1. Introduction

Optical science has just celebrated the 50th Anniversary of the first laser [1] 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 [2]. One of the most important achievements in the field of laser materials during the last 15 years is the development of oxide laser ceramics [3], 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 [8]. 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 [9]. Furthermore it is possible to reach the ultraviolet part of the spectrum with a single step of frequency doubling [10]. This allows for a wide variety of applications such as display technology [11], fluorescence microscopy [12] or bio-medical applications [13].

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^- [4], CaF₂-SrF₂:Yb³⁺ [5], CaF₂:Yb³⁺ [6], SrF₂:Nd³⁺ [7]. 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 [14] 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.

 

Fig. 1 Room temperature absorption spectra of Pr³⁺ ions in SrF₂.

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Fig. 2 Room temperature fluorescence spectrum of Pr³⁺ ions in SrF₂.

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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 [15], to be 4% which is similar to the loss value of about 3% obtained in case of SrF₂:Nd³⁺ ceramic [16]. 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.

 

Fig. 3 Photo of visible (639 nm) operation of SrF₂:Pr³⁺.

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Fig. 4 Input-output characteristics of SrF₂:Pr³⁺ ceramics and crystal samples for laser operation at 639 nm.

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Fig. 5 Oscillation spectrum of SrF₂:Pr³⁺ ceramics.

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3. Conclusions

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.