Chirped microlens arrays for diode laser circularization and beam expansion Peter Schreiber, Peter Dannberg, Bernd Hoefer, Erik Beckert Fraunhofer Institute for Applied Optics and Precision Mechanics Albert-Einstein-Strasse 7, D-07745 Jena, Germany ABSTRACT Single-mode diode lasers are well-established light sources for a huge number of applications but suffer from astigmatism, beam ellipticity and large manufacturing tolerances of beam parameters. To compensate for these shortcomings, various approaches like anamorphic prism pairs and cylindrical telescopes for circularization as well as variable beam expanders based on zoomed telescopes for precise adjustment of output beam parameters have been employed in the past. The presented new approach for both beam circularization and expansion is based on the use of microlens arrays with chirped focal length: Selection of lenslets of crossed cylindrical microlens arrays as part of an anamorphic telescope enables circularization, astigmatism correction and divergence tolerance compensation of diode lasers simultaneously. Another promising application of chirped spherical lens array telescopes is stepwise variable beam expansion for circular laser beams of fiber or solid-state lasers. In this article we describe design and manufacturing of beam shaping systems with chirped microlens arrays fabricated by polymer-on-glass replication of reflow lenses. A miniaturized diode laser module with beam circularization and astigmatism correction assembled on a structured ceramics motherboard and a modulated RGB laser-source for photofinishing applications equipped with both cylindrical and spherical chirped lens arrays demonstrate the feasibility of the proposed system design approach. Keywords: microlens arrays; diode laser; laser beam shaping; circularization; astigmatism correction
1. INTRODUCTION Beam expansion as well as circularization and correction of astigmatism are common tasks in laser beam shaping. A typical approach to realize these functions are telescopes with optional anamorphic imaging. There are two different basic setups, the Keplerian and the Galilean telescope (Fig.1).
Fig. 1: Pricipal beam expander layout, realized as Kepler (left) and Galilei telescope (right).
The Galileian expander provides shortest system length, while the Kepler-type enables the integration of a spatial filter to clean the beam, if required. If variable output beam parameters and/or compensation of source waist size tolerances are required, a zoom function is added to the telescope, usually by splitting the first lens into at least two parts (Fig. 2). Compared to the simple telescopes shown in Fig. 1 the variable beam expanders suffer from enhanced number of lenses required (at least one additional element), increased system length and difficulties in ensuring diffraction limited operation over the whole zoom range. Usually, a trade-off between aberration correction, zoom range and complexity has to be made during the system design.
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Laser Beam Shaping VI, edited by Fred M. Dickey, David L. Shealy, Proceedings of SPIE Vol. 5876 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.616988
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Fig. 2: Basic layout of variable beam expanders realized as zoomed Kepler (left) or Galilei telescope (right). Maximum and minimim expansion are shown in the upper and the bottom row, respectively.
In this work an alternative approach based on microlens arrays with varying focal width from lenslet to lenslet - referred to as chirped arrays in the framework of the paper - is proposed. The main idea realized as Galileian beam expander is sketched in Fig. 3. To achieve variable expansion ratio of the telescope, we change the focal length of the first lens by selecting the proper lenslet of the chirped array for the required expansion ratio. An analogous Keplerian telescope with a positive chirped array works as well.
Fig. 3: Principle layout of the variable Galileian telescope with a chirped lens array. Large beam expansion is shown left and smallest expansion right. The arrows visualize the required movement of the array to switch from large to smallest expansion.
The chirped array approach permits a telescope with a stepwise variable expansion ratio. The use of microlenses for the chirped array is crucial, to achieve acceptable lateral system dimensions. A comparison between the common zoomed telescope and the chirped array telescope is shown in Tab. 1.
Min. number of lenses Min. number of moved lenses Direction of movement Expansion variation System length
Zoomed telescope 3 2 axial continuous enhanced
Chirped array telescope 2 1 axial and lateral stepwise minimum
Tab. 1: System comparison between classical zoomed telescope and chirped array telescope.
2. CHIRPED LENS ARRAY MANUFACTURING 2.1 Mastering Master structures of spherical and cylindrical chirped lens arrays were generated using photolithography and a reflow process1. The main advantage of lithographical patterning is the high lateral precision (crucial for pitch matching, stacking of arrays, and hybrid integration) as well as the parallel processing of a huge number of elements in an arbitrary, user specified layout on a wafer scale2. Typically, the feature height in a binary lithographic process is related to the resist thickness which is constant across a given sample. Nevertheless, lens parameters like radius of curvature Rc, lens sag or numerical aperture can be varied via the subsequent photoresist reflow. I.e. the radius Rc after reflow depends on the lens width/footprint as well as on the available amount of photoresist for the reflow. It turned out, that both footprint and resist volume can be used to generate a definite variation/chirp of the microlens parameters across an array. The first approach consists in a simple increase of the width of a cylindrical lens (or the diameter of a spherical lens) on the photomask leading to an increased focal length at almost constant lens sag. This approach is beneficial especially in the case of moderate parameter variations. But with increasing focal length the footprint becomes larger leading to
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higher form deviations (compared to a smaller lenslet). The minimum possible gap between neighboured lenslets is determined by the resist thickness, enabling minimum gap sizes of about 1/10 of lens sag height. The constraining parameters for the reflow lenses are maximum element sag height of about 90µm and contact angles (angle between the tangent of the lens profile and the substrate at the rim of the lenslet) in the range of about 1°... 17°. Thus, the possible range of variation in this approach is mainly limited by space consumption on the wafer, changes of the lens aperture and lens aberrations. Fortunately, the smaller lens apertures for smaller focal lengths fit well for the use as first lens of a beam expander: For small input beams small apertures are sufficient while the required large expansion ratio is realized by the associated small focal width of the first lens of the telescope. In Fig. 4 part of a master containing two sets of chirped cylindrical and spherical lens arrays is shown. It turned out, that the ratio of the different focal lengths on a chip corresponds very closely to the variation on the photomask (relative focal length error
