Femtosecond Laser Direct Writing Research Articles

2 µm band four-wavelength-switchable narrow linewidth fiber laser enabled by fs-laser direct-written polarization-dependent parallel fiber Bragg gratings

We propose and experimentally demonstrate a four-wavelength-switchable single-longitudinal-mode (SLM) narrow linewidth thulium–holmium co-doped fiber laser (THDFL) using two polarization-dependent parallel fiber Bragg gratings (PD-PFBGs). The PD-PFBGs, fabricated using femtosecond (fs) laser direct-writing technology in a standard single-mode fiber (SMF) via a point-by-point method, are used as a four-channel reflection filter. Two FBGs are inscribed in parallel in the fiber core along the axial direction and are uniquely positioned symmetrically on either side of the centerline. This configuration enables polarization-dependent multi-channel filtering capability, which further allows for polarization-control-based four-wavelength-switchable operations of the THDFL. SLM lasing is accomplished by utilizing a simple dual-ring sub-cavity filter. An exceptional output performance of the THDFL is achieved, including an optical signal-to-noise ratio (SNR) of >72 dB, maximum power and wavelength fluctuations of 0.350 dB and 0.024 nm, respectively, and a linewidth of <2 kHz, for all four single-wavelength operations lasing at ∼2000 nm. These performance indicators suggest that the THDFL can be applied in free-space optical communication, atmospheric monitoring, and Lidar.

Monolithic 2-µm single-frequency linearly-polarized gain-switched distributed feedback fiber laser by femtosecond laser direct-writing.

We report a single-frequency, linearly polarized gain-switched, distributed feedback (DFB), 2-µm thulium doped silica fiber laser (TDFL), with an effective cavity length of 2.5 mm. The cavity is based on a heavily thulium doped non-polarization-maintaining silica fiber and composed of a π-phase-shifted fiber Bragg grating (FBG) with a total FBG length of 35 mm. The DFB FBG was written by femtosecond-laser point-by-point (PbP) method. In-band pumping scheme is chosen with a 1550 nm nanosecond pulsed erbium-doped silica fiber laser pump. Single-longitudinal, linearly polarized, gain-switched TDFL at 2002 nm, with a recorded shortest pulse duration of 4.7 ns, a repetition rate of 20 kHz, a maximum peak power of 170 W, and single pulse energy of 0.8 µJ, has been obtained, benefitting from the ultrashort DFB cavity made by the femtosecond laser direct-writing method.

Room-temperature laser operation at 1.8 μm in Tm:GdVO4 waveguides

We report on the first experimentally demonstrated Tm-doped gadolinium vanadate waveguide laser operating at 1.8 μm. Cladding waveguides with a diameter of 60 μm are fabricated by femtosecond laser direct writing (FsLDW) in 3 at. % Tm:GdVO4 crystal. A continuous-wave (CW) Ti:sapphire laser is used as the pump source for waveguide laser generation. The impacts of waveguide lengths and cavity conditions on the obtained lasing performance are well studied. For the Tm:GdVO4 waveguide with a total length of 2.8 mm, the laser output at 1812 nm is achieved with a lasing threshold of 116.6 mW and a slope efficiency of 23.2% when utilizing the π-polarized pump source at 799 nm. And the maximum output power of 179 mW is determined at a pump power of 932 mW, which corresponds to an overall optical-to-optical efficiency of 19.2%. The experimental results achieved in this work suggests that compact and low-threshold Tm:GdVO4 waveguide lasers can be very promising for applications in mid-infrared photonic devices with compact structures.

Femtosecond‐Laser‐Direct‐Writing Micro‐Scale Soft Actuators with Controllable Shape Morphing

AbstractMicro‐scale soft actuators with controllable shape‐morphing are the focus of advanced technological fields, ranging from advancing sensing, industrial robotics, and digital manufacturing to medical devices. Particularly, there is a growing interest in the scientific community to leverage liquid crystal polymers (LCPs) to fabricate such soft actuators, because LCPs can offer reversible, programmable deformations under external stimuli. However, pattern micromachining of LCPs into micro‐scale remains a daunting challenge. Herein, a femtosecond laser direct writing (FsLDW) method for cross‐linked LCP (CLCP) microstructure construction is reported that enables arbitrary pattern machining with a minimum size of 40 µm and average heat‐affected zone (HAZ) below 8 µm through optimization of processing parameters. Light‐driven behaviors of CLCP microstructures are systematically characterized through analyzing the effects of film thickness, length‐width ratio, light irradiation time, incident angle, light intensity, and cutting direction on bending and twisting behaviors. Finally, a light‐driven micromirror system is demonstrated, which can achieve not only a controllable swing but also a rotation of the mirror surface with a maximum scanning frequency of ≈2 Hz.

