Published papers

A repetition-rate-tunable dual-comb laser is critical for alias-free ranging, yet delay lines consistently introduce errors by travel uncertainty. Here, we demonstrate bidirectional polarization-multiplexed Lyot filtering with only 0.11% error compared with the previous PZT solution reported.

TA15 alloy has received widespread attention due to its excellent properties. The native oxide film limits its application. Laser cleaning can remove the native oxide film. However, research on the surface modification of materials by laser cleaning is rarely discussed. In this paper, the effects of different energy densities (E) on the improvement of surface morphology were studied. The wear morphology and microstructure were observed in situ. The difference between the wear mechanism between the remelted layer and the base material at different wear depths was analyzed. The results show the native morphological defects are transformed into a homogeneous crater after laser cleaning. With increasing energy, the crater morphology changes to a striation morphology. Tip growth of the supercooled melt and the cumulative of multiple processes results in morphology evolution. Compared to the base material, the coefficient of friction of the remelted layer decreased to 40%. Observing the microstructure in situ, the average grain area of the remelted layer decreased by about 40%. The large-angle grain boundary ratio increased. As E increases, the thickness of the remelted layer increases. Abrasion to 10 μm below the surface, the untreated sample showed furrow, scaly flaking, and pitting spalling. The laser-cleaned sample surface showed abrasions and plastic smearing. Abrasion to 50 μm below the surface, the surface of the remelted layer was fine abrasive particles. The surface of the untreated sample was larger abrasive particles and fatigue delamination.

The natural oxide film of TA15 titanium alloy hinders its serviceability for subsequent process applications. Nanosecond pulsed laser removal of oxide layers is feasible, while the nanosecond scale and high pulse number cause challenges for mechanistic studies. Therefore, we research the laser removal mechanism of the oxide layer in TA15 titanium alloy by analyzing in-situ high-speed camera and product characteristics in this paper. As the energy density (E) increases, the fluctuation degree of the molten pool intensifies. Below 17.5 J/cm2, only the molten pool and plasma variations were observed. With 17.5 J/cm2 and above, ejected block and splash occurred. As E increases, it causes the surrounding blocks to move or even “expand” before breaking. It is demonstrated the existence of the boundary between the pure thermal mechanism and the thermal-inpact mechanism between 8.75 J/cm2 to 17.5 J/cm2. The vaporization pressure was obtained by the finite difference method and the transition threshold was fitted as 14.08 J/cm2. The block ejection velocity was coincided by calculating the buckling velocity. The collection of process products also verified the in-situ camera results for the transition mechanism. Below 17.5 J/cm2, only TiO2 particles with hundred nanometer to micron sizes were collected. With 17.5 J/cm2 and above, the number of particles increased and the blocks are collected. In addition, flocculent agglomerates wrapped around particles and block agglomerates with attached particles appeared. The morphology and composition of the products were discussed with the variation of energy input.

TA15 alloy is susceptible to form bluish-purple thermo-oxidized layer during thermal processing. It is accompanied by surface morphological defects. Laser ablation can remove the thermo-oxidized layer and modify the morphology. However, the pulse width is only a hundred nanoseconds and the spot is in the micron range. It makes the laser-matter interaction investigations extremely challenging. In this paper, a multiphase flow model is established to simulate the removal mechanism of thermo-oxidized layer and surface morphological evolution by nanosecond pulsed laser ablation with different ablation energy densities (E). The critical removal conditions of the thermo-oxidized layer were explored. The critical removal window of the thermo-oxidized layer was determined between 0.1 J/cm2 and 0.35 J/cm2. In conjunction with thermodynamic calculations, the reasons for the minimum surface oxygen content between 3.5 J/cm2 and 8.75 J/cm2 were discussed. A multiphase flow model was used to analyze the formation of crater under time-dependent. Initially, the molten pool is mainly influenced by the recoil pressure and expands outward. Subsequently, the recoil pressure weakens. Instead, convection caused by surface tension and thermal buoyancy forms double vortex. Finally, depending on the low-temperature matrix, the molten pool solidifies rapidly and forms crater.

