This PDF file contains the editorial “Annual highlights” for OE Vol. 56 Issue 02

Aiming at the problem of low segmentation accuracy and high computation time by applying existing segmentation methods for underwater color images, this paper has proposed an underwater color image segmentation method via RGB color channel fusion. Based on thresholding segmentation methods to conduct fast segmentation, the proposed method relies on dynamic estimation of the optimal weights for RGB channel fusion to obtain the grayscale image with high foreground-background contrast and reaches high segmentation accuracy. To verify the segmentation accuracy of the proposed method, the authors have conducted various underwater comparative experiments. The experimental results demonstrate that the proposed method is robust to illumination, and it is superior to existing methods in terms of both segmentation accuracy and computation time. Moreover, a segmentation technique is proposed for image sequences for real-time autonomous underwater vehicle operations.

Illumination estimation is an important component of color constancy and automatic white balancing. According to recent survey and evaluation work, the supervised methods with a learning phase are competitive for illumination estimation. However, the robustness and performance of any supervised algorithm suffer from an incomplete gamut in training image sets because of limited reflectance surfaces in a scene. In order to address this problem, we present a constrained low-rank gamut completion algorithm, which can replenish gamut from limited surfaces in an image, for robust illumination estimation. In the proposed algorithm, we first discuss why the gamut completion is actually a low-rank matrix completion problem. Then a constrained low-rank matrix completion framework is proposed by adding illumination similarities among the training images as an additional constraint. An optimization algorithm is also given out by extending the augmented Lagrange multipliers. Finally, the completed gamut based on the proposed algorithm is fed into the support vector regression (SVR)-based illumination estimation method to evaluate the effect of gamut completion. The experimental results on both synthetic and real-world image sets show that the proposed gamut completion model not only can effectively improve the performance of the original SVR method but is also robust to the surface insufficiency in training samples.

Recently, image encryption has been significantly improved by methods originating from optics. Phase retrieval is one method potentially applicable to image encryption. It is the process of algorithmically finding solutions to a phase loss problem due to light detectors only capturing the intensity of lights. Because the phase retrieval can constrain information to the phase with a specified condition, we propose using it to encrypt an image and compress it as phase information simultaneously. Moreover, the encryption will involve a customized key known only to authorized users. The encryption strategy provides dual protection of the encrypted image from unauthorized access. Experiments have been conducted to demonstrate the performance of the encryption method.

A method for computer-generated hologram (CGH) compression and transmission using a quantum back-propagation neural network (QBPNN) is proposed, with the Fresnel transform technique adopted for image reconstruction of the compressed and transmitted CGH. Experiments of simulation were conducted to compare the reconstructed images from CGHs processed using a QBPNN with those processed using a back-propagation neural network (BPNN) at the optimal learning coefficients. The experimental results show that the method using a QBPNN could produce reconstructed images with a better quality than those obtained using a BPNN despite the use of fewer learning iterations at the same compression ratio.

We propose a simple gradation representation method for a reconstructed three-dimensional (3-D) image without controlling the brightness of the reference light. In the proposed method, we use multiple bit planes comprised of binary-weighted computer-generated holograms (CGHs) with various light transmittances. Binary-weighted CGH is generated by changing the white in the conventional binary CGH to gray. The light transmittance of a binary-weighted CGH is less than that of a conventional binary CGH. The object points of a 3-D object are assigned to multiple bit planes according to the gray level of the object points. The multiple bit planes are displayed sequentially in a time-division multiplex manner. Consequently, the proposed method realizes a gradation representation of a reconstructed 3-D object.

In the fabrication process of aspheric glass lens and molds, shape characterization is a fundamental task to control geometrical errors. Nevertheless, the more significant geometrical functional aspect related to the optical properties is the curvature, which is rarely investigated in the manufacturing process of lenses. Algorithms for the assessment of shape and curvature errors on aspheric surface profile are presented. The method has been investigated on profiles measured before and at different steps of the membrane polishing process. The results show how surface roughness, shape, and curvature change during the polishing process as a function of the machining time.

