Moore, N., Clarkson, W.A. and Hanna, D.C.
Efficient 1123nm diode-bar pumped Nd:YAG laser
At Thirteenth National Quantum Electronics Conference (QE13), United Kingdom.
08 - 13 Sep 1997.
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Laser emission at 1123nm has a number of possible applications, the most notable being differential absorption water vapour LIDAR, to remotely sense concentrations of water vapour in the atmosphere For this application use is made of two different wavelengths, one, at 1123.06nm which coincides with a peak absorption peak of water vapour, while the other at 1123.2nm which falls in a region of very low water absorption. Nd YAG provides the possibility for laser emission at both of the required wavelengths, and has been recognised as a suitable laser source for such a system. However the emission cross-section of Nd:YAG at 1123nm is very small, approximately one fifteenth that of the 1064nm line, hence resulting in very low gain. Thus to achieve efficient laser operation at 1123nm, a very bright pump source is required so that intense pumping can be achieved. In this paper we describe a diode-pumped Nd:YAG laser pumped by such a bright source. The pump-diode used was a 7W diode-bar operating at 807nm, manufactured by the Opto-Power Corporation. The diode emission, which came from a 1.6mm wide region. was reshaped using the previously reported two-mirror 'beam-shaping' technique. The resulting beam had 5.6W of power, was approximately circular, and had M2 values of ~20 and ~40 in orthogonal directions. This was used to end-pump a 10mm long Nd:YAG rod, housed in a water-cooled heat-sink. To obtain intense pumping, the pump light was focused to a spot size of radius 130µm. The small spot-size results in a strong thermal lens in the laser rod, measured to have a focal length of ~55mm, but also being significantly aberrated, and thus requiring care over the choice of appropriate resonator. This Nd:YAG laser produed an output of 1.6W at 1123nm, in a linearly polarised, diffraction limited gaussian beam, from a standing wave cavity. This is around a five-fold increase in power on previous reports for this wavelength narrow linewidth emission is required for a working LIDAR, thus a unidirectional ring cavity was constructed to enable single frequency operation. This produced 180mW of single frequency diffraction-limited output, which could be tuned to the wavelengths of interest. Owing to a lack of suitable components (e.g. a low loss Faraday rotator), this performance is far from optimal. Further improvements will be discussed
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