N. M. Lyndin

Modal analysis and suppression of the Fourier modal method instabilities in highly conductive gratings

The Fourier modal method (FMM), often also referred to as rigorous coupled-wave analysis (RCWA), is known to suffer from numerical instabilities when applied to low-loss metallic gratings under TM incidence. This problem has so far been attributed to the imperfect conditioning of the matrices to be diagonalized. The present analysis based on a modal vision reveals that the so-called instabilities are true features of the solution of the mathematical problem of a binary metal grating dealt with by truncated Fourier representation of Maxwells equations. The extreme sensitivity of this solution to the optogeometrical parameters is the result of the excitation, propagation, coupling, interference, and resonance of a finite number of very slow propagating spurious modes. An astute management of these modes permits a complete and safe removal of the numerical instabilities at the price of an arbitrarily small and controllable reduction in accuracy as compared with the referenced true-mode method.

Subwavelength cylindrical grating by holistic phase-mask coordinate transform

A periodic grating with an integer number of periods is fabricated at the resist-coated wall of a cylinder by exposing a circularly symmetrical planar high index phase mask to a cylindrical wave. This extends the spatial coherence features easily achievable in a planar 2D space to the 3D space of cylindrical waves and elements.

Enhancement of the Mode Area and Modal Discrimination of Microchip Lasers Using Angularly Selective Mirrors

Angularly selective mirrors (ASMs) are proposed as a means to expand the mode area and modal discrimination of microchip lasers. ASMs used as output couplers selectively reflect incoming k vectors over a narrow angular range, while they transmit more inclined components. The eigenvalue problem of a microchip resonator equipped with a Gaussian ASM is solved analytically in the paraxial optics approximation using the ABCD matrix formalism. The narrow angular distribution of the reflected beam produces, through the laws of diffraction, a significant increase of the mode size and improved transverse mode discrimination, at the expense of higher oscillation threshold due to larger output coupling losses. Simulations performed using the parameters of Yb3+-doped YAG material show that one order of magnitude increase of the mode area can reasonably be achieved without causing overheating and thermal fracture. ASMs can be directly deposited on the active material in the form of a resonant grating mirror. This technology involves only planar batch processes that retain the mass production advantage of microchip lasers. The significant increase of brightness of microchips expected from this innovation will give rise to more effective and more compact devices and new applications.

Unidirectional waveguide grating coupling by means of parallelogrammic grooves

The blazing effect of parallelogramic grooves is analyzed theoretically and demonstrated experimentally in the case of TE and TM modes in large guidance waveguides. A novel fabrication method is proposed, modellized and demonstrated.

Resonant grating pulse compression element with 99% flat top efficiency for high average power femtosecond laser machining

Top hat diffraction efficiency in an all-dielectric SiO2/HfO2 grating femtosecond pulse compression grating is demonstrated with a close to 100% flat top over more than 20 nm around 800 nm wavelength. New perspectives are open for high average power femtosecond laser machining.

1-nm Line Width, Flux-Resistant Laser Mirror Using a Resonant Grating

A corrugated structure comprising a multidielectric mirror, a waveguide resonant mirror and a buffer is shown to exhibit close to 100% abnormal reflection close to normal incidence at 1064 nm wavelength thanks to the excitation of a guided mode of the structure. A solution is found to overcome the laser damage problem caused by high electric field strength in the narrow band high index metal oxide slab waveguide reflector.

Transverse-mode selective resonant grating-mirrors for high power and high brightness emission.

The finite angular spectral width of a 2D resonant grating mirror is adjusted to select the fundamental transverse mode of a laser and to filter out higher order modes. The selection principle is explained phenomenologically on a simplified 1D model. The 2D design is made so as to sustain the large field concentration in the grating slab-waveguide mirror, and the technology permitting to obtain the resonant reflection within the gain bandwidth of two types of laser is described. The blank experimental measurements by means of a white light supercontinuum are shown to match the targeted specifications on the resonance spectral position and angular width.

Wafer-scale measurement of the modes of a dielectric multilayer

Prism and grating coupling techniques can be used to retrieve the refractive index and thickness of a thin film or a stack of layers from the measurement of the effective index of the guided modes they propagate. These techniques are discussed as possible means to assess the wafer scale index and thickness uniformity in the prespective of the batch manufacturing of resonant gratings. The application example considered is a grating polarizer used as a microlaser polarizing mirror.