WO2004092809A1

WO2004092809A1 - Microphotonic filter

Info

Publication number: WO2004092809A1
Application number: PCT/AU2004/000504
Authority: WO
Inventors: Kamran Eshraghian; Kamal Alameh
Original assignee: Edith Cowan University
Priority date: 2003-04-16
Filing date: 2004-04-16
Publication date: 2004-10-28
Also published as: AU2003901878A0

Abstract

A filter system (1) having a light beam generator (19) to generate a plurality of light beams which are then received by a multi-cavity optical substrate (22). Light beams (21) in the multi-cavity optical substrate (22) are propagated and delayed therein. Defractive optical elements (19) steer the light beams within the cavities of the multi-cavity optical substrate (22). Samples of the light beams from the multi-cavity optical substrate (22) are detected by a photoreceiver array (20). The photoreceiver array (20) generates output electrical signals corresponding to the delayed samples of the light beams (21).

Description

"Microphotonic Filter"

Field of the Invention

The present invention relates to optical (or photonic) processing of radio frequency (RF) signals, and more particularly, to photonic filtering of RF signals.

Background Art

Broadband receivers, such as those used in radio telescopes and microwave radars, typically operate in dense signal and noise environments. The function of a filtering system is to sort, classify and remove unwanted signals from signals of interest.

It has previously been proposed to electronically filter signals. However, it has been found that electronic filtering has a number of disadvantages including but not limited to phase distortion, non-linear effects and noise corruption. Therefore, other techniques such as photonic filtering of signals have been proposed.

Photonic signal processors based on amplified fibre Bragg grating cavities have been previously proposed for broadband transversal RF filters. However, these filter structures have two fundamental problems, namely: (i) they generate substantial phased-induced intensity noise caused by the long coherence length of laser sources; and (ii) their transfer characteristics cannot be arbitrarily reconfigured.

The use of multiple wavelength sources of variable power levels and separations has been proposed for generating reconfigurable filter characteristics. However, for high-resolution filter characteristics, this technique is impractical because it requires a large number of wavelengths of accurate separation and dynamically- apodized Bragg gratings for controlling the power levels of each wavelength source component. There is therefore a need for an adaptive filter system which is compact and compatible with other integrated optoelectronic systems.

Disclosure of the Invention

According to one aspect of the present invention, there is provided a filter system comprising:

beam generating means for generating a plurality of light beams;

an optical structure comprising a plurality of cavities for receiving said light beams, and propagating and delaying said light beams;

steering means for steering the plurality of light beams within the plurality of cavities; and

detecting means for detecting delayed samples of said light beams from said plurality of cavities of said optical structure and generating output electrical signals corresponding to the delayed samples of said light beams.

Preferably, each of said light beams is modulated by a spectral component of an input radio frequency (RF) signal spectrum

Preferably, the means for generating the plurality of RF-modulated light beams comprises a laser array. Preferably, the wavelength of the laser array is 850 nm. Preferably, the laser array is a vertical cavity surface emitting laser (VCSEL) array, a vertical external cavity surface emitting laser (VECSEL) array or a surface mounted laser array.

Preferably, the optical structure comprises a transparent substrate. Preferably, the transparent substrate is glass or sapphire.

Preferably, the plurality of cavities in the optical structure have different lengths defined by the pairs of spaced apart reflectors, the length of the shortest cavity corresponding to the high-frequency band of the input RF signal and the length of the longest cavity corresponding to the low-frequency band of the input RF signal such that, together, the plurality of cavities cover the total bandwidth of the input RF signal.

Preferably, the means for respectively steering the plurality of RF-modulated light beams comprises optics means. Preferably, the optics means comprise diffractive optical elements, macro lenses, or a microlens array. Preferably, the optics means collimates the plurality of RF-modulated light beams and steers the collimated light beams. Preferably, the optics means is integrated on the optical structure.

Preferably, at least one of the pair of reflectors in each cavity is at least partially transmissive. Preferably, the reflectors comprise a 100% reflectivity mirror and a 95% reflectivity partial reflector. Preferably, the partial reflector is provided by a coating on the optics means.

