Photonic system comprising a laser source capable of emitting multispectral optical radiation - Patents.com

Abstract

The invention relates to a photonic system (S) comprising a laser source (1) capable of emitting at least one multispectral optical radiation. The photonic system comprises photonic components (MRA1, MRA2) having a number of tunable optical filters (MR) each having a number of resonant frequencies. The photonic system also comprises photodetectors (P1, P2) arranged downstream of the components for establishing a signal representative of the multispectral radiation (V) and a modulator (M) associated with a bank (B) of tunable lasers. The laser source (1) also comprises a locking device (R) configured to process the signal representative of the multispectral radiation (VI, NT) and to tune the resonant frequencies of the tunable optical filters (MR) of the photonic components (MRA1, MRA2) to the emission frequencies of the optical radiation of the lasers of the bank (B).

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 given spectral interval. Such a source is particularly useful in the field of wavelength division multiplexed communications. [Background technology]

[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 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 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 a radiation emitted by a bank of lasers).

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

[0005] Thus, the spectral range of optical radiation provided by state-of-the-art multispectral laser sources is poorly controlled and prone to drift during operation of the 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, March 6-10, 2022, proposes a solution to compensate for this limitation. The approach adopted in this paper aims to lock the filters to the emission frequency of a multispectral laser source. For this purpose, the resonant frequency of the filter is modulated to generate a feedback signal from the resonant frequency of the filter. However, this approach requires a calibration of the device, which makes the solution particularly laborious to operate. The choice of modulation frequency proposed in this literature would introduce an error term in the feedback signal. This configuration runs the risk of locking several modulators to a single transmission frequency.

[0007] Object of the invention An object of the present invention is to provide at least a partial solution to this problem. More specifically, an object of the present invention is to propose a laser source capable of providing a multispectral optical radiation that is better controlled than the spectral ranges present in the optical radiation generated by prior art laser sources. Another object of the present invention is to provide a laser source for locking the tunable resonant frequency of at least one filter of a photonic component. Summary of the Invention

[0008] To this end, the object of the invention is to propose a photonic system comprising a laser source capable of emitting at least one multispectral light beam having a plurality of spectral lines, the photonic system comprising: a laser bank including a plurality of lasers, the spectral lines of the multispectral optical emission corresponding to the emission frequencies of the lasers in the bank; a modulator associated with the bank of lasers, 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 arranged downstream of the laser bank and comprising a number of tunable optical filters, each having a number of resonant frequencies; a photodetector arranged downstream of the photonic component to establish a signal representative of the multispectral radiation generated by the photonic component; a locking device configured to process the signal representative of the multispectral radiation and to tune the resonant frequency of the tunable optical filter of the photonic component to the radiation frequency of the laser of the bank.

[0009] According to other advantageous, non-limiting characteristics of the present invention, taken alone or according to any technically feasible combination, the following: - the photonic component of the tunable optical filter is a ring microresonator, the tunable optical filters each comprise a device for adjusting their resonant frequency, The resonant frequency adjustment device is a heater, the laser is a distributed feedback laser or a distributed Bragg reflector laser, a locking device configured to control the modulator and to select, by a selection signal, a laser from the bank to which the modulation signal is applied; a modulator generating a plurality of distinct modulation signals, the modulation signals being applied to the lasers in the bank; and a resonant frequency of the tunable optical filters being adjusted, one optical filter at a time. the modulator is configured to generate a sinusoidal modulated signal having a modulation frequency; the locking device is configured to establish a measurement representative of the power present at a second harmonic of the modulation frequency of the signal representative of the multispectral radiation; the locking device is configured to establish a measurement representative of the power present in a main component of the modulation frequency of a signal representative of multispectral radiation, the locking device is configured to establish a measurement representative of a phase of a main component of the modulation frequency of a signal representative of multispectral radiation, the photonic component is a radio frequency modulator, The photonic component is a radio frequency demodulator. [Brief description of the drawings]

[0010] 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.

