Coupled Optoelectronic Oscillator Research Articles

Ultralow-phase-noise and broadband frequency-hopping coupled optoelectronic oscillator under quiet point operation

Advancements in microwave photonics have yielded novel approaches for generating high-purity microwave sources. Among these, optoelectronic oscillators (OEOs) and coupled optoelectronic oscillators (COEOs) have demonstrated the capability to generate frequency-independent microwaves with exceptionally low phase noise. Nonetheless, the tunability of the oscillators is rather limited due to the necessity for narrowband electronic bandpass filters, presenting challenges in achieving both wide and rapid tuning capabilities. Here, we present a COEO featuring ultralow phase noise, flexible tuning capability, and high robustness. This is achieved through a quiet point (QP)-operated harmonic mode-locked fiber laser, which effectively mitigates optical amplifier noise and supermode competition, thus significantly diminishing the necessity for ultra-narrow electronic filters. Due to the liberated tuning ability, we present an oscillator that can be tuned from 2 GHz to 18 GHz, with phase noise as low as −140 dBc/Hz at 10 kHz under the QP operation. We then illustrate the practical application of the proposed oscillator in generating frequency-hopping signals with consistent spurious modes less than −85 dBc, absolute phase noise below −135 dBc/Hz at 10 kHz, hopping resolution of 1.25 MHz, and fractional frequency stability below 6.1×10−12 at 1 s averaging time when locked to a reference. The presented COEO structure emerges as a compelling solution for agile and low-noise microwave sources in advanced wireless communication and radar systems.

Enhancing sensitivity of trace copper detection based on coupled optoelectronic oscillator

We propose to employ coupled optoelectronic oscillator (COEO) for trace copper detection to achieve an enhanced sensitivity and stability. By replacing the optical source of the traditional OEO with an optical feedback loop, the oscillation frequency is determined by the cavity loop length in conjunction with the laser output wavelength. The sensing head, which is fabricated by inserting a fusion spliced fiber with large lateral offset fusion splicing in a Sagnac loop, act as a wavelength selection component in the laser loop. Hence, the oscillation frequency changes with the varying solution concentration. Benefitting from the matching of the laser loop and the OEO loop, the final oscillation mode is more stable. By comparing the performance of the COEO-based, single-loop and dual-loop OEO-based sensors, the highest sensitivity of 41.1 Hz/(uMol/L) is obtained by the COEO-based sensor. Since the positive feedback of two loops, the single mode oscillation can be implemented without additional devices.

Faraday Rotation Angle Measurement based on Coupled Optoelectronic Oscillator with Temperature Compensation

Faraday Rotation Angle Measurement based on Coupled Optoelectronic Oscillator with Temperature Compensation

GHz repetition rate tunable optical pulses generation using a SBS-based coupled optoelectronic oscillator

An optical pulse generator with a tunable repetition rate of GHz is proposed employing a coupled optoelectronic oscillator (COEO). The tunability is achieved with an equivalent tunable RF bandpass filter which is realized through phase modulation and stimulated Brillouin scattering (SBS) in a high nonlinear fiber. An injected seed laser is applied to stabilize the central wavelength of the mode-locked loop. The repetition rate of the generated optical pulses is the frequency difference between the Stokes wave and the seed laser. It is adjusted by changing the wavelength of the pump laser for SBS effect. In the experiments, optical pulse trains with a repetition rate of 7.3 GHz–9.3 GHz are generated, and RF oscillating signals at the corresponding frequency are simultaneously produced. The side-mode suppression ratios (SMSRs) of the generated RF signals are all higher than 50 dB, and the phase noise is lower than −80dBc/Hz @10-kHz offset frequency. The tunable range of the optical pulse rate is limited by the bandwidth of the used RF phase shifter in the oscillating loop.

