Modern optical systems are evolving towards miniaturization, integration and three-dimensionality, while precision metrology systems put forward stringent requirements for the recurrence and traceability of micro-nano-scale measurements. Traditional lithography, wet etching, mechanical grinding and other processes often rely on mask plates, multi-step chemical treatment, it is difficult to build a three-dimensional optical path inside the transparent medium, the processing process is also easy to introduce thermal distortion, edge defects, periodic errors, restricting the accuracy of micro-nano optical elements and measurement standard sample blocks. Relying on the femtosecond laser with ultra-short pulse cold processing characteristics, with nonlinear multi-photon effect, the internal modification of materials, surface micro-nano etching and two-photon three-dimensional additive molding are realized, which opens up a new path for the manufacture of optical chips, micro-optical components, measurement standard substances and calibration targets.
Femtosecond laser pulse width is extremely short, the energy is concentrated in a very short time, and the heat is too late to diffuse to the periphery, forming a processing effect that is almost free of heat-affected zone. In transparent media, femtosecond laser relies on multi-photon absorption effect, only in the focal position of the refractive index change or material removal, not limited by linear absorption, can go deep into the glass, sapphire, crystal and other optical substrates to complete "in vivo processing", without damaging the surface optical surface shape. This non-contact, mask-free processing method can not only reduce the material etching of metal, medium and semiconductor materials, but also realize the three-dimensional structure forming with sub-micron resolution through two-photon polymerization, which is suitable for the strict requirements of structure profile, cycle uniformity, edge sharpness and long-term stability in the field of optics and metrology.
Femtosecond laser direct writing optical waveguide is the most representative application direction in integrated photonics and micro optical path manufacturing. Focusing femtosecond laser on transparent materials such as fused silica, borosilicate glass, sapphire, etc., the refractive index of the focus area will be controlled to increase, and a single-mode or multi-mode optical waveguide will be directly formed. The optical path can be arbitrarily bent, crossed and arranged in layers inside the material, breaking through the limitation that the traditional planar photonic chip can only be wired on the surface layer. Based on this technology, passive photonic devices such as beam splitters, couplers, arrayed waveguide gratings, and interference cavities can be fabricated in one step for optical communication wavelength division multiplexing, quantum optical chips, spectral detection and astronomical optical detection. Fiber Bragg gratings also use a large number of femtosecond laser point-by-point direct writing processes to write periodic modulation structures inside the fiber core. Compared with ultraviolet writing processes, femtosecond gratings are resistant to high temperature and irradiation, and have stronger sensing stability in high temperature and strong radiation environments, and are widely used in optical fiber sensing and precision spectrum measurement scenarios.
The processing of micro-optical components and super-surface structures is another important landing scene of femtosecond laser. Microlens array, Fresnel lens, diffractive optical element, antireflection and polarization control micro-nano grating can be formed on the surface of optical glass, infrared crystal and polymer optical material by femtosecond laser. Two-photon polymerization technology further expands the design freedom of micro-optics, and can directly print micro-mirror groups containing multiple aspheric lenses, with a size of only 100 microns, for endoscopic imaging, micro-spectrometers, and on-board sensing optical systems. Laser-induced periodic surface nanostructures can also be prepared on the surface of the optical window sub-wavelength grating, to achieve broadband anti-reverse, polarization selection, structural color and other functions, without the need for multi-layer coating process, structural stability is stronger. Artificial micro-nano structures such as photonic crystals and supermaterials are difficult to achieve three-dimensional orderly arrangement by relying on traditional processes, and femtosecond two-photon direct writing can directly re-engrave complex three-dimensional periodic topology, providing support for the research and development of super-resolution imaging and photonic control devices.
In the field of precision metrology, femtosecond laser is responsible for the preparation of standard sample blocks, calibration targets and microscale scales. All kinds of microscopes, profilers, laser interferometers and scanning probe equipment need to be calibrated regularly. The one-dimensional and two-dimensional grating standard materials, step height sample blocks, cross-dividing targets and micro-hole reference plates used for calibration have extremely high requirements on period consistency, line width uniformity and edge verticality. Femtosecond cold-formed micro-grooves and periodic gratings have no molten recast layer at the edge, the periodic error can be controlled at a very low level, and the standard sample size is stable, which is suitable as a carrier for micro-nano-scale value transfer. The reference microholes, alignment marks and positioning cross structures of various measuring instruments are also often processed by femtoseconds. The inner wall of the microholes is smooth and free of burrs, avoiding stray light from interfering with the optical measurement optical path and ensuring the accuracy of brightness measurement, spectral calibration and distortion correction of imaging system. At the same time, the femtosecond laser inner engraving mark can write the traceability code inside the optical reference block, the mark is buried inside the material, does not damage the optical working surface, is wear-resistant and does not react with air and contact medium, and meets the requirements of the whole life cycle traceability management of the measuring device.
Optical sensing and microfluidic optical detection chips also benefit from femtosecond micromachining. The microfluidic channel and the detection optical waveguide are synchronously written inside the glass substrate, so that the optical path and the flow channel are naturally integrated to form an on-chip spectral detection, fluorescence detection and refractive index sensing chip. When the sample flows in the microchannel, the optical signal is directly coupled and collected inside the chip, thus eliminating complicated external optical path alignment. This kind of integrated chip has compact structure and anti-vibration, which is suitable for field rapid detection and portable spectrum instrument development.
Of course, femtosecond lasers still have obvious technical bottlenecks in the field of optical and metrology micromachining. Two-photon polymerization processing speed is low, large-area array device production efficiency is limited, high numerical aperture processing field of view is small, splicing processing will introduce splicing error, different optical crystals, special glass modification mechanism is complex, refractive index control window is narrow, process debugging cycle is long. At the same time, the high-end femtosecond processing system is costly, and the production of metrological standard samples also requires an ultra-high precision displacement platform and an in-situ online detection system, which requires harsh environmental temperature, humidity and vibration control. With the continuous iteration of high-frequency femtosecond light source, space light field shaping technology, parallel multi-focus processing scheme, the processing efficiency continues to improve, the process stability is constantly optimized, femtosecond micromachining is gradually moving from laboratory prototype preparation to small batch precision device mass production.
Facing the development trend of photon integration, nano-metering and micro-photoelectric instruments, femtosecond laser is not only a micro-processing means, but also a core technology to open up the integrated chain of optical design, device preparation and value calibration. In the future, femtosecond laser will be deeply integrated with optical frequency comb, super-resolution detection and intelligent closed-loop process control. It will continue to make breakthroughs in the cutting-edge directions of three-dimensional integrated photonic chips, traceable nano-grating standards, on-chip quantum optical devices and micro super-resolution optical systems, pushing optical manufacturing and precision metrology towards smaller scale, higher precision and higher integration.
