Photonics — Open Access Journal

We report a significant improvement in the diagnosis of cervical cancer through a combined application of principal component analysis (PCA) and support vector machine (SVM) on the average fluorescence decay profile of Fluorescence Lifetime Images (FLI) of epithelial hyperplasia (EH) and CIN-I cervical tissue samples, obtained ex-vivo. The fast and slow components of double exponential fitted fluorescence lifetimes were found to be higher for EH compared to the lifetimes of CIN-I samples. Application of PCA to the average time-resolved fluorescence decay profiles showed that the 2nd PC, in combination with 1st PC, enhanced the discrimination between EH and CIN-I tissues. Fluorescence lifetime and PC scores were then classified separately by using SVM support vector machine to identify the two. On applying SVM to a combination of fluorescence lifetime and PC scores, diagnostic capability improved significantly. Full article

In this paper, we design and numerically demonstrate an ultra-thin super-oscillatory metalens with a resolution below the diffraction limit. The zones of the lens are implemented using metasurface concepts with hexagonal unit cells. This way, the transparency and, hence, efficiency is optimized, compared to the conventional transparent–opaque zoning approach that introduces, inevitably, a high reflection in the opaque regions. Furthermore, a novel two-step optimization technique, based on evolutionary algorithms, is developed to reduce the side lobes and boost the intensity at the focus. After the design process, we demonstrate that the metalens is able to generate a focal spot of 0.46λ₀ (1.4 times below the resolution limit) at the design focal length of 10λ₀ with reduced side lobes (the side lobe level being approximately −11 dB). The metalens is optimized at 0.327 THz, and has been validated with numerical simulations. Full article

Electromagnetic waves at frequencies below the X-ray region strongly couple to the optical vibrational modes in a solid. These coupled excitations have been called phonon polaritons. The relationship of the polariton frequency versus the polariton wavevector shows a remarkable dispersion, especially in the vicinity of the transverse and longitudinal optical mode frequencies. The significant frequency dependence enables valuable applications such as a tunable terahertz radiation source. The polariton dispersion relations of technologically important dielectric and ferroelectric crystals were reviewed in the broad terahertz range using terahertz time-domain spectroscopy, far-infrared spectroscopy, and Raman scattering spectroscopy. Full article

The decomposition of the Mueller matrix of blood films has been carried out using differential matrices with polarized and depolarized parts. The use of a coherent reference wave is applied and the algorithm of digital holographic reconstruction of the field of complex amplitudes is used. On this basis, the 3D Mueller-matrix diffuse tomography method—the reconstruction of distributions of fluctuations of linear and circular birefringence of depolarizing polycrystalline films of human blood is analytically justified and experimentally tested. The dynamics of the change in the magnitude of the statistical moments of the first-fourth order, which characterize layer-by-layer distributions of fluctuations in the phase anisotropy of the blood film, is examined and analyzed. The most sensitive parameters for prostate cancer are the statistical moments of the third and fourth orders, which characterize the asymmetry and kurtosis of fluctuations in the linear and circular birefringence of blood films. The excellent accuracy of differentiation obtained polycrystalline films of blood from healthy donors and patients with cancer patients was achieved. Full article

In this paper, spatial pulse position modulation (SPPM) is used as a case study to investigate the performance of the optical spatial modulation (SM) technique in outdoor atmospheric turbulence (AT). A closed-form expression for the upper bound on the asymptotic symbol error rate (SER) of SPPM in AT is derived and validated by closely-matching simulation results. The error performance is evaluated in weak to strong AT conditions. As the AT strength increases from weak to strong, the channel fading coefficients become more dispersed and differentiable. Thus, a better error performance is observed under moderate-to-strong AT compared to weak AT. The performance in weak AT can be improved by applying unequal power allocation to make free-space optical communication (FSO) links more distinguishable at the receiver. Receive diversity is considered to mitigate irradiance fluctuation and improve the robustness of the system to turbulence-induced channel fading. The diversity order is computed as half of the number of detectors. Performance comparisons, in terms of energy and spectral efficiencies, are drawn between the SPPM scheme and conventional MIMO schemes such as repetition coding and spatial multiplexing. Full article

