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.

A 16 × 256 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 µm highvoltage 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 receiver circuit for pulsed time-of-flight line profiling laser radar has been designed in 0.35µm CMOS technology. The receiver consists of a line detector array with 256×8 SPADs with a pitch of 41.6µm. The diameter of the SPAD's active area is 25.6µm, which gives a fill factor of 0.35. Each column has 8 SPADs and a TDC with a range and resolution of 640 ns and 20 ps, respectively. The TDCs are based on the Nutt interpolation method where cyclic converters are utilized as fine converters. A compact demonstrator system has been built with a horizontal FOV of about 37 • , which gives a column-wise angular resolution of about 0.15 •. This solid-state system uses a pulsed laser diode with cylindrical transmitter optics to illuminate the full FOV of the receiver with a single laser pulse. The system can measure distances up to 30 m @ 28 fps with an accuracy and precision of few millimeters in typical indoor lighting conditions. Outdoors, in direct sunlight conditions, the measurement range is decreased to 5 m. Outdoor range, however, can be increased if frame rate is lowered and more laser shots are accumulated per frame. Index Terms-Time-to-digital converter (TDC), single photon avalanche diode (SPAD), time-of-flight (ToF), light detection and ranging (LiDAR). I. INTRODUCTION P ULSED time-of-flight (TOF) laser radar operates on the principle of sending a high power (>10W) and short (<5ns) laser pulse towards the target, and then measuring the time-of-flight by measuring the time difference between the transmitted pulse and the reflected echo pulse [1]-[4]. Since the velocity of light is relatively constant, the distance to the target can be calculated based on the measured transit time. These techniques have important advantages compared to other optical time-of-light methods, e.g. to the measurement of the phase difference between the transmitted continuous-wave (CW) optical signal and the reflected echo. For a pulsed TOF system, the non-ambiguous measurement range is limited basically by the pulsing rate of the transmitter and can thus be very long (i.e. 100m with 1MHz) whereas in the CW phase comparison techniques there is a trade-of between the non-ambiguous

A single-chip receiver for pulsed laser direct time-of-flight 3-D imaging applications has been realized in a 0.35-µm HV CMOS technology. The chip includes a 32 × 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 × 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 [∼150ps 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 ∼42 • ×21 • 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. Index Terms-3-D imager, CMOS, direct time-of-flight (dTOF), single-photon avalanche diode (SPAD), solid state, time gating, time-to-digital converter (TDC).

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