A 18 mJ femtosecond Ti: sapphire amplifier at 100 Hz repetition rate

从目前 技术手段来看, 阿秒脉冲主要来源于强飞秒激光与气 体作用产生的高次谐波(high-order harmonic generation) [5~7] .从高次谐波的发射过程来看, 它实际由多个 具有相同时间间隔的阿秒脉冲(即阿秒脉冲链)构成.

In this letter, a sub-pm linewidth, high pulse energy and high beam quality microsecond-pulse 766.699 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At an incident pump energy of 824 mJ, the maximum output energy of 132.5 mJ at 766.699 nm with linewidth of 0.66 pm and a pulse width of 100 µs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 766.699 nm with pulse width of hundred micro-seconds for a Ti:sapphire laser. The beam quality factor M2 is measured to be 1.21. It could be precisely tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm. The wavelength stability is measured to be less than ±0.7 pm over 30 min. The sub-pm linewidth, high pulse energy and high beam quality Ti:sapphire laser at 766.699 nm can be used to create a polychromatic laser guide star together with a home-made 589 nm laser in the mesospheric sodium and potassium layer for the tip-tilt correction resulting in the near-diffraction limited imagery on a large telescope.

Beam quality M2 factor of the single-aperture coherent laser beam synthesis system is a major problem to be solved. Based on the definition of second-order moment, beam quality M2 factors of the coherently combined beam supposed by the basic model TEM00, TEM01and TEM10 are presented by some analytical expressions in this paper. And, the effects produced by the beam waist width, the propagation distance, the ratio of the two amplitudes and the distance between the two coherent sources on the beam quality M2 factor are numerically analyzed. Some results show that the beam quality M2 factor of the coherently combined beam is unchanged when the propagation distance parameter d1100. And the experiment is carried to verify some theories of the two TEM00 coherently combined beam.

In this letter, a high pulse energy and high beam quality 819.7 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At incident pump energy of 774 mJ, the maximum output energy of 89 mJ at 819.7 nm with a pulse width of 100 μs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 819.7 nm with pulse width of hundred microseconds for a Ti:sapphire laser. The beam quality factor M2 is measured to be 1.18. This specific wavelength with the high pulse energy and high beam quality at 819.7 nm is a promising light source to create a polychromatic laser guide star together with a home-made 589 nm laser via exciting the sodium atoms in the mesospheric atmosphere.

<p indent="0mm">Attosecond light source is a new type of light source that was born at the beginning of the 21st century, which has a short pulse, broad spectrum, high temporal and spatial coherence and wide tunability, thus being widely employed in various research fieds. From ultrafast motion of electrons in atoms to charge transfer in biological macromolecules, attosecond pulses is currently the only tool that can track and capture these ultrafast dynamics. Attosecond pulse enables us to investigate ultrafast dynamics of micro-world both in nanometer and attosecond scales. However, the mechanism of attosecond pulse generation is completely different from general ultrafast lasers. Instead, attosecond pulse is generated by a highly nonlinear interaction between strong ultrafast femtosecond laser and matter, which is called high-order harmonic generation (HHG). The mechanism of HHG can be understood by a classical three step model. First, an electron is ionized from an atom by a strong laser electric field through tunnel ionization. Then the free electron is accelerated by the laser field and gains energy. Finally, the electron recombines with the parent ion when the laser field changes its sign with emission of a photon, whose energy equals its kinetic energy gained in the laser field plus ionization potential of the atom. Apparently, HHG is strongly affected by the laser waveform. Several key parameters of driving laser, such as wavelength, intensity, and carrier envelope phase strongly affect the process of HHG. Thus, the characteristics of attosecond pulses are determined by the driving laser. The rapid development of attosecond pulse technology strongly depends on the development of driving laser technology. In the beginning, chirped pulse amplification (CPA) technology greatly promoted the development of attosecond light sources. The femtosecond CPA systems based on Ti:sapphire crystal has been the main driving laser to generate attosecond light pulses. The driving laser wavelength is in the near infrared region near <sc>800 nm,</sc> which generally has a pulse duration of multiple optical cycles. By employing post-compression technology to shorten pulse durations of CPA systems to few-cycle, isolated attosecond pulses in the extreme ultraviolet (XUV) can be generated, which is called the first generation of attosecond light source. Recently, optical parametric amplification (OPA) systems have been widely used as driving laser due to its flexible wavelength tunability. Using OPA, longer driving laser wavelength up to the midinfrared (MIR) can be obtained, which pushes the attosecond pulses to the soft X-ray region, and has been called the second generation of attosecond light sources. Due to broad spectrum and higher photon energy, attosecond pulses have a shorter duration in the soft X-ray region compared with XUV region, given that the temporal chirp is properly compensated. In 2017, reseachers generated soft X-ray isolated attosecond pulses, which was driven by mid-infrared pulses centered at <sc>1.8 µm.</sc> These attosecond pulses, whose duration reaches 53 as and spectrum surpasses carbon K-edge, provide a tool for studying the ultrafast dynamics of diamond, graphene and other carbon materials. In this paper, we start with the principle of attosecond pulses generation based on HHG. Then, we introduce different technologies and their development for obtaining driving laser pulses for HHG. Finally, we introduce prospect on development of driving laser pulses for attosecond pulses generation.

