影响石英单模光纤数值孔径测量的因素及分析

本文采用远场光斑法和光强法测量了石英单模光纤的数值孔径.实验结果表明,由光纤耦合条件不同导致的光斑形状变化,对数值孔径的测量影响不大,单模光纤的纤芯直径小,导致出射光存在较强的衍射现象,对数值孔径的测量造成较大的影响,无论采用光斑法还是光强法,需以衍射第1次极大为计算标准,尽量选择光斑中心为亮斑时测量其数值孔径.此外,...     

High-numerical-aperture focusing of radially polarized doughnut beams with a parabolic mirror and a flat diffractive lens

Recently, Dorn et al. [Phys. Rev. Lett. 91, 233901 (2003)] demonstrated the significance of radially polarized doughnut beams in obtaining very small focal spots (with an area of approximately 0.26 lambda2) with high-numerical-aperture (NA) aplanatic microscope objectives. We propose two simple alternative ways to focus such radially polarized beams: a parabolic mirror and a flat diffractive lens. Because of their large apodization factor for a high NA, a significant further reduction in spot area (up to a factor of 1.76 at a NA of 1) compared with the aplanatic system can be achieved.     

Spring constant modulation in a zone plate tweezer using linear polarization

At large NAs a micro-Fresnel zone plate produces a focal spot that is more elliptical than that produced by an objective lens with the same NA. Using this anisotropy we demonstrate a method for modulating the spring constant of an optical trap by rotating the linear input polarization. The focal spot ellipticity is enhanced by the apodization factor of the zone plate and its extremely high NA. By measuring the positions of trapped particles we obtain two-dimensional histograms of particle position. These indicate that the trap spring constant is 2.75 times larger perpendicular to the incident polarization than along it. The elliptical focal spot distribution can be rotated by rotating the incident polarization, allowing the spring constant along a given direction to be modulated.     

Estimation of the laser beam enabling rectangular processing by the inverse diffraction theory

A small rectangular laser spot has been widely studied for laser processing and laser repair technology. It is usually made by converging the input beam with the lens. Faithful reproduction of the laser spot is dependent on NA of the imaging lens. The small rectangular spot can be obtained by high NA lens, which is limited by many factors, such as high energy loss due to the reflection on the surface, large mass and volume, and strong sensitivity to aberrations and misalignment. On the other hand, the beam cannot be faithfully reproduced because of the diffraction with the low NA lens which has no such the limitations. One of the alternative ways to produce small rectangular profiles by using the low NA lens system is to estimate the input beam profile leading to the output profile of the sharp rectangular shape.     

Toward the subdiffraction focusing limit of optical superresolution

A superresolving three-zone plate is applied to a Fresnel diffractive lens. It is shown that for radial incident polarization this combination produces a focal spot approaching superresolution allowed subdiffractive limit of 0.36lambda/NA for focusing. For media responsive to longitudinal field component only, our phase engineering scheme results in a focal spot size of 0.368lambda/NA. When used with a solid immersion lens, the scheme can generate the smallest focal spot available for passive optics.     

Generation of sub-wavelength and super-resolution longitudinally polarized non-diffraction beam using lens axicon

It is well known that a light spot of sub-wavelength will diverge in all directions. In this letter, A method is presented for generating sub-wavelength (0.44λ) longitudinally polarized beam, which propagates without divergence over lengths of about 2λ in free space. This is achieved by a high numerical aperture (NA) lens axicon that utilizes spherical aberration to duplicate the performance of an axicon and to create an extended focal line.     

Controlling aspect ratio of focal spots of high numerical aperture objective lens in multi-photon absorption process

Three types of objective lens (OL), i.e., circular, annular and one-dark-ring, are presented to control the ratio, called aspect ratio (AR), of the longitudinal size and the transverse size of focal spots of micro-focus-region of high numerical aperture (NA) OL. The AR can be varied from 7.1 to 2.7 in the case of using a circular OL with NA changes from 0.7 to 1.4. By employing an annular OL, the transverse size of the central lobe of micro-focus-region decreases but its longitudinal size increases, so that the AR increases several times with respect to the case of using a circular OL. However, using a one-dark-ring OL, one can reduce either both transverse and longitudinal sizes or only the longitudinal size of focal spot, so that the AR obtained with a one-dark-ring OL is about 70% of that obtained with a circular OL. Such lenses can be useful for many applications such as sub-microfabrication and three-dimensional data storage using multi-photon absorption process.     

