Speckle wavemeter based on a multi-core fiber and compressive imaging.

Random speckle patterns contain valuable information about the incident light. Researchers have successfully constructed spectrometers and wavemeters by utilizing the speckles generated by inter-mode interferences of a multimode fiber (MMF). However, cameras were often employed to record the speckle data in previous reports. The camera's high cost (especially in the near-infrared range), large size, and low response speed limit the applications in optical communications, metrology, and optical sensing. A seven-core fiber (SCF) was fused with an MMF to capture the speckle pattern, where each core coupled part of the speckle field. Furthermore, we take advantage of the space division multiplexing capability of the SCF by incorporating an optical switch. This allows the variety of speckles generated by the incidence of different cores into the MMF. A convolutional neural network (CNN) regression algorithm was designed to analyze the complicated speckle data. The experimental results show that the proposed wavemeter can resolve adjacent wavelengths of 1 pm with an error of about 0.2 pm. We also discussed how different lengths of MMF influence the wavelength resolution. In conclusion, our research presents a robust and cost-effective approach to a wavelength measurement device by use of a seven-core optical fiber.

Machine learning enabled self-calibration single fiber endoscopic imaging.

Single fiber scanners (SFSs), with the advantages of compact size, versatility, large field of view, and high resolution, have been applied in many areas. However, image distortions persistently impair the imaging quality of the SFS, although many efforts have been made to address the problem. In this Letter, we propose a simple and complete solution by combining the piezoelectric (PZT) self-induction sensor and machine learning algorithms. The PZT tube was utilized as both the actuator and the fiber position sensor. Additionally, the feedback sensor signal was interrogated by a convolution neural network to eliminate the noise. The experimental results show that the predicted fiber trajectory error was below 0.1%. Moreover, this self-calibration SFS has an excellent robustness to temperature changes (20-50°C). It is believed that the proposed solution has removed the biggest barrier for the SFS and greatly improved its performance and stability in complex environments.

Deep-learning-assisted fiber Bragg grating interrogation by random speckles.

Fiber Bragg gratings (FBGs) have been widely employed as a sensor for temperature, vibration, strain, etc. measurements. However, extant methods for FBG interrogation still face challenges in the aspects of sensitivity, measurement speed, and cost. In this Letter, we introduced random speckles as the FBG's reflection spectrum information carrier for demodulation. Instead of the commonly used InGaAs cameras, a quadrant detector (QD) was first utilized to record the speckle patterns in the experiments. Although the speckle images were severely compressed into four channel signals by the QD, the spectral features of the FBGs can still be precisely extracted with the assistance of a deep convolution neural network (CNN). The temperature and vibration experiments were demonstrated with a resolution of 1.2 pm. These results show that the new, to the best of our knowledge, speckle-based demodulation scheme can satisfy the requirements of both high-resolution and high-speed measurements, which should pave a new way for the optical fiber sensors.

Rapid and label free detection of aflatoxin B1 in alcoholic beverages with a microfluid fiber device.

A rapid and label free aflatoxin B1 (AFB1) microfluid sensor was proposed and tested. The device was fabricated with hollow-core photonics crystal fiber infiltrated with the AFB1 solution. The autofluorescence emitting from the AFB1 molecules was detected. The sensor length was optimized. The AFB1 concentration was tested with a 4 cm long sensor. The best limit of detection was achieved as low as 1.34 ng/ml, which meets the test requirement of the national standards for AFB1 in food. The effectiveness of this sensor being applied in beer solution was also verified to be a little more sensitive than in aqueous solution. Compared with traditional AFB1 detection methods, the proposed single-ended device perfectly satisfies the demand of process control in alcoholic beverages manufacture.

Variable-fiber optical power splitter design based on a triangular prism.

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.

Study of a fiber spectrometer based on offset fusion.

A near-infrared spectrometer based on offset fused multimode fiber (MMF) is investigated in this study. The light spectrum is recovered by analyzing the speckle images when light is passing through the MMF. In order to generate adequate speckles, a polarization maintaining fiber (PMF) and a 30 cm long MMF are fused with a vertical offset. Seven different offset displacements are implemented in the fiber fusion. The follow-up experiments show that the fiber offset fusion has a significant influence on the spectral correlation and the resolution. Larger offset fusion can excite more high-order modes in the MMF, and it greatly improves the spectrometer's performance. The simulation results also show that more modes are excited in MMF, and the increase of mode number leads to lower correlation coefficients of the neighboring spectral channels. However, large offset fusion increases the fusion and the insertion loss of the whole system, which may bring difficulties in the low-light cases. In addition, an image denoising algorithm based on dynamic threshold filtering and a spectral reconstruction algorithm originated from complete orthogonal decomposition were used to remove the speckle pattern noise and recover the spectrum. The final speckle-based spectrometer has a spectral resolution of 0.6∼0.016nm depending on the different offset fusions.

PDMS-filled Fabry-Perot interferometer-based multipoint temperature measurement using an array-waveguide grating.

