Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

Buffer layers for the design of broadband optical filters

The use of buffer layers is a promising technique in the attempts developed for the design of optical multilayer coatings when two widely separated spectral regions are under consideration. Here we show that the buffer layer, associated with matching layers as in the application mentioned above, can also be exploited in a successive step process to broaden the spectral bandwidth of beam splitters or antireflection coatings.     

Photonic crystal beam splitters

This work studies two-dimensional photonic crystal beam splitters with two input ports and two output ports. The beam splitter structure consists of two orthogonally crossed line defects and one point defect in square-lattice photonic crystals. The point defect is positioned at the intersection of the line defects to divide the input power into output ports. If the position and the size of the point defect are varied, the power of two output ports can be identical. The beam splitters can be used in photonic crystal Mach-Zehnder interferometers or switches. The simulation results show that a large bandwidth of the extinction ratio larger than 20 dB can be obtained while two beams are interfered in the beam splitters. This enables photonic crystal beam splitters to be used in fiber optic communication systems.     

Bilayer Coatings for Light Refraction Without Change of Polarization

Abstract

Wave refraction without change of polarization at a dielectric—dielectric interface is achieved with a bilayer coating of two transparent thin films. For given refractive indices of all media, the thicknesses of the two films are determined as functions of the angle of incidence. The polarization-independent reflectance and the differential reflection phase shift of the coated substrate are also calculated. Examples are presented of MgF₂—ZnS and Ge—MgF₂ bilayer coatings on a Ge substrate that refract infrared radiation without change of polarization. Such coatings are useful in the construction of polarization-preserving beam deflectors and beam splitters.     

Guided wave acousto-optic and electro-optic tunable filter designs with relaxed beam-splitter requirements

New designs for acousto-optic tunable filters and electro-optic tunable filters with the Mach-Zehnder configuration are proposed. The new designs differ from conventional designs in three respects: (1) polarizing beam splitters are not required, (2) the optical path difference for the waveguides between the beam splitters is a half-wavelength, and (3) the relative positions of the polarization coupling regions in the two waveguides are displaced in the propagation direction by half of the spatial period of the perturbation responsible for the coupling. Because the new designs provide an additional degree of freedom in achieving the required beam-splitter performance, they are expected to be much easier to fabricate than conventional filter designs with polarizing beam splitters.     

Holographic Optical Beam Splitters in Dichromated Gelatin

Abstract

Holographic beam splitters can split an incoming beam into several partial beams travelling at different angles with arbitrary intensities, thus offering high design flexibility. In this paper the fabrication of beam splitters as multiplex holograms in dichromated gelatin is reported. Shrinking or swelling of the gelatin was avoided by careful process control, reciprocity law failure (as expected from silver halide holograms) was not observed, and, for example, a 1 : 2 beam splitter diffracting 47·4% and 47·0% of the incoming light into two higher orders was realized.     

Silicon beam splitter for far-infrared and terahertz spectroscopy

Silicon beam splitters several millimeters thick offer numerous advantages over thin freestanding dielectric beam splitters. For routine spectroscopy for which resolutions of better than 1 cm(-1) are not required, a silicon beam splitter can replace several Mylar beam splitters to span the entire far-infrared region. In addition to superior long-wavelength performance that extends well into the terahertz region, the silicon beam splitter has the additional advantage that its efficiency displays little polarization dependence.     

Symmetrically coated pellicle beam splitters for dual quarter-wave retardation in reflection and transmission

A trilayer pellicle that consists of a high-index center layer that is symmetrically coated on both sides by a low-index film can be designed to produce differential reflection and transmission phase shifts of +/- 90 degrees at oblique incidence and equal throughput for the p and the s polarizations. Such a device splits a beam of incident linearly polarized light into two orthogonal circularly polarized components that travel in well-separated angular directions. Examples of infrared dual quarter-wave retarders that use a symmetrically coated Ge pellicle at 77 degrees angle of incidence are presented. A 50-50% splitter requires a symmetric pellicle with at least five layers. Error analysis shows that the thicknesses of the high-index layers must be tightly controlled. These circular polarization beam splitters are intended for operation with a well-collimated light source and can be used as the basis of a novel circular polarization Michelson interferometer.     

New method for the fabrication of stratified gratings and their applications

Anew method for the preparation of stratified light-sensitive film is developed, and the stratified gratings (SG's) are recorded in the film. The sensitive layers on both sides of a dichromated cellulose triacetate film are produced simultaneously through chemical reaction and not with the conventional coating technique. Compared with SG's in other materials made with coating techniques, double-layer SG's in the film have, to my knowledge, the highest experimental diffraction efficiency (~54%) in addition to their having a simple recording optical system. The diffraction efficiency and the periodic Bragg selectivity of the SH in the film is given. Based on the SG's of the film, several beam splitters with 2, 3, 4, or 7 fan-outs and higher than 80% total diffraction efficiencies are realized experimentally. The advantages of this method as compared with others, such as the method based on volume holographic beam splitters, are explained.     

Diffractive interconnection between a high-power Nd:YAG laser and a fiber bundle

Beam splitters are used to couple the light of high-power Nd:YAG lasers into a fiber bundle. By the fibers the light is delivered to the workstations in the factory. This technique is used to reduce the costs of the light and to simplify the logistics within the fabrication process. Typically, dielectric beam splitters are used. However, they suffer from a polarization-sensitive splitting ratio. Because the polarization content of a high-power Nd:YAG-laser is not constant in time, polarization-independent beam splitters are of concern. The feasibility of diffractive beam splitters is investigated.     