We present the principle of a terahertz-wave radar and its proof-of-concept experimental verification. The radar is based on a 522 GHz resonant-tunneling-diode oscillator, whose terahertz output power can be easily modulated by superimposing the modulation signal on its bias voltage. By using one modulation frequency and measuring the time delay of the returning signal, a relative measurement of the propagation distance is possible; adding a second modulation frequency removes the ambiguity stemming from the periodicity of the modulation sine wave and allows an absolute distance measurement. We verified this measurement method experimentally and obtained a submillimeter precision, as predicted by theory. Full article

Directly modulated long-wavelength vertical cavity surface emitting lasers (VCSELs) are considered for the implementation of sliceable bandwidth/bitrate variable transceivers for very high capacity transmission (higher than 50 Gb/s per wavelength) in metropolitan area systems characterized by reduced cost, power consumption, and footprint. The impact of the frequency chirp measured for InP VCSELs with different kinds of design (high-bandwidth very short cavity and widely-tunable with micro electro-mechanical systems (MEMS) top mirror) is analyzed in case of discrete multitone (DMT) direct modulation in combination with 25-GHz wavelength selective switch (WSS) filtering. The maximum transmitted capacity for both dual side- and single side-band DMT modulation is evaluated as a function of the number of crossed nodes in a mesh metro network, comparing VCSEL based transmitters performance also with the case of external electro-absorption modulator use. Finally, the maximum reach achieved based on the received optical signal to noise ratio (OSNR) and the fiber span length is discussed. The results confirm the possibility to use directly-modulated long-wavelength VCSELs for the realization of sliceable bandwidth/bitrate variable transmitters targeting 50-Gb/s capacity per polarization, also in the presence of 5 crossed WSSs for reaches of hundreds of kilometers in multi-span Erbium-doped fiber amplified (EDFA) metro links supported by coherent detection. Full article

In terahertz-band communication using ultra-high frequencies, compound semiconductors with superior high-frequency performance have been used for research to date. Terahertz communication using the 300 GHz band has nonetheless attracted attention based on the expectation that an unallocated frequency band exceeding 275 GHz can be used for communication in the future. Research into wireless transceivers using BiCMOS integrated circuits with silicon germanium transistors and advanced miniaturized CMOS integrated circuits has increased in this 300 GHz band. In this paper, we will outline the terahertz communication technology using silicon integrated circuits available from mass production, and discuss its applications and future. Full article

The evolution of radio access networks is towards a centralized architecture (C-RAN), with massive antenna deployments and large radio-frequency bandwidths. In the next future, traditional optical transport technologies based on digital radio over fiber will no longer be able to support the mobile fronthaul traffic connecting antennas hosted at remote radio units and centralized baseband units. Analog radio over fiber can be selected to support the mobile fronthaul (MFH) network and, in particular, pulse width modulation (PWM) is a viable alternative for analog signal transport. In order to increase the MFH spectral efficiency, we propose to exploit multilevel PWM (M-PWM) in a wavelength division multiplexing-passive optical network (WDM-PON) network, comparing its performance with a conventional 2-level PWM solution. Experimental results show successful transmission over 7.5-km standard single mode fiber (SSMF) of up to 16 aggregated LTE-like 20-MHz signals with 64-QAM on each subcarrier, while up to eight aggregated LTE-like 20-MHz signals with 256-QAM could be supported. M-PWM thus allows either using higher order modulation formats or aggregating a higher number of LTE channels. Full article

The modeling and design of fiber lasers facilitate the process of their practical realization. Of particular interest during the last few years is the development of lanthanide ion-doped fiber lasers that operate at wavelengths exceeding 2000 nm. There are two main host glass materials considered for this purpose, namely fluoride and chalcogenide glasses. Therefore, this study concerned comparative modeling of fiber lasers operating within the infrared wavelength region beyond 2000 nm. In particular, the convergence properties of selected algorithms, implemented within various software environments, were studied with a specific focus on the central processing unit (CPU) time and calculation residual. Two representative fiber laser cavities were considered: One was based on a chalcogenide–selenide glass step-index fiber doped with trivalent dysprosium ions, whereas the other was a fluoride step-index fiber doped with trivalent erbium ions. The practical calculation accuracy was also assessed by comparing directly the results obtained from the different models. Full article

