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ANALYSIS OF MEMS DEFORMABLE MIRROR TECHNOLOGY Chandran VinothKumar - A0065626B
Department of Electrical and Computer Engineering, NUS, Singapore
Adaptive Optics (AO) technology originally invented for improving astronomical imaging have found many other applications like retinal imaging, microscopy, laser communication, defence, etc. over the decade. MEMS Deformable Mirrors (DM) are the most commonly used wavefront corrector in most of AO applications due to their versatility, the high resolution correction of wavefront aberrations and advanced technology. The imaging application plays a major role in advancement in deformable mirror technology. The need for high resolution images demands for very advanced AO enabled imaging systems. Retinal imaging is the major system currently using AO technology with it. AO technology holds promise to the doctors to change the way they diagnose disease. Recent research found that, retinal imaging technology is to predict hypertension, diabetes, stroke, heart disease and other risk factors. In this paper, the use of DM in retinal imaging has been demonstrated first. Further different possible DM configurations have been discussed. Also the scope of DM in other types of imaging techniques and its future direction has been discussed.
Keywords - Mems, deformable mirror, adaptive optics, retinal imaging, wavefront correction.
Leading vision scientists believe that the retina of human eye will be a window for human health for someday. The capability to resolve photoreceptors or individual retina cells and ocular blood flow using microscopic view will allow scientists to diagnose changes in patient health. This gives promise to detect, diagnose, and treat the major eye diseases such as diabetic retinopathy, glaucoma, and age related macular degeneration noninvasively. An ultra high resolution image of the human retina has not been achieved since due to the imperfections of the eye within itself, leading to wavefront aberrations.
AO system corrects the wavefront distortions caused by the cornea and crystalline lens and provides high contrast levels and retinal resolution levels. The two major techniques for retinal imaging using AO are confocal scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT). The working principle of Confocal SLO is, when laser light is focussed on the retina, an image is created using scanning. The system without AO produces resolution levels in the range of 5-10 µm scale and this resolution is not sufficient to detect cells of about 3 µm. AO retinal imaging can give 1 µm resolution levels. In OCT, an interferometric imaging technique is used which creates 3D images.
2. Typical Retinal Imaging System
The most commonly used retinal imaging system is Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO). This operates under the principle of confocal microscopy, where eye is used as the objective lens and retina is taken to be the sample.
2.1 Adaptive Optics
Adaptive Optics (AO)is an optical system whichadaptsto compensate for optical effects (normally wavefront aberrations) introduced by the medium between the object and its image. Here AO measures and compensates for the blur (or say aberrations) in an optical imaging system. Fig.1 shows typical adaptive optics system which consists of three main components - a wavefront sensor to measure distortion, a wavefront corrector (MEMS DM) to compensate for the distortion and control system to calculate the correction needed and necessary signal to apply to the wavefront corrector to get proper shape.
2.1.1. Wavefront Sensor: Hartmann-Shack wavefront sensor is normally used to measure the wavefront aberrations of the human eye. This sensor provides a fast, precise, and objective measurement of the wavefront aberrations of the eye. The information from this sensor is very much essential for deformable mirror to correct the wavefront errors.
A point source (light) is imaged on the retina, and the reflected wavefront is observed by a wavefront sensor which uses lenslet array conjugate to the pupil plane of the eye. Depending on the aberrated wavefront, the positions of the spots taken by the lenslet array are displaced. The deviations of the spots are reconstructed to the aberrated wavefront.
2.1.2. Control System: The aberrated wavefront from wavefront sensor is given as input to the control system. This system compares the measured wavefront with that of required wavefront without aberrations and the difference between the two wave fronts is the error signal for compensation. This error signal is converted into electrical signals to operate wavefront corrector i.e. deformable mirror.
