Direct-detection Microwave Photonic Links Research Articles

Broadband Spurious-Free Dynamic Range Expander for Microwave Photonic Links Based on Optical Distortion Control

A wideband spurious-free dynamic range (SFDR) expander that uses optical distortion control technique is proposed in this study for microwave photonic links. A third-order intermodulation distortion (IMD3), which is sensed by extracting the modulated optical carrier, will be monitored and attenuated adaptively by controlling the optical power injected into the photodetector. Therefore, the IMD3 can be suppressed below the noise floor (NF), and the SFDR can be expanded further. In addition, the sensing signal stems from the modulated optical carrier, which is independent of radio frequency. Thus, our system is essentially a broadband system whereby the dynamic range expander is applied to classical intensity modulation and direct detection microwave photonic link. Experimental results demonstrate that the SFDR is improved from 46.84 to 67.98 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> with a -70 dBm noise power. The SFDR can even reach 150 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> with a bandwidth of 36 GHz with a 1 Hz NF.

Dynamic-Range Enhancement for a Microwave Photonic Link Based on a Polarization Modulator

A microwave photonic link (MPL) with an increased spurious-free dynamic range (SFDR) using a polarization modulator is proposed and experimentally demonstrated. The fundamental concept of the proposed approach to increase the SFDR is to reduce the third-order intermodulation distortion (IMD3) and to improve the modulation efficiency by partial carrier suppression, which is realized here by producing two channels of intensity-modulated signals with one having only the upper sidebands and the other having the optical carrier that is partially suppressed and the upper sidebands using two optical filters. If the optical powers into the two channels are kept identical, the combination of the two signals at a photodetector will fully cancel the IMD3 terms. In addition, the modulation efficiency is optimized due to the partial carrier suppression, which also leads to an increased SFDR. The proposed MPL is theoretically studied and experimentally demonstrated. An SFDR of ${\hbox{123.48 dB}}\cdot{\hbox{Hz}}^{2/3}$ at a noise floor of $-{\hbox{166 dBm/Hz}}$ is achieved, which is 15 dB higher than that of a regular intensity-modulation and direct-detection MPL.

基于微波光链路对激光器相对强度噪声的测量

This paper proposes and presents the experimental veri cation of a new method for measuring the relative intensity noise (RIN) of a laser. As an important microwave-photonic link (MPL) module, the RIN of a laser contributes to the systematic noise of the intensity modulation and direct detection MPL, thus increasing the phase noise of the transmitted carrier. This physical phenomenon is used to measure the RIN of a laser. Compared with previous methods, the proposed technique is less strongly in uenced by near-DC colored noise. Experimental results show that the MPL-based measurement method proposed in this paper is more accurate in the near-DC frequency band than traditional approaches.

Tunable microwave photonic transversal filter

We present a tapped tunable delay line filter for radiofrequency (RF) photonic filtering applications, capable of rapid tunability over a wide RF bandwidth limited only by the optical components’ losses, while maintaining independence from polarization state. Multiple fiber taps with contrasting dispersion slopes are used in intensity-modulated direct detection microwave photonic links. A temporal delay is generated between the signals within each arm of the link. Once a signal is received using balanced differential detection, nulls are generated as a function of the laser's operating wavelength. Tuning of the laser allows for a rapid shifting of the nulls in the RF spectrum to dynamically mitigate co-site interference. Through this method we demonstrate the potential for rapid tunability over the RF spectrum by the variation of the operating wavelength of the optical carrier.

Noise figure of microwave photonic links operating under large-signal modulation and its application to optoelectronic oscillators.

The noise performance of intensity-modulation direct-detection microwave photonic links (MWPL) operating under large-signal conditions has been studied in this paper. A sinusoidal signal plus narrowband white Gaussian noise is applied at the radio frequency input of the link, and the output spectrum is derived using a nonlinear analytical approach. We show that the output SNR can be severely affected by the interaction of signal and noise due to the nonlinearity of the MWPL combined with the large input modulating signal. It is shown that the large-signal noise figure (NF) of an MWPL depends on the input power, a dependence that is not readily apparent under small-signal conditions, due to two unavoidable issues appearing in the large-signal conditions: (1) the link power gain is a function of its input power, and (2) the link power gain is not the same for the signal and noise due to the capture effect. We also have observed that if shot noise or laser relative intensity noise (RIN) is the dominant source of noise, link large-signal NF increases as the input signal power increases. We have shown that, when the MWPL is operating in the linear regime, our theoretical predictions approach the already published results on small-signal NF, which are verified by experimental data. We have shown that large-signal NF affects the noise performance of optoelectronic oscillators because they contain MWPLs at saturation.

Digital Phase Noise Cancellation for a Coherent-Detection Microwave Photonic Link

An intensity-modulation and coherent-detection (IM/CD) microwave photonic (MWP) link with digital signal processing (DSP)-based phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, the optical carrier is intensity-modulated by a radio-frequency (RF) signal and sent to a coherent receiver over a single-mode fiber (SMF). At the receiver, the intensity-modulated optical signal is coherently detected using a local oscillator (LO) laser source. The phase noise introduced by both the transmitter laser source and the LO laser source is converted into an amplitude noise at the output of the coherent receiver, which would be added to the in-phase component and quadrature component from the coherent receiver. The noise can be completely cancelled by summing the squared magnitudes of the in-phase and quadrature components from the coherent receiver, which can be implemented using a DSP unit. Error-free transmission of a 1.25-Gb/s quadrature phase shift keying modulated RF signal with a center frequency of 1.6 GHz over 25-km SMF is experimentally demonstrated. For a signal with a bit rate of 834 Mb/s, the receiver sensitivity can reach ${-}{\rm 24.5}~{\rm dBm}$ which is 20 dB better than that based on an intensity-modulation and direct-detection MWP link.

Theoretical investigation of the capture effect in intensity-modulation direct-detection microwave photonic links

We introduce the capture effect concept in microwave photonic links (MWPLs) for the first time to our knowledge. The capture effect or the small-signal suppression is the change in the amplitude ratio of the two signals between input and output of the intensity-modulation direct-detection (IMDD) MWPLs. An analytical explanation of the performance of external IMDD MWPLs due to the effects of nonlinearity combined with sum of several input sinusoidal signals is given. We have investigated the suppression of a weaker signal in these links. General analytic expression for the small-signal suppression is derived using a nonlinear analytical approach. We show that the small-signal suppression is quite dependent on the input back-off, the power ratio of input signals, and on the number of input sinusoidal signals. The theoretical maximum possible signal suppression was found to be 6 dB. This analytical asymptotic value is verified by numerical results. We show the influence of the capture effect of the nonlinear MWPL on the optoelectronic oscillator operation that is verified by experimental data in the literature that has already been published.

A photonic chip based frequency discriminator for a high performance microwave photonic link

We report a high performance phase modulation direct detection microwave photonic link employing a photonic chip as a frequency discriminator. The photonic chip consists of five optical ring resonators (ORRs) which are fully programmable using thermo-optical tuning. In this discriminator a drop-port response of an ORR is cascaded with a through response of another ORR to yield a linear phase modulation (PM) to intensity modulation (IM) conversion. The balanced photonic link employing the PM to IM conversion exhibits high second-order and third-order input intercept points of + 46 dBm and + 36 dBm, respectively, which are simultaneously achieved at one bias point.