METHOD FOR MEASURING THE FREQUENCY MODULATION OF A LASER SOURCE The field of the invention is that of the measurement and possibly of the control of the frequency modulation of a laser source. Up to now, the measurement of the frequency modulation of a laser source was most often achieved using a Michelson or Mach-Zehnder interferometer one of the two arms of which included an acousto-optical modulator. An example of a system of this type is shown in figure la. It comprises: ■ - a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint f o (t), said controller being equipped with a unit 111 for storing digital setpoint s and a converter 112 for converting these digital setpoints into analog signals fo (t); a coupler 12 that samples some of the light emitted in order to send it to an interferometer 2; a two-arm Mach-Zehnder interferometer 2 with, in one arm, a delay line 21 and, in the other, an acousto-optical modulator for "AOM") 22 itself associated with an RF generator 221, and two couplers, one 23 allowing splitting, preferably into two equal portions", and the other 24 allowing light that has passed through the two arms to be recombined; a photodiode 3 able to convert the light-intensity signal of a beat generated by the interferometer into an analog electrical signal; a device 4 for measuring the signals delivered by the photodiode 3, which includes a converter 41 for converting these analog signals into digital signals, a converter 42 for converting the analog signals of the generator into digital signals and reciprocally connected to the generator 221, and a unit 43 for storing, at preset times, digital signals generated by the converters 41 and 42; a unit 5 for processing the stored signals, and transmitting a set voltage to the controller 11; and a synchronizing device 6 between the storing unit 43, the acousto-optical modulator 22 (via the converter 42 and the generator 221) and the voltage controller 11. The frequency is determined by analyzing the signal output from the interferometer; it is a question of a beat signal between the two signals respectively emerging from the two arms. The signal measured by the photodiode (excluding any DC component) is then: where ip\f} is the phase of the laser source, where fm is the frequency of the acousto-optical modulator and i is the delay induced by the optical fiber and corresponding to the path difference between the two arms of the Mach-Zehnder interferometer 2. The phase difference f'uO — p{i ~ T) is characteristic of the frequency /(f) of the laser according to the following relationship: #ft) - &($ - T) = 2JT jf_ f(t}dt ^ 2mf{i} {1) . To evaluate the frequency of the laser, it is therefore advisable to calculate: "sflff' then to apply a low-pass filter of cut-off frequency lower than Jm&t. ZW is then found such that: The evaluation of the complex argument of 2\rj then 5 finally allows the frequency of the laser to be deduced according to equation (1). This method relies on the frequency translation induced by the acousto-optical modulator. Acousto-optical modulators are components that are liable to directly penalize the size, weight, 5 electrical power consumption, reliability and cost of the systems in which they are used. These penalties may also be indirect. For example, it may be necessary to electromagnetically shield the detection chain because of interference caused by the acousto-optical 10 modulator. In addition, it may also be noted that working at high intermediate frequencies requires a more complex detection chain to be used. Other solutions allow the frequency modulation of the laser source to be measured. The simplest solution 15 is based on the use of an interferometer that is "unambiguous" in the vicinity of the phase quadrature, such as for example a Mach-Zehnder interferometer with a very short delay or an optical resonator of large free spectral range. An example of a system of this 20 type, equipped with a Fabry-Perot resonator is shown in Figure lb. It comprises: a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint fo(t) , equipped with a unit 111 for 25 storing digital setpoints and a converter 112 for converting these digital setpoints into analog signals f0 (t) ; a coupler 12 that samples some of the light emitted in order to send it to an Interferometer 30 2; a Fabry-Perot resonator 2; a photodiode 3 able to convert the light- intensity signal generated by the resonator 2 into an analog electrical signal; 35 a device 4 for measuring the signals delivered by the photodiode 3, which includes a converter 41 for converting these analog signals into digital signals, and a unit 4 3 for storing, at preset times, the digital signals generated by the converter 41; a unit 5 for processing the stored signals, and for transmitting a set voltage to the controller 11; and a synchronizing device 6 between the storing unit 43 and the voltage controller 11. In this case, the signal output from the interferometer or the resonator and measured by the photodiode may be written: r(t) = A'F(/(0) where A is a proportionality factor depending on the injected power and F a function that is monotonic (and therefore . invertible) over the possible range of excursion of the frequency f\tj ~ JJ^y lhij/(£) of the laser. For example, in the case of the short-delay interferometer, if the powers are perfectly balanced, we have: x(f) cc cos(^(t) — (p{t - T)) -f I — cos(2in/(t)} T 1. A necessary condition for the function to be invertible is for T to be sufficiently small that |2rcA/(Q?