In the fiber optic gyroscope (FOG) single-axis rotation inertial navigation system (SRINS), the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<Z</mml:mi< </mml:mrow< </mml:math< </inline-formula<-axis gyro drift (<inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mi<ϵ</mml:mi< <mml:mi<z</mml:mi< </mml:msub< </mml:mrow< </mml:math< </inline-formula<) dramatically limits its navigation precision and the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<Z</mml:mi< </mml:mrow< </mml:math< </inline-formula<-axis gyro scale factor error (<inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<δ</mml:mi< <mml:msub< <mml:mi<K</mml:mi< <mml:mrow< <mml:mi<G</mml:mi< <mml:mi<z</mml:mi< </mml:mrow< </mml:msub< </mml:mrow< </mml:math< </inline-formula<) can cause greater navigation error because of the rotation process compared with the strap-down inertial navigation system. Hence, identification and compensation for the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mi<ϵ</mml:mi< <mml:mi<z</mml:mi< </mml:msub< </mml:mrow< </mml:math< </inline-formula< and <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<δ</mml:mi< <mml:msub< <mml:mi<K</mml:mi< <mml:mrow< <mml:mi<G</mml:mi< <mml:mi<z</mml:mi< </mml:mrow< </mml:msub< </mml:mrow< </mml:math< </inline-formula< are important in SRINS. An approach based on the azimuth error model of open-loop algorithm is proposed. This approach considers azimuth angle as a measurement and uses a least recursive square algorithm for identifying the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mi<ϵ</mml:mi< <mml:mi<z</mml:mi< </mml:msub< </mml:mrow< </mml:math< </inline-formula< and <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<δ</mml:mi< <mml:msub< <mml:mi<K</mml:mi< <mml:mrow< <mml:mi<G</mml:mi< <mml:mi<z</mml:mi< </mml:mrow< </mml:msub< </mml:mrow< </mml:math< </inline-formula<. Compared with the traditional method, which takes velocity and position errors as measurements, the time required for identifying <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mi<ϵ</mml:mi< <mml:mi<z</mml:mi< </mml:msub< </mml:mrow< </mml:math< </inline-formula< with the proposed method is only approximately 10 min, while the traditional method requires 6 h to 12 h. Experimental results from an SRINS with FOGs demonstrate that the accuracy of identifying the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mi<ϵ</mml:mi< <mml:mi<z</mml:mi< </mml:msub< </mml:mrow< </mml:math< </inline-formula< reaches 0.002°/h and that of the <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<δ</mml:mi< <mml:msub< <mml:mi<K</mml:mi< <mml:mrow< <mml:mi<G</mml:mi< <mml:mi<z</mml:mi< </mml:mrow< </mml:msub< </mml:mrow<

Interpixel capacitance (IPC) is a deterministic electronic coupling resulting in a portion of signal incident on one pixel of a hybridized detector array being measured in adjacent pixels. Data collected by light sensitive HgCdTe arrays that exhibit this coupling typically goes uncorrected or is corrected by treating the coupling as a fixed point spread function. Evidence suggests that this coupling is not uniform across signal and background levels. Subarrays of pixels using design parameters based upon HgCdTe indium hybridized arrays akin to those contained in the James Webb Space Telescope’s NIRcam have been modeled from first principles using Lumerical DEVICE Software. This software simultaneously solves Poisson’s equation and the drift diffusion equations yielding charge distributions and electric fields. Modeling of this sort generates the local point spread function across a range of detector parameters. This results in predictive characterization of IPC across scene and device parameters that would permit proper photometric correction and signal restoration to the data. Additionally, the ability to visualize potential distributions and couplings as generated by the models yields insight that can be used to minimize IPC coupling in the design of future detectors.

A glass birefringence measurement system utilizing the reflective laser feedback (RLF) effect is presented. The measurement principle is analyzed based on the equivalent cavity of a Fabry–Perot interferometer, and the experiments are conducted with a piece of quartz glass with applied extrusion force. In the feedback system, aluminum film used as a feedback mirror is affixed to the back of the sample. When the light is reflected back into the cavity, as the reinjected light is imprinted with the birefringence information in the sample, the gain and polarization states of the laser are modulated. The variation of optical power and polarization states hopping is monitored to obtain the magnitude of the stress. The system has advantages such as simplicity and low-cost with a precision of 1.9 nm. Moreover, by adjusting the position of the aluminum, large-area samples can be measured anywhere at any place.

