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We report on the first results of diode pumped laser operation of Pr3+:LaF3 in a quasi continuous wave (qcw) mode with average output powers of up to 80.0 mW (≈ 161.3 mW qcw) and a maximum slope efficiency of 37% at 719.8 nm. Furthermore it was possible to operate the laser at 537.1 nm and 635.4 nm and to tune the emission wavelength from 609 nm to 623 nm. The pump source was an InGaN laser diode with a maximum output power of 1 W at a central emission wavelength of 442 nm.
© 2012 Optical Society of America
1. Introduction
The trivalent praseodymium ion (Pr3+) is well known for its various fluorescence transitions in the visible spectral region, starting in the cyan, ranging through green, orange and red into the deep red part of the spectrum. It can furthermore exhibit cross sections in the order of 10−19 cm2 and is thus an excellent candidate for an active ion to produce visible laser radiation without the need of frequency doubling of near infrared laser light.
During recent years, much progress has been made in building highly efficient and compact lasers based on Pr3+-doped hosts, emitting in the visible spectral range [1, 2] or, via a single step of intra-cavity frequency doubling, in the UV spectral region [3]. An important factor was the availability of efficient, blue emitting InGaN laser diodes (LDs), which allowed to replace flash lamps [4], Ar-Ion- [5] and frequency doubled Nd-lasers [6, 7] as the pump source [8, 9]. However, the central emission wavelength of commercially available LDs can vary by about 10 nm between diodes and they exhibit an emission spectrum with a full width at half maximum (FWHM) of ≈ 2 nm. For the most commonly used Pr3+ host, LiYF4, with its narrow absorption line around 443.9 nm, this means, that a costly selection of diodes has to be made. It is therefore worthwhile to investigate other host lattices that offer broader absorption lines.
In this paper, we present to the best of our knowledge for the first time diode pumped laser operation as well as investigations of the ground state aborption, the stimulated emission, and the excited state absorption spectra of Pr3+:LaF3. Solomon and Mueller demonstrated pulsed stimulated emission at 598.5 nm for the first time in 1963 by employing a xenon flash lamp and cooling the gain medium to cryogenic temperatures [10]. In recent years, stimulated emission at room temperatur was demonstrated on several other wavelengths by employing dye lasers as pump sources [11].
2. Crystal growth
LaF3 is a simple fluoride with a triagonal structure which belongs to the space group P3̄c1. The unit cell contains six formula units where the La-ion occupies only a single site. The lattice constants are 7.19 Å and 7.37 Å for the a- and c-axis, respectively [12]. For the thermal conductivity varying values can be found. They range from κ||c = 2.1 W m−1 K−1[13] to κav = 5.1 W m−1 K−1[14] were it has to be noted, that the first value given is parallel to the c-axis while the second is an averaged value.
For the spectroscopic investigations and the laser experiments, two Pr3+:LaF3 crystals with doping concentrations of 0.35 at.% and 0.7 at.% in the melt were fabricated. Commercial high purity powders (99.999 %) were mixed and placed in a vitreous carbon crucible with a diameter of 35 mm and a height of 38 mm which was insulated by carbon foam. Early experiments showed a strong evaporation of LaF3, which made it impossible for us to employ the Czochralski method. In order to surpress this evaporation, a lid of carbon foam was placed on the crucible. The furnace was evacuated and filled with a 60 %/40 % Ar/CF4 mixture. The crucible was heated by inductive heating with a mid-frequency generator to the melting point of LaF3 of about 1493 °C. It was then cooled down to room-temperature over a period of 72 h.
The resulting boules contained pieces of up to 5 cm3 of single crystalline material with no visible inclusions. It has to be noted though, that at these high temperatures, the crucible degraded during the growth and microscopic carbon impurities may be present. The crystals were orientated, cut and polished for spectroscopic investigations and laser experiments. The doping concentration of a sample with nominally 0.7 at.%-doping was determined to be 0.4 at.% by microprobe analysis. This yields an ion concentration of 7.3 · 1019 cm−3. The Pr3+-concentration of other samples could then be calculated from the determined absorption cross sections.
