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Bidirectional low-noise Raman amplification and simplex coding based on the return-to-zero modulation format are optimized through numerical simulations for long-range Brillouin optical time-domain analysis sensing. Experimental results are reported on sensing capabilities along 120-km distance with 1-m spatial resolution, and worst-case temperature and strain resolution values of 1.3 degrees C and 26 mu epsilon, respectively.

This paper aims to present the design and the achieved results on a CMOS electronic and photonic integrated device for low cost, low power, transparent, mass-manufacturable optical switching. An unprecedented number of integrated photonic components (more than 1000), each individually electronically controlled, allows for the realization of a transponder aggregator device which interconnects up to eight transponders to a four direction colorless-directionless-contentionless ROADM. Each direction supports 12 200-GHz spaced wavelengths, which can be independently added or dropped from the network. An electronic ASIC, 3-D integrated on top of the photonic chip, controls the switch fabrics to allow a complete and microsecond fast reconfigurability.

We present a Fiber Bragg Grating sensor interrogator in silicon-on-insulator (SOI) platform based on interferometric wavelength-shift detection. A commercial FBG readout unit is used as a reference for validation of dynamic strain measurement.

In this paper we present a validation of distributed Raman temperature sensing (RDTS) at the CERN high energy accelerator mixed field radiation test facility (CHARM), newly developed in order to qualify electronics for the challenging radiation environment of accelerators and connected high energy physics experiments. By investigating the effect of wavelength dependent radiation induced absorption (RIA) on the Raman Stokes and anti-Stokes light components in radiation tolerant Ge-doped multi-mode (MM) graded-index optical fibers, we demonstrate that Raman DTS used in loop configuration is robust to harsh environments in which the fiber is exposed to a mixed radiation field. The temperature profiles measured on commercial Ge-doped optical fibers is fully reliable and therefore, can be used to correct the RIA temperature dependence in distributed radiation sensing systems based on P-doped optical fibers.

We propose and experimentally demonstrate the feasibility of a highly efficient FBG-based quasi-distributed sensing system employing dual-wavelength cyclic pulse coding. Significant improvement in the measurement range, resolution and TDM multiplexing capabilities can be achieved, as well as crosstalk reduction with respect to a single wavelength TDM-based FBG interrogation scheme. The mechanism of noise reduction by quasi-periodic cyclic coding is experimentally demonstrated, pointing out significant improvement in accuracy with respect to dual-wavelength single pulse TDM-based FBG interrogation. The proposed technique can also enhance the sensing range of hybrid fiber optic sensor systems in which continuous monitoring of distributed and discrete points are simultaneously measured over the same sensing fiber.

We propose and experimentally demonstrate a Distributed Acoustic Sensor exploiting cyclic Simplex coding in a phase-sensitive OTDR on standard single mode fibers based on direct detection. Suitable design of the source and use of cyclic coding is shown to improve the SNR of the coherent back-scattered signal by up to 9 dB, reducing fading due to modulation instability and enabling accurate long-distance measurement of vibrations with minimal post-processing.

Silicon photonics enables the fabrication of photonic integrated circuits with high bandwidth density, making it suitable for computercom applications. In multicore computing systems, the communications between cores and memory can be supported by optical networks-on-chip (NoC) realized with photonic integrated circuits (PIC). While different optical NoC topologies have been proposed in the past, only few NoC were fabricated and tested. This paper aims at comparing the performance of two PIC NoC with a bus and a ring topology. First, a framework is presented for passing from the theoretical analysis of silicon photonics basic building blocks like waveguides and microrings, to the PIC design and to the NoC performance derivation, using the scattering matrix method. Based on this framework, the two NoC topologies are simulated, designed, fabricated in silicon photonics, and experimentally characterized for comparison. Spectral performance validates the theoretical model with minor deviations due to the fabrication inaccuracies and limitations. Bit error rate performance at 10 Gb/s demonstrates the capability of simultaneous transmissions in both topologies with limited or negligible crosstalk. Moreover, ring NoC is shown to slightly outperform the bus NoC thanks to the filtering properties of the central microring.

