Before the 1960s, all anti-Stokes emissions, which were known to exist, involved emission energies in excess of excitation energies by only a few kT. They were linked to thermal population of energy states above excitation states by such an energy amount. It was the well-known case of anti-Stokes emission for the so-called thermal bands or in the Raman effect for the well-known anti-Stokes sidebands. Thermoluminescence, where traps are emptied by excitation energies of the order of kT, also constituted a field of anti-Stokes emission of its own. Superexcitation, i.e., raising an already excited electron to an even higher level by excited-state absorption (ESA), was also known but with very weak emissions. These types of well-known anti-Stokes processes have been reviewed in classical textbooks on luminescence.1 All fluorescence light emitters usually follow the well-known principle of the Stokes law which simply states that excitation photons are at a higher energy than emitted ones or, in other words, that output photon energy is weaker than input photon energy. This, in a sense, is an indirect statement that efficiency cannot be larger than 1. This principle is

Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light-matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label-free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.

Whispering gallery microcavity lasers

Sol-gel silica/titania-on-silicon Er/Yb-doped waveguides for optical amplification at 1.5 μm

A comprehensive summary is provided on the subject of heavy metal fluoride glasses. This includes discussions on glass compositions, preparation, structure, fiber drawing characteristics, and intrinsic and extrinsic optical properties. Other relevant glass and fiber characteristics such as strength, durability, and thermal Shock resistance are also outlined. Particular emphasis is given to the measurements of optical attenuation in fluoride glasses and their potential for use in ultralow-loss fiber optic waveguides. An excellent bibliography of recent work in the area of heavy metal fluoride glasses has been generated for those seeking more detailed information on this topic.

Heavy metal fluoride glasses and fibers: A review

The dependence of erbium-doped fiber amplifier (EDFA) performance on the erbium ion concentration is studied experimentally and theoretically. The quantum efficiency of the amplifier is found to he strongly dependent on the erbium ion concentration, the signal wavelength, and the relative propagation direction of the pump and signal beams. This dependence is fully explained by the presence of an upconversion mechanism between ions residing in pairs or larger clusters and suggests that other sources of amplifier performance degradation (back-ground loss, excited state absorption, homogenous upconversion) are negligible. The experimental data show that in the present EDFA designs with over 80% quantum/conversion efficiency, the aluminum co-doped fibers with erbium ion concentration less than 20/spl times/10/sup 24/ m/sup -3/ (900 molar ppm Er/sup 3+/) are most suitable.

Effects of concentration on the performance of erbium-doped fiber amplifiers

It is shown that in low-concentration erbium-doped silicate fibers (below 1000 p.p.m.), a residual absorption at 980 nm cannot be saturated. Usual models for upconversion of Er/sup 3+/ cannot match this behavior nor explain a fluorescence lifetime independent of pump power and erbium concentration. A phenomenon, the pair induced quenching (PIQ), that is compatible with the previous experimental results is exhibited. The influence of erbium and aluminum concentration on the ion pair proportion is shown. >

Modeling of pair-induced quenching in erbium-doped silicate fibers

Bragg gratings have been written within four germanosilicate fibers either by a pulsed or by a continuous-wave exposure of each fiber to a coherent UV two-beam interference pattern. These gratings have been exposed under steady state conditions to /spl gamma/-ray doses as high as 10/sup 4/ Grays. The dose rates ranged between 10 Gy/h and 1.3/spl times/10/sup 2/ Gy/h. The transmission spectra of the fibers have been recorded during and after the /sup 60/Co irradiation, near the grating Bragg wavelengths. Whereas the induced loss reached 600 dB/km near 1.3 /spl mu/m, no significant change in the spectral characteristics of the gratings could be detected within the experimental accuracy, enabling their future use in a nuclear environment. >

Behavior of Bragg gratings, written in germanosilicate fibers, against /spl gamma/-ray exposure at low dose rate

The design of quadruple-clad profiles, leading to (truly) monomode fibres is described: an explicit set of parameters is proposed. Experimental results are presented, concerning a fibre with a dispersion smaller than ±0.5 ps/km nm and an attenuation around 0.3 dB/km, over the 1.5?1.65 ?m wavelength interval.

Design of monomode quadruple-clad fibres

Optical equalisation of CSO degradation induced by laser chirp and standard fibre dispersion is experimentally demonstrated. This equalisation is based on the combination of the standard singlemode fibre with a large negative dispersion fibre. With a 10 km standard fibre span the optical fibre equaliser allows the degraded CSO to be reduced back to the value of the intrinsic laser CSO. A shorter standard fibre span for loop distribution was also compensated by using this technique. The linear behaviour of the optical fibre equaliser is demonstrated.

Fibre-equaliser second order distortion compensation in 1.55 μm lightwave CATV transmission system

Fluoride glasses in the ZrF4-BaF2-RF3 system (R=Gd, Yb) have been investigated as potential low loss materials for achievement of fibres transmitting wavelengths at about 3–5 μm. Optical measurements confirm the interest in such components and PbF2 doping allows substantial refractive index increases, making for feasible doped core glass preforms.

J.F. Bayon

Papers

Modeling of pair-induced quenching in erbium-doped silicate fibers

Behavior of Bragg gratings, written in germanosilicate fibers, against /spl gamma/-ray exposure at low dose rate

Design of monomode quadruple-clad fibres

Fibre-equaliser second order distortion compensation in 1.55 μm lightwave CATV transmission system

Fluoride glasses for infra-red optical fibres