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In attosecond and strong-field physics, the acquisition of few-cycle laser sources open up a new area. We report a nonlinear pulse compression technology combining multi-thin-plate spectral broadening and nonlinear self-compression that generates a 0.52 mJ, good spatial quality characteristics and a spectral bandwidth supporting a 14 fs Fourier transform limited duration at 1 kHz repetition rate and at a center wavelength of 1.9 μm. The total energy transfer efficiency is up to 83%. Pulse to pulse stability of the energy output is 0.7% (RMS). The pulse duration is near 3 optical cycles. This pulse compression approach can be a key-enabling technology for the next generation of extreme photonics, attosecond research and coherent x ray-science, and it also can be further extended to mid-infrared lasers with longer wavelengths and higher peak power.

In this article, we report robust vector soliton bunches arising in a passively mode-locked fiber laser based on carbon nanotube (CNT). By introducing a polarization-route-assisted module (PRAM) and finely adjusting the cavity birefringence, the soliton bunches can be flexibly manipulated to operate with controllable soliton numbers and positions. Moreover, the potential bandwidth resource provided by the concept of soliton bunches for the ease of breaks a new path for the enhancement of data-carrying capacity. Our research is helpful to extend the theory of vector soliton dynamics in fiber lasers and promotes the promising application in optical communication systems.

A near-infrared (NIR) supercontinuum and ultrashort pulses are generated via phase-mismatched cascaded frequency conversion in DSTMS (4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2, 4, 6-trimethylbenzenesulfonate) crystal. NIR pulses with a central wavelength of 1.5 μm and 63 fs pulse duration are incident into the crystal, and the spectrum is broadened to over 1200-2100 nm. Compensating the dispersion by fused silica, ultrashort NIR pulses with 16.6 fs pulse duration are obtained. Meanwhile, intense single-cycle terahertz pulses are generated via optical rectification in DSTMS crystal.

In attosecond and strong-field physics, the acquisition of few-cycle laser sources open up a new area. We report a nonlinear pulse compression technology combining multi-thin-plate spectral broadening and nonlinear self-compression that generates a 0.52 mJ, good spatial quality characteristics and a spectral bandwidth supporting a 14 fs Fourier transform limited duration at 1 kHz repetition rate and at a center wavelength of 1.9 μm. The total energy transfer efficiency is up to 83%. Pulse to pulse stability of the energy output is 0.7% (RMS). The pulse duration is near 3 optical cycles. This pulse compression approach can be a key-enabling technology for the next generation of extreme photonics, attosecond research and coherent x ray-science, and it also can be further extended to mid-infrared lasers with longer wavelengths and higher peak power. [ABSTRACT FROM AUTHOR]

In this article, we report robust vector soliton bunches arising in a passively mode-locked fiber laser based on carbon nanotube (CNT). By introducing a polarization-route-assisted module (PRAM) and finely adjusting the cavity birefringence, the soliton bunches can be flexibly manipulated to operate with controllable soliton numbers and positions. Moreover, the potential bandwidth resource provided by the concept of soliton bunches for the ease of breaks a new path for the enhancement of data-carrying capacity. Our research is helpful to extend the theory of vector soliton dynamics in fiber lasers and promotes the promising application in optical communication systems. [ABSTRACT FROM AUTHOR]

A near-infrared (NIR) supercontinuum and ultrashort pulses are generated via phase-mismatched cascaded frequency conversion in DSTMS (4-N, N-dimethylamino-4’-N’-methyl-stilbazolium 2, 4, 6-trimethylbenzenesulfonate) crystal. NIR pulses with a central wavelength of 1.5 μm and 63 fs pulse duration are incident into the crystal, and the spectrum is broadened to over 1200–2100 nm. Compensating the dispersion by fused silica, ultrashort NIR pulses with 16.6 fs pulse duration are obtained. Meanwhile, intense single-cycle terahertz pulses are generated via optical rectification in DSTMS crystal. [ABSTRACT FROM AUTHOR]

Nano-crystals were formed in the exposed regions of photo-thermo-refractive glass undergoing irradiation with zeroth order chirp-controlled ultrafast laser Bessel beams and subsequent heat treatment. Effects of various writing powers, pulse durations and heat treatment time on the distribution and the size of the nano-crystals were investigated. The results show that nano-crystals’ distribution depended on the laser power density spatial shape, while the size of the nano-crystals is quasi-independent. However, the average diameter of the nano-crystals was affected by the heat treatment time, decreasing from 175 to 105 nm with the time halved. In addition, using crystallographic characterization by X-ray diffraction, the nano-crystal composition in the laser-exposed regions was detected to be sodium fluoride.

