The editor-in-chief explains the discontinuation of the print edition of Optical Engineering.

We propose a photonic approach to generate triangular waveforms based on nonlinear polarization rotation (NPR) effect in a highly nonlinear fiber. A continuous wave probe light is polarization rotated by an intensity-modulated control light through the NPR effect. By tuning the cascaded polarization controller, the undesired second-order harmonic is well suppressed after photodetection. A 90-deg hybrid coupler is used to compensate the phase mismatch between the fundamental and third-order harmonic components, so that triangular waveforms are generated successfully. The proposed approach is theoretically analyzed and experimentally verified. Triangular waveforms at repetition rates of 3, 4, and 5 GHz with full-duty cycle are experimentally generated.

Within the last two decades, terahertz systems have become a well-established tool in scientific laboratories and industrial research and development departments. Unlike the methods of pulsed terahertz radiation that mainly rely on ultrafast optical technology, the technology behind continuous-wave terahertz sources and detectors has a long history involving many different types of technical schemes. The intent is to provide a brief review of the continuous-wave terahertz systems that are based on photomixing and current application trends in the field of terahertz technology.

We present an improved calibration method to optimize and simplify the current principal point estimation model and the zoom lens calibration process. A Euclidean distance weighting model (EDWM) for the principal point is constructed, which can more accurately estimate the principal point, that is, the distortion center. In addition, the camera has been calibrated by Zhang’s method in several zoom settings to get the intrinsic parameters, which can be used to simplify the distortion model in zoom processing. As a result, the discrepancy between the principal point determined by the EDWM and Zhang’s approach is 2.7484 pixels on average, and the reprojection root-mean-square error in the image distortion correction experiment is 0.1157 pixels. The results demonstrate that the proposed method is precise and effective in identifying the principal point while simplifying the process of zoom lens calibration. Therefore, the approach is suitable for close-range vision measurement tasks that require large-scale zoom camera calibration.

We present a method based on image pixel value to evaluate the characteristics of the corona discharge of polymer insulators. To capture the images of the solar-blind ultraviolet (SBUV) band, an ultraviolet (UV) imaging detector based on an image intensifier is designed. First, through the experiments’ results with the UV light source, the proposed method based on the image pixel value is proved to detect solar-blind signals more effectively than the photon counting method. Then, by conducting experiments with the insulator at a fixed observation distance, the corona characteristics of the insulator can be described through analyzing the images captured by the detector. Finally, the experiments at different observation distances are conducted, and the image pixel value ratios of the recorded images with certain observation distance are computed. The results reveal that the developing stages of different corona discharges can be uniquely described. Based on the method, the characteristics of a corona discharge in the SBUV band can be described accurately and objectively.

We describe the process of preparing an experimental study of a coded aperture in the visible spectral range. In the experiments, shadowgrams are supposed to be recorded with a conventional imaging sensor. A feature of the work is a proposed coded aperture implementation method based on surface evaporation by laser ablation. As a coded aperture base material, a glass substrate with a titanium film 500-nm thick deposited on it is used. To characterize and validate the fabricated coded apertures, a dual-metric method based on image processing techniques was developed. The method was used to optimize laser parameters, including the pulse power, the spot moving speed, and the pulse repetition rate. The coded aperture implemented with the parameter values determined was then used as the imaging device in the experimental stand and proved to form a high-contrast image. The double metric method allows one to optimize other laser parameters, along with the studied ones, for the given experimental conditions.

Infrared (IR) images are not affected by factors such as illumination and have the ability to work all day long, which is of great significance for night detection of unmanned platforms. We propose a multiview infrared target detection and localization algorithm (MVIDL), a complete sensory-fusion framework that uses IR images and lidar point cloud to detect and locate infrared targets (pedestrian and vehicle). MVIDL is a two-stage pipeline with an IR camera and three-dimensional lidar information as input. First, we introduce an infrared region proposal method that fuses lidar point cloud cluster results and IR image cluster results to obtain target regions and their position. In the second stage, an aggregate feature is proposed and extracted from the target regions, after which SVM is adopted to classify. Experimental results demonstrate that this algorithm can effectively detect targets and precisely get their position and size.

