Speckle formation is a limiting factor when using coherent sources for imaging and sensing, but can provide useful information about the motion of an object. Illumination sources with tunable spatial coherence are therefore desirable as they can offer both speckled and speckle-free images. Efficient methods of coherence switching have been achieved with a solid-state degenerate laser, and here we demonstrate a semiconductor-based degenerate laser system that can be switched between a large number of mutually incoherent spatial modes and few-mode operation.
Our system is designed around a semiconductor gain element, and overcomes barriers presented by previous low spatial coherence lasers. The gain medium is an electrically-pumped vertical external cavity surface emitting laser (VECSEL) with a large active area. The use of a degenerate external cavity enables either distributing the laser emission over a large (~1000) number of mutually incoherent spatial modes or concentrating emission to few modes by using a pinhole in the Fourier plane of the self-imaging cavity. To demonstrate the unique potential of spatial coherence switching for multimodal biomedical imaging, we use both low and high spatial coherence light generated by our VECSEL-based degenerate laser for imaging embryo heart function in Xenopus, an important animal model of heart disease. The low-coherence illumination is used for high-speed (100 frames per second) speckle-free imaging of dynamic heart structure, while the high-coherence emission is used for laser speckle contrast imaging of the blood flow.
Hui Cao, Sebastian Knitter, Changgeng Liu, Brandon Redding, Mustafa Kezar Khokha, Michael Andrew Choma, and Wen Xiong, "Coherence switching of a vertical-cavity semiconductor-laser for multimode biomedical imaging (Conference Presentation)," Proc. SPIE 10075, Biophysics, Biology and Biophotonics II: the Crossroads, 1007502 (Presented at SPIE BiOS: January 28, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2251513.5381802478001.
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