Open AccessArticle
Evanescently Coupled Rectangular Microresonators in Silicon-on-Insulator with High Q-Values: Experimental Characterization
Photonics 2017, 4(2), 34; doi:10.3390/photonics4020034 -
Abstract
We report on evanescently coupled rectangular microresonators with dimensions up to 20 × 10 μm2 in silicon-on-insulator in an add-drop filter configuration. The influence of the geometrical parameters of the device was experimentally characterized and a high Q value of 13,000 was
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We report on evanescently coupled rectangular microresonators with dimensions up to 20 × 10 μm2 in silicon-on-insulator in an add-drop filter configuration. The influence of the geometrical parameters of the device was experimentally characterized and a high Q value of 13,000 was demonstrated as well as the multimode optical resonance characteristics in the drop port. We also show a 95% energy transfer between ports when the device is operated in TM-polarization and determine the full symmetry of the device by using an eight-port configuration, allowing the drop waveguide to be placed on any of its sides, providing a way to filter and route optical signals. We used the FDTD method to analyze the device and e-beam lithography and dry etching techniques for fabrication. Full article
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Open AccessArticle
Coupling of Surface Plasmon Polariton in Al-Doped ZnO with Fabry-Pérot Resonance for Total Light Absorption
Photonics 2017, 4(2), 35; doi:10.3390/photonics4020035 -
Abstract
Al-doped ZnO (AZO) can be used as an electrically tunable plasmonic material in the near infrared range. This paper presents finite-difference time-domain (FDTD) simulations on total light absorption (TLA) resulting from the coupling of a surface plasmon polariton (SPP) with Fabry-Pérot (F-P) resonance
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Al-doped ZnO (AZO) can be used as an electrically tunable plasmonic material in the near infrared range. This paper presents finite-difference time-domain (FDTD) simulations on total light absorption (TLA) resulting from the coupling of a surface plasmon polariton (SPP) with Fabry-Pérot (F-P) resonance in a three-layer structure consisting of an AZO square lattice hole array, a spacer, and a layer of silver. Firstly, we identified that the surface plasmon polariton (SPP) that will couple to the F-P resonance because of an SPP standing wave in the (1,0) direction of the square lattice. Two types of coupling between SPP and F-P resonance are observed in the simulations. In order to achieve TLA, an increase in the refractive index of the spacer material leads to a decrease in the thickness of the spacer. Additionally, it is shown that the replacement of silver by other, more cost-effective metals has no significance influence on the TLA condition. It is observed in the simulations that post-fabrication tunability of the TLA wavelength is possible via the electrical tunability of the AZO. Finally, electric field intensity distributions at specific wavelengths are computed to further prove the coupling of SPP with F-P resonance. This work will contribute to the design principle for future device fabrication for TLA applications. Full article
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Open AccessArticle
XL-SIM: Extending Superresolution into Deeper Layers
Photonics 2017, 4(2), 33; doi:10.3390/photonics4020033 -
Abstract
Of all 3D-super resolution techniques, structured illumination microscopy (SIM) provides the best compromise with respect to resolution, signal-to-noise ratio (S/N), speed and cell viability. Its ability to achieve double resolution in all three dimensions enables resolving 3D-volumes almost 10× smaller than with a
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Of all 3D-super resolution techniques, structured illumination microscopy (SIM) provides the best compromise with respect to resolution, signal-to-noise ratio (S/N), speed and cell viability. Its ability to achieve double resolution in all three dimensions enables resolving 3D-volumes almost 10× smaller than with a normal light microscope. Its major drawback is noise contained in the out-of-focus-signal, which—unlike the out-of-focus signal itself—cannot be removed mathematically. The resulting “noise-pollution” grows bigger the more light is removed, thus rendering thicker biological samples unsuitable for SIM. By using a slit confocal pattern, we employ optical means to suppress out-of-focus light before its noise can spoil SIM mathematics. This not only increases tissue penetration considerably, but also provides a better S/N performance and an improved confocality. The SIM pattern we employ is no line grid, but a two-dimensional hexagonal structure, which makes pattern rotation between image acquisitions obsolete and thus simplifies image acquisition and yields more robust fit parameters for SIM. Full article
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Open AccessArticle
Photothermal Microscopy for High Sensitivity and High Resolution Absorption Contrast Imaging of Biological Tissues
Photonics 2017, 4(2), 32; doi:10.3390/photonics4020032 -
Abstract
