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Fiber Frequency Combs
Jennifer Black
EE230 Final Presentation
Mode-Locked Lasers
• Laser that produces a series of ultrashort
pulses (infinite pulse train)
• Two techniques:
– Active: uses optical modulator
– Passive: may use a saturable absorber
Mode-locking
Pictures from Rick Trebino lecture notes
Passively Mode-Locked Lasers
Use of a saturable absorber (SA) in the cavity creates the pulse train. SA are materials
with non-linear optical properties that attenuate low optical intensities.
SA
[1]
Gain
Fourier Transform!
Mirror
Output
Coupler
Frequency Combs
[2]
• If the carrier
envelop offset can
be set then the
comb is stabilized
• Frequencies used
as optical ruler
• Beat notes are
measured
The frequency comb (red) can be beat against an
unknown frequency (blue). If the comb frequencies are
known, then the unknown frequency can be
determined.
fn = nfr + foffset
Applications
Breath analyzer, NIST
Optical clock,
NIST
Astronomical measurements,
Max Planck Institute
Atomic spectroscopy, KSU
Table Top vs. Fiber lasers
• Optical fibers can be
used as waveguides
for lasers that are:
– Cheap
– Portable
– Robust
– “Easy”
Table top frequency comb at Center for
Quantum Technologies in Singapore
Fiber frequency comb shipped via
FedEx (worked first try)!
CNT Fiber Laser
CNT!
~ 1550nm
EDF
CNT Deposition
• Process:
•10mW at 1560 nm
through SMF and put
into solution for 30s
•Out of solution for 1
min
• Throughput checked
•Continue until loss =
-3dB
•Put into fiber laser
cavity and mode-locks
Nanotubes about fiber taper
LD
EDFA
CNT/ethanol solution:
•0.5mg CNT
•20mg Ethanol
CNT Fiber Frequency Combs
• PROS: High rep
rate compared to
highly nonlinear
optical fiber
(HNLF)
• CONS: Low power
threshold
VS.
HNLF
Damaged Nanotubes!
Photonic bandgap fibers
n1 = 1.5
n2 = 1.52
Core
Cladding
Standard optical fiber: total internal reflection
n1 = 1.5
n2 = 1.0
Core
Cladding
Hollow capillary fiber: 4% loss per bounce.

~1.1
1.0

PBG fiber
Bragg scattering forbids radial
propagation
--or-Photonic crystal forbids
propagation everywhere except at
defect.
Photonic Bandgap Optical Fibers (PBG)
•Using 10 µm inner
diameter PBG fiber
•Want CNT/PMMA
solution in center hole
d = 10 mm
• Have PBG guide like
solid core fiber
Method
• Taper PBG fiber:
• Cleave fiber where photonic crystal has
collapsed and center hole is all that is left
open:
Method
• Apply vacuum to
cleaved end of
PBG while
cleaved end is in
CNT/PMMA
solution
CNT/PMMA solution
Microscope
Vacuum
chamber
Vacuum Chamber
CNT/PMMA solution
F
I
B
E
R
Conclusion
• Frequency combs used for precision
measurements.
• Fiber offers robust, “easy”, cheap and portable
frequency combs.
• Design challenges remain for desired threshold
powers.
• Work continues on CNT/PMMA SA at KSU
Questions?
References
• [1]: http://www.optik.unierlangen.de/mpf/php/abteilung2/index.php?show=res
earch&in=precisionmeasurements&and=rim
• [2]: http://www.rpphotonics.com/frequency_combs.html
• [3]: http://www.justmeans.com/Carbon-Nanotubebased-Batteries-for-HEVs/11428.html
• [4]: Sze Y. Set, H. Yaguchi, Y. Tanaka, M. Jablonski.
Ultrafast Fiber Pulsed Lasers Incorporating Carbon
Nanotubes. IEE Journal of Selected Topics in Quantum
Mech., Vol. 10, No. 1
Single Walled Carbon Nanotubes
Single wall carbon nanotubes have
semiconductor, semimetal or metallic
properties depending on the chiral vector of
the nanotube
C  na1  ma 2
Metallic
n  m  integer multiple of 3
Semiconductor n  m  integer multiple of 3
Semimetal
nm 0
Excitonic absorption in the semiconductor
nanotube is responsible for the saturable
absorption property
Ultrafast recovery of the saturable absorber
is due to metallic nanotubes serving a
recombination centers
Carbon Nanotubes (CNT)
Transmission vs. Wavelength Curves
for CNT of Different Mean Diameter
[4]
Mean diameter = 1.35 nm
Mean diameter = 1.2 nm
[3]
• Diameter of
CNT change
transmission
wavelength
dependence
• How to
incorporate
CNT into fiber
laser?
CNT/Polymer Solution
• Polymer (we use PMMA) used to disperse CNT
homogenously – nPMMA = 1.49
• Put inside of PBG fiber and guide like solid core
fiber
Step 1: 3mg CNT and 10mL
of a solvent are sonicated
for 3 hours
Step 2: 37mg of PMMA
are added and sonicated
for an additional 2.5 hours
Solvents used:
- Acetone
- Anisole
A few
days
later
Carbon Nanotube
precipitant
Testing the Fibers
• Test small piece of sample in a pre-existing fiber laser
Laser LD
Diode
Butt-couple SA (..?) into
laser cavity
Output
Coupler
Gain
Testing the Fibers
Cleaved
fiber
from
the
laser
Fiber
Laser
PBG
sample
Sample is butt-coupled on both sides to a pre-existing fiber laser
Optical Spectra
0.1
• Mode-locked
– Broad spectra
Power (mW/nm)
0.01
1E-3
1E-4
1E-5
1E-6
1540
1550
1560
1570
Wavelength (nm)
• Continuous Wave (CW)
– Sharp peak
1580
1590
Results for Acetone
• Acetone Sample:
– Lasing CW but not
mode-locking…
– Not stable
– Poor solvent for this
process
– Laser possibly boiling
away solvent – optical
limiting
– … Try a different
solvent!
Pout = 0.5 mW; length = 4 cm
Results for Anisole
• Anisole Sample:
– Also lasing CW
– Not mode-locking
Pout =
120 µW
Length =
3.0 cm
Pout = 0.8 mW
Length = 2.8 cm
Conclusions
“Clean PBG”
PBG with solution