We investigated the possibility of making ever-shorter optical pulses by using the nonlinearity of ambient air. We produced a broad spectrum consisting of mutually coherent optical sidebands via collinear Raman generation driven by two picosecond laser pulses that are Raman-resonant with molecular vibrations of nitrogen. We demonstrated the ability to adjust the sideband phases via dispersion control which we accomplished by changing the optical path length of the generated multi-color beam through a pair of tilted glass plates. The resultant measured phases suggest the generation of a 3-fs optical pulse train. [ABSTRACT FROM AUTHOR]

We demonstrate a scheme for correcting the angular dispersion of the 3.1- $\mu$m idler beam from a high repetition rate, KTiOAsO $_4$ (KTA) based noncollinear optical parametric amplifier pumped at 1.03 $\mu$m and seeded by broadband signal pulses at 1.55 $\mu$m. Following angular dispersion correction, successful temporal chirp compensation was possible, and a close to transform-limited pulse duration of 70 fs and a pulse energy of 30 $\mu$J at a repetition rate of 100 kHz were obtained. The angular chirp compensated KTA-based scheme is scalable to higher pulse energies and average powers and has various advantages compared to periodically poled and angle-tuned LiNbO$_3$ -based amplifiers, which exhibit low resistance to photorefractive damage.

An optical time-stretch flow imaging system enables high-throughput examination of cells/particles with unprecedented high speed and resolution. A significant amount of raw image data is produced. A highspeed cell recognition algorithm is, therefore, highly demanded to analyze large amounts of data efficiently. A high-speed cell recognition algorithm consisting of two-stage cascaded detection and Gaussian mixture model (GMM) classification is proposed. The first stage of detection extracts cell regions. The second stage integrates distance transform and the watershed algorithm to separate clustered cells. Finally, the cells detected are classified by GMM. We compared the performance of our algorithm with support vector machine. Results show that our algorithm increases the running speed by over 150% without sacrificing the recognition accuracy. This algorithm provides a promising solution for high-throughput and automated cell imaging and classification in the ultrafast flow cytometer imaging platform. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the imaging condition of asymmetric pulses on a temporal imaging (TI) system comprising two optical fibers and a phase modulator. We introduced a fitness function to evaluate the difference between target and output waveforms to quantitatively estimate TI characteristics. We successfully determined the optimal combination of phase modulation index and phase deviation to provide good TI by simulating the fitness function. We generated chirp-free pulses with symmetric and asymmetric waveforms using an in-line waveguide pulse synthesizer, i.e., an optical pulse synthesizer, based on pulse shaping and experimentally confirmed the TI condition in a time lens system. Moreover, TI was achieved not only at a theoretical value of the modulation index satisfying the TI condition but at a much smaller value than the theoretical value for a time-reversal mode as well. Measured waveforms of the experimental output agreed with simulation results. This may be important for practical applications because a TI system requires a much smaller modulation power.

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.

We investigated the possibility of making ever-shorter optical pulses by using the nonlinearity of ambient air. We produced a broad spectrum consisting of mutually coherent optical sidebands via collinear Raman generation driven by two picosecond laser pulses that are Raman-resonant with molecular vibrations of nitrogen. We demonstrated the ability to adjust the sideband phases via dispersion control which we accomplished by changing the optical path length of the generated multi-color beam through a pair of tilted glass plates. The resultant measured phases suggest the generation of a 3-fs optical pulse train.

The optical layout of soft X-ray grating compressors designed to provide both positive and negative group-delay dispersion (GDD) is discussed. They are tailored for chirped-pulse-amplification experiments with seeded free-electron laser sources. Designs with plane or concave gratings are discussed, depending on the sign of the GDD to be introduced. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We demonstrate a scheme for correcting the angular dispersion of the 3.1- $\mu$m idler beam from a high repetition rate, KTiOAsO $_4$ (KTA) based noncollinear optical parametric amplifier pumped at 1.03 $\mu$m and seeded by broadband signal pulses at 1.55 $\mu$m. Following angular dispersion correction, successful temporal chirp compensation was possible, and a close to transform-limited pulse duration of 70 fs and a pulse energy of 30  $\mu$J at a repetition rate of 100 kHz were obtained. The angular chirp compensated KTA-based scheme is scalable to higher pulse energies and average powers and has various advantages compared to periodically poled and angle-tuned LiNbO$_3$ -based amplifiers, which exhibit low resistance to photorefractive damage. [ABSTRACT FROM PUBLISHER]

