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.

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.

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.

Pulse repetition rate tuning of a harmonically mode-locked ring fiber laser using resonant optical injection.

We report on a new, to the best of our knowledge, technique enabling fine-tuning of the pulse repetition rate (PRR) of a soliton harmonically mode-locked (HML) fiber laser built on the nonlinear polarization evolution (NPE). Optical injection of an external continuous wave (CW) into the HML laser cavity is used for this purpose enabling precise PRR tuning with the elementary step equal to the fundamental PRR one-by-one. The effect exhibits strong resonance dependence on the CW laser wavelength and available in both positive and negative directions. Our findings offer important insights into the HML laser dynamics associated with the birth and annihilation of solitons in the cavity.

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.

Brillouin-like amplification in rare-earth-doped optical fibers.

We present a theoretical formalism to describe the amplification of two monochromatic waves counter-propagating in a rare-earth-doped optical fiber amplifier. Interaction of the waves through a dynamical population inversion grating inscribed in the active fiber by the waves during their amplification results in a strong power transfer from one wave to another providing a preferable amplification of one wave at the expense of another. In this sense, the effect is similar to stimulated Brillouin scattering and is expected to be observed with both pumped and unpumped rare-earth-doped fibers possessing a finite polarizability difference between the excited and ground states.

Detuning effects in Brillouin ring microresonator laser.

Brillouin lasers, with their unique properties, offer an intriguing solution for many applications, yet bringing their performance to integrated platforms has remained questionable. We present a theoretical framework to describe Brillouin lasing in integrated ring microcavities. Specifically, a general case of a mismatch between the Brillouin shift and the microresonator inter-mode spacing is considered. We show that although the lasing threshold is increased with the frequency detuning, a significant enhancement of the laser power in comparison with the pure resonant interaction could be achieved. Moreover, there is an optimal pump frequency detuning from the resonant mode frequency, when the effect is most pronounced. An increase of the Brillouin threshold with the pump frequency detuning is accompanied by narrowing the pump frequency range available for lasing. Importantly, at the optimal value of the pump frequency detuning when the Brillouin signal is maximal, Brillouin signal noise level is minimal. Analytical results obtained in the steady-state approach are in quantitative agreement with the results of numerical simulations.

Mode-locking evolution in ring fiber lasers with tunable repetition rate.

We have applied a simple approach to analyze behavior of the harmonically mode-locked fiber laser incorporating an adjustable Mach-Zehnder interferometer (MZI). Our model is able to describe key features of the laser outputs and explore limitations of physical mechanisms responsible for laser operation at different pulse repetition rates tuned over a whole GHz range. At low repetition rates the laser operates as a harmonically mode-locked soliton laser triggered by a fast saturable absorber. At high repetition rates the laser mode-locking occurs due to dissipative four-wave mixing seeded by MZI and gain spectrum filtering. However, the laser stability in this regime is rather low due to poor mode selectivity provided by MZI that is able to support the desired laser operation just near the lasing threshold. The use of a double MZI instead of a single MZI could improve the laser stability and extends the range of the laser tunability. The model predicts a gap between two repetitive rate ranges where pulse train generation is not supported.

Electronic and thermal refractive index changes in ytterbium-doped fiber amplifiers.

We develop a theoretical framework to analyze the mechanism of refractive index changes (RIC) in double-clad Yb³âº doped optical fibers under resonant core or clad pumping, and with signal amplification. The model describes and compares thermal and electronic contributions to the phase shifts induced on the amplified signal at 1064 nm and the probe signal at 1550 nm, i.e. located inside and outside of the fiber amplification band, respectively. The ratio between the thermal and electronic phase shifts is evaluated as a function of the pump pulse duration, the gain saturation, the amplified beam power and for a variety of fiber parameters.

Modeling of CW Yb-doped fiber lasers with highly nonlinear cavity dynamics.

We develop a theoretical framework for modeling of continuous wave Yb-doped fiber lasers with highly nonlinear cavity dynamics. The developed approach has shown good agreement between theoretical predictions and experimental results for particular scheme of Yb-doped laser with large spectral broadening during single round trip. The model is capable to accurately describe main features of the experimentally measured laser outputs such as power efficiency slope, power leakage through fibre Bragg gratings, spectral broadening and spectral shape of generated radiation.

Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber.

We observed an unusually narrow spectrum of Stokes field and Gaussian statistics of Stokes power for the stimulated Brillouin scattering (SBS) process in 300-m single-mode optical fiber with high Rayleigh losses. The measured characteristics of the Stokes radiation indicate that SBS lasing took place in the fiber. The effect is explained as the result of dynamic distributed feedback that is due to double Rayleigh scattering (RS) of the Stokes field. The results of numerical simulation of the cooperative SBS-RS process in fiber are in good agreement with experimental results.