CSEM Scientific and Technical Report 2008

Toward High-performance Frequency Comb for Metrology Applications
S. Lecomte
In the last decade, optical frequency combs based on mode-locked femtosecond lasers have revolutionized the optical frequency metrology field.
Different laser technologies have been investigated but the lack of a high-performance convenient frequency comb is still an issue for widespread
application of the technology. In the frame of a stay at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, the author
participated in the development of a fully-stabilized high-performance frequency comb based on diode-pumped passively mode-locked solid-state
laser technology.
The optical frequency comb generated by a stabilized
femtosecond laser is now a widely used tool in the field of
optical frequency metrology. It effectively replaces the old,
complicated, and cumbersome frequency chain developed
only in a few national metrology laboratories [ 1] .The 2005
Nobel Prize in Physics was even partially awarded to Theodor
W. Hänsch and John Hall for the invention of the stabilized
frequency comb. Many new technologies and applications
already profit from the extremely-precise set of optical
frequencies generated by a frequency comb. Some relevant
examples include optical clock clockwork [ 2] , parallel multiwavelength
spectrometry [3] , arbitrary optical pulse waveform
generation, etc. Another very interesting opportunity is the
generation of ultra-low phase noise microwaves [4] .
The CSEM Time and Frequency division is actively planning
activities in the field of stabilized femtosecond lasers. In this
frame, the author spent three months in 2008 in the group of
S. A. Diddams of the NIST Time and Frequency division in
Boulder, Colorado. Dr. Diddams is one of the worldwide
leaders in the area of stabilized frequency combs and their
applications. Its contributions to the field are numerous and
imply the development of new comb sources as well as the
implementation of the technology in different applications.
Moreover NIST is a unique place where many of the best
narrow linewidth continuous wave lasers as well as microwave
and optical clocks are located and pushed to extreme limits.
During the NIST stay, emphasis was put on the development
of a frequency comb based on diode-pumped passively modelocked
solid-state laser technology. This approach potentially
combines the advantages of the Ti:sapphire and fiber lasers.
A diode-pumped solid-state laser could hence offer the
excellent optical and microwave spectral purity, high repetition
rate, and high-average output power of Ti:sapphire lasers as
well as the small form factor, low-cost, and efficiency of fiber
lasers.
In previous work on Yb:KYW femtosecond laser, the NIST
group reported the detection and locking of the carrierenvelope
offset frequency (fCEO) [ 5] . The octave-spanning
spectrum generated in a microstructured fiber covers the
spectral range from 650 nm to 1400 nm. fCEO is then detected
using an f-2f interferometer. The integrated phase noise of
fCEO from 1 Hz to 1 MHz is 0.3 radians, which is better than the
values reported for Er:fiber combs [6] .
In the current work one line of the comb is locked to a cavity
stabilized laser at 1126 nm, which is the Hg + -ion fundamental
clock laser [ 7] . This control leads to a fully-stabilized comb,
which should possess 1-s fractional frequency instability at the
level of 1x10 -15 in both the optical and microwave outputs. As
a test of the microwave output of the stabilized frequency
comb, the short term (1 s) instability on the laser repetition
rate (frep) was detected. The measured instability is currently
limited by the RF reference (H-maser) at the level of 2-3x10 -13
from 1 s to 50 s (see Figure 1 below). For longer integration
times, the drift of the optical reference cavity is observed.
Figure 1: Measured Allan deviation of the frequency-stabilized laser
repetition rate (frep) vs. a H-maser.
The presented results demonstrate the potential of the
technology [8] .
Funding for a new dedicated laboratory for optical frequency
comb developments at CSEM has been awarded and
activities in the field should start soon.
[1] H. Schnatz, et al., “First Phase-Coherent Measurement of Visible
Radiation”, Phys. Rev. Lett., 76 (1996), 18-21
[2] D. J. Jones, et al., “Carrier-Envelope Phase Control of
Femtosecond Mode-Locked Lasers and Direct Optical
Frequency Synthesis”, Science 288 (2000), 635-639
[3] S. A. Diddams, et al., “Molecular Fingerprinting with Spectrally-
Resolved Modes of a Femtosecond Laser Frequency Comb”,
Nature 445 (2007), 627-630
[4] J. J. McFerran, et al., “Low Noise Synthesis of Microwave
Signals from an Optical Source”, Electron. Lett. 41 (2006), 36-37
[5] S. A. Meyer, et al., “A Diode-Pumped Yb:KYW Femtosecond
Laser Frequency Comb with Stabilized Carrier-Envelope Offset
Frequency”, Eur. Phys. J. D. (2008), 1-8
[6] J. J. McFerran, et al., “Elimination of Pump-Induced Frequency
Jitter on Fiber-Laser Frequency Combs”, Opt. Lett. 31 (2006),
1997-1999
[7] T. Rosenband, et al., “Frequency Ratio of Al + and Hg + Single-Ion
Optical Clocks; Metrology at the 17th Decimal Place”, Science
319 (2008), 1808-1812
[8] S. A. Meyer, et al., “An Optically-Stabilized Yb:KY(WO4)2
Frequency Comb”, submitted to EFTF 2009
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