We present a new detection instrument for chemical/biological fluorescence lifetime-based sensors. The instrument comprises a primary, closed loop with a secondary loop controlling a variable phase delay within the primary loop. The primary loop consists of a fluorescence excitation light source, a fiber-optic delay line (with a gap for placement of a fluorescent sensor), an electronic phase shifter, a photo-detector, and a resonance-type RF amplifier. The secondary loop consists of a long-wavelength-pass optical filter, multimode fiber, a PMT, and an electronic phase detector (which is connected to the phase shifter of the primary loop). The system exhibits self-oscillations in the form of RF sinusoidal intensity modulation with frequency dependent on the fluorescence lifetime. Since the primary loop does not contain an optical filter, it is easier to obtain self-oscillations (compared to single loop systems). The feedback also improves the stability of the detection platform. The detection system is simple, inexpensive, and scalable for sensor array purposes. We demonstrate the use of a cost-effective, multi-channel, computer-algorithm-based frequency counter with this new system. We illustrate the detection capabilities of this detection system with the pH-sensitive, fluorescent probe carboxy seminaphthofluorescein (SNAFL-2) and an immunosensor based on fluorescence resonance energy transfer.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.