Global motion estimation is a fundamental tool for image processing and computer vision. Since low-cost real-time devices have limited computational resources, the computational burden of global motion estimation remains considerable for low-cost devices. Hence, a fast global motion estimation algorithm for single instruction multiple data (SIMD) processors with which a CPU in modern mobile computing devices is commonly equipped is proposed. Conventional projection-based global motion estimation is modified to improve the prediction accuracy for videos containing large motions. To significantly increase the parallelism, the exact sizes of the variables in each processing step of the proposed algorithm are determined by considering the size of an SIMD register. Simulation results demonstrate that the proposed scheme is considerably fast and accurate, even for videos containing large motions.

In order to optically refocus three-dimensional (3-D) objects on their real depths, we design an adaptive periodic δ-function array (A-PDFA)-based 3-D optical refocusing method. An A-PDFA kernel with adaptively determined impulse number is first generated, and next by convolving this A-PDFA with the elemental image array (EIA) captured using an integral imaging system, the depth-filtered EIA is obtained to finally refocus the objects on their real depths. Based on simulations and experiments, the performance of the A-PDFA-based 3-D optical refocusing method is confirmed with the widest effective process zone and the highest image quality compared to other classical 3-D optical refocusing methods. Thus, it is believed the proposed method potentially can be used in various 3-D imaging and display applications.

We experimentally investigate the performance of co-optimized hybrid optical–digital imaging systems based on binary phase masks and digital deconvolution for extended depth-of-field (DoF) under narrow-band illumination hypothesis. These systems are numerically optimized by assuming a simple generic imaging model. Using images of DoF targets and real scenes, we experimentally demonstrate that in practice, they actually reach the DoF range for which they have been optimized. Moreover, they are shown to be robust against small mask manufacturing errors and residual spherical aberration in the optical system. These results demonstrate that the optical/digital optimization protocol based on generic imaging model can be safely used to design DoF-enhanced imaging systems aimed at real-world applications.

Tunable diode laser absorption spectroscopy (TDLAS) has been an effective technology to measure the velocity of the combustion flow in hypersonic engine. The measurement theory is based on the relative Doppler shifts of the absorption transition for the upstream and downstream beams in the flow. The resolution of the TDLAS system for velocity measurement, which is defined as the minimum velocity change value that the TDLAS system can identify, is analyzed in three aspects: the structure of the beam path, the absorption transition from the laser, and the quantization of the digital signal sampling. The measured accuracy of velocity is also investigated based on the study of the resolution. An experiment was executed on a scramjet engine with a TDLAS system to validate the resolution and the accuracy of the measured velocity.

We design a simple visible light communication (VLC) system to test the attenuation of the transmitted signal in a smoke environment. A closed glass box filled with smoke is used to simulate the smoke environment, and an air quality detector is used to measure the particle concentration in the air. The signal attenuation at different smoke concentrations is investigated. The experimental results show that, with the increase of the smoke concentration, the signal attenuates exponentially until it is completely submerged by noise signal. When a cigarette is burned in a 0.027-m³ closed glass box, the signal is almost completely attenuated. This experiment provides a reliable basis for studying the VLC technology in a smoke environment.

There is a need to remotely measure the full phase and amplitude information of small-scale acousto-seismic vibrations in order to detect the presence of buried objects (e.g., tunnels, etc.), or for other purposes. This remote sensing information may need to be collected with a large area coverage rate and at a safe standoff distance. To accomplish this, we have implemented a shearographic imaging system that incorporates phase stepping in a novel way, automatically separating random speckle noise from surface motion, without requiring an intermediate unwrapping step. This method, which we call surface-phase-resolved shearography, is especially effective for very low-amplitude motions that generate less than one light-wavelength of phase change. In laboratory studies, we have demonstrated sensitivity of two nanometers RMS with 532-nm-wavelength light.