3. Spectroscopy
3.1. Absorption
Polarized ground state absorption (GSA) measurements were performed with a Varian Cary 5000 spectrophotometer. The resolution was set to 0.45 nm and the stepwidth to 0.15 nm. The sample on which the microprobe analysis was performed was used. It had a thickness of 1.22 mm. It was c-cut to allow polarized measurements with the electric field vector of the probe light parallel to the crystallographic a- or c-axis.
The corresponding spectra are displayed in Fig. 1. The peaks in the region between 440 nm and 490 nm can be assigned to the transitions 3H4→3PJ,1I6, whereas the peaks between 580 nm and 605 nm originate from the transition 3H4→1D2. In the blue region, the well known fingerprint of the Pr3+-ion can be seen. Strong absorption peaks are present on both polarization axis with peak cross sections as high as 1.9 · 10−20 cm2 at 479 nm (σ-polarization). Furthermore, the absorption band around 442 nm (σ-polarization) has a FWHM of more than 6 nm. Comparing these values with those of the absorption band of Pr3+:LiYF4 which is used for diode pumping, one can see, that while the linewidths are more than three times bigger (Pr3+:LiYF4 ≈ 2 nm) the maximum peak cross section is in turn lower by a factor of ≈ 5.6 (Pr3+:LiYF4: σGSA@443.9 nm = 8.9 · 10−20 cm2). Additional values can be found in Tab. 1.
Table 1:. Values of peak absorption and emission cross sections in Pr3+:LaF3.
3.2. Emission
Polarized emission spectra of Pr3+:LaF3 were obtained by exciting a sample with a doping concentration of 0.22 at.% with an InGaN LD with a maximum output power of 1 W and a central emission wavelength of λL = 442 nm. The beam of the LD was furthermore chopped. A sample with low doping concentration was chosen in order to minimize reabsorption occuring around 485 nm, 600 nm, and 610 nm. The fluorescence of the sample was focused onto the slit of a SPEX 1000M 1 m monochromator and detected by a Si-diode. The signal of the diode and the reference signal of the chopper were fed into a Lock-In amplifier which lead to an excellent signal-to-noise ratio. A polarizer was placed in front of the slit to allow polarized measurements. The obtained spectra were corrected with respect to the sensitivity of the detector and for the optical set-up. The Füchtbauer-Ladenburg-method was used to calculate the emission cross sections [15]. The lifetime of the 3PJ-manifold was taken as 51 μs [16].
The obtained spectra can be seen in Fig. 2. It shows the emission bands of the Pr3+-ion with a maximum emission cross section of 6.6 · 10−20 cm2 at a wavelength of 719.8 nm (π) and with further peaks at 537.1 nm, 600.1 nm, 610.0 nm, and 635.4 nm. Additional information on these peaks can be found in Tab. 1.
For the laser experiments it should be taken into consideration that the reabsorption from the 3H4 ground state into the 1D2 level may affect a laser operating on 600.1 nm or 610.0 nm. The absorption cross sections at these wavelengths are 2.11 · 10−21 cm2 and 0.13 · 10−21 cm2, respectively. An interesting feature of the maximum at 610 nm is its comparatively large width of ≈ 5.6 nm, which may enable a wider tunability on this peak compared to other Pr3+-doped materials.
3.3. Excited state absorption
A phenomenon that often occurs in Pr-doped materials and which can have detrimental effects on the laser parameters is excited state absorption (ESA) from the 3PJ-multiplet into the 4f5d-states [17]. Due to their interconfigurational nature, 4f4f → 4f5d-transitions exhibit very broad absorption and emission bands with cross sections which are orders of magnitude larger than the intraconfigurational 4f4f → 4f4f -transitions.
In order to determine if there is ESA occuring on either the pump or any of the prospective laser wavelengths, ESA measurements were performed by employing the pump and probe technique as described by Koetke and Huber [18]. As pump source the InGaN LD was used. A 450 W Xenon lamp acted as the probe source. The resolution for this measurement was 0.8 nm, the step width was 0.25 nm.
