Development of a tunable ultrashort dye laser system by using molecular photonic processes and a nanosecond pumping laser

ndard 1 cm × 1 cm spectroscopic cell (DL-
type Hellma), filled with solution of Rhodamine 6G in ethanol (4.10-4 M/l) and is transversally 
pumped at 10 times above threshold with 6 ns pump pulses. The dye cell constitutes a short and 
low-Q cavity with two un-coated side faces of 0.04 reflectivity. Figure 2 shows the spectral 
evolution in the broadband laser emission from such a dye laser by solving the rate equations 
extended to wavelengths [9 - 11] and using the dye molecular parameters [12, 13]: 
1
1
0
1
1 1 NINII
t
N n
i
iei
n
i
iaipap ⎟⎠
⎞⎜⎝
⎛ +−⎟⎠
⎞⎜⎝
⎛ += ∑∑
==
στσσ∂
∂
( )[ ] 1012 NAT
I
lNN
t
I
i
i
iaiei
i +−−= ασσ∂
∂ 
It should be noted that on the short-wavelength wing of the broadband laser spectrum, the laser 
oscillation lasts inherently for a short duration. This single short pulse can be obtained by a 
simple spectral se lec t ion . In practice, therefore, the STS method is to select, with an extra 
cavity filter, a spectral narrow band on the short-wavelength wing of the broadband laser 
emission from a short and low-Q dye laser oscillator. Its STS laser output is single short pulses, 
for current dyes, of less than 100 ps [9 - 11]. 
In our laser, the STS dye laser (Part 1- Fig. 1) is pumped by 6 ns pulses at 532 nm from the 
Nd:YAG laser. The broadband laser emission from O1 dye cell (Rh6G /ethanol solution, 4.10-4 
M/l) is collimated and directed to a small grating G (600 grooves/mm, 1 cm × 2 cm), the second 
order diffracted beam is imaged and a spectral narrow band is selected with slit S of 0.3 mm 
wide. By that way, single short laser pulses of 75 ps and an energy of 15 nJ are produced. 
Le H. Hai, et al Development of a tunable ultrashort dye laser system by using molecular... 
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Sequently, the pulse is amplified in a 6-pass amplifier to 30 µJ. The amplifier used a bow-tie 
configuration with 1-mm thick dye cell (Rh6G/ethanol solution, 3.10-4 M/l, circulated). Such a 
STS dye laser generated single laser pulses of 75 ps (Fig. 3, assuming sech2 pulse shape), an 
pulse energy of 30 µJ at 562 nm and a spectral linewidth of 0.5 nm (FWHM). 
Fig. 3: Autocorrelation trace of output pulses from the Part 1 
We have successfully operated the picosecond STS laser with different dyes (in liquid and solid 
state matrices) pumped at 355 nm or 532 nm by the third and second harmonic generation of the 
Nd:YAG laser in order to produce picosecond dye laser pulses (< 100 ps) spectrally adjustable 
from 386 nm to 860 nm [9 - 11]. In comparison with the mode-locked lasers that are used as 
optical pumping sources, these STS picosecond dye lasers have advantages such as compactness 
(25 cm × 35 cm), ease of operation, economy, possible use of many pump wavelengths and 
tunability of the output wavelength. 
2.2 Part 2: Generation of subpicosecond pulses 
Part 2 is to convert the 75 ps pulses generated from the STS dye laser into sub-picosecond laser 
pulses (Fig. 1). For this purpose, we used an ultralow-Q micro-cavity dye laser (MC) to produce 
a chain of ultrashort pulses (spikes) and the highly- saturated dye amplifier and absorber for 
extra-microcavity pulse treatment. The ultralow-Q micro-cavity dye laser (MC) is made of two 
wedged silica plates (preferably wedged to avoid parasitic reflection) pressed together with a 
very thin layer of solution of Rhodamine 640/methanol (3.10-2 M/l). Similar small-path cells are 
commercially available (e.g., Hellma). MC is pumped longitudinally by the at 562 nm, 75 ps 
pulse that emitted from Part 1 through a 10 cm focal length lens. 
