Thomas Hall

Off-axis negative-branch unstable resonator in rectangular geometry

The application of an off-axis negative-branch unstable resonator to an active medium of rectangular geometry is examined. The presented unstable resonator consists of spherical mirrors and a scraper mirror. The adaptation to the rectangular cross section is performed by the scraper, which takes two different shapes. One shape resembles a rectangular bracket [ and the other resembles the letter L. The [ and L configurations correspond to a shift of the optical axis away from the center of the cross section, toward one of the edges or toward one of the corners, respectively. Both scraper setups are examined numerically and experimentally. Experiments are performed with a multikilowatt class chemical oxygen iodine laser. The active medium is characterized by a low amplification coefficient. Measured results of the intensity distribution in the far field and of the phase distribution in the near field are shown for both resonator configurations. Using the same resonator magnification, the setup with the L-shaped scraper has a lower output coupling and, therefore, a higher output power and a slightly higher beam divergence. The L-shaped scraper configuration is able to cover the gain medium completely.

Numerical studies on hybrid resonators for a medium-sized chemical oxygen iodine laser

Off-axis negative-branch and off-axis positive-branch hybrid resonators, suitable for a laser with large medium cross section and a small output coupling, are explored numerically. The basis of our theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to mirror tilting, the calculations confirm that in the unstable direction, the off-axis negative-branch resonator is less sensitive to mirror misalignments than the off-axis positive-branch one. In the stable direction, the sensitivity to mirror misalignment is larger than in the unstable direction and is dependent on the mirror curvature. The fabrication of cylindrical mirrors for such hybrid resonators is difficult, and deviations in mirror radius of curvature are possible. The effects of such deviations are addressed. Near the nominal mirror parameters, the off-axis negative-branch unstable resonator is insensitive to mirror cur- vature variations if the resonator length is appropriately adapted to the mirror curvatures.

Laser-Based Space Debris Monitoring

The extended use of outer space left remarkable amounts of objects uncontrollably orbiting the earth. These objects, hereafter denoted as “space debris”, impose reasonable risk on present space missions, as they hold relative velocities on the order of 10 km/s. Due to their high kinetic energies, objects up from 1 cm size have to be monitored in order to prevent collisions with satellites. For the purpose of accuracy and effectiveness, we present a concept for a laser‐based space debris monitoring system. The chosen approach is primarily based on laser ranging of optically non‐cooperative targets. The involved physical processes such as optical turbulence, absorption, and scattering are modeled and the results will define the design of a future system. In addition, short range on‐ground tracking and ranging will be conducted where downscaling of photon densities simulates the strong signal attenuation and system feasibility will be proven.

Unstable resonator with reduced output coupling

The properties of a laser beam coupled out of a standard unstable laser resonator are heavily dependent on the chosen resonator magnification. A higher magnification results in a higher output coupling and a better beam quality. But in some configurations, an unstable resonator with a low output coupling in combination with a good beam quality is desirable. In order to reduce the output coupling for a particular resonator, magnification fractions of the outcoupled radiation are reflected back into the cavity. In the confocal case, the output mirror consists of a spherical inner section with a high reflectivity and a flat outer section with a partial reflectivity coating. With the application of the unstable resonator with reduced output coupling (URROC), magnification and output coupling can be adjusted independently from each other and it is possible to get a good beam quality and a high power extraction for lasers with a large low gain medium. The feasibility of this resonator design is examined numerically and experimentally with the help of a chemical oxygen iodine laser.

Simulations of wavefront correction of distorted laser beams

The transverse beam profile of solid-state lasers is a key issue for determining its propagation properties. Herein, we address laser beam distortions caused by thermal lensing and birefringence. For the simulations a finite difference method is used for the description of beam propagation problems. The method is an improved approach to the solution of the wave equation in paraxial approximation. Provided that steady-state conditions prevail, in comparison with the Fresnel-Kirchhoff integral description this method allows for a fully 3 D treatment of the aforementioned laser-optical phenomena.

