PHOTICAL SYSTEM WITH A LASER SOURCE WITH THE CAPABILITY TO EMITE MULTISPECTRAL LIGHT RADIATION

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to a laser source for emitting light radiation exhibiting a plurality of spectral lines separated by a determined spectral interval. Such a source finds particular application in the field of wavelength-division multiplexing communications. ARRIERE PLAN TECHNOLOGIQUE DE L'INVENTION

[0002] The paper "WDM Source Based on High-Power, Efficient 1280-nm DFB Lasers for Terabit Interconnect Technologies," by B. Buckley, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 22, NOVEMBER 15, 2018, proposes a laser source consisting of a bank of distributed feedback lasers. Each laser comprises a Bragg grating distributed along the laser cavity. The lasers emit light at stepped wavelengths, typically separated by 100 GHz or 50 GHz.

[0003] The light emitted by the lasers is propagated to the input ports of a passive optical mixer. This mixer produces, at its output ports, a plurality of light beams, each combining the light beams supplied at its input ports. The output beams produced at these output ports are therefore multispectral (in spectral combs, each line of the comb corresponding to the radiation emitted by a laser from the bank).

[0004] Due to inaccuracies and variability in laser manufacturing processes, their emission wavelengths are poorly controlled. This leads to variability in the spectral range between two spectral lines of multispectral radiation, on the order of plus or minus 5 to 10% of the expected spectral range, or even more depending on the range. The wavelengths of the light emitted by lasers are also sensitive to the operating temperature, which can vary, for example, from 0 to 80°C.

[0005] The spectral range of the light radiation provided by a state-of-the-art multispectral laser source is therefore poorly controlled and likely to drift during the operation of this source.

[0006] The paper "Streamlined Architecture for Thermal Control and Stabilization of Cascaded DWDM Micro-Ring Filters Bus," presented by Maarten Hattink at the Optical Fiber Conference held from March 6 to 10, 2022, proposes a solution to overcome this limitation. The approach adopted in this paper aims to lock filters to the emission frequencies of a multispectral laser source. To this end, the resonant frequencies of the filters are modulated to produce feedback signals that control their resonant frequencies. However, this approach requires device calibration, making the solution particularly complex to implement. The choice of modulation frequencies proposed in this paper introduces error terms into the feedback signals. This configuration presents the risk of locking multiple modulators to a single emission frequency.

[0007] Documents WO2013044863A1, US20090232493A1 and EP2573961A1 propose solutions for detecting the wavelength drift of a light source and locking it to a chosen wavelength. OBJET DE L'INVENTION

[0008] One object of the invention is to provide at least a partial solution to this problem. More specifically, one object of the invention is to provide a laser source capable of supplying multispectral light radiation with a spectral range that is more precisely controlled than that of light radiation produced by prior art laser sources. Another object of the invention is to provide a laser source that allows locking a tunable resonant frequency of at least one filter of a photonic component. BREVE DESCRIPTION DE L'INVENTION

[0009] To achieve this goal, the object of the invention proposes a photonic system comprising a laser source capable of emitting at least one multispectral light radiation exhibiting a plurality of spectral lines, the photonic system comprising: A laser bank comprising a plurality of lasers, a spectral line of multispectral light radiation corresponding to an emission frequency of a laser in the bank; a modulator associated with the laser bank, the modulator being configured to generate a modulation signal and to modulate the emission frequency of at least one laser in the bank; a photonic component disposed downstream of the laser bank and comprising a plurality of tunable optical filters having respectively a plurality of resonance frequencies; a photodetector disposed downstream of the photonic component to establish a signal representative of multispectral radiation produced by the photonic component; a locking device configured to process the signal representative of the multispectral radiation and adjust the resonance frequencies of the tunable optical filters of the photonic component to the emission frequencies of the lasers in the bank.

[0010] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: The tunable optical filters of the photonic component are ring microresonators; the tunable optical filters are each equipped with devices for adjusting their resonant frequency; the resonant frequency adjustment devices are heaters; the lasers are distributed feedback lasers or distributed Bragg reflector lasers; the locking device is configured to control the modulator and select, using a selection signal, a laser from the bank to which the modulation signal is applied; the modulator generates a plurality of distinct modulation signals, the modulation signals being applied to the lasers in the bank; the adjustment of the resonant frequencies of the tunable optical filters is performed one optical filter at a time; the modulator is configured to produce a sinusoidal modulation signal with a modulation frequency;The locking device is configured to establish a representative measurement of the power present in a second harmonic of the modulation frequency of the signal representative of the multispectral radiation; the locking device is configured to establish a representative measurement of the power present in a principal component of the modulation frequency of the signal representative of the multispectral radiation; the locking device is configured to establish a representative measurement of the phase of the principal component of the modulation frequency of the signal representative of the multispectral radiation; the photonic component is a radio frequency modulator; the photonic component is a radio frequency demodulator. BREVE DESCRIPTION DES FIGURES

