10 Gbit/s tunable dual-wavelength nonreturn-to-zero-to-return-to-zero data-format transformer based on a non-direct-current-biased fabry-perot laser diode

We demonstrate the conversion of a nonreturn-to-zero (NRZ)-formatted electrical data stream into a wavelength-tunable return-to-zero (RZ)-formatted optical pulse code by externally seeding a synchronously sinusoidal-modulated Fabry-Perot laser diode (FPLD) with optical pseudorandom binary-sequence data at 10 Gbits/s (Gbps). The FPLD without a dc-biased current was modulated by use of a power-amplified sinusoidal wave signal (approximately 25.6 dBm) as an NRZ-to-RZ data-format transformer, which is regeneratively amplified by a closed-loop erbium-doped fiber amplifier. The gain switching and on-off keying operation of the FPLD is initiated under the seeding of self-feedback and external-injection signals. A maximum wavelength tuning range of 30 nm with a side-mode suppression ratio of greater than 36 dB is obtained. The power penalty of the NRZ-to-RZ data-format conversion at 10 Gbps is 1.5 dB.     

Photonic crystal fibre based all-optical modulation format conversions between NRZ and RZ with hybrid clock recovery from a PRZ signal

Several all-optical modulation format-converting schemes are described for non-return-to-zero (NRZ) and return-to-zero (RZ) modulation formats that make use of spectral filtering of either self-phase modulation (SPM) or cross-phase modulation (XPM) broadened signal spectrum in a highly-nonlinear dispersion-flattened photonic-crystal fibre. Format conversions have been performed between the most widely used modulation formats - NRZ and RZ. In addition, a hybrid clock recovery scheme is proposed to obtain the data rate of the NRZ signal for NRZ-to-RZ format conversion. All format-converting schemes are based on the extraction of the spectral components in a nonlinear phase modulation broadened signal spectrum. In NRZ-to-RZ format conversion, a periodic pulse train, at a repetition rate similar to the NRZ data rate, is used as a control that induces a nonlinear phase shift to the NRZ probe signal and broadens its spectrum. The spectral components, contributed by different time instances of the control pulse, can be extracted as the converted RZ signal output. In RZ-to-NRZ format conversion, the RZ signal serves as a control that induces a nonlinear phase shift to a continuous wave probe light, where a logic-inverted NRZ signal can be extracted by filtering out the chirped components. Format conversions between NRZ and RZ signals at 9.95328 GB/s (OC-192) are demonstrated. As the proposed optical signal-processing schemes make use of the fibre nonlinearity (SPM/XPM), it is possible to extend it to a high-speed operation <160 Gb/s. Therefore the proposed format-converting schemes can serve as a format converter between the optical time-division multiplexed networks and the wavelength division multiplexed networks     

偏振模色散对高速光码的影响

研究一阶、二阶偏振模色散（PMD）对10 Gb/s和40 Gb/s的光传输系统性能的影响，考虑PMD的统计特性，基于基本偏振态（PSP）理论数值模拟了非归零码（NRZ）和归零码（RZ）在传输过程中产生的脉冲畸变以及系统Q值的变化。结果表明，传输速率越高PMD对系统性能的影响越显著，二阶PMD也将不可忽略。另外，RZ码传输性能明显优于NRZ码并且可通过预啁啾进行改善。     

All-optical wavelength reuse with simultaneous upstream data and PPS timing signal transfer for flexible optical access networks

All-optical wavelength reuse is a viable approach for realization of low cost colourless ONUs. We experimentally demonstrate a novel all-optical wavelength reuse technique with simultaneous upstream data and pulse-per-second signal transfer, exploiting EDFA gain saturation with a holding beam. A DFB laser is modulated with 8.5 Gbps data and transmitted downstream over 24.7 km fibre. A saturated EDFA located at the ONU is adopted to reduce the extinction ratio of the downstream data from 6.2 dB to 839.1 mdB. This allows for data rewrite and wavelength reuse for upstream transmission. Receiver sensitivities of ?20.19 dBm and ?19.60 dBm are achieved at back-to-back analysis and 24.7 km downstream link respectively. A holding beam is further exploited to attain simultaneous carrier reuse and PPS clock upstream transfer. PPS jitter stability of 1.01 × 10^-08 ns and 6.64 × 10^-08 ns are attained respectively. This work offers a convenient all-optical wavelength reuse solutions for optical access networks.     

