Simon Joly

Singular topology of optical absorption in biaxial crystals

We show for the first time that biaxial crystals exhibit continua of directions of propagation where the absorption coefficient is the same for the two associated polarization modes. This statement is supported by both calculations and experimental data obtained in Nd:YCOB.

Laser and self-doubling properties of a Nd:YCOB crystal cut as a sphere and inserted in a cavity

We propose what we believe to be for the first time a method for the direct characterization of the angular distribution of laser and self-doubling properties. It is based on the rotation of a sample cut as a sphere and inserted between two plane mirrors. We studied the monoclinic crystal Nd:YCOB. The only data that are mandatory to know are the crystal class and the orientation of the sphere in the dielectric frame.

Refined modeling of angular distributions of linear absorption and fluorescence in biaxial crystals

We provide a deeply detailed description of linear absorption and fluorescence angular distributions in biaxial media; we properly take into account the distributions of the polarization eigenmode vectors around the principal plane of the dielectric frame that contains the optical singularities of the optical axes. By introducing a non-zero angular aperture of experimental detection setups and a related angular integration of absorption and fluorescence angular distributions, we provide for the first time a complete interpretation and refined modeling of former experimental measurements. Such developments should be considered to perform relevant metrology of low-symmetry biaxial materials for optics.

Rotation of the absorption frame as a function of the electronic transition in the Nd 3+ :YCa 4 O(BO 3 ) 3 monoclinic crystal

We report the first measurements of the angular distribution of absorption under polarized light corresponding to seven electronic transitions of Nd(3+) ions in the monoclinic crystal Nd:YCOB, revealing a rotation of the symmetry axes of the different patterns.

Layer-Inversion Zones in Angular Distributions of Luminescence and Absorption Properties in Biaxial Crystals

We numerically depict the complete angular distributions of luminescence and absorption properties in biaxial media, by calculating the imaginary part of the optical index for all directions of propagation. Our simulations show a double-layer surface with specific topology and symmetry properties that greatly differ from those of the refractive index surface. Our calculations show that the two layers intersect and inverse themselves along continuous loci related to polarization-independent luminescence or absorption properties. Specificities related to the orthorhombic, monoclinic and triclinic biaxial crystal systems are discussed. Such theoretical developments should be considered to fully exploit innovating luminescent materials.

Enabling patterning of polymer optical devices working at visible wavelength using thermal nano-imprint lithography

Thermal Ultraviolet NanoImprint Lithography is a fast and reliable process to manufacture large scale integrated polymer-based optical components from a soft stamp. This technology already provides polymer integrated components for optical communications operating in the infrared region. However, several fabrication issues must be addressed to enable reliable mass production of optical components working in the visible region, especially when patterning large devices with nanometric features. In this work, we report our fabrication results on grating coupler and optical microring resonators with SU-8 resist. The device is conceived for monomode visible wavelength operation dedicated to future optical sensing applications. For this purpose, sub-micron waveguides are needed. We reported two main defects on soft stamp fabrication: partial sidewall detachment and shifted or double embossing of the features. Nanoimprinting SU-8 waveguides was achieved with the operational devices of the soft stamp.

Accurate electro-optical characterization of high power density GaAs-based laser diodes for screening strategies improvement

In this study, we report on a methodology based on reverse and forward current-voltage curves (I-V) and on Degree of Polarization (DoP) of electroluminescence measurements on 980 nm laser diodes chip-on-submount (CoS) for the improvement of screening tests. Current-voltage curves are performed at reverse bias up to breakdown voltage (VBR) using both a high current accuracy (< 1 pA) and high voltage resolution (< 10 mV) at different submount-temperatures (20-50°C). The DoP of luminescence of such devices, related to strains in materials and effect of shear strain on the birefringence, is calculated from the simultaneous measurement of TE (LTE) and TM (LTM) polarized light emissions. We observe that application of high reverse voltages occasionally produces significant micro-plasma (MP) pre-breakdown on reverse I-V characteristics as recently observed in InGaN/GaN LEDs and assumed to be a response of electrically active defects. Comparisons between breakdown voltages and number of MP, and changes of leakage current at low forward voltage (< 0.1 V) are considered. DoP measurements are also analyzed versus temperature. Finally the usefulness of these measurements for effective screening of devices is discussed.

Seed laser diodes in pulsed operation: limitations and reliability

Using InGaAs/AlGaAs 1060nm commercial Laser Diodes (LDs) and Laser Diode Modules (LDMs) under high current (> 2A) and nanosecond pulsed conditions offers a large flexibility for fiber Laser seeding applications. Nevertheless, the behavior and long term reliability of these LDs under such conditions is not well established. Our work focuses on determining the reliability of 1060nm seed LDs under such conditions by the extraction of Electro-Optical (EO) characteristics and the monitoring of their evolution during pulsed ageing tests. In this context, two segments of parasitic oscillations (“A” and “B” type) were observed in the optical response of LDs and LDMs driven under such conditions, with their associated threshold current (IthA and IthB) on three batches of Laser diodes, with different packaging. We observed a strong part-to-part variation in the value of IthA and a package dependency for IthB. We conducted a near-field and a time spectral analysis of the LDs optical responses. A near field widening, associated with A-type oscillations, and a temporal spectral broadening, associated with A-type and B-type oscillations, were highlighted. Step-stress ageing tests were then carried out on two batches of three LDMs each, with different values of IthA (3.9A, 7A and 11.2A for the first batch, 2.8A, 7.4A and 9.3A for the second). The modules with the lowest IthA values were the first to fail suggesting this IthA threshold as a key parameter for early failure detection. One module exhibited a gradual drop of the measured optical power. No variation of the optical power or of the IthA and IthB value were observed on the other modules, which were able to withstand the chosen ageing conditions without any noticeable decrease of their performances.

Analysis of optical responses of 1060nm seed laser diodes under overcurrent and short-pulse conditions for reliability investigations

New phenomena are observed in the optical response of InGaAs/AlGaAs 1060nm Laser Diodes (LDs) and Laser Diode Modules (LDMs) driven under high peak current condition: two segments of parasitic oscillations appear in the optical response of every tested LD and LDM, when increasing the current above two respective thresholds. In order to understand their origins and to discuss their influence over the operation range and reliability of seed LDs, we designed a test bench, based on an Electro-Optical Modulator, devoted to the time-spectral analysis of LD optical responses under such conditions. A correlation was found between the presence of the first segment of oscillations on the optical response and a temporal broadening of the LD spectrum. The presence of the second segment of oscillations is associated with a red-shift of the LD spectrum, but not with a significant spectral broadening, suggesting different causes for these phenomena. Step-stress ageing tests have then been carried out, in order to estimate the evolution of those parasitic oscillations over the lifetime of these LDs for reliability investigations.

Laser and self-doubling operations in a Nd:YCOB sphere

We report experimental results on a laser based on a Nd:YCOB crystal cut as a sphere inserted between two plane mirrors. It is being used as a unique tool for the direct characterization of self-doubling.