A Tm:fiber-laser-pumped Ho:YLF non-planar ring laser with fractional image rotation (~77.45 degree per round trip) is presented. The ring laser cavity consists of six flat mirrors with an incident angle on all six mirrors of ∼32.7 degree with a total cavity length of 222 mm. To make the resonator stable, an intracavity lens has been used with different focal lengths of 75, 100 and 300 mm resulting in different TEM00 spot radii. With this configuration, a maximum laser power of 17.3 W was obtained at a wavelength of 2064 nm, corresponding to a slope efficiency of 28% (57%) with respect to incident (absorbed) pump power. The beam quality factor M2 was measured to be 6.4, 5.2 and 1.7 with focal length of 75, 100 and 300 mm, respectively. Near and far field images show, that the mode exhibits a strong narrow central spot with some ring structure around it, which can be understood as a superposition of Laguerre-Gaussian modes (l, p) with l = 0. Identical experiments have been carried out with a planar ring configuration without beam rotation resulting in comparable power performance but somewhat better beam quality with M2 values of 5.2, 3.5 and 1.4, respectively. The intensity distribution of near and far field in case of a planar cavity is more or less Gaussian.
We report on the growth, spectroscopy, and laser emission of two fluoride materials, LiLuF 4 and KY 3 F 10 , doped with Ho 3+ . In particular, laser emission has been obtained for the first time from a Ho 3+ :KY 3 F 10 crystal, grown with the Czochralski technique. Utilizing the micropulling down (μ-PD) method, we grew a Ho 3+ :LiLuF 4 crystal, which allowed us to obtain an efficient laser emission in the 2-μm spectral region. The KY 3 F 10 has a cubic symmetry and it is particularly suitable for technological applications, while the μ-PD technique is appealing for high-power applications. We present the spectroscopic characterization of the two crystals as a function of temperature in the range 10 to 300 K and few laser results.
The paper describes two laser prototypes devoted to the jamming or the damaging of heat-seeking missiles for use in field trials. The semi-ruggedized compact jamming prototype is based either on an OP-GaAs or a ZnGeP2 (ZGP) OPO directly pumped by a 2.09 μm Q-switched Ho3+:YAG laser with up to 20 W of average power around 2.1 μm and an M2 of less than 1.1. For jamming in band II, up to 3.5 W of average power were obtained and repetition rates from 20 kHz to 100 kHz were achieved. For 3.5 W of averaged output power, the M2 of the signal and idler beams were estimated to be less than 1.2. The destruction laser consists of a Ho3+:LLF MOPA laser system which is used to pump a ZGP OPO. The maximum pulse energy of the Ho3+:LLF MOPA was 82 mJ at a repetition rate of 100 Hz. The pump beam quality was measured to M2x = 1.01 and M2y = 1.03 at a wavelength of 2053 nm. The total 3-5 μm energy obtained for destruction was 23.4 mJ, corresponding to an optical-to-optical conversion efficiency of 51 %. The M2 values of the signal were M2x = 1.81 and M2y = 1.98. The M2 values of the corresponding idler beam were M2x = 1.91 and M2y = 1.94, respectively. ISL is also currently working on new laser sources and non linear conversion setups for proposing new versions that should be more compact, more efficient and more integrable.
High pulse energies in the mid-IR with a comfortable tuning possibility are required in a number of areas, including remote sensing, medicine and counter measure applications. Frequency converters based on the crystal ZnGeP2 (ZGP) are widely used for generating tunable IR radiation in the 2.5 μm - 12 μm region. ZGP possesses high nonlinearity, however, due to its limited transparency range it requires pump wavelengths longer than 2 μm. A significant drawback of ZGP and of most other IR non-linear crystals is a low damage threshold of ~ 1 J/cm2 for nanosecond pulses. Therefore nanoseconds OPOs at pulse energies in the 10 mJ – 100 mJ range require large diameter pump beams to reduce the risk of optical damage to the crystal. We used the “Rotated-Image, Singly resonant, Twisted-RectAngle” (RISTRA) ring cavity concept to improve the beam quality of the OPO stage. A Tm3+ fiber laser pumped Ho3+:LuLiF4 (Ho3+:LLF) MOPA system was developed delivering > 100 mJ at 2053 nm in TEM00 operation at a repetition rate of 100 Hz. Using a RISTRA as a single OPO stage we demonstrated an overall OPO pulse energy of 23.8 mJ in the wavelength region 3 μm - 5 μm at a pump energy of about 45.6 mJ and a repetition rate of 100 Hz. The beam quality was measured to be M2 ~ 2.5.
