Arnaud Gardelein
email: arnaud.gardelein@univ-nantes.fr.

Sylvain Le Tacon

Eric Tanguy
email: eric.tanguy@univ-nantes.fr

Nicolas Breuil

Tchanguiz Razban
email: tchanguiz.razban@univ-nantes.fr

Electrooptic microwave antenna using organic poled polymers

Keywords: Electrooptic, organic polymer, characterization, antenna

INTRODUCTION

Fiber optic is well suited to transmit high bit rate between country long hall or between two cities medium hall, but fiber optic is an expensive channel to connect every building, as fiber to the home (FTTH). Microwave communications, through LMDS channel, is a relevant approach to transmit data to the home. Optoelectronic transmitters are expensive to produce and present high electrical power consumption, we propose a new approach based on a low cost polymer electro-optic antenna, as shown on figure 1, to receive radiofrequency signal in the 0.1 -30 GHz range. Here the operating frequency is 6.3 GHz to meet Wi-Fi requirements. The trend on home connexions is to use increasing frequencies. Thus the purpose of the device is to enhance sensitivity by using resonant phenomena both in microwave and optical domain. Our current researches focus on the receiving part of the antenna, the microwave to optical part, where the user sends the signal to our optoelectronical transmitter. The purpose is then to receive the information carried out on microwave signal, and to transmit directly to optical one. The free space radiofrequency signal is captured and condensed inside the device.

Input light

Incident radiofrequency wave

Moduled output light

The latter uses the Pockels effect in materials : the electrically-induced index modulation. The optical index's change modifies the propagating optical waves speed, then results in a variation of the polarization. The polarization modulation can be converted in amplitude modulation by using polarizer or other means.

Two electrooptic-enabled materials are under interest. The older one is lithium niobate, which presents high electrooptic coefficient (35 pm/V at 1550 nm), but high dielectric constants (ε r ≈ 43 and 28) and index (n ≈ 2.2), and has a high production cost. Nevertheless electrooptical modulator with lithium niobate have been proven to work up to 40 GHz. By the way, the bandwidth of this kind of modulator is limited due to the high velocity mismatch between optical and radiofrequency waves.

Poled polymers are promising electrooptical material. The low permittivity (ε r ≈ 3) and optical index (n ≈ 1.6) reduce the velocity mismatch inside the material, thus providing a bandwidth up to 102 GHz. [START_REF] Kuzyk | Characterization techniques and tabulations for organic nonlinear optical materials[END_REF][START_REF] Chang | Time stretching of 102ghz millimeter waves using novel 1.55-um polymer electrooptic modulator[END_REF] Whereas actually electrooptical polymer properties and stability are lower than lithium niobate ones, the gap between those technologies is decreasing. The first polymer we use in this work is a PGMA backbone, associated with a DR1 side chain chromophore with epoxide cross-linkable function. [START_REF] Foll | Preparation of curable polymers for use in nonlinear optical devices[END_REF] This polymer has the advantage to combinate the well-known PGMA/DR1 electrooptic properties with long-term stability provided by cross-linking.

The optimal size of the device depends mainly on the electrooptic and dielectric polymer's properties. In the following, we will present the electrooptic and micro-wave frequencies characterizations of the material, and an overview of the probe design.

ELECTROOPTICAL CHARACTERIZATION

Since poled polymer films have uniaxial anisotropy, and by symetry considerations, we need to know only the r 31 and r 33 elements of the electrooptical tensor. [START_REF] Kuzyk | Characterization techniques and tabulations for organic nonlinear optical materials[END_REF] We use a reflexion technique, first proposed by Teng and Man. [START_REF] Teng | Simple reflection technique for measuring the electro-optic coefficient of poled polymers[END_REF] The goal of this free-space method, presented figure 2, is to provide an estimation of the final electrooptic coefficient. Among other assumptions, this technique states that r 33 = 3r 31 and there are no other reflexion than the one at the top electrode. By the way it has been shown that the factor would be between 3 and 6 rather than 3. In this experiment, we measure the modulated portion of the detected optical signal I m , and I max the intensity just before the analyser. Knowing the modulation voltage V m , the polymer optical index n, the incidence angle θ and the wavelength λ, we obtain the electrooptic coefficient r 33 :

r 33 = 3λ 2πV m n 2 -sin 2 θ n 2 sin 2 θ I m I max (1)

Samples preparation

A glass subtrate was first coated with 1-µm-thick ZnO as bottom electrode. Then the polymer, a crosslinkablee PGMA/DR1 side-chain system, dissolved in (1,1,2-Trichloroethane) was spun-cast on the ZnO. The samples were then poled by corona effect, by applying 5 kV over 1 cm between the pin and the ground electrode and warming the sample up to the glass transition temperature and then cross-linking temperature. Thereafter, the top aluminium electrode was evaporated. The layer superposition is shown figure 3.

1

Experimental setup

The electrooptic coefficient characterization setup we used is presented Fig. 4. First, laser beam goes throught a fiber optic an a fiber collimator. Two IR wavelength can be selected using this fiber : 1310 nm and 1550 nm. After the collimator, the laser beam propagates in free space. A polariser, sets the laser beam polarization at +45°of sample's axis. The Soleil-Babinet-Bravais compensator cancels the static relative phase retardation caused by the polymer anisotropy.

Then the beam goes through the sample, reflects over the aluminium top electrode. The analyzer, set in quadrature of the polarizer, changes the polarization modulation in intensity modulation. The photodetector, a PDA400 from Thorlabs, is used to convert the optical modulation in a electric modulation. The detected signal is measured by a voltmeter to get the continuous component of the signal, and a lock-in amplifier to measure the modulated part. An oscilloscope simplifies the sample alignment and displays the modulating voltage.

Results

The measurement results over 4 samples are reported in Fig. 5. The incidence angle is 35°, where the detected signal amplitude is maximum. We used a laser optical power of 1 mW, thus the signal amplitude detected with the lock in amplifier was in the 0 -50 µV range.

Analysis

The values of electrooptic coefficient are lower than the 10 pm/V expected : we measured values around 3.2 pm/V at 1320 nm. For reference we reported the electrootpic coefficient values measured using the same samples at Thales Research & Technology (TRT), Orsay, France. At the common wavelength, 1310 nm, values are in good agreement. Since there was 8 months between the two measurements, we can conclude the polymer stability is good over time.

We also compared those results to litterature, [START_REF] Kuzyk | Characterization techniques and tabulations for organic nonlinear optical materials[END_REF] where the polymer, the sample preparation and the measurement technique are quite similar : a sidechain PMMA/DR1 polymer, poled using the corona method and measured using throughplane method. Our measurement is in good agreement with the reported value of 2.6 pm/V at 1550 nm.

MICROWAVE CHARACTERIZATION

The purpose of the microwave characterization is to know the value of the relative dielectric constant of electrooptics polymers. However, this kind of material can be only deposited in thin layer in the range of 1 to 5 µm. Above this range, we have to face with fissures appearing at drying phase.

To solve this problem, we use a two-steps method to get the relative dielectric constant. First we determine an approximation of the low frequency dielectric constant, and this value is then used to size an high-frequency characterization structure.

