Ultrashort pulse (USP) laser processing has great potential for precise microfabrication. Toward higher quality and productivity, we have developed the data-driven USP laser processing in which process parameters can be controlled based on data such as in-process monitoring and artificial intelligence (AI) optimization. In this work, stable formation of laser-induced periodic surface structure (LIPSS) in nanoscale on transparent glass materials has been demonstrated by the data-driven UPS laser processing.
We report on recent research on the development of data-driven ultrashort pulse laser processing to achieve higher productivity and quality. We are developing in-process monitoring, artificial intelligence (AI) optimization, and fast active control of the laser based on them. These key technologies are introduced for micro-drilling of metals and transparent materials and laser-induced periodic surface structure (LIPSS) formation on a silica glass. We demonstrate a fast pulse-to-pulse modulation of the fluences to control the ablation efficiency. A deep neural network was utilized to predict the 3-dimensional shapes of the ablation craters depending on the laser parameters (fluence and pulse duration). The scheme was extended to 10 sequential modulations of fluences. An in-process monitoring of the crack formation on glasses was implemented by optical transmission imaging with deep neural network. The optical reflection/transmission technique was also employed to probe the quality of the LIPSS formation on a silica glass.
Recently, we started a new project toward data-driven laser processing, in which real-time monitoring data are delivered to AI system for high-speed optimization. Then, the optimized laser parameters are transferred to the ultrafast laser processing system. To follow the optimized parameters instantly, it’s important to develop a new laser processing system with high-speed modulation as well as wide-range variable parameters.
In this work, effect of ultrafast laser parameters including high-speed modulation on ablation volume was discussed for micro-drilling of glass materials. The drilling of silica glass was performed under three types of modulation conditions with the same total input power and number of pulses: monotonic increase in the pulse energy, constant energy, monotonic decrease. As a result, the holes drilled with monotonic decrease became much deeper compared to samples drilled under monotonic increase. The usefulness of the high-speed laser modulation was demonstrated.
Laser-induced periodic surface structure (LIPSS) in nanometer scale formed by femtosecond (fs) laser pulses depends strongly on laser parameters such as fluence and superimposed pulse number, as well as the surface condition of target materials. Process control based on in-process monitoring is one of the solutions to form the nanostructure stably and homogeneously. However, it is difficult to monitor the nanostructure in the process, because it is much smaller than the light wavelength. This paper reports on a new technique for in-process monitoring of the periodic nanostructure using its anti-reflection property. As a target, we used a synthetic quartz plate. The linear-polarized 1030-nm, 250-fs laser pulses from a Yb fiber laser amplification system operated at a repetition rate of 20 kHz were focused with an objective lens and scanned on the target surface. Microscopic images of the target surface with coaxial epi-illumination and transillumination were acquired with two CMOS cameras. From these images, the surface reflectivity and transmittance were evaluated. After the ablation experiment, the surface morphology was observed with a scanning electron microscope. The surface of which transmittance increased as reflectivity decreased, had a line-like periodic nanostructure with a period of ~200 nm and a depth of ~1 μm. On the other hand, the surface of which both transmittance and reflectivity decreased did not have the nanostructure. These results demonstrate that an observation technique using anti-reflection property is much more effective in monitoring fs-laser-induced nanostructure on glass in the process.
Ultrashort laser processing has great potential for precise microfabrication of transparent materials. For optimizing laser parameters, the quick survey of laser parameters is vital. We have developed Yb-doped fiber chirped-pulse amplification system that can control various laser parameters in a wide range (pulse duration: 0.4 to 400 ps, etc.). The laser micro-drilling and imaging under 450 conditions were performed for about 0.5 hour, the 2D mapping of ablation volume with pulse duration and fluence was also accomplished by automatic confocal laser scanning microscopic measurements for several hours. The fast and comprehensive survey of ultrafast laser processing of glasses was demonstrated.
Micro/nanoscale surface patterning of zirconia ceramic is needed for surface functionalization and performance enhancement, such as improved biocompatibility of medical devices, as well as device miniaturization. Therefore, formation of laser-induced periodic surface structures (LIPSS) with periods shorter than the laser wavelength on a zirconia ceramic was carried out using an ultrashort pulsed laser. In this case, it is important to shorten the processing time required for forming the LIPSS without deteriorating the processing quality. Using the various-parameters-controlled laser processing and observation system, we optimized LIPSS formation by changing parameters such as pulse duration, repetition frequency, number of shots, and fluence.