High performance flexible PVDF film pressure sensor fabricated by femtosecond laser

Flexible pressure sensors have shown great potential for applications in detection and medical fields due to their unique piezoelectricity and high sensitivity. In this paper, micro-nano-perforated structures with different periods and diameters are fabricated on the polyester (PET) films by femtosecond laser direct writing (FsLDW), and then the perforated structures are embossed on polyvinylidene fluoride (PVDF) films to form sensors with circular truncated cone (CTC) structures. The effect of period and diameter on the sensitivity of the sensor is studied numerically and experimentally. The results show that the high sensitivity of 8.98 mV/KPa can be achieved in a PVDF film pressure sensor with a period of 50 μm and a diameter of 30 μm, which is about 10.9 times that of the structureless sensor. This work not only provide a fast fabricated method for the pressure sensor based on the femtosecond laser and hot embossing, but also has potential applications in the detection of the micro-signals.

Superhydrophobic and Anti‐Icing Surface by Femtosecond Laser Direct Writing

Glass fiber‐reinforced polymer (GFRP) is formed with glass fiber as the reinforcing material and resin as the matrix. It is widely used in wind turbine blades because of its lightweight, high strength, and corrosion resistance properties. Herein, a method to prepare superhydrophobic GFRP surfaces by femtosecond laser direct writing combined with fluoroalkylsilane modification is demonstrated. The prepared GFRP surface has excellent superhydrophobicity with contact angle of 163.9° and sliding angle of 3.8°. In the ice resistance tests, the icing delay time is extended from 33 to 273 s at −5 °C. The ice adhesion strength is reduced from 217.4 to 40.3 kPa. The surface still has superhydrophobicity and ice adhesion strength of less than 100 kPa after ten cycles of the test. The laser exposure conditions are optimized for water/ice repelling and are at high intensity of 4 TW cm−2 pulse−1 and 2.5 m s−1 beam travel speed, which make the presented approach efficient for fabrication over industrially large areas.

Nd:YSAG waveguide-grating vortex laser: design and implementation

In this work, we fabricate a hybrid waveguide-grating vortex laser in Nd:YSAG by using femtosecond laser direct writing (FsLDW). The detailed parameters of the hybrid structure are fixed by optical simulation. In experiments, an efficient vortex beam is produced in the passive operation at 1064 nm. Under optical pumping at 808 nm, a dual-wavelength waveguide laser at 1060/1062 nm as well as a waveguide-grating vortex laser at 1060 nm are obtained. The laser performance and diffraction properties of the generated vortex laser are detailed, studied, and discussed, providing meaningful reference results toward the practical applications of FsLDW and waveguide-grating structures in integrated photonics.

Customized Micron-Scale Waveguide-Grating Structure for Vortex Laser Emission in Tm:YLF Processed by Femtosecond Laser Direct Writing

We report a micron-scale tailored waveguide-grating device designed for vortex laser generation in the Tm:YLF crystal inscribed by femtosecond laser direct writing (FLDW). The confocal micro-Raman measurement clarifies the microscopic mechanisms of the laser modification on the crystal lattice and the formation of the waveguide and the fork grating. The guiding performance and diffraction properties of the integrated generator are measured by the end-face coupling system with a 1064 nm continuous laser. A polarization-dependent vortex laser array at 1877 nm is pumped by the laser at 792.8 nm. Our results reveal that the hybrid structure combining waveguide and fork grating Tm:YLF crystal has great potential for the application of the vortex laser generator in the mid-infrared region.

A Brief Review on Nonlinear Photonic Crystals Induced by Direct Femtosecond Laser Writing

Direct femtosecond laser writing or inscription is a useful technique, and it has been employed to engineer various materials in many applications including nonlinear photonic crystals, which are of periodically patterned second-order nonlinearity to get and control the coherent light at new frequencies. By manipulation of second-order nonlinearity, either erased or poled, quasi-phase matching has been achieved in several crystals, especially three-dimensional nonlinear photonic crystals have been originally proposed and proved to be truly three-dimensional. Here we shortly review on the recent advances in the research field of nonlinear photonic crystals inscribed by femtosecond laser, as well as look into the future in this field.

