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In this paper, we report the use of crossed polarizer technique to measure the differential group delay (DGD) of few-mode optical fiber (FMF). The windowed Fourier transform (WFT) is applied in the analysis of beat length measurement in the spectral domain to obtain the dependence of DGD as a function of wavelength. The birefringence of polarization-maintaining fiber (PMF) and the DGD of FMF are measured by applying our method. We discuss the noise background, the width of DGD peaks, and the possible errors introduced in the optical path in the modified crossed polarizer technique. [ABSTRACT FROM AUTHOR]

We investigate, based on laser performance, the fractional thermal loadings in a [Yb.sup.3+]/[Er.sup.3+] codoped phosphate glass laser. The results indicate that fractional thermal loading increases with [Er.sup.3+] doping concentration nonlinearly. Above an [Yb.sup.3+] doping level, the increase of [Yb.sup.3+] cannot promote laser performance but induces a more serious heat effect. We investigate the main factors that simultaneously affect laser and thermal performances. The results show that the ratio of laser output power to heat is sensitive to beam overlap efficiency, the laser extraction efficiency, especially to quantum efficiency of laser material. The optimized doping concentrations of [Yb.sup.3+] and [Er.sup.3+] are deduced. [c] 2010 Optical Society of America OCIS codes: 140.6810, 140.3500, 000.2690.

In this work, the relationship among the thermal history, refractive index (RI), and structures of Yb3+/Al3+/P5+/F−‐co‐doped silica glass is studied. We reveal the structural origin of the thermally induced RI change in the Heraeus F300 and Yb3+/Al3+/P5+/F−‐co‐doped silica glasses with different states of thermal history by advanced solid‐state nuclear magnetic resonance and Raman scattering techniques, and the effect of the thermal history on the beam quality of Yb3+‐doped large‐mode‐area photonic crystal fiber (LMA PCF). The structural analysis demonstrates that the RI change in the glass is inextricably linked to the change in the AlPO4 domains, coordination number of the Al and Si–O–Si rings. An intimate and direct connection of the thermally induced physical behaviors is developed with the structural pictures of glasses, and the beam quality of the LMA PCF is greatly improved by proper thermal annealing. The results provide a significant reference for the precise manipulation of the numerical aperture of Yb3+‐doped active fiber. [ABSTRACT FROM AUTHOR]

Based on the material properties of 47 Yb3+‐doped phosphate fibers with different sizes and lengths, a laser performance estimation model was established to estimate the laser wavelength just above the threshold (λth), the laser threshold (Pth), and the laser slope efficiency (η). This model was solved via a numerical computing method with an accuracy of 0.01. Under the material constraints of this model, λth, Pth, and η were investigated by sensitivity analysis and Monte Carlo numerical simulation. The results show that short fibers with a small core diameter may more easily produce a shorter λth. Additionally, for constant material properties, fibers with a longer λth may have a lower Pth and higher η than other fibers. Verifying the above conjecture, an output power of 8.7 W with 35% enhancement in slope efficiency was obtained from an optimized phosphate fiber for an optical path in which the output of the short‐wave laser was inhibited. This model can be extended to simulate the lasing wavelength of multi‐composition fibers, providing a theoretical basis for special laser bands, laser material preparation, and fiber structure design. [ABSTRACT FROM AUTHOR]

Based on the Boltzmann distribution and multi-phonon relaxation probability criterion, an original Yb3+ population equation was proposed to describe the population distribution before and after laser generation, and the population distribution of Yb3+ under different pump ratios and temperatures was investigated by numerical simulation. The simulation results indicated that the laser wavelength of Yb3+-doped modified phosphate fibers have a high probability of being in the range of 1019 nm--1056 nm under the conventional pump ratio. Additionally, fibers lasing at a longer wavelength may have a lower laser threshold. For ultra-high pump ratio or high fiber temperature, the laser operation state changes from a quasi-four-level to a quasi-three-level scheme, and the laser wavelength may blue-shift. Experimental results verify the above simulation results, and in addition demonstrate an output power of 9.38 W with a slop efficiency of 27.4% in an Yb3+-doped phosphate modified fiber with a length of 35.4 cm and diameter of 280 μm from an optical path with a refrigeration patch and suppressing short-wave laser output. The results show that the laser performance of Yb3+-doped fibers can be improved by reducing the operating temperature and inhibiting short-wave laser output. [ABSTRACT FROM AUTHOR]

