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.

This paper discusses the construction principles and performance of a pulsed time-of-flight (TOF) laser radar based on high-speed (FWHM $100 ps) and high-energy ($1 nJ) optical transmitter pulses produced with a specific laser diode working in an "enhanced gain-switching" regime and based on single-photon detection in the receiver. It is shown by analysis and experiments that single-shot precision at the level of 2W3 cm is achievable. The effective measurement rate can exceed 10 kHz to a noncooperative target (20% reflectivity) at a distance of 9 50 m, with an effective receiver aperture size of 2:5 cm 2 . The effect of background illumination is analyzed. It is shown that the gating of the SPAD detector is an effective means to avoid the blocking of the receiver in a high-level background illumination case. A brief comparison with pulsed TOF laser radars employing linear detection techniques is also made.

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A multiple quantum well laser diode utilizing an asymmetric waveguide structure with a large equivalent spot size of ∼3 μm is shown to give high energy (∼1 nJ) and short (∼100 ps) isolated optical pulses when injected with <10 A and ∼1-ns current pulses realized with a MOS driver. The active dimensions of the laser diode are 30 μm (stripe width) and 3 mm (cavity length), and it works in a single transversal mode at a wavelength of ∼0.8 μm. Detailed investigation of the laser behavior at elevated temperatures is conducted; it is shown that at high enough injection currents, lasers of the investigated type show low temperature sensitivity. Laser diodes of this type may find use in accurate and miniaturized laser radars utilizing single photon detection in the receiver.Index Terms-Semiconductor lasers, quantum well lasers, optical pulses, gain switching, laser radar.