Single-sideband Phase Noise Research Articles

Microwave Photonic Frequency Multiplier with Low Phase Noise Based on an Optoelectronic Oscillator

A microwave photonic frequency multiplier with low phase noise based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. In this scheme, a dual-parallel Mach–Zehnder modulator (DPMZM) is employed to generate the third-harmonic frequency of the input radio frequency (RF) signal, while the oscillation frequency of the OEO is also three times the RF signal frequency. By adjusting the bias voltages of different arms in the DPMZM, a triple-frequency signal with a high side-mode suppression ratio of 64.8 dB can be obtained. The experimental results indicate that the output of the frequency-multiplier has a better single-sideband phase noise value, for instance, −126 dBc/Hz@10 kHz at 20.019 GHz. It has improvements of 34 dB and 43.5 dB compared with the input RF signal and the simulated electrical frequency tripler module, respectively.

Robust self-injection locking to a non-confocal monolithic Fabry–Perot cavity

We demonstrate an efficient simultaneous self-injection locking of two semiconductor lasers to high-order modes of a standalone monolithic non-confocal Fabry–Perot cavity. The lasers are used to generate a low-noise microwave signal on a fast photodiode. The overall improvement of the laser spectral purity exceeds 80 dB. The observed single-sideband phase noise of X- to W-band signals is at the −110 dBc/Hz level and is limited by the fundamental thermorefractive noise of the cavity. The demonstrated cavity–laser configuration can be tightly packaged and is promising for the generation of high-frequency RF signals as well as for referencing optical frequency combs.

Parity time symmetry all-optical microwave oscillator using a polarization dependent Sagnac loop

An all-optical microwave oscillator based on parity-time symmetry was proposed and experimentally demonstrated. It mainly consists of a big feedback loop, an active small loop and a polarization-dependent Sagnac loop. The microwave seed signal originates from the beating between the continuous wave and the Stokes wave. The function of microwave envelope detection and feedback modulation are implemented through cross gain modulation of two semiconductor optical amplifiers. By employing the parity time symmetry loops and the Vernier effect, the single mode selectivity capability of the oscillator has been enhanced, and a single mode microwave signal with high quality can be generated. In the experiment, a highly quality microwave signal with a frequency of 10.71 GHz was generated. The single side band phase noise can reach −94.1 dBc/Hz@10 kHz and the side mode suppression ratio of the generated microwave signal reaches 41 dB. Compared to the previously reported all-photonic microwave oscillator, the single side band phase noise is improved by 7 dB at a frequency offset of 10 kHz, and by 20 dB at a frequency offset of 100 Hz. The results indicate that our proposed scheme significantly improves the quality of the generated microwave signals.

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.

Dual-frequency fundamental-mode NPRO laser for low-noise microwave generation.

Monolithic nonplanar ring oscillators (NPROs) have achieved great success in industry, scientific applications and space missions due to their excellent narrow-linewidth, low-noise, high beam-quality, lightweight and compact performances. Here, we show that stable dual-frequency or multi-frequency fundamental-mode (DFFM or MFFM) laser can be stimulated directly by tunning pump divergence-angle and beam-waist injected to NPRO. The DFFM laser has a frequency deviation of one free spectral range of the resonator and thus can be utilized for pure microwave generation by common-mode-rejection. To demonstrate the purity of the microwave signal, a theoretical phase noise model is established, and the phase noise and the frequency tunability of the microwave signal are experimentally studied. Single sideband phase noise for a 5.7 GHz carrier is measured as low as -112 dBc/Hz at 10 kHz offset, and -150 dBc/Hz at 10 MHz offset in the free running condition of the laser, which outperforms its counterparts from dual-frequency Laguerre-Gaussian (LG) modes. The frequency of the microwave signal can be efficiently tunned through two channels, with frequency tunning coefficients of 15 Hz/V by piezo, and -60.5 kHz/K by temperature, respectively. We expect that such compact, tunable, low-cost and low-noise microwave sources can facilitate multiple applications including miniaturized atomic clocks, communication and radar, etc.

