Some Practical Applications Of Lithium Niobate Integrated Optics Technology

ABSTRACT Some of the practical applications that have been envisaged for Lithium Niobate Integrated Optics are discussed in the light of developments in the technology. Much of the basic physics that governs devices is now fairly well understood and with attention coming to focus on practical engineering developments a number of systems demonstrations are being made which illustrate well the capabilities of the technology. From these demonstrations it is clear that major opportunities exist for integrated optics devices, although the successful exploitation of these depends strongly on continued progress in engineering the devices for use in a broad range of non-research laboratory environments. 1. INTRODUCTION It is now some thirteen years since the initial demonstration of an active integrated optical device in a research laboratory and about three since the first emergence of commercial devices. Most of the intervening time has been spent developing material, processing techniques and basic physics associated with the technology, but more recently attention has moved towards practical engineering aspects and particularly towards packaging the devices in a way that can cope with the physical demands that are made in non-research laboratory environments. As yet, components are still produced in small volume but batch processing techniques are being developed along with tolerant manufacturing steps to enable the device technology to move forward, while attention to the way in which the chips will be used is leading gradually to more advanced packaging technology where real operating problems caused by laboratory temperature drifts or humidity changes can be dealt with by stabilised Peltier coolers and hermetic sub-modules.The major driving force behind the development of integrated optics devices has been the desire to utilise further the potential of fibre-optic communication systems. Several aspects can be readily observed. Firstly, in the signal coding itself, direct modulation of lasers is still only practicable up to a few GHz, for a variety of reasons; therefore, for higher speed of modulation some external device such as the Mach-Zehnder Interferometer looks promising. Secondly, for routing signals traffic, conversion of optical signals to electronic, routing via conventional technology then translation back into the optical domain is an operation which is clumsy. Thirdly, for certain applications coherent signalling systems offer distinct benefits. The need here is for good control of operating wavelength along with reasonably narrow bandwidth in the source, hence external modulation is recruired, but there is also aneed for control of the polarisations at the mixing stage on the receiver. Furthermore, particularly for space-borne systems where Doppler-induced frequency shifts might be considerable, some means of externally adjusting the frequency of the signals might be require. In summary, the requirements of advanced optical communication systems demand a range of devices which actively control various physical parameters of the light itself, or else control the route the light is taking.A second and significant impetus for the development of integrated optics comes from fibre-based sensor systems. The principle application envisaged is the fibre gyro, where the general desire is to measure the rotation-induced path-length difference between counter-propagating laser beams. However, further uses for integrated optics have been outlined in such areas as phased array radar and more recently several other fields such as strain and temperature sensing have been considered. As the technology gains acceptance there is no doubt that more applications areas will be exploited.Before discussing the principal applications in greater detail, the status of the device technology is briefly reviewed to bring out the current developmental position. This includes both chip fabrication and packaging and from this is drawn a summary of basic performance details available from commercial devices. Application areas are then considered and some estimates made of future trends in integrated optics based on both anticipated system requirements and advances in the technology itself.