Guided Acoustic-wave Brillouin Scattering Research Articles

GAWBS Noise in Digital Coherent Transmission

This paper presents recent progress on guided acoustic wave Brillouin scattering (GAWBS) noise in single-core fiber (SCF), multi-core fiber (MCF), and few-mode fiber (FMF). First, we describe theoretical analyses of GAWBS phase noise generated in these various types of fiber. By using three types of SCF, namely ultra-large area fiber (ULAF), standard single-mode fiber (SSMF), and dispersion-shifted fiber (DSF), we show that the GAWBS phase noise depends strongly on the effective core area A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> . We also describe the GAWBS phase noise in four-core fiber (4CF), where it is shown that GAWBS noise in the off-center core is quite different from that in SCF. Furthermore, we describe the GAWBS phase noise in FMF supporting two LP modes. Then, we compare the GAWBS noise powers induced in SCF, MCF, and FMF. Next, we describe experiments on GAWBS phase noise in various fibers, which agree well with the theory. We also measured and analyzed the GAWBS phase noise correlation between the cores in a 4CF, where it is shown that the correlation between cores strongly depends on the core position. Finally, we describe the influence of GAWBS phase noise on digital coherent transmission. We formulate the GAWBS phase noise based on the Gaussian distribution and evaluate its influence on the BER characteristics in a coherent QAM transmission. We found that it is difficult to realize a transoceanic-class QAM transmission with a QAM multiplicity of higher than 64 even without other impairments. Furthermore, we evaluate the SNR penalty induced by the GAWBS noise in a long-haul transmission by taking account of the amplified spontaneous emission (ASE) and nonlinear interference noise, which are unavoidable in actual systems. The calculated results show that the influence of the GAWBS noise is smaller than the other impairments, but it cannot be neglected in system evaluations.

High-Temperature Sensing Based on GAWBS In Silica Single-Mode Fiber.

High temperature detection is a constant challenge for condition monitoring under harsh environments in optical fiber sensors research. In this study, the temperature response characteristics of guided acoustic wave Brillouin scattering (GAWBS) spectra in silica single-mode fiber (SMF) up to 800 °C are experimentally investigated, demonstrating the feasibility of the method for high-temperature monitoring. With increasing temperature, the resonance frequency of GAWBS spectra increases in a nearly linear manner, with linearly fitted temperature-dependent frequency shift coefficients of 8.19 kHz/°C for TR2,7 mode and 16.74 kHz/°C for R0,4 mode. More importantly, the linewidth of the GAWBS spectra is observed to narrow down with increasing temperature with a linearly fitted rate of -6.91 × 10-4/°C for TR2,7 modes and -8.56 × 10-4/°C for R0,4 modes. The signal-to-noise ratio of the GAWBS spectra induced by both modes increase by more than 3 dB when the temperature rises from 22 °C to 800 °C, which indicates that the proposed sensing scheme has better performance in high-temperature environments, and are particularly suitable for sensing applications in extreme environments. This study confirms the potential of high-temperature sensing using only GAWBS in silica fibers without any complex micromachining process, which has the advantages of strong mechanical strength, simple structure, easy operation, and low cost.

GAWBS noise correlation between cores in multi-core fibers

We describe experimental and analytical results regarding the guided acoustic-wave Brillouin scattering (GAWBS) noise correlation between cores in a multi-core fiber. First, we evaluated the GAWBS phase noise correlation between cores in an uncoupled four-core fiber (4CF) spaced at equal intervals of 90 degrees. We clarified that there are strong correlations in the TR2,m and TR4,m modes because the profile of the TR n ,m mode has a torsional dependence given by cos(nθ) and sin(nθ). In contrast, the TR3,m modes showed a strong correlation in an uncoupled 19-core fiber (19CF) with a hexagonal close-packed structure since a strong interaction occurs where the cores are arranged at 60 degrees from each other. We also clarified that the correlation strength of the TR3,m mode depends on the distance of the core from the fiber center because the amplitude of the TR3,m mode changes in a radial direction.

Scaling properties of guided acoustic-wave Brillouin scattering in single-mode fibers.

