FIMMPROP

A Lensed Optical Fiber (Fiber to Chip Coupler)

Simulations with FIMMPROP software

FIMMPROP was used to model a fiber-to-chip coupler based on a lensed fiber design. In this model a standard single mode fibre is tapered and lensed to focus light into a InGaAsP/InP ridge waveguide chip, whose position is adjusted for optimal transmission.

Focusing and coupling to planar waveguide after a lensed fiber in FIMMPROP

This example illustrates the following capabilities for FIMMPROP:

• The calculation is extremely fast: a 3D fully vectorial solution is obtained in half a minute on a 4-core computer!

• FIMMPROP's rigorous full-vector 3D model is able to simulate diffraction effects as waveguide dimensions are tapered to very small dimensions. FIMMPROP can model both the small scale (lensed fiber thin film layers in the planar waveguides) and the large scale (input fiber).

• FIMMPROP can design optical tapers and lensed waveguides and simulate them using its adaptive taper algorithm.

• FIMMPROP can take advantage of FIMMWAVE's cylindrical solvers to model fully vectorial propagation in optical fibers extremely quickly. The variety of mode solvers associated with FIMMPROP allows it to efficiently calculate the coupling from fiber modes to planar waveguide modes, as FIMMPROP can combine cylindrical and Cartesian coordinate systems.

• FIMMPROP's Free Space Joint allows you to combine EigenMode Expansion (EME) in the waveguide sections with analytical Plane-Wave Expansion (PWE) in the free space region, allowing you to take advantage of the speed of PWE in regions where this method is suitable. 

Description of the structure

The specifications of the fibre are given in the table below; it is a typical single-mode fibre with an operating wavelength of 1.545um. 

This fibre is pulled linearly over a length of 100um, where the cladding diameter reaches 10um and the core diameter is 1.33um; the fibre is then rounded off to create a hemispherical lens. This lens is used to focus the light over a small air gap (5-10um) into the InGaAsP/InP waveguide.

The full structure is shown below. We combined FIMMPROP's fiber editor with the taper and lensed section design tools. Note how FIMMPROP is able to handle the variation in size along the structure. We used the semi-analytical and fully vectorial GFS Solver to solve the modes in the fiber sections, and the Cartesian FDM Solver in the planar waveguide section.

Full design shown in FIMMPROP. The fiber cross-section is shown at the left-hand side
and the planar waveguide cross-section in the insert.

The propagation in the air gap between the tip of the lensed fiber and the chip is simulated using FIMMPROP's Free Space Joint, which propagates an analytical plane-wave expansion of the near field. The Free Space Joint's bidirectional algorithm can also model thin film filter and AR coatings; in this case you could also study the effect of introducing an AR coating on the chip facet.

Simulation results

For this structure, FIMMPROP was able to calculate the scattering matrix and plot the intensity profile in ~half a minute on a 4-core i7-2600 PC. The intensity profile and the scattering matrix are shown below. At the end of the taper you can see the excitation of the high order modes of the fiber.

Intensity profile and scattering matrix

FIMMPROP was used to scan the evolution of transmission with distance between the tip of the lensed fiber and the chip facet, in order to optimise the focusing. The results are shown below - we find an optimal coupling of 34.0% for a gap of 7.0um. The oscillations correspond to constructive and destructive interference in the cavity formed by the two "facets".

Transmission (fraction of input power) versus gap length (in microns)

The vertical alignment was optimised in a similar way.

Similar simulations

Note that FIMMPROP can also model other types of fiber to chip couplers, including surface-emitting gratings, and various mode size converters (or spot size converters) such as grating-assisted mode size converters and inverted tapers.