WO2003036828A1

WO2003036828A1 - Method and arrangement for microwave signals transmission through a multimode fibre

Info

Publication number: WO2003036828A1
Application number: PCT/NL2002/000624
Authority: WO
Inventors: Antonius Marcellus Jozef Koonen
Original assignee: Technische Universiteit Eindhoven
Priority date: 2001-09-26
Filing date: 2002-09-26
Publication date: 2003-05-01
Also published as: NL1019047C2

Abstract

Method and arrangement for transferring electrical signals, in particular microwave radio signals, via a multi-mode optical fibre, by generating an optical signal which periodically sweeps a predetermined wavelength range. The optical signal is amplitude-modulated by the electrical signal to be transferred. The optical signal that has been transferred via the optical fibre is optically filtered by an optical filter having a number of passbands, the mutual distance between which is smaller than the wavelength range swept by the optical signal, after which the filtered optical signal is converted into an electrical signal.

Description

METHOD AND ARRANGEMENT FOR MICROWAVE SIGNALS TRANSMISSION THROUGH A MULTIMODE FIBRE

Description. The invention relates to a method and an arrangement for transferring electrical signals, in particular radio signals, through a multi-mode optical fibre.

Wireless radio communication has witnessed rapid growth in the past few years. Not only for public mobile telephony and data transmission, but also for use as wireless local networks to be used for business and private purposes. The growing number of users, in particular for business appli ations, as well as the bandwidth required for each user, necessitate ever greater increases of the transmission capacity of wireless systems. The current wireless local networks ("Local Area Networks

(LANs)") operating in the ISM frequency band of around 2.4 GHz offer transmission capacities around 11 bit/s per carrier frequency. The so- called HIPERPLAN/2 systems are suitable for information transport of up to 54 Mbit/s per carrier frequency in the 5.2 GHz band, and currently a study is being made with regard to LANs capable of providing a transmission capacity of more than 100 Mbit/s at carrier wave frequencies far in excess of 10 GHz.

Wireless connections at high bit rates make it necessary to use relatively high carrier frequencies and small radio cells. A base station fitted with an antenna is needed for each of the cells, and the high transmission rates enable an advantageous transfer of information signals to the base stations via optical fibre links.

In order to provide adequate coverage, a relatively large number of cells is needed even in house and small office buildings, which leads to an extensive' wiring network for connecting the base stations associated with the cells. Accordingly, installing the wiring and extending the network should be as easy and as simple as possible. Polymer-based optical fibres meet these requirements because of their relatively large core diameter and high numerical aperture and partly because of the fact that these optical fibres are easy to bend, split and connect to other devices. The limited bandwidth- length product of this type of fibre is a point of special attention in connection with the transportation of signals at a high bit rate. On the other hand, the distances indoors are relatively small, so that this limitation of this type of fibre as such does not need to prevent the use thereof in a cable network. In practice, use is made inter alia of optical heterodyne techniques for the direct transfer of radio signals via mono-mode optical fibres, which techniques enable direct generation of microwave frequencies for the transmission of radio signals via an antenna.

Multi-mode optical fibres, in particular polymer-based optical fibres, however are not suitable for the use of heterodyne techniques, due to the spreading in the phase relations between the various propagation modes.

Consequently it is an object of the invention to provide a solution for the direct transportation of analog electrical signals, in particular radio signals, via a multi-mode optical fibre, so that it will be possible to transmit the transferred radio signal without any further signal processing at the base stations. Especially if a large number of base stations are used, as in the case of wireless networks as explained in the foregoing, it will be advantageous to keep the base stations as simple as possible. Not only does this provide a direct financial benefit as regards the cost of the base station itself, but it also enables a considerable saving on maintenance and repair costs of the network.

The invention, in a first aspect thereof, provides a method for transferring electrical signals, in particular radio signals, through a multi-mode optical fibre, which method is characterized by the steps

Of ;

- generating an opti cal signal whi ch peri odi cal ly sweeps over a predetermined wavelength range,

- amplitude modulating the optical signal by the electrical signal to be transferred,

- transferring the optical signal through the optical fibre,

- optically filtering the transferred optical signal in a number of passbands having a mutual distance less than the wavelength range over which the optical signal sweeps, and

- converting the filtered optical signal into an electrical signal.

