Mode-locked Optoelectronic Oscillators Research Articles

Generation of frequency-shift keying (FSK) and amplitude-shift keying (ASK) RF signals by diode-tuned Fourier domain mode-locked opto-electronic oscillator

Generation of frequency-shift keying (FSK) and amplitude-shift keying (ASK) RF signals by diode-tuned Fourier domain mode-locked opto-electronic oscillator

Generation of short microwave pulse trains based on a hybrid mode-locked optoelectronic oscillator

A novel approach, to the best of our knowledge, for generating short microwave pulse trains based on a hybrid mode-locked optoelectronic oscillator (HML-OEO) is proposed and demonstrated. In the proposed scheme, a saturable absorber (SA) device is inserted into the active mode-locked OEO (AML-OEO) to compress the pulse width of the microwave pulse trains. Numerical simulations and experimental results show that the HML-OEO generates a short microwave pulse train with a repetition rate of 98.994 kHz through fundamental frequency mode locking, and its pulse width is compressed by about 50% compared to the AML-OEO. Additionally, in the experiment, microwave pulse trains with different repetition rates are generated by second-, third-, fourth-, and fifth-order harmonic mode locking, respectively. Compared to the AML-OEO, the HML-OEO achieves pulse compression effects of 49.3%, 49.8%, 49.4%, and 49.9%, respectively. Notably, compared to the AML-OEO, the proposed scheme also exhibits outstanding performance in frequency stability.

Enhancing long-term frequency stability of an actively mode-locked optoelectronic oscillator via a phase-locked loop.

A method to improve the frequency stability of microwave frequency comb (MFC) signals generated by an actively mode-locked optoelectronic oscillator (AML-OEO) is proposed and experimentally demonstrated. In the experiment, fundamental mode locking in the constructed AML-OEO is achieved, producing MFC signals with a center frequency of 2.165 GHz and a repetition rate of 396.629 kHz. By locking the MFC comb tooth to a stable reference source using a phase-locked loop (PLL), the stability of MFC's center comb tooth reaches 2.4 × 10-13@1000s and the frequency stability improves by more than two orders of magnitude on a second-level time scale compared to the free-running AML-OEO. The improved frequency stability of MFC signals makes AML-OEO suitable as high-performance microwave sources for various critical applications.

High resolution frequency shift measurement based on the time domain mode-locked optoelectronic oscillator

High resolution frequency shift measurement based on the time domain mode-locked optoelectronic oscillator

Generation of a Long Linearly Chirped Microwave Waveform Based on a Fourier Domain Mode-Locked Optoelectronic Oscillator Incorporating a Frequency Shifting Loop

Generation of a Long Linearly Chirped Microwave Waveform Based on a Fourier Domain Mode-Locked Optoelectronic Oscillator Incorporating a Frequency Shifting Loop

Dual-loop fourier domain mode-locked optoelectronic oscillator based on three-section mutual-injection integrated semiconductor laser

A directly-modulated Fourier domain mode-locked optoelectronic oscillator (FDML_OEO) based on three-section mutual-injection integrated semiconductor laser (TS_MIISL) with dual-loop feedback structure is proposed and experimentally demonstrated in this paper. By driving the TS_MIISL to work under the period-one (P1) state and modulating the TS_MIISL with low-repetition-rate sawtooth drive signal, a linear frequency-modulated (LFM) signal can be generated. Through adjusting the delay time of the feedback loops to match with the time duration of drive signal, the LFM signal can be significantly optimized in terms of spectral purity. In such a proposed FDML_OEO, no external modulator, polarization controller (PC) and filter are utilized, leading to an outstanding advantage in terms of stable performance and compact structure. The high-purity LFM signal with center frequency and bandwidth of 14 GHz and 8 GHz is obtained. The maximum time-bandwidth product (TBWP) of generated LFM signal could reach up to 48800. Due to the introduction of dual feedback loop, the frequency comb contrast and linewidth could reach up to 45 dB and 5 kHz, respectively. The center frequency and bandwidth of the generated signals are both tunable. A demonstration is made to prove that the pre-compensation processing could improve the linearity of LFM signals.

