J.R. Lincoln

Fabrication and optical properties of lead‐germanate glasses and a new class of optical fibers doped with Tm3+

In this article we present a study of a new class of optical fibers based on lead germanate glass. The maximum vibrational frequency of this glass is intermediate between silica and zirconium barium lanthanum aluminum fluoride glass, causing a beneficial change in nonradiative decay and therefore quantum efficiency for particular laser transitions. Fabrication of high-strength, low-loss fibers of this glass has been achieved by modification of the composition to produce optimal physical properties for fiber drawing, while retaining the useful vibrational properties of the original PbGeO2 glass. Measurements of both the thermal and optical properties are described. The fibers produced are ideal for many applications in fiber devices.

Time resolved and site selective spectroscopy of thulium doped into germano- and alumino-silicate optical fibres and preforms

Abstract Optical properties of thulium ions doped in varying concentrations into germano- and alumino-silicate glass fibres and their preforms are studied. Absorption and fluorescence spectra are shown to vary little with thulium concentrations up to 12 000 ppm, but vary drastically with germanium or aluminium codopant. Resonant fluorescence studies of Tm3+ show correlations in the movements of the Stark levels of the ground state, indicating systematic correlations between the ion site and Stark level position. An upper limit of 10 cm-1 for the homogeneous line width of thulium in alumino-silicate glass is established at 9 K. The lifetimes of the 1G4 and 3F4 levels are studied and are related to codopant influence and cross-relaxation which are linked to fibre drawing and Tm3+ concentration. A significant lifetime shortening is observed on drawing preform to fibre. A model is presented for non-exponential decay of the 3F4 level. Three- and four-level rate equations have been successfully used to simulate the observed rise of the 3F4 fluorescence and decay of the 1D2 fluorescence, excited resonantly and by up-conversion processes, respectively. These models yield both the excitation pump rate and cross-relaxation rates.

Defect production in silica fibers doped with Tm3

Irradiation of Tm3+ fibers with blue light at 476 nm induces a broad-bandwidth loss in these fibers. We have measured the spectral dependence of the loss for both silica–germania and silica–alumina fibers and show through micro-Raman studies of the core regions of the fibers that this induced loss is correlated with the production of structural defects in the glass host.

Optically written waveguides in ion implanted Bi4Ge3O12

We report the first observation of optically written channel waveguides in ion implanted Bi4Ge3O12 (BGO). Raman spectroscopy has been used to investigate the changes occurring due to both the original ion implantation and the subsequent optical writing of channel waveguides

Probing molecules by surface-enhanced Raman spectroscopy

A new class of Surface-Enhanced Raman Scattering (SERS) substrates have been engineered by exploiting both Photonic Crystal (PC) and semiconductor technologies. Gold coated inverted pyramids nanotextured substrates allow reproducibility <10% and enhancement factors > 106 over large areas. Modelling and optical characterization of the engineered structures is demonstrated. Examples of applications to amino acids and illicit drug detection are given. Concentrations as low as ppm-ppb (mg/mL to ng/mL) have been measured depending on the adsorbed analytes. Information on structure and conformation of the molecule is inferred due to the richer nature of SERS spectra.

Spectroscopic evaluation of the vibrational coupling of Er3+ ions in phospho-aluminosilicate fibres and an explanation of compositional variations in Er-Yb 1.5 μm amplifier performance

Evidence is presented that erbium will occupy one of two sites in phospho-aluminosilicate glass dependent on the Al : P ratio. Although high energy P=O vibrational modes are created we suggest these are localised and only couple to one type of Er site. Decay rates and Er-Yb amplifier performance are then critically determined by the Al : P ratio.

Thulium-doped silica fiber lasers

The trivalent thulium ion is an interesting activator for silica fiber lasers because of the near infrared transition which is broadband tunable ( diode pumpable can be operated with photon conversion efficiencies greater than 100 and has yielded in excess of 1W output power when pumped by a cw Nd:YAG laser. The paper will review progress on this system and indicate some potential future developments.

Fluorescent lifetime of Er3+4I132 level in BK-7 borosilieate glass

It is established that the B3+ ions exist only in the form of [BO4] units in BK-7 borosilicate glass. This limits the maximum vibrational energy at peak to 1200 /cm rather than 1400 /cm, as is found in borate glasses, and explains the long lifetime of the Er3+ 4 I (13/2) level observed in BK-7 borosilicate glass.

Octave-wide continuum generation in high-index planar waveguide by 1.5-μm femtosecond pump

Ultra-high bandwidth continuum generation has been attracting enormous interest for applications in optical frequency metrology, low-coherence tomography, laser spectroscopy, dispersion measurements, sensor techniques and others. The acceptance of this new technology would greatly benefit from the availability of compact and user-friendly sources. High index planar devices provide a versatile and unique approach to continuum generation. The dispersion can be carefully engineered by choosing the material and the geometry of the waveguides. Optical integration can also be provided on the same platform. Hundreds of different waveguides having different and calibrated dispersions can be integrated in few tens of millimeters. Input and output of the 2D guides can be tailored to provide mode matching to fibers and pump lasers by means of single element bulk optics. In this paper for the first time we demonstrate a low-noise, ultra-high bandwidth continuum at 1.55 μm. A bandwidth in excess of 390 nm is obtained by launching energy as low as 50 pJ in a 12 mm short tapered planar waveguides. The pump wavelength was in the normal dispersion regime and was provided by a compact, fiber-based sub-100 femtosecond source.

Low-noise self-phase modulation continuum generation in high index tapered planar waveguide at 1040 nm

Continuum Generation (CG) in optical waveguides has been recently attracting widespread interest in fields requiring large spectral bandwidth such as metrology and Optical Coherence Tomography (OCT). Real time and in-vivo tissue imaging with cell resolution (Δz<1μm) is rapidly becoming the ultimate frontier of several OCT medical applications. CG wavelength and bandwidth are the pertinent criteria to obtain ultra high imaging resolution. The axial resolution in tissues is inversely proportional to the bandwidth whereas the central wavelength is chosen according to the minimum absorption of water and hemoglobin. Therefore optimal candidates for OCT low coherence sources1 are continua around 1μm as this is the zero group velocity dispersion wavelength of water. In this work we demonstrate for the first time a low-noise continuum at very low powers in high index planar waveguides pumped at 1.04 μm. Bandwidths in excess of 150 nm at -3dB are generated with launching energies <1nJ/pulse in a ~2μm2 single mode ridge waveguides pumped in the normal dispersion regime. Self-Phase Modulation (SPM) had proven to be the only nonlinear process responsible for the CG. The polarization of the generated continua is highly preserved. Great flexibility in engineering waveguide dispersion, mode matching and optical functionality on chip is demonstrated by the planar approach.