The invention is based on frequency multiplication by means of a periodically frequency or wavelength-varied optical source at the transmission end and an optical ulti -passband filter at the receiving end. By filtering the transferred optical signal in a number of passbands, the mutual distance between which is smaller than the wavelength range swept by the optical signal, frequency multiplication of the electrical signal converted from the filtered optical signal is provided.

The frequency multiplication factor depends on the filter properties or the number of passbands of the filter swept by the optical signal, and on the rate or frequency at which this happens.

If the optical signal periodically sweeps the wavelength range at a (sweeping) frequency f_$u and the optical filter comprises a number of N passbands, the optical signal will sweep 2N passbands during each period, which results in a frequency of the filtered optical signal, and consequently in the electrical signal obtained therefrom, in the order of f_m = 2N-f_m.

The intensity-modulated electrical signal, which is the envelope of the swept optical signal, is not up-converted, so that an electrical signal having a frequency f_m * 2N-f_8W is obtained after conversion.

A suitable selection of the sweeping frequency and of the number of filter bands makes it possible to present the obtained electrical signal directly, after suitable amplification, if necessary, to an antenna for transmission in the form of a radio signal thereof.

In another embodiment of the method according to the invention, the transferred electrical signal is bandpass-filtered, at a frequency suitable for direct transmission of the electrical signal, in order to suppress undesirable higher harmonics generated by the frequency multiplication process. The bandpass filter not only suppresses higher harmonics, but it also reduces noise. The invention also relates to an arrangement for transferring electrical signals, in particular radio signals, through a multi-mode optical fibre, which system comprises:

- first means for generating an optical signal that periodically sweeps over a predetermined wavelength range, - modulator means for amplitude modulating the optical signal by the electrical signal to be transferred,

- optical filtering means for filtering the transferred optical signal in a number of passbands having a mutual distance less than said wavelength range over which said optical signal sweeps, and - first means for converting the filtered optical signal into an electrical signal.

In an embodiment of the system according to the invention, the first means for generating the optical signal are comprised of a tunable fast first laser diode, to which oscillator means for driving the laser diode with a periodically varying current connect. The laser diode is preferably suitable for emission in the 1.3 μm wavelength range of polymer-based optical fibres, in which wavelength range the fibres exhibit relatively low losses. The current of the oscillator means may have any suitable shape, such as a sinusoidal or triangular shape, with a duty cycle of 50% or otherwise.

In another embodiment of the invention, it is preferred to use a symmetrically driven Mach-Zehnder modulator, by means of which additional chirp effects are minimised, for realising the amplitude- modulation of the optical signal delivered by the first laser diode.

According to yet another embodiment of the invention, a Fabry-Perot (FP) etalon can be used advantageously as the optical filter means, which Fabry-Perot (FP) etalon is preferably disposed between two lenses for projecting the image of the relatively large core of the multi-mode polymer-based optical fibres on the relatively narrow active area of the first photodiode.

In another embodiment of the system according to the invention, the filtered optical signal is converted by a fast first photodiode into an electrical signal which, after bandpass filtering, is directly fed to an RF power amplifier for transmitting the respective electrical signal in question at the frequency f_m = 2N*f_sw via antenna means connected to the amplifier. In yet another embodiment of the system according to the invention, the transfer of signals in two directions via the multi-mode fibre is effected in that the modulator means are connected to an input of first optical splitter means, to an output of which second means connect for converting an optical signal into an electrical signal, and in that the optical filter means are connected to a first output of second optical splitter means, to an input of which second means for generating an optical signal are connected, which first and second optical splitter means are arranged for connection thereof by means of a multi-mode optical fibre. Arranging the first means for generating an optical signal and the second means for generating an optical signal to operate at different wavelength ranges makes it possible to provide a bidirectional system for transferring electrical signals, in particular radio signals for use in a time-division multiplex transmission system. In a practical embodiment of the bidirectional system according to the invention, the second means for generating an optical signal comprise a second laser diode, whose electrical input connects to a mixer circuit for converting a received (by the antenna means) radio signal to baseband.

