Highly efficient grating couplers

The first step in increasing efficiency is to better match the mode profile of the grating coupler to the fiber profile by no longer making the grating strictly periodic. Using an algorithm developed at INT, the ridge and groove width of the coupler is optimized for efficiency.

When diffracting at the grating, higher diffraction orders lie outside the acceptance angle of the waveguide and are normally radiated through the coupler into the substrate.

These losses are significantly reduced by a rear mirror coating if the light reflected by the mirror interferes constructively with the directly incident light.

The standard grating couplers with a width of approx. 15 µm must be aligned with the actual waveguide with a width of approx. 400 nm via taper structures. To minimize losses during the transition, the tapers are very long (typically 400 µm), which requires a considerable amount of space.

The bars and grooves of the diffraction grating can also be curved so that the diffracted wavefront converges at a focal point. This allows the taper to be significantly shorter, at approx. 10 µm in length, as the focused wavefront already causes a narrowing of the mode cross-section.

Grating couplers only achieve their maximum coupling efficiency at a specific wavelength. Efficiency decreases significantly at longer and shorter wavelengths. A grating coupler is therefore characterized not only by its maximum coupling efficiency but also by its 1 dB or 3 dB bandwidth.

The highly efficient couplers developed at INT have 1 dB bandwidths in the range of 35 nm. For many applications, however, it is advantageous if the couplers have as constant a coupling efficiency as possible over a much larger range. Therefore, grating couplers are also specially optimized for this property.

In optical communications, polarization multiplexing is often used to double the data rate on a fiber. To do this, the two data channels are modulated onto two orthogonal polarization states of the same carrier wavelength. In the receiver, the two polarization states must be separated again.

Conventional grid couplers have high coupling efficiency for one input polarization, while the other polarization is virtually not coupled. The power is thus lost. However, with appropriate dimensioning of the grating, it is possible to achieve efficient coupling of both polarizations, whereby the propagation directions in the waveguide are then opposite. This divides the two polarization directions spatially between two waveguides, with a TE wave propagating in one waveguide section and a TM wave in the other.