Laser source having a plurality of spectral lines separated by determined spectral intervals

Abstract

The invention relates to a laser source (1) for emitting a multispectral optical radiation (RLM) having a plurality of spectral lines separated by a determined spectral interval. The laser source (1) comprises a bank of tunable lasers (B), an optical filter (MR) having a plurality of resonant frequencies, a photodetector (PD) arranged downstream of the optical filter (MR) for establishing a signal representative of the multispectral radiation (V), and a modulator (M) associated with the bank of tunable lasers (B) and capable of modulating the emission frequency of the optical radiation emitted by at least one tunable laser (La, Lb, Lc) of the bank (B) at a modulation frequency (Vd; Vda, Vdb, Vdc). The laser source (1) also comprises a locking device (R) of at least one tunable laser configured to process the signal representative of the multispectral radiation (V) and to tune the emission frequency to the resonant frequency of the filter.

Description

[Technical field]

[0001] The present invention relates to a laser source for emitting optical radiation having a plurality of spectral lines separated by a determined spectral interval. Such a source finds very particular application in the field of communications using wavelength division multiplexing. [Background technology]

[0002] The article "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, presents a laser source formed from a bank of distributed feedback lasers. Each laser contains Bragg gratings distributed along the laser cavity. The lasers emit optical radiation at stepped wavelengths, typically spaced apart from each other by 100 GHz or 50 GHz.

[0003] The optical radiation emitted by the lasers is propagated to the input ports of a passive optical mixer, which generates a plurality of optical radiations at its output ports, each combining the optical radiations fed to its input ports. The output radiation generated at these output ports is therefore multispectral (in a spectral comb, each line of the comb corresponds to the radiation emitted by a bank of lasers).

[0004] Due to inaccuracies and variability in laser manufacturing methods, the laser emission wavelength is not well controlled. This results in variability in the spectral interval between two spectral lines in the multispectral emission, which can be on the order of ±5-10% of the expected spectral interval, or even greater depending on the interval. The wavelength of the optical radiation emitted by a laser is also sensitive to the operating temperature, which can vary, for example, from 0°C to 80°C.

[0005] Thus, the spectral range of the optical radiation provided by state-of-the-art multispectral laser sources is poorly controlled and prone to drift during operation of the source.

[0006] Document WO2013044863 presents a transmitter formed by a multispectral laser source composed of several lasers with adjustable emission frequencies. A generator associated with the lasers generates a pilot signal that modulates the laser emission at a low frequency. The multispectral emission generated by the transmitter is guided by an optical fiber to a remote etalon filter. Optical splitters, respectively placed before and after the etalon filter, feed a signal to a regulator. The regulator generates an adjustment signal that is added to the modulation signal of the lasers, locking the emission of each laser to the wavelength defined by the etalon filter.

[0007] It should be noted that in the solution presented in this document, the etalon filter, remote from the laser source, is not subject to the same temperature excursions as the laser source. This filter defines the absolute etalon frequency to which the laser radiation frequency is respectively adapted by adjusting their power supply. This solution is unsatisfactory, since when the radiation frequency deviates significantly from the standard frequency, it can lead to the generation of high amplitude laser adjustment signals, which affect the power of the radiation emitted by the multispectral laser source and make it variable beyond the permissible threshold.

[0008] Object of the invention It is an object of the present invention to provide at least a partial solution to this problem. More specifically, it is an object of the present invention to present a laser source capable of providing multispectral optical radiation having better controlled spectral ranges than those present in the optical radiation produced by prior art laser sources. Summary of the Invention

[0009] To this end, the object of the invention is to provide a laser source for emitting at least one multispectral optical radiation having a plurality of spectral lines separated by a predetermined spectral interval, the laser source comprising: a bank of tunable lasers, the spectral lines of which correspond to frequencies, called "radiation frequencies", of the optical radiation emitted by the tunable lasers of the bank; - means for adjusting the emission frequency of the tunable laser; an optical filter having a plurality of resonant frequencies, two successive resonant frequencies being separated by a determined spectral interval, said optical filter being arranged optically downstream of the bank of tunable lasers and comprising a device for tuning the plurality of resonant frequencies; a photodetector arranged optically downstream of the optical filter to establish a signal representative of the multispectral optical radiation transmitted by the filter; a modulator associated with the means for adjusting the emission frequency of the tunable laser, the modulator being configured to generate a modulation signal and to modulate the emission frequency of the optical radiation emitted by at least one tunable laser of the bank; a locking device connected to the means for adjusting the radiation frequency of the tunable laser and to the device for adjusting the multiple resonant frequencies, the locking device being configured to process a signal representative of multispectral radiation and to lock the radiation frequency of the tunable laser to the resonant frequencies of the filter.

