Spatial Coherence Degradation Research Articles

Soft X-ray grating monochromators as a source of spatial coherence degradation: A wave-optical approach

We present wave-optical simulations and a coherence analysis of the photon beam transported through a soft X-ray beamline, paying particular attention to a focusing varied line spacing (VLS) plane grating monochromator (PGM). We show that this beamline optical element used in several soft X-ray beamlines at synchrotron sources and free-electron lasers can cause a non-negligible spatial coherence degradation. We demonstrate that the origin of this effect arises from the coupling between spatial and spectral properties of the photon beam generated by the grating. The latter implies that space and frequency dependencies are not separable after such a dispersive element. It is shown which parameters are essential for this effect and how they are linked to each other.

Thermodynamic and nonlinear optical analysis of solid-state multipass cells for compression of picosecond pulses in the 2−μm range

Multipass cells (MPCs) recently emerged as a new femtosecond pulse compression technique for lasers with high pulse energy. While most of the work focused on the compression of near-infrared laser systems so far, we address the case of holmium-based gain materials, which offer an enormous potential in terms of pulse energy, yet can only host a very narrow bandwidth in the 2-$\ensuremath{\mu}\text{m}$ wavelength range. Compression into the few-cycle regime, therefore, requires at least a hundredfold spectral broadening. Specifically, we investigate the utility of four different solid-state materials in the MPC, namely, fused silica, sapphire, YAG, and diamond. Spectral broadening dynamics is numerically investigated using the $2D+1$-unidirectional pulse propagation equation. To this end, we put particular emphasis on the thermal properties of the nonlinear optical material, which turn out as a critical issue above 2-$\ensuremath{\mu}\text{m}$ wavelength. Solving the heat equation for each of the materials, we then estimate the maximum temperature at beam center. These considerations show that outside the near infrared, thermodynamic parameters of nonlinear optical materials become at least equally important as their optical properties. Among the four materials under test, in particular, diamond stands out as it combines highly favorable thermal properties with a large optical nonlinearity. Finally, a concomitant slow degradation of spatial coherence is monitored. Our findings provide an effective guideline for the design of high pulse-energy compressors at wavelengths of $2\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\text{m}$ and beyond.

Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor.

Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed.

Spatial acquisition scheme with ultrawide FOV for atmospheric optical links: based on a fish-eye lens and a sinusoidal amplitude grating

We present a spatial acquisition scheme based on a fish-eye lens and a sinusoidal amplitude grating that can acquire laser beacons in an ultrawide field of view for atmospheric optical links. In this scheme, an incoming laser beacon can be imaged into three diffraction spots. The laser direction and the wavelength can be measured from the center position of the spots and the distance between two adjacent spots, respectively. The total optical transfer function can be described by the combined effect of the acquisition system itself and the spatial coherence degradation due to atmospheric turbulence. Spot spread and spot dancing effects are both considered for long-term exposure and short-term exposure. We investigate how root mean square errors of the measured laser direction and wavelength vary with the ratio of the pupil diameter D2 to atmospheric coherence length r0, without consideration of aberrations of the fish-eye lens. The results show that with the limitation of r0 the performance cannot be infinitely improved by enlarging D2. When considering aberrations of the fish-eye lens, the imaged spot is gradually blurred and deteriorated with the increase of the field angle, since aberrations are enlarged with the increase of the field angle. We study how the measured errors of laser direction and wavelength vary with r0 and incident angles (with different aberrations). A severe turbulence with a small value of r0 and a large incident angle will cause a bad acquisition performance.

Effect of spatial coherence on damage occurrence in multimode optical fibers.

We investigate the influence of spatial coherence on damage occurrence in highly multimode optical fibers using ultraviolet (UV) nanosecond pulses, with the aim of delivering high fluence in the UV. In some cases, the optical damage is initiated below the fiber facet damage threshold and takes place along the propagation path; such damage is believed to be caused by local constructive interference, creating "hot spots." In order to reduce the degree of spatial coherence, we used a large-diameter core (1.5mm) fiber as a mode scrambler. Different lengths of this large core fiber were used to deliver energy to a fiber core with a smaller diameter (0.6mm), in which the damage occurrence was observed. The experimental results indicate that there is a correlation between the degree of spatial coherence and the occurrence of optical damages, typically observed a few millimeters from the fiber facet. Numerical simulations, based on the beam-propagation method, support the degradation of spatial coherence, due to the excitation of high-order modes. Finally, by degrading the spatial coherence of the beam, we establish a new record by delivering more than 100mJ via a 1.5mm core diameter fiber in the UV, corresponding to ∼26 times the critical power for self-focusing. Our work sheds light on the ability to deliver high energies of nanosecond-pulsed UV laser radiation through multimode optical fibers.