[0011] [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] FIG. 1 shows a first embodiment of the present invention. [Figure 2b] FIG. 1 shows two variants of the first embodiment of the present invention. [Figure 2c] FIG. 1 shows two variants of the first embodiment of the present invention. [Figure 3a] FIG. 13 illustrates a transition from a natural state to a locked state. [Figure 3b] FIG. 2 shows advantageous features of the filter transmission function of a laser source according to a first aspect of the invention. [Figure 3c] 4 illustrates a calibration of the filters of a laser source according to a first embodiment of the present invention. [Figure 4] 2 shows a further embodiment of the first aspect of the present invention. [Diagram 5] 2 shows a further embodiment of the first aspect of the present invention. [Figure 6] As shown in the embodiment of FIG. 5, the signal provided by the photodetector in the frequency domain is shown. [Figure 7] 1 illustrates a variation applicable to all embodiments of the first aspect of the invention. [Figure 8a] 2 illustrates a second embodiment of the present invention. [Figure 8b] 2 illustrates a second embodiment of the present invention. [Figure 8c] 2 illustrates a second embodiment of the present invention. [Figure 9a] 4 shows a modulator that can be used in a photonic system according to a second aspect of the present invention. [Figure 9b] 4 illustrates the calibration of a modulator according to a second embodiment of the present invention. [Figure 9c] 4 shows a demodulator that can be used in a photonic system according to a second aspect of the present invention. [Figure 9d] 4 illustrates a calibration of a demodulator according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Principles of the invention 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, optical radiation from a tunable laser La is directed to a ring resonator MR forming a filter with a resonant frequency FO, and to a photodetector PD downstream of the resonator MR.

[0013] The term "tunable laser" refers to a laser that produces light whose frequency ("emission frequency") can be adjusted. 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 be provided with multiple means for adjusting its emission frequency, such as adjustable current sources and heaters for changing the operating temperature of the laser.

[0014] The modulator M connected to the laser emission frequency adjustment means is configured to modulate the emission frequency Fia of the optical radiation emitted by the tunable laser La by a modulation frequency Fd. This modulation frequency Fd is relatively low, for example 5 kHz, compared to the laser emission frequency, for example 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 light 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 Fia+A.cos(2πFd.t).

[0015] Fig. 1b shows the frequency domain transfer function T of the filter MR, whose spectrum TF has a resonant frequency FO, and the signal V delivered by the photodetector PD when the radiation frequency FLa of the tunable laser La is not locked to the resonant frequency FO of the resonator MR, but has a radiation frequency Fia higher than this resonant frequency FO. Since the radiation frequency of the tunable laser La is located in a relatively linear part of the transfer function of the resonator MR, the signal V delivered by the photodetector PD has, in the frequency domain, a relatively large main component Fd (corresponding to the modulation frequency) for its harmonics, in particular for its second harmonic 2*Fd. Furthermore, the phase of the main component Fd of the signal V is reduced, i.e. this main component is in phase with the signal V delivered by the photodetector PD.

[0016] Similarly to Fig. 1b, Fig. 1c shows the transfer function T of a filter MR whose spectrum TF has a resonant frequency FO, and the signal V delivered by this photodetector PD when the emission frequency FLa' of a tunable laser La is matched with 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 delivered by the photodetector PD has a relatively large 2*Fd harmonic content in the frequency domain relative to the modulation frequency Fd.

[0017] Finally, FIG. 1d shows the frequency domain transfer function T of a filter MR whose spectrum TF has a resonance frequency FO, and the signal V delivered by a tunable laser La, whose emission frequency FLa is not matched to the resonance frequency F0 of the resonator MR, but has an emission frequency FLa″ lower than this resonance 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 delivered by the photodetector PD has, in the frequency domain, a relatively large principal component Fd for its harmonics, in particular for its second harmonic 2*Fd. Furthermore, the phase of the principal component Fd of the signal V is important, i.e. it is in antiphase with the signal V delivered by the photodetector.

