Supermode Noise Research Articles

Passive harmonic mode-locking in Tm–Ho doped fiber laser with MoS2–ZnO saturable absorber deposited on the arc-shaped fiber

This work reports a harmonic mode-locked Tm–Ho doped fiber laser (THDFL) using newly fabricated MoS2–ZnO as a saturable absorber (SA). It generates harmonic mode-locking (HML) operation with a center wavelength of 1911.4 nm and a 3-dB bandwidth of 3.02 nm. The fundamental repetition rate and pulse width were 14.1 MHz and 2.05 ps, respectively. Based on the radio frequency (RF) spectrum, the observed signal-to-noise (SNR) of the fundamental HML pulse was 67.22 dB. Increasing the pump power with fine-tuning of the polarisation controller (PC), higher order HML pulses were also observed using the MoS2–ZnO SA. The highest stable HML frequency was 300 MHz, observed at the 21st HML. The central wavelength and 3-dB bandwidth of the HML pulse were 1915.2 nm and 2.29 nm, respectively. The 21st HML pulses demonstrated a SNR of 40.09 dB. No supermode noise was observed in the RF spectrum, indicating that the 21st HML pulses were relatively stable. With industry demand, this high repetition rate HML with MoS2–ZnO SA opens new avenues for comprehension and manipulation of electronic phenomena, paving the way for future innovations in ultrafast lasers.

Harmonic active mode-locking optoelectronic oscillator with suppressed supermode noise based on pulse intensity feedback.

A harmonic active mode-locking optoelectronic oscillator (HAML-OEO) with pulse intensity feedback is proposed and experimentally demonstrated. It is capable of generating microwave pulses characterized by suppressed supermode noise, uniform intensity, and tunable repetition rates. Unlike traditional HAML-OEOs, active mode-locking and pulse intensity feedback are simultaneously achieved through the use of a dual-drive Mach-Zehnder modulator (DDMZM). By synchronously feeding back the generated microwave pulses to the DDMZM, each pulse undergoes a loss proportional to its intensity, facilitating pulse intensity equalization and supermode noise suppression. In the experiment, intensity-equalized microwave pulse trains with repetition rates of 499 kHz and 998 kHz are generated by the 5th- and 10th-order HAML-OEOs, respectively, with the measured supermode noise suppression ratios exceeding 40 dB.

Dual-band tunable microwave pulse signals generation based on a time domain mode locked optoelectronic oscillator

We propose and experimentally demonstrate a dual-band tunable microwave pulse signals generation method based on a time domain mode locked optoelectronic oscillator (TDML-OEO). The dual-band radio frequency (RF) combs are generated via injecting two single-tone signals and a low frequency signal. The two single-tone signals are injected to optimize the aimed center frequencies gain, while the low frequency signal is used to accomplish the multi-mode oscillation mode locking process. Furthermore, the dual-band RF combs with variable repetition rates of 3.69 MHz, 9.22 MHz, and 18.45 MHz are obtained with an over 72 dB ultra-high suppression of super mode noise as the TDML-OEO operates in the varied harmonic multiples mode locking state. Besides, by adjusting the amplitude and central frequency of the two injected local oscillator signals (LOs), a controllable output power and frequency tunability ranged from 11.1 GHz to 13.8 GHz are achieved for the generated dual-band RF combs. The proposed dual-band tunable microwave pulse generation method has great potential applications in multiband radars and electronic warfare.© 2023 Optica Publishing Group under the terms of the Optica Publishing Group Open Access Publishing Agreement.

Tunable High-Purity Microwave Frequency Combs Generation Based on Active Mode-Locking OEO

A novel approach to generate tunable high-purity microwave frequency combs (MFCs) based on active mode-locking optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. In the proposed method, one cavity mode of the OEO is injection-locked by a local oscillator (LO) signal resulting in single-mode oscillation. Under the assistance of an additional radio frequency signal, the MFCs generation is realized by loss modulation the single-mode oscillation. A key significance of our scheme is that no tunable filter is needed in OEO cavity. By altering the frequency of the LO signal, the MFCs center frequency can be flexibly precisely tuned over a bandwidth of 1.6 GHz. Furthermore, the high suppression of the supermode noise is obtained due to the injection locking of the OEO decreasing the required loop gain. The experimental results show that the generated MFCs maintain a large supermode noise suppression ratio over the entire tuning range.

