Simultaneous demodulation and dispersion compensation of WDM DPSK channels using optical ring resonator

We have introduced and comprehensively analyzed a novel scheme of simultaneous demodulation and dispersion compensation of wavelength division multiplexed (WDM) non-return-to zero (NRZ) differential phase shift keying (DPSK) optical link using an optical ring resonator (ORR) based filter. Using extensive numerical simulation we have demonstrated the transmission of 10.7 Gb/s WDM DPSK channels having 50 GHz and 100 GHz spacing over 400 km of unrepeatered reach at 20 dB optical-signal-to-noise-ratio (OSNR) to achieve a bit error rate (BER) of 10^? 3.     

Design of multi-layer mirrors for measuring optical phase modulation at 1300 nm

A theoretical design and experimental realization of multi-layer mirrors for Fabry–Perot interferometry and optical telecommunications is described in this work. The mirrors were designed and fabricated by 13 successive thin layers to achieve very high reflectance at optical wavelengths around 1300 nm. Thin layers are ZnS and MgF₂ presenting high and low refractive index, respectively. Layer thickness λ_o/2 at λ_o=656 nm. Experimental results include characterization of transmittance of mirrors around 1300 nm. Additionally, the mirrors were integrated in a Fabry–Perot interferometer to characterize optical sources emitting at 1300 nm. Finally to show a practical application, optical phase modulation was analyzed, using the fabricated mirrors through a scanning Fabry–Perot interferometer acting as high-resolution optical spectrum analyzer (OSA).     

Performance evaluation of 64 × 10 Gbps and 96 × 10 Gbps DWDM system with hybrid optical amplifier for different modulation formats

In this paper, we investigated the performance of multi terabits DWDM system consisting of hybrid optical amplifier RAMAN-EDFA for different data format such as non-return to zero (NRZ), return to zero (RZ) and differential phase shift keying (DPSK). We find that in 64 × 10 and 96 × 10 Gbps, RZ is more adversely affected by nonlinearities, where as NRZ and DPSK is more affected by dispersion. We further show that RZ provide good quality factor (13.88 dB and 15.93 dB for 64 and 96 channels), less eye closure (2.609 dB and 3.191 dB for 64 and 96 channels) and acceptable bit error rate (3.89 × 10⁸ and 1.24 × 10⁹ for 64 and 96 channels) at the respective distance as compare to other existing modulation format. We further investigated the maximum single span distance covered by using existing data formats.     

Remote sensing estimation of Chlorophyll a and suspended sediment concentration in turbid water based on spectral separation

Chlorophyll a and suspended sediment are important indicators of water quality, and remote sensing estimation of them is difficult due to the optical complexity of turbid water. The spectrum above water surface is influenced by phytoplankton, suspended sediment and colored dissolved organic material in water, thus spectral separation is important before estimating one specific component. Based on the field experiment of pond water and Taihu lake, China, this study calculated the Gaussian parameters of Chlorophyll a (Chla) and suspended sediment (SS) through spectral decomposition, and then these parameters were used to separate the mixed spectrum of water samples from pond water and Taihu lake. After spectral separation, the Chla estimation model based on the peak height at 650 nm has high accuracy (R² = 0.78, RMSE = 4.80 mg/m³), better than the band-ratio model; the SS estimation model based on the peak height at 811 nm (R² = 0.82, RMSE = 6.80 mg/L) performs better than the single-band model. Results in this study indicate that spectral separation based on Gaussian parameters is a good method for Chla and SS estimation in turbid lake water.     

Soliton propagation optimization and dynamic modulation in photonic crystal waveguide with polystyrene background

Bright optical soliton propagation properties near the left band edge of photonic crystal waveguide (PCW) are numerically investigated. Compared with the normal PCW with air background, by employing polystyrene as PCW background and adjusting the structure parameters simultaneously, the required soliton peak power sharply decreases from 8.63 × 10⁶ W/m to 9.98 × 10² W/m. The influence of optical loss on soliton propagation is numerically investigated. The dynamic modulation of the soliton propagation in PCW is realized, and a modulation range of 459 nm wavelength for the soliton transmission has been achieved. Simulation results show that the transmission wavelength, required soliton peak power and delay time decrease almost linearly as the external modulated voltage increases; the modulation sensitivities are 8.316 nm/V, 3.416 W/m/V and 16.6 ps/V, respectively.     

