Compound parabolic concentrators for narrowband wireless infrared receivers

The desire for inexpensive, high-speed wireless links has motivated recent interest in infrared free-space communication systems.15 As a medium for short-range wireless communication, infrared offers significant advantages over radio. The infrared spectrum represents an immense, unregulated bandwidth. Moreover, infrared radiation does not pass through walls, allowing the operation of at least one infrared link in every room of a building without interference. When an infrared link employs intensity modulation with direct detection (IMIDD), the short carrier wavelength and large square-law detector lead to efficient spatial diversity that prevents multipath fading.5 By contrast, radio links are typically subject to large fluctuations in received signal magnitude and phase. However, the infrared medium is not without drawbacks. In many indoor environments there exists an intense infrared ambient radiation, arising from sunlight, incandescent lighting, and fluorescent lighting, which induces noise in an infrared receiver. In many systems, it is desirable to employ nondirectional transmitters and receivers, alleviating the need for alignment between them, but this leads to a high path Abstract. We discuss the design of optical concentrators based on dielectric and hollow compound parabolic concentrators (CPCs), for use in free-space infrared communication receivers. In order to acheive a high signal-to-noise ratio in a direct-detection receiver, it is desirable to use an optical bandpass filter that passes the signal but attenuates ambient radiation. Placement of a planar bandpass filter at the entrance aperture of a CPC results in a receiver having a narrow passband and high gain, but a narrow acceptance angle. The addition of a second, angle-transforming CPC at the entrance aperture allows the receiver to achieve simultaneously a narrow passband and an acceptance angle approaching 90 deg. We have employed a Monte Carlo ray tracing method to calculate the optical gains of several optical concentrator designs. We find that the optical gains of single and double CPCs are, respectively, about 94% and 93% those of ideal optical concentrators, while addition of planar bandpass filters decreases these gains to about 88% and 86%, respectively. We compare the performance and size of CPC-based concentrators with those based on dielectric hemispheres fitted with hemispherical bandpass filters.