Range extension of the optical delay line in white light interferometry

We propose an effective and promising method to extend the continuous range of the optical delay line in white light interferometry. The method adopts the combination of switchable fixed delays and a continuous-scanning stage laid in one arm of the interferometer. Moreover, a fiber ring constructed by a fiber coupler is employed in the other arm to achieve self-calibration and improve the delay precision, voiding the use of a specialized measurement instrument. The range-extension method maintains the original performances of the stage just through the process of orderly copying and jointing. The setup with 16 parallel fixed delay paths is experimentally demonstrated. The insertion loss of the optical delay line will not increase as the range extension and keeps an average of 1.45 dB with a fluctuation of ±0.15  dB(3σ) over a delay range of 37.2 m without a blind zone. In addition, the delay precision as a function of the port number of the optical switch does not deteriorate and remains at a level of 10.21 μm with a standard deviation of 0.08 μm. The loss and delay precision features enable the proposed structure to be extended without limitation, which is particularly significant for practical applications.