This PDF file contains the editorial “A Message to SPIE Conference Presenters” for OE Vol. 55 Issue 04

Ridge and buried channel waveguides (BCWs) made using plasma-enhanced chemical vapor deposition SiO₂ were fabricated and tested after being subjected to long 85°C water baths. The water bath was used to investigate the effects of any water absorption in the ridge and BCWs. Optical mode spreading and power throughput were measured over a period of three weeks. The ridge waveguides quickly absorbed water within the critical guiding portion of the waveguide. This caused a nonuniformity in the refractive index profile, leading to poor modal confinement after only seven days. The BCWs possessed a low index top cladding layer of SiO₂, which caused an increase in the longevity of the waveguides, and after 21 days, the BCW samples still maintained ∼20% throughput, much higher than the ridge waveguides, which had a throughput under 5%.

Using additive manufacturing techniques in optical engineering to construct a gradient index (GRIN) optic may overcome a number of limitations of GRIN technology. Such techniques are maturing quickly, yielding additional design degrees of freedom for the engineer. How best to employ these degrees of freedom is not completely clear at this time. This paper describes a preliminary design philosophy, including assumptions, pertaining to a particular printing technique for GRIN optics. It includes an analysis based on simulation and initial component measurement.

Video-rate volumetric optical coherence tomography (vOCT) is relatively young in the field of OCT imaging but has great potential in biomedical applications. Due to the recent development of the MHz range swept laser sources, vOCT has started to gain attention in the community. Here, we report the first in vivo video-rate volumetric OCT-based microangiography (vOMAG) system by integrating an 18-kHz resonant microelectromechanical system (MEMS) mirror with a 1.6-MHz FDML swept source operating at ∼1.3  μm wavelength. Because the MEMS scanner can offer an effective B-frame rate of 36 kHz, we are able to engineer vOMAG with a video rate up to 25 Hz. This system was utilized for real-time volumetric in vivo visualization of cerebral microvasculature in mice. Moreover, we monitored the blood perfusion dynamics during stimulation within mouse ear in vivo. We also discussed this system’s limitations. Prospective MEMS-enabled OCT probes with a real-time volumetric functional imaging capability can have a significant impact on endoscopic imaging and image-guided surgery applications.

A spatial frequency sampling look-up table method is proposed to generate a hologram. The three-dimensional (3-D) scene is sampled as several intensity images by computer rendering. Each object point on the rendered images has a defined spatial frequency. The basis terms for calculating fringe patterns are precomputed and stored in a table to improve the calculation speed. Both numerical simulations and optical experiments are performed. The results show that the proposed approach can easily realize color reconstructions of a 3-D scene with a low computation cost. The occlusion effects and depth information are all provided accurately.

Geometric parameters that define the geometry of imaging systems are crucial for image reconstruction and image quality in x-ray computed tomography (CT). The problem of determining geometric parameters for an offset flat-panel cone beam CT (CBCT) system, a recently introduced modality with a large field of view, with the assumption of an unstable mechanism and geometric parameters that vary in each view, is considered. To accurately and rapidly find the geometric parameters for each projection view, we use the projection matrix method and design a dedicated phantom that is partially visible in all projection views. The phantom consists of balls distributed symmetrically in a cylinder to ensure the inclusion of the phantom in all views, and a large portion of the phantom is covered in the projection image. To efficiently use calibrated geometric information in the reconstruction process and get rid of approximation errors, instead of decomposing the projection matrix into actual geometric parameters that are manually corrected before being used in reconstruction, as in conventional methods, we directly use the projection matrix and its pseudo-inverse in projection and backprojection operations of reconstruction algorithms. The experiments illustrate the efficacy of the proposed method with a real offset flat-panel CBCT system in dental imaging.

We propose a simple yet effective method for long-term object tracking. Different from the traditional visual tracking method, which mainly depends on frame-to-frame correspondence, we combine high-level semantic information with low-level correspondences. Our framework is formulated in a confidence selection framework, which allows our system to recover from drift and partly deal with occlusion. To summarize, our algorithm can be roughly decomposed into an initialization stage and a tracking stage. In the initialization stage, an offline detector is trained to get the object appearance information at the category level, which is used for detecting the potential target and initializing the tracking stage. The tracking stage consists of three modules: the online tracking module, detection module, and decision module. A pretrained detector is used for maintaining drift of the online tracker, while the online tracker is used for filtering out false positive detections. A confidence selection mechanism is proposed to optimize the object location based on the online tracker and detection. If the target is lost, the pretrained detector is utilized to reinitialize the whole algorithm when the target is relocated. During experiments, we evaluate our method on several challenging video sequences, and it demonstrates huge improvement compared with detection and online tracking only.

