1. 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

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 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).

4. Higher-accuracy IR wavelength standard: nonlinear spectroscopy
Comité International des Poids et Measures, 2000
13C2H2 P(16) ± 100 kHz (2000)
Comb-based meas. ± 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)

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
10’s 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 widthc 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.

8. 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

9. 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

10. 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.

11. 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.

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
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.

14. 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.

15. 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
120mW
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)

16. 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

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