A common-path spatial phase shifting digital speckle pattern shearing interferometry setup is introduced for simultaneous measurement of in-plane and out-plane strain components under dynamic loading using two laser beams with different wavelengths that symmetrically illuminate the test object and a single detector. The simplicity, stability, and efficiency of the arrangement are provided by using a glass plate as a shearing device, which is capable of tuning the sensitivity continuously. The phase is recovered from a single frame by the Fourier method. In this setup, the spatial carrier frequencies can be adjusted independent of the amount of the lateral shear. Ultimately, the desired field of view can easily be achieved by simple imaging optics. To validate the feasibility and the flexibility of our technique, the proposed setup is used to evaluate the in-plane and out-plane strain maps of an aluminum plate, which is deformed under dynamic stress in its plane. The temporal phase stability of the proposed system is also investigated.

Currently, dual-energy x-ray phase contrast imaging is usually conducted with an x-ray Talbot-Lau interferometer. However, in this system, the two adopted energy spectra have to be chosen carefully in order to match well with the phase grating. For example, the accelerating voltages of the x-ray tube are supposed to be, respectively, set as 40 and 70 kV, with other energy spectra being practically unusable for dual energy imaging. This system thus has low flexibility and maneuverability in practical applications. Here dual energy x-ray phase-contrast imaging is performed in a grating-based noninterferometric imaging system rather than in a Talbot-Lau interferometer. The advantage of this system is that, theoretically speaking, any two separated energy spectra can be utilized to perform dual energy x-ray phase-contrast imaging. The preliminary experimental results show that dual-energy x-ray phase-contrast imaging is successfully performed when the accelerating voltages of the x-ray tube are successively set as 40 and 50 kV. Our work increases the flexibility and maneuverability when employing dual-energy x-ray phase-contrast imaging in medical diagnoses and nondestructive tests.

A high-frequency signal sampling approach by combining with compressive sensing and random equivalent sampling technology is proposed. The proposed sampling method achieves ∼10 times faster equivalent sampling rate by utilizing three parallel sub-Nyquist low-rate analog-to-digital converters (TPSL-ADC). By the union of the optical electric-field sensor with wide dynamic range and the proposed TPSL-ADC sampling method, a complete high-frequency electric-field signal measurement system has been realized. The high-frequency electric-field signal with an equivalent sampling rate of 1  GS  /  s is captured by utilizing three parallel ADCs with sampling rates of ∼32.3, 31.3, and 30.3  MS  /  s, respectively. The effectiveness of the proposed electric-field sensing and detecting system has been experimentally demonstrated.

Phase-shifting microwave holography utilizes the antennas to record the hologram. In this technique, the antenna has to perform two-dimensional (2-D) scanning over the test area corresponding to each phase shift in the reference wave. A numerical three-step phase-shifting is proposed for recording the microwave holograms in which only a single 2-D scan over an area is recorded experimentally whereas two remaining phase shifts are introduced numerically. This method is applied for imaging of metallic objects to inspect the feasibility of the proposed method. The qualitative results exhibit its effectiveness while reconstructing the amplitude and phase of an object at a microwave frequency of 8.5 GHz (or λ  =  35.29  mm).

In order to obtain the precise three-dimensional (3-D) points of underwater objects, we propose a corresponding pixel matching algorithm based on coplanar constraint for underwater laser scanning systems. It uses three coplanar constraints of left camera, right camera, and laser plane to achieve high-precision and high-speed pixel matching according to the characteristics of underwater laser scanning system. Results show that the measurement error is <3  mm for the bat placed 3 m away and compared with a method using epipolar constraints. The proposed method can greatly reduce the measurement error by 89.64%. In addition, the average processing time of each image is 5 ms, and the generated 3-D points are uniform and dense, which means the system can realize real-time processing and good robustness with high precision.

