In industry and architecture, real-time and complete measurement of structures has become a research frontier since it is directly related to the safety of life and property. Optical fibers have the potential to monitor constructions due to advantages such as small size, anti-electromagnetic interference, etc. However, existing research on optical fiber sensors pays attention to the sensing fiber itself and ignores the layout of the optical fibers. As a result, currently available fiber sensors are only suitable for monitoring one-dimensional rigid structures such as surgical robots and puncture needles. They may face challenges when measuring the freeform surfaces encountered in industrial and architectural scenarios. In this paper, a cross-orthogonal measurement method based on optical fiber shape sensors is proposed to meet the shape measurement requirements of freeform surfaces. The cross-orthogonal network allows global measurement and performs surface reconstructions more easily as compared to single-fiber arrays. It also divides the sensing fiber into several segments that are reconstructed separately (with different initial frames), avoiding the cumulative error at the end of the sensing fiber. Results of verification experiments using four-core fibers indicate that the presented method can improve measurement accuracy.

With the developments of the tunable laser source (TLS), there are increasing demands for high-resolution dynamic wavelength calibration in recent years. Considering mutual constraints between wide measurement range and high calibration resolution, we propose a dynamic wavelength calibration method based on an auxiliary Mach-Zehnder interferometer (MZI) and the synchrosqueezed wavelet transform (SSWT). Our proposed method can achieve a calibration resolution of 5 fm and a tuning range of 10 nm. Moreover, the measurement range and spatial resolution of the optical frequency domain reflectometer (OFDR) system are improved to ∼80 m and ∼mm, respectively. Our proposed approach can substantially reduce the subtle spectrum distortion (tens of fm) in coherent optical spectrum analyzer (COSA) systems.

Optical fiber measurement technology is widely used in the strength testing of buildings, the health testing of industrial equipment, and the minimally invasive surgery of modern medical treatment due to its characteristics of free calibration, high precision, and small size. This paper presents an algorithm that can improve the range and stability of strain measurements in order to solve the problems of the small range and measurement failure of optical fiber strain sensors based on optical frequency-domain reflectometry (OFDR). Firstly, a Rayleigh scattering model based on the refractive index perturbation of an optical fiber is proposed to study the characteristics of Rayleigh scattering and to guide the strain demodulation algorithm based on the spectral shift. Secondly, a local similar scanning method that can maintain a high similarity by monitoring local Rayleigh scattering signals (LSs) before and after strain is proposed. Thirdly, a generalized cross-correlation algorithm is proposed to detect spectral offset, solving the problem of demodulation failure in the case of a Rayleigh scattering signal with a low signal-to-noise ratio. Experiments show that the proposed method still has high stability when the spatial resolution is 3 mm. The measurement precision is 6.2 με, which proves that the multi-peaks or pseudo-peaks of the traditional algorithm in the case of a large strain, the high spatial resolution, and the poor signal-to-noise ratio are solved, and the stability of the strain measurement process is improved.

The laser scanning interferometry system has been successfully applied to many measurement fields because of its efficient measurement ability. However, the practical application ability of this measurement method is restricted due to the laser tuning nonlinearity. In this paper, the fiber ring resonator is equidistant in the frequency domain, which is used as the external clock signal to resample the main interference signal so as to realize the equifrequency sampling of the laser scanning interference system and correct the tuning nonlinearity. The final experimental result shows that this method can effectively reduce the phase noise caused by tuning nonlinearity and improve the performance of the system.

Optical spectral analysis is essential to demonstrate the light-matter interactions from the frequency domain. Although the existing coherent spectrum analyzers (COSA) have already achieved resolutions of ∼50 fm, the mirror effect limits the further improvement in spectral resolution and thus the detection of low-concentration, small-scale analytes. Focusing on this problem, we first formulate the resolution degradation of COSA caused by the mirror effect, indicating the tuning nonlinearity of the local oscillator could introduce a non-stationary disturbance to the spectral data sampled at equal time intervals. Subsequent theoretical and experimental works have proved that the combination of dynamic wavelength calibration and spectral deconvolution can suppress the non-stationary disturbance. And the spectral resolution could reach 8 fm without extra compressions. This deconvolution-based scheme also avoids the long fibers used in scattering compressed schemes, improving the system's environmental stability.

This paper addresses the issue of reduced measurement stability in low-coherence measurement techniques when applied to glass-based dispersive media. A high-precision, low-coherence interferometric measurement method is proposed based on synchrosqueezed Wavelet Transform (SSWT). First, the Schott dispersion formula is used to analyze the impact of glass-based dispersive media on the phase of the interference spectrum. Wavelet transform is then employed to extract the chirp information of the interference spectrum, enabling the separation of optical path lengths for different wavenumbers. Building upon this, SSWT is introduced to enhance time-frequency resolution further, improving the measurement performance of the system. An experimental setup is constructed to validate the effectiveness of the proposed method. Compared to traditional methods, SSWT optimizes the distribution of wavelet coefficients, concentrating signal energy and significantly improving instantaneous frequency capture accuracy. Experimental results show that, in addition to effectively capturing chirp characteristics and reducing phase noise, SSWT achieves a 3-fold and 7-fold improvement in peak full-width at half-maximum compared to conventional Fourier Transform (FT) and Continuous Wavelet Transform (CWT), respectively, and a 9.1-fold and 17.1-fold improvement in standard deviation over 20 measurements, demonstrating superior noise resistance and measurement precision. In conclusion, the SSWT-based white-light interferometric measurement method provides a high-precision, reliable solution for dimensional measurements in industrial applications.