High-throughput microscopy in the sense of large areas imaged at high-resolution demands costly hardware such as objective lenses with high numerical aperture and high sensitivity cameras, typically combined with lateral mechanical scanning of the sample. The field of view and the resolution of an imaging system depend strongly on the applied objective lens, with higher resolution coming at the cost of a smaller field of view. To address this limitation of conventional microscopes, both aperture synthesis and phase retrieval techniques are combined in the recent computational imaging approach of Fourier Ptychographic Microscopy (FPM). Gigapixel space-bandwidth product of FPM is obtained by combining low-resolution images obtained with illumination diversity through phase retrieval, which is facilitated by ensuring that the input images overlap in the Fourier domain. In practice, the illumination is achieved using one lamp at a time from an LED array. A drawback of FPM is that it requires long acquisition times and has significant computational cost. Here, we present a refined FPM procedure by using Fresnel propagation and reducing the number of exposures by multiplexing and symmetry considerations, thus slashing the amount of data and the processing time. The multiplexing strategy works by illuminating groups of three LEDs that are chosen from one-half plane of the LED array – an approach valid for pure amplitude samples. We have experimentally demonstrated that the FPM recovered image has approximately the same resolution as recovery based on one exposure from each of the LEDs.
Piezoelectrically actuated microelectromechanical systems (MEMS) lens structures can be composed of a clamped square elastic diaphragm partially covered with a thin piezoelectric film leaving a circular transparent region to form a lens pupil. To model these lenses’ linear static optoelectromechanical performance, the displacement can be approximated by a linear combination of basis functions, e.g., weighted Gegenbauer polynomials that satisfy clamped boundary conditions along the diaphragm edges. However, such a model needs as much as 120 degrees of freedom (DOFs) to provide a good approximation of the lens optical performance. To improve on this, we here consider approximating the deflection by an expansion using piecewise smooth functions that have different forms in the pupil and the actuator regions. We use exact solutions for the elastic plate differential equation over circular and annular subdomains, and weighted Gegenbauer polynomials in the remaining region. The latter enforces the boundary conditions. We have found that the larger the diaphragm area with exact plate solutions is, the lower is the number of DOFs needed to predict mechanical and optical quantities accurately. For example, a model with 10 DOFs achieves accuracies of 5.1% and 2.1% for RMS wavefront error and reciprocal F-number, respectively, for all pupil openings of interest.
Shifted superimposition is a resolution-enhancement method that has gained popularity in the projector industry the last couple of years. This method consists of shifting every other projected frame spatially with subpixel precision, and by doing so creating a new pixel grid on the projected surface with smaller effective pixel pitch. There is still an open question of how well this technique performs in comparison with the native resolution, and how high the effective resolution gain really is. To help investigate these questions, we have developed a framework for simulating different superimposition methods over different image contents, and evaluate the result using several image quality metrics (IQMs). We have also performed a subjective experiment with observers who rate the simulated image content, and calculated the correlation between the subjective results and the IQMs. We found that the visual information fidelity metric is the most suitable to evaluate natural superimposed images when subjective match is desired. However, this metric does not detect the distortion in synthetic images. The multiscale structural similarity metric which is based on the analysis of image structure is better at detecting this distortion.
A square geometry for the diaphragm of microelectromechanical systems (MEMS) tunable lenses with circular pupils is often used. This square diaphragm introduces aberrations that would not be present for a circular diaphragm and the question arises of whether it could be advantageous to also consider other than circular geometries for the pupil. To investigate this question, we have extended a previously established modeling framework for piezoelectrically actuated MEMS tunable lenses to devices with general polygonal-shaped pupils. It models the static optoelectromechanical coupling for symmetric configurations based on laminated-plate theory, linear piezoelectricity, and ray tracing. The framework helps to find geometrical parameters that give a diffraction-limited tunable lens with a minimum F-number. The tunable lens’ optical performance and its focusing capability, alone and in combination with a fixed lens, were calculated in terms of object distances and actuation voltages. Using the modeling framework, we show that the modulation transfer function of the tunable lens and the fixed lens combination remains the same up to a ±10-deg field of view after voltage adjustment to refocus on near objects. In addition, we found that pupil masking of the tunable lens can provide a beneficial tradeoff between the lens dioptric power and its RMS wavefront error.
