A digital signal processing (DSP) scheme based on Volterra equalizer (VE) combined with adaptive noise-whitening post-filter and maximum likelihood sequence detection (MLSD) is proposed to mitigate nonlinear impairments in vertical-cavity surface-emitting lasers (VCSEL) multimode fiber (MMF) system. Successfully transmission of 108 Gb/s, 100 Gb/s and 60 Gb/s 4-ary pulse amplitude modulation (PAM4) signal over 5 m, 160 m and 460 m OM3-MMF is demonstrated below the 7% overhead hard-decision forward error correction (HD-FEC) bit error rate (BER) threshold by using a 20-GHz class VCSEL at 850 nm. Linear pre-equalization is applied to mitigate severe bandwidth limitation of the system. Our experimental results show that the scheme can well mitigate modulation nonlinearity induced by VCSEL and fiber nonlinearity induced by MMF. The BER decreases about two order of magnitude compared to linear equalizer after 100 m OM3-MMF transmission for 100 Gb/s PAM4 signal.
We have experimentally demonstrated a direct-detection (DD) 112-Gbit/s 16 quadrature amplitude modulation (QAM) transmission over single-span 140-km standard single mode fiber (SSMF) with Kramers-Kronig receiver and a sparse I/Q Volterra filter (VF). The sparse I/Q VF was proposed in our previous work and it is based on dual-input real-valued Volterra series and ℓ1-regularization method. In this paper it is used for compensating the nonlinear distortion in a short-reach DD optical 16-QAM signal transmission system. In back to back case, sparse I/Q VF represents the great compensation ability to the saturation effect of the electrical amplifiers and the nonlinear sinusoidal transfer function of I/Q modulator. It provides around 1-order magnitude improvement of BER when reducing 84% complexity from full I/Q VF. For fiber transmission case, sparse I/Q VF can mitigate the fiber nonlinearity effectively and it achieves single-span 140-km transmission at hard-decision forward error correction (HD-FEC) threshold of 3.8 ×10-3 with less than half complexity of full I/Q VF. Besides, optical signal noise ratio (OSNR) performance at 120 km is measured and sparse I/Q VF reduces the required OSNR at HD-FEC threshold by 1.3 dB. In a word, we investigate the performance of sparse I/Q VF in short-reach optical 16-QAM transmission system and sparse I/Q VF reveals its potential in the growing short-reach applications, such as data center inter-connection and metropolitan area network.
We propose and demonstrate a multifunction-stabilized photonic link, which is capable of transmitting wideband time signal and stable frequency signal between the central station and the remote end bidirectionally over a single-fiber link. Experimentally, 3.95-GHz frequency signal and pulsed time signal are delivered to the remote end with frequency stability of 3.6×10−16 and time jitter of 0.45 ps at 4000 s average time, respectively. Also, a downlink radio frequency signal is transferred from the remote end back to the central station with suppressed delay variation.
By cascading two standard Mach–Zehnder modulators (MZMs), we propose and demonstrate a scheme to effectively eliminate the cross-modulation distortion (XMD), which results from the out-of-band interference in multicarrier intensity-modulation direct-detection (IMDD) analog photonic links. When the bias angle of the cascaded modulator is specifically designed, the XMDs, both from the photonic link itself and from the nonlinear electrical amplifiers, are well suppressed. Our proposal is theoretically analyzed, and the performance of the cascading system is experimentally demonstrated. A suppression ratio of more than 30 dB is achieved by the cascading scheme. By using the high-performance pre- and postamplifiers, the measured link gain and noise figure are improved by 62 and 32 dB, respectively.
KEYWORDS: Analog electronics, Digital signal processing, Modulators, Intermodulation, Data communications, Signal processing, Nonlinear optics, Optical engineering, Broadband telecommunications, Telecommunications
The multicarrier analog photonic links suffer from both the traditional third-order intermodulation distortions (IMD3) and the cross-modulation distortions (XMDs), severely limiting the dynamic range of the links. This paper proposes and demonstrates an effective technique based on a single modulator and photodetector to simultaneously realize the downconversion and receiving of multiple radio frequency signals, as well as suppress the nonlinearities, including the IMD3 and XMD. In the scheme, the nonlinear compensation information is directly obtained from hardware then the distortion compensation is carried out in the digital domain. Experimental results show that the XMD and IMD3 distortions are suppressed with 36.6 and 25.8 dB, respectively, and the link dynamic range is improved by 25 dB, preventing the degradation of the dynamic range of the link. Moreover, the structure of our scheme can eliminate the stringent requirement for hardware.
The envisioned C-RAN concept in wireless communication sector replies on distributed antenna systems (DAS) which consist of a central unit (CU), multiple remote antenna units (RAUs) and the fronthaul links between them. As the legacy and emerging wireless communication standards will coexist for a long time, the fronthaul links are preferred to carry multi-band multi-standard wireless signals. Directly-modulated radio-over-fiber (ROF) links can serve as a lowcost option to make fronthaul connections conveying multi-band wireless signals. However, directly-modulated radioover- fiber (ROF) systems often suffer from inherent nonlinearities from directly-modulated lasers. Unlike ROF systems working at the single-band mode, the modulation nonlinearities in multi-band ROF systems can result in both in-band and cross-band nonlinear distortions. In order to address this issue, we have recently investigated the multi-band nonlinear behavior of directly-modulated DFB lasers based on multi-dimensional memory polynomial model. Based on this model, an efficient multi-dimensional baseband digital predistortion technique was developed and experimentally demonstrated for linearization of multi-band directly-modulated ROF systems.
We demonstrate a photonic-assisted broadband radio frequency (RF) channelization scheme based on dual coherent optical frequency combs (OFCs). The advantages include coarse optical alignment requirement, ideal rectangular frequency response in each channel without any ultra-narrow optical filters, and digitalized output for further processing. Meanwhile, the channel frequency response and crosstalk of the scheme are also evaluated experimentally.
In order to integrate local ultra-wideband (UWB) environment into fixed wired networks or wireless wide-area
infrastructures and eliminate the high cost of microwave electrical circuits or devices, UWB-over-fiber systems have
emerged to exploit the advantages offered by optical fiber. We experimentally demonstrate a photonic UWB monocycle
pulse generation and pulse position modulation (PPM). The UWB monocycle pulse with inverse polarity can be
generated based on the two differently-biased Mach-Zehnder modulators, subsequently the PPM of the UWB monocycle
pulse is realized through the nonlinear polarization rotation (NPR) arising in a semiconductor optical amplifier (SOA).
We proposed and experimentally demonstrated an effective procedure to generate and maintain a linear output state of
polarization (SOP) during the course of the nonlinear polarization rotation (NPR) of an SOA. High performance of
polarization switching can be achieved. The proposed scheme is more preferable for the UWB-over-fiber applications
due to its compact size and high stability.
In this paper, a novel method to generate both monocycle and doublet UWB pulses is demonstrated, where pulse shape
modulation(PSM) can be easily implemented. Only two wavelengths and two modulators (one dual-in dual-out
modulator) are applied to achieve PSM. The data driving the first modulator is set to be 250Mbit/s 107-1 pseudo-random
bit sequence (PRBS). The 1GHz pulse pattern is synchronised with the data. The electrical spectrum of the signals
processes the centre frequency of 4GHz and -10dB bandwidth of 5.9GHz. The fractional bandwidth is about 147.5%,
which matches the FCC standard.
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