FR2642923A1 - Device for optical signal processing, especially for microwave signals, with evanescent-wave optical couplers, and use of such a device - Google Patents

Abstract

The invention relates to an optical signal processing system, especially for subtracting or multiplying signals, using incoherent guided optical waves. The aim of the invention is to provide an optical signal processing device capable of operating in a simple, reliable manner, with a good signal-to-noise ratio. The device of the invention includes a set of two optical couplers 20, 17, the said couplers 20, 17 being mounted in opposition and connected to one another by their two parallel paths 11, 12, and in that at least one of the said optical couplers 20 is an evanescent-wave optical coupler.

Description

OPTICAL SIGNAL PROCESSING DEVICE, PARTICULARLY FOR
BYPERFREQUENCY SIGNALS, WITH OPTICAL WAVE COUPLERS
EVANESCENT, AND USE OF SUCH A DEVICE.

 The field of the invention is optical signal processing.

 More specifically, the invention relates to an optical signal processing system, in particular for the subtraction or multiplication of signals, using optical waves which are incoherent between them. The applications relate in particular to the processing of microwave signals, but can be transposed to other uses.

 In most signal processing operations, it is necessary to perform signal subtraction. However, performing a subtraction operation on an optical signal requires a particular approach, the intensity of the light can only be positive.

Three methods are known to overcome this drawback, as described in the article: "Fiber Optic
Delay line Signal Processors ", KP Jackson et al., In" Optical Signal Processing ", JL Horner, Academic Press, Inc .:
A first method is to produce optical systems using only additions. This solution, which is suitable for a certain number of applications (filtering, etc.), is clearly not applicable in each case of signal processing.

 The second method consists in using two opto-electronic detectors and in performing the subtraction on the electrical signals. In addition to the complexity, the disadvantage is that once the subtraction has been carried out, the signal is no longer available in optical but electrical form, with the problems of loss appearing at the level of the opto-electronic transducers, and possibly of the electro-transducers. resulting signal conversion optics in optical form.

 The third known solution consists in using the properties of coherence of light. Although this approach is attractive, it remains complex because it requires that the optical paths are stable with an accuracy less than the wavelength of the light used. In the microwave domain, it would indeed be necessary to produce devices having an optical length of approximately 100 meters, with an accuracy of the order of a micron.

 A method based on this operating principle and allowing self-compensation of the optical paths has been proposed, as described in French patent document 8704725 concerning a "coherent processing device using an optical delay line". The recommended method nevertheless has the disadvantages of being complex and of producing a poor signal-to-noise ratio.

 The objective of the invention is to provide an optical signal processing device capable of operating in a simple, reliable manner, with a good signal to noise ratio.

The device of the invention also makes it possible to efficiently perform not only subtraction operations on optical signals, for example to perform filtering processing, but also signal multiplication operations, which can be used for interrelation processing or still spectral analysis of signals.

 More precisely, in the case of spectral analysis of a signal, we know that the intercorrelation of a signal s (t) with a sinusoidal signal of frequency x is equal to the value of the Fourier transform of the signal to this frequency x.

This well known property is widely used in spectrum analyzers. The disadvantage of this method is that it requires a frequency synthesizer for each of the frequencies analyzed or with variable frequency. The device of the invention allows in particular to overcome this drawback.

 The device of the invention has the advantage of being an element that can be integrated into more complex signal routing and processing systems.

These objectives, as well as others which will appear subsequently, are achieved by means of an optical signal processing device characterized in that it comprises a set of two optical couplers of the type comprising on the one hand at at least one input and / or output channel, and on the other hand two parallel output and / or input channels, the couplers being mounted in opposition and connected to each other by their two parallel input channels and / or exit,
in that the input channel of the first coupler is supplied by an optical source, the output channel of the second coupler providing an optical signal representative of the signal processing carried out by the device,
and in that at least one of said optical couplers is an evanescent wave optical coupler.

