Dynamic phase extraction in high-SNR DAS based on UWFBGs without phase unwrapping using scalable homodyne demodulation in direct detection.

We demonstrate a Distributed Acoustic Sensor (DAS) based on Ultra-Weak Fiber Bragg Gratings (UWFBGs) using a scalable homodyne demodulation in direct detection. We show that a distributed interferometric system using delay and mixing of backscattering from consecutive identical gratings can be combined with a Phase-Generated Carrier Differentiate and Cross-Multiply (PGC-DCM) demodulation algorithm to perform dynamic measurements with high SNR, employing a simple narrowband laser and a pin photodiode. The proposed homodyne demodulation technique is suitable for real-time monitoring using distributed measurements, as it does not require computationally costly phase unwrapping common in conventional schemes and is robust against detrimental harmonic distortions, while not requiring additional mechanisms to handle division-by-zero operations. The demodulation scheme is also scalable, as it involves symmetric ordinary differentiation and integration operations suitable for processing with FPGA-based or analogue systems which, thanks to readily realizable schemes for implementing fractional order calculus, are also candidates for small-scale integration. We experimentally demonstrate the effectiveness of the technique by monitoring the dynamic response of a generic 2.5 kHz vibration applied to a PZT placed at the end of a sensing fiber comprised of a 1 km array of 200 UWFBGs each with a reflectivity of ~-43 dB written at a spacing of 5 m, with an SNR of ~34.52 dB.

Dynamic phase extraction in a modulated double-pulse ϕ-OTDR sensor using a stable homodyne demodulation in direct detection.

We propose and experimentally demonstrate a stable homodyne phase demodulation technique in a ϕ-OTDR using a double-pulse probe and a simple direct detection receiver. The technique uses selective phase modulation of one of a pair of pulses to generate a carrier for dynamic phase changes and involves an enhanced phase demodulation scheme suitable for distributed sensing by being robust against light intensity fluctuations, independent of the modulation depth, and convenient for analogue signal processing. The capability of the technique to quantify distributed dynamic phase change due to a generic external impact is experimentally demonstrated by measuring the phase change induced by a nonlinear actuator generating a 2 kHz perturbation at a distance of 1.5 km on a standard singlemode fiber with an SNR of ~24 dB. The demodulated nonlinear response is shown to have a spectrum consistent with one obtained using an FBG sensor and a commercial reading unit.

Fiber-Optic Liquid Level Sensing by Temperature Profiling with an FBG Array.

We describe a fiber-optic system to measure the liquid level inside a container. The technique is based on the extraction of the temperature profile of the fiber by using a fiber Bragg grating (FBG) array. When the temperatures of the liquid and the gas are different, the liquid level can be estimated. We present a physical model of the system and the experimental results and we compare different algorithms to extract the liquid level from the temperature profile. We also show how air convection influences the temperature profile and the level of estimation accuracy. We finally show dynamic response measurements which are used to obtain the response time of the sensor. Turbomachinery monitoring is proposed as one possible application of the device.

Hybrid distributed acoustic and temperature sensor using a commercial off-the-shelf DFB laser and direct detection.

We demonstrate a hybrid distributed acoustic and temperature sensor (DATS) using a commercial off-the-shelf (COTS) distributed feedback (DFB) laser, a single-mode optical fiber, and a common receiver block. We show that the spectral and frequency noise characteristics of the laser, combined with a suitable modulation scheme, ensure the inter-pulse incoherence and intra-pulse coherence conditions required for exploiting the fast denoising benefits of cyclic Simplex pulse coding in the hybrid measurement. The proposed technique enables simultaneous, distributed measurement of vibrations and temperature, with key industrial applications in structural health monitoring and industrial process control systems. The sensor is able to clearly identify a 500 Hz vibration at 5 km distance along a standard single-mode fiber and simultaneously measure the temperature profile along the same fiber with a temperature resolution of less than 0.5°C with 5 m spatial resolution.

