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Mode-locked lasers with low noise performance, such as low timing jitter and relative intensity noise are essential for high-precision applications. In this letter, we demonstrated a 122-MHz all-polarization-maintaining mode-locked Er-doped fiber laser based on a nonlinear amplifying loop mirror. A hybrid component integrating a wavelength division multiplexer and a phase shifter is applied to increase the repetition rate. The relative intensity noise is 0.004% and 0.76% rms (integrated from 1 MHz to 100 Hz) from two output ports. The timing jitter characteristic is measured by the delayed optical heterodyne method. The timing jitter is ~1.9 fs rms (integrated from 1 MHz to 10 Hz) of both two output ports. The origin of the timing jitter of this laser is analyzed numerically based on the well-established theoretical model using the laser’s repetition rate, cavity dispersion, etc.

When a laser cavity supports the propagation of several ultrashort pulses, these pulses interact and can form compact bound states called soliton molecules. Soliton molecules are fascinating objects of nonlinear science, which present striking analogies with their matter molecules counterparts. The soliton pair, composed of two identical pulses, constitutes the chief soliton molecule of fundamental interest. The relative timing and phase between the two propagating pulses are the most salient internal degrees of freedom of the soliton molecule. These two internal degrees of freedom allow self-oscillating soliton molecules, which have indeed been repeatedly observed, whereas the lowdimensional chaotic dynamics of a soliton-pair molecule remains elusive, noting that it would require at least three degrees of freedom. We here report the observation of chaotic soliton-pair molecules within an ultrafast fiber laser, by means of a direct measurement of the relative optical pulse separation with sub-femtosecond precision in real time. Moreover, we demonstrate an all-optical control of the chaotic dynamics followed by the soliton molecule, by injecting a modulated optical signal that resynchronizes the internal periodic vibration of soliton molecule., 11 pages, 5 figures

We demonstrate that our theoretical scheme developed in the previous study on the caustics of the abruptly autofocusing vortex beams [Xiao et al., Opt. Express 29, 19975 (2021)10.1364/OE.430497] is universal for all the axially symmetric vortex beams. Further analyses based on this method show the complex compositions of the vortex caustics in real space. Fine features of the global caustics are well reproduced, including their deviations from the trajectories of the host beams. Besides, we also show the possibility of tailoring the vortex caustics in paraxial optics based on our theory. The excellent agreements of our theoretical results with both numerical and experimental results confirm the validity of this scheme.

Optical soliton molecules in ultrafast lasers present striking analogies with their matter molecule counterparts, such as internal vibrations. However, the vibrations of soliton molecules are nonlinear, with frequencies that are sensitive to the system parameters, thus presenting an opportunity of control. Here, we experimentally demonstrate the synchronization of the internal vibrations of self-excited vibrating soliton molecules through a modulated optical injection. We show efficient sub-harmonic, fundamental and super-harmonic synchronization, forming a pattern of Arnold tongues with respect to the injection strength. Our observations are supported by numerical simulations., This work is supported by the National Natural Science Foundation of China (Grant 61975144, 61827821); Ph.G. acknowledges support from the EIPHI Graduate School (ANR-17-EURE-0002) and from PIA3 ISITE-BFC (ANR-15-IDEX-0003); O. Gat acknowledges support from the Israel Science Foundation (ISF) (Grant No. 2403/20)

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN’s impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

Based on femtosecond laser ablated processing, one-dimensional arrays with different line spacings were fabricated on titanium disulfide (TiS2) nanosheet. Optical modulated measurements with these devices were demonstrated. Photo-induced complex conductivity changes of the TiS2 nanosheet devices were studied by an optical-pump THz-probe (OPTP) measurement system. For the 700 μm line spacing device, the optically introduced modulation depth (MD) reached up to 70%. Our experimental results have demonstrated the ultrafast dynamics and photoconductivity for the TiS2 nanosheet devices in the THz frequency range. TiS2 nanosheet based devices would be suitable as the photoelectric modulators of THz wave.

