Bidirectional Mode-locked Fiber Laser Research Articles

Bidirectional mode-locked erbium-doped fiber laser based on an all-fiber gold nanofilm saturable absorber.

We demonstrate a bidirectional mode-locked erbium-doped fiber laser by incorporating gold nanofilm as a saturable absorber (SA). The gold nanofilm SA has the advantages of high stability and high optical damage threshold. Besides, the SA exhibits a large modulation depth of 26% and a low saturation intensity of 1.22 MW/cm2 at 1.56 μm wavelength band, facilitating the mode-locking of bidirectional propagating solitons within a single laser cavity. Bidirectional mode-locked solitons are achieved, with the clockwise pulse centered at 1568.35 nm and the counter-clockwise one at 1568.6 nm, resulting in a slight repetition rate difference of 19 Hz. Moreover, numerical simulations are performed to reveal the counter-propagating dynamics of the two solitons, showing good agreement with the experimental results. The asymmetric cavity configuration gives rise to distinct buildup and evolution dynamics of the two counter-propagating pulses. These findings highlight the advantage of the gold nanofilm SA in constructing bidirectional mode-locked fiber lasers and provide insights for understanding the bidirectional pulse propagation dynamics.

Single-cavity bidirectional Raman-assisted mode-locked fiber laser

Bidirectional mode-locked fiber lasers are promising dual-comb light sources. Stimulated Raman scattering (SRS) effect can significantly extend the wavelength range of mode-locked fiber lasers. In this work, we propose and demonstrate a single-cavity bidirectional Raman-assisted mode-locked fiber laser based on nonlinear multimode interference effect. By adjusting the polarization of the cavity, we obtain synchronous and asynchronous pulse trains in counterclockwise (CCW) direction and clockwise (CW) direction, as a consequence of changes in the dispersion and nonlinear effects. Due to the extension of the wavelength range of single-cavity bidirectional mode-locked fiber laser, we believe that this work will hold potential for future applications in dual-comb spectroscopy, asynchronous sampling and other areas.

An Information Coding System Based on Bidirectional Mode-Locked Fiber Laser

As a carrier of the information transmission based on the optical signal, the laser communication is a communication mode by using the laser to transmit information. It is also a promising method in the field of optical communication. We report a bidirectional mode-locked fiber laser with graphene-oxide-based modelocker that allows bidirectional conventional solitons in the L-band, and design an information coding system based on pulse trains from this bidirectional mode-locked fiber laser by taking advantage of the periodicity of the pulse trains of the bidirectional mode-locked fiber laser in the time domain, which could be applied to optical communication with bidirectional mode-locked fiber laser.

Multi-dimensional multiplexed, tri-comb and quad-comb generation from a bidirectional mode-locked fiber laser.

Tri-comb and multi-comb techniques could enable many advanced measurement applications beyond the reach of traditional dual-comb schemes. However, the sophisticated and bulky control systems of the conventional schemes based on three comb lasers render them impractical for many potential applications. Like their dual-comb counterparts, tri-comb and multi-comb lasers are being investigated as attractive alternatives. In contrast to previous dual-comb lasers using only one multiplexing dimension of optical pulses, this work simultaneously leverages multiplexing methods in three physical dimensions, i.e. wavelength, polarization, and direction, to generate triple to quadruple asynchronous pulse trains in a bidirectional mode-locked fiber laser. Because of the unique cavity structure studied here, both wavelength-multiplexed and polarization-multiplexed dual-comb generation from a completely shared-cavity and wavelength/polarization-multiplexed multi-comb generation from a bidirectional partially shared-cavity are achieved. Good relative stability among the generated combs of the fiber laser is demonstrated, as well as proof-of-concept dual-comb spectroscopy measurements, which validates the mutual coherence between the combs. The analysis of the experimental results further reveals interesting performance comparisons between combs from different multiplexing schemes, thanks to the special laser design used here that allows a side-by-side dual-comb demonstrations from different combinations of outputs from the same laser. Our investigation could facilitate multi-comb generation based on one light source for field-deployable multi-comb applications.

