Patent ID: 6292611
Filing Date: 2001-09-18
Classification: G02B

Abstract:
An optical fiber cable comprising:a longitudinally extending central strength member;at least one substantially fluid impervious, flexible plastic buffer tube S-Z wound around said central member in an alternating hand helix, said alternating hand helix having a lay length of from about 400 mm to about 600 mm, said tube having an axis and having a wall around the axis with a thickness and an inner and an outer diameter;a plurality of superimposed ribbon units in a stack loosely received in said tube, each of said ribbon units comprising a plurality of optical fibers held together in side-to-side relation in a non-conductive material, said stack of superimposed ribbon units having a longitudinal axis, having an axial length from about 0.1% greater than the axial length of said tube to about 0.2% less than the axial length of said tube and having a width transverse to the stack axis and a thickness transverse to the stack width; said plurality of ribbon units being helically twisted around the axis of the stack with a pitch of from about 250 mm to about 1000 mm in said tube;wherein, the ratio of said pitch of said plurality of superimposed ribbon units to said lay length of said tube is from about 0.5 to about 1.2;wherein there is a clearance C between said plurality of superimposed ribbon units and the inner diameter of said tube wall is between about 1 mm and about 2 mm with the relation;C=(T.sub.I.sup.2 -W.sub.R.sup.2).sup.1/2 -H.sub.Rwhere T.sub.I is the inner diameter of said tube, W.sub.R is the width of said stack and H.sub.R is a thickness of said stack;a sheath of plastic material encircling said central strength member and said tube wound around said central member; andwherein said ratio of said ribbon pitch to said tube lay length, said clearance and said axial length of said stack are selected to provide optical signal attenuation in the optical fibers of less than 0.30 average dB/km at a 1550 nm wavelength.