Additive-pulse limiting

Generation of 583 fs optical pulses at 10 GHz from a regeneratively mode-locked fiber laser combining nonlinear polarization evolution

A regeneratively mode-locked erbium fiber laser was numerically investigated and experimentally demonstrated, which was able to generate a 583 fs pulse train at 10 GHz via intracavity pulse compression with nonlinear polarization evolution (NPE). To excite the NPE at such a high repetition rate, a dispersion map was intentionally introduced to obtain short pulses accompanied by high peaks through soliton-like pulse shaping. Numerical simulations indicated that steady-state oscillation with pulses below 1 ps can be successfully established using this laser configuration. Experimentally, we obtained a pulse duration of 583 fs and a 3 dB spectral width of 4.5 nm at an average output power of 15.6 mW. Simultaneously, supermode suppression of more than 80 dB was also obtained from the appropriate biased NPE.

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.

Sidemode suppression for coupled optoelectronic oscillator by optical pulse power feedforward

Multiple sidemodes have been observed in a coupled optoelectronic oscillator (COEO) when the contained actively mode-locked fiber ring laser employs erbium-doped fiber (EDF). We propose that such sidemodes can be suppressed significantly by an optical pulse power feedforward scheme, through which the mode-locked optical pulse is reversely intensity-modulated by itself, resulting in a fast power limiting. Experimentally we show that sidemodes are suppressed as much as 40 dB in a 10-GHz COEO. The additional noise induced by the power feedforward technique is analyzed numerically. We show that for a COEO with a typical cavity length, the feedforward contribution to final single-side band (SSB) noise is minor and neglectable.

High frequency optoelectronic oscillators based on the optical feedback of semiconductor mode-locked laser diodes

Optical self seeding feedback techniques can be used to improve the noise characteristics of passively mode-locked laser diodes. External cavities such as fiber optic cables can increase the memory of the phase and subsequently improve the timing jitter. In this work, an improved optical feedback architecture is proposed using an optical fiber loop delay as a cavity extension of the mode-locked laser. We investigate the effect of the noise reduction as a function of the loop length and feedback power. The well known composite cavity technique is also implemented for suppressing supermode noise artifacts presented due to harmonic mode locking effects. Using this method, we achieve a record low radio frequency linewidth of 192 Hz for any high frequency (>1 GHz) passively mode-locked laser to date (to the best of the authors' knowledge), making it promising for the development of high frequency optoelectronic oscillators.

On wave-breaking free fiber lasers mode-locked with two saturable absorber mechanisms

We propose a hybrid mode-locking scheme for wave-breaking free fiber lasers based on a saturable Bragg reflector and the nonlinear polarization evolution in the fiber section. With this scheme, the self-starting operation is ensured by the saturable Bragg reflector while the nonlinear polarization evolution acts as an additional pulse shaper in the steady state. Owing to the sensitivity of the pulse dynamics to filtering effects, a tuning range of more than 10nm as well as the suppression of undesired modes of operation became possible. The impact of the modulation depth and the non-saturated losses is discussed via comparative measurements with different saturable Bragg reflectors.

Direct generation of a 10 GHz 816 fs pulse train from an erbium-fiber soliton laser with asynchronous phase modulation

By employing the technique of asynchronous mode locking, we have successfully demonstrated direct generation of stable 10 GHz 816 fs pulse trains with a supermode-suppression ratio >70 dB from a hybrid mode-locked Er-fiber laser. When the modulation frequency deviates from the cavity harmonic frequency by 15-40 kHz, stable femtosecond soliton pulses are formed. Our results demonstrate that asynchronous mode locking can act as an effective mechanism for achieving a shorter pulse width and for stabilizing high-repetition-rate pulse trains in soliton fiber lasers.

Suppression of self-pulsing behavior in erbium-doped fiber lasers with a semiconductor optical amplifier

We experimentally demonstrate that the stability of cw and mode-locked erbium-doped fiber ring lasers can be improved significantly with a semiconductor optical amplifier (SOA) inside the cavity. The fast saturable gain of the SOA suppresses significantly the self-pulsing that is due to ion pairs in the erbium-doped fiber, which acts as a saturable absorber. A linear stabilization analysis of the laser system agrees with our experimental results.

