FR3014272A1 - DEVICE AND METHOD FOR DEMODULATING A MODULE SIGNAL ON AN OPTICAL CARRIER - Google Patents

Abstract

Device for demodulating (1) a signal (E1) modulated on an optical carrier, said device being characterized in that it comprises an optical loop (2) comprising: - an opto-electronic modulator (5) adapted to modulate in phase or amplitude a signal that it receives as input, - optical frequency transposition means (3) adapted to shift by a frequency step (fAOM) the frequency of an optical signal they receive as input, and an amplifier (4).

Description

The present invention relates to a device for demodulating a modulated signal on an optical carrier.

There are different techniques for demodulating an optical carrier signal, especially at very high frequency, i.e. greater than 100 gigahertz (GHz), which is modulated on an optical carrier. One of these techniques is, for example, direct demodulation. According to this technique, the modulated optical signal is sent to a very fast photodetector. The very high frequency signal is emitted by an antenna attached to the photodetector which can then be demodulated (i.e., multiplied) by a very high frequency reference signal. Although the previously described architecture seems simple, this direct demodulation of the very high frequency signal is difficult to implement. On the one hand, photodiodes and standard mixers are limited in terms of maximum frequency (typically 200 GHz max). In addition, very high frequency reference signal generators are expensive and inefficient. An alternative solution to direct demodulation is RF (Radio Frequency) harmonic demodulation. The diagram is identical to the previous case except that the reference signal is a radiofrequency signal (typically less than 20 GHz). This technique requires the use of specific mixers and can demodulate signals typically up to 500 GHz, the "effective" demodulation signal being a harmonic, ie an integer multiple, of the reference signal. However, the efficiency is lower and the power of the reference radiofrequency signal must be large (typically 1 W). In addition photodiodes for detecting signals beyond 200 GHz are not standard components. Another known technique is the optical harmonic demodulation according to which the very high frequency signal on optical carrier is provided at the input of an optoelectronic modulator RF powered by a powerful RF signal, typically of characteristics such as 1 Watt - 20 GHz. At the output of the opto-electronic modulator, the optical signal is converted into an electrical signal by a photodiode. The bandwidth requirements of the photodiode are much lower than in the previous cases since the signal is already demodulated. Optoelectronic modulators are generally very non-linear: for example a commercial modulator of the MachZehnder type has a sinus response. This technique can demodulate signals up to about 200 GHz. The maximum demodulation frequency can be increased by serializing several opto-electronic modulators whose RF signals are controlled in phase. The required RF injection power then increases and the optical insertion losses of the opto-electronic modulators become a limiting obstacle. These losses can however be compensated by an amplifier. The present invention aims at providing a stable solution, with reduced complexity, for demodulating a modulated signal on an optical carrier and for demodulating signals well above 100 GHz. For this purpose, according to a first aspect, the invention proposes a device for demodulating a signal modulated on an optical carrier of the aforementioned type, characterized in that it comprises an optical loop comprising: an opto-electronic modulator adapted to modulate in phase or amplitude a signal which it receives as input, - optical frequency transposition means adapted to shift by a frequency step the frequency of an optical signal they receive as input, and - an amplifier.

In embodiments, the device for demodulating a signal modulated on an optical carrier according to the invention further comprises one or more of the following characteristics: the device is adapted to demodulate a signal of frequency greater than 100 GHz and modulated on an optical carrier and wherein the opto-electronic modulator is a radio frequency modulator; the optical frequency transposition means comprise an acousto-optical modulator; the device comprises means for measuring the power of a component of the signal extracted from the loop and of frequency fdemod = foot ± kfRF + nfAoM -, where foot is the frequency of the optical carrier, fRF is the frequency of the opto modulator -electronics, fAoM is the frequency step of the optical frequency transposition means and k, n, k ', n' are positive integers; - the frequency of the opto-electronic modulator is in the range [10 GHz - 20 GHz]; the opto-electronic modulator is adapted to modulate signals of first polarization and not to modulate signals of second polarization, said first and second polarizations being crossed; the device comprises a first polarization separator and a second polarization separator, the first polarization separator being adapted to selectively supply: to the optical loop the first polarization portion of the modulated signal, and to the input of the second polarization separator the second polarization portion of the modulated signal, at least a portion of the signal being extracted from the optical loop and further provided at the input of the second polarization separator; the device is adapted to polarize the component of the signal of frequency equal to the optical frequency with respect to another component of the signal modulated by the optical frequency, and to supply the said crossed polarization components at the input of the optical loop; the optical loop further comprises an optical coupler.

