Picosecond laser cutting and drilling of thin flex glass

We investigate the feasibility of cutting and drilling thin flex glass (TFG) substrates using a picosecond laser operating at wavelengths of 1030 nm, 515 nm and 343 nm. 50 μm and 100 μm thick AF32^®Eco Thin Glass (Schott AG) sheets are used. The laser processing parameters such as the wavelength, pulse energy, pulse repetition frequency, scan speed and the number of laser passes which are necessary to perform through a cut or to drill a borehole in the TFG substrate are studied in detail. Our results show that the highest effective cutting speeds (220 mm/s for a 50 μm thick TFG substrate and 74 mm/s for a 100 μm thick TFG substrate) are obtained with the 1030 nm wavelength, whereas the 343 nm wavelength provides the best quality cuts. The 515 nm wavelength, meanwhile, can be used to provide relatively good laser cut quality with heat affected zones (HAZ) of <25 μm for 50 μm TFG and <40 μm for 100 μm TFG with cutting speeds of 100 mm/s and 28.5 mm/s, respectively. The 343 nm and 515 nm wavelengths can also be used for drilling micro-holes (with inlet diameters of ⩽75 µm) in the 100 μm TFG substrate with speeds of up to 2 holes per second (using 343 nm) and 8 holes per second (using 515 nm). Optical microscope and SEM images of the cuts and micro-holes are presented.     

Multiple-objective optimization in precision laser cutting of different thermoplastics

Thermoplastics are increasingly being used in biomedical, automotive and electronics industries due to their excellent physical and chemical properties. Due to the localized and non-contact process, use of lasers for cutting could result in precise cut with small heat-affected zone (HAZ). Precision laser cutting involving various materials is important in high-volume manufacturing processes to minimize operational cost, error reduction and improve product quality. This study uses grey relational analysis to determine a single optimized set of cutting parameters for three different thermoplastics. The set of the optimized processing parameters is determined based on the highest relational grade and was found at low laser power (200 W), high cutting speed (0.4 m/min) and low compressed air pressure (2.5 bar). The result matches with the objective set in the present study. Analysis of variance (ANOVA) is then carried out to ascertain the relative influence of process parameters on the cutting characteristics. It was found that the laser power has dominant effect on HAZ for all thermoplastics.     

Development of an efficient coherent optical source at 6.04 μm

Efficiency as high as 26% is obtained for generation of mid-infrared radiation at 6.04 μm by frequency doubling of ammonia laser emission at 12.08 μm in a 15 mm long type-I cut AgGaSe₂ crystal. The NH₃ laser used for this work is optically pumped by a commercial TEA CO₂ laser operating on 9.22 μm and produces pulsed output of ∼210 mJ with a duration of ∼200 ns at 12.08 μm. The generated radiation at 6.04 μm is separated out from the residual radiation at 12.08 μm by exploiting the principle of polarization dependent diffraction of reflection grating.     

Influence of the p-type doping on the radiometric performances of MWIR InAs/GaSb superlattice photodiodes

In this paper, quantum efficiency (QE) measurements performed on type-II InAs/GaSb superlattice (T2SL) photodiodes operating in the mid-wavelength infrared domain, are reported. Several comparisons were made in order to determine the SL structure showing optimum radiometric performances: same InAs-rich SL structure with different active zone thicknesses (from 0.5 μm to 4 μm) and different active zone doping (n-type versus p-type), same 1 μm thick p-type active zone doping with different SL designs (InAs-rich versus GaSb-rich and symmetric SL structures). Best result was obtained for the p-type doped InAs-rich SL photodiode, with a 4 μm active zone thickness, showing a QE that reaches 61% at λ = 2 μm and 0 V bias voltage.     