[0011] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which: [ Fig. 1a ] ] Fig. 1b ] ] Fig. 1c ] ] Fig. 1d ] ] Fig. 1e ] THE figures 1a, 1b , 1c, 1d And 1e represent diagrams of the principles underlying the invention; [ Fig. 2a ] There figure 2a represents a first aspect of the description; Fig. 2b ] ] Fig. 2c ] There figure 2b and the figure 2c represent two variants of the first aspect; [ Fig. 3a ] There figure 3a represents the transition from a natural state to a locked state; [ Fig. 3b ] There figure 3b represents an advantageous characteristic of a transmission function of a filter of a laser source conforming to the first aspect; [ Fig. 3c ] There figure 3c illustrates a calibration of a laser source filter conforming to the first aspect; [ Fig. 4 ] ] Fig. 5 ] THE figures 4 And 5 represent other examples of the first aspect; [ Fig. 6 ] There figure 6 represents the signal provided by the photodetector in the frequency domain, within the framework of the embodiment of the figure 5 ; Fig. 7 ] There figure 7 represents a variant applicable to all examples of the first aspect; [ Fig. 8a ] ] Fig. 8b ] ] Fig. 8c ] THE figures 8a,8b,8c illustrate a second aspect of the description corresponding to the invention; [ Fig. 9a ] There figure 9a represents a modulator that can be used in a photonic system according to the invention; [ Fig. 9b ] There figure 9b illustrates a calibration of a modulator; [ Fig. 9c ] There figure 9c represents a demodulator that can be used in a photonic system according to the invention; [ Fig. 9d ] There figure 9d illustrates the calibration of a demodulator. DESCRIPTION DETAILLEE DE L'INVENTION Principes de l'invention

[0012] THE figures 1a, 1b , 1c, 1d And 1eThese are diagrams of the principles underlying the invention. Regarding the architecture of the figure 1a , a light beam from a tunable laser La is guided towards a ring resonator MR, constituting a filter having a resonance frequency F0, and towards a photodetector PD arranged downstream of the resonator MR.

[0013] A "tunable laser" is a laser that produces light whose frequency (the "emission frequency") can be adjusted. For example, a means of adjusting the emission frequency of a laser might include a device configured to modify its supply current, temperature, refractive index, or free carrier concentration. A tunable laser may be equipped with multiple means of adjusting its emission frequency, such as an adjustable current source and a heater to modify the laser's operating temperature.

[0014] A modulator M, connected to a laser emission frequency adjustment means, is configured to modulate the emission frequency Fla of the light emitted by the tunable laser La by a modulation frequency Fd. This modulation frequency Fd, for example 5 kHz, is relatively low compared to the laser emission frequency, for example 200 terahertz. The amplitude of this modulation is also small. For example, a 1 mA modulation amplitude of the supply current can lead to a variation in the emission frequency FLa of the tunable laser La of the order of ±1 GHz. The light emitted by the tunable laser La therefore varies at a very low frequency Fd and with a small amplitude A (1 GHz) around its fundamental frequency FLa. The laser frequency thus varies as Fla + A cos(2πFdt).

[0015] On the figure 1b In the frequency domain, we have represented the transmission function T of the MR filter, whose spectrum TF has a resonance frequency F0, and the signal V provided by this photodetector PD in the case where the emission frequency FLa of the tunable laser La is not locked to a resonance frequency F0 of the MR resonator, but has an emission frequency Fla higher than this resonance frequency F0. Since the emission frequency of the tunable laser La is located in a relatively linear section of the transmission function of the MR resonator, the signal V provided by the photodetector PD has, in the frequency domain, a relatively large principal component Fd (corresponding to the modulation frequency) compared to its harmonics, and in particular compared to its second harmonic 2*Fd.Furthermore, the phase of the main component Fd of the signal V is reduced, that is to say that this main component is in phase with the signal V provided by the photodetector PD.

[0016] On the figure 1c , we have represented, similarly to the figure 1b The transmission function T of the MR filter, whose spectrum TF exhibits a resonance frequency F0, and the signal V provided by this photodetector PD in the case where the emission frequency FLa' of the tunable laser La is aligned with the resonance frequency F0 of the MR resonator. In this case, the emission frequency FLa' of the tunable laser La is located in a relatively nonlinear section of the transmission function of the MR resonator. Consequently, the signal V provided by the photodetector PD exhibits, in the frequency domain, relatively significant 2*Fd harmonic components compared to the modulation frequency Fd.

[0017] Finally, on the figure 1d The transmission function T of the MR filter, whose spectrum TF exhibits a resonance frequency F0, is represented in the frequency domain. The signal V provided by this photodetector PD is also represented in the case where the emission frequency FLa of the tunable laser La is not aligned with a resonance frequency F0 of the MR resonator, but has an emission frequency Fla'' lower than this resonance frequency F0. Since the emission frequency of the tunable laser La lies in a relatively linear section of the MR resonator's transmission function, the signal V provided by the photodetector PD exhibits, in the frequency domain, a relatively large principal component Fd compared to its harmonics, and particularly compared to its second harmonic 2*Fd. Furthermore, the phase of the principal component Fd of the signal V is significant; that is, this principal component is in opposite phase to the signal V provided by the photodetector.

[0018] There figure 1e summarizes the results of figures 1b , 1c et 1d and represents, in the upper graph, the evolution of the power present in the principal component Fd and in the second harmonic 2*Fd of the signal V provided by the photodetector when the emission frequency FLa of the tunable laser La is modified and the resonance frequency of the filter remains fixed (or vice versa). figure 1e also represents, in the lower graph, the evolution of the phase of the principal component Fd of the signal V provided by the photodetector.

