In recent years, attacks from pseudo base stations and other kinds of illegal wireless access have emerged one after another, and more and more telecom frauds are causing numerous property losses. There is an urgent need for real-time monitoring of the utilization of wireless spectrum, so as to identify illegal signals and take appropriate protective measures promptly. Traditional wireless spectrum monitoring solutions used to be narrow-band, and due to the insufficient capabilities of data acquisition equipment, the amount of equipment needed is huge, which will lead to the increase of cost, energy consumption and maintenance complexity. We need to transmit signals from various areas to a specific place, where there is a global and full-spectrum perception device that could process and analyze wide-band signals. Therefore, a wireless channel monitoring architecture is proposed and experimentally demonstrated. Based on microwave photonics, this architecture is consisted of an optical-wireless converged network and an all-optical channelized receiver. The frequency error of the channelized receiver is no more than 50 kHz, meeting the requirement for signal analysis in 5G communications. The experiment results show that the proposed architecture could effectively monitor collected signals of different channels.
A dither-free bias controller, locking a Mach-Zehnder modulator (MZM) working at the null bias point, is proposed and experimentally demonstrated. In microwave photonics systems, the null bias point of the MZM enables double sideband with suppressed carrier (DSB-SC) modulation utilizing for many applications. The output optical power of the MZM is monitored to dynamically lock the bias point using the hill-climbing algorithm. The bias controller is a feedback circuit consisting of a monitor photodetector (PD), a gain-controlled trans-impedance amplifier (TIA), a digital-to-analog converter (ADC), an analog-to-digital converter (DAC), an operational amplifier, and a microcontroller unit (MCU). To improve the accuracy of data processing, an adaptive magnification factor of the detection signal is added to the algorithm. In the experiment, the drift degree of the bias point with and without the bias controller has been compared in 150 minutes. The stability of the proposed bias controller is verified by using a 10 GHz RF signal. With the proposed controller, the bias point of MZM is stabilized within ±1 degree, and the optical carrier suppression ratio reaches more than 25dB. No dither is introduced into the spectrum hence the realization of frequency multiplication with no spurs, which has a broad prospect in microwave photonics.
An optoelectronic oscillator (OEO) is a microwave photonic system that produces microwave signals with ultralow phase noise using a high-quality-factor optical energy storage element. This type of oscillator is desired in various practical applications, such as communication links, signal processing, radar, metrology, radio astronomy, and reference clock distribution. Recently, new mode control and selection methods based on Fourier domain mode-locking and parity-time symmetry have been proposed and experimentally demonstrated in OEOs, which overcomes the long-existing mode building time and mode selection problems in a traditional OEO. Due to these mode control and selection methods, continuously chirped microwave waveforms can be generated directly from the OEO cavity and single-mode operation can be achieved without the need of ultranarrowband filters, which are not possible in a traditional OEO. Integrated OEOs with a compact size and low power consumption have also been demonstrated, which are key steps toward a new generation of compact and versatile OEOs for demanding applications. We review recent progress in the field of OEOs, with particular attention to new mode control and selection methods, as well as chip-scale integration of OEOs.
Erbium-doped fiber amplifier (EDFA) is widely used in the optical communication system, and it is usually used as a linear amplifier in most scenarios. However, in analog photonic links, we observe the nonlinear spurs caused by EDFA. In the analog photonic link, the dynamic range is one of the most important characteristics while the nonlinear spurs set the upper limit of input signal power. We study the nonlinearity caused by EDFA in analog photonic links when the input power of EDFA fluctuates at a low frequency. We have theoretically and experimentally studied the characteristics of the nonlinear spurs caused by EDFA.
A novel photonics channelization spectrum stitching technique in a dual-OFC-based photonic channelizer for receiving a wide-band signal is proposed. By studying the slow and rapid variant characteristics of the channel differences separately, all the channels’ responses are precisely estimated and stitched in the frequency domain. A proof-of-concept experiment is performed and signals with 3 GHz bandwidth are recovered with an SNR loss of 1.01 dB.
We proposed a mode-locked all-polarization-maintaining erbium-doped fiber laser base on a nonlinear amplifying loop mirror (NALM). The laser can generate 1.6 ps pulses at 1550 nm with the energy of 1 nJ that can be compressed down to 100 fs with the compressor outside the cavity. The repetition rate of the output pulse is 12MHz. Such configuration of laser is easier controlled and self starting long term operation, and is highly desirable for industrial applications, such as micro-machining.
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.
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