M. Faheem, R. Thapa, and Kristan L. Corwin Kansas State University Spectral Hole Burning of Acetylene Gas inside a Photonic Bandgap Optical Fiber M. Faheem, R. Thapa, and Kristan L. Corwin Kansas State University Department of Physics

Outline Acetylene as a frequency standard moderate accuracy (~100 MHz) high accuracy (~kHz) Advantage of PBG fiber cells Other uses of acetylene in PBG fiber Observations of 25 MHz-wide lines efforts at noise reduction Requirements to further narrow transitions.

Existing portable wavelength references for the telecom industry laser or LED C2H2 Line centers:±130 MHz or ±13 MHz Used to calibrate optical spectrum analyzers (OSA’s) Line widths ~5 GHz (OSA resolution) pressure → broadening & shift W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc. Am. B, 17, 1263 (2000).

Higher-accuracy IR wavelength standard: nonlinear spectroscopy Comité International des Poids et Measures, 2000 13C2H2 P(16) ± 100 kHz (2000) Comb-based meas. ± 2 kHz (2005) Great Britain, Japan, Canada, Japan 100 mW 8.5 mW Cavity: Long interaction length High intracavity power Fragile Cavity and laser locked to resonance independently Figure from: K. Nakagawa, M. de Labachelerie, Y. Awaji, and M. Kourogi, JOSAB 13, 2708 (1996)

Goal: create high-accuracy, portable optical frequency references Solution: perform nonlinear molecular absorption inside optical fibers. Advantages: High intensities Long effective path lengths More portable Easy to align

Popularity of acetylene in photonic bandgap fiber Advantages of photonic bandgap fiber: long interaction lengths high laser intensities Megawatt solitons (Cornell, Corning Inc. 2003) Gas sensors (Helsinki U. of Tech., Crystal Fibre 2004) Slow light (Cornell, Corning, 2005) Optical frequency standards (Bath, 2005) Doppler-broadened lines 10’s of kHz stability Sealed PBG fiber cells All above applications have Collinear geometry Figure from F. Benabid et al., Nature (2005).

Saturated Absorption Doppler-broadened line width c d frequency (MHz) Pump Probe Saturated Absorption Doppler-broadened line width l Fractional absorption Sub-Doppler line width Pump and probe at same frequency Frequency (MHz) Pump burns hole in velocity distribution, probe samples different velocity class, except when on resonance.

Filling the fiber diode laser 20 mm core, 60 cm length To pump Gas Inlet Gas Inlet Hollow optical fiber Probe diode laser 1 mW C2H2 molecules C2H2 molecules 20 mm core, 60 cm length Fiber fills to 2 Torr in ~ 10 s Chambers Equilibrate in days! 20 mm core, 60 cm length Fiber fills to 2 Torr in ~ 10 s Chambers Equilibrate in days! photonic bandgap fiber photonic bandgap fiber

Filling the fiber Probe Pump diode laser Ultimately: Probe To pump Gas Inlet Gas Inlet Hollow optical fiber Probe diode laser 1 mW C2H2 molecules C2H2 molecules Ultimately: Probe Pump

Observing the Signal To pump Gas Inlet Gas Inlet diode laser Probe EDFA PBS Hollow optical fiber Pump 10% PBS 1 mW Probe 90% C2H2 molecules Interference between pump and probe beams observed on probe photodetector.

Observing the Signal To pump Gas Inlet Gas Inlet diode laser Probe EDFA PBS Hollow optical fiber Pump 10% 2x AOM PBS 1 mW Probe 90% C2H2 molecules Interference between pump and probe beams observed on probe photodetector. AOM added to put interference at 40 MHz, too fast to detect.

Saturation feature observed in hollow fiber Significant signal strength at 10 and 20 mW pump powers! 10 mm core Noise: residual interference between pump and probe beams

Eliminating the noise To pump Gas Inlet Gas Inlet diode laser Probe l/2 EDFA PBS Hollow optical fiber Pump 10% 2x AOM PBS 1 mW PD d2 50/50 1 Mirror 1% d1 l/4 Probe 90% C2H2 molecules Wave plates added to keep pump and probe polarizations orthogonal. Michelson interferometer allows sweep calibration.

Observed signals Beer’s Law I = I0 e-al (n) Dal width ~500 MHz Probe only Pump and Probe Dal width ~500 MHz 20 mm core fiber, BlazePhotonics 35 cm long Pressure ~ 615 mTorr, Pump power ~35 mW Michelson interferometer fringes calibrate sweep.

Line widths depend on pressure, power 500 1000 1500 2000 20 25 30 35 40 45 50 55 Width(MHz) Pressure (mT) Fit: Psat = 75 mW P(11) @ 120mW P(13) @ 28mw slope: 11 MHz/Torr observed 11.4 MHz/Torr expected (NIST) Intercept: 23 MHz observed, ~18 MHz expected (transit time) saturation power: 75 mW observed ~300 mW expected (Nakagawa)

Conclusions and future directions Saturated absorption is readily achievable in photonic bandgap fibers with power <20 mW. Linewidth dominated by transit-time broadening. larger-core photonic bandgap fibers desirable. Counter-propagation prone to noise careful polarization control required Future: Characterize linewidth and signal size vs pressure, power, fiber geometry Narrow the line (Target ~1 MHz) larger core size, coated cell? Measure frequency shift and stability frequency comb

Acknowledgements Acetylene inside pbg fibers is a promising system for stable, high-accuracy frequency standards in the near IR. Funding generously provided by: AFOSR NSF CAREER Kansas NSF EPSCoR program Kansas Technology Enterprise Corporation Kansas State University Thanks to: Sarah Gilbert Greg Johnson Dirk Müller Ahmer Naweed Bill Swann Kurt Vogel Brian Washburn Mikes Wells and JRM staff