An anamorphic beam concentrator for linear laser-diode (LD) bar is presented. It consists of a tapered SiO2-rod with skewed and curved surface. The principle and applicability of this device are numerically investigated by ZEMAX and experimentally illustrated for the specific example of the linear LD bar. Results show that a relative symmetrical output beam spot is produced at the output facet of the rod and the intensity and spatial fluctuations in the input beam are compensated at distance of 20cm from the output facet.
© 2007 Optical Society of America
Laser beam shaping has many applications in different fields, including material processing, imaging, optical inspection and metrology, optical communication and laser pumping of high power solid-state lasers and fiber lasers. Generalized laser beam shaping techniques commonly use diffractive optical elements with complicated phase functions [1–6] or micro-optic-element groups [7–9]. However, these laser beam shaping systems are difficult to adjust and cost much to fabricate. As the high-power solid-state lasers and fiber lasers become more and more important in military, deep space laser communication system and laser machining, the anamorphic beam shaping of the high-power pump sources such as linear and stack LD bar attract more and more interests. A simple, cheap and compact package high-power laser system can be realized by using a non-imaging optics concentrator scheme such as the lens duct [10, 11] and the gradient-index rod [12, 13]. However, the lens duct and gradient-index rod cannot provide an anamorphic, adiabatic shaping, so there is no coupling between the two transverse directions (viz.fast axis and slow axis) of the LD bar, which causes that the brightness cannot be transferred between them and an output beam with uniform intensity and angular distributions cannot be produced . Therefore, a simple anamorphic shaping optical element for concentration of linear LD bar is present in this paper. It consists of a tapered SiO2-rod with skewed and curved surface, hence enabling coupling between the horizontal and vertical directions of the guided LD beam. Because it not only achieves an anamorphic transformation, but also produces different optical magnification and minification along mutually perpendicular radii just like an anamorphic lens, we call it anamorphic beam concentrator.
2. Principle and Fabrication
In our technique, the beam shaping is achieved with a tapered SiO2-rod with skewed and curved surface. Its input facet is a rectangular cross section with 1:10 aspect ratio, which is match to the output facet of the linear LD bar. And the output facet is a rhombic cross section. The design is presented in Fig. 1.
The four structural parameters (the area of input facet, the area of output facet, the taper angle and the length of the rod) of the device are optimized by the ZEMAX. The areas of input and output facets are 1.2×12mm2 and 2×2mm2 respectively. And the total length of the tapered SiO2-rod with the taper angle of 14.2mrad is 30cm. The shape of a guide beam is changed and concentrated by tapering the shape of this SiO2-rod along its longitudinal axis slowly. If the transformation of taper is slow enough, the term adiabatic can be used and then the total (four-dimensional) phase-space volume (PSV) of the beam is indeed conserved, as was demonstrated in .Therefore, the length of the device has to be lengthened if we want to concentrate the light into a smaller region, like 1×1mm2 or 0.5×0.5mm2. Whereas, overlong length will lead to excess reflections, which may cause too much leakage of the power and increase the diverging angle of the final concentrated beam. This beam shaping method relies on the fact that for each transverse direction (say fast or slow axis of the linear LD bar), the slow changes of the cross-section and shape of the beam guide ensure adiabaticity and hence conservation of the beam quality in that direction (discussion in more details can be referred to ). Furthermore, the skewed and curved surface of the SiO2-rod provides the necessary anamorphic transformations where coupling between the two transverse directions may exist to increase the brightness in one transverse direction and compensate for angular and spatial fluctuations in the input beam to produce a uniform output beam.
The device is fabricated by a curved surface grinder and an optical polishing lathe. First of all, a pure cylindrical SiO2-rod, whose length is 30cm and radius of cross section is 10mm, is prepared; and then it is ground and shaped into a tapered rod with skewed and curved surface by using a curved surface grinder; finally, the front and rear surface and the side surface of the taper are polished by the optical polishing lathe.
