Octave-spanning Ti:Sapphire laser with repetition rate >4 GHz

Summary form only given. Ultra-broadband laser frequency combs operating at multi-gigahertz repetition rate are important for applications in frequency metrology, as well as time-resolved and frequency-domain spectroscopy. A compact Ti:Sapphire laser (depicted in Fig. 1(a)) is a promising candidate for implementing such a frequency comb. Recently a 10 GHz self-referenced Ti:Sapphire was shown to provide an ultra-broadband optical spectrum after external broadening in a microstructure fiber [1]. This provided an octave spectrum for detecting the carrier-envelope offset frequency [1,2]. However, the use of nonlinear fiber with high average power introduces challenges for long-term stability. Thus, it is interesting to consider the possibility of directly generating octave spectra [3,4] at repetition rates that could ultimately approach 10 GHz. To our knowledge, the highest repetition rate obtained with an octave-spanning Ti:Sapphire ring laser, is 2.166 GHz [4]. Here we report the achievement of a 4.3 GHz octave-spanning laser frequency comb.To achieve an octave at the direct output of the laser, optimization of the different elements of the cavity is required. A combination of broad bandwidth chirped mirrors and an additional fused silica plate are used in the cavity to bring the average group-delay dispersion (GDD) close to 0 fs2. The optical spectra at different repetition rates are shown in Fig.1 (b). With mode-locked operation, the total output power is 850 mW for frep = 2.5 GHz and decreases to around 750 mW for frep = 4.3 GHz. The broadband spectrum at frep = 4.3 GHz was input to a f-2f interferometer, and we were are able to measure light at 560 nm as well as the 1120 nm light doubled to 560 nm with a PPKTP crystal (See Fig.1 (c)). The low light powers and high frequency range (up to 2.15 GHz) of f0 makes its detection difficult. Thus far, using an avalanche photodiode with bandwidth of 1.3 GHz we have managed to detect the offset frequency f0 at a maximum re- etition rate of 3 GHz (see Fig.1 (d)). With additional optimization of the laser, the f-2f interferometer, and photodiode bandwidth, it seems feasible to detect and stabilize the offset frequency of this comb with repetition rate >4 GHz.