This paper presents experimental studies on heterodyne Frequency Modulated Continuous Wave (FMCW) signal reception for different optical heterodyne configurations including internal and external mixing between an incoming signal and a local oscillator. Signals and potential noise sources from a fibered FMCW Mach-Zehnder Interferometer (FMCW MZI) are theoretically evaluated. These optical estimations (signal and noise) of various power spectral densities (PSD) are converted into electrical unities to be compared to the measurements.The PSD are validated by using a known alternating voltage with controlled frequency and amplitude. This validation is used to compare the experimental and theoretical detection limits of different FMCW photodetectors, including a Photonic Integrated Circuit (PIC) detector developed and produced at CEA. The detection limit achieved with this PIC module closely matches with the expected theoretical performances. It validates the optical and electronic architecture and the achievements of CEA’s design. The miniaturization of this operational detection module is underway. In the future, it will be located on a single chip alongside two Optical Phased Arrays (OPA), one for emission and the other for reception.
CEA aims at developing a compact 1550 nm Frequency Modulated Continuous Wave (FMCW) LiDAR on chip. In this paper, individual demonstrators, corresponding to three main components of a LiDAR (Light Distance And Ranging) system, are combined in a test bench: a FMCW laser source, an emission and reception Optical Phased Array (OPA) and an optical heterodyne detection module. Each component has been individually tested, but also evaluated in order to derive the system performance of a complete LiDAR. A test platform has been developed to calibrate an OPA fabricated at CEA platform, either in emission or in reception mode. The tested OPA includes 256 channels based on grating antennae, with 1.5 μm pitch and 256 thermo-optic phase shifters. More recently, this platform has been completed with a FMCW interferometer, where the FMCW LiDAR detection can be evaluated through a mixed propagation setup, composed of optical fibers and free space. Then, the OPA may be inserted into this setup. Therefore, the optical fiber FMCW interferometer has been optimized to detect the lowest signal (typically less than one hundred fW) and to estimate the signal-to-noise ratio (up to almost 30 dB) with low noise photodiodes. Performance has been compared to theoretical predictions. Then, our custom OPA is included inside this experimental setup in a free space propagation environment. The performance measurements extracted from the spectral analysis are in agreement with the expectations.
In order to produce a powerful, single and low divergence output beam for 3D sensing applications, integrated Optical Phased Arrays (OPA) must have a large number of closely spaced optical antennas. This high density leads to specific constraints in component design compared to devices for optical transceivers. Furthermore, OPA characterization requires significant adaptations compared to traditional photonic wafer level measurement systems. In this presentation, we will focus on some key components used in a large scale OPAs, describing specific challenges and solutions. We will show characterization results of single components as well as active beam-steering with OPA circuits using our modified wafer-scale prober setup.
An Optical Phased Array (OPA) is similar to a one dimensional (1D) dynamic diffraction array. The phase law of the emitters is numerically programmable and enables to form a beam, that point towards a targeted direction. OPAs have a high potential for a new generation of LiDAR (Light Detection and Ranging) systems, since they avoid mechanical beam scanning. For the development of such LiDAR, many characterizations are essential to optimize the OPA and to get a full control of their performance. To carry out these tests, CEA-Leti has developed a modular optical bench designed to characterize large scale 1D-OPAs in free space. This bench allows beam-forming calibration at various angles thanks to an optical setup based on far-field imaging in the Fourier plane. This set up directly analyses a field of view of 22° (-11°/+11°) and can rotate in the azimuthal plane of the OPA to cover angles ranging from -50° to +50°. The OPA board is mounted on an additional rotation stage to match the OPA beam output with the beam forming set-up optical axis. For practical use, the optical axis is parallel to the floor (i.e. to the optical table). Moreover, after calibration, additional options allow to switch the setup for practical operations, as the OPA use in real space, e.g. for aiming at a target. A Peltier and a regulation loop allow testing the OPA at various temperatures. Fast photodiodes have been implemented to measure the switching time between distinct angular positions. In this paper, we report data acquired with this setup on a 256 channels OPA operating at @1550 nm, that is based on grating antennas with 1.5 μm pitch and thermo-optic phase shifters.