Synchronously-tunable bidirectional mode-locked fiber laser based on Lyot filtering effect

We report a widely tunable mode-locking in a bidirectional fiber laser, where the wavelength tuning is achieved by a birefringent filter. The central wavelength of the CW pulse can be tuned within 1549.5 nm-1572.7 nm, synchronously, the central wavelength of the CCW pulse changes within 1548.3 nm-1573.1 nm. The relative wavelength difference of the bidirectional pulses is controllable within a certain range, which supports the switching of the laser output between asynchronous and synchronous pulses. The bidirectional output pulses do not exhibit significant variations both in spectral shapes and central wavelengths in 20 min. In addition, we record a novel output with bidirectional soliton-type switchability. Our work achieves a wide range of CW and CCW pulses with tunable center wavelengths simultaneously based on the bidirectional laser and Lyot filtering effect, which may facilitate the design of dual-wavelength fiber lasers and satisfy different application requirements.

Intracavity-loss controlled wavelength-tunable bidirectional mode-locked erbium-doped fiber laser.

Bidirectional wavelength-tunable mode-locked fiber lasers have demands for many applications. In our experiment, two frequency combs from a single bidirectional carbon nanotube mode-locked erbium-doped fiber laser are obtained. Continuous wavelength tuning is demonstrated in the bidirectional ultrafast erbium-doped fiber laser for the first time. We utilized the microfiber assisted differential loss-control effect on both directions to tune operation wavelength and it presents different wavelength tuning performances in two directions. Correspondingly, the repetition rate difference can be tuned from 98.6 Hz to 32 Hz by applying strain on microfiber within 23 µm stretching length. In addition, a minor repetition rate difference variation of 4.5 Hz is achieved. Such technique may provide possibility to expand wavelength range of dual-comb spectroscopy and broad its application fields.

All-fiber bidirectional mode-locked laser to generate cylindrical vector beam asynchronous noise-like pulses.

We proposed and demonstrated a bidirectional mode-locked fiber laser to generate cylindrical vector beam (CVB) asynchronous pulses based on a graded index multimode fiber. A homemade fused taper two-mode fiber optical coupler (TMF-OC) is employed as a mode converter. The central wavelength for clockwise (CW) pulses can be tuned from 1030.32 nm to 1041.04 nm due to the filtering effect based on multimode interference, that of counterclockwise (CCW) pulses is from 1030.81 nm to 1039.28 nm. When the central wavelengths are 1033.22 nm and 1032.71 nm for CW direction and CCW direction respectively, CVB asynchronous noise-like pulses with a repetition rate difference of ∼436.9 Hz can be obtained. The purity of CVB in CW direction and CCW direction is 95.7% and 93.4% respectively. This bidirectional mode-locked fiber laser with CVB output can be better applied to laser gyroscopes, asynchronous sampling, and dual-comb technique, and impel the interdisciplinary studies in the future.

Bidirectional mode-locked fiber laser based on nonlinear multimode interference

Bidirectional mode-locked lasers are an ideal light source for dual-comb technique for their system compactness and common noise cancellation. Here, we report an all-fiber bidirectional mode-locked laser based on nonlinear multimode interference. Asynchronous pulse trains in different directions with a repetition rate difference of 7 Hz are obtained. Besides, we record the pulse trains and spectra at different pump powers. In our experiment, the counter-propagating pulse (CPP) trains can operate in different states, such as the counterclockwise (CCW) direction works in single pulse while the clockwise (CW) direction works in multi-pulse or harmonic mode-locking. We believe that the work will provide a new platform for studying the dynamic characteristics of bidirectional mode-locked lasers.

Mutual dynamics between synchronous solitons in a bidirectional mode-locked fiber laser.

In this Letter, the mutual dynamics between synchronous solitons in a bidirectional mode-locked fiber laser are studied via dispersive Fourier transform methodology. We explore the spectral evolution and the statistical correlations between solitons with bidirectional propagation, indicating the low and high mutual linear dependences of the spectral energy jitters in stable and breathing mode-locking states, respectively. Moreover, to the best of our knowledge, the oscillating and sliding phase dynamics are experimentally revealed by the interference between bidirectional breathing solitons in ultrafast fiber lasers for the first time. Our findings enrich the understanding of the internal mutual dynamics between bidirectional solitons, which guides the extension of their potential applications, such as Sagnac-effect-based optical sensing.

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.