Tight Focusing of Radially and Azimuthally Polarized Vortex Beams through a Dielectric Interface

Based on vectorial Debye theory, tight focusing of radially and azimuthally polarized vortex beams passing through a dielectric interface are studied. The intensity distribution in the focal region is illustrated by numerical calculations. We show the influence of numerical-aperture （NA） on the full-width at half maximum （FWHM） of the focal spot or the focal hole. It has been found that compared with the azimuthally polarized Besse~Gaussian （BG） beams, the longitudinal component in the z direction of the radially polarized BG beams has no influence on the FWHM of the focal spot and hole, but enhances the total light intensity.     

Characteristics of filament induced Dammann gratings fabricated using femtosecond laser

To establish optimal processing conditions for direct write fabrication of diffractive optical elements such as gratings, waveguides, lenses, we have investigated the effect of process parameters such as scan speed, numerical aperture (NA) of objective lens, pulse energy on the characteristics of the filament induced inside fused silica with a femtosecond Ti:sapphire laser. The optimum process parameters were used to fabricate a number of Dammann gratings, 6×6 array, having different thicknesses and number of layers. The performance of these optical elements was evaluated by measuring their diffraction efficiencies. All gratings fabricated were strongly birefringent, the zero order spot with high intensity was not separated from the spot array, and the intensity distribution of 6×6 spot array exhibited some degree of nonuniformity. The single layer Dammann grating fabricated with a thickness of 80 μm attained a maximum diffraction efficiency of 38.8%.     

Superresolution-focal-volume induced 3.0 Tbytes/disk capacity by focusing a radially polarized beam

This paper reports on the study of the superresolution volume of the focal spot by focusing a radially polarized beam. This feature is achieved by increasing the inner radius of a high NA annular objective to break the diffraction-limited volume of a focal spot. The application of this finding into two-photon induced three-dimensional optical data storage leads to an enhanced photoreduction threshold effect in recording. As such, multilayer subdiffraction optical recording is experimentally demonstrated with an equivalent capacity of 3.0 Tbytes/disk.     

Focusing properties of Gaussian beam by a nonspiral phase plate system

In this article, a nonspiral plate is investigated numerically by vector diffraction theory to observe the focusing properties of Gaussian beams. Both low- and high-numerical-aperture (NA) optical systems are considered in the investigation. It is found that the parameter of NA and the phase vary rate of the phase plate influence the focal intensity distribution considerably. When a nonspiral plate is used to provide linear phase variation on one half of the Gaussian beam, it may adjust the focal spot considerably and conveniently. Changing the vary rate of the phase plate or the parameter of NA can alter optical intensity distribution; some novel focal spots and focal switch may also occur.     

Superhigh numerical aperture (NA > 1.5) micro gradient-index lens based on a dual-material approach

We describe a novel scheme for obtaining a superhigh numerical aperture gradient-index (SHNA GRIN) lens from multiple thin layers of two or more materials with large refractive-index contrast. Design procedures for the lens are described, including variation of the layer thickness to achieve focusing and of the thickness limit to reduce scattering loss. We use an exact numerical solution by the finite-difference time-domain method to evaluate the lens's performance. Specific examples of a SHNA GRIN lens with a SiO2-TiO2 material system designed for fiber coupling to a nanowaveguide are shown to have focusing FWHM spot sizes of 0.53-0.7 microm at lambda =1.55 microm (corresponding to a NA of approximately 1.6-1.1) with 2.7-2.4% more loss than an ideal continuous index profile GRIN lens. With this approach, a SHNA GRIN lens with a NA of > 1.5 and a length of <20 microm can be achieved with currently available thin-film deposition techniques.     

Tight focusing of circularly polarized beam over high NA lens axicon with a diffractive optical element

We propose to use diffractive optical element in combination with high NA lens axicon to achieve a high depth of focus when illuminated by a circularly polarized beam. With this kind of system, the focal depth is increased to 12.816λ and the magnetic spot size is reduced to 0.3764λ. However, in the conventional lens with same NA, the FWHM of the magnetic spot is found to be 0.4308λ and its corresponding magnetization depth is only 0.888λ. The author expects that such a high focal depth strong longitudinal magnetic field with large magnetization depth can be widely used in high density magneto optic recording, laser machining, laser cutting and the scanning near-field magnetic microscope.     

