Cw Laser Research Articles

Distributed Temperature Sensing through Network Analysis Frequency-Domain Reflectometry.

In this paper, we propose and demonstrate a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed temperature measurements at high spatial (down to ≈3 cm) and temperature resolution. The system makes use of a frequency-stepped, continuous-wave (cw) laser whose output light is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude of the beat signal formed by coherently mixing the Rayleigh backscattered light with a local oscillator. The system is capable of attaining high measurand resolution (≈50 mK at 3-cm spatial resolution) thanks to the high sensitivity of coherent Rayleigh scattering to temperature. Furthermore, unlike the conventional optical-frequency domain reflectometry (OFDR), the proposed system does not rely on the use of a tunable laser and therefore is less prone to limitations related to the laser coherence or sweep nonlinearity. Two configurations are analyzed, both numerically and experimentally, based on either a double-sideband or single-sideband modulated probe light. The results confirm the validity of the proposed approach.

Cladding-pumped laser and amplifier for E- and S-bands based on multimode bismuth-doped graded-index fibers: toward "watt-level" output power.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

Surface roughness and microhardness improvement of laser cladding stainless-steel 316 by laser polishing based on multiple remelting

Laser polishing (LP) has emerged as a promising alternative to traditional post-treatment technologies. Nonetheless, the LP process encounters challenges when dealing with surfaces of high roughness, since the excessive thermal input could induce undesirable effects, including the formation of mid-frequency waviness, expansion of heat-affected zone (HAZ), etc. In this study, we investigated the surface roughness and the characters of the remelted layer and HAZ of polishing the laser-cladding 316 stainless steel layer deposited on the surface of a 45# steel cylinder by a 1064 nm hundred-watt pulsed laser and a 1080 nm thousand-watt CW laser respectively. It is observed that the combination of low power density and extended interaction time failed to produce minimum roughness by the pulse laser, contradicting existing LP theories. To address this, we developed a multi-physics model to elucidate the underlying mechanisms and propose an LP scheme based on multiple remelting cycles with the pulsed laser. The Sa roughness is reduced from 20.958 μm to 0.242 μm, decreasing 98.8 %. The processing quality is equivalent to the surface of the high-power CW laser (Sa of 0.283 μm), while the remelted layer and HAZ are significantly smaller. The pulse laser polishing based on multiple remelting increased the surface microhardness by 22 % to 560 HV, while the CW laser remelting reduced it by 33 % to 309 HV. And the HAZ microhardness all decreased in all cases. This study can provide a theoretical basis and solution for the surface finishing of additive manufacturing parts.

Optical Manipulation of Fibroblasts with Femtosecond Pulse and CW Laser

Using tight focusing light, optical tweezers (OT) are tools that can manipulate and capture microscopic particles and biological cells as well as characterize a wide range of micro and nanomaterials. In this paper, we focused on fibroblasts, which are widely used in the biomedical area for a variety of purposes, including promoting human wound healing and preventing the early proliferation of tumor cells. We first built an optical tweezer experimental platform, using an 808 nm continuous-wave laser as the capture light source, to confirm that the device can precisely control the movement of single or multiple particles as well as fibroblasts. Then, a 1030 nm femtosecond laser was employed as the capture light source to study the manipulation of microparticles and fibroblasts at different powers. Lastly, a protracted manipulation protocol was used to prevent the fibroblasts from adhering to the wall. This method can be used to isolate and precisely block adherent growth of fibroblasts in cell populations. This experimental result can be further extended to other biological cells.

Vacuum partial pressure measurement using low-budget dual comb system

Dual-comb spectroscopy is a powerful tool for gas sensing but its expensive cost is limiting its practical application in many fields. In terms of this issue, we present a low-cost and effective dual-comb generation method without complicated feedback lock electronics and nonlinear optics. The proposed dual combs are seeded with the same CW laser, and modulated by electric-optic modulators driven by a pair of pulse trains with slightly different repetition rates of 200 MHz and 200.25 MHz to generate tooth resolved spectrum. A pair of vacuum partial pressure measurements were performed to demonstrate its feasibility and ability. In the experiment, the built dual-comb apparatus is applied to analyze two standard gas mixtures containing carbon monoxide and carbon dioxide respectively. The relative deviations of measurement results against their certified values for carbon monoxide and carbon dioxide are both below 2%. Furthermore, by averaging a set of measurement results at different wavenumbers, the deviation could further be reduced, indicating that the proposed manner is capable of enabling gas partial pressure measurement with good accuracy and low cost.

