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This paper reviews the experimental component of the US Air Force-supported Multi-University Research Initiative (MURI) on femtosecond laser filamentation in transparent dielectric media. The program comprises a coordinated effort by research groups from 6 US universities and from the multi-terawatt laser laboratory at the Kirtland Air Force Base. The goal of the program is to conduct fundamental basic research on femtosecond laser filamentation, to develop new first-principle-based models of ultrashort and ultraintense pulse propagation in transparent dielectrics and to validate these models though a comprehensive experimental campaign.

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our approach is based on the dependence of single-shot laser ablation threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser filaments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the external beam focusing. The limiting value of the peak intensity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f-number above ∼500), is about 55 TW/cm2.

We report experiments and numerical simulations on the self-focusing and filamentation of ultraintense femtosecond Airy waveforms in water. The accelerating property of Airy waveforms results in the generation of distinct features in the angularly resolved spectra of forward-propagating supercontinuum emission. Fitting these features with appropriate phase-matching conditions allows for the quantification of the propagation history of the waveforms.

We report experimental results on the dual laser-pulse plasma excitation in various gases at atmospheric pressure. Dilute plasma channels generated through filamentation of ultraintense femtosecond laser pulses in air, argon, and helium are densified through the application of multi-Joule nanosecond heater pulses. Optical breakdown in atomic gases can be achieved for considerably longer delays between femtosecond and nanosecond pulses compared to that in molecular gases. The densification of the seed channel in molecular gases is always accompanied by its fragmentation into discrete bubbles, while in atomic gases the densified channel remains smooth and continuous.

We report experiments and numerical simulations on supercontinuum generation with femtosecond Airy pulses in a highly nonlinear optical fiber. The ability of the Airy waveform to regenerate its dominant intensity peak results in the generation of distinct spectral features. Airy pulses and other self-healing temporal waveforms may be useful for the generation of spectra with desired properties.

Funds from this one-year grant supported one dedicated graduate student (Craig Ament) and the PI at 50%. The experimental research under this program was supporting the ongoing AFOSR MURI program on theoretical and numerical studies of femtosecond laser filamentation. The particular projects that we pursued in the course of this program are: 1. Creation of extended and dense plasma channels in air and other gases using a nanosecond-femtosecond dual-pulse laser excitation scheme. 2. Generation of regular arrays of plasma channels using femtosecond Bessel beams of higher order 3. Application of accelerating Airy temporal waveforms to ultrafast nonlinear optics 4. Generation of harmonics of tightly focused mid-IR pulses in ambient air.

We report experiments on the generation of third and fifth harmonics of millijoule-level, tightly focused, femtosecond laser pulses at 2.2 {\mu}m wavelength in air. The measured ratio of yields of the third and fifth harmonics in our setup is about 2 \cdot 10-4. This result contradicts the recent suggestion that the Kerr effect in air saturates and changes sign in ultra-intense optical fields., Comment: 3 pages, 2 figures

In this talk, we will review recent and on-going experiments on femtosecond laser filametation at the University of Arizona. Our overall approach is based on the utilization of various beam shapes for controlling filamentation in gases and liquids. In particular, Bessel beams with their extended linear foci facilitate the creation of longitudinally extended filaments [1,2]. Femtosecond Airy beams allow the generation of curved filaments that follow bent trajectories of the main intensity features of these beams [3]. The broadband conical emission by curved filaments in this case allows for the detailed study of pulse evolution along the optical path [4]. We currently extend this work onto the generation of regular arrays of filaments by using intense femtosecond Bessel beams of higher order.

