K.M. Golant

Double clad tapered fiber for high power applications.

We report a novel type of active fiber - tapered double clad fiber suitable for pumping by low brightness sources with large beam parameter product of 50/300 mm x mrad. Ytterbium double clad all-silica fiber (core/1(st) clad/2(nd) clad diameters 27/834/890 mum, NA(core)=0.11, NA(clad)=0.21), tapered down by a factor 4.8 for a length of 10.5 m was drawn from a preform fabricated by plasma chemical technologies. At a moderate Yb-ion concentration and 1:31 core/cladding ratio, the tapered double clad fiber demonstrates 0.9 dB/m pump absorption at 976 nm and excellent lasing slope efficiency. An ytterbium fiber laser with 84 W of output power and 92% slope efficiency, a 74 W superfluorescent source with 85% slope efficiency and amplifiers operating both in CW and pulsed regimes have been realized. All devices demonstrated robust single mode operation with a beam quality factor of M(2)=1.07.

Refractive index dispersion of doped silica for fiber optics

The spectral dependencies of refractive index have been measured in Ge-, P-, N-, Cl-, B-, F-, and Al-doped silica glasses as well as in undoped silica glasses using bulk prism samples cut from optical fiber preforms. The latter have been fabricated by MCVD-, PCVD-, and SPCVD-processes. Based on the experimental results, material dispersion in the glasses has been analyzed, which is one of the most important parameters for fiber optics. An assumption has been made regarding the origin of the significant discrepancy of the zero-dispersion wavelength of nominally identical glass compositions in different publications. The effect of chlorine admixture on the dispersion curves has been investigated. Nitrogen-doped silica is shown to be a promising material for broadband graded-index multimode fibers.

Performance of Bragg and long-period gratings written in N- and Ge-doped silica fibers under /spl gamma/-radiation

In-fiber Bragg and long-period gratings as well as Mach-Zehnder interferometers based on germanium- and nitrogen-doped silica fibers have been investigated under /spl gamma/-rays. The majority of the experimental results suggest that both types of gratings in both types of fibers are stable with respect to /spl gamma/-ray doses of up to 1.47 MGy.

γ‐radiation resistance of aluminum‐coated all‐silica optical fibers fabricated using different types of silica in the core

Induced loss spectra in four Al‐coated large‐core optical fibers are measured in the range 380–1000 nm during and after 60Co γ irradiation. These fibers had F‐doped‐silica claddings and synthetic silica core materials produced by different technologies. Measurements were carried out independently in Washington and Moscow using differing techniques but yielding very similar results. The highest radiation resistance is exhibited by two fibers with extremely low OH and Cl contents in their cores, one with a core material containing 0.5 mass % of fluorine, fabricated specially for this study, and one based on KS‐4V silica, an original technology developed in Russia. The latter fiber demonstrates a rather flat induced loss spectrum in the visible region at a level of 2–2.5 dB/m for γ‐ray doses of ∼10 MGy(Si).

Bismuth activated alumosilicate optical fibers fabricated by surface-plasma chemical vapor deposition technology

Plasma chemical technology is experimentally applied to the fabrication of a Bi-activated alumosilicate-core pure-silica-cladding fiber preform. To the best of our knowledge, this is the first time this technology has been applied in this way. We measure gain efficiency at pumping by a 1058 nm wavelength Yb fiber laser in a piece of a newly obtained fiber 20 m in length within 100-1200 nm wavelengths band. The gain efficiency reaches as high as 0.2 dB/mW. Bi-activated alumosilicate-core pure-silica-cladding fiber that is not more than 12 m in length serves a basis for a 1 W output power fiber laser emitting at the wavelength of 1160 nm with 8% slope efficiency. We also measure the photoluminescence spectrum and kinetics of Bi centers responsible for laser emission under the excitation of 193 nm wavelength ArF laser pulses.

Highly efficient 750 W tapered double-clad ytterbium fiber laser

The results of theoretical and experimental studies of active tapered double-clad fibers, intending the optimization of its imperative parameters--tapering ratio, longitudinal profile, core/cladding diameters ratio, are presented. Using a refined taper geometry we have demonstrated power scaling of a ytterbium fiber laser pumped by low-brightness, cost-effective laser diodes up to 750 W, with 80% efficiency.

Super-high-strength metal-coated low-hydroxyl low-chlorine all-silica optical fibers

High-purity KS-4V synthetic silica developed in the Silicate Chemistry Institute of the Russian Academy of Sciences is tested as the core material for radiation hardened optical fibers. Pure-silica-core fluorine-doped-silica-cladding optical fibers with polymer (acrylate) or metal (aluminum) coating are produced as the experimental samples. The light-reflecting fluorine-doped silica cladding is synthesized by the plasma outside deposition process. The aluminum coating technology used provides a very high strength of the fibers, unattainable for polymer coatings, and expands the fiber operating range up to 400/spl deg/C. It is established that the metal coating application can result in the annealing of the drawing-induced color centers with an absorption peak at 630 nm. Post-/spl gamma/-irradiation loss spectra in KS-4V-based fibers measured in 1-2 hours after 2 MGy irradiation at a dose rate of 8.3 Gy/s in the spectral range 350-700 nm are discussed. The 630 nm absorption peak is practically absent from the post-irradiation loss spectra of aluminum-coated KS-4V-based fibers.

/spl gamma/-radiation-induced absorption in pure-silica-core fibers in the visible spectral region: the effect of H/sub 2/-loading

/spl gamma/-radiation-induced absorption in the visible spectral region is studied in H/sub 2/-loaded and untreated optical fibers with high-OH low-Cl KU core and low-OH low-Cl KS-4V core. H/sub 2/-loading is found to suppress the low-dosage transient absorption band centered at 670 nm in KS-4V fibers. After irradiation to 1.7 MGy(Si), H/sub 2/-loaded fibers show losses many times lower than the untreated fibers. H/sub 2/-loaded and irradiated fibers are found to retain very high radiation resistance under subsequent irradiations even in the absence of molecular hydrogen. It is concluded that H/sub 2/-loading followed by pre-irradiation can be used as an efficient radiation hardening technique for pure-silica optical fibers.

Mode-field converters and long-period gratings fabricated by thermo-diffusion in nitrogen-doped silica-core fibers

Summary form only given. Novel nitrogen-doped silica-core pure-silica-cladding fibers provide exceptional possibilities to achieve abrupt variations of the refractive-index profile by thermo-diffusion of nitrogen. A high diffusion coefficient of nitrogen in silica makes it possible to create deep changes in refractive index by means of short-time heat action on a fiber section with a length comparable to the fiber diameter. Thus, N-doped silica fibers are most appropriate for various fiber-optic devices produced by thermo-diffusion.

Actively Q-switched 1.6-mJ tapered double-clad ytterbium-doped fiber laser.

We have demonstrated an actively Q-switched tapered double-clad fiber laser capable of single-shot generation of 1.6-mJ, 64-ns pulses. The active medium based on tapered double-clad fiber is shown to exhibit a reduced level of amplified spontaneous emission which allows for high-energy pulse extraction at extremely low repetition rates.