WO2008119958A1

WO2008119958A1 - Apparatus and method relating to time-delay systems for phased-array antennas

Info

Publication number: WO2008119958A1
Application number: PCT/GB2008/001076
Authority: WO
Inventors: Kamal Alameh; Khalid Al-Begain
Original assignee: Ugcs (University Of Glamorgan Commercial Services) Ltd
Priority date: 2007-03-30
Filing date: 2008-03-28
Publication date: 2008-10-09
Also published as: GB2462027A; GB2462027B; GB0919123D0; GB0706307D0

Abstract

A time-delay system (10) for a phased-array antenna receiver comprises: a plurality of planar delay elements (50a-z), each arranged to be connected to a different antenna element (30a- z) of the phased-array antenna and each arranged to provide an electrical output signal, and a combiner (70) arranged to receive the electrical output signals from each of the plurality of planar delay elements (50a-z) and to produce a composite output signal. Each planar delay element (50a-z) comprises: an input for receiving an electrical input signal from the antenna element (30a-z) to which the planar delay element (50a-z) is arranged to be connected, a laser (100) for producing a light beam; a modulator arranged to modulate the laser' s light beam according to the electrical input signal received from said antenna element (30a-z), and a planar waveguide (110) on a first substrate (140). The waveguide (110) is arranged to guide the laser's light beam. The waveguide (110) includes a plurality of optical taps (158) for transmitting from the waveguide (110) a fraction of the guided light beam. The taps (158) are spaced along the waveguide (110), so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap (158). Each planar delay element (50a-z) also comprises: a plurality of photodetectors (168), each photodetector (168) being arranged to detect light transmitted by a one of the taps (158) and to generate an electrical photodetector output signal in response to the detected light; and a combiner for combining the photodetector output signals to form the electrical output signal of the planar delay element (50a-z).

Description

Apparatus and Method Relating to Time-delay Systems for

Phased-array Antennas

Field of the Invention

This invention relates to the field of time-delay systems for phased-array antennas. More particularly, although not exclusively, the present invention relates to time-delay systems for broadband phased-array antennas. Even more particularly, although not exclusively, the present invention relates to photonics-based signal processors for providing adaptive beamforming with beam-steering capabilities.

Background Art

Antenna arrays generally outperform conventional, single antennas because the shape of their spatial output can be controlled to a greater extent, which provides enhanced directionality; for example, the output can be controlled to form a beam. That offers potential benefits for many kinds of devices that use antennas, including mobile phones, radio telescopes, and various microwave devices. For example, broadband smart antennas (that is, antennas in which the direction of the generated beam is changed dynamically, in response to changes in local conditions) are expected to enable a new generation of cellular communications equipment to better access the internet and download larger amounts of data than is possible with earlier antenna designs. Smart, beam-steering antennas can synthesise a directed beam that is adapted to achieve maximum signal-to-noise ratio. Dynamic adaptation of beam direction enables the smart antenna to follow a moving transmitter in real time. In computer networking applications , there is the potential for Gbit/sec connection speeds and potential bandwidths are much wider than in existing wireless LANs. Moreover, the directionality offered by these techniques offers improved privacy over conventional broadcast systems.

The directivity pattern generated from an antenna aperture is at least approximately related by a Fourier transform to the electric or magnetic field distributions in the aperture. In antenna arrays, the field distributions are controlled by providing a plurality of antenna elements, which together make up the antenna aperture. Varying the field in each element varies the overall field patterns within the antenna aperture. The overall field patterns are therefore determined by the amplitude and phase of the field pattern of each element, and in particular by the relative amplitudes and relative phases between the elements.

An antenna beam points in a direction normal to its phase front, and so varying the phase of the elements varies the direction of the beam. For beam steering (Fig. 9), one needs to generate a linear delay profile that equalises the delay profile ti.-.tu received by the antenna elements 930. The desired delay increment t_n-t(_n-i) depends on the desired direction of the beam 900 and hence of the wavefront 940, the spacing between the antenna elements 930, and the frequency of the RF signal. By changing the delay increment between the antenna elements 930 one can form beams at different directions . In the device of Fig. 9, the delays are achieved by converting the RF signal into an optical signal in electrical-optical converters 910, providing the delay in optical fibre delay line 920, converting the optical signal back into an electronic signal in optical-to-electrical converter 960, and then selecting the desired delay by switching on the appropriate one of variable- gain amplifiers 970, the different potential signals being combined in combiner 970.

If the transmitted signal is narrowband, electronic phase shifters can be used for beam forming. However, phase shifters provide frequency-dependent time delays, which restricts their use to narrowband applications. For example, if an RF carrier wave sin(wt) (where w is angular frequency, i.e. 2πf, and t is time) is shifted by an amount φ in phase, it becomes sin(wt-φ) = sin[w(t- φ/w) ] = sin [w (t-to) ] ; i.e., it is delayed by an amount to = φ/w. The time delay to is dependent on the frequency w; hence, if the RF signal is broadband, phase shifting causes signal distortion.

Thus, relying on simple phase variation to achieve a desired delay is problematic for wide-band signals because the correct delay will only be achieved at a single wavelength, causing problems including a narrowing of the available bandwidth and "squint" (in which different frequency components of the antenna output point in different directions) . Those problems can be ameliorated by using "true time delay" techniques, in which group delays rather than phase delays are used to control the antenna elements.

