Five-channel spectral beam combining (SBC) using volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR)
glass with 0.5 nm spectral separation between channels and combined power >750 W has been recently reported. We
report on improvements in this technique with the use of thermal control of VBGs that allows precise high-power
alignment required for dense SBC with 0.25 nm spectral separation of channels. Experimental results of passive coherent
beam combining (CBC) of fiber lasers using multiplexed VBGs are presented and analyzed. Methods for achieving
100 kW class systems using novel hybrid architectures that combine both coherent and spectral beam combining are
discussed.
Chirped Bragg Gratings (CBGs) recorded in photo-thermo-refractive (PTR) glass have been successfully used as
ultrashort pulse stretchers and compressors in a variety of solid-state and fiber chirped pulse amplification (CPA) laser
systems. Compared to traditional pairs of surface gratings, CBG-based stretchers and compressors offer significant
advantage in compactness and robustness. They are insensitive to polarization, require virtually no alignment and can
handle high average and peak power. At the current technology stage PTR-glass CBGs can provide up to 30 nm spectral
bandwidth and up to 300 ps stretched pulse duration. In this paper we propose a concept of sectional CBGs, where
multiple CBGs with different central wavelengths recorded in separate PTR-glass wafers are stacked and phased to form
a single grating with effective thickness and bandwidth larger than each section. We present results of initial experiment
in which pulses from a femtosecond oscillator centered at 1028 nm are stretched by a 32-mm thick CBG to about 160 ps
and recompressed by a monolithic 32-mm CBG with 11 nm bandwidth and by a sectional CBG with two 16-mm thick
sections each having ~ 5 nm bandwidth and offset central wavelengths: 1025.5 and 1031 nm. In both cases, compressed
pulse duration of 350-400 fs, ~ 1.1 × transform-limit was obtained. These results allow CBG-based pulse stretchers and
compressors with high stretch ratio and wide bandwidth to be constructed from multiple sections.
We introduce a novel technique of coherently locking fiber lasers using volume Bragg gratings (VBGs) recorded in
photo-thermo-refractive (PTR) glass as a passive multiplexer between channels. A two-channel coherently-locked Ybdoped
fiber laser system with a narrow linewidth (~2.5 pm) and linear polarization (PER >20 dB) is demonstrated at a
level of ~ 4 W (limited by pump). Scaling of this technique to coherently lock multiple (>2) fiber laser channels is
discussed.
A tabletop kW-level spectral beam combining (SBC) system using volume Bragg gratings (VBGs) recorded in photothermo-
refractive (PTR) glass was presented at the last meeting [1]. Diffraction efficiency of VBGs close to 100% was
demonstrated. However, when using VBGs for spectral beam combining, it is important to ensure high diffraction
efficiency for the diffracted beam and low diffraction efficiency for the transmitted beams simultaneously. The unique,
unmatched properties of VBGs allow spectral beam combining achieving this condition at wavelengths with less than
0.25 nm separation. We present modeling of reflecting VBGs for high power SBC that takes into account laser spectral
bandwidth, beam divergence, PTR-glass scattering losses, and grating non-uniformity. A method for optimization of
VBG parameters for high-efficiency SBC with an arbitrary number of channels is developed. Another important aspect
of spectral beam combiner design is maintaining high diffraction efficiency as the temperature of beam-combining
VBGs changes during operation due to absorption of high power radiation. A new technique of thermal tuning of large
aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide
range of laser power, is developed. Finally, these tools are used to demonstrate a robust and portable 5-channel SBC
system with near diffraction limited spectrally-combined output beam.
KEYWORDS: Fiber Bragg gratings, Fiber lasers, Glasses, High power lasers, Beam controllers, Laser systems engineering, Diffraction gratings, Collimators, Output couplers, Chemical elements
Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass are used in a wide range of high-power
laser applications due to their unique spectral response and excellent optical and thermo-mechanical properties.
