Increase of photosensitivity in Ge-doped fibers under strain

Straining a Ge-doped optical fiber increases its UV photosensitivity. The speed of grating fabrication in a standard AT&T telecommunication fiber increases by 18 times, while the final refractive-index change is enhanced by a factor of 5. Measurements reveal opposite changes in the stress of the fiber core induced by UV light in strained and unstrained fibers.     

Contribution of photoinduced densification to refractive-index modulation in Bragg gratings written in Ge-doped silica fibers

We have computed the contribution of UV-light-induced densification to the refractive-index modulation of fiber Bragg gratings. Our results confirm that, for strong gratings written in Ge-doped silica fibers with 248-nm UV light, density changes account for a major part of the photosensitivity effect.     

Supercontinuum generation in highly Ge-doped core Y-shaped microstructured optical fiber

We report experimental results on supercontinuum generation in a highly Ge-doped core Y-shaped microstructured optical fiber using pump pulses of 9 ns duration at 1064 nm. The fiber is easy to fabricate due to its simple structure and shows good compatibility with standard fiber. The generation of nonlinear effects takes advantage of the large nonlinear refractive index and Raman gain of the Ge-doped core, as well as the air hole structure that surrounds the core. Although the fiber was pumped in normal dispersion and relatively far from the zero dispersion wavelength, flat and smooth supercontinuum in the fundamental mode from 550 nm to beyond 1750 nm was generated with a rather moderate value of fiber length and pump peak power product of 13 kW?m.     

Hydrogen loading for fiber grating writing with a femtosecond laser and a phase mask

The threshold for the fabrication of fiber Bragg gratings with ultrafast 800-nm radiation and a phase mask was studied in SMF-28 and all-silica core fiber by use of 125-fs pulses. High-pressure molecular hydrogen loading (H2 loading) was observed to significantly lower the grating writing threshold in standard Ge-doped telecommunication fiber. No reduction was observed with all-silica core fiber. The index change appeared to be confined to the Ge-doped core region of the fiber. Gratings in H2-loaded SMF-28 had thermal annealing behavior similar to UV-induced gratings. Unlike UV-induced H2-loaded gratings, no absorption associated with Ge-OH defect formation was observed.     

Internal Features of Fiber Fuse in a Yb-Doped Double-Clad Fiber at 3 kW

We study internal features of fiber fuse in a Yb-doped double-clad fiber. The samples of fiber fuse are acquired at the power level of 3 kW in an all-fiber forward-pumped master oscillator power amplifier configuration fiber laser that is built specially for fiber fuse analyses. At this high power level, drastic refractive-index redistribution arises in an expended high refractive index area around the bullet-shaped voids of fiber fuse. Electron spin resonance analyses on post-fiber-fuse samples of the Yb-doped double-clad fiber indicate rising Frenkel defect concentration, meanwhile showing a new resonance center that is different from the ones of the Ge-doped fibers studied previously. This new resonance center probably suggests the generation of Al-oxygen hole center, a kind of defect formed during the catastrophic fuse process.     

A 168-km ultra-long-range OTDR measurement at a wavelength of 1.55 μm

A Rayleigh backscattered signal and Fresnel reflection over a 167.6-km and a 251.2-km long pure silica-core fiber (PSF) with fiber loss 0.176 dB/km, and a 138.6-km and a 206.5-km long Ge-doped core dispersion shifted fiber (DSF) with fiber loss 0.218 dB/km have been observed with ultra-long-range optical time domain reflectometry (OTDR) using a 1.55-μm high-power semiconductor laser (LD) and InGaAs-APD with a low dark current at room temperature. The Rayleigh backscattered signal level and fiber loss are strongly dependent on the fiber dopant materials and fiber structural parameters. DSF gives a better SNR for short length fibers because the Rayleigh backscattered signal level is larger than PSF, but because the DSF loss is also larger, PSF gives a better SNR for fibers longer than the critical length. Accordingly, the dynamic range of OTDR in the different fiber types such as PSF and DSF becomes different. One-way backscattered dynamic ranges of 29.5 dB for the PSF and about 30.2 dB for the DSF at room temperature have been achieved.     