A Symmetric Parity–Time Coupled Optoelectronic Oscillator Using a Polarization–Dependent Spatial Structure

We propose and experimentally investigate a symmetric parity-time (PT) coupled optoelectronic oscillator (COEO) based on a polarization-dependent spatial structure. In such a COEO system, the gain/loss and coupling coefficients of two orthogonal polarization optical waves can be controlled by adjusting the polarization controller (PC) and the bias voltage of a Mach-Zehnder modulator (MZM). The single-mode selection of a microwave signal can be implemented by the PT symmetry breaking of a special mode. The performance of the proposed COEO is experimentally examined, and a 10.0 GHz microwave signal with a phase noise of −109.1 dBc/Hz @ 10 kHz and a side mode suppression ratio of 51.4 dB is generated. Moreover, an optical frequency comb with a comb tooth spacing of 10.0 GHz and a bandwidth of 100 GHz within a 10 dB amplitude variation can be simultaneously generated.

Finely Tunable Coupled Optoelectronic Oscillator Based on Injection Locking and Phase Locked Loop

Finely Tunable Coupled Optoelectronic Oscillator Based on Injection Locking and Phase Locked Loop

Highly sensitive torsion sensor based on a coupled optoelectronic oscillator incorporating nonlinear polarization rotation.

We propose and demonstrate a new, to the best of our knowledge, technique to implement a high-speed and highly sensitive torsion sensor based on a coupled optoelectronic oscillator (COEO) incorporating nonlinear polarization rotation (NPR). The COEO consists of a mode-locked laser loop and an OEO loop. In the laser loop, the NPR effect effectively induces intensity- and wavelength-dependent loss, which acts as a Lyot birefringent fiber filter. When twisting the polarization-maintaining fiber (PMF), the transmission of the filter varies as well as the laser output wavelength. In the OEO loop, the optical source is provided by the output signal of the mode-locked laser. The variation in the optical carrier wavelength changes the time delay and the oscillation frequency of the OEO loop. The oscillation frequency shift is a linear function of the twist angle. Sensitivities of -60.006 Hz/deg over 360° for a 48 cm PMF and -180.996 Hz/deg over 92° for a 22 cm PMF are achieved.

A Polarimetric Fiber Ring Laser Incorporating a Coupled Optoelectronic Oscillator and Its Application to Magnetic Field Sensing

A novel configuration for a polarimetric fiber ring laser incorporating a coupled optoelectronic oscillator (COEO) is proposed and experimentally demonstrated, and its application to magnetic field sensing is studied. The COEO-based polarimetric fiber ring laser has two mutually coupled loops: the fiber ring laser loop and the OEO loop. In the fiber ring laser loop, longitudinal modes break up into orthogonal polarization modes because of birefringence. The frequency of the polarization mode beat (PMB) signals is determined by the cavity birefringence. In the OEO loop, a microwave signal with its frequency equal to the PMB signal is generated. By feeding the oscillation mode to modulate the optical loop, mode-locking can be achieved, rendering the mode spacing of the laser equal to the frequency of the oscillating OEO mode. We can estimate the birefringence variation by measuring the oscillating frequency of the COEO. To validate the proposed sensing system, a circular birefringence change is introduced in a magneto-optic crystal via the Faraday rotation effect. Then, the magnetic field sensing is implemented. Such configuration can achieve single longitudinal oscillation and realize high-speed and high-precision measurements.

Optical fiber strain sensor with high precision and extended dynamic range based on a coupled optoelectronic oscillator.

We have proposed and experimentally demonstrated an optical fiber strain sensor with high precision and extended dynamic range based on a coupled optoelectronic oscillator (COEO). The COEO is a combination of an OEO and a mode-locked laser, sharing one optoelectronic modulator. The feedback between the two active loops makes the oscillation frequency equal to the mode spacing of the laser. It is equivalent to a multiple of the natural mode spacing of the laser, which is affected by the applied axial strain to the cavity. Therefore, we can evaluate the strain by measuring the oscillation frequency shift. Higher sensitivity can be obtained by adopting higher frequency order harmonics owing to the accumulative effect. We carry out a proof-to-concept experiment. The dynamic range can reach 10000 μ ε. Sensitivities of 6.5 Hz/μ ε for 960 MHz and 13.8 Hz/μ ε for 2700 MHz are obtained. The maximum frequency drifts of the COEO in 90 mins are within ±148.03 Hz for 960 MHz and ±303.907 Hz for 2700 MHz, which correspond to measurement errors of ±22 μ ε and ±20 μ ε. The proposed scheme has the advantages of high precision and high speed. The COEO can generate an optical pulse whose pulse period is influenced by the strain. Therefore, the proposed scheme has potential applications in dynamic strain measurement.

