Fluidic fibre dye lasers.

Abstract

We report the demonstration of compact fluidic fibre lasers based on capillary tubes and photonic crystal fibres, featuring single channel and multiple laterally integrated fluidic lasers respectively. Their preparation was based on capillary action and lasing occurred without the need for external mirrors or lithographically defined microstructures. The fibre lasers were found to be tunable by varying the chromophore density in the liquid core and a functional wavelength selectivity mechanism inherent in both types of lasers provided a long free spectral range that does not correspond to the length of the fibres. The enhanced mode spacing is attributed to a Vernier resonant effect.

Full text

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Fluidic fibre dye lasers
A. E. Vasdekis, G. E. Town,1 G. A. Turnbull, I. D. W. Samuel
Organic Semiconductor Centre & Ultrafast Photonics Collaboration, SUPA, School of Physics and Astronomy,
University of St Andrews, St Andrews, Fife, KY16 9SS, UK
idws@st-andrews.ac.uk
http://www.st-andrews.ac.uk/~osc

1Department of Electronics and Centre for Lasers & Applications,
Macquarie University, NSW 2109, Australia.
http://www.elec.mq.edu.au/gwopr

Abstract: We report the demonstration of compact fluidic fibre lasers based
on capillary tubes and photonic crystal fibres, featuring single channel and
multiple laterally integrated fluidic lasers respectively. Their preparation
was based on capillary action and lasing occurred without the need for
external mirrors or lithographically defined microstructures. The fibre lasers
were found to be tunable by varying the chromophore density in the liquid
core and a functional wavelength selectivity mechanism inherent in both
types of lasers provided a long free spectral range that does not correspond
to the length of the fibres. The enhanced mode spacing is attributed to a
Vernier resonant effect.
©2007 Optical Society of America
OCIS codes: (140.2050) Dye lasers; (140.3510) Lasers, fiber; (140.4780) Optical resonators;
(230.3990) Microstructure devices; (230.7370) Waveguides;



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1. Introduction
In recent years, opto-fluidic devices have emerged as a tool for sensing, imaging and
spectroscopy application as they uniquely combine the non-intrusive interaction capabilities
of photons and the fluidic delivery of biological samples [1]. Several types of devices have
been demonstrated including adaptive lenses, optical manipulators of particles and sensors
based on photonic crystal fibres and waveguides [2-5]. The practicality of most systems is
significantly enhanced by cost-effective fabrication techniques, such as soft lithography [6, 7].
Increasing interest has been focused additionally on fluidic organic lasers that can act as
efficient sources of narrow linewidth emission in the visible spectral range [8-11]. These
sources can be tuned over 45 nm for a single chromophore and their integration within fluidic
networks can be easily implemented achieving significant progress towards lab-on-a-chip
applications.
In this paper, we demonstrate fluidic lasers based on optical fibres. We explore two
geometries realised in single and multiple cores fibres based on low cost and commercially
available capillaries and photonic crystal fibres respectively [12]. The latter demonstrates the
potential of multiple laterally integrated fluidic lasers, approximately 50 in our experiments.
In both structures, lasing is achieved without the need of external mirrors or lithographically
defined microstructures, featuring thus a rapid fabrication technique of compact laser sources.
Long interaction lengths are achieved in both geometries, even for the small volumes of the
gain medium (~ nL), where in addition an advantageous spectral selectivity is observed
despite the long cavity lengths.
2. Methodology
The single core capillaries had a core and fused silica cladding diameters of 2 μm and 128 μm
respectively (Polymicro Technologies). The photonic crystal fibre was the ESM-12-01
(Crystal Fibre), which consists of a hexagonal array of capillaries with diameter of 3.68 μm at
a period of 8 μm, a defect silica core and a silica cladding of 12 μm and 125 μm in diameter
respectively. The inner side of the capillary walls was not chemically treated and the fibres’
coating was stripped both in the single and multiple cores (polyimide and acrylate coating
respectively). Both types of fibres were examined using an SEM and shown in Fig. 1.


