Changgeng Ye

Ytterbium-doped fibers fabricated with atomic layer deposition method

We report on a new fabrication method of producing ytterbium doped fibers by atomic layer deposition (ALD) in combination with the conventional modified chemical vapor deposition (MCVD) technique. An MCVD soot-preform with a porous layer of SiO(2) is coated with layers of Yb(2)O(3) and Al(2)O(3) prior to sintering, using the gas-phase ALD method. An SEM/EDS material analysis study shows that the dopants successfully penetrate the full thickness of 320 µm of the soot layer. An Yb-doped fiber fabricated by this technique shows a background attenuation of 20 dB/km, a uniform longitudinal Yb-doping profile, and good laser characteristics with a slope efficiency of 80%. Furthermore, we present a comparison in terms of photodarkening between the MCVD-ALD fiber and a solution doped fiber, fabricated with the same MCVD recipe. The new MCVD-ALD fiber appears to be more photodarkening resistant.

Ag nanoparticles embedded in glass by two-step ion exchange and their SERS application

Silver nanoparticles embedded in glass are prepared by a two-step ion exchange process, where silver ions are introduced into glass in silver ion exchange, and reduced into metallic silver in subsequent potassium ion exchange. The formation of the particles can be explained by the combination effect of the galvanic replacement reaction and the electrolytic deposition. The formed particles are characterized by optical absorption measurements, transmission electron microscopy and atomic force microscopy. Their application in SERS is demonstrated, and the optimal surface features in terms of SERS enhancement are also discussed.

Fiber amplifier utilizing an Yb-doped large-mode-area fiber with confined doping and tailored refractive index profile

Power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities. Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of the fibers. Both fibers exhibited a mode field diameter comparable to a 40μm step index fiber with 0.07 NA. The fibers were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to the fibers.

Refractive index change in ytterbium-doped fibers induced by photodarkening and thermal bleaching

High-accuracy measurements and analysis of refractive index change induced by photodarkening and thermal bleaching in ytterbium-doped fibers are presented, based on a modal interference method. Photodarkening-induced refractive index change is positive at the ytterbium lasing wavelengths near 1080 nm, and it approaches a saturated level, which is in the order of 10(-6)∼10(-5) for the tested fiber samples. It is found that the value of this refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. Thermal bleaching can only partially erase the photodarkening-induced refractive index change, leaving a residual index change of (0.2∼0.3)×10(-5). The influence of the photodarkening-induced refractive index change on fiber lasers is discussed.

Analysis of temperature dependence of photodarkening in ytterbium-doped fibers

We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.

Confined-doped ytterbium fibers for beam quality improvement: fabrication and performance

By confining the Yb doping within a smaller radius in the center of the core of the few-mode large-mode-area fiber, the fundamental mode, which overlaps better with the Yb ions, sees higher gain than the higher order modes, and dominates the output. Hence, improved beam quality can be achieved. A confined-doped fiber with 41/395μm core/cladding diameters is fabricated by Direct Nanoparticle Deposition (DND) process. The fiber is characterized in an amplified spontaneous emission (ASE) source setup. Near-diffraction-limited beam quality (M2~1.3) is experimentally demonstrated.

Measurement of photodarkening-induced refractive index change in ytterbium doped fibers by modal interference

By using a modal interference method, photodarkening-induced refractive index change in ytterbium doped fibers is measured with high accuracy. This refractive index change is positive at the ytterbium lasing wavelengths near 1080nm, and it gradually approaches a saturated level over time. It is found that the refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. The maximum refractive index change in our experiment is close to 1×10-5. The origin and influence of the photodarkening-induced refractive index change on fiber lasers is discussed.

On the measurement of fundamental mode bend loss in large-mode-area optical fibers

The measurement of fundamental mode bend loss is thoroughly studied in large-mode-area few-mode optical fibers. The influencing factors, including spectral properties of the light source, modal power content and cladding light, are experimentally investigated. Monte Carlo simulations are performed to help in understanding and illustrating the distribution of the variations. Practical guidelines and an example setup are provided for precise and accurate measurements.

A novel all-vapor phase fabrication process for ytterbium-doped fibers with atomic layer deposition method

We present a novel all-vapor phase fabrication method for producing Yb-doped fibers by atomic layer deposition (ALD) in combination with the well-established modified chemical vapor deposition (MCVD) technique.

Atomic layer deposition for fabrication of ytterbium doped fibers

Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO2 doped with GeO2 was coated with layers of Yb2O3 and Al2O3 prior to sintering, using the ALD method. ALD is a surface controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of 320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses, lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating Yb-doped fibers with high concentration of dopants.