We investigate guided acoustic-wave Brillouin scattering (GAWBS) in single-mode optical fibers. By treating the discrete GAWBS spectrum as a continuum, we propose a closed-form formula to evaluate the GAWBS variance due to radial modes, showing its dependence on the inverse of the fiber effective area. The formula is checked against an experimental validation at variable signal power. The almost flat experimental results with power suggest that GAWBS dominates stimulated electrostriction. We also compare GAWBS with the Kerr effect showing their common grounds and their main differences. The phase noise characteristics of the nonlinear interference induced by GAWBS is investigated, showing that in ultra-long links GAWBS can be safely taken as a circular white noise. The theoretical results can be used for quick estimations of the GAWBS penalty with any fiber effective area. In particular, we estimated a GAWBS penalty on the signal-to-noise ratio (SNR) of 0.5 dB after 6000 km with fibers of effective area 150 μm2.

Chromatic dispersion dependence of GAWBS phase noise compensation with pilot tone

We describe experimental and numerical results regarding the influence of chromatic dispersion in optical fibers on guided acoustic-wave Brillouin scattering (GAWBS) phase noise compensation with a pilot tone (PT). We compared the compensation performance for GAWBS phase noise generated in an ultra-large-area fiber (ULAF) where DULAF = 21 ps/nm/km with that in a dispersion-shifted fiber (DSF) where DDSF = -1.3 ps/nm/km and found that the performance depends strongly on chromatic dispersion. The numerical analysis shows that the group delay between the signal and PT caused by chromatic dispersion reduces the GAWBS noise correlation between them, which degrades the compensation performance for GAWBS phase noise. It is clarified that a condition for effective GAWBS compensation is that the group delay between the signal and PT should be less than half the period of the GAWBS phase noise component.

Design, Acceptance and Capacity of Subsea Open Cables

This article will discuss the collaboratively formed cross-industry open cables concept for characterizing optical performance of undersea cables with the intent of assessing and understanding their capacity potential. The article proposes definitions of two critical nonlinear and linear performance metrics for open cables: GSNR (Gaussian or generalized signal to noise ratio) and SNR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ASE</sub> (Signal to noise ratio amplified spontaneous emission), including effects such as GAWBS (guided acoustic wave Brillouin scattering) and signal droop. Measurement methodologies for these metrics are proposed, with considerations for limitations and impact of the test conditions and characteristics of the transponders used. Expanded definitions are offered to enable variable symbol rate transponders to be used for measurement, with considerations for scaling of SNR values. Considerations for using these metrics for capacity assessment and applying these techniques to concatenated multi-segment systems are introduced. Recommendations on key parameters for system specification, system characterization, and proposals for SNR-based performance budgeting tables are also discussed as foundational elements to enabling accurate estimation of the capacity potential of a subsea open cable.

Precise measurements and their analysis of GAWBS-induced depolarization noise in various optical fibers for digital coherent transmission.

We undertake precise measurements of guided acoustic wave Brillouin scattering (GAWBS) depolarization noise caused by the TR2,m mode (torsional and radial mode) in various fibers and analyze the results. And we describe the influence of the noise on digital coherent transmission. We first show that the TR2,m mode is distributed over a wider bandwidth when the effective core area Aeff of the fiber is smaller. We then describe the strong mode-number dependence of the depolarization power generated from the profile of the refractive index change induced by the TR2,m mode. We also use two methods to measure the polarization crosstalk (XT) induced by the depolarization, namely, a direct detection method with a photodiode and a conventional power detection method with an optical spectrum analyzer. The results of the two methods agree well, and the XT increase is inversely proportional to the fiber Aeff and proportional to fiber length. Finally, we evaluate the influence of the GAWBS-induced XT on the BER characteristics in a coherent QAM transmission, where we find that the influence of the TR2,m mode is much weaker than that of the R0,m mode (radial mode). That is, the error-free transmission distance in standard single-mode fiber is extended to 8600 km for 256 QAM signal assuming hard-decision FEC with a 7% overhead. This distance is seven times longer than that obtained with the R0,m mode.

GAWBS phase noise characteristics in multi-core fibers for digital coherent transmission.

We describe the guided acoustic-wave Brillouin scattering (GAWBS) phase noise characteristics in multi-core fibers (MCFs) used for a digital coherent optical fiber transmission both experimentally and analytically. We first describe the GAWBS phase noise in an uncoupled four-core fiber with a 125 µm cladding and compare the phase noise spectrum with that of a standard single-mode fiber (SSMF). We found that, unlike SSMF where the R0,m mode is dominant, off-center cores in MCF are affected by higher-order TRn,m modes. We then report measurement results for GAWBS phase noise in a 19-core fiber with a 240 µm cladding. The results indicate that the cores exhibit different spectral profiles depending on their distance from the center of the fiber, but the amount of phase noise generated in each core is almost identical. These results provide a useful insight into the space division multiplexing transmission impairments in digital coherent transmissions using MCF.

