Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Patent
1994-08-31
1995-11-14
Ullah, Akm E.
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
G02B 61, G02F 1035
Patent
active
054674140
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a device for modulated optical signals for coherent optical generation of modulated phase-controlled RF charge carriers and for summing such signals on a plurality of optical carriers.
Optical devices of this kind have been disclosed by the following publications: Wale, M. J. and Birkmayer, W. S. "Coherent optical techniques in antenna beamforming for satellite communications," Colloquium on Optical Intersatellite Links and Onboard Techniques, IEEE, London, Jan. 12, 1990 (IEE Digest 1990/8), as well as Birkmayer, W. S. and Wale, M. J. "Optical BFN for telecom satellites and/or SAR applications; analysis and results," in Proc. ESA Workshop on Advanced Beamforming networks for Space Applications, November 1991, ESA publication WPP-030. FIG. 3 shows the block diagram of an example of an optical beamforming network in which the optical device is incorporated into a combiner. In the example shown, an optical carrier is superimposed with two modulated optical carriers. In particular, the publications discuss the problem that occurs in conjunction with the use of coherent optical methods to generate and control several modulated RF carriers, e.g., in producing antenna lobes in phase-controlled antennas.
For the arrangement with an RF carrier, the required RF signal is obtained by mixing two optical carriers with frequencies w.omega..sub.1, .omega..sub.L0, separated by the required superimposition frequency .omega..sub.RF. Since any phase or amplitude encoding on .omega..sub.1 or .omega..sub.L0 is transferred by the mixing process in the optical receiver directly to the RF carrier with a frequency .omega..sub.RF, phase or amplitude control can be used in the optical range to control the corresponding properties of the RF signal. This is the source of the advantages in the described system presented in the above publications. The basic configuration, as used to control phase and amplitude in several phase-controlled antenna elements, is shown in FIG. 3. Here the two optical carriers with frequencies .omega..sub.1, .omega..sub.L0 are generated by a pair of lasers operating under the control of an electronic monitoring circuit (PLL). The latter monitors the relative frequency and/or the phase of the RF carrier and transmits a feedback signal to one or both lasers, so that a constant frequency ratio, in some cases a constant phase ratio as well, is maintained between the measured RF carrier and a reference frequency from an external control oscillator.
The above-mentioned publications describe an embodiment which is concerned primarily with providing an optical output for the measurement process, with this output having certain properties. In order to permit accurate RF phase control, the temperature coefficient of the phase of the differential frequency applied to the output must be as low as possible and the control signal must travel a path other than the one followed by the antenna signals which are controlled independently with the smallest possible expense. The signal applied to the monitoring output must not be highly modulated, since data modulation sidebands interfere with the operation of the feedback control circuit of the laser for phase/frequency and result in coupling between data modulation and the controlled RF frequency or phase. In the special case of a modulation circuit with suppressed carrier, such as QPSK, the carrier is not present in the spectrum of the modulated signal at all and must be regenerated in the laser control circuit, for example by nonlinear signal processing. Consequently, however, the electronics must be designed in a very costly fashion, despite which the accuracy and temperature stability of the system can be considerably adversely affected.
The goal of the present invention is to provide an optical device of the species recited at the outset in which the previous cost of specially designed optical and mechanical components and the consumption of electrical power are considerably reduced and the signal
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patent: 5408544 (1995-04-01), Seino
"Optical BFN for Telecom Satellites and/or SAR Applcations: Analysis and Results," W.Ss.> Birkmayer et al., Proc. ESA Workshop on Advanced Beamforming Networks for Space Applications, Nov. 1991, ESA Publication WPP-030.
"Coherent Optical Techniques in Antenna Beamforming for Satellite Communications," M. J. Wale et al., Colloquim on Optical Intersatellite Links and Onboard Techniques, IEEE, London Jan. 12, 1990 (IEE Digest 1990/8). pp. 17/1-17/4.
Birkmayer Wolfram
Edge Colin
Wale Michael J.
Deutache Aerospace AG
GEC-Marconi Ltd.
Ullah Akm E.
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