Optical and dielectric studies of solution grown glycinium maleate single crystal

Bulk single crystals of glycinium maleate have been grown from aqueous solution by slow evaporation method by optimizing the growth parameters within a period of 15 days. From X-ray diffraction analysis, the crystal was found to crystallize in monoclinic structure (space group C_2/c) with a = 17.866 Å, b = 5.684 Å, c = 17.408 Å and β = 112.65°. Presence of characteristic functional groups was confirmed in FTIR analysis. UV–Vis spectral analysis has revealed the absence of any high absorbance region between the wavelengths ranging from 300 to 900 nm. The optical band gap was calculated and found to be 3.91 eV. The activation energy for conduction at different frequencies was calculated and found to decrease from 0.54 eV to 0.41 eV as frequency increased from 100 Hz to 2 MHz. The dielectric behavior, conduction mechanism and the optical characterization of the glycinium maleate single crystals are being reported for the first time.     

Electrical and optical properties of the GaInAsSb-based heterojunctions for infrared photodiode and thermophotovoltaic cell application

The electrical end optical characteristics of a type II double heterojunction (DH) in the GaSb/GaInAsSb/GaAlAsSb system with staggered band alignment were studied. An analysis of the photodiodes performance through the investigation into electrical and optical characteristics was carried out. The dark current mechanisms in the heterostructures were investigated at several temperatures. The experimental results show that at the low temperature region, the tunneling mechanism of the current flow dominates in both forward and reverse biases. At high temperatures region and in the range of voltage from 0.1 V to 1 V, the reverse current was defined by generation of carriers in the depletion region. Have been estimated the temperature coefficient of the shift of the long-wavelength edge of the spectral sensitivity at half-maximum as Δλ/ΔT = 1.6 nm/K. Quantum efficiency of 0.6–0.7 for the investigated photodiodes was reached without any antireflection coating. For GaSb/GaInAsSb/GaAlAsSb TPV cells, the internal quantum efficiency of 90% was achieved at wavelengths between 1.2 and 1.6 μm.     

An optical OFDM multiplexer and 16 × 10 Gb/s OOFDM system using serrodyne optical frequency translation based on LiNbO3 phase modulator

This paper proposes an all-optical orthogonal frequency division multiplexing (OFDM) multiplexer based on sawtooth wave driving LiNbO₃ phase modulators by using serrodyne optical frequency translation. This multiplexer has high integration ability. The designing concept and implementation method have been discussed. A 16 × 10 Gb/s optical OFDM system is designed based on this multiplexer. Transmission characteristics, including tolerances of polarization mode dispersion (PMD) and nonlinear impairments have been studied numerically. Simulations show that the PMD tolerance is about 42 ps. The spectral efficiency reaches 1 bit/s/Hz with binary modulation format.     

Synthesis,growth and characterization of a semiorganic nonlinear optical single crystal: L-Valine cadmium bromide

A semiorganic nonlinear optical material L-valine cadmium bromide was synthesized and single crystal grown from an aqueous solution by the method of slow evaporation technique at room temperature. The grown crystal was characterized by Powder X-ray diffraction and FT IR studies. The LVCB crystallizes in monoclinic system with cell parameters a = 10.144(2) Å, b = 5.54(1) Å, c = 12.07(2) Å, β = 109.115(2)° with space group P₂₁. Thermal behavior and stability of crystal were studied using thermogravimetric analyses (TGA) and differential thermal analysis (DTA) techniques. The suitability of this material for NLO application was studied by optical absorption studies and second harmonic generation (SHG) efficiency measurement by Kurtz–Perry powder method.     

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5 THz and 1.1 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1.325 THz to 1.38 THz. These results will be applied to THz modulation and tunable filtering.     

Synthesis,crystal growth and characterization of a semiorganic material: Calcium dibromide bis(glycine) tetrahydrate

Single crystals of semiorganic material calcium dibromide bis(glycine) tetrahydrate were grown from aqueous solution. The crystal belongs to monoclinic system, with a = 13.261(5) Å, b = 6.792(2) Å, c = 15.671(9) Å and β = 91.68(4)°. The presence of the elements in the title compound was confirmed by energy dispersive X-ray analysis. The solubility and metastable zone width were found. The grown crystals were tested by powder XRD, FTIR, Thermo Gravimetric and Differential Thermal Analysis, UV–vis–NIR analysis, dielectrical and mechanical studies. The transmittance of calcium dibromide bis(glycine) tetrahydrate crystal has been used to calculate the refractive index n, the extinction coefficient K and both the real ɛ_r and imaginary ɛ_i components of the dielectric constant as functions of wavelength. The optical band gap of calcium dibromide bis(glycine) tetrahydrate is 3.23 eV.     

