Search

International audience; Indium nitride (InN)-based semiconductor saturable absorbers have previously shown advantages for application in near-IR fiber lasers due to their broad modulation depth, ultrafast nonlinear response and thermal stability. However, up to now all demonstrated saturable absorber elements based on InN (either transmissive or reflective) have shown limited performance due to poor coupling and insertion losses. We present here a simple mode-locking device based on a GRIN-rod lens in conjunction with an InN semiconductor saturable absorber mirror (SESAM) for its use in a passively mode-locked all-fiber laser system operating at telecom wavelengths. Our results demonstrate that this coupling element ensures not only a compact, turnkey and alignment-free design but also a highly-stable optical femtosecond pulse train. The reduction of insertion losses (3.5 dB) enables the generation of 90-fs ultrafast pulses with an average power of 40 mW and up to 7 nJ of pulse energy without the need for additional amplification.

We demonstrate a spectral compression technique (equivalent to a temporal expansion) that allows a simple time-domain distributed interrogation of fibers with high resolution (~2 cm in this work) and using only low-bandwidth photodetection (400 KHz).

9 pags., 7 figs., Dual frequency comb (DFC) spectroscopy using electro-optic comb generators stands out for its flexibility, easy implementation, and low cost. Typically, two combs with different line spacing are generated from a common laser using independent electro-optic comb generators. This approach minimizes the impact of laser phase noise; however, the distinct paths followed by the two combs ultimately limit the attainable signal-to-noise ratio and long-term stability of the system. In this work, a common-path DFC is generated using a single modulator driven by an arbitrary waveform generator, thus enabling a remarkable increase of the system stability (up to 0.8 s of integration time) while maintaining high flexibility. The proposed technique is experimentally validated by implementing a dual frequency comb with 3000 lines, covering an optical bandwidth of 4.5 GHz, and demonstrating an optical-to-radiofrequency compression factor of 7500. Our system is able to measure extremely narrowband optical features (in the MHz range) with an accuracy only limited by the master laser stability., MAS acknowledges AC3E Basal Project FB0008 and thanks “Becas Iberoamérica Santander Universidades Convocatoria 2018” for supporting his research stay at Universidad de Alcalá, Spain

Conventional chirped-pulse (CP) phase-sensitive optical time-domain reflectometry (CP φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high accuracies of millikelvin or nanostrain. With respect to standard coherent-detection φ-OTDR, it shows increased robustness to coherent fading and allows a linear and quantitative monitoring of the perturbations acting on the fiber. Its spatial resolution, however, remains a critical parameter and new techniques allowing its improvement without reducing significantly other performances (or increasing the setup complexity) are constantly being researched. In this paper, we propose a method to improve the spatial resolution of CP φ-OTDR without reducing the input pulse width, by means of sub-bands processing. The method is based on adding an optical carrier to the input pulse. Using digital filtering, the spectrum of the fiber backscatter can be split into multiple sub-bands. Each of these sub-bands corresponds to the fiber response generated by a short optical pulse, chirped over a smaller frequency range. This way each sub-band results in φ-OTDR measurements with high spatial resolution, but with a reduced SNR. A dedicated post-processing methodology is proposed to mitigate the SNR reduction obtained from each sub-band, while securing high-resolution measurements of the perturbations acting on the fiber. Experimental results demonstrate the possibility of achieving CP φ-OTDR measurements with a 15-fold spatial resolution improvement over the conventional CP analysis, at the expense of an SNR reduction lower than a factor 2., FINESSE MSCA-ITN-ETN-722509; DOMINO Water JPI; Spanish MINECO and Italian Ministry of University and Research; Comunidad de Madrid and FEDER; MINECO; Fondazione Ing; 10.13039/501100006302-Universidad de Alcalá

When a disturbance acts on a fiber it induces a change in the local refractive index that influences the fiber backscattering trace. If a chirped pulse φ-OTDR setup is used to interrogate the fiber, this refractive index change appears as a local shift of the received trace, linear to the acting perturbation. However, the refractive index change influences the round trip time of all the backscattering components generated by further fiber sections as well. Due to the high sensitivity of chirped pulse φ-OTDR, the change in the round trip time of the backscattering components, which is usually negligible, may appear as a virtual perturbation in certain conditions. In this letter we derive a mathematical model for the virtual perturbation induced by a disturbance acting on the fiber, when the measurement is performed by a chirped pulse φ-OTDR. We experimentally validate the model by inducing a temperatura change on a known span of fiber while monitoring its effects in a further fiber section kept at rest. The experimental results are then analyzed and compared with the theoretical ones., European Commission, Ministerio de Economía y Competitividad, Agencia Estatal de Investigación, Comunidad de Madrid