Photothermal microscopy is useful to visualize the distribution of non-fluorescence chromoproteins in biological specimens. Here, we developed a high sensitivity and high resolution photothermal microscopy with low-cost and compact laser diodes as light sources. A new detection scheme for improving signal to noise
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Photothermal microscopy is useful to visualize the distribution of non-fluorescence chromoproteins in biological specimens. Here, we developed a high sensitivity and high resolution photothermal microscopy with low-cost and compact laser diodes as light sources. A new detection scheme for improving signal to noise ratio more than 4-fold is presented. It is demonstrated that spatial resolution in photothermal microscopy is up to nearly twice as high as that in the conventional widefield microscopy. Furthermore, we demonstrated the ability for distinguishing or identifying biological molecules with simultaneous muti-wavelength imaging. Simultaneous photothermal and fluorescence imaging of mouse brain tissue was conducted to visualize both neurons expressing yellow fluorescent protein and endogenous non-fluorescent chromophores. Full article
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Open AccessArticle
High Harmonics with Controllable Polarization by a Burst of Linearly-Polarized Driver Pulses
Photonics 2017, 4(2), 31; doi:10.3390/photonics4020031 -
Abstract
We theoretically explore a scheme for generation of bright circularly and elliptically polarized high-order harmonics by bursts of linearly polarized pulses with a rotating polarization axis. Circularly polarized harmonics are formed if the bursts are comprised of N pulses that uphold an N
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We theoretically explore a scheme for generation of bright circularly and elliptically polarized high-order harmonics by bursts of linearly polarized pulses with a rotating polarization axis. Circularly polarized harmonics are formed if the bursts are comprised of N pulses that uphold an N-fold rotational symmetry, for N > 2. Rotating the polarization axes of the comprising pulses can generate elliptical harmonics with a collectively tunable ellipticity, from circular through elliptic to linear. The method preserves the single-cycle, single-atom and macroscopic physics of ‘standard’ linearly polarized high harmonic generation, with a high yield and cutoff energy. We investigate the method from a time-domain perspective, as well as a photonic perspective, and formulate the energy and spin-angular momentum conservation laws for this scheme. We find that the case of N = 4 is optimal for this method, resulting with the highest conversion efficiency of elliptical photons. The new features of this source offer new applications to helical ultrafast spectroscopy and ellipsometry. Full article
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Open AccessArticle
Polarization Characterization of Soft X-Ray Radiation at FERMI FEL-2
Photonics 2017, 4(2), 29; doi:10.3390/photonics4020029 -
Abstract
The control of polarization state in soft and hard X-ray light is of crucial interest to probe structural and symmetry properties of matter. Thanks to their Apple-II type undulators, the FERMI-Free Electron Lasers are able to provide elliptical, circular or linearly polarized light
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The control of polarization state in soft and hard X-ray light is of crucial interest to probe structural and symmetry properties of matter. Thanks to their Apple-II type undulators, the FERMI-Free Electron Lasers are able to provide elliptical, circular or linearly polarized light within the extreme ultraviolet and soft X-ray range. In this paper, we report the characterization of the polarization state of FERMI FEL-2 down to 5 nm. The results show a high degree of polarization of the FEL pulses, typically above 95%. The campaign of measurements was performed at the Low Density Matter beamline using an electron Time-Of-Flight based polarimeter. Full article
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Open AccessArticle
High-Resolution Arrayed-Waveguide-Gratings in Astronomy: Design and Fabrication Challenges
Photonics 2017, 4(2), 30; doi:10.3390/photonics4020030 -
Abstract
A comprehensive design of a folded-architecture arrayed-waveguide-grating (AWG)-device, targeted at applications as integrated photonic spectrographs (IPS) in near-infrared astronomy, is presented. The AWG structure is designed for the astronomical H-band (1500 nm–1800 nm) with a theoretical maximum resolving power R = 60,000 at
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A comprehensive design of a folded-architecture arrayed-waveguide-grating (AWG)-device, targeted at applications as integrated photonic spectrographs (IPS) in near-infrared astronomy, is presented. The AWG structure is designed for the astronomical H-band (1500 nm–1800 nm) with a theoretical maximum resolving power R = 60,000 at 1630 nm. The geometry of the device is optimized for a compact structure with a footprint of 5.5 cm × 3.93 cm on SiO2 platform. To evaluate the fabrication challenges of such high-resolution AWGs, effects of random perturbations of the effective refractive index (RI) distribution in the free propagation region (FPR), as well as small variations of the array waveguide optical lengths are numerically investigated. The results of the investigation show a dramatic degradation of the point spread function (PSF) for a random effective RI distribution with variance values above 10-4 for both the FPR and the waveguide array. Based on the results, requirements on the fabrication technology for high-resolution AWG-based spectrographs are given in the end. Full article
