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The miniaturization of semiconductor chips creates discrepancies between the designed node size and physical values. It has resulted in a tightened on-product overlay (OPO) budget and increased the demand for improved measurement noise reduction and accuracy in optical systems. A solution utilizing moiré targets can address such challenges by enabling the amplification of small misalignments that cannot be achieved with conventional overlay targets using an image-based overlay (IBO) estimator. However, moiré patterns formed within a layer introduce noise sources and problems owing to interference from the reflected light, adversely affecting the precision of overlay measurements and limiting the effective utilization of moiré patterns. We investigate the problems associated with moiré patterns in the IBO measurement method and propose a novel overlay measurement algorithm to mitigate the problems. The proposed algorithm increases the accuracy of the filtering method in the spatial frequency domain and improves the overlay precision by approximately 2% compared with conventional measurement algorithms. The proposed low-frequency selection algorithm and signal indexing algorithm effectively address the challenges posed by high-frequency problems and signal strength degradation in moiré patterns. The proposed practical solution achieves more accurate overlay measurements in semiconductor manufacturing, enabling better control and optimization of chip fabrication processes. [ABSTRACT FROM AUTHOR]

Infrared search and track (IRST) systems require reliable detection of small targets in complex backgrounds. Outlier-based methods are prone to high false positive rates due to the resemblance of point-like background features to small targets. The difference image-based method is an effective approach for suppressing point-like background interference; however, it has limitations in detecting slow-moving targets. In this letter, a novel sparse trajectory is proposed for moving target detection in IR videos. With a trajectory growing strategy, two kinds of trajectories from difference images, namely, short sparse trajectories and long sparse trajectories, are correlated to avoid the slow-moving targets being dismissed. The strategy matches the trajectories based on the sparse trajectory intensity composed of similarity measures and optical flow consistency. Finally, real targets are extracted from candidate trajectories using trajectory filtering. Experimental results show that, in the scene with point-like background features, our method achieves the best detection rate and lowest false alarm compared to the state-of-the-art methods. [ABSTRACT FROM AUTHOR]

As the semiconductor manufacturing process is moving towards the 3 nm node, there is a crucial need to reduce the edge placement error (EPE) to ensure proper functioning of the integrated circuit (IC) devices. EPE is the most important metric that quantify the fidelity of fabricated patterns in multi-patterning processes, and it is the combination of overlay errors and critical dimension (CD) errors. Recent advances in machine learning have enabled many new possibilities to improve the performance and efficiency of EPE optimization techniques. In this paper, we conducted a survey of recent research work that applied machine learning/ deep learning techniques for the purposes of enhancing virtual overlay metrology, reducing overlay error, and improving mask optimization methods for EPE reduction. Thorough discussions about the objectives, datasets, input features, models, key findings, and limitations are provided. In general, the results of the review work show a great potential of machine learning techniques in aiding the improvement of EPE in the field of semiconductor manufacturing. [ABSTRACT FROM AUTHOR]

Multitask measurements with different measuring ranges and resolutions have put forward higher requirements for force sensors. This article presents a novel variable configuration approach to develop an adjustable-resolution force sensor with a compact force-sensitive structure. Unlike the traditional strain gauge-based force measurement, the external force is detected through measuring the displacement of the deformed structure with a precision optical linear encoder. This approach can enable variable measuring range and force resolution through the continuous adjustment of structural stiffness. Specifically, the stiffness adjustment is implemented by changing the second moment of area of the sensor structure through rotating its build-in flexure beam shafts. Analytical models are established to provide the theoretical basis for analysis and design of the sensor. Furthermore, the validity of the design is evaluated with finite element simulations. Finally, a sensor prototype is developed for performance evaluation, and applied for the wire bond pull tests to evaluate the bond strength on the printed circuit board assembly. Experimental results show that it is effective to change the measuring range and force resolution of the developed force sensor through the stiffness adjustment. [ABSTRACT FROM AUTHOR]

The increasing requirement for reliable power demands robust protection with excellent waveform sensors. The presence of nonlinear elements in the modern-day loads brings challenges in sensing the current waveform, which is different from the conventional sinusoids. This research article proposes a novel current waveform sensing philosophy with the help of five numbers of series-connected wire-wound Hall current sensors. Signal conditioning circuits and their mathematical expressions are also discussed here to estimate the current waveform with 9.6% ± 2.4% linearity and 2.1%/A ± 1.3%/A sensitivity. The sensing technology senses the current waveform with a root mean square error (RMSE) value of 0.2. Apart from sensing the typical sinusoidal current waveform, the proposed sensing philosophy also senses the impulse and switching current waveforms with RMSE values 2.7 and 0.65, respectively. These results have a broad scope of improvement by implementing more wire-wound Hall current sensors in series. Digital output features and the optical link between the sensor and receiver offer necessary isolation to the microcontroller against harmful transients and other waveform distortions. The experimental verification and required data analytics confirm the applicability of the sensor with substantial accuracy, sensitivity, and linearity. [ABSTRACT FROM AUTHOR]

The performance and applications of the optical inline-Sagnac current sensor/transducer in utility high voltage alternating current measurements (metering and protection) and high voltage direct current measurements (control, metering, and protection) are presented. New measurements of commercially available sensors are presented for a 216 kApeak transient current, scale factor linearity for alternating currents from 5 mArms to 32 kArms and for direct currents from 100 mA to 27 kA, and temperature performance of ±0.11% over the temperature range of −60 °C to +65 °C. New properties of polarizer temperature-wavelength effects on the scale factor of the sensor are presented. Power quality measurements are discussed over the frequency range of 16.66 Hz to 5 kHz. A discussion of the limitations of optical noise on low current measurements is given. The experimental and calculated sensor response functions are compared. The limitations for low current measurements due to the IEC 61869 standard digital output are presented. [ABSTRACT FROM AUTHOR]

