Strain and temperature measurement discrimination with forward Brillouin scattering in optical fibers.

A novel method that enables simultaneous and discriminative measurement of strain and temperature using one single optical fiber is presented. The method is based on the properties of transverse acoustic mode resonances (TAMRs) of the optical fiber. In particular, it is based on the different sensitivity to temperature and strain that exhibit the radial modes R0,m and a family of torsional-radial modes denoted as T R2,m(1). We show that the resonance frequencies of both types of resonances shift linearly with temperature and strain, but at different rates. By the combined use of the different sensitivities of the two families of TAMRs, we experimentally demonstrate discriminative measurements of strain and temperature. A detection limit of strain and temperature better than 25 µÎµ and 0.2 °C is achieved.

High accuracy measurement of Poisson's ratio of optical fibers and its temperature dependence using forward-stimulated Brillouin scattering.

Transverse acoustic mode resonances enable a high accuracy determination of Poisson's ratio and elastic properties of optical fibers. An all-optical pump and probe technique is used for efficient excitation and accurate characterization of both, radial and torsional-radial acoustic resonances of optical fibers. Simple and precise algebraic expressions for the frequencies of high order acoustic resonances are derived, enabling a rigorous analysis of the experimental data using standard least squares fitting. Following this approach, the determination of Poisson's ratio does not require the measurement of any physical length, but only frequency measurements are required. An accuracy better than 1 ‰ is achieved. The dependence of the fiber Poisson's ratio with temperature is also determined experimentally.

Measurement of phase and group refractive indices and dispersion of thermo-optic and strain-optic coeffients of optical fibers using weak fiber Bragg gratings.

In this work we report on the measurement, with record accuracy, of the absolute modal effective refraction index (phase index) of single-mode optical fibers by using Bragg gratings. We also demonstrate a new method to measure the group index of the fibers from the grating's Bragg wavelength. We present as well the characterization of the thermo-optic and strain-optic coefficients as a function of the wavelength; the values we have obtained are the closest to those of pristine fiber measured with gratings technology so far. The phase index is measured with a set of gratings in the wavelength ranges from 1509 to 1563 nm, and the group index is obtained from the wavelength dependence of the phase index. Very weak gratings with reflectivity down to 10-3 have been used in order to minimize the perturbation of the pristine fiber. Results are presented at a temperature of 22°C and zero strain after preliminary calibration of the thermo-optic and strain-optic coefficients as a function of the wavelength. Accuracies better than 3×10-5 and 7×10-4 have been achieved for the phase and group indices, respectively. It is also shown that the main source of error relates to the uncertainty in pitch of the phase masks used for grating inscription. The technique is useful for testing different kinds of fibers including telecommunication, amplifying, and polarization maintaining.

Efficient interrogation method of forward Brillouin scattering in optical fibers using a narrow bandwidth long-period grating.

A new technique, to the best of our knowledge, for the characterization of the effective refractive index modulation in optical fibers due to transverse acoustic mode resonances excited by electrostriction is reported. The resonances excited by an optical pulse are probed by a narrow bandwidth long-period grating (LPG) inscribed in the fiber, which is interrogated by a continuous wave (CW) beam. The LPG used in this experiment has a narrow bandwidth and high sensitivity to small mode index perturbations, allowing the measurement of index variations from below 10-9 to 10-6. Radial and torsional-radial acoustic modes were characterized up to 1.1 GHz. The linewidth of resonances was found to be much shorter than in previous reports in which long fiber lengths are typically required, obtaining Q factors as high as 5000.

Broadband tuning of polarization modulation instability in microstructured optical fibers.

The wideband tuning of strong bands generated through polarization modulation instability (PMI) in microstructured optical fibers (MOFs) is reported. Tunability is achieved by exploiting the dependence of the phase-matching condition on the fiber's chromatic dispersion and birefringence, which is particularly sensitive when the fiber is pumped near the zero-dispersion wavelength. MOFs designed to accomplish PMI phase-matching when they are infiltrated with ethanol and pumped at 1064 nm were designed and fabricated. Taking advantage of the large thermo-optic coefficient of ethanol, both chromatic dispersion and birefringence were varied through temperature. Wavelength shifts from 937 nm to 863 nm (anti-Stokes) and from 1231 nm to 1387 nm (Stokes) are demonstrated. Such a tuning range corresponds to a frequency shift between the pump and a PMI sideband ranging from 1274cm-1 to 2189cm-1.

