Many imaging techniques require highly sensitive optical systems including detectors capable of measuring
extremely small fluctuations in the detected incident light. Such systems use a modulated light source (at
frequencies up to 100's of kHz) in combination with optics that induce a change in the amplitude and/or phase
of the modulation in response to changes in the sample being imaged. These signals are usually demodulated
using a point detector and a lock-in amplifier. However, this technique is not suitable for the fast acquisition of
2D images.
Using a modified active pixel sensor architecture, cameras with resolutions up to 256 x 256 pixels which are
capable of demodulating optical signals with frequencies up to 1 MHz and have been designed and fabricated.
Each demodulation pixel consists of a photodiode, a reset switch, four independently controlled shutter switches
and four supplementary well-boosting capacitances that improve both linearity and signal to noise ratio. The
reset and shutter switches are implemented with 5 V thick oxide transistors to maximize the dynamic range of
the sensor. Demodulation is achieved by rapidly acquiring four images at 90 degree intervals of the modulation
period, then applying simple post processing to extract the modulation amplitude, phase, and DC level of the
optical signal. The camera outputs 16 parallel analogue channels and can deliver total pixel rates of up to 160
Mega pixels per second.
In imaging systems where demodulation is not necessary, the camera can be clocked to behave as a
conventional DC camera capable of taking four images with independent exposure periods allowing for
advanced multi-parametric imaging.
In pump-probe type experiments the signal of interest is often a very small fraction of the overall light intensity reaching
the detector. This is beyond the capabilities of conventional cameras due to the necessarily high light intensity at the
detector and its limited dynamic range. To overcome these problems, phase-sensitive or lock-in detection with a single
photodiode is generally used. In phase-sensitive detection, the pump beam is modulated and the probe beam is captured
with a photodiode connected to a lock-in amplifier running from the same reference. This provides very narrowband
detection and moves the signal away from low frequency noise. We have developed a linear array detector that can
perform shot-noise limited lock-in detection in 256 parallel channels. Each pixel has four independent wells to allow
phase-sensitive detection. The depth of each well is massively increased and can be controlled on a per-pixel basis
allowing the gain of the sensor to be matched to the incident light intensity, improving noise performance. The array
reduces the number of dimensions that need to be sequentially scanned and so greatly speeds up acquisition. Results
demonstrating spectral parallelism in pump-probe experiments are presented where the a.c. amplitude to background
ratio approaches 1 part in one million.
Many optical measurements that are subject to high levels of background illumination rely on phase sensitive lock-in
detection to extract the useful signal. If modulation is applied to the portion of the signal that contains information, lockin
detection can perform very narrowband (and hence low noise) detection at frequencies well away from noise sources
such as 1/f and instrumental drift. Lock-in detection is therefore used in many optical imaging and measurement
techniques, including optical coherence tomography, heterodyne interferometry, optoacoustic tomography and a range of
pump-probe techniques. Phase sensitive imaging is generally performed sequentially with a single photodetector and a
lock-in amplifier. However, this approach severely limits the rate of multi-dimensional image acquisition. We present a
novel linear array chip that can perform phase sensitive, shot-noise limited optical detection in up to 256 parallel
channels. This has been achieved by employing four independent wells in each pixel, and massively enhancing the
intrinsic well depth to suppress the effect of optical shot noise. Thus the array can reduce the number of dimensions that
need to be sequentially scanned and greatly speed up acquisition. Results demonstrating spatial and spectral parallelism
in pump-probe experiments are presented where the a.c. amplitude to background ratio approaches 1 part in one million.
Complementary metal oxide semiconductor (CMOS) cameras that can measure the phase and amplitude of periodically
modulated optical signals have been developed. These allow parallel lock-in imaging at up to 256 x 256 pixels resolution
without the need for slow and costly mechanical scanning. In conjunction with a differential surface plasmon resonance
(dSPR) system, spatially resolved SPR imaging has been achieved with sensitivities of better than 10 microRIU per pixel
per second. Results demonstrating the performance of modulated light cameras for dSPR imaging and high resolution
SPR microscopy are presented and discussed.