We utilize singular spectrum analysis (SSA) for post-processing distance measurement data obtained by a dual-comb femtosecond laser rangefinder. Signal denoising technique based on SSA allows detection of low amplitude vibrations from time series of distance data with limited signal-to-noise ratio (SNR). Various one-dimensional vibrations, including harmonic, amplitude modulated harmonic, and sinc-function vibrations, with micrometer amplitude comparable with measurement uncertainty of laser rangefinder, have been imposed on the target mirror by a piezo positioner. These vibrations have been extracted by SSA and the obtained vibration amplitudes agree with that imposed on the target very well. A commercial interferometer is used to evaluate the accuracy of SSA. Standard deviations of residuals between the SSA extractions and interferometer measurements are smaller than 0.6 $\mu$ m. This technique is promising for complex moving pattern detection in object tracking and remote sensing.

As an imperative method of investigating the internal mechanism of femtosecond lasers, traditional femtosecond laser modeling relies on the split-step Fourier method (SSFM) to iteratively resolve the nonlinear Schrodinger equation suffering from the large computation complexity. To realize inverse design and optimization of femtosecond lasers, numerous simulations of mode-locked fiber lasers with different cavity settings are required further highlighting the time-consuming problem induced by the large computation complexity. Here, a recurrent neural network is proposed to realize fast and accurate femtosecond mode-locked fiber laser modeling for the first time. The generalization over different cavity settings is achieved via our proposed prior information feeding method. With the acceleration of GPU, the mean time of the artificial intelligence (AI) model inferring 500 roundtrips is less than 0.1 s. Even on an identical CPU-based hardware platform, the AI model is still 6 times faster than the SSFM method. The proposed AI-enabled method is promising to become a standard approach to femtosecond laser modeling.

Although higher order Poincare (HOP) beams are attractive due to their unique advantages and various efforts have been made for the generation of HOP sphere beams, the current generation technologies suffer from both limited output power and narrow bandwidth, preventing further practical applications. In this paper, we report on a femtosecond optical parametric oscillator (OPO) which can deliver high average power, on-demand arbitrary HOP sphere beams tunable across a wide spectral range in the near-infrared. The desired beam mode can be achieved by manipulating the geometric phase inside the OPO cavity to map the polarization to orbital angular momentum (OAM). Pumped by a Gaussian beam from an Yb-fiber laser, the generalized vector vortex femtosecond signal beams with spectral coverage from 1367 to 1650 nm are obtained. The device can produce over 200 mW structured beam signal power cross the complete tuning range with as much as 377 mW at 1480 nm for 2 W of input pump power. The proposed configuration, which shows superiorities of high average output power, flexible wavelength tunability and complete mode controlling property, may have direct implications in applications such as quantum optics and material processing.

We investigate on bound-state mode-locking in the close-to-zero dispersion regime of an Er-fiber laser. A two-soliton bound state with 82 fs pulse duration and 0.58 ps pulse separation is achieved. Compound 2+1 type bound state solitons with ~100 fs pulse duration has also been observed. Both the double-pulse and the 2+1 type bound states could experience period-doubling bifurcations and exhibit the pumping hysteresis characteristics as the single pulse. To the best of our knowledge, this work provides the first observation of sub-100 fs bound solitons in a fiber laser. Such ultra-short bound state solitons show potential applications of multilevel coding unit for high-capacity telecommunication systems.

We demonstrate a dual-wavelength, compact femtosecond optical parametric oscillator (OPO) with high-repetition-rate, which is harmonically pumped by a ~101 MHz Yb-doped fiber laser system. The dual-wavelength output is realized by exploiting the bidirectional off-axis pumping technique, and the multiplication of repetition rate is derived by placing a nonlinear crystal in a miniaturized V-type cavity with only 16.5 cm length. By adjusting the detuning degree of the pump at one end, the maximum dual-wavelength gap is 66 nm from 1497 to 1563 nm. The time interval between the dual wavelengths can be adjusted freely. This is the first report on a dual-wavelength with the high-repetition-rate of GHz-level, femtosecond OPO system.

