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Hollow cathode (HC) lasers usually operate in a single axial mode without any optical selection. This is attributed to the large homogeneous linewidth of the gain curve due to the relatively high filling pressure of these lasers. Collisional and Doppler broadening (ΔνC and ΔνD) of the Cd+ 636 nm and Cd+ 538 nm lines (laser transitions of the HeCd+ laser) excited in a HC discharge tube were determined using a Fabry–Perot interferometer technique. It was found that in the pressure range 7–25 mbar ΔνD was nearly constant, while, as expected, ΔνC increased linearly with pressure. The broadening constants were α(636 nm)= (47±2) MHz/mbarand α(538 nm)=(11.8±0.5) MHz/mbar. The first constant is large enough to explain single-mode operation of the red HeCd+ laser; but in the case of the green laser, the exact reason for the single-mode operation remained unclear.

The pressure dependence of the cathodic current density in the case of an electrolyte and a metal cathode glow discharge operating in air and helium atmospheres was investigated. In all cases, at pressures above 100-200 mbar, violation of the similarity law was observed, which was attributed to the occurrence of the dissociative recombination of molecular ions of the applied gases (O, and ). Taking into consideration this process, the relation was deducted, which was found to be in accordance with experimental data. In the case of an electrolyte cathode this relation was found to be valid also at low pressures.

cm2, which indicates a possibility for laser action on the 210 nm Zn II line. The maximum laser power obtained was 78 mW for the infrared lines and 72 mW for the blue lines.

The role of the internal anode geometry and the influence of an axial magnetic field were investigated in a high voltage hollow cathode 469.4 nm HeKr laser. The discharge tube consisted of two 7 cm active length segments having internal anodes with two and four dots, respectively. The discharge was excited by 5 Hz, 250 mu s duration pulses. No significant difference was found in discharge and laser operation parameters using the two or four slotted internal anodes. The axial magnetic field increased tube voltage but decreased the laser output power. Operating simultaneously the two segments an output power at mW level was observed at an unusually high Kr partial pressure of 0.9 mbar. The maximum output power obtained was 12 mW.

Strong recombination peaks were observed at He-, Ne-, and Ar-ion lines in the afterglow of a pulsed, helical hollow-cathode discharge excited by 10 Hz, 0.5–100 µs duration and 1–23 A rectangular current pulses. The intensity of the peaks depended strongly on the high-voltage leading part of the exciting pulse. An optimum was found in the peak intensity as a function of pulse length. The peaks could not be observed at pulses exceeding a certain duration. From results obtained using double-pulse excitation, the peaks are considered to be due to recombination of doubly ionized atoms produced by the high-voltage leading edge of the exciting pulse with thermal-energy electrons. Line intensity investigations using an optical resonator indicate the possibility of gain at the He II 468.6 nm transition.

A review of noble gas mixture ion lasers operating cw in hollow cathode discharges is given. The basic properties of hollow cathode discharges and the main principles of tube construction are described. For the He−Kr, He−Ar and He−Ne−Xe systems laser transitions, excitation mechanisms, operation of the lasers and questions of lasers for practical use are discussed. The single frequency operation of the lasers and some problems connected to this are also dealt with.

The spectral line shapes of the Ar+ 476.5 nm and 480.6 nm lines, excited in a He-Ar hollow cathode (HC) discharge, were measured using the Fabry-Perot technique. The collisional and Doppler linewidths were determined for the two lines. The collisional broadening constants are estimated to be (5±3) MHz/mbar and (6±3) MHz/mbar, respectively. The temperature obtained from the two Ar ion transitions was found to be 260 K higher than that expected for the rest of the gas mixture from earlier measurements. The possibility is discussed, that this excess temperature is caused by Ar ions partially created in the HC discharge by charge transfer collisions with He2+molecules.

Operation characteristics of a short, 10 cm active length hollow cathode He-Kr+ laser were investigated. The laser operated at 469.4 nm and was excited by a 50 Hz near-square wave, maximum 10 A, 500 mu s duration current pulses. The lowest threshold current observed was 0.9 A and a maximum laser power of 2.7 mW was obtained. Laser oscillation was also observed at high He pressures, at 104 mbar the output power was 1.5 mW. The shape of the laser pulse showed a characteristic change with increasing He pressure. Saturation of laser power with increasing discharge current was observed even at high He pressures. The saturation is considered to be caused by the appearance of sputtered Al vapour.