The struggle against tuberculosis is one of the World Health Organization priorities. Identifying in a short time, patients
with active tuberculosis, would bring a tremendous improvement to the current situation. Recovering from this infectious
and deadly disease (2 million of death per year) is possible with a correct diagnosis to give an appropriate treatment.
Unfortunately, most common tuberculosis diagnoses have few drawbacks:
- skin tests: not reliable at 100% and need an incubation of 2 days before the diagnosis,
- blood tests: costly and sophisticated technology,
- chest X-ray: the first step before the sputum tests used for a bacterial culture with a final diagnosis given within
2 weeks.
A tuberculosis test based on exhaled breath analysis is a prospective and noninvasive solution, cheap and easy to use and
to transport. This test lies on a fluoregenic detection of niacin, a well-known mycobacterium tuberculosis specific
metabolite. In this paper, it is assumed that the selected probe is specific to niacin and that exhaled breath does not
contain any interfering species.
To address this problem, a fluorimeter is developed with a cheap and cooled CCD ( 2k$) as a sensor, to easily
determine the suitable “fluorescent zone”. In comparing aqueous solutions with and without niacin, 250 pM of niacin
have been detected. With a commercial fluorimeter (Fluorolog from Horiba), only 200 nM of niacin are detected. The
present detection remains 10 times above the estimated targeted value for a tuberculosis test.
The excitation source is a LED, which typically emits 20 W at 265 nm through an optical fiber. The emission signal is
detected around 545 nm. A typical light exposure lasts 700 seconds.
Analysis of biomarkers with a liquid fluorimeter is generic and promising as health diagnosis.
In this paper, defect counting results are presented both on silicon substrates and on EUV mask blanks with their equivalent PSL size distribution. The measurements are achieved with our counting defect prototype COMNET but also with a commercial tool. Improvements of COMNET have been performed to reach sensitivities (PSL equivalent size on silicon) of 80 nm on silicon substrate, 90 nm on EUV mask blank and 125 nm on glass substrate. A first analysis about COMNET repeatability is presented and the impacts of known errors on defect number and size values are evaluated.
Typical counting data measured on a EUV mask blank having an added defect density inferior to 1 def/cm2 (defect size superior to 90 nm) are presented. The analysis of these data shows two main populations of added defects which are characteristic of the deposition process: one close to 100 nm and the other in the micron range. Each group represents half of the total number of added defects. The origins of these defects are finally discussed on the basis of specific experiments chosen to highlight the possible role of the different process steps.
The characteristics of a defect counting tool, COMNET< based on scattering light measurement, is presented. This prototype supports the development of defect-free EUV blanks. Thanks to new improvements, it becomes possible to detect PSL particles having a diameter as low as 100 nm, with a video CCD camera, on silicon substrates or on EUV blanks. To reach this sensitivity, one of the enhancements consists in a laser irradiation close to 65°. The present configuration and the use of a CCD camera, with a variable exposure time, should lead to the detection of 80 nm PSL particles deposited on silicon substrate. This extrapolation is based on experimental results and on a simple model. To detect 100 nm particle and smaller particles, it is essential to reduce the level of stray light and to increase the signal to noise ratio. In our application, the stray light essentially comes from three sources: the noise induced by the roughness of the sample, the Rayleigh scattering of the atmosphere, and the stray light in the room. The restrictions induced by these phenomena are described in some detail. All the improvements are not only available for the characterization of silicon substrates but also for transparent blank substrates and for EUV mask blanks. The additional noise induced by the tranparent substrate is analyzed. The defects, whatever the compoent sizes and the component shapes can be detected. A cross characterization achieved with a commercial tool on silicon substate is reported. Counting measurements performed on EUV blanks are shown. Furthermore, a more explicit definition of added defects is proposed.
