Microwave Frequency Comb Generation Research Articles

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Generation of microwave frequency combs based on dual-frequency injected optoelectronic oscillator

Generation of microwave frequency combs based on dual-frequency injected optoelectronic oscillator

Tunable microwave frequency comb generation via periodic relaxation oscillation in directly modulated laser

In this paper, a novel and simple generation scheme based on a directly modulated laser (DML) is proposed for tunable microwave frequency combs (MFCs). When the input modulated RF power is intentionally controlled to be sufficiently high to drive the DML into its nonlinear operating region periodically, relaxation oscillations will be excited in the same period. The modulated light from the DML is subsequently detected by a photodetector, and a special kind of microwave signal source can be optically produced and shows up in the form of MFC in the frequency domain. In the proof-of-concept experiment, tunable MFCs are demonstrated with the bandwidth of more than 25 GHz, the comb spacing of 0.5 ∼ 3 GHz, and the flatness of less than ± 2 dB with the frequency from 3 GHz to 18 GHz. The bandwidth of the generated MFC is in accordance with the relaxation oscillation frequency of the DML. The comb spacing is strictly equal to the frequency of the modulated RF signal and thus flexibly adjustable. The carrier-to-noise ratio can be up to 48 dB, and the single-sideband phase noise is measured to be lower than − 90 dBc/Hz@1 kHz for MFCs with comb spacings of 0.7 ∼ 3 GHz.

Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

A novel photonics-assisted method for tunable microwave frequency comb (MFC) generation based on an actively mode-locked optoelectronic oscillator (AML-OEO) is proposed and experimentally demonstrated in this letter. Two key factors for the proposed system, periodic gain modulation and widely tunable center frequency, are implemented by an externally introduced intensity-modulated optical signal and a microwave photonic filter (MPF) based on the phase-shifted fiber Bragg grating (PS-FBG) respectively. Unlike a conventional OEO with only one dominant oscillation mode, an AML-OEO operates in a stable multimode oscillation state and thus can generate MFC signal. Through simply adjusting the center frequency of the PS-FBG-based MPF, the center frequency of the generated MFC changes accordingly. In the proof-of-principle experiment, two sets of MFC signals with center frequency tuning from 3.5 GHz to 6.5 GHz and frequency intervals of 1.02 MHz and 340.46 kHz are generated.

Tunable microwave frequency comb generation using a Si3N4-MDR based actively mode-locked OEO.

Microwave frequency combs (MFCs) have important applications in communication and sensing owing to their characteristics of large number of comb lines, wide frequency range, and high precision of comb spacing. In many applications, MFCs are required to emit signals with tunable center frequency and variable comb spacing to accommodate different operating frequency bands and accuracies. Here, we demonstrate a tunable MFC by injecting a low-frequency electrical signal into a tunable optoelectronic oscillator (OEO). Tuning of MFC's center frequency and comb spacing are realized, allowing a frequency tuning range from 1 to 22 GHz and 50 comb lines within a 5 MHz bandwidth obtained in the MFC generator. In addition, the introduction of the silicon nitride micro-disk resonator (Si3N4-MDR) in the system paves the way for the integration of MFC generator.

On-Chip Microwave Frequency Combs in a Superconducting Nanoelectromechanical Device.

Nanomechanical resonances coupled to microwave cavities can be excited, measured, and controlled simultaneously using electromechanical back-action phenomena. Examples of these effects include sideband cooling and amplification, which are commonly described through linear equations of motion governed by an effective optomechanical Hamiltonian. However, this linear approximation is invalid when the pump-induced cavity microwave field is large enough to trigger optomechanical nonlinearities, resulting in phenomena like frequency combs. Here, we employ a niobium-based superconducting electromechanical device to explore the generation of microwave frequency combs. We observe the formation of combs around a microwave resonant frequency (3.78 GHz) with 8-MHz frequency spacing, equal to the mechanical resonant frequency. We investigate their dynamics for different optomechanical parameters, including detuning, pump powers, and cavity decay rates. Our experimental results show excellent agreement with numerical modeling. These electromechanical frequency combs can be beneficial in nanomechanical sensing applications that require precise electrical tracking of mechanical resonant frequencies.

Broadband Tunable Microwave Frequency Comb Generation Based on Modulated Optical Injection Semiconductor Laser

Broadband Tunable Microwave Frequency Comb Generation Based on Modulated Optical Injection Semiconductor Laser

Microwave Frequency Comb Generation by Gain-Switching Versus Relaxation Oscillations

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> of the laser leading to microwave combs in two distinct regimes. For <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> close to the threshold current <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J_{th}$ </tex-math></inline-formula> , the pulse repetition rate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm{ rep}}$ </tex-math></inline-formula> is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching and leads to comb spacings in the tens of MHz range. In contrast, for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J \simeq 2J_{th}$ </tex-math></inline-formula> , the repetition rate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm{ rep}}$ </tex-math></inline-formula> is observed to be related to the relaxation-oscillation frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm{ RO}}$ </tex-math></inline-formula> . The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy.