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.

Turnkey Coupled Optoelectronic Oscillator Based on Non-Polarization-Maintaining Fiber Ring Cavity

Turnkey Coupled Optoelectronic Oscillator Based on Non-Polarization-Maintaining Fiber Ring Cavity

Adjustable Coupled Optoelectronic Oscillator Based on Double Frequency Output

At present, the research and development of high-speed communication technology and the more detailed division of communication frequency band have become the hot spots all over the world, which also puts forward a greater technical challenge to the down-conversion system in the receiver. However, the key to the signal receiving performance of down-conversion system is the local vibration performance. Low phase noise, tunable wideband and strong anti-electromagnetic interference have become the new directions for the future research on oscillator. Compared with the traditional microwave local oscillator, the technical advantage of the optoelectronic oscillator is more obvious, and more research and development space is left for the development of the new frequency band of communication technology in the future. The tunable coupling optoelectronic oscillator based on the double frequency output designed in this paper uses the equivalent light source of the ring path to replace the laser, and the wavelength interval of the output spectrum is changed by adjusting the length difference of the optical fiber cavity of the ring path filter. At the same time, by adjusting the bias voltage of the double-output intensity modulator, the oscillator suppresses the fundamental frequency signal and achieves the output of the two-frequency signal. After the test, it can be known that when the optical carrier wavelength of the oscillator is 1551 nm, the tuning range of the output fundamental frequency is 1.8–9 GHZ; The output range of double frequency is 3.6–18 GHz, the output power is greater than 10 dBm, and the phase noise changes slightly at different output frequencies, which is about –102 dBc/Hz.

A Computationally Efficient Integrated Coupled Opto-Electronic Oscillator Model

Coupled opto-electronic oscillators can generate microwave signals with very low phase noise. Several approaches for modelling these oscillators exist, but none of them include the fast carrier dynamics of the mode-locked laser, due to computational limitations. In this work, we present a new model, that is based on a computationally efficient model for semiconductor mode-locked lasers, which is capable of simulating both the fast laser dynamics and the long term phase stability. We use it to model the phase noise of a hybridly integrated coupled opto-electronic oscillator. The results show that a timing jitter of down to $\mathbf {\sim} \text{0.2}$ $\text{ps}$ in the integration range from $\text{10}$ $\text{kHz}$ to $\text{10}$ $\text{MHz}$ is achievable when using an integrated delay line of $\text{10.2}$ $\text{m}$ in the opto-electronic feedback path. It is also shown that by increasing both gain and bandwidth of the opto-electronic feedback link, the timing jitter is reduced. Together, these results present a path towards low phase noise performance of hybridly integrated coupled opto-electronic oscillators, and the model serves as a new design tool. Such oscillators might increase, for example, the spectral efficiency in future data communication systems and thus enable higher data rates.

Coupled Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

A tunable optoelectronic oscillator (TOEO) with coupled dual-loop (CDL) based on stimulated Brillouin scattering (SBS) is proposed. In our scheme, the CDL is constructed by cascading a dual-output Mach–Zehnder intensity modulator (DOMZM) and a single output Mach–Zehnder intensity modulator (MZM). One optical output of the DOMZM is directly injected to the MZM, and the other optical output of the DOMZM is transformed into microwave signals to modulate the MZM. The narrow gain spectrum of SBS is used to select the oscillation frequency and realize frequency tunability. By joining the CDL, we can not only increase side mode suppression ratio (SMSR) and decrease phase noise, but also improve stability of the TOEO. Experimental results show that microwave signals from 2 GHz to 18 GHz with the SMSR as high as 60 dB and the phase noise as low as −95 dBc/Hz at 10 kHz frequency offset are achieved. Furthermore, the frequency drift is less than 0.3 ppm and the power drift is lower than 0.2 dB at 10 GHz within 30 min in lab condition.

