[Supercontinuum Generation in Tapered Microstructure Fibers with Different Taper Length by Using Femtosecond Laser].

Tapered microstructure fibers with different taper lengths and waist diameters are pumped with femtosecond laser for supercontinuum generation. With "fast and cold tapered method", home made microstructure fiber with air-hole pitch Λ=6.53 µm and normalized air-hole diameter d/Λ=0.79 were tapered to 6, 8, 10 mm taper length while keeping d/Λ unchanged. Numerical simulations show that the zero dispersion wavelength shifts to blue when the taper waist shrinks. The zero dispersion wavelengths for untapered and 6, 8, 10 mm length tapered fiber were 1 029, 885, 806, and 637 nm, respectively. In the experiment, 120 fs pulses centered at 810 nm, which is generated by mode-locked Ti:sapphire laser at a repetition rate of 76 MHz, is coupled into the tapered microstructure fiber. With the tapered length of 6 mm, the center wavelength of the pump light locates in the normal dispersion region of the fiber and near the zero dispersion wavelength of the tapered waist. The main factors causes spectra broaden are intrapulse Raman scattering and cascaded four-wave mixing. When the pump power reaches 450 mW, continuous spectra with -5 dB flatness are generated at 390~461 and 1 134~1 512 nm. With 500 mW pump power, supercontinuum spans from 366 to 2 450 nm, which has already covered ultraviolet, visible, near-infrared and mid-infrared. This broadband spectrum almost reaches the red and blue edges of the microstructure fiber's transmission bandwidth. With 8mm tapered length and 450 mW pump power, the blue edge of the continuous spectrum shifts down to 366 nm as a result of group velocity match and group acceleration mismatch, a 9 nm deeper blue shift compared to results from 6mm tapered length. With the tapered length of 10 mm, because the zero dispersion wavelength of the waist also moves to visible region, phase matching condition can still be satisfied in that region. Due to the effect of cascaded four-wave mixing, the frequency up conversion is realized in visible region. When pump power reaches 500mW, up conversion frequency lies in 30 nm band from 382 to 412 nm, the conversion efficiency is up to 27.7%.

[Study on phase-matching of four-wave mixing spectrum in photonic crystal fiber].

In the present paper, the four-wave mixing principle of fiber was analyzed, and the high-gain phase-matching conditions were shown. The nonlinear coefficient and dispersion characteristics of photonic crystal fibers were calculated by multipole method. The phase mismatch characteristics of fibers with multiple zero-dispersion wavelengths were analyzed for the first time. The changing rules of phase matching wavelength with the pump wavelength and the pump power were obtained, and the phase matching curves were shown. The characteristics of phase matching wavelengths for different dispersion curves were analyzed. There are four new excitation wavelengths of four-wave mixing spectrum in two zero-dispersion wavelength photonic crystal fiers. Four-wave mixing spectroscopy of photonic crystal fibers with two zero-dispersion wavelengths was obtained in the experi-ent, which is consistent with the theoretical analysis, and verified the reliability of the phase matching theory. The fiber with multiple zero-dispersion wavelengths can create a ricbhphase-matching topology, excite more four-wave mixing wavelengths, ena-ling enhanced control over the spectral locations of the four-wave mixing and resonant-radiation bands emitted by solitons and short pulses. These provide theoretical guidance for photonic crystal fiber wavelength conversion and supercontinoum generation based on four-wave mixing.

[Research on spectral characteristics of Yb3+ doped double-cladding large-mode-area micro-structured optical fiber].

Yb3+ doped double-cladding large-mode-area micro-structured optical fibers (Micro-structured fibers, MSF) are the ideal medium for the super high-power optical fiber laser applications. In the present paper, the authors fabricated the Yb3+ doped silica-based glass using the method of non-chemical vapor deposition, and fabricated the Yb3+ doped double-cladding large-mode-area MSF by stack-drawing method using this glass as the core of MSF, according to the design requirements. Fluorescence spectrum of the MSF was obtained using Ti: sapphire femtosecond laser with the wavelength of 975 nm and LD laser with the wavelength of 980 nm as pumping source. The experimental results show that the optical fiber has strong fluorescence at the wavelength of 1 050 nm, and it can inhibit generation of cooperative luminescence effectively.

[Investigation on the fabrication and spectrum properties of Yb3+ -doped silicate laser glasses].

Two kinds of Yb3+ doped silicate laser glass with little difference were produced by high temperature of melting process. The absorption and emission spectra of the two glass samples were tested by the correlative spectrographs; the integral absorption cross section, stimulated emission cross section, fluorescence line-width, fluorescence lifetime, least particle count, saturation pump intensity and least pump intensity of the Yb3+ -doped laser glasses were calculated respectively, and by comparison it was found that the chart of the absorption cross section is similar to the stimulated emission cross section calculated by the reciprocity method, and is very different from the stimulated emission cross section calculated by the Fuchbauer-Ladenburger method. This result is precisely in line with the theoretical analysis. The line-types of the absorption spectra of the two glass samples are almost the same, and the first peak value of absorption is located at 975 nm while the second peak value is at 908 nm. As the two components of the samples are not very different, the accord of the line-types of the absorption spectra indicates that the makeup of the glass material is the primary factor influencing the line-type of the absorption spectra. The fluorescence spectra of the two glass samples are very different, and the first fluorescence peak value of sample one is located at 993 nm with the second peak value at 1029 nm, while the first fluorescence peak value of sample two is located at 1 035 nm with the second peak value at 994 nm. The cause of the major difference in the fluorescence spectra of two samples lies in the different doping density of Yb3+. By comparison we found that the laser performance of sample two is better than that of sample one. The test shows that both samples are suitable for drawing fiber.

Study of the sensitivity of gas sensing by use of index-guiding photonic crystal fibers.

We demonstrate an absorption transmission spectrum of CH(4) in a 16.9 cm long index-guiding photonic crystal fiber (PCF) fabricated in our laboratory. One of the main factors to improve the sensitivity is to increase the fraction of power in PCF cladding air holes. We study the fraction of power in PCF cladding air holes as a function of the index-guiding PCF parameters. We found that a PCF with small spacing and a large air-filling ratio has a higher fraction of power in its cladding air holes. At the same time the mode area in this PCF is small and would generate strong nonlinear effects in the fiber. If we use a PCF in which the core is formed by missing seven air holes, it is immediately obvious that the PCF used as a sensor has higher sensitivity and a larger mode area.