Generation of 840 mW of red light by frequency doubling a diode-pumped 1342 nm Nd:YVO4 laser with periodically-poled LiTaO3

We report an efficient generation of red light in a periodically-poled LiTaO₃ (PPLT) crystal by extracavity single-pass frequency doubling of a diode-pumped, Q-switched Nd:YVO₄ laser at 1342 nm. The sample used in the experiment is 20 mm in length and 14.77 μm in period. An average power of 840 mW of the 671 nm red light is obtained with a 808 nm pump of 12.3 W, the overall optical-to-optical efficiency being 6.8%. The measured effective nonlinear coefficient of the sample is ∼3.8 pm/V. The high conversion efficiency and output power demonstrate that the periodically-poled crystal serving as a frequency conversion device may be used in practice to construct an all-solid-state red laser based on an extracavity single-pass quasi-continuous scheme. Received: 17 September 2001 / Revised version: 23 January 2002 / Published online: 8 May 2002     

20-Hz operation of an eye-safe cascade Raman laser with a Ba(NO3)2 crystal

Operation of a 1.598-μm eye-safe third-Stokes Raman laser with a Ba(NO₃)₂ crystal pumped by a 1.064-μm Nd:YAG laser is described. We observed a substantial decrease in the output energy during the first 50 s of the continuous operation at 20 Hz. The energy drop is ∼76% of the initial third-Stokes output. We confirmed negative thermal lensing and thermally induced birefringence in the crystal. With a concave cavity mirror at a matched curvature to the thermal lensing, we obtained an output energy of 11 mJ at 20 Hz. TEM₀₀ output was also obtained with a smaller pump-beam diameter with a highest conversion efficiency of 15.5% for a pumping power of only 45 MW/cm² (0.9 J/cm²). Received: 20 November 2001 / Revised version: 20 February 2002 / Published online: 2 May 2002     

Adaptive shaping of two-cycle visible pulses using a flexible mirror

We present a double-pass non-collinear optical parametric amplifier (NOPA) that delivers 4-fs visible–near-infrared pulses. The paper discusses geometrical and temporal properties of the NOPA configuration, which allow us to broaden the gain band of simultaneous parametric amplification in excess of 250 THz. The key elements of the bandwidth enhancement include (i) the use of a broadband second-harmonic pump, (ii) optimization of the incidence angle of all monochromatic spectral components of the pump in order to produce a favorable offset of the corresponding phase-matching curves, and (iii) selection of the chirp rate and time delay between the stretched pump and seed pulses. We next devise a two-stage compressor that incorporates a flexible mirror for adaptive pulse shaping and develop a simple and trustworthy feedback loop based on a one-dimensional spectral measurement. Our rapid numerical algorithm for adaptive control of the flexible membrane is found superior to more complex search routines that are less resistant to the fluctuations of the laser intensity. The automated optimization procedure results in the generation of two-cycle pulses with a carrier wavelength at ∼600 nm. The absence of deep modulation on the parametrically amplified spectrum in combination with the adaptive phase correction lead to a high quality of the temporal profile and allow concentrating 90% of the pulse energy within only a 7.5-fs time window. Received: 8 April 2002 / Revised version: 26 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Present address: Institut für Photonik, Technische Universit?t Wien, Gusshausstrasse 27, 1040 Vienna, Austria RID="**" ID="**"Corresponding author. Fax: +81-3/5841-4165, E-mail: kobayashi@phys.s.u-tokyo.ac.jp     

Photoemission study of clean and c(4×2)-2CO-covered Pt (111) using high-harmonic radiation

An extreme ultraviolet (EUV) laser light source based on high-harmonic generation is presented. Coherent radiation in the photon energy range hν=20–120 eV is produced in the conversion media argon, neon and helium. High-harmonic radiation in the energy range 20–50 eV is applied to investigate photoemission spectra of Pt (111) and CO/Pt (111). In the photoemission spectra of the clean surface, new secondary electron emission structures are found which influence the cross section analysis of the CO states. When taking these Pt resonances into consideration, the 4σ and 5σ CO shape resonances are found at photon energies of 37 eV and 28 eV, respectively. Additionally, a resonance at hν=31 eV is also observed for the CO 1π state, in contrast to formerly published experimental data. Experimental and theoretical data suggest that this resonance is not connected to the well-known shape resonances in the σ-channel. Based on theoretical approaches, it is identified as an autoionization resonance. Received: 8 April 2002 / Accepted: 22 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-251/833-3604, E-mail: kutzner@uni-muenster.de     

