We demonstrate nano-structuring and the reduction of mid-spatial-frequency (MSF) errors using femtosecond laser figuring and finishing. We established a method to create periodic nanostructures using femtosecond-laser-based ablation. In addition, a laser-created sinusoidal pattern with five cycles/mm was generated to mimic the mid-spatial frequency error that often occurs s during conventional optics fabrication. A deterministic material removal having nanometer-scale precision was performed to remove the mid-spatial-frequency error from 17 nm to one nanometer in magnitude. The resulting surface remains single-digit-nanometer surface roughness after the MSF error correction.
We have demonstrated, for the first time to our knowledge, the simultaneous figuring and finishing of glass substrates using a femtosecond laser. Deterministic material removal with nanometer precision was achieved while maintaining optical surface quality (<1 nm RMS roughness). We have identified a metric that enables high-precision material removal and scaling up the material removal towards larger processing tasks and flexible geometrical features. The controllability of material removal was demonstrated using various laser parameter combinations. Furthermore, we created a four-step staircase structure to demonstrate the highly deterministic figuring and finishing process. The controllable femtosecond laser figuring with optical quality surface demonstrates a novel pathway for precision figuring and finishing of optics.
We present a new experimental method for assessing subsurface damage (SSD) on optical glasses processed using fs-laser. The method employed nanoindentation and Raman spectroscopy to characterize material damage. Statistically, our method shows over 95% confidence for SSD depths of over 55 nm. According to our results, the fs-laser processed surfaces with optimized processing parameters revealed no detectable SSD, thus establishing the feasibility of fs-laser polishing for precision optical manufacturing.
We investigated femtosecond laser inscription of IG2 waveguides and photonic structures for mid-infrared applications. 1 x 4 beamsplitters are realized, achieving normalized splitting ratios of more than 90% among the four arms. In addition, both fundamental mode and multimode guiding in the mid-IR regime at 4.55 µm are demonstrated. The Mid-IR waveguiding properties at 4.55 µm will be presented for both single-mode and multimode waveguides with as low as 0.8 dB/cm propagation loss for the single-mode guiding and controllable mode field diameter.
This work is supported by the US National Aeronautics and Space Administration under STTR Contracts 80NSSC20C0027, 80NSSC21C0638, and 80NSSC22PA936.
Ultrafast laser processing of glass material was investigated to address the need for precise, flexible figuring and finishing tools with minimal thermal effects. A dynamic pulse propagation model was established and used to predict the spatial and temporal distribution of surface temperature, through which a set of optimum laser processing conditions was determined. The impact of laser parameters on material removal and surface quality was experimentally investigated for Borofloat glass. Linearly controllable material removal is achieved with the determined laser parameters, and the processed area maintains the optic-quality surface roughness for various material removal depths.
This research was supported by the US National Science Foundation I/UCRC Center for Freeform Optics (IIP-1822049 and IIP-1822026).
We compare the double-track waveguide lasers inscribed inside Nd:YAG crystals using femtosecond laser pulses at 515 nm and 1030 nm. For both source wavelengths, the highly efficient, continuous wave, single mode and stable lasing at 1064 nm has been achieved in a monolithic cavity formed by the crystal with dielectric coatings at the input and output end facets. In addition to source wavelength, the lasing performance and guiding properties for track separations of 15 µm and 30 µm are compared.
We present a dynamic pulse propagation modelling and experiments for femtosecond laser bonding of glass to glass/metal. The modelling provides quantitative estimates of the heat affected zone and weld geometry, incorporating the nonlinear electron dynamics along with temperature-dependent thermal properties. The model numerically determines the desired relative position between the geometrical focus of a femtosecond-laser-pulse and the interface of the two substrates to be welded, for the first time to our knowledge. The welding results of two similar and/or dissimilar materials using the model-predicted weld geometry and offset distance will be presented. This research was supported by NASA SBIR contract 80NSSC20C0432.
Ultrafast lasers enable non-contact, waste free, precise material removal. We have demonstrated single-digit nanometer precision polishing of optical material using a femtosecond laser. For ultrafast-laser-based waveguide writing, we study the underlying physics behind nonlinear optical dynamics during the femtosecond laser processing of crystalline materials. Unidirectional pulse propagation equation simulation is carried out to study the evolution of energy, fluence, plasma generation, and beam waist of a femtosecond pulse along the propagation direction under different energy and focusing conditions. Waveguides having a loss of 0.21 dB/cm are obtained. A Nd:YAG based waveguide laser with lasing threshold of 50 mw was demonstrated.
