Voltage-controlled photonic oscillator

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.

W-Band Photonic Receiver for Compact Cloud Radars

We introduce an RF-photonics receiver concept enabling the next generation of ultra-compact millimeter wave radars suitable for cloud and precipitation profiling, planetary boundary layer observations, altimetry and surface scattering measurements. The RF-photonics receiver architecture offers some compelling advantages over traditional electronic implementations, including a reduced number of components and interfaces, leading to reduced size, weight and power (SWaP), as well as lower system noise, leading to improved sensitivity. Low instrument SWaP with increased sensitivity makes this approach particularly attractive for compact space-borne radars. We study the photonic receiver front-end both analytically and numerically and predict the feasibility of the greater than unity photonic gain and lower than ambient effective noise temperature of the device. The receiver design is optimized for W-band (94 GHz) radars, which are generally assessed to be the primary means for observing clouds in the free troposphere as well as planetary boundary layer from space.

A low-noise photonic heterodyne synthesizer and its application to millimeter-wave radar

Microwave photonics offers transformative capabilities for ultra-wideband electronic signal processing and frequency synthesis with record-low phase noise levels. Despite the intrinsic bandwidth of optical systems operating at ~200 THz carrier frequencies, many schemes for high-performance photonics-based microwave generation lack broadband tunability, and experience tradeoffs between noise level, complexity, and frequency. An alternative approach uses direct frequency down-mixing of two tunable semiconductor lasers on a fast photodiode. This form of optical heterodyning is frequency-agile, but experimental realizations have been hindered by the relatively high noise of free-running lasers. Here, we demonstrate a heterodyne synthesizer based on ultralow-noise self-injection-locked lasers, enabling highly-coherent, photonics-based microwave and millimeter-wave generation. Continuously-tunable operation is realized from 1-104 GHz, with constant phase noise of -109 dBc/Hz at 100 kHz offset from carrier. To explore its practical utility, we leverage this photonic source as the local oscillator within a 95-GHz frequency-modulated continuous wave (FMCW) radar. Through field testing, we observe dramatic reduction in phase-noise-related Doppler and ranging artifacts as compared to the radar’s existing electronic synthesizer. These results establish strong potential for coherent heterodyne millimeter-wave generation, opening the door to a variety of future applications including high-dynamic range remote sensing, wideband wireless communications, and THz spectroscopy.

Photonics-based radars offer intriguing potential but face tradeoffs in tunability, complexity, and noise. Here the authors present microwave generation in a photonics platform by heterodyning of two low-noise, self-injection-locked lasers, and demonstrate its advantages in an FMCW radar system.

High sensitivity microwave phase noise analyzer based on a phase locked optoelectronic oscillator

Phase noise is a key parameter to evaluate the short-term stability of a microwave oscillator. This metric is of major concern for many applications. A phase locked loop (PLL) is widely used to extract the phase noise. However, due to the limitation of the phase noise of the reference, it is still a technical challenge to precisely characterize the phase noise of a high frequency carrier. To address this issue, we propose a high sensitivity microwave phase noise analyzer by using a photonic-based reference. By combining an optoelectronic oscillator (OEO) and a direct digital synthesizer, we achieve a 9 GHz to 11 GHz frequency tunable reference with phase noise of -140 dBc/Hz at 10 kHz offset, side-mode suppression ratio of 128 dB, and frequency switching time of 176 ns. Thanks to this low phase noise reference, we attain an X-band phase noise analyzer with an excellent sensitivity of -139 dBc/Hz at 10 kHz offset without cross-correlation. This is the first time to realize a PLL-based phase noise analyzer utilizing an OEO. We thoroughly present a theoretical analysis of our proposed system. Benefiting from the OEO's phase noise independent of frequency, the operation frequency of our proposed system can be extended to the millimeter-wave range while maintaining high sensitivity.

