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optical frequency, frequency comb, atom interferometer, optical frequency comb, fibre laser, clock transition, frequency measurements, phase noise, laser frequency, optical frequency standards, atomic clocks, frequency synthesizer, optical frequency metrology, yag laser, himeji institute of technology
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Document Sample
A transportable optical frequency comb
based on a mode-locked fibre laser
B. R. Walton, H. S. Margolis, V. Tsatourian
and P. Gill
National Physical Laboratory
Joint meeting for Time and Frequency Club and Location
and Timing KTN, 27 November 2007
Wednesday, 14 May 2008
Motivation
• Frequency combs have revolutionised
optical metrology.
• So far frequency combs are too large to
move between labs → reduced flexibility.
• Therefore need to find a robust system with
small footprint, low weight, to make
transportable.
2
Wednesday, 14 May 2008
Outline
• An introduction to frequency combs
• A description of the transportable frequency comb
system
• Long-term frequency measurements
• A ‘comb comparison’ between the transportable
comb and another frequency comb system
3
Wednesday, 14 May 2008
Optical femtosecond frequency combs
• Based on a mode-locked pulsed laser: generates a ‘comb’ of
phase-coherent optical modes, equally spaced in frequency.
• The mode frequencies can be locked to a stable frequency
standard (e.g. a hydrogen maser).
• Can be used to measure the frequency of almost any stable
optical laser source.
• Conversely, can be used to generate stable signals at
almost any optical frequency.
4
Wednesday, 14 May 2008
Optical femtosecond frequency combs
• Modes are spaced by
the repetition rate of
the pulses (frep ∝ 1 / Lcavity )
• Dispersive elements in
the cavity lead to a
carrier-envelope phase
slip.
• This manifests as an
offset of the mode
spectrum by f0.
• Frequency of nth mode is therefore fn = n frep + f0
5
Wednesday, 14 May 2008
Finding the offset frequency f0
• frep is found from inter-mode RF-frequency beats on a photodetector.
• f0 can be found if the comb spectrum covers at least one octave.
• This usually requires the comb spectrum to be broadened by passage
through a non-linear microstructured fibre.
‘f:2f interferometer’
6
Wednesday, 14 May 2008
Microstructured fibre
• The beam is tightly guided in the core by a high refractive index contrast.
• This enhances optical non-linear effects which broaden the spectrum.
• Coherent non-linear effects
preserve the comb structure
(i.e. frep, f0 are unchanged).
7
Wednesday, 14 May 2008
Optical frequency measurement
• The frequency flaser of a stable laser may be measured by
beating it with the nearest comb mode.
• More generally:s apples
flaser = nfrep ± f0 ± fbeat
• The optical frequency flaser may therefore be related in a single
step to countable RF frequency signals.
• These signals can be referenced to the signal from a caesium
fountain or hydrogen maser, providing traceability
to the SI second.
8
Wednesday, 14 May 2008
Combs at NPL
• A comb based on a Ti:sapphire laser is currently in use at NPL
• It can be used to measure
frequencies between ~500 –
1000 nm.
• Prisms for dispersion
compensation require long
cavity → large footprint
(frep ~ 90 MHz).
• Small cavity (frep ~ 800 MHz) comb under development at NPL
(compensation with chirped mirrors) → reduced footprint.
• The high pump laser power (several Watts) makes
Ti:sapphire lasers unsuitable for transportable combs.
9
Wednesday, 14 May 2008
Comb measurements at NPL
• The Ti:sapphire comb has been used to determine absolute
frequencies of several optical frequency standards at NPL:
– Trapped single-ion standards:
• 88Sr+ quadrupole transition at 674 nm
(with an uncertainty of 3.4 ×10-15 )
• 171Yb+ octupole transition at 467 nm
– Experiments probing 2S–nS,D
transitions in the hydrogen atom to
determine the Rydberg constant via
2-photon interactions
10
Wednesday, 14 May 2008
Comb measurements at NPL
• The Ti:sapphire comb has been used to determine absolute
frequencies of several optical frequency standards at NPL:
– Acetylene gas-cell standard at (1.5 μm – outside comb
spectral range – transfer lasers at 1542 and 771 nm were
required),
– Calibration of iodine-stabilized
helium neon lasers at 633, 594
and 543 nm.
11
Wednesday, 14 May 2008
Why a transportable comb?
