Performance analysis of NRZ, RZ, CRZ and CSRZ data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD

This paper presents the performance analysis of non-return-to-zero (NRZ), return-to-zero (RZ), chirped return-to-zero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in optical soliton transmission link under the impact of chirp and third-order dispersion (TOD). The performance of these data formats has been analyzed on the basis of certain performance metrics, viz, bit error rate (BER), Q² (dB), OSNR, eye opening, etc. It has been reported here that the performance of CRZ and CSRZ modulation format is better as compared to NRZ and RZ in a soliton transmission link. Further, CSRZ modulation format has been found to deliver optimum performance on the basis of performance evaluation metrics reported in this paper. In case of NRZ and CSRZ, comparatively narrow power spectrum has been observed. Best eye opening, highest value of Q² (dB) of 18 dB and lowest value of BER of the order of 10⁻¹⁶ has been reported in case of CSRZ among the considered data formats. The results have been obtained by varying noise figure from 3.0 to 9.0. No considerable effect of noise was observed. It was observed that at very narrow and ultra short pulse width, OSNR value suffers heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The results were obtained by varying chirp factor from −0.6 to +0.6. Negative chirp resulted in improved OSNR as compared to positive chirp. RZ data format yielded a broader optical spectrum, comparatively low spectral efficiency and poor OSNR thus it was found that RZ format is not suitable for optical soliton transmission under the impact of chirp and TOD.     

Third order optical nonlinearities of eight-β-octa-octyloxy-phthalocyanines

The third order nonlinear optical properties of the eight-β-octa-octyloxy-phthalocyanines in chloroform were investigated by Z-scan using ps laser at 532 nm. The excited state absorption cross section and the nonlinear refractive cross-section were determined. Eight-β-octa-octyloxy-phthalocyanine free base and eight-β-octa-octyloxy-nikle-phthalocyanine both posses large σ_es/σ₀ ratio and positive σ_r, and the introduction of nikle ion reduces these values. Optical limiting performance was studied using multi-level model, the result shows that eight-β-octa-octyloxy-phthalocyanine free base has a much lower limiting threshold.     

Enhanced nonlinear optical responses in metal-glass nanocomposites

The “engineered” nonlinear nanocomposite materials with extremely large values of optical Kerr susceptibility and fast temporal responses that can be precisely tuned to satisfy the requirements of switching applications is of current interest in photonics. Metal quantum-dot composite glasses can exhibit enhanced optical susceptibility, χ⁽³⁾, whose real and imaginary parts are related to the intensity-dependent refractive index and two-photon absorption coefficient, respectively. Classical (dielectric) and quantum confinement effects come into play in the nonlinear optical responses of these nanocomposites. Metal nanocluster-glass composites have been synthesized by 200 keV Cu⁺ and 1.5 MeV Au⁺ ion implantations in fused silica glasses at a dose of 3 × 10¹⁶ ions/cm², followed by thermal annealing in reducing atmosphere to promote cluster growth. UV-Visible spectroscopy has revealed prominent linear absorption bands at characteristic surface plasmon resonance (SPR) frequencies signifying appreciable formation of copper and gold colloids in glass matrices. Third-order optical properties of the composite materials have been studied by Z-Scan and Anti-Resonant Interferometric Nonlinear Spectroscopy (ARINS) measurements. The sign of nonlinear refraction is readily obtained from the Z-Scan signatures. The ARINS technique utilizes the dressing of two unequal-intensity counter-propagating pulsed light beams with differential nonlinear phases, which occur upon traversing the sample if it exhibits nonlinear optical response. This difference in phase manifests itself in the intensity-dependent transmission. The nonlinear refractive index, nonlinear absorption coefficient, the real and imaginary parts of the third-order optical susceptibility have been extracted.     

Nonlinear optical studies of a newly synthesized copolymer containing oxadiazole and substituted thiophenes

We investigated the third-order nonlinear optical properties of a newly synthesized soluble copolymer containing oxadiazole and thiophene units using Z-scan and Degenerate Four Wave Mixing (DFWM) techniques. The measurements were performed at 532 nm with 7 ns pulses from a Nd:YAG laser. We found a good agreement between the values of χ⁽³⁾ determined from both experiments. Z-scan results indicate a negative nonlinear refractive index, n₂, whose magnitude is of the order of 10⁻¹⁰ esu. The copolymer exhibits strong nonlinear absorption and good optical limiting properties at 532 nm, and hence may be a potential material for optical limiting applications.     