[0010] According to other advantageous, non-limiting characteristics of the present invention, taken alone or according to any technically feasible combination, the following: the laser source comprises an optical mixer associated with the bank of tunable lasers, for combining optical radiation emitted by the tunable lasers of the bank and providing a multispectral optical radiation to the filter; The optical filter is a microring resonator. The device for adjusting multiple resonant frequencies is a heater, The means for adjusting the emission frequency of the tunable laser is selected from the following list: current source, heater, free carrier injection / depletion device. The lasers in the bank of tunable lasers are either distributed feedback lasers or distributed Bragg reflector lasers. the locking device is configured to control the modulator and to select, by means of a selection signal, a tunable laser to which the modulation signal is applied; a modulator generating a plurality of distinct modulation signals, the modulation signals being applied to a tunable laser; the modulator is configured to generate a sinusoidal modulating signal having a modulation frequency; the locking device is configured to establish a measurement representative of the power present in the second harmonic and / or in the principal component and / or a measurement representative of the phase of the principal component of the modulation frequency of the signal representative of multispectral radiation, A bank of tunable lasers and optical filters are integrated on / within the same substrate of a photonic chip, The temperature drift coefficient of the tunable laser emission frequency and the temperature drift coefficient of the resonant frequency are identical within 10%.

[0011] According to another aspect, the present invention provides a method for using a laser source, the method being implemented by a locking device, comprising: a control stage for operating a device for tuning the resonant frequencies of the optical filter; a locking step for locking the optical emission frequency of a selected tunable laser to the resonant frequency of the filter.

[0012] Advantageously, the locking step is performed after the control step. [Brief description of the drawings]

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings. [Figure 1a] FIG. 1 shows a schematic diagram of the principle on which the present invention is based. [Figure 1b]FIG. 1 shows a schematic diagram of the principle on which the present invention is based. [Figure 1c] FIG. 1 shows a schematic diagram of the principle on which the present invention is based. [Figure 1d] FIG. 1 shows a schematic diagram of the principle on which the present invention is based. [Figure 1e] FIG. 1 shows a schematic diagram of the principle on which the present invention is based. [Figure 2a] 1 shows a first embodiment of the present invention. [Figure 2b] 3 shows a modification of the first embodiment of the present invention. [Figure 3a] 4 illustrates the transition from a natural state to a locked state of a laser source according to the present invention. [Figure 3b] 4 shows advantageous features of the filter transfer function of the laser source according to the invention. [Figure 3c] 1 illustrates a calibration of filters of a laser source according to one embodiment of the invention. [Figure 4] 4 illustrates a further embodiment of the present invention. [Diagram 5] 4 illustrates a further embodiment of the present invention. [Figure 6] 5 shows the signal provided by the photodetector in the frequency domain. [Figure 7] Variations that are applicable to all embodiments of the present invention are shown. [Figure 8] 4 illustrates the use of a source according to a second embodiment of the invention for tuning a modulator array. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Figures 1a, 1b, 1c, 1d and 1e are schematic diagrams of the principle on which the invention is based. In the architecture shown in [Figure 1a], the optical radiation from a tunable laser La is guided to a ring resonator MR constituting a filter with a resonant frequency F0 and to a photodetector PD located downstream of the resonator MR.

[0015] The term "tunable laser" refers to a laser that generates optical radiation at an adjustable frequency ("emission frequency"). By way of example, the means for adjusting the emission frequency that a laser may be provided with may comprise devices configured to change its supply current, temperature, refractive index, and free carrier concentration. A tunable laser may comprise multiple means for adjusting its emission frequency, for example adjustable current sources and heaters for changing the operating temperature of the laser.

[0016] The modulator M connected to the laser emission frequency adjustment means is configured to modulate the emission frequency Fla of the optical radiation 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, which is 200 THz. The amplitude of this modulation is also small. As an example, a supply current modulation amplitude of 1 mA can result in a variation of the emission frequency FLa of the tunable laser La of the order of ±1 GHz. The optical radiation emitted by the tunable laser La therefore varies at a very low frequency Fd with a low amplitude A (1 GHz) around its fundamental frequency FLa. The laser frequency therefore varies as Fla+A.cos(2pi.Fd.t).

[0017] FIG. 1b shows the frequency domain transfer function T of a filter MR whose spectrum TF has a resonant frequency F0, and the signal V provided by this photodetector PD when the emission frequency FLa of a tunable laser La is not locked to the resonant frequency F0 of the resonator MR, but has a higher emission frequency FLa than this resonant frequency F0. Since the emission frequency of the tunable laser La is located in a relatively linear part of the transfer function of the resonator MR, the signal V provided by the photodetector PD is linear in the frequency domain for its harmonics, in particular for its second harmonic 2. * PD has a relatively large principal component Fd (corresponding to the modulation frequency) with respect to Fd. In addition, the phase of the principal component Fd of the signal V is reduced, i.e., this principal component is in phase with the signal V provided by the photodetector PD.

[0018] Similarly to [Fig. 1b], [Fig. 1c] shows the transfer function T of a filter MR whose spectrum TF has a resonant frequency F0, and the signal V provided by this photodetector PD when the emission frequency FLa' of a tunable laser La is locked to the resonant frequency F0 of the resonator MR. In this case, the emission frequency FLa' of the tunable laser La is located in a relatively nonlinear part of the transfer function of the resonator MR. As a result, the signal V provided by the photodetector PD has a relatively large harmonic content 2 in the frequency domain relative to the modulation frequency Fd. * Has Fd.

