Commutation-Q | EPFL Graph Search

La commutation-Q (Q-switching en anglais), également appelée formation de grandes pulsations, est une technique permettant à un laser de produire un rayon sortant pulsant. Elle autorise la création de pulsations de lumière de haute puissance (de l’ordre du gigawatt), beaucoup plus que si le laser produisait une onde de sortie continue (mode constant). Comparée au blocage de mode, qui est une autre technique pour produire des pulsations avec un laser, la commutation-Q permet de plus faibles fréquences de répétition, de plus grandes énergies, ainsi que de plus longs temps de pulsations. Les deux techniques sont parfois utilisées conjointement. La commutation-Q fut pour la première fois proposée en 1958 par Gordon Gould, et découverte et démontrée indépendamment en 1961 ou 1962 par R.W. Hellwarth et F.J. McClung, en utilisant des cellules de Kerr à commutation électrique comme volets sur un laser à rubis. Principe de la commutation-Q La commutation-Q est réalisée en intégra

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

Proof of concept of a dual-band circularly-polarized RF MEMS beam-switching reflectarray

Caner Güçlü, Julien Perruisseau-Carrier

In this communication we propose the concept of a circularly polarized reflectarray (RA) antenna capable of independent beam-switching in both K and Ka bands. The RA unit cell comprises one microstrip ring per each operation frequency. Each ring is integrated with six equally spaced series RF micro electro-mechanical systems (RF MEMS) switches, which allows implementing the sequential rotation principle formerly used in circularly-polarized RA for single-frequency operation. A detailed design is proposed, considering the best relative arrangement of the rings corresponding to each frequency, the accurate modeling of the RF MEMS switches, and the full-wave simulation of the full array. The designed RA is implemented on a 4-inch quartz wafer and comprises 109 K-band and 124 Ka-band split-rings. Two prototypes representing two frozen states of the reconfigurable antenna are fabricated and measured. The designed RA can +/- 120 degrees provide progressive phase difference in both operation bands exhibiting beam switching to +/- 35 degrees and +/- 24 degrees off the broad-side in K and Ka bands respectively. The performance of the designed antenna is verified by the agreement of the measured and simulated radiation patterns.

Institute of Electrical and Electronics Engineers2012

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

Sébastian Favre

EPFL2001