Miniature fiber optic isolators with diameters of <4 mm are available from Oz Optics Ltd. The company says they block reflected signals in fiber optic systems and eliminate feedback problems. The devices offer isolation levels as high as 60 dB, insertion losses as low as 0.6 dB and a return loss of -60 dB. Polarization-maintaining and polarization-independent models are available for several wavelengths, including 1300 and 1550 nm. The input and output fibers can be either single-mode or polarization-maintaining fibers.