Voltage-current density (V-j) characteristics were studied in the cathode region of noble gas mixture glow discharges. Measurements were carried out in He-X (X=Ne,Ar,Kr,Xe) mixtures in plane cathode and hollow cathode arrangements. The partial pressure of the admixed gas was at a maximum of 2% of the total pressure. It was found that at constant voltage the current density increased by mixing Ar to He, while it decreased when Ne, Kr or Xe were added. Above a certain partial pressure value the current density was found to increase again in the case of Ne and Kr admixtures. In He-Xe it decreased, however, in the whole range investigated. These observations are explained, taking into account ionization processes and the differences in secondary electron emission coefficients.

Investigations were carried out on the Ar-II 476.5 nm laser to obtain information on possible excitation mechanisms. The He-Ar hollow cathode discharge was excited by square wave current pulses. From the results of measurements it could be concluded that collisions of the second kind between He 23S metastable atoms and ground state Ar ions are dominant in the excitation of the upper laser level. This is also supported by results obtained in the case of the He-Kr hollow cathode laser, where the upper level of the Kr-II 469.4 nm laser is excited by a similar collision process.

Collisional and Doppler linewidths (ΔvCand ΔvD) of the 469.4 nm and 473.9 nm Kr ion lines were measured in a He-Kr hollow cathode discharge using Fabry-Perot technique. A linear dependence of ΔvCon He pressure was found for both lines. Significant differences were found between the temperature values deduced from the ΔvD-s of the two lines, and an unexpected temperature dependence of the broadening parameter for the Kr+ 469 nm line was also observed. The temperature difference between the two lines is explained by excitation of the upper level of the 469 nm line by second kind collisions between metastable He atoms and ground-state Kr ions, while the temperature dependence of the broadening parameter of the Kr+ 469 nm line is suggested to be due to the inverse process.

The optimum gas pressure of noble gas mixture and cathode sputtered hollow cathode lasers was found to increase with increasing discharge current. This observation is explained taking into consideration the change of the spatial distribution of the negative glow in the cathode hollow occuring due to the higher density of fast electrons present at increased discharge current and tube voltage. The influence of higher discharge current on excitation processes is also considered.

A method is proposed for determining the gain of gas mixtures. The results are interpreted taking into account the influence of a transverse inhomogeneity of the active medium. It is shown theoretically how the refractive index and gain inhomogeneities influence differently the modes, the eigenfrequencies, and the losses of a stable resonator with circular spherical mirrors. An equation is obtained for describing the dynamics of the complex radiation field amplitude.

Laser action was obtained at the Cu-II 781 nm transition in a slotted hollow cathode discharge having a short active length of 3 cm. A threshold current of 0.3 A was observed which compares well with the value of 0.2 A reported earlier by Warner et al. The laser could be operated at discharge currents considerably higher than threshold, the maximum laser power was 1.6 mW at a discharge current of 2 A. Results on the dependence of laser power on discharge parameters are presented.

Laser oscillation at the 572.7 nm, 531.4 nm, 526.2 nm and 486.3 nm transitions of Xe II was investigated as a function of discharge parameters in a slotted hollow cathode He-Ne-Xe discharge. Problems connected to assignment of the 572.7 nm and 526.2 nm transitions are discussed and cleared. The 572.7 nm and 526.2 nm lines oscillated in a different spatial region of the discharge and showed a different dependence on He and Ne pressure, than was observed at 531.4 nm and 486.3 nm. The excitation mechanism is considered to be second kind collisions between different Ne 1s metastables and ground state Xe ions.

Laser operation parameters were investigated in He-Ar, He-Kr and He-Ne-Xe slotted hollow cathode discharges, where laser oscillation occurred at visible transitions of the Ar, Kr, and Xe ions. The discharge was excited by square wave current pulses of 300 μs duration. The effect of voltage and cataphoresis on the laser pulse shape was studied. Enhancement of the laser power during the fall of the current pulse was observed, this being explained by lower level depopulation. The He-Ar and He-Ne-Xe lasers exhibited single axial mode oscillation. Maximum peak output power was 120 mW at 469.4 nm in He-Kr, 8 mW at 476.5 nm in He-Ar and 5 mW at 531.4 nm in He-Ne-Xe.

