FR2572806A1 - METHOD FOR DETERMINING THE SIGN AND VALUE OF A FREQUENCY OFFSET - Google Patents

Abstract

IN A PROCESS FOR DETERMINING THE SIGN AND THE VALUE OF A FREQUENCY OFFSET, A MOVING OBJECT IS SCAN BY TWO LIGHT WAVES OF DIFFERENT FREQUENCIES. THE LIGHT WAVES THE FREQUENCY OF WHICH IS SHIFTED BY THE DOPPLER EFFECT ARE COMBINED WITH PART OF ONE OF THE TWO LIGHT WAVES. THE TWO LIGHT WAVES OF DIFFERENT FREQUENCIES ARE GENERATED IN A SINGLE AND SINGLE RESONATOR, BY FORCING TWO OSCILLATIONS OWN OF THE RESONATOR.

Description

Method for determining the sign and value of a

frequency offset.

  The present invention relates to a method for determining the sign and the value of a frequency offset by means of a heterodyne receiver. Using superheterodyne optical reception, that is to say by mixing a signal wave with a carrier frequency wave (hereinafter referred to as "local oscillator wave"), it is possible, by mixing two light waves

  high frequency, to highlight the different frequency

  and process it electronically; a summing frequency occurring then also cannot be

  recorded, since its value is always considered

  significantly larger than the detector bandwidth.

  The measured differential frequency is always positive, i.e. it is not possible to distinguish

  if the frequency of the signal wave is higher or lower

  lower than the frequency of the local oscillator wave.

  This process is called "homodyne process".

  To obtain information relating to the sign of the differential frequency, the frequency spectrum must be shifted entirely in the direction of the positive axis, which is termed "heterodyne process". The intermediate frequency appearing during such an offset will be designated hereinafter by "offset frequency" fOF 'The optical superheterodyne reception is, for example, used during the measurement of speeds by taking advantage of the Doppler effect and by using only a laser, that is to say during what is called a speed measurement "laser-Doppler". In this case, a discrete and sufficiently stable laser frequency fo is offset as a result of the Doppler effect on a moving object (for example a particle), that is to say that the displaced object causes a Doppler shift of this laser frequency f o. This frequency offset A fD 'which is directly proportional to the speed v of a particle, obeys equation (1): AfD = (2fo / c) .v (1),

  in which c denotes the speed of light.

  When the frequency (fo + a fD) with Doppler shift of the signal wave thus generated is mixed with the frequency fLO = f of the local oscillator, the spectrum obeys, as illustrated diagrammatically in FIG. 1, the equations (2 ) following: (fo + tAfD) + fL0 = 2fo + fD (fo + fD) - fL0 = fD = fhom (2) where fhom designates the frequency resulting from homodyne reception. Admittedly, the frequency 2fo + A fD could make it possible to obtain unequivocal information concerning the sign of the speed; however, this cannot be demonstrated because of the magnitude of the frequency, i.e. because of its height. Consequently, the frequency fhom does not give

  no indication of speed direction.

  When, on the other hand, the frequency (fo + t fD) with Doppler shift of the signal wave is mixed with the frequency (fLO + fOF) of a local oscillator with offset frequency, the spectrum thus obtained can, as shown diagrammatically on Figure 2, be expressed by the following equations (3): (fo + - fD) fL0 = fD = fhom (fLo + fOF) - fL0o fOF (3) (fo + fD) fLO + fOF) = fOF + fD 'or , respectively, the frequency (f0F + AfD) contains information as to the value of the speed of movement

and as for its direction.

  In the case of an insufficient overlap, for example as a result of a defective adjustment, the following spectrum is also obtained: (fo0 fD fOF) - fLO- fOF fD0 (4) '(f0 fD + fOF) - (fLO + fOF) = fD 'In this case, due to the frequency (fFt TO fD), the information relating to the sign of the speed again partially escapes. The frequencies (fOF + a fD) and (fOF + fD) contain, each as far as it is concerned, information relating to the sign of the speed as long as only one of these two frequencies has a greater amplitude. If the two frequencies have the same amplitude, the spectrum contains the same information as during homodyne reception. As a result, during heterodyne reception, it

  it is necessary to ensure good cohesion between the

  local oscillator, as well as a scru-

  pulpy. The uniqueness of the speed vector sign

  to be measured can be checked, for example by generating

  drant a reference signal using, for example, a

  disc rotating in a known direction.