According to a second aspect, the present invention proposes a method for demodulating a modulated signal on an optical carrier, comprising the steps of receiving said modulated signal and then of demodulating said signal, characterized in that the demodulation step comprises the provision of the signal received at an optical loop comprising an opto-electronic modulator adapted to modulate in phase or amplitude a signal which it receives as input, optical frequency transposition means adapted to shift by a frequency step the frequency of a optical signal they receive as input, and an amplifier. These features and advantages of the invention will appear on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: FIG. 1 represents a demodulator in one embodiment of FIG. the invention; FIG. 2 represents a demodulator in another embodiment of the invention; FIG. 3 represents a demodulator in another embodiment of the invention; FIG. 4 represents a demodulator in another embodiment of the invention; FIG. 5 represents steps of a demodulation method in one embodiment of the invention. Figures 1 to 4 show a demodulator in different embodiments of the invention. This demodulator is adapted to demodulate a modulated signal on an optical carrier. The demodulator, referenced 1, 10 or 11 according to the figures, comprises an optical loop, referenced 2, 20 or 21 according to the figures, and an optical coupler 6.

The optical coupler 6 is adapted to receive, at the input 7, an optical signal supplied at the input of the demodulator and to mix it with a signal coming from the input optical loop 7b of the coupler, then to inject a part of this mixed signal into the optical loop through the output 8b of the coupler and to output 8 of the coupler, the other part of this mixed signal. The optical loop, referenced 2, 20 or 21 according to the figures, comprises an opto-electronic modulator referenced 5 or 50 according to the figures, an amplifier 4 and an optical frequency transposition means (for example an acoustooptic modulator) referenced 3. In FIG. the optical loops considered with reference to the figures, the optical frequency transposition means 3 follows the amplifier 4, which itself follows the opto-electronic modulator 5 or 50, but these modules are arranged in a different order in others embodiments of the invention. The optical frequency transposition means 3 is adapted to receive an optical signal and to transpose the frequency of the received optical signal using a fAoM transposition frequency.

The amplifier 4 is adapted to receive an optical signal and to amplify the received signal, ideally to compensate for the losses incurred by the optical signal in the optical loop and to compensate for the extraction of a portion of the signal at each turn to the output 8. This amplification makes it possible in particular to preserve the components resulting from the modulation by fRF during several turns in the optical loop.

The amplifier may be a fiber amplifier or a semiconductor amplifier, for example. The opto-electronic modulator 5 or 50 is adapted to receive an optical signal and then to modulate the optical signal received, in amplitude or in phase, by means of a signal. The frequency of the signal for modulating the optical signal has a frequency lower than the carrier of the received optical signal. For example, in this case, this signal is a radiofrequency signal of frequency called fRF and the value of fRF is between 10 and 20 GHz. A portion of the optical signal thus received at the input 7 is injected into the optical loop, while the other portion of the signal is delivered at the output 8.

This optical signal received at the input 7 injected into the optical loop makes one turn of the optical loop and successively undergoes the processing performed by the optoelectronic modulator 5 or 50, the amplifier 4 and the optical frequency transposition means 3 and is received on entry 7b. At the end of each turn, a portion of the processed signal is thus fed back into the optical loop to perform an additional loop of optical loop, while the other portion of the processed signal is output 8.

Referring now more specifically to FIG. 1, the demodulator 1, in one embodiment of the invention, comprises the coupler 6 and the optical loop 2 comprising the optical frequency transposition means 3, which is in FIG. this case an acousto-optic modulator, the amplifier 4 and the opto-electronic modulator 5, which is in this case an electro-optical amplitude modulator of the Mach-Zehnder type. A signal El is supplied to the input of the demodulator 1 and applied to the input 7 of the coupler 6. A view of the spectrum of the signal E1 is shown at the top left of FIG. 1. It comprises a component at the frequency of the optical carrier fopt and further comprises two components corresponding to a very high frequency modulation defining the useful signal, ie bearing the information, at the respective frequencies fopt fThz and fopt fThz, where fThz is the frequency of the useful signal. For example, the value of f -Thz is a very high frequency, equal here to 1 Thz. A signal S1 coming from the output 8 of the coupler 6 is delivered at the output of the demodulator 1. Let us consider the processing carried out on the part of the signal El injected into the optical loop during its first loop run, ie n = 1. step 101 implemented by the electro-optic modulator 5, the modulation at the frequency fRF of the components foot and foot ± - fTHz, THz, due to the non-linearities of the modulator 5, gives components to the frequencies foot ± fRF, fopt ± 2 fRF fopt ± k fRF, and fopt ± fTHz t fRF, fopt tfTHz ± 2 fRF, - - fopt ± fTHz tk fRF with any integer k. In a step 102, the signal in the loop is amplified by the amplifier 4. In a step 103 implemented by the acousto-optic modulator 3, the optical frequencies are shifted by fAOM.

