High speed, high capacity optical communication systems require high performance devices that introduce minimum degradation. Such devices are required, for example, to introduce minimal insertion losses and should show no spurious reflection peaks or unwanted dispersion slopes. The adoption of wavelength-division-multiplexing (WDM) technology, as a means of increasing the optical system bandwidth and speed, is increasingly shifting the emphasis towards the use of multiband optical devices. The implementation of a 128-channel optical network, for example, will require the development of complex devices with up to 128 different transmission bands.
Multiband reflective optical devices have already been successfully demonstrated by using fibre Bragg grating technology. Multiband operation can be achieved by multi-element arrays formed by splicing together a series of single-band gratings with different central wavelengths and strong side-lobe suppression. Such an approach can result in high total insertion loss, due to finite splice losses. Their performance can also be compromised by residual backreflections introduced by the splices, especially if the individual gratings are written in fibres with different characteristics, such as different numerical apertures or core/cladding compositions, in order to increase photosensitivity and/or eliminate short-wavelength cladding-mode losses.
Multiband operation can also be achieved by overwriting and essentially superimposing different gratings, corresponding to the different reflection bands, on the same fibre length. However, such an approach quickly saturates the available fibre photosensitivity and results in a small number of bands with relatively small reflectivity. Also, since this process involves multiple exposures, any error during the writing of a certain grating (e.g., due to different exposure conditions and UV-fluence stability) is quite likely to affect the other gratings as well.
Finally, multiband operation can also be achieved by single, complex superstructured gratings, such as sampled or sinc-apodised superstructured gratings. These complex-grating structures can be viewed as resulting from a linear, coherent superposition of the individual gratings that correspond to each different band. Such linear coherent superposition is essentially an additive process and results in complex refractive-index-variation patterns and very large required peak refractive index changes. This can potentially limit the number or types of photosensitive fibres that can be used. Also, it can severely limit the maximum achieved reflectivity at each band.
None of these prior-art approaches achieve multiband operation provide multi-channel, high-reflectivity devices (>50%) having low dispersion. This is a serious problem for high-speed (eg 10 GB/s and 40 GB/s) communication systems.
An aim of the present invention is to improve the performance of gratings that reflect optical radiation at more than one wavelength.