2.1.3. Wavefront Corrector - Deformable Mirror (DM): Mems DMs are one of the advanced wavefront control devices that consist of a mirror membrane with an underlying actuator array. Each actuator in the array can be deflected independently by piezo-electric or electrostatic actuation in order to obtain the desired pattern of deformation of the mirror membrane. Electrostatic actuation-free mirror deformation but still other types of actuators also in use.
The optics from retina is aberrated due to internal optics problem. So a deformable mirror usually deformed in such a way that output from deformable mirror has no aberrations providing high resolution image of the retina.
The actuation signals are given from control system so as to provide proper deformation of mirror to obtain aberration free wavefront as output. The system now becomes closed loop one since the output from DM is again fed back to the wavefront sensor. Thus the closed loop system maintains constant aberration free output. The output from DM is fed to the high definition camera to image the sample (retina).
220.127.116.11. Deformable Mirror Parameters:
Number of actuatorsdetermines the number of degrees of freedom i.e. wavefrontinflections that the mirror can correct.
Actuator pitch is the distance between the two actuator centres. DM with large actuator pitch along with large number of actuators seems to become expensive and expensive.
Actuator stroke is the possible displacement of the actuator at the maximum which will be positive or negative values with respect to the central null position. An actuator stroke typically ranges from ±1 to ±10.
Actuator couplingis the indication of disturbance caused by movement of one of the actuator with its neighbours.
Hysteresisis the nonlinear effects of actuator which affects the precision of the response time of the DM. The hysteresis normally varies from practically zero (for electrostatic actuated mirrors) to tens of percent for mirrors (for piezoelectric actuators). Hysteresis is defined as the positional error compared from previous position commands of actuator and hence this factor has to be limited for better operation of AO.
Response timeis the indication of how fast the mirror will react towards the control signal and it is normally in the range of microseconds.
3. Root of Deformable mirror
The major use of Mems DM is in adaptive optics systems. The root ofthis AOtechnology is astronomy field. It was introduced in 1950s to improve the astronomical images by correcting the atmospheric aberrations. Today, all of ground based telescopes around the world are using this AO system along with deformable mirrors which are providing reliable high resolution images of the target. MEMS DMs are used as wavefront corrector in telescopes and have resulted in 2-3X gains of resolution. Now the major astronomy related research projects are using this Mems DMs as wavefront correctors.
4. MEMS Deformable Mirror - Configurations
Mems DM have five different configurations which are continuous facesheet, segmented facesheet, membrane, bimorph and ferrofluidic.
Segmented DM is formed by independent flat mirror segments (Fig. 4b). Each of the mirror segments can move a small distance to back and forth with respect to the wavefront correction signals. These individual mirrors have zero or very low cross-talk in between the actuators. But stepwise approximation not works well for smooth continuous wavefronts. And it is seen from fig. 4b, that gaps between sharp edges of the segments and the segments results in light scattering, such as limiting the applications of this type to those not sensitive to scattered light. An improvement in the performance of segmented DM can be made by insisting three degrees of freedom per segment. But these mirrors require three times more actuators than that of piston based segmented mirrors. As shown in fig 4, local influence of deflection on its neighbouring actuators is ~20% of maximum stroke for continuous facesheet type and 0% of maximum stroke for a segmented DM. The segmented mirror was normally used for fabrication of largely segmented primary mirrors for thetelescopes.
Continuous DM is formed by single thin deformable membrane. The shape of the membrane is varied by discrete actuators which are fixed in its back side. The deformation or shape of the mirror depends on the forces applied to the plate supporting the mirror, boundary conditions (way the plate is fixed to the mirror) and the material of the plate. This type of mirror is taken to be the best since this provides smooth wavefront control with very large degrees of freedom.
Membrane based mirrors are made by a thin conductive as well as reflective type membrane stretched on solid flat frame. Normally the membrane uses electrostatic based deformation by applying voltages to the electrode of the actuator which can be placed over the membrane.
