Reflective optical switch offers digital operation for DWDM

May 1, 2001

Yvonne Carts-Powell

A titanium-doped lithium niobate device acts as a reflective optical switch with promise for DWDM applications, including fiber-to-the-home. Roger Krähenbühl at Virginia Tech (VPI; Blacksburg, VA) and coworkers at SFA (Largo, MD) and the Naval Research Laboratory (Washington, DC) developed a reflective digital optical switch (RDOS) with high-speed polarization-independent reflection modulation and 30-dB off-on ratios over a wavelength range from 1520 to 1570 nm.¹

Reflective switches based on Mach-Zehnder interferometers are analog devices, and require sophisticated optical controls to make them work like digital devices. Mechanical optical switches have speed limitations.

The RDOS is based on a Y-branch (transmissive) digital optical switch, but the reflectivity offers some novel applications for use of this device in DWDM passive optical network for fiber-to-the-home. The device consists of a Y-branch waveguide structure made from LiNbO₃ with Ti-doped waveguide regions. Three electrodes are formed on top of the device: two ground electrodes to the outside of the Y, and a signal electrode in the center of the Y (see Fig. 1). By applying a voltage to electrodes on the surface of the device, the light can be directed into one waveguide or the other. The end of one of the waveguides is mirrored, reflecting the light back to the input port. The waveguide is coupled to single-mode fiber, with a circulator at the input side.

Over a 50-nm wavelength range, applying a +/-4V voltage to the center electrode resulted in on-off ratios of more than 30 dB for the back reflected signals and 20 dB for transmitted signals. Excess losses for the device were 9 dB for the back signal and 5 dB for the through signal because the SiO₂ buffer layer between the waveguide and the electrodes was very thin. The researchers tested the device at 2.5 MHz, but expect that it could operate at speeds up to several hundred megahertz.

The researchers envision the device finding use in fiber-to-the-home systems. The central office would transmit many channels at once using WDM, and at some remote office the wavelengths would be split off and one channel would be delivered to each customer (see Fig. 2). Instead of using a dedicated laser at a particular wavelength at the subscriber's end of the network, the subscriber could send signals back up the network using an RDOS to modulate and reflect light back into the network. (The subscriber would be limited to transmitting at the speed of the RDOS, but signals from the central office could be faster.)

Yvonne Carts-Powell

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