As a participant of the French Extreme UltraViolet (EUV) program PREUVE, our laboratory has been involved for one year in the development of Mo/Si blanks free of defects using an Ion Beam Sputtering deposition technique (IBS). Among the different tasks to end at an EUV stepper, the achievement of the blank and its characterization in terms of defect counting is probably one of the most challenging. This is the reason why a characterization tool based on the well known scattering method is under progress in our laboratory. In addition to defect counting, the possibility of cleaning a sample (substrate, blanks,.) with a powerful laser source is allowed on the same facility. This set up is fully computerized. Its first performances are illustrated with several examples on our blanks and silicon substrates. Our system is designed to optimize the laser cleaning parameters (wavelength, incidence, polarization,.) and to decrease the detection limit size. Today, our facility currently enables the detection of the defect as small as 200 nm in size. Our strategy is to reach the defect size as low as possible and at least until 50 nm, which is the size of interest for the EUV lithography. The major developments under consideration in order to lessen the detection limit size are described in details in this paper. Moreover, the roughness of the EUV blanks may be evaluated with our device. This surface parameter has to be optimized, at least for two reasons : to reduce the optical losses and to decrease the number of defects. The mean to quench the roughness signal in order to detect the smallest defect size and the possibility to evaluate the component roughness are combined on the same facility. The major trumps of our experiment are the visualization of the defects in live, the measurement of the defect sizes and the quick evaluation of the laser cleaning efficiency versus various experimental conditions. The cleaning part of the set up is built up to use various powerful laser sources and to shoot, with the laser, only the locations where the defects remain. Finally, this facility allows to evaluate and to optimize our defect free blank process.
We describe the experimental set-up used to determine the optical densities and the laser damage threshold of laser goggles. Optical densities have been measured at 1064 nm using 3ns laser pulses on an upgraded automatic test bench, which also allows for the determination of laser damage threshold of otpical components. We have performed experiments at constant fluence by varying different parameters such as the angle of incidence, the light polarization, the laser repetition rate, and the spot size. Multiple shot data are obtained in real time, which enables us to detect optical density variations during the laser exposure. From our study, we can conclude that a few goggles show the well known effect of saturable absorbers. This means that, above a given thresholds energy, the goggles become transparent. Finally, we will present a simple model which allows to understand the behavior of the laser damage threshold of polycarbonate goggles under these various conditions.
Very high damage threshold Hafnia Silica mirror coating have been developed successfully thanks to a clear understanding of the main limitations of the available mirrors. Coating with a 3ns pulse width threshold above 100J/cm2 are reported. These coatings proved to be much more resistant than very high quality bare silica substrate tested the same way. The limitation of these coatings is still extrinsic. The potential of this technology for large size high damage mirrors for fusion laser application is discussed.
Understanding the 'extreme statistics' of failure at a weak link allows extrapolation of the results of small area laser damage tests to predict damage levels for the large areas pertinent to NIF/LMJ. Conceptually, it is important to focus on the fluence dependence of the surface density of damage sites. Results of different types of damage tests can be reported in terms of this sample characteristic property.
The automatic damage test benches allow to obtain more data in less time but also to imagine new laser damage tests. Thereby, new questions can be asked and more answers can be provided. In this paper, the 3 YAG automatic damage test facilities build up at the laboratory will be shown. There are based on well-known principles : an half wave plate to adjust the wished peak fluence and a scattered light measurement in real time to detect the damage appearances. Many tests are developed around these benches as the usual hon-i, N-on-i, R-on-1, S-on-i tests and raster scans previously proposed by other laboratories. On all our test benches, these standard tests are driven by the same software designed at the laboratory : "Wintfl". But new tests and characterizations are also added. Indeed, on these thcilities the aging ofthe optics at various repetition rate under constant fluence (like S-on-i with or without a limit number of shots) are carried out. An automatic defect count has been also implemented. This tool is able to evaluate the number of defects after cleaning or after coating but also between laser irradiation. New tests are also imagined. The so-called HR-on-i test (High Ramp), which is a R-on4 (Ramp) test without beginning at zero has been achieved. It allows to compare 1-on-I and R-on-1 distributions performed with exactly the same damage criterion. Other parameters can be investigated as the speed of the motor ramp during the R-on-1 test, the pulse repetition frequency during the S-on-i test,... In this article, after a short presentation of our test bench facilities, the convenient possibilities of automatic YAG test benches are illustrated with experimental results.
