Laura Abrardi

Detrimental effect of modulation instability on distributed optical fiber sensors using stimulated Brillouin scattering

Modulation instability can limit the resolution and the range of distributed fibre sensors based on stimulated Brillouin scattering. In this paper we analyse this process and suggest adequate methods to overcome it.

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.

The effects of gamma-radiation on the properties of Brillouin scattering in standard Ge-doped optical fibres

We have experimentally studied the effects of gamma-radiation up to very high total doses on the physical properties of Brillouin scattering in standard commercially available optical fibres. A frequency variation of about 5 MHz for both Brillouin frequency and linewidth has been measured at the total dose of about 10 MGy. The radiation-induced shift has a negligible practical impact and makes Brillouin scattering very immune to radiation, so that distributed sensors based on this interaction exhibit an interesting potential for use in nuclear facilities.

Spectrally-bounded continuous-wave supercontinuum generation in a fiber with two zero-dispersion wavelengths

A common issue in fiber-based supercontinuum (SC) generation under continuous-wave pumping is that the spectral width of the resulting source is related to the input power of the pump laser used. An increase of the input pump power leads to an increase of the spectral width obtained at the fiber output, and therefore, the average power spectral density (APSD) over the SC spectrum does not grow according to the input power. For some applications it would be desired to have a fixed spectral width in the SC and to increase the average PSD proportionally to the input pump power. In this paper we demonstrate experimentally that SC generation under continuous-wave (CW) pumping can be spectrally bounded by using a fiber with two zero-dispersion wavelengths (ZDWs). Beyond a certain pump power, the spectral width of the SC source remains fixed, and the APSD of the SC grows with the pump power. In our experiment we generate a reasonably flat, spectrally-bounded SC spanning from 1550 nm to 1700 nm. The spectra l width of the source is shown to be constant between 3 and 6 W of pump power. Over this range, the increase in input power is directly translated in an increase in the output APSD. The experimental results are confirmed by numerical simulations, which also highlight the sensitivity of this configuration to variations in the fiber dispersion curve. We believe that these results open the way for tailoring the spectral width of high-APSD CW SC by adjusting the fiber dispersion.

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.

\mu \hbox{m}

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.

Generated in a Stepwise Dispersion-Decreasing-Fiber Arrangement

Distributed fibre optic sensors are being evaluated by the nuclear industry for monitoring purpose. We evaluate the radiation tolerance of distributed Brillouin sensors up to very high total gamma doses.

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

A four-wavelength low-power continuous-wave frequency laser reference system has been realised in the 935.4-nm range for water vapour differential absorption lidar (DIAL) applications. The system is built around laboratory extended-cavity and DFB diode lasers. Three lasers are directly locked to three water vapour absorption lines of different strength, whereas the wavelength of the fourth laser lies out of any absorption line (offline). On-line stabilisation is performed by wavelength modulation spectroscopy technique, while precise offline stabilisation is realised by an offset locking at 18.8 GHz. Offset frequency larger than 320 GHz has also been demonstrated at 1.55 μm, based on an all-fibre optical frequency comb. First steps towards the use of a photonic crystal fibre as ultra compact reference cell with long optical pathlength were realised. The developed techniques for direct and offset-lock laser stabilisation can also be applied to other gases and wavelengths, provided the required optical components are available for the laser wavelength considered.

Study of the radiation effects on the properties of Brillouin scattering in standard Ge-doped optical fibres

Supercontinuum sources generated by continuous-wave excitation are very promising for many applications as they present in general higher spectral power density then their pulsed counterparts. On the other hand, the properties of supercontinuum are very difficult to be controlled as the initial broadening is driven by modulation instability. This latter one breaks-up the CW radiation into a train of ultra-short pulses whose peak power, spectral length and shape strongly depend on the power, coherence and noise of the pump and on the fiber properties. In this paper, we present a preliminary work on the role of chromatic dispersion on supercontinuum spectral broadening in order to study how to optimize SC spectral width under CW regimes. By means of a home-made tunable high-power laser we induce supercontinuum generation by pumping at different dispersion values of the fiber. We show that at low injected powers the wider spectrum is obtained when pumping just above the zero-dispersion wavelength of the fiber. By contrast, for higher injected powers, wide and squared-shaped spectra can be obtained when pumping over a larger range of anomalous dispersion values. These results seem to be very promising for a number of applications requiring smooth, squared and high-power SC spectral profiles such as optical coherence tomography.

Frequency-stabilised laser reference system for trace-gas sensing applications from space

We present a multiwavelength fiber source based on cascaded of four-wave mixing in two semiconductor optical amplifiers followed by further four-wave mixing in an optical fiber enhanced by Raman amplification. The multiwavelength source is generated by two initial frequencies detuned 200 GHz and referenced in the absorption lines of the acetylene 12C2H2, which sweep in frequency keeping the detuning of the lasers constant. With this configuration, we have achieved a high resolution source with a spectrum of 36 channels centered with adjustable peaks separation. The source can be employed to interrogate a fiber Bragg grating sensors network and in gas spectroscopy applications.