A flexibly frequency switchable active mode-locking optoelectronic oscillator with supermode noise suppression

In this paper, we propose and experimentally demonstrate a flexibly frequency switchable active mode-locking optoelectronic oscillator (FSAML-OEO) in which a multi-passband microwave photonic filter (MPF) with two polarization-maintaining fibers (PMF) based Solc-Sagnac interferometer is used. Microwave frequency combs (MFCs) signals with four switchable central frequencies of 217 MHz, 453 MHz, 662 MHz and 1101 MHz are generated by controlling the polarization state of the Sagnac interferometer via rotating the polarization controller (PC). By cascading a dual-loop structure, MFCs signals with adjustable comb spacing of 1 MHz, 3 MHz, and 5 MHz are obtained with fundamental and harmonic FSAML-OEO, in which the supermode noise suppression ratios of the dominate modes and the super-modes are measured to be about 30 dB.

Polarization maintaining harmonically mode-locked fiber laser with suppressed supermode noise due to continuous wave injection

We report on experimental studies of a SESAM mode-locked Er-doped fiber laser completely spliced from polarization-maintaining fiber components. With the fiber mode spot properly aligned to the SESAM, the laser operates harmonic mode-locking (HML) available in a whole range of pump powers up to ∼355mW with the supermode suppression level (SSL) less than 25 dB. Importantly, the HML laser generates the pulses in a linear polarization state and enables the pulse repetition rate (PRR) up to ∼1145 MHz. In our experiments we show that an optical injection of an external continuous wave (CW) into the laser cavity improves the stability of the laser operation in HML regime resulting in an increase of the SSL by 20–30 dB. Besides, the CW injection is able to expand the range of pump powers available with HML laser providing an increase of the maximal laser PRR up to ∼2195 MHz. Importantly, the implemented optical injection does not affect high purity of the laser polarization state. The presented numerical simulations enable qualitative explanations of the effects observed experimentally.

Supermode noise mitigation and repetition rate control in a harmonic mode-locked fiber laser implemented through the pulse train interaction with co-lased CW radiation: publisher's note.

This publisher's note contains corrections to Opt. Lett.47, 5236 (2022)10.1364/OL.472780.

Supermode noise mitigation and repetition rate control in harmonic mode-locked fiber laser implemented through the pulse train interaction with co-lased CW radiation.

We report on new, to the best of our knowledge, techniques enabling both the mitigation of supermode laser noise and highly precise setting of the pulse repetition rate (PRR) in a soliton harmonically mode-locked (HML) fiber laser employing nonlinear polarization evolution (NPE). The principle of operation relies on resonant interaction between the soliton pulses and a narrowband continuous wave (CW) component cooperatively generated within the same laser cavity. In contrast to our recent findings [Opt. Lett.46, 5747 (2021)10.1364/OL.441630 and Opt. Lett.46, 5687 (2021)10.1364/OL.443042], the new methods are implemented through the specific adjustment of the HML laser cavity only and do not require the use of an external tunable CW laser source.

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.

Resonantly induced mitigation of supermode noise in a harmonically mode-locked fiber laser: revealing the underlying mechanisms.

We have performed experimental and numerical studies enabling clear insight into the physical mechanisms underlying the super-mode noise mitigation in harmonically mode-locked (HML) fiber lasers using the resonant continuous wave (CW) injection. New experiments have refined the requirements to the positions inside the laser spectrum assigned to the injected CW component, a Kelly sideband, and the transparency peaks of the birefringent fiber filter. In particular, we have proved experimentally that the noise mitigation effect is dominating with the CW injected to the long-wavelength side of laser spectrum. Injection to the opposite side destroys the HML operation regime. Our numerical simulations confirm these specific features. To get the result, we have simulated phase-locking between the CW and a single soliton. Then, the developed model has been applied to the laser cavity operating multiple pulses in the presence of the gain depletion and recovery mechanism responsible for harmonic pulse arrangement. We clearly demonstrate how the CW injection accelerates or slows down the HML process enabling the generation of additional inter-pulse forces.

Timing jitter reduction of silicon hybrid harmonically mode-locked ring laser using an integrated sampled-grating DBR.

An integrated sampled-grating silicon hybrid mode-locked ring laser is proposed for 20-GHz super-mode noise suppression. The dynamics, phase noise, and timing jitter are investigated using a delay differential equation (DDE) model. The results show that the 20-GHz harmonic regime of the proposed structure is significantly increased when the sampled-grating structure is included. Additionally, a better selectivity of the optical modes by a sampled-grating distributed Bragg reflector (SGDBR) leads to more suppression of super-mode noise compared to the long-ring structures with a spectral filter which were previously reported. Compared to previous works, the phase noise is improved by approximately 6 dB and the timing jitter is decreased to less than a third.

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.