A high-sensitivity and high-resolution refractive index (RI) sensor with ultrawide measurement range based on no core fiber (NCF) with large offset splicing and microwave photonic filter (MPF) interrogation is demonstrated theoretically and experimentally for the first time. In the sensor, an inline Mach–Zehnder interferometer (MZI) based on NCF with large offset splicing is used as the sensing head, and a single-passband MPF is designed to interrogate the sensing signal. Compared with the traditional interrogation in optical domain, the MPF interrogation converts the periodic interference pattern in optical spectrum into the nonperiodic bandpass frequency response in radio frequency (RF) domain, so the measurement range is unlimited in principle. Owing to the large offset splicing in the inline MZI structure, the sensitivity of the sensor as high as −1.1054 GHz/RIU is obtained in the experiment. More importantly, MPF interrogation improves the resolution by an order of magnitude to $9\times10$ −7 RIU. Meanwhile, the sensitivity and the resolution of the sensor are easily adjusted by changing the length of dispersive media. The high sensitivity and high resolution of the sensor have potential applications in ultralow concentration biochemical detection field. [ABSTRACT FROM AUTHOR]

Vector quantization is a viable solution to the problem as to full utilization of randomness provided by statistically dependent channel measurements in secret key extraction from randomly fluctuating channels. By aiming at atmospheric optical wireless channels, new vector quantization schemes based on sample grouping (SG) are proposed by using different ways, specifically, the $K\text {-dimensional}$ tree (KDT) partition and random placement, to separate samples of the dimension-reduced channel coefficient vector into a given number of groups, with each containing equal number of samples. Use of the KDT partition leads to both the basic KDT-partition-based quantization (B-KDTPQ) and improved KDT-partition-based quantization (I-KDTPQ) schemes. On the other hand, utilization of the random placement brings forth the random equisized grouping based quantization (REGQ) scheme. Performance evaluation shows that, due to use of quantization symbol modification, the I-KDTPQ scheme always has lower quantization symbol disagreement rate (QSDR) than the B-KDTPQ scheme; compared with the I-KDTPQ scheme, the REGQ scheme has more flexibility in accommodating itself to various conditions of channel coefficient fluctuations and noise levels. [ABSTRACT FROM AUTHOR]

This letter proposed a method to achieve angular selectivity of electromagnetic waves. Based on odd-mode resonant frequency selective surface (FSS), we can obtain varying frequency responses under different incident angles and polarizations. Angular selectivity for a given angle can be achieved at a fixed frequency. To further improve the angular selectivity, quarter-wave stacks principle is used to design the odd-mode FSS into angular selective surface (AGSS) with several bilayers. Multi bilayers AGSSs are analyzed and compared which indicates that our proposed method can effectively achieve angular selectivity from 20° to more than 70° for TE or TM polarizations. A 6-bilayer AGSS with perfectly transmission angle at 30° under TE polarization has been measured and the measurement agrees well with the simulation which further verifies the feasibility of our method. [ABSTRACT FROM AUTHOR]

In this work, by sandwiching a p-AlGaN layer into the n-AlGaN layer, we generate an energy barrier in the n-AlGaN layer to improve the current spreading effect of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). The abrupt energy barrier can balance the resistivity in the n-AlGaN layer, and this leads to the better lateral current distribution in the p-type hole injection layer. Numerical and experimental results show that the homogenized current distribution enables the enhanced external quantum efficiency (EQE) for the proposed DUV LED. In addition, the better conductivity modulation arising from the more homogenized current distribution decreases the forward voltage and suppresses the self-heating effect. [ABSTRACT FROM AUTHOR]

This article presents a novel and cost-effective vector network analyzer (VNA)-based phase-compensated channel sounder operating in the frequency range of 10–50 GHz using radio-over-fiber (RoF) techniques, which can support multilink/channel long-range phase-coherent measurements. The optical cable enables long-range channel measurements with a dynamic range of 115.7 dB at 30 GHz (for the back-to-back connection). The phase compensation scheme is utilized for stabilizing the inherent phase variations introduced by the optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. A novel optical delay line and combiner scheme is proposed and implemented to separate the signals, thereby saving the port resource on the VNA for multilink/channel measurements. The proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with the presence of thermal changes and mechanical stress. The phase change could be maintained within 3° at 10–30 GHz and 7° in 30–50 GHz compared to the over 80° phase variation introduced by the cable effects at 10–50 GHz, demonstrating the robustness and effectiveness of the developed channel sounder in practice. With the proposed optical delay line and combiner scheme, multiple channels can be measured simultaneously with minimal VNA ports, significantly reducing the measurement cost and time for multilink channel measurements. Two multilink indoor channel measurements are conducted and analyzed using a virtual uniform rectangular array (URA) at 28-GHz bands. The results demonstrate the capability of the proposed channel sounder to perform high-fidelity long-range ultrawideband multilink channel measurements. [ABSTRACT FROM AUTHOR]

The optical return-stroke speed calculation is vital to analyze the lightning development process. The traditional calculation methods of lightning optical return-stroke speed, including the slope-intercept and percentage-of-peak methods, usually have considerable calculation errors in noise cases. In this article, we generate the simulated optical return-stroke signal within 100 m above the starting point along the lightning channel and use four cross-correlation methods and six traditional methods to calculate the return-stroke speed at different observation points along the channel. We optimize the cross-correlation method and reduce the calculation error to within ±10% by selecting a part of the waveform to participate in the cross-correlation operation and performing the exponential operation on the frequency-domain cross-correlation sequence. The calculation results of the quadratic-correlation method and two optimized cross-correlation methods do not change significantly at different observation points along the channel. Different channel-base waveforms affect the return-stroke speed variation trend. The smaller the dispersion factor λp is, the more pronounced the variation of the calculation result is, and the greater the calculation error is. The optical waveform with generated Gaussian noise will significantly affect the traditional calculation method. In contrast, the cross-correlation calculation method with noise has only a tiny calculation error. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) detection from protrusion defects is performed in an actual gas-insulated switchgears (GISs) based on a built-in sensor integrated with fluorescent fiber and ultrahigh frequency (UHF) antenna. The characteristics of both optical and UHF signals are analyzed in terms of inception voltage, pulse distribution, and statistical characteristics including the phase-resolved partial discharge (PRPD) pattern and PD number. In the experiment, protrusions with different curvatures are adopted to investigate the reason for the differences in the characteristics of UHF and optical signals. The results show that UHF and optical methods have different detection sensitivity for different discharge types owing to the different energies of electromagnetic waves and optical signals radiated in different discharge types. The optical method can easily detect avalanche discharge, while the UHF method can easily detect streamer discharge. This difference leads to the difference of the PD characteristics obtained by the two partial methods. For example, for the PD of small curvature protrusions, the PD inception voltage (PDIV) of the optical method is lower than that of the UHF method, because the defect triggers avalanche discharge. Moreover, the optical and UHF signal pulses are not always simultaneous in the time domain because they detect different discharge types. The PRPD patterns obtained by the optical method have a wider phase distribution than the UHF method because the optical method can detect avalanche discharge under low voltage. [ABSTRACT FROM AUTHOR]

