| Summary: | 碩士 === 國立交通大學 === 影像與生醫光電研究所 === 107 === In theory, an ideal ultrafast laser pulse is generated from broadband frequencies with both constant amplitude and constant phase in the frequency domain. If there exists spectral phase variation, it will distort the pulse shape in the time domain and increase the pulse duration. The spectral phase variation after propagating through a bulk of material is the dispersion phenomenon, which can be described as a frequency-dependent propagation constant . However, every material, even air, is dispersive medium and will inevitably induce spectral phase variation in ultrafast laser. Therefore, the measurement and the compensation of spectral phase play a significant role in ultrafast applications. One significant influence is in temporal focusing multiphoton microscopy (TFMPM). Unlike conventional point-scanning multiphoton microscopy, TFMPM can induce widefield multiphoton excited fluorescence while maintaining the axial sectioning ability. In TFMPM, each frequency component of the ultrafast laser is first separated by a blazed grating. After passing through a 4-f system, each frequency component will overlap as an area at the focal plane of the objective lens. The overlapping plane is called the temporal focusing plane and only at this plane can the high-peak-power pulse be composed. Thus, TFMPM can excited widefield fluorescence with axial confinement.
However, the dispersion in optical system itself and the turbid biospecimen will induce spectral phase variation, which distorts the pulse shape and reduce the excitation efficiency. Moreover, it also induces the displacement of temporal focusing plane, leading to the mismatch of fluorescence excitation plane and image formation plane. With the intention of conquering the image degradation caused by the above reasons, an adaptive optics system (AOS) for spectral phase compensation is developed in this thesis. In order to measure the spectral phase, a lab-built non-collinear autocorrelator is modified to be a scanning frequency-resolved optical gating (FROG). With a scanning resolution of 1 μm, the scanning FROG can acquire FROG traces at around 3.3 fs temporal resolution. By the FROG algorithm, the spectral phase can be retrieved from the FROG trace and utilized as the reference for compensation. Moreover, for the purpose of faster measurement, a single-shot FROG is applied in the system where the FROG trace can be directly aperture as an image by a USB camera without mechanical scanning. As for the optical setup of AOS, it is mainly based on the structure of TFMPM. At the spectral expansion plane of TFMPM, a 61-channel deformable mirror acts as the actuator to individually modulate the phase of different frequency components. The overall AOS is integrated by a data acquisition card, a personal computer, and a MATLAB program. The theoretical control loop can achieve a 5.4 Hz cyclic speed, which is mainly limited by the 8 Hz camera frame rate.
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