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Multiple optical scattering occurs when light propagates in a non-uniform medium. During the multiple scattering, images were distorted and the spatial information they carried became scrambled. However, the image information is not lost but presents in the form of speckle patterns (SPs). In this study, we built up an optical random scattering system based on an liquid crystal display (LCD) and an RGB laser source. We found that the image classification can be improved by the help of random scattering which is considered as a feedforward neural network to extracts features from image. Along with the ridge classification deployed on computer, we achieved excellent classification accuracy higher than 94%, for a variety of data sets covering medical, agricultural, environmental protection and other fields. In addition, the proposed optical scattering system has the advantages of high speed, low power consumption, and miniaturization, which is suitable for deploying in edge computing applications.

An interferometric sensor based on grapefruit photonic crystal fiber (PCF) is proposed and demonstrated for several physical parameter sensing. The PCF is sandwiched between two single-mode fibers (SMFs) through a short multimode fiber (MMF) at its both ends to form a Mach-Zehnder interferometer (MZI) with a total length of less than 2 mm. Combined with femtosecond laser micromachining and selectively filling technique, the micro-channels in the PCF are flexibly used for highly sensitive strain, gas refractive index/pressure, and temperature measurements. Moreover, device can also be used for hydrogen sensing with a high sensitivity of ~3.00 nm/% (vol. %) by coating Pt-loaded WO3 on its lateral surface. Meanwhile, the unique signal demodulation technique based on inverse Fourier transform is adopted to extract out desired information. The proposed device has promising potential in many physical parameter sensing, environmental monitoring, and hydrogen energy measurement owing to its high sensitivity, versatile sensing capability, and compact size.

Recovering the wavelengths from disordered speckle patterns has become an exciting prospect as a wavelength measurement method due to its high resolution and simple design. In previous studies, panel cameras were often utilized as the speckle images receiver. However, high cost (especially at near infrared range), large size and low speed limit its application in optical communications, metrology and optical sensing. In this work, the speckle patterns were highly compressed into four intensities by using a quadrant detector (QD). And wavelengths still can be recovered through only four pixels instead of millions of pixels of a camera. A new CNN based demodulation algorithm, shallow residual network (SRN), was proposed to recognize the wavelengths from the highly compressed speckle images. Finally, a wavelength precision of 4 fm (~ 0.5 MHz) with an updating speed of ~ 1 kHz was achieved in the demonstrations. In addition, the SRN shows a broad measurement range and good noise robustness. Compared with camera based system, the QD detection scheme associated with the CNN algorithm provides a compact, high speed, and low cost method to examine the speckle patterns, which opens new routes in many other fields.

Phase-insensitive amplifiers (PIAs), as a class of important quantum devices, have found significant applications in the subtle manipulation of multiple quantum correlation and multipartite quantum entanglement. Gain is a very important parameter for quantifying the performance of a PIA. Its absolute value can be defined as the ratio of the output light beam power to the input light beam power, while its estimation precision has not been extensively investigated yet. Therefore, in this work, we theoretically study the estimation precision from the vacuum two-mode squeezed state (TMSS), the estimation precision of the coherent state, and the bright TMSS scenario, which has the following two advantages: it has more probe photons than the vacuum TMSS and higher estimation precision than the coherent state. The advantage in terms of estimation precision of the bright TMSS compared with the coherent state is researched. We first simulate the effect of noise from another PIA with gain M on the estimation precision of the bright TMSS, and we find that a scheme in which the PIA is placed in the auxiliary light beam path is more robust than two other schemes. Then, a fictitious beam splitter with transmission T is used to simulate the noise effects of propagation loss and imperfect detection, and the results show that a scheme in which the fictitious beam splitter is placed before the original PIA in the probe light beam path is the most robust. Finally, optimal intensity difference measurement is confirmed to be an accessible experimental technique to saturate estimation precision of the bright TMSS. Therefore, our present study opens a new avenue for quantum metrology based on PIAs.

