Dissipative Kerr solitons in optical microresonators with Raman effect and third-order dispersion

Using the mean-field normalized Lugiato-Lefever equation,we theoretically investigate the dynamics of cavity soliton and comb generation in the presence of Raman effect and the third-order dispersion.Both of them can induce the temporal drift and frequency shift.Based on the moment analysis method,we analytically obtain the temporal and frequency shift,and the results agree with the direct numerical simulation.Finally,the compensation and enhancement of the soliton spectral between the Raman-induced self-frequency shift and soliton recoil are predicted.Our results pave the way for further understanding the soliton dynamics and spectral characteristics,and providing an effective route to manipulate frequency comb.

Sensitivity Improvements for Picosecond Ultrasonic Thickness Measurements in Gold and Tungsten Nanoscale Films

Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practicability of this technique,is often limited by the weak intensity of the ultrasonic signal.To solve this problem,we investigate the distinct mechanisms involved in picosecond ultrasonic thickness measurement for two types of metals,namely tungsten(W)and gold(Au).For thickness measurement in W films,theory and simulation show that optimizing the pump and probe laser wavelengths,which determine the intensity and shape of the ultrasonic signal,is critical to improving measurement sensitivity,while for Au film measurements,where acoustic-induced beam distortion is dominant,the signal intensity can be optimized by selecting an appropriate aperture size and sample position.The above approaches are validated in experiments.A dual-wavelength pump-probe system is constructed based on a passively mode-locked ytterbium-doped fiber laser.The smoothing method and multipeak Gaussian fitting are employed for the extraction of ultrasonic time-of-flight.Subnanometer measurement precision is achieved in a series of W and Au films with thicknesses of 43-750 nm.This work can be applied to various high-precision,noncontact measurements of metal film thickness in the semiconductor industry.

Spatiotemporal optical vortices generation in the green and ultraviolet via frequency upconversion[Invited]

As a newly discovered type of structured light,a spatiotemporal optical vortex(STOV),which is remarkable for its space–time spiral phase and transverse orbital angular momentum(OAM),has garnered substantial interest.Most previous studies have focused on the generation,characterization,and propagation of STOVs,but their nonlinear frequency conversion remains largely unexplored.Here,we experimentally demonstrate the generation of green and ultraviolet(UV)STOVs by frequency upconversion of a STOV carried near-infrared(NIR)pulse emitted by a high repetition rate Yb-doped fiber laser amplifier system.First,we verify that the topological charge of spatiotemporal OAM(ST-OAM)is doubled along with the optical frequency in the second-harmonic generation(SHG)process,which is visualized by the diffraction patterns of the STOVs in the fundamental and second-harmonic field.Second,the space–time characteristic of NIR STOV is successfully mapped to UV STOV by sum-frequency mixing STOV at 1037 nm and Gaussian beams in the green band.Furthermore,we observe the topological charges of the ST-OAM could be degraded owing to strong space–time coupling and complex spatiotemporal astigmatism of such beams.Our results not only deepen our understanding of nonlinear manipulation of STOAM spectra and the generation of STOVs at a new shorter wavelength,but also may promote new applications in both classical and quantum optics.

Two-channel, dual-beam-mode, wavelength-tunable femtosecond optical parametric oscillator

Optical vortices,which carry orbital angular momentum,offer special capabilities in a host of applications.A single-laser source with dual-beam-mode output may open up new research fields of nonlinear optics and quantum optics.We demonstrate a dual-channel scheme to generate femtosecond,dualwavelength,and dual-beam-mode tunable signals in the near infrared wavelength range.Dual-wavelength operation is derived by stimulating two adjacent periods of a periodically poled lithium niobate crystal.Pumped by an Yb-doped fiber laser with a Gaussian(lp?0)beam,two tunable signal emissions with different beam modes are observed simultaneously.Although one of the emissions can be tuned from1520 to 1613 nm with the Gaussian(ls?0)beam,the other is capable of producing a vortex spatial profile with different vortex orders(ls?0 to 2)tunable from 1490 to 1549 nm.The proposed system provides unprecedented freedom and will be an exciting platform for super-resolution imaging,nonlinear optics,multidimensional quantum entanglement,etc.

Widely tunable,high-power,femtosecond noncollinear optical parametric oscillator in the visible spectral range

Ultrafast visible radiation is of great importance for many applications ranging from spectroscopy to metrology.Because some regions in the visible range are not covered by laser gain media,optical parametric oscillators offer an added value.Besides a high-power broadband laser source,the ability to rapidly tune the frequency of pulses with high-power spectral density offers an extra benefit for experiments such as multicolor spectroscopy or imaging.Here,we demonstrate a broadband,high-power,rapidly tunable femtosecond noncollinear optical parametric oscillator with a signal tuning range of 440–720 nm in the visible range.The oscillator is pumped by the third harmonic of an Yb-fiber laser at 345 nm with a repetition rate of 50.2 MHz.Moreover,the signal wavelength is tuned by changing the cavity length only,and output powers up to 452 m W and pulse durations down to 268 fs are achieved.This is,to the best of our knowledge,the first demonstration of a quickly tunable femtosecond optical parametric oscillator that covers nearly the entire visible spectral range with high output power.

Generation of sub-three-cycle pulses at 53 MHz repetition rate via nonlinear compression in optical parametric oscillator

We report an experimental generation of few-cycle pulses at 53 MHz repetition rate. Femtosecond pulses with pulse duration of 181 fs are firstly generated from an optical parametric oscillator(OPO). Then, the pulses are compressed to subthree-cycle with a hybrid compressor composed of a commercial single-mode fiber and a pair of prisms, taking advantage of the tunability of the OPO and the numerical simulating of the nonlinear compression system. Our compressed optical pulses possess an ultrabroadband spectrum covering over 470 nm bandwidth(at-10 dB), and the output intensity fluctuation of our system is less than 0.8%. These results show that our system can effectively generate few-cycle pulses at a repetition rate of tens of megahertz with excellent long-term stability, which could benefit future possible applications.