초고속 3차원 영상 획득을 위한 단일 펄스 간섭계 연구

Abstract

An optical imaging technology has been drastically developed in science and industrial fields to observe very tiny features of objects, dissimilarity of materials and quantitative dimensions of a specimen with high resolution and precision. Most of previous researches took efforts to improve the lateral and axial resolutions, which are critical system parameters to obtain clear and obvious reconstruction of targets. On the other hand, fast optical imaging techniques have been recently introduced to capture an instant image from several natural phenomena such as physical and chemical reactions. Furthermore, they are essential tools to understand the light-matter interactions generated by combustion, laser manufacturing, laser-induced nuclear fusion and thermal reactions. The fundamental functionalities of the fast optical imaging mainly depends on the characteristics of the optical source and detectors. Traditional stroboscopic techniques, which modulate the intensity of the light and synchronize its operating frequency with triggering of the detector, are good examples to capture the instant images. However, its electronic operation of the intensity modulation inherently has the limitation to hardly shorten the duration of the exposure time below a few picoseconds. Even though the short exposure time by electronics is possibly implemented, the optical energy in this short duration is not enough to be detected by the detectors to obtain images. Therefore, the applicability of typical stroboscopic imaging techniques are limited to obtain relatively slow imaging and it is hard to be applied to capture the instant images, which includes transient information of phenomena during ultrashort time duration less than a few picoseconds. The advent of a femtosecond pulse laser is greatly beneficial in several research areas such as optical frequency metrology, dimensional metrology, manufacturing, communication, etc. By the mode-locking scheme, tremendous longitudinal laser modes are phase-locked to each other and the output can provide an ultrashort pulse train in the time domain and discrete broadband spectrum in the optical frequency domain. Especially, the ultrashort pulse duration less than 1 ps. and the concentrated optical energy within a pulse are very attractive to be applied in the fast optical imaging to overcome the limitations of previous works. On the other hand, optical imaging can be roughly classified into 2D and 3D imaging. In order to observe the physical phenomena such as various chemical reactions, fluorescence reactions, spintronics, acoustics and fluids and plasmas, it is necessary to acquire 3D information for analysis. In general, the 3D optical imaging requires additional techniques such as phase shifting for detecting the phase of the object to be measured. Since the phase shifting method shifts the phase in time, however, it is difficult to be applied to the instantaneous optical imaging. For instantaneous 3D optical imaging, a technique capable of acquiring 3D information from a single image is needed. In this thesis, I propose a single pulse interferometry (SPI) which can instantly collect 3D images based on ultrashort optical pulses. SPI is capable of freezing and capturing 3D images at a specific moment for repeatable and non-repeatable timely varying situations with ultrashort pulse duration even though an imaging device has much longer exposure time. By synchronizing the repetition rate of the pulse train and the frame rate of camera, only a single pulse is used as the illumination light and an interferometric configuration based on spatial carrier fringe analysis technique can acquire the phase information of a target. In order to verify the performances of SPI, feasible experiments were performed with ultrashort pulse lasers in comparison with continuous wave (cw) optical sources. Furthermore, I have measured and analyzed thermoelastic wave with high frequency characteristics of several tens of MHz using a Q-switched laser and a mode-locked laser as a light source, respectively. For the successful operation of SPI, the leakage lights to prevent imaging at the specific moment were analyzed and the amplified spontaneous emission (ASE) of the mode-lock laser was considered as the dominant limiting factor. In order to eliminate the ASE interferogram in SPI, three types of rigorous single pulse extraction methods, the time averaged phase modulation, unbalanced interferometric configuration and second harmonic imaging, were proposed and explained in this investigation. In the experimental results, it was confirmed that they had the capabilities to remove the ASE effect and the efficiencies were also discussed. In comparison, each method was reviewed in the view of the advantages and limitations. The single pulse interferometry proposed in this thesis is expected to be applicable not only for the performance evaluation of ultrafast pulses and real-time monitoring/control fields, but also for basic researches such as mechanics and physics with fast dynamics field.