High Repetition-rate Mode-locked Solid-state Lasers
Date Issued
2004
Date
2004
Author(s)
Liu, Tzu-Ming
DOI
en-US
Abstract
High repetition rate (HRR) mode-locked solid-state lasers have increasing importance in many applications. In optical communications, it’s a potential source for the hybrid wavelength-division multiplexed (WDM) and optical time-division multiplexed (OTDM) systems. In frequency metrology, HRR lasers can increase the resolution and signal to noise ratio (SNR) of frequency measurement. In the domain of measuring applications, HRR lasers reduce the optical pulse energy while maintaining a high average power, which is important for achieving high SNR. Although HRR mode-locked lasers are advantageous to these applications, there are still some hindrances for better development. In WDM/OTDM hybrid communication system, it still lacks a high power HRR multi-channel source to exploit the spectral domain resources. In frequency metrology, there are still some spectral black holes corresponding to specific transition level of atoms or molecules. Besides, once a HRR mode-locked solid-state laser is built, the repetition rate can’t be easily multiplied without changing the cavity geometry.
In this thesis, we circumvented these hindrances and made HRR mode-locked lasers more promising and more flexible in many HRR applications. Complete information of HRR pulses can be characterized with developed auxiliary technique. For the problem of spectral black holes, we employed frequency conversion such as second harmonic generation (SHG) to achieve HRR femtosecond lasers at red and blue wavelengths. Based on an 110-MHz Cr:forsterite laser, we first used intracavity frequency doubling to obtain HRR femtosecond pulses around 620-nm. Red pulses with 170-fs pulse width were obtained with 24-mW average power. For the frequency conversion of short cavity length, it’s better to employ external resonant cavity to enhance the conversion efficiency. As a consequence, based on a 2-GHz Ti:sapphire laser, we demonstrated a 2-GHz-repetition-rate high-power femtosecond blue sources for the first time. Pumped by the 2-GHz Ti:sapphire laser with 740-mW output power, 150-mW femtosecond pulses at 409nm can be efficiently generated from the resonant cavity.
For the development of high power WDM/OTDM sources, we exploit the broadband nature of HRR femtosecond lasers and demonstrated first high power HRR multi-channel sources. By inserting an intracavity etalon into a HRR femtosecond Cr:forsterite laser, 12 phase-locked channels with 9–19-ps pulse width near 1230 nm could be generated. Average output power of 280-mW can be obtained from a single laser oscillator. By tuning the etalon bandwidth, we can shorten the pulse width in a specific channel to 1.8 ps.
For the improvement of repetition rate flexibility, we invented an intracavity flat surface technique to multiply the repetition rate. We first built a compact self-started HRR Cr:forsterite laser at 100-MHz for the application of portable clinical use operating around 1230 nm. Instead of prism pairs, double-chirped mirrors were employed to compensate the group delay dispersion of the laser cavity. The laser mode-locking was self-started with the help of a semiconductor saturabe absorber mirror. By adopting an intracaity flat surface with low reflectivity and controlling the ratio of subcavity lengths, the repetition rate of this compact femtosecond Cr:forsterite laser can be multiplied from 100 MHz to 500 MHz in femtosecond regime. Repetition rate higher than 1GHz can also be achieved. Compared with the conventional coupled cavity method, this newly developed technique provides a flexible and phase insensitive way to increase the repetition rate of femtosecond solid-state lasers.
For the diagnosis of ultrashort HRR pulses, we developed a new technique. By adding spectral information into triple-optical-autocorrelation measurements, we made the triple-autocorrelation method capable of providing complete knowledge of HRR laser pulses. With the measured temporal intensity of an optical pulse and its corresponding spectral intensity obtained with a spectrometer, exact intensity and phase variations in time can all be recovered with the Gerchberg-Saxton algorithm through an iterative calculation with an O(n) complexity.
Subjects
鎖模雷射
飛秒光學
固態雷射
脈衝量測
phase retrival
High repetition-rate
Mode-locked laser
femtosecond optics
Type
thesis
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