[0019] Returning to the description of the basic diagram of the figure 1a A locking device R receives the signal V established by the photodetector and processes it to generate a control signal CLa for the tunable laser La, aiming to tune its emission frequency to the resonant frequency F0 of the resonator MR. The processing implemented by the locking device R exploits the results presented in the figures 1b , 1c , 1d , 1e and determines the control to be applied by adjusting the laser emission frequency, which optimizes the proportion of the signal present in the main component or in the second harmonic 2*Fd of the signal provided by the PD photodetector. It should be noted that the proportion of the signal present in the main component and the proportion of the signal present in the second harmonic 2*Fd are related to each other, as can be clearly seen in the figure 1e Therefore, one can choose to use one or the other depending on the need. The phase can also be used, as illustrated previously.

[0020] As an illustration, the locking device R can apply a series of incrementing CLa commands to the adjustment device, progressing from a minimum to a maximum value, so that the laser emission frequency La is adjusted, in a series of steps, from a minimum emission frequency to a maximum emission frequency. At each step, the locking device R applies a frequency transformation to the signal provided by the photodetector (for example, a Fourier transform) to determine the proportion of the signal present in the second harmonic and / or the principal component. At the end of these steps, the step and its associated CLa command that resulted in a maximum signal in the second harmonic (or a minimum signal in the principal component) are identified. This associated CLa command is the one that best matches the laser emission frequency La and the resonant frequency of the MR filter.This command is then applied to the laser emission frequency adjustment device La to lock the system. Other approaches are also possible, for example, by exploiting, in a control loop that increments or decrements the Cla command supplied to the frequency adjustment device by a predetermined step, the phase information of the main frequency Fd of the signal provided by the photodetector PD. As already noted, this information indicates whether the laser emission frequency is lower than the laser's resonant frequency (high phase) or higher than this frequency (zero or low phase).

[0021] One can attempt to match the laser emission frequency La with the resonant frequency of the MR filter, as just presented as an example. Alternatively, one can aim to position them relative to each other by a predetermined distance. Generally, the control signal CLa is determined by optimizing a function that takes as an argument the proportion of the signal present in the principal component and / or the second harmonic 2*Fd of the control signal V. The optimization criterion can be that the function reaches a target value, either below a predetermined ceiling value or above a predetermined threshold value. One can also consider exploiting the phase of the principal component and / or the second harmonic 2*Fd of the control signal V.

[0022] As an example, one can thus seek to fix the ratio between the part of the signal present in the second harmonic and the part of the signal present in the main component so that it is equal to a target value or to maximize it.

[0023] For the sake of precision, we will say that the system is "locked" when the chosen optimization criterion is satisfied. This can correspond to the situation in which the emission frequency Fla and the resonance frequency F0 coincide or when these frequencies are shifted from each other by a determined distance.

[0024] It is noted that by applying a sinusoidal modulation signal, the appearance of harmonics in the signal provided by the photodetector PD is limited (compared to square wave modulation for example), the harmonics detected by the locking device R then being well representative of the quality of the locking between the emission frequency and the resonance frequency.

[0025] It is also noted that the same locking principles are applicable to a configuration in which the adjustment device is associated with the filter in such a way as to adjust its resonant frequency. Premier aspect de la présente description

[0026] According to a first aspect of this description, which is not the subject of the invention, the principles just presented are exploited to provide a laser source of multispectral light, thus exhibiting a plurality of spectral lines, these spectral lines being separated by a controlled spectral interval. For example, for applications in the field of wavelength division multiplexing transmission, the aim is to provide a laser source of multispectral light whose spectral lines are precisely separated (within 5%) by an interval of, for example, 100 GHz or 50 GHz.

[0027] With reference to the figure 2a As a first example, such a source 1 comprises a bank B of tunable lasers La, Lb, Lc. By way of example, the tunable lasers in bank B can be distributed feedback lasers. As is well known, each laser comprises a Bragg grating distributed along a laser cavity. Each La, Lb, Lc laser in bank B is associated with a current source Sa, Sb, Sc, enabling its power supply and the generation of light radiation. As already noted, the emission frequency of a distributed feedback laser is dependent on its supply current. By adjusting this current, the emission frequency can be adjusted, which makes these lasers "tunable" in the sense of this description. Bank B can contain any number of tunable lasers, typically between 4 and 100. Of course, the invention is by no means limited to a bank of distributed feedback lasers and applies to any tunable laser.In a complementary example, it could be a DBR type laser (for "Distributed Bragg reflector laser").

[0028] The lasers in Bank B are designed to emit light radiation with stepped emission frequencies, typically spread across a spectral range of 100 GHz for WDM applications, as previously explained. However, and as is made apparent on the left side of the figure 3a (where the frequencies Fla, Flb, Flc and the transfer function of the FT filter are represented), the variability of the manufacturing process of bank B does not allow for perfect control of the spectral interval separating the emission frequencies FLa, FLb, FLC of the lasers in bank B. The spectral interval separating two successive lasers (ordered by emission frequency) is therefore variable, and this variation, in the absence of any locking mechanism, can be on the order of or greater than + / -20%. It should also be noted that the operating temperature of bank B can affect and cause drift in the emission frequencies of the lasers.

