Various patterns of device-to-device (D2D) communication, from Bluetooth to Wi-Fi Direct, are emerging due to the increasing requirements of information sharing between mobile terminals. This paper presents an innovative pattern named device-to-device visible light communication (D2D-VLC) to alleviate the growing traffic problem. However, the occlusion problem is a difficulty in D2D-VLC. This paper proposes a game theory–based solution in which the best-response dynamics and best-response strategies are used to realize a mode-cooperative selection mechanism. This mechanism uses system capacity as the utility function to optimize system performance and selects the optimal communication mode for each active user from three candidate modes. Moreover, the simulation and experimental results show that the mechanism can attain a significant improvement in terms of effectiveness and energy saving compared with the cases where the users communicate via only the fixed transceivers (light-emitting diode and photo diode) or via only D2D.

We present the use of a q-plate device operating at the 1550 nm telecommunications wavelength. A prototype liquid-crystal device from Citizen Holdings Co. is demonstrated to be useful for the generation of vector beams and orbital angular momentum transfer at this important wavelength.

This PDF file contains the editorial “Special Section Guest Editorial:Microwave Photonics: Deep Interactions between Microwaves and Lightwaves” for OE Vol. 55 Issue 03

The multicarrier analog photonic links suffer from both the traditional third-order intermodulation distortions (IMD3) and the cross-modulation distortions (XMDs), severely limiting the dynamic range of the links. This paper proposes and demonstrates an effective technique based on a single modulator and photodetector to simultaneously realize the downconversion and receiving of multiple radio frequency signals, as well as suppress the nonlinearities, including the IMD3 and XMD. In the scheme, the nonlinear compensation information is directly obtained from hardware then the distortion compensation is carried out in the digital domain. Experimental results show that the XMD and IMD3 distortions are suppressed with 36.6 and 25.8 dB, respectively, and the link dynamic range is improved by 25 dB, preventing the degradation of the dynamic range of the link. Moreover, the structure of our scheme can eliminate the stringent requirement for hardware.

By cascading two standard Mach–Zehnder modulators (MZMs), we propose and demonstrate a scheme to effectively eliminate the cross-modulation distortion (XMD), which results from the out-of-band interference in multicarrier intensity-modulation direct-detection (IMDD) analog photonic links. When the bias angle of the cascaded modulator is specifically designed, the XMDs, both from the photonic link itself and from the nonlinear electrical amplifiers, are well suppressed. Our proposal is theoretically analyzed, and the performance of the cascading system is experimentally demonstrated. A suppression ratio of more than 30 dB is achieved by the cascading scheme. By using the high-performance pre- and postamplifiers, the measured link gain and noise figure are improved by 62 and 32 dB, respectively.

A fiber-wireless sensor system based on a power-over-fiber technique is developed to offer a flexible, distributed sensing ability over a middle distance, especially under environments that are sensitive to electromagnetic interference. In this system, the optical energy of a high-power laser in the base station is transmitted via a fiber and then converted into electrical energy by a photovoltaic power converter (PPC) in the remote unit. This optically power-supplied remote unit operates as the coordinator in the wireless sensor network (WSN) and exchanges the sensing information with the base station via another fiber. In our demonstration system, the sensing information can be collected by a WSN 2 km away and be transmitted back. In order to improve the power supply ability of PPC, a maximum power point tracking technique is applied. More than 80% of PPC’s maximum output power can be obtained. Moreover, to reduce the power consumption of the remote unit and the sensor nodes, a simple and stable low-power communication protocol is designed.

We propose a tunable fractional-order photonic differentiator based on a distributed feedback semiconductor optical amplifier (SOA) working in reflection mode. The phase shift at the resonant wavelength can be adjusted by controlling the current injected into the DFB-SOA, which can implement the fractional-order differentiation. A 60 ps Gaussian pulse is temporally differentiated with a tunable order range from 0.7 to 1.3.

We propose and analyze a photonic method of generating frequency-quadrupling millimeter-wave signal. This scheme is realized by using a single LiNbO₃ intensity modulator (IM) and a Faraday mirror based transverse-electrical and transverse-magnetic mode converter in a Sagnac loop without using an optical filter or an electrical microwave phase shifter. Making use of the intrinsic polarization dependence and the velocity phenomenon of the IM, a special double sideband modulation is implemented, which ensures that the optical carrier can be effectively cancelled employing polarization manipulation. A linear polarizer is used as the polarization selection element to choose the second-order sidebands from the modulated light. After beating at the photodiode, a frequency-quadrupled millimeter-wave signal with <30  dB radio frequency spurious suppression ratio is generated. The imperfection of the devices is considered when estimating the system performance.

We have proposed and experimentally demonstrated a fiber-optic temperature sensing system based on an optoelectronic oscillator (OEO). A part of the fiber in the oscillator loop is used as the sensing fiber. As the free spectrum range (FSR) of the OEO is determined by the length of the oscillator loop, when the temperature of the sensing fiber is changed, the refractive index of the fiber varies; thus, the optical path length changes as well as the FSR of the OEO. By tracking one peak of the OEO harmonics, the temperature variation can be monitored. Therefore, the temperature variation can be converted to the frequency shift of the radio frequency signals. In the experiment, we have measured the spectra of the OEO with different temperatures, which show good sensing linearity. We have also measured the relationship between the sensing responsivity and the tracked frequency (1.0, 1.5, 2.0, and 3.0 GHz) in the experiment, which shows that the harmonics at a higher frequency produce a higher sensing responsivity. The proposed temperature sensing system exhibits good tunability, stability, linearity tailorable sensing responsivity, and ease of implementation, thus it shows good application potential in remote fiber-optical temperature sensing and monitoring.

