Mode-Locked Yb-Fiber Laser with Saturable Absorber Based on
Carbon Nanotubes
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An ytterbium fiber laser with the self mode locking using a saturable
absorber based on carbon nanotubes is developed. Original films that contain
carbon nanotubes make it possible to generate pulses with a duration of 16 ps
and a mean power of up to 10 mW at a wavelength of 1058 nm and a repetition rate
of 125MHz.
Mode locked fiber lasers that generate ultrashort pulses are interesting for
scientific study and technological applications [1]. One of the most widely used
methods for the initiation of mode locking employs intracavity elements that
allow the self starting of the mode locked lasing. Semiconductor saturable
absorbers are often employed as mode lockers [2]. However, the disadvantages are
a relatively high price and the damage induced by the high intensity radiation
upon the development of mode locking. In the fiber lasers with the semiconductor
saturable absorbers, the energy of the femtosecond pulses can reach 18 nJ at a
mean output power of 1.2 W [3]. Note that the nonlinear polarization rotation
can also be used for the mode locking in fiber lasers [4, 5]. Such a regime
employs the nonlinear effects in the optical fiber and makes it possible to
generate pulses with relatively high energies (greater than 1 μJ) in relatively
simple laser systems [6, 7]. However, the nonlinear evolution of polarization
upon mode locking leads to long term instabilities. The nonlinear polarization
rotation depends on both temperature and tuning of the polarization controllers.
Normally, the long term stability of the mode-locking regime is impossible when
the operation of the fiber polarization controllers is based on the mechanical
deformation of fiber. Optical fiber is an amorphous medium, and its initial
mechanical deformation exhibits a tendency towards plastic variations with time.
Therefore, the nonlinear polarization rotation does not provide the long term
stability of mode locking in a fiber laser.
In this regard, it is expedient to search for reliable devices that allow the
mode locking in fiber lasers owing to the disadvantages of the existing
semiconductor saturable absorbers and the absence of the long term stability in
the lasers with the nonlinear evolution of polarization. One of the promising
approaches for the further improvement of mode locking in fiber lasers involves
the analysis of new methods and devices based on carbon nanotubes [8].
Mode locked fiber lasers using saturable absorbers based on carbon nanotubes
were demonstrated in several works [9–15]. Note that the technologies that make
it possible to create nanotubes and absorbing films consisting of nanotubes are
substantially different, so that the resulting laser parameters are different
and it is expedient to analyze each specific system.
In this work, we present a mode locked Yb fiber laser with an original
saturable film absorber based on carbon nanotubes.
SATURABLE FILM ABSORBER WITH CARBON NANOTUBES
We developed and implemented a technology that allows the fabrication of the
polyvinyl alcohol films containing carbon nanotubes. The volume concentration of
nanotubes can be controlled in the highly homogeneous films. We use single wall
carbon nantubes (SWCNs) with a diameter of 0.8 nm, a meanlength of 1 μm, and the
saturable absorption in the range 900–1100 nm to fabricate saturable absorbers
that make it possible to initiate and to maintain the self mode locking in the
Yb fiber laser over a relatively long time interval.
The physicochemical and optical properties of SWCNs [16, 17]
show that they can be used instead of semiconductor saturable absorbers in
the mode locked fiber lasers [18] and the corresponding Raman converters and
amplifiers [19, 20]. SWCNs exhibit relatively high nonlinearities and
saturation recovery times of less than 1 ps. Note also that the SWCNs are
relatively inexpensive and are stable against high intensity radiation
(i.e., the damage threshold of carbon nanotubes is significantly higher than
the level above which the nanotubes exhibit the nonlinear optical
properties).
The SWCN based saturable absorbers can be created using
several methods: for example, sputtering or direct synthesis on the
substrate, deposition on the end surfaces of optical fibers, and dissolution
and mixing with polymers (e.g., polyvinyl alcohol, polymethyl
methacrylate, polycarbonate, etc.). Such methods for the fabrication of
saturable absorbers are appropriate for the lasers that work in wide
wavelength ranges (1– 2 μm) with various SWCNs.
The main problem in the creation of the SWCN based saturable absorber is
related to the linking of nanotubes in solution. The linked nanotubes give
rise to the optical inhomogeneity and induce scattering in the films made of
such liquids. Therefore, the maximum homogeneity of solution must be reached
when the nanotubes are dissolved in liquid.
