H. J. Eichler

Dual-wavelength Q-switched Er:YAG laser around 1.6 μm for methane differential absorption lidar

A pulsed Er:YAG laser with resonant in-band diode pumping was demonstrated. Laser pulses with 6.6 mJ energy and 50 ns pulse duration were generated. With an intra-cavity etalon a broad wavelength tuning range from 1643.47 to 1646.78 nm was obtained. A stable dual-wavelength operation was observed by fine tuning of the etalons. The separation of the two wavelengths was 0.78 nm. One of them was located within a methane absorption line and the other was off the line. The frequency stability of the narrow linewidth Er:YAG laser was less than 40 MHz. It is a very compact laser source for methane differential absorption lidar.

Resonantly diode pumped Er:YAG laser systems emitting at 1645?nm for methane detection

We report on the development of compact and frequency-stable Er:YAG laser systems emitting in the eye-safe spectral region. Resonant cw diode pumping provides 4.5 W output power in cw operation and 2.2 mJ in Q-switched operation with pulse duration of about 140 ns. The application of intra-cavity etalons allows for wavelength tuning from 1645.22 to 1646.33 nm while the frequency stability accounts for less than 50 MHz. The potential of the erbium laser sources in terms of methane detection was evaluated under laboratory conditions by absorption measurements employing a multi-pass absorption cell. The experimental investigations were accompanied by theoretical studies on the influence of pressure broadening on the absorption behavior of methane. (Some figures may appear in colour only in the online journal)

Multi-octave frequency comb generation by χ(3)-nonlinear optical processes in CVD diamond at low temperatures

We study stimulated Raman scattering (SRS) and Raman four-wave mixing (RFWM) processes in chemical vapour deposition (CVD) diamond crystals. The strong interaction of picosecond laser pulses at 1.064 and 0.532u2009µm wavelength with the dominant Raman-active F2g mode results in a more than two-octave-spanning frequency comb, ranging from the UV to the NIR spectral region. In addition to spectroscopic analysis of the χ(3)-nonlinear emission at room temperature, comparative measurements have been carried out at cryogenic temperature of 10u2009K. The Raman shift and the Raman gain are found to change only weakly. Moreover, the potential of SRS frequency comb generation in CVD diamond for ultra-short pulse synthesis is discussed.

Frequency conversion concepts for the efficient generation of high power 935 - 942 nm laser radiation

The three-dimensional measurement of the global water vapor distribution in the atmosphere considerably improves the reliability of the weather forecast and climate modeling. A spaceborne Differential Absorption Lidar (DIAL) is able to per-form this task by use of suitable absorption lines of the broad absorption spectrum of water vapor. Because no interference with the absorption of other molecules exists, the range of 935/936 nm, 942/943 nm are the most preferred wavelength ranges for a water vapor DIAL. The challenge is to develop a dedicated efficient high power laser source emitting at these wavelengths. The comparison between frequency converters based on stimulated Raman scattering (SRS) and Ti:Sapphire and the directly generated Mixed Garnet laser shows the favorable properties of each concept and helps to evaluate the most suitable concept. Development of Raman frequency converters for high pulse energies concentrates on linear resonator de-signs and seeding using the Raman material as a direct amplifier based on Raman four-wave-mixing. In addition a seeded and frequency stabilized pulsed Ti:Sapphire laser system with output pulses up to 22 mJ injection-seeded at the water vapor absorption line at 935.684 nm with a spectral purity up to 99.9 % has been developed. Direct generation of the wavelengths 935/936 nm and 942/943 nm required for water vapor detection is possible with diode-pumped, Nd-doped YGG- and GSAG-crystals. First experiments resulted in pulse energies of 18 mJ in Q-switched and 86 mJ in free-running operation at 942 nm wavelength.

Lithium silicate, LiAlSi4O10 (petalite)—a novel monoclinic SRS-active crystal

The discovery of χ (3)–nonlinear lasing by stimulated Raman scattering (SRS) in natural monoclinic crystal of petalite, LiAlSi4O10, is reported. All recorded Stokes and anti-Stokes lasing sidebands under picosecond laser pumping in the visible are identified and attributed to the two occurring SRS-promoting phonon modes with ω SRS1 ≈ 490 cm−1 and ω SRS2 ≈ 357 cm−1. A short review of known SRS-active crystalline minerals and observed manifestations of their χ (2)– and χ (3)–nonlinear interactions is given as well.

