Complex spatio-temporal dynamics can be observed in single broad-area semiconductor lasers under external optical feedback. Yet, non-linear dynamics is mostly unexplored in large 1D-arrays of lasers. In our recent investigations, we demonstrated both numerically and experimentally that single-mode and broad-area laser arrays in a V-shape external cavity can generate complex spatio-temporal dynamics with typical frequencies in the GHz range as well as periodic and chaotic phase-locking. Feedback misalignment and feedback strength are key parameters to warrant that diode lasers in the array display a variety of dynamics. Potential applications may include directed energy, LIDAR, and random number generators.
We present theoretical and computational investigations of the nonlinear dynamics and heterogeneity-promoted synchronization of diode laser arrays with decayed non-local coupling topology. The diode laser array exhibits a wide variety of dynamical behaviors as laser and coupling parameters vary. Here, we explore the dynamics for the intermediate and large coupling feedback strengths and further analyze the phase diagram and power spectrum as a function of the feedback strength, coupling topology, and misalignment introduced in the array. The dynamics induced by intermediate feedback are complex for small values of external cavity misalignment but display a clustering phenomenon consisting of several separate groups showing incoherent and coherent dynamics for the appropriate value of the cavity misalignment parameter. Furthermore, the dynamics and the power spectrum in the stronger feedback regime show frequency and phase-locking as the amount of misalignment disorder increases.
Compact, repetitively fired, gain-saturated x-ray lasers have been limited to wavelengths above λ=8.85 nm. Here we discuss their extension to λ = 6.85 nm by transient traveling wave excitation of Ni-like Gd ions in a plasma created with an optimized pre-pulse followed by rapid heating with an intense sub-ps pump pulse. Isoelectronic scaling also produced strong lasing at 6.67 nm and 6.11 nm in Ni-like Tb, and amplification at 6.41 nm and 5.85 nm in Ni-like Dy.
Plasma-based soft x-ray lasers (SXRL) enable experiments requiring bright, high energy, soft x-ray laser pulses to be conducted in compact facilities. Recent advances in high energy, high repetition rate, ultrashort pulse solid state lasers now make it possible to extend their wavelenght range, increase their repetition rates, and improve their efficiency. Recently we extended the operation of gain-saturated compact repetitive x-ray lasers down to λ=6.85 nm in Ni-like Gd [1]. Isoelectronic scaling of these collisionally excited laser transitions produced strong lasing at 6.67 nm and 6.11 nm in Ni-like Tb and amplification at 6.41 nm and 5.85 nm in Ni-like Dy [1]. This recent progress will be summarized, and efforts to further extend laser operation to shorter wavelengths will be reviewed. We will also discuss the possibility of increasing the efficiency of plasma-based soft x-ray lasers by controlling the pulse shape of the pump pulses. Prospects of these SXRL will be discussed.
Work supported by the US Department of Energy Basic Energy Sciences and by the National Science Foundation
[1] A. Rockwood, Y. Wang, S. Wang, M. Berrill, V.N. Shlyaptsev, and J.J. Rocca, "Compact gain-saturated x-ray lasers down to 6.85 nm and amplification down to 5.85 nm". Optica. 5, 257, (2018).
We will review recent progress in the development of high repetition, high average power rate soft x-ray lasers at 10-20 nm wavelength at Colorado State University, and the compact diode- pumped solid state lasers that drive them. The latter includes the development of a 1 J picosecond laser capable of operating at 500 Hz repetition rate. Results that demonstrate soft x-ray laser operation at the highest repetition rate reported to date: 400 Hz, and prospects of the use of these lasers in applications are discussed.
The table-top generation of high average power coherent soft x-ray radiation in a compact set up is of high interest for numerous applications. We have demonstrated the generation of bright soft x-ray laser pulses at 100 Hz repetition rate with record-high average power from compact plasma amplifiers excited by an ultrafast diode-pumped solid state laser. Results of compact λ=18.9nm Ni-like Mo and λ=13.9nm Ni-like Ag lasers operating at 100 Hz repetition rate are discussed.
We discuss recent advances in the development of high repetition rate table-soft soft x-ray lasers resulting from
research conducted at Colorado State University. Advancing saturated table-top lasers to shorter wavelengths we report
the operation of gain-saturated sub-10 nm table-top lasers at 1 Hz repetition rate. We also present experimental results
that show that injection-seeding of solid-target soft x-ray plasma amplifiers reduces the far field divergence by an order
of magnitude and to allow for control of the far-field beam characteristics by tailoring the divergence of the seed. We
finally discuss progress towards the development of high repetition rate compact all-diode-pumped soft x-ray lasers. We
have operated the front end of the diode-pumped soft-ray laser driver at 100Hz repetition rate, obtaining sub-5 ps optical
laser pulses of 100 mJ energy.
