Joël Fontaine

Design of an off-axis see-through display based on a dynamic phase correction approach

The design and the analysis of an off-axis (50°) diffractive imaging optical system is presented in this paper. A 10°x15° field of view is considered. The optical system is composed of two diffractive optical elements. A static diffractive optical element having a frozen phase transfer function is used to perform a virtual point in the considered field of view. A dynamic diffractive optical element having an adapted calculated phase transfer function is used to compensate for aberrations of the static element. Using a sequential creation of virtual image points and considering human eye characteristics, it is shown that a nine points virtual image can be obtained with current technology. Moreover, it is presented that aberrations can be compensated whatever the position of the virtual point in the 10°x15° field of view. Finally, using rigorous coupled wave analysis, it is shown that an average diffraction efficiency of 79% can be reached across the considered field of view with a standard deviation of nearly 5%.

Ultra-narrow photonic nanojets through a glass cuboid embedded in a dielectric cylinder

A glass cuboid, embedded inside a dielectric cylinder is studied when illuminated with a monochromatic plane wave. A photonic nanojet (PNJ) with a full-width at half-maximum (FWHM) waist of around 0.25λ0 is obtained outside the external surface of the cuboid. The influence of the parameters of a square section cuboid is studied. Three particular phenomena can be obtained and are discussed: an ultra-narrow PNJ on the external surface of the cuboid, a long photonic jet and the excitation of whispering gallery modes (WGMs). A parametric study, over the width and the height of a rectangular section cuboid, shows that these parameters can be used to control the photonic jet properties. We also study several other geometries of the insert, which shows that the key parameter is the refractive index of the inserted material. Finally, we show that by changing the incident angle we can obtain a curved photonic jet.

Design of a large field-of-view see-through near to eye display with two geometrical waveguides

A novel waveguide near to eye display (WGNED), with new in-coupling and propagation subsystems, is proposed for the first time, to our knowledge, to enlarge the vertical field-of-view (FOV) and the vertical size of the eye box. Two waveguides are stacked-one is for in-coupling and the other for out-coupling. A freeform prism is used to correct the aberrations. These components are combined to form the WGNED. We have simulated such a system; as a result, we show that it achieves a FOV of 30°horizontal (H)×60°vertical (V) and an eye box of about 15  mm (H)×12  mm (V). The modulation transfer function of the system is larger than 0.3 at 33  lp/mm and the distortion is smaller than 5%.

Filter design for dynamic frequency calibration of an external cavity diode tunable laser using frequency comb

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.

Low energy sub-micron laser machining using photonic nanojet with shaped optical fiber tip (Conference Presentation)

A photonic nanojet is a highly concentrated laser beam observed in the vicinity of dielectric micro-objects such as glass micro-spheres. Thanks to the concentration of the beam beyond the diffraction limit, giving a spot with a width smaller than a half-wavelength, the incident power density can be multiplied by a factor larger than 200. Photonic jet obtained with microspheres has been applied successfully to material ablation. It has been demonstrated that the ablation on metal or glass can have a half-wavelength width using a common infrared nanosecond pulse laser. However, the spheres in contact with the sample are difficult to move to achieve an industrial process and are disturbed by the removed material at the beginning of the process. Recently we have shown that photonic nanojet obtained in the vicinity of shaped optical fiber tip is an alternative to overcome these limitations. Sub-micron etchings have been obtained on metals, semiconductors and ITO using multimode optical fibers with a numerically designed shaped tip. The possibility to perform not only ablation, but also to generate self-organized micro-peaks, has also been experimentally demonstrated. Besides the small size of the processed area, our talk will focus on the low energy required for material removal. Due to the high energy concentration, the required energy to ablate is already 20 times smaller than in a classical process. Finally, we will show how the energy coupling in the fiber is a parameter as important as the tip shape to decrease the energy required to reach ablation.

Photonic jet etching: Justifying the shape of optical fiber tip

Photonic jet (PJ) is a low diverging and highly concentrated beam in the shadow side of dielectric particle (cylinder or sphere). The concentration can be more than 200 times higher than the incidence wave. It is a non-resonance phenomenon in the near-field can propagate in a few wavelengths. Many potential applications have been proposed, including PJ etching. Hence, a guided-beam is considered increasing the PJ mobility control. While the others used a combination of classical optical fibers and spheres, we are concerned on a classical optical fiber with spherical tip to generate the PJ. This PJ driven waveguide has been realized using Gaussian mode beam inside the core. It has different variable parameters compared to classical PJ, which will be discussed in correlation with the etching demonstrations. The parameters dependency between the tip and PJ properties are complex; and theoretical aspect of this interaction will be exposed to justify the shape of our tip and optical fiber used in our demonstrations. Methods to achieve such a needed optical fiber tip will also be described. Finally the ability to generate PJ out of the shaped optical fiber will be experimentally demonstrated and the potential applications for material processing will be exposed.

One step marking process with fiber laser and diffractive optical elements

In this paper we propose a one step marking process using a high power near infrared fiber laser and diffractive optical elements or gratings as an alternative to traditional scanning systems. The impact of the gratings fill factor, the power level on the diffracted beams and the quality of the reconstructed images are studied. We present simulation results of gratings diffraction efficiencies, experimental observations of gratings and DOEs behaviour illuminated with a fiber laser beam. We demonstrate that the fiber laser, showing good beam quality, can be used with standard diffractive structures with interesting results.

Fabrication of multilevel reflective diffractive optical elements by means of laser ablation lithography

Multilevel Diffractive Optical Elements for high-power laser beam shaping have been designed and produced. The originality of the proposed approach rests on the realization of four phase levels reflection elements with a single etching step. Thus, if the phase change of 270° is obtained by etching a pixel element, intermediary phase difference is obtained by etching subwavelength structures of the same height with appropriate filling factor. Sizing of the subwavelength gratings required with this theory has been carry out for TE and TM polarized radiation with a rigorous electromagnetic model, the Finite Difference Time Domain method. In a first step, a test component has been realized using photolithography to validate the sizing of the subwavelength gratings. In a second step we have adapted a specific fabrication tool based on laser ablation and direct writing for greater flexibility. Characterization has been carried out with Coherence Probe Microscopy. The technique has been applied to the fabrication of a diffractive element used with a high-power CO2 laser beam for surface marking.

Association of acousto-optic and microscanning mirrors for diffractive memory high-speed reading

We present an investigation that has been carried out on the design of a high speed scanning system for a data storage application. Polypeptide material is used to store data by the angular multiplexing process. This material presents many advantages compared with others. To address the optical memory, our set-up is composed of micro-scanning mirrors (MEMS) and an acousto-optic deflector (AOD). This association leads to an addressing set-up with a very good performance in terms of the access time and the angular bandwidth. Expermental tests made with micro-scanning mirrors (MEMS) of 3 x 3 mm2 are described. Problems fo synchronization between the different elements and the influences of MEMS deformation are also discussed.

Design of a large spatial and temporal bandwidth beam deflecting system for display addressing

We present the investigation which was conducted to improve the spatial and temporal performances of a deflecting system for display addressing. In a first part we describe the operation principle used to increase the optical deflection angle of an acousto-optic deflector. We show that it is possible to obtain an angular amplification simply by using a grating at grazing incidence angle; the diffraction is then close to 90 degrees. Experimental tests made with a 1200 gr/mm grating have shown that an amplification of the deflection angle as high as 10 can be obtained. In a second part we introduce a complete set-up composed by the association of MEMS, Acousto-optic and grating. All those components are used to achieve a compact addressing system.