The constitutive relation of geological mechanics similar material is the basis of the geological mechanics experiment. It is obtained using stress curves and strain curves from the uniaxial compression test of square specimen. However, the traditional measuring method exhibits a nonignorable error for similar material owing to its boundary effect. An approach based on embedding a bare fiber Bragg grating (FBG) sensor into a specimen is presented for measuring the constitutive relation of the similar material. The error of the traditional approach was examined by simulation, and the results of measurement in different frictions were compared. The simulation result revealed that the error increases with friction when the sensor was pasted on the surface. When the sensor was embedded in the middle of the specimen, the friction was less effective. Two similar FBG sensors were used in the measurement of similar material for verification: one embedded into the specimen and another pasted on the surface. The friction was varied using silicone oil. Experimental results agreed well with the simulation results, indicating that the approach of embedding bare FBG into a specimen can measure the constitutive relation precisely, and it is more accurate than the traditional approach.

A scheme using timed random frequency hopping and signal logarithmic mean method is proposed and experimentally demonstrated in a coherent optical time domain reflectometry (OTDR) system to reduce the fading noise of the OTDR trace and simplify the signal processing procedure. The timed random frequency hopping is realized by randomly changing the driving current of the laser at certain time points. By this method, the fading noise of OTDR trace can be reduced to be 1/5 of that without using it. Also, a radio frequency power detector (RFPD), whose output voltage has linear relationship with the input logarithmic RF power, is used to extract the power of the RF signals from the balanced photodetector. Then, a data acquisition card directly captures and adds the digital voltage signals from the RFPD to reduce the fading noise and improve the measurement dynamic range. Compared with synchronous and asynchronous frequency hopping scheme, the proposed method is of high efficiency.

The computation time of wavefront reconstruction is decreased by sampling the difference fronts in the present study. The wavefront can be reconstructed with high accuracy up to 64 Zernike terms with only 32×32 sampled pixels. Furthermore, the computational efficiency can be improved by a factor of more than 1000, and the measurement efficiency of lateral shearing interferometry is improved. The influence of the terms used to reconstruct the wavefront, the grid size of the test wavefront, the shear ratio, and the random noise on the reconstruction accuracy is analyzed and compared, when the difference fronts are sampled with different grid sizes. Numerical simulations and experiments show that the relative reconstruction error is <5% if the grid size of the sampled difference fronts is more than four times the radial order of difference Zernike polynomials with a reasonable noise level and shear ratio.

Typical fully distributed optical fiber sensors (DOFS) with dozens of kilometers are equivalent to tens of thousands of point sensors along the whole monitoring line, which means tens of thousands of data will be generated for one pulse launching period. Therefore, in an all-day nonstop monitoring, large volumes of data are created thereby triggering the demand for large storage space and high speed for data transmission. In addition, when the monitoring length and channel numbers increase, the data also increase extensively. The task of mitigating large volumes of data accumulation, large storage capacity, and high-speed data transmission is, therefore, the aim of this paper. To demonstrate our idea, we carried out a comparative study of two lossless methods, Huffman and Lempel Ziv Welch (LZW), with a lossy data compression algorithm, fast wavelet transform (FWT) based on three distinctive DOFS sensing data, such as Φ-OTDR, P-OTDR, and B-OTDA. Our results demonstrated that FWT yielded the best compression ratio with good consumption time, irrespective of errors in signal construction of the three DOFS data. Our outcomes indicate the promising potentials of FWT which makes it more suitable, reliable, and convenient for real-time compression of the DOFS data. Finally, it was observed that differences in the DOFS data structure have some influence on both the compression ratio and computational cost.

