RESEARCH

Optical fiber random lasers

The development of photonic devices with flexible structure is important in the field of display technology and optical source. Flexible random laser structures also need much attention in the imaging field because of its ability to produce speckle free images. The development of fluorescent polymer optical fiber opens a new window to the field of flexible random lasers due to their flexibility, low cost, ease of fabrication and can provide highly directional optical output. Over other host materials, polymer matrix has better chemical ompatibility with organic dyes

Normalised random lasing emission spectra from (Red) ZnO nanorods and (Blue) Au Nano urchins at a pump pule energy of 3.1mJ

An ion sensing using optcal fibers

Over recent years in our lab, focus has increasingly shifted towards development of optical fiber sensors for the detection of chemical species like anions (with special attention to fluoride) and biochemical indicators like cholesterol, glucose and dextrose. We studied the response of evanescent wave absorption (straight and U shaped) and fluorescence (tapered fiber) based sensors with the use of natural dye Curcumin as the sensing agent. Evanescent wave fluorescence based optical fiber sensor using the diminishing fluorescence of Curcumin in presence of fluoride was implemented using silica multimode fiber chemically etched at the end face to form a tapered tip.

(a) Sensor probe configuration for the excitation and collection of fluorescence from Curcumin (b) Light path in acetonitrile (A, B) and anisole (C, D) with A,C - Curcumin alone in the solvents and B, D- Curcumin in the presence of high concentration TBAF

Light amplification using doped polymer optical fibers

Since the microlaser emission from POFs has very low energy, it needs to be amplified for any practical application. The amplification can be done using another dye doped POF. The microlasing modes generated from one fiber is coupled to another fiber. The microlasing modes get amplified while propagating through the second fiber. The first fiber is called the lasing fiber and the second is called the amplifying fiber. Both fibers are pumped transversely using the second harmonic output of a Q-switched Nd: YAG laser.

Experimental setup for fibre amplifier.

Graded Index and hollow optical fibres

International School of Photoincs have successfully fabricated and demonstrated the whispering Gallery mode (WGM) laser emission from RhB dye doped Graded Index (GI) optical fiber. The confinement and propagation of the WGMs inside the GI fiber are also reported. A variation in the FSR from 0.29 to 1.24 nm has been obtained along the length due to the confinement of WGMs in the GI fiber. It was also found that the whispering gallery lasing envelope shifted from 598 to 615 nm (Stokes shift) along the length of the GI fiber. The dye doped GI fiber may become a potential device in photonic applications like wideband tunable, low threshold lasers and distributed sensing.

(a) Confinement and propagation of Whispering Gallery Spirals (modes) when excited with a pump laser beam. (b) Lasing spectrum from the RhB doped Graded Index optical fibre.

[Linslal, Kailasnath et.al OPTICS LETTERS, 2013]

Another important result is that a strongly modulated laser emission has been observed from rhodamine B doped microring resonator embedded in a hollow polymer optical fiber under transverse optical pumping. The doped microring resonator fabricated on the inner wall of a hollow polymer fiber offered highly sharp lasing lines, strong mode selection, along with a collimated laser beam.

(a) Cross-section of different microring embedded hollow polymer preforms (left) and photographic and SEM images of the microring embedded hollow polymer fiber (right) (b) integrated emission intensity of the peaks as a function of pump pulse energy and the inset shows the intensity profile of the emitted laser beam. [Linslal, Kailasnath APPLIED PHYSICS LETTERS 2015]

Nearly single mode lasing with a side mode suppression ratio of up to 11.8 dB was obtained from the strongly modulated lasing spectrum. The microring embedded hollow polymer fiber laser has shown efficient lasing characteristics even at a propagation length of 1.5 m.

(a) Modulated multimode lasing spectrum and (b) schematic cross section of the microring showing the interference of the modified WGMs at the interface. Strongly modulated lasing mode spectra collected from two different microring fibers and the insets show the magnified version of the highest intensity lasing lines with Gaussian fit. [Linslal, Kailasnath ,APPLIED PHYSICS LETTERS 2015]

Tuning whispering Gallery modes - Hollow optical fibre as a sensor.

WGMs are easily tunable by applying a compressive or stretching force to the microcavity. The tuning of modes is a result of the deformation of the cavity shape and the refractive index change (elasto-optic effect) by the applied force . Stress/strain tuning of WGMs has been reported with microspheres, microbubbles , microdroplets etc. WGM lasing emissions are highly sensitive to the gain medium, refractive index, and the size and shape of the optical cavity.

(a) Whispering gallery lasing spectra from RhB-doped hollow polymer fiber with different elongation in the axial direction, and (b) shift of three whispering gallery lasing modes as a function of elongation. The inset shows the cross-sectional view of the hollow polymer fiber. Linslal C.L, M.Kailasnath ,2016, Optics Letters.

Ag nanowire-assisted low threshold lasing

Whispering Gallery Mode (WGM) emission has been observed from Ag nanowire doped polymer optical fiber laser. Low threshold lasing and high photostability of the active medium has been noticed with a given concentration of Ag nanowires in the microcavity of the fiber. Quantum yield and lifetime measurements of the dye active medium with and without nanowires confirm that presence of nanowires enhance the rate of radiative decay of the fluorophore, thereby providing low pump pulse energy for the excitation of lasing modes in the cavity, as compared with a bare dye-doped polymer fiber laser. The Rh6GAg nanowire system showed enhanced emission intensity with increased quantum yield and reduced lifetime as compared with bare Rh6G-doped polymer fiber.

(A) WGM lasing from Ag nanowire-doped POF laser (B)The threshold for lasing has been significantly reduced from 10 mJ to 0.5 mJ (20 times) for A1 ( Ag concentration 0.32 x10-3 M ) as compared with POFs containing Rh6G only (R), Suneetha Sebastian , M. Kailasnath , Optics Letters 2017.