FR2580391A1 - Method and device for heterodyne detection using an avalanche photodiode, of amplitude-modulated light radiation, and applications to telemetry - Google Patents

Abstract

The device comprises a photodetector 1 consisting of an avalanche photodiode included in a biasing electrical circuit, at least two light sources 3, 4 emitting, simultaneously towards the avalanche photodiode 1, respectively two light radiations which are amplitude modulated at the frequencies f1 and f0, and a detection means 5 connected to a terminal of the avalanche photodiode 1 for detecting the interference (beat) electrical signal having the intermediate frequency f1-f0 present on the said terminal of the photodiode.

Description

Method and device for heterodyne detection, using an avalanche photodiode, of amplitude modulated light radiation, and application to telemetry.

 The present invention relates to a heterodyne detection method and device, using an avalanche photodiode, of amplitude modulated light radiation and it also relates to the application of said method and device to telemetry.

 The main object of the invention is to enable heterodyne detection to be carried out in a simple and easy to implement manner, which allows the production of inexpensive and robust devices.

 The invention also aims, in the context of one of the possible applications of the invention which is telemetry, to propose a method for determining the distance separating two objects, which can be implemented in a particularly effective manner, both with regard to the precision and the dynamics of the measurement, using an original measurement device which is particularly robust and capable of being used in harsh environments and under severe conditions of use.

 According to a first aspect of the invention) a method of heterodyne detection of at least one light radiation, modulated in amplitude at the frequency (fl), is characterized in that it comprises the steps consisting in - directing on a photoreceptor, constituted by an avalanche photodiode included in an electric polarization circuit, simultaneously the above-mentioned radiation to be detected and another light radiation amplitude modulated at a known frequency (fO) relatively close to the frequency (fl), - and detect, on a terminal of the avalanche photodiode, the electrical beat signal having the intermediate frequency (fl - fO).

 To determine the distance d between at least two light sources respectively emitting two amplitude modulated light radiations at the respective frequencies (fl) and (fi), the steps of the preceding method are implemented and then the electrical beat signal is processed. collected on the terminal of the avalanche photodiode to extract an output signal representative of the above distance d.

 In particular, in order to determine the distance d between two objects, two light sources respectively emitting two light sources emitting amplitude modulated light radiation are then determined, then the distance between the two light sources is determined as indicated above.

 In either case, to process the electrical beat signal, the amplitude modulation signals, of respective frequencies (fl) and (fi), of the two light sources are mixed, the beat signal is detected. having the intermediate frequency (fl - fO) serving as a reference, the phase of the reference beat signal and the phase of the beat signal detected on the terminal of the avalanche photodiode are compared, and the distance to be measured is deduced therefrom. difference of these phases.

 Such a method can be extended to any number (at least three) of light sources by detecting electrical beat signals relating to pairs of light sources combined in pairs, it is thus possible to determine the mutual distances between the different light sources, and therefore the mutual distances between the objects with which the different light sources can be associated.

 According to a second aspect, the invention relates to a device for heterodyne detection of a light radiation modulated in amplitude at the frequency (wire) which is characterized in that it comprises in combination - a photoreceptor constituted by an avalanche photodiode included in an electric polarization circuit, - at least two light sources emitting simultaneously, in the direction of the avalanche photodiode, respectively, two light radiations modulated in amplitude at the frequency (fl) and (fi), - and detection means connected to a terminal of the avalanche photodiode to detect the electric beat signal having the intermediate frequency (f1-f0) present on said terminal of the photodiode.

 Preferably, each light source comprises a laser diode and an electric oscillator whose output is connected to the corresponding light source to modulate in amplitude the radiation emitted.

 In a preferred application of this device cpncernemetemetry, -the fact that the electrical beat signal present on the terminal of the avalanche photodiode contains information relating to the distance d separating the two light sources- to the above detection means are connected means for processing the detected electric beat signal, arranged to provide an output signal representative of said distance d.

 Advantageously, the processing means comprise - a mixer circuit for mixing the modulation signals of the light sources, of respective frequencies (fl) and (fi), - a first filtering circuit connected to the output of the mixer circuit and arranged to select the intermediate frequency beat signal (fl-fO), serving as a reference, - a phase comparator circuit whose inputs are respectively connected to the means for detecting the beat signal delivered by the photodiode and to the first filtering circuit, and arranged to provide an output signal representative of the phase difference between the two input signals, itself representative of the distance separating the two light sources, - and a receiver circuit connected to the output of the phase comparator circuit and arranged for process said output signal from the phase comparator circuit and extract the distance information d therefrom.