Fast femtosecond laser ablation for efficient cutting of sintered alumina substrates

Fast, accurate cutting of technical ceramics is a significant technological challenge because of these materials' typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material with small kerf and little to no collateral damage to the surrounding material. The main drawback to femtosecond laser machining of ceramics is slow processing speed. In this work we report on the improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with F_th,1=6.0 J/cm² (±0.3) and F_th,∞=2.5 J/cm² (±0.2). The focal length and depth, laser power, number of passes, and material translation speed were optimised for ablation speed and high quality. Optimal conditions of 500 mW power, 100 mm focal length, 2000 µm/s material translation speed, with 14 passes, produced complete cutting of the alumina substrate at an overall processing speed of 143 µm/s – more than 4 times faster than the maximum reported overall processing speed previously achieved by Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby reducing costs, making femtosecond laser machining a more viable option for industrial users.     

Compact sensor for methane detection in the mid infrared region based on Quartz Enhanced Photoacoustic Spectroscopy

A compact system for methane sensing based on the Quartz-Enhanced Photoacoustic Spectroscopy technique has been developed. This development has been taken through two versions which were based respectively on a Fabry Perot quantum wells diode laser emitting at 2.3 μm, and on a quantum wells distributed feedback diode laser emitting at 3.26 μm. These lasers emit near room temperature in the continuous wave regime. A spectrophone consisting of a quartz tuning fork and one steel microresonator was used. Second derivative wavelength modulation detection was used to perform low methane concentration measurements. The sensitivity and the linearity of the QEPAS sensor were studied. A normalized noise equivalent absorption coefficient of 7.26 × 10⁻⁶ cm⁻¹ W/Hz^1/2 was achieved. This corresponds to a detection limit of 15 ppmv for 12 s acquisition time.     

Porous microstructures induced by picosecond laser scanning irradiation on stainless steel surface

A study of porous surfaces having micropores significantly smaller than laser spot on the stainless steel 304L sample surface induced by a picosecond regenerative amplified laser, operating at 1064 nm, is presented. Variations in the interaction regime of picosecond laser pulses with stainless steel surfaces at peak irradiation fluences(F_pk=0.378–4.496 J/cm²) with scanning speeds(v=125–1000 μm/s) and scan line spacings(s=0–50 μm) have been observed and thoroughly investigated. It is observed that interactions within these parameters allows for the generation of well-defined structured surfaces. To investigate the formation mechanism of sub-focus micropores, the influence of key processing parameters has been analyzed using a pre-designed laser pulse scanning layout. Appearances of sub-focus ripples and micropores with the variation of laser peak fluence, scanning speed and scan line spacing have been observed. The dependencies of surface structures on these interaction parameters have been preliminarily verified. With the help of the experimental results obtained, interaction parameters for fabrication of large area homogeneous porous structures with the feature sizes in the range of 3–15 μm are determined.     

CO2 laser cutting of ultra thin (75 µm) glass based rigid optical solar reflector (OSR) for spacecraft application

Present study reports for the first time laser cutting of multilayered coatings on both side of ultra thin (i.e., 75 µm) glass substrate based rigid optical solar reflector (OSR) for spacecraft thermal control application. The optimization of cutting parameters was carried out as a function of laser power, cutting speed and number of cutting passes and their effect on cutting edge quality. Systematic and in-detail microstructural characterizations were carried out by optical and scanning electron microscopy techniques to study the laser affected zone and cutting edge quality. Sheet resistance and water contact angle experiments were also conducted locally both prior and after laser cut to investigate the changes of electrical and surface properties, if any.     

A tunable and single-longitudinal-mode Ho:YLF laser

A 1.94 μm Tm-doped fiber laser pumped tunable single-longitudinal-mode Ho:YLF laser with double etalons was reported for the first time. The maximum single-longitudinal-mode output power of 345 mW at 2051.6 nm was achieved at the absorbed pump power of 11.9 W, corresponding to a slope efficiency of 5.5% and an optical conversion efficiency of 2.9%. By regulating the angle of the F–P etalons, the output wavelength of the laser can be tuned from 2051.6 nm to 2063.3 nm. The single-longitude-mode Ho:YLF laser operating at 2 μm can be used as the seed laser source of coherent Doppler lidar, differential absorption lidar and so on.     

