Activity

The recent emergence of efficient O-band amplification technologies has enabled the consideration of O-band transmission beyond short reach. Despite the O-band being a low chromatic dispersion (CD) window, the impact of CD will become increasingly significant when extending the reach of direct-detection (DD) systems. In this work, we first numerically investigate the 3-dB bandwidth of single-mode fibers (SMF) and the CD-restricted transmission reach in intensity-modulation DD systems, confirming the significant difference between low- and high-dispersion O-band wavelengths. We then carry out experimental transmission studies over SMF for distances of up to 70 km at two different wavelengths, the low-dispersion 1320 nm and the more dispersive 1360 nm, enabled by the use of an O-band bismuth-doped fiber amplifier as a preamplifier at the receiver. We compare three 50-Gb/s optical DD formats, namely, Nyquist on-off keying (OOK), Nyquist 4-ary pulse amplitude modulation (PAM4) and Kramers-Kronig detection-assisted single-sideband quadrature phase shift keying (KK-QPSK) half-cycle subcarrier modulation. Our results show that at both wavelengths, OOK and QPSK exhibit better bit error rate performance than PAM4. When transmitting over 70-km of SMF at the less dispersive wavelength of 1320 nm, 50-Gb/s OOK modulation offers more than 1.5-dB optical power sensitivity improvement at the photodiode (PD) compared to 50-Gb/s QPSK. Conversely, at 1360 nm, the required optical power to the PD can be reduced by more than 3 dB by using QPSK instead of OOK.A novel, to the best of our knowledge, two-layer hybrid solid wedged etalon was fabricated and combined with a traditional imager to make a compact computational spectrometer. The hybrid wedge, comprised of $\rm Nb_2\rm O_5$ and Infrasil 302, was designed to operate from 0.4-2.4 µm. Initial demonstrations, however, used a complementary metal-oxide semiconductor (CMOS) imager and demonstrated operation from 0.4-0.9 µm with spectral resolutions $\lt\;30\;\rm cm^- 1$ from single snapshots. The computational spectrometer itself operates similarly to a spatial Fourier transform spectrometer (FTIR), but rather than use conventional Fourier-based methods or assumptions, the spectral reconstruction used a non-negative least-squares fitting algorithm based on analytically computed wavelength response vectors determined from extracted physical thicknesses across the entire two-dimensional wedge. This new computational technique resulted in performance and spectral resolutions exceeding those that could be achieved from Fourier processing techniques applied to this wedge etalon. With an additional imaging lens and translational scanning, the system can be converted into a hyperspectral imager.The bandwidth and stability limits of the stochastic parallel gradient descent (SPGD) algorithm used for coherent beam combination is investigated by deriving an analytical model for the phase control loop. The analytical model is compared to experiments and numerical simulations using a laboratory tiled coherent beam combination setup. The setup consisted of four sub-beams from fiber-optic collimators and used a backreflected signal as feedback. A rotating phase plate was used to induce phase disturbances into the system. The analytical model compared favorably to numerical simulations and experiments as well as to other studies found in the literature. The results can be used to provide an estimate of the achievable phase control bandwidth of coherent beam combination systems using SPGD as a control algorithm.Deep learning using convolutional neural networks (CNNs) has been shown to significantly outperform many conventional vision algorithms. Despite efforts to increase the CNN efficiency both algorithmically and with specialized hardware, deep learning remains difficult to deploy in resource-constrained environments. In this paper, we propose an end-to-end framework to explore how to optically compute the CNNs in free-space, much like a computational camera. Compared to existing free-space optics-based approaches that are limited to processing single-channel (i.e., gray scale) inputs, we propose the first general approach, based on nanoscale metasurface optics, that can process RGB input data. Our system achieves up to an order of magnitude energy savings and simplifies the sensor design, all the while sacrificing little network accuracy.Speckle noises widely exist in optical coherence tomography (OCT) images. We propose an improved double-path parallel convolutional neural network (called DPNet) to reduce speckles. We increase the network width to replace the network depth to extract deeper information from the original OCT images. In addition, we use dilated convolution and residual learning to increase the learning ability of our DPNet. We use 100 pairs of human retinal OCT images as the training dataset. Then we test the DPNet model for denoising speckles on four different types of OCT images, mainly including human retinal OCT images, skin OCT images, colon crypt OCT images, and quail embryo OCT images. We compare the DPNet model with the adaptive complex diffusion method, the curvelet shrinkage method, the shearlet-based total variation method, and the OCTNet method. We qualitatively and quantitatively evaluate these methods in terms of image smoothness, structural information protection, and edge clarity. Our experimental results prove the performance of