Image sensor-based visible light positioning can be applied not only to indoor environments but also to outdoor environments. To determine the performance bounds of the positioning accuracy from the view of statistical optimization for an outdoor image sensor-based visible light positioning system, we analyze and derive the maximum likelihood estimation and corresponding Cramér–Rao lower bounds of vehicle position, under the condition that the observation values of the light-emitting diode (LED) imaging points are affected by white Gaussian noise. For typical parameters of an LED traffic light and in-vehicle camera image sensor, simulation results show that accurate estimates are available, with positioning error generally less than 0.1 m at a communication distance of 30 m between the LED array transmitter and the camera receiver. With the communication distance being constant, the positioning accuracy depends on the number of LEDs used, the focal length of the lens, the pixel size, and the frame rate of the camera receiver.

Pedestrian detection is one of the most critical but challenging components in advanced driver assistance systems. Far-infrared (FIR) images are well-suited for pedestrian detection even in a dark environment. However, most current detection approaches just focus on pedestrian patterns themselves, where robust and real-time detection cannot be well achieved. We propose a fast FIR pedestrian detection approach, called MAP-HOGLBP-T, to explicitly exploit the scene context for the driver assistance system. In MAP-HOGLBP-T, three algorithms are developed to exploit the scene contextual information from roads, vehicles, and background objects of high homogeneity, and we employ the Bayesian approach to build a classifier learner which respects the scene contextual information. We also develop a multiframe approval scheme to enhance the detection performance based on spatiotemporal continuity of pedestrians. Our empirical study on real-world datasets has demonstrated the efficiency and effectiveness of the proposed method. The performance is shown to be better than that of state-of-the-art low-level feature-based approaches.

The physical imaging model, which is based on atmospheric absorption and scattering, plays an important role in single-image dehazing. It is critical that the transmission is accurately estimated for the dehazing algorithm based on the physical imaging model. A self-adaptive weighted least squares (AWLS) model is proposed to refine the rough transmission, which is extracted by the dark channel (DC) model. In our model, the gray-world hypothesis and a smoothing technique with edge preservation are integrated to optimize the transmission and remove the artifacts which are brought by the DC model. The self-AWLS model has higher computational efficiency and can prevent the distortion of the recovered image better when the hazy image contains sky region, while many other dehazing techniques are not applicable for such images. Experimental results show that the proposed model is both effective and efficient for the haze removal application.

We describe a compliant static deformable mirror approach to reduce the wavefront concavity at the Navy Precision Optical Interferometer (NPOI). A single actuator pressing on the back surface of just one of the relay mirrors deforms the front surface in a correcting convex shape. Our design uses the mechanical advantage gained from a force actuator sandwiched between a rear flexure plate and the back surface of the mirror. We superimpose wavefront contour measurements with our finite element deformed mirror model. An example analysis showed improvement from 210-nm concave–concave wavefront to 51-nm concave–concave wavefront. With our present model, a 100-nm actuator increment displaces the mirror surface by 1.1 nm. We describe the need for wavefront improvement that arises from the NPOI reconfigurable array, offer a practical design approach, and analyze the support structure and compliant deformable mirror using the finite element method. We conclude that a 20.3-cm-diameter, 1.9-cm-thick Zerodur^® mirror shows that it is possible to deform the reflective surface and cancel out three-fourths of the wavefront deformation without overstressing the material.

Adaptive optics (AO) in conjunction with subsequent postprocessing techniques have obviously improved the resolution of turbulence-degraded images in ground-based astronomical observations or artificial space objects detection and identification. However, important tasks involved in AO image postprocessing, such as frame selection, stopping iterative deconvolution, and algorithm comparison, commonly need manual intervention and cannot be performed automatically due to a lack of widely agreed on image quality metrics. In this work, based on the Laplacian of Gaussian (LoG) local contrast feature detection operator, we propose a LoG domain matching operation to perceive effective and universal image quality statistics. Further, we extract two no-reference quality assessment indices in the matched LoG domain that can be used for a variety of postprocessing tasks. Three typical space object images with distinct structural features are tested to verify the consistency of the proposed metric with perceptual image quality through subjective evaluation.