Parity–Time-Symmetric Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

Parity–time (PT) symmetry has been intensively exploited in optoelectronic oscillators (OEOs), enabling mode selection in mutually coupled feedback loops. However, two mutually coupled feedback loops are usually established to achieve a spatial PT-symmetric architecture, which increases system complexity and instability. Here, we propose and experimentally prove a PT-symmetric OEO based on stimulated Brillouin scattering (SBS). The PT symmetry is implemented in an overlapping spatial loop in which the gain and loss are realized based on SBS gain and SBS loss, respectively. By adjusting the pump power and the polarization states of the probe and pump lights, the gain and loss can be balanced, thus achieving stable single-mode oscillation. In experiment, a 9.66 GHz microwave signal with a side-mode suppression ratio of 41 dB is generated. At 10 kHz offset frequency, the single-sideband phase noise reaches as low as −120.1 dBc/Hz. This demonstration opens new avenues to enhance mode selectivity in an OEO and can potentially be integrated on a chip.

High power flatness and wide spectrum range microwave frequency combs generation based on an actively mode-locked optoelectronic oscillator with intensity feed-forward path

An actively mode-locked optoelectronic oscillator (OEO) with intensity feed-forward path is proposed and experimentally demonstrated to generate microwave frequency combs (MFCs) with high power flatness and wide spectrum range. An intensity feed-forward path with the same time delay as the delay of the main loop is inserted into an actively mode-locked OEO, which optimizes the power flatness and spectrum range of the generated MFCs signal. Experimental results show that comb lines within ± 10 dB power variations increase from 15 to 78 and power difference decreases from 8.2 dB to 1.5 dB when the intensity feed-forward path is working in the loop. Moreover, the performance of single sideband (SSB) phase noise with different lengths of SMF inserted in the proposed scheme are also investigated.

All-optical microwave oscillator based on a mutual-injection coupling between DFB-LDs.

An all-optical microwave oscillator (AOMO) is proposed and experimentally demonstrated. It contains only a mutual injection loop consisting of two distributed feedback laser diodes (DFB-LDs) and a few passive components. In this AOMO, the microwave seed signal originates from the period-one (P1) oscillation in one of the DFB-LDs. The functions of microwave envelope detection and feedback modulation are implemented by the other DFB-LD. Due to the optical injection locking and the optical-optical modulated effect in DFB-LD, the P1 signal is enhanced, and the stability of the P1 signal can be improved by coupling the P1 signal with a resonant mode of the mutual injection loop. Meanwhile, since the P1 oscillation is sensitive to injected light, the frequency of the P1 signal can be easily adjusted, which makes the AOMO easy to be tuned. In the experiment, a highly stable single-mode microwave signal with a frequency of 16.69 GHz and a single-sideband (SSB) phase noise of -90.7 dBc/Hz@10 kHz is generated. The frequency can be tuned from 14.48 to 21.45 GHz by adjusting the parameters of DFB-LDs and the injection light.

Semiconductor laser and polarization modulator for W-band signal generation

A photonic microwave harmonic up-conversion is experimentally demonstrated using the local oscillator (LO) to directly modulate the distributed feedback laser and externally modulate the polarization modulator simultaneously. By feeding the initial radio frequency signal generated by the LO into the system, the second and fourth harmonic photonic microwaves are eventually available through the separation of the polarization beam splitter. The steady second and fourth harmonic photonic microwaves have a linewidth of about 100 Hz and a discrepancy of up to 60 dB between the central peak and the noise floor. Undesired harmonics and sidebands in the 30 GHz, 60 GHz, and 80 GHz fourth harmonic photonic microwaves can be suppressed without the use of optical amplifiers, resulting in a clear display of both tones. The single-sideband (SSB) phase noise of this proposed system adds about 6 dB of SSB phase noise in each cumulative harmonic, which is close to the ideal state.

