Portable, robust optical
Frequency standards in hollow
optical fiber
Mohammad Faheem,
Rajesh Thapa, Ahmer Naweed,
Greg Johnson, and
Kristan Corwin
Motivation
Develop high accuracy Portable Wavelength
Standards for Telecommunication Industry.
Outline
Introduction
Broadening Mechanisms and Saturation
Spectroscopy
Frequency measurement.
Previous Work
Our Approach
Experimental Set-up
Results
Limitations
Future Work
Over View
C 2H 2
Laser
frequency
50 Torr
?
Frequency measurement
Frequency comb
Wavelength Division Multiplex
Demultiplexer
Fiber
Separation between
channel is 1 nm
Coupler
Sources
Output Fibers
- Channel adjustment in WDM (1525-1565 nm) system.
- Calibration of wavelength measurement devices.
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
» Hydrogen cyanide
» Rubidium
(1510-1540 nm)
(1530-1565 nm)
(1560-1590 nm)
Acetylene ( Also called Ethyne)
H
C
H
C
- Colorless and extremely flammable
- 50 transition lines, spaced by 60-80 GHz extending from
1515nm-1540 nm
H
C
CH
n1
C
H
Symmetric C- H bond Stretching
CC
n2
n1+n3 lies in 1.5 mm region
CH
n3
Stretching Vibrations
Anti-symmetric C- H bond Stretching
Important Broadening In IR Spectroscopy
Absorption (Beer’s law)
z
‘ ‘is the absorption cofficient

Ne
 / 2
4 o mc ( wo  w) 2  ( / 2) 2
Arbitrary Axis
I  I o e  ( w ) z
0
IO
I
-1
-2
-3
0
2
Doppler Broadening
500
1000
1500
2000
Frequency in MHz
Absorption sample
Power Broadening
 power   1  S
2wo
kT
 Doppler 
2 ln 2
c
m
C2H2 at room temp. ~ 500 MHz
P
S
Ps
 power ~ 2 max .
Laser spectroscopy by Wolfgang Demtroder
Important broadening near IR spectroscopy
Pressure broadening
and shift
Line Broadening

 Pressure  Nv  s where  s   (sin ( R)) RdR
0
C2H2 broadening P(10) ~ 11.6 MHz/Torr
Line Shift
w    2 Nv  b
p