[0029] Returning to the description of the example of the figure 2a The tunable La, Lb, and Lc lasers in the bank are coupled to an optical MO mixer via waveguides. This MO mixer produces at least one multispectral RLM light beam, with a spectral line of this beam corresponding to an emission frequency of the light beam emitted by a tunable laser from bank B. The MO mixer can provide a plurality of identical multispectral light beams.

[0030] The multispectral light radiation (or a plurality of such radiations) constitutes the so-called "useful" radiation of source 1, that is, the radiation that can be exploited by other elements, optical modulators, optical routers, etc., when, for example, source 1 forms a component of a communication system. According to one aspect of the invention, at least a portion of a "useful" multispectral radiation is taken to allow the alignment of the emission frequencies of the tunable lasers of bank B onto a frequency comb having a predetermined spectral range.

[0031] This portion of the multispectral light radiation is guided to an MR optical filter with a TF transfer function, defining a frequency comb template with the determined spectral range DF, as shown on the figure 3a In other words, two successive resonance frequencies F0i, F0j, F0k of the MR filter are separated by a defined spectral interval DF. For example, the optical MR filter can be a resonator, such as a micro ring resonator or a Fabry-Perrot type resonator, which allows for precise control, for example to within 5%, of the spectral interval DF between two resonance frequencies F0i, F0j, F0k. Regardless of its nature, the optical MR filter is placed downstream of the bank B of tunable lasers, and more precisely downstream of the optical mixer MO, to receive the multispectral light radiation RLM. In order to discriminate with sufficient sensitivity a frequency deviation imposed by the modulation, the transfer function of the MR filter must be particularly narrow, preferably exhibiting a slope greater than 6 dB / GHz when deviating by one gigahertz or more from one of its resonance frequencies.Such a feature is represented on the . figure 3b .

[0032] The optical MR filter may include a device for adjusting the plurality of its resonant frequencies F0i, F0j, F0k. Thus, when the MR filter is implemented by a ring resonator, this device may be a heater H allowing the frequency shift of the natural frequency comb, as will be explained in detail in a later section of this description.

[0033] Source 1 is shown on the figure 2a also includes a PD photodetector positioned downstream of the MR optical filter to establish a V signal representative of the multispectral MLR light radiation.

[0034] It also includes a modulator M associated with the bank of tunable lasers B, the modulator M being controllable via a selection signal Sel. The modulator M's function is to provide a signal Vd that modulates the emission frequency of the light emitted by a tunable laser with a frequency modulation signal Fd. The modulation signal Vd, whose general form is of the type cos(2πFdt), has a relatively low modulation frequency Fd, on the order of a few kHz to a few MHz, typically on the order of 5 kHz, a few 10 kHz, or even 1 MHz or more. The amplitude of the modulation signal Vd is chosen so that the frequency deviation of the emission frequency of the light emitted by a tunable laser is on the order of 1 GHz or more. The selection signal Sel of this embodiment allows the selection of the tunable laser from bank B to which the modulation frequency Fd will be applied.

[0035] In practice, this frequency modulation can be applied by modulating the current produced by the current source Sa, Sb, Sc associated with the selected tunable laser La, Lb, Lc with the modulation signal Vd. Other methods of modulating the laser's emission frequency can be used. These include applying the modulation signal Vd to a heater associated with the laser, or to a device injecting / depleting free carriers into the laser. Generally, therefore, the modulator M is electrically connected to the laser bank so as to apply the modulation signal to a means of adjusting the emission frequency of the selected tunable laser.

[0036] Finally, the laser source 1 shown on the figure 2a includes a locking device R for a tunable laser from bank B. This locking device R is connected to laser bank B via controls CLa, CLb, CLc, which are respectively connected to the current sources Sa, Sb, Sc. It is also connected to the photodetector PD to receive the signal V established by this element and, if present, to the heater H of the filter MR to control it. The locking device R is configured to control the modulator M and select, using the selection signal Sel, the tunable laser to which the modulation signal Vd is applied. The locking device R is also configured, in this aspect of the invention, to implement a control loop for tuning the emission frequency of the selected tunable laser Fla, Flb, Flc and locking it to a resonant frequency of the filter Foi, Foj, Fok. This control loop implements the principles set forth in relation to the description of... figures 1a à 1c It can, in particular, perform a Fourier transform (or any other transformation in the frequency domain) of the signal V produced by the photodetector PD and determine the power component present in the modulation frequency Fd and its harmonics, especially the second harmonic. Based on this, the locking device R can generate the control signal associated with the selected tunable laser, allowing its emission frequency to be adjusted to lock onto a resonant frequency of the MR filter.

[0037] In the embodiment shown in the figure 2a The current sources Sa, Sb, and Sc are adjustable, and the emission frequency of a laser is adjusted by controlling, through feedback, its average supply current provided by the associated adjustable current source. As previously stated, this average current, that is, the DC portion of the laser's supply current, affects the laser's emission frequency.