  The offset frequency (fOF) can be

  generated in different ways. We will establish fundamental-

  Below is the difference between two methods depending on whether, to generate and stabilize the offset frequency (fOF), it is necessary to have components other than optical (as for example in a hybrid system), or vice versa

  (as for example in an intrinsic system).

  In the case of hybrid systems, essentially two processes are known. In the first method, a second coherent light source is coupled by means of an electronic discriminator circuit at the frequency of the first light wave, at a distance from the frequency

  offset fOF 'which is for example described in "Appl. Opt."

  Flight. 20, 4, page 579 (1981) by RL Schwiesow and RECupp, and in US Pat. No. 4,169,906. However, in order to obtain by this process a sufficient degree of efficiency to detect very weak signals, a considerable electronic and optical complexity must be foreseen. Indeed, the electronic circuit must include a reaction characteristic adapted to the optical properties of the first light source (that is to say rapid rise times, delay-free actuators, etc.). For reasons of cohesion (that is to say for example concerning the divergence, the polarization, the phase position, etc.), it is necessary that the optical properties of the two

  light sources are in harmony.

  In the second known method, the frequency of part of the light power of the source is shifted using an optoacoustic modulator, as described for example in "J. Phys." E 13, page 982 (1980), by

  W.R.M. Pomeroy et al. This process requires on the one hand a complete

  considerable adjustment capacity because, for example, the angles of incidence and the angles of emergence of the modulator must

  be respected with precision; and, on the other hand, the long

  optical strength and the poor performance of the active part of the electronic circuit cause strong distortion of the wave fronts of the light source. In addition, in this method, the value of the offset frequency fOF is fixed according to the type of embodiment. To obtain other values which may be more suitable for the application, the offset frequency fOF must, for example, do more

  the subject of electronic sub-combinations.

  In a third method, a frequency offset is obtained using the Doppler effect via a mechanical device, for example a rapidly rotating wheel. However, the applications are essentially limited by the magnitude of the offset frequency fOF thus made possible, and as a result of the restrictions imposed on the mechanical production, which in turn influences the quality of the superposition.

  In this context, we do not yet know

  that now intrinsic systems, that is to say

  processes which can dispense with an external command.

  According to the invention, the disadvantages of conventional methods and devices must be overcome as widely as possible, and the invention aims to provide

  a process by which the sign and the value of a decimal

  The frequency can be determined by means of a relatively simple arrangement and easy adjustment

  having the form of a heterodyne receptor.

  According to the invention, in a process of the aforementioned kind, this object is achieved by the fact that an object

  in motion is swept by two light waves of frequency

  these different; and by the fact that the light waves

  whose frequency is shifted by Doppler effect are mixed

  part of one of the two light waves.

  According to advantageous improvements of the invention

  tion: - the two light waves of different frequencies can be generated, in a single resonator, by constraining two own oscillations of the resonator (resonance modes); - Two transverse resonance modes can be constrained by obscuring one or more filiform zones in the resonator, these filiform zones being mutually perpendicular and perpendicular to the axis of the resonator;

  - one or more filiform areas of a resonance mirror

  partially transparent reflector can be reflected, or the reflections of one or more threadlike areas of a resonator mirror can be suppressed - two transverse resonance modes can be constrained by obscuring one or more annular areas in the resonator, these areas annulars being mutually concentric and their axis of symmetry coinciding with the axis of the resonator; one or more annular zones of a partially mixed resonator mirror can be reflected, or else the reflections of one or more annular zones of a resonator mirror can be eliminated; - to generate one of the parts of one of the two

  light waves, the second surface of a quarter-

  wave may be partially metallized; - To generate one of the parts of one of the two light waves, a partially metallized surface can be arranged after the quarter-wave plate; and - to mix with a part of one of the initial waves the light waves with frequency shifted by Doppler effect, the light waves with frequency shift can be partially reflected on the second surface of a resonator output mirror, and partially return to this resonator. In a process according to the invention, advantage is taken of the fact that different discrete natural oscillations can form in a resonator; however, only natural oscillations are possible, the frequency of which lies within the limits of the bandwidth of the resonator. These "resonance modes" are disruptive for most applications and, therefore, they are suppressed in different processes by devices

  corresponding, such as standards and screens.