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Received 22 January 2007; revised 9 March 2007; accepted 10 March 2007
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The gain medium was a toluene solution of the ionized organic dye Perylene Red (BASF)
at a concentration of 0.5 mg/ml (Fig. 1(c)). The solvent was chosen based on its higher
refractive index than fused silica and its appropriate wetting properties with the capillary
walls. The preparation of the fibre lasers involved the cleavage of the fibres at appropriate
lengths and their subsequent infilling by capillary action from dipping them in the dye
solution. The infilling process was relatively rapid and was measured to be 1 μm/sec on
average for a 25 mm long test capillary tube with a 5 μm core diameter. This technique is
promising for high throughput preparation; however it is characterised by the presence of on
average 0.5 mm long air-plugs on either side of the fibres, resulting in a mismatch of
termination of the fluidic regions with respect to the fibre facets [13]. To delay the solvent
evaporation and the photo-oxidation of the organic chromophore, the fibre ends were end-
capped by dipping the fibres in a viscous solution of the transparent polymer CYTOP®.
The optical pump source was a frequency-doubled diode pumped Nd: YVO4 laser with
repetition rate and pulse duration of 200 Hz and 7.5 nsec respectively. The laser beam was
coupled to an 8 μm core diameter silica fibre and butt-coupled to the fluidic lasers to
longitudinally excite them (Fig. 1(d)). It was found that the diameter of the pump fibre was
critical both in terms of threshold and quantum efficiency of the fluidic lasers. The 8 μm core
diameter pump fibre provided the optimal combination for both the single core capillary and
the holey fibre. For the aforementioned concentration, the excitation is absorbed in the first
800 μm of propagation.
3. Characterisation
The axial emission from the fibres was focused using a 25x microscope objective on to a fibre
coupled CCD spectrometer with a 40 pm resolution. Typical emission spectra above and
below threshold are depicted in Fig. 2 for a 41 mm long single core capillary (a) and a 39 mm
long photonic crystal fibre (b). As both types of lasers exceed threshold, the broadband
emission becomes characterised by a series of narrow linewidth peaks. The near-field images
above and below threshold for the multi-channel fluidic laser are shown in (c). The transverse
intensity variation above threshold designates the localization of the light within the cores,
indicative of stimulated emission.

Fig. 1. (a), (b). SEM images of the fibres; inset shows the lateral dimensions of the cores.
(c). the molecular structure of the organic dye Perylene Red. (d). A schematic of the set-
up for the longitudinal excitation of the fluidic fibres.
20 μ μm
20 μ μm
3.7
μ μm
2
μ μm
(a)
(b)
pump fibre fluidic fibre
Nd: YAG
emission
butt coupled
(d)
(c)
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Received 22 January 2007; revised 9 March 2007; accepted 10 March 2007
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The threshold energy and the slope efficiency were found to be different for the single and
multiple core fluidic lasers of similar lengths (Fig. 3(left)). A number of factors may
contribute to such an effect, including the efficiency of the coupling of the pump light into the
liquid core and also the fact that in the photonic crystal fibre the active area is larger and
multiple cores are excited simultaneously. In addition, the laser threshold and slope efficiency
were found to be dependent on the fibre length. This effect is shown in Fig. 3(right), where
the input-output curves are plotted for the photonic crystal fibre lasers of different length.
Such behaviour is indicative of the length mismatch between the amplification section and the
total propagation length. The emitted photons experience gain only in the initial section of the
fibre where the excitation is absorbed, while the rest of the liquid waveguide acts as a source
of loss due to re-absorption, reducing thus the output intensity.

The laser emission was also found to be tunable by varying the concentration of the dye in
the liquid gain medium. An increase in the chromophore density resulted in increased
absorption losses at the shorter wavelengths of the fluorescence spectrum due to the finite
Stokes’ shift in the electronic structure of the dye molecules. The narrowing of the
fluorescence spectra for increasing dye concentration is illustrated in Fig. 4(a). Such red-shift
of the blue-end of the edge-emitted fluorescence results in a convenient gain-tuning
mechanism since the gain peak red-shifts accordingly for increasing chromophore densities
[14]. The tuning of the centre wavelength of the laser emission spectrum is illustrated in Fig.
4(b). At different chromophore densities, lasing occurs at the vibronic of the highest net-gain,
Fig. 3. Comparative study of the input-output relationship for a single and multi-
channel fibre laser (left) and for a multi-channel laser of different lengths (right). In
the right figure the black arrow designates the threshold energies for the three lasers.
040080012001600
0
2
4
6
8