Collision refers to a striking nonlinear interaction in dissipative systems, revealing the particle-like properties of solitons. In dual-wavelength mode-locked fiber lasers, collisions are inherent and periodic. However, how collisions influence the dynamical transitions in the dual-wavelength mode-locked state has still not been explored. In our research, dispersion management triggers the complex interactions between solitons in the cavity. We reveal the smooth or reversible Hopf-type bifurcation transitions of dual-color soliton molecules (SMs) during collision by real-time spectral measurement technique of TS-DFT. The reversible transitions from stationary SM to vibrating SM, revealing that cavity parameters pass through a bifurcation point in the collision process without active external intervention. The numerical results confirm the universality of collision-induced bifurcation behavior. These findings provide new insights into collision dynamics in dual-wavelength ultrafast fiber lasers. Furthermore, the study of intermolecular collisions is of great significance for other branches of nonlinear science., Comment: 11 pages, 4 figures

We develop a single cavity, single crystal, bidirectional-pumped femtosecond optical parametric oscillator. Unlike the conventional single-pumped OPO, the pump pulses are injected into the OPO cavity from the front and reverse directions, respectively, which can double the tolerable maximum pump power onto the nonlinear crystal. By means of synchronizing the two signals generated from the bidirectional pump pulses, distinct improvements for optical-to-optical conversion efficiency, maximum signal power, and operational stability in the bidirectional pumped OPO have been demonstrated.

We report a single method for measuring the effective electro-optics (EO) coefficient only by spectral intensity modulation with a terahertz time-domain spectroscopy (THz-TDS) device. Compared with conventional methods, the measuring device based on THz-TDS by probing the spectral intensity modulation is simple, rapid, and highly sensitive. It does not require special preparation process, electrode contact or applying high voltage on the sensor samples. The EO coefficients of four kinds of materials, namely, ZnTe and GaP crystals, DAST wafer and GaSe film, are measured at 1040 nm. The results showed good agreement with literature data by the conventional methods.

Structured light beams that possess unique polarization distribution could offer a new degree of freedom for a variety of applications, and hence its flexible polarization manipulation is necessary. Here we experimentally report a heterodyne interference-based method for generating femtosecond cylindrical vector beam (CVB) with high-speed controllable rotated polarization states. The femtosecond CVBs are created through the superposition of two optical vortices with opposite handedness. The use of two acoustic-optical modulators (AOMs) with frequency differences allows to achieve polarization rotation in a hopping-free scheme at on demand speed. Up to 1 MHz of the rotation frequency is demonstrated by visualizing the fast rotation events through a fast-frame-rate CCD camera. Moreover, we show our method can be readily extended to produce higher order CVBs with more complex rotated polarization distributions. Such a simple yet versatile femtosecond polarization-controlled laser system has the capability to act as a nonlinear trapping platform, thus opening tremendous potential opportunities in the fields of micromachining, nanofabrication, and so force.

In this letter, we propose and demonstrate a practically compact 1-GHz repetition-rate femtosecond optical parametric oscillator (OPO) based on MgO:PPLN crystal, which is harmonically pumped by a 53 MHz Yb-doped fiber laser. The multiplication of the repetition rate is derived by placing the nonlinear crystal in a miniaturized V-type cavity with 13.9 cm length. The near-infrared signal output pulses are tunable from 1350 nm to 1650 nm. The signal pulse duration across the complete tuning range is measured to be around 125 fs. Thanks to the high repetition rate, broadband tenability, and simplicity of the optical setup, this type of OPO holds promise for many applications.

We demonstrate the generation of few cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical, and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.