A simple tractable model based on a thermomechanical approach of nodule ejection is developed. Our main purpose is to investigate the nodule critically according to the depth of the seed inside of the coating, in order to understand why the deepest nodules located close to the substrate interface are the most critical. A summary of the experimental results concerning the behavior of irradiated nodules is achieved. Then, the model is checked thanks to experimental data. A critical location of the nodules close to the top surface of the coating is found. A supplemental ingredient is added to understand why the nodules close to the substrate interface are critical.
The laser damage threshold determination is linked to many factors as the damage criteria, the test conditions, etc. Among them, the spatial beam characterization as well as the power or the energy in the beam plays a key role. Indeed, an appropriate value has to be assigned to each tested site. This number is important to certify a component or to compare the components. This paper is focused on determining the beam parameters for 2 kinds of laser damage. The first part addresses the peak fluence measurement on YAG laser damage test facilities. The proposed method is applied to stable laser. Stable lasers mean the spatial profile is similar during the experiments. Before the laser damage tests, the spatial beam is visualized with a CCD camera. Them, the spatial beam profile is cautiously processed to extract the relation between the peak fluence and the energy, which is easy to measure in real time. This method is illustrated at 1064 nm. Finally, this procedure is validated on silicon wafer. Indeed, at 1064 nm, the theoretical laser damage threshold of silicon wafer is well known. Furthermore, it is independent of the pulse length in the nanosecond regime. The second part will be shortly devoted to a more specific problem: the CW damage threshold of CO2 mirrors. It has been previously shown that the 'short' term threshold of these cooled components is well described by a critical temperature. Due to the test geometry, the temperature rise is proportional to the beam power divided by a value homogeneous to the beam radius. In this article, it is sown the knife-edged method allows to literally extract the beam size parameter which appears in the theoretical formula of the critical temperature. The beam just needs to have a symmetry of revolution.
A variety of 1064-nm laser conditioning methods were investigated to find the optimum method for production of large aperture (0.25 m2) multilayer coatings for the National Ignition Facility and Laser MegaJoule. Two conditioning methods were explored, multi-step and single-step on two different laser systems. Off-line conditioning was done on PLATO, a small beam diameter ('-1 mm) raster scanning system. On-line conditioning was done on Beamlet, a single beam prototype of the National Ignition Facility with a large rectangular beam (35 cm x 35 cm). Concurrent with this work, coating process development for low-defect density high damage threshold coatings was realized by switching from hafnia to hafnium starting materials. The results of this study suggest that single-step raster-scan off-line laser conditioning is an effective method to raise the damage threshold of multilayer mirrors deposited from hafnium and silica by reactive electron beam deposition.
Since the value and the shape of the pulse widths and also the spatial profile vary from one test bench to another, it appears interesting to see how the damage threshold evolves with these two parameters. The scaling laws have 2 main impacts: the possibility to compare easily the results between the laboratories and to help the understanding of the damage phenomena. But it can be also a great help on the laser damage threshold specifications of the optical components. Moreover, the spatial law versus the component area could become a request in the future specifications. The first part of this paper is devoted to experimental damage result performed with YAG laser at 1.064 micrometers in R- on-1 mode and in raster scan. 2 pulse widths close to a Gaussian profile is used: around 3 ns and 12.5 ns. Furthermore, 3 laser spot sizes with a shape close to a Gaussian beam are used. Their impact on the laser damage threshold is studied. The results are analyzed with the purpose of finding a tool to extrapolate the result at larger components. To test full large optics is time consuming. A primary automatic test is suggested too quickly eliminate the components below the specifications.
