Abstract:The optical frequency comb (OFC) has experienced more than 20 years of rapid development since 1999. From the initial laboratory system to the widely used high-tech products, the optical frequency comb based on femtosecond laser has played an irreplaceable role in many frontier research fields, such as frequency metrology, ultrafast spectroscopy, optical frequency standards, generation of attosecond pulses, and multi-pulse time domain synthesis. Especially after the femtosecond Ti: sapphire laser frequency comb and the femtosecond fiber laser frequency comb, the all-solid-state femtosecond laser frequency comb directly pumped by diode laser has attracted the interest of many research groups and made a series of meaningful progress because it has the common advantages of low noise, high repetition rate, compact structure and high electro-optical efficiency. This review summarizes the development and typical applications of all-solid-state optical frequency combs, and looks forward to the future development of all-solid-state optical frequency combs based on the research results achieved by the authors research group, so as to provide reference for the development of all-solid-state femtosecond mode-locked oscillators.

Abstract:As the aircraft's speed increases, the research on hypersonic aerodynamics, hypersonic combustion flow field, and aircraft aerodynamic characteristics has become more and more important. The conventional velocity measurement methods are increasingly limited in the complex hypersonic flow field, and new measurement techniques need to be investigated to meet the demand for the accurate measurements of the flow. Therefore, the molecular tagging velocimetry is becoming a hot research topic because of its advantages of non-intrusiveness and no limitation of following behaviors. This paper describes the basic principles, main parameters, and operating characteristics of nanosecond laser molecular tagging velocimetry and femtosecond laser molecular tagging velocimetry, analyzes the challenges faced by these techniques in measurement and discusses the prospects of their application in science and engineering fields. This paper provides a reference for the development of velocity measurement techniques in the hypersonic complex flow field.

Abstract:As a necessary tool for the calibration of astronomical spectrograph, the astrocomb can help to solve a lot of major problems in astrophysics. This paper reviews the requirements of astronomical spectrograph for astrocombs and the current status of technologies for generating astrocombs, analyses the advantages and defects of those astrocomb technologies. The prospect of astrocombs is also given.

Abstract:Processing an ultra-stable cavity using ultra-low-expansion (ULE) material with an ultra-low coefficient of thermal expansion and controlling its temperature at the zero-thermal-expansion is the key to obtain a low-frequency drift rate. The measurement of the zero-thermal-expansion temperature of the ultra-stable cavity plays a key role in the research field of ultra-stable lasers. The ultra-stable laser with a low drift rate at the zero-thermal-expansion temperature is of great significance for the precise measurement of the energy level transition spectral lines. This paper expounds the basic principle of PDH (Pound-Drever-Hall) laser frequency stabilization based on an ultra-stable cavity, introduces the methods to measure the important specifications of an ultra-stable laser, and introduces in details the three methods to measure the zero-thermal-expansion temperature of the ultra-stable cavity, including reference cavity beat frequency method, femtosecond optical comb measurement method, and the optical clock measurement method. Then the paper analyzes their operating conditions and working principles, and completes the experimental verifications. Finally, the paper summarizes and prospects the future development direction of the precision measurements of zero-thermal-expansion temperature. It provides a reference for the further development of the ultra-stable laser technology.

Abstract:Ultrashort laser pulses are important driving light sources in the fields of ultrafast optics and high-field laser physics, and can usually be obtained by pulse post-compression methods. This paper introduces in details the development status, principles and related technologies of ultrashort pulses post-compression, including bulk solid material compression, flake group compression, multi-pass cavity compression, hollow waveguide compression, and photonic crystal fiber compression. By summarizing the existing pulse post-compression technology, theoretical guidance is provided for the future research and development direction.

