Experimental investigation and numerical simulation of the influence of resonator-length detuning on the output power,pulse duration and spectral width of a cw mode-locked picosecond optical parametric oscillator

We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency.     

Multiple Window and Cluster Size Scan Procedures

Researchers apply scan statistics to test for unusually large clusters of events within a time window of specified length w, or alternatively an unusually small window w that contains a specified number of events. In some cases, the researcher is interested in testing for a range of specified window lengths, or a set of several specified number of events k (cluster sizes). In this paper, we derive accurate approximations for the joint distributions of scan statistics for a range of values of w, or of k, that can be used to set an experiment-wide level of significance that takes into account the multiple comparisons involved. We use these methods to compare different ways of choosing the window sizes for the different cluster sizes. One special case is a multiple comparison procedure based on a generalized likelihood ratio test (GLRT) for a range of window sizes. We compare the power of the GLRT with another method for allocating the window sizes. We find that the GLRT is sensitive for very small window sizes at the expense of moderate and larger window sizes. We illustrate these results on two examples, one involving clustering of translocation breakpoints in DNA, and the other involving disease clusters.     

Laser excited fluorescence of CH in a low pressure flame

Production of CH(X²Π) in a low pressure H/O/C₂H₂ flame has been observed by laser excited fluorescence in the 0, 0 band of the A² Δ ← X²Π system at 431 nm. Laser excitation spectra have been recorded of the P, Q, and R branches with a bandwidth of 0.03 nm and of the head of the Q branch with a bandwidth of 0.0009 nm. Values of the spin—orbit and spin—rotation coupling constants in the A²Δ state have been obtained.     

Tunable UV radiation at short wavelengths (188–240 nm) generated by sum-frequency mixing in lithium borate

Sum-frequency mixing (₃=₁+₂) of UV laser radiation (₁=266 nm and 213 nm) and tunable coherent infrared light (₂=1.2–2.6 m) in lithium borate (LBO) generates radiation at short wavelengths (₃=188–242 nm). The UV radiation at ₁ is produced by the fourth and fifth harmonic of a pulsed Nd-YAG laser. The infrared light is generated with an optical parametric oscillator of beta barium borate. The phase-matching angle is measured as function of ₃ and compared with calculated values. For UV laser radiation at shorter wavelengths (173 nm₁213 nm) the calculations predict an extension of the tuning range of the sum-frequency generated at ₃ to wavelengths as short as the LBO transmission cutoff at 160 nm.     

Nanosecond optical parametric oscillator based on large-aperture periodically poled RbTiOAsO(4)

We report on optical parametric oscillators (OPO's) based on periodically poled RbTiOAsO(4) (PP RTA), which are pumped by Q -switched solid-state lasers. With a diode-pumped Nd:YVO(4) laser (pulse energy, 800microJ ; pulse duration, 5.5 ns; repetition rate, 1 kHz) the PP RTA OPO generated 1.58-microm signal and 3.26-microm idler radiation with a signal pulse energy of 45microJ . The large aperture of 3 mmx3 mm of the PP RTA crystal also permitted OPO operation with pump pulse energies as high as 65 mJ, provided by a flash-lamp-pumped Q -switched Nd:YAG laser (pulse duration, 20 ns; repetition rate, 10 Hz). With this pump source the OPO generated signal pulse energies as high as 17 mJ, corresponding to an efficiency of 26%. The performance of this OPO shows that large-aperture PP RTA crystals are well suited for pulsed nanosecond OPO operation with pump pulse energies of tens of millijoules.     

Tunable XUV radiation generated by nonresonant frequency tripling in neon

Nonresonant frequency tripling of ultraviolet (UV) radiation (λ_uv = 216–223 nm, generated by sum frequency mixing of the outputs of a frequency doubled pulsed dye laser and of the Nd-YAG pump laser) generates in neon coherent light in the extreme ultraviolet (XUV). The XUV radiation is tunable in spectral regions of negative dispersion λ_xuv = 72.05- 73.58 nm and λ_xuv = 74.3–74.36 nm at the high energy side of the transitions $\text{2p-3s′[}\text{built1}\text{2}\text{, 1]}$ and $\text{2p-3s[}\text{built3}\text{2}\text{, 1]}$. At UV input powers of 0.1-0.3 MW the generated XUV power was typically P_xuv = 0.1-0.4 W (1.5–6 x 10⁸ photons/pulse). Since present UV dye laser systems provide at λ_uv pulse powers of almost 1 MW the XUV output could easily be increased by more than one order of magnitude.     

