Efficient 1 GHz Ti:sapphire laser with improved broadband continuum in the infrared

Abstract

We demonstrate a 1 GHz prismless femtosecond Ti:sapphire ring laser which emits 890 mW for 7.6W of pump power over a continuum extending from 585 to 1200 nm at -20 dB below the maximum. A broadband continuum is obtained without careful mirror dispersion compensation, with the net cavity group-delaydispersion having -50 to +100 fs 2 oscillations from 700 to 900 nm. Further broadening is obtained by use of a slightly convex cavity mirror that increases selfphase modulation. 17% (75%) of the intracavity (output) power is generated in single-pass through the crystal, outside the cavity bandwidth and concentrated in the low gain infrared region from 960 to 1200 nm. This laser seems well suited for optical frequency metrology, possibly allowing easier stabilization of the carrier-to-envelope offset frequency without use of photonic fibers.OCIS codes: 140.7090, 320.7160, 120.3940 Femtosecond lasers are the basis of optical frequency combs, which have revolutionized frequency metrology and precision measurements in the recent past years 1 . They have been used, for example, for direct measurements of frequencies of several hundred THz 2 , in optical atomic clocks 3 and for phase-sensitive nonlinear optics experiments 4 . For measurement and control of the carrier-to-envelope offset frequency 1 (f ceo ) in optical frequency combs, a femtosecond laser whose spectrum covers one octave is desirable. Although the use of a microstructure fiber offers a solution to this, there are problems for continuous and long-term operation, which is important for clocks and other applications. In this way, the advent of femtosecond lasers (in particular Ti:sapphire) emitting a broadband continuum 5,6,7,8,9 has attracted interest. These lasers have been demonstrated with stationary-wave, longer cavities operating with prisms in order to compensate for group delay dispersion (GDD), or with short, traveling-wave, prismless ring cavities that use chirped mirrors. In the first case, repetition rates are typically smaller, near 100 MHz or below, while the second case allows for repetition rates of one Gigahertz or higher, which are often preferred for optical frequency measurements. Measurement and stabilization of f ceo has also been demonstrated with such lasers 7,8,9 . In this letter, we report what to our knowledge is the most broadband femtosecond spectrum to date, obtained from a Ti:sapphire laser whose spectrum covers from 585 to 1200 nm at a level of -20 dB below the maximum. It operates stably with an output power of 890 mW for 7.6 W of pump power, and a repetition rate near 1 GHz. We discuss its characterization and the points which we found to be important for stable ultra broadband operation.The main laser cavity described here consists of a 4-mirror bow-tie ring cavity comprising two curved mirrors (including the input coupler (IC)), with broadband high reflecting (HR) coatings and radius of curvature of 3 cm, a HR flat mirror and a flat output coupler (OC) with a transmission of 2 % from 730 to 870 nm. All mirrors are commercia...

Dispersion Compensation Devices

“…However, in the octave-spanning lasers, particularly in the high repetition rate (~1 GHz) ring cavity lasers (Fortier et al 2006;Nogueira et al 2006), there are three CMs and one output coupler. Two of the CMs are usually paired, but the other one has no mirror to pair with.…”

“…Two of the CMs are usually paired, but the other one has no mirror to pair with. Therefore, the total dispersion is usually heavily oscillated, and the compensation is incomplete in such a laser (Fortier et al 2006;Nogueira et al 2006). The same difficulty also happens to the linear cavity, where the end mirrors offer one bounce reflection while other CMs experience twice which can be designed in pair.…”

“…M1 is the pump mirror. M3 may be a convex mirror (Fortier et al 2006;Nogueira et al 2006). The optimization procedure for two mirrors was similar to that in reference (Crespo et al 2008), but with more steps as described below.…”

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

“…[6], to cause further broadening of this spectrum. In our 1 GHz laser [5], a broadband spectrum with about 350 nm bandwidth (see Fig. 2, black curve) is easily achieved, as this is the preferred condition for operation of the laser.…”

“…The repetition rate can be easily adjusted form 750 MHz to 1 GHz. 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 microstructure fibers [5]. The reduced peak power associated with increasing the repetition rate maintains stable Kerr lens mode-locking and also keeps enough self-phase modulation to maintain the significant spectral broadening.…”