Results for new fluoride ceramics doped with praseodymium ions in which, to the best of our knowledge, for the first time a laser in the visible spectral range (639 nm) was obtained under GaInN blue laser diode optical pumping. In our experiments, CW operation with lasing threshold less than 100 mW of absorbed pump power and slope efficiency exceeding 9% was realized.
© 2011 OSA
Optical science has just celebrated the 50th Anniversary of the first laser  which opened a new era in optics and photonics. High interest to ceramic materials follows from the fact that only four years later the first CaF2:Dy2+ ceramic laser was demonstrated . One of the most important achievements in the field of laser materials during the last 15 years is the development of oxide laser ceramics , which is suitable to realize solid-state lasers in the near infrared (IR) spectral region. During the last 5 years also fluoride laser ceramics with F2- color centers, ytterbium and neodymium rare earth ions (RE) were developed [4–7] demonstrating some unique properties. Fluoride laser ceramics demonstrates the same high thermal conductivity as analogous single crystals, low energy phonons and a very broad transparency range from the ultraviolet to the far infrared. Fluoride laser ceramics are also characterized by large size and scalability of the active elements, high fracture toughness and cleavage resistance, and even higher laser damage threshold compared to the single crystals . In the present work, we report on the first successful development of fluoride laser ceramics doped with praseodymium (Pr3+) ions and the first realization of laser oscillation using ceramics in the visible spectral range (639 nm) .
Lasers based on the trivalent praseodymium ion are highly interesting for solid-state lasers in the visible due to the various transitions in the blue, green, orange and red spectral range . Furthermore it is possible to reach the ultraviolet part of the spectrum with a single step of frequency doubling . This allows for a wide variety of applications such as display technology , fluorescence microscopy  or bio-medical applications .
During the last years the interest for rare-earth (RE) doped fluoride ceramics has arisen due to their improved thermo mechanical properties compared to analogous single crystals. Within a short period of time different types of fluoride nanoceramics were developed and rather efficient diode pumped lasing was demonstrated for LiF:F2- , CaF2-SrF2:Yb3+ , CaF2:Yb3+ , SrF2:Nd3+ . The common feature for all these laser ceramics was that laser operation only in the near IR spectral region around 1-1.2 μm was obtained. It is well known that the optical losses in ceramic host material (multiphoton absorption, scattering on grain boundaries, pores and inclusions) should strongly increase for shorter wavelengths. From this point of view the development and the investigation of laser ceramics which can oscillate at visible wavelengths is of great importance. The recent success in visible light generation with Pr3+ ions  has become possible due to the development of efficient blue GaInN diodes. In this work the results on optical and oscillation properties of the new Pr3+ doped SrF2 ceramics are firstly presented.
2. Experimental results
The SrF2:Pr3+ ceramics were developed using a hot pressing technique from a SrF2:Pr3+ single crystalline precursor similar as described in [4,5]. The Pr3+ ions concentration in ceramic sample was 0.3 at.% and 0.5 at.% for single crystalline sample. Rather low Pr3+ concentration was chosen to avoid formation of low symmetry paired centers which is known to start in SrF2:Nd3+ crystal for concentrations exceeding 0.8 at.%. The absorption spectrum near the pump wavelength of 444 nm and the fluorescence spectrum (corrected to the spectral sensitivity of the measuring system) of Pr3+ ions in SrF2 crystal are presented in Fig. 1 and Fig. 2 respectively. As follows from Fig. 1 the maximum absorption of Pr3+ ions in SrF2 is peaking at 443 nm. The absorption maximum is seen to be very broad allowing pumping wavelength fluctuations from 438 to 446 nm. As can be seen in Fig. 2, the sharp and strong maximum of room temperature fluorescence is observed in the red spectral range at 639 nm, which is very important for various applications.
The lasing properties of SrF2:Pr3+ ceramics were tested under GaInN laser diode pumping with a central emission wavelength of 444 nm and a maximum output power of 1W. The tested ceramic sample was only about 3 mm thick, so only about 9% of the 444 nm pump radiation was absorbed while for the single crystal also used in the experiments the thickness was 5 mm resulting in about 28% of pump absorption. For the experiments, a hemispherical cavity was formed by the plane input coupling mirror with anti-reflection coating for the pump wavelength and with high reflectivity around 640 nm (3P0-3F2 transition of the Pr3+-ion) and an output mirror with a radius of curvature of 50 mm. The pump beam was focused into the active medium by a lens with a focal length of 40 mm.
