Pekka Keranen

Bio: Pekka Keranen is an academic researcher from University of Oulu. The author has contributed to research in topics: Time-to-digital converter & Raman spectroscopy. The author has an hindex of 9, co-authored 24 publications receiving 326 citations.

Fluorescence suppression in Raman spectroscopy using a time-gated CMOS SPAD

Abstract: A Raman spectrometer technique is described that aims at suppressing the fluorescence background typical of Raman spectra. The sample is excited with a high power (65W), short (300ps) laser pulse and the time position of each of the Raman scattered photons with respect to the excitation is measured with a CMOS SPAD detector and an accurate time-to-digital converter at each spectral point. It is shown by means of measurements performed on an olive oil sample that the fluorescence background can be greatly suppressed if the sample response is recorded only for photons coinciding with the laser pulse. A further correction in the residual fluorescence baseline can be achieved using the measured fluorescence tails at each of the spectral points.

Wide-Range Time-to-Digital Converter With 1-ps Single-Shot Precision

Abstract: A high-resolution time-to-digital converter (TDC) was designed and tested. The converter is based on the fundamental method of counting the full clock cycles of a low-phase-noise reference clock and using a single-stage interpolating method employing time-to-amplitude converters that are based on Miller integrators. Counters and other control logic were implemented on a field-programmable gate array, and the interpolation units were constructed using discrete components. The single-shot precision of the uncompensated converter is about 1.8 ps over a time interval range of 0 to 328 μs. Single-shot precision is limited by the nonlinearities of the interpolators. These measurement errors caused by the nonlinearities are systematic, and thus, precision can be improved to 1 ps by a simple integral nonlinearity compensation. Other important factors that contribute to single-shot precision are the N -cycle jitter of the reference clock and the noise generated by the TDC circuit itself. By careful design, these errors can be made small enough to achieve picosecond-level precision.

A $16\times256$ SPAD Line Detector With a 50-ps, 3-bit, 256-Channel Time-to-Digital Converter for Raman Spectroscopy

Abstract: A $16\times256$ element single-photon avalanche diode array with a 256-channel, 3-bit on-chip time-to-digital converter (TDC) has been developed for fluorescence-suppressed Raman spectroscopy. The circuit is fabricated in $0.35~\mu \text{m}$ high-voltage CMOS technology and it allows a measurement rate of 400 kframe/s. In order to be able to separate the Raman and fluorescence photons even in the presence of the unavoidable timing skew of the timing signals of the TDC, the time-of-arrival of every detected photon is recorded with high time resolution at each spectral point with respect to the emitted short and intensive laser pulse (~150 ps). The dynamic range of the TDC is set so that no Raman photon is lost due to the timing skew, and thus the complete time history of the detected photons is available at each spectral point. The resolution of the TDC was designed to be adjustable from 50 ps to 100 ps. The error caused by the timing skew and the residual variation in the resolution of the TDC along the spectral points is mitigated utilizing a calibration measurement from reference sample with known smooth fluorescence spectrum. As a proof of concept, the Raman spectrum of sesame seed oil, having a high fluorescence-to-Raman ratio and a short fluorescence lifetime of 1.9 ns, was successfully recorded.

A 32 × 128 SPAD-257 TDC Receiver IC for Pulsed TOF Solid-State 3-D Imaging

Abstract: A single-chip receiver for pulsed laser direct time-of-flight 3-D imaging applications has been realized in a 0.35- $\mu \text{m}$ HV CMOS technology. The chip includes a $32 \times 128$ single-photon avalanche diode (SPAD) array [35% fill factor (FF)] and 257 time-to-digital converters (TDCs) with a ~78-ps resolution. Two adjacent rows ( $2 \times 128$ SPADs) at a time can be selected for simultaneous measurement, i.e., 16 measurement cycles are needed to cover the whole array. SPADs are capable of operating in a gated mode in order to suppress dark and background light-induced detections. The IC was designed to be used in a solid-state 3-D imaging system with laser illumination concentrated in both time (short sub-ns pulses) and space (targeting only the active rows of the SPAD array). The performance of the receiver IC was characterized in a solid-state 3-D range imager with flood-pulsed illumination from a laser diode (LD)-based transmitter, which produced short [~150-ps full-width at half-maximum (FWHM)] high-energy (~3.8-nJ pulse/~14-W peak power) pulses at a pulsing rate of 250 kHz when operating at a wavelength of 810 nm. Two detector/TDC ICs formed an 8k pixel receiver, targeting a field-of-view of $\sim 42^{\circ } \times 21^{\circ }$ by means of simple optics. Frame rates of up to 20 fps were demonstrated with a centimeter-level precision in the case of Lambertian targets within a range of 3.5 m.

