Advances on Portable Frequency Advances on Portable Frequency References in LUMOSReferences in LUMOS
Kansas State UniversityKansas State UniversityKevin KnabeKevin Knabe
Advising Professors: Kristan Corwin & Brian WashburnAdvising Professors: Kristan Corwin & Brian WashburnColleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-RawhiColleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-Rawhi
OutlineOutline
GoalsGoals Overview of Frequency StandardsOverview of Frequency Standards Saturated Absorption SpectroscopySaturated Absorption Spectroscopy Fiber CellsFiber Cells Reflected Pump Setup (single Reflected Pump Setup (single
beam!)beam!) LockingLocking Update on the Cr:F LaserUpdate on the Cr:F Laser
GoalsGoals
Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)
Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber ap (PBG) Fiber and and SSingle ingle MMode ode FFiber (SMF)iber (SMF)
Lock a laser to absorption signalLock a laser to absorption signal
Acetylene as Frequency Acetylene as Frequency ReferenceReference
Moderate accuracy (± 100 MHz)Moderate accuracy (± 100 MHz) High accuracy ( ±2 kHz)High accuracy ( ±2 kHz)
>500 MHz
Complex, fragile
1- 40 MHz
W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc. Am. B, 17, 1263 (2000).
100 mW
8.5 mW
Figure from: K. Nakagawa et al., JOSAB 13, 2708 (1996)
Our Goal: Combine portability with improved accuracy
Saturation spectroscopy – pump-probe techniqueSaturation spectroscopy – pump-probe technique Long interaction lengthsLong interaction lengths
Portable, robust -> Large linewidths
SMFPBG Fiber
Splice Splice
SMF
Saturated AbsorptionSaturated Absorption
Pump burns hole in velocity distribution, probe samples different velocity class, except when on resonance.
Frequency (MHz)
Fra
cti
on
al ab
sorp
tion Doppler-broadened line width
Sub-Doppler line width
Pump and probeat same frequency
l“Bennett Hole” or Saturated Absorption Feature
Theory of Saturated Theory of Saturated AbsorptionAbsorption
Pump
Probe
Pump and probe burn independent holes while scanning velocity classes
The only time they see each other is when they are resonant
Ipump >> Iprobe (Iprobe < (0.05) Ipump)
Gas Cell
Beer’s Law
Ab
so
rptio
n
Pump-Probe SetupPump-Probe Setup
ECDL EDFA2x
AOM 70%
30% PC
PBGF
PBS PBS 2
λ
PD
40%
60%
Iso .
Diagnostics
VC4
λ
2
λ
Michelson Interferometer for
Frequency Calibration
Pump
Probe
PBGF acts like a waveplate!!
Extended Cavity Diode Laser
-Tunable from 1510 nm to 1580 nm
-Can sweep using external voltage control
Sweep ~ 4 GHz at 1530 nm
Erbium Doped Fiber Amplifier - Can amplify up to 500
mW
Photonic Bandgap FibersPhotonic Bandgap Fibers
““10 10 m fiber”- 7 missing cellsm fiber”- 7 missing cells 7.5 7.5 μm mode field diameter mode field diameter
““20 20 m fiber”- 19 missing cellsm fiber”- 19 missing cells 13.5 13.5 μm mode field diameter mode field diameter
Images by Crystal Fibre A/S
Allows for long interaction lengths!
Typical DataTypical Data
Uneven background in 10 μm attributed to coupling into
surface modes due to small mode field diameter
-750 -500 -250 0 250 500 750
0.0
0.5
1.0
1.5
2.21 torr 0.93 torr 0.66 torr 0.47 torr 0.22 torr
L
(u
nit
less
)
Frequency Offset (MHz)
20 m diameter, 0.80 m long, 29 mW Pump Power
-750 -500 -250 0 250 500 750-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
L
(u
nit
less
)
10 m diameter, 0.90 m long, 30 mW Pump Power
0.84 torr 0.79 torr 0.60 torr 0.48 torr 0.40 torr 0.31 torr
Frequency Offset (MHz)
Good signal quality, small widths
Problem: Splices are not commercially available!!
Pressure ResultsPressure Results
R. Thapa, K. Knabe, M. Faheem, A. Naweed, O. L. Weaver, and K. L. Corwin, "Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber," Opt. Lett. 31, 2489 (2006).
20 μm fiber shows lower fundamental width at zero pressure – larger transit time, so less broadening!
Mode field radius of
fiber
Power ResultsPower Results
Power broadening widens transition, but discrimination keeps going up
Because of availability of additional laser sources, staying under 50 mW may be a requirement (EDFA’s are expensive)
0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
D (
khz-1
)
Power (mW)
GoalsGoals
Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)
Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber ap (PBG) Fiber and and SSingle ingle MMode ode FFiber (SMF)iber (SMF)
Lock a laser to absorption signalLock a laser to absorption signal
“Arc fusion splicing of hollow-core photonic bandgap fibers for gas-filled fiber cells”R. Thapa, K. L. Corwin, and B. R. Washburn, Accepted to Optics Express 2006
LUMOS Spliced Fiber (2005) - 20 μm core
Splice Loss:PBG ->SMF ~ 2.0 dB (30%)SMF ->PBG ~ 0.5 dB (10%)
The Splice is RightThe Splice is Right
20 μm core PBG supports more than 1 mode!