This article proposes an absolute-type inductive angle sensor for detecting the position of the YRT turntable bearing. The sensor comprises a multispeed incremental measuring unit and a single-speed absolute measuring unit sharing the same stator, rotor, and excitation circuit. The stator is composed of a ferromagnetic gear groove, two groups of excitation coils, and one group of induction coils arranged according to certain rules. The end cover of the YRT turntable bearing is machined as rotor and it rotated eccentrically relative to the rotation of the stator. A bridge circuit is used to acquire and process the sensor signal from the single-speed measuring unit. Compared with the traditional dual-channel absolute measurement, the proposed measurement method avoids the effects of different magnetic circuit disturbances on signal quality and measurement accuracy, and has more advantages in the volume and weight of the sensor. The structure and working principles of the sensor are presented in detail, the feasibility of sensing scheme is verified via simulation, and a sensor prototype is designed for the final experiment. Experimental results show that the accuracy of the sensor prototype is –0.06758° to 0.1203° before compensation and –0.001139° to 0.0015° after compensation in the range of [0°, 360°], respectively. [ABSTRACT FROM AUTHOR]

Recently, the rapidly developing Internet of Things (IoT) needs to support massive connectivity with high transmission data rate, which requires IoT transmitters with high speed and low power consumption. In this work, non-polar micro-LED operating at low current density of 10 A/cm $^{2}$ can still maintain a high bandwidth of 508 MHz and near-peak external quantum efficiency (EQE), making it promising for low-power, energy-efficient visible light communication (VLC) to address the challenge. At 10 A/cm $^{2}$ , non-polar micro-LEDs can achieve a low power consumption and high energy efficiency, still maintaining the data rate of 0.25 Gbps. In addition, data rates of 2 Gbps and 1.1 Gbps were also successfully achieved for non-polar micro-LED operating at 320 and 50 A/cm $^{2}$ , respectively, to evaluate the relationship between power consumption and achievable data rate. The capability of non-polar micro-LED that can realize low-power consumption and high bandwidth at low current density is expected to play a great role in the field of energy-efficient VLC, multi-function display and IoT in the future. [ABSTRACT FROM AUTHOR]

Infrared thermography is a widely used noncontact temperature measurement method. However, the infrared temperature measurement result is susceptible to interference factors. To overcome the infrared temperature measurement errors caused by the simultaneous influence of multiple interference factors, this article takes the measuring distance and dust as specific interference factors and proposes an intelligent compensation method for the infrared temperature measurement errors caused by the measuring distance and dust. First, the influence of the measuring distance and dust is analyzed from the perspective of the infrared temperature measurement principle. Second, a weighted ensemble stacked denoising autoencoder (WE-SDAE) is proposed and used to construct an intelligent compensation model. Then, the measuring distance, dust transmittance, and measured temperature are used as the input of the compensation model to estimate the errors caused by the measuring distance and dust. Finally, the estimated error is used to compensate for the original infrared temperature measurement results under multifactor interference. Experimental results in two cases show that the proposed method is effective in compensating for the infrared temperature measurement errors caused by the measuring distance and dust, which is beneficial to ensure the accuracy of the infrared thermal imager in complex temperature measurement scenarios. [ABSTRACT FROM AUTHOR]

By combining information such as genomics, proteomics, and metabolomics data, multiomics methodologies have the potential to reveal the otherwise invisible processes of carcinogenesis. Using these data, a new-generation point-of-care (POC) device may support patient care through rapid diagnosis involving accurate and simultaneous multiomics measurements. In this work, we present a proof-of-concept portable system capable of measuring a metabolite (glutamate) and a protein [proteolytically active prostate-specific antigen (aPSA)] simultaneously on a single device in a 2-min interval using on-chip optical methods. The device has the potential to improve the current diagnostic pathways of prostate cancer (PCa) by reducing the high false-positive rate with the widely used total prostate-specific antigen (tPSA) test. We demonstrate a simultaneous assay of glutamate and aPSA in buffer and synthetic urine. For the glutamate assay, the limit of detection (LOD) and resolution were found to be 0.9 and $2.7 {\boldsymbol {\mu }}\text{M}$ , respectively. The LOD and resolution for aPSA were 0.5 and 2.0 ng $/$ ml, respectively. The LOD and resolution achieved using this device are, therefore, in line with human physiological ranges. The platform is suitable for accommodating two independent optical assays, and, therefore, chemistry can be modified to measure different analytes regardless of their nature. This technological breakthrough has the potential to deliver an accurate multiomics portable device for PCa and may be extended to other diseases. [ABSTRACT FROM AUTHOR]

This letter presents a new Si CMOS linear-mode avalanche photodiode (APD) based on an electric field distribution formed by field-line crowding. In this structure, a spherical avalanching electric field is enforced by field-line crowding due to the curvature of the half-sphere cathode (n-well). The electric field extends radially and, therefore, the entire low-doped epitaxial layer serves as charge collection zone. This APD can provide high responsivity and bandwidth due to its thick absorption zone and drift-based carrier transport. Measurements using a 675 nm laser source at 200 nW optical power show a maximum bandwidth of 1.6 GHz while the responsivity is 32 A/W. In addition, a maximum responsivity of 3.05 $\times \,\,10^{3}$ A/W at 5 nW optical power is achieved. Due to the high avalanche gain, large bandwidth, and CMOS compatibility without any process modification, this APD is a promising optical detector for many applications. [ABSTRACT FROM AUTHOR]

The measurement precision of the optical encoder is influenced by coupling interferences from the system and the environment. To further improve the measurement precision of the optical encoder, in this article, a nonlinear error compensation method is proposed to extract the optimal underlying trend of the measurement error, which is based on the empirical mode decomposition (EMD) with a local-sinusoidal-assisted (LSA) scheme and an adaptive intrinsic mode function (IMF) selection scheme. To solve the inherent problem of mode-mixing phenomena existing in the decomposed IMFs from the measurement error, an LSA scheme is proposed to adaptively change the extreme distribution of the measurement error, which is based on an abnormal segment discrimination rule. An adaptive IMF selection scheme is proposed to construct the optimal underlying trend of the measurement error, which is first formulated as an optimization problem. Next, a significant factor is defined to select the eligible IMFs with more contributions to the underlying trend. Comparison experiments indicate that the eligible IMFs can be adaptively selected, and the proposed compensation method achieves an excellent compensation performance with the root-mean-square error of 0.380 $\mu$ m in the 95% confidence interval of $-$ 0.017–0.016 $\mu$ m, which is superior to the previous EMD-based compensation methods. [ABSTRACT FROM AUTHOR]