Method for the generation of microwave frequency combs based on a Vernier optoelectronic feedback loop.

Microwave frequency combs (MFCs) with flexible tunability and prominent phase noise performance are of importance to many applications, including consumer electronic product, fundamental research and military defence. It is difficult for traditional electronic signal sources to meet the imperative demand in terms of high frequency scale, due to a challenging problem of deteriorating phase noise performance with increasing frequency. Photonics-assisted methods have capacity of implementing the generation of microwave signals with high frequency and low phase noise. Here we report a novel photonics-assisted MFC generation method utilizing an optoelectronic feedback loop with a Vernier configuration. The proposed MFC generation system features self-sustained oscillation, inherent multiple-mode oscillation and low phase noise level. In the proof-of-principle experiment, the MFC generation system based on a dual-path Vernier optoelectronic feedback loop is demonstrated, and the comb spacing tuning from 3.072 to 4.710 GHz and the single sideband phase noise of -99.60 dBc/Hz at 10 kHz offset from the carrier are achieved.

Controllable microwave frequency comb generation in a tunable superconducting coplanar-waveguide resonator* *Project supported by the Science Challenge Project (Grant No. TZ2018003), the National Key Research and Development Program of China (Grant No. 2016YFA0301200), the National Natural Science Foundation of China (Grant Nos. 62074091, 11934010, U1801661, and U1930402), and the BAQIS Research Program (Grant No. Y18G27).

Frequency combs are useful in a wide range of applications, such as optical metrology and high-precision spectroscopy. We experimentally study a controllable frequency comb generated in a tunable superconducting coplanar-waveguide resonator in the microwave regime. A two-tone drive is applied on one of the resonance modes of the resonator and comb generation is observed around the resonance frequency of the resonator. Both central frequency and teeth density of the comb are precisely controllable, and the teeth spacing can be adjusted from Hz to MHz. Moreover, we show that a few hundreds of sidebands can be generated using a sufficiently strong drive power and the weakest drive power needed to generate the comb can be reduced to approach the quantum limit. These experimental results can be qualitatively explained via theoretical analysis.

Microwave frequency comb generation through optical double-locked semiconductor lasers

A microwave frequency comb (MFC) consists of equally spaced discrete microwave frequency components that have small magnitude variations across its bandwidth. In this work, a scheme comprising of a slave laser that is optically injected by a strongly intensity-modulated master laser is proposed and numerically investigated to generate MFCs. Based on the double-locking characteristics of the periodic oscillations induced by strong intensity-modulated optical injection, locked comb structures are invoked when the modulation frequency is at a submultiple frequency of the period-one oscillation frequency. Furthermore, broad microwave frequency comb bandwidths are achieved through the bandwidth enhancement effect of optically injected lasers at appropriate operating conditions. The demonstrated MFC source has a continuously tunable comb-line spacing between 1–10 GHz reaching bandwidths that are six times the relaxation resonance frequency of the free-running semiconductor laser.

Digitally controlled microwave frequency comb based on Mach–Zehnder modulator and pulse signals

We propose a scheme of a digitally controlled microwave frequency comb (MFC) based on a Mach–Zehnder modulator (MZM) and pulse signals. In the proposed scheme, an MZM is driven by a periodic pulse signal that is controlled with binary sequences and then outputs an optical frequency comb (OFC). After the beat frequency occurs among modes of the OFC, an MFC with a wide spectrum coverage and small power variation is obtained using a photodetector. The theoretical model of the MFC generation based on pulse signals is developed. The performance of the MFC generation scheme is investigated. The results show that the minimum power variation and maximum spectrum bandwidth of the generated MFC are 0.31 dB and 300 GHz, respectively. By programming the binary sequence, the frequency spacing of MFC and the number of microwave signals can be tuned in the range of 1.25 to 10 GHz and 31 to 255, respectively.

Ultra-flat broadband microwave frequency comb generation based on optical frequency comb with a multiple-quantum-well electro-absorption modulator in critical state

In this paper, we proposed a novel ultra-flat broadband microwave frequency comb (MFC) generation based on optical frequency comb (OFC) with a multiple-quantum-well electro-absorption modulator (MQW-EAM) in critical state. The scheme is simple and easy to adjust. The performance of the MFC generation scheme is investigated using software Optisystem. The results show that the comb spacing of MFC can be adjusted from 5 to 20 GHz by changing RF signal’s frequency and the MFC is almost independent on the linewidth of the tunable laser diode. The performance of the MFC can be improved by reasonably increasing the voltage of the RF, the small-signal gain of the Erbium-doped fiber amplifier (EDFA) and the responsivity of the photodetector. The MFC generated by this scheme has 300 GHz effective bandwidth with 15 comb lines, whose power variation is 0.02 dB, when the components’ parameters in the Optisystem are set as follows: the power of tunable laser diode (TLD) is 0 dBm, the wavelength is 1552.52 nm, and linewidth is 1 MHz; RF signal’s frequency is 20 GHz and the voltage is 10 V; the reverse bias voltage of MQW-EAM is 6.92 V; the small-signal gain of the EDFA is 40 dB; the responsivity of the photodetector (PD) is 1 A/W.

