FR3136070A1 - Method for spectral positioning of a photonic system - Google Patents

Abstract

The invention relates to a method for spectral positioning of a photonic system, the photonic system implementing the operations of conditioning a control signal (V) leading to producing a digital control signal, the implementation of a first operation (OP1) for processing the digital control signal to produce at least one digital signal, called “first locking signal”, representative of the power of a second harmonic of the modulation frequency (Fd) present in the signal digital control, the implementation of a second operation (OP2) for processing the first locking signal (Vr1) to produce a digital adjustment command, the second operation (OP2) aimed at optimizing the amplitude of the first signal locking (Vr1), and conditioning the digital adjustment command to produce the adjustment command (CLa) and apply it to the adjustment device (H1). Figure to be published with the abstract: Fig. 1 a

Description

Method for spectral positioning of a photonic system FIELD OF INVENTION

A photonic system is disclosed. More precisely, the invention relates to a method for spectral positioning of a photonic device comprising, at least, a laser source and a filter. The filter may be a resonant ring filter, a Mach Zehnder (MZ) interferometer type filter or form, at least in part, a resonant ring modulator or an MZ modulator.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Numerous methods are known in the state of the art aimed at aligning the resonance frequency of an optical filter and the emission frequency of a laser source. Adjustment of the resonance frequency or the emission frequency can be done using a heater placed near the filter or the laser. The heater is controlled so as to “lock” the system, that is to say to match the emission frequency to the resonance frequency.

In the document "Error-free operation of a polarization-insensitive 4λ; x 25 Gbps silicon photonic WDM receiver with closed-loop thermal stabilization of Si microrings," Opt. Express 24, 13204-13209 (2016) a photodetector and a heater are associated with each filter in the system. A regulator measures at regular intervals the current supplied by the photodetector and incrementally adjusts the control of the heater in order to maximize the current produced, and thus seek to align the emission frequency of the laser and the resonance frequency of the filter. Each filter (or groups of filters) in the system includes its own control loop (including a heater and a photodetector) and is therefore individually locked.

The paper “Wavelength Locking and Thermally Stabilizing Microring Resonators Using Dithering Signals,” in Journal of Lightwave Technology, vol. 32, no. 3, pp. 505-512, Feb.1, 2014 also proposes a method for spectral positioning of a similar photonic system, including a plurality of filters equipped with heaters and photodetectors. According to the approach disclosed by this document, the resonance frequencies of the filters are modulated using a slot modulation signal. For each filter, an analog circuit performs the product between the signal supplied by the photodetector and the modulation signal, filters the harmonics to retain only the static part of this product to identify, according to the sign of this static part, if the frequency emission is lower or higher than the resonance frequency. This information is used to adjust the control applied to the heater in order to reduce the difference between these two frequencies.

According to this approach also each filter of the photonic system includes its own regulation loop (including a heater and a photodetector) and is therefore individually locked.

When the photonic system has several channels, that is to say several resonance frequencies and/or several emission frequencies that must be locked together, these methods require as many photodetectors as channels. The presence of a photodetector for each channel to be locked makes these approaches particularly complex to implement, particularly when the number of channels is large, for example greater than 10 or 50.

The document “Simultaneous wavelength locking of microring modulator array with a single monitoring signal,” Opt. Express 25, 16040-16046 (2017) proposes a technique allowing the simultaneous locking of the resonant frequencies of a network of ring modulators on the spectral lines of light radiation in a frequency comb. As is well known in itself, a ring modulator allows both filtering a wavelength chosen from several wavelengths and allows the modulation of this chosen wavelength. In the aforementioned document, locking is carried out using a single photodetector sampling the power of the radiation circulating in an optical bus to which the modulators are coupled and in which a modulated WDM optical signal circulates. An optimization algorithm exploits the RF optical power part of the radiation, that is to say the power generated by the modulators, to establish the heater control signals respectively associated with these modulators, the controls aiming to maximize the power optical RF present in the optical bus.

When initializing the optimization algorithm, each modulator is adjusted consecutively during an exhaustive search until there is a noticeable gradient in the measured RF power. At this point, a gradient method takes over for rapid convergence to the best heating bias. During this process, each modulator is locked. Once the last ring modulator is adjusted, the algorithm uses the endless gradient method on each ring to track the temperature drift.

The approach followed by this document is based on the hypothesis that the RF optical power is maximum when the resonance frequencies of the modulators are well aligned with the emission frequencies forming the spectral lines of the radiation. For this hypothesis to be satisfied, it is particularly essential that the data modulated by each modulator be uncorrelated. This approach also requires that the transmission frequencies be well separated from each other, by at least 50 GHz.

Another approach based on a single photodetector and the modulation of the resonance frequencies of the filters is presented in the document “Streamlined Architecture for Thermal Control and Stabilization of Cascaded DWDM Micro-Ring Filters Bus” presented by Maarten Hattink at the Optical conference Fiber Conference, held from March 6 to 10, 2022. This approach, however, requires calibration of the device which makes the solution particularly cumbersome to operate. The choice of modulation frequencies proposed by this document leads to the introduction of error terms on the servo signal.