In order to synthesise a beam (i.e. a main signal lobe) of very wideband RF signals in an arbitrary, desired direction, variable "true-time" delay is required. If true time delay is used, the delayed signal becomes sin[w(t-to)] where t₀ is a constant independent of w. True-time delay therefore results in a phase shift w*t₀ that is proportional to the frequency, thus making the beamformer broadband.

Thus, an RF signal given by f(t) becomes f (t-to) after true-time delaying, i.e. an undistorted replica of f (t) but shifted in time. In contrast, as mentioned above, using electronic phase-shifting results in an RF signal represented as f(t) not becoming f(t-t₀), where t₀ is constant; rather, f (t) will be distorted and the distortion becomes significant if f(t) is broadband.

Analogue-to-digital converters generally offer insufficient bandwidth for digital signal processing to be used to provide the delays. Conventional RF signal processing is also limited by loss in coaxial cables at RF frequencies.

Carrying out photonic, rather than electronic, signal processing also allows wideband RF signals to be delayed with negligible loss. Use of metallic waveguides results in a very large loss that is proportional to the frequency (due to the well-known skin effect) and the length of the metallic medium, whereas optical media can have nearly negligible optical loss. More generally, photons do not lose energy as quickly as electrons, and can pass near one another without generating crosstalk. Future beam-steering smart antennas can be expected to comprise many antenna elements, for example more than four; each is required to generate and combine many delayed versions of the RF signal. In order to achieve that, there is a need for a means to cost-effectively generate several delayed versions of the RF signals arriving at each antenna element. Use of photonics is desirable for broadband signal processing and use of microelectronics is cost- effective for reconfigurable processing. It has been recognised that hybrid electronic/photonic RF signal processing has the potential to overcome the existing electronic bottlenecks for processing high-bandwidth signals. Various systems for providing suitable time delays have been proposed. One group of systems utilises bulk optics, i.e. discrete optical elements such as lenses and beamsplitters. Another group utilises optical fibres.

For example, US 4,271,413 (US Army) describes an antenna controller including a mask for inserting deep nulls into a far-field antenna pattern. The arrangement described uses a bulk optical system, and involves inserting a filter including the nulls at a Fourier plane of the optical system.

That arrangement is of rather limited application. It is bulky and is essentially an analog, rather than digital, approach .

US 5,461,687 (TRW Inc) describes a true-time-delay generator that incorporates a dispersive optical element to send different optical frequencies along paths of different lengths, in order to generate a desired group delay. In one embodiment, a dispersive element is used to reflect different wavelengths of the input beam at different angles. In another embodiment, an optical fibre is provided with a plurality of gratings that are reflective for different wavelengths. The use of discrete dispersive elements or fibres again makes that a bulky arrangement. The passive approach described in the document does not address the need for agile switching in smart antennas .

US 5,623,360 (Essex Corp) describes a time-delay generator for use in antenna beam-steering applications. The generator utilises an interferometer and bulk optics with acousto-optic Bragg cells or optical fibres including a resonant cavity.

Again, that arrangement is bulky, relying on an interferometer or optical fibres.

WO01/97326 (US Navy) describes a wideband array antenna beamformer using cascaded, chirped optical-fibre gratings in a distributed architecture. The optical signal is distributed to each feed of the array. The signal in each feed traverses a multiport optical circulator and is reflected off a number of the gratings, the number being proportional to their position in the array. The signal then passes back through the circulator and is fed to the appropriate antenna element. The use of optical circulators makes that arrangement bulky and expensive.

An attempt to provide free-space (that is, unguided) propagation of the signal light beam within a device that is more compact than devices based on bulk optics is described in WO 2004/092809 (Edith Cowan University) . That document describes a filter system in which a plurality of signal light beams are propagated and delayed inside a multi-cavity optical substrate. Diftractive optical elements steer the light beams within the cavities. Samples of the light beams from the multi-cavity optical substrate are detected by a photoreceiver array. The photoreceiver array generates output electrical signals corresponding to the delayed samples of the light beams . That approach offers compact , digital control of the light signals, suitable for smart beam-steering applications. However, it has been found that it is difficult to achieve accurate steering by the diffractive optical elements of the light beams within the cavities . Another general approach has been to waveguide the signal light beams, but in one or more compact silica waveguides, rather than in relatively bulky optical fibres.

For example, EP 0890854 (TRW Inc) describes a true-time- delay system for use with a phased-array antenna system. The system is integrated on a planar silica waveguide. The system utilises a polarisation-sensitive beamsplitter or Mach-Zender interferometer, together with a spiral waveguide to provide the optical delays. Bragg reflectors are written into the spiral waveguide to reflect optical signals of different frequencies by different amounts.

That approach is rather complex.

US 6,114,994 (USAF) describes a photonic time-delay beam- steering system in which an array of progressively spaced reflective Bragg gratings form a grating prism. The gratings are provided on optical fibres or on waveguides formed on a semiconductor substrate. An optical signal is introduced simultaneously into each fibre or waveguide, and a series of reflected waveforms are produced having time delays proportional to the optical wavelength of the signal. The reflected signals are combined and used to control the antenna elements of a phased array RF antenna system.

