Bidirectional Fiber Laser Research Articles

364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber

Raman fiber laser (RFL) based on distributed Raman gain along optical fiber is an ideal platform for realizing ultra-long-cavity laser, which is of great interest in fundamental laser science and multidisciplinary fundamental studies such as complex systems exhibiting turbulence-like characteristics. RFL based on standard single-mode fiber sets the record of the longest laser cavity reaching 270 km with a clearly resolvable mode structure. Here, we explore the effect of pumping scheme and key fiber parameters to further extend the ultimate length of RFL. We theoretically analyze and experimentally demonstrate that by adopting higher-order pumping and ultra-low-loss fiber (ULLF) with low transmission loss and low Rayleigh backscattering coefficient, the laser cavity length of RFL with resolvable mode structure can be significantly increased. As a result, a 364 km-long bi-directional pumped 6th-order ultra-long Raman fiber laser (URFL) based on ULLF is demonstrated with resolvable longitudinal mode, an extension of 35 % over the prior 270 km record. This work paves a way to lengthen cavity length of URFL and provides a new platform for fundamental laser physics study and the ultra-long fiber laser applications in communication and sensing.

Spotlighting the Simultaneous Formation of Coherent and Incoherent Dissipative Solitons in an Er-Doped Bidirectional Ultrafast Fiber Laser

The emergence of localized structures originating from instabilities is of particular interest to scientists interested in exploring the rich nonlinear dynamics associated with complex dynamical systems. In the context of ultrafast lasers, the localized structures can be broadly divided into two categories: coherent structures of the dissipative soliton (DS) and incoherent structures such as the noiselike pulse (NLP). These two different types of structures have unique properties and applications and their physical origins are markedly different. Having both in a single fiber laser is an intriguing but challenging problem. Here we demonstrate the formation of the DS and NLP in a bidirectional ultrafast fiber laser and study their buildup dynamics from $Q$-switched instabilities. The clockwise light is a DS while the counterclockwise light is a NLP. The synchronized buildup dynamics of a DS or NLP from $Q$-switched instabilities provides vital evidence of the $Q$-switched instabilities as a universal indicator for the generation of localized structures in ultrafast lasers. The evenly spaced $Q$-switched spikes before the onset of a DS (NLP) exhibit intensity fluctuations, which is strikingly different than in the mode-locking regime in unidirectional oscillators, hence providing decisive evidence of the interaction between the counterpropagating light fields in the bidirectional lasers. The spectra cross-correlation and the histogram of the spectral intensity confirm the concurrent formation of a DS and NLP. Numerical simulations confirmed experimental observations. These findings help deepen understanding of the behavior of bidirectional ultrafast lasers and also demonstrate their potential as multifunctional ultrafast laser sources.

2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser.

Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with a reflectivity of about 10%. The pump power provided by the 976 nm laser diodes was injected into a double-clad Yb-doped fiber with core/cladding diameters of 20/400 µm. At the maximum pump power, the bidirectional output powers were 2025 W and 1948 W, respectively, and the output laser beam quality (M2 factor) at both ends was about 1.5. For the first time, to the best of our knowledge, the feasibility of a bidirectional output fiber laser that can achieve double high (2-kW-level) power was verified. Compared with a traditional unidirectional output laser, this type of bidirectional output laser can achieve a double high-power laser by employing a laser resonant cavity. Thus, the average cost and structure size can be further reduced in mass production.

Transient switching dynamics between asynchronous and synchronous modes in a bidirectional ultrafast fiber laser

Continuous interest in bidirectional ultrafast fiber lasers not only reinforces the practical applications in coherent dual-comb spectroscopy but also enriches the understanding of soliton dynamics in nonlinear systems. However, the transient dynamics of bidirectional solitons are still unexplored. Here, we report on an observation of the switching dynamics in a net-anomalous-dispersion bidirectional ultrafast fiber laser which can operate in both asynchronous and synchronous operations. Transient switching dynamics between the asynchronous and synchronous states are demonstrated, which is accompanied by switchable and weak interaction between bidirectional solitons. The dynamical evolution and output properties of the asynchronous and synchronous bidirectional solitons are investigated respectively. Stable breathing solitons in the synchronous operation show opposite behaviors in both spectra and pulse energy dynamics, which vary with different pump powers. Our findings provide insight into the transient dynamics in ultrafast lasers and contribute to the understanding of nonlinear systems in general.

Dynamics of breathing dissipative soliton pairs in a bidirectional ultrafast fiber laser.