A series of Sn gratings in terahertz band is additive fabricated by femtosecond laser with a repetition rate of 50 MHz and central wavelength of 1040 nm. Different manufacturing parameters were applied to explore the relationship between linewidth and processing condition during the sintering of Sn power with 50 nm granularity. Furthermore, polarization modulation effect caused by Sn gratings are measured and evaluated by transmission experiment in a THZ-TDS system.

Although the Raman fiber amplifiers are attractive for their unique advantages and various Raman fiber amplifiers in continuous wave (CW) regime were built, there are a few fundamental limitations for short-pulsed Raman fiber amplifiers. In this paper, we present a theoretical and experimental study on a novel method termed picosecond (ps) coherent Raman fiber amplification, where the stimulated Raman scattering is employed to generate high-quality femtosecond pulses from narrowband ps pump pulses, with high conversion efficiency and compact construction. Detailed numerical simulation reveals the nonlinear dynamics of the ps-coherent Raman fiber amplification in an Yb-doped gain fiber. The spectral filtering effect induced by finite Raman gain bandwidth significantly improves the temporal quality of the generated Raman pulses, similar to the well-known Mamyshev regenerator. By employing this method in a 4.5 m Yb-doped polarization maintaining double-clad fiber, 67 nJ, 76 fs nearly transmission limited Raman pulses are generated from narrowband (

We propose an effective scheme to interpret the abruptly autofocusing vortex beam. In our scheme, a set of analytical formulae are deduced to well predict not only the global caustic, before and after the focal plane, but also the focusing properties of the abruptly autofocusing vortex beam, including the axial position as well as the diameter of focal ring. Our analytical results are in excellent agreement with both numerical simulation and experimental results. Besides, we apply our analytical technique to the fine manipulation of the focusing properties with a scaling factor. This set of methods would be beneficial to a broad range of applications such as particle trapping and micromachinings.

We numerically and experimentally investigate the multi-pulsing mechanism in a dispersion-managed mode-locked Yb-doped fiber laser. Multi-pulsing occurs primarily owing to the inherent filtering effect of the chirped fiber Bragg grating. The spectral filtering effect restricts the spectral broadening induced by self-phase modulation and causes extra loss, leading to a decreased pump power threshold for the multi-pulsing state. Numerical simulations show that multi-pulsing emerges at a lower pump power when the spectral filter bandwidth becomes narrower. In the experiment, the spectral width increases as the net cavity dispersion approaches zero. Pulses with wider spectral widths experience more loss from the spectral filtering effect, leading to a decreased pump power threshold for multi-pulsing. Therefore, the net cavity dispersion also has an impact on the multi-pulsing threshold. Based on this conclusion, we devise a strategy to obtain single-pulsing operation with the shortest pulse width and the highest pulse energy.

We demonstrate a direct time-of-flight approach that utilizes dual-comb electronically controlled optical sampling (ECOPS) to measure small displacements. ECOPS is enabled by electrically controlling the repetition rate of one laser via an intracavity electric-optical modulator (EOM). The acquisition rate is set by the EOM modulation frequency, which is much higher than commonly used asynchronous optical sampling (ASOPS). In a proof-of-principle experiment, an 80-kHz acquisition rate is obtained with a pair of ∼105 MHz repetition rate Er-fiber lasers. At an average time of 30 ms, a measurement precision evaluated with Allan deviation reaches 26.1 nm for a 40-µm static displacement. In a dynamic measurement, a 500-Hz sinusoidal vibration with 15 µm amplitude has also been identified. The high-precision and high-speed displacement measurement technique can be potentially used in 3D surface profilometry of microelectronic step-structures and real-time monitoring of high frequency mechanical vibrations, etc.

A simple and practical apparatus enabling repetition rate (frep) noise, carrier-envelope frequency (fceo) noise and nth optical comb mode (νn) noise spectra measurements with high precision is established. The frep and νn noise spectra are measured by a fiber delay line interferometer, while fceo noise spectrum is measured by an f-2f interferometer. We utilize this apparatus to characterize the noise performance of an Er-fiber optical frequency comb (OFC) and analyze the origin of dominant noise sources. Moreover, this apparatus provides a powerful tool for diagnosing noise dynamics intrinsic in mode-locked lasers and OFCs. To this end, we uncover the anti-correlation between frep and fceo noise as well as the impact of servo loops on noise characteristics in the stabilized OFC.

