Rational Harmonic Mode-locking Research Articles

Ultra-Short Pulse-Train Generation of 30-GHz Repetition Rate Using Rational Harmonic Mode Locking and Nonlinear Polarization Rotation

A 30-GHz pulse-train is generated using the rational harmonic mode-locking technique, experimentally, using a Mach–Zehnder Lithium Niobate modulator. The width of the pulses is then reduced from 5.8-ps to 1.9-ps by incorporating nonlinear polarization rotation. This phenomenon arises due to the very high nonlinear behavior of the photonic crystal fiber (PCF) added to the ring laser cavity. Numerically solving the Generalized Nonlinear Schrödinger Equation provided insights into the pulse evolution behavior. The relative polarization angle and length of the PCF were varied to study their effects on the pulse-width.

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.

Mode-Locked Fiber Ring Laser Using Graphene Nanoparticles as Saturable Absorbers

Mode locked fiber ring laser using graphene nanoparticles as saturable absorbers to compress the pulse width has been studied. An experimental method to demonstrate the generation of pulse train with 50 GHz repetition rate with an ultrashort pulse width has been demonstrated. This method uses a combination of rational harmonic mode-locking (RHML) and a saturable absorber in the fiber ring laser. The pulse width using saturable absorbers is shorter by about a factor of 2 compared to that without a saturable absorber. Pulse generation using fiber ring laser with a saturable absorber has been analyzed. The experimental results are in agreement with the results from the simulation.

20 GHz actively mode-locked thulium fiber laser.

A high repetition-rate actively mode-locked thulium fiber laser is demonstrated where an electro-optic lithium niobate phase modulator is used to synchronize the longitudinal modes in the cavity. The repetition rate of the actively mode-locked laser is tunable from 14.6 MHz to 19 GHz, where the 19 GHz pulses exhibit a super-mode suppression ratio of 46 dB. Furthermore, the output pulse width could be manipulated through finely controlling the detuning frequency or repetition rate. We have also experimentally observed rational harmonic mode-locking in the laser and obtained 14 GHz and 21 GHz repetition rate pulses using a 7 GHz modulating signal. To the best of our knowledge, we have improved the repetition-rate of actively mode-locked thulium fiber laser by more than one order of magnitude. Such a high repetition source can be readily synchronized to a master clock, which makes it very suitable for high speed optical data processing, communication and remote sensing.

Up to 400 GHz burst-mode pulse generation from a hybrid harmonic mode-locked Er-doped fibre laser

By inserting a birefringence filter with FSR = 100 GHz inside a hybrid mode-locked Er-doped fibre laser, we successfully generate ps to sub-ps optical burst pulses with the intra-burst pulse rate up to 400 GHz. Multiplication of the intra-burst pulse rate is attributed to a new effect analogous to rational harmonic mode-locking, which occurs due to the relative alignment of the cavity harmonic frequencies, the external phase modulation induced frequencies, and the filter-selected frequencies.

High-repetition-rate ultrashort pulsed fiber ring laser using hybrid mode locking.

We propose and demonstrate a hybrid mode-locked erbium-doped fiber ring laser by combining the rational harmonic mode-locking technique and passive mode locking based on nonlinear polarization rotation in a highly nonlinear photonic crystal fiber. By carefully adjusting the modulation frequency and the polarization controllers in the cavity, a 30 GHz pulse train with improved stability and narrower pulse width is generated. The pulse width at 30 GHz using rational harmonic mode locking alone is 5.8 ps. This hybrid scheme narrows the pulse width to 1.9 ps at the repetition rate of 30 GHz. Numerical simulations are carried out that show good agreement with the experimental results.

High-speed ultrashort pulse fiber ring laser using charcoal nanoparticles.

A mode-locked erbium-doped fiber ring laser that is easy to set up is proposed and experimentally demonstrated to generate a high-repetition-rate optical pulse train with an ultrashort pulse width. The laser combines a rational harmonic mode-locking technique and charcoal nanoparticles as saturable absorbers. Compared to a solely active mode-locking scheme, the scheme with charcoal nanoparticles can remove the supermodes and narrow the pulse width by a factor of 0.57at a repetition rate of 20GHz. Numerical simulation of the laser performance is also provided, which shows good agreement with the experimental results.

