Microwave Photonic Signal Research Articles

Narrow-linewidth microcavity Brillouin laser based on pump-locked high-Q silica microsphere resonator

Microcavity-based Brillouin lasers are promising high-performance light sources for integrating photonics and optoelectronics. One method to lock the pump light frequency is to utilize a complex system with optoelectronic feedback, which requires a high-cost narrow-linewidth pump laser and limits the application of microlasers in integrated optoelectronic systems. Another method reported recently is all-optical feedback to achieve the locking of microcavity laser. We propose to utilize Rayleigh scattering of microcavities to lock the frequency of the pump laser to the resonant frequency of the Brillouin laser microcavity with the all-optical method. While compressing the linewidth of the pump laser, it can greatly improve the long-term stability of the optically pumped microcavity Brillouin laser. In the experiment, the linewidth of the semiconductor pump laser is compressed from the MHz level to the kHz level. The microcavity Brillouin laser achieves an ultra-narrow intrinsic linewidth of 100 Hz, with an ultra-low frequency noise of 35 Hz2/Hz. The constructed microlaser obtains a locking time up to 1 h, which does not require any temperature control or vibration isolation of the laser system. This work demonstrated an optically pump-locked microcavity Brillouin laser, which provides a stable and reliable low-cost experimental platform for ultra-narrow-linewidth lasers, precision laser sensors, microwave-photonic signal synthesizer, and optomechanical systems.

Two-dimensional nanoscale position sensing using microwave photonic signal induced by period-one dynamics of semiconductor laser

Two-dimensional nanoscale position sensing using microwave photonic signal induced by period-one dynamics of semiconductor laser

Electrically interfaced Brillouin-active waveguide for microwave photonic measurements

New strategies for converting signals between optical and microwave domains could play a pivotal role in advancing both classical and quantum technologies. Traditional approaches to optical-to-microwave transduction typically perturb or destroy the information encoded on intensity of the light field, eliminating the possibility for further processing or distribution of these signals. In this paper, we introduce an optical-to-microwave conversion method that allows for both detection and spectral analysis of microwave photonic signals without degradation of their information content. This functionality is demonstrated using an optomechanical waveguide integrated with a piezoelectric transducer. Efficient electromechanical and optomechanical coupling within this system permits bidirectional optical-to-microwave conversion with a quantum efficiency of up to −54.16 dB. Leveraging the preservation of the optical field envelope in intramodal Brillouin scattering, we demonstrate a multi-channel microwave photonic filter by transmitting an optical signal through a series of electro-optomechanical waveguide segments, each with distinct resonance frequencies. Such electro-optomechanical systems could offer flexible strategies for remote sensing, channelization, and spectrum analysis in microwave photonics.

Versatile quantum microwave photonic signal processing platform based on coincidence window selection technique

Versatile quantum microwave photonic signal processing platform based on coincidence window selection technique

Flexible and reconfigurable integrated optical filter based on tunable optical coupler cascaded with coupled resonator optical waveguide.

Reconfigurable optical filter can satisfy diverse filtering requirements in different application scenarios and shorten development cycle. However, it is still challenging to achieve multi-functional filtering richness with high performance. Here, based on a tunable optical coupler cascaded with a coupled resonator optical waveguide (CROW), a highly flexible and reconfigurable integrated optical filter is proposed and demonstrated on the low-loss silicon nitride platform. Both single injection and double injection configurations can be deployed to obtain rich spectral responses. For the single injection configuration, flat-top bandpass filter was experimentally achieved, whose shape factor could be as low as 1.648 and extinction ratio (ER) can be 37.5 dB with a bandwidth tuning range from 2.12 to 4.01 GHz. For the double injection configuration, Lorentz, triangular, sinusoidal, square, tangent-like, and interleaver spectral responses have been reconfigured by controlling seven phase shifters. Moreover, both single and double free spectral ranges (FSR) can be obtained for a fixed ring perimeter in the double injection configuration. The measured ER for the notch filter of Lorentz responses with double FSR is 36.8 dB. We believe that the proposed device has great potential for reconfigurable photonic filtering and microwave photonic signal processing.