[0038] The laser source 1 of the example shown on the figure 2a is implemented by successively selecting a tunable laser to be locked from among the tunable lasers in bank B. Thus, the locking device R can include a state machine emitting a selection signal Sel that circularly selects one of the tunable lasers in bank B, for example, during successive locking periods whose duration can typically range from a few microseconds to a few milliseconds. During each locking period, the locking device R implements the processing that leads to locking the emission frequency of the selected tunable laser to the resonant frequency F0 closest to the optical filter MR. At the end of a complete cycle, each tunable laser is indeed locked to a resonant frequency of the optical filter. In this locked state of bank B, represented on the right-hand side of the figure 3a The multispectral RLM light radiation conforms well to the spectral pattern imposed by the MR optical filter: it exhibits a plurality of spectral lines FLa, FLb, FLc separated from each other by a spectral interval DF determined by the optical filter. Since the spectral interval DF separating two adjacent natural frequencies F0 of the filter is controlled, typically to within 5% or better, this property can be imparted to the emission frequencies of the tunable lasers in bank B.

[0039] By repeating the regulation cycles one after the other, in time multiplexing, it is possible to maintain the locked state of the bank of tunable lasers on the filter over time, and to compensate for any drifts, in particular those related to temperature variations of the lasers.

[0040] Optionally, the R locking device can implement another control loop to calibrate the resonance frequency comb of the MR optical filter and align it to target resonance frequencies in absolute value. For this purpose, and as shown in the figure 2b Light radiation from a standard laser Le is supplied to a complementary port of the MR filter. This standard laser Le has a modulated emission frequency, just like the other tunable lasers La, Lb, Lc in bank B. However, the standard tunable laser Le is not connected to the locking device R and its naturally stable emission frequency is not adjusted by this device.

[0041] The locking device R can extract from the Fourier transform of the signal V produced by the photodetector PD, the frequency components corresponding to the modulation frequency of the standard tunable laser, and the locking device R can control the heater H, or any other device for adjusting the plurality of resonant frequencies F0i, F0j, F0k of the filter, in order to shift the frequency range of this comb of natural frequencies and realign it with the target frequency provided by the standard laser. This operation is illustrated in the figure 3c . This MR filter calibration control loop and the tunable laser locking control loop are not necessarily distinct from each other, and according to one possible approach, the R locking device implements a single control loop or processing aimed at simultaneously optimizing the laser source 1 and the MR filter in order to produce RLM multispectral light radiation exhibiting a plurality of determined spectral lines, i.e., each of which is precisely positioned in the frequency domain.

[0042] There figure 2c presents another variant of this first aspect of the description. In this variant, the MR optical filter includes a tuning device H for the plurality of its resonant frequencies F0i, F0j, F0k, for example, a heater. The signal Vd provided by the modulator M is, in this variant, applied to the tuning device H of the MR optical filter. The application of this signal thus leads to the simultaneous modulation, at the modulation frequency Fd, of the plurality of resonant frequencies F0i, F0j, F0k of the MR optical filter. This variant has the advantage of greater simplicity in implementation. The laser source 1 of the figure 2c is implemented by successively selecting a tunable laser to lock from among the tunable lasers in bank B, similarly to the explanations given previously.

[0043] There figure 4 This represents another example of the implementation of laser source 1. In this example, each tunable laser La, Lb, Lc from bank B is equipped with a heater Ha, Hb, Hc. As is well known, a heater associated with a laser allows for fine control of the laser's emission frequency by varying its temperature. In this configuration of laser source 1, the commands CLa', CLb', CLc' generated by the locking device R are respectively connected to the heaters Ha, Hb, Hc in order to control them. The adjustment of the emission frequency of the tunable lasers in bank B is achieved via the heaters Ha, Hb, Hc, by controlling the temperature of the selected tunable laser, and not by controlling its average supply current as was the case in the first implementation.All other elements of the laser source 1 in the second example are identical to those in the first example, and for the sake of brevity, their description will not be repeated. It is of course possible to combine these two examples and adjust the emission frequency of the tunable lasers in bank B by controlling both the average supply current of the current source associated with a selected tunable laser and, simultaneously, by controlling the temperature of that laser using an associated heater.

[0044] In another variant of the laser source 1 shown on the figures 2a And 4Each La,Lb,Lc laser in bank B now includes a carrier injection / depletion device, for example, a waveguide positioned below the La,Lb,Lc laser. Similarly, the laser emission frequency can be precisely controlled by varying the concentration of free carriers in the device positioned below the laser. In the configuration of laser source 1 in this embodiment, the CLa', CLb', CLc' commands generated by the locking device R are respectively linked to the carrier injection / depletion devices.

[0045] As previously stated, each tunable laser in laser bank B can be equipped with multiple means of adjusting its emission frequency. In this case, it is not necessary for the same means to be used to modulate the laser's emission frequency and to adjust it to a resonant frequency of the MR filter. A first means can thus be used to modulate this emission frequency (for example, by applying the modulation signal Vd to the selected laser's power supply current source and thus modulating this supply current), and a second, different means can be used to adjust the laser's emission frequency to a resonant frequency of the filter (for example, by controlling the temperature produced by a heater associated with the laser). In the example of the figure 5 In frequency-division multiplexing, the modulator M generates a plurality of modulation signals Vda, Vdb, Vdc, each modulation signal being associated with a tunable laser La, Lb, Lc from bank B. Each modulation signal has a modulation frequency Fda, Fdb, Fdc distinct from the frequencies of the other modulation signals. These signals are applied simultaneously, and advantageously continuously, to the lasers to which they are respectively associated, in this case, to the current sources of these lasers. The modulator M thus allows the emission frequencies of the tunable lasers to be modulated via a modulation frequency specific to each tunable laser. For example, the modulation frequencies can be in the range of 1 kHz to 30 kHz. In this embodiment, it is therefore not necessary for the modulator M to be controllable via a selection signal.The rest of laser source 1 in this example is identical to the example in the . figure 2a , and its description will therefore be omitted here for the sake of brevity. One can notably plan, as in the first example, to inject radiation supplied by a standard laser into a complementary port of the MR filter, and to use a heater H associated with this MR filter, to precisely position each line of the RLM multispectral light radiation in the frequency domain.

