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This thesis proposes analytically developed aplanatic field correctors. To begin, an astigmatism correction method for aplanatic Gregorian telescopes was developed employing two lenses with spherical surfaces and a spherical GRIN medium. Sec ond, to expand the field of view of RC telescopes, a meniscus lens with aspherical surfaces has been designed. This meniscus can flatten the image surface while cor recting astigmatism. The meniscus, on the other hand, has lateral color. Finally, using the same principle as the meniscus, a refractive pair has also been designed to increase the FoV in RC telescopes. It does, however, correct the lateral color at the expense of introducing axial color. By adjusting the distance between the pair, this pair may function for various spectral bands individually. All of the methods mentioned above may be added to or removed from an existing aplanatic telescope without compromising image quality or changing the original telescope design. Fur thermore, their employment maintains the telescope’s aplanatic properties.

We present the formulation of a paraxial ray transfer or ABCD matrix for onion-type GRIN lenses. In GRIN lenses, each iso-indicial surface (IIS) can be considered a refracting optical surface. If each IIS is a shell or layer, the ABCD matrix of a GRIN lens is computed by multiplying a typically high number of translation and refraction matrices corresponding to the K layers inside the lens. Using a differential approximation for the layer thickness, this matrix product becomes a sum. The elements A, B, C, and D of the approximated GRIN ray transfer matrix can be calculated by integrating the elements of a single-layer matrix. This ABCD matrix differs from a homogeneous lens matrix in only one integration term in element C, corresponding to the GRIN contribution to the lens power. Thus the total GRIN lens power is the sum of the homogeneous lens power and the GRIN contribution, which offers a compact and simple expression for the ABDC matrix. We then apply this formulation to the crystalline lens and implement both numerical and analytical integration procedures to obtain the GRIN lens power. The analytical approximation provides an accurate solution in terms of Gaussian hypergeometric functions. Last, we compare our numerical and analytical procedures with published ABCD matrix methods in the literature, and analyze the effect of the iso-indicial surface’s conic constant (Q) and inner curvature gradient (G) on the lens power for different lens models., Ministerio de Ciencia e Innovación (PID2019-405 107058RB-100, PGC2018-095795B-I00); HORIZON EUROPE Marie Sklodowska-Curie Actions (956720).

International audience; We developed a flexible two-photon microendoscope (2P-FENDO) capable of all-optical brain interrogation at near cellular resolution in freely moving mice. The system performs fast two-photon (2P) functional imaging and 2P holographic photostimulation of single and multiple cells using axially confined extended spots. Proof-of-principle experiments were performed in freely moving mice co-expressing jGcaMP7s and the opsin ChRmine in the visual or barrel cortex. On a field of view of 250 µm in diameter, we demonstrated functional imaging at a frame rate of up to 50 Hz and precise photostimulation of selected groups of cells. With the capability to simultaneously image and control defined neuronal networks in freely moving animals, 2P-FENDO will enable to precisely define the functions of neurons in the brain and their interactions during naturalistic behaviours.

Integrated photonics devices, based in subwavelength grating (SWG) metamaterials, have shown unprecedented performance in a wide variety of situations. Since their proposal and first experimental demonstration in 2010 designers have made use of the new degrees of freedom provided by these structures to design advanced devices with improved capabilities. The extended design space provided by SWG structures has been successfully used to engineer the refractive index, the dispersion and, more recently, the waveguide birefringence, thus allowing novel advanced device design. In this invited talk we will review some of the advances made by our group in the field., Smart Photonic and Optoelectronic Integrated Circuits XXII, February 1-6, 2020, San Francisco, United States, Series: Proceedings of SPIE

Elastic metamaterials comprise a broad class of artificially engineered media designed to control the propagation of elastic waves at sub-wavelength scales. This unique ability stems from the presence of resonant inclusions embedded in an elastic matrix which affect the wave propagation at specific frequency ranges. Elastic metasurfaces are a special class of metamaterials, realized by arranging an array of resonant structures at the free surface of the medium or at an interface between two media. Metasurfaces of vertical resonators over an elastic half-space have been successfully utilized to control the propagation of in-plane surface waves, i.e., Rayleigh waves, for various applications across different length scales, as waveguiding or wave filtering. In this talk, we present an extension of the metasurface concept to Love waves, anti-plane surface waves existing in semi-infinite layered media. We investigate the interaction between Love waves and a metasurface of horizontal oscillators deriving an original analytical solution for its dispersion relation. By tuning the mass and the frequency of the resonators we achieve full control of the Love wave phase velocity, and thus on the related metasurface refractive index. We exploit the ability to manipulate the metasurface refractive index to design gradient index lenses (i,e. Luneburg and Maxwell lenses) for Love waves redirection. We analyse the performance of the designed lenses using full 3D FE simulations confirming the analytical predictions. Our work can serve as a guide for advanced Love wave-based devices and sensors as well as for the design of meter-scale barriers for low-frequency vibrations attenuation.

