Co-doped Silica Fiber Research Articles

Low photo darkening single mode RMO fiber

A new reduced mode overlap (RMO) single mode fiber design is proposed and demonstrated. For the first time saturated photo darkening operation is observed in a nominal 350 W Yb / Al co-doped silica fiber laser. After 1500 hours operation less than 7% slope efficiency degradation is found and further 500 hours operation show no degradation.

Brillouin spectroscopy of Nd–Ge co-doped silica fibers

Abstract The effect of Nd-doping on both the longitudinal acoustic velocity and material damping coefficient (Brillouin spectral width) of silica fiber are measured. We find that Nd-doping results in anomalously large changes in the acoustic properties of SiO2, such as that material damping is greatly exacerbated by the incorporation of Nd. We find that the spectral width increases by about 151% per wt% of Nd2O3 (for small [Nd]). This is very large compared to the effect of Ge-doping, as observed in this work. This may result from poor coordination in the SiO2/GeO2 matrix, and may be considerably different with the use of co-dopants such as Al that tend to improve the solubility of the rare earths. The acoustic velocity is found to decrease at a rate of about 6.1% per wt% of Nd2O3.

Visible and near infra-red up-conversion in Tm^3+/Yb^3+ co-doped silica fibers under 980 nm excitation

The spectroscopic properties of Tm(3+)/Yb(3+) co-doped silica fibers under excitation at 980 nm are reported. Three distinct up-conversion fluorescence bands were observed in the visible to near infra-red regions. The blue and red fluorescence bands at 475 and 650 nm, respectively, were found to originate from the (1)G(4) level of Tm(3+). A three step up-conversion process was established as the populating mechanism for these fluorescence bands. The fluorescence band at 800 nm was found to originate from two possible transitions in Tm(3+); one being the transition from the (3)H(4) to (3)H(6) manifold which was found to dominate at low pump powers; the other being the transition from the (1)G(4) to (3)H(6) level which dominates at higher pump powers. The fluorescence lifetime of the (3)H(4) and (3)F(4) levels of Tm(3+) and (2)F(5/2) level of Yb(3+) were studied as a function of Yb(3+) concentration, with no significant energy back transfer from Tm(3+) to Yb(3+) observed.

Investigation on the spectral characteristics of bismuth doped silica fibers

We investigated the fluorescence characteristics of the bismuth doped silica fibers with and without Al co-dopant which are fabricated by means of modified chemical vapour deposition (MCVD) technique. It was found that the bismuth and germanium codoped silica fiber exhibited a strong broad infrared emission centring at 1450 nm and covering 1300 nm when excited by the laser diodes working at 808 nm and 976 nm respectively. It was also found that for the Al and bismuth codoped silica fiber, the red (centring around 750 nm) and the infrared fluorescence (centring around 1100 nm) may originate from different emission centers in the fiber. Furthermore, strong up-conversion luminescence was also observed in both Al codoped and non-Al codoped bismuth doped fibers when the fibers were excited by an acoustic-optic Q-switched Nd:YVO 4 laser. It is thus concluded that the upper energy level absorption reported in the work of the bismuth doped silica fiber lasers may result from the fluorescence emission sites not responsible for the infrared emission.

Efficient Tm 3+, Ho 3+-co-doped silica fibre laser diode pumped at 1150 nm

High power and highly efficient operation of a Tm 3+, Ho 3+-doped silica fibre laser that is pumped with diode lasers operating at 1150 nm is demonstrated. Internal slope efficiencies approaching the Stokes limit were produced and the maximum output power was 2.9 W. High power diode lasers operating at 1150 nm are valuable pump sources for a range of fibre lasers offering output in the shortwave infrared spectrum.

Nonlinear transmission coefficient of ytterbium-holmium fiber at the wavelength 978 nm

An experimental and theoretical investigation of the nonlinear transmission coefficient of ytterbium-holmium codoped silica fiber (YHF) at continuous-wave 978-nm pumping is reported. An analysis of the fiber absorption and luminescence spectra under 978-nm pumping reveals a variety of processes participating in the mixed (Yb3+, Ho3+) system: the energy transfer Yb3+ → Ho3+, the up-conversion and excited-state absorption in Ho3+, and the luminescence quenching due to the presence of Yb3+-Yb3+ ion pairs. These processes are shown to notably affect the transmission coefficient of YHF, which is reflected in a pronounced modification of its dependences on pump power, fiber length, and dopant concentrations, in comparison with the case of purely ytterbium-doped fiber (YF). A modeling of the experimentally measured dependences of the YHF transmission coefficient on pump power and fiber length allows us to obtain the coefficients addressing the energy transfer process Yb3+ → Ho3+ in YHF. A comparison of YHF and YF at the same pumping conditions reveals that, in YHF, a considerable part of the pump power absorbed in the Yb3+ subsystem is transferred into the Ho3+ subsystem; this results in an effective population of its 5 I 7 state and makes YHF promising for 2-μm lasing and amplifying.