We propose a correspondence approximation approach between temporally adjacent frames for motion analysis. First, energy map is established to represent image spatial features on multiple scales using Gaussian convolution. On this basis, energy flow at each layer is estimated using Gauss–Seidel iteration according to the energy invariance constraint. More specifically, at the core of energy invariance constraint is “energy conservation law” assuming that the spatial energy distribution of an image does not change significantly with time. Finally, energy flow field at different layers is reconstructed by considering different smoothness degrees. Due to the multiresolution origin and energy-based implementation, our algorithm is able to quickly address correspondence searching issues in spite of background noise or illumination variation. We apply our correspondence approximation method to motion analysis, and experimental results demonstrate its applicability.

In order to improve the performance of conventional square markers widely used by marker-based augmented reality systems in aircraft assembly environments, an L-split marker is proposed. Every marker consists of four separate L-shaped parts and each of them contains partial information about the marker. Geometric features of the L-shape, which are more discriminate than the symmetrical square shape adopted by conventional markers, are used to detect proposed markers from the camera images effectively. The marker is split into four separate parts in order to improve the robustness to occlusion and curvature to some extent. The registration process can be successfully completed as long as three parts are detected (up to about 80% of the area could be occluded). Moreover, when we attach the marker on nonplanar surfaces, the curvature status of the marker can be roughly analyzed with every part’s normal direction, which can be obtained since their six corners have been explicitly determined in the previous detection process. And based on the marker design, new detection and recognition algorithms are proposed and detailed. The experimental results show that the marker and the algorithms are effective.

An automatic calibration method is proposed for a microlens-based plenoptic camera. First, all microlens images on the white image are searched and recognized automatically based on digital morphology. Then, the center points of microlens images are rearranged according to their relative position relationships. Consequently, the microlens images are located, i.e., the plenoptic camera is calibrated without the prior knowledge of camera parameters. Furthermore, this method is appropriate for all types of microlens-based plenoptic cameras, even the multifocus plenoptic camera, the plenoptic camera with arbitrarily arranged microlenses, or the plenoptic camera with different sizes of microlenses. Finally, we verify our method by the raw data of Lytro. The experiments show that our method has higher intelligence than the methods published before.

Compared with conventional optical elements, free-form surface optical elements, as a kind of nonrotationally symmetrical shaped component, can provide more freedom in optical design, optimize the structure of the optical system, and improve its performance. However, the difficulties involved in the measurement of free-form elements restrict their manufacture and application. A tilted-wave-interferometer (TWI) can achieve high precision in free-form surface measurement, but it requires higher space attitude error control. We analyze the relation between the alignment error and the measurement error introduced by the misalignment in free-form surface metrology with TWI. The attitude control method in the rotation direction is proposed based on the moire fringe technique. Then, combining it with the five-dimensional space attitude control method of aspherical elements, we put forward an alignment error control process in measuring the free-form surface. An experiment of measuring a free-form surface using TWI shows the effectiveness of our method.

An integrated navigation method based on the inertial navigational system (INS) and Lidar was proposed for land navigation. Compared with the traditional integrated navigational method and dead reckoning (DR) method, the influence of the inertial measurement unit (IMU) scale factor and misalignment was considered in the new method. First, the influence of the IMU scale factor and misalignment on navigation accuracy was analyzed. Based on the analysis, the integrated system error model of INS and Lidar was established, in which the IMU scale factor and misalignment error states were included. Then the observability of IMU error states was analyzed. According to the results of the observability analysis, the integrated system was optimized. Finally, numerical simulation and a vehicle test were carried out to validate the availability and utility of the proposed INS/Lidar integrated navigational method. Compared with the test result of a traditional integrated navigation method and DR method, the proposed integrated navigational method could result in a higher navigation precision. Consequently, the IMU scale factor and misalignment error were effectively compensated by the proposed method and the new integrated navigational method is valid.

Chirped laser dispersion spectroscopy (CLaDS) is a sensitive and robust method for gas detection. In CLaDS, a multitone laser beam is sent through the sample and focused onto the detector where RF beat note is generated. Spectroscopic information is subsequently retrieved through frequency demodulation of this beat note. When CLaDS is used for open-path sensing (i.e., when the pressure-broadened transitions are measured), demodulation at high frequency is required (1 GHz and beyond). We address this issue with a parametric downconversion of the beat note signal. This approach moves CLaDS toward compact, cheaper, and more robust implementations and can be used in both near- and mid-infrared regions. A setup operating at 1650 nm is presented and characterized. Ambient methane detection with an open-path multipass cell is demonstrated.