That approach requires many optical sources of different wavelengths in order to achieve a desired delay profile; that limits the system/ s reliability, because the failure of a single laser source (whether complete failure or a drift in performance) can be expected to lead to malfunction of the delay unit . WO93/11579 (Hughes Aircraft Company) describes several phased-array antenna beam steerers in which all of the operative elements, including lasers and modulators, delay- line waveguides, switches, photodetectors and electronics are monolithically integrated on a single substrate. The time delay network for each antenna element or group of antenna elements is provided as a single cascaded multi-stage network (that is, there is a serial arrangement of delay stages) . Silicon, GaAs and InP are suggested as alternative possible substrate materials for the monolithic integration. The apparatus described in this document is not practical, because it is very difficult to integrate the laser diodes (typically based on III-V semiconductors, such as InGaAsP) within the waveguides, if the waveguides are to be made of a material having a low optical loss, such as silica. If, on the other hand, a III-V semiconductor material is used as the waveguide material, losses will be high, and so the delays that the device can provide will be limited by those losses. Furthermore, insertion of the photodetectors within the silica waveguide is not straightforward, as the alignment of a plurality of different photodetectors is difficult, and likely to result in an expensive device. Moreover, use of different photodetectors of different lengths requires very precise control during manufacture. That tends to result in a lengthy fabrication process with a low yield, which is undesirable .

The present invention seeks to ameliorate at least some of the abovementioned problems.

Disclosure of the Invention

In a first aspect, the invention provides a time-delay system for a phased-array antenna receiver, the system comprising: a plurality of planar delay elements, each arranged to be connected to a different antenna element of the phased-array antenna and each arranged to provide an electrical output signal, a combiner arranged to receive the electrical output signals from each of the plurality of planar delay elements and to produce a composite output signal; each planar delay element comprising: an input for receiving an electrical input signal from the antenna element to which the planar delay element is arranged to be connected; a laser for producing a light beam; a modulator arranged to modulate the laser's light beam according to the electrical input signal received from said antenna element; a planar waveguide on a first substrate, the waveguide being arranged to guide the laser's light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate an electrical photodetector output signal in response to the detected light; and a combiner for combining the photodetector output signals to form the electrical output signal of the planar delay element.

Of course, the time-delay system may readily be adapted to operate as a transmitter. Thus, a second aspect of the invention provides a time-delay system for a phased-array antenna transmitter, the system comprising: a plurality of planar delay elements, each arranged to provide an electrical output signal to a different antenna element of the phased-array antenna; and a splitter arranged to receive an electrical input signal for transmission and to provide the input signal to each of the plurality of planar delay elements; each planar delay element comprising: an input for receiving the electrical input signal from the splitter; a laser for producing a light beam; a modulator arranged to modulate the laser's light beam according to the electrical input signal received from the splitter; a planar waveguide on a first substrate, the waveguide being arranged to guide the laser's light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate the electrical output signal in response to the detected light; and an output for providing the electrical output signal to the antenna element to which the planar delay element is arranged to be connected. A third aspect of the invention provides a planar delay element for a phased-array antenna, the element comprising: a laser for producing a light beam; an input for receiving an electrical input signal; a modulator arranged to modulate the laser's light beam according to the received electrical input signal; a planar waveguide on a first substrate, the waveguide being arranged to guide the laser' s light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate an electrical output signal in response to the detected light; and an output for outputting the electrical output 'signal.

The electrical input signal and the electrical output signal may for example be radio-frequency signals or microwave-frequency signals.

The laser may be a semiconductor laser. The laser may be a III-V semiconductor laser, for example a GaAs laser, a AlGaAs laser, a GaInAs, a InGaAsP laser, or an InP laser. The laser may be a microchip laser. The laser may be a vertical- cavity surface-emitting laser (VCSEL) . The laser may be an extended-cavity VCSEL (i.e. a vertical-extended-cavity surface-emitting laser, a VECSEL) . We do not believe that it is practical for the laser to be formed monolithically with the waveguides on the first substrate. The laser may be a surface-mounted laser. The laser may be surface-mounted on the first substrate. Alternatively, the laser may be on a second, different substrate.

The laser may include one or more p- and n-contacts on a single surface; thus, those co-sided contacts may be bonded to contact pads of a programmable logic controller (PLC) or a laser driver. Such an arrangement is especially relevant when the laser is surface mounted, as the contacts may then be on a surface opposite to the mounting surface, so that both are accessible for bonding.

The second substrate may be a III-V semiconductor, for example GaAs, AlGaAs, GaInAs, InGaAsP or InP. As is usual with semiconductor lasers, the laser may be driven by a driver. The modulator may be the laser driver; thus the modulator may modulate the laser's output beam by modulating an electrical supply of the laser in accordance with variations in the electrical input signal . The modulator may be an electronic driver used to drive the laser with a bias (DC) current and a radio-frequency current (such drivers are commercially available and may be adapted to provide modulation at a frequency of interest) . The modulator may be an electro-optic modulator, arranged to modulate the laser's output beam by causing the beam to experience an optical loss that varies with the electrical input signal. The modulator may be formed on the second substrate. The modulator may be integrated with the laser. The planar delay elements may further each comprise a lens or other focusing element arranged to focus the output beam of the laser into an input of the waveguide.

The first substrate may be silicon. The first substrate may be an insulator. The waveguide may be silica. The waveguide may be silicon.

The waveguide may be a solid. The waveguide may be surrounded by a homogenous cladding. The waveguide may guide by total internal reflection at the boundary between the waveguide and the cladding.