Time-expanded phase-sensitive optical time-domain reflectometry ( T E − Φ O T D R ) is a dual-comb-based distributed optical fiber sensing technique capable of providing centimeter scale resolution while maintaining a remarkably low (MHz) detection bandwidth. Random spectral phase coding of the dual combs involved in the fiber interrogation process has been proposed as a means of increasing the signal-to-noise ratio (SNR) of the sensor. In this Letter, we present a specific spectral phase coding methodology capable of further enlarging the SNR of T E − Φ O T D R . This approach is based on the use of a quadratic spectral phase to precisely control the peak power of the comb signals. As a result, an SNR improvement of up to 8 dB has been experimentally attained with respect to that based on the random phase coding previously reported.

EWOFS 2010, Limassol, Cyprus, 1 -4 October, 2019. -- https://www.cyprusconferences.org/ewofs2019/index.html. -- Confeencia invitada

Monitoring the presence of external electric fields over large distances and detecting losses along power transmission networks, is of extreme importance nowadays due to concerns with environment, ...

7th European Workshop on Optical Fibre Sensors (EWOFS'2019), 01/10/2019-04/10/2019, Limassol, Chipre., Any temperature change or strain acting on a section of the fiber induces a local variation of the refractive index. If the fiber is monitored by a chirped pulse φ-OTDR system, the variation of the refractive index causes a local shift of the backscattering trace and produces a change in the round trip time of the light coming from any further position of the fiber. While usually negligible, due to the high sensitivity of the chirped pulse φ-OTDR, in extreme occasions the distributed round trip time change may appear in the measurement as an undesired “virtual” perturbation. In this paper, we discuss and experimentally validate a mathematical model to account for (and eventually correct) the “virtual” perturbation., European Commission, Ministerio de Economía y Competitividad, Agencia Estatal de Investigación, Comunidad de Madrid

For some infrastructures like oil and gas extraction boreholes or radioactive waste repositories, where distributed optical fiber sensors are employed to grant the safety of the facilities, the presence of gas species such as hydrogen or deuterium is one more relevant parameter to monitor. The possibility of employing the same kind of sensors for this purpose is of special interest, reducing the cost by employing a single interrogator, able to measure more than one parameter by simply employing an adequate sensing fiber. To meet this goal, we present here a sensor based on Chirped Pulse Phase sensitive Optical Time Domain Reflectometry, which is able to detect these species while they diffuse into the silica fiber core. Its ability to measure the induced change of its refractive index with a sensitivity around 10 8 has allowed determining hydrogen concentration in the silica core with precision in the order of 10 3 mol/m3 and spatial resolution ~6 m, while also providing an indirect measurement of the solubility of deuterium in a standard telecom-grade optical fiber., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid, Agencia Estatal de Investigación

7th European Workshop on Optical Fibre Sensors (EWOFS'2019), , 01/10/2019-04/10/2019, Limassol, Chipre., We propose a fully distributed optical fiber sensor capable of performing spectrally-resolved detection of visible light radiation. The sensor is based on monitoring the temperature change between two optical fibers with different coating colors. In our implementation, the temperature is simultaneously monitored in a black-coated fiber (which is highly sensitive to all input wavelengths) and a color-coated fiber, which basically acts as an optical stop-band filter for a certain input color. By comparing the temperature behavior attained for each fiber, it is possible to obtain information of the wavelength/color of a given optical radiation present in the environment. Suitable calibration could lead to distributed colorimetry measurements., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid, Agencia Estatal de Investigación

This work demonstrates the capabilities of Chirped pulse, phase-sensitive optical time domain reflectometry (CP-PhiOTDR) to measure large dynamic strains. Benefitting from the use of incoherent detection and single shot measurements, this technique can sample the fiber under test at rates limited only by the fiber length. Yet, for large strains it relies on incremental measurements, where each trace is cross-correlated with its predecessor to determine the relative change of strain. The individual increments are then integrated to provide the strain at the spatial point of measurement. Here, a 4 m section (out of a 210 m spool) was subjected to longitudinal vibration by a mechanical shaker. With a pulse repetition rate of 200 kHz, we present, for the first time to our knowledge, Rayleigh backscattering based distributed measurements of large and fast dynamic strains: from 150 µepsilon at a vibration frequency of 400 Hz to 1190 µepsilon (peak to peak) at 50 Hz over long single-mode fibers. Along with a detailed description of the implemented data processing, we also discuss some of the associated limitations., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

