Optical frequency combs based on homemade high-repetition rate femtosecond Ti:sapphire lasers

Abstract

We report on two compact prismless femtosecond Ti:sapphire laser systems for use in optical frequency combs. One of them employes a 6-mirror cavity, has repetition rates adjustable from 750 MHz to 1 GHz, and is broadened into a microstructured fiber. The other uses a 4-mirror cavity, has repetition rates from adjustable 1 to 2.12 GHz and produces a broadband spectrum, extending from 635 to 1060 nm, and with an average power of 0.93 W for 8 W of pump power.Optical frequency combs consist of femtosecond lasers whose repetition rate and pulse-to-pulse optical phase (or carrier-envelope offset frequency) have been actively stabilized and connected to each other. They were introduced at the end of the 90s and since then have revolutionized the field of optical frequency (and time) metrology [1]. Optical frequency combs have been used, for example, for direct measurements of frequencies of several hundred TeraHertz [2], in the development of next generation optical atomic clocks [3] and for phase-sensitive nonlinear optics experiments, such as high harmonic and attosecond pulse generation [4].In this work, we report two versions of femtosecond Ti:sapphire lasers for use in optical frequency combs. They have a compact design leading to high-repetition rates, and use chirped mirrors instead of prisms for compensating for the group velocity dispersion. In one of the systems the laser produces pulses of 150 fs with a spectrum centered at 780 nm, and a width of 30 nm. We have used frequency resolved optical gating (FROG) to measure the pulse length and spectral phase of this laser (Fig.1, black and red curves, respectively). This is shown in Fig. 1. The spectrum of this laser has been broadened into a microstructure fiber, and covers an optical octave extending from 520 and 1040 nm. The repetition rate can be easily adjusted form 750 MHz to 1 GHz. -4 0 0 -3 0 0 -2 0 0 -1 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 0 ,0 0 ,2 0 ,4 0 ,6 0 ,8 1 ,0 -4 0 0 -3 0 0 -2 0 0 -1 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 -0 ,4 -0 ,2 0 ,0 0 ,2 0 ,4 0 ,6 phase (rad) Intensity tim e (fs )FIGURE 1. Pulse length (black) and spectral phase (red) of a 750 Mhz femtosecond Ti:sapphire ring laser, measured by FROG.In the second system, the Ti:sapphire laser has a repetition rate that can be changed from 1 to 2.12 GHz, and a spectrum that extends from 585 nm to 1200 nm at 20 dB below the maximum at 986 nm, without use of