[0046] The processes implemented by the R locking device are naturally adapted to this example, but are based on the same principles explained in the figures 1a à 1e In particular, the frequency domain analysis of the signal V provided by the photodetector reveals, as can be seen on the figure 6 , each of the modulation frequencies and their harmonics. With these modulation frequencies known, the locking device can be configured to identify them and implement the processing to adjust the emission frequency of the associated tunable laser.

[0047] On the representation of the figure 5 The control signals CLa, CLb, CLc prepared by the locking device R are respectively connected to the current sources Sa, Sb, Sc of the tunable laser bank. However, just as in the example of the figure 4 , it is conceivable within the framework of the example of the figure 5 , to control the emission frequency of tunable lasers using heaters respectively associated with these tunable lasers or any other means of adjusting the emission frequency of these lasers.

[0048] There figure 7 represents a variant applicable to the two examples just presented. In this variant, a single optical element (designated MO+MR in the figure) implements the functions of the MO mixer and the MR filter. This could be, for example, a multiplexer implemented by an array of waveguides (or "Arrayed waveguide grating") or by a ladder network. This element has a transfer function identical to that shown in the figure. figure 3a .

[0049] Regardless of the example chosen, a laser source 1 according to the invention can be implemented using silicon-based photonic technologies. According to these technologies, waveguides and other passive components are fabricated on a silicon substrate (and advantageously on a silicon-on-insulator substrate), and the other elements (laser sources, photodetectors, optical mixer, heaters) can be formed, by deposition or transfer, onto this substrate. This photonic chip can be combined with an electronic chip comprising some of the other electronic components of the laser source 1, such as the current sources or even the locking device. In some cases, a single chip can include both the photonic and electronic elements of the source 1.The locking device, if not integrated into one of the chips, can be implemented by a computing device (a microcontroller, a DSP signal processing computer or an ASIC) placed on a support and to which the chip(s) are electrically connected.

[0050] It is worth noting that a laser source 1 conforming to this first aspect of the description is particularly interesting, as it is possible to tune the emission frequency of the tunable lasers in bank B using a particularly simple circuit. This is especially true of the photonic part of the source, augmented by a single PD photodetector and a single MR filter, for example, a ring microresonator. This limits the number of additional interconnection pads required on the photonic chip to enable the locking functionality of the tunable lasers. Deuxième aspect et objet l'invention

[0051] The multispectral light radiation modulated by the modulation signal Vd, which forms the "useful" radiation provided by the source, can also be used for calibrating and / or locking photonic components (routers, modulators, demodulators, etc.) located downstream of the laser source 1, when this source 1 is used in a more complex photonic system S. According to the invention, a laser source providing multispectral light radiation with a poorly controlled spectral range can be used.

[0052] An example of such use of a laser source conforming to this aspect of the invention is shown in the figure 8a (in frequency multiplexing) and on the figure 8b (in time-division multiplexing). In these figures, a laser source 1 has at least one output port (two ports P1, P2 on the figures 8a,8b) each producing a multispectral RLM light radiation. This radiation is therefore spectrally composed of a plurality of lines separated by a spectral interval. At least one of these lines is frequency modulated, as has been explained in detail in relation to the explanation of each of the embodiments of source 1 (in time or frequency multiplexing).

[0053] A source 1 conforming to the invention includes a bank B of lasers La, Lb, Lc. Each laser in bank B is associated with a current source Sa, Sb, Sc, forming means for adjusting its emission frequency, connected to the modulator M. Thus, as in the first aspect of the description, at least one emission frequency of the RLM multispectral radiation emitted by source 1 can be modulated. The tunable lasers La, Lb, Lc of bank B are coupled to an optical mixer MO via an array of waveguides internal to source 1. The RLM multispectral light produced by the optical mixer MO is guided by the internal waveguide array to a port P1, P2 of source 1. It propagates in an external waveguide coupled to this port P1, P2.

[0054] The external waveguide is coupled to a photonic component MRA1,MRA2, which is optically positioned downstream of source 1 and laser bank B. This photonic component comprises a plurality of optical filters, each with a tunable resonant frequency. In such a photonic component, the resonant frequency of each filter is individually tunable. The photonic component MRA1,MRA2 conditions the multispectral light received at its input port according to the function performed by this component.

[0055] In the example according to the invention shown on the figures 8a, 8b Two radio frequency modulators MRA1, MRA2, each implementing an independent micro-resonator network, are respectively optically connected to two ports P1, P2 of the laser source 1. As is well known in the field of telecommunications, a radio frequency modulator makes it possible to condition each spectral line of the multispectral light radiation (here using micro-resonators respectively tuned to these lines) to transmit information signals in a frequency-multiplexed manner.