Micromachined silicon lenses for terahertz applications

Silicon microlenses are a very important tool for coupling terahertz (THz) radiation into antennas and detectors in integrated circuits. They can be used in a large array structures at this frequency range reducing considerably the crosstalk between the pixels. Drops of photoresist have been deposited and their shape transferred into the silicon by means of a Reactive Ion Etching (RIE) process. Large silicon lenses with a few mm diameter (between 1.5 and 4.5 mm) and hundreds of μm height (between 50 and 350 μm) have been fabricated. The surface of such lenses has been characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), resulting in a surface roughness of about ∼3 μm, good enough for any THz application. The beam profile at the focal plane of such lenses has been measured at a wavelength of 10.6 μm using a tomographic knife-edge technique and a CO₂ laser.     

Heat affected zone in aluminum single crystals submitted to femtosecond laser irradiations

An experimental procedure is developed to quantify the radial dimension of the heat affected zone (HAZ) in metals submitted to laser pulses. A cube oriented aluminum single crystal is highly deformed by plane strain compression, then micro-drilled by 200 fs or 8 ns laser pulses, and finally analyzed by the electron back scattering diffraction technique. Recrystallized and recovered zones are observed as signatures of the HAZ. A typical value of 1.5 μm is found in the femtosecond regime of illumination, whereas for nanosecond pulses a value of 25 μm is measured. These results are in accordance with previous experiments and numerical simulations.     

Investigation of optimum condition in oxygen gas-assisted laser cutting

Laser cutting characteristics including power level and cutting gas pressure are investigated in order to obtain an optimum kerf width. The kerf width is investigated for a laser power range of 50–170 W and a gas pressure of 1–6 bar for steel and mild steel materials. Variation of sample thickness, material type, gas pressure and laser power on the average cut width and slot quality are investigated. Optimum conditions for the steel and mild steel materials with a thickness range of 1–2 mm are obtained. The optimum condition for the steel cutting results in a minimum average kerf width of 0.2 mm at a laser power of 67 W, cutting rate of 7.1 mm/s and an oxygen pressure of 4 bar. A similar investigation for the mild steel cutting results in a minimum average kerf width of 0.3 mm at the same laser power of 67 W, cutting rate of 9.5 mm/s, and an oxygen pressure of 1 bar. The experimental average kerf is about 0.3 mm, which is approximately equal to the estimated focused beam diameter of 0.27 mm for our focusing lens (f=4 cm and 100 W power). This beam size leads to a laser intensity of about 1.74×10⁹ W/m² at the workpiece surface. The estimated cutting rate from theoretical calculation is about 8.07 mm/s (1.0 mm thickness and 100 W power), which agrees with the experimental results that is 7.1 mm/s for 1.0 mm thickness of mild steel at the laser power of 88 W.     

Integrated wavelength monitoring in a photonic-crystal tunable laser diode

A photonic-crystal tunable 1.55 μm laser diode is fitted with a wavelength monitor on its rear side. The 250-μm long laser based on a coupled-cavity design has approximately 15 nm tunability. The wavelength monitor collects and differentially feeds two-photodetecting areas, thanks to a mode conversion to a higher-order mode (a mini-stopband), followed by tunneling through a thin clad. The layout is numerically optimized to minimize unwanted reflections. Electrical cross-talk was prevented through guard rings and trenches. The correlation between wavelength and the monitor photocurrent ratio demonstrates a 10–20 GHz stabilization capability, or a 15 nm monitoring range.     

