Double Grating System Research Articles

Acoustic Angle-Selective Transmission Based on Binary Phase Gratings

Angle-selective transmission is the function of allowing a wave of certain incident angle to pass through the system. It is one of the essential functions in analog wave filtering and control systems. However, the incident-angle dependence and frequency dependence of wave transmission in existing systems are always coupled and cannot be controlled individually without influencing the other. This work reports the design of an angle-selection system that can decouple the incident-angle dependence and frequency dependence in acoustic transmission. The system is formed by the combination of two binary phase gratings with different grating periods. It selects not only the angle of incident wave, but also the frequency. The selected angle and frequency can be tuned independently by changing different geometric parameters in the binary phase gratings. The designed double grating system has the potential to serve as the building elements to construct arbitrary wave-filtering systems in the frequency-angle space, such as the broadband angle-selection system.

Spectral interval compressing structure in spectral beam combination system for diode laser arrays

An efficient spectral beam combining (SBC) system for diode lasers with compressed spectral interval is proposed and demonstrated experimentally. This structure consists of a transmission grating and a reflector paralleled to the grating, which can make the beam diffracted twice by only one transmission grating. The advantage of this configuration is that it compresses the spectral interval of the output beam into half compared to the traditional single-grating setup and improves the utilization rate of the grating element compared to the previous double grating system. The experimental results demonstrate that the spectral intervals of a 3-emitter diode laser array (DLA) and a 5-emitter DLA with traditional single-grating structure are 4.36 and 4.69 nm, respectively. In the modified system, the spectral intervals of the same DLA with 3 and 5 emitters are 2.15 and 2.35 nm, respectively, which shows that this method can compress the spectrum interval effectively and will be promising for a practical SBC system.

Lissajous figure-based single-frame collimation technique

An accurate collimation technique based on a double grating system is proposed. Transversal displacement of the grating is not required and then, automatic single-frame processing can be performed. Talbot self-images are projected onto a mask composed by several shifted diffraction gratings. A Lissajous figure is obtained with the signals acquired by a CMOS camera where the mask is simulated by software. The collimation degree is determined by measuring the ellipticity of the Lissajous figure. Visual or automatic procedures for simple and accurate collimation of a light source are proposed. Experimental results are obtained which show a resolution of δϕ≈4.16μrad in the divergence of the beam when a lens with focal length f=25mm and diameter D=20mm is used for collimation.

Collimation method using a double grating system

We present a collimation technique based on a double grating system to locate with high accuracy an emitter in the focal plane of a lens. Talbot self-images are projected onto the second grating producing moiré interferences. By means of two photodetectors positioned just behind the second grating, it is possible to determine the optimal position of the light source for collimation by measuring the phase shift between the signals over the two photodetectors. We obtain mathematical expressions of the signal in terms of defocus. This allows us to perform an automated technique for collimation. In addition, a simple and accurate visual criterion for collimating a light source using a lens is proposed. Experimental results that corroborate the proposed technique are also presented.

Compression of photon wave packets in the soft-x-ray region

Using a tapered undulator and a double grating system we have compressed soft-x-ray photon wave packets into 37% of the original length. To confirm the shortening of the length of wave packets, we employed a special analog method to measure two-photon correlation as it is without elongating the wave-packet length, while the previous two-photon correlation measurements had adopted the method to largely elongate the wave-packet length up to several hundred fs using a monochromator. The present method gives a new prospect in that the compressed wave-packet length is extremely short and of the order of 1 fs, which enables various soft-x-ray spectroscopic experiments to detect in the time domain fast elementary excitations in a solid.

Double grating systems with one steel tape grating

Steel tape gratings are used in different metrology applications. As the period of these gratings was large (around 100μm,), its analytical study has been performed, up to date, using a geometrical approach. Nowadays, steel tape gratings can be manufactured with lower periods, around 20–40 μm, and diffractive effects must be taken into account. Also, due to the roughness of the surface, statistical techniques need to be considered to analyze their behavior. In this work, an analysis of the pseudo-imaging formation in a double grating system including one steel tape grating is performed. In particular Moiré and Lau configurations are analyzed. We have found that roughness significantly affects to Moiré configuration. However, its effect is negligible in Lau configuration. Generalized grating imaging configuration is also studied in depth. It is shown that roughness does not affect to the contrast of pseudoimages, but it modifies their depth of focus.

Invariant grating pseudoimaging using polychromatic light and a finite extension source

The Talbot effect is a well studied phenomenon by which grating pseudoimages appear at certain periodic distances when monochromatic light is used. Recently, numerical simulations have shown a new phenomenon; when a polychromatic light beam is used in a double grating system, the intensity of the pseudoimages presents a transverse-profile that remains unaffected over a wide range of propagation distances. This effect can be used to increase the tolerances of gratings based optical devices, such as displacement measurement systems, interferometers, and spectrometers. The pseudoimages formation with a polychromatic and finite extension light source is analytically and experimentally demonstrated. Relatively simple analytical expressions for the intensity and the contrast allow us to predict when pseudoimages present a constant contrast and when they disappear. Furthermore, we experimentally obtain the pseudoimages using the proposed configuration, corroborating the theoretical predictions.

Grating pseudo-imaging with polychromatic and finite extension sources

The pseudo-imaging process in a system consisting of two periodic gratings and illuminated by an incoherent polychromatic and finite extension source placed at a finite distance from the gratings is studied. An analytical expression of the irradiance distribution on a plane, also located at a finite distance from the gratings, has been obtained from previous results on monochromatic illumination. In the analysis presented, different imaging regimes are found and related to the parameters which characterize the double grating system. The pseudo-imaging phenomenon strongly depends on both the spatial and temporal coherence of the incident illuminating field. Certain pseudo-images are observed with polychromatic and incoherent incident light under some restrictions. On the other hand, pseudo-image process analogous to Talbot effect appears only by monochromatic and plane illuminating wavefront.

Microwave transmission through a series of inclined gratings

A polarisation cascade matrix is derived which describes the transmission of an arbitrary elliptically polarised wave through an inclined grating of parallel wires. The terms of the matrix are the well established complex reflection and transmission coefficients for a linearly polarised wave incident on a single grating. Hence the matrix can be adapted for gratings with wires or strips of non-circular cross-section. The product of a series of matrices representing a series of gratings whose wires are at different angles to a fixed direction gives the total complex transmission and reflection coefficients. The results are shown to be in agreement with previously published work on a double-grating system, and are experimentally verified for a series of five gratings. These results confirm that such a series of gratings can be used to design broadband polarisation rotators for linear polarisation. A similar matrix analysis can be applied, for wider wire spacings, to the design of broadband circular or elliptical polarisers or analysers. A sharp resonance reflection condition, which occurs when the gratings are separated by half a wavelength, can have applications as a frequency filter.