Viktor O. Smolski

Photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. We adjusted the spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. Our results provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.

Multiwavelength Blue Light Source Based on MgO : PPLN SHG Using Vertically Stacked Grating-Coupled Surface-Emitting Lasers

In this letter, we present the first experimental results of frequency doubling the output of vertically stacked grating coupled surface-emitting laser/dual-grating reflector devices with different emitting wavelengths. We used a multigrating 5% magnesium-oxide-doped periodically poled lithium niobate (MgO : PPLN) crystal as the nonlinear conversion medium. In pulse operation, 0.6 W of multiwavelength total second-harmonic peak power was obtained. The individual emitter output is focused into each channel of the crystal by diffractive beam shaping optical element array to minimize the footprint area of the whole setup.

Reconfigurable photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Absorption filters commonly used to generate RGB color filters do not have the flexibility and narrow band filtering ability. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. This paper extends the previous work in adjusting spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. It was found that individual layer fluctuations have a minor impact on filter performance, whereas hole taper produces a large peak shift. The results in this paper provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.

Phase locked system for dual comb molecular spectroscopy at 2–6 µm based on tm-fiber laser

We demonstrate phase-coherent, frequency-stabilized dual-comb system at 2μm, extendable to mid-IR via phase-coherent frequency conversion in a doubly-resonant GaAs OPO. Results of dual-comb molecular spectroscopy with ~1M spectral points taken in ~1ms will be presented.

Blue Light Source Based on a ppKTP SHG Using a Grating-Coupled Laser Diode

In this letter, we present the first experimental results of a single-pass second-harmonic (SH) generation from a grating-coupled surface-emitting laser (GCSEL) with integrated dual-grating reflector. In pulse operation, the maximum SH peak power was obtained at 0.66 W, yielding a normalized conversion efficiency of 0.6times10-3 W-1. A high-level monolithical integration of the GCSEL device led to a compact blue light source, which was based on a frequency-doubled near-IR laser diode.

Recent progress in mid-IR materials and lasers based on Cr and Fe doped chalcogenides (Conference Presentation)

II-VI chalcogenides (e.g. ZnSe/S) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for fabrication of effective mid-IR gain media. TM:II-VI materials feature a favorable blend of laser spectroscopic parameters: a four-level energy structure, absence of excited state absorption, close to 100% quantum efficiency of fluorescence (for Cr doped II-VI media), broad mid-IR vibronic absorption and emission bands. This talk summarizes progress in fabrication of high quality Cr:ZnS/Se and Fe:ZnS/Se by cation vacancy and cation interstitial enhanced post growth thermal diffusion. We also describe recent breakthrough on recrystallization and effective doping of ZnS ceramics under hot isostatic pressing resulting in a large cm-scale monocrystalline domains formation and an increase of the Fe diffusion coefficient by three orders of magnitude. We report recent advances in high-power Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W near 2500 nm, 32 W at 2940 nm, and 35 W at 4300 nm with corresponding optical efficiencies of 62%, 29%, and 35%. Current improvements of output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime are reported: up to 2 W output power at 75-1200 MHz repetition rate, up to 2 cycle pulse duration (16 fs) with efficiency of 20-25% with regards to Er-fiber laser pump power. The effects of efficient up-conversion of mid-IR fs pulses in the laser medium as well as supercontinuum generation are demonstrated. Further extension of mid-IR spectral coverage to 3-8 m is demonstrated by Cr:ZnS KLM laser pumped degenerate (subharmonic) parametric oscillators (OPOs) based on based on quasi-phase matching in orientation-patterned gallium arsenide, and random phase matching in polycrystalline ZnSe.

Cr:ZnS laser-pumped subharmonic GaAs OPO with an instantaneous bandwidth 3.6–5.6 μm

High-power (110mW) mid-IR output suitable for ultra-broadband frequency comb generation was produced in a low-threshold (20mW) subharmonic GaAs optical parametric oscillator that was synchronously pumped (175MHz) by a compact 0.5-W femtosecond Cr:ZnS (2.38μm) oscillator.

Design and experimental study on the grating outcouplers providing the controlled 2D-intensity profile of the output beam from a broad area laser diode

We present a method of designing a grating outcoupler to obtain the desired 1D- and 2D-intensity profile of the optical beam emitted by a grating coupled surface emitting laser. The method is based on variation of the periodicity, duty cycle, and the groove tilt angle of the grating. Grating design involves numerical analysis of the optical field propagated through the grating, by applying the Rigorous Coupled Wave Approach method. Experimental evaluation of the designed grating components was done by fabrication and testing the broad area semiconductor lasers with the monolithically integrated grating outcouplers. We also present a grating design which provides the spreading of a single optical output into multi-beams at different outcoupling angles in the emitting plane.

Blue light source based on spectrally stabilized external dual grating reflector coupled surface emitter array

Second harmonic generation (SHG) from near infrared (IR) diode lasers is an attractive solution for blue-light sources with high peak power and narrow linewidth. IR sources based on broad stripe devices with narrow linewidth makes it possible to achieve a wide range of wavelengths throughout the blue region. This paper summarizes recent results utilizing a configuration of external dual grating reflector coupled surface emitting laser array for blue light generation.

2-D Stacked External Cavity Multiwavelength Surface-Emitting Laser Array

In this letter, we present a multiwavelength two-dimensional array based on vertically stacked grating coupled surface-emitting laser bars. The emitters in the array were individually wavelength stabilized using external Littrow gratings fabricated on silicon-based spacers. In pulse operation, lasing with a ~0.3-nm linewidth from each device and maximum total peak power of 120 W was achieved.