PHOTONIC INTEGRATED CIRCUITS FOR PHASED-ARRAY BEAMFORMING

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1 PHOTONIC INTEGRATED CIRCUITS FOR PHASED-ARRAY BEAMFORMING F.E. VAN VLIET J. STULEMEIJER # K.W.BENOIST D.P.H. MAAT # M.K.SMIT # R. VAN DIJK * * TNO Physics and Electronics Laboratory P.O. Box JG The Hague The Netherlands f.e.vanvliet@fel.tno.nl # Delft University of Technology Faculty of Electrical Engineering Mekelweg CD Delft The Netherlands J.Stulemeijer@its.tudelft.nl Photonic integration is very promising to bring down volume and weight of phased-array beamforming networks. In addition, photonics allows for increased functionality for widebandwidth systems. In this paper we demonstrate the feasibility of phase and amplitude control of a 16-element phased-array antenna with a single InP integrated circuit. The concept and design of an optical phase locked loop will be explained. Results of initial true-time delay experiments will also be shown. 1 Introduction Phased-array antennas are in widespread use since the beginning of the 70 s and are now becoming increasingly important for radar applications and in satellite and mobile communications. Phasedarray antennas have the advantage of 2-dimensional scanning without mechanical parts, accurate beampointing, and phase and amplitude control to reduce sidelobes in the overall antenna pattern. A drawback for broad application of active phased-array antennas is the voluminous and heavy RF electronic beamforming network. Photonics holds a great promise for reducing both weight and volume of these networks, by incorporating fibre optic and integrated optic components, and enables the use of antenna remoting and optical signal processing. Another advantage of the optical approach is the huge bandwidth which it offers: the response is flat from DC to tens of GHz; it is limited by the bandwidth of the photodetectors which can extend over 100 GHz. Other advantages of using photonics are the frequency independent low loss of optical fibres in comparison with coax cables, the insensitivity to electromagnetic interference (EMI) and the possibility of incorporating long true time delays, enabling large instantaneous bandwidth radar systems. A complete optical beamforming chip has been designed, realised and tested. A heterodyne detection technique is needed for the full benefits of this beamformer. Design considerations for an optical phase locked loop are presented but no results are available at this moment. Also a switched true time delay using fibre delay lines has been assembled. Measurement results are presented in paragraph 4. A.B. Smolders and M.P. van Haarlem (eds.) Perspectives on Radio Astronomy Technologies for Large Antenna Arrays Netherlands Foundation for Research in Astronomy

2 2 Optical Phase Locked Loop For the optical generation of microwave signals with high spectral purity, an Optical Phase Locked Loop (OPPL) is of interest because of its potential to significantly reduce the relative phase noise of a pair of lasers [1]. In Fig. 1 a schematic diagram of an OPLL is shown. In an OPLL the microwave signal is generated by mixing the output of two lasers onto a photodiode. This microwave signal is amplified and its phase is compared to a reference signal oscillator. The resulting phase error signal is used to tune the frequency of the slave laser, which is forced to track the master laser with a frequency offset equal to the reference signal. Using this scheme, the relative phase noise of the two lasers is significantly reduced. Figure 1: Optical phase locked loop (OPLL) An OPLL system is realized consisting of a New Focus 1.55 µm external cavity diode laser (linewidth <300 khz) operating as master laser, and a tunable 1.55 µm three section DBR laser diode (linewidth <5 MHz) acting as the slave laser in the loop. OPLLs based on semiconductor laser diodes offer the advantage of small, rugged devices but special attention is needed to the laser linewidth, tuning characteristics of the laser and total loop delay. In order to minimize the loop delay, micro-optic components are used for mixing of the optical signals and a special GaAs MMIC containing the electrical components like monolithic integrated photodiode, microwave mixer, amplifier and loop filter is designed and realised. An integrated MMIC, hence a small device, is necessary in this architecture in order to obtain the required very small loop delay. At the moment the complete OPLL system is being tested. Optical RF-signals of several GHz are realized by mixing the two lasers, additional tests are in progress. 3 Photonic Integrated Beamformer Chip Using a coherent detection scheme, phase and amplitude of an optical signal can be directly transferred to a microwave signal. In this way modulation of phase and amplitude of a microwave signal can be performed using optical phase and amplitude modulators. The bandwidth is almost unlimited because the frequency of the generated microwave signal is limited by the photodetector. Fig. 2 illustrates an integrated photonic beam control circuit for a 16-element phased-array microwave antenna which has been realized in InP, for operation in the long wavelength window (1550 nm) [2,3]. The dimensions of the chip are 8.5*8 mm 2, the optical excess loss is estimated to be 13.1 db, partly caused by erroneous processing. The chip has two inputs for two optical signals with a frequency difference equal to the radar RF signal (from an OPLL). The two inputs are fed into a 1x16 power splitting distribution network, the outputs are sorted in pairs. Each pair is connected to a phase and amplitude modulation section, after which the two signals are fed to a 3-dB-coupler. The RF-signals are obtained by coupling the signals coming out of the sixteen 3 db couplers to a series of 16 balanced detector pairs. 296