A 168-km ultra-long-range OTDR measurement at a wavelength of 1.55 μm

A Rayleigh backscattered signal and Fresnel reflection over a 167.6-km and a 251.2-km long pure silica-core fiber (PSF) with fiber loss 0.176 dB/km, and a 138.6-km and a 206.5-km long Ge-doped core dispersion shifted fiber (DSF) with fiber loss 0.218 dB/km have been observed with ultra-long-range optical time domain reflectometry (OTDR) using a 1.55-μm high-power semiconductor laser (LD) and InGaAs-APD with a low dark current at room temperature. The Rayleigh backscattered signal level and fiber loss are strongly dependent on the fiber dopant materials and fiber structural parameters. DSF gives a better SNR for short length fibers because the Rayleigh backscattered signal level is larger than PSF, but because the DSF loss is also larger, PSF gives a better SNR for fibers longer than the critical length. Accordingly, the dynamic range of OTDR in the different fiber types such as PSF and DSF becomes different. One-way backscattered dynamic ranges of 29.5 dB for the PSF and about 30.2 dB for the DSF at room temperature have been achieved.     

Brillouin gain spectrum characterization in Ge-doped large-mode-area fibers

The dependence of Brillouin gain spectrum (BGS) characteristics, including the Brillouin frequency shift (BFS) and the BGS bandwidth, on germanium concentration in large-mode-area Ge-doped passive fibers is investigated theoretically and experimentally. The simulation results show that the BFS is inversely proportional to GeO₂ concentration, and the BGS bandwidth initially increases with the augment of GeO₂ concentration, and then decreases. The BGSs of four fibers with core diameters of 10 μm and 20 μm for different GeO₂ concentrations are compared experimentally. Experimental results demonstrate that with the same core diameter, the variations of BFS and BGS bandwidths with GeO₂ concentration accord with the simulation results. Additionally, the BGS characteristics of three large-mode-area passive fibers with diameters of 10 μm, 25 μm, and 30 μm are measured, which confirm that the increasing of the fiber diameters will cause the BGS bandwidth to broaden. We believe that these results can provide valuable references for modulating the high-power narrow-linewidth fiber lasers and Brillouin fiber amplifiers.     

Sensing properties of fiber Bragg gratings in small-core Ge-doped photonic crystal fibers

We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented.     

Three-hole microstructured optical fiber for efficient fiber Bragg grating refractometer

We present a photosensitive three-hole microstructured optical fiber specifically designed to improve the refractive index sensitivity of a standard fiber Bragg grating (FBG) sensor photowritten in the suspended Ge-doped silica core. We describe the specific photowriting procedure used to realize gratings in such a fiber. We then determine their spectral sensitivity to the refractive index changes of material filling the holes surrounding the core. The sensitivity is compared with that of standard FBGs photowritten in a six-hole fiber with a larger core diameter. We demonstrate an improvement in the sensitivity by two orders of magnitude and reach a resolution of 3 x 10(-5) and 6 x 10(-6) around mean refractive index values of 1.33 and 1.40, respectively.     

Effect of radiation-induced mean wavelength shift in optical fibers on the scale factor of an interferometric fiber optic gyroscope at a wavelength of 1300 nm

In order to analyze the effect of wavelength-dependent radiation-induced attenuation (RIA) on the mean trans- mission wavelength in optical fiber and the scale factor of interferometric fiber optic gyroscopes (IFOGs), three types of polarization-maintaining (PM) fibers are tested by using a 60 Co γ-radiation source. The observed different mean wave- length shift (MWS) behaviors for different fibers are interpreted by color-center theory involving dose rate-dependent absorption bands in ultraviolet and visible ranges and total dose-dependent near-infrared absorption bands. To evaluate the mean wavelength variation in a fiber coil and the induced scale factor change for space-borne IFOGs under low radiation doses in a space environment, the influence of dose rate on the mean wavelength is investigated by testing four germanium (Ge) doped fibers and two germanium-phosphorus (Ge-P) codoped fibers irradiated at different dose rates. Experimental results indicate that the Ge-doped fibers show the least mean wavelength shift during irradiation and their mean wavelength of optical signal transmission in fibers will shift to a shorter wavelength in a low-dose-rate radiation environment. Finally, the change in the scale factor of IFOG resulting from the mean wavelength shift is estimated and tested, and it is found that the significant radiation-induced scale factor variation must be considered during the design of space-borne IFOGs.     

