Optical Correlation-domain Reflectometry Research Articles

Modified Expression for Spatial Resolution in Optical Correlation-Domain Reflectometry

Modified Expression for Spatial Resolution in Optical Correlation-Domain Reflectometry

High-speed high-resolution optical correlation-domain reflectometry without using electrical spectrum analyzer

We propose a newly configured optical correlation-domain reflectometry (OCDR) without the use of an electrical spectrum analyzer (ESA) for high-speed distributed reflectivity measurement with an enhanced spatial resolution. First, the operation of ESA-free OCDR is analyzed by numerical simulation, the results of which show the potential of the configuration to perform distributed reflectivity measurement with an improved spatial resolution. Subsequently, the simulation results are verified by comparison experiments, where distributed reflectivity measurements along a 32-m-long single-mode fiber joint with multiple connectors are performed using both conventional (with averaging) and ESA-free OCDR configurations (without averaging) at a repetition rate of 100 Hz. The widths of the reflectivity peaks experimentally obtained by the conventional and ESA-free OCDR configurations are 22.9 cm and 7.0 cm, respectively. Finally, we experimentally evaluated the performance of the ESA-free OCDR at higher repetition rates, achieving 6.3-cm spatial resolution at a repetition rate of 1 kHz. This result indicates that the ESA-free OCDR can perform a distributed reflectivity measurement with an improved spatial resolution at a higher operation speed than ever.

Compensation of noise floor distortion in long-range simplified optical correlation-domain reflectometry without the use of electrical spectrum analyzer

In simplified optical correlation-domain reflectometry (OCDR) without the use of an electrical spectrum analyzer (ESA), the noise floor of the reflectivity distribution is distorted in long-range measurement, which sometimes causes significant errors in the distributed reflectivity measurement. Herein, we experimentally analyze the distorted noise floor by high-speed ESA-free OCDR and clarify the origin of the noise floor. Then we show that this noise floor can be compensated by appropriate filtering. Consequently, distributed reflectivity measurement over a 1.1 km long sensing fiber is properly performed at a repetition rate of 100 Hz.

Super-simplified optical correlation-domain reflectometry

Optical correlation-domain reflectometry (OCDR), which is known as one of the fiber-optic techniques for distributed reflectivity sensing, conventionally included an acousto-optic modulator, a reference path, and erbium-doped fiber amplifiers in its setup. In this work, by removing all of these components simultaneously, we develop a super-simplified configuration of OCDR, which consists of a light source and a photodetector only. We experimentally show that this system can still perform distributed reflectivity sensing with a moderate signal-to-noise ratio, which will boost the portability and cost efficiency of the OCDR technology.

Proposal of external modulation scheme for fiber-optic correlation-domain distributed sensing

All the previous configurations of optical correlation-domain reflectometry (OCDR) have been based on the direct modulation of laser driving current. However, some lasers are not designed for high-speed large-amplitude modulation, and the modulation amplitude and frequency are mutually dependent. In this work, to mitigate these drawbacks, we propose an external modulation scheme for OCDR. After confirming its basic operation, we show that a non-specially designed laser can be employed. We also clarify that the interdependence of the modulation amplitude and frequency is negligibly small in this scheme, which is not the case for the conventional direct modulation scheme.

Brillouin Optical Correlation-Domain Technologies Based on Synthesis of Optical Coherence Function as Fiber Optic Nerve Systems for Structural Health Monitoring

Brillouin optical correlation-domain technologies are reviewed as “fiber optic nerve systems” for the health monitoring of large structures such as buildings, bridges, and aircraft bodies. The Brillouin scattering property is used as a sensing mechanism for strain and/or temperature. Continuous lightwaves are used in the technologies, and their optical coherence properties are synthesized to realize position-selective measurement. This coherence manipulation technology is called the “synthesis of optical coherence function (SOCF)”. By utilizing SOCF technologies, stimulated Brillouin scattering is generated position-selectively along the fiber, which is named “Brillouin optical correlation domain analysis (BOCDA)”. Spontaneous Brillouin scattering, which takes place at any portion along the fiber, can also be measured position-selectively by the SOCF technology. This is called “Brillouin optical correlation domain reflectometry (BOCDR)”. When we use pulsed lightwaves that have the position information, sensing performances, such as the spatial resolution, are inherently restricted due to the Brillouin scattering nature. However, in the correlation-domain technologies, such difficulties can be reduced. Superior performances have been demonstrated as distribution-sensing mechanisms, such as a 1.6-mm high spatial resolution, a fast measurement speed of 5000 points/s, and a 7000-με strain dynamic range, individually. The total performance of the technologies is also discussed in this paper. A significant feature of the technologies is their random accessibility to discrete multiple points that are selected arbitrarily along the fiber, which is not realized by the time domain pulsed-lightwave technologies. Discriminative and distributed strain/temperature measurements have also been realized using both the BOCDA technology and Brillouin dynamic grating (BDG) phenomenon, which are associated with the stimulated Brillouin scattering process. In this paper, the principles, functions, and applications of the SOCF, BOCDA, BOCDR, and BDG-BOCDA systems are reviewed, and their historical aspects are also discussed.

