Guido Toci

A retrieval algorithm to evaluate the Photosystem I and Photosystem II spectral contributions to leaf chlorophyll fluorescence at physiological temperatures

A new computational procedure to resolve the contribution of Photosystem I (PSI) and Photosystem II (PSII) to the leaf chlorophyll fluorescence emission spectra at room temperature has been developed. It is based on the Principal Component Analysis (PCA) of the leaf fluorescence emission spectra measured during the OI photochemical phase of fluorescence induction kinetics. During this phase, we can assume that only two spectral components are present, one of which is constant (PSI) and the other variable in intensity (PSII). Application of the PCA method to the measured fluorescence emission spectra of Ficus benjamina L. evidences that the temporal variation in the spectra can be ascribed to a single spectral component (the first principal component extracted by PCA), which can be considered to be a good approximation of the PSII fluorescence emission spectrum. The PSI fluorescence emission spectrum was deduced by difference between measured spectra and the first principal component. A single-band spectrum for the PSI fluorescence emission, peaked at about 735xa0nm, and a 2-band spectrum with maxima at 685 and 740xa0nm for the PSII were obtained. A linear combination of only these two spectral shapes produced a good fit for any measured emission spectrum of the leaf under investigation and can be used to obtain the fluorescence emission contributions of photosystems under different conditions. With the use of our approach, the dynamics of energy distribution between the two photosystems, such as state transition, can be monitored in vivo, directly at physiological temperatures. Separation of the PSI and PSII emission components can improve the understanding of the fluorescence signal changes induced by environmental factors or stress conditions on plants.

3-D numerical simulation of Yb:YAG active slabs with longitudinal doping gradient for thermal load effects assessment.

We present a study of Yb:YAG active media slabs, based on a ceramic layered structure with different doping levels. We developed a procedure allowing 3D numerical analysis of the slab optical properties as a consequence of the thermal load induced by the pump process. The simulations are compared with a set of experimental results in order to validate the procedure. These structured ceramics appear promising in appropriate geometrical configurations, and thus are intended to be applied in the construction of High Energy Diode Pumped Solid State Laser (DPSSL) systems working in high repetition-rate pulsed regimes.

Multilayered YAG-Yb:YAG ceramics: manufacture and laser performance

Thermal effects in transparent laser crystals and ceramics are generally an unwanted consequence of the pumping process: temperature gradients give rise to an unevenly distributed refractive index variation and a distortion of the optical surfaces crossed by the laser beam (thermal lens); birefringence due to thermomechanical stress can cause depolarization losses; and absorption from the ground level usually increases with temperature in quasi-three-level systems. All these effects can seriously impair laser performance, especially in high-power devices. Layered structures with a tailored modulation of the doping level can be used to reduce the peak temperature, the temperature gradients and also the thermally induced deformation of the laser material, thus mitigating the overall thermal effects. In the present work, structures comprising two and three layers of different compositions (pure YAG/10 at% Yb:YAG and pure YAG/10 at% Yb:YAG/pure YAG) were designed with a view to control deformation and stresses, and to reduce the thermal lensing effect. The multilayered samples were assembled by linear and cold isostatic pressing, and co-sintered under a high vacuum in a clean-atmosphere furnace. The microstructure of the layered samples obtained was characterized by FEG SEM, ESEM and TEM. The Yb diffusion profile across the doped/undoped interface was identified and related to the lasers output power. An internal optical transmittance up to 96% was obtained. A laser output power up to 5 W, with a slope efficiency as high as 74.3%, was also achieved.

A fluorescence LIDAR sensor for hyper-spectral time-resolved remote sensing and mapping

In this work we present a LIDAR sensor devised for the acquisition of time resolved laser induced fluorescence spectra. The gating time for the acquisition of the fluorescence spectra can be sequentially delayed in order to achieve fluorescence data that are resolved both in the spectral and temporal domains. The sensor can provide sub-nanometric spectral resolution and nanosecond time resolution. The sensor has also imaging capabilities by means of a computer-controlled motorized steering mirror featuring a biaxial angular scanning with 200 μradiant angular resolution. The measurement can be repeated for each point of a geometric grid in order to collect a hyper-spectral time-resolved map of an extended target.

Experimental evaluation of the cw lasing threshold for a Ce:LiCaAlF6 laser.

We present an experimental technique that allows the direct measurement of the continuous wave (cw) lasing threshold and the slope efficiency of a Ce:LiCaAlF6 (Ce:LiCAF) laser source by means of time-resolved measurement in the pulsed regime. We used a long-pulse-duration source to pump a tunable laser and a high-efficiency nondispersive laser in a quasi-stationary lasing regime. We compare the experimental results with earlier theoretical evaluations, and we demonstrate the feasibility of a cw Ce:LiCAF laser. Under the conditions discussed here, our technique can be applied to all the active media that achieved pulsed laser emission to investigate their potential as cw laser active media.