The breathing dissipative soliton as a dynamic solution to many nonlinear systems has induced substantial interest in nonlinear photonics and ultrafast laser science. However, the exotic breathing multi-soliton dynamics are still largely unexplored in the bidirectional fiber laser compared to the unidirectional laser. Here, we reveal nonequilibrium dynamics of a breathing soliton pair (BSP) with energy transfer in a bidirectional laser; in particular, the dissociation and annihilation of the BSP was triggered by control over intra-cavity polarization. Optical rogue waves were detected simultaneously, and the collision of breathers significantly increased the intensity of rogue waves, which is characteristic of the bidirectional laser. Further, the buildup dynamics of the BSP with nanosecond pulse separation and a breathing soliton molecule were observed. Multiple single soliton explosions and transient pulse splitting are distinct features of soliton molecule buildup compared to the soliton pair. These findings shed new insights into the multiple breather dynamics of nonlinear systems.

High-peak-power pump-modulated quasi-CW fiber laser.

Quasi-continuous wave (CW) laser output with high peak power and high energy is preferred in some industrial applications. Due to the non-linear effects and transverse mode instability, such high-peak-power laser output is difficult to achieve via monolithic fiber lasers in CW mode. For diode-pumped monolithic fiber lasers, by applying overshoot pulse modulation to the pumping diodes, we obtain a pulse laser output with a peak power much higher than that in CW mode. In this paper, it has been theoretically studied that stable pulses with the width of µs level can be generated with quasi-CW operation without distortion according to our simulation. We also experimentally demonstrate a bi-directional pumped quasi-CW monolithic fiber laser operating in CW and pulse modes. In quasi-CW mode, by applying overshoot pulse modulation to the diodes, with a frequency of 1 kHz and a pulse width of 100 µs, the peak power of the output laser reached 9713 W with an average power of 898 W and M2 of 2.4 and 2.3 in the two orthogonal directions, respectively. To the best of our knowledge, this is the very first quasi-CW fiber laser of a 10-kW level with the M2 level of 2.

Breathing Dissipative Soliton Molecule Switching in a Bidirectional Mode‐Locked Fiber Laser

Breathing optical solitons propagating in a dissipative nonlinear system can interact and bind stably, forming an optical soliton molecule that presents striking molecule‐like dynamics. To date, the breathing soliton pair has been mainly observed in the microcavity platform, and the peculiar dynamic evolution of breathing soliton molecules (BSMs) remains largely unexplored in mode‐locked fiber lasers. Herein, the transient switching dynamics of BSMs in a bidirectional ultrafast fiber laser is revealed, specifically triggered at different parameter spaces of saturable absorption with manipulation polarizations that maintain constant pulse separation or undergo strong repulsion and kink after switching. All‐optical switching of breather molecules and significantly increasing multiple soliton molecules’ switching of two or three solitons between states with different binding separations by applying a strong stimulus with periodic pump modulation is demonstrated. The instantaneous pulse breakup can be induced by the collision of bidirectional breathing solitons in each switching, which is characteristic of the breathing solitons in a bidirectional fiber laser and further corroborated by numerical simulation. The study unveils new perspectives into the ultrafast transient process of BSMs in various dissipative systems.

Microfiber Assisted Bidirectional Wavelength-Locked Ultrafast Erbium-Doped Fiber Laser

Microfiber Assisted Bidirectional Wavelength-Locked Ultrafast Erbium-Doped Fiber Laser

Mutually induced soliton polarization instability in a bidirectional ultrafast fiber laser.

The bidirectional ultrafast fiber laser is a promising light source for dual-comb applications. The counter-propagating geometry could lead to soliton interaction through gain sharing, as well as the possible outcome of polarization instability. However, the polarization dynamics hidden behind the soliton interaction process in bidirectional fiber lasers were rarely investigated. Herein, we report on the polarization instability induced by the mutual soliton interactions through fiber gain in a bidirectional mode-locked fiber laser. Depending on the adjustment of the intracavity birefringence, the polarization states of two counter-propagating solitons can exhibit similar periodical polarization switching behaviors with a polarization-rotating transition state. The successive interactions of the bidirectional solitons mediated by the polarization cross-saturation effect of gain fiber could be responsible for the soliton polarization instability. These findings, in addition to the fundamental interest of the soliton nonlinear dynamics in dissipative optical systems, also open up new possibilities for creating dynamical control of the soliton polarization state and performance improvement in bidirectional ultrafast fiber lasers.

Switchable generation of multi-wavelength continuous wave and square-wave pluses in a bidirectional Erbium-doped fiber laser

We present a switchable generation of multi-wavelength continuous wave (CW) and square-wave pluses (SWPs) in a bidirectional erbium-doped fiber laser. The variable attenuator (VA) is used to realize switchable generation of CW and mode-locking pulse. In the multi-wavelength loop (MWL), the laser using the Sagnac loop output single-wavelength, dual-wavelength and triple-wavelength CW by finely adjusting the polarization controllers (PC). In the mode-locked loop (MML), the stable SWPs and harmonic mode-lock (HML) SWPs can be obtained by nonlinear polarization rotation (NPR) technique.

