Time-spreading wavelength-hopping 2D-OCDMA system with optical hard limiter

The performance of time-spreading wavelength-hopping two-dimensional optical division multiple access (2D-OCDMA) system with optical hard limiter was analyzed firstly. Prime/OOC codes are employed as address codes. By dividing other codes into three groups according to the sum of shift cross-correlations and analyzing the probability distribution of the interference patterns, the bit error rate (BER) of the system was obtained.     

Reduction of nonlinear impairments in fiber transmission system using fiber and/or transmitter diversity

A multi-fiber architecture is proposed to mitigate the nonlinear impairments in fiber-optic systems. The power launched to each fiber is reduced in multi-fiber architecture as compared to the case of single fiber leading to reduction in nonlinear impairments. The optical pulses propagating in the fibers undergo different amount of phase shifts and timing delays. Optical and electrical equalization techniques to compensate for these channel effects are discussed. Our numerical simulation results show that for unrepeatered systems, the performance (Q factor) is improved by 6.2 dB using 8-fiber configuration as compared to single-fiber system. In addition, for multi-span system, the transmission reach at a bit error rate (BER) of 2.1 × 10^? 3 is quadrupled in 8-fiber configuration.     

Radio over fiber access network architecture employing RSOA with downstream OQPSK and upstream re-modulated OOK data

We have demonstrated a bidirectional reflective semiconductor optical amplifier (RSOA) based on wavelength division multiplexing ROF network utilizing an offset quadrature differential phase shift keying (OQPSK) signal for down-link and an on-off keying (OOK) signal re-modulated for up-link. A 50 km range colorless WDM-ROF without dispersion compensation was demonstrated for both 1 Gbit/s downstream and upstream signals. The BER performance of our scheme shows that our scheme is a practical solution to meet the data rate and cost-efficient of the optical links simultaneously in tomorrow's ROF access networks.     

Performance analysis of optical network based on optical add drop multiplexers with different MZI techniques

This paper presents an investigation on the performance of an optical network in terms of crosstalk based on optical add drop multiplexers with Mach–Zehnder interferometer (MZI), MZI-semiconductor optical amplifier (SOA) and MZI-fiber Braggs gratings (FBG) techniques obtained at 8 × 10 Gbps with 0.1 nm channel spacing wavelength division multiplexing (WDM) transmission with optical add drop multiplexer (OADM) placed at the 20 km point of a 40 km link. It is found that the signal can be transmitted with least BER and better Q-factor with MZI-FBG based OADM and the worst case is found with the MZI-SOA based OADM.     

Simulation of 16 × 10 Gb/s WDM system based on optical amplifiers at different transmission distance and dispersion

In this paper, the 16 channel WDM systems at 10 Gb/s have been investigated for the various optical amplifiers and hybrid optical amplifiers and the performance has been compared on the basis of transmission distance and dispersion. The amplifiers EDFA and SOA have been investigated independently and further compared with hybrid optical amplifiers like RAMAN-EDFA and RAMAN-SOA. It is observed that hybrid optical amplifier RAMAN-EDFA provides the highest output power (12.017 and 12.088 dBm) and least bit error rate (10^?40 and 9.08 × 10^?18) at 100 km for dispersion 2 ps/nm/km and 4 ps/nm/km respectively.     

Spectral efficient hybrid wireless optical broadband access network (WOBAN) based on transmission of wireless MIMO OFDM signals over WDM PON

In this paper, a spectral efficient hybrid wireless optical broadband access network (WOBAN) is proposed and demonstrated based on the transmission of wireless multi-input multi-output orthogonal frequency division multiplexing (MIMO OFDM) signals over wavelength division multiplexing passive optical network (WDM PON). By using radio over fiber (ROF) techniques, the optical fiber is well adapted to propagate multiple wireless services having different carrier frequencies. It is a known fact that multiple wireless signals having the same carrier frequency cannot propagate over a single optical fiber at the same time, such as MIMO signals feeding multiple antennas in fiber wireless (FiWi) system. A novel optical single-sideband frequency translation technique is designed and simulated to solve this problem. This technique allows four pairs of wireless MIMO OFDM signals with the same carrier frequency for each pair to be transmitted over a single optical fiber by using one optical source per wavelength. The crosstalk between the different MIMO channels with the same frequency is eliminated, since each channel is upconverted on specified wavelength with enough channel spacing between them. Also the maximum crosstalk level between the different MIMO channels with different frequencies is very low around ?76 dB. The physical layer performance of the proposed WOBAN is analyzed in terms of the bit error rate (BER), error vector magnitude (EVM), and signal-to-noise ratio (SNR). The proposed WOBAN achieves 7.68 Gb/s data rate for 20 km for the optical back-end and 240 Mb/s for the outdoor wireless front-end.     

