RESUMEN

A five fiber Bragg grating (FBG) array is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of a near-infrared femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The tunability of the center Bragg wavelength is achieved by a defocusing lens, and by translating the PM, which results in a different magnification of the PM. A first FBG is inscribed, followed by four cascading FBGs, which are inscribed exactly at the same spot only after the translation of the PM. The transmission and reflection spectra of this array are measured, showing a second-order Bragg wavelength at ~1.56 µm with a transmission dip of ~-8 dB. The spectral wavelength shift between each consecutive FBG is ~2.9 nm, and the total wavelength shift is ~11.7 nm. The reflection spectrum of the third-order Bragg wavelength is measured at ~1.04 µm, showing a wavelength separation of ~1.97 nm between neighboring FBGs, and the total spectral span between the first FBG and the last one is ~8 nm. Finally, the wavelength sensitivity to strain and temperature is measured.

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We present an efficient 976 nm laser generation from an ytterbium (Yb)-doped step-index multicore fiber (MCF) with six cores placed in a ring shape. Each of the six cores has a large-mode-area (LMA) and a low numerical aperture (NA), which makes the MCF equipped with the features of a large core-to-cladding area ratio and differential bending loss for wavelength and mode selection. Hence, the Yb-doped MCF benefits 976 nm laser generation by simultaneously suppressing unwanted 1030 nm emission and higher-order modes (HOMs). A 976 nm laser is obtained in a short piece (88 cm) of the Yb MCF, with a good slope efficiency of 46% with respect to launched pump power and the maximum output power of 25 W (pump power limited). A mode area of 1432 µm2 at the 976 nm is expected for the fundamental in-phase mode.

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The current laser atherectomy technologies to treat patients with challenging (to-cross) total chronic occlusions with a step-by-step (SBS) approach (without leading guide wire), are lacking real-time signal monitoring of the ablated tissues, and carry the risk for vessel perforation. We present first time post-classification of ablated tissues using acoustic signals recorded by a microphone placed nearby during five atherectomy procedures using 355 nm solid-state Auryon laser device performed with an SBS approach, some with highly severe calcification. Using our machine-learning algorithm, the classification results of these ablation signals recordings from five patients showed 93.7% classification accuracy with arterial vs nonarterial wall material. While still very preliminary and requiring a larger study and thereafter as commercial device, the results of these first acoustic post-classification in SBS cases are very promising. This study implies, as a general statement, that online recording of the acoustic signals using a noncontact microphone, may potentially serve for an online classification of the ablated tissue in SBS cases. This technology could be used to confirm correct positioning in the vasculature, and by this, to potentially further reduce the risk of perforation using 355 nm laser atherectomy in such procedures.

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We apply tiling and pattern theory in the design of hollow-core photonic crystal fibers for guiding light in multiple spectral bandgaps. By combining two different glass apexes in a single [36;32.4.3.4] 2-uniform tiling, transmission regions with fundamental, second and third harmonic wavelengths are supported. This cladding design may also be an excellent candidate for high power beam delivery of Er/Yb, Nd:YAG and Ti:Sapphire laser sources.

RESUMEN

We present an experimental characterization of the amplification of sub-nanosecond duration laser pulses at a wavelength of 1538 nm in short custom-made Er:Yb phosphate glass fibers with different core diameters. The fibers vary in their diameter from 100 µm (highly multi-mode) down to 12 µm (single-mode). The peak power, energy per pulse, and spectral shape of the amplified signal are presented. With our input pulses, the measurements show that the large core diameter fibers do not increase the amplification of the 1538 nm signal. We believe this is due to the high re-absorption of the Er3+ ions in the phosphate fiber. The optimal fiber geometry was found to have a core diameter of 20 µm with a length of 14 cm. The maximum peak power is 8.25 kW, corresponding to a net gain of 10.9 dB, with a pulse duration of 0.7 ns and a repetition rate of 40 kHz.