Investigation on the relative phase evolution between two bounded optical solitons is essential for its potential applications in development of larger telecommunication capacity of optical fiber transmission lines, resolution improvement in advancing ultrafast characterization approaches and development of all-optical information storage. Here we characterized relative comb-line phase jitter power spectral density (PSD) of a soliton molecule pair generated from a passively mode-locked Er:fiber laser. Through tracking the intensity difference between two selected wavelengths in one certain spectral interference fringe, the relative comb-line phase noise PSD is measured by balanced detection. The estimated measurement resolution is at 10^(-14) rad2/Hz level and the estimated integrated phase noise from 10 MHz to 100 Hz is 2.04 mrad. The estimated relative linewidth is far below 1 mHz. Comparison between phase noise PSD and intensity noise PSD indicates that AM-PM conversion plays an important role in relative phase jitter dynamics between the two solitons. Our spectral interference fringe tracking technique is attractive for its simplicity and shows potential in ultra-high resolution in phase noise measurement, providing an ultra-sensitive alternative approach for relative phase noise stabilization of optical soliton molecules, between two independent mode-locked lasers and optical length stabilization of interferometers., Comment: 9 pages, 5 figures, to be published in Optics Communications, CF-260

We study on the timing jitter characteristics of a passively mode-locked dual-wavelength dual-comb Er-fiber laser that generates two femtosecond optical frequency combs with an offset repetition rate in a shared cavity. The relative timing jitter is characterized directly in the time domain with tens of attoseconds precision by utilizing the intrinsic asynchronous optical sampling process between the two laser beams. This sensitive measurement reveals an amplified spontaneous emission (ASE) noise dominated random walk of the relative pulse timing, which has not been observed before in a dual-comb laser. The quantum-limited relative timing jitter behavior shows that the common-mode noise suppression based on a shared cavity does not apply to quantum noise. We further conduct a time-of-flight ranging experiment and a Fourier-transform spectroscopy simulation based on this dual-comb laser, showing that the quantum-limited relative timing jitter sets an ultimate limit on the reachable performance in time- and frequency-domain dual-comb applications particularly when the high acquisition rate is desired.

We studied how coating materials influence the thermal stress in the fiber core for three holding methods by simulating the temperature distribution and the thermal stress distribution in the photonic-crystal fiber laser. The results show that coating materials strongly influence both the thermal stress in the fiber core and the stress differences caused by holding methods. On the basis of the results, a two-coating PCF was designed. This design reduces the stress differences caused by variant holding conditions to zero, then the stability of laser operations can be improved.

We demonstrate the effective removal of Gordon–Haus (GH) jitter in mode-locked fiber lasers based on the narrow bandpass filtering, verified from large positive dispersion to large negative dispersion. Simulations and experiments indicate that there exists an optimum filtering bandwidth for the near-zero GH jitter along with the constant directly coupled jitter. The corresponding measured root-mean-square timing jitter (10 kHz to 10 MHz) is restrained within 0.6 dB, even though the net dispersion is largely varied by 70% of the intracavity fiber dispersion. The demonstrated effective GH jitter elimination free of the dispersion management gives an easy way for the low-jitter laser design and the related high-precision applications outside the laboratory.

Broadband nonlinear optical frequency conversion has attracted considerable interest in many applications, such as optical communication, frequency comb generation, and so on. In this letter, we report on the ultra-broadband second-harmonic generation (SHG) from the zinc oxide nano-tetrapod with less stringent phase-matching requirement. Using a super-continuum source together with an optical parametric oscillator as the excitation light, a broadband SHG with over-one-octave bandwidth spanning from the ultraviolet to near-infrared (375–1100 nm) is readily obtained. The SHG dependence of pump polarization and pump power is also characterized systematically. This nanoscale ultra-broadband optical converter may open a new way for various applications ranging from imaging to on-chip all-optical signal processing operations.