Optical 4-bit binary to gray and gray to binary code conversion has been demonstrated. The encoders are designed by implementing XOR gates in optical domain. Phase modulation in Mach–Zehnder interferometer has been exploited to achieve results at considerably high data rates of up to 60 Gbps. Also, the designs, being reversible in nature, will amount to less power consumption when embedded in an optical network. Performance parameters of the design, such as Q factor and extinction ratio, are analyzed based on simulation results carried out using comprehensive design suite OptiSystem-13.

Choosing an adequate initial design for optimization plays an important role in obtaining high-quality deep ultraviolet (DUV) lithographic objectives. In this paper, the grouping design method is extended to acquire initial configurations of catadioptric projection objective for DUV lithography. In this method, an objective system is first divided into several lens groups. The initial configuration of each lens group is then determined by adjusting and optimizing existing lens design according to respective design requirements. Finally, the lens groups are connected into a feasible initial objective system. Grouping design allocates the complexity of designing a whole system to each of the lens groups, which significantly simplifies the design process. A two-mirror design form serves as an example for illustrating the grouping design principles to this type of system. In addition, it is demonstrated that different initial designs can be generated by changing the design form of each individual lens group.

Motivated by the demand to minimize the mount-induced wavefront aberration of the large-aperture laser transport mirror, a low-stress flexure mounting configuration is proposed. Specific optomechanical analyses, including theoretical modeling, numerical analysis and field experiment, are presented. The mechanical properties of the flexure support were studied specifically. Besides, the relation between the mounting forces and the root-mean-square of the gradients (GRMS) value of the mirror surface is studied. Then, the appropriate value of the bolt preload is set to 500N, with which the GRMS value is just 5.35  nm/cm. The results indicate that the flexure mounting configuration is indeed a feasible and promising method to solve the mount-induced distortion problem of large-aperture optics.

We introduce a technique for illumination optimization based on a projector–camera system applied to a multireflective three-dimensional (3-D) scene. Simple mapping would fail if the scene is nonplane since they would translate to incoherent mapping between the projector and the camera. The correspondence between the pixels on the projector and camera in multireflective 3-D scene was indirectly established by surface orientation measurement. Once the correspondence was established, the local light was adapted by using the iteration in order to obtain the optimal illumination intensities at different surface orientations. User intervention for judging the desirable image is required in the iteration stage since there is no absolute reference for comparison to which the object must appear. According to the feedback from the camera, the projector is to compensate for surface reflectance for the most part by modulating the projection brightness at each pixel to be inversely proportional to the brightness of the pixel under uniform light. The time required for this adaptive illumination can be shortened to the inspection stage of 40 ms as long as the inspected object does not change in orientation or location. This proposed technique aims to enhance the illumination capabilities and presents a more unified system than existing methods.

A modified Mach–Zehnder interferometric phase sensitive amplifier (MMZI-PSA) configuration is proposed and simulated, which combined the four-wave mixing phase-sensitive amplifier (FWM-PSA) with the fiber interference phase-sensitive amplifier (FI-PSA). This modified scheme for an all-optical communication system can not only eliminate phase noise, but also reduce additional amplitude noise, reducing intrinsic amplitude noise and suppressing phase-to-amplitude noise conversion tremendously. Compared with the traditional FWM-PSA configuration, the proposed configuration can obtain higher gain, as well as reduce bit-error rate (BER) and error vector magnitude (EVM). The simulation results suggest that MMZI-PSA can effectively regenerate QPSK signals and obtain 6.9-dB gain. Furthermore, it can reduce BER to 3.98×10−6 and reduce EVM to 22%, which improves system noise tolerance.

We propose an all-fiber electric field sensor using a U-bent single-mode fiber integrated with liquid crystal. The sensing mechanism is based on the interference between whispering-gallery modes and core mode in U-shaped bent fibers. We have experimentally investigated the influences of applied electric voltage as well as polarization state of the incident light on the transmission spectral characteristics of the proposed electric field sensor. The experimental results indicate that the transmission spectra are highly sensitive to the applied voltage and the highest sensitivity reaches 20.4221  nm/kV. The dip wavelengths and transmission loss at transmission dip exhibit a periodic variation behavior in response to the light polarization state rotation by 360 deg, which is in agreement with our theoretical analysis.

