Polarization dependence in Ti-diffused LiNbO3 directional couplers.

Area of Science:

• Materials Science
• Optical Engineering
• Photonics

Background:

• Polarization dependence in directional couplers affects performance in optical systems.
• Titanium (Ti) diffusion in lithium niobate (LiNbO3) is a common fabrication method for optical waveguides.
• Optimizing splitting ratios is critical for applications like tunable filters.

Purpose of the Study:

• To investigate the impact of design and fabrication parameters on polarization-dependent splitting ratios in Ti:LiNbO3 directional couplers.
• To identify specific parameters that yield a polarization-independent splitting ratio (TE + TM = 1).
• To minimize transmission losses for both transverse electric (TE) and transverse magnetic (TM) polarizations.

Main Methods:

• Experimental investigation of directional couplers fabricated using Ti diffusion in LiNbO3.
• Parametric study varying waveguide separation, Ti film thickness, and diffusion time at 1025°C.
• Evaluation of splitting ratios for TE and TM polarizations at a 1.54 µm wavelength.
• Comparison of transmission losses for different input/output waveguide bending angles (0.6° vs. 1.0°).

Main Results:

• Specific fabrication parameters (3.5 mm long, 7 µm wide Ti strips, 11 µm separation, 11 h diffusion) yielded a sum of TE and TM splitting ratios close to unity.
• A diffusion temperature of 1025°C effectively suppressed outdiffusion effects.
• A 0.6° bending angle for input/output waveguides resulted in significantly lower transmission loss (>1 dB reduction) for both polarizations compared to a 1.0° angle.

Conclusions:

• Optimized Ti diffusion parameters enable the creation of polarization-independent directional couplers in LiNbO3.
• These couplers meet the relaxed beam-splitter requirements for electro-optic and acousto-optic tunable filters.
• Waveguide bending angles play a crucial role in minimizing insertion loss, with shallower angles being preferable.