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Register a new userStudy of longitudinal coherence properties of pseudo thermal light source as a function of source size and temporal coherence
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In conventional OCT, broadband light sources are generally utilized to obtain high axial resolution due to their low temporal coherence (TC) length. Purely monochromatic (i.e., high TC length) light sources like laser cannot be implemented to acquire high resolution optically sectioned images of the specimen. Contrary to this, pseudo thermal light source having high TC and low spatial coherence (SC) property can be employed to achieve high axial resolution comparable to broadband light source. In the present letter, a pseudo thermal light source is synthesized by passing a purely monochromatic laser beam through a rotating diffuser. The longitudinal coherence (LC) property of the pseudo thermal light source is studied as a function of source size and TC length. The LC length of the synthesized light source decreased as the source size increased. It is found that LC length of such light source becomes independent of the parent laser TC length for source size of greater than or equal to 3.3 mm and become almost constant at around 30 micron for both the lasers. Thus any monochromatic laser light source can be utilized to obtain high axial resolution in OCT system irrespective of its TC length. The maximum achievable axial resolution is found to be equal to 650 nm corresponding to 1.2 numerical aperture (NA) objective lens at 632 nm wavelength. The findings elucidate that pseudo thermal source being monochromatic in nature can improve the performance of existing OCT systems significantly.
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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 consider an inverse source problem for partially coherent light propagating in the Fresnel regime. The data is the coherence of the field measured away from the source. The reconstruction is based on a minimum residue formulation, which uses the authors recent closed-form approximation formula for the coherence of the propagated field. The developed algorithms require a small data sample for convergence and yield stable inversion by exploiting information in the coherence as opposed to intensity-only measurements. Examples with both simulated and experimental data demonstrate the ability of the proposed approach to simultaneously recover complex sources in different planes transverse to the direction of propagation.
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.
In absence of a lens to form an image, incoherent or partially coherent light scattering off an obstructive or reflective object forms a broad intensity distribution in the far field with only feeble spatial features. We show here that measuring the complex spatial coherence function can help in the identification of the size and location of a one-dimensional object placed in the path of a partially coherent light source. The complex coherence function is measured in the far field through wavefront sampling, which is performed via dynamically reconfigurable slits implemented on a digital micromirror device (DMD). The impact of an object -- parameterized by size and location -- that either intercepts or reflects incoherent light is studied. The experimental results show that measuring the spatial coherence function as a function of the separation between two slits located symmetrically around the optical axis can identify the object transverse location and angle subtended from the detection plane (the ratio of the object width to the axial distance from the detector). The measurements are in good agreement with numerical simulations of a forward model based on Fresnel propagators. The rapid refresh rate of DMDs may enable real-time operation of such a lensless coherency imaging scheme.
Quantum light sources are characterized by their distinctive statistical distribution of photons. For example, single photons and correlated photon pairs exhibit antibunching and reduced variance in the number distribution that is impossible with classical light. Most common realizations of quantum light sources have relied on spontaneous parametric processes such as down-conversion (SPDC) and four-wave mixing (SFWM). These processes are mediated by vacuum fluctuations of the electromagnetic field. Therefore, by manipulating the electromagnetic mode structure, for example, using nanophotonic systems, one can engineer the spectrum of generated photons. However, such manipulations are susceptible to fabrication disorders which are ubiquitous in nanophotonic systems and lead to device-to-device variations in the spectrum of generated photons. Here, we demonstrate topologically robust mode engineering of the electromagnetic vacuum fluctuations and implement a nanophotonic quantum light source where the spectrum of generated photons is robust against fabrication disorders. Specifically, we use the topological edge states to achieve an enhanced and robust generation of correlated photon pairs using SFWM and show that they outperform their topologically-trivial counterparts. We demonstrate the non-classical nature of our source using conditional antibunching of photons which confirms that we have realized a robust source of heralded single photons. Such topological effects, which are unique to bosonic systems, could pave the way for the development of robust quantum photonic devices.
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