Frank Kallmeyer

Single frequency operation of a tunable injection-seeded Nd:GSAG Q-switched laser around 942nm

Single frequency operation of a diode-pumped tunable injection-seeded Nd:GSAG Q-switched laser around 942nm was demonstrated. With a three-mirror ring cavity, the single frequency laser pulse with output energy of 13.2mJ was obtained at a repetition rate of 10Hz. The linewidth of the single frequency laser was less than 100MHz. The wavelength of the single frequency Nd:GSAG laser can be tuned from 942.38nm to 943.10nm.

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.

Efficient laser systems for 935 and 942 nm for water vapor lidar

Water vapour absorption wavelengths have been directly generated by diode pumped Nd:YGG crystals emitting at 935 nm and with Nd:GSAG crystals emitting at 942 nm in cw and pulsed operation. In addition the 1064 nm fundamental wavelength from Nd:YAG pump lasers with pulse lengths of 10 or 20 ns was shifted using Stimulated Raman Scattering (SRS) or Ti:Sapphire (TiSa) lasers. The potential of Nd:GSAG, Nd:YGG, SRS and TiSa laser systems is compared for future incorporation into a satellite based Lidar system. High output energies are possible by recent advances of fiber coupled diode sources allowing pulsed longitudinal pumping of Q-switched solid state lasers.

Tunable Nd:GSAG laser around 943 nm for water vapor detection

A Nd:GSAG laser operated at the 4F3/2 →4I9/2 transition was tuned by a FPI-etalon achieving a tuning range of 1.5 nm with a center wavelength at 942.7 nm. Three water vapor absorption wavelengths with different absorption strength as suitable for a water vapor LIDAR are within this tuning range and lasing could be achieved at all three wavelength. Q-switched pulse energies up to 26mJ were obtained as required for long range detection.

Determination of the Jones matrix of an LC cell and derivation of the physical parameters of the LC molecules

The wavelength-dependent Jones matrix representation of a twisted-nematic liquid crystal (TN-LC) cell contains four independent parameters. The absolute values of these parameters and two mutual sign relationships can be determined from comparatively simple transmission measurements of the TN-LC cells sandwiched between two rotatable polarizers. The physical parameters of the cell (twist angle α, director orientation ψ, birefringence β) can be retrieved if the Jones matrix is known for more than one wavelength. We have measured the Jones matrices of the TN-LC cells of a translucent Sony LCX-016 microdisplay for six wavelengths ranging from 488nm to 1064nm and determined the physical parameters of the cell. We have also measured the Jones matrices for one wavelength for a number of applied voltages. These experimental results show that it is not sufficiently exact to calculate the Jones matrix from the known physical parameters of the cell assuming a voltage-dependent birefringence only. We attribute the deviations from the theoretical model to edge effects which are not taken into account. The direct experimental determination of the Jones matrix components is therefore preferable and permits a more accurate simulation of the TN-LC microdisplay in experimental configurations involving other polarization-dependent optical components.

Nd:GSAG laser for water vapor detection by lidar near 942 nm

For weather forecast, especially for civil protection from high-impact weather events, measuring the three-dimensional distribution of water vapour by DIAL techniques is a fundamental concern. Especially for development and evaluation of atmospheric models, knowledge of water vapour distribution is important. Suitable wavelengths for a water vapour DIAL are e.g. around 943 nm. This region can be reached with well established technologies such as the optical parametric oscillator (OPO) and the Ti:Sapphire laser. But these systems suffer from low efficiency and complex set-up. In contrast the Nd:GSAG laser presented here can be directly pumped with 808 nm laser diodes. This supports the realisation of an efficient and compact laser system. Different oscillator and amplifier setups working at 943 nm were realised. An output energy of >17 mJ in a 100 ns pulse with 10 Hz repetition rate was demonstrated. In a MOPA system a double pass gain of 1.5 and an output energy of >18 mJ was achieved. The Nd:GSAG laser oscillator was successfully injection seeded with DFB laser diode from FBH-Berlin. Also the gain cross section in a Nd:GSAG laser crystal from 941-944 nm was measured. The FWHM of the homogeneous line is 2 nm with a peak stimulated emission cross section of 4.0•10-20 cm2 at 942.7 nm.

Blue Light Generation of Strongly Focused Gaussian Beams by Frequency Doubling of Nd:GSAG Laser

Frequency doubling of a focused Gaussian beam involves phase mismatching due to beam divergence. The conversion efficiency was calculated by integration second-harmonic (SH) field along the energy flow propagation path. The resulting SH field spatial distribution was discussed. We demonstrated the generation of blue light in critically type-I phase-matched LiB3O5 by frequency doubling of a diode-pumped electrooptic Q -switched Nd:GSAG laser at room temperature. Pulsed blue light with 8-mJ energy and 300-ns pulse duration at 471 nm was generated with 19-mJ incident radiation at 10 Hz. The maximum conversion efficiency was 42%. The experimental results agree well with our calculations.

Nd:GSAG Laser at 943 nm with High Pulse Energy

A Nd:GSAG laser end-pumped by pulsed laser diodes is presented. A maximum output energy of 160mJ at 10Hz repetition rate was obtained in free-running mode. In Q-switched mode an output energy of 31mJ was achieved.

Diode-pumped passively mode-locked Nd: GSAG laser at 942nm

Stable mode-locking laser pulse at 942nm by diode-pumped Nd:GSAG laser has been demonstrated for the first time. We obtained 8.7 ps pulses at repetition rate of 95.6 MHz and average output power of 220 mW.

Frequency doubling of focused laser beams for high pulse energy at 471nm

An actively Q-switched Nd:GSAG laser with 942nm wavelength was frequency doubled in a critically type-I phase-matched LBO. Maximum pulse energy of 8mJ with 300ns pulse duration at 471 nm was obtained with 19mJ incident radiation at 10Hz. The corresponding conversion efficiency was 42%. The frequency doubling of a focused Gaussian beam involves spatially dependent phase mismatching due to beam divergence. It decreases the conversion efficiency and deteriorates the beam quality. According to the theoretical calculation, elliptical focusing was used to improve the second harmonic beam quality and slightly increase the conversion efficiency.