The waveguide of each of the plurality of planar delay elements may be a different length from the waveguide of any other of the plurality of planar delay elements. The length of a shortest of the waveguides may correspond to the highest- frequency band of the input signal and a length of a longest of the waveguides may correspond to a lowest-frequency band of the input signal.

The waveguide may be an elongate waveguide that changes direction at a plurality of points along its length. The waveguide may comprise a plurality of segments , which may be straight segments, each joined end-to-end, the direction of alternate segments reversing; the waveguide may thus have a "zig-zag" shape. The length of each segment having the same direction may be the same, so that parallel segments have the same length. The length of all of the segments may be the same. The optical taps may be provided at the points at which the waveguide's direction changes. The optical taps may transmit the fraction of the guided beam in a direction lying in the plane of the planar waveguides; thus the transmitted light may for example be transmitted from or towards an edge of the planar waveguide.

The optical taps may transmit from the waveguide more than 0.1%, more than 0.5%, more than 1%, more than 2%, more than 5% or more than 10% of the guided light beam's power when it is incident on the tap. The optical taps may transmit from the waveguide less than 20%, less than 10%, less than 5%, less than 2%, less than 1% or less than 0.5% of the guided light beam's power when it is incident on the tap. Thus, for example, between 2% and 10% (for example 5%) of the incident power may be coupled out of the waveguide. The optical taps may be gratings, which may be written onto the waveguide.

The photodetectors may be wide-band photodetectors with transimpedance amplification. The bandwidth may for example be more than 0.5 GHz, or more than 1 GHz for 5.6 GHz operating frequency.

The photodetectors may be surface-mounted on the first substrate. The photodetectors may be on the second substrate. Thus, the laser and the photodetectors may be on the same substrate. The photodetectors may be photodiodes, which may be fabricated using Ge, or InGaAs. The photodetectors may be at the edge of the first or second substrate. The photodetectors may be on the third substrate. The waveguide may be static. Preferably, the planar delay element does not include moving parts. The presence of moving parts is expected to make it difficult to align optical components (e.g. waveguides) without introducing a significant optical loss. The waveguide's configuration may be fixed. It may be that the waveguide includes no moveable parts.

The planar delay element may further comprise an optical equaliser for compensating different losses associated with different ones of the delayed optical beams. For example, that may be achieved by using matching waveguides that attenuate one or more of the guided optical signals, or Bragg gratings may be designed with different reflectivities in order to equalise the optical power detected by the photodiode elements . All of the photodetectors may be identical, or substantially identical.

Each photodetector may comprise an amplifier for providing gain to the planar delay element's output signal. The amplifier may provide transimpedance amplification. Each amplifier may be associated with a switch. The switch may be a binary switch (i.e. the amplifier's gain may be OFF or ON) or it may be switchable to provide more than two gain levels. The gain of different photodetectors may be set to a different level to compensate for different losses experienced by the optical signal at different points in the waveguide. The system may further comprise a microprocessor arranged to switch the switches to selectively alter the gain of the amplifier associated with the switch. Thus, by switching the switches the microprocessor may control the relative levels of the photodetector output signals on each planar delay element. For example, the microprocessor may select one of the photodetector output signals to be ON, and the others to be OFF, on a first of the planar delay elements. The planar delay element's electrical output signal will then be a copy of the electrical input signal, delayed by an amount dependent on the selected photodetector' s position in the waveguide. A delay of a different amount may be obtained by selecting a different one of the photodiode output signals to be ON, or by selecting a photodiode output signal from a different one of the planar delay elements (which may have a waveguide of a different length) to be ON. By selectively switching the photodetector output signals on each planar delay element, the microprocessor can dynamically control the response of the antenna, according for example to changing environmental conditions or a predetermined scheme.

In practice, the same physical element may be used to perform the reverse function in the transmitter to that which it perforins in the transmitter; thus, for example, the splitter in the transmitter may be the combiner in the receiver and/or inputs in the receiver may be outputs in the transmitter (and vice versa) . Thus the time delay system may be configured to be switched from a receive configuration to a transmit configuration. It will be appreciated that features of the system described above in respect of the receive configuration may be employed, directly or with reversed function as appropriate, in the transmit configuration. A fourth aspect of the invention provides a method of operating a phased-array antenna system, the method comprising: connecting each of a plurality of planar delay elements, to a different antenna element of the phased-array antenna; and receiving electrical output signals from each of the plurality of planar delay elements and combining them to produce a composite output signal; wherein, the method further comprises, within each planar delay element: receiving an electrical input signal from the antenna element to which the planar delay element is connected; producing a light beam from a laser; modulating the light beam according to the electrical input signal received from said antenna element; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating electrical output signals in response to the detected light. It may be that light at a single carrier wavelength is used in the method; and combining the generated electrical output signals from each tap to form the electrical output signal of the planar delay element.

Similarly, a fifth aspect of the invention provides a method of operating a phased-array antenna system, the method comprising: connecting each of a plurality of planar delay elements, to a different antenna element of the phased-array antenna; and receiving an electrical input signal for transmission and providing the input signal to each of the plurality of planar delay elements; wherein, the method further comprises, within each planar delay element: receiving the electrical input signal for transmission; producing a light beam from a laser; modulating the light beam according to the received electrical input signal; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating an electrical output signal in response to the detected light; and providing the electrical output signal to the antenna element to which the planar delay element is connected.