Rational number harmonic mode-locked dual-loop optoelectronic oscillator with low supermode noise and low intermodulation distortions

A rational number harmonic mode-locked dual-loop optoelectronic oscillator (RHML-DL-OEO) is proposed and experimentally demonstrated. In the proposed system, an external radio frequency (RF) signal and a feedback oscillating microwave signal drive two arms of a dual-drive Mach-Zehnder modulator (DMZM). Mode locking is realized by frequency detuning. The larger effective free spectrum range (FSR) and higher side-mode suppression result from the Vernier effect effectively suppress supermode noise and intermodulation distortions (IMDs). Experimental results demonstrate that the microwave frequency comb (MFC) signals with repetition frequencies of 901.8 kHz, 2.3046 MHz and 5.3106 MHz are generated by 9th-, 23rd- and 53rd-order rational number harmonic mode-locking, respectively. Compared with the rational number harmonic mode-locked optoelectronic oscillator based on single-loop structure, the supermode noise suppression ratios of the scheme we propose are improved by 30.5 dB, 27.6 dB and 20.3 dB, respectively. Furthermore, the performance of single sideband (SSB) phase noise is also investigated.

Incoherent Rayleigh Scattering Noise Depression for Single Laser Stable Radio Frequency Transmission

The stable radio frequency (RF) transmission scheme involving single wavelength has received increasing attention due to the feature of accurate phase information extraction. However in single laser system, the single side band (SSB) phase noise of output signal will be deteriorated by Rayleigh scattering during long-distance optical transmission. In this work, we propose an incoherent Rayleigh scattering noise depression method in the passive phase compensation stable RF transmission scheme with single laser. The carrier of scattering light is suppressed with narrow-band optical filter based on the Stimulated Brillouin scattering (SBS) effect, so that the noise generated by the beat of scattering itself will be significantly reduced. As a result, a 5-GHz RF signal stability transmission over 50-km single mode fiber is demonstrated experimentally. The SSB phase noise at 10 KHz offset is 10.9 dB lower than the scenarios without incoherent Rayleigh scattering noise depression. Meanwhile, the phase jitter mean square error is 1.12 ps during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 10^{4}$ </tex-math></inline-formula> seconds.

An All-Optical Ka-Band Microwave Long-Distance Dissemination System Based on an Optoelectronic Oscillator

We proposed an all-optical Ka-band microwave long-distance dissemination system based on an optoelectronic oscillator (OEO). In this system, a single tone with high spectrum purity and low phase noise is excited by an injection-locked OEO, and distributed to the remote site through the fiber loop of the OEO at the same time. The second harmonic of the reference signal modulates the round-tripping optical signal at the local site, which achieves the phase conjugator at the optical domain. At the remote end, the phase-conjugated signal is photomixed with the OEO signal to eliminated the phase offset. The proposed scheme improves the phase noise of the high frequency signal received at the remote site and reduces the photoelectric conversion loss. We demonstrated the transmissions of a 36 GHz frequency-quadrupled RF signal along a 6 km optical fiber loop, and the frequency stability is 1.069 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−14</sup> /1 s and 3.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup> /1000 s. The single-sideband (SSB) phase noise of the received signal at the remote site is up to −130 dBc/Hz at 10 kHz offset frequency.

Tunable all-optical microwave signal generation based on period-one dynamics of an external optically injected distributed-feedback laser diode and feedback oscillation

A tunable all-optical microwave generator based on period-one (P1) dynamics of an external optically injected distributed-feedback laser diode (DFB-LD) is proposed and demonstrated. A DFB-LD implements the functions of light source, microwave frequency selection, and optical–optical modulation. To overcome the frequency drift in P1 oscillation process, a set of reference frequencies from a fiber ring laser (FRL) is introduced. Once the P1 oscillation signal and the modes of FRL are coupled, the whole system is closed and a stable frequency determined by P1 oscillation can be established. In the experimental verification, tunable photonic microwave signals with frequencies from 6.42 to 21.36 GHz are obtained. The measured single-sideband phase noise is −83 dBc / Hz at 10-kHz offset, which is operation frequency independent.

