Design and characterization of single mode circular photonic crystal fiber for broadband dispersion compensation

In this paper, we present a single mode circular photonic crystal fiber (C-PCF) for broadband dispersion compensation covering 1400 to 1610 nm wavelength band over the telecommunication windows. Investigations of guiding properties are carried out using finite element method (FEM) with circular perfectly matched layer boundary condition. Numerical study reveals that a negative dispersion coefficient of about −386.57 to −971.44 ps/(nm km) is possible to obtain over the wavelength ranging from 1400 to 1610 nm with a relative dispersion slope (RDS) of about 0.0036 nm⁻¹ at 1550 nm wavelength. In addition, the single mode behaviour of C-PCF is demonstrated by employing V parameter. According to simulation, it is found that the proposed C-PCF acts as a single mode fiber within 1340 to 1640 nm wavelength. Moreover, effective dispersion, relative dispersion slope, birefringence and confinement loss are also presented and discussed.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

LD end pumped, actively mode locked and passively Q-switched Nd:YAP laser at 1341 nm

An end pumped Nd:YAP laser at 1341 nm is actively mode locked and passively Q-switched. Pumping was done with a pulsed high power laser diode with maximum power 425 W. V³⁺:YAG with 61% initial transmission served as saturable absorber, and an acousto-optic modulator is used for active mode locking. The output pulse train with 69 ns duration has a total energy of 3.2 mJ with ±4% shot-to-shot fluctuation. The peak output energy of a single mode locked pulse is 0.25 mJ. The pulse duration of a single mode locked pulse is less than 800 ps. The output laser beam is nearly diffraction limited with 1.6 mm diameter, and beam propagation factor M² about 1.3.     

Design of single polarization single mode dispersion compensating photonic crystal fiber

In this paper, we present a photonic crystal fiber based on hexagonal structure for improved negative dispersion as well as high birefringence in the telecom wavelength bands. It is demonstrated that it is possible to obtain negative dispersion coefficient of −712 ps/(nm km) and relative dispersion slope (RDS) perfectly match to that of single mode fiber (SMF) of about 0.0036 nm⁻¹ at the operating wavelength 1550 nm. The proposed fiber exhibits high birefringence of the order 2.11 × 10⁻² with nonlinear coefficient about 57.57 W⁻¹ km⁻¹ at 1550 nm. Moreover, it is confirmed that the designed fiber successfully operates as a single mode in the entire band of interest.     

Demonstration of PMD compensation by using a DSP-Based OPMDC prototype in a 43-Gb/s RZ-DQPSK, 1200 km DWDM transmission

This paper reports the DSP-based prototype compensator we have made to compensate the polarization mode dispersion (PMD) in fibers. It was tested in one channel of a 40 × 43-Gb/s DWDM DQPSK system, which is the commercial product line, either in the back-to-back case by using a PMD emulator, or in a 1200-km transmission testbed. The prototype showed a good performance under the tests of fast SOP and PSP rotation, DGD jump variation, and moderate knock on the testbed in a period as long as 12 h.     

Broadband Er-Yb co-doped superfluorescent fiber source

In this paper, extensive experimental results on broad-band double cladding Er³⁺-Yb³⁺ co-doped superfluorescent fiber sources (SFSs), characterizing their output power, mean wavelength, and bandwidth (BW) stability with variations of pump power, pump wavelength, and fiber temperature, have been reported. For a 55-cm fiber, SFS power from 3.7755 (maximum BW condition of more than 80 nm) to 9.1837 mW (maximum power condition, BW is about 34 nm) has been achieved. The SFS mean wavelength dependence on pump wavelength is highly pump temperature sensitive, and can be reduced to zero in a chosen pump temperature field. The intrinsic variation of the SFS mean wavelength λ_m with fiber temperature is also measured, and a linear variation from 15 to 45 °C with a slop of −0.053 nm/°C for L_f = 100 cm and −0.04 nm/°C for L_f = 55 cm is found.     

Analysis and Compensation of Polarization Mode Dispersion in Single Channel,WDM and 32-channel DWDM Fiber Optic System

One of the most serious impairments which limit the data rate in long distance and high speed transmission systems is Polarization Mode Dispersion (PMD). PMD is negligible when data rate is low (i.e. in Mb/s or few Gb/s) but it will affect the high data rate transmission systems (10^s of Gb/s, Tb/s etc.), as the pulse broadening severely distorts the signal during transmission. Thus it is necessary to compensate the PMD in both single and multichannel fiber optic transmission system due to increase in the traffic demand. This paper deals with a Deterministic Differential Group Delay (DDGD) method to compensate the PMD in single channel, by delaying the fast polarization component and wavelength independent Polarization Maintaining Fiber (PMF) method for multichannel PMD compensation. The DDGD method efficiently compensates the PMD upto 45 ps in single channel 40 Gb/s transmission systems. The State of Polarization (SOP) before and after the PMD and after compensation is analyzed by means of Poincare Sphere. By using PM Fiber method, simultaneous and effective compensation of PMD in multichannel system is achieved. Here, the simulation has been carried out for 4-channel (40 Gb/s), 8-channel (80 Gb/s), 16-channel (160 Gb/s) WDM systems and 32-channel (320 Gb/s) DWDM fiber optic system with each channel having the data rate of 10 Gb/s and the results of PMD compensation for all the channels are analyzed. It is seen that the PMD compensation is achieved upto 90 ps 87 ps, 84 ps and 80 ps in 4-channel, 8-channel, 16-channel WDM systems and 32-channel DWDM systems respectively. As very high data rate of 100 Gb/s and above are in practice now-a-days, compensation of PMD is enhanced to 1.6 Tb/s (16 × 100 Gb/s) data rate for 16-channel by PMF method and 74 ps of broadening is compensated effectively.     

