the light 24 entering at end 26 to be compared and the phase angle relative to each other to be measured as the optical fiber tuft 12 is bent as shown in fig3 b responsive to aerodynamic flow separation the bent end portion 28 of the fiber tuft 12 causes a change in the relative length of the paths between the entering and exiting lights 24 24 a as illustrated by the arrows thus the light 24 a leaves the fiber tuft end 26 at a different phase angle relative to the source light 24 inasmuch as the effective path length of the light has been changed responsive to the bending of the optical fiber tuft 12 thus the obtained phase angle is an indicator of the amount of strain or stress being encountered by the structure mounting the tuft 12 as shown in fig4 of the drawing there is represented a birefringence active phase feedback system 30 wherein there is implemented an open loop phase measurement of the phase angle in essence the source light 32 enters the optic fiber tuft 34 at one end and exits at the opposite end 36 this end 36 being bendable in response to flow separation a phase comparator 40 which also receives source light 32 measures the differential between the entering and exiting fiber light 32 so as to determine the phase angle 38 alternatively in a closed loop phase measurement system 50 as illustrated in fig5 of the drawings a phase.