VCSEL-based differential modulation technique for high-speed gigabit passive optical networks

With the convergence towards 5G, optical networks need to be upgraded to support the emerging data tsunami. This work experimentally demonstrates the first real-time transmission of 20?Gbps over a class 10G 1550?nm vertical cavity surface emitting laser (VCSEL) in the context of on-off-keying (OOK) modulation format, by employing VCSEL differential drive mode technique, for adoption in high-speed gigabit passive optical networks (GPONs). Two OOK data streams each with 10?Gbps are differentially modulated onto a single VCSEL, therefore generating an aggregated data rate of 20?Gbps OOK signal. A receiver sensitivity of -13.36?dBm is experimentally achieved. Through extinction ratio optimization, an error free transmission over 24.7?km single mode fibre is attained, with a transmission penalty of 1.91?dB. Our proposed technique alleviates band-limitation of the VCSEL carrier, and doubles the channel data rate without replacing the transmitter optics.     

Performance of a QAM/FSO communication system employing spatial diversity in weak and saturation turbulence channels

In this paper, the bit error rate (BER) performance and outage probability of quadrature amplitude modulation free space optical (QAM/FSO) communications with spatial diversity in turbulent environments are investigated. The equal-gain combining (EGC) and selection combining (SelC) diversity techniques are considered to mitigate turbulence-induced signal fading in the proposed system. The average BER and outage probability expressions are derived for EGC diversity in weak and saturation turbulence channels. The results indicate that using EGC diversity can significantly improve the system performance compared to employing the SelC diversity or single monolithic aperture schemes. Specifically, approximately 4 and 9?dB lower signal-to-noise power ratios are required for the 1?×?4 EGC diversity system than for the 1?×?4 SelC and non-diversity systems at a BER of 10^?10. In addition, the use of diversity techniques also significantly decreases the outage probability. The proposed scheme can be helpful for establishing a spatial diversity FSO system with a low error rate and high transmission rate.     

Simulation and performance analysis of duobinary 40 Gbps optical link

This paper presents a system model to simulate and evaluate the performance of duobinary RZ (DB-RZ) and duobinary NRZ (DB-NRZ) modulation format in a 40?Gbps optical communication link. The design has been optimized to estimate the electrical and optical filter parameters for optimum performance under dispersion and self-phase modulation induced impairments. The presented model has been operated under various OSNR conditions to investigate the limitations of GVD and SPM on the BER-equivalent Q-factor for DB-NRZ and DB-RZ encoded optical data. The analysis reports the superiority of DB-NRZ over DB-RZ at higher OSNR. However, for a typical OSNR value a crossover optical filter bandwidth has been observed beyond which DB-RZ outperforms DB-NRZ.     

Use of return-to-zero formats in electrical dispersion compensated optical single sideband transmission systems

The use of return-to-zero (RZ) formats is proposed to enhance the dispersion tolerance of intensitymodulated carrier-unsuppressed (IM-CUs) optical single sideband (oSSB) systems. The performance of these systems, employing electrical dispersion compensation (EDC), is accessed using unipolar and polar RZ signalling formats, and compared with common non-return-to-zero (NRZ). Simulation results at 10.7 Gb/s reveal that oSSB systems with EDC using both NRZ and unipolar RZ signalling formats are limited to low extinction ratio (ER) values. However, the use of polar RZ allows increased ER and transmission distances, because of higher inter-symbol interference tolerance and the absence of discrete spectral tones. Considering a minimum ER of 9 dB and resort to forward error correction schemes, polar RZ allows single-channel error free transmission over 1400 km of standard single-mode fibre, substantially improving the 980 km obtained for NRZ with ER of 6 dB. Dense wavelength-division multiplexing transmission was also assessed, retrieving improved resilience of IM-CUs oSSB polar RZ.     

