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

2. 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

3. An Overview : Optical Frequency References
CH4
C2H2
C2HD Rb Ca I2 H
3390
1556
1064
778
657
532
486
Well studied optical frequency references
v1+v3 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

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

5. 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)

6. Hollow Core Photonic Band Gap Fiber
Blaze Photonics (
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)

7. Predicted loss from the fundamental mode in
ordinary hollow core fiber and PBG fiber
Knight, j.C., Photonic crystal fibers. nature, : p. 847.

8. 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).

9. 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, (2005)
See background noise,
We have at least 40 times higher signal to noise ratio
Fiber Diameter :- 10 µm
Width : MHz
Psat :- 23 mW
The improved SNR makes overtone transitions in the near-infrared
region accessible to frequency metrology.

10. 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

11. Splicing hollow-core photonic bandgap fibers for gas-filled
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

12. Pump Probe Spectroscopy
Theoretical Approach
Pump Probe Spectroscopy

13. Absorption Of Light Absorption ∆I I = -() ∆z Laser Fiber Molecules
(Beer’s law) ∆I I =
-() ∆z
‘a ‘is the absorption coefficient
True basically If, ∆z
This leaves most of population in Ground state
Or, I
Laser
Fiber
Molecules IO I ∆z ∆I

14. What if laser light is Intense
It Significantly begin to Deplete the Population of Ground State
N1(v)
N2(v) v
Doppler Broadened
Profile, ~ 500 MHz Wide

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

16. What a mess!!! Schematic Of Experimental Set up Experimental Set-Up
Diode
Laser
PBG Fiber
Photo Diode
Vacuum Chamber
Probe
Pump
AOM BS
EDFA
70%
30%
5 mW
400 mW
Diode
Laser
Michelson
Interferometer
Isolator
AOM
PBG Fiber
Glass Cell
Photo Diode
Vacuum Chamber
Probe
Pump
Squeezer
squeezer BS
EDFA
BS(30/70)
(10/90)
90%
70%
30%
Experimental Set-Up
What a mess!!!

17. Saturated absorption spectra as a function of 5 different pressures
P (13) line, 10µm diameter fiber
More Background Noise
Surface mode?
Transmission of light through glass
138 mT
219 mT
317 mT
435 mT
540 mT
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!!!

18. Our Fitting Equation Fitting Parameters
(Laser Spectroscopy, W. Demtroder)
True for s0<<1
Keep in Mind !!!

19. See Our Fit
It Seems Our Equation Works!!!

20. Pressure-Broadening measurement of the different lines.
10µm diameter fiber , Width varies from 35 to 45 MHz
20µm diameter fiber , Width varies from 20 to 35 MHz
~10 MHz/Torr of Pressure Broadening
What determines width?? Perhaps other broadening mechanism ???

21. Broadening Mechanism Transit time broadenings For 20µm diameter fiber
-Interaction time of the molecule with laser beam
In terms of frequency,
V, most probable velocity of molecule
For 20µm diameter fiber
For 10µm diameter fiber
Power broadenings
See some Power broadening effect!!!

22. Optimum Signal Size change in fractional transmission due to pump
signal width
Discrimination =

23. 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

24. Power-Broadening measurements
Ps ~ 20mW
Ps ~ 45mW
Why Psat different ??? A big question!!!

25. Calculation of Saturation Parameter
Theoretical Approach and Numerical Analysis
Calculation of Saturation Parameter
Remember:- Demtroder eq. is valid only for S<<1
But Psat is small so S is large !!!

26. 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
When S<<1,
, above eq reduces to,
Calculated By Demtroder
Which is exactly as
Now, we have our
Own Equation!!!

