For certain applications that involve faithfully transmitting microwave signals around or between military platforms, it is essential to use RF-on-fibre or an RF-photonic link. This is especially true for a broadband RF signal which, when used to modulate a 200 THz optical carrier wave, becomes effectively very narrow band and so does not suffer the effects of dispersion or frequency dependent loss. The main reason why RF-photonics is not more widely used is because it is difficult to achieve the often very severe dynamic range requirements of many military systems. In this paper we describe our approach to making highly linearised electrical-to-optical transducers (electro-optic waveguide modulators) using a slightly modified version of a high-throughput gallium arsenide p-HEMT fabrication process. To make high dynamic range linearised modulators requires the unprecedented precision in fabrication and manufacture that this approach allows. We describe our most recent results on tuneable linearised modulators.
Military applications such as electronic surveillance (ESM), intelligence (ELINT) and countermeasures (ECM) require
real-time analysis of broadband RF signals, often with very high frequency resolution. Direct sampling and digitisation,
followed by Fourier analysis and signal processing, will enable such signals to be analysed with much higher resolution
than can be achieved with conventional microwave techniques. Direct sampling and digitisation of signals at frequencies
above a few GHz is difficult to achieve electronically because the rise and fall times of electrical sampling gates are too
long. This can be overcome by using a low-jitter optical sampling pulse train to measure the voltage on an electro-optic
modulator. Optical sampling pulses can be very much shorter than electrical sampling pulses because of the large optical
carrier frequency. In this paper we describe a novel multiple wavelength optical sampling system architecture for the
real-time digitising of microwave signals between 1 and 20 GHz with a target resolution of more than 10 effective bits
(>60 dB spurious free dynamic range). We also describe our work on making highly linear electro-optic modulators for
this and other microwave-photonic applications.
We demonstrate a sixteen channel, GaAs/AlGaAs staring spectrum analyzer device based on a phased-array of sixteen electro-optic waveguide delay lines. The device is effectively a high resolution version of the widely reported optical wavelength division multiplexer phased-array devices, using electro-optic waveguides to adjust the optical phases across the array. The device comprises: a 1- to-16 way multimode interference coupler; 16 electro-optic phase controllers; 16 folded waveguide delay lines, from 0 to 1 nanosecond in equal steps; and a waveguide phased array output. By making use of novel waveguide and corner designs we demonstrate the device as a spectrum analyzer, the RF signal to be analyzed was superimposed on an optical carrier using a GaAs/AlGaAs electro-optic waveguide push-pull Mach Zehnder interferometer, and the intensity modulated light was then passed through the phased-array chip. The spectrum of the RF input signal was displayed in the far field of the phased-array as diffraction lines on either side of the main diffraction lines. By biasing the modulator to extinction, the CW carrier could be removed from the far field diffraction pattern so that only the RF spectrum was displayed. In this presentation we describe the device design, fabrication and testing including measurements of the dynamic range and resolution.
Opto-electronic devices offer a wide-band, low power, compact solution to a variety of functional requirements in both military and civilian systems. The applications include switching and modulation for telecommunications, microwave and mm wave radar and electronic warfare. The integration of opto-electronic devices on to waveguides promises the benefit of increased functionality and reduced cost systems. A key requirement of such integration is that devices be electrically isolated from each other but remain optically connected. This isolation may be achieved by etching a notch through the top conducting layer of a p-i-n structure, for example using reactive ion etching. This can be problematical owing to the need to control etch depths to high precision. We have developed a process for isolating hard walled AlGaAs/GaAs waveguides using proton implantation. The effect of the proton energy and the level and type of doping on the degree of isolation and transparency achievable have been investigated. Post implantation annealing has been found to have a significant effect. We have found that our process does not significantly alter the transparency of the waveguides and gives excellent electrical isolation. In contrast to the etched notch method our process also gives very good yield. Using this technique MachZehnder (MZ) modulators have successfully been fabricated and demonstrated. Keywords: Optoelectronics, proton isolation, waveguides, integration.
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