Search

In the past three decades, ultrafast pulse laser technology has greatly progressed and applied widely in many subjects, such as physics, chemistry, biology, materials, and so on. Accordingly, as well as for future developments, to measure or characterize the pulses temporally in femtosecond domain is indispensable but still challenging. Based on the operation principles, the measurement techniques can be classified into three categories: correlation, spectrogram, and spectral interferometry, which operate in time-domain, time-frequency combination, and frequency-domain, respectively. Here, we present a mini-review for these techniques, including their operating principles, development status, characteristics, and challenges.

The dynamical properties of InP-based quantum dot (QD) gain media are surveyed and analyzed for three time scales ranging from tens of picoseconds to less than 200 fs, where quantum coherent phenomena are observable. Each of these time scales determines the important properties of QD devices, i.e., modulation capabilities, nonlinear gain, and coherent light-matter interactions. Experimental investigations and modeling results are described, highlighting the state of the art in QD devices for the important 1550-nm telecom wavelength range.

Exploiting the characteristics of spatially chirped ultrafast pulses has been an increasingly active area of research. In this paper, we review the developments in the past year of the microscopy, materials processing, and micromachining fields, where the spatiotemporal structure of these beams is important. We also summarize progress in theoretical and experimental work to better control and characterize spatially chirped beams.

To exploit the potential of a dual-comb absolute distance ranging system outside a well-controlled laboratory, a compact dual-comb structure is proposed. A beat frequency generated by the repetition rates of two frequency combs serves as an electric reference, which is easily built and maintains a long range and high resolution compared with traditional dual-comb systems. The performance of the proposed method is compared with that of a heterodyne interferometer. The residuals range within -116.6 to 117.2 nm, the standard deviations vary from 46.3 to 137.9 nm, and the non-ambiguity range extension remains reliable throughout a 10-m test. Compared with Michelson-type dual-comb interferometers, this compact dual-comb system omits the redundant optical reference arm, promising practical applications of distance ranging.

Broadband supercontinuum (SC) spanning more than an octave was obtained in tapered seven-core photonic crystal fibers (PCFs) upon pumping with a 150-fs mode-locked fiber laser. The appearance of the second zero dispersion wavelength in the tapered section of the PCF limited the solitons' self-frequency shift. The simulations and experiments suggested that the unique group velocity in the tapered section could change the relative velocity of different spectral components, which resulted in some compression of the pulse in the time domain. It can be found that the spectral defects between the solitons were leveled up, and the visible region was enhanced remarkably, which was beneficial to control the SC generation. In addition, the interference fringes in the far field indicated the good spatial coherence of the output SC of the seven-core PCF.

High-order harmonic generation is an important mechanism to generate coherent radiation in the few–100-eV spectral range with ultrashort laser pulses. Moreover, a closer inspection of the measured spectra provides unique information about the underlying physics and allows deriving guidelines for improvements. The long-range modulation of the spectral envelope is linked to phase matching, and we will show how to improve it with a double-pulse excitation scheme. Additionally, the spectrum contains only every fourth harmonic, which can be well explained by the quantum interference of multiple scattered electrons, and two dominant electron trajectories were selected by X-ray parametric interaction.

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. [ABSTRACT FROM AUTHOR]

An approach for refractive index (RI) sensing is proposed using a waveguide-based optofluidic Mach–Zehnder interferometer (MZI). In this optofluidic MZI, the sensing arm is an epoxy-based negative photoresist (SU-8) core fabricated with femtosecond laser-induced two-photon polymerization technique, while the reference arm is the glass substrate. By integrating the SU-8 core with a microchannel, a RI sensor is realized with which the RI of sodium chloride (NaCl) solution has been measured. Effects of the chip length, microchannel length, and core size on the sensitivity of RI measurement have been revealed, indicating a shorter chip length, longer microchannel, and smaller core size can achieve a higher sensitivity, which is advantageous for sensing applications. [ABSTRACT FROM AUTHOR]

Time-resolved pump-probe microscopy experiments are presented to demonstrate the impact of laser- processing parameters onto the fundamental interaction processes between ultrashort laser pulses and glass. Our approach includes the implementation of polarization microscopy to detect the transient buildup of pressure waves and stress from single-shot and multi-pulse experiments. The presented data gives insight into the interplay between pulse duration and repetition rate. We see a decrease in length of the absorption zone for increasing pulse durations. The temporal delay between consecutive pulses changes the length of the modification and the strength. Additional experiments foresee the extension to Bessel-like beams and multiple focus spots. [ABSTRACT FROM AUTHOR]

