Frequency-tunable Optoelectronic Oscillator Research Articles

Wideband frequency-tunable optoelectronic oscillator with ultra-narrow linewidth based on stimulated Brillouin scattering

We propose and theoretically analyze a wideband frequency-tunable optoelectronic oscillator based on stimulated Brillouin scattering (SBS) and gain–loss superposition with ultra-narrow linewidth. Typically, the linewidth of optoelectronic oscillator (OEO) based on SBS is difficult to narrow because of the relatively fixed 3-dB bandwidth of SBS. In the structure, three generated new pump lights with certain frequency intervals generated by optical modulation on the pump laser light are launched in a highly nonlinear fiber (HNLF) from the direction opposite to the phase modulated signals, where the SBS effect occurs. The gain spectrum of one pump light is superimposed with the loss spectra of the other two pump lights. As a result the linewidth of the output signal reduces highly compared with conventional OEOs based on SBS. Consequently, a microwave signal with wide tunable range of 1 GHz to 40 GHz and near 60% linewidth reduction at most is realized in theory. Particularly, the theoretical simulation of the phase noise is about −112.5 dBc/Hz at 10 kHz offset when the wavelength of the continuous wave laser is 1550 nm and the frequency of the output signal is 35 GHz. In addition, the proposed method can be generalized, theoretically, to any OEO based on SBS to effectively reduce its linewidth. Overall, the proposed structure improves the quality of generated signals prominently and has vast potential in microwave generation.

Automatic Power Control for a Frequency Tunable Optoelectronic Oscillator

Automatic power control for a frequency tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO is implemented by a tunable microwave photonic filter using a phase-shifted fiber Bragg grating (PS-FBG) for frequency-tuning. Taking advantage of the automatic power control (APC), the problem of large power fluctuation in the OEO output signal during the frequency tuning or interfered by the external disturbance can be significantly improved and the functionality of equalization is realized. By adjusting the DC reference voltage of the automatic power control circuit, the tuning of RF signal power is conveniently achieved. To the best of the authors’ knowledge, this is the first time that the output power regulation of OEO has been implemented. For verification, the low-cost OEO at 2–7 GHz is established. Experiment results show that the output power fluctuation of less than 3 dB and a phase noise of −113 dBc/Hz@10 kHz are achieved.

Chip-Scaled Ka-Band Photonic Linearly Chirped Microwave Waveform Generator

Synthetic aperture radar (SAR) systems employ a Linearly Chirped Microwave Waveform Generator (LCMWG) with large time–bandwidth product (TBWP), to provide a wide range resolution. Photonics has now been recognized as a disruptive approach to achieve high performance at bandwidth of few tens of gigahertz, with light and compact architectures, due to the typical photonics benefits, such as electromagnetic interference immunity, small power consumption, small footprint, and high immunity to vibration/shock and radiation. In this article, we report on the photonic generation of a high-frequency LCMW, with a large TBWP (102–103), using a chip-scaled architecture, based on a frequency-tunable optoelectronic oscillator (OEO) and a recirculating phase modulation loop (RPML). A new configuration of the OEO employing an ultrahigh Q-factor resonator has been conceived to allow the oscillator working in Ka band at 40 GHz or even more, with very low phase noise. Key building block of the RPML is a phase modulator driven by an engineered parabolic split waveform. The ultra-large pulse compression rate (PCR) >> 102, together with large signal purity, was also obtained, making the proposed architecture particularly suitable for SAR systems with large range resolution demand, such as Earth surveillance and monitoring.

Parity-time symmetric tunable OEO based on dual-wavelength and cascaded PS-FBGs in a single-loop.

The ability to achieve low phase noise single-mode oscillation within an optoelectronic oscillator (OEO) is of fundamental importance. In the frequency-tunable OEO, the wide microwave photonic filter (MPF) bandwidth is detrimental to select single-mode among the large number of cavity modes, thus leading to low signal quality and spectral purity. Stable single-mode oscillation can be achieved in a large time delay OEO system by harnessing the mechanism from parity-time (PT) symmetry. Here, a PT-symmetric tunable OEO based on dual-wavelength and cascaded phase-shifted fiber gratings (PS-FBGs) in a single-loop is proposed and experimentally demonstrated. Combining the merits of wide frequency tuning of PS-FBG-based MPF and single mode selection completed by the PT-symmetric architecture of the OEO, where the gain and loss modes carried by dual-wavelengths to form two mutually coupled resonators in a single-loop, signals range from 1 GHz to 22 GHz with the low phase noise distributed in -122∼ -130 dBc/Hz at 10 kHz offset frequency are obtained in the experiment.

