Ana Carrasco-Sanz

Distributed Brillouin Fiber Sensor Assisted by First-Order Raman Amplification

Distributed optical fiber Brillouin sensors provide innovative solutions for the monitoring of temperature and strain in large structures. The effective range of these sensors is typically of the order of 20-30 km, which limits their use in certain applications in which the distance to monitor is larger. In this work, we have developed a new technique to significantly extend the measurement distance of a distributed Brillouin Optical Time-Domain Analysis (BOTDA) sensor. Distributed Raman Amplification in the sensing fiber provides the means to enhance the operating range of the setup. Three Raman pumping configurations are theoretically and experimentally investigated: co-propagating, counter-propagating and bidirectional propagation with respect to the Brillouin pump pulse. We show that some of the amplification schemes tested can extend the measurement range and improve the measurement quality over long distances.

Broadband spectrally flat and high power density light source for fibre sensing purposes

We present a new kind of broadband continuous-wave source which outperforms any other broadband superluminescent or amplified spontaneous emission source both in terms of output spectral density and bandwidth. Our source covers the wavelength band of interest for fibre applications (from 1450 to 1625 nm) and has an output power of approximately 1.3 W. The source is obtained by pumping a conventional non-zero dispersion-shifted fibre with a continuous-wave Raman fibre laser tuned to the region of small anomalous dispersion of the fibre. The laser beam undergoes an extreme spectral broadening in the fibre. Our experimental results show clearly that the modulation instability (MI)-induced soliton fission is the key element leading to this spectral broadening. Modulation instability is seeded by fast intensity instabilities present in the laser output. We show that this source features good power stability and we believe that it might have very interesting applications in fibre sensing, for instance to avoid the need of amplification in the interrogation of remote Bragg gratings or to improve the resolution and dynamic range of optical coherence tomography setups.

Experimental investigation of the effect of pump incoherence on nonlinear pump spectral broadening and continuous-wave supercontinuum generation

The development of high-power cw fiber lasers has triggered a great interest in the phenomena of nonlinear pump spectral broadening and cw supercontinuum generation. These effects have very convenient applications in Raman amplification, optical fiber metrology, and fiber sensing. In particular, it was recently shown that pump incoherence has a strong impact in these processes. We study experimentally the effect of pump incoherence in nonlinear pump spectral broadening and cw supercontinuum generation in optical fibers. We show that under certain experimental conditions an optimum degree of pump incoherence yields the best performance in the broadening process. We qualitatively explain these results, and we point out that these results may have important implications in cw supercontinuum optimization.

Optimized All-Fiber Supercontinuum Source at 1.3

In this paper, the generation of a continuous-wave (CW)-pumped supercontinuum (SC) source at 1.3 mum is described. The device makes use of a tunable Yb-doped fiber laser, a cascade of fiber Bragg grating mirrors, and a concatenation of standard silica fibers with stepwise decreasing dispersion. It is shown that the dispersion-decreasing-fiber set enhances the width of the generated SC, since it favors the fission of the CW input into high-order solitons. The generated SC spans from 1280 to 1513 nm, shows an average output power of 1.34 W, and exhibits >0-dBm/nm spectral density of over 200 nm.

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This paper describes the design, characterization and calibration of a high power transfer standard for optical power measurements in optical fibres based on an integrating sphere radiometer. This radiometer, based on two detectors (Si and InGaAs), can measure powers between 100 nW and 10 W within the wavelength range of (400–1700) nm. The radiometer has been calibrated over the total spectral range of use against an electrically calibrated pyroelectric radiometer and different fibre laser diodes and ion lasers. The total uncertainty obtained is lower than ±1.5% for these wavelengths and power ranges (excluding the water absorption region).

