355 nm Wind Lidar Rayleigh and Mie Return Signal Simulator

The conception and realisation of a prototype 355nm wind lidar return signal simulator capable of simulating the Rayleigh and the Mie return signals is described. The system is based on a commercial dual wavelength 1064nm/532nm diode pumped solid state laser with a monolithic non planar ring Nd:YAG cristal resonator and LBO frequency doubling. This laser is frequency locked to a low power high stability infrared Nd:YAG laser, with frequency offset capability of ± 50 GHz. This low power laser itself is locked to a very high stability Fabry-Perot inerferometer which is located in a thermally stabilised enclosure with a temperature stability of the order of some °mK. Dynamic amplitude shaping is performed with a low voltage electro-optic modulator. Dynamic frequency shaping is performed with four acouso-optic modulators in series, enshuring no beam walk during frequency shifting. The Rayleigh return signal is generated with a pulsed Xe flash lamp spectrally shaped with a grating followed by two air spaced etalons with adapted resolutions, the second, high resolution one, is pressure fine tuned for the exact frequency adjustment with respect to the laser frequency. The combined signal is injected in a 50μm core fiber conveying the signal to the detector test station. The test results are shown.

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Frequency-domain fluorescence lifetime imaging for endoscopic clinical cancer photodetection: Apparatus design and preliminary results

André Studzinski, Hubert van den Bergh, Georges Wagnières

We describe a new fluorescence imaging device for clin. cancer photodetection in hollow organs in which the tumor/normal tissue contrast is derived from the fluorescence lifetime of endogenous or exogenous fluorochromes. This fluorescence lifetime contrast gives information about the physicochem. properties of the environment which are different between normal and certain diseased tissues. The excitation light from a CW laser is modulated in amplitude at a radiofrequency by an electrooptical modulator and delivered by an optical fiber through an endoscope to the hollow organ. The image of the tissue collected by the endoscope is sepd. in two spectral windows, one being the backscattered excitation light and the other the fluorescence of the fluorochrome. Each image is then focused on the photocathode of image intensifiers (II) whose optical gain is modulated at the same frequency as the excitation intensity, resulting in homodyne phase-sensitive images. By acquiring stationary phase-sensitive frames at different phases between the excitation and the detection, it is possible to calc. in quasi-real time the apparent fluorescence lifetime of the corresponding tissue region for each pixel. A result obtained by investigating the endogenous fluorochromes present in the mucous membrane of an excised human bladder is presented to illustrate this method and most of the optical parameters which are of major importance for this photodetection modality have been evaluated.

Springer US1997

Endoscopic tissue fluorescence life-time imaging by frequency domain light-induced fluorescence

André Studzinski, Hubert van den Bergh, Georges Wagnières

An instrumentation is being developed to draw a fluorescence life-time map of tissue endoscopically. This fluorescence life-time of an endogenous or exogenous fluorochrome gives information about the physico-chemical environment which is thought to vary between normal and diseased tissue. The excitation light from a cw laser is modulated in amplitude at high frequencies by an electro-optic modulator and delivered to the endoscopic site through an optical fiber. The image of the tissue is spectrally split in two parts, the one being the backscattered excitation light, the other the fluorescence of the fluorochromes. Each image is focused on the photocathode of an image intensifier whose gain is modulated at the same frequency. By acquiring frames at different phases between the excitation and the emission, it is possible to calculate pixel by pixel the apparent fluorescence life-time of the corresponding tissue region.

1995

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

EPFL2001