We present a photonic based broadband DSSS system with photonic matched filter. The optical delay line and optical switch-based encoder/decoder is used in signal spreading/de-spreading. The simulation results show that the photonic matched filter could achieve fast acquisition of PN sequence.
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.
We report an optical vector network analysis (OVNA) based on optical suppressed carrier double-sideband (DSB) modulation and the Pound Drever Hall (PDH) technique. In this novel scheme, the optical carrier suppressed DSB modulation signal propagates through the high Q optical device, and then the double frequency of the driven radio frequency signal is detected, by which the frequency responses of the device can be accurately achieved. Comparing with the common DSB-based OVNA, by biasing the modulator at the minimum transmission point (MITP), the accuracy improvement can be realized since the errors caused by the even-order sidebands are eliminated. Moreover, the high stability of the proposed OVNA can also be achieved by using the PDH technique. In the proof-of-concept experiment, the magnitude and phase responses of the Fabry-Perot (FP) interferometer are realized with high accuracy when the modulation index is small. There is no repeated frequency response even if the test time is up to 30 minutes. The proposed scheme provides a novel strategy for high-accuracy frequency responses measurement, which can be potentially used in high Q optical devices characterization.
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.
A novel all-optical orthogonal frequency division multiplexing (AO-OFDM) scheme is proposed and demonstrated. Ultrashort optical pulses are used as samples for optical discrete Fourier transform (DFT) and the inverse DFT process. Different subcarrier channels can be parallelly processed by fiber Bragg gratings. A 20-Gb/s two subcarrier AO-OFDM experiment is carried out with narrowband filtering and optical cyclic postfixes (CP) inserted. Experimental results show that this scheme has good spectral efficiency. Furthermore, the received signals have better eye diagrams and bit error rate performance with the help of CP. This scheme can be used in high-speed optical transmission systems.
A pair of fiber Bragg grating (FBG) comb filters with different channel-spacings are
designed and fabricated to measure strain. The reflection peaks of one comb filter shift
when the strain changes. And the other comb filter is used as calibration standard. The
two comb filters have different channel-spacings, 1.00 nm and 0.95 nm, which can be
regard as the structure of vernier caliper. In our experiment, the strain resolution is about
41.7 με, with low demand of the demodulation devices. And it could be greatly improved
further in terms of Gaussian fitting technology of the peaks' intensity.
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