Due to the method of measurement, the obtained spectrum, which is shown in Fig. 3, is a combination of the spectra of GSA bleaching, stimulated emission (SE), and ESA. The first two are denoted by a postive sign and ESA by a negative sign. In the area around 442 nm the detector suffered from oversaturation by scattered pump light and this region was subsequently omitted.
Fig. 3 Unpolarized room temperature ESA spectrum of Pr3+:LaF3. Positive signal corresonds to bleaching of the ground state and/or to stimulated emission, negative to excited state absorption. The value ni denotes the population of the i-th level and ne = ∑i ni
It can be seen that ESA is present in the UV range up to a wavelength of ≈ 340 nm. Above this wavelength, the spectrum consists only of peaks which can be attributed either to bleaching of the GSA and/or to SE. This leads to the conclusion that all observed emission wavelengths exhibit gain and thus should in principle support laser operation. Furthermore, also the absorption peaks in the blue spectral range which are suitable for pumping with InGaN LDs are free of ESA.
4. Laser experiments
4.1. Free running mode
For the laser experiments, a 4.9 mm long, 0.42 at.%-doped crystal, which was cut parallel to the ac-plane, was polished plane parallel. It was placed in the focus of a V-type hemispherical resonator which consisted of a plane input coupling mirror (M1) which had a coating that was highly transmissive for the pump wavelength and highly reflective the respective laser wavelength, a plane folding mirror (M2) with the same coating as M1, and a curved output coupling mirror (M3) with a radius of curvature rOC of 100 mm and varying output coupling rates TOC. Each transition was addressed with separate output coupling mirrors. A schematic setup can be seen in Fig. 4. The overall length of the cavity was l ≈ 97 mm with lM1–M2 ≈ 37 mm and lM2–M3 ≈ 60 mm. The transversal and longitudinal multimode pump beam of the InGaN LD (Pout,max = 1 W; FWHM ≈ 2 nm) was shaped by an aspherical lens with f = 4.5 mm and an anamorphic cylindrical lens pair. The polarization was adjusted with a λ/2-waveplate. The LD pump light was focused by a lens (L) with a focal length of f = 50 mm through the input coupling mirror into the crystal. The spot size was estimated to have a waist of w0 = 45 μm. This design was chosen since it allows to accurately determine the absorbed pumped power during laser operation by measuring the residual pump light which is transmitted through M2.
In order to obtain stable laser operation, it was necessary to operate in a qcw regime. The diode was operated with a Thorlabs ITC4005 driver which allowed to adjust the duration and repetition rate of the LD pump light individually. The repetition rate was set to 620 Hz and the excitation interval had a duration of 800 μs which is an order of magnitude longer than the lifetime of the upper laser level. The qcw mode was verified by observing the relaxation oscillation of the laser with a fast Si-photodiode and an oscilloscope. The parameters correspond to a duty cycle of ≈ 1:2.
Laser oscillation was achieved and charactarized at wavelengths λL of 537.1 nm, 635.4 nm, and 719.8 nm (see Fig. 5). Thresholds were as low as 159.4 mW, 95.0 mW, and 10.0 mW of average absorbed pump power, respectively, with maximum slope efficiencies ηsl of 16 %, 16 %, and 37 %. The maximum average ouput powers were 15.1 mW, 22.6 mW, and 80.0 mW which corresponds to peak powers of 30.4 mW, 45.6 mW, and 161.3 mW.
Fig. 5 Summary of laser performances and highest slope efficiencies of 0.42 at.% Pr3+:LaF3 at different emission wavelengths. Each wavelength was realized with a separate output coupling mirror.
At higher repetition rates or longer pump durations, the laser output became increasingly unstable and even shut off in some cases.
