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We report on an approach for a compact ultrafast thin-disk multipass laser amplifier making use of a highly compact geometric folding scheme. The setup is also suitable to minimize the effects caused by natural convection of hot air in front of the thin-disk on the amplified laser beam as it facilitates to orient the laser disk with its axis in the vertical direction. The efficacy of this approach is analyzed with finite-element method simulations of the heated laser crystal in ambient air with different orientations of the thin laser disk. The experiments confirm a significant improvement of the amplifier performance in terms of stability and an increase of the output power with nearly diffraction-limited beam quality (M² ≤ 1.4) by a factor of 3 with respect to the conventional orientation of the laser disk. [ABSTRACT FROM AUTHOR]

We investigate an optical system for the absolute distance measurement of objects with rough surfaces, using the principle of asynchronous optical sampling (ASOPS). A telescope system collects the scattered laser light that returns from the rough surface and directs it into a collimator with high precision. The contrastive absolute distance measurement experiments are carried out using a plane mirror and objects with rough surfaces. The residual error is kept within 10 μm within a 2-ms sampling time for the rough surfaces. Our results indicate that this ASOPS method is a promising candidate for metrological applications in some industrial sites. [ABSTRACT FROM AUTHOR]

Ultrafast laser ablation may provide a treatment for vocal fold (VF) scarring. Optical properties of VFs must be known prior to clinical implementation to select appropriate laser surgery conditions. We present scattering lengths of epithelium ls,ep, superficial lamina propria ls;SLP, and ablation thresholds Fth of human and canine VF tissues. Our experimental approach involves an image-guided, laser-ablation-based method that allows for simultaneous determination of ls and Fth in these multilayered tissues. Studying eight canine samples, we found ls;ep ¼ 75.3± 5.7 μm, ls;SLP ¼ 26.1±1.2 μm, Fth;ep ¼ 1.58±0.06 J/cm2, and Fth;SLP ¼ 1.55±0.17 J/cm2. Studying five human samples, we found ls;ep ¼ 42.8±3.3 μm and Fth;ep ¼ 1.66±0.10 J/cm2. We studied the effects of cumulative pulse overlap on ablation threshold and found no significant variations beyond 12 overlapping pulses. Interestingly, our studies about the effect of sample storage on the scattering properties of porcine VF show a 60% increase in ls;ep for fresh porcine VF when compared to the same sample stored in isotonic solution. These results provide guidelines for clinical implementation by enabling selection of optimal laser surgery parameters for subsurface ablation of VF tissues. [ABSTRACT FROM AUTHOR]

Organic materials are attracting attention for their potential applications due to their large third-order optical nonlinearities. A passive fiber Q-switching laser has been demonstrated based on an erbium/ytterbium codoped amplifier and an acetone solution saturable absorber (SA). When pump power was increased from 120 to 155 mW, the repetition rate increased from 5.68 to 16.67 kHz, while the pulse width decreased from 24.1 to 15 µs. These results indicate that acetone solution can be used as an effective SA for pulsed lasers. [ABSTRACT FROM AUTHOR]

Conventional temporal focusing-based multiphoton excitation microscopy (TFMPEM) can offer widefield optical sectioning with an axial excitation confinement of a few microns. To improve the axial confinement of TFMPEM, a binary computer-generated Fourier hologram (CGFH) via a digital-micromirror-device (DMD) was implemented to intrinsically improve the axial confinement by filling the back-focal aperture of the objective lens. Experimental results show that the excitation focal volume can be condensed and the axial confinement improved about 24% according to the DMD holography. In addition, pseudouniform MPE can be achieved using two complementary CGFHs with rapid pulse-width modulation switching via the DMD. Furthermore, bioimaging of CV-1 in origin with SV40 genes-7 cells demonstrates that the TFMPEM with binary DMD holography can improve image quality by enhancing axial excitation confinement and rejecting out-of-focus excitation. [ABSTRACT FROM AUTHOR]

We present an investigation into the characteristics of a normal dispersion mode-locked Yb-doped fiber laser using a birefringent plate as a spectral filter. Before implementing mode locking using a birefringent plate, the transmission characteristics of the birefringent plate were analyzed. It was observed that the center wavelength and the full width at half maximum (FWHM) of the transmitted light varied according to the azimuthal angle and optical axis of the birefringent plate. The total dispersion value of the laser cavity was 0.04673 ps² at a center wavelength of 1.03 μm. The mode locking was therefore achieved in the normal-dispersion region. The total cavity length of the Yb-doped fiber laser was about 1.9 m including free space, and the repetition rate of the mode-locked pulse was ~104.2 MHz. The FWHM of the optical spectrum was ~18.09 nm, and the pulse width measured by an autocorrelator was ~1.71 ps assuming a Gaussian pulse shape. To obtain the transform limited pulse, a pulse was compressed using a pair of diffraction gratings outside the cavity. We achieved a compressed pulse of ~148 fs assuming a Gaussian pulse shape. The output characteristics of the mode-locked Yb-doped fiber laser were observed while rotating the optical axis of the birefringent plate in the laser cavity. It was found that the center wavelength and FWHM of the optical spectrum could be varied by rotating the optical axis of the birefringent plate. The center wavelength changed by about 18.7 nm from 1026.9 to 1045.6 nm during rotation of the optical axis of the birefringent plate. [ABSTRACT FROM AUTHOR]

