Raphael Lavi

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

An efficient pumping scheme that involves direct excitation of the upper lasing level of the Nd(3+) ion is demonstrated experimentally. The results obtained for direct upper laser level pumping of Nd:YAG R2 (869 nm) and Nd:YVO(4) (880 nm) were compared with traditional approximately 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:YVO(4), 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.

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

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 .

Thermally boosted pumping of neodymium lasers.

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 (lambda(pump) = 808 nm) and by 7% compared with ground-state direct pumping (lambda(pump) = 869 nm). The combined transition from the first and second thermally excited Stark components of the ground state (4I(9/2)) to the upper lasing level (4F(3/2)) 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.

Application of a unique scheme for remote detection of explosives

Abstract We report on an application for remote detection of explosives containing nitro groups. This is a step forward in implementing a unique scheme introduced recently as a remote, real-time, detection tool. The scheme is applied to a system for 2,4,6-trinitrotoluene (TNT) vapor detection. The sensitivity of our systems is better than 15 parts per billion (ppb) for near ambient conditions (i.e. atmospheric pressure and explosive sample holder at temperature of 28 °C) and at a range of 2.5 m. The response time of the system is about 15 s.

885 nm high-power diodes end-pumped Nd:YAG laser

Direct pumping from thermally excited ground levels directly to the upper lasing level of Nd:YAG was demonstrated using high-power, 885 nm, CW diodes. 14 W output power with 0.63 slope efficiency and 0.53 light-to-light efficiency were obtained.

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

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.

Reduction of thermally induced lenses in Nd:YAG with low temperatures

At high pump powers, Nd:YAG lasers suffer from thermally induced optical effects such as graded-index lenses, birefringence, and bi-focusing. Due to more favorable material properties at low temperatures, the thermal lenses are significantly reduced by cryogenic cooling. In the present study, we measured the thermal lens in a Nd:YAG rod to be reduced by more than an order of magnitude when cooled with liquid nitrogen.

Extractable energy from ytterbium-doped high-energy pulsed fiber amplifiers and lasers

An analytic model is developed to evaluate the extractable energy from high-energy pulsed ytterbium-doped fiber amplifiers and lasers. The energy extraction capabilities under the limitation of spurious lasing, due to amplified spontaneous emission, are mapped for various numerical apertures, single- and multitransverse-mode evolution, and operating wavelengths. The calculation results of the analytic model show a good match with experimental results carried out for various double-clad fiber amplifiers. The model provides an accurate assessment for the maximum pulse energy that can be extracted from a given ytterbium-doped fiber. In addition, for a specific pump power, the model can be used to determine the minimum repetition rate and optimal length, under which the laser source can be operated before spurious lasing occurs.

Nd:YAG laser pumped at 946 nm

A Nd:YAG laser crystal was pumped at 946 nm and lased at 1064 nm. This pump-lase format was investigated in order to reduce the quantum defect between the pump and laser photons as compared to other pump schemes of this material. To the best of our knowledge, this is the first realization of this scheme. A room temperature absorption coefficient and linewidth of approximately 0.075 cm(-1) and approximately 1 nm for 1% at. Nd(+3) concentrations were measured for the 946 nm absorption line. Those parameters impose both narrow-bandwidth pumping and a long absorption path. By increasing the laser crystal temperature above room temperature, the absorption cross sections at 946 and 938 nm increase due to enhanced thermal population of the upper energy level of the ground manifold. The possibility of exploiting this phenomenon to enhance the pump absorption is also discussed.

Highly efficient low-threshold tunable all-solid-state intracavity optical parametric oscillator in the mid infrared.

An intracavity critically phase-matched optical parametric oscillator (OPO) was demonstrated with a LiNbO(3) crystal placed inside the resonator of a diode side-pumped Q-switched Nd:YAG laser. The OPO acted as a singly resonating cavity on the signal with a threshold below 10 MW/cm(2). Output energy as high as 640 microJ was observed at the idler wavelength of 3.7 microm, which corresponds to a conversion efficiency of 1.8% from diode light to idler light. Tunability over the wavelength range 3.3-4.15 microm was obtained by rotation of the LiNbO(3) crystal.