The active titanium alloy is prone to form an oxide layer with poor thermal strength and fluffy during hot working. Native morphological defects also affect subsequent processing. In this study, the laser ablation was used to remove the thermally processing oxide layer and modified the surface morphology. The effect of the newly formed oxide layer on the surface thermal diffusivity was studied. As the average power increased, the native morphological defects were modified to the crater morphology. Finally, the wavy fringe morphology was formed. The thermal oxide layer of the untreated sample was composed of TiO2 (anatase), TiO2 (rutile), and Al2O3. After laser ablation, the content of TiO2 (rutile) increased. Cross-sectional morphology observation shows, above 12 W, the oxide layer appeared agitating morphology. It is proved that 12 W is the boundary between the thermal effect and the thermo-mechanical composite effect. The effectiveness of laser ablation in modulating the thermo-physical properties of the surface was verified by thermal diffusivity measurements.

The processes, testing and application technology research were carried out for the welding synchronous laser cleaning of aluminum alloy for rail transit vehicle carbodies. The process window of laser cleaning for oxide film of aluminum alloy was obtained through the study of process. The prediction model of laser cleaning parameters before matching different welding speeds was established. And the effect of laser cleaning on the superficial oxide film of aluminum alloy was analyzed. According to the actual working conditions, an integrated laser cleaning and arc welding synchronous device was designed. And the pre-welding synchronous laser cleaning process verification was carried out. High-quality and high-efficiency removal of oxide film before welding was achieved. By analyzing the mechanical properties of welded joints, detecting weld defects and observing the microstructure of the cross-section of the welded joint, the quality of welding synchronous laser cleaning welds was evaluated. The research shows that with 200 W pulsed laser, the cleaning speed is adjustable from 0.5 to 1.1 m/min. Welding synchronous laser cleaning of oxide film with a line width of 45 mm can be effectively achieved. After laser cleaning, the primary oxide film is completely removed and the effect of regenerative oxide film can be avoided. The shear strength and strain of the welded joint after laser cleaning before welding increased by 26.4% and 9.98% compared with the untreated joint. The shear strength and strain of welded joints after laser cleaning increased by 3.53% and 1.43% respectively compared to those after manual grinding. And the central microstructure of the weld was mainly composed of α matrix and β (Mg2Al3) phase. The performance of the welding met the requirements of rail transit vehicle carbody manufacturing. Laser cleaning with welding can effectively replace manual grinding technology.

We demonstrate a 783 MHz fundamental repetition rate mode-locked Er-doped all-fiber ring laser with a pulse width of 623 fs. By using carbon nanotubes saturable absorber, a relatively low self-starting pump threshold of 108 mW is achieved. The laser has a very compact footprint less than 10 cm × 10 cm, benefiting from the all-active-fiber cavity design. The robust mode-locking is confirmed by the low relative intensity noise and a long-term stability test. We propose a new scheme for generating high repetition rate femtosecond optical pulses from a compact and stable all-active-fiber ring oscillator.

We realize 783-MHz fundamental repetition rate in a carbon-nanotubes (CNT) modelocked Er-doped fiber ring laser with all active fibers. The proposed laser self-starts mode-locking at 108-mW pump power and delivers 623-fs pulses directly.

An all-polarization-maintaining tunable carbon nanotube mode-locked Er:fiber laser is demonstrated based on a compact reflective Lyot filter, with wavelength tuning slope of 0.62 nm/° C, 47.7 times higher than that of fiber grating based tunable lasers.

Wavelength tunable mode-locked fiber lasers have highly potential applications in precision spectroscopy, nonlinear microscopy and photonic sensing. Here, we present a compact and thermal-sensitive reflective Lyot filter and utilize it for all-polarization-maintaining efficiently wavelength-tunable Er-doped carbon-nanotube-mode-locked laser for the first time, to the best of our knowledge. The output wavelength of the laser can be tuned from 1544.46 nm to 1572.71 nm, with a wide tuning range of 28.25 nm, and a remarkable tuning efficiency of 0.589 nm/°C, when the angle-spliced fiber is only 8.2 cm and the free spectral range of the filter is 31.32 nm. Dual-wavelength mode-locking is also achieved at boundary temperatures when increasing the pump power. Due to its compact size and reflection configuration, the proposed reflective Lyot filter is promising for realizing highly efficient wavelength tuning and multiwavelength generation in all-polarization-maintaining fiber lasers where reflective filters are needed.

