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 250 W. 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.

1.2 kW clad pumped Raman all-passive-fiber laser with brightness enhancement.

An all-fiber clad pumped Raman fiber laser (FL) oscillator with a CW power of 1.2 kW and an efficiency of 85% is presented. To the best of our knowledge, this laser is the highest power and the highest efficiency Raman FL demonstrated in any configuration with brightness enhancement (BE). To the best of our knowledge, it is also the first greater than kilowatt FL of any kind that does not utilize rare-earth doping in the oscillator fiber (all passive fiber). The beam quality (BQ) of the Raman laser was M2=2.75 at 1 kW, and the pump-Stokes BE factor was approximately 7. The laser consists of a specially designed triple-clad fiber which confines the low BQ pump power into the multi-mode inner clad, while generating the Raman signal in the large-mode-area core. The second Stokes is inhibited by selecting the appropriate inner clad-to-core area ratio and by the oscillator's selective fiber Bragg grating reflectors.

Highly efficient all-fiber continuous-wave Raman graded-index fiber laser pumped by a fiber laser.

We report for the first time, to the best of our knowledge, an all-fiber Raman graded-index (GRIN) fiber laser pumped by a fiber laser. This configuration points to potential future power and brightness increases. Continuous-wave power of 135 W with an M2 value of 2.5 was obtained at a wavelength of 1081 nm with an optical-to-optical efficiency of 68%. A commercial GRIN core fiber acts as the Raman fiber in a power oscillator configuration that includes fiber Bragg gratings (FBGs) written onto the GRIN fiber. The efficiency and brightness demonstrated here are, to the best of our knowledge, the highest reported in any Raman GRIN fiber laser. A brightness enhancement of the pump beam by a factor of 5.6 is attained due to the transverse profiles of Raman gain and FBG reflection in the GRIN fiber.

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.

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.

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.

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.

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.

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.