Stimulated Raman scattering spectroscopy of Y0.4Gd0.6VO4 crystals with partly disordered zircon-type structure—observation of new χ(3)-nonlinear optical interactions

This letter presents the results of stimulated Raman scattering (SRS) spectroscopy of tetragonal Y0.4Gd0.6VO4 vanadate crystals having a partly disordered zircon-type structure. Picosecond laser pumping revealed previously unexplored χ(3)-nonlinear interactions comprising the generation of an almost two-octave spanning Stokes and anti-Stokes frequency comb, self-third harmonic generation as well as cross-cascaded processes involving four different Raman-active modes. The recorded spectral lines have been assigned to the participating molecular vibrational modes of the (VO4)3− structural units of the studied crystal. In addition, this work provides a brief review of all known SRS-active tetragonal and monoclinic vanadates of the form REVO4 including their χ(3)-nonlinear properties and steady-state Raman gain coefficients for first Stokes generation.

High aspect-ratio laser-drilling of micro-holes with a Nd:YAG master-oscillator power-amplifier (MOPA) system

A nanosecond Nd:YAG-MOPA-system with high average power at excellent beam qual-ity was used for laser drilling experiments into metals and ceramics. The output power of the oscillator (10u2005W@1064u2005nm) with a beam quality of M² = 1.3 was amplified to 95u2005W@1064u2005nm with M² = 2.3 and a pulse energy up to 500 mJ. Ablation rates in dependence of fluence, pulse-shape and ambient pressure were detected. The influence of beam quality on the geometrical shape of the hole was investigated. Additionally means for optimizing the drilling process such as burr reduction and cylindrical constancy were investigated. A maximum aspect-ratio of 1:200 was obtained.A nanosecond Nd:YAG-MOPA-system with high average power at excellent beam qual-ity was used for laser drilling experiments into metals and ceramics. The output power of the oscillator (10u2005W@1064u2005nm) with a beam quality of M² = 1.3 was amplified to 95u2005W@1064u2005nm with M² = 2.3 and a pulse energy up to 500 mJ. Ablation rates in dependence of fluence, pulse-shape and ambient pressure were detected. The influence of beam quality on the geometrical shape of the hole was investigated. Additionally means for optimizing the drilling process such as burr reduction and cylindrical constancy were investigated. A maximum aspect-ratio of 1:200 was obtained.

High-Speed Fabry–Pérot Optical Modulator in Silicon With 3-μm Diode

A silicon optical modulator using a novel node-matched-diode geometry inside a Fabry-Pérot nanowaveguide resonator is presented. The diode length is only 3 μm. An analysis of the dynamic spectral transmission behavior is given. Non-return-to-zero data transmission of 10 Gb/s is demonstrated.

Monoclinic m-LaVO4—a novel high Raman gain SRS-active crystal

Monoclinic m-LaVO4, having a monazite-type structure and being known as a host crystal for Nd3+-lasants, is an attractive χ(3)-nonlinear optical material for Raman laser converters. This paper presents the stimulated Raman scattering (SRS) spectroscopy of m-LaVO4 in the visible and near-IR spectral range using picosecond laser excitation. High-order SRS and Raman-induced four-wave parametric mixing processes result in an almost two-octave-wide Stokes and anti-Stokes frequency comb. All the measured spectral lines were assigned to the participating molecular vibration modes of the studied vanadate. Finally, an estimation of the steady-state Raman gain coefficient for first Stokes generation in near-IR was performed.

Efficient Er:YAG lasers at 1645.55 nm, resonantly pumped with narrow bandwidth diode laser modules at 1532 nm, for methane detection

Eye safe laser operation at 1645.55 nm (6077 cm-1) of resonantly pumped Er:YAG laser systems is demonstrated in cw and Q-switched operation. High brightness diode laser modules emitting at 1532 nm have been utilized as pump sources providing an absorption efficiency of up to 96%. This leads to an overall efficiency of the Er:YAG laser of 30%. For cw operation, 9 W output power is possible at pump power of 30 W while Q-switching results in generation of more than 7 mJ pulses with duration of 60 ns and repetition rate of 500 Hz. The Er:YAG laser systems have been applied for methane detection measurements demonstrating their feasibility for CH4-DIAL applications