Silica-based wavelength filter used for wide-channel-spacing applications
Researchers from the University of Cambridge (Cambridge, England) have used a new etching technique to produce filters for WDM. In a post-deadline paper at ECOC '01, they presented a two-dimensional integrated-optical system with a footprint of 4 x 3 mm to route four multiplexed incoming channels into four wavelength channels spaced 6-nm apart. This device, they said, already approaches the performance of commercial arrayed waveguide gratings (AWGs), and should do even better once optimized.
The principle the Cambridge researchers employed is not new (see figure): it consists of just two reflectors and an input beam. Light is coupled in, collimated, diffracted to split out the spectra, and then focused and channeled out of the device.1 What is new, however, is the material system used. Instead of the III-V semiconductor or polymer systems usually used for this type of application, the team wanted to build their devices out of silica. This material has the advantage of being low-loss, well index-matched to silica fiber, and relatively straightforward to manufacture. The problem had always been that it was difficult to produce the kind of deep, well-formed structures in silica that were required for this kind of application.
The new wavelength filter operates inside a slab waveguide, producing a folded two-dimensional optical system. After propagating from deep-etched waveguides, the light propagates (1) toward a parabolic mirror etched in the top of the slab (2). It is then, simultaneously, collimated and directed towards the diffraction grating (3) and then refocused (4) to exit into wavelength-selective channels at (5). The device is best-suited to applications with widely-spaced channels.
To solve this problem, the Cambridge team had to come up with a novel etching technique. They started with a silica-on-silica slab structure (with a high-index guiding layer on top of a thermally-grown cladding layer) and etched using a low-power, anisotropic, reactive ion etching process using a mixture of CFC, oxygen and an inert gas. Researchers say that this process represents a trade-off between the high definition masks and the depth required: they were able to etch to 8 µm with only a little damage at the top of the sidewalls.
The resulting device had an average insertion loss of 9.8 dB, with an insertion loss variation of 2.1 dB. The corresponding figures for a typical AWG, said the Cambridge researchers, are 5.5 dB and 1.5 dB respectively. For adjacent crosstalk, the silica-based device has an average of 21 dB, 24 dB for nonadjacent. These again compare favorably to figures for the AWG, quoted as being 25 and 30 dB respectively. For more information, contact Chris Morgan at e-mail: Chris.Morgan@altavista.net.
- C.N. Morgan et al., ECOC 2001, postdeadline paper PD.F.1.2.