[0019] Finally, FIG. 1d shows the frequency domain transfer function T of a filter MR whose spectrum TF has a resonant frequency F0, and the signal V provided by this photodetector PD when the emission frequency FLa of the tunable laser La is not locked to the resonant frequency F0 of the resonator MR, but has an emission frequency FLa″ lower than this resonant frequency F0. Since the emission frequency of the tunable laser La is located in a relatively linear part of the transfer function of the resonator MR, the signal V provided by the photodetector PD is, in the frequency domain, linear with respect to its harmonics, in particular its second harmonic 2 * It has a principal component Fd which is relatively large with respect to Fd. Furthermore, the phase of the principal component Fd of the signal V is important, i.e., it is in anti-phase with the signal V provided by the photodetector.

[0020] [Figure 1e] summarizes the results of Figures 1b, 1c and 1d, and shows that in the upper graph, the main component Fd and the second harmonic 2 of the signal V provided by the photodetector are changed when the emission frequency FLa of the tunable laser La is changed and the resonant frequency of the filter remains fixed (or vice versa). * FIG. 1e shows the evolution of the power present in Fd and in the lower graph the evolution of the phase of the principal component Fd of the signal V provided by the photodetector.

[0021] Returning to the description of the schematic diagram of [Fig. 1a], the locking device R receives the signal V established by the photodetector and processes it to generate a command CLa for tuning and locking the emission frequency of the tunable laser La to the resonant frequency F0 of the resonator MR. The processing carried out by the locking device R makes use of the results shown in Figs. 1b, 1c, 1d and 1e to determine the command to be applied to the means for adjusting the laser emission frequency, which is the second harmonic 2 of the signal provided by the photodetector PD. * The proportion of the signal present in Fd is maximized. As an example, the locking device R can apply to the adjustment device a series of commands CLa, which increment from a minimum value to a maximum value, such 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 (for example a Fourier transform) to the signal provided by the photodetector in order to detect the proportion of the signal present in the second harmonic. At the end of these steps, the step and the associated command CLa that resulted in a signal maximum in the second harmonic are identified, this associated command CLa being the command that best matches the laser emission frequency La and the resonance frequency of the filter MR. 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 using phase information of the main frequency Fd of the signal provided by the photodetector PD in a control loop that increments or decrements the command Cla provided to the frequency adjustment device by a predefined pitch. As already mentioned, this information indicates whether the laser emission frequency is lower (high phase) or higher (0 or low phase) than the laser resonance frequency.

[0022] The laser emission frequency La can be matched to the resonant frequency of the filter MR, as shown by way of example. Alternatively, they can be placed at a certain distance from each other. Generally speaking, the command CLa can be, for example, a signal that is generated by the main component and / or the second harmonic 2 of the control signal V. *The control signal V is determined by optimizing a function that takes into account the proportion of the signal present in Fd. The optimization criterion may be that the function reaches a target value, falls below a predefined upper limit, or exceeds a predefined threshold. * The phase of Fd can also be evaluated.

[0023] For example, the ratio between the share of the signal present in the second harmonic and the share of the signal present in the principal component can be set to equal or maximize a target value.

[0024] Therefore, for precision, we say that the system is "locked" when a selected optimization criterion is met. This may correspond to the situation where the radiation frequency Fla and the resonant frequency F0 coincide, or where these frequencies are offset from each other by a specified distance.

[0025] It should be noted that by applying a sinusoidal modulation signal, the appearance of harmonics in the signal provided by the photodetector PD is limited (compared to, for example, a square wave modulation), since the harmonics detected by the locking device R actually represent the quality of the lock between the radiation frequency and the resonant frequency.

[0026] It should also be noted that the same locking principle is applicable to a configuration in which a laser has a fixed emission frequency and a tuning device is associated with the filter to tune its resonant frequency.

[0027] The present disclosure utilizes the principles presented above to present a laser source of multispectral optical radiation, thus presenting a plurality of spectral lines, which are separated by a controlled spectral interval. As an example, in an application in the field of wavelength division multiplexing transmission, the aim is to provide a laser source of multispectral optical radiation, whose spectral lines are precisely (within 5%) separated by, for example, a 100 GHz or 50 GHz interval.

[0028] With reference to [Fig. 2a], which shows a first embodiment of the invention, such a source 1 comprises a bank B of tunable lasers La, Lb, Lc. For example, the tunable lasers in bank B can be distributed feedback lasers. As is well known, each laser comprises a Bragg grating distributed along the laser cavity. Each laser La, Lb, Lc of bank B is associated with a current source Sa, Sb, Sc for power and light generation. As already mentioned, the emission frequency of a distributed feedback laser depends on its supply current. By adjusting this current, this emission frequency can be adjusted, making these lasers "tunable" within the meaning of the present disclosure. Bank B can contain any number of tunable lasers, typically between 10 and 100. Of course, the invention is not limited to a bank of distributed feedback lasers, but applies to any tunable laser. A complementary example is the DBR laser (Distributed Bragg Reflector Laser).