Characterized by a high resolution, high throughput (large effective area), and large field of view, the Wolter-I microscope is the main grazing incidence imaging device for x-rays and neutrons. It is necessary to estimate the tolerance of surface deformation to achieve the required resolution in typical applications. Traditional tolerance estimation method based on Monte Carlo method required a large random sampling in a given sampling area and has a low efficiency. We derived the transverse ray aberration equations to investigate the image quality degradation caused by principal figure and alignment errors and proposed a line fitting tolerance estimation method. Finally, the figure and alignment error tolerances of a Wolter-I microscope are discussed.

We focus on the subaperture polishing tool concept development for suppressing midspatial frequencies (MSF) arising from computer numerical control machining of aspherical optical surfaces. The selective effect in the MSF range is achieved by setting the tool viscoelastic properties so that rigidity increases in the frequency-domain generated by the rotational tool movement over the aspheric surface with the MSF while the tool remains flexible in the lower frequency range associated with tool radial direction movement. The mechanism of MSF removal is discussed in detail, including in terms of elasticity module frequency dependences, and is also experimentally verified. In order to monitor and optimize viscoelastic properties, especially in regard to frequency dependences, the dynamic mechanical analysis method has been developed and applied and is presented in the paper as well.

An ultrawideband perfect absorber (UPA) is designed via a four-layer dielectric/refractory metal structure, which can produce near-unity absorption in the midinfrared region. The maximal absorption is up to 99.6%. Moreover, the absorber can maintain excellent absorption in a wide angle range, which indicates the angle-insensitive absorption and holds potential applications in complex electromagnetic situations. The ultrawideband spectral absorption mainly results from the intrinsic broadband plasmonic resonances by the refractory metals and the combination of the different plasmonic resonances by the resonators, and the cavity resonances by the layered nanostructures. Furthermore, due to the high melting point of titanium and chromium materials, the UPA is with highly thermal stability. The proposed absorber platform is therefore with both advantageous on the absorption properties and material features, which could pave ways for a wide application prospect in solar harvesting, infrared detection, and others.

A temperature-insensitive fiber Bragg grating (FBG) inscribed in ethanol-filled photonic crystal fibers (PCF) is designed and optimized. A structural PCF is proposed in which the diameters of the air holes of the first ring are larger than the other air holes. The structural PCF is designed and optimized via full-vector finite element method and genetic algorithms. The calculated Bragg wavelength shift (BWS) of FBG inscribed in ethanol-filled PCF is 31 pm while the temperature changes from −15  °  C to 55°C. When the temperature changes from 0°C to 40°C, the BWS is as small as 10 pm, and the thermal sensitivity of the Bragg wavelength is 0.25  pm  /    °  C.

An obstacle detection method based on multiframe point cloud fusion and ground plane estimation is proposed. First, the robot, three-dimensional lidar, and two-axis rotating platform motion models were established. With the pitch rotation of lidar and the movement of the robot, the multiframe point cloud data were superposed to obtain a dense point cloud. Then, the incremental linear fitting method was adopted to estimate the ground plane to eliminate the ground disturbance. Finally, the pairwise linkage clustering method could cluster multiple obstacles and the projection method could obtain the specific size information of obstacles. Therefore, verification of experiments was carried on the Robotics Lab Platform (BIT-NAZA), experimental results showed that multiframe point cloud data can be accurately integrated to obtain dense point cloud and then get information of obstacles. For some low-height obstacles with sparse density of point clouds, the detection accuracy of an object with a height of about 10 cm is about 95%, whereas for other higher objects, the detection rate will be higher. So the proposed algorithm can achieve its detection and related parameter acquisition efficiently.

