Mode-independent attenuation in evanescent-field sensors

Area of Science:

• Optical sensing
• Waveguide theory
• Spectroscopy

Background:

• Evanescent-field sensor waveguides typically exhibit nonproportional power attenuation due to differing modal attenuation constants.
• Direct absorbance measurements are complicated by waveguide sensitivity to input launching conditions.

Purpose of the Study:

• Derive a general asymptotic formula for modal power attenuation in asymmetric planar waveguides with weak absorption.
• Provide explicit expressions for various refractive-index profiles.
• Investigate the ratio of TM versus TE absorption.

Main Methods:

• Developed a general asymptotic formula for modal power attenuation.
• Derived explicit expressions for specific refractive-index profiles (linear, step, parabolic).
• Calculated the ratio of TM to TE absorption.
• Performed experimental verification for a second-order polynomial profile.

Main Results:

• The derived asymptotic formula for modal power attenuation shows good agreement with exact calculations, except near cutoff.
• Waveguides with a linear refractive-index profile exhibit mode-independent attenuation coefficients for each polarization.
• The asymptotic sensitivity is wavelength-independent, facilitating direct spectral absorption measurements.
• Experimental verification confirmed mode independence for profiles near linear distributions.

Conclusions:

• A new theoretical framework allows for accurate prediction of modal power attenuation in evanescent-field sensors.
• Linear refractive-index profiles are crucial for developing sensors capable of direct spectral absorption measurements.
• The findings contrast with step-profile waveguides, where attenuation is highly mode-dependent.