[0029] The lasers of Bank B are designed to emit light with stepped emission frequencies, typically within the 100 GHz spectral range for WDM applications, as mentioned above. However, as can be seen on the left side of [Figure 3a] (where frequencies Fla, Flb, Flc and filter transfer function FT are shown), the variability of the manufacturing method of Bank B means that the spectral interval separating the emission frequencies FLa, FLb, FLC of the lasers of Bank B cannot be fully controlled. Thus, the spectral interval separating two successive lasers (ordered by emission frequency) is variable, and this variation in the absence of a locking mechanism can be of the order of + / - 20% or more. It is also noted that the operating temperature of Bank B can affect the laser emission frequencies and cause them to drift.

[0030] Returning to the disclosure of [Fig. 2a], the tunable lasers La, Lb, Lc of the bank are coupled via waveguides to an optical mixer MO. This mixer MO generates at least one multispectral optical radiation RLM, whose spectral lines correspond to the radiation frequencies of the optical radiation emitted by the tunable lasers of the bank B. The mixer MO is capable of providing a plurality of mutually identical multispectral optical radiations.

[0031] The multispectral optical radiation RLM (or a number of such radiations) forms the so-called "useful" radiation of source 1, i.e. radiation that can be utilized by other elements, optical modulators, optical switches, etc., for example when source 1 forms a component of a communication system. At least a part of the "useful" multispectral radiation is sampled in order to match the radiation frequencies of the tunable lasers of bank B with a frequency comb having a specific spectral interval.

[0032] This sampled portion of the multispectral optical radiation is guided via a waveguide to an optical filter MR with a transfer function TF that defines a frequency comb template with a determined spectral interval DF, as shown in [Figure 3a]. In other words, two successive resonance frequencies F0i, F0j, F0k of the filter MR are separated by a determined spectral interval DF. By way of example, the optical filter MR can be a resonator, such as a microring resonator or a Fabry-Perot resonator, that allows the spectral interval DF between two resonance frequencies F0i, F0j, F0k to be precisely controlled, for example within 5%. Whatever its nature, the optical filter MR is placed downstream of the bank B of tunable lasers, more precisely downstream of the optical mixer MO, in order to receive the multispectral optical radiation RLM. In order to be able to discern with sufficient sensitivity the frequency shifts imparted by the modulation, the transfer function of the filter MR must be particularly narrow, preferably with a slope greater than 6 dB / GHz, when shifting from one of its resonance frequencies by more than 1 gigahertz. Such a characteristic is shown in [Figure 3b].

[0033] The optical filter MR may comprise a device for tuning its multiple resonant frequencies F0i, F0j, F0k, which may thus be a heater H for frequency shifting the eigenfrequency comb, when the filter MR is implemented by a ring resonator, as will be explained in detail in a later section of this disclosure.

[0034] The source 1 shown in FIG. 2a also comprises a photodetector PD arranged downstream of the optical filter MR for establishing a signal V representative of the multispectral light emission MLR.

[0035] It also comprises a modulator M associated with the bank B of tunable lasers, the modulator M being controllable via a selection signal Sel. The function of the modulator M is to provide a signal Vd for modulating the radiation frequency of the optical radiation emitted by the tunable lasers by means of a frequency modulation signal Fd. The modulation signal Vd has the general form cos(2.Pi * Fd * t) type, with a relatively low modulation frequency Fd of the order of a few kHz to a few MHz, typically of the order of 5 kHz, 10 kHz or even 1 MHz or more. The amplitude of the modulation signal Vd is selected such that the frequency deviation of the radiation frequency of the optical radiation emitted by the tunable laser is of the order of 1 GHz or more. The selection signal Sel in this embodiment makes it possible to select the tunable laser of bank B to which the modulation frequency Fd is applied.

[0036] In practice, this frequency modulation can be applied by modulating the current generated by the current sources Sa, Sb, Sc associated with the selected tunable laser La, Lb, Lc with a modulation signal Vd. Other means of modulating the laser emission frequency can also be used. This may involve applying the modulation signal Vd to a heater associated with the laser or to a free carrier injection / depletion device within the laser. Generally speaking, a modulator M is then electrically connected to the laser bank and applies the modulation signal to a means of adjusting the emission frequency comprised by the selected tunable laser.

[0037] Finally, the laser source 1 shown in [Fig. 2a] comprises a locking device R for the tunable lasers of the laser bank B. This locking device R is connected to the laser bank B via commands CLa, CLb, CLc connected to the current sources Sa, Sb, Sc, respectively. It is also connected to the photodetector PD to receive the signal V established by this element and to the filter MR adjustment device H to control it. The locking device R is configured to control the modulator M and to select, by means of a selection signal Sel, the tunable laser to which the modulation signal Vd is applied. The locking device R is also configured to implement a control loop for tuning the selected tunable laser emission frequencies Fla, Flb, Flc to the filter resonance frequencies Foi, Foj, Fok during the locking phase. This control loop implements the principle described in relation to Fig. 1a-1c. In particular, it is possible to perform a Fourier transform (or any other transform in the frequency domain) of the signal V generated by the photodetector PD and to determine the percentage of power present in the modulation frequency Fd and its harmonics, in particular the second harmonic. It is also possible to determine the phase of these signals. On this basis, the locking device R is able to generate a command associated with the selected tunable laser, enabling its emission frequency to be adjusted so as to lock onto the resonant frequency of the filter MR.