Low frequency estimation

A simple permittivity measurement method for thin film is a capacitance method. It involves the measurement of three parameters. Indeed, the dielectric constant ε r is related to the surface S the thickness d and the capacity C by :

C = ε 0 ε r S d (2)
Where ε 0 is the free space dielectric constant. We then have to mesure the capacity C, the surface S and the thickness d to quickly obtain the low frequency dielectric constant.

Sample preparation

The polymer is deposited on an aluminium layer over a glass substrate using spin coating at 4000 rpm during 30 seconds. Then 16 2-mm diameter aluminium electrodes are evaporated on the sample's polymer layer. For each small condensator we measured the capacitance at 1 MHz using an impedance/gain-phase HP4149A. The capacity measurement error is 4 % in the worst case. For thickness measurement, we use a destructive method. We cut other samples deposited using same parameters, and measure the thickness with SEM photography. The precision is ± 50 nm.

V

Results

As experience validation, we measured the dielectric constant of PMMA layers. There was 4 samples of 16 condensators, meaning 64 capacity measurements. Those measurements are presented Tab. 1. The evaluated thickness is 970 ± 50 nm. We reported table 1 the permittivity computed using this value. Therefore, the average permittivity is ε r = 2.79 ± 0.15. This value agrees with litterature's value of 2.6.

Microwave permittivity measurement

Since we will use our electrooptic antenna at discrete microwave frequencies, we can focus on resonant characterization methods, to get the dielectric constant only for working frequencies.

A simple microwave resonant structure is a patch resonator. Its resonant frequency is given by [START_REF] Lee | Handbook of microstrip antennas[END_REF] :

f mn = c 2π √ ε r • mπ L ef f 2 + nπ W ef f 2 (3)
With L ef f the electric length given by L ef f = L + ∆L, where ∆L is :

∆L = 0.412d (ε ef f + 0.3) L d + 0.264 (ε ef f -0.258) L d + 0.813 (4)
and ε ef f the effective permittivity :

ε ef f = ε r + 1 2 + ε r -1 2 • 1 + 10 d L (5)
The replacement of L by W in the previous equations gives W ef f . Unlike the capacitor method, the thickness is not really needed for thin films : when d goes to 0 in (4), ∆L tends to be 0. Then, to know the diectric constant, we only have to measure the first resonant frequency of the structure with a VNA, and using the length and the width of the patch, we can retrieve the microwave permittivity.

Sample preparation

The samples are made through the same process as the capacitors, but there is only one rectangular shaped top electrode.

The main drawback of the method is the way to put microwave energy in the structure : we cannot put a SMA connector directly onto the patch. We then use a 1 cm long microstrip line as an interface between the VNA and the patch. We solder a SMA connector to the VNA side of the line, and for the patch side, we solder a short thin wire. The wire's other end is connected to the patch using metallised paint. The patch groundplane is linked to the microstrip line groundplane by the same way. A prepared PMMA/DR1 sample is shown on photograph figure 7.

Results

Before applying this method on polymers substrates, we first tested it on thick substrates, i.e. we would like to prove the validity and to study the thin wire influence.

We measured the reflexion coefficient for two substrates : Rogers RT/Duroïd 6006 and Teflon glass (Fig 8). The reference plane used for calibration is the end of the microstrip line. Unlike the thin film case, here we have to take the substrate thickness in account. The measured frequency and the permittivities deduced from those measurement are shown Tab. For the teflon glass, measurement is in good agreement with constructor provided values of 2.55 ± 0.15. Whereas the measurement for the Rogers RT/Duroïd 6006 differs of 8% to 6.15, given by Rogers Corp. Thoses results show that this method can be used for material characterization. As first thin-layer experiment, we use the PMMA-DR1 sample shown on photogragh Fig. 7. The measured frequency, reported in Tab. 2, is GHz and 6.30 ± 50 MHz. The deduced permittivity is then between 2.52 ± 0.5. This value differs from 16% with the value of 3.01 found in litterature. [START_REF] Subramanyam | Microwave characterization of electrooptics polymers[END_REF] This result should be take carefully, as it is our first measurement using this method. We are currently working on the result accuracy improvement, by lowering the frequency measurement error.

THE ELECTROOPTIC ANTENNA

A metallic rectangular shape over a metallic substrate is a conformal patch antenna. Then the Teng and Man experimentation samples are a kind of first electrooptic antenna. At resonant frequency, the patch structure concentrates the electric field between the top electrode and the ground plane. The concentration may be as high as 1000:1, meaning 1 V/m amplitude incident wave generates 1 kV/m inside the structure. The choice we made here is then to reduce the device bandwith and then increasing the sensistivity inside the bandwith. Since the coating method and the synthetized polymer volume both limit the thickness of our samples, to enhance the performances, the idea is to increase interaction between the microwave signal and the optical beam inside the structure. The optical path length can be increased using Fabry-Perot effect inside the structure. [START_REF] Wang | Design and fabrication of polymer-based asymmetric Fabry-Pérot electro-optic modulators[END_REF] The total optical path length vary with cavity finesse, but we expect a ratio from 10 to 100 compared to a single reflexion structure.

Using this novel approach, combining microwave and optical resonant structures, we expected a overall sensitivity ratio of 10000:1 comparted to the non optimized case.

We are currently working on a quantification of this novel approach. Later, the antenna would be pigtailed to build an high-sensitivity electrooptic E-field sensor. [START_REF] Gaborit | Low invasiveness, high-bandwidth vectorial pigtailed electro-optic sensors for high power electromagnetics measurements[END_REF]

CONCLUSION

In this paper we focused on the characterization of organic polymer. We measured values up to 3.35 pm/V at 1310 nm and 1.98 pm/V at 1550 nm for optical coefficient of crosslinkable PGMA/DR1 polymer and proved the stability of the material over 8 months. We also presented two dielectric constant measurement methods. The low frequency capacitybased permittivity measurement was demonstrated using a PMMA polymer and a value of 2.79 ± 0.15 was found in good agreement with litterature. We achieved microwave permittivity measurement using a resonant patch method. The result of 2.52 ± 0.5 for PMMA/DR1 is in the order of magnitude of litterature's values.

Our future work will concern the enhancement of the electrooptic antenna, using a Fabry-Perot cavity in optical domain and a patch antenna in microwave domain to incresase the sensitivity of the antenna.

Figure 1 .

 1 Figure 1. Electrooptic antenna

1

 1

Figure 2 .

 2 Figure 2. Teng and Man principle

  Photo-50 µV/300 msLock-in amplifier

Figure 4 .

 4 Figure 4. Teng and Man experimental setup as implemented in IREENA labs.

Figure 5 .

 5 Figure 5. Electrooptic coefficient measurement results.

Figure 6 .

 6 Figure 6. Low frequency capacity measurement principle

Figure 7 .

 7 Figure 7. Picture of a PMMA/DR1 sample with Al evaporated top electrode. Electrode is 15 mm wide, 12 mm long.

Figure 8 .

