KEYWORDS: Mode locking, Microwave radiation, Microsoft Foundation Class Library, Optoelectronics, Harmonic generation, Oscillators, Signal generators, Tunable filters, Pulse signals, Photonics
The optoelectronic oscillator (OEO) is a typical time-delay system with rich nonlinear dynamical characteristics. Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay, which makes it difficult to realize mode locking without additional phase-locking mechanisms. We have achieved, for the first time to our knowledge, a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale. In the experiment, the self-mode-locking OEOs with fundamental mode, second-order harmonic, and sixth-order harmonic were realized by adjusting the system parameters, all of which produced uniform square wave signals with tunable duty cycles, steep rising and falling edges, and periods of less than 20 ns. The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized. Furthermore, the experiment revealed the self-hybrid mode-locking OEO, which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity, demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals. The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation, offering promising applications in areas such as atomic clocks, radars, communications, and optoelectronic neural networks.
High-range resolution microwave radars are greatly important for automatic drive, security check, topographic mapping, etc. However, the generation and processing of broadband signals still challenge to state-of-the-art electronics. Besides, the broadband signal with a continuous spectrum is susceptible to interference from complex electromagnetic signals in the environment. Here, we present a microwave photonic radar with a sparse stepped-frequency chirp (SFC) signal, aiming to realize a high-range resolution detection with enhanced anti-jamming capability. With a recirculating frequency-shifting loop modulated by MHz radio frequency (RF) signal, a sparse SFC signal with a bandwidth of several GHz can be easily produced for radar target detection. In addition, microwave photonic de-chirping and compressive reconstruction are performed to achieve fast high-range resolution detection. Experimental results show that the proposed radar could provide a sub-centimeter level range resolution and a millimeter-level precision.
High-accuracy measurement of optical fiber transfer delay (OFTD) is essential to applications such as distributed optical fiber sensing, radio over fiber networks, and optically controlled phased array radars. However, it is difficult to simultaneously acquire high accuracy and high spatial resolution transfer delay distributed in different positions along the optical fiber path. Here, we propose an approach to measure the distributed OFTD based on optical stepped frequency chirp signal (OSFC) enabling sub-millimeter-level spatial resolution and sub-picosecond-level accuracy. Thanks to an optical frequency shifting loop, an OSFC signal with ultra-wide bandwidth of up to hundreds of GHz can be obtained. By coherent de-chirping and fusing the OSFC signal, high-accuracy and high-spatial-resolution distributed OFTD measurement can be achieved. In a proof-of-concept experiment, an OSFC signal with a bandwidth of up to 320 GHz was generated, achieving 0.29-mm spatial resolution and ±0.05 ps accuracy distributed OFTD measurement.
An optoelectronic oscillator (OEO) based on a double-Brillouin-frequency shifter (DBFS) is proposed and experimentally demonstrated. Two orders of stimulated Brillouin scattering are realized in the DBFS, and the narrow gain bandwidth of the second-order Stokes wave is utilized to implement the phase modulation to intensity modulation conversion. Only one laser is used to provide both signal and pump waves. A k-band microwave signal with a harmonic suppression ratio of 61 dB and a side-mode suppression ratio of 45 dB is experimentally generated, and 400-MHz tunability is realized by adjusting the laser wavelength. Because of the simple structure and high-frequency oscillation, the DBFS-based OEO can find applications for wireless communications and civil radar systems.
A novel broadband microwave frequency divider enabled by an optoelectronic oscillator (OEO) is proposed, which features large bandwidth, low noise and compact structure. The Mach-Zehnder modulator in the optoelectronic oscillator operates at the minimum transmission point to perform the optical carrier-suppressed double sideband modulation and avoid the free-running oscillation of the OEO. In a proof-of-concept experiment, the proposed frequency divider successfully divides the microwave frequency from 12 GHz to 6 GHz.
A wideband signal upconversion and phase shifting scheme based on a frequency tunable optoelectronic oscillator (OEO) are proposed and demonstrated. The OEO performs simultaneously tunable high-quality local oscillator (LO) signal generation, wideband frequency upconversion, and phase shifting within the whole 2π range. With the generated LO tuning from 9.549 to 11.655 GHz, wideband square signals are successfully upconverted to the X band. The phase of the upconverted signal is tuned from 0 to 360 deg. The phase noise of the oscillation signal is about −104 dBc/Hz at 10 kHz offset with or without the injected baseband signal.
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