Bruce E. Perilloux

Discrete thin-film layer thickness modulation

A novel analytical thin-film design method is presented that is based on electrical engineering communication theory. The proposed thickness modulation describes the thickness modulation of discrete, homogeneous thin-film layers of a multilayer coating. One modulation scheme, amplitude modulation, is presented in which analytical equations determine individual layer thicknesses for a given modulation amplitude, carrier frequency (f(c)), direct-current bias, as well as several layers and refractive indices. The spectral performance (especially stop bands) of multilayer coatings with alternating layers of two refractive indices is presented for different carrier frequencies and modulation amplitudes. For f(c) </= 1, the ratio of the center frequencies of the first-order (f(1)) and the next present stop band (f(2)) is determined analytically from the modulation frequency for which f(2)/f(1) = 2f(c) + 1. Particular cases of the carrier frequency produce virtual stop bands below the spectral frequency of the first-order stop band as well as high-frequency harmonics. Degenerate and other cases of thickness-modulated designs are presented, along with other modulation methods.

Al-glass kW fibre laser end-pumped by MCCP-cooled diode stacks

High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic “all-bibre” architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. [1]. The gain fiber utilizes a low index polymer coating to provide the wave-guiding for the multimode pump as well as for compatibility with the NA increase (typically 0.22–0.45) which occurs in the fused taper combiners. While this all fibre approach has been shown to be viable, it is not trivial to implement at power levels in excess of several hundred watts Issues include polymer coating degradation, transverse mode-coupling induced instability at splices or FBGs, grating walk-off, and modal instability, [2]. The latter issue arises because these fiber laser designs are focused on single-transverse-mode operation, [3.4], even though the fibres themselves are multimode to avoid nonlinear impairments. This is despite the face that most cutting and welding applications actually utilize a multimode fibre for delivery to the cutting head. The BBP of such systems is typically 2.5mm.mRad at a wavelength around 1080nm. However, single mode operation allows power scaling by incoherently combining several lower power fiber lasers into a single beam with that BPP.

Infrared thin film polarization preserving reflectors

The design of ideal infrared thin-film polarization preserving reflectors requires the equalization of the p and s polarization reflectances and zero differential phase shift between them. Depending on system design requirements for absolute reflectivity, either non-absorbing substrates such as zinc selcnide, or metallic films such as silver or gold are commonly utilized. In addition, a few dielectric layers are deposited onto the substrate for reflection enhancement and phase correction. This paper will investigate the design of enhancement layers with refractive indices n1 and n2, onto various substrates for a single wavelength and in most cases, 45 degrees angle of incidence. Indices of actual film materials such as thorium fluoride, germanium and zinc sulfide will be utilized to demonstrate actual design performance. Also, an equation is presented that is used to predict the differential phase shift sensitivity to wavelength centering of a quarter wave stack. In this case, the film indices determine the incidence angle sensitivity. Next, the incidence angle sensitivity of some designs is investigated. Some designs act as polarization preserving reflectors from normal incidence to nearly 80 degrees angle of incidence. A brief summary of conventional enhanced metal coatings is presented, along with design methods. Some applications of polarization preserving reflectors are described, especially for C02 lasers.

Altered stopband positions of thin-film thickness modulated designs based on numerical layer periodicity

Thickness modulated designs are investigated for stopband positions versus layer periodicity. Plots of TMD amplitude reflection coefficients show zeros. TMD with non-integer multiples of layer periodicity have altered spectral stopband positions.

Helium neon laser optics: scattered light measurements and process control

Output coupler (OC) and high reflector (HR) thin-film coatings and substrates that are employed in 632.8 nm helium-neon (HeNe) lasers are investigated for optical scatter. The measurement of scatter in this paper is in terms of bidirectional reflectance distribution function and calculated total integrated scatter, or BRDF and CTIS, respectively. Laser output power will be briefly reviewed as a function of total scatter loss from the OC and HR. Increasing amounts of loss (scatter), in individual sets of OCs and HRs, reduces the output power of each laser tube from a theoretical optimum output, for the same amount of loss. Sample optics were measured for BRDF and compared with visual microscope inspection. Statistical analysis of many thousands of coated optics provides insight to controlling causes of scatter in the manufacturing process. Here, the average and standard deviation of BRDF scatter data for many optics provide important data for process control. Various scatter data is presented form OC and HR production runs.