Coherence Length Of Radiation Research Articles

On an interferometric method for determining the X-ray coherence length of radiation

On an interferometric method for determining the X-ray coherence length of radiation

Optical fluxes propagation in the planar transport layer of multilayer coatings

The article is concerned with peculiarities study of the quasimonochromatic optical fluxes propagation through thin planar transparent layer of multilayer coating. There is shown that these fluxes can be transported by the layer in process of its multiple consequtive total internal reflection or by the waveguide-resonance propagation manner depending on correlation between the layer width and the radiation coherence length half of transported fluxes. Efficiency comparison of these radiation transportation mechanisms showed that the waveguide-resonance propagation approach is more adequate for results description of the optical waveguides functioning. It allowed to conclude that optical waveguides (fibers) function in frame of the waveguide-resonance paradigm and the waveguide-resonance mechanism is responsible for the light fluxes transportation on great distances.

Partial Coherence of Laser Diode Radiation as a Cause of Generation Channels Formation

The modeling of high-power wide-contact laser diodes is discussed taking into account the limitations imposed by the short coherence length of radiation. The relationships between coherence, spectrum, and nonlinear properties of a laser resonator are considered.

The coherence of powerful laser diodes and formation of lasing channels

The effect of the radiation coherence length on the lasing channels lateral dimensions in a planar laser diode with a wide contact is considered. The relationship between the coherence of high-power laser diode radiation, its spectrum and the nonlinear optical properties of the laser resonator active medium is discussed.

High Brightness SASE Operation of X-ray FELs

A method to achieve High-Brightness Self-Amplified Spontaneous Emission (HB-SASE) in the Free Electron Laser (FEL) is described. The method is a development of earlier work in which a series of unequal delays to the electron beam was shown to extend the SASE temporal coherence. It is now shown that the enhanced electron-radiation slippage delocalises the collective FEL interaction and breaks the dependence of the radiation coherence length on the FEL cooperation length. The method requires no external seeding or photon optics and is therefore applicable at any wavelength or repetition rate. Simulations show that the radiation coherence length can be increased by approximately two orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown where HB-SASE may generate near transform-limited X-ray FEL pulses. The dependence of the output on the undulator module length and the choice of delay sequence are discussed.

Transform-Limited X-Ray Pulse Generation from a High-Brightness Self-Amplified Spontaneous-Emission Free-Electron Laser

A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research.

Propagation-based phase-contrast enhancement of nanostructure images using a debris-free femtosecond-laser-driven cluster-based plasma soft x-ray source and an LiF crystal detector

We demonstrate in-line phase-contrast imaging of nanothickness foils by using a relatively large, polychromatic, debris-free femtosecond-laser-driven cluster-based plasma soft x-ray source, and a high-resolution, large dynamic range LiF crystal detector. The spatial coherence length of radiation in our setup reached a value of 5 microm on the sample plane, which is enough to observe phase-contrast enhancement in the images registered by the detector placed only a few hundred micrometers behind the object. We have developed a tabletop soft x-ray emission source, which emits radiation within a 4pi sr solid angle, and which allows one to obtain contact and propagation-based phase-contrast imaging of nanostructures with 700 nm spatial resolutions. This advance could be of utility for metrology applications.

Application of Young—Michelson and Brown—Twiss interferometers for determining geometric parameters of nonplanar rough objects

The possibility of using Young—Michelson and Brown—Twiss interferometers for measuring the angular dimensions and parameters of the surface shape of remote passively scattering and self-luminous nonplanar rough objects by optical radiation propagating from them is substantiated. The analysis is based on the properties of approximate transverse functions of field coherence Bt and Bt' and intensity coherence Bti and Bti' formed by the time averaging of the products of fields and intensities taken at two points of a receiving aperture (the prime denotes self-luminous objects). The averaging time is set to be much longer than the coherence time of radiation propagating from an object. It is shown that for the radiation coherence length much smaller than the depth of the visible region of the object, the functions Bt and Bt' are proportional to the Fourier transform of the intensity distribution in the image of a remote object, which is the generalisation of the Van Cittert—Zernicke theorem to the case of a nonplanar object, while functions Bti and Bti' are proportional to the squares of the modulus of the Fourier transform of this distribution. It is also shown that the recording of functions Bt and Bt' with a Young—Michelson interferometer gives only the angular dimensions of the visible region of objects, whereas the recording of functions Bti and Bti' with a Brown—Twiss interferometer allows one to find these dimensions and the radius of curvature of the object surface.

Phase-contrast imaging of nanostructures by soft x rays from a femtosecond-laser plasma

The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ≃1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ≃100 nm.

Role of coherence in interaction of opticalradiation with macromolecules

Based on analysis of the properties of macromolecules in a coherent optical radiation field and taking into account experimentally established data on the specificity of the interaction between laser radiation and biomolecules (the dependence of the efficacy of the interaction on the coherence length, the presence of an effect in a region of the spectrum far away from the absorption band), we propose a mechanism for wave interaction of coherent optical radiation with macromolecules, and we construct a very simple mathematical model for such interaction. Using the mathematical model, we calculate the dependence of the vibrational energy of the macromolecule in a coherent radiation field on the coherence time and the intramolecular relaxation rate. We show that the increase in the vibrational energy of the macromolecules strongly depends on the radiation coherence length. When exposed to incoherent radiation, the vibrational energy of the biomolecules remains practically constant, while when exposed to laser radiation (coherence length ≈3 cm), the vibrational energy of the atoms increases by 2–4 orders of magnitude, leading to a change in the conformation of the biomolecules and the activity of enzymes.

Influence of the coherence length of radiation on phase noise in a fiber-optic rotation sensor

An investigation was made of a fiber-optic rotation sensor with a semiconductor light source. The length of a single-mode fiber waveguide was 1 km, its linear losses were ~ 2 dB/km at λ = 0.84 μ, the coil diameter was 19 cm, and the sensitivity of the sensor was 0.5 deg/h (1.1 × 10−5 rad) for an integration time of 0.5 sec. The drift of the zero setting in 80-min operation was ~ 6 deg/h when the fiber waveguide coil was screened from external magnetic fields and the integration time was 1 sec. A comparison was made with a similar rotation sensor based on a He–Ne laser. A new method for suppressing nonreciprocal phase noise by a vibration device simulating the Sagnac phase shift in a wide spectral range was proposed and demonstrated.