A field trial has been performed at the CERN high-energy accelerator-mixed (CHARM) field facility, newly developed for testing devices within accelerator representative radiation environments, to validate the use of Raman-based optical fiber sensors for distributed temperature measurements in highly radiative environments. Experimental results demonstrate that Raman distributed temperature sensors, operating in loop configuration on radiation-tolerant optical fibers, are able to compensate for wavelength-dependent losses and are, therefore, robust to harsh environments, in which a mixed-field radiation, including protons, neutrons, photons, and other particles, is potentially altering the fiber material properties. The temperature profile measured on commercial radiation-tolerant optical fibers shows that a temperature resolution

We propose and experimentally demonstrate a fully hybrid distributed sensing scheme that uses a single narrow-band laser to perform fast measurement of the BFS using BOTDA and simultaneous temperature measurement based on spontaneous Raman scattering over 10 km of single mode fiber. The use of cyclic pulse coding effectively reduces the pump peak power levels required for accurate Raman-based distributed temperature measurement, enhancing at the same time the speed of the BFS measurement in the BOTDA technique.

We propose a new wavelength-division-multiplexed (WDM) optical amplifier cascade, based on erbium-doped fiber amplifiers, gain equalizing filters, and arrays of integrated waveguide amplifiers, to ensure good performance in wavelength-routed optical transport networks without the need for a complex centralized network control. To prove its efficiency, we propose this configuration for the National Science Foundation Network and we analyze its performance under critical conditions of link failure restoration. Computed results demonstrate that the self-regulating properties of this WDM amplifier cascade make these networks robust against link failures.

We propose and experimentally demonstrate a Brillouin Optical Time Domain Analysis sensor combining adaptive linear prediction with cyclic pulse coding for fast and accurate strain measurement over 10km standard singlemode fiber with meter-scale spatial resolution.

In this paper we perform field validation of distributed Raman temperature sensing (RDTS) for oil leakage detection in soil. The capability of the distributed Raman sensor in detecting and locating, with high accuracy and spatial resolution, drop leakages in soil is demonstrated through a water leakage simulation in a field trial. The future trends and the high potential of the Raman DTS technology for oil and gas leakage detection in long pipelines is then outlined in this paper by reporting lab experiments demonstrating accurate meter scale temperature measurement over more than 50 km of standard single mode fiber. The proposed solution, based on distributed Simplex coding techniques, can be competitive in terms of cost and performance with respect to other distributed sensing technologies.

This book chapter describes Raman Based Distributed Optical Fiber Sensors and their main industrial applications. In particular, after a brief introduction on the basic physical mechanisms, and a theoretical description of distributed optical fiber sensors based on spontaneous Raman scattering, we have focused our attention on the most attractive applications in terms of market opportunity: fire detection in tunnels and industrial plants and leakage detection in pipelines. We show that in order to achieve long sensing distances with high temperature resolution and good reliability, most standard Raman based distributed temperature sensors make use of graded-index multi-mode fibers, in double-ended or looped configuration. Advanced pulse coding techniques can however be effectively applied to make Raman sensors based on standard single mode fibers competitive with respect to other distributed optical fiber sensing technologies, such as Brillouin-based optical fiber sensors. Future developments of Raman based distributed temperature sensors will likely exploit standard single mode fibers, achieving meter-scale spatial resolutions over ultra-long distances, with high temperature resolution, fast response time, high robustness and reliability.

For the first time [to our best knowledge] a high-index-contrast z-cut Er:LiNbO(3) photonic wire waveguide laser, optically pumped at 980 nm wavelength, is designed for continuous-wave operation. Waveguide modes and laser characteristics are numerically computed using a developed full vectorial finite-element method based tool. In order to maximize the output power of the laser, the active cavity length and output mirror's reflectivity have been optimized, considering different pump power and waveguide background losses. Efficient laser emission is theoretically predicted at 1531 nm wavelength for the fundamental TE mode and a value of threshold pump power as low as 0.2 mW has been computed.