OBJECT OF THE INVENTION

An aim of the invention is to propose a method for spectral positioning of a photonic system which does not have the limitations of the state of the art and which is distinct from it. More precisely, an aim of the invention is to propose a spectral positioning system which does not require as many photodetectors as channels to be locked.

BRIEF DESCRIPTION OF THE INVENTION

With a view to achieving this goal, the object of the invention proposes a method for spectral positioning of a photonic system, the photonic system comprising

• at least one laser source producing radiation having at least one emission frequency;
• at least one filter having a resonant frequency;
• at least one waveguide configured to optically couple the radiation produced by the laser source to the filter;
• a photodetector optically coupled to the waveguide to receive filtered radiation, the photodetector producing a control signal;
• at least one adjustment device associated with the laser source or the filter, the adjustment device being controlled to modulate, at a modulation frequency, and adjust the emission frequency of the laser source or the resonance frequency of the filter .

The process implements the following operations:

• conditioning the control signal leading to producing a digital control signal;
• a first processing operation of the digital control signal to produce at least one digital signal, called “first locking signal”, representative of the power of a second harmonic of the modulation frequency present in the digital control signal;
• a second operation of processing the first locking signal to produce a digital adjustment command, the second operation aiming to optimize the amplitude of the first locking signal;
• conditioning the digital adjustment command to produce the adjustment command and apply it to the adjustment device.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the second operation aims to maximize the amplitude of the first locking signal;
• the first locking signal is formed from the ratio between the powers, in the digital control signal, of the signal share in the second harmonic and in the fundamental frequency, the second operation aimed at optimizing the amplitude of the first locking signal for that it is equal to a determined value;
• the modulation applied by the adjustment device is spectrally pure;
• the first operation produces at least a second locking signal representative of the phase of the digital control signal;
• the first operation comprises a Fourier transform of the digital control signal;
• the photonic system comprises a plurality of selectively controllable adjustment devices and respectively associated with a plurality of filters or with a plurality of laser sources, the method comprising a step of selecting only one of the adjustment devices and applying the command thereto adjustment;
• the selection step is repeated to successively select each of the adjustment devices and successively apply the adjustment commands thereto;
• the photonic system comprises a plurality of adjustment devices respectively associated with a plurality of filters or with a plurality of laser sources, the modulation frequencies of the adjustment devices being distinct from each other;
• the modulation frequency of each adjustment device is also distinct from a second harmonic of the modulation frequencies of the other adjustment devices;
• the first operation produces a plurality of locking signals, each locking signal being associated with a second harmonic of the modulation frequency of an adjustment device;
• a plurality of second operations associated with the plurality of latch signals concurrently produces a plurality of digital adjustment commands.

According to another aspect, the object of the invention proposes a photonic system for spectral positioning of a photonic device comprising:

• at least one laser source producing radiation having at least one emission frequency;
• at least one filter having a resonant frequency;
• at least one waveguide configured to optically couple the radiation produced by the laser source to the filter;
• a photodetector optically coupled to the waveguide to receive filtered radiation, the photodetector producing a control signal;
• at least one adjustment device associated with the laser source or the filter, the adjustment device being controlled to modulate, at a modulation frequency, and adjust the emission frequency of the laser source or the resonance frequency of the filter ;
• a locking device configured to implement the following operations:
  1. conditioning the control signal leading to producing a digital control signal;
  2. a first processing operation of the digital control signal to produce at least one digital signal, called “first locking signal”, representative of the power of a second harmonic of the modulation frequency present in the digital control signal;
  3. a second operation of processing the first locking signal to produce a digital adjustment command, the second operation aiming to maximize the amplitude of the first locking signal;
  4. conditioning the digital adjustment command to generate the adjustment command and apply it to the adjustment device.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the photonic system comprises a single photodetector;
• the photonic system comprises a filter comprising a plurality of coupled elementary filters;
• the photonic device is a tunable laser.
• the photonic device is an optical switch.
• the photonic device includes a plurality of lasers emitting a plurality of emission frequencies.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

Figures 1a, 1b, 1c, 1d and 1e represent diagrams of the principles underlying the invention;

There represents a locking device capable of implementing a method according to the invention;

Figures 1g, 1h represent two variants of the treatments implemented by the locking device of the ;

Figures 2a and 2b represent two variants of a first embodiment of the invention, in time multiplexing;

Figures 3a and 3b represent two variants of a second embodiment of the invention, in frequency multiplexing;

Figures 4a, 4b represent a first application of a method according to the invention;

Figures 5a, 5b represent a second application of a method according to the invention;

Figures 6a, 6b illustrate a third application of a process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Principles of the invention

Figures 1a, 1b, 1c, 1d and 1e are diagrams of the principles underlying the invention. On the architecture of the photonic system taken as an example on the , light radiation from a laser source La is guided towards a ring resonator MR, constituting a filter, and having a resonance frequency F0, and towards a photodetector PD placed downstream of the resonator MR. For this purpose, the photonic system comprises a waveguide WG configured to optically couple the radiation produced by the laser source La to the filter MR and to the photodetector PD.