 Preferably, the device comprises two electrical oscillators of respective frequencies (wire) and (fO), the outputs of which are respectively connected to the light sources and the mixing circuit has two inputs connected respectively to the outputs of said oscillators.

 Such a device can then be used to measure the distance between two objects, the two aforementioned light sources being associated respectively with the two objects.

 It is also possible to constitute a device for measuring the mutual distances between several amplitude modulated light sources, several detection means being connected to the terminal of the avalanche photodiode, to detect the beat frequencies corresponding respectively to pairs of light sources and a plurality of electrical signal processing means being respectively connected to said detection means to respectively supply output signals representative of the distances separating the light sources from the pairs of respective light sources.

 Thanks to the arrangements provided according to the invention in the context of the specific application to telemetry, it is possible to produce a telemetry device capable, for example, of measuring a range of distances of the order of 10 cm. at 1 m (dynamic range of 10) with an error of at most 10% and a dynamic range of the photodiode current of only 3.

 It is thus possible, thanks to the invention to carry out measurements of absolute or relative distances.

 It is also possible to carry out simultaneous measurements of several distances. In this context, the implementation of the invention may extend to all the fields of relative positioning of two, or more than two, objects in particular in the field of automatic machines and robots, for example for the mutual positioning of tools and / or for positioning tools with respect to parts to be machined and / or to be treated and / or to be handled, in the field of aeronautics, for example to constitute altimeters, in the space field, etc.

 Of course, other applications can be envisaged modeled on those using heterodyne detection with electric local oscillator, such as for example the characterization of emitters by phase, etc.

The invention will be better understood on reading the detailed description which follows of some of its embodiments given only by way of illustrative examples; in this description, reference is made to the appended drawings in which
- Figure 1 is a diagram illustrating a heterodyne detection device arranged in accordance with the invention
- Figure 2 is a diagram illustrating a device arranged in accordance with the invention for measuring the distance separating two light sources amplitude modulated at its different frequencies
FIG. 3 is a diagram illustrating a device arranged in accordance with the invention for measuring the mutual distances between several light sources amplitude modulated, and
- La'figure 4 is a diagram illustrating a retroreflective telemetric device arranged in accordance with the invention.

 Referring first to FIG. 1, the heterodyne detection device according to the invention comprises a photoreceptor constituted by an avalanche photodiode 1 included in a bias circuit. For example, its cathode is brought to a potential + VB while its anode is connected to ground via a load resistor 2.

 A first light source 3, which advantageously can be a laser diode, emits light radiation modulated in amplitude at the frequency f 1, in the direction of the avalanche photodiode 1. The frequency f 1 constitutes, for example, the frequency of the radiation at detect.

 A second light source 4, which in advance, can also be a laser diode, also emits light radiation modulated in amplitude at the frequency f03 in the direction of the avalanche photodioid 1.

The frequency f0 constitutes, for example, the frequency of the reference radiation.

 The frequencies f1 and f0 are not too close in order to avoid an optical deflection between the two light rays.

 When the two light sources simultaneously emit radiation of frequencies f 1 and f 0, the applicant has been able to observe that an electrical signal of frequency (fl 1 appears on the anode (in the case of the assembly shown) of the avalanche photodiode 1 'fi), which therefore constitutes the electrical beat signal of the two optical radiations of frequencies' fl and fO. This beat signal is detected by detection means 5.

 The circuit which has just been described is the basic circuit which can give rise to numerous applications. Among these, one which seems particularly interesting concerns telemetry and is based on the fact that the phase of the beat signal collected at 5 contains the distance information d separating the two light races.

 Figure 2 shows a circuit, including the circuit of Figure 1, to determine the distance d above. In Figure 2, the elements identical to those of Figure 1, are designated by the same reference numerals.

 The detection means 5 are constituted by a bandpass filter filtering the frequency (f1-f0) of the beat signal collected on the anode of the photodiode 1.

The output of the bandpass filter 5 is connected to an input of a phase comparator 6.

In the example shown, the light sources 3 and 4 are assumed to be laser diodes excited by respective electric oscillators 7 and 8, providing electrical modulation signals calibrated respectively on the frequencies f0 and fl. Two couplers 9, connected to the respective outputs of the generators 7 and 8, are connected to two inputs of a mixer circuit 10 whose output signal notably contains the frequency (fl-f o
The output of the mixer 10 is therefore connected to the input of a band-pass filter intended to isolate the frequency (f1-f0) and the output of the filter 11 is connected to a second input of the phase comparator 6.

 The frequency signal (l-fO) applied to the second input of the phase comparator 6 constitutes a phase reference to which the phase of the signal of the same frequency (f1-f0) consisting of the beat signal from the photodiode is compared. 1, the latter signal having undergone a phase variation which is a function of the distance d separating the two light sources 3 and 4.