Large aperture multilayer hafnia silica high reflector coatings at 1064 nm, deposited by reactive electron-beam deposition, were prepared to examine different laser conditioning methods for manufacturing high fluence optics in the National Ignition Facility. Laser conditioning is a process where the damage threshold of the coating is increased or the damage that is created is minimized so that it does not grow upon further irradiation. Two laser conditioning methods were examined for coatings deposited from only oxide starting materials. Off-line laser conditioning consists of raster scanning a mirror past a 1 mm diameter Gaussian beam over the entire clear aperture; a process that takes approximately 24 hours per scan. On-line laser conditioning consisted of a large aperture 300 mm X 300 mm beam from the Beamlet laser that irradiated the entire full clear aperture of a series of mirrors; a process that was limited by a 2 - 4 hour shot rate. In both cases a six-step process was used with the mirror first irradiated at a low fluence, then successively higher fluences increased in equal increments up to the peak laser operating fluence. Mirrors that were only partially laser conditioned damaged catastrophically while fully conditioned mirrors survived fluences exceeding the safe operating Beamlet fluence. An alternative off-line laser conditioning method was examined for coatings deposited from hafnia or metallic hafnium sources. Single-step laser conditioning consists of off-line raster scanning an optic at the peak operating fluence, thus decreasing the laser conditioning cost by reducing the number of scans and required laser conditioning stations to process all the mirrors for the National Ignition Facility. Between pulses the optic is stepped approximately one fourth of the 1/e2 Gaussian beam diameter so each area of the coating is irradiated by different segments of the beam starting at a low fluence at the outer edge of the beam diameter and increasing to the peak fluence in the center of the beam. The one-step conditioning results appear positive, but the influence of the coating improvements due to the metallic hafnium process on laser conditioning is undefined.
The demonstration of the technical feasibility of very high damage threshold mirrors deposited by an ion beam sputtering process is made. The two chosen materials are Yttrium fluoride for the high index and Lithium fluoride for the low index. The threshold of these two materials at 355nm 3ns is higher than 20J/cm2. High reflection stacks have been deposited with a reflection level of 98.5 percent and a threshold which meets the laser megajoule requirements.
The possibilities of very high damage threshold optics at the Nd:YAG third harmonic are discussed for Megajoule applications. Only two possibilities which are either electron beam oxides materials like Al2O3/SiO2 or IBS fluorides like YF3/LiF seem to be able to meet the specifications. Current state of the art of the two solutions is given as well as their possible improvements.
As the size of optics has increased, so has the difficulty in effectively measuring and defining their laser damage threshold. This is exemplified in the case of optical coatings being developed for the National Ignition Facility in the USA and the laser megajoules in France. Measuring the threshold on small witness samples rather than on the full aperture optic presents obvious advantages. In this article, the threshold of large-area components is being addressed in two general ways that both use experimental mapping data. First, a model based on the fruitful concept of the R-on-1 threshold distribution is shown to predict the threshold of a large optic with a high degree of confidence. At the same time, it is determined that the average R-on-1 threshold provides a reliable and accurate value to evaluate the coating improvements. To acquire the essential data, an automated damage test bench has been developed by the laboratory of 'Couches minces pour l'Optique' at the CEA. Secondly, the damage threshold has to be defined according to the final use of the component. To address this issue, the Lawrence Livermore National Laboratory has defined a functional damage threshold to set limits on the maximum damage size. An empirical power law dependence of average damage size on peak fluence was found. This relation can be used to predict the damage behavior of large-aperture optics exhibiting the same damage morphology.
The damage threshold specifications for the National Ignition Facility will include a mixture of standard small- area tests and new large-area tests. During our studies of laser damage and conditioning processes of various materials we have found that some damage morphologies are fairly small and this damage does not grow with further illumination. This type of damage might not be detrimental to the laser performance. We should therefore assume that some damage can be allowed on the optics, but decide on a maximum damage morphology limit. A new specification of damage threshold termed 'functional damage threshold' was derived. Further correlation of damage size and type to system performance must be determined in order to use this measurement, but it is clear that it will be a large factor in the optics performance specifications. Large-area tests have verified that small area testing is not always sufficient when the optic in question has defect-initiated damage. This was evident for example on sputtered polarizer and mirror coatings where the defect density was low enough that the features could be missed by standard small-area testing. For some materials, the scale-length at which damage non- uniformities occur will effect the comparison of small-area and large-area tests. An example of this was the sub- aperture tests on KD*P crystals on the Beamlet test station. The tests verified the large-area damage threshold to be similar to that found when testing a small-area. Implying that for this KD*P material, the dominate damage mechanism is of sufficiently small scale-length that small-area testing is capable of determining the threshold. The Beamlet test station experiments also demonstrated the use of on- line laser conditioning to increase the crystals damage threshold.