A sixth aspect of . the invention provides a method of operating a phased-array antenna system, the method comprising receiving an electrical input signal and providing the input signal to each of a plurality of planar delay elements; wherein, the method further comprises, within each planar delay element: receiving the electrical input signal; producing a light beam from a laser; modulating the light beam according to the received electrical input signal; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating an electrical output signal in response to the detected light.

It may be that light at a single carrier wavelength is used in the method. It will also be appreciated that features of the present invention described in relation to the methods of the present invention are equally applicable to the apparatuses of the present invention and vice versa.

Brief Description of the Drawings

Certain illustrative embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying schematic drawings, in which:

Fig. 1 is an example of a time-delay system according to an example embodiment of the invention; Fig. 2 is a plan view of one planar delay element from the system of Fig. 1;

Fig. 3 shows steps in a basic waveguide delay-unit fabrication process; Fig. 4 is a block diagram of a photodiode amplifier used in the system of Fig. 1;

Fig. 5 is a circuit diagram of a transimpedance amplifier and post-amplifier circuits used in the system of Fig. 1;

Fig. 6 is a circuit diagram of a circuit used to select photodetectors in the system of Fig. 1;

Fig. 7 is a circuit diagram of an active RF combiner sued in the system of Fig. 1;

Fig. 8 is a schematic diagram showing how the system of Fig. 1 can be switched between a transmit mode and a receive mode/ and

Fig. 9 is a schematic illustration of the principle of photonic beam-forming.

Detailed Description

The directional characteristics of an N-element phased antenna array is given by:

where :

x = expykdsin(#)) d is the antenna element spacing, k = wave number = ω/c, and X_n= x(θ_n) = zero of D_N corresponding to a null at angular coordinate θ_n. Note that a change of even one zero affects all the weights, W_m. Note also that an N-element smart antenna can synthesise only (N-I) zeros, as evident from Equation (1) .

As an example, for a 4-element antenna, and nulls located along angular coordinates, θi, θ₂, and θ₃, we have :

D₄ (θ) = x³ - x² (e^Jωτ» + e^Jωτ» + e^Jωτ" )+ x(e^jωτ" + e^jωτ« + e^jωτ" )- e^Jωτ°' _{( 3 }}

where τi_j are function of the null directions. These 2^4-1-l = 7 Tij delays can be selected from many delays available from the delay system. Negative coefficients are obtained by using an inverting low-noise RF amplifier at the even-numbered antenna elements. The gain of an amplifier integrated to a photoreceiver element can independently be adjusted to equalize the amplitude of the delayed RF signal detected by that photoreceiver. Note that, in a narrow-band application, a phase shifter can be used to obtain a typical coefficient, W_n. However, the adaptive steering of broadband nulls is much more difficult than that of narrowband nulls. Generally, for an N-element broadband smart antenna, the synthesis of (N-I) zeros is feasible if the antenna beamformer can adaptively generate and combine (2¹^-I) delayed versions of the RF signals received by the antenna elements.

MicroPhotonic structures that integrate a laser array, a 2S ultra-wideband photoreceiver array and a multi-waveguide planar optical circuit can potentially achieve substantially arbitrary, high-resolution RF true-time delay. By detecting delayed samples of RF-modulated laser beams propagating inside optical waveguides of different lengths, many delayed version of the RF signal are generated. By adding these delayed RF signals, a broadband adaptive antenna directivity pattern can be realised. In a simple approach, the waveguides can be designed to generate fixed true-time delay profiles, while the amplifiers of the photoreceiver array can be reconfigured to adjust the amplitudes of the delayed samples. In an example of a system 10 according to an embodiment of the invention (Fig. 1), an RF signal 20 is detected by an antenna array is made up from a plurality of antenna elements 30a-z (typically, there will be 4 to 8 elements) . (The antenna array may be, for example, a telescope, mobile or radar antenna.) Each antenna element 30a-z produces an electrical signal which is amplified in a low-noise amplifier 40a-z associated with the antenna element 30a-z, to compensate for subsequent reduction in signal levels due to splitting and to boost the subsequent modulation efficiency. The signal is then passed into a planar delay element 50a-z associated with the relevant low-noise amplifier. In each planar delay element 50a-z, the input RF signal is converted to the optical domain, producing one or more RF modulated light beams. The RF modulated beams are then processed in the optical domain, before being converted back into RF electrical signals. Each planar delay element 50a-z is arranged to produce an RF electrical output signal along an associated one of intermediate co-axial cables 60a-z. The coaxial cables βOa-z are connected to a combiner 70, which combines the signals from each planar delay element into a composite RF electrical output signal, on output coaxial cable 80.

Within each planar delay element (e.g. element 50b, Fig. 2) , the signal from the low noise amplifier 40b passes to a laser driver (not shown) arranged to modulate the electrical supply of an AlGas VECSEL laser 100. The laser 100 outputs a light beam which, by virtue of the modulation of the laser's electrical supply, carries the RF signal as a modulation. The laser 100 is connected to a waveguide 110, so that the modulated light beam from laser 100 is guided within the waveguide 110. Waveguide 110 is a silica waveguide formed on a silicon substrate 140. The waveguide 110 has a "zig-zag" shape, being formed from a plurality of straight sections 120, 130, which alternate in direction, such that sections 120 are parallel to each other and sections 130 are parallel to each other, and each section 120 is joined at one of its ends to a different one of sections 130. Thus the modulated light from laser 100 passes into the first of sections 130, propagates in that section to the far side of substrate 140 where it is reflected into the first of sections 120, in which it propagates until it reaches the end of that section and is reflected into the second of sections 110.

