Polarization aberration of hyper-numerical-aperture projection optics should be measured accurately in order to control it exactly and ensure favorable imaging performance. A hybrid calibration method combining the Fourier analysis method and the eigenvalue calibration method is proposed. A wide-view-angle Mueller polarimeter (WMP) is exemplified to demonstrate the capability of the proposed calibration method, which can calibrate the polarimeter and determine the error budget of polarizing elements in the polarimeter. In addition, an experimental setup and a WMP are developed in-house to implement the hybrid calibration method.

A differential phase extracting method based on self-copy-shift for distributed acoustic sensing is proposed. Heterodyne and optical hybrids are used to realize high signal-to-noise ratio in-phase and quadrature-phase (IQ) signal measurement. The measured signals are self-copied and shifted for certain data points, and then they are digitally mixed with the original signals to construct the differential phase. The four produced signals are then combined to carry out IQ demodulation. An experiment with strain having an amplitude modulation waveform is carried out. The results showed that waveform information can be recovered well, and the signal-to-noise ratio achieves 32.8 dB.

A multi-channel synchronous demodulation system of a polarized low-coherence interferometer (PLCI) based on a matrix charge-coupled-device (CCD) is proposed and demonstrated. By using special designs, the system allows the signals from different channels to be received and demodulated synchronously. Multichannel air pressure experiments were implemented to verify the effectiveness of the proposed system. The experiment results showed that the Fabry–Perot (F–P) sensors could be demodulated synchronously with a high tolerance for light sources and sensors, which indicated that any sensor and light source that can be demodulated by PLCI were allowed to be employed, leading to a wide application in the field of multichannel synchronous measurement.

Until now, a high-efficiency demodulation method for fiber Bragg grating (FBG) sensors has been a challenge. In this Letter, by employing multi-peak FBGs, an FBG sensor with a partial wavelength scan is proposed and initially demonstrated. By demodulating a near-symmetrical multi-peak FBG and an asymmetrical multi-peak FBG in the strain experiment, sensor sensitivities of 1.02 pm/με and 1.01 pm/με are measured for the interrogation system, respectively. The average demodulation deviations for the two sensors are 1.81% and 0.4%, respectively. The proposed method is expected to realize high-efficiency and low-cost FBG interrogators.

An asymmetrical tapered singlemode–multimode–singlemode (SMS) fiber coupler based on two parallel physical contact SMS fiber structures was proposed. Since the coupler includes modes both from fiber core and cladding, two dips of the transmission spectrum exhibit different sensing characteristics to the surrounding temperature and refractive index (RI) change, which allows the use of the standard matrix inversion method to determine temperature and RI simultaneously. The temperature sensitivities of 0.0498 and 0.0324 nm/°C and RI sensitivities of 1151.76 and 1325.66 nm/RIU have been achieved, respectively. For biosensing application, with the functionalized fiber coupler sensor, a human chorionic gonadotropin concentration of 0.05 mIU/mL has been detected for a wavelength shift of 0.2 nm with good stability and excellent selectivity. The developed tapered SMS fiber coupler structure has advantages of simultaneous measurement of two independent parameters, simple configuration, low cost, and good repeatability that offer a great potential for medical diagnostics.

In order to achieve the accurate measurement of displacement, this Letter presents a self-mixing interference displacement measurement method suitable for the speckle effect. Because of the speckle effect, the amplitude of the self-mixing interference signal fluctuates greatly, which will affect the measurement accuracy of displacement. The ensemble empirical mode decomposition is used to process the interference signal, which can filter out high-frequency noise and low-frequency noise at the same time. The envelope of the self-mixing interference signal is extracted by Hilbert transform, and it is used to realize the normalization of the signal. Through a series of signal processing, the influence of speckle can be effectively reduced, and the self-mixing interference signal can be transformed into standard form. The displacement can be reconstructed by fringe counting and the interpolation method. The experimental results show that the method is successfully applied to the displacement measurement in the presence of speckle, which verifies the effectiveness and feasibility of the method.