RESUMEN

We demonstrate a direct inscription of a fiber Bragg grating (FBG) in the active cores of an Yb-doped large mode area multicore fiber (MCF). An ultrashort pulsed laser is used to inscribe the FBG simultaneously in all six cores. In order to validate the FBG reflection and uniformity, the FBG is incorporated as a rear mirror in a fiber laser oscillator setup. The MCF, which has been fabricated in-house, has six cores located in a hexagonal-ring shape, each with a 19 µm diameter and an NA of ∼0.067. A reflection of ∼96% was measured at a center Bragg wavelength of ∼1062nm for the inscribed FBG. The laser performance of the MCF with the femtosecond inscribed FBG at its end shows a similar performance to lasing with a free-space commercial volume Bragg grating as the rear-reflector. A slope efficiency of ∼72.4% and a maximum (pump limited) output power of 51.8 W have been obtained for the FBG setup. An effective M2 of 3.88, indicating a somewhat multimode operation and a narrow bandwidth of ∼0.19nm, has been measured for this fiber laser.

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We have measured the fluorescence quantum efficiency in Ti3+:sapphire single crystals between 150 K and 550 K. Using literature-given effective fluorescence lifetime temperature dependence, we show that the zero temperature radiative lifetime is (4.44 ± 0.04) µs, compared to the 3.85 µs of the fluorescence lifetime. Fluorescence lifetime thermal shortening resolves into two parallel effects: radiative lifetime shortening, and non-radiative transition rate enhancement. The first is due to thermally enhanced occupation of a ΔE = 1,700 cm-1 higher (top) electronic state of the upper multiplet, exhibiting a transition oscillator strength of f = 0.62, compared to only 0.013 of the bottom electronic state of the same multiplet. The non-radiative rate relates to multi-phonon decay transitions stimulated by the thermal phonon occupation. Thermal enhancement of the configuration potential anharmonicity is also observed. An empiric expression for the figure-of-anharmonicity temperature dependence is given as [Formula: see text] (T) = [Formula: see text] (0)(1 + ß exp(-ℏωco /kBT )), where [Formula: see text] (0) = 0.276, ß = 5.2, ℏωco = 908 cm-1, and kB is the Boltzmann constant.

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We present a comparative experimental investigation of vibrational stimulated Raman scattering in hollow-core photonic bandgap fibers pressurized with sulfur hexafluoride (${{\rm SF}_6}$SF6) and tetrafluoromethane (${{\rm CF}_4}$CF4) gases. Nanosecond-duration pulses at a wavelength of 1030 nm are coupled into the gas-filled fiber, and the first and second Stokes orders are measured at the fiber output. We characterize the conversion process as a function of gas, fiber length, and input power. With a 15 m fiber filled with ${{\rm SF}_6}$SF6, we obtain conversion efficiency to the first Stokes of 55.7% at an input peak power of 0.63 kW. In comparison, with ${{\rm CF}_4}$CF4, we obtained a higher conversion threshold and maximum conversion efficiency of 45.4%. To the best of our knowledge, this is the first reported conversion experiment with hollow-core fibers filled with ${{\rm SF}_6}$SF6 gas.

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Fiber Bragg grating (FBG) inscription in standard fibers with femtosecond (fs) laser pulses was first reported nearly two decades ago. FBG fs inscription through the fiber polymer coating was recently demonstrated with a phase mask (PM) and High Numerical Aperture (High-NA) cylindrical lenses. In this work, we report on a new technique for FBG inscription through the acrylate polymer coating of optical fibers using a Low-NA lens and the PM. The FBGs were inscribed through the polymer coating of the fiber after a suitable fs photo-treatment process that was done to the polymer coating. We experimentally demonstrate inscription of high-quality FBGs yet with some damage to the coating. We characterize the wavelength sensitivity to strain and temperature of the inscribed FBGs, and compare them to FBGs that were inscribed in fibers that have undergone stripping, inscription, and recoating. The technique may simplify FBGs inscription through the coating especially in large mode area fibers and double clad fibers for laser applications in the future.