CW laser operation was observed at transitions of Kr II, Ar II and Xe II in noble gas mixture hollow cathode discharges. The laser transitions are excited in two steps: 1) ionization of atoms, 2) excitation of ions to the upper laser state. It is shown that in contrast to the pulsed positive column system the dominant process of ionization of Kr atoms is not Penning collisions but electron impact. This result is found to be valid also for the CW He−Ar and He−Ne−Xe hollow cathode ion lasers. Excitation of ground state ions to the upper laser state is considered to be second kind collisions with metastable atoms.

Decreasing the pressure in the hollow cathode discharge, simultaneously ensuring room for the expanding negative glow, allows the discharge to transit, from the hollow cathode type into a high voltage discharge where the cathode cavity is filled with the cathode glow. Such an arrangement seems to be very effective for endoergic charge transfer excitation as it was observed in a He-Cu system. Voltage-current characteristics of the transition region are also presented.

Design principles, construction and operation characteristics of a pulsed 469.4 nm He-Kr laser are described. The laser was operated in a 20 cm active length transverse hollow cathode discharge and was excited by 50 Hz repetition rate 0.1–1 ms duration square wave current pulses. Cataphoresis of Kr ions was found to play an important role in forming the laser pulse shape. By increasing the voltage a considerable increase of peak laser power occurred, the maximum power obtained was 80 mW. The sealed-off laser was operated using 400 μs, 4 A current pulses, pulsed laser power being 20 mW, average power 0.4 mW, respectively. The lifetime of the laser was 440 h, tube failure being due to gas cleanup caused by cathode sputtering.

A hollow cathode discharge tube with internal anodes was found to operate at significantly higher discharge voltages than a conventional HCD. A laser tube having internal anodes was constructed and laser operation was investigated in He-Kr, He-Ar and He-Ne-Xe gas mixtures. Due to the increased voltage low threshold currents and increased output power were obtained at the 4694 A Kr ion and the 4765 A Ar ion transitions. CW laser oscillation was observed for the first time at the 5314 A and 4863 A transitions of Xe II.

The possibilities of high-current laser operation in Ne-TlCl and He-Kr discharges are studied using a multiple-anode helical hollow cathode design of the laser tube. Lasing on two thallium and two krypton-ion lines is obtained. The processes limiting the c.w. mode of operation are discussed.

The intensity of Cu-II lines with upper level energies near and above those of the He ion was measured as a function of He pressure in a Cu hollow cathode tube. In this tube at low pressures the negative glow could expand above the cathode. The maximum intensity of the Cu-II 493.1 nm line was found in the low voltage, high pressure hollow cathode discharge region in accordance with a resonant charge transfer excitation process. Enhancement of the intensity of the Cu-II 436.5 nm and 417.9 nm lines was observed in the cathode glow at low pressures. Excitation of these lines is attributed to endoergic charge transfer collisions between He ions accelerated by the 2 kV tube voltage and ground state Cu atoms. The cross-section for this reaction exciting the 436.5 nm line was estimated to be of the order of 10−17 cm2.

Measurements have been carried out on the spatial coherence of a He−Ne laser with the help of a Mach—Zehnder interferometer. The results obtained from the measurements verify the complete spatial coherence expected from theory for single mode operation of a laser.

Measurements have been carried out on the output power, gain and oscillation bandwidth of a continuous wave 4416 A Cd ion laser. Optimum parameters for laser operation are obtained, the results of measurements indicate the possibility of two processes in the excitation mechanism.

Measurements carried out on a 50 Hz a.c. excited He−Cd laser operating at 4416 A indicated the possibility that two processes are involved in the excitation mechanism for this laser transition. It is suggested that the two processes are (1) Penning ionization of neutral Cd atoms by collisions with He metastables, and (2) electron — Cd ion collisions. The theory developed on the basis of the suggested excitation mechanism gives quantitative data on operation parameters of the laser.

Various features of the photoelectric effect were examined in a direct manner by irradiating very short light pulses of low intensity on to the photo-cathode of a photomultiplier. It was shown that the probability of the emission of a photo-electron during a time τ is proportional to the light energy absorbed by the photo-cathode, even when the absorbed energy is less thanhv. The possibility of light energy accumulation occurring in the photoelectric effect process was also investigated. It was verified that during time intervals of the order of 10−9 sec such an energy accumulation phenomenon does not exist. Time statistical measurements of the emitted photo-electrons showed that these are released from the photo-cathode at random.

Optimum discharge conditions, gain and oscillation linewidth were determined for the blue (4416 A), green (5378, 5337 A) and the red (6360, 6355 A) transitions of a hollow cathode He-Cd laser. A total peak output power of 75 mW has been obtained at 5378 and 5337 A. Mode investigations showed that the green and red transitions operate in a single axial mode, while the blue one does not.