Theoretical and experimental analyses of GAWBS phase noise in various optical fibers for digital coherent transmission.

We describe the fiber structural dependence of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. We present theoretical and experimental analyses of GAWBS phase noise spectra in three types of optical fibers and show that the GAWBS resonant modes are distributed over a wider bandwidth as the effective core area of the fiber becomes smaller. We also use a vector signal analysis to show phase fluctuations caused by GAWBS. On the basis of these analyses, we show that the GAWBS phase noise fluctuation has a Gaussian distribution, which is used to evaluate its influence on the BER characteristics in a coherent QAM transmission. As a result, we found that the error-free transmission distance in SSMF is limited to 4600, 1200, and 340 km with 64, 256, and 1024 QAM, respectively, assuming a hard-decision FEC with a 7% overhead. These results provide useful insights into the influence of GAWBS on digital coherent transmission.

Suppression of large error floor in 1024 QAM digital coherent transmission by compensating for GAWBS phase noise.

There is a large error floor in an ultra multi-level digital coherent transmission signal of 1024 QAM or higher, and we have yet to determine its origin. In this paper, we show that this large error floor results from guided acoustic-wave Brillouin scattering (GAWBS) phase noise. We prove experimentally that such an error floor can be greatly reduced by compensating for the GAWBS noise with a phase modulation technique. We show that the BER of a 1024 QAM signal was reduced from 8.7 × 10-4 to 2.7 × 10-4 after a 160 km transmission with GAWBS noise compensation. Furthermore, we successfully extend the transmission distance from 160 to 240 km with a 7% overhead forward error correction.

Simultaneous generation of guided-acoustic-wave Brillouin scattering and stimulated-Brillouin-scattering in hybrid As2Se3-PMMA microtapers.

We demonstrate simultaneous generation of Stimulated Brillouin Scattering (SBS) and guided-acoustic-wave Brillouin scattering (GAWBS) from electrostriction of optical waves in a 60cm As2Se3-PMMA microtaper waveguide. The GAWBS in the microtaper couples with SBS through a complex energy transfer between weak Stokes and Anti-Stokes (AS) continuous waves in the presence of a high power pulsed pump wave. This results in an amplification of Stokes wave at 7.4 GHz due to modulation of the optical fiber by GAWBS at 211 MHz generated by the pump in addition to a strong Stokes peak at 7.62 GHz and a secondary peak at 7.8 GHz that are contributed by SBS for a 2μm As2Se3 core radius. Such strong coupling of forward and backward Brillouin scattering due to large acoustic impedance between the core and cladding in such compact, highly nonlinear fibers plays a vital role in simultaneously sensing longitudinal and trasnverse strain within the core as well as its surroundings.

Experimental observation of spontaneous depolarized guided acoustic-wave Brillouin scattering in side cores of a multicore fiber

Spontaneous depolarized guided acoustic-wave Brillouin scattering (GAWBS) was experimentally observed in one of the side cores of an uncoated multicore fiber (MCF). The frequency bandwidth in the side core was up to ∼400 MHz, which is 0.5 times that in the central core. The GAWBS spectrum of the side core of the MCF included intrinsic peaks, which had different acoustic resonance frequencies from those of the central core. In addition, the spontaneous depolarized GAWBS in the central/side core was unaffected by that in the other core. These results will lead to the development of polarization/phase modulators using an MCF.

Observation of guided acoustic-wave Brillouin scattering noise and its compensation in digital coherent optical fiber transmission.

We describe the first observation of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. GAWBS noise, which is a forward lightwave generated by thermally excited vibration modes in a cylindrical fiber structure, occurs coherently not only in a signal at a single carrier frequency, but also in modulated wide-band optical signals. Since the signal-to-GAWBS-noise ratio is independent of signal power, it has caused problems in various fields including quantum optics. We point out that GAWBS noise exists even in a digital coherent transmission system such as quadrature amplitude modulation (QAM) and degrades the transmission performance since the phase noise is inevitably included within the bandwidth of the transmitted data. We propose two analogue and one digital method to compensate for the GAWBS noise and demonstrate improved performance in a QAM digital coherent transmission.

