Micromachining Applications Research Articles

Influences of size effect and workpiece temperature during cryogenic micro milling of soft viscoelastic polymer: an experimental assessment

Application of tool-based micromachining in soft polymer has been limited due to high adhesion and low elastic modulus of this material at room temperature. This study aims to evaluate the machinability of a typical viscoelastic soft polymer and understand the effect of material and process parameters on machining performances. In this study, a micro-milling process using cryogenic-assisted cooling is proposed and the importance of temperature control toward glass transition zone was particularly addressed. To identify insight of machinability in micro domain, this article also determines minimum uncut chip thickness (MUCT) and size effects by considering the variations of cutting force and surface integrity with the ratio of h/re (uncut chip thickness (h) to cutting edge radius (re)). The experimental results reveal that consideration of size effect during micromilling of soft viscoelastic polymer helps in reduction of machined surface roughness (Sa) value. Based on the cutting force pattern, it is evaluated that higher machining stability can be achieved during cryogenic machining by reduction of specific cutting force value. Moreover, temperature range around glass transition zone yields more promising experimental outcomes, outperforming other cooling zones.

Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear-induced Side-chain Liberation.

Molecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p-TIPS-DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α-phase, the p-TIPS-DSB crystal undergoes an irreversible martensitic transition to the β-phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side-chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9 (32) MK-1, particularly in the actuation direction from 175.0 (7) to 291.7 (20) MK-1, enabling about 4.5% elongation/contraction. As demonstrated here, p-TIPS-DSB exhibits a decent force density (> 1.4 × 107 N m-3) and precise motion control capabilities due to its hysteresis-free and non-abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.

Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear‐induced Side‐chain Liberation

Molecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p‐TIPS‐DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α‐phase, the p‐TIPS‐DSB crystal undergoes an irreversible martensitic transition to the β‐phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side‐chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9 (32) MK‐1, particularly in the actuation direction from 175.0 (7) to 291.7 (20) MK‐1, enabling about 4.5% elongation/contraction. As demonstrated here, p‐TIPS‐DSB exhibits a decent force density (> 1.4 × 107 N m‐3) and precise motion control capabilities due to its hysteresis‐free and non‐abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.

Capillary trapping of various nanomaterials on additively manufactured scaffolds for 3D micro-/nanofabrication

High-precision additive manufacturing technologies, such as two-photon polymerization, are mainly limited to photo-curable polymers and currently lacks the possibility to produce multimaterial components. Herein, we report a physically bottom-up assembly strategy that leverages capillary force to trap various nanomaterials and assemble them onto three-dimensional (3D) microscaffolds. This capillary-trapping strategy enables precise and uniform assembly of nanomaterials into versatile 3D microstructures with high uniformity and mass loading. Our approach applies to diverse materials irrespective of their physiochemical properties, including polymers, metals, metal oxides, and others. It can integrate at least four different material types into a single 3D microstructure in a sequential, layer-by-layer manner, opening immense possibilities for tailored functionalities on demand. Furthermore, the 3D microscaffolds are removable, facilitating the creation of pure material-based 3D microstructures. This universal 3D micro-/nanofabrication technique with various nanomaterials enables the creation of advanced miniature devices with potential applications in multifunctional microrobots and smart micromachines.

Observation of Boyer-Wolf Gaussian modes

Stable laser resonators support three fundamental families of transverse modes: the Hermite, Laguerre, and Ince Gaussian modes. These modes are crucial for understanding complex resonators, beam propagation, and structured light. We experimentally observe a new family of fundamental laser modes in stable resonators: Boyer-Wolf Gaussian modes. By studying the isomorphism between laser cavities and quadratic Hamiltonians, we design a laser resonator equivalent to a quantum two-dimensional anisotropic harmonic oscillator with a 2:1 frequency ratio. The generated Boyer-Wolf Gaussian modes exhibit a parabolic structure and show remarkable agreement with our theoretical predictions. These modes are also eigenmodes of a 2:1 anisotropic gradient refractive index medium, suggesting their presence in any physical system with a 2:1 anisotropic quadratic potential. We identify a transition connecting Boyer-Wolf Gaussian modes to Weber nondiffractive parabolic beams. These new modes are foundational for structured light, and open exciting possibilities for applications in laser micromachining, particle micromanipulation, and optical communications.