The spectrum of the signal S1 delivered by the demodulator 1 at the end of this first loop turn by S1 thus comprises the following components: fopt fAOM, fopt ± frhz fAOM, and fopt ± fRF + fAOM, fopt ± 2 fRF fAOM, - - -, fopt ± k fRF fAOM, - - - and foot ± fTHz ± fRF + fAOM, foot ± fTHz ± 2 fRF + fAOM, - - - fopt ± fTHz fk fRF + fA0M- In a second round (n = 2) in the optical loop, the signal re-injected after a first turn undergoes a new modulation by fRF and a new fAOM shift. It should be noted that the frequencies from one turn of the signal (any n) are shifted by n fAoM and can not interfere with a frequency of another turn. In a general manner, during a round loop disruption by a signal considered, for a value of n greater than 1, considering an optic frequency of the input signal of this d to f th loop round equal to reference to Figure 5, the following steps are implemented.

In a step 101 implemented by the electro-optical modulator 5, the modulation at the frequency fRF gives rise to a signal comprising components at frequencies f ktk'n-1 = fkn-1 ± k'f RF, with k ' any integer. In a step 102, this signal in the loop is amplified by the amplifier 4.

In a step 103 implemented by the acousto-optical modulator 3, the frequency components of the signal are translated from fAoM to give the components at frequencies f - k + k'n = fk ± k'n-1 + fAOM - A new An optical loop is made by a portion of the optical signal present in the loop, ie n = n + 1 (step 104).

Finally, the loop contains the following frequency components, denoting N the set of any positive natural integers: Components A: f0pt ± kfRF + nfAoM (k EN and ne N) Components B: fopt ± f-rHz ± k'fRF In one embodiment, the demodulator according to the invention comprises a photodiode adapted to measure the power of a signal at a low frequency f - demod (typically less than 10d / min). fAoM), which is the demodulated signal. As a reminder, the beat signal, corresponding to the interference between components, has the power of the product of the powers of these components and has for frequency the difference of the frequencies of the components. The photodiode measures the power of the frequency beat signal fdemod, between two of these respective components A and B delivered at the output 8. Indeed, certain particular values of k, k ', n, n' make it possible to obtain as desired a low - frequency demodulation signal f - demod such that f - demod = fopt ± kfRO - nfAom - (fopti - fi - Hzi - ORF - Fn'fAom). The power of this demodulation signal is measured with the photodiode. The present invention makes it possible to demodulate signals whose frequency f - THz is much greater than 100 GHz, and is for example beyond THz.

Since the acousto-optical modulator shifts the frequencies from one revolution to the next, there is no need to tune the optical frequency fopt and the length of the optical loop: the choice of the optical frequency is thus not imposed. . Since a demodulator according to the invention is not optically resonant, in one embodiment, several multiplexed optical carrier modulated signals are injected into the demodulator and are demodulated in parallel.

Other embodiments of the invention are shown with reference to FIGS. 2 to 4. Identical references in different figures denote similar elements.