In this work, we dissolve SWCNs in the polyvinyl alcohol. An ultrasonic bath
is used for the homogenizing, and the residual linked nanotubes are removed
owing to the additional filtering. Then, the polymer is added to the
suspension and the liquid is dried in a mold until solidification occurs. We
use a Shimadzu UV 3600 spectrometer to measure the absorption and film
thickness. The absorbance of the samples ranges from 50 to 90% in the
wavelength interval 1000 – 1100 nm, and the film thickness ranges from 6 to
20 μm.
EXPERIMENT
Figure 1 demonstrates the block diagram of the Yb fiber laser. The ring
cavity predominantly contains polarization maintaining components, so that the
nonlinear evolution of polarization is minimized. An Yb1200 6/125DC PM
polarization maintaining fiber serves as the active medium. The radiation is
outcoupled from the cavity using a 30% beam splitter. The active medium is
pumped by a laser diode at a wave length of 975 nm via a fiber combiner. The
maximum pumping power at the exit of the single mode fiber is 150 mW.
The film with the carbon nanotubes is placed between two FC/APC fiber
connectors. The mode locking is self started at a mean output power of 2 mW and
is maintained when the power increases to 13 mW. At intermediate output powers
(2–13 mW), the laser generates almost bandwidth limited pulses with a duration
of 16 ps (Fig.
2) and a spectral width of 0.15 nm (Fig.
3) at a repetition rate of 130 MHz (Fig.
4). For a spectral width of 0.15 nm (Fig.
3), the duration of the bandwidth limited pulse with the sech2 shape is 8
ps. This circumstance indicates the phase modulation of the laser pulses. At a
relatively long working time (greater than 30 min) and a mean output power of
greater than 10 mW, the instability of lasing emerges due to the heating of the
film. At a mean output power of greater than 13 mW, the Q switching is observed.
When the mean output power ranges from 2 to 10 mW, the laser exhibits stable
mode locking over several hours or days and even the reliable self starting of
the regime upon the on off switching in the absence of additional tuning.
CONCLUSIONS
A picosecond ring Yb fiber laser with the SWCN based saturable absorber is
demonstrated. A method for the fabrication of the SWCN films under laboratory
conditions is presented. The results prove the advantages of the SWCN based
saturable absorbers that can be used as reliable mode lockers in fiber lasers.
REFERENCES 1. A. Tünnermann, J. Limpert, and S. Nolte, “Ultrashort Pulse Fiber
Lasers and Amplifiers, in Femtosecond Technology for Technical and Medical
Applications
(Springer, Berlin, Heidelberg, 2004), vol. 96, pp. 35–54.
2. U. Keller, “Semiconductor Nonlinearities for Solid State Laser Modelocking
and Q Switching,” in Semiconductors and Semimetals (Academic, Boston, 1999),
59A.
3. Y. J. Song, M. L. Hu, C. L. Gu, L. Chai, C. Y. Wang, and A. M. Zheltikov,
Laser Phys. Lett. 7, 230 (2010).
4. V. J. Matsas, T. P. Newson, D. J. Richardson, and D. N. Payne, Electron.
Lett. 28, 1391 (1991).
5. S. Kobtsev, S. Kukarin, S. Smirnov, S. Turitsyn, and A. Latkin, Opt. Express
17, 20707 (2009).
6. S. M. Kobtsev, S. V. Kukarin, S. V. Smirnov, and Y. S. Fedotov, Laser Phys.
20, 351 (2010).
7. B. N. Nyushkov, V. I. Denisov, S. M. Kobtsev, V. S. Pivtsov, N. A. Kolyada,
A. V. Ivanenko, and S. K. Turitsyn, Laser Phys. Lett. 7, 661 (2010).
8. T. Hertel, Nature Photon. 4, 77 (2010).
9. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, IEEE J. Sel. Top. Quantum
Electron. 10, 137 (2004).
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and S. Y. Set, Opt. Lett. 29, 1581 (2004).
11. S. Yamashita, Y. Inoue, K. Hsu, T. Kotake, H. Yaguchi, D. Tanaka, M.
Jablonski, and S. Y. Set, Photon. Technol. Lett. 17, 750 (2005).
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Konyashchenko, P. G. Kryukov, and E. M. Dianov, Quantum Electron. 37, 847
(2007).
13. S. Kivistö, T. Hakulinen, A. Kaskela, B. Aitchison, D. P. Brown, A. G.
Nasibulin, E. I. Kauppinen, A. Härkönen, and O. G. Okhotnikov, Opt. Express 17,
2358 (2009).
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Popov, and J. R. Taylor, Appl. Phys. Lett. 95, 111108 (2009).