Laser damage threshold measurement is statistical in nature. For a commercial qualification or for a user, the threshold determined by the weakest point is a satisfactory characterization. When a new coating is designed, threshold mapping is very useful. It enables the technology to be improved and followed more accurately. Different statistical parameters such as the minimum, maximum, average, and standard deviation of the damage threshold as well as spatial parameters such as the threshold uniformity of the coating can be determined. Therefore, in order to achieve a mapping, all the tested sites should give data. This is the major interest of the R-on-1 test in spite of the fact that the laser damage threshold obtained by this method may be different from the 1-on-1 test (smaller or greater). Moreover, on the damage laser test facility, the beam size is smaller (diameters of a few hundred micrometers) than the characteristic sizes of the components in use (diameters of several centimeters up to one meter). Hence, a laser damage threshold mapping appears very interesting, especially for applications linked to large optical components like the Megajoule project or the National Ignition Facility. On the test bench used, damage detection with a Nomarski microscope and scattered light measurement are almost equivalent. Therefore, it becomes possible to automatically detect on line the first defects induced by YAG irradiation. Scattered light mappings and laser damage threshold mappings can therefore be achieved using a X-Y automatic stage (where the test sample is located). The major difficulties due to the automatic capabilities are shown. These characterizations are illustrated at 355 nm. The numerous experiments performed show different kinds of scattering curves, which are discussed in relation with the damage mechanisms.
The CMO YAG laser damage test facility, which is equipped with a 30 Hz laser, is presented in this paper. The main points are described: (1) The characteristics of the laser beam and the in situ damage detection technique (a scattered light measurement system) are perfectly suited to work up to the frequency of the laser. They are monitored in real time, and work at three wavelengths: 1064 nm, 532 nm, 355 nm. (2) The laser beam characteristics are preserved during the laser damage tests even for only one shot or frequencies lower than 30 Hz due to a fast shutter. (3) With this same shutter, it is possible to automatically stop the laser on the pulse which induces the first damages. These automatic capabilities enable the samples to be tested quickly. (4) A Nomarski microscope supplied with a 16-bit CCD camera enables the test sites to be photographed before and after the laser interaction. Image processing enables us to extract the first damages. (5) Six pulse widths are available (between 3 ns and 13 ns). Therefore, with all these characterization tools, many kinds of laser tests may be considered. These different features are illustrated by experimental results (1-on-1 test or R-on-1 test).
The morphology of the first damage occurring on either single layer materials or multilayer HR coatings are studied using Scanning Electron Microscopy. For single layers obtained by electron beam evaporation or ion beam sputtering, the damages are always centered on a small size center of about 0.1 micrometers . This center triggers the degradation which presents a well defined shape and size. Two kinds of damage mechanisms occur, either melting of the layer or mechanical breaking of the layer. The same behavior is observed on HR coatings with damage centers located at the interfaces between the high and low index materials of the stack. From these experiments an impurity dominated model is proposed which enables the main features of UV damages to be understood.
In this paper, the destruction of the mirrors under continuous wave (cw) laser irradiation is studied. The existence of a critical temperature, at which the mirrors are destroyed, has been already established. But two damage processes are associated with this temperature: the threshold and the ageing. The threshold occurs when the mirror reaches this critical temperature at the steady-state. The threshold essentially depends on the thermal conductivity of the substrate. The ageing processes begin after the thermal equilibrium time and depend essentially on the physical properties of the thin films. The ageing procedure proposed is used in order to compare the mirrors lifetime.
This paper investigates the laser damage threshold under C.W. CO2 laser irradiation. It has been already shown that the destruction of each mirror is characterized by a critical temperature: the mirror is destroyed when this temperature is reached. The aim of this paper is to compare the laser damage threshold and the critical temperature.
From experimental results we characterize the catastrophic damage threshold of mirrors irradiated with cw CO2 laser by a critical temperature. Analytical expressions of the mirror temperature versus beam radius are developed. A comparison between experiments and theory shows that P/R determined in test is the good ratio to compare mirror coatings and to scale test.
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