Kennedy Thorndike on a small satellite in low earth orbit Length Standard Development Shally Saraf for the JCOE Team Nice, 2013 1
STAR conceptual diagram 2
ministar conceptual diagram CUT 3
Optical cavity design at 10-15 stability Procure and shape material with minimal creep and ultralow expansion like ULE glass Coeff. of thermal exp. < 10ppb/K Develop supermirrors to obtain Finesse > 10 5 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 4
optical cavity in thermal enclosure Cavity 500g Enclosure 8750g 5
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 mk of expansion null (~ 16-19 C nom) External strain attenuation: > 10 12 Stiffness: L/L < 10-9 per g, 3-axis Implied spacer: ULE glass Mirrors: Fused silica with ULE backing rings 6
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 orbit Thermal gradient ~ 10-9 K/cm at orbit Maintain cavities temperature to 1 mk 7
thermal shield attenuation factors COMSOL MODEL 8
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. 9
deflection of cavities with acceleration worst case lab accelerations ~ 10-3 ms -2 at 30 Hz vertically Horizontal deflection per ms -2 4 nm 0.8 nm/div -4 nm a L top Vertical deflection per ms -2 21 nm a L botto m Short cavity ~ 200 khz/ms -2 0.5 nm/div 25 nm Support near geometrical center for CMRR Vertical orientation for symmetry DL cavity ~ pm 10
frequency/acceleration sensitivity 10 MHz/ ms -2 a = 0.11 * L 1 2200 khz/ms -2 0.1 150 khz/ms -2 a = 0.577* L a 0.01 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 100 2 L 11
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 12
strain attenuation model - FEA Estimated strain attenuation: > 10 3 per can Extrapolating to entire enclosure: > 10 15 Exceeds requirement by x1000 13
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 14
possible mstar cavity designs (GRACE FO) 15
fiber coupling: ray tracing for fiber GRIN-lens system x1 x 1 4 3 2 1 d 1 z d 2 x x 2 2 1 0 1 cos cos 0 0 cos cos sin cos sin cos cos cos 0 0 cos cos 1 0 1 16
fiber-lens assembly at 1550nm 0.9mm 42mm 20mm Fiber Pigtail Grin Lens f = 1.9mm Regular Lens f = 12 mm Backup Tube Measured w 0 = 378.5 μm @ 20 mm after lens 2 (Could be adjusted to ~200 mm for longer working distance) Optimal w 0 = 375 μm for 10 cm cavity with flat & curved mirror (Rcc = 1 m) Total distance from fiber pigtail to second lens is ~42 mm 17
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. 18
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. 19
basic fiber-coupled cavity layout 20
ministar optical diagram 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 21
optical cavity work at Stanford 1064nm ULE cavities with fused silica mirrors and ULE backing rings operating in a 2-layer thermal enclosure at10-9 torr. Currently tracking CTE null Beat note between cavity and I 2 @ 10-12 stability 22
iodine MTS setup at Stanford 1110 line R(56)32-0 is best Lock laser to the a 10 HFS Sub-doppler detection Modulation Transfer Spectroscopy (MTS) Natural Linewidth ~ 400KHz Broadened line < 1MHz Investigate narrower lines at ~508 nm 23
iodine setup at Stanford 24
mstar instrument concept in 3U CubeSat configuration Motherboard, CPU, Radio PDH, counter boards chassis Thermal & magnetic enclosure for optics Spherical optical cavity Thermal enclosures for cavity Electrical power system w/ batteries (30 W hr) AOM x2 Circulator Optical bench Cobolt 04 01 532nm laser Iodine cell (2cm long) 25
double conical optical cavity within UV-LED footprint 26
project schedule 27