Patent GB-1182189-A (GB)
Title: Automatic Control of Strain Modulator.
IPC: G02F1/01
Filed: 1967-07-27
Abstract: 1,182,189. Light modulators. CARY INSTRUMENTS. 27 July, 1967 [7 April, 19671, No. 34562/67. Heading H4F. [Also in Division G1] Apparatus for modulating the ellipticity of polarization of a monochromatic light beam whose wavelength varies, so that the ellipticity varies cyclically and so that the amplitude of the cyclical variation of the ellipticity is maintained at an optimum level, comprises a modulator operable on a linearly polarized light beam to produce at least one emergent beam having cyclically varying ellipticity, and means operable upon one such emergent beam for deriving a modified beam whose intensity varies cyclically as a function of the cyclically varying ellipticity which is utilized to control the modulator to maintain the amplitude of the cyclical variation of the ellipticity at the predetermined optimum level. In the embodiment described the modulator comprises a strainbirefringence modulator although it may comprise an electro-optic modulator, and the modulated light beam is used to measure the circular dichroism of a sample for different wavelengths although it may be used to determine the optical rotation produced by specimens. Also, the modulated and modified beams comprise the ordinary and extraordinary beams of the input light beam, whereas they may comprise the same beam a beam splitting device being used. The strain birefringence modulator, Fig. 2, comprises a fused silica plate 10 which is cyclically stressed along its X axis by means of piezoelectric elements 13, which receive a sinusoidal drive signal from amplifier-oscillator 21 and cause the plate to vibrate longitudinally at its fundamental resonant frequency. The plane polarized light beam 11 having electric vector E is incident over small cross-section 11a and the amplitude of the electrical oscillations from amplifier oscillator 21 is such as to maintain the peak retardation i.e. peak birefringence magnitude, at 11a constant. Plate 10 is arranged to oscillate in a self-resonant manner by means of a piezoelectric sensor 15 attached to the plate. The amplitude of oscillation of the plate is controlled by a control signal 22, which causes the amplification of the amplifier to vary so as to maintain the peak strain of each cycle of vibration of the plate at such a magnitude as is required to maintain constant peak retardation independent of wavelength. To measure the circular dichroism of a sample 25, Fig. 3, and its variation with wavelength, a scan drive 28 is coupled to a monochromotor 27 to cause it sequentially to select different narrow wavelength bands from a light source 26 for transmission at 29. The light beam is linearly polarized 30 and transmitted at 31 and 60 as the ordinary and extraordinary beams respectively. The cyclically varying elliptically polarized light leaving modulator 10 at 33 is incident upon sample 25 which absorbs unequally the circularly polarized components of opposite sense i.e. circular dichroism, and phototube 34 detects the resulting fluctuating light flux components and the D.C. component. The D.C. component I 2  corresponds in magnitude to the sum of the intensities of the two circular components transmitted by the sample, and the fluctuating components I 1  are approximately sinusoidal A.C., one component of frequency equal to the fundamental frequency of the modulator plate and the other components having frequencies which are odd multiples of the fundamental and corresponding in magnitude to the difference between the transmission levels for the circularly polarized components of opposite sense. The phototube output is fed to read-out circuitry 37 together with a synchronizing input signal from amplifier 21 and the ratio I 1  to I 2  is derived which is very nearly proportional to the value of circular dichroism and is recorded on " circular dichroism versus wavelength " chart paper 43. If the D.C. component is held constant by means of an A.G.C. arrangement then the A.C. component itself is a measure of the circular dichroism. To provide automatic control of peak retardation at the modulator the extraordinary beam 60 is passed twice through the modulator by means of mirror 62 and via a quarter-wave plate 64 and an analyser 66 to produce the modified beam 108 whose intensity varies cyclically and contains the fundamental frequency of the phase delay fluctuations of the modulator multiplied by the Bessel function at twice the differential phase delay. Thus when the fluctuating phase delay reaches about 110 degrees the fundamental component of the modified light beam intensity fluctuation is reduced to zero and with further increase beyond 110 degrees of peak optical phase delay fluctuation the fundamental in intensity fluctuation reappears with reversed sign providing a null in the fundamental component of intensity at about 110 degrees peak optical phase delay. The output of photomultiplier 67 which receives modified beam 108 drives amplifier 71 containing a synchronous rectifier, the timing for which is derived from oscillator amplifier 21 on lead 72. The output 75 is compared at 73 with a reference standard input voltage 74 and the comparator produces a difference output at 22 to control oscillator amplifier 21 so that the system reaches an equilibrium condition with a peak optical phase delay of approximately 110 degrees.