In this work we study the photorefractive and electro-optical properties of Zirconium-doped congruent lithium
niobate (LN) crystals. In order to set the ground for the utilization of these crystals in nonlinear wavelengthconversion
devices, we investigate the dependence of the photorefractive properties of the crystals on dopant
concentration and incident power. In our experiments the birefringence variations induced by a 532-nm laser beam
are measured by using the Sénarmont method, in the ZrO2 concentration range 0-3mol% and intensity range 155-
1800 W/cm2. In order to investigate photorefractivity at high intensities, we have also utilized the direct observation
of the distortion of the light spot transmitted by the crystal. In presence of photorefractivity, the transmitted light
spot becomes smeared and elongated along the c-axis. Our data show that the threshold ZrO2 concentration can be in
the range 2.5-3mol%. Considering that the growth of large homogeneous Zr:LN crystals should be easier than for
Mg:LN, and that electrical poling of these crystals has already been demonstrated, Zr-doped LN could represent a
more convenient choice than Mg:LN for the realization of room-temperature wavelength converters.
We describe an experiment in which a supercontinuum spectrum is generated by exciting the third-order mode of a
highly nonlinear photonic crystal fiber (PCF). Our experiment consists of launching a train of femtosecond pulses into a
45-cm-long span of a PCF by means of an offset pumping technique that can selectively excite higher-order modes. For
input wavelengths below 810 nm, the fiber was found to allow for the propagation of higher-order modes. When exciting
the third-order mode we were able to generate an almost purely visible supercontinuum even with pulse energies below
100 pJ. Although the spectrum broadens on the short-wavelength side down to the blue region, no components at
wavelengths larger than the pump wavelength were observed. The mechanism behind the spectral broadening is mainly
ruled by soliton propagation leading to the generation of a blue-shifted dispersive wave. The fact that higher-order modes
have a cut-off wavelength plays a fundamental role that accounts for the observed asymmetry of spectral broadening.
Our experimental results are compared with the numerical solutions of the nonlinear Schrödinger equation. Good
agreement between experimental and numerical results is found.
Trapping and manipulation of microscopic objects using fiber optical traps is gaining considerable interest, as these objects can be manipulated inside complex environments, thus removing the limitation of short working distance of the conventional optical tweezers. We show that an axicon like structure built on the tip of a single mode optical fiber produces a focused beam shape with a central hole, implying a very small fraction of the power traveling with rays nearly parallels to the optical axis. Interesting transportation behavior of polystyrene particles using the scattering forces from such an axicon tip fiber was observed. As the distance of the particle from the fiber tip increased, since almost no rays interact with the particle, the scattering forces decreased substantially. Therefore, velocity of the particle at different distances was found to depend much more critically on the particle size in contrast to the beam generated by the bare fiber. While the speed of transport could be increased linearly by increasing the laser power in both axicon tipped fiber and bare fiber, increased speed was observed for particles of larger sizes for both the fiber types. However, the fractional increase in speed for increased size of particles was found to be quite large for axicon tipped fiber as compared to the bare fiber. Use of the observed differences in speed of transportation of microscopic objects could be used to sort them based upon their size.
Since the low index particles are repelled away from the highest intensity point, trapping them optically requires either a rotating Gaussian beam or optical vortex beams focused by a high numerical microscope objective. However, the short working distance of these microscope objectives puts a limit on the depth at which these particles can be manipulated. Here, we show that axicon like structure built on tip of a single mode optical fiber produces a focused beam that is able to trap low index particles. In fact, in addition to transverse trapping inside the dark conical region surrounded by high intensity ring, axial trapping is possible by the balance of scattering force against the buoyancy of the particles. The low-index particle system consisted of an emulsion of water droplets in acetophenone. When the fiber was kept horizontal, the low index spheres moved away along the beam and thus could be transported
by influence of the scattering force. However in the vertical position (or at an angle) of the fiber, the particles could be trapped stably both in transverse and axial directions. Chain of such particles could also be trapped and transported together by translation of the fiber. Using escape force technique, transverse trapping force and thus efficiency for particle in Mie regime was measured. Details of these measurements and theory showed that trapping of Raleigh particle is possible with such axicon-tip fibers. This ability to manipulate low-index spheres inside complex condensed environments using such traps will throw new insights in the understanding of bubble-bubble and bubble-wall interactions, thus probing the physics behind sonoluminescence and exploring new applications in biology and medicine.