Atmospheric propagation of high power laser radiation at different weather conditions

Applications based on the propagation of high power laser radiation through the atmosphere are limited in range and effect, due to weather dependent beam wandering, beam deterioration, and scattering processes. Security and defense related application examples are countermeasures against hostile projectiles and the powering of satellites and aircrafts. For an examination of the correlations between weather condition and laser beam characteristics DLR operates at Lampoldshausen a 130 m long free transmission laser test range. Sensors around this test range continuously monitor turbulence strength, visibility, precipitation, temperature, and wind speed. High power laser radiation is obtained by a TruDisk 6001 disk laser (Trumpf company) yielding a maximum output power of 6 kW at a wavelength of 1030 nm. The laser beam is expanded to 180 mm and focused along the beam path. Power and intensity distribution are measured before and after propagation, providing information about the atmospheric transmission and alterations of diameter and position of the laser beam. Backscattered laser light is acquired by a photo receiver. As a result, measurements performed at different weather conditions show a couple of correlations to the characteristics of the laser beam. The experimental results are compared to a numerical analysis. The calculations are based on the Maxwell wave equation in Fresnel approximation. The turbulence is considered by the introduction of phase screens and the “von Karman” spectrum.

Negative-branch unstable resonator in off-axis configuration for rectangular cross-sections

An off-axis configuration of the negative-branch confocal unstable resonator is examined numerically and experimentally for a gain medium with rectangular cross-section. Due to less diffraction effects such a configuration yields lower beam divergences than the standard on-axis resonator. The output coupling and the adaptation to the geometry of the gain medium are attained by a scraper. Two different scraper profiles are examined. One profile resembles to a rectangular bracket [ and the other profile resembles to the letter L. The experiments are performed with a 10 kW class chemical oxygen iodine laser (COIL), which has a medium of low gain. Both scraper profiles are applied to a resonator of the same magnification. Measurements of the intensity distributions in the near field and in the far field are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output power and a lower beam divergence, whereas the setup using the L-shaped scraper makes use of the complete gain medium. Furthermore, the L-shaped scraper is reusable for different resonator magnifications.

Investigation of beam quality and gain behavior in a large aperture e-beam controlled CO2 laser

An investigation of the parameters influencing the small- signal gain and the beam quality of the laser emission from a stable TEM00 cavity within a large aperture e-beam controlled carbon-dioxide laser discharge unit was performed. An almost linear dependency of the small-signal gain as a function of the energy loading was monitored. Small-signal gains of up to 2.4%/cm in the center of the discharge region were found for an energy loading of up to 140 J/(1 bar) at a wavelength of 10.59 micrometer. The beam quality evaluation was done by M2 parameter measurements according to the ISO standard. The fluence distribution was recorded with a beam analyzing system of 100 micrometer spatial resolution. No increase of the M2 parameter was found within the limits of measurement accuracy for a stable resonator configuration when the energy loading almost doubled from 61.7 J/(1 bar) to 112.5 J/(1 bar).

Parametric investigation of the small signal gain in a large aperture e-beam-controlled CO2 laser

In an e-beam controlled carbon-dioxide laser unit the spatial, temporal and spectral properties of the small-signal gain were investigated in a single pass geometry. A smooth spatial distribution was found with peak values of 2.9%/cm near the cathode at 112.5 J/(1 bar). No reduction in the gain value was monitored up to repetition rates of 60 Hz when circulating the laser gas with a gas flow velocity of 100 m/s. A HeNe laser probe beam, being collinear to the carbon-dioxide laser beam, was used to monitor shock waves or mechanical vibrations by measuring its deflection. This investigation showed, that the small-signal gain measurements were not interfered by any disturbances.

Unstable resonator with high magnification and low output coupling

The application of standard unstable resonators does not allow for an independent adjustment of the resonator magnification and the output coupling. Either you get a high magnification together with a high output coupling, or vice versa. Certain laser types, like e.g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order to apply unstable resonators with a high magnification also to low gain media an additional mirror surface retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently, whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 μm and demonstrate the feasibility of this resonator design.