Rayleigh, Brillouin and Raman scatterings in fibers result from the interaction of photons with local material characteristic features like density, temperature and strain. For example an acoustic/mechanical wave generates a dynamic density variation; such a variation may be affected by local temperature, strain, vibration and birefringence. By detecting changes in the amplitude, frequency and phase of light scattered along a fiber, one can realize a distributed fiber sensor for measuring localized temperature, strain, vibration and birefringence over lengths ranging from meters to one hundred kilometers. Such a measurement can be made in the time domain or frequency domain to resolve location information. With coherent detection of the scattered light one can observe changes in birefringence and beat length for fibers and devices. The progress on state of the art technology for sensing performance, in terms of spatial resolution and limitations on sensing length is reviewed. These distributed sensors can be used for disaster prevention in the civil structural monitoring of pipelines, bridges, dams and railroads. A sensor with centimeter spatial resolution and high precision measurement of temperature, strain, vibration and birefringence can find applications in aerospace smart structures, material processing, and the characterization of optical materials and devices.

Recent progress in distributed fiber optic sensors.

Optical Waveguide Theory

Robotic medical instrument systems and associated methods utilizing an optical fiber sensors such as Bragg sensor optical fibers. In one configuration, an optical fiber is coupled to an elongate instrument body and includes a fiber core having one or more Bragg gratings. A controller is configured to initiate various actions in response thereto. For example, a controller may generate and display a graphical representation of the instrument body and depict one or more position and/or orientation variables thereof, or adjust motors of an instrument driver to reposition the catheter or another instrument. Optical fibers having Bragg gratings may also be utilized with other system components including a plurality of working instruments that are positioned within a sheath lumen, an instrument driver, localization sensors, and/or an image capture device, and may also be coupled to a patient's body or associated structure that stabilizes the body.

Robotic instrument systems and methods utilizing optical fiber sensors

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

We present a polarization-sensitive optical coherence-domain reflectometer capable of characterizing the phase retardation between orthogonal linear polarization modes at each reflection point in a birefringent sample. The device is insensitive to the rotation of the sample in the plane perpendicular to ranging. Phase measurement accuracy is ±0.86°, but the reflectometer can distinguish local variations in birefringence as small as 0.05° with a distance resolution of 10.8 μm and a dynamic range of 90 dB. Birefringence-sensitive ranging in a wave plate, an electro-optic modulator, and a calf coronary artery is demonstrated.

Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging

A coherent multi-wavelength Brillouin random fiber laser with randomly distributed Rayleigh scattering feedback was demonstrated. Up to six orders Stokes random lasing radiations were generated with an unprecedented high optical signal-to-noise ratio of ∼47dB.

Multi-wavelength coherent Brillouin random fiber laser with high optical signal-to-noise ratio

Fiber-optic sensors fabricated by core-offset splicing method with 2 to 12 unequal-length single-mode fiber (Corning SMF-28) segments are demonstrated for the ultrasound detection. High-order modes are created in the air and silica cladding to enhance multi-mode interference, leading to large spectrum range comparing to that case with the same number of equal fiber segments. This leads to larger wavelength tuning range which overcomes the free-spectrum-range limitation in Fabry-Perot interferometer with multiple equal fiber segments. Furthermore, wavelength shift associated with power change, and characterized as high spectrum slope defined as 1st derivative of the reflection spectrum, is realized based on these unequal-length and asymmetric waveguides, which enhances the sensitivity of ultrasound detection. Using piezoelectric transducer as ultrasound generator at 260 kHz with 10 V peak-to-peak driving voltage, we detect ultrasound response based on the fiber devices with 2 to 12 segments as sensing unit or as feedback/sensor in a fiber laser. In the former sensor case, the ultrasound response via photo-detector could be increased from 15.5 mV for the core-offset fiber with 2 unequal-length segments to 115.3 mV for that with 12 segments, corresponding to a factor of 7.4 times improvement due to narrow linewidth of reflection spectrum in the fiber-optic sensor with more unequal-length fiber segments. The fiber laser sensor for the ultrasound detection brings improvement from 31.6 mV to 207.1 mV with the enhancement of 6.6 times, thanks to higher quality factor (Q) due to the optical filter formed by unequal-length core-offset fiber.

Fiber-Optic Sensor Based on Core-Offset Fused Unequal-Length Fiber Segments to Improve Ultrasound Detection Sensitivity

A novel polarization mode dispersion (PMD) emulator is presented which accurately follows the dynamics of PMD in field optical fiber. A modification on a dynamic mode coupling wave-plate model is presented to model the emulator. It is found that the emulator and model can accurately describe the dynamical behaviour of the state of polarization (SOP) and thus PMD in a fiber.

Novel dynamical polarization mode dispersion emulator

Random fiber gratings (RFGs) have shown great potential applications in fiber sensing and random fiber lasers. However, a quantitative relationship between the degree of randomness of the RFG and its spectral response has never been analyzed. In this paper, two RFGs with different degrees of randomness are first characterized experimentally by optical frequency domain reflectometry (OFDR). Experimental results show that the high degree of randomness leads to low backscattering strength of the grating and strong strength fluctuations in the spatial domain. The local spectral response of the grating exhibits multiple peaks and a large peak wavelength variation range when its degree of randomness is high. The linewidth of its fine spectrum structures shows scaling behavior with the grating length. In order to find a quantitative relationship between the degree of randomness and spectrum property of RFG, entropy was introduced to describe the degree of randomness induced by period variation of the sub-grating. Simulation results showed that the average reflectivity of the RFG in dB scale decreased linearly with increased sub-grating entropy, when the measured wavelength range was smaller than the peak wavelength variation range of the sub-grating. The peak reflectivity of the RFG was determined by κ2LΔP (where κ is the coupling coefficient, L is the grating length, ΔP is period variation range of the sub-grating) rather than κL when ΔP is larger than 8 nm in the spatial domain. The experimental results agree well with the simulation results, which helps to optimize the RFG manufacturing processes for future applications in random fiber lasers and sensors.

https://www.mdpi.com/1424-8220/20/21/6071/pdf

Random Fiber Grating Characterization Based on OFDR and Transfer Matrix Method.

A mechanism of Kerr-lens mode locking in fiber laser is proposed and analyzed. It could be realized in active photonic-crystal fibers with finite number of the air holes hexagonally arranged around the core with refractive index larger than refractive index of the glass matrix and relies at decrease of the mode confinement loss for higher intensity and mode transformation from leaky to guided when refractive index in the core increases owing to Kerr self-focusing.

Liang Chen

Papers

Multi-wavelength coherent Brillouin random fiber laser with high optical signal-to-noise ratio

Fiber-Optic Sensor Based on Core-Offset Fused Unequal-Length Fiber Segments to Improve Ultrasound Detection Sensitivity

Novel dynamical polarization mode dispersion emulator

Random Fiber Grating Characterization Based on OFDR and Transfer Matrix Method.

Feasibility of Kerr-lens mode locking in fiber lasers