GB2143338A - Transversal filter for electromagnetic waves - Google Patents

Abstract

The transversal filter makes it possible to reject or pass a frequency ( omega o) with which an electromagnetic wave is intensity-modulated. To accomplish this, the transversal filter (2, 3, 4, 5) is tuned to the frequency of the modulating signal rather than the frequency of the carrier wave as is the case in the prior art. In the case of electromagnetic waves whose frequencies lie in the optical frequency range, a frequency discriminator can be implemented simply by placing an optical-to-electric transducer (7) behind the transversal filter (2,3,4,5). <IMAGE>

Description

SPECIFICATION Transversal filter for electromagnetic waves The present invention relates to a transversal filter for electromagnetic waves.

Transversal filters are generally known; they are described, for example, in an article by H.E. Kallmann, "Transversal Filters", Proceedings of l.R.E., July 1940, pages 302 to 310. There are many publications describing how the element values for transversal filters must be chosen to obtain the desired passband or bandstop. Such transversal filters are used to select carrier waves.

In receiver signal processing circuitry, it is customary to separate carrier waves of different frequency which are modulated with one or more modulating signals, to demodulate these separated carrier waves, and then to split the demodulated signal into its individual modulating signals. In the case of carrier waves at frequencies in the optical range, the optical signal must first be converted into an electric signal, which is done in an optical-to-electric transducer. Such transducers are mostly nonlinear and frequently produce a cross-modulation, which shows up as interfering frequencies in the following signal processing circuits. This is the case particularly if two or more coherent carrier waves (e.g. in the RF range) are modulated and then used to intensity-modulate an additional carrier wave (e.g. in the optical frequency range).

The signal processing could be improved by rejecting the frequencies causing the interference (or individual information-containing frequency bands) at the beginning of the signal processing as far as possible, or by passing only the desired frequency (frequencies). In the case of carrier waves at frequencies in the optical range, this would have to be done prior to the conversion of the optical signal into the electric signal. This, however, is possible neither with conventional transversal filters nor with other conventional filters.

The recognition that it is advantageous during signal processing to reject interfering frequencies (or select intelligence frequencies) prior to demodulation, resulted in the present invention which seeks to provide a filter with which this is possible.

According to the invention there is provided a transversal filter for electromagnetic waves wherein the electromagnetic waves have different propagation times by propagating in at least two paths between a first coupler and a second coupler, the first coupler splitting the electromagnetic wave into two or more partial waves, and the second coupler recombining the partial waves, characterised in that the propagation times and the attenuations of the propagation paths are so chosen that a predetermined passband characteristic is obtained for one or more frequencies of at least one signal with which the electromagnetic wave is intensity-modulated.

With the filter of the present invention, it makes no difference whether the carrier wave intensitymodulated with intelligence and/or interfering signals has a frequency in, e.g. the RF range or the optical range. The filter can be implemented as a bandpass filter or a bandstop filter.

The transversal filter of the present invention is very well suited to filtering out two or more coherent and modulated carrier waves which, in turn, have intensity-modulated a further incoherent carrier wave.

The transversal filter can be used to special advantage in a frequency discriminator in which the frequency-modulated signal is converted into a n an amplitude-modulated signal for demodulation. The frequency swing of the frequency-modulated signal can be very large (e.g. equal to half the frequency of the carrier wave). A frequency discriminator with such a filter is very easy to implement.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which: Figures 1 to 4 are schematic representations of four different embodiments of transversal filter constructed in accordance with the invention, Figure 5 shows the transfer function of the transversal filter for a sinusoidally intensity-modulated electromagnetic wave, Figure 6 shows the transfer function of the transversal filter for a square-wave-modulated electromagnetic wave, Figure 7 is a vector diagram for illustrating the response of transversal filters constructed in accordance with the invention, and Figures 8 to 11 show transfer functions for different filter arrangements.

In the following description, the intensity-modulated electromagnetic wave to be filtered is assumed to be a light wave. Those skilled in the art will be readily able to modify the embodiments so as to make them suitable for electromagnetic waves with frequencies in other frequency ranges and such versions are intended to fall within the scope of this invention.

In the embodiment of Figure 1, an optical waveguide 1 runs to an optical directional coupler 2. From the coupler 2, two optical waveguides 3 and 4 of different length run to a second optical directional couplers, which is connected to an optical-to-electric transducer 7 by an optical waveguide 6. Due to the different lengths (and possibly other quantities), the light waves require different times to propagate along the two optical waveguides. The optical-to-electric transducer delivers an electric signal to a line 8. The components used are generally known. Optical directional couplers are described, for example, in an article by F.Auracher, "Prinzipien und Eigenschaften von Abzweigen fur Multimodefasern", Frequenz, No.34, 1980, pages 52 to 57.