Experimental femtosecond laser photodisruption of rabbit sclera for minimally invasive laser sclerostomy: An in vitro study

Femtosecond laser technology, used as a minimally invasive tool in intrastromal refractive surgery, may also have potential as a useful instrument for glaucoma filtration surgery. The purpose of the present study was to evaluate the feasibility of minimally invasive laser sclerostomy by femtosecond laser photodisruption and seek the appropriate patterns of laser ablation and relevant laser parameters. A femtosecond laser (800 nm/50 fs/1 kHz), focused by a 0.1 numerical aperture (NA) objective lens, with different pulse energies and exposure times was applied to ablate hydrated rabbit sclera in vitro. The irradiated samples were examined by scanning electron microscopy (SEM). By moving a three-dimensional, computer-controlled translation stage to which the sample was attached, the femtosecond laser could produce three types of ablation patterns, including linear ablation, cylindrical aperture and rectangular cavity. With pulse energies ranging from 37.5 to 150 μJ, the linear lesions were consistently observed at the inner surface of sclera, whereas it failed to make any photodisruption if pulse energy was below the threshold value of 31.25 μJ, with the corresponding threshold intensity of 4.06×10¹⁴ W/cm². The depths of the linear lesions increased linearly with both pulse energy (37.5–150 μJ) and exposure time (0.1–0.4 s). Histological examination showed the incisions produced by femtosecond laser photodisruption had precise geometry and the edges were sharp and smooth, with no evidence of collateral damage to the surrounding tissue. Our results predict the potential application of femtosecond laser pulses in minimally invasive laser sclerostomy for glaucoma treatment.     

Use of thermal imaging to characterize laser light reflection from thermoplastics as a function of thickness,laser incidence angle and surface roughness

Laser light reflection during the laser transmission welding (LTW) of thermoplastics has the potential to overheat and/or cause unintentional welding of adjacent features of the part being welded. For this reason, and in order to assess how much light is being absorbed by the transparent part (after measurement of the light transmitted through the transparent part), it is important to be able to quantify the magnitude and distribution of reflected light. The magnitude and distribution of the reflected light depends on the total laser input power as well as its distribution, the laser incidence angle (angle between the normal to the transparent part surface and the laser beam), the laser light polarization as well as the surface and optical properties of the transparent part. A novel technique based on thermal imaging of the reflected light was previously developed by the authors. It is used in this study to characterize the magnitude and distribution of reflected light from thermoplastics as a function of thickness (1–3.1 mm), laser incidence angle (20–40°) and surface roughness (0.04–1.04 μm). Results from reflection tests on nearly polished nylon 6 (surface roughness between 0.04 and 0.05 μm) have shown that, for the various thicknesses tested (1–3.1 mm), the total reflection was larger than the specular top surface reflection predicted via the Fresnel relation. From these observations, it is conjectured that, in addition to top surface reflection, the bulk and/or bottom surface also contribute to the total reflection. The results also showed that reflection decreased slightly with increasing thickness. As expected, for the p-polarized light used in this study, the reflection decreased with increasing angle of incidence for the range of angles studied. It was also found that when the surface roughness was close to zero and when it was close to the wavelength of the input laser beam (i.e. 940 nm), the reflectance values were close and reached a minimum between these two roughness values.     

Modeling and optimization on Nd:YAG laser turned micro-grooving of cylindrical ceramic material

Nd:YAG laser turning is a new technique for manufacturing micro-grooves on cylindrical surface of ceramic materials needed for the present day precision industries. The importance of laser turning has directed the researchers to search how accurately micro-grooves can be obtained in cylindrical parts. In this paper, laser turning process parameters have been determined for producing square micro-grooves on cylindrical surface. The experiments have been performed based on the statistical five level central composite design techniques. The effects of laser turning process parameters i.e. lamp current, pulse frequency, pulse width, cutting speed (revolution per minute, rpm) and assist gas pressure on the quality of the laser turned micro-grooves have been studied. A predictive model for laser turning process parameters is created using a feed-forward artificial neural network (ANN) technique utilized the experimental observation data based on response surface methodology (RSM). The optimization problem has been constructed based on RSM and solved using multi-objective genetic algorithm (GA). The neural network coupled with genetic algorithm can be effectively utilized to find the optimum parameter value for a specific laser micro-turning condition in ceramic materials. The optimal process parameter settings are found as lamp current of 19 A, pulse frequency of 3.2 kHz, pulse width of 6% duty cycle, cutting speed as 22 rpm and assist air pressure of 0.13 N/mm² for achieving the predicted minimum deviation of upper width of ?0.0101 mm, lower width 0.0098 mm and depth ?0.0069 mm of laser turned micro-grooves.     