Observation of depolarized guided acoustic-wave Brillouin scattering in partially uncoated optical fibers

We observed the depolarized guided acoustic-wave Brillouin scattering (GAWBS) spectrum in a highly nonlinear fiber with a partially side-stripped polymer coat. The linewidth of the GAWBS spectral line at 941 MHz was measured to be 6.4 MHz, which was 0.9 times that of a coated fiber.

Temperature coefficient of sideband frequency produced by polarized guided acoustic-wave Brillouin scattering in highly nonlinear fibers

We measured the temperature dependence of the polarized guided acoustic-wave Brillouin scattering (GAWBS) spectrum using a highly nonlinear fiber. The temperature coefficient is 168 kHz/K, which is 1.7 times larger than that of small-core photonic crystal fibers. This result indicates that highly temperature-sensitive GAWBS-based sensing is feasible.

Experimental study on depolarized GAWBS spectrum for optomechanical sensing of liquids outside standard fibers.

We report an experimental study on the spectral dependence of depolarized guided acoustic-wave Brillouin scattering (GAWBS) in a silica single-mode fiber (SMF) on acoustic impedance of external materials. The GAWBS spectrum was measured when the acoustic impedance was changed from 1.51 to 2.00 kg/s·mm2. With increasing acoustic impedance, the linewidth increased; the dependence was almost linear with an acoustic impedance dependence coefficient of 0.16 MHz/kg/s·mm2. Meanwhile, with increasing acoustic impedance, the central frequency linearly decreased with an acoustic impedance dependence coefficient of -0.07 MHz/kg/s·mm2. These characteristics are potentially applicable to acoustic impedance sensing.

Observation of Backward Guided-Acoustic-Wave Brillouin Scattering in Optical Fibers Using Pump–Probe Technique

We experimentally demonstrate the observation of backward (i.e., opposite of the direction of the pump) guided-acoustic-wave Brillouin scattering (GAWBS) using the pump–probe technique and investigate the spectral dependence on the pump–probe light power. We show that the signal-to-noise ratio (SNR) of the backward GAWBS measurement can be drastically improved by using this technique. We also observe a nonmonotonic dependence of the SNR on the probe power, which indicates that optimal probe power exists for implementing GAWBS-based distributed sensors.

Depolarized guided acoustic wave Brillouin scattering in hollow-core photonic crystal fibers

By performing quantum-noise-limited optical heterodyne detection, we observe polarization noise in light after propagation through a hollow-core photonic crystal fiber (PCF). We compare the noise spectrum to the one of a standard fiber and find an increase of noise even though the light is mainly transmitted in air in a hollow-core PCF. Combined with our simulation of the acoustic vibrational modes in the hollow-core PCF, we are offering an explanation for the polarization noise with a variation of guided acoustic wave Brillouin scattering (GAWBS). Here, instead of modulating the strain in the fiber core as in a solid core fiber, the acoustic vibrations in hollow-core PCF influence the effective refractive index by modulating the geometry of the photonic crystal structure. This induces polarization noise in the light guided by the photonic crystal structure.

Guided acoustic-wave Brillouin scattering characteristics of few-mode fiber.

We experimentally investigate the modal dependence of the guided-acoustic wave Brillouin scattering (GAWBS) characteristics in a few-mode fiber (FMF). We clarify that launching higher-order modes into FMF induces different peak frequencies from those of single-mode fiber (SMF) in the GAWBS spectrum and that the GAWBS spectrum strongly depends on the mode excitation power ratio between the LP(01) and LP(11) modes. We evaluate the mode excitation power ratio at a splice between an FMF and an SMF by utilizing the GAWBS spectrum, which we realize by detecting only the fundamental mode at the output of the FMF and without the use of a mode demultiplexer.

Generation of squeezed vacuum pulses at 810 nm using a 40-cm-long optical fiber

We experimentally demonstrate the generation of a squeezed vacuum pulse at 810 nm with a fiber polarization interferometer. During femtosecond laser pulse propagation through an optical fiber in the normal dispersion regime, only self-phase modulation within a short length contributes to pulse squeezing since the laser pulse is immediately broadened. Guided acoustic-wave Brillouin scattering (GAWBS) noise that increases in proportional to the fiber length is also lower with shorter fibers. Consequently, a maximum noise reduction of 2.1 dB (4.8 dB when corrected for losses) is obtained using a 40-cm-long single mode optical fiber.