Carbon Nanotube Polymer
Composite Saturable Absorbers in
Photonic Bandgap Fibers
Jennifer Black
2010 Kansas State University
Advisor: Dr. Brian Washburn
Overview
• Goal: Create a unique saturable absorber to
mode-lock a fiber laser
• How: Inserting a carbon nanotube polymer
solution into the center hole of a photonic
bandgap fiber
• Why: To mode-lock a laser with high repetition
rate and increase intercavity energy
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
Gain
[1]
Mirror
Output
Coupler
Fourier Transform!
Frequency Combs
[2]
• The carrier envelop
offset and can be set
and the comb
stabilized
• Applications of stable
combs:
–
–
–
–
–
Metrology
Optical clock
Telecommunications
Doppler lidar
Spectroscopy
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
Passively Mode-Locked Fiber lasers
• Optical fibers can be
used as waveguides
for lasers that are:
–
–
–
–
Cheap
Portable
Robust
“Easy”
• Carbon Nanotube
Fiber Lasers:
– High rep rate vs. HNLF
– 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
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
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
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
Conclusion
• Believe solution
is in fiber:
• Possible
Problems:
– Not enough
CNT per
sample length
• What next…?
– CNT fluoresce
– Change
composition
CNT/PMMA
solution
“Clean PBG”
PBG with solution
Acknowledgments
• Jinkang Lim, Shun Wu, Andrew Jones, Rajesh
Thapa, May Ebbeni, Chenchen Wang
• Dr. Washburn, Dr. Corwin, Dr. Weaver
• Mike Wells and Scott Chainey
• NSF; ASOFR
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
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
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