Voltage sensitive fluorescent dyes have long been used to measure physiological voltages in live cell cultures. However
dyes suffer from poor contrast and limited recording duration due to photobleaching. A photostable voltage sensitive
cellular label, such as a noble metal nanoparticle, would potentially allow for indefinite recording from neural and other
live cell cultures. Noble metals possess an inherent voltage sensitivity: their optical properties depend on their density of
free electrons, which can be modulated in an aqueous environment by charging or discharging the double layer
capacitance with an applied voltage. This manuscript contains a simple analysis of the expected voltage sensitivity using
gold nanospheres and nanoshells in both darkfield and photothermal detection modalities and concludes that high
bandwidth voltage measurement is fundamentally achievable.
A CMOS camera for a pyramid wavefront sensor (PWS) based adaptive optics system (AOS) is proposed in this paper.
The designed camera has four 32 x 32 active reset pixel arrays with a common addressing to produce synchronous
analogue outputs. The custom CMOS sensor has a layout area of 12.6 sq. mm, has a pixel fill factor of 56% and was
fabricated using Austria Micro Systems 0.35 &mgr;m C35B4 CMOS process. The sensor consumes about 56 mW of power
during the readout. The camera chip is enclosed in a box of dimensions 6 cm x 5.5 cm x 5 cm, which makes it suitable
for use in an optical system. A high speed camera reading system has been developed that can read up to 1000
frames/second. The camera exhibits a quantum efficiency of 0.24 and the estimated minimum detectable flux is 8.69
nW/mm2 on the surface of the sensor. The complete camera system detects the four pupil images produced by the PWS
and can be used in the AOS.
A monolithically integrated, high speed optical front-end for optical sensing application in standard 0.35-micron CMOS technology is reported. The proposed receiver consists of an integrated photodiode, a transimpedance amplifier, a mixer, an IF amplifier and an output buffer. By treating the n-well in standard CMOS technology as a screening terminal to block the slow photo-generated bulk carriers and interdigitizing shallow p- junctions as the active region, the integrated photodiode operates up to several gigahertz with no process modification. With multi-inductive-series peaking technique, the improved regulated cascade (RGC) transimpedance amplifier achieves an experimentally measured -3 dB bandwidth of more than 6 GHz and a transimpedance gain of 51 dB(omega), which is the fastest reported TIA in CMOS 0.35-micron technology. The 5 GHz broadband mixer produces a conversion gain of 13 dB which greatly minimizes the noise contribution from the IF amplification stage. The optical front-end of the active pixel demonstrates a -3 dB bandwidth of 4.9 GHz while consuming a current of 40 mA from 3.3 V power supply. This work presents the highest bandwidth for fully integrated CMOS optical receivers reported to date.
Extracting light that has maintained its original polarization state can be used to improve the image resolution in imaging
or localize the volume probed in spectroscopy. This paper describes polarization dependent instrumentation and
modelling methods used in the imaging and spectroscopy of scattering media. The use of integrated optical sensors in
imaging the polarization difference signal is also demonstrated.
Fluctuations in the polarization state of light scattered by ensembles of very small, randomly oriented spheroids and ellipsoids are calculated. For the regime of Gaussian single scattering, applicable to dilute particle suspensions and aerosols, it is shown how a goniometric measurement of the cross-correlation coefficient of light scattered into orthogonal polarization states by such particles could be used to estimate their mean aspect ratio, even in the presence of considerable shape polydispersity.
KEYWORDS: Sensors, Diffraction, Modulation, Phase shifts, Monte Carlo methods, Phase shift keying, Wave propagation, Light, Diffusion, Near field optics
Scalar diffraction theory is used to model the propagation of diffuse photon density waves in heavily scattering media. The principal difference between this and previous approaches is that the three classical boundary conditions have been considered. The model is shown to qualitatively demonstrate the correct trends for diffraction by planar objects embedded at different depths within the scattering medium. Quantitatively the three scalar diffraction boundary conditions are shown to provide different results in both amplitude and phase of the diffraction pattern at the detector plane. Preliminary results indicate that the Kirchoff boundary condition provides the best match to Monte Carlo simulations.
A semi-analytic diffusion model to describe frequency domain imaging through heavily scattering media has been validated by a quantitative comparison with Monte Carlo simulations and experimental results. Although the model is not an exact solution it can deal with situations where exact analysis fails. The method lends itself to modeling responses from restricted objects and deals with the fact that the object is embedded in a finite medium. The presence of inhomogeneities is accounted for by a two stage propagation of Green's functions from source to object to detector. Using a combined photon flux and photon density term at the object plane to represent the radiance provides the best match to Monte Carlo simulations.
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