The in vivo detection of biomarkers in a liquid environment is very important for the early diagnosis of diseases. Spectroscopy methods are employed in ultraviolet-visible-infrared wavelengths, fluorescence or Raman spectra are detected for clinical diagnose. The dual-mode image can provide more diagnostic information and has been realized in some research work. However, there is still lacking simple and sensitive dual-mode sensors to satisfy the in vivo detecting demands. In this paper, a dual-mode fiber sensor for Surface-enhanced Raman Scattering (SERS) and fluorescence detection is proposed. The sensor is formed by a tapered optical fiber, half of the fiber tip surface is coated with Ag nanoparticles. In the detection of Rhodamine 6G (R6G) aqueous solution, the minimum detectable concentrations in SERS and fluorescence tests are of the same order of magnitude. By combining the Raman spectral features and the fluorescence intensity, the recognition and quantitation of target molecules were obtained reliably. It is the first time, to our knowledge, that the Raman-fluorescence dual-mode detection is realized in one single fiber, which was manufactured with micro-machinery techniques. It is a label-free, general-purpose fiber sensor, which can be applied for liquid biopsy, helping to diagnose and treat diseases in vivo.

In this paper, a fiber sensing system combined with fiber loop ring down (FLRD) technique to detect aflatoxin B1 (AFB1) in food is proposed. The single-mode fiber (SMF) taper was set as the sensor head, and the waist diameter of it was determined to be $17.1~\mu \text{m}$ by the refractive index (RI) measurement experiments. When a light pulse enters and circulates in the fiber loop, it experiences a little loss each time as it passes through the sensor head. The ring-down time of the system would be altered by intensity loss near the sensor head, which is related to the concentration of the AFB1 solution. The experimental results showed that the sensitivity of AFB1 was - $0.755~\mu $ s/( $\mu \text{g}$ /ml) in the concentration range of 0- $5~\mu $ g/ml with a precision of $0.016~\mu \text{g}$ /ml. Compared with other AFB1 sensors, the system has the advantages of simple structure, low cost, fast response and insensitive to light source fluctuation.

Fiber optical power splitters (OPSs) have been widely employed in optical communications, optical sensors, optical measurements, and optical fiber lasers. It has been found that OPSs with variable power ratios can simplify the structure and increase the flexibility of optical systems. In this study, a variable-fiber OPS based on a triangular prism is proposed and demonstrated. By adjusting the output beam width of the prism, the power ratio can be continuously tuned. The optical simulations show that the horizontal displacement design is better than the traditional tilt angle design. Our scheme combines a dual-fiber collimator, a focus lens, and a triangular prism with a vertex angle of 120°. By changing the axial displacement of the prism, the power splitting ratio can be altered from 50:50 to 90:10. The polarization and wavelength dependence of the variable OPS were also investigated.

Random speckles are proposed to demodulate Fabry–Perot (FP) sensors in this study. A piece of multimode fiber is used to interrogate the FP transmission spectrum, and tiny spectral changes lead to significant variations in the generated speckle patterns. In the demonstration experiments, the pressure resolution of 0.001 MPa can be obtained from an open cavity FP sensor based on the convolutional neural network (CNN) demodulation algorithm. It is worth noting that the spectral differences in neighboring orders can be precisely distinguished due to the high sensitivity of speckles. Thus, the fringe-order ambiguity problem is solved and the dynamic measurement range can be greatly improved. The speckle-based demodulation scheme provides a new way to balance resolution, dynamic range, speed, and cost of FP sensors.

A fiber microfluidic velocity sensor based on a bare fiber adapter (BFA) was proposed and demonstrated. Ceramic end face in BFA was covered by polyethylene (PE) film, in which the inserted single mode fiber (SMF) can be moved freely. The end face of the SMF and the PE film composed a Fabry-Perot (F-P) construction. Due to PE film sustaining different press with different microfluidic velocity, the F-P cavity length was changed. The velocity can be measured by monitoring the reflection spectrum. In our experiment, three cavity lengths were measured. The experimental results showed that the achieved maximum sensitivity to microfluidic velocity is 0.048 dB/(µL/min) or 0.0059 nm/(µL/min). Also, responses of PE film thickness and microfluidic concentration to the proposed sensor have been measured in the experiment. The sensor can be used for low velocity with high sensitivity, especially in micro-channels such as biological blood vessels.