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.     

Advanced multifunctional optical switch for multimode optical fiber networks

In this work, an advanced multifunctional optical switch based on multimode fibers is proposed. It can work as a 3 × 1 optical multiplexer/combiner, a 2 × 2 optical switch, a variable optical attenuator and a variable optical power splitter. All these functionalities can be developed in the same device without any hardware modification, only by using the proper ports and control electronics.The proposed switch has been developed for being used in the visible and near infrared wavelength range: 450–650 nm for optical fiber automobile applications, 650–850 nm for home broadband applications; and 850–1300 nm for multimode fiber access networks. Up to three different types of twisted nematic liquid crystal cells have been designed and fabricated for fulfilling these different wavelength ranges as part of the proposed device.The multifunctional switch has been implemented and experimentally tested. Crosstalk usually better than ? 15 dB at 532 nm, 660 nm and 850 nm, in any state has been measured. Switching is achieved at voltage levels of 4 Vrms. Fiber to fiber insertion losses when operating as a 2 × 2 optical switch, range from 10 to 15 dB within 200 nm wavelength range; with a non-optimized optics for collimation and coupling.     

Adaptive upstream rate adjustment by RSOA-ONU depending on different injection power of seeding light in standard-reach and long-reach PON systems

The wavelength division multiplexing-time division multiplexing (WDM-TDM) passive optical network (PON) using reflective semiconductor optical amplifier (RSOA)-based colorless optical networking units (ONUs) is considered as a promising candidate for the realization of fiber-to-the-home (FTTH). And this architecture is actively considered by Industrial Technology Research Institute (ITRI) for the realization of FTTH in Taiwan. However, different fiber distances and optical components would introduce different power budgets to different ONUs in the PON. Besides, due to the aging of optical transmitter (Tx), the power decay of the distributed optical carrier from the central office (CO) could also reduce the injection power into each ONU. The situation will be more severe in the long-reach (LR) PON, which is considered as an option for the future access. In this work, we investigate a WDM-TDM PON using RSOA-based ONU for upstream data rate adjustment depending on different continuous wave (CW) injection powers. Both standard-reach (25 km) and LR (100 km) transmissions are evaluated. Moreover, a detail analysis of the upstream signal bit-error rate (BER) performances at different injection powers, upstream data rates, PON split-ratios under stand-reach and long-reach is presented.     

A long-reach WDM passive optical network enabling broadcasting service with centralized light source

We propose a long-reach wavelength-division-multiplexed (WDM) passive optical network (PON) to provide conventional point-to-point (P2P) data and downstream broadcasting service simultaneously by superimposing, for each WDM channel, the differential-phase-shift-keying (DPSK) broadcasting signal with the subcarrier multiplexing (SCM) modulated downstream P2P signal, at the optical line terminal (OLT). In the optical network units (ONUs), by re-modulating part of the downstream signal with a reflective semiconductor optical amplifier (RSOA), we realize color-less ONUs for upstream data transmission. The proposed scheme is numerically verified with a 5 Gb/s downstream P2P signal and broadcasting services, as well as 2.5 Gb/s upstream data through a 60 km bidirectional fiber link. In particular, the influence of the downstream lightwave's optical carrier–subcarrier ratio (OCSR) on the system performance is also investigated.     

A novel 2D-OCDMA system with dynamic optimum decision threshold by estimating the number of active users

A novel two dimensional optical code division multiple access (2D-OCDMA) system with dynamic optimum decision threshold by estimating the number of active users is proposed. The bit-error rates (BER) of the system is analyzed. By using one wavelength as the estimator of the number of active users, which is not assigned to any address code, the number of active users can be obtained. The dynamic optimum decision threshold can be obtained according to the multiple user interference (MUI), which can be obtained by cross-correlation characteristics between address codes and the number of active users.     

