Elsevier

Optical Fiber Technology

Volume 75, January 2023, 103216
Optical Fiber Technology

Birth and annihilation of solitons in harmonically mode-locked fiber laser cavity through continuous wave injection

https://doi.org/10.1016/j.yofte.2022.103216Get rights and content

Highlights

  • We have performed the experimental and numerical studies of birth and annihilation of solitons in fiber laser.

  • We present experimental observations of transition dynamics associated with the changes of the number of solitons in the cavity.

  • We found that external CW injection allows one-by-one change of the number of solitons in the laser cavity.

  • The CW injection enables repetition rate tuning of the HML laser with an elementary step equal to the fundamental frequency.

Abstract

We have performed the experimental and numerical studies exploring dynamical processes in a harmonically mode-locked fiber laser responsible for birth and annihilation of solitons in the laser ring cavity. The injection of external continuous wave (CW) allows one-by-one change of the number of solitons in the laser cavity thus enabling the fine-tuning of the pulse repetition rate. We present new experimental observations of the laser transition dynamics associated with the changes of the soliton numbers and give clear insight into the possible physical mechanisms responsible for these effects.

Introduction

Passively mode-locked fiber lasers have become a deserving alternative to semiconductor and solid-state lasers ensuring reliability, compactness, convenient output and single-mode beam quality inherent to fiber lasers. Soliton fiber lasers being in focus of multiple studies for a few decades have proven to be reliable sources of sub-picosecond pulses. Depending on a comprehensive interaction via nonlinearity, dispersion, gain and loss, various multi-pulse states such as soliton molecules and crystals, pulse bunches, noise-like pulses can be formed in the laser cavity making the fiber lasers to be a versatile platform for studies of dissipative system with complex dynamics [1], [2], [3], [4]. A special multi-pulse regime of harmonic mode-locking (HML) enabling uniform distribution of multiple solitons in the cavity is used in a wide range of practical applications [5], [6], [7].

To describe the physical mechanisms responsible for laser operation in different multi-pulse regimes the specific features of the laser dynamics associated with pulse interaction within the laser cavity, transitions between different laser steady-states, soliton birth and annihilation have been intensively studied [8], [9]. The role of background radiation as a mediator providing the equalizing interaction between pulses has been intensively discussed in this context [5], [10], [11], [12]. Recent studies of the transit processes in the HML laser have confirmed an importance of the pulse interaction with the background radiation in the build-up or annihilation of soliton pulses [9]. In our direct experiment, we have studied the effect of the CW injected into the laser cavity from an external narrow-band laser source on the HML laser operation. We demonstrate that under a proper pump power adjustment the optical injection could trigger a complex transition process in the laser cavity resulting in the birth or annihilation of individual solitons. The effect exhibits a strong resonant dependence on the wavelength of the injected CW and makes no impact on other laser performance characteristics. We have also explored a possible application of the observed effect for precise tuning of the HML laser pulse repetition rate (PRR) [13]. Commonly, with perfect adjustment of the pump power, the number of solitons in the HML laser cavity could be changed simultaneously by a large number. It is known as a part of the hysteresis effects typical for soliton lasers [14], [15]. Surprisingly, the technique based on the CW injection enables PRR tuning with an elementary step equal to the fundamental frequency. However, the physical mechanisms underlying these laser properties have not been considered yet. In this work, we have performed new studies on dynamical processes in a HML fiber laser, in particular, considering the birth and annihilation of individual solitons in the laser ring cavity. We report on new experimental findings in the laser transition dynamics associated with the changes of numbers of solitons inside the cavity and propose numerical model of the laser operation exploring the possible physical mechanisms responsible for these effects.

Section snippets

Experiment

In this section, we describe the results of our recent experiments demonstrating the effects of birth and annihilation of the solitons in the laser cavity triggered by the resonant injection of an external CW light. The experimental configuration of an Er-doped soliton nonlinear polarization evolution (NPE) mode-locked fiber ring laser shown in Fig. 1 is not so different from one used in Ref. [13]. The laser cavity consists of two types of fibers: 0.8 m length of heavily erbium-doped fiber

Numerical model

The configuration of fiber ring laser with CW injection used for numerical analysis is close to the one described earlier [19]. It is depicted schematically in Fig. 6. The configuration comprises a gain fiber (or EDF amplifier), a polarization controller (PC), a piece of passive SMF, a polarizer, and an output coupler. We assume that the light propagating in the gain fiber is linearly polarized, whereas the light propagating in SMF could possess an elliptical polarization. For this reason, a

Simulation of a system without CW injection

Without injection of the external CW component, the considered laser model describes dynamics of a soliton fiber laser. We explore the laser system dynamics established after ∼2500 roundtrips in the cavity. Since the saturation energy Eg is proportional to the pumping strength [14], the increasing Eg corresponds to the increasing pump power in the experiments. In simulations, the self-started mode locking is achievable at Eg>3 pJ. Similar to the experiment, an increase of Eg leads to an

Simulation of pulse birth/annihilation in the system with injected CW

In this section we apply the proposed laser model to explore the effect of the CW injection on the single soliton birth/annihilation processes in the HML laser cavity. We have performed a series of simulations similar to those reported in the previous section to explore the laser system states established after ∼2500 roundtrips in the cavity. For simplicity, we restrict our consideration to the simplest case ofωCW=0, when the CW injection wavelength coincides with the active fiber gain line

Discussion and conclusions

In the experiment with a ring fiber HML laser we have observed the hysteresis in the dependence of the laser PRR on the pump power. Up to the maximum PRR of ∼6.75 GHz the dependence possesses typical soliton hysteresis areas with the PRR jumps of hundreds of MHz corresponding to the simultaneous birth/annihilation of tens of pulses in the cavity. Using the CW injection into the cavity, we are able to change the number of solitons in the cavity. The effect is observed near critical pump power

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work is supported by the Ministry of Science and Higher Education of the Russian Federation (Megagrant program, #075-15-2021-581) and Russian Science Foundation (grant #19-72-10037P). A.A.F. is supported by the European Union’s Horizon 2020 research and innovation programme (H2020-MSCA-IF-2020, #101028712).

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