 The use of an optical coupler with evanescent waves has the advantage of allowing the modulation of the optical signal passing through the device in the microwave domain, and in particular up to frequencies of the order of 15 GHz. In addition, the properties of optical couplers with evanescent waves apply surprisingly well for carrying out signal processing operations as diverse as the subtraction of two signals or even the product of two signals, in particular, as will be seen below.

 According to an important characteristic of the invention, an incoherent processing is carried out on at least one of the delx parallel channels for connecting the couplers, in particular so as to couple two incoherent optical signals in the output blow of the device.

 According to a first embodiment of the device according to the invention, in particular for carrying out signal subtraction operations, the device, which may have means for carrying out incoherent processing on each of the two coupler connecting paths, comprises l one of the couplers being an evanescent wave coupler, and the other of said couplers being a passive coupler, the output of the device being representative of the subtraction of the incoherent treatments carried out on each of said parallel coupler connecting channels. In this embodiment, the incoherent processing means advantageously belong to the group comprising the interferometers (but used in incoherent mode) and the recirculation lines.

 According to a second advantageous embodiment of the invention, in particular for carrying out operations for multiplying two signals, the two couplers are optical couplers with evanescent waves, and a first modulation signal is applied to the first coupler, and to the second coupler a second modulation signal, the output signal of said device being representative of the product of said first and second modulation signals.

In the case of the application of such a device to the spectral analysis of a signal, each of said coupler connecting channels includes an optical bandpass filter around a frequency W0 and a width of 6 W, and said couplers are modulated by the same modulation signal,
the output of said device being representative of the spectral density of the signal at the frequency W0.

 In all cases (multiplication and subtraction), said optical source is advantageously a light source with a low coherence length, such as a superluminescent diode, and said coupler connecting channels have between them a difference in length of their optical paths greater than the coherence length of said source.

Other characteristics and advantages of the invention will appear on reading the following of some preferred embodiments of the invention given by way of illustration and not limitation, and of the appended drawings in which
Figure 1 shows schematically the operation of an optical coupler with evanescent waves, of the Y coupler type, used in the device of the invention;
FIG. 2 shows diagrammatically the structure of an embodiment of the device according to the invention, in a use implementing a subtraction operation, for example for a filtering processing of a microwave signal;
Figure 3 shows schematically the structure of an embodiment of the device according to the invention, for the implementation of a product operation of two microwave signals
. FIG. 4 illustrates the structure of an embodiment of the device of the invention, applicable to the intercorrelation calculation of two signals;
. FIG. 5 illustrates an embodiment of the structure of the device of the invention, for the simultaneous and parallel calculation of the intercorrelation of two signals, for several delays;
FIG. 6 illustrates an embodiment of the structure of the device according to the invention, for the spectral analysis of a signal.

The device of the invention consists in making assemblies using optical couplers with evanescent waves, such as for example the "COBRA" (M. Papuchon,
Rev. Tech. Thomson-CSF, vol 6, 1974, p. 983) or the "Y couplers (S. Thaniyavarn Electronics Letters, August 28, 1986, Vol. 22, no. 8). These couplers have mainly been used until now for switching operations on the two parallel output channels" right left".

 As shown schematically in Fig. 1, these couplers 10 have the following characteristics.

 They make it possible to modulate an optical wave 13 up to frequencies of the order of 15 GHz, by an electrical modulation signal 14.

They have two optical outputs 11, 12, the intensities of which are respectively
Io- re + s (t) 3 and Io [i - s (t)), where
Io is the light intensity of the optical wave 13 supplied by an optical source 15 at the input of the integrated optical element 10
s (t) is the microwave signal 14 applied to the modulator of the coupler 10.

 These couplers therefore make it possible to have signals s (t) and -s (t) in parallel.

 This property of the evanescent wave optical couplers, as well as others which will appear subsequently, are used according to the invention for carrying out signal processing operations, in particular but not exclusively of microwave signals, such as signaling operations. subtraction or multiplication, and derived processing using these arithmetic operations.

Signal subtraction
FIG. 2 illustrates an embodiment of the invention making it possible to perform a filtering operation on a microwave signal 24.

