1. Portable, robust optical Frequency standards in hollow optical fiber
Mohammad Faheem,
Rajesh Thapa, Ahmer Naweed, Greg Johnson, and
Kristan Corwin

2. Motivation
Develop high accuracy Portable Wavelength Standards for Telecommunication Industry.

3. Outline Introduction Broadening Mechanisms and Saturation Spectroscopy
Frequency measurement.
Previous Work
Our Approach
Experimental Set-up
Results
Limitations
Future Work

4. Over View
C2H2
Laser
50 Torr ?
Frequency measurement Frequency comb

5. Wavelength Division Multiplex
Sources
Coupler
Fiber
Demultiplexer
Output Fibers
Separation between
channel is 1 nm
- Channel adjustment in WDM ( nm) system.
- Calibration of wavelength measurement devices.

6. Recipe for a Wavelength Standard
Atomic or molecular absorption lines
- Absolute frequency reference.
- Very stable under changing environmental
conditions
- Good references in the 1500 nm region
Good references in 1500 nm region
Acetylene ( nm)
Hydrogen cyanide ( nm)
Rubidium ( nm)

7. Acetylene ( Also called Ethyne)
- Colorless and extremely flammable
50 transition lines, spaced by GHz extending from
1515nm-1540 nm C C H H CH n1
Symmetric C- H bond Stretching CC n2
n1+n3 lies in 1.5 mm region CH n3
Anti-symmetric C- H bond Stretching
Stretching Vibrations

8. Important Broadening In IR Spectroscopy
Absorption
(Beer’s law) z IO I
‘a ‘is the absorption cofficient
Absorption sample
Doppler Broadening
Power Broadening
C2H2 at room temp. ~ 500 MHz
Laser spectroscopy by Wolfgang Demtroder

9. Important broadening near IR spectroscopy
Pressure broadening
and shift
Line Broadening
C2H2 broadening P(10) ~ 11.6 MHz/Torr
Line Shift
C2H2 line shift P(10) ~ 0.29 MHz/Torr
Transit-Time Broadening
Transit time T=d/v
500 KHz for 0.94 mm dia cavity
Laser spectroscopy by Wolfgang Demtroder

10. Saturation Spectroscopy
Eliminates Doppler width
Requires high Power (Typically 300 mW
for acetylene)
Dominant Line width
Pressure broadening (~11 MHz/Torr)
Transit-time broadening (coherence
time between laser and molecules)
Power Broadening
Signal Size
Depends linearly on pressure
Depends linearly on sample length
Laser
90%
B.S
10%
Pump Beam
Probe Beam M2 M1
Cell
Det.

11. Frequency Measurement
Frequency = Cycles/second
Definition of time
Caesium 133 atom
Duration of period
Of the radiation corresponding to
the transition between two hyperfine Level of the ground state of Cs atom.
Optical frequency In hundreds of THz
Its easy to measure in THz ?
Photo Detector In GHz
What we need to do?
Mode Locking
Frequency Comb

12. Time-Frequency Correspondence
2Df
tr.t = 1/fr t
E(t) Df
Fourier transform of periodic signal discrete frequency components.
I(f) fo fr f
fn = n fr + fo
fr Laser repetition rate
fo Offset
D. J. Jones, et al. Science 288, 635 (2000)

13. Measurement of fr and fo
Repetition Rate
fr can be measured with photo-detector in optical path
Offset
I(f) fo fr f
f2n fn
2fn-f2n = 2(nfrep+fo) - (2nfrep+fo) = f0
Octave Spanning
- Microstructure fiber
- Laser Cavity
D. J. Jones, et al. Science 288, 635 (2000)

14. A. Czajkowski,J. E Bernard,A. A. Madej,R. S
A.Czajkowski,J.E Bernard,A.A.Madej,R.S.Winler Self reference frequency comb
Unknown signal f fr fo f
Unknown signal
App.Phys.B79,45-20 (2004)

15. Solid core microstructure Fiber
Fused silica core
Cladding
Core
1.7 m
laser
spectrum
100 mW
(1 nJ)
Spectrum Broadening
20 mW
(0.2 nJ)
Wavelength (nm)
Relative Power (db)

16. Frequency comb Set-up Fiber in Fiber out Fiber Laser 10 W 1075 nm HNLFSC BS
Cr:forsterite Laser DM
nonlinear
crystal
stabilized optical
frequency comb
Synthesizer f
rep
Loop
Filter
Phase
Detector

17. Previous work: K.Nakagawa, M.de Labachelerie, Y.Awaji
and Kourogi
(J.opt.soc.Am.B/Vol.13,No.12/December1996)
Cavity :
Long interaction length.
High intracavity power (100 mw).
Fragile.
Cavity and laser locked to
resonance independently.
Signal Measurement :
Two photon Rb (778 nm) transition as
a reference.
Hydrogen Cyanide(1556 nm, P(27)) as a
Intermediate reference.