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Open AccessArticle
Ultra-High-Capacity Optical Packet Switching Networks with Coherent Polarization Division Multiplexing QPSK/16QAM Modulation Formats
Photonics 2017, 4(2), 27; doi:10.3390/photonics4020027 -
Abstract
Optical packet switching (OPS) networks and its subsystems, like the burst-mode receiver, are an essential technology currently used in passive optical networks (PONs). Moreover, OPS may play a fundamental role on future hybrid optical circuit switching (OCS)/OPS networks and datacenter networks. This paper
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Optical packet switching (OPS) networks and its subsystems, like the burst-mode receiver, are an essential technology currently used in passive optical networks (PONs). Moreover, OPS may play a fundamental role on future hybrid optical circuit switching (OCS)/OPS networks and datacenter networks. This paper focuses on two fundamental subsystems of packetized optical networks: the digital coherent burst-mode receiver and the electro-optical switch. We describe and experimentally characterize a novel digital coherent burst-mode receiver that makes uses of the Stokes parametrization to rapidly estimate the state of polarization (SOP) and optimize the equalizer convergence time. This burst-mode receiver is suitable for optical packetized networks that make use of advanced modulation formats such as quadrature amplitude modulation (QAM). We study the suitability of (Pb,La)(Zr,La)O3 (PLZT) optical switches for amplitude-variable coherent polarization division multiplexing (PDM) 16QAM modulation format and demonstrate a switching capacity of 10.24 Tb/s/port. We demonstrate a full 2 × 2 OPS node with a control plane capable of solving packet contention by means of packet dropping or buffering with a switching capacity of 10.24 Tb/s/port. Finally, we demonstrate the operation of the 2 × 2 OPS node with a record capacity of 12.8 Tb/s/port plus 100 km of dispersion-compensated fiber transmission. Full article
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Open AccessReview
Generation and Applications of Extreme-Ultraviolet Vortices
Photonics 2017, 4(2), 28; doi:10.3390/photonics4020028 -
Abstract
Vortex light beams are structures of the electromagnetic field with a spiral phase ramp around a point-phase singularity. These vortices have many applications in the optical regime, ranging from optical trapping and quantum information to spectroscopy and microscopy. The extension of vortices into
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Vortex light beams are structures of the electromagnetic field with a spiral phase ramp around a point-phase singularity. These vortices have many applications in the optical regime, ranging from optical trapping and quantum information to spectroscopy and microscopy. The extension of vortices into the extreme-ultraviolet (XUV)/X-ray regime constitutes a significant step forward to bring those applications to the nanometer or even atomic scale. The recent development of a new generation of X-ray sources, and the refinement of other techniques, such as harmonic generation, have boosted the interest of producing vortex beams at short wavelengths. In this manuscript, we review the recent studies in the subject, and we collect the major prospects of this emerging field. We also focus on the unique and promising applications of ultrashort XUV/X-ray vortex pulses. Full article
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Open AccessFeature PaperReview
Generation of Attosecond Light Pulses from Gas and Solid State Media
Photonics 2017, 4(2), 26; doi:10.3390/photonics4020026 -
Abstract
Real-time observation of ultrafast dynamics in the microcosm is a fundamental approach for understanding the internal evolution of physical, chemical and biological systems. Tools for tracing such dynamics are flashes of light with duration comparable to or shorter than the characteristic evolution times
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Real-time observation of ultrafast dynamics in the microcosm is a fundamental approach for understanding the internal evolution of physical, chemical and biological systems. Tools for tracing such dynamics are flashes of light with duration comparable to or shorter than the characteristic evolution times of the system under investigation. While femtosecond (fs) pulses are successfully used to investigate vibrational dynamics in molecular systems, real time observation of electron motion in all states of matter requires temporal resolution in the attosecond (1 attosecond (asec) = 10−18 s) time scale. During the last decades, continuous efforts in ultra-short pulse engineering led to the development of table-top sources which can produce asec pulses. These pulses have been synthesized by using broadband coherent radiation in the extreme ultraviolet (XUV) spectral region generated by the interaction of matter with intense fs pulses. Here, we will review asec pulses generated by the interaction of gas phase media and solid surfaces with intense fs IR laser fields. After a brief overview of the fundamental process underlying the XUV emission form these media, we will review the current technology, specifications and the ongoing developments of such asec sources. Full article
