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Reflector yields tunablity triumph

08 Jun 2005

A wavelength tuning scheme that outperforms thermo-optic or electro-optic techniques is revealed.

A tunable waveguide mirror that operates over a giant wavelength range of 160 nm could be the key to making a new breed of broadly tunable devices for telecom networks, say Japanese scientists. Possibilities include tunable filters, lasers and dispersion compensators that out perform today’s designs and suit use in future ultrawide WDM networks.

The so-called “hollow waveguide Bragg reflector” is the brainchild of researchers from the Tokyo Institute of Technology and was presented as a post-deadline paper (CPDA6) at last month’s CLEO conference in Baltimore, US.

“Our proposed tuning scheme can induce a propagation constant change of several tens of percent -- this change is much larger than that of thermo-optic or electro-optic tuning,” said Yasuki Sakurai from the Institute’s Microsystem Research Center. “Because our devices are based on a waveguide structure we can easily form compact tunable devices and integrated optical circuits.”

The team’s device consists of a 2 mm-long hollow silicon-slab waveguide that is formed by an air-gap sandwiched between two distributed Bragg reflectors (DBRs). Each DBR is a multilayer mirror made of 6 pairs of Si/SiO2 layers. In addition, the surface of one of the DBRs features an etched diffraction grating to reflect the light back out of the waveguide.

Changing the size of the air-gap between the two DBRs from 1.8 to 10.7 µm tunes the peak reflection of the device from 1400 and 1560 nm. At the moment the gap-distance is controlled by a piezoelectric actuator but in the future the team are thinking of replacing it with a MEMs actuator to create a monolithic design that supports tuning speeds in the microsecond regime.

The researchers are now optimizing their design and say that calculations suggest that it should be possible to maintain a reflectivity of 93% over a 200 nm tuning range. “If the grating structure is optimally designed we can expect a small insertion loss of less than 1 dB including coupling losses with singlemode fibre,” Sakurai told Optics.org. “By using a narrower air-core we can easily expand the tuning range. The limiting factor is the reflection bandwidth of the multilayer mirrors [DBRs].”

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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