Chromatic dispersion monitoring for high-speed WDM systems using two-photon absorption in a semiconductor microcavity

This paper presents a theoretical and experimental investigation into the use of a two-photon absorption (TPA) photodetector for use in chromatic dispersion (CD) monitoring in high-speed, WDM network. In order to overcome the inefficiency associated with the nonlinear optical-to-electrical TPA process, a microcavity structure is employed. An interesting feature of such a solution is the fact that the microcavity enhances only a narrow wavelength range determined by device design and angle at which the signal enters the device. Thus, a single device can be used to monitor a number of different wavelength channels without the need for additional external filters. When using a nonlinear photodetector, the photocurrent generated for Gaussian pulses is inversely related to the pulsewidth. However, when using a microcavity structure, the cavity bandwidth also needs to be considered, as does the shape of the optical pulses incident on the device. Simulation results are presented for a variety of cavity bandwidths, pulse shapes and durations, and spacing between adjacent wavelength channels. These results are verified experimental using a microcavity with a bandwidth of 260 GHz (2.1 nm) at normal incident angle, with the incident signal comprising of two wavelength channels separated by 1.25 THz (10 nm), each operating at an aggregate data rate of 160 Gb/s. The results demonstrate the applicability of the presented technique to monitor accumulated dispersion fluctuations in a range of 3 ps/nm for 160 Gb/s RZ data channel

Correction of errors and harmonic distortion in pulse-width modulation of digital signals

Article number 153991Pulse-Width (PW) modulation is widely used in those applications where an analog or digital signal has to be encoded in the time domain as a binary stream, such as switched-mode power amplifiers in transmitters of modern telecommunication standards, high-resolution digital signal conversion using single-bit digital-to-analog converters, and many others. Due to the fact that digital signals are sampled in the time domain, the quality of the resulting PW modulated waveforms is worsened by harmonic distortion. Multilevel PW modulation has been proposed to reduce these adverse effects, but the modulated waveform is no longer binary. In this paper, the mechanisms by which harmonic distortion is produced are analyzed. As a result, the distortion terms are mathematically quantified and used to correct the errors. Note that a correction network based on a simple subtraction of the distortion terms from the PW modulated signal would produce a waveform that would no longer be binary. The proposed correction network is implemented in the digital domain and, by means of a sigma-delta modulator, preserves the binary feature of the PW modulated output.Ministerio de Ciencia, Innovación y Universidades (España) RTI201- 099189-B-C2

All-semiconductor High Power Mode-locked Laser System

All-optical synchronization and its application in advanced optical communications have been investigated in this dissertation. Dynamics of all-optical timing synchronization (clock recovery) using multi-section gain-coupled distributed-feedback (MS-GC DFB) lasers are discussed. A record speed of 180-GHz timing synchronization has been demonstrated using this device. An all-optical carrier synchronization (phase and polarization recovery) scheme from PSK (phase shift keying) data is proposed and demonstrated for the first time. As an application of all-optical synchronization, the characterization of advanced modulation formats using a linear optical sampling technique was studied. The full characterization of 10-Gb/s RZ-BPSK (return-to-zero binary PSK) data has been demonstrated. Fast lockup and walk-off of the all-optical timing synchronization process on the order of nanoseconds were measured in both simulation and experiment. Phase stability of the recovered clock from a pseudo-random bit sequence signal can be achieved by limiting the detuning between the frequency of free-running self-pulsation and the input bit rate. The simulation results show that all-optical clock recovery using TS-DFB lasers can maintain a better than 5 % clock phase stability for large variations in power, bit rate and optical carrier frequency of the input data and therefore is suitable for applications in ultrafast optical packet switching. All-optical timing synchronization of 180-Gb/s data streams has been demonstrated using a MS-GC DFB laser. The recovered clock has a jitter of less than 410 fs over a dynamic range of 7 dB. All-optical carrier synchronization from phase modulated data utilizes a phase sensitive oscillator (PSO), which used a phase sensitive amplifier (PSA) as a gain block. Furthermore, all-optical carrier synchronization from 10-Gb/s BPSK data was demonstrated in experiment. The PSA is configured as a nonlinear optical loop mirror (NOLM). A discrete linear system analysis was carried out to understand the stability of the PSO. Complex envelope measurement using coherent linear optical sampling with mode-locked sources is investigated. It is shown that reliable measurement of the phase requires that one of the optical modes of the sampling pulses be locked to the optical carrier of the data signal to be measured. Carrier-envelope offset (CEO) is found to have a negligible effect on the measurement. Measurement errors of the intensity profile and phase depend on the pulsewidth and chirp of the sampling pulses as well as the detuning between the carrier frequencies of the data signal and the center frequency of the sampling source. Characterization of the 10-Gb/s RZ-BPSK signal was demonstrated using the coherent detection technique. Measurements of the optical intensity profile, chirp and constellation diagram were demonstrated. A CW local oscillator was used and electrical sampling was performed using a sampling scope. A novel feedback scheme was used to stabilize homodyne detection