| 10% A/c, but little relatively typically <1%, where c is the speed of light); and the measurements carried out at the time ti in a 5 given period are reiterated at t±+kT, with k^l and in that the delay T has varied from one iteration to the next. This method allows the modulation frequency of a .0 laser source to be measured with a good compromise between precision and dynamic range using a simple two-arm interferometer that does not include any acousto-optical modulators. This allows drawbacks associated with the use of this component (cost, bulk, L5 reliability, etc.) to be avoided. Furthermore, the proposed solution is based on an analysis of a signal that may be low-frequency, thereby allowing certain constraints on the detection chain and processing of the signal, such as constraints on the sampler, to be >0 relaxed. The calculation preferably includes: organizing reiterated measurements that are homologous from one period to the next in the >5 form of vectors x(t), 0 ^ t < T; these vectors x(t) describing an elliptical cylinder, calculating the axis w0 of the cylinder; and projecting, along the axis w0, onto a determined BO plane, this projection being parameterized by an angle that is a function of f(t). In practice, this function is advantageously developed to the first order and the projection is then parameterized by an angle proportional 35 to fft). The period T is typically about a few us (from 5 us to 1 ms) , and the delay x typically varies over a duration varying from one-hundred milliseconds to one minute (from 100 ms to 1 mn). According to one variant of the invention, the variation as a function of time of the delay i is stimulated by means of a piezoelectric device. The invention may be used to calibrate the control signal in order to get as close as possible to a frequency modulation defined beforehand by the user. To this end, the invention also relates to a method for calibrating the frequency of the laser source of a lidar to a setpoint f0 (t), which comprises the following steps: modulating the frequency of the laser source by means of a preset periodic control voltage U(t); defining a linear transformation between f (t) and U(t), which transformation may for example be obtained by measuring the transfer function of the frequency modulation, which is designated FTM; calculating a first control voltage Ui (t) from fo(t) and said linear transformation; i=l and iterating the following steps: o measuring the frequency fi ft) of the laser source as indicated above; o calculating the error Af± (t) = fi (t) -fo (t) and a correcting control voltage from Afi (t) and said linear transformation; o defining a new control voltage Ui+i (t) from the preceding control voltage Ui (t) and the correcting control voltage; o i=i+l. The number of iterations is generally lower than 10. WO 2016/150783 - 10 - PCT/EP2016/055639 Another subject of the invention is a computer program, said computer program comprising code instructions allowing the steps of the method such as described to be carried out when said program is 5 executed on a computer. The invention also relates to a system for measuring the frequency modulation f(t) of a laser source that comprises: -0 - the laser source associated with a modulation controller; a two-arm interferometer with a delay line in one of the arms; a device for measuring beat signals generated _5 by the interferometer; a unit for processing the measured signals; and a synchronizing device that is connected to the modulation controller and to the processing unit; 10 characterized in that the processing unit is suitable for implementing the described method. The interferometer is for example of Mach-Zehnder or Michelson type. Advantageously, the interferometer does not 15 include any acousto-optical modulators. Other features and advantages of the invention will become apparent on reading the following detailed description, which is given by way of nonlimiting 30 example with reference to the appended drawings, in which: figures la to lc schematically show examples of systems for measuring the frequency modulation of a laser source according to the prior art, with a two-arm 35 Mach-Zehnder interferometer equipped with an AOM (figure la) , with an optical resonator (figure lb), or with a two-arm interferometer able to measure the phase component and the quadrature component of the interferometric signal (figure lc); figures 2a and 2b schematically show an example of a system for measuring the frequency modulation of a laser source able to implement the method according to the invention, using a Mach-Zehnder interferometer (figure 2a) or a Michelson interferometer (figure 2b); figures 3a and 3b schematically show an example of the path of the vector representative of the i measurements obtained over 2 periods with then an elliptical path (figure 3a) and the transformation of this path into a circle so as to directly obtain the frequency to within a constant (figure 3b); figure 4a schematically shows the projection into J a three-dimensional space composed of three main components of an example path of the vector representative of the measurements obtained over 400 periods with then a cylindrical path of elliptical base, figure 4b schematically shows the path of figure ) 4a projected onto a plane that is almost perpendicular to the axis of the cylinder and normalized to a circle, and the corresponding frequency reconstruction is shown in figure 4c; figure 5 illustrates various steps of a method for ) calibrating the frequency of a laser source according to the invention; and figure 6 schematically shows an example of modulation errors obtained after i iterations. In all the figures, elements that are the same ) have been referenced with the same references. A first example of a measuring system able to implement the method according to the invention will now be described with reference to figure 2a. It 5 comprises: a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint fo(t) , which is equipped with a unit Ill for storing digital setpoints and a converter 112 for converting these digital setpoints into analog signals fo(t); a