The use of laser-induced breakdown spectroscopy (LIBS) as a low-cost, nondestructive method for detecting counterfeit coins was examined. A pulsed laser was used to evaporate a minute amount of coin surface, and the emanating plasma was interrogated with an entry-level spectrometer. The spectra produced showed evidence of lead content in six of the eight counterfeits examined. Thus, LIBS could offer a viable low-cost technique for identifying a significant number of fake coins.

Reported herein is the design, characterization, and demonstration of a laser interrogation and response optical discrimination system based on large-area corner-cube retroreflective films. The switchable retroreflective films use light-scattering liquid crystal to modulate retroreflected intensity. The system can operate with multiple wavelengths (visible to infrared) and includes variable divergence optics for irradiance adjustments and ease of system alignment. The electronic receiver and switchable retroreflector offer low-power operation (<4  mW standby) on coin cell batteries with rapid interrogation to retroreflected signal reception response times (<15  ms). The entire switchable retroreflector film is <1  mm thick and is flexible for optimal placement and increased angular response. The system was demonstrated in high ambient lighting conditions (daylight, 18k lux) with a visible 10-mW output 635-nm source out to a distance of 400 m (naked eye detection). Nighttime demonstrations were performed using a 1.5-mW, 850-nm infrared laser diode out to a distance of 400 m using a night vision camera. This system could have tagging and conspicuity applications in commercial or military settings.

We propose a technique that improves the channel capacity of an optical wireless orthogonal frequency division multiplexing (OFDM) transmission, which employs a visible light-emitting diode. An OFDM waveform encoded by quadrature phase shift keying (QPSK) or 16-quadrature amplitude modulation is compressed and then transformed into a sparse waveform using a proposed advanced systematic sampling. At the optical wireless receiver, the original waveform is recovered by L₁-minimization based on a Bayesian compressive sensing. Our experimental results show the significant increase in the channel capacity from 31.12 to 51.87  Mbit/s at forward error correction limit (i.e., error vector magnitude of 32%) in case of QPSK symbols.

The thulium fiber laser (TFL) is being studied as an alternative to the standard holmium:YAG laser for lithotripsy. The TFL beam originates within an 18-μm-core thulium-doped silica fiber, and its near single mode, Gaussian beam profile enables transmission of higher laser power through smaller (e.g., 50- to 150-μm core) fibers than possible during holmium laser lithotripsy. This study examines whether the more uniform TFL beam profile also reduces proximal fiber tip damage compared with the holmium laser multimodal beam. Light and confocal microscopy images were taken of the proximal surface of each fiber to inspect for possible laser-induced damage. A TFL beam at a wavelength of 1908 nm was coupled into 105-μm-core silica fibers, with 35-mJ energy, and 500-μs pulse duration, and 100,000 pulses were delivered at each pulse rate setting of 50, 100, 200, 300, and 400 Hz. For comparison, single use, 270-μm-core fibers were collected after clinical holmium laser lithotripsy procedures performed with standard settings (600 mJ, 350  μs, 6 Hz). Total laser energy, number of laser pulses, and laser irradiation time were recorded, and fibers were rated for damage. For TFL studies, output pulse energy and average power were stable, and no proximal fiber damage was observed at settings up to 35 mJ, 400 Hz, and 14 W average power (n=5). In contrast, confocal microscopy images of fiber tips after holmium lithotripsy showed proximal fiber tip degradation, indicated by small ablation craters on the scale of several micrometers in all fibers (n=20). In summary, the proximal fiber tip of a 105-μm-core fiber transmitted up to 14 W of TFL power without degradation, compared to degradation of 270-μm-core fibers after transmission of 3.6 W of holmium laser power. The smaller and more uniform TFL beam profile may improve fiber lifetime, and potentially translate into lower costs for the surgical disposables as well.

Taguchi’s method is introduced to perform multiobjective optimization of fiber Raman amplifier (FRA). The optimization requirements are to maximize gain and keep gain ripple minimum over the operating bandwidth of a wavelength division multiplexed (WDM) communication link. Mathematical formulations of FRA and corresponding numerical solution techniques are discussed. A general description of Taguchi’s method and how it can be integrated with the FRA optimization problem are presented. The proposed method is used to optimize two different configurations of FRA. The performance of Taguchi’s method is compared with genetic algorithm and particle swarm optimization in terms of output performance and convergence rate. Taguchi’s method is found to produce good results with fast convergence rate, which makes it well suited for the nonlinear optimization problems.