Experimental investigation and numerical simulation of the spatio-temporal dynamics of the light-pulses in nanosecond optical parametric oscillators

This paper reports on the experimental investigation and the numerical simulation of the spatio-temporal dynamics of the pulse formation in nanosecond optical parametric oscillators. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD-camera. The measurements are performed for a signal-resonant nanosecond pulsed optical-parametric-oscillator (OPO) which consists of a 12 mm long, critically phase-matched beta-barium-borate (BBO)-crystal in a 40 mm long cavity of two plane mirrors. The OPO was pumped by the third harmonic of a flashlamp-pumped Nd:YAG-Laser at a repetition rate of 10 Hz. At pump energies close to the OPO threshold the emitted OPO-radiation has an almost gaussian intensity distribution throughout the entire pulse. The beam quality factor M² remains below 2.2. At high pump energies the OPO oscillation also starts with an almost gaussian beam-profile. During the build-up and parametric amplification of the pulse the fields experience, however, a spatially inhomogeneous gain, caused by walk-off in the birefringent crystal, pump-depletion, and back-conversion. The spatial intensity distribution thus becomes asymmetric and the M² value increases. The measurements are compared with the results of detailed numerical calculations. The model takes the amplification of the OPO-radiation in the nonlinear crystal, and the properties of the OPO cavity as well as the diffraction of the beams during propagation into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results. PACS 42.65.Sf; 42.65.Yj     

A sensitive non-destructive method for measuring the Nd doping concentration in Nd : YAG with high spatial resolution

This paper reports on an experimental method for measuring in Nd:YAG the Nd doping concentration (C) with high sensitivity (0.01 at.%Nd) and high spatial resolution (50 m). The method is based on the measurement of the fluorescence lifetime _f of the upper Nd laser level. Additional parameters required to determine C are the intrinsic fluorescence lifetime ₀ and the quenching parameter Q. The measured values of _f, ₀ (256.47±0.14 s) and Q (4.45±0.40×10²⁰ cm^-3) allow us to calculate the Nd concentration C with an absolute accuracy of 0.1 at.%Nd. By using this method C is measured at the outer surfaces of standard laser rods and at the faces of boule slices (with diameters of up to 50 mm). The obtained results demonstrate a detection sensitivity of 0.01 at.%Nd. PACS 42.70.Hj; 81.70.Fy     

Amplification and wave mixing of induced and spontaneous emission in semiconductor laser amplifiers

We theoretically describe and experimentally investigate the spatio-spectral wave mixing of induced and spontaneous emission in large-area InGaAs-semiconductor laser amplifiers. The dynamic light-matter-coupling is described by a spatially resolved theory based on Maxwell–Bloch–Langevin equations, taking into account many-body-carrier interactions, energy transfer between the carrier and phonon systems and, in particular, the spatio-temporal interplay of stimulated and amplified spontaneous emission and the noise caused by spontaneous emission. Our numerical model reveals the fundamental physical processes which are responsible for the spectral power distribution of the amplified laser light and predicts the emission properties of high-power semiconductor laser amplifiers, such as emission spectra and input power–output power characteristics. Received:30October2002/Revisedversion:21November2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-711/6862-349, E-mail: Edeltraud.Gehrig@dlr.de RID="**" ID="**"Also at: Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101, Tampere, Finland RID="***" ID="***"Present address: Heidelberger Druckmaschinen AG, Speyerer Strasse 6, 69115 Heidelberg, Germany RID="****" ID="****"Present address: Lightbit Corporation, 411 Clyde Avenue, Mountain View, CA 94043, USA     

Spectral properties and numerical modelling of a critically phase-matched nanosecond LiB3O5 Optical Parametric Oscillator

We report on the spectral properties and numerical modelling of a singly resonant Optical Parametric Oscillator (OPO) of Lithium-triBOrate (LBO). The OPO is pumped by the second, third or fourth harmonic of an injection-seeded,Q-switched Nd:YAG laser. The measured OPO parameters are the tuning range, the threshold, the spectral linewidth, the efficiency and the output power. For the LBO-OPO critical type-I phase-matching (xy-plane) provides a wide tuning range and optimum values of the effective nonlinear coefficient. Pumped, for example, by the 355nm third harmonic the spectral range extends from 414 nm to 2.47 µm. With a 15 mm long crystal the OPO generates a total output-pulse energy of 77 mJ with an efficiency of 45%. For 266 nm pump radiation the signal wave is in the near ultraviolet at 307–325 nm. If pumped at 532 nm the OPO generates simultaneously two pairs of signal and idler waves in the infrared (0.707–2.15 µm). The pulse-energy fluctuations of the two pairs are correlated. If, however, the OPO is injection-seeded at one of the signal waves the two wavelength pairs are anti-correlated. The observed wavelength tuning as well as the measured spectral line-widths, threshold energies and efficiencies are in agreement with the values predicted by computer simulation.