This paper introduces three different dispersion compensation methods based on superstructure fiber Bragg grating (SSFBG) and an injection distributed feedback (DFB) laser. First, an SSFBG with a nonlinearity group delay spectrum was designed to achieve tunable dispersion compensation. Second, an approach is proposed for realizing single-sideband modulation with an optimum optical carrier to sideband ratio for maximizing the transmission performance of a radio-over-fiber (RoF) system based on a strong optical injection-locked DFB laser. Finally, a broadband chromatic dispersion compensation scheme using an optical phase conjugator based on a DFB semiconductor laser is proposed and experimentally demonstrated in RoF links.

An electrical method is proposed for the microwave characterization of dual-drive Mach–Zehnder modulators based on heterodyne mixing. The proposed method utilizes the heterodyne products between the two-tone modulated optical sidebands and frequency-shifted optical carrier, and achieves calibration-free and bias-drift-free microwave measurement of dual-drive Mach–Zehnder modulators with high resolution electrical-domain techniques. Our method avoids the extra calibration for the photodetector and reduces half the bandwidth requirement for the photodetector and the electrical spectrum analyzer through carefully choosing a half frequency relationship of the two-tone modulation. Moreover, our measurement avoids the bias drifting problem due to the insensitivity to the bias phase of the modulator under test. The frequency-dependent modulation depths and half-wave voltages are measured for a commercial dual-drive Mach–Zehnder modulator with our method, which agree well with the results obtained by the conventional optical spectrum analysis method.

Stimulated Brillouin scattering (SBS) is one of the most dominant nonlinear effects in standard single-mode fibers and its unique spectral characteristics, especially the narrow bandwidth, enable many different applications. Most of the applications would benefit from a narrower bandwidth. Different methods for the bandwidth reduction of SBS in optical fibers are presented and discussed. A bandwidth reduction down to 17% of the natural gain can be achieved by the superposition of the gain with two losses or the utilization of a multistage system. Furthermore, applications in the field of microwave photonics and optical signal processing like high-resolution spectroscopy of communication signals, the storage of optical data packets as well as the processing of frequency combs including generation of millimeter waves and ideal sinc-shaped Nyquist pulses are presented.

We propose and demonstrate a multifunction-stabilized photonic link, which is capable of transmitting wideband time signal and stable frequency signal between the central station and the remote end bidirectionally over a single-fiber link. Experimentally, 3.95-GHz frequency signal and pulsed time signal are delivered to the remote end with frequency stability of 3.6×10−¹⁶ and time jitter of 0.45 ps at 4000 s average time, respectively. Also, a downlink radio frequency signal is transferred from the remote end back to the central station with suppressed delay variation.

A scheme to generate a flat optical frequency comb (OFC) is proposed and experimentally demonstrated based on a directly modulated distributed feedback (DFB) laser cascaded with a polarization modulator (PolM). In the proposed scheme, the DFB laser is optically injection-locked by a tunable laser source and directly modulated by a radio frequency (RF) signal, which is amplified by a microwave power amplifier. The optical signal is then sent to PolM via a polarization controller (PC) and modulated by the amplified and phase-shifted RF signal from the same source. The optical signal is finally received and measured by an optical spectrum analyzer (OSA) after transmitting through another PC and a polarizer. Here, the OFC with their power variation within 3 dB is desired, and four OFCs with 6, 6, 5, and 4 comb lines are generated using the RF signals with different frequencies, which have a flatness of, respectively, 2.4, 2.5, 0.7, and 0.6 dB. Here, the number of comb lines is decreased, which is due to the RF signal power decrease while its frequency is raised.

We present a review of physical networking options and enabling system technologies for efficient unification of optical and wireless access backhaul infrastructure. Different physical layer integration architectures are reviewed in conjunction with the possible wireless signal transport schemes. We also review some of our research work as one possible physical layer integration technique using RF-overlay passive optical network incorporating an optical tandem single sideband (OTSSB) modulation scheme for simultaneous transport of wired and wireless signals.

We propose and demonstrate for the first time an optical single-sideband (OSSB) modulation generation based on silicon-on-insulator coupled-resonator optical waveguides (CROWs) capable of operating at wide bandwidth with enhanced sideband suppression. The optimum order of the CROW filter was synthesized based on comprehensive performance analysis including optical sideband suppression and electrical power variation. Experimental results demonstrate an OSSB signal with sideband suppression as large as 23 dB. The performance of the proposed OSSB was demonstrated via compensating radio frequency (RF) power degradation in the transmission of the RF signal within the fiber.

A 2-bit photonic digital-to-analog conversion unit is proposed and demonstrated based on polarization multiplexing. The proposed 2-bit digital-to-analog converter (DAC) unit is realized by optical intensity weighting and summing, and its complexity is greatly reduced compared with the traditional 2-bit photonic DACs. Performance of the proposed 2-bit DAC unit is experimentally investigated. The established 2-bit DAC unit achieves a good linear transfer function, and the effective number of bits is calculated to be 1.3. Based on the proposed 2-bit DAC unit, two DAC structures with higher (<2) bit resolutions are proposed and discussed, and the system complexity is expected to be reduced by half by using the proposed technique.

