Metin S. Mangir

Power-scalable phase-compensating fiber-array transceiver for laser communications through the atmosphere

We report laboratory experiments demonstrating a phase-compensating 70-mm-diameter aperture transceiver that comprises a hexagonally close-packed array of seven 23-mm-diameter fiber collimator subapertures. Other than the collimators, the transceiver uses only fiber optics, connected as a master oscillator-multiple amplifier. The master oscillator is a fiber-coupled 1.5-µm diode laser, which is split and fed to 1-W fiber amplifiers before it exits the collimators. To obtain a phase-coherent far field we control each subapertures phase by adjusting the current to its amplifiers pump diodes in a multidither arrangement, maximizing the signal at the receiver. We achieve a diffraction-limited coherent beam combination in the far field that produces 1.4 W of power in the main lobe, in agreement with theory.

200 W self-organized coherent fiber arrays

We report producing 200 W coherent fiber laser arrays without active control. This outcome is obtained via self-organization using a non-fiber coupler for two- to ten-laser arrays.

Energy storage and heating measurements in flashlamp-pumped Cr:Nd:GSGG and Nd:YAG

The authors have experimentally measured the energy stored and the heat generated in flashlamp-pumped Cr:Nd:GSGG for three Cr/sup 3+/ concentrations in the range of 1-2*10/sup 20/ ions/cm/sup 3/. It has been found that the energy storage efficiency in these samples is 1.7 times greater than that of the Nd:YAG sample, and the normalized heating parameter chi , defined as the heat deposited per unit of stored energy, is 2.5 under the specified pumping conditions, with no evident dependence on the Cr/sup 3+/ concentration. It has been been found that the measured chi value for the sample of Nd:YAG is 2.9 for the same pumping conditions. These observed chi values exceed expected values by factors of approximately 1.1 and 2 for the Cr:Nd:GSGG and Nd:YAG samples, respectively. The thermal focal length in the two materials was measured showing that the lensing is shorter in GSGG by a factor of 2.6 for the same available output power, or a factor of 4.5 for the same input pump power. The expected thermal lensing was determined using measured heat loads with no adjustable parameters, achieving satisfactory agreement with measured lensing values. >

Self-organized coherence in fiber laser arrays

Self-organized coherence between fiber lasers has been reported both via all-fiber 2x2 directional coupler trees and in spatially multi-core fibers. We have taken this a major step forward, coupling together a number of independent fiber lasers to obtain a spatially and spectrally coherent far field, with no active length, polarization, or amplitude control. The near field output comes from a spatial array rather than from a single fiber, making this approach scalable to extremely high power.

Stimulated Brillouin scattering gain variation and transient effects in a CH 4 : He binary gas mixture

Experimental measurements are presented that show that the stimulated Brillouin scattering gain of methane, and hence the nature of the competition between stimulated Brillouin scattering and stimulated Raman scattering, can be systematically varied by the addition of helium. The gain variation is explained in terms of a transient gain model in which the dominant effect of the helium is to increase the Brillouin linewidth (acoustic attenuation). Broadening mechanisms are discussed. At methane and helium partial pressures of 19 and 41.8 kTorr (25 and 55 atm), respectively, backward stimulated Raman scattering dominates stimulated Brillouin scattering when the 532-nm pump energy exceeds 160 mJ.

High-power Yb:YAG rod oscillators and amplifiers

We report 0.95 kW average output power from a single cw-diode pumped Yb:YAG power oscillator. The 3-mm diameter solid-state laser rod is side pumped by three sets of cw diode arrays each of which has an electrical-to-optical efficiency of up to 50%. Our phase-conjugate master oscillator, power-amp architecture will incorporate this pump cavity as one of the power amplifiers for multi-kW average power, good beam quality laser applications.

Efficient anti-Stokes Raman conversion by four-wave mixing in gases

We describe a means for efficient, energy-scalable generation of first-order anti-Stokes light by four-wave mixing, using collimated pump and first-order Stokes seed beams. We performed a one-dimensional, plane-wave analysis of the process, assuming steady-state conditions and monochromatic beams. A prescription for designing the optimum anti-Stokes converter with this technique is derived from this analysis, and we show that energy conversion efficiencies from pump light to anti-Stokes light of ~30% might be possible. We also report an unoptimized, proof-of-concept experiment that produced 40 mJ of anti-Stokes-shifted light at 432 nm from 1.2 J of 527-nm pump light in a 60-cm-long hydrogen cell.

A stimulated Brillouin scattering phase-conjugate mirror having a peak-power threshold < 100 W

Abstract We have demonstrated stimulated Brillouin scattering phase conjugation in a hollow glass lightguide filled with CS 2 . Threshold peak powers were as low as 70 W, and the reflectivity and conjugation fidelity reached 50% and 80%, respectively. The SBS threshold and reflectivity were unchanged with average input powers up to 20 W.

Laser-related properties of chromium-and neodymium-doped gadolinium scandium aluminum garnet (Cr:Nd:GSAG)

The spectroscopic, lasing, and thermal properties of chromium- and neodymium-doped gadolinium scandium aluminum garnet (Cr:Nd:GSAG) are reported. The Judd–Ofelt parameters, radiative lifetime, and branching ratios were determined, and an effective emission cross-section value of 0.92 × 10−19 cm2 was derived at 1.06 μm from our spectroscopic measurements. The temporal behavior of the Cr3+ fluorescence intensity agreed with the Inokuti–Hirayama model for the chromium–neodymium energy-transfer mechanism, which invokes a static dipole-dipole interaction and no migrational energy transfer. Under identical flash-lamp-pumping conditions the measured pumping efficiency of Cr:Nd:GSAG for threshold lasing is nearly equal to the pumping efficiency of codoped gadolinium scandium gallium garnet (Cr:Nd:GSGG). From the threshold-lasing data, an effective emission cross section of 0.7 × 10−19 cm2 for Cr:Nd:GSAG was independently determined, as well as a value of 1.3 × 10−19 cm 2 for Cr:Nd:GSGG. The normalized heating parameter χ, defined as the heat deposited per unit stored energy, experimentally ranges from 2.1 to 2.7, depending on the effective emission cross-section value. Our calculated value of χ is 1.9 ± 0.3. Based on our heating-efficiency measurements, calculated Cr:Nd:GSAG thermal lensing values are ~2.1–2.6 D/kW, which indicates a thermally induced focal length that is weaker than that of Nd:YAG by approximately a factor of 2 at the same output power. Finally, the benefits and trade-offs of laser-diode pumping are discussed.

Vector phase conjugation with loop laser geometry

3. In (l) , the output fluence decreases when the depolarization induced by the h/4 into the loop increases, whereas in (2) the output fluence remains almost constant. The configuration (2) not only compensates for the phase aberrations but also for the depolarization aberrations. Indeed, in (2), both [loo] and [ O l O ] types of absorbing dipoles are sampled, which can lead to equal conjugate reflectivities along those two orthogonal axes forming a base, and thus to vectorial phase conjugation (VPC)? In (I) , only the [ loo] type of dipoles is sampled, as a consequence, VPC is not performed. In conclusion, Cr4+:YAG used in its specific configuration which can correct polarization aberrations appears as a promising nonlinear medium to realize SPPCM in NdYAG loop resonators. Such resonators produce selfQswitching by dynamic absorption grating formation, achieve greater-than-unity reflectivities, and can compensate for both intracavity phase and polarization aberrations.