Femtosecond regenerative amplification in Cr:forsterite at 5–10 kHz

We developed a high-power oscillator–regenerative amplifier femtosecond laser system. Based on chromium-doped forsterite. The system is operating near 1.25 μm at a 5–10 kHz repetition rate. Chirped-pulse amplification produced 0.86 W (0.75 W) of average power, or 465 mW (400 mW) after compression at 5 kHz (10 kHz). Nearly bandwidth-limited pulses of duration 135 fs (shortest) and 150 fs (typical) are available with an energy of 93 μJ and 40 μJ at 5 and 10 kHz, respectively. Received: 7 June 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-30/63921289, E-mail: petrov@mbi-berlin.de     

A dispersionless Michelson interferometer for the characterization of attosecond pulses

A novel application of a free-standing transmission grating as a beam splitter in a Michelson-type interferometer is described. The arrangement can operate in the XUV and soft X-ray spectral region and, therefore, it is well suited for the characterization of attosecond pulses. Using ray-tracing codes, we have analyzed three different setups in which spherical mirrors are employed in conjunction with the transmission grating and have investigated in detail their dispersive characteristics. It is shown that such an arrangement can be made to exhibit group-delay dispersion of ∼1 as² while it provides two co-propagating and converging beams. Received: 12 October 2001 / Revised version: 17 December 2001 / Published online: 7 February 2002     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca     

High-resolution sum-frequency generation from alkylsiloxane monolayers

During the last decade, infrared optical sum-frequency generation (SFG) has been demonstrated to be a valuable technique for acquiring vibrational spectra of molecules at interfaces. However, the instrumentation that most frequently has been employed, which is based upon picosecond-scale pulse-width lasers, is fundamentally limited in the spectral resolution that can be achieved (typically ∼6 cm^-1). We have optimized an older technology, based on a nanosecond-scale pulse-width laser, which enables the infrared frequency to be specified to within 0.2 cm^-1. Exploiting this advantage, we have observed previously unrecognized features in the SFG spectrum of self-assembled monolayers. We demonstrate this through experiments performed on rubbed octadecylsiloxane (ODS) on glass, a system of considerable importance in liquid-crystal-display technology. Received: 14 August 2001 / Revised version: 15 January 2002 / Published online: 14 March 2002     

Pulse-parameter dependence of the configuration characteristics of a micro-structure in fused SiO2 induced by femtosecond laser pulses

The fabrication of a micro-structures in fused SiO₂ by femtosecond laser pulses was studied in the range of pulse duration 150∼500 fs, pulse energy 60 nJ to ∼7 μJ and wavelengths 400 nm and 800 nm. The characteristics of the cross section of the micro-structure were explained with the multiphoton-ionization and avalanche-ionization processes in the femtosecond laser-induced breakdown. The characteristics of the longitudinal length of the micro-structure relating to the direct-focusing and self-action effects are also discussed. Received: 10 January 2001 / Accepted: 7 April 2001 / Published online: 23 May 2001     

Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone

A pulsed optical parametric oscillator (OPO) with intracavity sum frequency mixing was developed generating energies of up to 16 mJ in the 281–293 nm wavelength range. Both OPO process and sum frequency mixing are pumped by the harmonics of a single, medium-sized Nd:YAG laser. The system is characterized by a high overall efficiency (∼4% conversion from 1064 nm to the UV), a very compact set-up and stable and reliable operation. This system was successfully employed to measure tropospheric ozone using the differential absorption lidar (DIAL) technique and shows much promise as a lidar transmitter in airborne case studies as well as in unattended lidar systems for long-term monitoring. An unattended ozone profiling system could already be successfully realized. Received: 1 April 2002 / Revised version: 30 May 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-8153/28-1271, E-mail: Andreas.Fix@dlr.de     

Generation of 100 μJ pulses at 82.8 nm by frequency tripling of sub-picosecond KrF laser radiation

. Investigations of the efficient generation of powerful coherent radiation at 82.8 nm by frequency tripling of short-pulse KrF laser radiation are presented. Argon gas is selected as nonlinear medium due to the resonantly enhanced 3rd-order susceptibility χ⁽³⁾(-3ω,ω,ω,ω). Pulse energies of 100 μJ at 82.8 nm have been measured for a pump pulse energy of 14 mJ. An upscaling to more than 500 μJ is expected with available more powerful pump lasers. Features of this XUV source and possible applications are discussed. Received: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-511/7622211, E-mail: reinhardt@iqo.uni-hannover.de     

A 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator with cw diode laser injection seeding