<p indent="0mm">Since the laser was invented, harmonic generation and wavelength conversion have been important research topics in nonlinear optics regime. In 1987, McPherson A et al. used sub-picosecond KrF laser <sc>(248 nm</sc> wavelength) to interact with noble gas to obtain high-order harmonics for the first time. In 2001, the first single attosecond <sc>(10^–18 s)</sc> pulse with a pulse width of 650 as was measured experimentally by Krausz et al. by using the broadband high-order harmonic generation. Since then, high-order harmonics and attosecond pulses have been widely used in atomic and molecular physics, materials science and other fields. However, due to the low efficiency of attosecond pulses generated by the gas media, the energy of attosecond pulses is limited, which limits the detection methods of attosecond time dynamics research (currently mainly IR+XUV pumping/detection) and its applications in many fields. How to obtain high-brightness, high-energy attosecond pulses has always been the pursuit in this field since the generation of the first attosecond pulse. With the development of femtosecond laser technology, the output peak power of the femtosecond laser system has been rapidly increased in the past two decades. At present, the laser system can output the pulse with a peak power of up to 10 PW (1 PW<sc>=10¹⁵ W).</sc> Its intensity can be much higher than <sc>10¹⁸ W/cm²</sc> and its interaction with the overdense plasma can efficiently generate high-order harmonics at the level of hundreds of eV or even keV, which brings new opportunities for the development and application of high-order harmonics and attosecond pulses. Generally speaking, the interaction of high peak power femtosecond laser pulses with solid-density plasmas has unique advantages in high-luminance, high-energy high-order harmonics and attosecond pulse generation. In the ELI-ALPS plan, solid plasma high-order harmonics and attosecond pulse generation are very important parts, especially to promote their high brightness and their high photon energy. In terms of the attosecond pulse energy, the goal of their first phase is that the attosecond pulse energy reaches the μJ level, which will be advanced to the mJ level in the next phase, and the photon energy of the attosecond pulse will reach a few keV. This review gives a brief introduction to its physical mechanisms and their current experimental research progress. Three mechanisms (coherent wake emission (CWE), relativistic oscillating mirror (ROM), coherent synchrotron emission (CSE)) will be discussed. They are working under different laser intensity. CWE can emit high efficiency harmonics at the laser intensity below <sc>10¹⁸ W/cm².</sc> But when the peak laser intensity is very high, the research of the CSE mechanism shows the generation of high-luminance high-order harmonics under super-relativistic light intensity, which can be used to produce extremely high field-strength pulse of attoseconds, even tens of zeptoseconds. If the laser parameters and the plasma condition can be well controlled, the laser pulse may even generate higher harmonic radiation with field strength much higher than the pump laser field strength, which may be a possible way to study the 39th question (“What is the most powerful laser researchers can build? ”) among the 125 most challenging scientific questions published by <italic>Science</italic> on its 125th anniversary.

