MEMS tunable VCSEL with polarization selected by sub-wavelength pitch grating

Area of Science:

• Optoelectronics
• Microelectromechanical Systems (MEMS)
• Nanophotonics

Background:

• Vertical-cavity surface-emitting lasers (VCSELs) are crucial for optical applications.
• Controlling polarization in VCSELs is essential for advanced functionalities.
• MEMS technology enables tunable VCSELs for wavelength control.

Purpose of the Study:

• To investigate a novel P-VCSEL design incorporating a sub-wavelength grating for polarization control.
• To understand the mechanism of polarization selection in the P-VCSEL.
• To evaluate the tuning range and suitability for optical coherence tomography (OCT).

Main Methods:

• Fabrication of a P-VCSEL by thermocompression bonding a 1/2-VCSEL with a semiconductor distributed Bragg reflector (DBR) to a silicon MEMS mirror.
• Etching a sub-wavelength pitch grating onto the 1/2-VCSEL surface.
• Optical layer stack simulations to predict tuning behavior.
• Experimental characterization of wavelength tuning and polarization behavior.

Main Results:

• Optical simulations accurately predicted wavelength tuning but not polarization behavior.
• Polarization selection is achieved through polarization-dependent loss in the sub-wavelength grating, not gain asymmetry.
• Rough grating sidewalls are identified as the cause of polarization-dependent loss.
• A tuning range of 91 nm in the 1050 nm band was demonstrated.
• High polarization-dependent loss reduced output power, particularly at shorter wavelengths.

Conclusions:

• The sub-wavelength grating effectively controls polarization in MEMS-tunable VCSELs.
• Polarization-dependent loss, attributed to grating sidewall roughness, is the key mechanism for polarization selection.
• The P-VCSEL demonstrates significant wavelength tuning, making it suitable for OCT applications despite power output limitations.