Optical fibers are inherently designed to allow no interaction between the guided light and the surrounding optical radiation. Thus, very few optical fiber-based technologies exist in the field of optical radiation sensing. Accomplishing fully-distributed optical radiation sensing appears then as even more challenging since, on top of the lack of sensitivity explained above, we should add the need of addressing thousands of measurement points in a single, continuous optical cable. Nevertheless, it is clear that there exist a number of applications which could benefit from such distributed sensing scheme, particularly if the sensitivity was sufficiently high to be able to measure correctly variations in optical radiation levels compatible with the earth surface. Distributed optical radiation sensing over large distances could be employed in applications such as Dynamic Line Rating (DLR), where it is known that solar radiation can be an important limiting factor in energy transmission through overhead power cables, and also in other applications such as thermo-solar energy. In this work, we present the proof-of-concept of the first distributed bolometer based on optical fiber technology and capable of detecting absolute changes of irradiance. The core idea of the system is the use of a special fiber coating with high emissivity (e.g. carbon coating or black paint). The high absorption of these coatings translates into a temperature change that can be read with sufficiently high sensitivity using phase-sensitive reflectometry. To demonstrate the concept, we interrogate distinct black-coated optical fibers using a chirped-pulse PhiOTDR, and we readily demonstrate the detection of light with resolutions in the order of 1% of the reference solar irradiance, offering a high potential for integration in the aforementioned applications., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

A method to extend the operation of traditional single-frequency phase-sensitive optical time-domain reflectometry ( $\Phi $ OTDR) to the monitoring of distributed temperature gradients along an optical fiber is proposed and experimentally validated. The measurement principle is derived from the perturbation response of a single-wavelength $\Phi $ OTDR signal, which is analyzed as a unidimensional speckle pattern. The method could be implemented in parallel to standard $\Phi $ OTDR systems used for distributed vibration sensing with a close to zero cost and without affecting its operation, as it only requires a low computational cost post-processing of the traces which are already acquired. Frequency scanning of the laser, heterodyning or additional hardware are not required. The distributed detection of a temperature gradient of 2.5 °C over 10 min is demonstrated.

We present an overview of the recently developed III-nitride-based optical waveguides for application in ultrafast signal processing at telecom wavelengths. We focus on different active and passive optical devices for further implementation within all-optical integrated circuits. Optical waveguides based on GaN/AlN quantum dots have been demonstrated to act as saturable absorbers requiring ∼3 pJ of input pulse energy to reach +3 dB transmittance contrast for TM-polarized light. On the contrary, sputtered-InN-based devices show −3 dB transmittance contrast associated to two-photon absorption for input pulse energies of ∼1 pJ, making them suitable to act as highly-efficient reverse saturable absorbers. Finally, the passive optical nature of waveguides based on sputtered AlN at 1.55 μm makes them suitable for further connections between different III-nitride-based active devices.

4 pags., 4 figs., 1 tab., Phase birefringence in optical fibers typically fluctuates over their length due to geometrical imperfections induced from the drawing process or during installation. Currently commercially available fibers exhibit remarkably low birefringence, prompting a high standard for characterization methods. In this work, we detail a method that uses chirped-pulse phase-sensitive optical time-domain reflectometry to directly measure position-resolved linear birefringence of single-mode optical fibers. The technique is suitable for fiber characterization over lengths of tens of kilometers, relying on a fast measurement (∼1 s) with single-ended access to the fiber. The proposed method is experimentally validated with three different commercial single-mode optical fibers., Ministerio de Economía y Competitividad (TEC 2015-71127-C2-2-R); Ministerio de Ciencia, Innovación y Universidades (FJCI-2016-27881, IJCI-2017-33856, RTI 2018-097957-B-C31); Comunidad de Madrid (SINFO TON2-CM:P2018/NMT-4326); European Commission (PCIN-2015-219 DOMINO,MSCA-ITN-ETN-722509).