[0018] Figure 1e summarizes the results of Figures 1b, 1c and 1d and shows, in the upper graph, the evolution of the power present in the main 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 changed and the resonant frequency of the filter remains fixed (or vice versa), and also shows, in the lower graph, the evolution of the phase of the main component Fd of the signal V provided by the photodetector.

[0019] 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 commands CLa for tuning the tunable laser La to the resonant frequency FO of the resonator MR. The process implemented by the locking device R uses the results presented in FIGS. 1b, 1c, 1d, 1e to determine the control applied to the laser emission frequency adjustment means, 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 photodetector PD. 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 FIG. 1e, so that one and / or the other can be utilized as required. The phase can also be utilized as indicated above.

[0020] As an example, the locking device R can apply a series of commands CLa to the adjustment device, which increase from a minimum to a maximum value, so that the laser emission frequency La is adjusted in a series of steps from the minimum emission frequency to the maximum emission frequency. In 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 and / or in the main component. At the end of these steps, the step and the associated control CLa that resulted in a signal maximum in the second harmonic (or a signal minimum in the main component) are identified, this associated command CLa being the command that best matches the laser emission frequency La and the resonant frequency of the filter MR. This command is then applied to the device for adjusting the laser emission frequency La in order to lock the system. Other approaches are also possible, for example by using the phase information of the main frequency Fd of the signal provided by the photodetector PD in a control loop that increments or decrements the Cia command provided to the frequency adjustment device by a given step. As already mentioned, this information indicates whether the laser emission frequency is below (high phase) or above (0 or low phase) the laser resonance frequency.

[0021] 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 located at a certain distance from each other. Generally speaking, the control CLa is determined by optimizing a function that takes into account the proportion of the signal present in the main component and / or the second harmonic 2*Fd of the control signal V. The optimization criterion may be that the function reaches a target value, falls below a predefined upper limit, or exceeds a predefined threshold. It may also be provided to use the phase of the main component and / or the second harmonic 2*Fd of the control signal V.

[0022] For example, the ratio of the percentage of the signal present in the second harmonic to the percentage of the signal present in the principal component can be set to be equal to or maximized to a target value.

[0023] For accuracy, the system is said to be "locked" when a selected optimization criterion is met. This may correspond to the situation where the radiation frequency F and the resonant frequency F coincide, or where these frequencies are offset from each other by a specified distance.

[0024] 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 are a good indicator of the quality of the lock between the transmission frequency and the resonant frequency.

[0025] It should also be noted that the same locking principles are applicable to configurations in which a tuning device is associated with the filter for tuning the resonant frequency of the filter. First aspect of the present invention

[0026] According to a first aspect, the present description utilizes the principle just presented to propose 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, for example, by an interval of 100 GHz or 50 GHz.

[0027] 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 in bank B is associated with a current source Sa, Sb, Sc, which makes it possible to power the laser and generate optical radiation. 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" in the sense of the present specification. Bank B can contain any number of tunable lasers, typically between 4 and 100 lasers. Of course, the invention is not limited to a bank of distributed feedback lasers, but applies to any tunable laser. It may be a DBR (Distributed Bragg Reflector Laser).

[0028] The Bank B lasers, as mentioned above, are designed to emit light with staggered emission frequencies, typically within the 100 GHz spectral range for WDM applications. However, as seen on the left side of FIG. 3a (showing the transfer function of frequencies Fla, Flb, Flc and filter FT), the variability of the Bank B fabrication process means that the spectral interval separating the emission frequencies FLa, FLb, FLc of the Bank B lasers cannot be fully controlled. Thus, the spectral interval separating two consecutive lasers (ordered by emission frequency) is variable, and this variation in the absence of a locking mechanism can be roughly + / - 20% or more. It is also noted that the operating temperature of Bank B can affect and drift the laser emission frequencies.

[0029] Returning to the description of the embodiment of Fig. 2a, the tunable lasers La, Lb, Lc of the bank are coupled via waveguides to an optical mixer MO, which generates at least one multispectral light beam RLM, one spectral line of which corresponds to the radiation frequency of the light beam emitted by the tunable laser from bank B. The mixer MO is capable of providing a plurality of mutually identical multispectral light emissions.

