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In this paper we report the fabrication of a short, robust, single-mode Er/sup 3+/ fiber-grating laser. This laser is proven by the successful outcome of a long-term bit-error-rate test at the 10/sup -15/ level in a 5- G/s fiber transmission experiment. In the process we present a new pumping geometry, the elucidation of one origin of relaxation oscillations that have plagued previous lasers and the demonstration of a simple electronic feedback scheme for suppressing them. This prototype source shows real promise for use in a high-speed communications system. >

We report the fabrication of low loss (

Upon irradiation with laser light of specific wavelengths and intensities optical fibers can undergo a slow (minutes to hours) permanent change, which results in new optical phenomena that were not present before irradiation. These phenomena include photoinduced refractive index gratings 1,2 and photoinduced second harmonic generation 1-4 in doped silica fibers. Since these phenomena are the result of a photoinduced periodic modification of the optical properties of the material with a periodicity that is determined by the wavelength(s) of irradiation, they are examples of photoinduced organization in glasses. Although the physics of these phenomena is not understood in detail, there is ample experimental evidence that the presence of optically active centers in the glass is of crucial importance for the observation of these phenomena. The optically active centers are due to defects or dopants in the glass. Here we discuss photoinduced SHG in Ge-doped silica fibers and the use of luminescence spectroscopy to characterize different types of Ge-related defects in the fiber.

We have measured the efficiency of photoinduced second-harmonic generation (SHG) in Ge-doped silica fibers containing different concentrations of 'GeO' and 'GeH' defects.The relative concentrations of these defects were varied by heat treatments in H2 anddetermined using luminescence spectroscopy. The results show that the GeH defect center, which can be excited in the visible (around 500 nm) and luminesces around 700nm, plays a role in the photoinduced SHG process. On the other hand, the results do notallow us to rule out contributions from the GeO defects. 1. INTRODUCTIONAlthough photoinduced second-harmonic generation (SHG) in optical fibers was discovered in 1986, many aspects of this phenomenon are still not understood1 . One of the important questions concerns the role of defects in the fiber, more specifically the roleof Ge-related defects in the case of SHG in germania doped silica fibers. Two crucial - and in many ways related - issues are i) the role which the defects play in the physicalmechanism giving rise to the second-order susceptibility (x2)

Permanent X(2) gratings can be written in doped silica fibers by coherent irradiation with light at ω and 2ω. The X(2) gratings give rise to phase-matched second harmonic generation (SHG), i.e. subsequent irradiation of the prepared fiber with light at ω results in an output at 2ω. The efficiency with which the gratings can be written depends on the nature and concentration of defect states induced by dopants in the, glass. We present results for fibers doped with Ge and rare earth ions and discuss the implications of our results for the various mechanisms that have been proposed to explain photoinduced SHG in fibers.

A refractive index grating has been side written in the core of a hydrogen loaded germanosilicate optical fiber using a phase mask and 351 nm light from an Ar ion laser, the longest wavelength yet reported, to the best of our knowledge, for a grating of this type. The grating strength was ∼1.25 dB, indicating an index change of ∼2×10−4. The fiber was sensitized by low temperature hydrogen loading followed by a brief exposure to a CO2 laser, resulting in the formation of high levels of germanium–oxygen deficiency centers. Fluorescence changes and UV transmission changes in hydrogen loaded germanosilicate glasses exposed to 325 nm and combined 351/364 nm radiation are also reported.

High pressure 'hydrogen loading' has been used to sensitise standard singlemode fibres, resulting in the largest reported UV induced index changes for low GeO/sub 2/ fibres. Grating bandwidths of 4 nm and peak nu ns of 5.9*10/sup -3/ have been reproducibly achieved. Substantial index changes have also been achieved by rapidly heating H/sub 2/ loaded fibres of various compositions.< >

Germanosilicate glasses soaked in high pressure hydrogen at low temperature exhibit markedly increased photoinduced refractive index changes when exposed to 248 nm UV radiation. The mechanisms involved include both photolytically and thermally driven reactions between hydrogen and germania, forming OH and deep UV absorbing species.

UV (248 nm) induced changes in the UV spectrum (165–300 nm) of optical fibre preform cores have been measured which can account for refractive index changes observed on writing Bragg gratings in optical fibres. The changes are thermally reversed at 900°C. A fibre grating has been thermally erased and rewritten with no significant change in fibre photosensitivity.

A new technique has been developed to measure directly the UV absorption spectrum of optical fibers to wavelengths as short as 200 nm. Both the shape and the intensity of the UV absorption bands found in the fiber correspond to those measured in preforms. No bands other than those attributed to GeO (240 and 330 nm) were observed.