 8 Figure 8. Reflexion coefficient measurement for two substrates thickness : mm-thickness as Teflon glass and RT/Duroïd 6006, µmthickness as PMMA/DR1.

  Sample cut. Polymer is deposited over ZnO glass subrate. An Al electrode is evaporated onto the polymer layer.

	1 µm	Al
	1 µm	Polymer
	1 µm	ZnO
	mm	Glass subtrate
	Figure 3.

Table 1 .

 1 [START_REF]Polymer Handbook[END_REF] Mean capacity and per-sample estimated permittivity for each sample, using 970 ± 50 nm for thickness.

	Sample Mean capacity	Permittivity
	1	77.22 ± 0.55 pF 2.71 ± 0.17
	2	85.63 ± 1.12 pF 2.99 ± 0.20
	3	77.21 ± 3.21 pF 2.69 ± 0.26
	4	79.46 ± 2.12 pF 2.77 ± 0.22

Table 2 .

 2 2. Measured frequencies and deduced dielectric constants for two thick substrates. Frequency values given at ± 30 MHz for thick subtrate and ± 50 MHz for thin subtrate.

	Substrate	Measured	Permittivity
		frequency
	Thick substrates
	Teflon glass 0.6 mm thick	7.654 GHz 2.54 ± 0.02
	Rogers RT/Duroïd 6006 1.27 mm thick 5.043 GHz 5.62 ± 0.07
	Thin subtrate
	PMMA-DR1	6.30 GHz	2.52 ± 0.5

ACKNOWLEDGMENTS

The authors wish to thanks C. Monnereau, A. Scarpaci, E. Blart, F. Odobel from LSO, Nantes for synthetising the polymer, R. Seveno, D. Averty and H. Gundel from IREENA, Nantes for polymer depositing and poling and N. Barreau from LAMP, Nantes for depositing the aluminium Layer. Cookson Electronics, Cholet, France which realized the electrode evaporation masks is thereby granted. P. Le Barny and M. Vergnolles from Thales Research and Technology should also be granted for the help in electrooptic measurements. And last, Paul Molina, IRCCyN, is acknowledged for the building of some vital pieces for the electrooptic characterization experiment.


Jean-Marie Gauthier
email: jmgauthi@femto-st.fr

Fabrice Bouquet
email: fbouquet@femto-st.fr

Ahmed Hammad
email: ahammad@femto-st.fr

Fabien Peureux
email: fpeureux@femto-st.fr

Verification and Validation of Meta-Model Based Transformation from SysML to VHDL-AMS

Keywords: SysML, VHDL-AMS, Verification, Validation, Unit Testing, Meta-Model Transformation, Code Generation

This paper proposes an approach to verify SysML models consistency and to validate the transformation of SysML models to VHDL-AMS code. This approach is based on two main solutions: the use of model-to-model transformation to verify SysML models consistency and writing unit tests to validate model transformations. The translation of SysML models into VHDL-AMS simulable code uses MMT (Model to Model Transformation) ATL Atlas Transformation Language and M2T (Model To Text) Acceleo tooling. The test validation of the model transformations is performed using EUNIT framework.

CONTEXT AND MOTIVATION

Critical heterogeneous systems design is a complex activity since such systems are often composed of mixed hardware and software components. This complexity makes it difficult to validate the conception of the entire system in regards of its requirements. Moreover, strengthened by the growing complexity and intensive use of such systems, the validation activity still defines a key challenge for engineering practices. It is then crucial to set up a process in order to verify and simulate the system model at the earliest stage of the design process. In this context, the Object Management Group (OMG) promotes the SysML language [START_REF] Friedenthal | A Practical Guide to SysML: The Systems Modeling Language[END_REF] to model heterogeneous systems, while the IEEE association proposes the standard VHDL-AMS [START_REF] Bakalar | VHDL-AMS-a hardware description language for analog and mixedsignal applications[END_REF] to describe and simulate hardware components. In this paper, we propose an approach based on Model-Driven Engineering (MDE) techniques to verify and transform SysML models into VHDL-AMS code.

The first goal of this approach was to define a subset of the SysML meta-model and to design the VHDL-AMS meta-model addressed by the SysML subset. The transformation between these meta-models, called SysML2VHDLAMS, was introduced in [START_REF] Gauthier | Transformation of SysML structure diagrams to VHDL-AMS[END_REF]. The second goal is to verify the SysML model consistency in regards of design rules. The third goal is to validate the transformation process using unit testing techniques. These last two goals define the contributions of this paper.

The paper is decomposed into five main parts. Section 2 introduces some backgrounds about languages and MDE techniques. Section 3 describes the approach we used to verify the SysML model consistency in regards of design rules. In Section 4, we present white-box testing method to validate the transformation. Finally, Section 5 and 6 give some related works, conclude and outline our future work.

PRELIMINARIES

This section introduces languages, tools and concepts of model-driven engineering in order to generate VHDL-AMS code from SysML models.

SysML Language

SysML [START_REF] Friedenthal | A Practical Guide to SysML: The Systems Modeling Language[END_REF] is a profile of UML 2.0 [START_REF] Rumbaugh | The Unified Modeling Language Reference Manual[END_REF] standardized by the Object Management Group (OMG) and the International Council on Systems Engineering (INCOSE). SysML consists of several diagrams, which are requirement, use case, sequence, activity, block, internal block, constraint blocks, parametric, state machine and allocation. It supports the specification, design, verification and the validation of a broad range of systems.

We use a subset of SysML expressiveness to represent requirements and to model components of a system with their interactions. Therefore, we use elements that are part of the Requirements, Block Definition, Internal Block and Parametric Diagrams.

VHDL-AMS Language

VHDL-AMS [START_REF] Bakalar | VHDL-AMS-a hardware description language for analog and mixedsignal applications[END_REF] is the IEEE standard modeling language created to provide compatibility and capability in an open language for modern analog, mixed-signal and multi-domain designs. It provides both continuous-time and eventdriven modeling semantics. VHDL-AMS also facilitates modeling of multi-domain systems that can include (among others) a combination of electrical, mechanical, thermal, hydraulics, and magnetic models. This allows the entire system to be modeled, simulated, analyzed, verified and optimized.

The ATL Language

ATL (ATL, ) is a model transformation language and toolkit initiated by the AtlanMod team (previously called ATLAS Group). In the field of MDE, ATL provides ways to produce a set of target models from a set of source models. An ATL transformation program is composed of rules that define how source model elements are matched and navigated to create and initialize the elements of the target models. An ATL transformation can be decomposed into three parts: a header, helpers and rules. Rules are the heart of ATL transformations because they describe how output elements (based on the output meta-model) are produced from input elements (based on the input meta-model). They are made up of bindings, each one expressing a mapping between input and corresponding output element.

Acceleo Language

Acceleo (Acceleo, ) is a language specialized on code generation based on Eclipse Modeling Framework (EMF) models. The Acceleo language defines templates to control the output generation. Most of the time, the output is a programming language. Acceleo requires the definition of EMF meta-models and one or more templates that translate the model into text.