[0056] We have thus represented in more detail on the figure 9a An example of such a radio frequency modulator is shown. A ring array of microresonators MR1-MR3 is arranged between a first and a second waveguide WG1, WG2. The first waveguide WG1 is a through-port between an input port Pin, to which the multispectral light to be modulated is supplied, and an output port Pt, also known as a "through port," to which the modulated light is produced. The second waveguide WG2 is connected to a drop port Pd. Each microresonator is used to modulate an emission frequency from the source 1 via a radio frequency signal RF1-RF3. Each MR1-MR3 ring micro-resonator is associated with a means of adjusting H1, H2, H3 its resonant frequency F01, F02, F03, for example a heater.In such a radio frequency modulator MRA, we seek to position the resonance frequency F01,F02,F03 of each micro resonator MR1-MR3 relative to a spectral line FLa,FLb,FLc of the multispectral radiation produced by the source 1, according to the configuration of the . figure 1d that is, in a linear part of the TF1-TF3 transfer function of the MR1-MR3 microresonator. We naturally seek to allocate a distinct emission line to each microresonator. As illustrated in the figure 9b , we exploit the H1-H3 adjustment means associated with the MR1-MR3 micro resonators to adjust the original resonance frequencies F01,F02,F03 towards the optimized frequencies F01',F02',F03'.

[0057] Returning to the description of figures 8a,8b , the MR micro-resonators of the micro-resonator network of the radio frequency modulator MRA1, MRA2 are respectively associated with H adjustment devices allowing their resonance frequency to be adjusted independently, in particular to lock them onto the spectral lines to which they are to be associated.

[0058] It should be noted that this association can be achieved with considerable flexibility, for example by combining the microresonator and the laser whose resonance and emission frequencies are closest. State-of-the-art solutions in which the modulation signal is applied to the microresonators do not allow for such flexibility, because it is impossible to determine which laser a microresonator is associated with.

[0059] A PD1, PD2 monitoring photodetector is also provided optically downstream of the MRA1, MRA2 photonic component, here on a transmission output port of the MRA1, MRA2 photonic component, in order to establish an electrical signal representative of the RLM1, RLM2 multispectral radiation. A small portion of this RLM1, RLM2 multispectral radiation can be sampled for this purpose and coupled to the PD1, PD2 monitoring photodetector, as shown in the diagrams. figures 8a,8b .

[0060] In a variant represented in figure 8c , and in order to avoid taking part of the multispectral radiation RLM1,RLM2 propagating from the transmission port Pt of the photonic component MRA1,MRA2, it is possible to couple the monitoring photodetector PD1,PD2 directly to a loss port Pt of the photonic component MRA1,MRA2.

[0061] A locking device R receives the V1, V2 signal provided by the monitoring photodetector PD1, PD2 and produces the control signals Cd11, Cd12, Cd13; Cd21, Cd22, Cd23, allowing independent control of the adjustment devices H of the optical filters MR composing the photonic component MRA1, MRA2, and thus adjusting their resonant frequencies to a given emission frequency. The locking device R exploits the same principles as those presented in figures 1a à 1e to determine these control signals. When the photonic component MRA1,MRA2 is a modulator, and as previously mentioned, it is preferable to position an emission frequency of the RLM multispectral radiation within a linear portion of the transfer function of the filter formed by the microresonator. This allows us to aim to fix the ratio between the signal component present in the second harmonic and the signal component present in the principal component so that it equals a target value or is maximized.

[0062] This R-locking device can be used in time-division multiplexing ( figure 8b ) in which it is configured to control the modulator M and select, using the selection signal Sel, the tunable laser La,Lb,Lc to which the modulation signal Vd is applied.

[0063] The R locking device can also be used in frequency multiplexing ( figure 8a in which the modulator M generates a plurality of distinct modulation signals Vda, Vdb, Vdc, the modulation signals being applied simultaneously to the tunable lasers La, Lb, Lc. This frequency mode is particularly interesting to exploit, because each filter of the photonic component can be associated and allocated to a specific emission frequency of the multispectral radiation received at the input port.

[0064] There figure 9c presents a demodulator, this demodulator forming another type of optical component MR1,MRA2 that can be calibrated / locked according to the principles of the invention. In such a demodulator, each microresonator of the ring microresonator array MR1-MR3 is arranged between a first and a second waveguide WG1,WG2. The first waveguide WG1 has a Pin input port to which the multispectral light to be demodulated can be supplied and a Pt transmission port. The loss ports of the microresonators MR1,MR2,MR3 are connected to demodulation photodetectors PDa, PDb, PDc. The PD1 photodetector is placed on the first waveguide WG1, at the Pt transmission port.Each MR1-MR3 microresonator filters and extracts radio frequency signals RF1, RF2, and RF3 from the multispectral light radiation. These signals are carried by the emission frequencies (referred to as "carrier frequencies" in this context) FLa, FLb, and FLc of the multispectral light radiation. Each MR1-MR3 microresonator also includes a means for adjusting the resonance frequency F01-F03, such as a heater.

[0065] In figure 9d The relative positioning of the resonance frequencies F01, F02, F03 of the filters MR1, MR2, MR3 and the carrier frequencies FLa, FLb, FLc, before adjustment is shown on the left of this figure. The positioning of the resonance frequencies F01, F02, F03 of the filters MR1, MR2, MR3 and the carrier frequencies FLa, FLb, FLc, before adjustment is also shown on the right of this figure. figure 9d, the relative positioning of the adjusted resonance frequencies F01',F02',F03' and the carrier frequencies FLa,FLb,FLc, after adjustment and in the case where, voluntarily, it is chosen to lock the micro-resonators MR1,MR2,MR3 on the nearest optical carriers FLa,FLb,FLc.