CW laser operation on near infrared transitions of Cu II was investigated in a high voltage hollow cathode discharge tube of 19 cm active length. The high voltage was obtained by a special anode system placed inside the cathode. Threshold current for the strongest 7808 A laser line was 0.4 A. At a discharge current of 2.4 A a multiline output power of 30 mW was obtained on six transitions between 7404 and 7896 A.

Laser oscillation at the 469.4 nm and 431.8 nm transitions of Kr II was obtained in a d.c. hollow cathode He-Kr discharge. Optimum He and Kr partial pressures were found to depend on discharge current; a gain of 6.7%/ m and an output power of 5 mW was measured at 469.4 nm.

In recent gas laser research important development has occured due to the application of hollow cathode discharges (HCD). Several new laser systems were discovered by using HCD excitation. The main advantage of the HCD is that the cathode fall results in a component of fast electrons which increases excitation of high lying ion levels.

Laser operation parameters were investigated in a 469 nm He−Kr hollow anode-cathode laser. In this type of discharge tube voltage could be changed by varying the number of internal anode rods. Increasing tube voltage resulted in higher output power and lower threshold current. Optimum He and Kr partial pressure values were found to depend on discharge current. A maximum output power of 75 mW and a gain of 16%/m was obtained.

Experiments carried out up to now related to the hollow cathode He-Kr laser were done using for excitation half-wave rectified a.c. pulses with a low repetition rate in order to overcome problems associated with Kr cataphoresis, impurities, unwanted transitions of the glow discharge into an arc and to avoid overheating of the discharge tube, which may cause damage of it. In the present paper a report is given on the investigations performed to develope a discharge tube for laser operation in a d.c. discharge.

Laser oscillation was investigated at the 469.4 nm and 431.8 nm transitions of Kr II in a d.c. excited hollow cathode He-Kr discharge. A water-cooled discharge tube was used in the experiment with an active length of 40 cm and an inner diameter of 3 mm. The laser was operated at a maximum current of 4 A, voltage being 240 V. Optimum He and Kr partial pressures were found to depend on discharge current. A gain of 6.7%/m and an output power of 5 mW was measured at 469.4 nm. Single axial mode operation was observed without using any special mode selection technics.

A discharge tube with a slitlike hollow cathode and a large number of anodes was constructed for the investigation of metal vapor lasers. The intensities of the lines emitted by a helium-cadmium ion laser were determined as a function of the metal vapor pressure, gas pressure, and discharge current when these parameters were varied independently within a wide range.

Pulsed laser oscillation at the 488.0 and 514.5 nm transitions of Ar II was observed in an Ar-He pseudospark discharge. The time delay between current rise and laser pulse of the 488.0 nm line was 2.5 μs.

CW laser oscillation was observed at the 7808 A transition of Cu II in HeNeCu, HeArCu, and HeKrCu hollow cathode discharges. The role of the noble gas mixture in obtaining laser action is discussed.

It is shown that the anomalous output power-discharge current dependence observed on a 50 Hz a.c. excited 4416 A HeCd laser is due to the appearance of plasma oscillations, which cause an increase of He metastable density.

Laser operation on transitions of Ar II was investigated in a pulsed HeAr discharge. Oscillation at 4765 A in the afterglow indicates collisions between He 2 3 S metastables and ground state Ar ions being involved in obtaining population inversion.

A description is given of a spectroscopic method for the determination of the composition and pressure of a gas in a sealed helium-neon laser tube. The method is based on the measurement of the relative intensities of the He (λ = 501.6 and 587.6 nm) and Ne (λ = 585.2 nm) lines.

Laser oscillation on transitions of Al II was investigated in a large diameter hollow cathode discharge where Al vapour was produced by cathode sputtering. The special internal anode system used considerably increased the tube voltage and resulted low threshold currents.

A relationship between laser intensity and side light variations is examine experimentally. On the basis of the measuring method it is possible to estimate the lifetimes of the laser levels.

Measurements have been carried out on a He-Cd laser operated with 1 μsec duration pulses. A time delay of the order of 20 μusec was observed in the onset of laser oscillation on the 5378 A and 5337 A transitions of Cd(II).

Measurements of spatial coherence in the modes TEM00 and TEM10 showed this to be very nearly ideal. A phase difference of π was observed between the two parts of the mode pattern TEM10.