We propose and demonstrate a radio-over-fiber system to generate an optical millimeter wave (MMW) and realize wavelength reuse for an uplink connection. A tunable optical comb generated by a single Fabry–Perot laser serves as the optical source. The central carrier is separated by an optical circulator cascaded with a fiber Bragg grating. For the downlink, the unmodulated central carrier is coupled with one subcarrier, which has been modulated with 2.5-Gb/s data. Then, different MMWs can be generated by choosing different subcarriers. While for the uplink, the same central carrier is reused for an uplink connection with 1.25-Gb/s data. In the scheme, a 60-GHz MMW is obtained and the bidirectional data are simultaneously transmitted over 60-km transmission with <0.5-dB power penalty. This system shows a simple cost-efficient configuration and good performance over long-distance delivery.

Tunable optoelectronic oscillator (OEO) with an embedded delay-line oscillator (EDO) for fine steps is presented. A delay-line oscillator consisting of an amplifier, phase shifter, and electrical delay line is embedded in the general single loop OEO. The oscillation frequency is controlled by the EDO loop phase, which is adjusted by the phase shifter. Without any narrow radio frequency bandpass filter, about 340 MHz tunable range with a fine step of ∼100  kHz is realized by directly tuning the direct current voltage of the phase shifter for 8 GHz oscillation. We also experimentally demonstrate κ-band tunable microwave signal generation within 22.18 to 23.32 GHz. The tunable accuracy and tunable range have a better performance compared to other OEO schemes with a fine frequency tunable step. The phase noise of the generated 22.25 GHz microwave signal is −108.7  dBc/Hz at 10 kHz offset.

This paper is regarding the concept and development of a photonics-based tunable and broadband radio frequency converter (PBRC). It employs an external modulation technique to generate and reconfigure its output frequency, a digital circuit to manage the modulators’ bias voltages, and an optical interface for connecting it to optical-wireless networks based on radio-over-fiber technology. The proposed optoelectronic device performs photonics-based upconversion and downconversion as a function of the local oscillator frequency and modulators’ bias points. Experimental results demonstrate a radiofrequency (RF) carrier conversion with spectral purity over the frequency range from 750 MHz to 6.0 GHz, as well as the integration of the photonics-based converter with an optical backhaul based on a 1.5-km single-mode fiber from a geographically distributed optical network. Low phase noise and distortion absence illustrate its applicability for convergent and reconfigurable optical wireless communications. A potential application relies on the use of PBRC in convergent optical wireless networks to dynamically provide RF carriers as a function of the telecom operator demand and radio propagation environment.

We propose and demonstrate an all‐optical correlator based on modal dispersion in a multimode fiber. Many of the modes can be excited in a large core high numerical aperture step‐index multimode fiber under overfilled launching. The incident optical signal is copied into different modes and a time delay is introduced by the modal dispersion. We use a mask with slits as a space filter to select the modes needed. A correlator of the mask and input signal is achieved at the output end. Thanks to the use of modal dispersion, the correlator is nearly independent of the wavelength and bandwidth of the input signal. By adjusting the slits on the mask, the target patterns of the correlator can be changed easily. A radio frequency signal detection is also experimentally demonstrated with this construction.

A reconfigurable microwave photonic filter based on a polarization modulator (PolM) is proposed and experimentally demonstrated. The PolM together with a polarization controller (PC) and a polarization beam splitter (PBS) implements two complementary intensity modulations in two separated branches. Then, optical components are inserted in the two branches to realize a bandpass filter and an allpass filter, respectively. When the two branches are combined by a second PBS, a filter with a frequency response that equals the subtraction of the frequency responses of the allpass filter and bandpass filter is achieved. By adjusting the PCs placed before the second PBS, a notch filter with a tunable notch depth or a bandpass filter can be achieved.

Various multifunctional operations are performed by proposing designs of optical adder, subtractor, comparator, and decoder at 60  Gb/s. In all operations, constructive interference is produced by choosing optimized parameters, i.e., optical pulse generator power, input power, semiconductor optical amplifier-Mach–Zehnder interferometer parameters, and so on, for delivering a true output signal. An optical pulse-generated signal is required for all operations except addition, subtraction and equal to in a comparator.

Dim targets are extremely difficult to detect using methods based on single-frame detection. Radiation accumulation is one of the effective methods to improve signal-to-noise ratio (SNR). A detection approach based on radiation accumulation is proposed. First, a location space and a motion space are established. Radiation accumulation operation, controlled by vectors from the motion space, is applied to the original image space. Then, a new image space is acquired where some images have an improved SNR. Second, quasitargets in the new image space are obtained by constant false-alarm ratio judging, and location vectors and motion vectors of quasitargets are also acquired simultaneously. Third, the location vectors and motion vectors are mapped into the two spaces, respectively. Volume density function is defined in the motion space. Location extremum of the location space and volume density extremum of motion space will confirm the true target. Finally, actual location of the true target in the original image space is obtained by space inversion. The approach is also applicable to detect multiple dim targets. Experimental results show the effectiveness of the proposed approach and demonstrate the approach is superior to compared approaches on detection probability and false alarm probability.