We use double reversing wavefront Michelson interferometer to measure the spatial coherence properties of high power broad area Fabry Perot edge emitting blue laser. The measurements are done at different bias current, from lasing threshold up to 1A. The laser is driven in both continuous-wave, CW and pulsed-wave modes. It is found that the magnitude of complex degree of coherence decrease when the laser is driven in pulsed mode. Among driving currents for each mode just for 700 mA bias current more uniform and rather visible interference fringes attained.
We investigated the speckle contrast reduction characterization of high power broad-area edge-emitting red and blue
laser. The speckle contrast is measured with different pulse driving conditions. It can be observed from the measurement
that the speckle contrast reduces with the increasing of pulse duration and pulse amplitude. The highest speckle contrast
reduction that can be achieved with the red and the blue lasers is 27.9% and 10.4% respectively. The reduction of
speckle contrast is due to the shift of emission wavelength and the incoherence of the laser beam at high driving current.
A speckle reduction apparatus is proposed by using an optical interferometer to introduce temporally changing
interference fringes. With the help of the vibration of a mirror, diverse interference fringes and speckle patterns are
added together in the intensity basis during the exposure time of the CCD camera, which result in a summed speckle
image having lower speckle contrast. Experimentally, we have demonstrated the speckle reduction efficiency by this
method, and compared with another approach by only using the modulation beam. The obtained speckle contrast is 0.66
after using both the modulation beam and the reference beam, which is lower than the 0.77 speckle contrast by only
using the modulation beam. We conclude that the introduction of interference fringes helps the speckle reduction.
This paper describes a novel idea for reduction of speckle contrast in laser display projectors using the rotation of a diffraction pattern whose zeroth order has been canceled out without loosing power. The feasibility of the proposed method was investigated by illuminating gratings with a sinusoidal phase on two spatial light modulators (SLMs) in series for minimal intensity modulation, where the phase grating pattern was rotated with respect to the previous one on both SLMs. Two series of measurements were done with different periods of the sinusoidal grating. For each series, an image of the speckle pattern was recorded at discrete rotation angles of the phase grating, and then an average image was calculated. Experimental results were compared with a new theoretical model for speckle contrast of N partially correlated speckle patterns. The experimental measurement results compare well with the theoretical predictions resulting in a minimum speckle contrast of 0.36, with further reduction possible. Parameters necessary to achieve target contrast (0.08 or less) are discussed.
Speckle suppression in projection displays with the laser light source can be achieved by imaging a changing diffuser
with random phase cells onto the screen. Theoretical expressions for the speckle contrast in this method have been earlier
obtained in the case when different realizations of the phase diffuser produce statistically independent patterns of the
light field on the screen [J. W. Goodman]. In the present paper, these expressions are generalized in the case when
different realizations of the phase diffuser produce partly correlated speckle patterns. Both cases when the diffuser just
fills or overfills the projection optics are considered. Possible structure of a motionless changing diffuser is presented. It
includes the dynamic diffractive optical element (DDOE) and a light homogenizer. The DDOE can be based on the
electrically controlled spatial light modulator with a deformable polymer layer (SLMDPL). The SLMDPL can handle
high light power and therefore, can be used in projection displays with powerful laser beams.
It has been suggested to use Hadamard matrices H(M) of order M for speckle reduction in laser based projection displays
by creating a set of M two-dimensional phase masks from rows or columns of the H(M) and introducing them sequentially into the intermediate image plane of the laser projector. The speckle contrast reduction can reach M-1/2. In
this paper, we have analyzed the contrast reduction. The result presents that any matrices can be used to form phase mask
as long as its columns are orthogonal to each other, such as the parts of columns of Hadamard matrix. The speckle contrast reduction is determined by the number of projection resolution elements lying in single camera resolution
element. To get high quality image with low speckle contrast reduction by Hadamard matrix, its order should be as high
as possible. However, it is impossible to implement by vibrating diffuser with high order due to the large vibration
amplitude. The motionless time-vary diffuser with Hadamard matrix phase pattern based on MEMS technology and
Electro-optical effect can be a good choice.
We report a polymer based multiple diffraction modulator, in which PDMS (polydimethylsiloxane) is utilized as the
actuation material, for speckle reduction. The properties of the PDMS are characterized based on its response time and
deformability, which are the key properties concerned in this work. The structure dependent properties of PDMS are
discussed. Using the described technique, the PDMS satisfy the system demand.