[0030] 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 used by other elements, such as optical modulators, optical routers, etc., when source 1 forms, for example, a component of a communication system. According to one aspect of the invention, at least a part of the "useful" multispectral radiation is sampled to enable the radiation frequencies of the tunable lasers in bank B to be matched with a frequency comb having a specific spectral interval.

[0031] This sampled portion of the multispectral optical radiation is guided 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 Fig. 3a. In other words, two successive resonance frequencies FOi, FOj, FOk of the filter MR are separated by a predefined 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 the two resonance frequencies FOi, FOj, FOk to be precisely controlled, for example to 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 and must preferentially exhibit a slope greater than 6 dB / GHz when shifting more than 1 gigahertz from one of its resonance frequencies. Such a characteristic is illustrated in Fig. 3b.

[0032] The optical filter MR may include a device for tuning its multiple resonant frequencies FOi, FOj, FOk, which may thus be a heater H for frequency shifting the eigenfrequency comb, as will be described in more detail in a later section of this specification, if the filter MR is implemented by a ring resonator.

[0033] The source 1 shown in FIG. 2a also includes a photodetector PD arranged downstream of the optical filter MR for establishing a signal V representative of the multispectral light emission MLR.

[0034] It also includes a modulator M associated with the tunable laser bank B, which is 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 laser with a frequency modulation signal Fd. The modulation signal Vd, whose general form is of the cos(2.Pi*Fd*t) type, has a relatively low modulation frequency Fd of approximately a few kHz to a few MHz, typically 5 kHz, a few tens of kHz or 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 approximately 1 GHz or more. The selection signal Sel in this embodiment selects the tunable laser from the bank B to which the modulation frequency Fd is applied.

[0035] 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.

[0036] Finally, the laser source 1 shown in FIG. 2a includes a locking device R for a tunable laser from the laser bank B. This locking device R is connected to the laser bank B via controls 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 set 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 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 in this aspect of the invention to implement a control loop whose purpose is to tune the selected tunable laser emission frequency Fla, Flb, Flc and lock it to the filter resonance frequencies Foi, Foj, Fok. This control loop implements the principle described in relation to FIGS. 1a-1e. 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. On this basis, the locking device R is able to generate a control 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.

[0037] In the embodiment shown in Fig. 2a, the current sources Sa, Sb, Sc are adjustable and adjustment of the emission frequency of the laser is achieved 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.

[0038] The laser source 1 of the embodiment shown in Fig. 2a is implemented by successively selecting a tunable laser to be locked from the tunable lasers of the bank B. The locking device R may therefore comprise, for example, a state machine that emits a selection signal Sel that cyclically selects one of the tunable lasers from the 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 FO 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 the bank B shown on the right side of Fig. 3a, the multispectral light emission RLM follows the spectral template imposed by the optical filter MR. The multispectral light emission RLM presents a number of spectral lines FLa, FLb, fLC separated from each other by a spectral interval DF determined by the optical filter. This property can be imparted to the emission frequencies of the tunable lasers in bank B because the spectral spacing DF between two adjacent eigenfrequencies FO of the filter is controlled, typically to within 5% or better.

[0039] By repeating the control cycle one after the other in time division multiplexing, the locked state of the bank of tunable lasers on the filter can be maintained over time and any drift, particularly that caused by variations in laser temperature, can be compensated for.

[0040] Optionally, the locking device R can implement a further control loop to calibrate the resonant frequency comb of the optical filter MR and align it to an absolute target resonant frequency. For this purpose, as shown in FIG. 2b, optical radiation from a standard laser Le is fed into a port complementary to the filter MR. This standard laser Le has a modulated emission frequency, as do the other tunable lasers La, Lb, Lc in the 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 generated by the photodetector PD a frequency component corresponding to the modulation frequency of the tunable standard laser, and the locking device R can control any other device to adjust the heater H or the multiple resonant frequencies FOi, FOj, FOk of the filter in order to frequency shift this comb of natural frequencies and realign it with the target frequency delivered by the standard laser. This operation is illustrated in FIG. 3. This calibration control loop of the filter MR and the tunable laser locking control loop do not necessarily have to be separate 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 beam RLM presenting multiple determined spectral lines, i.e. each of whose spectral lines is precisely positioned in the frequency domain.