3 Experimental and numerical results
Figure 2 shows the output intensity image of the linear LD bar, whose diverging angle and width of the fast axis are 3.5mrad and 1mm respectively after collimating, when the anamorphic beam concentrator is not used. An elliptical beam spot with much high aspect ratio is obtained and the beam size in the slow axis of the LD bar is as high as 50-60mm when the distance z between the output facet of the LD bar and the image plane is only 30cm. Furthermore, the distribution of the output intensity is not homogeneous. Obviously, this kind of irregular beam with high aspect ratio and uneven intensity distribution is difficult to be applied in the practical applications such as end pumping of solid-state lasers and fiber lasers, material processing and optical imaging. Therefore, a novel anamorphic beam concentrator is applied to shape the output beam of the linear LD bar for obtaining a relative symmetric and uniform beam spot.
Figure 3 is the ray tracing of the linear LD bar, which is shaped and concentrated by the tapered SiO2-rod. The LD bar is assumed that its fast axis has been collimated by a cylindrical lens and the slow axis has a FWHM divergence of~0°. And the total area of the emitting plane of the LD bar is 1mm×10mm. Therefore, we design a tapered SiO2-rod whose input facet is 1.2mm×12mm for maximum coupling efficiency and that concentrates the light into a rhombic cross section with side length of 2mm. A non-sequential ray-tracing model of the ZEMAX (a kind of optical design software) is used to optimize the coupling efficiency and to permit calculations of the energy distribution at the receiving plane. As showed in Fig. 3, rays of the LD-bar are guided along the longitudinal direction of the rod and concentrated on the output plane.
A diffuse screen is placed immediately at the output facet of the SiO2-rod, and the intensity images are recorded with a digital camera in Figs. 4 and 5. Because an equipartition occurs between different directions of the linear LD bar, resulting in coupling between fast axis and slow axis, the input laser beam with high aspect ratio of 1:10 is concentrated into a rhombic beam spot with side length of 2mm and the most of light intensity is concentrated into an elliptical beam with aspect ratio of 1:2. Because the receiving screen is placed immediately at the output facet, the size of the beam image in Fig. 5 is equal to the size of the output facet (2×2mm2) of the SiO2-rod.
Then the receiving screen is removed to the different distances (6cm, 10cm, 15cm and 20cm) from the output facet of the rod respectively, and the intensity distributions are recorded in Figs. 6 and 7. We expect that the light intensity will become uniform along with the increase of the radiation distance, provided that there are a “sufficient” number of reflections inside the rod. The simulated and experimental results of the output intensity distributions are showed in Figs. 6 and 7 respectively. In Fig. 6, the rhombic beam spot at the output facet of the device transforms into a rectangular spot with aspect ratio of 1:4 gradually, meanwhile, the light intensity distribution becomes more and more homogeneous as the rays propagate. And in Fig. 7, the beam image sizes are 3.2×1.6mm2, 4×1.5mm2, 4.9×1.2mm2 and 6×1.1mm2 when the receiving screen is move to 6cm,10cm,15cm and 20cm respectively.
Figure 8 depicts the variations of the intensity distribution: when the receiving screen is moved from 6cm to 20cm, the peak irradiance of the output beam varies from 4.9219W/cm2 to 1.4931W/cm2, meanwhile, the aspect ratio varies from 1:2 to 1:4 and a uniform intensity distribution is obtained. The reason for this kind of homogenization is that the SiO2-rod, having curved and slanted surface, achieves some coupling between the fast axis and slow axis directions of the linear LD bar, which compensates for intensity and spatial fluctuations in the input beam and produces a relative uniform and symmetrical output beam spot. The output power of the LD bar, the output power of the concentrator and the slope efficiency of the concentrator are showed in Fig. 9 together. The low efficiency (<40%) is caused by three kinds of loss of the incident power, which are reflection loss at the front and rear surface of the taper, scattering loss along the side surface of the taper and leakage loss of the refraction.
A tapered SiO2-rod with skewed and curved surface is proposed as an anamorphic beam concentrator for the linear LD bar. The geometrical shape and size of the SiO2-rod are designed and optimized. And the simulated and experimental results of the output intensity distribution at the output facet of the rod and four different distances from it are investigated. Because a mutual coupling between the fast and slow axis directions of the linear LD bar is achieved by the SiO2-rod with a curved and slanted surface, a relative symmetrical output beam spot with small aspect ratio of 1:2 is produced at the output facet of the rod and the intensity and a beam spot with uniform intensity distribution is obtained at a distance.
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