A Mach–Zehnder interferometer constructed by femtosecond-laser written in-fiber waveguides in no-core fiber (NCF) is demonstrated. Such an interferometer device consists of a section of the NCF in which a waveguide has been inscribed, fusion spliced with single-mode fiber at its two ends. One of the interferometer arms is formed by the waveguide inscribed and the other follows the diverging light path inside the NCF before arriving at the end of single-mode fiber core. Such an internal structure-based device can be used not only for strain sensing but also for external refractive index measurement.

A compact and simple surface plasmon resonance sensor based on coreless fiber for refractive index measurement is developed. Compared with common SPR sensors, the proposed SPR sensor has a shorter length of 1 cm, avoids a taper and can be used to detect a wider range of 1.335–1.4019. The resonant wavelength shifts ∼200 nm to the longer wavelength in the refractive index range of 1.3335–1.4019. The average refractive sensitivity of 2836 nm/RIU is obtained in the range of 1.3335–1.4019, and the highest sensitivity can reach 4328.2 nm/RIU. Furthermore, our device has the advantages of small size, wide measuring range, excellent stability and short response time, which is appropriate for biological sensing.

A novel volatile organic compound (VOC) sensor based on poly (dimethylsiloxane) (PDMS) coated Fabry–Perot interferometer (FPI) with Vernier effect was experimentally demonstrated. The proposed sensor, total length is ~1 mm, is composed of a segment of large mode area fiber (LMAF) and a segment of hollow-core fiber (HCF). The HCF and the LMAF form two Fabry–Perot cavities and achieve the Vernier effect. The proposed sensor head was coated with PDMS film so it is not affected by humidity. The high sensitivity of the proposed VOC sensor will be achieved by monitoring the interference spectral envelope of the cascaded FPI due to the Vernier effect, even the VOC sensor is short. The experimental results show that the sensitivity of isopropanol is 20 pm/ppm with the range from 0 to 500 ppm and the response time is ~50 s. The sensor has high sensitivity, waterproofness, and gas selectivity.

A fiber optic isopropanol sensor based on polypyrrole-coated polarization maintaining fiber (PMF) is demonstrated. The sensor configuration is based on the combination of the Vernier effect of a single fiber Sagnac loop and the isopropanol sensitive material of polypyrrole, which swells in corresponding to the change of isopropanol concentration. And this swell effect can induce the strain effect on the fiber Sagnac loop. In this paper, the sensitivity of the proposed sensor exhibits ∼4.2 times (239 pm/ppm) higher than the conventional Sagnac interferometer (56.89 pm/ppm) from the range of 0 to 42 ppm with a satisfactory stability and linearity.

A novel fiber-optic surface plasmon resonance sensor for detection of Cu2+ ions in an aquatic environment is proposed and experimentally demonstrated. The proposed sensor is fabricated by splicing no-core fiber between multimode fibers and then functionalizing the sensor surface with gold film and chitosan (CS)/poly(acrylic acid) (PAA) bilayers. The gold film is used to stimulate surface plasmon waves and make the sensor sensitive to changes in refractive index induced by Cu2+ ions absorption on the CS/PAA bilayers. The proposed sensor has a linear relationship between the wavelength shifts and the concentrations of Cu2+ ions from 0.2 to 50 μ M and 50 to 500 μ M, respectively. It is also observed that the sensor's performance is better at low concentrations, and its sensitivity decreases with the increasing concentration. Moreover, we find the proposed sensor can express the self-assembly process between CS and PAA clearly, which shows us a new and easy way to investigate the polyelectrolyte self-assembling.

A liquid surface deformation sensor with high sensitivity based on a Mach–Zehnder interferometer (MZI) was demonstrated. The MZI was constructed by cascading two Bowknot Type Tapers (BTTs) with an ...

Fiber Bragg grating (FBG) demodulators based on tunable lasers have been extensively applied in many fields. However, the tunable lasers can hardly avoid wavelength nonlinear tuning. Such problems and unexpected wavelength fluctuations may cause serious measurement errors. In this study, we propose a wavelength calibration scheme to solve this problem by incorporating a magneto-optic wavemeter into the interrogation system. We present experimental verification of real-time wavelength calibration of tunable lasers and measurement corrections. In the temperature measurement demonstrations, the experimental results show that the FBG interrogation accuracy was raised from ±3 pm to ±1 pm, and better long-term stability was achieved, corresponding to the temperature accuracy of ±0.1 °C.

