Yuan Bao

Nonlinearity mitigation for high-speed optical OFDM transmitters using digital pre-distortion

Optical orthogonal frequency-division multiplexing (OOFDM) signal is sensitive to nonlinear distortions induced by optical modulators. We propose and experimentally demonstrate a digital pre-distortion (DPD) algorithm to linearize the optical modulators including electro-absorption modulated lasers (EML) and Mach-Zehnder modulators (MZM) used in high-speed OOFDM transmitters. By using an adaptive DPD algorithm with a learning structure, the inverse transfer function of a modulator, which is based on a polynomial model, has been obtained. In the experiment, the performance improvements with and without considering the memory effects of the DPD model are illustrated. The two typical kinds of high-speed OOFDM signals with a bit rate up to 30-Gb/s have been implemented experimentally. The results show that the nonlinear distortion induced by optical modulators can be compensated by using the DPD algorithm to substantially improve the optical modulation index.

Advanced modulation formats for 400-Gbps short-reach optical inter-connection

Besides the long-haul optical networks covering over thousands of kilometers for backbone transmission, short reach optical networks (SR-ONs) are widely deployed in metro-area for aggregation and accessing. The SR-ONs include the metro optical transport networks (Metro-OTN), optical access networks or other optical inter-connection systems with even shorter distance. As predicted, the growing bandwidth demanding from SR-ONs will be much more than that from the long-haul optical networks in the near future. Besides, there are tremendous amounts of optical terminals and end-users in SR-ONs compared with the long-haul transmission systems and thus will induce large cost and huge energy consumption. So, the power and cost efficiency should be the key consideration for SR-ONs besides the transmission performance. To improve the power and cost efficiency in SR-ONs, advanced modulations and detection techniques based on low power, low cost and integrated optical modulators should be utilized. In this paper, different advanced modulation formats have been discussed. 56Gbps PAM4, 112Gbps poly-binary and 100Gbps DMT that can be used to realize 400-Gbps SR-ONs for different applications have also been demonstrated respectively. In addition, low-cost and low-power opto-electronic components suitable for SR-ONs, the impairments induced by all kinds of defects and bandwidth limitation of opto-electronic components and the corresponding compensation techniques based on DSP algorithms have also been discussed in the experiments.

Optical SEFDM System; Bandwidth Saving Using Non-Orthogonal Sub-Carriers

We propose and demonstrate a new optical spectrally efficient frequency division multiplexing (O-SEFDM) system, where non-orthogonal and overlapping sub-carriers are employed to provide higher spectral efficiency relative to optical-orthogonal frequency division multiplexing (O-OFDM). The O-SEFDM technique can increase spectral efficiency in both the electrical and optical domains. It is experimentally shown that for bandwidth saving up to 25%, we can achieve the same performance as O-OFDM. This is the first experimental verification of 25% optical faster than the Nyquist rate. Furthermore, for approximately the same spectral efficiency, 4QAM O-SEFDM outperforms standard 8QAM by 1.6 dB. It is experimentally shown that a lower-order modulation format can achieve a better performance by replacing a higher one.

Bidirectional Hybrid OFDM-WDM-PON System for 40-Gb/s Downlink and 10-Gb/s Uplink Transmission Using RSOA Remodulation

We experimentally demonstrate a bidirectional hybrid orthogonal frequency-division-multiplexing wavelength-division-multiplexing passive optical network (OFDM-WDM-PON) system. The proposed scheme uses OFDM signals occupying different portions of the available signal spectrum for uplink and downlink signal transmissions to reduce the effects of Rayleigh backscattering, chromatic dispersion, and power fading. A 40-Gb/s downlink signal transmission based on a single sideband modulated OFDM signal, and a 10-Gb/s uplink signal transmission using remodulation of a reflective semiconductor-optical-amplifier are realized.