We had proved that a kind of graphene nanomaterial “carboxyl-functionalized graphene oxide (GO-COOH)” possessed nonlinear saturable absorber (SA) property. The modulation depth of a GO-COOH water solution was measured as ∼8%. Moreover, a GO-COOH-based SA device was made and applied in an erbium-doped fiber laser. In this fiber laser, Q-switching pulses and mode-locked pulses were both obtained. With an increase in the pump power, the pulse width of Q-switching pulses decreased from 9.05 to 2.49  μs. The mode-locked pulse width was 844 fs, and the corresponding spectral bandwidth was 3.64 nm. Moreover, polarization adjusting or control was not needed during the whole process of mode locking. It illustrated that the proposed fiber laser incorporating GO-COOH could endure bigger intracavity birefringence. Our results indicated that the GO-COOH nanomaterial was a promising SA for generating high-performance pulse lasers. [ABSTRACT FROM AUTHOR]

When ultrashort laser pulses are focused inside a glass at a high repetition rate, structural changes occur because of the heat accumulation from the train of laser pulses. This report describes phase measurement of the structural changes induced inside glass by phase-shifting digital holographic microscopy. Two-dimensional phase distribution across the structural change for static exposure is retrieved. By focusing femtosecond laser pulses at the interface between two glass substrates, melted materials can directly weld glass plates. Quantitative phase measurement of welded glass substrates revealed that the refractive index decreased in the laser-irradiated zone. [ABSTRACT FROM AUTHOR]

Soliton self-frequency shift (SSFS) is a nonlinear effect that generates wavelength-shifted laser pulses in optical fibers. Based on SSFS, we report the generation of ultrashort pulses emitting at 1.7-µm band in a common dispersion shifted fiber (DSF) pumped by an ultrafast Er-doped fiber laser. Through optimizing the DSF length, sub-200 fs pulses in the range from 1.70 to 1.74 µm have been achieved with >76% conversion efficiency. The maximum output power is 26.8 mW and the single pulse energy is up to 0.7 nJ. Numerically, simulations have been carried out to map the SSFS pulse evolution in DSF. The simulation results are in a good agreement with the experiment results, giving the feasibility of this method. [ABSTRACT FROM AUTHOR]

The present study describes utilization of two photon excitation fluorescence (2PE) microscopy for visualization of the hemoglobin in human and porcine erythrocytes and their empty membranes (i.e., ghosts). High-quality, label- and fixation-free visualization of hemoglobin was achieved at excitation wavelength 730 nm by detecting visible autofluorescence. Localization in the suspension and spatial distribution (i.e., mapping) of residual hemoglobin in erythrocyte ghosts has been resolved by 2PE. Prior to the 2PE mapping, the presence of residual hemoglobin in the bulk suspension of erythrocyte ghosts was confirmed by cyanmethemoglobin assay. 2PE analysis revealed that the distribution of hemoglobin in intact erythrocytes follows the cells' shape. Two types of erythrocytes, human and porcine, characterized with discocyte and echinocyte morphology, respectively, showed significant differences in hemoglobin distribution. The 2PE images have revealed that despite an extensive washing out procedure after gradual hypotonic hemolysis, a certain amount of hemoglobin localized on the intracellular side always remains bound to the membrane and cannot be eliminated. The obtained results open the possibility to use 2PE microscopy to examine hemoglobin distribution in erythrocytes and estimate the purity level of erythrocyte ghosts in biotechnological processes. [ABSTRACT FROM AUTHOR]

We present a numerical model of plasma formation in ultrafast laser ablation on the dielectrics surface. Ablation threshold dependence on pulse duration is predicted with the model and the numerical results for water agrees well with the experimental data for pulse duration from 140 fs to 10 ps. Influences of parameters and approximations of photo- and avalanche-ionization on the ablation threshold prediction are analyzed in detail for various pulse lengths. The calculated ablation threshold is strongly dependent on electron collision time for all the pulse durations. The complete photoionization model is preferred for pulses shorter than 1 ps rather than the multiphoton ionization approximations. The transition time of inverse bremsstrahlung absorption needs to be considered when pulses are shorter than 5 ps and it can also ensure the avalanche ionization (AI) coefficient consistent with that in multiple rate equations (MREs) for pulses shorter than 300 fs. The threshold electron density for AI is only crucial for longer pulses. It is reasonable to ignore the recombination loss for pulses shorter than 100 fs. In addition to thermal transport and hydrodynamics, neglecting the threshold density for AI and recombination could also contribute to the disagreements between the numerical and the experimental results for longer pulses. [ABSTRACT FROM AUTHOR]

