Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere
require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus
and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width
below 50 MHz at 30 ns pulse duration. Transferred to the fundamental wavelength of the laser systems the stability
requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the
laser system is much higher in airborne and spaceborne systems, especially at high frequencies of some hundred Hertz or
even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations
sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration
frequencies of the order of the pulse repetition frequency.
In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the
cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-
Fire - is theoretically and experimentally investigated and compared.
The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators,
which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver
diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition
The Australian Consortium for Interferometric Gravitational wave Astronomy (ACIGA) is carrying out research on the detection of gravitational waves using laser interferometry. Here we discuss progress on each of the major sub systems: data analysis, lasers and optics, isolation suspension and thermal noise, and configurations, and report on the development of a high optical power test facility in Gingin, Western Australia.
The rod geometry for high power solid state lasers has been proven to be both, reliable and reasonably inexpensive. On the other hand, setting up rod lasers with excellent beam quality implies a number of problems that have to be tackled. One of the most serious problems for isotropic crystals like Nd:YAG might be the effect of depolarization and bifocusing due to thermally induced birefringence (tib). The effect of tib can be compensated by a 90 degree(s) polarization rotation between two identical rods. However, because of the finite length of the laser rods in an optimized compensation scheme, matching the two paths in the two laser rods becomes necessary. By doing this, one wide stability region is yielded. The laser behaves like a single rod laser, and the beam path for the radial and tangential eigenmode becomes the same. Thereby, excellent beam qualities at high output powers can be achieved. Our Nd:YAG double rod system provides a maximum average output power of 180 W with a beam propagation factor of M2< 1.2. It is quasi cw diode pumped with a repetition rate of around 1 kHz and a pump pulse length of 250 microsecond(s) . The laser heads contain 3 star like arranged sets of diode bars being set up in a complementary position to each other. The maximum average pump power per laser head is 800 W.
Current commercially available diode lasers with output powers above a few watts lack beam quality, i.e. they have only limited possibility of small foci in combination with long Rayleigh lengths. Recent advances in coherent coupling of such lasers open view to a new generation of high power, high beam quality, low cost lasers suitable for a wide range of technical applications such as microshaping or cutting. Therefore, we performed experiments to couple the 25 diode lasers of a bar with specially coated low-reflection front facets. Mutual coherence can be improved in external resonators as opposed to the internal resonator absent in our case. Additional elements like mode stops can improve beam quality. Here we present results on the coupling of gain- guided broad-area diode lasers in external resonators, both of single emitters and bars of 25 emitters. In the single emitter case we achieved output powers up to 0.8 W at a beam quality of M2 equals 16 or 0.4 W with M2 equals 3.5 along slow axis. For the bars we achieved 10 W with M2 equals 304.
Current commercially available diode lasers with output powers above a few watts lack beam quality, i.e. the ability to be precisely focused. Recent advances in coherent coupling 1,2 of such lasers open view to a new generation of high power, high beam quality, low cost lasers suitable for a wide range of technical applications such as microshaping or cutting. Therefore, we couple bars of 25 diode lasers with total output powers of 25-40 Watts and specially coated lowreflection front facets. Mutual coherence is achieved in external resonators as opposed to the internal resonator absent in our case. Additional elements like mode stops can improve beam quality. Here we present results on the coupling of gain-guided broad-area diode lasers in external resonators, both of single emitters and bars of 25 emitters. Also numerical simulations concerning the mutual coherence of the single emitters have been performed.
Many applications of solid state lasers in research, material processing and especially in micromachining and medicine require high brightness. Resonators for solid state lasers with phaseconjugate mirrors based on stimulated Brillouin scattering (SBS) allow significant improvement of the beam quality up to theoretical diffraction limit at high average powers. We could demonstrate operation of a flashlamp-pumped Nd:YAG laser with SBS mirror with an average output power of more than 30 W and TEMoo-mode. The beam diameter in the laser rod of 8 mm diameter was larger than 5 mm.
For industrial applications of high-average power Nd:YAG- lasers the laser power is usually transmitted through all- silica optical fibers. The transmission properties of different types of step index and graded-index fibers are investigated, using a multimode high-power Nd:YAG rod laser with 2000 W output power in CW and Q-switch mode. The fibers are step index and graded-index fibers with 400 and 600 micrometers core diameters, different cladding to core ratios and different types of coating materials. The dependence of the output beam parameters, waist diameter and divergence, and the resulting power transmission are given. The upper limits for the maximum beam parameters and maximum laser powers which can be coupled into fibers per limits for the maximum beam parameters and maximum laser powers which can be coupled into fibers without loss, as well as the dependence of the output beam profiles upon the intensity distribution at the fiber input are briefly discussed. The end faces of polished and cleaved fibers are compared. Different set-ups for coupling several laser beams into one fiber, in order to increase the maximum laser power to be transmitted, are discussed. By these means more than 6kW laser power could be transmitted.
The transmission properties of different types of all-silica fibers are investigated, using a multimode high-power cw Nd:YAG rod laser with 2000 W output power. The dependence of the output upon the input beam parameters, waist diameter and divergence, and the resulting power transmission are given. The upper limits for the maximum beam parameters and maximum laser powers, which can be coupled without loss into fibers with certain numerical apertures, as well as the dependence of the output beam profiles upon the intensity distribution at the fiber input are briefly discussed. An experimental set-up is presented, whereby the beam parameters are chosen and the measuring procedures are performed as suggested by the ISO. The waist diameter at the fiber end face, dependent on the laser power, is observed, so controlling the influence of the spherical aberrations of the fiber coupling lens systems. The end faces of polished and cleaved fibers are compared.