We use numerical simulations to investigate the effects of photodarkening (PD) on pulse compression and coherent combination following chirped-pulse amplification in Yb-doped fiber amplifiers. Even though we increase the pump power to keep the pulse energy constant, PD degrades the pulses due to phase distortions resulting from heating acting already within a pulse as well as to increased self-phase modulation if the fiber length is optimized for the PD-free fiber at beginning of life. By contrast, in a fiber shorter than this, the drop in peak power is a relatively modest 35% even at a PD propagation loss as large as 5 dB, thus providing effective mitigation against PD-induced phase distortions. The improvements extend to the combination efficiency of beam-combined systems. [ABSTRACT FROM AUTHOR]

Advanced applications of attosecond pulses require the implementation of experimental techniques for a complete and accurate characterization of the pulse temporal characteristics. The method of choice is the frequency resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB), which requires the development of suitable reconstruction algorithms. In the last few years, various numerical techniques have been proposed and implemented, characterized by different levels of accuracy, robustness, and computational load. Many of them are based on the central momentum approximation (CMA), which may pose severe limits in the reconstruction accuracy. Alternative techniques have been successfully developed, based on the implementation of reconstruction algorithms which do not rely on this approximation, such as the Volkov-transform generalized projection algorithm (VTGPA). The main drawback is a notable increase of the computational load. We propose a new method, called refined iterative ptychographic engine (rePIE), which combines the advantages of a robust algorithm based on CMA, characterized by a fast convergence, with the accuracy of advanced algorithms not based on such approximation. The main idea is to perform a first fast iterative ptychographic engine (ePIE) reconstruction and then refine the result with just a few iterations of the VTGPA in order to correct for the error introduced by the CMA. We analyse the accuracy of the novel reconstruction method by comparing the residual error (i.e., the difference between the reconstructed and the simulated original spectrograms) when VTGPA, ePIE, and rePIE reconstructions are employed. We show that the rePIE approach is particularly useful in the case of short attosecond pulses characterized by a broad spectrum in the vacuum-ultraviolet (VUV)–extreme-ultraviolet (XUV) region. [ABSTRACT FROM AUTHOR]

We demonstrate a broad, flat, visible supercontinuum spectrum that is generated by a dispersion-engineered tapered photonic crystal fiber pumped by a 1 GHz repetition rate turn-key Ti:sapphire laser outputting $\sim$ 30 fs pulses at 800 nm. At a pulse energy of 100 pJ, we obtain an output spectrum that is flat to within 3 dB over the range 490–690 nm with a blue tail extending below 450 nm. The mode-locked laser combined with the photonic crystal fiber forms a simple visible frequency comb system that is extremely well suited to the precise calibration of astrophysical spectrographs, among other applications. [ABSTRACT FROM AUTHOR]

ReS2 films with different thickness are fabricated by a vacuum filtration from liquid exfoliated ReS 2 suspension. The saturable absorption properties of these films are investigated by an open-aperture Z-scan technique with a femtosecond laser. The saturable absorption coefficients ${{{\alpha }}_{{\rm{NL}}}}$ and the imaginary part of the third-order optical susceptibility ${\rm{Im}}{{{\chi }}^{(3)}}$ are from −61.5 to −171.8 cm/GW and from $- {\text{2.24}}\times {\text{10}}^{- 11}$ to $- {\text{6.27}}\times {\text{10}}^{- 11}$ esu with the change of thickness. The figure of merit increases with the increasing of the thickness of ReS2 films, which suggests the possible layer-coupling influence to the nonlinear optical properties. A double-covered ReS2 microfiber as a saturable absorber is then fabricated. We obtain stable mode-locking pulses with high-damage threshold (410 mW), which are centered at 1563.3 nm with the duration of 3.8 ps and the fundamental repetition of 1.78 MHz. Our results provide complementary information for the reliable ReS2 film fabrication with good nonlinear optical property for the mode-locking fiber lasers. [ABSTRACT FROM AUTHOR]

