Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1997-05-28
1998-06-09
Nelms, David C.
Optics: measuring and testing
By particle light scattering
With photocell detection
356358, G01B 902
Patent
active
057643601
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electro-optical measuring device for absolute distances to a target point having means for producing a collimated, linear-polarized laser beam, electro-optical modulation means for modulating and demodulating the polarization of the laser beam with a specific sequence of modulation frequencies, a reflector which is coupled to the target point, electro-optical detection means for determining the value of the modulation phase in the region of minimum brightness between the transmitted, modulated laser beam and the demodulated laser beam received from the target point, and for determining the associated modulation frequency, as well as having means for calculating the distance from two adjacent modulation frequencies with the modulation phase zero.
Such a measuring device is disclosed in EP 0 205 406 B1. The principle on which this measuring device is based corresponds to the so-called toothed-wheel method according to Fizeau. In this case, a light beam was originally interrupted cyclically by a toothed wheel, then transmitted to a reflector, and finally interrupted cyclically a second time on the toothed wheel. If the returning light beam is extinguished, its propagation time can be determined from the rotation speed of the toothed wheel by comparison with the delay time between a tooth of the toothed wheel and the next tooth gap.
In the case of the measuring device which is disclosed in EP 0 205 406 B1, an electro-optical crystal is used as the modulator instead of the toothed wheel. In this case, the measurement beam is no longer interrupted periodically, but elliptical polarization is varied periodically. If a linear-polarized light beam, whose polarization plane with respect to the crystal axes is located suitably, is modulated in the electro-optical crystal with a sinusoidal signal at several 100 MHz, and if reflected beam elements experience a further modulation when they pass through the crystal a second time in the opposite direction, the linear polarization, which was originally constant with respect to time, is obtained again at certain modulation frequencies, and brightness cancellation is observed on a suitable polarization analyzer.
In this case, at any given moment, precisely an integer number of modulation wavelengths occur on twice the measurement distance between the modulator crystal and the reflector. If no brightness minimum is observed, then such a minimum can be induced by varying the measurement distance or the modulation wavelength. The modulation wavelength is varied via the modulation frequency, which is proportional to the propagation time of the modulation wavelengths. The condition for an integer number of modulation wavelengths on the measurement path can clearly be set for different modulation frequencies. The absolute distance can then be determined unambiguously from two such adjacent modulation frequencies.
Lithium-tantalate crystals are preferably used as modulators since they require a relatively low modulation voltage for the polarization modulation. The AC modulation voltage is in this case normally applied to the crystal inductively by arranging it in a microwave cavity resonator. However, one disadvantage of these crystals is that the static double refraction, which produces the modulation, is very severely temperature-dependent. This means that the brightness minimum which is evaluated for the measurement principle is flattened as a function of the temperature and can even become the maximum, and the distance measurement: in consequence becomes imprecise or impossible.
In order to compensate for the abovementioned temperature influences, a quarter-wave plate, which is matched to the light wavelength of the measurement beam, may be positioned downstream of the modulator crystal. The measurement beam passes through this quarter-wave plate a second time on its return from the reflector, there being a polarization rotation through 90.degree. during the second passage, and the temperature-dependent double refraction effects of the modulator crys
REFERENCES:
patent: 5493395 (1996-02-01), Otsuka
patent: 5521704 (1996-05-01), Thiel et al.
patent: 5523839 (1996-06-01), Robinson et al.
Kim Robert
Leica AG
Nelms David C.
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