Generated in a Stepwise Dispersion-Decreasing-Fiber Arrangement

The influence of chromatic dispersion on continuous-wave (CW)-pumped supercontinuum (SC) generation in kilometer-long standard fibers is experimentally investigated. We perform our study by means of a tunable, high-power fiber ring laser pumping a dispersion-shifted fiber in the wavelength range of small and medium anomalous dispersion. Our results show that, at low input powers, chromatic dispersion plays a dominant role on nonlinear pump spectral broadening, giving rise to a broader spectrum when pumping just above the zero-dispersion wavelength of the fiber. At higher input powers, however, the width of the generated SC spectrum is mostly due to the Raman effect, hence more independent of the value of the chromatic dispersion coefficient. We show that, in this case, the optimum pumping wavelengths for SC generation are not so close to the zero-dispersion wavelength of the fiber as in the previous case. In these conditions, as the chromatic dispersion grows, we can obtain square-shaped and high-power density spectra, which seem extremely promising for applications in optical coherence tomography.

An integrating sphere radiometer as a solution for high power calibrations in fibre optics

We describe the design, characterization, and calibration of a high-power and high-accuracy transfer standard for optical power measurements in fibers based on an integrating sphere radiometer working from -50 to +30 dBm. The integrating sphere radiometer has been calibrated in the spectral range 1250-1650 nm by use of an electrically calibrated pyroelectric radiometer and four tunable laser diodes. The total uncertainty obtained is less than +/-0.8% for these wavelength and power ranges.

Experimental Study on the Role of Chromatic Dispersion in Continuous-Wave Supercontinuum Generation

Optical frequency combs are a useful tool for measuring reference laser frequency whose uncertainty depends on the stability and accuracy of the reference clock. The relative uncertainty of the laser frequency measurements in the optical telecommunication band with the frequency comb technique is estimated around 10-12. In this paper, we present the development and implementation of a filtering technique on an optical frequency comb based on an Erbium optical fiber oscillator using Brillouin scattering amplification. This filtering technique allows us to isolate and transmit frequencies generated by a stabilized optical frequency comb. This method has been developed for the remote comparison of frequency combs. Finally, we present the characterization of the optical frequency comb and its application to the calibration of high wavelength resolution meters in the optical telecommunications window. Measurements with uncertainties under the resolution of the own instrument were achieved using stabilized lasers at molecular absorptions. The result is a significant improvement of the measurement capability given by the current equipment.

High-power and high-accuracy integrating sphere radiometer: design, characterization, and calibration

Supercontinuum (SC) generation in optical fibers and waveguides is a phenomenon of increasing interest that has found applications in fields like time-resolved spectroscopy, ultrashort pulse compression, multiwavelength optical sources for WDM and optical frequency metrology. Most of the experiments performed up to now have been accomplished using femtosecond or picosecond-pulsed laser sources and special fibers such as highly-nonlinear photonic crystal fibers. Supercontinuum generation using continuous-wave laser sources was demonstrated only recently, but the initial results demonstrate that high power density (>1 mW/nm), broadband supercontinuums (more than 250 nm) can be achieved with good long-term stability. In this paper we show different experimental setups to produce continuous-wave supercontinuums in optical fibers. We show how the supercontinuum varies depending upon the pump source used in the experiment. We believe that such an incoherent source can have very interesting applications in optical fiber and component characterization, fiber sensing and optical coherence tomography for biomedical applications. As a sample application, we show that this source can be used to measure polarization mode dispersion (PMD) in optical fibers very accurately and with an extremely large dynamic range (>200 km).

Application of Brillouin scattering to optical frequency combs

A frequency comb is the optical spectrum formed by a train of optical pulses and comprises a series of equally spaced spectral lines. Ytterbium and Erbium based fiber lasers can produce frequency combs in the region of 1.0 and 1.5 μm respectively. The frequency comb based on an Erbium doped fiber is especially interesting because it emission is centered in the spectral region of optical fiber communications. In this work we use a frequency comb based on an Erbium doped fiber optically filtered by stimulated Brillouin scattering in an optical fiber to generate optical reference frequency in the regions of optical fiber telecommunications and microwaves. The study of the stability of the isolated tooth are performed, resulting two orders of magnitude better than the stability of the pump laser used. This result is the calibration of telecommunication wavelength meters. Also, we analyze some applications based on the Brillouin filtering technique.