Coherent beam combining of femtosecond third-harmonic generators: towards high-power, high-beam-quality UV light generation.

Coherent beam combining (CBC) of two femtosecond third-harmonic (TH) generators is proposed and demonstrated. By applying phase modulation to one of the fundamental laser pulses, the feedback loop effectively eliminates both phase and pointing errors between the two TH femtosecond laser beams. The system delivers 345-nm femtosecond laser pulses with 22-W average power at 1-MHz repetition rate. The average combining efficiency is 91.5% over approximately 1 h of testing. The beam quality of the combined ultraviolet (UV) laser beam is near-diffraction-limited with M2 factors of M X2=1.36, M Y2=1.24, which are similar to those of the individual channels. This scheme exhibits promising potential for increasing high-beam-quality UV laser power.

5-bit all-optical quantum random number generator based on a time-multiplexed optical parametric oscillator.

Random numbers are of critical importance in many applications, including secure communication, photonics computing and cryptography. Due to the non-deterministic nature of the quantum processes, a degenerate optical parametric oscillator (DOPO) constitutes a solution to produce true randomness. Nevertheless, one of the existing challenges for DOPO in this field is bit sequence scalability. Here, we experimentally report on the generation of 5-bit random number streams in a time-multiplexed femtosecond DOPO system. A multi-pass cell is added to elongate the OPO cavity to scale up the bit sequences. To this end, for a ∼15 m long all free space OPO cavity, resonating 5 signal pulses with a repetition rate of 50 MHz is demonstrated. The above-threshold binary phase nature originates from vacuum fluctuations of a DOPO ensuring the randomness of the system. The phase state of the output is characterized by the interference pattern between the output pulses and the fundamental pump pulses. Different bit sequences are presented here by turning on and off the OPO. Conditional probability is performed to verify the randomness of the output for 1200 bits. Our scheme provides a new direction for an all-optical random number generator.

Achieving desirable charge transport by porous frame engineering for superior 3D printed rechargeable Ni-Zn alkaline batteries.

Rechargeable 3D printed batteries with extraordinary electrochemical potential are typical contenders as one of the promising energy storage systems. Low-cost, high-safety, and excellent rechargeable aqueous alkaline batteries have drawn extensive interest. But their practical applications are severely hampered by poor charge carrier transfer and limited electrochemical activity at high loading. Herein, we report a unique structure-based engineering strategy in 3D porous frames using a feasible 3D printing technique and achieve 3D printed full battery devices with outstanding electrochemical performance. By offering a 3D porous network to provide prominently stereoscopic support and optimize the pore structure of electrodes, the overall charge carrier transport of engineered 3D printed Ni-Zn alkaline batteries (E3DP-NZABs) is greatly enhanced, which is directly demonstrated through a single-wired characterization platform. The obtained E3DP-NZABs deliver a high areal capacity of 0.34 mA h cm-2 at 1.2 mA cm-2, and an outstanding capacity retention of 96.2% after 1500 cycles is also exhibited with an optimal electrode design. Particularly, parameter changes such as a decrease in pore sizes and an increase in 3D network thickness are favorable to resultant electrochemical performance. This work may represent a vital step to promote the practical application progress of alkaline batteries.

High-capacity free-space optical communications using wavelength- and mode-division-multiplexing in the mid-infrared region.

Due to its absorption properties in atmosphere, the mid-infrared (mid-IR) region has gained interest for its potential to provide high data capacity in free-space optical (FSO) communications. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. We multiplex three ~3.4 µm wavelengths (3.396 µm, 3.397 µm, and 3.398 µm) on a single polarization, with each wavelength carrying two orbital-angular-momentum (OAM) beams. As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. The WDM channels are generated and detected in the near-IR (C-band). They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. We estimate that the system penalties at a bit error rate near the forward error correction threshold include the following: (i) the wavelength conversions induce ~2 dB optical signal-to-noise ratio (OSNR) penalty, (ii) WDM induces ~1 dB OSNR penalty, and (iii) MDM induces ~0.5 dB OSNR penalty. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link.