Microwave signal generation with discrete mode laser diode

Microwave signals with low phase noise can be broadly used in communication systems, satellite communications, and radar systems. Discrete mode laser diode (DM-LD) has the capacity to generate signals that meet the above requirements. Moreover, it has the following advantages such as stable operation over a wide temperature range, very high side mode suppression ratio, low sensitivity to optical feedback, and narrow linewidth. In this paper, we proposed and experimentally demonstrated DM-LD for generating the second harmonic signal in the self-starting microwave generation system. The results show that DM-LD effectively reduces the single sideband (SSB) phase noise of the fundamental signals and its second harmonic signals better than the same designed experimental system using a distributed feedback LD, which reduces the SSB noise of the fundamental signal/its second harmonic signal by over 16/22 dB, 18/22 dB, 13/19 dB at a frequency offset of 10 kHz, 100 kHz, and 1 MHz, respectively.

Polarization-insensitive coupled optoelectronic oscillator with low spurious tones and phase noise

A coupled optoelectronic oscillator (COEO) based on σ-shaped fiber ring structure and intra-cavity semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. The σ-shaped fiber ring structure is skillfully utilized in COEO to eliminate the harmful influence of polarization disturbance. The SOA is embedded for super-mode suppression due to the fast gain saturation effect. The eximious phase noise performance of COEO could be maintained by operating the SOA at the unitary gain regime. The stable operation of COEO is guaranteed by the immunity to polarization fluctuation and the greatly suppressed spurious-mode competition. As a result, a 10-GHz signal is generated featuring high spectral purity and ultra-low spurious tones as soon as the system is power-on, and can hold steady even if the polarization changes dramatically. The single sideband phase noise of the proposed COEO is about -133 dBc/Hz at 10-kHz offset frequency, and the spurious suppression ratio reaches more than 95 dB, which is 60-dB superior than the conventional COEO.

Broadband and tunable optical frequency comb based on 1550 nm verticalcavity surface-emitting laser under pulsed current modulation and optical injection

Optical frequency combs (OFCs) each consist of a set of equally spaced discrete frequency components, and they have been widely applied to many fields such as metrology, optical arbitrary waveform generation, spectroscopy, optical communication, and THz generation. In this work, we propose a scheme for generating broadband and tunable OFCs based on a 1550 nm vertical-cavity surface-emitting laser (VCSEL) under pulsed current modulation and optical injection. Firstly, a pulsed electrical signal is utilized to drive a 1550 nm-VCSEL into the gain-switching state with a broad noisy spectrum. Next, a continuous optical wave is further introduced for generating broadband and tunable OFC. Under injection light with power of 18.82 µW and wavelength of 1551.8570 nm, and pulsed electrical signal with a frequency of 0.5 GHz and pulse width of 200 ps, an OFC with a bandwidth of 82.5 GHz and CNR of 35 dB is experimentally acquired, and the single sideband phase noise at the 0.5 GHz reaches –123.3 dBc/Hz at 10 kHz. Moreover, the influences of injection light wavelength, frequency and width of pulse electrical signal on the performance of generated OFC are investigated. The experimental results show that OFCs with different comb spacings can be obtained by varying the frequency of pulsed electrical signal. For the frequency of pulsed current signal varying in a range of 0.25 GHz–3 GHz, the bandwidth of generated OFCs can exceed 60 GHz through selecting optimized injection optical wavelength and width of pulse electrical signal.

Stable microwave LO distribution via a phase-locked OEO assisted by passive compensation

We present a stable microwave local oscillator (LO) distribution scheme based on a phase-locked optoelectronic oscillator (OEO) assisted by passive compensation. By injecting the LO reference into the phase-locked OEO and locking it with the oscillation signal, a microwave LO with a low phase noise can be tapped at the remote site. Meanwhile, a frequency-mixing-based passive compensation system is utilized to efficiently prevent the remote LO signal from the slow phase drift caused by the temperature susceptibility of fiber. In the experiment, a 10.8 GHz LO signal is stably distributed along a 6.8-km ring fiber to the remote site that is 1.8 km away from the local site. The frequency stability of 3.8 × 10−14 at 1s and 2.3 × 10−17 at 10000 s can be attained. The single-sideband (SSB) phase noise of the received signal at the remote site is up to –110 dBc/Hz at 10 kHz offset frequency with the root-mean-square phase jitter of 0.023 rad.