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multi-photon ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light, and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments in air and argon. Our results consistently support the established paradigm., 4 pages, 4 figures

When ultra‐intense and ultra‐short optical pulses propagate in transparent dielectrics, the dynamic balance between multiple linear and nonlinear effects results in the generation of laser filaments. These peculiar objects have numerous interesting properties and can be potentially used in a variety of applications from remote sensing to the optical pulse compression down to few optical cycles to guiding lightning discharges away from sensitive sites. Materializing this practical potential is not straightforward owing to the complexity of the physical picture of filamentation. In this paper, we discuss recent experiments on using beam shaping as a means of control over the filament formation and dynamics. Two particular beam shapes that we have investigated so far are Bessel and Airy beams. The diffraction‐free propagation of femtosecond Bessel beams allows for the creation of extended plasma channels in air. These extended filaments can be used for the generation of energetic optical pulses with the duration in the few‐cycle range. In the case of filamentation of femtosecond Airy beams, the self‐bending property of these beams allows for the creation of curved filaments. This is a new regime of the intense laser‐pulse propagation in which the linear self‐bending property of the beam competes against the nonlinear self‐channeling. The bent filaments generated by ultra‐intense Airy beams emit forward‐propagating broadband radiation. Analysis of the spatial and spectral distribution of this emission provides for a valuable tool for analyzing the evolution of the ultra‐intense optical pulse along the optical path.

We report experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water. The supercontinuum radiation generated along the curved beam path is angularly resolved in the far field. Spectral maps of this radiation reveal the changing character of the laser-pulse evolution on propagation.

We report experimental results on ultrafast filamentation with temporally chirped femtosecond Bessel-Gauss beams. We find that by chirping the pulses, the longitudinal range of the generated plasma channels can be extended relative to filaments generated by fully compressed, transform-limited femtosecond pulses. We find a clear correlation between the extent of filamentation and the intensity of the on-axis emission by the femtosecond Bessel-Gauss beam. The on-axis emission is negligible for fully compressed pulses, but it can become quite substantial (up to 10% of the input pulse energy) when chirped pulses are used. Under certain conditions, the on-axis emission becomes sufficient for generating its own plasma channel thus resulting in extended filamentation. This effect may offer means of remote control over filament formation with femtosecond Bessel-Gauss beams. We identify a four-wave mixing process, enhancement of which is likely to result in a maximum of the on-axis emission, and derive a simple expression for estimating the duration of the chirped pulse that is required for such enhancement. Our estimations are in good agreement with the experiment.

We report experimental observation of laser filaments generated by intense, femtosecond, self-bending Airy beams in air and water. The generated curved filaments act as streak cameras for the forward-emitted broadband conical radiation.

We report an experimental study of scintillations in a free-space optical communication channel with turbulence. Using optimized multi-emitter beams results in a substantial reduction of scintillation index. Experimental results agree with calculations based on Rytov theory.

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.

We propose using high power, high brightness optically pumped vertical-external-cavity semiconductor lasers (VECSELs) as sources to build a partially coherent beam for laser communications applications. VECSELs are compact wavelength tunable, multi-Watt sources emitting light in a near TEM 00 mode. Our theory suggests that the scintillation index at a remote receiver can be significantly reduced by filling the transmitter aperture with an array of beams. An experiment will be reported on that confirms the theory predictions and demonstrates further that the reduction in scintillation index carried through to case of strong turbulence where our perturbation theory fails.

An all-fiber approach is utilized to phase lock and select the in-phase supermode of compact 19- and 37-core fiber lasers that are a few cm long, aligning-free in operation, and environmentally robust.

Viable laser transmitter for emerging optical undersea communications is reported. The high data-rate, picosecond fiber-laser system at 1.56µm is frequency-tripled into transparency window of seawater in cascade of two PPLN crystals operated at room temperature.

We report viable dual-band laser transmitter for emerging marine OCOMM applications. Eye-safe, picosecond fiber-laser system with pulse picking is frequency-tripled into transparency window of seawater in sequence of two PPLN crystals operating at room temperature.

The efficient generation of redshifted pulses from chirped femtosecond joule level Bessel beam pulses in gases is studied. The redshift spans from a few 100 cm⁻¹ to several 1000 cm⁻¹ corresponding to a shift of 50-500 nm for Nd:glass laser systems. The generated pulses have an almost perfect Gaussian beam profile insensitive of the pump beam profile, and are much shorter than the pump pulses. The highest measured energy is as high as 30 mJ, which is significantly higher than possible with solid state nonlinear frequency shifters.