Mid-infrared luminescence and energy transfer of Dy3+/Tm3+ doped fluorophosphate glass

This work reports the observation of emissions at 2.9 μm, 1.8 μm and 1.47 μm from Dy³⁺/Tm³⁺ codoped fluorophosphate glass upon excitation of a conventional 800 nm laser diode. Judd–Ofelt intensity parameters and radiative properties of Dy³⁺ ions in present glasses were calculated using the Judd–Ofelt theory. The mechanism and microparameters of energy transfer processes were investigated based on photoluminescence performance and lifetime measurements. The Dy³⁺/Tm³⁺ codoped fluorophosphate glass possessing advantageous spectroscopic characteristics as well as excellent thermal stability is a promising candidate for an efficient 2.9 μm laser.     

High power Nd:YAG lasers operating at 1.3 μm wave band

The laser properties of 1.3 μm spectral region in Nd:YAG crystal and their simultaneous dual wavelength threshold condition are investigated. Three types of high power 1.3-μm Nd:YAG quasi continuous wave (QCW) lasers, which operate at 1.319 μm or 1.338 μm single wavelength, 1.319 μm and 1.338 μm simultaneous dual wavelength, are achieved with a maximum average output power of 138 W, 132 W and 120 W, respectively.     

Optimization of microcrystalline silicon thin film solar cell isolation processing parameters using ultraviolet laser

This study used ultraviolet laser to perform the microcrystalline silicon thin film solar cell isolation scribing process, and applied the Taguchi method and an L₁₈ orthogonal array to plan the experiment. The isolation scribing materials included ZnO:Al, AZO transparent conductive film with a thickness of 200 nm, microcrystalline silicon thin film at 38% crystallinity and of thickness of 500 nm, and the aluminum back contact layer with a thickness of 300 nm. The main objective was to ensure the success of isolation scribing. After laser scribing isolation, using the minimum scribing line width, the flattest trough bottom, and the minimum processing edge surface bumps as the quality characteristics, this study performed main effect analysis and applied the ANOVA (analysis of variance) theory of the Taguchi method to identify the single quality optimal parameter. It then employed the hierarchical structure of the AHP (analytic hierarchy process) theory to establish the positive contrast matrix. After consistency verification, global weight calculation, and priority sequencing, the optimal multi-attribute parameters were obtained. Finally, the experimental results were verified by a Taguchi confirmation experiment and confidence interval calculation. The minimum scribing line width of AZO (200 nm) was 45.6 μm, the minimum scribing line width of the microcrystalline silicon (at 38% crystallinity) was 50.63 μm and the minimum line width of the aluminum thin film (300 nm) was 30.96 μm. The confirmation experiment results were within the 95% confidence interval, verifying that using ultraviolet laser in the isolation scribing process for microcrystalline silicon thin film solar cell has high reproducibility.     

Low-threshold all-fiber 1000 nm supercontinuum source based on highly non-linear fiber

We present an highly efficient all-fiber compact supercontinuum source that exhibits a nearly flat spectrum from 1.1 μm to 2.1 μm. This broadband infrared optical source is made-up of a highly non-linear fiber pumped by a 1.55 μm self-Q-switched Er-Brillouin nanosecond pulsed fiber laser, which in turn is pumped by a low-power 1480 nm laser diode. In this work we highlight the great potential of highly non-linear fiber for supercontinuum generation with respect to conventional dispersion-shifted fiber by demonstrating a significant 10 dB power enhancement in the short wavelength side of the supercontinuum.