(Tianjin University Precision Instrument and Optical Electronic Engineering Key Laboratory Key Laboratory Super Fast Laser Research Office)
This article is selected from "Physics" 2021, No. 11
Abstract Precision timing control is indispensable in the exploration of the scientific process, which is an important guarantee for high performance work under extreme conditions. The timing control technology of the Azu, provides a strong technical support for scientific detection devices to achieve more fine, faster, higher energy, and higher resolution. The article introduces the optical main clock generator of the Audi-Jitter, the timing signal of the Azu intensive, the distribution technology of the optical, radio signal source, and the scientific principle of the high-precision time error compensation technology of long links, and introduces these cut-edge technology X-ray free electronic lasers, super short laser devices, applications in large-scale radio telescope arrays.
Key words Azudi, timing synchronization, laser optical, femtosecond laser
Scientists continue to develop new technologies, develop new equipment to observe the world, pursue experiments with minimal size, fastest speed, strong energy, and farthest. In many front edge sciences, the time base within the experimental device needs to be very accurately assigned to achieve the ultimate detection effect. At present, in the scientific detection device that has been built or in construction, the aose (1 AS=10-18 S) level timing synchronization control can achieve the highest standard.
Many physical processes in the micro world, the chemical reaction is extremely short, in order to observe the motion state of the microscopic particles, it is necessary to adopt a pulse detection source with high strength and wide pulse width, to completely accurately analyze the kinetic problems. X-ray free electronic laser (XFEL) (Fig. 1 (a)) is capable of focusing on individual atoms, even focused on the internal interior of the complex system [1, 2], and observed electronics and nuclear movements on their inherent time scale. This is benefited from the substantial increase in X-ray source performance in the past ten years, which can provide unprecedented time resolution (about 50AS) and intensity, and can explore the acts of substances under extreme conditions . There are many large-scale XFEL devices built or built around the world, and atomic molecular physics [4-6], ultra-fast chemistry , and a series of research results [8, 9] and other research results. Document  reported the use of XFEL to generate an isolated soft X-ray artepath pulse, the light source has a pulse energy that is millions of pulse energy than any other isolated apocyphochid source than the soft X-ray spectrum, and the peak power exceeds 100 gw (1 GW)=109 w). This unique combination of high intensity, high photon energy, and ultra-short pulse duration, making X-ray nonlinear spectroscopy and monotomy-like electrodynamics research possible.  Document  demonstrates the electronic structure of the transition region molecule during the Ru surface CO oxidation by using XFEL ultrafast pump – detection. The literature  reported ionization of amino acid phenylalanine with isolated alchemine pulse, and ultra-fast kinetic detection was carried out at 4.5 fs (1 fs=10-15 s) time scale, and the molecule was observed. Charge migration. Completely coherent soft X-ray apocond pulse generally requires high gain harmonic amplification (hGHG) to produce [13, 14] by high gain harmonic amplification (HGHG) in the conditions of external seed pulse laser injection. To this end, some of the partial active devices (such as seed light sources, all level amplifiers, microwave networks) in XFEL must have a secondary timing synchronization accuracy . In addition, using XFEL as a detection light source to carry out pump – detection experimentally requires a secondary delay accuracy .
Figure 1 (a) Japanese physicochemical XFEL device local; (b) Shanghai super short laser experimental device; (c) Chile ALMA radio telescope antenna array of EHT eight-pointer
The extreme environment produced by the very high peak power laser pulses is also the interest of many scientists [17,18]. Super short laser can be seen as the brightest light source that can be generated in the laboratory , which is usually defined as: peak power is greater than 1 TW (1 TW=1012 W), and the pulse width is less than 100 fs . Shanghai super short laser experimental device "SULF) (Figure 1 (b)) outputs the highest peak power of 12.9 pW (1 pw=1015 w) in 2019, breaking The unit created in 2017 . Extreme Optical Infrastructure (ELI) of Europe is used to generate laser pulses with extreme peak power and focus strength, its peak power is expected to reach Eva (1 EW=1018 W) magnitude, pulse width is 10 fs  . Super short laser provides new technical means for new generation laboratories, material physics, high-energy physics, medical applications, etc., which have contributed to the intersection and development of disciplines. One way to produce super short laser is to utilize optical parameter 啁啾 pulse amplifier (OPCPA) technology, where the optical variable amplifier (OPA) needs to simultaneously inject signal light and pump laser into nonlinear media, requiring pump lasers. The synchronization accuracy of the 10 FS level is implemented with the seed source. In addition, coherent synthesis techniques using femtosecond fiber lasers can also produce super short laser , as an example, the basic method is the case, the basic method is thousands of femtosecond fiber Coherent synthesis of the laser performs super-short laser [24-26]. To achieve ultra-high pulse peak power and femtosecond pulse width, this process needs to precisely control the delay of multi-harm homologous femtosecond fiber lasers, and the jitter of the timing control system must be controlled at 10 AS levels.