Waveform-Diverse Stretch Processing

Stretch processing with the use of a wideband LFM transmit waveform is a commonly used technique, and its popularity is in large part due to the large time-bandwidth product that provides fine range resolution capabilities for applications that require it. It allows pulse compression of echoes at a much lower sampling bandwidth without sacrificing any range resolution. Previously, this technique has been restrictive in terms of waveform diversity because the literature shows that the LFM is the only type of waveform that will result in a tone after stretch processing. However, there are also many examples in the literature that demonstrate an ability to compensate for distortions from an ideal LFM waveform structure caused by various hardware components in the transmitter and receiver. This idea of compensating for variations is borrowed here, and the use of nonlinear FM (NLFM) waveforms is proposed to facilitate more variety in wideband waveforms that are usable with stretch processing. A compensation transform that permits the use of these proposed NLFM waveforms replaces the final fast Fourier transform (FFT) stage of the stretch processing configuration, but the rest of the RF receive chain remains the same. This modification to the receive processing structure makes possible the use of waveform diversity for legacy radar systems that already employ stretch processing. Similarly, using the same concept of compensating for distortions to the LFM structure along with the notion that a Fourier transform is essentially the matched filter bank for an LFM waveform mixed with an LFM reference, a least-squares based mismatched filtering (MMF) scheme is proposed. This MMF could likewise be used to replace thefinal FFT stage, and can also facilitate the application of NLFM waveforms to legacy radar systems. The efficacy of these filtering approaches (compensation transform and least-squares based MMF) are demonstrated in simulation and experimentally using open-air measurements and are applied to different scenarios of NLFM waveform to assess the results and provide a means of comparison between the two techniques