[0042] Fig. 2c shows another variant of this first aspect of the invention. In this variant, the optical filter MR comprises a device H, for example a heater, for tuning its multiple resonant frequencies FOi, FOj, FOk. A signal Vd provided by a modulator M is applied in this variant to the tuning device H of the optical filter MR. The application of this signal thus results in a simultaneous modulation at a modulation frequency Fd of the multiple resonant frequencies FOi, FOj, FOk of the optical filter MR. This variant offers the advantage of a simpler implementation. The laser source 1 shown in Fig. 2c is implemented by sequentially selecting the tunable laser to be locked from the tunable lasers in the bank B, as described above.

[0043] FIG. 4 shows another embodiment of the laser source 1. In this embodiment, each tunable laser La, Lb, Lc in the bank B is equipped 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 embodiment of the laser source 1, the commands CLa', CLb', CLc' generated by the locking device R connect for control purposes to the heaters Ha, Hb, Hc, respectively. Thus, in this embodiment, the emission frequency of the tunable lasers in 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 in the second embodiment are identical to those in the first embodiment, and for the sake of brevity, their description will not be repeated. Of course, it is also possible to combine these two embodiments and adjust the emission frequency of the tunable lasers in the bank B by controlling the average supply current of the current source associated with the selected tunable laser and at the same time controlling the temperature of this laser by the associated heater.

[0044] In another variation of the embodiment of the laser source 1 shown in Figures 2a and 4, each laser La, Lb, Lc in the bank B in turn includes a carrier injection / depletion device, such as a waveguide, located 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.

[0045] As already mentioned, each tunable laser in 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).

[0046] In the frequency division multiplexing mode shown in FIG. 5, 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. These signals are simultaneously, advantageously permanently, applied to the lasers with 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. For this embodiment, the modulator M does not need to be controllable via a selection signal. The remainder of the laser source 1 in this embodiment is identical to the first embodiment in FIG. 2a and therefore, for the sake of brevity, its description is omitted here. In particular, similar to the first embodiment, radiation provided by a standard 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.

[0047] The process implemented by the locking device R is naturally adapted to this embodiment, but is based on the same principles as described in Figures 1a to 1e. 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. As 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.

[0048] In the example of Fig. 5, the control signals CLa, CLb, CLc generated by the locking device R are respectively connected to the current sources Sa, Sb, Sc of the bank of tunable lasers. However, like 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 respectively associated with these tunable lasers, or any other means of adjusting the emission frequencies of these lasers.

[0049] Figure 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 a multiplexer implemented by an arrayed waveguide grating or a ladder network. This element has a transfer function identical to that shown in figure 3a.

[0050] Whatever embodiment is chosen, the laser source 1 according to the invention can be implemented by silicon-based photonic technologies. According to these technologies, the waveguides and other passive components are manufactured 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. This photonic chip can be combined with an electronic chip comprising some of the other electronic components of the laser source 1, for example 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. The locking device, if not integrated in one of the chips, can be implemented by a computing device (microcontroller, signal processing computer DSP or ASIC) placed on a support and to which one or more chips are electrically connected.

[0051] It is noted that the laser source 1 according to the invention is particularly useful since the emission frequency of the tunable lasers in the bank B can be tuned via a particularly simple circuit. This applies in particular to the photonic part of the source 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. Second Aspect of the Invention

[0052] The multispectral optical radiation modulated by the modulation signal Vd forming the "useful" radiation provided by the source may also be used for calibration and / or locking of photonic components (routers, modulators, demodulators, etc.) located downstream of the laser source 1 if this source 1 is used in a more complex photonic system S. According to this other aspect of the invention, the laser source may be used to provide multispectral light with poorly controlled spectral ranges.

