Heterodyne RF/Optical Links Utilizing Integrated Photonics

The integration scheme is based on use of asymmetric twin optical waveguides.

February 1, 2007

Army Research Laboratory, Adelphi, Maryland

A program of research and development has been directed toward the goals of demonstrating (1) ultra-low-noise communication links in which information is conveyed by phase-modulated radio-frequency (RF) carrier signals that are, in turn, used to modulate laser-generated optical carrier signals and (2) implementation of transmitters and receivers in such links by means of several key integrated optoelectronic and photonic components. Notably, the scheme for integrating these components is based on the use of asymmetric twin optical waveguides (see figure) that afford design versatility in that they enable the use of a broad range of components useful in RF/photonic applications.

The achievements of the program include the following:

The integration scheme was demonstrated by designing, constructing, and testing several archetypical asymmetric - twin - waveguide - based photonic integrated devices and associated electronic circuits, including not only waveguides but also laser/waveguide and photodetector/ waveguide units.
In This Example of Integration based on an asymmetric tapered optical waveguide, a laser (active waveguide) is integrated with a tapered coupler and a passive waveguide.
A fully integrated heterodyne receiver capable of receiving a 1.55-μmwavelength carrier signal modulated by an RF signal of 5 GHz was designed, constructed, and tested. A wide - band - rectifier/narrow - bandreceiver design was chosen to provide cancellation of phase noise. Advantages of using an integrated balanced photodiode pair and external modulation to reduce relative-intensity noise and link nonlinearity were experimentally demonstrated.
The use of a tunable 1.55-μm-wavelength laser as a local oscillator integrated in an asymmetric-twin-waveguide- based unit was demonstrated.
The highest-bandwidth heterodyne RF/optical links operating at 5 GHz and 16 GHz were demonstrated. These links were based on phase modulation and represent an extremely-high-sensitivity route to optical transmission of analog signals.

This work was done by Stephen R. Forrest of Princeton University for the Army Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Photonics category. ARL-0005