Recent advances of high power and narrow bandwidth laser diodes emitting at 1.9 μm open the path to direct diode
pumping of Ho3+:YAG lasers. The usual method to pump such laser is to use thulium fiber laser which has an excellent
beam quality with high power and narrow bandwidth emission. The draw back of this system is the low efficiency of this
fiber laser and the increased overall complexity. In this paper we present first results of resonantly diode pumping of a
Ho3+:YAG laser with fiberlike geometry. The fiber coupled diode modules used for pumping in this work (BrightLockTMUltra-500) produce each 25 W at 1.91 μm with 3 nm linewidth. The fiber has a core diameter of 600 μm with 0.22
numerical aperture. The Ho3+:YAG crystal has a diameter of 1.2 mm, a length of 60 mm, a doping concentration of 0.75
at.% and is symmetrically pumped by two diode modules from both ends. Total internal reflection on the polished rod
barrel allows a high pump intensity along the rod length. The Ho3+:YAG laser cavity is composed of a high reflective flat
mirror and a concave output coupler with a radius of curvature of 500 mm. With an output coupler of 50 % we measured
a threshold of 11 W. The maximum output power was 11.87 W with a wavelength of 2.09 μm. The incident power to
output power slope efficiency was 0.38 at currently 4 % of internal losses.
Efficient generation with good beam quality and high output power in the 3-5 μm wavelength range is desired and a 2 μm-pumped Optical Parametric Oscillator is a promising design for this purpose.
The low quantum defect of a resonant laser pumping of the Ho 5I7 manifold using 1.9 μm radiation leads to an efficient 2.09 μm laser. Due to a long fluorescence lifetime and a good energy storage capability, the system is well-suited for either increased average power at high repetition rates (more than tens of kHz) or high energy output at low repetition rates (several tens of Hz).
For both repetition rate ranges, we propose to evaluate critical components of the system such as the Tm pump laser source. Performances of a Ho:YAG laser pumped either by a 50 W Tm:fiber laser or by a 20 W Tm:YLF laser are compared. Efficiencies of the modulation devices are reported for an AOM and a RTP Q-switch cell.
Q-switched and diode-pumped 2 μm solid state lasers are becoming of increasing interest for efficient pumping of mid-infrared emitting optical parametric oscillators (OPOs). In particular, Thulium and Holmium rare earths seem to be most suited for systems with high efficiency due to their long upper state lifetime. Several works on Ho:YAG laser end-pumped by diode-pumped Tm:YLF laser have demonstrated high power operations. To simplify the set-ups, experiments with Tm-Ho intracavity lasers have been done; they demonstrated a 36.5% slope efficiency. Unfortunately these set-ups did now allow Q-switched operations and the thermal lens in the rods led to relatively poor beam quality (M2 ~ 5-6). We design an original intracavity configuration with a dichroic polarizing beamsplitter to decouple Tm:YLF and Ho:YAG cavities. This solution improves the beam quality and allows Q-switched operations. We obtained 1.9 W of 2.09 μm at the 17.3 W diodes pump level. The slope efficiency of the diode-pump to the Ho laser output and the optical-to-optical conversion efficiency achieved are respectively ~ 21.4% and ~ 11%. As anticipated, we experimentally scaled a shift of Tm:YLF emission from 1.908 to 1.953 μm that leads to an efficiency decrease for the Tm laser. In this intracavity geometry, Ho:YAG acted as a saturable absorber. Instead of a cw operation in free running, we observed random Tm:YLF laser pulses of ~ 2.5 μs that each resulted in a Ho pulse (~ 200-250 ns). When the acousto-optic modulator (AOM) worked, the Ho pulses did not follow the Q-switched frequency. In fact Ho emission depends on the Tm:YLF pump energy accumulated between two gates of the AOM. Possible ways to optimize the efficiency and avoid the passive Q-switching behaviour of Ho:YAG are suggested.
Average electron densities and temperatures were measured for both carbon and carbon-dioxide laser induced aluminum welding plasmas using spectroscopic techniques. The plasma temperature is smaller in the case of carbon laser welding, whereas the electron density is slightly higher. Therefore the absorption length (inverse bremsstrahlung) of the carbon laser radiation in the welding zone is a factor of 2 to 7 longer. This is the reason for the better weld seam quality obtained with the carbon laser.
