Amiel A. Ishaaya

Collapse dynamics of super-Gaussian Beams.

We investigate the self-focusing dynamics of super-Gaussian optical beams in a Kerr medium. We find that up to several times the critical power for self-focusing, super-Gaussian beams evolve towards a Townes profile. At higher powers the super-Gaussian beams form rings which break into filaments as a result of noise. Our results are consistent with the observed self-focusing dynamics of femtosecond laser pulses in air [1] in which filaments are formed along a ring about the axis of the initial beam where the initial beam did not form a ring.

Efficient selection of high-order Laguerre-Gaussian modes in a Q-switched Nd:YAG laser

Intra-cavity binary phase elements are incorporated into a Q-switched Nd:YAG laser resonator to obtain efficient high-order transverse mode selection. The resonator configuration is analyzed using the propagation-matrix diagonalization method and the Fox-Li algorithm, and a simple model for predicting the relative output powers of the selected modes is developed. The predicted results are verified experimentally with binary phase elements for selecting the TEM/sub 01/, TEM/sub 02/ and TEM/sub 03/, degenerate Laguerre-Gaussian modes. The output energy per pulse was 15 mJ for the TEM/sub 01/, 16.5 mJ for TEM/sub 02/ and 18.3 mJ for TEM/sub 03/, all higher than the 10 mJ for the TEM/sub 00/. The performance in Q-switched operation was found to be similar to that in free-running operation. The numerical calculations, experimental procedure and experimental results are presented.

Very high-order pure Laguerre-Gaussian mode selection in a passive Q-switched Nd:YAG laser

An intra-cavity phase element, combined with a passive Q-switch saturable absorber and a suitable intra-cavity aperture, can provide extremely high mode discrimination, so as to obtain laser operation with single, pure, very high order Laguerre-Gaussian mode. With a Nd:YAG laser setup, well controlled and extremely stable Q-switched operation in the degenerate Laguerre-Gaussian TEM04, TEM14, TEM24, TEM34, and TEM44 modes was obtained. The measured output energy per pulse for each of these modes was 5.2mJ, 7.5mJ, 10mJ, 12.5mJ, and 13.7mJ respectively, compared to 2.5mJ for the Gaussian mode without the phase element (more than a five fold increase in output energy). Correcting the phase for these modes, so that all transverse lobes have uniform phase, results in a very bright and narrow central lobe in the far field intensity distribution that can theoretically contain more than 90% of the output energy.

Intracavity coherent addition of 16 laser distributions

The efficient intracavity coherent addition of 16 separate laser Gaussian mode distributions is presented. The coherent addition is achieved in a multichannel pulsed Nd:YAG laser resonator by use of four intracavity interferometric beam combiners. The results reveal 88% combining efficiency with a combined output beam of nearly pure Gaussian distribution.

Conversion of a high-order mode beam into a nearly Gaussian beam by use of a single interferometric element

We present a new, compact, and practical optical mode converter that efficiently transforms a high-order Hermite-Gaussian (HG) laser beam into a nearly Gaussian beam. The mode converter is based on coherently adding different transverse parts of the high-order mode beam by use of a single planar interferometric element. The method, configuration, and experimental results obtained with a pulsed Nd:YAG HG TEM10 laser beam are presented. The results reveal that the efficiency of conversion of a HG beam to a nearly Gaussian beam can be as high as 90%.

Coherent addition of spatially incoherent light beams.

We report on efficient coherent addition of spatially incoherent multimode laser beam distributions. Such addition is demonstrated within a multi-channel laser resonator configuration, obtaining more than 90% combining efficiency while preserving the good beam quality. We explain the rather surprising physical phenomenon of coherently adding spatially incoherent light by self-phase-locking of each of the modal components within the multimode beams. Our approach could lead to significantly higher output powers concomitantly with good beam qualities than were hitherto possible in laser systems.

Efficient conversion of a Gaussian beam to a high purity helical beam

Abstract A method for efficiently converting a Gaussian beam into a helical Laguerre–Gaussian (LG) beam is presented. It is based on using a pair of axicons to produce a shifted-Gaussian (doughnut) intensity distribution that is then passed through a spiral phase element. It is shown that the conversion efficiency can be as high as ∼98%, and the calculated far-field intensity distributions of the output beams are very close to those of corresponding pure LG intensity distributions. The principle of the method, the needed optical arrangement, and calculated and experimental results are presented.

Intracavity coherent addition of Gaussian beam distributions using a planar interferometric coupler

We present a compact and practical combined laser resonator configuration in which several Gaussian beam distributions are efficiently combined. It is based on intracavity coherent addition of pairs of Gaussian beam distributions with a planar interferometric coupler. The principle, configuration, and experimental results using pulsed Nd:YAG laser beams are presented. The results reveal more than 92% combining efficiency with a nearly Gaussian output beam, in free running and Q-switched operation.

Wavelength conversion of nanosecond pulses to the mid-IR in photonic crystal fibers

We investigate degenerate four wave mixing with nanosecond pulses in fused silica photonic crystal fibers. Phase-matching curves are calculated taking into account the material and waveguide dispersion. Experiments with a nanosecond pulsed Nd:YAG pump laser and relatively short fiber lengths show more than an octave spanning conversion to idler and signal wavelengths at 3.105 μm and 0.642 μm, respectively. Conversion efficiency depends on the fiber length and pump intensity and is limited in our experiments by damage of the fiber input facet. Our results represent a new stretch towards the limit of the silica transmission window in the mid-infrared (IR).

Passive Laser Beam Combining With Intracavity Interferometric Combiners

In this paper, we present an approach for passive laser beam combining using plane-parallel intracavity interferometric combiners. With this approach, efficient coherent combining of Gaussian laser beams, transverse single high-order-mode laser beams, and even transverse multimode laser beams is possible. We experimentally obtained a combining efficiency of about 90% when coherently combining 16 solid-state laser channels, as well as when coherently combining four fiber laser channels. We also present an arrangement for simultaneous coherent combining and spectral combining, which could possibly overcome current upscaling limits. We present the basic coherent combining approach, review our past and recent investigations and results with both solid-state and fiber laser configurations, and discuss the possible upscaling and future prospects of this approach.