Saturation Spectroscopy

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Saturation Spectroscopy -Inside
Photonic Band Gap Fiber
Rajesh Thapa Kevin Knabe, Ahmer Naweed, Aaron
Pung Larry Weaver, Brian Washburn, Kristan
Corwin
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Outline

• An overview- optical frequency references
• IR wavelength standard
• Pump probe spectroscopy
• Observations of saturation spectra
• Modeling of Light Inside Fiber
• Conclusion and future direction

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An Overview Optical Frequency References
v1v3 combination band of acetylene
Robust and stable frequency communication and
navigation System - Frequency division
multiplexing - Spectrum analyzer

• Relevance to telecommunication industry.
• Well separated transitions.
• ms long lifetime, kHz linewidth.
• Lack of permanent dipole moment.
• Relatively immune to external field and shifts.

Sarah L. Gilbert, W.C.S., Acetylene 12C2H2
Absorption Reference for 1510 nm to 1540 nm
Wavelength Calibration-SRM 2517a. 2001
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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

Existing portable wavelength references for the
telecom industry
Line centers130 MHz or 13 MHz Used to
calibrate optical spectrum analyzers
(OSAs) pressure ? broadening shift
W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc.
Am. B, 17, 1263 (2000).
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Spectroscopy Inside Power Build up cavity
Basis for Highest-accuracy measurements
Cavity Long Interaction length High
Intracavity Power Cavity and laser locked to
resonance independently Not Portable, Fragile
Figure from K. Nakagawa, M. de Labachelerie, Y.
Awaji, and M. Kourogi, JOSAB 13, 2708 (1996)
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Hollow Core Photonic Band Gap Fiber
Blaze Photonics (www.blazephotonics.com)
J. C. Knight et al., Science, 282, 1476, 1998
photonic band gap fiber.
Advantages Long interaction length
High laser intensities More portable
Proximity of fiber surfaces to the molecules
Small beam size inside the fiber (15µm)as
compared to cavities (500µm)
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Predicted loss from the fundamental mode in
ordinary hollow core fiber and PBG fiber
Knight, j.C., Photonic crystal fibers. nature,
2003. 424 p. 847.
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How Popular is Acetylene in PBG Hollow Core
Fiber?

• Gas sensors (Helsinki U. of Tech., Crystal
• Fiber 2004)
• Optical frequency standards (Bath, 2005)
• Sealed PBG fiber cells

• Saturated absorption in hollow-core
• photonic bandgap fibers
• (J. Henningsen et al., 2005)

Figure from F. Benabid et al., Nature (2005).
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Recent Paper
Saturated absorption in acetylene and hydrogen
cyanide in hollow-core photonic bandgap
fibers Jes Henningsen, Jan Hald, and Jan C.
Peterson Opt. Express 13, 10475-10482 (2005)
See background noise, We have at least 40 times
higher signal to noise ratio
Fiber Diameter - 10 µm Width - 22.4 MHz Psat
- 23 mW
The improved SNR makes overtone transitions in
the near-infrared region accessible to frequency
metrology.
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Application and Motivation

• Basis of international frequency reference in
• near-IR region with accuracy in KHz limit .
• Basis of portable frequency reference for
• telecom industry .

Splice
Splice
Step Index, Single Mode Fiber (SMF)
Step Index, Single Mode Fiber (SMF)
PBG Fiber
Fiber Cell
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First Fiber Cell
LUMOS Spliced Fiber (2005) 74 coupling efficiency
splice loss - 1.6 dB
Splice Loss 1.3 dB
mechanical strength of the splices
80 bar -1 µbar
Evacuated up to the pressure of 10 mT for 14
hours
Compact, stable and efficient all-fibre gas
cells using hollow-core photonic crystal fibres
Benabid et al., Nature (2005).
Splicing hollow-core photonic bandgap fibers for
gas-filled optical frequency references to solid
core fiber using an arc fusion splicer R. Thapa,
K. L. Corwin, and B. R. Washburn, Submitted in
CLEO 2006
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Theoretical Approach
Pump Probe Spectroscopy
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Absorption Of Light

• Absorption

(Beers law)
a is the absorption coefficient
True basically If,
This leaves most of population in Ground state
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What if laser light is Intense
It Significantly begin to Deplete the Population
of Ground State
Doppler Broadened Profile, 500 MHz Wide
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Pump Probe Spectroscopy

• Pump burns hole in velocity distribution,
• probe samples different velocity class,
• except when on resonance.