Structure,optical spectroscopy and dispersion parameters of ZnGa2Se4 thin films at different annealing temperatures

Thin films of ZnGa₂Se₄ were deposited by thermal evaporation method of pre-synthesized ingot material onto highly cleaned microscopic glass substrates. The chemical composition of the investigated compound thin film form was determined by means of energy-dispersive X-ray spectroscopy. X-ray diffraction XRD analysis revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at T_a = 623 and 673 K have amorphous phase, while that annealed at T_a = 700 K is polycrystalline with a single phase of a defective chalcopyrite structure similar to that of the synthesized material. The unit-cell lattice parameters were determined and compared with the reported data. Also, the crystallite size L, the dislocation density δ and the main internal strain ε were calculated. Analyses of the AFM images confirm the nanostructure of the prepared annealed film at 700 K. The refractive index n and the film thickness d were determined from optical transmittance data using Swanepoel's method. It was found that the refractive index dispersion data obeys the single oscillator model from which the dispersion parameters were determined. The electric susceptibility of free carriers and the carrier concentration to the effective mass ratio were determined according to the model of Spitzer and Fan. The analysis of the optical absorption revealed both the indirect and direct energy gaps. The indirect optical gaps are presented in the amorphous films (as-deposited, annealed at 623 and 673 K), while the direct energy gap characterized the polycrystalline film at 700 K. Graphical representations of ε₁, ε₂, tan δ, ? Im[1/ε*] and ? Im[(1/ε* + 1)] are also presented. ZnGa₂Se₄ is a good candidate for optoelectronic and solar cell devices.     

Compact WDM demultiplexer for seven channels in photonic crystal

We demonstrate a new device concept for wavelength division demultiplexing based on planar photonic crystal waveguides. The filtering of wavelength channels is realized by shifting the cutoff frequency of the fundamental photonic bandgap mode in consecutive sections of the waveguide. The shift is realized by modifying the size of the border holes.The proposed demultiplexer has an area equal to (16.5 μm × 6.5 μm) and thus it is verified that this structure is very small and can be integrated easily into optical integrated circuits with nanophotonic technologies. The output wavelengths of designed structure can be tuned for communication applications, around 1550 nm. The wavelengths of demultiplexer channels are λ₁ = 1.590 μm, λ₂ = 1.566 μm, λ₃ = 1.525 μm, λ₄ = 1.510 μm, λ₅ = 1.484 μm, λ₆ = 1.450 μm, λ₇ = 1.400 μm respectively. Designs offering improvement of number of the separate wavelengths (seven), miniaturization of the structure (107.25 μm²) is our aim in this work.In our structure, we consider that the 2D triangular lattice photonic crystal is composed of air holes surrounded by dielectric. Its parameters are: radius of holes (r = 0.130 μm), lattice constant (a = 0.380 μm), and index of membrane (n = 3.181:InP). The numerical model used to simulate the structure of the demultiplexer is based on the finite difference time domain (FDTD).     

Excess currents larger than the point contact limit in normal-metal/superconducting junctions

In a point contact NS junction, perfect Andreev reflection occurs over a range of voltages equal to the superconducting energy gap, producing an excess current of I_exc =  (4 / 3)(2 eΔ / h). If the superconductor has a finite width, rather than the infinite width of the point contact, one cannot neglect superfluid flow inside the superconducting contact. The energy range available for perfect Andreev reflections then becomes larger than the superconducting gap, since superfluid flow alters the dispersion relation inside the finite width superconductor. We find a maximum excess current of approximately (7 / 3)(2 eΔ / h) when the width of the superconductor is approximately 7 / 3 times the width of the normal metal.     

Effect of annealing on structure and optical properties of Ga5Se95 films

To investigate the effect of annealing on the structural and optical properties of a binary compound Ga₅Se₉₅, thin films of Ga₅Se₉₅ have been deposited on quartz substrates at room temperature by the thermal evaporation technique. X-ray diffraction patterns showed that the films before and after annealing at 573 K have polycrystalline texture and exhibit tetragonal structure. The dependences of the optical constants, the refractive index n and extinction coefficient k were studied in the spectral range of 200 nm to 2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple–DiDomenco single-oscillator model. Analysis of absorption index data reveals that as-deposited Ga₅Se₉₅ films has indirect transitions with optical energy gap of 1.685 eV.     

