We report for first time, to our best knowledge, experimental measurement of the degree of spatial coherence of a λ532 nm laser source (of a DPSS type - frequency doubled Nd:YVO4 laser) by reversal shear interferometers developed in our laboratories not so long. The degree of a full-sized non-apertured laser beam turned out to be quite high viz. up to 0.6 which is comparable with the degree of spatial coherence of λ510 nm copper lasers we had measured repeatedly for last decade.
Experiments of focusing coherent laser beams being diffracted on annular aperture are carried out in order to verify issues of a two-dimensional Fast Fourier Transform simulation of coherent pure (dark) annular flat beams done and published before. The beam pattern at focal plane (aka the far-field pattern) is a prominent central peak and faint concentrical rings around it. In the course of simulation we calculate the fraction of the central peak power to the whole power of beam that gives a notion of power spread within the focal spot. This fraction is a function of beam annularity i.e. ‘inside diameter /outside diameter’ ratio. The experimentally-measured dependence of the central peak power to the whole power of beam versus the annularity of pure annular laser beams was the major target of the report. The purpose was to verify experimentally the issues of our earlier simulation. We found that from the five experimental points four of them are within 16% error as to the simulated dependence which is tolerable in such a measurement.
KEYWORDS: Hydrogen, Near field, Laser systems engineering, Beam analyzers, Transmission electron microscopy, Optical testing, Molybdenum, Near field optics, Optical amplifiers, Absorption
M2-factor of the master-oscillator power-amplifier (MOPA) CuBr laser emission compliant with ISO 11146 is studied
for first time. M2 is an invariant that gives how many times diffraction-limited is a laser beam compared to a perfect
Gaussian TEM00 beam. Statistical parameters of the near and far fields of MOPA laser radiation are measured by a beam
analyzing technique. Two patterns of the MOPA laser emission are examined: annular that is typical for lasers without
addition of hydrogen, and of filled-center (top-hat and Gaussian-like) with addition of hydrogen. 2D intensity profile
changes of the near and far fields are recorded as functions of delay time of laser excitation current pulses. The MOPA
gain curve is found and the influence of gain on the input signal (from MO into PA) due to the absorption/amplification
in PA on the field profiles is shown. The change of position and waveform of laser pulses is given too. For annular
radiation M2 range is from 13-14 (small delays) to 5-6 (large delays) and for filled-center radiation M2 is 6-7 (small
delays) and at the end of gain curve is as much as 4.
Great improvement of CuBr laser beam spatial coherence was made by a special design of the laser resonator, the generalized diffraction filtered resonator. Utilizing it diffraction-limited beam divergence can be easily obtained throughout the laser pulse. Since the spatial coherence is in inverse relation with the beam divergence, decreasing the latter we increase the former. The temporal evolution of beam divergence for the more intense green (λ=510nm) laser line was measured within laser pulse of MO (master oscillator) CuBr laser system fitted with a stable plane-plane resonator (PPR), a confocal unstable resonator of positive branch (PBUR) and a generalized diffraction filtered resonator (GDFR). With the MOPA (master oscillator power amplifier) system only GDFR was used. The estimations were verified by direct coherence measurements by means of a reversal shear interferometer that was a modified Michelson interferometer. The estimations as well as the direct measurement of spatial coherence show that coherence degree increases from PPR through PBUR to GDFR. Moreover, with GDFR it is time-independent. With MOPA system the coherence degree goes up further. So the degree of coherence measured interferometrically with MO is: for PPR - 0.16, for PBUR - 0.28 and for GDFR - 0.36. For MOPA the measured degree of coherence reaches 0.65. The estimated and the measured coherence trends show similarity. Based on the Michelson interferometer and having just four optical components (a spherical lens, an optical wedge and two plane mirrors), a new rigid instrument for spatial coherence analysis of optical beams was introduced as well.
