Fluorescence-lifetime spectroscopy has been recently used in different applications in rapid medical diagnoses for different diseases, e.g. cancer, oral carcinoma, actinic cheilitis and tissue diagnosis. In addition, it has various applications in biology, chemistry analysis, pharmaceutical applications and physics. The existing fluorescence spectrometer systems are bulky, contain complex optical setups, large and use either monochromators with movable parts, hence less mechanically stable, or use non-tunable bandpass filters. Therefore, the user has to adjust the filters while changing the fluorophore. In this work, we propose a spectrometer setup that can be tuned for different emission wavelength using spatial light modulators (SLM). SLMs use highly stable MEMS micromirrors in their architecture, hence, the system has no large moving parts and is mechanically stable. In addition, the system is suitable for more fluorophores and thus widens the application potentials of the system. Digital micromirror devices (DMD) are widely used in optical projection systems. However, some recent developments were made to use them in conventional optical spectroscopy which enabled using single point detectors to collect the spectrum instead of using linear or area camera sensor arrays. This is expected to have a big effect on the cost of the spectrometer. Using similar techniques, in this work, a light source, e.g. a supercontinuum laser was used along with dichroic filters to fix the excitation wavelength at a certain value. After that, the selected wavelength(s) pass through a beam splitter towards the sample. The beam splitter is installed to separate the emission and excitation light. The fluorescence emission is reflected from the beam splitter and is focused on a diffraction grating. The different wavelengths are projected to the DMD. The used DMD has a VGA resolution with a mirror pitch of 5.4µm. Each micromirror could be controlled individually into two states, on and off at ±17 degrees. The setup ensured that different wavelengths of light always fall on different mirror(s) and hence the desired wavelength(s) could be selected at once. A high-speed-gated single pixel CMOS detector with a large diameter was used to capture the reflected light with the desired wavelengths from the DMD. The system scans the mirrors and hence the wavelengths to readout the spectrum. In addition, the system measures the fluorescence lifetime of the used fluorophore at the emission wavelength. The proposed setup is compact and less complicated than conventional systems. Furthermore, the spectrometer, apart from the light source, has a great potential to be miniaturized. Thus, it can be used as hand-held system. The proposed system was characterized and the proof of concept of the setup was verified. Due to the small pitch of the DMD and the large number of mirrors, it was found that it has a high resolution especially in the VIS spectral range. The system is considered as semi-tunable as it can be tuned for different emission wavelengths and thus, can be used with different fluorophores. However, the light source along with the filters should be changed for different excitation wavelengths. or pulsed laser diodes can be used.
Time-gated image sensors with (sub-)nanosecond gating times have already found applications in multiple different domains such as 3D Time-of-Flight cameras, Fluorescence lifetime imaging (FLIM) and Tomography. Commercial timegated cameras are based on Image intensified CCDs (ICCD). The photomultiplier tubes used in these ICCDs have a limited quantum efficiency in visible and a fortiori in Near-Infrared (NIR). Furthermore, they are expensive, bulky, fragile and need high voltages to operate. We propose a time-gated camera based on the Current-Assisted Photonic Sampler (CAPS) which integrates the gating mechanism inside a silicon-based pixel without the need for photomultipliers. Due to particular pixel design, sub-nanosecond gating can be achieved while still attaining high quantum-efficiency even in NIR. A first proof-of-concept camera is demonstrated in this paper based on a 32x32-pixel CAPS array with specific timing circuitry to achieve precise and accurate high-resolution sensor gating. Quantitative results about the performance of the camera, such as gating speed and quantum efficiency will be presented and discussed. The cameras capabilities are demonstrated in two experimental setups. The first one: imaging a laser pulse traveling at the speed of light along the field of view. The second setup: making fluorescence lifetime images of two cuvettes containing fluorescent solutions with distinct lifetimes.
Fluorescence imaging using near-infrared (NIR) fluorescent contrast agents is increasingly being investigated as intraoperative tool to visualize, in real-time, tissues of interest such as tumors, lymph nodes or nerve bundles. Generally, spectral imaging systems are used that measure the intensity of fluorescent signals. However, to aid in a more specific detection of these fluorescent signals, fluorescence lifetime can be added to the image. The lifetime is independent of the intensity and in addition, multiple tracers emitting around the same wavelengths can still be distinguished based on their difference in lifetime. Imaging lifetimes, however, requires a much more advanced imaging system. None of the currently approved fluorescence guidance systems support fluorescence lifetime and today’s available lifetime imaging technology (TCSPC, ICCD) does not allow imaging the sub-nanosecond lifetimes of NIR dyes with the efficiency needed to reach video frame rates. In this paper, we present a 32×32-pixel proof-of-concept camera based on our novel CAPS-pixel based gated image sensor. This camera is specifically targeted at imaging fluorescence lifetimes at NIR wavelengths with high efficiency for the use in fluorescence-guided surgery and is a first step towards a full camera with video resolution at video frame rates. We describe the camera system and how it is used to image fluorescence lifetimes. Next, we show the lifetime imaging capability by imaging the lifetimes of two different nanosecond visible dyes (fluorescein and acridine orange) in cuvette and two more challenging (sub-)nanosecond NIR dyes (ICG and IRDye800CW). Lastly, we validate the camera by imaging NIR fluorescence phantoms in a mouse.
