This study addresses the challenges of adding functionality and hybridizing processes in additive manufacturing. It focuses on embedding a gold-coated optical fiber into an INOX structure, aiming to extend this process to optical sensors like fiber Bragg grating arrays. The primary concern is the sensor's resistance to high temperatures during metal deposition, while the second challenge involves the adhesion of filler material to the sensor and structure. The feasibility is assessed through a finite element thermal model and mechanical testing, confirming the process's viability. Successful light transmission through the fiber and tensile tests indicate structural integrity and reduced ductility, warranting further investigation under varying load conditions.
In coherent optical time-domain reflectometry, external modulation is used to maintain the coherence of laser probe pulses launched into optical fibers. However, the residual continuous wave (CW) component produced by modulation may considerably degrade the system sensitivity. The backscattered signal from the pulse must be dominant compared to the CW signal. We discuss the effects of the finite extinction ratio (ER) on the instrument’s sensing range. A model analyzing the impact of the CW component on the backscattered signal as a function of the ER, fiber length, and pulse widths is proposed. It is also shown that acousto-optic modulation is more suitable than electro-optic modulation in optical fiber for longer distances. The results are confirmed experimentally in a 31–km-long fiber. A 1-Hz vibration was applied at 25.5 km, and the resulting signal-to-noise ratio of ∼13 dB was measured using 75-ns pulses. This result is in agreement with the performance predicted by the model.
The utilization of an external modulation allows the coherence of laser probe pulses launched to the optical fiber to be maintained in coherent optical time domain reflectometry. However, continuous-wave (CW) component product of external modulation could considerably degrade the sensitivity of the system. In this paper, we discuss the effects of finite extinction ratio in different external modulation methods and the attempts that have been made in order to overcome these effects. A model is proposed to analyze the impact of external modulation CW component on backscattered optical signal of probe pulses from optical fiber as a function of extinction ratio, fiber length and pulse widths. External modulation can be achieved by acousto-optic modulation or electro-optic modulation. From the results obtained with the model, the advantages and drawbacks of each technology are discussed, subsequently leading to required prospective developments and refinements.
Additive manufacturing is more and more used in optics to produce opto-mechanical components as well as light transmission mediums, either for prototype evaluation or for functional part generation. It was previously shown that optical systems can benefit from the geometrical accuracy of the printed parts. Intrinsic defects such as surface roughness or volume birefringence can also be exploited for optical component design. We here present such use of particular properties of an additive manufacturing process based on photopolymerization. The final goal of the work is the design of a force sensor for collaborative robotics. More precisely, the aim is to design an optical force sensor to control the contact force between a human body and a magnetic source controlled by a robot for medical purpose. Optical sensors are known to have major interests in harsh environments where classical electrical sensors cannot be used due to, like here, electromagnetic compatibility issues. Two 3D-printed designs of optical force sensors are compared. The first one, conceptually developed in a previous work, is using polarization modulation due to force-induced birefringence to modify optical transmission in a sensor based on a monolithic original geometry. For such a case, additive manufacturing appears as a powerful production technique as the 3D part must be transparent and at the same time obtained with an accurate complex geometry. The second design is based on the volume scattering properties of printed transparent parts. For the first time to our knowledge, we show that the optical system made out of a beam expander and a cylindrical lens, necessary to achieve an optical line, can be replaced by a simple prismatic 3D-printed element. Using the Polyjet technology developed by Stratasys Ltd, a line can simply be obtained using the 1D volume light scattering inside the printed medium. The variation of line properties is then related to the mechanical strain induced by the force to be measured. In other words, the optical properties we rely on are linked to the bulk liquid material, its photopolymerization during printing and finally the impact of mechanical stress on the printed component. The sensitive element in the force sensor can be seen as a metamaterial with properties which depend on its micrometric structuration. The micro-structuration size is not related to the standard minimum feature size as claimed by the manufacturer but to the additive manufacturing process itself. In our case, a Stratasys Connex 350 printer has been used with an acrylate transparent material. Opto-mechanical properties such as birefringence, surface roughness, elasto-optic coefficients have been measured. The ability to generate an optical line using natural 1D volume light scattering in a printed parallelepiped with polished surfaces is experimentally demonstrated. As potential application, the parallelepiped is used to replace a cylindrical lens in an amplitude modulation force sensor. The sensor response is measured. Thus, additive manufacturing appears to be a promising technique to achieve optical components and to integrate optical sensors in future 3D-printed mechatronic systems.
