ICTON 2008 223 We.D3.2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ This research has been supported by Enterprise Ireland under the Commercialisation Fund Proof of concept Phase, project number PC/2006/0192. 978-1-4244-2626-3/08/$25.00 ©2008 IEEE Polarization Dependent Gain in Pr 3+ /Yb 3+ Doped Fluoride Fibre Séverine Philippe, Michael Lynch, John F. Donegan* Semiconductor Photonics Group, School of Physics, Trinity College Dublin, Ireland *Tel: (+353) (1)8961987, Fax: (+353) (1) 6711759, e-mail: jdonegan@tcd.ie ABSTRACT Fibre amplifiers and lasers are formed by introducing rare-earth ions into silica and fluoride fibre material. The advantage of fluoride glasses is the much more efficient upconversion lasing in co-doped systems such as the Praseodymium/Ytterbium system used in this work. We investigate the gain properties of Pr 3+ /Yb 3+ doped fluoride fibres with the ultimate goal of developing a blue polarized fibre laser. When a probe with state of polarization parallel to the major axis of the elliptical fibre core is injected, no rotation of its output polarization is observed with pump injection and a higher gain is achieved for a given pump power. Moreover if both probe and pump polarization are aligned with the major axis, the onset of the gain is found to occur at lower pump power. Keywords: fluoride fibre, fibre amplifier, fibre laser, praseodymium, ytterbium, polarization. 1. INTRODUCTION The development of lasers emitting in the visible spectral range has been driven by a wide array of applications, including colour displays, data storage, printing, confocal microscopy and medicine. In particular sources in the blue region of the spectrum often rely on expensive and bulky gas lasers with high power consumption. Because they are cheaper and more efficient, fibre lasers are important alternative sources to the standard Argon-ion lasers. To date they suffer from a lack of polarization selectivity resulting in a large noise characteristic. Since the fibre material is uniform and regular fibres have circular symmetry, the output of fibre lasers are generally unpolarized, or randomly polarized. However introducing birefringence by using a fibre with an elliptical core allows stable propagation of linearly polarized signals aligned with the main axes of the fibre. If the gain of these two polarization modes can be made sufficiently different, then one of the modes will dominate and single polarization lasing will occur. The system under investigation is based on upconversion in a Pr 3+ /Yb 3+ co-doped fluoride fibre, which has shown potential for the development of lasers operating at a number of wavelengths in the visible region of the spectrum [1-3]. The praseodymium ions (Pr 3+ ) have a strong emission in the blue region of the spectrum around 490 nm but suffer from weak absorption at typical pump diode laser wavelengths, around 800 – 900 nm. Successful visible upconversion [4] can be obtained by co-doping the fibre with ytterbium ions (Yb 3+ ), which strongly absorb the diode laser pump and efficiently transfer the absorbed energy to the Pr 3+ . In this paper we investigate the polarization dependent gain in the blue region of the spectrum for a Pr 3+ /Yb 3+ co-doped fluoride fibre with an elliptical core. 2. EXPERIMENTAL SET-UP The fibre under test is a 50 cm long Pr 3+ /Yb 3+ co-doped fluoride fibre. The core diameter is about 1.4 μm with an ellipticity giving a beat length of 4.8 m. The active core doping concentrations are 3000 ppm Pr 3+ and 20000 ppm Yb 3+ . A pump probe set-up is used in order to investigate the polarization dependent gain provided by the blue transition in the Pr 3+ . The pump is provided by a Titanium:Sapphire laser set at 840 nm, while the probe is provided by the 488 nm line of an argon ion laser. The state of polarization (SOP) of the pump and probe signals can be controlled independently of one another. The output probe signal is measured by lock-in detection using a spectrometer and a photomultiplier tube. The measurements are performed as a function of pump and probe states of polarization as well as pump input power. The spectrum of the pumped fibre output is shown on Fig.1, as well as an energy diagram of the Pr 3+ . The fluorescence peaks corresponding to different transitions of the Pr 3+ can clearly be seen. These transitions occur from the thermally coupled 3 P 0 , 3 P 1 and 1 I 6 levels of Pr 3+ . The 489 nm, 520 nm, 603 nm and 634 nm transitions originate from the 3 P 0 level and terminate on the 3 H 4 , 3 H 5 , 3 H 6 and 3 F 2 levels respectively, while the 535 nm transition originates from the 3 P 1 level and terminates on the 3 H 5 level. As mentioned before one of the laser lines of the Argon ion laser lies at 488 nm, providing a very close match to the 3 P 0 ﾆ 3 H 4 transition. Authorized licensed use limited to: TRINITY COLLEGE LIBRARY DUBLIN. Downloaded on September 2, 2009 at 10:33 from IEEE Xplore. Restrictions apply.