27. Numerical Calculation of Psat Using Pump Propagation into account
Fiber P0 P1 P2 Pn
Pump Power
Pump
Small Finite Segment
Saturated absorption in acetylene and hydrogen cyanide
in hollow-core photonic bandgap fibers.
Jes Henningsen, Jan Hald, and Jan C. Peterson
Opt. Express 13, (2005)
Recent Paper
Psat~ 23 mW
We Know P0 and Pn, vary Ps until we get
measured pump output power.
Pump Power (mW)
Distance along fiber
Psat ~ 20 mW

28. Modeling Of Light inside the fiber
Pump P0 P1 P2 Pn
Pump Power
Small Finite Segment
Probe
P’1
P’2
P’n
Probe Power
P’0
From Beer’s Law (For Probe laser)
∆P’ P’ =
-S() ∆z
This Way we can find out Probe transmission
in presence of pump for entire frequency spectrum

29. Transmission effect along the fiber
Pump Power
Probe Power
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.

30. Psat vs. Pressure
P11 line, 10 micron, 0.9 m Fiber at 31 mW
P13 line, 20 micron, 0.8 m Fiber at 14 mW
P13 line, 10 micron, 1.9 m fiber at 60mW
P13 line, 10 micron, 1.9 m fiber at 20mW
We can not say anything about dependence of Psat on Pressure
Psat varies from 25 mW to 60 mW!!!

31. It seems there is linear dependence of Psat on Power.
Psat vs. Power
P-11, 20 um, 1T,0.8m
P-13, 20 um, 1T,0.8m
P-13, 10 um, 430 mT,1.92m
P-11, 10 um, 500 mT, 0.9m
It seems there is linear dependence of Psat on Power.
We don’t believe it, Should not Psat be constant?
An Open question!!!

32. Conclusions
Saturated absorption is readily achievable in photonic bandgap fibers with power <10 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.

33. We are on the process of making splice with CO2
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.

34. 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

37. Mode of vibration of acetylene
ν1(cm-1)
ν3(cm-1)
3373.7
3278
ν1+ν3=6651.7(cm-1)
Wavelength=1.5
V1 and v3 are doubly
degenerate (equal energy)
bending vibration.
υ1 mode alone is dipole-forbidden,
it can be excited in combination with the dipole-allowed υ3 mode excitation

40. [ R. F. Cregan et al., Science 285, 1537 (1999) ]
Photonic Band-gap (PBG) Fiber
10µm
5µm
[ R. F. Cregan et al., Science 285, 1537 (1999) ]

41. Guidance of light SMF Fiber PBG Fiber Total Internal Reflection
multiple Interference and scattering
at Bragg’s condition

42. Calculation Of Saturation Power
It Gives Sat. Power on resonance with out
taking Propagation effect into account.
Psat (from) calculation
Comparing Al vs. S
Psat (from) mathematica
by taking pump and probe
attenuation into account. S
P13 line, 20 micron, 0.8 m long Fiber.

43. Demtroder eq.
Fitting eq in origin
My Calculation
Larry’s Calculation
Or,
Our calculation
Demtroder Calculation

44. 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.
S1=Saturation Parameter due to probe
S2=Saturation 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

45. Transit time broadenings: a dominant factor in small core fiber
Molecules
Laser
FWHM
In terms of the beam diameter,
D=2w,
most probable velocity
In terms of frequency,
For acetylene, M= g/mole;
Room Temp, T=2950K
for 13x10-6m mode field diameter

46. Diode Laser EDFA Pump Probe PBG Fiber Vacuum Chamber BS Photo Diode BS
Isolator
AOM
Squeezer
70%
BS(30/70)
EDFA
Squeezer
30% BS
Diode
Laser
(10/90) BS
Isolator
Squeezer
Photo Diode
Glass Cell
90% BS
Michelson
Interferometer
Photo Diode
squeezer

47. Mode of vibration of acetylene
ν1(cm-1)
ν3(cm-1)
3373.7
3278
ν1+ν3=6651.7(cm-1)
Wavelength=1.5
V1 and v3 are doubly
degenerate (equal energy)
bending vibration.
υ1 mode alone is dipole-forbidden,
it can be excited in combination with the dipole-allowed υ3 mode excitation