Beat-length measurement in directional couplers by thermo-optic modulation

Area of Science:

• Integrated optics
• Waveguide optics
• Optical couplers

Background:

• Directional couplers are key components in integrated optical circuits.
• Understanding supermode propagation constants is crucial for device design.
• Existing methods for measuring propagation constant differences can be complex.

Purpose of the Study:

• To develop a novel, localized heating method for measuring propagation constant differences in directional couplers.
• To determine the modal beat length (Lambda) with high accuracy.
• To demonstrate the technique's effectiveness in K-ion exchanged glass waveguides.

Main Methods:

• Fabrication of integrated-optical directional couplers using K-ion exchange in glass.
• Localized heating of one waveguide using a fine spot to induce coupling.
• Scanning the heat spot along the coupler and measuring output power variations.
• Calculating the modal beat length from the periodicity of power variations.

Main Results:

• Periodic variations in output power were observed as the heat spot was scanned.
• The modal beat length (Lambda) was directly correlated to the measured power variations.
• Beat lengths ranging from 0.6 to 2.2 mm were accurately measured (+/-0.3%).

Conclusions:

• Localized heating provides an effective and accurate method for measuring modal beat lengths in directional couplers.
• The technique offers a new way to characterize integrated optical devices.
• This method is suitable for K-ion exchanged glass waveguides and potentially other integrated optical platforms.