Nonlinear Optical Crystals: A Complete Survey

Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals

We have designed and demonstrated a standoff Raman system for detecting high explosive materials at distances up to 50 meters in ambient light conditions. In the system, light is collected using an 8-in. Schmidt-Cassegrain telescope fiber-coupled to an f/1.8 spectrograph with a gated intensified charge-coupled device (ICCD) detector. A frequency-doubled Nd : YAG (532 nm) pulsed (10 Hz) laser is used as the excitation source for measuring remote spectra of samples containing up to 8% explosive materials. The explosives RDX, TNT, and PETN as well as nitrate- and chlorate-containing materials were used to evaluate the performance of the system with samples placed at distances of 27 and 50 meters. Laser power studies were performed to determine the effects of laser heating and photodegradation on the samples. Raman signal levels were found to increase linearly with increasing laser energy up to approximately 3 x 10(6) W/cm2 for all samples except TNT, which showed some evidence of photo- or thermal degradation at higher laser power densities. Detector gate width studies showed that Raman spectra could be acquired in high levels of ambient light using a 10 microsecond gate width.

Standoff detection of high explosive materials at 50 meters in ambient light conditions using a small Raman instrument.

A review of standoff detection technologies for explosives has been made. The review is focused on trace detection methods (methods aiming to detect traces from handling explosives or the vapours surrounding an explosive charge due to the vapour pressure of the explosive) rather than bulk detection methods (methods aiming to detect the bulk explosive charge). The requirements for standoff detection technologies are discussed. The technologies discussed are mostly laser-based trace detection technologies, such as laser-induced-breakdown spectroscopy, Raman spectroscopy, laser-induced-fluorescence spectroscopy and IR spectroscopy but the bulk detection technologies millimetre wave imaging and terahertz spectroscopy are also discussed as a complement to the laser-based methods. The review includes novel techniques, not yet tested in realistic environments, more mature technologies which have been tested outdoors in realistic environments as well as the most mature millimetre wave imaging technique.

Laser-based standoff detection of explosives: a critical review.

Near- and mid-infrared laser monitoring of industrial processes, environment and security applications

An efficient pumping scheme that involves direct excitation of the upper lasing level of the Nd3+ ion is demonstrated experimentally. The results obtained for direct upper laser level pumping of Nd:YAG R2 (869 nm) and Nd:YVO4 (880 nm) were compared with traditional ∼808-nm pump band excitation. A tunable cw Ti:sapphire laser was used as the pump source. In Nd:YAG, the oscillator slope efficiency increased by 10% and the threshold decreased by 11%. In Nd:YVO4, the slope efficiency increased by 5% and the threshold decreased by 11%. These results agree with theory. The increase in optical efficiency indicates that laser material thermal loading can be substantially reduced.

Efficient pumping scheme for neodymium-doped materials by direct excitation of the upper lasing level

Pumping at 885 nm from thermally excited ground-state levels directly to the Nd:YAG upper lasing level is experimentally demonstrated by use of a Ti:sapphire pump laser. This approach utilizes thermal energy contained within the laser medium to provide part of the pump energy required to achieve population inversion. Slope efficiency increased by 12% compared with traditional pump band excitation (λpump = 808 nm) and by 7% compared with ground-state direct pumping (λpump = 869 nm). The combined transition from the first and second thermally excited Stark components of the ground state (4I9/2) to the upper lasing level (4F3/2) has characteristics that make thermally boosted pumping a suitable candidate for use with diode lasers: reasonable absorption (1.8 cm-1) and bandwidth (2.7 nm FWHM). A model suggests that, compared with traditional 808-nm pumping, heat could be reduced by 40% by use of thermally boosted pumping.

Thermally boosted pumping of neodymium lasers

A unique scheme has been applied for sensitive remote detection of 2,4,6-trinitrotoluene (TNT) vapor trace amounts at atmospheric pressure and 24 degrees C. The detection concept is based on a single laser beam inducing a tandem process: photodissociation of TNT vapor followed by highly selective detection of its photofragments vibrationally excited NO, utilizing laser-induced fluorescence with the A2Sigma+(v' = 0) <-- X2Pi(v'' = 2) transition. A detection sensitivity of at least 8 parts in 10(9) of TNT vapor with a signal-to-noise ratio of approximately 10 has been experimentally verified for an unfocused approximately 5-mJ laser beam, measured at a distance of approximately 15 cm from the TNT sample.

Photodissociation followed by laser-induced fluorescence at atmospheric pressure and 24 degrees C: a unique scheme for remote detection of explosives.

An internal doubling efficiency of 64% at 2 MW/cm(2)was obtained in a single-pass configuration with an uncoated, 1-cm-long, bulk periodically poled KTP crystal placed outside the resonator of a pulsed, diode-pumped Nd:YAG laser. An average of 4.8W of green light was obtained from a 7.5-W pump beam inside the crystal. Doubling efficiency exceeded 50% at levels of 0.75 MW/cm(2). The measured thermal tolerance of the doubling process (FWHM) was 3.3( degrees )C cm, and the measured temperature tuning coefficient was 0.053 nm/( degrees )C .

Highly efficient doubling of a high-repetition-rate diode-pumped laser with bulk periodically poled KTP.

We have demonstrated the feasibility of cooling high-power solid-state lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than that of sapphire. An output power of 200 W was achieved from a single Nd:YVO/sub 4/ slab in a zigzag configuration when pumped with 600-W diodes at 808 nm. The maximum output power previously reported in the literature using conventional cooling schemes is only about 100 W. A 2.3/spl times/4/spl times/24 mm/sup 3/ slab was pumped from its broad side (4/spl times/24 mm/sup 2/) through a 0.3-mm-thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing, acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. By using a BBO Q-switch, 70-W average power was obtained at 20 kHz with a pulse width of 19 ns and with a beam quality of 3 and 12 times the diffraction limit in the zigzag and transverse directions, respectively. The output of a two-head configuration was 295 W. In addition, a cavity was designed to achieve increased beam quality and 133 W was accomplished with a beam quality of 2 and 7.5 times diffraction-limited on the zigzag and nonzigzag axes, respectively. Operating this cavity with an RTP Q-switch produced 114 W with a beam quality of 1.5 and 9.5 on the respective two axes.

Raphael Lavi

Papers

Efficient pumping scheme for neodymium-doped materials by direct excitation of the upper lasing level

Thermally boosted pumping of neodymium lasers

Photodissociation followed by laser-induced fluorescence at atmospheric pressure and 24 degrees C: a unique scheme for remote detection of explosives.

Highly efficient doubling of a high-repetition-rate diode-pumped laser with bulk periodically poled KTP.

Diamond cooling of high-power diode-pumped solid-state lasers