Transcript Slide 1
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
frequency
Over View
C 2 H 2 50 Torr Laser
?
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 C H
Colorless and extremely flammable
-
50 transition lines, spaced by 60-80 GHz extending from 1515nm-1540 nm
H C CH
n
1
C H
Symmetric C- H bond Stretching
CC
n
2
n
1 +
n
3 lies in 1.5
m
m region
CH
n
3 Stretching Vibrations Anti-symmetric C- H bond Stretching
Important Broadening In IR Spectroscopy
Absorption (Beer’s law)
I
I o e
(
w
)
z
‘
‘is the absorption cofficient
Ne
2 4
o mc
(
w o
w
) 2 / 2 ( / 2 ) 2
Doppler Broadening
Doppler
2
w o c
2 ln 2
kT m
C 2 H 2 at room temp. ~ 500 MHz I O I z 0 -1 -2 -3 0 500 1000 1500 Frequency in MHz 2000 Absorption sample
Power Broadening
power
1
S
power
~
S
P P s
2 max .
Laser spectroscopy by Wolfgang Demtroder
Important broadening near IR spectroscopy
Pressure broadening and shift Line Broadening
Pr
essure
N v
s where
s
0 (sin (
R
))
RdR
C 2 H 2 broadening P(10) ~ 11.6 MHz/Torr
w
Line Shift
C 2 H 2
2
N v
b where
b
0 ( 1 cos (
R
))
RdR
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
p
Laser spectroscopy by Wolfgang Demtroder
Saturation Spectroscopy
• • • •
Eliminates Doppler width Requires high Power for acetylene) ( Typically 300 mW Dominant Line width
Pressure broadening (~11 MHz/Torr )
Transit-time broadening ( coherence time between laser and molecules )
Power Broadening 90% Pump Beam Signal Size
Depends linearly on pressure
Depends linearly on sample length M 2 Det.
B.S
10% Probe Beam M 1 Cell
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
E
(
t
) f 2f
t
t r .
t = 1/f r
Fourier transform of periodic signal discrete frequency components .
I
(
f
)
f o f r f r f o
0
Laser repetition rate Offset
f
f n = n f r + f o
D. J. Jones, et al. Science 288, 635 (2000
)
Measurement of f r and f o Repetition Rate f r can be measured with photo-detector in optical path Offset
I
(
f
) 0
f o f r f n f 2n 2f n -f 2n = 2(nf rep +f o ) - (2nf rep +f o ) = f 0
f
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 f r f o
f
Unknown signal
App.Phys.B79,45-20 (2004)
Solid core microstructure Fiber
Cladding Fused silica core Core 1.7
m
m -20 -30 -40 -50 -60 -70 -80 400 Spectrum Broadening laser spectrum 600 800 Wavelength (nm) 1000 20 mW 100 mW (1 nJ) (0.2 nJ) 1200
Fiber in Fiber Laser 10 W 1075 nm
Frequency comb Set-up
Cr:forsterite Laser Fiber out SC BS HNLF stabilized optical frequency comb Synthesizer DM f rep Loop Filter f 0 Loop Filter nonlinear crystal Synthesizer Phase Detector
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 13 C 2 H 2
Optics Communications 234(2004) 259 268 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
.
Gas Inlet Probe 1 mW ultimately: Fiber in
Experimental Set-up
To vacuum pump Capacitive manometers Gas Inlet Hollow optical fiber Pump (15 - 300) mW C 2 H 2 molecules Fiber out
Setup- Optics
Diode Laser PBS 10/90 EDFA 30/70 PBS ISO BS C 2 H 2 Cell Squeezer PD Fringe width~156 MHz 50/50
Pump Beam
d 1 Mirror
Probe Beam
d 2 Mirror Fiber Probe PD ISO Squeezer λ/2 PBS Pump PD
Capillary Tube
l
Laser 2a
1 /
l atten
2
a
3
Too lossy Length 18 cm and dia 330 µm
Only 40% transmission
Doppler Broadened signal observed
No saturation signal
.
Power loss 1.0
0.8
0.6
0.4
0.2
0.0
1531.31 nm
50.3 Torr 27.9 Torr 12.3 Torr
300 600 900 Frequency in MHz 1200
Capillary tube
100 90 80 70 60 50 0 10 20 30 40 Pressure (mTorr) 50 60 20 18 16 14 12 10 8 6 4 2 0 10 20 30 Pressure (Torr) 40 Satisfy Beer’s law 50 10 µm PBF gives 50 MHz saturation dip.
300 µm should give 1.73 MHz saturation dip.
60
P s
I I s r
2 2
For saturation dip We need power 865 times
Photonic Bandgap fiber
10
m
m loss< 0.02 dB/m No total internal reflection Bragg’s reflection
10 µm Photonic Bandgap fiber
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
At 10 mW Pump & Probe Probe Reflection of Pump beam from fiber ends Saturation Dip -1500-1000 -500 0 500 1000 1500 Frequency (MHz)
10 µm Photonic Bandgap fiber
1.0
0.8
1.2 Torr of 12 C 2 H 2 at 1531.31 nm
112 mW (+ 0.4) 83 mW (+ 0.3) 40 mW (+ 0.2) 20 mW (+ 0.1) 10 mW 0.6
0.4
0.2
0.0
-1000 -500 0 500 Frequency (MHz) 1000 Significant signal strength at 10 and 20 mW pump powers!
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
112 mW (- 0.2) 83 mW (- 0.1) 40 mW (- 0.1) 20 mW (- 0.05) 10 mW -0.6
-400 -200 0 200 400 600 800 Frequency (MHz) 0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0 20 40 60 80 Power(mW) 100 120
10 µm Photonic Bandgap fiber
60 55 50 45 40 35 Wavelength 1531.31 nm 600 800 1000 Pressure in mTorr 1200 We are transit limited or pressure limited ?
pressure
P
N N v
s
Line width does not increase significantly with pressure which implies that it is transit time limit.
20 µm Photonic Bandgap fiber
20
m
m core, 60 cm length Fiber fills to 2 mTorr in ~ 10 s 20 µ m, 83 cm long PBF at 1531.20 nm
0 -2
Pump+Probe
-4
1000 Michelson's fringes Probe only 2000 Frequency (MHz) 3000 4000 1.0
0.8
0.6
0.4
0.2
0.0
0 20 40 60 Time (s) 80 100 0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
FWHM 29.6 MHz 1600 1800 2000 Frequency in MHz 2200
20 µm Photonic Bandgap fiber
20
m
m FBF 32 30 28 26 24 22 200 400 600 800 Pressure (mTorr) 1000
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
60 55 50 45 40 35 30 25 20 200 10
m
m PBF 20
m
m PBF 400 600 800 Pressure in mTorr 1000 1200 Transit Time
tt
4 (
v
/
d
)
To reduce Transit time Broadening
:
increase fiber hole size -or- find a heavier molecule -or- Decrease the velocity of molecule by cooling
2 ln( 2 )
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
m
m produce narrower feature than 10
m
m 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
Saturated absorption feature width
10
m
m 0.02
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
-1000 ~ 40 MHz -500 0 Frequency (MHz)
10 mW 20 mW
500 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.