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P (11) line, 20µm diameter fiber
Saturated absorption spectra as a function of 5
different pressures
We need some equation to fit this profile.
SEARCH!!!
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Our Fitting Equation
Fitting Parameters
(Laser Spectroscopy, W. Demtroder)
True for s0ltlt1 Keep in Mind !!!
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See Our Fit
It Seems Our Equation Works!!!
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Pressure-Broadening measurement of the different
lines.
10 MHz/Torr of Pressure Broadening
What determines width?? Perhaps other broadening
mechanism ???
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Broadening Mechanism

• Transit time broadenings

-Interaction time of the molecule with laser beam
V, most probable velocity of molecule
For 20µm diameter fiber
For 10µm diameter fiber

• Power broadenings

See some Power broadening effect!!!
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Optimum Signal Size
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Power broadening effect on the lineshape for P
(11) lines
Higher the power bigger the amplitude of narrow
feature- good for freq. reference
Higher the power wider the width of narrow
feature- bad for freq. reference
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Power-Broadening measurements
Why Psat different ??? A big question!!!
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Theoretical Approach and Numerical Analysis
Calculation of Saturation Parameter
Remember- Demtroder eq. is valid only for
Sltlt1 But Psat is small so S is large !!!
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The total absorption coefficient
Absorption cross-section of probe in presence of
pump.
Population change is due to Pump only
Integration over entire velocity spectrum gives,
This is true for any S0
Usual linear attenuation of the Weak probe beam
Change in signal induced by the Intense field, a
Lorentzian of width w
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Numerical Calculation of Psat Using Pump
Propagation into account
Small Finite Segment
We Know P0 and Pn, vary Ps until we get measured
pump output power.
Psat 20 mW
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Modeling Of Light inside the fiber
Fiber
Pump
Probe
Small Finite Segment
From Beers Law (For Probe laser)
This Way we can find out Probe transmission in
presence of pump for entire frequency spectrum
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Transmission effect along the fiber
Vary Ps , Calculate Signal height (Al), Compared
to Experiment , Repeat
We can find different S0 at each different
segment along the fiber and thus Psat.
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Psat vs. Pressure
We can not say anything about dependence of Psat
on Pressure Psat varies from 25 mW to 60 mW!!!
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Psat vs. Power
It seems there is linear dependence of Psat on
Power.
We dont believe it, Should not Psat be constant?
An Open question!!!
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Conclusions

• Saturated absorption is readily achievable in
  photonic bandgap fibers with power lt10 mW.
• We have characterized linewidth in terms of
  pressure and power.
• Linewidth dominated by transit-time broadening.
• larger-core photonic bandgap fibers desirable.
• Counter-propagation prone to noise- careful
  polarization control required
• We have got Satuation Power to be somewhere
  between
• 25 to 50 mW.
• We have also spliced the fiber outside the vacuum
  chamber and got very good splice.

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• Future
• We are on the process of making splice with CO2
  
• Laser to splice fiber inside the vacuum
  chamber.
• We are in final stage of generating frequency
  comb.
• - Measure frequency shift and stability
  of those narrow feature using
• frequency comb.
• We are also in the final stage to peak-lock
  these
• narrow feature.
• To Narrow the line (Target 1 MHz)
• larger core size, coated cell?
• To make fiber cell for portable frequency
  references.

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• Funding generously provided by
• AFOSR
• NSF CAREER
• Kansas NSF EPSCoR program
• Kansas Technology Enterprise Corporation
• Kansas State University
• Thanks to
• Sarah Gilbert
• Mohammad Faheem
• Dirk Müller
• Bill Swann
• Kurt Vogel
• Mikes Wells and JRM staff

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Mode of vibration of acetylene
?1?36651.7(cm-1)
Wavelength1.5
V1 and v3 are doubly degenerate (equal
energy) bending vibration.
?1 mode alone is dipole-forbidden, it can be
excited in combination with the dipole-allowed ?3
mode excitation
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Photonic Band-gap (PBG) Fiber
10µm
5µm
R. F. Cregan et al., Science 285, 1537 (1999)
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Guidance of light
SMF Fiber
PBG Fiber
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Calculation Of Saturation Power
Al
It Gives Sat. Power on resonance with out taking
Propagation effect into account.
S
P13 line, 20 micron, 0.8 m long Fiber.
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Demtroder eq.
Fitting eq in origin
My Calculation
Larrys Calculation
Or,
Demtroder Calculation
Our calculation
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Effect of Probe saturation upon Saturation Power
When Probe saturation is taken into account, the
total absorption must incorporate two different
Saturation Parameter due to both pump and probe.
S1Saturation Parameter due to probe
S2Saturation Parameter due to pump
There is almost no effect on Psat due to probe
power
Psat (mW) 20 mW of pump Input power 60 mW of Pump power
Without probe saturation 33.9 40.6
Probe saturation into consideration 33.5 40
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Transit time broadenings a dominant factor in
small core fiber
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Probe
Pump
PBG Fiber
Vacuum Chamber
BS
Photo Diode
BS
Photo Diode
Squeezer
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BS(30/70)
Squeezer
EDFA
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BS
BS (10/90)
Squeezer
Photo Diode
Glass Cell
BS
90
Michelson Interferometer
Photo Diode
squeezer
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Mode of vibration of acetylene
?1?36651.7(cm-1)
Wavelength1.5
V1 and v3 are doubly degenerate (equal
energy) bending vibration.
?1 mode alone is dipole-forbidden, it can be
excited in combination with the dipole-allowed ?3
mode excitation