Distributed optical fiber vibration sensing system is widely used as a monitoring system in communication cable and pipeline of long distances. When a vibration signal occurs at a particular position along the fiber, the response of the system, in the frequency domain, presents a series of periodic maxima and minima (or null frequencies). These minima depend on the position of the vibration signal along the fiber. Power spectral estimation methods are considered to denoise the power spectrum of the system and determine these minima precisely. The experimental results show higher accuracy of the position using a parametric model with appropriate selection of order p and q than just using fast Fourier transform algorithm.

The past decade has seen the phenomenal usage of orthogonal frequency division multiplexing (OFDM) in the wired as well as wireless communication domains, and it is also proposed in the literature as a future proof technique for the implementation of flexible resource allocation in cognitive optical networks. Fiber impairment assessment and adaptive compensation becomes critical in such implementations. A comprehensive analytical model for impairments in OFDM-based fiber links is developed. The proposed model includes the combined impact of laser phase fluctuations, fiber dispersion, self phase modulation, cross phase modulation, four-wave mixing, the nonlinear phase noise due to the interaction of amplified spontaneous emission with fiber nonlinearities, and the photodetector noises. The bit error rate expression for the proposed model is derived based on error vector magnitude estimation. The performance analysis of the proposed model is presented and compared for dispersion compensated and uncompensated backbone/backhaul links. The results suggest that OFDM would perform better for uncompensated links than the compensated links due to the negligible FWM effects and there is a need for flexible compensation. The proposed model can be employed in cognitive optical networks for accurate assessment of fiber-related impairments.

Coherent photonic radio frequency (RF) channelization based on dual coherent optical frequency combs (OFCs) and stimulated Brillouin scattering (SBS) is proposed and demonstrated. By using the dual OFCs to multicast a wideband RF signal and form a series of SBS gain responses simultaneously, the frequency components of the wideband RF signal are sliced into a number of parallel parts and converted into signals with the same center frequency. The bandwidth of each frequency slice is determined by the gain bandwidth of the SBS (tens of megahertz), and all the information in the RF signal is retained after the channelization. The channelization fineness can be tuned by adjusting the two OFCs’ free spectral ranges. An experiment is carried out. Photonic RF channelization with channel spacing of 80 MHz and a crosstalk suppression of more than 19.52 dB is realized.

Optical loss of fiber-optic connectors has a vital impact on fiber-optic-related systems. We analyze the contact loss caused by the endface geometry and the contact force. Based on analytical and simulated results, the analytical equations of the insertion loss (IL) and return loss (RL) as a function of the endface geometry and the contact force are derived. Then four fiber-optic connectors are experimentally tested. The experimental results are well consistent with the theoretical results, which show that there is an optimum relationship between the endface geometry and the contact force to ensure a low IL and a high RL.

We propose an orthogonal modulation scheme with 40  Gb/s Manchester-coded (MC) payload and 2.5  Gb/s frequency-shift keying (FSK) label. The high-speed FSK label is generated by an FSK modulator, which consists of two modulators, while MC payload is generated by an intensity modulator. Simulation results show that the available extinction ratio of FSK and amplitude-shift keying with MC payload can improve from 5.1 to 11.2 dB, compared with the traditional not-return-to-zero payload. The receiver sensitivities of both MC payload and FSK label show a remarkable improvement.

A cryogenic helium gas cooled Yb:YAG multislab amplifier with a longitudinal doping gradient concentration was proposed for developing high energy, high average power laser systems. As a comparison, the performance of the gradient doped amplifier was investigated with other constant and stepped doped amplifiers in terms of energy storage capacity, heat deposition, and amplification, based on the theory of quasi-three-level laser ions, Monte Carlo, and ray-tracing approaches. Improved lasing characteristics with more homogenous distributions of gain and heat load and higher efficiency was achieved in the gradient doped multislab amplifier while lower gain medium volume was required. It is shown that at the optimum operating temperature of 200 K, the maximum output energy of 867.76 J in the gradient doped amplifier was obtained, corresponding to an optical-to-optical efficiency of 22.41%.