NaSr3Be3B3O9F4 crystal growth and continuous-wave laser generation at 266 nm

In this paper, we have grown a large NaSr3Be3B3O9F4 (NSBBF) crystal with a top-seeded solution method and generated a continuous-wave (cw) laser radiation at 266 nm with a Brewster-cut NSBBF. With a high-power cw 532 nm and a frequency doubling stage, up to 120 mW cw UV laser was generated at 266 nm. A high stability at 50 mW was obtained more than 10 h. This cw UV laser source is applicable for self-calibrating solar irradiators.

Multi-kilowatt cw laser power measurement comparison between national standards

We present here the first comparison between National Metrology Institutes of high accuracy continuous wave optical power measurements in the kilowatt regime. The National Institute of Standards and Technology (NIST) performed measurements with a power meter relying on photon momentum. The Physikalisch-Technische Bundesanstalt (PTB) performed measurements with a modified off-the-shelf thermal power meter. The non-absorbing photon momentum measurement approach permits the two power meters to measure the same laser beam optical path simultaneously, resulting in a direct comparison of the meters supported by an optical system to accommodate differences in instrument settling times. The results show agreement within the expanded uncertainties for each instrument. NIST and PTB illustrate a degree of equivalence of 0.49% with an expanded uncertainty of 1.37% (k = 2) for an average result across all power levels.

Sonochemical‐Promoted Multicomponent Synthesis of a New Amidoalkyl Naphthol Derivative Catalyzed by [(Msim)Cl] Ionic Liquid and Assessment of Its Nonlinear Optical Properties

AbstractThe sonochemical‐assisted multicomponent reaction catalyzed by ionic liquid, solvent‐free synthesis, improving chemical structure, computational calculations, and nonlinear optical properties (NLO) assessment of a new amidoalkyl naphthol derivative 4, that is, [N‐((2‐hydroxynaphthalen‐1‐yl)(4‐nitrophenyl)methyl)‐2‐phenylhydrazinecarbothioamide (C24H20N4O3S), are introduced. The corrected structure of the synthesized derivative 4 is confirmed by different spectroscopic techniques viz., (1H and 13C) NMR, Infrared radiation (IR), Mass spectra, and UV‐vis. spectra. To obtain geometrical structure and confirm the experimental findings, DFT‐assisted B3LYP/6‐31G* method is used. The NLO properties of the synthesized derivative are studied using two visible laser beams where its nonlinear refraction index (NLRI) at 473 nm cw laser beam is obtained using diffraction patterns (DPs) and the Z‐scan techniques. As high as 5.312×10−11 m2/W of NLRI is obtained. The all‐optical switching (AOS) which is usually carried out utilizing two laser beams, the controlling and controlled, is generalized by the use of three laser beams, controlling, 473 nm beam and two controlled 532 nm and 635 nm beams.

Fs Laser Patterning of Amorphous As2S3 Thin Films.

This study investigates the morphological changes induced by femtosecond (fs) laser pulses in arsenic trisulfide (As2S3) thin films and gold-arsenic trisulfide (Au\As2S3) heterostructures, grown by pulsed laser deposition (PLD). By means of a direct laser writing experimental setup, the films were systematically irradiated at various laser power and irradiation times to observe their effects on surface modifications. AFM was employed for morphological and topological characterization. Our results reveal a clear transition threshold between photoexpansion and photoevaporation phenomena under different femtosecond laser power regimes, occurring between 1 and 1.5 mW, irrespective of exposure time. Notably, the presence of a gold layer in the heterostructure minimally influenced this threshold. A maximum photoexpansion of 5.2% was obtained in As2S3 films, while the Au\As2S3 heterostructure exhibited a peak photoexpansion of 0.8%. The study also includes a comparative analysis of continuous-wave (cw) laser irradiation, confirming the efficiency of fs laser pulses in inducing photoexpansion effects.

Study on multi-scale ablation behavior of C/SiC composites under high-energy CW laser irradiation

This work conducts ablation experiments and multi-scale numerical simulations of C/SiC composite materials under CW laser action. The highest surface temperature, macroscopic and microscopic ablation morphology, and approximate ablation threshold for C/SiC composites at different power densities were obtained. A multi-scale ablation calculation method for C/SiC composite materials under CW laser action was established using the level-set method. At the meso-scale, a macro-mesoscopic cross-scale surface ablation model was established, taking into account factors such as solid heat transfer, heat exchange of chemical reactions, differential temperature rise process between internal fibers and matrix, and differences in ablation behavior of two-dimensional plain weave fabrics. The experimental and microscale simulation results indicate that different ablation behaviors (oxidation and sublimation) occur in different regions, resulting in “shoot tip” fibers and residual matrix pore morphology. This work can provide theoretical and experimental basis for the design and precision machining of C/SiC materials.