[0053] Examples of the use of a laser source according to this aspect of the invention are shown in Fig. 8a (frequency division multiplexing) and Fig. 8b (time division multiplexing). In these figures, a laser source 1 has at least one output port (two ports P1, P2 in Fig. 8a, 8b), each generating a multispectral optical radiation RLM. This radiation is therefore spectrally composed of a number of lines separated by a spectral interval. At least one of these lines is frequency modulated, as explained in detail in connection with the description of the respective embodiment of the source 1 (in time division or frequency division multiplexing).

[0054] The source 1 according to this aspect of the invention comprises a bank B of lasers La, Lb, Lc. Each laser in the bank B is associated with a current source Sa, Sb, Sc forming means for adjusting its emission frequency and connected to a modulator M. In this way, it is possible to modulate at least one emission frequency of the multispectral radiation RLM emitted by the source 1, similar to the first aspect of the invention. The tunable lasers La, Lb, Lc of the bank B are coupled to an optical mixer MO via a waveguide network internal to the source 1. The multispectral optical radiation RLM generated by the optical mixer MO is guided by the internal waveguide network to ports P1, P2 on the source 1. It propagates in external waveguides coupled to this port P1, P2.

[0055] The external waveguide itself is coupled to a photonic component MRA1, MRA2 arranged optically downstream of the source 1 and the laser bank B, the photonic component comprising a number of optical filters each having a tunable resonant frequency. In such a photonic component, the resonant frequency of each filter is individually tunable. The photonic component MRA1, MRA2 adjusts the multispectral optical radiation received at its input port according to the function performed by this component.

[0056] Also, in the example shown in Figures 8a, 8b, two radio frequency modulators MRA1, MRA2, each implementing an array of independent micro-resonators, are optically connected to the two ports P1, P2, respectively, of the laser source 1. As is well known in the field of telecommunications, radio frequency modulators are used to tune each of the spectral lines of a multispectral optical radiation (here by means of micro-resonators tuned to these lines) in order to transmit information signals in a frequency division multiplexed manner.

[0057] An example of such a radio frequency modulator is shown in more detail in Fig. 9a. An array of micro-ring resonators MR1-MR3 is arranged between a first waveguide WG1 and a second waveguide WG2. The first waveguide WG1 passes between an input port Pin, at which the multispectral optical radiation to be modulated is supplied, and a so-called "transparent" output port Pt ("through port"), at which the modulated optical radiation is generated. The second waveguide WG2 connects to an output loss port ("drop port") Pd. Each micro-resonator operates to modulate the transmission frequency of the source 1 via a radio frequency signal RF1-RF3. Each micro-ring resonator MR1-MR3 is associated with a means H1, H2, H3, e.g. a heater, for adjusting its resonant frequency FO1, FO2, FO3. In such a radio frequency modulator MRA, the aim is to position the resonant frequencies FO1, FO2, FO3 of each microresonator MR1-MR3 relative to the spectral lines FLa, FLb, FLc of the multispectral radiation generated by the source 1 according to the configuration shown in Fig. 1d, i.e. in the linear parts of the transfer functions TF1-TF3 of the microresonators MR1-MR3. Naturally, the aim is to assign a separate radiation to each microresonator. As shown in Fig. 9b, the adjustment means H1-H3 associated with the microresonators MR1-MR3 are used to adjust the original resonant frequencies FO1, FO2, FO3 to the optimized frequencies FO1', FO2', FO3'.

[0058] Returning to the description of Figures 8a, 8b, the microresonators MR of the radio frequency modulator microresonator MRA1, MRA2 array are each associated with an adjusting device H for independently adjusting their resonant frequencies, and in particular for locking them to the spectral line with which they are associated.

[0059] It should be noted that this combination can be done with great flexibility, for example by associating the microresonators and lasers whose resonant and emission frequencies are closest to each other. Prior art solutions that apply modulation signals to the microresonators do not allow such flexibility, since it is not possible to determine which laser the microresonator is associated with.

