Harmonic Mode-locked Fiber Laser Research Articles

Polarization-Dependent Locking in SOA Harmonic Mode-Locked Fiber Laser

Polarization-dependent states of the output pulse train generated from an active harmonic mode-locked fiber laser by exploiting the birefringence of the fiber ring is reported for the first time. There exist two different fundamental frequencies for each polarization-mode lasing and hence the laser can be forced to mode-locking states of the two orthogonal polarization regimes by tuning or detuning the modulation frequency. Theoretical equations for predicting the tuning frequencies are described and good agreement with experimental results is obtained

Mode-locked erbium-doped fiber ring laser using intracavity polarization-maintaining loop mirror

Erbium-doped fiber lasers at repetition rate of 40 and 80 GHz are demonstrated in an injection-locked and rational harmonic mode-locked ring cavity. The cavity has a polarization-maintaining loop mirror. The injection-locking pulse is generated at repetition rates of 20 and 40 GHz from another rational harmonic mode-locked fiber laser. When these pulses are injected into the second fiber laser, pulses at twice the repetition rates viz. 40 and 80 GHz are produced

Development and performance comparison of two different approaches for stabilizing a harmonic mode-locked fiber laser at 40 GHz

What we believe to be a new and simple approach was developed for stabilizing a harmonic mode-locked fiber laser at 40 GHz. It uses a computer to tune the modulation frequency in a 100 kHz band near 40 GHz to follow variations in the length of the optical cavity. A second approach was also developed to compare with the new approach and to draw conclusions on its performance. Results for the pulse characteristics, side-mode suppression ratio, and timing jitter show that both approaches provide an efficient way of stabilizing a harmonic FM mode-locked fiber laser.

Eighth-order rational harmonic mode-locked fiber laser with amplitude-equalized output operating at 80?Gbits?s

We report the generation of an 80 Gbit/s pulse train by an eighth-order rational harmonic mode-locking technique. Equalized pulse-train amplitude is achieved by proper adjustment of the dc bias point and the 10 GHz modulation strength applied to the cavity-loss modulator. A numerical model is developed to simulate the properties of the generated 80 Gbit/s pulse train. The simulated results are well supported by the experimental measurements.

Suppression of phase and supermode noise in a harmonic mode-locked erbium-doped fiber laser with a semiconductor-optical-amplifier-based high-pass filter

By operating an intracavity semiconductor-optical-amplifier- (SOA-) based high-pass filter at the nearly transparent current condition, the supermode noise (SMN), the relaxation oscillation, and the single-sideband (SSB) phase noise can be simultaneously suppressed in an actively mode-locked erbium-doped fiber laser (EDFL). The SOA at the nearly transparent condition enhances the SMN suppression ratio of the EDFL from 32 to 76 dB at the cost of the phase noise degrading from -114 to -104.2 dBc/Hz and broadening the pulse width from 36 to 61 ps. With an optical bandpass filter, the SSB phase noise and the SMN suppression ratio can be further improved to -110 dBc/Hz and 81 dB, respectively. The EDFL pulse can be further shortened to 3.1 ps with a time-bandwidth product of 0.63 after compression.

Dispersion effects on the detuning properties of actively harmonic mode-locked fiber lasers

Detuning properties for actively harmonic mode-locked fiber lasers has been studied both theoretically and experimentally while taking into account of finite cavity dispersions. The theoretical work is based on the self- consistent time domain circulating pulse method. By keeping terms which are usually neglected in previous studies, we have derived an analytic formula which can predict the saturation behavior associated with large modulation frequency detuning. It is found that for the case of medium cavity dispersion, both the pulse-modulator RF phase lag and the optical carrier frequency of the circulating pulse will change significantly as a function of the modulation frequency detuning. The analytical results are supported by both the numerical simulations as well as the experimental measurements. Our theory can potentially serve as a design guidance for cavity length feedback control of harmonic mode-locked fiber lasers.

Experimental study on relaxation oscillation in a detuned FM harmonic mode-locked Er-doped fiber laser

We experimentally studied the relaxation oscillation characteristics in a FM harmonic mode-locked fiber laser when the modulation frequency has a small detuning. The relaxation oscillation can lower the laser output power and make the amplitudes of output pulses uneven both in normal and abnormal dispersion cavities. While combined with self-phase modulation and the optical filter, the abnormal intra-cavity dispersion can suppress the relaxation oscillation phenomenon.

Colliding-pulse passive harmonic mode-locking in a femtosecond Yb-doped fiber laser with a semiconductor saturable absorber

We demonstrate a passive harmonic mode-locked femtosecond Yb-doped fiber laser employing a semiconductor saturable absorber in a colliding-pulse configuration. 380-fs pulses at 605 MHz repetition rate with >60 dB supermode suppression is achieved.

Optical generation of amplitude-equalized pulses from a rational harmonic mode-locked fiber laser incorporating an SOA loop modulator

Rational harmonic active and passive mode locking has been demonstrated in an erbium-doped fiber laser using an all-optical approach. Based on the modulation and the self-switching effects of a semiconductor optical amplifier nonlinear loop mirror, hybrid mode locking and pulse-amplitude equalization are simultaneously achieved in the laser. In our setup, amplitude-equalized pulse trains up to about 12 GHz are obtained at 1.55 /spl mu/m using 3-GHz optical driving pulses.

Passive harmonic mode-locked soliton fiber laser stabilized by an optically pumped saturable Bragg reflector

Stabilization of passive harmonic mode locking is achieved for what is believed to be the first time in an Er-Yb soliton fiber laser by optical pumping of the semiconductor saturable absorber above the bandgap. The results show 35-dB mode suppression of undesired harmonics.

High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser

Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency f/sub m/=(n+1/p)f/sub c/, where n and p are both integers and f/sub c/ is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when f/sub m//spl ap/1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency f/sub m/=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier.

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.