In the example of the , the laser source La is a tunable laser. By “tunable laser” is meant a laser producing light radiation whose frequency (“the emission frequency”) can be adjusted via a device H for adjusting the emission frequency. This adjustment device H can be configured to modify the supply current of the source, its operating temperature, its optical index and/or the concentration of free carriers. A tunable laser can be provided with a plurality of devices for adjusting its emission frequency, for example an adjustable current source and a heater making it possible to modify its operating temperature.

A modulator M, connected to the device for adjusting the laser emission frequency, is configured to modulate the emission frequency Fla of the light 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 emission frequency of the laser, for example 200 terahertz. The amplitude of this modulation is also low, of the order of one gigahertz. For example, 1 mA of modulation amplitude of the power supply current of the laser source La can lead to a variation in the emission frequency FLa of the order of plus or minus 1 GHz. The frequency of the light radiation emitted by the tunable laser La therefore varies at a very low frequency Fd and with a low amplitude A (1GHz) around its fundamental frequency FLa. The laser frequency therefore varies as Fla + A.cos (2pi.Fd.t).

On the , we have represented in the frequency domain the transmission function T of the filter MR whose spectrum TF has a resonance frequency F0, and the signal V supplied by this photodetector PD in the case where the emission frequency FLa of the tunable laser La is not locked to a resonance frequency F0 of the resonator MR, but has an emission frequency Fla greater than this resonance frequency F0. The emission frequency of the tunable laser La being arranged in a relatively linear section of the transmission function of the resonator MR, the control signal V supplied by the photodetector PD presents, in the frequency domain, a main component Fd (corresponding to the modulation frequency) relatively important with respect to its harmonics and in particular with respect to its second harmonic 2*Fd. Furthermore, the phase Phi of the main component Fd of the control signal V is reduced, that is to say that this main component is in phase with the modulation signal supplied by the modulator M.

On the , we represented, similarly to the , the transmission function T of the filter MR whose spectrum TF has a resonance frequency F0 and the control signal V supplied by this photodetector PD in the case where the emission frequency FLa' of the tunable laser La is locked to the frequency resonance F0 of the resonator MR. In this case, the emission frequency FLa' of the tunable laser La is arranged in a relatively non-linear section of the transmission function of the resonator MR. Consequently, the control signal V supplied by the photodetector PD presents, in the frequency domain, a relatively large second harmonic component 2*Fd with respect to the modulation frequency Fd.

Finally, on the , we have represented in the frequency domain the transmission function T of the filter MR whose spectrum TF has a resonance frequency F0, and the control signal V supplied by this photodetector PD in the case where the emission frequency FLa of the laser tunable La is not locked to a resonance frequency F0 of the resonator MR, but has an emission frequency Fla'' lower than this resonance frequency F0. The emission frequency of the tunable laser La being arranged in a relatively linear section of the transmission function of the resonator MR, the control signal V supplied by the photodetector PD presents, in the frequency domain, a principal component Fd relatively large vis- with respect to its harmonics, and in particular with respect to its second harmonic 2*Fd. Furthermore, the phase Phi+Pi of the main component Fd of the control signal V is important, that is to say that this main component is in phase opposition with the modulation signal supplied by the modulator M.

Returning to the description of the principle diagram of the , a locking device R receives the control signal V supplied by the photodetector PD and processes it to develop a command CLa of the tunable laser La aimed at tuning its emission frequency to lock it on the resonance frequency F0 of the resonator MR.

There represents a locking device R which can be implemented within the framework of the present invention. This device firstly comprises a first section for conditioning the control signal V supplied by the photodetector PD, comprising an amplifier Am, a filter BP and a digital converter ADC and supplied a digital control signal Vn. In this section, the control signal V supplied by the photodetector PD is received by the amplifier Am, for example a transimpedance amplifier or a logarithmic amplifier. The amplified control signal V is applied to the BP filter, for example a bandpass filter extending over a range of working frequencies defined by a minimum cutoff frequency and a maximum cutoff frequency. The working frequency range is chosen to incorporate the modulation frequency and its second-order harmonic, that is, the maximum cutoff frequency is chosen to be at least twice the modulation frequency Fd. The signal produced by the filter is sampled by an analog-to-digital converter ADC, to provide a digital control signal Vn, the sampling frequency being chosen higher (for example 2 to 10 times higher) than twice the maximum cutoff frequency of the BP filter, therefore much greater than four times higher than the modulation frequency Fd. As an illustration, when the modulation frequency (or the maximum modulation frequency, when several frequencies are used as will be explained in a later section of this description) is 30kHz, we can choose a sampling frequency of 128kHz , or higher.