光波長多重及び光時分割多重ネットワークにおける光波長マルチキャスティング技術

The capacity of optical communication systems has shown an incredibly thriving growth from their inception to the last several decades. From the observations in traffic demand, the objectives of this thesis are to develop some key functions for improving the flexibility and efficiency of wavelength division multiplexing (WDM) and optical division multiplexing (OTDM) networks by using wavelength multicasting technique. Practically, at a photonic gateway, for the interconnection between WDM and OTDM networks, an nonreturn-to-zero (NRZ)-to-return-to-zero (RZ) waveform conversion is necessary due to the popular utilization of NRZ and RZ formats in WDM and OTDM networks, respectively. Moreover, if the waveform conversion combines with wavelength multicasting, multiple RZ signals will be generated, resulting in an increase of the throughput of network and the flexibility of wavelength assignment. A desirable stage after these conversions is to aggregate the higher bit-rates OTDM signals based on these lower bit-rates multicast RZ signals. The pulsewidth is one of the parameters to determine the bit-rates of OTDM signals. Therefore, to achieve the aggregate OTDM signals with flexible bit-rates adapting to specific network demand, it is necessary to manage the pulsewidth in a wide tuning range. In the first work, a NRZ data signal is injected into an highly nonlinear fiber (HNLF)-based four-wave mixing (FWM) switch with four RZ clocks compressed by a Raman amplification-based multiwavelength pulse compressor (RA-MPC).The pulsewidth of four multicast RZ signals is adjusted in a continuously large range from 12.17 to 4.68 ps by changing Raman pump power of RA-MPC. In addition, the sampling of optical signal waveform is necessary to monitor signals in optical network. The signals can always be analyzed off-line by capture-and-process-later techniques. However, it is challenging that these techniques are not compatible with instantaneous amplitude changes of signals as well as capturing the details and singular manners such as transient events which need real-time processing. Therefore, in the second work, an effort to characterize the waveform of signal in real-time using wavelength multicasting technique with multiwavelength sampling short-width pulses which are on the order of a few picoseconds is implemented. Using the short pulsewidths of the sampling pulses, it is possible to sample the signal precisely because its waveform does not change significantly in the sampling time. An all-optical waveform sampling of NRZ and RZ on-off-keying (OOK) signals is focused. The 4x10 GHz WDM sampling pulses are compressed with the pulsewidth which are less than 3 ps by RA-MPC and then interact with the input signal under test using FWM effect in an HNLF. Four obtained sampled signals result in a sampling rate of 40 GSample/s, therefore, the reconstructed waveforms are well-matched with the input signal waveforms. Moving to the phase-modulated signals, especially RZ-differential phase shift keying (DPSK) signal, it is attractive for RZ-DPSK signal due to its robust tolerance to the effects of some fiber nonlinearities, and the support of high spectral efficiency. Moreover, all-optical pulse compression has been widely investigated as one of the key elements to enable high bit-rate signals overcoming electronics limits. So far, pulse compression has often used before data modulation at the transmitter to generate high bit-rate signals. Our work, on the other hand, implements the pulse compression for RZ-DPSK signal for inline applications. A useful inline application of the data pulse compression is to generate an aggregate high-speed data rate based on optical time multiplexing of many channels with lower-speed data rates. The higher bit-rates of aggregate signals depend on the pulsewidths of lower bit-rate signals. Therefore, the compression of an inline 10 Gb/s RZ-DPSK signal using a distributed Raman amplifier-based compressor (DRA-PC) is done. The RZ-DPSK signal with pulsewidth of 20 ps after 30 km standard single mode fiber (SSMF) transmission is compressed down to in picoseconds duration such as 12, 7.0, and 3.2 ps. The pulse compression of the inline signal is applied in two works. In the first work, a compressed signal with the pulsewidth of 3.2 ps is multiplexed to a 40 Gb/s OTDM signal and then successfully de-multiplexed. The second application is wavelength multicasting of the inline compressed RZ-DPSK signal to get multicast signals with short-pulsewidths for increasing the throughput of network and wavelength resource. The DRA-PC compresses the inline RZ-DPSK signal with the obtained pulsewidths of 12, 7.0, and 3.2 ps which then interact with two continuous waves (CWs) in an HNLF-based FWM switch. Thus, the pulsewidths of the multicast signals were compressed down to 12.5, 7.89, and 4.27 ps. Finally, for networking between OTDM and WDM networks, an OTDM-to-WDM conversion is crucially required. However, it is given that in some cases, different WDM channels are expected to be generated in order to connect to each tributary of OTDM signal. In this work, a 20 Gb/s OTDM RZ-DPSK signal is converted to 4x10 Gb/s WDM RZ channels. One tributary of OTDM signal is converted to 2x10 Gb/s WDM RZ signals at two FWM products.電気通信大学201

Generation and optimization of picosecond optical pulses for use in hybrid WDM/OTDM networks