Tailorable ITO thin films for tunable microwave photonic applications

Tunability is a fundamental prerequisite for functional devices and forms the backbone of reconfigurable microwave photonic (MWP) signal processors. In this paper, we explore the use of indium tin oxide (ITO) thin films, notable for their combination of optical transparency and electrical conductivity, to provide tunability for integrated MWP devices. We study the impacts of post-thermal annealing on the structural, electrical, and optical properties of ITO films. The annealed ITO microheater maintains a low total insertion loss of just 0.1 dB while facilitating the tunability of the microring across the entire free spectral range (FSR) using less than half the voltage required by its non-annealed counterpart. Furthermore, the post-annealed ITO film exhibits a 30% improvement in response time, enhancing its performance as an active voltage-controlled microheater. Leveraging this advantage, we employed the post-annealed device to demonstrate continuous tunable radio frequency (RF) phase shifts from 0–330° across a frequency range spanning 15 GHz to 40 GHz with only 5.58 mW of power. The flexibility in modifying the ITO thin film properties effectively bridges the gap between achieving low-loss and high-speed thermo-optic based microheaters.

Slow-light-enhanced Brillouin scattering with integrated Bragg grating.

Advancements in photonic integration technology have enabled the effective excitation of simulated Brillouin scattering (SBS) on a single chip, boosting Brillouin-based applications such as microwave photonic signal processing, narrow-linewidth lasers, and optical sensing. However, on-chip circuits still require large pump power and centimeter-scale waveguide length to achieve a considerable Brillouin gain, making them both power-inefficient and challenging for integration. Here, we exploit the slow-light effect to significantly enhance SBS, presenting the first, to the best of our knowledge, demonstration of a slow-light Brillouin-active waveguide on the silicon-on-insulator (SOI) platform. By integrating a Bragg grating with a suspended ridge waveguide, a 2.1-fold enhancement of the forward Brillouin gain coefficient is observed in a 1.25 mm device. Furthermore, this device shows a Brillouin gain coefficient of 1,693 m-1W-1 and a mechanical quality factor of 1,080. The short waveguide length reduces susceptibility to inhomogeneous broadening, enabling the simultaneous achievement of a high Brillouin gain coefficient and a high mechanical quality factor. This approach introduces an additional dimension to enhance acousto-optic interaction efficiency in the SOI platform and holds significant potential for microwave photonic filters and high spatial resolution sensing.

Filterless Frequency Octupling Microwave Photonic Phase Shifter Based on Cascaded Mach–Zehnder Modulators

ABSTRACT In this paper, we propose a microwave photonic phase shifter (MPPS) without optical filtering to generate a frequency octupling microwave photonic (MWP) signal with 360º phase-shifting based on two cascaded Mach–Zehnder modulators (MZMs). Theoretical analysis is given to realize the high-quality frequency octupling MPPS by eliminating the undesired optical carrier and sidebands using an optical coupler (OC). Simulations show that the generated MMW signal has an electrical spurious suppression ratio (ESSR) higher than 33 dB. The impact of the non-ideal modulation index, extinction ratio, and phase drift is discussed and analyzed. Furthermore, the influence of different center frequencies, power, and linewidths of the laser is also analyzed.

High-speed data transmission based on mode-locked optical frequency comb

<sec>With the rapid development of emerging technologies such as multimedia services, live broadcasting, video conferencing, and high-definition television, traditional radio frequency communication is unable to meet people 's growing demand for communication capacity and transmission rate. In recent years, optical communication has received extensive attention from the industrial and scientific communities due to its advantages of large bandwidth, high speed, low power consumption, light weight, and strong anti-interference ability. As an emerging light source, the optical frequency comb (OFC) has a wide spectral range, multi-wavelength, high stability, and good phase coherence, providing a new idea for studying microwave signals with simple system structure, strong tunability and high frequency stability. At the same time, the multi-optical mode characteristics of OFC are compatible with the current communication system based on wavelength division multiplexing technology. Hundreds of laser arrays in a traditional communication system can be replaced by only one laser, which greatly reduces the power consumption of the system.</sec><sec>Combining the above advantages, in this paper, a large-scale parallel high-speed optical communication system based on mode-locked OFC is proposed. The linewidth of the OFC locked to the rubidium atomic clock can reach 1 Hz, which is sufficient to support the transmission of high-order modulation signals. The electro-optic modulators are used to adjust the amplitude and phase of each optical mode of the mode-locked OFC and self-coherently map to the RF domain. The high-speed high-order modulation signal with coded information is obtained by frequency screening through a narrow-band filter. The communication capability of the microwave photonic modulation signal in the 16 quadrature amplitude modulation (QAM) format is verified by simulation. The 16QAM communication with the rate of 2, 6, and 14 Gbit/s is realized by using the photonic microwave signal on the 100 m space optical link, and the bit error rate (BER) is less than 10<sup>–6</sup>. The proposed large-scale parallel optical communication system based on mode-locked OFC can achieve high-speed information transmission with a compact system structure, which is suitable for inter-satellite communication, emergency communication, military communication and other fields.</sec>

