External frequency conversion of cw mode-locked Ti:Al(2)O(3) laser radiation

Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler

Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media.

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.

High-efficiency blue generation by frequency doubling of femtosecond pulses in a thick nonlinear crystal

We have demonstrated highly efficient frequency doubling of femtosecond pulses in a thick, noncritically phase-matched KNbO(3) crystal under conditions of large group-velocity mismatch. At low power we observed a slope efficiency of ~300% nJ (-1) for harmonic conversion, and at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusing dependence for our conditions of large group-velocity mismatch is considerably different from that obtained for frequency doubling of continuous-wave light. We have also demonstrated that one can tune the spectral width of the generated blue light by varying the focusing conditions.

Tunable picosecond blue and ultraviolet pulses from a diode-pumped laser system seeded by a gain-switched laser diode

Picosecond pulses emitted from a gain-switched laser diode have been amplified in a Ti:sapphire regenerative amplifier indirectly pumped by a 4-W laser diode. This all-solid-state system produced microjoule pulses tunable from 803 to 840 nm at repetition rates up to 25 kHz with durations of 70-100 ps. By frequency doubling and tripling the output, we generated blue and UV pulses tunable from 401 to 420 nm and from 268 to 280 nm, respectively. Average powers larger than 4 mW were reached in these two wavelength regions.

Generation of the fourth harmonic of a femtosecond Ti:sapphire laser

A high-repetition-rate femtosecond laser system operating, for the first time to our knowledge, in the spectral region near 200 nm is described. Frequency quadrupling of a mode-locked Ti:sapphire laser results in maximum average powers of 6 mW (165-fs pulses) and 15 mW (340-fs pulses) at 82 MHz. The shortest wavelength achieved is 193.7 nm.

Generation of 16-fs pulses at 425 nm by extracavity frequency doubling of a mode-locked Ti:sapphire laser

An efficient doubling scheme capable of producing 16-fs pulses centered at 425 nm with an average power of 40 mW is described. The system uses 15-fs pulses from a continuous-wave mode-locked Ti:sapphire oscillator centered at 850 nm. The pulse characteristics resulting from doubling with beta-barium borate crystals of various lengths are presented. The results compare favorably with previous attempts at intracavity doubling and provide a more convenient route to femtosecond experiments with fully synchronized second-harmonic radiation.

Quantitative concentration measurements of atomic sodium in an atmospheric hydrocarbon flame with asynchronous optical sampling

We report the development of a pump-probe instrument that uses a high-repetition-rate (82-MHz) picosecond laser. To maximize laser power and to minimize jitter between the pump- and the probe-pulse trains, we choose the asynchronous optical sampling (ASOPS) configuration. Verification of the method is obtained through concentration measurements of atomic sodium in an atmospheric methane-air flame. For the first time to our knowledge, ASOPS measurements are made on a quantitative basis. This is accomplished by calibration of the sodium concentration with atomic absorption spectroscopy. ASOPS measurements are taken at a rate of 155.7 kHz with only 128 averages, resulting in a corresponding detection limit of 5 × 10(9) cm(-3). The quenching-rate coefficient is obtained in a single measurement with a variation of ASOPS, which we call dual-beam ASOPS. The value of this coefficient is in excellent agreement with literature values for the present flame conditions. Based on our quantitative results for detection of atomic sodium, a detection limit of 2 × 10(17) cm(-3) is predicted for the Q(1) (9) line of A (2)Σ(+) (v = 0)-X(2)II (v = 0) hydroxyl at 2000 K. Although this value is too large for practical flame studies, a number of improvements that should lower the ASOPS detection limit are suggested.

High-power second-harmonic generation with picosecond and hundreds-of-picosecond pulses of a cw mode-locked Ti:sapphire laser

A cw mode-locked Ti:sapphire laser was efficiently frequency doubled with a lithium triborate crystal in an external enhancement cavity. Second-harmonic output powers of 1.28 W and 860 mW have been generated at 399 nm with fundamental pulse widths of 1.5 and 320 ps, respectively. At the 1.5-ps pulse width the conversion efficiency was 75%. The spectral width of the 320-ps pulses, produced by modification of a laser cavity, is as narrow as the bandwidth of a normal cw, multimode, narrow-bandwidth Ti:sapphire laser. The frequency-doubled output can be thus used as a high-power, narrow-bandwidth light source in the UV region.

Intracavity frequency doubling in a Ti:sapphire laser: generation of 14-fs pulses at 416 nm

We demonstrate the use of intracavity frequency doubling in a self-mode-locked Ti:sapphire laser to generate pulses as short as 14 fs. We use a 100-microm-thick lithium triborate (LiB(3)O(5)) crystal in a standard Ti:sapphire laser cavity for the frequency doubling. The average blue output power is 30 mW in each of two beams directly from the laser and 11 mW after compression. The repetition rate is 91 MHz, and the spectral bandwidth is a near-transform-limited 19.2 nm centered at 416 nm. To our knowledge, these results represent the shortest pulse duration yet demonstrated in the blue region of the spectrum.

Terawatt KrF/Ti:sapphire hybrid laser system

A terawatt KrF/Ti:sapphire hybrid laser system with a high contrast between amplified spontaneous emission and the main pulses has been developed with an output energy of 130 mJ in 130-fs pulses. An efficient frequencytripling scheme with Ti:sapphire is demonstrated with an efficiency of 11% in the 100-fs regime with LiB(3)O(5) and beta-BaB(2)O(4).