Imaging with diffractive axicons rapidly milled on sapphire by femtosecond laser ablation

Abstract

Fabrication of large area (sub-1 cm cross-section) micro-optical components in a short period of time (~10 min) and with lesser number of processing steps is highly desirable and cost-effective. In the recent years, femtosecond laser fabrication technology has revolutionized the field of manufacturing by offering the above capabilities. In this study, a fundamental diffractive optical element, binary axicon -axicon with two phase or amplitude levels, has been designed in three configurations namely conventional axicon, photon sieve axicon (PSA) and sparse PSA and directly milled onto a Sapphire substrate. The fabrication results revealed that a single pulse burst fabrication can produce a flatter and smoother profile than pulse overlapped fabrication which gives rise to surface damage and increased roughness.The fabricated elements were processed in IsoPropyl Alcohol and Potassium Hydroxide to remove debris and redeposited amorphous Sapphire. An incoherent illumination was used for optical testing of the components and a non-linear optical filter was used for cleaning the noisy images generated by the diffractive optical elements.

Nonlinear Reconstruction of Images from Patterns Generated by Deterministic or Random Optical Masks—Concepts and Review of Research

“…The proposed direction of research using the above deterministic optical beams in an indirect-imaging framework may compactify the future optical systems by avoiding the imaging channel. It is convincing from the simulation and previous experimental studies [ 2 , 27 , 28 , 30 , 31 , 32 , 33 , 34 , 35 ] that the NLR, and raising the image to a power of p , can reconstruct a high-contrast object’s image faithfully and at the same time act as a spectral and spatial confocal system. Raising the image to a power of p improved the visibility with a slight decrease in the greyscale profile.…”

“…This process is suitable only for objects with binary values and is detrimental for objects with greyscale values. In fact, most, if not all, of the previous applications of NLR to stochastic—as well as deterministic—optical fields involved only binary objects such as standard-resolution targets [ 2 , 27 , 30 , 31 , 32 , 34 ]. The reconstructed point and normalized MTF after the application of the above method for p = 2 with NLR for eight cases of Figure 2 are shown in Figure 3 a.…”

“…A photon-sieve axicon is a binary axicon where the rings are composed of discs [ 31 ]. A photon-sieve axicon with a period of ~20 μm and diameter of 5 mm was fabricated using femtosecond ablation on a sapphire substrate with a thickness of 500 μm.…”

“…In all the cases, NLR performed significantly better than the above algorithms. Surprisingly, NLR also enabled the conversion of FINCH from a three-shot technique to a single-shot one [ 30 ] and successfully reconstructed images when the light was modulated by axicons and spiral elements [ 31 , 32 ]. Furthermore, NLR also opened the possibility of reimplementing the coded aperture consisting of a random array of pinholes for imaging applications, but only this time with a high SNR [ 33 , 34 , 35 ].…”

Laser 3D Printing of Inorganic Free-Form Micro-Optics

“…Another wide area of applications of calcinated microoptical elements is in specialized spectral applications, e.g., in astro-photonics where UV transparent micro-optical elements are required for coupling light into optical fibers [21]. Currently, such micro-optical elements can be made by laser ablation of UV-transparent sapphire [22]; however, a better optical quality of surfaces and unmatched freedom in 3D sculpting is achievable only via calcination.…”