This study concerns the determination of the diameter of an optical fiber by analysis of a 2D measured diffraction pattern relative to this linear object, falling within the scope of the Fraunhofer approximation. In this approach, when considering a small line-shaped aperture, with a radius α, or a diffractive object placed at the y-axis, an amplitude of the in-line Fraunhofer hologram can be achieved by a mathematical expression, for a given wavelength of the illuminating light λ and a distance z between the particle and the recording plane. The interferometric signal depends on an Airy curve expressed by a Sinc function whose determination of the zeros makes it possible to deduce an argument giving the radius of the fiber. The measurement is carried out for an object-CCD distance equal to z = 60 mm, with a wavelength of illumination λ = 635nm. The zeros of the Airy function appearing in the analytic expression of the interferometric signal allows us to achieve the value of the measured diameter. Knowing that the fiber radius is α = 62.5 μm, the measured value is acquired with an error of 1.7%.
The work developed and presented in this communication, relates to the restitution of frequency chirp of an interferometric signal deduced from a measured diffraction pattern relating to a spherical micro-particle. For this purpose, analysis were achieved by implementing a parametric method with a sliding window. These frequencies allows us to reconstruct the axial position of the corresponding object. The study, achieved in the far field approximation, allows us to validate preceding methods based on simulation results. The principle consists to generate optically in-line diffraction patterns of a spherical particle with radii of 39μm and measured with a microscope ZEISS. The collimated coherent light was generated from a He-Ne laser that the wavelength is λ = 632.8 nm. The generated diffraction pattern was recorded by using a 2D-CCD camera Ophir having 1024 x768 pixels with a pitch of 4.65 μm connected to a computer. Since the variation of the chirp frequency is linear, the knowledge of its variation slop, resulting from a linear fit, enables us to deduce the z-position of the particle. This is achieved with a resolution of 1.2 %.
We study the possibility of the fabrication of faceted structure with a standard additive fabrication technology using the 3D printer of Objet30 Prime series from Stratasys. The structure contains a small number of facets with size in millimeter: these facets are inclined in two directions. We use analytical expression to solve the tilt angles recorded into a matrix. After obtaining the desired illumination pattern, we try to fabricate the structure with a standard and commercial additive fabrication methods (3D printing) to make a master. We create the STL file usable for the 3D Printer. First tests were made with the printer of the Stratasys Model Objet30 Prime using the material of VeroWhite Plus FullCure 835 and the support model of FullCure 705 from FABLAB at INSA Strasbourg. The common dimension of the element is 6 × 6 facets, where one facet is in millimeter. As printing is conducted with the material of VeroWhite, the quality of the surface profile of the printed model is not sufficient for direct optical applications due to the porous property of the material. As a result, square silicon mirrors are cut and coated with a 100 nm aluminum layer in a second step. Professional adhesive of NHU series from Hart Kunststoff, Germany is used to glue the mirrors on the faceted structure with delicate operation. Then the surface profile of the glued mirrors on the faceted structure is measured with a Zygo Newview 7200 profilometer. A first optical test gives interesting results, but not sufficient for our applications, but this work is an innovation and can give new creativity at the frontier of design and optical applications.
This work focuses on the simulation and modeling of a passively Q-switched dual-cavity fiber laser doped Yb:Yb, to our knowledge, it is for the first time that the progressive wave model is applied to this type of laser, a good agreement between our simulation results and the experimental results published by another team is obtained, two pulse trains are produced by this laser, the first is at the wavelength of 1064 nm, and the second is at 1100 nm, we also show that certain parameters such as the concentration of Yb ions and their length are an important parameters for the optimization of the two laser signals at 1064 nm and 1100 nm.
In the present work, we propose a method to calibrate the instantaneous optical frequency of a tunable laser using frequency comb. The tunable laser is heterodyned with the equally spaced comb lines, and the heterodyne signal then passes through an electronic frequency selection unit. When the optical frequency of the tunable laser is in the vicinity of the comb lines, the output of the frequency selection unit delivers a peak. We analyzed the effect of the characteristics of the narrow bandpass filter (NBF) in the frequency selection unit. Simulated and experimental results show that the characteristic of the output peak is related to the normalized sweeping speed of the input tuning laser source. At small normalized tuning speed, the envelope of the filtered signal follows the amplitude-frequency response characteristic of the NBF. This shows that the filtered signal using Gaussian filter has broader peak than the one using Butterworth filter, due to the slower roll-off behavior in the transition band of Gaussian filters. At large sweeping speed, the envelope of the filtered signal deviates from the amplitude-frequency response character of the NBF. The peak intensity of the filtered signal is attenuated, and the bandwidth of full width at half maximum is broadened. Experiments were carried out to verify the simulated results. In the experiment, the instantaneous frequency of an external cavity laser diode was calibrated using the presented filtering method showing periodic non-linear tuning.