A double-ring resonator temperature sensing system based on a multiwavelength Brillouin fiber laser (MBFL) is proposed and demonstrated experimentally. In the two ring resonators, a 3.2-km long dispersion shifted fiber is used as the sensing fiber while a 3.2-km long dispersion compensating fiber is the reference. Second-order Stokes light generated by two gain fibers are used as the beat frequency to obtain the microwave signal. The frequency of the signal is 1.657 GHz, which avoids the low frequency noise region, and the signal has a high signal-to-noise ratio of 39 dB. A change in the center frequency of the beat frequency microwave signal accurately corresponds with the temperature changing. The sensitivity of temperature is 2.26  MHz  /    °  C in the condition of the sensing temperature being from 5°C to 85°C and the reference temperature being 25°C. Obviously, the MBFL can sense temperature not only below the reference but also above it; thus the scheme has a larger temperature sensing range.

The cross-sensitivity of fiber Bragg grating (FBG) corresponding to strain and temperature is common in practical applications. In order to achieve accurate and simultaneous measurement of temperature and strain, half size of apodized FBG is packaged on the equal strength beam by tin alloy, and the other half is in a free state. The independent temperature and strain can be obtained through a decoupling matrix using the sensitivity difference of the two parts. The experimental results show that the strain sensitivity of the metal-packaged part FBG is 1.04  pm  /  με, with a correlation coefficient of higher than 0.999. When temperature varies from −10  °  C to 80°C, the temperature sensitivity of the metal-packaged part and the free part of FBG are 35.5 and 11.2  pm  /    °  C, respectively. This method of using a single FBG to decouple temperature and strain has potential applications in optical fiber sensing.

Dual-frequency solid-state microchip lasers have advantages, such as large frequency differences, high pumping efficiency, high beam quality, and narrow laser linewidth. In addition, they are characterized by simplicity, compactness, stability, long lifetime, etc. However, the frequency difference of the dual-frequency solid-state microchip laser cannot be modulated and is only determined by its internal stress. The stress distribution inside the microchip laser is uneven and the stability of the frequency difference needs to be improved furthermore, which limits its applications. The structure of a Nd:YAG laser resonator, the mechanism of dual-frequency generation, and the output characteristics are studied. The relationship between the polarization direction of the pumping laser and the intensities of the dual-frequency components is theoretically and experimentally researched, realizing the maximum amplitude of the beat signal. Based on the photoelasticity theory, the relationship between the frequency difference and the external force is analyzed and the PID closed-loop frequency difference controlling system based on the piezoelectric transducer is proposed and realized. By applying the closed-loop control, the frequency difference of the Nd:YAG laser can be continuously modulated in the range from 24 to 30 MHz and the fluctuation is less than 450 kHz. The high-performance Nd:YAG dual-frequency solid-state microchip laser with stable and adjustable frequency difference is achieved.

We have experimentally obtained dispersion-managed solitons with sidebands in a passively mode-locked thulium-doped fiber laser. The stable single soliton with sidebands can be converted into two soliton pulses at the pump power of 867 mW with appropriate settings of the polarization controllers (PCs). By increasing the pump power and cautiously adjusting the PCs, the three, four, and five soliton pulses with nonuniform intensity operate with stability in the cavity due to the global soliton interaction caused by unstable continuous waves. Furthermore, the soliton bunch can be observed at the pump power of 1 W. The position of solitons in the soliton bunch is random with a fixed separation that is controllable by changing the linear phase delay. Our work gives insight into the dynamics of multipulse dispersion-managed solitons in a 2-μm mode-locked fiber laser.

The changes in the state of polarization of partially polarized, partially coherent pulsed electromagnetic beams propagating along a horizontal path through anisotropic turbulence are investigated. Unpolarized and partially polarized, partially coherent pulsed electromagnetic Gaussian Schell-model beams are compared in terms of the turbulence-induced changes in the spectral degree of polarization. Our analysis is focused on models pertaining to electromagnetic Gaussian Schell-model pulsed beams but can either be readily reduced to scalar or be readily generalized to other beam classes. Within the framework of the anisotropic generalized exponential spectrum, considering simultaneously the finite inner and outer scales of turbulence and the asymmetric property of turbulence eddies in the orthogonal xy-plane throughout the propagation path, we derive analytical expressions for the cross-spectral density matrix, spectral degree of polarization, orientation angle, and degree of ellipticity. Finally, we study the effects of the anisotropic turbulence parameter on the spectral degree of polarization, orientation angle, and degree of ellipticity of the beams. Our results can be useful for applications involving partially polarized, partially coherent pulsed beams propagating through atmospheric turbulence.