We measured the linewidth of a λ= 46.9 nm neon-like argon capillary discharge soft x-ray laser. A wavefront division
interferometer based on a pair of dihedrons was used to resolve the laser line measuring the variation in the interference
fringe visibility for different optical path differences. We measured a relative linewidths of Δλ/λ = 3-4 10-5. No
significant re-broadening was observed when the length of the laser medium was increased beyond the saturation length
due to effects that homogenizes the line profile.
We describe our recent progress in the investigation of the spectral properties of collisional XUV lasers, including both
experimental measurements and numerical calculations. Using a wavefront-division, variable path-difference
interferometer, we have characterized the temporal coherence and the spectral width of an injection-seeded transient
XUV laser emitted at 18.9 nm from a Ni-like Mo plasma. Our results show that the temporal coherence of the beam is
significantly increased by the injection-seeded operation, compared to the standard ASE mode, in agreement with
detailed numerical simulations. Using the PPP code we have calculated the intrinsic linewidth of the same lasing line
over a range of electron density and at temperatures that are relevant to transient collisional pumping. We discuss the
relative contributions of homogeneous and inhomogeneous broadening to the overall profile.
We report new advances in the development of high repetition rate table-top soft x-ray lasers. We have extended the gain-saturated operation of these lasers to 10.9 nm demonstrating a 1 Hz repetition rate laser operation in Ni-like tellurium with an average power of 1 microwatt. In a separate development we have demonstrated the first all-diodepumped soft x-ray laser. Lasing was achieved in the 18.9 nm line of Ni-like molybdenum in a plasma heated by a compact all-diode-pumped Yb:YAG laser. The solid state pump laser produces 8.5 ps pulses with up to 1 J energy at 10 Hz repetition rate. This diode-pumped laser has the potential to greatly increase the repetition rate and average power of soft x-ray lasers on a significantly smaller footprint. These compact soft X-ray lasers offer new scientific opportunities in small laboratory environments.
We have demonstrated ablation of holes with diameter as small as 82 nm in polymethyl methacrylate (PMMA) by
focusing the output of a capillary discharge soft x-ay laser with a Fresnel zone plate. We also report the first
demonstration of laser induced breakdown spectroscopy with soft x-ray laser light.
We have recently demonstrated high repetition rate tabletop lasers operating at wavelengths as short as 10.9 nm
based on collisional transient excitation of ions in plasmas created by laser heating of solid targets. As a further step in
the development of these lasers into very high brightness and fully coherent soft x-ray sources, we have demonstrated
injection seeding of the amplifiers with high harmonic seed pulses. We report results of an experiment in which a 32.6
nm Ne-like Ti amplifier was used to amplify a seed pulse from the 25th harmonic of Ti:Sapphire into the gain saturation
regime. Simultaneous amplification of the 27th harmonic at 30.1 nm was also observed. The seeded soft x-ray laser beam
was measured to approach full spatial coherence. We have demonstrated that this scheme is scalable to shorter
wavelengths and that is capable of producing extremely bright soft x-ray laser pulse with essentially full coherence.
We have demonstrated gain-saturated operation of high repetition rate table-top soft x-ray lasers producing microwatt average powers at wavelengths ranging from 13.2 to 32.6 nm in transitions of Ni-like and Ne-like ions. Lasing was also observed for shorter wavelength transitions of the 4d1S0→4p1P1 Ni-like isoelectronic sequence, with amplification approaching gain saturation in the 11.9 nm line of Ni-like Sn and progressively reduced gain for wavelengths as low as 10.9 nm in Ni-like Te. The results were obtained heating a pre-created plasma with a picosecond optical laser pulse with an energy of only 1 J impinging at optimized grazing angles of incidence for maximum pump energy deposition efficiency. This pumping geometry takes advantage of the refraction of the pump beam to increase the energy deposition efficiency of the pump beam into the gain region, making it possible to operate soft x-ray lasers in this wavelength range at significantly increased repetition rates. The results demonstrate the feasibility of producing high average powers of coherent radiation in the 100 eV spectral region for applications using a table-top source.
We report the generation and characterization of plasma waveguides in a highly ionized Ar plasma created by a fast capillary discharge. Their results are of interest for the efficient longitudinal laser excitation of collisional soft x-ray lasers. A discharge-driven hydrodynamic compression guides progressively lower order modes through a plasma with increasing density and degree of ionization. The mode structure and guiding properties were investigated using near and far field imaging, and transmission measurements. The lowest order mode (FWHM diameter ~ 50 um) is guided with an energy transmission of ~ 75% over a 5.5 cm long plasma a fraction of a ns before the discharge plasma column reaches the conditions for lasing in Ne-like Ar. The rapid expansion of the highly ionized plasma column after the pinch forms a significantly more leaky and absorbent waveguide.
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