A novel all optical method of implementing an n-bit wavelength encoded complete digital data comparator using nonlinear semiconductor optical amplifiers

Data comparator is the integral part of arithmetic and logical unit of any electronic or optical data processor. Due to some inherent limitations of electronics it cannot be possible to obtain a super fast operation (over terahertz limit) from electronic comparators. Again wavelength encoding technique has been established as an excellent one over other existing optical data encoding techniques. Semiconductor optical amplifier (SOA) technologies have shown their strong potentiality of realizing many all-optical systems. In this communication the authors have proposed a new scheme of developing all-optical wavelength encoded n bit binary comparator exploiting the four-wave mixing, wavelength filtering, wavelength conversion and nonlinear polarization rotation capabilities property of nonlinear semiconductor optical amplifiers. The scheme can be used for comparing signed and unsigned optical binary data of any bit wide numbers as well. The comparator is especially suitable for use as a building block in a larger optical circuit, such as in an all optical telecommunications switch.     

The nonlinear refraction and absorption dependence on the thermal effect for 4 ns pulse duration in binuclear Zn(II) phthalocyanine solution

The influence of thermal effect on the third-order nonlinear optical properties of binuclear Zn(II) phthalocyanine in chloroform solution was studied. The nonlinear refraction and absorption of the sample was measured by using Z-scan technique with 4 ns laser pulses at 532 nm wavelength. The opposite signs of the effective nonlinear refraction index were observed by changing the focal length of focusing lens from 10 cm to 20 cm in the experimental setup. Changing the focusing lens increased the beam waist radius from 7 μm to 20 μm. The nonlinear absorption coefficient was reduced about 200 times based on changing the fluence or beam waist radius. The drastic changes in the third-order nonlinear optical parameters were attributed to thermal effect. To investigate the role of thermal effects even further the effective nonlinear refraction and absorption coefficients were studied by using different repetition rates, input powers and concentrations.     

Cross-phase modulation induced performance degradation in nonlinear optical loop mirror switch

Performance deterioration induced by cross-phase modulation (XPM) in the basic nonlinear optical loop mirror (NOLM) switch unit in the digital optical communication system and analog microwave photonic communication system is studied by using the split-step Fourier method (SSFM) that can solve the coupled nonlinear Schrödinger equation (CNLSE). The simulation results show that the switching performance of basic NOLM unit is distorted by the XPM induced nonreciprocity when the splitting ratio of NOLM coupler f ≠ 0.5 and the distortion effect would be more serious with the increase of the working speed or the modulation bandwidth. In the digital optical communication system, the power transmission function of NOLM decreases in the range of 0 < f < 0.5 and increases in the range of 0.5 < f < 1. In the analog microwave photonic communication system, XPM induces the distortion of waveforms and the leak of side lobes. And the larger the modulation bandwidth is, the larger the leak of side lobes is.     

Optical switch with low-phase transition temperature based on thin nanocrystalline VOx film

An optical switch is fabricated by using micromachining technology, which is based on thin nanocrystalline vanadium oxide (VO_x) film, and it consists of four layers: a silicon (Si) substrate layer, a VO_x layer, a Si₃N₄ buffer layer, and an aurum (Au) electrode layer. By applying a switching power supply to a pair of the Au electrodes, the optical switch is controlled to exhibit from an “on” state with semi-conducting phase to an “off” state with metallic phase. The optical switch performance is investigated, and testing results show that its extinction ratio is about 14 dB, its switching response time can achieve about 1.5 ms, and the power dissipation required for stimulating switching to work can be below about 15 mW at least, which is lower than the power dissipation of conventional optical switches based on microstructure thin vanadium dioxide (VO₂) films. This kind of optical switch is potential to be applied as optical switch for optical communication.     