Effect of radiation-induced mean wavelength shift in optical fibers on the scale factor of interferometric fiber optic gyroscope at a wavelength of 1300 nm

In order to analyze the effect of wavelength-dependent radiation-induced attenuation (RIA) on the mean transmission wavelength in optical fiber and the scale factor of interferometric fiber optic gyroscopes (IFOGs), three types of polarization-maintaining (PM) fibers is tested by using a ⁶⁰Co γ -radiation source. The observed different mean wavelength shift (MWS) behaviors for different fibers are interpreted by color-center theory involving dose rate-dependent absorption bands in ultraviolet and visible range and total dose-dependent near infrared absorption bands. To evaluate the mean wavelength variation in fiber coil and the induced scale factor change for space-borne IFOG under low radiation dose in space environment, the influence of dose rate on the mean wavelength is investigated by testing four germanium (Ge) doped fibers and two germanium-phosphorus (Ge-P) codoped fibers irradiated at different dose rates. Experimental results indicate that the Ge-doped fibers show the least mean wavelength shift during irradiation and their mean wavelength of optical signal transmitting in fibers will shift to shorter wavelength in low-dose-rate radiation environment. Finally, the change in the scale factor of IFOG resulting from the mean wavelength shift is estimated and tested, and it is found that the significant radiation-induced scale factor variation must be considered during the design of space-borne IFOG.     

Fiber bragg gratings made with a phase mask and 800-nm femtosecond radiation

High-quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm. Induced index modulations of 1.9 x 10(-3) were achieved with peak power intensities of 1.2 x 10(13) W/cm2 without any fiber sensitization. The fiber gratings are stable and did not erase after 2 weeks at 300 degrees C. The primary mechanism of induced index change results from a structural modification to the fiber core.     

Effect of radiation-induced color centers absorption in optical fibers on fiber optic gyroscope for space application

The effects of color centers' absorption on fibers and interferometric fiber optical gyroscopes(IFOGs) are studied in the paper. The irradiation induced attenuation(RIA) spectra of three types of polarization-maintaining fibers(PMFs), i.e.,P-doped, Ge-doped, and pure silica, irradiated at 100 Gy and 1000 Gy are measured in a wavelength range from 1100 nm to1600 nm and decomposed according to the Gaussian model. The relationship of the color centers absorption intensity with radiation dose is investigated based on a power model. Furthermore, the effects of all color centers' absorption on RIA and mean wavelength shifts(MWS) at 1300 nm and 1550 nm are discussed respectively. Finally, the random walk coefficient(RWC) degradation induced from RIA and the scale factor error induced by MWS of the IFOG are simulated and tested at a wavelength of 1300 nm. This research will contribute to the applications of the fibers in radiation environments.     

Bragg grating fabrication in germanosilicate fibers by use of near-UV light: a new pathway for refractive-index changes

Using 334-nm light, we demonstrate side writing of Bragg gratings with an index change of ~10(-4) in germanium-doped fibers. No hydrogen loading of the fibers was required. These gratings have the same temperature stability as gratings fabricated with 240-nm light. Our results suggest that photoionization is not needed for formation of gratings in Ge-doped glass. We observe an enhancement of the 334-nm photosensitivity in boron-codoped fibers and suggest that B facilitates a structural transformation of the glass.     

Comparison of thermoluminescence energy response of optical fiber subject to photon irradiation between Monte Carlo simulation and experiments

This study is focused on energy response of Ge-doped silicon dioxide optical fiber subjected to photon irradiation. The TL responses for photon energies, ranging from 20 keV to 20MeV, were investigated as energy absorbed in the TL materials of Ge-doped silicon dioxide optical fiber. The simulation was performed using Monte Carlo N-particle transport code version 5 (MCNP5). The input parameters included in this study are source information, geometry specification, material information and tallies F6. Comparisons of energy response were done between simulation and previous results of experiments. A flat response can only be seen in the energy range of 200 keV to 10 MeV.     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

KrF准分子激光激发掺锗光纤折射率扰动的非线性过程

本文采用了一种简单的功率监测法,观察了KrF准分子激光对掺锗光纤折射率影响的非线性过程。观察到子纤芯折射率扰动过程中非稳定现象、饱和现象和折射率扰动的非线性。