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One sided optical mode-expanded SOA for efficient optical coupling between waveguide circuits and optical fibre Disclosure Number: IPCOM000008029D
Publication Date: 2002-May-13
Document File: 6 page(s) / 40K

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A semiconductor optical amplifier with a single mode expander can be used to couple optical signals from silica to semiconductor, and provide optical amplification of the signal

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‘One sided’ optical mode-expanded SOA for efficient

optical coupling between waveguide circuits and optical fibre

Semiconductor optical amplifiers (SOAs) are typically formed with a waveguide in an active semiconductor material. For operation at telecommunications wavelengths in bands centred at 1310nm and 1550nm this semiconductor is commonly based on indium phosphide (InP) substrates. The dimensions of the SOA waveguide are typically such that the size of the optical mode supported in the waveguide may be typically in the range 1-2mm. Thus is significantly smaller than the optical mode (commonly referred to as the optical mode field diameter (MFD)) in standard telecoms optical fibre (e.g. SMF-28), which is commonly in the range 9-10mm. To be useful in practical applications, SOAs typically need to be coupled to optical fibre. The efficiency of this coupling is important, because optical losses generally degrade the performance of the system, typically through increased noise figure or decreased optical power. A simple ‘butt coupling’ between an SOA and a standard cleaved optical fibre will generally result in an unacceptably large loss.

In practice efficient optical coupling between standard optical fibre and the waveguide within a SOA is typically done by using a lens, either as a separate piece between the fibre and the SOA or as a lens formed on the end of the fibre.

Another approach to obtaining efficient optical coupling between SOA and fibre is to expand the optical mode size in the SOA as it approaches the edge of the chip, so that the optical modes in SOA and fibre are approximately the same size and there is efficient optical coupling without the need for a lens. These approaches are shown schematically in Fig. 1.

A schematic geometry of mode-expanded and non-mode expanded SOAs are shown in Figs 2a and 2b. More details of one approach to forming the mode-expanding taper structure are shown in Fig 3.

Generally, SOAs are mode-expanded to allow efficient coupling to standard optical fibre at each end. In some circumstances it is desirable and useful to optically couple the SOA to a passive waveguide circuit at one end, while the other end is coupled to optical fibre. This may be the case...