Analyzing the Performance of Microcomb Based Microwave Photonic Signal Processors with Different Signal Waveforms

Analyzing the Performance of Microcomb Based Microwave Photonic Signal Processors with Different Signal Waveforms

Processing Accuracy of Microcomb-Based Microwave Photonic Signal Processors for Different Input Signal Waveforms

Microwave photonic (MWP) signal processors, which process microwave signals based on photonic technologies, bring advantages intrinsic to photonics such as low loss, large processing bandwidth, and strong immunity to electromagnetic interference. Optical microcombs can offer a large number of wavelength channels and compact device footprints, which make them powerful multi-wavelength sources for MWP signal processors to realize a variety of processing functions. In this paper, we experimentally demonstrate the capability of microcomb-based MWP signal processors to handle diverse input signal waveforms. In addition, we quantify the processing accuracy for different input signal waveforms, including Gaussian, triangle, parabolic, super Gaussian, and nearly square waveforms. Finally, we analyse the factors contributing to the difference in the processing accuracy among the different input waveforms, and our theoretical analysis well elucidates the experimental results. These results provide guidance for microcomb-based MWP signal processors when processing microwave signals of various waveforms.

Silicon-Organic Hybrid Electro-Optic Modulator and Microwave Photonics Signal Processing Applications.

Electro-optic modulator (EOM) is one of the key devices of high-speed optical fiber communication systems and ultra-wideband microwave photonic systems. Silicon-organic hybrid (SOH) integration platform combines the advantages of silicon photonics and organic materials, providing a high electro-optic effect and compact structure for photonic integrated devices. In this paper, we present an SOH-integrated EOM with comprehensive investigation of EOM structure design, silicon waveguide fabrication with Slot structure, on-chip poling of organic electro-optic material, and characterization of EO modulation response. The SOH-integrated EOM is measured with 3 dB bandwidth of over 50 GHz and half-wave voltage length product of 0.26 V·cm. Furthermore, we demonstrate a microwave photonics phase shifter by using the fabricated SOH-integrated dual parallel Mach-Zehnder modulator. The phase shift range of 410° is completed from 8 GHz to 26 GHz with a power consumption of less than 38 mW.

Wideband‐Tunable on‐Chip Microwave Photonic Filter with Ultrahigh‐Q U‐Bend‐Mach–Zehnder‐Interferometer‐Coupled Microring Resonators

AbstractMicrowave photonic filters are a key fundamental subsystem in microwave photonics. A wideband‐tunable on‐chip microwave photonic filter (MPF), which monolithically integrates a dual‐drive Mach‐Zehnder modulator (DDMZM), is proposed with ultrahigh‐Q Mach–Zehnder‐interferometer (MZI)‐coupled microring resonators (MRRs) in cascade and a high‐speed photodetector (PD). It is the first time an MPF has been developed by introducing U‐bend‐MZI‐coupled MRRs whose resonance wavelength and 3 dB bandwidth are tunable. In particular, the ultrahigh‐Q U‐bend‐MZI‐coupled MRRs are realized with ultralow loss broadened silicon waveguides based on a modified Euler curve. For the fabricated U‐bend‐MZI‐coupled MRRs, which have an effective radius of 29 µm, the Q‐factor is as high as 1.3 × 106, while the free spectral range (FSR) is as large as 113 GHz. Furthermore, the introduction of two cascaded U‐bend‐MZI‐coupled MRRs enables the MPF to have flat‐top bandstop (or bandpass) responses with a high extinction ratio (ER) of 20–35 dB as well as a tunable 3 dB bandwidth of 0.15–3 GHz. Meanwhile, the central frequency can be tuned thermally from 2 to 40 GHz. The work provides a feasible route to realize a compact, high spectral resolution, and wideband‐tunable MPF on silicon, enabling a range of RF applications from wireless communication to flexible high‐frequency microwave photonic signal processing.

Tunable bandpass microwave photonic filter with largely reconfigurable bandwidth and steep shape factor based on cascaded silicon nitride micro-ring resonators.

