OFDR shape sensor based on a femtosecond-laser-inscribed weak fiber Bragg grating array in a multicore fiber.

An optical frequency domain reflectometry (OFDR) shape sensor was demonstrated based on a femtosecond-laser-inscribed weak fiber Bragg grating (WFBG) array in a multicore fiber (MCF). A WFBG array consisting of 60 identical WFBGs was successfully inscribed in each core along a 60 cm long MCF using the femtosecond-laser point-by-point technology, where the length and space of each WFBG were 2 and 8 mm, respectively. The strain distribution of each core in two-dimensional (2D) and three-dimensional (3D) shape sensing was successfully demodulated using the traditional cross correlation algorithm, attributed to the accurate localization of each WFBG. The minimum reconstruction error per unit length of the 2D and 3D shape sensors has been improved to 1.08% and 1.07%, respectively, using the apparent curvature vector method based on the Bishop frame.

High-spatial-resolution φ-OFDR shape sensor based on multicore optical fiber with femtosecond-laser-induced permanent scatter arrays: publisher's note.

This publisher's note contains a correction to Opt. Lett.48, 3219 (2023)10.1364/OL.486644.

Ultra-linear broadband optical frequency sweep for a long-range and centimeter-spatial-resolution OFDR.

We demonstrated a long-range and centimeter-spatial-resolution optical frequency domain reflectometry (OFDR) system based on an ultra-linear broadband optical frequency sweep. The high nonlinear sweeping effect of the distributed feedback (DFB) diode laser was suppressed by a pre-distortion method, ensuring that the injection-locking process remained stable during fast tuning over a large span. An optical linear frequency sweep (LFS) with a sweep range and sweep rate of up to 60 GHz and 15 THz/s, respectively, was ultimately obtained by optimizing the injection-locking system. The high performance OFDR based on the proposed LFS achieved a sampling spatial resolution of 1.71 mm. Furthermore, distributed strain sensing was implemented with high-spatial resolutions of about 5 cm and 7 cm in the measurement range over 1 km and 2 km, respectively.

High-spatial-resolution φ-OFDR shape sensor based on multicore optical fiber with femtosecond-laser-induced permanent scatter arrays.

An optical fiber φ-OFDR shape sensor with a submillimeter spatial resolution of 200 µm was demonstrated by using femtosecond-laser-induced permanent scatter array (PS array) multicore fiber (MCF). A PS array was successfully inscribed in each slightly twisted core of the 400-mm-long MCF. The two-dimensional (2D) and three-dimensional (3D) shapes of the PS-array-inscribed MCF were successfully reconstructed by using PS-assisted φ-OFDR, vector projections, and the Bishop frame based on the PS-array-inscribed MCF. The minimum reconstruction error per unit length of the 2D and 3D shape sensor was 2.21% and 1.45%, respectively.

Wide-range OFDR strain sensor based on the femtosecond-laser-inscribed weak fiber Bragg grating array.

A wide-range OFDR strain sensor was demonstrated based on femtosecond-laser-inscribed weak fiber Bragg grating (WFBG) array in standard SMF. A WFBG array consisting of 110 identical WFBGs was successfully fabricated along a 56 cm-long SMF. Compared with SMF, the cross-correlation coefficient of WFBG array was improved to 0.9 under the strain of 10,000 µÎµ. The position deviation under the strain of 10,000 µÎµ, i.e., 2.5 mm, could be accurately obtained and compensated simply by using peak finding algorithm. The maximum measurable strain of single- and multi-point strain sensing was up to 10,000 µÎµ without using any additional algorithms, where the sensing spatial resolution was 5 mm.

Distributed Refractive Index Sensing Based on Etched Ge-Doped SMF in Optical Frequency Domain Reflectometry.

A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33-1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU.

Distributed high-temperature sensing based on optical frequency domain reflectometry with a standard single-mode fiber.

Distributed temperature sensing up to 600°C at a fiber length of 100.75 m based on optical frequency domain reflectometry (OFDR) was demonstrated using a standard single-mode fiber (SMF) without any treatment. The spatial resolution was 2.5 mm. An algorithm, instantaneous optical frequency resampling (IOFR), to eliminate the nonlinearity of the laser source was proposed and used to obtain calibrated reference and measurement signals that were used for temperature demodulation. Moreover, the temperature response stability of the annealed SMF was better than that of un-annealed SMF, where the temperature sensitivity was 1.96 GHz/°C at 600°C.

Submillimeter-spatial-resolution φ-OFDR strain sensor using femtosecond laser induced permanent scatters.