[0038] In the embodiment shown in FIG. 2a, the current sources Sa, Sb, Sc are adjustable and the emission frequency of the laser is adjusted by feedback control of the average supply current provided by the associated adjustable current sources. As already mentioned, this average current, i.e. the DC portion of the laser supply current, influences the emission frequency of the laser.

[0039] The laser source 1 of the embodiment shown in [Fig. 2a] is operated by successively selecting, during a locking phase, a tunable laser to be locked among the tunable lasers of bank B. The locking device R may thus comprise, for example, a state machine that emits a selection signal Sel that cyclically selects one of the tunable lasers of bank B during successive locking periods whose duration may typically be between a few microseconds and a few milliseconds. During each locking period, the locking device R implements the processes necessary to lock the emission frequency of the selected tunable laser to the natural frequency F0 closest to the optical filter MR. At the end of a complete cycle, each tunable laser is locked to a specific frequency of the optical filter. In this locked state of bank B shown on the right side of [Fig. 3a], the multispectral light emission RLM fits the spectral template imposed by the optical filter MR. The multispectral light emission RLM exhibits a number of spectral lines FLa, FLb, FLc separated from each other by a spectral interval DF determined by the optical filter. Since the spectral spacing DF between two adjacent eigenfrequencies F0 of the filter is typically controlled to within 5% or better, this property can be imparted to the emission frequencies of the tunable lasers of bank B.

[0040] In time division multiplexing, by repeating the control cycle one after the other, it is possible to maintain the locked state of the bank of tunable lasers on the filters over time and to compensate for any drift, particularly that associated with variations in laser temperature.

[0041] Optionally, the locking device R can implement another control loop to calibrate the resonant frequency comb of the optical filter MR and align it to the absolute target resonant frequency. For this purpose, as shown in [Fig. 2b], the optical radiation from the etalon laser Le is fed to the complementary port of the filter MR. This etalon laser has a modulated emission frequency, as do the other tunable lasers La, Lb, Lc of bank B. However, the tunable etalon laser Le is not connected to the locking device R and its naturally stable emission frequency is not adjusted by this device.

[0042] The locking device R can extract the frequency component corresponding to the modulation frequency of the tunable etalon laser from the Fourier transform of the signal V generated by the photodetector PD, and the locking device R can control the heater H, or any other device for adjusting the multiple resonant frequencies F0i, F0j, F0k of the filter, to frequency shift this natural frequency comb and realign it with the target frequency delivered by the etalon laser. This operation is illustrated in [Fig. 3c]. This calibration control loop of the filter MR and the locking control loop of the tunable laser do not necessarily have to be different from each other, and according to one possible approach, the locking device R implements a single control loop or process aimed at simultaneously optimizing the laser source 1 and the filter MR in order to generate a multispectral light emission RLM having multiple determined spectral lines, i.e. each of whose spectral lines is precisely positioned in the frequency domain.

[0043] It should be noted that the above-mentioned calibration of the resonant frequency comb of the optical filter MR is entirely optional, the main objective of the invention being to control the spectral interval existing between the spectral lines of the multispectral radiation generated by the laser source 1. In particular, it is entirely acceptable for the absolute value of the radiation frequency of each tunable laser in the source to drift, in particular under the influence of the operating temperature of the source, as long as the spectral interval existing between two adjacent spectral lines in the multispectral radiation remains controlled.

[0044] [Fig. 4] shows another example of the laser source 1. In this embodiment, each tunable laser La, Lb, Lc of the bank B is fitted with a heater Ha, Hb, Hc. As is well known, a heater associated with a laser allows fine control of the laser emission frequency by varying its temperature. In the configuration of the laser source 1 in this embodiment, the commands CLa', CLb', CLc' generated by the locking device R are connected to the heaters Ha, Hb, Hc, respectively, in order to control them. Thus, in this embodiment, the emission frequency of the tunable lasers of the bank B is adjusted via the heaters Ha, Hb, Hc by controlling the temperature of the selected tunable laser, and not by controlling its average supply current, as in the first embodiment. All other elements of the laser source 1 of the second embodiment are identical to those of the first embodiment, and for the sake of brevity, they will not be described again. Of course, it is also possible to combine these two methods to adjust the emission frequency of a tunable laser in Bank B by simultaneously controlling the average supply current of a current source associated with a selected tunable laser and controlling the temperature of this laser using an associated heater.

[0045] In another variant of the embodiment of the laser source 1 shown in Figures 2a and 4, each laser La, Lb, Lc of the bank B comprises in this case a carrier injection / depletion device, for example a waveguide, arranged under the laser La, Lb, Lc. Similarly, the laser emission frequency can be finely controlled by varying the concentration of free carriers in the device under the laser. In this embodiment of the configuration of the laser source 1, the commands CLa', CLb', CLc' generated by the locking device R are respectively connected to the carrier injection / depletion device.

