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Low noise microwave frequency has important applications in radar, long baseline interferometer and other fields. The phase noise of microwave signal generated by optical frequency comb is lower than –100 dBc/Hz at 1 Hz frequency offset and –170 dBc/ Hz at high frequencies (> 100 kHz), which is the lowest in the noise produced by all existing microwave frequency generation technologies. This paper introduces the basic principle of optical frequency comb generating microwave frequency, analyzes and summarizes various kinds of noise of microwave frequency signals and noise suppressing technologies. Then the low noise measuring methods are introduced, and several typical experimental devices generating microwave frequency and the obtained results are described. With the continuous improvement of optical frequency comb and noise suppression technology, microwave frequency source with very low noise will have wider application prospects and application fields.

With the unveiling of molecular and atomic dynamics, scientists crave finer and faster tools to communicate with the microworld. Attosecond pump-probe enjoys its reputation as the fastest camera, hinting ultrafast movements in the delay graph. To employ this camera, the stability and delay control should have very great accuracy comparable to the camera resolution. It is also of significant importance for stabilizing the carrier envelope phase (CEP) in few-cycle laser field. When dealing with a huge quantity of data, conventional Fourier transform algorism is challenging in high-speed control. Here we put forward the efficient calculation method, fast Fourier transform (FFT) algorism in Mach-Zehnder interferometer for arm length locking and f-2f for CEP locking. In the interferometer locking, 532 nm continuous wave laser is used in the Mach-Zehnder interferometer, and the phase of the FFT term corresponding to the delay between the two arms of the interferometer serves as a feedback signal on piezo transducer (PZT) in the delay line to reduce the change of the arm length. In the CEP control experiment, data to be analyzed are the f-2f spectrum interference fringes recorded by the spectrometer. The CEP values are obtained from the first order of FFT module output of the integrated spectrum interference fringes, and a labview program examines the relative phase drift and sends a feedback voltage signal to the PZT through the proportion integration differentiation module to compensate slow CEP drift after the chirped pulse amplification system by changing the insert length of a prism pair. The results show that the root mean square (RMS) of the arm length difference is 1.24 nm (4.1 attosecond for light to travel) per meter in the interferometer locking over 12 h, and the RMS of CEP is 227 mrad under 3 ms integration time in the CEP locking over 20 min. These results are able to meet the requirement of the accuracy for attosecond pulse generation and attosecond pump-probe experiments. We also use FFT to stabilize the CEP and relative time simultaneously in the waveform synthesis for 8 h (Huang P, Fang S, Gao Y, Zhao K, Hou X, Wei Z 2019 Appl. Phys. Lett. 115 031102), the phase-locking system results in a CEP stability of 280 mrad and a relative time stability of 110 as at a repetition rate of 1 kHz. These results imply that the FFT is versatile and reliable in ultrafast control.

Femtosecond pulse measurement of ultrafast spectrum is one of the important research directions in the ultrafast laser field. The conventional femtosecond pulse autocorrelation method is implemented by measuring the autocorrelated frequency-doubling signal, and the frequency-doubling signal has wavelength selectivity, so the femtosecond pulse measurement for the case of different central wavelengths needs to replace different frequency-doubling crystals, which is very inconvenient. This paper reports a kind of modified transient grating frequency resolution optical gating for measuring the femtosecond pulses. The method combines frequency-resolved optical gating (FROG) method with four-wave mixing. Its basic process is to divide the pulse to be measured into three beams. Two of the pulses can reach spatiotemporal coincidence on optical medium through precise delay control and focus. The other pulse interacts with the transient grating, and serves as the detection light to produce signal light. The spectrum and delay time of the signal light are measured by a spectrometer, and the spectrum and electric field information of the femtosecond pulse to be measured are obtained through the inversion iterative algorithm. Because this method only needs the power density of the measured light to reach the third-order nonlinear effect, it can be applied to the femtosecond pulse measurement of any central wavelength. We use this method to measure the femtosecond pulses with the central wavelengths of 800 nm and 400 nm respectively, and the ultra-wide spectrum femtosecond pulses with the period magnitude of sub-10 fs, and compare the measurement results with the results obtained with the conventional interferometric autocorrelation instrument. They are basically consistent. The experimental results show that our frequency-resolved optical switching method based on transient grating is very effective for measuring the femtosecond pulses with different central wavelengths and pulse widths.

Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of θ = 90° and φ = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of θ = 42.2° and φ = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.

The accurate and precise controlling of the attosecond time delay between the sub-pulses within a hundredth of an optical cycle is the key ingredient for the sophisticated custom-tailored coherent waveform synthesizer. The attosecond delay control technique commonly experiences the “complete” characterization of the ultrashort sub-cycle pulses, which includes the spatiotemporal pulse characterization of the synthesized waveform and the attosecond relative delay between the parent pulses. In this work, the relative time delay between spectrally separated ultrashort parent pulses is characterized in an interferometer scheme with a background-free transient-grating frequency-resolved optical grating (TG-FROG). The TG-FROG geometry accurately measures the full time-dependent intensity and phase of ultrashort laser pulses in a wide range of regime (from ultraviolet to infrared) and offers significant advantages over other nonlinear-optical processes geometries (i.e., the polarization-gate-FROG, the self-diffraction-FROG, the second-harmonic generation-FROG and the third-harmonic-generation-FROG). The attosecond measurement accuracy is achieved for the first time, to the best of our knowledge. In this experiment, the output of a carrier-envelope-phase-stable Ti:sapphire amplifier (sub-30-fs, over-1-mJ, 1 kHz) is spectrally broadened in a neon-filled hollow-core fiber with an inner diameter of 250μm. The transmission through the pressure-gradient hollow-core fiber results in an mJ-level octave-spanning whitelight supercontinuum, supporting a sub-3-fs Fourier transform-limited pulse. The supercontinuum is spectrally divided into two parent pulses by using a dichroic mirror. The sub-pulses are individually compressed by the custom-designed double-chirped mirrors and wedge pairs. The short and long wavelength pulses are separately compressed in few-cycle regime, yielding pulses with 6.7 fs and 9.8 fs, respectively. This technique overcomes the bottlenecks in the traditional delay measurement and should be applicable for many ultra-broadband pulse characterizations with extremely simple and alignment-free delay control device used. Furthermore, this new method will be easily adapted for the ultra-broadband two-dimensional electronic spectroscopy, the advanced temporal cloaking, and the field of sub-cycle arbitrary coherent waveform synthesizer for controlling strong-field interactions in atoms, molecules, solids, and nanostructures. We foresee that in the near future this novel technology will be very attractive for various applications in the next-generation light sources such as the Synergetic Extreme Condition User Facility in Beijing, China.

Coherent synthesis of laser pulses is a major trend in the generation of ultrafast pulse field. There is no good way to compensate for the whole spectrum when the spectrum of ultrashort pulses is wide enough to reach an octave, so it is difficult to realize a sub-cycle pulse in a single-path laser system even if the spectrum range is wide enough. In this paper, 0.8 mJ, 30 fs laser pulses at 1 kHz repetition rate with 790 nm center wavelength from a Ti:sapphire chirped pulse amplifier (CPA) system are focused into hollow fiber with an inner diameter of 250 μm and a length of 1 m to produce an octave-spanning white-light supercontinuum (450-950 nm). Using this supercontinuum, we conduct two sets of comparative experiments. 1) We split the supercontinuum into two pulses with different spectrum ranges (450-750 nm and 650-1000 nm) by a dichroic mirror (HR, 500-700 nm; HT, 700-1000 nm), and we compress the two pulses by the double-chirped mirrors and wedge pairs to generate two few-cycle pulses:the long and short wavelength yielding pulses are 7.9 fs and 6.1 fs, respectively. Then we coherently synthesize two pulses by using another dichroic mirror, and controlling the relative time delay between the two pulses, and thus we synthesize a pulse of 4.1 fs. 2) We directly compress the supercontinuum by the double-chirped mirrors and wedge pairs, and obtain an optimization result of 5.3 fs, of which the pulse duration is wider than that in experiment 1. In these comparative experiments, the advantage of coherent synthesis for shorter pulse duration is preliminarily verified. Besides, the balanced optical cross-correlator technique is used to lock the relative time delay between two pulses. The root-mean-square value of relative time delay drift is less than 80 as in the case with feedback control loop, which ensures the stability of coherent synthesis system. This scheme can be adopted to accurately compensate for the dispersion and obtain the sub-cycle synthesized pulse, which is useful for generating the high harmonic and atto-second pulse.