Preferably, the means for detecting and converting delayed samples of each light beam comprises a two-dimensional photoreceiver array. Preferably, the photoreceiver array is a wideband Si photoreceiver array with transimpedance amplification.

Preferably, the filter system further comprises means for converting the delayed samples of each light beam detected by the photoreceiver array into photocurrents and amplifying the photocurrents. Preferably, the filter system further comprises interface means to control the amplitudes of the amplified photocurrents. Preferably, the filter system further comprises means for selectively combining the amplified photocurrents by addition or subtraction.

Preferably, the optics means further comprises optical equalization means for maintaining the diameters of the beams falling on the photoreceiver array elements within a predetermined range.

Preferably, the filter system further comprises means for receiving, amplifying and splitting the input RF signal. ln a preferred embodiment, a VECSEL array and photoreceiver array are integrated into a single integrated circuit, and the optics means is integrated on the optical structure. This preferred embodiment provides a compact microphotonic adaptive RF filter which is compatible with other integrated optoelectronic systems.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a schematic diagram of a generic photonic filter system;

Figure 2 is a schematic perspective view of a filter system in accordance with a preferred embodiment of the invention;

Figure 3 is a schematic side view of the filter system shown in Figure 3;

Figure 4 is a schematic diagram of a diffractive optical element for use in a preferred embodiment of the invention;

Figure 5 are schematic side and top views of the optical structure in a preferred embodiment of the invention.

Best ode(s) for Carrying Out the Invention

Figure 1 illustrates the general principle of photonic RF signal processing. The antenna element 2, which may be a telescope, mobile, or radar antenna, receives the desired input RF signal 1 as well as unwanted interfering signals. These interfering signals, whose frequency content may be in the vicinity of the desired RF signal 1 , can make its detection extremely noisy. They may also occur at frequencies that change with time. The input RF signal 1 is converted to the optical domain via an electrical-to-optical conversion means 3 which produces one or many RF-modulated collimated optical beams 4. The RF-modulated beams are then processed directly in the optical domain via the photonic signal processor 5. Examples of processing functions performed by the photonic signal processor 5 include true-time delaying, attenuation, beam equalization, and optical routing. The optical receivers 7 convert the optically processed RF-modulated optical beams 6 into photocurrents and combine them to produce an output RF signal 8, whose frequency spectrum depends on the functions performed by the photonic signal processor.

The filter system of the present invention will now be described with particular reference to Figure 2.

Figure 2 shows a microphotonic RF filter system 1, in accordance with the preferred embodiment of the invention. An antenna element 2a receives the desired input RF signal (RFi_n) 2b as well as unwanted interfering signals that need to be filtered, as previously herein described with reference to Figure 1.

The filter system 1 comprises a VECSEL array 19 for generating a plurality of light beams, i.e. optical beams, a wideband photoreceiver array 20 and a multi- cavity optical substrate 22.

The filter system further comprises a low-noise amplifier (LNA) 13, an RF splitter 14 and a diffractive optical element (DOE) 18.

The RF splitter 14 equally splits the input RF signal 2b into N RF signals. However, the LNA 13 first pre-amplifies the input RF signal 2b, prior to it reaching the RF splitter 14, to compensate for subsequent RF splitting loss caused by the RF splitter 14 and also to boost the modulation efficiency of the subsequent VECSEL array 19. The N RF signals are transmitted via RF cables 16 to modulate the N elements of the 1xN 850nm wavelength VECSEL array 19. The use of VECSEL array 19 improves the brightness of the emitted optical beams and allows control of the loss discrimination between transverse modes so that the active diameter, and thus the output power, can be increased while maintaining fundamental mode oscillation. The diffractive optical element (DOE) 18 collimates and routes the Gaussian optical beams generated by the VECSEL array 19. Each of these RF-modulated collimated optical beams 21 propagates within the optical substrate 22 and undergoes several reflections in a cavity of the optical substrate 22.

