Shaul Pearl

Polarization-mixing optical parametric oscillator

We report the experimental realization of a new type of optical parametric oscillator in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear polarization mixing. The mixing is performed by a type II degenerate parametric downconversion in a periodically poled KTP crystal pumped by a 1064 nm pulsed Nd:YAG pump. A single, linearly polarized beam, precisely at the degenerate wavelength is generated. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1 nm) and is highly stable with respect to variations in the crystal temperature.

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.

Average power effects in periodically poled crystals

The introduction of periodically poled crystals with high non-linear coefficients has lowered significantly the threshold for parametric processes. This progress enables pumping frequency conversion devices with low pulse energy, Q-switched, diode-pumped, solid-state lasers. New non-linear optical ferroelectric materials, such as KTP and Stoichiometric Lithium Tantalate (SLT) were proven to exhibit adequate deff, higher optical damage resistance and lower photo-refractivity in comparison to well-known periodically poled Lithium Niobate. Advances in poling technology have enabled the production of relatively thick periodically poled crystals from those materials. Thus, in principal much higher average power levels can be converted. We have investigated the effects that limit frequency conversion efficiency as power levels are increased. Average power induced thermal lensing and thermal phase mismatching were considered. The resulting power limitations are discussed, and under some assumptions quantitative expressions for these limits were formulated. Thermal lensing imposes a limit on the local power density. Thermal phase mismatching imposes a limit on the overall power.

250 W clad pumped Raman all-fiber laser with brightness enhancement

We report a strictly all-fiber clad pumped Raman fiber laser with a CW power of 250xa0W. To the best of our knowledge, this is the highest power Raman fiber laser demonstrated in any configuration allowing brightness enhancement. In addition, this is the first report of a Raman clad pumped all-fiber laser. The brightness of the pump source was enhanced by a factor of ∼3.8. This result was achieved by the design of a novel triple-clad fiber, with tight pump power inner confining clad that both maximized the Raman gain and inhibited the second Stokes radiation. We discuss power-increase effects on the beam quality, efficiency, and brightness enhancement.

Single mode 1018nm fiber laser with power of 230W

We have developed a high power single-mode (SM) monolithic fiber laser 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 which take into account the various wavelength dependent parameters such as the fiber absorption and emission as obtained from the fiber manufacturers, and the cavity mirrors’ reflection, in addition to the fiber geometrical parameters. It was found that the major obstacle for increasing the power at 1018nm is the self-generation of amplified spontaneous emission at wavelengths of 1030-1040nm. If the laser is not designed properly these undesired wavelengths dominate the output spectrum.

Optical parametric generation at extremely low pump irradiance in a long periodically poled lithium niobate

Optical parametric generator (OPG) is a very attractive optical down-conversion configuration since it is a single pass process and no cavity mirrors alignment is required. Thus the system configuration is much more simple and robust. Traditionally, OPG processes were demonstrated using a pump source with a pulse length of the order of picoseconds or less. This is because GW/cm2 order of magnitude pump irradiance was required to excite an OPG process, and such irradiance in nanosecond long pulses commonly damages the non-linear crystal. The introduction of periodically poled crystals with high non-linear coefficients has significantly lowered the threshold for parametric processes. This progress in non-linear crystals enables exciting OPG processes at less than 100MW/cm2 irradiance, using nanoseconds long pulses from Q-switched lasers. We present an OPG with a threshold of less than 10 MW/cm2 using an 80 mm long Periodically Poled Lithium Niobate (PPLN) non-linear crystal. High signal conversion efficiency and high power were obtained at 25 nanosecond pulse length, 10 kHz repetition rate pumping without damaging the crystal. Theoretical approaches for explaining this OPG regime are discussed.

KW-class clad-pumped Raman all-fiber laser with brightness enhancement

The importance of average power scaling of fiber lasers (FL) is well known. However, power scaling is strongly limited by factors such as thermal load, and non-linear effects. An alternative path for reaching high powers utilizes the stimulated Raman scattering mechanism, and harnesses its power and brightness enhancement potential to reach high average power, high brightness FL. kW scale Raman FLs have been demonstrated, however they are in core-pumping configurations, meaning that they require an a-priori existing brighter kW laser that acts as their pump modules. There have been only a few publications of Raman FLs where the generated signal has a higher brightness than the pump source at levels of ≥100W, the highest, being at 250W. Here we report a strictly all-fiber clad pumped Raman FL with a CW power output of 800 W with a conversion efficiency of 80%. To the best of our knowledge this is the highest power and highest efficiency Raman FL demonstrated in any configuration allowing brightness enhancement (i.e clad pumped or graded index fiber, excluding step-index core pumped), thus being the first kW-class Raman FL with brightness enhancement. This result was achieved with a specially designed triple-clad fiber (TCF). The core was 25 μm, 0.065 NA, and the inner cladding was 45 μm 0.22NA. The choice of the small inner clad allows obtaining sufficient Raman gain without requiring too long a fiber, as well as being compatible with the waist size of the pump source fiber. In addition this diameter complies with the inner-clad/core ratio which prevents generation of a 2nd Stokes laser beam. Two fiber Bragg gratings at 1120 nm written onto the TCF, were employed as the oscillator’s reflectors. The cavity was pumped by a lower beam-quality source with an M2 of ~8 at 1070 nm. The Raman signal generated in the core, at the first Stokes wavelength of 1120 nm, showed an improved beam-quality in relation to the pump.

Terahertz Ultrashort Pulse Behavior: Near-Field and Far-Field Propagation

Abstract An investigation of beam propagation properties for terahertz ultrashort pulses emitted from a photoconductive switch antenna is presented. The propagation of terahertz pulsed beams is studied based on the diffraction theory of electromagnetic fields. It is shown that under the assumption that the pulse’s spectrum radiates from a common initiating beam waist, the terahertz temporal pulse shape can be predicted elsewhere without the need for paraxial or far-field approximations. An algorithm is given that enables simple calculation of a terahertz pulse and its spatial distribution, based on simplifications of previous works. A numerical simulation model is presented that predicts spatiotemporal behavior and shows excellent agreement with experimental results. The presented algorithm provides a practical tool for many relevant applications. It also assists in designing and understanding practical pulsed terahertz laboratory setups based on photoconductive switches and can be generalized to predict the behavior of ultrashort pulses in other spectral regions, such as the visible spectrum.