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

Time-resolved measurement enables us to reveal the dynamical evolution of the system. The ultrashort laser and pump-probe method provide a powerful tool for achieving the time resolved measurement. With the development of laser technology, e.g., Q -switching, mode-locking, chirped pulse amplification and the application of the Ti:Sapphire crystal, the pulse width of laser is getting shorter and shorter, and the pulse energy is getting higher and higher. It has brought tremendous changes to the field of optics and photonics. For example, in the 1980s, femtosecond (1 fs =10–15 s) lasers have been used to study the evolution of molecular intermediate states during chemical reactions, which revealed the dynamics of chemical reactions at the molecular level. Nevertheless, the electron inside atoms and molecules moves even faster on a timescale of “attosecond” (1 as =10–18 s). At the dawn of the 21st century, attosecond pulses were produced from the high harmonics generated in the interaction between intense femtosecond laser and novel atoms. In 2001, an isolated attosecond pulse with a duration of 650 as was produced in the experiment via a 7 fs few-cycle driving laser. In the same year, an attosecond pulse train with a duration of 250 as for single pulse was produced by a multicycle driving laser. These results marked that the pulse width of the laser broke through the femtosecond timescale, and opened the door to attosecond science. In the next decades, lots of ultrafast processes inside atoms and molecules, e.g., tunneling ionization, Auger decay of atoms and electron localization and internal charge transfer of molecules, have been invested by pump-probe measurement combining the attosecond pulses and few-cycle near infrared laser pulses. In addition, the applications of attosecond pulses have been gradually extended from gas to solid materials. By using the attosecond pulse, it is possible to manipulate the electron motion in the attosecond time scale, which corresponds to an extremely high frequency rate of petahertz. Such manipulation is considered to be the basis for developing the photoelectric information processing at petahertz. Attosecond science has become one of the most important achievements in the field of ultrafast optics in the past two decades. The crucial issue is the generation and control of attosecond laser pulses. Many schemes have been demonstrated to produce the isolated attosecond pulses. The most popular scheme is few-cycle driving laser, which requires carrier-envelope phase stabilized ultrashort laser pulse. On the other hand, since the generation efficiency of the attosecond pulse depends on the driving laser ellipticity, a polarization gating scheme has also been developed, which was later generalized to double optical gating. Another scheme is the two-color field scheme, which synthesizes the laser pulses with different wavelengths. The advantage of this method is that multi-cycle driving laser pulse can be used to produce isolated attosecond pulse. Recently, this method has been extended to the three-color cases. In addition, ionization gating and lighthouse schemes are also demonstrated in experiment. With these methods, the pulse duration of the attosecond pulse decreases gradually. At present, the shortest pulse width has reached 43 as and high power isolated attosecond pulse with a peak power of 2.6 GW has been produced. But how to improve the attosecond pulse energy and generation of circularly polarized attosecond pulses are still the hot topics currently.

High energy, high repetition rates laser with excellent beam quality are fundamental components for LiDAR. In particular, high pulse energy is benefit for long distance range imaging. Narrow pulse width and high repetition rates are benefit for high resolution and high refresh rate operation of LiDAR. However, increasing the laser repetition rates will cause the pulse energy decreasing and the pulse width expanding. In order to achieve the requirements at the same time, we propose a 885 nm laser diode end‐pumped RTP Q‐switched Nd:YAG laser. Pulse pumping scheme was used to avoid heat accumulation. With a master oscillator power amplifier (MOPA) system, 40.5 mJ, 7.5 ns of 1064 nm pulses were obtained at 2 kHz. The beam quality factor M2 was measured as 1.60/1.51. Moreover, by frequency doubling with a LBO crystal, 16.4 mJ, 6.5 ns of 532 nm laser with the M2 factor of 1.52 and 1.50 was obtained. The 1064 and 532 nm pulses are suitable for avalanche photodiode arrays LiDAR and streak tube imaging LiDAR, respectively. The laser system is shown to accomplish high stability from temperature of −10°C to 30°C and vibration frequency of 10 to 100 Hz.

In this work, we confirm a Pr3+:LiYF4 pulsed laser with high power and high energy at 639 nm based on the acousto-optic cavity dumping technique. The maximum average output power, narrowest pulse width, highest pulse energy and peak power of the pulsed laser at a repetition rate of 0.1 kHz are 532 mW, 112 ns, 5.32 mJ and 47.5 kW, respectively. A 639 nm pulsed laser with such high pulse energy and peak power has not been reported previously. Furthermore, we obtain a widely tunable range of repetition rates from 0.1 to 5000 kHz. The diffracted beam quality factors M2 are 2.18 (in the x direction) and 2.04 (in the y direction). To the best of our knowledge, this is the first time that a cavity-dumped all-solid-state pulsed laser in the visible band has been reported. This work provides a promising method for obtaining high-performance pulsed lasers.

We report on a SESAM-modelocked Yb:CaF2 thin-disk oscillator designed to generate more than 1 μJ of pulse energy at a moderate pulse repetition rate. The goal of our experiment was to explore the potential of Yb:CaF2 in a thin-disk laser (TDL) architecture for high power at pulse durations shorter than 300 fs as compared to other Yb-doped crystals exhibiting broad gain bandwidth. At a repetition rate of 10 MHz the laser produced an average output power of up to 17.8 W (1.78 μJ of pulse energy) with a beam quality factor M2 below 1.2. The pulse duration was measured to be 285 fs, which results in a peak power of 5.5 MW. To the best of our knowledge, this is the highest pulse energy and peak power demonstrated to date with sub-300 fs pulses generated by SESAM-modelocked oscillators, leading to the conclusion that Yb:CaF2 is a very promising crystal for TDL technology.