We report on an injection-seeded 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator (OPG) based on a 55 mm long crystal of periodically poled lithium niobate (PPLN) with a quasi-phase-matching (QPM) grating period of 29.75 μm. The OPG is excited by a continuously diode pumped mode-locked picosecond Nd:YVO₄ oscillator-amplifier system. The laser system generates 7 ps pulses with a repetition rate of 82.3 MHz and an average power of 24 W. Without injection-seeding the total average output power of the OPG is 8.9 W, which corresponds to an internal conversion efficiency of 50%. The wavelengths of the signal and idler waves were tuned in the range 1.57–1.64 μm and 3.03–3.3 μm, respectively, by changing the crystal temperature from 150 °C to 250 °C. Injection seeding of the OPG at 1.58 μm with 4 mW of single frequency continuous-wave radiation of a distributed-feedback (DFB) diode laser increases the OPG output to 9.5 W (53% conversion efficiency). The injection seeding increases the pulse duration and reduces the spectral bandwidth. When pumped by 10 W of 1.06 μm laser radiation, the duration of the signal pulses increased from 3.6 ps to 5.5 ps while the spectral bandwidth is reduced from 4.5 nm to 0.85 nm. Seeding thus improved the time-bandwidth product from 1.98 to a value of 0.56, much closer to the Fourier limit. Received: 29 April 2002 / Published online: 8 August 2002     

Ti:sapphire rib channel waveguide fabricated by reactive ion etching of a planar waveguide

We report, to our knowledge, the first active channel waveguide in Ti:sapphire. We have created ∼1.4-μm high ribs in a ∼10-μm thick Ti:sapphire planar waveguide by reactive ion etching. Following excitation by an Ar-ion laser, the rib structure showed channel-waveguide fluorescence emission. The mode profiles and the beam-parameter values (M²) were measured. The coupling efficiency of fluorescence emission into a single-mode fiber was an order of magnitude higher than for fluorescence from unstructured planar regions of the waveguide. Such devices are of interest as low-threshold tunable lasers and as broadband light sources in low-coherence interferometry. Received: 22 December 2002 / Revised version: 30 March 2002 / Published online: 8 August 2002     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Self-adaptive Nd:YLF holographic laser with selectable wavelength operation

We demonstrate, for the first time to our knowledge, operation of a self-adaptive holographic laser with wavelength selection, which uses Nd:YLF as the gain medium. The holographic laser was self-starting and, by virtue of the temporal dynamics in writing the gain grating, passively Q-switched the cavity to produce pulses with 20–50 ns duration. A pulse energy of 184 mJ was obtained from the system with beams having a TEM₀₀ spatial profile. Single-longitudinal-mode operation was observed with near-transform-limited bandwidth. Received: 30 July 2002 / Revised version: 1 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: s.y.lam@ic.ac.uk     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

Novel technique for the measurement of fiber dispersion properties

We present a novel method for measuring the linear and nonlinear dispersion properties of conventional and micro-structured fibers. It is based on the automated compensation of phase modulations using a high-resolution pulse-shaping device. No tunable laser source is required. Received: 20 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/947202, E-mail: stobrawa@ioq.uni-jena.de RID="**" ID="**"Present address: Carl Zeiss Meditec AG, G?schwitzer Strasse 51–52, 07745 Jena, Germany     

Phase-stabilized 4-fs pulses at the full oscillator repetition rate for a photoemission experiment

Compressing pulses of a mode-locked extended-cavity Ti:sapphire laser using a standard single-mode fiber and tilted-front-interface chirped mirrors yields phase-stabilized 4-fs, 3-nJ light pulses at the full, 24-MHz, repetition rate. The demonstrated source paves the way towards exploring interactions sensitive to the carrier-envelope-offset phase, such as, for example, photoemission from solid targets at moderate intensities (∼10¹² W/cm²). Received: 13 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/58801-38799, E-mail: apolonski@tuwien.ac.at     

Temperature-tuned difference-frequency mixing in periodically poled KTiOPO4

4 (PPKTP). We generated 12 μW of radiation tunable around 1.6 μm by difference-frequency mixing of the outputs of a frequency-doubled Nd:YLF laser at 523 nm (240 mW) and a tunable Ti:sapphire laser near 760 nm (340 mW). A temperature tuning rate of 0.73 nm/°C for the generated wavelength and a FWHM temperature acceptance bandwidth of 6.9 °C cm was observed. The effective d₃₃ coefficient was estimated to be ∼5 pm/V. Received: 02 September 1998