Heavily doped active fibers based on the soft phosphate glass offer an attractive gain medium for compact and high-power laser oscillators. We report a passively modelocked fiber oscillator at 1.5μm based on such active fiber. The standing-wave laser cavity consists of a 20cm-long piece of the side-pumped active phosphate fiber which is heavily co-doped with Er and Yb ions, and a low-ratio fused coupler. The length of the all-fiber laser cavity is 65cm. The modelocked operation of the oscillator is started and sustained by a Semiconductor Saturable Absorber Mirror (SESAM), and no additional pulse narrowing mechanism is used. In order to avoid a premature over-saturation of the SESAM, the fiber end which is butt-coupled to the SESAM is adiabatically tapered which expands the propagating fiber mode and decreases the power density incident on the absorber substantially. The stable modelocked operation of the laser oscillator occurs in the range between 0.65W and 2.3W of the average output power, which is limited by the maximum available pump power at 975nm. The peak pulse power is limited by the saturated SESAM at ~450W, and the pulse width grows from 11psec to 35psec as the pump power is increased. At the pulse repetition rate of 160MHz, the pulse energy reaches 14.4nJ. Our laser oscillator combines the convenience of the all-fiber construction with the power performance previously achievable only with the modelocked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.

Microstructured and multi-core fiber lasers were fabricated that have produced more than 1.3 W/cm at 1.5 /spl mu/m. Near 2 W single frequency, single-transverse-mode output was demonstrated.

We report an all-fiber source of high-power picosecond pulses at 1.5µm. Rapid amplification in heavily-doped phosphate fiber produces pulses with peak power of 16.6kW while pulse distortion is minimal in either temporal or spectral domain.

We report using short, heavily-doped active phosphate fiber for generation of picosecond pulses at 1.5 mum, with the peak power of 19 kW which results in a record-high aerial power density of 24 GW/cm2 in the fiber core.

We report passive harmonic mode locking of Er/Yr doped phosphate fiber ring laser. The laser can be mode-locked at repetition rates from 1.7 to 7.2 GHz producing pulses with durations from 300 fs to 570 fs and 1.1 W of average power.

We report an all-fiber laser oscillator producing as much as 1.9 W of single-frequency direct output at 1.5 microm. Spatial gain hole burning in the active fiber has been eliminated by use of a twisted-mode cavity approach. The two short pieces of a polarization-maintaining fiber that were spliced to the ends of the active fiber served as ultracompact quarter-wave plates. To our knowledge, the use of such a wave plate to manipulate the polarization state of light inside a fiber laser cavity is reported here for the first time. The laser output is linearly polarized and delivered through a polarization-maintaining fiber pigtail. We believe that the output power of our laser is the highest among all single-frequency fiber laser oscillators reported to date.

We report a high average power, all-fiber laser at 1.5µm, modelocked with a saturable semiconductor absorber mirror. The average output power reaches 2.2W, and the peak power is 540W at a repetition rate of 95MHz.

