Spectral Hole Burning of Acetylene Gas inside a Photonic Bandgap Optical Fiber

1
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

2
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.

3
Existing portable wavelength references for the
telecom industry
laser or LED
C2H2
Line centers130 MHz or 13 MHz Used to
calibrate optical spectrum analyzers (OSAs) Line
widths 5 GHz (OSA resolution) pressure ?
broadening shift
W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc.
Am. B, 17, 1263 (2000).
4
Higher-accuracy IR wavelength standardnonlinear
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

• 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)
5
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

6
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
• 10s of kHz stability
• Sealed PBG fiber cells
• All above applications have
• Collinear geometry

Figure from F. Benabid et al., Nature (2005).
7
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.
8
Filling the fiber
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
9
Filling the fiber
To pump
Gas Inlet
Gas Inlet
Hollow optical fiber
Probe
diode laser
1 mW
C2H2 molecules
C2H2 molecules
Ultimately
10
Observing the Signal
To pump
Gas Inlet
Gas Inlet
diode laser
Probe
Hollow optical fiber
PBS
Pump
10
PBS
1 mW
Probe
90
C2H2 molecules

• Interference between pump and probe beams
  observed on probe photodetector.

11
Observing the Signal
To pump
Gas Inlet
Gas Inlet
diode laser
Probe
Hollow optical fiber
PBS
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.

12
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

13
Eliminating the noise
To pump
Gas Inlet
Gas Inlet
diode laser
Probe
l/2
Hollow optical fiber
PBS
Pump
10
2x AOM
PBS
1 mW
l/4
Probe
90
C2H2 molecules

• Wave plates added to keep pump and probe
  polarizations orthogonal.
• Michelson interferometer allows sweep
  calibration.

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Observed signals

• Beers Law I I0 e-al

500 MHz

• 20 mm core fiber, BlazePhotonics
• 35 cm long
• Pressure 615 mTorr,
• Pump power 35 mW
• Michelson interferometer fringes calibrate sweep.

15
Line widths depend on pressure, power

• 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)

16
Conclusions and future directions

• Saturated absorption is readily achievable in
  photonic bandgap fibers with power lt20 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

17
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