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 (<2 nm) ps pump pulses, which have obvious pedestal. The corresponding peak power is close to 1 MW and the conversion efficiency reaches ∼30%. Simulations also indicate that this method can be trivially adapted to other passive fibers with normal group-velocity dispersion. This simple and compact scheme, which combines the Raman fiber amplifier and Mamyshev regenerator, could be attractive for applications in ultrafast science and technology. [ABSTRACT FROM AUTHOR]

Recent studies in spintronics (STs) have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz (THz) radiation sources. Such ST multilayers consist of ferromagnetic (FM) and non-magnetic (NM) thin films. When triggered by ultrafast laser pulses, they generate pulsed THz radiation due to the inverse spin-Hall effect—a mechanism that converts optically driven spin currents from the magnetized FM layer into transient transverse charge currents in the NM layer, resulting in THz emission. As THz emitters, FM/NM multilayers have been intensively investigated so far only at 800 nm excitation wavelength using femtosecond Ti:sapphire lasers. In this paper, we demonstrate that an optimized ST bilayer structure of 2 nm Fe and 3 nm Pt grown on 500 $\mu \text{m}$ MgO substrate is just as effective as a THz radiation source when excited either at $\lambda = 800$ nm or at $\lambda = 1550$ nm by ultrafast laser pulses from a femtosecond fiber laser (pulsewidth ~100 fs and repetition rate ~100 MHz). Even with low incident power levels, the Fe/Pt ST emitter exhibits efficient generation of THz radiation at both excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such ST emitters in THz systems driven by relatively low cost and compact femtosecond fiber lasers without the need for frequency conversion. [ABSTRACT FROM AUTHOR]

We report on the generation of optical rogue waves (RWs) during the process of the random dissipative soliton buildup in an ultrafast net-normal dispersion fiber laser. It is demonstrated that the random dissipative soliton buildup can be achieved under the multi-pulse regime by properly adjusting the cavity parameters, i.e., pump power level and polarization controller. The high amplitude waves accompanying with the dissipative soliton buildup process are analyzed by plotting the real-time spectral dynamics and the temporal pulse evolution, which is further confirmed as optical RWs by the statistical method. The obtained results provide a promising alternative for investigating the phenomenon of optical RW in ultrafast fiber lasers, and will be beneficial for further unveiling the physical mechanism of optical RWs. [ABSTRACT FROM AUTHOR]

A long-distance measurement system is proposed in this paper, which is based on optical sampling using a femtosecond fiber laser. The measurement principle and the importance of dispersion compensation are theoretically analyzed in the time domain. By sweeping the repetition rate periodically, stable cross-correlation patterns are acquired, and the unknown distance can be calculated from the interference information. A long optical delay line is placed in the signal path, which can eliminate the “dead zone” effect and improve the performance of long-range measurement. The experimental results show that the present system can realize an accuracy of 4 μm over a 60-m distance, corresponding to a relative precision of 6.7 × 10−8. The all-fiber optical path system provides a balance among long range, high resolution, and rapid measurement, with the potential for the development of instrumentation for precision engineering applications. [ABSTRACT FROM AUTHOR]

Tb3+-doped aluminum borosilicate glass is a promising magneto-optical material that can be used to fabricate Faraday rotating devices. In this paper, we report the fabrication of an optical ridge waveguide structure in the Tb3+-doped aluminum borosilicate glass by using a combination of femtosecond laser ablation and ion implantation. The end-face microscopic photographs of the waveguide were acquired by the optical microscope. The effective refractive indices of the modes and near-field intensity distributions were measured by the prism-coupling and end-face coupling techniques. The distributions of energy loss, refractive index, and light intensity were simulated by the stopping and range of ions in matter 2013, reflectivity calculation method, and finite-difference beam propagation method. The results provide a new idea for the fabrication of waveguide-type magneto-optical isolators. [ABSTRACT FROM AUTHOR]