The reflections at the edge of substrate 140 furthest from laser 100 (i.e. the first reflection, third reflection, fifth reflection and so on, at the "odd-numbered" corners of the waveguide 110) take place from Bragg gratings 151-158 formed in the waveguide there. (The reflections at the opposite, "even-numbered" corners are simple reflections from the surface of the waveguide 110.)

Every time the beam hits a grating 151-158, a small fraction of the power of the beam is transmitted for detection and amplification, while the remaining large fraction is reflected and routed for subsequent delayed photodetection . The Bragg gratings 151-158 are arranged in this example to transmit approximately 5% of the light that is incident on them. Thus, at each grating 151-158 a copy of the modulated light from laser 100 is coupled out of the waveguide. However, the copy coupled out at each successive Bragg reflector 151-158 is delayed relative to the copy coupled out at its predecessor, upstream in the waveguide 110. The delay between each copy is dependent on the length of waveguide between each Bragg reflector 151-158. In this example, that length is constant along the waveguide, and so the copy coupled out at Bragg reflector 158 will be delayed by seven delay "units" compared with that coupled out at reflector 151. So that all photodiodes 161-168 detect delayed RF signals having approximately equal amplitudes, all of the Bragg grating reflectors 151-158 are designed to have equal Bragg wavelengths but with slightly different reflectivities, so that they reflect an appropriate portion of the incoming signal and transmit the rest for photodetection; thus RF signal equalisation is achieved through varying the Bragg grating reflectivities.

The light coupled out at each detector 151-158 is directed onto a photodetector in the form of a photodiode 161- 168. The photodiodes 161-168 are surface mounted on the waveguide substrate using precision (i.e. sub-micron) alignment to ensure that optical coupling loss is minimal. In contrast to the system described in the Hughes Aircraft document discussed above, our approach is based on the attachment of a photoreceiver array having a spacing that matches the spacing of the waveguide ends, which can be achieved with commercially available flip-chip bonding technologies. The photodiode 161-168 generate an output electrical signal containing the RF signal that was modulated on the laser beam. The weights of the current impulses generated by the photodiodes 161-168 associated with the waveguide 110 can be changed by adjusting the gains of the photodiode 161-168. By equalising the amplitudes of the photodetected currents, and activating a single photodiode 161-168 at a time, a variable true-time delay unit is realised. By dedicating a delay unit to each antenna element, a beamformer is realised, which arbitrarily delays the various antenna signals to realise a directivity in an arbitrary direction. The gain of each photodiode 161-168 is switchable on or off, so that any one of the photodiodes 161-168 can be selected, by switching its gain on and the gain of the other photodiodes off. The switching is controlled by a microprocessor (not shown) , in order to provide a desired directionality or other property to the beam. The outputs from each photodiode 161-168 are multiplexed to form the electrical output signal of the planar delay element 20b on intermediate co-axial cable 60b. Waveguides 110 are of different lengths in each planar delay element 50a-z, so that each element 50a-z provides a different set of delays.

Thus, at each Bragg grating 151-158, a portion of the RF- modulated optical carrier is tapped and detected by a photodiode 161-168 which generates an RF signal whose delay is proportional to the length between the laser 100 and that Bragg grating. As a result, the detected photocurrents will be replicas of the RF signal but with different delays. By switching the photodiode gain between low and high gains, a single RF signal, with the appropriate delay, is generated at the output of the delay unit 50b. By using different delay units 50a-z, one can generate differently delayed signals for the antenna element 30a-z. For each delay unit (or antenna element) , a delay is generated by switching the appropriate photoreceiver element of that delay unit.

This approach allows the integration of the laser 100 and photodetectors 161-168 on one chip, to which the Si substrate of the waveguide cavity can be attached.

Manufacture of lasers and photodiodes is a well-known process, typically using several wafer-level processes such as photolithography, etching, and deposition.

An example of a process for fabricating the waveguide 110 is shown in Fig. 3. First, a layer 210 of silica (SiO₂) particles is deposited onto a silicon substrate 200using flame hydrolysis deposition. Next, a layer 220 of silica-germania (Siθ2~ GeO₂) particles is deposited on top of the silica layer 200, using the same technique. The particles are consolidated to form solid layers 210', 220'. The silica layer 210' forms an under-cladding layer, and the silica-germania layer 220' forms a core layer. Reactive ion etching is used to etch the cores 230 of the waveguides 110 out of the core layer 210' . A further layer of silica particles is then deposited by flame hydrolysis deposition and consolidated, to form an over-cladding layer 240' . Thus, each waveguide 110 is formed from a core 230 of silica-germania (which has a relatively high refractive index) encased within a cladding 210', 240' of silica (which has a relatively low refractive index) ; light is thus confined to the waveguides 110 by total internal reflection at the core-cladding interface.

After the true-time delay generation inside the waveguide 110, the light signal has to be converted back to the electrical domain. The photodiodes 161-168 transform the light intensity to a proportional current, which is then amplified and converted to a voltage for further signal processing. That current-to-voltage conversion is performed by a transimpedance amplifier. Further details of the circuitry used is described below (of course, the skilled person will readily be able to design suitable alternative circuitry) .