A scheme for Gaussian kernel-aided deep neural networks nonlinear predistortion (GK-DNNPD), which could effectively reduce the computational complexity of the receivers, is experimentally demonstrated. Compared with lookup table (LUT) PD, the GK-DNNPD could increase the Q-factor of 8 pulse amplitude modulation visible light communication (VLC) system by 1.56 dB at 1.335 Gbps. We experimentally proved that GK-DNNPD could increase the bitrate under hard-decision forward error correction from 1.335 to 1.385 Gbps. This is the first time that GK-DNNs are utilized for PD in the field of VLC systems. Meanwhile, GK-DNNPD requires less data for training than LUT, and the space complexity of the model is lower than LUT as well, which provides GK-DNNPD with the potential to be applied in practical VLC systems.

We present a detailed study of injection-locking (IL) and injection-pulling behavior of an IL single-loop optoelectronic oscillator (OEO) when injected by radio frequency (RF) signal. From the locking equation, the expression for the output spectrum of the IL-OEO under RF injection is derived in the time domain, to evaluate the spurious performance of the system. The frequency response of the spurious outputs due to RF injection signal is investigated in terms of the injection transfer function. Also, a dual-loop model of the IL-OEO is developed to study the phase noise performance of the system with a cofrequency RF injection signal. Also, the effect of Gaussian minimum shift keying signal interference on the IL-OEO output spectrum is investigated. An analytical expression for the side-mode attenuation is presented to describe the influence of IL on the side-modes of free-running OEO. The experimental results corroborating the theoretical findings are given.

Space-division multiplexing (SDM) has been proposed to increase fiber transmission capacity to cope with the increasing bandwidth requirements in the network. The use of multicore fiber (MCF) is considered to be a simple and feasible way to implement SDM technology. The intercore crosstalk (XT) in the MCF affects the performance of network transmission. Currently, bidirectional transmission of MCF has been proven to have lower XT than traditional unidirectional transmission of MCF. But there are few research studies on routing, core, and spectrum allocation strategies with bidirectional transmission. Thus, to get more XT reduction, we propose two different routing, core, and spectrum allocation strategies according to the traffic loads in the network based on the bidirectional transmission of MCF. The simulation results show that the proposed algorithm has lower blocking rate and higher resource utilization than the benchmark algorithm in the two stages of different traffic loads in the network.

We report efficient frequency downconversion of a continuous-wave low-power Raman laser using intracavity difference frequency mixing in a periodically poled lithium niobate (PPLN)-adopted optical parametric oscillator (OPO). The Raman laser was obtained using a 1060-nm fiber laser and 137-m passive fiber. Its central wavelength was fixed at 1111 nm with the power ranging from 320 mW to 5.984 W. The 1060- and 1111-nm pump beams were incident into the OPO at the same time. The high-power 1060-nm pump beam built parametric oscillation first, and the difference frequency generation (DFG) occurred between the low-power Raman laser and intracavity signal laser. The PPLN temperature was properly controlled at 45°C to ensure both the OPO and DFG processes synchronously satisfy phase matching conditions. Benefiting from intracavity high signal power, the Raman laser was successfully converted to the 3560-nm midinfrared radiation under every investigated pump power level. The maximum 3560-nm idler power reached 1.026 W, indicating a 17.4% pump-to-idler slope efficiency and about 15% optical-to-optical conversion efficiency. The comparative experiments also verified that the phase matching conditions were satisfied maximally at 45°C.