Kerr nonlinear switch based on ultra-compact photonic crystal directional coupler

In this paper, an all optical switch based on nonlinear photonic crystal directional coupler has been simulated and analyzed by the finite difference time domain (FDTD) method. The lunched pump signal increases the refractive indices of the central row of the coupler, due to nonlinear Kerr effect, hence the coupler works in the nonlinear conditions and lightwave guides to the other output port. We have tried to increase the coupling efficiency and reduce the required power in the nonlinear status by optimizing the bends structure and increasing the interaction between dielectric and lightwave signal. Therefore, the input signal beam can be controlled to be exchanged between two output ports to earn the highest output power ratio and the smallest amount of power required for nonlinear performance, the physical length of the coupler is determined to be 20a, where a is the structure lattice constant.     

The refractive nonlinearities of InAs/GaAs quantum dots above-bandgap energy

The third-order optical nonlinear refractive properties of InAs/GaAs quantum dots grown by molecular beam epitaxy have been measured using the reflection Z-scan technique at above-bandgap energy. The nonlinear refractive index and nonlinear absorption index of the InAs/GaAs quantum dots were determined for wavelengths from 740 to 777 nm. The measured results are compared with the nonlinear refractive response of several typical III-V group semiconductor materials. The corresponding mechanisms responsible for the large nonlinear response are discussed.     

Nonlinear responses and optical limiting behavior of fast green FCF dye under a low power CW He-Ne laser irradiation

The nonlinear responses and optical limiting performance of a dye type acid, called fast green FCF, are investigated under irradiation of 35 mW continuous wave He-Ne Laser. The second order refractive index and nonlinear absorption coefficient are measured by use of z-Scan technique. The optical limiting behavior is investigated by transmission measurement through the sample. Linear absorption coefficient (α), nonlinear absorption coefficient (β) and second order refractive index (n₂) of fast green FCF are measured at different concentrations.     

Performance evaluation of bi-directional passive optical networks in the scenario of triple play service

In this paper, the performance of bi-directional passive optical network (BPON) has been evaluated and compared at different bit rates in the scenario of triple play service. The triple-play service is realized as a combination of data, voice and video signals. This architecture is investigated for symmetrical data traffic for uplink and downlink transmission and its performance is also evaluated in terms of Q-factor and eye height at different transmission distance. The Q-factor results show the acceptable performance at 10 Gbps data rate for downstream and upstream transmission, as it accommodates 128 optical network units (ONUs). Further the proposed system's performance is compared with the current state-of-the-art PON architectures.     

Measurement of nonlinear properties and optical limiting ability of Rhodamine6G doped silica and polymeric samples

The third order nonlinear optical properties of Rhodamine6G (Rh6G) doped silica and polymeric samples have been investigated using single beam z-scan technique under excitation by the second harmonic of Nd:YAG laser beam (532 nm). The nonlinear refractive index, nonlinear absorption coefficient, real and imaginary parts of third order nonlinear susceptibility in the samples of silica and poly-methylmethacrylate (PMMA) matrices are measured. Thermal contribution to the nonlinear refractive index in case of undoped silica samples has been calculated in order to have better accuracy of the material response contribution to third order nonlinearity. The comparative study of the optical limiting performance of Rh6G doped silica and polymeric samples show that Rh6G doped silica is relatively superior for optical limiting applications.     

Simulative analysis of pre- and post-compensation using CRZ format in WDM optical transmission link

This paper reports the effects of pre- and post-compensation using CRZ modulation format in long-haul WDM optical transmission link using wavelengths in three bandwidths viz. 1537.4; 1550; 1562.6 nm at per channel bit rates of 10 Gbit/s. It has been investigated here that optimization of dispersion map results in improved management of nonlinear effects in long-haul light wave systems operating in the quasi-linear regime. In addition, pre- and post-dispersion compensation was applied at the transmitter and receiver depending on the signal wavelength, which resulted in improvement of performance metrics viz. Q² (dB), BER and OSNR over longer transmission distances. It is reported here that optimum values of Q² dB of 17.1 dB, BER of 8.4933e⁻⁰¹⁵ and OSNR of 30.1 dB are obtained at 1550 nm at a transmission distance of 7360 km with pre- and post-compensation using CRZ modulation format.     