When a short laser pulse passes through transparent medium, the spectrum may be broadened due to nonlinear optical effects, and a coherent octave supercontinuum may be generated under certain conditions. Such a supercontinuum may be compressed into a femtosecond few-cycle pulse, which has many applications in ultrafast optics and beyond. Spectral broadening has been achieved experimentally in gases, liquids, and solids. Current mainstream technique of supercontinuum generation is to send multi-cycle femtosecond pulses through inert-gas-filled hollow-core fibers. However, due to the limitation of the core diameter, the hollow-core fiber cannot work with high-energy laser pulses. With a much higher nonlinear index of refraction, solid-state material is naturally a more promising candidate for supercontinuum generation, but it is difficult to obtain a near-octave spectrum in one piece of solid without filamentation. The optical Kerr effect in solids triggers self-phase modulation (SPM) which induces desired spectral broadening as well as self-focusing, thus causing the laser intensity to rise drastically with substaintial multiphoton excitation and ionization leading to plasma formation. This behavior results in filamentation and optical breakdown, and eventually permanent damage to the material occurs if the laser pulse energy is high enough. Using a thin plate of dielectrics may minimize the effect of self-focusing-the beam exits from the nonlinear medium before it starts to shrink and causes damage. However, one thin plate does not provide enough nonlinear effect to generate a broad spectrum. To prevent disastrous self-focusing while achieving spectral broadening, using multiple Kerr elements has been proposed theoretically and demonstrated experimentally at microjoule to millijoule level. In such a configuration, a femtosecond laser pulse is being spectrally broadened via SPM in the thin plates, while self-focusing converges the beam in each plate but the focal spot is located outside the plate. Once the converging beam passes through its focal spot in air, the beam diverges and enters the next plate to repeat this process until the spectral broadening stops after several elements. Using this method, octave supercontinuum with energies at microjoule to millijoule level has been experimentally obtained in a spectral range covering near-ultraviolet to mid-infrared. In this paper, we review the development of supercontinuum generation in multiple thin solid plates, outline the principle of supercontinuum generation in this new type of thin solid medium, brief the experiments using this new method in recent years, and look into the prospects for its development.

This paper reports that a 5-cm length birefringent photonic crystal fibre is used to tune the output frequency of unamplified 10-fs Ti:sapphire pulses. The zero dispersion of the fibre is at 823 nm and 800 nm for slow and fast fundamental modes, respectively. It is demonstrated that efficient upshift of the output frequency can be achieved when the pumped radiation is polarized along the slow axis of the fibre. When the average input power reaches 320 mW, about 60% of the output energy is located in one peak at 600 nm and is accompanied by depletion of the pulse inside the anomalous dispersion region.

A compact femtosecond Ti:sapphire laser resonator consisting of three chirped mirrors and one output coupler was designed. By accurately balancing the intracavity dispersions between Ti:sapphire crystal, air and chirped mirrors, we directly generated the laser pulse shorter than 7 fs at the average power of 340 mW with 3.1 W pump. The repetition rate of the laser oscillator is 173 MHz at the centre wavelength of 791 nm, and the ultrabroaden spectrum covers from 600 nm to 1000 nm. To the best of our knowledge, this is the simplest laser resonator capable of generating sub-10 fs laser pulse.