Ion conduction in proton- and related defect (super) ionic conductors: Mechanical,electronic and structure parameters

We find the relationships among optical dielectric constant ε_∞, activation energy E_ac, averaged atomic mass per a formula unit, ∑_jm_j / N, volume V and transition temperature T_c for various type ion conductors with forms of E_ac = α / (ε_∞ ? β), E_ac = A₀ + δ / [(∑_jm_j / N) ? d], E_ac = A_v / V^2/3, and ln(T_c) = g ? hln(∑_jm_j), where α, β, δ, A₀, d, A_v, g and h are constants depending on the kinds of conduction elements. We derive those proportional forms from a simple equation of motion under the assumption of ion hopping assisted by enhanced vibration displacement of host lattice. The enhancement is induced from the large fourth-order term of the host lattice potential originating from the electronic shielding effect of Coulomb force, heavy atomic mass of constituent ion, and volume expansion under the long-range periodicity of crystal structure. This mechanism is ascertained from characteristic phenomena of various kinds of conduction elements. For impurity-type H⁺-ion or defect conductor, the proportional form is shifted from that of superionic conductor because of weakened effect of host lattice vibration mode on H⁺-ion or O-ion defect. Photo-induced spectra of mobile ion in AgCl are understood, and a small quantum effect of H⁺ -ion is suggested.     

Titanium oxide thin layers deposed by dip-coating method: Their optical and structural properties

TiO₂ thin films were prepared by sol-gel method. The structural investigations performed by means of X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) showed the shape structure at T = 600 °C. The optical constants of the deposited film were obtained from the analysis of the experimentally recorded transmittance spectral data in the wavelength of 200–3000 nm range. The values of some important parameters of the studied films are determined, such as refractive index n and thickness d. In this work, using the transmission spectra, we have calculated the dielectric constant (ε_∞) for four layered TiO₂ films; a simple relation is suggested to estimate the third-order optical nonlinear susceptibility χ⁽³⁾. It has been found that the dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high-frequency dielectric constant were determined. The estimations of the corresponding band gap E_g, χ⁽³⁾ and ε_∞ are 2.57 eV, 0.021 · 10⁻¹⁰ esu and 5.20, respectively.     

Optical millimeter-wave signal generation by frequency quadrupling using one dual-drive Mach–Zehnder modulator to overcome chromatic dispersion

We propose a novel approach to generate quadrupling-frequency optical millimeter-wave using a dual-drive Mach–Zehnder modulator (MZM) in radio-over-fiber system. By properly adjusting the phase difference in the two modulation arms of MZM, the direct current (DC) bias, the modulation index and the gain of base-band signal, the quadrupling-frequency optical millimeter-wave with signal only carried by one second-order sideband is generated. As the signal is transmitted along the fiber, there is no time shift of the codes caused by chromatic dispersion. Theoretical analysis and simulation results show that the eye diagram keeps open and clear even when the quadrupling-frequency optical millimeter-wave are transmitted over 110 km and the power penalty is about 0.45 dB after fiber transmission distance of 60 km. Furthermore, due to another second-order sideband carrying no signals, a full duplex radio-over-fiber link based on wavelength reuse is also built to simplify the base station. The bidirectional 2.5 Gbit/s data is successfully transmitted over 40 km standard single mode fiber with less than 0.6 dB power penalty in the simulation.     

Optical parameters of In–Se and In–Se–Te thin amorphous films prepared by pulsed laser deposition

The thin films of materials based on In–Se are under study for their applicability in photovoltaic devices, solid-state batteries and phase-change memories.The amorphous thin films of In₂Se_3−xTe_x (x=0–1.5) and InSe were prepared by pulsed laser deposition method (PLD) using a KrF excimer laser beam (λ=248 nm, 0.5 J cm⁻²) from polycrystalline bulk targets. The compositions of films verified by energy-dispersive X-ray analysis (EDX) were close to the compositions of targets. The surfaces of PLD films containing small amount of droplets were viewed by optical and scanning electron microscopy (SEM).The optical properties (transmittance and reflectance spectra, spectral dependence of index of refraction, optical gap, single-oscillator energy, dispersion energy, dielectric constant) of the films were determined.The values of index of refraction increased with increasing substitution of Te for Se in In₂Se₃ films, the values of the optical gap decreased with increasing substitution of Te for Se in In₂Se₃ films.     

Spectral sensitivity dependencies of backside illuminated planar MCT photodiodes

Spectral sensitivity dependencies of Hg_1−xCd_xTe (0.20 ⩽ x ⩽ 0.25) backside illuminated planar photodiodes were investigated at T = 80 K to study their longwavelength edge features. It was shown that the longwavelength part of these spectral dependencies is mainly formed by the exponential wavelength dependence of the optical transitions. Empirical dependencies of cut-off wavelengths at different values (λ_max, λ_0.5, λ_0.9) were obtained. The influence of the epitaxial layer thickness on the maximum sensitivity position was also studied.