Low-spatial-coherence liquid crystal based random fiber laser for speckle-free imaging applications
Tzu-Hsuan Yang1*, Chun-Wei Chen2, Hung-Chang Jau1, Chih-Wei Wu1, Iam Choon Khoo2, Tsung-Hsien Lin1
1Department of Photonics, National Sun Yat-Sen University, Kaohsiung, Taiwan
2School of Electrical Engineering and Computer Science, Pennsylvania State University, Pennsylvania, USA
* presenting author:Tzu-Hsuan Yang, email:pascale93lion@gmail.com
Fiber lasers are ideal light sources for imaging applications (e.g. optical coherence tomography) because of their brightness, directionality and flexibility. The quality of laser imaging is, however, severely restricted by speckles and strong diffractions that originate from high spatial coherence. Random lasers are a particular class of laser sources, in which the resonant feedback comes from multiple scattering. One random laser comprises a massive amount of tiny little laser sources, thus featuring low spatial coherence. Liquid crystals have been reported as one promising branch of random laser materials that enable polarized emission, directional output, low lasing threshold and tunable laser properties. Here, we propose a Liquid Crystal based Random Fiber Laser (LC-RFL), which emits in-line and low-coherence random lasing, and demonstrate its feasibility in speckle-free laser imaging.
The LC-RFL is fabricated by sandwiching a fluorescent nematic liquid crystal in between two multimode fiber with surface-aligning facets; the active region is clad in a glass capillary. Detailed underlying mechanisms for the random laser action in a partially ordered liquid crystal will be presented, as well as the optical characteristics and features of the LC-RFL.
An important part of the presentation will be devoted to the demonstration of laser imaging and associated speckle characterization. Both the images from a pulsed Nd:YAG laser and continuous-wave He-Ne laser passing through 0.5 m of an optical fiber are filled with speckles; the corresponding speckle contrasts are ~ 0.5. However, if replaced with a LC-RFL, the image becomes smooth and uniform; also, the speckle contrast is below ~ 0.04, under which the speckles cannot be detected by human eyes.

Keywords: Random laser, Liquid crystal, Fiber laser, Speckle-free imaging, Spatial coherence