20th International Conference on Transparent Optical Networks (ICTON 2018), 01/07/2018-05/07/2018, Bucarest, Rumania., We show a technology capable of performing position-resolved measurement of nanostrain perturbations along a conventional optical fiber cable, with meter-scale resolution. This technology can be deployed in already existing networks to achieve a dense and ultrasensitive seismic sensor array. Applications ranging from seismic protection to chemical sensing are explored., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

We demonstrate a technique to reduce the sidelobes in optical pulse compression reflectometry for distributed acoustic sensing. The technique is based on using a Gaussian probe pulse with linear frequency modulation. This is shown to improve the sidelobe suppression by 13 dB compared to the use of square pulses without any significant penalty in terms of spatial resolution. In addition, a 2.25 dB enhancement in signal-to-noise ratio is calculated compared to the use of receiver-side windowing. The method is tested by measuring 700 Hz vibrations with a 140 nϵ amplitude at the end of a 50 km fiber sensing link with 34 cm spatial resolution, giving a record 147,058 spatially resolved points., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

The Optical Networking and Communication Conference & Exhibition,, 11/03/2018-15/03/2018, San Diego, Estados Unidos., We demonstrate a technique allowing to perform distributed wind speed measurements over >17 km with, European Commission, MInisterio de Economía y Competitividad, Comunidad de Madrid

Chemical sensing using optical fibers is often challenging, as it is generally difficult to achieve strong interaction between the guided light and the analyte at the wavelength of interest for performing the detection. Despite this difficulty, many schemes exist (and can be found in the literature) for point chemical fiber sensors. However, the challenge increases even further when it comes to performing fully distributed chemical sensing. In this case, the optical signal which interacts with the analyte is typically also the signal that has to travel to and from the interrogator: for a good sensitivity, the light should interact strongly with the analyte, leading inevitably to an increased loss and a reduced range. Few works in the literature actually provide demonstrations of truly distributed chemical sensing and, although there have been several attempts to realize these sensors (e.g. based on special fiber coatings), the vast majority of these attempts has failed to reach widespread use due to several reasons, among them: lack of sensitivity or selectivity, lack of range or resolution, cross sensitivity to temperature or strain, or need to work at specific wavelengths where fiber instrumentation becomes extremely expensive or unavailable. In this work we provide a preliminary demonstration of the possibility of achieving distributed detection of gas presence with spectroscopic selectivity, high spatial resolution, potential for long range measurements and feasibility of having most of the interrogator system working at conventional telecom wavelengths. For a full exploitation of this concept, new fibers (or more likely, fiber bundles) should be developed capable of guiding specific wavelengths in the IR (corresponding to gas absorption wavelengths) with good overlap with the analyte while also having a solid core with good transmission behavior at 1.55 μm, and good thermal coupling between the two guiding structures., European Commission, Ministerio de Economía y Competitividad

We demonstrate a technique allowing to develop a fully distributed optical fiber hot-wire anemometer capable of reaching a wind speed uncertainty of ~ ± 0.15 m/s (± 0.54 km/h) at only 60 mW/m of dissipated power in the sensing fiber, and within only four minutes of measurement time. This corresponds to similar uncertainty values than previous papers on distributed optical fiber anemometry but requires two orders of magnitude smaller dissipated power and covers at least one order of magnitude longer distance. This breakthrough is possible thanks to the extreme temperature sensitivity and single-shot performance of chirped-pulse phase-sensitive optical time domain reflectometry (PhiOTDR), together with the availability of metal-coated fibers. To achieve these results, a modulated current is fed through the metal coating of the fiber, causing a modulated temperature variation of the fiber core due to Joule effect. The amplitude of this temperature modulation is strongly dependent on the wind speed at which the fiber is subject. Continuous monitoring of the temperature modulation along the fiber allows to determine the wind speed with unprecedented low power injection requirements. Moreover, this procedure makes the system immune to temperature drifts of the fiber, potentially allowing for a simple field deployment. Being a much less power-hungry scheme, this method also comfortably allows for monitoring over much longer distances, in the orders of 10s of km. We expect that this system can have application in dynamic line rating and lateral wind monitoring in railway catenary wires., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid, Universidad de Alcalá

26th International Coference on Optical Fiber Sensors OFS-26, 24/09/2018-28/09/2018, Lausanne, Suiza., Chirped-pulse ΦOTDR provides distributed strain measurement via a time-delay estimation process. We propose a lower bound for performance, after reducing sampling error and compensating phase-noise. We attempt to reach the limit, attaining unprecedented pε/√Hz sensitivities., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

26ª edición del congreso internacional Optical Fiber Sensors (OFS26), 24/09/2018-28/09/2018, Lausanne, Suiza., It is experimentally demonstrated that a chirped pulse phase-sensitive OTDR can measure large and fast dynamic strains (~100’s of µε, ~100’s of Hz) with SNR of ≥ 24dB. Signal smoothing and impact of error accumulation are also discussed., European Commission