The spatial correlation extensively exists in the multiple-input multiple-output (MIMO) free space optical (FSO) communication systems due to the channel fading and the antenna space limitation. Wilkinson’s method was utilized to investigate the impact of spatial correlation on the MIMO FSO communication system employing multipulse pulse-position modulation. Simulation results show that the existence of spatial correlation reduces the ergodic channel capacity, and the reception diversity is more competent to resist this kind of performance degradation.

A spatial circuit switching system based on a beam steering application for W-band wireless links is proposed and experimentally demonstrated. The system enables two simultaneous transmissions of a 2.5  Gbit/s data signal over a carrier of 81 GHz, while allowing the receiver to dynamically switch between them. The performance of the system is tested with the real-time measurements of the BER, achieving values below the FEC limit for 7% of overhead and serving to prove the viability of wireless spatial circuit switching in the next generation of wireless access networks.

Line surveillance and management information in erbium-doped fiber amplifiers (EDFAs) can be broadcast by modulating the amplitude of the low-frequency lightwave information signal, the process termed as overmodulation in the literature. This paper presents systematic solutions for the overmodulated pump and information signal transfer functions for EDFA. It includes amplified spontaneous emission (ASE) that has an impact on outcomes in the high-gain system. To the extent of our belief, the methodical model simulated with the current approach leads to a distinct perspective of an outcome in the respective field. The test bed described here is realistic. It specifically represents the overmodulation behavior in an EDFA under the influence of ASE.

Laser safety regulating the deployment of kW-class high-energy laser (HEL) technologies in outdoor applications can rapidly cause significant planning and operations issues due to the ranges involved. Safety templates based on a simplistic approach of assuming a continuous wave laser beam incident on a highly reflective totally flat solid surface of infinite size can easily result in ranges of tens of kilometers for kW-class lasers. Due to the complexity of HEL-matter interactions, the assumptions underlying the aforementioned approach are, however, deemed inappropriate. We identify a more suitable approach, which assumes a time-variant reflection pattern as well as a change in the variance of beam divergence as it reflects from the target’s surface. Based on experimental results, we instead propose to assess the nominal ocular hazard distance by applying the American National Standard Institute rules for time-variant multipulse laser exposure and using measured divergence angles from the target’s surface. The resulting safety templates, thus, exhibit a higher fidelity with respect to the behavior of the reflection patterns while reducing the hazard zones.

A compact dual-loop optoelectronic oscillator (OEO) employing a dual-output Mach–Zehnder intensity modulator (DOMZM) and a balanced photodetector (BPD) is theoretically analyzed and experimentally demonstrated. The fundamental idea of the scheme is based on double loops formed by two complementary output ports of DOMZM and two input ports of BPD, which could be naturally combined without any additional optical coupler or polarization beam splitter devices. This simple structure makes it possible to enhance side-mode suppression ratio (SMSR) and reduce phase noise of the OEO. Compared with the traditional dual-loop OEO (SO-OEO) based on single-output Mach–Zehnder modulator (MZM), optical coupler, and BPD, the advantages of our proposed dual-loop OEO (DO-OEO) with DOMZM and BPD are presented. Experimental results show that a 16-GHz single-mode OEO is obtained with measured SMSR of 72 dB and phase noise of −133.2  dBc/Hz at 10-kHz frequency offset.

A low-cost transistor outline-CAN (TO-CAN) package, which is combined with flexible printed circuit board (PCB) and hard PCB, has been developed for a 25-Gb/s optical subassembly module. On the flexible PCB, the transmission line structure used top ground microstrip line, and the wider transmission bandwidth can be obtained. Using ground pads and ground notch technologies, the impedance of connection between flexible PCB and hard PCB was designed to match with the impedances of signal traces of the flexible and hard PCBs. In the TO-CAN package, a TO-46 header was used, and the header needs to closely connect with the flexible PCB. The bandwidth of TO-46 package combined with flexible and hard PCBs can achieve above 23 GHz. The clear 25-Gb/s transmission eye diagram was also measured, and the rise time, fall time, and Q-factor of the eye diagram are 13.78, 13.56 ps, and 8.76, respectively. The TO-46 package combined with flexible and hard PCBs has been verified to be suitable for application in 25-Gb/s optical subassembly modules.