We present a new experimental method for measuring subsurface damage (SSD) on ground surfaces of single crystal germanium and borosilicate glass BK-7 based on the morphology and mechanical properties dependence on depth into the material. The material selection allows us to compare crystalline and non-crystalline materials. We use spots of different penetration depths on ground surfaces by Magnetorheological Finishing (MRF) spots to remove part or all of the damaged layer, and then evaluate the surface roughness, fracture toughness and material removal rate (MRR) of MRF at the deepest point penetration of MRF fluid into the spot. The dependence of these results on penetration depth into the material reveals the subsurface damage of the surfaces. It is shown that the subsurface damage depths revealed by each property (surface roughness, MRR, fracture toughness) match each other.
Femtosecond laser ablation has a wide variety of applications, from re-shaping the cornea of the eye to micro-machining electronic devices. It is imperative to understand the dynamics of ablation from energy absorption to surface vaporization. In this work, time- and space-resolved microscopy is used to analyze the ablation dynamics induced by femtosecond laser pulses in single-crystalline silicon. These dynamics are revealed by capturing the surface images generated with probe pulse reflection at a variable delay time relative to the pump pulse. When the peak fluence of the incident laser pulse is near the ablation threshold, the transient surface reflectivity initially changes from low to high due to electron-hole plasma formation and then exhibits dynamic Newton-ring patterns with increasing numbers of fringes. When the peak fluence exceeds two times the ablation threshold, surface reflectivity first increases, then significantly decreases (to a level lower than the initial value), and increases again with a growing Newton-ring pattern resulting from thermally induced material expansion. Finally, a crater is formed with two side bumps within which the absorption depth is reduced with spatial fluence level over two times the ablation threshold. The time-resolved silicon images and analyses describing the evolution of transient reflectivity and morphological will be presented.
In recent years, femtosecond-laser writing has recently emerged as one of the most versatile techniques for direct waveguide microfabrication of transparent optical materials. Femtosecond-laser-based fabrication of three-dimensional silicon waveguide enables compact silicon photonics and their integration as large third-order nonlinearity and the high refractive index of silicon allows for tightly confining optical waves to a sub-micron region. The writing process is however challenging because the unique features exhibited by the semiconductor crystal, such as two-photon absorption, free-carrier absorption / dispersion, anisotropic and dispersive third-order nonlinearity, which may drastically influence the writing process at high intensities required for the femtosecond-laser writing. In this work, we provide a detailed description of the underlying physics behind nonlinear optical dynamics in femtosecond laser processing of silicon waveguides, considering the generation of free carriers induced by various absorption mechanisms, plasma formation, refractive index change and their impact on the waveguide microfabrication and performance.
NASA is working with US industry and academia to develop Photonic Integrated Circuits (PICs) for: (1) Sensors (2) Analog RF applications (3) Computing and free space communications. The PICs provide reduced size, weight, and power that is critical for space-based systems. We describe recent breakthrough 3D monolithic integration of photonic structures, particularly high-speed graphene-silicon devices on CMOS electronics to create CMOS-compatible highbandwidth transceivers for ultra-low power Terabit-scale optical communications. An integrated graphene electro-optic modulator has been demonstrated with a bandwidth of 30 GHz. Graphene microring modulators are especially attractive for dense wavelength division multiplexed (DWDM) systems. For space-based optical communication and ranging we have demonstrated generating a variable number of channels from a single laser using breadboard components, using a single-sideband carrier-suppressed (SSBCS) modulator driven by an externally-supplied RF tone (arbitrary RF frequency), a tunable optical bandpass filter, and an optical amplifier which are placed in a loop. We developed a Return--to-Zero (RZ) Differential Phase Shift Keying (DPSK) laser transmitter PIC using an InP technology platform that includes a tunable laser, a Semiconductor Optical Amplifier (SOA), high-speed Mach-Zehnder Modulator (MZM), and an electroabsorption (EAM) modulator. A Silicon Nitride (SiN) platform integrated photonic circuit suitable for a spectrally pure chip-scale tunable opto-electronic RF oscillator (OEO) that can operate as a flywheel in high precision optical clock modules, as well as radio astronomy, spectroscopy, and local oscillator in radar and communications systems is needed. We have demonstrated a low noise optical frequency combs generation from a small OEO prototypes containing very low loss (~1 dB) waveguide couplers of various shapes and sizes integrated with an ultrahigh-Q MgF2 resonators. An innovative miniaturized lab-on-a-chip device is being developed to directly monitor astronaut health during missions using ~3 drops of body fluid sample like blood, urine, and potentially other body fluids like saliva, sweat or tears. The first-generation system comprises a miniaturized biosensor based on PICs (including Vertical Cavity Surface Emitting Laser – VCSEL, photodetector and optical filters and biochemical assay that generates a fluorescent optical signal change in response to the target analyte.
The effect of increasing Q on image interpretability is explored, and the fidelity of general image quality equation (GIQE) predictions is assessed for Nyquist-sampled (Q=2) imagery at low signal-to-noise ratio. A digital image chain simulation is developed and validated against a laboratory test bed using objective and subjective assessments. Using the validated model, additional test cases are simulated to study the effects of increased detector sampling on image quality with operational considerations for space-based remote sensing. Variants of the GIQE are evaluated against subject-provided ratings, and modifications that increase prediction accuracy for Q=2 imagery are proposed. Finally, using the validated simulation and modified image quality equation, trades are conducted to ascertain the feasibility of implementing Q=2 designs in future electro-optical systems.