In a practical embodiment of the bidirectional system according to the invention, the power amplifier, the antenna means and an input of the mixer circuit connect to a circulator, with a second input of the mixer c rcui being connected to the power amplifier and an output of the mixer circuit being connected, through an amplifier circuit and a low-pass filter, for driving the second laser diode.

In this embodiment, the -transferred radio signal is also used for down-converting the radio signal arriving on the antenna from, for example, a mobile station to baseband by means of a mixer circuit, in which baseband it can be transferred by the second laser diode, via the optical fibre, to a central processing device or the like comprising second means for converting an optical signal into an electrical signal, such as a second photodiode, and a low-pass filter and amplifier circuit connected to the electrical input of the second photodiode.

The invention also relates to a radio access unit or base station comprising one or more of the above-disclosed means for converting a received optical signal into a radio signal and/or means for providing bidirectional signal transfer.

The invention also relates to a head end station or processing station comprising one or more of the above-disclosed means for converting a radio signal to be transferred into an optical signal and/or means for converting an optically transferred baseband signal into a radio signal.

The invention will be discussed in more detail hereinafter with reference to the appended Figures, in which:

Figure 1 shows an embodiment of the system according to the invention which is suitable for a unidirectional signal transfer. Figure 2 illustrates a bandpass characteristic of an optical filter suitable for use in the system according to the invention.

Figures 3 and 4 show the waveform of optical signals generated in the system according to the invention.

Figure 5 shows an embodiment of the system according to the invention which is suitable for bidirectional signal transfer.

Figure 1 shows a unidirectional embodiment of an arrangement 1 for transferring an electrical signal, in particular a radio signal in the microwave range, via a multi-mode optical fibre, such as a polymer-based optical fibre 2.

Radio signals are transferred to various base stations or radio access unit ("Radio Access Points (RAPs)") 4 from an end station or processing station 3, for example a so-called "head end" station, via the optical fibre 2.

Although only one RAP 4 is shown in Figure 1, it will be appreciated by those skilled in the art that several RAPs may be connected to the optical fibre 2, as is schematica ly illustrated by means of the branches 5, 6, 7 of the optical fibre 2.

The end station 3 comprises first means 10 for generating an optical signal, such as a first tunable laser diode. The laser diode 10 is electrically driven via an oscillator 11, which is arranged for generating a periodically varying current control signal i_βw at a frequency f_8V(, by means of which signal the laser diode 10 is excited. As a result, the laser diode 10 will deliver an optical signal which periodically varies or sweeps a predetermined wavelength range A_Q.

Figure 3 graphically shows a cycle time T-normalized, triangul r periodic current signal i_s having a duty cycle of 50% as well as the optical signal u generated by the laser diode 10, which varies over a wavelength range λo between 1309.6 and 1310.5 nm.

Figure 4 shows a graphical representation similar to Figure 3, in which the current signal i_sw exhibits a sinusoidal variation, however. The duty cycle of the periodic current signal i_sw can be varied for the purpose of the invention, it does not necessarily have to be 50%. The optical signal generated by the laser diode 10 is optically amplitude-modu ated, by modulator means 12, with the electrical signal to be transferred. A syrnmetrically controlled Mach-Zehπder modulator comprising + data-in and - dat-in inputs can be used for this purpose, by means of which additional chirp effects are minimised. Such a modulator is known per se in practice and need not be explained in more detail to those skilled in the art.

The optical signal that has been modulated in this way is transferred to the radio access unit 4 via the optical fibre 2 and its branches 5, 6, 7.

The radio access unit 4 comprises optical filter means 13, such as a Fabry-Perot (FP) etalon, which is disposed between a first lens 14 having a focal distance fl_a and a second lens 15 having a focal distance fl₂ in the illustrated embodiment. The lenses 14, 15 have been selected so that the image of the relatively large core of the multi-mode optical fibre is projected on the relatively narrow field of first means 16, such as a fast photodiode, for the purpose of converting the received optical signal into an electrical signal.

For the purpose of the invention, the optical filter 13 comprises a number of passbands or transmission bands 17, the mutual distance Δλ_fSR ("Free Spectral Range") between which is smaller than the bandwidth range λo over which the optical signal u generated by the laser diode 10 varies.