Single-pixel imaging (SPI) represents a promising approach to multispectral imaging. We investigated the interband similarity of color images among red, green, and blue bands and found that it is highly possible for their wavelet coefficients to be at the same locations due to edges and transactions. Accordingly, we constructed a multiple measurement vectors model that includes a constraint under which the sparse coefficients of different bands have the same sparse structure, and then joint reconstruction is performed for all bands. We ran both simulated and actual experiments to validate the feasibility and effectiveness of the proposed method and found that compared with similar methods, it significantly improves the reconstruction quality of color SPI.

Identifying phase discontinuity locations is a necessary and complex step in the phase unwrapping process, and it becomes more challenging when dealing with noisy wrapped phase maps that are produced through shearography or other speckle-based interferometry methods. Recently, the task of identifying phase discontinuities has been formulated into a two-class classification problem, where the phase discontinuities are identified by a complex neural network trained on plenty of image patches taken from wrapped phase maps. A simple but efficient classification framework is proposed for the phase discontinuities identification task. Six features are first designed to describe the characteristics of discontinuous and continuous pixels. Then, the naive Bayes classifier, working on these features, is employed as the classifier of our framework. Finally, a thinning procedure is performed on the classification results to get the one-pixel-width discontinuity location map which can be used for further phase unwrapping. The experiments on simulated wrapped phase maps are performed to validate the performance of the proposed approach. The experimental results show that the proposed approach can identify phase discontinuities in the wrapped phase maps well and has more robust performances when the signal-to-noise ratios of the phase maps are low.

The wavefront sensor is used in adaptive optics (AO) to detect the atmospheric distortion, which feeds back to the deformable mirror to compensate for this distortion. While the Shack–Hartmann sensor has been widely used, the plenoptic sensor was proposed in recent years. The two different wavefront sensing methods have different interpretations and numerical consequences, though they are both slope-based. The plenoptic sensor is compared with the Shack–Hartmann sensor in a closed-loop AO system. Simulations are performed to investigate their performances under closed-loop conditions. The plenoptic sensors both without and with modulation are discussed. The results show that the closed-loop performance of the plenoptic sensor without modulation is worse than that of the Shack–Hartmann sensor when the star for observation is brighter than magnitude 7, but better when the star is fainter. The closed-loop performance of the plenoptic sensor could be improved by modulation, except for the faint star. In summary, the limiting magnitude of the astronomical AO system may be improved by using the plenoptic sensor instead of the Shack–Hartmann sensor, and the modulation of the plenoptic sensor is more suitable for the bright star.

A practical technique is presented based on DH, for the reconstruction of a wavefront from three recorded intensity images. Combining the off-axis Fourier spatial filtering (OFSF) technique with iterative phase retrieval algorithms, it is shown how the twin image can be eliminated. The proposed method overcomes system geometry constraints and improves both the flexibility and resolution associated with OFSF-based DH. It also overcomes the cost problem associated with phase-shifting interferometry-based DH. In order to demonstrate the performance of the proposed DH method, both simulation and experiment results for objects having smooth and rough surfaces are presented.

Complex surface forms are becoming increasingly prevalent in optical designs, requiring advances in manufacturing and surface metrology to maintain the state of the art. Non-null interferometry extends the range of standard interferometers to test complex shapes without the need for complicated and expensive compensating elements. However, non-null measurements will accumulate significant retrace errors, or interferometer-induced errors, which can be difficult to isolate from surface figure errors. Methods discussed in the literature to correct for retrace errors in a reflection-based interferometer are computationally intensive and limited in spatial resolution. A method is presented for reconstructing complex surface shapes in a reflection-based non-null interferometer configuration, which is computationally efficient, easy to implement, and can produce high spatial resolution surface reconstructions. The method is verified against simulated surfaces that contain more than 200  μm of surface departure from a null configuration. Examples are provided to demonstrate the effects of measurement noise and interferometer model uncertainties, as well as an experimental validation of the method.

In real-time phase measuring profilometry based on color-encoded sinusoidal three-step phase-shifting algorithm, three frames of red (R), green (G), and blue (B) individual channel color-encoded sinusoidal gratings with an equivalent shifting phase of 2π/3 are combined to be a color fringe pattern. While this color fringe pattern is projected onto the measured object rapidly and sequentially by a special digital light projector, a high-speed monochrome camera synchronized with the projector signal is used to capture the corresponding deformed patterns in R, G, and B channels. Due to the monochrome camera’s different sensitivity to R, G, and B light, there will be a grayscale imbalance among the captured phase-shifting deformed patterns from R, G, and B channels. So, a grayscale imbalance correcting method based on fringe normalization is proposed. The experimental results verify the feasibility of the proposed method. The three-dimensional (3-D) shape information can be obtained in real-time. It is suitable for real-time 3-D measurement.

When testing lenses with Hartmann methods, a wave aberration function W is typically estimated. This W represents the deviations of the wavefront surface w with respect to an ideal wavefront E. In this test, the distance r from the observation screen to the second lens surface is considered, and, as in the case of W, by considering paraxial approximations, two estimations of w can be directly constructed from Hartmann test data without calculating W. We have compared these two estimations by taking into account small r values; a possible and suitable condition to measure some relatively high-power lenses. The importance of estimating w can be useful for improving some optical measurements as power map reconstructions.

