Selective Two-photon Excitation by Parametrically Shaped Laser
Pulses after an Optical Fiber
Albrecht Lindinger
Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
Keywords: Pulse Shaping, Optical Fibers, Multi-photon Processes.
Abstract: Laser pulse shaping through optical fibers is reported for applications on two-photon processes in dye
molecules. The presented method utilizes pre-compensation of the optical fiber properties by analytical
pulse shaping in order to receive specific parametric pulse forms after the fiber including polarization
modulation. Particularly phase-tailored pulse shapes at the distal fiber end are employed for two-photon
fluorescence of dyes in a liquid environment in order to improve the contrast between dye markers having
similar excitation spectra. This will lead to new endoscopic imaging applications with an increased
fluorescence contrast.
Coherent control of photo-induced molecular
processes by shaped laser pulses has attained
considerable success in recent years. It became most
exciting when self-learning feedback loop
algorithms were employed where tailored laser
pulses can be generated, which drive the induced
processes at a maximum yield along desired paths
(Judson and Rabitz, 1992); (Brixner and Gerber,
2003). An important issue in this regard is the
information coded in the optimized laser pulse shape
which supplies insight about the underlying
processes (Schäfer-Bung et al., 2004).
In the last years polarization pulse shaping was
explored in order to consider the vectorial character
of the light field (Brixner and Gerber, 2001);
(Polachek et al., 2006). Novel pulse shaper schemes
for simultaneous phase, amplitude, and polarization
pulse control were designed, and a parametric sub
pulse encoding was developed (Weise and
Lindinger, 2010). In this approach, the physically
intuitive parameters energies, distances, and chirps,
as well as the states of polarization of the sub pulses
can be controlled. This yields new perspectives of
adding the polarization and hence utilizing all
properties of the light field in the pulse modulation.
Recently, pulse shaping methods were
increasingly used in life sciences in order to
investigate biologically relevant systems (Herek et
al., 2002). Promising applications are imaging using
multi-photon microscopy, spectroscopy, and
photodynamic therapy (Myaing et al., 2006);
(König, 2000); (Tsen et al., 2007). Laser pulse
shaping was moreover applied to multi-photon
excited fluorescence where interference effects are
utilized (Meshulach and Silberberg, 1998). This
enables to achieve narrow band transitions and can
be used to selectively excite different biologically
relevant fluorophores with close lying excitation
bands (Lozovoy et al., 2002); (Ogilvie et al., 2006).
In order to operate in biologically relevant
environments, in vivo applications are desired were
the light is guided to the place of interest. This can
be realized by transmission of light through optical
fibers for endoscopic applications, which is a novel
topic of large relevance e.g. for medical treatments.
Yet, it is challenging to transfer femtosecond laser
pulses through fibers due to the occurring distortion
of the intense light pulses by linear and nonlinear
optical fiber properties (Agrawal, 2001). Successful
guidance of ultrashort laser pulses would however
enable novel endoscopic imaging and therapeutic
In this contribution, laser pulse shaping will be
combined with optical fibers and biologically
relevant multi photon spectroscopy. Tailored laser
pulses are guided through hollow core photonic
crystal fibers, and predetermined parametrically
shaped laser pulses in phase, amplitude and
polarization are achieved after the fiber by
Lindinger A..
Selective Two-photon Excitation by Parametrically Shaped Laser Pulses after an Optical Fiber.
DOI: 10.5220/0004337400190024
In Proceedings of the International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2013), pages 19-24
ISBN: 978-989-8565-44-0
2013 SCITEPRESS (Science and Technology Publications, Lda.)
considering the linear, nonlinear, and polarization
fiber properties. This enables to steer photoinduced
processes by utilizing these pulses for multi-photon
excitation in molecular systems. The application of
phase-tailored pulses for imaging contrast
enhancement is finally demonstrated for dye
molecules in a liquid after transmitting a hollow core
photonic crystal fiber.
The experimental setup consists of a femtosecond
laser system (Mira oscillator, RegA amplifier,
Coherent Inc.) which delivers 60 fs pulses at 805 nm
central wavelength with a repetition rate of 76 MHz
for the oscillator and 286 kHz for the amplifier.
Pulse shaping is performed by a spatial light
modulator (SLM640, CRi) placed in the Fourier
plane of a 4f-setup for the dye experiments and
combined with another modulator (SLM256, CRi)
and a polarizer in between for the polarization
modulated pulses. The center pixels of the pulse
modulator are aligned to the central wavelength of
805 nm. A wave plate is placed in the beam after the
pulse shaper in order to align the light polarization
parallel to the optical axes of the fiber. The pulses
are subsequently coupled into a hollow core
photonic crystal fiber (HC-800-1, NKT Photonics).
