3 GHz Yb-fiber laser based femtosecond sources and frequency comb

Abstract

Many applications require femtosecond lasers of high repetition rate. In the time domain, a higher repetition rate means more pulses in a fixed time period. For nonlinear bio-optical imaging in which photo-induced damage is caused by pulse energy rather than average power, increasing pulse repetition-rate will improve signal-to-noise ratio and reduce data acquisition time. In the frequency domain, a higher repetition rate means the comb line spacing is larger. This permits access to and manipulation of each individual comb line. Such capabilities have opened numerous frequency-domain applications including optical arbitrary waveform generation, high-speed photonic analog-to-digital conversion, and high-resolution spectroscopy. Femtosecond optical sources in the wavelength range between 0.7 [mu]m and 1.55 [mu]m have found many applications such as optical coherence tomography, high speed optical sampling, photonic analog-to-digital conversion, and multi-photon spectroscopy. To date, femtosecond solid-state lasers are mainly employed in these applications. Take two most common femtosecond solid-state lasers for example: a Ti:sapphire laser can cover 0.7-1.1 [mu]m and is useful for optical coherence tomography and multi-photon biological imaging. A Cr:forsterite laser can operate in the wavelength range 1.15-1.35 [mu]m, an important wavelength range for multi-photon microscopy because light in this wavelength range experiences lower scattering loss and higher penetration depth for most biology samples. Nonetheless, solid-state lasers are usually expensive, bulky, and prone to misalignment. The gain crystals often requires water cooling. These disadvantages hamper their wide usage outside the lab environment. The mentioned versatile applications can be even more widespread used if we are able to make the femtosecond laser sources less expensive, flexible, and easy to maintain. In this dissertation, 3-GHz femtosecond laser sources are demonstrated. These sources are useful for applications in optical coherence tomography, optical frequency metrology, multi-photon biological imaging, photonic analog-to-digital conversion, etc. First, a 3-GHz fundamentally mode-locked Yb-fiber laser is demonstrated with the highest rep-rate among all femtosecond Yb-fiber lasers to date. We then numerically and experimentally study the optimization of femtosecond YDFAs in order to achieve both high-quality and high-power compressed pulses in the 3 GHz high power fiber laser system. Using the 3 GHz high power femtosecond Yb-fiber laser system, a few-cycle ultrafast source at the Ti:sapphire laser wavelength is demonstrated as a promising substitute of multi-GHz mode-locked Ti:sapphire lasers. In addition, a watt-level, femtosecond Raman soliton source wavelength-tunable from 1.15 [mu]m to 1.35 [mu]m is implemented. Such a Raman soliton source exhibits both the highest repetition rate and highest average power to the best of our knowledge. Finally, preliminary work on a 3 GHz passive frequency comb via difference frequency generation at 1.5 [mu]m is demonstrated. This is currently the highest rep-rate frequency comb in the telecommunication wavelength range.