We present a simple mathematical method for phase shifting that overcomes some phase shift errors and limitations of commonly used methods. The method is used to generate a sequence of phase-shifted interferograms from a single interferogram. The generated interferograms are employed to reconstruct the wavefront aberrations, as an application. The approach yields results with only very small deviations compared to both simulated wavefront aberrations, including the first 25 Zernike polynomials (0.05%) and those measured with a Shack-Hartmann sensor (0.5%).

Automatic guided vehicle (AGV) as a kind of mobile robot has been widely used in many applications. For better adapting to the complex working environment, more and more AGVs are designed to be omnidirectional by being equipped with Mecanum wheels for increasing their flexibility and maneuverability. However, as the AGV with this kind of wheels suffers from the position errors mainly because of the frequent slipping property, how to measure its position accurately in real time is an extremely important issue. Among the ways of achieving it, the photoelectric scanning methodology based on angle measurement is efficient. Hence, we propose a feasible method to ameliorate the positioning process, which mainly integrates four photoelectric receivers and one laser transmitter. To verify the practicality and accuracy, actual experiments and computer simulations have been conducted. In the simulation, the theoretical positioning error is less than 0.28 mm in a 10  m × 10  m space. In the actual experiment, the performances about the stability, accuracy, and dynamic capability of this method were inspected. It demonstrates that the system works well and the performance of the position measurement is high enough to fulfill the mainstream tasks.

Contact lens performance depends on a number of lens properties. Many metrology systems have been developed to measure different aspects of a contact lens, but none test the surface figure in reflection to subwavelength accuracy. Interferometric surface metrology of immersed contact lenses is complicated by the close proximity of the surfaces, low surface reflectivity, and instability of the lens. An interferometer to address these issues was developed and is described here. The accuracy of the system is verified by comparison of glass reference sample measurements against a calibrated commercial interferometer. The described interferometer can accurately reconstruct large surface departures from spherical with reverse raytracing. The system is shown to have residual errors better than 0.05% of the measured surface departure for high slope regions. Measurements made near null are accurate to λ/20. Spherical, toric, and bifocal soft contact lenses have been measured by this system and show characteristics of contact lenses not seen in transmission testing. The measurements were used to simulate a transmission map that matches an actual transmission test of the contact lens to λ/18.

For high-speed three-dimensional shape measurement, techniques with projector defocusing have become increasingly important. When the projector is defocused, structured patterns used for phase unwrapping are blurred. These blurred patterns need to be normalized into binary distributions in actual measurement. We propose a method to normalize these blurred structured patterns. The retrieved absolute phase is correct even when the measured objects contain discontinuous parts or contrasting colors.

Odd-order surfaces have begun to be used in optics. In order to investigate the aberration characteristics of such surfaces, Zernike expansion is widely used since it directly and explicitly corresponds to wavefront aberrations. Since the Zernike expansion of an odd-order surface contains an infinite number of terms, the convergence of the expanded sum and the possibility of termwise derivatives are not explicitly guaranteed mathematically. We give a complete proof for these problems. For an application of this result, we analyze the aberration characteristics of odd-order surfaces and present their effectiveness in optical design.

Recently, annular beams have been developed to rapidly fabricate microscope tubular structures via two-photon polymerization, but the distribution of the light field is limited to a ring pattern. Here a Fresnel lens is designed and applied to modulate the light field into a uniform quadrangle or hexagon shape with controllable diameters. By applying a spatial light modulator to load the phase information of the Fresnel lens, quadrangle and hexagon structures are achieved through single exposure of a femtosecond laser. A 3×6 array of structures is made within 9 s. Comparing with the conventional holographic processing, this method shows higher uniformity, high efficiency, better flexibility, and easy operation. The approach exhibited a promising prospect in rapidly fabricating structures such as tissue engineering scaffolds and variously shaped tubular arrays.

A design is proposed to enhance the quality factor of a photonic crystal ring resonator. A super defect is employed inside the ring resonator, which consists of variation of hole dimensions inside the ring resonator in such a way that the radiation field components of the resonant nanocavity are forced to get cancelled in order to reduce radiation loss. After this forced cancellation, the improved Q factor is calculated as 18,000. This photonic crystal ring resonator can be used for sensing applications like force sensing, pressure sensing, biochemical sensing, and communication applications like demultiplexing.

We demonstrate, theoretically and experimentally, a new method to measure high-order topological charges (TCs) of Laguerre–Gaussian vortex beams, including the magnitude and the sign, by analyzing the interference intensity patterns. The magnitude is determined by analyzing the interference intensity patterns between the vortex beam and its conjugate beam, and using an improved Mach–Zehnder interferometer with a dove prism. Counting the number of interference bright petals attests to the magnitude of high-order TCs through half of the bright petal number. After the TC is modulated by a spiral phase plate, the sign is acquired by comparing the counting results of two charge-coupled devices. Just by this method, we have been able to measure both the magnitude and the sign of the TCs up to l=±90. Our experimental results are in good agreement with the numerical simulations.