We investigated the influence of back-reflected pump power in a high-power Yb:YAG thin-disk laser. In such lasers, there is usually a fraction of pump power that is not absorbed in the Yb:YAG crystal reflected back via the initial path and incident on the pumping laser diode. In a high-power laser, such a reflection may cause catastrophic optical damage in laser diodes. The unabsorbed pump power was calculated using rate equations. The experimental results are in agreement with those obtained through calculations. Furthermore, two methods for back-reflected pump power suppression were introduced. We have shown that with a thin-film polarizer and quarter wave plate, unabsorbed power incident on the laser diode can be reduced significantly.

A microwave photonic filter (MPF) based on four-wave mixing (FWM) is proposed and experimentally demonstrated. Two single-frequency laser beams produce the four-wave-mixing effect in a highly nonlinear fiber and generate a multiwavelength optical signal output. The multiwavelength optical signal is used to generate the multitaps of the MPF. The wavelength spacing of the multiwavelength optical source is equal to the frequency difference of the two input laser beams. By changing the frequency difference of the two input laser beams, wavelength spacing of the multiwavelength optical source output can be continuously tuned in the range of 0.36 to 1.6 nm. Thus the center frequency of the filter can be continuously tuned within the range of 6.914 to 30.729 GHz.

An experimental platform was built for the atmospheric turbulence simulation upon a liquid crystal spatial light modulator. The phase-only spatial light modulator is driven by a series of dynamic phase screens that have non-Kolmogorov turbulence characteristics. After passing through the simulated non-Kolmogorov turbulence, the angle-of-arrival (AoA) and light intensity of the laser beam were analyzed statistically. The variation trends of the AoA fluctuation and the scintillation index exhibited by the experimental results are in accordance with the theoretical predictions. Although there was some divergence between the experiments and theory that needs to be improved further, the experimental data demonstrate the feasibility of using the platform to simulate the turbulence influence on laser communication in the lab.

Static position errors are usually ineluctable during the assembly of a satellite optical communication terminal, which will lead to pointing errors and make it difficult to establish an optical communication link. The model resulting from static position errors of satellite optical communication terminal has been efficiently established through the transformation matrix between the coordinate system of an optical terminal and satellite. Hence, the static position errors for a satellite optical communication terminal can be well predicted. Our results show that the pointing error can be reduced from thousands of microradians to hundreds of microradians, which has been also validated in orbit to greatly improve the quality of optical communication link of HY-2 satellite.

A simple tunable fiber comb filter composed of two quarter-wave plates and a section of polarization-maintaining fiber is proposed. The states of polarization are investigated through theoretical analysis, and we present the means for continuous wavelength tunability using analytic expression. The optimal angle combination of each birefringent device could be deduced so that an arbitrary wavelength could be a channel center of the comb filter without transmission loss, which is verified experimentally.

Realizing high flux of hyperentangled photons requires collecting photon pairs simultaneously entangled in multiple degrees of freedom over relatively wide spectral and angular emission ranges. We consider the hyperentangled photons produced by superimposing noncollinear spontaneous parametric down conversion (SPDC) emissions of two crossed and coherently pumped nonlinear crystals. We present an approach for determining the directional-spectral relative-phase and time-delay maps of hyperentangled photons all over the SPDC emission cone. A vectorial representation is adopted for all parameters of concern. This enables us to examine unconventional arrangements such as the autocompensation of relative-phase and time-delay via oblique pump incidence. While prior works often adopt first-order approximation, it is shown that the actual directional relative-phase map is very well approximated by a quadratic function of the polar angle of the two-photon emission while negligibly varying with the azimuthal angle.

