Profile formation in holographic diffraction gratings

A theoretical method is outlined for the calculation of the profiles of holographic diffraction gratings produced in photo-resist. This method is shown to give results in agreement with all experimental observations of such profiles. Consideration is then given to the flexibility of arrangements for the formation of holographic gratings and an experimental system is discussed which fully utilizes this flexibility. The wide range of profiles which may be generated with such a system is demonstrated. Finally a theoretical programme of investigation is suggested which may well make possible the formation of holographic gratings of greatly improved spectral performance.     

Phonon diffraction gratings

A two-dimensional diffraction grating for phonons is described. It scatters thermal phonons strongly when their dominant wavelength is equal to the grating's lattice constant. Through measurements of the Kapitza resistance between the silicon crystal carrying the grating and liquid helium, it is shown that the scattering by the grating is predominantly coherent, as expected for phonon diffraction.     

Ray picture of diffraction gratings

We elaborate on a geometrical picture of diffraction gratings. Exact paraxial propagation including coherence effects is obtained by allowing the geometrical rays to transport Wigner function instead of simply specific intensity. We apply this formalism to perfect and less than perfect gratings, illuminated by plane and Gaussian waves.     

Thermomechanical modification of diffraction gratings

The most accurate approaches to fabrication of diffraction gratings are known to be the lithographic and holographic methods. The lithographic methods allow fabrication of arbitrarily chirped gratings whose performance, however, is degraded by stitching errors. The holographic methods are free from stitching errors; however, they are limited in the achievable spatial variations of their grating periods. We suggest a method of diffraction grating modification by nonuniform heating and stretching that is much more flexible than the holographic approach and does not suffer from the problem of stitching error. We demonstrate our approach for quartz phase masks that have a characteristic grating period of 1 microm and a length of several centimeters. Our approach allows the grating periods of the phase masks to vary in a range from a few picometers to a few nanometers and a spatial resolution of a few millimeters. It is shown that the grating period can be modified with a negligible effect on the profile of the gratings.     

中皮秒二波耦合及其应用

本文报道在InP:Fe中皮秒二波耦合的实验研究.文中测量了瞬态二波耦合增益随泵浦光与探测光相对延迟时间改变而变化的曲线,分析了瞬态二波耦合中各种过程的特性,最后还利用皮秒二波耦合实现了低开启光强、高信噪比,透过率达17%的皮秒光开关.     

Nonstationary gain processes of holographic gratings by two-wave interaction in photorefractive crystals

The transformation processes of holograph gratings in photorefractive crystals from two-wave interaction with detuning have been studied. For weak interactions an analytic solution has been obtained. Analysis of the gain, both for a purely nonlocal response, and for phase-shifted responses has been performed. Gain thresholds have been determined in terms of their dependence on materials parameters and the conditions of interaction. It is shown that the main effects in the gain process are a rotation of the grating vector towards its Bragg orientation and the appearance of a strong nonuniformity in the grating in proportion to the gain. As a result, for significant increases in the thresholds, the growth of the diffraction field of a non-Bragg-matched grating is larger than for diffraction of a Bragg-matched grating.Tomsk State Academy of Linear Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 77–83, July, 1995.     

Calculations of phase reflection diffraction gratings

Intensities of principal diffraction maxima for a phase reflection diffraction grating with a rectangular groove profile were calculated by the Fresnel method, using a special algorithm. The procedure under consideration enables us to perform high-efficiency calculations of diffracted-beam characteristics and can be used extensively to optimize diffraction-grating parameters.__________This revised version was published online in May 2005 with a correction on affiliation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 38–41, November 2004.     

Shaping of photorefractive two-wave coupling by fast phase modulation

Using the distinctive features of the photorefractive nonlinearity, we derive a general self-consistent set of equations to describe two-wave coupling in the presence of fast and arbitrary strong-phase modulation. By considering a number of important particular cases, we show that phase modulation is a powerful and useful tool for shaping the characteristics of two-wave coupling such as the value of the energy exchange, the diffraction efficiency of the recorded grating, and the structure of the grating fringes. Finally, we analyze the role of the phase modulation in the active stabilization of wave coupling by means of an electronically introduced phase feedback.     

Microwave pulse compression using diffraction gratings

Electromagnetic pulse compressors employing gratings of high breakdown strength in the autocollimation regime at the center frequency of the pulse for p polarization are investigated theoretically. A theory is devised on the basis of a numerical solution of the electromagnetic field equations in an integral-equation formulation. The case of the aberrationless approximation of the compression of a bell-shaped pulse with a quadratically modulated phase is considered analytically. A numerical experiment demonstrates the presence of strong distortions in the pulse after compression, which are caused by the cubic and higher terms in the expansion of the frequency dependence of the phase acquired by the wave in the compressor. The conditions for optimal pulse compression with allowance for aberrations are investigated. Zh. Tekh. Fiz. 68, 82–90 (February 1998)     

Light diffraction on acoustophotorefractive holographic gratings

The intermediate regime of light diffraction on acoustophotorefractive holographic gratings written in cubic photorefractive crystals according to the synchronous-detection mechanism is studied. It is shown that the diffraction efficiency in sillenite-type gyrotropic crystals only slightly depends on the incident light polarization and the external electric field. The highest diffraction efficiency in nongyrotropic crystals was achieved for p-polarized writing and reading light and at a considerable external electric field strength.     

Producing binary diffraction gratings in the double-diffraction system

In this paper, a method — based on the diffraction of light — for producing binary diffraction gratings is presented. It is shown that using two master gratings of the Ronchi type and utilizing the results of the double diffraction system it is possible to produce binary gratings with an arbitrary opening ratio. The results have been extended for a two-dimensional case and it is shown that producing binary grid gratings with arbitrary opening ratios in both x and y directions is also possible.     

Diffraction efficiency of holographic blazed diffraction gratings

Holographic blazed diffraction grating is analysed as a periodic relief of an inclined sinusoidal curve in the shallow profile approximation. Under the condition that the rate of the development is a linear function of the development dose the evolution of the relief during the development is discussed. Theoretical curves of diffraction efficiency are compared with measurements on gratings made by direct holographic method for uv spectral region.     

Numerical analysis of apodized fiber Bragg gratings formation using phase mask with variable diffraction efficiency

Scalar diffraction theory is applied to analyze the intensity distribution in the fiber core during fiber Bragg grating (FBG) fabrication using an apodized phase mask. The averaged diffraction field distribution in fiber core was calculated as a function of optical fiber position. The results show that in a case of apodized FBGs fabrication, averaged field intensity profile, and thus refractive index changes in fiber core have complex form. Moreover, it was shown that the influence of optical fiber position behind the phase mask on average intensity distribution in fiber core decreases with increasing of its diameter.     

Formation of holographic diffraction gratings in photoresist

The exposure-development process for the fabrication of holographic diffraction gratings for integrated optics in the positive photoresist Shipley AZ-1350 is theoretically and experimentally investigated. An analysis of the light intensity distribution is carried out, taking into account the reflectivity at the photoresist-substrate interface and the attenuation in the photoresist for two holographic exposure arrangements. The influence of the exposure energy, the development time for a fixed concentration of the developer and the initial photoresist thickness are described. Various grating profiles are calculated for photoresist films, coated on matched and reflective substrates. Scanning electron micrographs demonstrate very good agreement between the actual and calculated gratings profiles.