Download Determination of bandwidth and beamwidth of a

Practicum Spectroscopy
DLA
HS 2010
Determination of bandwidth and
beamwidth of a Rhodamine 6G dye laser
using different optical setups
Jorge Ferreiro, study degree Chemistry, 5th semester, [email protected]
Alex Lauber, study degree Chemistry, 5th semester, [email protected]
Assistant: Dr. Julie Michaud
Abstract:
Using different experimental setups the band- and beamwidths
of a Rhodamine 6G dye laser were investigated. For the fluorescence spectra a
bandwidth of 9.19 ± 0.06 nm was calculated and pump power dependence... The
two-mirror resonator lead to a bandwidth of 8.54 ± 0.04 nm. For the Littrow
and Littman the bandwidth was 4.05 ± 0.03 nm and 3.69 ± 0.04 nm respectively,
for the beamwidth 275 ± 6 µm and 284 ± 6 µm respectively. To convert all
spectra from pixel to wavelength a calibration was performed, resulting in a linear
dependence y = 0.0550(1)x + 670.60(7).
Zürich, the 16.12.2010
J. Ferreiro
A. Lauber
1 Introduction
The acronym LASER stands for Light Amplification by Stimulated Emission of
Radiation. In contrast to conventional light sources a laser is monochromatic and
coherent over a long distance. These characteristics are usefull for many applications in science where e.g. constant beamwidths are demanded. The basic building
blocks of a laser are: an initial excitacion, an active medium and a resonator. The
initial excitation can be performed by electric discharge (mostly with solid active
media), by a flash lamp or a pumping laser (mostly in gaseous and liquid active
media). In a simple model an active medium can consist of two energy states E1
and E2 . Equations (1)-(3) of [1] describe the transition rates between both states.
The Einstein coefficients Aij and Bij describe the propability of a specific transition
to occour.
N2
, because spontanous emission remains
Increasing ρν results in a bigger ratio N
1
constant while absorption and stimulated emission grow. Saturation is achieved
when the absoprtion is constant while spontanous and stimulated emission are
increased. To amplify an enduring stimulated emission a enduring perturbation
of the population distribution is requiered, which is only possible for a short time.
Therefore no enduring laser pulse is achievable in a two-level system. A threelevel system can be a reasonable solution to such a problem because the population accumaltes in a third level which lays between E1 and E2 and enduring
stimulated emission is possible since stimulated emission is slower than absorption. However in a three-level system the population inversion will be degraded at
some point which leads to a further modification: the four-level system (Fig.1).
A resonator is necessary to arrange a multiple passage of light back and forth
Fig. 1
The four-level system of Rhodamine 6G:
The ongoing stimulated emission doesn’t lead to an
accumulation of the poplation in the ground state
since the lasing process occurs between the two states
E3 and E4 . The abreviations stand for SR solvent
relaxation, A absorption, IC internal conversion, Fl
fluorescence, IE stimulated emission, Ph Phosphorescence, S singlet and T triplet. [1]
2
through the active medium such that with every passage the intensity is increased
by further stimulated emission. To achieve resonance, constructive interference is
requiered which is given by
2·L=k·λ
(1)
with k ∈ N, L being the length of the resonator and λ the wavelength. The effective
linewidth of the resonator is enlarged by effects like radiation loss or disperision in
the fluorescence beam of the dye cell. Therefore it’s important to use optical components like mirrors, lenses or gratings to focus the beam such that the effective
linewidth complies with the expected linewidth. To acheive a stable laser oscillation the gain has to be ajusted. According to [1] the laser has a minimal linewidth
which can’t be under-runned known as the threshold gain. Optical effects like
scattering losses are considered as absorption. The pumping power Ppump defines
the excitation of the active medium and thus the lasing output. The output power
is proportional to Ppump . Increasing the transmittance of the mirrors slowely leads
to higher output powers but at some point saturation is achieved.
2 Experimental [2]
2.1 General
For initial excitation a nitrogen Everett Research Laboratory, C950 Pulsed
Gas Laser was used. Rhodamine 6G in methanol (R-phrases: 22; S-phrases: 26,
36/37/39) was used as active medium with excitation wavelength of 514 nm or
shorter. Spectra were recorded with a spectrometer Trias Series 320, CCD
Camera (1024 pixels). Each measurement was performed three times and the
evaluation was done with the program R. All acquired spectra were first converted
from pixel to wavelength by the obtained calibration equation (section 3). For the
calculation of the bandwidth the FWHM1 was evaluated with a algortihm in R (see
Appendix).