A DC current sensor based on an optically pumped atomic magnetometer is proposed. It has a high linearity in a wide operation range, since the magnetometer measures the absolute magnitude of the magnetic field produced by the current to be measured. The current sensor exhibits a high accuracy with a non-moment solenoid and magnetic shielding to suppress the influence from the environment. The absolute error of the measured current is below 0.08 mA when the range is from 7.5 mA to 750 mA. The relative error is 5.54 × 10?5 at 750 mA.

Measuring the microscopic temperature of graphene is challenging. We used cholesteric liquid crystal microcapsules (CLCMs) as temperature sensors to detect the local temperature of three-dimensional porous graphene through quantitative visualization. Based on a CLCM (～20 μm in size), we determined the temperature variation in a small area with an accuracy of 0.1°C. By analyzing the color changes between two CLCMs, we demonstrated the temperature changes dynamically in a region with a diameter of approximately 110 μm. Furthermore, by comparing the color evolution among the three CLCMs, we visualized the anisotropic thermal properties in the micro-zone. This convenient and low-cost temperature measurement method is expected to further improve graphene-based devices.

We propose a new non-intrusive flow measurement method using the distributed feedback fiber laser (DFB-FL) as a sensor to monitor flow in the pipe. The relationship between the wavelength of the DFB-FL and the liquid flow rate in the pipeline is derived. Under the guidance of this theory, the design and test of the flow sensor is completed. The response curve is relatively flat in the frequency range of 10 Hz to 500 Hz, and the response of the flow sensor has high linearity. The flow from 0.6 m3/h to 25.5 m3/h is accurately measured under the energy analysis method in different frequency intervals. A minimum flow rate of 0.046 m/s is achieved. The experimental results demonstrate the feasibility of the new non-intrusive flow measurement method based on the DFB-FL and accurate measurement of small flow rates.

A novel phase-shifted long-period fiber grating (PS-LPFG) for the simultaneous measurement of torsion and temperature is described and experimentally demonstrated. The PS-LPFG is fabricated by inserting a pre-twisted structure into the long-period fiber grating (LPFG) written in single-mode fiber (SMF). Experimental results show that the torsion sensitivities of the two dips are ?0.114 nm/(rad/m) and ?0.069 nm/(rad/m) in the clockwise direction, and ?0.087 nm/(rad/m) and ?0.048 nm/(rad/m) in the counterclockwise direction, respectively. The temperature sensitivities of the two dips are 0.057 nm/°C and 0.051 nm/°C, respectively. The two dips of the PS-LPFG exhibit different responses to torsion and temperature. Simultaneous measurement of torsion and temperature can be implemented using a sensor. The feasibility and stabilization of simultaneous torsion and temperature measurement have been confirmed, and hence this novel PS-LPFG demonstrates potential for fiber sensing and engineering applications.

An all-reflective self-referenced spectral interferometry based on the transient grating (TG) effect is proposed for single-shot measuring of the amplitude and phase of ultrashort pulses in a broadband spectral range. Except for a thin third-order nonlinear medium, which was used to generate the TG signal, no transmitted optics were used in the proposed device, and few-cycle pulses in a broad spectral range from deep UV to mid-IR can be characterized. With a homemade compact and alignment-free device, a 5.0 fs pulse at 800 nm corresponding to about two cycles and a 14.3 fs pulse at 1800 nm corresponding to less than three cycles were successfully characterized.