Optical Waveguides Fabricated via Femtosecond Direct Laser Writing: Processes, Materials, and Devices

AbstractAs the most fundamental unit of photonic integration, optical waveguides offer the possibility of developing efficient and practical photonic chips by facilitating the on‐chip integration of devices with different functions. Femtosecond direct laser writing (FsDLW), as a burgeoning 3D microfabrication technology, can realize the rapid formation of arbitrary optical waveguides without any mask inside versatile materials, such as crystalline dielectric crystals, glasses, polymers, and photoresists. This significant material‐independence allows FsDLW to combine different materials and manufacturing processes to implement a cross‐platform solution. This review first describes the different optical loss types and suitable measurement methods for different applicable scenes, then summarizes the two fabrication mechanisms and points out the general direction for adjusting the waveguide properties by fabrication processes. Finally, the application fields and development prospects of different materials are discussed by overviewing the characteristics of different materials and the material selection preferences of different devices. With a panoramic view of the development, it is concluded with insights and perspectives of the future development of optical waveguides and processing technology via FsDLW.

Femtosecond laser direct-writing of perovskite nanocrystals in glasses

Femtosecond laser direct-writing of perovskite nanocrystals in glasses

Recent Developments of Femtosecond Laser Direct Writing for Meta-Optics.

Micro-optics based on the artificial adjustment of physical dimensions, such as the phase, polarization, and wavelength of light, constitute the basis of contemporary information optoelectronic technology. As the main means of optical integration, it has become one of the important ways to break through the future bottleneck of microelectronic technology. Geometric phase optical components can precisely control the polarization, phase, amplitude and other properties of the light field at the sub-wavelength scale by periodically arranging nanometer-sized unit structures. It has received extensive attention in the fields of holographic imaging and polarization optics. This paper reviews the physical mechanism of micro-nano structure modification, research progress of femtosecond laser direct-writing photoresist, femtosecond laser ablation of metal thin films, femtosecond laser-induced nanograting, and other methods for preparing polarization converters and geometric phase optics. The challenges of fabricating ultrafast optical devices using femtosecond laser technology are discussed.

Photon propagation control on laser-written photonic chips enabled by composite waveguides

Femtosecond laser direct writing (FsLDW) three-dimensional (3D) photonic integrated circuits (PICs) can realize arbitrary arrangement of waveguide arrays and coupling devices. Thus, they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation. However, the propagation constant β is limited to a narrow range in single-mode waveguides by solely changing the processing parameters, which greatly hinders the design of FsLDW PICs. This study proposes a composite waveguide (CWG) method to increase the range of β, where a new single-mode composite waveguide comprises two adjacent circular waveguides. As a result, the photon propagation can be controlled and the variation range of β can be efficiently enlarged by approximately two times (Δβ∼36 cm−1). With the CWG method, we successfully realize the most compact FsLDW directional couplers with a 9 μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array. Thus, the study represents a step further toward the fine control of the coupling between waveguides and compact integration of FsLDW PICs.

Silicon Vacancy Color Centers in 6H-SiC Fabricated by Femtosecond Laser Direct Writing

As a single photon source, silicon vacancy (VSi) centers in wide bandgap semiconductor silicon carbide (SiC) are expected to be used in quantum technology as spin qubits to participate in quantum sensing and quantum computing. Simultaneously, the new direct femtosecond (fs) laser writing technology has been successfully applied to preparing VSis in SiC. In this study, 6H-SiC, which has been less studied, was used as the processed material. VSi center arrays were formed on the 6H-SiC surface using a 1030-nm-wavelength fs pulsed laser. The surface was characterized by white light microscopy, atomic force microscopy, and confocal photoluminescence (PL)/Raman spectrometry. The effect of fs laser energy, vector polarization, pulse number, and repetition rate on 6H-SiC VSi defect preparation was analyzed by measuring the VSi PL signal at 785-nm laser excitation. The results show that fs laser energy and pulse number greatly influence the preparation of the color center, which plays a key role in optimizing the yield of VSis prepared by fs laser nanomachining.

Si-Cr Nano-Alloys Fabricated by Direct Femtosecond Laser Writing.

Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots and utilised in opening ∼400 nm nano-holes in a Cr etch mask that was tens-of-nm thick. The ablation threshold was found to be 2.3 nJ/pulse, double that of plain silicon. Nano-holes irradiated with pulse energies below this threshold produced nano-disks, while higher energies produced nano-rings. Both these structures were not removed by either Cr or Si etch solutions. Subtle sub-1 nJ pulse energy control was harnessed to pattern large surface areas with controlled nano-alloying of Si and Cr. This work demonstrates vacuum-free large area patterning of nanolayers by alloying them at distinct locations with sub-diffraction resolution. Such metal masks with nano-hole opening can be used for formation of random patterns of nano-needles with sub-100 nm separation when applied to dry etching of Si.

