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A Laser, a Transition, and a Counter

This presentation outlines the research project with an overall goal to create an atomic clock using a laser, transition, and counter elements. It covers the background of atomic transitions in Rubidium-87, challenges faced, and future work on finding resonances with new VCSEL lasers. The use of a Vertical-cavity surface-emitting laser (VCSEL) is highlighted, along with issues encountered during the project such as exceeding maximum reverse voltage and current limitations. The presentation showcases the progress made in measuring resonances and the need for further experimentation with the new VCSEL. Future plans include modulating the VCSEL and fabricating a box for the rf modulator. Acknowledgements to REU program, Prof. Kossler, NSF Grant, Prof. Irina Novikova, and Prof. Eugeniy Mikhailov are mentioned.

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A Laser, a Transition, and a Counter

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  1. A Laser, a Transition, and a Counter Nathan Belcher Prof. Irina Novikova William and Mary REU Talk 8.2.07

  2. Acknowledgements • REU program and Prof. Kossler • NSF Grant No. PHY-0453502 • Prof. Irina Novikova • Prof. Eugeniy Mikhailov

  3. Outline • Overall goal of project • Background • The VCSEL • Rubidium Resonances • Trouble • Future Work

  4. Overall Goal of Project • Create an atomic clock • Need: laser, transition, counter, laser frequency lock • Have: • transition (Rubidium-87 at 780 or 794 nm) • counter (piece of equipment that is bought) • Working on: • laser (VCSEL) • laser frequency lock (optical setup)

  5. Background • Atomic transition in Rubidium-87 is the hyperfine splitting at 780 or 794 nm • Known as Lambda system

  6. Background continued • Use electromagnetically induced transparency to measure transmitted light • The closer to hyperfine splitting resonance, the more transmission • Counter locked to maximum transmission which corresponds to clock frequency

  7. Background continued • Lambda system requires two lasers at different frequencies • Problem: inherent in lasers are small random shifts in frequency around a set frequency (“jumps”) • Bigger problem: if two lasers are physically separate, the “jumps” are random

  8. Background continued • Solution: use phase modulation to create two lasers out of one physical laser • Why? Both lasers “jump” with each other so relative frequency can be set by external generator • Creates carrier with sideband comb

  9. Background continued • Three phase modulation formulas

  10. Background continued • Carrier to sideband spacing determined by input frequency

  11. The VCSEL • Vertical-cavity surface-emitting laser • Why? • Low power consumption • Easy to modulate current for phase modulation • Already in use in miniature atomic clocks

  12. VCSEL continued • What it looks like…

  13. VCSEL continued • Where it is housed..

  14. VCSEL continued • On this heat sink…

  15. VCSEL continued • In this laser system…

  16. VCSEL continued • Previous work: • Characterization of polarizations and how they interact with changing current • Calculating sideband/carrier ratios at different frequencies and modulation powers • Seeing resonances in a Rubidium vapor cell

  17. VCSEL continued • How we took the polarization and modulation data:

  18. VCSEL continued • Polarizations with Roithner at 1.35 mA Y-Axis: Amplitude, Arbitrary Units X-Axis: Time, 10 ms scale

  19. VCSEL continued • Polarizations with Roithner at 2.09 mA Y-Axis: Amplitude, Arbitrary Units X-Axis: Time, 10 ms scale

  20. VCSEL continued • Sideband/carrier ratios for 3.4 and 6.8 GHz with Roithner VCSEL

  21. VCSEL continued • Carrier and sideband combs at 3.4 GHz, -9 dBm, 2.25 mA Y-Axis: Amplitude, Arbitrary Units X-Axis: Time, 1 ms scale

  22. VCSEL continued • Carrier and sideband combs at 6.834 GHz, 14 dBm, 2.25 mA Y-Axis: Amplitude, Arbitrary Units X-Axis: Time, 1ms scale

  23. Rubidium Resonances • Roithner VCSEL not able to get to transition at 780 nm • 5 nm away, needs to be heated to 80 degrees Celsius • Can’t get there because of current limits in peltier and temperature controller

  24. Rubidium Resonances continued • How we measured the resonances:

  25. Rubidium Resonances continued • Switch to Ulm VCSEL #1 • Rated for 795 +/- 1 nm • Measured at 795 nm • Need 794.7 nm for resonance

  26. Rubidium Resonance continued • Resonance with Ulm VCSEL #1

  27. Rubidium Resonance continued • Ulm VCSEL #2 • Rated for 795 +/- 1 nm • Measured at 792.9 nm • Again, need 794.7 nm

  28. Rubidium Resonance continued • Resonance with Ulm VCSEL #2

  29. Trouble • Battery-powered constant current source • Forward current protection with limiting resistor • No protection from exceeding maximum reverse voltage

  30. Trouble continued • While looking at resonances, turned amplitude up on function generator too much • Exceeded maximum reverse voltage • Destroyed lasing cavity

  31. Trouble continued • Installed Ulm #2 • Saw resonances • Used clamp instead of zip tie and double-sided tape to hold Bias-T in place • Grounded Bias-T to heat sink

  32. Trouble continued • In circuit design, laser in feedback of operation amplifier • Current flowing through case of Bias-T • When shorted, approximately 10 mA of forward current through VCSEL • Absolute maximum rating: 3 mA

  33. Trouble continued • Too much forward current through laser • VCSEL now just fluoresces at some wavelength • Have to wait for more Ulm VCSELs to arrive

  34. Protection Circuit • Changed circuit to protect VCSEL

  35. Future Work • Find resonances with new Ulm VCSEL • Modulate new Ulm VCSEL • See electromagnetically induced transparency • Fabricate box for rf modulator

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