2.2 Fluorescence of Rhosdamin 6G
The nitrogen laser pump (power = 15 kV, Gain=1.0) was focused by using a
spherical and a cylindrical lens on the dye cell to achieve a maximal fluorescence
intensity and the bandwidth was calculated. (Fig.2)
1
FWHM = f ull width at half maximum
3
2.3 Two-mirror resonator
To achieve a lasing acitvity of the dye cell a out-coupling mirror and a fully reflecitve mirror were placed in the setup (Fig. 3) and spectra were recorded. To
study the relation between pump power and output intensity, spectra at different
pumping powers were recorded (PPump : 13.0, 13.2, 13.5, 13.7, 14.0 kV).
Fig. 2 Experimental setup for fluorescence of dyeFig. 3
Experimental setup for two-mirror rescell: The black lines indicate fully reflective mirrors.onator: The beam is partially reflected by the 95%
reflective mirror and a stimulated emission is created.
2.4 Littrow scheme
The fully reflective mirror of the resonator was replaced by a grating (Fig. 3) which
was placed such that the 0th order reflection was tilted back towards the dye cell.
Turning the grating in both possible directions on each threshold the spectra were
recorded by so determining the tuning range. To measure the beamwidth the
knife-edge technique [1] was used. A razor blade was placed between the 95%
mirror and the first redirective mirror and then moved such that the beam was
fully interrupted at some point. After every step towards the beam, spectra were
recorded.
2.5 Littman scheme
In addition to the Littrow scheme an additional mirror is placed above the grating such that the beam is directed away and back to the grating (Fig.5). The
tuning range of the setup was determined and the respective FWMH aswell. The
beamwidth was again measured with the knifge-edge technique.
4
2.6 Calibration
The spectra of 15 different target positions within (670 nm - 730nm) were recorded
in order to obtain a calibration function to convert all spectra from pixel to wavelength in nm.
Fig. 4 Experimental setup Littrow scheme: TheFig. 5 Experimental setup for Littmann scheme:
black lines indicate fully reflective mirrors. The grat-The black lines indicate fully reflective mirrors. Same
ing is placed in such a manner that the 0th ordersetup as for Littrow with additional mirror above
reflection is redirected to the dye cell.
grating.
3 Results and discussion
All results for beam- and bandwidth for the different setups are listed in the table
below (Tab.1). The figures can be found in the appendix (Fig. 6 - Fig. 14).
Obviously the bandwidth achieves smaller values using more optical components
Setup
Fluorescence
Two-mirror resonator
Littrow scheme
Littman scheme
Commercial dye laser
bandwidth \ nm
9.19 ± 0.06
8.54 ± 0.04
4.05 ± 0.03
3.69 ± 0.04
0.015
beamwidth \ µm tuning range \ nm
275 ± 6
18.45 ± 0.02
284 ± 6
16.72 ± 0.02
-
Tab. 1: Results for beam- and bandwidth of Rhodamine 6G laser with different experimental
setups according to section 2: Experimental schemes with more optical components
achieve better values for the bandwidth. The beamwidth for the Littrow and Littman
scheme are nearly equal.
due to the better focus of the laser beam. Since fluorescence is very dispersive
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the focussing enhances clearly, when gratings are used, i.e. in the Littrow an
Littman scheme. The tuning range of the Littman scheme seems to be asymmetric,
but no possible reason could be pointed out. Another way of optimizing the
experiment more is using optical fibres which lower dipsersive losses of radiation.
To reach a similar bandwidth to commercial lasers much better gratings (with
more lines per cm) and lenses have to be used. As expected from theory with
Pump power \ kV
13.0
13.2
13.5
13.7
14.0
FWHM \ nm
0
0
8.2 ±0.4
8.27 ±0.09
9.5 ±0.5
Tab. 2: FHWM of laserbeam with different pump power values. As the pump power increases
the bandwidth increases too.
increasing pump power Ppump the output power of the laser increases linearly
until saturation is reached. The maximal value for Ppump is estimated to be ≈
13.8 kV. So in order to optimize the lasing process at all the pumping power
should as well be taken into account as the optical components to focus the beam.
The calibration showed a linear dependence of the wavelengths and the pixels (Fig.
14). The calculated function for the linear regression is
y = 0.0550(1)x + 670.60(7)
(2)
4 References
[1] J. Michaud, Instruction Manual, ETH Zürich, Version 4. November 2010
[2] R Development Core Team (2010). R: A language and environment for statistical computing. Foundation for Statistical Computing, Vienna, Austria. ISBN
3-900051-07-0, URL http://www.R-project.org.
