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Saidur Rahaman, Zenon Witowski, Hong Fu; Precise Laser Pulse Characterization for Ophthalmology Applications. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4857.
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© ARVO (1962-2015); The Authors (2016-present)
The laser pulse duration is a key factor for femtosecond laser corneal incisions. For a given spot-size, shorter pulses generally ablate at lower energy levels and generally present lower collateral damage than longer pulses. Recent state of the art advancements in fiber laser technology allow for a compact laser system design for ophthalmology applications. However, laser pulse artifacts such as satellite pulses, are a common occurrence with this technology and are undesirable. In this study Frequency Resolved Optical Gating (FROG) method was used to temporally characterize the beam and identify the levels of energy contained within the satellite pulses.
For number of years, before the advancement of FROG technology, pulse duration was estimated using an autocorrelator (AC). However, AC measurements do not fully reveal the presence of satellite pulses in ultrafast pulse regime. Thus, FROG pulse measurement method was introduced to address this issue. In FROG measurements, the fundamental pulse is split into two and one is variably delayed with respect to other. The two pulses are then combined in a nonlinear second-harmonic-generation (SHG) crystal. The SHG crystal produces “signal light” at twice the frequency of the input light when both pulses are overlapped in time and space. Intensity and spectrum of the resulting light signal is then recorded at each delay point using a spectrometer and a 2D spectrogram is plotted as a function of wavelength and delay. Spectrogram includes essential information to characterize the laser pulse. Effectively, it also reveals the levels of the satellite pulses.
The pulse duration and the amount of the satellite pulses have been measured using FROG. An integral value was computed over the main pulse and the satellite pulses. The ratio of the satellite pulses to the main pulse was approximately 10%. The data was taken at the focus of the system and the laser was optimized to minimize pulse duration, satellite pulses, and to pre-compensate the dispersion of the delivery optics. Ultimately the FROG measurements and system optimization lead to a better pulse quality at the focus.
Ophthalmology applications are demanding optimum ultrafast fiber laser based solutions. FROG is an essential tool for a full laser pulse characterization and system optimization to achieve these demanding results.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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