15. A. Schmidt, Opt. Express 17, 20109 (2009).
16. Y. C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y. P. Zhao, T. M.
Lu, G. C. Wang, and X. C. Zhang, Appl. Phys. Lett. 81, 975 (2002).
17. P. L. McEuen, Physics World 13, 31 (2000).
18. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I.
Milne, and A. C. Ferrari, Nature Nanotechnol. 3, 738 (2008).
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(2010).
Saturable absorber relies on carbon nanotubes John Wallace
A saturable absorber is a nonlinear optical material that becomes more
transparent as the intensity of light falling upon it increases. Passive
saturable absorbers can be integrated into laser systems to provide modelocking
and into fiberoptic systems for passive optical regeneration. The traditional
saturable absorber is a multiple-quantum-well (MQW) structure that requires
expensive equipment for fabrication—cleanroom-housed metal-oxide chemical-vapor
deposition or similar approaches to create the structure itself, and ion
implantation to reduce the device's saturation recovery time from the nanosecond
to the more practical picosecond range.
Researchers at Alnair Labs (Saitama-ken, Japan), the National Institute of
Advanced Industrial Science and Technology (Ibaraki, Japan), Tokyo Metropolitan
University (Tokyo, Japan), and the Research Center for Advanced Science and
Technology Tokyo, Japan) have created a saturable absorber from a layer of
single-walled carbon nanotubes sandwiched between two pieces of glass, termed a
saturable absorber incorporating nanotubes (SAINT). The fabrication process is
simple, consisting of spraying nanotubes onto glass. Because carbon nanotubes
are chemically stable, no hermetic sealing is required. The optical damage
threshold of the device is higher than that of a MQW saturable absorber; in
addition, a SAINT works in transmission—an easier-to-work-with mode of operation
than the reflective mode required for a MQW device.
The nanotubes themselves are synthesized by ablating a metal-catalyzed carbon
target with a Nd:YAG laser in 500 Torr of argon gas. The resulting tubes, with a
mean diameter of 1.1 nm, are dispersed in ethanol and sprayed onto 1-mm-thick
substrates with an airbrush (see figure). The researchers used the SAINTs for
two purposes: a noise-suppressing saturable absorber for 1550-nm light, and a
modelocked fiber laser operating in the same 1550-nm spectral region.
Single-walled carbon nanotubes are imaged by an atomic-force microscope (left)
and a transmission electron microscope (right). When sprayed onto a glass
substrate, these nanotubes exhibit saturable absorption of light, a property
useful in fiberoptic systems for noise suppression and laser modelocking.
Click here to enlarge image
The light source for the saturable-absorber setup was a fiber laser producing
1-ps pulse bunches (of about 120 pulses at a time) at an 80-GHz repetition rate.
A fiber collimator and an aspheric lens brought the light to a focus on the
SAINT. Varying the spot size by shifting the nanotube sample along the optical
axis varied the intensity of light falling on the carbon-nanotube film. At a
maximum peak intensity of 5.8 MW/cm2, the device reached a transmission of
almost 69%, whereas transmission dropped to about 63% at lower powers. Spectral
measurements indicate an inhomogeneously broadened absorption that responds on a
1-ps timescale. The performance of this first SAINT device is still far from its
full potential, the researchers note.
Two configurations
Modelocking a fiber laser is ordinarily done with a semiconductor saturable
absorber mirror (SESAM). For their modelocking experiment using a SAINT instead
of a SESAM, the researchers put together a fiber laser in two different
configurations, one with a ring geometry and the other of linear orientation.
The erbium-doped ring laser was backward pumped with a 980-nm laser diode, with
two optical isolators ensuring operation in one direction only. The SAINT and
associated optics were simply inserted in a break in the ring—a geometry
impossible with an ordinary reflective SESAM.
The ring laser began to modelock at a pump power of 18 mW, which could then be
backed off to 14 mW, with the laser operating at 6.1 MHz in single-pulse mode
(higher pump powers resulted in multiple-pulse operation at harmonics of 6.1
MHz). The resulting soliton pulses had a full-width half-maximum (FWHM) width of
1.1 ps, and were somewhat chirped—though the chirping could be reduced with the
use of low-dispersion fiber. When the SAINT was removed from the laser cavity,
all modelocking stopped, even at high pump powers.
The linear version of the modelocked fiber laser produced nonsoliton pulses at a
repetition rate of 9.85 MHz, a FWHM width of 318 fs, a 3-dB spectral width of
13.6 nm, and an average power of almost 1 mW. "We believe this is the first
demonstration of using carbon nanotubes for practical applications in the field
of applied optics," said Sze Set, general manager of research and development at
Alnair Labs, who noted that there was great interest in this material at this
year's Optical Fiber Communications Conference (Atlanta, GA; March 23–28).
