Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1996-09-26
1998-10-27
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250236, 356328, 356334, G01J 318
Patent
active
058280618
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an apparatus for detecting a rotation angle of a rotatable diffraction grating generally used for an optical apparatus and, in particular, an apparatus for detecting a rotation angle of a diffraction grating applicable to an optical spectrometer apparatus (optical spectrum analyzer), a tunable wavelength light source, etc.
BACKGROUND ART
The diffraction grating used in the field of an optical apparatus is rotatable as well known, has a great number of grooves provided on its surface in a predetermined interval, and is composed of an element for generating diffracted light due to an interference of light reflected at those smooth surfaces between grooves in accordance with an incident angle of light.
As an optical apparatus using such diffraction grating there has been known a light spectrum analyzer as shown in FIG. 8 (see Anritsu Technical Review No. 13, July 1991, pp 36 to 49).
When a wavelength distribution, etc., of light under consideration is measured in the optical spectrum analyzer, a diffraction grating 31 has its rotation controlled by a motor 33 while its rotation angle is detected by an angle detector 32, such as an encoder, potentiometer, etc.
The light to be measured is split by the diffraction grating 31.
The exit light split by the diffraction grating 3 is narrowed down, by a slit 34 in a slit mechanism 34, on a focusing position and then received by a light receiving unit 35 and displayed as spectra on a display unit, not shown.
Upon the measurement of the waveform distribution (spectra) of the light to be measured in this kind of a light spectrum analyzer, the absolute value of the waveform relative to the to-be-measured light is first found from a theoretical diffraction angle inherent in the diffraction grating 31.
The rotation angle of the diffraction grating 31 is controlled by setting the absolute value of the waveform so as to correspond to the pulse number of an encoder as the angle detector 32 or to a voltage of potentiometer. Stated in more detail, a number is allocated to a pulse output from the encoder as the angle detector 32, or to a voltage of the potentiometer, for each predetermined angle in a rotatable angle range of the diffraction grating 31, so that the absolute value of the waveform is allocated to the corresponding number.
FIG. 9 shows the diffraction order of the diffracted light by the diffraction grating 31 of this kind and its diffraction.
For example, let it be assumed that light of wavelength 1 .mu.m is incident at an angle of incidence of 15.degree. with respect to a normal N. Then the light is diffracted in 10 directions, that is, with the order of diffraction, m=-3, -2, -1, 0, 1, 2, 3, 4, 5, 6.
The light receiving unit 35 of the light spectrum analyzer as shown in FIG. 8 receives diffracted light of the order m=1, 2.
In this case, the angle of diffraction, .beta.m, of the incident light is found from the following equation (1).
Further, a tunable wavelength light source is known as an optical unit utilizing this kind of diffraction grating (see Optical Fiber Telecommunications II, pp 533 to 536, Academic Press, Inc. 1988).
This tunable wavelength light source is such that exit light from a laser diode 36 is reciprocably moved back and forth relative to a diffraction grating 31 and resonated light is output from the other end of the laser diode 36.
Such an external cavity type tunable light source is such that the wavelength of output light from the other end of the laser diode 36 is variably controlled by varying the rotation angle of the diffraction grating 31 by means of a motor 33.
Like the above-mentioned optical spectrum analyzer, the diffraction grating 31 has its rotation controlled by the motor 33 while rotation angle is detected by an encoder and an angle detector 32 such as a potentiometer.
In the above-mentioned optical spectrum analyzer and tunable wavelength light source, however, if the environmental condition, such as the ambient temperature, humidity and pressure, varies, then the diffracti
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Anritsu Corporation
Lee John R.
Nelms David C.
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