  In the case of a laser resonator, only the modes included in the line width of this resonator are amplified, which is for example usually included

  between 200 and 500 MHz for a CO2 laser. The frequency gap

  this 4 fax of two axial resonance modes is indicated, ax in a first approximation, by the following equation (5): Afax = c / 2L (5) in which c represents the speed of light in the medium and L indicates the length of the resonator. The frequency difference A ftr of two transverse modes is notably more modest and lies between a TEMnm mode and the basic nm mode, neglecting losses by XnmC 2 diffraction (6)

= ftr () -

  2fo I d.n o Xnm denotes the m th zero point of the nth order Bessel function, n indicates the refractive index of the amplification medium and d corresponds to the free diameter of

the optics of the resonator.

  - Thus, for a line width of 300 MHz for example, an offset frequency fOF from 1 to 250 MHz is possible by the superposition of two different modes, in which case the length of the resonator is 1 m, the free diameter d of l he optic of the resonator measures 2.54 cm, the refractive index n is equal to 1 and the length of the laser waves represents 10.6 μm. The magnitude of the offset frequency foF can be influenced by a modification of the geometry of the resonator, for example a variation of the length of the resonator, a variation of the free diameter, etc., or even by varying the pressure of a gas. filling, which leads for example to a variation in the refractive index of the medium. In the simplest case, to achieve this end, the optical axis of the resonator is tilted relative to the mechanical axis of the latter; it is thus possible to obtain, in a very simple manner, a variation

modal volume.

  The advantages of this embodiment, compared to the first two methods described above, are as follows: the fact that only one atomic source is

  necessary to form the offset frequency fOF 'the properties

  tees of the two modes, and therefore the characteristics

  of the transmitter and the local oscillator, coincide to a large extent (for example with regard to their polarization, their divergence, etc.) or show firm cohesion (for example with regard to their phase position) . Compared to the second process described in the preamble, the complexity required by adjusting all of the

  system is considerably less and it also occurs

  in general, less distortion of the wave fronts; this results in a heterodyne yield overall

  better, and therefore also a better signal / noise ratio.

  In addition, the use of a single source represents a considerable saving of money, and the elimination of mechanical, electrical and optical components.

  provides a lower need for electrical power

  than the whole system. Furthermore, such an arrangement can be generally achieved in a significantly more compact manner. The invention will now be described in more detail by way of non-limiting examples, with reference to the appended drawings in which: FIG. 1 shows a frequency spectrum of a

  homodyne reception both in the presence of a positive component

  ve that a negative component of the speed of movement; FIG. 2 illustrates a frequency spectrum of a heterodyne reception for a positive component and a negative component of the speed of movement; Figure 3 is a schematic representation of

  the realization aiming at an implementation of the compliant process

  mement to the invention for heterodyne reception; and Figure 4 is a schematic representation of an alternative embodiment for the implementation

  of the method for heterodyne reception.

  To verify and implement the process

  In accordance with the invention, a homodyne arrangement, existing for the intrinsic heterodyne process described above, has been slightly modified. Figure 3 schematically illustrates the embodiment used in this case. A source 1 is formed by a stabilized frequency CO2 laser which, for a wavelength of 10.59 pm (line P-20), delivers a continuous power (or CW power) of 3 W. A portion of the power transferred by the laser, of the order of approximately 5 mW, is directly directed, via a partially reflecting mirror 2, onto a detector 3 of the Pb-Sn-Te type in which it is combined to a signal wave. In FIGS. 3 and 4, for illustrative purposes, the trajectory of the wave of the local oscillator is indicated in dashes, while the trajectory of the signal wave is

marked by mixed lines.

  In this case, to form the signal wave, the part of the radiation emanating from the laser 1 and having been able to pass through the mirror 2 is focused, via

  of a telescope 4, on a dispersing device 5 accom-

  pleating a movement, preferably a rotation. The part dispersed by the device 5 towards the telescope 4 is again collected by this telescope 4, and now crosses

  in the opposite direction the partially transparent mirror 2

  rent. The signal wave is then partly reflected in itself on an output mirror of the laser 1 not shown in detail, and partly amplified parametrically in the

  laser 1; finally, this signal wave follows the path

  re of the local oscillator wave, to be coherently superimposed on this local oscillator wave on the detector 3. Although this is not shown in more detail in FIG. 3, the signal of the detector 3 is electronically amplified, as a result of which the frequency spectrum

can be analyzed.