Three-dimensional on-chip mode converter.

The implementation of transverse mode, polarization, frequency, and other degrees of freedom (d.o.f.s) of photons is an important way to improve the capability of photonic circuits. Here, a three-dimensional (3D) linear polarized (LP) LP11 mode converter was designed and fabricated using a femtosecond laser direct writing (FsLDW) technique. The converter included multi-mode waveguides, symmetric Y splitters, and phase delaying waveguides, which were constructed as different numbers and arrangements of circular cross section waveguides. Finally, the modes (LP11a and LP11b) were generated on-chip with a relatively low insertion loss (IL). The mode converter lays a foundation for on-chip high-order mode generation and conversion between different modes, and will play a significant role in mode coding and decoding of 3D photonic circuits.

Design and fabrication of Mach-Zehnder interferometers in soda-lime glass for temperature sensing applications.

High sensitivity represents one of the main goals that sensing devices need to satisfy for their applications. This work presents to the best of our knowledge the first integrated Mach-Zehnder interferometer (MZI) embedded in soda-lime glass with comparable sensitivity to silicon-on-insulator (SOI) devices. We manufactured the MZIs by the femtosecond direct laser writing (FDLW) technique and characterized them with temperature. Four buried MZIs were manufactured by slightly increasing the optical path due to separation between the arms of the interferometer (Δ s). We achieved a fringe shift of ∼8n m for an increase of 0.18µm. We have characterized one of these devices with temperature from 30°C to 70°C obtaining a sensitivity of ∼28p m/ ∘ C. We improved the sensitivity of the device to ∼54p m/ ∘ C due to the advantage of the unique three-dimensional (3D) capabilities that FDLW provides, overcoming the characteristically low thermo-optic coefficient of soda-lime glass just by rotating the MZI structure 11°.

A tailored 100%-efficient 532/1064-nm demultiplexer in y-cut LiNbO3 crystal.

In this Letter, we report a tailored 532/1064-nm demultiplexer based on a multimode interference (MMI) coupler with an efficiency of 100%. The device structure is designed according to the self-imaging principle, and the propagation and the wavelength division performance are simulated by the beam propagation method. The demultiplexer is fabricated in a y-cut LiNbO3 crystal by femtosecond laser direct writing (FLDW) combined with the ion implantation technique. The end-face coupling system is used to measure the near field intensity distribution, and the spectra collected from the output ports are obtained by spectrometers. The simulated and the experimental results indicate that the customized demultiplexer in the LiNbO3 crystal presents excellent wavelength division performance operating at 532 nm and 1064 nm. This work demonstrates the application potential of FLDW technology for developing miniaturized photonic components and provides a new strategy for fabricating high-efficiency integrated wavelength division devices on an optical monocrystalline platform.

High‐Efficiency and Low‐Intensity Threshold Femtosecond Laser Direct Writing of Precise Metallic Micropatterns on Transparent Substrate

AbstractLaser direct writing (LDW) is a promising approach for fabricating metallic micropatterns on transparent substrates for transparent electronic circuits that satisfy both electronic and optical criteria. However, high efficiency and precision patterning remain a challenge for both photochemical and photothermal LDW. Herein, a novel method is proposed with a femtosecond laser to achieve a highly‐efficient photothermal process via single‐photon absorption by photosensitive particles (SPA‐FsLDW). The dispersive photosensitive particles act as numerous heating sources, enabling simultaneous multiple‐location photothermal reactions and highly‐efficient metallization due to heat‐induced metal ion reduction. The new approach effectively exploits the excellent heat‐input regulation with the ultrashort pulse of the femtosecond laser to achieve great temperature controllability and precision. It is shown that, with a deposition rate of ≈107 µm3 s−1 and electrical resistivity of ≈10−7 Ω m, SPA‐FsLDW improves efficiency and electrical resistivity by at least one order of magnitude compared to previously reported FsLDW. A self‐powered sensor is fabricated using SPA‐FsLDW, demonstrating its practical applicability.

On‑Line Monitoring of Optical Waveguide Micro‑Structure Fabricated With Femtosecond Laser Direct Writing Based on OCDR

On‑Line Monitoring of Optical Waveguide Micro‑Structure Fabricated With Femtosecond Laser Direct Writing Based on OCDR