1,720,974 research outputs found
Photochromic dynamics and non-linear transmission at modulated cw blue/green wavelengths in germanosilicate optical fibres
No full textPhotochromic dynamics in germanosilicate fibres at quasi-CW blue/green wavelengths are investigated using sinusoidally modulated Ar+ laser light. The induced loss has both transient and permanent features, and gives rise to strongly nonlinear transmission
A colour-centre model for second-order optical nonlinearities in germanosilicate optical fibres
No full textSecond-order nonlinearities can be induced in germanosilicate (GS) glass fibers in a variety of ways: by coherent superposition of pump and second-harmonic light (a third-order dc polarization Pdc is induced that breaks the inversion symmetry); by excitation poling with blue light and an external dc field, Epol ; and by strong Epols 's in the absence of light. There is as yet no satisfactory agreement over the nature of the underlying alignment process. From related detailed studies of color-center behavior in GS fiber, we have formulated a self-consistent model as developed in the following points. &more..
Spontaneous relaxation processes in irradiated germanosilicate optical fibres
No full textMany potential applications requiring high-power transmission at short wavelengths are hampered by colour-centre formation. It is perhaps not generally realised that the induced absorption continues to evolve after the laser light is blocked. This relaxation phenomenon is studied in germanosilicate fibres at blue/green wavelengths, and fitted to a three-rate analytical model
Two-photon-induced losses in germanosilicate optical fibers: relaxation processes
No full textExperimental data on the spontaneous color-center relaxation processes that ensue after exposure of germanosilicate fibers to blue/green light are fitted to a three-rate analytical model
Enhanced photorefractivity in germanosilicate fibres: Effects of bleaching with 488nm Light
No full textPermanent index changes, Δn, of the order of 10-4 at 633 nm are induced in germanosilicate optical fibers by exposure to light at 488,266, and 240 nm. This photorefractivity has great potential in the fabrication of efficient grating-based devices for a wide range of WDM and lasing applications. It has been explained in terms of alterations in the UV absorption spectrum yielding (through the Kramers-Kronig relation) almost dispersion-free Δn values between 500 nm and 1.5 µm. The predominant cause is the movement of electrons from broken oxygen deficient Ge-Si bonds (associated absorption peak at 240 nm) to Ge(2) traps (when occupied by an electron an absorption peak appears at 213 nm) (see Fig. 1). The Ge-Si bonds can be broken by single-photon absorption of 240-or 266-nm light (permitting gratings of any period to be fabricated by side writing), or (much more slowly) by two-photon absorption (TPA) of 488-nm light. However, an undesirable side effect of 266-nm treatment is that the absorption induced for approximately the same Δn value is some two orders of magnitude larger than that obtained with 488-nm light (30 dB/m at 633 nm). This absorption is attributed to the creation of Ge(1) color centers, with a broad absorption peak centered at 281 nm and extending into the visible (Fig. 1). Because the balance between bleaching and trapping at Ge(1)and Ge(2) dopant sites is different for 266-nm light, it seemed possible that the population of Ge(1) centers could be depleted by 488-nm light, perhaps even enhancing the induced Δn
Non-linear transmission in germanosilicate fibres at blue/green wavelengths
No full textMany fibre-based devices rely on the ability to transmit light at high intensities down single-mode fibres. Examples are fibre lasers, amplifiers, second-harmonic generators and simple fibre-based high power delivery systems. It is thus important to know whether any non-linear processes exist that could restrict the usefulness of the fibres used in these applications. Brown et al have reported that non-linear effects of this kind do indeed occur at Argon ion wavelengths in germanosilicate fibre, limiting the power that can be delivered to some tenths of a W over only few-metre lengths of single-mode fibre. In this paper we report the chief results of an extensive investigation into this effect in germanosilicate HiBi fibres designed to transmit single-mode blue/green light. These observed non-linearities in the transmission of CW blue/green light cannot be attributed to conventional non-linear processes (such as stimulated Raman or Brillouin scattering) because only short (few-metre) fibre lengths are needed, the thresholds are low, and the effect is insensitive to laser line-width. We have indicated that this non-linear behaviour can be explained by the creation (via two-photon absorption - TPA) and bleaching (via normal absorption) of colour-centres. TPA enables blue/green photons to reach UV energy levels that are sufficient to disrupt the glass matrix, and even though the TPA rate is very low at intensity levels where non-linear transmission is significant, the long path lengths in the fibre, added to the high likelihood of colour-centre formation by quanta at double the photon energy, mean that colour-centres created by TPA can have a dramatic effect on transmission
Nonlinear transmission and colour-centre dynamics in germanosilicate fibres at 420-540 nm
No full textWe report evidence in support of the view that induced loss and non-linear transmission in pure germanosilicate fibers at blue/green wavelengths are governed by the formation (via two-photon absorption), spontaneous and stimulated transformation and bleaching (via single-photon events) of Ge(1), Ge(2) and Ge(3) colour-centres. Using a tunable pulsed dye laser, the excitation spectrum of the induced absorption, its spectral attenuation and the effects of germania concentration and thermal annealing are investigated
Dynamics of color-center induced nonlinear transmission in GeO<sub>2</sub>-SiO<sub>2</sub> fibres
No full textWe show that the predominant factor limiting high power transmission in the blue/green spectral region in germanosilicate core fibres is the creation of germania-related color-centres via two-photon absorption
Tunable holographic second-harmonic generators in high birefringence optical fibres
No full textThe formation of efficient holographic second-harmonic generation in high-birefringence phosphorus-doped germanosilicate fibers is reported. The influence of optical polarisation on the nonlinear writing and read-out processes in explored. Fiber birefringence permits phase-matched second-harmonic conversion at wavelengths within ±125 /cm (±14nm) of the writing wavelength (1.061 µm)
Frequency doubling, absorption and grating deformation in glass fibres: effective defects or defective effects?
No full textThe present understanding of colour centres in germanosilicate glass fibres and the diverse effects attributed to colour centre activity are reviewed. Drawing on a wide range of up-to-date research results, an attempt is made to piece together as far as possible a unified picture of the defect processes behind second harmonic generation, nonlinear transmission and photorefractive grating formation in optical fibres
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