Single-mode Bragg gratings in tapered few-mode and multimode fibers.

We propose the implementation of fiber Bragg gratings in tapered few-mode and multimode fibers to accomplish single-mode operation by reducing the core diameter, while preserving the core-cladding structure. The gratings present a single reflection band, and the device shows low insertion losses after the taper fabrication and the fiber Bragg gratings inscription. The excitation of high-order odd modes in the core of the fiber has been identified as the main loss mechanism; it can be prevented by means of symmetric illumination of the fibers. We also demonstrate the excitation of high-order cladding modes (cladding-air modes) along the taper transitions; these modes can be removed without a significant increment of the insertion loss.

Measurement of UV-induced absorption and scattering losses in photosensitive fibers.

UV-assisted fabrication of gratings using photosensitive fibers is a well-established technique, based on the UV-induced permanent modification of the refractive index of the fiber material. As a result, the absorption coefficient is also changed. Here, we exploit the thermal sensitivity of whispering gallery mode resonances of the fiber itself to measure the profile of the heating along an irradiated fiber versus the input power and for different UV radiation intensities. Our technique allows discriminating between the absorption and scattering contributions to the overall losses, by comparing the results obtained with our technique with direct transmission measurements. Different photosensitive fibers are characterized by means of this technique. Nonuniform UV irradiation of photosensitive fibers can be characterized with this technique.

Fabrication of long period fiber gratings of subnanometric bandwidth.

This Letter reports on the fabrication of long period fiber gratings having subnanometric bandwidths in the 1500 nm spectral region. Large gratings have been photo inscribed in a high NA fiber; the grating pitch and the order of the HE cladding mode are optimized to produce gratings with a large number of periods and prevent the coupling to TE, TM, or EH modes. Resonances with a FWHM of 0.83 and 0.68 nm have been achieved for gratings 15 and 20 cm long, respectively; the free spectral range between the transmission notches is 125 nm. The polarization effects and the sensitivity of the gratings to temperature and to strain variations are presented as well.

Measurement of Pockels' coefficients and demonstration of the anisotropy of the elasto-optic effect in optical fibers under axial strain.

The elasto-optic effect in optical fibers under axial strain can be characterized by means of the whispering gallery mode (WGM) resonances of the fiber itself. This technique enables a direct measurement of the anisotropy, the determination of the individual Pockels' coefficients, and the study of the wavelength dependence. The method is based on a rigorous theoretical study of WGM resonances in cylindrical microresonators. The shift of the WGM resonances as a function of strain was measured for the TE and TM modes, showing a strong modal anisotropy. In particular, the shift rate for TE modes was 1.84 times the one for TM modes. From these measurements, experimental values for the Pockels' coefficients were obtained: p11=0.116 and p12=0.255 at 1531 nm, and p11=0.131 and p12=0.267 at 1064 nm. The dispersion of p44 with wavelength was shown to be 5% µm-1.

Measurement of temperature profile induced by the optical signal in fiber Bragg gratings using whispering-gallery modes.

The temperature sensitivity of whispering-gallery mode resonances of an optical fiber is exploited to measure thermal effects induced by an optical signal of moderate power along a fiber Bragg gating (FBG). The UV inscription technique used for the fabrication of FBG introduces a permanent change in the absorption coefficient of the fiber; thus, thermal effects are expected. The resonance wavelength shift of whispering-gallery modes provides information about the temperature change in the fiber, point to point. We present the experimental characterization of the thermal effects in FBG as a function of the wavelength and the power of the launched optical signal through the grating.

Waveguiding properties of a photonic crystal fiber with a solid core surrounded by four large air holes.

The polarization-dependent guiding properties of a hexagonal-lattice photonic crystal fiber with a solid-core surrounded by four large air holes are investigated. The appearance of a polarization dependent cutoff frequency, together with several parameters as the birefringence, the modal effective area, the group velocity dispersion and the polarization dependent loss are analyzed. A collection of fibers with different structural parameters were fabricated and characterized. An effective anti-guide structure from at least 450 nm to 1750 nm, a polarizing fiber with a polarization dependent loss of 16 dB/m at 1550 nm, and an endlessly singlemode polarization-maintaining fiber with group birefringence of 2.1x10(-3) at 1550 nm are reported. Experimental results are compared with accurate numerical modeling of the fibers.

Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating.

We report active Q-switching of an all-fiber laser using a Bragg grating based acousto-optic modulator. Q-switching is performed by modulating a fiber Bragg grating with an extensional acoustic wave. The acoustic wave modulates periodically the effective index profile of the FBG and changes its reflection features. This allows controlling the Q-factor of the cavity. Using 1 m of 300 ppm erbium-doped fiber and a maximum pump power of 180 mW, Q-switch pulses of 10 W of peak power and 82 ns wide were generated. The pulse repetition rate of the laser can be continuously varied from few Hz up to 62.5 kHz.

Mode locking of an all-fiber laser by acousto-optic superlattice modulation.

Active mode locking of an erbium-doped all-fiber laser with a Bragg-grating-based acousto-optic modulator is demonstrated. The fiber Bragg grating was acoustically modulated by a standing longitudinal elastic wave, which periodically modulates the sidebands at twice the acoustic frequency. The laser has a Fabry-Perot configuration in which cavity loss modulation is achieved by tuning the output fiber Bragg grating to one of the acoustically induced sidebands. Optical pulses at 9 MHz repetition rate, 120 mW peak power, and 780 ps temporal width were obtained. The output results to be stable and has a timing jitter below 40 ps. The measured linewidth, 2.8 pm, demonstrates that these pulses are transform limited.

Tunable and reconfigurable microwave filter by use of a Bragg-grating-based acousto-optic superlattice modulator.

We present an all-optical novel configuration for implementing multitap transversal filters by use of a broadband source sliced by fiber Bragg grating arrays generated by propagating an acoustic wave along a strong uniform fiber Bragg grating. The tunability and reconfigurability of the microwave filter are demonstrated.