Integrated TE and TM optical circulators on ultra-low-loss silicon nitride platform.

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.

Numerical study of high-index-contrast Er:LiNbO3 photonic wire lasers optically pumped at 980 nm.

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.

Optimization of a DPP-BOTDA sensor with 25 cm spatial resolution over 60 km standard single-mode fiber using Simplex codes and optical pre-amplification.

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 µÎµ.

Hybrid Raman/fiber Bragg grating sensor for distributed temperature and discrete dynamic strain measurements.

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.

Integrated hybrid Raman/fiber Bragg grating interrogation scheme for distributed temperature and point dynamic strain measurements.

We propose and experimentally demonstrate the feasibility of an integrated hybrid optical fiber sensing interrogation technique that efficiently combines distributed Raman-based temperature sensing with fiber Bragg grating (FBG)-based dynamic strain measurements. The proposed sensing system is highly integrated, making use of a common optical source/receiver block and exploiting the advantages of both (distributed and point) sensing technologies simultaneously. A multimode fiber is used for distributed temperature sensing, and a pair of FBGs in each discrete sensing point, partially overlapped in the spectral domain, allows for temperature-independent discrete strain measurements. Experimental results report a dynamic strain resolution of 7.8 nε/√Hz within a full range of 1700 µÎµ and a distributed temperature resolution of 1°C at 20 km distance with 2.7 m spatial resolution.

Optimization of long-range BOTDA sensors with high resolution using first-order bi-directional Raman amplification.

In this paper we perform an optimization of Brillouin optical time-domain analysis (BOTDA) sensors for achieving high resolution over long sensing ranges using distributed Raman amplification. By employing an optimized first-order bi-directional Raman amplification scheme and combining high-power fiber-Raman lasers and Fabry-Pérot lasers with low relative-intensity-noise (RIN), we demonstrate distributed sensing over 120 km of standard single-mode fiber with 2 meter spatial resolution and with a strain/temperature accuracy of 45µÎµ/2.1°C respectively.

Design of transverse electric ring isolators for ultra-low-loss Si3N4 waveguides based on the finite element method.

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.

Long-range simplex-coded BOTDA sensor over 120 km distance employing optical preamplification.

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 µÎµ respectively.

Raman-based distributed temperature sensor with 1 m spatial resolution over 26 km SMF using low-repetition-rate cyclic pulse coding.

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.

Analysis of pulse modulation format in coded BOTDA sensors.

A theoretical and experimental analysis of the impact of pulse modulation format on Brillouin optical time-domain analysis (BOTDA) sensors using pulse coding techniques has been carried out. Pulse coding with conventional non-return-to-zero (NRZ) modulation format is shown to induce significant distortions in the measured Brillouin gain spectrum (BGS), especially in proximity of abrupt changes in the fiber gain spectra. Such an effect, as confirmed by the theoretical analysis, is due to acoustic wave pre-excitation and non-uniform gain which depends on the bit patterns defined by the different codewords. A successful use of pulse coding techniques then requires to suitably optimize the employed modulation format in order to avoid spurious oscillations causing severe penalties in the attained accuracy. Coding technique with return-to-zero (RZ) modulation format is analyzed under different duty-cycle conditions for a 25 km-long sensing scheme, showing that low duty-cycle values are able to effectively suppress the induced distortions in the BGS and allow for spatially-accurate, high-resolution strain and temperature measurements being able to fully exploit the provided coding gain (approximately 7.2 dB along 25 km distance) with unaltered spatial resolution (1 meter). Although Simplex coding is used in our analysis, the validity of the results is general and can be directly applied to any intensity-modulation coding scheme.

Simplex-coded BOTDA fiber sensor with 1 m spatial resolution over a 50 km range.

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.

Analysis of optical pulse coding in spontaneous Brillouin-based distributed temperature sensors.

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.