Eout (nJ)
Epump (nJ)
ESM-12-01, 39 mm
Cap. 2 μm, 41 mm
400800
Epump (nJ)
12001600
0
10
20
30
Eout (nJ)



ESM-12-01:

18 mm

25 mm

39 mm
Fig. 2. The emission spectra at threshold and 1.26 times above threshold for the 2
μm diameter capillary (a) and for the photonic crystal fibre (b). In (c), the near
field images are depicted below (i) and above (ii) threshold.
(a)
(b)
(i)
(ii)
(c)
610615620625630
0
2
4
intensity (a.u.)



wavelength (nm)
1.26 x threshold
1.0 x threshold
610615620625630
0
1
2
1.26 x threshold
1.0 x threshold
wavelength (nm)
intensity (a.u.)


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Received 22 January 2007; revised 9 March 2007; accepted 10 March 2007
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Page 5

while the laser performance does not degrade significantly as indicated by the slope efficiency
measurements for the same concentrations (Fig. 4(c)).

4. Spectral selectivity
Both in the single and in the multi-core fluidic lasers, the measured free spectral range is
approximately 300 times longer than expected for a Fabry-Perot resonator formed by the
liquid waveguide and the solvent-air interfaces. The laser emission appears to be strongly
modulated by an envelope function, allowing the appearance of only certain longitudinal
modes. This intracavity wavelength selectivity mechanism is unanticipated though
conveniently provides a functional mode spacing that does not correspond to the fibre length.
A wide range of processes have been reported to affect the mode spacing in waveguide
lasers. One is the excitation diffusion along the cavity axis, which in our experiments is very
limited and thus ineligible as an origin of this effect [15]. The presence of an additional
resonance constraint that imposes this broader spectral selectivity is more likely. Similar
spectral observations have been reported for lasers based on a dye solution in a cuvette and a
quantum-dot waveguide, and in both cases the lateral dimensions of the waveguide were
considered responsible for this wavelength selectivity [16,17]. In other work, longitudinal
mode grouping has been attributed to interference effects with substrate modes leading to a
wavelength dependent depletion of the net-gain, but also linked to the linewidth of the
homogenously broadened laser transition [18,19]. Saturable absorber based resonances have
been also considered as a linewidth-narrowing mechanism in fibre lasers [20].
In order to further investigate the effect in our system, we measured the dependence of the
laser mode spacing with the length of the liquid waveguide. The effective optical cavity
lengths were determined for both the single and multiple core lasers by Fourier transforming
the emission spectra above threshold and are plotted in Fig. 5 as a function of the physical
length of the fibres [21]. In contrast to the aforementioned reports, we found a linear
dependence between the mode separation and the fibre length, indicating a primarily
longitudinal origin of this spectral selectivity. The linearity is present in both the single
capillary and photonic crystal fibre lasers at approximately the same rate. The observed linear
dependence indicates that the spectral selectivity is due to a Vernier type of resonant effect
[22]. Such a resonant selection mechanism requires the simultaneous presence of two (or
more) cavities of different free spectral ranges. In such a ‘superstructure’, the spectral overlap
of the resonances for each cavity can lead to a combined response of longer modal spacing.

Fig. 4. For increasing chromophore densities, the blue-end of the fluorescence
spectra red-shifts (a) and the centre wavelength of the laser emission spectrum is
accordingly tuned (b). Under the same conditions, the laser efficiency does not vary
significantly (c).
550600650700750
0
4
8(a)
intensity normalised
increasing
concentration
wavelength (nm)
normalisation
level


0.06 mg/ml
0.45 mg/ml
0.87 mg/ml
2.00 mg/ml
4.20 mg/ml
01234
0
1
2
(b)
(c)

λlaser (nm)
concentration (mg/ml)
620
640
660


efficiency (%)
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Received 22 January 2007; revised 9 March 2007; accepted 10 March 2007
2 April 2007 / Vol. 15, No. 7 / OPTICS EXPRESS 3966