Single-frequency all-polarization-maintaining ytterbium-doped bidirectional fiber laser.

We reported an all-polarization-maintaining single-frequency ytterbium-doped bidirectional fiber laser for the first time, to the best of our knowledge. Single-frequency operation was achieved by a stable dynamic grating in the active fiber of a proper length owing to the bidirectional operation of the laser. The fiber laser possesses a linewidth of 7.43 kHz, a slope efficiency of 47.9%, and a great long-term stability.

Carbon nanotube mode-locked fiber lasers: recent progress and perspectives

Abstract Carbon nanotubes (CNTs) possess remarkable nonlinear optical properties; a particular application is to function as a mode locker used in ultrafast fiber lasers to produce ultrashort optical pulses. Various types of CNT saturable absorbers (SAs) and ultrafast fiber lasers have been demonstrated. In this review, typical fabrication process and development of CNT SAs are discussed and we highlight the recent investigation and progress of state-of-the-art ultrafast fiber lasers covering GHz, bidirectional ultrafast fiber lasers, vectorial mode fiber lasers, comb systems, and mode-locking dynamics. Our perspectives of ultrafast fiber lasers based on CNT SAs are given finally.

Breathing dissipative soliton explosions in a bidirectional ultrafast fiber laser

Soliton explosions, among the most exotic dynamics, have been extensively studied on parameter invariant stationary solitons. However, the explosion dynamics are still largely unexplored in breathing dissipative solitons as a dynamic solution to many nonlinear systems. Here, we report on the first observation of a breathing dissipative soliton explosion in a net-normal-dispersion bidirectional ultrafast fiber laser. The breathing soliton explosions could be stimulated by the soliton buildup process or alteration of polarization settings. Transient breathing soliton pairs with intensive repulsion that is sensitive to initial conditions can also be triggered by multiple soliton explosions in the soliton buildup process instead of being triggered by varying polarization settings. The high behavior similarity also exists in the breathing soliton buildup and explosion process owing to the common gain and loss modulation. In addition, dissipative rogue waves were detected in the breathing soliton explosion, and the collision of breathing soliton significantly enhanced the amplitude of rogue waves, which is characteristic of the breathing solitons in a bidirectional fiber laser. These results shed new insights into complex dissipative soliton dynamics.

Complex pulsating dynamics of counter-propagating solitons in a bidirectional ultrafast fiber laser.

Bidirectional ultrafast fiber lasers capable of generating counter-propagating (CP) coherent solitons are promising to be served as a dual-comb light source for the applications in spectroscopy and gyroscope. In the absence of efficient numerical model, the understanding of the operation of bidirectional fiber lasers is very limited. In this paper, we experimentally explore the pulsating dynamics of CP solitons in a bidirectional mode locked fiber laser and present a set of rich complex dynamics of CP solitons, revealing for the first time, to the best of our knowledge, the periodic breathing and acyclic pulsating dynamics of CP solitons. With a bi-directional pumping configuration, the impacts of gain distribution along the fiber on the dynamics of CP solitons have been investigated and discussed. These results provide further evidence of the universality of breathing dynamics of solitons. More importantly, the abundant dynamical behavior of CP solitons demonstrated in this paper, collaborating with a handful of previous reports on the buildup dynamics of CP solitons in bidirectional fiber lasers, underline further the independent evolving dynamics of CP solitons. These findings contribute to the understanding of how bidirectional lasers work and, consequently, will accelerate the development of bidirectional lasers in the applications such as gyroscope.

Bidirectional mode-locked all-normal dispersion fiber laser

The bidirectional mode-locked oscillator is an emerging light source architecture suitable for dual-comb applications. Therein, bidirectional mode-locked fiber lasers (MLFLs) are particularly promising for their cost effectiveness, system compactness, and environmental robustness. However, the pulse energy has been limited to tens of picojoules, restricting practical dual-comb applications, especially in the nonlinear regime. In this paper, we break the pulse energy limit by devising the first bidirectional all-normal dispersion MLFL with an artificial saturable absorber (ASA). Bidirectional dissipative solitons are generated with > 5 -THz bandwidths and > 1 -nJ pulse energies. Free-running laser performance is extensively characterized, and the physical mechanism for bidirectional ASA mode-locking is studied. Last but not least, a proof-of-concept dual-comb spectroscopy measurement is demonstrated. Our work paves the way for a new class of bidirectional MLFLs benefiting dual-comb applications in both linear and nonlinear regimes.