Self-starting stretched-pulse fiber laser mode locked and stabilized with slow and fast semiconductor saturable absorbers

A self-starting stretched-pulse mode-locked erbium-doped fiber laser that uses fast and slow semiconductor saturable absorbers is described. By using two absorbers, we obtained reliable operation at a fundamental repetition rate with 250 pJ of pulse energy without multiple-pulse breakup. External chirp compensation was used to compress the highly chirped pulses to durations of 135 fs.

Gigahertz-repetition-rate mode-locked fiber laser for continuum generation

We report direct generation of <500-fs pulses at a 1-GHz rate from a self-starting passively mode-locked fiber laser by regeneratively synchronizing the pulses with a phase modulator. The pulses are amplified and passed through a dispersion-decreasing fiber and a normal-dispersion supercontinuum fiber. The resulting continuum is wider than 350 nm.

Stabilization of an active harmonically mode-locked fiber laser using two-photon absorption

Two-photon absorption provided by a semiconductor mirror structure is shown to reduce amplitude fluctuations significantly in a harmonically mo e-locked fiber ring laser. Pulse dropouts are eliminated in a laser that produces picosecond pulses at a repetition rate of 2 GHz.

Dispersion-tuned harmonically mode-locked fiber ring laser for self-synchronization to an external clock

The stable operation of a high-repetition-rate dispersion-tuned harmonically mode-locked erbium-doped fiber ring laser is reported for the first time to our knowledge. The dispersion causes a round-trip delay that varies with wavelength, so synchronism with the amplitude modulator is passively maintained at a wavelength that is synchronous with the modulation frequency (external clock). One can tune the laser by simply changing the cavity length. A 10-GHz train of transform-limited, 6.5-ps duration pulses is obtained over an 8-nm tuning range. Pulse energy equalization can be achieved by the interplay of normal dispersion, self-phase modulation, and amplitude modulation.

Pulse energy equalization in harmonically FM mode-locked lasers with slow gain

Harmonic mode locking in lasers with slow gain relaxation times requires fast intensity-dependent loss (fast loss) to equalize the pulse energies. A practical method to obtain fast loss is to use self-phase modulation and spectral filtering (SPM+F). We show that in a frequency modulation mode-locked laser SPM+F produces a fast loss only when the cavity dispersion is anomalous. In the absence of SPM the dispersion creates a gain imbalance between the pulsing modes in the upchirped and downchirped modulation cycles, which causes one mode to dominate over the other.

Cladding-pumped passive harmonically mode-locked fiber laser

A passive harmonically mode-locked fiber laser cladding pumped by a broad-area diode-laser array is described. Harmonic mode locking is obtained in a frequency range from 33.3 to 128.6 MHz, where the higher frequency limit is imposed because of insufficient available pump power. The maximum pulse jitter in one cavity roundtrip time is between 300 and 50 ps in the whole frequency range, and the sidebands in the frequency domain are suppressed by as much as 50 dB. 600-fs bandwidth-limited pulses with pulse energies of as much as 20 pJ are obtained, giving rise to an average output power as great as 2.5 mW.

Dual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmission

I show how removal of the central optical frequencies from the output of an externally phase-modulated, single-frequency laser results in a dual-wavelength source of high-quality pulse trains for wavelength-division-multiplexed soliton transmission and describe successful experimental demonstration of such sources in the 2.5-12.6-Gbit/s range.

Asynchronous soliton mode locking

We report a harmonically mode-locked erbium-doped fiber ring laser with short solitons. A phase modulator running asynchronously with the pulses provides pulse start-up, continuous background cleanup, and a new form of pulse timing stabilization. Steady-state operation of picosecond solitons at a repetition rate of 1 GHz was obtained with no need for modulator drive frequency stabilization.

Additive-pulse mode-locking/limiting storage ring

A pulse-storage ring is an optical device capable of memorizing a pulse sequence of 0's and l's. The soliton ring of Nakazawa et al. [Electron. Lett. 29, 729 (1993)] with nearly unlimited transmission distance is such a device. We propose and experimentally demonstrate a different type of pulse-storage ring that uses intensity-dependent interferometric action to create a stronger intracavity transmission differential between the 0's and the l's.