SBIR-STTR Award

Opportunities exist for analog links to be used in a broad range of RF, antenna and communications applications. However, performance and cost has been a drawback. We propose to incorporate the most advanced existing technology into analog link design, such as well developed techniques for impedance matching, plus contribute to the technology by developing simulation models and by novel application of a unique predistortion technique. A laser diode displays non-linear characteristics which can be understood by modelling the input impedance of the laser due to change of its bias with RF signal. This variable load impedance working against a fixed source impedance produces distortion. Feedforward techniques work well with near ideal and very wide bandwidth loop components. This scheme is good only up to frequencies for which the phase shift in the loop components is minimal. Our new approach uses the Gilbert technique of predistortion - a technique developed in the late sixties to linearlize the transconductance of a forward biased p-n junction. Th emethod we propose to model and employ, will pre-distort the modulating input in a manner opposite to the non-linear function, such that when the predistorted drive is applied to the laser diodes non-linearitites, the optical output will linearlize back and recover the original drive signal. The technique is remarkable in that it approaches the diode from a purely circuit perspective, not the physics perspective traditionally taken. This combination of applied circuit techniques will significantly advance the state-of-the-art by providing net link gain, SFDR above 120dB/rtHz 2/3 and NF less than 3 dB. Anticipated

Benefits:
Capitalizing on this technology by commercialization of high performance, low cost analog fiber optic links will expand the domestic photonics manufacturing industry. The specific markets to benefit are the booming commercial wireless markets such as cellular microcell remoting and PCS distributed antenna, as well as federal and defense related antenna and RF applications such as GPS and radar remoting. The added performance will expand the opportunities for this technologies insertion. The technique proposed to be developed will utilize conventional optoelectronic components with proven reliability and will significantly enhance their performance with innovative circuit design.

Keywords:
Laser Diode Analog Non-Linearities Predistortion Gilbert Impedance Matching

The feasibility of achieving the required performance for directly modulated Laser diode analog fiber optic links was proven in the Phase I effort. The performance level required of typical systems such as Radar and cellular antenna remoting calls for spurious free dynamic range of better than 120 dB/Hz (2/3) a noise figure of less than 4 dB and net link gain. We intend to further our work so as to develop a line of commercial fiber optic link products. We will refine the techniques used successfully in the Phase I effort, such as predistortion and linear amplifier designs, while adding new circuit techniques such impedance matching. Furthermore, we intend to extend the use of this technology, for the first time ever, to a new band of frequencies in the 2 GHz range. The end result will be the highest performance ever achieved for commercial RF fiber optic links operating in 450 to 2000 MHz range.

Keywords:
Laser Diode, Analog, Rf, Predistortion, Impedance Matching, Dynamic Range, Radar, Cellular