Laser operation was also observed and characterized at 612.0 nm with a maximum average output power of 20.1 mW, a maximum qcw power of 40.5 mW, and a maximum slope efficiency of 15 %. Using highly reflective mirrors for the 610 nm region, the wavelength changed to 619.2 nm. The reason for this shift to an emission peak with lower cross sections is reabsorption on the 3H4→1D2-transition. For low inversion densities this leads to a higher gain at 619.2 nm. A summary of these values can be found in Tab. 2, the performance curves for highest slope efficiencies can be seen in Fig. 5.
Table 2:. Laser parameters obtained with a 4.9 mm long 0.42 at.% Pr3+:LaF3 crystal.
The beam quality factors at each wavelength were measured with a Spiricon M200s at their respective maximum output powers. All beams were near diffraction limited and exhibited a almost Gaussian beam profile. The M2-values are given in Tab. 2, exemplary mode profiles can be seen in Fig. 6.
Fig. 6 Exemplary mode profiles of the laser operating at (a) 537.1 nm (b) 612.0 nm (c) 635.4 nm (d) 719.8 nm.
The unstable cw operation is not yet understood but may be attributed to thermal effects occuring in the active medium and/or the formation of color centres. Further investigations are necessary in order to fully understand this problem. As a remark it should be noted, that laser oscillation was observed on up to four wavelengths simultaneously (537 nm, 612 nm, 616 nm, and 619 nm) by employing a set of broad band highly reflective mirrors.
4.2. Wavelength tuning experiments
In order to investigate the spectral tunability of the laser operating on the 3P0→3H6 transition, a linear hemispherical cavity was set up. The beam of the InGaN LD again was focused with a 50 mm lens through a plane input coupling mirror M1 into the crystal. The output coupling mirror M2 had a radius of curvature of 100 mm. Both mirrors had an anti reflective coating for the pump wavelength. The reflectivity throughout the tuning range can be seen in Fig. 7. A 2.5 mm thick quartz plate was inserted into the cavity at Brewster’s angle to act as a birefringent filter. It was observed, that the tuning range expanded by shortening the duration of the excitation to 280 μs.
By rotating the plate around the normal of the surface, the emission wavelength could be tuned from 609.2 nm to 622.7 nm with two small gaps around 613.5 nm and 619.5 nm. The corresponding output power of the laser for one period of the birefringet filter can be seen in Fig. 7. The output power was measured behind the output coupling mirror but substantial outcoupling in the order of a few times the power behind the mirror occured at the quartz plate which indicates a depolarization of the beam. It has to be noted, that different areas of the quartz plate allowed different tuning ranges which indicates inhomogeneities of the plate. At some areas a gapless tuning was possible but with a narrower over-all tuning range.
5. Summary
Investigations of the GSA of Pr3+:LaF3 show maximum absorption cross sections of 1.9 · 10−20 cm2 at 479.0 nm. Furthermore, the absorption band around 442 nm has a FWHM of more than 6 nm and thus offers an excellent feature regarding pumping with InGaN LDs. Emission cross sections reach values of up to 6.6 · 10−20 cm2 at 719.8 nm. The obtained ESA spectra reveal, that no ESA occurs at wavelengths larger than ≈ 340 nm. All absorption and emission peaks which are relevant for the laser experiments are therefore free of ESA.
We demonstrated diode pumped qcw laser action of Pr3+:LaF3 in the green, orange, red and deep red spectral range. Maximum slope efficiencies and average output powers of up to 37 % and 80.0 mW were realized, threshold average pump powers were as low as 10.0 mW. Towards cw operation the laser operation becomes increasingly unstable to the point were, in some cases, it shuts off. The reason for this behaviour is not yet understood and may be attributed to thermal problems and/or the formation of color centers.
Tunability in the orange-red spectral region was achieved in a range between 609.2 nm and 622.7 nm. To the best of our knowledge, this represents the widest tunability of a laser based on a Pr3+-doped crystalline material.
Acknowledgments
The authors gratefully acknowledge the financial support of the German science foundation (DFG) within the graduate school 1355 “Physics with new advanced coherent radiation sources”, the Landesexzellenzinitiative “Frontiers in Quantum Photon Science” of the city of Hamburg and the Joachim Herz Stiftung.
References and links
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