Ion-exchange is a relatively mature technology for the production of optical waveguides in glass substrates, and it appears as the most convenient approach to the development of a number of passive integrated optical devices, due to the low loss and process robustness. The purpose of this paper is to give a quick overview of the status of this technology, with particular attention devoted to the problems of manufacturing tolerances and of fabrication and characterization reproducibility.
Different issues in design of passive integrated optics devices based on glass waveguide technologies are reviewed. The established fabrication technologies, ion exchange into substrate glasses, and deposited doped-silica waveguides on silicon wafers, are described in terms of waveguide engineering for differing needs. The status of various optical guided-wave propagation methods and tools are discussed. Design aspects for two most important categories of glass waveguide devices, M x N couplers and wavelength division demultiplexers/multiplexers, are considered. Particularly, designs with insensitivity to fabrication process tolerances and optical signal bandwidth and polarization state are emphasised. A brief look is also taken on design of complicated guided-wave circuits.
Silica-on-silicon integrated optics is a key technology for passive optical components, as required for the next generation of fibre optics communications systems. This paper reviews progress on silica-on-silicon technology and key devices developed to date. Waveguide fabrication technology is described, with particular attention to deposition processes. Fibre pigtailing technology is described, and the relative merits of active and passive alignment are discussed. A range of passive components are detailed, concentrating on couplers, splitters and wavelength division multiplexers. Recent developments in rare-earth doping of silica-on-silicon waveguides are also covered. The paper concludes with some discussion on future directions for silica-on-silicon research and development.
The sol-gel process allows the synthesis of a wide variety of amorphous as well as crystalline materials which can exhibit useful passive or active optical properties. The process offers many advantages, such as low-temperature synthesis, excellent control and flexibility over composition and design. For applications in future integrated optics, the sol-gel process is flexible in making various kinds of optical components which either have been successfully made or have potential in their realization. This paper examines a number of the state-of-the-art optical components fabricable by the sol-gel process for glass integrated optics. Major examples furnished are in glass substrates and waveguides, third-order nonlinear materials, lasers and optical amplifiers, optical fibers, and gradient-index lenses. The benefits as well as limitations by using the sol-gel approach will be critically presented.
Discussion has been made on two kinds of sol-gel derived silica-based fibers: fibers drawn from sol-gel derived silica preform and fibers directly drawn from sols. The former fibers can be used as optical fibers and have been developed in that direction. The optical loss of the sol-gel derived fiber has been reduced to 1 dB/km level for pure silica and 10 dB/km level for Ge-doped silica. The latter directly drawn fibers have been developed as heat resistant fibers.
Integrated-optical devices in rare-earth-doped glasses have emerged as an attractive new technology on the threshold of wide-scale manufacturing and commercial insertion. These devices can be used both as laser oscillators and optical amplifiers. They have been formed by a number of fabrication methods including ion exchange and thin-film deposition. Active integrated-optical devices are expected to be important elements in future optical fiber networks. Rare-earth-doped optical fiber devices provide nearly perfect amplification of signals in optical fibers. The performance of these rare-earth-doped optical fibers is so promising that researchers started investigating whether similar performance could be achieved in planar waveguides. The combination of passive integrated-optical components and rare-earth ions has produced many devices with impressive performance.
Recent developments of ion-exchanged glass waveguide devices for optical communications are reviewed. The focus is on waveguide configurations for passive devices and on their optical characteristics. First the ion-exchanged glass waveguide structures are briefly described. The most important ion-exchanged passive components, power splitters and dual-wavelength multi/demultiplexers (WDM), are discussed in detail. We also discuss the recent advancement on other components such as narrowband WDM-devices, modefield transformers and ring resonators. A view to the important issue of fiber attachment and device packaging is briefly given, and finally the integration of several ion-exchanged components (e.g. wavelength multi/demultiplexers and splitters) in a single glass chip is briefly discussed.
Integrated optics in glass are establishing as a reliable new technique with a wide range of applications. The most advanced applications are in the sensor and in the telecommunication field. The paper describes a number of different sensors based on interferometry, absorption effects, spectroscopy, Faraday rotation etc.