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.

Highly sensitive PDMS-filled Fabry-Perot interferometer temperature sensor based on the Vernier effect.

A highly sensitive temperature sensor is demonstrated experimentally, which is fabricated based on a Fabry-Perot interferometer (FPI) filled with polydimethylsiloxane (PDMS). The sensor's sensitivity is -0.653 nm/°C by utilizing the thermal expansion effect of PDMS, which has been greatly improved compared to that of the traditional FPI temperature sensor. Moreover, in order to further improve the sensitivity, a scheme where two parallel FPI structures are used to form the Vernier effect is proposed, which are a sensing FPI and reference FPI, respectively. Such a temperature sensor based on the FPI filled with PDMS and the Vernier effect exhibits a high temperature sensitivity of 17.758 nm/°C. Meanwhile, the proposed sensors show the advantages of high sensitivity, simplicity, and low cost.

Nonlinear impairment compensation using expectation maximization for dispersion managed and unmanaged PDM 16-QAM transmission.

In this paper, we show numerically and experimentally that expectation maximization (EM) algorithm is a powerful tool in combating system impairments such as fibre nonlinearities, inphase and quadrature (I/Q) modulator imperfections and laser linewidth. The EM algorithm is an iterative algorithm that can be used to compensate for the impairments which have an imprint on a signal constellation, i.e. rotation and distortion of the constellation points. The EM is especially effective for combating non-linear phase noise (NLPN). It is because NLPN severely distorts the signal constellation and this can be tracked by the EM. The gain in the nonlinear system tolerance for the system under consideration is shown to be dependent on the transmission scenario. We show experimentally that for a dispersion managed polarization multiplexed 16-QAM system at 14 Gbaud a gain in the nonlinear system tolerance of up to 3 dB can be obtained. For, a dispersion unmanaged system this gain reduces to 0.5 dB.

Core-shell NaErF4@NaYF4 upconversion nanoparticles qualify as a NIR speckle wavemeter for a visible CCD.

Speckle patterns have been widely employed as a method for precisely determining the wavelength of monochromatic light. In order to achieve higher wavelength precision, a variety of optical diffusing waveguides have been investigated with a focus on their wavelength sensitivity. However, it has been a challenge to find a balance among the cost, compactness, precision, and stability of the waveguide. In this work, we designed a compact cylindrical random scattering waveguide (CRSW) as the light diffuser by mixing TiO2 particles and ultra-violate adhesive. In the CRSW, speckle patterns are generated by input light scattering off TiO2 particles multiple times. Additionally, a thin layer of upconversion nanoparticles (UCNPs) was sprayed on the end face of CRSW to allow near-infrared (NIR) light to be converted to visible light, breaking the imaging limitations of visible cameras in the NIR range. We, then, further designed a convolution neural network (CNN) to recognize the wavelength of the speckle patterns with excellent robustness and ability to transfer learning. This resulted in the achievement of a high wavelength precision of 20 kHz (∼0.16 fm) at around 1550 nm with a temperature resistance of ±2 °C. Our results demonstrate a low-cost, compact, and simple NIR wavemeter, which is capable of ultra-high wavelength precision and good temperature stability. It has significant value for applications in high-speed and high-precision laser wavelength measurements.

High-sensitivity hydraulic pressure sensor based on Fabry-Perot interferometer filled with polydimethylsiloxane film.

A high-sensitivity hydraulic pressure sensor is proposed, which consists of a Fabry-Perot interferometer (FPI) filled with a polymer film of polydimethylsiloxane (PDMS). The FPI structure is fabricated by splicing a section of hollow core fiber (HCF) to the end-face of a lead-in single mode fiber (SMF). Then, the PDMS is filled into the HCF which acts as a light reflector and a diaphragm to detect external pressure variation. As a result, the length of the FPI cavity and the thickness of the PDMS are 137.8 µm and 33.8 µm, respectively. Experimental results indicate that the sensor's wavelength exhibits a linear response to the hydraulic pressure, which function is described as y = -7.35 × 10-3x + 1536.395. Here, x and y represent the hydraulic pressure and the wavelength, respectively. The pressure sensitivity is up to -7.35 nm/kPa. Besides, a temperature compensation method based on a fiber Bragg grating is proposed to eliminate the influence of temperature. Experiments show that the scheme can effectively eliminate the influence of temperature and achieve accurate measurement of hydraulic pressure.

Kinetic process of UV Cu(+) laser in Ne-CuBr longitudinal pulsed discharge.

The kinetic process of Cu(+) UV laser in Ne-CuBr longitudinal pulsed discharge is analyzed and a comprehensive self-consistent physical model is developed. The temporal evolutions of discharge parameters, main particle densities, the electron temperature, and the laser pulse intensity are numerically calculated. The model results illustrate the process of population inversion and the lasing mechanism. The calculations on the influences of the tube radius and Br atoms on the laser output characteristic well explain the experimental results.