26th International Conference on Optical Fiber Sensors OFS-26, 24/09/2018-28/09/2018, Lausanne, Suiza., The statistical properties of SNR are analyzed for chirped-pulse (CP-)ΦOTDR and compared with traditional ΦOTDR using coherent-detection. CP- ΦOTDR is showed to have drastically narrower (lower variance) SNR distribution, thus providing higher reliability measurements., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

We use the Dynamic Density-Functional Formalism and the Fundamental Measure Theory as applied to a fluid of parallel hard squares to study the dynamics of heterogeneous growth of non-uniform phases with columnar and crystalline symmetries. The hard squares are (i) confined between soft repulsive walls with square symmetry, or (ii) exposed to external potentials that mimic the presence of obstacles with circular, square, rectangular or triangular symmetries. For the first case the final equilibrium profile of a well commensurated cavity consists of a crystal phase with highly localized particles in concentric square layers at the nodes of a slightly deformed square lattice. We characterize the growth dynamics of the crystal phase by quantifying the interlayer and intralayer fluxes and the non-monotonicity of the former, the saturation time, and other dynamical quantities. The interlayer fluxes are much more monotonic in time, and dominant for poorly commensurated cavities, while the opposite is true for well commensurated cells: although smaller, the time evolution of interlayer fluxes are much more complex, presenting strongly damped oscillations which dramatically increase the saturation time. We also study how the geometry of the obstacle affects the symmetry of the final equilibrium non-uniform phase (columnar vs. crystal). For obstacles with fourfold symmetry, (circular and square) the crystal is more stable, while the columnar phase is stabilized for obstacles without this symmetry (rectangular or triangular). We find that, in general, density waves of columnar symmetry grow from the obstacle. However, additional particle localization along the wavefronts gives rise to a crystalline structure which is conserved for circular and square obstacles, but destroyed for the other two obstacles where columnar symmetry is restored., Comment: 15 pages, 16 figures

We report on the design, fabrication, and optical characterization of InN-based optical waveguides aiming at their application as all-optical limiters at 1.55 $\mu \text{m}$ . The InN guiding layers are grown by radio frequency (RF) sputtering on sapphire substrates. Experimental cutback method and nonlinear optical transmittance measurements were performed for the developed devices. The waveguides present nonlinear behavior associated with two photon absorption process. A nonlinear absorption coefficient ranging from $\sim 43$ to 114 cm/GW is estimated from optical measurements. These results open the possibility of using RF sputtering as a low cost and thermally harmless technique for the development and overgrowth of InN-based optical waveguides in future III-nitride all-optical integrated circuits working at telecom wavelengths.

25th International conference on Optical Fiber Sensors, , 24/04/2017-28/04/2017, Jeju, Corea del Sur., We review our recent work on chirped-pulse phase-sensitive optical time-domain reflectometry along optical fibers and its application in the measurement of true nanostrain variations in the kHz frequency range over km-long fibers. We also show how this technique can be used to perform distributed photothermal measurements of gas presence in suitable microstructured fibers., European Commission, Ministerio de Economía y Competitividad

Recently, it has been demonstrated that by recovering the amplitude and phase of the backscattered optical signal, a ΦOTDR using pulse coding can be treated as a fully linear system in terms of trace coding/decoding, thus allowing for the use of tens of thousands of bits with a dramatic improvement of the system performance. In this communication, as a continuation of previous work by the same authors, a preliminary study aiming at characterizing the limits of the system in terms of maximum usable code length is presented. Using a code exceeding 1million bits over a duration of 0.26ms, it is observed that fiber optical path variations exceeding π occurring over a time inferior to the pulse code length can lead to localized fading in the ΦOTDR trace. The occurrence, positions and form of the fading points along the ΦOTDR trace is observed to be strongly dependent on the type, frequency and amplitude of the perturbations applied to the fiber., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

25th International Conference on Optical Fiber Sensors, 24/04/2017-28/04/2017, Jeju, Corea del Sur., European Commission, Ministerio de Economía y Competitividad

A method for distributed birefringence measurements is proposed based on the interference pattern generated by the interrogation of a dynamic Brillouin grating (DBG) using two short consecutive optical pulses. Compared to existing DBG interrogation techniques, the method here offers an improved sensitivity to birefringence changes thanks to the interferometric effect generated by the reflections of the two pulses. Experimental results demonstrate the possibility to obtain the longitudinal birefringence profile of a 20 m-long Panda fibre with an accuracy of ∼10-8 using 16 averages and 30 cm spatial resolution. The method enables inf-metric and highly-accurate distributed temperature and strain sensing.