The digital control signal Vn is processed by a calculation device MP which may correspond to a microcontroller, a signal processing microprocessor, an FPGA or any other calculation device. The calculation device produces at least one digital signal, called a “locking signal”, intended to form the adjustment command CLa of the frequency adjustment device. In the example shown on the , this adjustment command CLa is formed by a second conditioning section of a digital adjustment command CLan, the second conditioning section here consisting of a digital-to-analog converter DAC.

Of course, the first and second packaging sections could include other elements, in addition or in replacement, than those represented by way of example on the . It is possible in particular for the ADC, DAC converters to be integrated into the MP calculation device and not in the signal conditioning sections.

In very general terms, the calculation device MP implements processing operations exploiting the results presented in Figures 1b, 1c, 1d to determine a command CLa to be applied to the adjustment means H of the laser emission frequency aimed at maximizing the part of the signal present in the second harmonic 2*Fd of the control signal supplied by the photodetector PD. As explained in relation to the description of these figures 1b, 1c, 1d, when this portion of signal present in the second harmonic 2*Fd is maximum, the system is well locked, that is to say that the emission frequency Fla and the resonance frequency F0 correspond.

Even more generally, the determination of the control CLa is carried out by optimization of a function taking as an argument the part of the signal present in the second harmonic 2*Fd of the control signal V. The optimization criterion can correspond to what the function reaches a target value, either lower than a predetermined ceiling value or higher than a predetermined threshold value.

For example, we can thus seek to fix the ratio between the portion of signal in the second harmonic and the portion of the signal present in the fundamental frequency so that it is equal to a target value or to maximize it.

Returning to the description of the , the processing implemented by the MP calculation device includes:

• A first operation OP1 for processing the digital control signal to produce at least one digital signal Vr1, called “first locking signal”, representative of the power of a second harmonic of the modulation frequency present in the digital control signal Vn ;
• A second processing operation OP2 of the first locking signal Vr1 to produce a digital adjustment command CLan, the second processing operation OP2 aiming to optimize an optimization function taking the first locking signal Vr1 as an argument.

For example, the second processing operation OP2 may aim to maximize the amplitude of the first locking signal Vr1.

There represents the operations OP1, OP2 implemented by the calculation device MP according to a first approach. The first operation OP1 for processing the digital control signal Vn thus comprises a first time windowing step W followed by a second transformation step T in the frequency domain. This second step T can thus be a fast Fourier transform, but any other digital transformation in the frequency domain can be suitable. This transformation can provide a spectral distribution of power and phase, although only the power information is exploited in the first approach shown on the . As is well known in itself, the windowing step W aims to determine the portion of the digital control signal Vn to which the frequency transformation is applied and, possibly, to condition this sequence. It can thus be a simple rectangular window, a Hanning or Hamming window. The length of this time window, that is to say the number of time samples retained in the portion of the digital control signal Vn to which the frequency transformation is applied, defines the frequency resolution of the signal produced at 'outcome of the transformation step T, and therefore the possibility of discriminating the power present in a range of frequencies. As an illustration, a length of 1024 samples makes it possible to obtain a resolution of 125 Hz in the digital signal produced by the transformation step T when the sampling frequency is 125kHz. Taking up the description of the first operation OP1 represented on the , the transformation step T is followed by a selection step S during which the sample corresponding to the second harmonic 2Fd of the modulation signal Fd is selected from the signal produced by the transformation step. This selection step S therefore leads to producing a digital signal Vr1 called "first locking signal" in the remainder of this description, this first locking signal being representative of the power of a second harmonic of the modulation frequency present in the digital control signal Vn. When the control signal V incorporates several modulation signals, as will be detailed later in this description, the selection step produces a plurality of samples, each sample corresponding to the second harmonic of one of the modulation signals. The first operation OP1 in this case provides a plurality of first locking signals, each signal being associated with a particular modulation signal.

The first locking signal Vr1 can correspond to the power of a second harmonic of the modulation frequency present in the digital control signal Vn. More generally, this locking signal can correspond to any function taking this second harmonic power as an argument. This function can in particular be the ratio between the part of the signal in the second harmonic and the part of the signal present in the fundamental frequency.

There also represents the second operation OP2 implemented by the calculation device MP according to the first approach. This second operation OP2 processes the first locking signal Vr1 produced during the first operation OP1. During a first comparison step, the value of the sample Vr1(n), produced at time n, of the first locking signal Vr1, is compared with the value of the sample Vr1(n-1) produced at the previous instant n-1, which has therefore been memorized. Depending on the result of this comparison, we increment or decrement by a predetermined increment value Delta, the digital adjustment command Clan produced at the previous time n-1 to develop this digital command at time n. We therefore understand that this second operation OP2 aims to maximize the amplitude of the first locking signal Vr1 (representative of the power of a second harmonic of the modulation frequency in the digital control signal Vn), by changing, at the up or down, the Clan digital adjustment control. When the control signal V incorporates several modulation signals, and therefore as presented in the previous paragraph the first operation produces a plurality of first locking signals Vr1, a plurality of second operations are executed concurrently, each second operation processing a first locking signal Vr1 among the plurality produced by the first operation.

