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
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
; T = 120 fs
;λ = 20 nm
λ0 = 1,070 nm
Pulse characteristics are measured by frequency resolved
1,090 1,070 1,050
–500 0 500
than by the fiber frequency comb operated only at 30 Watts—
40 percent of its maximum power.
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.
Axel Ruehl ( email@example.com) is a postdoc in the department of physics and astronomy at VU University Amsterdam, the Netherlands. Member
[ References and Resources ]
>> A. Ruehl et al. Opt. Lett. 35, 3015 (2010).
>> A. Cingöz et al. Opt. Lett. 36, 743 (2011).
>> L. Nugent-Glandorf et al. Opt. Lett. 36, 1578 (2011).
>> A. Ruehl et al. Phys. Rev. A 84, 011806 (2011).
>> C. Benko et al. arXiv:1202.5199v1 (2012).
>> A. Cingöz et al. Nature 482, 68 (2012).