Figure 4 shows a block diagram of the photodiode amplifier 300. It consists of two stages: the first stage 310 is the transimpedance stage, and the second 320 is the post- amplifier stage. The amplifier topology selected is a differential amplifier, because it is suitable for CMOS implementation, especially when using a low-level input signal under low-supply voltage in submicron CMOS. Single-ended topology is not generally considered suitable because it is very susceptible to supply noise and plagued by stability problems stemming from parasitic feedback paths, despite the advantages of high-gain, high-bandwidth, and low-power consumption. When there are other high-speed circuits present on the same substrate, large substrate cross-talk noise may significantly deteriorate the performance of the whole system. (Differential receiver designs attempt to remove noise from their inputs by using parallel signal paths, with a 180-degree phase difference, so that the same noises are picked up, subtracted, and thus cancelled during the amplification process . )

Figure 5 shows the circuit implementation of the transimpedance amplifier 400 and the post-amplifier circuits 410. In the transimpedance stage, PMOS transistors operating in the triode region are used as loads (M2 & M4) . Those transistor loads allow the output signal to swing up to Vdd because the gate-drain voltage (VGD) of the input transistor is almost zero, and transconductance could be maximised with the least compromise of the feedback resistance. The differential topology also helps in enhancing the overall noise performance by providing a better common-mode rejection ratio. The post-amplifier stage employs the Cherry-Hooper topology with fT doubler scheme. The Cherry-Hooper stage provides the required gain, and the fT doubler serves as an output buffer which halves the input capacitance while maintaining the same overall transconductance.

Since there are many photodiode amplifiers in smart antenna circuits, the current source is turned on and off, so that only a selected cell is driven at a time. That reduces the power consumed by the system, enabling the photoreceiver array to be in ON-OFF mode and for the control system to adaptively select the cell needed for beamsteering weight generation as illustrated in Figure 6. RF signal combiners are required to combine the output RF signals from the photodiode amplifiers. Low loss, compact size, system compatibility and good isolation between signal branches are desirable for RF combiners. Several RF signal combining techniques are known, for example, Resistive Adders, Wilkinson Combiners and Active RF Signal Adders. Resistive adders isolate different branches by attenuating each input signal with an isolation equals to twice the value of the insertion loss for each path. A resistive adder is a broadband topology, but its power loss is quite high.

Wilkinson combiners provide a relatively low loss, but are very sensitive to the lengths of the interconnections between different antenna paths, and create routing issues if many RF signals are combined. On the other hand, active RF combining has the ability to add gain to the output. Figure 7 illustrates an active RF combiner designed for the smart antenna. This circuit has the capability of controlling the bias current to adjust the gain for weight generation needed for beamsteering . The control signal comes from an adaptive/control system. By tuning the voltage of the bias terminals, the amplitude of each RF signal from every photodiode amplifier can be adjusted so that any non-uniformity of the signal strength from different photodiodes 161-168 can be compensated for, and the signals to be combined for beamsteering can be selected.

In this example embodiment, the photodetectors 161-168 are formed on the opposite side of substrate 140 from the laser 100. In an alternative embodiment, the photodetectors are formed on the same side as the laser, and are formed monolithically with the laser, on a single substrate. In that case, the Bragg reflectors are formed on the even-numbered corners of the waveguide, rather than the odd-numbered corners . The planar delay elements 50a-z may also be used in a transmit configuration, as illustrated by way of example in Fig. 8, in respect of element 50b. The element 50b is switched from receive to transmit mode by using two switches 810,820. The first switch 810 switches the input to the laser driver, from the antenna element 30b to a source 830 of the RF signal to be transmitted. The second switch 820 switches the output from the planar element 50b, from the combiner 70 to the antenna element 30b. Thus, in receive mode the electrical signal received by the antenna element 30b is fed into the delay element 50b via the laser 100 (which converts the electrical signal into an optical signal) , is output (having been converted back to an electrical signal) from the planar delay element 50b as a delayed signal, and the delayed signals from all of the planar delay elements 50a...z are combined in the combiner 70 to produce a composite received signal. In transmit mode, the signal to be transmitted is split in a splitter into multiple copies and one copy is fed to the laser driver of the laser 100 in each planar element 50a...z. The signal is converted into an optical signal in the planar delay element 50b, delayed therein, and then output from the planar delay element as a delayed electrical signal. The delayed electrical signal is directed to the antenna element 30b associated with the planar delay element 50b, where it is broadcast. The antenna elements 30a...z thus broadcast delayed versions of the signal, so that the composite broadcast signal has the desired properties (e.g. directionality).

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Some examples of such variations and alternatives have been described above. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Claims

30Claims

1. A time-delay system for a phased-array antenna receiver, the system comprising: a plurality of planar delay elements, each arranged to be connected to a different antenna element of the phased-array antenna and each arranged to provide an electrical output signal, a combiner arranged to receive the electrical output signals from each of the plurality of planar delay elements and to produce a composite output signal; each planar delay element comprising: an input for receiving an electrical input signal from the antenna element to which the planar delay element is arranged to be connected; a laser for producing a light beam; a modulator arranged to modulate the laser' s light beam according to the electrical input signal received from said antenna element; a planar waveguide on a first substrate, the waveguide being arranged to guide the laser' s light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate an electrical photodetector output signal in response to the detected light; and a combiner for combining the photodetector output signals to form the electrical output signal of the planar delay element . 31

2. A system as claimed in claim 1, in which the laser is a semiconductor laser.

3. A system as claimed in claim 2, in which the semiconductor laser is a VCSEL.

4. A system as claimed in any preceding claim, in which the laser is surface-mounted on the first substrate.

5. A system as claimed in any of claims 1 to 3, in which the laser is on a second, different substrate.

6. A system as claimed in claim 5, in which the second substrate is a III-V semiconductor.

7. A system as claimed in any preceding claim in which the modulator is a driver for the laser.

8. A system as claimed in any preceding claim, in which the planar delay elements further each comprise a lens or other focusing element arranged to focus the output beam of the laser into an input of the waveguide.