Summary form only given. Microstructured fibers and fiber lasers are providing new approaches for achieving single transverse mode guiding in large core fibers. We have successfully fabricated microstructured fibers from phosphate glass that allows the same high doping levels as in our step index fibers. We developed a two step draw-stack-redraw process that enables the realization of various microstructured cladding and multiple core designs. With our first active microstructured fibers we obtained the following important results. We fabricated and tested the first phosphate glass microstructured fiber lasers with large Er-Yb-codoped cores. For an 11-cm-long cladding-pumped fiber laser, more than 3 W continuous wave output powers has been demonstrated and single-mode beam quality was obtained for an active core area larger than 400 /spl mu/m/sup 2/. We performed, to the best of our knowledge, the first systematic study on how a negative core-cladding index difference influences microstructured optical fiber's modal behavior. Single-mode lasing has been realized for short-length cladding-pumped phosphate glass fibers with large depressed-index Er-Yb-codoped cores. Large core microstructured fibers allowed us to construct even shorter fiber lasers at Watt level output power. A 3.5 cm short microstructured fiber laser produced 5 W cw output power at a pump-to-signal conversion efficiency of 20%. To scale the output power of compact fiber lasers systems to even higher levels, we are working on phase-locked arrays of single mode fiber lasers. This work was initiated through theoretical modeling. Only recently, we were able to use the flexibility of our two-step fiber drawing process to fabricate fibers with multiple cores that can be used to test phase-locking techniques. Initial experiments in this direction has been started. As an example, a microscope image of our microstructured fiber with 12 active cores is shown. Another image demonstrates the lasing action of all 12 cores just above threshold indicating quality and homogeneity of the multi-core fiber. To avoid thermal issues, the first 2 fibers feature a 4 times lower (2 wt%) Yb/sub 2/O/sub 3/ doping level compared to our record breaking step index fibers leading to reduced pump absorption and efficiency. However, even with these test fibers we achieved more than 2 W of combined output power with more than 10% conversion efficiency in a 10 cm fiber laser.

The paper reports on the experimental results showing that short length (7 cm) phosphate fiber lasers generates more than 9.3 W multimode and 4.0 W single mode output power. To the authors' knowledge, the power generated per unit fiber length of 1.33 W/cm represents a record in fiber laser field. This research opens possibilities to develop on-chip watt-level single-frequency light sources at the 1550 nm wavelength band.

We consider propagation of a pair of ultrashort coherent pulses in V-type medium. Both pulses are supposed to be resonant with corresponding atomic transitions and medium not prepared in a coherent state. It is shown that, in the system under consideration, lasing without inversion is impossible. However, the resonant character of the atom-field interaction yields several novel phenomena. If the strong pump pulse is taken in the form of a 2/spl pi/-soliton, and the medium is prepared in the ground state, then weak probe pulse propagates there without any absorption and dispersion. The system yields a pulse locking, in which the two pulses propagate with equal group velocities. The probe pulse evolves towards the locked state independently of its initial shape. If the medium is prepared in a state with small Raman inversion, then the probe pulse experiences high resonantly enhanced Raman gain, still accompanied by vanished linear absorption and dispersion. We demonstrate that, in the nonlinear stage of probe amplification, gain for the probe does not disappear or decrease, but considerably increases. Raman amplification of the probe pulse ends with complete vanishing of the pump, and all photons of the pump exchange on those of the drive one by one. Since the atom-field interaction is resonantly enhanced, this Raman scattering requires much weaker pump intensity in comparison with conventional Raman laser schemes working with far-detuned fields. We study various practical aspects of frequency up- and down-conversion utilizing resonant Raman scattering of ultrashort pulses. The physics behind the effect of pulse locking and resonant Raman amplification are also discussed. The proposed effect holds promise to yield coherent radiation in new frequency domains and improve performance of modern Raman lasers.

We report experimental results on micromachining of borosilicate glass slides with femtosecond higher-order Bessel beams. Transverse intensity profiles of these beams comprise concentric rings that are maintained over extended linear focus zones, facilitating machining geometries with large working distances and high aspect ratios. Both single-shot and multi-shot front-surface machining and multi-shot back-surface processing are experimentally investigated. Material removal in the latter case is furnished through the immersion of the back side of the glass slide in water. Under certain conditions, we observe evidence of self-focusing and azimuthal breakup of the initially smooth ring intensity features of the beams near the glass–water interface. These beam dynamics result in the formation of beaded ring features on the back surface of the glass slide. In the case of multi-shot front-surface machining, pillar-like structures can be fabricated.