The optimization of a primary mirror support system is one of the most critical problems in the design of large telescopes. Here, we propose a hybrid optimization methodology of variable densities mesh model (HOMVDMM) for the axial supporting design, which has three key steps: (1) creating a variable densities mesh model, which will partition the mirror into several sparse mesh areas and several dense mesh areas; (2) global optimization based on the zero-order optimization method for the support of primary mirror with a large tolerance; (3) based on the optimization results of the second step, further optimization with first-order optimization method in dense mesh areas by a small tolerance. HOMVDMM exploits the complementary merits of both the zero- and first-order optimizations, with the former in global scale and the latter in small scale. As an application, the axial support of the primary mirror of the 2.5-m wide-field survey telescope (WFST) is optimized by HOMVDMM. These three designs are obtained via a comparative study of different supporting points including 27 supporting points, 39 supporting points, and 54 supporting points. Their residual half-path length errors are 28.78, 9.32, and 5.29 nm. The latter two designs both meet the specification of WFST. In each of the three designs, a global optimization value with high accuracy will be obtained in an hour on an ordinary PC. As the results suggest, the overall performance of HOMVDMM is superior to the first-order optimization method as well as the zero-order optimization method.

Predictable and stable fabrication processes are essential for reliable cost and quality management in optical fabrication technology. This paper reports on strategies to generate and control optimum sets of process parameters for, e.g., subaperture polishing of small optics (featuring clear apertures smaller than 2 mm). Emphasis is placed on distinguishing between machine and process optimization, demonstrating that it is possible to set up the ductile mode grinding process by means other than controlling critical depth of cut. Finally, a recently developed in situ testing technique is applied to monitor surface quality on-machine while abrasively working the surface under test enabling an online optimization of polishing processes eventually minimizing polishing time and fabrication cost.

A high-birefringence photonic quasi-crystal fiber (HB-PQF) based on SiO₂ is proposed. The relationships between birefringence and structure parameters and between beat length and structure parameters are researched by finite difference beam propagation method. With the optimization of fiber structure parameters, the birefringence is 1.4207×10−², which is two orders of magnitude higher than the normally used fiber when the wavelength is 1.55  μm. The radius of the fiber is 6.5  μm. The HB-PQF in a communication sensor will have important application prospects.

We propose an asymmetric 3×2 multi-input multi-output (MIMO) system for polarization division multiplexing (PDM) transmission in visible light communication (VLC). In PDM transmission, independent channels are constructed by polarization orthogonality, which is vulnerable to misalignment of polarization between the transmitter and the receiver. Although PDM-based VLC system may not maintain polarization orthogonality, the proposed asymmetric MIMO system can achieve the polarization diversity required for a multichannel system. We experimentally demonstrate the performance enhancement of PDM VLC transmission using the proposed asymmetric 3×2 MIMO technique.

An investigation on the effect of practically possible upper and lower parabolic dips in the refractive index profile of the inner core of the coaxial fiber Raman gain amplifier is reported using matrix method for single pump. It is seen that for lower parabolic dip, the tolerable limits of dip parameters correspond to dip depth of 0.25% and dip width of 25% of the respective parameters for ideal step index profile case and agree with the earlier predicted linear dip. However, for upper parabolic dip, one gets higher gain and better flatness at these limits. Even up to 1% of the dip depth for 25% of dip width or 75% of dip width for 0.25% of the dip depth or 0.5% of dip depth and 50% of dip width, one can expect performance as good as that of the ideal one. However, since system designers will be aimed to produce ideal profile, our recommendation is to keep tolerable limits within 0.25% of dip depth and 25% of dip width of respective parameters. But one can accept profile with upper parabolic dip if there is deviation within the above relaxation limits for such dip.

The effect of directional image extension on the total pumping spot in thin disk lasers is reviewed. Three modified pumping setups that can compensate this effect and improve the absorption pumping area are presented, and the efficiency, advantages, and disadvantages of each modified configuration are discussed. Numerical comparison between absorption intensity profiles confirms an increment of maximum pump absorption density at the central region of the pumping area with respect to normal setup. Experimental investigation for the last modified pumping arrangement is in good agreement with simulation results. Measuring laser power output with a similar simple I-shaped resonator shows an enhancement of slope efficiency up to 2.5% together with a lower laser threshold for our modified setup.

The effect of a gradual reduction of the fiber diameter on the acousto-optic (AO) interaction is reported. The experimental and theoretical study of the intermodal coupling induced by a flexural acoustic wave in a biconical tapered fiber shows that it is possible to shape the transmission spectrum, for example, substantially broadening the bandwidth of the resonant couplings. The geometry of the taper transitions can be regarded as an extra degree of freedom to design the AO devices. Optical bandwidths above 45 nm are reported in a tapered fiber with a gradual reduction of the fiber down to 70  μm diameter. The effect of including long taper transition is also reported in a double-tapered structure. A flat attenuation response is reported with 3-dB stopband bandwidth of 34 nm.

We propose and experimentally demonstrate time division multiplexed orbital angular momentum (OAM) access system to increase transmission capacity and spectral efficiency. In this system, data carried on different time tributaries share the same OAM mode. Multiple time division multiplexed OAM modes are multiplexed to realize two-dimensional (time dimension and OAM dimension) multiplexing. Therefore, the capacity and spectral efficiency of the access system will increase. The orthogonality between optical time division multiplexing (OTDM) and OAM techniques is also verified in our experiment. In a proof-of-concept experiment, 2×5-Gbps return-to-zero signal over OAM mode +4 is transmitted and investigated. The bit error ratio performance after transmission in this system can be smaller than 1×10−⁹. Results show that the proposed time division multiplexed OAM access system is suitable for future broadband access network.