A Wide Range, 4.2 ps(rms) Precision CMOS TDC With Cyclic Interpolators Based on Switched-Frequency Ring Oscillators

Abstract: A time-to-digital (TDC) converter based on cyclic interpolators has been designed. The TDC is designed to be used in a pulsed time-of-flight laser radar, where a long measurement range is required. The TDC's two interpolators provide a picosecond level resolution, which is combined with a main reference clock counter to give a measurement range of 327 $\mu{\rm s}$ . The interpolators measure time intervals with a switched-frequency ring oscillator. The frequency switching is used as a mechanism to amplify the quantization error in the cyclic interpolator. A digital calibration scheme is used for radix extraction. The interpolators' worst case INL is $\pm$ 4.5 ps. Due to the interpolator's INL, the TDC's RMS precision is about 4.2 ps, while the worst and best case single-shot precisions are 5.5 ps and 1.7 ps, respectively. The overall accuracy of the TDC is better than 5 ps in a temperature range of $-$ 30 C to 70 C. The TDC is designed in 0.35 $\mu{\rm m}$ CMOS technology and consumes 80 mW at 0.8 MHz measurement rate.

EXPRESS: Portable Spectroscopy:

Abstract: Until very recently, handheld spectrometers were the domain of major analytical and security instrument companies, with turnkey analyzers using spectroscopic techniques from X-ray fluorescence (XRF) for elemental analysis (metals), to Raman, mid-infrared, and near-infrared (NIR) for molecular analysis (mostly organics). However, the past few years have seen rapid changes in this landscape with the introduction of handheld laser-induced breakdown spectroscopy (LIBS), smartphone spectroscopy focusing on medical diagnostics for low-resource areas, commercial engines that a variety of companies can build up into products, hyphenated or dual technology instruments, low-cost visible-shortwave NIR instruments selling directly to the public, and, most recently, portable hyperspectral imaging instruments. Successful handheld instruments are designed to give answers to non-scientist operators; therefore, their developers have put extensive resources into reliable identification algorithms, spectroscopic libraries or databases, and qualitative and quantitative calibrations. As spectroscopic instruments become smaller and lower cost, “engines” have emerged, leading to the possibility of being incorporated in consumer devices and smart appliances, part of the Internet of Things (IOT). This review outlines the technologies used in portable spectroscopy, discusses their applications, both qualitative and quantitative, and how instrument developers and vendors have approached giving actionable answers to non-scientists. It outlines concerns on crowdsourced data, especially for heterogeneous samples, and finally looks towards the future in areas like IOT, emerging technologies for instruments, and portable hyphenated and hyperspectral instruments.

Single-photon avalanche diode imagers in biophotonics: review and outlook

Abstract: Single-photon avalanche diode (SPAD) arrays are solid-state detectors that offer imaging capabilities at the level of individual photons, with unparalleled photon counting and time-resolved performance. This fascinating technology has progressed at a very fast pace in the past 15 years, since its inception in standard CMOS technology in 2003. A host of architectures have been investigated, ranging from simpler implementations, based solely on off-chip data processing, to progressively "smarter" sensors including on-chip, or even pixel level, time-stamping and processing capabilities. As the technology has matured, a range of biophotonics applications have been explored, including (endoscopic) FLIM, (multibeam multiphoton) FLIM-FRET, SPIM-FCS, super-resolution microscopy, time-resolved Raman spectroscopy, NIROT and PET. We will review some representative sensors and their corresponding applications, including the most relevant challenges faced by chip designers and end-users. Finally, we will provide an outlook on the future of this fascinating technology.

Review of Fluorescence Suppression Techniques in Raman Spectroscopy

Abstract: Raman spectroscopy is an important and powerful technique for analyzing the chemical composition of biological or nonbiological samples in many fields. A serious challenge frequently encountered in Raman measurements arises from the existence of the concurrent fluorescence background. The fluorescence intensity is normally several orders of magnitude larger than the Raman scattering signal, especially in biological samples. Such fluorescence background must be suppressed in order to obtain accurate Raman spectra. Several different techniques have been explored for this purpose. These techniques could be generally grouped into time-domain, frequency-domain, wavelength-domain, and computational methods in addition to various Raman enhancement techniques and other unconventional methods. This review briefly describes the fundamental principles of each group of methods, reports the most recent advances, and makes comparison across those major categories of techniques in terms of cost and performance i...

Raman spectroscopy: techniques and applications in the life sciences

Abstract: Raman spectroscopy is an increasingly popular technique in many areas including biology and medicine. It is based on Raman scattering, a phenomenon in which incident photons lose or gain energy via interactions with vibrating molecules in a sample. These energy shifts can be used to obtain information regarding molecular composition of the sample with very high accuracy. Applications of Raman spectroscopy in the life sciences have included quantification of biomolecules, hyperspectral molecular imaging of cells and tissue, medical diagnosis, and others. This review briefly presents the physical origin of Raman scattering explaining the key classical and quantum mechanical concepts. Variations of the Raman effect will also be considered, including resonance, coherent, and enhanced Raman scattering. We discuss the molecular origins of prominent bands often found in the Raman spectra of biological samples. Finally, we examine several variations of Raman spectroscopy techniques in practice, looking at their applications, strengths, and challenges. This review is intended to be a starting resource for scientists new to Raman spectroscopy, providing theoretical background and practical examples as the foundation for further study and exploration.