Abstract:The invention of passive solid-state mode-locked lasers has promoted the rapid development of high-precision timing detection technology. In the past three decades, a variety of timing detection techniques have emerged to reduce the measurement noise floor, and keep approaching the high-frequency timing jitter measurement limit of mode-locked lasers (far less than 1 fs), with significant improvements in terms of the resolution, robustness, stability, power consumption and many other aspects. In this paper, different kinds of optical timing detection methods including direct detection, BOC technology, AOM detection, optical heterodyne technology are reviewed. The advantages and application scenarios of various timing detectors are discussed, and the important role of high-precision timing detection technology in large-scale scientific instruments such as coherent pulse synthesis and free electron lasers are introduced. Finally, the development direction of optical timing detectors is prospected. The purpose of this paper is to provide technical reference for promoting the development of high precision and cutting-edge fields such as attosecond science and timing synchronization by summarizing various optical timing detection technologies.

Abstract:In recent years, the dual optical comb technique has become a hot topic in the field of scientific research due to its ultra-high frequency resolution and detection accuracy. In this paper, the methods of generating dual-combs using electro-optical modulation, micro-resonator, two phase-locking independent mode-locked lasers and single-cavity dual-comb laser are introduced respectively. The advantages of the dual optical comb generation method based on single-cavity dual wavelength laser, such as high integration, simple structure and low cost, are emphasized. After that, the working principle and the latest research progress of the multiplexing methods for the dual-wavelength operation in single-cavity dual comb lasers, such as pulse waveform multiplexing, space multiplexing, direction multiplexing, wavelength multiplexing and polarization multiplexing are described in details. Finally, the applications of single-cavity dual optical combs in spectroscopy, ranging, fiber sensing and other fields are presented, and future research and development directions of single-cavity dual-combs are prospected, which can provide reference for promoting the further improvement of technical performance and wide application of single-cavity dual-optical comb.

Abstract:Laser-induced breakdown spectroscopy (LIBS) is an effective tool for element analysis, which has the advantages of being fast, simple and real-time. It can be used for qualitative analysis and quantitative detection of various elements in solids, liquids and gases. However, the traditional LIBS and filament-induced breakdown spectroscopy (FIBS) have intensity clamping and their sensitivity is difficult to improve, which leads to certain limitations in practical application. This has become a major technical bottleneck since the invention of LIBS technology. From the perspective of excitation sources, this paper discusses the problems of LIBS and FIBS, and introduces the research progress of optimizing LIBS in recent years. The plasma grating formed by multi-beam interference is applied to LIBS. On the basis of retaining its original advantages, the plasma shielding effect and matrix effect are effectively overcome, the intensity clamping is broken, the spectral line signal is enhanced, and the detection sensitivity and quantitative analysis ability are improved. These studies will promote the practical application of LIBS technology in various fields, and also provide research ideas for the combination of femtosecond laser plasma and other technologies.

Abstract:This paper reviews the measurements of absolute frequency of the transition of calcium ion optical frequency standard, made by the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, and describes the history of the absolute frequency measurement results accepted by the Consultative Committee for Time and Frequency (CCTF) of the International Committee for Weights and Measures (CIPM). During the years between 2011 and 2020, based on the laboratory optical frequency standard and the transportable optical clock, by using the femtosecond optical frequency comb both referenced to the SI second through the satellite links, and directly traced to a cesium fountain clock made by the National Institute of Metrology (NIM), the absolute frequency of the clock transition for the calcium ion optical frequency standard was measured for a few times, and the measurement uncertainty was gradually improved from the order of 10^-15 to the order of 10^-16. Four measurement results were adopted by the CCTF to participate in the calculation of the internationally recommended frequency of the calcium ion optical clock transition, and the recommended value was updated for four times in 2012, 2015, 2017 and 2021, respectively. The calcium ion optical clock transition was recommended in 2021 as a new "secondary representation of the SI second".