A Dual-Wavelength Rational Harmonic Mode-Locked Fiber Ring Laser With Different Repetition Frequencies

A dual-wavelength rational harmonic mode-locked fiber ring laser with different repetition frequencies is theoretically analyzed and experimentally demonstrated. In this scheme, two wavelength channels are inserted into the laser loop to form two resonant cavities. By tuning the length of each channel, required rational harmonic mode-locking conditions can be achieved, and dual-frequency mode-locked signals are simultaneously generated at different wavelengths. In addition, the supermode suppression ratio is further improved due to the mode competition caused by this dual-cavity configuration. In the experimental demonstration, when the fourth-order (or fifth-order) harmonic mode-locked signal is generated in Cavity 1, the first-, second-, and third-order (or fourth-order) harmonic mode-locked signals, respectively, can be generated in Cavity 2 by tuning its length. The measured radio frequency spectra and waveforms show that the signals have high quality. Compared with a conventional rational harmonic mode-locked laser (RHMLL), more than 10 dB supermode suppression ratio improvement is obtained. Since this system can flexibly implement multiwavelength and multifrequency signal generation without adding much cost, it may have wide applications in optical communication system, photonic microwave signal generation, and optical waveform synthesizers.

Dynamics of solid-state lasers pumped by mode-locked lasers.

We analyze the dynamics of mode-locked pumped solid-state lasers focusing on the transition between mode-locked and CW behavior. Where the ratio of the pump and laser cavity lengths is a rational number, 'rational-harmonic mode-locking' is obtained. When the cavity length is detuned away from such resonances, modulated continuous output is generated. The transition from mode-locked to modulated CW operation is explored experimentally for a Ce:LiCAF laser operating at 290 nm and pumped by a 78.75 MHz mode-locked frequency quadrupled Nd:YVO(4) laser. Both CW output and mode-locked output with pulse repetition rates up to 1.1 GHz were achieved. A rate equation model is developed to predict optimum cavity lengths for achieving CW output with minimized modulation.

Over 10-GHz Peak-Equalized Pulse Train Source With Pulsewidth Shortening

Peak-flattened and width-shortened pulses have been successfully generated in an improved rational harmonic mode-locking (RHML) fiber laser in which an intensity modulator is driven by square waveform signals at gigahertz (GHz) repetition rates rather than typically used sinusoidal radio frequency (RF) signals. The repetition frequency of a fifth-order RHML under a driving signal at 3.125 GHz has been up to 15.7 GHz in our system. The equalized peaks of pulses generated reached to a minimum standard deviation of 0.2%, which is the best performance to our knowledge. The pulsewidth is shortened to 17.8 ps in the same RHML fiber laser driven by the square-wave signals compared with 30.3 ps driven by traditional sine-wave signals. The 3-dB spectral width of the RHML pulses was increased almost two times and the spectrum has a clear comb-like structure which can be developed into a frequency comb with GHz frequency spacing.

Second-order fractional Talbot effect induced frequency-doubling optical pulse injection for 40 GHz rational-harmonic mode-locking of an SOA fiber laser

A second-order fractional Talbot effect induced frequency-doubling of a 10 GHz optical pulse-train is demonstrated to backward injection mode-lock a semiconductor optical amplifier fiber laser (SOAFL) for 40 GHz rational-harmonic mode-locking (RHML). That is, a real all-optical gain-modulation of the SOAFL can be created by injecting such a time-multiplexed but pseudo-frequency-doubled pulse-train into the cavity. The time-multiplexing pulse-train can thus be transformed into a frequency-multiplied pulse-train via cross-gain modulation (XGM). The optical pulse-train at 10 GHz is generated by nonlinearly driving an electro-absorption modulator (EAM), which experiences the second-order fractional Talbot effect after propagating through a 4 km long dispersion compensation fiber (DCF). The DCF not only plays the role of frequency-doubler but also compensates the frequency chirp of the 10 GHz optical pulse-train. The pulsewidth broadening from 22 to 60 ps for initiating the time-domain Talbot effect is simulated by the nonlinear Schrödinger equation. With careful detuning of the RF modulation power of the EAM at 5 dBm, the generated 20 GHz optical pulse-train exhibits a positive frequency chirp with minimum peak-to-peak value of 2 GHz, and the peak-amplitude fluctuation between adjacent pulses is below 1.4%. In comparison with the SOAFL pulse-train repeated at 40 GHz generated by the fourth-order purely RHML process, the optimized second-order fractional Talbot effect in combination with the second-order RHML mechanism significantly enhances the modulation-depth of RHML, thus improving the on/off extinction ratio of the 40 GHz SOAFL pulse-train from 1.8 to 5.6 dB. Such a new scheme also provides a more stable 40 GHz RHML pulse-train from the SOAFL with its timing jitter reducing from 0.51 to 0.23 ps.