Astronomers hopes to see the distant uglyhood in the universe, such as the black hole that is scattered in the universe. The scientists of the US National Sports Observatory announced the photos of the M87 garage center black hole and published a series of articles recorded this breakthrough scientific research. It is not an easy thing to take pictures for distant black caves, and you need to shoot the camera with a sufficiently high angular resolution. Considering the size and distance from the black hole, the black hole of the M87 galaxy is one of the most convenient observations, but still needs to provide a camera to provide 22 phenoncained angular resolution. This is equivalent to distinguishing the object on the earth’s surface tennis size, requires a telescope that requires a large diameter to observe. For the black hole taking a picture of the event vision telescope (EHT) , it consists of 8-poinsed telescope dispersed around the world (Fig. 1 (c)), its principle is based on very long baseline interferometry (VLBI): Ideally The two telescopes that are far from the distance are simultaneously received, and the super-diameter telescope equivalent to the two telescope spacing is equivalent to a diameter of two telescope spacings. EHT’s 8 sets of electrical telescope is close to the diameter of the earth, and all scientists have used EHT for several days of synchronous observations. Through two years of data analysis, I finally got photos of the M87 galaxy center. It is worth noting that VLBI technology needs to interfere and analyze the beam from different telescopes, meaning that the telescope array needs to have a precise timing signal to perform the probe task. For the shorter optical telescope array synchronous accuracy of the wavelength, the working status is to be equivalent to a virtual large caliber telescope, and each sub-telescope must construct a timed synchronization control network of femtosecond or even a second. [28, 29].
Most of the scientific detection instruments used to detect extreme conditions are distributed configuration. As science and technology have continuously in-depth of the world, the synergy between different terminal devices in advanced scientific detection devices needs to rely on the secondary constant synchronous control. Otherwise, the high performance of the overall system cannot be played. Next, the timing synchronization technology and the application of timing synchronization techniques in advanced scientific detection devices.
A second timing synchronization technology
The timing synchronization of the Azu is completed under the support of the two key technologies of the primary clock generator, super high stability clock distribution system. The main clock generator acts as a metronome, providing a timing baseline for the entire scientific detection device. Unlike the metronome in the instrument, the scientific device is extremely demanding on the timing of timing jitter, and the traditional RF clock is difficult to meet the requirements. The femtosecond laser produces a periodic ultra-short pulse sequence, which carries the microwave signals with extremely low jitter, which can be used as a super-steady "metronome" of the advanced science device, called an optical main clock oscillator (Optical Master Oscillator, Omo .
The omo timing synchronization signal must be transmitted to different terminal devices through the fiber link, and strictly calibrate the laser within the terminal device, the timing of the microwave source can achieve ultra-high stability clock distribution, so that the terminal device and the fiber link are detected. The timing jitter is extremely important. The following two key technologies are introduced separately from OMO and super-stable clock distribution systems.
2.1 Optical main clock oscillator based on femtosecond laser
The femtosecond laser produces a ultra-short pulse sequence, a single pulse width as a few tens of femtoseconds, and the repetition frequency of the pulse is in the MHz-GHz level, thereby providing a natural source. The spacing of the femtosecond pulse is not strict, affected by quantum noise, environmental noise, the pulse sequence produces random jitter, which is expressed as the random time deviation of the center of gravity of each pulse envelope in the sequence. There is no feedback control in the freely running femtosecond laser, and the deviation amplitude of the pulse center relative to the ideal time increases as time increases, and the situation of random swing is present . As shown in FIG. 2, the periodic red burst is an ideal position of femtosecond laser pulses in the case of noise, and its pulse interval is strict. Blue burst indicates that the laser exists in the influence of the laser, there is a range of light pulses that appears a certain jitter. As time goes by, the timing uncertainty of the pulse is gradually expanded, and the repeating frequency signal (microwave signal, yellow curve) of femtosecond laser pulse bearer will also generate jitter. Since the photon in each pulse is highly densely aggregated within the time window of narrow to the femtose, the extremely high peak power is provided, so that the effect of random photonic noise such as spontaneous radiation (ASE) on femtosecond laser pulses is minimal, The timing jitter of the quantum limit is low until the anchiose level [31-33]. Despite this, the long-term length of the femtosecond laser is inevitably fluctuated with ambient temperature, vibration, and other environmental noise, so that the pulse sequence produces additional timing jitter.