Experimental results of applications of narrow-band reflecting VBGs to spectral beam combining (SBC) and wavelength
control of fiber lasers are presented. Output power of 770 W from a system combining five fiber lasers with 91.7%
efficiency is demonstrated with spectral separation between channels of 0.5 nm around 1064 nm and no distortions in
diffracted beams. Similar system with 0.25 nm channel separation around 1550 nm is demonstrated with the same
efficiency and M2 of the spectrally-combined beam < 1.15. A novel compact monolithic multi-channel beam combiner
based on stacked tilted VBGs is suggested. Absolute efficiency exceeding 90% is reported for a four-channel device with
0.7 nm spectral separation of channels. We show that a linear stack of monolithic combining elements enables compact
spectrally-combined laser systems with output power of 10-100 kW. A common-cavity approach to multi-channel
spectral beam combining of high-power lasers is demonstrated. In this configuration wavelengths of the sources are
passively controlled by a combination of a common output coupler and intra-cavity VBGs, which also act as combining
elements. Laser wavelengths are automatically selected to match resonant wavelengths of respective gratings and
provide maximum combining efficiency. Stable operation of a passively-controlled system combining two amplifiers
with 0.4 nm spectral separation is demonstrated. Wavelengths of amplifiers are shown to automatically follow Bragg
condition of VBGs during heating of gratings.
KEYWORDS: Glasses, Fiber lasers, Fiber Bragg gratings, Diffraction gratings, High power lasers, Absorption, Diffraction, High power fiber lasers, Laser systems engineering, Automatic control
Spectral beam combining (SBC) has been extensively used for power scaling of laser systems. SBC is an incoherent
technique of combining laser radiation from multiple sources with offset wavelengths into a single near-diffractionlimited
beam with increased energy brightness. SBC by means of volume Bragg gratings (VBGs) recorded in photo-thermo-
refractive (PTR) glass has been shown to be a simple and robust technique for combining high-power laser
radiation. High-efficiency large-aperture VBGs were fabricated in PTR glass wafers. While being photosensitive in the
UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Excellent mechanical properties
and refractive index independent of temperature enable VBGs in PTR glass to withstand high-power laser radiation,
making them ideal elements for high-power SBC. We report spectral combination of five randomly polarized fiber
lasers with 0.5 nm spectral separation between channels around 1064 nm using reflecting VBGs in PTR glass.
Maximum output power of the system is 773 W, corresponding to 91.7% combining efficiency. It is shown that VBGs
introduce no significant beam distortions under high-power operation. Additionally, a common-cavity configuration for
SBC with automatic wavelength control of sources by intra-cavity VBGs is suggested. Two fiber lasers are combined
using this technique and automatic wavelength control is demonstrated. We show how simple power scaling allows
obtaining multi-kW near-diffraction-limited laser radiation via SBC with volume Bragg gratings in PTR glass.
Incoherent spectral beam combining (SBC) by means of volume Bragg gratings (VBGs) has been shown to be a simple
and robust technique for generating high-power laser radiation. Combination of laser radiation from multiple sources
into a single near-diffraction-limited beam results in energy brightness increase, while spectral brightness is preserved.
High-efficiency VBG recording in photo-thermo-refractive (PTR) glass has been recently developed. While being
photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Moreover,
this glass has excellent mechanical properties and refractive index independent of temperature. These features enable
VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We
present experimental results of successful 5-channel SBC with reflecting VBGs in PTR glass with small channel spacing
(~0.43 nm around 1064 nm). Absolute system efficiency of 93.5% is demonstrated. Combined beam is shown to be near-diffraction-limited with M2=1.11. Behavior of narrow-band reflecting VBGs in high-power beams is studied. VBGs are shown to withstand 570 W CW radiation around 1064 nm with diffraction efficiency in excess of 92%. Pathway to near-diffraction-limited high-power laser systems via SBC with VBGs is shown. High-efficiency SBC system with 0.2 nm channel spacing is designed.
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.
Femtosecond ablation has several distinct advantages: the threshold energy fluence for the onset of damage and ablation is orders of magnitude less than for traditional nanosecond laser machining, and by virtue of the rapid material removal of approximately an optical penetration depth per pulse, femtosecond machined cuts can be cleaner and more precise than those made with traditional nanosecond or longer pulse lasers. However, in many materials of interest, especially metals, this limits ablation rates to 10-100 nm/pulse. We present the results of using multiple pulse bursts to significantly increase the per-burst ablation rate compared to a single pulse with the same integrated energy, while keeping the peak intensity of each individual pulse below the air ionization limit. Femtosecond ablation with pulses centered at 800-nm having integrated energy of up to 30 mJ per pulse incident upon thin gold films was measured via resonance frequency shifts in a gold-electrode-coated quartz-crystal oscillator. Measurements were performed using Michelson-interferometer-based burst generators, with up to 2 ns pulse separations, as well as pulse shaping by programmable acousto-optic dispersive filter (Dazzler from FastLite) with up to 2 ps pulse separations.
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