The core of this fiber measures 9.2 μm for the short
axis and 9.5 μm for the long axis. The transmission
window is centered at 830 nm and about 70 nm
wide. The zero dispersion wavelength is located at
805 nm and the third order Taylor term of the
dispersion function dominates for these pulses. Laser
pulses utilized in this contribution exhibit negligible
non-linear effects in the fiber. A telescope is used to
modify the beam diameter for adequate coupling
into the fiber. After the fiber, the laser beam is
widened by a telescope for the dye experiment and
focused into a cuvette containing rhodamine B
molar) and coumarin 1 (7.5·10
solved in ethanol.
Since the exact spectral phase of the laser pulses
at the cuvette is crucial for the measurements, its
distortion by the optical elements is compensated by
writing the corresponding phase retardances on the
pulse modulator. The fluorescence light is collected
by two lenses and then detected by a fiber
spectrometer. The fluorescence of the two dyes is
spectrally well separated, which enables to record
the excitation efficiency by integrating from 410 nm
to 550 nm for coumarin 1 and 560 nm to 700 nm for
rhodamine B.
The shaped pulses are detected after the fiber by
using a time resolved ellipsometry scheme. It is
based on a sum frequency generation cross
correlation setup using a BBO crystal (Plewicki et
al., 2006). The shaped pulse is convoluted with a
short reference pulse received directly from the
laser. The BBO crystal is polarization sensitive and
selects only the intensity in one polarization
direction. A set of cross correlation traces is
recorded by rotating the polarization of the shaped
pulse using a half-wave plate. For each time step the
instantaneous ellipse of the electrical field can be
determined. This data is presented in a single graph
in which the time-dependent intensity and
polarization state including ellipticity and orientation
are displayed. The ellipticity is defined as the ratio
of the major to minor axis. A three-dimensional
representation of the pulse is calculated based on the
measured data.
Ultrashort laser pulses guided through optical fibers
are affected by the intrinsic optical fiber properties
which have influence on the spectral distribution and
the temporal evolution of the transmitted pulses.
Particularly the dispersion, birefringence and
nonlinearity of the fiber have to be considered.
These properties can be determined by utilizing the
pulse shaper. Generally, spatial light modulator
phase values are searched for in order to pre-
compensate the laser pulse such that one receives a
short pulse after the fiber.
Novel hollow core photonic crystal fibers are
utilized here since they are advantageous for guiding
ultrashort pulses due to the light propagation in the
hollow core and hence their minor dispersion and
nonlinearity (Russel, 2003). The asymmetric shape
of the hollow fiber core leads to large birefringence.
This causes a temporal separation of the components
after propagation through the fiber with each
component chirped differently. Therefore, the fiber
can be described by two perpendicular optical axes,
which have different dispersions. These axes are
denoted as fast (f) and slow (s) axes due to the
different group velocities. If light is linearly
polarized along one of these axes, the state of
polarization is not altered by the fiber. The overall
orientation of the polarization shaped pulse is
determined by the orientation of the optical axis of
the fiber. Both sub pulses are still linearly polarized
and oriented along the respective optical axes after
propagation through the fiber. The temporal
intensity profiles are asymmetric and broadened due
to chromatic dispersion of the fiber. This broadening
and asymmetry, which is characteristic for second
and third order phase functions, can be compensated
for by applying a phase function of the opposite sign
on each pulse in order to produce short sub pulses
after the fiber. These values corresponding to the
respective axes are written as an offset on the
modulator. The delay between the differently
polarized pulse components is attributed to the
difference in the group velocity and can as well be
pre-compensated with the pulse shaper. Finally, the
relative phase between the two pulses is adjusted to
generate a single linearly polarized output pulse
oriented at 45°. Having this compensation
determined, the pulse which is transmitted through
the fiber can be arbitrarily controlled in phase,
amplitude, and polarization.
Figure 1: Block diagram of the parametric pulse shaping
method through optical fibers.
For the birefringent hollow core fiber the
orientation of the output pulse can be changed by
altering the relative amplitudes of the two
polarization components. The shift of the relative
phase between the components results in a change of
the ellipticity (Weise et al., 2010). This procedure
can be extended to pulse sequences consisting of a
variable number of sub-pulses. Any individual sub-
pulse is generated by two perpendicularly polarized
pulses whose energy ratio, relative phase, and delay
are set. Parametric sub-pulse sequences for each
polarization component are produced by following
the method described in ref. (Weber et al., 2005).