 The phase comparator 6 therefore provides an output signal representing the phase difference between the two signals present on its inputs, which signal is applied to the input of a user or display circuit 12 quiet calibrated to display directly the value d to be measured.

The advantage of the telemetric device which has just been described is to be able to carry out a measurement
continuous distance d, which gives a large pre
cision on the mutual localization of the two light sources
neuses 3 and 4, for example when one of them is mobile.

Another advantage of this circuit is that it
easily ready for simultaneous measurements of several dis
tances, by multiplying the number of light sources.

Such a circuit is represented in the figure 3-on
which elements identical to those of Figure 2
are designated by the same reference numerals.

This circuit includes a plurality of sources
luminous, here four in number, respectively: the
source 4 emits amplitude modulated radiation at fre
quence (fO) and sources 31 32, 33, emit rays
amplitude modulated at the respective frequencies (f1, f2, f3) sources 4 and 31, 32, 33 being excited
by electric oscillators respectively 8 and
7 73. The frequencies (f0, f1, f2, f3) of the oscillators
8, 71, 72, 73 are, for example, chosen so that
fO>f1>f2> f3 '
-This assembly can give rise to the measurements of the six
distances between the light sources taken two to
two, either
dl between sources 32 and 33
d2 between sources 31 and 33
d3 between sources 4 and 33 d4 between sources 33 and 32
d5 between sources 4- and
d6 between soruces 4 and 31.

Of course, in the case of light sources,
the number of distances that can be measured is given by
the number Cn2 of the combinations of the n sources taken two by two, that is

 For the measurement of each of these distances, a phase-shift measurement circuit is used such as that repk ~ feels at fgure i.

 In other words, six mixer circuits 101 to 10 are used, connected by couplers to the corresponding outputs of oscillators 3 and 71 to 73. The outputs of mixers 10 1 to 10 are connected to the input of bandpass filters 111 to 116 tuned on the beat frequency to be isolated, the outputs of these filters are connected to the respective inputs of phase comparators 6là 66 whose second inputs are connected, by means of ban pass filters 51 to 55 also tuned to the beat frequency to be isolated , at the anode of the avalanche photodiode 1.

 The phase comparators 61 to 66 provide an output signal representative of the distances dl to d6, respectively.

 The circuits shown in FIGS. 2 and 3 make it possible to measure the distance between any two objects by fixing a light source on each of the objects.

 Another possible arrangement for measuring the distance between two objects consists in placing the entire circuit according to the invention on one of the objects and in operating by retro-reflection on the other object. Such an arrangement is shown in Figure 4, in which the elements identical to those of Figure l are designated by the same reference numerals.

 The distance d to be measured separates an object 13 (shown schematically by a dashed rectangle) in which the entire measuring circuit is arranged, and an object 14 which is provided with a retroreflector 15.

The frequency reference is constituted by the frequencies (fO) supplied by the light source 4 (laser diode for example) the light signal being routed opposite the photodiode 1 by a fiber optic conduit 16. Optionally, a filter 17 is arranged next to photodiode 1 to eliminate para-radiation
The measurement light signal, of frequency f1 is emitted by the light source 3, by means of a focusing optic 18, in the direction of the retroreflector 15 where it is returned to the photodiode 1.

 It will be noted that the filter 17 and the optics 18 are located in the plane of a face of the object, which constitutes for example the origin of the distance d measured.

 Of course, in this case, the phase comparator 6 provides a signal representative of twice the distance d to be measured and the circuit 12 is shaped to display the value d.

 By associating three or four circuits such as that of FIG. 4 (the light source 4 providing the frequency f0 being common to these circuits), it is possible to locate the position of the object 14 in space (for example for an application in the field of robots).

In a typical embodiment, the light sources consist of laser diodes
C 86030 E sold by the firm RCA; the photodetector consists of an avalanche photodiode
C 30872 also from the firm RCA, polarized under a stabilized constant-voltage of the order of 360 to 390 volts with a background noise current of '5 p A; the frequency f1 to be detected is for example 100 MHz, the current in the avalanche photodiode was 68 p AA; the local frequency f0 (for example 100.003 MHz) is then chosen so that the frequency of the beat signal or intermediate frequency collected is a few kilohertz (in this case 3 KHz), the current in the avalanche photodiode being 146 p A at 10 cm.

 A distance d between 10 cm and approximately 2 m could be measured with an error of less than 10% over the entire range, the error being less than 5% between 50 cm and 2 m and less than or equal to 1% between 1.10 m and 2 m, the current in the avalanche photodiode varying from 231 A (at 210 cm) to 449 A (at 10 cm), the ambient temperature being 20 C.