Applying the same type of calculations of the rate equation system extended to wavelengths, we 
have computed the spectral evolution of the laser emissions from the 50 µm low-Q dye laser 
cavity, as shown in Fig. 4, which is a chain of sub-picosecond pulses (spikes). Such a pulse 
structure well presents the spiking phenomenon in the laser relaxation oscillations of an 
ultralow-Q ultrashort laser cavity and, in particular, that well agrees with experimental one 
measured by a streak camera (Fig. 5). The MC laser emission spectrum could be changed by 
translation or rotation the MC in respect to the pumping beam axis (Fig. 4b). 
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 a) b) 
Fig. 4: Laser emission from a 50 µm microcavity filled with Rh640/methanol (C = 3.10-2 M/l) 
pumped by Gaussian shape pulses of 100 ps at 6 times above laser threshold. a) MC is 
perpendicular to the pumping beam; b) MC is tilted 40 
Fig. 5: Typical emission from a 25 µm cavity 
In order to obtain single ultrashort laser pulses by extra-micro cavity pulse treatment, the micro 
cavity dye laser (MC) is followed by a multi-pass saturable dye amplifier (MSA), its 
configuration is similar to MPA in Part 1 and the Rh640/ethanol solution (3.10-4 M/l) is used as 
a gain medium. Because of highly-saturated gain, this amplifier functions mainly as a pulse 
shaper, the chain of spikes is strongly distorted pass by pass and the first spike becomes 
dominant in the amplified pulse chain. This pulse-shaping process is demonstrated by solving 
the following rate equation for gain G at each pass [14 - 16] and the results obtained are 
presented in Fig. 6. This pulse-shaping process is still effective until the sixth pass, that is why a 
six - pass saturable amplifier (MSA) configuration was used in our laser. 
( ) ( ) ( )[ ] [ ]NLGGGtIGettIGv
dt
dG
ass
NL
pases στσσ
γκ +−−−−∆++= − ln11)(1)( 00 
Le H. Hai, et al Development of a tunable ultrashort dye laser system by using molecular... 
 280
Fig. 6: a) The input signal; b) – h) Evolution of gain (dotted curves) and amplified signal 
intensity (solid curves) after each pass. (the left vertical axis: Signal intensity 
[photons.cm-2.s-1]; The right vertical axis: Gain) 
The distorted output from MSA is focused into a saturable absorber (SA). Due to absorption 
saturation, the first spike is isolated from the chain of spikes and well passed SA. This provides 
us single sub-picosecond pulses. Fig. 7 presents the computation results for this pulse isolation 
process using a 1 mm dye cell filled with Malachite Green (MG) in methanol as a saturable 
absorber [17]. As analyzed in [11], suitable selection of concentration, pumping energy and 
input signal intensity for MSA and SA is a critical task due to their significant influences on 
pulse distortion. 
In our laser system, the 25 µm MC is used and filled with Rh640/methanol solution (2.10-2 M/l) 
and pumped by 75 ps pulses from Part 1. The MC laser output is collimated and sent to MSA 
(Rh640/ethanol solution, 3.10-4 M/l). MSA is pumped semi-longitudinally by pulses of 6 ns 
(FWHM) and 3 mJ at 532 nm from the Nd:YAG laser. The synchronization between the signal 
and the pump pulses is performed by an optical delay line. The MSA output is of ~30 µJ with an 
amplified spontaneous emission energy of ~ 50%. Adequate focusing of the MSA output pulse 
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into the Malachite Green saturable absorber cell (5.10-4 M/l) wipes out the spike tail and a single 
ultrashort spike - the first strongest one of ~1 µJ is obtained. 
Fig. 7: Action of saturable absorber in extracting single sub-ps pulses. a) output; b) input; c) 
optical density of absorber 
2.3 Part 3: Power amplifier 
The single ultrashort laser pulse at 606 nm from Part 2 is amplified with three double-pass 1 cm 
amplifiers containing a circulated Rh640/ethanol solution (5.10-5 M/l). The second and the third 
amplifiers are separated by a 1 mm Malachite Green saturable absorber cell in order to eliminate 
ASE. The pumping pulse energies for the amplifiers are of 8, 10 and 10 mJ, respectively. Such a 
power amplification chain provides an amplifying factor of more than 100 that led to the output 
pulse energy of ~125 µJ. 
