A time-resolved Raman spectrometer is demonstrated based on a 256×8 CMOS SPAD line sensor and a 573 nm fiber-coupled diamond Raman laser delivering pulses with duration below 100 ps FWHM. The collected back scattered light from the sample is dispersed on the line sensor using a custom volume holographic grating having 1800 lines/mm. Efficient fluorescence rejection in the Raman measurements is achieved due to a combination of time gating on sub-100 ps-time scale and a 573 nm excitation wavelength. To demonstrate the performance of the spectrometer, fluorescent oil samples were measured. For organic sesame seed oil having a continuous wave mode fluorescence-to-Raman ratio of 10.5 and a fluorescence lifetime of 2.7 ns, a signal-to-distortion value of 76.2 was achieved. For roasted sesame seed oil having a continuous wave mode fluorescence-to-Raman ratio of 82 and a fluorescence lifetime of 2.2 ns, a signal-to-distortion value of 28.2 was achieved. In both cases, the fluorescence-to-Raman ratio was reduced by a factor of 24-25 owing to time gating. For organic oil, spectral distortion was dominated by dark counts while for the more fluorescent roasted oil, the main source of spectral distortion was timing skew of the sensor. With the presented post-processing techniques, the level of distortion could be reduced by 88-89 % for both samples. Compared to common 532 nm excitation, approximately 73 % lower fluorescence-to-Raman ratio was observed for 573 nm excitation when analyzing the organic sesame seed oil. Index Terms-Fluorescence rejection, Raman laser, Raman spectrometer, Raman spectroscopy, SPAD sensor, time-correlated single photon counting, time gating, timing skew I. INTRODUCTION AMAN spectroscopy is used in a wide range of fields including food and oil industries, mining industry, medical diagnostics, pharmacy, forensic science and archaeometry [1]
Abstract-We report a compact and efficient picosecond diamond Raman laser at 573 nm wavelength. The laser consists of a 0.5 mm thick single-crystal synthetic diamond coated to form a plane-plane laser resonator, and pumped at 532 nm by a frequency-doubled Q-switched microchip laser system. The pump delivers 85 ps pulses at 100 kHz repetition rate at a maximum average power of ~500 mW. We demonstrate 1st Stokes emission from the diamond Raman laser with maximum power of 175 mW, corresponding to a conversion efficiency of 47% and a pulse duration of 71 ps. Substantial pulse shortening is obtained by proper adjustment of the pump spot diameter on the diamond sample. A minimum pulse duration of 39 ps is reported for a conversion efficiency of 36% and 150 mW output power. The simplicity of the architecture makes the system highly appealing as a yellow picosecond laser source.
We report a disk laser using two quantum-dot semiconductor gain elements, resulting in what we believe is the first demonstration of intracavity frequency conversion with these active media. Output power of 6 W has been obtained in dual-gain configuration at a wavelength of 1180 nm, while single-gain lasers produced up to 3 and 4 W individually, limited by thermal rollover in the output characteristics. The gain enhancement achieved with two active elements comprising 39 layers of Stranski-Krastanov InGaAs quantum dots allows for intracavity frequency doubling delivering 2.5 W of orange radiation.
We report a monolithic 1240 nm diamond Raman laser producing pulses with duration of 42-62 ps at 100 kHz repetition rate, and maximum average power of 246 mW. The Raman laser is formed by a 0.5-mm thick planar diamond, coated on both sides and pumped by ~100 ps pulses from a Q-switched 1064 nm laser. The maximum conversion efficiency from 1064 nm to 1240 nm was about 25%. The 1240 nm signal was frequency-doubled in single-pass configuration through a 10-mm long LBO crystal, enabling generation of pulses with a duration of 29-46 ps at 620 nm. The maximum average power at 620 nm was 128 mW, and the maximum conversion efficiency from 1240 nm to 620 nm was 50%. The Raman laser provides an efficient and flexible way to extend short pulse operation to wavelengths in spectral domains difficult to reach, such as 620 nm and in addition provides a simple pulse shortening mechanisms.
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