For higher cell-to-module efficiency in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, it is important to reduce the loss of active area due to integrated connection. The integrated connection contains three scribing processes: P1 (back contact insulation), P2 (electrical connection) and P3 (transparent conductive oxide, shortly TCO front contact insulation). In this work, we focused on ultrashort-pulse laser scribing (λ=1034 nm, Δτ=300 fs) of TCO via lift-off process as damage-less P3 scribing of CIGS thin-film solar cells. The lift-off of TCO was caused by laser ablation of only an upper part of CIGS light-absorbing layer. The dependence of lift-off behavior on the laser pulse energy and TCO film thickness has been investigated. It was observed that the lift-off of TCO formed a heat-affected zone (HAZ) with a thickness up to 250 nm beneath the trench bottom, where the CIGS experienced to melt. Further, thinner TCO film required lower laser energy threshold for the TCO lift-off, which is favorable to higher solar cell efficiency due to smaller HAZ. Using the TCO liftoff as P3, a submodule with an active area of approximately 3.5 cm2 made by all laser scribing exhibited the conversion efficiency of 11.6 %. After post-annealing at 85 °C for 15 h in vacuum for recovering laser-induced damages, the efficiency was successfully improved to 15.0 %, which is comparable to mechanically-scribed one.
An ultrashort pulse laser system with precisely controlled output-timing and carrier-envelope phase (CEP) is reported.
Recently developed technology Ofl CEP control of a mode-locked laser not only introduced an optical frequency comh
in frequency domain hut also gave us a way to generate optical pulses whose oscillating electric field is under a fixed
phase relation with the intensity shape. Fortunately, recent advances on optical physics have also showed that sonic types
of light-matter interactions become sensitive to the field shape when the pulse approaches a few cycles in duration and
has a high peak intensity. Owing to those advances, field-controlled ultrashort pulse generation, based on
suh-femtosecond resolution timing-control and sub-radian CEP control of femtosecond lasers, becomes an attractive
challenge. Our final goal is to realize a shaped electric field within optical-cycle time scale br researches on light-matter
interaction and other future application.
CEP control Ofl a mode-locked Ti:sapphire laser is the first step of such a laser system. Trade-off between the
accuracy and robustness of the control, and the monitoring technique of CEP br amplilication, will he discussed.
Amplification of a CEP-controlled pulse, which is necessary for most of time-domain application, is successfully
performed by the CEP monitoring technique. Our chirped-pulse amplifier, that includes a grating-based
stretcher/compressor, has a potential to achieve higher-energy amplification of a fixed CEP pulse. Multichannel phase
control of spectrally divided ultrashort pulses is applied to dynamic control of pulse-timing and CEP of amplifled pulses.
Related results on short-pulse, sub-l3fs, generation by a chirped-pulse Ti:sapphire amplifier, and multicolor
phase-coherent pulse sources will be also discussed briefly, showing our on-going efforts to approach the final goal.
Recent progress on ultrashort pulse lasers has made it possible to control the pulse timing and optical phases from different cavities very precisely. By applying detection and control techniques of the carrier-envelope phase to multicolor pulse sources with synchronized pulse timing, we obtained phase-coherent pulse-trains in different wavelengths. The pulse-trains have sufficient stability to achieve Fourier synthesis among optical fields. One of our test sources was a femtosecond optical parametric oscillator (OPO). Using a specially designed OPO whose signal and idler are sub-harmonics of the pump frequency, we obtained long-term stabilization of the optical phase among those three waves. Since the OPO also generates sum-frequencies of the three waves, six-color coherent pulse-trains from 425nm to 2550nm are available. Another test source was a passively synchronized mode-locked laser with two kinds of gain media, Ti:sapphire and Cr:forsterite. Although it is harder to reduce phase-noise between the different color pulses than in the OPO example, we can expect shorter pulse duration and higher average power from this type of coupled laser. These coherent multicolor pulse sources will be applied not only to shorter-pulse generation by field summation, but also to some applications that include competing non-linear processes among multicolor pulses.
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