Photonic Magnetometry at a (Short) Distance

Transcription

1 Photonic Magnetometry at a (Short) Distance Chris Sataline IEEE Reliability Boston Section 13 February, 2013 This work is sponsored by the Air Force under Air Force Contract FA C Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government.

2 Acknowledgements Lincoln Scholars Program Jon Ashcom James Breen Mike Grzesik Kevin Holman Sumanth Kaushik Madhavi Seetamraju Chris Semisch Prof. Sergienko Eric Statz Matt Stowe RSAM- 2

3 1859: Geomagnetic storm disrupts telegraph systems RSAM- 3

4 Space Weather Events 1859: Geomagnetic storm takes down telegraph system 1972: Lethal doses of energetic particles detected 1989: Hydro-Quebec transformer damage takes down Quebec power grid for nine hours (cost: $10M) 1997: Coronal mass ejection permanently damages AT&T satellite with estimated cost of $200M Space weather can damage technology RSAM- 4 _events

5 Purpose of Space-borne Magnetometry Understand the interaction Predict the disaster X X X X X X X Solar Wind Earth Atmosphere L1 Lagrange Point Magnetometry aids reliability analysis RSAM- 5 X: Satellite magnetometers

6 Satellite Death Atmospheric drag Single event upset Differential charging Bulk charging RSAM- 6

7 Coronal Mass Ejection (CME) Earth X X: GOES-07 RSAM- 7

8 Space Weather Measurements: 1989 geomagnetic storm CME impacts Earth X-ray Protons Cosmic rays GOES-07 Magnetometer nt RSAM- 8 Image:

9 Space Weather Science GOES SOHO Oersted Understanding space weather critical to satellite performance & survival RSAM- 9

10 System Concept Satellite electromagnetic systems contaminate magnetic field measurement Boom Magnetometer RSAM- 10

11 System Concept Satellite electromagnetic systems contaminate magnetic field measurement Laser Passive Vapor Cell Eliminate isolating boom. Interrogate passive magnetic sensor by laser. RSAM- 11

12 Outline Introduction Background Material Experimental Results Summary RSAM- 12

13 Magnetic Yardstick Human Biophysical Signals Space Weather Fields of Interest ( nt) Earth s Geomagnetic Field (~50 ut) Strongest manmade magnets (MRI) Refrigerator magnets Neutron Stars pt nt μt mt Telsa (T) kt MT Magnetometers: SQUID ft / (Hz) Cryogenics, highly sensitive Fluxgate pt/ (Hz) Low SWaP, less sensitive Atomic ft / (Hz) High sensitivity with low SWaP RSAM- 13

14 Crash Course in Quantum Optics ( Semester in a slide ) Rabi Oscillation Von Neumann/Liouville Equation E 2 λ~e 2 -E 1 E 1 e - Resonant oscillating electromagnetic field (laser) affects atomic electron distributions Atomic populations evolve according to Hamiltonian RSAM- 14

15 Coherent Population Trapping (CPT) 3> λ 1 λ2(t) Laser lines resonant with two atomic transitions - Rabi oscillation - Electrons pumped into dark state - Dark state: not resonant with either lase field - Decrease in atomic absorption 1> 2> Sweep λ 2 (sideband) through resonance - Absorption varies versus RF sideband frequency - FM laser spectroscopy Coherent population trapping produces narrow optical (two-photon) resonances RSAM- 15

16 Rubidium-87 Atomic Transitions Fine Structure 5 2 P 3/2 5 2 P 1/ nm (D1 line) 3> 1> F = 2 F = 1 F = 2 Hyperfine Structure 3-level system 5 2 S 1/2 6.8 GHz Energy/ Frequency 2> F = 1 RSAM- 16

17 Zeeman Splitting Hyperfine levels contain 2F+1 magnetic sublevels Zeeman sublevels shift in external magnetic field m F = -1 m F = 0 m F = +1 F = 2 6.8GHz 2gμB 6.8GHz + 2gμB 5 2 S 1/2 6.8 GHz hyperfine splitting F = 1 RSAM- 17

18 CPT Magnetometry Photodiode Signal m= -1 m= 0 m= 1 δ RF RF Sideband Detuning (khz) Change in magnetic field causes coherent population trapping dark state at different sideband separation RSAM- 18

19 Outline Introduction Background Material Experimental Results Summary RSAM- 19

20 RSAM- 20 Optical Magnetometry Testbed

21 Transmit and Receive Optical System φ Beam Expander TX 1 meter standoff QWP Photodiode RX Rb-87 Target Cell Retro Frequency Stabilization: Rb-87 Lock Cell Balanced Photodiodes HWP Isolator Laser Laser 10% Tap Mirror Attenuation RSAM- 21

