Optics & Photonics News

The key issue for supercontinuum generation is the preservation of
coherence and comb structure.

generate high harmonics in noble gases. This approach relies on high-power frequency combs that are fully compatible with low-noise phase control.

Chirped pulse amplification in large-mode-area fibers allows for easy power scaling of high-repetition-rate fiber laser systems. However, low noise optical phase control in frequency combs also requires pedestal-free ultra-short pulses. This specification is difficult to fulfill because not only the mismatch in group-delay between stretcher and compressor need to be compensated but material dispersion introduces significant contributions in fiber laser systems.

Fortunately, tailored dispersion properties are available from many fibers, such as high-order mode fibers or depressed cladding fibers. A combination of these can be used to compensate almost arbitrary dispersion profiles and provide an integrated and alignment-free platform unique for high-power chirped pulse amplification.

An 80-W fiber frequency comb based on the oscillator design we described here was recently demonstrated. It highlighted the unique power scaling capability of Yb:fiber frequency combs. By carefully adjusting the length of the stretcher fibers, the researchers optimized the pulse compression until they achieved 120-fs pulses. The pulse characteristic shown on the right was measured by frequency-resolved optical gating and revealed that only 14 percent of the pulse energy remains in the pedestal. The deviations from the ideal phase—especially at the blue side of the optical spectrum—are attributed to the residual fourth-order dispersion, which, according to theoretical estimations, is only compensated by 90 percent.

Due to the strictly linear amplification, the B-integral ( representing the integrated nonlinear phase shift) is as low as 0.2. The pulse characteristic is independent of the output power, highlighting the amplification at negligible nonlinear phase shifts. Such linear cladding-pumped fiber amplifier schemes are fully compatible with low-noise phase control. This was confirmed in long-term measurements where the frequency comb was phase-locked to an Rb-microwave clock for more than eight hours. Both in-loop signals were simultaneously counted with 1-s gate time and exhibited an instability of 0.88 mHz RMS for f0 and 0.35 mHz for frep.

The laser system described here was used to demonstrate the first full repetition rate XUV frequency comb covering wavelengths down to 40 nm, pushing the boundaries of frequency comb technology to unprecedented wavelength ranges. By isolating single harmonics, it was possible to resolve high-energy transitions of argon at 82 nm and neon at 63 nm revealing an upper boundary for the width of individual comb teeth of less than 10 MHz. The performance of this XUV comb is currently limited by the enhancement cavity, rather

1.0

2

Intensity [a.u.]

0.0

Measured Gaussian ;t

Retrieved Intensity

Phase Retrieved

; T = 120 fs

Phase [rad]

;λ = 20 nm

λ0 = 1,070 nm
- 4

Pulse characteristics are measured by frequency resolved optical grating.

1,090 1,070 1,050

Wavelength [nm]
–500 0 500

Time [fs]

than by the fiber frequency comb operated only at 30 Watts— 40 percent of its maximum power.

Summing up

Fiber laser technology provides a unique technological platform for integrated and alignment-free laser setups. Frequency combs based on ultrafast fiber lasers became an indispensable tool for many emerging applications in fundamental and applied science, including optical metrology, atomic clock calibration, low-noise frequency synthesis and especially high-precision spectroscopy. Due to the huge potential for power scaling, Yb:fiber frequency combs are ideally suited for frequency conversion schemes extending the wavelength coverage of frequency combs beyond spectral regions accessible by laser gain materials. t

The contributions of I. Hartl and M.E. Fermann at IMRA America Inc., U.S.A.; A. Gambetta and M. Marangoni at Politecnico di Milano, Italy; K.S.E. Eikema at VU University, Amsterdam, the Netherlands; and M.J. Martin, C. Benko, D.C. Yost, A. Cingöz and J. Ye at JILA, University of Colorado and NIST, U.S.A., are acknowledged and greatly appreciated.