The second comparison step could be made different from the simple comparison between two successive samples presented in this example leading to maximizing the amplitude of the first locking signal Vr1. We will choose this comparison step according to the optimization criterion chosen, that is to say depending on whether we seek to maximize, minimize or make the locking signal equal to a determined value.

There represents the operations OP1, OP2 implemented by the calculation device MP according to a second approach. The first operation OP1 for processing the digital control signal Vn comprises the same first windowing step W and second transformation step T in the frequency domain as that of the first approach. These steps are followed by two concurrent selection steps S1, S2 producing two locking signals Vr1, Vr2. The first selection step S1 producing the first locking signal Vr1 is identical to the selection step of the first approach. During the second selection step S2, the sample corresponding to the phase of the modulation frequency Fd (the fundamental) is selected from the signal produced by the transformation step. This selection step S2 therefore leads to producing a digital signal Vr2 called “second locking signal” in the remainder of this description, this second locking signal being representative of the fundamental phase of the modulation frequency present in the digital signal of Vn control.

There also represents the second operation OP2 implemented by the calculation device MP according to the second approach. This second operation preserves the principle explained in the detailed description of the first approach according to which we seek to maximize the amplitude of the first locking signal Vr1, by changing, upwards or downwards, the digital adjustment control Clan . We clearly find the step of incrementing or decrementing a Delta increment value of the digital adjustment command developed at the previous time n-1 to develop this digital control at time n. According to this second approach, however, the sign Si applied to the increment value Delta is determined by a step of comparison between the second locking signal Vr2 with the phase Phi of the modulation signal. As previously presented, this comparison on the phases makes it possible to determine the relative position of the emission frequency and the resonance frequency of the filter. The Delta increment value is for its part determined via a metric function f which applies to the first locking signal Vr1. For example, this function may consist of determining the relative proportion of the power present in the second harmonic with respect to the total power measured in the spectrum or the power present in the fundamental frequency.

When the control signal V incorporates several modulation signals, the same principles as those exposed in the context of the first approach apply, these principles leading to providing, at the end of the first operation OP1, a plurality of first and second locking signals Vr1, Vr2, and each pair of locking signals Vr1, Vr2 being processed by an instance of the second operation OP2, these instances of the second operation OP2 executing concurrently.

To conclude this section relating to the general principles exploited by the invention, we note that by applying, using the modulator M, a sinusoidal modulation signal, that is to say by applying a spectrally pure modulation to the emission frequency, the appearance of harmonics in the control signal V supplied by the photodetector PD is limited (by comparison with slot modulation for example). The harmonics detected by the locking device R are then representative of the quality of the locking between the transmission frequency and the resonance frequency.

We also note that the same locking principles are applicable to a configuration in which the laser has a fixed emission frequency, and in which the adjustment device H is associated with the filter MR so as to adjust its resonance frequency. This configuration is shown on the . The adjustment device H in this case can be a heater, making it possible to adjust the operating temperature of the filter and therefore its resonance frequency.

Finally, it is not necessary for the modulator M to be a separate element of the photonic system 1: it can for example be integrated into the locking device R, the latter therefore producing the modulation signal and applying it to the locking device. H adjustment used to modulate the emission or resonance frequency. If the same adjustment device H is used both to modulate and adjust the emission frequency of a laser or the resonance of a filter, the locking device R can simply proceed to the sum of the modulation signal and of the control signal, and the signal resulting from this sum applied to this unique adjustment device H.

The principles which have just been presented are of particular interest when a plurality of elements in the photonic system 1 must be locked together. This is particularly the case when the photonic system 1 is composed of a plurality of filters, as is the case in Figures 2a and 3a. This is also the case when the photonic system is composed of a filter composed of a plurality of elementary filters coupled together as shown in Figures 2b and 3b. In all cases, a single photodetector PD is preferably used to adjust and lock together at least one emission frequency of at least one laser source La and at least one resonance frequency of at least one filter MR.

Time multiplex locking

In this first embodiment, the photonic system 1 comprises a plurality of adjustment devices H selectively controllable and respectively associated with a plurality of filters or with a plurality of laser sources. The spectral positioning method of the photonic system 1 comprises a step of selecting only one of the adjustment devices H therefore associated with a single filter on which the locking operations are implemented.