Optimizing 10 Gbps optical communication system with duty cycle selection of return to zero pulse

We have investigated the return-to-zero (RZ) pulse duty cycle for single-channel Standard Single mode fiber (SSMF), Non Zero Dispersion shifted fibers (normal NZDSF and anomalous NZDSF fiber) for 10 Gbps optical fiber communication system. We give a comprehensive look on the behavior of variable duty cycle optical RZ pulse indicating that lowest bit error rate for duty cycle 0.8 among the duty cycle values 0.2, 0.4, 0.6 and 0.8 investigated for the case of SSMF. The single repeaterless mode fiber length is increased from existing 55 km at duty cycle 0.2 to fiber length 85 km by keeping duty cycle at 0.8. The result is also emphasized through the 10 dB Q value improvement and corresponding improvement in average eye opening diagram. The normal NZDSF show similar improvement but at greater fiber length, it offers BER 10⁻⁹ at length 110 km with duty cycle 0.2. NZDSF operating length can further be increased to length 160 km by keeping duty cycle 0.8. The corresponding 8 dB Q value improvement and Average eye opening improvement also supports the result through its graphical variation. Thirdly Anomalous NZDSF for same optical communication system showed that 0.2 duty cycle value give operational length of 130 km which could be extended to 160 km if 0.8 duty cycle is kept. The corresponding 8 dB Q value improvement, average eye-opening improvement endorsed the fact in the graphs.     

Effects of polarization mode dispersion on cross-phase modulation in a WDM optical transmission system

The cross-phase modulation (XPM) changes the state-of-polarization (SOP) of various channels and leads to amplitude modulation of the propagating waves in a WDM system. Due to the presence of polarization mode dispersion (PMD), the angles between the SOP of the channels change randomly and cause the modulation amplitude fluctuation random in the perturbed channel. We analytically derive the dynamic equation of the perturbed channel, determine the combined probability density function of the random angle between the SOP of pumps and probe channel and evaluate the effects of PMD on XPM for a 4-channel IM-DD WDM system in terms of BER at bit rate of 10 Gb/s per channel. We also simulate the impact PMD on XPM for a 4-channel WDM system in terms of eye diagram and found that eye opening penalty is 1.85 dB higher when the fiber PMD coefficient increases from 0.5 to 1.5 ps/√km.     

Very large mode area optical fiber with complex ring cores

A novel kind of single-mode large-mode-area optical fiber is presented in this paper. The proposed fiber core is composed of high-index central rod and the surrounding multilayer rings. The mode characteristics are discussed considering the fiber structure parameters. The calculation results show that the proposed fiber possesses extreme large mode area of 2975 μm² with single mode operation at the wavelength of 1.08 μm. Even larger mode area of the complex ring core fiber with single mode output can be achieved by coiling the fiber, due to the significant difference of bending loss between the fundamental mode and the higher-order transverse modes. Such fibers are expected to find applications in the field of fiber lasers and amplifiers.     

High speed all-silicon optical modulator

Electrorefractive effect is experimentally demonstrated in an all-silicon optical structure. A highly doped Si P⁺ layer is embedded in the intrinsic region of a PIN diode integrated in a SOI waveguide. Holes are confined at equilibrium around the P⁺ layer. By applying a reverse bias to the diode, electrical field sweeps the carriers out of the active region. Free carrier concentration variations are responsible for local refractive index variations leading to an effective index variation of the waveguide optical mode and to an optical absorption variation. As a figure of merit, the product V_πL_π, determined from the measured effective index variation, is equal to 3.1 V cm. Furthermore, the device performances have theoretically been investigated. Estimations show that V_πL_π as small as 1 V cm are feasible using optimized structures. Response times lower than 2 ps are predicted, which gives the possibility to achieve very high-speed modulation. Furthermore, a temperature increases from 300 to 400 K does not change the index variation amplitude, and despite the carrier mobility reduction, response times are still lower than 2 ps.     

High-speed transmission limitations due to Polarization Mode Dispersion

In this paper, the impacts of Polarization Mode Dispersion (PMD) on the performance of high-speed optical communication system have been reported at different bit rates. The two systems are modeled using older fibers with same PMD coefficient at different bit rates and third is with the new fiber with less PMD coefficient than that of the previous two. The attenuation, chromatic dispersion and non-linear effects have been disabled, so that all the variation of the results is due to PMD. The bit rate is varied from 2.5 to 40 Gbps and the length is varied from 1000 to 20,000 km. It is shown that the impact of PMD increases with the bit rate of system. It is also reported that the impact of PMD becomes intolerable at the bit rates of more than 40 Gbps. And also the PMD produces very minute impact on the system performance for same bit rate with the variation in the fiber length.     