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We suggest a novel method to classify the type of tissue that is being ablated, using the recorded acoustic sound waves during pulsed ultraviolet laser ablation. The motivation of the current research is tissue classification during vascular interventions, where the identification of the ablated tissue is vital. We classify the acoustic signatures using Mel-frequency cepstral coefficients (MFCCs) feature extraction with a Support Vector Machine (SVM) algorithm, and in addition, use a fully connected deep neural network (FC-DNN) algorithm. First, we classify three different liquids using our method as a preliminary proof of concept. Then, we classify ex vivo porcine aorta and bovine tendon tissues in the presence of saline. Finally, we classify ex vivo porcine aorta and bovine tendon tissues where the acoustic signals are recorded through chicken breast medium. The results for tissue classification in saline and through chicken breast both show high accuracy (>98%), based on tens of thousands of acoustic signals for each experiment. The experiments were conducted in a noisy and challenging setting that tries to imitate practical working conditions. The obtained results could pave the way towards practical tissue classification in various important medical procedures, achieving enhanced efficacy and safety.

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RESUMEN

Two slightly shifted gratings are inscribed, one over the other, while exploiting fiber strain in a single-mode fiber. The inscription is done with a near-infrared femtosecond laser, a phase mask, and a cylindrical focusing lens. The first fiber Bragg grating (FBG) is inscribed under normal fiber tension, while the second overlapping FBG is inscribed under higher fiber tension. The transmission spectrum of the complex structure is similar to that of a phase-shifted grating, yet the fabrication process is much faster and simpler compared to other standard methods. A high-quality phase-shifted grating with two -30 dB transmission dips, a 25 dB transmission peak, and <50 pm transmission bandwidth is achieved. We observe polarization-dependent transmission in the phase-shifted gratings.

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Various types of pre- and post-treatments to optical fibers are typically used to improve and/or change the properties of fiber Bragg grating (FBG). Here, we investigate experimentally the effects of NIR femtosecond (fs) laser pulses used for pre- and post-treatment on the refractive index of the fiber, and the resulting center wavelength shift of an fs inscribed FBG. We observe "red"-shift when applying a suitable pre-treatment, and both "blue"- and "red"-shifts when applying post-treatment. We characterize the photo-treatment parameters and compare to an FBG inscribed on a fresh fiber without any treatment. We also show that when the photo-treatment is saturated the result is a phase-shifted grating. Our results give further insight into the process of fs photo-treatments and quantify the effects in the case of fs FBG inscription.

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We investigate mode-area scaling by means of supermode operation in an all-solid multicore fiber. To obtain a large-mode area (LMA), we designed and fabricated an active double-clad multicore fiber, where each ytterbium-doped core is 19 µm in diameter and has a numerical aperture of 0.067, comparable to the core of the largest available commercial LMA fibers. The six large cores are stacked tightly in a ring structure to enable phase locking of the core fields and supermode operation. The fiber laser performance was investigated in a linear laser cavity with an external Talbot resonator for mode selection. The highest output power achieved was 115 W with an overall 61% slope efficiency corresponding to the pump power. The measured M2 was 1.43 for the central lobe with nearly 70% of the total power.

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We demonstrate, for the first time to our knowledge, fast all-optical switching in standard silica fibers, based on a transient Bragg grating. The grating is implemented in the fiber using an immunization photo-pretreatment process, followed by side illumination with femtosecond laser pulses through a suitable phase mask. Each pulse is nonlinearly absorbed, creating a thermal grating that is washed out by thermal diffusion. Reflections measured from such gratings are characterized by a very fast rise time, nanoseconds duration, and a high extinction ratio.

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Peripheral arterial disease (PAD), caused by atherosclerotic processes, is allied with an increased risk of ischemic events, limb loss, and death. Recently, the use of a solid-state laser at 355 nm within a hybrid catheter was suggested for that purpose. In this work, short nanosecond pulses of a solid-state laser at 355 nm delivered through a hybrid catheter, composed of optical fibers and a blunt mechanical blade, are used to conduct a pre-clinical study and two clinical cases. The pre-clinical study consisted of an atherosclerotic calcified cadaveric leg and a porcine in vivo trial within the iliac artery, respectively. The clinical cases include chronic total occlusions with a calcified lesion. The occluded cadaveric leg is recanalized successfully and no evidence of thermal necrosis is indicated in the histopathology analysis of the porcine study. No arterial wall damage is demonstrated on the animals' treated arteries and no significant impact on blood count and biochemistry analysis is noted in the animal trial. Successful recanalization of the occluded arteries followed by balloon angioplasty is obtained in both clinical cases. Our work constitutes a proof of concept for using a solid-state pulsed laser at 355 nm in atherectomy.