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Open AccessArticle
Optimization and Characterization of High-Harmonic Generation for Probing Solid Density Plasmas
Photonics 2017, 4(2), 25; doi:10.3390/photonics4020025 -
Abstract
The creation of high energy density plasma states produced during laser–solid interaction on a sub-picosecond timescale opens a way to create astrophysical plasmas in the lab to investigate their properties, such as the frequency-dependent refractive index. Available probes to measure absorption and phase-changes
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The creation of high energy density plasma states produced during laser–solid interaction on a sub-picosecond timescale opens a way to create astrophysical plasmas in the lab to investigate their properties, such as the frequency-dependent refractive index. Available probes to measure absorption and phase-changes given by the complex refractive index of the plasma state are extreme-UV (EUV) and soft X-ray (XUV) ultra-short pulses from high harmonic generation (HHG). For demanding imaging applications such as single-shot measurements of solid density plasmas, the HHG probe has to be optimized in photon number and characterized in intensity and wavefront stability from shot-to-shot. In an experiment, a coherent EUV source based on HHG driven by a compact diode-pumped laser is optimized in photons per pulse for argon and xenon, and the shot-to-shot intensity stability and wavefront changes are characterized. The experimental results are compared to an analytical model estimating the HHG yield, showing good agreement. The obtained values are compared to available data for solid density plasmas to confirm the feasibility of HHG as a probe. Full article
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Open AccessArticle
Study of the Effect of Nematic Order Degradation in Liquid Crystal-Based Surface Plasmon Resonance Sensors
Photonics 2017, 4(2), 24; doi:10.3390/photonics4020024 -
Abstract
This paper presents a new analysis of optical sensors based on surface plasmon resonance (SPR) phenomenon and nematic liquid crystal (LC) sensitive layer in the partially ordered state. In particular, the paper studies the influence of degradation in the LC ordering state on
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This paper presents a new analysis of optical sensors based on surface plasmon resonance (SPR) phenomenon and nematic liquid crystal (LC) sensitive layer in the partially ordered state. In particular, the paper studies the influence of degradation in the LC ordering state on the behavior of the plasmon resonance parameters. The degradation in the LC ordering is represented by the order parameter. The explicit treatment of the order parameter is critical when trying to differentiate between a change in alignment and a degradation of alignment in LC in response to the presence of an external stimulus in LC based sensors. When a reduction in ordering occurs, ignoring the order parameter can produce misleading results. This sensor has potential applications in chemical and biological systems. The paper presents a tracking method for the state of alignment and degree of ordering of the partially ordered LC film. This can be achieved via the SPR propagation constant and the critical angle at the interface between a metal and an LC film. Full article
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Open AccessArticle
Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating
Photonics 2017, 4(2), 23; doi:10.3390/photonics4020023 -
Abstract
Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by
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Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time (τ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature (Te 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing. Full article
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Open AccessArticle
Minimizing Blocking Probability in Elastic Optical Networks by Varying the Bandwidth Granularity Based on Optical Path Fragmentation
Photonics 2017, 4(2), 20; doi:10.3390/photonics4020020 -
Abstract
Elastic optical networks (EONs) based on orthogonal frequency-division multiplexing (OFDM) are considered a promising solution for the next optical network’s generation. These networks make it possible to choose an adequate portion of the available spectrum to satisfy the requested capacity. In this paper,