 For this application, each of the outputs 11, 12 of the evanescent wave coupler is connected to a fiber optic filter 21, 22 consisting of Mach-Zehnder interferometers and / or recirculating lines. Each of these filters 21, 22 allows only positive coefficients to be produced.

However, if their outputs 25, 26 are connected using a passive coupler 17, the output 23 of this coupler 17 will be proportional to the subtraction of the two transfer functions of the filters 21, 22 which indirectly amounts to producing coefficients negative.

Indeed, if we ask
AS (w) = 5 s (t) eiwt dt (Fourier transform of the modulation signal s (t) 24)
Rtw) =) e (t) eiwt dt (Fourier transform of the output signal R (t) 23)
so we have
R (w) = tH1 (w) - H2 (w)) s (w) + CH1 (0) + H2 (0)) d (w) with d (w): Dirac function
H1 (w): transfer function of channel 21
H2 (w): transfer function of channel 22
The global transfer function H1 (w) - H2 (w) therefore makes it possible to produce negative coefficients.

 The advantage of this approach is that it allows great flexibility for the realization of filters 21, 22. It makes it possible to combine the advantages of incoherent systems, such as for example the realization of recursive filter, while freeing from the constraint of only be able to carry out additions or multiplications by positive coefficients. At present, this is much more difficult to achieve with systems using the coherence properties of light.

MultiPlication of signals
L1invention allows the realization of a product operation of two signals, including microwave signals, by the cooperation of two evanescent wave couplers. Indeed, if we cascade two evanescent wave couplers 31, 32 (Fig. 3), it is possible to carry out the multiplication of two microwave signals s (t) 33 and s2 (t) 34. I1 suffices that each of the two signals 33, 34 is applied to one of the couplers 31, 32 so that the output 35 of this system contains information on the product of these signals.

 In addition, if the detection of the optical signal is carried out with a detector 36 with low bandwidth, the result which is obtained is the average value of this signal over a time interval of the order of the inverse of the bandwidth . It is advantageous that the detector 36 has a low pass band since this limits the photonic noise which, if not, may be important for fiber optic systems since the powers detected are generally low.

A mathematical proof can be as follows:
. s1 (t), sz (t): microwave signals,
. dF: detector detection bandwidth 36.

The output 35 of the detector 36 is proportional to
2/6 Re [# wso H (w) S1 (w) S * (w) dw] + H (O) where
. s1 (w) is the Fourier transform of 81 (t)
. S2 (w) is the complex conjugate of that of S2 (t), and
. Re [X] represents the real part of X.

 This property of the association of two evanescent wave couplers 31, 32 finds application in a certain number of uses, two of which will be presented without implied limitation.

 FIG. 4 represents an embodiment of the device of the invention for cross-correlation operations.

The optical source supplies a first coupler 41 with evanescent waves, and a second coupler 42 with evanescent waves, mounted head to tail. The two couplers 41, 42 are supplied respectively by a modulation signal sl (t) 45 and s2tt) 46. The two connecting links 43, 44 introduce a delay
Z. The output signal 47 R (t) of the second coupler 42 supplies a detector 48.

The output of detector 48, which functions as an integrator,
# 1 / d dR (t) dt is proportional to: + 1F F s1 (t) s2 (t + t) dt
The system working in inconsistent optics, a great flexibility of implementation is allowed. For example, the value of the cross-correlation can be known simultaneously for several delays (zl, z2, z3, ... zn), using the assembly with a single input coupler 51, and multiple output couplers 52 - 52 connected each. has a separate delay line, as shown in Figure 5.

 The invention also applies to the analysis of a signal in a spectral window.

The corresponding device is shown in FIG. 6, and has two identical filters 67 of transfer function
H (w) on the connecting channels 63, 64 of the evanescent wave couplers 61, 62. The real part of the transfer function H (x) performed by the optical fiber filters acts as a bandpass filter around a frequency wOet of widths w and the two modulators are controlled by the same signal s (t) 65, 66. In this case the output of the system is a linear function of the spectral density of the signal at the frequency wo. -
For the device of the invention to function properly, it is essential that the two optical waves are inconsistent with one another. This can be achieved, for example, by using a source with a low coherence length and by imposing a slight difference in length for the different optical paths of the filters. This difference must be greater than this coherence length. The coherence lengths of the superluminescent diodes (DSL) are so small (~ 40 ssm) that the condition which has just been stated is easily fulfilled and does not have any consequences on the desired signal processing operation.