Phase-sensitive optical time-domain reflectometry (φOTDR) is widely used for the distributed detection of mechanical or environmental variations with resolutions of typically a few meters. The spatial resolution of these distributed sensors is related to the temporal width of the input probe pulses. However, the input pulse width cannot be arbitrarily reduced (to improve the resolution), since a minimum pulse energy is required to achieve a good level of signal-to-noise ratio (SNR), and the pulse peak power is limited by the advent of nonlinear effects. In this Letter, inspired by chirped pulse amplification concepts, we present a novel technique that allows us to increase the SNR by several orders of magnitude in φOTDR-based sensors while reaching spatial resolutions in the centimeter range. In particular, we report an SNR increase of 20 dB over the traditional architecture, which is able to detect strain events with a spatial resolution of 1.8 cm., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

We demonstrate a simple scheme that allows performing distributed Brillouin phase spectrum (BPS) measurements with very high spatial resolution (~7 cm) over long (~4.7 km) optical fibers. This is achieved by inserting a Sagnac interferometer (SI) in a Brillouin optical correlation domain analysis (BOCDA) configuration. Over its already-presented time-domain equivalent (SI-BOTDA), this approach reduces the main source of noise (coherent backscatter noise) thanks to the low-coherence nature of the used signals. On the other hand, over the most usual schemes used for distributed BPS measurements, this implementation presents the key advantage of not requiring high-bandwidth detection or complex modulation while reaching unprecedented values of spatial resolution and number of resolved points for this type of measurement. Thanks to the linear dependence of the BPS feature around the Brillouin frequency shift, this scheme could also have the advantage of requiring shorter scanning ranges than amplitude-based configurations., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

The impact of Brillouin scattering on the polarisation of pump and probe in a Brillouin optical-time domain analyser is investigated. Experimental results indicate that the Brillouin interaction integrated along the entire sensing fibre produces a polarisation pulling force that changes the pump polarisation. Although this force can be compensated to first order by dual-sideband probes, the Brillouin gain/loss experienced by probe bands induce a net pulling that leads to uncompensated polarisation fading and reduced signal-to-noise ratio. While the use of two simultaneous orthogonally-polarised probes alleviates fading issues, spectral distortions originate from the coupling between Brillouin gain and polarisation pulling., European Commission, Swiss Commission for Technology and Innovation, Israel Science Foundation

The use of linearly chirped probe pulses in phase sensitive-(Phi)OTDR technology has been recently demonstrated to allow for high-resolution, quantitative and dynamic temperature or strain variation measurements in a simple and very robust manner. This new sensing technology, known as chirped-pulse PhiOTDR, had a maximum reported sensing range of 11 km. In this paper, a 75 km sensing range with 10 m spatial resolution is demonstrated by using bidirectional first order Raman amplification. The system is capable of performing truly linear, single-shot measurements of strain perturbations with an update rate of 1 kHz and 1 nepsilon resolution. The time-domain trace of the sensor exhibits a signal to noise ratio (SNR) in the worst point of >3 dB, allowing to monitor vibrations up to 500 Hz with remarkable accuracy. To demonstrate the capabilities of the proposed system, we apply 20 dB (with only 300 ms analysis window and no post-processing) and no evidence of nonlinearity in the acoustic response. The optical nonlinear effects that the probe pulse could suffer along the sensing fiber are thoroughly studied, paying special attention to potential distortions of the pulse shape, particularly in its instantaneous frequency profile. Our analysis reveals that, for proper values of peak power, the pulse does not suffer any major distortion and therefore the system performance is not compromised., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

25th International conference on Optical Fiber Sensors, , 24/04/2017-28/04/2017, Jeju, Corea del Sur., European Commission, Ministerio de Economía y Competitividad

The growing demand of ultrafast mode-locked fiber lasers in the near infrared has boosted the research activity in this area. One of the most convenient ways to achieve passive mode locking consists of inserting a semiconductor saturable absorber in the laser cavity to modulate the losses. However, in such a configuration, the limited power range of operation is still an unsolved issue. Here we report the fabrication of an ultrafast, high-power, widely power-tunable and non-polarization-dependent mode-locked fiber laser operating at 1.55 μm, using an InN layer as saturable absorber. With post-amplification, this laser delivers 55-fs pulses with a repetition rate of 4.84 MHz and peak power in the range of 1 MW in an all-fiber arrangement., Ministerio de Economía y Competitividad, Comunidad de Madrid

