Mechanism of noise-like pulse in all-normal dispersion all-fiber laser based on nonlinear polarization rotation.

We investigate the mechanism of changing the polarization state to generate noise-like pulses (NLPs) in the all-normal dispersion (ANDi) all-fiber laser based on nonlinear polarization rotation (NPR). Numerical simulations show that the intracavity positive and negative feedback states change with the polarization state, the peak power of the pulse will be clamped when the negative feedback comes into play, thus facilitating the transition from dissipative soliton (DS) to NLP. Experimentally, the observation of wavelength switching and transition between DS and NLP by simply adjusting the polarization state matches the numerical simulation results. This study contributes to a deeper understanding of the mechanism for generating NLP by changing the intracavity polarization state in ANDi all-fiber lasers based on NPR and offers new possibilities for pulse-switchable light sources.

All-normal dispersion widely tunable dual-wavelength mode-locked fiber laser based on NALM.

We experimentally and numerically demonstrate the all-normal dispersion (ANDi) ytterbium (Yb)-doped fiber laser based on nonlinear amplifying loop mirror (NALM) mode-locked, which allows tunable single-wavelength and dual-wavelength outputs. The pulses tuning ranges of the dual-wavelength are from 1032.24 nm to 1053.13 nm and from 1047.94 nm to 1069.05 nm, and the repetition frequency difference varies from 1766Hz to 1834Hz. To our knowledge, this is the widest dual-wavelength tuning range of Yb-doped fiber lasers based on NALM mode-locked. We test for 90 minutes and have high stability in both single-wavelength and dual-wavelength. In addition, the pulsed collision dynamics between two solitons at different wavelengths are numerically studied. Numerical results show that during the pulse collision, the two solitons pass through each other and maintain their properties, which also confirms the particle nature of the isolated wave. Our research contributes to the dynamics of dual-wavelength solitons collision in NALM mode-locked fiber laser and provides what we believe to be is a new idea for tunable Yb-doped dual-comb sources.

170-W Nd:YAG InnoSlab laser at 1319†nm.

In this paper, a continuous-wave Nd:YAG InnoSlab laser at 1319 nm with high output power and high beam quality is demonstrated. The maximum output power of 170 W at 1319-nm single wavelength is obtained with an optical-to-optical efficiency of 15.3% from absorbed pump power to laser output and the corresponding slope efficiency of 26.7%. The beam quality factors of M2 are 1.54 and 1.78 in the horizontal and vertical directions, respectively. To the best of our knowledge, this is the first report on Nd:YAG 1319-nm InnoSlab lasers with such high output power and good beam quality.

Observation of four self-sweeping regimes in a single-mode bi-directional ytterbium-doped fiber ring laser.

In this work, for the first time, four self-sweeping regimes in a single-mode bi-directional ytterbium-doped fiber ring laser are observed by adjusting the polarization controller (PC): normal self-sweeping, reverse self-sweeping, mixed state, and wavelength stop state. In addition, regulating the PC can artificially selectively make the laser operate in normal self-sweeping or reverse self-sweeping within a certain pump power range, and their self-sweeping characteristics (e.g., sweeping rate, sweeping range, etc.) and intensity dynamics are investigated in detail, respectively. In conclusion, we can flexibly regulate the sweeping direction and sweeping characteristics of the bi-directional self-sweeping fiber ring laser in a simple approach by adjusting the PC, which is potentially valuable for its practical application.

Sideband-free tunable and switchable dual-wavelength mode-locked fiber laser based on the Lyot filter and spontaneous radiation peaks.

We demonstrate a compact tunable and switchable dual-wavelength fiber laser based on the Lyot filtering effect and the spontaneous radiation peaks of gain fiber. By introducing a period of polarization-maintain Er-doped fiber (PM-EDF), stable dual-wavelength pulses can operate in both the anomalous dispersion region and the normal dispersion region. The corresponding repetition frequency difference of the dual wavelengths has excellent stability while the relative center wavelength can be adjusted in the range of 5 nm to 13 nm. There is no existence of significant sidebands in the optical spectrum during the whole tuning process. This dual-wavelength laser based on two spontaneous radiation peaks in the shorter wavelength direction has great application potential. Our work provides a new design solution for dual-comb sources (DCSs).