A photo of the cavity with blue beam pumping and red SrF2:Pr3+ visible operation is shown in Fig. 3 . In Fig. 4 the input-output laser characteristics of a SrF2:Pr3+ ceramic are presented for several output mirror transmittance. As can be seen from Fig. 4, the maximum output CW power obtained was about 7.5 mW with a slope efficiency of about 9% with respect to absorbed pump power. Using the data for oscillation thresholds for different output couplers the cavity losses can be estimated, using the Findlay-Clay method , to be 4% which is similar to the loss value of about 3% obtained in case of SrF2:Nd3+ ceramic . In the same conditions the results with an output mirror transmittance of 1.8% are also presented. The laser threshold realized in the case of the ceramic sample with the same output mirror was slightly lower with similar values of slope efficiencies. In Fig. 5 the measured oscillation spectrum of SrF2:Pr3+ laser ceramics is demonstrated. As could be seen from the figure the shape of the spectrum is not simple and could be approximated by a combination of two oscillating lines with maxima peaking at approximately 638.5 nm and 639.5 nm which were observed also in fluorescence spectrum of Pr3+ ions in SrF2.
Thus for the first time laser grade Pr3+ doped SrF2 fluoride ceramics were developed using a hot pressing technique and visible 639 nm red laser oscillation was obtained in a ceramic material. Under LD pumping the CW mode of operation without any special cooling with low oscillation threshold and slope efficiency about 9% was realized. The estimated optical loss values for visible radiation in the SrF2:Pr3+ ceramic sample were in the range of the Fresnel reflection losses expected for a beam propagating through two ceramic sample surfaces. These values are similar to the ones obtained for previously tested SrF2:Nd3+ ceramics in the near IR. With respect to the optical quality of the ceramics, which can be regarded as similar to a well prepared crystalline material, a considerable enhancement of the laser characteristics can be expected in the future.
References and links
1. T. H. Maiman, “Stimulated optical radiation in ruby,” Nature 187(4736), 493–494 (1960). [CrossRef]
2. S. E. Hatch, W. F. Parsons, and R. J. Weagley, “Hot-pressed polycrystalline CaF2:Dy2+ laser,” Appl. Phys. Lett. 5(8), 153–154 (1964). [CrossRef]
3. A. Ikesue and Y. L. Aung, “Ceramic laser materials,” Nat. Photonics 2(12), 721–727 (2008). [CrossRef]
4. T. T. Basiev, M. E. Doroshenko, V. A. Konyushkin, V. V. Osiko, L. I. Ivanov, and S. V. Simakov, “Lasing in diode-pumped fluoride nanostructure F2-:LiF colour centre ceramics,” Quantum Electron. 37(11), 989–990 (2007). [CrossRef]
5. T. T. Basiev, M. E. Doroshenko, P. P. Fedorov, V. A. Konyushkin, S. V. Kuznetsov, V. V. Osiko, and M. Sh. Akchurin, “Efficient laser based on CaF2-SrF2-YbF3 nanoceramics,” Opt. Lett. 33(5), 521–523 (2008). [CrossRef] [PubMed]
6. O. K. Alimov, T. T. Basiev, M. E. Doroshenko, P. P. Fedorov, V. A. Konyushkin, S. V. Kouznetsov, A. N. Nakladov, V. V. Osiko, and O. V. Shlyakova, “Spectroscopic and laser properties of Yb3+ ions in BaF2-SrF2-CaF2 single crystals and nanoceramics,” presented at International Conference on Luminescence and Optical Spectroscopy of Condensed Matter (ICL), Lyon, France, 7–11 July 2008.
8. P. P. Fedorov, V. V. Osiko, T. T. Basiev, Yu. V. Orlovskii, K. V. Dukel’skii, I. A. Mironov, V. A. Demidenko, and A. N. Smirnov, “Optical fluoride nanoceramics,” Russ. Nanotechnol. 2(5–6), 95–105 (2007).
9. T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+:LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm, and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011). [PubMed]
10. A. Richter, N. Pavel, E. Heumann, G. Huber, D. Parisi, A. Toncelli, M. Tonelli, A. Diening, and W. Seelert, “Continuous-wave ultraviolet generation at 320 nm by intracavity frequency doubling of red-emitting Praseodymium lasers,” Opt. Express 14(8), 3282–3287 (2006). [CrossRef] [PubMed]
11. P. Semenza, “Can anything catch TFT LCDs?” Nat. Photonics 1(5), 267–268 (2007). [CrossRef]
13. D. Graham-Rowe, “A new light in dentistry,” Nat. Photonics 2(12), 705–707 (2008). [CrossRef]
14. N.-O. Hansen, A.-R. Bellancourt, U. Weichmann, and G. Huber, “Efficient green continuous-wave lasing of blue-diode-pumped solid-state lasers based on praseodymium-doped LiYF4.,” Appl. Opt. 49(20), 3864–3868 (2010). [CrossRef] [PubMed]
15. D. Findlay and R. A. Clay, “The measurement of internal losses in 4-level lasers,” Phys. Lett. 20(3), 277–278 (1966). [CrossRef]