Controllable high-speed rotated femtosecond cylindrical vector beam based on optical heterodyne interference.

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.

Study on the Differences of Chemical Structures and Pyrolysis Characteristics between the Jurassic and Carboniferous Coking Coals.

Using Jurassic coking coals and Carboniferous coking coals as raw materials, carbonization experiments were carried out on the cokes produced by them in a self-made furnace in a laboratory-scale coking furnace, finding that the coke quality of the Jurassic fat coals and coking coals was obviously inferior to that of the Carboniferous coking coals of the same brand. In this study, the reasons for this phenomenon were studied by elemental analysis, Fourier transform infrared spectroscopy analysis, and thermogravimetric analysis of experimental coal samples and by combining the differences in chemical structures of experimental coal samples with pyrolysis characteristic parameters. It was found that the key factor affecting the quality of cokes made from the Jurassic fat coals, coking coals, and highly volatile coking coals was that the coals contained too many oxygen-containing functional groups, which were decomposed into reactive oxygen species in the main pyrolysis stage of coal. These reactive oxygen species would consume too much free-moving hydrogen and then trigger a large number of condensation and cross-linking reactions, resulting in poor plastic mass and coke quality.

Orbital-angular-momentum-resolved diagnostics for tracking internal phase evolution in multi-bound solitons.

The generation of multi-bound solitons is a fascinating subject of investigation in many conservative and dissipative systems, such as photonics, fluid mechanics, Bose-Einstein condensates, and so on. In this study, we demonstrate the successful extraction of phase dynamics between solitons in bound multiple solitons with up to seven constituents in a mode-locked Er laser system. By mapping the internal phase motions of multi-bound solitons to the spatial phase movement of cylindrical vector beams using orbital angular momentum (OAM)-based diagnostics, different categories of internal pulsations are revealed. We show that bound state of four solitons exhibits linear drifting relative phase evolution dynamics; while for bound multiple solitons with constituents from five to seven pulses, stationary relative phase dynamics are observed. These findings highlight the possibility of the OAM-based method access to the internal motion of multi-soliton molecules with more freedom of degrees and fuel the analogy with research on chemistry molecule complex.

Single-shot compressed ultrafast photography based on U-net network.

The compressive ultrafast photography (CUP) has achieved real-time femtosecond imaging based on the compressive-sensing methods. However, the reconstruction performance usually suffers from artifacts brought by strong noise, aberration, and distortion, which prevents its applications. We propose a deep compressive ultrafast photography (DeepCUP) method. Various numerical simulations have been demonstrated on both the MNIST and UCF-101 datasets and compared with other state-of-the-art algorithms. The result shows that our DeepCUP has a superior performance in both PSNR and SSIM compared to previous compressed-sensing methods. We also illustrate the outstanding performance of the proposed method under system errors and noise in comparison to other methods.

Orthogonally polarized tunable dual-wavelength femtosecond optical parametric oscillator.

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.

Manipulation of infrared dispersive waves in customized microstructured optical fibers for 1.7 and 2.0 µm light sources.

We demonstrate the controllable generation of infrared dispersive waves (DWs) from customized, in-house fabricated silica microstructured optical fibers (MOFs) by manipulating the location of zero dispersion wavelength (ZDW) through the structure of the fibers. The highly enriched shaping mechanism of arrested soliton in the MOFs with two ZDWs provides a technique for efficient energy transfer into the targeted eye-safe wavelengths at 1.7 and 2.0 µm by the virtue of DW formation.

Dual-mode and two-signal-wavelength femtosecond optical parametric oscillator based on LiB3O5.