Bimorph conceptmirrors are made by two or more layers of different forms of materials. These layers are fabricated from an electrostrictive or piezoelectric material. When a voltage is applied to its electrodes, the mirror gets deformed letting them to expand laterally, which in turn provides local mirror curvature.
Ferrofluidconceptmirrors are new concept of DM which is actually a liquid deformable mirrors made with small (10 nm in dia) ferromagnetic nanoparticles which are dispersed in a liquid carrier. On the application of external magnetic field, these ferromagnetic nanoparticles gets aligned with that of field, now the liquid becomes magnetized and the liquid surface gets a shape (as shown in fig. 5) due to the equilibrium condition between the magnetic, surface tension and gravitational forces.
5. MEMS Deformable Mirror - Actuators
Currently, Mems DMs are mainly actuated piezoelectrically, electrostatically or electromagnetically. Since piezoelectric actuated mirrors have the drawbacks of hysteresis and large size, bimorph actuated mirrors have drawbacks of slow response and therefore hysteresis-free electrostatically actuated mirrors become the most interesting and attractive ones. Many developed electrostatic DMs are based on parallel plate actuator type, and the only disadvantage of this type of actuator is the existence of pull in instability. This gives limits to the actuator's stroke to 1/3 of the separation between the plates (or gap) and this allows the use of other type of actuators too.
5.1. Electrostatic actuation:
The Mems DM with electrostatic actuator (fig. 6) consists of actuator electrodes under a double cantilever flexure which is isolated electrically from of the electrodes and it is maintained at ground potential. The electrostatic actuator supports the flexible facesheet mirror through an attachment post at the centre of each actuator. This post is responsible for translation of actuator motion to a deformation of mirror surface. As shown in fig.6 it is evident to see that the single actuator deflection influences only its near neighbours. And it does not cause deformations on the entire aperture as like in membrane concept mirrors. Likewise, high order aberrations present in the optical path can be corrected by using these DMs. Fig.7shows the surface measurements of DM with single actuator deflected and with a pattern of actuators deflected.
Electrostatics principle is used in order to achieve deformation of mirror at each point of actuation using an actuator, as shown in Fig. 8. The actuator has an initial gap as "g", between the flexure and the electrode.
With the application of potential, V, between electrode and flexure, an attractive electrostatic force created which bends the actuator membrane to downward direction. As the flexure bends, elastic (or mechanical) restoring force acts in the opposite direction. These two forces balance at equilibrium and the deflection at the membrane midspan in this condition is z. The equilibrium deflection is actually a nonlinear increasing function of V as shown in fig. 9. The equilibrium is stable until the voltage is raised to a point where the equilibrium deflection is equal to approximately ½ of the initial gap (g). Above that voltage, electrostatic forces are so large, for which they cannot be balanced by mechanical restoring forces, and so the actuator membrane crashes unstably with that of the fixed electrode at bottom. This unstable region should be avoided generally in practical.
5.2. Piezoelectric Actuation:
The piezoelectric actuated Mems DM consists of a continuous facesheet mirror in which piezoelectric PZT films are deposited on its backside. The piezoelectric actuator array is a unimorph structure of PZT films and a Si device layer. The bottom electrode is taken to be the common electrode at the interface Si and PZT film. And 19 individual electrodes are placed on PZT. The individual electrodes are made out as concentric circles of segments as shown in fig. 9(a). The voltage is applied to the film which provides a transverse strain and this converted into vertical deflection with respect to the Si elastic layer restriction. Both concave and convex deformations will be obtained by changing the polarity of the voltage applied. When compared to the conventional electrostatic Mems DMs as explained before, the thickness of the membrane of piezo type is greater since because piezoelectric force is normally larger than the electrostatic force. Thus we can say that the
(a) Plane view of the piezoelectric actuator array
(b) Displacement profiles of the actuators
piezoelectric DM is more stable even in the presence of disturbance and noise from vibrations (external). The displacement profiles are examined on the actuators 1, 2 and 3 as shown in fig. 9(b).