Performance of line codes in optical direct detection continuous phase frequency shift keying transmission system impaired by polarization mode dispersion

Small deviations from perfect circular symmetry in the core region of single mode fiber (SMF) cause optical pulses to become broadened as they propagate. This phenomenon is referred to as polarization mode dispersion (PMD), which leads to intersymbol interference and becomes a major impediment for the high speed long-haul fiber-optic links. We present here the theoretical complement for evaluating the performance of a line-coded continuous phase frequency shift keying (CPFSK) optical transmission system with a direct detection receiver. The analysis is carried out for two different line-coding schemes, i.e. alternate mark inversion (AMI) and order-1 coding, to investigate the effectiveness of the line coding in counteracting the effect of PMD in a CPFSK direct detection transmission system in the presence of group velocity dispersion (GVD) and receiver noise in a single mode fiber. The average bit error rate (BER) performances are evaluated without and compared to that of line codes at a bit rate of 10 Gb s^?1 considering Maxwellian distribution for the mean differential group delay (DGD). We found that the amount of power penalty improvement of line-coded systems is within 0.65 to 2.25 dB with respect to NRZ data at a BER of 10^?9.     

New approach for the design of an optical square pulse generator

What is believed to be a new approach for the design and analysis of a reconfigurable optical square pulse generator using the concept of temporal optical integration and the digital signal processing method is presented. The reconfigurable square pulse generator is synthesized using compact active semiconductor-based waveguide technology, and it consists simply of the cascade of a tunable microring resonator (or a tunable all-pole filter) and a tunable asymmetrical Mach-Zehnder interferometer (or a tunable all-zero filter). The reconfigurable generator can convert an input picosecond pulse (i.e., soliton or Gaussian pulse) into an optical square pulse. The pulse width of the generated square pulse can be adjusted by controlling the time delay of a variable delay element in the tunable all-zero filter. The reconfigurable generator can convert an input picosecond pulse train into return-to-zero (RZ) and non-return-to-zero (NRZ) signals with square pulse shapes. The repetition rates of the generated RZ and NRZ signals can be varied by adjusting the bit period of the input picosecond pulse train, the input pulse width, and the time delay of the variable delay element. The effect of the deviation of the parameter values on the generator performance is also studied.     

All optical active high decoder using integrated 2D square lattice photonic crystals

The paper introduces a novel all optical active high 2 × 4 decoder based on 2D photonic crystals (PhC) of silicon rods with permittivity of ε = 10.1 × 10^?11 farad/m. The main structure of optical decoder is designed using a combination of five nonlinear photonic crystal ring resonator, set of T-type waveguide, and line defect of Y and T branch splitters. The proposed structure has two logic input ports, four output ports, and one bias input port. The total size of the proposed 2 × 4 decoder is equal to 40 μm × 38 μm. The PhC structure has a square lattice of silicon rod with refractive index of 3.39 in air. The overall design and the results are discussed through the realization and the numerically simulation to confirm its operation and feasibility.     

Analysis of optical sensing behavior of a novel optical bean-shaped resonator biosensor integrated with MZI configuration

A novel optical bean-shaped resonator (BR) biosensor integrated with Mach–Zehnder Interferometer structure based on a silicon-on-isolator platform has been proposed and investigated theoretically and numerically. By characterizing and optimizing the structure, our bean-shaped device exhibits a high extinction ratio over 50?dB and a high Q factor of about 5.46?×?10⁴ in a wide wavelength span. The quasi-free spectral range (FSR) is about 469?nm. The sensitivity of 688.6?nm/refractive index unit (RIU) is achieved for bulk changes of refractive index and the detection range varies from 10^?6 to 0.689 RIU. More complex bean-shaped structures can also be cascaded by adding more bending waveguides in BR to obtain wider quasi-FSR range.     

