Patent GB-1149190-A (GB)
Title: A device for the frequency stabilization of a gas laser oscillator
IPC: H01S3/22
Filed: 1968-01-16
Abstract: 1,149,190. Lasers. TURLABOR A.G. 16 Jan., 1968 [8 Feb., 1967], No. 2307/68. Heading H1C. The output frequency of a helium-neon gas laser is stabilized by passing generated radiation through an absorption tube containing isotopically pure neon at constant gas pressure, a gas discharge being produced in the absorption tube which has an applied magnetic field so as to cause the right and left circularly polarized components of the generated radiation to be absorbed to different extents depending on the radiation frequency, this difference being detected and utilized as an error signal for adjusting the spacing of the laser resonator mirrors. As shown in Fig. 1, laser tube 2 and absorption tube 6 containing helium-neon and isotopically pure neon, respectively, are located within a resonator formed by mirrors 4 and 5, the axial position of mirror 5 being determined by a tubular piezo-electric element 15. Radiation is emitted from both ends of the resonator in the form of an output beam 17 and a reference beam 19. Due to the Zeeman effect in the absorption tube, the circular polarization components L and R of the beams become unequal when the generated frequency deviates from the required value. These components are converted into two linearly polarized components R<SP>1</SP> and L<SP>1</SP>, respectively, by a double refracting plate 20, and are then passed through a polarizer such as a Nichol prism which is rotated by a synchronous motor 27. The resulting beam 1911 is detected by an electron multiplier 22, and is modulated at twice the rotational frequency of the polarizer when the components R<SP>1</SP> and L<SP>1</SP> differ due to frequency drift of the laser output. The detected error signal is analysed by a circuit comprising a narrow band synchronized amplifier 23 and a phase-sensitive demodulator 24, the demodulator being supplied from the synchronous motor power source 26 in order to determine which of the components R<SP>1</SP> and L<SP>1</SP> is the larger. The output signal of the demodulator is passed to the piezo electric element 15 and causes mirror 5 to be adjusted to a position giving the correct radiation output frequency. An alternative arrangement is shown in Fig. 3 which comprises a helium-neon laser 1 having fixed and adjustable mirrors 4, 5. This laser is arranged coaxially with a further resonator formed by fixed and adjustable mirrors 29, 30, the further resonator containing a helium-neon laser amplifier 33 and the absorption tube 6. Laser 1 supplies an output beam 17 and a reference beam 19 both linearly polarized by providing the laser tube 2a with Brewster angle windows 28, the reference beam passing through an optical attenuator 34 to the further resonator where it is amplified. The amplified reference beam is modulated by applying an alternating voltage from a source 26 to a tubular piezo-electric element 35 controlling the resonator dimensions, the mean resonator dimension being determined by a D.C. signal originating in a demodulator 24a. The alternating dimension of the resonator causes time-spaced circularly polarized components L and R to be produced which are detected by a photo-electron multiplier 22 and appear as spaced output pulses L<SP>11</SP>, R<SP>11</SP>. These pulses are equal if laser 1 generates the required output frequency, but become unequal if the output frequency changes, the sense of the deviation being denoted by the predominance of L or R polarization. The pulses L<SP>11</SP>, R<SP>11</SP> pass through an equalizing amplifier 38 to give equal amplitude reference pulses L<SP>111</SP>, R<SP>111</SP> which then pass through a synchronized amplifier 23a to the phase-sensitive demodulator 24a. Also the pulses L<SP>11</SP>, R<SP>11</SP> have their amplitude difference increased by an amplifier 39 and are then applied through a synchronized amplifier 23b to a demodulator 24b. Both demodulators are connected to the alternating source 26 and produce D.C. output signals applied to piezo-electric elements 15 and 35 to control the respective resonator dimensions; the circuit to element 35 including a superheterodyne network 40.