On-Chip Photonic Generation of Tunable Wideband Phase-Coded Linearly-Chirped Microwave Waveforms

On-chip photonic generation of tunable wideband phase-coded linearly-chirped microwave waveforms (PC-LCMWs) is proposed and experimentally demonstrated. The on-chip generation system consists of two main parts. One part is a hybrid Fourier-domain mode-locked opto-electronic oscillator (FDML-OEO) for a wideband linearly-chirped microwave waveform (LCMW) generation; the other part is a high-speed microwave photonic phase shifter (MPPS) to perform the phase-coding. By synchronizing the two parts, a PC-LCMW is finally generated. In the proposed chip, a thermally-tunable ultrahigh-Q micro-ring resonator (MRR) is a key component which is incorporated in the FDML-OEO loop to select the oscillation frequency, and an electrically-tunable micro-disk resonator (MDR), as another key component, is used to code the phase of the generated microwave signal while maintaining the amplitude by controlling the free carrier concentration in the lateral PN junction of the disk. With the use of the two key component chips, an experiment is performed and different PC-LCMWs are generated with a tunable center frequency from 12 to 16 GHz, a bandwidth from 2 to 6 GHz, and phase code of 7-bit and 13-bit Barker code. In particular, a PC-LCMW with a maximum bandwidth as wide as 6 GHz and an ultra-large time-bandwidth product (TBWP) as large as 1.96×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> is experimentally demonstrated. The proposed integrated PC-LCMW generation system holds great advantages including ultracompact configuration and fast tunability, which holds a high potential for applications in modern high-resolution multi-functional radar system.

Generation of Phase-Coded LFM Signals Based on Fourier Domain Mode-Locked Optoelectronic Oscillator

Phase-coded linearly frequency modulated (LFM) signals play essential roles in enhancing the performance of radar as well as exploring wireless communication. To generate phase-coded LFM signals, we propose to launch a phase-coded signal into an optoelectronic oscillator (OEO). When the OEO is Fourier domain mode-locked (FDML), a phase-coded LFM signal is generated. The center frequency, bandwidth, coding pattern, and bit rate of the generated signals can be flexibly adjusted. In the experiment, a phase-coded LFM microwave signal with a bandwidth of 5.0 GHz is generated, corresponding to a time bandwidth product (TBWP) of 2.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . The center frequency and bandwidth of the generated signals are adjusted from 9.7 to 10.7 GHz and from 5.0 to 3.0 GHz, respectively. Meanwhile, different coding patterns and rates are also implemented, indicating that the phase-coded LFM signal can be used in wireless communications. The proposed approach can potentially be applied to multifunctional integration in radar.

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.

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.

Dual-loop diode-tuned Fourier domain harmonically mode-locked opto-electronic oscillator with over 50 dB side-mode spur reduction.

We propose and demonstrate a dual-loop harmonic Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) for drastically reducing the side-mode spurs. The frequency domain mode-locking is achieved by synchronizing the scanning period of the filter to an integer fraction of the round trip times of the two loops with a self-made low cost diode-tuned RF filter. We found, for the first time to the best of authors' knowledge, that the frequency scanning bandwidth (FSBW) of the mode-locked output is strongly affected by the length mismatch between the two fiber loops. By using the phase noise of FDML OEO's delayed self-heterodyne signal as a performance indicator, we found experimentally that both the locking bandwidth and the FSBW of the device are inversely proportional to the length mis-match of the two loops. Finally, with dual-loop fiber lengths of 2041m and 2449.2m, including 2039m common fiber loop, we successfully obtained linearly chirped microwave signals around 9GHz with a phase noise of -127dBc/Hz at 10kHz offset from the 9GHz carrier, a FSBW of 0.4GHz, and a chirp rate of 200THz/s at 500.38 kHz repetition rate. More impressively, the side-mode spur ratio of the linear frequency modulated (LFM) signal is reduced to less than -83dB, the lowest ever achieved for a FDML OEO to the best of authors' knowledge, which is more than 50dB improvement over that achieved with a single loop FDML OEO reported previously.