Figure 2 Feedback laser pulse jitter
The jitter caused by environmental noise typically has a limited bandwidth, which can be effectively compensated by an electronographic locked loop. The repetition frequency of the femtosecond laser is locked using the frequency reference such as the atomic clock, and the effect of environmental noise can be eliminated, so that the femtosecond laser obtains the same frequency stability as the 铷 atom clock. Quantum noise cannot be completely eliminated by feedback control, but the remaining high frequency jitter is extremely low (usually <1 fs), so that the reference to the atomic frequency reference femtosecond laser can be competent to provide an optical main clock oscillator that provides a supercompens "beat" signal. Omo.
2.2 Ultra high stability clock distribution system
A variety of lasers and microwave sources are distributed in a huge scientific detecting device, and the omo timing signal needs to be distributed through the fiber link to these signal sources to make the scientific device to operate normally. To this end, it is necessary to solve the laser and the omo timing synchronization (simultaneous synchronization of light-light timing), microwave source and omo timing synchronization (referred to as light-microwave timing synchronization), and build key issues such as superhang OMO clock distribution link.
2.2.1 Based on balanced optical cross-correlation light – light timing synchronization technology
Balance Optical Cross-Cross – related (BOC) technology can achieve timing synchronization of two separate femtosecond lasers. The BOC method is the measurement sensitivity is directly determined by the pulse width by the time error between the nonlinear optical effect of the frequency (SFG), and the optical pulses output from the femtosecond laser (SLAVE). Figure 3 is a schematic diagram of a single crystal BOC. Two-bundle pulses (red) orthogonal to the polarization direction generated by the omo and slave lasers enters the BOC measurement system at a specific time difference Δτ, corresponding to the initial position 1. Reached the position after the two-color mirror 1 is reached 2. When the two bundles of pulses are transmitted in nonlinear crystals, their overlapping portions produce and frequency signals (green), and finally reachabited position 3. It is worth noting that the orthogonal two bundles of pulses are different in the refractive index of the crystals, and the optical paths are different, affecting the delay of the two bundles of pulses. Since the pulse in the horizontal direction is more "fast" in the crystal, it is assumed that the specific initial pulse interval is Δτ0 (determined by the crystal thickness), when the position 3 is reached, the two pulses are completely aligned, that is, the delay is zero. The frequency signal transmits a two-color mirror 2 to one end of the balance detector, and the baseband is reflected by the two-chromorative mirror 2, and then undergoes one and frequency process, the delay of the two bundles is exactly -Δτ0, the resulting frequency signal Received from the other end of the balance detector. Since the two and frequency processes are identical (because the relative delay of the pulse is changed from Δτ0 to 0, the balance detector is subtracted, the two signals are subtracted, the zero level, the pulse itself is fluctuated. Flat amplitude fluctuations are automatically eliminated by balance detection. For any pulse relative delay Δτ, the balance cross-correlation curve shown in the right side of Fig. 3 is obtained. There is a region with a larger slope and a better linear feature near the zero zero Δτ0 of the curve, the level value of the balance detector is proportional to (Δτ – Δτ0), and the detection sensitivity is up to the vetum. The level signal feedback by the BOC is given the chamber length of the femtosecond laser, and the pulse sequence output from the laser can be achieved with the strict synchronization of OMO.
Figure 3 Balance optical cross-correlation measurement principle
2.2.2 Light – Microwave Timing Synchronization Technology
The light-microwave phase measurement method can synchronize the microwave signal source in the terminal device to OMO. In the optical-microwave network, the omo timing signal needs to be converted into a microwave timing signal in a certain manner to provide a synchronous microwave source, and the traditional direct use of the photodetector extracts the timing signal is large because the photoelectric detection process will The introduction of new timing noise is unable to play the advantage of the omo extremely low jitter . In order to solve this problem, people have developed a variety of optical-microwave synchronization techniques based on optical-microwave phase detection, such as balanced light-microwave phase detectors (BOMPD), fiber loop light-microwave phase (FLOM-PD) Wait [35-37], has a timing accuracy of femtoseconds. The basic idea of ??these synchronization techniques is to use the omo output low jitter ultra-short pulse laser sequence as a probe, detect the time error between the pulse and the microwave signal (Fig. 4), and transformed into a level signal, feedback control microwave The signal source transmits the repetitive frequency stability of the pulse sequence to the microwave signal, so that the direct photodelectrode detection additional phase noise is cleverly avoided.