Minor side pulses still occur for the employed pulse
shaper setup, yet they can be removed by a more
advanced pulse shaper design (Weise and Lindinger,
The determined fiber precompensation phase
functions and the fiber phase difference are included
in the calculation of the phase retardances of the
liquid crystal arrays by adding the corresponding
phase values. The required electric fields at the
proximal end of the fiber can be generated with the
appropriate phase retardances of the liquid crystal
arrays. Hence, combining the desired sub-pulse
sequence phase values with the pre-compensation
phase parameters controls the shape of the laser
pulse after the fiber (see Figure 1 for a
comprehensive method description).
Parametric control of sub-pulses in a pulse
sequence delivered by the Mira oscillator is
illustrated for a series of double pulses, where one
parameter of the pulse sequence is independently
controlled (Figure 2). The double pulses are
constructed from three sub pulse components. The
first sub-pulse in these pulse sequences is linearly
polarized parallel to the slow axis of the fiber. The
two other sub pulse components yield the second
sub-pulse. The experimental application of a linear
chirp on the second sub-pulse and the associated
increase in pulse duration is presented in Figure 2(a).
The rotation of a sub-pulse is demonstrated in Figure
2(b). Both sub-pulses are linearly polarized while
the orientation of the second sub-pulse is changed
from 0 to 150°. The control of the ellipticity from
linear to circular is visualized in Figure 2(c) in
which the second sub-pulse is oriented at 45°
relative to the slow axis of the fiber. These data
prove the good agreement of the set parameters with
the measured parameters of the shaped pulses.
This procedure is not limited to double pulses
and can be extended to a larger number of sub-
pulses and higher complexity, which is exemplified
in Figure 3 where a triple pulse sequence is depicted
with differently chirped and polarized sub-pulses.
The example shows the potential of laser pulse
shaping after optical fibers which can be employed
for various optical applications. Even nonlinear fiber
properties leading dominantly to self-phase
modulation can be controlled by pre-calculating the
input laser pulse shape with back-propagating the
nonlinear Schrödinger equation (Tsang et al., 2003);
(Pawłowska et al., 2012). In the following, pulse
shaping via fibers applied to two-photon dyes will
be demonstrated for one polarization direction in the
linear regime, but it can in principle be extended to
differing polarization directions and to the nonlinear
Figure 2: Measured parametric pulse shapes after the
hollow core fiber where the linear chirp (a), the orientation
(b), and the ellipticity (c) of the second sub-pulse are
varied. An exemplary 3-dimensional representation of an
experimentally recorded double pulse is depicted in (d)
where s and f denote the slow and fast axes, respectively.
Figure 3: Shape of a complex laser pulse after the fiber.
The left side displays a 3-dimensional representation of
the measured laser pulse and the right side shows the time-
dependent intensity, ellipticity, and orientation.
In two-photon processes the photon interference can
be utilized by tailored laser pulses to achieve a
spectrally narrow two-photon excitation (Meshulach
and Silberberg, 1998). This enables selective
excitation of different dyes with partially
overlapping excitation spectra, which can be applied
to two-photon fluorescence microscopy (Ogilvie et
al., 2006). The effective two-photon field is due to
interference of different components within the laser
spectrum E(
) and has the form
i ))()((
with the phase function (
) and the center
. If the phase function is antisymmetric
, the exponent vanishes and leaves for
) the same result as a transform limited pulse
with a flat spectral phase (Meshulach and
Silberberg, 1998).
In the present contribution sinus phase and third
order phase are employed as antisymmetric phase
functions. They exhibit constructive interference at
the center frequency and partially destructive
interference at other frequencies (see Figure 4). With
the two photon spectrum and the two-photon cross
sections of the involved dyes it is possible to
simulate the excitation frequency dependent relative
fluorescence intensity I of each dye. This permits to
calculate the frequency dependent contrast (I
) between the examined dyes
rhodamine B (rhB) and coumarin 1 (cou1).
For the experiment it is very important to
precisely control the spectral phase after the fiber by
adjusting the pulse shaper settings and carefully
detecting the pulse shape since the measurements are
phase sensitive. The measurements were performed
Figure 4: Simulated two-photon field for two
antisymmetric phase functions around 815 nm. (a) shows a
third order phase function and (b) a sinus phase function.
Both two-photon spectra exhibit a sharp peak at the point
of antisymmetry at 815 nm which allows selective
with amplified laser pulses since the non-linear
excitation requires substantial pulse energy. The
obtained well separated fluorescence spectra of the
two dyes enable to record the specific excitation
efficiencies by integrating the spectral intensity from
410 nm to 550 nm for coumarin 1 and from 560 nm
to 700 nm for rhodamine B.