A φ-optical frequency domain reflectometry (OFDR) strain sensor with a submillimeter-spatial-resolution of 233 µm is demonstrated by using femtosecond laser induced permanent scatters (PSs) in a standard single-mode fiber (SMF). The PSs-inscribed SMF, i.e., strain sensor, with an interval of 233 µm exhibited a Rayleigh backscattering intensity (RBS) enhancement of 26 dB and insertion loss of 0.6 dB. A novel, to the best of our knowledge, method, i.e., PSs-assisted φ-OFDR, was proposed to demodulate the strain distribution based on the extracted phase difference of P- and S-polarized RBS signal. The maximum measurable strain was up to 1400 µÎµ at a spatial resolution of 233 µm.

High-quality fiber Bragg grating inscribed in ZBLAN fiber using femtosecond laser point-by-point technology.

We demonstrate for the first time, to the best of our knowledge, the fabrication of a high-quality fiber Bragg grating (FBG) in ZBLAN fiber by using an efficient femtosecond laser point-by-point technology. Two types of FBG, e.g., high coupling coefficient and narrow bandwidth grating, are successfully obtained. The coupling coefficient is strongly dependent on the grating order and pulse energy. A second-order FBG with an ultrahigh coupling coefficient of 325 m-1 and reflectivity of 97.8% is inscribed in the ZBLAN fiber. A pair of FBGs with a narrow FWHM of 0.30 and 0.09 nm are also demonstrated.

A Nondestructive Measurement Method of Optical Fiber Young's Modulus Based on OFDR.

A nondestructive measurement method based on an Optical frequency domain reflectometry (OFDR) was demonstrated to achieve Young's modulus of an optical fiber. Such a method can be used to measure, not only the averaged Young's modulus within the measured fiber length, but also Young's modulus distribution along the optical fiber axis. Moreover, the standard deviation of the measured Young's modulus is calculated to analyze the measurement error. Young's modulus distribution of the coated and uncoated single mode fiber (SMF) samples was successfully measured along the optical fiber axis. The average Young's modulus of the coated and uncoated SMF samples was 13.75 ± 0.14, and 71.63 ± 0.43 Gpa, respectively, within the measured fiber length of 500 mm. The measured Young's modulus distribution along the optical fiber axis could be used to analyze the damage degree of the fiber, which is very useful to nondestructively estimate the service life of optical fiber sensors immersed into smart engineer structures.

CT Diagnosis of the Fracture of Anterior Nasal Spine.

OBJECTIVE: To study anterior nasal spine fractures, including the incidence, missed diagnosis rates, and relationship with shapes using computed tomography (CT). METHODS: Two hundred cases of axial CT images performed for maxillofacial trauma were reviewed. The incidence, correct, and missed diagnosis rates of anterior nasal spine fractures were studied. The relationship between the fracture and the shape of the anterior nasal spine was also analyzed. RESULTS: The rate of anterior nasal spine fractures was 22.00% (44 of 200). The diagnostic accuracy was 4.55% (2 of 44) and the missed diagnosis rate was 95.45% (42 of 44). The fracture rates of the double rod, single rod, triangle, and irregular anterior nasal spine were 33.85% (22 of 65), 32.26% (10 of 31), 12.24% (12 of 98), and 0.00% (0 of 6), respectively. The double and single rod types of anterior nasal spine were most likely to be fractured than the type of triangle (χ2 = 11.05, 6.67, P < .0167). No fracture was found in the irregular type of anterior nasal spine. CONCLUSION: Anterior nasal spine fractures are not rare and the high missed diagnostic rate results from unfamiliarity with the structure. Double and single rod types of anterior nasal spines are easy to fracture. Bony reconstruction and thin thickness of CT images are necessary for diagnosis.

Quasi-Distributed Temperature and Strain Sensors Based on Series-Integrated Fiber Bragg Gratings.

Two types of series-integrated fiber Bragg gratings (SI-FBGs), i.e., strong and weak SI-FBGs, were inscribed in a standard single-mode fiber (SMF) using the femtosecond laser point-by-point technology. In the SI-FBGs inscribing system, the grating pitch of each FBG and the distance between the two adjacent FBGs in the SI-FBGs can be flexibly controlled by adjusting the inscription parameters. The strong SI-FBGs with different grating pitches and the weak SI-FBGs with an identical grating pitch were employed to successfully measure the temperature distribution in a tube furnace and the strain distribution on a cantilever beam, respectively. A high spatial resolution of less than 1 mm was achieved during the distributed temperature sensing experiment. Moreover, the spatial resolution could be improved by decreasing the distance between the two adjacent FBGs, i.e., decreasing the FBG length and the space between the two adjacent FBGs. Hence, the inscribed high-quality SI-FBGs have great potential to be developed as various quasi-distributed sensors with a high spatial resolution.