Optical whispering-gallery-mode (WGM) micro-resonators based on SiO2

The FBH develops disk-type micro-resonators based on SiO2-on-silicon to replace macroscopic optical resonators of modest linewidth and to provide high-Q optical cavities for micro-integration into electro-optical modules.

The Joint Lab Laser Metrology  is in charge of component simulation and design as well as optical characterization and hybrid integration into modules. The development of the corresponding process technology is carried by the Process Technology Department .

Functional principle

REM-disk
REM-picture of a SiO2 disk resonator fabricated by FBH. The thin SiO2 disk (thickness ~2 µm) sits on top of a Si pedestal
Schematic for laser scheme with frequency stabilization
Schematic for laser scheme with frequency stabilization through resonant feedback from an external cavity (MIW)
resonance profile
Resonance profile of a typical WGM excited in a SiO2 disk resonator measured with a tuneable diode laser @ 780 nm

In thin disks of SiO2 with diameters ranging from a few tens to several hundreds of micrometers, modes of the optical field are guided close to the circumference by total internal reflection. These modes are known as whispering-gallery modes. The resonators are fabricated in-house from SiO2 on Si wafers. Due to the very good optical quality of thermally grown SiO2 and the excellent quality of the lithographic manufacturing process, material and guiding losses are so small that the light can propagate along the circumference for tens of nanoseconds before being absorbed or scattered. These disks hence constitute high-Q optical resonators with quality factors of more than 106 and corresponding resonance linewidths of less than 100 MHz. FBH focuses on technology approaches that are compatible with its III-V semiconductor process capabilities. The main challenge here is to integrate the light coupling optics and the resonator monolithically, i.e., “on chip” in order to allow for hybrid integration of chip-scale micro-resonators into electro-optical modules.

Application: Ultra-narrow linewidth laser module

Within the context of the EFRE-funded project iMiLQ a demonstrator for the SiO2 technology platform is being developed together with partners from the Humboldt Universität zu Berlin and two local SMEs, Brilliance Fab Berlin and PicoQuant.

A high-Q optical microresonator is used to provide resonant optical feedback to a laser diode to lock the laser frequency to that of the microresonator. As a result the frequency noise and corresponding laser linewidth is reduced and the laser frequency stability is improved by several orders of magnitude. The laser module will be fully micro-integrated into a custom made, CTE-matched butterfly housing without any moving parts.

Typical wavelengths

  • 780 - 1100 nm

Technology and packaging

  • semiconductor layers with metalorganic vapor phase epitaxy (MOVPE)
  • beam projection lithography
  • wet and dry etching

Funding

This work was supported by the European Fund for Regional Development (ERDF).