With reference to FIG. 2, the demodulator 1 is similar to that of FIG. 1, the optical signal El supplied at the input being replaced by the optical signal E2 with two optical frequencies, foot and fopt + fTHz separated by a very high difference. frequency (eg fTHz = 1 THz). The spectrum of the signal S2 delivered by the demodulator 1 thus comprises the following components: fopt ± kfRF + nfAom (ke N and n N) fopt2 ± ORF + r1'fAom (k'E N and n) with fopt2 = fopt A view of a part of the spectrum of the signal, called S2n, delivered by the demodulator 1 is schematically shown in the bottom left of Figure 2 (to facilitate the reading of the spectrum, the effect of the frequency shifts by the acousto-optic modulator was not shown). The corresponding power involves the crossed products of the frequency components described above and in particular those corresponding to values of k, k ', n, n' such that fdemod = foot - ± kfRF + nfA0M- (fopt + fTHz ± ICRF + 11 The power of the fodder frequency signal f demod is, for example, measured with a photodiode as indicated previously. Referring now to FIG. 3, a demodulator 10 in one embodiment of the invention comprises an optical loop. , in which an electro-optical phase modulator 50 is disposed in place of the opto-electronic modulator 5 of the optical loop 2 of the embodiments previously considered with reference to FIGS. 1 and 2. And with respect to the signal E2, in the E3 signal provided input of the demodulator 10, the two foot and foot fThz frequencies have been crossed in polarization. The phase modulator 50 is adapted to modulate only the signals whose polarization is equal to only one of the polarizations among those of the foot and foot fThz frequencies.

Thanks to the birefringence of the phase modulator 50, only one of the foot and foot fThz components is efficiently modulated in the optical loop 20. A view of a part of the signal spectrum, named S3 ,, is schematically represented in the bottom left of 2 (to facilitate the reading of the spectrum, the effect of frequency shifts by the acousto-optic modulator has not been shown). The demodulator 11 in the embodiment of the invention shown in FIG. 4 has an optical loop 21 similar to the optical loop 20 of Figure 3. It further comprises two polarization beam splitters 12, 13, or "polarization beam splitter" or "PBS".

The PBS 12 is arranged at the input of the demodulator 11, upstream of the coupler 6. This PBS 12 separates the light signal received at the input of the demodulator 11 into distinct beams, such that the frequencies of two distinct beams are different and the polarizations of two distinct beams are orthogonal, and the PBS 12 selectively provides one of the components, for example, the fopt frhi component, at the input of the coupler 6, while the other component, in this case, the fopt component, at the The polarization separator 13 is disposed after the output of the coupler 6, downstream of the output of the demodulator 11. The PBS 13 is adapted to recombine into a single light signal separate light beams which are provided at input and whose respective polarizations are orthogonal. Thus, let us consider a signal E4 similar to the signal E3, supplied at the input of the demodulator 11. The PBS 12 is adapted to deliver, at the input 7 of the coupler, for example the input frequency component fmt + frhz of the signal E4, while the component of the signal E4 of frequency foptz is, it provided, input of PBS 13.

The processing performed by the optical loop 21 is thus performed only on this component of the signal E4 of frequency fopt of the signal E4. A view of a part of the signal spectrum, named S4n, delivered by the coupler 6 at the output 8 of the coupler at the end of an nth turn of the optical loop 21 and supplied at the input of the PBS 13, is schematically represented in FIG. bottom left of Figure 4 (to facilitate the reading of the spectrum, the effect of frequency shifts by the acousto-optic modulator was not shown). It thus comprises components at frequencies, fopt + fi-Hz ± kfRF + nfAom (k = 0,1,2, ... and n = 0,1,2, ...). The PBS 13 receives as input the frequency component f0pt-f-rhz of the signal E4 and the signal S4n, and outputs a signal S5n combining the optical signals received at the input. The power modulation induced by the crossed products involves case of components at frequencies:. This configuration has the advantage of not being sensitive to the residual modulation of the component (fopt-fTHz) within the phase modulator.

The present invention is particularly applicable to the demodulation of signals of very high frequency (i.e. greater than 100 GHz) on optical carrier, in the context of applications such as: the generation of millimeter and terahertz signals with high spectral purity; the generation of RF signals with high spectral purity by non-resonant optical division; the processing of signals at very high frequencies; optical frequency metrology. It will be noted that other means of transposition (in English "frequency shifter") of the optical frequency that the acousto-optical modulator could be used, for example the so-called electro-optical frequency translation equipment. reference to the figures, the signal from the demodulation delivered by the demodulator is output 8 of the coupler 6. In another embodiment, instead of the output 8 of the coupler 6, to take a part of the signal corresponding to the signal demodulated, an additional coupler is inserted in the optical loop, or the second output of a dual output MZM is used, or the output "order 0" of a dual output AOM.