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.

Generating narrow bandwidth pulses in a hybrid mode-locked fiber laser.

We demonstrate for the first time, to our knowledge, an all-fiber erbium-doped mode-locked laser in which mode-locking (ML) is realized by the combination of nonlinear polarization rotation and a saturable dynamic filtering effect, thereby generating nearly transform-limited ultrashort pulses with a pulse duration and spectral width of 45.2 ps and 0.0775 nm, respectively. The laser achieves both ML and harmonic ML by increasing the pump power. Simultaneously, the filtering function is maintained by the saturable dynamic induced grating (SDIG) throughout the power-modulation process. Furthermore, numerical simulations are used to analyze the pulse energy evolution in the cavity, revealing the advantages of hybrid ML in decreasing the pulse duration and time-bandwidth product under narrow filtering status. This work proposes a practical method to achieve ultrafast laser pulses with a narrow bandwidth, solving the problem that the SDIG has a hard time realizing a stable ML sequence.

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.

Stable-, period-N- and multiple-soliton regimes in a mode-locked fiber laser with inconsistently filtered central wavelengths.

We systematically study the stable-, period-N- and multiple-soliton regimes in an Erbium-doped fiber laser effectively mode-locked by nonlinear polarization rotation technique. In the stable mode-locked regime, an invariant soliton with 497 fs pulse duration and 6.9 nm optical spectrum are obtained. With a larger pump power of 180 mW, the period-N state (in which the pulse intensity returns to its original value after N cavity-roundtrips) emerges, accompanied by sub-sideband generation on the first Kelly sideband and spectrum shift. Considering the inconsistent central wavelengths between gain and polarization-dependent isolator (PD-ISO) firstly, to our knowledge, the numerical results are in good agreement with the experiment and reveal the composite filtering of gain and PD-ISO takes major responsibility for spectrum shift, which causes group velocity offset simultaneously. Further study shows the continued increase of pump power can lead to the laser operating in the unstable multi-pulse state and the narrow spectral width contributes to stabilizing the multi-pulse state. Our work can promote the understanding of soliton dynamics and filtering in ultrafast fiber lasers.

Asymmetric localization and symmetric diffraction-free transmission in synthetic photonic lattice with anti-parity-time symmetry.

We study, to the best of our knowledge, the first observations of light propagation in synthetic photonic lattice with anti-parity-time symmetry by tuning the gain or loss of two coupled fiber rings alternatively and corresponding phase distribution periodically. By tuning the phase φ and the wave number Q in the lattice, asymmetric transmission of the light field can be achieved for both long and short loops when φ≠nπ/2 (n is an integer). Further investigations demonstrate that asymmetric localization of the light field in the long loop and symmetric diffraction-free transmission in two loops can both be realized by changing these two parameters. Our work provides a new method to obtain anti-parity-time symmetry in synthetic photonic lattice and paves a broad way to achieve novel optical manipulation in photonic devices.

Black phosphorus flakes covered microfiber for Q-switched ytterbium-doped fiber laser.

We demonstrate a passively Q-switched ytterbium-doped fiber laser based on black phosphorus (BP) flakes covered microfiber. The BP saturable absorber is fabricated by sandwiching a microfiber between two pieces of polydimethylsiloxane supported BP flakes film, which is prepared by the mechanical exfoliation method. In this case the BP flakes can be well protected from the action of air and moisture. By incorporating BP flakes covered microfiber into a ytterbium-doped ring fiber laser, stable and reliable Q-switched operation at 1064 nm can be realized via interaction between few-layers BP flakes and the evanescent field of the laser. The laser allows Q-switched pulse generation with a repetition rate in the range of 26-76 kHz and a pulse duration in the range of 5.5-2.0 µs, by varying the pump power from 38 mW to 100 mW.