Photonic microwave synthesizer based on optically referenced sub-sampling phase-locked optoelectronic oscillator

Wideband and low phase noise microwave synthesizers are of critical significance for various applications. Phase-locked loop (PLL) based microwave synthesizers are widely used. Normally, the phase noise of the PLL based microwave synthesizer is limited by the phase noise of the reference, phase detector, frequency divider and voltage-controlled oscillator (VCO). It is challenging to achieve a pure electronic microwave synthesizer with wide frequency coverage and low phase noise, simultaneously. Here, we propose a photonic microwave synthesizer which incorporates an ultra-low phase noise fiber mode-locked laser (MLL) based reference and a wideband frequency tunable optoelectronic oscillator (OEO) based VCO in a hybrid optoelectronic sub-sampling PLL. The frequency of the OEO can be locked to the harmonics of the repetition-rate of the fiber MLL, which combines the distinct advantages of the low phase noise of MLL and the wideband frequency tuning of OEO. A sub-sampling phase detector formed by a balanced optical microwave phase detector (BOMPD) eliminates the use of RF frequency divider and maintains low residual phase noise. We attain an X- and Ku-band photonic microwave synthesizer with over an octave frequency coverage from 8 GHz to 18 GHz, frequency synthesis step of 60.74 MHz and phase noise of -80 dBc/Hz at 100 Hz offset and -100 dBc/Hz at 1 kHz offset for 10 GHz output. The characteristics of the optoelectronic sub-sampling phase detector, the phase-locking dynamics of the hybrid optoelectronic PLL, and the phase noise of the synthesized RF signals are both theoretically and experimentally investigated. The experimental results agree well with the theoretical models. Our approach is promising to achieve wideband and low phase noise microwave synthesis.

Parity-time symmetric frequency-tunable optoelectronic oscillator based on a Si3N4 microdisk resonator.

The optoelectronic oscillator (OEO) generates low-phase noise and high-frequency microwave signals thanks to a high Q-factor cavity with long and low-loss fiber delay. Traditionally, for the desired mode selection from the ultradense cavity modes, a narrowband electrical filter is needed, whose frequency tuning is very limited. On the other hand, for a tunable OEO offered by a microwave photonic filter (MPF), a paradox existed between the large number of cavity modes and the wide MPF bandwidth. Here, we achieve a tunable OEO using the mode-selection mechanism of parity-time symmetry, which overcomes the paradox. A high Q-factor silicon nitride microdisk resonator (Si3N4 MDR) is introduced to achieve frequency filtering and tuning. Moreover, the experimental results reveal that the tunable OEO generates a signal range from 3 GHz to 20 GHz with a phase noise about -120dBc/Hz at a 10 kHz offset frequency.

Ultrawide and tunable self-oscillating optical frequency comb generator based on an optoelectronic oscillator

A new mechanism for ultra-wide and tunable self-oscillating optical frequency comb (OFC) based on a novel frequency tunable optoelectronic oscillator (OEO) is proposed using cascaded configuration of optical phase-modulator (PM) and electro-absorption-modulator (EAM). The proposed scheme does not require a dedicated RF source to drive the optical-modulators. The OEO is configured by incorporating the self-oscillation loop with cascaded modulators. A high-speed PIN-photodetector (PD) with a dark current of 10nA, bandwidth of 20 GHz, and responsivity of 1A/W in correspondence with an electrical-amplifier (EA), and bandpass Bessel filter (BPBF) is utilized to convert the optical signal into a microwave signal. The PD detected signal is amplified by the EA. The BPBF is used to extract a 10 Mhz bandwidth from the amplified signal with tunable central frequencies. Part of the extracted OEO signal is directly connected to the PM, whereas, a 180-degree phase-shift is added to the signal before transmitting into the EAM. The PM produces ~19 carrier frequencies, whereas, the generated tunable carriers by the EAM ranges from 85 to 35carrriers with an overall bandwidth of ~1.28THz. The negligible power-fluctuations, good number of generated carriers, and high tone-to-noise-ratio of around 40 dB endorse the applicability of the proposed scheme.

