JEOS:RP - Journal of the European Optical Society Rapid publications
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Using penetration depth for phase matching in photonic crystal waveguides
Get PDFA new method of design for the phase-matching in waveguides is suggested. The approach is based on utilizing the concept of the penetration depth of light into the waveguide walls. The lateral components of wavevectors are employed to adjust the phase-matching condition in the propagation direction. The method is demonstrated in two systems: one using single and the other using double photonic-crystal mirrors
Modelling and measurement of polishing tool influence functions for edge control
Get PDFWe present progress on our development of edge control for fabrication of mirror segments potentially for the European Extremely Large Telescope (E-ELT). Zeeko’s Bonnet polishing technology has been adopted to achieve the form correction target. Like other Computer Numerical Controlled (CNC) polishing, accurate and stable tool Influence Functions (IFs) are important. Particular challenges arisen when polishing up to edges where the geometry of the IFs created by a bonnet changes. We described a model based on measured IFs data that allowed us accurately to predict the edge profile. To obtain IFs at the edge, data from interferometers and profilometers have been stitched. Numerical models with empirical Ifs as input data have been used to predict edge profiles with some preliminary success
HVPE growth and characterization of GaP on different substrates and patterned templates for frequency conversion devices
Get PDFThis article describes efforts to achieve fast deposition of thick Quasi-Phase-Matched (QPM) GaP structures with high surface and structural quality on oriented patterned (OP) templates in a Hydride Vapor Phase Epitaxial (HVPE) process. These QPM structures will be incorporated in devices for conversion of frequencies from the near infrared to the mid infrared and THz regions, where powerful and tunable sources are in great demand for both military and civilian applications. In contrast with GaAs—the most studied OP QPM material—the two-photon absorption of GaP is predicted to be extremely low, which allows pumping with a number of convenient sources between 1 – 1.7 µm. Unpatterned GaP layers up to 370 µm thick were grown with growth rates up to 93 µm/hr with high reproducibility on bare substrates. The layers demonstrated smooth surface morphology with RMS < 1 nm and high structural quality with FWHM equal to 39 arcsec for layers grown on GaP and 112 arcsec for those grown on GaAs. Growth on OP-GaP templates resulted in 142 µm thick QPM structures deposited at a growth rate of 71 µm/h with good vertical (normal to the layer surface) propagation of the initial pattern. When the growth was performed on OP-GaAs one of the domains showed a trend toward a faceting growth. Further investigations are in progress to equalize the vertical and lateral growth of the two domains, and determine the best orientation of the substrate and pattern in order to achieve structures thick enough for high power nonlinear applications
Effect of fill-factor on the Talbot effect of diffraction gratings
Get PDFWe analyze the effect of the fill factor of binary diffraction gratings on the near field propagation. We show that the location of the best-visibility planes changes with the configuration parameters of the grating, that is, the amplitude and phase modulation and the fill factor. Moreover, different intensity patterns and contrast shapes can be obtained when different configurations are used. Analytical expressions describing the contrast of the diffracted field are obtained, that agree with numerical simulations
Equivalent temperature of nonlinear-optical crystals interacting with laser radiation
Get PDFTemperature calibrated piezoelectric resonances of internal acoustic vibration modes of a nonlinear-optical crystal during its heating by high-power laser radiation are used for noncontact highly accurate measurements of both the non-uniform temperature distribution in the crystal volume and in the surrounding air. A novel notion of equivalent temperature of a crystal heated by laser radiation is introduced in laser physics. The true non-uniform crystal thermodynamic temperature at a given laser power is substituted by the measured equivalent crystal temperature, which is constant at that laser power. Using appropriate laser heating model the measured value of the equivalent crystal temperature allows one to calculate the unknown linear and nonlinear optical absorption coefficients as well as the heat transfer coefficient of the crystal with the surrounding air
Dispersion management in nonlinear photonic crystal fibres with nanostructured core
Get PDFThe subwavelength structure of the core of a photonic crystal fibre can modify its dispersion characteristic and significantly shift the zero dispersion wavelength. The dispersion properties of photonic crystal fibres with core structures made of a 2D lattice of subwavelength air holes and various glass inclusions are studied. We show that a modification of the core structure can give flat dispersion over a range of over 300 nm and can shift the zero dispersion wavelength over 700 nm while the core diameter and photonic cladding remain unchanged. The developed photonic crystal fibre with nanorod core has successfully demonstrated supercontinuum generation in NIR
One-step fabrication of polymer components for microphotonics by gray scale electron beam lithography
Get PDFWe demonstrate an application of gray scale electron beam lithography (EBL) for fabrication of three dimensional polymer waveguides and grating output couplers using the SU-8 resist. The groove depths of structure are controlled by choosing a proper exposure dose. Unlike reactive ion etching which is limited by the lag effect, the gray scale EBL allows free combination of groove widths and depths. Shrinking effect which is critical in polymer couplers' writing is taken into account and can be compensated. For better fabrication feasibility, the grating couplers can be simultaneously produced with waveguides with no inter-step alignment required. Therefore, this is a promising technique in manufacturing grating output couplers for polymer based waveguides with high performance in terms of mode matching/confinement and coupling efficiency
Removal of mid spatial-frequency features in mirror segments
Get PDFBoX™ grinding technology has been adopted in our E-ELT segment process. The mid-spatial frequency features generated can be removed by several ‘smoothing’ processes. We have reported here a novel method that can smooth these features whilst avoiding edge down-turn. This process can be scaled up to E-ELT segment fabrication time-scale. It has been experimentally demonstrated that the surface quality is good enough for subsequent Zeeko form correction technology to achieve form specifications
Enhancement of photodetector responsivity in standard SOI CMOS processes by introducing resonant grating structures
Get PDFA new photodetector concept is described which is fully compatible with the standard SOI CMOS process and does not require any post-processing steps. Our simulations are based on two-dimensional RCWA (Rigorous Coupled Wave Analysis) and local absorption theory (K.-H.Brenner, "Aspects for calculating local absorption with the rigorous coupled-wave method" Optic Express 2010, accepted). The simulations show that optimized lateral grating structures are able to enhance the absorption efficiency of thin semi-conductor detectors by a factor of 32 compared to nonenhanced approaches
Assessment of the scattering by sub-micron particles in inland waters
Get PDFCurrent protocols prescribe 0.2 µm filters for absorption measurements of Coloured Dissolved Organic Matter (a(CDOM)). However, a large historical dataset of a(CDOM) was obtained over 0.7 µm filters. Since small particles may pass 0.7 µm filters, a(CDOM) was probably overestimated because of residual scattering. Furthermore, current protocols require suspended matter to be filtered over 0.7 µm. So the 0.2 µm - 0.7 µm fraction is missing from the particles and attributed to enhanced a(CDOM), potentially leading to poor optical closure. In this paper natural water samples from a eutrophic lake were analysed with both filter pore sizes and two different absorption determination methods. Besides a standard bench spectrometer with a cuvette a calibrated Point-Source integrating-Cavity Absorption Meter (PSICAM) was used to gauge the absorption measurements. After filtration over 0.7 µm we found a systematic 8% overestimation of a(CDOM) measured in the cuvette caused by residual scattering. By filtering over 0.2 µm the a(CDOM) cuvette values are 6% underestimated because also a significant fraction of absorption was removed from the sample. As a consequence we recommend testing for coloured residues on the 0.2 µm filter after prime filtration with a GF/F filter. In case of colouration, the method presented in this paper is a fast and simple check to correct the CDOM measurements