[0046] As already mentioned, each tunable laser of the laser bank B may comprise several means for adjusting its emission frequency. In this case, it is not necessary to use the same means for modulating the emission frequency of this laser and adjusting it to the resonance frequency of the filter MR. Thus, a first means may be used to modulate this emission frequency (for example by applying a modulation signal Vd to the supply current source of the selected laser and thus modulating this supply current) and a second means, different from the first means, may be used to adjust the emission frequency of this laser to the resonance frequency of the filter (for example by controlling the temperature generated by a heater associated with the laser).

[0047] In the embodiment of [Fig. 5], in the frequency division multiplexing mode, the modulator M generates a number of modulation signals Vda, Vdb, Vdc, each of which is associated with a tunable laser La, Lb, Lc from the bank B. Each modulation signal has a modulation frequency Fda, Fdb, Fdc that differs from the frequency of the other modulation signals. During the locking phase, these signals are simultaneously, advantageously permanently, applied to the lasers to which they are respectively associated, in this case to the current sources of these lasers. The modulator M thus modulates the emission frequencies of the tunable lasers via a modulation frequency specific to each tunable laser. For example, the modulation frequency may be in the range of 1 kHz to 30 kHz. It is therefore not necessary for the modulator M to be controllable via a selection signal for this embodiment. The remaining parts of the laser source 1 in this embodiment are identical to the first embodiment in [Fig. 2a] and therefore will not be described here for the sake of brevity. In particular, as in the first embodiment, the radiation provided by the etalon laser can be injected into a complementary port of a filter MR, and a heater H associated with this filter MR can be used to accurately position each line of the multispectral light radiation RLM in the frequency domain.

[0048] The process implemented by the locking device R is of course adapted to this design, but is based on the same principles as described in Figures 1a to 1c. In particular, a frequency domain analysis of the signal V provided by the photodetector reveals the modulation frequencies and each of their harmonics, as can be seen in Figure 6. Since these modulation frequencies are known, the locking device can be configured to implement a process for identifying them and adjusting the emission frequency of the associated tunable laser.

[0049] In the depiction of Fig. 5, the control signals CLa, CLb, CLc generated by the locking device R are connected to the current sources Sa, Sb, Sc, respectively, of the tunable laser bank. However, as in the embodiment shown in Fig. 4, in the embodiment shown in Fig. 5, it is also possible to control the emission frequencies of the tunable lasers using heaters associated with each of these tunable lasers, or any other means of adjusting the emission frequencies of these lasers.

[0050] [Fig. 7] shows a variant applicable to both of the above embodiments. In this variant, a single optical element (indicated in the figure as MO+MR) implements the functions of the mixer MO and the filter MR. It may be, for example, a multiplexer implemented by an arrayed waveguide grating or a ladder network. This element has the same transfer function as shown in [Fig. 3a].

[0051] Very advantageously, the locking device R of the laser source 1 according to the invention also makes use of an adjustment device H of the multiple resonant frequencies of the optical filter MR, MO+MR, in order to lock the radiation frequency of the optical radiation of the tunable laser. This approach can be deployed for all the above-mentioned embodiments and does not require the use of an etalon laser. The invention aims at controlling the spectral interval between two adjacent spectral lines of the multispectral radiation without imposing a precise position in absolute terms of these spectral lines. In this way, it is possible to "float" the spectral positioning of the multispectral radiation while controlling the spectral interval between two adjacent spectral lines. This therefore avoids overloading the means for adjusting the radiation frequency of the tunable laser by trying to force these frequencies to absolute frequencies, which may affect and excessively change the power emitted by the laser (for example, when the adjustment means are constituted by a laser current source) or lead to excessive energy consumption by the source (for example, when the adjustment means are constituted by a heater). It should be noted that in some cases, adjusting the emission frequency of a laser when seeking to impose an absolute emission frequency may lead to the requirement to cool this source, which is not always easily possible.

[0052] The locking device R is configured, during a control phase that may precede the locking phase, to activate a device H for adjusting the resonant frequencies of the optical filter MR in order to position these resonant frequencies relative to the laser emission frequency in a so-called "average" configuration that tends to bring the two frequency combs closer to each other. This average configuration allows, for example, that the means for adjusting the emission frequency of the tunable laser are used less intensively to lock the system during the locking phase.

[0053] A number of optimization criteria can be deployed by the locking device R during the control phase in order to establish this average configuration. This may for example involve optimizing the sum of the respective differences between the emission frequencies of the tunable lasers and the resonant frequencies of the filters. This may be a quadratic sum or it may be an absolute or relative optimization of the maximum value of these deviations.

[0054] As in the locking stage, the locking device R uses the signal V established by the photodetector to determine the power present in the second harmonic and / or the fundamental of the modulated signal, as well as the phase information of the modulated signal. This data can be used to determine the difference between the emission frequency of the laser and the corresponding resonant frequency of the filter, for example using the graph in [Figure 1e].

