Determination of bandwidth and beamwidth of a Rhodamine 6G dye laser using different optical setups

Transcription

1 Practicum Spectroscopy HS 2 Determination of bandwidth and beamwidth of a Rhodamine 6G dye laser using different optical setups DLA Jorge Ferreiro, study degree Chemistry, 5 th semester, fjorge@student.ethz.ch Alex Lauber, study degree Chemistry, 5 th semester, allauber@student.ethz.ch 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 9.9 ±.6 nm was calculated and pump power dependence... The two-mirror resonator lead to a bandwidth of 8.54 ±.4 nm. For the Littrow and Littman the bandwidth was 4.5 ±.3 nm and 3.69 ±.4 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 =.55()x (7). Zürich, the J. Ferreiro A. Lauber

2 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 E and E 2. Equations ()-(3) of [] describe the transition rates between both states. The Einstein coefficients A ij and B ij describe the propability of a specific transition to occour. Increasing ρ ν results in a bigger ratio N 2 N, because spontanous emission remains 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 E and E 2 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.). A resonator is necessary to arrange a multiple passage of light back and forth Fig. 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 E 3 and E 4. The abreviations stand for SR solvent relaxation, A absorption, IC internal conversion, Fl fluorescence, IE stimulated emission, Ph Phosphorescence, S singlet and T triplet. [] 2

3 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 () 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 [] 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 P pump defines the excitation of the active medium and thus the lasing output. The output power is proportional to P pump. Increasing the transmittance of the mirrors slowely leads to higher output powers but at some point saturation is achieved. 2 Experimental [2] 2. General For initial excitation a nitrogen Everett Research Laboratory, C95 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 54 nm or shorter. Spectra were recorded with a spectrometer Trias Series 32, CCD Camera (24 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 FWHM was evaluated with a algortihm in R (see Appendix). 2.2 Fluorescence of Rhosdamin 6G The nitrogen laser pump (power = 5 kv, Gain=.) 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) FWHM = full width at half maximum 3

4 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 (P Pump : 3., 3.2, 3.5, 3.7, 4. kv). Fig. 2 Experimental setup for fluorescence of dyefig. 3 Experimental setup for two-mirror resonator: The beam is partially reflected by the 95% cell: The black lines indicate fully reflective mirrors. 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 th 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 [] 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

5 2.6 Calibration The spectra of 5 different target positions within (67 nm - 73nm) 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-thing is placed in such a manner that the th ordersetup as for Littrow with additional mirror above black lines indicate fully reflective mirrors. Same 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.). The figures can be found in the appendix (Fig. 6 - Fig. 4). Obviously the bandwidth achieves smaller values using more optical components Setup bandwidth \ nm beamwidth \ µm tuning range \ nm Fluorescence 9.9 ± Two-mirror resonator 8.54 ± Littrow scheme 4.5 ± ± ±.2 Littman scheme 3.69 ± ± ±.2 Commercial dye laser Tab. : 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 5

6 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 FWHM \ nm ± ± ±.5 Tab. 2: FHWM of laserbeam with different pump power values. As the pump power increases the bandwidth increases too. increasing pump power P pump the output power of the laser increases linearly until saturation is reached. The maximal value for P pump is estimated to be 3.8 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. 4). The calculated function for the linear regression is y =.55()x (7) (2) 4 References [] J. Michaud, Instruction Manual, ETH Zürich, Version 4. November 2 [2] R Development Core Team (2). R: A language and environment for statistical computing. Foundation for Statistical Computing, Vienna, Austria. ISBN , URL 6

7 5 Appendix 5. Figures and Tables 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 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

8 .5.5 Fig. 8 Averaged spectra for Littrow scheme of Rhodamine 6G laser: The main peak shows the maximal intensity in at 75 nm. The other peaks are the maximal achivable limits changing the position of the grating x mm 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 (. 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. 8

9 .5.5 Fig. Averaged spectra for Littman scheme of Rhodamine 6G laser: The main peak shows the maximal intensity in at 7 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 x mm Fig. 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 (. 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. 9

10 Fig. 2 Calibration curve to determine wavelength of measurements. The equation for the linear regression is y =.55()x (7) x Pixels.5 P Pump = 3.,3.2kV P Pump = 3.5kV P Pump = 3.7kV P Pump = 4.kV Fig. 3 Effect of pump power on laser process. With increasing pump power the output power raises until saturation (flat top) is achieved

11 .5 Two mirror resonator Littman scheme Littrow scheme.5 Fluorescence Fig. 4 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.