Hemmo Tuovinen

Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser

We report a first dissipative dispersive-managed soliton fiber laser operating at 2 μm. The cavity comprised of all-anomalous-dispersion fiber employs chirped fiber Bragg grating, which ensures net-normal cavity dispersion and semiconductor saturable absorber for mode-locking.

LINEAR AND SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF ARRAYS OF NONCENTROSYMMETRIC GOLD NANOPARTICLES

The optical properties of nanoscopic arrays of metal particles are dominated by plasmon resonances and electromagnetic interaction between the particles. We use electron-beam lithography to prepare arrays of noncentrosymmetric gold particles and study their linear and second-order nonlinear optical properties. By varying the orientation of the particles in a fixed lattice, we observe shifts in the polarized linear extinction spectra. The second-harmonic generation efficiencies of the two types of samples differ by up to 60%. The results show that the properties of the samples are sensitive to the smallest details of their structure.

Inverse polarizing effect of subwavelength metallic gratings in deep ultraviolet band

Using subwavelength structures to manipulate the polarization of deep ultraviolet light is generally known as difficult with the current nano-fabrication technologies. An aluminum grating with period close to the incident wavelength is designed and experimentally evidenced to exhibit a pronounced inverse polarizing effect in the deep ultraviolet band. By using the Fourier modal method and planar waveguide theory, we show that the main contributor to the inverse polarizing effect is the excitation of surface plasmons on the front surface of the grating.

Dispersion compensation technologies for femtosecond fiber system

Developed highly chirped broadband fiber Bragg gratings and suspended-core fibers are shown to offer flexible dispersion management and allow for femtosecond oscillators with transform-limited pulse quality. Such dispersion compensators could have potential, particularly for all-fiber chirped-pulse amplification systems.

Asymmetry of two-wave coupling in cubic photorefractive crystals

Analysis of two-wave coupling in cubic crystal classes 23 and 4¯3m is presented for arbitrary orientation of the grating vector and the applied electric field. We show that the most efficient two-wave coupling is achieved when both the grating vector and the external electric field are parallel to the 〈111〉 axis in the (110) crystal cut. However, beam coupling is asymmetric in this geometry: The enhancement coefficient may differ by orders of magnitude whether interacting beams propagate in the 〈110〉 direction or backwards. Because of this asymmetry, the far-field fanning patterns of photorefractive Bi12TiO20 crystals strongly depend on the direction of the pump-beam propagation. A theoretical model is able to explain main features of the observed fanning patterns if the piezoelectric and photoelastic effects are incorporated into the system of vectorial coupled-wave equations.

Double phase conjugation in Bi12TiO20 photorefractive fiber-like crystal

Abstract An experimental investigation of phase conjugation of two mutually incoherent laser beams in the photorefractive Bi 12 TiO 20 fiber is presented. Phase conjugate wave reflectivity close to 7% and with a response time of a few seconds was obtained at λ = 632.8 nm with a 24 mW power HeNe laser under an alternating external electric field.

Holographic recording of complex images by double phase conjugation in Bi(12)TiO(20) fiberlike crystal.

Volume holographic recording of complex images with a double phase-conjugation mirror in a fiberlike Bi(12)TiO(20) single crystal, mutually pumped by two separate He-Ne lasers, is experimentally demonstrated. Complex images can be stored and reconstructed by use of even a partial input image. The advantages of the small-cross-section, light-confining photorefractive sample in comparison with bulk crystal are a wider spatial bandwidth of recorded images and the possibility of recording holograms by application of smaller ac voltages.

Fabrication of terahertz wire-grid polarizers

Wire-grid polarizers for terahertz region were fabricated by manufacturing triangular grating using a ruling-based, ultraprecision diamond machining process and replicating the pattern into polymethylpentene (TPX) and cyklo-olefin copolymer (COC) sheets using hot embossing. On top of the imprinted structures, aluminum was evaporated in an oblique angle, forming an aluminum wire grid. The achieved extinction rate was over 150 for TPX polarizers and near 1000 for COC polarizers.

High efficiency half-wave retardation in diffracted light by coupled waves

We demonstrate high-efficiency half-wave retardation in diffracted light in the 2nd order Littrow mounting. The diffracting structure is a slanted crossed grating with subwavelength period in the direction of the second grating vector, which makes it possible to mix the polarization states of the input light inside the grating layer, and hence to create the half-wave retardation. We present an experimental result with 58.9 % diffraction efficiency and a near perfect half-wave retardation. We explain the effect qualitatively using the classical coupled-wave approach.

Phase conjugate mirrors on the base of Bi12TiO20 photorefractive fibre

Abstract The distribution of two-wave mixing coefficient as a function of pump beam incidence has been measured for the Bi12TiO20 photorefractive fibre. Double phase-conjugate mirror with its conversion efficiency higher than 5% has been recorded in a wide range of the pumps incidence angles at λ=632.8 nm for a 2×3 mW input light power. Self-pumped phase-conjugate mirror with efficiency of 3% was obtained by using internal reflections of fanning beam inside the fibre.