Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-03-04
2004-11-30
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
Reexamination Certificate
active
06825455
ABSTRACT:
The invention concerns a method and an apparatus for determining the phase and amplitude information of an electromagnetic wave.
The term phase here generally stands for phase transit time and for the designation transit time which is also used according to the respective signal shape involved.
Hereinafter reference is made to a light wave instead of an electromagnetic wave. That however does not denote a restriction only to the spectral range of visible electromagnetic waves, but is only for the purposes of simplification.
For the measurement of frequency components in terms of amplitude and phase in wide-band and high-frequency signals, the electronic measuring art and communication art frequently use phase detectors which multiply or mix the unknown signal with a sine oscillation and determine the steady component which occurs in the presence of a signal component of the same frequency by integration or low-pass filtering.
That procedure produces the correlation function of the unknown signal with the mixing signal for a given, adjustable relative phase position. By altering the mixing frequency (sweep) the unknown signal can be broken down into its spectral components. Steady component, varying amplitude and phase of the unknown frequency component of the same frequency can be determined by at least three phase positions.
The investigation of corresponding optical signals which have acquired increasing significance in the measuring and communication arts is implemented nowadays inter alia by way of wide-band photodetectors as electro-optical transducers with subsequent electronic measurement value ascertainment, as previously described for electrical signals.
Because of the high level of expenditure involved those methods and the corresponding measurement apparatuses are usually only of a one- or two-channel nature. In the case of optical signals however very many parallel channels—in particular entire image sequences—with high frequency components frequently have to be surveyed simultaneously.
Besides the spectral modulation properties of two-dimensional light waves, an aspect of increasing interest is the rapid run of the envelope in space and time. In addition there is a wish to provide for rapidly and accurately surveying 3D-objects, for example by way of optical radar processes, which requires very fast detectors in the sub-nanosecond range, as a result of the light speed of the echo signals. At the same time they should be available as a detector array if there is desire to avoid the time-consuming operation of scanning the actively or passively bright 3D-objects.
Such a 3D-camera is proposed in DE 44 39 298 A1 which the present invention takes as its basic starting point.
FIG. 10
is intended to illustrate that 3D-camera which is based on the echo transit time or phase transit time process. The HF-modulated light wave
101
which is irradiated by a modulated light transmitter
107
and
103
and reflected by the 3D-object
100
contains all the depth information in the delay in respect of the phase front. If the incident light wave is modulated once again in the reception aperture
102
with a two-dimensional, optical mixer
104
of the same frequency, which corresponds to a homodyne mixing or demodulation process, the result is a steady high-frequency interferogram.
That HF-interferogram can be recorded with a conventional CCD-camera
105
and subjected to further processing with an image processing arrangement
106
. Integration of the steady component of the mixed product in the CCD-photoelectric charge corresponds to the formation of the correlation function of the two mixing signals. The distance-related phase delays due to the echo transit times and the amplitudes can be calculated pixel-wise from three or more interferograms by virtue of different phases of the demodulating mixing frequency, for example 0°, 120° and 240° or 0°, 90°, 180° and 270°, and thus the 3D-depth image can be reconstructed.
The two-dimensional optical mixer
103
or
104
which is also referred to as a spatial light modulator or SLM comprises in that case for example a Pockel cell which has a series of serious disadvantages which are described in the literature.
Further implementation options are afforded by LCD-windows which are admittedly inexpensive but which are about a factor of 1000 too low in terms of the desired band width.
The use of a so-called microchannel plate, as is used in image amplifiers, is also expensive and costly. The gain can be modulated by modulation of the acceleration voltage which is applied to the microchannels and which influences the secondary electron emission in the microchannels.
Furthermore, the state of the art sets out a proposal for a 2D-correlator based on a CCD-photodetector array: “The Lock-In CCD-Two-Dimensional Synchronous Detection of Light” by Spirig, Seitz et. al., published in IEEE Journal of Quantum Electronics, Vol. 31, No. 9, September 1995, pages 1705-1708. There, a photopixel is interrogated by way of four transfer gates in order to ascertain the phase of sine-modulated light. For each sine period, a respective equidistant sample is taken with the four transfer gates, whereby the phase can be easily calculated. That procedure is too slow for the indicated problems as the harmonic light signal is firstly integrated on during a scanning duration which significantly delimits the band width. It is only then that the desired mixing process is implemented with the stored charge being taken over as the scanning sample.
The object of the present invention is therefore that of providing a method and an apparatus for determining the phase and/or amplitude information and thus the envelope of a light wave, which permit a simpler, wider-band and less expensive correlator concept and rapid 3D-object surveying by way of a predeterminable lighting.
The above-indicated object is now attained by the method as set forth in claim
1
and by the photonic mixing element as set forth in claim
14
, by the mixing element arrangement set forth in claim
20
and by the apparatus set forth in claim
23
.
The principle according to the invention s based on a drift produced by the modulation photogate voltage and separation of the minority charge carriers photo-generated by the light wave in the material beneath at least two adjacent light-sensitive modulation photogates. In this case those charge carriers drift under the influence of the modulation photogate voltages U
am
(t) and U
bm
(t) applied to the modulation photogates, depending on the respective polarity or phase involved, to the accumulation gates which are biased with preferably double the dc voltage U
a
and U
b
. The modulation photogate voltages U
am
(t) and U
bm
(t) are preferably complementarily applied and are preferably composed of a bias voltage U
0
and the modulation voltage +U
m
(t) and −U
m
(t) respectively superimposed in push-pull relationship. The two modulation photogates together preferably form a square surface. A pixel with only two modulation photogates can also be referred to as a dual pixel.
This principle according to the invention presupposes the photoelectric quantum effect, caused by electromagnetic waves. Nonetheless, reference will always be made to light waves, without this being interpreted as a limitation.
The actual mixing or multiplication process lies in the modulation voltage-dependent or phase-dependent drift of the photo-generated charge carriers to the right or to the left side of the modulation photogate (“charge swing”). In that respect the charge difference between the charge carriers which are separated in that way and collected under the accumulation gates and transmitted to the electronic reading-out system, having regard to integration in a predetermined time, represents a measurement in respect of the correlation function of the envelope of the incident modulated light signal and the modulation voltage U
m
(t).
At the same time the charge sum of those charge carriers which have drifted to the accumulation gates and passed on remains uninfluenced
Luu Thanh X.
The Maxham Firm
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