Parity-time-symmetric frequency-tunable optoelectronic oscillator with a single dual-polarization optical loop.

We propose and experimentally demonstrate a parity-time (PT)-symmetric frequency-tunable optoelectronic oscillator (OEO) in which the PT symmetry is implemented based on a single dual-polarization optical loop. By employing the inherent birefringence of a z-cut lithium niobate (LiNbO3) phase modulator (PM), two mutually coupled optoelectronic loops supporting orthogonally polarized light waves with one experiencing a gain and the other a loss are implemented. By controlling the gain, loss, and the coupling coefficients between the two loops, the PT symmetry breaking condition is met, which enables the OEO to operate in single mode without using an ultranarrow passband optical or microwave filter. The frequency tunability is realized using a microwave photonic filter (MPF) implemented using the PM and a phase-shifted fiber Bragg grating (PS-FBG). The proposed PT-symmetric OEO is experimentally evaluated. A stable and frequency-tunable microwave signal from 2 to 12 GHz is generated. The phase noise of the generated signal at 11.8 GHz is measured, which is -124dBc/Hz at a frequency offset of 10 kHz.

Simple frequency-tunable optoelectronic oscillator using integrated multi-section distributed feedback semiconductor laser.

A novel approach to realizing an optoelectronic oscillator (OEO) based on an integrated multi-section (IMS) distributed feedback (DFB) laser is proposed and experimentally demonstrated. Our scheme adopts the method of direct modulation and a built-in microwave photonic filter (MPF), making the structure simpler and more flexible than an external modulator and electrical bandpass filter (EBPF). The IMS-DFB laser, which can overcome the drawbacks of using discrete lasers, is the key device in the scheme. Further, the two DFB sections, which are fabricated by Reconstruction Equivalent Chirp (REC) technique, are injected mutually. The SSB phase noise of the generated signal at the frequency of 20.3 GHz is -115.3 dBc/Hz@10kHz and -92.9 dBc/Hz@1kHz. The sidemode suppression ratio (SMSR) is 60.94 dB, which is a 40 dB improvement over a single loop. Furthermore, we demonstrate that the phase noise improves about 8 dB at the frequency offset of 1 kHz, when employing 13 km and 5.4 km fibers as the dual loop. The simple and compact structure, which consists of an IMS-DFB laser with high wavelength controlling accuracy and low process requirement, is a promising development for OEO integration.

Wideband frequency-tunable optoelectronic oscillator with a directly modulated AlGaInAs/InP integrated twin-square microlaser.

We propose and demonstrate an optoelectronic oscillator with a directly modulated AlGaInAs/InP integrated twin-square microlaser for generating wideband frequency-tunable microwave signals with low phase noise. Apart from the relaxation oscillation peak, the modulation response of the twin-square microlaser working at the mutual optical injection state exhibits a significant enhancement around the beating frequency of the lasing modes in the two square cavities owing to the photon-photon resonance. A self-sustaining oscillation can be generated around the modulation response peak with the lowest loop loss occurring at the relaxation oscillation frequency or the beating frequency, depending on the practical state of the twin-square microlaser. High-quality tunable microwave signals ranging from 2.22 to 19.52 GHz are generated with single sideband phase noises below -110 dBc/Hz at the 10 kHz offset frequency and side-mode suppression ratios of approximately 40 dB by tuning the injection currents of the twin-square microlaser.

Stable and Compact Dual-Loop Optoelectronic Oscillator Using Self-Polarization-Stabilization Technique and Multicore Fiber

We experimentally demonstrate a stable and compact dual-loop optoelectronic oscillator (OEO) using multicore fiber and self-polarization-stabilization technique. Two fiber cavities using two standard single mode fibers with different length in traditional dual-loop OEOs are replaced by only one multicore fiber where several cores are linked together to form the short and the long cavity simultaneously. Thanks to this fiber-integrated medium, the OEO relative stability is enhanced and the required cavity length is decreased. To avoid the influence from unstable polarization states of optical signals in these two fiber cavities, a self-polarization-stabilization technique is proposed. Therefore, no additional polarization controller and polarization maintaining fiber are required, contributing to further shorter fiber length in cavities and more compact structure. Moreover, the frequency and power stability of the generated RF signal in OEO is improved a lot compared with the traditional dual-loop OEO using the polarization division multiplexing technique. The experimental results show that the frequency and power drift of the generated RF signal with frequency of 7.8 GHz are within 0.59 ppm and 0.44 dB during 1 h, respectively. Furthermore, by introducing a tunable microwave photonics filter, a frequency tunable OEO is achieved. The experimental results show that the frequency of the generated RF signal could be tuned from 3.5 to 17.1 GHz, the phase noise performance at 10 kHz frequency offset is around −100 dBc/Hz in whole frequency range, and the side mode suppression ratio is 61 dB.