The pulse duration is measured with the autocorrelator which used a non-collinear second 
harmonic generation with a BBO nonlinear crystal. The autocorrelation traces (Fig. 8) are 
corresponded to two positions of the Malachite Green saturable absorber (SA). In both cases, 
output pulse duration is about 500 fs assuming the sech2 pulse shape. However, the obtained 
autocorrelation traces present the different actions of extra-cavity pulse treatment. For example, 
as the saturable absorber is at the tightly focused position of the input beam, the output could be 
multiple pulses corresponding to the enlargement of the autocorrelation trace on its foot (Fig. 
8a). The trace in Fig. 8b corresponds to the shift of the saturable absorber 0.3 mm away from the 
previous position. At this position, the MG absorber is only saturated with the first and strongest 
spike, therefore, the output is the single pulses. 
In brief, at the output of Part 3 we have obtained the single dye laser pulses of ~500 fs in 
duration, 125 µJ in energy at 606 nm. 
Le H. Hai, et al Development of a tunable ultrashort dye laser system by using molecular... 
 282
 a) b) 
Fig. 8: Autocorrelation traces of output laser pulses from Part 3 
3. SUPERCONTINUUM GENERATION AND WIDELY TUNABLE 500 FS PULSE 
A white light continuum is generated by focusing the ~100 µJ - 500 fs pulse into 2 cm water cell 
with a 10 cm focal length lens. As a result of self-phase modulation, the supercontinuum 
spectrum is obtained, as presented in Fig. 9, and strongly broadened to cover over the UV, 
visible and near infrared spectral regions. The energy of the white light supercontinuum varies 
by 4 orders of magnitude from the central wavelength to both continuum wings (345 and 860 
nm). 
Significantly, the white light continuum generation enables one to produce widely tunable 
ultrashort pulses. By selecting a narrow band in the supercontinuum with an interferential filters 
of 1 nm FWHM bandwidth, we have produced nearly transform-limited subpicosecond pulses at 
a desired wavelength. Furthermore, widely and continuous tunability of ultrashort laser pulse 
from 540 nm to 720 nm is obtained by using suitable spectral filters and several dyes in dye 
amplifiers pumped at 532 nm. With the 355 nm pumping by the third harmonic of the same 
Nd:YAG laser, the system generates tunable sub-picosecond pulses between 540 nm to 450 nm 
with Coumarin dyes. Detail presentations of spectral selection and amplification of a desire 
spectral band from the supercontinuum spectrum were showed [18, 19]. 
Fig. 9: Supercontinuum Spectrum of 500 fs laser pulse after passing through a water cell of 2 
cm long 
4. CONCLUSION 
In this paper we have presented a simple, low-cost and relatively compact (80 cm wide × 100
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cm long) sub-picosecond dye laser system using a standard nanosecond pumping laser and 
photonic processes of organic dye molecules. The ultrashort dye laser system generates 
simultaneous 100 ps pulses (Part 1), single powerful 500 fs pulses at 606 nm (Part 3) and 
tunable laser 500 fs pulse resulted from the white light supercontinuum generation. Such an 
ultrafast laser does not need any expensive and specific optical components and therefore, it is 
reproducible at the laboratories of the developing countries and directly applied to time – 
resolved laser spectroscopic measurements and applications. 
ACKNOWLEDGEMENT 
This work was financially supported by the Laser Project of Vietnamese Academy of Science 
and Technology (VAST) and the Fundamental Research Program (Natural Sciences - Physics). 
The authors wish to acknowledgement the supports from Laboratoire de Photophysique 
Moleculaire (CNRS France) and the Research Grant Agreement (02-548 Rg/Phys/As) of The 
Third World Academy of Science (TWAS). 
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