Bandpass microwave photonic filter (MPF) can be achieved based on the well-known phase to intensity conversion method by using phase modulation and single micro-ring resonator (MRR) notch filter. Since MRR could introduce residual phase in handling one optical sideband, the out-of-band radio frequency (RF) rejection ratio and the shape factor of the bandpass MPF are very limited. Here, by introducing another MRR to handle the other optical sideband, the residual phase can be greatly suppressed, thus the filter's performance can be greatly improved. The proposed bandpass MPF was both verified theoretically and experimentally. Compared with the single MRR, the out-of-band RF rejection ratio and the shape factor were improved by 20 dB and 1.67, respectively. Furthermore, the bandpass MPF's bandwidth is reconfigurable by adjusting the optical carrier's frequency or the two MRRs' amplitude coupling coefficients. The bandpass MPF's center frequency is also tunable by changing the resonant wavelengths of two MRRs in the opposite direction simultaneously. Experimentally, bandwidth reconfiguration from 0.38 GHz to 15.74 GHz, the shape factor optimization from 2 to 1.23, and frequency tuning from 4 GHz to 21.5 GHz were achieved. We believe that the proposed bandpass MPF has great potential for microwave photonic signal processing.

On-chip non-uniformly spaced multi-channel microwave photonic signal processor based on an ultrahigh-Q multimode micro-disk resonator.

Multi-channel microwave photonic (MWP) signal processing can simultaneously perform different task operations on multiple signals carried by multiple wavelengths, which holds great potential for ultrafast signal processing and characterization in a wavelength-division-multiplexed (WDM) network. As emerging telecommunication services create more data, an elastic optical network, which has a flexible and non-uniform spectrum channel spacing, is an alternative architecture to meet the ever-increasing data transfer need. Here, for the multi-channel ultra-fast signal processing in the elastic optical network, we propose and demonstrate an on-chip non-uniformly spaced multi-channel microwave photonic signal processor based on an ultrahigh-Q multimode micro-disk resonator (MDR). In the proposed signal processor, an MDR supporting multiple different order whispering-gallery modes (WGMs) with an ultrahigh Q-factor is specifically designed. Benefiting from the large and different free spectral ranges (FSRs) provided by the different order WGMs, a non-uniformly spaced multi-channel microwave photonic signal processor is realized, and various processing functions are experimentally demonstrated including bandpass filtering with a narrow passband of 103 MHz, a rejection ratio of 22.3 dB and a frequency tuning range from 1 to 30 GHz, multiple frequency measurement with a frequency measurement range from 1 to 30 GHz, a frequency resolution better than 200 MHz and a measurement accuracy of 91.3 MHz, and phase shifting with a phase tuning range from -170°∼160°, an operational bandwidth of 26 GHz from 6 GHz to 32 GHz and a small power variation of 0.43 dB. Thanks to the coexistence of different order WGMs supported by the MDR, non-uniformly spaced multi-channel signal processing is enabled with the key advantages including a broad operation bandwidth, an ultra-narrow frequency selectivity, and a large phase tuning range with a small power variation. The proposed signal processor is promising to be widely used in future elastic optical networks with flexible spectrum grids.

Acousto-optic reconfigurable filter based on vector mode fusion in dispersion-compensating fiber.

An acousto-optic reconfigurable filter (AORF) is proposed and demonstrated based on vector mode fusion in dispersion-compensating fiber (DCF). With multiple acoustic driving frequencies, the resonance peaks of different vector modes in the same scalar mode group can be effectively fused into a single peak, which is utilized to obtain arbitrary reconfiguration of the proposed filter. In the experiment, the bandwidth of the AORF can be electrically tuned from 5 nm to 18 nm with superposition of different driving frequencies. The multi-wavelength filtering is further demonstrated by increasing the interval of the multiple driving frequencies. The bandpass/band-rejection can also be electrically reconfigured by setting the combination of driving frequencies. The proposed AORF gains the feature of reconfigurable filtering types, fast and wide tunability, and zero frequency shift, which is advantageous for high-speed optical communication networks, tunable lasers, fast optical spectrum analyzing and microwave photonics signal processing.

Broad Tunable and High-Purity Photonic Microwave Generation Based on an Optically Pumped QD Spin-VCSEL with Optical Feedback

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) with birefringence-induced polarization oscillations have been proposed to generate desired photonic microwave signals. Here, we numerically investigate the generation of photonic microwave signals in an optically pumped quantum dot (QD) spin-VCSEL. First, the influence of intrinsic key parameters on period-one (P1) oscillations and microwave properties is discussed. Second, the difference between microwave generation based on the quantum well (QW) and QD spin-VCSELs is analyzed by controlling the carrier capture rate that is described in the spin-flip model. The QD spin-VCSEL shows superior microwave quality in the low-frequency range (e.g., 10 GHz~20 GHz) compared with the QW spin-VCSEL. Finally, to boost the performance of the generated photonic microwave signal, optical feedback is introduced. The results show that dual-loop feedback can simultaneously narrow the microwave linewidth and suppress the side modes that are derived from the external cavity mode.