Characterization of thermal (in)stability and temperature-dependence of type-I and type-IIA Bragg gratings written in B–Ge co-doped fiber

The temperature-dependence and the thermal stability of UV-exposed type-I and type-IIA fiber Bragg gratings written in boron–germanium co-doped silica fiber were examined, in terms of their refractive-index modulations and the change in the Bragg wavelengths. The decay in the reflectivity of the gratings with time at different temperatures was studied and modelled according to a power-law and a master aging curve approach. The distribution of the activation energy, for these types of gratings, was evaluated to describe their thermal stability and then the comparison of their stabilities has been made. Temperature-induced reversible and irreversible changes in the grating characteristics were observed. The temperature-induced reversible spectral characteristics of type-I and type-IIA gratings were found to be different in nature. All these results were discussed within the framework of the current theoretical approaches for predicting the stability and operational lifetime of these types of gratings.

Optical amplification in Yb3+-codoped thulium doped silica fiber

Ytterbium–thulium-codoped silica fiber is proposed and studied for the first time. We first established a novel model for Yb 3+ –Tm 3+ -codoped silica fiber amplifier based on the Yb 3+ to Tm 3+ energy transfer. Using appropriate fiber parameters and energy transfer parameters, we analyzed and solved the coupled rate equations. We have obtained improved gain results in comparison to silica fiber amplifiers doped with Tm 3+ only. The results demonstrated that the ytterbium to thulium energy transfer process in the material is significant and beneficial.

Lasing Characteristics of Tm3+:Ho3+ Codoped Silica FibreLaser Pumped by 1.18-μm Raman Fibre Laser

The fundamental characteristics of Fabry–Perot-cavity continuous-waveTm–Ho codoped silica fibre lasers pumped by a 1.18 μm Raman fibre laserare presented. A maximum output power of 930 mW at 1880 nm is generatedfor a fibre length of 1 m from the transition of Tm3+, with aslope efficiency of 32.4%. For a 3-m-long fibre, the maximum outputpower decreases to 650 mW at 1960 nm due to the laser emission fromHo3+ with a lower slope efficiency of 25%, which clearly showsthe effective energy transfer from Tm3+ to Ho3+. Thewavelength red-shifting of the laser emission originates from thetransition competition and the emission cross section differencebetween Tm3+ and Ho3+.

Secondary energy transfer and nonparticipatory Yb^3+ ions in Er^3+-Yb^3+ high-power amplifier fibers

The energy exchange processes within Er3+–Yb3+-codoped silica fibers manufactured for high-power optical amplifiers are investigated through luminescent decay measurements and rate-equation modeling. The Er3+ and Yb3+ luminescence, as functions of pump power, exhibit an unquenchable Yb3+ to Er3+ cooperative energy transfer (CET) that does not subside with saturation of the Er3+ inversion. Consistent with the experimental results is the occurrence of secondary energy transfer to excited Er3+ ions in their metastable 4I13/2 state. The transfer coefficient for this secondary 4I13/2 CET is determined to be of the same order of magnitude as that for the initial Yb3+ to ground-state Er3+ CET. Additionally, the decay measurements and modeling indicate that a fraction of Yb3+ ions does not participate in energy exchange with the Er3+. These nonparticipatory Yb3+ ions amounted to ~15% of the total Yb3+ concentration and could constitute Yb3+ clusters. Both secondary CET and nonparticipatory Yb3+ will lower Er3+–Yb3+ fiber amplifier efficiencies.

Analysis of thermal decay and prediction of operational lifetime for a type I boron-germanium codoped fiber Bragg grating.

The thermal decay of a type I fiber Bragg grating written at 248 nm in boron-germanium codoped silica fiber was examined in terms of its reflectivity and Bragg wavelength change. In addition to the decay in reflectivity, which was observed, a shift in Bragg wavelength over the temperature range considered was seen. A mechanism for the decay in the reflectivity was developed and modeled according to a power law, and the results were compared with those from the aging curve approach. The wavelength shift was simulated by modification of the power law, which was also found to fit well to the experimental data. Temperature-induced reversible and irreversible change in the grating characteristic were observed and considered to be a means to predict the working lifetime of the grating at comparatively low temperatures. Accelerated aging was also reviewed and compared in terms of reflectivity and Bragg wavelength shift. It was shown that the temperature-induced irreversible shift in the Bragg wavelengths could not be predicted by use of the isothermal decay of the refractive-index modulation. The results were discussed within the framework of the current theoretical approaches for predicting the stability of gratings of this type.

Broadband amplified spontaneous emission from an Er(3+)-Tm(3+)-codoped silica fiber.