The modulator is used to create real-time diffraction patterns by dynamic gratings formed by flexible PDMS. The
diffracted light passes through a diffuser, which is placed after the modulator, and induces speckle patterns on the screen.
Speckle-reduction is achieved by adding the time-varying speckle patterns in the integration time of the detector. It is
observed that using the modulator which has two gratings, the speckle contrast ratio reaches to 50%, which shows fair
agreement with the simulation.
Barker and Barker-like binary phase codes have previously been used for speckle reduction in line-scan laser
projectors, and a speckle contrast factor decreased down to 6% has been theoretically achieved assuming ideal
imaging conditions. In this article, it is shown by theoretical simulations that the speckle reduction performance
of the the binary phase codes is adversely affected by the finite numerical aperture and the aberrations of the
projections lens. A minimum numerical aperture of the projection lens is needed to faithfully reproduce the
binary phase code placed at an intermediate image plane onto the final display screen. The effects of the cell
dimension of the binary phase code are also considerd in simulations.
An array of diffraction gratings and a Random Phase Plate (RPP) are used to suppress laser speckle effect. Dynamic
diffraction spots are generated on the surface of the RPP, after which the scattering lights are perceived by a detector.
Speckle Contrast Ratio (CR) and Number of Independent Speckle Patterns (NISP) with different gratings rotation
orientations (θ), gratings frequencies (grooves per millimeter: f), diameters of laser beam (D), and distances between the array of diffraction gratings and the RPP (Z) are calculated based on ZEMAX simulations, and an optimized model is proposed.
Barker binary phase code of maximum length 13 has previously been used for speckle reduction in line-scan laser
projectors, and a speckle contrast factor decreased down to 13% has been achieved. In this article, Barker-like
binary phase codes of length longer than 13 are used at an intermediate image plane. It is shown by theoretical
calculation that much better speckle reduction with speckle contrast factor up to 6% can be achieved by using
longer binary phase codes other than the Barker code. Preliminary experimental results are also presented
indictaing good speckle reduction.
Speckle is the constructive and destructive interference pattern observed from an optically
rough surface when a highly coherent laser light is used as the illumination source. In this article, we
present some methods to reduce the undesired laser speckle in laser display projector and show
measurement results indicating reduction in speckle contrast obtained by using a dynamical flexible
polymer-based diffraction grating phase modulator. Light is diffracted into multiple orders by the dynamic
polymer diffraction grating. This diffracted light, after collection and homogenization, is used as the
illumination source for displaying picture information. Due to the time-varying phase of the diffraction
grating, the diffraction pattern changes thus creating time-varying speckle patterns on the rough screen. The
time-varying speckle is captured by a CCD camera with appropriate integration time to mimic the
averaging action of the human eye. Thus the time-integrated speckle is minimized by the intensity
averaging of many independent speckle patterns. Different configurations of the diffraction grating along
with random phase plate and beam combining lens were explored to ascertain the corresponding reduction
in speckle contrast and its dependence on the time-period of the driving voltage of the diffraction grating.
We report on device properties of tunable spatial light modulators for high-resolution optical applications by a novel
fabrication process. Thin polydimethylsiloxane (PDMS) films (4ìm-13ìm) were sandwiched between a flexible gold
film(50nm) and a rigid substrate with a comb-like electrode either by compression molding or spin coating. By applying
voltage between the upper gold film and underlying electrode, the initial plane PDMS surface changes into a form of
grating. Far-field scattering pattern with high order light components was observed by illumination at the continuously
reflective gold film with laser beam. Characterization was done by measuring the grating profile of the PDMS and the
response time. The PDMS deformation was demonstrated to increase with driving voltage. The deformation for 6ìm
thick PDMS is measured around 100nm when driving voltage is applied as 230V. Modeling and simulation of the
modulator electro-mechanical behavior was done for varies structure design. The simulation results showed fair
agreement with the experimental results. The response time, which defines how fast the PDMS response to the applied
voltage, was measured as a function of the driving voltage. The measured rise time is around 1 micorseconds and the fall
time is around 0.2 microseconds.