Claims (13)

 1. A method for heterodyne detection of at least one amplitude modulated light radiation, at the frequency (leak), characterized in that it comprises the steps consisting in - directing on a photoreceptor, constituted by an avalanche photodiode included in a electric polarization circuit, simultaneously the above-mentioned radiation to be detected and another light radiation amplitude modulated at a known frequency (f0) relatively close to the frequency (fl), - and detect, on a terminal of the avalanche photodiode, the signal electric beat having the intermediate frequency (fl - f0).  2. Method for determining the distance between ent.re at least two light sources emitting respectively two light radiations modulated in amplitude at the respective frequencies (fl) and (f0), characterized in that it comprises the steps of the method according to claim 1 and in that then the sinal-electric beat collected on the terminal of the avalanche photodiode is processed in order to extract an output signal representative of the above distance d.  3. Method for determining the distance d between at least two objects, characterized in that two light sources emit two light sources respectively  so much amplitude modulated light radiation at  different frequencies and in what we then implement  the method according to claim 2.  4. Method according to claim 2 or 3, characterized  in shell, to process the electrical beat signal, the modulation signals, of respective frequencies (wire) and (f (fr)) are mixed,  of the two light sources, in that the siqnal is detected  beat having the intermediate frequency (fl-0) ser  front of reference, in that we compare the phase of the signal  reference beat and the phase of the beat signal detected on the terminal of the avalanche photodiode, and in that the distance d to be measured is deduced from the difference of these phases.  5. Method according to claim 4, in which the light sources are controlled respectively by electric oscillators of frequencies (fl) and (fi). characterized in that the electrical output signals of the oscillators are mixed.  6. Method for determining the distances separating several (at least three) light sources, characterized in that electrical beat signals are detected relating to pairs of light sources combined two by two by implementing, in each case, the method according to any one of claims 2 to 5.  7. Heterodyne detection device of a light radiation amplitude modulated at the frequency (fl), characterized in that it comprises in combination - a photoreceptor (1) constituted by an avalanche photodiode included in an electric polarization circuit, - at least two light sources (3, 4) emitting, if simultaneously towards the avalanche photodiode (1), respectively two light radiations modulated in amplitude at frequencies (fl) and (f0) ', - and means < 5) (5) of detection connected to a terminal of the avalanche photodiode (1) to detect the electric beat signal having the frequency -intermediate '(fl- f0) present on said terminal of the photodiode.'  8. Device according to claim 7, characterized in that each light source (3, 4) comprises a laser diode and an electric oscillator (7, 8) whose output is connected to the corresponding light source (respectively 3, 4).  9. Device according to claim 7 or 8, characterized in that the electrical beat signal present on the terminal of the avalanche photodiode (1) containing information relating to the distance d separating the two light sources (3, 4) - to the above detection means (6) are connected means (6, 9, 10, 11, 12) for processing the detected electric beat signal, arranged to provide an output signal representative of said distance d.  10. Device according to claim 9, charac  terized in that the processing means include - a mixer circuit (10) for mixing the modulation signals (fl) and (fi), - a first filter circuit (11) connected to the output of the mixer circuit and arranged to select the intermediate frequency beat signal (fl - fi) 'serving as a reference, - a phase comparator circuit' (6) whose inputs are connected respectively to the beat signal detection means (5) and to the first wire circuit  trage (11), and arranged to provide an output signal representative of 'the phase difference between the two input signals, itself representative of the distance, separating the two light sources,  - And a receiver circuit (12) connected to the output of the phase comparator circuit (6) and arranged to process  said output signal from the phase comparator circuit and extracting the distance information d therefrom.  11. Device according to claim 10, charac  characterized in that it has two electric oscillators  (7, 8) of respective frequency (wire) and (f) and whose  outputs are respectively connected to the sources read  miners (3, 4) and in that the mixer-circulator has two inputs connected respectively to the outputs of said 'oscillators (7, 8).  12. Device for measuring the distance between  two objects, characterized in that it is constituted according to  any one of claims 7 to 11 and in that the two aforementioned light sources are secured respectively to the two objects spaced from the distance d to be measured.  13. Device for measuring mutual distances between several amplitude modulated light sources (4, 31 '32' 33), characterized in that. is constituted according to claim 7 or 8, several detection means (51-56) being connected to the terminal of the avalanche photodiode (1) for detecting the beat frequencies corresponding respectively to pairs of light sources combined in pairs and in that several electrical signal processing means are connected respectively to said detection means to respectively supply output signals representative of the distances separating the light sources from the pairs of respective light sources.