Novel adaptive laser measurement techniques
The development of adaptive optics systems is a rapidly growing field. Adaptive optics systems consist of three main components. They include a sensor that captures optical wavefronts (e.g., a Hartmann-Shack camera), an electrical control unit, and a light modulator that corrects the distorted optical wavefronts. Deformable mirrors, segmented micromirror arrays, or liquid crystal arrays are often used as the light modulator. Although adaptive optics systems have so far been mostly used in astronomy applications, progress is now also being made in developing these systems for laser measurement techniques.
Astronomy applications of adaptive and active optics have already led to a renaissance of large Earth-based telescopes.1 In these cases, adaptive optics are used to correct the optical distortions of a light path towards an angular resolution that reaches the optical diffraction limit. The distortions are caused by turbulent fluctuations of the light path as it passes through the atmosphere. The planned flagship European Extremely Large Telescope will have a primary mirror diameter of 39m. More than 6000 actuators will be used for the deformable mirror to correct light distortions with a temporal resolution that is better than 1ms. Uses of adaptive optics outside of astronomy include laser systems for free-space communication, laser material processing, lithography, and biomedical microscopy. An additional, intriguing possibility in ophthalmology is to use adaptive optics to resolve retinal cells (cones and rods) based on the correction of aberrations inside the eye,1 which could provide an early detection mechanism for heart attacks and strokes.
The development of application-based intelligent photonics systems and adaptive optics has enabled their use in new areas. We have designed two new adaptive measurement techniques that both exhibit unique advantages. First, we use adaptive lenses for confocal microscopy with an agile scanner that does not involve mechanical movements. In the second technique we use an adaptive mirror for interferometric velocity measurements. This intelligently programmable photonic system can be used to make precise measurements through disturbed fluid boundaries. We have harnessed the power of various programmable photonics devices (computational optics) with both measurement techniques. Our work represents a paradigm shift from static to dynamic optical elements.
In conventional confocal microscopy, the axial measurement position is scanned mechanically. However, lenses that can be tuned electrically are now available and they present an opportunity to make scans without mechanical movements. Systems such as ours (see Figure 1)2 that include these lenses can be made smaller, lighter, and with higher tuning speeds and flexibility. To get depth information with our system we tune the axial measurement position by applying a voltage to a piezoelectric actuator.3 We have corrected the hysteresis of the piezoelectric technique by using a lens-integrated pressure sensor, which allows us to control the focal length accurately, and to increase the reliability of the lens and the microscope as a whole. We combine lenses that can be tuned electrically with lenses of fixed focal length to guarantee high-resolution measurements with our novel confocal microscope. Some example measurements are shown in Figure 2. The capabilities of our microscope are illustrated in these images, where test chart structures—500nm in size—can observed. The use of electrically tunable lenses in confocal microscopy offers great potential for the creation of hand-held devices that are fast, mobile, and agile.
Our second novel technique was designed to overcome a shortcoming of existing laser interferometers in fluid dynamics applications.4 Transmitted laser beams can be distorted due to the fluctuating interfaces between media with different refractive indices. Temporal fluctuations of these distortions from jet flows and thin-film flows cause deterioration of laser interferometers and result in increased measurement uncertainties, or inability to make successful measurements. With our adaptive interferometry technique it is possible to make fluid flow velocity measurements behind a rapidly fluctuating phase boundary, as shown in Figure 3.
The adaptive optics we use are based on a low-order distortion correction system, which consists of a Hartmann-Shack camera, a PC, and a Flexible Optical B.V. OKOTech microelectromechanical system (MEMS) light modulator. This modulator is constructed from a 17-electrode deformable mirror that is mounted on a two-axis piezoelectric tip-and-tilt stage. We are therefore able to achieve both beam stabilization and wavefront shape control to compensate for the dynamic optical distortions that are mostly induced by capillary-wave fluctuations at the gas-liquid phase boundary. Our new methodology provides an opportunity to make several types of optical flow measurements in complex environments, such as thin-film convection processes in turbo machines.
We have used various programmable photonics devices to design and develop two new smart laser measurement techniques. The speed and agility of our confocal microscopy technique can be improved for biomedical and other applications. Our next step will be to reduce the aberrations of the adaptive lenses that we use, which will improve the imaging quality of the microscope. Our adaptive interferometer can be used to make flow velocity measurements through fluctuating fluid interfaces. This technique is valuable for understanding fundamental convection processes in cooling liquid films. We plan to improve the interferometer by dynamically correcting the large stroke aberrations. Both of our adaptive measurement systems have a minimalistic design and employ only a few agile components. We expect adaptive optics to now be used in other measurement applications as the technologies and methodologies involved continue to improve.
Juergen Czarske is a full professor and the head of the laboratory for measurement techniques. He studied electrical engineering and physics at University of Hannover, Germany, and completed his PhD in the field of laser measurement systems in 1995.