Burst-mode pulse generation in passively mode-locked all-fiber green/orange lasers at 543 nm and 602 nm

We report on the experimental realization of, to the best of our knowledge, the first green and orange passively mode-locked all-fiber lasers. Stable mode-locking in the burst-pulse status is achieved at the wavelengths of 543.3 nm and 602.5 nm. The figure-9 cavity comprises the fiber end-facet mirror, gain fiber (Ho3+:ZBLAN fiber or Pr3+/Yb3+:ZBLAN fiber), and fiber loop mirror (FLM). The FLM with long 460 HP fiber is not only used as an output mirror, but also acts as a nonlinear optical loop mirror for initiating visible-wavelength mode-locking. The green/orange mode-locked fiber lasers with the fundamental repetition rates of 3.779/5.662 MHz produce long bursts containing ultrashort pulses with the 0.85/0.76 GHz intra-burst repetition rates, respectively. The ultrashort intra-burst pulses stem from the dissipative four-wave-mixing effect in the highly nonlinear FLM as well as the intracavity Fabry–Perot filtering. Long bursts of 22.2/11.6 ns with ultrashort pulses of 87/62 ps are obtained in our experiment. The visible-wavelength passively mode-locked lasers in an all-fiber configuration and burst-mode would represent an important step towards miniaturized ultrafast fiber lasers and may contribute to the applications in ablation-cooling micromachining, biomedicine imaging, and scientific research.

Nonlinear generation of vector beams by using a compact nonlinear fork grating

Vectorial beams have attracted great interest due to their broad applications in optical micromanipulation, optical imaging, optical micromachining, and optical communication. Nonlinear frequency conversion is an effective technique to expand the frequency range of the vectorial beams. However, the scheme of existing methods to generate vector beams of the second harmonic (SH) lacks compactness in the experiment. Here, we introduce a new way to realize the generation of vector beams of SH by using a nonlinear fork grating to solve such a problem. We examine the properties of generated SH vector beams by using Stokes parameters, which agree well with theoretical predictions. Then we demonstrate that linearly polarized vector beams with arbitrary topological charge can be achieved by adjusting the optical axis direction of the half-wave plate (HWP). Finally, we measure the nonlinear conversion efficiency of such a method. The proposed method provides a new way to generate vector beams of SH by using a microstructure of nonlinear crystal, which may also be applied in other nonlinear processes and promote all-optical waveband applications of such vector beams.

Generation for high-dimensional caustics and artificially tailored structured caustic beams

We theoretically propose and demonstrate topological parabolic umbilic beams (PUBs) with high-dimensional caustic by mapping catastrophe theory into optics. The PUBs are first experimentally observed via dimensionality reduction. Due to the high-dimensionality, such light beams exhibit rich caustic structures characterized by optical singularities where the high-intensity gradient appears. Further, we propose an improved caustic approach to artificially tailored structured beams which exhibit significant intensity gradient and phase gradient. The properties can trap and drive particles to move along the predesigned trajectory, respectively. The advantages for structured caustic beams likely enable new applications in flexible particle manipulation, light-sheet microscopy, and micromachining.

Mode-locked laser with multiple timescales in a microresonator-based nested cavity

Mode-locking techniques have played a pivotal role in developing and advancing laser technology. Stable fiber-cavity configurations can generate trains of pulses spanning from MHz to GHz speeds, which are fundamental to various applications in micromachining, spectroscopy, and communications. However, the generation and exploitation of multiple timescales in a single laser cavity configuration remain unexplored. Our work demonstrates a fiber-cavity laser configuration designed to generate and control pulse trains from nanosecond to picosecond timescales with a broadband output and a low mode-locking threshold. Our approach exploits a frequency mode-locking mechanism that simultaneously drives the modes of an integrated microring resonator nested within an external fiber-loop cavity, guaranteeing ultra-stable operation. By selectively filtering the nested cavity modes, we can transition from nanosecond pulses to pulse burst trains in which nanosecond and picosecond components coexist. Our laser configuration produces a train of pulses with durations of 5.1 ns and 3.1 ps at repetition rates of 4.4 MHz and 48.7 GHz, with time-bandwidth products close to the transform-limited values of 0.5 and 0.46, respectively. Moreover, in the absence of frequency modulation, we demonstrate the generation of comb spectra with an adjustable central wavelength. Our findings have the potential to significantly contribute to the development of cutting-edge technologies and applications, harnessing the distinct advantages of mode-locked pulses across various scientific and engineering disciplines.