9. A system as claimed in any preceding claim, in which the first substrate is silicon.

10. A system as claimed in any preceding claim, in which the waveguide is silica or silicon.

11. A system as claimed in any preceding claim, in which the waveguide of each of the plurality of planar delay elements is a different length from the waveguide of any other of the plurality of planar delay elements.

12. A system as claimed in any preceding claim, in which the waveguide comprises a plurality of segments, each joined end- to-end, the direction of alternate segments reversing.

13. A system as claimed in any preceding claim, in which the optical taps are provided at points at which the waveguide's direction changes .

14. A system as claimed in any preceding claim, in which the optical taps transmit from the waveguide more than 0.1% of the guided light beam's power when it is incident on the tap. 32

15. A system as claimed in any preceding claim, in which the optical taps transmit from the waveguide less than 20% of the guided light beam's power when it is incident on the tap.

16. A system as claimed in any preceding claim, in which the optical taps are gratings .

17. A system as claimed in any preceding claim, in which the photodetectors are surface-mounted on the first substrate.

18. A system as claimed in any preceding claim, in which the photodetectors are on the second substrate.

19. A system as claimed in any preceding claim, in which each photodetector comprises an amplifier for providing gain to the planar delay element's output signal.

20. A system as claimed in claim 15, in which the gain of different photodetectors is set to a different level to compensate for different losses experienced by the optical signal at different points in the waveguide.

21. A system as claimed in claim 15 or claim 16, in which each amplifier is associated with a switch.

22. A system as claimed in claim 17, in which the switch is a binary switch.

23. A system as claimed in claim 17 or 18 further comprising a microprocessor arranged to switch the switches to selectively alter the gain of the amplifier associated with the switch.

24. A method of operating a phased-array antenna system, the method comprising: connecting each of a plurality of planar delay elements, to a different antenna element of the phased-array antenna; and receiving electrical output signals from each of the plurality of planar delay elements and combining them to produce a composite output signal; wherein, the method further comprises, within each planar delay element: 33

receiving an electrical input signal from the antenna element to which the planar delay element is connected; producing a light beam from a laser; modulating the light beam according to the electrical input signal received from said antenna element; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating electrical output signals in response to the detected light; and combining the generated electrical output signals from each tap to form the electrical output signal of the planar delay element.

25. A time-delay system for a phased-array antenna transmitter, the system comprising: a plurality of planar delay elements, each arranged to provide an electrical output signal to a different antenna element of the phased-array antenna; and a splitter arranged to receive an electrical input signal for transmission and to provide the input signal to each of the plurality of planar delay elements; each planar delay element comprising: an input for receiving the electrical input signal from the splitter; a laser for producing a light beam; a modulator arranged to modulate the laser's light beam according to the electrical input signal received from the splitter; 34

a planar waveguide on a first substrate, the waveguide being arranged to guide the laser' s light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate the electrical output signal in response to the detected light; and an output for providing the electrical output signal to the antenna element to which the planar delay element is arranged to be connected.

26. A method of operating a phased-array antenna system, the method comprising: connecting each of a plurality of planar delay elements, to a different antenna element of the phased-array antenna; and receiving an electrical input signal for transmission and providing the input signal to each of the plurality of planar delay elements; wherein, the method further comprises, within each planar delay element: receiving the electrical input signal for transmission; producing a light beam from a laser; modulating the light beam according to the received electrical input signal; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a 35

signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating an electrical output signal in response to the detected light; and providing the electrical output signal to the antenna element to which the planar delay element is connected.

27. A planar delay element for a phased-array antenna, the element comprising: a laser for producing a light beam; an input for receiving an electrical input signal; a modulator arranged to modulate the laser' s light beam according to the received electrical input signal; a planar waveguide on a first substrate, the waveguide being arranged to guide the laser's light beam, the waveguide including a plurality of optical taps for transmitting from the waveguide a fraction of the guided light beam, the taps being spaced along the waveguide, so that in use a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; a plurality of photodetectors, each photodetector being arranged to detect light transmitted by a one of the taps and to generate an electrical output signal in response to the detected light; and an output for outputting the electrical output signal.

28. A method of operating a phased-array antenna system, the method comprising receiving an electrical input signal and providing the input signal to each of a plurality of planar delay elements; wherein, the method further comprises, within each planar delay element: receiving the electrical input signal; producing a light beam from a laser; 36

modulating the light beam according to the received electrical input signal; guiding the light beam in a planar waveguide on a first substrate; tapping from the waveguide a fraction of the guided light beam, using optical taps spaced along the waveguide so that a signal carried on the transmitted light has been delayed by a different amount when it reaches each tap; detecting any light transmitted by each tap and generating an electrical output signal in response to the detected light.