Adaptive time-domain equalizer (TDE) is an important module for digital optical coherent receivers. From an implementation perspective, we analyze and compare in detail the effects of error signal feedback delay on the convergence performance of TDE using either least-mean square (LMS) or constant modulus algorithm (CMA). For this purpose, a simplified theoretical model is proposed based on which iterative equations on the mean value and the variance of the tap coefficient are derived with or without error signal feedback delay for both LMS- and CMA-based methods for the first time. The analytical results show that decreased step size has to be used for TDE to converge and a slower convergence speed cannot be avoided as the feedback delay increases. Compared with the data-aided LMS-based method, the CMA-based method has a slower convergence speed and larger variation after convergence. Similar results are confirmed using numerical simulations for fiber dispersive channels. As the step size increases, a feedback delay of 20 clock cycles might cause the TDE to diverge. Compared with the CMA-based method, the LMS-based method has a higher tolerance on the feedback delay and allows a larger step size for a faster convergence speed.

We experimentally demonstrated the generation of dispersive waves (DWs) at the visible wavelength by coupling femtosecond pulses into the anomalous dispersion region of the second-order mode of a homemade photonic crystal fiber. When center wavelengths of the pump pulses are located at 800 and 850 nm and input average powers P_av are increased from 300, to 400, and to 500 mW, the blue-shifted DWs can be generated during the soliton dynamics and are tunable within the wavelength range of 614 to 561 nm. Moreover, the conversion efficiency η_DW of DWs is enhanced from 5% to 21%, and the corresponding bandwidth B_DW is broadened from 17 to 30 nm. It is believed that the DWs can be used as the ultrashort pulse source for visible photonics and spectroscopy.

We present the experimental results on the laser characteristics of diode-pumped passively Q-switched Tm,Ho:GdVO₄ and Tm,Ho:YVO₄ lasers with a Cr²⁺:ZnS saturable absorber emitting in the 2-μm range. The Tm,Ho:GdVO₄ laser exhibits better performance than the Tm,Ho:YVO₄ laser. The minimum pulse duration of 32.7 ns is obtained with the pulse energy of 0.30 mJ, corresponding to the peak power of 9.1 kW. The slope efficiencies of continuous wave and passively Q-switched Tm,Ho:GdVO₄ lasers are 49.9% and 36.5%, corresponding to the Q-switching efficiency of 70.2%.

In this study, we propose and experimentally demonstrate a simple direct detection passive optical network (PON) uplink transmission scheme based on frequency division multiplexing and polarization division multiplexing. Two optical network units (ONUs) are assigned to two different frequency bands at two different orthogonal polarization directions. At the optical line terminal, both ONU signals can be simultaneously detected by a single photodiode without utilizing any polarization control, polarization selection, or complicated polarization demultiplexing algorithms. As a proof-of-concept, the 2×ONU 80 Gbps 32-ary quadrature amplitude modulation Nyquist single carrier signals are successfully transmitted over 2 km standard single mode fiber or 20 km large effective area fiber with the assistance of frequency domain equalization and decision-directed least-mean-square. The measured bit error rate can be below the 7% pre-forward error correction threshold of 3.8×10−³. Meanwhile, this scheme is compatible with the widely used wavelength-division multiplexed PON, which shows the promising potential and feasibility of this proposal.

A reflectance-based surface plasmon resonance (SPR) fiber sensor with enhanced sensitivity for biochemical sensing is reported after comparing its result with the transmittance-based SPR optical fiber sensors. The fabricated SPR sensor contains a gold-coated multimode fiber with the implementation of a standard source-sensor-spectrometer interrogation system. As the refractive index of the liquid under test is increased, a redshift of the SPR is observed. The coupling of the source to the fiber sensor is optimized by investigating the effect of an intentional misalignment in transmission-based setup. When a fiber tip coated with the silver mirror and the bifurcated fiber bundle is used, an alignment-free disposable sensor probe is achieved. A comprehensive characterization of the proposed reflectance-based SPR probe is discussed. The maximum sensitivity of 3212.19  nm/refractive index unit (RIU) is obtained.

We demonstrate a gain-switched Ho:YAG ceramic laser in-band pumped by an acousto-optically modulated thulium fiber laser at ∼1908  nm. The laser pulse repetition rate could be tuned continuously from 60 to 100 kHz with the pulse energy kept constant for a certain pump power level. The shortest pulse width of 204 ns and a maximum peak power of 75 W have been obtained at 60 kHz under the maximum pump power level of 11 W. A maximum average output power of 1.4 W has been achieved with a pulse repetition rate of 100 kHz, corresponding to a slope efficiency of 57% with respect to the incident pump power. The prospects for further improvement in laser performance are discussed.