Frequency-Tunable OEO Using a DFB Laser at Period-One Oscillations With Optoelectronic Feedback

A simple frequency-tunable optoelectronic oscillator based on a distributed-feedback (DFB) laser under optical feedback at period-one (P1) oscillation is proposed and experimentally demonstrated. The directly modulated DFB laser functions as an active tunable microwave photonic filter and a high-efficiency modulator due to the high-efficiency response at the P1 oscillation frequency, eliminating the needs of the narrowband electrical bandpass filter and high-gain RF amplifier. By tuning the optical feedback ratio, tunable microwaves ranging from 8.6 to 15.2 GHz with single sideband phase noise below −110.88 dBc/Hz at a 10-kHz offset from the carrier were realized.

Multi-format signal generation using a frequency-tunable optoelectronic oscillator.

A novel photonic approach for multi-format signal generation based on a frequency-tunable optoelectronic oscillator (OEO) is proposed using a dual-polarization quadrature phase shift-keying (DP-QPSK) modulator. The upper dual-parallel Mach-Zehnder modulator (DP-MZM) integrated in the DP-QPSK modulator is properly biased to serve as an equivalent phase modulator, which functions in conjunction with a phase-shifted fiber Bragg grating (PS-FBG) in the OEO loop as a high-Q microwave photonic band-pass filter. The lower DP-MZM in the DP-QPSK modulator injected by the oscillation signal functions as a frequency multiplier, a phase-coded microwave signal generator or an optical frequency comb generator, respectively, with different signal injection methods. An experiment is performed. When the lower DP-MZM serves as a frequency multiplier, tunable frequency-doubled and quadrupled microwave signals up to 40 GHz are generated without using an optical notch filter; and if it functions as a phase-coded microwave signal generator, fundamental and frequency-doubled binary phase-coded microwave signals are generated with a tunable frequency. Furthermore, tunable five-line optical frequency combs are also generated using the compact system without an external RF source. The performance of the generated signals is also investigated.

Widely Tunable Optoelectronic Oscillator Using a Dispersion-Induced Single Bandpass MPF

A widely frequency tunable optoelectronic oscillator (OEO) based on a dispersion-induced single bandpass microwave photonic filter (MPF) is proposed and demonstrated. The MPF is implemented using a broadband optical source, a dual-output Mach–Zehnder modulator ( $1\times 2$ MZM) with unbalanced output fiber lengths into the upper and lower arms of the $1\times 2$ MZM, and a linearly chirped fiber Bragg grating. Through adjusting the difference between the two output fiber lengths after the $1\times 2$ MZM, the oscillation frequency of the OEO can be tuned by incorporating the MPF into an optical domain dual-loop OEO. A proof-of-concept experiment is performed. A microwave signal with a frequency tunable from 4.078 to 13.05 GHz is generated and its performance is investigated.

Frequency-tunable Optoelectronic Oscillator With Synchronized Dual-Wavelength Narrow-Linewidth Laser Output

Synchronized dual-wavelength narrow-linewidth lasers and a wideband frequency-tunable optoelectronic oscillator are simultaneously realized using mutual-injection-locked distributed feedback (DFB) lasers. The two laser modes serve as the seeding light and a microwave photonic filter for the optoelectronic oscillation. Mutual-injection locking between the two DFB lasers through a delay fiber loop results in synchronized narrow-linewidth operation. Microwave tuning from 19 to 41 GHz has been obtained through thermal-tuning of the DFB lasers, with the single-sideband phase noises below −100 dBc/Hz at a 10-kHz frequency offset from the carrier. Laser linewidth has been reduced from several-MHz to kHz with a linewidth reduction factor over 104.

