This paper demonstrates the wavelength locking of a coherent random lasing system, i.e. an Erbium-doped random fiber laser with a disordered array of fiber Bragg gratings. To lock lasing modes of the disordered system, an external seed light from a tunable laser was introduced into the cavity. It was found that different emission wavelengths/modes can be selected to emit separately through injection locking. The wavelength fluctuation of the output is less than 0.01%, and the power fluctuation is less than 4%. The proposed method is also applicable to general disordered systems, providing an efficient way to control or select light emission in these systems.

Contrary to the conventional detection method like radiography, the near infrared light source has been demonstrated to be suitable for dental imaging due to different reflectivity among enamel, dentin, and caries lesion. In this paper, three light sources with different bandwidths based on a transillumination method are compared. The contrast among enamel, dentin, and caries lesion is calculated in different situations. The experimental results show that the random fiber laser has the best comprehensive quality in dental imaging due to its high spectral density, low coherence, and deep penetration. This work provides a guidance for light source selection in dental imaging.

Decoherence of fiber laser sources is of great importance in imaging applications, and most current studies use ordinary multi-mode fibers (MMFs). Here, a newly designed single-trench fiber (STF) is investigated to reduce the spatial coherence of fiber light source and compared with MMFs. By bending two fibers with different turns, speckle contrast of a 0.8-m-long STF can be reduced from 0.13 to 0.08, while a 0.8-m-long MMF shows an inverse result. Through speckle contrast and decoupling-mode analysis, the reason of this inverse trend is revealed. Firstly, the STF supports more modes than the MMF due to its larger core diameter. Secondly, mode leak from the first core of the STF can couple to the second core when bending the STF. Thus, power distribution among high and low-order modes become more even, reducing the spatial coherence considerably. However, in the MMF, high-order modes become leaky modes and decrease slightly when bending the fiber. This work provides a new method to modulate coherence of light source and a new angle to study decoherence principle using special fibers.

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