This proposed piezoelectric actuated Mems DM shows that hysteresis effects can be reduced by low application voltage, but still not compared to hysteresis free electrostatic actuator.
5.3. Electromagnetic Actuation:
In this type electromagnetic energy is used to deform the DM. It can be used where severe aberrations need to be corrected. This type of actuator have good optical quality, high linearity and lack of hysteresis (but not like electrostatic), wavelength and intensity independence, which makes it suitable for wide range of applications
6. MEMS Deformable Mirror Future Direction
Due to the wide application area for the use of AO, Mems DM is expected to grow more rapidly and will find more applications in future. Some of the possible applications are discussed here.
6.1. Hard X-ray Focusing:
Hard X-rays exceptional properties make its use in chemical, elemental and structure analysis of matter. Although 1nm resolutions in various hard X-ray methods are theoretically possible, the fabrication of focusing optics for such systems remains the main hurdle. Optical systems used inside the microscopes owe to imperfections caused during fabrication and this will degrade the quality of the focused beam. Therefore, the use of AO comes into picture to get 1nm resolution images. The system uses same components as shown in fig. 1 with Mems DM as aberration corrector.
The surface profile of the DM is deformed (fig. 10) in order to compensate for the wavefront error which has been measured previously by wavefront sensor and control system. An ideally focused beam is produced finally and hence getting high resolution imaging.
6.2. Optical Communication:
Free-space communication technology has an exciting potential since a new method of data transmission has been proposed without wires or fibres. In this system, data is actually transmitted in the form of laser light in the atmosphere. A small number of commercial system provide this method, but with a limited range of 1km. When the laser beam of data has been sent over longer distances, atmospheric distortions begin to affect the laser beam, thus producing a data loss.So an AO system with Mems DM is to be used to compensate for atmospheric distortions, by providing a high-speed transmission, error-free data and long range data link.
6.3. Laser Pulse Shaping:
Laser pulse shaping applications usually use liquid crystal modulators (LCMs), to convert optical spread into optical pulses with short timing (femto second). Mems DM has been found to be the best alternative for LCMs due to several advantages over LCMs, such as higher optical efficiency, wavelength independence and cost.
6.4. Space applications:
Rather than ground based telescopes, Mems DM can also be used inspace based high-contrast imaging systems for wavefront control, but this application needs high actuator count for high definition images. But the growth of fabrication technology promises to this application soon.
The future directions can be more than indicated since there is no limitation for Mems DM due to its versatility for wide range of applications.
Thus it has been studied that Mems DM are the most effective part as wavefront correction in most of the AO applications. It has been shown the use of DM in retinal imaging system (i.e. AOSLO) and its use in getting high quality image of human eye. Also the scope of DM in other types of imaging techniques and its future direction has been discussed. The different parameters and different configurations of DM commercially used nowadays have been discussed. Also the different type of actuators used for DM has been studied along with their pros and cons. The analysis proves that there are wide areas for its application and it has been studied that still fabrication improvements have to be taken for high actuator array DMs.
1. "A MEMS micromirror driven by electrostatic force", Fangrong Hu; Jun Yao; Chuankai Qiu; Hao Ren; Journal of Electrostatics, 2010.
2. "Retina imaging in vivo with the adaptive optics confocal scanning laser ophthalmoscope", Jing Lu; Hao Li; Ling Wei; Guohua Shi; Yudong Zhang; Proceedings of SPIE, 2009.
3. "A 4096 Element Continuous Facesheet MEMS Deformable Mirror for High-Contrast Imaging", S.A. Cornelissen; P.A. Bierden1; T.G. Bifano; Proceedings of SPIE, 2008.
4. "Development of Deformable Mirror Composed of Piezoelectric Thin Films for Adaptive Optics", Isaku Kanno; Takaaki Kunisawa; Takaaki Suzuki; Hidetoshi Kotera; IEEE Journal, 2007.