In the field of scintillators, high scintillation and light production performance require high-quality crystals. Although the composition and structure of crystals are fundamental in this direction, their ultimate optical performance is strongly dependent on the surface finishing treatment. This paper compares two surface finishing methods in terms of the final structural condition of the surface and the relative light yield performances. The first polishing method is the conventional “Mechanical Diamond Polishing” (MDP) technique. The second polishing technique is a method applied in the electronics industry which is envisaged for finishing the surface treatment of scintillator crystals. This method, named “Chemical Mechanical Polishing” (CMP), is efficient in terms of the cost and material removal rate and is expected to produce low perturbed surface layers, with a possible improvement of the internal reflectivity and, in turn, the light collection efficiency. The two methods have been applied to a lead tungstate PbWO₄ (PWO) single crystal due to the wide diffusion of this material in high energy physics (CERN, PANDA project) and diagnostic medical applications. The light yield (LY) values of both the MDP and CMP treated crystals were measured by using the facilities at CERN while their surface structure was investigated by Scanning Electron Microscopy (SEM) and Grazing Incidence X-ray Diffraction (GID). We present here the corresponding optical results and their relationship with the processing conditions and subsurface structure. Full article

A Pancharatnam–Berry optical element is designed, fabricated, and optically characterized for the demultiplexing of beams with different polarization and orbital angular momentum states at the telecom wavelength of 1310 nm. The geometric phase control is achieved by fabricating properly-oriented subwavelength gratings on a silicon substrate, inducing a spatially-variant form birefringence. The digital grating pattern is transferred to the silicon substrate with a two-step nanofabrication protocol, using inductively coupled plasma reactive ion etching to transfer the resist pattern generated with high-resolution electron beam lithography. The optical characterization of the sample confirms the expected capability to sort circularly polarized optical beams with different handedness and orbital angular momentum. Encompassing optical element design and silicon photonics, the designed silicon metasurface paves the way to innovative devices for total angular momentum mode division multiplexing with unprecedented levels of integration. Full article

The optical access network is currently driving studies on transmissions beyond 10 Gbit/s. This paper reports an analysis of Pulse Amplitude Modulation (PAM), seen as a promising candidate for future Passive Optical Networks (PON). Previous 25 Gbit/s real-time PAM4 results are extrapolated here with simulations to higher bit rates and a higher number of PAM levels. Our main goal is to evaluate the compliancy of PAM with the existing standards and legacy networks as far as fiber length, optical budget class, and wavelength plan are concerned. The simulations enlighten us as to the challenges of multilevel modulation formats, such as noise and jitter, compared to the currently adopted Non-Return-to-Zero (NRZ). Full article

This paper presents a pre-processing method to remove multiple scattering artifacts in spectroscopic optical coherence tomography (SOCT) using time–frequency analysis approaches. The method uses a multiple multipole expansion approach to model the light fields in SOCT. It is shown that the multiple scattered fields can be characterized by higher order terms of the multiple multipole expansion. Hence, the multiple scattering artifact can thus be eliminated by applying the time–frequency transform on the SOCT measurements characterized by the lower order terms. Simulation and experimental results are presented to show the effectiveness of the proposed pre-processing method. Full article

Spectrally pure semiconductor lasers produced via self-injection locking to high quality factor monolithic optical resonators demonstrate sub-kHz instantaneous linewidth. The lasers are used in photonic sensor systems and microwave photonic oscillators benefitting from the improved spectral purity, the stability and the reduced environmental sensitivity of the lasers. The laser frequency stability is defined by both the optical resonator and the optical path of the entire system comprising the laser, the resonator, and the miscellaneous optical components. The impacts of the various destabilization factors are usually convoluted, and it is hardly possible to separate them. In this paper, we report on an experimental study of an influence of the variations of the optical path on the laser frequency stability. We have created a whispering gallery mode optical resonator having the record small thermal sensitivity, on the order of 0.1 ppm/${}^{\circ }$C, and demonstrated a self-injection locked laser based on this resonator. The measured laser stability is characterized with 1 s Allan deviation of ${10}^{-12}$, limited by the thermal sensitivity of the optical path between the laser and the resonator. The thermal stabilization on the order of 10$\mathsf{\mu }$K at 1 s is achieved using a standard thermo-electric element. The long term drift of the laser frequency is determined by both the fluctuations of the atmospheric pressure in the laboratory impacting the monolithic resonator and by the optical path instability. Full article