Wideband tunable optoelectronic oscillator based on a microwave photonic filter with an ultra-narrow passband

A novel wideband tunable optoelectronic oscillator based on a microwave photonic filter (MPF) with an ultra-narrow passband is proposed and experimentally demonstrated. The single-passband MPF is realized by cascading an MPF based on stimulated Brillouin scattering and an infinite impulse response (IIR) MPF based on an active fiber recirculating delay loop. The measured full width at half-maximum bandwidth of the cascaded MPFs is 150 kHz. To the best of my knowledge, this is the first time realizing such a narrow passband in single-passband MPF. The oscillation frequency of the OEO can be tuned from 0 to 40 GHz owing to the wideband tunability of the MPF. Thanks to the ultrahigh mode selectivity of the IIR filter, the mode hopping is successfully suppressed. A stable microwave signal at 8.18 GHz is obtained with a phase noise of -113  dBc/Hz at 10 kHz, and the side mode noise is below -95  dBc/Hz. The signal-to-noise ratio exceeds 50 dB during the tuning process.

Supermode noise suppression with mutual injection locking for coupled optoelectronic oscillator

The coupled optoelectronic oscillator (COEO) is typically used to generate high frequency spectrally pure microwave signal with serious sidemodes noise. We propose and experimentally demonstrate a simple scheme for supermode suppression with mutual injection locking between the COEO (master oscillator with multi-modes oscillation) and the embedded free-running oscillator (slave oscillator with single-mode oscillation). The master and slave oscillators share the same electrical feedback path, which means that the mutually injection-locked COEO brings no additional hardware complexity. Owing to the mode matching and mutually injection locking effect, 9.999 GHz signal has been successfully obtained by the mutually injection-locked COEO with the phase noise about -117 dBc/Hz at 10 kHz offset frequency. Besides, the supermode noise can be significantly suppressed more than 50 dB to below -120 dBc.

Wideband tunable optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering

A wideband tunable optoelectronic oscillator (OEO) based on the deamplification of stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. A tunable single passband microwave photonic filter (MPF) utilizing phase modulation and SBS deamplification is used to realize the tunability of the OEO. Theoretical analysis of the MPF and phase noise performance of the OEO are presented. The frequency response of the MPF is determined by the + 1st sideband attenuation due to SBS deamplification and phase shift difference between the two sidebands due to chromatic dispersion and SBS. The close-in (< 1 MHz) phase noise of the proposed OEO is shown to be dominated by the laser frequency noise via phase shift of SBS. The conversion of the laser frequency noise to the close-in phase noise of the proposed OEO is effectively reduced compared with the OEO based on amplification by SBS. Tunable 7 to 40 GHz signals are experimentally obtained. The single-sideband (SSB) phase noise at 10 kHz offset is -128 dBc/Hz for 10.30 GHz signal. Compared with the OEO based on SBS amplification, the proposed OEO can achieve a phase noise performance improvement beyond 20 dB at 10 kHz offset. The maximum frequency and power drifts at 10.69 GHz are within 1 ppm and 1.4 dB during 1000 seconds, respectively. To achieve better close-in phase noise performance, lower frequency noise laser and higher pump power are preferred. The experimental results agree well with the theoretical models.

Rapidly and continuously frequency-scanning opto-electronic oscillator

An opto-electronic oscillator (OEO) scheme which operates at "chirp oscillation" mode and generates low-phase-noise, frequency-swept microwave is proposed and experimentally demonstrated. This frequency-swept OEO is achieved by embedding a rapidly frequency-scanning microwave filter in an opto-electronic cavity. The filter has fixed passband while its center frequency scans rapidly and periodically at cavity round-trip time, covering a large frequency range (~GHz). Experimentally, the generated frequency-swept microwave is linear frequency-modulated continuous wave (FMCW) which centers at 7 GHz with 1-GHz bandwidth. Its instantaneous frequency varies linearly from 6.5 GHz to 7.5 GHz, back and forth, in a period of 12.8 μs, resulting in a frequency scanning rate of ~156 MHz/μs. The single-side-band (SSB) noise of the generated FMCW is -104 dBc/Hz at 10 kHz offset frequency, which is much lower than that from a commercial electronic arbitrary waveform generator (E-AWG). Improvement as large as 23 dB is experimentally reported.

Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO₃ and LiTaO₃ are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>10⁸) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO₃ resonator (Q∼10⁸). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.

Chip Scale Atomic Resonator Frequency Stabilization System With Ultra-Low Power Consumption for Optoelectronic Oscillators

We present a long-term chip scale stabilization scheme for optoelectronic oscillators (OEOs) based on a rubidium coherent population trapping (CPT) atomic resonator. By locking a single mode of an OEO to the (85)Rb 3.035-GHz CPT resonance utilizing an improved phase-locked loop (PLL) with a PID regulator, we achieved a chip scale frequency stabilization system for the OEO. The fractional frequency stability of the stabilized OEO by overlapping Allan deviation reaches 6.2 ×10(-11) (1 s) and  ∼ 1.45 ×10 (-11) (1000 s). This scheme avoids a decrease in the extra phase noise performance induced by the electronic connection between the OEO and the microwave reference in common injection locking schemes. The total physical package of the stabilization system is [Formula: see text] and the total power consumption is 400 mW, which provides a chip scale and portable frequency stabilization approach with ultra-low power consumption for OEOs.

Optically controlled low-power on-off mode resonant tunneling oscillator with a heterojunction phototransistor switch

We report an optically controlled low-power on-off mode oscillator based on a resonant tunneling diode (RTD) that is monolithically integrated with a heterojunction phototransistor (HPT) optical switch. In order to achieve a low-power operation at a wavelength of 1.55 μm an InP-based quantum-effect tunneling diode is used for microwave signal generation based on a unique negative differential conductance (NDC) characteristic of the RTD at a low applied voltage. In addition, the high-gain HPT is used for converting incident optical data to an electrical data signal. The fabricated on-off mode oscillator shows a low-power consumption of 5 mW and a high-data-rate of 1  Gb/s at an oscillation frequency of 4.7 GHz. A good energy efficiency of 5  pJ/bit has been obtained due to the low DC power consumption along with high-data-rate performance of the RTD-based optoelectronic integration scheme.

Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

Ultrahigh-quality (Q) factor microresonators have a lot of applications in the photonics domain ranging from low-threshold nonlinear optics to integrated optical sensors. Glass-based whispering gallery mode (WGM) microresonators are easy to produce by melting techniques, however they suffer from surface contamination which limits their long-term quality factor to a few 10⁸. Here we show that an optical gain provided by erbium ions can compensate for residual losses. Moreover it is possible to control the coupling regime of an ultrahigh Q-factor three port microresonator from undercoupling to spectral selective amplification by changing the pumping rate. The optical characterization method is based on frequency-swept cavity-ring-down-spectroscopy. This method allows the transmission and dispersive properties of perfectly transparent microresonators and intrinsic finesses up to 4.0 × 10⁷ to be measured. Finally we characterize a critically coupled fluoride glass WGM microresonator with a diameter of 220 μm and a loaded Q-factor of 5.3 × 10⁹ is demonstrated.

Microwave synthesizer using an on-chip Brillouin oscillator

Low-phase-noise microwave oscillators are important to a wide range of subjects, including communications, radar and metrology. Photonic-based microwave-wave sources now provide record, close-to-carrier phase-noise performance, and compact sources using microcavities are available commercially. Photonics-based solutions address a challenging scaling problem in electronics, increasing attenuation with frequency. A second scaling challenge, however, is to maintain low phase noise in reduced form factor and even integrated systems. On this second front, there has been remarkable progress in the area of microcavity devices with large storage time (high optical quality factor). Here we report generation of highly coherent microwaves using a chip-based device that derives stability from high optical quality factor. The device has a record low electronic white-phase-noise floor for a microcavity-based oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of a photonic-based, microwave frequency synthesizer. The synthesizer performance is comparable to mid-range commercial devices.