Abstract:Temporal parameters of ultrashort pulse laser and ultrashort pulse laser measuring instrument cannot be traced to the International System of Units for a long time due to the lack of measurement standards. This paper introduces the measurement standard for time-domain parameters of ultrashort pulse laser at the National Institute of Metrology (NIM), China. An instrument based on Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) was constructed. The spectral phase of femtosecond pulse laser was automatically and accurately extracted with wavelet-transform technology, and the time-domain waveform of the ultrashort pulses was precisely reconstructed with the inverse Fourier-transform technique. The expanded uncertainty is U_rel=8% (k=2) in a measurement range of 10~500 fs. A large range autocorrelator was developed. The accurate measurement and traceability of ultrashort pulse width in the range of 10 fs~50 ps are realized, and the expanded uncertainty is U_rel=10% (k=2). The national measurement standard "Calibration setup for ultrashort optical pulse autocorrelators” was established, and the expanded uncertainty is U_rel=10% (k=2) in a measurement range of 10 fs~12 ps. The measurement standard provides an accurate traceable standard for ultrashort pulse measurement instruments, and plays an important role in ensuring the accuracy and consistency for the measurement results of ultrashort pulse temporal parameters.

Abstract:Accurate measurement of the timing jitter of femtosecond laser is an important prerequisite to promote its high-precision application. In order to achieve high-precision and no-reference measurement of timing jitter, a measurement system based on fiber delay line was designed and built. Based on the structure of asymmetric Michelson interferometer, a long fiber delay line is added into one arm to identify the time error and improve the measurement accuracy of timing jitter. The timing jitter power spectrum of a lab-made mode-locked fiber laser with a repetition rate of 82 MHz was measured, and the RMS timing jitter was 10.1 fs in the integrated range of 100 Hz to 10 kHz.

Abstract:The characteristic absorption spectra of a large number of substances are in the terahertz range, so the development of terahertz spectroscopy applications has attracted much attention in recent years. Compared to existing commercial spectrometers, spectral measurement methods based on tunable single-mode lasers offer the advantages of high accuracy and high spectral acquisition speed. Terahertz quantum cascade laser (QCL) is an ideal candidate for tunable laser sources. Its tuning characteristics need to be studied before it can be used to achieve spectral measurements, but the existing measurement methods are limited by accuracy. The study finds that the corresponding beatnote signals can be obtained in the microwave band by beating the terahertz quantum cascade optical frequency comb and the single-mode laser. When the single-mode laser is tuned, the beatnote signals shift accordingly. Therefore, combined with the self-detection technology of QCLs, the tuning coefficient of the single mode laser can be measured with high precision by measuring the microwave beatnote signals using a spectrum analyzer. The resulting coefficient of the measured terahertz single-mode laser were 53 MHz/K (temperature tuning) and 2.7 MHz/mA (current tuning).

Abstract:As the core components of wavelength multiplexed single-cavity dual-comb light source, fiber filters and mode-locking devices have attracted extensive attention of researchers. In order to solve the problems that the traditional filter is sensitive to polarization and the fabrication process is complex, a composite device based on micro-nano fiber is developed. The micro-nano device is fabricated by means of fused biconical taper, the filter effect is generated by the inter mode interference, and the photo-deposition is realized by the thermophoretic effect. The composite device is applied to a ring cavity, the multi-wavelength mode-locking at 1532,1543,1555 nm is achieved, and the wavelength shift is not higher than 0.2 nm at different polarization states. The composite device provides a new solution for the realization of single-cavity dual-comb and single-cavity tri-comb, which is of great significance for promoting the application of multi-comb technology in precision measurement and other fields.

Abstract:The theoretical and experimental research based on the oblique apodized fiber Bragg gratings (FBG) is carried out, in order to solve the crosstalk to the main peak from the high reflected sidelobe of the uniform fiber Bragg gratings written by femtosecond laser point-by-point. According to the characteristics of the refractive index induced by femtosecond laser, the AC coupling coefficient in the coupled mode model is modified. The apodization type of the oblique apodization method is verified to be Gaussian by finite element simulation. Then the key parameters affecting the apodization effect are studied experimentally. The results show that the experimental results are basically consistent with the simulation results, and when the initial lateral displacement is between 5 μm and 10 μm and the asymmetric offset is less than ± 2 μm, the side mode suppression ratio can be improved to 15 dB. The research results provide important theoretical support and practical guidance for improving the side mode suppression ratio, and are of great significance for promoting the development of fabrication technology of FBG.