Investigation of the effects of SOA locations in the linear cavity of an O-band Brillouin SOA fiber laser

An investigation into the effect of semiconductor optical amplifier (SOA) location in an O-band Brillouin SOA fiber laser (BSFL) was performed. Better output peak power flatness was generated by placing the SOA after the nonlinear medium, which is a 20?km true wave fiber (TWF) than placing it before the TWF. A maximum power of six flat output peaks with average power of ?22.0?dBm for a BP (Brillouin pump) wavelength of 1320?nm was obtained, generated from a BSFL with a SOA located after the TWF, compared with three flat Stokes signals with the SOA before the TWF at a BP wavelength of 1310?nm. The flat peak power output for the O-band Brillouin fiber laser is important, especially in producing a good O-band source.     

Field enhancement and large optical nonlinearity in Ag nanocomposite polymer film

We report a simple in situ synthesis for Ag nanocomposite polymer film. The extinction spectrum and the distribution of the local field intensities for Ag nanoparticles are performed by means of the dipole discrete approximation. The local field intensity is enhanced over 20 times to that of the incident light at the peak wavelength of the extinction spectrum. Nonlinear optical measurements, performed by using the Z-scan techniques, are presented afterwards. Giant enhancement of nonlinear optical responses is found for Ag/PMMA film compared with pure PMMA (polymethyl methacrylate) film. The nonlinear refractive index γ of the Ag/PMMA film is measured to be 3.708 × 10^?2 cm² GW^?1. The enhanced optical properties are due to the surface plasmon resonance of Ag nanoparticles. These results are in agreement with the previous field calculations. Analyzed with respect to Stegeman figures of merit, Ag/PMMA nonlinear polymer film shows promise for practical use in ultrafast optical devices.     

All-optical NRZ-to-RZ format conversion at 10 Gbit/s with 1-to-4 wavelength multicasting exploiting cross-phase modulation & four-wave-mixing in single dispersion-flattened highly nonlinear photonic crystal fiber

All-optical NRZ-to-RZ format conversion with a function of wavelength multicasting is proposed in this paper, which is realized by exploiting cross-phase modulation (XPM) and four-wave-mixing (FWM) in a dispersion-flattened highly nonlinear photonic crystal fiber (DF-HNL-PCF). The designed format converter is experimentally demonstrated, for which the 1-to-4 wavelength multicasting is achieved simultaneously by filtering out two FWM idler waves and both blue-chirped and red-chirped components of the broadened NRZ spectrum induced by XPM. Moreover, the wavelength tunability and dynamic characteristics of the proposed NRZ-to-RZ format converter are also exploited using the different central wavelengths of an optical clock signal and varying the input optical power at a DF-HNL-PCF in our experiment. It is shown that the designed format converter can possess a wide range of operational wavelength over 17 nm, an optimal extinction ratio of 11.6 dB, and a Q-factor of 7.1, respectively. Since the proposed scheme uses an optical fiber-based configuration and is easy for implementation, it can be very useful for future applications in advanced fiber-optic communication networks.     

A high capacity data centre network: simultaneous 4-PAM data at 20 Gbps and 2 GHz phase modulated RF clock signal over a single VCSEL carrier

Optical fibre communication technologies are playing important roles in data centre networks (DCNs). Techniques for increasing capacity and flexibility for the inter-rack/pod communications in data centres have drawn remarkable attention in recent years. In this work, we propose a low complexity, reliable, alternative technique for increasing DCN capacity and flexibility through multi-signal modulation onto a single mode VCSEL carrier. A 20 Gbps 4-PAM data signal is directly modulated on a single mode 10 GHz bandwidth VCSEL carrier at 1310 nm, therefore, doubling the network bit rate. Carrier spectral efficiency is further maximized by modulating its phase attribute with a 2 GHz reference frequency (RF) clock signal. We, therefore, simultaneously transmit a 20 Gbps 4-PAM data signal and a phase modulated 2 GHz RF signal using a single mode 10 GHz bandwidth VCSEL carrier. It is the first time a single mode 10 GHz bandwidth VCSEL carrier is reported to simultaneously transmit a directly modulated 4-PAM data signal and a phase modulated RF clock signal. A receiver sensitivity of ?10. 52 dBm was attained for a 20 Gbps 4-PAM VCSEL transmission. The 2 GHz phase modulated RF clock signal introduced a power budget penalty of 0.21 dB. Simultaneous distribution of both data and timing signals over shared infrastructure significantly increases the aggregated data rate at different optical network units within the DCN, without expensive optics investment. We further demonstrate on the design of a software-defined digital signal processing assisted receiver to efficiently recover the transmitted signal without employing costly receiver hardware.     

