A high-power multi-transverse modes random fiber laser (RFL) is investigated by combining a master oscillator power-amplifier (MOPA) configuration with a segment of extra-large mode area step-index multimode fiber (MMF). Spatial coherence of the highpower multi-transverse modes RFL has been analyzed, which shows that speckle contrast is reduced dramatically with the output power increasing. In this way, considerably low speckle contrast of ~0.01 is achieved under high laser power of ~56 W, which are the records for multi-transverse modes RFLs in both spatial coherence and output power. This work paves a way to develop high-power RFLs with very low spatial coherence for wide-range speckle-free imaging and free-space communication applications.
Memory-effect-based speckle correlation is one of the most practical
techniques for imaging through scattering opaque media, where a light
source with low spatial coherence and moderate bandwidth plays a
pivotal role. Usually, a rapidly rotating diffuser is applied to make
the light source spatially decoherent. Here, an all-fiber-based
low-spatial-coherence light source is proposed and demonstrated for
such speckle-correlated imaging. The illumination structure is greatly
simplified, the lightening efficiency is enhanced, and the wavelength
is extended to the near-infrared band, which is favorable for a larger
memory effect range and deeper penetrating depth through opacity.
Moreover, the proposed local illumination method can identify the
orientation of the object, which has not been revealed by former
methods. This work would facilitate the research in optical biomedical
imaging and broaden the applications of multimode random fiber
lasers.
Control of the properties of speckle patterns produced by mutual interference of light waves is important for various applications of multimode optical fibers. It has been shown previously that a high signal-to-noise ratio in a multimode fiber can be achieved by preferential excitation of lower order spatial eigenmodes in optical fiber communication. Here we demonstrate that signal spatial coherence can be tailored by changing relative contributions of the lower and higher order multimode fiber eigenmodes for the research of speckle formation and spatial coherence. It is found that higher order spatial eigenmodes are more conducive to the final speckle formation. The minimum speckle contrast occurs in the lower order spatial eigenmodes dominated regime. This work paves the way for control and manipulation of the spatial coherence of light in a multimode fiber varying from partially coherent or totally incoherent light.
In this work, a backscattering imaging method based on near infrared random fiber laser is shown to provide a high contrast optical image between carious and sound enamel. The obtained contrast is 0.70, which is more than 8 times higher than the contrast obtained from radiographic imaging. Caries and cracks in enamel could clearly be identified against healthy enamel using the optical system. The near infrared wavelength, high spectral density and low coherence of random fiber laser contribute to its deep penetration, high brightness and low speckle contrast, using the method in a backscattering configuration opens potential clinical use.
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