6,306 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
The Role of Evidence in Establishing Trust in Repositories
No full textThis article arises from work by the Digital Curation Centre (DCC) Working Group examining mechanisms to roll out audit and certification services for digital repositories in the United Kingdom. Our attempt to develop a program for applying audit and certification processes and tools took as its starting point the RLG-NARA Audit Checklist for Certifying Digital Repositories. Our intention was to appraise critically the checklist and conceive a means of applying its mechanics within a diverse range of repository environments. We were struck by the realization that while a great deal of effort has been invested in determining the characteristics of a 'trusted digital repository', far less effort has concentrated on the ways in which the presence of the attributes can be demonstrated and their qualities measured. With this in mind we sought to explore the role of evidence within the certification process, and to identify examples of the types of evidence (e.g., documentary, observational, and testimonial) that might be desirable during the course of a repository audit.
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
, Ross Laird
No full textRoss Laird, PhD RCC is a clinical consultant focused on trauma, addictions, and social vulnerability. He is also a best-selling author, award-winning scholar and educator, and clinical supervisor for BC’s largest licensed non-profit program in addictions, trauma, and mental health. Dr. Laird focuses particularly on traumatized and marginalized client populations — those navigating homelessness, mental illness, and complex trauma — and provides professional development training for organizations that serve them: social service agencies, first responders, cultural groups, nonprofits, and educational institutions. He also works extensively with organizations in arts and culture and Indigenous communities to develop trauma-informed practices for cultural programming, museum exhibitions, and community initiatives
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
Optical control of electric-field poling in LiTaO<sub>3</sub>
No full textWe present a room temperature technique for optically inducing periodic domain-inverted structures in bulk (0.2mm thick) LiTaO3. By simultaneous application of an electric field and patterned illumination using UV wavelengths (351nm and 364nm) we demonstrate modulation of the resulting domain profile. We discuss the origins of the observed optical effect and describe our results from repeated domain switching, by cycling the electric field
Periodically poled lithium niobate optical parametric oscillator pump-tuned by a single-axial-mode Ti:sapphire laser
No full textWe demonstrate a pulsed optical parametric oscillator (OPO) based upon periodically poled lithium niobate, and pumped by a Ti:sapphire laser. Agile and continuous OPO tuning is achieved by tuning the pump laser, with no need for OPO crystal translation or temperature change. The Ti:sapphire laser uses a coupled cavity grazing incidence resonator for efficient, single-axial-mode operation within the OPO spectral acceptance bandwidth. The OPO tuning range is 1200 to 2400 nm, limited only by the available cavity mirrors
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