Integrating an optical receiver in CMOS optimized for near infrared light (NIR) remains appealing but at the same time challenging due to the deep photon penetration depth. A novel implementation of a light detector is demonstrated in a 350 nm CMOS technology, whereby, through adding a majority current with associated electric field distribution in the silicon detection volume, photo-generated minority electrons get quickly guided to the center of this volume. In the center, a tiny PN junction collects the photo-electrons. The detection speed subsequently increases, NIR light is received with improved responsivity and the detector capacitance gets drastically reduced to femtofarad level. The latter improvement also increases signal-to-noise performance and can be used to trade-off with other design parameters to improve global performance of the opto-electronic system. An optical datacom receiver at 1 Gbps is demonstrated at NIR-wavelength for proving useful Current-Assisted Photodiode detector operation in an actual CMOS system
Imaging based on fluorescence lifetime is becoming increasingly important in medical and biological applications. State-of- the-art fluorescence lifetime microscopes either use bulky and expensive gated image intensifiers coupled to a CCD or single-photon detectors in a slow scanning setup. Numerous attempts are being made to create compact, cost-effective all- CMOS imagers for fluorescence lifetime sensing. Single-photon avalanche diode (SPAD) imagers can have very good timing resolution and noise characteristics but have low detection efficiency. Another approach is to use CMOS imagers based on demodulation detectors. These imagers can be either very fast or very efficient but it remains a challenge to combine both characteristics. Recently we developed the current-assisted photonic sampler (CAPS) to tackle these problems and in this work, we present a new CAPS with two detection taps that can sample a fluorescence decay in two time windows. In the case of mono-exponential decays, two windows provide enough information to resolve the lifetime. We built an electro-optical setup to characterize the detector and use it for fluorescence lifetime measurements. It consists of a supercontinuum pulsed laser source, an optical system to focus light into the detector and picosecond timing electronics. We describe the structure and operation of the two-tap CAPS and provide basic characterization of the speed performance at multiple wavelengths in the visible and near-infrared spectrum. We also record fluorescence decays of different visible and NIR fluorescent dyes and provide different methods to resolve the fluorescence lifetime.
Coronary artery disease (CAD) contributes to millions of deaths each year. The identification of vulnerable plaques is essential to the diagnosis of CAD but is challenging. Molecular probes can improve the detection of these plaques using intravascular imaging methods. Fluorescence lifetime sensing is a safe and robust method to image these molecular probes. We present two variations of an optical system for intravascular near-infrared (NIR) fluorescence lifetime sensing through a multimode fiber. Both systems are built around a recently developed fast and efficient CMOS detector, the current-assisted photonic sampler (CAPS) that is optimized for sub-nanosecond NIR fluorescence lifetime sensing. One system mimics the optical setup of an epifluorescence microscope while the other uses a practical fiber optic coupler to separate fluorescence excitation and emission. We test both systems by measuring the lifetime of several NIR dyes in DMSO solutions and we show that these systems are capable of detecting lifetimes of solutions with concentrations down to 370 nM and this with short acquisition times. These results are compared with time-correlated single photon counting (TCSPC) measurements for reference.
Range-imaging is a measurement technique able to generate an image which contains the distance information from the camera to all the points of a scene. This distance information can be captured by, amongst others, the Time-of-Flight principle which measures the time a light pulse needs to travel back and forth from the camera to the scene and converts this time into a depth value. For a good operation of the Time-of-Flight principle, a high-power, fast-modulated light source is required. Currently, most 3D cameras use laser diodes or LEDs. Moreover, most systems use square-wave modulation of the light source, requiring high bandwidths of the optical driver. To enhance both bandwidth and optical power, we developed a light source consisting of 16 high-power (50 mW) laser diodes using GHz laser drivers, combined with GHz buffers. Moreover, this light source can be integrated in a Time-of-Flight camera. Specifically, we designed and experimentally validated this new light source, based on ultra-fast laser diodes, allowing an increased performance of the current Time-of-Flight cameras. In this paper, we first discuss the development of a high-power illumination board, with a large beam divergence and suitable for high-speed square-wave modulation with a chosen duty-cycle. Our light source can be modulated faster than 1 GHz, which corresponds to optical pulses shorter than 500 ps. Moreover, the pulses can be shifted in time with sub-nanosecond precision. Secondly, we integrated this light source into a Time-of-Flight setup, able to measure the distances of objects behind a semi-transparent surface. The resulting images are compared with the image quality of commercially available Time-of-Flight cameras. From these results, we can conclude that our light source is suitable for Time-of-Flight measurements and gives a low-cost alternative for imaging purposes. Moreover, it can handle both pulsed as continuous-wave Time-of-Flight, to allow a broader range of applications.