An effective method to control dichromatic pulse encodings is reported. The spectral ripples are enhanced via a twistable tapered-fiber Lyot filter, thereby degrading the actual saturable absorption and thus causing multi-pulse participations.

An effective method to control dichromatic pulse encodings is reported. The spectral ripples are enhanced via a twistable tapered-fiber Lyot filter, thereby degrading the actual saturable absorption and thus causing multi-pulse participations.

Ultrafast laser is an ideal playground for the study of precise spectroscopy and microscopy that heavily rely on the high-performance optical frequency combs, which is the unique nature of mode-locked fiber lasers. In particular, dual-comb fiber lasers can greatly improve system performance while reducing complexity by developing self-referencing combs and coupling pulse trains with different repetition rates. Whereas the repetition rate is generally determined for a certain lasing resonator, which creates additional constraints in dual-comb applications. Here, we developed a bidirectional polarization-maintaining fiber laser, where the repetition rate of mode-locked pulse trains can be tuned continuously inside the cavity. The proposed laser outputs mutually orthogonal polarization states in opposite directions, and the maximum tuning range of the repetition rate reaches 681.6 kHz. Compared with referencing CW loop, the RF difference reaches 978.2 kHz, almost equal to 1 MHz. Meanwhile, the two directions generate pulses with nearly the same central wavelength, enabling excellent capacity in self-referencing dual-comb generation. This tunable-repetition-rate dual-comb source is promising for two-photon fluorescence microscopy, precise metrology of wavelength and synchronization of other combs based on fiber lasers with different repetition rate, etc.

Comb-mode-resolvable fiber lasers output a set of evenly spaced and detectable frequencies, enabling precision applications in frequency metrology and high-resolution spectroscopy. Here, we report a narrow-bandwidth down to GHz level, polarization-maintaining mode-locked fiber laser with comb-mode resolvability. Under narrow-bandwidth filtering, the laser owns a minimum spectral width of 12.1 pm (1.49 GHz) at 1560 nm, with a pulse width of 437 ps and a comb spacing of 21 MHz. The comb spectrum is successfully resolved by using low-bandwidth electronics, owing to the high coherence of the combs. The output bandwidth (12.1 pm–3 nm) and center wavelength (1530 nm–1565 nm) can be easily tuned in a large range by setting the intracavity programmable pulse shaper. Therefore, the laser can operate at a broadband range despite its narrow bandwidth. The proposed widely tunable narrow-bandwidth laser has the potential for precise frequency comb metrology and spectroscopy with only a simple detection end.

We demonstrate a comb-based photonic processor that performs temporal convolution in the optical domain and directly converts the analog computation result into a 300 GHz wireless signal. A dissipative Kerr soliton microcomb with a 300-GHz free spectral range provides parallel wavelength channels, in which convolution kernels are encoded as comb-line weights and processed via dispersion-induced delays. The convolution output is optically heterodyned in a uni-traveling-carrier photodiode, generating a 300 GHz carrier whose amplitude envelope represents the convolution result. We experimentally demonstrate real-time optical convolution at 1.2 Gbaud and a successful free-space THz link of the processed signal in the terahertz band. Offloading the convolution to the optical domain reduces the estimated computational cost by 23.1% relative to a purely electronic implementation. A latency estimate further suggests that direct optical-to-terahertz transmission can alleviate the I/O overhead by avoiding local digitization and bus-based data transfer.

A compact Lyot-filter/SA hybrid that offers extra tunable degrees of freedom is proposed to support continuous wavelength tuning and dual-wavelength mode-locking with low free-running wavelength shift in an all-polarization-maintaining fiber laser.

A compact Lyot-filter/SA hybrid that offers extra tunable degrees of freedom is proposed to support continuous wavelength tuning and dual-wavelength mode-locking with low free-running wavelength shift in an all-polarization-maintaining fiber laser.