Historically, adaptive or computer controlled active optical components, were used primarily in astronomy, but today adaptive optical components are increasingly being incorporated into more traditional static optical designs; greatly enhancing optical performance and functionality by transcending the traditional limitations on resolution and field of view. Recently this was demonstrated in the development of Adaptive Scanning Optical Microscopy (ASOM).
As early as 1953, active optical compensation was suggested by Babcock as a way of overcoming the inherent challenges in astronomical imaging. Turbulence or variations in the density of air creates a refractive index gradient, which changes the path length of the incoming light, resulting in amplitude and phase distortions of the wavefront. This produces shimmering bright or dark regions in the image (i.e. the star “twinkles”). For ground-based telescopes this limits the angular resolution. To compensate for this Babcock suggested the use a deformable mirror with a computer-controlled surface profile. Though the details of Babcock’s design were not used, his concept of using a mirror with a deformable surface profile was adopted into standard astronomical, and today that concept is being adopted into other applications such as ASOM and optical coherence tomography (OCT).
A Collaboration with Ben Potsaid at RPI
The original adaptive scanning optical microscope was developed in 2005 by Ben Potsaid, John Wen, and Yves Bellouard at Rensselear Polytechnic Institute (RPI; Troy, NY), and more recently a new commercial design (ASM9600) was developed through a joint RPI-Thorlabs collaboration. The true advantage of the ASM9600 is the ability to surpass one of the major limitations of traditional microscopy — the trade-off between resolution and the field of view. This new microscope design combines a fast scanning mirror (FSM) and a MEMS deformable mirror (DM) with an ensemble of computer controlled electrostatic actuators used to optimize the image in real-time, producing micron-level image resolution with a large effective field of view—up to 1250 mm2. This system was recently recognized for innovation by winning the 2007 PhAST/Laser Focus World Innovation Award.
The system shown in Figure 1 consists of a custom-designed scan lens, a fast steering mirror (FSM), a 4.4 mm by 4.4 mm DM with a 12 x 12 grid of electrostatic actuators and a CCD camera.
Figure 1. The Schematic (a) and photo (b) of ASOM (Thorlabs ASM9600)
In a traditional microscope, the field of view is small and limited by the objective lens. In order to image large samples at high resolution, the objective must be scanned across the sample (either by moving the microscope or the sample with a moving stage). In contrast, the ASM9600 uses the FSM to scan the across a large diameter objective lens. Off-axis light experiences significant wavefront aberrations from the objective lens resulting in image distortion. However, by using a MEMS DM with real-time control, the surface of the mirror is optimized to correct these wavefront distortions (see Figure 2); providing uniform resolution and diffraction-limited imaging over an extended field of view. This wavefront correction is demonstrated in Figure 3, which shows the same Air Force target imaged using a flat mirror (a) or the optimized DM (b).
Figure 2. Example deformable mirror profiles used for correcting different field positions
The DM and FSM design incorporated in the new Thorlabs’ commercial ASM900 system makes pathology screening quicker, easier, and more accurate. By providing cellular-level resolution and a large field of view, the full plane of the pathology slice can be rapidly imaged, eliminating the need for multiple scans with a moving stage, enabling high throughput slide processing. The high resolution obtained in a single scan provides facile determination of cell abnormalities for accurate clinical diagnoses.
Figure 3. An air force target images without (a) and with (b) the use of the deformable mirror. The smallest lines are separated by 2 µm.
To image a large sample area the ASM9600 uses a rapid multi-tile scanning technique which creates an image mosaic. Light is scanned across a single square aperture 720 by 540µm, and the full multi-tile image is created in less than 20 seconds. Table 1 shows the actual specifications of the ASM9600 design. The total field of view is over 1250 mm2 with a 1.5 µm resolution. This is particularly significant in that traditional “flat field” microscope designs become increasingly complex and expensive as one tries to combine high resolution with a large field of view. For very large fields with high numerical apertures, the optical complexity can approach that of a lithography projection lens.
This new ASOM technology is easily adopted for a variety of applications, from developmental biology and mobility experiments of live samples, to pathology and microelectronics. Thorlabs’ new ASM9600 has been shown to be useful instrument in both research and industrial environments. For sample applications click the links on the left side of this page.
1. Babcock, Publications of the Astronomical Society of the Pacific 65(386), (Oct., 1953)
2. Potsaid et al., Optics Express 13(17) (Aug. 22, 2005)
3. B Cense, et al., “Retinal Imaging at 850nm with Swept Source Optical Coherence Tomography and Adaptive Optics,”ARVO 2007.
4. Y. Zhang, et al., Optics Express. 14, 4380 (2006)