Efficient laser sources in the 3 - 5 μm wavelength range are needed for directed infrared countermeasures, but also have
applications in remote sensing, medicine and spectroscopy. We present results and discuss the possibilities of a tandem
optical parametric oscillator (OPO) scheme for converting the radiation from a 1.06 μm Nd<sup>3+</sup>-laser to the mid-infrared.
Our setup uses type I quasi phase-matched (QPM) crystals in a near degenerate OPO to generate 2.13 μm radiation. The
QPM crystal provides higher nonlinearity and longer interaction lengths, because walk-off is avoided, compared to
conventional bulk crystals. This is an advantage especially in low pulse energy applications. To make the 2.13 μm
radiation usable for pumping a second OPO a volume Bragg grating is used as a cavity mirror to limit the bandwidth,
which in a conventional QPM OPO at degeneracy can be several hundred nanometers. The acceptance bandwidth for
efficient OPO operation of a 14 mm long ZnGeP<sub>2</sub> (ZGP) crystal is approximately 5 nm, which makes the need for
bandwidth limiting clear. The majority of the signal energy from a periodically poled KTiOPO<sub>4</sub> (PP KTP) OPO with a
volume Bragg grating output coupler was found to be in a single longitudinal mode, whereas the idler bandwidth was
measured to 19 GHz (FWHM). A volume Bragg grating resonant near 2124 nm was chosen so that the signal and idler
were separated by 9 nm. This OPO output has been used as a pump source for a conventional ZGP OPO demonstrating
efficient conversion and providing broadband tunable output in the mid-infrared.
We present results of a measurement system designed for detecting the fluorescence spectrum of individual aerosol particles of biological warfare agents excited with laser pulses at wavelengths around 290 or 340 nm. The biological aerosol is prepared and directed into a narrow air beam. A red laser is focused on the aerosol beam and a trigger photomultiplier tube monitor the presence of individual particles by measuring the scattered light. When a particle is present in the detection volume, a laser pulse is triggered from an ultraviolet laser and the fluorescence spectrum is acquired with a spectrometer based on a diffraction grating and a 32 channels photomultiplier tube array with single-photon sensitivity. The spectrometer measures the fluorescence spectra in the wavelength region from 300 to 800 nm. In the experiment we used different simulants of biological warfare agents. These bioaerosol particles were excited by a commercial available gas laser (337 nm), or a laser (290 nm) that we have developed based on an optical parametric oscillator with intracavity sum-frequency mixing. In the analysis of the experiments we compare the measured signals (fluorescence spectra, total fluorescence energy and the scattered energy) from the individual bioaerosol particles excited with the two different ultraviolet wavelengths.
We present initial results of a measurement system designed for detecting the fluorescence spectrum of individual particles of biological warfare agent (BWA). A compact optical parametric oscillator with intracavity sum-frequency mixing and a commercial Nitrogen gas laser was used as excitation sources to generate 293 nm or 337 nm UV laser irradiation. The pulsed lasers and a photomultiplier tube (PMT) array based spectrometer were triggered by a red laser-diode and a PMT detector that sensed the presence of a particle typical of size 5-20 μm in diameter. The spectral detection part of the system consisted of a grating and a PMT array with 32 channels, which measured fluorescence in the wavelength from 280 nm to 800 nm. The detector system was used to demonstrate the measurement of laser induced fluorescence spectra of individual BWA simulant particles by excitation of single UV laser pulses. The spectrum obtained by averaging spectra from several BWA aerosol simulant particles were found generally similar, but not identical, to the fluorescence spectrum obtained from water solutions containing the same particles dissolved.
A novel, compact and robust UV laser has been developed for laser induced fluorescence spectroscopy of biomolecules in the spectral region from 290 nm to 345 nm. It was based on a frequency-doubled passively Q-switched Nd:YAG laser, emitting at 532 nm, which was pumping a periodically poled KTiOPO<sub>4</sub> optical parametric oscillator with intra-cavity sum-frequency mixing in a BBO crystal. The output was generated in two branches in the UV, 293 nm and 343 nm, with pulse widths of 1.8 ns and pulse repetition rate of 100 Hz. These wavelengths were then used for fluorescence experiments of bioagents.
A compact parametric oscillator (OPO) with intracavity sum-frequency generation (SFG) to generate 293 nm UV laser irradiation, was developed. The OPO/SFG device was pumped by a 100 Hz Nd:YAG laser (1064 nm) of own design, including subsequent second harmonic generation (SHG) in an external periodically poled KTiOPO<sub>4</sub> (KTP) crystal. The whole system could be used to deliver more than 30 μJ laser irradiation per pulse (100 Hz) at 293 nm. The UV laser light was introduced in an optical fiber attached to a sample compartment allowing detection of fluorescence emission using a commercial spectrometer. Aqueous samples containing biomolecules (ovalbumin) or bacteria spores (<i>Bacillus subtilis</i>) were excited by the UV-light at 293 nm resulting in strong fluorescence emission in the range 325 - 600 nm.
Optical parametric oscillator (OPO) and sum-frequency mixing (SFM) devices are useful tools for constructing ultraviolet (UV) laser sources for fluorescence spectroscopy. Here, a compact UV-laser sources based on frequency conversion of an actively Q-switched Nd:YAG laser is presented. The second harmonic generation from a Nd:YAG laser was utilized as pump radiation for a periodically poled KTiOPO<sub>4</sub> nanosecond optical parametric oscillator. The OPO-signal and the remaining pump were spatially mode-matched for Type I SFM in a β-barium borate (BBO) crystal and UV radiation at 293 nm could be generated. This corresponds to a conversion efficiency of 2% with respect to the 532 nm harmonic radiation. The wavelength region accessible with this UV source is useful for chemical and biological sensing. Excitation of tryptophan at 293 nm for detection of fluorescence emission in ovalbumin and transthyretin was demonstrated.