Passively aligned integrated regenerator offers low coupling loss
At ECOC 2002 (Copenhagen, Denmark), Graeme Maxwell and others at Corning Research Centre (Martlesham Heath, Ipswich, England) reported integrating a low-coupling-loss all-optical regenerator using several passive alignment techniques.1 The researchers know of no similar devices that have coupling losses as low as their device's 4.37 dB. Using the device, the researchers demonstrated penalty-free 2R regeneration at 40 Gbit/s. Other optical processing units such as nanosecond switch arrays, could be integrated using similar hybrid techniques.
Integration reduces the cost, size, and power consumption of optical systems, but the labor-intensive active alignment of integrated optics is expensive. The Corning researchers used passive alignment methods that simplify assembly. "It should now be possible to build a wide variety of hybrid modules using the same technique and we should now be able to use automated equipment for the assembly," said Maxwell.
The regenerator is an integrated nonlinear Mach-Zehnder interferometer. The integration techniques include flip-chip bonding of active devices onto silicon substrates, silicon-submount technology of daughterboards onto a motherboard, and micromachined V-groove technology.
Semiconductor optical amplifiers (SOAs) were flip-chipped onto the silicon daughterboards. The micromachined daughterboards enable passive positioning in both lateral and vertical directions to align the SOAs with waveguides on the motherboard (see figure). Another, arrow-head-shaped silicon micromachined V-groove assembly was used to provide passive pigtailing of fiber arrays to the motherboard waveguides.
The separation of alignment functions between the silica-on-silicon planar waveguide motherboard and silicon daughterboards allows each board to be optimized for higher yield. The top surface of the motherboard was chosen as the reference surface for alignment. This simplifies the processing required for the silica waveguide motherboard and allows good vertical alignment as long as the cladding thickness and uniformity, and bowing in the wafer are well controlled. Lateral positioning is handled both by using mechanical end stops patterned on the top of the cladding and holes in the motherboard designed to accommodate the SOAs. The V-groove's depth was chosen so that the fiber core height after assembly is equal to the distance between the center of the waveguide core and the top of the cladding on the motherboard.
Light coupled from an input fiber to a waveguide to an SOA and out to a waveguide and output fiber demonstrated total losses as low as 4.37 dB. The 2R regenerator (which restores both the amplitude and shape of the signal) demonstrated penalty-free operation at 40 Gbit/s using only 6-dBm total input signal power.
For more information contact Graeme Maxwell at firstname.lastname@example.org.
- G. Maxwell et al., ECOC 2002, Postdeadline paper 3.5, Copenhagen, Denmark (Sept. 8-12, 2002).