Figure 2 shows an example of a periodic optical filter comprising passbands numbered 0, 1, 2, ... N. Such optical filters are known per se in practice and need not be explained in more detail to those skilled in the art.

During each period of the exciting current i_sw of the laser diode 10, the signal sweeps N filter transmission periods of the filter 13 twice, so that the intensity of the optical signal hitting the photo diode 16 fluctuates at a frequency 2N*f_SH. This results in an electrical signal having a frequency f_m = 2N-f_5W, which is delivered by the photo diode 16.

Besides the radio signal at a carrier wave frequency f_m> also higher harmonics are generated, which are suppressed by means of a bandpass filter 8PF 18. The radio signal that thus remains can be directly supplied to a power amplifier 19 without further processing for transmission thereof via an antenna 20. Besides the suppression of undesirable higher harmonics, the bandpass filter 18 also reduces the amount of noise in the electrical signal.

The intensity-modulated electrical signal, which is the envelope of the optical frequency-swept signal, is likewise detected by the photodiode 16 but it is not up-converted.

Since the electrical signal, such as a radio signal, can be directly recovered from the optical signal in the radio access unit at a carrier wave frequency suitable for transmission, as already explained above, the radio access unit 4 can be of very simple design, in fact only comprising an optical filter 13, lenses 14, 15, if necessary, the photo diode 16 or other means for converting the optical signal into an electrical signal, and electrical filter means, such as the bandpass filter 18, a power amplifier 19 and an antenna 20 for transmitting the radio signal. In particular in the case of radio signals in the GHz range, the antenna 20 may be a short rod antenna or even a strip on a printed circuit board. The radio access units 4 can thus be manufactured in a very cost-effective manner. This renders these radio access units 4 specially suitable for use in wireless networks comprising relatively small cells, i.e. high frequencies and higher transmission capacities as explained in the foregoing. The cable network from the end station 3 to the radio access units 4 can advantageously be built up of multi-mode optical fibres, such as polymer-based optical fibres, the installation of which does not need to take place in accordance with any special requirements. In English professional literature such fibres are also referred to by the acronym GIP0F.

Figure 5 shows another embodiment of the arrangement 0624

10 according to the invention, which is suitable for bidirectional signal transfer.

In addition to the unidirectional arrangement according to the invention as shown in Figure 1, the bidirectional arrangement comprises a return path from the radio access unit 27 to the end unit 22 for signals received from the antenna 20.

In the embodiment of the bidirectional arrangement 21 as shown in Figure 5, the end station 22 includes an optical power splitter 23, to an input of which the modulator means 12 are connected, and to an output of which second means for converting the received optical signal into an electrical signal, such as a second photodiode 24, are connected. The electrical signal delivered by the second photodiode 24 is delivered to an output data-out by means of a low-pass filter LPF 25 and a second amplifier 26 for further processing of the signal. The radio access unit 27 includes second optical power splitter means 28, to an output of which the optical filter 13 comprising the lenses 14, 15 is connected, and to an input of which second means 29 for converting an electrical signal into an optical signal, such as a second laser diode 29, are connected. The first and the second optical power splitters 23 and 28 are arranged for coupling to a multi-mode fibre 2, such as a polymer-based multi-mode optical fibre.

Present at the electrical input of the photodiode 29 is a low-pass filter LPF 30 comprising a fron -connected amplifier 31, which is connected to an output of a mixing circuit or mixer 32. A first input of the mixer 32 is connected to the antenna

20 via a circulator 33. In the embodiment of the radio access unit 27, the output of the power amplifier 19 is likewise connected to the antenna 20 via the circulator 33. The circulator 33 is arranged so that a radio signal from the amplifier 19 is delivered to the antenna 20 and that radio signals received by the antenna 20 end up on the mixer 32 via the circulator 33. A further input of the mixer 32 is connected to the output of the amplifier 19. In the circuit that is thus obtained, a signal arriving on the antenna 20, for example a radio signal from a mobile station, is down-converted to baseband and converted into an optical signal by the second laser diode 29, which signal is transferred to the second photodiode in the end station 22 via the power splitter 28, the multi-mode fibre 2 and the power splitter 23 and converted into an electrical signal for further processing.

