We propose a scheme based on a type of continuous-phase encoding grating for the generation of multiple curved Bessel-like beams simultaneously. It is shown that multiple identical curved Bessel-like beams that diverge from a common center can be generated by overlapping a specially designed phase with a continuous-phase grating, and multiple high-order curved Bessel-like beams can be generated when another spiral phase is embedded. As an example, a 5×5 symmetric continuous-phase grating embedded with the special designed phase and a spiral phase is demonstrated based on a spatial light modulator. The experimental results show that 5×5 square arrays of bright- and dark-center curved Bessel-like beams were well generated. The proposed method provides an interesting method for obtaining simultaneously multiple curved Bessel-like beams, which should be of high interest for its promising applications in parallel optical manipulation, optical guiding, laser machining or laser surgery, particle acceleration, etc.
Displacement laser interferometers and grating interferometers are two main apparatus for the micron-nanometer displacement measurement over a long range. However, the laser interferometers, whose measuring scale is based on the wavelength, are very sensitive to the environment. On the contrast, the grating interferometers change the measuring scale from wavelength to grating period, which is much stable for the measurement results. But the resolution of grating interferometer is usually lower than that of laser interferometer. Therefore, further investigation is needed to improve the performance of grating interferometer. As we known, the optical subdivision is a main factor that affects the measurement resolution. In this paper, a grating interferometer with high optical subdivision is presented based on the Littrow configuration. We mainly use right angle prisms accompanied with plane mirrors to make the measuring lights diffracted by the grating scale for many times. An optical subdivision factor of 1/24 can be obtained by this technique. A main difficulty of this technique is that the grating scale should be with high diffraction efficiency. Fortunately, the measuring light is incident on the grating scale at the Littrow angle, the grating scale can be designed with very high efficiency easily in this condition. Compared with traditional grating interferometers, this kind of grating interferometer can greatly increase the measuring resolution and accuracy, which could be widely used in nanometer-scale fabrications and measurements.
For higher-density integration and acceleration of operating speed in Si ICs, 3D integration of wafers and/or dies is essential. Fabrication of current 3D ICs relies on 3D assembly which electrically connects stacked chips to form a single circuit. A key technology for the 3D assembly is TSVs which are vertical electrical connections passing completely through silicon chips to electrically connect vertically assembled Si ICs. Typical TSVs have wide features, with diameters of a range from several microns to 50 μm and depths up to 500 μm with aspect ratios up to 15 depending on the application and integration scheme. In this work, we present high-throughput, taper-free TSVs fabrication using femtosecond Bessel beams operated at different wavelengths from 400 nm to 2.4 μm. Furthermore, special phase filters are designed to suppress the damages induced by the side-lobes of Bessel beams for high-quality TSVs fabrication. Our technique can be potentially used for 3D assembly in manufacture of 3D silicon integrated circuits.
We propose a method for generating axial multifocal spots (AMS) with a high numerical aperture (NA) objective. The AMS is generated by using phase-only modulation at the back aperture of the objective. Without using any iteration algorithm, the modulated phase distribution is directly calculated by an additional phase analytical formula with different focal distances. By dividing the back aperture of the objective into multi sectorial zones and applying the corresponding additional phase with different focal distances, the AMS can be created. Numerical simulation shows that the numbers of the axial focus depends solely on the different phase distribution calculated by different focal distances. By engineering the phase pattern with different focal distances, axial multifocal spots with different spacing can be realized. Furthermore, combined with vortex phase, the AMS with specific shape spots also can be created. In addition, the AMS focused by incident beams of circular polarization, radial polarization and angular polarization are also studied. This kind of AMS may be found applications in optical imaging, especially in three-dimensional (3D) biological imaging, and also be attractive in mult-plane optical trapping.
This paper will report our recent works on fabrication, evaluation, and applications of gratings. We are using the Dammann parallel laser writing facility for fabrication of gratings. High-efficiency reflective gratings and large-sized grating are fabricated. We have fabricated high-power reflective laser vortex grating with expectation of a new laser drilling effect for laser fusion in the future, which is just evaluated by our developed method. These gratings are essential elements for high-power laser systems and other high-demanding metrology applications.
Optical encoders and laser interferometers are two primary solutions in nanometer metrology. As the precision of encoders depends on the uniformity of grating pitches, it is essential to evaluate pitches accurately. We use a CCD image sensor to acquire grating image for evaluating the pitches with high precision. Digital image correlation technique is applied to filter out the noises. We propose three methods for determining the pitches of grating with peak positions of correlation coefficients. Numerical simulation indicated the average of pitch deviations from the true pitch and the pitch variations are less than 0.02 pixel and 0.1 pixel for these three methods when the ideal grating image is added with salt and pepper noise, speckle noise, and Gaussian noise. Experimental results demonstrated that our method can measure the pitch of the grating accurately, for example, our home-made grating with 20μm period has 475nm peak-to-valley uniformity with 40nm standard deviation during 35mm range. Another measurement illustrated that our home-made grating has 40nm peak-to-valley uniformity with 10nm standard deviation. This work verified that our lab can fabricate high-accuracy gratings which should be interesting for practical application in optical encoders.
This paper summarized our work on three-dimensional optical technologies using Dammann gratings, e.g., threedimnensional Dammann gratings, three dimensional imaging using a Dammann grating, etc.. We developed threedimensional Dammann grating which can produce three-dimensional array with equal distance and equal intensity with a high-numerical-aperture lens. As we know, a lens usually has a single focal point. Fresnel zone plate can generate several axial focal points, but the intensity between them is unequal. By introducing the concept of Dammann grating into the circular phase plate, we invented Dammann zone plate(DZP) which can produce a series of axial focal points with equal intensity. Combining DZP with a Dammann grating, three-dimensional Dammann array will be generated, which is highly interesting for various applications. We also built a three–dimensional measuring system using a Dammann grating, with two cameras as the right eye and right eye, respectively. We used a 64×64 Dammann grating for generation of a square array of light spots for parallel capturing the three-dimensional profile of an object. The two cameras and the illuminating part are packaged together. After scanning the object, its three-dimensional profile will be obtained. Experimental results demonstrated the effectiveness of this technique.
The focal shift effect and axial dispersion property of binary pure-phase filters (BPFs) in focusing systems are described
in the regime of the scalar Debye theory. By expanding the formula of the electric field in focal region into a summation
of a polynomial series, the axial behavior and the general rule of the focal shift effect of BPFs are analytically studied.
The small focal shift formula of BPFs is derived based on the second-order approximation, and its scope of validity is
also discussed. Based on this small focal shift formula, the property of axial dispersion for a given BPF is also
analytically discussed. Furthermore, numerical results of 2-zone and 3-zone BPFs with various normalized radii and
phase shift values are also given for verifying these analytical results. At last, as an example, the application of BPFs
with negative axial dispersion in compensation of chromatic aberration of a single lens in an ultrashort laser focusing
system is presented. The numerical results show that the chromatic aberration induced by the single lens is compensated
well nearly for the whole spectral bandwidth of the ultrashort laser. Therefore, this focal shift effect induced by BPFs
should be of great interest for its potential applications in compensation of chromatic aberration and compact tunable
focal modulation in some special cases.