Sensitive heterodyne detection with lasers applied .to radar and satellite communication is seriously hampered by the large electronic bandwidth due to random Doppler shift and frequency instability. These drawbacks can be circumvented by dual signal heterodyne detection. The system consists of mixing the local oscillator with two
signal beams and the lock-in amplification at the beat frequency of the two intermediate frequencies. The signal modulation is demodulated by the lock-in amplification. A detailed analysis of the accompanying noise is given. The derived NEP is much smaller than that of a conventional heterodyne system. As examples, the system is
applied to radar and space communication.
The intermediate power compact continuous wave (CW) CO2 waveguide (WG) lasers have been studied and developed. Most attention is devoted to small-bore slab WG devices. The results of studies of space discharge electric and laser parameters, effects of pumping frequency, electrical matching of RF supply and laser head load as well as cavity optimization are briefly described and taken into account in practical design. Two models of slab lasers are described and compared with some other known models.
A theory for the beam pointing variation of the optical beam from a pulsed XeCl* excimer laser equipped with a positive branch unstable resonator is introduced. The measurements are in good agreement with the calculations.
An ArF excimer laser has been operated using four different spiker-sustainer excitation circuits. We report on the large differences in the laser output energy caused by nanosecond- scale variations in the preionization timing with each circuit.
The discharge conditions of the multi-atmospheric e-beam sustained Ar-Xe laser are investigated. It is observed that the quasi-stationary period of a laser pulse depends on the e-beam current, the discharge power deposition and the gas density. The laser efficiency can be as high as 8%. The pulse energy with optimum efficiency depends strongly on the gas density. The best results are obtained for 4 bar with an input power of 8 MW/l. The pulse duration with corresponding output energies are 12 microsecond(s) with 10 J/l and 16 microsecond(s) with 16 J/l for e-beam currents of 0.4 and 0.9 A/cm2 respectively. An analysis of the quasi-steady state conditions that include the effects of electron collision mixing and atomic quenching is presented. The effects of output power saturation by the fractional ionization and atomic collisions are in agreement with the observations. The analysis clarifies the optimum performance conditions.
For high repetition operation of excimer-lasers care has to be taken of the changing performance of the electrical circuit, gas dynamic effects and contamination of the gas mixture to avoid deterioration of the laser performance. The parameters that influence the stability of the discharge are discussed. With the proper settings 1 kW of average power can be reached.
High energy extraction from UV pre-ionized molecular fluorine lasers is hampered by the short period of discharge stability and the spatial inhomogeneity of the discharge. So far stable discharge operation is reported for small discharge cross sections with an area less than 0.8 cm2 and efficiencies in the order of 0.15%. To increase the energy extraction of the molecular fluorine laser the discharge cross section should be enlarged. Here we report on the successful operation of a molecular fluorine laser with a large discharge cross section of 1.5 multiplied by 2.4 cm2 (electrode spacing times discharge width) which operates at a high intrinsic efficiency of 0.45%. Crucial for obtaining this result is the development of a short pulse, high intensity x-ray preionization source.
Experimental and theoretical results are presented of a XeF(C- A) blue green laser driven by 5 kJ energy. The laser was pumped by a ferrite induced discharge of 90 cm long. An output energy of 0.22 J has been obtained with a plane-parallel resonator. A code to simulate the laser has been developed. Numerical results for a wide range of conditions are compared with experiments performed by us and other authors. It is found that the intracavity refractive losses limit the laser operation for XeF2 pressures above 3 Torr. The laser output depends strongly on the discharge-cavity length ratio. Possible ways to increase the laser power and efficiency are discussed.
The focusability of a long pulse XeCl excimer laser has been improved using confocal positive branch unstable resonators where the outcoupling is done through the convex mirror. For the outcoupler different reflectivity profiles are used. A near diffraction limited output beam is obtained from hard edge unstable resonators. A beam with only one central spot in the focus of a lens can be obtained with a resonator fitted with a Gaussian outcoupling mirror.