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We investigate the influence of the cladding diameter of an optical delivery fiber on the ablation dynamics of porcine aorta immersed in tetracycline antibiotic solution using 355 nm nanosecond pulses. We manipulate the pressure transients by enforcing a rear rigid interface (applied by an enlargement of the cladding diameter) to the ablated area, which leads to enhanced ablation efficiency along with a reduction in tissue disruption effects. Numerical simulations, based on the finite elements method, are used to study the propagation of the pressure transients within the suggested scheme. Ultrasonic transducers are used for measuring the increased pressure in front of the fiber's facet and the reduced pressure at the fiber's circumference in the presence of large diameter cladding. The increase and decrease are both found to be by a factor of ˜1.8. The width of the cavitation bubble is measured by high-speed photography. An enlargement of 13.8% is demonstrated, at the expense of backward expansion along the fiber's axis. A histopathological in vitro study demonstrates an average enhancement of 12.27% in the diameter of the ablated crater, as well as significant reduction in the disruption effects. Our study sheds light on the potential to improve the ablation efficiency without additional energy cost, along with attaining improved safety for interventional medical procedures.

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Two slightly shifted gratings are inscribed, one over the other, in an SMF fiber with a femtosecond laser and a phase mask. The transmission spectrum of the complex structure is similar to that of a phase-shifted grating; yet, the fabrication process is fast and simple compared to standard methods. High-quality semi-phase-shifted gratings with -24 dB transmission loss and <100 pm transmission bandwidth are presented. Their application as highly narrow micro-resonators and notch filters seems feasible.

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Pre- and post-treatment of optical fibers is typically used to improve the fiber Bragg grating (FBG) fabrication process. Here, we investigate experimentally the effects of femtosecond photo-treatment on the ability to inscribe and erase FBGs in standard, non-sensitized, silica SMF fibers. We observe "immunity" to inscription after applying a suitable pre-treatment to the fiber and full "erasure" of the FGB after applying a suitable post-treatment. We characterize the required photo-treatment parameters and compare to FBG inscription on an untreated fiber. We believe that pre/post-photo-treatment of the fibers with fs pulses may have practical advantages such as modifying standard grating structures or observing ultrafast transient effects more clearly.

RESUMEN

BACKGROUND: Current lead extraction (LE) devices can harm the blood vessel endothelium, increasing the risk of perforation. OBJECTIVE: Proof of concept for using a solid-state pulsed laser at 355 nm with a hybrid catheter in LE. METHODS: A hybrid catheter was used comprising optical fibers for the delivery of 355 nm laser pulses at 30 Hz and 6 J/cm(2) combined with a blunt mechanical blade. Specific parameters were chosen to enable selectivity in ablation, thereby reducing the risk of blood vessel perforation. The design exploits differences in the mechanical properties of the fibrotic tissue and the normal blood vessel. Ex vivo ablation was performed to evaluate a hybrid catheter operation on various tissues. Two ex/in vivo pig studies used a free-floating electrode to which three bovine tendon pieces were glued. Finally, two in vivo dog model studies were performed on specimens with 4-5-year-old pacing lead implants, followed by a histopathology study. RESULTS: Catheter penetration rate in the ex vivo experiments was 0.1 mm/seconds for bovine tendon, 0.025 mm/seconds for porcine superior vena cava and 0.033 mm/seconds for porcine aorta. In the ex/in vivo pig study, the three tissue blocks were successfully dissected. In the in vivo dog study, the two leads were successfully extracted. In all in vivo tests, hemodynamic stability was maintained. Gross histopathology did not reveal any injury. CONCLUSIONS: Ablation using 355 nm laser pulses in combination with a mechanical blunt blade may potentially constitute a viable alternative for LE.

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We investigate an innovative multipass-pumping technique in a short photonic crystal fiber laser. The technique is based on simple optical elements and shows, in a power oscillator configuration, highly efficient CW and Q-switched operation of an extremely short fiber laser. We report significantly higher absorption of the pump light, compared to a single-pass configuration, which results in an impressive fourfold increase in slope efficiency, up to 60%. Moreover, we show pulse energy of 1.22 mJ and pulse duration shorter than 12 ns, corresponding to a peak power of more than 100 kW.