Defects related emission and nanosecond optical power limiting in CuS quantum dots

We report optical and nonlinear optical properties of CuS quantum dots and nanoparticles prepared through a nontoxic, green, one-pot synthesis method. The presence of surface states and defects in the quantum dots are evident from the luminescent behavior and enhanced nonlinear optical properties measured using the open aperture Z-scan, employing 5 ns laser pulses at 532 nm. The quantum dots exhibit large effective third order nonlinear optical coefficients with a relatively lower optical limiting threshold of 2.3 J cm⁻², and the optical nonlinearity arises largely from absorption saturation and excited state absorption. Results suggest that these materials are potential candidates for designing efficient optical limiters with applications in laser safety devices.     

Stability of widely tuneable,continuous wave external-cavity quantum cascade laser for absorption spectroscopy

The performance of widely tuneable, continuous wave (cw) external-cavity quantum cascade laser (EC-QCL) has been evaluated for direct absorption spectroscopy measurements of nitric oxide (NO) in the wavenumber range 1872–1958 cm^?1 and with a 13.5 cm long optical cell. In order to reduce the absorption measurement errors due to the large variations of laser intensity, normalisation with a reference channel was used. Wavelength stability within the scans was analysed using the Allan plot technique for the reduced wavenumber range of 1892.4–1914.5 cm^?1. The Allan variances of the NO absorption peak centres and areas were observed to increase with successive scan averaging for all absorption peaks across the wavelength scan, thus revealing short- and long-term drifts of the cw EC-QCL wavelength between successive scans. As an example application, the cw EC-QCL was used for NO measurements in the exhaust of an atmospheric pressure packed-bed plasma reactor applied to the decomposition of dichloromethane in waste gas streams. Etalon noise was reduced by subtracting a reference spectrum recorded when the plasma was off. The NO limit of detection (SNR = 1) was estimated to be ～2 ppm at atmospheric pressure in a 20.5 cm long optical cell with a double pass and a single 7 s scan over 1892.4–1914.5 cm^?1.     

Nuclear polarization by optical pumping in InP:Fe above liquid nitrogen temperature

Hyperpolarized nuclear spins are observed in optically pumped iron-doped InP from 70 K to 140 K. ³¹P NMR was carried out at 9.28 T (159.8 MHz) during optical excitation with circularly polarized light, using a laser diode (λ∼830 nm) as a source. The enhancement of the nuclear spin polarization by optical pumping at 70 K is estimated to be about 34 for those nuclei in the region of the sample absorbing light. This enhancement decreases with increasing temperature. As the direction of the enhanced nuclear spin polarization is found parallel or antiparallel to the travelling direction of the σ⁺ or σ⁻, the contact hyperfine interaction is dominant compared to the dipolar hyperfine interaction.     

Theoretical ultraslow bright and dark optical solitons in cascade-type GaAs/AlGaAs multiple quantum wells

We show the formation of ultraslow bright and dark optical solitons in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells (MQWs) structure based on the biexciton coherence in the transient optical response, and study analytically and numerically with Maxwell–Schrödinger equations. The calculated velocity of bright and dark optical solitons are V_g = 2.7 × 10⁴ ms^? 1 and V_g = 8.91 × 10⁴ ms^? 1, respectively. Such investigation of ultraslow optical solitons in MQWs may provide practical applications such as high-fidelity optical delay lines and optical buffers in semiconductor quantum wells structure, because of its flexible design.     

High-energy ion induced physical and surface modifications in antimony sulphide thin films

Sb₂S₃ thin films prepared by electrodeposition on indium tin oxide coated glass substrate were irradiated with 150 MeV Ni¹¹⁺ ions for various fluence in the range of 10¹¹–10¹³ ions/cm². The modifications in the structure, surface morphology and optical properties have been studied as a function of ion fluence. X-ray diffraction (XRD) analysis indicates a shift in the (2 4 0) peak position towards lower diffraction angle and a decrease in grain size with increase in ion fluence. Presence of microcracks due to irradiation induced grain splitting effect has been observed from the SEM micrograph at higher ion fluence. The optical absorbance spectrum revealed a shift in the fundamental absorption edge and the band gap energy increased from a value of 1.63 eV for as-deposited films to 1.80 eV for the films irradiated with 10¹³ ions/cm².     