A very important property which ensures the usefulness in practice is the reliability and the environmental stability.
Integrated optics is a generic term used to describe guided wave devices combining several functions on a single substrate. Passive integrated optics, or "planar", refers to a family of devices that requires no external power source to perform their functions. Their most promising short-term application is fiber-based telecommunications systems. Examples are power splitters and wavelength division multiplexers (WDMs). For the purposes of this review, we will consider devices in which light enters and exits via optical fiber pigtails.
Semiconductor quantum dots are being investigated as a new material system for a variety of optoelectronic applications. We will discuss applications of quantum dots for optical switching in waveguide structures, optical data storage, and potential light sources. We emphasize why quantum dots with discrete spectra are expected to be superior in many applications compared with other systems, such as bulk and quantum wells with continuum spectra. We also report on our fabrication of cadmium sulfide quantum dot sol-gel glass channel waveguides using the potassium ion-exchange technique. The waveguides were optically characterized and cross-correlation measurements showed significant pulse shaping, both spectrally and temporally, after femtosecond laser pulses were propagated through the waveguide.
Photosensitive glasses for applications in integrated optical devices have recently become a popular research topic, due primarily to the success of the germania doped silica fibre counterpart. Whole ranges of devices could in principle be fabricated utilising the photosensitivity phenomenon, from directly written waveguides to more complex grating structures. A key parameter in all applications is the change in material refractive index after light exposure. Considerable effort has been devoted to understanding mechanisms responsible for the phenonema with attempts to maximise the effect. To date index changes achieved are ~ 10-3 which is sufficient for the production of highly efficient holographic gratings within a waveguide, but not quite large enough for direct waveguide formation. Various writing schemes have been developed, but most are based on UV laser sources operating in the 240 - 270nm wavelength range to coincide with a germanium related defect absorption. High quality gratings, with reflection efficiencies of 14dB have been written. These gratings have been used as feed back elements in external cavity lasers and when incorporated in a Mach-Zender interferometer, a channel dropping filter has been demonstrated. With continued effort, more compact optical devices utilising gratings and the photosensitivity effect can be expected in parallel with the fibre development work.
Fabrication of high-quality glass waveguides with gratings is an important step in the use of glass in integrated optics. In this paper, some of the latest attempts in the fabrication of glass waveguides with gratings are reviewed. The design and fabrication of glass waveguides with ion-exchange and gratings are briefly described. A summary of some of the practical devices is given and the future directions are analyzed.
The practical usefulness of multimode guided wave optics has been overshadowed by the powerful trend of optical communications towards single mode technologies. Although multimode integrated optic technology lends itself to a very restricted palette of optical functions, it can perform elementary functions at a cost which falls dramatically as the number of identical samples increases unlike that of all-fibre and micro-optic components. Glass integrated optics brings the additional advantage of stability versus external physical parameters.
A variety of glass waveguide devices, including passive components (splitters and couplers), active network components (waveguide switches, lasers and amplifiers), and sensors for the measurement of pressure acceleration, position, rotation, etc, are in various stages of development, or have reached the production stage, at various companies. These components are expected to have a major impact on the implementation of fiber-optic (FO) systems for aircraft. Many new functions and applications can be realized with their introduction. This paper examines factors which have so far delayed the introduction of these products. Applications of FO and integrated-optic (10) technologies, and the potential of glass waveguide devices in aircraft systems, are discussed.
The technique of the fibre to waveguide connection are described and compared in terms of insertion loss, time requirement, return loss and environmental performance. The most promising techniques are outlined together with the latest technical improvements.
The theory and applications of optical fiber chemical sensors are discussed. Several transduction mechanisms for measuring the quantity of chemical species such as carbon monoxide, hydrogen sulfide, or toxic substances in the air or water are explained. A new frequency domain method which utilizes (chemo-optic) variable index material optical fiber couplers is also proposed for the application in distributed sensing.
An overview of all-fiber passive devices and components made by the fusion and tapering technique is given. Single tapered fiber are examined as well as 2 x 2, 3 x 3 and other simple star couplers, and interferometric structures made of these components. Some relevant applications are described with their theory of operation and illustrated with typical experimental results.