Abstract:We have combined Fourier-domain optical coherence tomography (FD-OCT) with a closed-loop adaptive optics (AO) system using a Hartmann-Shack wavefront sensor and a bimorph deformable mirror. The adaptive optics system measures and corrects the wavefront aberration of the human eye for improved lateral resolution (~4

The static and dynamic characteristics of a bimorph deformable mirror (DM) for use in an adaptive optics system are described. The DM is a 35-actuator device composed of two disks of lead magnesium niobate (PMN), an electrostrictive ceramic that produces a mechanical strain in response to an imposed electric field. A custom stroboscopic phase-shifting interferometer was developed to measure the deformation of the mirror in response to applied voltage. The ability of the mirror to replicate optical aberrations described by the Zernike polynomials was tested as a measure of the mirror's static performance. The natural frequencies of the DM were measured up to 20 kHz using both stroboscopic interferometry as well as a commercial laser Doppler vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model.

The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.

We built a Fourier domain optical coherence tomography (FD-OCT) system using a line scan CCD camera that allows real time data display and acquisition. This instrument is able to produce 2D B-scans as well as 3D data sets with human subjects in vivo in clinical settings. In this paper we analyze the influence of varying exposure times of the CCD detector on image quality. Sensitivity values derived from theoretical predictions have been compared with measurements (obtained with mirrors and neutral density filters placed in both interferometer arms). The results of these experiments, discussion about differences between sensitivity values, potential sources of discrepancies, and recommendations for optimal exposure times will be described in this paper. A short discussion of observed artifacts as well as possible ways to remove them is presented. The influence of relative retinal position with respect to reference mirror position will also be described.

This paper reports the results of interferometric characterization of a bimorph deformable mirror (DM) designed for use in an adaptive optics (AO) system. The natural frequencies of this DM were measured up to 20 kHz using both a custom stroboscopic phase-shifting interferometer as well as a commercial Laser Doppler Vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically-supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model. INTRODUCTIONOriginally developed to remove atmospheric distortion from astronomical imaging systems, adaptive optics (AO) has seen more recent application to ophthalmologic instruments and free-space optical communication systems. In each of these applications, the AO system uses a deformable mirror (DM) to correct for optical aberrations by removing phase distortions from the incident wavefront. Since the existing DM technology developed for astronomy is expensive and bulky, recent research has focused on using MEMS technology to create a more compact, low-cost DM. Several MEMS DM designs have been demonstrated, including: membrane-based (OKO Technologies Inc.) [1]; polysilicon surface-micromachined (Boston Micromachines Inc.) [2]; bulk silicon (Iris AO Inc.) [3]; and piezoelectric monomorphs (JPL) [4]. Many MEMS DM designs have been driven by the motivation to produce DMs with hundreds or thousands of actuators. For applications which require the correction of only low-order aberrations (such as defocus, astigmatism, coma, and spherical aberration), a DM with less than 100 actuators may be the best choice, as