We recently developed the tomographic ultrafast retrieval of transverse light E-fields (TURTLE) as a self-referenced method for retrieving vector-shaped ultrashort laser pulses. TURTLE combines three or more linear polarization projections of the pulse, which are measured with existing single-polarization pulse characterization techniques, to determine the full polarization state. We present in detail the two approaches for retrieving the full vector-shaped field: an analytic inversion and a parameter-fitting technique. By examining the impact of measurement noise on retrieval fidelity, we find that the fitting approach, which makes use of the additional redundancy inherent in some single-polarization measurements, is greatly superior to the analytic.

Adiagnostic systemusing three frequency-resolved optical gating (FROG) techniques-crosscorrelation, second harmonic generation, and self-diffraction-is reported for the reliable characterization of femtosecond laser pulses. The latter two FROG techniques are employed to evaluate suitability in measurements of the reference pulse. A train of optical pulses generated by the superposition of two femtosecond pulses emitting at 800 nmand 1180 nmhas been characterized by the cross-correlation FROG to evaluate the reliability of the present diagnostic system. [ABSTRACT FROM AUTHOR]

A spectral scanning filter method for decreasing the spectral phase modulation to improve the coherent intensity contrast in the chirped pulse amplification system is proposed and studied theoretically, which is made use of the frequency offset between the chirped signal pulse and the prepulse (or the postpulse). The spectral scanning filter based on the rapid Fabry–Pérot (F–P) interference for reducing the pre-pulse and post-pulse originated from the spectral phase modulation is analyzed theoretically and simulated numerically. The numerical simulation results show that the coherent pulse contrast can be improved by 0.41 F 2 for the chirped pulse and the compressed pulse, and the transmission efficiency of the chirped signal pulse can exceed 90% when the synchronizing time jitter ranges from −2 ps to 2 ps and the chirp rate parametric from −0.1 C / T 2 to 0.1 C / T 2 . [ABSTRACT FROM AUTHOR]

A prism compressor can compensate dispersion of femtosecond light pulses traveling in air for laser ranging. A general precise expression is acquired for the optical path length through a four-prism sequence in a special way. We improved the optical path calculating method to acquire the accurate expression of the group delay dispersion (GDD) of the prism compressor in arbitrary case, which apply to arbitrary incident angle and incident point conveniently. Influences of five parameters on GDD are simulated and analyzed for four kinds of material, respectively. The results demonstrate that, when experimental conditions change, GDD vary correspondingly, even from positive to negative, which are convenient to dispersion compensation of ultrashort laser pulses. When the incident angle and the apex angle of the prism are unsuitably selected, total reflection will occur in the prism, which should be avoided in application. [ABSTRACT FROM AUTHOR]

The speckle effect, generated by a liquid-crystal spatial light modulator (LCSLM), is studied using a femtosecond laser machining system. The speckle effect of the patterned beams produces hierarchical micro/nanostructures on the surface of polished stainless steel (SS 316L). The results show that the hierarchical micro/nanostructures are composed of micro-sized valley structures created by the LCSLM and nano-sized periodic structures induced by the femtosecond laser. By varying the azimuth of the pattern beams in the focal plane, the directions of the inner valley structures can be altered. As a result of these patterned beams with the speckle effect, the fabrication of hierarchical micro/nanostructures structures inside various shapes becomes a simple one-step procedure instead of a complex scanning process. These hierarchical micro/nanostructures can provide advantages for anti-friction, hydrophobic, and hydrophilic applications in surface treatment. [ABSTRACT FROM AUTHOR]

The state of the art of ultrafast Er:fiber technology is reviewed. Such lasers are increasingly used for generation of ultrabroadband and widely tunable pulse trains. Er:fiber sources prove to be flexible, compact and robust with important applications in fundamental and interdisciplinary sciences. After a short overview of different oscillator and amplifier designs the discussion focuses on coherent and tailored supercontinuum generation in highly nonlinear germanosilicate fibers. This approach enables a tuning range spanning from the visible to the mid infrared, synthesis of single-cycle light pulses and passive locking of the carrier-envelope phase. [ABSTRACT FROM AUTHOR]

