length standard development shally saraf for the jcoe team … · 2013. 11. 18. · shally saraf...

1Q2C6 Nice, October 16, 2013

Kennedy Thorndike on a small satellite in low earth orbit

Length Standard Development

Shally Saraf for the JCOE Team

Nice, 2013


STAR conceptual diagram


miniSTAR conceptual diagram

CUT


Optical cavity design at 10-15 stability

• Procure and shape material with minimal creep and ultra-low expansion like ULE glass – Coeff. of thermal exp. < 10ppb/K

• Develop supermirrors to obtain Finesse > 105 for high S/N– R>99.9995%

• Operate the cavities close to the CTE zero-crossing point Active thermal control – ULE backing rings + TEC’s + cooling + servos

• Develop multi-layer sub microkelvin thermal enclosures– Each layer attenuates thermal perturbations by >20X

• Develop fiber technologies to efficiently couple laser light into the cavity using an optical fiber – Grin lenses + pointing control + fiber phase compensation (?)

• Develop servos for locking cavity to Iodine-stabilized laser– FPGA control and science signal extraction


optical cavity in thermal enclosure

Cavity 500gEnclosure 8750g


optical cavity material

Key optical cavity parameters:

L/L < 10-17 at orbit and harmonics with 2 years of data

Derived requirements:

Expansion coefficient: < 10-9 per K

Operating temperature: within 1 mKof expansion null (~ 16-19°C nom)

External strain attenuation: > 1012

Stiffness: L/L < 10-9 per g, 3-axis

Implied spacer: ULE glass

Mirrors: Fused silica with ULE backing rings


thermal enclosure

Main Requirements:Thermal stability Stress attenuation Launch and space compatible

Thermal performance:Cavity L/L < 10-17 (2 yr data) at

orbital period and harmonics

Derived requirements (2 yr average):Thermal stability of 10-8 K at orbitThermal gradient ~ 10-9 K/cm at orbitMaintain cavities temperature to 1 mK


thermal shield attenuation factors

COMSOL MODEL


thermal modeling of 6-layer enclosure

Multi-stage thermal filtering can give excellent control even for an equatorial orbit.miniSTAR would need less layers for similar control.


~ 200 kHz/ms-2

4 nm

-4 nm

0.8 nm/div

a

Horizontaldeflection

per ms-2

Verticaldeflection

per ms-2

deflection of cavities with acceleration

Lbottom

a

Ltop

Short cavity

Support neargeometrical center for CMRR

Vertical orientationfor symmetry

DLcavity ~ pm

21 nm

25 nm0.5 nm/div

worst case lab accelerations ~ 10-3 ms-2 at 30 Hz vertically


a

L

0.01

0.1

1

10

5 6 7 8 91

2 3 4 5 6 7 8 910

2 3 4 5 6 7 8 9100

2

a = 0.11 * L

a = 0.577* L

MHz/ ms-2

2200 kHz/ms-2

150 kHz/ms-2

frequency/acceleration sensitivity


vibration–insensitive optical cavities

STRAIN DISTRIBUTION• Zero relative displacement at the ends of the optics axis• Static Load applied at the points marked on the perimeter


strain attenuation model - FEA

Estimated strain attenuation: > 103 per can Extrapolating to entire enclosure: > 1015

Exceeds requirement by x1000


fundamental frequencies

The first mode of the assembly was found to be 77 Hz

First lateral mode: 77.6 Hz First axial mode: 93.9 Hz


possible mSTAR cavity designs

(GRACE‐FO)


fiber coupling: ray tracing for fiber GRIN-lens system

1234

d1 d2z

1

1

xx

2

2

xx

10 1

coscos 0

0coscos

cossin

sin cos

coscos 0

0coscos

10 1


0.9mm

20mm

42mm

Fiber Pigtail

Grin Lens f = 1.9mm Regular Lens

f = 12 mm Backup Tube

fiber-lens assembly at 1550nm

• Measured w0= 378.5 μm @ 20 mm after lens 2 (Could be adjusted to ~200 mm for longer working distance)

• Optimal w0 = 375 μm for 10 cm cavity with flat & curved mirror (Rcc = 1 m)

• Total distance from fiber pigtail to second lens is ~42 mm


direct coupling into cavity

• Direct coupling arrangement• Alignment adjustment through two sets of set screws• Optical System is longer especially if transmitted light is

collected.


right angle prism solution

• Coupling into cavity through right angle prism

• Alignment adjustment through two sets of set screws

• Optical System can be made compact reducing size of thermal enclosure.


basic fiber-coupled cavity layout


• RF modulation obtained by frequency generation board• RF demodulation executed digitally by FPGA board (with RF ADC channels)• Digital VCO by onboard FPGA• Laser PZT and Temp controlled by FPGA• Science signal contained in the digital VCO error signal

miniSTAR optical diagram


optical cavity work at Stanford

1064nm

• ULE cavities with fused silica mirrors and ULE backingrings operating in a 2-layer thermal enclosure at10-9 torr.

• Currently tracking CTE null

• Beat note between cavity and I2 @ 10-12 stability


• 1110 line R(56)32-0 is best

• Lock laser to the a10 HFS

• Sub-doppler detection

• Modulation Transfer Spectroscopy(MTS)

• Natural Linewidth ~ 400KHz

• Broadened line < 1MHz

• Investigate narrower lines at ~508 nm

iodine MTS setup at Stanford


iodine setup at Stanford


Motherboard, CPU, Radio

Electrical power system w/ batteries (30 W hr)

Cobolt 04‐01 532nm laser

AOM x2 Circulator

Iodine cell (2cm long)

Optical bench

Spherical optical cavity

Thermal & magnetic enclosure for optics

Thermal enclosures for cavity

PDH, counter boards

chassis

mSTAR instrument concept in 3U CubeSat configuration


double conical optical cavity within UV-LED footprint


project schedule

length standard development shally saraf for the jcoe team … · 2013. 11. 18. · shally saraf...

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