Self-seeded reflective semiconductor optical amplifier based optical transmitter for up-stream WDM-PON link

A novel up-stream transmitter for wavelength division multiplexed-passive optical network (WDM-PON) using a self-seeded reflective semiconductor optical amplifier (RSOA) was proposed. The RSOA was self-seeded by an amplified spontaneous emission itself using a fibre Bragg grating without an additional optical source. The side mode suppression ratio of self-seeded output signal and the extinction ratio were ~28 and 8.1 dB, respectively. Required power for 10^-9 BER at 622 Mb/s was about -28 dBm and the power penalty after 20 km transmission was <2 dB for 30 nm range of wavelength     

偏振模色散对NRZ码和RZ码的40Gbit/s系统性能影响的仿真分析

偏振模色散(PMD)是进一步提高现有光纤通信系统单信道速率必须克服的障碍之一。本文基于已铺设光纤的典型值,针对偏振模色散对采用NRZ码和RZ码的40Gbit／s光纤通信系统的性能影响进行了分析与仿真。结果表明,考虑PMD的影响时,采用RZ码的系统并不一定比采用NRZ码的系统的传输性能好,而在占空比[0,1]之间存在一个最佳值,此时,高速系统受偏振模色散的影响最小。这对于研究、设计高速光纤通信系统时选择合适的码型和RZ码的占空比,具有重要的参考意义。     

The characteristics of transparent conducting Al-doped zinc oxide thin films deposited on polymer substrates

Al-doped zinc oxide (AZO) transparent, conductive thin films were deposited on inexpensive polyethylene terephthalate substrates, using radio frequency (rf) magnetron sputtering, with an AZO ceramic target (the Al₂O₃ content is approximately 2 wt%). This paper presents an effective method for the optimization of the parameters for the deposition process for AZO thin films with multiple performance characteristics, using the Taguchi method, combined with grey relational analysis. Using the Taguchi quality design concept, an L₉ orthogonal array was chosen for the experiments. The effects of various process parameters (rf power, substrate-to-target distance, substrate temperature and deposition time) on the electrical, structural, morphological and optical properties of AZO films were investigated. In the confirmation runs, using grey relational analysis, the electrical resistivity of the AZO films was found to have decreased from 5.0?×?10^?3 to 1.6?×?10^?3?Ω-cm and the optical transmittance was found to have increased from 74.39 to 79.40%. The results demonstrate that the Taguchi method combined with grey relational analysis is an economical way to obtain the multiple performance characteristics of AZO films with the fewest experimental data. Additionally, by applying an Al buffer layer, of thickness 10?nm, the results show that the electrical resistivity was 3.1?×?10^?4?Ω-cm and the average optical transmittance, in the visible part of the spectrum, was approximately 79.12%.     

Performance improvement of terrestrial free-space optical communications by mitigating the focal-spot wandering

Abstract

Focal-spot wandering is the main cause for the major power loss in free-space optical communications. Thus,mitigating it is a primary requirement for the successful performance improvement. In order to prove this prerequisite, an experimental set-up using 155-Mbps data transmission is built for the link range of 0.5 km at an altitude of 15.25 m. In the experiment, the receiver is equiped with a control system to stabilize the received optical propagation at the detector plane which is called as focal-spot wandering mitigation control so as to couple the power in bucket perfectly to the photodetector. The performance improvements due to mitigating focal-spot wandering are regressively investigated in terms of various quality assessment key parameters. Maximum radial distance of 0.25 mm, maximum effective scintillation index of 0.17, optical signal-to-noise ratio of 9 dB, minimum eye-opening of ±0.37 V, minimum eye-height of ±0.51 V, controlled bit-error-rate of 6.45 × 10^?9 to 7.09 × 10^?8 and the link margin of 1.83 dB are attained even during strong turbulence level while mitigating focal-spot wandering.