EUnit Framework

EUnit [START_REF] Garcia-Domnguez | EUnit: a unit testing framework for model management tasks[END_REF] is a unit testing framework based on the Epsilon platform. It is designed for testing model management tasks, such as model-to-model transformations, model-to-text transformations or model validations. It notably provides convenient tools for testing ATL transformation. Moreover, EUnit unit tests can be executed automatically in a continuous integration process. Figure 1 shows the workflow of the EUnit testing framework. Before writing a rule, the engineer creates a SysML model (test case) and a VHDL-AMS model (expected result of the transformation). These two models are next used by EUnit (by calling ATL). Finally, EUnit compares the result of the ATL transformation with the expected result: if they are equal, the verdict is true, false otherwise.

MODEL VERIFICATION

This section concerns the verification of the model consistency before generating VHDL-AMS code. The issue of the verification raises two questions:

• How to ensure that the generated VHDL-AMS code does not contain syntactic and semantic errors? For instance, the naming of a component with reserved words of the VHDL-AMS language (syntactic error) or the connection of two ports with different types (semantic error). • How to provide to the final user, i.e. system engineer, a SysML modeling guideline to generate correct VHDL-AMS code?

To answer these questions, we provide a verification approach based on the ATL technology by generating a list of problems written in a XMI file. This file is then parsed in order to show problems in an user interface. As ATL provides a model-to-model transformation based on meta-model, we define the metamodel that allows to represent a problem.

The Problem meta-model

The problem meta-model of the Figure 2 is the target of a transformation, which is called SysML2Problem. The meta-class Problem represents a problem with its location in the SysML file, its description and its severity. The severity of the problem is represented by the enumeration Severity, which could be warning, error or critic.

SysML to Problem Transformation

To illustrate this transformation, we propose to study the case of reserved words. We wish to forbid the naming of SysML elements with reserved words of VHDL-AMS. A rule named uncorrectNameReserved-Word is defined in the ATL file. This rule, see Figure 3, generates a problem only if the precondition um-lElement.isReservedWord() is evaluated to true. This precondition uses an ATL helper named isReserved-Word(), whose aim is to compare the name of umlElement with a set of reserved words.

Results

The SysML2Problem transformation is implemented as an Eclipse plug-in for the Topcased modeler (TOP-CASED, ). It includes 31 rules with 24 helpers, and provides a way to check that, for instance, identifiers are well formed, properties are typed or constants are always initialized. This kind of approach is well suited to check SysML models consistency in order to generate correct code. Indeed, it provides a way to write rules that automatically browse and check the SysML elements that can potentially bring errors in the generated VHDL-AMS code. Moreover, different levels of severity help the system engineer to adopt a specific methodology for modeling.

VALIDATION USING EUNIT

This section is about the validation of the SysML2VHDLAMS and SysML2Problem transformation with the framework EUnit. It also introduces a tool chain, which provides a transformation process applying a Test-Driven Development (TDD) strategy. The TDD approach advocates writing unit tests of a function before writing its code. TDD, and more generally unit testing, ensures that what is developed remains functional, and allows a very early detection of bugs in the development cycle. Figure 4 shows the BDD of this system. The block Vchirp represents the generator. It is linked by composition to the Resistor, VCVsin, PolySource2 and RampSource blocks. The block Resistor receives the chirp signal in order to display it during the simulation. The block VCVsin generates a sinusoidal signal. The component RampSource enables the amplitude of the signal to change over time, whereas the component PolySource2 enables the frequency to change over time. Finally, the block VSignal contains VHDL-AMS statements to initialize the simulation time.

The proposed MDE approach is based on two consecutive model transformations: the first one, called MMT (Model to Model Transformation), transforms SysML models into VHDL-AMS model representation using ATL, while the second one, called M2T (Model To Text), transforms the obtained VHDL-AMS models into VHDL-AMS source code using Acceleo. All the ATL and Acceleo transformation rules are available in [START_REF] Gauthier | Transformation of SysML structure diagrams to VHDL-AMS[END_REF]. As instance, Figure 5 shows the ATL transformation of SysML block into VHDL-AMS entity and architecture. A SysML block is transformed only if the precondition in the from section is true. The ATL transformation result takes thus the form of a VHDL-AMS model, which is then translated into VHDL-AMS code by the Acceleo engine. Figure 6 shows the main concept of Acceleo with the dedicated keyword template.

Validation of Transformations

We now illustrate the validation of three kinds of ATL rules. The first kind of rules (Figure 5) only contains a helper: notParentGeneralization(), which verifies that the block is not a generalization of another block. If the precondition of the from section is true, the section to of the rule is automatically executed by the ATL engine. In other words, if the precondition is true, the section to is covered. To test this rule, it is necessary to cover its associated control flow graph given in To create a test case, we create a SysML model (test input) and its expected transformation result (expected test result). This test case, which contains a block without generalization link, is therefore sufficient to cover the Block2Entity rule: path ab of the associated control flow graph.

The second category of rules contains if statements. For instance, the Figure 8 shows a rule with two if statements and the Figure 9 depicts its associated control flow graph. Note that diamonded nodes of this graph represent decision nodes. According to [START_REF] Myers | The Art of Software Testing[END_REF], the decision coverage usually can satisfy statement coverage. In our case, this criterion is sufficient (even if it is rather weak) because no variable is modified during the execution of ATL rules. Thus, we have to write enough test cases to ensure that each condition in decision has a true and a false outcome, and that each statement is executed at least once. From this point of view, we assess that decision coverage can be met by three SysML models covering paths abc, adef and adgh.

The last rule, given in Figure 10, has an or statement. It comes from the SysML2Problem transformation and prohibits the use of out and inout direction. Its control flow graph is shown in Figure 11. In this case, the decision coverage can be met by two SysML models covering paths abcd and abef.

Results

This section has presented a white-box testing method to validate ATL transformation from SysML to VHDL-AMS. We have created 41 unit tests (SysML models) to validate 34 ATL rules and 23 helpers of the SysML2VHDLAMS transformation. Concerning the SysML2Problem transformation, we wrote 41 unit tests which validate 31 rules and 24 helpers.

The use of decision coverage enables the complete coverage of our ATL rules both for SysML2Problem and SysML2VDHLAMS transformations. Moreover, unit tests are by an ANT file, which allows to validate transformations by running test cases in a continuous integration manner.

RELATED WORK

Further works have been studied in the model transformation testing domain, as introduced in [START_REF] Jochen | Validation of Model Transformations -First Experiences Using a White Box Approach[END_REF], by using white box approach for deriving test Some of these techniques concern the meta-model coverage testing. For example, the tests generation tool of [START_REF] Baudry | Model Transformation Testing Challenges[END_REF] is based on the idea that unit tests should cover the source meta-model. However, this approach does not take care of preconditions that define constraints of transformations. But, our approach aims at performing structural testing on ATL rules, which implies that the test cases have to be essentially based on the preconditions of ATL rules.

Fleurey et al. [START_REF] Fleurey | Validation in model-driven engineering: testing model transformations[END_REF] present an initial exploration of techniques for model transformation validation. They discuss two approaches to generate test data using systematic and bacteriologic algorithm. The systematic approach provides a way to build a model that covers as many items as possible of the effective meta-model. They compare this approach to the bacteriologic approach, which is an adaptation of genetic algorithms.