We report on the fabrication and characterization of KTiOPO₄ optical waveguides formed by Rb⁺_-K⁺ ion exchange in high-purity RbNO₃ melt at 340°C for 90 min and subsequent He⁺-ion irradiation at energy of 500 keV and fluence of 3×10¹⁶  ions/cm². The irradiation of KTiOPO₄ crystals with He⁺ ions was simulated using the stopping range of ions in matter (SRIM’2006) software. The dark mode spectra of the samples were measured with the prism coupling method. The reconstructed refractive index profiles of the planar waveguide show a barrier in the middle of the guiding region, and a refractive index enhancement region on each side of the barrier, indicating the formation of a double waveguide.

Laser beam far-field alignment as well as frequency-doubling and frequency-tripling crystal adjustment is very important for high-power laser facility. Separate systems for beam and crystal alignment are generally used while the proposed approach by off-axial grating sampling share common optics for these two functions, reducing both space and cost requirements. This detection system has been demonstrated on the National Laser Facility of Israel. The experimental results indicate that the average far-field alignment error is <5% of the spatial filter pinhole diameter, average autocollimation angle error of crystals is <10  μ rad, and average frequency-tripling conversion efficiency is 69.3%, which meet the alignment system requirements on the beam direction and crystals.

Cs diode pumped alkali laser (DPAL) operation using ethane, methane, and mixtures of these hydrocarbons with the noble gases He and Ar as a buffer gas for spin–orbit relaxation was studied in this work. The best Cs DPAL performance in continuous wave operation with flowing gain medium was achieved using pure methane, pure ethane, or a mixture of ethane (minimum of 200 Torr) and He with a total buffer gas pressure of 300 Torr.

We experimentally demonstrated a Q-switched mode-locked (QML) and a continuous-wave mode-locked (CWML) ytterbium-doped fiber lasers with topological insulator: Bi₂Se₃ as saturable absorber (SA) in all normal dispersion regime. The Bi₂Se₃-SA is conventionally composited by embedding Bi₂Se₃ nanoplatelets into polyvinyl alcohol thin film, which provides a modulation depth of 7.6% and a saturation intensity of 38.9  MW/cm². Based on this SA, with different cavity length, ytterbium-doped fiber laser can be operated at QML and CWML state, respectively. In the QML operation, a Q-switched envelope has the shortest pulse width of 1.12  μs and the tunable repetition rate from 96 to 175 kHz. The largest pulse envelope energy is 39.6 nJ, corresponding to average output power of 6.93 mW. In the CWML operation, an environmentally stable dissipative soliton laser pulse with pulse duration of ∼210  ps is obtained. The single pulse energy is 0.83 nJ with the repetition rate of 11.38 MHz at the wavelength of 1037 nm.

Photonic waveguides are fundamental components for photonic integrated circuits (PICs). Although a wide spectrum of nanophotonic structures, i.e., silicon waveguides and plasmonic waveguides, have been exploited for optical interconnects, these structures either can only support one polarization or they are not able to be integrated within a 1-μm scale due to strong crosstalk. The hurdle for high-density information transmission and waveguide integration is mainly the lack of a compact waveguide structure that can support different polarization states with low crosstalk. We propose and numerically demonstrate an ultralong-range waveguide that supports both transverse electric- and transverse magnetic-like polarizations. The propagation length of this waveguide is several decimeters with working bandwidths as great as 160 nm for both polarizations. In addition, this design is very compact with a small center-to-center distance of 1  μm between two adjacent waveguides while the isolation is as high as more than 69.3 dB. This waveguide is also able to guide light efficiently through a 90 deg bend with a 1-μm bending radius for both polarizations. Our work opens new perspectives for high-density waveguide integration in PICs, which would benefit various applications with limited physical space, such as on-chip information processing and sensing.

A type of extrinsic Fabry–Perot interferometer (EFPI) fiber optic sensor, i.e., the microcavity strain sensor, is demonstrated for embedded, high-temperature applications. The sensor is fabricated using a femtosecond (fs) laser. The fs-laser-based fabrication makes the sensor thermally stable to sustain operating temperatures as high as 800°C. The sensor has low sensitivity toward the temperature as compared to its response toward the applied strain. The performance of the EFPI sensor is tested in an embedded application. The host material is carbon fiber/bismaleimide (BMI) composite laminate that offer thermally stable characteristics at high ambient temperatures. The sensor exhibits highly linear response toward the temperature and strain. Analytical work done with embedded optical-fiber sensors using the out-of-autoclave BMI laminate was limited until now. The work presented in this paper offers an insight into the strain and temperature interactions of the embedded sensors with the BMI composites.

We demonstrate the manufacturing of embedded multimode optical waveguides through linking of polymethylmethacrylate (PMMA) foils and cyclic olefin polymer (COP) filaments based on a lamination process. Since the two polymeric materials cannot be fused together through interdiffusion of polymer chains, we utilize a reactive lamination agent based on PMMA copolymers containing photoreactive 2-acryloyloxyanthraquinone units, which allows the creation of monolithic PMMA-COP substrates through C-H insertion reactions across the interface between the two materials. We elucidate the lamination process and evaluate the chemical link between filament and foils by carrying out extraction tests with a custom-built tensile testing machine. We also show attenuation measurements of the manufactured waveguides for different manufacturing parameters. The lamination process is in particular suited for large-scale and low-cost fabrication of board-level devices with optical waveguides or other micro-optical structures, e.g., optofluidic devices.

