Harmonic femtosecond fiber laser based on supercontinuum generation with carbon nanotubes saturable absorber

Abstract

An ultrashort pulse fiber laser has been proposed due to the problem of bulky size and high cost of the Titanium Sapphire laser and other commercial ultrashort pulse fiber lasers. Thus, this study focused on the development of a robust, compact and stable femtosecond mode-locked fiber laser via optical telecommunication components. This laser was designed to have a high repetition rate (80 - 100 MHz) and average output power (30 - 50 mW), and also a narrow pulse width (< 100 fs) which are crucial for a laser source used in all-fiber terahertz time domain spectroscopy system. A short cavity was needed in order to get a high repetition rate while the effect of optical dispersion in the cavity was included in order to produce a narrow pulse width. This design employed a passive mode-locked technique with a carbon nanotube thin film as the saturable absorber. Initially, a diode laser of 980 nm wavelength was used as a pumping source and a 0.4 m long of highly erbium-doped fiber with 110 dB/m peak absorption at wavelength of 1530 nm was utilised as a gain medium. Then, in order to achieve the desired parameters, the pump power was increased to raise the repetition rate of the pulse laser and a supercontinuum generation technique was adopted to compress the pulse width. The preliminary results of the designed laser show a fundamental repetition rate of 67.8 MHz at mode-locking threshold pump power of 63.5 mW. The average output power and pulse width obtained are 0.77 mW and 410 fs respectively. The increment of pump power to 104.2 mW significantly increased the fundamental repetition rate to 193.5 MHz which corresponds to the 3rd order harmonic and compressed the pulse width to 70 fs. The average output power after compressing the pulse width is 4.27 mW. As the conclusion, two of the targeted parameters of the laser have been successfully attained. This design however has not been able to produce the targeted average output power and to operate with the desired parameters simultaneously.