Figure 4 Optical – Microwave Jense Principle Based on FLOM-PD
Figure 4 shows the working principle of an optical-microwave troroscopic based on FLOM-PD. The period of the femtosecond laser pulse sequence emitted by the omo is TREP (usually 10 ns), the cycle of the microwave signal is TRF (usually about 0.1 ns), and the remainder of the TREP and TRF corresponds to the phase difference of both. Δθ. The femtosecond laser pulse enters the ring from the 1 terminal, from the 2-terminal output into the 2 × 2 coupler, in the fiber Sagnac, two-way transmission, two beam lights in the SAGNAC ring Different phase modulations produce phase difference Δφ, Δφ derived from the π / 2 phase shifter and phase modulation associated with Δθ, and the two beam lights are returned to the 2 × 2 coupler and interference, the coupling output is two ways, All the way is IOUT2, and the other path Iout1 returns to the circular 2 port and outputs the 3 port output. Iout1, IOUT2 ??is done in the balance detector, output the level signal associated with Δθ. Near the zero level of the balance detector output, the output level value is proportional to Δθ, so as to feedback the microwave signal source as an error signal such that the frequency stability of the microwave signal is completely consistent with OMO.
2.2.3 Super Stability Clock Distribution Link
The femtosecond laser pulse sequence in the large science device is a laser, a microwave source transmitted ultra-stable time signal, and the fiber link replaces the impact of electromagnetic interference, and the transmission loss is lower. However, the fiber link will be affected by external factors such as ambient temperature, stress change, and introduce additional time errors. With the balanced optical cross-correlation (BOC) technology, this time error can be detected with high precision, and the process is shown in Figure 5: OMO is divided into two parts by the polarization beam splitter, and part of the polarization beam splitter enters the reference light path, reference The optical path passes this part of the OMO timing signal to the BOC. Another way of femtosecond laser is reflected by the polarization beam splitter, and entering the fiber link transmission, when the terminal device is reached, the partial mirror reflects back to a portion of OMO light, and then undergone the fiber link finally reaches the BOC. The BOC is aligned with the omo timing signal that undergoes a round-trip fiber link, which is real-time to get the timing error of the link introduced, and feed back to the servo control system. The servo control system controls the electric delay line and the optical fiber stretcher in the fiber link according to the feedback signal of the BOC, and the time error introduced by the active changing the optical path to correct the link being affected by the environment. Among them, the long-term time drift caused by factors such as electric delay line compensates temperature change, the fiber tensioner compensates for high frequency timing jitter caused by stress or the like.
Figure 5 Time noise compensation of the fiber link 
Application of Timed Synchronization Technology in Advanced Scientific Detection
3.1 Pumps Based on XFEL – Detection Experiment
One of the applications of ultra-high-time precision X-ray free lasers (XFEL) is time-distinguished pump – detection spectroscopy. The isolated apocose pulse generated by the high-end harmonic production (hHG) technology has opened the door to explore the sputum science. However, in order to detect inner electronics, photonic energy must reach a soft X-ray spectral segment. Currently, XFEL, which is capable of outputting soft X-ray apocose pulse is the only option to carry out such pump-detection experiments. XFEL is a large scientific device, high-energy pump laser pulse and X-ray apocytic pulses need to be synchronized on the km-level link. As shown in FIG. 6, the simplified XFEL pump-detection system is shown in Fig. 6, and the pump pulse is controlled by the delay device to reach the time of the sample, and the X-ray detection pulse periodically irradiates the sample to detect. In order to carry out an second pump-detection experiment, the entire device requires the timing synchronization design of Yafei second order [39,40]. To this end, microwave devices such as electron gun, accelerator, electron beam compressor in the XFEL device need to synchronize with OMO-like-microwave timing techniques by BOMPD; implanted lasers, seed lasers, pump lasers, pump lasers relative to seed lasers in XFEL Delayed by BOC and other light-optical timing techniques with OMO synchronization. At present, the XFEL pump-detecting the link synchronization accuracy of the experimental device can reach 5FS level .