By exploiting non-reciprocal magneto-optic micro-ring resonators, hybrid III-V semiconductor optical amplifiers can be effectively integrated to provide crosstalk-free bidirectional interleaved WDM communication between racks.

A two-ring add/drop switch is presented for scalable switching matrix architectures. The device performance is investigated using the transfer matrix method, showing low crosstalks, high out of band rejection and side-lobe suppression.

We show our progress on ring-based optical isolators and circulators, designed on both silica and silicon based photonic integration platforms. Isolation levels of better than -20 dB have been shown theoretically and -9 dB experimentally.

We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Raman optical time domain reflectometry and in-line time-division-multiplexing for fiber Bragg grating (FBG) dynamic interrogation. The highly integrated proposed scheme employs broadband apodized low reflectivity FBGs with a single narrowband optical source and a shared receiver block, allowing for simultaneous measurements of distributed static temperature and discrete dynamic strain, over the same sensing fiber.

We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Brillouin optical time-domain analysis and a time -domain multiplexing interrogation technique for Fiber Bragg Gratings (FBGs). The highly-integrated proposed scheme employs broadband apodized low-reflectivity FBGs with a single optical source and a shared receiver block, allowing for simultaneous measurements of distributed static and discrete dynamic temperature and strain, over the same sensing fiber. Keywords: Brillouin scattering, Fiber Bragg Gratings, fiber optic sensors, temperature sensing, strain sensing. 1. INTRODUCTION During the last years, distributed optical fiber sensing based on stimulated Brillouin scattering (SBS) has become an interesting approach for temperature and strain sensing in many industrial applications. In particular, distributed sensing with meter and sub-meter spatial resolution along several tens of km has been demonstrated using Brillouin optical time-domain analysis (BOTDA) technique [1]. Even though the sensing capabilities provided by BOTDA systems are attractive in many applications, the relatively long measurement time (of the order of a few minutes) makes this technology mainly suitable for static measurements, limiting its potential range of applications. Although the distributed information provided by BOTDA sensing is essential for static strain and temperature monitoring, there are several applications, such as for example in industrial Oil&Gas production plants monitoring, which also require dynamic strain measurements at specific critical points, ad ding in this way crucial information on the structure integrity (e.g. vibrations). Distributed sensing based on either spontaneous Brillouin scattering (SpBS) or SBS has been recently combined with point based Fiber Bragg Grating (FBG) sensors [3,4]. In particular, BOTDA technique has been combined with wavelength-division-multiplexed FBG point sensors, using different demodulation techni ques for distribute d and discrete sensing, as well as distributed Raman amplification, increasing cost and complexity of the whole sensor system [4]. In this paper we propose a highly-integrated hybrid sensing system that effectively combines the advantages of both standard BOTDA and in-line time-division-multiplexed (TDM) FBG-based sensing, allowing for simultaneous dynamic discrete strain measurements and distributed (static) strain/temperature sensing using a single optical fiber, a common pulsed narrowband optical source and a shared receiver unit.

