Fast - steering mirrors improve active beam stabilization

nce solely a research tool, the laser is now an enabling technology for a wide range of applications in semiconductor manufacturing and inspection, industrial marking, materials processing, biomedical systems, reprographics, information display, and telecommunications. Given today’s accelerated product-development schedules, laser-based optical systems are being pushed to produce finer resolution and greater throughput as soon as the application is developed. To fulfill these expectations, today’s laser systems must maintain precise optical alignment in demanding manufacturing environments that run continuously. Vibration or thermal-induced motion can blur precise images, reduce critical intensities, and misalign interfaces between laser and target, leading to severely compromised system performance. While using active and passive vibration isolation, athermalizing the design, and redesigning the mechanical structure can substantially reduce misalignment, none of these approaches is quick, simple, or guaranteed to keep the laser aligned to the optical axis. Dynamic laser-beam alignment actively reduces misalignment from vibration sources, thermal gradients, and mechanical creep. The success of this approach depends on the fast response characteristics of the active mirrors maintaining the alignment. Originally developed for military and aerospace applications, fast-steering mirrors (FSMs) have recently been commercialized and incorporated as active mirror elements in high-performance beam-stabilization and alignment modules. These modules are a simple, practical approach to system performance improvement. Sources of laser-beam misalignment Tilt errors that deviate a laser beam from the intended optical axis originate from three locations: internal to the laser, the optical beam-delivery system, and the external environment. Within the laser, the collimated beam can be subjected to strong thermal gradients, to vibration sources such as fans, shutters and cooling water, and to mechanical creep and aging optics. Tilt-error-generating thermal gradients, mechanical creep, motor vibration, mechanical impulse, and circulating fan turbulence can affect the optical beam-delivery system. External sources of shock and vibration from neighboring production equipment, loading and unloading parts, and environmental changes can drive resonances within the beam-delivery system and the laser. The net impact of all of these factors is to apply a spectrum of vibration to otherwisestatic optomechanical components, resulting in a temporally varying misalignment of the laser beam. Two dynamic parameters measure laser misalignment from the desired optical axis: lateral displacement and angular deviation. Each of these parameters is a function of time and as such can be represented by a temporal trace or by a power spectral density (PSD). The PSD is very useful because it graphically displays the angular misalignment (θ2/Hz) as a function of the error frequency. Typical optical systems have PSDs dominated by thermal drifts and low frequency vibrations, while PSD high frequencies are determined by the resonant characFast-steering mirrors improve active beam stabilization OPTICS: ALIGNMENT