The transmission delay in an accurate time-transfer system using an optical fiber is calculated by accurately measuring the roundtrip delay of the fiber. The delays in the forward and backward paths of the transmission line must be identical to achieve a highly accurate transmission delay compensation. The effect of chromatic dispersion, which occurs when the optical wavelengths of the forward and backward signals are different, and the effect of backscattering, which occurs in the fiber, are crucial and must be considered for higher accuracy. In this study, we present a fiber transmission delay measurement method that uses optical signals of the same wavelength for the forward and backward paths to overcome these problems. Signal separation can be achieved between the forward and backward signals and between the transmission and backscattered signals by shifting the modulation frequency of a sub-carrier of the optical signal at the remote station. The experimental results demonstrate that a highly accurate time-transfer of more than 1 ns can be expected over a 250 km fiber transmission. [ABSTRACT FROM AUTHOR]

Tunable 50/75 GHz millimeter wave (mmW) transmission system has been demonstrated employing self-injection locked quantum-dash laser diode comb source (SIL QDCS) in the challenging 1610 nm region. The generated 50/75 GHz mmW beat-tones exhibited low phase noise of −64 dB/Hz at 1 kHz offset and narrow radio-frequency (RF) linewidths of ~1 kHz at the transmitter end. A 4 GBd (8 Gb/s) quadrature phase-shift keying (QPSK) data stream is successfully transmitted over both mmW carriers and across various channels viz. 2 m wireless (WL) link, 11.6 km single-mode-fiber (SMF) – 2 m WL link, and 11.6 km SMF – 6 m free-space-optics (FSO) – 2 m WL link, thus affirming the potential of quantum-dash laser diode (QDLD) in high-frequency mmW applications. This work is a potential step towards realizing next-generation high-frequency tunable mmW optical-RF convergent hybrid networks operation in L-band, and one such possible proposal with a single QDCS is presented. [ABSTRACT FROM AUTHOR]

We propose a new scheme for polarization-sensitive distributed fiber-optic sensing, called polarization optical correlation-domain reflectometry (POCDR), and experimentally demonstrate its fundamental operations. In this scheme, the states of polarization (SOPs) of reference lights are controlled so that they are uniformly distributed on the Poincaré sphere using a calibration method, which cancels the effect of unknown birefringence in the fibers used in the system. Then the SOP of spontaneous Brillouin scattering in a sensing fiber is estimated using a least-squares method, by measuring the interference powers for the multiple reference lights with different SOPs. As a pilot demonstration, the SOP transitions along a fiber including a three-paddle polarization controller are measured under different conditions. We also measure the SOP transition of a fiber wound on a bobbin and calculate its polarization beat length, which agrees well with the theoretical value. By exploiting its unique nature called random accessibility, high-speed measurement of the roundtrip birefringence between two arbitrary points in a sensing fiber will be potentially achieved. [ABSTRACT FROM AUTHOR]

Simultaneously increasing the sensitivity and dynamic range of an optical fiber sensor is desired and yet challenging. In this article, we demonstrate an optical fiber curvature sensor based on a no-core fiber (NCF) cascaded with a hollow-core fiber (HCF), realizing simultaneously high sensitivity and a large dynamic range with the assistance of machine learning analysis. The sensor is fabricated by simply fusion splicing a section of NCF and HCF to two single-mode fibers (SMFs), forming the SMF-NCF-HCF-SMF hybrid structure. It is shown that the multimode interference in the NCF can increase the sensitivity of the device for curvature measurements, compared to the conventional SMF-HCF-SMF structure. However, the enhanced sensitivity poses a limitation on the dynamic range of the proposed curvature sensor. We propose the use of machine learning to analyze the measured spectra of the sensor device, achieving one-to-one mapping between the measured raw spectrum and the exerted curvature on the sensor, and thereby the issue of the limited dynamic range is resolved. The proposed strategy, enabling the co-existence of high sensitivity and a large dynamic range, is a highly generalizable technique and can be extended to other optical fiber sensors for measuring other physical, chemical, and biological quantities in different applications. [ABSTRACT FROM AUTHOR]

Spectroscopic ellipsometry is a very sensitive optical metrology technique commonly used in semiconductor manufacturing lines to accurately measure the thickness and refractive index of different layers present on specific dedicated metrology targets on the wafers. In parallel, optical defectivity techniques are widely implemented in production lines to inspect a significant amount of dies representative of the full wafer and detect physical and patterning defects. A new approach can then simply emerge which is to apply ellipsometry metrology techniques at a full or die wafer scale. This strategy, at the frontier between metrology and defectivity field is expected to bring solutions for certain types of process deviation. In our case, ellipsometry’s optical response was collected on large areas of product wafers to capture specific deviations such as film properties, thickness, and patterning variation. This is an innovative strategy that relies on a model-less approach to detect process drifts, using ellipsometry’s sensitivity to material properties and design architecture variations. In this paper, we will present this approach on three industrial cases. [ABSTRACT FROM AUTHOR]

The use of an active measurement system to localize contamination in concrete structures of nuclear facilities offers advantages such as an easy on-site analysis within a short measurement time. A measuring system based on a scintillation crystal connected to a long optical fiber with a large diameter could satisfy the demands of a measuring system for gamma-ray spectroscopy in boreholes. This work describes the development of a gamma-ray spectroscopy system using a fiber optic radiation sensor. A prototype was built with a GAGG scintillation crystal, which could reach an energy resolution of 33% at the 662-keV gamma-ray emission energy of 137Cs. A minimal time for the detection of a contamination of a 137Cs source with an activity of 21 kBq and a layer of 6-cm concrete between sensor and radiation source was determined to be under 1 min with three different evaluation methods. They were based on the decision threshold and the Kolmogorov–Smirnov test corresponding to the energy distribution and the distribution of time differences between detector events. In addition, it was possible to estimate the distance of a point-like source from the sensor by establishing a further analysis parameter comparing different energy regions of a spectrum. [ABSTRACT FROM AUTHOR]