[0055] The locking devices R can operate in a time-multiplexed or frequency-multiplexed manner during this control phase.

[0056] As an example, it may be desirable to position the resonant frequencies of the filter during this control phase so that they are each a small margin higher than the emission frequency of the tunable laser, and then, during the locking phase, these emission frequencies can be tuned upwards using heaters that operate only when necessary.

[0057] Regardless of the optimization criterion selected, the control device R controls the optical filter MR adjuster H during this control phase, which can be configured to, for example, scan its operating range. During this excursion, by time- or frequency-multiplexing the measurements, the control device R detects the difference between the resonance frequency of the filter MR and the emission frequency of the tunable laser of the bank. At the end of this excursion, the control device identifies the command of the optical filter MR adjuster H that best satisfies the selected optimization criterion and applies this command to the adjuster H to place the filter in the average configuration. However, the control phase can be performed using techniques other than the systematic technique of exploring the operating range of the adjuster H. For example, a successive optimization method can be applied based on the gradient of the optimization criterion, during which the command of the adjuster H of the optical filter MR is changed step by step to seek the optimum of the optimization criterion. In all cases, whatever technique is adopted, the control phase implemented by the control device R results in the generation and application of commands to the adjuster H aimed at placing the optical filter MR in the average configuration.

[0058] The control step may be preceded by a locking step, which may be repeated at selected times to take into account any variations in the operating point of the system. It is also possible to carry out the locking and control steps simultaneously, for example in a single control loop, especially if the control step uses a gradient-type continuous optimization technique. The laser emission frequency and collectively the filter resonance frequency are then adjusted simultaneously in order to match these frequencies as closely as necessary.

[0059] Regardless of the embodiment chosen, the laser source 1 according to the invention can be implemented using silicon-based photonic technologies. According to these technologies, the waveguides and other passive components are fabricated on a silicon substrate (advantageously on a silicon-on-insulator substrate) and the other elements (laser source, photodetector, optical mixer, heater) can be formed on this substrate by deposition or transfer. In particular, the bank B of tunable lasers, the optical filter MR with the tuning device, the photodetector and the waveguides connecting these elements can be formed on the same photonic chip, i.e. on / in the same substrate.

[0060] This photonic chip can be combined with some of the other electronic components of the laser source 1, such as an electronic chip comprising the current sources and even the locking device. In some cases, a single chip may comprise the optical and electronic elements of the source 1. If the locking device is not integrated into one of the chips, it can be implemented by a computing device (microcontroller, DSP signal processing computer, or ASIC) placed on the support and to which the chip is electrically connected.

[0061] It should be noted that since the bank B of tunable lasers and the optical filter MR are integrated on the same chip and on / in the same substrate, they undergo the same temperature variations. This operating temperature affects not only the resonance frequency of the filter MR, but also the emission frequency of the tunable laser, especially if the filter MR is formed by a ring resonator. Advantageously, these elements are configured such that the temperature drift of the emission / resonance frequency is identical or at least very similar. Thus, the temperature drift coefficient (mm / °C) of the tunable laser emission frequency and the temperature drift coefficient of the resonance frequency can be identical within 10%.

[0062] It should be noted that the laser source 1 according to the invention is particularly interesting since it is possible to tune the emission frequency of the tunable lasers of bank B using a particularly simple circuit. This applies in particular to the photonic part of the source, which is augmented by a single photodetector PD, such as a microring resonator, and a single filter MR. This limits the number of additional interconnect pads required on the photonic chip to enable the locking function of the tunable lasers.

[0063] The modulated multispectral optical radiation that forms the "useful" radiation provided by the source can also be used to calibrate and / or lock photonic components downstream of the laser source 1 (switches, modulators, etc.) if this source 1 is used in a more complex system. An example of the use of a laser source according to the invention is shown in [Fig. 8]. In this figure, the laser source 1 has at least one output port (two ports P1, P2 in [Fig. 8]), each generating a multispectral optical radiation RLM1, RLM2. This radiation is therefore spectrally composed of a number of lines separated by a specific spectral interval. At least one of these lines is frequency modulated, as explained in detail in connection with the description of each embodiment of the source 1 (in time or frequency multiplexing). The optical radiation RLM1, RLM2 generated by the ports P1, P2 of the source 1 propagates in a waveguide coupled to this port P1, P2.

[0064] The waveguide itself is coupled to a photonic component comprising a filter with a tunable resonant frequency, in this case two modulators MRA1, MRA2 each implementing a network of microresonators. As is well known in the field of telecommunications, this modulator MRA1, MRA2 allows tuning each spectral line of the multispectral optical radiation (here by a microresonator tuned to these respective lines) in order to transmit information signals S1, S2, S3 in a frequency division multiplexed manner. To enable the system shown in [Figure 8] to operate correctly, it may be advantageous to precisely tune the resonant frequencies of the resonators constituting the modulators MRA1, MRA2 to the emission frequency of the tunable laser of the source 1, i.e. to the spectral lines constituting the multispectral optical radiation RLM1, RLM2.

