Tension-adjusted in-line optical-fiber attenuator

We describe an in-line, single-mode fiber attenuator using a combination of polarization-preserving and single-polarization fibers. The attenuation is adjustable over a 30-dB range by varying the birefringence of a short lengthof fiber with tension. The insertion loss is 1.3 dB. The tension birefringence arises from the difference in Poisson's ratio between the noncircular stress cladding and the silica substrate glass.