Ivan Divliansky

Experimental observation of mode-selective anticrossing in surface- plasmon-coupled metal nanoparticle arrays

Surface plasmon excitation using resonant metal nanoparticles is studied experimentally. Geometry dependent reflection measurements reveal the existence of several optical resonances. Strong coupling of the in-plane nanoparticle plasmon resonance and propagating plasmons is evident from clear anticrossing behavior. Reflection measurements at high numerical aperture demonstrate the excitation of surface plasmons via out-of-plane particle polarization. The thus excited plasmons do not exhibit anticrossing in the considered frequency range. The results are explained in terms of the known surface plasmon dispersion relation and the anisotropic frequency dependent nanoparticle polarizability. These findings are important for applications utilizing surface-coupled nanoparticle plasmon resonances.

Electric-field-driven nano-oxidation trimming of silicon microrings and interferometers

Nanoscale disorder results in severe spectral misalignment of silicon microring resonators and Mach-Zehnder interferometers. We correct for such effects using electric-field-induced waveguide nano-oxidation, demonstrating a tuning wavelength range of several nanometers and 0.002 nm resolution without line shape degradation. Field-induced nano-oxidation is a permanent and precise technique and requires no new materials or high-temperature processing.

High-power spectral beam combining of fiber lasers with ultra high-spectral density by thermal tuning of volume Bragg gratings

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M2 of the individual beams, but the composite M2 after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths (>;100 m) of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <;2 dB/m is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 °C for several hours.

Scaling the spectral beam combining channels in a multiplexed volume Bragg grating

In order to generate high power laser radiation it is often necessary to combine multiple lasers into a single beam. The recent advances in high power spectral beam combining using multiplexed volume Bragg gratings recorded in photo-thermo-refractive glass are presented. The focus is on using multiple gratings recorded within the same volume to lower the complexity of the combining system. Combining of 420 W with 96% efficiency using a monolithic, multiplexed double grating recorded in PTR glass is demonstrated. A multiplexed quadruple grating that maintains high efficiency and good beam quality is demonstrated to pave a way for further scaling of combining channels.

Quantitative infrared imaging of silicon-on-insulator microring resonators.

There is considerable research activity in multiresonator optical circuits in silicon photonics, e.g., for higher-order filters, advanced modulation format coding/decoding, or coupled-resonator optical waveguide delay lines. In diagnostics of such structures, it is usually not possible to measure each individual microring resonator without adding separate input and output waveguides to each resonator. We demonstrate a non-invasive diagnostic method of quantitative IR imaging, applied here to a series cascade of rings. The IR images contain information on the otherwise inaccessible individual through ports and the resonators themselves, providing an efficient means to obtain coupling, loss, and intensity-enhancement parameters for the individual rings.

An ultra-narrow linewidth solution-processed organic laser

Optically pumped lasers based on solution-processed thin-film gain media have recently emerged as low-cost, broadly tunable, and versatile active photonics components that can fit any substrate and are useful for, e.g., chemo- or biosensing or visible spectroscopy. Although single-mode operation has been demonstrated in various resonator architectures with a large variety of gain media—including dye-doped polymers, organic semiconductors, and, more recently, hybrid perovskites—the reported linewidths are typically on the order of a fraction of a nanometer or broader, i.e., the coherence lengths are no longer than a few millimeters, which does not enable high-resolution spectroscopy or coherent sensing. The linewidth is fundamentally constrained by the short photon cavity lifetime in the standard resonator geometries. We demonstrate here a novel structure for an organic thin-film solid-state laser that is based on a vertical external cavity, wherein a holographic volume Bragg grating ensures both spectral selection and output coupling in an otherwise very compact (∼cm3) design. Under short-pulse (0.4 ns) pumping, Fourier-transform-limited laser pulses are obtained, with a full width at half-maximum linewidth of 900 MHz (1.25 pm). Using 20-ns-long pump pulses, the linewidth can be further reduced to 200 MHz (0.26 pm), which is four times above the Fourier limit and corresponds to an unprecedented coherence length of 1 m. The concept is potentially transferrable to any type of thin-film laser and can be ultimately made tunable; it also represents a very compact alternative to bulky grating systems in dye lasers.

Thermal tuning of volume Bragg gratings for high power spectral beam combining

A tabletop kW-level spectral beam combining (SBC) system using volume Bragg gratings (VBGs) recorded in photothermo- refractive (PTR) glass was presented at the last meeting [1]. Diffraction efficiency of VBGs close to 100% was demonstrated. However, when using VBGs for spectral beam combining, it is important to ensure high diffraction efficiency for the diffracted beam and low diffraction efficiency for the transmitted beams simultaneously. The unique, unmatched properties of VBGs allow spectral beam combining achieving this condition at wavelengths with less than 0.25 nm separation. We present modeling of reflecting VBGs for high power SBC that takes into account laser spectral bandwidth, beam divergence, PTR-glass scattering losses, and grating non-uniformity. A method for optimization of VBG parameters for high-efficiency SBC with an arbitrary number of channels is developed. Another important aspect of spectral beam combiner design is maintaining high diffraction efficiency as the temperature of beam-combining VBGs changes during operation due to absorption of high power radiation. A new technique of thermal tuning of large aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide range of laser power, is developed. Finally, these tools are used to demonstrate a robust and portable 5-channel SBC system with near diffraction limited spectrally-combined output beam.

Multiplexed Volume Bragg Gratings for Spectral Beam Combining of High Power Fiber Lasers

The recent development of kW fiber laser sources makes the concept of laser systems operating at power levels from tens of kilowatts up to 100-kilowatt levels a reality. The use of volume Bragg gratings for spectral beam combining is one approach to achieve that goal. To make such systems compact, lower the complexity and minimize the induced thermal distortions we propose and demonstrate the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The multiplexed volume Bragg gratings (MVBGs) were recorded in photo-thermo refractive glass and three beams with total power of 420 W were successfully combined using one MVBG. The combining efficiency was 97% and there was no significant beam quality degradation. The results demonstrated that the approach of using multiplexed volume Bragg gratings for spectral beam combining is an excellent extension to the current state of the art combining techniques. Especially valuable is the capability to reduce the number of optical elements in the system and while being able to manage the expected thermal load when kilowatt level sources are used for beam combining.

Three-dimensional low-index-contrast photonic crystals fabricated using a tunable beam splitter

This paper describes the design and implementation of a tunable beam splitter for fabricating three-dimensional low-index-contrast photonic crystals using four-beam interference. Here, a central console is used to split a single laser beam into four beams that are focused onto the sample in an umbrella-like configuration using three adjustable mirrors. The design facilitates simple and precise adjustments of the beam angles and polarization, which can be used to readily optimize fabrication conditions for different photosensitive materials and lattice structures. Structures fabricated using this tunable beam splitter at two different incident angles of 18.5° and 27° resulted in lattices with hexagonal symmetries having well-defined nanometre-scale features that are in close agreement with the feature sizes predicted theoretically.