A silicon device to simplify the coupling of multiple single-mode fibers to embedded single-mode waveguides has been developed. The silicon device features alignment structures that enable a passive alignment of fibers to integrated waveguides. For passive alignment, precisely machined V-grooves on a silicon device are used and the planar lightwave circuit board features high-precision structures acting as a mechanical stop. The approach has been tested for up to eight fiber-to-waveguide connections. The alignment approach, the design, and the fabrication of the silicon device as well as the assembly process are presented. The characterization of the fiber-to-waveguide link reveals total coupling losses of (0.45±0.20  dB) per coupling interface, which is significantly lower than the values reported in earlier works. Subsequent climate tests reveal that the coupling losses remain stable during thermal cycling but increases significantly during an 85°C/85 Rh-test. All applied fabrication and bonding steps have been performed using standard MOEMS fabrication and packaging processes.

We investigate a high-sensitivity sucrose sensor based on a standard erbium-doped fiber ring laser incorporating a coreless fiber (CF). A single-mode-coreless-single mode (SCS) structure with a very low insertion loss has been constructed. The SCS fiber structure performed dual function as an intracavity fiber filter and/or a sensing element. The gain medium (erbium-doped fiber) is pumped by a 975-nm wavelength fiber coupled diode laser. Laser emission around 1537 nm with −2  dBm peak output power is obtained when a CF in SCS structure has a diameter of 125  μm. The 3-dB line-width of the laser is <0.14  nm, which is beneficial to high precision sensing. The sucrose concentration varied from 0% to 60%, and the relationship between the lasing wavelength and the sucrose concentration exhibited linear behavior (R2=0.996), with sensitivity of 0.16 nm/% was obtained. To improve the measurement sensitivity, the CF is etched by hydrofluoric acid. The splice joint of etched CF with SMF is a taper, which improves its sensitivity to sucrose changes. An average sensitivity of 0.57 nm/% and a high signal-to-noise ratio of 50 dB make the proposed sensor suitable for potential applications.

A full-duplex radio over fiber (RoF) link with the generation of a 64-GHz millimeter wave (mm-wave) is investigated. This system is proposed as a solution to cope with the demands of a multi-Gb/s data transmission in the fifth generation (5G) and beyond for small cell networks. Cost reduction and performance improvement are achieved by simplifying the mm-wave generation method with an RoF technique. High-frequency radio signals are considered challenging in the electrical generation domain; therefore, our photonic generation method is introduced and examined. RoF design is proposed for mm-wave generation using both phase modulation and the effect of stimulated Brillouin scattering in the optical fiber for the first time. RoF system with transmission rates of 5  Gb/s is successfully achieved. In our scheme, one laser source is utilized and a fiber Bragg grating is used for wavelength reuse for the uplink connection. Stable mm-wave RoF link is successfully achieved in up to a 100-km fiber link length with high quality carrier. Simulation results show a reduction in fiber nonlinearity effects and the mm-wave signal has low noise equal to −75  dBm. This study ensures a practical mm-wave RoF link, and it could be appropriate for small cell 5G networks by reducing the installation cost.

A modulation format, polarity separating optical orthogonal frequency division multiplexing (PSO-OFDM), is proposed to mitigate the light-emitting diode (LED) nonlinearity for visible light communication systems. A polarity separator is used to divide the OFDM signal in time domain x(t) into two parts: x+(t) and x−(t), which will be transmitted parallelly from the different LEDs and overlap linearly in free space to realize PSO-OFDM. The experimental results demonstrate that PSO-OFDM has high spectral efficiency and suffers less nonlinear distortions than other methods. Employing PSO-OFDM, the modulation index and bit error rate performance can be significantly enhanced.

A flexible hybrid wavelength division multiplexing-time division multiplexing passive optical network architecture that allows dual rate signals to be sent at 1 and 10 Gbps to each optical networking unit depending upon the traffic load is proposed. The proposed design allows dynamic wavelength allocation with pay-as-you-grow deployment capability. This architecture is capable of providing up to 40 Gbps of equal data rates to all optical distribution networks (ODNs) and up to 70 Gbps of a asymmetrical data rate to the specific ODN. The proposed design handles broadcasting capability with simultaneous point-to-point transmission, which further reduces energy consumption. In this architecture, each module sends a wavelength to each ODN, thus making the architecture fully flexible; this flexibility allows network providers to use only required OLT components and switch off others. The design is also reliable to any module or TRx failure and provides services without any service disruption. Dynamic wavelength allocation and pay-as-you-grow deployment support network extensibility and bandwidth scalability to handle future generation access networks.