This paper, “Télèmetre utilisant une diode laser à balayage continu en fréquence et un double interférométre," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
Microscopic surface topography measurement is an important aspect of industrial inspection. Optical and near field
scanning techniques are increasingly replacing the use of the traditional mechanical stylus since they provide better lateral
resolutions and higher measurement speeds. The main far field optical techniques used are interference microscopy and
confocal microscopy, with the advantages of having larger fields of view and higher measurement speeds. Interference
microscopy is now widely used, mainly because of its nanometric axial measurement sensitivity and its ease of use but
suffers from a limitation in lateral resolution of about /2 due to diffraction. A new technique for high resolution 2D
imaging using a microsphere placed on the sample has been recently combined with interferometry by several groups to
greatly improve the lateral resolution. In this paper we present some of our own first results using glass microspheres with
a white light Linnik interferometer and demonstrate a lateral resolution of /4 and an axial measurement sensitivity of
several nm. Results are shown on calibrated square profile gratings with periods down to 400 nm, with a minimum feature
size of 200 nm and a height of 148 nm and a field of view of several μm. While these features are not visible directly with
the microscope objective, they become observable and measurable through the microsphere. An analysis using rigorous
electromagnetic simulations is also given to help better understand the imaging properties of the technique. These first
experimental and simulation results clearly indicate that this is an important new technique that opens new possibilities for
surface metrology with a lateral resolution well beyond the diffraction limit.
Temperature sensing is one of the key requirements for Structure Health Monitoring (SHM) in various applications. The intensity modulated optical fiber sensors are excellent candidate for this area of applications due to their relatively low cost, simple structure and diversity of applications. This work relates mainly to the feasibility evaluation of embedding optical fiber sensor into ceramic coating obtained by thermal spray process and the thermal response of the embedded sensor. The sensor principle and the specimen configuration are firstly presented, a 3D model is then built up in order to evaluate the effects of temperature variation on deformations of the optical fiber sensor which finally lead to the variation of optical intensity. First results of thermal response are discussed.
The in-situ detection of temperature or stresses produced by the thermal spraying process is important for both the optimization of the elaboration conditions and the subsequent service monitoring of these systems. Optical fiber sensors are excellent candidates for this area of application since they can be embedded into the layers of several dissimilar materials of smart structures. This work relates mainly to the process of embedding optical fibers into ceramic coatings and to the characteristics of the embedded fiber. Firstly, thermal flame spraying is chosen as the elaboration process. Next, a thermal model is proposed in order to evaluate the thermal strain variation with the temperature during the elaboration process in the structure. Finally, a microscopic observation of the embedded optical fiber in the ceramic coating is reported, the mechanical adhesion strength of the embedded fiber is evaluated and the results of the optical attenuation change during the elaboration process are given. They show that no significant fluctuation of the optical power transmitted in the fiber is observed.
In the field of areal surface roughness measurement, characterization using several techniques can be helpful to better understand the performance of each technique and to improve the overall precision. Measuring exactly the same area with different techniques in practice is not easy. Such studies are of great interest in order to characterize and to understand important new materials today such as semiconductor alloys and graphene for silicon technologies, or biomaterials such as hydroxyapatite for use in human implants. In this work, two types of samples based on a silicon wafer were made by marking with a fractal, multi-scale photolithographic mask and etching. The first sample consisted of a bare silicon wafer with a pattern consisting of 2.4 μm deep numbered square features. The second sample was a rough layer of hydroxyapatite deposited from a solution of simulated body fluid on a similarly etched silicon wafer. The same zone of several squares on the two samples were measured by interference microscopy, AFM and ESEM. The 2D cross sectional profiles and 3D views from the different results were then compared using different analytical measurement software tools. While the general shapes of the measured microstructures were similar, several differences also appeared. Variations were found of up to 7 % in the depths of the etched features measured with the different techniques. This is ascribed to instrumentation calibration errors, probe/surface interactions and to differences in measurement procedures between the software used. Artifacts were also visible at square edges due to probe/source interactions.
Coherence scanning interferometry (CSI) is an optical profilometry technique that uses the scanning of white light
interference fringes over the depth of the surface of a sample to measure the surface roughness. Many different types of
algorithms have been proposed to determine the fringe envelope, such as peak fringe intensity detection, demodulation,
centroid detection, FFT, wavelets and signal correlation. In this paper we present a very compact and efficient algorithm
based on the measurement of the signal modulation using a second-order nonlinear filter derived from Teager-Kaiser
methods and known as the five-sample adaptive (FSA) algorithm. We describe its implementation in a measuring system
for static surface roughness measurement. Two envelope peak detection techniques are demonstrated. The first one,
using second order spline fitting results in an axial sensitivity of 25 nm and is better adapted to rough samples. The
second one, using local phase correction, gives nanometric axial sensitivity and is more appropriate for smooth samples.
The choice of technique is important to minimize artifacts. Surface measurement results are given on a silicon wafer and
a metallic contact on poly-Si and the results are compared with those from a commercial interferometer and AFM,
demonstrating the robustness of the FSA algorithm.