Room-temperature continuous wave (CW) operation of a tunable external-cavity quantum cascade laser (EC-QCL) at center wavelength around 7.2 μm is presented. The EC-QCL was implemented in a Littrow configuration. The gain chip is based on a diagonal bound-to-continuum design with a high-reflection coating on the back facet. A two-layer antireflection (AR) coating consisting of Al₂O₃ and ZnSe was designed and deposited on the front facet of the chip to suppress the Fabry–Pérot modes. With this AR coating, single-mode tuning range of 128  cm−¹ was achieved, from 1346.7 to 1475.3  cm−¹ (6.78 to 7.43 μm). High side-mode suppression ratio over 30 dB was achieved near the center gain region. A very low-threshold current density of 0.89  kA/cm² and a high output power of 50 mW were obtained when the EC-QCL was operated in CW mode at 20°C.

Cylindrical vector (CV) beams have found increasing applications in physics, biology, and chemistry. To generate CV beams, interferometric technique is popularly adopted due to its flexibility. However, most interferometric configurations for the generation of CV beams are faced with system instability arising from external disturbance, limiting their practical applications. A common-path interferometer for the generation of radially and azimuthally polarized beams is proposed to improve the system stability. The optical configuration consists of a vortex phase plate acting to tailor the phase profile and a cube nonpolarizing beamsplitter to split the input beam into two components with mirror-like spiral phase distribution. The generated CV beams show a high quality in polarization and exhibit a better stability of beam profile than those obtained by noncommon-path interferometric configurations.

An interleaver with three cascaded 3×3 single-mode fiber couplers and one 2×2 single-mode fiber coupler is presented. Under certain values of physical length differences of the interferometer arms, 3-dB passband in odd channels and in even channels of the interleaver can realize an asymmetric passband wavelength response. Through a mathematical algorithm, the optimum splitting ratios, which are the relationship among the splitting ratios of the four couplers, were determined to achieve a flattop response within the passband for the interleaver. Based on the optimum splitting ratios, the output spectrums of the interleaver were analyzed and discussed. The results indicate that the amplitude of ripples in the “flat” region of the flattop passband wavelength response in odd channels and even channels are almost zero, the interleaver can achieve a perfect smoothing flattop response and good uniform channel passband, and the tolerance characteristics of the splitting ratios is favorable. In fabrication recipe, splitting ratios of the four couplers in the interleaver can be accurately controlled, so the degree of manufacturing difficulty is reduced.

A waveguide-based antireflection structure is proposed. The device consists of two polarization rotators (PRs), two polarization-distinguished 90-deg phase delay units (PDUs), and a polarization beam combiner (PBC). The PR and PDU, providing the same function as a quarter wave plate in free-space optics, convert a linearly polarized light into a circularly polarized light. Upon reflection from an isotropic homogenous interface, the returned light is converted back into a linearly polarized light in its perpendicular direction. Through the PBC placed at the input port, the returned light is then redirected into a different port for further use or discard. Our three-dimensional mode-matching method-based simulation shows that, on the silicon-on-insulator waveguide platform, the total device length can be made as short as 10.5  μm.

The influence of the phosphorus pentoxide (P₂O₅) content on the material properties of Er,Yb-doped potassium-lanthanum phosphate glass was studied. Glass samples of the following nominal composition 35.0K₂O-6.8Yb₂O₃-8.0La₂O₃-0.2Er₂O₃-50.0P₂O₅ (in mol%) were prepared from starting materials mixed with five additional amounts of P₂O₅ (0, 7.5, 15.0, 30.0, and 45.0 mol% related to the nominal glass composition). The P₂O₅ addition influence on properties of prepared glasses was studied using Raman, absorption, and fluorescence spectroscopy. The glass residual IR absorption and Judd–Ofelt intensity parameters together with absorption and emission cross sections were estimated. The results showed the increasing polymerization of glass and the P─O bond shortening with P₂O₅ content increase. The spectroscopy of Er³⁺ and Yb³⁺ ions was affected only marginally by the glass composition. It was found that fluorescence decay time corresponding to upper-laser-level ⁴I_13/2 increased with the decrease of P₂O₅ content in the glasses, which was related to increasing OH− contamination of the glass. The laser action at 1.53  μm under 975-nm pulsed laser diode pumping was successfully demonstrated. Low threshold and laser slope efficiency up to 21% in respect to absorbed pumping power were obtained.