Abstract: 2D plasmons are of particular interest in the exploration of light–matter interactions in 2D materials. In 2D plasmonic materials, response time (in sub‐picosecond scale) of free carriers is order of magnitude faster than excitonic recombination in semiconductors, making them highly attractive for realizing faster optical switching and modulation in nanoscale devices. However, the small carrier density in gapless 2D plasmonic materials like graphene has strongly limited the strength of light–matter interaction and the operative spectral range. Here, it is shown that in optically activated plasmonic molybdenum oxide (MoO3) sheets of atomic thickness, the nonlinear optical (NLO) absorption, characterized by a saturable behavior in the near‐infrared region, is notably enhanced by the plasmon resonance, giving a modulation depth of 34.96%. Ultrafast pump‐probe spectroscopy reveals that the transient photobleaching in plasmonic MoO3 nanosheets associated with the relaxation of hot electrons recovers in a timescale less than 200 femtoseconds (fs). This ultrafast NLO response allows the development of an optical switch based on saturable absorption that enables the generation of mode‐locked laser pulses from a fiber laser operating at 1.0 µm region. The results may have strong implications for the application of 2D plasmonics based on 2D MoO3 in nanophotonics. [ABSTRACT FROM AUTHOR]

A piezotronic pulsed GaN-based photoconductive TeraHertz (THz) emitter is proposed and simulated for the first time. In this paper, we benefit from high break down-voltage, thermal conductivity, and saturation velocity of GaN, to design a pulsed antenna-less THz emitter. The proposed emitter consists of asymmetric metal-semiconductor-metal structure. Moreover, strong coupling of semiconducting/piezoelectric properties in GaN, allows modulation of the dissimilar Schottky barriers, leading to tunable THz output power and bandwidth. Our simulation results demonstrate that applying an external strain of about 4% results in 26% enhancement in the radiated THz power, while output bandwidth is improved about 4.2%. The proposed structure is promising for emerging reconfigurable THz emitters, capable of being tunable by external strain as a controlling gate. [ABSTRACT FROM AUTHOR]

The performance of fiber laser systems has drastically increased over recent decades, which has opened up new industrial and scientific applications for this technology. However, currently a number of physical effects prevents further power scaling. Coherent combination of beams from multiple emitters has been established as a power scaling technique beyond these limitations. It is possible to increase the average power, and for pulsed laser systems, also parameters such as the pulse energy and the peak power. To realize such laser systems, various aspects have to be taken into account that include beam combination elements, stabilization systems, and the output parameters of the individual amplifiers. After an introduction to the topic, various ways of implementing coherent beam combination for ultrashort pulses are explored. Besides the spatial combination of beams, the combination of pulses in time will also be discussed. Recent experimental results will be presented, including multidimensional (i.e., spatial and temporal) combination. Finally, an outlook on possible further developments is given, focused on scaling the number of combinable beams and pulses. [ABSTRACT FROM AUTHOR]

A compact, high-average-power, sub-picosecond, two-color (1025 and 1085 nm) fiber-coupled, chirped pulse amplification Yb:fiber laser is reported. Two colors are selected from a continuum using a chirped fiber Bragg grating, which are then amplified in a two-stage Yb:fiber amplifier system. The preamplifier is fully fiber-coupled, improving significantly the stability and efficiency compared to the former bulk-optic system. The system delivers two-color pulses with the total average power of 2.3 W (1.9 W at 1025 nm and 0.4 W at 1085 nm) at a 65 MHz repetition rate. The FWHM duration of the final compressed output pulses of each color is 900 and 600 fs. The amplified spectrum showed insignificant spontaneous emission between the colors, indicating that the separation could be further increased. [ABSTRACT FROM AUTHOR]

We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared (deep-IR) based on the nonlinear optical crystal, CdSiP2 (CSP), pumped directly by a Ti:sapphire laser, for the first time. By pumping CSP at <1 μm, we have achieved practical output powers at the longest wavelengths generated by any Ti:sapphire-pumped OPO. Using a combination of pump wavelength tuning, type-I critical phase matching, and cavity delay tuning, we have generated continuously tunable radiation across 6654–8373 nm (1194–1503 cm−1) at 80.5-MHz repetition rate, providing up to 20 mW of average power at 7314 nm and >7 mW beyond 8000 nm, with idler spectra exhibiting bandwidths of 140–180 nm across the tuning range. Moreover, the near-IR signal is tunable across 1127–1192 nm, providing up to 37 mW of average power at 1150 nm. Signal pulses, characterized using intensity autocorrelation, have durations of ∼260–320 fs, with corresponding time-bandwidth product of ΔυΔτ ∼ 1. The idler and signal output exhibit a TEM00 spatial profile with single-peak Gaussian distribution. With an equivalent spectral brightness of ∼6.68 × 1020 photons s−1 mm−2 sr−1 0.1% BW−1, this OPO represents a viable alternative to synchrotron and supercontinuum sources for deep-IR applications in spectroscopy, metrology, and medical diagnostics. [ABSTRACT FROM AUTHOR]