Laser dynamics of asynchronous rational harmonic mode-locked fiber soliton lasers

Laser dynamics of asynchronous rational harmonic mode-locked (ARHM) fiber soliton lasers are investigated in detail. In particular, based on the unique laser dynamics of asynchronous mode-locking, we have developed a new method for determining the effective active modulation strength in situ for ARHM lasers. By measuring the magnitudes of the slowly oscillating pulse timing position and central frequency, the effective phase modulation strength at the multiplication frequency of rational harmonic mode-locking can be accurately inferred. The method can be a very useful tool for developing ARHM fiber lasers.

A Comprehensive Study of Actively Mode-Locked Fiber Optical Parametric Oscillators for High-Speed Pulse Generation

We perform a comprehensive study on the implementation of active harmonic and rational harmonic mode-locking of a fiber optical parametric oscillator (FOPO). In particular, the effect of pump parameters and cavity designs on the characteristics of the generated pulses are analyzed. Two methods of frequency multiplication in FOPOs are discussed and compared and we show that we can generate optical pulses with very high-repetition-rate (up to 240 GHz) using rational harmonic mode-locking. We find that optical pulses with short duration can be obtained using pump pulses with small duty cycle, large pump power, and an intra-cavity optical filter with proper bandwidth. In addition, optical pulses with low root-mean-square timing jitter are achievable when no pump phase modulation is applied and hybrid gain of erbium-doped fiber amplifier and parametric amplifier is used. High supermode noise suppression ratio is achieved by taking advantage of the fast parametric gain and the combined effect of self-phase modulation and spectral filtering. We conclude that synchronous-pumping of an actively mode-locked FOPO offers many advantages in terms of reducing the cavity loss, increasing the modulation frequency, enabling rational harmonic mode-locking, and applications to all-optical clock recovery.

Repetition rate and wavelength-tunable all-optical actively mode-locked fibre ring laser based on a reflective semi-conductor optical amplifier

A repetition rate- and wavelength-tunable fibre ring laser that utilises a reflective semi-conductor optical amplifier as both a gain and mode-locking element is presented. Moreover, 10.3 ps pulses at 10 GHz repetition rate are obtained across a 30 nm tuning range covering the entire C-band. The repetition rate is tunable up to 30 GHz via rational harmonic mode-locking. A comparison is made using a conventional semi-conductor optical amplifier as the gain and mode-locking element. It is shown that the reflective semi-conductor optical amplifier requires in excess of 5 dB less average pump pulse power in order to achieve the optimum mode-locking condition, for achieving the narrowest output pulse width, thereby relaxing the requirement for a complex and expensive pump pulse source.

Rational Harmonic Mode-Locking of a Fiber Optical Parametric Oscillator at 30 GHz

We demonstrate rational harmonic mode-locking of a fiber optical parametric oscillator (FOPO) in which the parametric gain serves as an all-optical mode-locker. A pulse train with a repetition rate from 10 to 30 GHz is generated using a 10-GHz optical clock signal. The generated pulses have a pulsewidth of ~ 9 ps, a root-mean-square (RMS) timing jitter of ~ 300 fs, and a wavelength tunable to more than 20 nm.

High-order rational harmonic mode-locking and pulse-amplitude equalization of SOAFL via reshaped gain-switching FPLD pulse injection

The 40-GHz rational harmonic mode-locking (RHML) and pulse-amplitude equalization of a semiconductor optical amplifier based fiber-ring laser (SOAFL) is demonstrated by the injection of a reshaped 10-GHz gain-switching FPLD pulse. A nonlinearly biased Mach-Zehnder modulator (MZM) is employed to detune the shape of the double-peak pulse before injecting the SOA, such that a pulse-amplitude equalized 4th-order RHML-SOAFL can be achieved by reshaping the SOA gain within one modulation period. An optical injection mode-locking model is constructed to simulate the compensation of uneven amplitudes between adjacent RHML pulse peaks before and after pulse-amplitude equalization. The indirect gain compensation technique greatly suppresses the clock amplitude jitter from 45% to 3.5% when achieving 4th-order RHML, and the amplitude fluctuation of sub-rational harmonic modulating envelope is attenuated by 45 dB. After pulse-amplitude equalization, the pulsewidth of the optical-injection RHML-SOAFL is 8 ps, which still obeys the trend predicted by the inverse square root of repetition rate. The phase noise contributed by the residual ASE noise of the RHML-SOAFL is significantly decreased from -84 to -90 dBc/Hz after initiating the pulse-amplitude equalization, corresponding to the timing jitter reduction from 0.5 to 0.28 ps.