coupler 12 that samples some of the light emitted in order to send it to an interferometer 2; a two-arm Mach-Zehnder interferometer 2, with a delay line 21 in one of its arms, and two couplers, one 23 allowing splitting, preferably into two equal portions, and the other 2 4 allowing light that has passed through the two arms to be recombined; a photodiode 3 able to convert the light-intensity signal generated by the interferometer 2 into an analog electrical signal; a device 4 for measuring the signals delivered by the diode 3, which includes a converter 41 for converting these analog signals into digital signals, and a unit 43 for storing, at preset times, the digital signals generated by the converter 41; a unit 5 for processing the stored signals and for transmitting a set voltage to the controller 11; and a synchronizing device 6 between the storing unit 4 3 and the voltage controller 11, which is also connected to the processing unit. Another example of a measuring system able to implement the method according to the invention, in which the Mach-Zehnder interferometer of the preceding example is replaced by a Michelson interferometer, will now be described with reference to figure 2b. This system comprises: a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint fo10% A/c, where c is the speed of light and X the wavelength of the source) but varies little relatively (typically less than 1% i.e. (Al /T)< 0.01, AT being the variation in T over a period T). Since the frequency f (t) of the laser is proportional to the derivative of the phase: - is calculated then diagonalized in order to define the eigenvectors vi and the eigenvalues \±: T vi = \i v± Ai^A2^...^Am>0: eigenvalues (vi, v2, ..., vm) : orthonormal basis of lffl (eigenvectors) . In practice, only the 3 largest eigenvalues are non-negligible. Therefore, the projection of x in the sub-space formed by (vl, v2, v3) is calculated, thereby allowing the dimensionality of the problem to be decreased. An example of an experimental result for the path {x(t), 0 < t < T} of the vector x(t) in this sub-space is presented in figure 3b: this path is obtained for m = 400 periods of 200 us measured over about 10 s with a sampling frequency of 125 MHz i.e. about 25000 points per period (a satisfactory ■ result is obtained with 5000 points per period or more). Typically the period T is comprised between 10 to 800 us and the delay x typically varies over a duration comprised between 1 s and 30 0 s. The points are organized into an elliptical cylinder of axis w0- To determine the axis Wo, of the ellipse, it is sought to minimize a criterion C (w) such as the variance of the norm relative to the square of the norm (the projection plane is chosen in order to obtain the most circular shape possible): where p„(x) designates the projection of x along the axis w. By projecting the points x along w0, a slightly elliptical shape is obtained that, after re-normalization, as may be seen from figure 4a, allows: a(t] = 2ttT/(t], and therefore the frequency over time as illustrated in figure 4b, to be deduced. It has been possible to simultaneously evaluate various frequencies f(t) in this way, using this technique, for example by implementing a complex frequency f(t) comprising over a given period T a portion that is - Constant - Sinusoidal - Parabolic - Triangular. A method for treating the signals xi (t) based on organization thereof in a vector form has been described. Other processing methods may be envisaged, such as, for example: an iterative linear regression; a simulated anneal; or recursive, genetic or Monte Carlo algorithms taking into account all of the measurements. This method may in particular be used to calibrate the frequency of the laser source of a lidar to a setpoint fo(t)f without using any AOMs. Such a procedure allows possible drifts in the transfer function of the laser (related to temperature, to the ageing of the diode, etc.) to be avoided. The main calibrating steps described with reference to figure 5 are carried out as follows. A first step consists in defining a linear transformation between the control voltage and the frequency of the laser. This linear transformation may advantageously be obtained by measuring the transfer function of the frequency modulation. This is then done by using a known white noise (for example in a frequency band comprised between 0 and 150 kHz) as the control voltage of the modulation of the form where the Ok are independent random phases, and by measuring the emitted frequency, using the method described above. The modulation transfer function is obtained with the relationship: The calibrating process is then iterative in order to take into account the (experimentally observed) nonlinearity in this transfer function: from the frequency setpoint, a first voltage to be applied to the laser diode is calculated using a linear transformation of this setpoint, for example using the modulation transfer function, such that: the emitted frequency fi (t) is measured using the method described above; the error in frequency with respect to the setpoint Afi(t) = fi(t) - fo(t) is deduced from the preceding measurement; this error allows a correction of the control voltage defined from Af±(t) and the function defined above (for example the FTM) to be defined: 5 tf(+I(£) = a,(t) - TF-*{TF{Aft(t% X FTM"\v) }. the system repeats the preceding 3 points in order to refine the required control voltage and therefore the emitted frequency. Two iterations generally allow a satisfactory 10 result be obtained and, typically, 3 to 4 iterations are sufficient to achieve the minimal accessible error (i.e. about 1 minute) as illustrated in figure 6. These calibrating and measuring methods allow the 15 AOM found in the examples of the prior art to be omitted. However, use thereof is not excluded; specifically an AOM may optionally be added to one of the arms of the interferometer in order to avoid low-frequency noise. 