In [START_REF] Erwan | Metamodel-based Test Generation for Model Transformations: an Algorithm and a Tool[END_REF], the authors propose an algorithm and a prototype that build test models from the input meta-model and a set of model fragments. The set of model fragment is automatically deducted from the meta-model with a test criterion provided by the user. Constraints of transformation are not taken into account, which implies that test cases do not cover the transformation code with a white-box testing approach.

Sen et al. [START_REF] Sagar | On Combining Multi-formalism Knowledge to Select Models for Model Transformation Testing[END_REF] and [START_REF] Sagar | Automatic Model Generation Strategies for Model Transformation Testing[END_REF] have developed a tool called Cartier that generates hundreds of models conforming to the input domain and guided by different strategies. However, this tool is based on a black-box testing method that does not take the internal model transformation design requirements into account.

Finally, [START_REF] Mottu | Model transformation testing: oracle issue[END_REF] identify and discuss important issues that are relevant to define transformation testing oracles. Among them, we can underline the model comparison techniques with an oracle using an expected output model. This technique is very close to our approach since EUnit provides an engine that compares (using EMFCompare framework) expected data with the obtained transformation result. WORKS Verification of SysML consistency and validation of model transformation allow to improve the quality of critical systems design process. In this paper, we have presented an approach to verify the model consistency in regards of design rules. We have used an ATL transformation to generate problems from SysML models. Indeed, ATL provides an engine that automatically browses the source meta-model to execute the transformation process. Consequently, this technique is relevant to verify the modeling process in order to generate correct code, i.e. which does not contain any syntactic or semantic errors.

An approach to validate model-to-model transformation with EUnit has been presented. This approach is part of a larger work and provides a way to validate our transformation toolchain by using white-box testing method. The decision coverage criterion used and presented in this paper allows to ensure the exhaustive coverage of ATL rules. Moreover, unit tests could be automatically executed in a continuous integration manner and Test Driven Development context.

Further challenges have been identified in the domain of model transformation testing. The main limit of this approach is that the user has to define and write test cases and expected outputs. The automatic generation of test cases for model transformation has been discussed in several works and it would be possible to implement such a tool to generate test models with the same coverage criteria for ATL rules. For instance, the automatic generation of SysML models from the interpretation of header and from section of ATL rules.

Finally, the Acceleo model-to-text transformation has not been validated by unit tests, even if it has been validated with several experiments. It would be interesting to define a dedicated unit testing approach to validate Acceleo templates.

Figure 1 :

 1 Figure 1: EUnit workflow.

Figure 2 :

 2 Figure 2: Problem meta-model.

Figure 3 :

 3 Figure 3: ATL rule for reserved words.

Figure 4 :

 4 Figure 4: BDD of the chirp signal generator.

4. 1

 1 SysML Model to VHDL-AMS CodeThis subsection explains the transformation of SysML model into VHDL-AMS code using MDE technologies. The SysML model typically uses the Block Definition Diagram (BDD), in the Internal Block Diagram (IBD) and in the parametric diagram. To illustrate this transformation, we introduce the model of a chirp signal generator, i.e. a system which generates a chirp signal. A chirp signal has frequency and/or amplitude that changes over time. The purpose of such a generator is to enable the activation of systems such as Inter Digital Transducer (IDT).

Figure 5 :

 5 Figure 5: ATL rule Block2Entity.

[

  template generateEntity(du : Sequence(DesignUnit))] ENTITY [entity.name/] IS END ENTITY [entity.name/]; [generateArchitecture(entity.architecture)/] [/template] [template generateArchitecture(archi : Architecture)] ARCHITECTURE [archi.name/] OF [archi.entity.name/] IS BEGIN END ARCHITECTURE [archi.name/]; [/template]

Figure 6 :

 6 Figure 6: Acceleo templates.

  Figure 7. Note that squared nodes of this graph represent boolean OCL expressions.

Figure 7 :

 7 Figure 7: Control flow graph of the rule (i).

Figure 8 :

 8 Figure 8: ATL rule with an "if" statement.

Figure 9 :

 9 Figure 9: flow graph of the rule (ii).

  Figure 10: ATL rule with an "or" statement

ACKNOWLEDGEMENTS

We would like to acknowledge the support of the Regional Council of Franche-Comté with the SyVad project (http://syvad.univ-fcomte.fr/syvad/) and the ANR-2011-BS-03-005 Smart Blocks project.


Vu Nguyen

Nam Man Hoai

Anh Tran Kim

Minh Le Quoc

Eric Tanguy

Optical properties of Eu3+ doped Y2O3 nanophors

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J Charrier

C Lupi2

L Hajf1

E Tanguy2

C Boisrobere

GRADED AND STPE INDEX BURIED OPTICAL WAVEGUIDES FABRICATED FROM P-TYPE POROUS SILICON: PROCESSING AND MEASUREMENTS

The realisation of optical buried waveguides fabricated from porous silicon layers is presented. The refractive index of porous silicon layer varies according to its porosity and its oxidisation process conditions. So either step or graded index waveguides are achieved. These waveguides are formed by a localised anodisation of heavily doped P-type silicon wafers. Measurements at a wavelength of l.ôum yield waveguide losses below 4 dB/cm.

Introduction

Photonic integrated circuits on silicon require waveguiding through a material compatible with silicon integrated circuit technology. Porous silicon (PS), whose refractive index varies according to its porosity [1], is a good candidate for use in optical interconnect technology. Many results on the optical properties of oxidised or unoxidised porous silicon have been reported and optical waveguides have been fabricated and characterised [2-3-4]. In this paper, the fabrication of different optical waveguides from PS layers is reported. Buried waveguides are realised using a simple photolithographic process. The lowest porosity layer, the waveguiding medium, is formed on top of a cladding porous layer. To consider the propagation of light within an optical waveguide it is necessary to take the refractive index of the medium into account. So this refractive index i.e. the porosity of the waveguiding layer has to be perfectly controlled. The refractive index of waveguiding layer is modulated ill order to modify the propagation zone. The shape of the propagation zone and the optical loss measurements for these different waveguides are described below.

Sample processing and experimental arrangement

Buried waveguides were fabricated by the localised anodisation of heavily doped (0.007 n.cm) p-type (lOO)-oriented silicon wafers to form PS using an electrolyte composition of HF:H20:ethanol (2:5:1). The mask consisted in two-layers of Si02 and amorphous silicon (100 of and 400 nm thickness respectively). Windows apertures were opened by photolitographic patterning and reactive ion etching. The layers of different porosities (i.e. different refractive indexes) were formed by abruptly changing the anodisation current to provide the waveguide structure. Refractive indexes of PS layers were modulated by varying the current intensity step by step. The time of each step lasted 70 s. The thickness of the waveguiding and cladding layers was controlled by the anodisation time and was equal to 10 urn. Some of the prepared structures were oxidised at 800DC during 30 minutes in dry O2 following a preoxidisation step at 300DC for one hour. The characteristics of different waveguides are summarised in table 1.