Tunable microwave frequency comb generation based on double-loop mixing-frequency optoelectronic oscillator

With the development of wireless communication technology and micro-cell technology, optical-borne microwave technology, specially optical-borne multi-carrier technology has become one of the most important trends for generating high-quality sources. Therefore, the efficient generation of high-quality microwave signals has always been a requirement in wireless communication systems. Due to its low-noise and high-frequency output characteristics, photoelectric oscillator is widely used to generate high-quality microwave frequency sources in communication systems. Combining the advantages of photoelectric oscillator's low-noise output and direct-modulated laser's gain-switching state characteristics, a tunable optical-borne microwave frequency comb scheme based on dual-loop mixing-frequency photoelectric oscillator is proposed in this paper. And a direct-modulated laser operating in a gain-switching state is used to generate the original optical-borne microwave frequency comb signals. The dual-loop adjacent resonant frequencies are separated by two different high-frequency microwave bandpass filters. The beat frequency of adjacent frequencies mentioned above is injected back into laser to form photoelectric resonance, and thus enhancing the generated original optical-borne microwave frequency comb signals. To suppress the side modes caused by long resonant cavity, a polarized dual-loop structure is used in the system, and thus improving the noise characteristics of output signals. After experimental analysis, the dual-loop filtered resonant microwave signals and low-phase-noise microwave comb signals with a frequency interval of 797.4 MHz are all obtained. The microwave output side-mode suppression ratio after polarized dual-loop adjustment is improved to 47 dB. And microwave comb signal's first-order carrier phase noise is lower than-101.7 dBc/Hz at 10 kHz,-115.2 dBc/Hz at 50 kHz. In addition, higher-order carriers all come from the light multiplication of first-order carrier, they share the same low noise characteristics with first-order microwave comb signal. The output power of first-to-fourth, fifth-to-thirteenth order carriers are balanced to 10 dB by photoelectric resonance injection. And their side-mode suppression ratios are all better than 40 dB. Furthermore, theoretically, the comb interval can be adjusted to any frequencies by changing the center frequencies of two high-frequency bandpass microwave filters. Therefore, optical-borne multi-carrier microwave signals are generated efficiently and cost-effectively by this tunable optical-borne microwave frequency comb scheme, and the generated low-noise multi-carrier frequency sources meet the demand of an optical-borne microwave wireless communication system.

Tunable and Broadband Microwave Frequency Comb Generation Using Optically Injected Semiconductor Laser Nonlinear Dynamics

Based on an optically injected semiconductor laser (OISL) operating at period-one (P1) nonlinear dynamical state, tunable and ultrabroadband microwave frequency combs (MFCs) are generated experimentally through further current-modulating the OISL. First, by introducing an injection light with an injection power of $P_{\rm{i}}\,= \,12.42$ mW, whose wavelength is identical to the central wavelength of a free-running distributed feedback semiconductor laser, the OISL can be driven into P1 state with a fundamental frequency of $f_{{0}}\,= \,26.44$ GHz. Next, further modulating the OISL with a modulation frequency of $f_{\rm{m}}\,= \,3.3$ GHz and a modulation power of $P_{\rm{m}}$ = 22.0 dBm, an MFC with a bandwidth of 59.4 GHz within a 10 dB amplitude variation is experimentally obtained, and the single sideband phase noise at offset frequency 10 kHz for all comb lines contained within the bandwidth is below −95.0 dBc/Hz. Finally, through varying the modulation frequency, the MFCs with different comb spacing can be obtained, and the influences of relevant operation parameters on the performances of the generated MFCs have been analyzed.

Microwave frequency-comb generation in a tunneling junction by intermode mixing of ultrafast laser pulses

We present a method for hyper-spectral characterization of the nonlinear effects in a tunneling junction. Harmonics up to 1 GHz were measured in a frequency comb in the tunneling current when 15-fs laser pulses at a repetition rate of 74.25 MHz were focused on the tunneling junction of a scanning tunneling microscope. The typical output power is −120 dBm at the fundamental frequency, which is the pulse repetition rate and decreases by several dB for the higher harmonics. The observed square-law dependence of the signal power on the tunneling current and incident laser power is in good agreement with theoretical predictions.