An optical hydrogen sensor based on PDMS-formed double-C type cavities with embedded Pt-loaded WO3/SiO2 has been proposed. The sensor is fabricated by fusion splicing a single-mode fiber (SMF) with a short segment of hollow-core fiber (HCF), the total length is only ∼123 μm. The HCF is separated into the double-C type cavities by a PDMS film, inner C-type cavity forms a micro-cavity Fabry-Perot interferometer (FPI) while outer C-type cavity with embedded Pt-loaded WO3/SiO2 powder serves as a sensitive area. When the sensor is exposed to air mixed with hydrogen, redox reactions result in an increase in ambient temperature leading to the length change of the micro-cavity due to the thermal expansion of the PDMS. The hydrogen sensitivity is −15.14 nm/% within the hydrogen concentration range of 0%–1%. The response time is short, ∼23 s. The sensor is compact, high sensitivity, fast response. Most of all, the novel double-C type structure effectively solves the problem that sensitive material falls off easily, so it can be long-term used and has great application potential.

We demonstrate a new type of whispering gallery mode microsphere resonator with microsphere-microsphere coupling. The device is based on two cascaded microspheres glued on a single mode fiber tip. The incident light from the fiber core diverges in the first microsphere and part of light is coupled to the surface of the second microsphere, exciting whispering gallery mode resonance when the angle between the connection line of the two microsphere centers and the fiber axis is appropriate. Such a whispering gallery mode microsphere resonator is robust in structure, compact in size, easy in fabrication and stable in operation.

A temperature sensor by using an alcohol-filled side-hole fiber (SHF) based Mach-Zehnder Interferometer (MZI) is proposed and experimentally demonstrated. The SHF-MZI is fabricated by splicing one section of alcohol-filled SHF between two single mode fibers (SMFs). The interference pattern between the core mode and the cladding modes excited at the first fusion splicing point shifts with temperature, due to the temperature-dependent refractive index of alcohol in the SHF, as well as thermo-optic effect and thermal expansion effect of the SHF. Temperature measurement with sensitivity of 105 pm/°C is therefore achieved by detecting resonant wavelength shift of the interference pattern. Meanwhile, the interference properties of the alcohol-filled SHF is investigated and discussed.

A rapid and label free aflatoxin

In this paper, a multipoint temperature measurement scheme based on Fabry–Perot interferometers (FPIs) multiplexing is proposed. The FPI sensor is constructed as a section of hollow-core fiber (HCF) partially filled with polydimethylsiloxane (PDMS) spliced to a single-mode fiber. An array-waveguide grating with 16 channels is used for the FPI sensors’ multiplexing and demultiplexing, and a broadband source is used as the light source. The corresponding theoretical model was built for analysis of the scheme, and the simulation results shown the FPI working principle can be simplified as a dual-beam interference. Two channels connected to two FPI sensors were experimentally tested for the concept verification. The temperature sensitivities of the proposed two sensors are 1.090 dB/°C and 1.210 dB/°C from 30°C to 40°C, respectively. There is no interchannel cross talk observed. Hence, FPI temperature sensors can work simultaneously in this structure, proving the validity of the multipoint temperature measurement concept.

In microfluidic chips applications, the monitoring of the rate and the direction of a microfluidic flow is very important. Here, we demonstrate a liquid flow rate and a direction sensor using a partially gold-coated tilted fiber Bragg grating (TFBG) as the sensing element. Wavelength shifts and amplitude changes of the TFBG transmission resonances in the near infrared reveal the direction of the liquid flowing along the fiber axis in the vicinity of the TFBG due to a nanoscale gold layer over part of the TFBG. For a device length of 10 mm (and a diameter of 125 µm for easy insertion into microfluidic channels), the flow rates and the direction can be detectable unequivocally. The TFBG waveguiding properties allow such devices to function in liquids with refractive indices ranging from 1.33 to about 1.40. In addition, the proposed sensor can be made inherently temperature-insensitive by referencing all wavelengths to the wavelength of the core mode resonance of the grating, which is isolated from the fiber surroundings.