Si:H/GaAs heterojunction bipolar transistor (HBT) are investigated through detailed numerical simulations. The electronic properties of the a-Si:H layer, as the distributed density of states typical of amorphous materials, have been carefully considered. The tuning of the simulator and the reliability test of its output have been performed on experimental results obtained through the fabrication of a-Si:H/GaAs p-i-n diodes. The study shows that limiting the number of defects located at the amorphous/crystalline interface below a critical level would dramatically improve the minority carrier injection ratio at the heterojunction. The current thin film silicon technology would allow the fabrication of a transistor performing a DC current gain close to 3000 and a cut-off frequency close to 10 GHz. Due to the simplicity of fabrication, such a device could represent an effective way to add a bipolar stage to a GaAs MESFET IC without recurring to AlGaAs/GaAs heterostructures.
Based on the total internal reflection (TIR) phenomenon and the thermo-optic effect in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si), a symmetric rib optical waveguide integrated switch is proposed and theoretically discussed. The device exploits the similar refractive index coupled to the different thermo-optic coefficient in the two materials. The possibility of alloying and doping for the band-gap engineering of a-Si:H, by means of the gas phase composition during the modern plasma enhanced chemical vapour deposition process, which takes place at temperatures as low as 220 degrees C, makes this semiconductor ideal for this type of application. In particular the refractive index at room temperature of the amorphous film can be properly tailored to match that of c-Si in order to achieve the light switching when the device experiences a given temperature change. TIR may be achieved however at the interface by acting on the temperature, because the two materials have different thermo-optic coefficient. The integrated single-mode rib waveguide is 4 μm wide and 3 μm high. The substrate is a SOI wafer with an oxide thickness of 500 nm. The switch has a quite short operation length of about 280 μm. The device performance is analyzed at the wavelength of 1.55 μm. It shows that the output crosstalk and insertion loss are less than -26.9 dB and 3.5 dB, respectively.
A new type of non-perturbing electromagnetic power sensor for microwaves and millimeter-waves, based on the thermo- optical effect in a silicon interferometric etalon cavity is presented. The incident field power is partially dissipated into the all-silicon metal-less etalon, constituting the sensing element of the detector, so causing its temperature increase. This, in turn, induces the intensity modulation of a probe laser beam reflected by the cavity after a multiple beam interference process. The sensing element is directly connected to an optical fiber for remote interrogation, so avoiding the use of perturbing coaxial cables. The performances of such a new class of non-perturbing and wideband probes, in terms of sensitivity and resolution are discussed in detail. The experimental results concerning the characterization of a preliminary prototype sensor are presented and compared with theoretical data. The dependence of the sensor response on the electromagnetic frequency and on the sensing element characteristics is finally discussed.
The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly-doped silicon substrate, using ion-implantation to form the lower cladding, is reported. We exploited a standard microelectronic process step, followed by a calibrated thermal treatment in order to activate and drive-in the implanted impurities, so obtaining a spatially confined lower cladding. The implanted buffer layer enhances the vertical confinement and improves the propagation characteristics. The waveguides were designed with a cross-section comparable in size to the mode-field- diameter of standard single-mode optical fiber, so reducing the fiber-waveguide coupling losses. Propagation losses of about 1.2 dB/cm, for (lambda) equals 1.3 micrometers , in the single mode regime, have been measured. This attenuation is about one order of magnitude lower respect to similar standard all-silicon waveguides. This is the best value of attenuation, to our knowledge, for all-silicon single-mode small-cross-section waveguides reported in literature. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles, both for (lambda) equals 1.3 micrometers and (lambda) equals 1.55 micrometers . As a result of the presence of the ion implanted buffer layer, a strong reduction of propagation losses and an increase of the fundamental mode confinement have been shown. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low-cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally- controlled active optical devices.
The careful design of active and passive silicon-based optoelectronic devices requires the precise knowledge of the refractive index and its modification with temperature. The thermo-optic coefficient ((partial)n/(partial)T) of silicon has been therefore measured in the temperature range from 30 to 300 degree(s)C at the important wavelength of 1.5 micrometers for fiber-optic communication applications. The adopted technique is very simple and reliable, and is based on the measurement of the modal shift induced by temperature variations in a Fabry-Perot resonant cavity made of the material to be characterized, namely silicon. As an interferometric measure scheme, the technique provides high precision and sensitivity. Various samples of silicon were characterized, differing in doping type, doping level, and crystal plane orientation. The reported data allows to rule out the temperature dependence of the thermo-optic coefficient of silicon on the considered technological parameters.