Differential pulse-width-pair BOTDA technique is combined with bi-directional Raman amplification and Simplex coding to achieve sub-meter spatial resolution over very long sensing distances. Numerical simulations are used to optimize the power levels of the Raman pumps, avoiding nonlinear effects and pump depletion. Distortions in the Brillouin gain spectrum due to acoustic-wave pre-excitation are also avoided by numerical optimization of the pulse width and duty cycle of return-to-zero Simplex coding, providing significant SNR enhancement. We demonstrate 50 cm spatial resolution over 93 km of standard SMF with a strain/temperature accuracy of 34 0 /1.7 °C, respectively. Keywords: fiber optic sensors, Brillouin scattering, Raman scattering, temperature sensing, strain sensing. 1. INTRODUCTION Brillouin optical time domain analysis (BOTDA) has became one of the most adopted approaches in distributed sensing due to its capability of measuring strain and temperature simu ltaneously. The possibility of this technique to achieve long sensing ranges with high spatial resolution using standard single-mode fiber (SMF) enables many practical applications in structural health monitoring and civil engineering. In some applications, such as crack detections in large civil structures, spatial resolution values ranging in the cm-scale ar e required over a few kilometers distance. Other industrial fields, like pipelines monitoring in the oil&gas industry, would strongly benefit from temperature and strain distributed sensing capabilities over tens or hundreds of km SMF with tens of cm spatial resolution. However, spatial resolution in standard BOTDA systems is limited to 1 m due to the acoustic-phonon lifetime (~10 ns), inducing a broadening of the Brillouin gain spectrum when pulse widths shorter than 10 ns are used, thus leading to reduction of Brillouin peak gain [1] and to inaccuracies in Brillouin fre quency shift (BFS) measurements. In order to achieve spatial resolutions smaller than 1 m, one promising technique is given by BOTDA employing differential pulse-width pair (DPP–BOTDA). DPP-BOTDA has been successfully employed to attain sub-meter spatial resolution [1-3], although typically exhibiting limited sensing ranges due to a rather poor signal-to-noise (SNR) induced by the DPP subtraction process. Optical pulse coding has been applied to DPP-BOTDA [1,2] overcoming the SNR limitations, in order to achieve longer distances. On the other hand, an effective technique, based on bi-directional Raman amplification, has been applied in standard BOTDA to extend the sensing range over 120 km distance, although with spatial resolutions of the order of 2 meters [4]. In this paper we propose to apply low-RIN bi-directional Ra man amplification in combination with optical pulse coding to DPP-BOTDA schemes, thus providing distributed sensing over long sensing ranges together with sub-meter spatial resolution values. An effective implementation of such a sensor scheme is achieved through numerical optimization of the experimental test-bed. In particular, numerical simulations have been carried out in order to optimize all the coded Raman DPP-BOTDA system parameters in terms of pump-probe power levels, intensity noise (RIN) transfer to the probe signal, as well as in terms of modulation format. By combining the use of a return-to-zero (RZ)-format Simplex-coded DPP-BOTDA together with optimized low-RIN bi-directional Raman amplification, we finally experimentally demonstrate long-range distributed measurements over ~93 km of SMF, achieving 50-cm spatial resolution and worst-case strain-temperature resolutions of 34 0 - 1.7°C respectively.

Sub-meter distributed optical fiber sensing based on Brillouin optical time-domain analysis with differential pulse-width pairs (DPP-BOTDA) is combined with the use of optical pre-amplification and pulse coding. In order to provide significant measurement SNR enhancement and to avoid distortions in the Brillouin gain spectrum due to acoustic-wave pre-excitation, the pulse width and duty cycle of Simplex coding based on return-to-zero pulses are optimized through simulations. In addition, the use of linear optical pre-amplification increases the receiver sensitivity and the overall dynamic range of DPP-BOTDA measurements. Experimental results demonstrate for first time a spatial resolution of ~25 cm over a 60 km standard single-mode fiber (equivalent to ~240 k discrete sensing points) with temperature resolution of 1.2°C and strain resolution of 24 με.

We present advanced techniques for high performance Raman distributed temperature sensing systems, based on graded index multi-mode and standard single-mode fibers. Advanced coding allows for ultra-long sensing capabilities with meter or sub-meter scale spatial resolution

In this Letter we present the design of a novel (to our best knowledge) integrated TE isolator realized using ultra-low-loss Si(3)N(4) waveguides. The device is made of two straight waveguides coupled to an array of ring resonators including a Ce:YIG garnet grown on their internal side. The analysis demonstrates advantages in loss, isolation, and passband width as the number of rings is increased.

In this paper we demonstrate distributed Raman temperature sensing (RDTS) in a loop scheme employing anti-Stokes light intensity only. Using a single-channel receiver and anti-Stokes traces measured in loop configuration, we implement RDTS with inherent compensation of fiber wavelength-dependent losses, as well as local external perturbations. Experimental results show a signal-to-noise ratio enhancement with respect to a standard RDTS in loop configuration, providing a robust and reliable high-performance sensor for long sensing ranges. Keywords: Raman scattering, fiber optic sensors, temperature sensing.