Silver(I)-induced anisotropy in the plasmon-driven growth of nanoscale gold

ABSTRACT The plasmon-mediated synthesis of anisotropic gold nanoparticles (AuNPs) has only recently been investigated and achieved. This process typically utilizes the surface plasmon resonances (SPRs) of the nanoparticles themselves to drive metal deposition and growth. Many of the traditional methods for shape control, including use of functional surfactants and etchants, have yet to be utilized within the context of light-driven synthesis. Here, the underpotential deposition of Ag(I) is utilized as a shape-controlling strategy in a hot electron-driven photochemical synthesis to grow anisotropic gold nanoparticles. The strategy also relies on L-pyroglutamic acid, a natural amino acid metabolite, as a photochemical relay aiding in the transport and localization of plasmon-generated hot electrons. By carefully adjusting reagent concentrations, the morphologies of the resulting nanoparticles can be facilely tuned from spheres to nanostars and nanoflowers. The high extinction in the NIR region of the ‘ideal’ gold nanostars synthesized advocates their use as photothermal therapy agents, supported by a sufficient photothermal conversion efficiency (η = 30.8%) under 808 nm CW laser irradiation. This contribution expands the repertoire of gold nanostructures accessible to visible-light-driven plasmonic photochemistry. Different gold nanoparticle shapes have been accessed using a plasmon-mediated growth technique. Ag(I) ions induce symmetry breaking, yielding nano-sized gold spheres, stars, and flowers. A specialized photoreactor, equipped with 505 nm LED illumination, facilitates the transfer of hot electrons from a pre-made gold nanocube seed surface. Gold reduction and deposition occur, and anisotropic structures grow with the aid of L-pyroglutamic acid and Triton X-100 as coordinating and shape-directing agents.

Enhanced optical third-harmonic generation in phase-engineered nanostructured Zn1−x Cd x S thin films for optoelectronic device applications

A polycrystalline nanostructured thin film of zinc cadmium sulfide was meticulously fabricated on a glass substrate using the thermal evaporation method physical vapor deposition within a vacuum chamber. Different doping concentrations were introduced by varying the cadmium (Cd) content, resulting in Zn1-x Cd x S films with Cd concentrations ranging from x = 0.00–0.20 wt %. The impact of Cd doping on the third-order nonlinear optical (TONLO) properties of these films was thoroughly studied using the Z-scan method, employing a diode-pumped solid-state continuous-wave laser. To gain insight into the structural characteristics, the Zn1-x Cd x S thin films underwent analysis through x-ray diffraction. Optical studies confirmed the tunability of the optical band gap (Eg) in the Zn1-x Cd x S films, ranging from 3.88 eV for undoped ZnS to 2.80 eV for the film fabricated with 20 wt. % of Cd-content. This significant reduction in ‘Eg’ renders the films highly suitable for use as absorbing layers in applications such as solar cells and optoelectronics. Surface morphology analysis, performed via field emission scanning electron microscopy, revealed noticeable alterations with increased Cd doping. Significantly, the doped films exhibited a substantial redshift in the band edge and an increase in transmittance within the visible and near-infrared regions. The investigation of TONLO properties, including the nonlinear absorption coefficient (β), nonlinear refractive index (n 2) and susceptibility χ(3), yielded values ranging from 3.15 × 10−3 to 8.16 × 10−3 (cm W−1), 1.65 × 10−8 to 7.45 x 10–8 (cm2 W−1), and 3.12 × 10−5 to 7.86 × 10−5 (esu), respectively. These results indicate the presence of self-defocusing nonlinearity in the films. Overall, the outcomes underscore the potential of Cd-doped ZnS nanostructures in modifying surface morphology and enhancing NLO characteristics. Zn1-x Cd x S thin films exhibit promise for applications in nonlinear optical devices, as evidenced by these encouraging findings.

Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses.

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35W) limited the cw powers we could achieve experimentally to 2.25W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10W level.

Highly efficient, powerful Nd:YAG laser CW mode-locked with a monolayer CVD graphene saturable absorber mirror

Graphene-mode-locked solid-state lasers usually have a low optical efficiency (&lt;10%) or a low output average power (&lt;1W) due to losses in the graphene saturable absorber and the thermal load of the laser crystal. We demonstrate what we believe to our knowledge is an improved design for a highly efficient, powerful, diode-end-pumped Nd:YAG laser mode-locked with graphene. Internal losses of the laser cavity were minimized by using a high-quality monolayer graphene saturable absorber applied directly on the cavity end mirror. In comparison to other graphene-mode-locked solid-state lasers, the record high optical-to-optical and slope efficiencies of 19.1 and 24.5%, respectively, in a continuous wave mode-locking regime were achieved. The highest output average power was as high as 1.7 W for the ultrashort pulse repetition rate of 82 MHz at the beam quality of M2=1.3×1.4.