It is standard to use and to characterize the beam quality of a non-circular symmetrical beam on its x-axis and y-axis orientation. However, we knew that the values of and are inconsistent if one selects a different coordinate system or measures beam quality with different experimental conditionals, even when analyzing the same beam. To overcome this, a new beam quality characterization method, the M2 factor matrix, is developed. It not only contains the beam quality terms, and , to characterize the beam quality along x-axis and y-axis orientation for the non-symmetric beam, but also introduces two additional cross terms, Mxy and Myx, which are used to characterize the location relationship between the principal axis of the test beam and coordinate system in experiment. Moreover, M2 factor matrix can be measured with a similar procedure to the traditional M2 factor whose measurement instructions are described in ISO11146 by adding some additional image and signal processing procedure. The measurement principle and method is present and the experiment system for beam quality M2 factor matrix is built to demonstrate the performance of M2 factor matrix with real experiments.

A real-time complex amplitude reconstruction method for determining the dynamic beam quality M2 factor based on a Mach-Zehnder self-referencing interferometer wavefront sensor is developed. By using the proposed complex amplitude reconstruction method, full characterization of the laser beam, including amplitude (intensity profile) and phase information, can be reconstructed from a single interference pattern with the Fourier fringe pattern analysis method in a one-shot measurement. With the reconstructed complex amplitude, the beam fields at any position z along its propagation direction can be obtained by first utilizing the diffraction integral theory. Then the beam quality M2 factor of the dynamic beam is calculated according to the specified method of the Standard ISO11146. The feasibility of the proposed method is demonstrated with the theoretical analysis and experiment, including the static and dynamic beam process. The experimental method is simple, fast, and operates without movable parts and is allowed in order to investigate the laser beam in inaccessible conditions using existing methods.

We demonstrated an efficient, high-power Ho:YAG master-oscillator power amplifier (MOPA) system and investigated its thermal-birefringence-induced depolarization. The maximum output power was 450 W with a depolarized power of 32.1 W and depolarization of 0.071 via three power amplifiers. To our knowledge, this is the highest average power generated from a Ho:YAG MOPA system. In theory, a simplified model was built to calculate the depolarization in the amplifier, and the theoretical results agreed with the actual value well. Moreover, the overall optical-to-optical efficiency of the MOPA system was near 60%, and the beam quality M2 factors of s-polarized laser were measured to be ∼ 1.8 at 400 W. In pulse operation, the per pulse energy was ∼ 11 mJ at the pulse repetition frequency of 40 kHz with the corresponding peak power of 220 kW.

Isolated attosecond pulses make it possible to study and control the ultrafast electron processes in atoms and molecules. High order harmonic generation (HHG) is the most promising way to generate such pulses, benefiting from the broad plateau structure of the typical HHG spectrum. In previous HHG studies on the polarization gating scheme, atomic ionization caused by the laser cycles before the polarization gate not only places a limit on the pulse width and intensity of the driving laser, but also affects the phase matching of harmonics generated in a polarization gate. According to these, in this paper we propose a new double optical gating scheme, in which the polarization of the laser pulse changes from linear to elliptical and back to linear again. Thus, only the linearly polarized field in the leading of the pulse contributes to high harmonic generation. By using a strong field approximation theory, we first simulate high order harmonic and attosecond pulse generation from helium atom irradiated by a double optical gating pulse based on the orthogonal polarization field. Here the orthogonal polarization field consists of two linearly polarized pulses with a certain time delay, orthogonal polarization directions and equal amplitudes. And for the double optical gating pulse, the second harmonic of the driving field is added to an orthogonal polarization field with an appropriate phase and energy. It is found that the high harmonic spectrum with higher efficiency and supercontinuum plateau is obtained by reasonably adjusting the parameters of the combined pulse. After inverse Fourier transform, an isolated 143-as pulse with higher intensity can be realized by superposing supercontinuum harmonics from the 50th to the 150th order. Compared with the double optical gating scheme proposed by Chang et al. (Zhao K, Zhang Q, Chini M, Wu Y, Wang X, Chang Z 2012 &lt;i&gt;Opt. Lett.&lt;/i&gt; &lt;b&gt;37&lt;/b&gt; 3891), our scheme not only overcomes the limit on the pulse duration and intensity of the incident pulse laser, but also avoids the harmonic phase mismatching in the process of the propagation due to unwanted ionization of the gas target caused by the laser cycles before the polarization gate.