Super-mode noise suppression of a silicon hybrid mode-locked ring laser using a symmetric Fabry–Perot filter

In this research, we propose a harmonically mode-locked hybrid silicon laser by introducing intracavity reflectors (ICRs) to suppress super-mode noise. Using a delay differential equation model, the dynamics, phase noise, and timing jitter of the proposed structure (PS) and the without-ICR structure (WOICRS) are investigated. The simulation results show a significant increase of 20 GHz harmonic regime of PS compared to WOICRS. Additionally, this regime of PS can be achieved at a lower current than WOICRS. The phase noise of the PS is improved to about 8 dB. Given the better selectivity of optical modes in PS, the phase noise is reduced compared to that of with-intracavity structure. Furthermore, numerical results show that timing jitter reduction has periodic behavior by changing the harmonic inter-spike interval time. The analyses demonstrate that harmonic mode-locking of a long cavity laser is a valid strategy for reducing phase noise and timing jitter. The simulation results of WOICRS are in a good agreement with experimental results.

Supermode noise suppression with polarization-multiplexed dual-loop for active mode-locking optoelectronic oscillator.

The active mode-locking (AML) technique has been widely used in erbium-doped fiber lasers to generate picosecond pulse trains. Here we propose a novel active mode-locking dual-loop optoelectronic oscillator (AML-DL-OEO), which can generate microwave frequency comb (MFC) signals with adjustable comb spacings. Based on this scheme, the order of harmonic mode-locking is dramatically decreased for a certain AML driving frequency compared with a single-loop AML-OEO. Thus, the supermode noise caused by harmonic mode-locking can be efficiently suppressed. In addition, the sidemodes are well suppressed by the dual-loop architecture. An experiment is performed. MFC signals with different comb spacings are generated under fundamental or harmonic mode-locking states. AML-DL-OEO systems with different length differences between two loops are implemented to evaluate supermode noise suppression capability. The performance of the generated MFC signals is recorded and analyzed.

Mitigation of the supermode noise in a harmonically mode-locked ring fiber laser using optical injection.

We report on a new, to the best of our knowledge, technique enabling mitigation of the supermode noise (and timing jitter) in a soliton harmonically mode-locked (HML) fiber laser built on the nonlinear polarization evolution (NPE). An optical injection of an external continuous wave (CW) into the HML laser cavity results in an increase of the supermode noise suppression level (SSL) by a two-three order of magnitude for harmonics between 25th and 135th. The operation mechanism involves phase-locking between the injected light and soliton pulses and exhibits strong resonant dependence on the CW laser wavelength. Our findings offer important insights into the HML laser dynamics associated with an interaction between solitons and CW background in the laser cavity.

Proposal of phase noise and jitter reduction in a long-ring hybrid silicon mode-locked laser by intracavity reflectors.

We propose a harmonically hybrid silicon mode-locked ring laser using intracavity reflectors (ICRs) to suppress supermode noise. The dynamic and noise properties of the structure are investigated using a delay differential equation model. The results of the dynamic show that a 20 GHz harmonic regime of the structure is significantly increased compared to a without intracavity-reflector structure. Additionally, the phase noise of the proposed structure (PS) is improved to about 8 dB. Furthermore, an analysis shows that timing jitter reduction has periodic behavior by changing the harmonic inter-spike interval time (TISI).

Actively mode-locked fiber laser at 2-μm for free-space data transmission with tunable repetition rate

We experimentally demonstrate a repetition rate controllable actively mode-locked fiber laser for 2-μm free-space data transmission. The actively mode-locking can be produced by a Mach-Zehnder modulator (MZM) driven by an arbitrary waveform generator (AWG). A Lyot filter and a polarization dependent isolator (PD-ISO) are used to suppress the super-mode noise and the ambient noise. The signal-to-noise ratio (SNR) of the mode-locked pulse with repetition rate of about 690MHz can be increased from 42.64dB to 49.45dB. By inserting a high-precision variable optical delay line (VODL) in the cavity, the fundamental frequency of the cavity can be adjusted from 18.21MHz to 18.13MHz with a tuning accuracy of 73.52Hz. The RF spectra of the 330th order harmonic mode-locked pulses with continuously tunable repetition rate can be observed. In addition, the mode-locked pulse sequence can be driven by sine wave, triangular wave and square-wave, and the duration of actively mode-locked pulse can be tuned independently by changing the type of the driven signal.

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.

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.

Low pump threshold CVD graphene based passively harmonic mode‐locked fibre laser

A low pump threshold passively harmonic mode-locked fibre laser is demonstrated. The key component is a chemical vapour deposition grown graphene saturable absorber to enable higher order harmonic mode-locking (HML) of the fibre laser with good output noise characteristics. With a relatively low pump power of 100 mW, the fibre laser generates ∼1 ps soliton pulses at the 21st order of HML with 40 dB of super-mode noise suppression.