We report on industrial-grade femtosecond Yb fiber lasers with >100μJ pulse energy and <300fs pulse duration using a tunable all-fiber pulse shaper. The rugged, compact phase modulator is a lossless addition to the standard chirped-pulseamplification scheme. The automated multichannel phase control across the optical bandwidth enables generation of near transform-limited pulses at the laser output, improves unit-to-unit reproducibility of laser pulse characteristics, and reduces laser build time.
With recent advances in high-power laser technology, Volume Bragg Gratings (VBG) have been recognized as
important elements in different types of beam-combining applications, such as, design of optical correlators, coherent
and incoherent power beam-combiners and in particular, spectral beam combiners (SBC) in which the output beams
from several distinct laser sources are combined into a single-aperture, diffraction-limited beam. The obvious advantage
of VBG's in these applications results from their narrow spectral and angular selectivity compared, for example, to any
type of surface gratings. Almost a two order magnitude difference in spectral efficiency (number of channels per usable
bandwidth) can potentially allow one to combine a much larger number of lasers into a single spot. The VBG recorded in
a photo-thermo-refractive (PTR) glass exhibit long-term stability of all its parameters in high-power laser beams. With
power density more than 1 MW/cm2 in the CW beam of total power on a kilowatt level the characteristics of these
elements appear to be stable. In order to increase the spectral efficiency of such a "beam-combiner" the overall loss
resulting from absorption and cross-talk between channels should be minimized. In this paper we consider architecturespecific
beam-combining scheme and address cross-talk minimization problem based on optimal channel positioning. A
mathematical model reveals the critical parameters for high efficiency spectral beam combining in which explicit
equations are derived to relate the spectral density to the total system efficiency. Issue of system scalability for up to 200
channels is addressed. Coupled wave theory of thick hologram gratings is used in this analysis to characterize.
Volume Bragg gratings (VBGs) have been recognized as critical elements in various types of beam-combining applications, such as, design of super-parallel holographic optical correlators, coherent power beam-combiners and couplers, and spectral beam combiners (SBC) in which the output beams from several distinct laser sources are combined into a single-aperture beam. The obvious advantage of VBG stems from extremely narrow spectral and/or angular selectivity compared, to any other surface grating. This feature of VBG enables combining of large number of laser beams within near-diffraction-limited divergence. The VBGs recorded in a photo-thermo-refractive (PTR) glass exhibit a long-term stability of all their parameters at total CW power at a multi-kilowatt level and have shown high-efficiency combining of high-power laser beams. In order to increase the spectral capacity of such a "beam-combiner", the overall loss resulting from absorption and cross-talk between channels should be minimized. This paper considers architecture-specific SBC scheme and addresses the cross-talk minimization problem based on optimal channel positioning. A mathematical model reveals the critical parameters for high efficiency spectral beam combining.
Photo-thermo-refractive (PTR) glasses have shown high efficiency and stability for different applications in laser systems. One of the applications of diffractive elements in PTR glasses is to use them for high power laser beam control and combining which requires an increase of the size of these elements. The opportunities of recording large aperture Bragg gratings by using a translational (multi-frame exposure) technique and by using an Ar<sup>+</sup> ion laser with higher power operating at 334.5 nm and 351 nm are studied. It is shown that photosensitivity of PTR glass at 334.5 nm and 351 nm is comparable to that of 325 nm. Because of higher power at 334.5 nm and 351 nm, the recording of large aperture holograms at these wavelengths is possible. Large-aperture holograms produced by multi-frame technique are demonstrated.
Multichannel resonators which should provide coherent emission of all its components are intensively studied for more than 25 years. However, no stable coherent emission with high efficiency was reported for such devices. The main problem preventing stable efficient coupling is a tendency of a multichannel system to switch between different modes of a complex resonator. This paper reports the use of thick Bragg grating recorded in a photothermo-refractive glass with an extremely narrow spectral width and angle selectivity that provided coherent coupling of two semiconductor laser diodes. In a described construction no oscillations of output power was observed and an interference patter with visibility close to unity was observed for a long time.
Results of a long-term research in spectral narrowing and transverse mode selection in semiconductor lasers by means of volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass are described. PTR glass is a multicomponent silicate optical glass which changes its refractive index after UV exposure followed by thermal development. This feature enables recording of volume holograms with efficiency exceeding 97% in visible and near IR spectral regions which tolerate high temperatures up to 400°C, high power laser radiation. Transmitting and reflecting volume Bragg gratings recorded in such manner have spectral and angular selectivity down to 0.01 nm and 0.1 mrad, respectively. These spectral and angular selectors were used as transmitting and reflecting elements of external resonators for high-power semiconductor laser diodes (LDs). Transmitting Bragg gratings provide tunability of LDs in the range up to 60 nm, spectral narrowing down to 200 pm, stabilization of wavelength within 500 pm. Reflecting Bragg gratings allow spectral narrowing down to 20 pm, stabilization of wavelength below 100 pm at temperature variations up to 75 K. A single transverse mode emission for wide stripe LDs is observed at pumping currents exceeding 10 thresholds. Narrowing and stabilization of emission spectra of LD bars is demonstrated. It is important that all these features are achieved by passive elements with efficiency exceeding 97% and unlimited lifetime while actual brightness increase exceeded two orders of magnitude.
We have studied the nonlinear optical properties of a high-index (n = 1.82) glass that is used as the core material in a commercially available fiber optic inverter, which is a coherent fiber bundle twisted 180 degrees to produce an inverted image. We have determined through open aperture Z-scan the two-photon absorption coefficient of the glass to be 0.8 cm/GW using 23 ps pulses (FWHM) at 532 nm, far from the linear absorption edge of 320 nm. For 5 ns (FWHM) pulses the nonlinear absorption is much larger, and is dominated by two-photon induced excited-state absorption. These effects contribute to the nanosecond optical limiting response that we have observed for the inverter using an F/5 focusing geometry.