6
5 Appendix
5.1 Figures and Tables
λ nm
695
λ nm
700
705
690
695
700
705
0.1
0.3
0.2
I AU
0.2
0.1
0
I AU
690
0
Fig. 6
Big Averaged fluorescence
line: The lineshape isn’t perfectly a
Lorentzian line and the fluctuations are
visible. Small Three fluorescence lines
to determine FWHM: The positions
where the bandwidth have been calculated are marked with a cross.
λ nm
690
695
700
705
0.2
0.1
I AU
I AU
0.2
0
0.1
0
670
675
680
685
690
695
700
705
710
715
720
725
λ nm
705
λ nm
710
715
1.5
1.5
695
700
705
710
715
1.5
1.2
1.2
0.9
0.9
I AU
700
0.6
0.6
0.3
0.3
0
0
I AU
λ nm
695
λ nm
1
695
700
705
710
715
1.5
0.9
0.6
I AU
I AU
1.2
0.3
0
0.5
0
670
675
680
685
690
695
700
705
710
715
720
725
λ nm
7
Fig. 7
Big Averaged fluorescence
line for two mirror resonator: The
line approaches more a Lorentzian lineshape. Small Three laserlines to determine FWHM: The positions where
the bandwidth have been calculated are
marked with a cross.
1.5
Fig. 8 Averaged spectra for Littrow
scheme of Rhodamine 6G laser: The
main peak shows the maximal intensity
in at ≈ 705 nm. The other peaks are
the maximal achivable limits changing
the position of the grating.
I AU
1
0.5
0
690
695
700
705
710
715
720
λ nm
1.5
Fig. 9 Representation of the maximum intensity of recorded spectra at
different positions of the razor blade.
In the beginning the blade is not interacting with the beam (1. set of white
points, dotted line) and therefore constant intensities are observed. At a certain point the blade starts cutting the
beam (red points, plain line), dimnishing its intensity. In the end the blade
obstructs totally the beam (2. set of
white points, dotted line) and therefore no intensity is measured. In order to determine the beamwidth, indipendent linear regressions were carried out for every set of mentioned
points. The intersection points of the
lines (dashed lines) represent therefore
the edges of the beam i.e. their difference the beamwidth.
I AU
1
0.5
0
0
1
2
3
x mm
8
1.5
Fig.
10
Averaged spectra for
Littman scheme of Rhodamine 6G
laser: The main peak shows the maximal intensity in at ≈ 700 nm. The other
peaks are the maximal achivable limits changing the position of the grating.
Compared to the Littrow scheme the
maximum peak is shifted to a slightly
lower wavelength.
I AU
1
0.5
0
690
695
700
705
710
715
720
λ nm
1.5
Fig. 11 Representation of the maximum intensity of recorded spectra at
different positions of the razor blade.
In the beginning the blade is not interacting with the beam (1. set of white
points, dotted line) and therefore constant intensities are observed. At a certain point the blade starts cutting the
beam (red points, plain line), dimnishing its intensity. In the end the blade
obstructs totally the beam (2. set of
white points, dotted line) and therefore no intensity is measured. In order to determine the beamwidth, indipendent linear regressions were carried out for every set of mentioned
points. The intersection points of the
lines (dashed lines) represent therefore
the edges of the beam i.e. their difference the beamwidth.
I AU
1
0.5
0
0
1
2
3
4
x mm
9
725
720
715
Fig. 12
Calibration curve to determine wavelength of measurements.
The equation for the linear regression
is y = 0.0550(1)x + 670.60(7).
710
λ nm
705
700
695
690
685
680
675
100
200
300
400
500
600
700
800
900
1000
x Pixels
1.5
Fig. 13 Effect of pump power on
laser process. With increasing pump
power the output power raises until saturation (flat top) is achieved.
PPump = 13.0,13.2kV
PPump = 13.5kV
PPump = 13.7kV
PPump = 14.0kV
I AU
1
0.5
0
670
675
680
685
690
695
700
705
710
715
720
725
λ nm
10
1.5
Two mirror resonator
Littman scheme
Littrow scheme
I AU
1
0.5
Fluorescence
0
690
695
700
705
710
715
λ nm
Fig. 14 Spectra for all experimental setups. Setups with more focussing elements result in a
thiner linewidth. Fluorescence without focussing elements shows the biggest amount of dispersion.
11