Wideband frequency tunable optoelectronic oscillator based on a dispersion compensated microwave photonic filter

A wideband tunable optoelectronic oscillator (OEO) based on a dispersion compensated phase modulated microwave photonics filter (MPF) is proposed and experimentally demonstrated. The MPF, consisting of a tunable laser source (TLS), a phase modulator, a fiber Bragg grating Fabry–Perot filter and a photodetector, is used as the oscillating mode selection device. Dual-loop configuration is employed to make sure the OEO operates at a single oscillating mode. Theoretically analysis is carried out to demonstrate the tuning range limitation due to the phase-modulation to intensity-modulation conversion caused by long single mode fiber (SMF). To overcome this disadvantage and expand the signal tuning range, a dispersion compensation fiber is introduced followed by the SMF to compensate the fiber dispersion. By simply adjusting the wavelength of the TLS, an ultra-wideband tunable microwave signal with the frequency tuning range from 2 to 44 GHz is obtained. The phase noise is also investigated and measured to be less than − 110 dBc/Hz at an offset of 10 kHz within the whole frequency range.

Tunable optoelectronic oscillator using a polarization division multiplexing emulator and a polarization modulator

This paper proposes a frequency-tunable optoelectronic oscillator (OEO) based on a single bandpass microwave photonic filter (MPF), which is implemented by a polarization division multiplexing emulator (PDME) with a tunable time delay difference between the two polarization states, a polarization modulator (PolM), a broadband optical source (BOS), and a linearly chirped fiber Bragg grating (LCFBG). The center frequency of the MPF can be shifted by adjusting the time delay difference between the two polarization states of the PDME. By incorporating the MPF into an OEO, the oscillation frequency of OEO is tuned. The proposed OEO is experimentally demonstrated. Frequency-tunable oscillation signals from 2.45 to 12.72 GHz are generated with their performance being investigated.

Photonic Generation of a Phase-Coded Chirp Microwave Waveform With Increased TBWP

Photonic generation of a phase-coded chirped microwave waveform with an increased time bandwidth product (TBWP) using a frequency-tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The frequency-tunable OEO is implemented using a tunable laser source (TLS), a phase modulator (PM), a phase-shifted fiber Bragg grating, and a photodetector (PD), with the frequency tuning realized by tuning the wavelength of the TLS. A frequency-tunable optical sideband with a frequency that is equal to that of the optical carrier plus the OEO oscillation frequency is generated by the OEO, which is then orthogonally polarization multiplexed with the optical carrier from the TLS at a polarization beam combiner, and applied to a polarization modulator, to which a binary phase-coded parabolic electrical signal is applied. By beating the two orthogonally polarized optical signals at a PD, a phase-coded chirped microwave waveform is generated. The TBWP is significantly increased due to the increase of the temporal duration of the microwave waveform. The proposed approach is experimentally demonstrated. Two phase-coded chirped microwave waveforms with TBWPs of 58.5 and 80 000 using two phase coding signals corresponding to a 13 Barker code and a 20480-bit pseudorandom sequence are generated.

Frequency-tunable optoelectronic oscillator using a dual-mode amplified feedback laser as an electrically controlled active microwave photonic filter.

A widely tunable optoelectronic oscillator (OEO) based on a self-injection-locked monolithic dual-mode amplified feedback laser (DM-AFL) is proposed and experimentally demonstrated. In the proposed OEO structure, the DM-AFL functions as an active tunable microwave photonic filter (MPF). By tuning the injection current applied on the amplifier section of the AFL, tunable microwave outputs ranging from 32 to 41 GHz and single sideband phase noises below -97 dBc/Hz at 10 kHz offset from the carriers were realized.

Frequency tunable optoelectronic oscillator based on a directly modulated DFB semiconductor laser under optical injection.

A frequency tunable optoelectronic oscillator based on a directly modulated distributed-feedback (DFB) semiconductor laser under optical injection is proposed and experimentally demonstrated. Through optical injection, the relaxation oscillation frequency of the DFB laser is enhanced and its high modulation efficiency can enable the loop oscillation with a RF threshold gain of less than 20 dB. The DFB laser is a commercial semiconductor laser with a package of 10 GHz, and its packaging limitation can be overcome by optical injection. In our scheme, neither a high-speed external modulator nor an electrical bandpass filter is required, making the system simple and low-cost. Microwave signals with a frequency tuning range from 5.98 to 15.22 GHz are generated by adjusting the injection ratio and frequency detuning between the master and slave lasers. The phase noise of the generated 9.75 GHz microwave signal is measured to be -104.8 dBc/Hz @ 10 kHz frequency offset.