The ability to precisely monitor and manipulate neural circuits is essential to understand the brain. Advancements over the last decade in optical techniques such as calcium imaging and optogenetics have empowered researchers to gain insight into brain function by systematically manipulating or monitoring defined neural circuits. Combining these cutting-edge techniques enables a more direct mechanism for ascribing neural dynamics to behavior. Here, we developed a miniaturized integrated microscope that allows for simultaneous optogenetic manipulation and cellular-resolution calcium imaging within the same field of view in freely behaving mice. The integrated microscope has two LEDs, one filtered with a 435-460 nm excitation filter for imaging green calcium indicators, and a second LED filtered with a 590-650 nm excitation filter for optogenetic modulation of red-shifted opsins. We developed and tested this technology to minimize biological and optical crosstalk. We observed insignificant amounts of biological and optical crosstalk with regards to the optogenetic LED affecting calcium imaging. We observed some amounts of residual crosstalk of the imaging light on optogenetic manipulation. Despite residual crosstalk, we have demonstrated the utility of this technology by probing the causal relationship between basolateral amygdala (BLA) -to- nucleus accumbens (NAc) circuit function, behavior, and network dynamics. Using this integrated microscope we were able to observe both a significant behavioral and cellular calcium response of the optogenetic modulation on the BLA-to-NAc circuit. This integrated strategy will allow for routine investigation of the causality of circuit manipulation on cellular-resolution network dynamics and behavior.

International audience; Linear and nonlinear optical imaging systems are currently being miniaturized to fit endoscopic size requirements of a diameter less than 2 mm. These systems are used for deep imaging and optical biopsy. We report our progress in the development of a multimodal microendoscope based on Gradient Index (GRIN) lenses and double clad fiber (DCF) for femtosecond pulse delivery and signal collection. Four commercial DCFs were identified from the literature and our choice was based on the simulation results for optimal pulse delivery -needed for efficient 2-photon excitation- and optimal fluorescence signal collection. The pulse delivery simulation was done using a GRISM line system for compensating linear and nonlinear effects inside the DCF. The effect of GRIN lens aberration on collection efficiency was estimated by simulation using Zemax. GRIN lenses were also validated for fluorescence lifetime, spectroscopic measurement and mouse tissue imaging.

A frequency control scheme is introduced where within each transmitter its frequency is referenced to a silica based optical frequency discriminator, which in addition is locked to an absolute reference frequency. An absolute stability of /spl plusmn/1 GHz was achieved. As frequency-determining elements, coupler-based Michelson interferometers or coated gradient-index (GRIN) lenses were used. The described set-up is employed in an OFDM based optical crossconnect-demonstrator (RACE 2065; COBRA).

International audience; Linear and nonlinear optical imaging systems are currently being miniaturized to fit endoscopic size requirements for clinical applications. These systems have demonstrated their potential for medical diagnostics of various diseases as a possible alternative to optical biopsy. Our work aims at transferring the abilities of non-linear microscopy into a miniaturized diagnostic tool for discriminating tumorous tissues with sub cellular resolution and different contrast mechanisms. In this context, we report our progress in the development of a multimodal microendoscope based on double clad fiber (DCF) for femtosecond pulse delivery and signal collection and Gradient Index lenses (GRIN). Grism-based stretcher was used to compensate for second and third orders of dispersion in the DCF and GRIN. Four commercial DCFs were identified from the literature and our choice was based on the simulation and experimental results for optimal pulse delivery and maximum fluorescence signal collection as well as two photon resolution. Pulse propagation in the stretcher and the DCFs was simulated by our Home-built Matlab program while collection efficiency was calculated using geometric ray tracing in Zemax. Finally, experiments showed that our GRIN lens broadened the pulses due to nonlinear effects, nonetheless our GRISM-based stretcher was able to compensate efficiently for this broadening as well as for pulse interactions in DCF. This allows the delivery of sub-30 fs pulses at the tissue site that are several folds shorter than those obtained in the literature using the same types of fibers.

