Apparatus and Method for Stabilizing Power to an Optical Multimode Receiver
One of the biggest challenges in free-space optical (i.e., laser) communications over long distances is that the varying atmospheric conditions between the transmitter and receiver cause large and rapid fluctuations in the power of the optical beam striking the receiver. To maximize signal availability, the receiver must provide the largest dynamic range possible.
Multimode-based receivers (as opposed to single-mode) are an appealing technology for moderate Gbps class data rates because the larger size of the multimode fiber relaxes the pointing and tracking requirements of the optical antennas and eliminates the need for expensive, high-end adaptive optic systems.
The Avalanche Photo Diode (APD), which operates at high voltages, is the most readily available device providing the best possible receiver sensitivity in a multimode-based architecture. An important consideration, however, is that these receivers have limited dynamic ranges and are susceptible to catastrophic damage from sudden excess optical power--thus their usability without protection is limited for free-space optical communications.
To address the limitations of the APD receiver, a variable optical attenuator (VOA) in front of the APD can be used to protect it from high optical power situations during normal operation. However, even with a VOA responding to power fluctuations, the control system has a finite response time and there are several conditions that the system must be designed to protect against. Examples of these include system startup with optical power present or the optical link dropping and then suddenly coming back online while the VOA attenuation is set to minimum because of the lack of an incoming signal.
Compounding the challenge in designing a robust multimode receiver architecture is the fact that multimode VOAs can have highly temperature-dependent, non-monotonic, non-linear attenuation responses. Because a control system prefers a linear function, such devices create high demands on the signal processor.
With a detailed analysis and careful design, however, researchers at the The Johns Hopkins University Applied Physics Laboratory have developed a patent-pending solution that includes control methods that protect the system during startup and link outages and an algorithm, based on the limited processing capabilities of an embedded 16-bit processor, that successfully stabilizes received optical power using non-linear, highly temperature-dependent optical attenuators and provides a 40-dB dynamic range and a 10-μs system response time. The design is translatable to even more limited 8-bit class processors.
This system has been proven in operational optical modems communicating at multi-gigabit bandwidths over distances exceeding 100 km.
Patent Status: U.S. patents pending.CONTACT:
Mr. M. T. Hickman