Edwin Jan Klein - Universiteit Twente

Chapter 7
largely solved, however, by placing the bus and add-drop waveguides in different
layers [146] The vertical distance between the add-drop and bus waveguides is 250
µm so that the device can be used in combination with standard fiber-arrays. The size
of the OADM, 1.5 mm by 0.2 mm, is mainly determined by this spacing. All
waveguides are horizontally tapered down to 1.5 µm at the facets. As shown in Figure
7.7 this can reduce the fiber chip-coupling losses from 6.5 to 6.0 dB for a standard
(SMF, Mode Field Diameter 10.2) fiber and from 4.3 to 3.9 dB for a small-core (HI-
1060, MFD 5.6) fiber.
For the resonators in the OADM the standard vertically coupled microring resonator
building block described in Chapter 6 was used, following the design parameters
described in Table 6.1 except for the thickness of the resonator which was set at 190
nm.
Fiber chip coupling loss (dB)
7
6
5
4
3
2
0 1 2 3 4 5 6
Waveguide width (µm)
164
Overlap with SMF
Overlap with smallcore
Figure 7.7. Fiber-chip coupling loss with a standard or a small-core fiber as a function of the
waveguide width for a Si3N4 SiO2 waveguide with a thickness of 140 nm.
The OADM was fabricated using the “Vertical I” fabrication process and
subsequently pigtailed and packaged so that the device could be fully characterized
[147]. Figure 7.6 shows a close-up of a fabricated OADM. The four add/drop and
central waveguides are clearly discernable as well as the omega shaped heater
elements on top of the four micro-ring resonators.
Two photographs of the pigtailed OADM chip are given in Figure 7.8 In these
photographs the fiber arrays, glued to both sides of the chip, can be seen. Also
discernable are the glass rails on top of the chip as well as the bond wires. The chip
measures 10 mm by 9 mm. Although the actual OADM measures only 10 mm by 1.5
mm, the width of 9 mm is required to provide the fiber arrays with enough space to be
glued onto. The chip that is shown therefore actually contains 4 OADMs.