High-sensitivity external modulator data link for high-energy particle detector diagnostics

An analog fiber-optic data link has been demonstrated for direct charge readout of wire chambers used in high energy particle detectors. The fiber link consists of a Nd:YAG laser carrier, a Mach-Zehnder external modulator, and a low-noise optical receiver. A charge pulse developed on a sensing wire flows directly into the electro-optic modulator with no preamplification. The substitution of passive modulators and fiber cable for active electronics and copper wire near large collider detectors has clear advantages with regard to radiation damage susceptibility, EMI immunity, physical size, and power consumption. Also, the modulator performance was unaffected for an applied one Tesla magnetic field. Reduction of noise contributions from the laser carrier and optical receiver and exploitation of the 'effective optical gain' properties of external modulators resulted in shot-noise-limited link sensitivity. Modulators were packaged with large termination resistors enabling increased charge sensitivity with an increased system risetime tradeoff. Energy resolution of the fiber link was comparable to preamps conditional on large terminations and subsequent slow readout. For shorter risetimes required in timing diagnostics, small termination resulted in sensitivity 6.8 times inferior to preamps. Several potential improvements in sensitivity are discussed.