[0066] Regardless of the nature of the photonic device(s) MRA1, MRA2 present in the photonic system, it can be advantageous to implement the system, during a startup phase, in frequency-division multiplexing operation where a distinct modulation frequency is applied to each emission / carrier frequency Fla, Flb, FLc of the RLM multispectral light radiation. For ease of implementation, the control signals for adjusting the resonance frequency of one optical filter at a time can be generated sequentially, independently controlling the tuning devices H of each microresonator MR1, MR2, MR3. As previously stated, this implementation method allows for precise control of the association of a microresonator with a carrier frequency, whereas prior art solutions could lead to associating several microresonators with the same carrier frequency.Once this start-up phase is completed, we can choose to produce the control signals concurrently.

[0067] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

[0068] An optical system according to the invention can exploit either of the aspects just presented. In particular, an optical system combining the two aspects of the invention can be provided, that is to say having a first locking device for adjusting the emission frequencies of the lasers of a bank to the resonance frequencies of a standard filter, and a second locking device for adjusting the resonance frequencies of the optical filters of a photonic component to the emission frequencies of the lasers of the bank.

Claims

1. A photonic system (S) comprising a laser source (1) capable of emitting at least one multispectral light radiation (RLM) having a plurality of spectral lines, the photonic system comprising: - a bank of lasers (B) comprising a plurality of lasers (La, Lb, Lc), a spectral line (FLa, FLb, FLc) of the multispectral light radiation (RLM) corresponding to an emission frequency of a laser (La, Lb, Lc) of the bank (B); - a modulator (M) associated with the laser bank (B), the modulator (M) being configured to generate a modulation signal (Vd; Vda, Vdb, Vdc) and to modulate the emission frequency of at least one laser (La, Lb, Lc) of the bank (B); - a photonic component (MRA1, MRA2) arranged downstream of the laser bank (B) and comprising a plurality of tunable optical filters (MR) each having a plurality of resonant frequencies; - a photodetector (PD1, PD2) arranged downstream of the photonic component (MRA1, MRA2) to establish a signal (V1, V2) representative of a multispectral radiation produced by the photonic component (RLM1, RLM2); - a locking device (R) configured to process the signal representative of the multispectral radiation (V1, V2) and to adjust the resonant frequencies of the tunable optical filters (MR) of the photonic component (MRA1, MRA2) to the emission frequencies of the lasers (La, Lb, Lc) of the bank (B).

2. The photonic system (S) according to the preceding claim, wherein the tunable optical filters (MR) of the photonic component (MRA1, MRA2) are ring micro-resonators.

3. The photonic system (S) according to one of the preceding claims, wherein the tunable optical filters (MR) are each provided with devices (H) for adjusting their resonant frequency.

4. The photonic system (S) according to the preceding claim, wherein the resonant frequency adjustment devices (H) are heaters.

5. The photonic system (S) according to one of the preceding claims, wherein the lasers (La, Lb, Lc) are distributed feedback lasers or distributed Bragg reflector lasers.

6. The photonic system (S) according to one of the preceding claims, wherein the locking device (R) is configured to control the modulator (M) and to select, by means of a selection signal (Sel), a laser (La, Lb, Lc) from the bank (B) to which the modulation signal (Vd) is applied.

7. The photonic system (S) according to one of claims 1 to 5, wherein the modulator (M) generates a plurality of modulation signals (Vda, Vdb, Vdc) which are distinct from one another, the modulation signals (Vda, Vdb, Vdc) being applied to the lasers (La, Lb, Lc) of the bank (B).

8. The photonic system (S) according to one of the preceding claims, wherein the adjustment of the resonant frequencies of the tunable optical filters (MR) is performed one optical filter at a time.

9. The photonic system (S) according to one of the preceding claims, wherein the modulator (M) is configured to produce a sinusoidal modulation signal (Vd; Vda, Vdb, Vdc) having a modulation frequency (Fd; Fda, Fdb, Fdc).

10. The photonic system (S) according to one of the preceding claims, wherein the locking device (R) is configured to establish a measurement representative of the power present in a second harmonic of the modulation frequency (Fd; Fda, Fdb, Fdc) of the signal (V1, V2) representative of the multispectral radiation.

11. The photonic system (S) according to one of the preceding claims, wherein the locking device (R) is configured to establish a measurement representative of the power present in a main component of the modulation frequency (Fd; Fda, Fdb, Fdc) of the signal (V1, V2) representative of the multispectral radiation.

12. The photonic system (S) according to one of the preceding claims, wherein the locking device (R) is configured to establish a measurement representative of the phase of the main component of the modulation frequency (Fd; Fda, Fdb, Fdc) of the signal (V1, V2) representative of the multispectral radiation.

13. The photonic system (S) according to one of the preceding claims, wherein the photonic component (MRA1, MRA2) is a radio-frequency modulator.

14. The photonic system (S) according to one of the preceding claims, wherein the photonic component (MRA1, MRA2) is a radio-frequency demodulator.