At the NCLR an industrial 1 kW XeCl excimer laser, delivering 1 J at 1 kHz, is under development. Particular emphasis is on reliable long-term operation with moderate operational and maintenance costs. The design challenges can be grouped into characteristic operational time durations. The initial step to scale a single shot laser to a kilohertz device concerns the attainment of a sufficient clearing ratio in the active gas. Furthermore, the optical quality of the laser gas must be maintained also at high frequencies, which requires a low degree of turbulence in the gas flow and minimization of shock waves generated by the discharge. Laser operation for several hours primarily demands a thermal management of all active components. In addition the gas quality needs to be maintained, which concerns the chemical composition, the amount of impurities and occurrence of dust in the gas. Finally, continued laser action for billions of shots puts tight requirements on the lifetimes of the gas, the electrodes, the mirrors and the excitation system. These issues are largely determined by proper choice of materials but to a major part also by the conceptual layout of the laser. Therefore, a choice of technology is required which is very different from the spark-preionized charge transfer excitation predominantly found in commercially available excimer lasers.
We consider the performance of high repetition rate XeCl excimer lasers by analyzing the constraints of the discharge parameters, the discharge technique, the shock wave formation and the scaling laws with respect to discharge dimensions and gas densities. It is shown that for high repetition rate the overshoot and charge mode are most suitable because of their lowest amount of fast switching energy that falls within the specifications of commercial long life thyratrons. Fast switching is not only required for better discharge quality but it also limits the heat dissipation of the saturable ferrite switches. An analysis is given of the shock wave development and its consequences on high repetition rate operation. It is found that the shock wave length relative to discharge width is independent on discharge volume. It depends only weakly on the discharge energy. The shock strength for a given discharge energy is inversely proportional to the initial gas density. Shock front disturbances will spread evenly as the shock propagates. For a homogeneous discharge they disappear roughly after a distance equal to the discharge width. With the present analysis scaling laws are derived which state that for optimized conditions the output power depends only on the blower capacity and on the square of the gas pressure.
KEYWORDS: Pulsed laser operation, Gas lasers, Molecular lasers, Neon, Vacuum ultraviolet, High power lasers, Electrons, Electron beams, Diodes, Resonators
A long pulse molecular F2 laser ((lambda) equals 157 nm) with an optical pulse width of 160 ns and an output energy of 1.7 J (4.6 MW/cm2) pumped by an electron beam has been realized. The only restriction for the optical pulse width of the laser seems to be the duration of the excitation pulse. No signs of self terminating laser pulses due to bottle-necking in the lower laser level have been observed.
The high pressure atomic xenon laser is becoming the most promising light source in the wavelength region of a few microns. The merits are high efficiency (so far up to 8 percent), high output energies (15 J/liter at 9 bar), high continuous output power (more than 200 W/liter), no gas dissociation and thermal heating of the lower laser level. Compared with the well-known low pressure xenon laser the power performance is now roughly a factor thousand higher. The operation of the system, based on three-body-collisions, uses the metastable state of the xenon atom as the ground state so that in the recirculation of energy a high quantum efficiency is obtained. Furthermore the homogeneous line broadening caused by the high collision frequency has also a strong beneficial effect on the efficiency. However, the required intense homogeneous excitation of the gas medium at high density is from a technical point of view a great challenge. From our experimental and theoretical work we found that at optimum performance the input power must be 1 to 2.5 [KW cm-3 atm-2]. We describe our results obtained with e-beam sustained and x-ray preionized systems delivering pulsed energies in the range of joules per liter. Furthermore we describe our recent results on continuous RF excited wave guide systems of about 37 cm length with output powers in the range of watts.
The progress of the development of an 1 kW, 1 kHz XeCl laser is reported. The laser consists of a low loss gas circulation system, a corona-plasma cathode x-ray pre-ionizer, and a pre- pulse main-pulse excitation circuit generating long optical pulses. The performances of the pre-ionizer and the electrical circuit are described.
