Scaling and passively Q-switch operation of a Nd:YAG laser pumped laterally through a YAG prism
Introduction
A main objective of the investigations that considered the laser-induced ignition of engines with internal combustion was to build a laser with a size comparable to that of an electrical spark plug. First experiments on laser ignition were performed with commercial lasers that delivered pulses with energy in the range of tens to a few hundreds of mJ and several-ns pulse duration [1], [2]; however, these lasers had large dimensions. Later, the experiments concluded that a suitable device for engine ignition can be realized by employing a Nd:YAG laser that is passively Q-switched by Cr4+:YAG saturable absorber (SA) [3], [4], [5]. An end- (or longitudinally-) pumped scheme with fiber-coupled diode lasers [3], [4], or side-pumping with array diode lasers [5] were the solutions used to build novel laser devices; still, these lasers had larger sizes than those of a classical (electrical) spark plug.
A Nd:YAG-Cr4+:YAG laser with dimensions comparable to those of an electrical spark plug was first realized by Tsunekane et al. [6]. The laser pulse features, suitable for ignition, were achieved by shortening the resonator length, and by maximizing the laser pulse energy following optimization of the pump conditions through right choice of the Nd:YAG parameters and of the Cr4+:YAG SA crystal initial transmission [7]. Furthermore, such a scheme was employed for efficient generation of laser radiation into visible and ultraviolet spectra by single-pass nonlinear conversion of the fundamental wavelength [8], [9], [10]. An Yb:YAG-Cr4+:YAG laser was developed recently by the same research group [11]. The passively Q-switched lasers mentioned above were realized with discrete Nd:YAG or Yb:YAG and Cr4+:YAG SA single-crystal media. On the other hand, the ceramic techniques allow obtaining of laser media with very good optical quality, featuring easy manufacturability at competitive prices. Consequently, optically-bonded, composite Nd:YAG/Cr4+:YAG all-poly-crystalline (or ceramic) media were used to build compact lasers with sizes comparable to that of a classical spark plug [12], [13], [14]. Such lasers outputted multiple beams and had maneuverability in varying distance between the ignition points as well as the depth of the focusing point.
These lasers have been realized using longitudinal pumping [3], [4], [6], [11], [12], [13], [14]. In general, in this arrangement the pump beam is delivered by a fiber-coupled diode laser and then it is transferred into the laser medium through one end of the laser rod. Typical coupling optics contains two lenses, and therefore the pump beam and the laser beam are collinear. Recently we have proposed a configuration that improves the compactness of a diode-pumped Nd:YAG laser [15]. This geometry employs a rectangular laser medium in which the pump beam is coupled directly from a fiber end through a single optical element, a prism. In the first experiments a diode-pumped YAG prism-Nd:YAG laser that in free-generation regime outputted pulses at 1.06 μm with energy Ep=2.1 mJ under the pump with pulses at 807 nm of energy Epump=9.9 mJ was demonstrated; the overall optical-to-optical efficiency (ηo) was 0.21. The laser slope efficiency (ηs) amounted to 0.22. Also, a passively Q-switched YAG prism-Nd:YAG-Cr4+:YAG laser that delivered laser pulses with low energy Ep=90 μJ and long duration tp=26 ns was built [15].
In this work we report on further investigations of this pumping geometry and obtain good improvements of the output performances. Laser pulses with Ep=22.1 mJ at optical efficiency ηo~0.49 were measured from a 10.0 mm long, 1.0-at.% Nd:YAG crystal of 1.5×1.5 mm2 cross section that was pumped through a YAG prism (i.e. the YAG prism-Nd:YAG laser). The slope efficiency was ηs=0.51. The emission characteristics delivered from various pumping arrangements are discussed. Furthermore, a passively Q-switched Nd:YAG/Cr4+:YAG laser that consisted of an optically-bonded, composite, ceramic structure was build for the first time (i.e. the YAG prism-Nd:YAG/Cr4+:YAG laser). Laser pulses with energy Ep=0.29 mJ and duration tp=12 ns were obtained by pumping this device through a YAG prism. Two geometries of such a Q-switched laser are presented and the corresponding experimental results are given.
Section snippets
The laser concept
Fig. 1 shows the laser configuration that will be discussed in this work. The Nd:YAG medium has square (t×t) transversal section, and a YAG prism is positioned near one of Nd:YAG ends. The prism is an isosceles triangle having a 90°-angle section, being in contact with Nd:YAG through one of the right angled surfaces; a glue of suitable refractive index is used to attach the prism to the laser medium. The pump beam is delivered from the fiber (which is placed close to the prism hypotenuses) and
Conclusions
In summary, we have reported on scaling the output performances of a laser geometry in which the pump beam is coupled directly from the diode-laser fiber into the laser medium through a prism optical element. Laser pulses with 22.1 mJ energy at optical efficiency of nearly 0.50 and 0.51 slope efficiency were obtained in free-generation operation from such a compact, coated Nd:YAG-YAG prism laser. In addition, an uncoated Nd:YAG crystal delivered pulses with energy of 17.8 mJ at 0.39 optical
Acknowledgments
This work was financed by the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project 58/2012 (PN-II-PT-PCCA-2011-3.2-1040) and project IDEI 37/2011 (PN-II-ID-PCE-2011-3-0801).
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