Optical waveguide properties were determined by loss measurements and by the near field guided mode profiles at À=1.3~m. Transmission loss in the waveguide is isolated from the coupling loss by the cut-back technique. In our experiments we used a waveguide test bench. The cleaved waveguide under observation is illuminated by light from a semiconductor laser (À=1.3~m) with the use of a microlensed fiber. Injection is optimised by monitoring the output signal of the guide with an IR camera. Then the light is collected at the output of the cleaved waveguide with x40-microscope objective and measured using a photodiode.

Results and discussion

Buried waveguide structures are fabricated utilising porous silicon multilayers. Indeed, the refractive index of PS decreases with increasing porosity. Moreover, the porosity increases with increasing current density for a given HF concentration [START_REF] Halimaoui | Porous silicon science and technology[END_REF]. So, by switching between high and low current densities during the anodisation process, layers of high and low porosities (low and high refractive indexes respectively) can be obtained. The geometry of the waveguides resembles that of a half cylinder (figure 1). A flat bottom is also observed and its size is dependent on the aperture mask. The thickness of each of the two layers constituting the waveguides is fixed by the anodisation time. Two types of buried waveguides were fabricated in order to modify the propagation zone of light. The first type (samples A as-anodised and B oxidised) was realised with two-PS-Iayers by applying two constant current intensities. Nevertheless, the surface in contact with the electrolyte was not constant during the anodisation process and increased so that the density of current decreased as the porosity decreased. This resulted in an increase in the refractive index as illustrated in figure 2. In this case, the induced radial porosity gradient involves an opposite radial variation of waveguiding layer refractive index. Thus, its highest value is near the waveguiding layer/cladding layer interface. .. 11-:.. Since the light will only propagate in the medium that has the highest refractive index, the propagation region will be near that of the interface. These waveguides have a non controled graded refractive index. Figure 2b illustrates the near field mode profiles of sample B. These experiments confirm the suspected variations in the refractive indexes with anodisation current and the localisation of the propagation region of light within the waveguide. A "flattened U" shape is obtained. The propagation zone is near the wageguiding/cladding interface.

The second type of sample, a step index buried waveguide (sample C), was realised by increasing the applied current intensity during the anodisation. Thus the current density is kept constant for each of the two layers of the waveguide as shown in figure 3a. A homogeneous near field profile is obtained inside the waveguiding layer (figure 3b).

-a- 11-:..

II-

Waveguiding] Cladding layer : layer

I . I X (urn)
Y (urn) The last type (sample D) is an intentionally graded index waveguide. The current density was kept relatively low at the initial time of PS formation and then increased as the PS grew. The increasing current density meant an increase in the porosity and so a decrease in the refractive index. The waveguiding layer index nïr) decreases with the radial distance from a maximum value of n, at the air/waveguiding layer interface to reach a constant value n2 at the waveguiding layer/cladding layer interface. The propagation region is suspected to be near the air/waveguiding interface.

The measured optical losses (table 1) for the oxidised PS waveguides range from 3.75 dB/cm to 9dB/cm which are much lower than that measured for unoxidised ones (22 dB/cm). In fact, the PS absorption coefficient is more important than that of the oxidised PS. Nevertheless, this does not account for this important loss difference between oxidised and unoxidised PS waveguides.

For the sample A, the propagation region is near the waveguiding/cladding interface and the optical losses are principally due to the roughness of this interface. The most important losses of Band C samples would be due to the appearance of interfaces (figure 1-b) during the formation of graded or step indexes by varying the current intensity. To reduce these losses a ramp of current intensity should be applied with a shorter time in order to reduce the effects of these interfaces with regard to the light wavelength.

Conclusion

The refractive index modulation is demonstrated.

Step and graded index waveguides are achieved. The propagation zone varies from air/waveguiding layer interface up to waveguiding layer/cladding layer interface. Optical losses remain important but the major loss mechanism is scattering due to the macro-roughness at different interfaces. It may possible to minimise these losses by better controlling the applied intensity current with a shorter time. Lastly, a better understanding of the PS oxidation process also leads to a reduction in optical losses.

Figure 1 :

 1 Figure 1 : Porous silicon buried waveguide, a) as anodised (sample A) and b) after oxidation (sample B).

Figure 2 :

 2 Figure 2 : a) Schematic representation of applied current intensity and refractive Index (samples Aand B); b) Near field profile for sample B.

Figure 3 :

 3 Figure 3 : a) Schematic representation of applied current intensity and refractive Index (sample C); b) Near field profile for sample C.

Table 1

 1 Experimental characteristics of samples. Iw and Ie are the applied current intensities used respectively to form the waveguiding layer and the cladding layer.

	sample	I	Iw	I	le	I oxidation	I	refractive index of I optical losses waveguiding layer (dB/cm)
	A		Constant		constant	no		Increasing index	22
	B		Constant		constant	yes		Increasing index	3.75
	C		Increasing		increasing	yes		Constant index	9
	D		Increasing		constant	yes		Decreasing index	7

Acknowledgements

The authors are grateful to K. Mehadji and M. Thual from FranceTelecom for the near field pattern measurements and the supplied micro lensed fibers.


C Lupi

D Leduc

B Froppier

E Tanguy

G Normand

C Boisrobert

OPTICAL LOW COHERENCE REFLECTOMETRY: NEW DEVELOPMENTS

Introduction

Optical low-coherence reflectometry (OLCR) is a technique based on interferometry which is design to detect and localize reflectors [l]. This technique has been widely investigated. It is now possible to scan samples with high spatial resolution and constant high speed [2]. However, the envelopes of the interferograms are usually considered as the essential part of the measurement results.

We report here on an OLCR experimental set-up designed to obtain simultaneously high accuracy, high stability and high reproducibility on the location of propagation defects along optical waveguides. One of our objectives is to get informations of the phase in the Fourier transform. We firstly describe the fringe counting system which is used to measure the length of the reflectometer reference arm and to trigger the interferograms sampling. We secondly describe the third interferometer coupled to a white light source whose response starts the fringe counting. We use our system as a spectrometer to test the stability and the reproducibility. Sources with very different broad spectra are investigated. One of them is filtered through a Fibre Bragg grating sample under stress and the results are plotted against the stress conditions.

Experimental set-up

The basic functions operate from three different Michelson interferometers (Fig. 1):

1. the reflectometer: the waveguide (or sample under test) is part of the test arm 2. the fringe counting for accurate length measurement: the light source coupled to the interferometer is a frequency stabilised HeNe 630 nm laser and the signal is sampled every 80nm 3. the high resolution "single" trigger signal is obtained at the output of an interferometer around the white light fringe: the light source is a broad band superluminescent LED. These three interferometers are coupled via a moving carriage as it can be seen on figure 1, so that the variations of the red and infrared optical pathlengths are correlated. Visible and infrared interferograms are detected respectively by Si and InGaAs PIN diodes connected to low noise amplifiers. The red fringe couting output is transformed into a TTL signal which samples the infrared interferograms before the data acquisition .. The white light interferograms obtained at the output of the third interferometer connected to the broadband superluminescent LED have a central fringe width less than .85 urn, This gives a very precise reference for the position of the moving carriage. ,.".\ ;' ,""' Iil ,1\\1111",,1,1,.