Planar concave grating wavelength demultiplexers with a flattened spectral response are realized based on SU-8 polymer waveguides. The flattened spectral response is accomplished by using an optimized multimode interference (MMI) coupler as the input aperture of the planar waveguide for all spectrally separated channels. The mode field distribution at the input of the planar waveguide is controlled by adjusting the width of the input taper connected to the MMI coupler. The devices are fabricated by cost-effective one-step UV lithography. Experimental results show that the desired flattened spectral response has been realized. The on-chip loss, crosstalk, and nonuniformity of the fabricated device are −14.8, −22, and 2.5 dB, respectively.

A type of high-birefringent terahertz (THz) photonic crystal fiber (PCF) with all circle air holes is proposed. The characteristics including birefringence, dispersion, and confinement loss are numerically analyzed in detail by using the finite element methods. Simulation results show that the proposed THz PCFs exhibit high birefringence on the level of 10⁻² in the frequency range of 2 to 4 THz, which is realized by the minor position adjustment of air holes in the first ring of the cladding. We believe that the proposed THz PCFs can be fabricated without complications due to their simple structure. In addition, two porous-core THz PCFs are proposed and the birefringence property is investigated.

This paper presents the first diode laser-based measurements of trace hydrogen sulfide at the 2646.35 nm absorption line and evaluates the feasibility of this line for natural gas sensing applications. The balanced detection technique is used to achieve a detection limit of 25 parts per million-meter H₂S in 150 Torr (T) balance N₂. Spectral interference effects of major natural gas constituents (methane, ethane, and propane) are also presented. It is shown that methane interference can be avoided by lowering the sample pressure, while propane has no interference. However, strong interfering lines of ethane are observed around the 2646.35 nm region. To our knowledge, this is the first report where individual ethane lines in this spectral region are observed. These observations make the 2646.35 nm H₂S line unfavorable for natural gas applications. This line, however, still remains attractive for H₂S sensing applications with low ethane content.

We study the potentials of a wide-aperture crystalline calomel-made acousto-optical cell. Characterizing this cell is nontrivial due to the chosen regime based on an advanced noncollinear two-phonon light scattering. Recently revealed important features of this phenomenon are essentially exploited in the cell and are investigated in more detail. These features can be observed more easily and simply in tetragonal crystals, e.g., calomel, exhibiting specific acousto-optical nonlinearity caused by the acoustic waves of finite amplitude. This parametric nonlinearity manifests itself at low acoustic powers in calomel possessing linear acoustic attenuation. The formerly identified additional degree of freedom, unique to this regime, is exploited for designing the cell with an eye to doubling the resolution due to two-phonon processes. We clarify the role of varying the central acoustic frequency and acoustic attenuation using that degree of freedom. Then the efficiency of calomel is exploited to expand the cell’s bandwidth with a cost of its efficiency. Proof-of-principle experiments confirm the developed approaches and illustrate their applicability to innovative techniques of optical spectrum analysis with the improved resolution. The achieved spectral resolution of 0.205 Å at 405 nm and the resolving power 19,800 are the best for acousto-optical spectrometers dedicated to space or airborne operations to date as far as we know.

Photonic multiline filters exhibiting periodic resonance lines on a dense spectral grid in a broad wavelength range are demonstrated. We design the filters using rigorous numerical methods and then proceed with experimental verification by patterning, etching, and collecting spectral data. We use standard double-side-polished 300-μm-thick silicon wafers for the experiments. On one side of the wafer, we place a shallow grating whereas the other side has a quarter-wave antireflection layer. An example filter with a ∼200-nm-deep TiO₂ grating on the wafer yields 12 narrow resonance peaks within a 10-nm wavelength range centered at 1550 nm. The spectral width of each filter peak is ∼0.1  nm with a free spectral range of ∼0.8  nm. Peak efficiency approaches 80% with low sidebands between the resonant filter lines. We discuss briefly design of polarization-independent periodic filters and the concept of Brewster-angle filters. Possible applications include spectral sampling and wavelength discretization as used, e.g., in sensing gas species and quantifying toxic gas concentrations.

A strip/slot hybrid horizontal silicon nitride slot waveguide is designed to provide an ultraflat and low dispersion. By optimizing the height and width of the structure, an ultraflat and low dispersion of ∼0±7  ps/nm/km over 812 nm wavelength range (from 1137 to 1949 nm) can be achieved. The waveguide with a 20-nm conformal overlayer has chromatic dispersion within ±1  ps/nm/km over 682-nm bandwidth. So the flatness is 0.0015, which is the lowest flatness in near-infrared regime of this kind of waveguide to our knowledge. The influence of the waveguide sidewall to dispersion is also discussed.

Several types of multilayer metamaterial devices are studied to achieve cross-polarization conversion as well as subwavelength transmission enhancement. The surface current of three metamaterial layers reveals that the induced current around the apertures of the middle metal layer is the key to the physical mechanism. Furthermore, the simulated results show that the proposed devices have good performance on both the polarization conversion ratio and transmission rate. Lastly, a tunable cross-polarization conversion design is proposed and researched. These results can help to facilitate the development of high-sensitivity sensors and terahertz technology.