We demonstrate a tunable femtosecond dual-beam-mode (cylindrical vector beam [CVB] and Gaussian beam [GB]), dual-signal-wavelength optical parametric oscillator based on a temperature-tuned lithium triborate crystal, synchronously pumped by a frequency-doubled mode-locked Yb-doped fiber laser. When fixing the CVB wavelength at 780 nm, the central wavelength of the GB signal could be continuously tuned from 664 to 722 nm. The maximum total signal output power is 515 mW at a 4 W pump with dual-wavelength operation (664 and 780 nm). All the measured signal pulse durations are around 150 fs. Moreover, sum-frequency-generation with Gaussian mode tuning from 548 to 588 nm is obtained, with the maximum power of 52 mW at 548 nm. Thanks to the dual-channel configuration, the wavelengths of a CVB and GB can be tuned independently. Such a flexible and versatile configuration makes it a practical tool for many applications such as high-resolution microscopy and high-capacity optical communication.

High-power femtosecond cylindrical vector beam optical parametric oscillator.

We report on high-power femtosecond cylindrical vector beam (CVB) generation from a Gaussian-pumped optical parametric oscillator (OPO). By introducing a half waveplate and a vortex half-wave plate of m = 1 to realize intracavity polarization modulation to the resonant Gaussian signal, the OPO could deliver broadband signal beam in CVB profile, i.e., radially and azimuthally polarized beam profile. The central wavelength of the generated CVB signals can be tuned continuously from 1405 to 1601 nm, while the corresponding pulse durations are all around 150 fs. A maximum average output power of 614 mW at 1505 nm is obtained. Moreover, our OPO cavity design can be extended to generate high order CVB by simply changing the vortex half-wave plate with different orders. Such a high-power CVB OPO configuration has the advantages of flexible control and wide tuning range, making it a practical tool for applications in super-resolution imaging, optical communication and quantum correlations.

Intracavity cylindrical vector beam generation from all-PM Er-doped mode-locked fiber laser.

We demonstrate a practical method that is used to generate on-demand first- and higher-order cylindrical vector beams, in the 1550 nm band, directly from an all polarization maintaining mode-locked Er-fiber laser. On demand typical 1st order CVBs, including the radially and azimuthally polarized beams, can be easily achieved by properly adjusting the angle of a half-wave plate with respect to the fast axis of the vortex wave plate. The spatial beam mode can be flexibly switched with no disturbance on the time domain mode-locking output. The laser outputs the desired vector beams at 1571 nm with a spectral bandwidth at full-width at half-maximum of 32 nm. The mode-locked laser pulses have a repetition rate of 74.9 MHz. Moreover, the proposed method can be easily extended to create higher-order CVBs. Our research provides a convenient way to generate ultrafast pulses in highly flexible-controlled structured modes, which is essential for optical fabrication and light trapping applications.

910-MHz, watt-level, signal-power-enhanced, compact femtosecond optical parametric oscillator based on bidirectional pumping technique.

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.

Generation of 3.9-cycle pulses from the coherent synthesis of two continuous-wave injection seeded optical parametric amplifiers at 53 MHz.

A high repetition-rate, few-cycle light pulse is of great importance due to its potential for a variety of applications, including two-dimensional infrared spectroscopy and time-resolved imaging of molecular structures, which benefit from its ultrabroadband spectrum and ultrashort pulse duration. The generation of an ultrabroadband coherent spectrum is one of the frontiers of ultrafast optics, and accessing such few-cycle pulses is presently under active exploration. Here, we demonstrate a simple yet effective pulse synthesizer. It is based on two continuous-wave (cw) injection-seeded high-repetition-rate optical parametric amplification systems and the following self-phase-modulation dominated spectra-broadening processes. The combined spectrum spans from 1250 to 1670 nm, and a near Fourier-transform-limited 3.9-cycle (19.2 fs) synthesized pulse with a central wavelength of 1470 nm is obtained accordingly.

Dielectric-mirror-less femtosecond optical parametric oscillator with ultrabroad-band tunability.