Autofocus and motion error compensation methods for gantry machine in ultra-precise laser machining applications

Error compensation by applying motion control methods makes auto-focus easy and reliable in micro-machining applications. We present a method for dynamic roll and yaw compensation in gantry machines.

Methods and Applications for Amplified Bursts of Picosecond-Spaced Ultrashort Pulses

Generating packages of picosecond-spaced ultrashort pulses yields various advantages in their application in nonlinear spectroscopy, micromachining, and plasma generation. We outline methods of burst amplification, with a focus on recent advancements in the generation and application of amplified pulse bursts.

Multipulse operation in Mamyshev oscillator: influence of external seed source

We study the influence of an external seed source on the startup of a fiber Mamyshev oscillator (MO). Depending on the seed source parameters, while keeping MO parameters fixed, we observe diverse mode-locked operations from the MO, generating stable single, double, triple and even up to a group of seven pulses. The probability of the occurrence of such mode-locked operations is studied. There can be different operating regimes in a particular multipulse operation. We find that there is an optimum spectral width of the seed source for which the probability of generating mode-locked stable train of single pulses from a fiber MO is maximum. Further, we find that the power of the seed source has less influence on the mode-locking state of the fiber MO; however, the probability of single or double pulse operation is relatively higher than that of multipulse operation at higher power of the seed source. These experimental observations will enrich the database of multipulse patterns and help in understanding pulse dynamics in the fiber MO. Suitably chosen seed source parameters can generate the desired pulse pattern. Such tailored outputs could have prospective applications in laser micromachining and high bit rate data transfer in optical networks.

Terahertz vector beams generated by rectangular multilayer transmission metasurface

We propose a multilayer unit structure with an “H” shape to construct a vector beam metasurface generator. Based on the principle that half-wave plate can modulate and change the polarization state of linearly polarized incident waves, we have designed a metasurface composed of rectangular units, which can convert linearly polarized beams into azimuthally or radially polarized vector beams at 0.1 THz. The theoretical transmission efficiency is about 60.92 %.We also have prepared a sample of the vector beam generating metasurface by printed circuit board (PCB) technology, and tested the intensity distribution and polarization state of the transmitted beam on the self-built terahertz testbed. The vector beam metasurface realizes the miniaturization and integration characteristics of the device, which has potential applications in microscopic imaging, biophotonics, quartentum information, laser acceleration, micromachining, and other fields.

Generation of a linear array of focal spots with prescribed characteristic using the radiation pattern from a tapered line source antenna

We report an approach for generating a linear array of focal spots with prescribed characteristics through the use of the antenna radiation theory and time reversal technology. A virtual line source antenna with a cosine-squared tapered distribution, placed at the vicinity of the common foci of a 4π-system, is employed to obtain the required illumination in the pupil plane. The polarization direction of the generated focal spots and the alignment direction of the linear array are determined by the spatial orientation of the line source antenna. The amount of focal spots depends on the period of the amplitude distribution. The position and spacing of the focal spots are related to the length and the period of the amplitude distribution. The generated linear arrays of focal spots have potential applications in laser micromachining, optical imaging and optical micromanipulation that require multiple focal points or synchronous optical operations.

Investigation of precession laser machining of microholes in aerospace material

Sidewall tapering is one of the main limitations in ultrashort pulse (USP) laser machining and is associated with the beam shape and self-limiting effect. Laser processing with a precession beam is a potential solution to overcome this limitation. A study into the effects of precession parameters on the taper angle in microhole drilling of a nickel alloy is reported in this paper. The effects of three key precession parameters, i.e., incident angle, relative distance between the focuses of the precession and individual beams, and scanning speed, have been investigated in detail. Experiments were performed to drill through holes with aspect ratios up to 20:1 and diameters ranging from 100 to 500 μm over 0.6–2 mm thick nickel alloy substrates. Experiment results showed that all the considered parameters/factors were significant and affected the hole tapering in different ways. In addition, there were important interaction effects between two of the factors, i.e., incident angle and focus position, in some cases. The optimal parameters to minimize the tapering effect are suggested, and the mechanism is discussed in detail. The precession laser machining showed clear advantages in overcoming the limitations to associated with conventional USP laser machining. Fabricating microholes with high geometrical accuracy, i.e., with straight side walls and zero taper angles, is feasible with the use of a precession beam. The results clearly show the potential of precession laser processing and the capabilities that the technology can offer for a range of laser micromachining applications in different industries, such as microelectronics, automotive, and aerospace.

