Esmaeil Mobini

Random lasing in an Anderson localizing optical fiber

A directional random laser mediated by transverse Anderson localization in a disordered glass optical fiber is reported. Previous demonstrations of random lasers have found limited applications because of their multi-directionality and chaotic fluctuations in the laser emission. The random laser presented in this paper operates in the Anderson localization regime. The disorder induced localized states form isolated local channels that make the output laser beam highly directional and stabilize its spectrum. The strong transverse disorder and longitudinal invariance result in isolated lasing modes with negligible interaction with their surroundings, traveling back and forth in a Fabry–Perot cavity formed by the air–fiber interfaces. It is shown that if a localized input pump is scanned across the disordered fiber input facet, the output laser signal follows the transverse position of the pump. Moreover, a uniformly distributed pump across the input facet of the disordered fiber generates a laser signal with very low spatial coherence that can be of practical importance in many optical platforms including image transport with fiber bundles.

Design of a wavelength-tunable optical fiber tweezer (Conference Presentation)

We present for the first time a wavelength tunable fiber-based optical tweezer using a graded index multimode optical fiber (GIMF). Optical fiber tweezer is a viable replacement for the bulky objective-based tweezers because of the low-cost and user-friendly operation and maneuverability. The proposed optical fiber tweezer consists of a GIMF spliced to a single mode fiber into which a wavelength tunable laser is launched. The exit field at the end-facet of the GIMF is used for tweezing. The GIMF setup is capable of generating a tunable-distance optical trap over a hundred microns by just tuning the laser wavelength. The position of the optical trap can also be customized with the proper design of the GIMF and straining the fiber. The length of the GIMF also plays an important role in the operation of the device. This length needs to be fined-tuned only over less than 500 microns due to the self-imaging properties of the beam propagating in a GIMF; therefore, the necessary length adjustments can be easily done by polishing the end-facet of the fiber. The numerical results also show that as the optical trap moves farther away from the GIMF tip, the optical trap gets weaker. The results also show that the minimum input power to meet the stability conditions for a particle with a radius of 0.1 micron is around 400 mW.

Spectral selectivity in optical fiber capillary dye lasers

We explore the spectral properties of a capillary dye laser in the highly multimode regime. Our experiments indicate that the spectral behavior of the laser does not conform to a simple Fabry-Perot (FP) analysis; rather, it is strongly dictated by a Vernier resonant mechanism involving multiple modes, which propagate with different group velocities. The laser operates over a very broad spectral range and the Vernier effect gives rise to a free spectral range, which is orders of magnitude larger than that expected from a simple FP mechanism. The theoretical calculations presented confirm the experimental results. Propagating modes of the capillary fiber are calculated using the finite-element method and it is shown that the optical path lengths resulting from simultaneous beatings of these modes are in close agreement with the optical path lengths directly extracted from the Fourier transform of the experimentally measured laser emission spectra.

Design of a Wavelength-Tunable Optical Tweezer Using a Graded-Index Multimode Optical Fiber

A wavelength-tunable optical fiber tweezer based on a graded index multimode fiber with a flat endface is proposed. The design offers a noncontact optical fiber tweezer generating a stable optical trap that can be tuned precisely over a large range using a common wavelength-tunable laser. This property comes from the wavelength dependence of the graded index multimode fiber design parameters, such as the numerical aperture. Using an optical fiber tweezer with a flat endface is also more desirable than the tapered one because of the easier fabrication process. Our analysis also shows that the setup can form a stable three-dimensional optical trap in the Rayleigh regime in the absence of any microfluidic flow force.