A novel reflected terahertz-emission microscopy is proposed and developed for improving the spatial resolution of THz imaging. When attaching a bow-tie antenna directly onto a thin generation crystal, the reflected THz waves can be collected and detected by a photoconductive antenna, and the spatial resolution is decided by the diameter of focused pump beam. In this way, the detected resolution can be largely improved and tunable. The configuration and characteristics of this microscopy are described in detail.

We report the properties of a compact diode-pumped continuous-wave Nd:GdVO4 laser with a linear cavity and different Nd-doped laser crystals. In a 0.2 at.% Nd-doped Nd:GdVO4 laser, 1.54 W output laser power is achieved at 912 nm wavelength with a slope efficiency of 24.8% at an absorbed pump power of 9.4 W. With 0.3 at.% Nd-doping concentration, we can obtain the either single-wavelength emission at 1064 nm or 912 nm or the dual-wavelength emission at 1064 nm and 912 nm by controlling the incident pump power. From an incident pump power of 11.6 W, the 1064 nm emission between 4F3/2 and 4I11/2 is suppressed completely by the 912 nm emission between 4F3/2 and 4I9/2. We obtain 670 mW output of the 912 nm single-wavelength laser emission with a slope efficiency of 5.5% by taking an incident pump power of 18.4 W. Using a Nd:GdVO4 laser with 0.4 at.% Nd-doping concentration, we obtain either the single-wavelength emission at 1064 nm or the dual-wavelength emission at both 1064 nm and 912 nm by increasing the incident pump power. We observe a strong competition process in the dual-wavelength laser.

The characteristics of hot electrons produced by p-polarized femtosecond laser-solid interactions are studied. The experimental results show that the outgoing electrons are mainly emitted in three directions: along the target surface, the normal direction and the laser backward direction. The electrons flowing along the target surface are due to the confinement of the electrostatic field and the surface magnetic field, while the electrons in the normal direction due to the resonant absorption.

We investigate the effects of confined laser ablation on laser plasma propulsion. Compared with planar ablation, the cavity ablation provides an effective way to obtain a large target momentum and a high coupling coefficient. When laser pulses are focused into a cavity with 1 mm diameter and 2 mm depth, a high coupling coefficient is obtained. By using a glass layer to cover the cavity, the coupling coefficient is enhanced by 10 times. Meanwhile, it is found that with the increase of the target surface size, the target momentum presents a linear increase.

Long plasma channels induced by femtosecond laser pulses in air are diagnosed using the sonographic method. By detecting the sound signals along the channels, the length and the electron density of the channels are measured. Refocusing is also observed at different laser energies and different focal lengths. We find that the sonographic method has manifest advantages compared to other techniques.

Spatial distribution of hot electrons with energies above 50 keV are investigated by an ethanol micro-droplet spray irradiated by linearly and elliptically polarized 150 fs laser pulses at an intensity of 1016W/cm2. Two symmetric hot electron jets with respect to the laser propagation direction are observed in the polarization plane for a linearly polarized laser field and in the plane of the long electric vector for an elliptically polarized laser field, respectively. Particle-in-cell simulations suggest that the resonance absorption on the spherical surface of the droplets is mainly responsible for the generation of the double-jet emission of hot electrons.