This work describes a novel ultrafast (

25th International Conference on Optical Fiber Sensors, 2017, Jeju, Corea del Sur., In this communication, the effect of the probe pulse shape on the backscattered power trace of phase-sensitive (φ-) OTDR is analyzed. In particular, the power traces obtained with rectangular and Gaussian-like probe pulses are compared, both numerically and experimentally. Our analysis reveals that the use of Gaussian-like pulses can increase the operation range and sensitivity of φOTDR-based sensors in at least two-fold as compared with that obtained from traditionally-employed rectangular-like pulses., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

Raman assistance is one of the most interesting routes toward extending the range of Brillouin optical timedomain analysis (BOTDA) systems. Unfortunately, the use of Raman amplification normally entails an important relative intensity noise (RIN) transfer toward the probe wave, which severely limits this approach. In this letter, we show that the RIN transfer problem can be effectively avoided using a slightly modified scheme comprising a dual-sideband modulated probe and balanced detection. Our measurements clearly indicate a nearly complete cancellation of the RIN noise in the detected trace signal. The use of balanced detection in our Raman-assisted BOTDA setup implies a fivefold increase in figure of merit compared with the same setup using conventional detection., European Commission, Ministerio de Economía y Competitividad

Systematic errors induced by distortions in the pump pulse of conventional Brillouin distributed fiber sensors are thoroughly investigated. Experimental results, supported by a theoretical analysis, demonstrate that the two probe sidebands in standard Brillouin optical time-domain analyzers provide a non-zero net gain on the pump pulse, inducing severe distortions of the pump when scanning the pump-probe frequency offset, especially at high probe power levels. Compared to the impact of non-local effects reported in the state-of-the-art, measurements here indicate that for probe powers in the mW range (below the onset of amplified spontaneous Brillouin scattering), the obtained gain and loss spectra show two strong side-lobes that lead to significant strain/temperature errors. This phenomenon is not related to the well-known spectral hole burning resulting from pump depletion, but it is strictly related to the temporal and spectral distortions that the pump pulse experiences when scanning the Brillouin gain/loss spectrum. As a solution to this problem, a novel scanning scheme for Brillouin sensing is proposed. The method relies on a fixed frequency separation between the two probe sidebands, so that a flat zero net gain is achieved on the pump pulse when scanning the pump-probe frequency offset. The proposed technique is experimentally validated, demonstrating its ability to completely cancel out non-local effects up to a probe power ultimately limited by the onset of amplified spontaneous Brillouin scattering. The method allows for one order of magnitude improvement in the figure-of-merit of optimized long-range Brillouin distributed fiber sensors, enabling measurements along a 100 km-long sensing fiber with 2 m spatial resolution and with no need of added features for performance enhancement., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

The phase shift feature of stimulated Brillouin scattering is rarely used for the development of sensors based on the Brillouin optical time-domain analysis (BOTDA). However, this feature shows significant interest for sensing thanks to its linear dependence around the Brillouin frequency shift. Until now, distributed Brillouin phase shift (BPS) measurements required complex modulations, high-bandwidth detection, and sharp filtering. Recently, we introduced the possibility of developing simple distributed measurements of BPS in BOTDA using the baseband technology. The basic idea was the use of a Sagnac interferometer coupled to a conventional BOTDA without filtering and incorporating balanced detection. Although the concept was demonstrated, the setup proposed (using a 2 × 2 optical coupler) had significant shortcomings, the main one being its lack of stability due to the need of setting the interferometer bias through the polarization of the interacting waves. This restricted severely the practicality of the method. In this letter, we propose a setup with enhanced stability thanks to the use of a 3 × 3 optical coupler. With this configuration, the impact of the drifts along the measurement is strongly reduced, and the quality of the measurements improves substantially. The method is theoretically studied and demonstrated experimentally., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

We evaluate the Brillouin frequency shift (BFS) determination error when using the Brillouin phase spectrum (BPS) instead of the Brillouin gain spectrum (BGS) in BOTDA setups. We compare the error obtained in the BFS determination in both cases, both with theoretical arguments and experimental data. In comparison to the gain, for an equal SNR and linewidth, the phase generally provides a better fit of the BFS for smaller frequency spans. This result opens a possible way to reduce the measurement time of certain BOTDA systems by using the phase feature, Ministerio de Economía y Competitividad, Comunidad de Madrid, Universidad de Alcalá, European Commission

Sixth European Workshop on Optical Fibre Sensors (EWOFS2016), 31/05/2016-03/06/2016, Limerick, Irlanda., Ministerio de Economía y Competitividad, Comunidad de Madrid, European Commission