Experimental study of an in-fiber acousto-optic tunable bandpass filter for single- and dual-wavelength operation in a thulium-doped fiber laser.

A tunable single- and dual-wavelength thulium-doped all-fiber laser is demonstrated based on the implementation of an in-fiber acousto-optic tunable bandpass filter (AOTBF). The AOTBF is fabricated to be operated in the 1.9 µm region, and takes advantage of the intermodal coupling effect produced by traveling flexural acoustic waves in an optical fiber. It exhibits a 3-dB bandwidth of 2.04 nm with an insertion loss of 4.75 dB. The tuning properties of the AO device allows a continuous-wave operation with characteristics of wide tuning range (211.5 nm), narrow linewidth (50 pm) and high signal-to-noise ratio (60 dB). In the dual-wavelength regime, the laser is capable of independent tuning of each of the laser lines, achieving a tunable dual-wavelength emission that extends from 1802.67 to 1932.75 nm. A controllable wavelength spacing with minimum and maximum separations of 1.04 and 130.08 nm is obtained.

Broadband tuning of a long-cavity all-fiber mode-locked thulium-doped fiber laser using an acousto-optic bandpass filter.

A long-cavity passively mode-locked thulium-doped all-fiber laser is reported incorporating a tapered acousto-optic tunable bandpass filter (AOTBF). The operation of the AOTBF relies on the intermodal coupling between core and cladding modes when a flexural acoustic wave propagates along an 80 µm tapered fiber. The filter works in transmission and exhibits a 3 dB bandwidth of 9.02 nm with an insertion loss of 3.4 dB. The laser supports ultrashort pulse generation at a low repetition rate of 784.93 kHz. Optical pulses with 2.43 nm of optical bandwidth and 2.1 ps pulse duration were obtained in a broad tuning range from 1824.77 to 1905.16 nm.

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.

Unrestricted generation of pure two-qubit states and entanglement diagnosis by single-qubit tomography.

We present an experimental proof-of-principle for the generation and detection of pure two-qubit states that have been encoded in degrees of freedom that are common to both classical-light beams and single photons. Our protocol requires performing polarization tomography on a single qubit from a qubit pair. The degree of entanglement in the qubit pair is measured by concurrence, which can be directly extracted from intensity measurements-or photon counting-entering single-qubit polarization tomography.

Long cavity ring fiber mode-locked laser with decreased net value of nonlinear polarization rotation.

We investigate a new configuration of a mode-locked fiber laser by using a nonlinear polarization rotation-based design to generate soliton pulses with low repetition rate. Unlike with previously reported configurations, we introduce a Faraday mirror after the first half of the cavity length to counteract the nonlinear polarization rotation effects. The total cavity length is 437 m including a 400-m long twisted SMF-28 fiber. The fiber was twisted to cancel the linear birefringence and to ensure that the polarization ellipticity is not altered as the pulse travels along the fiber. The strict control of polarization yields a stable relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. Depending on the polarization state we observed three different emission regimes, the single soliton regime (SR), conventional noise-like pulses (NLP) and noise-like square-waveform pulse (NLSWP). In the SR, a 467.2 kHz train of solitons was obtained with pulse duration of 2.9 ps at 1558.7 nm.

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.

Acousto-optic interaction in polyimide coated optical fibers with flexural waves.

Acousto-optic coupling in polyimide-coated single-mode optical fibers using flexural elastic waves is demonstrated. The effect of the polyimide coating on the acousto-optic interaction process is analyzed in detailed. Theoretical and experimental results are in good agreement. Although the elastic attenuation is significant, we show that acousto-optic coupling can be produced with a reasonably good efficiency. To our knowledge, it is the first experimental demonstration of acousto-optic coupling in optical fibers with robust protective coating.

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.

Instantaneous frequency measurement of dissipative soliton resonant light pulses.

We measured the instantaneous frequency profile of two different dissipative soliton resonant (DSR) light pulses, the usual flat-top and less-common trapezoid-shaped light pulses. The DSR light pulses were provided by an ytterbium-doped polarization-maintaining fiber ring passively mode-locked laser using the adequately selected amount of net-normal dispersion. We confirmed that the DSR light pulses have a (moderately) low linear chirp across the pulse, except at the edges, where the chirp changes exponentially. This unique instantaneous frequency behavior can be succinctly resumed by the following parameters: linear chirp slope and leading and trailing chirp lifetimes. As the pump power increases, the linear chirp slope decreases, whereas the leading and trailing chirp lifetimes do not show an appreciable change. The results are compared with previous theoretical works.

