Patent GB-1376228-A (GB)
Title: Analyzer employing magneto-optic rotation
IPC: G01J4/04
Filed: 1972-07-21
Abstract: 1376228 Measuring polarization photoelectrically HEWLETT-PACKARD CO 21 July 1972 [2 Aug 1971] 34234/72 Heading G1A In an apparatus for measuring the concentration of a magneto-optically active component in a sample cell 5, a plane polarized light beam is passed through the cell which is placed in a magnetic field pulsed at a frequency # 1 , the beam emerging from the cell has its polarization plane modulated at a frequency # 2 -at 25-and a demodulator arrangement receiving the output of a photo-cell 19 detects the sideband -# 1  # 2 - intensity to give an indication of the concentration of the component. As shown, U.V. from a deuterium lamp 2 is plane polarized at 4 and passes through a gas- or liquid-containing flow cell 5. A magnetic field H is applied along the flow cell axis. If the gas being tested contains an SO 2  component, say, then the polarization plane of the beam will be rotated through 90 degrees for a given wavelength range. Other gaseous components e.g. NO, NO 2  will produce the same 90 degrees rotation, but :for a different wavelength range - Fig. 2 (not shown). The magnetic field is pulsed at a frequency # 1  to provide a corresponding modulation in the output of a photo-multiplier 19. After leaving the cell 5 the beam passes through a reference modulator 25. This comprises either a reference cell containing the component being investigated or an acousto-optic filter, placed in a magnetic field which is pulsed at a frequency # 2 . Downbeam of the reference modulator is a polarizer 21 crossed with reference to polarizer 4 so that only polarized light rotated through 90 degrees in either the sample or reference modulator reaches the photo-cell. The photo-cell output contains sidebands at frequencies (# 1 # 2 ) and these are synchronously detected at 31 to give a D.C. output which is filtered, integrated and displayed at 33. This display is indicative of the concentration of the component in the sample. A plurality of components may be tested for in a single sample by providing a separate reference cell for each component. In one embodiment the reference cells are pulsed simultaneously at the same frequency # 2 , Fig. 4 (not shown). Light passing through the second polarizer is dispersed using a prism (41) or grating, and a plurality of detector/slit arrangements each receive light in a particular wavelength band in which one component is magneto-optically active. A switching arrangement (48) enables the detector outputs to be sampled sequentially. The plurality of photocells may be replaced by a single scanning photocell or a single photo-cell and a rotating prism or grating. A second synchronous detector (31) may be used in an a.g.c. loop. Alternatively three reference modulators may be pulsed in sequence and the output from a single photo-cell switched in synchronized sequence to one of three displays, Fig. 6 (not shown). By pulsing the reference cells at different frequencies and synchronously detecting the photo-cell output at the different sideband frequencies the concentrations of a plurality of gases may be displayed simultaneously, Fig. 5 (not shown).