Simulative demonstration of soliton pulse stability over the nonlinear regime in a birefringent optical fiber

In this paper, the results of numerical analysis are demonstrated for sech pulse (soliton) propagation in a birefringent optical fiber using computer modeling and simulation. Here, the initial pulse is polarized linearly and guided into the fiber at an angle of 45° to its polarization axes. The birefringence-induced time delay of 200 and 440 ps between X and Y polarization components has been reported at a fiber length of 631.72 km (10 soliton periods) by considering linear and nonlinear regimes, respectively. The Kerr nonlinearity, which stabilizes solitons against spreading due to GVD, also stabilizes them against splitting due to birefringence. A similar fact is true for the birefringent walk-off. Above a certain soliton order (N_th), the evolution scenario is qualitatively different and two orthogonally polarized components of the soliton move with a common group velocity despite their different modal indices or polarization mode dispersion (PMD) at a fiber length of 631.72 km (10 soliton periods) and 1264.344 km (20 soliton periods) over a nonlinear regime at θ≠45°. The physical effect responsible for this type of behavior is the cross-phase modulation (XPM) between the two polarization components.     

Bit-pattern-dependent polarization rotation in first-order PMD-compensated WDM systems

We demonstrate both numerically and experimentally that the phase-change due to fiber nonlinearities induces a bit-pattern-dependent rotation of the state-of-polarization which translates to uncertainty in the principal states of polarization. This effect severely limits the performance of the first-order PMD post-compensation and suggests the use of in-line compensators. Our simulation shows that fiber nonlinearities cause significant distortion (more than 4-dBm Q-penalty after 600-km transmission at 10 Gbit/s) after first-order PMD compensation. For optical powers as low as 3 dBm/channel in systems where PMD is not uniformly distributed along the transmission link, first-order PMD compensation may be ineffective.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Large negative dispersion in dual-concentric-core photonic crystal fiber with hybrid cladding structure based on complete leaky mode coupling

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm². The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.     

Transverse mode selection in a monolithic microchip laser

We outline an approach to mode selection in a microchip laser through judicious shaping of the pump light to create a high modal overlap with the desired mode. We demonstrate the principle by creating a donut-shaped pump profile in the focal plane of a converging lens, and use this profile to longitudinally pump a monolithic microchip laser where the output is a Laguerre-Gaussian mode of radial index p = 0 and azimuthal index l = 1 (LG_0l), or vortex beam, of power ~ 12 mW with a slope efficiency of 17%. This approach of diffractive pump shaping in the Fourier domain is advantageous as it allows for high pump intensity even with low pumping powers, thus ensuring sufficient gain is achieved for laser oscillation.     

Multiuser quantum key distribution over telecom fiber networks

We report five-user quantum key distribution (QKD) over switched fiber networks in both star and tree configurations, using the BB84-protocol [1] with phase encoding. Both setups implement polarization insensitive phase modulators, necessary for birefringent single mode fiber (SMF) networks. In both configurations we have achieved transmission distances between 25 km and 50 km with quantum bit error rates between 1.24% and 5.56% for the mean photon number μ=0.1. The measurements have showed feasibility of multiuser QKD over switched fiber networks, using standard fiber telecom components.     

A novel dispersion-shifted single mode optical fiber design with ultra-high-bit-rate and very low loss for long-haul communications

In this paper, a novel dispersion-shifted multi-clad optical fiber with very small bending loss and ultra-high bit-rate applicable for large capacity information transmission systems is presented. To decrease dispersion and higher-order dispersion effects at λ = 1.55 μm, a weighted pulse broadening factor and genetic algorithm (GA) optimization technique is used. Compared to the works reported previously, this method can precisely set the zero-dispersion wavelength. This kind of dispersion-shifted fibers has dispersion, dispersion slope, mode field diameter (MFD), effective area and quality factor within −1.40 × 10⁻⁴ to −8.44 × 10⁻² ps/km nm, 3.06 × 10⁻² to −4.07 × 10⁻² ps/km nm², 5.56−5.85 μm, 119.25−176.42 μm² and 3.49-5.27 at λ = 1.55 μm, respectively. Besides, by applying dispersion at λ = 1.55 μm as the cost function, dispersion of about 1.31 × 10⁻⁸ ps/km nm is obtained. Thus, this novel optical fiber can be used in long-haul high information-transmission capacity communication systems.     

Room-temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical properties

ZnO thin film with strong orientation (0 0 2) and smooth surface morphology was electrosynthesized on ITO-coated glass substrate at room temperature under pulsed voltage. Photoluminescence (PL) shows two obvious peaks: violet band and strong green band. The former is due to the free-excitonic transition and the latter is believed to arise from the single ionized oxygen vacancy (V_O⁺). Raman scattering reveals that the 580 cm⁻¹ mode and the shoulder peak mode at 550 cm⁻¹ originate from the N-related local vibration mode (LVM) and E₁ (LO) mode, respectively.