A new type of amplified spontaneous emission (ASE) based on an Er-Tm-codoped silica fiber is reported. When the fiber was pumped at 980 nm, the emission yielded a 3-dB bandwidth over 90 nm (1460-1550 nm) without any external filters, which is twice larger than that produced by conventional Er-doped fiber ASE sources. We believe that the superposition of two ASE bands, one from Er ions near 1530 nm and the other from Tm ions near 1450 nm, results in the broad bandwidth. Temperature-dependent ASE powers and fiber-length-dependent spectra strongly suggest that photon-assisted energy transfer between the sensitizing ion, Er, and the acceptor ion, Tm, plays an important role in changes in the bandwidth's size.

Characterization of broadband amplified spontaneous emission from an Er 3+–Tm 3+ co-doped silica fiber

We report spectral characteristics of amplified spontaneous emission (ASE) from an Er–Tm co-doped silica fiber. When pumped at 980 nm, the ASE yielded a 3 dB bandwidth over 90, 1460–1550 nm. The changes in ASE with regard to temperature, pumping scheme, and fiber lengths were measured experimentally, which strongly suggest that phonon-assisted energy transfer between the sensitizing ion, Er 3+, and the acceptor ion, Tm 3+, plays an important role in the bandwidth increment.

A new gas detection technique utilizing amplified spontaneous emission light source from a co-doped silica fibre in the 2.0 m region

A new technique for the measurement of the concentration of gas species is presented. The method is based on absorption spectroscopy in the infrared region utilizing a high-power broad band amplified spontaneous emission source from an optical fibre. Vibrational bands of gas in the range 1.9-2.1 m were measured and the relative intensities of bands were calibrated in terms of concentration. The amplified spontaneous emission from a co-doped silica fibre pumped near 800 nm was used as a light source that consisted of the transition of the ion and the transition of the ion with a full width at half maximum of 225 nm and total output power over 1 mW. The technique has potential for the simultaneous detection of multiple gas species due to its high spectral energy density over a wide wavelength band in the infrared where the vibrational overtones of gas molecules are located.

Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber

Fiber Bragg gratings (FBGs) have been written in silica fiber codoped with boron and germanium, using a continuous-wave (CW) 244-nm laser. The thermal decay of the gratings written with and without hydrogen loading is examined over a period in excess of 5000 h. It is shown that the thermal decay of the nonhydrogen loaded gratings is well explained by the power law model proposed by Erdogan. Gratings written in hydrogen loaded fiber, however, do not obey the power law model. A new model is presented based on a log time representation which can be used to predict the decay characteristics of gratings written in hydrogen loaded fiber.

Nd:YLF Pumped Grating-Feedback Er3+:Yb3+Co-Doped Silica Fiber Laser Operating at 1.49 μm

We demonstrate for the first time a 1.49-μm Er3+:Yb3+co-doped silica fiber laser with fiber Bragg grating feedback, pumped with a diode-pumped Nd:YLF laser at 1.047 μm, which can be used for remote pumping of EDFAs. We examine both the potential and limitations of this laser as a high power source and point out how an efficient, high power laser could be realized. The maximum demonstrated output power, 21 mW, was limited by the cavity losses, by the strong ASE at the peak gain wavelength 1.535 μm, and by a “bottleneck effect” in the energy transfer between ytterbium and erbium with a corresponding reduced pump absorption at higher pump powers. By overcoming these limitations an output power in excess of 500 mW with 1 W of pump power is predicted.

Continuous-wave oscillation of thulium-sensitized holmium-doped silica fiber laser

We have observed oscillation in a Tm(3+) and Ho(3+) codoped silica fiber from the (5)I(7) ? (5)I(8) transition of Ho(3+) in the 2-microm region, by pumping the fiber laser into a Tm(3+) absorption between 800 and 830 nm. By a change in the cavity length, the laser was tunable between 2.037 and 2.096 microm. When the laser was pumped with a Ti:sapphire laser at 820 nm, an absorbed threshold power extracted power of 214mW, a slope efficiency of 4.2%,and maximum extracted power of 12.5 mW were measured.

Yb 3+ sensitised Er 3+ doped silica optical fibre with ultrahigh transfer efficiency and gain

Efficient 1.54 μm emission under 1064 nm excitation of Er3+/Yb3+ codoped silica fibre is reported. The energy transfer efficiency from Yb3+ to Er3+, ∼95%, even under high inversion, is comparable to that in multicomponent glass fibres. The small signal gain of ∼45dB is measured and power amplifiers with 145 mW output power demonstrated.

Yb3+ Sensitised Er3+ Doped Silica Optical Fibre with Ultra High Transfer Efficiency and Gain

ABSTRACTEfficient 1.54 μm emission under 1064nm excitation of Er3+/Yb3+ codoped silica fibre is reported. The energy transfer efficiency from Yb3+ to Er3+, of at least 85%, even under high inversion, is comparable to that in multicomponent glass fibres. Fibre design parameters are discussed and results presented. Small signal gain of ˜ 45dB is also measured and a power amplifier giving 145mW output power demonstrated.