The impact of spectral phase noise on the temporal pulse contrast is explored experimentally by using two zero-dispersion stretchers (Offner stretcher and two-lens stretcher). The results show clearly that a long temporal pedestal appears when there exists far-field spectral phase noise, which greatly degrades the pulse contrast. In our proof-of-principle experiment, by using a two-lens stretcher, the far-field spectral phase noise is avoided and the noise pedestal of the amplified pulse in a Nd:glass laser system is reduced, with the contrast reaching a level of 10 –11 in the 60-ps time window before the main pulse. This simple method may pave the way in a PW-level Nd:glass laser by directly using pre-cleaning techniques. [ABSTRACT FROM AUTHOR]

We demonstrate a fiber-based femtosecond laser source, which can deliver 37 MHz, ~100 fs laser pulses tunable from 920 nm to 1030 nm with up to 10 nJ pulse energy. Pumped by an ultrafast Yb-doped fiber laser, a short piece of tapered photonic crystal fiber with lower dispersion was used to generate self-phase-modulation filtered for the tunable pulses. We believe that this new type of high-energy ultrafast fiber laser source can be an ideal solution for the two-photon fluorescence microscopy applications. [ABSTRACT FROM AUTHOR]

A soliton-locked Erbium-doped fiber laser is demonstrated for the first time using a pure Antimony (Sb) film as a saturable absorber (SA). The SA device is fabricated by depositing a 5 μm Sb layer onto a polyvinyl alcohol thin film via Physical vapor deposition process. Self-starting soliton mode-locked pulses train with a constant repetition rate of 0.99 MHz is obtained as the pump power increased from 101 mW to 142 mW. The laser operates at 1559 nm with a pulse width of 3.53 ps. At the pump power of 142 mW, the maximum peak power and pulse energy are calculated as 5.83 kW and 20.59 nJ, respectively. The obtainable pulse is very stable with a signal-to-noise ratio (SNR) of 70 dB. Our experimental results manifest that the pure Sb thin film-based SA is able to generate mode-locked pulsed laser at the 1.55-micron laser regime. [ABSTRACT FROM AUTHOR]

We report on the second harmonic generation of light in a cadmium silicon phosphide (CdSiP2, CSP) nanowire waveguide excited at the telecommunication wavelength of 1550 nm. This frequency-conversion process is studied by means of time-domain simulations that solve for the full vector fields of Maxwell’s equations and which incorporate the complete second-order nonlinear coefficient tensor of the CSP crystal. Over the short length of 30 $\mu \text{m}$ , the CSP nanowire waveguide produces electric field pulses having a peak-to-peak value of 116 kV/cm at a conversion efficiency of $1.6\times 10^{-3}$. This electric field value is five times larger than that obtained in a lithium niobate nanowire waveguide having the same footprint, whereas the conversion efficiency value is 11 times higher than what is achieved in the lithium niobate nanowire. These results suggest that CSP nanowire waveguides have the potential to be used for on-chip sources of coherent optical radiation that are highly efficient given their compact structure. [ABSTRACT FROM AUTHOR]

Optical tweezers have enabled important insights into intracellular transport through the investigation of motor proteins, with their ability to manipulate particles at the microscale, affording femto newton force resolution. Its use to realize a constant force clamp has enabled vital insights into the behavior of motor proteins under different load conditions. However, the varying nature of disturbances and the effect of thermal noise pose key challenges to force regulation. Furthermore, often the main aim of many studies is to determine the motion of the motor and the statistics related to the motion, which can be at odds with the force regulation objective. In this paper, we propose a mixed objective $H_2/H_\infty$ optimization framework using a model-based design, that achieves the dual goals of force regulation and real-time motion estimation with quantifiable guarantees. Here, we minimize the $H_\infty$ norm for the force regulation and error in step estimation while maintaining the $H_2$ norm of the noise on step estimate within user specified bounds. We demonstrate the efficacy of the framework through extensive simulations and an experimental implementation using an optical tweezer setup with live samples of the motor protein “kinesin”, where regulation of forces below 1 piconewton with errors below $\text{10}\%$ is obtained while simultaneously providing real-time estimates of motor motion. [ABSTRACT FROM AUTHOR]