SO2 and NO2 are the most important pollution in atmosphere. An optimized long path (LP) differential optical absorption spectroscopy (DOAS) system of high light intensity at an ultraviolet (UV) wavelength is proposed and used to measure the concentration of SO2 and NO2 simultaneously. In contrast to the traditional DOAS, the system adopted a Y-type optical fiber structure instead of a combination of mirrors in the telescope. The UV light intensity test shows that the light intensity of UV can arrive to above 80% of the max measuring range when the light path reaches 135 m, and the integral time of the spectrograph is only 15 ms. The system is proved to be efficacious through laboratory calibration. The maximum error of SO2 calibration is 4.19%, and is 5.22% for NO2. The error of the SO2 and NO2 mixture calibration is within 10%. Field measurement is implemented in a wastewater treatment plant in winter. The measurement light path is 738 m. The concentration of SO2 varies from 6 μg/m3 (2.26 ppb) to 20 μg/m3 (7.52 ppb), and the concentration of NO2 varies from 100 μg/m3 (53.2 ppb) to 200 μg/m3 (106.4 ppb) approximately. The results are in accordance with the data from a monitoring station nearby in magnitude order and variation tendency mostly.

We propose a novel modified frequency-shifted interferometer, where a Mach–Zehnder interferometer is added in order to obtain wavelength information. We use the Hilbert transform to extract the wavelength information from the phase of the interference pattern and construct the relationship between phase and wavelength. The laser wavelength measurement experiment is used to verify the compound interferometer. Experimental results demonstrated that our method could obtain the wavelength from the phase, which is of great significance for demodulation of the fiber Bragg grating based on a frequency-shifted interferometer.

This Letter presents a new type of optical fiber probe used to detect temperature, whose structure is very simple. The optical fiber probe is filled with cholesteric liquid crystals (CLCs) whose reflected light varies with temperature. The experimental results show that the proposed sensor can achieve a temperature sensitivity of 5.64 nm/°C in the temperature range of 18–40°C. The sensor has the advantages of simple structure, low cost, and easy mass manufacture. Its size is very tiny (the tapered structure, 125 μm in maximum diameter and <300 μm in length) and it is easy to integrate and measure. Meantime, the tapered structure of the probe is also ideal for measuring small samples such as cells and microfluidic channels, which will be a promising candidate for monitoring temperature fluctuations in small spaces.

In this Letter, a low-frequency acoustic sensor based on an extrinsic Fabry–Pérot (FP) interferometer with a silicon nitride (Si3N4) membrane is demonstrated. Using micromachining techniques, the 800 nm thick Si3N4 membrane is deposited on an 8 mm × 8 mm × 400 μm silicon (Si) substrate. All the assembly procedures of the sensor are focused on the substrate to avoid any damage to the membrane itself, compared to general membrane transfer methods. The frequency response of the proposed sensor is discussed theoretically and experimentally demonstrated. The sensor exhibits an excellent flat response to the tested acoustic frequency range of 1 Hz to 250 Hz. The phase sensitivity is around ?152 dB re 1 rad/μPa with sensitivity fluctuation less than 0.8 dB. The frequency response characteristic shows a promising potential of the sensor in low-frequency acoustic signal sensing applications.

In order to meet the practical needs of all-fiber conductivity-temperature-depth sensors with high sensitivity, compact structure, and easy packaging, this Letter uses a microfiber coupler combined with fiber loop (MCFL) reflective photonic device to conduct salinity, temperature, and deep sensing experiments. These MCFLs’ dynamic range and resolution of salinity, temperature, and depth can meet the requirements of actual marine environment monitoring. This structure opens up a new design idea for the practical research of microfiber coupler-based marine environmental parameter sensors.

Self-mixing interferometry (SMI) is an attractive sensing scheme that typically relies on mono-modal operation of an employed laser diode. However, change in laser modality can occur due to change in operating conditions. So, detection of occurrence of multi-modality in SMI signals is necessary to avoid erroneous metric measurements. Typically, processing of multi-modal SMI signals is a difficult task due to the diverse and complex nature of such signals. However, the proposed techniques can significantly ease this task by identifying the modal state of SMI signals with 100% success rate so that interferometric fringes can be correctly interpreted for metric sensing applications.