The main challenges of biomedical optical imaging methods are absorption and scattering of light by the tissue, which alter the signal and limit the imaging depth. Here we are interested in Optical Coherence Tomography (OCT), a technique that selects ballistic photons using low coherence interferometry in order to virtually slice inside the tissue. The so-called Full-Field approach achieves an isotropic resolution of 1 µm, but cannot image deeper than 1 mm. It is thus impossible to image internal organs in vivo and in situ: an endoscopic system is needed. The adaptation of Full-Field OCT for endoscopy had not yet been realized. We propose a Full-Field OCT system with a flexible endoscope based on the coupling of two interferometers, one external to the probe and the other situated at the tip of the probe in contact with the tissue. This makes the probe entirely passive and insensitive to environment-induced perturbations. The flexible endoscope could access organs such as the colon. However the probe is a bundle made of thousands of optical fibers, so that a parasitic interferometric signal arises from coupling between cores and modes. We have found and tested different solutions to get rid of these stray signals and allow imaging of tissue. Concerning rigid endoscopy we have built two setups, with either one or two interferometers, and compared both methods. The probe is an assembly of graded-index lenses, it can be applied in contact on areas like skin or it can be inserted as a needle inside organs like breast. This system has been demonstrated for in vivo human skin imaging.; L'imagerie optique biomédicale est confrontée aux phénomènes d'absorption et de diffusion de la lumière par les tissus qui atténuent et brouillent le signal au cours de sa propagation. La Tomographie de Cohérence Optique (OCT) filtre les photons balistiques par interférométrie en lumière temporellement incohérente. En particulier, l'approche Plein Champ atteint une résolution de 1 µm, mais sa profondeur de pénétration ne dépasse pas 1 mm. Il est donc impossible d'imager les organes internes in vivo et in situ : il faut pour cela disposer d'un instrument doté d'un endoscope, ce qui n'avait pas été réalisé à ce jour. Nous proposons un système d'OCT Plein Champ avec endoscope flexible qui repose sur le couplage de deux interféromètres, l'un extérieur à la sonde et l'autre situé à l'extrémité de la sonde en contact avec le tissu. Ainsi la sonde est entièrement passive et le signal interférométrique qui la traverse n'est pas perturbé par l'environnement. Sa flexibilité lui permettrait d'imager des organes internes comme le colon. Cependant l'utilisation d'un faisceau de plusieurs milliers de fibres optiques fait apparaître un signal d'interférence parasite dû aux phénomènes de couplage inter-coeurs et inter-modes. Des solutions sont proposées pour s'en affranchir et permettre l'imagerie d'échantillons de tissus. En imagerie par endoscopie rigide deux systèmes sont comparés, avec un ou deux interféromètres, et une sonde composée de lentilles à gradient d'indice. Cette sonde peut imager directement au contact des zones comme la peau, ou en pénétrant l'organe à la manière d'une aiguille, par exemple dans le sein. Elle est assez performante pour être utilisée in vivo sur la peau.

