1,721,039 research outputs found
Highly efficient self-pumped phase conjugation at near-infrared wavelengths by using nominally undoped BaTiO3
No full textUsing a nominally undoped crystal of photorefractive BaTiO3, we have examined self-pumped phase conjugation at near-infrared wavelengths. We report reflectivities as high as 74% between 720 and 800 nm. As expected, the crystal response time increases significantly at longer wavelengths. We believe that this value of reflectivity in the self-pumped geometry is the highest reported to date for this range of wavelengths with the use of a nominally undoped crystal
Induced absorption characterisation of infrared sensitive photorefractive BaTiO<sub>3</sub>
No full textPhotorefractive materials such as BaTiO3 have been extensively researched over the last fifteen years, and have shown themselves to be very efficient for generation of novel self-pumped and mutually pumped phase conjugate geometries. So far, most attention has been focussed on the visible spectral region, where the efficiency has tended to be highest, and the speed of response fastest. More recently however, interest has turned towards the near-infrared spectral region, due to the increasing availability and technological importance of very efficient solid-state laser diode sources operating at the ~800nm region. Such infrared active BaTiO3 crystals have been supplied for our use by Sandoz Huningue, and we have been investigating their unique properties for self-pumping, mutual pumping and two-beam coupling. The question of why these crystals, which are deep blue in colour, behave so well at these longer wavelengths is still open for discussion, but it undoubtedly involves their multiple dopant, many level impurity states which in turn involve both deep and shallow trap. We have characterised their behaviour therefore using simultaneous excitation from two different wavelength laser sources: a He-Ne, and a tunable Ti:sapphire, operating at ~800nm to simulate typical diode laser operation. This characterisation, together with a two-level model developed at Imperial College London, has allowed us to evaluate the relevant material parameters for these blue crystals, thus generating feedback for the crystal growers who seek to improve the material response out to beyond 1µm
Double phase conjugate mirror with x6 gain in photorefractive BaTiO<sub>3</sub> at near infrared wavelengths
No full textWe describe incoherent beam coupling using the double phase-conjugate mirror arrangement between a laser diode and a Ti:sapphire laser at the near-infrared wavelengths of ~800nm using a nominally undoped sample of BaTiO3. We report phase-conjugate reflectivities of greater than 6 times, which we believe to be the highest reported to date at these wavelengths. We also examine the fidelity of the phase-conjugate beam and the wavelength response of the double-color-pumped oscillator
Intensity-dependent absorption and its modelling in infrared sensitive rhodium-doped BaTiO<sub>3</sub>
No full textWe investigated the intensity-dependent absorption coefficient of photorefractive Rh:BaTiO3 from the maximum (633 nm) to the near infra-red end (1.06 µm) of this crystal's sensitivity. A numerical photorefractive model, incorporating dual-wavelength illumination and a secondary photorefractive centre, gives good agreement with experiment and shows that such a two-centre model is sufficient to explain the results obtained in the visible and infrared wavelength region
Strong photorefractive response of Rh:doped BaTiO<sub>3</sub> at red and infrared wavelengths
No full textBaTiO3 crystal is one of the most efficient photorefractive materials, showing strong beam coupling and phase conjugation effects. Its attractive properties are, however, normally limited to visible wavelengths. In order to improve its infrared response, a new type of BaTiO3 has been grown [1] with additional impurities, mainly rhodium. We have investigated this new Rh:BaTiO3 to characterise its response and optical parameters. Our first results show an enhanced absorption at red and infrared wavelengths [2] and high reflectivities in self-pumped phase conjugate configurations. Additionally, strong laser-induced effects have been observed [3]. The aim of the studies presented in this contribution was to provide information about photorefractive centres present in Rh:BaTiO3. Using dual-wavelength illumination we were able to investigate more thoroughly the laser-induced change in absorption. Laser-induced absorption and transparency was measured at both visible (514.5, 633 and 750 nm) and infrared (800 and 1060 nm) wavelengths. Further, we have successfully modelled the observed changes, using numerical simulation of photorefractive centres, and achieved a good agreement between theory and experiment. The strong changes of absorption influence the beam-coupling processes and phase conjugation. We will present the results of extremely high two-beam coupling gain, namely bigger than 20,000, obtained in only a 3 mm thick crystal. We believe that this is the highest photorefractive gain ever reported. We will discuss this experimental data together with theoretical modelling
Photorefractive processes at near infrared wavelengths
No full textPhase conjugation using photorefractive crystals has found many application in areas such as optical processing and distortion correction. The simplest and most elegant configuration (self-pumped phase conjugation, SPFC) allows phase conjugation without the need for additional optics - a single laser beam incident upon a cut, polished crystal induces a refractive index grating which in turn diffracts light to generate the precise phase conjugate of the incident beam. Observation of the effect has been carried out mainly in the visible part of the spectrum where crystal response has tended to be most efficient. However, with the availability of diode lasers operating at near infrared wavelengths, recent attention has been drawn to the possibility of extending the crystal response to allow observation of SPFC and other photorefractive processes at diode compatible wavelengths. Efficient SPPC has already been observed with reflectivities as high as 72% at 800nm, using a nominally undoped crystal of photorefractive BaTiO3, and there has been a concerted effort to extend and enhance the response in the near infrared region via the addition of different dopants
Competition and coexistence of multiple mutually pumped oscillations in the visible and infra-red
No full textA photorefractive oscillator. mutually pumped by three wavelengths is presented in various configurations and competition effects demonstrated. The theoretical model used to simulate the behaviour of the oscillation beams is in good agreement with experimental data
Optical control of domain structures in lithium tantalate
No full textThe patterning of domain structures in ferroelectric crystals continues to attract great interest for numerous non linear optical and photonic applications, including blue light generation from semi-conductor diode lasers, via quasi phase matching
Harmonic generation in X(2) photonic crystals
No full textRecently Berger [1] described theoretically the concept of a nonlinear photonic crystal. In such a crystal the nonlinearity varies periodically in multiple dimensions. These are the higher dimensional analogues of the well known quasi-phase matched crystals such as periodically poled lithium niobate (PPLN). We have fabricated what we believe to be the first example of a two dimensionally periodically poled sample of lithium niobate [2] in which the domains are arranged on a regular hexagonal lattice (see Fig. 1). The hexagonal shape of the poled domains is due to the crystal structure of lithium niobate. The scale of the particular hexagonal lattice was chosen phase match 2nd harmonic generation using light at 1536nm. In principle any lattice structure can be fabricated using standard photolithographic techniques. This nonlinear periodic crystal (NPC) had dimensions of 14mm x 7mm x 0.3mm and ~30% of the area was poled
PPLN optical parametric oscillator pump-tuned by a grazing incidence coupled cavity Ti:sapphire laser
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