Patent GB-959672-A (GB)
Title: Improvements in and relating to vapor cell light amplifiers and logical circuits utilizing such amplifiers
IPC: H03K19/02
Filed: 1960-06-13
Abstract: 959,672. Light, modulating. INTERNATIONAL BUSINESS MACHINES CORPORATION. July 13, 1960 [July 22, 1959], No. 24438/60. Headings H3R and H4F. [Also in Divisions G1 and G4] An optical device comprises a cell having transparent walls or parts of its walls transparent and containing a gaseous or vapour medium capable of splitting, when subject to a magnetic field, into a plurality of Zeeman magnetic sub-levels, means for subjecting the medium in the cell to a magnetic field, means for subjecting the medium in the cell to a pumping radiation such that electrons in at least one Zeeman sub-level absorb energy therefrom and undergo a transition to a higher energy state and electrons in at least another Zeeman sub-level do not absorb energy therefrom, means for subjecting the medium in the cell to a depumping radiation such that electrons in at least the said other Zeeman sublevel absorb energy therefrom, and means for controlling the intensity of at least the said depumping radiaton. In the preferred embodiment, sodium vapour contained in a transparent glass cell is subjected to a weak magnetic field to cause Zeeman splitting of the energy levels. On directing a beam of right circularly polarized D1 sodium line radiation-the pumping radiation-into the cell in a direction parallel to the magnetic field, energy is absorbed from the radiation to raise atoms from the ground S¢ state to the excited P¢ state according to the selection rule M F  = +1 and the cell is therefore substantially opaque to the radiation. Relaxation to the ground state occurs according to the selection rule M F  = +1,0,-1. Those atoms in the ground state for which M F  1, 0,- 1, - 2 may then be re-excited to the P¢ state, but atoms for which M F = + 2 are " trapped " in that level since there is no MF = + 3 level in the P¢ state. On the average 7 photons of the pumping radiation are required to land an atom in said M F = +2 level. Eventually, this M F  = + 2 level becomes highly populated and the cell becomes as a result substantially translucent to the pumping radiation. On directing a beam of right circularly polarized sodium D1 line radiation-the depumping radiation-into the cell in a direction perpendicular to the magnetic field, said " trapped " atoms may be raised to the excited P¢ level since the relevant selection rule is now other than M F  = +1. The cell then becomes again opaque to the pumping radiation, and since only one photon of depumping radiation is required to raise an a tom out of its '' trapped '' position, it is evident that a small modulation in the intensity of the depumping radiation effects a greater modulation in the intensity of the pumping radiation. The sodium vapour in the preferred cell is at a pressure of 2Î10<SP>-5</SP> mm. of Hg, at a temperature of 500  K and under the influence of a magnetic field of 10 gauss. Argon gas may be added to act as a buffer increasing the relaxation time of said " trapped " atoms, and the walls of the cell may be coated with a saturated long chain hydrocarbon such as polyethylene having all its electrons paired off for the same purpose. Other media suitable in place of sodium vapour are the alkali metal vapours-rubidium is specifically mentioned- and gases such as helium and hydrogen. The pumping radiation must be coupled with a selection rule of either M F  = +1 as in the case of right circularly polarized radiation or of M F =- 1 as in the case of left circularly polarized radiation. The depumping radion need not be in resonance with the vapour. However, if it is resonant and is of the same frequency as the pumping radiation it must not obey the same selection rule. The pumping and depumping radiation may be parallel with one another in the cell if either their frequencies or direction of circular polarization is different. Devices constructed in accordance with the preceding description may be used for light amplification -modulations in the intensity of the depumping radiation resulting in amplified modulations in the pumping radiation emerging from the cell. Such light amplifiers in which the frequencies of the pumping and depumping radiations are equal may be arranged in cascade (Fig. 3, not shown)-the pumping radiation output of the first amplifier being applied as the depumping input to the second, and like-wise with succeeding cells. An amplifier utilizing parallel entry of the pumping and depumping radiations is shown in Fig. 6. Here, radiation from a source 41-which may, for example, be a rubidium quartz lamp energized by a source of microwaves 40-is divided into two beams by a reflector 44. A first beam 42 is directed through a lens system 45, a circularly polarizing device 46, a filter 48, and into the cell 50 as the pumping radiation. A second beam 43 is directed along a path including a lens system 51, a reflector 52, lenses 53, 54, a " chopper " unit 55, a reflector 56, a circularly polarizing device 57 which polarizes the beam in a direction opposite to device 46, a beam splitter 47, and the filter 48 and is finally applied to the cell 50 as the depumping radiation. The output from the cell after passing through a lens system 58 and a polarizer 61 is then directed by means of a reflector 59 to a photo-multiplier 60 which detects the modulations of the pumping radiation output from cell 50. The " chopper " 55 may be replaced by a light valve (Fig. 7) comprising a gas or vapour cell 61 as previously described and a coil 62 through which a variable current is passed. If a beam of right circularly polarized light 43 of resonant frequency is directed into the cell parallel to a weak static magnetic field H then the cell becomes translucent. On reversing the field associated with the coil 62 thereby also reversing the resultant magnetic field (due to H and the coil 62) the signs of the magnetic quantum numbers change and the cell becomes opaque to the beam 43. Devices constructed according to the invention may also be employed, for example, in relay devices (Fig. 8, not shown), in flip-flop circuits (Fig. 4, not shown) and in data processing systems (Fig. 5, not shown).