20 The beat signal may be processed using hardware and/or software elements. This processing may be achieved using a computer-program product stored on a computer-readable medium, this computer program 25 comprising code instructions allowing the steps of the reconstruction method to be carried out. The medium may be electronic, magnetic, optical, electromagnetic or be a storage medium employing infrared. Such media are for example semiconductor memories (random access memories 30 (RAMs) , read-only memories (ROMs)), tapes, floppy disks, hard disks or optical disks (compact disc read-only memory (CD-ROM), compact disc - read/write (CD-R/W) and DVD). 35 Although the invention has been described with reference to particular embodiments, obviously it is in no way limited thereto and comprises any technical equivalent of the means described and combinations A method for measuring the modulation frequency f(t) of a laser source (1) that comprises the following steps: modulating the laser source over a period T, with a modulation controller (11); in a given period T, carrying out a plurality of measurements of a beat light intensity between two arms of an interferometer (2) located downstream of the laser source and able to introduce a delay i between the two arms, these measurements being synchronized with the control of the modulation; and calculating the frequency f (t) from the measurements; characterized in that during each period T, f(t); over a plurality of periods T, the delay T varies as a function of time, with AT>10% X/C and AI/T < 0.01 X/c, where c is "the speed of light and A the wavelength of the source; the measurements are carried out at the time ti in a given period and reiterated at ti+kT, with k>l the delay i having varied from ■ one iteration to the next; and the frequency modulation is calculated from all of the reiterated measurements obtained under distinct interference conditions because of the variation in T. The method for measuring the modulation frequency f(t) of a laser source as claimed in the preceding claim, characterized ■ in that the frequency modulation is calculated from a covariance matrix using a plurality of measurements. The method for measuring the modulation frequency f(t) of a laser source as claimed in either of the preceding claims, characterized in that the variation as a function of time of the delay i is stimulated by means of a piezoelectric device. The method for measuring the modulation frequency f (t) of a laser source as claimed in one of the preceding claims, characterized in that the calculation includes: organizing reiterated measurements that are homologous from one period to the next in the form of vectors x(t), 0 ^ t < T; these vectors x(t) describing an elliptical cylinder, calculating the axis w0 of the cylinder; and projecting, along the axis w0, onto a determined plane, this projection being parameterized by an angle that is a function of f(t). The method for measuring the modulation frequency f(t) of a laser source as claimed in the preceding claim, characterized in that function of f (t) of the angle is developed to the first order and in that the angle is proportional to f(t). The method for measuring the modulation frequency f(t) of a laser source as claimed in one of the preceding claims, characterized in that the period i is about a few us, and the delay varies over a duration varying from a few seconds to a few minutes. A method for calibrating the frequency of the laser source (1) of a lidar to a setpoint f0(t), which comprises "the following steps: modulating the frequency of the • laser source (1) by means of a preset periodic control voltage U (t); defining a linear transformation between f(t) and U(t); calculating a first control voltage Ui(t) from fo(t) and said linear transformation; i=l and iterating the following steps: o - measuring the frequency f± (t) of the laser source as claimed in one of the preceding claims; o calculating the error Af^(t) = fi(t) fo (t) and a correcting control voltage from Afi(t) and said linear transformation; o defining a new control voltage Ui+i(t) from the preceding control voltage Ui(t) and the correcting control voltage; o i=i+l. 8. The calibrating method as claimed in the preceding claim, characterized in that the linear transformation between f (t) and U (t) is obtained by measuring the transfer function of the frequency modulation, which is designated the FTM. 9. The calibrating method as claimed in either of claims 7 and 8, characterized in that the number of iterations is lower than 10„ 10. A computer-program product, said computer program comprising code instructions allowing the steps of the method as claimed in any one of claims 1 to 9 to be carried out when said program is executed on a computer. 11. A system for measuring the frequency modulation f(t) of a laser source that comprises: the laser source (1) associated with a modulation controller (11) ; a two-arm interferometer (2) with a delay line (21) in one of the arms; a device (4) for measuring beat signals generated by the interferometer; a unit (5) for processing the measured signal; and a synchronizing device (6) that is connected to the modulation controller (11) ' and to the processing unit; characterized in that the processing unit is-suitable for implementing the method as claimed in one of claims 1 to .10. The system for measuring frequency modulation as claimed in the preceding claim, characterized in that the interferometer (2) is of Mach-Zehnder or Michelson type. 5. The system for measuring frequency modulation as claimed in either of the preceding claims, characterized in that the interferometer (2) does not include any acousto-optical modulators.