Figure 6 Pump – detection experiment based on XFEL
3.2 Sub-cycle laser frequency domain coherence synthesis
With the extreme nonlinear effect of the sub-cycle laser pulse and atom, molecule, the medium, the electric field oscillation of the laser pulse produces an second X-ray [41, 42]. The wider the spectral, the narrower the degree of the domain pulse, so that the sub-cycle laser  can be generated by synthetic high-energy ultra-frequency band spectroscopy. Ultra-frequency spectrum spans ultraviolet, visible light, near infrared spectroscopy, is difficult to directly output through a laser. One way is to use the optical frequency domain coherence synthesis of multiple femtosecond lasers to achieve a flattening spectral spectrum, as shown in FIG. In order to establish a coherence between the femtosecond laser source, it is necessary to accurately control the pulse relative delay and spectral phase between the synthetic passage . Among them, the pulse synchronization accuracy must reach less than 1/10 optical cycle, BoC , spectral interference  is an important technical guarantee for achieving such high timing synchronization accuracy.
Figure 7 Multi-channel laser frequency domain coherence synthesis
3.3 Optical fiber super short laser time domain coherence synthesis
The International Coherent Zoom Network Project (ICAN) mentioned in the introduction is to break through the power output limit of the single femtosecond fiber laser, using the thousands of fiber flyosecond laser coherence synthesis to produce super short lasers (Fig. 8). In order to achieve coherent synthesis, the thousands of laser lasers come from the same seed source, and the seed source is insegrade and exports after the independent power amplification. In order to achieve the target of the output peak power, the narrow pulse width, the high-precision time between all femtosecond laser beams is also required, which is similar to the most highly high structure with several dice, and must ensure that all the center of all dice The same axis located vertically, like a sugar gourd. If you can’t do high-precision synchronization, it is similar to the center of this dice is not in an axis, but the dice is not a column, but a bunch, not narrow enough. Since the width of each bundle is extremely narrow, in order to achieve the best time domain coherence synthesis, the timing accuracy between the femtosecond laser pulses requires strict control in the sequin level, the delay control system relies on BOC , landscape shears Technical means for cutting interferometers  provide time error between pulses.
Figure 8 Multi-channel laser time domain coherence synthesis
3.4 Telecommunications Array Time Synchronization Technology
The work of the radio telescope array relies on a large-scale microwave synchronization network to realize the extremely low relative phase error  between the plurality of radio antennas, so that the specific astronomical event can be "simultaneously" to achieve the effect of the synthetic aperture. Synchronizing these antennas by using electronic phase detectors in the radio frequency domain, these antennas are limited to 50-100 fs, which may become potential obstacles for future ultra-high resolution distant celestial imaging. The femtosecond laser is used as an OMO, and optical-microwave synchronous control techniques such as BOMPD or FLOM-PD can significantly improve the timing accuracy of the antenna [29, 47], which is described with reference to Fig. 9. Reference to the omo of the microwave frequency reference to the target antenna by the fiber link, using BOC technology to eliminate the timing noise of the fiber link, and utilize the BOMPD technology to accurately detect the local clock and the OMO standard timing signal of the radio antenna terminal. The delay is calibrated by the servo system, so that all the terminal antennas can be synchronized to receive the radio signal transmitted by the observation object.
Figure 9 Synchronous microwave network timing for radio telescope arrays
Advanced scientific detection devices in the study of natural phenomena such as micro, strong field, and long distances, such as X-ray free electronic lasers, super short laser experimental devices, radio telescope arrays, etc., these large devices need multiple Terminal devices such as lasers and microwave sources strictly synchronously perform work commands to make the entire system play a role under the most limit of experimental conditions.
The timing stability of the femtosecond laser is extremely stable, which can become the odd-level shake of the new generation of secondary control system. With the "metronome" of OMO as an overall device, you can first synchronize the laser in the terminal device to the omo timing signal by BOC-based light-optical timing synchronization technique, in XFEL pump – detection experiment, sub-cycle pulse frequency domain Coherent synthesis experiments, time domain coherence synthesis experiments in fiber optic super short lasers. Secondly, based on BOMPD, FLOM-PD and other light-microwave timing synchronization techniques synchronize the microwave signal source in the terminal device to the OMO timing signal, applied in the synchronous XFEL, accelerator, electron beam compressor and other devices, and synchronous radio telescope Array each of the radio antenna. In addition, the noise of the optical fiber link between the OMO and the target device can not be ignored, and the additional noise of the link can be compensated by the BOC method. A second intensive timing control system, such as the central nervous system of the human body, controls the synergistic operation of each terminal module inside the scientific device, so that the large scientific device of the kilometer can take a small observation of the nano-scale micro-physical world, constructing extreme attire The environment, see an astronomical event that occurs within the distant black hole.
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