Roger Light

Multichannel, time-resolved picosecond laser ultrasound imaging and spectroscopy with custom complementary metal-oxide-semiconductor detector

This paper presents a multichannel, time-resolved picosecond laser ultrasound system that uses a custom complementary metal-oxide-semiconductor linear array detector. This novel sensor allows parallel phase-sensitive detection of very low contrast modulated signals with performance in each channel comparable to that of a discrete photodiode and a lock-in amplifier. Application of the instrument is demonstrated by parallelizing spatial measurements to produce two-dimensional thickness maps on a layered sample, and spectroscopic parallelization is demonstrated by presenting the measured Brillouin oscillations from a gallium arsenide wafer. This paper demonstrates the significant advantages of our approach to pump probe systems, especially picosecond ultrasonics.

Spatial modulation microscopy for real-time imaging of plasmonic nanoparticles and cells

Spatial modulation microscopy (SMM) is a technique originally developed for quantitative spectroscopy of individual nano-objects. Here, a parallel implementation of the SMM technique is demonstrated based on a line detector capable of demodulation at kHz frequencies. The capabilities of the imaging system are shown using an array of plasmonic nanoantennas and dendritic cells incubated with gold nanoparticles.

Highly parallel CMOS lock-in optical sensor array for hyperspectral recording in scanned imaging systems

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.

Widefield heterodyne interferometry using a custom CMOS modulated light camera

In this paper a method of taking widefield heterodyne interferograms using a prototype modulated light camera is described. This custom CMOS modulated light camera (MLC) uses analogue quadrature demodulation at each pixel to output the phase and amplitude of the modulated light as DC voltages. The heterodyne interference fringe patterns are generated using an acousto-optical frequency shifter (AOFS) in an arm of a Mach-Zehnder interferometer. Widefield images of fringe patterns acquired using the prototype MLC are presented. The phase can be measured to an accuracy of ±6.6°. The added value of this method to acquire widefield images are discussed along with the advantages.

Highly sensitive multipoint real-time kinetic detection of Surface Plasmon bioanalytes with custom CMOS cameras

Phase sensitive Surface Plasmon Resonance (SPR) techniques are a popular means of characterizing biomolecular interactions. However, limitations due to the narrow dynamic range and difficulty in adapting the method for multi-point sensing have restricted its range of applications. This paper presents a compact phase sensitive SPR technology using a custom CMOS camera. The system is exceptionally versatile enabling one to trade dynamic range for sensitivity without altering the optical system. We present results showing sensitivity over the array of better than 10−6 Refractive Index Units (RIU) over a refractive index range of 2×10−2 RIU, with peak sensitivity of 3×10−7 RIU at the center of this range. We also explain how simply altering the settings of polarization components can give sensitivity on the order of 10−8 RIU albeit at the cost of lower dynamic range. The consistent response of the custom CMOS camera in the system also allowed us to demonstrate precise quantitative detection of two Fibrinogen antibody–protein binding sites. Moreover, we use the system to determine reaction kinetics and argue how the multipoint detection gives useful insight into the molecular binding mechanisms.

Optical detection of ultrasound from optically rough surfaces using a custom CMOS sensor

The optical detection of ultrasound from optically rough surfaces is severely limited when using a conventional interferometric or optical beam deflection (OBD) setup because the detected light is speckled. This means that complicated and expensive setups are required to detect ultrasound optically on rough surfaces. We present a CMOS integrated circuit that can detect laser ultrasound in the presence of speckle. The detector circuit is based on the simple knife edge detector. It is self-adapting and is fast, inxepensive, compact and robust. The CMOS circuit is implemented as a widefield array of 32×32 pixels. At each pixel the received light is compared with an adjacent pixel in order to determine the local light gradient. The result of this comparison is stored and used to connect each pixel to the positive or negative gradient output as appropriate (similar to a balanced knife edge detector). The perturbation of the surface due to ultrasound preserves the speckle distribution whilst deflecting it. The spatial disturbance of the speckle pattern due to the ultrasound is detected by considering each pair of pixels as a knife edge detector. The sensor can adapt itself to match the received optical speckle pattern in less than 0.1 μs, and then detect the ultrasound within 0.5 μs of adaptation. This makes it possible to repeatedly detect ultrasound from optically rough surfaces very quickly. The detector is capable of independent operation controlled by a local microcontroller, or it may be connected to a computer for more sophisticated configuration and control. We present the theory of its operation and discuss results validating the concept and operation of the device. We also present preliminary results from an improved design which grants a higher bandwidth, allowing for optical detection of higher frequency ultrasound.

Embedded Supervisory Control and Output Reporting for the Oscillating Ultrasonic Temperature Sensors

Ultrasonic temperature sensors can potentially outperform conventional sensors because they are capable of very fast sensing across the complete ultrasound pathway, whilst conventional sensors only sense temperature at a single point and have substantial thermal inertia. We report recent developments in electronic instrumentation for oscillating ultrasonic temperature sensors with the aim of achieving high accuracy and low scatter at a low cost.

CMOS modulated light cameras for sensitive surface plasmon resonance imaging

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.

A custom CMOS sensor for pyramidal adaptive optics system

This paper reports a custom CMOS light sensor which can be used in a pyramid wavefront sensor based adaptive optics system for eye imaging. A pyramid sensor produces four pupil images and therefore, this CMOS light sensor consists of 4 arrays of 32 times 32 active reset pixels with synchronous analogue outputs. In an eye imaging adaptive optics system the light available for wavefront sensing is of the order of nW. Accordingly the photodiode capacitance size within the pixel is carefully chosen to detect this low light. The pixels were layed out with the fill factor of 56% and the chip consumes 56 mW of power during the read out. The custom CMOS sensor with an area of 12.6 sq. mm was fabricated using an Austria Micro Systems 0.35 mum C35B4 CMOS process. The sensor is mounted on a PCB and packaged in a box of dimensions 6 cm x 5.5 cm x 5 cm, which makes it feasible to install in an optical system. A dedicated high speed camera reading system has been constructed that can read upto 1000 frames per second. The realised complete camera system clearly detects the four pupil images created by a pyramid wavefront sensor.

Ultrastable heterodyne interferometer system using a CMOS modulated light camera

A novel ultrastable widefield interferometer is presented. This uses a modulated light camera (MLC) to capture and stabilise the interferogram in the widefield heterodyne interferometer. This system eliminates the contribution of piston phase to the interferogram without the need for common path optics and results in a highly stable widefield interferometer. The MLC uses quadrature demodulation circuitry built into each pixel to demodulate the light signal and extract phase information using an electronic reference signal. In contrast to the work previously presented [Opt. Express 19, 24546 (2011)], the reference signal is derived from one of the pixels on board the MLC rather than an external source. This local reference signal tracks the instantaneous modulation frequency detected by the other pixels and eliminates the contribution of piston phase to the interferogram, substantially removing the contributions of unwanted vibrations and microphonics to the interferogram. Interferograms taken using the ultrastable system are presented with one of the interferometer mirrors moving at up to 85 mm s(-1) over a variety of frequencies from 18 Hz to 20 kHz (giving a variation in optical path length of 220 μm, or 350 wavelengths at 62 Hz). This limit was the result of complex motion in the mirror mount rather than the stability limit of the system. The system is shown to be insensitive to pure piston phase variations equivalent to an object velocity of over 3 m s(-1).