Objective: Most methods for monitoring sweat gland activity use simple gravimetric methods, which merely measure the average sweat rate of multiple sweat glands over a region of skin. It would be extremely useful to have a method which could quantify individual gland activity in order to improve the treatment of conditions which use sweat tests as a diagnostic tool, such as hyperhidrosis, cystic fibrosis, and peripheral nerve degeneration. Methods: An optical method using an infrared camera to monitor the skin surface temperature was developed. A thermodynamics computer model was then implemented to utilize these skin temperature values along with other environmental parameters, such as ambient temperature and relative humidity, to calculate the sweat rates of individual glands using chemically stimulated and unstimulated sweating. The optical method was also used to monitor sweat pulsation patterns of individual sweat glands. Results: In this preliminary study, the feasibility of the optical approach was demonstrated by measuring sweat rates of individual glands at various bodily locations. Calculated values from this method agree with expected sweat rates given values found in literature. In addition, a lack of pulsatile sweat expulsion was observed during chemically stimulated sweating, and a potential explanation for this phenomenon was proposed. Conclusion: A simple, non-contact optical method to quantify sweat gland activity in-vivo was presented. Significance: This method allows researchers and clinicians to investigate several sweat glands simultaneously, which has the potential to provide more accurate diagnoses and treatment as well as increase the potential utility for wearable sweat sensors. [ABSTRACT FROM AUTHOR]

A novel and fast double-slope assisted Brillouin optical time domain reflectometry (DSA-BOTDR) with linear regions merging has been proposed to enlarge the measurement range. In the proposed method, the linear regions of the left and right slopes of the Brillouin gain spectrum (BGS) are both used, and the positions of the two linear regions can be controlled by setting a suitable bandwidth of band-pass filter in acquisition system. Thus the linear regions can be merged together and the measurement range can be enlarged greatly. In a proof-of-concept experiment, a frequency measurement range of about 190 MHz is obtained in DSA-BOTDR, which is about 3.8 times of the conventional slope-assisted BOTDR (SA-BOTDR). In addition, the temperature of an 800 m polyimide coated fiber is measured within 0.4 s with 3 m spatial resolution. Moreover, a measurement range of 100.3 °C with a temperature uncertainty of 1.0 °C is successfully proved. [ABSTRACT FROM AUTHOR]

Considering that the driver and passengers need different display content in the vehicle display, we propose a dual-view and multi-content head-up display (HUD). Based on the angle selectivity of volume holographic grating (VHG), the multiplexed VHGs diffract light beams at different angles to achieve dual-view display. Based on the wavelength selectivity of VHG, the multiplexed VHGs diffract light beams of different wavelengths that carry different information to realize multi-content display. The HUD prototype is composed of a miniature laser projector, a diffuser, and a holographic combiner. The holographic combiner with a size of 20 cm × 15 cm includes a red-green multiplexed VHG1, a red-blue multiplexed VHG2 and a glass substrate. The field of views (FOVs) and eye-boxs (EBs) of the two viewing zones are 10° × 5° and 8° × 4°, 147 mm × 81 mm and 105 mm × 87 mm, respectively. In addition, compared with the traditional dual-view HUD, the proposed HUD only requires one picture generation unit (PGU), which is beneficial for its application to compact vehicle or aviation displays. Moreover, the driver and passengers can see the red warning information displayed by the system at the same time, which can significantly improve driving safety. [ABSTRACT FROM AUTHOR]

In recent years, nanopore has promisingly emerged as a single molecule sensor and is able to detect biomolecules in a label-free manner. Much progress has been made in biotechnological applications using electrical measurement of temporal changes in ionic current. But it has its own limitation with respect to noise, throughput, spatial and temporal resolutions. By incorporating the optical techniques, we can acquire information about the intrinsic property and heterogeneity of a single molecule. Here we review the recent advancements in electrical and optical sensing in nanopores. [ABSTRACT FROM AUTHOR]

In this paper, a differential micro-opto-electro-mechanical system (MOEMS) accelerometer based on the Fabry-Pérot (FP) micro-cavities is presented. The optical system of the device consists of two FP cavities and the mechanical system is composed of a proof mass that is suspended by four springs. The applied acceleration tends to move the PM from its resting position. This mechanical displacement can be measured by the FP interferometer formed between the proof mass cross-section and the optical fiber end-face. The proposed sensor is fabricated on a silicon-on-insulator wafer using the bulk micromachining method. The results of the sensor characterization show that the accelerometer has a linear response in the range of $\boldsymbol {\pm }1\text{g}$. Also, the optical sensitivity and resolution of the sensor in the static characterization are 6.52 nm/g and $153 \boldsymbol {\mu }\text{g}$. The sensor sensitivity in the power measurement is 49.6 mV/g and its resonant frequency is at 1372 Hz. Using the differential measurement method increases the sensitivity of the accelerometer. Based on the experimental data, the optical sensitivity in static mode is two times as high as that of a similar MOEMS accelerometer with one FP cavity. [ABSTRACT FROM AUTHOR]

A disambiguation method for dual-comb sampling frequency measurement is proposed and experimentally demonstrated. For dual-comb sampling frequency measurement, multiple solutions may exist if only the frequency information of the output signal is employed for disambiguation. By further exploiting the phase relationship between the intermediate frequencies and the first harmonics of the two sampling rates, most of the ambiguous frequencies can be distinguished in our experiment. The experiment results also indicate that nonideal phase response of the devices will reduce the performance of the method especially in high frequencies. To solve the problem, an improved method, in which the variations of the phases are observed and used to solve the ambiguity, is given so that the method is free from the calibration of the nonideal phase response. [ABSTRACT FROM AUTHOR]

For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation. [ABSTRACT FROM AUTHOR]

In this letter, we present a novel invisible bilayer ZnSnOy/ZnSnOx memristor for photo sensing. The device exhibits excellent photo sensing features such as a highly stable electrical set and optical reset endurances at least 2200 cycles for blue light and 1800 cycles for green and red light without any deterioration. Moreover, the device shows excellent retention (105 s) at 90 °C without disruption and high transmittance of about 85%. These outstanding device properties would allow us to create either an optical to an electronic memory device or an always-on continuous measurement optoelectronic sensor. [ABSTRACT FROM AUTHOR]