Claims (6)

1. Demodulation device (1; 10) of a signal (E1; E2) modulated on an optical carrier, said device being characterized in that it comprises an optical loop (2) comprising: an opto-electronic modulator (5) adapted to modulate in phase or amplitude a signal which it receives as input, - optical frequency transposition means (3) adapted to shift by a frequency step (fAOM) the frequency of an optical signal which they receive as input, and - an amplifier (4). 2. Demodulation device (1; 10) according to claim 1, adapted to demodulate a signal of frequency greater than 100 GHz and modulated on an optical carrier and wherein the opto-electronic modulator (5) is a radiofrequency modulator. 3. Demodulation device (1; 10) according to claim 1 or 2, wherein the optical frequency transposition means (3) comprise an acoustooptic modulator. 4. Demodulation device (1; 10) according to one of claims 1 to 3, comprising means for measuring the power of a component of the signal extracted from the loop and frequency foemod = foot ± kfRF-FrifAom - (foot ÷ f-rHz ± k'fRF + raAom), where foot is the frequency of the optical carrier, fRF is the frequency of the opto-electronic modulator (5), fAoM is the frequency step of the optical frequency transposition means (3) and k, n, k ', n' are positive integers. 5. Demodulation device (1; 10) according to one of claims 2 to 4, wherein the frequency of the opto-electronic modulator (5) is in the range [10 GHz -20 GHz]. 6. Demodulation device (1; 10) according to one of the preceding claims, wherein the opto-electronic modulator (5) is adapted to modulate signals of first polarization and not to modulate signals of second polarization, said first and second polarizations being crossed ..- Demodulation device (11) according to claim 6, comprising a first polarization separator (12) and a second polarization separator (13), the first polarization separator being adapted to selectively provide: to the optical loop (21) the first polarization portion of the modulated signal, and - at the input of the second polarization separator, the second polarization portion of the modulated signal, at least a portion of the signal being extracted from the optical loop and being further provided at the input of the second polarization splitter. 8. Demodulation device (10) according to claim 6, adapted to cross polarize the component of the frequency signal equal to the optical frequency with respect to another component of the signal modulated by the optical frequency, and to provide said components of crossed polarizations at the entrance of the optical loop. 9. Demodulation device (1; 10) according to one of the preceding claims, wherein the optical loop further comprises an optical coupler (6). 10. A method for demodulating a signal (El; E2) modulated on an optical carrier, comprising the steps of receiving said modulated signal and then demodulating said modulated signal, characterized in that the demodulation step comprises providing the signal received at an optical loop (2) comprising an opto-electronic modulator (5) adapted to modulate in phase or amplitude a signal which it receives as input, optical frequency transposition means (3) adapted to shift a no frequency (fAoM) the frequency of an optical signal they receive as input, and an amplifier (4). 11. The demodulation method according to claim 10, wherein the signal has a frequency greater than 100 GHz and is modulated on an optical carrier and the opto-electronic modulator (5) is a radio frequency modulator. 12. Demodulation method according to claim 10 or 11, wherein the optical frequency transposition means (3) comprise an acoustooptic modulator. 13.- demodulation method according to one of claims 10 to 12, wherein the determination of the demodulated signal comprises a step of measuring the power of a component of the signal extracted from the loop and frequency f demod = foot ± kfRF + nfAOM - (fopt + frHz ± ORF-FrYfA0M), where fopt is the frequency of the optical carrier, fRF is the frequency of the opto-electronic modulator (5), fAoM is the frequency step of the optical frequency transposition means (3 ) and k, n, k ', n' are positive integers. 14. Demodulation method (1; 10) according to one of claims 10 to 13, wherein the frequency of the opto-electronic modulator (5) is in the range [10 GHz -20 GHz]. 15.- demodulation method according to one of claims 10 to 14, wherein the opto-electronic modulator (5) is adapted to modulate first polarization signals and not to modulate second polarization signals, said first and second polarizations being crossed. 16. A demodulation method according to claim 15, wherein a first polarization splitter selectively provides: to the optical loop (21) the first polarization portion of the modulated signal, and - at the input of the second polarization splitter, the portion second polarization of the modulated signal, at least a portion of the signal being extracted from the optical loop and being further provided at the input of the second polarization separator. 17. A demodulation method according to claim 15, comprising a polarization crossing step of the component of the frequency signal equal to the optical frequency with respect to another component of the signal modulated by the optical frequency, said crossed polarization components being provided at the input of the optical loop. 18. The demodulation method according to one of claims 10 to 17, wherein the optical loop further comprises an optical coupler (6).