Femtosecond laser pulses with sufficiently high peak power leave tracks of dilute plasma in their wakes. Potential use of this plasma for channeling electrical discharges in the atmosphere has been discussed and demonstrated in laboratory-scale experiments. However, the electron density in femtosecond laser-generated plasma decays rapidly on the nanosecond time scale, due to recombination and electron attachment to air molecules. The finite plasma lifetime limits the maximum extent of the guided electrical breakdown to a few meters. Here, we experimentally demonstrate that the limitation associated with the short plasma lifetime can be overcome though optical heating of the plasma filaments by an auxiliary energetic laser pulse with a duration in the nanosecond range. We show that the breakdown electric field can be reduced by up to a factor of 4 with a heater fluence of about 1 kJ/cm2. This approach could have applications in channeling long-range electrical discharges in the atmosphere and, potentially, in channeling lightning strikes.

A three-level atom having two close upper levels decaying to a common third level is considered. It is shown that interference between these two spontaneous transitions drastically changes the spectrum of emitted light. Particular attention is paid to the time evolution of the spectrum. This yields insight into a novel and counterintuitive aspect of the quantum theory of radiation.

The operation of the capacitive plasma probe commonly used for measurements of plasma density in laser filaments and sparks in gases is analyzed. The probe is employed to measure absolute mobilities of O2+ and O2− ions produced through femtosecond laser filamentation in air.

We report experiments on the generation of dense plasma channels in ambient air using a dual laser pulse excitation scheme. The dilute plasma produced through the filamentation of an ultraintense femtosecond laser pulse is densified via avalanche ionization driven by a co-propagating multi-Joule nanosecond pulse.

We report experiments on self-focusing of femtosecond diffraction-resistant vortex beams in water. These beams are higher-order Bessel beams with weak azimuthal modulation of the transverse intensity patterns. The modulation overrides the self-focusing dynamics and results in the formation of regular bottlelike filament distributions. The peak-power thresholds for filamentation, at a particular distance, are relatively accurately estimated by the adaptation of the Marburger formula derived earlier for Gaussian beams. The nonlinear conversion of the incident conical waves into the localized spatial wave packets propagating near the beam axis is observed.

Recent research in nonlinear optics with ultra-intense and ultra-short laser pulses has opened the door to applications in remote spectroscopy, communications and atmospheric science. This article reviews studies into the propagation of ultra-intense self-bending Airy beams in transparent dielectric media.

Simple diffraction does not establish the limits on lateral resolution. We can use our new knowledge of light propagation to develop ultrasensitive instruments.

Extending the longitudinal range of plasma channels created by ultrashort laser pulses in atmosphere is important in practical applications of laser-induced plasma such as remote spectroscopy and lightning control. Weakly focused femtosecond Gaussian beams that are commonly used for generating plasma channels offer only a limited control of filamentation. Increasing the pulse energy in this case typically results in creation of multiple filaments and does not appreciably extend the longitudinal range of filamentation. Bessel beams with their extended linear foci intuitively appear to be better suited for generation of long plasma channels. We report experimental results on creating extended filaments in air using femtosecond Bessel beams. By probing the linear plasma density along the filament, we show that apertured Bessel beams produce stable single plasma channels that span the entire extent of the linear focus of the beam. We further show that by temporally chirping the pulse, the plasma channel can be longitudinally shifted beyond the linear-focus zone, an important effect that may potentially offer additional means of controlling filament formation.

Experimental demonstration of an efficient energy extraction from heavily Er:Yb codoped phosphate-glass fiber is reported. In our experiments, sub-20-ns pulses at 1.5μm wavelength produced by an actively Q-switched all-fiber oscillator are amplified in 15cm long, side-pumped phosphate fiber doped with 1wt% of Er2O3 and 8wt% of Yb2O3. Two types of active fiber are used, with core diameters of 14 and 25μm. The pulse energy achieved with these fibers is 105 and 215μJ, respectively, with nearly diffraction-limited beam quality. These experiments indicate a potential for further energy scaling beyond the millijoule level by increasing the core size of the fiber.