Yb3+/Al3+-doped and Yb3+/Al3+/P5+-doped silica fibers with almost identical Yb3+ doping concentrations were prepared using modified chemical vapor deposition methods. A harsh experimental environment for simultaneous photo-darkening (PD) and radio-darkening (RD) of an ytterbium-doped silica fiber (YDF) was established. The RD, PD, and photo-radio-darkening (PRD) of the YDF were characterized at two dose rates of 0.1 rad(Si)/s and 1 rad(Si)/s. The laser performances before and after the darkening process were tested under 974-nm pumping. The results demonstrated that the PD largely dominated the PRD kinetics of the YDF at a low dose rate (e.g., that in space) and that the PRD was not a superposition of RD and PD owing to the pump light bleaching if the induced loss was larger than the PD equilibrium level. The induced loss and defect types were investigated through absorption spectra and electron paramagnetic resonance. The results revealed that the P doping could enhance the PD, RD, and PRD resistances by inhibiting the formation of aluminum–oxygen hole centers. [ABSTRACT FROM AUTHOR]

We report the demonstration of dual-wavelength laser output originating from excited Nd3+ ions and Yb3+ ions in Nd3+/Yb3+ co-doped phosphate glass fiber under 970-nm laser diode pumping. Using a free-space coupling method, laser bands at 1028 and 1053 nm were observed in a 35-cm-long fiber with the corresponding lasing thresholds of 4.3 and 31.5 W at room temperature. Emission and lasing spectra under different pump powers were tested by the optical spectrum analyzer. The 1053-nm laser threshold was found to be temperature sensitive. The mechanism of Yb3+ $\to$ Nd3+ energy transfer under 970 nm pumping was explored. It was proposed that Yb3+ $\to$ Nd3+ energy transfer process with the participation of phonon resulted in 1053-nm laser of Nd3+ ions in this co-doped fiber under 970 nm pumping. [ABSTRACT FROM PUBLISHER]

This paper reports, for the first time to our knowledge, a very large-mode-area, Nd-doped silicate glass all-solid large-pitch fiber (AS-LPF) with a core diameter of up to 75 μm. The concepts of the delocalization effect and differential loss were considered simultaneously to evaluate the single-mode operation of the AS-LPF. By reducing the symmetry of the fiber, the single-mode operation of the 75-μm AS-LPF was theoretically predicted using the finite-difference time-domain method and was experimentally fabricated. An output power of ∼9.3 W with a slope efficiency of 41% was obtained for a 60-cm-long fiber. [ABSTRACT FROM PUBLISHER]

Yb3+/Al3+ co-doped silica glass with the composition of 0.1Yb2O3-1Al2O3-98.9SiO2 was prepared by sol–gel method combined with high-temperature sintering. Using this glass as active core, a photonic crystal fiber (PCF) with 105- \mu \textm core was fabricated by the stack-capillary-draw technology. The refractive index fluctuation of this core rod is as low as 2 \times 10^\mathrm \mathbf -4 , implying that highly homogenous silica preform can be realized in this method. Furthermore, this PCF achieved a maximum amplified power of 255 W, with a slope efficiency of 65.7%. It indicates that Yb3+-doped silica preform prepared by the sol–gel method can be employed in the field of high-power laser. [ABSTRACT FROM PUBLISHER]

Single-mode ytterbium-doped phosphate all-solid photonic crystal fiber (AS-PCF) with 13.8 W output power and 32% slope efficiency was reported. By altering the diameter of the rods around the doped core and thus breaking the symmetry of the fiber, a polarization-maintaining AS-PCF with degree of polarization of >85% was also achieved, for the first time to knowledge, in a phosphate PCF. [ABSTRACT FROM AUTHOR]

Based on the sintering of ytterbium-doped nanoporous glass, a nonchemical vapor deposition process aimed at fabricating a large-mode area of photonic crystal fibers has been proposed for the preparation of active silica glass. With the relatively low doping concentration and boron in the silica glass as benefits, a close refractive index between the core and the cladding was obtained. The refractive index variation indicated good homogeneity of the doped fiber core. Up to 34.8 W laser output was demonstrated with the PCF laser and the slope efficiency was 71.3%. The output laser beam quality had slightly multimode behavior, with a mean M2 of 2.42. [ABSTRACT FROM AUTHOR]