A novel dual-band reflective optical sensing method for simultaneous measurement of hemoglobin (Hb) concentration and temperature by hybrid guide-mode resonance (GMR)-topological photonic state (TPS) sensor was proposed. The dual-band reflection peaks can be explained by two different mechanisms: one is attributed to the excitation of GMR, and the other is due to the generation of TPS. The GMR worked as the sensing channel of Hb concentration because it is in direct contact with the Hb. Moreover, the TPS worked as the sensing channel of temperature. When the Hb concentration and temperature variation, the two reflection peaks would all shift, but with different sensitivities. Excellent biosensor performance can be obtained by systematically adjusting structural parameters such as grating period (P), grating width (W), grating height (h), and spacer layer thickness (d). Notably, by using the dual-wavelength matrix method, the cross-interference caused by temperature variation is eliminated. The results concluded that the Hb concentration sensitivity and the temperature sensitivity of the proposed sensor could reach 264.44 nm/RIU and 0.061 nm/°C, respectively. The proposed sensor provides a promising sensing strategy for multi-parameter sensing. [ABSTRACT FROM AUTHOR]

Unknown micro-contact forces may affect operation accuracy in micro-operation technology and high-precision contact measurement. Existing micro force sensors are unable to meet the demand for high resolution, large range, and low cost. Therefore, a more economical solution for high-precision force measurement is needed. A high-precision optical force sensor (OPFS) with a simple sensing principle is proposed in this study. The OPFS is mainly composed of a laser diode, a force-sensitive structure, and a four-quadrant photodetector (QPD). When force is applied to the measuring platform, the force-sensitive structure produces the corresponding elastic deformation, which is sensed by the QPD. The multi-range and customized resolution of the OPFS can be achieved by adjusting the stiffness of the force-sensitive structure. The design and optimization of the OPFS are completed via theoretical modeling and simulation analysis. The performance of the OPFS is examined experimentally. The OPFS is found to have a sensitivity of 0.27 mV/ $\mu \text{N}$ , a measuring range of 0–2.71 mN, and a resolution of $32\mu \text{N}$. The measurement uncertainty of the OPFS due to nonlinearity, resolution, repeatability, and drift is $82\mu \text{N}$. The effectiveness of the OPFS in the measurement of micro-forces is verified. Finally, the OPFS is used to measure the contact force of a micro/nano contact probe. The results show that OPFS can be used for force calibration in micro/nano contact measurement instruments. [ABSTRACT FROM AUTHOR]

A novel technique is demonstrated for suppressing the relative intensity noise (RIN) and enhancing the high-speed transmission performance of 850 nm vertical-cavity surface emitting lasers (VCSELs). The orthogonal polarization suppression ratio (OPSR) of top-emitting high-speed 850 nm VCSELs with rectangular shaped mesas can be greatly enhanced by electroplating a copper substrate onto the backside, without any degradation in slope efficiency (output power). The enhancement of the OPSR results in a significant reduction of the RIN (around −130 vs. −145 dB/Hz) over a wide frequency range (near DC to 20 GHz) in comparison to the reference device without the additional copper substrate. Moreover, the structure of the demonstrated device can not only flatten the electrical-to-optical (E-O) responses but also narrow the spectral width due to the strain induced by the copper substrate. Overall, the lower RIN and flatter E-O frequency responses in turn lead to a significant improvement in the 25 Gbit/sec eye-opening and lower timing jitter in our demonstrated device. [ABSTRACT FROM AUTHOR]

This paper investigates the capacity of general multiple-input single-output (MISO) optical intensity channels (OICs) under per-antenna peak- and average-intensity constraints. We first consider the MISO equal-cost constrained OIC (EC-OIC), where, apart from the peak-intensity constraint, average intensities of inputs are equal to arbitrarily preassigned constants. The second model of our interest is the MISO bounded-cost constrained OIC (BC-OIC), where, as compared with the EC-OIC, average intensities of inputs are no larger than arbitrarily preassigned constants. By leveraging tools from quantile functions, stop-loss transform and convex ordering of nonnegative random variables, we prove two decomposition theorems for bounded and nonnegative random variables, based on which we equivalently transform both the EC-OIC and the BC-OIC into respective single-input single-output channels under a peak-intensity and several stop-loss mean constraints. Capacity lower and upper bounds for both channels are established, based on which the asymptotic capacity at high and low signal-to-noise-ratio are determined. [ABSTRACT FROM AUTHOR]

The uncontrolled growing number of resident space objects (RSOs) threatens the safe operation of space-related activities. Since the beginning of the Space Age, outer space is getting populated by objects emerging after breakup events; spare components through launching, orbiting, and aging of satellite missions; collisions between functional or defunct RSOs; or by missions that either completed or began their life cycle. One potential measure toward the sustainable use of outer space may start by preventing collisions between existing RSOs. Collisions between existing RSOs will only exacerbate the current situation, which could lead to a cascade effect known as the Kessler syndrome. In the context of such collisions, the enabling of optical daylight tracking has the potential to reduce the uncertainty of the estimated state vector for each RSO, thus aiding the planning and execution of efficient avoidance maneuvers when a collision is foreseeable, as well as benefiting just-in-time collision avoidance strategies in the future. This study starts by analyzing the impact of optical daylight observations, within the domain of defunct RSOs, with respect to the currently restricted nighttime observation windows, the type of observable acquired by the observing station, and the relative geometry between the Sun, the RSO, and the ground station. We highlight the role of key hardware components on each observing system deemed critical for current observing optical ground stations to enable daylight measurement acquisition. Once we have inspected all factors deemed crucial for daylight observations in our system, we present successful daylight observations, from which we derived angular observables, ranges, and apparent brightness. We additionally provide an example where the combination of measurements acquired by the different systems, operating in the optical regime only, contributed to partial disambiguation of the tumbling motion of a selected rocket body. All observations were conducted using a scientific complementary-metal-oxide-semiconductor (CMOS) sensor and a geodetic laser ranging system. Both systems make use of the 1-m Zimmerwald laser and astrometry telescope (ZIMLAT) for measurement acquisition and target RSO tracking at the Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald (SwissOGS), operated by the Astronomical Institute of the University of Bern, Switzerland. [ABSTRACT FROM AUTHOR]