In this Letter, we combine the use of optical preamplification at the receiver and optical pulse coding techniques with an optimized modulation format to effectively extend the sensing range of Brillouin optical time-domain analysis (BOTDA) sensors. Combining a return-to-zero modulation format with 25% duty cycle and linear gain preamplification allows for temperature and strain measurements over 120 km of standard single-mode fiber with 3 m spatial resolution and an rms strain-temperature accuracy of 3.1 °C/60 με respectively.

In this Letter, we propose the use of optical pulse coding techniques for long-range distributed sensors based on Brillouin optical time-domain analysis (BOTDA). Compared to conventional BOTDA sensors, optical coding provides a significant sensing-range enhancement, allowing for temperature and strain measurements with 1 m spatial resolution over 50 km of standard single-mode fiber, with an accuracy of 2.2 degrees C/44 muepsilon, respectively.

In this paper we demonstrate a long-range distributed strain and temperature optical fiber sensor using Brillouin optical time-domain analysis in conjunction with optical pulse coding. Compared to standard BOTDA, pulse coding provides higher signal-to-noise ratio at the same pump peak power, overcoming the limitations induced by pump depletion and modulation instability, and allowing to achieve a record of 1 me ter spatial resolution over 50 km of standard single mode fiber with temperature and strain resolution of 2.2 °C and 44 PH respectively. Keywords: Stimulated Brillouin scattering, optical fiber sensors, temperature sensing, strain sensing, Simplex coding.

In this paper we propose the use of multi-longitudinal mode lasers, such as Fabry-Perot (FP) lasers, for simultaneous distributed strain and temperature sensing based on hybrid Brillouin and Raman scattering. Fabry-Perot lasers allow for high-power Raman intensity measurement and the simultaneous detection of the Brillouin frequency shift parameter for all FP longitudinal modes, by using a self-heterodyne detection scheme based on a multi-wavelength optical local oscillator. Experiments point out significant improvement in hybrid sensor performance, also suggesting further potential benefits when combining the use of FP lasers with optical coding techniques and/or optical amplification.

We have experimentally characterized the benefits of pulse coding in coherent-detection Brillouin-based hybrid sensors, achieving 15 dB improvement in two-way sensing distance at same resolution in simultaneous strain and temperature sensing.

We propose the use of pulse Simplex-coding in distributed temperature sensors based on spontaneous Brillouin scattering. The analysis shows achieved performance improvement and the effect on stimulated Brillouin threshold, resulting in lower required input power.

In this paper, we will briefly outline our contributions for the physical realization of coded OTDR, along with its principles and also highlight recent key results related with its applications. For the communication network application, we report a multi-port / multi-wavelength, high-speed supervisory system for the in-service monitoring of a bidirectional WDM-PON system transmission line up to 16 ports x 32 nodes (512 users) capacity. Monitoring of individual branch traces up to 60 km was achieved with the application of a 127-bit simplex code, corresponding to a 7.5dB SNR coding gain effectively reducing the measurement time about 30 times when compared to conventional average mode OTDR. Transmission experiments showed negligible penalty from the monitoring system to the transmission signal quality, at a 2.5Gbps / 125Mbps (down / up stream) data rate. As an application to sensor network, a Raman scattering based coded-OTDR distributed temperature sensor system will be presented. Utilizing a 255-bit Simplex coded OTDR together with optimized sensing link (composed of cascaded fibers with different Raman coefficients), significant enhancement in the interrogation distance (19.5km from coding gain, and 9.6km from link-combination optimization) was achieved to result a total sensing range of 37km (at 17m/3K spatial/temperature resolution), employing a conventional off-shelf low power (80mW) laser diode.

In this paper we describe advanced bi-directional Raman pumping schemes for extending the maximum reach achievable in long-span unrepeated WDM transmission systems operating at 10 Gb/s. We describe both first- and higher-order bi-directional Raman pumping schemes, pointing out that co-pumping, although critical in terms of noise transfer from pump to WDM signals, can be very effective in extending the maximum system reach. New co-propagating Raman pump lasers, characterized by high power levels and low relative intensity noise, are also proposed to further increase the maximum achievable span loss, avoiding transmission penalties induced by relative intensity noise transfer.