A routing and wavelength assignment (RWA) algorithm against high-power jamming based on software-defined optical networks (SDONs) is proposed. The SDON architecture is designed with power monitors at each node, which can collect the abnormal power information from each port and wavelength. Based on the abnormal power information, a metric, the weighted attack probability (WAP), can be calculated. A WAP-based RWA algorithm (WAP-RWA) is proposed considering the WAP values of each link and node along the selected lightpath. Numerical results show that the WAP-RWA algorithm can achieve a better performance in terms of blocking probability and resource utilization compared with the attack-aware dedicated path protection (AA-DPP) RWA (AA-DPP-RWA) algorithm, while providing a protection comparable with the AA-DPP-RWA algorithm.

Passive phase locking of two multiwavelength fiber lasers has been demonstrated by mutual injection coupling and spatial filtering. Since the fiber loop mirrors are employed as two component lasers’ rear reflectors, multiple wavelengths operate simultaneously owing to their broadband reflection and the compound cavity’s mode selection effect. A single-mode filtering fiber is utilized to collect the expected spatial mode’s energy and feed them back into the coupled laser cavity, and stable and high visibility interference fringes have been obtained by the self-phasing process. A contrastive experiment has been made to investigate its coherent output properties when the rear mirrors are replaced by fiber Bragg gratings. Compared with the ordinary coherent combining of fiber lasers lasing at a single-wavelength, higher output power and efficiency have been obtained, and the cost is that the output beams’ coherence and stability decrease slightly.

The infrastructure essentialities for accurate time and stable frequency distribution are presented. Our solution is based on sharing fibers for a research and educational network carrying live data traffic with time and frequency transfer in parallel. Accurate time and stable frequency transmission uses mainly dark channels amplified by dedicated bidirectional amplifiers with the same propagation path for both directions of transmission. This paper targets challenges related to bidirectional transmission, particularly, directional nonreciprocities.

A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718  nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22  pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73  pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61×10−4  RIU and 6.81×10−4  RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication.

We present an arc flash sensor that can trace the arc event position as well as intensity by utilizing conventional plastic optical fibers (POFs). In order to check the possibility as a light-receiving sensor, we experimentally confirm that the externally irradiated flash light can be coupled into the fiber core through the surface of POF without any additional treatment. After the incident light is divided in two optical paths toward opposite directions, they have the different attenuation values determined by the propagation distance. Since the optical transmission loss of a POF is constant regardless of the irradiated energy, the intensity ratio for two signals measured at both fiber ends is given as a function of position. The experimental results show that we can successfully trace the event position from this intensity ratio. In addition, it is possible to define the illuminated energy by comparing the absolute value of the intensity measured from one side. According to the experimental results, the proposed sensor has a relatively fine spatial resolution, ±10  cm, despite having a simple structure.

A high-sensitivity sensing technique was demonstrated based on a flexible terahertz dual-band metamaterial absorber. The absorber has two perfect absorption peaks, one with a fundamental resonance ( f 1 ) of the structure and another with a high-order resonance ( f 2 ) originating from the interactions of adjacent unit cells. The quality factor ( Q ) and figure of merit of f 2 are 6 and 14 times larger than that of f 1 , respectively. For the solid analyte, the changes in resonance frequency are monitored upon variation of analyte thickness and index; a linear relation between the amplitude absorption with the analyte thickness is achieved for f 2 . The sensitivity ( S ) is 31.2% refractive index units ( RIU − 1 ) for f 2 and 13.7% RIU − 1 for f 1 . For the aqueous solutions, the amplitude of absorption decreases linearly with increasing the dielectric constant for the ethanol–water mixture of f 1