Continuous-wave pumped dispersion-compensated ultrashort-pulse doubly resonant oscillator studies

We have reported theoretical analysis of broadband phase-coherent spectral generation with stable short pulse output based on optical parametric oscillator (OPOs) pumped by low-cost single-mode continuous-wave (cw) laser considering the incorporation of mode-locking element in the cw dispersion-compensated OPO in doubly-resonant oscillator (DRO) configuration. Exploiting the control of dispersion in the cw mode-locked cavity, the intracavity round-trip phase shift, phase coherence among resonating waves, carrier envelope offset phase of mode-locked output and enhancement of efficiency of system using active mode-locking element driven at high modulation frequency has been studied. The study paves the way of realization of compact, low-cost, efficient ultrashort pulse OPO under cw pumping.

Laser isotope separation of 203Tl through pulsed laser optical pumping

Isotope separation of 203Tl through pulsed laser optical pumping into the 6s26p2P3/2∘ meta-stable state, followed by pulsed laser ionization, has been theoretically studied through density matrix formalism. The efficiency of the isotope separation process has been compared with the cw laser optical pumping followed by pulsed laser photoionization reported earlier. The effect of peak power density, bandwidth and number density of atoms on the degree of enrichment and the production rate has been systematically studied. The optimum parameters for the efficient laser isotope separation 203Tl have been derived. It has been shown that using this method, it is possible to produce about 1.5 kg/y of 203Tl isotope enriched to 90%.

Third order nonlinearity examined by pulsed and CW lasers: an organic urea barbituric acid (UBA) single crystal for optical limiting application with DFT study

Bulk size of urea barbituric acid single crystal which has not been reported earlier was successfully grown by slow evaporation solution growth method. Single-crystal XRD brought out the lattice constant and the crystal system is observed to be monoclinic with space group P. Functional group of UBA were determined by FT-IR. UBA crystal is entirely visible from 270 to 900 nm with linear optical energy gap value to be 4.50 eV. The observed HOMO–LUMO energy gap was 5.21 eV. Thermally, UBA crystal is found to be stable up to 184˚°C. The Cp value of UBA increases from 1.21 to 1.58 J g−1K−1 with the temperature difference from 30 to 100˚°C. UBA shows good photoconductive nature and it found to be positive and Laser Damage Threshold (LDT) value of UBA crystal is 0.97 GW cm−2. Hardness testing confirms that UBA crystal belongs to soft nature category. Dielectric properties of UBA crystal are studied as a function of frequency and temperature. Third order NLO properties of the UBA crystal were studied under CW and pulsed lasers (ns) regimes using Z-scan technique. Good Optical limiting (OL) response (2.2860 х 1012 W/m2) confirms the efficiency of UBA crystal to be used as optical limiters for protection towards short pulse lasers.

Experimental study on back-illuminated CMOS image sensor irradiated by CW laser

Experimental study on back-illuminated CMOS image sensor irradiated by CW laser

Photo‐Induced Evolution of Randomly Rough Surfaces of Amorphous Chalcogenide Films

Photoinduced (PI) evolution of statistically rough surfaces of amorphous chalcogenide films As20Se80 at room temperature has been studied by measuring the angular dependence of the intensity of light scattered from a surface illuminated by CW laser (λ = 660 nm). The interpretation of the scattering data based on the resonant scattering theory enables to confirm unequivocally the diffusion mechanism of PI mass transfer. It is detected that the change of the amplitude of a spatial harmonic in the roughness spectra strongly depends on its period ∧ . During illumination, the amplitude increases at ∧ &gt; ∧*, whereas harmonics with ∧ &lt; ∧* decreases by ∧*, which corresponds to zero evolution rate, is found to be 6.7 μm. In accordance with our theoretical prediction, both growth and decrease are exponential with the rates depending on ∧. As the result, the roughness with initial rms height of 50–70 nm transforms into quasiperiodic surface grating with the average amplitude of about 400 nm and average period close to 15 μm. From the kinetics of time variation of the scattered intensity, the PI diffusion coefficient D is calculated. When the laser intensity changes from 5.6 to 14 W cm−2, D is found in the range 1 × 10−13–3.4 × 10−13 m2 s−1.

Fully Optical Scanning Spectroscopy of the Anticrossing of Electron and Nuclear Spin Levels in a 4H-SiC Crystal

Transitions in a system of interacting electron and nuclear spins in color centers with S = 3/2 in a 4H-SiC crystal with the natural isotopic composition have been detected by fully optical methods at room temperature. Giant changes in the photoluminescence in a volume of about 1 μm3 under cw and pulsed laser excitation occur in the region of the anticrossing of electron and nuclear spin levels. An optical manifestation of the flip of the nuclear spin of the 29Si isotope with the conservation of the projection of the electron spin has been detected. All anticrossing points of the spin sublevels coupled by hyperfine interactions have been identified. This identification enables the observation of such effects in the family of quarter spin centers in other SiC polytypes.