B.S.T.J. Briefs: Optical Waveguides with Very Low Losses

B.S.T.J. BRIEF Optical Waveguides With Very Low Losses By W. G. F R E N C H , J. B. M A C C H E S N E Y , P. B. O ' C O N N O R , a n d G. W. T A S K E R (Manuscript received April 25, 1974) Low-loss optical fibers may be necessary for economical optical transmission systems. We have developed fibers t h a t exhibit losses of less t h a n 2 d B / k m , at 1.06 ^m. T h e fibers were made by a chemical vapor deposition (CVD) technique t h a t employs simultaneous reaction and fusion to a clear glassy core material. Two fiber compositions have been used. In t h e first fiber, a GeC>2doped fused-silica core is deposited inside a fused-quartz t u b e t h a t acts as the cladding after the t u b e is collapsed into a rod and pulled into a fiber. Figure 1 shows the loss spectrum of a fiber made in this manner. T h e fiber is 723 m long and has a core approximately 35 /xm in diameter. T h e numerical aperture is 0.235. T h e loss decreases by approximately X -4 , the expected Rayleigh scattering dependence, to a minimum just under 2 d B / k m at 1.06 jum. Hydroxyl-ion-related absorptions at 0.72, 0.88, and 0.95 jum are low, amounting to less t h a n 10 d B / k m at 0.95 nm. We believe t h a t the OH impurities causing these absorptions are due to siloxane present in the SiCl 4 starting material. This can be removed by fractional distillation, and loss peaks due to the hydroxyl-ionrelated absorptions as low as 2 d B / k m above background at 0.95 nm have been observed in similar fibers.