In Figure 5 frequency scans of the antisymmetric
center of the phase functions were performed after
the fiber for the sinus and third order phases. The
obtained contrast is plotted and shows a good
agreement with the simulation results. Higher
integrated rhodamine B fluorescence compared to
coumarin 1 leads to a contrast larger than zero. For
comparison, the contrast obtained for a transform-
limited pulse is depicted as a horizontal grey line.
The maximal contrast difference is about 0.4 in both
cases which allows a clear separation of the dyes.
The maximal contrast can slightly be increased by
adding higher Taylor terms to the phase functions
since this improves the condition of having
Figure 5: Frequency scans of the point of phase
antisymmetry recording the contrast of the two-photon
excited fluorescence of rhodamine B and coumarin 1. The
intensity is displayed in percentage of a short pulse. A
third order phase of 2·10
is used in (a) and a sinus
function with an amplitude of 25 rad and a wavenumber
of 0.0736 nm
is applied in (b). The experimental data are
in good agreement with the simulations.
constructive interference on one side of the spectrum
and destructive interference on the other side. These
phase scans on these typical dye markers
demonstrate the perspective of pulse shaping after
optical fibers for spectroscopic multi-photon
Applications of shaped laser pulses via optical fibers
were presented for two-photon processes in dye
molecules. The explained phase, amplitude, and
polarization forming method employs pre-
compensation of the optical fiber properties by
analytical pulse shaping to obtain tailored parametric
pulse forms at the distal fiber end. In a parametric
sub pulse encoding, the physically intuitive sub
pulse parameters including the polarization state
were individually controlled.
Moreover, anti-symmetrically phase-shaped
pulses after the fiber were applied for two-photon
fluorescence of dyes in liquids to enhance the
contrast between different dyes with similar
excitation spectra. The received experimental results
were found to be in good agreement with the
conducted theoretical simulations which
demonstrates the precision and reliability of the
presented method. This novel technique will have
perspectives in endoscopic imaging applications by
yielding an increased fluorescence contrast.
Agrawal, G. P. (2001) Nonlinear Fiber Optics, San Diego:
Brixner, T., Gerber, G., (2003) ChemPhysChem, 4, 418-
Brixner, T., Gerber, G., (2001) Opt. Lett, 26, 557-559.
Herek, J. L., Wohlleben, W., Cogdell, R. J., Zeidler, D.,
Motzkus, M., (2002) Nature, 417, 533-537.
Judson, R. S., Rabitz, H. (1992) Phys. Rev. Lett. 68, 1500-
Lozovoy, V. V., Pastirk, I, Walowicz, K. A., Dantus, M.
(2002) J. Chem. Phys., 118, 3187-3196.
König, K., (2000) J. Microsc., 200, 83-87.
Meshulach, D., Silberberg, Y. (1998) Nature, 396, 239-
Myaing, M. T., MacDonald, D. J., Li, X., (2006) Opt.
Lett., 31, 1076-1080.
Ogilvie, J. P., Debarre, D., Solinas, X., Martin, J.,
Beaurepaire, E., Joffre, M. (2006) Opt. Express, 14,
Pawłowska, M., Patas, A., Achazi, G., Lindinger, A.
(2012) 37, 2709-2711.
Plewicki, M., Weise, F., Weber, S. M., Lindinger, A.,
(2006) Appl. Opt. 45, 8354-8359.
Polachek, L., Oron, D., Silberberg, Y., (2006), Opt. Lett.
31, 631-633.
Russel, P (2003) Science, 299, 358-362.
Schäfer-Bung, B., Mitrić, R., Bonačić-Koutecký, V.,
Bartelt, A., Lupulescu, C., Lindinger, A., Vajda, Š,
Weber, S. M., Wöste, L. (2004) J. Phys. Chem. A, 108,
Tsang, M., Psaltis, D., Omenetto, F. G. (2003) Opt. Lett.
28, 1873-1875.
Tsen, K. T., Tsen, S. W. D., Chang, C. L., Hung, C. F.,
Wu, T. C., Kiang, J. G. (2007) J. Phys. Condens.
Matter, 19, 322102-322108.
Weber, S. M., Lindinger, A., Vetter, F., Plewicki, M.,
Merli, A., Wöste, L. (2005) Eur. Phys. J D, 33, 39-42.
Weise, F., Achazi, G., Lindinger, A. (2010) Phys. Rev. A,
82, 053827.
Weise, F., Lindinger, A. (2010) Appl. Phys. B, 101, 79-91.