where  b   (1  cos ( R)) RdR
0
C2H2 line shift P(10) ~ 0.29 MHz/Torr
Transit-Time Broadening
Transit time T=d/v
 tt  4(v / d ) 2 ln( 2)
500 KHz for 0.94 mm dia cavity
Laser spectroscopy by Wolfgang Demtroder
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
90%
B.S
Pump Beam
• Signal Size
 Depends linearly on pressure
 Depends linearly on sample length
10%
Probe Beam
M2
M1
Cell
Det.
Frequency Measurement
Frequency = Cycles/second
Definition of time
Caesium 133 atom
Optical frequency ----- In hundreds of THz
Duration of 9 192 631 770 period
Of the radiation corresponding to
the transition between two hyperfine
Level of the ground state of Cs atom.
Its easy to measure in THz ?
Photo Detector ------ In 40-100 GHz
What we need to do?
Mode Locking
Frequency Comb
Time-Frequency Correspondence
f
E(t)
2f
t
tr.t = 1/fr
Fourier transform of periodic signal
I(f)
0
fr
fo
Laser repetition rate
Offset
discrete frequency components.
fo
fr
fn = n fr + fo
f
D. J. Jones, et al. Science 288, 635 (2000)
Measurement of fr and fo
Repetition Rate
fr can be measured with photo-detector in optical path
Offset
I(f)
fo
fr
0
fn
f2n
f
2fn-f2n = 2(nfrep+fo) - (2nfrep+fo) = f0
Octave Spanning
- Microstructure fiber
- Laser Cavity
D. J. Jones, et al. Science 288, 635 (2000)
A.Czajkowski,J.E Bernard,A.A.Madej,R.S.Winler
Self reference frequency comb
Unknown signal
fr
fo
f
Unknown signal
App.Phys.B79,45-20 (2004)
Solid core microstructure Fiber
Fused silica core
Cladding
Core
Relative Power (db)
Spectrum Broadening
-20
-30
100 mW
20 mW (1 nJ)
(0.2 nJ)
-40
-50
-60
-70
laser
spectrum
-80
400
600
800
1000
Wavelength (nm)
1200
1.7 mm
Frequency comb Set-up
Fiber in
Fiber Laser
10 W
1075 nm
Fiber out
Cr:forsterite Laser
SC
BS
HNLF
stabilized optical
frequency comb
Synthesizer
frep Loop
Filter
nonlinear
crystal
Synthesizer
f0 Loop
Filter
Phase
Detector
DM
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.
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)
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) 259268
Saturation signal ~ 1 MHz
Measure Power shift 11.4 Hz/mw
Pressure Shift 230 Hz/mTorr
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.
Experimental Set-up
To vacuum pump
Gas Inlet
Capacitive
manometers
Gas Inlet
Hollow optical fiber
Probe
Pump
1 mW
(15 - 300) mW
ultimately:
Fiber in
C2H2 molecules
Fiber out
Setup- Optics
PD
Fringe width~156 MHz
Diode
Laser
Mirror
50/50
d2
d1
Mirror
BS
PBS
10/90
Pump Beam
Probe Beam
Fiber
EDFA
C2H2Cell
30/70
PBS
PD
ISO
Probe
Squeezer
PBS
ISO
Squeezer
Pump
λ/2
PD
Capillary Tube
l
Laser
2
a
Power loss
1531.31 nm
3
Too lossy
Length 18 cm and dia 330 µm
Only 40% transmission
Doppler Broadened signal
observed
No saturation signal.
1.0
Fractional Absoption
1 / latten
2a
0.8
50.3 Torr
27.9 Torr
12.3 Torr
0.6
0.4
0.2
0.0
300
600
900 1200
Frequency in MHz
Capillary tube
20
Absorption Cofficient (cm )
90
18
-1
Fractional Absorption in percentage
100
80
70
60
50
0
10
20 30 40
Pressure (mTorr)
50
60
16
14
Satisfy Beer’s law
12
10
8
6
4
2
0
10
20
30
40
50
60
Pressure (Torr)
10 µm PBF gives 50 MHz saturation dip.
300 µm should give 1.73 MHz saturation dip.
I 2 2
Ps r 
Is
For saturation dip
We need power 865 times
Photonic Bandgap fiber
10 mm
loss< 0.02 dB/m
No total internal reflection
Bragg’s reflection
Transmission (a.u)
10-µm Photonic Bandgap fiber
0.0
-0.2
-0.4
-0.6
At 10 mW
-0.8
Pump & Probe
Probe
-1.0
Reflection of Pump
-1.2
beam from fiber ends
Saturation Dip
-1.4
-1.6
-1.8
-2.0
-2.2
-1500-1000 -500 0 500 1000 1500
Frequency (MHz)
10-µm Photonic Bandgap fiber
at 1531.31 nm
Fractional Absorption
1.0
12C H
2 2
112 mW (+ 0.4)
83 mW (+ 0.3)
40 mW (+ 0.2)
20 mW (+ 0.1)
10 mW
0.8
0.6
0.4
0.0
-0.1
-0.2
-0.3
-0.4
112 mW (- 0.2)
83 mW (- 0.1)
40 mW (- 0.1)
20 mW (- 0.05)
10 mW
-0.5
-0.6
-400 -200 0 200 400 600 800
Frequency (MHz)
0.2
0.40
0.0
-1000
-500
0
500
Frequency (MHz)
1000
Significant signal strength at 10
and 20 mW pump powers!
Fractional Absorption
Fractional Absorption
1.2 Torr of
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
20
40 60 80
Power(mW)
100 120
Line width in MHz
10-µm Photonic Bandgap fiber
Wavelength 1531.31 nm
60
55
50
45
40
35
600
800
1000
1200
Pressure in mTorr
We are transit limited or pressure limited ?
 pressure  Nv  s
 P N
Line width does not increase
significantly with pressure which implies
that it is transit time limit.
20 mm core, 60 cm length
Fiber fills to 2 mTorr in ~ 10 s
20 µm, 83 cm long PBF at 1531.20 nm
1.0
0.8
0.6
0.4
0.2
0.0
0
20 40 60 80 100
Time (s)
0.45
Arbitrary units
Arbitrary Unit
0
Fract. Transmission
20-µm Photonic Bandgap fiber
Michelson's fringes
Pump+Probe
-2
Probe only
FWHM 29.6 MHz
0.40
0.35
0.30
0.25
0.20
0.15
0.10
1600
-4
1000
2000
3000
Frequency (MHz)
4000
1800
2000
Frequency in MHz
2200
20-µm Photonic Bandgap fiber
32
20 mm FBF
Line Width(MHz)
30
28
26
24
22
200
400
600
800
1000
Pressure (mTorr)
Pressure limited ?
Factor of 3 change in pressure gives a factor of
1.2 change in line width
Transit limited
10-µm, 20- µm PBF data Comparison
Line width in MHz
60
55
50
45
Transit Time
10 mm PBF
20 mm PBF
40
 tt  4(v / d ) 2 ln( 2)
35
30
25
20
200
400
600
800
1000
1200
Pressure in mTorr
To reduce Transit time Broadening:
increase fiber hole size
-or- find a heavier molecule
-or- Decrease the velocity of molecule by
cooling
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
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
Future Plan
Near-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.
Thank You
Photonic Bandgap fibers
Index guiding
Hollow Core guiding
10 mm
Fractional Absorption
Saturated absorption feature width
0.02
0.00
-0.02
-0.04
-0.06
-0.08
~ 40 MHz
-0.10
-1000
10 mW
20 mW
-500
0
500
Frequency (MHz)
1000
Transit time broadening: Naive
estimate
t= d/v = ~1/50 MHz
Pressure broadening:
11 MHz/Torr * 1.2 Torr = 13.2 MHz
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.