Hz-level cw-laser
Intensity [dB]
Numerical simulations
Experimental results
Raman
solitons
1,000 1,200 800
Phase
lock Second
harmonic
generation
Self-
referencing
& phase lock
Pump
wavelength
Zero dispersion
wavelength
600
Wavelength [nm] 1,400
1,600
Out-of-loop
measurement
1
Power [a.u.]
(b)
0.5
Exp. data
Lorentzian ;t
0
- 20 0 20
Fourier frequency [Hz]
Central wavelength [nm]
1,400
1,000
200 100 0
Tuning of the Raman solitons
40
1,800
(c)
60
Spectral width
FWHM [nm]
1. 5 Hz FWHM
20
Launched pump power [m W]
The superconinuum was created in a suspended-core fiber. (a) The pump
wavelength and the zero dispersion wavelength of the fiber are shown,
along with a schematic of the coherence measurement using two narrow-linewidth cw lasers. (b) Out-of-loop beat signal between the Yb:fiber
frequency comb and a cw laser at 1. 54 mm exhibiting 1. 5 Hz linewidth.
(c) Tuning characteristic of the longest wavelength Raman soliton.
heterodyne beat between the phase-locked Yb:fiber
frequency comb and a Hz-level cw laser pictured
to the left exhibits a linewidth of 1. 5 Hz. This corresponds to a coherence time of 200 ms or 3 × 107
pulses from the 154 MHz pulse train.
Detailed numerical simulations revealed no
changes in the phase standard deviation of the
supercontinuum comb lines, indicating that the
supercontinuum is not the limiting factor in this
measurement. This is supported by the fact that the
measured heterodyne beat signal corresponds to the
estimated optical linewidth of the 1. 54 mm laser.
Yb:fiber frequency combs can coherently bridge the
large spectral gap between the visible wavelengths
(where optical clocks operate) and the c-band of
optical telecommunication used for long-distance
frequency dissemination via fiber links in a simple
and robust scheme.
phase stabilization demonstrate that optical coherence can be
established anywhere within the spectral coverage, which is
significantly extended by supercontinuum generation.
Supercontinuum
Nonlinear spectral broadening in highly nonlinear fibers is
another building block for fiber frequency combs because
octave-spanning optical spectra required for self-referencing
are not directly available from fiber oscillators. Supercontinuum generation is also essential for all applications reliant on
wavelengths outside the laser gain bandwidth. The key issue
for supercontinuum generation is the preservation of coherence and comb structure.
When ultrashort pulses pump a nonlinear fiber in the
anomalous dispersion regime, the spectral broadening process is
an interplay among many nonlinear processes, such as self-phase
modulation, soliton fission, four-wave mixing and intrapulse
Raman scattering. For the supercontinuum shown in the figure,
we launched the pulse train at an energy of 1. 2 nJ into a 24-cm
long suspended-core fiber, and did numerical simulations on
the basis of a generalized nonlinear Schrödinger equation.
They showed a remarkable agreement with the experiment.
Several spectral features of the supercontinuum generation can
be clearly distinguished, including the Raman-shifted solitons
at long wavelengths and the corresponding dispersive waves
in the visible spectrum connected by four-wave mixing.
Optimization of both the pump pulse and the nonlinear
fiber enables preserving the phase coherence during spectral broadening. It can be optimized to transfer coherence
over the entire spectral width of the supercontinuum. The
Raman solitons
Raman solitons—spectral features of supercontinuum generation—are stable and temporally isolated
pulses that exhibit an interesting feature. Their tun-ability allows for pulse generation between 1. 1 and
1. 6 mm, phase-locked to the driving pulse as shown
in the figure on the left. The original pulse train at 1.05 mm
still exhibits Watt-level average powers. Such a coherent two-color source constitutes an ideal basis for difference frequency
generation. As both fields were generated with the same source,
the resulting idler output is offset-free, requiring only frep to
be phase-locked. This nonlinear mixing scheme enables a tunable frequency comb source in the MIR spectrum. It is highly
desirable for spectroscopy in the molecular fingerprint region,
such as trace gas detection. Based on difference frequency
generation in a gallium selenide crystal, we recently demonstrated a MIR source continuously tunable from 3 to 10 mm
at m W-power level.
Average power scaling
The true strength of Yb:fiber technology is its ability to scale
the average power of ultrafast laser systems to almost the
kilowatt level. This feature is particularly interesting for
frequency conversion schemes, pushing frequency comb technology into wavelength regions not accessible by lasers but
desired for spectroscopic measurements. The most demanding
of these schemes is high harmonic generation enabling coherent radiation in the XUV spectral region. The reduction of
the repetition rate to kHz-level is usually applied to reach the
required peak intensities of 1013 to 1014 W/cm2, but this can’t
be used with frequency combs.
Passive enhancement cavities provide a solution to this
problem, as they are capable of increasing the average power of
the driving frequency comb by a factor of several hundred. In
the intracavity focus, the peak intensities are high enough to
34 | OPN Optics & Photonics News
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