Abstract: Two micrometer emission of Ho3+-doped new fluorophosphate glass sensitized by Yb3+ upon excitation of a conventional 980nm laser diode is reported. Based on the measured absorption spectra, the Judd–Ofelt parameters and radiative properties were calculated and discussed. For Yb3+/Ho3+ codoped fluorophosphate glass, the predicted spontaneous transition probability and emission cross-section of Ho3+:5I7→5I8 reaches 73.54s−1 and 4.66×10−21 cm2 , respectively, are much higher than other reported glasses. Larger product of emission cross-section (σem ) and radiative lifetime (τrad ) in the prepared glass shows the prepared glass is expected to be better laser material. In addition, the σem and maximum gain coefficient of Ho3+ in Yb3+/Ho3+ codoped fluorophosphate glass is 12.02% and 12.12% larger than those of Ho3+ singly-doped sample, respectively. The results suggest Yb3+/Ho3+ fluorophosphate glass is a good candidate for efficient 2μm laser. [ABSTRACT FROM AUTHOR]

Highlights: [•] LMA PCF was prepared by sol–gel method and stack-capillary-draw technique. [•] The spectra properties of Yb3+, Al3+ co-doped silica glass were discussed. [•] The laser experiment and laser behaviors were illustrated. [ABSTRACT FROM AUTHOR]

Abstract: Tm3+ and Ho3+ co-doped TeO2–ZnO–Bi2O3 (TZB) glasses were prepared by the conventional melt-quenching method. Two micrometre emission properties of the TZB glasses exited by 808nm Laser Diode (LD) were analyzed and discussed. The Judd–Ofelt parameters, spontaneous radiative transition probabilities and radiative lifetime of Ho3+ were calculated based on the absorption spectra by using Judd–Ofelt theory. The absorption spectra and fluorescence spectra were measured. The predicted spontaneous radiative transition rate of 5I7 → 5I8 of Ho3+ is as high as 466.8 s−1. The result of fluorescence spectra indicates that the maximum emission intensity of Ho3+ 2.0μm and of Tm3+ 1.47μm emission can be achieved at the concentration of 1mol% Tm2O3 and 1mol% Ho2O3 in TZB glasses. The maximum absorption and emission cross-section of Ho3+ near 2.0μm are 0.87×10−20 cm2 at 1951nm and 1.07×10−20 cm2 at 2047nm, respectively. [Copyright &y& Elsevier]

Abstract: The lead-free boron silicon bismuthate glasses were investigated in terms of their physical, optical and structural properties. The physical properties of these glasses have been explored by compositional variation of X-ray diffraction (XRD), differential thermal analysis (DTA) and density. The optical properties of theses glasses have been investigated by UV–Vis and mid-infrared transmittance spectra, and refractive index. The structure of these glasses has been characterized by infrared (IR) absorption and Raman spectra. [Copyright &y& Elsevier]

Monolithic distributed Bragg reflector (DBR) cavity which directly integrates fiber Bragg gratings (FBGs) into the photosensitive RE-doped fibers is a promising configuration in constructing compact and efficient single frequency fiber lasers (SFFLs). Yet, the doping level of rare-earth (RE) ions has generally to be sacrificed in the classical Ge-photosensitized RE-doped silica fibers because of the dramatic refractive index increase caused by the introduction of Ge. Here, we demonstrate an approach to realize the trade-off between photosensitivity and RE doping concentration. We validate that the addition of a small amount of cerium (0.37wt.%) instead of Ge could photosensitize Yb 3+ -doped silica fiber (YDF), while maintaining fiber numerical aperture (NA) at 0.12 under a high 2.5-wt.% Yb doping level. Based on the short monolithic DBR cavity constructed by this germanium-free photosensitive highly YDF, a 1064 nm fiber laser with a 48.6% slope efficiency and an over 200 mW power on two orthogonally polarized modes could be realized. Further stable and linear-polarized 1064 nm SFFL is also demonstrated in a designed monolithic polarization maintaining cavity with an output power of 119 mW and an efficiency of 26.4%. Our results provide an alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.

Yb 3+ /Al 3+ co-doped silica fibers (YDFs) with almost identical core glass compositions were prepared using the sol-gel and modified chemical vapor deposition (MCVD) methods. The photodarkening (PD) and laser performance before and after the PD process were tested under 974 nm pumping. The doping homogeneity of Yb 3+ ions and clusters of Yb 3+ ions in preform slices of these two fibers were investigated via optical absorption spectroscopy, photoluminescence emission spectra, electron probe microanalysis (EPMA), and low-temperature (4 K) electron paramagnetic resonance (EPR). It is known that the PD resistance of YDFs prepared via the sol-gel method is significantly better than that of YDFs prepared via MCVD under the same test conditions. EPMA mapping reveals that the doping homogeneity of Yb 3+ ions in the sol-gel fiber core glass is better than that in the MCVD fiber. The low-temperature (4 K) EPR and cooperative luminescence spectra of Yb 3+ ions indicate that the clustering degree of Yb 3+ ions in the sol-gel fiber is lower than that in the MCVD fiber. In the absorption and emission spectra, small amounts of Yb 2+ ions are observed in the preform slice from the sol-gel method. A model of the color-center generation in the PD process was proposed to explain the mechanism of PD resistance improvement for the YDFs fabricated via the sol-gel method.