Thus, in the example shown in Figures 2a and 2b, a plurality of filters in the form of micro rings MR1, MR2, MR3 are coupled to a waveguide WG. We find in these examples, a laser source La upstream of a waveguide WG, WG1 and the photodetector PD downstream of this waveguide WG ( ) or a complementary WG2 waveguide ( ). In the example of the , the laser source La emits radiation having a plurality of emission frequencies, and we seek to tune each filter MR1, MR2, MR3 to one of the emission frequencies of the laser source La. In the example of the , the laser source emits light radiation which can have a single emission frequency, and we seek to tune each elementary filter MR1, MR2, MR3 to the emission frequency of the laser source to adjust the overall transfer function of the filter.

A modulator M produces a modulation signal Vd, preferably spectrally pure and therefore having a single modulation frequency Fd. This modulation signal is selectively applied to only one of the heaters H1, H2, H3 using a selection signal Sel.

At the same time, the locking device R receives the control signal provided by the photodetector PD and provides a control signal CL1, CL2, CL3 to the selected heater H1, H2, H3. The locking device R is thus configured to control the modulator M and select, using the selection signal Sel, the heater H1, H2, H3 to which the modulation signal Vd and the control signal CL1, CL2 are applied, CL3. The locking device thus chooses the filter MR1, MR2, MR3 whose resonance frequency is tuned.

The locking device R can be configured to execute, in addition to the first and second operations OP1, OP2, a state machine emitting a selection signal Sel circularly selecting one of the heaters H1, H2, H3 modulated and controlled, for example during successive locking periods whose duration can typically be between a few microseconds to a few milliseconds. During each locking period, the locking device R implements the locking operations explained previously and leading to locking the resonance frequency F0 of the filter or elementary filter selected on an emission frequency of the laser La. resulting from a complete cycle, each filter or elementary filter MR1, MR2, MR3 is well locked on a laser emission frequency La.

Frequency multiplex locking

In this second embodiment, the photonic system 1 also comprises a plurality of adjustment devices respectively associated with a plurality of filters or with a plurality of laser sources. The locking device produces a plurality of locking signals, each locking signal being associated with an adjustment device, and therefore with a filter. The locking device also produces, concurrently, a plurality of control signals. To enable this, the modulation frequencies applied to the adjustment devices H are distinct from each other.

Thus, we find in a second example conforming to this embodiment shown in Figures 3a and 3b, the photonic system 1 of the previous example comprising the plurality of micro-ring filters MR1, MR2, MR3, the heaters H1, H2,H3 associated with these filters. In the case of the , the laser produces radiation having a plurality of emission frequencies. In the case of the , the laser produces radiation having a single emission frequency. In both examples, the single photodetector PD produces the control signal V.

In this second embodiment of Figures 3a and 3b, the locking of the filters or elementary filters MR1, MR2, MR3 to an emission frequency of the laser La are carried out simultaneously, by frequency multiplexing. For this purpose, the system comprises a plurality of modulators M1, M2, M3, each modulator producing a modulation signal, preferably spectrally pure, therefore having a single modulation frequency. This single modulation frequency is distinct from one modulator to another, so that the resonance frequencies of the filters are modulated using distinct modulation frequencies, which makes it possible to identify them in the line spectrum developed by the locking device R, during the first operation OP1. For example, the modulation frequencies can be understood and scaled in the range extending from 1kHz to 30 kHz. In the case of this embodiment, it is therefore not necessary for the modulator M to be controllable via a selection signal. Care will be taken to choose the modulation frequencies so that they are not only distinct from each other, but also distinct from their second harmonics in order to avoid the spectral overlap of a modulation frequency and a second harmonic of another modulation frequency.

The treatments implemented by the locking device R are naturally adapted to this embodiment, but are based on the principles explained previously. In particular, the processing operation makes it possible to identify the lines (fundamental and harmonic) in the spectrum, to prepare a plurality of first locking signals and to associate them respectively with the filters. The locking device R is also configured to implement a plurality of operations, respectively associated with the locking signals, to concurrently produce a plurality of adjustment commands CL1, CL2, CL3 of the frequency adjustment devices H1, H2 ,H3.

First application example

There illustrates an example of application of a photonic device 1 which can benefit from a spectral positioning method according to the invention. In this illustration, the laser source La consists of a bank of lasers La1, La2, La3 each emitting radiation having an emission frequency F1, F2, F3 distinct from the other lasers. These radiations are recombined by a mixer MO producing a plurality of light radiation, each radiation combining the emission frequencies F1, F2, F3. One of these radiations is used by coupling to an MR filter, then to the PD photodetector via a WG waveguide. The MR filter has a plurality of regularly spaced resonance frequencies F0i, F0j, F0k.

Each laser La1, La2, La3 is associated with a respective current source S1, S2, S3 producing its supply current, each source forming a device for adjusting the emission frequency of a laser in the sense of invention. By modulating or adjusting the current produced by a current source, the frequency of the associated laser is modulated or adjusted. As has already been mentioned, the adjustment device can take other forms than the one taken here as an example.

