Polarization-dependent gain characterization in x-cut LNOI erbium-doped waveguide amplifiers

Area of Science:

• Photonics
• Materials Science
• Optical Engineering

Background:

• Erbium-doped waveguide amplifiers (EDWAs) are crucial for photonic integration.
• Current EDWAs predominantly use z-cut lithium niobate, which is incompatible with the mainstream x-cut platform.
• This incompatibility hinders the development of large-scale integrated photonic systems.

Purpose of the Study:

• To investigate the polarization-dependent optical gain in x-cut Erbium-doped lithium niobate on insulator (Er:LNOI).
• To theoretically analyze and experimentally validate the gain characteristics for different polarizations.
• To demonstrate the feasibility of optical amplification on the x-cut LNOI platform for advanced photonic applications.

Main Methods:

• Utilized Judd-Ofelt theory to analyze the influence of crystal orientation on TE-mode coupling with Er³⁺ ions.
• Calculated and compared transition strengths for alpha (α) and pi (π) polarizations.
• Conducted experimental measurements of absorption, emission cross-sections, and gain coefficients at 1531 nm and 1550 nm.

Main Results:

• Judd-Ofelt analysis predicted significant differences in transition strengths between α- and π-polarizations.
• Experimental results confirmed α-polarization exhibits 1.8 times larger absorption and emission cross-sections at 1531 nm.
• At 1550 nm, α-polarization achieved a gain coefficient of 3.3 dB/cm, compared to 2.2 dB/cm for π-polarization.
• The α-polarized amplifier demonstrated 32.01 dB signal enhancement and 11.18 dB internal net gain in the small-signal regime.

Conclusions:

• This work successfully demonstrates feasible optical amplification on the x-cut LNOI platform.
• The findings highlight the superior performance of α-polarization for optical gain in this material system.
• The results provide essential support for the development of large-scale photonic and microwave photonic systems utilizing x-cut LNOI.