Increasing the laser-induced damage resistance of optical components is one of the major challenges in the development of Peta-watt (PW) class laser systems. The extreme light infrastructure (ELI) beamlines project will provide ultrafast laser systems with peak powers up to 10 PW available every minute and PW class beams at 10 Hz complemented by a 5-TW, 1-kHz beamline. Sustainable performance of PW class laser systems relies on the durability of the employed optical components. As part of an effort to evaluate the damage resistance of components utilized in ELI beamlines systems, damage thresholds of several optical multilayer dielectric coatings were measured with different laser parameters and in different environments. Three coatings were tested with 10 Hz and 1 kHz pulse repetition rates, and the effect of a cleaning treatment on their damage resistance was examined. To explore the damage threshold behavior at different vacuum levels, one coating was subject to tests at various residual gas pressures. No change of damage threshold in a high vacuum with respect to ambient pressure was recorded. The effect of the cleaning treatment was found to be inconsistent, suggesting that development of the optimal cleaning treatment for a given coating requires consideration of its specific properties. [ABSTRACT FROM AUTHOR]

Ultrafast laser surgery of tissue requires precise knowledge of the tissue's optical properties to control the extent of subsurface ablation. Here, we present a method to determine the scattering lengths, ls, and fluence thresholds, Fth, in multilayered and turbid tissue by finding the input energies required to initiate ablation at various depths in each tissue layer. We validated the method using tissue-mimicking phantoms and applied it to porcine vocal folds, which consist of an epithelial (ep) layer and a superficial lamina propia (SLP) layer. Across five vocal fold samples, we found ls;ep = 51.0 ± 3.9 µm, Fth;ep = 1.78 ± 0.08 J/cm², ls;SLP = 26.5 ± 1.6 µm, and Fth;SLP = 1.14 ± 0.12 J/cm2. Our method can enable personalized determination of tissue optical properties in a clinical setting, leading to less patient-to-patient variability and more favorable outcomes in operations, such as femto-LASIK surgery. [ABSTRACT FROM AUTHOR]

We propose two schemes for achieving tungsten disulfide (WS2)-based saturable absorber (SA) and saturable absorber mirror (SAM). By utilizing the pulsed laser deposition method, we grow the WS2 film on microfiber to form an evanescent field interaction SA device. Incorporating this SA device into a common ring-cavity erbium-doped fiber (EDF) laser, stably passive mode-locking can be achieved with pulse duration of 395 fs and signal-to-noise ratio of 64 dB. We also produce a fiber tip integrated WS2-SAM by utilizing the magnetron sputtering technique (MST). This new type of SAM combines the WS2 layer as SA and gold mirror as high reflective mirror. By employing the WS2-SAM, we construct the linear-cavity EDF lasers, and achieve passive mode-locking operation with pulse duration of ~1 ns and SNR of ~61 dB. We further achieve stably passive Q-switching operation with pulse duration of ~160 ns and pulse energy of 54.4 nJ. These fiber-integrated SAs and SAMs have merits of compactness and reliability, paving the way for the development of new photonic devices such as SAs for pulsed laser technology. [ABSTRACT FROM AUTHOR]

We report on a method to enhance the temporal contrast of optical parametric chirped-pulse amplifiers (OPCPAs) by smoothing pump noise. The instantaneous parametric gain in OPCPA couples the temporal modulation on the pump pulses to spectral variations of the intensity of the stretched signal pulses being amplified. In this way, pump noise significantly degrades the temporal contrast of the amplified pulses after recompression. Cascaded second harmonic generation (SHG) is adopted to smooth modulation on the pump pulses in the proposed method. Apparent reduction of modulation on the pump pulses is observed in the experiments. Numerical simulation reproduces the experimental results. Simulation results show that cascaded SHG with stable output 2ω can enhance the temporal contrast for OPCPAs with four to five orders. It is believed that this new method can be widely adopted to build high-contrast OPCPA systems. [ABSTRACT FROM AUTHOR]