[0060] To establish an electrical signal representative of the multispectral radiation RLM1, RLM2, a monitor photodetector PD1, PD2 is also provided optically downstream of the photonic components MRA1, MRA2, here at the transmit output ports of the photonic components MRA1, MRA2. For this purpose, a small portion of this multispectral radiation RLM1, RLM2 may be sampled and coupled to the monitor photodetector PD1, PD2, as shown in Figures 8a, 8b.

[0061] In the variant shown in FIG. 8c, it may be possible to couple the monitoring photodetectors PD1, PD2 directly to the loss ports Pt of the photonic components MRA1, MRA2 in order to avoid picking up part of the multispectral radiation RLM1, RLM2 propagating from the transmit ports Pt of the photonic components MRA1, MRA2.

[0062] The locking device R collects the signals V1, V2 provided by the monitoring photodetectors PD1, PD2 and generates control signals Cd11, Cd12, Cd13. Cd21, Cd22, Cd23 allow the adjustment devices H of the optical filters MR constituting the photonic components MRA1, MRA2 to be controlled independently and thus to adjust their resonant frequencies to a given emission frequency. The locking device R uses the same principle as that shown in Fig. 1a-1e to determine these control signals. If the photonic components MRA1, MRA2 are modulators, as already mentioned, it is preferable to place the emission frequency of the multispectral radiation RLM in the linear part of the transfer function of the filter formed by the microresonator. In this way, the ratio between the proportion of the signal present in the second harmonic and the proportion of the signal present in the main component can be set to be equal to a target value or to be maximized.

[0063] This locking device R can be used for time division multiplexing (FIG. 8b) and is arranged to control a modulator M to select, by means of a selection signal Sel, a tunable laser La, Lb, Lc to which a modulation signal Vd is applied.

[0064] The locking device R can also be used for frequency multiplexing (FIG. 8a), where the modulator M generates a number of mutually distinct modulation signals Vda, Vdb, Vdc, which are simultaneously applied to the tunable lasers La, Lb, Lc. This frequency mode is particularly interesting to exploit, since each filter of the photonic component can be associated and assigned to a particular radiation frequency of the multispectral radiation received at the input port.

[0065] Fig. 9c shows a demodulator, which forms another type of optical component MR1, MRA2 that can be calibrated / locked according to the principles of this second aspect of the invention. In such a demodulator, each microresonator in the array of microring resonators MR1-MR3 is arranged between a first waveguide WG1 and a second waveguide WG2. The first waveguide WG1 has an input port Pin, to which the multispectral optical radiation to be demodulated can be supplied, and a transmission port Pt. The loss ports of the microresonators MR1, MR2, MR3 connect to demodulation photodetectors Pda, pDB, pDC. A photodetector PD1 is arranged on the first waveguide WG1 at the transmission port Pt. Each microresonator MR1-MR3 filters and extracts radio frequency signals RF1, RF2, RF3 from the multispectral optical beam. These radio frequency signals are carried by the respective emission frequencies (called "carrier frequencies" in this context) FLa, FLb, FLc of the multispectral optical radiation. Associated with each micro-resonator MR1-MR3 are means H1-H3, such as heaters, for adjusting the resonant frequencies FO1-FO3.

[0066] Figure 9d shows the relative placement of the resonant frequencies FO1, FO2, FO3 of the filters MR1, MR2, MR3 and the carrier frequencies FLa, FLb, FLc before tuning, and on the right side of Figure 9d shows the relative positions of the tuned resonant frequencies FO1', FO2', FO3' and the carrier frequencies FLa, FLb, FLc after tuning, when deliberately selected to lock the microresonators MR1, MR2, MR3 to the nearest optical carriers FLa, FLb, FLc.