To explain the principle of the filter of the present invention, a simple case will be assumed. The optical carrier wave is intensity-modulated with an interfering signal (frequency 0)o) and an intelligence signal (frequency )n) The difference between the propagation times in the two optical waveguides which is required to reject the interfering signal is then T/2, where T is equal to the period of the interfering signal. It should be emphasized that T is the period of the signal with which the light wave is intensity-modulated, not the period of the light wave, i.e. not of the carrier wave.If that is the case, and the light wave is split into two waves of the same intensity in the first coupler 2, the two partial waves will mix in the second coupler in such a way that the intensity modulation with the interfering signal (period TI of the light wave is eliminated, i.e. a bandstop filter for intensity modulation with the period T will be obtained. It is advantageous if the light wave is incoherent, for then it need not be taken into account as no subtraction takes place at the frequency of the existing light wave (nor at the modulating frequency).

If the light wave is sinusoidally intensity-modulated at a frequency toO, the cos-shaped transfer function shown in Figure 5 is obtained as a function of the modulating frequency (o.

If the light wave is intensity-modulated with square-wave pulses having a repetition frequency of (oO, the triangulartransferfunction shown in Figure 6 is obtained. The repetition frequency is equivalent to the frequency of a sinusoidal modulating signal.

In the embodiment shown in Figure 2, an optical waveguide 10 runs to a first coupler 11 which - unlike in the embodiment of Figure 1 - splits the light wave into three partial waves of different intensity, and these three partial waves propagate through three optical waveguides 12, 13, 14, of different length to a second coupler 15. The filtered signal is passed on via an optical waveguide 16.The propagation time through the longest optical waveguide 12 is chosen to be longer than that through the shortest optical waveguide 13 by T, and the propagation time through the optical waveguide 14, which has a medium length, is chosen to be longer than that through the shortest optical waveguide 13 by/2. If the intensity of the partial wave in the optical waveguide 14, which has the medium length, is designated I, the intensity of each of the partial waves in the two other optical waveguides 12 and 13 is 1/2. The desired energy distribution takes place in the coupler. It may also be produced by different attenuations in the light paths.

Figure 7 is a vector diagram of the sinusoidal modulating signals 12', 13', and 14', with which the partial waves propagating in the optical waveguides, 12, 13 and 14 are modulated. Since the partial wave in the optical waveguide 14 has twice the intensity of the two other partial waves, the magnitude of the modulating signal 14' is chosen to be twice the magnitudes ofthe modulating signals 12' and 13'. If the frequency of the signal with which the light wave is intensity-modulated increases, the vectors of the two modulating signals 12' and 13' will rotate in different directions relative to the vector of the modulating signal 14'. At the frequency w,, the direction of the two vectors 12' and 13' is opposite to that of the vector 14'.This results in a subtraction of the modulating signals and, hence, an elimination of the intensity modulation of the light wave with the frequency (oO.

In the embodiments of Figures 1 and 2, the optical waveguides may be implemented as optical fibres.

The transversal filter can also be implemented in various other ways. In all cases, however, at least two partial waves must be produced, and these must be recombined after different propagation times.

In the embodiment of Figure 3, the light wave travels through an optical waveguide 20 to a first beam splitter 21, which splits it into two partial waves 28 and 29. One of the partial waves is directed to a second beam splitter 26 via two mirrors 22 and 25. The two beam splitters 21 and 26 correspond to the two couplers 2 and 5 in the embodiment of Figure 1. The other partial wave travels to a pair of mirrors 23, 24. It is reflected back and forth between the mirrors several times, depending on the mirror arrangement chosen. After the last reflection, the partial wave is guided to the second beam splitter 26, where it is superimposed on the other partial wave. The lightwave produced by the superposition propagates down an optical waveguide 27.

With respect to the propagation-time difference to be chosen, the statements made in connection with the embodiment of Figure 1 apply analogously.

In the embodiment of Figure 4, a light wave propagates along an optical waveguide 40 to a beam splitter 41, which splits it into two partial waves 60 and 61. The partial wave 61 is reflected from a mirror 42 to a device 45. This device is a substrate in which several optical waveguides 46, 47, 48, 49 of different length are formed ("integrated optics"). The other partial wave 60 travels to a Bragg cell 44. It is known that a Bragg cell deviates a light beam by different amounts depending on the frequency of a control signal applied to the cell.