Growth and characterization of pure and doped bis thiourea zinc acetate: Semiorganic nonlinear optical single crystals

Metal–organic coordination complex single crystals bis thiourea zinc acetate (BTZA) and Cd²⁺ doped BTZA have been synthesized and grown successfully by slow-cooling technique from their aqueous solutions. Single crystals of pure and Cd²⁺ doped BTZA with dimensions of 35 × 4 × 2 mm³ and 10 × 5 × 6 mm³, respectively were obtained with well defined morphology. The as grown single crystals are characterized by single crystal XRD studies and melting point measurements which reveal the incorporation of metallic dopants has not changed the structure of the parent crystal. The powder X-ray diffractogram of the grown crystals has been recorded and the various planes of reflection identified shows shift in the peak positions. The metal coordination with thiourea through sulphur in pure and Cd²⁺ doped BTZA were ascertained by FTIR studies and optical absorption study to identify the UV cut-off range. The presence of metals in pure and Cd²⁺ doped BTZA crystal lattice were confirmed by atomic absorption spectroscopy (AAS). The thermal decomposition of pure and Cd²⁺ doped BTZA crystals were investigated by thermo gravimetric analyses (TGA) and differential thermal analysis (DTA) indicate that doped crystals are more stable than pure crystals. The dielectric response of the crystals were studied in the frequency range 100 Hz–5 MHz at different temperatures and the results are discussed. Second harmonic generation (SHG) measurement confirms that the pure and Cd²⁺ doped BTZA have nonlinear optical (NLO) property. Laser damage threshold value of 12.44 MW/cm² has been determined using Q-switched Nd:YAG laser operating at 1064 nm and with 8 ns pulses in single shot mode for pure BTZA single crystal is reported for the first time.     

Intermediate temperature proton conductors based on phosphonic acid functionalized oligosiloxanes

Fully immobilized phosphonic acid based proton conductors, where phosphonic acid groups are tethered to cyclic siloxanes via flexible alkane spacers, are synthesized. Unlike conventional hydrated ionomers containing sulfonic acid groups, which are commonly used as separator material in PEM fuel cells, the proton conductivity of these materials occurs within a dynamical hydrogen bond network formed by the protogenic groups (phosphonic acid), which are present at very high concentrations. Conductivities of up to 2 · 10^− 3 S cm^− 1 are obtained at T ≈ 130 °C and RH ≈ 37%. This is only slightly higher than the conductivity of similar imidazole based systems although neat phosphonic acid has a much higher proton conductivity compared to neat imidazole. The proton conductivity of phosphonic acid is more sensitive towards immobilization at cyclic siloxanes and the corresponding restrictions for hydrogen bond formation (aggregation).     

A proposed organic Schottky barrier photodetector for application in the visible region

The fabrication and characterization of an organic photodetector (OPD) in the form of ITO coated glass/polycarbazole (PCz)/Al Schottky contact is reported. The device has been fabricated in our laboratory for the first time using the polymer synthesized by us. The device has been subsequently characterized in respect of electrical and optical properties in order to explore its potential for possible use as a detector in the visible region at 650 nm. It is observed that the detector exhibits a reasonably high value of peak detectivity (∼6 × 10⁶ cm Hz^1/2 W⁻¹) near zero bias voltage at V = 0.2 V.     

Optical properties of amorphous Si/SiO2superlattice

Amorphous Si/SiO₂(a-Si/SiO₂) superlattices have been fabricated by the magnetron sputtering technique. The superlattice with an Si layer thickness of 1.8 nm has been characterized by transmission electron microscopy (TEM). The result indicates that most of the regions in the Si layer consist of amorphous phase, while regular structure appears in some local regions. This is in agreement with the Raman scattering spectroscopy. The optical absorption spectrum and photoluminescence (PL) spectrum have been measured. Moreover, the third-order optical nonlinearity χ⁽³⁾of this superlattice has been measured. To our knowledge, this is the first investigation of the nonlinear absorption and refractive index of an a-Si/SiO₂superlattice using the Z -scan technique. The real and imaginary parts of χ⁽³⁾have been found to be 1.316  ×  10 ^− 7eus and  − 5.596  ×  10 ^− 7eus, respectively, which are about two orders of magnitude greater than those of porous silicon. The results may be attractive for potential application in electro-optics devices.