Two planar dielectric reflectors with polarization-independent focusing ability have been designed. They are designed and compared on the base of the square lattice of circular two-dimensional (2-D) subwavelength gratings (SWGs) and the hexagonal lattice of circular 2-D SWGs on a silicon-on-insulator wafer.

A simple analytical model for the optical response and sensitivity of an ion-implanted MESFET has been developed with the presence of interface states and interfacial layer at the gate–channel contact. It is assumed that the applied drain field is very high and the device follows velocity saturation model. The mathematical expressions for drain current and threshold voltage of the device under optical irradiation have been derived. The dependence of drain current and threshold voltage as functions of drain bias and interface state density is studied for dark and illuminated conditions. The analytical expressions of optical sensitivities of drain current and threshold voltage are derived, and their dependence on different material and device parameters is discussed. It is observed that the device offers better optical sensitivity if excess carrier lifetime is high and for higher value of absorption coefficient optical sensitivity gradually degrades.

A ray-theoretic analysis of the problem of discontinuity in a linear guide is presented. The fields in the guides are expressed as superposition of plane wave components determined by subjecting the fields to the process of Fourier transformation. The reflected fields are finally synthesized by modifying these plane wave components by the corresponding Fresnel coefficients at the plane of discontinuity. The results obtained are compared with those obtained by other methods.

A multiplex localized surface plasmon resonance (LSPR) temperature sensor based on grapefruit fiber filled with a silver nanoshell (SNS) and liquid is designed and characterized by the finite element method. Due to the plasmon hybridization in SNS, the designed sensor supports two separate intense LSPR bands that can realize dual-channel temperature sensing at the same time by altering the light wavelength. When temperature increases from 20°C to 80°C, two LSPR bands shift to the shorter wavelength in good linear relationships with wavelength sensitivities of −1.14 and −0.86  nm  /    °  C. The maximum amplitude sensitivity can reach 0.77  °  C^−  1 for peak I with the resolution of 0.01°C. Moreover, the full width half maximum of peak I is <5  nm, exhibiting much better figure of merit than other works, which can provide a new way for multiplex temperature sensing of fiber sensors.

A highly sensitive optical fiber temperature sensor based on the thermal shift of the liquid–gas interface is proposed. A section of silica capillary tube (SCT) was spliced to a single mode fiber, and a small quantity of liquid polydimethylsiloxane was filled into the innermost part of SCT by using the offset capillarity method. Then, the end of the SCT was sealed by solidified UV curable adhesive. As a result, a liquid Fabry–Perot (FP) cavity, an air FP cavity, and a liquid–gas interface were formed simultaneously in the SCT. The lengths of the two cavities changed synchronously and complementarily along with the thermal shift of the liquid–air interface, which resulted in the opposite wavelength shifts of the corresponding FP interference fringes. The temperature sensitivities are 1.67 and −0.32  nm  /    °  C, respectively. A differential sensitivity of 1.99 nm/°C can be achieved by monitoring the two kinds of fringes synchronously.

We present interband cascade light-emitting devices with incoherent and broadband light emission peaked at a wavelength of around 3.7  μm. The substrate-side-emitting devices display higher wall plug efficiencies and maximum output powers than any earlier mid-infrared LEDs operating in continuous wave at room temperature. To reduce absorption losses, the epitaxial structures were grown on low doped (low absorbing) GaSb substrates. The nine active stages were positioned in different configurations to investigate the impact of constructive or destructive interference when reflected from the epitaxial-side metallization of the flip-chip mounted devices. A comparison shows improved electrical properties and outcoupling efficiencies when all active stages are centered within a single antinode of the optical field. The optimized voltage efficiency combined with low optical losses lead to a maximum wall plug efficiency of 0.7%. Flip-chip mounted devices with a 640-μm squared mesa reached output powers of up to 5.1 mW at ambient temperatures with driving current (voltage) of 0.6 A (5.1 V).