18. Previous Work: W.C. Swann and S.L. Gilbert. (NIST)
Pressure-induced shift and broadening of 1510–1540-nm acetylene wavelength calibration lines,
” Opt. Soc. Am. B, 17, 1263 (2000).
Pressure broadening & shift
For P(13) broadening 11.4 MHz/Torr
Line shift 0.27 MHz/Torr
Effect of Temp negligible effect
Used to calibrate Optical Spectrum Analyzers (OSA’s)

19. Previous Work :A.Czajkowski, A.A.Madej, P.Dube
Development and Study of a 1.5 um Optical frequency Standard referenced to p(16) Saturated
absorption line in the (V1+v3) overtone band of 13C2H2
Optics Communications 234(2004)
Saturation signal ~ 1 MHz
Measure Power shift Hz/mw
Pressure Shift Hz/mTorr

20. Our Approach
Develop high accuracy portable wavelength Standards for telecommunication industry.
Through existing Technology :
- Cavity based references are not Portable.
- Transitions in the glass cells can not be further narrowed.
Solution :
Use molecular absorption inside optical fiber.
Advantages:
Portable
Easy to align
Easier to get high intensities over long path.

21. Experimental Set-up Fiber in Fiber out ultimately: Probe (15 - 300) mW
Capacitive
manometers
Hollow optical fiber
Gas Inlet
1 mW
To vacuum pump
Probe
Pump
( ) mW
C2H2 molecules
ultimately:
Fiber in
Fiber out

22. Setup- Optics d2 d1 PD Pump Beam Probe Beam Fiber C2H2Cell PD PBS PD
Diode
Laser
Fringe width~156 MHz
Mirror
50/50 d2 d1
Mirror BS
PBS
Pump Beam
Probe Beam
10/90
Fiber
EDFA
C2H2Cell
30/70
PBS PD
ISO
Probe
Squeezer
PBS PD
ISO
Pump
Squeezer
λ/2

23. Capillary Tube Too lossy Only 40% transmission
Laser
Power loss 2a nm
Too lossy
Length 18 cm and dia 330 µm
Only 40% transmission
Doppler Broadened signal
observed
No saturation signal.

24. Capillary tube 10 µm PBF gives 50 MHz saturation dip.
Satisfy Beer’s law
10 µm PBF gives 50 MHz saturation dip.
300 µm should give 1.73 MHz saturation dip.
For saturation dip
We need power 865 times

25. Photonic Bandgap fiber
10 mm
loss< 0.02 dB/m
No total internal reflection
Bragg’s reflection

26. 10-µm Photonic Bandgap fiber

27. 10-µm Photonic Bandgap fiber
1.2 Torr of 12C2H2 at nm
Significant signal strength at 10 and 20 mW pump powers!

28. 10-µm Photonic Bandgap fiber
We are transit limited or pressure limited ?
Line width does not increase
significantly with pressure which implies
that it is transit time limit.

29. 20-µm Photonic Bandgap fiber
20 mm core, 60 cm length
Fiber fills to 2 mTorr in ~ 10 s
20 µm, 83 cm long PBF at nm

30. 20-µm Photonic Bandgap fiber
Pressure limited ?
Factor of 3 change in pressure gives a factor of
1.2 change in line width
Transit limited

31. 10-µm, 20- µm PBF data Comparison
Transit Time
To reduce Transit time Broadening:
increase fiber hole size
-or- find a heavier molecule
-or- Decrease the velocity of molecule by
cooling

32. Linewidth: (target < 1 MHz)
Ultimate limits
Signal strength:
optimal fiber length for pressure.
Noise
Interference (probe with stray/reflected pump)
laser intensity noise
Linewidth: (target < 1 MHz)
transit time broadening
pressure-broadening
To narrow the transition, we must:
reduce transit-time broadening
reduce the pressure
lengthen the fiber

33. Conclusions - Observed saturated absorption features in photonic
bandgap fiber for first time.
- Significant absorption fraction observed at low
power (< 20 mW), with 23 MHz-wide feature.
- Confirmed transit time broadened, 20 mm produce
narrower feature than 10 mm fibers

34. Future Plan Near-term: Longer-term: Make more portable, reduce noise.
Build frequency Comb for absolute measurement.
Observe dependence of different broadening
mechanisms.
Observe the shifts in Photonic bandgap fibers.
Longer-term:
Seal the fiber filled with gas. (Greg Johnson)
Narrow the transition
Explore larger photonic bandgap fibers
Explore other gases.

35. Thank You

37. Photonic Bandgap fibers
Index guiding
Hollow Core guiding

38. Saturated absorption feature width
10 mm
~ 40 MHz
Transit time broadening: Naive estimate
t= d/v = ~1/50 MHz
Pressure broadening:
11 MHz/Torr * 1.2 Torr = 13.2 MHz

40. Important Broadening In IR Spectroscopy
Doppler Broadening
Molecules are in motion when they absorb energy. This causes a change in
the frequency of the incoming radiation.
Pressure broadening
Produce by the shifts of energy levels by
interaction of radiating atom with near by
particles
Transit time Broadening
The interaction time of molecules with the
radiation field is small with the
spontaneous life time of excited levels
Power Broadening
Molecules absorb energy from intense laser. This causes a energy shift causing broadening.