This is used here to direct the partial wave to one of the optical waveguides 46 to 48, depending on the control signal. Each of the optical waveguides 46 to 48 is tuned to a different modulating frequency. This is particularly advantageous if the aim is to avoid suppression of an intensity modulation of the light wave but to pass on a given intensity modulation for further processing. If the optical waveguide 46 is used, the tuning condition is satisfied for a first modulating frequency; if the optical waveguide 47 is used, this condition is satisfied for a second modulating frequency, etc. It is thus readily possible to determine which of the modulating frequencies with which the light waves are intensity-modulated is to be passed on through the optical waveguide 51 for further processing. All components shown in Figure 4 can be constructed as an integrated optical device.

The element values for the novel transversal filter can be chosen in accordance with the known rules for conventional transversal filters. It is thus possible to realise filtsrs with desired bandstop or desired passbands, both with the desired slope. Since known rules can be used in selecting and combining the individual path lengths, the choice of element values will not be dealt with in detail; what is important is that this choice is determined not by the frequency of the carrier wave but by the frequency of the modulating signal with which the carrier wave is intensity-modulated. Care must also be taken to insure that, if the carrier wave is coherent, no undesired response will be caused by the interference between the partial waves. In the case of noncoherent carrier waves, the frequency of the carrier wave plays a part only in the selection of the components to be used.

In Figures 8 to 11, a few transfer functions are shown qualitatively as a function of the modulating frequency. The curve of Figure 8 is obtained with a filter as shown in Figure 1. With a filter as shown in Figure 2, the curve of Figure 9 is obtained, and Figures 10 and 11 show the curves obtained by cascading, respectively, two and four filters of the kind shown in Figure 2.

Figure 6 shows that transfer functions depending on the modulating frequency are obtained with transversal filters of the present invention. This is used to advantage for frequency demodulation. An especially good result will be obtained if the light wave is intensity-modulated with a square-wave signal frequency-modulated in itself, for in this case the transfer function is linear over a wide range. The centre frequency of the frequency at which frequency modulation occurs is chosen for example, to be equal to +2(1)o or o 2 2 for the transfer function given in Figure 6. After passing through the filter, the frequency modulation is changed to an amplitude modulation. If this signal is sent over an optical waveguide (Figures 1, 6) to an optical-to-electric transducer (7), the electric output signal of the optical-to-electric transducer corresponds to the signal with which the light wave was frequency-modulated.

With such a frequency discriminator, light waves frequency-modulated with a very high-frequency signal (e.g. 200 MHz) and a very large frequency swing of, e.g. t 100 MHz can be demodulated. The slope of the discriminator characteristic (Figure 6) depends, inter alia, on the propagation-time differences chosen for the different propagation paths for the light wave in the filter, and on where the centre frequency of the signal with which the frequency modulation is produced is located in the transfer function of Figure 6.

Claims (12)

1. A transversal filter for electromagnetic waves wherein the electromagnetic waves have different propagation times by propagating in at least two paths (3, 4) between a first coupler (2) and a second coupler (5), the first coupler (2) splitting the electromagnetic wave into two or more partial waves, and the second coupler (5) recombining the partial waves, characterised in that the propagation times and the attenuations of the propagation paths are so chosen that a predetermined passband characteristic is obtained for one or more frequencies of at least one signal with which the electromagnetic wave is intensity-modulated.

2. A transversal filter as claimed in claim 1, characterised in that the propagation paths are implemented as optical waveguides.

3. A transversal filter as claimed in claim 2, characterised in that the optical waveguides are optical fibres.

4. A transversal filter as claimed in any one of the preceding claims, characterised in that the transversal filter is tunable by switching to other frequencies.

5. A transversal filter as claimed in claim 2, characterised in that the optical waveguides are formed by an integrated optical device.

6. A transversal filter as claimed in any one of claims 1 to 4, characterised in that the propagation paths are formed at least in part by mirrors and beam splitters.

7. A transversal filter as claimed in any one of the preceding claims wherein the propagation paths are arranged to provide a relative delay of an odd number of half periods at the frequency of an interfering signal to be rejected.

8. A transversal filter as claimed in claim 7 wherein the relative delay is effected by providing path lengths which differ by an amount equivalent to an odd number of half periods at the frequency of the interfering signal to be rejected.

9. A transversal filter as claimed in claim 7 or 8, wherein the relative delay is an odd number of half periods of the carrier frequency of the electromagnetic wave.

10. A transversal filter substantially as described herein with reference to the drawings.

11. A frequency discriminator incorporating a transversal filter as claimed in any one of the preceding claims.

12. A frequency discriminator as claimed in claim 11 including an optical to electric transducer following the transversal filter.