An Ultra-Stable Optical Frequency Reference

for Space

1

Thilo Schuldt, Klaus Döringshoff, Evgeny Kovalchuk,

Julia Pahl, Martin Gohlke, Dennis Weise,

Ulrich Johann, Achim Peters, Claus Braxmaier

DLR – Institute of Space Systems, Bremen

ICSO Tenerife,

October 9th, 2014

• Motivation (w/ focus on mSTAR mission)

• Iodine-based frequency references

• Assembly-Integration Technology for Optical Systems

• Iodine-based frequency references on EBB & EM level

• Summary & Outlook

Contents

Highly stable optical clocks as key technology for

future space missions Earth observation

Science

Navigation

Here: We present an Iodine-based frequency

reference with a frequency stability at the 10-15 level

(at longer integration times), which can be developed

for space on a relatively short time scale.

NG2

Next Generation Gravity Mission

Part I: Technical Proposal

All the space you need

Astrium GmbHProposal No.: A2009-4028-0-1In Response to: ESA ITT AO/ 1-5914/ 09/ NL/ CT

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Motivation

Motivation: mSTAR

• mini SpaceTime Asymmetry Research

• Test of Special Relativity: Kennedy-Thorndike Experiment testing

the boost dependency of the speed of light

• Payload: Iodine-based and Cavity-based frequency references

• International collaboration

Saudi-Arabia:

- King Abdulaziz City of Science and Technology (KACST)

USA:

- Stanford University

- NASA Ames Research Center

Germany:

- DLR-Institute of Space Systems, Bremen

- Humboldt-University Berlin

- Center of Applied Space Technology and Microgravity (ZARM), Bremen

Motivation: mSTAR

DMU

Optical IF

Thermal Enclosure

Electrical IF

I/F to S/C: struts,

isostatic thermal washers

I/F

to

On

-Bo

ard C

om

pute

r

Laser

Control

Laser

AOMSHG

Iodine Clock Cavity Clock

Control

Electronics

Control

Electronics

• Phase A study ongoing investigating the

implementation on a SaudiSat 4 satellite bus

• Available payload budgets (TBC):

max 50 W

max 30 kg

JPL

Cavity Clock

Iodine Clock

DLR, HUB, HTWG

Iodine Frequency References

• NPRO-type Nd:YAG laser @ 1064nm

intrinsically high intensity and

frequency stability

frequency-doubled to 532nm

• hyperfine transition in molecular iodine taken as reference

(a10 component of R(56)32-0 near 532nm)

strong absorption

small natural linewidth (380kHz)

• State-of-the-art technology realized in various laboratories worldwide

Iodine Frequency References

Laboratory setup at the Humboldt-University Berlin

• Fiber-coupled setup

• Modulation transfer

spectroscopy

• 80 cm long iodine cell in

single-pass configuration

• Fibre EOM (low driving

voltage, low RAM due to low

temperature drift)

• Intensity stabilization of

pump and probe beams via

AOMs

• Noise-cancelling detection

Probe

PumpNd:YAG

AO

M

~

ref. beam

error signal

cold finger at -15°C

to beat measurement

IR

532 nm

80 cm iodine cell

275 kHz

ë/2 NC

ë/2

Intensity stabilization PDs

Pol

Pol

AO

M

fiber EOM

loo

p filt

er

T

PZT

Iodine Frequency References

For space applications, laboratory setup

... must be further developed with respect to

compactness

mechanical and thermal stability

Assembly-Integration Technology

Quasi-monolithic setup with optical components directly bonded to the

baseplate

improved pointing stability

AI technology suitable for space applications

Flight Model Optical Bench for LISA Pathfinder

Optical components are fixed to the baseplate

using hydroxide-catalysis bonding technology

Assembly-Integration Technology

Alternative Assembly-Integration Technology: adhesive bonding

• Use of space-qualified two component epoxy

• Use of a specifically designed alignment jig

• Used for glass-glass and metal-glass joints

• AI technology first demonstrated by realizing a heterodyne

interferometer setup with pm & nrad sensitivity (LISA context)

Spectroscopy Setup on EBB Level

• Compact and robust spectroscopy setup

dimensions: (600 x 300 x 100) mm3

• Layout based on the laboratory setup

• Use of a baseplate made of ultra-low expansion glass ceramics

Clearceram-HS by OHARA with a CTE of 2*10-8 K-1

• Wedged (25 x 35 x 8) mm3 optics made of fused silica

• Commercial 30 cm long iodine cell in triple-pass configuration

• Pairs of wedged glass plates for beam adjustment after integration

• Mechanical mounts made of Invar (e.g. for waveplates, polarizers,...)