As can be seen on the left side of the , the emission frequencies of the lasers in the bank are not necessarily uniformly frequency separated, and this variable spectral difference from one pair of lasers to another can be problematic. To correct this and obtain the configuration shown on the right side of the , the photonic device can be provided with a locking device R implementing the operations constituting the spectral positioning method of the invention. The locking device R can operate either according to the first embodiment (by time multiplexing) or according to the second embodiment (by frequency multiplexing). During operation of the photonic system thus constituted, the emission frequencies of the lasers are adjusted to the resonance frequencies of the MR filter, and the spectral difference is regularized.

Second application example

There represents another photonic device that can benefit from a spectral positioning method according to the invention. In this figure, this device consists of a tunable laser comprising a gain zone G and a phase zone P. These zones are supplied with current, as is well known per se, in order to cause the emission of radiation light guided by at least one waveguide WG1, WG2 between two reflectors PR defining a cavity, these reflectors being partial and allowing part of this radiation to pass, for example of the order of 90%. In this optical cavity, two elementary filters MR1, MR2 have been positioned, each of these filters being associated with a frequency adjustment device H1, H2, for example a heater. These elementary filters make it possible to precisely select one of the modes of the cavity, by adjusting each of the resonance frequencies of the elementary filters to the chosen mode of the laser emission frequency.

The photonic device can be equipped with the of a locking device R implementing the operations constituting the spectral positioning method of the invention. The locking device R can operate either according to the first embodiment (by time multiplexing) or according to the second embodiment (by frequency multiplexing). During the operation of the photonic system thus constituted, the resonance frequencies of the elementary filters MR1, MR2 are each locked to the chosen emission frequency CM (cavity modes) of the laser defining its emission mode, as shown. on the .

Third application example

There represents yet another photonic device that can benefit from a spectral positioning method according to the invention. In this figure, a photonic switch comprises a laser source, represented here one by an input coupler GCin making it possible to receive light radiation emitted by a laser external to the device. The received light radiation is guided by a waveguide WG towards a photodetector PD. Along the waveguide WG, a plurality of filters MR1, MR2, MR3 are arranged between this waveguide and output couplers GC1out, GC2out, GC3out. Each filter is associated with an adjustment device H1, H2, H3, for example a heater.

In operation, the photonic device makes it possible to address the light radiation received on the input coupler towards a chosen output coupler. To allow this choice, using the adjustment device, the resonance frequency of the filter associated with the chosen output coupler is locked to the emission frequency of the laser radiation. At the same time, the resonant frequencies of the other filters associated with the unselected output couplers are misaligned with the emission frequency of the laser source. This operation is shown on the , on which and by way of example, the light radiation is guided towards one of the output couplers GC3out by locking the emission frequency Fla to the resonance frequency of the filter MR3 associated with the chosen output coupler GC3out. Simultaneously, the two other filters MR1, MR2 are misaligned with the emission frequency Fla of the laser. In this way, the light radiation propagating in the waveguide is coupled to the single output coupler chosen.

The photonic device can be equipped with the of a locking device R implementing the operations constituting the spectral positioning method of the invention. The locking device R can operate either according to the first embodiment (by time multiplexing) or according to the second embodiment (by frequency multiplexing). During operation of the photonic system thus constituted, the resonance frequency of the filter associated with the chosen output coupler is locked to the emission frequency of the laser, and the other filters are on the contrary misaligned.

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

In particular, although we have taken in the examples filters in the form of resonant rings, this in no way constitutes a limitation of the invention which finds its application for any form of filter, for example a Mach Zehnder interferometer (MZ) type filter. This filter can form, at least in part, a modulator, for example a resonant ring modulator or an MZ modulator.

Furthermore, the waveguide can be an optical fiber and the laser can be modulated, for example an external laser whose intensity is modulated.

Claims (18)

Method for spectral positioning of a photonic system, the photonic system (1) comprising

• at least one laser source (La; La1, La2, La3) producing radiation having at least one emission frequency;
• at least one filter (MR1, MR2, MR3) having a resonance frequency;
• at least one waveguide (WG; WG1, WG2) configured to optically couple the radiation produced by the laser source (La; La1, La2, La3) to the filter (MR1, MR2, MR3);
• a photodetector (PD) optically coupled to the waveguide (WG;WG1,WG2) for receiving filtered radiation, the photodetector (PD) producing a control signal (V);
• at least one adjustment device (H1,H2,H3) associated with the laser source (La;La1,La2,La3) or with the filter (MR1,MR2,MR3), the adjustment device (H1,H2,H3 ) being controlled to modulate, at a modulation frequency (Fd), and adjust the emission frequency of the laser source or the resonance frequency of the filter;

the process implementing the following operations:

• conditioning the control signal (V) leading to producing a digital control signal (Vn);
• a first operation (OP1) for processing the digital control signal (Vn) to produce at least one digital signal (Vr1), called “first locking signal”, representative of the power of a second harmonic of the modulation frequency ( Fd) present in the digital control signal (Vn);
• a second operation (OP2) for processing the first locking signal (Vr1) to produce a digital adjustment command (CLan), the second operation (OP2) aimed at optimizing the amplitude of the first locking signal (Vr1);
• conditioning the digital adjustment command (CLan) to produce the adjustment command (CLa; CLa1, CLa2, CLa3) and apply it to the adjustment device (H1, H2, H3).

