Harmonic Mode-locked Pulses Research Articles

Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber

We propose an operation switchable ring-cavity erbium-doped fiber laser (EDFL) via intra-cavity polarization control. By using a semiconductor saturable absorber mirror in the EDFL cavity, stable Q-switching, Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic mode-locking pulses can be manipulated simply by detuning a polarization controller while keeping the pump power at the same level. All EDFL operation states can be obtained under the polarization angles detuning within 180 degrees. Continuous-wave mode-locking of EDFL with 800-fs pulsewidth repeated at 4 MHz has been obtained, for which the output pulse energy is 0.5 nJ and the peak power is 625 W. Interaction between solitons and the accompanied non-soliton component will lead to either pulse splitting or 5th-order harmonic mode-locking at repetition rate of 20 MHz.

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.

Different operation states of soliton pulses in an erbium-doped fibre ring laser

We report on the experimental observation of soliton pulses in an erbium doped fibre ring laser. The passive mode-locking is achieved using the nonlinear polarization rotation technique. By adjusting the pump power and the intracavity polarization controllers, a normal soliton, a stable 8th harmonic mode-locked pulse and a noise-like pulse have been observed in our laser. The experimental results revealed that the noise-like pulse is not suitable for the optical telecommunication, and in order to obtain the stable harmonic mode-locked soliton, a strong unstable CW laser field is necessary to mediate global soliton interaction. The formation mechanism of the harmonic mode-locked pulse has also been analysed.

NUMERICAL STUDY OF A RATIONAL HARMONICAL MODE-LOCKED SEMICONDUCTOR OPTICAL AMPLIFIER FIBER RING LASER

Based on the self-reproduction theory, a rational harmonic mode-locked semiconductor optical amplifier fiber ring laser was numerically investigated. The system parameters effects on pulse-amplitude equalization of rational harmonic mode locking was analyzed and the numerical result shows that the 2∼5 order rational harmonic mode-locked pulse with the small amplitude ripple can be obtained only by adjusting the system parameters.

Harmonic Mode-Locked Ytterbium-Doped Fibre Ring Laser

We demonstrate a harmonic mode-locked ytterbium-doped fibre ring laser,which consists of a polarization-sensitive isolator, two polarizationcontrollers, two 976 nm laser diodes as the pump source and atwo-segment ytterbium-doped fibre. Utilizing an additive pulse mode-lockedtechnique based on nonlinear polarization evolution, theytterbium-doped fibre laser can operate in mode-locked state byadjusting the position of polarization controllers. The cavityfundamental repetition rate is 23.78 MHz. We also observe the second-and third-harmonic mode locking in the normal dispersion region, andtheir repetition rates are 47.66 MHz and 71.56 MHz, respectively.Over-driving of the saturable absorber in the harmonic mode-lockingpulse is analysed and discussed in detail.

Numerical study of a harmonic mode-locked semiconductor optical amplifier fiber ring laser

An improved harmonic mode-locked Semiconductor optical amplifier (SOA) fiber ring lasers has been presented and numerically investigated based on the self-reproduction theory. The numerical result shows that a narrower optical pulse train with a more symmetrical-temporal shape can be obtained, when the modulation SOA is DC biased on high current, whereas the gain SOA is DC biased on the low current functions as a gain compensator in the experimental setup. Also, the system parameters effects on the characteristic of the harmonic mode-locked pulse have been investigated.

Amplitude-equalized high-order arbitrary numerator rational harmonic mode-locked pulse generation in fiber-ring lasers using nonlinear polarization rotation

We report the amplitude-equalized high order arbitrary numerator rational harmonic mode-locked (RHMLed) pulse generations based on the technique of nonlinear polarization rotation in the fiber-ring laser. The numerator effect on the amplitude uniformity of RHMLed pulse train is first investigated. It is demonstrated that the high order RHMLed pulse train with uniform amplitude can be achieved by choosing a proper numerator. The pulse amplitude equalization in up to the 10th order RHMLed pulse train is observed when the numerator is equal to 3 instead of 1. It is important that the technique may be useful to generate the high-speed pulse trains with uniform amplitude by using the high order RHMLing technique.

Generation of the 11th order rational harmonic mode-locked pulses with an arbitrary numerator in fiber-ring lasers

We report for the first time the generation of 5th, 10th and 11th order rational harmonic mode-locked pulse trains with the numerator of the detuning fraction be an arbitrary integer instead of unity used to date in conventional rational harmonic mode-locked fiber-ring lasers. Results show that the amplitude envelopes of these pulse trains are strongly modulated by some lower order rational harmonic mode locking with numerator equals to unity. This result makes it possible to generate dual- or multiple-pulse train when the detuning of the modulation frequency is close to the half of the cavity frequency. Choosing a proper numerator is also helpful in equalizing the amplitude distribution of the pulse train generated by high order rational harmonic mode locking.

Pulse-amplitude equalization of rational harmonic mode-locked fiber laser using a semiconductor optical amplifier loop mirror

The authors demonstrate, for the first time to our knowledge, the amplitude equalization of high-repetition-rate pulses generated from a rational harmonic mode-locked fiber laser using a semiconductor optical amplifier loop mirror. Here, the loop mirror acts as an all-optical pulse-amplitude equalizer. Optimized parameters of the loop mirror for pulse-amplitude equalization were determined by a simulation. The pulse-amplitude equalization was successfully demonstrated up to the thirteenth rational harmonic mode-locked pulse train (∼14 GHz).

Pulse-amplitude-equalized output from a rational harmonic mode-locked fiber laser

We report, for the first time to our knowledge, the demonstration of amplitude equalization of high-repetition-rate pulses generated from a rational harmonic mode-locked Er-doped fiber laser. The output pulses are injected into another fiber laser with a nonlinear optical loop mirror. This scheme provides pulse-amplitude equalization up to the ninth rational harmonic mode-locked pulse train.