--.---.. / D.t:.T.

figl : experimental set-up

The reflectometer center part is a broadband singlemode coupler. The sources we mostly use are 1.3 urn and Lôurn LEDs . The reference arm is in free space: the light emerging from the coupler is collimated by reflection on a parabolic gold mirror to avoid chromatic aberrations and dispersion, and then directed to a corner cube fixed on the moving carriage. The cornercube reflects it to the fixed plane gold mirror (see figure 1). The device under test is inserted in or coupled to the test arm.

Commercially available OLCR provide limited informations derived from the envelop of the interference patterns. They only detect and give the position of reflecting surfaces from the interferogram signals. When a more accurate description of the reflector is needed such as properties bound to the phase, the entire information contained into the signal must be taken into account [3][4][5][6]. The informations on the phase are also contained into the fringes : if x is the displacement of the moving carriage and crthe opposite of the wavelength, the detected intensity can be written as

1=11+12 +TF[S(O')D(O')r(O')]

where /1 and /2 are respectively the intensity in the reference and test arm.

The interference term is the Fourier Transform of the product of three functions :

• SeO') = Ip(O't is the source spectra, 0' is the inverse of the wavelength • D(O') = e-if3(a)aL describes the propagation ( and the dispersion) of the wave between the entrance of the component and the reflecting surface.

• r( 0' ) is the reflection coefficient, It depends on the nature• and the form of the reflecting surface. It is then possible, using inverse Fourier Transform, to measure the dispersion inside the device under test and the spectral signature of the reflectors.

Experimental results

We observed experimentally that the mechanical stability of our system is good: for a one meter displacement of a moving carriage the variation of the continuous signal is about 10%. But its main advantage lies in the linear sampling which leads to a perfect reproducibility of the experiments. An example of the degree of reproducibility is given on figure 3 : spectra of the source taken at different times are shown, with a fibre ended by a mirror in the test arm. Such a device can then be used as a spectrometer with a good resolution (about 30pm). In order to test this sensibility, the light reflected from a fibre bragg grating (FBG) has been used as a source for the reflectometer. This source is a sharp peak of 300pm width, centred at 1.55f.lm, as shown on figure 3 where is plotted a measured spectrum of the light reflected by the FBG. 'P""".lol-~c"J.e~••..• 'P""".lol ---. 'P""".lo4 ---'P""".lo5 --'P""".lo6 --::, ("~/ -~~\,/" "-"\ : We use the FBG as a source filter for the following reason: the centre wavelength (or Bragg wavelength) of the light reflected by the FBG depends on the period of the grating. So when the fibre is constrained, the variation of the grating pitch induces a variation of the Bragg wavelength, which order of magnitude is 1pm for lue strain.

: I \' :1 ' : :11 : \ ., -.------j---------l--------•t---------.
In our experiment the FBG is fixed on a plate. Stress are applied to the plate which undergoes strains ranging from 50~E to 300~E.We then observe, as expected, a linear displacement of the Bragg wavelength. This can be seen on the figure 4, where the Bragg wavelength is plotted versus the strain. •

We find a variation of almost 1.1pm/us which is in a very good agreement with the values found in the litterature [7].. The next step will be to test the system in the classical OLCR configuration to get full information on the phase. This will allow an accurate resolution in the localization of defects and a better understanding of their nature.

  fig2 : LED spectra fig3 : spectrum of FBG reflected light

  fig4 : Bragg wavelength versus strain

Acknowledgements

We gratefully acknowledge support from FRANCE TELECOM, BNM, ALCATEL OPTRONICS, HIGHWAVE OPTICAL TECHNOLOGIES. The authors wishes to thank P. Le Ray for his contribution.


Frédéric Lecoche
email: frederic.lecoche@orange-ftgroup.com

Benoit Charbonnier

Eric Tanguy

Hongwu Wu Li

Frederic Van Dijk

Alain Enard

Fabrice Blache

Michel Goix

Franck Mallecot

Comparison of two types of 60 GHz photonic millimeter-wave generation and distribution of a 3 Gb/s OFDM signal

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Introduction -Multiplication of connected devices and services (computers, media center, videophony, TVoIP etc…) [START_REF]European project ICT-FP7-ALPHA[END_REF] lead to a novel home network architecture and technology to enable wireless coverage offering a data rate above 1 Gbps (Figure 1) [START_REF] Charbonnier | Upcoming perspectives and future challenges for ROF[END_REF]. This Home Network architecture is based on a wired backbone network distributing local very high speed wireless connectivity in the different rooms of the house/building because the only radio technology whose capacity approaches some Gb/s uses the 60 GHz frequency window as for instance ECMA-387 [START_REF]High Rate 60GHz PHY, MAC and HDMI PAL[END_REF] or upcoming standards like IEEE 802.15.3c [START_REF]WPAN Task Group 3c[END_REF] or IEEE 802.11ad [START_REF]Task group AD[END_REF]. Such radio interfaces are inherently short reach (<15 m) and the radio signal is confined to within one single room. The radio home network then becomes a multicellular network where the cell interference and management issues are similar to that of larger scale mobile/radio networks. In this context, again, the use of the optical infrastructure with RoF, to link the different remote antennas to provide a cost effective and flexible solution, must be considered. In this paper, we present and compare the performance of two types of photonic system for 60 GHz millimeter-wave generation and distribution of a 3 Gb/s OFDM signal. The first one uses low cost well known components and the second one a new generation of optoelectronic components designed for 60 GHz applications.

I. RADIO OVER FIBRE SET-UPS

In this part, we show two RoF set-ups generating a 60 GHz millimeter-wave radio signal. For both set-ups, we use an OFDM QPSK signal compliant to IEEE802.15.3.c pre standard [START_REF]WPAN Task Group 3c[END_REF] (3 Gb/s).

A. Multi-Mode Fibre Set-up (Fig 2)

The OFDM signal is generated first on an intermediate frequency (IF=4.5 GHz) and is used to modulate directly a VCSEL (850 nm) which converts In this part, we show the results obtained for an OFDM modulation at 3 Gb/s and for each set-up.

A. Multi-Mode Fibre Set-up

The electrical power at the input of VCSEL is set to -10 dBm and the power at the output of the 20 dBi horn antenna is also +10 dBm. In figure 4 a) and b), we present respectively the spectrum of received OFDM signal and the associated QPSK constellation diagram obtained after demodulation. In spite of a strong disturbance of the spectrum, the mean computed EVM is 18.7% for a signal to noise ratio (SNR) of 23 dB. From the calculated EVM, a BER can be estimated to be around 10 -10 [6]. Theoretically, the measured SNR should provide an EVM of 7% [START_REF] Urick | Wide-Band QAM-Over-Fibre Using Phase Modulation and Interferometric Demodulation[END_REF]. The difference between the measured and theoretical values of EVM is attributed to the residual non-linearities of the system.