An autoregressive method to analyze the fringe pattern observed in holographic interferometry is reported. Considering the impact of a 30 dB signal-to-noise ratio, we have shown that the reconstruction of the simulated symmetric profiles with 3, 4, and 5 fringes produces a maximum error of 0.300, 0.520, and 1.015 rad, respectively. The reconstruction of an asymmetric profile gives a larger error. The method was also applied to a recent fringe pattern. Our results are in qualitative agreement with those obtained using other methods.
Numerous approaches to seismic detection have been proposed. Usual methods to sense seismic vibrations use either
accelerometers to measure the ground acceleration or geophones based on electro-dynamic actuator velocimeters. In this
paper, we present the design and the development of a polarimetric transducer using a single-mode optical fiber for low
level and low frequency vibration measurements such as those encountered in seismology. Polarimetric sensors can be
optimized to have a reduced sensitivity to temperature. The mechanical part of our one-dimensional seismic sensor is
based on a spring-mass device. A small section of the fiber is squeezed between a substrate connected to the ground and
the sprung mass. The resulting force acts along a vertical direction onto a small section of the optical fiber. The elastooptic
effect induces stress birefringence which varies temporally with the frequency of the applied force. We used a
polarized and single-mode laser diode source to couple light in the fiber. The induced polarization modulation measured
at the output of the fiber gives information of the seismic signals. The physical model of the developed inertial
seismometer has been considered as a mass-and-spring system with viscous damping. Firstly, we expose the principle
behind our optical fiber seismic sensor. Next, we computed the dynamic characteristics of the seismic sensor. Physical
simulation results obtained using the analytical model are presented and discussed. Finally, we present experimental
results measured with our seismic fiber sensor. Both model and experimental results demonstrate the potential of the
sensor for low level and low frequency vibrations characterization.
Performances of scanning wavelength systems are limited by several factors like the
sweeping range, the mode-hop-free tuning of the wavelength and the nonlinearities in the
sweeping speed. Nonlinearities are probably the last parameter to control to get the least
performances. In our absolute distance interferometer (ADI), we have observed that the
processing technique of the fringes in case of a double target system is very sensitive to
the type of nonlinearities and the final resolution of the ADI depends strongly on their
shape. Although the ideal sweeping speed should be perfectly linear, we have observed
that even in the presence of strong sinusoidal nonlinearities in the sweeping speed,
unexpected good results were obtained and were explained by the fact that these
nonlinearities act like some white noise whose contribution converges to zero as the
number of samples of the processed signal increases. In this paper we focus on the
symmetry of the spectrum, another consequence of these nonlinearities. We show how it
is possible to manipulate the spectrum by changing the sweeping speed. Results of
simulations as well as experimental measurements are presented.
This paper reports the optimization possibilities of some non-linear sources of limitations in the resolution and accuracy
of an Absolute Distance Interferometry setup using an External Cavity Laser Diode for wavelength scanning and a
fibered Mach-Zehnder interferometer as a reference. The system is able to measure one or two simultaneous targets with
a relative uncertainty of some 10-6 for distances of 1 to 20m. In order to achieve better performances, the experimental
non-linearities in the wavelength sweep are isolated and compared to different simulated sweeping models. This study
leads to the conclusion that accuracy and resolution could be improved by an optimal modulation of the wavelength
sweep. Another sensible point is the drift of the reference Optical Path Difference of the Mach-Zehnder with
temperature variations. This drift can be minimized by using an acrylate-coated fiber and a copper-coated fiber of
different lengths, adjusted by experimental measurements in a climatic chamber for a 10 to 40°C range.
This paper presents new results on a scanning wavelength interferometer for absolute distances measurements. We report its performances on one and two simultaneous targets with a Fourier Transform Technique (FTT) and a regular sampling. Then the limitations of the FTT are discussed, especially with regard to the tunable laser source performances. Finally it concludes on the new results acquired by the use of a non-uniform sampling technique which reduces the non-linearities of the sweeping speed and demonstrates a relative uncertainty of some 10-6 for distances of 0.25 to 1 m.
We propose an interferometric method for absolute distance measurement. This method uses the fact that the wavelength of a laser diode can be tuned continuously, by addition of an external cavity. The laser system, which has a small linewidth and therefore a large coherence length, is used to feed two interferometers. One of them serves as a reference distance. The phase variation of the signal is extracted by the Fourier transform technique. The magnitude of phase variation is proportional to the rate of measuring and reference distances. If the reference distance is known, a measuring distance can be obtained from measurement of the phase variation. We have done some fundamental experiments and shown that the precision is essentially limited by calibration of reference distance. The application of this technique has already allowed to measure distance of 3 m with relative accuracy of 1.8 10-4.
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