• Use of adhesive bonding AI technology

Spectroscopy Setup on EBB Level

55cm

25

cm

Laser System

• use of NPRO-type Nd:YAG laser @ 1064nm

this type of laser is also available in a space qualified version (TESAT)

• Second harmonic generation via fiber-coupled waveguide PPLNs

• Acousto-optic modulators used for intensity stabilization, frequency shift,

frequency modulation and RAM suppression

Spectroscopy Setup on EM Level

• Advanced setup based on the EBB experience

• Further improved with respect to compactness and mechanical stability

• Using adhesive bonding AI technology

• Implementation of a compact multipass gas cell with improved robust

cooling finger design

• Baseplate made of fused silica in order to guarantee high thermal stability

due to CTE matching with optical components and cell

• Performance of environmental tests (vibration test and thermal cycling)

Spectroscopy Setup on EM Level

Multi-pass gas cell

• (10 x 10 x 3) cm3 cell made of fused silica

• Windows optically contacted

• Designed for 9 pass operation

• Adapted robust cooling finger design

10cm

10

cm

pump

iodine cell coolingBS

probe

TFP

NC

I-stab

s-pol

I-stab

& RAM

L/2

L/2 L/2TFPTFP

BS

10%10%

L/2

TFP

L/2p-pol

Spectroscopy Setup on EM Level

Spectroscopy Setup on EM Level

Spectroscopy Setup on EM Level

Spectroscopy Setup on EM Level

Vibration Testing of the EM Setup

• Separate test of optical setup and iodine cell

• Lisa Pathfinder LTP specifications as basis

• All axis

Sine vibration up to 30 g

Random vibration up to 25.1 grms

Frequency Level Frequency Level

5 - 21 16.5 mm 20 - 100 +3 dB/oct

21 - 60 30 g 100 - 300 11 g²/Hz

60 - 100 9 g 300 - 2000 -5 dB/oct

Sine vibration (x,y,z axis) Random vibration (x,y,z axis)

1 sweep up, 2 oct./min 25.1 g rms, 2 min

Thermal Testing of the EM Setup

Specifications combined according to LTP and ECSS

Maximum: -20°C to +60°C

Temperature range -20°C - +60°C (±2 °C)

Ambient condition atmospheric, ambient pressure

# of cycles 8

Dwell time 120 minutes at min. and max. temperature

Rate 1 K/minute

Thermal cycling specifications

Environmental Testing

• Measurements before and after each test

• Performance unchanged

• Reproducibility < 250 Hz (8.8×10-13)

• Frequency offset between EBB and EM < 1.5 kHz

10-6

10-5

10-4

10-3

10-2

10-1

10010

0

101

102

103

104

105

Fourier Frequency ( Hz)

Fre

quen

cy n

ois

e A

SD

( H

z/

Hz)

10-6

10-5

10-4

10-3

10-2

10-1

10010

0

101

102

103

104

105

Fourier Frequency ( Hz)

Fre

quen

cy n

ois

e A

SD

( H

z/

Hz)

before environmental tests

after first shaker

after therm cycling

after second shaker

LISA

GRACE-FO

Frequency measurement

Shaker test: EM cell

Shaker test: EM bench

Thermal cycling: EM

Frequency measurement

Frequency measurement

Frequency measurement

Summary & Outlook

• Absolute frequency reference based on molecular Iodine

• Two setups realized on EBB and EM level

• Adhesive bonding used for assembly-integration

• EM setup environmentally tested (vibration, thermal cycling)

• Setups achieve frequency stability of < 4*10-15 @ 1.000 s,

fulfilling the requirements of future missions such as NGGM and eLISA

• mSTAR is one possible application, testing special relativity

• Outlook:

new laser system (diode laser based)

iodine transitions near 516nm/508nm with narrower linewidths

for enhanced frequency stability

more compact design

25

Thank you for your attention!

This work is supported by the German Space Agency DLR with funds

provided by the Federal Ministry of Economics and Technology

(BMWi) under grant numbers 50 QT 1102 and 50 QT 1201.