By Innofocus Photonics Technology Pty Ltd
Refractive index imaging is a method of quantifying the phase difference caused by difference in refractive index using computational holography, through spatial light interference, which further reproduces the three-dimensional refractive index distribution of the measured material. To measure the 3D refractive index, the material to be measured first needs to have a high transmittance in the wavelength region of the laser to be used. (Innofocus provides laser of multiple wavelength options including 450nm, 488nm, 532nm, 633nm etc., and can customize laser wavelength to be used according to customer requirement.) Therefore, in theory, the refractive index imaging can be performed as long as the material is transparent to the laser wavelength used. However, in practice, due to the need for high numerical aperture lenses (NA>0.5) to achieve high spatial resolution, there are certain requirements for the thickness of the material to be imaged. The thickness should be less than 2 mm and the distance from the fabricated structure to the material surface should be less than the working distance of the imaging microscope lens (typically <400 µm). Since the refractive index difference and spatial distribution are the same when processed with the same laser parameters on the same material, we can perform refractive index imaging on the materials in thin sheet first to quantify refractive index, optimize laser processing parameters, and use the optimized parameters on the final thick blocks of the materials. This allows for quick optimization of processing parameters and can be used on block materials of different thicknesses depending on the application requirements. This question hits on the difference between refractive index imaging and normal CCD in-situ observation. First of all, the major strength of refractive index imaging is to observe the distribution of the three-dimensional refractive index difference in the material, with quantitative accuracy up to 0.0001, and currently there is no other alternative that can achieve the same function. It is mainly for observing of the refractive index difference produced by using femtosecond laser processing in different optically transparent materials. This refractive index difference has irreplaceable importance in the application of optical waveguide and fiber grating. The refractive index imaging can be used in a wide range of crystalline and non-crystalline materials, including traditional polymers (e.g. PET, acrylic material and etc.), traditional optical crystal materials (e.g. quartz, optical glass), and new optical crystal materials (lithium niobate crystals, sapphire, diamond, BGO crystals). It can also be used to arbitrary rectangular, cubic and cylindrical shaped material, and is also suitable for fiber optic materials. The built-in camera in other laser processing system can display a two-dimensional picture of the process, or a modification of the material (such as an optical material showing a darker colour due to elevated absorption, or scattering created by glass or etching of the material, or areas of high transparency), and is a useful way to observe the process and the results in real time. However, in the need to observe laser-introduced refractive index differences, this traditional method faces two challenges: 1. If it is purely refractive index change, it does not introduce a change in transmittance or absorption, and it cannot be clearly distinguished when the refractive index change is entirely in the focal plane (e.g., the core-cladding boundary of an optical fiber cannot be distinguished when the fiber is accurately focused). Therefore, it is often necessary to move the introduced refractive index difference slightly away from the focal plane of observation to obtain some contrast by observing the colour streaks caused by this refractive index difference. However, since the diffracted image is observed, its width of lines is difficult to determine directly, and although deliberately moving the sample away from the focal plane is beneficial to the observation, it also causes some errors in the processing position, which is only suitable when the requirements for the processing position are not very precise (allowing a few hundred nanometers of error). 2. The observation of the three-dimensional spatial distribution of the refractive index difference is not possible with the conventional method. As mentioned in the first point, it is possible to see the effect of the introduced refractive index difference by observing diffraction pattern using the conventional observation method, but the diffraction pattern is controlled by the difference between the processing position and the position of the focal plane, and the three-dimensional spatial distribution cannot be fully determined. 3. Conventional methods cannot quantify the refractive index difference measured. Meanwhile, a quantitative refractive index difference has a decisive impact on the structural design and final device performance in fields of optical communication devices and optical fiber gratings. Therefore, it is extremely important to accurately quantify the three-dimensional refractive index distribution of the structure being processed. In this regard, Innofocus Photonics has developed the world's first and only intelligent laser nanofabrication system with in-situ refractive index imaging characterization capability, which is a ground-breaking invention. Based on this patented invention, they have achieved the first in-situ characterization of three-dimensional refractive index difference distribution, which is a fundamental breakthrough in the field of laser processing. To the best of our knowledge, Innofocus's commercialized nanofabrication equipment based on multiple patented technologies is the world first system to ever achieve in-situ laser nanofabrication characterization of three-dimensional refractive index distribution, which is a fundamental breakthrough in the field of laser processing. The traditional method of refractive index measurement by refractive index liquid is very time-consuming and laborious, and can only perform two-dimensional refractive index characterization, but not three-dimensional refractive index characterization. Reconstruction of refractive index consists of two steps: 1. Acquisition of interference fringe images with different illumination angles 2. Based on the images, conduct calculation to reconstruct the 3D refractive index distribution of the structures. Innofocus’s system has excellent performance in this area and completely automated the process for users, allowing automatic continuous image acquisition and computational reconstruction without multiple background acquisitions. The speed of the completed process depends on the configuration of the computer. A real-time reconstruction speed of 0.5 Hz can be achieved with the standard configuration of a small workstation. Speed can be increased with more advance-configured workstations. The most important purpose of using in-situ 3D refractive index reconstruction is to understand the refractive index difference resulting from the interaction between the femtosecond laser and the material. In addition, 3D refractive index reconstruction after laser processing not only measures the refractive index difference at the point of femtosecond laser interaction, but also demonstrates the entire specific morphology and spatial distribution of the laser-processed 3D structure, providing important information in line widths, positions, cross sections, etc.