Nonpolarizing and polarizing filter design

We describe a nonpolarizing filter design at oblique incidence and a polarizing filter design at normal incidence that use a uniaxially anisotropic layer. The phase thicknesses and the optical admittances of the layers are compensated for by the birefringent properties of a thin film at oblique incidence. This concept can be applied to the design of nonpolarizing bandpass and edge filters at oblique incidence and of polarizing beam splitters at normal incidence. Besides, the dependence of narrow-bandpass filters on normal incidence is discussed.     

A simple scheme with coupled down converters with type-I phase matching as resource for conditional preparation of macroscopic entangled states

It is shown that macroscopic entangled states can be generated using an experimental arrangement consisting of coupled spontaneous parametric down-converters with type-I phase matching (SPDCI) pumped simultaneously by optical fields in coherent state and two beam splitters. Two beam splitters in auxiliary generated modes are used to conditionally prepare macroscopic entangled states in output pumping modes of the studied system. Identification of two macroscopic entangled states is produced by use of photon number resolving detection. In contrast to all previous schemes, our scheme does not need Kerr-type nonlinear interaction and is purely based on second-order susceptibility of the crystal which is stronger for the Kerr nonlinearity. We calculate concurrence of the states as a measure of the amount of entanglement stored in the states and present analysis concerning ‘separation’ between components forming studied entangled states.     

Achieving stabilization in interferometric logic operations

Interferometric systems with amplitude beam splitters can implement reversible operations that, on detection, become Boolean operators. Being passive, they consume no energy, do not limit the operating bandwidth, and have negligible latency. Unfortunately, conventional interferometric systems are notoriously sensitive to uncontrolled disturbances. Here the use of polarization in a common-path interferometric logic gate with and without polarization beam splitters is explored as an attractive alternative to overcome those difficulties. Two of three device configurations considered offer significant stability and lower drive modulator voltage as advantages over the previous systems. The first experimental tests of such a system are reported. Common-path interferometry lends itself to even more stability and robustness by compatibility with no-air-gap, solid optics.     

Interferences in light deflection by ferroelastic domain walls

Deflection of light is studied in a crystal of glycine phosphite containing two twin walls. When the crystal is rotated in the incident laser beam, interferences are observed in both the direct beam and in the main deflected beam (A or B) for both polarizations of the incident light. The contrast is especially high, because the mutual tilt angle of the principal axes is close to 45 degrees in this twinned crystal. On this principle, fundamental-harmonic beam splitters could be built from as-grown twin crystals. Furthermore, the electrical modulation of the light deflected by ferroelectric-ferroelastic crystals can be now explained in terms of interference effects.     

Simple configuration to generate short bursts of picosecond optical pulses

We have developed an efficient method of generating short bursts of picosecond optical pulses. The multiplying stage consists of simple optical elements, including 50/50 beam splitters and a 100% reflecting mirror. Time delay between the pulses can be adjusted by changing the incident angle of the beam or the separation of the splitters. The multiplying scheme has been successfully applied to the output of a cw mode-locked Nd:YAG laser at 82 MHz. Multiplication of beam pulses up to a factor of 8 was demonstrated. This technique promises to increase the repetition rate of laser pulses up to hundreds of gigahertz.     

Theoretical analysis of two nonpolarizing beam splitters in asymmetrical glass cubes

The design principle for a nonpolarizing beam splitter based on the Brewster condition in a cube is introduced. Nonpolarizing beam splitters in an asymmetrical glass cube are proposed and theoretically investigated, and applied examples are given. To realize 50% reflectance and 50% transmittance at specified wavelengths for both polarization components with an error of less than 2%, two measurements are taken by adjusting the refractive index of the substrate material and optimizing the thicknesses of each film in the design procedures. The simulated results show that the targets are achieved using the method reported here.     

光束偏振的高精度实时检测技术

针对光束的偏振形态,本文设计了一种新型的光束偏振实时检测系统,该系统利用特殊设计的分光片和相位分光镜将被测入射光束分为四路的方法实现了光束偏振状态的实时测量。系统中添加有优化器件可调相位补偿器,用以优化系统仪器矩阵。系统具备优化功能,优化理论和公式被推导和给出。给出了一种理想的仪器矩阵,并实现了该系统,与理想偏差小于0.25%。系统具备优化功能,结构简单,对该系统的偏振检测能力进行了分析和测试,实验结果表明系统测试精度可达97%。     

Crystalline quartz as a far-infrared beam splitter for Fourier transform spectroscopy

We show that room-temperature, crystalline quartz is a useful material for a beam splitter for spectroscopy in the far infrared, ~60-mum wavelength and longer. We compare such a beam splitter with the traditional far-infrared candidates: Mylar, polarizing, and lamellar beam splitters.     

Plate beam splitters and symmetric multiports

Abstract

We show that for experimental arrangements with few nonvacuum inputs the fully symmetric multiport can be replaced by a simpler partially symmetric multiport. This device is equivalent to a beam splitter arrangement where the number of necessary beam splitters increases linearly with increasing number of inputs, that is it is considerably simpler to realize owning to the smaller number of components needed.