The burgeoning demand for broadband services such as database queries, home shopping, video-on-demand, remote education, telemedicine and videoconferencing will push the existing networks to their limits. This demand was mainly fueled by the brisk proliferation of Personal Computers (PC) together with the exceptional increases in their storage capacity and processing capabilities and the widespread availability of the internet. Hence the necessity, to develop high-speed optical technologies in order to construct large capacity networks, arises. Two of the most popular multiplexing techniques available in the optical domain that are used in the building of such high capacity networks, are Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM). However merging these two techniques to form very high-speed hybrid WDM/OTDM networks brings about the merits of both multiplexing technologies. This thesis examines the development of one of the key components (picosecond optical pulses) associated to such high-speed systems. Recent analysis has shown that RZ format is superior to conventional NRZ systems as it is easier to compensate for dispersion and nonlinear effects in the fibre by employing soliton-like propagation. In addition to this development, the use of wavelength tunability for dynamic provisioning is another area that is actively researched on. Self-seeding of a gain switched Fabry Perot laser is shown to one of the simplest and cost effective methods of generating, transform limited optical pulses that are wavelength tunable over very wide ranges. One of the vital characteristics of the above mentioned pulse sources, is their Side Mode Suppression Ratio (SMSR). This thesis examines in detail how the pulse SMSR affects the performance of high-speed WDM/OTDM systems that employ self-seeded gain-switched pulse sources

Techniques for Wideband All Digital Polar Transmission

abstract: Modern Communication systems are progressively moving towards all-digital transmitters (ADTs) due to their high efficiency and potentially large frequency range. While significant work has been done on individual blocks within the ADT, there are few to no full systems designs at this point in time. The goal of this work is to provide a set of multiple novel block architectures which will allow for greater cohesion between the various ADT blocks. Furthermore, the design of these architectures are expected to focus on the practicalities of system design, such as regulatory compliance, which here to date has largely been neglected by the academic community. Amongst these techniques are a novel upconverted phase modulation, polyphase harmonic cancellation, and process voltage and temperature (PVT) invariant Delta Sigma phase interpolation. It will be shown in this work that the implementation of the aforementioned architectures allows ADTs to be designed with state of the art size, power, and accuracy levels, all while maintaining PVT insensitivity. Due to the significant performance enhancement over previously published works, this work presents the first feasible ADT architecture suitable for widespread commercial deployment.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

A method for delivering spatio-temporally focused energy to a dynamically adjustable target along a waveguiding structure

It is possible to exploit the frequency-dependent velocity dispersion inherent to waveguiding structures to deliver spatio-temporally focused energy to a spatial target anywhere along the longitudinal extent of a waveguide. Such focusing of energy may have application to technologies as varied as nerve stimulation or chemical etching. A waveguide signal that effects this focused energy is conceptualized and derived. The spatial location of the target acted upon by the waveguide signal is demonstrated to be dynamically adjustable with a linear filtering step. Optimal parameters for waveguide signal generation are derived in the general case, allowing for application to a cross section of homogeneous waveguides. Performance is also considered in non-ideal, absorptive media. Numerical simulations are presented that indicate agreement with analytic results, and an evaluation of possible reduction to practice is presented

Waveform Conversion and Wavelength Multicasting with Pulsewidth Tunability Using Raman Amplification Multiwavelength Pulse Compressor

A combination of nonreturn-to-zero (NRZ)-to-return-to-zero (RZ) waveform conversion and wavelength multicasting with pulsewidth tunability is experimentally demonstrated. A NRZ data signal is injected into a highly nonlinear fiber (HNLF)-based four-wave mixing (FWM) switch with four RZ clocks compressed by a Raman amplification-based multiwavelength pulse compressor (RA-MPC). The NRZ signal is multicast and converted to RZ signals in a continuously wide pulsewidth tuning range between around 12.17 and 4.68 ps by changing the Raman pump power of the RA-MPC. Error-free operations of the converted RZ signals with different pulsewidths are achieved with negative power penalties compared with the back-to-back NRZ signal and the small variation among received powers of RZ output channels at a bit-error-rate (BER) of 10-9. The NRZ-to-RZ waveform conversion and wavelength multicasting without using the RA-MPC are also successfully implemented