Method according to the preceding claim in which the second operation (OP2) aims to maximize the amplitude of the first locking signal (Vr1). Method according to one of the preceding claims in which the first locking signal (Vr1) is formed from the ratio between the powers, in the digital control signal (Vn), of the signal share in the second harmonic and in the fundamental frequency , the second operation (OP2) aimed at optimizing the amplitude of the first locking signal so that it is equal to a determined value. Method according to one of the preceding claims in which the modulation applied by the adjustment device (H1, H2, H3) is spectrally pure. Method according to one of the preceding claims in which the first operation (OP1) produces at least a second locking signal (Vr2) representative of the phase of the digital control signal (Vn). Method according to one of the preceding claims in which the first operation (OP1) comprises a Fourier transform of the digital control signal (Vn). Method according to one of the preceding claims in which the photonic system (1) comprises a plurality of adjustment devices (H1, H2, H3) selectively controllable and respectively associated with a plurality of filters (MR1, MR2, MR3) or with a plurality of laser sources (La1,La2,La3), the method comprising a step of selecting only one of the adjustment devices (H1,H2,H3) and applying the adjustment command (CLa1,CLa2,CLa3) to it ). Method according to the preceding claim in which the selection step is repeated to successively select each of the adjustment devices (H1, H2, H3) and successively apply the adjustment commands (CLa1, CLa2, CLa3). Method according to one of claims 1 to 6 in which the photonic system comprises a plurality of adjustment devices (H1, H2, H3) respectively associated with a plurality of filters (MR1, MR2, MR3) or with a plurality of sources laser (La1,La2,La3), the modulation frequencies of the adjustment devices (H1,H2,H3) being distinct from each other. Method according to the preceding claim in which the modulation frequency of each adjustment device (H1, H2, H3) is also distinct from a second harmonic of the modulation frequencies of the other adjustment devices (H1, H2, H3). Method according to one of the two preceding claims in which the first operation (OP1) produces a plurality of locking signals (Vr1, Vr2), each locking signal being associated with a second harmonic of the modulation frequency of a device adjustment (H1,H2,H3). Method according to the preceding claim in which a plurality of second operations (OP2) associated with the plurality of locking signals (Vr1, Vr2) concurrently produces a plurality of digital adjustment commands (CLa1n, CLa2n, CLa3n). Photonic system (1) for spectral positioning of a photonic device comprising:

• at least one laser source (La; La1, La2, La3) producing radiation having at least one emission frequency;
• at least one filter (MR1, MR2, MR3) having a resonance frequency;
• at least one waveguide (WG; WG1, WG2) configured to optically couple the radiation produced by the laser source (La; La1, La2, La3) to the filter (MR1, MR2, MR3);
• a photodetector (PD) optically coupled to the waveguide (WG;WG1,WG2) for receiving filtered radiation, the photodetector (PD) producing a control signal (V);
• at least one adjustment device (H1,H2,H3) associated with the laser source (La;La1,La2,La3) or with the filter (MR1,MR2,MR3), the adjustment device (H1,H2,H3 ) being controlled to modulate, at a modulation frequency (Fd), and adjust the emission frequency of the laser source or the resonance frequency of the filter;
• a locking device (R) configured to implement the following operations:
  1. conditioning the control signal (V) leading to producing a digital control signal (Vn);
  2. a first operation (OP1) for processing the digital control signal (Vn) to produce at least one digital signal (Vr1), called “first locking signal”, representative of the power of a second harmonic of the modulation frequency ( Fd) present in the digital control signal (Vn);
  3. a second operation (OP2) for processing the first locking signal (Vr1) to produce a digital adjustment command (CLan), the second operation (OP2) aiming to maximize the amplitude of the first locking signal (Vr1);
  4. conditioning the digital adjustment command (CLan) to produce the adjustment command (CLa; CLa1, CLa2, CLa3) and apply it to the adjustment device (H1, H2, H3).

Photonic system (1) according to the preceding claim comprising a single photodetector (PD). Photonic system (1) according to one of claims 13 and 14 comprising a filter comprising a plurality of coupled elementary filters. Photonic system (1) according to the preceding claim in which the photonic device is a tunable laser. Photonic system (1) according to one of claims 13 and 14 in which the photonic device is an optical switch. Photonic system (1) according to one of claims 13 and 14 in which the photonic device comprises a plurality of lasers (La1, La2, La3) emitting a plurality of emission frequencies.