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A Study On New Technologies Of Single-shot Cross-correlator For Measuring Pulse Contrast

Posted on:2015-09-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:J G MaFull Text:PDF
GTID:1108330464460882Subject:Optics
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The development of high-intensity lasershas pushed optical-matter-interactions into relativistic regime which is one of the most frontier researches in the world involving numerous significant subjects including laser fusion, laser-based particles acceleration, high-energy radiation sources and so on. It will also promote the progress of fusion energy, national defense security and basic physical researches. These intense-laser based physical experiments present stringent demands on the signal-to-noise-ratio (i.e., contrast) of the driving laser, which is defined as the ratio of the peak intensity of the main pulse to its pedestal or pre-pulses. For a pulse with a low pre-contrast, once the intensity of its pedestal at leading edge or pre-pulses exceeds the threshold of producing pre-plasma (typically 1011W/cm2), they will interact with the target before the main pulse and modify the property of target. However the pulse contrast can be reduced significantly by many different factors during the generation, propagation, amplification and compression of high-intensity laser pulses. To meet the requirement of physical experiments and realize high pulse contrast, on one hand it needs efficient technologies for enhancing pulse contrast, on the other hand it calls for technologies for measuring pulse contrast. As the current high-intensity lasers usually operate with very low repetition rate or even in single-shot, the time-scanning cross-correlators based on multiple pulses have become inefficient and even invalid. Thus it is necessary to develop real-time measuring technologies, which is required to give the contrast information within a large temporal window as well as a high-dynamic range only according to one pulse. The single-shot measurement of pulse contrast is a big challenge in the world and was in the primary stage before our work. There is none commercial instruments and few related technologies. Actual demands make us to study and develop new technologies for measuring pulse contrast. In this thesis, we seriously analyze the existing problems in single-shot measuring technologies, and present a series of new technologies which can solve most technical problems and lay the foundation of the actual application of single-shot measurement.Noncollinear cross-correlation is a basic approach for measuring pulse contrast in single-shot. Based on the high-dynamic parallel detection system (a combination of a fiber array and a photomultiplier) which was presented in 2008 by our group, I mainly focus on how to enlarge the temporal window of single-shot measurement and how to fully optimize the performance of noncollinear cross-correlator. We also realize the really meaningful contrast is the contrast at the focus of ultrafast pulse. To measure it, we firstly develop a spatially-resolved cross-correlator working in the far-field, which can solve the problem of the contrast measured in the near-field can’t faithfully reflect the case in the far-field. The main contents and innovations of this thesis are as follows:1. Theoretically studied the pulse contrast enhancement effect in optical parametric amplifier (OPA).The content of this part is the theoretical base of the long-wavelength sampling technology in cross-correlator. We numerically studied the contrast enhancement effect of amplified signal and generated idler in a short-pulse-pumped OPA, and revealed their dependence on the parametric gain. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrast of the pump and signal pulse and increase with the gain. The effects of group velocity mismatch, group velocity dispersion and pump duration on the contrast enhancement are also investigated.2. Presented and verified the long-wavelength sampling technology.Second-harmonic generation (SHG) is the most commonly used sampling generated technology, which is simple and efficient. However SHG still has some disadvantages. Firstly if sum-frequency-generation (SFG) is adopted as the correlating process, the generated third-harmonic signal will locate in the ultraviolet region, which will be strongly absorbed by the fiber array. Secondly, if different-frequency-generation (DFG) is adopted as the correlating process, the generated correlating signal will has the same wavelength with the pulse under test, unable to effectively separate the scatter noise. Finally, the temporal window will be limited if a cross-correlator sampled by SHG. We noted that the sampling wavelength can be used as a design freedom. Long-wavelength sampling can push the wavelength of correlating signal into the low loss region of fiber array, and is benefit for spectral filtering. And if combined with quasi-phase matching (QPM) technology, the temporal window will increase with the sampling wavelength. Therefore, long-wavelength sampling technology both increases the temporal window and dynamic range. Experimentally, the long-wavelength sampling pulse can be produced by an OPA system pumped by the pulse under test.3. Presented and verified the high-order QPM-based cross-correlating technology.In a cross-correlator based on birefringent phase matching (BPM), the maximum noncollinear angle as well as the temporal window is limited by the phase matching condition. To overcome the limitation, we presented that the QPM can be used instead of the BPM. The introduction of reciprocal lattice vector can compensate the large phase mis-matching and allow larger noncollinear angle as well as the temporal window. Generally speaking, the shorter the poled period is, the larger the temporal window can be obtained. But the poled period is limited by the current processing technology. To solve the problem, we pointed out that the higher-order QPM can be used to provide much stronger compensating capability for phase-mismatching compared with the first-order QPM and support much larger noncollinear angle as well as the temporal window. Moreover, the flexible designability of QPM structure allows only poling a small portion on a large crystal substrate, which can simultaneously support high resolution and high fidelitymeasurements. We designed three kinds of cross-correlators based on high-order QPM technology, and only experimentally verified a novel side-sampled configuration. Experiments show the temporal window of this cross-correlator is 70 ps/cm, the dynamic range is 109 with high temporal resolution (sub-piscosecond) and fidelity.4. Developed a spatiotemporal cross-correlator capable of far-field contrast determination.All the current measuring method work in the near-field, however the target in physical experiments is located at the focus (far-field) of the driving laser. This raises a question if the near-field contrast can truly reflect the conditions in the far-field?The answer is no for some noises, such as the spatiotemporal noises produced by the imperfect surface quality of optics in stretcher and compressor of CPA system. These noises added in the near-field can have space-time coupling in the far-field, whichare swept across an area in the focal plane many times the size of the diffraction-limited spot. The space-time coupling of these noises in the far-field makes the measurement in the near-field invalid. It needs a spatially-resolved measuring scheme performing in the far-field. In order to solve the problem, we developed a spatiotemporal cross-correlator which could work in the far-field and experimentally verified its reliability. With such a spatiotemporal correlator, we could not only measure the spatiotemporal noise induced by the roughness of optical elements in the actual stretcher and compressor, but also identify their source origions. According to the measuring results, we suggest that the pulse contrat can be relevantly defined as the one on the far-field axis. Significant enhancement of this relevant contrat was experimentally demonstrated via avoiding the use of far-field optics in pulse stretcher. These results further our understanding of pulse contrast in its spatiotemporal nature and guide the high-contrast design of ultrahigh peak power lasers.
Keywords/Search Tags:pulse contrast, cross-correlator, single-shot measurement, quasi-phase matching, optical parametric amplifier, far-field
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