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Elastic optical networks (EONs) based on orthogonal frequency-division multiplexing (OFDM) are considered a promising solution for the next optical network’s generation. These networks make it possible to choose an adequate portion of the available spectrum to satisfy the requested capacity. In this paper, we consider the impact of spectrum fragmentation along the optical single/multipath routing transmission on the efficiency of the EONs. This involves reducing the fragmentation effects by dynamically updating and controlling the minimum bandwidth allocation granularity (ɡ). We adopt linear and nonlinear dynamic mechanisms, which are denoted as LDAɡ and NLDAɡ, respectively, to choose proper bandwidth granularities that are proportional to the optical link/path bandwidth fragmentation status. In order to avoid either splitting the capacity request over many routing paths, which would increase the management complexity, or encouraging single path transmission, the proposed schemes aim to choose a proper bandwidth allocation granularity (ɡ) for a predefined set of suggested values. Simulation results show that varying the bandwidth granularity based on the optical path fragmentation status can offer an improved performance over fixed granularity with respect to the bandwidth blocking probability, the number of path splitting actions, the throughput, and the differential delay constraint issue in terms of: the network bandwidth utilization and multipath distribution. Full article
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Open AccessArticle
Dynamics of the MnAs α/β-Striped Microstructure and of the Fe Magnetization Reversal in Fe/MnAs/GaAs(001): An Optical-Laser Pump–Free-Electron-Laser Probe Scattering Experiment
Photonics 2017, 4(2), 21; doi:10.3390/photonics4020021 -
Abstract
It was shown recently that the Fe magnetization reversal in the Fe/MnAs/GaAs(001) epitaxial system, attained by temperature control of the regular stripe pattern of the MnAs α- and β-phases, can also be driven by an ultrashort optical laser pulse. In the present time-resolved
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It was shown recently that the Fe magnetization reversal in the Fe/MnAs/GaAs(001) epitaxial system, attained by temperature control of the regular stripe pattern of the MnAs α- and β-phases, can also be driven by an ultrashort optical laser pulse. In the present time-resolved scattering experiment, we address the dynamics of the MnAs α-β self-organized stripe pattern induced by a 100 fs optical laser pulse, using as a probe the XUV radiation from the FERMI free-electron laser. We observe a loss in the diffraction intensity from the ordered α-β stripes that occurs at two characteristic timescales in the range of ~10−12 and ~10−10 s. We associate the first intensity drop with ultrafast electron-lattice energy exchange processes within the laser-MnAs interaction volume and the second with thermal diffusion towards the MnAs/GaAs interface. With the support of model calculations, the observed dynamics are interpreted in terms of the formation of a laterally homogeneous MnAs overlayer, the thickness of which evolves in time, correlating the MnAs microstructure dynamics with the Fe magnetization response. Full article
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Open AccessReview
LCoS SLM Study and Its Application in Wavelength Selective Switch
Photonics 2017, 4(2), 22; doi:10.3390/photonics4020022 -
Abstract
The Liquid-Crystal on Silicon (LCoS) spatial light modulator (SLM) has been used in wavelength selective switch (WSS) systems since the 1990s. However, most of the LCoS devices used for WSS systems have a pixel size larger than 6 µm. Although there are some
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The Liquid-Crystal on Silicon (LCoS) spatial light modulator (SLM) has been used in wavelength selective switch (WSS) systems since the 1990s. However, most of the LCoS devices used for WSS systems have a pixel size larger than 6 µm. Although there are some negative physical effects related to smaller pixel sizes, the benefits of more available ports, larger spatial bandwidth, improved resolution, and the compactness of the whole system make the latest generation LCoS microdisplays highly appealing as the core component in WSS systems. In this review work, three specifications of the WSS system including response time, crosstalk and insertion loss, and optimization directions are discussed. With respect to response time, the achievements of liquid crystal material are briefly surveyed. For the study of crosstalk and insertion loss, related physical effects and their relation to the crosstalk or insertion loss are discussed in detail, preliminary experimental study for these physical effects based on a small pixel LCoS SLM device (GAEA device, provided by Holoeye, 3.74 µm pixel pitch, 10 megapixel resolution, telecom) is first performed, which helps with predicting and optimizing the performance of a WSS system with a small pixel size SLM. In the last part, the trend of LCoS devices for future WSS modules is discussed based on the performance of the GAEA device. Tradeoffs between multiple factors are illustrated. In this work, we present the first study, to our knowledge, of the possible application of a small pixel sized SLM as a switching component in a WSS system. Full article
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Open AccessArticle
Echo-Enabled Harmonic Generation Studies for the FERMI Free-Electron Laser
Photonics 2017, 4(1), 19; doi:10.3390/photonics4010019 -
