Yariv Shamir

Beam quality analysis and optimization in an adiabatic low mode tapered fiber beam combiner

We study tapered fiber bundle based beam-combining structures that closely maintain their lowest supermodes in order to optimize the output beam quality. A method for efficiently producing structures with high beam quality is proposed. The method is based on monitoring the output powers of the tapered fiber bundle during fabrication and evaluating the beam properties at the central portion based on symmetry considerations. The method is demonstrated experimentally and supported by theoretical analysis. Several seven-fiber combiner configurations are demonstrated. The obtained beam qualities are close to the theoretical limits, as dictated by the brightness conservation law.

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 1011 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

Large-mode-area fused-fiber combiners, with nearly lowest-mode brightness conservation

We show the feasibility of producing a low-mode all-fiber combiner fabricated from a large core and extremely small NA fibers. Although these fibers support multiple modes, the combiner that we produce can be operated nearly at the single mode regime while preserving the brightness of the combined beam almost perfectly with respect to the inputs. The M-square parameter of the combined beam was 2.3 and the power transfer efficiency was close to 100%. Such an all-fiber beam combining device is a rugged solution for high-brightness, high-efficiency beam delivery.

High-average-power 2 μm few-cycle optical parametric chirped pulse amplifier at 100 kHz repetition rate

Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 μm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 μJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.

Table-top milliwatt-class extreme ultraviolet high harmonic light source

Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal to noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which delivers due to the drastic better single atom response for short wavelength drivers, an average output power of (832 +- 204) {\mu}W at 21.7 eV. This is the highest average power produced by any HHG source in this spectral range surpassing precious demonstrations by more than a factor of four. Furthermore, a narrow-band harmonic at 26.6 eV with a relative energy bandwidth of only {\Delta}E/E= 1.8 x 10E-3 has been generated, which is of high interest for high precision spectroscopy experiments.

Beam quality output of a few-modes fiber seeded by an off-center single-mode fiber source

We study the beam quality at the output of a multimode step-index optical fiber with a small number of modes following injection from a single-mode input fiber. It is shown that the output beam quality is optimized when the injecting fiber is offset from the center of the receiving core. Theoretical results demonstrating this phenomenon are in good agreement with an experimental demonstration. These results may have practical consequences in beam-coupling schemes and in applications emphasizing the importance of beam quality.

Spatial beam properties of combined lasers' delivery fibers.

The superiority of parabolic-index fibers over step-index fibers in delivering high-beam-quality light out of incoherently combined lasers is demonstrated numerically and experimentally. By utilizing the tapered fused bundle-combining approach and connecting it with delivery fiber, we point to an efficient, rugged, all-glass, integrated, and nearly brightness-preserving device that is capable of transmitting high-quality output beams.

A novel side coupling technique for rugged all-fiber lasers and amplifiers

A novel side coupling technique between two multimode high NA fibers is described. The technique is used to efficiently pump fiber lasers and amplifiers by low brightness fiber coupled pump diodes. With the presented technique, identical multimode fibers with 0.46NA and core diameters extending from 125&mgr;m to 400&mgr;m, can be coupled together, and provide pump coupling efficiency of >90%. Direct coupling to a rare-earth doped fiber is possible. In this configuration one fiber is used as the pump guiding fiber and the second fiber is the rare-earth doped double clad fiber. By utilizing the presented pump coupling technique, highly efficient, rugged and low cost short pulse and CW all-fiber lasers were implemented, with average output power extending to 300W and peak power of 600kW.

Single-mode 230 W output power 1018 nm fiber laser and ASE competition suppression

We present a high-power single-mode (SM) monolithic fiber laser centered at 1018 nm, producing 230 W CW, with an M2 of 1.17 and light to light efficiency of 75%. To the best of our knowledge this is the highest power described in the open literature from a SM fiber laser at this wavelength. Careful simulations were employed taking into account the various wavelength-dependent parameters, such as the doped fiber absorption, emission, saturation effects, and the cavity mirrors’ reflection, in addition to the fiber geometrical parameters. Parameters that were found to be most critical for suppressing the amplified spontaneous emission at higher wavelengths were the fiber length and the extinction ratio of the fiber Bragg grating reflectivity between 1018 nm and above 1030 nm.

Modeling the evolution of spatial beam parameters in parabolic index fibers

We analyze the evolution of beam quality when propagating through a parabolic index (PI) fiber. The deterioration in beam quality is expressed in terms of the fiber parameters, and a methodology for minimizing the deterioration is presented. The fiber optimization procedure is evaluated numerically for an application where the PI fiber is used to deliver the signal produced by a tapered fiber-bundle beam combiner. It was demonstrated that delivery with no beam quality deterioration can be achieved with proper fiber design.