Del Mar Photonics is involved in research of CNTs, graphene
nanoplatelets and graphene materials, develops advanced multifunctional
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Suspension.
3) Carbon Nanotubes.
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Trestles Ti:Sapphire laser
Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are
densely packed in a honeycomb crystal lattice. The term Graphene was coined as a
combination of graphite and the suffix -ene by Hanns-Peter Boehm,[1][2] who
described single-layer carbon foils in 1962.[3] Graphene is most easily
visualized as an atomic-scale chicken wire made of carbon atoms and their bonds.
The crystalline or "flake" form of graphite consists of many graphene sheets
stacked together.
The carbon-carbon bond length in graphene is about 0.142 nm. Graphene sheets
stack to form graphite with an interplanar spacing of 0.335 nm, which means that
a stack of 3 million sheets would be only one millimeter thick. Graphene is the
basic structural element of some carbon allotropes including graphite, charcoal,
carbon nanotubes, and fullerenes. It can also be considered as an indefinitely
large aromatic molecule, the limiting case of the family of flat polycyclic
aromatic hydrocarbons. The Nobel Prize in Physics for 2010 was awarded to Andre
Geim and Konstantin Novoselov "for groundbreaking experiments regarding the
two-dimensional material graphene".[4]
Graphene is a flat monolayer of carbon atoms tightly packed into a
two-dimensional (2D) honeycomb lattice, and is a basic building block for
graphitic materials of all other dimensionalities. It can be wrapped up into 0D
fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.[5]
References
[1] H. P. Boehm, R. Setton, E. Stumpp (1994). "Nomenclature and terminology of
graphite intercalation compounds". Pure and Applied Chemistry 66 (9): 1893–1901.
doi:10.1351/pac199466091893.
[2] H. C. Schniepp, J.-L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H.
Adamson, R. K. Prud’homme, R. Car, D. A. Saville, I. A. Aksay (2006).
"Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide".
The Journal of Physical Chemistry B 110 (17): 8535–8539. doi:10.1021/jp060936f.
PMID 16640401.
[3] H. P. Boehm, A. Clauss, G. O. Fischer, U. Hofmann (1962). "Das
Adsorptionsverhalten sehr dünner Kohlenstoffolien". Zeitschrift für anorganische
und allgemeine Chemie 316 (3-4): 119–127. doi:10.1002/zaac.19623160303.
[4] Nobel Foundation announcement
[5] Geim, A. K. and Novoselov, K. S. (2007). "The rise of
graphene". Nature Materials 6 (3): 183–191. doi:10.1038/nmat1849. PMID 17330084.
Carbon nanotubes (CNTs; also known as buckytubes) are allotropes of carbon
with a cylindrical nanostructure. Nanotubes have been constructed with
length-to-diameter ratio of up to 132,000,000:1,[1] which is significantly
larger than any other material. These cylindrical carbon molecules have novel
properties which make them potentially useful in many applications in
nanotechnology, electronics, optics, and other fields of materials science, as
well as potential uses in architectural fields. They may also have applications
in the construction of body armor. They exhibit extraordinary strength and
unique electrical properties, and are efficient thermal conductors.
Nanotubes are members of the fullerene structural family, which also includes
the spherical buckyballs. The ends of a nanotube may be capped with a hemisphere
of the buckyball structure. Their name is derived from their size, since the
diameter of a nanotube is on the order of a few nanometers (approximately
1/50,000th of the width of a human hair), while they can be up to 18 centimeters
in length (as of 2010).[1] Nanotubes are categorized as single-walled nanotubes
(SWNTs) and multi-walled nanotubes (MWNTs).
Chemical bonding in nanotubes is described by applied quantum chemistry,
specifically, orbital hybridization. The chemical bonding of nanotubes is
composed entirely of sp2 bonds, similar to those of graphite. These bonds, which
are stronger than the sp3 bonds found in diamonds, provide nanotubules with
their unique strength. Moreover, nanotubes naturally align themselves into
"ropes" held together by Van der Waals forces.
[1] Wang, X.; Li, Q.; Xie, J.; Jin, Z.; Wang, J.; Li, Y.; Jiang, K.; Fan, S.
(2009). "Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon
Nanotubes on Clean Substrates". Nano Letters 9 (9): 3137–3141.
doi:10.1021/nl901260b. PMID 19650638.
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