Transient and steady-state measurements of photo-induced birefringence variations in single domain and periodically poled lithium niobate (LN) crystals containing different non-photorefractive impurities are presented. The birefringence change is induced by a 532-nm laser beam in the intensity range 0-500 W/cm2, and is probed by 632.8- and 1523-nm beams. Data were taken at 25 and 50°C. We find that HfO2 doping is very effective in reducing the photorefraction. This is interesting also because it is known that Hafnium-doped LN crystals can be periodically-poled during growth. The analysis of the rise and decay of the induced birefringence shows that doping considerably increases both the photoconductivity and the dark conductivity of the LN crystals.
The present research work is devoted to the realization of an efficient fiber-waveguide optical coupling between single-mode fiber and rectangular waveguide. The outcomimg laser beam exiting from the fiber has a gaussian transversal field distribution. On the contrary, the single-mode waveguide has an asymmetric transversal field distribution in X and Y-axis. To transform the outcoming circular laser beam onto a rectangular, size adjusted, spot we have used a multilevel diffractive phase element fabricated directly on the top of the fiber by means of nanolitography. The diffractive phase element is calculated to focus and reshape the gaussian symmetric beam exiting a single-mode fiber into a desired asymmetric intensity distribution at the waveguide input plane. Phase modulation is obtained by multilevel profiling a polymeric material coated on the top of the fiber by means of a specific fabrications process including e-beam lithography and chemical etching. Experimental results obtained for fiber-waveguide coupling with a 20 microns diameter diffractive element are also presented.
Wavelength conversion is a key function in wavelength- division multiplexing. Frequency-shifting can be obtained through cascaded second-order nonlinear processes: a pump at (omega) is coupled into the waveguide, second harmonic is generated and made to interact with a coupled signal at (omega) -(Delta) (omega) so as to obtain a converted signal at (omega) + (Delta) (omega) via difference frequency generation. For practical applications, it is essential to achieve a good control in waveguide fabrication so as to be able to design a frequency-shifting device for specific pump and signal frequencies. In this work we report frequency- shifting based on cascaded second-order nonlinear processes obtained in simple planar Ti-undiffused LiBnO3 waveguides, where phase-matching is achieved by birefringence. A Y-cut planar waveguide, 17mm long, was fabricated by diffusing a 290-angstrom-thick titanium layer for 6 hours at a temperature of 1000 degrees C. Thanks to a good modeling of the fabrication process, the waveguide behavior could be predicted directly from the fabrication parameters. A converted signal at 1.100 micrometers was obtained from a pump at 1.104 micrometers and a signal at 1.108 micrometers at a working temperature of 85 degrees C. The phenomenon was observed with a reasonable efficiency and was highly reproducible. The experimental results were in very good agreement with the expected ones.
We report wavelength shifting (generation of a pulse at the wavelength (lambda) p - (Delta) (lambda) from a signal at (lambda) p + (Delta) (lambda) under the action of a pump at (lambda) p) and parametric amplification through a cascaded second order process in a periodically poled lithium niobate crystal and also in an organic crystal N-(4-nitrophenyl)-L- prolinol (NPP). NPP could convert signal pulse (from 1.16 to 1.14 micrometer) with unit gain under the action of a pump pulse (at 1.15 micrometer) of peak intensity as low as 9 MW/cm2. In the limit of negligible conversion, where the cascading effect can be described through an effective (chi) (3), we derive for NPP(chi) eff(3) approximately equals 2.4 X 10-17 m2/V2, which is approximately 102 larger than (chi) (3) of conjugated polymers or semiconductors. In a 19 mm long PPLN sample, at (lambda) p equals 1.8 micrometer and (Delta) (lambda) as large as 60 nm, we could obtain unit gain with a pump intensity of 6 MW/cm2, while amplification by a factor of 10 requires 18 MW/cm2. We also present a theoretical comparison between frequency mixing and cascading.
Semiconductor doped glasses have been studied by small angle neutron scattering. The results allow a precise determination of the average size of the microcrystallites and to evidence the volume depleted of semiconductor costituents that sorrounds each particle
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