First laser emission of Yb 0.15 :(Lu 0.5 Y 0.5 ) 3 Al 5 O 12 ceramics

We report the first laser oscillation on Yb_0.15:(Lu_0.5Y_0.5)₃Al₁₂ ceramics at room temperature. At 1030 nm we measured a maximum output power of 7.3 W with a corresponding slope efficiency of 55.4% by using an output coupler with a transmission of T = 39.2%. The spectroscopic properties are compared with those of the two parent garnets Yb:YAG and Yb:LuAG. To the best of our knowledge these are the first measurements reported in literature achieved with this new host.

Spectroscopic and laser characterization of Yb 0.15 :(Lu x Y 1-x ) 3 Al 5 O 12 ceramics with different Lu/Y balance.

We report a broad comparative analysis of the spectroscopic and laser properties of solid solution Lutetium-Yttrium Aluminum Garnet (LuYAG, (LuxY1-x)3Al5O12) ceramics doped with Yb. The investigation was mainly aimed to assess the impact of the Lu/Y ratio on the Yb optical and laser properties. Therefore we analyzed a set of samples with different Y/Lu balance, namely 25/75, 50/50 and 75/25, with 15% Yb doping. We found that the Yb absorption and emission spectra changed from YAG to LuAG when gradually increasing in Lu content. Regarding the laser emission, remarkable results were achieved with all samples. Maximum output power was 8.2 W, 7.3 W and 8.7 W for Y/Lu balance 25/75, 50/50 and 75/25 respectively, at 1030 nm; the slope efficiency and the optical-to-optical efficiencies approached or exceeded 60% and 50% respectively. The tuning range was investigated using an intracavity ZnSe prism. The broadest tuning range (998 nm to 1063 nm) was obtained with Y/Lu balance 75/25, whereas the emission of the other two samples extended from 1000 nm to 1058 nm. To the best of our knowledge, this is the first comparative analysis of Yb:LuYAG ceramics or crystals as laser host across such a broad range of Y/Lu ratios.

Automated Underwater Object Recognition by Means of Fluorescence LIDAR

This paper focuses on automated recognition of underwater objects by means of light detection and ranging (LIDAR) systems. Differently from most works involved in underwater object recognition with LIDAR, where objects are recognized by their shape, here the interest is distinguishing objects on the basis of physical/chemical properties of object materials. To this aim, laser-induced fluorescence (LIF) spectroscopy is exploited, and an ad hoc signal processing chain is presented to effectively analyze the LIF spectra extracted at the detected object-range. Specifically, the goal is that of automatically recognizing the detected object with respect to a database (DB) of objects of interest, which have been previously spectrally characterized by means of laboratory fluorescence measurements. To this aim, suitable physics-based methodologies are proposed to compensate the signal for water-column effects. A decision-theory-based framework is developed to approach spectral recognition of the detected object with respect to the object DB. Experimental results from a laboratory test-bed show that the proposed processing chain is effective at automatically recognizing objects submerged in an artificial water column at different depths, based on a diverse DB of sample materials. The presented approach is shown to provide great potential for automated object recognition in marine and other water environments.

Characterization of the lasing properties of a 5%Yb doped Lu 2 SiO 5 crystal along its three principal dielectric axes

The laser performance of a 5% Yb doped Lu2SiO5 (Yb:LSO) has been investigated in quasi continuous-wave pumping regime along the three principal dielectric axes of the crystal, to obtain a complete characterization of its laser properties. The comparison among the obtained results for differently polarized lasers, in term of relative slope efficiency and absolute efficiency, allows the exploitability of different orientations of the material in order to be determined to obtain efficient laser sources. The laser slope efficiency and the energy conversion efficiency were similar for emission polarized along the three indicatrix axes, with noticeable maximum values of slope efficiency around 90% for polarization along the Y and Z axes. Tunable laser action has been obtained in the range 990 nm - 1084 nm, with sizeable differences in the shape of the tuning curve for polarization along the X, Y and Z axes. In particular, the tuning for polarization along the Z axis is relatively flat and uniform in the range 1023 nm - 1083 nm.

Spectroscopy and CW first laser operation of Yb-doped Gd 3 (Al 0.5 Ga 0.5 ) 5 O 12 crystal

We present the spectroscopic characterization and laser operation of a 2%at. Yb doped Gd3(Al0.5Ga0.5)5O12 (Yb:GAGG) crystal, grown with the Czochralski method. We determined the absorption and the emission spectrum, the upper level lifetime, and the thermal conductivity. The internal disordered structure determines a significant broadening of the emission band (12.1 nm FWHM) with respect to the parent composition Yb:GGG (8 nm FWHM). The laser performances were evaluated on an end pumped cavity, using a CW semiconductor laser as the pump source. We obtained a maximum slope efficiency of 60.8% and an optical to optical efficiency of 46.0%, with a maximum output power of 4 W, limited only by the available pump power. The tuning range extends from 995 nm to 1050 nm. To our knowledge this is the first spectroscopic investigation and the first assessment of the laser performance of an Yb:GAGG crystal with this composition (i.e. Al/Ga balance = 0.5/0.5).