Patent GB-1007921-A (GB)
Title: Improvements in or relating to optical masers
IPC: H04B10/04
Filed: 1961-12-14
Abstract: 1,007,921. Lasers. WESTERN ELECTRIC CO. Inc. Dec. 14, 1961 [Dec. 28, 1960], No. 44748/61. Heading H3B. In a gaseous laser the effect of energetic electrons in impeding population inversion is reduced. Preferably a mixture of at least two gases is employed, the first gas having a metastable energy level to which it is excited by ionization, and the second gas having an upper energy level, close to the metastable level of the first gas, and a lower level whereby population inversion can be established in the second gas. In a first embodiment a glass envelope contains helium at 10 mms. pressure and neon at 0À1 mms. pressure, and radio-frequency energy at 2000 Mc/s. is applied to a resonant cavity surrounding the envelope. The R.F. energy ionizes some of the helium atoms and the energetic electrons produced excite un-ionized helium atoms to their meta-stable state E1 (Fig. 3). Energy is transferred by inelastic collisions to neon atoms to excite them to an energy level E 2 <SP>1</SP>, which thus becomes populated to a greater extent than level E 1 <SP>1</SP>. Stimulated emission is produced by setting up standing waves of the wavelength emitted when neon relaxes from the E 2 <SP>1</SP> to the E 1 <SP>1</SP> state, using planar or confocal mirrors at each end of the envelope. To prevent energetic electrons populating level E 1 <SP>1</SP> from the ground state the R.F. energy is supplied in pulses so that after each pulse free electrons quickly lose their energy. The metastable level, having a longer lifetime than the free electrons, remains populated for some time. The next R.F. pulse is supplied before the number of metastable atoms is reduced substantially, whereby continuous output may be maintained. Alternatively, an intermittent output may be produced by reducing the pulse frequency. In a modification (Fig. 2) ionization is produced by electrodes 21, 22 connected to voltage source 24. The electrons remain near wire electrode 21 whereas excited unionized helium atoms diffuse away and excite neon atoms in the absence of free electrons, particularly in the region of flat electrode 22. Alternatively, two wire electrodes may be used with an A.C. source whose frequency is sufficiently low to permit spacial separation of the free electrons. In a further embodiment, excitation of the helium atoms is produced by alpha-particles from polonium 210. It is stated that this gives rise to fewer energetic electrons than the application of voltages to the plasma. Use may also be made of the electron capture process by an ion, the electron being captured in a high energy level and dropping by radiative cascade from level to level. Atomic mercury is stated to have two energy levels of which the higher has a slower rate of decay than the lower, whereby population inversion may occur. An arrangement using a pulsed discharge in a helium-mercury mixture, making use of this process, is referred to. In a further embodiment neon is used alone, having a set of four close lying 1s levels, all metastable above 100 micron pressure, and a set of ten close lying higher energy 2p levels. A high electron-density discharge populates both sets of levels, and the pressure is made sufficient to cause population of the lower levels in each set at the expense of the upper levels, whereby the population of the lowest 2p level may exceed that of the highest 1s level, permitting maser action. A mercury-krypton system is also referred to, and the use of argon as a quenching gas for high power operation of the helium neon systems is mentioned. The laser mirrors may be aligned by initially setting them approximately parallel and then oscillating them about orthogonal axes, using magnetostrictive elements, until laser action occurs. Voltages proportional to the positions of the mirrors may be applied to an oscilloscope and the beam intensity may be modulated by the laser output to determine the optimum positions. Specification 985,204 is referred to.