B. Single-Mode Fibre Set-up

The level of radiated power is similar to the previous set-up at the output of the transmit horn antenna. In figure 5

III. CONCLUSION

In this paper, we presented two different set-ups for very high data rate wireless transmission using radioover-fibre at 60 GHz. The first one uses low cost commercial components (VCSEL emitting at 850 nm and 8.5 GHz photodiode) as well as Multi Mode Fibre and the up-conversion to 60 GHz is performed at the remote antenna site after the signal optical distribution. The second one uses a ML-FPL at 54.8 GHz which is able to up-convert directly the signal to 60 GHz before distribution over Single Mode Fibre. For both set-ups, we present the performance evaluations with an OFDM QPSK signal carrying 3 Gb/s. At 10 meters of wireless radio transmission, we report a computed EVM of 18.7% for the VCSEL set-up and 19% for ML-FPL. These values should lead to a BER around 10 -10 and show that both set-ups have quite similar performances. On one hand, the Multi-Mode Fibre Set-up has proven commercial availability but requires a more complex remote antenna station. On the other hand, the Single Mode Fibre Set-up allows the remote antenna station to be simplified but the components maturity is not yet achieved.

Fig 1 :

 1 Fig 1: RoF home architecture and solution for very high data rate wireless next generation the electrical signal into an optical one. After transport over 300 meters of multimode fiber (OM3 with 4000 MHz.km), an 8.5 GHz bandwidth photodiode converts the optical signal into an electrical one. After, the signal is sent into a low noise amplifier (LNA) and a mixer fed with a 54.5 GHz local oscillator (LO) to reach the 60 GHz radio frequency window. A high power amplifier (HPA) and a 20 dBi horn antenna are used for the subsequent wireless transmission (10 m). In the radio receiver, two LNA amplify the signal before frequency down-conversion and analysis by a Real Time Oscilloscope (RTO).

Fig 2 :

 2 Fig 2: VCSEL set-up with distribution of OFDM QPSK signal before up-conversion to 60 GHz B. Single Mode Fibre Set-up (Fig3) This set-up up-converts signal to 60 GHz before transmission over fiber using a 54.8 GHz Mode Locked Fabry Perrot Laser (ML-FPL). The IF OFDM signal modulates directly the ML-FPL. The laser pulses with a repetition rate of 54.8 GHz. Its

Fig 3 :

 3 Fig 3: ML-FPL set-up with OFDM QPSK signal upconversion to 60 GHz before distribution II. RESULTS

Fig 4 :

 4 Fig 4: Multi-mode fibre set-up received OFDM Spectrum (a) and respective constellation (b) (dots around coordinates [1,0] are pilot tones used for equalization).

Fig 5 :

 5 Fig 5: Mono-mode fibre set-up received OFDM Spectrum (a) and respective constellation (b).


E Tanguy"

N Huet+

A Vinçotte''

Lasers Cleaning of Patrimonial Plasters

Keywords:

The use of the lasers Q-switched Nd:YAG to remove the dust of the stone monuments especially sculpture gradually replaces the more abrasive technique of sandblasting. This tendency made us consider the lasers as solution for the cleaning of ceramics and the plasters. Indeed in museums, these materials are often covered with dirty mark (dust, grease, etc.) which is difficult to remove without damaging the object. This paper deals with the impact of different types of lasers (Nd:YAGfirst and third harmonic) irradiation on plaster and with the effects on its morphologyand its crystallography. Plaster is an interesting material because of its typical acicular crystals altered at low temperature. That is why synthesis samples were prepared, constrained in temperature then analysed by various processes (SEM, XRD, TGA... ). These results were compared with samples cleaned by laser. That enabled us to conclude that plasters cleaned by UV-laser (third harmonic of the Nd:YAG) underwent neither yellowing,nor morphological or crystallographic changes. It has to be opposed to the intense yellowing,and sometimes morphological destruction, which appears with an infrared wavelength (first harmonic of the Nd:YAG).

'Iamnr 'VV-S£ dd '61 °nns 'S'dUlW DEft' U! 'a1'1'!nd np a::JuBSS!BUUO;)''I0n0;) D °v £000 '16-£8 dd 'v %A ''d6V7U'dH IvJ,nnnO /0 IvUJ,no[ Ul 'aIql"l<Ul uo uoqB'ISTIl::JU8)[::JBIqUB8p 0'1 Ul8'1SAS18sBI DYA:PN p8lj::J'I!i\\S-\') B JO ::J!UOUlJBlj Pl£ 8lj'l pUB 'lSI 8lj'l JO uO!'IB::J!!ddB8lj'l uo Apn'lS 8AqBIBdUlO;) 'OIBpUB S!)[BlBW °D O£ £000 '9S(;-6V(; dd 'v %A ''d6V7~J,'dHIvJ,n71no /0 IvuJ,no[ U! 'satprus Imq8108lj'l pUB IB'I -U8Ulp8dX8 :S'IU8UI:âldJO uOqBlOIo::JS1PpUB p8Ba :

Introduction

Numerous plasters are found in museums (original sculptures, fine copies or simple casts). Plaster ends to get dirty very quickly what is due to its mineralogical and physical characteristics (dielectric constant ... ). Because of its natural fragility and sensitivity to moisture and physical actions, the removingof greasy dust is ordinary based on peeling methods or rubbing-out. Sometimesthese traditional techniquesturn out to be ineffectiveor damaging especiallywhen finenessof details excludephysicaloperation ar when the dust layers are too hard. In those cases, the use of lasers, already employed far stone [1],has then been considered.

Choice of the Wavelength

The first tests have shown that after cleaning with an infrared Nd:YAGlaser an intense yellowingappeared (Fig. la) while the use of the third harmonic of the Nd:YAG laser permits to recover a surface colour close to the initial A natural dust contamination would be too long and insufficientlyreproducible. We thus chose a deposit of carbon graphite reduced to powder and applied to the brush (Fig. 3). This powder forms part of porosities and makes it possible to obtain a fine and homogeneouslayer.

Treatment in Temperature

Laser ablation induced a fast but localizedrise in temperature in the medium. However the modifications of phase of the plaster occur for temperatures ranging between lOODC and 1200DCFig. 4. Plaster is an interesting material because of its typical acicular crystals (CaS04.2H20 P) altered at low temperature (120DC= CaS04.0,5H20 P) [4]. In order to detect possible modifications of phases of the plaster cleaned by laser, samples were first thermally treated, at various temperatures, then they were observed with the SEM and, eventually they were reduced to powder and analysed by x-rays diffraction (Fig. 5). These data will be employed as references and will be compared with the results obtained after the laser cleanings, The fine grains ----------------------------~3 000e H and the fine structure of the moulding or the surface quality is still preserved, even the fragile white coating.

Conclusion

The cleaning of the plaster using the third harmonic of Nd:YAGlaser modifies neither the colour, neither the crystallo-graphic structure nor the phase

Fig. 2 .Fig. 3 .

 23 Fig. 2. Crystal morphology of the face in contact with the air

Fig. 5 .Fig. 6 .(Fig. 10 .

 5610 Fig. 5. Modification of the plaster phases versus temperature at 20, 100 and 3000 C (DRX -SEM), G: gypsum, H: hemihydrate, A: anhydrite