Benefits provided by third-order Raman pumping in unrepeated WDM transmission systems are quantified in terms of BER performances at 10 Gb/s. Double-Rayleigh scattering noise induces transmission penalties at very high on-off Raman gain and must be kept under control.

We propose an original and simple geometry for endlessly single-mode photonic crystal fibers. Numerical results, based on a full-vectorial finite element analysis, point out single-mode operation for wavelengths greater than 100 nm.

The impact of Raman amplification and Simplex coding is studied in combination withdifferential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) to achievesub-meter spatial resolution over very long sensing distances. An optimization of the powerlevels for the Raman pumps, Brillouin pump and signal has been carried out through numericalsimulations, maximizing the signal levels and avoiding at the same time nonlinear effects andpump depletion. A reduction of acoustic-wave-induced distortions in the Brillouin gainspectrum down to negligible levels has also been achieved by numerical optimization of thepulse width and duty cycle of return-to-zero Simplex coding, providing significantsignal-to-noise ratio enhancement. Strain–temperature sensing over 93 km standard SMF isachieved with a strain / temperature accuracy of 34 μe/ 1.7 ◦ C, and 50 cm spatial resolutionthroughout the fiber length. Keywords: fiber optics, sensors, Brillouin scattering, coding(Some figures may appear in colour only in the online journal)

In this paper, we present two four-port optical circulators for TE and TM modes, respectively. Exploiting the recent technological development concerning Ce:YIG pulse laser deposition on silicon nitride platform, we design two integrated circulators, which can be used to implement several functions in integrated optics, such as de-interleavers, input/output amplifier isolators and output laser isolators. The proposed devices combine the benefit of low loss silicon nitride waveguides with the non-reciprocal properties of magneto-optical materials. The ring cross-section has been optimized in order to maximize the non-reciprocal phase shift and finally the scattering coefficients have been computed using the transfer matrix method. The material stability and refractive index regularity of silicon nitride, the small micro-ring footprint, and the high wavelength selectivity make these devices particularly attractive.

A theoretical model based on propagation-rate equations has been developed for modeling the continuous wave operations of Er 3+ /Yb 3+ co-doped fiber lasers. It includes uniform upconversion mechanisms from the 4 I 13/2 and 4 I 11/2 erbium levels and describes the coupling between Er 3+ and Yb 3+ ions through a pair-induced energy transfer process. The nonlinear differential equations of the model are numerically solved by means of a Runge-Kutta based iterative procedure. Numerical results, based on realistic input fiber parameters, show that slope efficiencies close to the quantum limit with threshold pump powers of few mW, can be achieved in short Er 3+ /Yb 3+ co-doped fibers equipped with input dielectric mirrors and output grating Bragg reflectors. Furthermore, we show that single frequency cw operations can also be performed, with lower efficiency, in Er 3+ /Yb 3+ co-doped Fabry-Perot fiber microlasers

Optical amplifiers have already deeply affected current thinking on optical fiber communications, and are likely to play an increasing role in the design of future systems. In this paper we review the principles of optical amplification and discuss briefly the operational characteristics of the main types of optical amplifiers and their main advantages and drawbacks.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We experimentally investigate the benefits of a new optical pulse coding technique for long-range, meter and submeter scale Raman-based distributed temperature sensing on standard single-mode optical fibers. The proposed scheme combines a low-repetition-rate quasi-periodic pulse coding technique with the use of standard high-power fiber lasers operating at 1550 nm , allowing for what we believe is the first long-range distributed temperature measurement over single-mode fibers (SMFs). We have achieved 1 m spatial resolution over 26 km of SMF, attaining 3°C temperature resolution within 30 s measurement time.

