Optical Filter Design and Analysis: A Signal Processing Approach

CHAPTER SEVEN

Optical Measurements and Filter Analysis

In this chapter, an introduction to optical measurements is given which focuses on the characterization of filters. For a broader discussion on optical measurements, see [1]. We begin with the direct measurement of loss versus wavelength followed by techniques for measuring polarization dependent loss. For planar waveguide filters, the uncertainty in the fiber-to-waveguide coupling loss makes it difficult to characterize loss over small lengths. An indirect method for characterizing loss and coupling ratios is presented that avoids the coupling loss uncertainty. Complete characterization of optical filters requires information on both the magnitude and phase (or group delay) response. Direct and indirect methods for group delay measurement are presented.

In Section 7.2, the inverse problem of filter analysis is treated, whereby measurements are made to infer the filter's constituent components. For a waveguide filter, the constituent elements are the coupling ratios and phases for each stage. For a reflective filter, the components are the partial reflectance values and phases for each stage. As discussed in Chapters 4–6, multi-stage narrowband filters are sensitive to fabrication variations. Filter analysis techniques combined with post-fabrication tuning methods are critical to successfully demonstrating complex optical filters with tight performance specifications. Three analysis algorithms are discussed which have been ...