  This simple layout makes it possible to obtain a good degree of heterodyne yield. Because the path of the local oscillator wave and the signal path coincide and, as a result, the otherwise conventional interferometer becomes superfluous, the complexity of the adjustment is reduced to a minimum. It only remains necessary to reproduce on the detector 3 the radiation of the local oscillator, by means of the partially transparent mirror 2, which, in practice, does not raise any kind of difficulty for the specialist. This particular embodiment of the superimposition geometry, suitable for homodyne arrangements, can be disadvantageous for heterodyne reception, since the angle between the wave of the local oscillator and the signal wave is zero at all times and any place, especially when, for example due to atmospheric turbulence, the phase ratio between the wave

  local oscillator and the signal wave is changed.

  Contrary to an opinion which has been expressed by

  O.E. DeLange in "IEE spectrum", page 77 (1968), a super

  position of two different modes remains, again, always

  possible in practice; in this case, however,

  should use an interferometer device with at least two additional degrees of freedom and shown

in detail in Figure 4.

  In the arrangement of FIG. 4, the laser radiation, polarized in parallel, passes unhindered; from the laser source 1, which is called a Brewster window 6, and then falls on a quarter-wave plate 7 which can be adjusted along two axes. The reflections of the two surfaces of this quarter-wave plate 7 have been eliminated, but said surfaces however have a reflection power R of approximately 0.5%. As in the arrangement of FIG. 3, the transmitted laser power is focused via a telescope 4 on a device

dispersion rotary 5.

  The part dispersed in the direction of the telescope 4 is collected by the latter, and crosses the quarter-wave plate 7 a second time. In this way, the signal wave is now polarized perpendicularly and it is reflected.

  at around 80% on slide 6 under brewster incidence.

  The part of the initial laser radiation reflected on the second surface of the quarter-wave plate 7 is also polarized perpendicularly, it is brought coherently to the superposition of the signal wave by a tilting of the plate 7, then is reproduced with

  this signal wave on the detector 3.

  Thanks to a rotation of the quarter-wave plate 7 around its optical axis and thanks to a tilting relative to the axis of the signal trajectory, it is always possible to adjust a condition in which the share of the power laser reflected on the second surface of the wafer 7 can be superimposed on the signal wave with correct phases, as local oscillation. We eliminate

  in this way the possibility of ambiguity mentioned above, affected

both heterodyne signals.

  The method according to the invention makes it possible, thanks to a heterodyne arrangement which can be established simply, to determine unequivocally the value and the direction of a vector component and, by a determination of at least three components, to establish sure the velocity vector of a movement. Furthermore, when this vector is known, it is possible to indicate whether the frequency fLO of the wave of the local oscillator is greater or less than the

  frequency f of the transmitter wave. -

  o It goes without saying that many modifications can be made to the process described and shown, without going out

of the scope of the invention.

? 2806

Claims (9)

  1. Method for determining the sign and the value   frequency offset using a hetero receiver   dyne, a process characterized by the fact that a moving object   is scanned by two light waves of different frequencies; and by the fact that the light waves whose frequency is shifted by Doppler effect are mixed with a   part of one of the two light waves.   2. Method according to claim 1, characterized in that the two light waves of frequencies   different are generated, in a single resonance   tor, by constraining two own oscillations of the resonator (resonance modes).   3. Method according to claim 2, characterized in that two transverse resonance modes are constrained by obscuring one or more filiform zones in the resonator, these filiform zones being mutually   perpendicular and perpendicular to the axis of the resonator.   4. Method according to claim 3, characterized in that one or more filiform zones of a partially transparent resonator mirror are reflected, or else that the reflections of one or more filiform zones   of a resonator mirror are removed.   5. Method according to claim 2, characterized in that two transverse resonance modes are constrained by obscuring one or more annular zones in the resonator, these annular zones being mutually concentric and their axis of symmetry coinciding with the axis of the resonator.   6. Method according to claim 5, characterized in that one or more annular zones of a partially transparent resonator mirror are reflected, or that the reflections of one or more annular zones of a   resonator mirror are removed.   7. Method according to claim 1, characterized in that, to generate one of the parts of one of the two light waves, the second surface of a wafer   quarter-wave is partially metallized.   8. Method according to claim 1, characterized in that, to generate one of the parts of one of the two light waves, a partially metallized surface   is placed after the quarter-wave plate.   9. Method according to claim 1, characterized in that, to mix with a part of one of the initial waves the light waves with frequency shifted by Doppler effect, the light waves with frequency shift are partially reflected on the second surface d 'an exit mirror of the resonator, and partially return to this resonator.