The second laser diode 29 and the second photodiode 24 are so arranged that they are operative for optical signals at a wavelength λ_x different from the wavelength ΛQ at which the first laser diode 10 and the first photo diode 16 are operative.

The bidirectional arrangement 21 as shown in Figure 5 enables the transfer of data traffic in time division duplex, i.e., information is alternately exchanged between the end station 22 and the radio access unit 27. The radio access unit 27 as shown in Figure 5 is of a very simple design to that end, in which the frequency f_m of the electrical signal delivered by the first photo diode 16 is used advantageously for down-converting the radio signal received from the antenna 20 to baseband. In the case of radio signals operating according to the time division duplex method, the carrier wave f_m is unmodulated in the time slots in which information is received by the antenna 20, because no information is transmitted from the end station 22 to the radio access station 27 during these time slots. This makes it possible to connect the output of the amplifier 19 directly to the input of the mixer 32. In the circuit according to the invention, the sweep frequency f_sw of the oscillator means 11 is limited by the speed at which the first laser diode 10 can be tuned, as well as by the dispersion of the multi-mode fibre 2. Owing to the very low material dispersion of polymer-based multi-mode fibres, multi-mode dispersion constitutes the main limitation in the use of these fibres. Recent developments in the field of polymer-based optical fibres show bandwidth-length products of 1 GHz.km, which enables sweep frequencies is of up to 2 GHz at fibre lengths of up to 500 . Recent development results for perfluoronate polymer-based fibres show bandwidth-length products of up to 10 GHz. m at a wavelength of 1.3 μm,

A Fabry-Perot filter having an etalon plate distance of, for example, 1.5 cm has a free spectral range of Δλ_reR - 10 GHz. By sweeping the wavelength λ<, of the laser diode 10 in the 1.3 μm range through, for example, 0.57 nm (corresponding to 100 GHz), a frequency multiplication factor of 2N=20 is obtained. A sweep frequency f_sw = 500 MHz thus gives a carrier wave frequency f_m = 10 GHz. A symmetrical, triangular wavelength sweep, i.e. with a duty cycle of 50%, as shown in Figure 3, yields a well-defined carrier wave frequency.

Preferably, the speed is such that the optical signal N sweeps entire transmission bands or N free spectral ranges Δλ_RR as shown in Figure 2. That is, the optical signal must sweep the optical filter in a mirror-symmetrical manner, for example in such a manner that the passbands swept by the signal are contiguous to each other, for example at the apexes thereof, as is indicated by the double arrow λ₀ in Figure 2. In other words, the laser wavelength must preferably sweep a whole number of passbands or free spectral ranges of the optical filter. If the sweep does not take place through a whole number of bands, the output signal of the photodiode will exhibit the periodicity equal to the sweep interval, which leads to spectral lines around the sweep frequency f^, near the fundamental component and higher harmonics. In the case of a sinusoidal sweep, many of these spectral lines will be present in the signal with a relatively high power, as a result of which higher quality demands are made on the bandpass filter 18 and the amplifier 19. Accordingly, a symmetrical, triangular wavelength sweep is preferred.

Of course it is possible to select one of the higher-order harmonics as the carrier wave frequency for the signal to be transmitted via the antenna 20, in particular if the harmonics are present with sufficient power in the electrical signal delivered by the first photo diode.

Although the invention has been illustrated by means of an application for wireless networks in the foregoing, it will be appreciated by those skilled in the art that the new and inventive method of transferring electrical signals via a multi-mode optical fibre can also be used for a large number of other applications, in particular for applications in which frequency multi lication of electrical signals is desired, for example in the case of microwave applications and radio wave applications.

Claims

1. Method for transferring electrical signals, in particular microwave radio signals, through a multi-mode optical fibre, characterized by the steps of:

- generating an optical signal which periodically sweeps over a predetermined wavelength range,

- amplitude modulating said optical signal by said electrical signal to be transferred, - transferring said optical signal through said optical fibre,

- optically filtering said transferred optical signal in a number of passbands having a mutual distance less than the wavelength range over which said optical signal sweeps, and - converting said filtered optical signal into an electrical signal.