3D vision systems are more and more required in a large variety of applications and mostly for mechanical and medical purposes. This paper presents the study and realization of a prototype of a structured light automated tilt-rotational 3D vision system for high-quality reconstructions of components of various sizes and in cases of freeform and complex surfaces. The main goal of this research work was to develop an instrument with the following main novelties: configurability for different object sizes, high precision and resolution levels and ability to automatically generate the mesh representing the full scanned objects without any intervention of the operator by means of a 2 degrees of freedom automated tilt-rotational mechanical positioning system. A detailed analysis of the instrument and the procedures and results of the performance tests are presented, together with the examination of possible strategies to obtain a better performance, especially by the calibration and the synchronization between the optical and the mechanical systems. As a result, the prototype and the performance parameters resulting from the experimental campaigns, are reported. Full article

Flip-chip truncated-pyramid-shaped blue micro-light-emitting diodes (μ-LEDs), with different inclinations of the mesa facets to the epitaxial layer plane, are studied by simulations, implementing experimental information on temperature-dependent parameters and characteristics of large-size devices. Strong non-monotonous dependence of light extraction efficiency (LEE) on the inclination angle is revealed, affecting, remarkably, the overall emission efficiency. Without texturing of emitting surfaces, LEE to air up to 54.4% is predicted for optimized shape of the μ-LED dice, which is higher than that of conventional large-size LEDs. The major factors limiting the μ-LED performance are identified, among which, the most critical are the optical losses originated from incomplete light reflection from metallic electrodes and the high p-contact resistance caused by its small area. Optimization of the p-electrode dimensions enables further improvement of high-current wall-plug efficiency of the devices. The roles of surface recombination, device self-heating, current crowding, and efficiency droop at high current densities, in limitation of the μ-LED efficiency, are assessed. A novel approach implementing the characterization data of large-size LED as the input information for simulations is tested successfully. Full article

In this paper, a high sensitivity, polarization preserving photonic crystal fiber (PCF), based on circular air holes for sensing in the terahertz (THz) band, is presented. The finite element method, a practical and precise computational technique for describing the interactions between light and matter, is used to compute the modal properties of the designed fiber. For the designed PCF, comprising of circular air holes in both the cladding and in the porous core, a relative sensitivity of 73.5% and a high birefringence of 0.013 are achieved at 1.6 THz. The all circular air-hole structure, owing to its simplicity and compatibility with the current fiber draw technique for PCF fabrication, can be realized practically. It is anticipated that the designed fiber can be employed in applications such as detection of biological samples and toxic chemicals, imaging, and spectroscopy. Full article

Pacemaker cardiomyocytes of the sinoatrial node (SAN) beat more rapidly than cells of the working myocardium. Beating in SAN cells responds to β-adrenergic and cholinergic signaling by speeding up or slowing, respectively. Beat rate has traditionally been assessed using voltage or calcium sensitive dyes, however these may not reflect the true rate of beating because they sequester calcium. Finally, in vitro differentiated cardiomyocytes sometimes briefly pause during imaging giving inaccurate beat rates. We have developed a MATLAB automation to calculate cardiac beat rates directly from video clips based on changes in pixel density at the edges of beating areas. These data are normalized to minimize the effects of secondary movement and are converted to frequency data using a fast Fourier transform (FFT). We find that this gives accurate beat rates even when there are brief pauses in beating. This technique can be used to rapidly assess beating of cardiomyocytes in organoid culture. This technique could also be combined with field scanning techniques to automatically and accurately assess beating within a complex cardiac organoid. Full article