We demonstrate the performance of an optical differentiation wavefront sensor (ODWS) relying on an optical system that images the pupil to a camera. A binary pixelated transmission filter with a linear amplitude-transmission gradient is located in a far field of the pupil. The ODWS uses the fluence data measured in the detection plane for two orthogonal orientations of the filter to determine wavefront-slope data along the two corresponding directions in the pupil plane. This technique allows for acquisition in real-time without moving parts, providing high resolution, high dynamic range, and achromatic wavefront sensing for astronomical imaging or metrology applications.
Advanced fabrication and finishing techniques are desired for freeform optics and integrated photonics. Methods including grinding, polishing and magnetorheological finishing used for final figuring and polishing of such optics are time consuming, expensive, and may be unsuitable for complex surface features while common photonics fabrication techniques often limit devices to planar geometries. Laser processing has been investigated as an alternative method for optic forming, surface polishing, structure writing, and welding, as direct tuning of laser parameters and flexible beam delivery are advantageous for complex freeform or photonics elements and material-specific processing. Continuous wave and pulsed laser radiation down to the nanosecond regime have been implemented to achieve nanoscale surface finishes through localized material melting, but the temporal extent of the laser-material interaction often results in the formation of a sub-surface heat affected zone. The temporal brevity of ultrafast laser radiation can allow for the direct vaporization of rough surface asperities with minimal melting, offering the potential for smooth, final surface quality with negligible heat affected material. High intensities achieved in focused ultrafast laser radiation can easily induce phase changes in the bulk of materials for processing applications. We have experimentally tested the effectiveness of ultrafast laser radiation as an alternative laser source for surface processing of monocrystalline silicon. Simulation of material heating associated with ultrafast laser-material interaction has been performed and used to investigate optimized processing parameters including repetition rate. The parameter optimization process and results of experimental processing will be presented.
Technologies including magnetorheological finishing and CNC polishing are commonly used to finish optical elements, but these methods are often expensive, generate waste through the use of fluids or abrasives, and may not be suited for specific freeform substrates due to the size and shape of finishing tools. Pulsed laser polishing has been demonstrated as a technique capable of achieving nanoscale roughness while offering waste-free fabrication, material-specific processing through direct tuning of laser radiation, and access to freeform shapes using refined beam delivery and focusing techniques. Nanosecond and microsecond pulse duration radiation has been used to perform successful melting-based polishing of a variety of different materials, but this approach leads to extensive heat accumulation resulting in subsurface damage. We have experimentally investigated the ability of femtosecond laser radiation to ablate silicon carbide and silicon. By substituting ultrafast laser radiation, polishing can be performed by direct evaporation of unwanted surface asperities with minimal heating and melting, potentially offering damage-free finishing of materials. Under unoptimized laser processing conditions, thermal effects can occur leading to material oxidation. To investigate these thermal effects, simulation of the heat accumulation mechanism in ultrafast laser ablation was performed. Simulations have been extended to investigate the optimum scanning speed and pulse energy required for processing various substrates. Modeling methodologies and simulation results will be presented.
KEYWORDS: Wavefronts, Wavefront sensors, Binary data, Sensors, Fourier transforms, Modulation, Photomasks, Near field optics, Reconstruction algorithms, Signal to noise ratio
High-resolution wavefront sensors are used in a wide range of applications. The Shack-Hartmann sensor is the industry standard and mostly used for this kind of analysis. However, with this sensor the analysis can only be performed for narrowband radiation, the recoverable curvature of the wavefront slopes is also restricted by the size of a single lens in the microlens array. The high-resolution Shack Hartmann wavefront sensor (>128×128) is also significantly expensive. The optical differentiation wavefront sensor, on the other hand, consists of only simple and therefore inexpensive components, offers greater signal to noise ratio, allows for high-resolution analysis of wavefront curvature, and is potentially capable of performing broadband measurements. When a transmission mask with linear attenuation along a spatial direction modulates the far field of an optical wave, the spatial wavefront slope along that direction can be recovered from the fluence in the near field after modulation. With two orthogonal measurements one can recover the complete wavefront of the optical wave. In this study the characteristics of such a wavefront sensor are investigated when the linear transmission modulation is implemented with a pixelated binary filter. Such a filter can be produced as a gray-scale quasi-continuous transmission pattern constructed using arrays of small (e.g., 10-micron) transparent or opaque pixels and therefore it can simply be fabricated by conventional lithography techniques. Simulations demonstrate the potential ability of such a pixelated filter to match the performance of a filter with continuously varying transmission, while offering the advantage of better transmission control and reduction of fabrication costs.
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