Patent GB-880478-A (GB)
Title: Improvements in microwave signal circuit-arrangements
IPC: H03H2/00
Filed: 1957-10-11
Abstract: 880,478. Masers. BLOEMBERGEN, M. Oct. 11, 1957 [Oct. 15, 1956], No. 31853/57. Class 40 (9). A maser utilizes a solid having at least three energy levels, an input signal effecting transitions between a first pair of non-adjacent levels, whereby an abnormal distribution between a second pair (one of which may be identical with one of the first pair) of energy levels, so that the system may be stimulated to emit radiation corresponding to the energy difference of the second pair. Thus, if the solid has energy states E 1 , E 2 , E 3 , Fig. 1, it may be subjected to a radiation of pump frequency (E 3  - E 1 )/h to raise it from the ground state E 1  to state E 3 . Owing to the resulting abnormal or " negative temperature " distribution between E 3  and E 2 , incident radiation of frequency (E 3  - E 2 )/h can cause transitions from state Eg to state Eg with resulting amplification of the input signal. Alternatively the transitions between E 3  and Eg may be allowed to take place unstimulated, thereby creating a negative temperature distribution between Eg and E 1  and the transitions between Eg and E 1  can be stimulated by the input signal which is required to be amplified. In a modification the substance can be raised from its ground state to a higher state by the application of two signals, one effecting the transition from the ground state to an intermediate state and the other effecting the transition from the intermediate state to the higher state. A negative temperature distribution set up between two states one in each of the intervals thus defined (or between a state existing in the higher interval and the ground state, or between a state existing in the lower interval and the highest state) can be used to obtain amplification. If suitable levels exist it is possible by this means to amplify a signal of frequency greater than that of either of the first mentioned signals. If the two intervals are equal then the same frequency can be used for affecting both the first mentioned transitions. Preferably, substances having crystalline field splitting are used in conjunction with a magnetic field of such strength as to produce a Zeeman splitting of comparable order. The magnetic field is arranged at an angle to the crystal axis. By this means several energy levels may be obtained from which suitable combinations may be obtained. Nuclear spin may also be utilized if desired, different levels being obtained e.g. by superimposing the nuclear by per fine splitting on the Zeeman splitting in a paramagnetic salt or an organic free radical, or the interaction of nuclear spin with crystal structure may be used in a substance such as iodine. One preferred form of solid consists of a single crystal of 95% zinc fluosilicate containing 5% isomorphous nickel fluosilicate which produces three energy levels. Another suitable substance is a crystal of 99% lanthanum ethyl sulphate and 1 % isomorphous gadolinium ethyl sulphate which provides four doublets. Both substances are preferably operated at a temperature of 2‹ K. In operation, Fig. 2, the crystal 14 of one of the above substances may be placed with its axis at a suitable angle to the magnetic field NS which may be of the order of 1000 gauss. The crystal is placed in a coaxial resonant cavity 11 having a fundamental resonant frequency (1,420 mc/s.) equal to that of the signal to be amplified, this signal being applied and extracted by probes 18, 19. In some cases a single probe may serve both purposes or may be omitted altogether if the device is required to operate at high gain as an oscillator. Tuning in the fundamental mode is effected by the screw 13. The cavity also has a higher mode resonating at the higher (energy supplying) frequency (10,000 mc/s.) which signal is applied via probe 17, the sector shaped piece of rutile 12 being used to tune the cavity at this frequency. The band width may be varied by varying the field NS. Other types of resonant cavity or even a straight through waveguide containing the crystal may be used. Modulation of the signal frequency may be effected by varying one of the operating parameters such as the amplitude of the pump frequency or the magnitude of the magnetic field. In operation as a radio receiver preamplifier the maser 23 is coupled to the antenna 21 and load 25 by a circulator or gyrator 22, the same probe being used for inserting and extracting the signal from the maser. The whole of the maser and circulator are maintained at 2‹ K. Specification 880,479 is referred to.