An rf-excited gas-dynamically cooled carbon monoxide laser with unstable resonator (M equals 2) has been developed. The extracted laser beam shows an astigmatic phase distortion which is corrected by use of an extracavity cylindrical mirror. A 4.5 kW laser beam with a total divergence of 1.3 mrad is obtained with an efficiency of 9% and a Strehl ration of 0.5. The beam quality is 1.3 times the diffraction limit. First welding experiments on the aluminum alloy AlMgSi1 (6082) show higher penetration depths compared to carbon-dioxide and Nd- YAG laser welding at the same power level. This is a result of the smaller focal spot size with the CO laser leading to higher intensities in the welding zone. The aluminum weld seams obtained with the CO laser are very homogenous and regular at the surface in contrast to the weld seams obtained with the CO laser. The process parameters in CO laser aluminum welding can be changed in a wide range. This is a consequence of the shorter 5 to 5.6 micrometer wavelength compared to the carbon-dioxide laser resulting in a reduced beam- plasma interaction. Spectroscopic investigation of the CO laser induced aluminum welding plasma show a strong decrease of the intensities of Al (II) lines and no appearance of Al (III) lines as in case of carbon-dioxide laser welding of aluminum.
A multi-kW CO laser system with nearly diffraction limited beam quality has been developed. The performance data are presented. Experiments in laser welding of the aluminum alloy AlMgSil (6082) has been carried out using the CO laser and a commercial industrial carbon- dioxide laser. The welding performance of both laser systems has been compared with results of Nd-YAG laser welding from literature. It has been shown that the penetration depths with the CO laser are higher compared with the results of carbon-dioxide and Nd-YAG laser at the same power level. This is on the one hand a consequence of the better beam quality of the CO laser compared to the Nd-YAG laser and on the other hand the shorter 5 to 5.6 micrometer wavelength compared to the carbon-dioxide laser resulting in a reduced beam-plasma interaction. Owing to the shorter wavelength of the CO laser the absorption of the radiation by laser induced plasma -- one major problem in deep penetration welding with lasers -- is drastically reduced. The aluminum weld seams obtained with the CO laser are very homogeneous and regular at the surface in contrast to the weld seams obtained with the carbon-dioxide laser. The process parameters in CO laser aluminum welding can be changed in a wide range. This is not possible using the carbon-dioxide laser because of the low threshold intensity for a shielding plasma.
An unstable resonator (M equals 2) has been applied to reduce the beam divergence of a gasdynamically cooled supersonic CO laser operating at 105 K in a semiclosed gas cycle. A 4.7 kW laser beam with a total divergence of 2.5 mrad is obtained with an efficiency of 9.4%. The results of welding experiments are compared with those using a CO2 laser. The weld depth obtained with the CO2 laser are drastically reduced using Argon as assist gas; whereas the results with the CO laser are independent of the assist gas because of the lower plasma absorption coefficient for the shorter CO laser wavelength. The beam quality of the CO laser is strongly influenced by water vapor absorption in the beam delivery system.
Two mid-IR gas lasers, the CO (5.3 micrometers ) and the HF (2.7 micrometers ) lasers afford higher absorptivities on metals, lower plasma absorption as compared to the CO2 laser. On the other hand, the resonant cavity can be improved in order to deliver smaller spots sizes than the Nd:YAG laser. A whole array of these four high power IR lasers in the kW range are available at the ISL. Comparative studies in metal processing with these lasers have been undertaken. It has been shown that, due to higher absorptivities, the remelting is obtained with lower laser power.
The RF-discharge region of a gasdynamically cooled carbon monoxide laser has been optimized. Thereby the laser power was increased from 2.5 kW to a value of 7 kW (stable resonator) with an efficiency of 14%. An unstable resonator has been applied to improve the beam quality resulting in a 4.7 kW laser beam with a total divergence of 2.5 mrad. The beam quality is strongly influenced by water vapor absorption in the beam delivery system. The design of the RF-excited laser will be described and experimental results will be presented.
Two high power mid-IR gas lasers, the CO laser and the chemical HF laser, can offer higher absorption for metals and better focusing, i.e. smaller spot areas than CO2 and Nd:YAG lasers. A comparison of the effects of three gas lasers is carried out at the ISL, using three high-power gas lasers: 1 kW (HF), 6 kW (CO) and 7.5 kW (CO2) and allows to compare their effects on different materials. The absorptivity of these laser beams for several metals has been measured, both with low energy measurement techniques and with high power laser beams; an increase in absorptivity has been demonstrated for the short wavelengths lasers. Plexiglass irradiated with these lasers beams exhibits a boiling-type destruction behavior. The threshold fluence for the occurrence of the bubbles has been measured and compared with a simple boiling model showing the importance of the spectral absorption coefficient.
A gasdynamically cooled CO-laser with a dielectrically stabilized rf-discharge in the subsonic region is described, and laser performance data are presented. The laser is based on the well proven design realized several years ago in a 1-kW-laser at DLR Stuttgart. The dimensions have been scaled up to enable laser output powers up to 5 kW. The laser is run in a blow- down gas system which allows operation times of about 10 seconds. At present a maximum output power of 3.2 kW can be achieved.