The authors report experimentally observed qualitative changes of the waveguiding properties in ytterbium-doped fibers with high concentration of doping when the fibers are optically pumped with light at the 975nm wavelength. They found that when concentration of Yb2O3 in a single-mode fiber exceeds ∼10wt.% and the pump power is gradually increased, the fiber first looses index-induced guidance, and then regains guidance as the population inversion in Yb+3 ions becomes sufficient for gain guiding. The associated variation of the mode-field diameter is a practically important effect that will complicate design and operation of fiber lasers and amplifiers based on highly Yb-doped glass.

Using numerical solutions of Maxwell's equations in conjunction with the Lorentz law of force, we compute the electromagnetic force distribution in and around a dielectric micro-sphere trapped by a focused laser beam. Dependence of the optical trap's stiffness on the polarization state of the incident beam is analyzed for particles suspended in air or immersed in water, under conditions similar to those realized in practical optical tweezers. A comparison of the simulation results with available experimental data reveals the merit of one physical model relative to two competing models; the three models arise from different interpretations of the same physical picture.

A short-cavity fibre laser oscillator emitting 12 ps-long pulses at the fundamental cavity frequency of 550 MHz is reported. The simple, stable and ultra-compact laser cavity consists of a short active fibre section which is spliced to a narrowband fibre Bragg grating, and a butt-coupled semiconductor saturable absorber mirror. Only 8 cm of the heavily Er/Yb co-doped phosphate-glass active fibre is sufficient to produce as much as 775 mW of average output power at 1.5 µm directly from the oscillator.

A single-shot optical pump-and-probe test system is reported. The system is designed for thermal characterization of thin-film samples that can change their phase state under the influence of a short and intense laser pulse on a subnanosecond time scale. In combination with numerical analysis, the system can be used to estimate thermal constants of thin films, such as specific heat and thermal conductivity. In-plane and out-of plane thermal conductivity can be estimated independently. The system is intended for use in research on optical data storage and material processing with pulsed laser light. The system design issues are discussed. As application examples, we report on using the system to study thermal dynamics in two different thin-film samples: a gold film on a glass substrate (a single-phase system) and the quadrilayer phase-change stack typical in optical data-storage applications.

We report a simple optical sensing device capable of measuring the refractive index of liquids propagating in microfluidic channels. The sensor is based on a single-mode optical fiber that is tapered to submicrometer dimensions and immersed in a transparent curable soft polymer. A channel for liquid analyte is created in the immediate vicinity of the taper waist. Light propagating through the tapered section of the fiber extends into the channel, making the optical loss in the system sensitive to the refractive-index difference between the polymer and the liquid. The fabrication process and testing of the prototype sensing devices are described. The sensor can operate both as a highly responsive on-off device and in the continuous measurement mode, with an estimated accuracy of refractive-index measurement of approximately 5 x 10(-4).

We report over 2 W of single spatial-mode output power at 1.5 microm from an 8-cm-long, large-core phosphate fiber laser. The fiber has a numerical aperture of approximately equal to 0.17 and a 25-microm-wide core, heavily doped with 1% Er(+3) and 8% Yb(+3). The laser utilizes a scalable evanescent-field-based pumping scheme and can be pumped by as many as eight individual multimode pigtailed diode laser sources at a wavelength of 975 nm. Nearly diffraction-limited laser output with a beam quality factor M2 approximately equal to 1.1 is achieved by use of a simple intracavity all-fiber spatial-mode filter. Both spectrally broadband and narrowband operation of the laser are demonstrated.

We report a single-frequency fiber laser with 1W output power at 1.5 mum which is to our knowledge, five times the highest power from a single-frequency fiber laser reported to-date. The short unidirectional ring cavity approach is used to eliminate the spatial gain hole-burning associated with the standing-wave laser designs. A heavily-doped phosphate fiber inside the ring resonator serves as the active medium of the laser. Up to 700mW of output power, the longitudinal mode hops have been completely eliminated by using the adjustable coupled-cavity approach. At higher power levels, the laser still oscillates at a single longitudinal mode, but with infrequent mode hops that occur at a rate of few hops per minute. Compared to the Watt-level single-frequency amplified sources, our approach is simpler and offers better noise performance.