A novel injection locking architecture is demonstrated to simultaneously improve the axial mode linewidth and stabilize the repetition rate of a monolithically integrated mode-locked laser. First, linewidth reduction is demonstrated via multitone injection locking. Then, the passive mode-locked laser is used as a source to make a coupled optoelectronic oscillator, and its RF spectral characteristics are investigated for different optical delays. A novel combination of these two techniques is established by creating a coupled optoelectronic oscillator via multitone injection locking, using an intracavity element as photodetector to generate the feedback signal to a Mach-Zehnder modulator. Using this combination of methods, the axial mode linewidth is reduced by over 6000× to $\sim$ 100 kHz and the RF linewidth is reduced by a factor of 70 at $-$ 30 dBc, resulting in a 3 dB RF linewidth of 400 Hz. The improvement in RF phase noise at 200 kHz offset is $>$ 40 dB. Finally, the system is referenced to a RF synthesizer using a variable voltage phase-shifter and a PID controller. Allan deviation measurements show a resolution limited stability in the repetition rate of $10^{-10}$ at 1 second following a $1/\tau$ trend. [ABSTRACT FROM AUTHOR]

We have used the ultrafast pump–probe technique known as picosecond ultrasonics to generate and detect surface acoustic waves on a structure consisting of nanoscale Al lines on SiO 2 on Si. We report results from ten samples with varying pitch (1000–140 nm) and SiO 2 film thickness (112 nm or 60 nm), and compare our results to an isotropic elastic calculation and a coarse-grained molecular dynamics simulation. In all cases we are able to detect and identify a Rayleigh-like surface acoustic wave with wavelength equal to the pitch of the lines and frequency in the range of 5–24 GHz. In some samples, we are able to detect additional, higher frequency surface acoustic waves or independent modes of the Al lines with frequencies close to 50 GHz. We also describe the effects of probe beam polarization on the measurement’s sensitivity to the different surface modes. [ABSTRACT FROM AUTHOR]

Developing efficient algorithms is an important task in the learning control of quantum systems, since most learning control problems for quantum systems involve a heavy requirement for computational resources. In this paper, we employ a learning control algorithm with an adaptive target state developed in the chemical physics community for several classes of quantum control problems. For these problems applied to some new quantum control tasks, we further demonstrate that the algorithm using an adaptive target state can be more efficient than traditional learning control algorithms using a fixed target state. In the algorithm, the target state is updated according to the renormalized fragmentary yield in the desired region (or subspace) throughout the learning iterations. The adaptive target scheme is applied to three significant quantum control tasks, including the slow collision of a sodium cation and an iodine anion, the orientation of a LiH molecule, and population transfer between subspaces. Shaped laser pulses are obtained using the learning control algorithm, and numerical results are presented to demonstrate the advantages of the adaptive target scheme over the algorithm using a fixed target state. The adaptive target scheme is especially useful for learning control problems of quantum systems where the target state is not unique or known. [ABSTRACT FROM AUTHOR]

To achieve complete security of communication, there is a need for “real-time” ultrafast physical random bit generators (RBGs). In the available physical RBGs, random bit extraction is effected in the electrical domain and, therefore, cannot directly function beyond the frequencies of 10 GHz or so in real time. Here, we present a fully photonic strategy for real-time random number generation and report a proof-of-concept experimental demonstration of an integrated 10 Gb/s RBG system (prototype) based on laser chaos. The use of an ultrastable mode-locked laser, together with ultrafast optical interactions, gives our method the capability to develop super-high-speed RBGs in practice. The photonic implementation is fully compatible with optical communications so that well-established optical multiplexing techniques can be used to realize Tb/s real-time random bit generation. [ABSTRACT FROM AUTHOR]