Perspective on ultrashort pulse laser micromachining

Ultrashort pulse lasers have found widespread applications in precise micromachining. Here, we present our brief perspective on the development of this innovative technology from the 1990s until today.

Customizing non-diffracting structured beams.

We demonstrate a universal approach to designing and generating non-diffracting structured light beams with arbitrary shapes. Such light beams can be tailored by predefining suitable spectral phases that match the corresponding beam shapes in the transverse plane. We develop a practical spectral superposition algorithm to discuss the non-diffracting properties and experimentally confirm our numerical results. Our proposed approach differs from that of classical non-diffracting beams, which are always constructed from wave equation solutions. The various non-diffracting structured beams could help manipulate particles following arbitrary transverse shapes and are likely to give rise to new applications in optical micromachining.

Self‐Referencing 3D Characterization of Ultrafast Optical‐Vortex Beams Using Tilted Interference TERMITES Technique

AbstractFemtosecond light pulses carrying optical angular momentums (OAMs), possessing intriguing properties of helical phase fronts and ultrafast temporal profiles, enable many applications in nonlinear optics, strong‐field physics, and laser micromachining. While generation of OAM‐carrying ultrafast pulses and their interactions with matters are intensively studied in experiments, 3D characterization of ultrafast OAM‐carrying light beams in spatiotemporal domain has, however, proved difficult to achieve. Conventional measurement schemes rely on the use of a reference pulsed light beam, which needs to be well characterized in its phase front and to have sufficient overlap and coherence with the beam under test, largely limiting practical applications of these schemes. Here a self‐referencing set‐up is demonstrated based on a tilted interferometer that can be used to measure complete spatiotemporal information of OAM‐carrying femtosecond pulses with different topological charges. Through scanning one interferometer arm, the spectral phase over the pulse spatial profile can be obtained using the tilted interference signal, and the temporal envelope of the light field at one particular position around its phase singularity can be retrieved simultaneously, enabling 3D beam reconstruction. This self‐referencing technique, capable of measuring spatiotemporal ultrafast optical‐vortex beams, may find many applications in fields of nonlinear optics and light–matter interactions.

Characterization of process parameters in micro EDM on stainless steel

A versatile operational technique called Micro-Electrical Discharge Machining (Micro-EDM) is employed to initiate miniature holes in hard materials to enable efficient conduction in the applications of micro machining. Short-term discharge of electric pulses of a high discharge current between the tool used and the work piece eliminate material on the surface. A study was performed to evaluate the impact of Micro-EDM process characteristics on Stainless Steel 304 with the help of a brass electrode the high utility levels of the metal in several industries. The current study undertakes to experiment with pulse durations of 3 μs, 4 μs, and 5 μs at a current intensity range of 20A with an emphasis on performance measurements including Metal Removal Rate (MRR) and Tool Wear Rate (TWR).This is achieved by drilling 1.0 mm and 1.5 mm micro holes on SS 304. The Taguchi approach uses a mixed 2–3 design to assess the findings. Subsequently, the outcomes of the study of tiny holes related to both diameters are contrasted to determine the significance of factor variables.

Sub-100 fs Kerr-lens mode-locked femtosecond Yb:CaYAlO<sub>4</sub> laser with GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens mode-locked (KLM) technique and passively mode-locking based on semiconductor saturable absorber mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made significant progress benefited from the high-power green pump lasers, and repetition rate up to 10 GHz has been obtained with the average power of 1.2 W. In the early 21st century, ytterbium ion (Yb<sup>3+</sup>) doped laser crystals and ceramics with emission wavelengths near 1 μm received attention due to their high conversion efficiency and broad gain-bandwidth. Combining the customized SESAM and high-power multimode fiber-coupled laser diodes (LDs), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (<i>M</i><sup> 2</sup> ～ 1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only ～1 W. As a result, the average power of GHz KLM femtosecond laser is typically limited to few tens of milliwatts, which limits the further applications. In this work, we report the first GHz high-power KLM Yb:CaYAlO<sub>4</sub> laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of <i>ABCD</i> matrix, a simple four-mirror bow-tie ring cavity is built so that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM is achieved, the laser has the average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to the peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for realizing the optical frequency combs and micro-machining applications.