Claims (12)

 1. Optical signal processing device, characterized in that it comprises at least one set of two optical couplers (20, 17; 31, 32; 41, 42; 51, 52t to 52n; 61, 62) of the type comprising each on the one hand at least one input and / or output channel (13), and on the other hand two parallel output and / or input channels (11, 12; 43,44; 63, 64) , said couplers (20, 17; 31, 32; 41, 42; 51, 521 to 52n; 61 62) being mounted in opposition and connected to one another by their two parallel paths (11, 12; 43, 44 63, 64) respective entry and / or exit, t  in that the input channel of the first coupler (20, 31, 41t 51, 61) is supplied by an optical source (15), the output channel of the second coupler (17, 32, 42, 521 to 52nr 62) providing an optical signal representative of the processing carried out by the device,  and in that at least one of said optical couplers (20; 31, 32; 41, 42; 51, 521 to 52n; 61, 62) is an evanescent wave optical coupler.  2. Device according to claim 1 characterized in that said optical coupler with evanescent waves comprises an input modulation of the optical signal which traverses it, by an electrical modulation signal (14; 24 t 33, 34 t 45, 46; 65, 66).  3. Device according to claim 1 characterized in that it comprises means (21, 22; 37; 67, 68) for carrying out an incoherent treatment on at least one of the two parallel ways of connecting the couplers  4. Device according to any one of claims 1 to 3, in particular for carrying out signal subtraction operations, characterized in that it comprises means (21, 22) for carrying out inconsistent processing of at least the two channels (11, 12) for connecting the couplers (20, 17),  and in that one (20) of the couplers is an evanescent wave coupler, and the other (17) of said couplers is a passive coupler,  the output (23) of the device being representative of the subtraction of the inconsistent treatments carried out on each of said parallel paths (11, 12) for connecting the couplers.  5. Device according to claim 4 characterized in that said means (21, 22) of incoherent processing belong to the group comprising the interferometers used in incoherent mode and the recirculating lines.  6. Device according to any one of claims 1 to 3, in particular for carrying out operations for multiplying two signals, characterized in that the two couplers (31, 32; 41; 51, 521 to 52n) are couplers evanescent wave optics, and in that a first modulation signal (33, 45) is applied to the first coupler, and a second modulation signal to the second coupler (32, 42, 521 to 52n), the output signal of said device being representative of the product of said first and second modulation signals.  7. Device according to claim 6 characterized in that an optoelectronic detector (36, 48, 69) with low passband is placed at the output of said device.  8. Device according to any one of claims 6 and 7, in particular for carrying out the spectral analysis of a signal, characterized in that each of said channels (43, 64) for connecting the couplers (61, 62) comprises a filter bandpass optics (67, 68) around a frequency W0 and width DW, and in that said couplers (61, 62) are modulated by the same modulation signal s (t) (65, 66),  the output of said device being representative of the spectral density of the signal s (t) at the frequency W0.  9. Device according to any one of claims 1, and 6 to 8 characterized in that said source (15) is a light source with a short coherence length, and in that said connecting channels and / or means for processing coupler connecting channels have between them a length difference in their optical paths greater than the coherence length of said source (15).  10. Device according to any one of claims 1, and 6 to 9 characterized in that said light source (15) consists of a superluminescent diode.  11. Use of the device according to any one of claims 1 to 10 for the signal processing operations belonging to the group comprising the subtraction of two signals, the filtering of a signal, the calculation of the product of two signals, itautocorrelation of a signal ', the intercorrelation of two signals, and the spectral analysis of a signal.  12. Use according to claim 11, for signal processing operations in the microwave domain.