We report a 30 cm long Nd 3+ /Yb 3+ co-doped silica glass fiber amplifier with well-maintained broadband gain through simultaneous dual-laser pumping at 808 and 975 nm. By controlling the ratio of the pump power at 975 (P 975 ) and 808 nm (P 808 ), the emission band of Yb 3+ /Nd 3+ and the energy transfer efficiency of Nd 3+ →Yb 3+ can be well controlled; thus, a tunable broadband and flat gain in the range of 1036-1080 nm are obtained. The theoretical calculation of the energy transfer efficiency of Nd 3+ →Yb 3+ with dual-laser pumping of 808 and 975 nm explains well the mechanism of broadband and flat gain formation in the Nd 3+ /Yb 3+ co-doped fiber.

Output power scaling of single mode large mode area (LMA) photonic crystal fiber (PCF) amplifiers urgently requires the low refractive index of Yb 3+ -doped silica glasses whilst maintaining high optical homogeneity. In this paper, we report on a promising alternative Yb 3+ /Al 3+ /F - /P 5+ -co-doped silica core-glass (YAFP), which is prepared by modified sol-gel method developed by our group and highly suitable for fabricating high power LMA PCF amplifiers. By controlling the doping combinations of Al 3+ /F - /P 5+ in Yb 3+ -doped silica glass,it not only ensures low refractive index (RI) but also maintains the excellent optical homogeneity and spectroscopic properties of Yb 3+ . The spectroscopic properties of Yb 3+ ions have not deteriorated by the co-doping of F - and P 5+ in YAFP glass compared with that of Yb 3+ /Al 3+ co-doped silica glass. A large-size (⌀5 mm × 90 mm) YAFP silica-core glass rod with low average RI difference of 2.6 × 10 -4 (with respect to pure silica glass), and low radial and axial RI fluctuations of ~2 × 10 -4 , was prepared. A LMA PCF with 50 µm core diameter was obtained by stack-capillary-draw techniques using YAFP core glass. Its core NA is 0.027. An average amplified power of 97 W peaking at 1030 nm and light-light efficiency of 54% are achieved from a 6.5 m long PCF in the pulse amplification laser experiment. Meanwhile, quasi-single-mode transmission is obtained with laser beam quality factor M 2 of 1.4.

For the first time, to the best of our knowledge, we report on the realization of a laser from a Yb-doped phosphate core/silicate cladding double-clad hybrid fiber. 5 W output power was extracted with 14.6% slope efficiency and a laser spectrum of a 1027 nm central wavelength from a 20 cm long single-mode fiber with a ∼10  μm core diameter in a 20%-4% laser cavity. The laser efficiency can be significantly enhanced by correspondingly adjusting and optimizing the laser oscillator.

This work reports the generation of an optical-power tunable 1 μm dual-wavelength fiber laser in Nd 3+ /Yb 3+ co-doped phosphate glass fiber under 970 nm and 808 nm laser diode pumping. A bidirectional free-space pump laser setup was adopted. An optical-power tunable dual-wavelength laser can be easily realized by the control of 808 nm pump power magnitude. Based on a series of laser tests under different pump conditions and theoretical calculation of the Nd 3+ →Yb 3+ energy transfer efficiency, the control mechanism was found to be the stimulated emission modulation of Nd 3+ on Yb 3+ lasing. This property is helpful to enhance the knowledge of the Nd 3+ /Yb 3+ co-doping system.

This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a Yb(3+)/Al(3+)/F(-) codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter >6  mm was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean M2<1.1 in this 50 μm core diameter fiber was achieved at a bend diameter of 0.35 m. The core glass with the refractive index nearly equal to pure silica glass is suitable for the fiber design, such as large-mode-area photonic crystal fiber.

An all-solid Yb(3+)/Er(3+) co-doped single-mode phosphate photonic crystal fiber (PCF) with Watt-level output power and 20 μm core diameter is demonstrated for the first time. A PCF whose refractivity of the active core is lower than that of the background glass is suggested and theoretically confirmed to be in single-mode operation at 40 μm core diameter.