Coherent optical fiber links recently demonstrate their ability to compare the most advanced optical clocks over a continental scale. The outstanding performances of the optical clocks are stimulating the community to build much more stable time scales, and to develop the means to compare them. Optical fiber link is one solution that needs to be explored. Here, we are investigating a new method to transfer time based on an optical demodulation of a phase step imprint onto the optical carrier. We show the implementation of a proof-of-principle experiment over 86-km urban fiber, and report time interval transfer stability of 1 pulse per second signal with sub-ps resolution from 10 s to one day of measurement time. Prospects for future development and implementation in active telecommunication networks, not only regarding performance but also compatibility, conclude this paper. [ABSTRACT FROM AUTHOR]

We show analytically and numerically that a practically realizable phase-shift fiber Bragg grating (PS-FBG) can function as a temporal first-order optical differentiator and a temporal first-order optical integrator at the same time. The PS-FBG working in reflection implements the differentiation and working in transmission implements the integration. We provide both the generalized conditions for a PS-FBG functioning as a first-order optical differentiator and a first-order optical integrator. The proposed PS-FBG can perform the time differential and integral of the complex envelope of an arbitrary input optical signal with high accuracy, respectively. [ABSTRACT FROM AUTHOR]

We report a simple fiber-implemented technique for complete reconstruction of intensity profile and phase of ultrashort laser pulses based on processing only pulse spectrum and two selfphase modulated spectra measured after a short piece of optical fiber. Its applicability is shown on an example of a fiber optical system in the telecommunication range. A retrieval algorithm in a dispersionless approximation and with considering dispersion effects is developed. The obtained results are confirmed by independent measurements using the second-harmonic generation frequency-resolved optical gating technique and by reconstructing purposely introduced signal features. We also provide estimates demonstrating great opportunities for implementing this technique in all-waveguide optical systems ranging from optical communications to nanophotonics with femtojoule pulses as well as to mid-IR photonics, where specialty fibers with huge optical nonlinearities can be used. [ABSTRACT FROM AUTHOR]

Building on the scientific understanding and technological infrastructure of single-mode fibers, multimode fibers are being explored as ameans of adding new degrees of freedom to optical technologies such as telecommunications, fiber lasers, imaging, and measurement. Here, starting from a baseline of single-mode nonlinear fiber optics, we introduce the growing topic of multimode nonlinear fiber optics. We demonstrate a new numerical solution method for the system of equations that describes nonlinear multimode propagation, the generalized multimode nonlinear Schrödinger equation. This numerical solver is freely available, implemented in MATLAB and includes a number of multimode fiber analysis tools. It features a significant parallel computing speed-up on modern graphical processing units, translating to orders-of-magnitude speed-up over the conventionally-used splitstep Fouriermethod.We demonstrate its use with several examples in graded- and step-index multimode fibers. Finally,we discuss several key open directions and questions, whose answers could have significant scientific and technological impact. [ABSTRACT FROM AUTHOR]

Wideband, octave spanning, coherent supercontinuum (SC) with high flatness is very useful for many applications, but it has been difficult to achieve such ideal SC. In this paper, we investigated the octave spanning, coherent SC generation based on Er-doped ultrashort pulse fiber laser system. First, we developed a high power ultrashort pulse system using a single-wall carbon nanotube fiber laser and similariton amplifier. A 1.0–2.2 μm octave-spanning coherent supercontinuum was successfully generated. Next, by use of a stabilized fiber laser comb system as the seed pulse, an octave spanning SC comb was demonstrated. An octave spanning, highly coherent comb with high flatness was generated with normally dispersive highly nonlinear fiber. The characteristics of the generated supercontinuum comb were examined via balanced heterodyne beat measurements with stable continuous-wave laser diodes. The SC comb was confirmed to have high coherence and low amplitude noise at the observed wavelengths. Finally, the SC generation in two kinds of highly nonlinear fibers with different dispersion properties and the characteristics of the generated comb were examined. The dependence on fiber length, wavelength, and dispersion properties were discussed. [ABSTRACT FROM AUTHOR]

A diode laser pumped carbon nanotube mode-locked Ho3+ -doped fluoride fiber laser at 1.2 μm was demonstrated for the first time. Stable mode-locked pulses with an average power of 1 mW at a repetition rate of 18.47 MHz were obtained at a pump power of 348 mW. The pulse energy and peak power of this mode-locked laser oscillator were about 54 pJ and 12.6 W, and were increased to 2.41 nJ and 0.54 kW, respectively, by using a 15-cm Ho3+ -doped fluoride fiber amplifier. The pulse duration was measured to be 4.3 ps by an autocorrelator. Because the operation wavelength is close to the zero-dispersion wavelength of 1.3 μm, this mode-locked fiber laser exhibits unique features. [ABSTRACT FROM AUTHOR]

