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Register a new userEfficient method for controlling the spatial coherence of a laser
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An efficient method to tune the spatial coherence of a degenerate laser over a broad range with minimum variation in the total output power is presented. It is based on varying the diameter of a spatial filter inside the laser cavity. The number of lasing modes supported by the degenerate laser can be controlled from 1 to 320,000, with less than a 50% change in the total output power. We show that a degenerate laser designed for low spatial coherence can be used as an illumination source for speckle-free microscopy that is 9 orders of magnitude brighter than conventional thermal light.
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Dielectric microstructures have generated much interest in recent years as a means of accelerating charged particles when powered by solid state lasers. The acceleration gradient (or particle energy gain per unit length) is an important figure of merit. To design structures with high acceleration gradients, we explore the adjoint variable method, a highly efficient technique used to compute the sensitivity of an objective with respect to a large number of parameters. With this formalism, the sensitivity of the acceleration gradient of a dielectric structure with respect to its entire spatial permittivity distribution is calculated by the use of only two full-field electromagnetic simulations, the original and adjoint. The adjoint simulation corresponds physically to the reciprocal situation of a point charge moving through the accelerator gap and radiating. Using this formalism, we perform numerical optimizations aimed at maximizing acceleration gradients, which generate fabricable structures of greatly improved performance in comparison to previously examined geometries.
We introduce a simplified version of the steady-state ab initio laser theory for calculating the effects of mode competition in continuous wave lasers using the passive cavity resonances. This new theory harnesses widely available numerical methods that can efficiently calculate the passive cavity resonances, with negligible additional computational overhead. Using this theory, we demonstrate that the pump profile of the laser cavity can be optimized both for highly multi-mode and single-mode emission. An open source implementation of this method has been made available.
The experimental characterization of the spatial and temporal coherence properties of the free-electron laser in Hamburg (FLASH) at a wavelength of 8.0 nm is presented. Double pinhole diffraction patterns of single femtosecond pulses focused to a size of about 10 microns by 10 microns were measured. A transverse coherence length of 6.2 microns in the horizontal and 8.7 microns in the vertical direction was determined from the most coherent pulses. Using a split and delay unit the coherence time of the pulses produced in the same operation conditions of FLASH was measured to be 1.75 fs. From our experiment we estimated the degeneracy parameter of the FLASH beam to be on the order of $10^{10}$ to $10^{11}$, which exceeds the values of this parameter at any other source in the same energy range by many orders of magnitude.
Coherence properties and wavelength of light sources are indispensable for optical coherence microscopy/tomography as they greatly influence the signal to noise ratio, axial resolution, and penetration depth of the system. In the present letter, we investigated the longitudinal spatial coherence properties of the pseudo-thermal light source (PTS) as a function of spot size at the diffuser plane, which is controlled by translating microscope objective lens towards or away from the diffuser plane. The axial resolution of PTS is found to be maximum ~ 13 microns for the beam spot size of 3.5 mm at the diffuser plane. The change in the axial resolution of the system as the spot size is increased at the diffuser plane is further confirmed by performing experiments on standard gauge blocks of height difference of 15 microns. Thus, by appropriately choosing the beam spot size at the diffuser plane, any monochromatic laser light source depending on the biological window can be utilized to obtain high axial-resolution with large penetration depth and speckle-free tomographic images of multilayered biological specimens irrespective of the source temporal coherence length. In addition, PTS could be an attractive alternative light source for achieving high axial-resolution without needing chromatic aberration corrected optics and dispersion-compensation mechanism, unlike conventional setups.
We design and fabricate an on-chip laser source that produces a directional beam with low spatial coherence. The lasing modes are based on the axial orbit in a stable cavity and have good directionality. To reduce the spatial coherence of emission, the number of transverse lasing modes is maximized by fine-tuning the cavity geometry. Decoherence is reached in a few nanoseconds. Such rapid decoherence will facilitate applications in ultrafast speckle-free full-field imaging.
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