Photonic Generation of 60 GHz Millimeter-Wave by Frequency Quadrupling Based on a Mode-Locking SOA Fiber Ring Laser with a Low Modulation Depth MZM

Generation and processing of millimeter-wave (MMW) signal based on photonic technologies is a new trend. A simple and effective optical technique to generate 60 GHz millimeter-wave signal by frequency quadrupling is proposed and demonstrated. In this approach, the frequency quadrupling efficiency is greatly improved by a semiconductor optical amplifier (SOA) fiber ring laser with 2nd-order rational harmonic mode-locking and a Mach-Zehnder modulator (MZM) which is biased at odd-order optical modulation side-bands suppressing state. A center-wavelength tunable comb filter outside the ring laser is tuned to select the optical sidebands separated by 60 GHz. The result shows that the power ratio between quadruple-frequency signal and double-frequency signal is improved at least 30 dB with the mode-locking ring laser when the modulation depth is less than 0.4 and the phase noise of the 60 GHz signal generated is better than –75 dBc/Hz at 1-kHz offset. The key advantage of the technique is that a high-frequency MMW signal can be generated with high efficiency by a low power microwave signal.

Competition of Rational Harmonic Mode-Locking and Continuous-Wave Lasing in Semiconductor Optical Amplifier Fiber Laser Under Optical Pulse Injection

Twentieth-order rational harmonic mode-locking (RHML) semiconductor optical amplifier fiber laser (SOAFL) pulses are demonstrated by using 1-GHz backward dark-optical comb injection. Maximum frequency detuning range of plusmn 300 Hz, broadened pulsewidth of 35 ps, and highest average power of 0.4 muW for twentieth-order RHML-SOAFL pulse are characterized. The extraordinary phenomena on the red-shifted wavelength from 1535.5 to 1541.5 nm and the corresponding spectral linewidth reduced from 12 to 3 nm are observed with the RHML order increasing to 8 or higher. Such a less pronounced RHML mechanism at higher orders occurred in the optically injection mode-locked SOAFL is mainly attributed to the weak mode-locking strength at high RHML orders as compared to continuous-wave lasing mechanism.

Peak equalization of rational-harmonic-mode-locking fiberized semiconductor laser pulse via optical injection induced gain modulation

Optical injection induced gain modulation of a semiconductor optical amplifier (SOA) is demonstrated to equalize the peak intensity of pulses generating from the rational-harmonic-mode-locking (RHML) SOA based fiberized semiconductor laser. This is achieved by adjusting the temporal shape of the injected optical signal generated from a Mach-Zehnder intensity modulator, in which the DC biased level exceeding Vpi and the electrical pulse amplitude of 1.5Vpi are concurrently employed. Numerical simulation on the injected optical signal profile and the SOA gain during the inverse-optical-pulse injection induced gain modulation process are also demonstrated. After a peculiar inverse-optical-pulse injection, each pulse in the 5th-order RHML pulse-train experiences different gain from temporally varied SOA gain profile, leading the pulse peak to equalize one another with a minimum standard deviation of 2.5% on the peak intensity variation. The optimized 5th-order RHML pulse exhibits a signal-to-noise suppression ratio of 20 dB and a reduced variation on temporal spacing from 11 to 4 ps. The clock amplitude jitter is compress from 35.3% to 7.3%, which is less than the limitation up to 10% for 5th order RHML generation.

Rational harmonic mode-locking pulse quality of the dark-optical-comb injected semiconductor optical amplifier fiber ring laser

We study the rational harmonic mode-locking (RHML) order dependent pulse shortening force and dynamic chirp characteristics of a gain-saturated semiconductor optical amplifier fiber laser (SOAFL) under dark-optical-comb injection, and discuss the competition between mode-locking mechanisms in the SOAFL at high-gain and strong optical injection condition at higher RHML orders. The evolutions of spectra, mode-locking and continuous lasing powers by measuring the ratio of DC/pulse amplitude and the pulse shortening force (I(pulse)/P(avg)(2) ) are performed to determine the RHML capability of SOAFL. As the rational harmonic order increases up to 20, the spectral linewidth shrinks from 12 to 3 nm, the ratio of DC/pulse amplitude enlarges from 0.025 to 2.4, and the pulse-shortening force reduces from 0.9 to 0.05. At fundamental and highest RHML condition, we characterize the frequency detuning range to realize the mode-locking quality, and measure the dynamic frequency chirp of the RHML-SOAFL to distinguish the linear and nonlinear chirp after dispersion compensation. With increasing RHML order, the pulsewidth is broadened from 4.2 to 26.4 ps with corresponding chirp reducing from 0.7 to 0.2 GHz and linear/nonlinear chirp ratio changes from 4.3 to 1.3, which interprets the high-order chirp becomes dominates at higher RHML orders.