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.

Dissipative soliton resonance in a full polarization-maintaining fiber ring laser at different values of dispersion.

We investigated the dissipative solitons resonance in an ytterbium-doped fiber ring laser in which all the elements are polarization maintaining (PM). A semiconductor saturable absorber mirror was used as a mode-locker. The cavity included a normal dispersion single-mode fiber (SMF) and an anomalous dispersion photonic crystal fiber. The change of the length of the PM SMF allows the variation of the net-normal dispersion of the cavity in the range from 0.022 ps2 to 0.262 ps2. As the absolute value of the net-normal dispersion increases from 0.022 ps2 to 0.21 ps2, a square-shaped single pulse transformed to a single right-angle trapezoid-shaped pulse, and, at the dispersion of 0.262 ps2, to multiple right-angle trapezoid-shaped pulses, per round-trip.

Accurate mode characterization of two-mode optical fibers by in-fiber acousto-optics.

Acousto-optic interaction in optical fibers is exploited for the accurate and broadband characterization of two-mode optical fibers. Coupling between LP01 and LP1m modes is produced in a broadband wavelength range. Difference in effective indices, group indices, and chromatic dispersions between the guided modes, are obtained from experimental measurements. Additionally, we show that the technique is suitable to investigate the fine modes structure of LP modes, and some other intriguing features related with modes' cut-off.

Mente Activa® Improves Impaired Spatial Memory in Aging Rats.

OBJECTIVES: Aging is accompanied by a decline in several aspects of the cognitive function, having negative personal and socioeconomic impacts. Dietary supplements could be beneficial for preventing age-related cognitive decline. In this context, we examined whether the nutritional supplement Mente Activa® has beneficial effects on aging-related cognitive deficits without inducing side effects. METHODS: Mente Activa® was administered to old rats (n= 30 treated rats and n= 30 control rats) during 5 months, and the Morris water maze was used to test the learning capacities of the animals. The first assessment was conducted before the nutritional intervention (age of 18-19 months), to determine the baseline of the performance of animals on this test, and the second assessment was performed at the end of the treatment (23-24 moths). In order to examine possible secondary effects of this nutritional supplement, plasma, heart anatomy and liver parameters were evaluated. RESULTS: Our data indicate that supplemented rats showed less escape latency, distance swum, higher use of spatial search strategies, and crossed the former platform location with higher frequency than control rats. These effects were specific of the treatment, indicating that this nutritional supplement has a beneficial effect on spatial memory. On the other hand, the regular intake of Mente Activa® did not induce any negative effects in plasma parameters and heart size. CONCLUSIONS: Aged rats under a sustained dietary intake of the nutritional supplement Mente Activa® displayed improved learning and memory abilities compared to the non-treated rats. These results suggest the therapeutic potential and safety of use of Mente Activa® for age-related cognitive deficits, particularly, in the onset of the first cognitive dysfunction symptoms.

Improved time-resolved acousto-optic technique for optical fiber analysis of axial non-uniformities by using edge interrogation.

The time-resolved acousto-optic technique demonstrated recently to be a very useful method for the analysis of fiber axial non-uniformities, able to detect variations of fiber diameter in the nanometric scale with a spatial resolution of few cm. An edge interrogation approach is proposed to improve further the performance of this technique. The detection of subnanometer fiber diameter changes or sub-ppm changes of the core refractive index is demonstrated.

Experimental analysis of distributed pump absorption and refractive index changes in Yb-doped fibers using acousto-optic interaction.

In-fiber acousto-optic interaction is used to characterize the refractive index changes at the C band in a single-mode ytterbium-doped optical fiber under 980 nm pumping. The transmission notch created by the acoustic-induced coupling between the core mode and a cladding mode shifts to longer wavelengths when the pump is delivered to the fiber. The electronic contribution to the refractive index change is quantified from the wavelength shift. Using a time-resolved acousto-optic method, we investigate the distribution of pump absorption, and the resulting refractive index change profile, along sections of ytterbium-doped fiber exceeding 1 m long under different pump power levels.