[0067] Whatever the nature of the photonic device(s) MRA1, MRA2 present in the photonic system, it may be advantageous to carry out the system implementation in a frequency multiplexed operation, in which during the start-up phase, a separate modulation frequency is applied to each emission / carrier frequency Fla, Flb, FLc of the multispectral optical radiation RLM. For simplicity of implementation, it may be decided that the control signals are generated one after the other, allowing independent control of the adjustment devices H of each microresonator MR1, MR2, MR3, in order to adjust the resonant frequency of one optical filter at a time. As already mentioned, this method of implementation allows to control the association of the microresonators with the carrier frequency, while prior art solutions may result in the association of several microresonators with the same carrier frequency. Once this start-up phase is completed, it may be decided to generate the control signals simultaneously.

[0068] Of course, the invention is not limited to the described embodiments, and alternative embodiments can be added without departing from the scope of the invention as defined by the claims.

[0069] An optical system according to the invention may utilize either or both of the above aspects, in particular an optical system combining both aspects of the invention may be provided, i.e. an optical system characterized by a first locking device for tuning the emission frequency of the laser in the bank to the resonant frequency of the standard filter and a second locking device for tuning the resonant frequency of the optical filter of the photonic component to the emission frequency of the laser in the bank.

Claims

1. A photonic system (S) comprising a laser source (1) capable of emitting at least one multispectral optical radiation (RLM) having a plurality of spectral lines, said photonic system comprising: a bank (B) of lasers comprising a plurality of lasers (La, Lb, Lc), the spectral lines (FLa, FLb, FLc) of said multispectral optical radiation (RLM) corresponding to the emission frequencies of the lasers (La, Lb, Lc) of said bank (B); a modulator (M) associated with said bank (B) of lasers, 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 said bank (B); a photonic component (MRA1, MRA2) arranged downstream of said bank of lasers (B) and comprising a number of tunable optical filters (MR), each having a number of resonant frequencies; - photodetectors (PD1, PD2) arranged downstream of said photonic components (MRA1, MRA2) and establishing signals (V1, V2) representative of the multispectral radiation generated by said photonic components (RLM1, RLM2); a locking device (R) configured to process the signals representative of the multispectral radiation (V1, V2) and to tune the resonant frequency of the tunable optical filters (MR) of the photonic components (MRA1, MRA2) to the radiation frequencies of the lasers (La, Lb, Lc) in the bank (B),

2. The photonic system (S) of claim 1, wherein the tunable optical filter (MR) of the photonic components (MRA1, MRA2) is a ring microresonator.

3. 3. Photonic system (S) according to claim 1 or 2, wherein said tunable optical filters (MR) each comprise a device (H) for adjusting their resonant frequency.

4. The photonic system (S) according to any one of claims 1 to 3, wherein the resonant frequency adjustment device (H) is a heater.

5. Photonic system (S) according to any one of claims 1 to 4, wherein said lasers (La, Lb, Lc) are distributed feedback lasers or distributed Bragg reflector lasers.

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

8. Photonic system (S) according to any one of claims 1 to 7, wherein tuning 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 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 (Fda, Fdb, Fdc).

10. 10. The photonic system (S) of any one of claims 1 to 9, wherein the locking device (R) is configured to establish a measurement representative of the power present at the second harmonic of the modulation frequency (Fd; Fda, Fdb, Fdc) of the signal representative of the multispectral radiation (V1, V2).

11. The photonic system (S) according to any one of claims 1 to 10, wherein the locking device (R) is configured to establish measurements representative of the power present in a main component of the modulation frequencies (Fd; Fda, Fdb, Fdc) of the signal representative of the multispectral radiation (V1, V2).

12. The photonic system (S) according to any one of claims 1 to 11, wherein the locking device (R) is configured to establish a measurement representative of the phase of the main components of the modulation frequencies (Fd; Fda, Fdb, Fdc) of the signal representative of the multispectral radiation (V1, V2).

13. Photonic system (S) according to any one of the preceding claims, wherein said photonic components (MRA1, MRA2) are radio frequency modulators.

14. Photonic system (S) according to any one of the preceding claims, wherein said photonic components (MRA1, MRA2) are radio frequency demodulators.

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