偏波ダイバーシティ技術と時間ゲート法を用いたブリルアン光相関領域反射計測法による距離21 kmでの歪み分布測定

We develop a Brillouin optical correlation domain reflectometry (BOCDR) system using polarization diversity method and temporal gating scheme. Polarization diversity method is used for detecting backscattered light with rotated polarization from single mode fiber (SMF), and temporal gating scheme is used for expanding the measurement range. A section with 3 m length and 6000 µε strain was experimentally measured at a distance of 21 km SMF with 20 cm nominal spatial resolution. Our result shows the figure of merit NR, which is a ratio between the measurement range and the spatial resolution, reaches to 105965.

Impact of Brillouin amplification on the spatial resolution of noise-correlated Brillouin optical reflectometry

To obtain high spatial resolution over a long sensing distance in Brillouin optical correlation domain reflectometry (BOCDR), a broad laser spectrum and high pump power are used to improve the signal-to-noise ratio (SNR). In this Letter, we use a noise-modulated laser to study the variation of the Brillouin spectrum bandwidth and its impact on the coherent length of BOCDR quantitatively. The result shows that the best spatial resolution (lowest coherent length) is achieved by the lowest pump power with the highest noise-modulation spectrum. Temperature-induced changes in the Brillouin frequency shift along a 253.1 m fiber are demonstrated with a 19 cm spatial resolution.

Optical Correlation Domain Reflectometry Based on Coherence Synchronization: Theoretical Analysis and Proof-of-Concept

We describe a novel optical correlation domain reflectometry technique based on coherence synchronization between probe and local lights. Coherence synchronization is realized by using artificially generated low coherence lights, which are controlled by arbitrary waveform generators. Thanks to the flexibility of the electronics, we can easily design the coherence characteristics of the artificially generated low coherence lights to yield a correlation gating that filters an autocorrelation of an original laser at an arbitrary delay. The correlation gating realized by coherence synchronization is effective as long as the measurement length is less than the coherence length of the original laser. We demonstrate the measurement of the Fresnel reflection at the end of an optical fiber by using coherence-synchronized optical frequency combs that have only one correlation peak in the fiber under test (FUT). The FUT is 100 m long, and the repetition rate of the optical frequency comb generated by cascaded electro-optic modulators is 9.5 GHz, which corresponds to a 10.5 mm spacing between the periodic correlation peaks. The proposed technique removes the restriction imposed by the periodic correlation peaks present in correlation-based measurement using an optical frequency comb.

Brillouin optical correlation domain reflectometry with lock-in detection scheme

We propose a Brillouin optical correlation domain reflectometry (BOCDR) technique with a lock-in detection scheme in this paper. By designing a new system using the lock-in detection scheme and amplifying a small spontaneous Brillouin signal with a lock-in amplifier, a Brillouin scattering spectrum with a stable shape is obtained. By further introducing a periodical on/off phase modulation for chopping for lock-in detection, the undesired optical background spectrum is effectively reduced, and a 20 cm section with 7,000 µε strain is clearly measured.

Simplified optical correlation-domain reflectometry without reference path

We develop a simplified configuration for optical correlation-domain reflectometry (OCDR) without an explicit reference path. Instead, the Fresnel-reflected light generated at the distal open end of the sensing fiber is exploited as a reference light. After the fundamental demonstration, the optimal incident power is found to be approximately 8dBm. We also show that the loss near the distal end should not be applied, unlike in the case of Brillouin-based OCDR.

Fiber distributed Brillouin sensing with optical correlation domain techniques

Fiber distributed Brillouin sensing is discussed, focusing mainly on optical correlation domain techniques. By synthesizing a delta-function like optical coherence function between pump and probe lightwave traveling along an optical fiber in opposite directions, which is realized by modulating laser source frequency by an appropriate waveform, stimulated Brillouin scattering can be selectively excited at one specific position along the fiber. The selected position can easily be swept by changing the modulation frequency, so the distributed measurement can be achieved. In the system, Brillouin Optical Correlation Domain Analysis (BOCDA), the position to be measured can be selected randomly along the fiber, which is a special feature of the system. Spatial resolution of 1.6mm and measurement speed of 1000samples/s have already been demonstrated. With a similar way, distribution of spontaneous Brillouin scattering can also be measured along the fiber. Spatial resolution of 10mm and measurement speed of 50samples/s have already been demonstrated in the system, Brillouin Optical Correlation Domain Reflectometry (BOCDR). Brillouin dynamic grating (BDG), which is acoustic-wave generated refractive-index grating caused in the stimulated Brillouin scattering process, was found to cause a Bragg reflection for the orthogonally polarized lightwave in an polarization maintaining fiber. By measuring both the BDG and the Brillouin scattering, discriminative distributed measurement of strain and temperature has been realized by the BOCDA scheme with a 10mm spatial resolution.