• Reviews the feasibility of mono-elemental materials for saturable absorber. • Reviews the synthesis method and material characterization of mono-elemental materials. • Reviews the fabrication of saturable absorber and its nonlinear optical characteristics. • Reviews the integration of saturable absorber for near-infrared mode-locked fiber laser. • Discusses the issues and challenges of mono-elemental saturable absorber in near-infrared mode-locked fiber laser. Two-dimensional mono-elemental material is an excellent saturable absorber candidate with low saturation intensity, large modulation depth, high nonlinearities, and fast recovery time of excited carriers. Typically, these mono-elemental material with two-dimensional structure possesses tunable bandgap from metallic to semiconducting according to different number of layers. The integration of these materials as the saturable absorber has exploited the development of near-infrared mode-locked fiber lasers. Therefore, this review is intended to provide an up-to-date information to the development of mono-elemental saturable absorber for the advances in near-infrared mode-locked fiber laser, with emphasis on their material properties, synthesis process and material characterization. Meanwhile, issues and challenges of the review research topic will be highlighted and addressed with recommendations. [ABSTRACT FROM AUTHOR]

We demonstrate a simple method for generating high-speed pulse trains from a relatively low-speed pulse source by spectral filtering using a high-finesse fibre Fabry-Pérot interferometer. Reasonably stable 40 GHz and 100 GHz pulse trains are obtained at 1550 nm from a 10 GHz mode-locked fibre ring laser. [ABSTRACT FROM AUTHOR]

A compact optical parametric chirped pulse amplification system has been demonstrated, in which a novel double-pass optical parametric chirped pulse amplifier was introduced. The signal was double-pass amplified in the second nonlinear crystal by a long pump pulse, and the signal and pump pulses of each pass were completely phase matched in the plane of the maximum effective nonlinearity. The superfluorescence was markedly suppressed to less than 1% of the total output pulse energy with a total saturation gain of ∼3 × 107 and single pulse energy fluctuation less than 3% rms. [ABSTRACT FROM AUTHOR]

We demonstrate ultralow phase noise 10-GHz microwave signal generation from a free-running mode-locked Er-fiber laser with -142- and -157-dBc/Hz single-sideband absolute phase noise at 10- and 100-kHz offset frequencies, respectively. The absolute RMS timing jitter is 1.5 fs when integrated from 1-kHz to 5-GHz (Nyquist frequency) offset frequency. In the 10-kHz to 10-MHz integration bandwidth typically used for microwave generators, the RMS integrated jitter is 0.49 fs. The Er-fiber laser is operated in the stretched-pulse regime at close-to-zero dispersion to minimize the phase noise of extracted microwaves. In order to suppress the excess phase noise in the optical-to-electronic conversion process, we synchronize a low-noise voltage-controlled oscillator to the fiber laser using a fiber Sagnac-loop-based optical-microwave phase detector.

Nano-crystals were formed in the exposed regions of photo-thermo-refractive glass undergoing irradiation with zeroth order chirp-controlled ultrafast laser Bessel beams and subsequent heat treatment. Effects of various writing powers, pulse durations and heat treatment time on the distribution and the size of the nano-crystals were investigated. The results show that nano-crystals' distribution depended on the laser power density spatial shape, while the size of the nano-crystals is quasi-independent. However, the average diameter of the nano-crystals was affected by the heat treatment time, decreasing from 175 to 105 nm with the time halved. In addition, using crystallographic characterization by X-ray diffraction, the nano-crystal composition in the laser-exposed regions was detected to be sodium fluoride. [ABSTRACT FROM AUTHOR]

Using amplitude modulation in a semiconductor saturable absorber mirror induced by optical clock signal, we have synchronized 600-fs pulse train from a mode-locked thulium–holmium fiber laser operating at 1.99 \mu\m to an external reference source. The optical source based on 1.56-\mu\m distributed-feedback diode laser driven by the clock signal was used as a seed source. The amplified seed pulses launched into the fiber cavity synchronized the mode-locked pulses through amplitude modulation in the absorber. The synchronization method allows us to operate the mode-locked laser at multiples of cavity harmonics and to increase the repetition rate. [ABSTRACT FROM PUBLISHER]