Significance: The creation of subepithelial voids within scarred vocal folds via ultrafast laser ablation may help in localization of injectable biomaterials toward a clinically viable therapy for vocal fold scarring. Aim: We aim to prove that subepithelial voids can be created in a live animal model and that the ablation process does not engender additional scar formation. We demonstrate localization and long-term retention of an injectable biomaterial within subepithelial voids. Approach: A benchtop nonlinear microscope was used to create subepithelial voids within healthy and scarred cheek pouches of four Syrian hamsters. A model biomaterial, polyethylene glycol tagged with rhodamine dye, was then injected into these voids using a custom injection setup. Follow-up imaging studies at 1- and 2-week time points were performed using the same benchtop nonlinear microscope. Subsequent histology assessed void morphology and biomaterial retention. Results: Focused ultrashort pulses can be used to create large subepithelial voids in vivo. Our analysis suggests that the ablation process does not introduce any scar formation. Moreover, these studies indicate localization, and, more importantly, long-term retention of the model biomaterial injected into these voids. Both nonlinear microscopy and histological examination indicate the presence of biomaterial-filled voids in healthy and scarred cheek pouches 2 weeks postoperation. Conclusions: We successfully demonstrated subepithelial void formation, biomaterial injection, and biomaterial retention in a live animal model. This pilot study is an important step toward clinical acceptance of a new type of therapy for vocal fold scarring. Future long-term studies on large animals will utilize a miniaturized surgical probe to further assess the clinical viability of such a therapy. [ABSTRACT FROM AUTHOR]

Ultrafast reflection and secondary ablation have been theoretically investigated with a Fresnel-Drude model in laser processing of transparent dielectrics with picosecond pulsed laser. The time-dependent refractive index has a crucial effect on the cascade ionization rate and, thereby, on the plasma generation. The relative roles of the plasma gas and the incident angle in the reflection are discussed in the case of the oblique incidence. The angular dependence of the reflectivity on the laser-excited surface for s- and p-polarization is significantly different from the usual Fresnel reflectivity curve in the low-fluence limit. A road map to the secondary ablation induced by the reflected pulse is obtained on the angles of the first and second incidence. It indicates that the laser-induced plasma plays a major role in the secondary ablation, which could overcome the saturation of the ablation crater depth or generate microcracks underneath the crater wall. [ABSTRACT FROM AUTHOR]

Toward developing a new method for restoring tissue viscoelasticity in scarred vocal folds, we previously proposed a method to localize biomaterials within subepithelial voids ablated using ultrafast laser pulses. The clini- cal implementation of this method necessitates the quantification of the laser parameters for ablating scarred tissue. Here, we present a comprehensive study of these parameters including ablation threshold and bubble lifetime in healthy and scarred tissues. We also present a new method for extracting tissue-specific ablation threshold and scattering lengths of different tissue layers. This method involves finding the ablation threshold at multiple depths and solving the equations based on Beer's law of light attenuation for each depth to estimate the unknown parameters. Measured threshold fluences were 1.75 J/cm² for vocal folds and 0.5 J/cm² for cheek pouches for 3-ps, 776-nm laser pulses. Scarred pouches exhibited 30% lower threshold than healthy pouches, possibly due to the degraded mechanical properties of scarred collagen during wound healing. The analysis of tissue architecture indicated a direct correlation between the ablation threshold and tissue tensile strength and that the bubble lifetime is inversely related to tissue stiffness. Overall, this study sheds light on the required laser parameters for successful implementation of ultrafast laser ablation for phonosurgery. [ABSTRACT FROM AUTHOR]

We demonstrate a saturable absorber (SA) based on cladding-filled graphene in a specially designed and manufactured photonic crystal fiber (PCF) for the first time. The saturation absorption property is achieved through the evanescent coupling between the guided light and the cladding-filled graphene layers. To boost the mutual interaction, the PCF is designed to contain five large air holes in the cladding and small-core region. Employing this graphene-PCF SA device, we construct an erbium-doped all-fiber laser oscillator and achieve mode-locked operation. This device can pave the way for high power and all-fiber applications of photonics with graphene with some unique advantages, such as single-mode operation, nonlinearity enhancement, high-power tolerance, environmental robustness, all-fiber configuration, and easy fabrication. [ABSTRACT FROM AUTHOR]

Calcium fluoride and zinc sulfide crystals exhibit broad multispectral transparency and are used for a multitude of optical applications. However, they both belong to a soft brittle crystal category and are mechanically challenging to process. Thus, a fabrication protocol using a single-point diamond turning method was developed, which can yield surfaces with optical quality for these materials. By using a diamond turning tool with low waviness (≤100-nm waviness) and by using feed rates of 1 and 0.2 μm / rev, CaF2 and ZnS surfaces with surface roughness of 6 and 5 nm were manufactured, respectively. Design and fabrication of a miniature objective with 0.5 NA are also presented, which was developed for the ultrafast laser microsurgery application. The objective consists of CaF2 and ZnS lenses to withstand the high power from ultra-short laser pulses and to provide a tightly focused beam with 1.23 μm 1 / e2 spot radius for laser ablation. Based on the spot size measurement of the objective, its performance was considered to be near diffraction limited. [ABSTRACT FROM AUTHOR]