Regeneratively mode-locked optoelectronic oscillator.

A regeneratively mode-locked optoelectronic oscillator is proposed to realize multi-mode phase locking in optoelectronic oscillators by coupling a modulating signal generation loop. To verify the feasibility of the scheme, a numerical simulation model is established. In addition, a proof-of-concept experiment is demonstrated in a single-loop optoelectronic oscillator with a 2.2 km polarization-maintaining single-mode fiber, using an electric amplitude modulator/Mach-Zehnder modulator as an active mode-locking device. The generated microwave pulse signal has a center frequency of 10 GHz and a repetition rate of 95 kHz. We believe this scheme can provide a new approach to overcome the problem of detuning between the modulating frequency and the mode spacing during long-term operation.

Multi-carrier broadband signal generation based on a mutually dual-domain mode-locked optoelectronic oscillator

A novel dual-domain mode-locked optoelectronic oscillator (DDML-OEO) is proposed and demonstrated to generate multi-carrier broadband signal. The DDML-OEO is built up by mode-locking a conventional OEO at the time domain and Fourier domain mutually. The Fourier domain mode locking (FDML) is achieved by driving the laser with a period triangular waveform voltage, of which the frequency is synchronized with the round-trip time of the OEO loop. Meanwhile, an electrical signal synchronizing with the free spectrum range (FSR) of the OEO loop is injected to realize the time domain mode locking (TDML) process. Through combined mechanisms of the FDML and TDML, the multi-carrier broadband signal can be directly generated via the DDML-OEO. Thanks to the TDML, different multi-carrier bands of the signal are coherent and the phase noise of the generated broadband waveform is strongly suppressed compared with free-running FDML-OEO. With the proposed scheme, an ultra-low phase noise of −111.1 dBc/Hz at 10 kHz offset for the generated multi-broadband signal waveform is achieved when the cavity length is about 3000 m. The bands interval, the bandwidth and the central frequency tunability are all discussed, which have great potential usage in multi-channel radar transmitter.

Tunable Microwave Pulse Generation Based on an Actively Mode-Locked Optoelectronic Oscillator

A tunable microwave-pulse-generation scheme is proposed and demonstrated by employing an actively mode-locked optoelectronic oscillator (OEO) based on a microwave photonic filter (MPF). The MPF mainly consists of a phase-shifted fiber Bragg grating (PS-FBG) and a phase modulator. The microwave pulse trains with variable repetition rates are achieved by injecting an external signal, of which the frequencies are equal to an integer multiple of the free spectrum range (FSR) of the OEO. The multi-mode oscillation mechanism is discussed in detail theoretically and experimentally. A microwave pulse train with a central frequency of 9.25 GHz and repetition rate of 1.68 MHz is demonstrated by setting the injecting signal frequency to be the same with the FSR of the OEO. A tunable center frequency of the microwave pulses from 5.47 GHz to 18.91 GHz can be easily generated by tuning the laser frequency benefit from adopting the MPF. Furthermore, the microwave pulses with different pulse periods of 297.62 ns, 198.69 ns, and 148.81 ns are also realized by harmonic mode-locking. The proposed tunable microwave-pulse-generation method has potential applications in the pulse Doppler radar and communications.

Active mode lock optoelectronic oscillator based on the simulated Brillouin scattering effect.

An active mode-locked optoelectronic oscillator (AML-OEO) based on the simulated Brillouin scattering (SBS) effect without an electrical filter is demonstrated here. By using phase modulation and SBS-based selective sideband amplification, the central frequency of the proposed SBS-AML-OEO is easily adjusted by simply changing the pump laser frequency instead of the filters. A microwave frequency comb signal with an adjustable central frequency and fixed bandwidth are generated by injecting a mode-locking external RF synchronizing with the free spectral ranging FSR. In addition, the harmonic SBS-AML-OEO is also achieved by harmonic signal injection. The proposed method reveals a simple solution to tune the central frequency and has the potential to be integrated on a chip since there is no structure changing in the scheme.

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.