We report a compact femtosecond optical parametric oscillator (OPO) with a tunable repetition rate that is harmonically pumped using a 52.5 MHz Yb-doped fiber laser system. By reducing the OPO cavity length, the repetition rate can be adjusted successively from 1.935 to 2.95 GHz at 52.5 MHz intervals. The multiplication of the repetition rate is achieved by placing a nonlinear crystal in a compact linear cavity, and the minimum cavity length is 47.8 mm. The maximum signal power (205.2 mW) at 2.04 GHz is obtained at an input pump power of 2.35 W and a central wavelength of 1472 nm. The central wavelength of the signal pulses can be tuned from 1383 to 1596 nm. To the best of our knowledge, 2.95 GHz is the highest repetition rate achieved using a femtosecond OPO based on an ultrashort cavity with a length that is N times (N is an integer) shorter than that of the pump cavity.

Soliton molecules in mode-locked lasers are expected to be ideal self-organization patterns, which warrant stability and robustness against perturbations. However, recent ultra-high resolution optical cross-correlation measurements uncover an intra-molecular timing jitter, even in stationary soliton molecules. In this work, we found that the intra-molecular timing jitter has a quantum origin. Numerical simulation indicates that amplified spontaneous emission (ASE) noise induces a random quantum diffusion for soliton pulse timing, which cannot be compensated by soliton binding mechanism. By suppressing indirectly coupled timing jitter at close-to-zero cavity dispersion, a record-low 350 as rms intra-soliton-molecular jittering is obtained from an Er-fiber laser in experiment. This work provides insight into the fundamental limits for the instability of multi-soliton patterns.

The coupling dynamics between a dispersion-managed soliton oscillator and a nonlinear amplifier in an all-photonic crystal fiber-based laser system is emphasized. The intracavity and output pulse evolution dynamics with the dispersion management by a pair of gratings in the oscillator are investigated numerically and experimentally in detail. Then the relationship between the net intracavity dispersion in the oscillator and properties of amplification pulses from the nonlinear amplifier is studied. We find that the best performance in the laser amplification system is significantly dependent on the time-bandwidth product of the seed pulses and the amplifier pump power. As a consequence, the optimized seed output from the DM soliton oscillator for the nonlinear amplifier should be as close to the transform-limited pulse as possible and the intracavity anomalous dispersion should be as large as possible. The narrowest pulse duration of 43 fs without a pedestal and an average power of 28 W, is obtained at a pump power of 60 W. At a pump power of 70 W, 50-fs pulses with a highest output power of 34 W, corresponding to a peak power of 16.2 MW, can also be obtained.

Optical rectification is one of the most widely used methods to generate broadband terahertz (THz) radiation using femtosecond laser pulses. In this work, we report the generation of THz radiation from GaP crystals based on optical rectification using a pulsed Bessel beam, i.e., a beam with a spatial profile described by a zeroth-order Bessel function of the first kind. It is shown that the nondiffracting feature of the central spot of the Bessel beam allowed an enhancement of the THz radiation generation efficiency. Compared with a Gaussian beam having the same beam size and pump power as its central spot, the Bessel beam enabled an average increase of the THz radiation power by factors of 1.37, 2.12, and 2.06 from GaP crystals of 1, 3, and 5 mm, respectively. Measurements showed that the spatial profile of the THz radiation followed a Gaussian distribution for both pump beams and increased linearly after the emitters.