Polarization-Dependent Anisotropy of LIPSSs' Morphology Evolution on a Single-Crystal Silicon Surface.

Utilizing the principle of laser-induced periodic surface structures (LIPSSs), this research delves into the morphological evolution of single-crystal silicon surfaces irradiated by a near-infrared picosecond laser through a scanning mode. With the increase in laser energy density, the nanostructure morphology on single-crystal silicon surfaces induced by incident lasers with different polarization directions sequentially produces high spatial-frequency LIPSSs (HSFLs) with a period of 220 nm ± 10 nm parallel to the laser polarization, low spatial-frequency LIPSSs (LSFLs) with a period of 770 nm ± 85 nm perpendicular to the direction of the polarization, and groove structures. Furthermore, by varying the angle between the laser polarization and the scanning direction, the study examined the combined anisotropic effects of the laser polarization scanning direction angle and the laser polarization crystal orientation angle on the genesis of LIPSSs on single-crystal silicon (100) surfaces. The experiments revealed polarization-related anisotropic characteristics in the morphology of HSFLs. It was found that when the polarization angle approached 45°, the regularity of the LSFLs deteriorated, the modification width decreased, and the periodicity increased. This is critical for the precise control of the LSFLs' morphology.

Slope efficiency over 30% single-frequency ytterbium-doped fiber laser based on Sagnac loop mirror filter.

A high-slope-efficiency single-frequency (SF) ytterbium-doped fiber laser, based on a Sagnac loop mirror filter (LMF), was demonstrated. It combined a simple linear cavity with a Sagnac LMF that acted as a narrow-bandwidth filter to select the longitudinal modes. And we introduced a polarization controller to restrain the spatial hole burning effect in the linear cavity. The system could operate at a stable SF oscillating at 1064 nm with the obtained maximum output power of 32 mW. The slope efficiency was found to be primarily dependent on the reflectivity of the fiber Bragg grating. The slope efficiency of multi-longitudinal modes was higher than 45%, and the highest slope efficiency of the single longitudinal mode we achieved was 33.8%. The power stability and spectrum stability were <2% and <0.1%, respectively, and the signal-to-noise ratio measured was around 60 dB.

Monolithic Saturable Absorber with Gallium Arsenide Nanowires Integrated on the Flexible Substrate for Optical Pulse Generation.

In this work, we demonstrated a kind of flexibly monolithic saturable absorber (SA) with GaAs nanowires (NWs) on polyimide (PI) plastic substrate for broadband optical modulation at 1.0 and 1.5 µm, separately. The monolithic SA sample was prepared by the metalorganic vapor phase epitaxy (MOVPE) method. The crystal structure and element analysis were examined carefully by high-resolution scanning transmission electron microscopy (HRSTEM) and energy-dispersive X-ray spectroscopy (EDX). We observed a high-density distribution of NWs on the flexible substrate by scanning electron microscopy (SEM). In addition, linear and nonlinear optical properties of the sample were examined by testing the photoluminescence and absorption properties, which showed its potential application as an optical switch due to the pure semiconducting properties. After the characterizations, we experimentally demonstrated this monolithic SA for laser modulation at 1.0 and 1.5 µm, which yielded the minimum optical pulse widths of 1.531 and 6.232 µs, respectively. Our work demonstrated such a kind of monolithic flexible NW substrate-integrated device used for broadband optical modulation, which not only eased the integration process of NWs onto the fiber endface, but also proved the potential of easily integrating with more semiconducting nanomaterials (e.g., graphene, MoS2, …) to realize monolithic active flexible photonic systems, such as a microscale phase modulator, delay-line, and so on, paving an easy avenue for the development of both active and flexible photonic devices.