Pulse-onset dynamics in a bidirectional mode-locked fibre laser via instabilities

Real-time observation of the emergence of coherent structures from noise via instabilities is of particular interest across disciplines ranging from biology to astrophysics. In the context of photonics, ultrafast fibre lasers provide an ideal test-bed for experimental observation of dynamical instabilities and generation of coherent structures of ultrashort pulses. Here we present experimentally obtained switch-on dynamics of counter-propagating ultrashort pulses in a bidirectional mode-locked fibre laser with delayed pulse formation via Q-switched and modulation instabilities, pronounced central wavelength drift, with the multiple-pulse formation. We define a localisation parameter using the round-trip resolved autocorrelation function to quantify the extent of the pulse formation, indicating an energy interchange between coherent features and background radiation. Furthermore, we report the formation of synchronised and unsynchronised dispersion waves. Our results reveal the complexity of the establishment of coherent features and their interaction with background radiation, contributing further towards the understanding of nonlinear systems in general.

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.

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.

Scalar and Vector Solitons in a Bidirectional Mode-Locked Fibre Laser

Recent discovery on scalar–vector features of ultrashort pulses sheds new light on the polarization dynamics of ultrafast fibre lasers. Here, we report on the generation of scalar and vector solitons in a bidirectional fibre laser based on nonlinear polarization rotation (NPR). Through introducing different polarization evolving mechanisms into the counter-propagation branches, the fibre laser respectively approaches the scalar-soliton regime and vector-soliton regime in the clockwise and counterclockwise directions. It is found that the strong local birefringence facilitates the energy to flow between two orthogonal polarization directions, consequently inducing the pulse evolution toward vector solitons even against obvious polarization discrimination effect introduced by the NPR mode-locking mechanism. Furthermore, the polarization resolved measurement is implemented to both analyze the scalar–vector feature of the emitted solitons and realize the manipulation of vector solitons. In addition, numerical simulations are carried out to clarify the manipulation process of the vector solitons. To our knowledge, this is the first observation of scalar–vector operation in a bidirectional fibre laser, which both enriches our understanding toward polarization dynamics of optical solitons and paves potential application scenarios of pulsed fibre lasers.

Controlling group and phase velocities in bidirectional mode-locked fiber lasers

A bidirectional mode-locked fiber laser producing two correlated frequency combs of the same repetition rate is demonstrated. The intensity dependence of pulse and phase velocities are measured simultaneously. As expected, the phase delay is determined by the linear and nonlinear indices of refraction. The nonlinear Kerr effect contribution to the pulse velocity is dwarfed by a contribution from saturable gain dynamics.

Free-running mode-locked laser based dual-comb spectroscopy.

We present a real-time dual-comb spectrometer operated from a bidirectional mode-locked fiber laser in the wavelength range of 1.9μm. Two pulsed signals emitted from a common cavity ensure mutual coherence and common mode noise rejection. The resulting spectrometer operates without any complex electronic feedback system.

Bi-Directional Mode-Locked Thulium Fiber Laser as a Single-Cavity Dual-Comb Source

We demonstrate the use of a bi-directional mode-locked thulium doped fiber ring laser as a single cavity free-running dual-comb source. We consider the effect of group-velocity dispersion on the repetition rate difference between the two combs and demonstrate a tunable repetition rate difference from 48–229 Hz. Dual-comb spectroscopy of atmospheric H2O near $1.87~\mu \text{m}$ is performed over a 1.47-m air path in laboratory with a peak single-shot signal-to-noise ratio of 27.5.

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1–2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

Bidirectional mode-locked thulium-doped fiber laser.

We report a bidirectional mode-locked thulium-doped fiber laser. Mode-locking is enabled by the combination of semiconductor saturable absorption and nonlinear polarization rotation. Two stable mode-locked picosecond pulse trains in opposite directions are generated with a fundamental repetition rate of ∼16.57 MHz. Output wavelengths are tunable over 35nm.

-Band Noise-Like Pulse Generation in a Bidirectional Mode-Locked Fiber Laser

We report on the generation of $L$ -band noise-like pulses (NLPs) in a bidirectional mode-locked fiber laser. Benefiting from the absence of the isolator and the in-band absorption of the erbium-doped fiber, mode-locked pulses at the waveband over 1600 nm are delivered from this bidirectional oscillator. With appropriate settings, conventional solitons can evolve into NPLs in both clockwise and counter clockwise directions, which are validated by the sideband-free spectral shape and the distinguishable autocorrelation trace. In addition, it is experimentally found that the $L$ -band NLP tends toward splitting due to the enhanced soliton collapse effect and the peak power clamping effect at the waveband around 1605 nm. Consequently, second–sixth harmonic mode locking of the NLP is, respectively, observed for the bidirectional operation. To the best of our knowledge, it is the first investigation of $L$ -band NLPs in a bidirectional mode-locked fiber laser.