High-sensitivity angular velocity measurement based on bidirectional coupled optoelectronic oscillator

Optical gyroscopes play important roles in the field of angular velocity measurement. Improving the accuracy of the Sagnac effect is always an attractive research topic. In this work, a new scheme is proposed and demonstrated by using a polarization-maintaining bidirectional coupled optoelectronic oscillator (COEO) incorporating a Sagnac loop structure. In COEO resonant cavity, optical carrier and one of the first-order sidebands are separated by polarization delay interferometers, and propagate in opposite directions in the Sagnac loop. The rotation-induced phase difference between optical carrier and first-order sideband produces a frequency shift in COEO system. Moreover, for bidirectional system, the trends of resonant frequencies shift are opposite. Therefore, the difference of bidirectional resonant frequencies is the summation of unidirectional rotation-induced frequency shift. And less-complex, high-resolution microwave detection technology is used to improve the detection resolution of frequency difference. Therefore, a high-sensitivity angular velocity measurement system is implemented, with a sensitivity scale of 172.04 kHz/(rad/s). Theoretically, a minimally detectable angular velocity is 1.2 °/h.

Highly Spectrally Pure 90-GHz Signal Synthesis Using a Coupled Optoelectronic Oscillator

A 90-GHz frequency reference is synthesized through harmonic generation from a 30-GHz coupled optoelectronic oscillator. The 30-GHz signal and its third harmonic feature an average power level of −5.6 and −15.8 dBm, respectively, at the photodiode output. Phase noise measurements at 30 and 90 GHz have been performed and phase noise levels as low as −114.4 dBc/Hz at 1-kHz offset for the fundamental and −104 dBc/Hz at 1-kHz offset frequency for the 90-GHz signal have been obtained.

A Coupled Optoelectronic Oscillator With Performance Improved by Enhanced Spatial Hole Burning in an Erbium-Doped Fiber

A coupled optoelectronic oscillator (COEO) with performance improved by the enhanced spatial hole burning (SHB) effect in an unpumped erbium-doped fiber (EDF) is proposed and demonstrated. In the proposed COEO, a strong standing-wave forming structure is incorporated to enable large SHB effect in the unpumped EDF, which significantly improves the supermode suppression ratio of the generated optical pulse, and the sidemode suppression ratio and phase noise performance of the generated electrical signal. A 10-GHz COEO is established. A stable optical pulse train is successfully generated with a supermode suppression ratio as high as 75 dB. The system is observed in the lab environment for 1.5 h with no significant variation. Without introducing any long fiber or multiloop structure in the optoelectronic oscillator loop, the sidemode suppression ratio of the 10-GHz RF output reaches 90 dB and the phase noise is about −130 dBc/Hz at 10-kHz offset. The influence of the optical power injected into the EDF, and the effects of the absorption coefficient and length of the EDF on the key parameters of the COEO are investigated and discussed.

Super-mode noise suppression for coupled optoelectronic oscillator with optoelectronic hybrid filter

A novel scheme to implement the super-mode noise suppression for coupled optoelectronic oscillator (COEO) utilizing an optoelectronic hybrid filter (OEF) is proposed and experimentally demonstrated. The OEF is made up of a multi-mode optoelectronic oscillator, incorporated in the COEO loop to serve as a channelized RF filter, while the fiber laser in the COEO can be viewed as another channelized RF filter of high Q. The cascading of the two equivalent channelized RF filters can greatly improve the out-of-band rejection ratio of the combined filter and restrain the adjacent mismatched modes in the oscillating loop. In the experiment, a 9.955 GHz RF signal with the side mode suppression ratio of larger than 82.6 dB is obtained, and the super-mode noise spur at an offset frequency of 5.9 MHz is suppressed 15 dB to -145 dBc/Hz level compared with the conventional COEO.

Polarization-maintained coupled optoelectronic oscillator incorporating an unpumped erbium-doped fiber

A polarization-maintained coupled optoelectronic oscillator (COEO) with its performance significantly improved by a short-length unpumped erbium-doped fiber (EDF) is reported and experimentally investigated. A 10 GHz optical pulse train with a supermode suppression ratio of 61.8 dB and a 10 GHz radio frequency signal with a sidemode suppression ratio of 94 dB and a phase noise of ?121.9 dBc/Hz at 10 kHz offset are simultaneously generated. Thanks to saturable absorption of the 1 m unpumped EDF, which introduces relatively large cavity loss to the undesired modes and noise, the supermode suppression ratio and the phase noise are improved by 9.4 and 7.9 dB, respectively.

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.