Ultra-short and ultra-intense laser is one of the hottest research spot of laser technology and strong field physics, due to its challenging and the frontier application research. As the key specification of ultrafast ultrahigh intensity laser pulse, the contrast ratio is very influential on the effect of laser-matter interaction. To perform the laser-matter interaction experiments at a high power level, the contrast is required to be as high as 1010 to prevent preplasma dynamics. To solve these problems, one has proposed many methods to improve the contrast of ultrafast laser, such as using the saturable absorbers, double chirped pulse amplification, plasma mirrors and the cross-polarized wave (XPW) generation. The XPW technology can not only enhance the contrast of the pulse by 3-4 orders of magnitude without introducing any space dispersion, but also extend the output spectrum to support shorter pulse duration. The XPW is a nonlinear filter technique in third-order nonlinear crystal with anisotropic susceptibility. Because of its simple and all-solid-state structure, the XPW technique has become one of the most effective methods to enhance the temporal pulse contrast and deliver shorter pulse duration in the field of high peak-power ultrafast lasers. This method has been used in many large laser facilities under construction or upgrades, such as the Apollon and ELI, the contrast ratio as high as 1010 has been achieved. It is known that the conversion efficiency and spectral characteristics of XPW have a strong dependence on the spatial and temporal magnitudes of the input driving pulse. In our experiment, it is found that the various changes of the driven pulse properties have different influences on the characteristics of XPW pulses. The relationship between the linear dispersion of driven pulse and temporal property of XPW is investigated theoretically. In addition, an experiment on verifying the theory is conducted by taking advantage of a programmable acousto-optic dispersion filter. The experimental results fit well to the theoretical results while some new phenomena emerge when the intensity in the BaF2 crystal reaches a saturation threshold. The spectral broadening capability of XPW becomes stronger and exceeds a theoretical upper limit. The pulse width can also be compressed to shorter than the theoretical limit. It is found that there are significant differences in spectral shape and conversion efficiency between the XPW signals by applying the opposite linear chirps to the driving pulse. A further analysis and theoretical explanation of these new phenomena are also presented.

With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system. High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit. Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.

Intense few-cycle pulses are widely used in transient light synthesis,high harmonic generation (HHG) and especially in isolated attosecond pulse generation.To obtain intense few-cycle pulses,the intense supercontinuum is needed.The traditional way to generate intense supercontinuum is using rare gas filled hollow-core fibers.Since the input energy of hollow-core fiber system is limited to a level of tens of mJ,it is necessary to find new ways to achieve energy scaling.In this paper we demonstrate the efficient generation of supercontinuum by solid thin plates,compression and its application in HHG. The Ti:sapphire laser used in the present experiment emits 0.8 mJ in energy with a duration of 30 fs at 1 kHz.After passing through a 3:1 telescope,the beam has a diameter changed from 12 mm to 4 mm.Then the laser is focused by an f=2000 mm lens into a 600 m-diameter spot.After propagating through 7 fused silica plates placed at Brewster's angle (55.5) with a thickness of 0.1 mm,the 0.7 mJ octave spanning supercontinuum is achieved,corresponding to an efficiency of 87.5%.The first three plates are placed at 31,11,2.5 mm in front of the beam waist,and the last four plates are placed at 2,7,12,17 mm behind the beam waist respectively.With a pair of wedges and 4 pairs of chirped mirrors,the 0.68 mJ supercontinuum is compressed to a duration of 6.3 fs,which is measured by TG-FROG. The 0.5 mJ,6.3 fs pulse is used to perform high-harmonic generation experiment.The beam diameter is 150 m when focused by an f=400 mm lens,with a laser intensity of 8.11014 W/cm2.The 1 mm Ne gas jet is used to perform HHG experiment with a back pressure of 300 mbar.To block the near-infrared light,a 150 m Zirconium foil is placed behind the gas jet.Then the XUV spectrum is detected by a spectrometer,which consists of a flat field grating and a CCD camera.For driving pulses of few-cycle regime without dispersion,the cutoff spectrum of HHG is continuous.But when the pulse is stretched by positive or negative dispersion,the cutoff spectrum turns discrete.The HHG result is that the cutoff region is continuous when the wedge is in a certain place.Then by increasing or reducing the insertion of the wedge,the cutoff spectrum becomes discrete.Our result is consistent with HHG generated by few-cycle pulses. In conclusion,we demonstrate high-harmonic generation based on supercontinuum generated by solid thin plates. The 0.7 mJ supercontinuum is achieved when 0.8 mJ pulses are injected to 7 thin fused silica plates.The supercontinuum is compressed to 0.68 mJ,6.3 fs.The 0.5 mJ,6.3 fs pulse is used to perform HHG experiments.The HHG result was consistent with few-cycle driving pulses.Our research indicates that solid state supercontinuum has great potential applications in HHG and isolated attosecond pulse generation.