We present the performance of an all polarization maintaining Erbium-doped fiber laser mode-locked by a multilayer graphene based saturable absorber. The cavity incorporates a Martinez-type compressor, based on a transmission grating, which allows for continuous tuning of the net cavity dispersion as well as spectral filtering. By adjusting the dispersion and the spectral bandwidth, we could operate the laser in the soliton, the dispersion managed soliton and the absorber-limited pulse regimes. The pulse durations from the laser ranged between 338 fs to 2.1 ps and the laser could be operated at nearly constant output powers in the anomalous and the normal dispersion regimes. We also conducted stability analyses based on dispersive Fourier transform measurements in the different operating regimes. Comparison of the collected spectra reveals that the soliton regime had the best overall stability. [ABSTRACT FROM AUTHOR]

Self-assembled quantum dots (QDs) as active media for ultrafast semiconductor disk laser offer large gain bandwidths, fast gain dynamics, and high temperature stability. We report on the shortest pulses and the highest pulse peak power from an optically pumped vertical external cavity surface emitting laser (VECSEL) based on QDs and optimized for passive modelocking at 1035 nm using a semiconductor saturable absorber mirror. We demonstrate 216-fs pulses with an average output power of 269 mW at a pulse repetition rate of 2.77 GHz and 396 W peak power. At a lower pulse repetition rate of 1.67 GHz, we achieve 193-fs pulses with 112 mW of average output power. We remark a higher optical-to-optical pump efficiency compared to our previous QW VECSELs in the sub-300-fs regime. This is further confirmed by a comparative analysis of the saturation recovery which reveals longer carrier lifetimes for the QD compared to QW VECSELs. [ABSTRACT FROM AUTHOR]

We report on the measurement and characterization of polarization properties of second harmonic (SH) light from starch granule using a Stokes vector based microscopy. The four-element Stokes parameters are the most complete description of the polarization state of SH light, including any depolarization effects. Various polarization parameters, such as the degree of polarization, the degree of linear polarization, the degree of circular polarization, as well as anisotropy are extracted by implementing a pixel by pixel image analysis from the 2D reconstructed SH Stokes images of starch granule. Furthermore, we implemented the technique by varying the polarization states of excitation light and recording the resulting Stokes parameters at micro-meter depths of resolution in order to investigate the molecular structure and orientation of the samples. [ABSTRACT FROM AUTHOR]

Tunable electrooptic combs are generated in an optoelectronic oscillator that is optically filtered using a 100 000 finesse Fabry–Pérot etalon. Optical and electrical outputs of the oscillator are combined in a series of external phase modulators to generate spectrally flat, broadband optical combs. Adjustment of a variable delay in the oscillator allows for tuning of output combline spacing to harmonics of the etalon’s 1.5-GHz free spectral range. Experimental results demonstrate regeneratively created electrooptic combs whose comb teeth spacing can be tuned to 7.5, 9, 10.5, and 12 GHz, with center comb tooth linewidths on the order of hundreds of Hertz. In the best case, the output electrooptic modulation comb consists of 52 individual comblines within a 10-dB amplitude deviation over a span of approximately 5 nm. Linear pulse compression is used to demonstrate ultrashort pulses with a 2.6-ps autocorrelation pulse width at a repetition rate of 10.5 GHz. [ABSTRACT FROM AUTHOR]

We proposed an improved femtosecond optical Kerr gate with double gate pulses (DOKG) using a femtosecond laser, in which an ultrashort switching time can be realized even when a relatively slow response optical Kerr medium of carbon disulfide was used. The results showed that when the time delay and the light intensity ratio of the two pump pulses were adjusted to be about 140 fs and 1:0.9, the switching time and the maximum transmission efficiency of the DOKG could be optimized to be about 130 fs and 37%. The DOKG is convenient to be built and offers a good choice for applications based on ultrafast optical switch. [ABSTRACT FROM AUTHOR]