1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Application of Femtosecond Laser Frequency Combs toward Frequency Stability Measurement for DPF Light Source

Shigeo Nagano, Hiroyuki Ito and Mizuhiko Hosokawa

Atomic Frequency Standards Project, Space-Time Standards Group,

National Institute of Information and Communications Technology (NICT)4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795 Japan

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Motivation・For DECIGO and DECIGO-pathfinder, the laser frequency stability of 1Hz/Hz1/2 level is required from 0.1Hz and 1Hz.

Such high stability was recently achieved in laboratory for clock lasers of the optical frequency standards.

・Reliable measurement of the laser frequency stability can provide technical feedbacks to the laser development, and thus the measurement can be one of the key point in order to succeed the DECIGO and DPF missions.

・To exclude the possibility of common mode noise in the frequency stability measurement, beat measurements were often taken between the lasers stabilized to the atomic resonance and Fabry-Perot cavity.

・By means of optical frequency comb based on femtoseond-pulse mode-locked lasers (OFCs), the laser frequency stability can be compared with cryogenic sapphire microwave oscillator with the measurement accuracy of 1Hz/Hz1/2 level. (Note that the frequency difference between light and microwave is ~105Hz.)

This is also good application of OFCs developed for the optical frequency standards in National Institute of Information and Communications Technology.

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Optical frequency standards in NICT・National Institute of Information and Communications Technology (NICT) is

Japanese official institution for the national frequency and time standards.

・Atomic frequency standards in NICT

- NICT-CsF1 : Cesium atomic fountain clock (microwave)

operation from 2007 with uncertainty of 1.9 x 10-15 and stability of 2 x 10-13 at 1s

- Optical clocks based on laser-cooled single 40Ca+ ion

and cold 87Sr atoms in optical lattice

potential for higher accuracy (10-18) than microwave standards

lead a new definition of SI second

new knowledge regarding fundamental physics

・Optical frequency comb based on femtosecond-pulse mode-locked laser (OFC)

serves as optical frequency counter and optical synthesizer.

OFC can be also applied to frequency stability measurement for DECIGO and DPF laser

with cryogenic sapphire microwave oscillator

Single 40Ca+ ion trap 87Sr optical lattice

Cs atomic fountain NICT-CsF1

CryogenicSapphireoscillator

Japan standard time(JST)

Sr atoms in MOT

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Frequency stability measurement -principle-

・Time domain

Carrier-envelope phase slip from pulse to pulse

Because grope velocity is not equal phase

velocity (vg≠vp)

・Frequency domain

m-th mode frequency:　fm = m frep + fCEO

frep : pulse repetition frequency

fCEO =∆φCEO/2πτ : carrier-envelope offset frequency

m : integer mode number

For frequency stability measurement, frep and fCEO must be measured and controlled.- frep: measured by direct detection of pulse train, controlled with stable microwave reference- fCEO : measured by the self-referencing technique, also controlled with stable microwave reference

Mode-lockedlaser

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Frequency stability measurement -principle-

・Time domain

Carrier-envelope phase slip from pulse to pulse

because of grope velocity is not equal phase velocity

・Frequency domain

m-th mode frequency:　fm = m frep + fCEO

frep : pulse repetition frequency

fCEO =∆φCEO/2πτ : carrier-envelope offset frequency

m : integer mode number

・Self-referencing technique: fceo is a frequency of heterodyne beat between the second harmonics of m-th mode and 2m-th mode. Need a very broadband spectrum (i.e. an octave) Spectrum broadening with photonic crystal fiber

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Frequency stability measurement -principle-

・Time domain

Carrier-envelope phase slip from pulse to pulse

because of grope velocity is not equal phase velocity

・Frequency domain

m-th mode frequency:　fm = m frep + fCEO

frep : pulse repetition frequency

fCEO =∆φCEO/2πτ : carrier-envelope offset frequency

m : integer mode number

For frequency stability measurement, a heterodyne beat measurementbetween CW laser and m-th mode of optical comb.

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Femtosecond laser frequency combs in NICT・ FC8003 (Menlo Systems GmbH)

Photonic crystal fiber for spectrum broadening over one octave

Straightforward now

Not reliable for long-term operation (> a few hours)

・Venteon OS (NanoLayers GmbH)

Pulse repetition frequency : 200MHz

Output spectrum coverage : 1 octave

Reliable for long-term operation

・ Gigajet 20W (Gigaoptics GmbH)

Pulse repetition frequency : 1GHz

Output spectrum coverage : 2/3 of one octave

Reliable for long-term operation

FC8003 (Menlo system GmbH)

Venteon OS (NanoLayers GmbH)

Gigajet 20W (Gigaoptics GmbH)

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Gigajet20W based optical frequency comb・Kerr-lens mode-locked

femtosecond-pulse Ti:sapphire laser

・Pulse repetition frequency : 1GHz

・Average output power : 800mW@8.5W pump power

・Pulse width : 26fs @951nm

・Output spectrum : 2/3 of one octave

・2f-to-3f self-referencing technique :

beat measurement of 960nm (THG)

and 640nm (SHG)

∵3 fm -2 f3m/2 = 3(m frep+fceo) – 2(3m/2 frep+fceo)

= fceo

・Stable long-term operation over 19 hours without any alignment

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Cryogenic sapphire oscillator・High quality HEMEX single-crystal sapphire

・Temperature : 7.3K

・High unloaded quality factor : ~109

・Whispering gallery resonant mode : WGH16,0,0

・Oscillation frequency : 11.2005GHz

Synthesis chain generates 1GHz signal

・Excellent short-term stability : <10-15 between 2s~700s, minimum 5x10-16 at 30s~60s

・Good frequency reference for optical frequency comb in order to evaluate the DPF light source

Originally introduced as flywheel oscillator for Cs atomic fountain

・Developed in University of Western Australia (UWA)

Sapphire and niobium cavity Baffles Liquid-helium dewar

F. Riehle, Frequency Standards, Wiley-VCH Verlag GmbH (2004)

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Frequency stability measurement by optical comb・Femtosecond laser frequency comb referenced to cryogenic sapphire oscillator

・Frequency stabilized laser for observation of atomic resonance (clock laser)

Wavelength 729nm, output power few mW

High-finesse Fabry-Perot cavity with ultralow expansion glass (ULE) spacer (F~150000)

Narrow line width (<25Hz)

・Measured frequency stability

5 x 10-15@1s~10s

Allan variance is a measure of frequency stability of oscillator

Vacuum chamber for ULE cavity

y2 =

⟨ y i1− y i2⟩

2, here y i≡

1∫ti

ti t 0

dt

Corresponding Allan variance of power spectral density of frequency noise δν(f)

y2 = 1

2 f

02

Inst

abilit

y

1st International LISA-DECIGO Workshop (November 12-13, 2008, Sagamihara, Japan)

Summary・National Institute of Information and Communications Technology plans to measure the frequency stability of the DPF light source by femtosecond laser optical frequency comb (OFC) referenced to cryogenic sapphire microwave oscillator.　

・The frequency stability of clock laser for optical frequency standard was measured by the OFC, and it was found to be 5 x 10-15 in root Allan variance between 1s~10s.

This is the first measurement with 10-15 level by the OFC referenced the CSO, to the best of our knowledge.

・Further improvement of the measurement accuracy is required to confirm the frequency stability of 1Hz/Hz1/2.