3 Figure 2: Photograph of integrated beamforming chip, dimensions 8.5*8 mm 2 The phase and amplitude of the optical signals can be controlled by using changes of the refractive index and in the absorption due to the electro-optic effect. The doping profile of the chip is chosen in such a way that the waveguiding layer gets depleted when a reverse bias is applied to the waveguide. At high voltages, the modulator acts as an electro-absorption modulator, due to the electrical field induced shift of the band edge. In the left side of Fig. 3 it can be seen that a phase shift of 180 degrees can be set with a voltage in the range of 0 to -5 V. The right side of Fig. 3b shows the attenuation as a function of the voltage. It is seen that an attenuation of over 15 db can be achieved with an applied voltage of -20V. Figure 3: Phase shift and attenuation as function of applied voltage A predefined value for amplitude and phase can be reached as follows. First the attenuation in both the interfering arms has to be set by applying the same voltage to the phase/attenuation sections in both arms. Next the phase can be adjusted by changing the voltage on both arms relative to each other with a small amount. Light from two different external cavity diode lasers was coupled into the two input waveguides of the chip. This was done by positioning two lensed fibres in front of the waveguides. The two lasers were tuned 250 MHz apart and the output level of 0 dbm was boosted to 13 dbm using erbium doped fibre amplifiers. At one of the outputs the light was coupled into a lensed fibre and guided to the detector. At the detector the light intensity was about -60 dbm. After the detector a 40 db electrical amplifier was used. 297

4 The frequency generated by mixing the two optical signals can be chosen at any frequency between a few MHz and hundreds of GHz (only limited by the detector). In our experiment the obtainable frequency was limited by the linewidth of the lasers (300 khz) and the FFT-analyzer (0.5 GHz). In this range the output level appeared to be virtually frequency independent. The SNR of the signals is very low because of the high losses which occurred in the measurement set-up in coupling light into and out of the chip. We are presently working on reducing the fibre to chip loss, by using spotsize converters, and the on-chip losses in order to reduce this problem. 4 True Time Delay Experiments To be able to realise phased array antennas with wide instantaneous bandwidth true time delay phase shifting elements are needed. Optical fibres offer the opportunity to obtain relatively long delay lines, in which the losses are independent of the size of the delay line. A 3-bit fibre optical true time delay (OTTD) architecture has been build using 4 2x2 Akzo Nobel Beambox integrated thermo-optic switches to demonstrate feasibility of fibre optic delay lines. A schematic drawing of this binary fibre optic delay line architecture is given in Fig. 4. Figure 4: Schematic drawing of a 3-bit OTTD architecture In this architecture, the optical signal is optionally routed through 3 fibre segments whose lengths increase successively by a power of 2. The required fibre segments are addresses using the 2x2 optical switches. Since each switch allows the signal to either connect or bypass a fibre segment, a delay time can be chosen between 0 T and 7 T, with increments of T. The unit time delay can be chosen as small as required because a differential phase shift between the two paths is of interest. In the OTTD experiment a delay increment T of 1.5 nsec was chosen, corresponding with a fibre segment increment of about 30 cm. In order to keep the delay line error below 5 psec, the fibre segments must be cut with mm precision. The time delays were measured using a network analyzer, the results of the second attempt are presented in Fig

5 Figure 5: Results of time delay measurement for a 3-bit OTTD component As can be seen from the figures, a time increment of exactly 1.5 nsec is achieved, with an error smaller than 5 psec. Typical optical losses of 12 db were measured, resulting from switch losses (2.5 db each) and fibre optic connector losses. With a proper waveguide system, which has sufficient low optical loss, optical delay line architectures can also be integrated on chip, offering the advantage of volume and weight reduction. 5 Conclusion Photonics offers great opportunities for phased array beamforming systems. Both coherent and incoherent techniques are of interest to antenna remoting and beamforming. To investigate its feasibility, research is carried out on optical microwave generation using an OPLL, optical integration of signal processing elements on a single InP chip, and architectures for optical true time delay lines. Phase locking of two lasers has not been achieved yet but the RF electronics and PIN photodiode has successfully been integrated onto one GaAs MMIC. The InP beamforming chip is capable of controlling phase and amplitude of 16 antenna channels. The maximum attenuation is 17 db and the phase can be adjusted to over 180. A 3-bit true time delay has been realised and tested with an architecture that exhibits an constant insertion loss for all possible states. The time delays have an error less than 5 ps. References [1] U. Gliese, T.N. Nielsen, M. Bruun, E. Lintz Christensen, K.E. Stubkjær, S. Lindgren, and B. Broberg, A Wideband Heterodyne Optical Phase-Locked Loop for Generation of 3-18 GHz Microwave Carriers, IEEE Photonics Technology Letters, Vol. 4, pp , [2] J. Stulemeijer, F.E. van Vliet, K.W. Benoist, D.H.P. Maat, M.K. Smit, Photonic Integrated Beamformer for Phased-Array Antennas, Proceedings Microwave Photonics conference MWP 98, 1998, pp ,

6 [3] J. Stulemeijer, F.E. van Vliet, K.W. Benoist, D.H.P. Maat, and M.K. Smit, Compact Photonic Integrated Phase and Amplitude Controller for Phased-Array Antennas, IEEE Photonics Technology Letters, Vol. 11, pp ,