We demonstrate that third-order dispersion (TOD) precompensation accompanied with nonlinear compensation can improve the pulse quality of high energy pulses in fiber chirped-pulse amplification (FCPA) system. The negative TOD fiber is implemented as the precompensation of TOD mismatch, resulting small required B-integral, which benefits the pulse quality. The conclusion is given by analytical study and numerical simulations. Based on this method, we obtain high-energy pulses up to 10.4 μJ. The quality of the dechirped pulse is high and >90% of pulse energy is contained in the main pulse. The dechirped pulse duration is ~280 fs, which is nearly the same as the transform-limited pulse of the oscillator. The system also shows a wide tunable range of pulse energy from 5 to 10 μJ with the good pulse quality.

We design and investigate the optical properties of a novel hollow-core antiresonant fiber with nested elliptical cladding elements. This new structure shows low confinement loss even with a relatively small core. The lowest confinement loss of this structure is more than two orders of magnitude smaller than its circular-element counterparts. When the size of the cladding elements is properly designed, higher order core modes resonantly couple with the cladding modes effectively, resulting in an extremely high higher order-mode extinction ratio ( $>$ $10^4$ ). This good single mode performance can be achieved for a wide range of cladding element parameters, which would increase the fabrication tolerance. The new fiber also shows low bending loss for a wide range of bend radii. Moreover, dispersion properties of the new structure are also investigated under both unfilled and filled conditions. Numerical results show that antiresonant structures with the same core diameter and strut thickness have similar dispersion curves. The zero dispersion wavelength within the fundamental antiresonant transmission band can be tuned over tens of nanometers by increasing the pressure up to 50 bar when the fiber is filled with argon.

We experimentally investigate the polarization dynamics of three kinds of gain fiber, the single-polarizing photonic crystal fiber, double clad polarization-maintaining large mode area fiber and double clad large mode area nonpolarizing fiber, in a continuous wave fiber laser oscillator under different initial polarization directions and pumping powers. The comparison results demonstrate that there are some distinguishable differences for the polarization evolutions inside the three kinds of active fibers. The polarization operation status of the fiber oscillator strongly depends on not only the gain fiber’s kinds, but also the length of the fiber, initial polarization directions and pumping power. The results suggest that polarization instability in a fiber laser should be an inherent reason for the higher operation noise.

We propose a high-efficiency and compact 910-MHz femtosecond optical parametric oscillator, which is harmonically pumped by a ∼101 MHz Yb doped fiber laser system. The OPO is capable of delivering watt-level, power-enhanced signals across the telecommunication waveband. The signal power enhancement is realized by exploiting the bidirectional pumping technique. A maximum signal power of 1.04 W at 1502 nm is obtained for an input pump power of 3.8 W. Tunable near-infrared signal pulses with a wavelength range between 1350 and 1610 nm are measured, and the pulse durations vary from 193 to 464 fs. This compact and economic design provides a solution for efficient high repetition rate pulse generation over a large wavelength span, which will be beneficial for a variety of practical applications.

Fluctuation of pump power is one of the major sources of temporal and intensity noise in femtosecond fiber lasers. In this work, the transfer functions between the relative intensity noise (RIN) of the pump laser diode (LD) and the output RIN, between the RIN of the pump LD and timing jitter of femtosecond fiber lasers are systematically studied. It is demonstrated, for the first time to our knowledge, that the amplitude of the pump RIN transfer function can be effectively decreased by an intra-cavity narrow band-pass filter. In particular, for normal-dispersion lasers, the 3-dB bandwidth of the transfer function can also be narrowed by two-thirds, with a steeper falling edge. Furthermore, with the narrow band-pass filtering, the transfer function is almost independent of the net intra-cavity dispersion due to amplifier similariton formation. The proposed scheme can effectively isolate the pump-induced noise without the need of complex active pump LD control and intra-cavity dispersion management, thus providing an easy way for practical high-power, high-stability femtosecond fiber laser design and related high-precision applications outside the laboratory.

