Brian J. Shortt

Heat-and-pull rig for fiber taper fabrication

We describe a reproducible method of fabricating adiabatic tapers with 3–4μm diameter. The method is based on a heat-and-pull rig, whereby a CO2 laser is continuously scanned across a length of fiber that is being pulled synchronously. Our system relies on a CO2 mirror mounted on a geared stepper motor in order to scan the laser beam across the taper region. We show that this system offers a reliable alternative to more traditional rigs incorporating galvanometer scanners. We have routinely obtained transmission losses between 0.1 and 0.3dB indicating the satisfactory production of adiabatic tapers. The operation of the rig is described in detail and an analysis on the produced tapers is provided. The flexibility of the rig is demonstrated by fabricating prolate dielectric microresonators using a microtapering technique. Such a rig is of interest to a range of fields that require tapered fiber fabrication such as microcavity-taper coupling, atom guiding along a tapered fiber, optical fiber sensing, and th...

Optical bistability in Er-Yb codoped phosphate glass microspheres at room temperature

We experimentally demonstrate optical bistability in Er3+-Yb3+ phosphate glass microspheres at 295 K. Bistability is associated with both Er3+ fluorescence and lasing behavior, and chromatic switching. The chromatic switching results from an intrinsic mechanism exploiting the thermal coupling of closely spaced energy levels, and occurs simultaneously with the intensity switching. A contrast ratio of 2.8 has been obtained for chromatic switching. The intensity switching shows ratios of 21 for 520 nm and 11 for 660 nm fluorescence emissions, and 11 for IR lasing at 1.5 μm. Concurrent with these observations, we investigate a temperature-dependent absorption of pump power, which exhibits bistable behavior. The influences of the host matrix on lasing and fluorescence mechanisms are highlighted.

An All-Fiber Coupled Multicolor Microspherical Light Source

We present results on the realization of an all-taper coupled, multicolor microspherical light source fabricated from the erbium-doped fluoride glass ZBLALiP. Whispering gallery mode lasing at 1555 nm and fluorescent emissions from the ultraviolet to the infrared (IR) have been observed. A tapered fiber is used to launch 980-nm pump light into the microresonator cavity and to collect the resulting IR lasing. A separate, half-taper fiber is used to outcouple the upconversion spectrum over several hundred nanometers. A number of discrete emissions from 400 to 850 nm have been observed in the upconversion spectrum, indicating the suitability of this scheme as a miniature device with a 450-nm-wide band and reasonable output coupling efficiency.

Characterization of Er:ZBNA microspherical lasers

We report here on efforts to characterise erbium-doped, heavy-metal fluoride glass microspherical lasers as a function of pump laser parameters, such as wavelength and power, and the temperature of the microsphere. The microspheres are fabricated from a novel material, ZBNA, optimised for its ability to act as a host for erbium and to favour laser emissions in the near infra-red region at 1.56 microns from the dopant. We work with microspheres that have typical diameters of ~80 microns. By attaching the microsphere to the tip of a narrow optical fibre a suitable method for manipulating the microsphere within the set-up has been developed. Pump light at 980 nm is coupled into the microsphere through evanescent wave tunnelling and propagates within the sphere in whispering gallery modes. A half-taper fibre is used as the coupling element. Strong green fluorescence at 540 nm has been observed. This indicates that the pump light is efficiently coupled into the sphere and that the alignment of the system is reasonable. We present an introduction to the fabrication of microsphere lasers, half-taper fibres and the physics of whispering gallery modes. In addition, we discuss the suitability of erbium as a lasing material when used in conjunction with the 980 nm pump light. Finally, we discuss our first experimental observations.

Spectral characterisation of erbium-doped microspherical lasers

We present continuing theoretical and experimental investigations into the characterisation of fibre taper coupled microspherical lasers. We investigate the spectral emissions of these micro resonators as a function of the wavelength and power of the pump radiation. We also consider various means of efficiently coupling radiation into and out of these devices. We are particularly interested in studying emissions about 1.55 μm with the aim of exploiting potential telecommunications applications of these devices. We study microspheres with diameters in the range 40-150 μm. Pump light about 980 nm or 1480 nm is coupled into the microsphere through evanescent wave coupling between microsphere and optical fibre. The resulting spatial and spectral distribution of radiation within the spheres is described by whispering gallery modes. Half and full-taper optical fibres are used to both couple the pump light into the sphere and to out couple the resulting infra red lasing. The physics of whispering gallery modes and the challenges of coupling to such modes is discussed. Experimental techniques and results will be presented and future directions indicated.

Precision control of magneto-optically cooled rubidium atoms (Invited Paper)

Research interest in designing sources of cold atoms has significantly increased during the past ten years with the development of suitable laser sources for magneto-optical trapping and the further mastering of evaporative cooling in order to achieve Bose-Einstein condensation. The magneto-optical trap is now viewed as a standard research facility worldwide and has opened up many new exciting research directions in atomic physics. One area of interest is that of combining spherical microcavities with cold atomic sources in order to achieve efficient photon exchange between the cavity and atom for further understandings of cavity quantum electrodynamics. This could eventually lead to atom entanglement via photon exchange which would have implications for quantum logic design. However, initial attempts to achieve such interactions have been hindered by inadequate control and manipulation of the cold atom source. Here, we present work on designing and building an ultra-stable source of magneto-optically cooled rubidium atoms with a temperature in the tens of microKelvin range. We discuss the different cooling mechanisms involved in the process and present a suitable experimental arrangement including details on the ultra-high vacuum chamber, the laser systems being used and the source of rubidium vapour. Finally, we discuss some future direction for the research including the diffraction of atoms from gratings and micron-sized objects and the parameter control of the cloud of atoms.

Atomic absorption from the evanescent field of a sub-micron fibre taper

In this paper, the interaction between cold rubidium atoms in a magneto-optical trap (MOT) and the evanescent field of a sub-micron fibre is studied. The fibre taper is fabricated from 780 nm single mode fibre using a heat-and-pull rig with a CO2 laser heat source.

Upconversion channels in Er:ZBLALiP: a multicolour, microspherical light source for microphotonics

We present results on the realization of an all-taper-coupled, microspherical light source fabricated from the erbium-doped fluoride glass ZBLALiP. We observe whispering gallery mode lasing at 1.55 mum, in addition to upconversion processes which give rise to fluorescent emissions at multiple wavelengths from the ultraviolet to the infrared. A single-mode, tapered fibre coupling scheme -which serves to both launch pump light at 980 nm into the microresonator cavity and collect the resulting infrared lasing -is employed. We demonstrate the use of a second, separate fibre taper to outcouple the upconversion spectrum over several hundred nanometres. Thirteen discrete emissions ranging from 320 to 850 nm have been observed in the upconversion spectrum.

Thermally induced optical bistability in Yb 3+ -Er 3+ co-doped phosphate glass microspheres at room temperature

This paper reports the first observation of chromatic and intensity optical bistability (OB) in a commercial, active, phosphate glass (Schott IOG-2) microsphere driven by thermal avalanche at room temperature. This is, to the best of our knowledge, the first report of simultaneous intensity OB (at multiple wavelengths) and chromatic OB in a single material.

Dynamics of Thermally Induced Optical Bistability in Yb 3+ -Er 3+ Co-Doped Phosphate Glass Microspherical Lasers

We theoretically study and experimentally demonstrate thermally induced optical bistability in Yb³⁺-Er³⁺ phosphate glass microspheres at 295 K. Thermal avalanche in Yb³⁺ ions is concomitant with bistability in Er³⁺ fluorescence and lasing behavior, and chromatic switching.