Instrumentation of the Very Forward Region at Future Linear Colliders - design and R&D by the FCAL Collaboration

Two finely segmented and very compact calorimeters are foreseen for the instrumentation of the very forward region of the ILC and the CLIC detectors; a luminometer to measure the rate of low angle Bhabha scattering events with a precision better than $10^{-3}$ and, at lower polar angles, a detector for fast luminosity estimate. Both devices will improve the hermeticity of the detector in searches for new particles. Due to the expected high occupancy, fast front-end electronics is needed. The calorimeter closest to the beampipe will be hit by a large amount of beamstrahlung remnants, so that radiation hard sensors are required. Results on the performance of the sensor and readout electronics prototypes are summarized.