Compressing picosecond laser pulses to the femtosecond level is an attractive shortcut for obtaining femtosecond laser pulses. However, dechirped pulses generated by nonlinear compression with self-phase modulation (SPM) show obvious pedestals, which are induced by nonlinear chirp accumulation in spectral broadening process and cannot be easily suppressed. Here, we report systematic numerical simulations and experimental studies on self-similar amplification of picosecond pulses in a short gain fiber for obtaining ~100-fs laser pulses with nearly transform-limited (TL) temporal quality. It is demonstrated that self-similar amplification with picosecond seed pulses is only sensitive to pulse duration and pulse energy. Based on this optimization guideline, we built a compact self-similar amplification fiber system with a picosecond fiber laser as the seed source. This system outputs 66-fs pulses with 6.1-W average power at a repetition rate of 30 MHz. Due to the linear chirp produced in self-similar evolution process, compressed pulses show nearly TL temporal quality. It promises an efficient way of obtaining high-quality femtosecond laser pulses from a picosecond laser source.

We demonstrate an all polarization-maintaining (PM) fiber based dual-wavelength mode-locked Er-fiber laser. A nonlinear amplifying loop mirror (NALM) with an intracavity nonreciprocal phase shifter is used for self-started mode-locking. A short segment of PM fiber is angle-spliced to the NALM, functioning as a PM Sagnac loop filter, thus enabling dual-wavelength mode-locking. The wavelength separation is solely determined by the angle-spliced PM fiber length. Stable dual-wavelength mode-locking operation is switchable between 1570/1581 nm and 1581/1594 nm. The two-color pulse trains oscillating in the same cavity have an inherent offset repetition rate of ~1 kHz owing to cavity dispersion, allowing future high precision dual-comb applications with a simple and robust configuration.

In this study, we successfully demonstrate the achievement of 1.37 W average power for a picosecond deep ultraviolet (DUV) laser at 213 nm with a 1 MHz repetition rate using β-BaB2O4 (BBO) crystals, which is the highest output power of a 213 nm picosecond laser system with an all-solid-state setup so far. The laser system generates over 1.3 μJ 213-nm pulse energy. The fifth harmonic is generated by sum-frequency generation of the 532 and 355 nm beams based on a “2 + 3” scheme. BBO crystals with lengths of 6, 8, and 10 mm are investigated. Furthermore, the DUV laser system produces a high-beam quality and narrow linewidth output. The DUV system can be stably maintained over 100 h with expanded beam sizes of 532 and 355 nm at 800 mW, and without requiring a change in the positions and temperatures of nonlinear crystals.

On the basis of a home-made femtosecond Yb-doped fiber laser, we designed a compact and efficient third harmonic generation scheme by a simple compensation plate of β-BaB2O4 crystal. The compensation plate is optimized through its thickness and cutting angle to reverse both spatial and temporal walk-off. By optimizing the parameters of the compensation plate and incident light intensity, a maximum output of 2.23 W with a repetition rate of 1 MHz at 345 nm is obtained, which implies a conversion efficiency of 23% from the infrared to ultraviolet.

Terahertz (THz) grating is a widely used as a modulation device. In this paper, we propose a low cost and fast fabrication method of THz diffraction grating, which is utilized by a conventional mask as mold composed with metallic micro- and nano- particle fillers. Three kinds of metal microparticle gratings based on Cu, Sn and Ni were prepared by this method. Experimental and computational results demonstrated that the first-order diffraction efficiency for the metal microparticle gratings could be significantly improved than that for the conventional rectangular metal gratings. The physical mechanism is attributed to the enhanced transmittance and the concentration feature within the metallic microparticle strips of the gratings.

We demonstrate a femtosecond optical parametric oscillator that can generate orthogonally polarized dual-wavelength femtosecond pulses. Two periodically poled lithium niobate (PPLN) crystals with mutually orthogonal crystal axes are pumped by a single femtosecond fiber laser. The central wavelength of the two orthogonally polarized signal pulses can be continuously tuned from 1387 to 1588 nm with a maximum frequency separation of 27 THz. Because of the orthogonal dual-crystal scheme, the system is immune to the coherent coupling effect, thus overcoming the limitation of minimum frequency separation.