Operation and management of Wavelength Division Multiplexing (WDM) systems require the monitoring of optical channel frequency and power. In this paper we propose a simple and low-cost solution for tracking the frequency of WDM channels, based on the thermo-optic tuning of single or coupled-cavities Fabry-Perot silicon optical filters. The fabricated structures are single-cavity filters exhibiting, thanks to a suitable coating stack on both cavity sides, high finesse and narrow bandwidth. Moreover, the free spectral range is large compared to the channel spacing, allowing the monitoring of one carrier frequency at a time. By means of a heater we change the cavity refractive index and move the transmission peaks, thus scanning the WDM frequency set. In particular, we demonstrate the possibility to resolve up to seven 50-GHz-spaced channels, with a crosstalk of -10 dB, at wavelengths around 1550 nm. Better performances, in terms of resolvable channels and cross-talk can be obtained by using two coupled-cavities, having a common resonance peak and different free spectral ranges. The global optical transfer function of such a cascade shows only one transmission peak in a frequency range of about 30 nm, and can be thermally tuned in this range.
The present work reports on our recent achievements in the exploitation of a simple technology for the fabrication of hydrogenated amorphous silicon (a-Si:H) based low-loss rib waveguides. In particular, waveguides with various widths have been fabricated out of an a-SiC:H/a-Si:H stack deposited by plasma enhanced chemical vapor deposition at the relatively low temperature of 220 degrees C. The ribs were defined by an anisotropic, CH4-based, reactive ion etching process. The devices have been subsequently characterized by cut-back technique. Even though a dependence of attenuation parameter on the waveguide width was observed, propagation losses as low as 0.7 dB/cm could be measured at λ=1.3 μm, in good agreement with he theoretical estimations based on the intrinsic absorption of the material. Starting from the same structure, a Fabry- Perot thermo optical modulator has been also fabricated and tested at the communication wavelength of 1.3 μm.
In the past, absolute quantification of tissue chromophore concentrations has not been possible using near infrared spectroscopy (NIRS). On the contrary, the Critikon 2020 Cerebral Redox Monitor (Johnson and Johnson Medical Ltd) provides real time quantified value for oxyhemoglobin (Hb02), deoxyhemoglobin (HbH) and total hemoglobin (THb) concentrations. It achieves quantification by incorporating the following features : • a 2 detectors approach (to delete extracranial contribution) • an algorithm based on quantified absorption spectra • inclusion of an optical pathlenght and a wavelength dependent scattering factors
A theoretical study aimed at the optimum design of thermo- optic guided-wave switches realized in silicon-on-oxide technology is reported. The heat transfer processes occurring inside such devices are analyzed by means of a 3D finite element thermal simulator. In particular, in rib waveguiding structures, we demonstrate that the optimum values of the oxide layer thickness and of the outer rib height are a function of the heating pulse power used to drive the device. The theoretical results carried out are consistent with the experiments reported in literature.
All silicon Fabry-Perot modulators, operating at the wavelengths of 1.3 and 1.55 micrometers , have been fabricated by VLSI technology. The planar channel cavities, with lengths ranging between 50 and 100 micrometers , have been obtained by plasma etching and not by cleavage of the chips. Therefore, the devices are really integrable with microelectronic circuits or with other photonics components. the modulators, based on the thermo-optic effect, are electrically driven and operate at repetition frequencies up to about 2 MHz. This is by far the best result obtained in thermo-optic effect based modulators. Optimixed version of the devices could find application in low-cost optoelectronic systems at the subscriber units in fiber-to-the-home networks.
A coupled cavity Fabry-Perot silicon electro-optic modulator for X 3 and 1. 5 jun is proposed and analyzed. The modulator consists of alternated slabs with high (silicon) and low (Si02 or air) refractive iixlex arI can be realized by standard microelectronic technique. Varying the optical properties of the silicon slabs by carrier injection it is possible to modulate tl light intensity transmitted across the device. The small size and the low current values requiitd to drive it ai compatible with a small scale integration.
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