Flexible Ultra-Wide Electro-Optic Frequency Combs for a High-Capacity Tunable 5G+ Millimeter-Wave Frequency Synthesizer

This paper presents a new scheme of a cost-effective tunable millimeter-wave (MMW) frequency synthesizer based on an ultra-wideband electro-optic frequency comb. The architecture for the quasi-tunable millimeter-wave frequency synthesizer mainly consists of a compact ultra-wide flat electro-optic frequency comb and a multi-tone frequency generator, which only includes a quantum dot mode-locked laser, a LiNbO3 dual-driving Mach–Zehnder modulator (DD-MZM) and Uni-traveling-carrier photodiode (UTC-PD). MMW signals generated with a quasi-tunable frequency are experimentally demonstrated. The difference in power is obtained for the different frequencies. The linewidth of the quasi-tunable frequency signals is less than 273 Hz. In addition, the single side band (SSB) phase noise of the 25, 37.5, 50 and 75 GHz is measured as −115, −106, −102 and −95 dBc/Hz at an offset of 1 kHz, respectively. The proposed frequency synthesizer has ultra-low phase noise, quasi-tunable frequency and simple structure. The research results of the frequency synthesizer are applied for 5G+ transmission with radio wave working at K-band and V-band. The flexible, compact and robust MMW frequency synthesizer is suitable for the future of ultra-high capacity 5G+ communication.

Low jitter microwave pulse train generation based on an optoelectronic oscillator.

We demonstrate an approach to ultra-short pulse train generation with a low time jitter based on pulse compression of a frequency comb generated by a dual-loop optoelectronic oscillator (OEO). The proposed dual-loop OEO consists of two feedback loops, with one having a long loop length and the other a short loop length. In the long loop, a phase modulator (PM) cascaded with a Mach-Zehnder modulator (MZM) are employed, and in the short loop, only the MZM is included. Due to the Vernier effect, the use of the dual-loop structure can facilitate mode selection to generate a single-frequency microwave carrier with multiple optical sidebands corresponding to an optical comb. By adjusting the phase relationship between the optical sidebands using a dispersion compensating fiber (DCF), a stable optical pulse train is generated. Thanks to the low phase noise nature of an OEO, the generated pulse train has a low time jitter. The proposed approach is evaluated experimentally. A pulse train with a repetition frequency of 2.023 GHz and a pulse width of 40 ps is generated. The single-sideband (SSB) phase noise of the carrier frequency generated by the OEO is measured to be -118 dBc/Hz at a 10-kHz offset frequency, corresponding to a time jitter of the pulse train of 391.2 fs. The phase noise can be further reduced if an active cavity stabilization mechanism is adopted, enabling further reduction in the time jitter to the order of tens of femtoseconds.

Tunable Broadband Optoelectronic Oscillator Based on Integrated Mutually Coupled Distributed Feedback Lasers

A photonic approach to generating an optoelectronic oscillator (OEO) based on an integrated mutually coupled (IMC) distributed feedback (DFB) laser is reported. Compared with the traditional construction of an OEO, the IMC-DFB has functions of the laser source, intensity modulator, and microwave photonic filters (MPFs), which could simplify the OEO scheme. An optical feedback loop (O-Loop) and an optoelectronic feedback loop (OE-Loop) are used simultaneously in the proposed scheme. The optical loop was used to reduce the optical linewidth of the IMC-DFB to improve the quality of the laser source in the OE loop. The IMC laser consisted of two distributed feedback (DFB) laser sections with a semiconductor optical amplifier (SOA) section in between. The optical linewidth of the IMC-DFB is reduced from 5.86 MHz to 3.94 kHz due to the feedback of the optical loop. By tuning the current of the SOA and DFB sections, tunable microwaves ranging from 24.9 to 46.5 GHz with single-sideband (SSB) phase noise below −110 dBc/Hz at a 10 kHz offset from the carrier were realized.