Multiple streams of high definition television (HDTV) and improved home-working infrastructure are currently driving
forces for potential fiber to the home (FTTH) customers . There is an interest to reduce the cost and physical size of
the FTTH equipment. The current fabrication methods have reached a cost minimum. We have addressed the costchallenge
by developing 1310/(1490)/1550nm bidirectional diplexers, by monolithic seamless integration of lasers,
photodiodes and wavelength division multiplexing (WDM) couplers into one single InP-based device. A 250nm wide
optical gain profile covers the spectrum from 1310 to 1550nm and is the principal building block. The device fabrication
is basically based on the established configuration of using split-contacts on continuos waveguides. Optical and electrical
cross-talks are further addressed by using a Y-configuration to physically separate the components from each other and
avoid inline configurations such as when the incoming signal travels through the laser component or vice versa. By the
eliminated butt-joint interfaces which can reflect light between components or be a current leakage path and by leaving
optically absorbing (unpumped active) material to surround the components to absorb spontaneous emission and nonintentional
reflections the devices are optically and electrically isolated from each other. Ridge waveguides (RWG) form
the waveguides and which also maintain the absorbing material between them. The WDM functionality is designed for a
large optical bandwidth complying with the wide spectral range in FTTH applications and also reducing the polarization
dependence of the WDM-coupler. Lasing is achieved by forming facet-free, λ/4-shifted, DFB (distributed feedback
laser) lasers emitting directly into the waveguide. The photodiodes are waveguide photo-diodes (WGPD). Our seamless
technology is also able to array the single channel diplexers to 4 to 12 channel diplexer arrays with 250μm fiber port
waveguide spacing to comply with fiber optic ribbons. This is an important feature in central office applications were
small physical space is important.
In this paper, the complete design and modeling of a dual-magnification 8 - 12 μm infrared Galilean telescope along with the associated focusing optics and scanning mechanism is described. Preliminary calculations are done to determine the first-order parameters of the narrow and the wide-field modes. Initially, the telescope (narrow and wide-angle modes) and the focusing optics are designed as individual subsystems. Later, they are combined together and the complete optical system is modelled and optimized as one unit to get the required final performance. It is shown that this final optimization step is very important to access the overall performance of the complete system. We can actually reduce the total number of lenses or the number of conic surfaces used in the final system by modelling and optimizing it as one unit. This reduces the cost and the complexity of the overall system. The final optical designs along with their aberrations curves and MTF plots are presented showing excellent performance in both the high and the low magnification modes.
In this paper, the design of a dual field-of-view telescope for 8 - 12 μm imaging waveband is described. Preliminary calculations are done to determine the first-order parameters of the narrow and wide-field modes. To achieve a switchable dual field-of-view system, an optical configuration based on the axial motion of a single lens group along the optical axis is used. The same lens is also used for focusing at near objects and for athermalization by very small axial movement. A total of 6 lenses with one conic surface are used in the design making it cost effective and lightweight. The final optical design is presented along with the aberrations curves and the MTF plots showing excellent performance in both field-of-views.
In this paper, the design of a dual field-of-view optical system for 3-5 μm infra-red focal-plane arrays is described. Preliminary calculations are done to determine the first-order parameters of the narrow and the wide-field modes. To achieve a switchable dual field-of-view system, two different optical configurations, one based on the axial motion of a lens group and the other based on a roate-in motion of two separated lens groups, are studied and compared. Diffractive and conic surfaces are used to control the color and the monochromatic aberrations with less number of total lenses used. Paraxial and real-ray modelling of the Narcissus effect is described. It is shown that the rotate-in scheme achieves better optical performance in both the narrow and the wide-fifeld modes. The axial-motion scheme suffers from poor lateral color in the wide-angle mode. The final optical designs along with the aberrations curves and MTF plots are presented showing excellent performance.
In this paper, the design of a dual field-of-view optical system for 3-5 μm infra-red focal-plane arrays is described. Preliminary calculations are done to determine the first-order parameters of the narrow and the wide-field modes. To achieve a switchable dual field-of-view system, two different optical configurations, one based on the axial motion of a lens group and the other based on a rotate-in motion of two separated lens groups, are studied and compared. Diffractive and conic surfaces are used to control the color and the monochromatic aberrations with less number of total lenses used. Paraxial and real-ray modeling of the Narcissus effect is described. It is shown that the rotate- in scheme achieves better optical performance in both the narrow and the wide-field modes. The axial-motion scheme suffers from poor lateral color in the wide-angle mode. The final optical designs along with their aberrations curves and MTF plots are presented showing excellent performance.