22 System Block Diagram Electrical Optical Frequency generator Tunable 795nm laser Power Amplifier Phase Modulator (φ) Rubidium-87 Vapor Cell One meter Separation RF Synthesizer (6.8 GHz +/- ) Oscilloscope/ Lock-in Amplifier Photodiode RSAM- 22

23 RF Sideband Modulation RF Detuning [MHz] 6.8GHz Center wavelength fixed on D1 optical transition Phase modulator creates RF sidebands Phase modulation frequency swept through = 900 khz detuning 6.8GHz - 6.8GHz + Time [ms] Sweep RF sideband through two-photon resonance optical power 795 nm carrier wavelength RF sideband RSAM- 23

24 Basic CPT Data ( No magnetic field, Zeeman sublevels degenerate) Narrow linewidths enable small frequency shift measurement RSAM- 24

25 Magnetic Environment Blue curve Broadened from magnetic environment in laboratory Red curve Narrower linewidth with less averaging Facilitates ideal system characterization RSAM- 25

26 Magnetic Field Control μ-metal Helmholtz Coils High magnetic susceptibility alloy shields contents from external magnetic fields Used only for system characterization Current through symmetric coils forms uniform field in center 3-axis control Used for producing artificial magnetic fields RSAM- 26

27 Outline Introduction Background Material Experimental Results Summary RSAM- 27

28 RSAM- 28 No Magnetic Field

29 Magnetic Field < 1μT ~ 4uT Magnetic field causes measureable Zeeman shift RSAM- 29

30 Polarization Effects 0-field Left-handed Right-handed Dipole matrix elements squared: 1/2, 1/4, 1/12 RSAM- 30

31 Transit-Time Broadening t 1 Rubidium-87 Atoms Photodiode t 2 Measurement time limited by atoms path through laser RSAM- 31

32 Transit-Time Limited Linewidth CPT Resonance [khz] Minimum CPT linewidth: 30 khz Optimal sweep rate: 100 Hz Sweep Rate [Hz] ~1 cm beam width, Rb thermal velocity (290 m/s) matches theory* RSAM- 32 * Schmidt, et al. Phys. Rev. A, 53, R27, 1996

33 Field Determination Photodiode Signal [V] 35 khz Sideband Spacing [khz] RSAM- 33

34 1-meter Magnetometry 15.0 khz/μt 13.6 khz/μt RSAM- 34

35 Outline Introduction Background Material Experimental Results Summary RSAM- 35

36 Summary CPT magnetometry testbed established Sensitivity matches theory (unbuffered vapor, 30 khz) 1 meter standoff measurements Zeeman shift tracks prediction (7 khz/μt) RSAM- 36

37 Future Work Peak finding algorithm Sensitivity better than quoted Buffer gas improvements Helium at ~10 torr Better sensitivity VCSEL Direct current injection modulation up to 10 GHz Control over sideband conversion efficiency Gradiometry Removes common mode noise CCD imaging capability Time-varying fields RSAM- 37

38 RSAM- 38 Questions

39 RSAM- 39 Backup Slides

40 Atomic Structure Fine structure Relativistic corrections Spin-orbit coupling J = L + S Hyperfine structure Electron spin/nuclear spin coupling F = J + I Rubidium-87 (alkali metal) Atomic number 37, nuclear spin I = 3/2 e - configuration: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 5s 1 Valence e - ground state: 5s ( s L=0) Fine structure ground state: J = 0+½ 5S 1/2 Hyperfine ground state: F = {3/2 + ½, 3/2 ½ } = {2,1} RSAM- 40

41 F = 2-2gμB 6.8 GHz +2gμB 5 2 S 1/2 m= -1 m= 0 m= +1 F = 1 RSAM- 41

42 RSAM- 42 Electron Cloud Distributions

43 Boom Expense 8 meters Model: (8 m) * Scissor Geometry = (315 inches) * (1.4) = 441 in (½ in x 1 in) Boom Aluminum Beryllium metal alloy (AlBeMet) - Low weight - High stiffness (x2) Total Volume: ~440 in 3 Density: lb/in 3 Total Weight: 33 lb Satellite launch cost (to LEO): Raw material costs: x $5000 per pound x $409 per pound Lightweight 8 meter boom (w/o design): $178,500 RSAM- 43