a transportable optical frequency comb based on a mode ... · a transportable optical frequency...

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

2

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.


3

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


4

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.


5

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


6

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’


7

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).


8

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.


9

Combs at NPL •

A comb based on a Ti:sapphire laser is currently in use at NPL

•

It can be used to measurefrequencies 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.


10

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


11

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.


12

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.


13

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.


14

MenloSystems FC1500 •

A diode-pumped erbium-doped fibre laser system operating at 1.5 μm, with frep = 100 MHz.


15

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.


16

Reference signal performance

0 5 10 15 20-4x10 -12

-2x10 -12

0.0

2x10 -12

4x10 -12

Time (days)

Nor

mal

ized

freq

uenc

y de

viat

ion

from

10

MH

z ov

er a

3 h

our a

vera

ging

per

iod

101 102 103 104 105 106

10-14

10-13

10-12

10-11

Alla

n de

viat

ion

Time interval τ (s)

Quartz + Rubidium + GPS Rubidium + GPS H Maser


17

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.

Laser & EDFA

2 EDFAs, IR broadening

f:2f interferometer

Frequency doubler

Visible broadening


18


•

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.


19


•

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.


20

Long-term frequency measurements•

The frequency of a commercial stabilized594 nm helium-neonlaser was measuredover an 18 hour period.

•

A stable HeNe

is usuallycalibrated against an iodine-stabilised HeNe, however a 594 nm I2

-stabilized laser was not available.

0

5

10

15

20

25

0 5 10 15 20

Time (hours)

Freq

uenc

y - 5

0461

8992

MH

z (M

Hz)


21

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.


22

Comb comparison results

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.

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.)


23

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.

a transportable optical frequency comb based on a mode ... · a transportable optical frequency...

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