Silicide Schottky-barrier midwave infrared detectors for the 3 to 5  μm waveband typically use a 2 to 8 nm metal-silicide layer grown on silicon substrates as an absorption layer. We demonstrate the use of an SiO₂-film-coated subwavelength grating as an antireflection incident plane in such detectors to enhance absorption at the 3 to 5  μm waveband in the metal-silicide layer for improving the external quantum efficiency (EQE). Taking a PtSi/p-Si structure as an example, we fabricate samples and build a test platform to characterize the EQE of a PtSi/p-Si Schottky-barrier detector. Simulation results show that low reflection efficiency (<9%) for the subwavelength-grating incident plane and factors of 1.5 to 1.8 enhancement in absorption of the PtSi layer are achieved at normal incidence across the 3 to 5  μm waveband. Measurement results show that this increase translates into factors of 1.3 to 1.7 enhancement in EQE. This improvement in EQE results from absorption enhancement due to antireflection effects and the forward scattering of incident infrared radiation in the subwavelength grating, which increases the effective optical path in the detector. The results also suggest that fabricating a subwavelength-grating incident plane is a general, low-cost method to enhance EQE in various infrared material platforms used for back-illuminated detectors.

We present an electrically actuated tunable optical grating based on dielectric elastomer actuators. A simple fabrication protocol is presented, which integrates the grating with the actuating mechanism both made of soft elastomers, improving the tunability of the grating. The device is designed to be operated in the transmission mode. It exhibits a continuous period tunability of 34.4% at an actuation voltage of 5.5 kV, which is an improvement over reported tunable optical gratings.

KLa (MoO₄)₂ : Eu³+ phosphors were prepared by the hydrothermal method. The after tuning of synthesis time and the ratio of the ethylene glycol to water ratio made the phosphor present different morphologies, including peanut-like shape and spheres. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), diffuse reflectance spectrum, and fluorescence spectrum. Under the excitation of 397 nm near-ultraviolet, the typical red emission produced by Eu³+ ions can be observed. And the phosphors show strong red light around 612 nm, attributed to ⁵D₀ → ⁷F₂ transition of Eu³+ ion. The luminescence properties of the as-prepared phosphors were studied based on changing the synthesis condition. It is found that the synthesis time and the changing of the ratio of ethylene glycol to water play the crucial role in the formation of morphology. The optimum dopant concentration of Eu³+ ions in KLa (MoO₄)₂ : Eu³+ is around 7 mol. %. Moreover, the fluorescence decay curve and thermal stability of luminescence were also investigated in detail. The Commission International de I’Eclairage coordinates of KLa (MoO₄)₂ : 0.07Eu³+ located in the red reddish region. All the results suggest that KLa (MoO₄)₂ : 0.07Eu³+ might be a promising reddish-orange emitting phosphor used in white light-emitting diodes (w-LED).

Recent improvements in optics miniaturization have led to the development of multiple techniques in order to perform minimally invasive endoscopic procedures. Among them, the wide-angle lens is often used for its simplicity and ability to collect a large amount of information in one capture. We report the design and analysis of two versions of a wide-angle endoscope with a full field of view of 180 deg. The difference between the two designs resides in the direct control over distortion that was used in order to achieve foveated imaging. By doing so, the local magnification at the center of the image can be controlled during the design process, allowing for a better resolution in the center of the field of view. The design is also folded in order to address certain issue concerning the region of interest during an endoscopic procedure.

An octagonal photonic crystal fiber (O-PCF) for numerical structure design and analysis of some particular properties are presented in this paper. The proposed design is suitable for residual dispersion compensation (RDC) with polarization maintaining (PM) applications as it offers extremely high-negative flattened average chromatic dispersion (D_T) and absolute dispersion variation (ΔD) of around −(708±10)  ps nm−¹ km−¹ and average high birefringence (B) of the order 10−² for the wavelength limits of 1.46 to 1.67  μm (bandwidth of 210 nm that covers S+C+L+U bands in the infrared region of the optical third window). In addition, it exhibits very low confinement loss of 10−^3.5 to 10−^2.5  dB/m for that bandwidth. Moreover, to evaluate the sensitivity of the fiber properties (D_T and B) during fabrication, ±0.02  μm variation in the optimum parameters is also studied.

Experimental results show that the use of film-forming solutions based on zinc and aluminum nitrates and high molecular polyvinylpyrrolidone allows obtaining thin oxide coatings having unusually large bandgap values and transparent in a wide spectral range. The values of the bandgaps of coating materials are significantly higher than the bandgap of bulk ZnO, and it is varied in the range from 3.46 to 4.16 eV, resulting from the metastable structure of the coating.

This PDF file contains the errata for “OE Vol. 55 Issue 04 Paper OE-2016-0328-ERR” for OE Vol. 55 Issue 04