We demonstrate a high average power, widely tunable, dielectric-mirror-less optical parametric oscillator (OPO) based on MgO:PPLN (MgO-doped periodically poled lithium niobate), which is synchronously pumped by a 1040 nm femtosecond fiber laser. The OPO does not require any dielectric coating mirrors. By exploiting the four-prism sequence system, combined with the gold mirrors, the oscillating laser pulses could span the spectral regions in both the signal and idler, and the output pulses of OPO can be tuned across 1367-1914 nm in the signal, and 2152-4480 nm in the idler as well. This device can deliver as much as 1.2 W of average power at 1482 nm in the signal and up to 411 mW at 3487 nm in the idler, respectively. The ultrabroad-band spectra tunability, along with the high average output property, makes the dielectric-mirror-less OPO an attractive alternative to conventional OPOs.

Intensity and temporal noise characteristics in femtosecond optical parametric amplifiers.

We characterize the relative intensity noise (RIN) and relative timing jitter (RTJ) between the signal and pump pulses of optical parametric amplifiers (OPAs) seeded by three different seed sources. Compared to a white-light continuum (WLC) seeded- and an optical parametric generator (OPG) seeded OPA, the narrowband CW seeded OPA exhibits the lowest root-mean-square (RMS) RIN and RTJ of 0.79% and 0.32 fs, respectively, integrated from 1 kHz to the Nyquist frequency of 1.25 MHz. An improved numerical model based on a forward Maxwell equation (FME) is built to investigate the transfers of the pump and seed's noise to the resulting OPAs' intensity and temporal fluctuation. Both the experimental and numerical study indicate that the low level of noise from the narrowband CW seeded OPA is attributed to the elimination of the RIN and RTJ coupled from the noise of seed source, being one of the important contributions to RIN and timing jitter in the other two OPAs. The approach to achieve lower level of noise from this CW seeded OPA by driving it close to saturation is also discussed with the same numerical model.

Noise characteristics of high power fiber-laser pumped femtosecond optical parametric generation.

We study, both numerically and experimentally, the relative intensity noise (RIN) and timing jitter characteristics of optical parametric generation (OPG) process in MgO-doped periodically poled LiNbO3 (MgO:PPLN) pumped by fiber femtosecond laser. We directly characterize the RIN, and measure timing jitter spectral density of the OPG process based on the balanced optical cross-correlator (BOC) technique for the first time as well, which are both in a fairly good agreement with numerical simulation. Both the numerical and experimental study reveals that OPG can suffer from a smaller intensity fluctuation but a lager temporal jitter when it is driven into saturation. Furthermore, we demonstrate that with a 30 mW CW diode laser injection seeding the OPG output results in superior noise performance compared to the vacuum fluctuations seeded OPG.

Programmable controlled mode-locked fiber laser using a digital micromirror device.

A digital micromirror device (DMD)-based arbitrary spectrum amplitude shaper is incorporated into a large-mode-area photonic crystal fiber laser cavity. The shaper acts as an in-cavity programmable filter and provides large tunable dispersion from normal to anomalous. As a result, mode-locking is achieved in different dispersion regimes with watt-level high output power. By programming different filter profiles on the DMD, the laser generates femtosecond pulse with a tunable central wavelength and controllable bandwidth. Under conditions of suitable cavity dispersion and pump power, design-shaped spectra are directly obtained by varying the amplitude transfer function of the filter. The results show the versatility of the DMD-based in-cavity filter for flexible control of the pulse dynamics in a mode-locked fiber laser.

Extended femtosecond laser wavelength range to 330 nm in a high power LBO based optical parametric oscillator.

We experimentally demonstrate a compact tunable, high average power femtosecond laser source in the ultraviolet (UV) regime. The laser source is based on intra-cavity frequency doubling of a temperature-tuned lithium tribotate (LBO) optical parametric oscillator (OPO), synchronously pumped at 520 nm by a frequency-doubled, Yb-fiber femtosecond laser amplifier system. By adjusting crystal temperature, the OPO can provide tunable visible to near-infrared (NIR) signal pulse, which have a wide spectral tuning range from 660 to 884 nm. Using a ß-barium borate (BBO) crystal for intra-cavity frequency doubling, tunable femtosecond UV pulse are generated across 330~442 nm with up to 364 mW at 402 nm.