Estimation of coupling efficiency of optical fiber by far-field method

Coupling efficiency to a single-mode optical fiber can be estimated with the field amplitudes at far-field of an incident beam and optical fiber mode. We call it the calculation by far-field method (FFM) in this paper. The coupling efficiency by FFM is formulated including effects of optical aberrations, vignetting of the incident beam, and misalignments of the optical fiber such as defocus, lateral displacements, and angle deviation in arrangement of the fiber. As the results, it is shown the coupling efficiency is proportional to the central intensity of the focused spot, i.e., Strehl intensity of a virtual beam determined by the incident beam and mode of the optical fiber. Using the FFM, a typical optics in which a laser beam is coupled to an optical fiber with a lens of finite numerical aperture (NA) is analyzed for several cases of amplitude distributions of the incident light.     

4π Focusing with single paraboloid mirror

Creating small bright or dark focal spots with good spherical symmetry requires 4π focusing, usually achieved by using two counter-propagating beams and two high NA lenses that need to be positioned with interferometric accuracy. We investigate a simple alternative way of focusing light to a small and uniform bright or dark focal spot. It only requires a single parabolic mirror illuminated with a single beam from one direction, resulting in a robust and compact setup. The focal spot distribution is tailored by optimizing the polarization, intensity and helicity of the incoming illumination.     

Sub wavelength multiple focal spot generation of high NA objective lens using diffractive optical element

In this article, based on the vector diffraction theory, the effect of specially designed phase modulating optical element by means of an incident tightly focused azimuthally polarized Bessel Gaussian beam in the focal region of high NA lens is investigated numerically. It is observed, that a specially designed diffractive optical modulation element (DOE) can generate multiple focal spot segment of transversely polarized in the focal region by controlling the angles of DOE. Such kind of sub wavelength transversely polarized focal spots segment may find wide applications in multiple optical traps and optical manipulation technology.     

Wafer-Scale Manufacturing of Near-Infrared Metalenses

Metalenses have revolutionized optical technologies with their superior ability to manipulate light and the potential to replace conventional, bulky optical components. However, their commercialization is hindered by limitations in conventional manufacturing techniques, such as small patterning areas, low throughput, and high cost. In this study, two methods are introduced for scalable and wafer-scale manufacturing of metalenses operating in the near-infrared region, aimed at overcoming the abovementioned challenges. The first type of metalens is polarization-independent and constructed using hydrogenated amorphous silicon cylindrical structures fabricated through direct photolithography. This metalens has a diameter of 1 cm and numerical aperture (NA) of 0.53. The focusing efficiency is confirmed at a 940 nm wavelength, and the focal spot profile approaches the diffraction limit. The second metalens is polarization-dependent and fabricated using silicon nanoparticle-embedded-resin rectangular structures through a cost-effective nanoimprinting method. This process can produce metalenses with a diameter of 5 mm and an NA of 0.53. Both types of metalenses demonstrate high-resolution capabilities when imaging a 1951 USAF resolution test target and bioimaging. This research offers innovative pathways for the mass production and large-scale fabrication of metalenses. It is believed that the work will accelerate the industrialization of metalenses, fostering further advances in optical technology.     

Generation of sub wavelength focal spot with large depth of focus generated by radially polarized beam

We investigate the focusing properties of a radially polarized Bessel Gaussian beam by a high numerical aperture (NA) lens based on vector diffraction theory. We observe that our proposed system generates a sub wavelength focal spot of 0.42λ having large uniform focal depth of 6.45λ. The authors expect such a long depth of focus have great potential for use in optical, biological, high-resolution and atmospheric sciences.     

Laser writing system for fabrication of diffractive optics elements

We report a laser writing system for fabrication of diffractive optical elements with He-Cd laser. The wavelength of the light source is 441.6 nm. The output beam is collimated into parallel light with uniform intensity distribution after passing through the spatial filter with a pinhole of 25μm and the collimating device. A microscopy objective lens with numerical aperture (NA) of 0.65 is used to focus the beam into a small diffraction spot. Any pattern can be written with this system. Experimental results are presented.The written gratings and the phase patterns were verified with a conventional optical microscopy and the Taylor Hobson equipment.     

Creation of Multiple Subwavelength Focal Spot Segments Using Phase Modulated Radially Polarized Multi Gaussian Beam

Based on the vector diffraction theory,the effect of complex phase filters on intensity distribution of a radially polarized multi Gaussian beam in the focal region of high NA lens is theoretically investigated.It is observed that a properly designed multi belt complex phase filter can generate subwavelength novel focal patterns including splitting of focal spots and generation of multiple focal spot segments such as eight,six and four focal spots along the optical axis are obtained.We expect that such an investigation is useful for optical manipulation and material processing,multiple high refractive index particle trapping technologies.