The Thermal Infrared Sensor 2 (TIRS-2) payload for the Landsat 9 mission closely follows the design of the TIRS instrument currently flying aboard Landsat 8. The TIRS-2 instrument, however, incorporates an important design change to mitigate the stray light issue that plagued the TIRS instrument. Shortly after the launch of Landsat 8 in 2013, calibration errors due to stray light artifacts were observed in Earth imagery from TIRS with magnitudes of 4% (10.8 $\mu \text{m}$ band) and 8% (12.0 $\mu \text{m}$ band). Out-of-field scans of the Moon were conducted to map the angles from which off-axis radiance was detected on the focal plane arrays. Optical modeling, constrained by reverse ray traces of the lunar data, identified the primary scattering sources within the TIRS telescope, and these results informed the locations and design of mitigating baffles for TIRS-2. The effect of the modifications to the TIRS-2 instrument was tested preflight through thermal vacuum (TVAC) characterization tests, and the optical models were updated to be consistent with the measured data. Preliminary assessments indicated at least an order of magnitude reduction of the total signal due to scattering in TIRS-2. On-orbit lunar scans provided the final confirmations and demonstrated that the new design changes to TIRS-2 have reduced the primary out-of-field scattering by over 40x from the original TIRS design bringing the total scattering to 1% or less. More importantly, Earth imagery produced by Landsat 9 TIRS-2 does not show any stray-light-related artifacts, as was prevalent in the Landsat 8 TIRS imagery. [ABSTRACT FROM AUTHOR]

Objective: Diffuse correlation spectroscopy (DCS) is an optical technique that allows for the non-invasive measurement of blood flow. Recent work has shown that utilizing longer wavelengths beyond the traditional NIR range provides a significant improvement to signal-to-noise ratio (SNR). However, current detectors both sensitive to longer wavelengths and suitable for clinical applications (InGaAs/InP SPADs) suffer from suboptimal afterpulsing and dark noise characteristics. To overcome these barriers, we introduce a cross correlation method to more accurately recover blood flow information using InGaAs/InP SPADs. Methods: Two InGaAs/InP SPAD detectors were used for during in vitro and in vivo DCS measurements. Cross correlation of the photon streams from each detector was performed to calculate the correlation function. Detector operating parameters were varied to determine parameters which maximized measurement SNR.State-space modeling was performed to determine the detector characteristics at each operating point. Results: Evaluation of detector characteristics was performed across the range of operating conditions. Modeling the effects of the detector noise on the correlation function provided a method to correct the distortion of the correlation curve, yielding accurate recovery of flow information as confirmed by a reference detector. Conclusion: Through a combination of cross-correlation of the signals from two detectors, model-based characterization of detector response, and optimization of detector operating parameters, the method allows for the accurate estimation of the true blood flow index. Significance: This work presents a method by which DCS can be performed at longer NIR wavelengths with existing detector technology, taking advantage of the increased SNR. [ABSTRACT FROM AUTHOR]

In this article, a cheap and simple method to determine the electron density of atmospheric microwave air plasma torch (AMAPT) is proposed, which avoids the complex and costly spectrum diagnosis methods. First, the incident and reflected microwave (MW) power of the AMAPT are measured to obtain the MW power reflection ratios. Then, a 3-D Gaussian electron density distribution is built to simulate the AMAPT, especially in calculating the MW power reflection ratios. By adjusting the electron density distribution to make the calculated MW power reflection ratios equal to the measured results, the electron density can get calculated. Finally, to verify this calculation, a system of a 2.45-GHz AMAPT is built up to measure the electron density with optical emission spectroscopy (OES) in different gas flow rates and gas compositions. The results show that the electron density determined by this method agrees well with the experimental measurements by OES with a relative error of 8% which is competent for engineering applications. The simplicity and accuracy make it a promising approach to obtaining the electron density of MW plasma in engineering applications. [ABSTRACT FROM AUTHOR]

In this article, an $SE(3)$ -constrained extended Kalman filter is proposed in continuous time as well as in a more practical continuous-discrete framework. The filter allows for the state estimation of the 6-DOF rigid body motion while accounting for measurement error statistics and using the rotation matrix instead of quaternions or other attitude parameterizations. The proposed filter differs from the recently proposed $SO(3)$ -constrained attitude filter in that only a subset of the configuration states are constrained in the present filter. Its effectiveness is demonstrated in a numerical example in which its performance is compared with that of an existing $SE(3)$ estimator from the literature and a Monte Carlo simulation is carried out to provide credence to the accuracy of the proposed filter. [ABSTRACT FROM AUTHOR]

The motion of electric charge is involved in several processes such as an electrochemical reaction taking place at a metal/electrolyte interface, the measurement of electrical impedance of a sample between a couple of electrodes or the absorption of a photon in a semiconductor and its conversion into an electron/hole pair (Fig. 1). Consequently, the quantification of the amount of charge collected at a sensing electrode is relevant in a broad variety of applications. This brief tutorial focuses on the challenges and techniques of measuring charge, primarily at the micro-scale, with application in the bio-medical field. [ABSTRACT FROM AUTHOR]

We present ultra-broadband flat-topped supercontinuum laser sources with programmable repetition rate, spectral bandwidth, and wavelength regime. Supercontinuum sources with multiband coverage can be achieved by merging high-repetition-rate electro-optic frequency comb techniques with optical line-by-line pulse shaping. Because nonlinear broadening in the optical wave-breaking regime does not provide the required flatness and spectral bandwidth to the comb-based sources, we iteratively manipulate the spectral amplitude and phase of the optical pulse shaper. We implemented here a smooth flat-topped supercontinuum with a 97 nm bandwidth at 10 dB and programmable repetition rate up to 50 GHz by applying a bandwidth-limited quasi-super-Gaussian apodised pulse train to a highly nonlinear medium with programmable super-Gaussian and phase coefficients. This work opens new possibilities for generating robust and coherent multiple optical carriers with further C-bands in optical networks. We believe that the supercontinuum sources based on electro-optic frequency comb and pulse shaping techniques enable the provision of coherent flat-topped optical sources, at least from the L- to S-band with a programmable repetition rate and spectral bandwidth. [ABSTRACT FROM AUTHOR]