The cavities of the optical substrate differ in size from one another in at least one of their dimensions. This dimension can be described as the length. The length of each cavity is defined by the distance between a respective mirror 23, of each cavity and the DOE 18. Each of the mirrors 23 and respective cavities has a corresponding element in the VECSEL array 19. A partial reflectivity reflector is provided as a partial reflective coating on the DOE 18. Every time an optical beam 21 in the optical substrate hits the DOE 18, a small fraction of the power of that optical beam 21 is transmitted through the DOE 18 and detected by an element of the wideband photoreceiver array 20. The photoreceiver array 20 generates a photocurrent corresponding to each small fraction of the power of each optical beam 21 transmitted through the DOE 18 and amplifies these photocurrents. The remaining large fraction of each optical beam 21 is reflected, by the partial reflectivity reflector coating on the DOE 18, and routed back within the multi-cavity optical substrate 22 for subsequent delayed photodetection by the photoreceiver array 20.

An interface device (not shown) is provided to independently control the amplitudes of the photocurrents amplified by the photoreceiver array 20. The interface device may be an electronic board that allows the filter system 1 to be controlled by a PC. The interface device controlling the amplitudes of the amplified photocurrents may be custom designed to control the gains of the amplifiers of the photoreceiver array 20.

An RF combiner 22a adds (or subtracts) the amplified RF photocurrents, generated by the photoreceiver array 20 and generates the output RF signal 24.

The VECSEL array 19 and wideband photoreceiver array 20 may be integrated on a VECSEL/photoreceiver chip 17. The VECSEL array 19 may be a GaAs VECSEL array and photoreceiver array 20 may be Si photoreceiver array. The integration of the GaAs VECSEL array 19 and the Si photoreceiver array 20 can be accomplished by mounting both arrays onto a common base substrate which can support electrical microstrips between the two chips. T This can be done by bonding both to the base substrate with their device sides up and then electrically connecting them by wire-bonding both chips to the microstrips and their associated bonding pads on the base substrate. Another promising technique, which emerged from the technology of multi-chip module (MCM) fabrication, is to place the chips device-side down (flip-chip) and bump bond them to the carrier.

The ability of the preferred embodiment of the microphotonic RF filter system 1 to reconfigure its transfer characteristics can be understood by examining the impulse response of the filter by driving the antenna 2 with an input RF impulse. In this case, each VECSEL element of the VECSEL array 19 generates an optical impulse of power P₀. The optical impulse propagating inside an optical cavity is

sampled at a frequency of ^{; cos}^ < ^'¥ , where L, is the cavity length, θ. is the incidence angle, and v is the speed of light inside the cavity. The power of the reflected impulse after n photodetections is P₀R" and that of the transmitted impulse is P₀(l -R)R" , where R is the reflectivity of the DOE 18. For example, if R = 95% and P_Q = 10mW, then the power of the optical impulse detected by the first photoreceiver element is 0.5mW while the power of optical impulse detected by the last photoreceiver element is 0.1 mW. The weights of the current impulses generated by the photoreceiver elements associated with an optical cavity can be changed by adjusting the gains of those photoreceiver elements. By combining the arbitrarily weighted current impulses, an adaptive Finite Impulse Response (FIR) may be produced, or equivalently an adaptive RF filter may be realised.

The filter system 1 , shown in Figure 2, will now be described with reference to Figure 3. Figure 3 uses different reference numbers for the features of the filter system 1 shown in Figure 3 so as to distinguish between the descriptions of these figures. Figure 3 schematically illustrates the interface between the VECSEL/photoreceiver chip 112 and the optical substrate 108 in the preferred embodiment of the invention. It also illustrates the propagation of the optical beams 106 inside the multi-cavity optical substrate 108 in the preferred embodiment. The VECSEL element 101 , of the VECSEL/photoreceiver chip 112, generates the high-power VECSEL beam 102 using an external reflector 103. A glass layer 104 is provided over the VECSEL/photoreceiver chip 112 for protection. The DOE 109 is inserted between the VECSEL/photoreceiver chip 112 and the optical substrate 108.