Spatial Coherence Degradation of Light Influenced by Temperature and Aerosol by Use of Atmospheric Turbulence Chamber

The spatial coherence of light influenced by temperature and aerosol is measured. An atmospheric turbulence chamber is used in experiments. The spatial coherence of light degrades with increase of turbulence in accordance with the rise of temperature and the increase of aerosol. In the experiment for wavelength dependency, the degradation is larger for a shorter wavelength. The experimental results are in good agreement with theoretical results currently available.

Effects Of Aerosols On Imaging Through The Atmosphere: A Review Of Spatial Frequency And Wavelength Dependent Effects

Experiments carried out over diagonal lines of sight through the entire atmosphere support the concept of spatial coherence degradation through forward scattering as described by an aerosol transfer function that strongly affects the wavelength dependence of imaging through the atmosphere. Airborne particulate size and concentration are affected strongly by wind strength and soil moisture. Changes in weather that result in changes in average particulate size of airborne soil-derived particulates also strongly change the wavelength dependence of resolution through the atmosphere as a result of changes in the wavelength dependence of the scattering coefficient. Knowledge of such effects can therefore permit prediction of spectral regions most suitable for imaging through the atmosphere. The roles of aerosols in determining optimum spectral regions for imaging through the atmosphere are quite significant since the turbulence modulation transfer function displays very little spectral dependence.

Heterodyne detection of optical signals propagating through the atmosphere in the presence of scattering and turbulence

The effects of the spatial coherence degradation of optical signals on optical heterodyne detection are investigated. The expression of the signal-to-noise (SNR) is derived, taking into account the receiving optics, the atmosphere turbulence, and the scattering, in a form suitable for numerical computation. This expression is in terms of the mutual intensity function of the homogeneous signal field. It is shown that the effective detector radius is nearly equal to that of the Airy disk, and that the effective radius of the receiving aperture approximately equals the coherence length of the signal on the aperture plane.

Heterodyne detection through rain, snow, and turbid media: effective receiver size at optical through millimeter wavelengths

Both scattering and turbulence can effect the spatial coherence of short wavelength signals propagating through the open atmosphere. In this paper, the influence of forward scattering on heterodyne receiver performance is investigated, taking into account turbulence. It is shown that the effect of forward scattering is to reduce the effective heterodyne receiver area through spatial coherence degradation. A common approach to scattering as an attenuation phenomenon is not always valid. Generally, this approach underestimates the SNR. The accuracy of the attenuation approach depends on the ratio R of the actual receiver diameter to the scattering particle diameter. If R >100, scattering is essentially large angle and the typical treatment of scattering as an attenuation effect is indeed justified. However, for small R, forward scattering is primarily small angle, field coherence is noticeably affected by forward scattering, and the attenuation approach is not valid. Further, it is shown that the SNR is improved when the ratio of the scattering particulate size to turbulence coherence diameter decreases. From the practical point of view, the most important result of this study is that small receivers use their area more effectively than large receivers. Thus, an array of several small receivers may perform better than one large receiver with the same total area. The treatment here is particularly relevant for coherent detection through clouds, fog, precipitation, and turbid media in general, including liquid media.

Spatial-frequency- and wavelength-dependent effects of aerosols on the atmospheric modulation transfer function

Experiments carried out over diagonal lines of sight through the entire atmosphere support the concept of spatial-coherence degradation through forward scattering as described by an aerosol modulation transfer function that strongly affects the wavelength dependence of imaging through the atmosphere. Airborne-particulate size and concentration are affected strongly by wind strength and soil moisture. Changes in weather that result in changes in average particulate size of airborne soil-derived particulates also strongly change the wavelength dependence of resolution through the atmosphere as a result of changes in the wavelength dependence of the scattering coefficient.