A transverse RF excited gas discharge has been successfully used to produce a CW Ar-He-Xe laser. A maximum output power of 330 mW has been obtained from an experimental device with 37 cm active length and a 2.25 (DOT) 2.25 cm2 cross-section. This corresponds to a specific output power of about 175 mW/cm3. Under these preliminary optimum conditions the gas pressure was 85 Torr (Ar:He:Xeequals59:40:1). The laser output spectrum consisted of 5 atomic xenon lines (2.03, 2.63, 2.65, 3.37 and 3.51 micrometers ). The 2.03 micrometers and 2.65 micrometers lines were the strongest ones. Complementary to this device a quartz capillary was tested as laser tube for the atomic Xe laser. With this configuration it was possible to sustain a longitudinal DC as well as a transversal RF discharge in the laser gas mixture. Combined excitation was also possible for this device. This enabled us to compare the laser performance in both the DC and the RF mode in the same device. Preliminary measurements showed us that the highest output power in the DC mode was less than 1 mW, while the RF excited laser yielded about 130 mW. The gain coefficient was found to be extremely high. Laser generation was obtained for a wide range of reflectivities R of the outcoupling mirror. At the minimum reflectivity of 5% an output power of 20 mW was obtained. Results obtained from both systems are discussed.
An investigation and optimization of a single channel transverse RF excited CW sealed CO2 waveguide laser is presented. The laser performance has been studied as a function of various parameters like the excitation frequency, gas pressure, gas mixture composition, and cooling temperature for two pairs of metal electrodes with equivalent sizes, but made of different material - gold plated copper and aluminum. The waveguide structure used was metal-ceramic with an active discharge volume of 2.5X2.5X370 mm3. Single-pass small-signal gain measurements for the two sets of electrodes have been performed, as well. The experiments show that the influence of the electrode material on the laser behavior is significant, while it was generally accepted as a factor of no importance. The best result we obtained with the Al electrodes was a specific output power of 0.78 W/cm with an efficiency of 11% at 125 MHz excitation frequency and 140 Torr of 1:1:5+5% (CO2:N2:He+Xe) gas mixture, which is very close to the highest specific power of 0.85 W/cm, previously reported. With the gold plated electrodes a specific output power of 1.1 W/cm with an efficiency of about 13% was achieved at 190 MHz and 100 Torr 1:1:5+5% gas mixture. This improvement is most likely related to the catalytic properties of the gold layers. This favorable process is accelerated at elevated temperatures, so that an intensive cooling is not necessary for good laser performance. The gain measurements confirmed this behavior. With gold plated electrodes at certain experimental conditions an increase of the gain of a factor 2 was observed.
A thermionic X-ray generator using Philips scandate cathodes is described for the preionization of a high repetition rate XeCl laser. Currents as high as 40A from an 8 cm long line source can be extracted from an array of 10 cathodes each having an emitting surface of 0.011 cm2 (360 A/cm2).
Results on a high efficiency excimer laser operating according to the prepulse-mainpulsetechnique are reported. The laser volume of about 90 cm3 is X-ray preionized. The mainpulse obtained from a PFN is switched onto the discharge by means of a racetrack saturable inductor (magnetic switching)connected to the laser head with low inductance. Laser output energy has been measured as a function of gas mixture and delay between X-ray and prepulse.
An efficient atomic Xe laser pumped by a combination of an e-beam and an electric discharge has been made. In the present study we investigated the laser operation as a function of gas pressure. The best results were obtained at a pressure of 7-8 bar under optimized excitation conditions. The specific output energy reached 10 J/l and the specific laser power 12 MW/1. The efficiency is about 2%. When both e-beam and sustainer current are present simultaneously the efficiency can rise to 7-9% in a 0.25 |?s interval.
Using high quality low loss ferrite, a single stage magnetic pulse compression network has been demonstrated working at 1 kHz PRF. A pulse compression factor of 4 has been achieved, delivering a 50 ns pulse as prepulse in the excitation circuit for a high power XeCl-excimer laser
The self-sustained discharge of excimers is analyzed. Several excitation schemes that have been successfully applied are compared. For high repetition rate operation not only the discharge stability and its efficiency are important selection criteria but more important is the potential of fast discharge switching with minimum pulse energy. Pulse compression plays a key role in the laser performance. A technology for low energy compression is described.
Design considerations concerning a high power XeCl excimer laser are described. The subsystems to be investigated are the gas flow circulating system with a homogeneous velocity profile and thin boundary layers near the electrodes, a pulse power circuit scheme that takes care of an efficient stable discharge and the X-ray preionization source with a high repetition rate electron beam cathode. Pulse compression technology at high repetion rate plays a key role in the laser performance.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.