Micro-optical mode matching optimises waveguide coupling

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For interconnecting passive and active devices, interfaces have been developed and standardised mainly based on mechanical connectors of single-mode fibres. But individual devices can consist of various fibre-optic and planar-integrated waveguides, such as single-mode fibres, diode lasers, integrated planar amplifiers or diffraction gratings. They may feature clearly different geometries of their guided transverse mode, as can be seen in Figure 1.

Consequently, within a system the specific advantages and the high functionality of individual waveguide types can only be used with restrictions, since mode geometry differences can cause severe coupling losses.

For example, when a diode laser beam is coupled into a fibre and from there into a planar waveguide, the overall coupling efficiency may be <10%. When several systems of that kind are connected in series with each other, then often just a fraction of the input power can be detected at the output.

A possible compromise for a high coupling efficiency is tailoring the mode geometry by modification of waveguide shape. This option is used, for example, when fibres with a mode diameter of 10µm are coupled to arrayed waveguide gratings that feature waveguide entries with an increased cross section of approximately 10x10µm.

However, this type of mode matching can also have a significant negative effect on the function, particularly of active waveguide components. The performance of high-speed switches in optical router switching matrices, based on electro-optical modulators, is clearly reduced if a higher control voltage is required for compensation of a wider electrode distance, due to larger mode dimensions.

The reliable way to supply the M3, or matching mode micro-optics, with precise positioned fibres is through V-Groove arrays. These are structured in glass and have terminal expansion similar to M3 optics and the particular waveguide design.

We believe that this high-stability assembly from Limo is the only commercially available, industrial-scale technique to provide fibre for a waveguide with mode matching.

A solution for efficient waveguide coupling — especially in data storage, optical information and sensor technologies — is found in the use of suitable arrangements of refractive micro-lenses that enable a low-loss interconnection for almost all kinds of optical waveguides. Typically, the losses are below 1dB. The various coupling requirements can be accomplished by the so-called M3 systems.

These have been founded on the following five principles:

  1. Mode matching with anamorphotic micro-lens systems consisting of acylindrical lenses with diffraction-limited beam-forming.
  2. Parallel signal processing by coupling of arrays consisting of fibres, planar waveguides and micro-optics (Figure 2).
  3. Reliable supply of fibre through V-Grooves manufactured with alignment of the M3-optic.
  4. Thermal and mechanical system stability through selection of suitable optical glass materials and micro-lenses with integrated assembling surfaces, in favour of long-term stability and reproducible assembly.
  5. Manufacturing of micro-lenses on a wafer base, which has already been the standard in microelectronics for many years. This enables flexible up-scaling of production capacities.

Combination of all five guidelines during development and production of M3 systems allows for coupling of any kind of waveguide components with symmetrical or elliptical mode geometries.

What can be achieved with aspherical or acylindrical micro-optics would often hardly be manufactured at all in a comparable way with conventional macro-optics. Due to optimised coupling efficiencies, the use of active optical elements for the compensation of dissipated power usually becomes obsolete.

Therefore, the complete system can be manufactured considerably more economically. Especially in the case of modulators and switches, signals such as those from circular fibres can be coupled into planar waveguides with a mode field diameter of, for example, 2x10µm.

The user of this technology can optimise the performance of modulators without needing to consider the geometry of subsequent waveguides. The application of micro-lenses as an interface between different types of waveguides extends system design flexibility and reveals new options in optical signal processing and transmission.

Dirk Hauschild is VP Micro Optics at Limo (Lissotschenko Mikrooptik), a 10-year-old private company with 130 employees. Contact details: Bookenburgweg 4, 44319 Dortmund, Germany. Tel: +49 231 97 53 24-0. www.limo.de

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