• Gives laboratories without a frequency comb access
to optical frequency metrology.
• Enables NPL to expand scope of comb applications,
e.g. dimensional metrology, spectroscopy.
12
Wednesday, 14 May 2008
Transportable combs – requirements
• Compact. Either high repetition-rate (small cavity →
small footprint) or fibre system (can coil fibre).
• Robust. Fibre systems have advantage - fibre
coupling reduces alignment drift.
• Low pump power → low power supply weight.
• Large spectral measurement range → maximise
usefulness and flexibility of the system.
13
Wednesday, 14 May 2008
MenloSystems FC1500
• A diode-pumped erbium-doped fibre laser system
operating at 1.5 μm, with frep = 100 MHz.
14
Wednesday, 14 May 2008
GPS-disciplined reference signal
• The GPS network provides a timing signal that enables comb
measurements to be traced to the SI second.
• The transportable comb uses a 10 MHz reference signal from a
Rapco quartz oscillator which is locked to a rubidium oscillator
which is in turn locked to the GPS signal.
15
Wednesday, 14 May 2008
Reference signal performance
-11
10 MHz over a 3 hour averaging period
-12 10
4x10
Normalized frequency deviation from
Quartz + Rubidium + GPS
Rubidium + GPS
H Maser
2x10-12 10
-12
Allan deviation
0.0
-13
10
-2x10-12
-14
10
-4x10-12
0 5 10 15 20 10
1 2
10 10
3
10
4
10
5
10
6
Time (days) Time interval τ (s)
16
Wednesday, 14 May 2008
Mounting arrangement
• The system is mounted on a wheeled aluminium frame
measuring 1.21 m (height), 0.95 m (width) and 1.72 m (length).
• Robust – system has performed measurements with minimal
readjustment after transportation over rough ground.
Frequency doubler
2 EDFAs, IR broadening
Laser & EDFA
f:2f interferometer Visible broadening
17
Wednesday, 14 May 2008
Long-term frequency measurements
• Long-term measurements were undertaken to determine the
stability and robustness of the system.
• The transportable frequency comb was used to measure the
frequency of a high-finesse-cavity-stabilised diode laser at
674 nm.
• A tracking oscillator (TO) was locked to fbeat to amplify the beat
without giving additional noise.
• Measurements were referenced to a hydrogen maser signal.
18
Wednesday, 14 May 2008
Long-term frequency measurements
• A continuous
measurement of the
laser frequency was
performed for more
than 60 hours.
• Drift in frequency due
to variation in high-
finesse cavity length.
19
Wednesday, 14 May 2008
Long-term frequency measurements
• The frequency of a 25
commercial stabilized
Frequency - 504618992 MHz (MHz)
594 nm helium-neon 20
laser was measured
over an 18 hour period. 15
• A stable HeNe is usually 10
calibrated against an
iodine-stabilised HeNe, 5
however a 594 nm
I2-stabilized laser was 0
0 5 10 15 20
not available.
Time (hours)
20
Wednesday, 14 May 2008
Comb comparison
• A concurrent measurement of the same frequency by
a separate comb system is an important check of the
comb’s accuracy and stability.
• A measurement of the frequency of a 934 nm beam
from a cavity-stabilised CW Ti:sapphire laser was
performed.
• The measurement was performed simultaneously by
the transportable comb and by the Ti:sapphire comb.
21
Wednesday, 14 May 2008
Comb comparison results
Stability
• Transportable comb:
σ (10 s) = 1.0×10-12 (GPS ref.)
N.B. GPS-disciplined oscillator:
σ (10 s) = 1×10-12
σ (10 s) = 2.4×10-13 (Maser ref.)
• Ti:Sapphire comb:
σ (10 s) = 1.8×10-13 (Maser ref.)
Accuracy
Mean GPS-referenced comb measurement differed by
+9.0×10-13 compared with Ti:sapphire comb. Accuracy of
GPS-disciplined oscillator: ~4×10-12 over a few hours.
Good enough for most applications.
22
Wednesday, 14 May 2008
Summary
• A new transportable frequency comb system is in use
at NPL.
• The system has been used to perform continuous
measurements over more than 60 hours.
• A comparison between this system and another comb
has shown that the transportable comb has a GPS-
reference-limited stability of 1.0×10-12 at 10 seconds,
and an accuracy of approximately 4×10-12 when
averaged over a few hours.
23
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