Parameter design and performance analysis of a ultrafast all-optical XOR gate based on quantum dot semiconductor optical amplifiers in nonlinear mach-zehnder interferometer

This paper presents the parameter design and performance analysis of a 160 Gb/s all-optical XOR gate based on cross-gain modulation (XGM) in a nonlinear Mach-Zehnder interferometer (MZI) with quantum dot semiconductor optical amplifiers (QD-SOAs). Detailed numerical simulations of the QD-SOA parameters and optical signal parameters are performed to elevate the gate performance. With the optimized parameters, a Q factor over 8 dB is obtained. The possibility of operating at higher speed of the XOR gate is demonstrated as well. The results will be helpful for the design and performance analysis of practical quantum dot devices.     

The N-soliton solution of a two-component modified nonlinear Schrödinger equation

The N-soliton solution is presented for a two-component modified nonlinear Schrödinger equation which describes the propagation of short pulses in birefringent optical fibers. The solution is found to be expressed in terms of determinants. The proof of the solution is carried out by means of an elementary theory of determinants. The generalization of the 2-component system to the multi-component system is discussed as well as a (2+1)-dimensional nonlocal equation arising from its continuum limit.     

Performance evaluation of path-averaged soliton pulses in loss-managed 10-Gbps soliton transmission link over a long haul

In this paper, the performance evaluation of path-averaged soliton transmission link for various performance measures viz. OSNR, optical power, extinction ratio, bit error rate (BER) and Q factor at different levels of noise figure and values of pulse width (FWHM) has been carried out. The performance of soliton transmission link is studied, taking into account soliton interaction, amplified spontaneous emission (ASE) noise and noise figure. The model presented considers interaction in a random sequence of solitons and the effect of the ASE noise added in each amplification stage. The influence of ASE noise, noise figure and pulse width with different amplifier spacing on the BER and quality factor has been investigated. It has been shown that these play dominant roles in degrading the performance measures. We have demonstrated the capability of path-averaged (guiding-centre) soliton for a long-haul distance of 17,000 km at a bit rate of 10 Gbps without ASE effect and noise figure in each amplifier span length of 500 km. The average value of quality factor is found to be 16.6 dB and the average BER is of the order of 10⁻¹² over the transmission distance of 17,000 km. Further, it has been investigated that a severe system penalty results on the inclusion of ASE effect and noise figure in order to achieve the same level of performance. Thus, the investigations ascertain that in order to maintain the same level of BER and Q factor, the amplifier spacing and total transmission distance reduce considerably.     

Broadband optical limiting performance of polymer-wrapped carbon nanotubes in the orange-NIR region

Soluble multi-walled carbon nanotubes (MWNTs) have been obtained by noncovalent modification with poly [2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV). For the composite MWNT/MEH-PPV, there is π-π interaction between the MEH-PPV and MWNTs in addition to the wrapping of the polymer. The nonlinear optical transmittance was measured using a nanosecond optical parametric oscillator pumped with a Nd:YAG system. Excellent optical limiting performance of the composite MWNT/MEH-PPV was observed both in the visible region of 590-680 nm and at the wavelength of 1064 nm. By means of time-correlated single-photon counting fluorescence measurement, an explanation based on the nonlinear absorption of MWNT dominated by the intermolecular energy transfer was proposed.     

Synthesis and photoluminescent and nonlinear optical properties of manganese doped ZnS nanoparticles

In this work we synthesized ZnS:Mn²⁺ nanoparticles by chemical method using PVP (polyvinylpyrrolidone) as a capping agent in aqueous solution. The structure and optical properties of the resultant product were characterized using UV-vis optical spectroscopy, X-ray diffraction (XRD), photoluminescence (PL) and z-scan techniques. UV-vis spectra for all samples showed an excitonic peak at around 292 nm, indicating that concentration of Mn²⁺ ions does not alter the band gap of nanoparticles. XRD patterns showed that the ZnS:Mn²⁺ nanoparticles have zinc blende structure with the average crystalline sizes of about 2 nm. The room temperature photoluminescence (PL) spectrum of ZnS:Mn²⁺ exhibited an orange-red emission at 594 nm due to the ⁴T₁-⁶A₁ transition in Mn²⁺. The PL intensity increased with increase in the Mn²⁺ ion concentration. The second-order nonlinear optical properties of nanoparticles were studied using a continuous-wave (CW) He-Ne laser by z-scan technique. The nonlinear refractive indices of nanoparticles were in the order of 10⁻⁸ cm²/W with negative sign and the nonlinear absorption indices of these nanoparticles were obtained to be about 10⁻³ cm/W with positive sign.