Abstract
Studying ultrafast processes on the nanoscale with element specificity requires a powerful femtosecond source of tunable extreme-ultraviolet (XUV) or x-ray radiation, such as a free-electron laser (FEL). Current efforts in FEL development are aimed at improving the wavelength tunability and multicolor operation, which
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Studying ultrafast processes on the nanoscale with element specificity requires a powerful femtosecond source of tunable extreme-ultraviolet (XUV) or x-ray radiation, such as a free-electron laser (FEL). Current efforts in FEL development are aimed at improving the wavelength tunability and multicolor operation, which will potentially lead to the development of new characterization techniques offering a higher chemical sensitivity and improved spatial resolution. One of the most promising approaches is the echo-enabled harmonic generation (EEHG), where two external seed lasers are used to precisely control the spectro-temporal properties of the FEL pulse. Here, we study the expected performance of EEHG at the FERMI FEL, using numerical simulations. We show that, by employing the existing FERMI layout with minor modifications, the EEHG scheme will be able to produce gigawatt peak-power pulses at wavelengths as short as 5 nm. We discuss some possible detrimental effects that may affect the performance of EEHG and compare the results to the existing double-stage FEL cascade, currently in operation at FERMI. Finally, our simulations show that, after substantial machine upgrades, EEHG has the potential to deliver coherent multicolor pulses reaching wavelengths as short as 3 nm, enabling x-ray pump–x-ray probe experiments in the water window. Full article
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Open AccessArticle
A Highly Sensitive Gold-Coated Photonic Crystal Fiber Biosensor Based on Surface Plasmon Resonance
Photonics 2017, 4(1), 18; doi:10.3390/photonics4010018 -
Abstract
In this paper, we numerically demonstrate a two-layer circular lattice photonic crystal fiber (PCF) biosensor based on the principle of surface plasmon resonance (SPR). The finite element method (FEM) with circular perfectly matched layer (PML) boundary condition is applied to evaluate the performance
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In this paper, we numerically demonstrate a two-layer circular lattice photonic crystal fiber (PCF) biosensor based on the principle of surface plasmon resonance (SPR). The finite element method (FEM) with circular perfectly matched layer (PML) boundary condition is applied to evaluate the performance of the proposed sensor. A thin gold layer is deposited outside the PCF structure, which acts as the plasmonic material for this design. The sensing layer (analyte) is implemented in the outermost layer, which permits easy and more practical fabrication process compared to analyte is put inside the air holes. It is demonstrated that, at gold layer thickness of 40 nm, the proposed sensor shows maximum sensitivity of 2200 nm/RIU using the wavelength interrogation method in the sensing range between 1.33–1.36. Besides, using an amplitude interrogation method, a maximum sensitivity of 266 RIU−1 and a maximum sensor resolution of 3.75 × 10−5 RIU are obtained. We also discuss how phase matching points are varied with different fiber parameters. Owing to high sensitivity and simple design, the proposed sensor may find important applications in biochemical and biological analyte detection. Full article
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Open AccessArticle
Experimental Evaluation of Impairments in Unrepeatered DP-16QAM Link with Distributed Raman Amplification
Photonics 2017, 4(1), 16; doi:10.3390/photonics4010016 -
Abstract
The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase noise on the signal due to relative intensity noise (RIN) transfer from the pump are compared for two pumping configurations:
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The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase noise on the signal due to relative intensity noise (RIN) transfer from the pump are compared for two pumping configurations: first-order backward pumping and bi-directional pumping. Experimental results indicate that with increased Raman backward pump power, though the optical signal-to-noise ratio (OSNR) is increased, so is the pump-induced amplitude and phase noise. The transmission performance is firstly improved by the enhanced OSNR at a low pump power until an optimum point is reached, and then the impairments due to pump-induced noise start to dominate. However, the introduction of a low pump power in the forward direction can further improve the system’s performance. Full article
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Open AccessArticle
XUV Transient Absorption Spectroscopy: Probing Laser-Perturbed Dipole Polarization in Single Atom, Macroscopic, and Molecular Regimes
Photonics 2017, 4(1), 17; doi:10.3390/photonics4010017 -
Abstract
We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is
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We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules. Full article
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