The main challenges of biomedical optical imaging methods are absorption and scattering of light by the tissue, which alter the signal and limit the imaging depth. Here we are interested in Optical Coherence Tomography (OCT), a technique that selects ballistic photons using low coherence interferometry in order to virtually slice inside the tissue. The so-called Full-Field approach achieves an isotropic resolution of 1 µm, but cannot image deeper than 1 mm. It is thus impossible to image internal organs in vivo and in situ: an endoscopic system is needed. The adaptation of Full-Field OCT for endoscopy had not yet been realized. We propose a Full-Field OCT system with a flexible endoscope based on the coupling of two interferometers, one external to the probe and the other situated at the tip of the probe in contact with the tissue. This makes the probe entirely passive and insensitive to environment-induced perturbations. The flexible endoscope could access organs such as the colon. However the probe is a bundle made of thousands of optical fibers, so that a parasitic interferometric signal arises from coupling between cores and modes. We have found and tested different solutions to get rid of these stray signals and allow imaging of tissue. Concerning rigid endoscopy we have built two setups, with either one or two interferometers, and compared both methods. The probe is an assembly of graded-index lenses, it can be applied in contact on areas like skin or it can be inserted as a needle inside organs like breast. This system has been demonstrated for in vivo human skin imaging.; L'imagerie optique biomédicale est confrontée aux phénomènes d'absorption et de diffusion de la lumière par les tissus qui atténuent et brouillent le signal au cours de sa propagation. La Tomographie de Cohérence Optique (OCT) filtre les photons balistiques par interférométrie en lumière temporellement incohérente. En particulier, l'approche Plein Champ atteint une résolution de 1 µm, mais sa profondeur de pénétration ne dépasse pas 1 mm. Il est donc impossible d'imager les organes internes in vivo et in situ : il faut pour cela disposer d'un instrument doté d'un endoscope, ce qui n'avait pas été réalisé à ce jour. Nous proposons un système d'OCT Plein Champ avec endoscope flexible qui repose sur le couplage de deux interféromètres, l'un extérieur à la sonde et l'autre situé à l'extrémité de la sonde en contact avec le tissu. Ainsi la sonde est entièrement passive et le signal interférométrique qui la traverse n'est pas perturbé par l'environnement. Sa flexibilité lui permettrait d'imager des organes internes comme le colon. Cependant l'utilisation d'un faisceau de plusieurs milliers de fibres optiques fait apparaître un signal d'interférence parasite dû aux phénomènes de couplage inter-coeurs et inter-modes. Des solutions sont proposées pour s'en affranchir et permettre l'imagerie d'échantillons de tissus. En imagerie par endoscopie rigide deux systèmes sont comparés, avec un ou deux interféromètres, et une sonde composée de lentilles à gradient d'indice. Cette sonde peut imager directement au contact des zones comme la peau, ou en pénétrant l'organe à la manière d'une aiguille, par exemple dans le sein. Elle est assez performante pour être utilisée in vivo sur la peau.

Fiber-Optic Variable Optical Attenuators (VOAs) are demonstrated using Liquid Crystals (LC) for broadband as well as wavelength tunable applications. Attenuation is achieved by using a beam spoiling approach implemented via electrically reconfigurable non-pixelated no moving parts Nematic LC deflectors. The VOAs feature in-line architecture and polarization insensitive design without the use of bulky polarization splitting and combining optics. The proof-of-concept VOAs in the 1550 nm band demonstrate >30 dB attenuation ranges, low polarization dependent losses and low power consumption. Applications for these VOAs include agile wavelength tunable secure data communications networks and RF sensor systems.

A high-speed programmable optical attenuator (POA) is introduced that can provide high dynamic range and high resolution attenuation control. An acousto-optic Bragg cell has been used to realize the desired attenuator. Various practical architectures have been proposed to prove the concept. A typical acousta-optic deflector (AOD) based design offers a dynamic range of 46.11 dB. The average resolution is 3.08 dB/volt whereas the best resolution achieved is 0.39 dB/volt. The response time of the AO attenuator is in the sub CLS regime, and was observed as low as 0.175 mu s. The polarization dependent loss (PDL) was about 0.7 dB, whereas the excess losses were about 1.5 dB.A high-speed programmable optical attenuator (POA) is introduced that can provide high dynamic range and high resolution attenuation control. An acousto-optic Bragg cell has been used to realize the desired attenuator. Various practical architectures have been proposed to prove the concept. A typical acousta-optic deflector (AOD) based design offers a dynamic range of 46.11 dB. The average resolution is 3.08 dB/volt whereas the best resolution achieved is 0.39 dB/volt. The response time of the AO attenuator is in the sub CLS regime, and was observed as low as 0.175 mu s. The polarization dependent loss (PDL) was about 0.7 dB, whereas the excess losses were about 1.5 dB.

Novel two-dimensional (2-D) optical polarization switching array-based photonic time delay units (PTDUs) have been introduced for phased array antenna and wideband signal processing applications. The use of low loss optical fibers allows remoting of the photonic beamformer, along with providing a compact, lightweight, and low electromagnetic interference (EMI) microwave frequency signal interconnection and distribution method, such as needed for very large aperture wide instantaneous bandwidth phased array antennas/radars. However, there are losses associated with multiple fiber interconnects that limit the maximum number of array channels in these systems. Thus, accurate analysis of such losses is crucial to the design of an optimal photonic fiber-based system. In this paper, we present theoretical design and simulation results on optical fiber array interconnects for our 2-D N bit M channel photonic beamformer for wideband phased array antennas. In addition, we discuss an alignment technique for the large channel count fiber arrays proposed for our beamformer that uses V-grooved silicon wafers. Note that these precise V- groove structures are fabricated via crystallographic perfection of the substrate, accurate alignment of the etch pattern with respect to the crystal planes, and optimized etch conditions. This paper discusses these and other fiber array issues.