Characterization of Passive Optical Components by DSP-Based Optical Channel Estimation

Passive optical components with fine structures require a characterization method with high-resolution and fast measurement speed. In this letter, we extend the digital signal processing-based optical channel estimation to measure their transfer functions, including the intensity and phase responses. In the experiment, by using 16384-point fast Fourier transform and a narrow line-width laser, we demonstrate sub-MHz resolution with a measurement speed bounded by the theoretical limit.

A distributed fiber vibration sensor utilizing dispersion induced walk-off effect in a unidirectional Mach-Zehnder interferometer

We propose and experimentally demonstrate a novel ultra-long range and sensitive distributed fiber vibration sensor. Only one unidirectional Mach-Zehnder interferometer (MZI) is employed in this scheme as the sensing element. In this sensor structure, we utilize chromatic dispersion-induced walk-off effect between the vibration signals sensed by two distributed feedback (DFB) lasers at different wavelengths to locate the vibration position. Vibration signals with frequencies up to 9 MHz can be detected and the spatial resolution of 31 m is achieved over 320 km of the standard single mode fiber. Monitoring multiple vibration sources can also be realized using this scheme.

High-resolution optical spectrum characterization using optical channel estimation and spectrum stitching technique

A technique is proposed to measure the high-resolution and wide-band characterization of amplitude, phase responses, and polarization property of optical components. This technique combines the optical spectrum stitching and optical channel estimation methods. Two kinds of fiber Bragg grating based Fabry-Perot cavities with ultrafine structures have been characterized based on this technique. By using 1024 point fast Fourier transform and a narrow linewidth, wavelength-tunable laser source, a frequency resolution of ~10 MHz is realized with an optical measurement range beyond 250 GHz.

Characterization of passive optical components with ultra-fast speed and high-resolution based on DD-OFDM.

The passive optical components with very fine structures in wavelength domain are very sensitive to the mechanical vibrations or thermal fluctuations. If the measurement speed is lower than the temperature and mechanical fluctuation, we cannot measure the dynamic characteristics of the optical components. In this paper, we propose and demonstrate a novel method with ultra-fast measurement speed and high-resolution based on optical channel estimation using direct-detected orthogonal frequency division multiplexing (DD-OFDM) signal, which can be used to measure the dynamic characteristics and fine structure of the passive optical components. In experiment, by using fast Fourier transform (FFT) and a low-cost electro-absorption modulated laser (EML), we can achieve the transfer function characteristics with 3.9 MHz resolution. Compared with the optical channel estimation using coherent OFDM signal reported before, the proposed measurement technique is cost-effective.

Optical spectrally efficient FDM system for electrical and optical bandwidth saving

A newly proposed optical-spectrally efficient frequency division multiplexing (O-SEFDM) system reduces the required communication spectrum by employing non-orthogonal and overlapping sub-carriers. This results in higher spectral efficiency relative to an equivalent optical-orthogonal frequency division multiplexing (O-OFDM) delivering the same data rate. O-SEFDM technique can save spectrums in both the electrical and optical domains. However, due to the loss of orthogonality, detection of O-SEFDM signals becomes more complicated. In this work, we employ a hybrid soft Iterative Detection (ID) together with fixed sphere decoder (FSD), concurrently optimizing performance and complexity. We show that for Bandwidth Compression Factor (BCF) of up to 25 percent, we can achieve the same performance as O-OFDM. This verifies Mazos rates of transmission 25 percent faster than the Nyquist rate. We report a 4QAM system occupying approximately the same bandwidth as that of an 8QAM with 1.6 dB improved error performance for the same transmission rate. The same system shows only minor power penalty (1 dB) relative to its 4QAM OFDM bit rate equivalent but with the advantage of 30% bandwidth saving. This study reports1 optical and electrical bandwidth saving with minor error performance degradation and paves the way for practical.

Ultra-fine optical spectrum microscope using optical channel estimation and spectrum fusion technique

Ultra-fine optical spectrum microscope is demonstrated based on optical channel estimation using coherent optical OFDM signals and optical spectrum fusion technique. The frequency resolution and measurement range of this technique are 10.7-MHz and 250-GHz respectively.