An investigation is performed of the possibility of achieving simultaneous distributed strain and temperature sensing based on hybrid Raman–Brillouin scattering with the use of multi-wavelength optical sources such as common Fabry–Perot (FP) lasers. By employing a self-heterodyne detection scheme based on a multi-wavelength optical local oscillator, the benefits of FP lasers are fully exploited, allowing for high-power Raman intensity measurements and a simultaneous high-accuracy detection of the Brillouin frequency shift parameter for each FP longitudinal mode. Experimental results point out a significant reduction of coherent Rayleigh noise, and highlight the enhanced performance in hybrid Raman–Brillouin sensing when using FP lasers; in particular using standard FP lasers at 1550 nm results in about 12 dB (7 dB) temperature (strain) accuracy improvement at 25 km sensing distance with respect to the use of standard distributed feedback lasers.

In this paper we describe and implement a long-range Brillouin optical time-domain analysis (BOTDA) sensor, for both temperature and strain measurements, using optical pulse coding techniques. A theoretical analysis of Simplex coding applied to BOTDA systems is presented and experimentally demonstrated for both Brillouin loss and Brillouin gain configurations. With the proposed technique, ~7.1 dB and ~10.3 dB of signal-to-noise ratio improvements are demonstrated in BOTDA measurements using 127-bit and 511-bit Simplex codes, respectively. This feature allows us to extend the dynamic range of the measurements, overcoming the limitations to the maximum usable optical power imposed by pump depletion and modulation instability; thus, the sensing range can be extended by several tens of kilometers while keeping meter-scale spatial resolution. Experimental results show the capabilities of optical pulse coding techniques to achieve 1 m spatial resolution over 50 km of standard single-mode fiber enabling temperature and strain resolutions equal to 2.2 °C and 44 µe, respectively.

A theoretical and experimental analysis of optical pulse coding techniques applied to distributed optical fiber temperature sensors based on spontaneous Brillouin scattering using the Landau-Placzek ratio (LPR) scheme is presented, quantifying in particular the impact of Simplex coding on stimulated Brillouin and Raman power thresholds. The signal-to-noise ratio (SNR) enhancement and temperature resolution improvement provided by coding are also characterized. Experimental results confirm that, differently from Raman-based sensors, pulse coding affects the stimulated Brillouin threshold, resulting in lower optimal input power levels; these features allow one to achieve high sensing performance avoiding the use of high peak power pulses.

We propose the use of standard multiwavelength Fabry–PErot (FP) lasers for distributed strain and temperature measurement in hybrid Raman- and Brillouin-based fiber-optic sensors. The multiple longitudinal modes of FP lasers allow for accurate spontaneous Raman scattering measurement and for simultaneous high-sensitivity coherent detection of the Brillouin frequency shift parameter for all longitudinal modes. Experimental results confirm great performance improvement in simultaneous strain and temperature measurement using a single optical fiber.

A long-range fast-measurement Raman-based distributed temperature sensor (RDTS) with a meter spatial resolution exploiting a quasi-periodic pulse-coding technique is proposed in this paper. We experimentally demonstrate the feasibility of temperature sensing over 26 km standard single-mode fiber with 1 m spatial resolution obtaining 3 °C of temperature resolution within 30 s of measurement time. Experimental results employing 1023-bits codewords are also reported, showing a notable range enhancement and enabling distributed sensing over 58 km distance with 2 m spatial resolution and 4 °C of temperature resolution within 5 min measurement time.

Pulse coding techniques are proposed in combination with the use of Fabry-Perot lasers for simultaneous strain and temperature sensing based on hybrid spontaneous Raman-Brillouin scattering. Combination of both techniques allows for ∼0.27K / ∼30μe resolution along 25-km sensing range.

In this paper we present a Brillouin optical-time domain analysis (BOTDA) sensor combining optimized optical pulse coding with return-to-zero (RZ) modulation format and first-order bi-directional Raman amplification, using a single, shared, high relative-intensity-noise (RIN) pump source. We show that coding provides great benefits in sensing performance, even when employing noisy high-RIN Raman fiber-lasers degrading single-pulse BOTDA-schemes. Combining pulse coding with the use of a single high-power source for Raman amplification allows us to achieve distributed sensing over 120 km SMF, achieving 2.3 m spatial resolution with 52µe/2.6°C strain/temperature resolutions.