This article put forwards a simple microwave photonics measurement method for microwave doppler frequency shift (DFS) and angle of arrival (AOA), which obtains the information conveniently and precisely by comparing the frequency and phase shift between two channels signals. The DFS direction can be judged more intuitively thanks to the introduction of the reference signal. The system uses two parallel dual parallel Mach-Zehnder modulators (DPMZMs) for carrier suppression single-sideband (CS-SSB) modulation. A proof-of-concept experiment was taken, showing that during ±100 kHz, the DFS measurement error is <±0.1 Hz over a 40 GHz frequency range as well as power sensitivity is down to −50 dBm. What is more, the measurement error of AOA is < ±1° from 0° to 90°. This simple system acquits excellent width band performance without the optical filter, providing a reliable and more conducive to human-computer interaction solution for future measurement and sensing applications. [ABSTRACT FROM AUTHOR]

Computer-generated hologram (CGH) is a diffractive optical element that has been commonly used as wavefront corrector in the interferometer and can be accurately manufactured to meet the requirements of high-precision detection. But the transmitted wavefront error (TWE) of the CGH substrate and the adjustment errors of cylindrical surface are influence factors of high accuracy measurement. In this paper, we propose the method of calibrating the CGH substrate TWE. After calibrating, the substrate TWE is 0.0052λ (RMS). Meanwhile, we obtain the general expression for the misalignment aberrations by describing the relationship between the wavefront of the measured cylindrical mirror and the corresponding adjustment errors. Then the adjustment errors of the test cylindrical surface can be completely separated by subtracting the fitting function from the measurement on the basis of least-square algorithm. After that, the PV value changes from 3.0565λ to 0.9825λ, and the RMS value changes from 0.5278λ to 0.1273λ, the measurement results have been greatly improved. [ABSTRACT FROM AUTHOR]

Transceiver imperfections become the primary source of impairment as baud rate and modulation order grow in advanced optical coherent communications. Thus, transceiver imperfections, both linear and nonlinear, need to be appropriately characterized, measured, and specified. Treatments for linear imperfections are relatively mature. This study reviews the transceiver's linear imperfection modeling, characterization, and measurement technologies. In practical applications, in-field measurement using the transceiver and a few low speed additional devices is preferred. In the case of nonlinear imperfections, the situation is complex. One important task is to estimate the nonlinear system performance from the nonlinear characteristics of the devices. In this study, we attempt to establish a connection between them by examining different technologies. Although the orthogonal component has a good correlation with nonlinear system performance, its measurement is prohibitively complex. In the nonlinear noise to power ratio measurement, a certain frequency component of the input signal is notched, and the re-growth component at the notch frequency is measured at the nonlinear device output. The ratio between the re-growth component power and the output signal power is the noise to power ratio. While this method is easy to carry out, it does not correctly estimate nonlinear impairment in general. The reason for this is that the signal incurs different nonlinear responses in two conditions, i.e., with or without a notch. This method accurately estimates the nonlinear impairment in some special but useful cases, such as Gaussian input signals and nonlinear systems whose dominant nonlinear term is the even order term. [ABSTRACT FROM AUTHOR]

The traditional way to detect the vertical structure of seawater optical properties and chlorophyll-a is mainly through shipboard discrete observations or biogeochemical-argo profiling floats, which requires considerable time to cover a limited area. In this study, the vertical distribution of seawater optical properties and chlorophyll-a concentration across two different optically contrasted sea areas from the East China Sea (ESC) to the South China Sea (SCS) were obtained for the first time using a shipboard integrated Mie–Raman–fluorescence Light Detection and Ranging (LiDAR) for large-scale observations, with a total observation distance of over 3700 km. More than 74000 LiDAR profiles were obtained from September 5 to September 15, 2020. In general, the LiDAR-estimated inherent optical properties (IOPs) and chlorophyll-a values decreased from turbid water in the ECS to clear water in the SCS. Subsurface scattering layers were often observed at depths ranging from 10 to 20 m along the SCS coast. Subsequently, the LiDAR-derived results were compared against in situ measurements. In addition, the diurnal hourly variation in IOPs and chlorophyll-a by LiDAR at a fixed coastal station was monitored for the first time, which was relatively lower in the early morning and midday yet was higher in the evening, while the relative tide height showed the reverse change trend, which revealed that the tide possibly impacted the diurnal variation in IOPs and chlorophyll-a on the SCS coast. Overall, our results indicate that the LiDAR remote sensing technique is effective and feasible to monitor large-scale and long-term subsurface phytoplankton structure over different optically contrasted sea regions, and integration of multiple detection mechanisms will enhance the monitoring capacity. [ABSTRACT FROM AUTHOR]

In this article, a constrained image-based visual servoing control method for the shipboard landing problem of unmanned helicopters is proposed. First, the pitch and roll motion of ship are predicted by an autoregressive (AR) model to determine an appropriate period for landing. Subsequently, a novel robust sliding mode controller without linear velocity measurements is developed on the basis of the perspective image feature in a virtual image plane. Meanwhile, a modified Chebyshev neural network (CNN) is proposed to estimate the uncertainties, including the linear acceleration of ship and translational perturbation, while an adaptive law is employed to compensate the influence of rotational disturbances. The whole controller only requires the measurements feedback of a vision sensor and an inertial measurement unit (IMU). Ulteriorly, to prevent the visual target on the ship from going beyond the field of view of camera, the constrained controller is developed by a control barrier function and a quadratic programming, where the unknown relative velocity is estimated by a velocity observer. Finally, simulations are implemented to substantiate the capability of the presented shipboard landing control method. [ABSTRACT FROM AUTHOR]

A simplified Doppler frequency shift (DFS) measurement approach based on Serrodyne optical frequency translation is reported. A sawtooth wave with high amplitude is sent to one phase modulation arm of a Mach–Zehnder modulator in conjunction with the transmitted signal to implement the Serrodyne optical frequency translation, as well as the optical phase modulation of the transmitted signal on the frequency-shifted optical carrier. The echo signal is applied to the other arm of the modulator. The lower first-order optical sidebands from the modulator are selected by an optical bandpass filter and detected in a photodetector. By measuring the frequency of the output low-frequency signal, the value and direction of the DFS can be determined. DFS from −100 to 100 kHz is measured for 6- to 17-GHz microwave signals with a measurement error of ±0.03 Hz and a measurement stability of ±0.015 Hz in 30 min when a 500-kHz sawtooth wave is used. [ABSTRACT FROM AUTHOR]