Impressive improvements in the quality of laser beam and the electric discharge properties of CuBr laser are reported lately as results of our continuing research on the hydrogen effect in copper halide-based lasers. Laser output power, Pout >= 10 W and beam divergence, (theta) <EQ 90 (mu) rad are available from a single-unit medium-scale (2 cm- bore x 50 cm) medium-power (10 - 20 W) CuBr laser fitted with a positive-branch unstable resonator of medium (M equals 73 - 100) magnification. Laser spatial intensity, Pout/(theta) 2 increases 5 times compared with the case of no hydrogen added (1390W.mrad-2 and 280W.mrad-2, respectively). Electrical field potential measurements in Ne-H2 mixtures show that in comparison with the pure-Ne-gas case, the near-electrode E- fields are nearly of a factor of 2 lower whereas the far- from-electrode E-field is almost of a factor of 6 higher as the pressure of H2 additive reaches 0.37 Torr. Energy dissipation in the CuBr laser electrodes has been studied in a typical gaseous environment and with discharge length scaling. The power lost in the electrodes significantly increases not only with a decrease in electrode separation but also more compounds are added to the medium. The total energy dissipated in both electrodes varies between 10% and 30% of the storage capacitor energy.
Photoinduced changes in the structure of amorphous chalcogenide films reflect in their optical transmission as photodarkening (PD) or photobleaching (PB). As a rule As- chalcogenides photodarken while Ge-chalcogenides photobleach when illuminated by band-gap light. Our previous investigations have shown that in GexAs40-xS60 films PD and PB depend on the prevalence of the As- or Ge- content, respectively. In the present work we show for the first time that the appearance of PB or PD in the Ge-As-S films depends not only on the composition, but also on the characteristics of the irradiation source. Pulsed and CW lasers as well as HBO 500 lamp were used to specify the necessary experimental conditions. Possible explanations of the differences in the light effects are discussed.
The Hydrogen influence on (Ne+ H2) hollow cathode discharge plasma characteristics has been studied. The discharge voltage/current characteristics have been registered and the electron temperatures and concentrations have ben measured by using Langmuir's probe technique in both pure Ne and (Ne + H2) hollow cathode discharges as a function of H2 concentration. The Hydrogen negative ion concentration has been obtained by the laser photodetachment method. The results show that small (up to 10%) concentrations of H2 to Ne hollow cathode plasma improve the plasma conductivity and decrease the electron temperature. Negative hydrogen ions occur in (Ne + H2) hollow cathode discharge. Their density with (Ne + 10%H2) discharge is 2 X 108 cm-3 and they should be taken into account for a description of the plasma studied.
Copper vapor and copper halide lasers are the most efficient high-power lasers that operate directly in the visible spectral region. In order to scale these lasers to high average powers, it is important to determine the maximum output power that can be generated from a given volume of the active medium, and the discharge conditions for which it can be achieved. In this paper we describe the attainment of a record average specific output power for self-terminating atomic copper lasers of any type. We have performed our experiments with a narrow-bore (4.5 mm) copper bromide laser. From the active volume of 4.77 cm3 a maximum average output power of 6.7 W was reached, which corresponds to the record specific average power of 1.4 W/cm3. The discharge was excited with a pulse recurrence frequency of 52 kHz. The tube was sealed with pressure of 20 torr neon, to which was added 0.3 torr of hydrogen. We describe the design and construction of the laser tube, the excitation circuit and the discharge conditions which allowed these results to be obtained.
This paper discusses the design and performance of CuBr lasers. It is demonstrated that a sealed off CuBr laser tube of 1000 h operation is allready realizable. Investigation of the output parameters in laser tubes with a limited discharge zone shows that the output power and efficiency increase 2 times when a small amount of hydrogen is added to the neon buffer gas. We obtained from a typical CuBr laser tube with an electrode spacing of 50 cm and an i.d. of 2 cm an average output power as high as 20 W with a plug-in HV efficiency of 1.5%. At a repetition frequency of 16 kHz this implies 8 mJ/l laser energy extraction from unit active volume or 0.13 W/cm3.
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