Microwave Photonic Temperature Sensing Based on Fourier Domain Mode-Locked OEO and Temperature-to-Time Mapping

We report a novel temperature sensor based on a Fourier domain mode-locked optoelectronic oscillator (FDML OEO) and temperature-to-time mapping. In the proposed scheme, a narrow bandpass electrical filter is inserted into the FDML OEO cavity. The temperature information is mapped to the time interval of the output pulses due to the temperature-sensitive bidirectional scanning property of the FDML OEO. Compared with other OEO-based temperature sensors, the proposed scheme avoids the needs of wideband electrical spectrum analyzers, since the temperature-sensitive output pulses can be interrogated using a low-speed oscilloscope working under sub-Nyquist sampling condition. In the proof-of-concept experiment, a high sensitivity of 8.166 μs/°C and resolution of 0.03°C is achieved, demonstrating a good temperature sensing performance.

High-sensitivity optical fiber sensor based on a Fourier domain mode-locked optoelectronic oscillator

We propose and experimentally demonstrate an optical fiber sensor based on a Fourier domain mode-locked optoelectronic oscillator (FDML-OEO), which is achieved by synchronizing the period of the driving current of the laser with the round-trip time of the OEO loop. Pulsed microwave output can be obtained when a narrow-band electrical filter with the central frequency located within the frequency scanning range of the FDML-OEO is employed in the cavity. Thanks to the triangle waveform of the laser driving signal, two microwave pulses can be observed in one period. The wavelength shift of the phase-shift fiber Bragg grating (PS-FBG) in the cavity will produce a shift of the frequency scanning range, resulting in the pulse interval variation of the OEO output. The experimental results indicate that the pulse interval of the generated signal has a linear relationship with the notch wavelength shift of the PS-FBG. By applying fiber axial strain to the PS-FBG, we obtained a high strain measuring sensitivity of 0.817 μs/με. The sensor reveals the advantages of highly sensitive and easy demodulation, providing great potential for practical applications.

Broadband pulse generator based on a self-injection-locked Fourier domain mode-locked optoelectronic oscillator

A novel broadband pulse generator based on a Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) is proposed and experimentally demonstrated. The system consists of a dual-loop self-injection-locked OEO with an on-off keying (OOK) bias-controlled Mach–Zehnder modulator (MZM) as an optical switch. By applying a square-wave voltage onto the bias port of the MZM the broadband pulses are directly generated by slicing the outputs of the FDML-OEO. The duty cycle can be easily changed by just tuning that of the square wave. Different microwave pulses with the duty cycles of 1:1, 1:10 and 1:100 are demonstrated. In particular, the tunability of the central frequency and the bandwidth of the generated signal is also displayed. Furthermore, the amplitude coherence superposition of 8 pulses is increased by ∼16 times and the frequency modulated pulse with different central frequencies are also demonstrated. The proposed broadband microwave signal pulse generator based on the injection-locked FDML-OEO can generate signals with large time bandwidth product (TBWP), and the linearly chirped microwave waveforms (LCMWs) are widely used in modern radar systems to achieve high-resolution detection and imaging.

Harmonically mode-locked optoelectronic oscillator with ultra-low supermode noise

A harmonically mode-locked optoelectronic oscillator (OEO) based on a dual-loop architecture is proposed to generate a microwave pulse train with ultra-low supermode noise. Through using a dual-loop architecture, the unwanted longitudinal modes can be weakened by the “Vernier effect” at the early stage of the oscillation, which is beneficial for suppressing the supermode noise originating from phase locking among the unwanted modes in a harmonically mode-locked OEO. The proposed scheme is numerically and experimentally demonstrated. In the experiment, microwave pulse trains with repetition rates of 179 kHz and 186 kHz are generated through 2nd- and 5th-order harmonic mode locking, where the supermode noise suppression ratios are measured to be beyond 60 dB and 55 dB, respectively. The effective suppression of supermode noise can greatly improve the output power stability and the correlation between different pulses. Under 2nd-order harmonic mode locking, the output power drifts of the single-loop and the dual-loop architectures are measured to be 38.13 dB and 0.97 dB, respectively.