Patent GB-1099394-A (GB)
Title: Optically pumped atomic diffusion maser with separate pumping and observation regions
IPC: H01S3/06
Filed: 1965-08-18
Abstract: 1,099,394. Masers. VARIAN ASSOCIATES. Aug. 18, 1965 [Aug. 25, 1964], No. 35474/65. Heading H1C. [Also in Divisions G1 and G3] An optically pumped gas maser includes an envelope in two parts, an optical pumping region 2 and an observation region 3 to which is coupled an electric circuit arranged to cause regenerative stimulated emission of radiation from gas in the observation region, the two regions being separated by an arrangement for constricting diffusion therebetween. In chamber 2 He<SP>3</SP> atoms are ionized by an R.F. discharge and the resulting electrons excite the He<SP>3</SP>(<SP>1</SP>S 0 ) state to the metastable He<SP>3</SP>(<SP>3</SP>S 1 ) state which is optically pumped by circularly polarized light from He<SP>4</SP> lamp 7 so that the nuclear spins are aligned in field H 0 . Collisions with He<SP>3</SP>(<SP>1</SP>S 0 ) states then transfers the spin orientation to the latter states, the lifetime of these nuclear spin orientations being up to 55 sees. The aligned nuclei then diffuse through tube 4 and a constriction 5 into chamber 3. Coil 15 coupled to chamber 3 and connected in parallel with capacitor 5 resonates at the resonance frequency of the emission of radiation. An alternative construction (Figs. 3 and 4, not shown) for the gas envelope includes a cylindrical pumping chamber surrounded by a toroidal observation chamber and joined to it by a plurality of radial tubes. The R.F. ionizing energy may be supplied by means of electrodes on the ends of the cylinder. The resonant output circuit may be coupled to a Q-multiplying circuit (Fig. 5, not shown), via terminals 17 (Fig. 1), the circuit including two triodes connected in series, with positive feedback to the first valve just below the level of self-oscillation. Applications. The maser may be used as a magnetometer (Fig. 6, not shown), the output signal being amplified, frequency multiplied in the case of a nuclear resonance oscillation, and supplied to a counter. A gradiometer consists of two similar observation chambers, one attached to each side of a common pumping chamber. The output signals from the two observation chambers are mixed and the difference frequency is measured. In a device for measuring average field strength (Fig. 8) a second observation chamber 63 is attached to one side of the main observation chamber 55 but has no output circuit coupled to it. It is arranged that the diffusion time between chambers 55 and 63 is short compared with the radiation time of the particles. The maser may also be used as a tunable filter or amplifier if input and output windings are coupled to the observation chamber. The maser parameters are selected to be below the maser threshold except when an input signal is supplied. The device is tuned by varying the field H 0 , e.g. by varying the current in solenoid 6 (Fig. 1), or energizing a bucking coil. The pass band can be altered by changing the dimensions of constriction 5, or increasing the magnetic field inhomogeneity across the maser, or by raising the temperature. In addition, the bandwidth is also broadened by use of an output circuit having a Q of one or less. This may necessitate increasing the filling factor and/or the polarization of the atoms to maintain maser conditions, unless the temperature is reduced, e.g. by immersing the observation chamber in liquid nitrogen. A low-Q output circuit may be realized by omitting capacitor 16 and connecting coil 15 to a load, either directly or through a variable series capacitor for tuning out the coil inductive reactance. Hydrogen maser (Fig. 13). When the active material is atomic hydrogen the envelope should be lined with inert material to prevent relaxation of excited atoms due to wall collisions. The observation chamber 82 is surrounded by a cavity resonator 86. By using the field-independent hyperfine transition #M = 0, #F = 1 the device may be used as a frequency standard. The field-dependent #M=1, #F=1 transition permits use as a magnetometer. In this case the alternating magnetic field component of the resonant mode of the resonator is arranged to be perpendicular to the polarizing field H 0  instead of parallel with it. The arrangement of Fig. 16 (not shown), uses Zeeman transitrons coupled to the output circuit by means of a coil 15, as in Fig. 1. In Fig. 17 two separate chambers 88, 81 communicating with each other through a small orifice are provided, in one of which 88 the R.F. discharge is set up and the other of which 81 is irradiated with pumping light. Magnetic field stabilizer (Fig. 18, not shown). The amplified output from the maser of Fig. 1 is applied to a lock-in detector to which is also connected a reference frequency synthesizer. The resulting D.C. error signal is amplified and phase-shifted (to prevent selfoscillation) and applied to coils for generating bucking or boosting fields upon the field H 0  to be stabilized. The level of stabilization is controlled by adjusting the reference frequency. Additional parallel feedback paths may be provided having wider band-pass characteristics to provide an initial coarse adjustment. Current regulator. This device is similar to the field stabilizer, the amplified output of the lock-in detector being applied to a current control regulator, e.g. a transistor or valve, in series with the circuit through which current is to be regulated. This circuit also includes a solenoid providing the field H 0  in which the maser is immersed. The device is shielded from stray fields. Gyromagnetic resonance spectrometer (Fig. 20). The maser output is applied to coil 114 adjacent sample 111. The receiver coil 115 of the spectrometer is connected to coil 15 on a maser amplifier bulb 117 coupled to the same pumping chamber 2 as bulb 3. The amplified output is further amplified and detected and the output is displayed on an oscilloscope and recorder 122. Solenoid 112 may be of the superconducting type. Connecting tube 4 may be dispensed with so that bulbs 2 and 3 are joined directly together. Other active media include Hg<SP>199</SP>, Hg<SP>201</SP> and mixtures thereof, Rb<SP>85</SP>, Rb<SP>87</SP> and mixtures thereof.