the DPNet model, and it allows us to batch and quickly process different types of poor-quality OCT images without any parameter fine-tuning under a time-constrained situation.We design, fabricate, and characterize a multilayer nanophotonic structure that couples light from standard optical fiber to an integrated photonics chip with unprecedented efficiency. The structure comprises a multilayer waveguide array tapering into a single waveguide supporting only fundamental TE- and TM-like modes. Measurements reveal a record-setting fiber-to-chip coupling efficiency of $98.3\% \;\pm\;0.3\%$ per facet at a 1575 nm wavelength that remains greater than $92.8\% \;\pm\;0.4\%$ across the 1550-1600 nm wavelength range. This approach is tailorable to any material platform, fiber type, or operating wavelength and represents a significant step forward for the accessibility of integrated photonics.The weak-value-amplification technique has shown great importance in the measurement of tiny physical effects. Here we introduce a polarization-dependent angular velocity measurement system consisting of two Glan prisms and a true zero-order half-wave plate, where a non-Fourier-limited Gaussian pulse acts as the meter. The angular velocities measurements results agree well with theoretical predictions, and its uncertainties are bounded by the Cramér-Rao bound. We also investigate uncertainties of angular velocities for different numbers of detected photons and the smallest reliable postselection probability, which can reach $3.42*10^- 6$.An elegant breadboard prototype of the Aerosol Limb Imager (ALI) has been developed to meet key performance parameters that will meet requirements for the retrieval of aerosol from the upper troposphere and stratosphere from limb scattered sunlight radiance measurements. Similar in concept to previous high altitude balloon-based generations, this instrument pairs a liquid crystal polarization rotator with an acousto-optic tunable filter to capture polarimetric multi-spectral images of the atmospheric limb. This design improves the vertical resolution, signal-to-noise ratio, and athermalization, all of which will facilitate observation from a moving high altitude aircraft platform, which provides a platform analogous to the spatially varying measurements that would be made from a satellite. Finally, a preliminary design is presented for a satellite-based generation of ALI.At present, accurate wavelength calibration plays an important role in laser spectrum measurements. Although the wavelength calibration methods have been investigated for a long time, there are no techniques that are particularly designed for laser spectral calibration to the best of our knowledge. A mathematical model for calibrating a pulse laser wavelength is first established, to the best of our knowledge. According to the analysis formula of dispersion aberration, a flat-field concave grating in the near-infrared band is designed. Then, a wavelength calibration model based on concave grating spectroscopy is proposed. Through adjusting the spectra of each pixel, we design a calibration algorithm based on the cubic spline interpolation and kernel regression methods. By compensating and interpolating spectral data, accurate wavelengths are obtained. Finally, some experiments verify the calibration performance of the proposed method. Meanwhile, the uncertainty of measurement is also analyzed.Nonintrusive three-component (3C) velocity measurements of free jet flows were conducted by stereoscopic picosecond laser electronic excitation tagging (S-PLEET) at 100 kHz. The fundamental frequency of the burst-mode laser at 1064 nm was focused to generate the PLEET signal in a free jet flow. A stereoscopic imaging system was used to capture the PLEET signals. The 3C centroids of the PLEET signal were determined by utilizing simultaneous images from two cameras placed at an angle. The temporal evolutions of the centroids were obtained and used to determine the instantaneous, time-resolved 3C velocities of the flows. The free jets with various inlet pressures of 10-40 bars exhausting into atmospheric pressure air (i.e., underexpanded free jet with large pressure ratios; Reynolds numbers from the jet ranged from 39,000 to 145,000) were measured by S-PLEET. Key 3C turbulent properties of the free jets, including instantaneous and mean velocities, were obtained with an instantaneous measurement uncertainty of about $\rm\pm 10\;\rmm/s$, which is about 2% of the highest velocities measured. Computation of higher-order statistics including covariances related to turbulent kinetic energy and the Reynolds stress component was demonstrated. The 3C nonintrusive and unseeded velocimetry technique could provide a new tool for flow property measurements in ground test facilities; the measured high-frequency turbulence properties of free jet flows could be useful for turbulence modeling and validations.A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. Selleck SC144 The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550-800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the $\rmN_\rmII(565\;\rmnm)$ and $\rmN_\rmII(594\;\rmnm)$ emissions is presented. In studying line emission ratios for FAR calculation, it is found that $\rmH_\alpha(656\;\rmnm)/\rmN_\rmII(568\;\rmnm)$ is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while $\rmH_\alpha(656\;\rmnm)/\rmO_\rm I(777\;\rmnm)$ is optimal for the ps-LIBS and fs-LIBS cases.