[0065] To enable this adjustment, the resonators of the networks MRA1, MRA2 are associated with heaters H11, H12, H13 to adjust their resonant frequencies to the spectral lines with which they are associated, thus tuning the optical components. Monitor photodetectors P1, P2 are also provided, coupled to the waveguides for establishing electrical signals representative of the multispectral radiation.

[0066] The regulator R' collects the signals V1, V2 provided by the monitoring photodetectors P1, P2 and generates control signals Cd11, Cd12, Cd13; Cd21, Cd22, Cd23 for controlling the heaters H11, H12, H13 and -H21n, H22, H23 of the modulators MRA1, MRA2 and thus for adjusting the resonant frequencies of the micro-resonators. To determine these control signals, the controller uses the same principles as those shown in Figures 1a-1e and 3c.

[0067] Naturally, the invention is not limited to the described embodiments and variants can be added thereto without departing from the scope of the invention as defined by the claims.

Claims

1. A laser source (1) for emitting at least one multispectral optical radiation (RLM) having a plurality of spectral lines separated by a determined spectral interval, a bank (B) of tunable lasers, the spectral lines of which correspond to frequencies, called "radiation frequencies", of the optical radiation emitted by the tunable lasers (La, Lb, Lc) of said bank (B); - means for adjusting the emission frequency of the tunable laser; an optical filter (MR; MO+MR) having a plurality of resonant frequencies, two successive resonant frequencies being separated by said determined spectral interval, said optical filter (MR; MO+MR) being arranged optically downstream of said bank of tunable lasers (B), said optical filter (MR; MO+MR) being provided with a device (H) for tuning said plurality of resonant frequencies; a photodetector (PD) arranged optically downstream of said optical filter (MR; MO+MR) in order to establish a signal (V) representative of said multispectral optical radiation transmitted by said filter (MR; MO+MR); a modulator (M) associated with the means for adjusting the radiation frequency of the tunable laser and configured to generate a modulation signal (Vd), the modulator (M) being configured to generate a modulation signal (Vd; Vda, Vdb, Vdc) and to modulate the radiation frequency of the optical radiation emitted by at least one tunable laser (La, Lb, Lc) of the bank (B); a locking device (R) connected to the means for adjusting the radiation frequency of the tunable laser and to the device (H) for adjusting the resonant frequencies, the locking device (R) being configured to process the signal representative of the multispectral radiation (V) and to lock the radiation frequency of the tunable laser to the resonant frequency of the filter (MR; MO+MR).

2. 2. The laser source (1) according to claim 1, comprising an optical mixer (MO) associated with the bank (B) of tunable lasers, for combining the optical radiation emitted by the tunable lasers of the bank and for providing the multispectral optical radiation (RLM) to the filter (MR).

3. 3. The laser source (1) according to claim 1 or 2, wherein the optical filter (MR) is a micro-ring resonator.

4. The laser source (1) according to any one of the preceding claims, wherein the device (H) for tuning the multiple resonant frequencies is a heater.

5. Laser source (1) according to any one of the claims 1 to 4, wherein the means for adjusting the emission frequency of the tunable laser are selected from a current source, a heater, a free carrier injection / depletion device.

6. The laser source (1) according to any one of claims 1 to 5, wherein the lasers (La, Lb, Lc) of the bank (B) of tunable lasers are distributed feedback lasers or distributed Bragg reflector lasers.

7. The laser source (1) according to any one of claims 1 to 6, wherein the locking device (R) is configured to control the modulator (M) and to select, by means of the selection signal (Sel), the tunable laser (La, Lb, Lc) to which the modulation signal (Vd) is applied.

8. The laser source (1) according to any one of claims 1 to 6, wherein the modulator (M) generates a plurality of different modulation signals (Vda, Vdb, Vdc), which are applied to the tunable lasers (La, Lb, Lc).

9. The laser source (1) according to any one of claims 1 to 8, wherein the modulator (M) is configured to generate a sinusoidal modulation signal (Vd; Vda, Vdb, Vdc) having a modulation frequency (Fd; Fda, Fdb, Fdc).

10. 10. The laser source (1) according to claim 9, wherein the locking device (R) is configured to establish a measurement representative of the power present in a second harmonic and / or in a principal component and / or a measurement representative of the phase of the principal component of the modulation frequency (Fd; Fda, Fdb, Fdc) of the signal of the multispectral radiation (V).

11. The laser source (1) according to any one of the preceding claims, wherein the bank of tunable lasers (B) and the optical filter (MR) are integrated on / in the same substrate of a photonic chip.

12. 12. The laser source (1) according to claim 11, wherein the temperature drift coefficient of the tunable laser radiation frequency and the temperature drift coefficient of the resonant frequency are identical to within 10%.

13. A method of using a laser source (1) according to any one of claims 1 to 12, said method being implemented by means of said locking device (R), a control step for operating said device (H) to adjust said resonant frequencies of said optical filter (MR); a locking step for locking the optical emission frequency of the selected tunable laser to a resonant frequency of the filter.

14. The method of claim 13, wherein the locking step is operated after the controlling step.

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