Build-up dynamics in bidirectional soliton fiber lasers

Bidirectional ultrafast fiber lasers present an attractive solution, enabling the generation of two mutually coherent ultrashort pulse trains in a simple and turnkey system. Still, the lack of a comprehensive numerical model describing steady-state bidirectional generation, and even less ultrafast soliton breakdowns and collisions, is obstructing the achievement of the performance compared with unidirectional lasers. In this paper, we have experimentally investigated real-time build-up dynamics of counter-propagating solitons in an ultrafast ring Er-doped fiber laser via the dispersive Fourier transform methodology. We parade that counter-propagating pulses experience independent build-up dynamics from modulation instability, undergoing breathing dynamics and diverging subordinate pulse structure formation and annihilation to a stable bidirectional pulse train. Yet, the interaction of pulses in the cavity presents the key underlying phenomenon driving formation evolution distinct from unidirectional pulse build-up. Our findings will provide physical foundations for bidirectional ultrafast fiber laser design to carry forward their application.

Observation of reverse self-sweeping effect in an all-polarization-maintaining bidirectional ytterbium-doped fiber laser.

In this article, we report, to the best of our knowledge, the first observation of the reverse self-sweeping phenomenon in an all-polarization-maintaining bidirectional ytterbium-doped fiber laser. Conventional behaviors, including the dependence of sweeping range, sweeping rate and average pulse repetition rate on the pump power, can be observed in our fiber laser. Two couplers with ratio of 50/50 and 10/90 are respectively employed as the output coupler in fiber laser, which generates the reverse self-sweeping phenomenon for comparison.

Wavelength-tunable bidirectional passively Q-switched Er-doped fiber laser incorporating a single-walled carbon nanotube and tunable bandpass filter.

We demonstrated a continuously wavelength-tunable bidirectional passively $ Q $Q-switched fiber laser based on a single-walled carbon nanotube saturable absorber and a bandpass filter (BPF). By tuning the commercial BPF, the $ Q $Q-switched pulse can be obtained in the same cavity at the same pump power in clockwise (CW) and counter-clockwise (CCW) directions. The central wavelength can be continuously tuned from 1520.88 to 1568.56 nm in the CW direction, and from 1520.96 to 1568.64 nm in the CCW direction. A wide tuning range of 48 nm is achieved for two directions in the passively $ Q $Q-switched fiber laser. During the tuning process of the fiber laser, the output pulses with the same central lasing wavelength can be obtained in both oscillation directions owing to the BPF. By increasing the pump power from 130 to 350 mW, the cavity delivers a $ Q $Q-switched pulse with the central wavelength of 1560 nm whose repetition rate changes from 9.64 to 59.18 kHz for the CW direction. In the CCW direction, the repetition rate of the $ Q $Q-switched pulse changes from 10.26 to 61.03 kHz. To the best of our knowledge, it is the first time that continuous wavelength-tunable passively $ Q $Q-switched pulses have been achieved in a bidirectional erbium-doped fiber laser.

Real-time observation of the optical Sagnac effect in ultrafast bidirectional fibre lasers

Active ring laser gyroscopes (RLG) operating on the principle of the optical Sagnac effect are preferred instruments for a range of applications, such as inertial guidance systems, seismology, and geodesy, that require both high bias stability and high angular velocity resolutions. Operating at such accuracy levels demands special precautions like dithering or multi-mode operation to eliminate frequency lock-in or similar effects introduced due to synchronisation of counter-propagating channels. Recently proposed bidirectional ultrafast fibre lasers can circumvent the limitations of continuous wave RLGs. However, their performance is limited due to the nature of the highly-averaged interrogation of the Sagnac effect. In general, the performance of current optical gyroscopes relies on the available measurement methods used for extracting the signal. Here, by changing the paradigm of traditional measurement and applying spatio-temporal intensity processing, we demonstrate that the bidirectional ultrafast laser can be transformed to an ultrafast gyroscope with acquisition rates of the order of the laser repetition rate, making them at least two orders of magnitude faster than commercially deployed versions. We also show the proof-of-principle for dead-band-free round trip time-resolved spectral domain measurements using the Dispersive Fourier Transform, further enhancing the gyroscopic sensitivity. Our results reveal the high potential of application of novel methods of signal measurements in mid-sized ultrafast fibre laser gyroscopes to achieve performances that are currently available only with large-scale RLGs.

CFBG-Based Bidirectional Mode-Locked Fiber Laser Emitting Conventional and Dissipative Solitons

We report on the multi-state soliton generation in a chirped fiber Bragg grating (CFBG) based bidirectional mode-locked fiber laser. In particular, a $\theta $ -shape associate cavity is designed to introduce reverse dispersion distributions in counter-propagation directions through incorporating the bidirectional reflection of the CFBG into the lasing oscillator. With a relatively compact cavity configuration, the clockwise (CW) and counterclockwise (CCW) directions are managed into large anomalous and normal dispersion regimes, respectively. Consequently, conventional solitons (CSs) and dissipative solitons (DSs) can be emitted from the counter-propagation directions. To the best of our knowledge, it is the first report of CS/DS generation in a CFBG based bidirectional fiber laser. This investigation can contribute to research community of multiplexed ultrafast fiber lasers.