European Workshop on Optical Fibre Sensors (EWOFS2016), 31/05/2016-03/06/2016, Limerick, Irlanda., A method to evaluate distributed temperature gradients along an optical fiber using phase-sensitive optical time domain reflectometry (PhiOTDR) with direct detection is proposed and experimentally validated. The measurement principle derives from the perturbation response of a single-wavelength PhiOTDR signal, which is analyzed as a unidimensional speckle pattern. Our method can be implemented in real-time, relies solely on a low-cost post-processing of the standard PhiOTDR traces and requires no scanning of the laser frequency. This post-processing method can be implemented over a conventional PhiOTDR system used for distributed intrusion detection, without affecting its operation or requiring any additional hardware., Ministerio de Economía y Competitividad, Comunidad de Madrid, Universidad de Alcalá, European Commission

We evaluate the Brillouin frequency shift (BFS) determination error when utilizing the Brillouin phase spectrum (BPS) instead of the Brillouin gain spectrum (BGS) in BOTDA systems. Systems based on the BPS perform the determination of the BFS through a linear fit around the zero de-phase frequency region. An analytical expression of the error obtained in the BFS determination as a function of the different experimental parameters is provided and experimentally validated. The experimental results show a good agreement with the theoretical predictions as a function of the number of sampling points, signal-to-noise ratio (SNR) and Brillouin spectral linewidth. For an equal SNR and linewidth, the phase response only provides a better BFS estimation than the gain response when the fit is performed over a restricted frequency range around the center of the spectral profile. This may reduce the measurement time of specific BOTDA systems requiring a narrow frequency scanning. When the frequency scan covers most of the Brillouin spectral profile, gain and phase responses give very similar estimations of the BFS and the BPS offers no crucial benefit., Ministerio de Economía y Competitividad, Comunidad de Madrid, European Commission

A fundamental limitation in high-resolution and long-range Brillouin optical timedomain systems is overcome in this paper, enabling 500k resolved points over 25 km of SMF with 5 cm spatial resolution in a usual acquisition time.

We present the structural and optical properties of (0001)-oriented nanocolumnar films of InN deposited on c-sapphire substrates by radio-frequency reactive sputtering. It is observed that the column density and dimensions are highly dependent on the growth parameters of the buffer layer. We investigate four buffer layers consisting of (i) 30 nm of low-growth-rate InN, (ii) 30 nm of AlN deposited on the unbiased substrate (us), (iii) 30 nm of AlN deposited on the reverse-biased substrate (bs), and (iv) a 60-nm-thick bilayer consisting of 30-nm-thick bs-AlN deposited on top of 30-nm-thick us-AlN. Differences in the layer nucleation process due to the buffer layer induce variations of the column density in the range of (2.5-16)×109 cm-2, and of the column diameter in the range of 87-176 nm. Best results in terms of mosaicity are obtained using the bs-AlN buffer layer, which leads to a full width at half-maximum of the InN(0002) rocking curve of 1.2°. A residual compressive strain is still present in the nanocolumns. All samples exhibit room temperature photoluminescence emission at ∼1.6 eV, and an apparent optical band gap at ∼1.7 eV estimated from linear optical transmittance measurements., Ministerio de Economía y Competitividad, Comunidad de Madrid, Universidad de Alcalá, European Commission

So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4nε resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

Raman assistance in distributed sensors based on Brillouin optical time-domain analysis can significantly extend the measurement distance. In this paper, we have developed a 2 m resolution long-range Brillouin distributed sensor that reaches 100 km using first-order Raman assistance. The estimated uncertainty in temperature discrimination is 1.2 °C, even for the position of worst contrast. The parameters used in the experiment are supported by a simple analytical model of the required values, considering the main limitations of the setup., Ministerio de Ciencia e Innovación, Ministerio de Fomento, Comunidad de Madrid

We present a detailed investigation on the influence of deposition conditions on morphological, structural and optical properties of InN films deposited on Si(1 1 1) and GaN-on-sapphire templates by reactive radio-frequency (RF) sputtering. The deposition parameters under study are nitrogen content in the sputtering gas, substrate–target distance, substrate temperature and RF power. X-ray diffraction measurements confirm the (0 0 0 1) preferred growth orientation and the wurtzite crystallographic structure of the material. For optimized deposition conditions, InN on Si(1 1 1) substrates presents smooth surface with root-mean-square roughness ∼1 nm. Surface quality of the InN films can be further improved by deposition on GaN-on-sapphire templates, achieving root-mean-square roughness as low as ∼0.4 nm, comparable to that of the underlying substrate. The room-temperature absorption edge is located at 1.70 eV. Intense low-temperature photoluminescence peaking at 1.60 eV is observed.