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Advances on Portable Frequency References in LUMOS

Advances on Portable Frequency References in LUMOS. Kansas State University Kevin Knabe. Advising Professors: Kristan Corwin & Brian Washburn Colleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-Rawhi. Outline. Goals Overview of Frequency Standards Saturated Absorption Spectroscopy

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Advances on Portable Frequency References in LUMOS

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  1. Advances on Portable Frequency References in LUMOS Kansas State University Kevin Knabe Advising Professors: Kristan Corwin & Brian Washburn Colleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-Rawhi

  2. Outline • Goals • Overview of Frequency Standards • Saturated Absorption Spectroscopy • Fiber Cells • Reflected Pump Setup (single beam!) • Locking • Update on the Cr:F Laser

  3. Goals • Studying ν1 + ν3 vibrational band of acetylene (optimize signal for locking) • Create an acetylene cell using Photonic BandGap (PBG) Fiber and Single Mode Fiber (SMF) • Lock a laser to absorption signal

  4. Acetylene as Frequency Reference Moderate accuracy (± 100 MHz) High accuracy ( ±2 kHz) >500 MHz W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc. Am. B, 17, 1263 (2000). 1- 40 MHz Splice Splice SMF PBG Fiber SMF 8.5 mW 100 mW Figure from: K. Nakagawa et al., JOSAB 13, 2708 (1996) Portable, robust -> Large linewidths Complex, fragile Our Goal: Combine portability with improved accuracy • Saturation spectroscopy – pump-probe technique • Long interaction lengths

  5. Fractional absorption “Bennett Hole” or Saturated Absorption Feature Frequency (MHz) Saturated Absorption Pump burns hole in velocity distribution, probe samples different velocity class, except when on resonance. Doppler-broadened line width l Sub-Doppler line width Pump and probe at same frequency

  6. Pump Probe Theory of Saturated Absorption Gas Cell Beer’s Law • Pump and probe burn independent holes while scanning velocity classes • The only time they see each other is when they are resonant • Ipump >> Iprobe (Iprobe < (0.05) Ipump)

  7. Erbium Doped Fiber Amplifier - Can amplify up to 500 mW Extended Cavity Diode Laser Probe . Iso 40% PC 30% 2x -Tunable from 1510 nm to 1580 nm -Can sweep using external voltage control Sweep ~ 4 GHz at 1530 nm EDFA ECDL AOM 70% Pump PBS PBS VC PBGF λ λ λ 2 4 2 PD Michelson Interferometer for Frequency Calibration 60% Diagnostics Pump-Probe Setup PBGF acts like a waveplate!!

  8. Photonic Bandgap Fibers • “10 mm fiber”- 7 missing cells • 7.5 μm mode field diameter • “20 mm fiber”- 19 missing cells • 13.5 μm mode field diameter Allows for long interaction lengths! Images by Crystal Fibre A/S

  9. Problem: Splices are not commercially available!! Typical Data Uneven background in 10 μm attributed to coupling into surface modes due to small mode field diameter Good signal quality, small widths

  10. Pressure Results • 20 μm fiber shows lower fundamental width at zero pressure – larger transit time, so less broadening! Mode field radius of fiber R. Thapa, K. Knabe, M. Faheem, A. Naweed, O. L. Weaver, and K. L. Corwin, "Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber," Opt. Lett. 31, 2489 (2006).

  11. Power Results • Power broadening widens transition, but discrimination keeps going up • Because of availability of additional laser sources, staying under 50 mW may be a requirement (EDFA’s are expensive)

  12. Goals • Studying ν1 + ν3 vibrational band of acetylene (optimize signal for locking) • Create an acetylene cell using Photonic BandGap (PBG) Fiber and Single Mode Fiber (SMF) • Lock a laser to absorption signal

  13. The Splice is Right “Half Cell” LUMOS Spliced Fiber (2005) - 20 μm core Splice Loss: PBG ->SMF ~ 2.0 dB (30%) SMF ->PBG ~ 0.5 dB (10%) 20 μm core PBG supports more than 1 mode! “Arc fusion splicing of hollow-core photonic bandgap fibers for gas-filled fiber cells” R. Thapa, K. L. Corwin, and B. R. Washburn, Accepted to Optics Express 2006

  14. Vacuum Chamber filled to a low pressure with C2H2 Arc Fusion Splicer Aaron Pung CO2 Laser Making Fiber Cells SMF PBG SMF We will have a robust portable fiber cell!

  15. Goals • Studying ν1 + ν3 vibrational band of acetylene (optimize signal for locking) • Create an acetylene cell using Photonic BandGap (PBG) Fiber and Single Mode Fiber (SMF) • New Goal: Check quality of half cells • Lock a laser to absorption signal

  16. Saturated Absorption Spectroscopy With Only 1 Beam in PBG Fiber* *Patent pending

  17. . Iso PC EDFA EDFA ECDL 2x AOM PBS PBS Splice VC High Loss PBGF SMF λ λ λ 2 4 2 PD Diagnostics Low Loss Pump-Probe Setup for Spliced Half-Cells Pump Probe

  18. . Iso PC EDFA EDFA ECDL PBS PBS Splice VC High Loss PBGF SMF λ λ λ 2 4 2 PD Diagnostics Low Loss Pump-Probe Setup for Spliced Half-Cells – No AOM Pump Probe

  19. 40% EDFA ECDL BS Splice VC PBGF SMF PD 60% λ Diagnostics 2 Reflected Pump Setup Pump Probe

  20. R-Pump Results: Signal Quality • Keeping saturated absorption feature centered with Doppler broadened profile has locking benefits • Wings exhibit very small interference pattern which has a free spectral range associated with the length of the PBG fiber • Reflections occurring at front end of splice, causing interference

  21. R-Pump Results: Comparison Of Widths R-Pump : P11 Line : Pressure = 0.5 torr Pump-Probe : P11 Line : Pressure = 1 torr R-Pump : P11 Line : Pump Power = 30 mW Pump-Probe : P11 Line : Pump Power = 29 mW

  22. Goals • Studying ν1 + ν3 vibrational band of acetylene (optimize signal for locking) • Create an acetylene cell using Photonic BandGap (PBG) Fiber and Single Mode Fiber (SMF) • Check quality of half cells • Lock a laser to absorption signal, then compare with locked Cr:F frequency comb

  23. Frequency domain I(f) fo fr EDFA f 0 fn = nfr + fo Locking • Lock laser to sat. abs feature. • Measure with comb referenced to GPS. • Lock comb to fiber-based reference, output stable microwaves. 1550nm ECLD Locking Electronics Probe Pump AOM PBG Cell C2H2 molecules

  24. Chromium Forsterite Update

  25. Cr:F Spectra Supercontinuum Using HNLF to generate supercontinuum from ~1000nm to ~2250 nm Laser out of oscillator 70 nm BW @ 1280 nm center λ

  26. PPLN 40 80 0 120 f (MHz) f-2f Interferometer • Using Periodically Poled Lithium Niobate to get 2nd harmonics • Got 47 dB beat note signal • Have also locked repetition rate • Currently Rajesh is playing with pump power modulation and other servo controls to lock f0

  27. Conclusion • Characterization has been done on acetylene filled PBG fibers • Advances have been made on making gas filled cells • Discovery of the Reflected-Pump technique • Cr:F is close to being locked! • Next: Lock Laser to saturated absorption signal

  28. Thanks • Funding Agencies: • AFOSR • NSF CAREER • Kansas NSF EPSCoR program • Kansas Technology Enterprise Corporation • Kansas State University • Mike Wells and the JRM Staff

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