2, Method according to claim 1, further characterized by the step of bandpass filtering of said transferred electrical signal at a radio frequency suitable for transmission thereof,

3. Method according to claim 2, characterized in that said optical signal sweeps over said wavelength range at a frequency fsw, in that said optical signal is optically filtered in a number of N passbands, and in that said electrical band-pass filter is tuned at a frequency fmm^N-fsw.

4. Arrangement for transferring electrical signals, in particular microwave radio signals, through a multi-mode optical fibre, comprisi g;

- first means for generating an optical signal that periodically sweeps over a predetermined wavelength range, - modulator means for amplitude modulating said optical signal by said electrical signal to be transferred,

- optical filtering means for filtering said transferred opti cal signal in a number of passbands having a mutual di stance l ess than said wavelength range over which said optical signal sweeps, and

- first means for converting said fi ltered optical signal into an electrical signal .

5. Arrangement according to cl aim 1, further characterized by el ectrical band-pass filter means for band-pass filtering of said transferred el ectri cal signal .

6. Arrangement according to cl aim 5, characterized i n that said fi rst means for generating said optical signal are arranged for generating an optical signal whi ch sweeps over said wavelength range at a frequency fsw, in that said optical fi lter means comprise a number of N filter pass-bands, and n that said band-pass fi lter means are arranged for band-pass filtering said el ectrical signal at a frequency fmm=2N-fsw.

7. Arrangement according to claim 4, 5 or 6, characterized i n that said first means for generating said optical signal comprise a tunable fast l aser diode and oscil lator means, connected to said laser diode, for driving said laser diode with a periodical ly varying current.

8. Arrangement accordi ng to claim 4, 5, 6 or 7, characterized in that said modulator means comprise a symmetrical ly driven Mach-Zehnder modulator.

9. Arrangement according to claim 4, 5, 6, 7 or 8, characterized in that said fi rst means for converting sai d fi ltered optical signal into an electri cal signal compri se a fast photodiode.

10. Arrangement according to claim 9, characterized in that said opti cal fi lter means compri se a periodic band-pass fil ter, arranged between two lenses for imaging said image of said optical fibre on the active area of said photodiode.

11. Arrangement according to claim 10, characterized in that said periodic band-pass fi lter compri ses a Fabry-Perot etal on.

12. Arrangement according to cl aim 4, 5, 6, 7, 8, 9, 10 or 11, characteri zed by transmitter means, having an RF-power ampl i fier and antenna means for ampl ifying and transmitting said band-pass filtered transferred electrical signal.

13. Arrangement according to claim 4, 5, 6, 7, 8, 9, 10, 11 or 12, characterized in that said modulator means connect to an input of first optical splitter means, to an output of which second means connect for converting said optical signal into an electrical signal, and in that said optical filter means connect to a first output of second optical splitter means, to an input of which second means connect for generating an optical signal, which first and second optical splitter means are arranged for connecting same using a multi-mode optical fibre,

14. Arrangement according to claim 13, characterized in that said first means for generating an optical signal and said second means for generating an optical signal are arranged to operate at different wavelength ranges.

15. Arrangement according to claim 14, characterized in that said second means for generating an optical signal comprise a second laser diode, an electrical input of which connects to a mixer circuit for converting said received radio signal to baseband.

16. Arrangement according to claim 15, characterized in that said power amplifier, said antenna means and an input of said mixer circuit connect to a circulator, a second input of said mixer circuit connects to said power amplifier and in that an output of said mixer circuit connects for driving said second laser diode through an amplifier circuit and a band-pass filter.

17. Arrangement according to claim 14, 15 or 16, characterized in that said second means for converting said optical signal into an electrical signal comprise a second photodiode and a band-pass filter and an amplifier circuit connected to an electrical output of said second photo diode.

18. Radio access unit, comprising a least one of said means according to cl ims 4 - 17, for converting a received optical signal into a radio signal .

19. Radio access unit according to claim 18, further comprising at least one of said means according to claims 13 - 17, for converting a received radio signal to baseband and for converting same into an optical signal .

20. Head end station, comprising at least one of said means according to claims 4 - 17, for converting said optical signal into a radio signal .

21. Head end station, according to claim 20, further comprising at least one of said means according to claims 13 - 17, for converting said optically by transferred baseband signal into a radio signal.