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.

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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.

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A simplest saturable absorber, in the form of an unpumped section, is introduced into a Fabry-Perot semiconductor laser with a strongly asymmetric broadened waveguide structure incorporating a relatively thick (80 nm) active layer. This allows for suppression of trailing oscillations and a decrease in the optical pulse width compared to the uniformly biased structure. Single optical pulses of ~80 ps full width at half maximum (FWHM) and ~35 W peak power (~3 nJ pulse energy, E(opt)), practically without trailing edge oscillations, were experimentally achieved under room temperature conditions by absorber-assisted gain-switching, using pumping current pulses of ~1.3 ns FWHM and ~17 A amplitude. The laser emission has a narrow (13 degrees FWHM in the transverse direction) far field.

In this work, we utilize a short-wavelength, 532-nm picosecond pulsed laser coupled with a time-gated complementary metal-oxide semiconductor (CMOS) single-photon avalanche diode (SPAD) detector to acquire Raman spectra of several drugs of interest. With this approach, we are able to reveal previously unseen Raman features and suppress the fluorescence background of these drugs. Compared to traditional Raman setups, the present time-resolved technique has two major improvements. First, it is possible to overcome the strong fluorescence background that usually interferes with the much weaker Raman spectra. Second, using the high photon energy excitation light source, we are able to generate a stronger Raman signal compared to traditional instruments. In addition, observations in the time domain can be performed, thus enabling new capabilities in the field of Raman and fluorescence spectroscopy. With this system, we demonstrate for the first time the possibility of recording fluorescence-suppressed Raman spectra of solid, amorphous and crystalline, and non-photoluminescent and photoluminescent drugs such as caffeine, ranitidine hydrochloride, and indomethacin (amorphous and crystalline forms). The raw data acquired by utilizing only the picosecond pulsed laser and a CMOS SPAD detector could be used for identifying the compounds directly without any data processing. Moreover, to validate the accuracy of this time-resolved technique, we present density functional theory (DFT) calculations for a widely used gastric acid inhibitor, ranitidine hydrochloride. The obtained time-resolved Raman peaks were identified based on the calculations and existing literature. Raman spectra using non-time-resolved setups with continuous-wave 785- and 532-nm excitation lasers were used as reference data. Overall, this demonstration of time-resolved Raman and fluorescence measurements with a CMOS SPAD detector shows promise in diverse areas, including fundamental chemical research, the pharmaceutical setting, process analytical technology (PAT), and the life sciences.Graphical abstractTime-resolved Raman measurement of a pharmaceutical sample using the complementary metal-oxide semiconductor (CMOS) single photon avalanche diode (SPAD) detector technologyElectronic supplementary materialThe online version of this article (doi:10.1007/s00216-015-9156-6) contains supplementary material, which is available to authorized users.

An analytical model for internal optical losses at high power in a 1.5 μm laser diode with strong n-doping in the n-side of the optical confinement layer is created. The model includes intervalence band absorption by holes supplied by both current flow and two-photon absorption (TPA), as well as the direct TPA effect. The resulting losses are compared with those in an identical structure with a weakly doped waveguide, and shown to be substantially lower, resulting in a significant improvement in the output power and efficiency in the structure with a strongly doped waveguide.