The DOE 109 comprises two parts, or sections. The first section is the VECSEL collimator 105, which is a hologram capable of collimating and steering VECSEL beam 102 by an appropriate angle and in an appropriate direction so that the optical beam 102 hits active areas of the photoreceiver elements 111. The second section of the DOE 109, shown by hatched lines, is an optical equalization means and acts as a lens relay that prevents the cavity beam 106, i.e. the beam 102 once it is inside the multi-cavity optical substrate 108, from diverging as it propagates along its optical path in the multi-cavity optical substrate 108, and also maintains its diameter within an adequate range.

The DOE 109 can be appropriately coated to provide any desired level of reflectivity. As the cavity beam 106 hits the DOE 109, a large portion of its power is reflected inside the multi-cavity optical substrate 108 and its diameter is equalized for subsequent propagation, while a small fraction of its power is transmitted through the DOE 109 and the glass layer 104 and then detected by one of the photoreceiver elements 111.

For a cavity length L and a photoreceiver element 111 spacing d, the steering angle θ, of the VECSEL collimator 105, is arctan(d/2L).

The diffractive optical element (DOE) 18 and 109, shown in Figure 2 and 3, respectively, will now be described with reference to Figure 4. Figure 4 uses different reference numbers for the features of the DOE 200 shown in Figure 4 so as to distinguish between the descriptions of these figures. Figure 4 schematically illustrates the preferred embodiment of the design of the diffractive optical element (DOE). The DOE plate 200 comprises (i) VECSEL DOEs 201 which are used to steer and collimate the VECSEL beams, and (ii) beam equalization DOEs 202 which maintain the diameters of the beams propagating in the cavity within an adequate range, as previously herein described. The DOEs can have a very high spatial resolution (eg. 1200 lines per mm) and they can be coated for the required level of reflection.

The multi-cavity optical substrate 22 and 108, included in Figures 2 and 3, respectively, will now be described with reference to Figure 5. Figure 5 uses different reference numbers for the features of the multi-cavity optical substrate 301 shown in Figure 5 so as to distinguish between the descriptions of these figures.

Figure 5 schematically illustrates the design of the multi-cavity optical substrate 301 in the preferred embodiment of the invention. It consists of several cavities of different lengths, as previously herein described, and a cavity length increment δL. The shortest cavity (length = L|) controls the high-frequency band of the RF filter response, whereas the longest cavity (length = L_N) adjusts the response of the low-frequency band of the RF filter. Two reflectors are used for each cavity, namely, the respective 100% reflectivity mirror 305 of each cavity and the partial reflectivity coating on the DOE plate 304. The height, H, and the width, W, of the optical substrate 301 must be higher or equal to the height and width of the VECSEL/photoreceiver chip 306, which integrates both the 2D photoreceiver array 302 and the 1 D VECSEL array 303.

The preferred embodiment of the invention provides a compact microphotonic adaptive RF filter which is compatible with other integrated optoelectronic systems. The adaptive processing of very wideband RF signals directly in the optical domain is advantageous because photons have higher information- carrying capacity, do not lose energy as quickly as electrons, and can pass near one another without generating crosstalk. The incorporation of microelectronics in photonic signal processing has the potential of overcoming the existing electronic bottlenecks for processing high bandwidth signals, and also provides intelligence that is directly integrated with the photonic processor.

Preferred embodiments of microphotonic structures that integrate a VECSEL array, a two-dimensional ultra-wideband photoreceiver array and a multi-cavity optical substrate, can achieve arbitrary high-resolution RF filter transfer characteristics, with no phase-induced intensity noise. By detecting delayed samples of RF-modulated laser beams propagating inside optical cavities of different lengths, an adaptive transversal photonic RF filter may be achieved in preferred embodiments. Each optical cavity can realize a Finite Impulse Response (FIR) filter of centre frequency inversely proportional to the length between the reflectors of that cavity. By adding the responses of many optical cavities of different lengths, a broadband adaptive RF filter may be realized in preferred embodiments. The tuning of the filter may be achieved in preferred embodiments by adjusting the power levels of the delayed samples. The optical cavities 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. This approach is attractive because the VECSEL array, photodetector array and broadband RF amplification can be cost-effectively integrated into a single chip in preferred embodiments and easily attached (or glued) to a multi-cavity optical substrate.