Photonic Generation of Background-Free Phase-Coded Pulses With Low Power Fluctuation Over the Multi-Octave Frequency Range

In this article, a photonic microwave signals generation scheme for background-free phase-coded (PC) radar pulse with multioctave tuning is experimentally demonstrated. Firstly, an optical frequency comb (OFC) demultiplexing technology based on optical injection locking (OIL) provides a power-equalized coherent optical local oscillator (LO) with ultra-broadband tuning capability. Then, a phase-modulated optical signal is produced based on the conversion intensity modulation to phase modulation. Finally, a background-free phase-coded signal can be obtained by beating the optical signal with LO at a balanced photodetector (BPD). The proposed scheme features a flat frequency response over an ultra-wideband frequency range without needing a broadband modulator and radio frequency (RF) synthesizer. A proof-of-concept experiment demonstrates that the power fluctuation of 0.5 Gbps background-free PC pulses is lower than 3.12 dB within 3∼24 GHz carrier frequency. Furthermore, the pulse compression performance and system stability are investigated, respectively.

Specific Features of Designing Microwave Photonic Receiving and Transmitting Channels of Onboard Systems for Communication, Radar and Radio Monitoring

Introduction. Designers of modern on-board systems for communication, radar, and radio monitoring face the problem of improving their qualitative characteristics, including the operating frequency, instantaneous bandwidth, receiver sensitivity, and electromagnetic compatibility. In addition, the dimensions, weight, and power of such systems, as well their cost, should be minimized. However, the current semiconductor microwave electronics has reached its limits in terms of frequency and dynamic characteristics. A possible solution consists in the implementation of microwave photonic transmission lines in the design of on-board systems for communication, radar, and radio monitoring on the basis of modulation of laser radiation by means of electro-absorption.Aim. To study the transfer characteristics and noise figure of a microwave photonic transmission line realized based on the modulation of laser radiation by means of electro-absorption. To compare the results of theoretical calculations and experimental investigations.Materials and methods. The research methodology involved external modulation using an electro-absorption modulator (EAM), mathematical representation of the transmission coefficient, as well as comparison of the theoretical and practical results.Results. Theoretical values of the transmission coefficient and noise figure for a microwave photonic transmission line based on the external modulation method using an EAM were obtained. Experimental values of the transmission coefficient and noise figure for a microwave photonic line in the frequency range from 100 MHz to 16 GHz were presented. The obtained data were compared with those of the nearest mass-produced products of foreign production and those presented in domestic publications on microwave photonic signal transmission lines.Conclusion. The use of an EAM, whose main advantage consists in the possibility of integration with a laser emitter, allowed the authors to design and manufacture a small-sized industrial prototype of a radio-photonic transceiver, capable of transmitting a radio signal over tens of kilometers in the frequency range from 100 MHz to 12 GHz with a transmission coefficient of at least −3 dB and a noise figure no more than 36 dB at the upper operating frequency. At the same time, the closest analogue manufactured by Emcore with similar dimensions has a transmission coefficient of −30 dB and uses direct modulation of laser radiation as a transmission method, which significantly reduces the transmission range of the microwave signal.

Numerical investigation of photonic microwave generation in an optically pumped spin-VCSEL subject to optical feedback.

Photonic microwave generation based on period-one (P1) dynamics of an optically pumped spin-polarized vertical-cavity surface-emitting laser (spin-VCSEL) is investigated numerically. Here, the frequency tunability of the photonic microwave generated from a free-running spin-VCSEL is demonstrated. The results show that the frequency of the photonic microwave signals can be widely tuned (from several gigahertz to hundreds of gigahertz) by changing the birefringence. Furthermore, the frequency of the photonic microwave can be modestly adjusted by introducing an axial magnetic field, although it degrades the microwave linewidth in the edge of Hopf bifurcation. To improve the quality of the photonic microwave, an optical feedback technique is employed in a spin-VCSEL. Under the scenario of single-loop feedback, the microwave linewidth is decreased by enhancing the feedback strength and/or delay time, whereas the phase noise oscillation increases with the increase of the feedback delay time. By adding the dual-loop feedback, the Vernier effect can effectively suppress the side peaks around the central frequency of P1, and simultaneously supports P1 linewidth narrowing and phase noise minimization at long times.