“Half Cell”
Making Fiber CellsMaking Fiber Cells
SMFPBGSMF
Vacuum Chamber filled to a low
pressure with C2H2
CO2 Laser
Arc Fusion Splicer
We will have a robust portable fiber cell!Aaron
Pung
GoalsGoals
Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)
Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber and ap (PBG) Fiber and SSingle ingle MMode ode FFiber (SMF)iber (SMF)
New GoalNew Goal: Check quality of half : Check quality of half cellscells
Lock a laser to absorption signalLock a laser to absorption signal
Saturated Saturated Absorption Absorption
Spectroscopy With Spectroscopy With Only 1 Beam in PBG Only 1 Beam in PBG
Fiber*Fiber*
*Patent pending
ECDL EDFA EDFA
PC
PBGF
PBS PBS 2
λ
PD
Iso .
Diagnostics
VC 4
λ
2
λ
SMF
Splice
2x AOM
Pump-Probe Setup for Pump-Probe Setup for Spliced Half-CellsSpliced Half-Cells
High Loss
Low Loss
Pump
Probe
-800 -600 -400 -200 0 200 400 600 800
0.0
0.2
0.4
0.6
0.8
Ab
sorp
tio
n (
Arb
. u
nit
s)
Frequency Offset (MHz)
P11 line at 0.9 torr, 23 mW pump power
Pump-Probe Setup for Pump-Probe Setup for Spliced Half-Cells – No Spliced Half-Cells – No
AOMAOM
ECDL EDFA EDFA
PC
PBGF
PBS PBS 2
λ
PD
Iso .
Diagnostics
VC 4
λ
2
λ
SMF
Splice
-500 0 500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Frequency Offset (MHz)
High Loss
Low Loss
Pump
Probe
ECDL EDFA
PBGF
BS 2λ
PD
40%
60%
Diagnostics
VC SMF
Splice
Reflected Pump SetupReflected Pump Setup
Pump
Probe
-500 0 500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
*L
(Arb
. Uni
ts)
Frequency Offset (MHz)
R-Pump Results: R-Pump Results: Signal QualitySignal Quality
-1000 -500 0 500 1000
0.0
0.2
0.4
0.6
0.8
*L
(uni
tless
)
Frequency (MHz)
0.31 t 0.39 t 0.45 t 0.50 t 0.55 t 0.66 t
Keeping saturated absorption feature centered with Doppler broadened profile has locking benefits
Wings exhibit very small interference pattern which has a free spectral range associated with the length of the PBG fiber
Reflections occurring at front end of splice, causing interference
R-Pump Results: R-Pump Results: Comparison Of WidthsComparison Of Widths
R-Pump : P11 Line : Pump Power = 30 mW
Pump-Probe : P11 Line : Pump Power = 29 mW
0.00 0.25 0.50 0.75 1.00
20
25
30
35
L (M
Hz)
Pressure (torr)
R-Pump : P11 Line : Pressure = 0.5 torr
Pump-Probe : P11 Line : Pressure = 1 torr
0 25 50 75 100
25
30
35
40
45
L (M
Hz)
Power (mW)
GoalsGoals
Studying Studying νν11 + + νν33 vibrational band of vibrational band of acetylene (optimize signal for acetylene (optimize signal for locking)locking)
Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber and ap (PBG) Fiber and SSingle ingle MMode ode FFiber (SMF)iber (SMF)
Check quality of half cellsCheck quality of half cells Lock a laser to absorption signal, Lock a laser to absorption signal,
then compare with locked Cr:F then compare with locked Cr:F frequency combfrequency comb
LockingLocking
– Lock laser to sat. abs feature.Lock laser to sat. abs feature.– Measure with comb referenced to GPS.Measure with comb referenced to GPS.– Lock comb to fiber-based reference, output Lock comb to fiber-based reference, output
stable microwaves.stable microwaves.
Frequency domain
0 fn = nfr + fo
I(f)
f
fo fr
EDFA
C2H2 molecules
AOM
PBG Cell
PumpProbe
1550nmECLD Locking
Electronics
-500 0 500
0.0
0.2
0.4
0.6
0.8
*L
(u
nitl
ess
)
Frequency (MHz)
0.31 t 0.39 t 0.45 t 0.50 t 0.55 t 0.66 t
Chromium Forsterite Chromium Forsterite UpdateUpdate
Cr:F SpectraCr:F Spectra
Supercontinuum
Using HNLF to generate supercontinuum from
~1000nm to ~2250 nm
Laser out of oscillator
70 nm BW @ 1280 nm center λ
PPLN
f-2f Interferometerf-2f Interferometer
Using Periodically Poled Lithium Niobate to get 2nd harmonics
Got 47 dB beat note signal
Have also locked repetition rate
Currently Rajesh is playing with pump power modulation and other servo controls to lock f0 1200 40 80f (MHz)
ConclusionConclusion
Characterization has been done on Characterization has been done on acetylene filled PBG fibersacetylene filled PBG fibers
Advances have been made on Advances have been made on making gas filled cellsmaking gas filled cells
Discovery of the Reflected-Pump Discovery of the Reflected-Pump techniquetechnique
Cr:F is close to being locked!Cr:F is close to being locked! Next: Lock Laser to saturated Next: Lock Laser to saturated
absorption signalabsorption signal
ThanksThanks
Funding Agencies: Funding Agencies: – AFOSRAFOSR– NSF CAREERNSF CAREER– Kansas NSF EPSCoR programKansas NSF EPSCoR program– Kansas Technology Enterprise Kansas Technology Enterprise
CorporationCorporation– Kansas State UniversityKansas State University
Mike Wells and the JRM StaffMike Wells and the JRM Staff