An advantageous possible future application for preferred embodiments of the present invention is in Square Kilometre Array (SKA) radio telescopes. Future SKA will comprise many antenna arrays (~1000) and each array will have tens of thousands of antenna elements. Each antenna element will require an adaptive RF filter to mitigate human-generated RF interference over the 0.15 - 20 GHz band. In order to achieve this there is a need for a means to cost-effectively process broadband RF signals arriving at each antenna element. The use of photonics is vital for broadband signal processing and the use of microelectonics is cost-effective for reconfigurable processing. The embodiment has been described by way of example only and modifications and variations apparent to a skilled addressee are possible and are deemed to be within the scope of the present invention.

Throughout the specification, unless the context requires otherwise, the word "comprise" and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A filter system characterised in that it comprises:

beam generating means for generating a plurality of light beams;

an optical structure comprising a plurality of cavities for receiving said light beams and propagating and delaying said light beams;

s.teering means for steering said light beams within said plurality of cavities; and

2. A filter system according to claim 1 , characterised in that each of said light beams is modulated by a spectral component of an input radio frequency (RF) signal spectrum.

3. A filter system according to claim 1 or 2, characterised in that said beam generating means comprises a laser array.

4. A filter system according to claim 3, characterised in that said laser array is a vertical cavity surface emitting laser (VCSEL) array, a vertical external cavity surface emitting laser (VECSEL) array, or a surface mounted laser array.

5. A filter system according to any one of the preceding claims, characterised in that said optical structure comprises a transparent substrate.

6. A filter system according to claim 5, characterised in that said transparent substrate comprises glass or sapphire.

7. A filter system according to anyone of the preceding claims, characterised in that said optical structure comprises spaced apart first and second reflector means.

8. A filter system according to claim 7, characterised in that said plurality of cavities of said optical structure have different lengths defined by spacings between said first reflector means and second reflector means.

9. A filter system according to claim 8, characterised in that the length of the shortest cavity of said plurality of cavities corresponds to the high-frequency band of an input RF signal received by said filter system and the length of the longest cavity of said plurality of cavities corresponds to the low-frequency band of the input RF signal such that, together, the plurality of cavities cover the total bandwidth of the input RF signal.

10. A filter system according to any one of the preceding claims, characterised in that said filter system further comprises optics means and said optics means comprises said steering means.

11. A filter system according to claim 10, characterised in the that said optics means collimates said light beams.

12. A filter system according to any one of the preceding claims, characterised in that said steering means comprises defractive optical elements (DOEs).

13. A filter system according to any one of claims 10 - 12, characterised in that said optics means comprises macrolenses or a microlens array to collimate said light beams.

14. A filter system according to any one of claims 10 -13, characterised in that said optics means is integrated on said optical structure.

15. A filter system according to any one of claims 7 - 14, characterised in that said first reflector means comprises a plurality of mirrors, a respective one of said mirrors being associated with each cavity of said plurality of cavities.

16. A filter system according to any one of claims 7 - 15, characterised in that said second reflector means comprises a partial reflector.

17. A filter system according to claim 16, wherein said partial reflector is provided as a coating on said optics means.

18. A filter system according to claim 17, wherein said coating is provided on at least a portion of the defractive optical elements (DOEs).

19. A filter system according to any one of the preceding claims, characterised in that said detecting means comprises a photoreceiver array.

20. A filter system according to claim 19, characterised in that said photoreceiver array is a wideband Si photoreceiver array with transimpedence amplification.

21. A filter system according to claim 19 or 20, characterised in that said photoreceiver array amplifies said photocurrents.

22. A filter system according to claim 21 , further comprising interface means to control the amplitudes of the photocurrents amplified by the photoreceiver array.

23. A filter system according to claim 21 or 22, characterised in that it further comprises combiner means to selectively combine the photocurrents amplified by the photoreceiver array by way of addition or subtraction.

24. A filter system according to any one of claims 10 - 23, characterised in that said optics means further comprises optical equalisation means to maintain the diameters of the beams falling on said detecting means within a predetermined range.

5. A filter system according to any one of claims 19 to 24, characterised in that said laser array and said photoreceiver array are integrated into a single integrated circuit, and said optics means is integrated on said optical structure.